meier2005.pdf

Conjugated Oligomers

Conjugated Oligomers with Terminal DonorAcceptorSubstitutionHerbert Meier*

AngewandteChemie

Keywords:absorption conjugation intramolecular charge transfer nonlinear optics oligomers

H. MeierReviews

2482 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/anie.200461146 Angew. Chem. Int. Ed. 2005, 44, 2482 2506

1. Introduction

Conjugated oligomers are target compounds for numer-ous applications in materials science because of their inter-esting electrical, optical, and optoelectronic properties andthey are also model compounds for the correspondingconjugated polymers.[1] A topic of high topicality in terms ofnonlinear optics and electroluminescence concerns p systemssubstituted with donor and acceptor groups in which con-jugated oligomers form the p-electron linker. The compoundscan have a linear or a star-shaped architecture. Scheme 1provides an overview of the most important structural types.

The push-pull effect of this class of compounds depends onthe strength of the donor and acceptor groups; however, italso depends on the conjugated p system, to which azwitterionic resonance structure should contribute(Scheme 2). The energy of the dipolar resonance structure is

determined by the charge separation as well as the change inthe p system. The latter influence is certainly smaller for anoligoene chain (1) than for repeat units consisting of aromaticrings (2), whose zwitterionic resonance structures have p-quinoid character.

Several parameters, such as BLA,[25] MIX,[6] and c2,[79]

have been suggested for quantification of the contribution ofzwitterionic resonance structures. This will be discussedfurther in Section 5. However, it should be noted here thatthe weight of resonance structures depends on externalfactors such as the solvent or an applied electrical field.

[*] Prof. Dr. H. MeierJohannes Gutenberg-UniversittDuesbergweg 101455099 Mainz (Germany)Fax: (+49)6131-392-5396E-mail: [email protected]

Conjugated oligomers represent a prominent class of compoundsfrom a viewpoint of their theory, synthesis, and applications inmaterials science. Push-pull substitution with an electron donor Dat one end of the conjugation and an electron acceptor A at theother end results in them having outstanding optical and elec-tronical properties. This Review highlights fundamental syntheticstrategies for the preparation of such oligomers with n repeat units(n= 1, 2, 3, 4, ) and the rules that govern their linear andnonlinear optical properties (absorption, frequency doubling andtripling). The unification of chemical, physical, and theoreticalaspects with an interdisciplinary image of this class of compoundsis attempted herein.

From the Contents

1. Introduction 2483

2. Long-Wavelength ElectronTransitions in Conjugated Oligomers 2484

3. Push-Pull-Substituted Oligomers:Synthetic Concepts and AbsorptionBehavior 2486

4. Nonlinear Optics in Series ofOligomers with DonorAcceptorSubstitution 2496

5. VB and MO Models of D-p-ASystems 2499

6. Summary and Outlook 2502

Scheme 1. Construction of donoracceptor-substituted conjugatedoligomers consisting of donor groups D, a p-electron linker, andacceptor groups A; selected examples are shown.

Scheme 2. Participation of zwitterionic resonance structures for theillustration of the push-pull effect in conjugated oligomers.

Conjugated OligomersAngewandte

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2483Angew. Chem. Int. Ed. 2005, 44, 2482 2506 DOI: 10.1002/anie.200461146 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

A special case, which is occasionally referred to in thisarticle, is represented by the symmetrical, charged, all-E-configured polymethines 3a and 3b (Scheme 3). At largervalues of n (beyond the so-called cyanine limit) it needs to be

considered[10] whether the resonance should be substituted bya fast equilibration (automerization) as soon as the C2v sym-metry is abandoned in favor of a Cs symmetry.

[1016]

A special aspect of series of conjugated oligomers is givenby the expectance that certain properties P(n) convergetowards a limiting value P for increasing numbers n of repeatunits, or at least their derivatives dP(n)/dn converge towardsP. The long-wavelength electron transition S0!S1 providesan example of the first case [lmax(n)!l],[1722] while thehyperpolarizability of second order g is an example of thelatter case [dg(n)/dn!g].[23] In most cases lmax(n) increasesmonotonously with n and reaches the limiting value l at theso-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 oligomerswith terminal donoracceptor substitution can exhibit amonotonously decreasing value for lmax with increasingnumbers n of repeat units;[25] the behavior of the hyper-polarizabilities b and g of such series is currently unknown.Both effects will be discussed in Sections 3 and 4, whilequantum mechanical models for D-p-A systems will bediscussed in Section 5.

2. Long-Wavelength Electron Transitions inConjugated Oligomers

As already expressed in the Introduction, one expects thelowest electron excitation energies E(n) for conjugated

oligomers to converge towards a certain limiting value Efor n!. The hyperbolic function described by Equation (1)

En c f 1n 1 1

seems to be adequate for polyenes. However, simple HMOtheory [Eq. (2)] supplies the limiting value (zero). Only the

limn!1

En c 0 2

consideration of perturbation theory of first or second orderresults in a limiting value which is different from zero[Eq. (3)].[26] Db is the difference in the resonance integrals of

limn!1

En 4pDb > 0 3

neighboring bonds. The perturbation is based on the fact thatpolyenes have alternating single and double bonds of differ-ent lengths and consequently different b values.

Wenz, Wegner et al. derived a function on the basis of theelectron gas theory[27,28] as Equation (4) with a limiting value

En c f 1n 0:5 4

of c6 0.[29] Root laws, such as Equation (5) used by Drefahl

lmaxn c bn

p 5

and Pltner[30] for the long-wavelength absorption maxima,and the corresponding functions for l2 suggested by Lewis andCalvin[31] on the basis of coupled oscillator models weremodified by Hirayama[32,33] to Equation (6) and revised by

l2maxn c b an 6

Dhne und Radeglia.[34] Since a< 1, Equation (6) yields afinite limiting value as shown by Equation (7). Equation (5)[30]

limn!1

lmax c

p7

and the related Equation (8)[23] are in principle suited for an

En E1nn 8

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 procedurescorrectly reflect the saturation phenomenon for series ofoligomers having high numbers (n) of repeat units. The OPV

Herbert Meier was born 1939 in Prague. Hestudied chemistry and mathematics at theUniversity of Tbingen and the Free Univer-sity in Berlin and in 1968 he completed hisdoctoral thesis in organic chemistry withProf. E. Mller in Tbingen. After a habilita-tion in organic chemistry and photochemis-try he became a docent in 1972 and a fullProfessor in 1975. In 1982 he accepted thechair of Prof. L. Horner at Mainz University.His research focuses on organic compoundswith interesting properties for materials sci-ence and on heterocycles with a possibleactivity spectrum. He is co-author of thetextbook Spectroscopic Methods in OrganicChemistry.

Scheme 3. Symmetrical charged polymethines (cyanines).

H. MeierReviews

2484 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org Angew. Chem. Int. Ed. 2005, 44, 2482 2506

series 4[18, 19,36] will be used here as an example. Figure 1demonstrates that a linear function of type (1) fits for the firstmembers (n= 18) of the series, but it is not suitable for thehigher oligomers (n= 11,15) or for the extrapolation to the

(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] Theeffective conjugation length amounts to Equation (12).

En E1 E1E1ean1 10

lmaxn l1l1l1ebn1 11

nECL lnl1l1

6 1 12

Aggregation has to be avoided, especially for UV/Vismeasurements of higher oligomers, which means that series ofdecreasing concentrations need to be measured in a goodsolvent. Comparative measurements with a constant productof molar concentration and cell thickness, namely, cd=(101c)(10d)= (102c)(100d), proved to be particularly suc-cessful. Even a minor influence of the aggregation results indeviation of the absorption curves. Aggregates whose absorp-tions differ little from the monomer absorption are partic-ularly tricky. Figure 2 shows the modification of the tran-sitions S0!S1 when aggregation occurs. To simplify matters itis assumed that the transition moments M of aggregatedmolecules lie along their longitudinal axes. The van der WaalsinteractionW1 leads to an energy level which is subjected to aDavidov splitting W2. The allowed transition corresponds tothe sumM+M, and the forbidden transition to the differenceMM= 0. The transition energy E depends on the orienta-tion of the molecules in the aggregate; E is lowest for pureJ aggregates (a= 08) and highest for pure H aggregates (a=908). The functionW2(a) in Figure 2 illustrates thatW2 is zeroat the magic angle (a= 54.738), which means there is no

discernible energy change. Aggregation can also lead to asteric effect, with the molecules less distorted and conse-quently absorb at longer wavelengths on aggregation.

