Large quadratic hyperpolarizabilities with donor-acceptor polyenes functionalized with strong donors. Comparison with donor-acceptor diphenylpolyenes

Chemical Physics ELSEVIER

Chemical Physics 199 (1995) 253-261

Large quadratic hyperpolarizabilities with donor-acceptor polyenes functionalized with strong donors. Comparison with donor-acceptor diphenylpolyenes Mireille B l a n c h a r d - D e s c e a,b C l a u d e Runser c, A l a i n Fort c Marguerite B a r z o u k a s c

Jean-Marie Lehn b, Vincente Bloy b, Valtrie Alain

a

a D#partementde Chimie, Ecole Normale Sup3rieure, (URA 1679 CNRS), 24 rue Lhomond, F-75231 Paris Cedex 05, France Collbge de France, Chimie des Interactions Mol3culaires (UPR 285 CNRS), 11 Place Marcelin Berthelot, F-75005 Paris, France c lnstitut de Phyoique et Chimie des Mat#riaux de Strasbourg, Groupe d'Optique nonlin~aire et d'Opto3lectronique (UM 046 CNRS), 23 rue du Loess, F-67084 Strasbourg Cedex, France

Received 21 April 1995

Abstract Donor-acceptor polyenes and camtenoids of defined and increasing length, bearing electron-donating ferrocene or julolidine moieties, and formyl or dicyanovinyl electron-withdrawing end groups have been prepared in order to achieve enhanced quadratic optical nonlinearities. The variation of their quadratic hyperpolarizability/3 in solution was investigated using the electric field induced second harmonic (EFISH) generation technique, and was compared to the behavior of two series of soluble push-pull diphenylpolyenes. The chainlength behavior for each series of homologous compounds can be modeled by /x/3(0) = kna relationships with respect to the number n of double bonds in the polyenic chain, the exponent value a depending markedly on the end groups. Steeper increases were observed with the ferrocene donor moiety as compared to the strong julolidine donor group, and with the stronger dicyanovinyl acceptor (a = 2.4 and 1.45), as compared to the weak formyl acceptor (a = 1.6 and 1.3). In contrast, the series of push-pull diphenylpolyenes display the weakest dependencies, and show very little influence of the terminal acceptor substituents on the length behavior (a ~- 1).

1. Introduction Due to their efficiency and their chemical flexibility, organic derivatives have received considerable attention in the field of nonlinear optics [1-7]. In terms of quadratic effects such as second harmonic generation (SHG) or electro-optic modulation, it is now a c o m m o n knowledge that molecules containing an electron-donating and an electron-withdrawing group interacting through a conjugated system ex-

hibit large quadratic hyperpolarizabilities • [8-10]. Such so-called " p u s h - p u l l " compounds are of particular interest for the design of materials with enhanced quadratic responses. Early experimental studies performed on benzene and stilbene derivatives demonstrated that increasing the donor-acceptor strength resulted in a steady enhancement of /3, and that extension of the conjugated path linking the electron-donating and electron-withdrawing substituents led to a pronounced

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M. Blanchard-Desce et al./ Chemical Physics 199 (1995) 253-261

increase in /3 values [i 1-13]. In particular, several recent experimental studies conducted on different series of donor-acceptor polyenes have stressed the particular efficiency of the polyenic system in terms of quadratic nonlinearities [14-23]. These investigations yielded different superlinear dependencies of/3 on the length. In addition, very long push-pull derivatives obtained through push-pull functionalization of the carotenoid backbone were shown to display enhanced quadratic hyperpolarizabilities [15-16,21-23]. Examination of the various experimental trends observed indicates that the chainlength behavior depends on the nature of the end groups and calls for additional systematic experimental investigations in order to understand the factors which influence the length dependence. This would eventually allow to identify and design combinations leading to even larger hyperpolarizabilities and to reach the strongly enhanced quadratic nonlinearities required for appli-

~ A

A=

~A

~ 14 a

x ~ ( cN CN b

Scheme 2. Structural formulae of the series of push-pull julolidinylpolyenes (lla and lib) investigated in the present work.

