Visible light photoinitiating systems based on squaraine dye: kinetic, mechanistic and laser flash photolysis studies

Abstract

New two-component photoinitiator systems for radical polymerization of acrylates are presented. The systems discussed comprise a synthetic dye 1,3-bis(p-bromophenylamino)squaraine and borate and onium salts as coinitiator. The effect of the composition of the system on the photopolymerization kinetics was analyzed. To this end, the photophysics and photochemistry of the dye under polymerization conditions were explored by means of stationary and time-resolved spectroscopic methods. The action mechanism of the different photoinitiators systems is discussed.

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1. Introduction

The development of new materials by means of photopolymerization requires investigations aimed at discovering more efficient photoinitiator systems.¹ Most traditional photoinitiating systems employ UV radiation to generate active species. Different classes of synthetic dyes play very important roles in polymer chemistry. They are very often used as photosensitizers and make the polymerization process possible in the visible light region. Therefore, the development of photoinitiators based on synthetic dyes and coinitiators has been the subject of a large amount of work.

For example, ammonium, phosphonium, sulfonium, iodonium salts, arsonium, pyridinium and organoborate salts are capable of undergoing photochemical decomposition, producing active species suitable to initiate polymerization. When these coinitiators may act as an electron acceptor, photosensitization was proposed to take place by reduction and posterior fragmentation of the cation.^1–4 On the other hand a coinitiator may also play a role of an electron donor, as in a case of alkyltriphenylborate salts.⁵ The first group of coinitiators (onium salts) may act as a source of free radicals, radical cations, or Brönsted acids when exposed to light¹ and may be employed as efficient photoinitiators of radical, cationic, or mixed polymerization.^4,6–10

Recently, the following synthetic chromophores: camphorquinone,¹¹ anthracene derivatives,¹² pyrene derivatives,¹³ indanedione derivatives,¹⁴ N-substituted quinoxalinobenzothiazine derivatives,¹⁵ thiobarbituric acid derivative,¹⁶ chalcone derivatives,¹⁷ acridinedione derivatives,¹⁸ naphthalimide derivatives,¹⁹ diketopyrrolopyrrole-thiophene or diketopyrrolopyrrole-furan derivatives,^20,21 violanthrone-79,²² NIR sensitized polymethine dyes⁴ and other acting as photosensitizers for onium salts were used in dye mediated photoinitiating systems.²³

The squarylium dyes have been very rare used as photosensitizers in photopolymerization process.

In 2004 Yong He and co-workers described the application of two squaraine dyes: bis(1,2,3,3-tetramethylindolenium-2-ylidene)squaraine and bis(3-methylbenzothiazol-2-ylidene)squaraine as sensitizers for three (p-octanoxyphenyl)phenyliodonium salts varying with a type of counterion, in radical polymerization of methyl methacrylate.²⁴

In 2013 and 2015 Lalevée and co-workers presented the photoinitiating abilities of 2,2,3-trimethylindolenine-based squaraine dye incorporated in multicomponent systems for cationic polymerization of an epoxide or a vinyl ether as well as radical polymerization of TMPTA.^25,26 In 2016 our research group described the application of 1,3-bis(phenylamino)squaraine and conventional free radical sources, such as tetramethylammonium n-butyltriphenylborate, diphenyliodonium chloride and diphenyliodonium hexafluorophosphate for initiation of photopolymerization occurring via radical or cationic mechanism.²⁷

From our knowledge the 1,3-bis(p-bromophenylamino)squaraine has not been studied in the photoinitiating systems, yet. Because, the dye under study absorbs in the region from 350 nm to 450 nm an application of the blue light sources is possible.

In the present paper, we will focus on the research carried out on the two-component photoinitiating systems for free radical polymerization of multifunctional acrylates containing three different coinitiators. We will describe two-component photoinitiating systems employing 1,3-bis(p-bromophenylamino)squaraine (SQ) as a sensitizer adequate for blue light and following coinitiators: tetramethylammonium n-butyltriphenylborate (B2), diphenyliodonium chloride (I1) and N-methoxy-4-phenylpyridinium tetrafluoroborate (NO). The photoinitiating ability of new photoinitiating systems, acting in UV-Vis light region, for initiation of free radical polymerization of di- and triacrylates was also compared with few commercially used photoinitiators.

2. Experimental

2.1. Materials

Monomers: 1,6-hexanediol diacrylate (HDDA), pentaerythritol triacrylate (PETA), 2-ethyl-2-(hydroxymethyl)-1,3-propanediol triacrylate (TMPTA), coinitiators (diphenyliodonium chloride (I1) and N-methoxy-4-phenylpyridinium tetrafluoroborate (NO)) and solvents (spectroscopic grade) were purchased from Aldrich (Poland) and used without further purification. 1,3-Bis(p-bromophenylamino)squaraine (SQ) and tetramethylammonium n-butyltriphenylborate (B2) were synthesized in our laboratory by methods described in literature.^28–30

2.2. Spectroscopic measurements

Absorption and emission spectra were recorded at room temperature using an Agilent Technology UV-Vis Cary 60 Spectrophotometer, a Hitachi F-7000 spectrofluorimeter and UV-VIS-NIR Fluorolog 3 Spectrofluorimeter (Horiba Jobin Yvon), respectively. The spectra were recorded in following solvents: water (H₂O), dimethylsulfoxide (DMSO), acetonitrile (CH₃CN), N,N-dimethylformamide (DMF), 1-methyl-2-pyrrolidinone (MP), methanol (MeOH), ethanol (EtOH), acetone, tetrahydrofuran (THF) and diethyl ether. The final concentration of dye in solution was 1.0 × 10⁻⁵ M. The spectroscopic measurements were performed in mentioned above solvents containing 10% of 1-methyl-2-pyrrolidinone. For this purpose a suitable amount of the dye was dissolved in 1-methyl-2-pyrrolidinone, than 2.0 mL of the concentrated (ca. 1 mM) stock solution was added to a 10 mL volumetric flask containing spectroscopic grade solvents under the study.