The extension of conjugation by increasing numbers ofrepeat units n normally leads to a monotonously decreasingexcitation energy E(n) which converges towards E.

[18, 37] Theexponent a in Equation (10) determines the rate of conver-gence. Some time ago we found series of conjugatedoligomers which show a monotonous hypsochromiceffect.[25,3841] Such a behavior is typical for certain p linkers(see Section 3.1) with strong donors D and strong acceptors Ain the terminal positions. The convergence can then also bedetermined by an equation of the form (10) or (11). Theenergy EDA(n) [Eq. (13)] of an electron transition in D-p-Asystems can be split into two parts; the first part ES(n)[defined by Eq. (14)] takes the extension of conjugation in thepurely donor- or purely acceptor-substituted[42] system intoconsideration, the second termDEDA [defined by Eq. (15)] is acorrection term for the push-pull effect in series with terminaldonoracceptor substitution.[25]

EDAn ESnDEDAn 13

ESnE1 ES1E1ean1 14

DEDAn ES1EDA1eDan1 15

[ES(n)E] has a monotonously declining and DEDA(n)a monotonously rising fitting function. Both approach to zerofor increasing numbers n of repeat unitsclearly, this is alsovalid for their sum [EDA(n)E]. Figure 3 shows differentcases of summation. A monotonously decreasingEDA(n) value results for type (a), which means such anoligomer series exhibits a uniform bathochromic effect:EDA(n)EDA(n+1)E. An oligomer series with a uniformhypsochromic effect is realized in type (b): EDA(n)EDA-(n+1)E. The borderline case (a)/(b) between (a) and (b) ispresent for EDA(n)E, that is, when the energy of the

Figure 1. Energies of the long-wavelength absorption maxima of 4ajand 4p in chloroform and their exponential fit function (dotted line),which approaches the value of the corresponding polymer 4p. Thelinear function of (n+1)1 furnishes an erroneous limiting value.

Figure 2. Electron transitions in aggregates, visualized for aggregatedmolecule pairs, whose transition moments M lie along the longitudinalmolecular axis. The energy of the allowed transition (c) and of theforbidden transition (a) depends on the stacking angle a. Jelleyaggregates (J, a=0) exhibit a bathochromic shift (nn).


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2485Angew. Chem. Int. Ed. 2005, 44, 2482 2506 www.angewandte.org 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

electron transition is nearly independent of n (of the size ofthe chromophore). A rapidly decreasing term [ES(n)E]with increasing numbers n can also lead to the fact thatEDA(n)goes through a minimum before it approaches to E(type (c)). Examples of oligomer series D-p-A for theconsidered cases are given in the following sections; thetheoretically imaginable case, in which EDA(n) goes through amaximum, is to my knowledge not unequivocally provenexperimentally to date.

3. Push-Pull-Substituted Oligomers: SyntheticConcepts and Absorption Behavior

3.1. Linear Oligomers D-p-A

The push-pull effect has a strong influence on the long-wavelength electron transitions in conjugated oligomers withterminal donoracceptor substituents (see Section 5). Table 1shows as an example the trans-stilbenes 5af (Scheme 4)which bear a branched dialkylamino group in the 4-positionand various substituents R in the 4-position.[21,25,39] Comparedto 5a with R=H, the compounds with acceptor groups Rexhibit a bathochromic shift, which is more and more

pronounced as the acceptor strength increases. Since thecorresponding excitation S0!S1 is connected to an intra-molecular charge transfer (ICT), the long-wavelength band iscalled a charge transfer band.

An exciting question is how does the intramolecularcharge transfer change when the distance between donor andacceptor groups increases, that is, when the number n ofrepeat units in the p linker is increased. Since dialkylaminogroups with long alkyl chains have a solubilizing character, asystematic study of the oligo(1,4-phenylenevinylene)s(OPVs) 5ae, 6ae, 7ae, and 8ae could be performed.[21,25]

Compounds 5c8c were constructed from 9 by means of aWittigHorner reaction and a simple protecton strategy(Scheme 5). Phosphonate 10 served as an extensionreagent. After a condensation reaction in an alkalinemedium, the deprotection of the masked formyl group in 10

Figure 3. Variants for the convergence of excitation energies EDA(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 afterpassing through a minimum of EDA(n).

Table 1: Absorption in CHCl3 and color of the crystals of the trans-stilbenes 5a5 f.[a]

Compound 5 R lmax [nm] Crystal color

a[21,25] H 366 colorlessb[21, 25] CN 401 yellowc[21, 25] CHO 423 oranged[21, 25] NO2 461 rede[39] HC=C(CN)2 525 dark redf[39] C(CN)=C(CN)2 670 blue

[a] Since the e values are not known for many UV/Vis data discussed inthis article, they are omitted completely.

Scheme 4. Push-pull-substituted oligo(1,4-phenylenevinylene)s (OPVs)58/bf and the comparitive series 5a8a.

Scheme 5. Coupled convergent synthetic strategy for the OPV series58/ae.

H. MeierReviews


occurred directly in the acidic work-up, so that a freealdehyde function was available for the next extension step.The compounds 9, 5c, 6c, and 7c were then reacted in anend-capping process with the phosphonates 11a,b,d to givethe series 5a8a, 5b8b, and 5d8d.[21,25] A condensationreaction of the aldehydes 5c8c with malononitrile 12 can berecommended for the end capping for the preparation ofthe series 5e8e.[39] It was possible to obtain the series of fiveoligomers through a minimum number of synthetic steps byapplying this coupled, convergent synthetic strategy.

Figure 4 depicts the maxima of the long-wavelengthabsorptions of the OPVs 58ae measured in CHCl3. A

pronounced bathochromic effect can be realized for 5a8a, adecreased bathochromic effect for 5b8b, lmax values of 5c8c which are fairly independent of the size of the chromo-phore, a hypsochromic effect for 5d8d, and an even strongerhypsochromic effect for 5e8e.

The evaluation according to Equations (13)(15) is dem-onstrated in Figure 5. The extension of the conjugation leads

to a bathochromic shift, which is shown by a decreasingdifference of ESE for the series 5a8a. The push-pull-substituted OPV series 5b8b bearing the relatively weakacceptor R=CN is characterized by a correction termDEDAwhich weakens the bathochromic shift. In the series 5d8d,with the nitro group as strong acceptor, the term ESE isover-compensated by the term DEDA; thus, a hypsochromiceffect results. The two terms generally cancel each other outin 5c8c (formyl series), so that the absorption maxima arealmost independent of the length of the chromophore.[43]

The compounds 58/ae show, without exception, positivesolvatochromic effects, which originate from intramolecularcharge transfer (ICT). As soon as the push-pull effect issuspended by protonation of the amino group, the batho-chromic shift in the series 5b8b is strengthened and thehypsochromic shift in the series 5d8d is reversed to abathochromic shift (Figure 6). However, the entire absorption

range is located at essentially higher energy when the ICT iscancelled out (see Section 5).

An extension of conjugation in push-pull-substitutedOPVs results in a bathochromic shift, but the decrease ofthe ICT and its effect on the absorption causes an oppositehypsochromic shift (see Section 5). Depending on thestrength of the acceptor, a bathochromic or hypsochromicnet effect results for systems with the same donor; thisincludes the case in which both effects cancel each other out.Exclusive bathochromic effects were found for OPV linkerswith weaker donors, such as alkoxy groups. Compounds13[4446] 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 inpush-pull-substituted OPVs.

The trans-configured double bond in the repeat unit of 58/ae is replaced in the donoracceptor-substituted oligo(1,4-phenyleneethynylene)s (OPEs) 1518/ae shown in Scheme 7

Figure 4. Maxima of the long-wavelength absorptions in the OPVseries 58/ae in CHCl3.

Figure 5. Partition of the energies of the electron transition S0!S1 intoa term (EsE) which reflects the bathochromic shift caused by theextension of conjugation and a term DEDA which takes the decreaseof the ICT and its consequence on the absorption into account. Themeasured data of 58/ad in CHCl3 shown in Figure 4 are the basisfor this distribution.[43]

Figure 6. Absorption maxima in the OPV series 5d8d (n=14); topcurve: measurement in CHCl3, bottom curve: measurement in CHCl3/CF3COOH (10:1).


