The publication deals with polymeric pA●pT and oligomeric A20●T20 DNA duplexes whose fluorescence is studied by time-correlated single photon counting. It is shown that their emission on the nanosecond timescale is largely dominated by high-energy components peaking at a wavelength shorter than 305 nm. Because of their anisotropy (0.02) and their sensitivity to base stacking, modulated by the duplex size and the ionic strength of the solution, these components are attributed to mixed ππ*/charge transfer excitons. As high-energy long-lived excited states may be responsible for photochemical reactions, their identification via theoretical studies is an important challenge.
Keywords: DNA; excitons; hypochromism; intrinsic fluorescence; time-resolved fluorescence spectroscopy.