meier2005.pdf

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Conjugated Oligomers

Conjugated Oligomers with Terminal Donor–Acceptor

Substitution

Herber

Herbert t Meier*

Meier*

A

n

g

e

w

a

n

d

t

e

Chemie

Keywords: Keywords: absorption · conjugation · absorption · conjugation · intramolecular charge transfer · intramolecular charge transfer · nonlinear optics ·

nonlinear optics · oligomers

oligomers

2482

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1.

1. Introduction Introduction

Conjugated oligomers are target compounds for Conjugated oligomers are target compounds for numer-ous applications in materials science because of their ous applications in materials science because of their inter-esting electrical, optical, and optoelectronic properties and esting electrical, optical, and optoelectronic properties and th

they ey are are alalso so momodedel l cocompmpououndnds s fofor r ththe e cocorrrrespesponondidingng conjugated polymers.

conjugated polymers.[1][1] _{A topic of}_{A topic of high topical}_{high topicality in terms}_{ity in terms of}_of nonlinear optics and electroluminescence concerns

nonlinear optics and electroluminescence concerns p p systems systems

substituted with donor and acceptor groups in which substituted with donor and acceptor groups in which conjugated oligomers form the

jugated oligomers form thepp-electron linker. The compounds-electron linker. The compounds

can have a linear or a star-shaped architecture. Scheme 1 can have a linear or a star-shaped architecture. Scheme 1 provides an overview of the most important structural types. provides an overview of the most important structural types.

The push-pull effect of this class of compounds depends on The push-pull effect of this class of compounds depends on the strength of the donor and acceptor groups; however, it the strength of the donor and acceptor groups; however, it al

also so dedepependnds s on on ththe e conconjujugagatetedd ppsysyststemem, , to to whwhicich h aa

zw

zwititteteririononic ic reresosonanancnce e ststruructcturure e shshouould ld cocontntriribubutete (Scheme 2). The energy of the dipolar resonance structure is (Scheme 2). The energy of the dipolar resonance structure is

determined by the charge separation as well as the change in determined by the charge separation as well as the change in the

the p p system. The latter influence is certainly smaller for an system. The latter influence is certainly smaller for an

oligoene chain (

oligoene chain (11) than for repeat units consisting of aromatic) than for repeat units consisting of aromatic

rings (

rings (22), whose zwitterionic resonance structures have), whose zwitterionic resonance structures have pp

--quinoid character. quinoid character.

Several parameters, such as BLA,

Several parameters, such as BLA,[2–5][2–5] _MIX,_MIX,[6][6] _{and c}_{and c}22_,,[7–9][7–9] have been suggested for quantification of the contribution of have been suggested for quantification of the contribution of zwit

zwitteriterionionic c resresonaonance nce strstructuctureures. s. ThThis is wilwill l be be disdiscuscussedsed further in Section 5. However, it should be noted here that further in Section 5. However, it should be noted here that the “weight” of resonance structures depends on external the “weight” of resonance structures depends on external factors such as the solvent or an applied electrical field. factors such as the solvent or an applied electrical field.

[*] Prof. Dr. H. Meier [*] Prof. Dr. H. Meier Johannes Gutenberg-Universitt Johannes Gutenberg-Universitt Duesbergweg 10–14 Duesbergweg 10–14 55099 Mainz (Germany) 55099 Mainz (Germany) FFax: ax: (( 49)6131-392-539649)6131-392-5396 E-mail:

E-mail: [email protected]@mail.uni-mainz.demainz.de

C

onjugated oligomers represent a prominent class of compoundsonjugated oligomers represent a prominent class of compounds from

from a a viewpoint viewpoint of of their their theory, synthesis, theory, synthesis, and and applications applications inin materials science. Push-pull substitution with an electron donor D materials science. Push-pull substitution with an electron donor D at one end of the conjugation and an electron acceptor A at the at one end of the conjugation and an electron acceptor A at the other end results in them having outstanding optical and other end results in them having outstanding optical and elec-tronical properties. This Review highlights fundamental synthetic tronical properties. This Review highlights fundamental synthetic strategies for

strategies for the preparation the preparation of such of such oligomers witholigomers with

n

repeat units repeat units (

(

nn

= = 1, 2, 3, 4, …) and the rules that govern their linear and1, 2, 3, 4, …) and the rules that govern their linear and

nonlinear optical properties (absorption, frequency doubling and nonlinear optical properties (absorption, frequency doubling and tripling). The unification of chemical, physical, and theoretical tripling). The unification of chemical, physical, and theoretical aspects with an interdisciplinary image of this class of compounds aspects with an interdisciplinary image of this class of compounds is attempted herein.

is attempted herein.

