Efficient intersystem crossing in 2-aminopurine riboside probed by femtosecond time-resolved transient vibrational absorption spectroscopy

The photophysical dynamics of 2-aminopurine riboside (2APr) in CHCl3 have been studied following excitation at (pump) = 310 nm by means of femtosecond transient vibrational absorption spectroscopy (TVAS) aided by quantum chemical density functional theory (DFT) and ab initio calculations. The experiments identified numerous vibrational marker bands in the regions of the NH2 stretch and the 2AP ring vibrations which could be assigned to the bleach of the S-0 electronic ground state (GS) and to transient populations in the (1)* and (3)* excited electronic states. The temporal evolution of the transient vibrational bands shows that the decay of the (1)* population is accompanied by a partial recovery of the GS and a concurrent population of the (3)* state with a time constant of (2) = 740 +/- 15 ps. The ensuing electronic relaxation is concluded to proceed via the (1)n* state as intermediate state. The absence of observable transient vibrational bands of this state hints at an upper limit for its lifetime of < 100 ps. The triplet quantum yield is found to be phi(T) = 0.42 +/- 0.07.