The photooxidative sensitization of bis(p-substituted diphenyl)iodonium salts in the radical polymerization of acrylates

The ability of two-component dyeing photoinitiating systems for the radical polymerization of 1,6-hexanediol diacrylate (HDDA) and 2-ethyl-2-(hydroxymethyl)-1,3-propanediol triacrylate (TMPTA) is presented. The systems under study comprised a hemicyanine dye as a sensitizer and iodonium salts that played a role of a coinitiator. The kinetic parameters of the polymerization reaction, such as the rate of polymerization (Rp) and the degree of conversion of monomer (C%), were estimated. The thermodynamic feasibility of an electron transfer process in the systems studied was verified and calculated using the Rehm–Weller equation. It was found that a benzoxazole derivative in the presence of iodonium salts effectively initiated the polymerization of acrylate monomers. The polymerization rates of about 10−7 s−1 and the degree of conversion of acrylate groups from 20% to 50% were observed. The effects of photoinitiator structures on the initiating ability and spectroscopic properties of sensitizers are described in this article.


Introduction
Photopolymerization is one of the popular technologies used for the production of various types of polymer materials, which are used in various elds. In recent years, interest towards the photoinitiated polymerization has rapidly increased due to many advantages, such as low energy consumption, the possibility of using non-solvent composition and efficiency. 1,2 This process is suitable for curing of dental llings, fabrication of paints, coatings, lacquers, 3D objects and many others. 3 Materials obtained by free radical polymerization (FRP) enable their further practical applications.
It is expected that these materials will be relatively cheap, compatible and have not undergone any negative changes upon the exposure to radiation and high temperatures. 4,5 The photoinitiating system (PIS) plays a key role in FRP. For this reason, the design and development of high-performance systems, which allow the high values of kinetic parameters of the process, seem extremely important. Due to the harmful effects of ultraviolet radiation on the human body, scientists are still looking for new initiating systems for polymerization in the visible region of the spectrum. 5,6 Free radical polymerization involves three steps: initiation, growth of the polymer chain (propagation) and termination. 7 Usually, the initiating step of polymerization requires the application of an appropriate photoinitiator. The introduction of a dye as an absorber of light (photosensitizer) to the system shis the sensitivity of PIS towards longer wavelengths. This molecule aer absorption reaches a higher energy state (excited single or triplet state) and then decomposes or reacts with another molecule, which leads to the formation of active radicals. 8 In general, the PISs may be classied into three groups: one-component (Type I): the radicals are formed via a homolytic a-cleavage of bonds, two-component (Type II): the generation of active species is related to the process of energy, electron or hydrogen atom transfer or is based on the process of the photoinduced cleavage of bonds via electron transfer, multi-component: they are composed of three or more compounds and characterized by a more complex mechanism comprising many reactions. 9 In dyeing photoinitiating systems, there are two types of sensitization, namely, photoreducible and photooxidative, that occur via photoinduced electron transfer (PET) (Scheme 1). It should be noted that the dye molecules in the presence of appropriate coinitiators are capable of undergoing electron transfer reactions in the photoexcited state. The dyes may act as an electron donor or an acceptor. 7,10 As shown in Scheme 1, the dye is a sensitizer (chromophore) molecule, for example, a hemicyanine dye. The symbols D and A correspond to electron donor (for example, alkyltriphenyl borate salt) and electron acceptor (for example, diphenyliodonium salt) molecules, respectively.
There are many photoinitiating systems based on dye molecules, as described in the literature. The synthetic chromophores, such as xanthenic dyes, 11,12 camphorquinone, 13,14 ketocoumarin derivatives, 15 pyrromethenes, 16 polymethines, 17 and others, [18][19][20][21][22][23][24][25][26] are used as absorbing species. Another example of sensitizers are neutral hemicyanine dyes. Benzothiazole-, benzoxazole-and anapthiazole-based hemicyanines are used with different coinitiators for the initiation of the radical polymerization of triacrylates under irradiation using a 360 nm light source. 27 Those chromophores were paired with thiophenoxyacetic acid, phenoxyacetic acid, N-phenylglycine, tetramethylammonium butyltriphenylborate, N,N 0 -dimethoxybipyridinium tetrauoroborate, and N,N 0 -diethoxybipyridinium tetrauoroborate. We did not nd any information about using them with diphenyliodonium salts. In the past decades, we can nd some examples for the application of diphenyliodonium salts for the initiation of photopolymerization. For example, Xiao et al. described the photoinitiating abilities of two-and three-component systems for the radical polymerization of TMPTA, containing naphthalimide derivatives as sensitizers, and diphenyliodonium hexauorophosphate, N-vinylcarbazole, Nmethyldiethanolamine and 2,4,6-tris(trichloromethyl)-1,3,5-triazine as coinitiators. High values of the rate of polymerization and the degree of monomer conversion above 50% were achieved. 28 In 2019, Chen et al. presented new Type II photoinitiators based on benzophenone and thioxanthone, exhibiting low migration in photocuring systems. It was shown that with the increase in the light intensity from 10 mW cm À2 to 50 mW cm À2 , the degree of monomer conversion increased from 56% to 89%. 29 Lalevée et al., in 2019, 30 proposed two coumarins as highperformance visible light photoinitiators in the presence of an iodonium salt or with an amine, for both the free radical polymerization (FRP) of (meth)acrylates and the cationic polymerization (CP) of epoxides upon visible light exposure using an LED at 405 nm. Coumarin-based systems are high-performance photoinitiators that may be used for new photosensitive 3D printing resins.
The aryliodonium ylides described by the same authors 31 are new interesting and efficient additives for the photoinitiators of radical polymerization composed of camphorquinone and amines.
In 2019, the iodonium salt as an acceptor and indoles as donors are described as stable dual thermal and photochemical free radical polymerization (FRP) initiators for benchmarked methacrylates. 32 Due to the lack of reports on neutral hemicyanine dye/ iodonium salt pairs, we decided to develop our study on dyeing photoinitiating systems acting under visible light, which are composed of neutral hemicyanine and diphenyliodonium salts. For this purpose, we have synthesized benzothiazolebased hemicyanine dyes as a blue-light-sensitive sensitizer and used several iodonium salts containing the electrondonation or electron-withdrawing groups for the polymerization of di-and triacrylates.
The uorescence quantum yield of dyes in different solvents was determined by comparison with the solution of coumarin 1 in ethanol serving as the reference (l ex ¼ 366 nm, The uorescence spectra of dilute dye solution (A ¼ 0.1) were registered by excitation at the maximum band of reference's absorption. This parameter was estimated using eqn (1): 34 where F dye is the uorescence quantum yield of dye, F ref is the uorescence quantum yield of reference, I dye and I ref are the integrated emission intensities of the dye and reference, A dye and A ref are absorbances of the dye and reference at the excitation wavelength, n dye and n ref are the refractive indexes of solvents used for the dye and reference, respectively.

