Roberts, Gareth M., Williams, Craig A., Young, Jamie D., Ullrich, Susanne, Paterson, Martin P. and Stavros, Vasilios G. (2012) Unraveling ultrafast dynamics in photoexcited aniline. Journal of the American Chemical Society, Vol.134 (No.30). pp. 12578-12589. doi:10.1021/ja3029729 ISSN 1520-5126.

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Abstract

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.

Item Type:	Journal Article
Divisions:	Faculty of Science, Engineering and Medicine > Science > Chemistry
Journal or Publication Title:	Journal of the American Chemical Society
Publisher:	American Chemical Society
ISSN:	1520-5126
Official Date:	2012
Dates:	Date Event 2012 Published
Volume:	Vol.134
Number:	No.30
Number of Pages:	12
Page Range:	pp. 12578-12589
DOI:	10.1021/ja3029729
Status:	Peer Reviewed
Publication Status:	Published
Access rights to Published version:	Restricted or Subscription Access

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