Orientation factor in Förster energy transfer with photoselection of donor and acceptor Original Research Article

The relative orientation of donor and acceptor in Förster energy transfer is no longer completely random, if both donor and acceptor are photoselected as in the energy transfer S1 + T1 → S0 + Tn between molecules in their lowest excited singlet state S1 and triplet state T1. The average 〈Ψ〉 of the orientation factor Ψ depends on the relative intramolecular orientation of the transition moment vectors active in photoselection and energy transfer. The deviation of 〈Ψ〉 from 23 is always small. Distribution functions w(Ψ) have been calculated analytically and/or numerically for several cases of uniformly and nonuniformly polarized electronic transitions. The most useful numerical procedure is based on Monte Carlo integrations. A pecularity of w(Ψ) for the energy transfer S1 + S1 → S0 + Sm is treated. The boundary condition responsible for the anti-Smoluchowski behavior of the delayed fluorescence due to triplet-triplet sensitively depends on w(Ψ).