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by a triple bond, and the didodecylamino group serves as asolubilizing donor function.[41] The preparation of theoligomer series 1518/ae again takes place by a coupled,convergent strategy. The SonogashiraHagihara reaction anda simple protection strategy form the preparative basis.[41]

Starting from 19 and 20, the auxiliary series 15 f17 f and15g17g were first prepared (Scheme 8). The extension

reagent 20 was utilized for the Pd-catalyzed CC couplingstep; the subsequent alkaline deprotection left the ethynylcomponent open for the next extension step. The Sonoga-shiraHagihara 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 OPEseries 15e18e could be obtained by the condensationreaction of 15c18c and malononitrile.[46]

The long-wavelength absorption data for compounds 1518/ae are summarized in Table 2.[41] The evaluation of theUV/Vis spectra is somewhat more difficult in the OPE seriesthan in the OPV series because the long-wavelength absorp-tion band (S0!S1) is superimposed by the higher energyelectron transition S0!S2 (Figure 7 demonstrates this using

16d as an example). A separation of the bands can beperformed for example with an algorithm based on Gaussfunctions.[49] Since non-overlapping absorption bands at longwavelengths are also slightly unsymmetric in these series,exponential functions of type (16) proved to be a success.

e~nn emax exp~nnmax~nn~nnmax

~nn D~nn

16

with D~nn j0:5~nn2~nn1j 17

and e~nn1 e~nn2 emax e1 18

Scheme 7. Push-pull-substituted OPE series 1518/be, comparitiveseries 15a18a, and precursors 15 f17 f and 15g17g.[41, 48]

Scheme 8. Coupled convergent synthetic strategy for the OPE series1518/ae with the precursor series 15 f17 f and 15g17g.

Table 2: Long-wavelength UV/Vis absorption of the OPE compounds1518/ae in CHCl3.

Compound n nmax [103 cm1] lmax [nm]

15a 1 29.330.07 34116a 2 26.740.07 37417a 3 26.280.18 379[a]18a 4 26.160.30 378[a]15b 1 25.840.07 38716b 2 25.840.07 38717b 3 25.970.18 384[a]18b 4 26.090.30 379[a]15c 1 25.000.06 40016c 2 25.640.06 39017c 3 26.000.20 382[a]18c 4 26.020.25 388[a]15d 1 23.150.06 43216d 2 24.810.06 40317d 3 26.150.30 380[a]18d 4 26.150.30 382[a]15e 1 20.450.05 48916e 2 22.620.07 44217e 3 24.751.00 384[a]18e 4 26.80.30 373[a]

[a] The lmax values differ in these cases from 1/nmax of the separated long-wavelength band because of the superposition of the bands.

Figure 7. UV/Vis spectrum of 16d in CHCl3 (c) and its dissectioninto two bands according to Equations (16)(18).[41]

Scheme 6. Push-pull-substituted OPVs with alkoxy groups as donorgroups; absorption maxima in CHCl3.

H. MeierReviews


The evaluation of the data of 1518/ad is visualized inFigure 8, which corresponds more or less to Figure 5 for theanalogous donoracceptor-substituted OPV systems 58/ad.[50] The interpretation of Figure 8 corresponds to theinterpretation of Figure 5. The bathochromic effect resulting

from the extension of conjugation is surpassed in the NO2series by the hypsochromic effect, which arises from thedecrease in the ICT; the same is true to a lesser extent in theCHO series 15c18c ; the two effects cancel themselves outalmost completely in the CN series 15b18b. Altogether, theOPE linker is still somewhat more prone than the OPV linkerto exhibit the unusual hypsochromic effect with increasingnumbers n.[51] This situation has the consequence that evenmethoxy groups (as weaker donors) do not show a red-shiftwhen in combination with strong acceptors such as NO2.Compounds 22 and 23 in Scheme 9 are shown here as

examples. Whereas lmax(2)lmax(1) amounts to + 16 nm for22a/23a, a value of 3 nm was found for 22b/23b.[46] Thedialkylamino group (a strong donor) does not effect aninversion of the red-shift in the series 15g17g bearing theethynyl group (a weak acceptor group).

Only single examples of push-pull-substituted oligoenes(OEs) of type 1 (polymethine dyes) or 24a,b29a,b (n= 1,2,3,) are known; an exception is represented by the aldehydes24c29c (R=CH3),

[52ac] which were prepared from theZincke aldehyde by chain extensions with Grignard reagentsand hydrolysis of the corresponding cyanines (Scheme 10).[52a]

The absorption spectra of 24c29c measured in CH2Cl2show a pronounced bathochromic shift for increasing num-bers n of repeat units. Even the stronger electron-withdrawingdicyanovinyl group does not change this effectnor when thetrans double bonds are fixed in a transoid arrangement byincorporation in rings.[52d] Bathochromic effects were alsoobserved by Lehn, Blanchard-Desce, Zyss, and co-workers inthe series 30a32a and 30b32b, which contain carotinoidunits as p linkers (Scheme 11). The synthesis of these com-

pounds was performed by one-sided Wittig and WittigHorner reactions, respectively, of the carotinoid dialdehydesand the corresponding phosphorus reagents of 1,3-benzodi-thiole followed by subsequent condensation of the still-present aldehyde function with malononitrile.[53,54]

Figure 8. Partition of the electron transition energies (S0!S1) of 1518/ad into a term (ESE) which reflects the bathochromic shiftcaused by the extension of conjugation and a term DEDA which takesthe decrease of the ICT and its consequence on the absorption intoaccount.

Scheme 10. Push-pull-substituted oligoenes: Maxima of the long-wave-length absorption in CH2Cl2.

Scheme 9. Maxima of the long-wavelength absorptions of push-pull-substituted OPEs with methoxy groups as donors and CN or NO2groups as acceptors (CHCl3 as solvent).

Scheme 11. Carotinoid push-pull compounds and the maxima of theirlong-wavelength absorption in CHCl3.

[5355]


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The bathochromic shift for increasing length of thechromophore was also found by Blanchard-Desce, Barzou-kas, Marder, and co-workers who studied the series 3335,which includes an aromatic or heteroaromatic ring in thep linker at the donor end (Scheme 12).[3, 56,57] The syntheticstrategy for 3335 is depicted in Scheme 13. The aldehyde

series 33a, 34a, and 35a with the corresponding donor groupsD acted as the central series and were constructed by meansof the Wittig reaction with the extension reagent 36. Thecondensation reaction with the active methylene components37be was then selected as the end capping.[57] The maximaof the long-wavelength absorptions of a selection of com-pounds 3335 are listed in Table 3. For constant values of n,the lmax value becomes higher as the donor and acceptor

strength increases; lmax(n+1)> lmax(n) is valid within eachseries. Analogous bathochromic effects were measured forseries of compounds with 1,3-benzodithiole donor groups andcarotinoid linkers which contain an aromatic or heteroaro-matic ring (4-nitrophenyl, 4-cyanophenyl, 4-pyridyl) at theacceptor end.[53,55]

Push-pull-substituted oligoenes bearing aromatic rings atboth chain ends[53, 55,58b,59] show a diminished bathochromiceffect. A comparison of the series 38a41a and 38b41b(Scheme 14) shows a characteristic result.[46,58b,59] Compounds

analogous to 38b40b, but with a CN group instead of theNO2 group, were investigated in the context of their dualfluorescence and twisted intramolecular charge transfer(TICT) states;[6066] however, a discussion of these states isbeyond the scope of the present Review.

Push-pull-substituted oligoynes (OIs) are scarcelyreported in the literature to date.[67] The aminoketones 42and 43 and the aminonitro compounds 44 and 45 are givenhere as examples (Scheme 15). Benzene rings at the ends ofthe p linker cause a hypsochromic effect for neither the push-pull-substituted oligoenes nor the corresponding oligoynesin contrast with the accordingly substituted OPV and OPEsystems.

Scheme 12. Oligoenes with (hetero)aromatic rings as donors as well asvarious acceptors.

Scheme 13. Synthetic strategy for the series of compounds 3335.[57]

Table 3: Maxima of the long-wavelength absorption lmax [nm] of theoligoenes 33b (n=04), 33d (n=03), 34a (n=14), 34b (n=05),34 f (n=03), and 35c (n=02) in CHCl3.

n 33b[56] 33d[57] 34a[56] 34b[56] 34 f[56] 35c[57]

0 443 603 458 494 5121 507 695 413 531 542 6192 540 773 446 572 556 7103 571 826 469 594 5664 586 486 6065 613

Scheme 14. Maxima of the long-wavelength absorption of oligoeneswith terminal dimethylamino/nitro substitution, which include abenzene ring in the p linker on the donor side (38a41a) as wellas on the donor and acceptor sides (38b41b); measured in CHCl3.

