HOW EFFICIENT IS DIFFUSION-CONTROLLED TRIPLET ENERGY TRANSFER

The efficiency with which 2,5-dimethyl-2,4-hexadiene (Et ˂ 58 kcal) quenches the triplet-state photo-elimination of valerophenone (ET ˃ 72 kcal) has been measured in different solvents. In three tertiary alcohols, kqT = 160/n cP M-1. In primary alcohols more viscous than 1-pentanol, kqr = 125/n cP M-1. In the less viscous primary alcohols and in alkanes and cycloalkanes, kqτ rises as n decreases, but kqτ is not a linear function of 1/n. These results suggest that, in solvents where n ˃ 3 cP, the rate of exothermic triplet-energy transfer from ketones to conjugated dienes is truly limited by and is very nearly equal to the average rate of diffusion in the solution. In less viscous solvents rates of energy transfer are slower than rates of diffusion, indicating that there is enough inefficiency in the energy-transfer process that diffusion apart of triplet ketone and diene molecules can compete with energy transfer during the lifetime of a solution encounter. When the probability that energy transfer will occur during the lifetime of a solution encounter is close to unity, Stern-Volmer quenching plots should curve sharply upward at high quencher concentrations due to nearest neighbor “static” quenching. Quenching of y-methylvalerophenone by 2,4-hexadien-1-ol in t-butyl alcohol displays just such behavior, whereas quenching of γ-phenylbutyrophenone by 1,3-pentadiene in pentane does not. These observations provide evidence that triplet energy transfer is not totally diffusion controlled in solvents of low viscosity. © 1968, American Chemical Society. All rights reserved.