Hyperpolarizability bond order alternation

Quantum mechanical analysis based on a simple two-level model [22] and bond-order alternation (BOA) principle exploiting aromaticity [23] have worked surprisingly well in providing useful structure/property relationships for the design of chromophores with ever improving molecular hyperpolarizability. Table 1 provides some representative examples with improved molecular optical nonlinearity developed over the past decade. It has been shown that very large nonlinearities... 

Figure 2 Dependence of the first and second hyperpolarizabilities on bond-order alternation (BOA) calculated for a donor-acceptor polyene under the influence of an external electric field. The left-hand side of each plot corresponds to a structure dominated by the neutral resonance structure, whereas the right-hand side corresponds to a zwitterionic structure. In a two-state model BOA = 0 corresponds to an equal mixture of the two resonance forms.

Luo et al. have investigated the influence of the solvent polarity on the NLO properties of a simple donor-acceptor polyene molecule (Scheme 4) at the ab initio level of theory applying the continuum model of solvent [107]. It has been shown that the evolution of the TPA cross section with respect to the bond length alternation (BLA) closely follows that of the static first-order hyperpolarizability, p. The TPA cross section is strongly dependent on the geometrical changes. Moreover, these authors have noticed that the solvent effect on the TPA cross section (for the CT excited state) in tlie typical donor-acceptor polyene molecule exhibiting the positive solvatochromism is smaller than the influence of the solvent on the values of p. 

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