Push−Pull Molecules in the Gas Phase: Stark-Effect Measurements of the Permanent Dipole Moments of p-Aminobenzoic Acid in Its Ground and Electronically Excited States

Abstract

Stark effect measurements of the permanent electric dipole moments of p-aminobenzoic acid (PABA) in the gas phase are reported based on studies of its fully resolved S1<--S0 electronic spectrum in the presence of an electric field. Ground-state (S0) PABA has mu = 3.3 D, whereas excited-state (S1) PABA has mu = 4.4 D. Despite PABA's reputation as a "push-pull" molecule, the photon-induced change in both the magnitude and orientation of mu is relatively small. Possible reasons for this behavior are discussed.