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 pi pi* 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 n pi* electronic state, which is connected to the excited and ground states via conical intersections.