Liaw, B.; Orchard, S.W.; Kutal, C.
Photoisomerization of cis- and trans-piperylene can be sensitized with high quantum efficiency by Cu(diphos)BH/sub 4/ (diphos is 1,2-bis(diphenylphosphino)ethane) and Cu(prophos)BH/sub 4/ (prophos is 1,3-bis(diphenylphosphino)propane). Sensitization is accompanied by quenching of the emissive /sup 3/(/sigma/-a/sub /pi//) excited state in each copper(I) complex, and the two processes occur with identical Stern-Volmer kinetics. Measurements of the trans/cis diene ratio at the photostationary state can be used to estimate the triplet-state energy as 60-61 kcal for Cu(diphos)BH/sub 4/ and > 61 kcal for Cu(prophos)BH/sub 4/; additional evidence suggests that 66-67 kcal is a reasonable value for the latter complex. Collectively, the results support the assignment of triplet-triplet energy transfer as the primary mechanism for sensitization and quenching in these systems. This mechanism also can accommodate earlier reports that Cu(diphos)BH/sub 4/ and Cu(prophos)BH/sub 4/ sensitize the valence isomerization of norbornadiene to quadricyclene with markedly different quantum efficiencies. 17 refs., 6 figs., 2 tabs.
Extensive data from Stern-Volmer, Lamola-Hammond, and Ilenda-Daughenbaugh-Cristol quenching kinetics have now been accumulated on photosolvolysis in t-butyl alcohol for benzyl chloride and a number of meta and para substituted benzyl chlorides. Evidence for the existence of two triplet states, one relatively short-lived (tau 0-2 nsec) which gives solvolysis product and a second, relatively long-lived (tau 5-26 nsec), which does not give product, but instead is energy wasting, has been accumulated. The system, p-acetobenzyl chloride, has been investigated in detail. A method for quenching of singlet states for measurement of singlet lifetimes in the 100 picosecond to nanosecond range is being developed. Preliminary work on benzyl acetate photosolvolysis has been conducted. Some work on the goemetrical requirements for intra-molecular excitation transfer in bichromophoric molecules has been conducted. Several dienes related to norbornadiene have been prepared and preparative photoisomerizations to quadricyclene analogues have been carried out. Considerable attention has been given to certain di-..pi..-methane rearrangements, work on most of which is still in progress. One system, the ethyl ester of dibenzobarrelene-7-carboxylic acid, has been scrutinized in detail.