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

Abstract

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-d(4) and -d(5) isotopomers, using two-color resonant two-photon ionization spectroscopy at 0.3 and 0.045 cm(-1) resolution. The S(1) ← S(0) 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 S(1) ← S(0) spectra of 9H-2AP, 9H-2AP-d(4), and 9H-2AP-d(5) show numerous low-frequency bands that can be systematically assigned to overtone and combinations of the out-of-plane vibrations ν(1)', ν(2)', and ν(3)'. The intensity of these out-of-plane bands reflects an out-of-plane deformation in the (1)ππ∗(L(a)) state. Approximate second-order coupled-cluster theory also predicts that 2-aminopurine undergoes a "butterfly" deformation in its lowest (1)ππ∗ state. The rotational contours of the 9H-2AP, 9H-2AP-d(4), and 9H-2AP-d(5) 0(0)(0) bands and of eight vibronic bands of 9H-2AP up to 0(0)(0) + 600 cm(-1) exhibit 75%-80% in-plane (a∕b) polarization, which is characteristic for a (1)ππ∗ excitation. A 20%-25% c-axis (perpendicular) transition dipole moment component may indicate coupling of the (1)ππ∗ bright state to the close-lying (1)nπ∗ dark state. However, no (1)nπ∗ vibronic bands were detected below or up to 500 cm(-1) above the (1)ππ∗ 0(0)(0) band. Following (1)ππ∗ excitation, 9H-2AP undergoes a rapid nonradiative transition to a lower-lying long-lived state with a lifetime ≥5 μs. The ionization potential of 9H-2AP was measured via the (1)ππ∗ 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)ππ∗ state. Time-dependent B3LYP calculations predict the (3)ππ∗ (T(1)) state 1.12 eV below the (1)ππ∗ state, but place the (1)nπ∗ (S(1)) state close to the (1)ππ∗ state, implying that the long-lived state is the lowest triplet (T(1)) and not the (1)nπ∗ state.