Stable high-spin polycarbene, polybrominated 1,3,5-tris[4-(phenylcarbeno)-phenylethynyl]benzene in a glass matrix was studied by Electron Spin Transient Nutation (ESTN) spectroscopy. 2D-ESTN spectra of the polycarbene were measured in order to identify a molecular spin multiplicity of the high-spin ground state. The observed natation frequencies were assigned to those arising from the allowed ESR transitions of the septet ground state (S=3). It indicates that the high-spin state is generated by ferromagnetic interaction between stable bromo-substituted diphenylmethylenes (S=1) through a 2D topological spin coupling unit with C-3 symmetry, 1,3,5 -tris(ethynyl)benzene. The observed fine-structure parameters of the septet state. D = -0.040 cm(-1) and \E\ = 0.007 cm(-1), were well interpreted in terms of an intramolecular spin-spin interaction model. The observed small and negative D value was shown to arise from the C-3 symmetric molecular structure. The high-spin polycarbene has survived after annealing of the glass matrix. The stable triplet carbene unit is supposed to serve as a useful building block for extended super high-spin molecular systems and organic superparamagnets.