S_0 and S_1 state structure, methyl torsional barrier heights, and fast intersystem crossing dynamics of 5-methyl-2-hydroxypyrimidine

摘要

We report the analysis of the S_1 ← S_0 rotational band contours of jet-cooled 5-methyl-2-hydroxypyrimidine (5M2HP), the enol form of deoxythymine. Unlike thymine, which exhibits a structureless spectrum, the vibronic spectrum of 5M2HP is well structured, allowing us to determine the rotational constants and the methyl group torsional barriers in the S _0 and S_1 states. The 0, 6a_0~1, 6b_0~1, and 14_0~1 band contours were measured at 900 MHz (0.03 cm~(-1)) resolution using mass-specific two-color resonant two-photon ionization (2C-R2PI) spectroscopy. All four bands are polarized perpendicular to the pyrimidine plane (>90% c type), identifying the S_1 ← S_0 excitation of 5M2HP as a ~1nπ* transition. All contours exhibit two methyl rotor subbands that arise from the lowest 5-methyl torsional states 0″ and 1E″. The S_0 and S_1 state torsional barriers were extracted from fits to the torsional subbands. The 3-fold barriers are V _3″ ≥ 13 cm~(-1) and V_3′ ≥ 51 cm~(-1); the 6-fold barrier contributions V_6″ and V_6′ are in the range of 2 - 3 cm~(-1) and are positive in both states. The changes of A, B, and C rotational constants upon S _1 ← S_0 excitation were extracted from the contours and reflect an "anti-quinoidal" distortion. The 0_0~0 contour can only be simulated if a 3 GHz Lorentzian line shape is included, which implies that the S_1(~1nπ*) lifetime is ～55 ps. For the 6a_0~1 and 6b _0~1 bands, the Lorentzian component increases to 5.5 GHz, reflecting a lifetime decrease to ～30 ps. The short lifetimes are consistent with the absence of fluorescence from the ~1nπ* state. Combining these measurements with the previous observation of efficient intersystem crossing (ISC) from the S_1 state to a long-lived T _1 (~3nπ*) state that lies ～2200 cm ~(-1) below [S. Lobsiger, S. et al. Phys. Chem. Chem. Phys.2010, 12, 5032] implies that the broadening arises from fast intersystem crossing with k_(ISC) 2×10~(10) s~(-1). In comparison to 5-methylpyrimidine, the ISC rate is enhanced by at least 10 000 by the additional hydroxy group in position 2.