Using high resolution electronic spectroscopy to probe the effects of ring twist on charge transfer in 2-phenylindole and N-phenylcarbazole.

High resolution electronic spectra of 2-phenylindole (PI) and N-phenylcarbazole (PC) have been recorded in the collision-free environment of a molecular beam. Inertial defects determined from fits of the spectra were used to determine the twist angles between the two chromophores and their attached benzene rings in the ground (S0) and excited (S1) electronic states. PI was found to be significantly more planar than PC, especially in the S1 state. Stark-effect measurements of the permanent electric dipole moments of both molecules in both states show that significantly more charge is transferred from the phenyl group to the chromophore in PI (0.13e) than in PC (0.076e) when the photon is absorbed. Thereby demonstrated for the first time is a direct connection between photo-induced geometry change and charge transfer on excitation of an isolated molecule by light.