Energy Change For Electron Transfer Research Articles

Role of Substituent Position in Coumarin Derivatives during their Interaction with TiO2 Nano Particles.

The effect of position of benzo group in coumarin derivatives, 5,6 benzo-4-azidomethyl coumarin (5BAMC) and 7,8 benzo-4-azidomethyl coumarin (7BAMC) during their interaction with TiO2 nanoparticles in ethyl acetate, tetrahydrofuran, butan-1-ol and acetonitrile solvents has been studied using different spectroscopic methods and electrochemical analysis. Benesi-Hildebrand plots indicate that nature of interaction between 7BAMC and TiO2 is 1:2 in solvent with low dielectric constant whereas for 5BAMC and TiO2, it is 1:1 in all the solvents. From the fluorescence quenching study and binding equilibria analysis, it is observed that interaction between 5BAMC and TiO2 depends on the dielectric constant of the solvent. Time resolved quenching study reveals that quenching is dynamic for 5BAMC in solvent with high dielectric constant. Whereas for 7BAMC, it is dynamic in solvent with low dielectric constant. Hence the nature of interaction of these two coumarin derivatives with TiO2 NPs is different. From electrochemical analysis, it is observed that, free energy change for electron transfer is more negative for 5BAMC-TiO2 compared to 7BAMC-TiO2 therefore quenching is more efficient for 5BAMC-TiO2 compared to 7BAMC-TiO2 system, which is also confirmed from fluorescence quenching studies. Non-radiative energy transfer rate is more than radiative energy transfer rate for both the systems according to FRET study.

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

The 5-(4-substituted-arylidene)-1,3-dimethylpyrimidine-2,4,6-triones were tested as visible light sensitizers for phenyltrialkylborate salts applied to initiate polymerization processes. The initiation occurs as a result of photoinduced electron transfer from the borate salt to the merocyanine dye. The main factor that facilitates the step of the reaction is the free energy change for electron transfer. Its value is favorable according to the reduction properties of the dyes influenced by the type of amino groups and the oxidation potentials of the borate salts. The observed bleaching of the dyes during photopolymerization affects the yield of both the alkyl radical and sensitizer-based radical formation and thus the efficiency of the photopolymerization.

Effect of stereo electronic factors of coumarin derivatives during their interaction with TiO2 nanoparticles

The effect of stereo electronic factors of coumarin derivatives, 6-methoxy-4-azido methyl coumarin (6MCA) and 7-methoxy-4-azido methyl coumarin (7MCA) during their interaction with the TiO2 nanoparticles in ethyl acetate, tetrahydrofuran, methanol, dimethyl formamide and acetonitrile solvents has been studied using uv-vis absorption spectroscopy, fluorescence quenching, FRET and electrochemical analysis. Benesi-Hildebrand plots confirm that interaction between 6MCA and TiO2 is stronger in solvents with low dielectric constant whereas for 7MCA and TiO2 system, interaction is stronger in solvents with high dielectric constant. The steady state and time resolved S-V plots are linear for both the systems. For 6MCA system, quenching rate parameter is decreasing with increasing dielectric constant of the solvent and for 7MCA system these values are in increasing order with increasing dielectric constant of the solvents. FRET study also confirms that nature of interaction between these two systems is different. Electrochemical analysis reveals the role of electron transfer from coumarin derivative to TiO2 nanoparticles and shows that azido group (-N3) of 7MCA and 6MCA are responsible for electron transfer. The magnitude of free energy change for electron transfer confirms that quenching is more efficient for 7MCA system. The higher magnitude of quenching rate parameter of 7MCA compared to 6MCA is due to its stable and polar structure.