The fluorescence lifetimes were measured using a single-photon counting system UV-VIS-NIR Fluorolog 3 Spectrofluorimeter (Horiba Jobin Yvon). The apparatus utilizes for the excitation a picosecond diode laser generating pulses of about 55 ps at 370 nm. Short laser pulses in combination with a fast microchannel plate photodetector and ultrafast electronics make a successful analysis of fluorescence decay signals with a resolution of few picoseconds possible. The dye was studied at concentration able to provide equivalent absorbance at 370 nm (0.2 in the 10 mm cell) to be obtained. The fluorescence decay was fitted to two exponentials.

The fluorescence quenching measurements were performed using a single-photon counting system UV-VIS-NIR Fluorolog 3 Spectrofluorimeter (Horiba Jobin Yvon). The apparatus uses a picosecond diode laser (370 nm) generating pulses of about 50 ps for the excitation. Short laser pulses in combination with a fast microchannel plate photodetector and ultrafast electronics make a successful analysis of fluorescence decay signals in the range of single picoseconds possible. The dye was studied at a concentration able to provide equivalent absorbance at 370 nm (0.2 in the 10 mm cell). The rate constant for quenching of 1,3-bis(p-bromophenylamino)squaraine by all quenchers under studies were determined in 1-methyl-2-pyrrolidinone. The concentration of dye was 2 × 10⁻⁵ M and that of quenchers was in the range from 1 × 10⁻⁴ M to 5.0 × 10⁻³ M. The fluorescence quenching at 440 nm was measured in deaerated solution by bubbling with argon.

2.3. Polymerization measurements

The kinetics of polymerization of all monomers photoinitiated by: 1,3-bis(p-bromophenylamino)squaraine/tetramethylammonium n-butyltriphenylborate (SQ/B2), 1,3-bis(p-bromophenylamino)squaraine/diphenyliodonium chloride (SQ/I1) and 1,3-bis(p-bromophenylamino)squaraine/N-methoxy-4-phenylpyridinium tetrafluoroborate (SQ/NO) was measured using Differential Scanning Calorimeter TA DSC Q2000 Instrument and TA Q PCA photo unit equipped with a high-pressure mercury lamp (Photo-DSC). The heat of the photoinitiated polymerization reaction measured by means of a photo differential scanning calorimeter, is a good control of the reaction temperature. UV-visible light (300 nm < λ < 500 nm) was applied from a high pressure mercury lamp at a constant intensity of 30 mW cm⁻² for several min under a nitrogen flow of 50 mL min⁻¹ at a prescribed temperature (isothermal mode). The weight of the samples 30 ± 0.1 mg was placed into an open aluminum liquid DSC pan. The measurements were carried out under identical conditions. The sample was maintained at a prescribed temperature for 2 min before each measurement run began. Measurements were recorded at a sampling interval of 0.05 s per point. The polymerizing solution was composed of 1.8 mL of monomer, 0.2 mL of 1-methyl-2-pyrrolidinone and appropriate amount of photoinitiator. The using of 1-methyl-2-pyrrolidinone was necessary due to poor solubility of dye in the monomers studied.

The reaction heat liberated in the polymerization is directly proportional to the number of acrylates reacted in the system. By integrating the area under the exothermic peak, the conversion of the acrylate groups (C%) or the extent of the reaction was determined according to eqn (1):

where ΔH_t is the reaction heat evolved at time t and ΔH₀ is the theoretical heat for complete conversion. A reaction heat for an acrylate double bond polymerization of ΔH₀ = 78.0 kJ mol⁻¹ was used. The rate of polymerization (R_p) is directly related to the heat flow (dH/dt) as in eqn (2):

2.4. Cyclic voltammetry measurements

The electrochemical measurements were evaluated by Cyclic Voltammetry (CV). Cyclic voltammetric measurements were made with ER466 Integrated Potentiostat System (eDAQ, Poland) in a three-electrode configuration. The electrolyte was 0.1 M tetrabutylammonium perchlorate in dry acetonitrile. Platinum 1 mm disk electrode was applied as working electrode and platinum and Ag/AgCl were used as auxiliary and reference electrodes, respectively. All solutions were deoxygenated with N₂ for at least 15 min prior to measurements. The computer-controlled potentiostat was equipped with EChem Software.

2.5. Nanosecond laser flash photolysis

Transient absorption spectra and decay kinetics were studied out using the nanosecond laser flash photolysis method. The nanosecond laser flash photolysis experiments were performed using a LKS.60 Laser Flash Photolysis apparatus (Applied Photophysics). Laser irradiation at 355 nm from the third harmonic of the Q-switched Nd:YAG laser from a Lambda Phisik/model LPY 150 operating at 65 mJ per pulse (pulse width about 4–5 ns) was used for the excitation. Transient absorbances at preselected wavelengths were monitored by a detection system consisting of a monochromator, a photomultiplier tube (Hamamatsu R955) and a pulsed xenon lamp (150 W) as a monitoring source. The signal from the photomultiplier was processed by a Helwett-Packard/Agilent an Agilent Infiniium 54810A digital storage oscilloscope and an Acorn compatible computer.

