Rotationally resolved UV spectroscopy of the rotamers of indole-4-carboxylic acid: Evidence for charge transfer quenching.

Rotationally resolved electronic spectra of two conformational isomers of jet-cooled indole-4-carboxylic acid (I4CA) and the deuterated forms of the acid (-COOD) and amide (-ND) groups have been obtained using a UV laser/molecular beam spectrometer. The in-plane orientation of the acid group defines the two lowest energy rotamers of I4CA. The S ← S origin bands of the two rotamers and four isotopologues have been fit to asymmetric rotor Hamiltonians in both electronic states. From the best-fit parameters, the positions of the H-atoms in the principal axis frames of each conformer have been determined and serve to unambiguously identify the syn forms (i.e., COH⋯O) of the cis and trans rotamers. The experimental S and S inertial parameters, hydrogen atom positions, and transition dipole moment (TDM) orientations are compared with the results of theoretical calculations. The TDM orientation indicates that the S state is the L state in contrast to most substituted indoles. The molecular orbital properties and natural charges are investigated to better understand the L/L state reversal and the extent of photoinduced intramolecular charge transfer that impacts the rotamer-dependent fluorescence lifetimes.