Abstract

A metal-free sonophotocatalytic system based on a biopolymer-derived hybrid catalyst is reported for the selective C – C coupling of lignin-derived acetovanillone to the pharmaceutically relevant dimer diapocynin. The system employs a frequency-optimized chitosan – lignin sonophotocatalyst (Syn-340) under synergistic 365 nm UV‑A irradiation (~300 W m‑2) and high-frequency ultrasound (500 kHz), affording diapocynin in 33% yield with >97% selectivity at ambient temperature and pressure after 15 h. The lower photon energy of UV‑A relative to UV‑C suppresses non-selective substrate photolysis, thereby preserving the >97% C – C selectivity. Selective excitation suppresses over-oxidation of phenoxyl radicals to phenoxenium ions, preventing ring opening and mineralisation. The high primary amine density of Syn-340 promotes single-electron oxidation. Radical mechanism was confirmed by TEMPO inhibition, while scavenger tests identified •OH and O2•⁻ as the dominant reactive species, with h⁺ contributing least. A mechanistic investigation combining acoustic pressure mapping, time-resolved mass spectrometry, and luminol imaging reveals a frequency- dependent trade-off between reactive species generation and cavitation stability. Although 500 kHz enables the reaction, 22 kHz provides greater stability and reproducibility for future optimization. This work demonstrates a metal-free route to lignin-derived acetovanillone and offers design insights for future lignin valorization using biopolymer-based sonophotocatalysts.