Hyperpolarizability push-pull polyenes

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.

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)... 

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. 

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. 

Chen, and W. A. Goddard III, Hyperpolarizabilities of push-pull polyenes experimental results and a new two-state model, Polym. Prepr. i5(2) 148 (1995). 

The most easily accessible experimental approach to this kind of modulation is offered by the study of solvent effects. This problem has recently been successfully faced by several authors [153,255,256]. In Fig. 28.33 it is possible to observe the modulation of the NLO responses obtained by changing the polarity of the solvent. It can be shown that a similar modulation is observed when the vibrational method is used to estimate molecular hyperpolarizabilities. Obviously, not every push-pull polyene exhibits solvent-dependent behavior. The analysis of the intensity behavior will reveal which systems are more suitable for modulation of bond alternation. Again it is the acceptor-donor pair that makes the molecule more responsive to the action of the solvent. In the polyene regime the chain structure is less affected by the interaction with the solvent. This can be rationalized if one remembers the two limiting canonical structures whose weighted combination reproduces the molecular structure. The interaction with the solvent is reflected in stabilization of the zwitterionic structure. 

Brotin, T, et al., [nJ-Polyenovanillins (n= 1-6) as New Push-Pull Polyenes for Nonlinear Optics Synthesis, Structural Studies, and Experimental and Theoretical Investigation of Their Spectroscopic Properties, Electronic Structures, and Quadratic Hyperpolarizabilities. Chemistry of materials, 1996, 8(4), 890-906. 

Jayaprakash et al. [198] reported the series of push-pull bimetallic polyene complexes [(CO)5M=C(OCH3)(-CH=CH-) (t C5H4)Fe(r C5H5)] (M = W, Cr n = 1-4) with ferrocene as donor and a carbene complex of Cr or W as acceptor. The jSi o6(HRS) values increase with increasing the conjugation of the n linker for the same n linker, the chromophore based on W exhibits higher second-order NLO responses than that based on Cr, as expected for a more efficient n backdonation from the W atom. The quadratic hyperpolarizability of these bimetallic push-pull... 

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