Metal-Free Photocatalytic Aryl Radical Coupling for Donor–Acceptor Molecules

Palladium-catalyzed Suzuki–Miyaura and Buchwald–Hartwig couplings remain central to the synthesis of biaryl and donor–acceptor conjugated molecules. Although efficient, these methods require transition-metal catalysts and purification to remove residual metal species, a concern for organic electronic materials. Thus, metal-free aryl–heteroaryl bond formation is needed.

Herein, we investigate photocatalytic aryl radical cross-coupling as an alternative to palladium-based coupling. Aryl iodides were used as radical precursors, and electron-rich heteroarenes were examined as donor components to construct donor–acceptor-type aryl–heteroaryl frameworks. 10-Phenyl-10H-phenothiazine was employed as an organic photocatalyst under 380 nm irradiation. Electron-deficient aryl iodides bearing acetyl or ester groups were evaluated as acceptors, while alkylthiophenes and alkylindoles were screened as donors.

Model studies with iodobenzene confirmed C–I bond activation under the conditions. Solvent screening showed that reaction outcomes depend on solvent polarity, substrate solubility, and radical trapping efficiency. Acetonitrile served as a diagnostic medium for confirming aryl radical formation through hydrodehalogenation. Electron-deficient aryl iodides showed more favorable tendencies than simple aryl iodides. Carbonate and phosphate bases improved reaction behavior, presumably by facilitating deprotonation and rearomatization after radical addition to heteroarene donors. Reaction progress was monitored by GC–MS and NMR spectroscopy.

These results indicate that acceptor electronics, donor reactivity, solvent environment, and base additives govern the competition between cross-coupling and hydrodehalogenation. This study provides design principles for metal-free photocatalytic aryl radical coupling and suggests a sustainable platform for donor–acceptor organic electronic materials.