~o

~A ~e

F,o

A=

|

H" •

CN b

Scheme I. Structural formulae of the seriesof push-pull ferrocenyIpolyenes (la and lb) investigatedin the presentwork,

cations such as electro-optic modulation. Within this context, the push-pull functionalization of the polyenic chain with either the ferrocene moiety (Scheme 1) or the efficient electron-donating julolidine group (Scheme 2) as donor end groups was studied. The ferrocene donor has been shown to act as an effective donor [24] and to lead to compounds exhibiting large SHG powder efficiency [25,26]. Likewise, the julolidine is known to behave as an even stronger donor than the classical dimethylanilino moiety [16,27,22]. Two different acceptor end groups were used: a weak one (formyl) and a strong one (dicyanovinyl). Compounds of increasing length (up to n = 7 or 9, where n is the number of conjugated double bonds in the polyenic chain) were prepared (Schemes 1 and 2). Their efficiencies was compared to those obtained with "classical" push-pull diphenylpolyenes (i.e. vinylogous derivatives of stilbenes) of increasing length (up to n = 7). Since diphenylpolyenes are of

M. Blanchard-Desce et al. / Chemical Physics 199 (1995) 253-261

Bu2N-~

255

[30]. In this particular case, incorporation of part of the conjugated path into ring structures was chosen in order to increase stability (through impeding of cis-trans isomerization). However, comparison of the absorption characteristics of these compounds with those of unsubstituted [17] or even carotenoid [21] analogs indicates that such structures lead to decreased electronic delocalization, probably through torsional defects.

BuzN'--.~I

BuzN-,,~

Bu2N.,~.~'~,.

2. Experimental 2.1. Materials

Bu2Nx

A:

CN c

NO'z d

Scheme 3. Structural formulae of the series of push-pull diphenylpolyenes (lllc and llId) investigated in the present work,

poor solubility, the dibutylamino substituent was chosen as a satisfying donor ensuring sufficient solubility for the longest molecules. This is critical in terms of experimental accuracy, since the solubility of polyenic derivatives decreases with increasing length. Both types of molecules bearing a moderate acceptor (the cyano substituent) and a strong one (the nitro substituent) were investigated (Scheme 3). As indicated in Schemes 1 and 2, the longest derivatives prepared (n >/4) bear methyl substituents onto the polyenic chain. The presence of such side groups confers sufficient solubility to polyenic systems and is a structural feature inspired from the conjugated skeleton of naturally occurring polyenes, the carotenoids. Functionalization of the carotenoid backbone has been shown to lead to very high quadratic hyperpolarizabilities [15,16,18,21-23,28, 29]. However, the characteristic methyl groups could be responsible for distortion effects due to steric hindrance, and resulting in deviation from planarity and concomitant decrease in effective conjugation along the conjugated chain. Planar-locked push-pull polyenes have recently been prepared and studied

The synthesis of the push-pull derivatives containing carotenoid polyenic backbones was performed through functionalization of conjugated symmetrical dialdehydes using Wittig, Horner-Emmons-Wittig and Knoevenagel reactions according to a previously described strategy [15,21,31,32]. The short unsubstituted push-pull polyenes (n = 1-3) of series I-III were synthesized stepwise. Endfunctionalized polyenals were prepared from the generic compounds (n = 0) by repetitive Wittig homologations as reported in Ref. [33], leading to molecules I-IIa (n = 1-2) as well as well pcyanophenyl and p-nitrophenyl polyenals. Knoevenagel condensation of aldehydes of series I-IIa (n = 0-2) with malonodinitrile yielded dicyanovinyl derivatives of series Ib and lib (n = 1-3). Compounds of series IIIc and IIId (n = 1-3) were obtained via Wittig condensation of p-cyanophenyl or p-nitrophenyl polyenals (n = 0-2) with the phosphonium salt prepared in a one pot reaction by reacting dibutylaniline with equivalent amount of formaldehyde, potassium iodide and triphenylphosphine [3435]. All compounds were purified by column chromatography on silica gel followed by crystallization leading to all-trans compounds of stereochemical purity higher than 95%. All new compounds were characterized by NMR, mass spectroscopy and gave correct elemental analysis. 2.2. Absorption spectra The wavelengths A~ of the absorption maxima were determined with a Beckmann DU 600 spec-

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Table 1 Experimental results of the EFISH experiments performed at 1.907 l~m on series la and Ib (push-pull ferrocenylpolyenes)

Table 2 Experimental results of the EFISH experiments performed at 1.907 Izm on series lla and lib (push-pull julolidinylpolyenes)

Series

na

AI b (nm)

/zfl(2 to) b (10 -48 esu)

p,fl(0) c (10 -48 esu)