3. Results and discussion

The schematic structures of all compounds involved in the photoinitiating systems studied are presented in Table 1.

Table 1 Structures, names and abbreviations of sensitizer, electron donor, electron acceptors and monomers

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The synthetic dye applied in new photoinitiating systems for radical polymerization of multifunctional acrylates belongs to squaraine dyes. Generally, squaraines are a family of chromophores containing structures such as cyanine dyes, two donor groups conjugated to an electron deficient oxocyclobutenolate core, leading to highly electron delocalized structure that can be exemplified as zwitterions. Due to their planar structures and zwitterionic properties, squaraine dyes exhibit strong absorption (ε > 10⁵ dm³ mol⁻¹ cm⁻¹) and emission. By modifying the aromatic or heterocyclic donor moiety it is easy to modify the chromophore structure to tune the optical properties.³¹ The dye under study possess a strong withdrawing group (–Br) in para position of phenyl ring.

3.1. Spectroscopic properties

One of the parameters that may have a significant influence on the photophysical properties of photosensitizer is a solvent medium. The solvent polarity may play a significant role in dictating spectral shifts of the absorption band of squaraine dyes.³² The influence on the absorption properties of squaraine dye studied of different polarity solvents is shown in Fig. 1.

Fig. 1 The electronic absorption spectra of 1,3-bis(p-bromophenylamino)squaraine recorded at room temperature in solvents of different polarity.

(SQ) exhibits a pronounced, well-defined absorption band with maximum from 400 nm to 415 nm. The value of molar absorption coefficient depends on the polarity of solvent and ranging from 0.56 × 10⁴ dm³ mol⁻¹ cm⁻¹ to 4.95 × 10⁴ dm³ mol⁻¹ cm⁻¹ from high polar to nonpolar solvents. The position of an absorption band only slightly depends on a type of solvent. Significant variation of solvent polarity, from diethyl ether to 1-methyl-2-pyrrolidinone to water, led to negligible hypsochromic shifts (415 nm → 405 nm → 398 nm). More polar solvents lead to the blue-shift of an absorption band by 10–15 nm. Moreover, no changes in the spectral bands were observed with increasing squaraine dye concentration, thus indicating the ability of dye to remain in monomeric form even at relatively high concentrations. Similar results were observed by P. V. Kamat et al. for tetrahydroquinoxaline-based squaraine dyes.³² Other structurally modified squaraines undergo H- and J-type aggregation upon an increase of concentration.^33–35

Emission measurements revealed sharp fluorescence spectra with relatively high Stokes shifts (Δν_max ≈ 5000 cm⁻¹) and fluorescence quantum yields equal 5 × 10⁻⁴ and 34.5 × 10⁻⁴ in diethyl ether and 1-methyl-2-pyrrolidinone, respectively.

As is seen, 1,3-bis(p-bromophenylamino)squaraine exhibit low fluorescence quantum yield and short emission lifetime about 10 ns. Fig. 2 shows the fluorescence decay observed for 1,3-bis(p-bromophenylamino)squaraine in 1-methyl-2-pyrrolidinone solution.

Fig. 2 The fluorescence decay recorded for 1,3-bis(p-bromophenylamino)squaraine in 1-methyl-2-pyrrolidinone as a solvent, λ_EX 370 nm.

The fluorescence decay observed was fitted to two exponential curve. The dye exists in two conformers that differ in fluorescence lifetime described by the corresponding components of two exponential models, such as τ₁ and τ₂ together with the corresponding amplitudes (B₁ and B₂). This feature results from interaction of 1,3-bis(p-bromophenylamino)squaraine with surrounding solvent. On the basis of fitting of two exponential curve, the fluorescence lifetimes of two conformers were designated. The fluorescence lifetime of conformer appearing predominantly (over 95%) is 10.8 ns. While, the fluorescence lifetime of shorter living conformer is equal 2.7 ns.

Taking into account very small value of the fluorescence quantum yield one can conclude that there are other deactivation processes (radiative and/or nonradiative) which squaraine undergoes in its excited state. As it was mentioned in our previous paper,³⁶ the excited state of squaraine molecule may be quenched by different molecules in bimolecular reaction. The following compounds: tetramethylammonium n-butyltriphenylborate (B2), diphenyliodonium chloride (I1) and N-methoxyphenyl-4-phenylpyridinium tetrafluoroborate (NO) were used as a quencher. The fluorescence quenching measurements were confirmed an electron transfer between coinitiator and photosensitizer. For this purpose, the changes in fluorescence lifetime under increasing concentration of quencher (coinitiator) was studied.^37,38 The influence of a selected quencher on fluorescence lifetime and Stern–Volmer relationship are presented in Fig. 3, respectively.

Fig. 3 Left: The effect of tetramethylammonium n-butyltriphenylborate (B2) on the fluorescence lifetime of 1,3-bis(p-bromophenylamino)squaraine in 1-methyl-2-pyrrolidinone as a solvent. Right: The Stern–Volmer plots for the quenching of fluorescence of 1,3-bis(p-bromophenylamino)squaraine by borate salt (B2), diphenyliodonium salt (I1) and N-alkoxypyridinium salt (NO), respectively.

