Mechanisms of Photochemical Reactions in Solution. XXII.¹ Photochemical cis-trans Isomerization

Flavin-photosensitized reactions of retinal and stilbene

“…The triplets have similar lifetimes and reactivities but different energies, and there is therefore a systematic variation of the triplet properties. Some of the complexity of the relationship found by Hammond et al (1964) may be caused by the diverse properties of the sensitizers used. Another possible explanation is that the triplet energies we used were for the phosphorescence transition and not for the 0-0 band.…”

“…605nm, 46.5kcal/mol) (Gordon-Walker et al 1970) it can be seen that classical energy transfer from the flavin triplet to the c8-stilbene is unlikely to occur. To explain the sensitized isomerization of the cwa isomer one must postulate non-classical excitation and 'phantom triplets' (Hammond et al 1964). If one also postulated that the main decay to the ground state is from the 'phantom triplet' then the following scheme for the mechanism can be suggested, where Fs is the flavin excited singlet, F, c and t are the flavin, ci8- …”

“…For these flavins an approximate final ratio was obtained and then mixtures of the two stilbenes near this ratio (total concentration 10-15mM) were made up and illuminated for short times to give the exact photostationary-state ratio of isomers (Hammond et al 1964). The photostationary-state mixture was always approached from both sides to give an accurate value and the final ratios were averages of two or three experiments for each flavin.…”

“…The photosensitized isoinerizations of stilbene have been studied (Hammond et al 1964; Posthuma & Berends, 1966) and are postulated to proceed by a triplet mechanism perhaps involving 'phantom triplets' (Hammond et al 1964). These are defined as triplets with twisted configurations that have a lower energy than the classical triplets (Hammond et al 1964).…”

“…These are defined as triplets with twisted configurations that have a lower energy than the classical triplets (Hammond et al 1964). A further suggestion is that nonvertical excitation can occur from the ground state to the 'phantom triplet' (Hammond et al 1964).…”