Excited state decay pathways of some photochemical reactions

In the last few decades, several photo-chemical reactions have been thoroughly investigated through high-level quantum mechanical techniques. These include photo-isomerizations of retinylidene, azobenzene, stilbene, neutral polyene, spiropyran-merocyanine conversion, channel 3-decay of benzene, non-radiative decay of nucleic acid bases and so on. The common feature of these photo-conversion reactions is the involvement of fast non-radiative decay paths through conical intersections of the states involved in the process. Topography of these conical intersections can be peaked (faster process) or sloped (slower process). The barrierless ultrafast 11-cis to all-trans photo-isomerization of the retinal protonated Schiff base (RPSB) chromophore in the visual pigment rhodopsin and the all-trans to 13-cis isomerization of this chromophore in bacteriorhodopsin are few well-known ultra-fast isomerizations where conical intersection plays a major role. Involvement of H-out-of-plane (HOOP) wagging mode and torsional motions are reported in these retinal pigments. Competitions between the volume conserving Hula-twist (HT) (and bicycle pedal BP) and volume demanding one-bond-flip (OBF) motions along the conical intersection path in the photo-isomerizations of polar and non-polar polyenes are well-studied by different groups. Conversion of trans to cis azobenzene under UV-radiation and the reverse thermal or photo-isomerization under visible light is another example where numerous studies on rotation, inversion, concerted inversion and inversion-assisted rotation leading to the conical intersection channels have been reported