Series

na

Ai (nm)

/~/3(2 to) b (10 -48 esu)

/x/3(0) c (10 -48 esu)

la la la la lb lb lb Ib

1 2 4 6 1 2 3 5

367,480 330,494 398.und d 430,und d 320,526 370,556 406,568 458,und d

60 215 560 1150 92 420 1120 4600

42-49[45] 147-1841165] 440 d 870 d 60-80{70] 250-340{300] 660-875[770] 3300

IIa lla Ila lla llb lib lib llb lib

1 4 6 8 1 2 5 7 9

401 468 482 498 453 520 570 572 572

210 1330 2310 3550 450 1500 5500 11000 13700

165 950 1610 2400 330 975 3220 6410 7980

a n: number of conjugated double bonds in the polyenic chain. b The maximum absorption wavelengths A] and the /x/3(2to) values were measured in acetone. c Since the UV-visible absorption spectra exhibit two distinct bands, the two-level model is not relevant for the calculation of the static p43(0) values. However, it is helpful to indicate the range of p,fl(0) values derived from the two-level model [12], using the two absorption maxima. An approximated /zfl(0) value can than be estimated by taking the average and is indicated between square brackets, d The two absorption bands tend to be closer together with increasing polyenic chainlength. As a result, the two bands overlap for the longest derivatives and the maximum of the lowest energy band cannot be determined.

trophotometer using spectroscopic grade acetone. The absorption spectra for all p u s h - p u l l m o l e c u l e s display broad and intense bands in the near U V or visible region characteristic o f charge transfer transitions. All c o m p o u n d s exhibit positive s o l v a t o c h r o m i c b e h a v i o r indicating an increase o f the dipole m o m e n t upon photoexcitation. In each series o f h o m o l o g o u s c o m p o u n d s , a b a t h o c h r o m i c shift o f the main absorption band is o b s e r v e d with increasing p o l y e n i c chain length (Tables 1-3). 2.3. E F I S H m e a s u r e m e n t s fl m e a s u r e m e n t s w e r e p e r f o r m e d using the electric field induced second h a r m o n i c ( E F I S H ) generation t e c h n i q u e [ 3 6 - 3 8 ] . T h e E F I S H e x p e r i m e n t allows the determination o f the m e a n m i c r o s c o p i c hyperpolarizability Y0: "/0 = " Y ( - 2 t o ;

to, to, 0)

+ ] x / 3 ( - - 2 t o ; to, t o ) / 5 k T .

number of conjugated double bonds in the polyenic chain. b The maximum absorption wavelengths A1 and the /x/3(2to) values were measured in acetone. c The static /x/3(0) values were calculated by using the two-level model [12]. a n:

The first term is the scalar part o f the cubic hyperpolarizability tensor, whereas the second originates from the partial orientation o f the p e r m a n e n t dipole m o m e n t /x in the static field. T h e orientational contribution is usually a s s u m e d to be the predominant c o m p o n e n t in the case o f polar c h a r g e transfer molecules. The product /x/3(2 to) - w h e r e /3(2to) (a short hand notation f o r / 3 ( - 2 t o ; to, to)) is the vector part o f the hyperpolarizability tensor - is thus directly inferred.

Table 3 Experimental results of the EFISH experiments performed at 1.907 p,m on series Illc and llld (push-pull diphenylpolyenes) Series

na

'~l b (nm)

p,fl(2 to) b (10 -48 esu)

,U43(0) c (10 -48 esu)

lllc lllc lllc lllc lllc llld llId llld Illd

1 2 3 5 7 1 2 3 5

400 418 436 462 479 448 458 468 486

360 890 I 160 1395 2000 700 1345 2190 2600

285 685 870 1005 1400 515 975 1560 1800

a n : number of conjugated double bonds in the polyenic chain. b The maximum absorption wavelengths AI and the p.fl(2to) values were measured in acetone. c The static /.~/3(0) values are calculated by using the two-level model [12].

M. Blanchard-Desce et al. / Chemical Physics 199 (1995) 253-261

EFISH measurements were conducted with a Qswitched Nd 3+ :YAG laser emitting pulse trains of about 90 ns envelope duration (individual pulses of 160 ps duration) and operating with the first Stokes radiation, at 1.907 p,m, of the YAG 1.064 i~m emission generated in an hydrogen Raman cell at 40 bar. These experiments were performed using, for each molecule, solutions of increasing concentration in acetone. The measurements are calibrated relative to a quartz wedge. For the quartz reference, the experimental value of the quadratic susceptibility dlt = 1.2 × 10 -19 e s u determined at 1.06 I~m was used. To account for dispersion, this value is extrapolated to d l l = 1.1 10 -19 e s u at 1.91 i~m. The experimental accuracy ranges between 5 and 20%.

