title: Singlet excited-state behavior of uracil and thymine in aqueous solution: a combined experimental and computational study of 11 uracil derivatives.
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 S(0)/S(1) 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 S(1) 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.
authors.affiliation: Laboratoire Francis Perrin, CEA/DSM/DRECAM/SPAM - CNRS URA 2453, CEA Saclay, Gif-sur-Yvette, France. thomas.gustavsson@cea.fr
authors.forename: Thomas
authors.initials: T
authors.lastname: Gustavsson
authors.forename: Akos
authors.initials: A
authors.lastname: Bányász
authors.forename: Elodie
authors.initials: E
authors.lastname: Lazzarotto
authors.forename: Dimitra
authors.initials: D
authors.lastname: Markovitsi
authors.forename: Giovanni
authors.initials: G
authors.lastname: Scalmani
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Frisch
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Barone
authors.forename: Roberto
authors.initials: R
authors.lastname: Improta
mesh_terms: D008958:Models, Molecular; D012996:Solutions; D013050:Spectrometry, Fluorescence; D013816:Thermodynamics; D013941:Thymine / Q000737:chemistry*; D014498:Uracil / Q000031:analogs & derivatives* / Q000737:chemistry*; D014867:Water / Q000737:chemistry
chemical_list: D012996:Solutions; D014867:Water; D014498:Uracil; D013941:Thymine