Ultraviolet relaxation dynamics of aniline, N, N-dimethylaniline and 3,5-dimethylaniline at 250 nm

Time-resolved photoelectron imaging was used to investigate the electronic relaxation dynamics of gas-phase aniline, N, N-dimethylaniline, and 3,5-dimethylaniline following ultraviolet excitation at 250 nm. Our analysis was supported by ab initio coupled-cluster calculations evaluating excited state energies and (in aniline) the evolution of a range of excited state physical properties as a function of N-H bond extension. Due to a lack of consistency between several earlier studies undertaken in aniline, the specific aim of this present work was to gain new insight into the previously proposed non-adiabatic coupling interaction between the two lowest lying singlet excited states S-1(pi pi*) and S-2(3s/pi sigma*). The methyl-substituted systems N, N-dimethylaniline and 3,5-dimethylaniline were included in order to obtain more detailed dynamical information about the key internal molecular coordinates that drive the S-1(pi pi*)/S-2(3s/pi sigma*) coupling mechanism. Our findings suggest that in all three systems, both electronic states are directly populated during the initial excitation, with the S-2(3s/pi s*) state then potentially decaying via either direct dissociation along the N-X stretching coordinate (X = H or CH3) or internal conversion to the S-1(pi pi*) state. In aniline and N, N-dimethylaniline, both pathways most likely compete in the depletion of S-2(3s/pi s*) state population. However, in 3,5-dimethylaniline, only the direct dissociation mechanism appears to be active. This is rationalized in terms of changes in the relative rates of the two decay pathways upon methylation of the aromatic ring system. (C) 2015 AIP Publishing LLC.