Polyenes donor-acceptor substituted

Fig. 3.21 Non-polar valence bond (at left) and zwitterionic full charge transfer (at right) forms of a donor-acceptor substituted polyene.

Table 3. Ground-state structure of a series of donor-acceptor substituted polyenes with increasing ground-state polarization, and the corresponding values and wavelengths of maximum absorption Amax in CHCI3.

Bourhill, G., Bredas, J.L., Cheng, L.-T., Marder, S.R., Meyers, F., Perry, J.W., Tiemann, B.G. Experimental demonstration of the dependence of the first hyperpolarizability of donor-acceptor-substituted polyenes on the ground-state polarization and bond length alternation. J. Am. Chem. Soc. 116, 2619-2620(1994)... 

LANGMUIR-BLODGETT FILMS FROM DONOR-ACCEPTOR SUBSTITUTED POLYENES... 

We have prepared ultra-thin films with the Langmuir-Blodgett technique from donor-acceptor substituted polyenes. To improve the formation of such well-ordered structures we have synthesized polyenes to which long aliphatic chains were attached ... 

In order to study highly ordered structures of donor-acceptor substituted polyenes, we synthesized (7) in all-trans configuration. The polyene (7), substituted with two aliphatic chains, showed a of 18 dyn/cm. At higher surface pressure the films seem to be stable but consist of, at least partly, folded floes (dashed part of curve in Fig. 5). At the transfer pressure of 13 dyn/cm the film was very insensitive to mechanical vibrations. The transferred film was characterized by IR and UV spectroscopy. 

Pucetti and cow orkers [6] carried out a detailed study of both symmetrically (donor-donor and acceptor-acceptor) and asymmetrically (donor-acceptor) substituted polyenes and found very large values of y for the polyenes bearing strong electron donor groups with no... 

The desire to increase P values above those of molecules used in the earliest materials has led to the exploration of organic compounds as the active components of second order NLO devices. Considerable effort has been expended in the synthesis and analysis of candidate molecules, which are largely donor-acceptor substituted conjugated n systems. (2) Examples of compound classes whose members display large nonresonant P include azo dyes, stilbenes, polyenes, merocyanines, stilbazolium salts, and quinoid... 

Springborg and Kirtman have investigated the impact of donor/acceptor substitutions on the linear and nonlinear response properties of long polyenes and have demonstrated that, for sufficiently long chains, the responses per unit of a push-pull system becomes independent of the donor and acceptor groups. This conclusion, which concerns both the electronic and structural/geometrical relaxations, implies that, in that case, material properties cannot be improved upon substitution. Their predictions have been further illustrated and analyzed by adopting a Huckel like model. 

Zyss has established a relationship between various molecular stractures and optical nonUnearities by quantum mechairical calculations. Molecular hyperpolarizabiUty of monosubstituted benzenes and donor-acceptor-substituted phenyl-polyenes was evaluated from the finite field (FF) perturbed intermediate neglect of differential overlap (INDO) approach. From the calculations, the roles of charge transfer and chain length in NLO processes of conjugated molecules were confirmed. Oudar and Chemla su ested the two-state model for the design of NLO chromophores. In this model. 

The same method has been applied to the donor-aeeeptor substituted eumulenes and has re-established the conclusion that these molecules will have higher /S-values than the polyenes, other things being equal. Donor-acceptor quinoid molecules have been analysed in the same way. Comparisons are made between the non-hnear responses of these molecules and the polyenes. 

Another aspect worth mentioning is that it is usually the case that the 2Ag state has a covalent (neutral) character while the 1BU state has a more ionic character. As a result, in polyenes end-capped by strong donor and acceptor groups, the 1 Bu state becomes very much stabilized and passes below the 2Ag state24 substitution of the... 

Wang et al have used the AMl/FF approach for static properties and ZlNDO/SCl to study the -hyperpolarizabdity of some azulene derivatives. High values are obtained and it is concluded that the azulene ring is a more efficient conjugation bridge between donor and acceptor than either benzene or thiophene. They use the same methods in a study of push-pull polyenes containing nonaromatic cycfic olefines and for tetra-substituted benzenes and pyrazines. 

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