Singlet Excited-State Behavior of Uracil and Thymine in Aqueous Solution: A Combined Experimental and Computational Study of 11 Uracil Derivatives Thomas Gustavsson,* ,² A Ä kos Ba ´ nya ´ sz, ²,‡ Elodie Lazzarotto, ² Dimitra Markovitsi, ² Giovanni Scalmani, § Michael J. Frisch, § Vincenzo Barone, | and Roberto Improta* ,|,∇ Contribution from the Laboratoire Francis Perrin, CEA/DSM/DRECAM/SPAM - CNRS URA 2453, CEA Saclay, F-91191 Gif-sur-YVette, France, Gaussian, Inc., 340 Quinnipiac St. Bldg 40,Wallingford, Connecticut 06492, Dipartimento di Chimica, UniVersita Federico II, Complesso UniVersitario Monte S. Angelo, Via Cintia, I-80126 Napoli, Italy, and Istituto Biostrutture e Bioimmagini/CNR, V. Mezzocannone 6-80134 Napoli, Italy Received September 8, 2005; E-mail: thomas.gustavsson@cea.fr Abstract: The excited-state properties of uracil, thymine, and nine other derivatives of uracil have been studied by steady-state and time-resolved spectroscopy. The excited-state lifetimes were measured using femtosecond fluorescence upconversion in the UV. The absorption and emission spectra of five representative compounds have been computed at the TD-DFT level, using the PBE0 exchange-correlation functional for ground- and excited-state geometry optimization and the Polarizable Continuum Model (PCM) to simulate the aqueous solution. The calculated spectra are in good agreement with the experimental ones. Experiments show that the excited-state lifetimes of all the compounds examined are dominated by an ultrafast (<100 fs) component. Only 5-substituted compounds show more complex behavior than uracil, exhibiting longer excited-state lifetimes and biexponential fluorescence decays. The S0/S1 conical intersection, located at CASSCF (8/8) level, is indeed characterized by pyramidalization and out of plane motion of the substituents on the C5 atom. A thorough analysis of the excited-state Potential Energy Surfaces, performed at the PCM/TD-DFT(PBE0) level in aqueous solution, shows that the energy barrier separating the local S1 minimum from the conical intersection increases going from uracil through thymine to 5-fluorouracil, in agreement with the ordering of the experimental excited-state lifetime. 1. Introduction Nucleic acids are known to undergo ultrafast internal conver- sion after photoexcitation in the UV-visible region. 1 This mechanism is supposed to have an enormous biological relevance by providing a natural limiting effect to photochemical damage due to UV absorption. However, despite numerous studies, the primary photoinduced processes in nucleic acids remain poorly understood. As a first step toward a full understanding of the photoexcited processes occurring in the double helix, a large number of experimental and theoretical studies have thus been devoted to characterize the photophysical behavior of their building blocks (nucleobases, nucleosides, and nucleotides). All the bases absorb strongly in the UV region, and thus, a significant amount of energy is deposited in the absorbing excited state. However, such as in the case of nucleic acids, the nucleobases appear to be remarkably stable toward photodegradation, suggesting that any possible photochemical processes are efficiently prevented by a fast nonradiative decay to the ground state. Indeed, femtosecond time-resolved fluorescence and transient absorption studies agree in assigning subpicosecond lifetimes to the bright excited state of nucleobases in room-temperature aqueous solution, implying very efficient internal conversion processes. 2 In particular, fluorescence upconversion experiments 3-7 on the monomeric DNA constituents have revealed that the fluorescence decays are extremely fast (<1 ps) and cannot be described by single exponentials, hinting at complex nonradia- tive deactivation processes occurring in the excited state(s). In parallel to recent experimental work on the nucleobases, theoretical efforts have indeed begun to constitute an important source of valuable information, complementing the experimental studies in the characterization of the nature and the properties of the lowest electronically excited states and on the mechanism ² Laboratoire Francis Perrin. ‡ Present address: Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, P.O. Box 49, Budapest, Hungary H 1525. § Gaussian, Inc. | Universita Federico II. ∇ Istituto Biostrutture e Bioimmagini. (1) Bensasson, R. V.; Land, E. J.; Truscott, T. G. Excited states and free radicals in Biology and Medicine; Oxford University Press: Oxford, 1993. (2) Crespo-Hernandez, C. E.; Cohen, B.; Hare, P. M.; Kohler, B. Chem. ReV. 2004, 104, 1977-2020. (3) Peon, J.; Zewail, A. H. Chem. Phys. Lett. 2001, 348, 255-262. (4) Gustavsson, T.; Sharonov, A.; Markovitsi, D. Chem. Phys. Lett. 2002, 351, 195-200. (5) Gustavsson, T.; Sharonov, A.; Onidas, D.; Markovitsi, D. Chem. Phys. Lett. 2002, 356, 49-54. (6) Onidas, D.; Markovitsi, D.; Marguet, S.; Sharonov, A.; Gustavsson, T. J. Phys. Chem. B 2002, 106, 11367-11374. (7) Sharonov, A.; Gustavsson, T.; Carre ´, V.; Renault, E.; Markovitsi, D. Chem. Phys. Lett. 2003, 380, 173-180. Published on Web 12/17/2005 10.1021/ja056181s CCC: $33.50 © 2006 American Chemical Society J. AM. CHEM. SOC. 2006, 128, 607-619 9 607