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Substituent Characterization of Series B Pyrylium Photoredox Catalysts

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Pyrylium compounds are important photoredox catalysts that can be utilized in a variety of applications due to their easily modified structure. Photoredox catalysts undergo single electron transfer with organic substrates to convert light energy to chemical energy. The dyes have one pyrylium group and can either become cyanine dyes or squaraines depending on the reacting amines. The main advantages of pyrylium dyes are their high absorption and fluorescence induction to remove background colors from other dyes (3). In turn, proteins with pyrylium dyes keep their native charges and isoelectric points during electrophoresis more efficiently than other dyes (1). For this reason, pyrylium dyes are most effective for protein labeling such as in pre-stain capillary electrophoresis and SDS-protein electrophoresis (2).

Pyrylium derivatives can be synthesized to investigate the possibility of finding more photoredox catalysts that are more sustainable than other common photocatalysts with iridium and ruthenium bases and are still effective oxidizing agents (3). Further compilation of quantitative data on these compounds will allow for identification of structural properties that can be used for specific functions due to the varying substituents causing changes in the oxidation activity. The excited state reduction potential (E*½) data is what shows the different oxidizing activity of these pyryliums. The more negative reduction potentials indicate that the pyrylium is more easily oxidized, and therefore is a more strongly reducing compound. Using this the overall state potential is found from a reference value against the saturated calomel electrode (SCE) effectiveness of the pyrylium dye can be found (4). This research focuses on analyzing possible differences in oxidizing activity based on electronic properties of different substituents on pyrylium molecules.

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Chalcone was synthesized using 4- fluoroacetophenone and 4’-methylbenzaldehyde to synthesize pyrylium B2. The 1H NMR spectrums of the synthesized chalcone and pyrylium both closely match that of the literature spectra, which indicates they contained the correct substituents (5).

The maximum absorbance wavelength was 460.1 nm and the absorbance value at that wavelength was -0.014. The maximum emission wavelength was 472.9nm and the fluorescence value at that wavelength was 0.026. Both the wavelengths of absorbance and emission were obtained in order to assess the amount of energy required to excite an electron in each compound. Both of these values have a relative increasing trend across the series as a result of different substituent characteristics. According to the data, the substituents become more electron rich and electron donating I the order of B2, B3, B1. Compound B2 absorbed light at the highest wavelength while compound B1 absorbed light at the lowest wavelength. Fluorine is an electron withdrawing group, so it should have a higher wavelength of maximum absorbance. It makes the compound electron poor, which means the HOMO is lowered and the LUMO is raised. Therefore, less energy is required to excite the electrons and a larger wavelength of light would be absorbed as a result. The opposite trend seen in this experiment could result from changes in steric properties (2). The phenyl groups attached to the center ring can twist in orientation, which changes molar absorptivity values and absorbance patterns (2). The maximum emission wavelength was also obtained as a measure of excitation because electrons will only produce a fluorescent signal when going from the LUMO to the ground state. Absorbance values can register for all levels of excited states rather than the LUMO alone. The trends in absorbance values serve to confirm that B2 contained one altered substituent group while trends in emission values confirmed B3 had an altered substituent.

The ground state reduction potential (E1/2) values become less negative from B1, B3, B2. A more negative value indicates that the ground state is at a lower energy level. The excited state reduction potential (E*1/2) and excited state energy (E0,0) values vary only slightly in B1 and B2. However, B3’s data is lower in value. There seems to be a slight decrease in E0,0 and E*1/2 values across the series. The lack of a definitive trend for the E*1/2 values indicates that the electronic properties of the varied substituents have a negligible effect on the oxidizing activity of these pyryliums. The less negative trend seen in E1/2 values works alongside the decrease in E0,0 values in order to produce the trend for E*1/2. The compound with the most negative E1/2 value, B1, also had the highest excited energy state (E0,0). When adding these two values together for E*1/2 calculation, the resulting value closely matches that of the other two compounds due to this counteractive effect. This result means that the three pyryliums have similar oxidizing activity and can be used as equally effective photoredox catalysts for synthesis of complex compounds.

The percent yield for pyrylium was relatively high (87%). However, reasons yield would not be at 100% could be poor isolation of product as a result of the suction filtration technique used. As a result, the solution may not be completely purified when the filtration is completed. This type of error is systematic because it affects the final product in a single way. Other sources of error in this experiment include not producing the correct product and changing methods to produce the correct one. In the future, placing the varying substituents at different positions may reveal even greater changes in oxidizing activity for pyryliums. And while the chalcone intermediate can be produced a faster and more efficient way of producing the pyrylium is the 2:1 ratio of 4- fluoroacetophenone and 4’-methylbenzaldehyde in order to minimize the amount of steps and purifications and immediately end up with product.