3. Results and discussion

The results are collected in Tables 1-3. Given along with the /z/3(2to) values are the static values /z/3(0) calculated by using the two-level model since /3(2 to) values are significantly affected by dispersion enhancement. The two-level quantum model takes only into account the predominating charge transfer process that occurs between donor and acceptor groups interacting through a conjugated system, and has proved appropriate to describe push-pull stilbenes [12]. Although the number of excited states contributing to the hyperpolarizability of extended conjugated systems is expected to increase with length [39], the two-level dispersion factor should here yield a good estimate of the static values since the longest maximum absorption wavelength A, is far below the second harmonic wavelength (i.e. 953 rim).

In the case of the ferrocene derivatives, the calculation of the static /x/3(0) values is problematic since their UV-visible absorption spectra exhibit two distinct bands, the blue-shifted one being sharper and more intense. For this reason, the use of the two-level model does not seem fully relevant. However, it is helpful to calculate the range of /x/3(0) values derived from the two-level model using the two absorption maxima. The difference between the two calculated values being mostly within the experimental uncertainty, approximated static values can then be obtained by taking the average.

257

As a result of the quasi-one-dimensional nature of the 'rr-electron network, the /3ijk tensor of the pushpull molecules investigated here is expected to be essentially one-dimensional along the charge transfer molecular long axis, /3 and /x being nearly parallel. In addition, from the /x values reported in the literature for several series of short push-pull polyenes [14,17,40], as well as from calculations [41,42], the dipole moments of homologous push-pull compounds can be considered to exhibit a very weak length dependence. The variations of the /x/3 (respectively /z/3(0)) values, in each series of homologous compounds of increasing size, will therefore reflect, for the most part, the length dependence of 13 (respectively /3(0)). In addition, p,/3 is the relevant figure of merit for using such molecules as guests in poled polymers films since it combines both the quadratic nonlinearity and the ability of the molecule to couple with an external electric field.

3.1. Chainlength dependence of the quadratic hyperpolarizability As seen in Tables 1-3, for each series of homologous compounds, lengthening the polyenic chain results in a significant increase in both /x/3 and /x/3(0) values. This leads to particularly large /x/3(0) values for the longest molecules. For instance, the /x/3(0) value for molecule lib (n = 9) amounts to 17 times that of DANS (4-dimethylamino-4'-nitrostilbene) [43], a common benchmark for quadratic effects. The enhancement of /x/3(0) as a function of the number n of double bonds of the polyenic linker is shown in Fig. 1 for series Ia, Ib, IIa and lib evidencing linear dependencies of log(p,/3(0)) on log(n). This leads to /x/3(0)=kn" relationships, with a values of 1.6, 2.4, 1.3 and 1.45 for series Ia, Ib, IIa and lib respectively. A linear dependence of log(/x/3(0)) on log(n) is also observed for series IIIc and IIId, as shown in Fig. 2, leading in both cases to a/x/3(0) ~ n relationships. However, in contrast to series I and II, where the carotenoid derivatives (i.e. n >1 4) show no apparent deviation from a linear behavior, carotenoid compounds of series IIIc and IIId clearly fall out of line. In that case, the /x/3(0) values corresponding to n - - 5 or 7 are lower than those expected from extrapolation of the data for the shorter unsubstitued

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M. Blanchard-Desce et al. / Chemical Physics 199 (1995) 253-261

10000 1000 ¢I:X ::L

100 l0 10

l n

Fig. 1. Plot of the /z/3(0) values versus the number n of double bonds in the polyenic chain for the series of ferrocenylpolyenes (la: open squares and Ib: full squares) and julolidinylpolyenes (lla: open triangles and lib: full triangles) using logarithmic scales; all ~,8(0) values are expressed in 10-48 esu.

oligomers (n = 1-3). Such behavior indicates a reduction in the electronic transmission process. Tapering-off of the chain length dependence could be a possible explanation. Actually, examination of the semi-empirical calculations conducted on a series of push-pull polyenes of increasing length [39] indicates a linear dependence of log(/3) upon log(n) but a decrease in the slope occurs above n = 10 [15]. This behavior suggests a critical size corresponding approximately to n = 7 in terms of total geometrical length in the case of push-pull diphenylpolyenes. The non-alignment of the experimental points corresponding to the carotenoid derivatives could also result from a drop in effective conjugation along the polyenic chain due to a structural distortion induced by the methyl side groups. It has been shown that these groups are responsible for in-plane bending of 10000

1000

tO0 1

10

n

Fig. 2. Plot of the /~/3(0) values versus the number n of double bonds of the polyenic chain for the series of push-pull diphenylpolyenes Illc (full triangles) and Illd (full circles) using logarithmic scales; all /z/3(0) values are expressed in 10 - 4 8 esu.

the polyenic chain in the solid state in the case of natural carotenoids [44]. In addition, the crystal structure of a functionalized synthetic carotenoid displays slight deviations from planarity. The polyenic chain is fully extended in the solid state but shows small torsions around the carbon-carbon single bonds adjacent to the methyl side groups [45]. Such factors in releasing steric hindrance are likely to occur also in solution and conformational twisting could be significant for " f r e e " molecules. It is interesting to note that the carotenoid backbone seems to bring no apparent perturbation in the case of push-pull phenylpolyenes (series IIa and lib) or ferrocenylpolyenes (series Ia and Ib). Actually, not only length dependence but also onset of saturation as well as torsional defects could depend on the end groups. In particular, push-pull diphenylpolyenes, ferrocenylpolyenes and phenylpolyenes (such as those bearing the julolidine moiety) might exhibit different behavior.

3.2. Effect of the ferrocene moiety. Comparison with the julolidine donor As indicated by the respective exponent values obtained for the series Ia (1.6) and Ib (2.4), as compared to series IIa (1.3) and lib (1.45), the rise in /x/3(0) is steeper with the ferrocene moiety than with the julolidine group. As seen in Fig. 1, the difference in nonlinear efficiency between ferrocene and julolidine analogous derivatives of similar size decreases with increasing polyenic chainlength and even vanishes when comparing series Ib and lib. Although short ferrocenyl derivatives exhibit moderate nonlinear efficiencies, the ferrocene donor group leads to very large /x/3 values in the case of elongated push-pull ferrocenylpolyenes, in particular with the strongly electron-withdrawing dicyanovinyl acceptor (Table 1). Not only steeper variations but also larger dispersion of the exponent values are observed with the ferrocene donor moiety. This suggests that long ferrocenylpolyenes bearing even stronger acceptor terminal moieties could lead to exceptionally large quadratic nonlinearities. Small push-pull metallocene derivatives have already been studied [24-26]. They were found to exhibit lower molecular quadratic nonlinearity than

M. Blanchard-Desceet al./ ChemicalPhysics 199 (1995) 253-261 expected based upon the donor strength estimated from oxidation potential alone. It was suggested that this behavior was related to a poor electronic coupling between the metal center and the organic fragment. Actually, the ferrocenyi derivatives exhibit two absorption bands, the less intense red-shifted one being attributable to a metal-to-ligand charge transfer (MLCT) transition centered on the ferrocene entity, and the sharper and more intense blue-shifted one to a -rr'rr * charge transfer transition [24]. In the case of the two series of push-pull ferrocenylpolyenes investigated in the present work (Ia and Ib), the two characteristic absorption bands present positive solvatochromic behavior and tend to be closer together with increasing polyenic chain length. Both bands are red-shifted but the higher energy one moves more rapidly than the lower energy one which gains in relative intensity. As a result, these two bands overlap for long enough ferrocenyipolyenes. This suggests a mixing that could be responsible for increased coupling between the metal center and the acceptor, and could explain the enhanced quadratic nonlinearities observed with the longest derivatives in particular for series lb. 3.3. Effect of the acceptor strength Both larger /z/3(0) values and steeper increases with increasing length were obtained with the stronger dicyanovinyl acceptor, as compared to the weak formyl acceptor, both with the ferrocene (a = 2.4 and 1.6) and the julolidine (a = 1.45 and 1.3) donor moieties. This phenomenon compares with the conclusions that can be derived from the data reported for different series of push-pull phenylpolyenes bearing various acceptor [17,22]. According to the reported /3(0) values, again steeper increases are obtained with stronger acceptors. It should be noted that a previous study carried out on two other series of push-pull phenylpolyenes also bearing formyl or dicyanovinyl acceptor end groups indicated an opposite trend [21]. In that particular case, a steeper increase was obtained with the lbrmyl derivatives. However, EFISH measurements were carried out at different wavelength for the two different series (1.9 I~m for the dicyanovinyl molecules and 1.34 I.~m for the formyl series). The dispersive enhancement might have been underesti-

