Applying triplet-triplet annihilation upconversion in degradation of oxidized lignin model with good selectivity

Abstract Extraction of higher value products from lignin via photoredox catalysis is crucial to the economic viability of integrated biorefineries. Triplet–triplet annihilation upconversion (TTA-UC) as a photon modulation technique, including a sensitizer and an emitter, could convert of long-wavelength light into higher-energy photons. In this work, photocatalysts based on TTA-UC are used in oxidized lignin model degradation for the first time. Under the irradiation of light with wavelength > 510 nm, TTA-UC catalyst comprised of 5,10,15,20-tetra(N,N-diethylaniline)porphyrin palladium (PdTPNEt2P) and perylene achieves a comparable high degradation efficiency with that perylene as catalyst used directly (irradiated at λ > 420 nm), but a higher product selectivity is obtained. Compared with conventional tetraphenyl porphyrin palladium (PdTPP), PdTPNEt2P is a better sensitizer of TTA-UC catalyst with a 13 times improvement on the lignin degradation efficiency (from 7% to 99%). That is because the introduction of four electron-donating groups (N,N-diethyl amino) onto PdTPNEt2P could remarkably reduce its oxidation potential and thus the electron transfer from reductive sacrificial reagents to PdTPNEt2P is efficiently hindered and ensured the occurrence of TTA-UC. It suggests that the redox potential of TTA-UC sensitizer should be considered when applying it to redox reactions. This work also indicates that TTA-UC catalyst could enhance the product selectivity with a relatively lower energy photon excitation via avoiding side reactions induced by high energy excitation. Considering the flexibility of the molecular structure of the TTA-UC sensitizer and emitter, the application of TTA-UC in lignin photocatalytic decomposition should be very promising.