Unraveling Ultrafast Dynamics in Photoexcited Aniline

A combination of ultrafast time-resolved velocity map imaging (TR-VMI) methods and complete active space self-consistent field (CASSCF) ab initio calculations are implemented to investigate the electronic excited-state dynamics in aniline (aminobenzene), with a perspective for modeling (1)pi sigma* mediated dynamics along the amino moiety in the purine derived DNA bases. This synergy between experiment and theory has enabled a comprehensive picture of the photochemical pathways/conical intersections (CIs), which govern the dynamics in aniline, to be established over a wide range of excitation wavelengths. TR-VMI studies following excitation to the lowest-lying (1)pi pi* state (1(1)pi pi*) with a broadband femtosecond laser pulse, centered at wavelengths longer than 250 nm (4.97 eV), do not generate any measurable signature for (1)pi sigma* driven N-H bond fission on the amino group. Between wavelengths of 250 and >240 nm (<5.17 eV), coupling from 1(1)pi pi* onto the (1)pi sigma* state at a 1(1)pi pi*/(1)pi sigma* CI facilitates ultrafast nonadiabatic N-H bond fission through a (1)pi sigma*/S-0 CI in <1 ps, a notion supported by CASSCF results. For excitation to the higher lying 2(1)pi pi* state, calculations reveal a near barrierless pathway for CI coupling between the 2(1)pi pi* and 1(1)pi pi* states, enabling the excited-state population to evolve through a rapid sequential 2(1)pi pi* -> 1(1)pi pi* -> (1)pi sigma* -> N-H fission mechanism, which we observe to take place in 155 +/- 30 fs at 240 nm. We also postulate that an analogous cascade of CI couplings facilitates N-H bond scission along the (1)pi sigma* state in 170 +/- 20 fs, following 200 nm (6.21 eV) excitation to the 3(1)pi pi* surface. Particularly illuminating is the fact that a number of the CASSCF calculated CI geometries in aniline bear an exceptional resemblance with previously calculated CIs and potential energy profiles along the amino moiety in guanine, strongly suggesting that the results here may act as an excellent grounding for better understanding (1)pi sigma* driven dynamics in this ubiquitous genetic building block.