Reviews
H. Meier
Conjugated Oligomers
Conjugated Oligomers with Terminal Donor–Acceptor Substitution Herbert Herbert Meier*
Keywords:
absorption · conjugation · intramolecular charge transfer · nonlinear optics · oligomers
An A ngewandte
Chemie
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2005 Wiley-VCH Wiley-VCH Verlag Verlag GmbH GmbH & Co. KGaA, KGaA, Weinheim Weinheim
DOI: 10.1002/anie.200461146
Angew. Chem. Int. Ed. 2005, 44, 44 , 2482 2482 – 2506 2506
Conjugated Oligomers
onjugated oligomers represent a prominent class of compounds C onjugated from a viewpoint of their theory, synthesis, and applications in materials science. Push-pull substitution with an electron donor D at one end of the conjugation and an electron acceptor A at the other end results in them having outstanding optical and electronical properties. This Review highlights fundamental synthetic strategies for the preparation of such oligomers with n repeat units ( n 1, 2, 3, 4, …) and the rules that govern their linear and nonlinear optical properties (absorption, frequency doubling and tripling). The unification of chemical, physical, and theoretical aspects with an interdisciplinary image of this class of compounds is attempted herein. =
1. Introduction Conjugated oligomers are target compounds for numerous applications in materials science because of their interesting electrical, optical, and optoelectronic properties and they they are also also mode modell comp compou ound ndss for for the the corr corresp espon ondi ding ng [1] conjugated polymers. A topic of high topicality topicality in terms of nonlinear optics and electroluminescence concerns p systems substituted with donor and acceptor groups in which con jugated oligomers form the p-electron linker. The compounds can have a linear or a star-shaped architecture. Scheme 1 provides an overview of the most important structural types.
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From the Contents 1. Introduction
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2. Long-Wavelength Long-Wavelength Electron Transitions in Conjugated Oligomers 2484 3. Push-Pull-Substituted Oligomers: Synthetic Concepts and Absorption Behavior
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4. Nonlinear Optics in Series of Oligomers with Donor–Acceptor Substitution
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5. VB and MO Models of D- p -A -A Systems
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6. Summary and Outlook
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The push-pull effect of this class of compounds depends on the strength of the donor and acceptor groups; however, it also also depe depend ndss on the the conju conjuga gate tedd p syst system em,, to whic whichh a zwit zwitte teri rion onic ic reso resona nanc ncee stru struct ctur uree shou should ld cont contri ribu bute te (Scheme 2). The energy of the dipolar resonance structure is
Scheme 2. Participation of zwitterionic resonance structures for the
illustration of the push-pull effect in conjugated oligomers.
determined by the charge separation as well as the change in the p system. The latter influence is certainly smaller for an oligoene chain ( 1) than for repeat units consisting of aromatic rings (2), whose zwitterionic resonance structures have pquinoid character. Several parameters, such as BLA,[2–5] MIX,[6] and c2,[7–9] have been suggested for quantification of the contribution of zwitteri zwitterioni onicc resona resonance nce struct structure ures. s. This This will will be discus discussed sed further in Section 5. However, it should be noted here that the “weight” of resonance structures depends on external factors such as the solvent or an applied electrical field.
Scheme 1. Construction of donor–acceptor-substituted conjugated oligomers consisting of donor groups D, a p -electron linker, and
acceptor groups A; selected examples are shown. Angew. Chem. Int. Ed. 2005, 44, 44 , 2482 2482 – 2506 2506
[*] Prof. Dr. H. Meier Johannes Gutenberg-Universitt Duesbergweg 10–14 55099 Mainz (Germany) Fax: ( 49)6131-392-5396 E-mail:
[email protected] [email protected]
DOI: 10.1002/anie.200461146
2005 Wiley-VCH Wiley-VCH Verlag Verlag GmbH & Co. KGaA, Weinheim Weinheim
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Reviews
H. Meier
A special case, which is occasionally referred to in this article, is represented by the symmetrical, charged, all- E configured polymethines 3 a and 3 b (Scheme 3). At larger values of n (beyond the so-called cyanine limit) it needs to be
oligomers to converge towards a certain limiting value E for n ! . The hyperbolic function described by Equation (1)
¥
¥
ð Þ ¼ c þ f ðn þ1 1Þ
ð1Þ
E n
seems to be adequate for polyenes. However, simple HMO theory [Eq. (2)] supplies the limiting value (zero). Only the E ðnÞ ¼ c ¼ 0 lim !1
ð2Þ
n
Scheme 3. Symmetrical charged polymethines (cyanines).
considered[10] whether the resonance should be substituted by a fast equilibration (automerization) as soon as the C 2v symmetry is abandoned in favor of a C s symmetry.[10–16] A special aspect of series of conjugated oligomers is given by the expectance that certain properties P(n) converge towards a limiting value P for increasing numbers n of repeat units, or at least their derivatives dP( n)/dn converge towards P . The long-wavelength electron transition S 0 !S1 provides an example of the first case [ lmax(n)! l ],[17–22] while the hyperpolarizability of second order g is an example of the latter case [dg(n)/dn !g ]. [23] In most cases l max(n) increases monotonously with n and reaches the limiting value l at the so-called effective conjugation length nECL.[1a,18] In contrast, the slope of the curves g(n) and logg(n), respectively, decreases with increasing n .[23,24] Recently it was found that certain conjugated oligomers with terminal donor–acceptor substitution can exhibit a monotonously decreasing value for lmax with increasing numbers n of repeat units; [25] the behavior of the hyperpolarizabilities b and g of such series is currently unknown. Both effects will be discussed in Sections 3 and 4, while quantum mechanical models for D-p-A systems will be discussed in Section 5. ¥
’
¥
¥
consideration of perturbation theory of first or second order results in a limiting value which is different from zero [Eq. (3)].[26] D b is the difference in the resonance integrals of 4 E ðnÞ ¼ D b > 0 lim p !1
neighboring bonds. The perturbation is based on the fact that polyenes have alternating single and double bonds of different lengths and consequently different b values. Wenz, Wegner et al. derived a function on the basis of the electron gas theory[27,28] as Equation (4) with a limiting value
’
¥
ð Þ ¼ c þ f ðn þ10:5Þ
of c ¼ 6 0.[29] Root laws, such as Equation (5) used by Drefahl
ð Þ ¼ c þ bp nffi ffi
ð5Þ
lmax n
and Pltner[30] for the long-wavelength absorption maxima, and the corresponding functions for l2 suggested by Lewis and Calvin[31] on the basis of coupled oscillator models were modified by Hirayama [32,33] to Equation (6) and revised by l2max n
ð Þ ¼cþba
ð6Þ
n
Dhne und Radeglia. [34] Since a < 1, Equation (6) yields a finite limiting value as shown by Equation (7). Equation (5) [30]
2. Long-Wavelength Electron Transitions in Conjugated Oligomers As already expressed in the Introduction, one expects the lowest electron excitation energies E (n) for conjugated Herbert Meier was born 1939 in Prague. He studied chemistry and mathematics at the University of Tbingen and the Free University in Berlin and in 1968 he completed his doctoral thesis in organic chemistry with Prof. E. Mller in Tbingen. After a habilitation in organic chemistry and photochemistry he became a docent in 1972 and a full Professor in 1975. In 1982 he accepted the chair of Prof. L. Horner at Mainz University. His research focuses on organic compounds with interesting properties for materials science and on heterocycles with a possible activity spectrum. He is co-author of the textbook “Spectroscopic Methods in Organic Chemistry”.
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ð4Þ
E n
¥
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ð3Þ
n
p ffiffi
l ¼ c lim !1 max
n
ð7Þ
and the related Equation (8) [23] are in principle suited for an
ð Þ ¼ E ð1Þ n
E n
ð8Þ
n
interpolation but not for the extrapolation ( n ! ). Finally, the matrix method, conceived by Pade, [35] could also succeed [Eq. (9)]. ¥
However, it turned out that none of these procedures correctly reflect the “saturation phenomenon” for series of oligomers having high numbers ( n) of repeat units. The OPV
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Conjugated Oligomers
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series 4[18,19,36] will be used here as an example. Figure 1 demonstrates that a linear function of type (1) fits for the first members (n = 1–8) of the series, but it is not suitable for the higher oligomers ( n = 11,15) or for the extrapolation to the
Figure 2. Electron transitions in aggregates, visualized for aggregated molecule pairs, whose transition moments M lie along the longitudinal
molecular axis. The energy of the allowed transition ( ) and of the forbidden transition ( ) depends on the stacking angle a . Jelley aggregates (J, a = 0) exhibit a bathochromic shift ( n < n) and H aggregates (a = 90 ) hypsochromic shifts (n > n). c
a
’’
’’’
Figure 1. Energies of the long-wavelength absorption maxima of 4 a– j and 4 p in chloroform and their exponential fit function (dotted line), which approaches the value of the corresponding polymer 4 p. The linear function of (n 1)1 furnishes an erroneous limiting value.
(infinitely long) polymer chain. Exponential functions E (n) and lmax(n) can be used here according to Equations (10) and (11) as natural growth functions for such a case. [18,19] The effective conjugation length amounts to Equation (12).
