Efficient Synthesis and Electropolymerization of Meso-Carbazole and Meso-Pyrene BODIPY Dyes for Optoelectronic Applications

Significance 

Boron dipyrromethene (BODIPY) dyes exhibit high molar absorptivities and near-unit quantum yields which makes them highly desirable in bioimaging, phototherapy, and dye-sensitized solar cells (DSSCs) applications. They also possess versatile structure which allows extensive post-functionalization and enable the fine-tuning of their spectroscopic properties which expand their use. Despite their promising attributes, several challenges remain in the wide application of BODIPY dyes including stability of these dyes when incorporated into devices or used in harsh environments. For instance, the formation of stable films on substrates such as indium tin oxide (ITO) glass slides is important for applications in optoelectronics and DSSCs, however, the process of electropolymerization, which can be used to form such films often leads to the degradation of the BODIPY core when using dyes with extended conjugation and compromises the photophysical properties of BODIPY dyes. To this end, new study published in the Journal Molecules led by Professor Shawn Swavey and graduate student Alex Wright from the Department of Chemistry at University of Dayton synthesized a new series of meso-carbazole and meso-pyrene BODIPY dyes using a simplified, two-step method that avoided the need for catalysts and oxidizing agents with enhanced stability and integrity of the BODIPY core during film formation on ITO glass slides.

The researchers began by synthesizing a series of meso-carbazole and meso-pyrene BODIPY dyes using a mechanochemical technique for BDP1 and a solvent-free reaction method for BDP2 and BDP3. For BDP1, they combined commercially available N-(4-formylphenyl) carbazole and 2,4-dimethyl pyrrole in the presence of trifluoroacetic acid (TFA) and p-chloranil as an oxidizing agent and then triethylamine and boron trifluoride etherate were added with a BDP1 yield of a 28% after chromatography. They also synthesized BDP2 and BDP3 by dissolving naphthyl or fluoranthene pyrrole with N-(4-formylphenyl) carbazole in chloroform, which resulted in yields of 16.5% and 10.0%, respectively. Afterward, the authors characterized the synthesized dyes using UV/vis spectroscopy and demonstrated strong absorption peaks in the visible region. Next, the researchers investigated the electrochemical properties of the BODIPY dyes using cyclic voltammetry which showed intense oxidation waves coupled with weaker reduction waves and BDP2 and BDP3 displayed additional reduction waves indicative of their more extensive conjugation.  Moreover, upon repeated anodic scanning, the synthesized dyes formed films on glassy carbon electrodes, attributed to the electropolymerization of the carbazole moieties. When these electrodes were cycled in pure supporting electrolyte solutions the redox couples observed which suggested adsorbed reversible electron transfer. Furthermore, the researchers conducted bulk electrolysis of millimolar solutions of BDP1, BDP2, and BDP3 containing TBAPF6 as a supporting electrolyte to evaluate the ability of the dyes to form stable films on ITO glass slides and observed the buildup of BDP films on ITO slides. According to the authors, UV/vis spectroscopy confirmed the formation of BDP films and BDP1 maintained its BODIPY-core absorption characteristics post-electropolymerization. However, BDP2 and BDP3 lost their distinct absorption peaks, possibly due to the oxidative decomposition of the BODIPY structure during film formation. The researchers also studied the pyrene-substituted isoquinol BDP dye (BDP4) to evaluate its film formation on ITO slides. BDP4 incorporated highly conjugated pyrene moieties which can form conductive films upon anodic oxidation. The cyclic voltammetry of BDP4 indicated off-scale current in the anodic direction, and subsequent electrolysis confirmed the film formation on ITO slides. The UV/vis spectrum of the BDP4-coated ITO slide retained the characteristic absorption bands of the dye and demonstrated the potential for maintaining the BODIPY structure during electropolymerization.

In conclusion, Swavey and Wright developed a simplified, catalyst-free, and oxidant-free synthetic method for producing meso-carbazole and meso-pyrene BODIPY dyes. The new approach enhanced the efficiency of the synthesis process and reduced the environmental impact and managed to avoid using hazardous chemicals. The successful formation of stable films with BDP1 and the promising results with the pyrene-substituted BDP4 demonstrate the potential in synthesis of advanced materials that retain desirable photophysical properties. These materials could be critical in developing new optoelectronic devices such as organic light-emitting diodes and organic photovoltaic cells. Moreover, the new BODIPY dyes have shown promise in DSSCs due to their high molar absorptivities and robust photostability and the authors findings on the stability of these dyes during film formation suggest that with further optimization, BODIPY dyes could significantly improve the efficiency and longevity of DSSCs which makes solar energy more viable and cost-effective. Additionally, the new method can provide a foundation for synthesis new dyes with enhanced stability and tailored photophysical properties which will ultimately lead to more effective diagnostic and therapeutic applications.

About the author

Shawn Swavey joined the Department of Chemistry at the University of Dayton in 2002. His research interests include photodynamic therapy, developing small fluorescent molecules for bioimaging, electrocatalysis, and solar energy conversion. With support from Propel Dayton, the entrepreneurial program at the University of Dayton, he is cofounder of a small biotech company, Biodye, focusing on small fluorescent organic and inorganic molecules for a wide variety of applications.

About the author

Alexa Wright is an honors student in the Department of Chemistry at the University of Dayton with interests in sustainability, energy conversion, and biochemistry.

Reference

Swavey S, Wright A. Electropolymerization on ITO-Coated Glass Slides of a Series of π-Extended BODIPY Dyes with Redox-Active Meso-Substituents. Molecules. 2023 Dec 15;28(24):8101. doi: 10.3390/molecules28248101.

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