From the comparison of the results obtained for the same chromophore and chromophore in the presence of quenchers the shortening of the average fluorescence lifetime was observed. The shortening of the fluorescence lifetime is interpreted as a result of interaction of dye with quencher molecule.

An addition of borate salt, iodonium salt and N-alkoxypyridinium salt results in a significant decrease in a lifetime of the excited singlet state of dye. Basing on this, it should be noted, that in a presence of suitable quencher the fluorescence state of 1,3-bis(p-bromophenylamino)squaraine is quenched. The results obtained from fluorescence quenching experiments were analyzed with use the Stern–Volmer relationship eqn (3).

where: I₀ and I are the fluorescence intensities of squarylium dye in absence and presence of quencher, respectively; K_SV is the Stern–Volmer constant, characterized the collision interaction of quencher molecules (Q) with the excited state of fluorophore, k_q is the quenching rate constant.

From the fluorescence lifetime τ and the slope of the linear relationship of Stern–Volmer plot (Fig. 3 right), one can calculated the k_q value.

As it was previously observed for other squaraine,³⁶ the rate of dynamic quenching of the excited singlet state depends on the type of quencher used. Form the data presented in Fig. 3, it is seen the linear relationship between changes in the fluorescence lifetime and concentration of quencher. The slopes of Stern–Volmer linear relationship are as follows: 473.24, 398.33, and 328.92 for (B2), (I1) and (NO), respectively. From Stern–Volmer equation plot, the rate constants of fluorescence quenching reaction were obtained to be 8.19 × 10¹⁰ M⁻¹ s⁻¹, 6.89 × 10¹⁰ M⁻¹ s⁻¹ and 5.69 × 10¹⁰ M⁻¹ s⁻¹ for (B2), (I1) and (I2), respectively, which are nearly one order of magnitude larger than that of diffusion controlled bimolecular reaction constant (∼2 × 10⁹ M⁻¹ s⁻¹).

These results confirmed that the coinitiators under study are very effective fluorescence quenchers for excited (SQ) dye and the fast quenching occurs predominantly through the intramolecular ion-pair pathway.

The influence of borate salt, iodonium salt and N-alkoxypyridinium salt on the rate of fluorescence decay of dye suggests that the primary photoreaction occurs between the dye and coinitiator in ground state. This phenomena may be a result of photoinduced electron transfer process occurred between excited dye molecule and quencher in ground state. During photoinduced electron-transfer process squaraine may act as an electron donor or an electron acceptor depending on the electrochemical properties of both dye and quencher.

In conclusion, owing to the energy of excited (SQ) dye lower than that of borate and all coinitiators used, the energy transfer from excited (SQ) to coinitiator is impossible. Therefore, it is reasonable to consider that upon irradiation the photoinduced electron transfer reaction between excited squaraine dye and coinitiator occurs via intramolecular pathway, which has a large reaction rate as fluorescence quenching experiments described above.

3.2. Electrochemical measurements

In order to confirm, that the primary photochemical process in photoinitiating systems studied is an electron transfer, the redox properties of all components of photoinitiator were studied by cyclic voltammetry. In order to estimate thermodynamically, the activity of the squaraine dye/borate salt, squaraine dye/iodonium salt and squaraine dye/N-alkoxypyridinium salt photoreaction, the values of free energy change (ΔG_el) for an electron transfer reaction were calculated according to the Rehm–Weller equation, eqn (4).³⁹

ΔG_el = E_ox(D˙⁺/D) − E_red(A/A˙⁻) − Ze²/εa − E₀₀ (4)

in which E_ox(D˙⁺/D) is the oxidation potential of an electron donor molecule, E_red(A/A˙⁻) is the reduction potential of electron acceptor, Ze²/εa is the coulombic energy, normally considered negligible in high-dielectric solvents, and E₀₀ is the singlet energy of the squaraine dye.

It should be noted, that in photoinitiator systems under study, the photoreducible and photooxidizable sensitization occurs. Therefore, photoexcited dye may act as an electron acceptor or an electron donor. In a case of tetramethylammonium n-butyltriphenylborate, the squaraine plays a role of an electron acceptor. On the other hand, squaraine dye acts as an electron donor for diphenyliodonium salt and N-alkoxypyridinium salt.

For the calculation of the value of free energy change for electron transfer process the both oxidation and reduction potentials of photosensitizer must be measured. To establish the redox properties of squaraine, we used cyclic voltammetry experiment.

The electrochemical oxidation of several symmetrical squaraine dyes has been investigated earlier by Law and co-workers.⁴⁰ Most of squaraines exhibit two reversible oxidations, and their oxidation potentials are influenced by type of substituent.⁴¹ Dye (SQ) exhibits only one very weak irreversible oxidation peak at 1.24 eV and reduction peak at −0.252 eV, in acetonitrile. The cyclic voltammogram of this dye in shown in Fig. 4.

Fig. 4 Cyclic voltammograms of 1,3-bis(p-bromophenylamino)squaraine dyes in 0.1 M tetrabutylammonium perchlorate solution in dry acetonitrile as the supporting electrolyte. Inset: the normalized absorption and fluorescence spectra of sensitizer recorded at room temperature in acetonitrile as a solvent.

The higher value of oxidation potential than that observed for other squaraine dyes³² reflects a decrease in the charge-transfer character of the molecule due to the presence of zwitterionic structure of dye.

