Role of the triplet state in retinal photoisomerization as studied by laser-induced optoacoustic spectroscopy

Laser-induced optoacausiic spectroscopy (LIOAS) was applied to the study of 13-cis (13RT), 11-cis (11RT), and all-trans (trRT)-retinal photoisomerization in various n-alkanes. From the analysis of the LIOAS data in aerated and N-2-saturated solutions of the three isomers, it is concluded that their triplet states undergo different relaxation pathways, Whereas trRT and 11RT produce long-lived triplet states that are quenched by oxygen, yielding O-2((1) Delta(g)) with high efficiency, the 13RT triplet rapidly releases its excess energy by isomerization to the trRT isomer. Direct measurement of the near-IR emission from O-2((1) Delta(g)) supports these conclusions. The quantum yield for O-2((1) Delta(g)) production is 0.61 +/- 0.05 for trRT, 0.27 +/- 0.05 for 11RT (in each case similar to the respective yield of triplet formation, as detected by LIOAS), and <0.15 for the 13RT isomer (i.e., much smaller than its triplet yield). With this combination of techniques and the use of Literature data, it was possible to elucidate the role of the triplet state in the photoisomerization reaction, which turned out to be different for each retinal isomer: the trRT triplet is nonreactive and loses all its excess energy by intersystem crossing to the ground state, the 11RT triplet isomerizes to the trRT triplet, and the 13RT triplet isomerizes to the trRT ground state, Furthermore, the LIOAS data for 13RT appear to be solvent dependent in the range from 12-pentane to n-hexadecane, which might be interpreted in terms of a structural volume change for the cis --> trans (13RT --> trRT) photoisomerization, Depending on the interpretation of the data, a contraction with a value between 33 and 130 mL/mol was calculated, However, the validity of this analysis was difficult to establish, and alternative explanations of the solvent dependence such as a variation of the isomerization quantum yield could not be ruled out, due to the intrinsic, relatively large error of the quantum yield determination.