On the Participation of Photoinduced N-H Bond Fission in Aqueous Adenine at 266 and 220 nm: A Combined Ultrafast Transient Electronic and Vibrational Absorption Spectroscopy Study

A combination of ultrafast transient electronic absorption spectroscopy (TEAS) and transient vibrational absorption spectroscopy (TVAS) is used to investigate whether photoinduced NH bond fission, mediated by a dissociative (1)sigma* state, is active in aqueous adenine (Ade) at 266 and 220 nm. In order to isolate UV/visible and IR spectral signatures of the adeninyl radical (Ade[-H]), formed as a result of NH bond fission, TEAS and TVAS are performed on Ade in D2O under basic conditions (pD = 12.5), which forms Ade[-H]- anions via deprotonation at the N7 or N9 sites of Ades 7H and 9H tautomers. At 220 nm we observe one-photon detachment of an electron from Ade[-H]-, which generates solvated electrons (e(aq)) together with Ade[-H] radicals, with clear signatures in both TEAS and TVAS. Additional wavelength dependent TEAS measurements between 240260 nm identify a threshold of 4.9 +/- 0.1 eV (similar to 250 nm) for this photodetachment process in D2O. Analogous TEAS experiments on aqueous Ade at pD = 7.4 generate a similar photoproduct signal together with eaq after excitation at 266 and 220 nm. These e(aq) are born from ionization of Ade, together with Ade+ cations, which are indistinguishable from Ade[-H] radicals in TEAS. Ade+ and Ade[-H] are found to have different signatures in TVAS and we verify that the pD = 7.4 photoproduct signal observed in TEAS following 220 nm excitation is solely due to Ade+ cations. Based on these observations, we conclude that: (i) NH bond fission in aqueous Ade is inactive at wavelengths =220 nm; and (ii) if such a channel exists in aqueous solution, its threshold is strongly blue-shifted relative to the onset of the same process in gas phase 9H-Ade (=233 nm). In addition, we extract excited state lifetimes and vibrational cooling dynamics for 9H-Ade and Ade[-H]-. In both cases, excited state lifetimes of <500 fs are identified, while vibrational cooling occurs within a time frame of 45 ps. In contrast, 7H-Ade is confirmed to have a longer excited state lifetime of similar to 510 ps through both TEAS and TVAS.