ð Þ ¼ E 1 þ ðE 1 E 1Þeað 1Þ
ð10Þ
ð Þ ¼ l1 ð l1 l1 Þebð 1Þ
ð11Þ
¼ lnð l16 l1 Þ þ 1
ð12Þ
n
E n
lmax n nECL
n
Aggregation has to be avoided, especially for UV/Vis measurements of higher oligomers, which means that series of decreasing concentrations need to be measured in a good solvent. Comparative measurements with a constant product of molar concentration and cell thickness, namely, c d = (101 c)(10 d) = (102 c)(100 d), proved to be particularly successful. Even a minor influence of the aggregation results in deviation of the absorption curves. Aggregates whose absorptions differ little from the monomer absorption are particularly tricky. Figure 2 shows the modification of the transitions S0 !S1 when aggregation occurs. To simplify matters it is assumed that the transition moments M of aggregated molecules lie along their longitudinal axes. The van der Waals interaction W 1 leads to an energy level which is subjected to a Davidov splitting W 2. The allowed transition corresponds to the sum M M , and the forbidden transition to the difference M M = 0. The transition energy E depends on the orientation of the molecules in the aggregate; E is lowest for pure Jaggregates (a = 0 ) and highest for pure H aggregates ( a = 90 ). The function W 2(a) in Figure 2 illustrates that W 2 is zero at the magic angle ( a = 54.73 ), which means there is no Angew. Chem. Int. Ed. 2005, 44, 2482 – 2506
discernible energy change. Aggregation can also lead to a steric effect, with the molecules less distorted and consequently absorb at longer wavelengths on aggregation. The extension of conjugation by increasing numbers of repeat units n normally leads to a monotonously decreasing excitation energy E (n) which converges towards E .[18,37] The exponent a in Equation (10) determines the rate of convergence. Some time ago we found series of conjugated oligomers which show a monotonous hypsochromic effect.[25,38–41] Such a behavior is typical for certain p linkers (see Section 3.1) with strong donors D and strong acceptors A in the terminal positions. The convergence can then also be determined by an equation of the form (10) or (11). The energy E DA(n) [Eq. (13)] of an electron transition in D- p-A systems can be split into two parts; the first part E S(n) [defined by Eq. (14)] takes the extension of conjugation in the purely donor- or purely acceptor-substituted [42] system into consideration, the second term DE DA [defined by Eq. (15)] is a correction term for the push-pull effect in series with terminal donor–acceptor substitution. [25] ¥
ð13Þ
ð Þ ¼ E S ðnÞDE DA ðnÞ
E DA n
ð14Þ
ð ÞE 1 ¼ ½E S ð1ÞE 1 eað 1Þ n
E S n
ð15Þ
ð Þ ¼ ½E S ð1ÞE DA ð1Þe að 1Þ D n
DE DA n
[E S(n)E ] has a monotonously declining and DE DA(n) a monotonously rising fitting function. Both approach to zero for increasing numbers n of repeat units—clearly, this is also valid for their sum [E DA(n)E ]. Figure 3 shows different cases of summation. A monotonously decreasing E DA(n) value results for type (a), which means such an oligomer series exhibits a uniform bathochromic effect: E DA(n) E DA(n 1) E . An oligomer series with a uniform hypsochromic effect is realized in type (b): E DA(n) E DA(n 1) E . The borderline case (a)/(b) between (a) and (b) is present for E DA(n) E , that is, when the energy of the
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¥
¥
¥
¥
¥
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Reviews
H. Meier Table 1 : Absorption in CHCl3 and color of the crystals of the transstilbenes 5 a–5 f .[a]
Compound 5
R
lmax [nm]
Crystal color
a[21,25] b[21,25] c[21,25] d[21,25] e[39] f [39]
H CN CHO NO2 HC=C(CN)2 C(CN)=C(CN)2
366 401 423 461 525 670
colorless yellow orange red dark red blue
[a] Since the e values are not known for many UV/Vis data discussed in this article, they are omitted completely.
Scheme 4. Push-pull-substituted oligo(1,4-phenylenevinylene)s (OPVs) 5–8/b–f and the comparitive series 5 a–8a.
Figure 3. Variants for the convergence of excitation energies E DA(n) E of the long-wavelength electron transition in series of push-pull-sub-
!
¥
stituted conjugated oligomers: a) uniform bathochromic behavior, b) uniform hypsochromic behavior, c) hypsochromic convergence after passing through a minimum of E DA(n).
electron transition is nearly independent of n (of the size of the chromophore). A rapidly decreasing term [ E S(n)E ] with increasing numbers n can also lead to the fact that E DA(n) goes through a minimum before it approaches to E (type (c)). Examples of oligomer series D-p-A for the considered cases are given in the following sections; the theoretically imaginable case, in which E DA(n) goes through a maximum, is to my knowledge not unequivocally proven experimentally to date. ¥
¥
pronounced as the acceptor strength increases. Since the corresponding excitation S0 !S1 is connected to an intramolecular charge transfer (ICT), the long-wavelength band is called a charge transfer band. An exciting question is how does the intramolecular charge transfer change when the distance between donor and acceptor groups increases, that is, when the number n of repeat units in the p linker is increased. Since dialkylamino groups with long alkyl chains have a solubilizing character, a systematic study of the oligo(1,4-phenylenevinylene)s (OPVs) 5 a–e, 6 a–e, 7 a–e, and 8 a–e could be performed. [21,25] Compounds 5 c–8 c were constructed from 9 by means of a Wittig–Horner reaction and a simple protecton strategy (Scheme 5). Phosphonate 10 served as an “extension reagent”. After a condensation reaction in an alkaline medium, the deprotection of the masked formyl group in 10
3. Push-Pull-Substituted Oligomers: Synthetic Concepts and Absorption Behavior 3.1. Linear Oligomers D- p -A
The push-pull effect has a strong influence on the longwavelength electron transitions in conjugated oligomers with terminal donor–acceptor substituents (see Section 5). Table 1 shows as an example the trans-stilbenes 5 a–f (Scheme 4) which bear a branched dialkylamino group in the 4-position and various substituents R in the 4 -position.[21,25,39] Compared to 5 a with R = H, the compounds with acceptor groups R exhibit a bathochromic shift, which is more and more ’
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Scheme 5. Coupled convergent synthetic strategy for the OPV series 5–8/a–e.
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Conjugated Oligomers
occurred directly in the acidic work-up, so that a free aldehyde function was available for the next extension step. The compounds 9, 5 c, 6 c, and 7 c were then reacted in an “end-capping process” with the phosphonates 11 a,b,d to give the series 5 a–8 a, 5 b–8 b, and 5 d–8 d.[21,25] A condensation reaction of the aldehydes 5 c–8 c with malononitrile 12 can be recommended for the “end capping” for the preparation of the series 5 e–8 e.[39] It was possible to obtain the series of five oligomers through a minimum number of synthetic steps by applying this coupled, convergent synthetic strategy. Figure 4 depicts the maxima of the long-wavelength absorptions of the OPVs 5–8 a–e measured in CHCl 3. A
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to a bathochromic shift, which is shown by a decreasing difference of E SE for the series 5 a–8 a. The push-pullsubstituted OPV series 5 b–8 b bearing the relatively weak acceptor R = CN is characterized by a correction term DE DA which weakens the bathochromic shift. In the series 5 d–8 d, with the nitro group as strong acceptor, the term E SE is over-compensated by the term DE DA; thus, a hypsochromic effect results. The two terms generally cancel each other out in 5 c–8 c (formyl series), so that the absorption maxima are almost independent of the length of the chromophore. [43] The compounds 5–8/a–e show, without exception, positive solvatochromic effects, which originate from intramolecular charge transfer (ICT). As soon as the push-pull effect is suspended by protonation of the amino group, the bathochromic shift in the series 5 b–8 b is strengthened and the hypsochromic shift in the series 5 d–8 d is reversed to a bathochromic shift (Figure 6). However, the entire absorption ¥
¥
Figure 4. Maxima of the long-wavelength absorptions in the OPV series 5 –8/a–e in CHCl3.
pronounced bathochromic effect can be realized for 5 a–8 a, a decreased bathochromic effect for 5 b–8 b, l max values of 5 c– 8 c which are fairly independent of the size of the chromophore, a hypsochromic effect for 5 d–8 d, and an even stronger hypsochromic effect for 5 e–8 e. The evaluation according to Equations (13)–(15) is demonstrated in Figure 5. The extension of the conjugation leads
Figure 5. Partition of the energies of the electron transition S0 S1 into a term (E s E ) which reflects the bathochromic shift caused by the extension of conjugation and a term DE DA which takes the decrease
!
¥
of the ICT and its consequence on the absorption into account. The measured data of 5 –8/a–d in CHCl3 shown in Figure 4 are the basis for this distribution.[43] Angew. Chem. Int. Ed. 2005, 44, 2482 – 2506
Figure 6. Absorption maxima in the OPV series 5 d–8 d ( n = 1–4); top
curve: measurement in CHCl3, bottom curve: measurement in CHCl3/ CF3COOH (10:1).
range is located at essentially higher energy when the ICT is cancelled out (see Section 5). An extension of conjugation in push-pull-substituted OPVs results in a bathochromic shift, but the decrease of the ICT and its effect on the absorption causes an opposite hypsochromic shift (see Section 5). Depending on the strength of the acceptor, a bathochromic or hypsochromic net effect results for systems with the same donor; this includes the case in which both effects cancel each other out. Exclusive bathochromic effects were found for OPV linkers with weaker donors, such as alkoxy groups. Compounds 13[44–46] and 14[46,47] in Scheme 6 illustrate this statement. Among the depicted variants E (n) in Figure 3, the cases (a) and (b) as well as the borderline case (a)/(b) are realized in push-pull-substituted OPVs. The trans-configured double bond in the repeat unit of 5– 8/a–e is replaced in the donor–acceptor-substituted oligo(1,4phenyleneethynylene)s (OPEs) 15–18/a–e shown in Scheme 7
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The long-wavelength absorption data for compounds 15 – summarized in Table 2.[41] The evaluation of the UV/Vis spectra is somewhat more difficult in the OPE series than in the OPV series because the long-wavelength absorption band (S0 !S1) is superimposed by the higher energy electron transition S0 !S2 (Figure 7 demonstrates this using 18/a–e are
Table 2: Long-wavelength UV/Vis absorption of the OPE compounds 15–18/a–e in CHCl3. Scheme 6. Push-pull-substituted OPVs with alkoxy groups as donor
groups; absorption maxima in CHCl3.
