Hemicyanine and Its Hexyltriphenyl Borate Ion Pairs as Red and Near-IR Light-Absorbing Photoinitiators for Efficient Radical Photopolymerization

A series of coumarin-derived hemicyanine dyes (C-1–C-4) with red/near-infrared (NIR) absorption were developed as novel photoinitiators (PIs) for free-radical photopolymerization (FRP) to address the low electron transfer efficiency of hemicyanines caused by their short singlet excited-state lifetimes. Tetrabutylammonium n-hexyltriphenylborate (NB) was employed as the co-initiator, and pre-associated hemicyanine–borate ion pairs (e.g., C-1–NB) were prepared via two strategies: in situ ion-exchange upon mixing of hemicyanines with NB (without tedious isolation) and ion-exchange, followed by separation of the pure ion-pair complex from the mixture. Fluorescence and photobleaching experiments (Stern–Volmer constant >104 M–1) confirm that the formation of the ion pair significantly enhances fluorescence quenching and photobleaching. We propose tight ion-pair formation in nonpolar solvents (e.g., toluene) to accelerate intra-ion-pair electron transfer, while loose ion-pair formation in polar solvents (e.g., acetonitrile), where the electron transfer efficiency decreases. Nanosecond transient absorption (ns-TA) spectra detected hemicyanine neutral radicals (τ = 96.0 μs), and femtosecond transient absorption (fs-TA) spectroscopy confirmed intra-ion-pair electron transfer. Radical photopolymerization of trimethylolpropane triacrylate (TMPTA) in the presence of the PIs upon low light intensity irradiation at 620 nm demonstrates that the C-1–NB ion pair exhibits optimal photopolymerization efficiency, giving C═C double-bond conversion up to 80% within 50 s of photoirradiation, while the mixture of C-1 and NB gives slightly poorer performances. Moreover, the photobleaching of the red/NIR-absorbing initiators yields colorless transparent polymers, suitable for 3D volumetric displays and holographic recording. This work provides a new strategy for developing efficient red/NIR FRP systems and presents insights into the photoinitiation mechanism.