The singlet excited-state energy (E₀₀) for 1,3-bis(p-bromophenylamino)squaraine was determined from the crossover point between the normalized absorption and emission spectra and equals 2.877 eV for ¹SQ*.

Next, the values of free energy change for an electron transfer process were calculated. For this purpose the oxidation potential of 1,3-bis(p-bromophenylamino)squaraine (E_ox = 1.24 eV) and reduction potentials of diphenyliodonium chloride (E_red = −0.494 eV) and N-methoxyphenyl-4-phenylpyridinium tetrafluoroborate (E_red = −0.594 eV) must be used. The thermodynamic parameters calculated (−110.31 kJ mol⁻¹ and −100.66 kJ mol⁻¹) indicate that both SQ/I1 and SQ/NO combination systems possess high driving force, ΔG_el. In the presence of tetramethylammonium n-butyltriphenylborate (B2), squaraine dye undergoes photoreduction process. In such a case the reduction potential of dye (E_red = −0.252 eV) and the oxidation potential of coinitiator (E_ox = 1.153 eV) were used for the calculation of ΔG_el value, that is equal −142.06 kJ mol⁻¹. Negative values of ΔG_el indicate, that for all photoinitiating systems under study an electron transfer reaction yielding free radicals is thermodynamically allowed.

3.3. Kinetics of polymerization of multifunctional acrylates

The polymerization of different monomers, such as: 1,6-hexanediol diacrylate (HDDA), pentaerythritol triacrylate (PETA), 2-ethyl-2-(hydroxymethyl)-1,3-propanediol triacrylate (TMPTA), with photosensitizer (squaraine dye) was performed to examine the efficiency of three different coinitiators, such as tetramethylammonium n-butyltriphenylborate (B2), diphenyliodonium chloride (I1) and N-methoxy-4-phenylpyridinium tetrafluoroborate (NO) in radical polymerization of HDDA, PETA and TMPTA in the presence and absence of active species sources: coinitiators. The photopolymerizations were conducted using light wavelengths in the range from 300 nm to 500 nm and an irradiation intensity of 30 mW cm⁻².

In order to evaluate the optimum conditions for the polymerization process, the effect of the concentration of photoinitiator on the rate of polymerization process was studied.

To examine the effect of free radical source concentration on the initiator efficiency, polymerization with different concentration of coinitiator changing from 5 × 10⁻⁴ M to 5 × 10⁻³ M was done. Fig. 5 shows the kinetics of TMPTA polymerization with squaraine dye in presence of N-methoxy-4-phenylpyridinium tetrafluoroborate (NO) at four different concentrations.

Fig. 5 Time-conversion curves recorded during the polymerization of TMPTA with 1,3-bis(p-bromophenylamino)squaraine at 300 nm < λ < 500 nm irradiation in presence of different concentration of N-methoxy-p-phenylpyridinium tetrafluoroborate (NO) (marked in the figure). Inset: the influence of concentration of coinitiator on the kinetic of polymerization process.

The concentration of all components of photoinitiating system has a significant impact on the rate of polymerization. As the concentration of photosensitizer and coinitiator increased from 5 × 10⁻⁴ M to 5 × 10⁻³ M, the maximum polymerization rate was observed to increase from 5 × 10⁻⁴ M to 2 × 10⁻³ M in the case of N-methoxy-p-phenylpyridinium tetrafluoroborate used as coinitiator.

It is well known, that in the conventional UV/Vis photopolymerization, R_p increases when more initiator is used, however it decreases rapidly if too much initiator is added. This effect is attributed to the “inter filter effect” and becomes more significant for photoinitiators with high molar extinction coefficient (for squaraine tested, ε is about 4 × 10⁴ dm³ mol⁻¹ cm⁻¹).

From the data presented in Fig. 5, it is evident that as the photoinitiator concentration is increasing, the initial rate of polymerization increases and reaches a maximum followed by a continuous mild decrease. For the tested photoinitiators under irradiation conditions the highest rate of polymerization was observed at the photoinitiator concentration of about 2 × 10⁻³ M.

In absence of active species source (coinitiator), the photopolymerization of all monomers studied was carried out with 1,3-bis(p-bromophenylamino)squaraine (SQ) (5 × 10⁻³ M) at irradiation ranging from 300 nm to 500 nm and no monomer conversion was observed.

Fig. 6–8 show the kinetic curves recorded during photopolymerization of radically polymerizable monomers: 1,6-hexanediol diacrylate (HDDA), pentaerythritol triacrylate (PETA) and 2-ethyl-2-(hydroxymethyl)-1,3-propanediol triacrylate (TMPTA), and, as well as the degree of double bonds conversion as a function of irradiation time.

Fig. 6 The kinetic and time-conversion curves recorded during radical polymerization of HDDA initiated by 1,3-bis(p-bromophenylamino)squaraine in presence of different coinitiators (marked in the figure) at ambient temperature; [SQ] = 2 × 10⁻³ M; [coinitiator] = 2 × 10⁻³ M. Light intensity was equal 30 mW cm⁻².

Fig. 7 The kinetic and time-conversion curves recorded during radical polymerization of PETA initiated by 1,3-bis(p-bromophenylamino)squaraine in presence of different coinitiators (marked in the figure) at ambient temperature; [SQ] = 2 × 10⁻³ M; [coinitiator] = 2 × 10⁻³ M. Light intensity was equal 30 mW cm⁻².

