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 donors, namely, N‑ethylcarbazole (NEC), p-dimethoxybenzene (DMB) and diphenyla-cetylene (DPA), were prepared for the first time by solvent evaporation method and characterized by single-crystal X-ray diffraction, UV-Vis, NMR spectroscopy and theo-retical 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 com-plex 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 ex-hibited photoinitiation in the cationic polymerizations of cyclohexene oxide and iso-butyl vinyl ether, under visible, white and near-infrared light, attributed to their high electron-donating ability and favorable co-facial stacking with. In contrast, DPA-DDQ complex failed to initiate electron transfer, thus, 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. This comprehensive study provides design principles for engineering solid-state CT photoinitiators via do-nor choice and crystal packing.