ELECTRIC-FIELD MODULATED NONLINEAR-OPTICAL PROPERTIES OF DONOR-ACCEPTOR POLYENES - SUM-OVER-STATES INVESTIGATION OF THE RELATIONSHIP BETWEEN MOLECULAR POLARIZABILITIES (ALPHA, BETA, AND GAMMA) AND BOND-LENGTH ALTERNATION

We investigate the influence of an external, static electric field upon the relationship between the structure, spatial distribution of electron density, and linear and nonlinear polarizabilities for a set of pi-electron chromophores. The bond length alternation (BLA) and the pi-electron bond order alternation (BOA) in the molecular structure are calculated as a function of the external electric field strength so that the dependence of the linear polarizability, alpha, second-order polarizability, beta, and third-order polarizability, gamma, can be expressed in terms of the BLA or BOA structural parameters. The calculated effect of the external electric field on the structure and electronic properties of the chromophores is similar to that resulting from donor-acceptor substituents on a segment of the chromophore or from changing the dielectric properties of the environment around the molecule, e.g., increasing the polarity of the solvent. The incorporation of a static electric field into the calculations provides a more realistic treatment of the nonlinear optical properties of the chromophore in the condensed phase and suggests strategies to optimize these properties by adjusting the dielectric properties of the medium around the chromophore.