Fig. 8 The kinetic and time-conversion curves recorded during radical polymerization of TMPTA initiated by 1,3-bis(p-bromophenylamino)squaraine in presence of different coinitiators (marked in the figure) at ambient temperature; [SQ] = 2 × 10⁻³ M; [coinitiator] = 2 × 10⁻³ M. Light intensity was equal 30 mW cm⁻².

As it is seen from the kinetic data presented above (Fig. 6–8 and Table 2) the rate of polymerization and degree of double bond conversion depend on the type of monomer and coinitiator. The best kinetic results were achieved for photoinitiator system composed of N-methoxy-4-phenylpyridinium tetrafluoroborate (NO) as a coinitiator. The highest efficiency for radical polymerization was observed in the case of two-functional monomer: 1,6-hexanediol diacrylate (HDDA). The radical polymerization of HDDA occurs with the degree of double bond conversion from 24% to 73% and is about two-times higher than that observed for trifunctional monomers: pentaerythritol triacrylate (PETA) and 2-ethyl-2-(hydroxymethyl)-1,3-propanediol triacrylate (TMPTA). The rates of polymerization of HDDA are higher than that achieved for triacrylates and ranging from 0.6 mmol s⁻¹ to 11.96 mmol s⁻¹. The radical polymerization of triacrylates occur with the lowest rates for all coinitiators used. The efficiency of photoinitiator system composed of diphenyliodonium chloride (I1) to initiation of polymerization of 1,6-hexanediol diacrylate is similar to that observed when N-methoxy-4-phenylpyridinium tetrafluoroborate was used as coinitiator. The monomer conversion was achieved values 73.3% and 71.8% for iodonium and N-alkoxypyridinium salt, respectively.

Table 2 Thermodynamic and kinetic parameters of photoinitiating systems under study

Photoinitiator	ΔGel [kJ mol^−1]	HDDA		PETA		TMPTA
		Rp [mmol s^−1]	Monomer conversion [%]	Rp [mmol s^−1]	Monomer conversion [%]	Rp [mmol s^−1]	Monomer conversion [%]
SQ/B2	−142.06	0.6	24.12	1.02	23.54	1.0	6.9
SQ/I1	−110.31	9.90	73.3	2.03	27.05	2.6	36.7
SQ/NO	−100.66	11.96	71.8	9.22	45.43	7.42	47.3

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On the other hand, the lowest efficiency of radical polymerization of all monomers studied was observed for tetramethylammonium n-butyltriphenylborate used as coinitiator.

The maximum conversion of PETA obtained during the polymerization initiated by squaraine dye in presence of (B2), (I1) and (NO) was found as 23.54%, 27.05% and 45.43%, respectively. The rate of polymerization is about 2 times higher for diphenyliodonium chlorine in comparison with borate salt. Similar results were obtained in the case of 2-ethyl-2-(hydroxymethyl)-1,3-propanediol triacrylate. The photoinitiating systems possessing N-alkoxypyridinium salt as an electron acceptor initiate radical polymerization about 7–9 times faster than systems composed of borate salt.

Basing on the kinetic results, the following order of coinitiators activity may be proposed: borate salt (B2) > iodonium salt (I1) > N-methoxypyridinium salt (NO). This trend can be explained by the difference in the decomposition rate constant and the reactivity of active species formed toward the functional group of monomer.

Therefore, N-methoxy-4-phenylpyridinium tetrafluoroborate is relatively more efficient radical source than others to accelerate the rate of polymerization. The higher activity of N-alkoxypyridinium salts can be explained by high efficiency of alkoxy radicals formation.^42–48

The different kinetic results obtained may be also related to different properties of monomers used. It should be also noted, that the rate of radical polymerization ranges from 0.6 mmol s⁻¹ to 11.96 mmol s⁻¹ and is greater than that obtained for other squaraine dye-based photoinitiating systems composed of 1,3-bis(phenylamino)squaraine and borate salt and diphenyliodonium salts.²⁷ As it will be show below, the polymerization of radically polymerizable monomers is initiated by butyl, phenyl and methoxy radicals. The differences in photoinitiation ability observed between all coinitiators used may be related to different redox potentials of coinitiators, resulting in different values of free energy change for an electron transfer process.

In next step, for fully demonstration the interest of proposed approach, the photoinitiating ability was compared to commercially available photoinitiators. It is well known, that the photoinitiating ability is highly dependent on the nature of the light source, its intensity and wavelength, as well as on reactivity of the formulation.

For example, 1,6-hexanediol diacrylate (HDDA) polymerizes in presence of a initiator: bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (TMBAPO, Irgacure 819) (0.1% w/w) at light intensity 0.6 mW cm⁻² with total monomer conversion about 12%.⁴⁹ HDDA under 30 min of UV-irradiation initiated by triaryl sulfonium hexafluoroantimonate salt gives the monomer conversion about 50%.⁵⁰ In 2016 Wu and co-workers described the polymerization of 1,6-hexanediol diacrylate initiated by visible light one-component photoinitiating systems composed of 4-[(methyl)-(9-oxo-9H-thioxanthen-2-yl)amino]methyl phenyl acrylate. An addition of tertiary amine leads to the 60% of monomer conversion.⁵¹

The polymerization of pentaerythritoltriacrylate (PETA) in presence of bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide as photoinitiator under irradiation of UV LED with emission maximum at 385 nm gives the total monomer conversion about 35% and 60%.⁵²