From the Contents From the Contents

1.

1. Introduction Introduction 24832483

2.

2. Long-Wave Long-Wavelength length ElectronElectron

Transitions in Conjugated Oligomers Transitions in Conjugated Oligomers 2484 2484

3.

3. Push-Pull-Substituted Oligomers: Push-Pull-Substituted Oligomers: Synthetic Concepts and Absorption Synthetic Concepts and Absorption Behavior

Behavior 2486 2486

4.

4. Nonlinear Optics in Series of Nonlinear Optics in Series of Oligomers with Donor–Acceptor Oligomers with Donor–Acceptor Substitution

Substitution 2496 2496

5.

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

Systems

Systems 2499 2499

6.

6. Summary and Outlook Summary and Outlook 25022502

Scheme 1.

Scheme 1. Construction of donor–acceptor-substituted conjugated Construction of donor–acceptor-substituted conjugated oligomers consisting of donor groups D, a

oligomers consisting of donor groups D, a p p-electron linker, and-electron linker, and

acceptor groups A; selected examples are shown. acceptor groups A; selected examples are shown.

Scheme 2.

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

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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

considered[10]_{whether the resonance should be substituted by} a fast equilibration (automerization) as soon as the C 2v

sym-metry 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 S₀

!

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

hyper-polarizabilities 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.

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

oligomers to converge towards a certain limiting value E ¥

for n

_!

¥. The hyperbolic function described by Equation (1)

E

_ð

n

ð

3

Þ

neighboring bonds. The perturbation is based on the fact that polyenes have alternating single and double bonds of differ-ent lengths and consequdiffer-ently differdiffer-ent 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}

E

_ð

n

_{Þ ¼}

c

_þ

f

_ð

1

n

_þ

0:5

Þ

ð

4

Þ

of c

_¼

₆

0.[29]_{Root laws, such as Equation (5) used by Drefahl}

lmax

ð

n

Þ ¼

c

þ

b

p

ffiffiffi

n

ð

5

Þ

and Pltner[30] _{for the long-wavelength absorption maxima,} and the corresponding functions for l2suggested by Lewis and

Calvin[31] _{on the basis of coupled oscillator models were} modified by Hirayama[32,33] _{to Equation (6) and revised by}

l2_max

_ð

n

_{Þ ¼}

c

_þ

b an

ð

6

Þ

Dhne und Radeglia.[34] _{Since a}_<_{1, Equation (6) yields a} finite limiting value as shown by Equation (7). Equation (5)[30]

lim

n_!1 lmax

¼

ffiffiffi

c

p

ð

7

Þ

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

E

_ð

n

_{Þ ¼}

E

_ð

1

_Þ

nn

ð

8

Þ

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

Herbert Meier was born 1939 in Prague. He studied chemistry and mathematics at the University of Tbingen and the Free Univer-sity in Berlin and in 1968 he completed his doctoral thesis in organic chemistry with Prof. E. Mller in Tbingen. After a habilita-tion in organic chemistry and photochemis-try 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 sci-ence and on heterocycles with a possible activity spectrum. He is co-author of the textbook “Spectroscopic Methods in Organic Chemistry”.

Scheme 3. Symmetrical charged polymethines (cyanines).

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series 4[18,19,36] will be used here as an example. Figure 1

1

ð

12

Þ

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= (101_c₎₍₁₀_d₎₌₍₁₀2_c₎₍₁₀₀_d_{), proved to be particularly}

suc-cessful. Even a minor influence of the aggregation results in deviation 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 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 energyE depends on the

orienta-tion of the molecules in the aggregate; E is lowest for pure

Jaggregates (a=0 =

90

=

discernible energy change. Aggregation can also lead to a steric effect, with the molecules less distorted and conse-quently 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 conver-gence. 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 termDE DA[defined by Eq. (15)] is a correction term for the push-pull effect in series with terminal donor–acceptor substitution.[25]

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



E ¥. An oligomer series with a uniform

hypsochromic effect is realized in type (b): E DA(n)



E DA -(n

_

E ¥. The borderline case (a)/(b) between (a) and (b) is

present for E DA(n)



E ¥, that is, when the energy of the

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.}

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 (c) and of the

forbidden transition (a) depends on the stacking angle a. Jelley

aggregates (J, a=0) exhibit a bathochromic shift (n’’<n) and H

aggregates (a=90 >_n_).