Scheme 15. Maxima of the long-wavelength absorption of oligoyneswith push-pull substitution.[6871]

H. MeierReviews


Oligo(1,4-phenylene)s (OPs) differ from the OPVs,OPEs, OEs, and OIs discussed so far as a result of a strongtorsion of the benzene rings along the chain. Torsional anglesbetween 30 and 408 can be assumed, which considerablyinfluence the conjugation and the ICT.[71, 72] Since the reso-nance integrals do not only depend on the different atomicdistances in the p linker but also on the torsion of thep orbitals, an acceleration of the convergenceE(n)!E can inprincipal be expected for increasing torsion angles. Aconsiderable planarization of the 1,4-phenylene chain byanelated five-membered rings results in a bathochromicshift;[7173] Table 4 shows a comparision of biphenyls 46a49a and fluorenes 46b49b (see Scheme 16 for structures).

The synthetic strategy of oligo(1,4-phenylene)s is basedon usual Pd-catalyzed arylaryl CC coupling reactions suchas the Suzuki, Negishi, Stille, Yamamoto, or Kumadareactions.[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

construction of the auxiliary series 52. The primaryinsertion of the Pd into the CBr bond of 51 is decisive forthis step. Compounds 51 and 52 were then subjected to across-coupling reaction with 53.[74]

Since the range of the ICT is considerably shorter thanthe conjugation (see Section 5), the continuous torsions alongan oligo(1,4-phenylene) chain can lead to a fast decrease inthe ED(n)E value (see Figure 3c). Consequently, theenergy EDA(n) for the electron transition S0!S1 can passthrough a minimum, and lmax(n) accordingly through amaximum (type (c), Figure 3)in particular, when the cor-rection termDEDA of the ICT is large, that is, when a push-pulleffect of a strong donor and a not too weak acceptor ispresent. This is realized for 54 and 55 (Table 5); the maximum

value of lmax is found in both cases for n= 2, but can be solventdependent. When the amino function is substituted by theless-strong donor OCH3, shifts to longer wavelengths areobtained for the series with A=NO2 as well as for the serieswith A=CN with increasing values of n (type (a), Figure 3).

The introduction of thiophene or furan rings instead ofbenzene rings in the p linker results in the absorption maximashifting to longer wavelengths (Scheme 18 shows someexamples).[71] Push-pull-substituted oligomers whose p link-ers consist exclusively of five-membered-ring heterocycles,were studied in particular for the thiophene series. Table 6offers a comparison of bithiophenes with various donor andacceptor groups; it can be seen that the combination of a 1-pyrrolidine group and a nitro group in particular results in afar-red-shifted CT band. Thus, an interesting dependence ofthe absorption on the number n of repeat units can beexpected for the push-pull-substituted oligothiophenes (OT;oligo(2,5-thienylene)s) studied by Effenberger and Wrth-ner.[7981] Table 7 shows a comparison of the methoxy-nitroseries 66ad (n= 14) and the 1-pyrrolidino-nitro series 67ad (n= 14). Whereas a monotonous bathochromic shift withincreasing n was found for 66ad, the lmax value of 67adpasses through a maximum at n= 3.[82] Hence, the latteroligomer series belongs to type (c) in Figure 3 and resemblesthe corresponding OPs 54ad and 55ad. To date, there are

Table 4: Maxima of the long-wavelength absorption of thebiphenyls 46a49a and comparison with the correspondingfluorenes 46b49b.[71, 73]

R1 R2 Biphenyl driva-tives

lmax [nm] Fluorene deriva-tives

lmax [nm]

H H 46a 252[a] 46b 262[a]

H NO2 47a 304[a] 47b 328[a]

N(CH3)2 H 48a 301[b] 48b 310[b]

N(CH3)2 NO2 49a 390[b] 49b 417[b]

[a] Measurement in 1,4-dioxane. [b] Measurement in CHCl3.

Scheme 16. Planarization of the torsional angles of biphenyls influorenes (see Table 4 for R1, R2).

Scheme 17. Synthetic strategy for the construction of donoracceptor-substituted OPs 54.[74]

Table 5: Maxima of the long-wavelength absorption of donoracceptorsubstituted oligo(1,4-phenylene)s D-[-C6H4-]n-A: 54ad,

[75] 55ad,[71, 76]

56ac,[71] and 57ac.[46, 77,78]

Compound A D n lmax [nm] Solvent

54a CN N(CH3)2 1 290 CCl454b 2 342 CCl454c 3 332 CCl454d 4 314 CCl455a NO2 NH2 1 373 EtOH55b 2 378 EtOH55c 3 358 EtOH55d 4 340 EtOH56a NO2 OCH3 1 302 1,4-dioxane56b 2 332 1,4-dioxane56c 3 340 1,4-dioxane57a CN OCH3 1 247 DMSO57b 2 292 DMSO57c 3 302 DMSO


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few D-p-A series with repeat units consisting of thiophenerings and CC double or triple bonds.[83] Oligo(2,5-thienyle-nevinylene)s (OTVs) which bear 4-diethylaminophenylgroups as the donor and NO2, CHO, or CH=C(CN)2 groupsas the acceptor have to be mentioned in this context; all thesecases correspond to bathochromic series (type (a)).[83a,b]

The oligo(2,5-thienyleneethinylene)s (OTEs) 68ac and

69ad, which were recently prepared by a SonogashiraHagihara reaction, should also be mentioned (Scheme 19).[46]

The series 68ac belongs to type (a) in Figure 3, whereas theseries 69ad exhibits an S0!S1 transition, which is independ-ent of the length of the chromophore.

It remains to state at the end of this section that D-p-Asystems can also be generated by protonation of suitable D-p-D systems. Not only does the protonation of terminal aminogroups have to be considered, but also the thiophene ringitself, as demonstrated in Scheme 20. Protonation of 70a,b inCD2Cl2/CF3COOH leads to a shift of more than 200 nm tolonger wavelengths.[84]

3.2. Oligomers with D-p-A-p-D or A-p-D-p-A Structures

Conjugated oligomers with a donoracceptordonorstructure require bidentate acceptor groups in the center ofthe molecule. The presence of carbonyl and related groups inthis position leads to cross-conjugation. An example ispresented by Michlers ketone 72 (m= n= 1) and its higherhomologues, though little is known about them.[85] Singleexamples of linearly conjugated D-p-A-p-D compounds existin the series of azobenzenes 73,[46,8688] pyridazines 74,[89]

pyrazines 75,[90] and 1,2,4,5-tetrazines 76 ;[46, 91] a systematicstudy was only performed for the squaraine series 77ad[92]

(Scheme 21).Compounds 77, which are obtained by coupling the

corresponding resorcinols and squaric acid, show an absorp-

Scheme 18. Red-shift of the long-wavelength absorption band onreplacement of the benzene rings in the p linker by thiophene or furanrings.

Table 6: Maxima of the long-wavelength absorption of donoracceptor-substituted bithiophenes in n-hexane.[79,80]

Compound D A lmax [nm]

63 OCH3 CHO 37264 N(CH3)2 CHO 42165 SCH3 NO2 39166b OCH3 NO2 40861 N(CH3)2 NO2 466

67b NO2 499

Table 7: Maxima of the long-wavelength absorptions of the oligothio-phenes 66ad and 67ad in n-hexane.[7981]

Compound D A n lmax [nm]

66a OCH3 NO2 1 34066b 2 40866c 3 44266d 4 454

67a NO2 1 408

67b 2 49967c 3 50567d 4 497

Scheme 19. Maxima of the long-wavelength absorption of donoracceptor-substituted oligo(2,5-thienyleneethynylene)s 68ac and69ad (measured in CHCl3).

[46]

Scheme 20. Protonation of symmetrical oligothiophenes for thegeneration of D-p-A systems.

H. MeierReviews


tion with a strong red-shift on going from n= 0 to n= 1; theblue dye 77a is thereby converted into the NIR dye 77b.[92]

The electron transition which is predominantly localized inthe squaraine ring[93] in 77a becomes a transition in a push-pull-substituted stilbenoid compound and hence results in apronounced hypsochromic effect for n= 2,3.[92] This effectoccurs only when the measurements are made in an organicsolvent such as CHCl3 (Figure 9)when an acidic mediumsuch as CF3COOH is used, the amino groups becomeprotonated and consequently lose their donor character; thegenerated cations then have an A-p-A-p-A structure andshow the expected bathochromic shift (from n= 0 to n= 3).

An extended planarization of the chromophore, in 77through the formation of intramolecular hydrogen bridges, isan essential precondition for an efficient push-pull effect.Figure 10 shows, using compound 78 as an example, how theabsorption is shifted from the NIR range to the UV range

(Dlmax> 450 nm) by the addition of ethanol which acts as ahydrogen bridge donor.[94] The benzene rings turn out of theplane of the squaraine rings when intermolecular hydrogenbridges are formed.

