A Laser Flash Photolysis Study of 11-Cis-Locked Retinal Analogues

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Abstract

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, T1. 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 sp2 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 (τ ≈ 110 ns) transoid ground state species.

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