Effect of amino anilines on the fluorescence of coumarin derivative

The effect of stereo electronic factors of amino anilines such as ortho-phenylenediamine (OPD) and para-phenylenediamine (PPD) on the fluorescence of 5,6-benzo-4-azidomethyl coumarin (5BAMC) in polar aprotic and polar protic solvents has been studied. From UV absorption and 1 H NMR studies, observed blue shift in the presence of PPD is due to H-bonding with 5BAMC. Fluorescence quenching studies indicate the role of static and dynamic quenching mechanisms for both the quenchers and larger quenching efficiency of PPD than OPD. The sphere of action static quenching and finite sink approximation models reveal static effect in the excited state and diffusion limited bimolecular quenching reactions. Electrochemical analysis indicates that quenching efficiency of PPD is more than OPD based on the magnitudes of free energy change for electron transfer. Binding equilibria analysis confirms that the magnitude of binding between PPD and 5BAMC is stronger compared to OPD. It is also noticed that the binding between PPD and 5BAMC in polar protic solvents is stronger than that in aprotic solvents. Lastly, from the magnitudes of thermodynamic parameters, interaction between PPD and 5BAMC is hydrophobic and spontaneous. Thus from the present investigation, it is concluded that the separation between two –NH2 groups in amino anilines plays role in their interaction with coumarin derivative 5BAMC.

Blinking Suppression in Highly Excited CdSe/ZnS Quantum Dots by Electron Transfer under Large Positive Gibbs (Free) Energy Change.

Semiconductor quantum dots with stable photoluminescence are necessary for next generation optoelectronic and photovoltaic devices. Photoluminescence intensity fluctuations of cadmium and lead chalcogenide quantum dots have been extensively investigated since the first observation of blinking in CdSe nanocrystals in 1996. In a quantum dot, blinking originates from stochastic photocharging, nonradiative Auger recombination, and delayed neutralization. So far, blinking is suppressed by defect passivation, electron transfer, and shell preparation, but without any deep insight into free energy change of electron transfer. We report real-time detection of significant blinking suppression for CdSe/ZnS quantum dots exposed to N, N-dimethylaniline, which is accompanied by a considerable increase in the time-averaged photoluminescence intensity of quantum dots. Although the Gibbs (free) energy change (Δ Get = +2.24 eV), which is estimated electrochemically and from density functional theory calculations, is unfavorable for electron transfer from N, N-dimethylaniline to a quantum dot in the minimally excited (band-edge) state, electron transfer is obvious when a quantum dot is highly excited. Nonetheless, Δ Get crosses from the positive to negative scale as the solvent dielectric constant exceeds 5, favoring electron transfer from N, N-dimethylaniline to a quantum dot excited to the band-edge state. Based on single-molecule photoluminescence and ensemble electron transfer studies, we assign blinking suppression to the transfer of an electron from N, N-dimethylaniline to the hot hole state of a quantum dot. In addition to blinking suppression by electron transfer, complete removal of blinking is limited by short-living OFF states induced by the negative trion.

