Low-lying excited states and nonradiative processes of the adenine analogues 7H- and 9H-2-aminopurine.

We have investigated the UV vibronic spectra and excited-state nonradiative processes of the 7H- and 9H-tautomers of jet-cooled 2-aminopurine (2AP) and of the 9H-2AP-d4 and -d5 isotopomers, using two-color resonant two-photon ionization spectroscopy at 0.3 and 0.045 cm-1 resolution. The S1 ← S0 transition of 7H-2AP was observed for the first time. It lies ∼ 1600 cm-1 below that of 9H-2AP, is ∼1000 times weaker and exhibits only in-plane vibronic excitations. In contrast, the S1 ← S0 spectra of 9H-2AP, 9H-2AP-d4, and 9H-2AP-d5 show numerous low-frequency bands that can be systematically assigned to overtone and combinations of the out-of-plane vibrations ν1′, ν2′, and ν3′. The intensity of these out-of-plane bands reflects an out-of-plane deformation in the 1ππ*(La) state. Approximate second-order coupled-cluster theory also predicts that 2-aminopurine undergoes a 'butterfly' deformation in its lowest 1ππ* state. The rotational contours of the 9H-2AP, 9H-2AP-d4, and 9H-2AP-d5 0₀⁰ bands and of eight vibronic bands of 9H-2AP up to 0₀⁰+600 cm-1 exhibit 75%-80% in-plane (a/b) polarization, which is characteristic for a 1ππ* excitation. A 20%-25% c-axis (perpendicular) transition dipole moment component may indicate coupling of the 1ππ* bright state to the close-lying 1nπ* dark state. However, no 1nπ* vibronic bands were detected below or up to 500 cm-1 above the 1ππ* 0₀⁰ band. Following 1ππ* excitation, 9H-2AP undergoes a rapid nonradiative transition to a lower-lying long-lived state with a lifetime >=5μs. The ionization potential of 9H-2AP was measured via the 1ππ* state (IP = 8.020 eV) and the long-lived state (IP > 9.10 eV). The difference shows that the long-lived state lies >=1.08 eV below the 1ππ* state. Time-dependent B3LYP calculations predict the 3ππ* (T1) state 1.12 eV below the 1ππ* state, but place the 1nπ* (S1) state close to the 1ππ* state, implying that the long-lived state is the lowest triplet (T1) and not the 1nπ* state. [ABSTRACT FROM AUTHOR]