Elodie Lazzarotto

Fluorescence of the DNA double helices (dAdT)n·(dAdT)n studied by femtosecond spectroscopy

Polymeric and oligomeric DNA helices, poly(dAdT).poly(dAdT) and (dAdT)(10).(dAdT)(10), composed of 200-400 and 20 adenine-thymine base pairs, respectively, are studied by fluorescence upconversion. Fluorescence decays, anisotropy decays and time-resolved spectra, obtained for this alternating base sequence, are compared with those determined previously for the homopolymeric sequence (dA)(n).(dT)(n). It is shown that identical fluorescence decays may correspond to quite different anisotropy decays and vice versa, both varying with the emission wavelength, the base sequence and the duplex size. Our observations cannot be explained in terms of monomer and excimer emission exclusively, as concluded in the past on the basis of steady-state measurements. Excitons also contribute to the fluorescence. These are rapidly trapped by excimers, characterized by long-lived weak emission.

A combined femtosecond fluorescence and TD-DFT study of uracyl derivatives in aqueous solution

This chapter provides several interesting indications on the excited-state behavior of uracil derivatives in aqueous solution. The excited-state lifetimes of all the compounds examined, except 5-fluoro uracil, are dominated by a component shorter than 250 femtosecond. Calculations indicate that the relevant state for discussing the absorption and fluorescence spectra of uracil derivatives in aqueous solution is the ππ*. The key steps in the ultrafast internal conversion are the pyramidalization of C5 and the out of plane motion of C5 substituent, which explains why the excited-state lifetime depends mainly on the nature of the C5 substituent. The shape of the potential energy surface of the S 1 electronic states seems consistent with a very fast nonradiative decay. The result strongly supports the reliability of the general picture of the ultrafast ground state recovery of excited uracils in aqueous solution.