Planarizing cytosine: The S-1 state structure, vibrations, and nonradiative dynamics of jet-cooled 5,6-trimethylenecytosine

摘要

We measure the S-0 -> S-1 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 similar to 500 cm(-1) above its 0(0)(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 S-1 state vibrations and S-0 -> S-1 geometry changes in detail. The TMCyt S-1 state out-of-plane vibrations nu(1)', nu(3)', and nu(5)' lie below 420 cm(-1), and the in-plane nu(11)', nu(12)', and nu(23)' vibrational fundamentals appear at 450, 470, and 944 cm(-1). S-0 -> S-1 vibronic simulations based on SCS-CC2 calculations agree well with experiment if the calculated nu(1)', nu(3)', and nu(5)' frequencies are reduced by a factor of 2-3. MS-CASPT2 calculations predict that the ethylene-type S-1 (sic) S-0 conical intersection (CI) increases from + 366 cm(-1) in cytosine to > 6000 cm(-1) in TMCyt, explaining the long lifetime and extended S-0 -> S-1 spectrum. The lowest-energy S-1 (sic) S-0 CI of TMCyt is the "amino out-of-plane" (OPX) intersection, calculated at + 4190 cm(-1). The experimental S-1 (sic) S-0 internal conversion rate constant at the S-1(v' = 0) level is k(IC) = 0.98-2.2.10(8) s(-1), which is similar to 10 times smaller than in 1-methylcytosine and cytosine. The S-1(v' = 0) level relaxes into the T-1((3)pi pi*) state by intersystem crossing with k(ISC) = 0.41-1.6.10(8) s(-1). The T-1 state energy is measured to lie 24 580 +/- 560 cm(-1) above the S0 state. The S-1(v' = 0) lifetim