Epigenetic modifications impart important functionality
to nucleic
acids during gene expression but may increase the risk of photoinduced
gene mutations. Thus, it is crucial to understand how these modifications
affect the photostability of duplex DNA. In this work, the ultrafast
formation (<20 ps) of a delocalized triplet charge transfer (CT)
state spreading over two stacked neighboring nucleobases after direct
UV excitation is demonstrated in a DNA duplex, d(G5fC)9•d(G5fC)9, made of alternating
guanine (G) and 5-formylcytosine (5fC) nucleobases. The
triplet yield is estimated to be 8 ± 3%, and the lifetime of
the triplet CT state is 256 ± 22 ns, indicating that epigenetic
modifications dramatically alter the excited state dynamics of duplex
DNA and may enhance triplet state-induced photochemistry.