Compounds of the type A-p-D-p-A require bidentatedonors such as O, S, or NH. No oligomer series of this typecurrently exists. Recently, the two first members (n= 1, 2) ofthe series 79 (Scheme 22) with ferrocene as a strong donorwere studied; they exhibit a bathochromic effect: lmax(2)>lmax(1).

[95]

3.3. Star-Shaped Compounds A-(p-D)3 and D-(p-A)3

Tridentate cores have to be considered in addition tothe bidentate central acceptors or donors described inSection 2.3. Scheme 23 shows some central building blocks.

Scheme 21. Examples of D-p-A-p-D systems.

Figure 9. Maxima of the long-wavelength absorption of the squaraines77ad (n=03) in CHCl3 ~ and in CF3COOH &.

Figure 10. Absorption of squaraine 78 in CHCl3 and in CHCl3/EtOHmixtures.

Scheme 22. Maxima of the long-wavelenth absorption of A-p-D-p-Acompounds with ferrocene as the central donor (measured in CHCl3).


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The series of methylium salts 80ad (Scheme 24) wasprepared; they can be regarded as higher homologues of thewell-known triphenylmethane dyes.[96] The synthesis of 80adwas realized from the corresponding carbinol bases, whose

treatment with acid led to the elimination of the OH groupbound on the central carbon atom. The cations 80ad, strictlyas their carbinol bases, exhibit monotonously growing lmaxvalues with nof course shifted by the push-pull effect fromthe UV/Vis region to the Vis/NIR region (Figure 11). Thesame is valid for methylium ions which are linked throughpolyene chains -(CH=CH)n- to ferrocene as the terminaldonor group; the lmax value rises from 618 to 1187 nm ongoing from n= 2 to 14.[97]

In contrast to the weakly pH-dependent alkoxy-substi-tuted color salts 80ad and 81a,b, the dialkylamino com-pounds 82a,b and 83ac (Table 8) exhibit absorptions whichdepend strongly on the concentration of H+ ions.[98101] Theformation of the cations from the carbinols or their ethers oninteraction with strong acids leads first to absorption maximawhich are located far in the NIR region. A lmax value of1003 nm is found for 82b,[100] and further addition ofCF3COOH induces a decrease in the intensity of this band

and a slight blue-shift. Simultaneously, a new band appears atlmax= 530 nm, which increases strongly and is red-shifted to615 nm at high excess of CF3COOH.

[100] Table 8 shows thelmax values at the end of this titration when all threeterminal amino groups of 82b are protonated. Hence, thepush-pull character in the three arms is lost. Thus, it isunderstandable that the lmax values of the methoxy com-pounds 81a,b with n= 1 are higher than those of thedialkylamino compounds 82a,b and 83a.

Of the central acceptors for three-star oligomers shown inScheme 21, the 1,3,5-triazines deserve special mention. Thealkoxy-substituted compounds 8486 and the dialkylaminocompounds 87 and 88were prepared by alkaline condensationreactions of 2,4,6-trimethyl-1,3,5-triazine with the corre-sponding aldehydes (Scheme 25).[101] The influence of thepush-pull effect in 84 and the even stronger effect in 87 can beseen by comparison to the unsubstituted 2,4,6-tristyryl-1,3,5-triazine, which has a lmax value of 327 nm. The absorption for

Scheme 24. Methylium salts with OPV chains which bear terminaldonor groups.

Figure 11. Maxima of the long-wavelength absorptions of thetrifluoroacetates 80ad (n=14) in CHCl3/CF3CO2H (7:3) and thecorresponding carbinols (bottom curve) in CHCl3. Extrapolation to lby application of Equation (11).

Table 8: Maxima of the long-wavelength absorption of the colored salts81a,b, 82a,b, and 83ac.

R D n lmax[a] [nm]

81a[97] H OCH3 1 71881b[99] CH3 OCH3 1 73382a[97] H N(CH3)2 1 60982b[99] CH3 N(CH3)2 1 61583a[100] H N[CH2CH(C6H13)2]2 1 62283b[100] H N[CH2CH(C6H13)2]2 2 74083c[100] H N[CH2CH(C6H13)2]2 3 790[a] End values of lmax at a high excess of CF3COOH.

Scheme 23. Central building blocks for conjugated three-armoligomers: a) acceptor groups for star-shaped compoundsA-(p-D)3, b) donor groups for star-shaped compounds D-(p-A)3.

H. MeierReviews


the series 86ad is red-shifted as the length of the conjugatedarms increases. The absorption quickly approaches (nECL= 7)a limiting value of l= 427 nm (Figure 12). An interestingfeature is given by the indicator behavior of 87. Unexpect-edly, the first protonation on addition of trifluoroacetic acidoccurs at the triazine ring, even though, for example, N,N-dimethylaniline has a higher basicity than 1,3,5-triazine. Theyellow solution in CHCl3 turns deep violet (lmax= 549 nm).Further addition of CF3COOH leads to a protonation of theterminal amino groups and the solution bleaches (lmax=365 nm).[102] The primary red-shift is based on an increase inthe push-pull effect. However, as soon as the amino functionsbecome protonated, their donor character is lost and an A-(p-A)3 system is obtained.

Since the protonated 1,3,5-triazine ring is a strongacceptor and the amino group a strong donor, an extensionof the chromophore should result in a hypsochromic effect.We established this relationship by comparing 88a and 88b(Figure 13). Compound 88a, like 87, shows a red-shift uponweak acidification, and afterwards a blue-shift on increasedacidification. The higher homologue 86b behaves in exactly

the opposite way: the primary blue-shift is followed by a red-shift. The logical explanation is the following: the extension ofthe chromophore causes a bathochromic effect (445!458 nm) for 88 itself; a much stronger push-pull effect ispresent in the species with a protonated 1,3,5-triazine ring andthe extension of the chromophore leads to a hypsochromiceffect (551!394 nm); there is no push-pull effect in thecompletely protonated compounds; the normal extension ofthe conjugation then results in a bathochromic shift (368!459 nm).[103]

The benzene system seems to be the most interestingamong the central donor groups listed in Scheme 23; how-ever, until now only a few examples, such as 89a,b(Scheme 26), have been prepared and studied.[104] Compound89 and analogous benzene derivatives bearing three acceptorgroups and three conjugated arms with terminal donorsubstituents (see Scheme 23) can be regarded as parentsystems of hexasubstituted benzenes 91 having an octupolarcharacter and therefore special significance for nonlinearoptics.[70] Scheme 27 shows a synthetic approach to 91 basedon the trimerization of alkynes. To my knowledge, series ofconjugated compounds of type 91 with systematicallyextended p linkers are currently unknown.

Figure 12. Top: long-wavelength absorption bands of the 1,3,5-triazines86ad (n=14) in CH2Cl2; bottom: extrapolation of the lmax valuesaccording to Equation (11) and determination of the effectiveconjugation length according to Equation (12).

Scheme 25. 1,3,5-Triazines with donor-substituted OPV chains and themaxima of their long-wavelength absorption (measurement of 8486in CH2Cl2, of 87 and 88 in CHCl3).

[101]


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4. Nonlinear Optics in Series of Oligomers withDonorAcceptor Substitution

Nonlinear optical properties (NLO) of organic materialsare of great interest for optical data storage, data processing,and data transfer,[1p,105] and conjugated NLO chromophoreswith a pronounced push-pull character are of high signifi-cance. Figure 14[106] provides an explanation for this: if light is

shining on a compound consisting of D-p-A molecules, theE vector causes a high polarization P(E). The periodicity ofE(t) of the light wave corresponds to the periodicity P(t);however, the function P(t) is not a sine function. Its Fouriertransformation leads to a progression (19), which contains theoptical susceptibilities c(n) of nth order. For a single molecule,this corresponds to Equation (20) for the induced dipolemoment.

P e0 c1E c2E2 c3E3 . . . 19

mind: aE bE2 gE3 . . . 20

Apart from the linear polarizability a, there are hyper-polarizabilities b and g (of first and second order, respec-tively) which are many magnitudes smaller. Since b and g arevery small (factors of 1010, 1017), high intensity laser light isneeded to measure the frequency doubling and tripling. Theadvantage of donoracceptor-substituted conjugated p sy-stems arises from the fact that the shift of electrons from the

Scheme 26. Three-star compounds 89a,b with central donor andterminal acceptor groups (absorption maxima in CHCl3).

Scheme 27. Octupole 91 with a benzene core obtained bycyclotrimerization of alkynes 90 with push-pull character.

