Singlet excited-state behavior of uracil and thymine in aqueous solution: a combined experimental and computational study of 11 uracil derivatives

J Am Chem Soc. 2006 Jan 18;128(2):607-19. doi: 10.1021/ja056181s.

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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Models, Molecular
  • Solutions
  • Spectrometry, Fluorescence
  • Thermodynamics
  • Thymine / chemistry*
  • Uracil / analogs & derivatives*
  • Uracil / chemistry*
  • Water / chemistry

Substances

  • Solutions
  • Water
  • Uracil
  • Thymine