Ultrafast spin crossover in 4-thiothymidine in an ionic liquid

The excited-state dynamics of 4-thiothymidine in the ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) imide, is revealed. Branching of the initial excited-state population results in efficient intersystem crossing to the triplet manifold with a rate constant of 1.6 x 10(12) s(-1) and close to unity triplet yield after UVA excitation at 340 or 360 nm.