High-spin organic molecules have attracted much attention as spin sources for magnetic materials. In this paper, a novel high-spin organic molecule was designed according to a logical strategy. A 2,2-dimethylcyclopentane-1,3-diyl diradical unit was utilized as a spin source and a spin-bridged fragment. In addition, nitroxides were introduced as spin sources. Quintet electron-spin resonance (ESR) signals, which were attributed to the 1,3-di(3′-nitoroxyphenyl)-2,2-dimethylcyclopentane-1,3-diyl tetraradical (TR), were observed in a 2-methyltetrahydrofuran glassy matrix at 80 K after the photodenitrogenation of a newly synthesized azoalkanes AZ. A simulation of the signal provided the zero-field splitting parameters D (|D|/hc = 0.0127 cm À1) and E (|E|/hc = 0.0014 cm À1) for the quintet state of the tetraradical TR. The parameter D of the quintet state of TR was found similar to those of the previously reported quintet spin states of tetraradical TR1 with two cyclobutane-1,3-diyl units (0.0190 cm À1) and TR2 with two 2,2-dimethylcyclopentane-1,3-diyl diradicals (0.0116 cm À1).