A femtosecond time-resolved investigation of dual fluorescence from N6,N6-dimethyladenine

Abstract

The excited electronic state dynamics of N⁶,N⁶-dimethyladenine (DMAde), a molecule known to emit dual fluorescence, has been studied in aqueous solution using femtosecond fluorescence up-conversion spectroscopy. Time profiles of the fluorescence of DMAde excited at λ = 258 nm were measured at a series of wavelengths in the range 320 nm ≤ λ_fl ≤ 650 nm. At wavelengths in the near UV (λ_fl ≤ 420 nm), the time profiles were dominated by a very short-lived decay component with a lifetime between 0.2 ps ≤ τ₁ ≤ 0.6 ps, depending on the detection wavelength. Two other components with lifetimes of τ₂ ≈ 3.5 ps and τ₃ ≈ 60 ps gave minor contributions. In the emission observed at longer wavelengths (λ_fl ≥ 500 nm), which appeared slightly delayed compared to the UV fluorescence, the long-lived fluorescence component (τ₃) dominated, the second component (τ₂) disappeared. The results are consistent with the assumption that DMAde is primarily excited to a short-lived local excited (LE) electronic state that fluoresces mostly in the UV and decays rapidly, on a ∼0.5 ps timescale, to an intramolecular charge transfer (ICT) state that emits only at longer wavelengths in the visible spectrum. The fluorescence–time profiles and transient fluorescence spectra reconstructed from the time profiles provided further information on secondary relaxation processes within and between the excited states and their non-radiative relaxation to the electronic ground state.