Push-pull polyenes

The asymmetrical D-ji-A dyes, often referred to as push-pull polyenes, are an additional class of cyanine-like molecules of interest. Due to their dipolar nature, the linear and nonlinear optical properties of this series of dyes can be strongly influenced by solvent polarity [84]. The structures of a series of such dyes (G19,... 

Of special interest are push-pull polyenes composed of a polyolefinic chain bearing a donor group on one end and an acceptor on the other (see 96). Push-pull carotenoid derivatives (Figure 20) are highly polarizable conjugated polyenes that also represent polarized molecular wires. 

Fig. 29. Optical switching of electronic conduction (left) and non-linear optical properties (right) respectively in redox and push-pull polyene molecular wires /J is the hyperpolarizability.

Figure 11.44 Push-pull polyenes in which an organic electron donor and acceptor are connected by a molecular wire.

Since the vibrational spectra obtained for these two structures (iii) are very similar, we will describe in detail only the results obtained for the stacking geometry (Figure 4.17). This architecture seems to be a better model mimicking the arrangement of strongly interacting dimers as observed in crystals of similar push-pull polyenes [2],... 

Suitieri97 has also evaluated the anharmonic contributions to the nuclear relaxation y for some push-pull polyenes using analytical methods in a valence bond charge transfer model. Saal and Ouamerali98 have investigated the vibrational ft of N-fluorophemyl-2,5-dimethypyrrole in the double harmonic... 

Marder. S.R., Cheng, L.-T.. Tiemann, B.G., Friedli. A.C., Blanchard-Desce, M., Perry, J.W., Skindhoj, J. Large first hyperpolarizability in push-pull polyenes by tuning of the bond length alternation and aromaticity. Science 263, 511-514 (1994)... 

Barzoukas and Blanchard-Desce proposed an approach of molecular engineering using multivalence-bond state models [55]. Push-pull polyenes were shown also to present an enhancement of the TPA response and a loss of transparency of molecules, as a function of the increase of the polyenic chain length [56,57]. Trends observed in these polyenic systems are supported by the large third-order optical nonlinearities measured in asymmetric carotenoids, in which the role of the large value of dipole moment difference A/z was shown [58]. 

NLO properties. For different polyene series (Figure 1.13), the MLCT bands unexpectedly blue-shift as the number of ethylene units increases from one to three, and / o maximises at = 2. This behaviour is in marked contrast with known push-pull polyene organic chromophores, in which the tt-tt intramolecular charge transfer (ICT) bands red-shift as the number of ethylene units increases and the values increase steadily with Time-dependent (TD)-DFT calculations corroborate the... 

A model exact solution of PPP model was utilized to study the nature of the singlet excited state of push-pull polyenes [46]. Indeed, the studies revealed that the push-pull polyenes acquire a highly polarized or zwitterionic nature in their singlet excited state. A large change in the dipole moment was also noted in their states at rotational angle of 90°. 

Fig. 36.18. Scheme of interactions of end groups in push-pull polyenes of increasing size. 

Del Zoppo et al.197 have made an ab initio study of a model push-pull polyene and calculated the dependence of the polarizability and first hyperpolarizability on the bond alternation. Vibrational and solvent effects are simulated in the calculation. 

This description of quantum mechanical methods for computing (hyper)polarizabilities demonstrates why, nowada, the determination of hyperpolarizabilities of systems containing hundreds of atoms can, at best, be achieved by adopting, for obvious computational reasons, semi-empirical schemes. In this study, the evaluation of the static and dynamic polarizabilities and first hyperpolaiizabilities was carried out at die Time-Dependent Hartree-Fock (TDOT) [39] level with the AMI [50] Hamiltonian. The dipole moments were also evaluated using the AMI scheme. The reliability of the semi-empirical AMI calculations was addressed in two ways. For small and medium-size push-pull polyenes, the TDHF/AMl approach was compared to Hartree-Fock and post Hartree-Fock [51] calculations of die static and dynamic longitudinal first hyperpolarizability. Except near resonance, the TDHF/AMl scheme was shown to perform appreciably better than the ab initio TDHF scheme. Then, the static electronic first hyperpolaiizabilities of the MNA molecule and dimer have been calculated [15] with various ab initio schemes and compared to the AMI results. In particular, the inclusion of electron correlation at the MP2 level leads to an increase of Paaa by about 50% with respect to the CPHF approach, similar to the effect calculated by Sim et al. [52] for the longitudinal p tensor component of p-nitroaniline. The use of AMI Hamiltonian predicts a p aa value that is smaller than the correlated MP2/6-31G result but larger than any of the CPHF ones, which results fi-om the implicit treatment of correlation effects, characteristic of die semi-empirical methods. This comparison confirms that a part of die electron... 

Bishop and Bouferguene have also attempted to partition molecular polarizabilities and hyperpolarizabilities into localized group contributions over the molecular frame. The theory has been applied to a series of push-pull polyenes. An interesting conclusion is that there is little similarity between the distributions for the vibrational and electronic contributions. 

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. 

Zhu and Jiang have systematically employed the AMI hamiltonian with point eharge fields represented by sparkles to calculate static -tensors. They have investigated push-pull polyenes and correlate the polarizabilities and hyperpolarizabilities with the net charge on the polyene bridge between donor and acceptor. 

Luis et a/. have shown that, in evaluating the contribution to the static vibrational hyperpolarizability that comes from the normal co-ordinate derivatives of the zero point energy, it is possible to replace the sum over 3N — 6 normal coordinates by transforming to a set of field induced co-ordinates, when only one term remains. The method has been applied to a typical push-pull polyene, NH2-(C=C)-N02. 

Here we discuss a joint theoretical and experimental study of the influence of solvent polarity on the second-order molecular polarizability p of p-nitroaniline and the push-pull polyene l,l-dicyano-6-dibutylamine-hexa-triene [144]. The calculations are carried out at the Har-tree-Fock ab initio level on the basis of an expanded self-consistent reaction field approach and are compared to hyper-Rayleigh scattering measurements performed in solvents with a wide range of dielectric constants. 

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