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

Type of publication Peer-reviewed
Publikationsform Original article (peer-reviewed)
Author Trachsel Maria A., Wiedmer Timo, Blaser Susan, Frey Hans-Martin, Li Quansong, Ruiz-Barragan Sergi, Blancafort Lluis, Leutwyler Samuel,
Project Spektroskopie, Photophysik und Photochemie von Molekülclustern in Überschallmolekularstrahlen
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Original article (peer-reviewed)

Journal J. Chem. Phys.
Volume (Issue) 145(13)
Page(s) 134307-1 - 134307-15
Title of proceedings J. Chem. Phys.
DOI 10.1063/1.4964091

Abstract

We have investigated the S-0 -> S-1 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 1 pi pi* excitation predicted by the calculations. The methyl torsion and nu(1)' (butterfly) vibrations are strongly coupled, in the S-1 state. The S-0 -> S-1 vibronic spectrum breaks off at a vibrational excess energy E-exc similar to 500 cm(-1), indicating that a barrier in front of the ethylene-type S-1 (sic) S-0 conical intersection is exceeded, which is calculated to lie at E-exc = 366 cm(-1). The S-1 (sic) S-0 internal conversion rate constant increases from k(IC) = 2 . 10(9) s(-1) near the S-1(v = 0) level to 1 . 10(11) s(-1) at E-exc = 516 cm(-1). The 1 pi pi* state of 1MCyt also relaxes into the lower-lying triplet T-1 (3 pi pi*) 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 k(ISC) = 2 . 10(8) s(-1) near S-1(v = 0) to k(ISC) = 2 . 10(9) s(-1) at E-exc = 516 cm(-1). The T-1 state energy is determined from the onset of the time-delayed photoionization efficiency curve as 25 600 +/- 500 cm(-1). The T-2 (3 eta pi*) state lies >1500 cm(-1) above S-1(v = 0), so S-1 (sic)T-2 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 S-0 -> S-1 spectrum. The effect of methylation on the radiationless decay to S-0 and ISC to T-1 is small, as shown by the similar break-off of the spectrum and the similar computed mechanisms.
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