Scheme 7. Push-pull-substituted OPE series 15–18/b–e, comparitive series 15 a–18a, and precursors 15 f –17 f and 15 g–17g.[41,48]
by a triple bond, and the didodecylamino group serves as a solubilizing donor function.[41] The preparation of the oligomer series 15–18/a–e again takes place by a coupled, convergent strategy. The Sonogashira–Hagihara reaction and a simple protection strategy form the preparative basis. [41] Starting from 19 and 20 , the “auxiliary series” 15 f –17 f and 15g–17g were first prepared (Scheme 8). The extension
Compound
n
n˜ max [103 cm1]
lmax [nm]
15a 16a 17a 18a 15b 16b 17b 18b 15c 16c 17c 18c 15d 16d 17d 18d 15e 16e 17e 18e
1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
29.33 0.07 26.74 0.07 26.28 0.18 26.16 0.30 25.84 0.07 25.84 0.07 25.97 0.18 26.09 0.30 25.00 0.06 25.64 0.06 26.00 0.20 26.02 0.25 23.15 0.06 24.81 0.06 26.15 0.30 26.15 0.30 20.45 0.05 22.62 0.07 24.75 1.00 26.8 0.30
341 374 379[a] 378[a] 387 387 384[a] 379[a] 400 390 382[a] 388[a] 432 403 380[a] 382[a] 489 442 384[a] 373[a]
[a] The lmax values differ in these cases from 1/n˜ max of the separated longwavelength band because of the superposition of the bands.
Figure 7. UV/Vis spectrum of 16 d in CHCl3 (
) and its dissection into two bands according to Equations (16)–(18). [41] Scheme 8. Coupled convergent synthetic strategy for the OPE series 15–18/a–e with the precursor series 15 f –17 f and 15 g–17g.
reagent 20 was utilized for the Pd-catalyzed C C coupling step; the subsequent alkaline deprotection left the ethynyl component open for the next extension step. The Sonogashira–Hagihara reaction with the corresponding iodobenzene, which contained the desired p -substituent (R = H, CN, CHO, NO2), was used then as the “end-capping” step. [41] The OPE series 15 e–18e could be obtained by the condensation reaction of 15 c–18 c and malononitrile. [46] 2488
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c
16 d as
an example). A separation of the bands can be performed for example with an algorithm based on Gauss functions.[49] Since non-overlapping absorption bands at long wavelengths are also slightly unsymmetric in these series, exponential functions of type (16) proved to be a success.
n
ð Þ ¼ emax exp
~ en
ð nmax Þ
~max n ~
~
~ D n ~ n
ð16Þ
with D n~ ¼ j0:5ðn~2 n~1 Þj
ð17Þ
and e ðn~1 Þ ¼ eðn~2 Þ ¼ emax e 1
ð18Þ
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The evaluation of the data of 15–18/a–d is visualized in Figure 8, which corresponds more or less to Figure 5 for the analogous donor–acceptor-substituted OPV systems 5–8/a– d.[50] The interpretation of Figure 8 corresponds to the interpretation of Figure 5. The bathochromic effect resulting
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Only single examples of push-pull-substituted oligoenes (OEs) of type 1 (polymethine dyes) or 24 a,b–29a,b (n = 1,2,3, …) are known; an exception is represented by the aldehydes 24 c–29 c (R = CH3),[52a–c] which were prepared from the Zincke aldehyde by chain extensions with Grignard reagents and hydrolysis of the corresponding cyanines (Scheme 10). [52a]
Scheme 10. Push-pull-substituted oligoenes: Maxima of the long-wave-
length absorption in CH2Cl2. Figure 8. Partition of the electron transition energies (S0 S1) of 15 – 18/a–d into a term ( E S E ) which reflects the bathochromic shift caused by the extension of conjugation and a term DE DA which takes
!
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the decrease of the ICT and its consequence on the absorption into account.
from the extension of conjugation is surpassed in the NO 2 series by the hypsochromic effect, which arises from the decrease in the ICT; the same is true to a lesser extent in the CHO series 15 c–18 c ; the two effects cancel themselves out almost completely in the CN series 15 b–18 b. Altogether, the OPE linker is still somewhat more prone than the OPV linker to exhibit the unusual hypsochromic effect with increasing numbers n.[51] This situation has the consequence that even methoxy groups (as weaker donors) do not show a red-shift when in combination with strong acceptors such as NO 2. Compounds 22 and 23 in Scheme 9 are shown here as
The absorption spectra of 24 c–29 c measured in CH2Cl2 show a pronounced bathochromic shift for increasing numbers n of repeat units. Even the stronger electron-withdrawing dicyanovinyl group does not change this effect—nor when the trans double bonds are fixed in a transoid arrangement by incorporation in rings.[52d] Bathochromic effects were also observed by Lehn, Blanchard-Desce, Zyss, and co-workers in the series 30 a–32 a and 30 b–32 b, which contain carotinoid units as p linkers (Scheme 11). The synthesis of these com-
Scheme 9. Maxima of the long-wavelength absorptions of push-pull-
substituted OPEs with methoxy groups as donors and CN or NO 2 groups as acceptors (CHCl3 as solvent).
Scheme 11. Carotinoid push-pull compounds and the maxima of their
examples. Whereas lmax(2) lmax(1) amounts to 16 nm for 22a/23a, a value of 3 nm was found for 22 b/23 b.[46] The dialkylamino group (a strong donor) does not effect an inversion of the red-shift in the series 15 g–17 g bearing the ethynyl group (a weak acceptor group).
pounds was performed by “one-sided” Wittig and Wittig– Horner reactions, respectively, of the carotinoid dialdehydes and the corresponding phosphorus reagents of 1,3-benzodithiole followed by subsequent condensation of the stillpresent aldehyde function with malononitrile. [53,54]
Angew. Chem. Int. Ed. 2005, 44, 2482 – 2506
long-wavelength absorption in CHCl3.[53–55]
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The bathochromic shift for increasing length of the chromophore was also found by Blanchard-Desce, Barzoukas, Marder, and co-workers who studied the series 33–35, which includes an aromatic or heteroaromatic ring in the p linker at the donor end (Scheme 12). [3,56,57] The synthetic strategy for 33–35 is depicted in Scheme 13. The aldehyde
Table 3: Maxima of the long-wavelength absorption lmax [nm] of the oligoenes 33 b (n = 0–4), 33 d (n = 0–3), 34 a (n = 1–4), 34 b (n = 0–5), 34 f ( n = 0–3), and 35 c ( n = 0–2) in CHCl3.
n
33b[56]
33d [57]
0 1 2 3 4 5
443 507 540 571 586
603 695 773 826
34a[56]
34b[56]
34 f [56]
35c [57]
413 446 469 486
458 531 572 594 606 613
494 542 556 566
512 619 710
strength increases; lmax(n 1) > lmax(n) is valid within each series. Analogous bathochromic effects were measured for series of compounds with 1,3-benzodithiole donor groups and carotinoid linkers which contain an aromatic or heteroaromatic ring (4-nitrophenyl, 4-cyanophenyl, 4-pyridyl) at the acceptor end.[53,55] Push-pull-substituted oligoenes bearing aromatic rings at both chain ends [53,55,58b,59] show a diminished bathochromic effect. A comparison of the series 38 a–41a and 38b–41b (Scheme 14) shows a characteristic result.[46,58b,59] Compounds
Scheme 14. Maxima of the long-wavelength absorption of oligoenes
with terminal dimethylamino/nitro substitution, which include a benzene ring in the p linker on the donor side ( 38a–41a) as well as on the donor and acceptor sides (38b –41b); measured in CHCl3. Scheme 12. Oligoenes with (hetero)aromatic rings as donors as well as
various acceptors.
Scheme 13. Synthetic strategy for the series of compounds 33 –35.[57]
series 33 a, 34 a, and 35 a with the corresponding donor groups D acted as the central series and were constructed by means of the Wittig reaction with the extension reagent 36. The condensation reaction with the active methylene components 37b–e was then selected as the “end capping”. [57] The maxima of the long-wavelength absorptions of a selection of compounds 33 –35 are listed in Table 3. For constant values of n , the lmax value becomes higher as the donor and acceptor 2490
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analogous to 38 b–40 b, but with a CN group instead of the NO2 group, were investigated in the context of their dual fluorescence and twisted intramolecular charge transfer (TICT) states;[60–66] however, a discussion of these states is beyond the scope of the present Review. Push-pull-substituted oligoynes (OIs) are scarcely reported in the literature to date. [67] The aminoketones 42 and 43 and the aminonitro compounds 44 and 45 are given here as examples (Scheme 15). Benzene rings at the ends of the p linker cause a hypsochromic effect for neither the pushpull-substituted oligoenes nor the corresponding oligoynes— in contrast with the accordingly substituted OPV and OPE systems.
Scheme 15. Maxima of the long-wavelength absorption of oligoynes
with push-pull substitution.[68–71]
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Conjugated Oligomers
Oligo(1,4-phenylene)s (OPs) differ from the OPVs, OPEs, OEs, and OIs discussed so far as a result of a strong torsion of the benzene rings along the chain. Torsional angles between 30 and 40 can be assumed, which considerably influence the conjugation and the ICT. [71,72] Since the resonance integrals do not only depend on the different atomic distances in the p linker but also on the torsion of the p orbitals, an acceleration of the convergence E (n)!E can in principal be expected for increasing torsion angles. A considerable planarization of the 1,4-phenylene chain by anelated five-membered rings results in a bathochromic shift;[71–73] Table 4 shows a comparision of biphenyls 46 a– 49a and fluorenes 46 b–49 b (see Scheme 16 for structures). ¥
Table 4: Maxima of the long-wavelength absorption of the biphenyls 46 a–49a and comparison with the corresponding fluorenes 46 b–49 b.[71,73]
R1
R2
Biphenyl driva- lmax [nm] Fluorene deriva- lmax [nm] tives tives
H H N(CH3)2 N(CH3)2
H NO2 H NO2
46a 47a 48a 49a
252[a] 304[a] 301[b] 390[b]
262[a] 328[a] 310[b] 417[b]
46b 47b 48b 49b
[a] Measurement in 1,4-dioxane. [b] Measurement in CHCl3.