) and highest for pure H aggregates (a

). 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

1) 1)

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electron transition is nearly independent of n (of the size of

the chromophore). A rapidly decreasing term [E S(n)



E ¥]

with increasing numbers ncan 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.

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 long-wavelength 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

pronounced as the acceptor strength increases. Since the corresponding excitation S0

!

S1 is connected to an intra-molecular 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

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).

Table 1 : Absorption in CHCl3 and color of the crystals of the

trans-stilbenes 5 a–5 f .[a]

Compound 5 R lmax[nm] Crystal color

a[21,25] H 366 colorless b[21,25] CN 401 yellow c[21,25] CHO 423 orange d[21,25] NO2 461 red e[39] HC=_C(CN)₂ ₅₂₅ _{dark red} f [39] C(CN)=_C(CN)₂ ₆₇₀ _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.

Scheme 5. Coupled convergent synthetic strategy for the OPV series

5–8/a–e.

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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 CHCl3. A

pronounced bathochromic effect can be realized for 5 a–8 a, a

decreased bathochromic effect for 5 b–8 b, lmax values of 5 c–

8 c which are fairly independent of the size of the

chromo-phore, 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 dem-onstrated in Figure 5. The extension of the conjugation leads

to a bathochromic shift, which is shown by a decreasing difference of E S



E ¥ for the series 5 a–8 a. The

push-pull-substituted 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 S



E ¥ 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 batho-chromic 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

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(1,4-phenyleneethynylene)s (OPEs) 15–18/a–e shown in Scheme 7

Figure 4. Maxima of the long-wavelength absorptions in the OPV series 5–8/a–e in CHCl3.

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]

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/

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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

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 Sonoga-shira–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]

The long-wavelength absorption data for compounds 15– 18/a–e are 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 absorp-tion band (S0

!

S1) is superimposed by the higher energy electron transition S₀

_!

S₂ (Figure 7 demonstrates this using

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. e

_ð

~nn

_{Þ ¼}

emax exp





~nnmax~

ð

n Dn~nn



n~nnn~max

Þ



ð

16

Þ

with D~nn

_{¼ j}

0:5

_ð

nn~2



~nn1

Þj

15–18/a–e with the precursor series 15 f –17 f and 15 g–17g.

Table 2: Long-wavelength UV/Vis absorption of the OPE compounds

15–18/a–e in CHCl3.

Compound n n˜ max [103cm1] lmax [nm]

0.30 373[a]

[a] The lmax values differ in these cases from 1/n˜ max of the separated

long-wavelength band because of the superposition of the bands.

Figure 7. UV/Vis spectrum of 16 d in CHCl3 (c) and its dissection

into two bands according to Equations (16)–(18).[41]

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

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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

from the extension of conjugation is surpassed in the NO₂ 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 NO₂. Compounds 22 and 23 in Scheme 9 are shown here as

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).

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]

The absorption spectra of 24 c–29 c measured in CH2Cl2 show a pronounced bathochromic shift for increasing num-bers 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-pounds was performed by “one-sided” Wittig and Wittig– Horner reactions, respectively, of the carotinoid dialdehydes and 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 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

the decrease of the ICT and its consequence on the absorption into account.

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 NO2

groups as acceptors (CHCl3 as solvent).

Scheme 11. Carotinoid push-pull compounds and the maxima of their 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, Barzou-kas, 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

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 com-pounds 33–35 are listed in Table 3. For constant values of n,

the lmax value becomes higher as the donor and acceptor

strength increases; lmax(n > 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 heteroaro-matic 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

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 thep linker cause a hypsochromic effect for neither the

push-pull-substituted oligoenes nor the corresponding oligoynes— in contrast with the accordingly substituted OPV and OPE systems.

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]

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] 34a[56] 34b[56] 34 f [56] 35c[57] 0 443 603 458 494 512 1 507 695 413 531 542 619 2 540 773 446 572 556 710 3 571 826 469 594 566 4 586 486 606 5 613

Scheme 14. Maxima of the long-wavelength absorption of oligoenes with terminal dimethylamino/nitro substitution, which include a benzene ring in the plinker on the donor side (38a–41a) as well as on the donor and acceptor sides (38b–41b); measured in CHCl3.

Scheme 15. Maxima of the long-wavelength absorption of oligoynes with push-pull substitution.[68–71]

2490  2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org Angew. Chem. Int. Ed. 2005, 44, 2482 – 2506

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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} reso-nance integrals do not only depend on the different atomic distances in the plinker 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).

