Synthesis and characterization of some 2,4,6-triarylpyri̇di̇ne derivatives for dye-sensitized solar cells

While the global population has been increasing rapidly recently, the production of the needed energy has become a big problem. The increase in migration from rural areas to technologically developed places has increased the energy demand and electrical energy load. Considering these problems, safe, environmentally friendly, and efficient energy sources are needed more than ever for a sustainable and healthy society. A large portion of the energy used worldwide is provided by fossil fuels such as coal, oil, and natural gas. However, it is known that the reserves of these fuels are limited and cause global warming and climate change by creating greenhouse gases when used. Worldwide energy scarcity, environmental concerns, and the increasing depletion of resources have made it very urgent to seek renewable and sustainable energy solutions for a safe and healthy future. Solar energy, one of the renewable energy sources, is the most abundant natural resource. Sunlight can be directly converted into electricity using photovoltaic (PV) devices. Today, the photovoltaics market is dominated by silicon-based solar panels. The difficult production processes of these panels, which have an energy conversion efficiency of approximately 26%, the use of toxic chemicals during the production phase, and quite expensive maintenance processes restrict their further commercial spread. Dye-sensitized solar cells (DSSCs), discovered by Gratzel and his group in 1991, have low production costs, environmental sensitivity, and an innumerable variety of dyes. These cells, which contain various advantages, have attracted the attention of scientists as a promising alternative to silicon-based solar cells. One of the most important components of DSSCs, which consist of five main parts, is the dye molecule used as a sensitizer. In these devices, the dye coated on the surface of semiconductor metal oxide (TiO2, etc.) converts photons into excited electrons and causes current to occur. The light-harvesting capacity of the dye molecule determines the upper limit of the electrical conversion efficiency of DSSC. In other words, the dye absorbs light and plays a direct role in the production and transfer of electrons. These reasons have made the structural design of dye molecules to be used in DSSCs very important. Metal-complex, natural, and metal-free organic dyes are generally used as sensitizers in DSSCs. Metal-free organic dyes, which have easily tunable molecular structures and high molar absorption coefficients (), attract much attention due to their low toxicity and relatively easy synthesis. These dyes generally have the donor-π-acceptor (D-π-A) structure. It is composed of an electron-rich electron-donating donor (D), a conjugated aromatic or heteroaromatic π-bridge that allows easy transport of electrons, and an electron-accepting group called the acceptor (A). Frequently used as donors are triphenylamine, diphenylamine, dialkylamine, phenothiazine or carbazole, etc. groups are preferred. Carboxylic acids or cyanoacrylic acid, which can form ester bonds with the semiconductor surface to increase electron conduction, are generally used as acceptors. The aromatic π-bridge, which is effective in intramolecular charge transfer and determines the absorption capacity of the molecule, is the main compound of metal-free organic dyes. The fact that donor and acceptor groups are relatively limited further increases the importance of the π-bridge to increase dye diversity and obtain new dyes. In order to provide high current from DSSCs, it is very important that the dye used receives as much sunlight as possible. Sensitizers are generally designed to contain thiophene units to absorb in a wide region and cover more area in the visible region. Due to their good charge carrier properties, the most efficient DSSCs mostly contain thiophene units. Because when thiophene moieties are incorporated into the structure, the planarity increases, the absorption spectrum expands and shifts to the red region, and cell performance improves