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

Abstract

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 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(v = 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(v = 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(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.

Locating Cytosine Conical Intersections by Laser Experiments andAb InitioCalculations

“…Nanosecond laser two-color resonant two-photon ionization (2C-R2PI) spectra of the amino-keto tautomers of (a) 6-methylCyt, (b) 1-ethylCyt, , (c) Cyt (with atom numbering), − (d) 1-methylCyt, , (e) 5-methylCyt, (f) 5-fluoroCyt, (g) 5,6-trimethyleneCyt, and (g) 1-methyl-5,6-trimethyleneCyt. Ionization wavelength: 226 nm for panels c and e, 223 nm for panel d, 220 nm for panels a and b, and 245 nm for panels f–h.…”

“…The spectroscopic and lifetime measurements of cold gas-phase cytosines are advantageous because they can be conducted tautomer-specifically, ,,− and the molecules are excited in a narrow energy window of ∼10 cm –1 (1.2 meV). Our much better energy resolution allows direct comparison to high-level excited-state calculations of the decay paths, and the nanosecond (ns) laser spectra with highly resolved vibronic bands yield detailed information on the structure of the optically excited S 1 ( ππ *) state, the vibrations that are excited upon S 0 → S 1 excitation, and the direction of the transition dipole moments. − …”

Ultrafast excited‐state dynamics of gas‐phase 5‐methylcytosine tautomers

“…At λ pump $ 290 nm, the photon energy is close to the origin of enol-5mCy with a minimal excitation probability due to poor Franck-Condon (FC) factors. [17,18] With λ pump < 290 nm, the 1 ππ* state of both enol and imino forms should become energetically accessible. Table 3 also lists the oscillator strengths calculated at the PBE0/ def2-TVZP level.…”

“…The fastest decay component (τ 1 ) is present in transients measured at all λ pump 's and exhibits a robust time constant of about 0.25 ps. Because it has been shown that the excited-state deactivation rates of Cy derivatives are very sensitive to the excess energy above the origin, [9,12,[15][16][17]26] this rather independence of the τ 1 component on λ pump over a large energy range ($0.7 eV) suggests that it is not likely to be due to an excited-state population decay of 5mCy tautomers. Indeed, its time scale suggests that it is more likely to be due to an initial intramolecular vibrational-energy redistribution (IVR) occurring in excited 5mCy tautomers.…”

“…[3][4][5][6] Among all nucleobases, cytosine (Cy) and its derivatives are probably the most studied system experimentally in the gas phase. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] Ultrafast excited-state dynamics of Cy in the gas phase has been explored by several groups. [22][23][24][25][26] Our group has previously used femtosecond pump-probe photoionization spectroscopy to investigate the ultrafast excited-state dynamics of Cy and its microsolvated complexes.…”