The solvent effect and identification of a weakly emissive state in nonradiative dynamics of guanine nucleosides and nucleotides - a combined femtosecond broadband time-resolved fluorescence and transient absorption study

A combined method of femtosecond broadband time-resolved fluorescence (fs-TRF) and transient absorption (fs-TA) was employed to investigate the excited state dynamics of 2'-deoxyguanosine (dG) and 2'-deoxyguanosine 5'-monophosphate (dGMP). Comparative fs-TRF and fs-TA measurements were conducted on dG and dGMP in neutral water, deuterated water, and methanol with excitation wavelengths of 245, 267 and 285 nm. Very similar results were observed with dG and dGMP. The data provide compelling evidence for the co-existence of two nonradiative pathways. One is the generally recognized L-a pi pi(star) mediated channel, the other involves an unprecedented weakly emissive state which plays a significant role in the overall deactivation processes. The L-a pi pi(star) channel features biphasic dynamics with time constants (tau(1)/tau(2)) of similar to 0.2/0.8 ps in water and similar to 0.25/1.0 ps in methanol. The biphasic decay arises due to a partial transfer with tau(1) of the L-a pi pi(star) population to the newly identified state followed by conversion in tau(2) of the remaining L-a pi pi(star) molecules into the electronic ground state. The channel mediated by the weakly emissive species shows solvent-dependent dynamics with time constants (tau(3)) of similar to 2.0 ps in water, similar to 2.3 ps in deuterated water, and similar to 4.1 ps in methanol. The species features absorption at UV wavelengths (similar to 300-400 nm) and exhibits deeply red-shifted fluorescence (lambda(max) similar to 520 nm) with polarization direction varied markedly from that of the L-a pi pi(star) but close to the L-b pi pi(star). This species acts as an effective quenching state to the radiative decay of the brightly emissive L-a pi pi(star) and L-b pi pi(star). It sets in promptly (