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(La) 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.
All Science Journal Classification (ASJC) codes
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry