We report on the electronic and triplet state ODMR spectroscopy of s-tetrazme, 3,6-dimethyl-, 3,6-dimethoxy-and 3,6-bis(methylthio)-s-tetrazines.Zero field splitting parameters were obtained by ODMR monitoring both the delayed fluorescence and the phosphorescence m neat single crystals. The S, and T, exciton electronic origins are reported. The zero field splitting tensor is nearly axially symmetric with 1 D 1 approximately 0.4 cm-'. This value is found to be in good agreement with calculattons based on magnetic dipole-dipole coupling in a 'BS, (nx") state. The calculations predict a positive value for D with the major axis passing through the C atoms at the 3 and 6 positions. In~~ue~onA great deal of interest has been devoted to the electronic spectroscopy and photochemistry of s-tetrazine ( 1,2,4,5_tetraazabenzene, 1). Early studies of the vapor phase absorption spectra [ l-51 revealed two clear absorption band systems; an intense system near 250 nm was assigned to the analogue of the 'BZuelAg (X--X*) transition of benzene, while bands of a weaker system near 550 nm have been shown to belong to a iB3ut 'A, (n-x*) transition #'. A broad absorption at = 350 nm has been assigned [ 31 as a second (n-rc*) transition. Fluorescence of 1 ( 'BsU-+ 'A,) was observed by Chowdhury and Goodman in a glass at 77 K [ 7 1, who also noted the absence of any detectable phosphorescence accompa-' Now at: Institut fur Klmische Radiologie, Klinikum Mannheim, Mannhelm, Germany. 2 On leave from the Chemistry Department of the University of California, Davis, USA. #' The symmetry designations are based on the molecular axis system used m the early spectroscopic work [ 2-51, i.e., the z-axis goes through the CX and C6 positions of s-tetrazine and the x-axis IS normal to the molecular plane. A more recent axis system conventton (see, for Instance ref.[ 6 ] ) ts based on the transformation, (x, y, Z) -+ (y, .a. x), in which case the excited state designations would be 'Blu(~-rr*) and 'B,(n-n*). In this paper, we retain the earlier axis convention. nying excitation in the singlet manifold. The weak 'Bj,,t 'A, system of 1 was identified by Livak and Innes [S] in the vapor phase absorption spectrum; the corresponding phosphorescence was observed by McDonald and Brus [ 91 following direct excitation in the Tl+-SO system. These authors also noted the apparent lack of efftcient Si -+T, intersystem crossing (isc) in 1. Single crystal absorption spectra of 1 [ lo] and the related compound 3,6-dimethyl-s-tetrazine, 2 [ 111, yielded 3B3Ut 'Ap band systems in absorption, and phosphorescence was observed upon excitation in this band. The lack of perceptible isc in 1 was also pointed out by Hochstrasser and King [ 10 J. The phosphorescence lifetimes in the gas phase r=72+2 11s [9], and in the neat crystal at ~4.2 K, r=96.8f2.1 ys [lo] are similar. Their magnitude implies the presence of efficient radiationless deactivation.Vemulapalli and Cassen [ 121 reported an extremely low vapor phase fluorescence quantum yield for 1, implying efficient radiatio...

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The fluorescence and phosphorescence of kekulene in a host matrix of polycrystalline tetrachlorobenzene are investigated, together with the triplet zero field splitting parameters IDI and lEI obtained by ODMR in zero field. The D value is also calculated within a semi-empirical 1T-theory and compared with experiment. It could be shown that the triplet state energies of a number of different sites of kekulene in the host matrix and the zero field splitting parameters are related, in first order, by spin orbit interaction.

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