We have investigated the S0 → S1 UV vibronic spectrum and time-resolved S1 state 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−1 about 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 upon 1ππ∗ excitation predicted by the calculations. The methyl torsion and (butterfly) vibrations are strongly coupled, in the S1 state. The S0 → S1 vibronic spectrum breaks off at a vibrational excess energy Eexc ∼ 500 cm−1, indicating that a barrier in front of the ethylene-type S1⇝S0 conical intersection is exceeded, which is calculated to lie at Eexc = 366 cm−1. The S1⇝S0 internal conversion rate constant increases from kIC = 2 ⋅ 109 s−1 near the S1(v = 0) level to 1 ⋅ 1011 s−1 at Eexc = 516 cm−1. The 1ππ∗ 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 ⋅ 108 s−1 near S1(v = 0) to kISC = 2 ⋅ 109 s−1 at Eexc = 516 cm−1. The T1 state 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−1 above S1(v = 0), so S1⇝T2 ISC 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 → S1 spectrum. The effect of methylation on the radiationless decay to S0 and ISC to T1 is small, as shown by the similar break-off of the spectrum and the similar computed mechanisms.
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7 October 2016
Research Article|
October 07 2016
The excited-state structure, vibrations, lifetimes, and nonradiative dynamics of jet-cooled 1-methylcytosine
Maria A. Trachsel;
Maria A. Trachsel
1Department of Chemistry and Biochemistry,
University of Bern
, Freiestrasse 3, CH-3012 Bern, Switzerland
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Timo Wiedmer;
Timo Wiedmer
1Department of Chemistry and Biochemistry,
University of Bern
, Freiestrasse 3, CH-3012 Bern, Switzerland
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Susan Blaser;
Susan Blaser
1Department of Chemistry and Biochemistry,
University of Bern
, Freiestrasse 3, CH-3012 Bern, Switzerland
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Hans-Martin Frey;
Hans-Martin Frey
1Department of Chemistry and Biochemistry,
University of Bern
, Freiestrasse 3, CH-3012 Bern, Switzerland
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Quansong Li;
Quansong Li
2Key Laboratory of Cluster Science of Ministry of Education, School of Chemistry,
Beijing Institute of Technology
, 100081 Beijing, China
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Sergi Ruiz-Barragan;
Sergi Ruiz-Barragan
3Lehrstuhl für Theoretische Chemie,
Ruhr-Universitat Bochum
, 44780 Bochum, Germany
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Lluís Blancafort
;
Lluís Blancafort
a)
4Institut de Química Computacional i Catàlisi and Departament de Química,
Universitat de Girona
, Facultat de Ciències, C/ M. Aurèlia Campmany 69, 17003 Girona, Spain
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Samuel Leutwyler
Samuel Leutwyler
b)
1Department of Chemistry and Biochemistry,
University of Bern
, Freiestrasse 3, CH-3012 Bern, Switzerland
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a)
E-mail: lluis.blancafort@udg.edu
b)
E-mail: leutwyler@dcb.unibe.ch
J. Chem. Phys. 145, 134307 (2016)
Article history
Received:
July 15 2016
Accepted:
September 21 2016
Citation
Maria A. Trachsel, Timo Wiedmer, Susan Blaser, Hans-Martin Frey, Quansong Li, Sergi Ruiz-Barragan, Lluís Blancafort, Samuel Leutwyler; The excited-state structure, vibrations, lifetimes, and nonradiative dynamics of jet-cooled 1-methylcytosine. J. Chem. Phys. 7 October 2016; 145 (13): 134307. https://doi.org/10.1063/1.4964091
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