The excited state lifetimes of uracil, thymine and 5-fluorouracil have been measured using femtosecond UV fluorescence upconversion in various protic and aprotic polar solvents. The fastest decays are observed in acetonitrile and the slowest in aqueous solution while those observed in alcohols are intermediate. No direct correlation with macroscopic solvent parameters such as polarity or viscosity is found, but hydrogen bonding is one key factor affecting the fluorescence decay. It is proposed that the solvent modulates the relative energy of two close-lying electronically excited states, the bright ππ* and the dark nπ* states. This relative energy gap controls the non-radiative relaxation of the ππ* state through a conical intersection close to the Franck-Condon region competing with the ultrafast internal conversion to the ground state. In addition, an inverse isotope effect is observed in D2O where the decays are faster than in H2O. © 2008 Elsevier B.V. All rights reserved.

*** IBB - CNR ***

Laboratoire Francis Perrin, CEA/DSM/DRECAM/SPAM - CNRS URA 2453, CEA/Saclay, F-91191 Gif-sur-Yvette, France

Department of Chemistry, Indian Institute of Technology, Kharagpur, 721 302 WB, India

Dipartimento di Chimica, Universita Federico II, Complesso Universitario Monte S. Angelo, Via Cintia, I-80126 Napoli, Italy

Istituto Biostrutture e Bioimmagini, CNR, V. Mezzocannone 6, 80134 Napoli, Italy

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