Scheme 16. Planarization of the torsional angles of biphenyls in
fluorenes (see Table 4 for R 1, R 2).
The synthetic strategy of oligo(1,4-phenylene)s is based on usual Pd-catalyzed aryl–aryl CC coupling reactions such as the Suzuki, Negishi, Stille, Yamamoto, or Kumada reactions.[1k] The preparation of the series 54 with D = N(CH3)2 and A = CN are described here as an example (Scheme 17). Negishi couplings of 50 with 51 led to the
Scheme 17. Synthetic strategy for the construction of donor–acceptorsubstituted OPs 54.[74] Angew. Chem. Int. Ed. 2005, 44, 2482 – 2506
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construction of the “auxiliary series” 52. The primary insertion of the Pd into the C Br bond of 51 is decisive for this step. Compounds 51 and 52 were then subjected to a cross-coupling reaction with 53 .[74] Since the “range” of the ICT is considerably shorter than the conjugation (see Section 5), the continuous torsions along an oligo(1,4-phenylene) chain can lead to a fast decrease in the E D(n)E value (see Figure 3c). Consequently, the energy E DA(n) for the electron transition S 0 !S1 can pass through a minimum, and lmax(n) accordingly through a maximum (type (c), Figure 3)—in particular, when the correction term DE DA of the ICT is large, that is, when a push-pull effect of a strong donor and a not too weak acceptor is present. This is realized for 54 and 55 (Table 5) ; the maximum ¥
Table 5: Maxima of the long-wavelength absorption of donor–acceptor substituted oligo(1,4-phenylene)s D-[-C6H4-]n-A: 54 a–d,[75] 55a–d,[71,76] 56a–c,[71] and 57 a–c.[46,77,78]
Compound
A
D
n
lmax [nm]
Solvent
54a 54b 54c 54d 55a 55b 55c 55d 56a 56b 56c 57a 57b 57c
CN
N(CH3)2
NO2
NH2
NO2
OCH3
CN
OCH3
1 2 3 4 1 2 3 4 1 2 3 1 2 3
290 342 332 314 373 378 358 340 302 332 340 247 292 302
CCl4 CCl4 CCl4 CCl4 EtOH EtOH EtOH EtOH 1,4-dioxane 1,4-dioxane 1,4-dioxane DMSO DMSO DMSO
value of lmax is found in both cases for n = 2, but can be solvent dependent. When the amino function is substituted by the less-strong donor OCH 3, shifts to longer wavelengths are obtained for the series with A = NO2 as well as for the series with A = CN with increasing values of n (type (a), Figure 3). The introduction of thiophene or furan rings instead of benzene rings in the p linker results in the absorption maxima shifting to longer wavelengths (Scheme 18 shows some examples).[71] Push-pull-substituted oligomers whose p linkers consist exclusively of five-membered-ring heterocycles, were studied in particular for the thiophene series. Table 6 offers a comparison of bithiophenes with various donor and acceptor groups; it can be seen that the combination of a 1pyrrolidine group and a nitro group in particular results in a far-red-shifted CT band. Thus, an interesting dependence of the absorption on the number n of repeat units can be expected for the push-pull-substituted oligothiophenes (OT; oligo(2,5-thienylene)s) studied by Effenberger and Wrthner.[79–81] Table 7 shows a comparison of the methoxy-nitro series 66 a–d (n = 1–4) and the 1-pyrrolidino-nitro series 67 a– d ( n = 1–4). Whereas a monotonous bathochromic shift with increasing n was found for 66 a–d, the lmax value of 67 a–d passes through a maximum at n = 3.[82] Hence, the latter oligomer series belongs to type (c) in Figure 3 and resembles the corresponding OPs 54 a–d and 55 a–d. To date, there are
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which were recently prepared by a Sonogashira– Hagihara reaction, should also be mentioned (Scheme 19). [46] The series 68 a–c belongs to type (a) in Figure 3, whereas the series 69 a–d exhibits an S 0 !S1 transition, which is independent of the length of the chromophore.
Scheme 18. Red-shift of the long-wavelength absorption band on replacement of the benzene rings in the p linker by thiophene or furan
rings.
Table 6: Maxima of the long-wavelength absorption of donor–acceptorsubstituted bithiophenes in n -hexane.[79,80]
Compound 63 64 65 66b 61
D OCH3 N(CH3)2 SCH3 OCH3 N(CH3)2
67b
A
lmax [nm]
CHO CHO NO2 NO2 NO2
372 421 391 408 466
NO2
499
Scheme 19. Maxima of the long-wavelength absorption of donor– acceptor-substituted oligo(2,5-thienyleneethynylene)s 68 a–c and 69a–d (measured in CHCl3).[46]
It remains to state at the end of this section that D- p-A systems can also be generated by protonation of suitable D- pD systems. Not only does the protonation of terminal amino groups have to be considered, but also the thiophene ring itself, as demonstrated in Scheme 20. Protonation of 70 a,b in CD2Cl2/CF3COOH leads to a shift of more than 200 nm to longer wavelengths. [84]
Table 7: Maxima of the long-wavelength absorptions of the oligothiophenes 66 a–d and 67 a–d in n-hexane.[79–81]
Compound
D
A
66a 66b 66c 66d
OCH3
NO2
67a
NO2
67b 67c 67d
n
lmax [nm]
1 2 3 4
340 408 442 454
1
408
2 3 4
499 505 497
3.2. Oligomers with D- p -A- p -D or A- p -D- p -A Structures
few D-p-A series with repeat units consisting of thiophene rings and CC double or triple bonds. [83] Oligo(2,5-thienylenevinylene)s (OTVs) which bear 4-diethylaminophenyl groups as the donor and NO 2, CHO, or CH=C(CN)2 groups as the acceptor have to be mentioned in this context; all these cases correspond to “bathochromic series” (type (a)).[83a,b] The oligo(2,5-thienyleneethinylene)s (OTEs) 68 a–c and 2492
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Scheme 20. Protonation of symmetrical oligothiophenes for the generation of D-p-A systems.
Conjugated oligomers with a donor–acceptor–donor structure require bidentate acceptor groups in the center of the molecule. The presence of carbonyl and related groups in this position leads to cross-conjugation. An example is presented by Michlers ketone 72 ( m = n = 1) and its higher homologues, though little is known about them. [85] Single examples of linearly conjugated D- p-A-p-D compounds exist in the series of azobenzenes 73,[46,86–88] pyridazines 74,[89] pyrazines 75,[90] and 1,2,4,5-tetrazines 76 ;[46,91] a systematic study was only performed for the squaraine series 77 a–d[92] (Scheme 21). Compounds 77, which are obtained by coupling the corresponding resorcinols and squaric acid, show an absorp-
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An extended planarization of the chromophore, in 77 through the formation of intramolecular hydrogen bridges, is an essential precondition for an efficient push-pull effect. Figure 10 shows, using compound 78 as an example, how the absorption is shifted from the NIR range to the UV range
Scheme 21. Examples of D-p-A-p-D systems. Figure 10. Absorption of squaraine 78 in CHCl3 and in CHCl3/EtOH
mixtures.
tion with a strong red-shift on going from n = 0 to n = 1; the blue dye 77a is thereby converted into the NIR dye 77b.[92] The electron transition which is predominantly localized in the squaraine ring [93] in 77 a becomes a transition in a pushpull-substituted stilbenoid compound and hence results in a pronounced hypsochromic effect for n = 2,3.[92] This effect occurs only when the measurements are made in an organic solvent such as CHCl3 (Figure 9)—when an acidic medium such as CF3COOH is used, the amino groups become protonated and consequently lose their donor character; the generated cations then have an A- p-A-p-A structure and show the expected bathochromic shift (from n = 0 to n = 3).
(D lmax > 450 nm) by the addition of ethanol which acts as a hydrogen bridge donor. [94] The benzene rings turn out of the plane of the squaraine rings when intermolecular hydrogen bridges are formed. Compounds of the type A- p-D-p-A require bidentate donors such as O, S, or NH. No oligomer series of this type currently exists. Recently, the two first members ( n = 1, 2) of the series 79 (Scheme 22) with ferrocene as a strong donor were studied; they exhibit a bathochromic effect: lmax(2) > lmax(1).[95]
Scheme 22. Maxima of the long-wavelenth absorption of A- p-D-p-A
compounds with ferrocene as the central donor (measured in CHCl3).
3.3. Star-Shaped Compounds A-( p -D)3 and D-( p -A)3
Figure 9. Maxima of the long-wavelength absorption of the squaraines 77a–d ( n = 0–3) in CHCl3 ~ and in CF3COOH & . Angew. Chem. Int. Ed. 2005, 44, 2482 – 2506
Tridentate “cores” have to be considered in addition to the bidentate central acceptors or donors described in Section 2.3. Scheme 23 shows some central building blocks.
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Scheme 23. Central building blocks for conjugated three-arm
oligomers: a) acceptor groups for star-shaped compounds A-(p-D)3, b) donor groups for star-shaped compounds D-(p-A)3.
The series of methylium salts 80 a–d (Scheme 24) was prepared; they can be regarded as higher homologues of the well-known triphenylmethane dyes. [96] The synthesis of 80 a–d was realized from the corresponding carbinol bases, whose
Figure 11. Maxima of the long-wavelength absorptions of the trifluoroacetates 80 a–d (n = 1–4) in CHCl3/CF3CO2H (7:3) and the
corresponding carbinols (bottom curve) in CHCl3. Extrapolation to l by application of Equation (11).
¥
Table 8: Maxima of the long-wavelength absorption of the colored salts 81a,b, 82 a,b, and 83 a–c.
