Highly symmetric high-spin polycarbenes of pi-conjugation, which are the first and second generations of the spin-mediated dendrimers based on phenylacetylenes, have been studied by cw and pulsed ESR spectroscopy. We measured pulsed ESR-based two-dimensional electron-spin transient nutation (2D-ESTN) spectra of the stabilized polycarbene systems connecting the phenylacetylene moieties at the meta-positions of benzene rings, and we determined their spin multiplicity with the help of a transition moment analysis. A septet ground state of the triscarbene system, which is the first generation of the phenylacetylene dendrimers, was unequivocally discriminated. It is indicated that the stable diphenylcarbene serves as a useful building block and the phenylacetylene dendrimer as a ferromagnetic coupling unit with robust spin polarization for super high-spin polymeric systems and super paramagnetic materials.