New concepts in photochemistry and photophysics: Photochromic and other type molecules

It is now clear for the first time that photochemistry can compete with vibrational relaxation within a singlet state. This possibility has not been generally considered, or believed possible, prior to the considerations given herein. It is also evident that both the nature of the vibrational/vibronic mode and the vibrational level excited within a given mode can markedly affect the experimentally determined quantum yields of photochemistry ΦPC(n) and fluorescence ΦF(n) in multi-atom systems, where n is the nth level. There is not just one quantum yield of fluorescence (F) or photochemistry (PC), but there are many that can exist. Since the yields of PC depend on the vibrational/vibronic level excited, the new concepts of ΦV and ΦPC inevitably arise ΦV = kV/(kV + kPC) and ΦPC = kPC/(kPC + kV). ΦPC measures the efficiency of photochemistry from a given level within a given mode in competition with vibrational relaxation between the given level and a lower one. ΦV measures the efficiency of vibrational relaxation between a given level and a lower one. The magnitude of the photochemical yield is largely dictated by the nature of the mode excited within a given electronic state as well as the level that is excited. ΦV and ΦPC have no parallel in previously existing experimental or theoretical photophysics/photochemistry. In general, relaxation from the 0 level of an upper state Sx (x = 2, 3, …) does not occur via vibrational levels of any lower energy states.

► Equations for dependence of ΦF on vibrational mode and level (n) excited. ► Experimental ΦPC dependence on vibrational mode and level (n) excited. ► New concepts of quantum yield of vibrational relaxation ΦV and ΦPC. ► Relaxation only occurs between vibrational levels of same mode. Generally no upper state relaxation via levels of states below.