Modulating the Rate of Charge Recombination in Bichromophore End-Capped Hairpin DNA Sequences

Understanding hole transport across DNA is essential for improving its performance in devices by tuning charge separation and recombination via suitable donor-acceptor systems. We investigate the modulation of charge recombination in naphthalimide-perylenimide dyad end-capped hairpin DNA, where the dyad is separated from GC base pairs by AT base pairs. Transient absorption measurements reveal single-step tunneling for 1-3 base-pair separations and multistep hopping for ≥4 base pairs. The perylenimide radical anion lifetime upon naphthalimide excitation, compared to direct perylenimide excitation, increases up to ∼37-fold for separations up to 3 base pairs. In short DNA bridges, charge recombination is further modulated via a tandem electron-transfer pathway involving a secondary acceptor. Femtosecond fluorescence upconversion and TCSPC measurements, supported by molecular dynamics and electronic coupling calculations, reveal four distinct lifetime components arising from conformational heterogeneity and corresponding to unstacked, end-stacked, partially intercalated, and intercalated structures. This work demonstrates distance- and excitation-dependent control of charge recombination in DNA-based donor-acceptor assemblies.