Aryl benzobisoxazole and aryl benzobisthiazole compounds in the solid state and in solution have completely different photoreactivity. In the solid state, intermolecular pi-stacking interactions between these molecules lead to excimer formation. The excimer undergoes a photoinduced electron transfer to generate an ion radical pair. In the presence of oxygen, the anion radical transfers an electron to molecular oxygen to form superoxide. The cation radical undergoes bond cleavage followed by hydrogen abstraction and other reactions to form benzobisoxazoles (or benzobisthiazoles), benzonitriles, and benzamides. In solution, aryl benzobisoxazole and aryl benzobisthiazole molecules cannot readily pi-stack, and therefore photoinduced electron transfer and its subsequent reactions do not occur. The compounds in solution are stable even after prolonged irradiation, suggesting that intermolecular pi-stacking plays a very important role in photoreactivity for these molecules. Reversible redox reagents, such as ferrocene compounds, are found to retard the subsequent reactions from the photoinduced electron-transfer reaction and hence improve the photostability of poly{(benzo[1,2-d:5,4-d']bisoxazole-2,6-diyl)-1,4-phenylene}.