title: The excited-state structure, vibrations, lifetimes, and nonradiative dynamics of jet-cooled 1-methylcytosine.
abstract: We have investigated the S0→ S1UV vibronic spectrum and time-resolved S1state dynamics of jet-cooled amino-keto 1-methylcytosine (1MCyt) using two-color resonant two-photon ionization, UV/UV holeburning and depletion spectroscopies, as well as nanosecond and picosecond time-resolved pump/delayed ionization measurements. The experimental study is complemented with spin-component-scaled second-order coupled-cluster and multistate complete active space second order perturbation ab initio calculations. Above the weak electronic origin of 1MCyt at 31 852 cm-1about 20 intense vibronic bands are observed. These are interpreted as methyl group torsional transitions coupled to out-of-plane ring vibrations, in agreement with the methyl group rotation and out-of-plane distortions upon1ππ∗excitation predicted by the calculations. The methyl torsion and ν1'(butterfly) vibrations are strongly coupled, in the S1state. The S0→ S1vibronic spectrum breaks off at a vibrational excess energy Eexc∼ 500 cm-1, indicating that a barrier in front of the ethylene-type S1⇝S0conical intersection is exceeded, which is calculated to lie at Eexc= 366 cm-1. The S1⇝S0internal conversion rate constant increases from kIC= 2 ⋅ 109s-1near the S1(v = 0) level to 1 ⋅ 1011s-1at Eexc= 516 cm-1. The1ππ∗state of 1MCyt also relaxes into the lower-lying triplet T1(3ππ∗) state by intersystem crossing (ISC); the calculated spin-orbit coupling (SOC) value is 2.4 cm-1. The ISC rate constant is 10-100 times lower than kIC; it increases from kISC= 2 ⋅ 108s-1near S1(v = 0) to kISC= 2 ⋅ 109s-1at Eexc= 516 cm-1. The T1state energy is determined from the onset of the time-delayed photoionization efficiency curve as 25 600 ± 500 cm-1. The T2(3nπ∗) state lies >1500 cm-1above S1(v = 0), so S1⇝T2ISC cannot occur, despite the large SOC parameter of 10.6 cm-1. An upper limit to the adiabatic ionization energy of 1MCyt is determined as 8.41 ± 0.02 eV. Compared to cytosine, methyl substitution at N1 lowers the adiabatic ionization energy by ≥0.32 eV and leads to a much higher density of vibronic bands in the S0→ S1spectrum. The effect of methylation on the radiationless decay to S0and ISC to T1is small, as shown by the similar break-off of the spectrum and the similar computed mechanisms.
authors: Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland.
authors: Maria A
authors: MA
authors: Trachsel
authors: Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland.
authors: Timo
authors: T
authors: Wiedmer
authors: Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland.
authors: Susan
authors: S
authors: Blaser
authors: Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland.
authors: Hans-Martin
authors: HM
authors: Frey
authors: Key Laboratory of Cluster Science of Ministry of Education, School of Chemistry, Beijing Institute of Technology, 100081 Beijing, China.
authors: Quansong
authors: Q
authors: Li
authors: Lehrstuhl für Theoretische Chemie, Ruhr-Universitat Bochum, 44780 Bochum, Germany.
authors: Sergi
authors: S
authors: Ruiz-Barragan
authors: Institut de Química Computacional i Catàlisi and Departament de Química, Universitat de Girona, Facultat de Ciències, C/ M. Aurèlia Campmany 69, 17003 Girona, Spain.
authors: Lluís
authors: L
authors: Blancafort
authors: Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland.
authors: Samuel
authors: S
authors: Leutwyler