259

mated in the case of the formyl series due to the proximity of the second harmonic wavelength (670 nm) to the charge transfer absorption bands. This leads to overestimated static /x/3(0) values, the corresponding error becoming more important as length increases due to the bathochromic shift of the charge transfer absorption band. Therefore, the exponent value calculated for the formyl series [21] was most probably overestimated. Such a source of error is also apparent when comparing the different exponent values reported for an identical series of push-pull diphenyipolyenes in two independent experimental studies. The EFISH experiment conducted at 1.9 Ixm [17] afforded a smaller exponent value than when performed at 1.06 I.zm [14]. This underlines the limitation of the two level model in the case of long conjugated systems and emphasizes the need for measurements to be performed at the longest wavelength available. 3.4. Buffer effect of the terminal phenyl rings The series of push-pull diphenyipolyenes IIIc and IIId beating the dibutylamino donor group and the cyano or nitro acceptor groups display the smallest exponent values (a ~ 1). This behavior indicates a quasi-linear dependence of the /x/3(0) values on the number of conjugated double bonds of the polyenic chain. The stronger nitro acceptor always leads to larger/x/3(0) values than the moderate cyano acceptor. However, the two different electron-withdrawing substituents exhibit very little influence on the length behavior, similar exponent values being obtained. Comparison of the various chainlength behaviors reported in previous experimental studies [17] also shows both weaker length dependence and weaker dispersion of the exponent values, in the case of push-pull diphenylpolyenes as compared to pushpull phenylpolyenes. This distinctive behavior indicates that the presence of the phenyl terminal groups results in a decrease in the donor-acceptor interaction through the conjugated system. This compares with the fact that the polyenic system displays higher nonlinear efficiency than the polyphenyl system according to both calculations and experimental studies [17,39]. Likewise, examination of the experimental results reported for homologous push-pull stilbenes and

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styrenes [12], or polyenes and phenylpolyenes [17,20], shows that the relative contribution of the phenyl moiety to the conjugated path in terms of quadratic nonlinearity can be slightly superior, comparable or even inferior to one double bond. The presence of the phenyl rings can even be detrimental as indicated by comparison of homologous push-pull phenylpolyenes and diphenylpolyenes [19]. All these results taken together show that the presence of a phenyl ring in a polyenic system (in between the donor or the acceptor substituent and the polyenic chain for instance) affects the electron transmission process. The aromaticity of the benzenic ring is responsible for electron localization, therefore reducing the effective conjugation length. As a result, the phenyl ring behaves as a buffer for donor/acceptor interaction.

4. Conclusion In each series of homologous push-pull compound studied here, lengthening the conjugation path results in a pronounced increase in /2/3 values. This behavior can be modeled by /2/3(0)= kn a relationships with respect to the number n of double bonds in the polyenic chain, the exponent value a depending markedly on the end groups. In the case of the diphenylpolyenes only, the presence of methyl side groups characteristic of the carotenoid polyenic chain results in smaller /.t/3(0) values. This phenomenon indicates a reduction in the electron transmission process, either from torsional effects and concomitant loss of effective conjugation, or through tapering-off of the D / A interaction. Both steeper increases and larger dispersion of the exponent values were observed with the ferrocenylpolyenes. The ferrocene moiety behaves as an effective donor and leads to enhanced quadratic nonlinearities when associated with a strong acceptor, in particular in the case of long polyenic derivatives. Larger /z/3(0) values and steeper increases were obtained with the stronger dicyanovinyl acceptor, as compared to the weak formyl acceptor, both with the ferrocenyl (a = 2.4 and 1.6) and the julolidine (a = 1.45 and 1.3) donor moieties. In contrast, the series of push-pull diphenylpolyenes display the lowest exponent values. They ex-

