Electro-optic response of chiral helicenes in isotropic media

Correlated quantum-chemical calculations are used to estimate the second-order electro-optic response of chiral helicene molecules in isotropic media. Dephasing-induced terms are included in a sum-over-states (SOS) perturbation formalism derived from the density-matrix expression. Besides the usual resonances associated with intrinsic electronic excitations of the material, resonant peaks are found at optical frequencies corresponding to energy differences between intermediate excited states. The mechanism for such resonance phenomena in helicenes is analyzed in the framework of a configuration interaction description of the singlet excited states. The possibility is discussed that in such a resonance regime the optical field can experience gain without linear loss. (C) 1998 American Institute of Physics. [S0021-9606(98)02704-4].