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

We have investigated the S 0 → S 1 UV vibronic spectrum and time-resolved S 1 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 1 ππ ∗ excitation predicted by the calculations. The methyl torsion and ν 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 Eexc ∼ 500 cm−1, indicating that a barrier in front of the ethylene-type S 1⇝S 0 conical intersection is exceeded, which is calculated to lie at Eexc = 366 cm−1. The S 1⇝S 0 internal conversion rate constant increases from kIC = 2 ⋅ 109 s−1 near the S 1(v = 0) level to 1 ⋅ 1011 s−1 at Eexc = 516 cm−1. The 1 ππ ∗ state of 1MCyt also relaxes into the lower-lying triplet T 1 (3 ππ ∗) 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 S 1(v = 0) to kISC = 2 ⋅ 109 s−1 at Eexc = 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 nπ ∗) state lies >1500 cm−1 above S 1(v = 0), so S 1⇝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.