  The spintronic properties of magnetic molecules have attracted significant
scientific attention. Special emphasis has been placed on the qubit for quantum
information processing. The single molecule magnet, bis(phthalocyaninato (Pc))
Tb(III) (TbPc2), is one of the best examined cases in which the delocalized
{\pi}-radical electron spin of the Pc ligand plays the key role in reading and
intermediating the localized Tb spin qubits. We utilized the electron spin
resonance (ESR) technique implemented on scanning tunneling microscope (STM)
and use it to measure local ESR of single TbPc2 molecule decoupled from the
Cu(100) substrate by 2 monolayers NaCl film to identify the {\pi}-radical spin.
We detected the ESR signal at the ligand positions at the resonance condition
expected for the S = 1/2 spin. The results reveal that the {\pi}-radical
electron is delocalized within the ligands and exhibits intramolecular coupling
susceptible to the chemical environment.
