We measure the S → S spectrum and time-resolved S state nonradiative dynamics of the "clamped" cytosine derivative 5,6-trimethylenecytosine (TMCyt) in a supersonic jet, using two-color resonant two-photon ionization (R2PI), UV/UV holeburning, and ns time-resolved pump/delayed ionization. The experiments are complemented with spin-component scaled second-order approximate coupled cluster (SCS-CC2), time-dependent density functional theory, and multi-state second-order perturbation-theory (MS-CASPT2) ab initio calculations. While the R2PI spectrum of cytosine breaks off ∼500 cm above its 0 band, that of TMCyt extends up to +4400 cm higher, with over a hundred resolved vibronic bands. Thus, clamping the cytosine C-C bond allows us to explore the S state vibrations and S → S geometry changes in detail. The TMCyt S state out-of-plane vibrations ν, ν, and ν lie below 420 cm, and the in-plane ν, ν, and ν vibrational fundamentals appear at 450, 470, and 944 cm. S →  S vibronic simulations based on SCS-CC2 calculations agree well with experiment if the calculated ν, ν, and ν frequencies are reduced by a factor of 2-3. MS-CASPT2 calculations predict that the ethylene-type S ⇝ S conical intersection (CI) increases from +366 cm in cytosine to >6000 cm in TMCyt, explaining the long lifetime and extended S → S spectrum. The lowest-energy S ⇝ S CI of TMCyt is the "amino out-of-plane" (OP) intersection, calculated at +4190 cm. The experimental S ⇝ S internal conversion rate constant at the S(v=0) level is k=0.98-2.2⋅10 s, which is ∼10 times smaller than in 1-methylcytosine and cytosine. The S(v=0) level relaxes into the T(ππ*) state by intersystem crossing with k=0.41-1.6⋅10 s. The T state energy is measured to lie 24 580±560 cm above the S state. The S(v=0) lifetime is τ=2.9 ns, resulting in an estimated fluorescence quantum yield of Φ=24%. Intense two-color R2PI spectra of the TMCyt amino-enol tautomers appear above 36 000 cm. A sharp S ionization threshold is observed for amino-keto TMCyt, yielding an adiabatic ionization energy of 8.114±0.002 eV.

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 00 (0)+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 00 (0) rotational band contour is 75% in-plane (a/b) polarized, characteristic for a dominantly long-axis (1)ππ(*) excitation. The residual 25% c-axis polarization may indicate coupling of the (1)ππ(*) to the close-lying (1)nπ(*) state, calculated at 4.00 and 4.01 eV with the CC2 method. However, the CC2 calculated (1)nπ oscillator strength is only 6% of that of the (1)ππ(*) transition. The (1)ππ(*) 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 (1)ππ(*) state. This gives rise to many torsion-vibration combination bands built on out-of-plane fundamentals, which are without precedence in the (1)ππ(*) 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 00 (0) contour of 9M-2AP, the (1)ππ(*) lifetime is τ ⩾ 120 ps, reflecting a rapid nonradiative transition.