The wavelengths of maximum absorbance and emission overall increased across the B pyrylium series as depicted in table 1. The E1/2 values generally became less negative from B1 to B3 as a result of differing electronic properties on the substituents. The differences in electron withdrawing or donating ability caused this shift in oxidizing power. The E*1/2 values had a very small trend between B1 and B2 with a larger change in B3, which indicates changing the substituents at the specific position shown in Table 1 have different but still slight effects on these values. Pyryliums with alterations in substituent properties at the position investigated provide a wide selection of molecules to begin the synthesis of more complex compounds. This finding allows researchers to utilize a wider variety of more accessible and sustainable molecules. Combining the data collected from this research with further testing will confirm the viability of using these pyrylium compounds as a safer and more efficient method of synthesizing other compounds. Future experiments could utilize different starting materials in order to investigate substituent effects at alternative positions on the pyrylium molecules. New synthetic routes should be explored in order to increase the yield of the pyryliums. These experiments may result in a pyrylium molecule with a higher reduction potential and increased oxidative ability than those analyzed for this research.

Experimental for Compound B2:

Chalcone Synthesis

To a flask 4-fluoroacetophenone (1.72 mL, 14.4 mmol), 4’-methoxybenzaldehyde (1.70 mL, 14.4 mmol), ethanol (15 mL), and a stir bar was added. Sodium hydroxide (4.5 mL, 4M) was added dropwise to the flask while stirring. The reaction ran for 25 minutes. The flask was put in an ice bath for 2 minutes. The product was isolated using suction filtration and rinsed with cold ethanol.

Pyrylium Synthesis

In a flask the chalcone (0.673 g, 2.8 mmol), 4-fluoroacetophenone (0.33mL, 2.7 mmol), and concentrated hydro sulfuric acid (0.25 mL) were heated at 100°C for 45 minutes. The flask was removed from heat and cooled to room temperature. Then placed into an ice bath for 5 minutes. Ethyl acetate (3 mL) was then added to help precipitate the product. Suction filtration was used to isolate the product.

Synthesis of more Pyrylium Salt

The wrong chalcone was originally made, therefore the correct pyrylium salt needed to be synthesized. To a flask 4-fluoroacetophenone (0.65 mL, 5.4 mmol), 4’-methylbenzaldehyde (0.32 mL, 2.7 mmol), and concentrated hydro sulfuric acid (0.25 mL) were heated at 100°C for 40 minutes. The flask was removed from heat and placed into an ice bath for 5 minutes. Ethyl acetate (3 mL) was then added to help precipitate the product. Suction filtration was used to isolate the product.

1H NMR Preparation

The synthesized chalcone (30mg) was dissolved in deuterated chloroform until a saturated solution was obtained. The solution was transferred to an NMR tube and analyzed using a NMReady 60 Nanalysis NMR at 60 MHz. A second spectrum was obtained for pyrylium. The synthesized pyrylium (30 mg) was dissolved in DMSO. It was analyzed in the same instrument.

UV-Vis and Fluorescence Preparation

A stock solution was prepared by dissolving pyrylium (5 mg, 0.01 mmol) in acetonitrile (10 mL). A 16 μM dilution sample was prepared by adding stock solution (146 μL) to acetonitrile (10 mL). A fluorescence value was obtained from a Vernier ultraviolet-visible spectrophotometer using this dilution sample. The low concentration of the sample inhibited obtainment of an absorbance value. A new 0.547 mM sample was prepared by adding diluted sample (5 mL) to acetonitrile (5 mL). Absorbance and emission values was obtained using this new sample.

CV Preparation

Pyrylium (35 mg, 0.08 mmol) was dissolved in 0.1 M electrolyte tetrabutylammonium hexafluorophosphate solution (10 mL). The solution was analyzed by Pine Wavenow cyclic voltammetry using a Potentiostat analog. The ground state reduction potential was obtained using the resulting voltammogram. Resources 1. Romero, N. A.; Nicewicz, D. A. Chemical Reviews 2016, 116(17), 10075–10166. 2. Perkowski, A. J.; Cruz, C. L.; Nicewicz, D. A. Journal of the American Chemical Society 2015, 137(50), 15684–15687. 3. Joshi-Pangu, A.; Levesque, F.; Roth, H. G.; Oliver, S. F.; Campeau, L.-C.; Nicewicz, D.; DiRocco, D. A. Acridinium-Based Photocatalysts: A Sustainable Option in Photoredox Catalysis. J. Org. Chem 2016. 4. Riener, M.; Nicewicz, D. A. ChemInform 2013, 44(47). 5. Spectral Database for Organic Compounds; National Institute of Advanced Industrial Science and Technology, Japan.

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