Importance of π-Stacking in Photoreactivity of Aryl Benzobisoxazole and Aryl Benzobisthiazole Compounds

Abstract

Aryl benzobisoxazole and aryl benzobisthiazole compounds in the solid state and in solution have completely different photoreactivity. In the solid state, intermolecular π-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 π-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 π-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}.

Photodegradation mechanism and stabilization of polyphenylene oxide and rigid‐rod polymers

“…PBO and PBZT, from here on represented as PBX, have a unique extended rigid‐rod‐like configuration and are called rigid‐rod polymers 16. Photophysics and photochemistry of PBX have been reported by workers at The Dow Chemical Company, and they also proposed a PET mechanism for PBX photo‐oxidation 12, 13. This article compares the similarities and differences of PPO and PBX under photo‐oxidative conditions.…”

“…PET is a key step in biological systems, organic reactions, organometallic and inorganic chemistry, catalysis and molecular‐level electronics 8, 9. Poly(2,4‐dimethyl‐1,4‐phenylene oxide) (PPO),10, 11 poly(benzo[1,2‐ d :5,4‐ d ′]bisoxazole‐2,6‐diyl‐1,4‐phenylene) (PBO) and poly (benzo[1,2‐ d :4,5‐ d ′]bisthiazole‐2,6‐diyl‐1,4‐phenylene) (PBZT)12, 13 (Scheme ) have been reported to undergo self‐sensitized PET. In this mechanism, an excited polymer repeating unit undergoes electron transfer with another unit to generate a radical‐cation and radical‐anion pair.…”

Rigid‐Rod Polymers

“…The compounds in solution are stable even after prolonged irradiation, suggesting that intermolecular π-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 PBO photostability (353).…”

Study on photoaging of poly(p‐phenylene benzobisoxazole) fiber

“…Some studies of the degradation of PBO fibers have been reported, including thermal degradation, hydrolysis, and photodegradation 9–15. Walsh et al found that exposure to moisture results in loosening of the PBO fiber morphology, leading to an increase in the number and size of defects 10.…”

“…UV–Vis radiation primarily hydrolyzes the material near the fiber surface with the attendant formation of amide linkages. So et al found that aryl benzobisoxazole and aryl benzobisthiazole compounds both in the solid state and in solution have completely different photoreactivities 11. In the solid state, excimers form and undergo photo‐induced electron transfers to generate an ion radical pair.…”