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

Abstract

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 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 ν′1, ν′3, and ν′5 lie below 420 cm−1, and the in-plane ν′11, ν′12, and ν′23 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 ν′1, ν′3, and ν′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” (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⋅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 kISC=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 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