The synthetic strategy of oligo(1,4-phenylene)s is based on usual Pd-catalyzed aryl–aryl C

_

C 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

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 S0

!

S1 can pass through a minimum, and lmax(n) accordingly through a maximum (type (c), Figure 3)—in particular, when the cor-rection termDE 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

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 OCH₃, 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 thep linker results in the absorption maxima

shifting to longer wavelengths (Scheme 18 shows some examples).[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 6 offers a comparison of bithiophenes with various donor and acceptor groups; it can be seen that the combination of a 1-pyrrolidine 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 Wrth-ner.[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 l_max 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

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-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 in fluorenes (see Table 4 for R1_{, R}2_).

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

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 CN N(CH3)2 1 290 CCl4 54b 2 342 CCl4 54c 3 332 CCl4 54d 4 314 CCl4 55a NO2 NH2 1 373 EtOH 55b 2 378 EtOH 55c 3 358 EtOH 55d 4 340 EtOH

56a NO2 OCH3 1 302 1,4-dioxane

56b 2 332 1,4-dioxane

56c 3 340 1,4-dioxane

57a CN OCH3 1 247 DMSO

57b 2 292 DMSO

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few D-p-A series with repeat units consisting of thiophene

rings and C

_

C double or triple bonds.[83] Oligo(2,5-thienyle-nevinylene)s (OTVs) which bear 4-diethylaminophenyl groups as the donor and NO₂, 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

69 a–d, 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 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-A

systems can also be generated by protonation of suitable D-p

-D 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]

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

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-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–acceptor-substituted bithiophenes in n-hexane.[79,80]

Compound D A lmax [nm] 63 OCH3 CHO 372 64 N(CH3)2 CHO 421 65 SCH3 NO2 391 66b OCH3 NO2 408 61 N(CH3)2 NO2 466 67b NO2 499

Table 7: Maxima of the long-wavelength absorptions of the oligothio-phenes 66 a–d and 67 a–d in n-hexane.[79–81]

Compound D A n lmax [nm] 66a OCH3 NO2 1 340 66b 2 408 66c 3 442 66d 4 454 67a NO2 1 408 67b 2 499 67c 3 505 67d 4 497

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]

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

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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} push-pull-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).

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

(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: l_max(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 to the bidentate central acceptors or donors described in Section 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 squaraines

77a–d (n=0–3) in CHCl3~ and in CF3COOH &.

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

mixtures.

Scheme 22. Maxima of the long-wavelenth absorption of A-p-D-p-A

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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

treatment with acid led to the elimination of the OH group bound on the central carbon atom. The cations 80 a–d, strictly

as their carbinol bases, exhibit monotonously growing lmax values with n—of course shifted by the push-pull effect from

the UV/Vis region to the Vis/NIR region (Figure 11). The same is valid for methylium ions which are linked through polyene chains -(CH=CH)_n- to ferrocene as the terminal donor group; the lmax value rises from 618 to 1187 nm on going from n=2 to 14.[97]

In contrast to the weakly pH-dependent alkoxy-substi-tuted color salts 80 a–d and 81a,b, the dialkylamino

com-pounds 82 a,b and 83 a–c (Table 8) exhibit absorptions which

depend strongly on the concentration of H ions.[98–101] _The formation of the cations from the carbinols or their ethers on interaction with strong acids leads first to absorption maxima which are located far in the NIR region. A lmax value of 1003 nm is found for 82 b,[100] and further addition of

CF₃COOH induces a decrease in the intensity of this band

and a slight blue-shift. Simultaneously, a new band appears at

lmax=530 nm, which increases strongly and is red-shifted to 615 nm at high excess of CF₃COOH.[100] _{Table 8 shows the}

lmaxvalues at the “end of this titration” when all three terminal amino groups of 82 b are protonated. Hence, the

push-pull character in the three arms is lost. Thus, it is understandable that the lmax values of the methoxy com-pounds 81 a,b with n=1 are higher than those of the dialkylamino compounds 82 a,b and 83 a.

Of the central acceptors for three-star oligomers shown in Scheme 21, the 1,3,5-triazines deserve special mention. The alkoxy-substituted compounds 84–86 and the dialkylamino

compounds 87and88 were prepared by alkaline condensation

reactions of 2,4,6-trimethyl-1,3,5-triazine with the corre-sponding aldehydes (Scheme 25).[101] _{The influence of the} push-pull effect in 84 and the even stronger effect in 87 can be

seen 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 terminal donor groups.