Polymerization measurements
The kinetic parameters of the free radical polymerization of multifunctional acrylate monomers were determined using a Differential Scanning Calorimeter TA DSC Q2000 Instrument equipped with a high-pressure mercury lamp (Photo-DSC). The heat evolved during reaction was registered for radiation range 300-500 nm and at a constant intensity of 30 mW cm À2 . The measurements were performed at a sampling interval of 0.05 s per point in isothermal conditions under a nitrogen ow of 50 mL min À1 . Due to the poor solubility of the dye in monomers, 1methyl-2-pyrrolidinone was used as the solvent. The polymerization mixture was composed of 1.8 mL of the monomer, 0.2 mL of 1-methyl-2-pyrrolidinone, the sensitizer and an appropriate coinitiator. The concentration of photoinitiators was 5 Â 10 À3 M. The polymerizing solution without a coinitiator was used as the reference sample. The degree of conversion (C % ) is directly proportional to the number of reactive groups (double bonds) in the monomer molecule. This parameter was calculated using eqn (2): where DH t is the reaction heat evolved at time t and DH 0 is the theoretical heat for the complete degree of conversion (for acrylates: DH 0 ¼ 78.0 kJ mol À1 ). The rate of polymerization (R p ) is derived from the amount of heat released during the process, which is expressed by eqn (3): where dH/dt is a heat ow in the polymerization reaction. Moreover, the overall ability to the initiation reaction was also calculated using eqn (4): where I p is the photoinitiation index, R p (max) is the maximum rate of polymerization and t max is the time required for the maximum rate of heat release.