even more. Pyridine is a six-membered heterocyclic compound consisting of five carbon atoms and one nitrogen atom. The arrangement of atoms is similar to the benzene molecule; the only difference is that one of the carbon-hydrogen ring groups is replaced by a nitrogen atom. Pyridine rings are found in the structure of many natural products, including vitamins such as niacin and B6, coenzymes such as nicotinamide adenine dinucleotide (NAD), and alkaloids such as trigonelline. The structure of many drugs and pesticides contains a pyridine moiety. There are many studies on the antimicrobial, antiviral, antioxidant, antidiabetic, anti-malarial, anti-inflammatory, psychopharmacological antagonist, and antiamoebic properties of pyridines, which have a critical role in medicinal chemistry. Thanks to their thermal stability and extended π-conjugated systems, 2,4,6-triaryl pyridines have attracted great attention as versatile starting materials for the preparation of fluorescent small molecules, polymers, supramolecules, and fluorescent sensors for ion detection. However, the importance of 2,4,6-triaryl pyridines is further enhanced by their excellent coordination abilities towards various metal centers, which enable the design of new catalysts and advanced device components (OLEDs and LEDs). With Kröhnke pyridine synthesis, used in the synthesis of 2,4,6-triaryl pyridine compounds, takes place between pyridinium methyl ketone salts, chalcone compounds, and ammonium acetate to obtain pyridine derivatives with good properties. When the literature is examined, there is no study in which 2,4,6-triaryl pyridine compounds, which have good fluorescence properties, are included as π bridges in dyes used as sensitizers in DSSCs. In metal-catalyzed coupling reactions in which a metal is used as a catalyst, two parts, the nucleophile and the electrophile, are combined. The 2010 Nobel Prize in Chemistry was awarded to Heck, Negishi, and Suzuki, who carried out carbon-carbon coupling reactions using palladium metal as a catalyst. Richard Heck, who discovered the coupling reactions of aryl halides and alkene compounds in the presence of palladium metal, pioneered the use of palladium in organic synthesis. Later, many coupling reactions using palladium were reported, in which new carbon-carbon and carbon-heteroatom bonds were formed. One of the most effective ways to create new carbon-carbon bonds using metal-catalyzed coupling reactions is the palladium-catalyzed Suzuki-Miyaura coupling reaction. In this study, used to be used in dye-sensitized solar cells two novel dyes with a D-π-A structure (DC6 and DC7) were synthesized and structurally characterized. These dyes contain ethoxy and dimethylamino groups as donors, a 2,4,6-triaryl pyridine group with a thiophene moiety as a π-bridge, and a cyanoacrylic acid or methylenemalononitrile group as acceptors. Commercially available 4-(dimethylamino)benzaldehyde, 4-ethoxyacetophenone and 2-acetylthiophene compounds were used as starting compounds and 3-(4-(dimethylamino)phenyl)-1-(4-ethoxyphenyl)prop-2-en-1-one (DC1), 1-(5-bromothiophen-2-yl)ethan-1-one (DC2), 1-(2-(5-bromothiophen-2-yl)-2-oxoethyl)pyridin-1-ium iodide (DC3), 4-(2-(5-bromothiophen-2-yl)-6-(4-ethoxyphenyl)pyridin-4-yl)-N,N-dimethylaniline (DC4), 4-(5-(4-(4-(dimethylamino) phenyl)-6-(4-ethoxyphenyl)pyridin-2-yl)thiophen-2-yl)benzaldehyde (DC5), 2-cyano-3-(4-(5-(4-(4-(dimethylamino)phenyl)-6-(4-ethoxyphenyl)pyridin-2-yl)thiophen-2-yl)phenyl)acrylic acid (DC6) and 2-(4-(5-(4-(4-(dimethylamino) phenyl)-6-(4-ethoxyphenyl)pyridin-2-yl)thiophen-2-yl)benzylidene) malononitrile (DC7) compounds were synthesized. While obtaining these compounds, chalcone preparation, bromination, pyridinium methyl ketone iodide salt preparation, Kröhnke pyridine synthesis, Suzuki-Miyaura coupling reaction, and Knoevenagel reactions were used. Using the 2,4,6-triaryl pyridine group as an π-bridge in sensitizers for the first time is important in terms of the study's originality. The characterization of all compounds, including the synthesized intermediates, was confirmed by taking and evaluating FT-IR, 1H NMR, 13C NMR and LC-MS spectra.