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-d ₄, and 9H-2AP-d₅ show numerous low-frequency bands that can be systematically assigned to overtone and combinations of the out-of-plane vibrations ₁′, ₂′, and ₃′. The intensity of these out-of-plane bands reflects an out-of-plane deformation in the ¹(L_a) state. Approximate second-order coupled-cluster theory also predicts that 2-aminopurine undergoes a butterfly deformation in its lowest ¹ 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 (ab) polarization, which is characteristic for a ¹ excitation. A 20-25 c-axis (perpendicular) transition dipole moment component may indicate coupling of the ¹ bright state to the close-lying ¹n dark state. However, no ¹n vibronic bands were detected below or up to 500 cm^-1 above the ¹ 000 band. Following ¹ excitation, 9H-2AP undergoes a rapid nonradiative transition to a lower-lying long-lived state with a lifetime 5s. The ionization potential of 9H-2AP was measured via the ¹ 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 ¹ state. Time-dependent B3LYP calculations predict the ³ (T₁) state 1.12 eV below the ¹ state, but place the ¹n (S₁) state close to the ¹ state, implying that the long-lived state is the lowest triplet (T₁) and not the ¹n state.