hibit a quasi-linear dependence of the /.,/3(0) values on the number of conjugated double bonds in the polyenic chain, and show no apparent influence of the acceptor terminal substituents on the length dependence. This behavior suggests that the presence of a phenyl ring as an intermediate between the acceptor (and the donor) and the polyenic chain affects the electron transmission process. This is consistent with the benzenic ring acting as a buffer for electron delocalization due to aromatic stabilization. Actually, the steepest increases and the largest dispersion of the exponent values obtained up to now have been observed for two series of short push-pull polyenes where donor and acceptor substituents were directly grafted on the opposite ends of the polyenic chain [20]. Finally, comparison with other elongated pushpull systems indicates that the polyenic system is yet unequalled in terms of quadratic hyperpolarizability. Even efficient push-pull molecules derived from oligothiophenes [46] or oligo(vinylthiophene) [47-49] display smaller quadratic nonlinearities for compounds of similar sizes. However, for such systems, the loss in nonlinear efficiency can be balanced by improved stability which is critical for practical applications.

Acknowledgement The Centre National de la Recherche Scientifique (CNRS) is acknowledged for financial support and Hoffmann-La Roche AG, Basel (Switzerland) for gift of starting materials used in the syntheses.

References [1] D.S. Chemla and J. Zyss, eds., Nonlinear optical properties of organic molecules and crystals, Vols. 1 and 2 (Academic Press, New York, 1987); J. Zyss, ed., Molecular nonlinear optics: materials, physics and devices (Academic Press, New York, 1987). [2] A.J. Heeger, J. Oreinstein and D.R. Ulrich, eds., Nonlinear optical properties of polymers, MRS Syrup. Proc. 109 (Mater. Res. Soc., Pittsburg, 1988). [3] P.N. Prasad and D.R. Ulrich, eds., Nonlinear optical and electroactive polymers (Plenum, New York, 1988). [4] R.A. Harm and D. Bloor, eds., Organic materials for nonlinear optics (Roy. Soc. Chem., London, 1989).

M. Blanchard-Desce et al. / Chemical Physics 199 (1995) 253-261 [5] J. Messier, F. Kajzar, P.N. Prasad and D.R. Ulrich, eds., nonlinear optical effects in organic polymers (Kluwer, Dordrecht, 1989). [6] S.R. Marder, J.E. Sohn and G.D. Stucky, eds., Materials for nonlinear optics: chemical perspectives, ACS Symp. Ser. 455 (Am. Chem. Soc., Washington, 1991). [7] P.N. Prasad and D.J. Williams, eds., Introduction to nonlinear optical effects in molecules and polymers (Wiley, New York, 1991). [8] J.-L. Oudar and D.S. Chemla, J. Chem. Phys. 66 (1977) 2664. [9] B.F. Levine, Chem. Phys. Letters 37 (1976) 516. [10] S.J. Lalama and A.F. Garito, Phys. Rev. A 20 (1979) 1179. [11] J.-L. Oudar and H. Le Person, Opt. Commun. 15 (1975) 258. [12] J.-L. Oudar, J. Chem. Phys. 67 (1977) 446. [I 3] A. Dutcic, C. Flytzanis, C.L. Tang, D. P6pin, M. F6tizon and Y. Hoppilliard, J. Chem. Phys. 74 (1981) 1559. [14] R.A. Huijts and G.L.J. Hesselink, Chem. Phys. Letters 156 (1989) 209. [15] M. Barzoukas, M. Blanchard-Desce, D. Josse, J.M. Lehn and J. Zyss, Chem. Phys. 133 (1989) 323. [16] M. Barzoukas, M. Blanchard-Desce, D. Josse, J.-M. Lehn and J. Zyss, in: Materials for non-linear and electro-optics, ed. M.H. Lyons, Inst. Phys. Conf. Set. 103 (lOP, Bristol, 1989) p. 239. [17] L.T. Cheng, W. Tam, S.R. Marder, A.E. Stiegman, G. Rikken and C.W. Spangler, J. Phys. Chem. 95 (1991) 10643. [18] J. Messier, F. Kajzar, C. Sentein, M. Barzoukas, J. Zyss, M. Blanchard-Desce and J.-M. Lehn, Nonlinear Opt. 2 (1992) 53. [19] B.G. Tiemann, L.-T. Cheng and S.R. Marder, J. Chem. Soc. Chem. Commun. (1993) 735. [20] S.R. Marder, C.B. Gorman, B.G. Tiemann and L.-T. Cheng, J. Am. Chem. Soc. 115 (1993) 3006. [21] M. Blanchard-Desce, J.-M. Lehn, M. Barzoukas, I. Ledoux and J. Zyss, Chem. Phys. 181 (1994) 281. [22] S.R. Marder, L.-T. Cheng, B.G. Tiemann, A.C. Friedli, M. Blanchard-Desce, J.W. Perry and J. Skindhoj, Science 263 (1994) 511. [23] M. Blanchard-Desce, J.-M. Lehn, M. Barzoukas, C. Runser, A. Fort, I. Ledoux and J. Zyss, Nonlinear Opt. 9 (1995) in press. [24] J.C. Calabrese, L.-T. Cheng, J.C. Green, S.R. Marder and W. Tam, J. Am. Chem. Soc. 113 (1991) 7227. [25] M.L.H. Green, S.R. Marder, M.E. Thompson, J.A. Bandy, P.V. Kolinsky and R.J. Jones, Nature 330 (1987) 360. [26] S.R. Marder, J.W. Perry, W.P. Schaefer and B.G. Tiemann, Organometallics 10 (1991) 1896.

