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

Year: 2026

Authors: Qin XQ., Jiang PW., Sambucari G., Ramos TJD., Hou YQ., Huang HJ., Dong X., Zhao JZ., Versace DL., Di Donato M.

Autors Affiliation: Dalian Univ Technol, Frontiers Sci Ctr Smart Mat Oriented Chem Engn, Sch Chem Engn, State Key Lab Fine Chem, Dalian 116024, Peoples R China; LENS European Lab Nonlinear Spect, I-50019 Sesto Fiorentino, Firenze, Italy; CNR, INO, I-50019 Sesto Fiorentino, Italy; Dalian Univ Technol, Sch Chem Engn Ocean & Life Sci, Panjin 124221, Peoples R China; Ningbo Sunny Automot Optech Co Ltd, Ningbo 315400, Yuyao, Peoples R China; Univ Paris Est Creteil UPEC, Inst Chim & Mat Paris Est ICMPE, UMR, CNRS 7182, F-94320 Thiais, France; CNR, ICCOM, I-50019 Sesto Firorentino, Italy.

Abstract: 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 >10(4) 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 (tau = 96.0 mu s), 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.

Journal/Review: MACROMOLECULES

Volume: 59 (9)      Pages from: 5347  to: 5361

More Information: J.Z. thanks the NSFC (W2521100, U25A20619 and 22473021), the Research and Innovation Team Project of Dalian University of Technology (DUT2022TB10), the Fundamental Research Funds for the Central Universities (DUT22LAB610), and Ningbo Sunny Automotive Optech Co., Ltd. for financial support. M.D.D. thanks the European Union’s Horizon 2020 Research and Innovation Program through the project LASERS4 EU (grant agreement no. 101131771) for the support.
KeyWords: Electron-transfer Photoinitiators; Trimethylolopropane Triacrylate Tmpta; Cyanine-borate; Monomethine Dyes; Polymerization; Salts; Systems; Fluorescence; Sensitizers; Initiators
DOI: 10.1021/acs.macromol.6c00065