Photooxygenation of Hydrocarbons with Molecular Oxygen By Electron Transfer

Phenol is an very important bulk organic compound, having a variety of applications in industry. Phenol is commercially produced through three-step cumene process via cumene hydroperoxide, which is the key intermediate. In addition acetone is produced as a co-product and a-methylstyrene is produced as a byproduct together with phenol. Thus, phenol synthesis from benzene in a one-step reaction with high benzene conversion and high phenol selectivity is highly desired from the viewpoints of an environmentally benign green process and economical efficiency. Extensive efforts have so far been devoted to develop the one-step direct hydroxylation of benzene using various oxidants. we would like to focus on the catalytic mechanisms of selective hydroxylation of benzene to phenol. First, catalytic mechanisms of liquid phase hydroxylation of benzene with H2O2 using metal complexes are discussed by identifying the reactive intermediates, which act as strong electrophiles for the benzene hydroxylation. Next, photochemical hydroxylation of benzene with an organic oxidant via benzene radical cation produced in photoinduced electron transfer from benzene to the excited state of the organic oxidant is discussed to clarify the reason of the selective hydroxylation to phenol. The reason why phenol is not further oxidized is discussed based on the Marcus theory of electron transfer. Finally, the photocatalytic hydroxylation of benzene with O2 to phenol is made possible by using an appropriate organic photocatalyst. The photochemical hydroxylation of benzene with an organic oxidant to phenol is also combined with the catalytic reoxidation of the reduced organic oxidant by O2 to make the selective hydroxylation of benzene with O2 to phenol catalytic. How overoxidation of phenol can be avoided is discussed in both the thermal and photochemical catalytic hydroxylation of benzene. Photochemical hydroxylation of benzene with 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) and water occurs under visible light irradiation to yield phenol and DDQH2. The yield of phenol was 99% with 99% conversion (>99% selectivity) [1]. The mechanism of the photochemical hydroxylation of benzene with DDQ was clarified by time-resolved transient absorption spectroscopy. The photochemical reaction was initiated by the photoinduced electron transfer from benzene to 3DDQ* to produce benzene p-dimer radical cation and DDQ•−. The free energy change for electron transfer from benzene to 3DDQ* is largely negative (DG et = −0.70 eV), as determined from the one-electron oxidation potential of benzene (E ox = 2.48 V vs. SCE) and the one-electron reduction potential of 3DDQ* (E red = 3.18 V vs. SCE). Then, benzene radical cation reacts with H2O to produce an OH-adduct radical of benzene. Finally, phenol was produced by hydrogen atom transfer from the OH-adduct radical to DDQ•–. Photoinduced oxygenation of neat cyclohexane in the presence of O2 also occurred under visible light irradiation of DDQ which acts as a super photooxidant. The products detected by GC and NMR analyses were cyclohexanol, cyclohexanone and cyclohexane hydroperoxide with 70, 40, 210% yields, respectively. When cyclohexane was replaced by n-butane, n-pentane and 3-methylpentane, the oxygenation reactions also took place to form the corresponding oxygenated products under the otherwise same photochemical reaction conditions [2-5]. [1] Ohkubo, K.; Fujimoto, A.; Fukuzumi, S. J. Am. Chem. Soc. 2013, 135, 5368. [2] Ohkubo, K.; Hirose, K.; Fukuzumi, S. Chem.–Eur. J. 2015, 21, 2855. [3] Ohkubo, K.; Hirose, K.; Fukuzumi, S. Photochem. Photobiol. Sci. 2016, 15, 731. [4] Ohkubo, K.; Hirose, K.; Fukuzumi, S. Chem. Asia J. 2016, 11, 2218. [5] Fukuzumi, S.; Ohkubo, K. Asian J. Org. Chem. 2015, 4, 836 (Focus Review).

Bromide Photo-oxidation Sensitized to Visible Light in Consecutive Ion Pairs.

The titration of bromide into a [Ru(deeb)(bpz)2]2+ (Ru2+, deeb = 4,4'-diethylester-2,2'-bipyridine; bpz = 2,2'-bipyrazine) dichloromethane solution led to the formation of two consecutive ion-paired species, [Ru2+, Br-]+ and [Ru2+, 2Br-], each with distinct photophysical and electron-transfer properties. Formation of the first ion pair was stoichiometric, Keq1 > 106 M-1, and the second ion-pair equilibrium was estimated to be Keq2 = (2.4 ± 0.4) × 105 M-1. The 1H NMR spectra recorded in deuterated dichloromethane indicated the presence of contact ion pairs and provided insights into their structures and were complimented by density functional theory calculations. Static quenching of the [Ru(deeb)(bpz)2]2+* photoluminescence intensity (PLI) by bromide was observed, and [Ru2+, Br-]+* was found to be nonluminescent, τ < 10 ns. Further addition of bromide resulted in partial recovery of the PLI, and [Ru2+, 2Br-]* was found to be luminescent with an excited-state lifetime of τ = 65 ± 5 ns. Electron-transfer products were identified as the reduced complex, [Ru(deeb)(bpz)2]+, and dibromide, Br2•-. The bromine atom, Br•, was determined to be the primary excited-state electron-transfer product and was an intermediate in Br2•- formation, Br• + Br- → Br2•-, with a second-order rate constant, k = (5.4 ± 1) × 108 M-1 s-1. The unusual enhancement in PLI for [Ru2+, 2Br-]* relative to [Ru2+, Br-]+* was due to a less favorable Gibbs free energy change for electron transfer that resulted in a smaller rate constant, ket = (1.5 ± 0.2) × 107 s-1, in the second ion pair. Natural atomic charge analysis provided estimates of the Coulombic work terms associated with ion pairing, ΔGw, that were directly correlated with the measured change in rate constants.

