On the structure and excited electronic state lifetimes of cytidine self-assemblies with extended hydrogen-bonding networks

The nucleoside cytidine (C) assembles in extended hydrogen-bonded aggregates in neat apolar solvents like n-hexane, when carrying inert hydrophobic groups at the 2′,3′,5′-oxygen atoms of the ribose moiety. The ensuing structures, which constitute model systems for DNA super-structures such as triplexes or quadruplexes, were elucidated by FTIR spectroscopy and further characterized by UV spectroscopy. The lifetimes of the optically excited electronic states of the aggregates were investigated using femtosecond UV fluorescence up-conversion spectroscopy. Time profiles were measured after excitation at a number of pump wavelengths between 296 nm ≥ λpump ≥ 262 nm. The bi-exponential decay curves were characterized by time constants (with 2σ error limits) of τ1,C = 0.58(1) ps (88–95% fractional amplitudes) and τ2,C = 19.4(13) ps (12–5%) independent of the pump wavelength. The results indicate that the C multimers compared to the monomer do not gain photostability by coupled excited-state electron–proton transfer in the H-bonded networks, in contrast with recent findings for G·G and the G·C Watson–Crick dimer.