Solvent effects on the two lowest-lying singlet excited states of 5-fluorouracil
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Solvent effects on the two lowest-lying singlet excited states of 5-fluorouracil Carlos Bistafa • Sylvio Canuto
Received: 23 July 2012 / Accepted: 6 November 2012 / Published online: 30 November 2012 Ó Springer-Verlag Berlin Heidelberg 2012
Abstract Different solvation models based on the sequential-QM/MM methodology are used to investigate the two lowest n–p* and p–p* transitions of 5-fluorouracil in water and acetonitrile. Electronic polarization of the solute, use of discrete and explicit solvent models and different QM models ranging from semiempirical, timedependent DFT, size-extensive CI and equation of motion are considered. The results show that DFT-based methods provide good results for the energy transitions, but fail to describe the relative energy shifts. Very good and equivalent shifts are obtained using CIS(D) and EOM-CCSD methods combined with the polarizable continuum model solute polarization and discrete solvent description. Our best results give the p–p* transition lower than the n–p* by 0.1 eV in water and, in acetonitrile, place these two states essentially as degenerate in the Franck–Condon region of the ground state. The relative position of these two excited states is of crucial importance for understanding the photophysics of 5-fluorouracil in solvent environment. Keywords Solvent effects 5-Fluorouracil Absorption spectrum Solvatochromic shifts
1 Introduction Recent data of the World Health Organization show that cancer is now among the three main causes of death in the world [1]. One important possible treatment uses 5-fluorouracil (5FU, Fig. 1) because of its great efficiency in two C. Bistafa S. Canuto (&) Instituto de Fı´sica, Universidade de Sa˜o Paulo, CP 66318, Sa˜o Paulo, SP 05314-970, Brazil e-mail: [email protected]
cytotoxic effects: the RNA falsification and the inhibition of the TYMS enzyme [2]. This has increased the interest for more experimental and theoretical investigations on 5FU. Recent works have experimentally studied the absorption, emission and NMR spectra. Theoretical works have considered the isolated molecule or included the solvent effects using the continuum model [3–6]. In the aftermath of the work of Barone, Improta and coworkers [7], there is considerable interest in the photophysics of 5FU, and one central aspect is the location of the low-lying singlet excited states. The two lowest-lying singlet excited states of 5FU are of n–p* and p–p* character, respectively. In the gas phase (isolated molecule) the n–p* should lie lower than the p–p* state. As it is normally expected, the solvent effect will blue shift the lowest n–p* and red shift the p–p*. Depending on the amount of the shifts, there may be a crossing and thus an inversion of the two states in solvent environment. Indeed, recent theoretical and experimental results suggest that there is such an inversion of the n–p* and p–p* transitions of the 5FU in solvent environment, compared to the gas phase [7]. Santoro et al. [7] used a cluster model composed of four water molecules
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