Room-Temperature Phosphorescence of the DNA Monomer Analogue 4-Thiothymidine in Aqueous Solutions after UVA Excitation

Room-temperature phosphorescence of the 4-thiothymidine DNA analogue after UVA excitation has been revealed in aqueous buffer solutions at pH 7.4. Femtosecond broad-band transient absorption measurements in combination with quantum chemical calculations that include explicit and implicit solvent effects are used to rationalize this remarkable phenomenon. It is shown that the initial excited singlet-state population bifurcates on the sub-240 fs time scale to populate the triplet state in high yield. A negligibly small fraction of the excited singlet-state population decays by photon emission.