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 ¹πσ* 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 ¹ππ* state (1¹ππ*) with a broadband femtosecond laser pulse, centered at wavelengths longer than 250 nm (4.97 eV), do not generate any measurable signature for ¹πσ* driven N–H bond fission on the amino group. Between wavelengths of 250 and >240 nm (<5.17 eV), coupling from 1¹ππ* onto the ¹πσ* state at a 1¹ππ*/¹πσ* CI facilitates ultrafast nonadiabatic N–H bond fission through a ¹πσ*/S₀ CI in <1 ps, a notion supported by CASSCF results. For excitation to the higher lying 2¹ππ* state, calculations reveal a near barrierless pathway for CI coupling between the 2¹ππ* and 1¹ππ* states, enabling the excited-state population to evolve through a rapid sequential 2¹ππ* → 1¹ππ* → ¹πσ* → 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 ¹πσ* state in 170 ± 20 fs, following 200 nm (6.21 eV) excitation to the 3¹ππ* 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 ¹πσ* driven dynamics in this ubiquitous genetic building block