Low-lying excited states and nonradiative processes of 9-methyl-2-aminopurine.

The UV spectrum of the adenine analogue 9-methyl-2-aminopurine (9M-2AP) is investigated with one- and two-color resonant two-photon ionization spectroscopy at 0.3 and 0.05 cm^-1 resolution in a supersonic jet. The electronic origin at 32 252 cm^-1 exhibits methyl torsional subbands that originate from the 0A₁″ (l = 0) and 1E″ (l = ±1) torsional levels. These and further torsional bands that appear up to 0₀⁰+ 230 cm^-1 allow to fit the threefold (V₃) barriers of the torsional potentials as ∣V₃′∣ = 50 cm^-1 in the S₀ and ∣V₃′∣ = 126 cm^-1 in the S₁ state. Using the B3LYP density functional and correlated approximate second-order coupled cluster CC2 methods, the methyl orientation is calculated to be symmetric relative to the 2AP plane in both states, with barriers of V₃″= 20 cm^-1 and V₃′ = 115 cm^-1. The 0₀⁰rotational band contour is 75% in-plane (a/b) polarized, characteristic for a dominantly long-axis ¹ππ* excitation. The residual 25% c-axis polarization may indicate coupling of the ¹ππ* to the close-lying ¹nπ* state, calculated at 4.00 and 4.01 eV with the CC2 method. However, the CC2 calculated ¹nπ oscillator strength is only 6% of that of the ¹ππ* transition. The ¹ππ* vibronic spectrum is very complex, showing about 40 bands within the lowest 500 cm^-1. The methyl torsion and the low-frequency out-of-plane ν₁′ and ν₂′ vibrations are strongly coupled in the ¹ππ* state. This gives rise to many torsion-vibration combination bands built on out-of-plane fundamentals, which are without precedence in the ¹ππ* spectrum of 9H-2-aminopurine [S. Lobsiger, R. K. Sinha, M. Trachsel, and S. Leutwyler, J. Chem. Phys. 134, 114307 (2011)]. From the Lorentzian broadening needed to fit the 000 contour of 9M-2AP, the ¹ππ* lifetime is τ ≥ 120 ps, reflecting a rapid nonradiative transition. [ABSTRACT FROM AUTHOR]