A laser flash photolysis study of 11-cis-locked retinal analogues

The photophysics and photochemistry of cis-locked retinal analogues have been investigated by steady-state and by picosecond and nanosecond time-resolved techniques. We have found that retinal analogues locked into the 11,12-cis configuration via bridged 8- and 9-membered rings, Ret-8 and Ret-9, respectively, are restricted in twisting about the 11,12 double bond in the first triplet state, T-1. The twisting is restricted to different extents depending on the size of the ring. In the case of Ret-8, no geometric isomerization occurs following photoexcitation. This contrasts cis-cyclooctene derivatives which can be photoisomerized to thermodynamically less favored trans-cyclooctene, The result is rationalized by noting that in Ret-8 the ring incorporates four consecutive sp(2) carbons whereas the latter has only two. For Ret-9 which has one extra carbon in the ring, however, ''cis-trans isomerization'' becomes possible resulting in shorter-lived excited states and a detectable transient on the nanosecond time scale which is assigned to a short-lived (tau approximate to 110 ns) transoid ground state species.