Introduce of Triplet Excited State in DNA and RNA

This chapter introduces the significance of triplet excited states in DNA and RNA. While ultraviolet (UV) radiation poses a significant threat to genomic stability, the unique structure and ultrafast deactivation pathways (picosecond timescale) of nucleobases typically dissipate absorbed energy from singlet excited states, minimizing photodamage. However, triplet states, formed via intersystem crossing (ISC) from singlets, play crucial but complex roles. Characterized by parallel electron spins (S = 1), long lifetimes (microseconds to milliseconds), and “dark” nature due to spin-forbidden transitions, triplet states were first identified in nucleic acids via low-temperature phosphorescence spectroscopy in the 1960s. Studying them at room temperature is challenging due to low extinction coefficients and quantum yields. Triplet sensitization techniques and advances in ultrafast spectroscopy and quantum chemistry since the 1980s have been pivotal in elucidating triplet dynamics and their involvement in DNA photodamage (e.g. cyclobutane pyrimidine dimer formation) and photosensitized therapies. This chapter reviews historical milestones and current research frontiers (including ultrafast ISC mechanisms, theoretical simulations, and lesion formation pathways) and outlines the book’s structure, aiming to integrate experimental and theoretical findings into a comprehensive model of triplet state behavior in nucleic acids.