Figure 14. Description of nonlinear optics of D-p-A systems (SHG:second harmonic generation, THG: third harmonic generation).[106]

Figure 13. Top: bathochromic shift of the absorptions band of 88a(c) by protonation of the 1,3,5-triazine ring (d) and subsequenthypsochromic shift by complete protonation (g) in CHCl3/CF3COOH; bottom: hypsochromic shift of the absorption band of 88b(c) by protonation of the 1,3,5-triazine ring (d) and subsequentbathochromic shift by complete protonation (g) in CHCl3/CF3COOH.

H. MeierReviews


donor to the acceptor is highly efficient (Figure 14); thatbecomes particularly apparent in the b values. Although D-p-A molecules are not centrosymmetric, they can crystallize incentrosymmetric space groups. The centrosymmetry mustthen also be valid for the function P(E) [Eq. (21)]. This

PE PE 21

requires that c(2)= b= 0 in the progressions (19) and (20).Many compounds with donoracceptor substitution unfortu-nately crystallize in centrosymmetric crystal classes. Star-shaped systems of type 89 or 91 avoid this.

The influence of the substitution, in particular of the push-pull substitution, on b and g becomes evident in the trans-stilbene derivatives in Table 9. Strong donors and strongacceptors enhance the b and g valuesin analogy to thedipole moment m. However, a direct relationship between mand b or g does not exist, as the comparison of 13a and 96 inTable 9 reveals.

Since strengthening the push-pull effect also shifts the CTband to higher lmax values (see Section 3.1), a power law b ~lkmax or logb ~ loglmax seems to apply.

[107] The second orderhyperpolarizability g of trans-stilbenes scales with b.[107] Thepush-pull compounds N,N-dimethyl-4-nitroaniline and (E)-4-dimethylamino-4-nitrostilbene (DANS; 38b) represent NLOstandards that are often used.

Incorporation of a triple bond instead of a trans-config-ured double bond results in b and g decreasing considerablyand m decreasing to a small extent. For 4-dimethylaminophenyl-4-nitrophenyle-thyne b= 46 1030 esu, g= 151 1036 esu,and m= 6.1 1018 esu (CHCl3).

[71] Theincorporation of benzene rings alsoproved to be unfavorable relative to equallyextended p linkers consisting of olefinicdouble bonds (Scheme 28).

The b values, which were obtained, forexample, by the EFISHG method (electricfield induced second harmonic generation),depend somewhat on the applied wave-length. A simple correction to wavelength-

independent, so-called static, b0 values can be made on thebasis of the two-level model[108,109] which works for D-p-Asystems because of the domination of the CT transitions.[110]

The deterioration of the conjugation as a consequence ofthe torsion in oligo(1,4-phenylene)s (OPs) is alreadyexpressed in the lmax values, but it is also noticeable in thehyperpolarizabilities b. Table 10 shows a comparison ofbiphenyls and the corresponding fairly planar fluorenes forthis purpose. The correlation is much more complex for theg values, as the comparison of 47a,b and 56a,b demonstrates.

The extension of the p linker in D-OP-A systems canresult 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 caseis realized with 56ac (D=OCH3/A=NO2) and the latterwith 55ad (D=NH2/A=NO2). The g values always increasewith increasing n. The effects are less pronounced in each casecompared to the oligoenes 101 and 102 (Table 11)evenwhen the oligoenes bear benzene rings on one or both endsof the p linker. Consequently, oligoenes form thefocus in NLO investigations of push-pull-substitutedoligomers.[3,53,54,56,57,58a,b,71,113119] Some examples are summar-ized in Table 12.

Analogous results are obtained for OEs with carotinoidlinkers (see Section 3.1).[54,113,115,116,117,119] The incorporation offive-membered-ring heterocycles, such as furan or thiophene,in the p linker generates higher hyperpolarizabilities b than

Scheme 28. Comparison of the b0 values of D-p-A compounds havingthe same length of p linker but a different number of benzenerings.[58b]

Table 10: Comparison of dipole moments m and hyperpolarizabilities b and g of biphenyls and thecorresponding fluorenes.[71]

R1 R2 Biphenyl[a] 1018 m[esu]

1030 b[esu]

1036 g[esu]

Fluorene[a] 1018 m[esu]

1030 b[esu]

1036 g[esu]

H H 46a 0 0 10 46b 0 0H NO2 47a 3.8 4.1 15 47b 4.1 5.1 29OCH3 NO2 56a 4.5 9.2 39 56b 4.7 11 28N(CH3)2 NO2 49a 5.5 5.0 130 49b 6.0 55

[a] Measurement of 46a,b, 47a,b, 56a,b in 1,4-dioxane, of 49a,b in CHCl3.

Table 9: Dipole moments m and hyperpolarizabilities b and g of trans-stilbenes and their derivatives which bear a donor group in the 4-positionand/or an acceptor group in the 4-position.[107]

Compound Solvent 4-R 4-R 1018 m[esu][a]

1030 b[esu][a]

1036 g[esu][a]

92 CHCl3 H H 0 0 2693 1,4-dioxane N(CH3)2 H 2.1 10 6494 1,4-dioxane H NO2 4.2 11 6138b CHCl3 N(CH3)2 NO2 6.6 73 22595 CHCl3 NH2 NO2 5.1 40 14713a CHCl3 OCH3 NO2 4.5 34 9396 CHCl3 SCH3 NO2 4.3 34 10097 CHCl3 N(CH3)2 CN 5.7 36 12598 CHCl3 OCH3 CN 3.8 19 54

[a] esu: electrostatic units; m : 1030 Cm=0.29981018 esu=0.2998 D;b : 1050 Cm3V2=2.6941030 esu; g : 1060 Cm4V3=8.0781036 esu.


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for the corresponding benzene systems with comparablelinker lengths, but lower b values than analogous compoundswith diene building blocks. Table 13 shows comparisons ofcompounds 52/61, 38b/59/58, and 56c/60/66c/102b.

Empirical laws were proposed many times on the basis ofmeasured data for the hyperpolarizabilities b and g. Theselaws reflect the influence of the substituents[71] and theinfluence of the number n of repeat units in the oligo-mers,[54, 56,57,110,112,116,129] and include equations such as (22) and(24). Additional power laws derived on a theoretical or

b0 nk or logb0 logn 22

e:g: k 1:9,562:0,110 2:454

23

g n or logg logn 24

e:g: 4:2,56 2:7116 25

semitheoretical basis also exist.[23] The rare case in which thevalue of b does not increase monotonously with n, as forexample in the OP series 55ad, cannot be covered byEquation (22). The individual exponents k und refer to themeasurement of a certain series of donoracceptor-substi-tuted oligomers under certain conditions; moreover, theyalways refer to a narrow range of repeat units 1 n 5. Theexpression > k is always valid for an oligomer series, whichmeans that the g values increase more strongly than theb values for increasing numbers n. The exponents k and should virtually be functions of n : k(n), (n). However, anessential distinction arises from the fact that b0 shouldapproach a limiting value b for high numbers n, whereasthis 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 notexist, nor for systems which are nearly as large. Theconvergence problem of b and g/n can be comparedanalogously to lmax!l by the aid of exponential func-tions.[122] Since the length L of the chromophore in conjugatedoligomers is a linear function of n, b(n) and g(n) can bedescribed 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 S0!S1, the correspond-ing transiton moment m01, and the difference Dm of the dipolemoments m(S1) and m(S0)

[123] by applying the two-level modelsuggested by Oudar and Chemla.[108,109] Equation (26) pro-

b0 6m201Dml

2max

h2c226

vides the possibility to determine b0 for the normallyaccessible region located away from the limiting value bynormal absorption measurements of lmax and m01 as well as by

Table 11: Hyperpolarizabilities b and g of push-pull-substitutedoligo(1,4-phenylene)s (55, 56) and oligoenes with one (101) or twoterminal benzene rings (102).[3,71]

Compound D A n Solvent 1030 b[esu]

1036 g[esu]

56a OCH3 NO2 1 1,4-dDioxane 5.1 1056b 2 9.2 3956c 3 11.055a NH2 NO2 1 NMP 10 2155b 2 24 9655c 3 16 12455d 4 11 133101a N(CH3)2 CHO 1 CHCl3 30 63101b 2 52 140101c 3 88 257102a OCH3 NO2 1 CHCl3 34 93102b 2 47 130102c 3 76 230102d 4 101

Table 12: Dipole moments m and hyperpolarizabilities b0 and g ofoligoenes.[56] (Structures shown in Scheme 12.)