Scheme 24. Methylium salts with OPV chains which bear terminal
donor groups.
81a[97] 81b[99] 82a[97] 82b[99] 83a[100] 83b[100] 83c[100]
R
D
n
lmax[a] [nm]
H CH3 H CH3 H H H
OCH3 OCH3 N(CH3)2 N(CH3)2 N[CH2CH(C6H13)2]2 N[CH2CH(C6H13)2]2 N[CH2CH(C6H13)2]2
1 1 1 1 1 2 3
718 733 609 615 622 740 790
[a] End values of l max at a high excess of CF3COOH.
treatment with acid led to the elimination of the OH group and a slight blue-shift. Simultaneously, a new band appears at bound on the central carbon atom. The cations 80 a–d, strictly lmax = 530 nm, which increases strongly and is red-shifted to as their carbinol bases, exhibit monotonously growing lmax 615 nm at high excess of CF 3COOH.[100] Table 8 shows the values with n —of course shifted by the push-pull effect from lmax values at the “end of this titration” when all three the UV/Vis region to the Vis/NIR region (Figure 11). The terminal amino groups of 82 b are protonated. Hence, the same is valid for methylium ions which are linked through push-pull character in the three arms is lost. Thus, it is polyene chains -(CH=CH)n- to ferrocene as the terminal understandable that the lmax values of the methoxy comdonor group; the lmax value rises from 618 to 1187 nm on pounds 81 a,b with n = 1 are higher than those of the going from n = 2 to 14.[97] dialkylamino compounds 82 a,b and 83 a. In contrast to the weakly pH-dependent alkoxy-substiOf the central acceptors for three-star oligomers shown in tuted color salts 80 a–d and 81a,b, the dialkylamino com- Scheme 21, the 1,3,5-triazines deserve special mention. The pounds 82 a,b and 83 a–c (Table 8) exhibit absorptions which alkoxy-substituted compounds 84–86 and the dialkylamino depend strongly on the concentration of H ions.[98–101] The compounds 87 and 88 were prepared by alkaline condensation formation of the cations from the carbinols or their ethers on reactions of 2,4,6-trimethyl-1,3,5-triazine with the correinteraction with strong acids leads first to absorption maxima sponding aldehydes (Scheme 25). [101] The influence of the which are located far in the NIR region. A lmax value of push-pull effect in 84 and the even stronger effect in 87 can be 1003 nm is found for 82 b,[100] and further addition of seen by comparison to the unsubstituted 2,4,6-tristyryl-1,3,5CF3COOH induces a decrease in the intensity of this band triazine, which has a lmax value of 327 nm. The absorption for 2494
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Figure 12. Top: long-wavelength absorption bands of the 1,3,5-triazines 86a–d (n = 1–4) in CH2Cl2 ; bottom: extrapolation of the lmax values Scheme 25. 1,3,5-Triazines with donor-substituted OPV chains and the maxima of their long-wavelength absorption (measurement of 84 –86 in CH2Cl2, of 87 and 88 in CHCl3).[101]
the series 86 a–d is red-shifted as the length of the conjugated arms increases. The absorption quickly approaches ( nECL = 7) a limiting value of l = 427 nm (Figure 12). An interesting feature is given by the “indicator behavior” of 87 . Unexpectedly, the first protonation on addition of trifluoroacetic acid occurs at the triazine ring, even though, for example, N ,N dimethylaniline has a higher basicity than 1,3,5-triazine. The yellow solution in CHCl 3 turns deep violet ( lmax = 549 nm). Further addition of CF 3COOH leads to a protonation of the terminal amino groups and the solution bleaches ( lmax = 365 nm).[102] The primary red-shift is based on an increase in the push-pull effect. However, as soon as the amino functions become protonated, their donor character is lost and an A-( pA)3 system is obtained. Since the protonated 1,3,5-triazine ring is a strong acceptor and the amino group a strong donor, an extension of the chromophore should result in a hypsochromic effect. We established this relationship by comparing 88 a and 88b (Figure 13). Compound 88 a, like 87 , shows a red-shift upon weak acidification, and afterwards a blue-shift on increased acidification. The higher homologue 86b behaves in exactly ¥
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according to Equation (11) and determination of the effective conjugation length according to Equation (12).
the opposite way: the primary blue-shift is followed by a redshift. The logical explanation is the following: the extension of the chromophore causes a bathochromic effect (445 ! 458 nm) for 88 itself; a much stronger push-pull effect is present in the species with a protonated 1,3,5-triazine ring and the extension of the chromophore leads to a hypsochromic effect (551!394 nm); there is no push-pull effect in the completely protonated compounds; the normal extension of the conjugation then results in a bathochromic shift (368 ! 459 nm).[103] The benzene system seems to be the most interesting among the central donor groups listed in Scheme 23; however, until now only a few examples, such as 89a,b (Scheme 26), have been prepared and studied. [104] Compound 89 and analogous benzene derivatives bearing three acceptor groups and three conjugated arms with terminal donor substituents (see Scheme 23) can be regarded as parent systems of hexasubstituted benzenes 91 having an octupolar character and therefore special significance for nonlinear optics.[70] Scheme 27 shows a synthetic approach to 91 based on the trimerization of alkynes. To my knowledge, series of conjugated compounds of type 91 with systematically extended p linkers are currently unknown.
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Scheme 27. Octupole 91 with a benzene core obtained by cyclotrimerization of alkynes 90 with push-pull character.
4. Nonlinear Optics in Series of Oligomers with Donor–Acceptor Substitution Nonlinear optical properties (NLO) of organic materials are of great interest for optical data storage, data processing, and data transfer,[1p,105] and conjugated NLO chromophores with a pronounced push-pull character are of high significance. Figure 14[106] provides an explanation for this: if light is
Figure 13. Top: bathochromic shift of the absorptions band of 88 a
( ) by protonation of the 1,3,5-triazine ring ( ) and subsequent hypsochromic shift by complete protonation ( ) in CHCl3/ CF3COOH; bottom: hypsochromic shift of the absorption band of 88 b ( ) by protonation of the 1,3,5-triazine ring ( ) and subsequent bathochromic shift by complete protonation ( ) in CHCl3/ CF3COOH. c
d
g
c
d
g
Figure 14. Description of nonlinear optics of D- p-A systems (SHG:
second harmonic generation, THG: third harmonic generation).[106]
shining on a compound consisting of D- p-A molecules, the E vector causes a high polarization P (E ). The periodicity of E (t ) of the light wave corresponds to the periodicity P (t ) ; however, the function P (t ) is not a sine function. Its Fourier transformation leads to a progression (19), which contains the optical susceptibilities c(n) of nth order. For a single molecule, this corresponds to Equation (20) for the induced dipole moment. P
¼ e0 ð cð1Þ E þ cð2Þ E 2 þ cð3Þ E 3 þ Þ ...
¼ aE þ bE 2 þ gE 3 þ
mind:
Scheme 26. Three-star compounds 89 a,b with central donor and
terminal acceptor groups (absorption maxima in CHCl3).
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...
