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 <= lambda(exc) <= 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 tau(1) = (0.34 +/- 0.07) ps at the shorter excitation wavelengths and tau(1) = (0.67 +/- 0.14) ps at the longer excitation wavelengths and, independent of the excitation wavelength, a slow decay time of tau(2) = (8.4 +/- 0.8) ps. The two values were assigned to the canonical 9H-Ade tautomer (tau(1)) and the less stable 7H-Ade tautomer (tau(2)) 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 pi pi* state, in contrast to a report for the 0-0 level in the gas phase (greater than or similar to 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 tau = (0.31 +/- 0.05) ps. The results are consistent with fast radiationless electronic relaxation from the excited pi pi* to the S-0 ground state. The apparent step in the lifetime of 9H-Ade centered at = 265 nm might be interpreted as evidence for the opening of an additional radiationless electronic relaxation pathway, which could arise from the pi sigma* state predicted at about that energy. (c) 2005 Elsevier B.V. All rights reserved.