Planarizing cytosine: The S state structure, vibrations, and nonradiative dynamics of jet-cooled 5,6-trimethylenecytosine.

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.