Naphthoylenebenzimidazolone dyes as one‐component photoinitiators

The appropriate substituents have been added to a 7H‐benzimidazo[2,1‐a]benz[de]isoquinolin‐7‐one skeleton in order to obtain a one‐component initiator system. The absorption, fluorescence, and electrochemical properties of these naphthoylenebenzimidazolone dyes and their usefulness as visible light initiators of free radical polymerisation of trimethylolpropane triacrylate were examined. The initiation efficiency of these dyes was compared with that of the parent 7H‐benzimidazo[2,1‐a]benz[de]isoquinolin‐7‐one in the presence of a coinitiator. Results show that these bifunctional dyes are able to initiate polymerisation without an additional coinitiator. Photopolymerisation studies and the relationship between the rate of polymerisation and the free energy change for electron transfer revealed that, depending on the concentration, initiating radicals are generated via intra‐ and intermolecular electron transfer.

Effect of donor and acceptor properties of solvents on the kinetics of photoreduction of sterically hindered о-benzoquinones

Effect of the solvent nature on the kinetics of photoreduction of 3,6-di-tert-butyl-1,2-benzoquinone and its six derivatives in the presence of N,N-dimethylaniline and 4-(N,N-dimethylamino)benzaldehyde has been investigated. It has been found that for the о-quinone—amine pair, for which the free energy change of electron transfer is ΔGe > +0.11 eV, the rate constant of о-quinone photoreduction kH decreases proportionally to the increase in the acceptor number of the solvent. For the о-quinone—amine pair with ΔGe < +0.11 eV, the kH value decreases proportionally to the increase in the donor number of the solvent. It has been established that the enhancement of the electron-acceptor properties of the solvent leads to the emergence of kinetic isotope effect for the reactant pairs of 3,6-di-tert-butyl-1,2-benzoquinone and 4,5-dimethoxy-3,6-di-tert-butyl-1,2-benzoquinone with N,N-di-methylaniline (ΔGe = +0.11 and +0.22 eV, respectively).

Quenching of diphenylmethyl radical fluorescence by cyanoaromatics and phenols

We have used double-laser nanosecond laser flash photolysis to study the kinetics of bimolecular quenching of the lowest doublet excited state of the transient diphenylmethyl radical (2Ph2CH·) in acetonitrile at 25 °C. We chose a series of cyanoaromatic compounds as acceptor-type quenchers and a series of 4-substituted phenols as donor-type quenchers. The observed bimolecular quenching rate constants (k Q) are in the range 2 × 106–2 × 1010 M −1 s−1 for cyanoaromatics and 1 × 105–3 × 109 M −1 s−1 for phenols. The Hammett plots of the k Q values for these two series have positive and negative slopes, respectively, and establish the enhanced behavior of the photo-excited diphenylmethyl radical as both a donor and an acceptor. For cyanoaromatics, the trend in the dependence of 2Ph2CH·* quenching rate constants on the free energy change of electron transfer (ΔG el) coincides well with that of Rehm–Weller data on acceptor-type and donor-type quenchers of singlet excited states. An approximate fit of the k Q data for cyanoaromatics to the Agmon–Levine model of electron transfer gave a value of 4.5 kcal mol−1 for the free energy of activation at ΔG el = 0. For phenols, the trend in the dependence of k Q on the free energy change of electron transfer deviates significantly from that of cyanoaromatics data. This deviation was traced to the use of irreversible oxidation potentials of phenols to calculate ΔG el. No isotope effect is observed from substitution of phenolic hydrogen by deuterium, i.e., k Q,H/k Q,D ≈ 1. Phenolate ions quench 2Ph2CH·* with large rate constants, 1–2 × 1010 M −1 s−1, i.e., well in the limit of diffusion control.

