Substituent effects in molecular electronic relaxation dynamics via time-resolved photoelectron spectroscopy:: ππ* states in benzenes

We study the applicability of femtosecond time-resolved photoelectron spectroscopy to the study of substituent effects in molecular electronic relaxation dynamics using a series of monosubstituted benzenes as model compounds. Three basic types of electronic substituents were used: C=C (styrene), C=O (benzaldehyde), and CdropC (phenylacetylene). In addition, the effects of the rigidity and vibrational density of states of the substituent were investigated via both methyl (alpha-methylstyrene, acetophenone) and alkyl ring (indene) substitution. Femtosecond excitation to the second pipi* state leads, upon time-delayed ionization, to two distinct photoelectron bands having different decay constants. Variation of the ionization laser frequency had no effect on the photoelectron band shapes or lifetimes, indicating that autoionization from super-excited states played no discernible role. From assignment of the energy-resolved photoelectron spectra, a fast decaying component was attributed to electronic relaxation of the second pipi* state, a slower decaying component to the first pipi* state. Very fast electronic relaxation constants (< 100 fs) for the second pipi* states were observed for all molecules studied and are explained by relaxation to the first pipi* via a conical intersection near the planar minimum. Although a "floppy" methyl substitution (a-methylstyrene, acetophenone) leads as expected to even faster second pipi* decay rates, a rigid ring substitution (indene) has no discernible effect. The much slower electronic relaxation constants of the first pipi* states for styrene and phenylacetylene are very similar to those of benzene in its first pipi* state, at the same amount of vibrational energy. By contrast, the lifetime of the first pipi* state of indene was much longer, attributed to its rigid structure. The second pipi* state of benzaldehyde has a short lifetime, similar to the other derivatives. However, the relaxation of its first pipi* state is orders of magnitude faster than that of the non-carbonyl compounds, due to the well-known presence of a lower lying pipi* state. Methylation (acetophenone) leads to still faster first pipi* state relaxation rates. These results fit very well with the current understanding of aromatic photophysics, demonstrating that time-resolved photoelectron spectroscopy provides for a facile, accurate and direct means of studying electronic relaxation dynamics in a wide range of molecular systems.