Excited state processes of nitrobenzaldehydes probed by ultrafast fluorescence and absorption spectroscopy

We have investigated the isomers ortho-, meta-, and para-nitro-benzaidehyde (NBA) by means of ultrafast emission and absorption spectroscopy. The fluorescence dynamics of all three molecules are very similar and feature bi-phasic emission decays with time constants of <100 fs and similar to 1 ps. The first process goes along with a strong loss of oscillator Strength, which we assume is the result of a non-radiative (1)pi pi* -> 1 pi pi* transition. These commonalities are contrasted by qualitatively different transient absorption signatures: For the photo-reactive isomer o-NBA, signatures of a ketene intermediate in the electronic ground state are observed. Its formation time matches the slower component of the emission decay. The ketene afterwards experiences pronounced cooling dynamics on the picosecond timescale followed by its decay in the nanosecond regime. For the non-reactive isomers m- and p-NBA, time constants with values similar to the ones derived from the emission experiment are found in the absorption experiment. In addition, at later delay times, a distinct spectral signature is observed for both isomers which decays within similar to 1 ns and is assigned to a triplet state. This triplet decay is preceded by a process on the 10 ps timescale which we tentatively assign to a transition within the triplet manifold. (C) 2008 Elsevier B.V. All rights reserved.