6‐Pyridinium benzo[a]phenazine‐5‐oxide derivatives as visible photosensitisers for polymerisation

Several 6‐pyridinium benzo[a]phenazine‐5‐oxide derivatives have been synthesised and characterised by proton nuclear magnetic resonance spectroscopy and mass spectrometry. The spectroscopic and electrochemical properties of these dyes were examined. The dyes were used as reducible sensitisers for selected electron donors (phenylthioacetic acid, phenoxyacetic acid, N‐phenylglycine, and ethyl 4‐N,N‐dimethylaminobenzoate) and as oxidisable sensitisers for electron acceptors (onium and N‐alkoxypyridinium salts). These photoredox pairs were found to be effective visible‐wavelength photoinitiators for the free radical polymerisation of trimethylolpropane triacrylate under visible light. The cationic photopolymerisation of cyclohexene oxide by the studied dyes and the onium salt photoredox pairs was ineffective. The obtained results are discussed on the basis of both free energy change for electron transfer to or from the benzo[a]phenazine dyes and the photochemical properties of the dyes, particularly their photobleaching. The proposed mechanism of dye fading is supported by density functional theory calculations and spectroscopic characterisation of the radical cation of the dye.

Factors affecting the TMPTA radical polymerization photoinitiated by phenyltrialkylborates paired with tri-cationic hemicyanine dye. Kinetic studies

This article introduces tri-cationic hemicyanine dye employed as a visible-light photoinitiator of acrylic monomers polymerization. This dye, in combination with borate anions, was found to be a very effective photoinitiating system. The kinetics of trimethylolpropane triacrylate polymerization was studied by a microcalorimetric method. The photoredox pair concentration, the co-initiator structure as well as the light intensity strongly affected the progress of the polymerization, leading, for example, to an increase in the polymerization rate and quantum yield of the process. The efficiency of these photoinitiators was discussed on the basis of the free energy change for electron transfer from a borate anion to an excited hemicyanine dye cation. The ∆G el values were estimated for photoredox pairs containing a series of phenyltrialkylborate anions and one styrylpyridinium dye cation. The relationship between the rate of polymerization and the free energy of activation for electron transfer reaction gives the dependence predicted by the classical theory of electron transfer. The photoinitiating abilities of the selected novel photoredox pairs (BPB61, BPB7, BPB8, and BPB9) are comparable with the photoinitiating efficiency of commercially available photoinitiators.

N-substituted quinoxalinobenzothiazine/iodonium salt systems as visible photoinitiators for hybrid polymerization

Abstract Sensitizers based on the 12-substituted 12H-quinoxalino-[2,3-b][1,4]-benzothiazine skeleton were synthesized and characterized using 1H NMR spectroscopy, mass spectrometry and elemental analysis. Their electrochemical and spectral properties, including absorption and emission spectra, fluorescence quantum yield and singlet lifetime were also measured. These benzothiazines combine with commercially available diphenyliodonium hexafluorophosphate to create novel visible initiator systems of free radical/cationic hybrid polymerization of glycidyl methacrylate. The efficiency of these initiator systems is discussed on the basis of the free energy change for electron transfer from the benzothiazines to the iodonium compound. During the photopolymerization photobleaching of the benzothiazines was observed; a mechanism to account for this bleaching is proposed.

