DEPENDENCE OF THE TRIPLET YIELD ON EXCITATION ENERGY IN ALL‐TRANS AND 11‐CIS RETINAL*

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Abstract
The triplet‐triplet absorption of all‐trans and 11‐cis retinal was measured as a function of the exciting radiation from 423 nm to 365 nm in a glass of 3‐methylpentane at 77 K. This experiment was also accomplished with all‐trans retinal in hexane at ambient temperature. The relative triplet formation quantum yields of all‐trans and 11‐cis retinal at 77 K were found to be independent (±10%) of the frequency of the exciting radiation. At room temperature we measured an increase in this relative quantum yield for all‐trans retinal from 1.0 at 365 nm to 1.82 at 423 nm [Bensasson et al. (1975) measured an absolute quantum yield of 0.45 at 353 nm]. These results are used to evaluate previous interpretations for photophysical decay processes in all‐trans retinal, and previous suggestions for wavelength dependent radiationless transitions are shown to be unacceptable. High energy excitation of 300 K solutions of all‐trans retinal produce excited states that result in less efficient intersystem crossing. These states appear to be inaccessible in the 77 K matrices. We suggest that steric restrictions introduced by the retinal matrix interaction at 77 K are able to block this new internal conversion pathway back to the ground state.