Raman intensities induced by electrostatic intermolecular interaction and related nonlinear optical properties of a conjugated π-electron system:: A theoretical study

Raman intensities of a charged conjugated,pi-electron system induced by electrostatic intermolecular interaction are studied theoretically. By using a simple Hamiltonian based on a two-state model, in which the response of the system to an electric field is taken into account, the formulas for the polarizability derivative and related quantities are derived. These formulas are applied to the case of a pentamethine streptocyanine dye, which has a symmetric conjugated chain consisting of four CC bonds with one NC bond on each end. It is shown that a reasonable magnitude of electrostatic interaction with a counterion induces Raman intensities on the order of 10(2) Angstrom(4) amu(-1) for the modes with large contributions from the vibration along the bond-alternation coordinate of the chain, explaining the appearance of the 1574- and 1207-cm(-1) bonds in the Raman spectrum measured in solution. A slight deformation of the conjugated chain along the bond-alternation coordinate induced by electrostatic interaction and the strong electron-vibration interaction are responsible for these Raman intensities. The formulas derived in this study are then used for evaluating the electronic and vibrational contributions to the first hyperpolarizability. It is concluded that the vibrational contribution is on the same order of magnitude as (but smaller than) the electronic one, at least in the case of typical charged conjugated pi-electron systems.