Femtosecond fluorescence up-conversion spectroscopy of adenine and adenosine: experimental evidence for the πσ* state?

Femtosecond time-resolved fluorescence up-conversion spectroscopy has been used in a study of the excited electronic state lifetimes of the purine base adenine (Ade) and its ribonucleoside adenosine (Ado) in aqueous solution. The molecules were excited at wavelengths in the range 245 ⩽ λexc ⩽ 280 nm using tunable UV pulses from a frequency doubled non-collinear optical parametric amplifier (NOPA). The observed biexponential temporal fluorescence profiles of Ade could be fitted using a fast decay time between τ1 = (0.34 ± 0.07) ps at the shorter excitation wavelengths and τ1 = (0.67 ± 0.14) ps at the longer excitation wavelengths and, independent of the excitation wavelength, a slow decay time of τ2 = (8.4 ± 0.8) ps. The two values were assigned to the “canonical” 9H-Ade tautomer (τ1) and the less stable 7H-Ade tautomer (τ2) which are known to be present in aqueous solution. The excited state lifetime of 9H-Ade in H2O is thus sub-picosecond even around the electronic origin of the first excited ππ* state, in contrast to a report for the 0-0 level in the gas phase (≳9 ps). The fluorescence decay profiles of Ado, in which the 9H atom is substituted by the ribose, could be described assuming monoexponential behavior with a lifetime τ = (0.31 ± 0.05) ps. The results are consistent with fast radiationless electronic relaxation from the excited ππ* to the S0 ground state. The apparent step in the lifetime of 9H-Ade centered at λexc ≈ 265 nm might be interpreted as evidence for the opening of an additional radiationless electronic relaxation pathway, which could arise from the πσ* state predicted at about that energy.