Rational design of molecules with large hyperpolarizabilities. Electric field, solvent polarity, and bond length alternation effects on merocyanine dye linear and nonlinear optical properties

The effects of applied electric fields and solvent polarity on the linear and nonlinear optical (NLO) properties of three prototypical merocyanine (donor-conjugated pathway-acceptor) chromophores of differing architectural types are analyzed using semiempirical INDO/1 calculations in the presence of imposed static electric fields, using finite-field self-consistent field (FFSCF), and in the presence of solvent dielectrics, self-consistent reaction field (SCRF), models. The NLO properties are computed using the computationally efficient correction vector approach. Both applied electric fields and solvent are found to affect the extent of charge separation induced in the ground states of these molecules. This charge separation leads to a geometric distortion, measured by the bond-length alternation (BLA) parameter, which indexes the geometrical evolution of the molecular architecture from a neutral polyenic structure to a partially ionic cyanine-like structure to an ionic polymethine-like structure. These geometric variations lead to large changes in the linear as well as NLO response properties, which are different, both qualitatively and quantitatively, at the three different structural limits. The applied electric field is found to produce large variations in the structural, electronic, linear, and NLO properties. However, an analysis based on the Onsager reaction field model shows that in general the electric field produced by even the most polar solvents is inadequate to produce large geometric distortions. This trend is also observed experimentally in solvent-dependent electronic and optical properties, where even the most polar solvents produce effects equivalent only to those produced by a very small electric field. However, variations in structural and optical properties are also found to be highly architecture specific and large variations are possible in chromophore structures stabilized on charge separation. The field at which the chromophore geometry attains the cyanine-like structure is not coincident with the field at which the SHG coefficient goes to zero; an asymmetry in the BLA pattern is required to attain the zero-p limit. Optimal NLO response occurs when the geometry of the conjugation pathway lies between the polyenic and cyanine-like structures.