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

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 cm¹ above its 0⁰₀ band, that of TMCyt extends up to +4400 cm^-1 higher, with over a hundred resolved vibronic bands. Thus, clamping the cytosine C⁵-C⁶ bond allows us to explore the S₁ state vibrations and S₀→S₁ geometry changes in detail. The TMCyt S₁ state out-of-plane vibrations v'₁, v'₃, and v'₅ lie below 420 cm^-1, and the in-plane v'₁₁, v'₁₂, and v'₂₃ vibrational fundamentals apper at 450, 470, and 944 cm^-1. S₀ → S₁ vibronic simulations based on SCS-CC2 calculations agree well with experiment if the calculated v'₁, v'₃ , and v'₅ 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" (OP_X) intersection, calculated at +4190 cm^-1. The experimental S₁ S₀ internal conversion rate constant at the S₁(v' = 0) level is k_IC = 0.98-2.2 10⁸ s^-1, which is ~10 times smaller than in 1-methylcytosine and cytosine. The S₁(v' = 0) level relaxes into the T₁(³ππ*) state by intersystem crossing with k_ISC = 0.41-1.6 10⁸ 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 = 2.9 ns, resulting in an estimated fluorescence quantum yield of ϕ_fl = 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. [ABSTRACT FROM AUTHOR]