But, the photopolymerization of an trimethylolpropane triacrylate (TMPTA) in presence of phosphine oxide as the photoinitiator leads to the 5% of double bond conversion under air and irradiation with Xe lamp.⁵³ Addition of 3% w/w of tris(trimethylsilyl)silane causes in an increase of conversion about 40% under the same conditions.⁵³ Monoacylphosphine oxides such as Speedcure TPO initiates radical polymerization of TMPTA under irradiation with LED at 400 nm and light intensity equal 85 mW cm⁻². The maximum double bond conversion was about 15%.⁵⁴ Arsu and co-workers⁵⁵ shown that the polymerization of TMPTA using BAPO (0.2 wt%) in the absence and presence of 2-mercaptothioxanthone (0.02 wt%), during irradiation with a medium-pressure mercury lamp with a light intensity of 20 mW cm⁻², occurs with conversion about 30% and 38%, respectively.⁵⁵ Shi et al. in 2011 described a series of benzophenone-terminated hyperbranched polyester, bearing amine moieties as photoinitiator for radical polymerization of trimethylolpropane triacrylate. The maximal conversion achieved is in range from 44% to 59%.⁵⁶ Yang and co-workers studied the kinetics of radical polymerization using UV-lamp (200–400 nm) as a light source whit an intensity 20 mW cm⁻². The dibenzoyl peroxide does not initiate polymerization of TMPTA under these conditions. But application of isopropyl thioxanthone or thioxanthone-based N-phthalimidoamino acid ammonium salt as a photoinitiator gives the final conversion about 40% and 83%, respectively.⁵⁷ Arsu and co-workers studied an amine linked benzophenone photoinitiator for free radical polymerization of triacrylates.⁵⁸ A medium pressure mercury arc lamp (220–400 nm) giving light intensity of 40 mW cm⁻² was used as a light source. The final conversion changes in the range from 12% to 55%.

The kinetic results shown also, that the systems under study initiate polymerization process faster than other squaraine-based photoinitiators. For example, for two-component photoinitiating systems composed of bis(1,2,3,3-tetramethylindolenium-2-ylidene)squaraine or bis(3-methylbenzothiazol-2-ylidene)squaraine, and (p-octanoxyphenyl)phenyliodonium salt the maximum double bond conversion of methyl methacrylate is in the range from 10% to 14% after 4 h of irradiation time.²⁴ But photopolymerization of TMPTA in laminate initiated by 1,3,3-trimethylindolenine-based squaraine dye and diphenyliodonium chloride (0.5%/2%, w/w) leads to the conversion of monomer about 40%.²⁶ When camphorquinone and diphenyliodonium chloride (0.5%/2%, w/w) were used to initiation of TMPTA polymerization under halogen lamp as a light source, final monomer conversion obtained in laminate was about 18%.⁵⁵ The application of 1,3-bis(phenylamino)squaraine in presence of tetramethylammonium n-butyltriphenylborate for initiation of polymerization of different acrylates leads to the total monomer conversion about 10.5%, 21.6% and 27.3% for TMPTA, PETA and HDDA, respectively. The polymerization of triacrylates initiated by system composed of 1,3-bis(phenylamino)squaraine in presence of diphenyliodonium chloride as a coinitiator leads to about 20% of total monomer conversion using irradiation of high-pressure mercury lamp (300–500 nm) with light intensity 30 mW cm⁻².²⁷

The total conversion obtained for photoinitiating systems under study are about 73.3%, 45.43% and 47.3% for HDDA, PETA and TMPTA, respectively.

3.4. Investigation of the initiation mechanism

As it was mentioned above the three different compounds were used as coinitiators in polymerization systems under study. Therefore, various species are products of primary and secondary photochemical reactions. The initiating radicals (derived from corresponding coinitiators) are different for all photoinitiating systems under study. Taking this into account, the activity of the photoinitiating systems studied depends on the reactivity and the efficiency of initiating radicals formation.

Irradiation of 1,3-bis(p-bromophenylamino)squaraine in acetonitrile solution, with 5 ns laser pulse results in instantaneous appearance of its excited state, which is characterized by absorption at 380 nm. The rate constant of excited state formation is equal 2.48 × 10⁸ s⁻¹. The time of decay of excited state, and the rate constant of the decay of excited state are about 2.33 μs and 4.29 × 10⁵ s⁻¹, respectively. The lifetime of SQ* is decreasing as concentration of borate salt increases. This mean, that SQ* is quenched by tetramethylammonium n-butyltriphenylborate, and a new transient with absorption at 480–500 nm is simultaneously formed. The new transient can be assigned to radical anion of squaraine dye. At the same time the disappearance of the band corresponding to the absorption of the dye in the ground state is observed (Fig. 9).

Fig. 9 Transient absorption spectra of 1,3-bis(p-bromophenylamino)squaraine in presence of tetramethylammonium n-butyltriphenylborate (B2) recorded: 2 ns (squares, circles) and 100 ns (triangles) after laser pulse. Inset: transient absorption spectra of squaraine dye recorded 100 ns after laser pulse (circles) in acetonitrile solution. Coinitiator concentration is marked in the figure.