Compound n m [D] 1030 b0 [esu] 1036 g [esu]

33b 0 8.8 741 9.3 195 3782 9.8 361 17243 10 642 73634 10 1229

34b 0 9.7 631 9.0 195 3952 10.2 4233 10.5 8104 11 10435 11 1530

Table 13: Comparison of the hyperpolarizabilities b0 of D-p-Acompounds 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

H. MeierReviews


electrooptical absorption measurements (EOAM) of Dm. Ifm01(n), Dm(n), and lmax(n) all increase with n, it is also valid forb0(n).

[79,81] The situation is more critical when m201Dm increasesbut l2max decreases. We obtained b0 values of 198, 287, and346 1050 Cm3V for the series 5d7d (n= 13), which meansthat the value of b0 also increases in the case of ahypsochromic series.[25] The fact that b0 increases with n,irrespective of whether lmax increases or decreases, revealsthat a fitting function b0(lmax) is generally not meaningful.However, the linear function logb= f(lmax) can give evidencefor substituent effects in compounds (for example, trans-stilbenes) which have the same p linker (see compounds inTable 9).[107] Substituent effects, but not necessarily the push-pull effect, have an effect on the size of g. Hence, there areexamples for which the g values of D-p-A compounds are inbetween the g values of D-p-D and A-p-A compounds.[116]

The power laws (27) and (28) suggested by Flytzanis and

c3 L6d orlog c3 logLd

27

c3 l6max orlog c3 log lmax

28

co-workers[124, 125] only make sense when lmax increases withthe so-called delocalization length Ld.

[126] The relation (27) isproblematic for short p linkers with Ld n[105a] and thecorrelation of c(3) or g with lmax is also not generally valid.In the hypsochromic series 15d, 16d, , lmax(n) decreaseswith growing n, but g(n) increases.[127] The relation (29) is

g nm or logg m logn 29

even reliable in such a case, in which m(n) decreases with nand approaches the limiting value of 1.[105a] This convergencedefines an effective conjugation length n0ECL, which can,however, be different from nECL obtained from the conver-gence of the long-wavelength absorption. The absorptiontakes S0 and S1 into account, and an essential-state model ofthree, four, or more states is taken as the basis for the THG.

To summarize Section 4, the statement can be made thatin addition to the substituent effects of D and A on g and thepush-pull effect on b, the nature of the p linker (type, length)plays a decisive role in the size of the b and g values. Even thesame linker has a different effect when its polarization isdifferent with various D/A pairs. An explanation for this isprovided in Section 5 in the context of VBtheory.

5. VB and MO Models of D-p-ASystems

As already stated in Scheme 2, themodel usually applied for D-p-A systemsin the literature is a valence-bond model. Itdescribes the ground state S0 and the firstelectronically excited singlet state S1 of suchcompounds by a linear combination of a

zwitterionic Z and an electroneutral resonance structure N[Equations 30 and 31].

D-p-A Z $ D-p-A N

yS0 cyZ1c2

pyN

30

yS1 1c2

pyZ cyN 31

Several suggestions were made for the determination ofthe weights of the two limiting structures. Wortmann andco-workers[79] proposed the parameter c2, which can beobtained by Equation (32); the participation of Z and N is

c2 12

1 Dm

4m201 Dm2q

32

considered therein to be included in the difference in thedipole moments m(S1)m(S0)=Dm and the transition momentm01. Integration of the absorption curve or an approximationformula provide m01; Dm is accessable for example by electro-optical absorption measurements (EOAM). The difference Dof the dipole moments of Z and N is related to Dm and c2

according to Equation (33). Barzoukas et al.[6] used sinq/2

Dm

D 12c2 33

instead of c in Equation (30) and defined the mixedparameter MIX as cosq which corresponds to (2c21) inEquation (33).

Marder et al.[25] introduced the parameter BLA for thealternation of bond lengths in linear D-p-A chains. Theparameter BOA for the alternating bond orders is closelyconnected to the alternation of bond lengths. BLA isaccessible from X-ray data and is empiricly related to MIX[Eq. (34)].[6] However, BLA is not very useful if the p linker

BLA in 0:11MIX 34

contains aromatic rings, because aromatic rings will keep theirtypical adjusted bond lengths.

Table 14 shows the relationship between the polarizabil-ities a, b, and g and the discussed parameters c2, MIX, andBLA. The parameter f proposed by Lu, Chen et al.,[128] wasalso included in the table; f and MIX are connected by

Table 14: Dependence of the polarizabilities a, b, and g on the parameters c2, MIX, f, and BLA with thecorresponding weigths of the resonance structures Z and A.

c2 [a] Weights N/Z or Z/N MIX[b] f[a] Weights N/Z or Z/N BLA[c] []

amax 0.5 50:50 0 0.5 50:50 0b=0 0.5 50:50 0 0.5 50:50 0jb jmax 0.211 79:21 1p55 0.50.224 72:28 0.049g=0 1p55 0.50.224 72:28 0.049jg1 jmax

37

q0.50.327 83:17 0.072

jg2 jmax 0 0.5 50:50 0[a] 0< c2

Equation (35). Moreover, for a better illustration, theweights N/Z (and equivalent Z/N) valid for c2 and for

f MIX 12

35

MIX and f are included in Table 14. The maximum of a isreached for a 50:50 ratio of Z and A, that is, for a completeequalization of the bonds (cyanine limit). Here, b= 0 (inreality b is very small). The maximum of b is reached for anN/Z ratio of about 3:1 (and its reciprocal); but then g= 0. Aparameter value close to the maximum of g can berecommended for a simultaneous optimization of b and g.[129]

The parameters proved to be successful for relativelyshort D-p-A systems with OE linkers. These parameters arenot adequate when c2 or f are close to 0 or 1 (MIX close to1or + 1), and the VB model with the y functions (30) and (31)is generally not suitable for these cases.[21] An S1 structurewith predominant Z character (1c2 0.9) has a smallprobability for an extended chromophore which has apredominant N character in S0 (c

2 0.1). The resonancestructure Z should have a similar energy as the resonancestructure N so that the resonance Z$N is combined with anoticeable energy gain. The latter is certainly not realizedwhen the p linker consists of repeat units which containbenzene rings or other aromatic ring systems.

An MOmodel appears to be much more suitable for sucholigomers. In this model, partial dipole moments are presenton the donor and the acceptor sides, and each partial momentis itself composed of an intrinsic part (mD, mA) and a part whichis induced in each case by the dipole on the other chain end(mDi, mAi). The induced parts become smaller with increasingseparation of the donor and acceptor. The extent of thepolarization from the ends of the chain in the direction of itscenter should rapidly decrease. This was also demonstrated bymeans of BLAvalues and partial charges obtained by a DFT/B3LYP/6-31G* calculation.[130]

Semiempirical methods need to be considered for thecalculation of higher oligomers. We chose the AM1 methodfor the optimization of the geometry and the INDO/S methodfor the electron transitions.[21, 22] Scheme 29 shows the polar-ization derived from AM1) calculations of the olefinic doublebonds of the OPVs 5d7d and the corresponding triple bondsof the OPEs 15d17din each case expressed in terms of thecharge differences Dq and Dq, respectively. The positive andnegative partial charges are related to the standards trans-stilbene and tolane, respectively.[131] The Dd and Dd valuesreflect the differences in the chemical shifts of the 13C nucleiof the double and triple bonds. A direct correlation can beseen: a large Dq() value results in a large Dd() value. Theessential polarization can be found on the chain ends.[132] Theeffect decreases strongly towards the center of the chain. Thistrend of Dq() is even more pronounced for longer chains (n4).[21,22] 13C NMR signals are a good indicator of partialcharges. The correlations of the 13C chemical shifts and theirdifferences Dd() (obtained by the aid of the 2D HMBCtechnique) agree very well with theDq() values, and thereforesupport nicely the MO model. The decreasing significance ofthe zwitterionic resonance structure Z with increasing dis-

tance of D and A can be also recognized in the compounds(CH3)2N-OE-CHO 24c26c, in which the rotational barrierof the CN bond decreases strongly with increasing values ofn.[52b,c]

Whereas the electron transitions S0!S1 in trans-stilbeneand tolane are still almost pure HOMO!LUMO transitions,other orbitals mix in for the D-OPV-A and D-OPE-Asystems, even for n= 1. The energy of HOMO1 approachesconstantly closer to the HOMO, and the LUMO+ 1 con-stantly closer to the LUMO, as n increases. The transitionsHOMO1!LUMO, HOMO!LUMO+ 1, andHOMO1!LUMO+ 1 become more and more importantfor the long-wavelength absorption, especially since theoverlap density of the HOMO and LUMO becomes smallerand smaller. Figure 15 shows as an example the OPEs 15d(n= 1) and 18d (n= 4), with the AM1-INDO/S calculationperformed with dimethylamino groups instead of didodecyl-amino groups, which are necessary for solubilization. TheHOMO!LUMO transitions are characterized by a strongintramolecular charge transfer (ICT) from the donor to theacceptor side. Figure 15 reveals that the ICT plays a veryminor role in the other transitions. The long-wavelengthabsorption band (the charge transfer band) of D-p-A systemsis essentially determined by the ICT.

