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

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 La-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.