ð19Þ ð20Þ
Apart from the linear polarizability a, there are hyperpolarizabilities b and g (of first and second order, respectively) which are many magnitudes smaller. Since b and g are very small (factors of 10 10, 1017), high intensity laser light is needed to measure the frequency doubling and tripling. The advantage of donor–acceptor-substituted conjugated p systems arises from the fact that the shift of electrons from the www.angewandte.org
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donor to the acceptor is highly efficient (Figure 14); that becomes particularly apparent in the b values. Although D- pA molecules are not centrosymmetric, they can crystallize in centrosymmetric space groups. The centrosymmetry must then also be valid for the function P (E ) [Eq. (21)]. This
ð Þ ¼ P ðEÞ
ð21Þ
P E
requires that c( 2) = b = 0 in the progressions (19) and (20). Many compounds with donor–acceptor substitution unfortunately crystallize in centrosymmetric crystal classes. Starshaped systems of type 89 or 91 avoid this. The influence of the substitution, in particular of the pushpull substitution, on b and g becomes evident in the transstilbene derivatives in Table 9. Strong donors and strong Scheme 28. Comparison of the b0 values of D-p-A compounds having acceptors enhance the b and g values—in analogy to the the same length of p linker but a different number of benzene [58b] dipole moment m . However, a direct relationship between m rings. and b or g does not exist, as the comparison of 13 a and 96 in Table 9 reveals. independent, so-called static, b0 values can be made on the Since strengthening the push-pull effect also shifts the CT basis of the two-level model [108,109] which works for D- p-A band to higher l max values (see Section 3.1), a power law b systems because of the domination of the CT transitions. [110] The deterioration of the conjugation as a consequence of lkmax or log b log lmax seems to apply. [107] The second order hyperpolarizability g of trans-stilbenes scales with b.[107] The the torsion in oligo(1,4-phenylene)s (OPs) is already push-pull compounds N ,N -dimethyl-4-nitroaniline and ( E )-4- expressed in the lmax values, but it is also noticeable in the dimethylamino-4 -nitrostilbene (DANS; 38 b) represent NLO hyperpolarizabilities b. Table 10 shows a comparison of standards that are often used. biphenyls and the corresponding fairly planar fluorenes for Incorporation of a triple bond instead of a trans-config- this purpose. The correlation is much more complex for the ured double bond results in b and g decreasing considerably g values, as the comparison of 47 a,b and 56 a,b demonstrates. and m decreasing to a small extent. For 4dimethylaminophenyl-4 -nitrophenyleTable 10: Comparison of dipole moments m and hyperpolarizabilities b and g of biphenyls and the thyne b = 461030 esu, g = 1511036 esu, corresponding fluorenes.[71] and m = 6.11018 esu (CHCl3).[71] The incorporation of benzene rings also proved to be unfavorable relative to equally extended p linkers consisting of olefinic R1 R2 Biphenyl[a] 1018 m 1030 b 1036 g Fluorene[a] 1018 m 1030 b 1036 g double bonds (Scheme 28). [esu] [esu] [esu] [esu] [esu] [esu] The b values, which were obtained, for H H 0 0 10 0 0 46a 46 b example, by the EFISHG method (electric H 47 b NO2 47a 3.8 4.1 15 4.1 5.1 29 field induced second harmonic generation), OCH3 56 b NO2 56a 4.5 9.2 39 4.7 11 28 depend somewhat on the applied wave- N(CH3)2 NO2 49a 5.5 5.0 130 6.0 55 49 b length. A simple correction to wavelength- [a] Measurement of 46 a,b, 47 a,b, 56 a,b in 1,4-dioxane, of 49 a,b in CHCl3. ~
~
’
’
Table 9: Dipole moments m and hyperpolarizabilities b and g of transstilbenes and their derivatives which bear a donor group in the 4-position
and/or an acceptor group in the 4 -position.[107] ’
Compound 92 93 94 38b 95 13a 96 97 98
Solvent
4-R
4 -R
1018 m [esu][a]
1030 b [esu][a]
1036 g [esu][a]
CHCl3 1,4-dioxane 1,4-dioxane CHCl3 CHCl3 CHCl3 CHCl3 CHCl3 CHCl3
H N(CH3)2 H N(CH3)2 NH2 OCH3 SCH3 N(CH3)2 OCH3
H H NO2 NO2 NO2 NO2 NO2 CN CN
0 2.1 4.2 6.6 5.1 4.5 4.3 5.7 3.8
0 10 11 73 40 34 34 36 19
26 64 61 225 147 93 100 125 54
’
[a] esu: electrostatic units; m : 10 30 Cm = 0.299810 18 esu = 0.2998 D; b : 1050 Cm3 V2 = 2.6941030 esu; g : 1060 Cm4 V3 = 8.078 36 10 esu. Angew. Chem. Int. Ed. 2005, 44, 2482 – 2506
The extension of the p linker in D-OP-A systems can result in an increase in the b values;[71,111,112] similar to lmax(n), it is possible to pass through a maximum of b(n). The first case is realized with 56a–c (D = OCH3/A = NO2) and the latter with 55 a–d (D = NH2/A = NO2). The g values always increase with increasing n. The effects are less pronounced in each case compared to the oligoenes 101 and 102 (Table 11)—even when the oligoenes bear benzene rings on one or both ends of the p linker. Consequently, oligoenes form the focus in NLO investigations of push-pull-substituted oligomers.[3,53,54,56,57,58a,b,71,113–119] Some examples are summarized in Table 12. Analogous results are obtained for OEs with carotinoid linkers (see Section 3.1).[54,113,115,116,117,119] The incorporation of five-membered-ring heterocycles, such as furan or thiophene, in the p linker generates higher hyperpolarizabilities b than
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Table 11: Hyperpolarizabilities b and g of push-pull-substituted oligo(1,4-phenylene)s (55, 56 ) and oligoenes with one ( 101) or two terminal benzene rings (102).[3,71]
Compound
D
A
n
OCH3
56a 56b 56c 55a 55b 55c 55d 101a 101b 101c 102a 102b 102c 102d
NO2 1 2 3 NH2 NO2 1 2 3 4 N(CH3)2 CHO 1 2 3 OCH3 NO2 1 2 3 4
Solvent 1,4-dDioxane NMP
CHCl3 CHCl3
1030 b [esu]
1036 g [esu]
5.1 9.2 11.0 10 24 16 11 30 52 88 34 47 76 101
10 39 21 96 124 133 63 140 257 93 130 230
Table 13: Comparison
of the hyperpolarizabilities b0 of D-p-A compounds with and without heterocycles in the p linker. Compound
Structural formula
1030 b0 [esu]
52[79,81]
33
61[79,81]
54
38b[71]
73
59[71]
83
58[71]
98
56c[71]
11
60[71]
40
66c[81]
41
102b[71]
47
Table 12: Dipole moments m and hyperpolarizabilities b0 and g of
oligoenes.[56] (Structures shown in Scheme 12.) Compound
n
m [D]
1030 b0 [esu]
33b
0 1 2 3 4 0 1 2 3 4 5
8.8 9.3 9.8 10 10 9.7 9.0 10.2 10.5 11 11
74 195 361 642 1229 63 195 423 810 1043 1530
34b
1036 g [esu] 378 1724 7363 395
semitheoretical basis also exist.[23] The rare case in which the value of b does not increase monotonously with n, as for example in the OP series 55a–d, cannot be covered by Equation (22). The individual exponents k und ‘ refer to the measurement of a certain series of donor–acceptor-substituted oligomers under certain conditions; moreover, they always refer to a narrow range of repeat units 1 n 5. The expression ‘ > k is always valid for an oligomer series, which means that the g values increase more strongly than the b values for increasing numbers n. The exponents k and ‘ should virtually be functions of n : k(n), ‘(n). However, an essential distinction arises from the fact that b0 should approach a limiting value b for high numbers n, whereas this is only valid in the case of g for dg/dn.[23,24] A calculation (ZINDO, CEO) was performed for oligoene linkers up to n = 40.[120,121] Experimental values for such D- p-A systems do not exist, nor for systems which are nearly as large. The convergence problem of b and g/n can be compared analogously to lmax ! l by the aid of exponential functions.[122] Since the length L of the chromophore in conjugated oligomers is a linear function of n, b(n) and g(n) can be described as functions b (L) and g (L). A simple calculation of b0 can be made with the energy hc/ lmax of the long-wavelength transition S 0 !S1, the corresponding transiton moment m01, and the difference D m of the dipole moments m (S1) and m(S0)[123] by applying the two-level model suggested by Oudar and Chemla. [108,109] Equation (26) pro¥
for the corresponding benzene systems with comparable linker lengths, but lower b values than analogous compounds with diene building blocks. Table 13 shows comparisons of compounds 52 /61, 38 b/59/58, and 56 c/60/66 c/102b. Empirical laws were proposed many times on the basis of measured data for the hyperpolarizabilities b and g. These laws reflect the influence of the substituents[71] and the influence of the number n of repeat units in the oligomers,[54,56,57,110,112,116,129] and include equations such as (22) and (24). Additional power laws derived on a theoretical or b0
ð22Þ
nk or log b0 log n
e:g: k ¼ 1:9,½56 2:0,½110 2 :4½54 g
n
‘
ð23Þ ð24Þ
or log g log n
ð25Þ
e:g: ‘ ¼ 4:2,½56 2 :7½116
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¥
2
2
max ¼ 6 m01hD2 ml c2
b0
ð26Þ
vides the possibility to determine b0 for the normally accessible region located away from the limiting value by normal absorption measurements of lmax and m01 as well as by
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Conjugated Oligomers
electrooptical absorption measurements (EOAM) of D m. If m01(n), D m(n), and lmax(n) all increase with n, it is also valid for b0(n).[79,81] The situation is more critical when m201D m increases but l2max decreases. We obtained b0 values of 198, 287, and 3461050 Cm3 V for the series 5 d–7 d (n = 1–3), which means that the value of b0 also increases in the case of a “hypsochromic series”.[25] The fact that b0 increases with n, irrespective of whether lmax increases or decreases, reveals that a fitting function b0( lmax) is generally not meaningful. However, the linear function log b = f ( lmax) can give evidence for substituent effects in compounds (for example, transstilbenes) which have the same p linker (see compounds in Table 9).[107] Substituent effects, but not necessarily the pushpull effect, have an effect on the size of g . Hence, there are examples for which the g values of D- p-A compounds are in between the g values of D-p-D and A- p-A compounds. [116] The power laws (27) and (28) suggested by Flytzanis and
L6d or log cð3Þ log Ld cð3Þ l6max or log cð3Þ log lmax cð3Þ
ð27Þ ð28Þ
co-workers[124,125] only make sense when lmax increases with the so-called delocalization length L d.[126] The relation (27) is problematic for short p linkers with Ld n[105a] and the correlation of c(3) or g with lmax is also not generally valid. In the “hypsochromic series” 15 d, 16 d, …, l max(n) decreases with growing n, but g(n) increases.[127] The relation (29) is g
n
m
or log g m log n
ð29Þ
Angewandte
Chemie
zwitterionic Z and an electroneutral resonance structure N [Equations 30 and 31] . Dþ -p-A ðZÞ $ D-p-A ðNÞ
ð30Þ
p ffiffi ffi ffi ffiffi ffi yðS0 Þ ¼ cyZ 1c2 yN p ffiffi ffi ffi ffi ffi yðS1 Þ ¼ 1c2 yZ þ cyN
ð31Þ
Several suggestions were made for the determination of the “weights” of the two limiting structures. Wortmann and co-workers[7–9] proposed the parameter c 2, which can be obtained by Equation (32); the participation of Z and N is c2 ¼
1 D m 1 2 4 m201 þ D m2
q ffiffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi
ð32Þ
considered therein to be included in the difference in the dipole moments m(S1) m(S0) = D m and the transition moment m01. Integration of the absorption curve or an approximation formula provide m 01; D m is accessable for example by electrooptical absorption measurements (EOAM). The difference D of the dipole moments of Z and N is related to D m and c2 according to Equation (33). Barzoukas et al. [6] used sin q/2 D m D
ð33Þ
¼ 12c2
instead of c in Equation (30) and defined the “mixed parameter” MIX as cosq which corresponds to (2c21) in Equation (33). Marder et al.[2–5] introduced the parameter BLA for the alternation of bond lengths in linear D- p-A chains. The parameter BOA for the alternating bond orders is closely connected to the alternation of bond lengths. BLA is accessible from X-ray data and is empiricly related to MIX [Eq. (34)].[6] However, BLA is not very useful if the p linker
even reliable in such a case, in which m(n) decreases with n and approaches the limiting value of 1. [105a] This convergence defines an effective conjugation length n0ECL, which can, however, be different from nECL obtained from the convergence of the long-wavelength absorption. The absorption ð34Þ takes S0 and S 1 into account, and an essential-state model of BLA ðin Þ ¼ 0:11MIX three, four, or more states is taken as the basis for the THG. To summarize Section 4, the statement can be made that contains aromatic rings, because aromatic rings will keep their in addition to the substituent effects of D and A on g and the typical adjusted bond lengths. push-pull effect on b, the nature of the p linker (type, length) Table 14 shows the relationship between the polarizabilplays a decisive role in the size of the b and g values. Even the ities a, b, and g and the discussed parameters c 2, MIX, and same linker has a different effect when its polarization is BLA. The parameter f proposed by Lu, Chen et al., [128] was different with various D/A pairs. An explanation for this is also included in the table; f and MIX are connected by provided in Section 5 in the context of VB Table 14: Dependence of the polarizabilities a, b, and g on the parameters c 2, MIX, f, and BLA with the theory. corresponding “weigths” of the resonance structures Z and A.
