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

Abstract

The photophysical dynamics of 2-aminopurine riboside (2APr) in CHCl₃ have been studied following excitation at λ_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 S₀ electronic ground state (GS) and to transient populations in the ¹ππ* and ³ππ* excited electronic states. The temporal evolution of the transient vibrational bands shows that the decay of the ¹ππ* population is accompanied by a partial recovery of the GS and a concurrent population of the ³ππ* state with a time constant of τ₂ = 740 ± 15 ps. The ensuing electronic relaxation is concluded to proceed via the ¹nπ* state as intermediate state. The absence of observable transient vibrational bands of this state hints at an upper limit for its lifetime of τ < 100 ps. The triplet quantum yield is found to be ϕ_T = 0.42 ± 0.07.