Triplet Excited State in Canonical Nucleobases and Their Derivatives

This chapter introduces the triplet excited states in canonical nucleobases and their derivatives DNA and RNA. The discovery of triplet excited states in nucleobases using low-temperature flash photolysis is briefly reviewed, followed by the advances in the field using femtosecond transient absorption spectroscopy in the past two decades. Next, the triplet excited state dynamics in thio- and selenium-substituted nucleobases are summarized. Due to the heavy-atom effect, they generally exhibit much higher triplet excited quantum yields compared to the unmodified nucleobase monomers. Ultrafast spectroscopy reveals that intersystem crossing (ISC) in those bases could take place in several hundreds of femtosecond to several picosecond timescale after photo-excitation. Finally, triplet excited state dynamics in nitrogen-substituted nucleobases are also reviewed. Although their triplet quantum yields are lower compared with those of the thio- and selenium-substituted nucleobases, the effect cannot be neglected as they are commonly used as chemotherapy medicines.