5. VB and MO Models of D- p -A Systems As already stated in Scheme 2, the model usually applied for D- p-A systems in the literature is a valence-bond model. It describes the ground state S 0 and the first electronically excited singlet state S1 of such compounds by a linear combination of a Angew. Chem. Int. Ed. 2005, 44, 2482 – 2506
amax b = 0 b max g=0
j j
jg j jg j
c2[a]
Weights N /Z or Z /N
MIX[b]
f [a]
Weights N /Z or Z /N
BLA[c] []
0.5 0.5 0.211
50:50 50:50 79:21
0 0
0.5 0.5 0.5 0.224 0.5 0.224 0.5 0.327 0.5
50:50 50:50 72:28 72:28 83:17 50:50
0 0 0.049 0.049 0.072 0
1 max
0
2 max
1 5 1 5
p p q ffiffi 3 7
2
[a] 0 < c < 1, 0 < f < 1. [b] 1 < MIX < 1. [c] BLA is varied between the bond lengths L(C=C) = 1.34 and L (=CC=) = 1.45 by about 0.11 ; if Equation (33) is not used, optimum BLA values of 0.04 0.01 and 0.06 0.01 , respectively, are proposed for j b j max and j c1 j max.[129] www.angewandte.org
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Equation (35). Moreover, for a better illustration, the “weights” N/Z (and equivalent Z/N) valid for c 2 and for f ¼
MIX þ 1 2
ð35Þ
MIX and f are included in Table 14. The maximum of a is reached for a 50:50 ratio of Z and A , that is, for a complete equalization of the bonds (cyanine limit). Here, b = 0 (in reality b is very small). The maximum of b is reached for an N/ Z ratio of about 3:1 (and its reciprocal); but then g = 0. A parameter value close to the maximum of g can be recommended for a simultaneous optimization of b and g.[129] The parameters proved to be successful for relatively short D-p-A systems with OE linkers. These parameters are not adequate when c 2 or f are close to 0 or 1 (MIX close to 1 or 1), and the VB model with the y functions (30) and (31) is generally not suitable for these cases. [21] An S1 structure with predominant Z character (1c2 0.9) has a small probability for an extended chromophore which has a predominant N character in S0 (c2 0.1). The resonance structure Z should have a similar energy as the resonance structure N so that the resonance Z$N is combined with a noticeable energy gain. The latter is certainly not realized when the p linker consists of repeat units which contain benzene rings or other aromatic ring systems. An MO model appears to be much more suitable for such oligomers. In this model, partial dipole moments are present on the donor and the acceptor sides, and each partial moment is itself composed of an intrinsic part ( mD, mA) and a part which is induced in each case by the dipole on the other chain end ( mDi, m Ai). The induced parts become smaller with increasing separation of the donor and acceptor. The extent of the polarization from the ends of the chain in the direction of its center should rapidly decrease. This was also demonstrated by means of BLA values and partial charges obtained by a DFT/ B3LYP/6-31G* calculation.[130] Semiempirical methods need to be considered for the calculation of higher oligomers. We chose the AM1 method for the optimization of the geometry and the INDO/S method for the electron transitions. [21,22] Scheme 29 shows the polarization derived from AM1) calculations of the olefinic double bonds of the OPVs 5 d–7 d and the corresponding triple bonds of the OPEs 15 d–17 d—in each case expressed in terms of the charge differences D q and Dq , respectively. The positive and negative partial charges are related to the standards transstilbene and tolane, respectively. [131] The Dd and Dd values reflect the differences in the chemical shifts of the 13 C nuclei of the double and triple bonds. A direct correlation can be seen: a large Dq( ) value results in a large Dd( ) value. The essential polarization can be found on the chain ends. [132] The effect decreases strongly towards the center of the chain. This trend of Dq( ) is even more pronounced for longer chains ( n 4).[21,22] 13C NMR signals are a good indicator of partial charges. The correlations of the 13C chemical shifts and their differences Dd( ) (obtained by the aid of the 2D HMBC technique) agree very well with the Dq( ) values, and therefore support nicely the MO model. The decreasing significance of the zwitterionic resonance structure Z with increasing dis’
’
’
’
’
’
’
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Scheme 29. MO model for conjugated oligomers with donor–acceptor substitution: Calculated charge differences D q at the double bonds of the OPVs 5 d–7 d (n = 1–3) and D q at the triple bonds of the OPEs 15d–17 d ( n = 1–3); the differences D d and D d of the 13 C chemical ’
’
shifts were measured in CDCl3.[21,22]
tance of D and A can be also recognized in the compounds (CH3)2N-OE-CHO 24c–26 c, in which the rotational barrier of the CN bond decreases strongly with increasing values of n.[52b,c] Whereas the electron transitions S0 !S1 in trans-stilbene and tolane are still almost pure HOMO !LUMO transitions, other orbitals mix in for the D-OPV-A and D-OPE-A systems, even for n = 1. The energy of HOMO1 approaches constantly closer to the HOMO, and the LUMO 1 constantly closer to the LUMO, as n increases. The transitions HOMO1!LUMO, HOMO !LUMO 1, and HOMO1!LUMO 1 become more and more important for the long-wavelength absorption, especially since the overlap density of the HOMO and LUMO becomes smaller and smaller. Figure 15 shows as an example the OPEs 15d (n = 1) and 18d ( n = 4), with the AM1-INDO/S calculation performed with dimethylamino groups instead of didodecylamino groups, which are necessary for solubilization. The HOMO !LUMO transitions are characterized by a strong intramolecular charge transfer (ICT) from the donor to the acceptor side. Figure 15 reveals that the ICT plays a very minor role in the other transitions. The long-wavelength absorption band (the charge transfer band) of D- p-A systems is essentially determined by the ICT. D and A not only influence the orbital energies, they also alter the electron correlation. In terms of a self-consistent field (SCF) approximation, both the differences in the orbital energies D E 0 and also the configuration interactions of oneelectron functions need to be considered for the excitation energies E (S0 !S1). Equation (36) contains the Coulomb E
¼ DE 0 J þ 2 K
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Chemie
Figure 15. Calculation of the frontier orbitals of the OPEs 15 d and 18 d
and their partipication in the electron transition S 0 !S1 by the AM1INDO/S method.
repulsion integral J and the exchange integral K. [133] E < DE 0 is always valid because 0 % 2K < J. A charge transfer within the chromophore diminishes the negative portion (2 K J), because the overlap density decreases. Thus, E becomes smaller, which means that the band of the electron transition is shifted to longer wavelengths. A reduction in the ICT effects accordingly shifts the band to shorter wavelengths (s ee Figure 3). The classical example is given by anthracene and azulene. Although DE 0 is the same, the excitation energy E of the blue azulene is essentially smaller than E of the colorless anthracene because an ICT from the five-membered ring to the seven-membered ring takes place in the S 0 !S1 transition of azulene. This is not possible in the symmetric anthracene.[134] Figure 16 illustrates the relationship of Equation (36) in a simplified picture of a HOMO !LUMO transition.
Figure 17. Bottom: partipication of the HOMO LUMO transition in the long-wavelength absorption of 15 d–18d (n = 1–4) calculated by AM1-INDO/S; middle: corresponding decrease in the term DE DA(n);
!
top: decrease in the calculated difference of the dipole moments D m(n) = m(S1) m(S0).
Figure 16. Influence of the donor–acceptor substitution in D- p-A
systems on the energy of the long-wavelength electron excitation. (Exclusive regard of HOMO and LUMO, D E 0 as the difference of the ionization energy and negative electron affinity EA according to Koopmans theorem.)
The stronger the electron-releasing effect of D and the electron-withdrawing effect of A is, the lower is the transiton energy E (S0 !S1) of D-p-A systems with a constant number n of repeat units. The decrease of the ICT for increasing numbers n and a certain terminal D/A substitution is expressed in the DE DA(n) term in Equation (13). Figure 17 Angew. Chem. Int. Ed. 2005, 44, 2482 – 2506
shows the calculated decrease of the HOMO !LUMO fraction of S 0 !S1 resulting from increasing numbers n in the series 15d–18 d (D = N(CH3)2, A = NO2, n = 1–4).[22] Simultaneously, the values of D E DA(n) and D m = m(S1)m (S0) decrease. As already explained in Section 2, the DE DA term is able to diminish, cancel out, or reverse (to hypsochromic) the bathochromic effect in a series of conjugated oligomers. Analogous results were obtained for compounds 5 d–8 d with an OPV linker instead of the OPE linker. [21] Therefore, it is not necessary to show here a picture of the highest occupied and the lowest unoccupied orbitals. [21,25] The stronger the donor and acceptor are, the faster the participation of the HOMO !LUMO transition decreases for the long-wavelength absorption, the more the topology of the other involved orbitals determines the decrease of the ICT, and the more rapidly the ICT-affected correction term DE DA(n) approaches zero (Figure 18).