D and A not only influence the orbital energies, they alsoalter the electron correlation. In terms of a self-consistentfield (SCF) approximation, both the differences in the orbitalenergies DE0 and also the configuration interactions of one-electron functions need to be considered for the excitationenergies E(S0!S1). Equation (36) contains the Coulomb

E DE0J 2K 36

Scheme 29. MO model for conjugated oligomers with donoracceptorsubstitution: Calculated charge differences Dq at the double bonds ofthe OPVs 5d7d (n=13) and Dq at the triple bonds of the OPEs15d17d (n=13); the differences Dd and Dd of the 13C chemicalshifts were measured in CDCl3.

[21, 22]

H. MeierReviews


repulsion integral J and the exchange integral K.[133] E

6. Summary and Outlook

Conjugated oligomers with terminal donoracceptor sub-stitution are examples of molecular wires[135] which arepredestinated to have special optical and optoelectroniceffects. The preparation of such an oligomer series with anincreasing number of repeat units in the p linker is beststarted on the donor side which bears solubilizing groups(such as NR2, OR). The p linker is then extended by theapplication of an extension reagent and a protection groupstrategy until the acceptor group is attached in an end-cappingstep. This approach is recommended for the construction ofparallel series with different acceptor groups to generate acentral series from which other series are accessible bycoupled, convergent syntheses.

The excitation of electrons in the linear or star-shapedoligomers of this type is characterized by an intramolecular

charge transfer (ICT). The charge-transfer band reacts quitedifferently upon the extension of the chromophore, because ahypsochromic effect resulting from the decrease in the ICT isopposed to the bathochromic effect caused by the extensionof the conjugation. This becomes apparent as soon as thenumber n of repeat units is increased, and consequently thedistance between the donor and acceptor enhanced. Thesuperposition of both influences leads to the variants for theS0!S1 transitions which are summarized in Table 15. Inaddition to the strength of donor and acceptor in the presentD/A pair, the type of p linker plays a decisive role. Until now,exclusively bathochromic series exist for the oligoenes (D-OE-A) and oligoynes (D-OI-A; type (a): lmax(n)lmax(n+1) l). A certain weakening of the donor oracceptor strength may then give rise to a borderline case, inwhich the long-wavelength absorption is independent of thesize of the chromophore (type (a)/(b): lmax(n) lmax(n+1) l). OPV and OPE series with weak donors and weakacceptors show a bathochromic behavior. Oligo(1,4-phenyl-ene)s (D-OP-A) and oligo(2,5-thienylene)s (D-OT-A) arespecial cases of type (c), in which lmax(n) passes through amaximum. Torsions along the p linker have a decisiveinfluence on the CT band.[136] The fourth imaginable case, inwhich lmax passes through a minimum, has not been unequiv-ocally proven to date.[137] The number of investigatedcompounds of oligo(2,5-thienylenevinylene)s (D-OTV-A) oroligo(2,5-thienyleneethynylene)s (D-OTE-A) is too small fora reliable conclusion to be drawn.

The superposition of conjugation effect and oppositeICT effect can be rationalized by the superposition of twoexponential functions (Figure 3); both effects exhibit aconvergence behavior (n!). The intramolecular chargetransfer diminishes the electron correlation, which is ofspecial importance for the HOMO!LUMO excitation.However, semiemprical quantum mechanics (AM1, INDO/S) studies on D-OPV-A and D-OPE-A systems reveal thatthe HOMO!LUMO fraction of the long-wavelength absorp-tion decreases continuously as n grows, that is, as the size ofthe chromophore grows (Figure 15). Together with this, Dm=m(S1)m(S0) and the correction term of the ICT in theabsorption decrease (Figures 17 and 18).

The VB model D-p-A$D+-p-A often used in theliterature is primarily suitable for the case in which shortoligoenes are used as a p linker. This model with itszwitterionic resonance structure is not very useful for otherp linkers, particularly not for p linkers which contain aro-matic rings. In contrast, anMOmodel proved to be successful,which implies a decreasing polarization from the donor aswell as from the acceptor side towards the center of thep linker (Scheme 29).

In the field of nonlinear optics (NLO), laws were oftentemptatively proposed that correlated the hyperpolarizabi-lites b and g with lmax values. This approach, however, fails forhypsochromic series. Power laws or logarithmic functionscan 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 repeatunits (AM1-INDO/S calculation); b) dependence of the ICT term DEDAon the fraction of H!L transition in the push-pull series (H3C)2N-OPV-NO2 and (H3C)2N-OPV-CN.

H. MeierReviews


or the number n of repeat units in their argument. In contrastto lmax and b, g does not approach a limiting value for n!,but g/n does.[23] The effective conjugation lengths nECLrequired for nonlinear optical properties are much greaterthan for linear optics. Therefore, an experimental proof of theconvergence of b and g/n is much more difficult.

Besides the selected D/A combination, the nature andlength of the p linker are decisive for b and even to a greaterextent for g. 1,4-Phenylene units in an OP linker show atorsion along the chain which is unfavorable for largehyperpolarizabilities. On the whole, a comparison of b andg values of various series D-p-A is essentially more difficultthan a comparison of linear optical properties. The errors ofthe methods are considerably larger than, for example, forlmax values, even when static, that is, wavelength-independentb0 values and nonresonant g values are used. Relative quan-tities related to a standard, which is measured at the sameconditions, for example [Eq. (37)], are recommended formaterials science applications.

brel b0compound

b04-nitroaniline molecular mass 4-nitroanilinemolecular mass compound 37

Altogether, considerable deficits can be recognized forthe experimental determination of b and g values of oligomerseries. Numerous theoretical attempts have been made fortheir linear (Section 2) and nonlinear optics (Sections 4 and5).[138,139] A new approach in the synthetic area is offered bystar-shaped compounds (Section 3.3)[140] and dipolar struc-tures such as 101, in whichcontrary to the resonancedescribed in Scheme 3-the aromatic and dipolar resonancestructures are compatible (Scheme 30).[141] The preparation of

D-p-A structures tailored for cer-tain electrooptical applications(molecular engineering) representthe final, highly promising goal.[142]

I am grateful to the Deutsche For-schungsgemeinschaft, the Volkswa-gen Foundation, the Fonds derChemischen Industrie, and theCenter of Materials Science of theUniversity of Mainz for financialsupport of my own work cited in thisarticle.

Received: July 1, 2004

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Table 15: Classification of the absorption behavior of series of donoracceptor-substituted conjugatedoligomers according to their dependence on the D/A combination and the p linker.

p

linkerMonotonously batho-chromic series type (a)

Borderlinetype (a)/(b)

Monotonously hypso-chromic series type (b)

Series with an energyminimum type (c)

OPV NR2/CN NR2/CHO NR2/NO2OR/NO2 NR2/CH=C(CN)2OR/CHO NR2/squaraineOR/squaraine NR2/1,3,5-triaziniumferrocene/NO2OR/C+

NR2/1,3,5-triazineOR/1,3,5-triazine

OPE NR2/CCH OR/NO2 NR2/NO2OR/CN NR2/CN NR2/CH=C(CN)2

NR2/CHOOE[a] all investigated D/A com-

binationsOI NR2/NO2OP OR/NO2 NR2/NO2

OR/CN NR2/CNOT OR/NO2 NR2/NO2OTV[b] NR2/NO2

NR2/CH=(C(CN)2OTE SR/NO2 OR/NO2

[a] D: NR2, OR, SR, ferrocene, A: NO2, CHO, CN, CH=CR2 containing electron-withdrawing groups R,C+; the OE linkers also include benzene or thiophene rings on one or both ends of the chain. [b] The OTVlinker includes a terminal benzene ring on the donor side.

Scheme 30. Resonance forms of a quinoid, electroneutral, and anaromatic, zwitterionic resonance structure as an example that isopposed to the usual resonance of an aromatic, electroneutral, and aquinoid, zwitterionic limiting structure.


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