Cytosine excited state dynamics studied by femtosecond fluorescence upconversion and transient absorption spectroscopy

We report a femtosecond spectroscopic study of the DNA base cytosine in aqueous solution at room temperature. Two different experimental techniques were used, fluorescence upconversion and transient absorption, providing complementary information on the excited state relaxation. While the fluorescence decay is clearly bi-exponential, with an ultrafast (0.2 ps) and a slower (1.3 ps) component, the decay of the transient absorption signal is mono-exponential with a 1.1 ps characteristic time. In addition, the fluorescence anisotropy is also found to decay in a bi-exponential manner. The results are discussed in terms of possible non-radiative relaxation processes that may intervene in the deactivation of the excited state.