Time resolved velocity map imaging of H-atom elimination from photoexcited imidazole and its methyl substituted derivatives

Abstract

The photoresistive properties of DNA bases, amino acids and corresponding subunits have received considerable attention through spectroscopic studies in recent years. One photoresistive property implicates the participation of ¹πσ* states, allowing electronically excited states to evolve either back to the electronic ground state or undergo direct dissociation along a heteroatom–hydride (X–H) coordinate. To this effect, time-resolved velocity map imaging (TR-VMI) studies of imidazole (a subunit of both adenine and histidine) and methylated derivatives thereof have been undertaken, with the goal of understanding the effects of increasing molecular complexity, through methylation, on the dynamics following photoexcitation at 200 nm. The results of these measurements clearly show that H-atom elimination along the N–H coordinate results in a bimodal distribution in the total kinetic energy release (TKER) spectra in both imidazole and it's methylated derivatives: 2-methyl, 4-methyl and 2,4-dimethylimidazole. The associated time constants for H-atoms eliminated with both high and low kinetic energies are all less than 500 fs. A noticeable increase in the time constants for the methylated derivatives is also observed. This could be attributed to either: ring methylation hindering in-plane and out-of-plane ring distortions which have been implicated as mediating excited state dynamics of these molecules or; an increase in the density of vibrational states at 200 nm causing an increased sampling of orthogonal modes, as opposed to modes which drive any dynamics that cause subsequent H-atom elimination. The results of these findings once again serve to illustrate the seemingly ubiquitous nature of ¹πσ* states in the photoexcited state dynamics of biomolecules and their subunits.