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We measure the S0 → S1 spectrum and time-resolved S1 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(-1) above its 00(0) band, that of TMCyt extends up to +4400 cm(-1) higher, with over a hundred resolved vibronic bands. Thus, clamping the cytosine C(5)-C(6) bond allows us to explore the S1 state vibrations and S0 → S1 geometry changes in detail. The TMCyt S1 state out-of-plane vibrations ν1('), ν3('), and ν5(') lie below 420 cm(-1), and the in-plane ν11('), ν12('), and ν23(') vibrational fundamentals appear at 450, 470, and 944 cm(-1). S0  →  S1 vibronic simulations based on SCS-CC2 calculations agree well with experiment if the calculated ν1('), ν3('), and ν5(') frequencies are reduced by a factor of 2-3. MS-CASPT2 calculations predict that the ethylene-type S1 ⇝ S0 conical intersection (CI) increases from +366 cm(-1) in cytosine to >6000 cm(-1) in TMCyt, explaining the long lifetime and extended S0 → S1 spectrum. The lowest-energy S1 ⇝ S0 CI of TMCyt is the "amino out-of-plane" (OPX) intersection, calculated at +4190 cm(-1). The experimental S1 ⇝ S0 internal conversion rate constant at the S1(v(')=0) level is kIC=0.98-2.2⋅10(8) s(-1), which is ∼10 times smaller than in 1-methylcytosine and cytosine. The S1(v(')=0) level relaxes into the T1((3)ππ*) state by intersystem crossing with kISC=0.41-1.6⋅10(8) s(-1). The T1 state energy is measured to lie 24 580±560 cm(-1) above the S0 state. The S1(v(')=0) lifetime is τ=2.9 ns, resulting in an estimated fluorescence quantum yield of Φfl=24%. Intense two-color R2PI spectra of the TMCyt amino-enol tautomers appear above 36 000 cm(-1). A sharp S1 ionization threshold is observed for amino-keto TMCyt, yielding an adiabatic ionization energy of 8.114±0.002 eV.

Planarizing cytosine: The S1 state structure, vibrations, and nonradiative dynamics of jet-cooled 5,6-trimethylenecytosine