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

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.