Highly effective quenching of the ultrafast radiationless decay of photoexcited pyrimidine bases by covalent modification: photophysics of 5,6-trimethylenecytosine and 5,6-trimethyleneuracil.Phys Chem Chem Phys. 2007 Jul 07; 9(25):3206-9.PC
5,6-Trimethylenecytosine (TMC) and 5,6-trimethyleneuracil (TMU), in which the twist of the C5-C6 bond (or the pyrimidalization of C5) is strongly hindered, do not exhibit the subpicosecond excited-state lifetime characteristic of the naturally occurring pyrimidine bases. This result demonstrates the important role the out-of-plane deformation of the six-membered ring plays in the ultrafast (subpicosecond) internal conversion of photoexcited nucleobases. The dramatically shorter fluorescence lifetime of TMU (approximately 30 ps) relative to TMC (approximately 1.2 ns), in aqueous solution at room temperature, is attributed to the presence in TMU of an efficient, secondary nonradiative decay channel of S(1)(pipi*) involving a low-lying (1)npi* state.