Excited-State Dipole Moments, Polarizabilities, and Local Fields in Molecular Crystals; Polarizabilities of Benzophenone in its Lowest Singlet and Triplet nπ Excited States

Abstract

A refined calculation that accounts for molecular polarizabilities, crystal reaction fields and resulting field induced moments in polar molecular crystals allows excited-state dipole moments and polarizabilities to be derived from optically determined Stark splitting versus applied electric field results. The determination of these excited-state properties in a dipolar lattice is particularly dependent on the evaluation of the local field at a molecular site. For this case, the local field is evaluated by introduction of an ellipsoidal cavity approximation and by using the Ewald-Kornfeld lattice summation method for dipolar arrays. The dipolar benzophenone crystal was chosen to demonstrate this method. The dipole moments of the lowest 1nπ* and 3nπ* states of benzophenone have been determined and are 1.23 D and 1.72 D, respectively; these are the zero field values and refer to the benzophenone molecules in the absence of the crystal environment. The polarizabilities of benzophenone in its ground, 1nπ* and 3nπ* states are, in units of 10−23 cm3, 0.532, 0.898, and 0.56, respectively. The excited-state moments are nearly parallel to the ground state moment in the crystal environment and the benzophenone dipole does not reverse direction in going from the ground to the lowest singlet and triplet states.