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 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 ¹ππ^∗ excitation predicted by the calculations. The methyl torsion and ν₁^' (butterfly) vibrations are strongly coupled, in the S₁ state. The S₀ → S₁ vibronic spectrum breaks off at a vibrational excess energy E_exc ∼ 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 E_exc = 366 cm^-1. The S₁⇝S₀ internal conversion rate constant increases from k_IC = 2 ⋅ 10⁹ s^-1 near the S₁(v = 0) level to 1 ⋅ 10¹¹ s^-1 at E_exc = 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 k_IC; it increases from k_ISC = 2 ⋅ 10⁸ s^-1 near S₁(v = 0) to k_ISC = 2 ⋅ 10⁹ s^-1 at E_exc = 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.