Solvent Effect on the Singlet Excited-state Dynamics of 5-Fluorouracil in Acetonitrile as Compared with Water

Abstract

Abstract. The excited state dynamics of 5-fluorouracil in acetonitrile has been investigated by femtosecond fluorescence upconversion spectroscopy in combination with quantum chemistry TD-DFT calculations ((PCM/TD-PBE0). Experimentally it was found that when going from water to acetonitrile solution the fluorescence decay of 5FU becomes much faster. The calculations show that this is related to the opening of an additional decay channel in acetonitrile solution since the dark n/π* excited state becomes near degenerate with the bright π/π* state, forming a conical intersection close to the FranckCondon region. In both solvents a S 1 -S 0 conical intersection, governed by the out-of-plane motion of the fluorine atom is active, allowing an ultrafast internal conversion to the ground state.

Fluorescence of the DNA Double Helix (dA)20·(dT)20 Studied by Femtosecond SpectroscopyEffect of the Duplex Size on the Properties of the Excited States

“…The maximum of the (dA) 20 . (dT) 20 spectrum is located at the same position as that of TMP (330 ± 1 nm) but at somewhat longer wavelength than that of poly(dA).poly(dT), which peaks at 327 6 18/02/2010 ± 1 nm. The similarity between the peak position of the fluorescence spectrum of poly(dA).poly(dT), dissolved in sodium cacodylate, with that of TMP, had lead in past to the conclusion that emission of the polymer originates only from excited states localized on thymines.…”

Influence of base stacking on excited-state behavior of polyadenine in water, based on time-dependent density functional calculations

“…Even if a number of important aspects have not yet been convincingly assessed, there is general consensus regarding the mechanisms that explain the photostability of isolated nucleobases. Indeed, time-resolved spectroscopic studies (1) and quantum mechanical calculations (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11) agree in suggesting the existence of an ultrafast nonradiative pathway between the bright excited state and the ground electronic state for both pyrimidine and purine nucleobases, which can explain their extremely short (Յ1-ps) excited-state lifetimes. However, the excited-state behavior of DNA and nucleobase multimers (the systems of actual interest in vivo) appears much more complex, showing multiexponential decays with very different time constants (12)(13)(14)(15)(16)(17)(18).…”

Excited states and energy transfer among DNA bases in double helices

“…The nπ* state was experimentally shown to be the lowest in energy for adenine in the gas phase. 43,44 When TMP and dAMP are dissolved in bulk water, the n-π* transitions shift to higher energies 45,46 and ultrafast emission corresponding to the allowed π-π* transitions is observed. [47][48][49] It would not be surprising that the specific environment created by the double helix, and in particular the presence of multiple electric charges on the phosphate moieties, affects the relative positions of the nπ* and ππ* states.…”