Double Role of Diphenylpyridine Derivatives as Fluorescent Sensors for Monitoring Photopolymerization and the Determination of the Efficiencies of the Generation of Superacids by Cationic Photoinitiators

Abstract

Novel fluorescent sensors with electron-donating or electron-withdrawing substituents incorporated into a chromophore group based on 2,6-diphenylpyridine were designed and synthesised. The spectroscopic properties of these compounds were studied. Moreover, the positive solvatochromism of 2,6-bis-(4-methylsulphanylphenyl)pyridine (PT-SCH3) in selected solvents was studied by measurement of the absorption and emission spectra and analysed using the Dimroth–Reichardt solvent parameter set. After that, the performance of a series of 2,6-diphenylpyridine derivatives as fluorescent molecular sensors for monitoring free-radical and cationic photopolymerization processes by the Fluorescence Probe Technique (FPT) was studied. As a consequence of this stage of research, the effect of substituents on the sensitivity of the 2,6-diphenylpyridine derivatives as sensors during photopolymerization has been evaluated and discussed. It has been found that compounds containing strong electron-donating substituent (PT-SCH3) slightly shift their fluorescence spectrum during the free-radical polymerization of monomer, which enables the monitoring of the polymerization progress using the fluorescence intensity ratio measured at two different wavelengths as the progress indicator. The position of the fluorescence spectrum of 2,6-diphenylpyridine derivatives with electron-withdrawing substituents is practically insensitive to changes occurring in their environment. Hence, it is recommended to use these compounds with different indicators of the progress of the photopolymerization process based on normalised intensity of fluorescence (Imax/I0). Among the compounds studied, 2,6-bis(4-methylsulphanylphenyl)pyridine (PT-SCH3) turned out to be the best fluorescent sensor for the purpose of monitoring free-radical polymerization by FPT. Consequently, the dual application of the selected 2,6-diphenylpyridine derivatives is proposed: (a) as fluorescent sensors for monitoring the free-radical photopolymerization progress, and (b) as spectroscopic sensors for the determination of efficiencies of the generation of superacids by cationic photoinitiators during the cationic photopolymerization process. Finally, a new method for determining the relative efficiency of the photogeneration of superacids during the photo cleavage of onium salt has been devised and applied for the evaluation of the performance of 2,6-diphenylpyridine derivatives.

Review of quantitative and qualitative methods for monitoring photopolymerization reactions

“…The high importance of the RT-FTIR technique has been proved by numerous studies in photopolymerization systems. 71–78…”

“…The high importance of the RT-FTIR technique has been proved by numerous studies in photopolymerization systems. [71][72][73][74][75][76][77][78] RT-FTIR spectroscopy, according to numerous literature sources, has been and remains a useful technique for studying the kinetics of photopolymerization reactions according to radical, cationic, and hybrid mechanisms, in addition to various exposure conditions, types of light source used, coating thickness, monomer and oligomer reactivity or efficiency of newly developed photoinitiating systems. [79][80][81][82][83][84][85] For free-radical photopolymerization, RT-FTIR measurement consists of repeated scanning of the sample at short intervals and observing in real-time changes in the IR absorption spectrum, for example, changes in both the intensity and position characteristic of (meta)acrylic groups, i.e.…”

Moving Towards a Finer Way of Light-Cured Resin-Based Restorative Dental Materials: Recent Advances in Photoinitiating Systems Based on Iodonium Salts

“…Diaryliodonium salts, with a weakly nucleophilic counter ion, are efficient photoinitiators for cationic photopolymerization. Due to the low C-I binding energy, which is 26-27 kcal/mol, after irradiation, diaryliodonium salts are broken down to a radical-cation, and reactive aryl radical and an anion [193][194][195] (Figure 21).…”

“…Diaryliodonium salts, with a weakly nucleophilic counter ion, are efficient photoinitiators for cationic photopolymerization. Due to the low C-I binding energy, which is 26-27 kcal/mol, after irradiation, diaryliodonium salts are broken down to a radical-cation, and reactive aryl radical and an anion [193][194][195] (Figure 21). However, commercial iodonium salts currently used in the industry have light absorption characteristics in the UV-C range, i.e., λmax = 220-280 nm, and that have very low or zero light absorption in the long-term UV-A range (λ > 300 nm).…”

New horizons for carbon dots: quantum nano-photoinitiating catalysts for cationic photopolymerization and three-dimensional (3D) printing under visible light

“…Scientists around the world are currently working on new molecules, the so-called iodine salt photosensitizers, to efficiently transfer electrons to the iodine salt and thus effectively trigger light-initiated polymerization. 19 Many types of compounds are available as effective photosensitizers of iodine salts, including biphenyl, 20,21 terphenyl, 1,22,28 thioxanthone, 23 pyridine [24][25][26] or 1,8-naphthalimide 27 derivatives. However, it is a constant challenge to develop more "greener" sensitizers derived from natural resources that are non-toxic and highly effective and can be successfully exploited in biomedical applications.…”