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The excited-state structure, vibrations, lifetimes, and nonradiative dynamics of jet-cooled 1-methylcytosine

We have investigated the S0 → S1 UV vibronic spectrum and time-resolved S1 state dynamics ofjet-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 timeresolvedpump/delayed ionization measurements. The experimental study is complemented withspin-component-scaled second-order coupled-cluster and multistate complete active space secondorder 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 torsionaltransitions coupled to out-of-plane ring vibrations, in agreement with the methyl group rotationand out-of-plane distortions upon 1ππ∗ excitation predicted by the calculations. The methyl torsionand ν′1 (butterfly) vibrations are strongly coupled, in the S1 state. The S0 → S1 vibronic spectrumbreaks off at a vibrational excess energy Eexc ∼ 500 cm−1, indicating that a barrier in front of theethylene-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-lyingtriplet T1 (3ππ∗) state by intersystem crossing (ISC); the calculated spin-orbit coupling (SOC) valueis 2.4 cm−1. The ISC rate constant is 10–100 times lower than kIC; it increases from kISC = 2 · 108 s−1near S1(v = 0) to kISC = 2 · 109 s−1 at Eexc = 516 cm−1. The T1 state energy is determined from theonset 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 SOCparameter of 10.6 cm−1. An upper limit to the adiabatic ionization energy of 1MCyt is determinedas 8.41 ± 0.02 eV. Compared to cytosine, methyl substitution at N1 lowers the adiabatic ionizationenergy 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 bythe similar break-off of the spectrum and the similar computed mechanisms

This research has been supported by the Schweiz. Nationalfonds (Grant Nos. 121993 and 132540), the Agència de Gestió d’Ajuts Universitaris i de Recerca (AGAUR) from Catalonia (Spain) (Grant No. 2014SGR1202), the Ministerio de Economía y Competividad (MINECO) from Spain (Grant No. CTQ2015-69363-P), and the National Natural Science Foundation of China (Grant No. 21303007)

American Institute of Physics (AIP)

Autor: Trachsel, Maria A.
Wiedmer, Timo
Blaser, Susan
Frey, Hans-Martin
Li, Quansong
Ruiz-Barragán, Sergi
Blancafort San José, Lluís
Leutwyler, Samuel
Resum: We have investigated the S0 → S1 UV vibronic spectrum and time-resolved S1 state dynamics ofjet-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 timeresolvedpump/delayed ionization measurements. The experimental study is complemented withspin-component-scaled second-order coupled-cluster and multistate complete active space secondorder 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 torsionaltransitions coupled to out-of-plane ring vibrations, in agreement with the methyl group rotationand out-of-plane distortions upon 1ππ∗ excitation predicted by the calculations. The methyl torsionand ν′1 (butterfly) vibrations are strongly coupled, in the S1 state. The S0 → S1 vibronic spectrumbreaks off at a vibrational excess energy Eexc ∼ 500 cm−1, indicating that a barrier in front of theethylene-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-lyingtriplet T1 (3ππ∗) state by intersystem crossing (ISC); the calculated spin-orbit coupling (SOC) valueis 2.4 cm−1. The ISC rate constant is 10–100 times lower than kIC; it increases from kISC = 2 · 108 s−1near S1(v = 0) to kISC = 2 · 109 s−1 at Eexc = 516 cm−1. The T1 state energy is determined from theonset 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 SOCparameter of 10.6 cm−1. An upper limit to the adiabatic ionization energy of 1MCyt is determinedas 8.41 ± 0.02 eV. Compared to cytosine, methyl substitution at N1 lowers the adiabatic ionizationenergy 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 bythe similar break-off of the spectrum and the similar computed mechanisms
This research has been supported by the Schweiz. Nationalfonds (Grant Nos. 121993 and 132540), the Agència de Gestió d’Ajuts Universitaris i de Recerca (AGAUR) from Catalonia (Spain) (Grant No. 2014SGR1202), the Ministerio de Economía y Competividad (MINECO) from Spain (Grant No. CTQ2015-69363-P), and the National Natural Science Foundation of China (Grant No. 21303007)
Accés al document: http://hdl.handle.net/2072/272665
Llenguatge: eng
Editor: American Institute of Physics (AIP)
Drets: Tots els drets reservats
Matèria: Ionització
Ionization
Radiació ultraviolada
Ultraviolet radiation
Títol: The excited-state structure, vibrations, lifetimes, and nonradiative dynamics of jet-cooled 1-methylcytosine
Tipus: info:eu-repo/semantics/article
Repositori: Recercat

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