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charge transfer (ICT). The charge-transfer band reacts quite differently upon the extension of the chromophore, because a hypsochromic effect resulting from the decrease in the ICT is opposed to the bathochromic effect caused by the extension of the conjugation. This becomes apparent as soon as the number n of repeat units is increased, and consequently the distance between the donor and acceptor enhanced. The superposition of both influences leads to the variants for the S0 !S1 transitions which are summarized in Table 15. In addition to the strength of donor and acceptor in the present D/A pair, the type of p linker plays a decisive role. Until now, exclusively bathochromic series exist for the oligoenes (DOE-A) and oligoynes (D-OI-A; type (a): lmax(n) < lmax(n 1) l ). In contrast, oligo(1,4-phenylenevinylene)s (D-OPV-A) and oligo(1,4-phenyleneethynylene)s (D-OPEA) bearing strong donors and strong acceptors as end-groups can exhibit a hypsochromic behavior (type (b): lmax(n) > lmax(n 1) l ). A certain weakening of the donor or acceptor strength may then give rise to a borderline case, in which the long-wavelength absorption is independent of the size of the chromophore (type (a)/(b): l max(n) lmax(n 1) l ). OPV and OPE series with weak donors and weak acceptors show a bathochromic behavior. Oligo(1,4-phenylene)s (D-OP-A) and oligo(2,5-thienylene)s (D-OT-A) are special cases of type (c), in which lmax(n) passes through a maximum. Torsions along the p linker have a decisive influence on the CT band. [136] The fourth imaginable case, in which l max passes through a minimum, has not been unequivocally proven to date.[137] The number of investigated compounds of oligo(2,5-thienylenevinylene)s (D-OTV-A) or oligo(2,5-thienyleneethynylene)s (D-OTE-A) is too small for a reliable conclusion to be drawn. The superposition of “conjugation effect” and opposite “ICT effect” can be rationalized by the superposition of two exponential functions (Figure 3); both effects exhibit a convergence behavior (n ! ). The intramolecular charge transfer diminishes the electron correlation, which is of special importance for the HOMO !LUMO excitation. However, semiemprical quantum mechanics (AM1, INDO/ S) studies on D-OPV-A and D-OPE-A systems reveal that the HOMO !LUMO fraction of the long-wavelength absorption decreases continuously as n grows, that is, as the size of the chromophore grows (Figure 15). Together with this, D m = m(S1) m(S0) and the correction term of the ICT in the absorption decrease (Figures 17 and 18). The VB model D-p-A$D -p-A often used in the literature is primarily suitable for the case in which short oligoenes are used as a p linker. This model with its zwitterionic resonance structure is not very useful for other p linkers, particularly not for p linkers which contain aromatic rings. In contrast, an MO model proved to be successful, which implies a decreasing polarization from the donor as well as from the acceptor side towards the center of the p linker (Scheme 29). In the field of nonlinear optics (NLO), laws were often temptatively proposed that correlated the hyperpolarizabilites b and g with lmax values. This approach, however, fails for “hypsochromic series”. Power laws or logarithmic functions can be used which contain the length L of the chromophores ¥
¥
¥
Figure 18. Partipication of the HOMO
!LUMO transition in the long-
wavelength absorption of OPVs with an increasing number n of repeat units (AM1-INDO/S calculation); b) dependence of the ICT term D E DA on the fraction of H !L transition in the push-pull series (H 3C)2NOPV-NO2 and (H3C)2N-OPV-CN.
6. Summary and Outlook Conjugated oligomers with terminal donor–acceptor substitution are examples of “molecular wires” [135] which are predestinated to have special optical and optoelectronic effects. The preparation of such an oligomer series with an increasing number of repeat units in the p linker is best started on the donor side which bears solubilizing groups (such as NR2, OR). The p linker is then extended by the application of an “extension reagent” and a protection group strategy until the acceptor group is attached in an end-capping step. This approach is recommended for the construction of parallel series with different acceptor groups to generate a central series from which other series are accessible by coupled, convergent syntheses. The excitation of electrons in the linear or star-shaped oligomers of this type is characterized by an intramolecular 2502
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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¥
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Chemie
Table 15: Classification of the absorption behavior of series of donor–acceptor-substituted conjugated oligomers according to their dependence on the D/A combination and the p linker.
Monotonously batholinker chromic series type (a) p
OPV
OPE
NR2/CN OR/NO2 OR/CHO OR/squaraine ferrocene/NO2 OR/C NR2/1,3,5-triazine OR/1,3,5-triazine NR2/CCH OR/CN
Borderline type (a)/(b)
Monotonously hypsochromic series type (b)
NR2/CHO
NR2/NO2 NR2/CH=C(CN)2 NR2/squaraine NR2/1,3,5-triazinium
OR/NO2 NR2/CN
NR2/NO2 NR2/CH=C(CN)2 NR2/CHO
OE[a]
Series with an energy minimum type (c)
D-p-A structures tailored for certain electrooptical applications (molecular engineering) represent the final, highly promising goal. [142] I am grateful to the Deutsche For schungsgemeinschaft, the Volkswa gen Foundation, the Fonds der Chemischen Industrie, and the Center of Materials Science of the University of Mainz for financial support of my own work cited in this article.
Received: July 1, 2004
all investigated D/A combinations NR2/NO2 OI OP OR/NO2 OR/CN OT OR/NO2 OTV[b] NR2/NO2 NR2/CH=(C(CN)2 OTE SR/NO2 OR/NO2
NR2/NO2 NR2/CN NR2/NO2
[1] Selected monographs and reviews: a) H.-H. Hrhold, M. Helbig, D. Raabe, J. Opfermann, U. Scherf, R. Stockmann, D. Weiß, Z. Chem. 1987, 27 , 126– 137; b) J. L. Brdas, R. Silbly, [a] D: NR2, OR, SR, ferrocene, A : NO2, CHO, CN, CH=CR2 containing electron-withdrawing groups R, Conjugated Polymers, Kluwer, Dordrecht, 1991; c) K. Mllen, C ; the OE linkers also include benzene or thiophene rings on one or both ends of the chain. [b] The OTV linker includes a terminal benzene ring on the donor side. Pure Appl. Chem. 1993, 65, 89– 96; d) W. R. Salaneck, I. Lundstrm, B. R. Rnby, Conjugated Polymers and Related Materials, or the number n of repeat units in their argument. In contrast Oxford University Press, Oxford, 1993 ; e) J. M. Tour, Chem. to l max and b, g does not approach a limiting value for n , Rev. 1996, 96, 537–553; f) R. Giesa, J. Macromol. Sci. Rev. but g/n does.[23] The effective conjugation lengths nECL Macromol. Chem. Phys. 1996, 36, 631– 670; g) J. S. Moore, Acc. Chem. Res. 1997, 30, 402– 413; h) L. Jones II, J. S. Schumm, required for nonlinear optical properties are much greater J. M. Tour, J. Org. Chem. 1997, 62, 1388–1410; i) J. Roncali, than for linear optics. Therefore, an experimental proof of the Chem. Rev. 1997, 97 , 173–205; j) A. Kraft, A. C. Grimsdale, convergence of b and g /n is much more difficult. A. B. Holmes, Angew. Chem. 1998, 110, 416–443; Angew. Besides the selected D/A combination, the nature and Chem. Int. Ed. 1998, 37 , 403–428; k) Electronic Materials: The length of the p linker are decisive for b and even to a greater Oligomer Approach (Eds: K. Mllen, G. Wegner), Wiley-VCH, extent for g. 1,4-Phenylene units in an OP linker show a Weinheim, 1998 ; l) T. M. Swager, Acc. Chem. Res. 1998, 31, torsion along the chain which is unfavorable for large 201 – 207; m) P. F. H. Schwab, M. D. Levin, J. Michl, Chem. Rev. 1999, 99, 1863–1933; n) U. Scherf, Top. Curr. Chem. 1999, 201, hyperpolarizabilities. On the whole, a comparison of b and 163–222; o) R. E. Martin, F. Diederich, Angew. Chem. 1999, g values of various series D- p-A is essentially more difficult 111, 1440–1469; Angew. Chem. Int. Ed. 1999, 38 , 1350– 1377; than a comparison of linear optical properties. The errors of p) J. J. Wolff, R. Wortmann, Adv. Phys. Org. Chem. 1999, 32 , the methods are considerably larger than, for example, for 121–217; q) U. H. F. Bunz, Chem. Rev. 2000, 100, 1605–1644; lmax values, even when static, that is, wavelength-independent r) J. L. Segura, N. Martin, J. Mater. Chem. 2000, 10, 2403 – 2435; b0 values and nonresonant g values are used. Relative quans) G. Hadziioannou, P. F. van Hutten, Semiconductivity Polymers, Wiley-VCH, Weinheim, 2000 ; t) J. Roncali, Acc. Chem. tities related to a standard, which is measured at the same
!
¥
conditions, for example [Eq. (37)], are recommended for materials science applications. brel
ðcompoundÞ molecular mass ð4-nitroanilineÞ ¼ b0bð04-nitroaniline Þ molecular mass ðcompoundÞ
ð37Þ
Altogether, considerable deficits can be recognized for the experimental determination of b and g values of oligomer series. Numerous theoretical attempts have been made for their linear (Section 2) and nonlinear optics (Sections 4 and 5).[138,139] A new approach in the synthetic area is offered by star-shaped compounds (Section 3.3) [140] and dipolar structures such as 101, in which—contrary to the resonance described in Scheme 3–-the aromatic and dipolar resonance structures are compatible (Scheme 30). [141] The preparation of Angew. Chem. Int. Ed. 2005, 44, 2482 – 2506
Scheme 30. Resonance forms of a quinoid, electroneutral, and an
aromatic, zwitterionic resonance structure as an example that is opposed to the usual resonance of an aromatic, electroneutral, and a quinoid, zwitterionic limiting structure.
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Res. 2000, 33, 147–156; u) J. M. Tour, Acc. Chem. Res. 2000, 33,
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