Determination of the geometry change of 5-cyanoindole upon electronic excitation from a combined Franck–Condon/rotational constants fit

Abstract

The geometry change of 5-cyanoindole upon electronic excitation from the ground to the lowest excited singlet state has been determined from a combined fit of the rotational constant changes upon excitation and the vibronic intensities in various fluorescence emission spectra using the Franck–Condon principle. The so determined geometry change is compared to the results of ab initio calculations and points to an excited state geometry, which is L_a-like in the nomenclature of Platt. A mode selective coupling of vibronic bands to higher-lying excited states is discussed on the basis of Herzberg–Teller contributions to the Frank–Condon intensities.