On the Reaction of Lupulones, Hops β-Acids, with 1-Hydroxyethyl Radical

Lupulones, hops β-acids, are one of the main constituents of the hops resin and have an important contribution to the overall bacteriostatic activity of hops during beer brewing. The use of lupulones as natural alternatives to antibiotics is increasing in the food industry and also in bioethanol production. However, lupulones are easy oxidizable and have been shown to be very reactive toward 1-hydroxyethyl radical with apparent bimolecular rate constants close to diffusion control k = 2.9 × 10(8) and 2.6 × 10(8) L mol(-1) s(-1) at 25.0 ± 0.2 °C in ethanol-water solution (10% of ethanol (v/v)) as probed by EPR and ESI-IT-MS/MS spin-trapping competitive kinetics, respectively. The free energy change for an electron-transfer mechanism is ΔG° = 106 kJ/mol as calculated from the oxidation peak potential experimentally determined for lupulones (1.1 V vs NHE) by cyclic voltammetry and the reported reduction potential for 1-hydroxyethyl radical. The major reaction products identified by LC-ESI-IT-MS/MS and ultrahigh-resolution accurate mass spectrometry (orbitrap FT-MS) are hydroxylated lupulone derivatives and 1-hydroxyethyl radical adducts. The lack of pH dependence for the reaction rate constant, the calculated free energy change for electron transfer, and the main reaction products strongly suggest the prenyl side chains at the hops β-acids as the reaction centers rather than the β,β'-triketone moiety.

Diazobenzo[a]fluorene derivatives as visible photosensitizers for free radical polymerization

Several benzophenazine dyes containing a diazobenzo[a]fluorene moiety have been synthesized and characterized by 1H NMR spectroscopy and mass spectrometry (CI MS). The spectroscopic and electrochemical properties of these dyes were examined. These compounds were evaluated as potential light absorbing chromophores for free radical polymerization. The results are discussed on the basis of the free energy change for electron transfer from the diazobenzo[a]fluorene dyes to the electron donors/acceptors. The kinetic studies of the photoinitiated polymerization of trimethylolpropane triacrylate (TMPTA) using electron donors, such as phenylthioacetic acid, phenoxyacetic acid, N-phenylglycine and ethyl 4-N,N-dimethylaminobenzoate, and electron acceptors, such as 1-methoxy-4-phenylpyridinium tetrafluoroborate and 1-ethoxy-2-methylpyridinium hexafluorophosphate, have shown that these dyes are efficient photoinitiators for free radical polymerization in visible light. The heavy atoms present in the chemical structure may lead to excited triplet states within the dye facilitating electron transfer from these states.

Phenyltrialkylborates as co-initiators with cyanine dyes in visible light polymerization of acrylates

Photoredox pairs consisting of selenocarbocyanine dye cations and phenyltrialkylborate anions were employed as the novel, effective visible-wavelength initiators of the radical polymerization of acrylic monomer. The influence of the sensitizers and electron donor structure on the photopolymerization kinetics of multiacrylate monomer was investigated by photo-DSC. It was found that the polymerization rate and the final conversion degree were dependent on both dye and borate structure. The kinetic studies of the free radical polymerization revealed an increase in the polymerization rate with a decrease of the borate oxidation potentials which was additionally reflected by the linear relationship between the Hammett constant and rate of polymerization. The efficiency of these initiators was discussed on the basis of the free energy change for electron transfer from an excited cyanine dye cation to a borate anion. The Δ G el values were estimated for photoredox pairs containing a series of phenyltrialkylborate anions and one selenocarbocyanine dye cation. The relationship between the rate of polymerization and the free energy of activation for electron transfer reaction gives the dependence predicted by the classical theory of electron transfer. The photoreduction of cyanine phenyltrialkylborate complex was studied using nanosecond laser flash photolysis. The dye triplet was found to be quenched by the electron donors via an electron transfer process. Rate constants ( k q ) for the quenching of the excited states were high and approached diffusion-controlled limits and were found to depend on the borate structure.

