A theoretical analysis of the two-photon properties of linear polyenes and the visual chromophores

Abstract

A self‐consistent field molecular orbital formalism for calculating molecular two‐photon absorptivities is presented based on the combined use of the Pariser–Parr–Pople π‐electron method including full single and double excitation configuration interaction and Monson and McClain’s two‐photon orientational averaging procedures. The formalism is applied to a series of linear, nonlinear, and retinyl polyenes to study the effect of chain length, conformation, and polarity on the calculated two‐photon absorptivities for various photon polarization and propagation relationships. The calculations indicate that the low‐lying ’’¹A_g^*−’’ covalent state should be strongly two‐photon allowed in virtually all polyenes, whether polar, nonpolar, linear, or nonlinear, provided a strongly one‐photon allowed ’’¹B_u^*+’’ state is nearby. The two‐photon absorptivity of the ’’¹A_g^*−’’ state for two linearly polarized photons is predicted to increase with increasing polyene chain length. Linearly polarized light produces the strongest two‐photon absorption for polyenes with four or more conjugated double bonds. The two‐photon absorptivity and polarization ratio of the ’’¹B_u^*−’’ state is predicted to be proportional to the degree of nonlinearity introduced into the polyene chain as a result of cis linkages. Accordingly, this state is responsible for the ’’cis band’’ in two‐photon spectroscopy.