Figure 2.

Transient triplet absorbance of thioxanthone after 351-nm laser flash photolysis of peptide A(n = 4) in ethanol. The laserflash was applied at t = 0. The decrease in thioxanthone triplet absorbance at 620 nm (○) can be described by a single-exponential process with a rate constant of 1.4 ± 0.2 × 10⁷ s⁻¹ (solid line). Because intermolecular triplet-energy transfer was shown to be diffusion-controlled, this rate corresponds to the rate of intrachain diffusion between donor and acceptor. The peptide concentration was 40 μM. Under these conditions, the half-life of thioxanthone triplets in peptides containing no acceptor was 30 μs (data not shown). The experiments were carried out in ethanol at 22°C. About 30% of the thioxanthone molecules remain in the triplet state after the equilibrium is reached, because of the small difference in triplet energy between donor and acceptor in polar solvents. These equilibrated triplet states decay on a much slower time scale and with the same rate as decay of the acceptor triplets occurs (1/k ≈30 μs; data not shown).