Structure–reactivity relationships in DDQ-based charge-transfer complexes: Single-crystal XRD, spectroscopic, DFT studies and photoinitiation efficiency in cationic polymerization

Charge-transfer (CT) cocrystals of the strong π-acceptor 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) with a series of aromatic electron donor molecules, namely, N-ethylcarbazole (NEC), p-dimethoxybenzene (DMB) and diphenylacetylene (DPA), were prepared for the first time by solvent evaporation method and characterized by single-crystal X-ray diffraction, UV–Vis, NMR spectroscopy and theoretical studies. According to conducted measurements, all complexes crystallize in 1:1 donor–acceptor stoichiometry, with nearly planar π-systems that stack in alternating columns. While NEC-DDQ and DMB-DDQ pairs form strong face-to-face π–π stacks with centroid–centroid distances of 3.28 Å and 3.39 Å, respectively, DPA–DDQ complex shows a slipped geometry with partial overlap (larger shift distance) and weaker π–π interaction with centroid-centroid distance of 3.84 Å. NEC and DMB donors exhibited photoinitiation in the cationic polymerizations of cyclohexene oxide (CHO) and isobutyl vinyl ether (IBVE), under visible, white and near-infrared irradiation, attributed to their high electron-donating ability and favorable co-facial stacking with DDQ. In contrast, DPA-DDQ complex failed to initiate photocationic polymerization due to weak donor strength and poor π–π stacking. No complexes initiated ε-caprolactone (ECL) polymerization, indicating a need for stronger Lewis acid for the initiation. This study provides design principles for engineering solid-state CT photoinitiators via donor choice and crystal packing.