Geminate Charge Recombination in Polymer/Fullerene Bulk Heterojunction Films and Implications for Solar Cell Function

We have studied the influence of three different fullerene derivatives on the charge generation and recombination dynamics of polymer/fullerene bulk heterojunction (BHJ) solar cell blends. Charge generation in APFO3/[70]PCBM and APFO3/[60]PCBM is very similar and somewhat slower than charge generation in APFO3/[70]BTPF. This difference qualitatively matches the trend in free energy change of electron transfer estimated from the LUMO energies of the polymer and fullerene derivatives. The first order (geminate) charge recombination rate is significantly different for the three fullerene derivatives studied and increases in the order APFO3/[70]PCBM < APFO3/[60]PCBM < APFO3/[70]BTPF. The variation in electron transfer rate cannot be explained from the LUMO energies of the fullerene derivatives and single-step electron transfer in the Marcus inverted region and simple considerations of expected trends for the reorganization energy and free energy change. Instead we suggest that geminate charge recombination occurs from a state where electrons and holes have separated to different distances in the various materials because of an initially high charge mobility, different for different materials. In a BHJ thin film this charge separation distance is not sufficient to overcome the electrostatic attraction between electrons and holes and geminate recombination occurs on the nanosecond to hundreds of nanoseconds time scale. In a BHJ solar cell, we suggest that the internal electric field in combination with polarization effects and the dynamic nature of polarons are key features to overcome electron-hole interactions to form free extractable charges.

Fluoflavin dyes as electron transfer photosensitizers for onium salt induced cationic photopolymerization

Abstract Photoredox pairs consisting of fluoflavin dyes and commercially available diphenyliodonium salts and triarylsulfonium salts are effective visible wavelength initiators of the cationic polymerization of cyclohexene oxide. The efficiency of these initiator systems is discussed on the basis of the free energy change for electron transfer from fluoflavin dyes to onium compounds. Photosensitization occurs through electron transfer, which is confirmed by the observation of a radical cation of the fluoflavin dyes. The singlet state of the dyes is involved in the photosensitization of triarylsulfonium salts, whereas both the singlet and triplet states of the dyes are involved in the photosensitization of iodonium compounds. The experimental results show that the photoinitiating ability of the tested photoredox pairs strongly depends upon the rate of electron transfer, which is reflected in the rate of radical cation formation. This determines the subsequent release of protic acid, a crucial component for the application of these systems as cationic initiators.

12H-Quinoxalino[2,3-b][1,4]benzothiazine derivatives as novel visible photosensitizers in cationic photopolymerization

Photoredox pairs consisting of dyes based on a 12H-quinoxalino[2,3-b][1,4]benzothiazine skeleton and iodonium salt are found to be effective initiation systems for cationic polymerization of epoxide monomers and tetrahydrofuran using VIS light. The efficiency of these initiator systems is discussed on the basis of the free energy change for electron transfer from benzothiazine dyes to the iodonium compounds. Three products, phenyl derivatives of dye 1 were isolated after photolysis of 1/onium salt system. These compounds were characterising by chemical ionization mass spectra. The reactions created the phenylated analogues of dye 1 and protons were proposed.

Synthesis and photochemical reaction of novel, visible-wavelength oxidizable polymerization sensitizer based on the 12H-quinoxalino[2,3-b][1,4]benzothiazine skeleton

Novel dyes based on the 12H-quinoxalino[2,3-b] [1,4]benzothiazine skeleton were synthesized and subsequently characterized using 1H NMR. Their electrochemical and spectral properties, such as absorption and emission spectra, quantum yield of fluorescence, and quantum yield of singlet oxygen formation, were measured. These compounds were evaluated as sensitizers for alkoxypyridinium salt photodecomposition, and the results are discussed on the basis of the free energy change for electron transfer from benzothiazine dyes to alkoxypyridinium compounds. Benzothiazine dyes are useful oxidizable sensitizers for N-alkoxypyridinium photoinitiators. The mechanism of the dye photobleaching is supported by time-dependent density functional theory (TD-DFT) calculations and the quantum yields of sensitized proton formation. Photoredox pairs consisting of benzothiazine dyes and alkoxypyridinium salt were found to be effective initiation systems for free radical polymerization of methyl acrylate and trimethylolpropane triacrylate (TMPTA) using visible light.