LASER PHOTOLYSIS OF RETINAL AND ITS PROTONATED AND UNPROTONATED n‐BUTYLAMINE SCHIFF BASE

Abstract—The pulsed ruby laser (347 nm) flash photolysis technique has been used to measure the triplet‐triplet absorption spectra and triplet lifetimes of trans ‐retinal, N‐frans‐retinylidene‐n ‐butylamine (NRBA), and protonated NRBA (NRBAH+) at room temperature. In methylcyclohexane solution, the triplet lifetimes are in the range 10–20 μs and decrease in the order NRBAH+ > NRBA > trans ‐retinal. Intersy stem‐crossing efficiencies (φISC) were determined by a comparison technique using anthracene and 1,2‐benzanthracene as reference compounds. For trans ‐retinal, φISC is 0–50 pM 0–05 in methylcyclohexane and 0–08 in methanol, which confirms that earlier values of 0–11 and 0–017 in these solvents are in error. For NRBA and NRBAH+ in methylcyclohexane, ΦISC values are 0008 and < 0–001, respectively. Evidence is presented for a significant solvent effect in the isomerization of retinal via the triplet state, and that cisφtrans isomerization occurs from the triplet state of NRBAH+. The relation between the intersystem‐crossing properties of model compounds and the photochemistry of rhodopsin is discussed.

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