High-spin ground-state polyradicals are an important platform due to their potential applications in magnetic and spintronic devices. However, a low high-to-low spin energy gap limits the population of the high-spin state, precluding their application at room temperature. Also, design strategies delineating control of the ground electronic state from a closed-shell low-spin to open-shell polyradical character with a high-spin ground state are not well established. Here, we report indacenodinaphthothiophene isomers fused with a 6,6-dicyanofulvene group showing a high-spin quintet ground state. Density functional theory calculations indicate that the syn - and anti -configurations have a closed-shell low-spin singlet ground state. However, the linear -configuration displays a high-spin quintet ground state, with the energy difference between the high-spin quintet to the nearest low-spin excited states calculated to be as large as 0.24 eV (≈5.60 kcal mol −1 ), exhibiting an exclusive population of the high-spin quintet state at room temperature. These molecules are compelling synthetic targets for use in magnetic and spintronic applications.