Photochemistry in jet-cooled aniline derivatives Original Research Article

High-resolution laser-induced fluorescence excitation spectra of four para-substituted pyrrolidino- and dimethylamino-benzoic acid nitriles and esters have been compared under jet-cooled conditions and the fluorescence decay profiles of jet-cooled dimethylaminobenzoic acid nitrile (DMABN) and methyl ester (DMABME) have been determined using synchrotron radiation. The resonant two-photon ionization mass spectra (REMPI-TOF-MS) of DMABME revealed the presence of monomeric and dimeric DMABME and of van der Waals complexes with water. The loss of a well-resolved B-type rotational structure in the LIF excitation spectra for DMABME as compared to DMABN is indicative of state mixing between the initially populated 1Lb and the energetically close-lying 1La excited state and is paralleled by the onset of continua in the excitation spectra and associated ‘anomalous’ red-shifted emission and non-exponentiality in the fluorescence decays. However, the REMPI-TOF-MS spectra of jet-cooled DMABME show that complexes with water make a major contribution to the underlying continuum and the associated red-shifted fluorescence and serve to emphasise the essential role of the ‘solvent’. A further contribution comes from DMABME dimers. The results regarding the enhancement of red fluorescence of DMABME with respect to DMABN are discussed in terms of an adiabatic photochemical reaction, i.e. formation of a rotamer on the excited state hypersurface and nearly full electron transfer from the donor to the acceptor moeity, the so-called twisted intramolecular charge transfer (TICT) state. Microsolvation and state mixing in DMABME facilitate this process.