The rate constant, k_q, for the quenching of the excited state of squaraine dye by borate salt was determined in MeCN solution. The k_q value was obtained by monitoring the excited state absorption decays of SQ* at 380 nm for various quencher concentrations by employing the Stern–Volmer equation. The established value of this rate constant is equal 3.31 × 10⁷ mol⁻¹ s⁻¹. The time of formation of squaraine dye-based radical anion and its disappearance is equal 10 nm and 1.14 μs, respectively.

The excited state of photosensitizer is also quenching by N-methoxy-p-phenylypyridinium tetrafluoroborate, as well as diphenyliodonium chloride. In both cases the formation of new absorption band at 480–500 nm is observed. The time of formation of new product and its disappearance is equal 20 ns and 1.47 μs, 14 ns and about 200 ns for N-alkoxypyridinium salt and diphenyliodonium salt, respectively. New absorption band is attributed to the absorption of squaraine dye-based radical cation formation. Simultaneously the disappearance of the band at 380 nm is observed. The established value of quenching rate constant is equal 2.31 × 10⁶ mol⁻¹ s⁻¹. Fig. 10 shows the kinetic traces recorded at 480 nm for different delay times: 2 ns and 2 μs, that present the formation and disappearance of squaraine dye radical cation.

Fig. 10 The kinetic traces recorded at 480 nm for different delay times: 2 ns and 2 μs observed after irradiation of 1,3-bis(p-bromophenylamino)squaraine in presence of N-methoxy-p-phenylpyridinium tetrafluoroborate. Coinitiator concentration was 5 × 10⁻⁴ M.

On the basis of the above experiments, it appears that n-butyltriphenylborate anion is oxidized, but N-methoxy-p-phenylpyridinium cation and diphenyliodonium cation are reduced by excited state of squaraine dye. These reactions yield ground state squaraine dye, n-butyltriphenylboranyl radical, N-methoxy-p-phenylpyridyl radical and diphenyliodonium radical. The radicals formed undergo fast and irreversible fragmentation giving: n-butyl radical and triphenylboron, methoxy radical and p-phenylpyridinium and phenyl radical and iodobenzene.

Basing on the laser flash photolysis results and thermodynamic parameters for the electron transfer process, one can conclude, that there is the possibility of donating an electron from borate salt to squaraine dye. This process is thermodynamically allowed, ΔG_el = −142 kJ mol⁻¹. On the other hand the squaraine dye studied can also act as an electron donor for both onium salts studied. In summary, photoinitiating systems studied may act via photoreducible or photooxidizable mechanism.

On the basis of the nanosecond laser flash photolysis and the thermodynamical analysis presented in this paper, the following mechanism for the primary and secondary reactions was proposed (Scheme 1) for sensitized generation of free radicals.

Scheme 1 The primary and secondary processes occurring in the photoinitiating systems under study after irradiation with visible light.

After irradiation of the photoinitiating system with a visible light, the excited state of squaraine (SQ*) is formed. The deactivation of excited state occurs by radiative and nonradiative processes. One of the nonradiative processes is an electron transfer. In presence of borate salts the squaraine dye undergoes one-electron reduction. The dye radical anion and boranyl radical are formed. The boranyl radical undergoes carbon–boron bond cleavage, giving a butyl radical that can start the polymerization reaction. However, in the presence of diphenyliodonium salt or N-methoxy-p-phenylpyridinium salt, an electron transfer from the excited state of squaraine dye to the ground state of onium salts occurs, giving squaraine radical cation and diphenyliodonium radical or N-methoxy-p-phenylpyridinium radical. The lasts undergo fast fragmentation as a result of carbon-iodide or nitrogen–oxygen bond cleavage, forming the iodobenzene and phenyl radical or p-phenylpyridinium and methoxy radical.

4. Conclusions

In this paper, the efficiency of visible light induced polymerization of multifunctional acrylates in the presence of two-component photoinitiating systems based on squaranine dye and tetramethylammonium n-butyltriphenylborate, diphenyliodonium chloride and N-methoxy-p-phenylpyridinium tetrafluoroborate was ascertained.

1,3-Bis(p-bromophenylamino)squaraine could successfully initiate photopolymerization of 1,6-hexanediol diacrylate (HDDA), trimethylolpropane triacrylate (TMPTA) and pentaerythritol triacrylate (PETA) under high-pressure mercury lamp light exposure (300 nm < λ < 500 nm) in the presence of N-methoxy-4-phenylpyridinium tetrafluoroborate, diphenyliodonium chloride and tetramethylammonium n-butyltriphenylborate. The best photoinitiating ability was observed for photoinitiator composed of squaraine dye as a photosensitizer and N-methoxy-p-phenylpyridinium tetrafluoroborate as a coinitiator. The rate of polymerization depends on the type of coinitiator and type of monomer. The highest values of monomer conversion were observed for diacrylate monomer polymerization. The max values reached above 73%.

Basing on the kinetic results and the laser flash photolysis experiment the mechanism of reactions occurring in new photoinitiating system composed of squaraine dye was proposed. The free radicals formation involves both, an electron transfer from borate salt to the dye and an electron transfer from dye to onium salt, as the primary photochemical reactions. Borate salt is an electron donor reducing dye, but diphenyliodonium salt and N-alkoxypyridinium salt are the electron acceptor oxidizing the excited dye. These reactions generate free radicals, which can start the polymerization chain reaction via photoreducible or photooxidizable series mechanism.

Acknowledgements

This work was supported by The National Science Centre (NCN) (Cracow, Poland), Grant No. 2013/11/B/ST5/01281.

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