New Insights into the Photophysics of DNA Nucleobases

We report the results of an extended time-resolved study of DNA nucleobases in aqueous solutions conducted in the deep UV using broad-band femtosecond transient absorption and electronic two-dimensional spectroscopies. We found that the photodeactivation in all DNA nucleobases occurs in two steps: fast relaxation (500–700 fs) from the excited state ππ* to a “dark” state and its depopulation to the ground state within 1–2 ps. Our experimental observations and performed theoretical modeling allow us to conclude that this dark state can be associated with the nπ* electronic state, which is connected to the excited and ground states via conical intersections.