Photochemical Hydrogen Abstraction Reactions of 9,10-Dihydro-9,10-o-benzenoanthracene-1,4-dione Derivatives with Xanthene: Intramolecular Charge Transfer Quenching

Intramolecular charge transfer interaction in triptycene system was investigated. 9,10-Dihydro-9,10-o-benzenoanthracene-1,4-dione (triptycenequinone) and its analogs were readily photoreduced with xanthene to give the corresponding hydroquinones and the adducts with two xanthenyl radicals. The chemically induced dynamic nuclear polarization examination and the corresponding thermal reaction proved the adducts to be originated from free radical coupling of xanthenyl radicals and quinones. The quantum yield for disappearance of the quinones depended upon both the substituents and solvents. Kinetic analysis by applying Stern–Volmer plot, using anthracene as a quencher, revealed that the quantum yield of intersystem crossing and the lifetime of the triplet state decreased with increasing electron-donating power of the aromatic rings in triptycenequinones. These solvent and substituent effects suggest that intramolecular charge transfer quenching of both the singlet and the triplet excited states of quinones by neighboring aromatic rings occurs in the triptycenequinone system. Such intramolecular charge transfer interaction would lower the rate constants of hydrogen abstraction reaction from xanthene via the triplet states of triptycenequinone derivatives.