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 |$0A_{1}^{{\prime \prime }}$|$0A1′′$ (l = 0) and 1E^″ (l = ±1) torsional levels. These and further torsional bands that appear up to |$0_0^0+230$|$000+230$ cm⁻¹ allow to fit the threefold (V₃) barriers of the torsional potentials as |$|V^{\prime \prime }_3|\break = 50$|$|V3′′|=50$ cm⁻¹ in the S₀ and |$|V^{\prime }_3 |=126$|$|V3′|=126$ 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 |$V^{\prime \prime }_3=20$|$V3′′=20$ cm⁻¹ and |$V^{\prime }_3\break =115$|$V3′=115$ cm⁻¹. The |$0_0^0$|$000$ rotational 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 |$\nu ^{\prime }_1$|$ν1′$ and |$\nu ^{\prime }_2$|$ν2′$ 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 |$0_0^0$|$000$ contour of 9M-2AP, the ¹ππ^* lifetime is τ ⩾ 120 ps, reflecting a rapid nonradiative transition.