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 E _exc ∼ 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 E _exc = 366 cm ^-1 . The S ₁ ⇝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 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 k _IC ; 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 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.