| Autor: |
Trachsel, Maria A., Wiedmer, Timo, Blaser, Susan, Frey, Hans-Martin, Quansong Li, Ruiz-Barragan, Sergi, Blancafort, Lluís, Leutwyler, Samuel |
| Předmět: |
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| Zdroj: |
Journal of Chemical Physics; 2016, Vol. 145 Issue 13, p1-15, 15p, 3 Diagrams, 6 Charts, 11 Graphs |
| Abstrakt: |
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(ν = 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(ν = 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(ν = 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. [ABSTRACT FROM AUTHOR] |
| Databáze: |
Complementary Index |
| Externí odkaz: |
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