Effect of substituents on the excited-state dynamics of the modified DNA bases 2,4-diaminopyrimidine and 2,6-diaminopurine

To explore the excited state dynamics of pyrimidine derivatives, we performed a combined experimental and theoretical study. We present resonant two-photon ionization (R2PI) and IR-UV double resonance spectra of 2,4-diaminopyrimidine and 2,6-diaminopurine seeded in a supersonic jet by laser desorption. For 2,4-diaminopyrimidine (S₀→ S₁ 34 459 cm^-1), we observed only the diamino tautomer with an excited state lifetime bracketed between experimental limits of 10 ps and 1 ns. For 2,6-diaminopurine, we observed two tautomers, the 9H- (S₀→ S₁ 34 881 cm^-1) and 7H- (S₀→ S₁ 32 215 cm^-1) diamino forms, with excited state lifetimes of 6.3 ± 0.4 ns and 8.7 ± 0.8 ns, respectively. We investigated the nature of the excited state of 2,4-diaminopyrimidine by means of multi-reference ab initio methods. The calculations of stationary points in the ground and excited states, minima on the S₀/S₁ crossing seam and connecting reaction paths show that several paths with negligible barriers exist, allowing ultrafast radiationless deactivation if excited at energies slightly higher than the band origin. The sub-nanosecond lifetime found experimentally is in good agreement with this finding.