S0 and S1 State Structure, Methyl Torsional Barrier Heights, and Fast Intersystem Crossing Dynamics of 5-Methyl-2-hydroxypyrimidine

摘要: We report the analysis of SI So rotational band contours jet-cooled 5-methyl-2-hydroxypyrimidine (5M2HP), enol form deoxythymine. Unlike thymine, which exhibits a structureless spectrum, vibronic spectrum 5M2HP is well structured, allowing us to determine constants and methyl group torsional barriers in S-0 S-1 states. The 0(0)(0), 6a(0)(1), 6b(0)(1), 14(0)(1) 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 pyrimidine plane (>90% c type), identifying <- excitation as 1n pi* transition. exhibit two rotor subbands that arise from lowest 5-methyl states 0A '' 1E ''. state extracted fits subbands. 3-fold V-3 = 13 cm(-1) V3' cm(-1); 6-fold barrier contributions V-6 V-6' range 2-3 positive both changes A, B, C upon reflect an "anti-quinoidal" distortion. 0(0)(0) contour can only be simulated if 3 GHz Lorentzian line shape included, implies S-1(1n pi*) lifetime similar 55 ps. For 6a(0)(1) 6b(0)(1) bands, component increases 5.5 GHz, reflecting decrease 30 short lifetimes consistent with absence fluorescence state. Combining these measurements previous observation efficient intersystem crossing (ISC) Si long-lived T-1((3)n lies 2200 below [S. Lobsiger, S. et al. Phys. Chem. 2010, 12, 5032] broadening arises fast k(ISC) approximate 2 x 10(10) s(-1). In comparison 5-methylpyrimidine, ISC rate enhanced by least 10 000 additional hydroxy position 2.