and Z-6 these should stem from destabilizing steric congestion in the S0 state. Yet, the E(T1) is also low for E-6 where the low energy instead reveals hyperconjugative stabilization of both the alkyl and the allyl radical moieties of the T1 state. Differences in the rate constants of triplet quenching have earlier been observed among different dienes as quenchers, and Sandros and Bäckström concluded that the triplet energy transfer is a diffusion-controlled process. Changes in the steric hindrance of the photosensitizer have also been shown to not affect the energy transfer from carbonyl compounds to different substrates. We previously identified the rate-determining step to be the addition of a T1 state diene to an S0 state diene leading to the triplet bis(allyl) radical pair intermediate. The most reactive part of each triplet 1,3-diene is the alkyl radical moiety.