Exploring the Dynamics of the Photoinduced Ring-Opening of Heterocyclic Molecules

Abstract

Excited states formed by electron promotion to an antibonding σ* orbital are now recognized as key to understanding the photofragmentation dynamics of a broad range of heteroatom containing small molecules: alcohols, thiols, amines, and many of their aromatic analogues. Such excited states may be populated by direct photoexcitation, or indirectly by nonadiabatic transfer of population from some other optically excited state (e.g., a ππ* state). This Perspective explores the extent to which the fast-growing literature pertaining to such (n/π)σ*-state mediated bond fissions can inform and enhance our mechanistic understanding of photoinduced ring-opening in heterocyclic molecules.

Photoinduced C–H bond fission in prototypical organic molecules and radicals

“…7,10 nσ*/σ* excited states have also been implicated in UV photoinduced ring-opening processes, the dynamics of which are also now attracting interest. 11 follows. The highest (indeed the only) occupied molecular orbital (HOMO) in the ground (X 1 g + ) state of H2 is the 1σg orbital.…”

Effects of probe energy and competing pathways on time-resolved photoelectron spectroscopy: the ring-opening of 1,3-cyclohexadiene

“…1. [9][10][11] Absorption of UV light at approximately 266 nm places the wavepacket on the steeply repulsive potential energy surface of the optically bright 1B 1 pp* state, with the wavepacket accelerated ballistically towards the diabatic 2A spectroscopically dark state on a o50 fs timescale. It returns to the 1A electronic ground state through a conical intersection, leading to either the ring-open 1,3,5-hexatriene (HT) photoproduct or the ring-closed CHD molecule.…”

“…12 Interest in this reaction is driven by its role as a model system for modern mechanistic photochemistry. [9][10][11] The CHD ringopening is a prototypical pericyclic reaction and has contributed to the development and corroboration of the Woodward-Hoffman rules, 13 which predict the stereochemical outcome for pericyclic reactions by correlating the first excited state of the reactant with the first excited state of the product and the ground state of the reactant with a doubly excited state of the product, and vice versa. This yields a pericyclic minimum in the first excited state, which acts as photochemical funnel that provides an efficient path for internal conversion.…”

Protomer-Dependent Electronic Spectroscopy and Photochemistry of the Model Flavin Chromophore Alloxazine

“…It is notable that HNCO loss is well-known to be associated with photoexcitation of single-ring uracil [ 41 , 42 , 43 ], and the AL∙H + transition accessed close to 5 eV appears to be associated with almost localized excitation to the uracil moiety of AL. Although there have been a considerable number of experimental and theoretical studies to understand ring-opening dynamics over recent years [ 44 ], these studies have focused on single-ring systems. The tri-ring flavin group, present in the important biologically active flavins such as flavin adenine dinucleotide (FAD) and FMN, offers an important target system in which to extend these simpler studies.…”