261

[27] A. Hassner, D. Bimbaum and L.M. Loew, J. Org. Chem. 49 (1984) 2546. [28] H. Ikeda, Y. Kawahe, T. Sakai and K. Kawasaki, Chem. Letters (1989) 1285. [29] S. Gilmour, S.R. Marder, B.G. Tiemann and L.-T. Cheng, J. Chem. Soc. Chem. Commun. (1993) 432. [30] I. Cabrera, O. Althoff, H.-T. Man and H.N. Yoon, Advan. Mater. 6 (1994) 43. [31] M. Blanchard-Desce, I. Ledoux, J.-M. Lehn, J. MalthSte and J. Zyss, J. Chem. Soc. Chem. Commun. (1988) 737. [32] M. Blanchard-Desce, J.-M. Lehn, 1. Ledoux and J. Zyss in: Organic materials for nonlinear Optics, eds. R.A. Harm and D. Bloor (Roy. Soc. Chem., London, 1989) p. 170. [33] C.W. Spangler and R.K. McCoy, Synth. Commun. 18 (1988) 51. [34] G. Puccetti, M. Blanchard-Desce, 1. Ledoux, J.-M. Lehn and J. Zyss, J. Phys. Chem. 97 (1993) 9385. [35] M. Bredereck, G. Simchen and W. Griebenow, Chem. Ber. 106 (1973) 3732. [36] B.F. Levine and C.G. Bethea, J. Chem. Phys. 63 (1975) 2666. [37] I. Ledoux and J. Zyss, Chem. Phys. 73 (1982) 203. [38] M. Barzoukas, D. Josse, P. Fremaux, J. Zyss, J.-F. Nicoud and J.O. Morley, J. Opt. Soc. Am. B 14 (1987) 977. [39] J.O. Morley, V.J. Docherty, and D. Pugh, J. Chem. Soc. Perkin Trans. 2 (1987) 1351. [40] M.H. Hutchinson and L.E. Sutton, J. Chem. Soc. (1958) 4382. [41] F. Meyers, J.-L. Bredas and J. Zyss, J. Am. Chem. Soc. 114 (1992) 2914. [42] F. Meyers and J.-L. Bredas, in: Organic materials for nonlinear optics Ill, eds. R.A. Hann and D. Bloor (Roy. Soc. Chem., London, 1993) p. 1. [43] M. Barzoukas, A. Fort, G. Klein, A. Boeglin, C. Serbutoviez, L. Oswald and J.F. Nicoud, Chem. Phys. 164 (1992) 395. [44] B.C.L. Weedon, in: Carotenoids, ed. O. lsler (Birkhauser, Basel, 1971) p. 285, and references cited therein. [45] M. Bianchard-Desce, T.S. Arrhenius and J.-M. Lehn, Bull. Soc. Chim. France 130 (1993) 266. [46] F. Wiirthner, F. Effenberger, R. Wortmann and P. Kramer, Chem. Phys. 173 (1993) 305. [47] A.K.-Y Jen, V. Pushkara Rao, K.Y. Wong and K.J. Drost, J. Chem. Soc. Chem. Commun. (1993) 90. [48] V. Pushkara Rao, A.K.-Y Jen, K.Y. Wong and K.J. Drost, Tetrahedron Letters 34 (1993) 1747. [49] V. Pushkara Rao, A.K.-Y Jen, K.Y. Wong and K.J. Drost, J. Chem. Soc. Chem. Commun. (1993) 1118.