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 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⋅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.
Highlights
Computational studies of the excited-state dynamics and nonradiative decay of amino-keto Cyt10–20 have located three different conical intersections (CIs) between the ground and lowest excited 1ππ∗ states
000 of band is identified the S0 → S1 spectrum is dominated by fundamentals, overtone excitations, and combination bands of four out-of-plane vibrations
Based on the energetic sequence of the spin-component scaled CC2 (SCS-CC2) calculated vibrational frequencies and on their predicted Franck-Condon factors, we assign these as ν1, ν3, ν5, and ν7
Summary
Computational studies of the excited-state dynamics and nonradiative decay of amino-keto Cyt have located three different conical intersections (CIs) between the ground and lowest excited 1ππ∗ states. The lowest of these, which dominates the S1 S0 nonradiative decay, is called (Eth)X since the intersection structure is similar to the CI structure of ethylene. This CI is characterized by a puckering of the C6 atom and a twist around the C5–C6 bond, with a H–C5–C6–H torsional angle of ∼120◦.10–13,15,16,19,21,22. The higher CI involves an N3 out-of-plane bending and a large out-of-plane amino deformation and is called (OP)X .10,16,19,22. The third CI, called (nO, π∗)X , has a semi-planar structure with sp hybridization of the C6 atom, shortening of the C2–N3 bond, and stretching of the C2-O bond relative to the ground state minimum.. This CI is characterized by a puckering of the C6 atom and a twist around the C5–C6 bond, with a H–C5–C6–H torsional angle of ∼120◦.10–13,15,16,19,21,22 The higher CI involves an N3 out-of-plane bending and a large out-of-plane amino deformation and is called (OP)X .10,16,19,22 The third CI, called (nO, π∗)X , has a semi-planar structure with sp hybridization of the C6 atom, shortening of the C2–N3 bond, and stretching of the C2-O bond relative to the ground state minimum.
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