Results and discussion
The structures of all compounds used as components in the photoinitiating systems for polymerization experiments are depicted in Table 1.
The synthetic dye 2-(p-N,N-dimethylaminostyryl)benzoxazole (R1) used as the sensitizer in the photoinitiating systems belongs to hemicyanines. The functional dyes belonging to this group possess a characteristic donor-p-acceptor (D-p-A) structure and show specic, useful properties, such as high extinction coefficients, 35 wide range of absorption and emission radiation, good affinity for biomolecules, 36 and good chemical activity. 37 For recent years, the photophysical properties of cyanine dyes were intensively studied due to their applications in different areas, such as biological imaging, 38 molecular electronics, 39 nonlinear optics, 40 and textile industry. 41 There are many commercially available coinitiators, initiating both radical and cationic polymerization. 42 The iodonium salts belong to the most commonly used coinitiators in the polymerization process. [43][44][45] In our study, we present the application of series of p-substituted diphenyliodonium salts acting as an electron acceptor in PISs. The spectroscopic properties of the benzoxazole derivative R1 and the possibility of its application as a sensitizer in photoinitiating systems are also presented. The mixture was heated at 195-200 C for 5 h. The obtained compound (yellow solid) was recrystallized from ethanol and dried at ambient temperature. 46 The structure of dye was conrmed by nuclear magnetic resonance spectroscopy. 1

Spectroscopic studies
The data characterizing the spectroscopic properties of the synthesized dye are presented in Table 2.
In Table 2, the symbols l ab , FWHM ab , 3, l  , FWHM  , F dye and E 00 are dened as the maximum of absorption band, full width at half maximum of absorption, molar extinction coefficient, maximum of uorescence band full width at half maximum of emission, uorescence quantum yield and excitation energy, respectively.
As shown in Fig. 1 (L), the dye under study has a single pronounced absorption band with maximum (l ab ) located from 380 nm to 399 nm. This band is attributed to the p / p* transition. 38 The position of the band of absorption depends on the polarity of the solvent. When the polarity increases, the absorption band shis towards a higher wavelength (batochromic shi) from non-polar diethyl ether (Et 2 O) to polar aprotic dimethylsulfoxide (DMSO). The molar extinction coef-cients (3) of dye achieve relatively high values. This parameter ranges from 2.82 Â 10 4 M À1 cm À1 to 4.39 Â 10 4 M À1 cm À1 . The value of Stokes shis (Dn max ) is equal to ca. 5000 cm À1 .
The emission spectra ( Fig. 1 (R)) are broad with the single maximum of uorescence (l  ) at about 455-500 nm. In this case, the maximum of uorescence shis also towards higher wavelength values as the polarity of solvent increases. The hemicyanine dye displays low values of uorescence quantum yields of about 1 Â 10 À3 .
The values of excited-state energy level (E 00 ) increase as the polarity of solvent decreases and are in the range from 2.76 eV to 2.97 eV.
The effect of the presence of coinitiators on the absorption properties of the sensitizer was studied, and the results obtained are shown in Fig. 2. 47 For this purpose, the absorption spectra of R1 dye, iodonium salt I1 and the mixture of dye/iodonium salt in acetonitrile were recorded. The concentration was 1 Â 10 À5 M and 2 Â 10 À5 M for the dye and coinitiator, respectively.
As shown in Fig. 2, there are two bands of absorption: (i) iodonium salt in the UV region with l ab ¼ 229 nm and (ii) hemicyanine dye in the Vis region with l ab ¼ 386 nm. The presence of coinitiator does not have any inuence on the shape and position of the absorption band of the sensitizer. The overlap of the absorption region of iodonium salt I1 and the emission of a light source is not observed. Therefore, it can be concluded that the sensitizer absorbs only the radiation emitted by the light source used, e.g. 300-500 nm.

