Excited-state hydrogen detachment and hydrogen transfer driven by repulsive 1πσ* states: A new paradigm for nonradiative decay in aromatic biomolecules

Ultrafast Excited-State Dynamics of Kynurenine, a UV Filter of the Human Eye

“…Another proposed mechanism involves a repulsive πσ* state connecting the lowest singlet excited state to the ground state via conical intersections. 15 A similar mechanism involving a conical intersection between the S 1 and S 0 states was also suggested to account for the subpicosecond fluorescence lifetime of uracil and its derivatives. 16 The rate of nonradiative S 1 f S 0 transitions can also be significantly affected by intra-and intermolecular hydrogen bonds.…”

On the Participation of Photoinduced N–H Bond Fission in Aqueous Adenine at 266 and 220 nm: A Combined Ultrafast Transient Electronic and Vibrational Absorption Spectroscopy Study

“…There has been some debate regarding the dominant pathway for re-accessing S0 after UV excitation in 9H-Ade, although the general consensus is that out-ofplane ring distortion at either the C2 or C6 sites (see Scheme 1) leads to conical intersections which facilitate ultrafast (and near barrierless) internal conversion (IC) back to S0; we note for completeness that recent work suggests direct 1 ππ* → S0 IC is likely to be the dominant process, 9,21,26,31 rather than a sequential 1 ππ* → 1 nπ* → S0 process. 12,26 In addition to these out-of-plane 'ring-puckering' conical intersections, Domcke and co-workers 14,32 first proposed that at shorter excitation wavelengths, competing relaxation mechanisms, mediated by conical intersections lying along alternative co-ordinates, may also become active; namely, ring-opening along the C8-N9 bond 14 and homolytic bond fission along the N9-H coordinate (or N10H2 amino group). 14,17,18,21 Both of these channels are driven by the presence of dissociative 1 πσ* states, which are ubiquitous in aromatic molecules containing heteroatoms (O, N, S etc.)…”

“…and have become an intense area of study within the chemical dynamics community. [32][33][34] With reference to the schematic potentials in Figure 1, Domcke and co-workers predicted that upon excitation to the 1 ππ* state(s) at energies >5.5 eV (<225 nm), population could couple through a 1 ππ*/ 1 πσ* conical intersection at elongated N9-H distances to access a dissociative 1 πσ* state. 14 Once on that 1 πσ* state, population may then traverse a lower energy 1 πσ*/S0 conical intersection at further extended N9-H bond lengths and either (i) directly form adeninyl radical photoproducts, Ade[-H], in coincidence with translationally excited H-atoms, or (ii) form vibrationally hot Ade in its S0 state.…”

GPU-Accelerated State-Averaged Complete Active Space Self-Consistent Field Interfaced with Ab Initio Multiple Spawning Unravels the Photodynamics of Provitamin D3

“…Excited-state potential energy surfaces are quite complex, often exhibiting multiple avoided crossings or conical intersections (CIs) between two or more electronic states. [1][2][3][4][5] Furthermore, photochemical reactions occur under nonequilibrium conditions, typically on the femtosecond to picosecond timescale. [6][7] Although advances in ultrafast spectroscopy have enabled detailed studies of such reactions, 6 these experiments are not straightforward to perform and their results can be difficult to interpret.…”