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. R D n lmax[a][nm] 81a[97] H OCH3 1 718 81b[99] CH3 OCH3 1 733 82a[97] H N(CH3)2 1 609 82b[99] CH3 N(CH3)2 1 615 83a[100] H N[CH2



CH(C6H13)2]2 1 622 83b[100] H N[CH2



CH(C6H13)2]2 2 740 83c[100] H N[CH2



CH(C6H13)2]2 3 790

[a] End values of lmax at a high excess of CF3COOH.

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.

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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.

Unexpect-edly, 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 CHCl3 turns deep violet ( lmax=549 nm). Further addition of CF₃COOH 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-(p

-A)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

the opposite way: the primary blue-shift is followed by a red-shift. 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; how-ever, 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.

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

according to Equation (11) and determination of the effective conjugation length according to Equation (12).

Scheme 25. 1,3,5-Triazines with donor-substituted OPV chains and the maxima of their long-wavelength absorption (measurement of 84–86

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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 signifi-cance. Figure 14[106]_{provides an explanation for this: if light is}

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.

very small (factors of 1010_{, 10}17_{), high intensity laser light is} needed to measure the frequency doubling and tripling. The advantage of donor–acceptor-substituted conjugated p

sy-stems arises from the fact that the shift of electrons from the Scheme 26. Three-star compounds 89 a,b with central donor and

terminal acceptor groups (absorption maxima in CHCl3).

Scheme 27. Octupole 91 with a benzene core obtained by cyclotrimerization 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 88 a

(c) by protonation of the 1,3,5-triazine ring (d) and subsequent

hypsochromic shift by complete protonation (g) in CHCl₃/

CF3COOH; bottom: hypsochromic shift of the absorption band of 88 b

(c) by protonation of the 1,3,5-triazine ring (d) and subsequent

bathochromic shift by complete protonation (g) in CHCl3/

CF3COOH.

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donor to the acceptor is highly efficient (Figure 14); that becomes particularly apparent in the b values. Although D-p

-A 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

P

_ð

E

_{Þ ¼ }

P

_ð

E

_Þ

_ð

21

_Þ

requires that c( 2)

= b=0 in the progressions (19) and (20). Many compounds with donor–acceptor 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 strong acceptors enhance the b and g values—in analogy to the

dipole moment m. However, a direct relationship between m

and b or g does not exist, as the comparison of 13 a and 96 in

Table 9 reveals.

Since strengthening the push-pull effect also shifts the CT band to higher lmax values (see Section 3.1), a power law b~

lk_max or log b~log l_max seems to apply.[107] The second order

hyperpolarizability g of trans-stilbenes scales with b.[107]The

push-pull compounds N ,N -dimethyl-4-nitroaniline and (E

)-4-dimethylamino-4’-nitrostilbene (DANS; 38 b) represent NLO

standards that are often used.

Incorporation of a triple bond instead of a trans

-config-ured double bond results in b and g decreasing considerably

and m decreasing to a small extent. For

4-dimethylaminophenyl-4’

-nitrophenyle-thyne b=461030esu, g=1511036esu, and m=6.11018esu (CHCl3).[71] The incorporation of benzene rings also proved to be unfavorable relative to equally extended plinkers consisting of olefinic

double bonds (Scheme 28).

The b values, which were obtained, for

example, by the EFISHG method (electric field 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 the basis of the two-level model[108,109] _{which works for D-}_p_-A systems because of the domination of the CT transitions.[110] The deterioration of the conjugation as a consequence of the torsion in oligo(1,4-phenylene)s (OPs) is already expressed in the lmax values, but it is also noticeable in the hyperpolarizabilities b. Table 10 shows a comparison of

biphenyls and the corresponding fairly planar fluorenes for this purpose. The correlation is much more complex for the

g values, as the comparison of 47 a,b and 56 a,b demonstrates.

