πσ* excited states in molecular photochemistry

Abstract

The last few years have seen a surge in interest (both theoretical and experimental) in the photochemistry of heteroaromatic molecules (e.g. azoles, phenols), which has served to highlight the importance of dissociative excited states formed by electron promotion to sigma* molecular orbitals. Such excited states--which, for brevity, are termed pi sigma* states in this Perspective article--may be populated by direct photo-excitation (though the transition cross-sections are intrinsically small), or indirectly, by non-adiabatic coupling from an optically 'bright' excited state (e.g. an excited state resulting from pi* <--pi excitation). The analogous pi sigma* excited states in prototypical hydride molecules like H(2)O and NH(3) have long been recognised. They have served as test-beds for developing concepts like Rydbergisation, conical intersections (CIs) between potential energy surfaces, and for investigating the ways in which non-adiabatic couplings at such CIs influence the eventual photofragmentation dynamics. This Perspective article seeks to highlight the continuity of behaviour revealed by the earlier small molecule studies and by the more recent studies of heteroaromatic systems, and to illustrate the photochemical importance of pi sigma* excited states in many broad families of molecules. Furthermore, the dynamical influence of such excited states is not restricted to closed shell species; the Article concludes with a brief consideration of the consequences of populating sigma* orbitals in free radical species, in molecular cations, and in dissociative electron attachment processes.

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

“…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.…”

UV Photodissociation of Pyrroles: Symmetry and Substituent Effects

“…excited states formed via σ*n or σ*π electron promotions) in promoting such bond fissions, and much recent research has sought to explore the importance (or otherwise) of such processes following UV photoexcitation of larger, more biochemically relevant molecules. [1][2][3][4] Increasing size is accompanied by an increase in the vibrational (and in many cases electronic) state density and, typically, in the number and efficiency of alternative, radiationless, pathways by which excited state population can be channeled back to the ground (S 0 ) electronic state.…”

Following the excited state relaxation dynamics of indole and 5-hydroxyindole using time-resolved photoelectron spectroscopy

“…22 In the case of indole, the 1 πσ* state has been identified as having 1 A ′′ symmetry and significant Rydberg (3s) character, with much of the electron density being localised on the NH group and exhibiting a node along the N-H bond. [23][24][25] The 1 πσ* ← S 0 transition possesses little or no oscillator strength and so is essentially "optically dark" to single photon absorption.…”