Paramagnetic Resonance of the Triplet State of Tetramethylpyrazine

EPR measurements are reported for solid solutions of tetramethylpyrazine (TMP) in durene under ultraviolet irradiation at 77°K. The fine structure of the paramagnetic resonance spectrum observed at 9.2 and 35 GHz may be described by the spin Hamiltonian H=gβH·S+DSz2+E(Sx2−Sy2) in which S=1, D/hc=±0.0990±0.0005 cm−1, E/hc=±0.0043±0.0007 cm−1, and g(isotropic) =2.003±0.001. No hyperfine structure is observed in crystals doped with either normal TMP or perdeuterated TMP. The triplet emission spectrum is also reported for TMP measured in an argon and a hydrocarbon matrix at 4.2°K. The 0–0 band of the T→S emission of TMP is at 26 050±30 cm−1 in both matrices. The emission decay constant is 0.41±0.03 sec in both matrices at 4.2°K. The EPR measurements and the emission studies indicate that the lowest triplet state of TMP is of π—π* character. This is in contrast with the known n—π* character of the triplet state of the parent compound pyrazine.