Item


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 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 timeresolved 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 ν′1 (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

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)

© Journal of Chemical Physics, 2016, vol. 145, núm. 13, p. 134307

American Institute of Physics (AIP)

Author: Trachsel, Maria A.
Wiedmer, Timo
Blaser, Susan
Frey, Hans-Martin
Li, Quansong
Ruiz-Barragán, Sergi
Blancafort San José, Lluís
Leutwyler, Samuel
Date: 2016 September 1
Abstract: 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 timeresolved 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 ν′1 (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
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)
Format: application/pdf
ISSN: 0021-9606 (versió paper)
1089-7690 (versió electrònica)
Document access: http://hdl.handle.net/10256/13493
Language: eng
Publisher: American Institute of Physics (AIP)
Collection: MINECO/PE 2015-2017/CTQ2015-69363-P
Reproducció digital del document publicat a: http://dx.doi.org/http://dx.doi.org/10.1063/1.4964091
Articles publicats (D-Q)
Is part of: © Journal of Chemical Physics, 2016, vol. 145, núm. 13, p. 134307
Rights: Tots els drets reservats
Subject: Ionització
Ionization
Radiació ultraviolada
Ultraviolet radiation
Title: The excited-state structure, vibrations, lifetimes, and nonradiative dynamics of jet-cooled 1-methylcytosine
Type: info:eu-repo/semantics/article
Repository: DUGiDocs

Subjects

Authors