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). Theg 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 plinker. 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} summar-ized 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

Scheme 28. Comparison of the b₀ values of D-p-A compounds having

the same length of plinker but a different number of benzene

rings.[58b]

Table 10: Comparison of dipole moments m and hyperpolarizabilities b and g of biphenyls and the corresponding 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 46 b 0 0 H NO2 47a 3.8 4.1 15 47 b 4.1 5.1 29 OCH3 NO2 56a 4.5 9.2 39 56 b 4.7 11 28 N(CH3)2 NO2 49a 5.5 5.0 130 49 b 6.0 55

[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 trans

-stilbenes and their derivatives which bear a donor group in the 4-position and/or an acceptor group in the 4’-position.[107]

Compound Solvent 4-R 4’-R 1018 m [esu][a] 10 30_b [esu][a] 10 36_g [esu][a] 92 CHCl3 H H 0 0 26 93 1,4-dioxane N(CH3)2 H 2.1 10 64 94 1,4-dioxane H NO2 4.2 11 61 38b CHCl3 N(CH3)2 NO2 6.6 73 225 95 CHCl3 NH2 NO2 5.1 40 147 13a CHCl3 OCH3 NO2 4.5 34 93 96 CHCl3 SCH3 NO2 4.3 34 100 97 CHCl3 N(CH3)2 CN 5.7 36 125 98 CHCl3 OCH3 CN 3.8 19 54

[a] esu: electrostatic units; m: 1030_Cm

=0.29981018esu=0.2998 D;

b: 1050_Cm3_V__{2 =}_2.69410_30_esu; _g_: ₁₀_60_Cm4_V__{3 =}_8.078

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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

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-substi-tuted 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

func-tions.[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 S0

!

S1, the correspond-ing 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- b0

¼

6 m2₀₁D ml2max

h2_c2

ð

26

Þ

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

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 Solvent 1030_b

[esu]

1036_g

[esu]

56a OCH3 NO2 1 1,4-dDioxane 5.1 10

56b 2 9.2 39 56c 3 11.0 55a NH2 NO2 1 NMP 10 21 55b 2 24 96 55c 3 16 124 55d 4 11 133 101a N(CH3)2 CHO 1 CHCl3 30 63 101b 2 52 140 101c 3 88 257 102a OCH3 NO2 1 CHCl3 34 93 102b 2 47 130 102c 3 76 230 102d 4 101

Table 12: Dipole moments m and hyperpolarizabilities b0 and g of

oligoenes.[56]_{(Structures shown in Scheme 12.)}

Compound n m[D] 1030_b 0[esu] 1036g [esu] 33b 0 8.8 74 1 9.3 195 378 2 9.8 361 1724 3 10 642 7363 4 10 1229 34b 0 9.7 63 1 9.0 195 395 2 10.2 423 3 10.5 810 4 11 1043 5 11 1530

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

compounds with and without heterocycles in the p linker.

Compound Structural formula 1030_b

0 [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

(18)

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 l2_max decreases. We obtained b0 values of 198, 287, and 3461050_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} _b

0 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, trans

-stilbenes) which have the same p linker (see compounds in

Table 9).[107]_{Substituent effects, but not necessarily the} push-pull 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]

co-workers[124,125]_{only make sense when} _l

max increases with the so-called delocalization length Ld.[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, …, lmax(n) decreases with growing n, but g(n) increases.[127] The relation (29) is

g

_

nm _{or log}_g



mlogn

_ð

29

_Þ

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 n0_ECL_{, which can,}

however, be different from nECL obtained from the conver-gence of the long-wavelength absorption. The absorption takes S₀ and S₁ into account, and an essential-state model of three, four, or more states is taken as the basis for the THG.

To summarize Section 4, the statement can be made that in addition to the substituent effects of D and A on g and the

push-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 the

same linker has a different effect when its polarization is different with various D/A pairs. An explanation for this is provided in Section 5 in the context of VB

theory.

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 S0 and the first electronically excited singlet state S1 of such compounds by a linear combination of a

zwitterionic Z and an electroneutral resonance structure N

[Equations 30 and 31] .

¼

12



1

_

D m

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

₄_m2 01

þ

D m2

q



ð

32

Þ

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 m01; D m is accessable for example by electro-optical 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

¼

1



2c

2

ð

33

Þ

instead of c in Equation (30) and defined the “mixed parameter” MIX as

_

cosq which corresponds to (2c2

_

1) 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

BLA

_ð

in 

_{Þ ¼}

0:11MIX

_ð

34

_Þ

contains aromatic rings, because aromatic rings will keep their typical adjusted bond lengths.

Table 14 shows the relationship between the polarizabil-ities a, b, and g and the discussed parameters c2, MIX, and

BLA. The parameter f proposed by Lu, Chen et al.,[128] _was also 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 the}

corresponding “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]_[]

g2

j

max 0 0.5 50:50 0

[a] 0<c2<1, 0<f <1. [b]

_

1<MIX<1. [c] BLA is varied between the bond lengths_L(

_

C=C

_

)=1.34 

and L(=C

_

C=)=1.45  by about 0.11 ; if Equation (33) is not used, optimum BLA values of 0.04