Curcuminoid‐Based Difluoroboron Dyes as High‐Performance Photosensitizers in Long‐Wavelength (Yellow and Red) Cationic Photopolymerization

Abstract

derivatives (DAAQs) have a remarkable influence on the initiating efficiency of the DAAQs-based photo initiating systems. [5] In such photo initiating systems, dyes play a significant role in absorbing light and interacting with other additives to generate initiating species. Dyes determine the suitable spectral scope and initiating efficiency of the photoinitiating systems. The design of facilely prepared dyes with excellent light absorption properties at long-wavelength and the development of high-performance dye-sensitized cationic photoinitiating systems still remain challenging. Difluoroboron β-diketonate dyes are highly luminescent organboron complexes that exhibit wide applications in biosensing, bioimaging, and optoelectronic devices due to their rich photophysical properties (strong fluorescence, large extinction coefficients, and two-photon excited fluorescence). [6] However, to the best of our knowledge, no attempts have been made to use these dyes in cationic polymerization. Bellinger et al. reported a series of curcuminoid and hemicurcuminoid-based difluoroboron dyes (BF 2 Curs). [7] The reported dyes exhibit excellent light absorption ability at 450−650 nm and suitable redox potentials, which endow them with the potential for use in dye-sensitive long-wavelength photopolymerization.In the present paper, four BF 2 Curs were prepared (Scheme 1). Their light absorption properties were investigated. Furthermore, the photoinitiating efficiency of BF 2 Curs in the presence of an iodonium salt (ONI) for the CP of diglycidyl ether of bisphenol A (DGEBA) and tri(ethylene glycol) divinyl ether (DVE-3) was analyzed upon exposure to yellow and red LEDs. For comparison, the reported OBN-based photoinitiating systems were tested under the same experimental conditions. The photochemical reactivity between BF 2 Curs and iodonium salts was investigated. The mechanisms involved in the formation of initiation species were proposed, and structure−activity relationships of the BF 2 Curs-based photo initiating systems were discussed.The UV-vis absorption spectra of BF 2 Curs (BF 2 Cur1, BF 2 Cur2, BF 2 Cur3, and BF 2 Cur4) and OBN in DCM are presented in Figure 1. Their absorption maxima (λ max ) and extinction coefficients (ε) at λ max and at the maximum emission wavelengths of different LEDs are summarized in Table 1. As illustrated, BF 2 Curs showed much higher light absorption intensity than OBN at λ max . Notably, BF 2 Curs demonstrated satisfactory overlap with the emission wavelengths of yellow (585 nm) and red (625 nm) LEDs. The extinction coefficients of Photopolymerization Difluoroboron β-diketonate dyes are reported to exhibit excellent photophysical properties (e.g., broad absorption and large extinction coefficients) and have the potential to act as high-performance photosensitizers in cationic photopolymerization (CP). In this study, four curcuminoid-based difluoroboron dyes (BF 2 Curs) are prepared. Their ability to initiate the CP of epoxides or vinyl ethers in combination with an iodonium salt under yel...

Highly Fluorescent Dianionic Polymethines with a 1,3,2-Dioxaborine Core

“…Typically, dioxaborine is the terminal electron-acceptor group in merocyanines (D−π–A) or anionic polymethines (A−π–A), whereas dyes with the dioxaborine ring incorporated into the polymethine chain (D−π–A−π–D) have only recently gained much attention . Most of the D−π–A−π–D dioxaborines are difluoroborinated curcuminoids, used in bioimaging, fluoride-anion detection, and as photosensitizers in long-wavelength cationic photopolymerization …”

“…10 Most of the D−π−A−π−D dioxaborines are difluoroborinated curcuminoids, used in bioimaging, 11 fluoride-anion detection, 12 and as photosensitizers in long-wavelength cationic photopolymerization. 13 Synthesis of difluoroborinated curcuminoids usually involves condensation of the boron difluoride complex of acetylacetone with benzaldehydes. 14 However, this synthetic route is illsuited for obtaining dyes with stronger electron-donating heterocyclic end groups, and hence, polymethines with the central dioxaborine core remain scarce.…”

“…This is most likely due to relatively fast rotamerization around the C−C bonds connecting the tricyanofuran end groups and polymethine chain. 17,18 Analysis of the 13 C NMR data has revealed the relation between the chemical shift of the dioxaborine meso-C atom and the terminal groups' electronaccepting abilities. Thus, for malononitrile-based dye 3, the chemical shift δ C of C meso is equal to 76.3 ppm.…”

“…However, the correlation is violated for dianions 7 and 8, the C meso signals of which are even more downfield than that of dianion 5, while the electronaccepting abilities of their end groups are similar or weaker than that of the thiobarbituric moiety. 2, 18 The explanation here is typical in 13 C NMR spectroscopy long-range structural effects, since dyes 3−6 has dimethine and 7, 8 trimethine chains between the dioxaborine core and their end groups. The 19 F NMR chemical shifts, though minor, depend more regularly on the electron-accepting properties of end groups.…”

Highly Effective Sensitizers Based on Merocyanine Dyes for Visible Light Initiated Radical Polymerization

“…The systems included acridine and xanthene dyes. Since then, many other dyestuffs have been designed as photosensitizers [6] that extend the spectral range of the photopolymerization to the visible region [7][8][9][10][11][12][13][14] because this light is cheap, safe and possesses higher penetration in comparison to UV light [15].…”