Kinetics of the polymerization of acrylates
The inuence of the combinations of sensitizer R1 and various structure diphenyliodonium salts on the kinetics parameters of the polymerization process was estimated by differential scanning calorimetry technique. As shown in Table 1, the coinitiators used in the photopolymerization experiment differ in the type of a substituent attached to the phenyl ring in the para position. In general, these compounds may be divided into three groups: coinitiators without substituents in the p-position: I1, I2. coinitiators with an activating substituent: I77, I78, I83, I86, I90, I92.
It was found that the neutral hemicyanine dye R1 in the presence of diphenyliodonium salts is an efficient photoinitiator of HDDA and TMPTA polymerization (Tables 3 and 4).
Depending on the type of the coinitiator used, different values of such parameters as the heat emitted during the polymerization reaction that were directly related to its rate (R p ) as well as the degree of conversion of acrylate groups (C % ) for both monomers were observed.
The data presented below clearly indicated that the efficiency of the photoinitiating system depends on both the type of the coinitiator and the monomer used in polymerization experiments. Fig. 3 (L) shows that in the case of systems consisting of coinitiators with an activating substituent, the highest amount of heat released during the polymerization reaction was observed for the R1/I86 combination. This system exhibited the highest polymerization rate of 7.37 Â 10 À4 s À1 . The values of the degree of monomer conversion were in the range from 33.50% to 46.49%. Furthermore, in this case, 2-methylbenzoxazole derivative in the presence of diphenyliodonium chloride (I1) did not initiate HDDA polymerization.
From the kinetics curves (Fig. 4) and data summarized in Table 4, it can be concluded that the highest nal monomer conversion was obtained for the systems, containing coinitiators with electron-withdrawing substituents. The maximum rate of polymerization oscillated about 3-4 Â 10 À4 s À1 and the degree of double bond monomer conversion achieved values from 30% to even 40%. Similar to the polymerization of HDDA, the sensitizer R1 in the presence of diphenyliodonium chloride (I1) is not an effective photoinitiator of the polymerization of TMPTA. The effectiveness to the initiation of polymerization is higher for systems, containing coinitiators with strong deactivating substituents, such as -CN and -NO 2 . For both cases, the R p value achieved ca. 4.30 Â 10 À4 s À1 .  The values of photoinitiation index (I p ) show that all of the compounds used as coinitiators in the polymerization process exhibit similar effectiveness to the generation of active centers. This parameter achieved values of about 10 À7 s À2 .
In order to explain the relation between the structure of the coinitiator used and the ability of the system to initiate free radical polymerization, the dependence of the polymerization rate on the Hammett substituent constants is presented (Fig. 5).
A linear correlation of the R p values with the Hammett constants s for the deactivating substituents was observed. The correlation coefficient R 2 was determined to be 0.89. The system containing a coinitiator with a deactivating substituent   characterized by a higher value parameter of s effectively initiated the polymerization of TMPTA. Accordingly, the highest R p value was reached for the combination of 2-(4-N,N-dimethylaminostyryl)benzoxazole/(4-methoxyphenyl)-(4-nitrophenyl)iodonium p-toluenesulfonate (R1/I81). In general, in dye-sensitized photoinitiating systems, the photoinduced electron transfer process plays a key role in the formation of free radicals.
In this endothermal process, the absorbed light quantum initiates an electron transfer from the donor to the acceptor. 5,9 In order to conrm that in photoinitiating systems under study, an electron transfer is possible from a thermodynamic point of view, the change of free energy for electron transfer process was calculated on the basis of Rehm-Weller equation (eqn (5)). 48 The obtained data are summarized in Table 5.
where: DG et is the free energy change for an electron transfer reaction, E ox is the oxidation potential of an electron donor, E red is the reduction potential of an electron acceptor, E 00 is the excited-state energy level, C is a constant depending on the degree of charge separation (negligible value for polar solvents).
Analyzing the data presented in Table 5, it can be seen that the free energy change for electron transfer process ranges from À0.744 eV to À1.789 eV. The negative values of DG et demonstrated that as a result of an electron transfer reaction between   Table 2). the dye and the coinitiator for all studying photoinitiating systems the radicals initiating polymerization are formed. Moreover, the iodonium salts are an electron acceptor and the dye acts as an electron donor. Based on the nanosecond laser ash photolysis results described earlier, the mechanism of the formation of radicals in two-component photoinitiating systems composed of hemicyanine dye and iodonium salt has been proposed (Scheme 3). 7 Aer the irradiation of the polymerizing mixture, the excited state of the dye is formed. In the presence of iodonium salts, the deactivation of the excited state of the sensitizer via electron transfer from the dye molecule to the coinitiator occurs, and diphenyliodonium radical decomposes via the cleavage of the carbon-iodide bond. The p-substituted iodobenzene and psubstituted active phenyl radicals, which initiate the polymerization reaction, are formed.

Conclusions
The benzoxazole derivative in combination with iodonium salts is a high-performance photoinitiator for the free radical polymerization of both HDDA and TMPTA. The synthesized dye intensively absorbs radiation in the UV-Vis region; therefore, this molecule may be used as a sensitizer in PISs. The photoinitiating ability is related to the composition of photoinitiating system. It depends on both the structure of coinitiators and the type of monomers used in the experiment. The rate of polymerization increases with the increase in Hammett constants. The initiating radicals are formed via a photoinduced electron transfer process. The mechanism of the formation of radical sources is presented. The mechanism of the generation of active centers involves an electron transfer from the excited state of the dye to the coinitiator molecule. The obtained results allowed us to evaluate the effectiveness of new systems that can be used as photoinitiators of the free radical polymerization of acrylates in the UV-Vis region.

Conflicts of interest
There are no conicts to declare.