Isophlorins are generally unstable toward oxidation because of their antiaromatic character and high-energy HOMO levels. Previous studies have indicated that electron-withdrawing substituents at the meso-positions are crucial for stabilizing these compounds. Therefore, we synthesized four S2O2Iphs derivatives (1a-d) with varying steric properties of the electron-withdrawing meso-substituents to systematically investigate their effects on antiaromaticity and stability.