Femtosecond spectroscopy of p-dimethylaminocyanostilbene in solution—no evidence for dual fluorescence

The photo-induced relaxation of p-dimethylaminocyanostilbene (DCS) in acetonitrile was studied with broadband transient fluorescence and absorption spectroscopy. The dynamic Stokes shift of fluorescence was measured to be 4000 cm−1. The ‘dual fluorescence’ of DCS, reported previously, is shown to be an artefact resulting from excited state re-absorption of fluorescence. The shift has a characteristic time of 100 fs and is governed by solvation in acetonitrile. Together with results from quantum chemical calculations it is concluded that twisted intramolecular charge transfer does not contribute measurably to the evolution of fluorescence and that the dynamic Stokes shift reflects pure polar solvation.