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

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 ’’1Ag*−’’ covalent state should be strongly two-photon allowed in virtually all polyenes, whether polar, nonpolar, linear, or nonlinear, provided a strongly one-photon allowed ’’1Bu*+’’ state is nearby. The two-photon absorptivity of the ’’1Ag*−’’ 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 ’’1Bu*−’’ 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.