Synthesis and Properties of Styrylpyridinium and Styrylquinolinium Derivatives with Various Anions
Authors: Tanaka Yuka , Yamakado Ryohei , Okada Shuji 
Published: February 2020

Introduction: In conventional organic solar cells, electron excitation of a donor by light absorption and electron transfer to an acceptor occur stepwise. During this two-step process, energy is often lost via photophysical and thermal manners. On the other hand, direct intermolecular electron excitation from the donor to the acceptor may solve this problem, and high open circuit voltage may be realized. In order to prepare such systems, two molecular species must be close each other even in the ground state, and the electrostatic force between a cation and an anion can be used. Many of ionic compounds are 1:1 or 1:2 complexes of cations and anions, and their crystal structures can be controlled to show semiconducting properties appropriate for photovoltaics by proper ionic design and combination. In this study, we synthesized ionic compounds combining acceptor cations (substituted styrylpyridinium and styrylquinolinium) and donor anions (p-toluenesulfonate, sulfanilate, 1,3-bis(dicyanomethylene)indanide and TCNQ radical anion), and their properties were examined.

Experimental: Styrylpyridinium or styrylquinolinium derivatives substituted by nitro, cyano or dimethylamino group were used as cations, and their iodide salts were prepared. p-Toluenesulfonate, sulfanilate, 1,3-bis(dicyanomethylene)indanide and TCNQ radical anion were selected as anions, and silver salts of toluenesulfonate and sulfanilate, sodium salt of indanide and lithium salt of TCNQ were also prepared. The solutions of cationic and anionic species were mixed with 1:1 molar ratio in methanol. After a few minutes of stirring at room temperature, microcrystalline solids appeared. The solids were filtered off and dried in vacuum. Thus, we successfully synthesized various ionic compounds with high yield as shown in Figure 1 (a). Formation of the 1:1 complexes was mainly confirmed by 1H-NMR. Other characterizations of the compounds were performed by UV-vis-NIR spectra in the solution states, Vis-NIR diffuse reflection spectra of the crystalline solids, electron spin resonance (ESR) spectra, and single-crystal X-ray diffraction. All single crystals used for X-ray crystallographic analysis were obtained by the vapor diffusion method, in which acetone or methanol was used as good solvent and hexane was used as poor solvent.

Results and discussion: Vis-NIR diffuse reflectance spectroscopy was used to analyze the powdered samples, which were supported on filter paper. The spectra of most of the compounds were able to be explained as summation of the spectra of the starting compounds, i.e., iodide of the corresponding cationic species and metal salt of the corresponding anionic species. However, in some spectra, new absorption bands appeared. In Figure 1 (b), the vis-NIR spectrum of cyano-substituted styrylpyridinium TCNQ radical anion (PC-TCNQ, red line) is shown as an example. A broad absorption band appeared between 800 and 2000 nm, which was not observed for the spectra of cyano-substituted styrylpyridinium iodide salt (PCI, blue line) and TCNQ lithium salt (Li-TCNQ, green line). This broad band is originated from charge transfer in the complex. From the Tauc plots, the optical band gap of PN-TCNQ was calculated to be 0.70 eV. This value is within the typical range of semiconductors. Single-crystal X-ray crystallographic analysis of PN-TCNQ revealed that it had a monoclinic unit cell structure with a = 25.405(8), b = 13.170(4), and c = 13.347(4) Å, and β = 99.062(7)°. As shown in Figure 1 (c), cyano-substituted styrylpyridinium cations are stacked with overlap of the pyridinium and phenyl rings to form a one-dimensional array along c-axis. And dimeric TCNQ radical anions are also stacked along c-axis. The cationic columns and the anionic columns are alternately arranged along a-axis as well as b-axis. Although π-π interaction between cationic and anionic species was not observed in this crystal structure, charge transfer between these species was confirmed by the vis-NIR diffuse reflectance spectra as mentioned above. Structures and properties of this complex as well as those of other compounds prepared will be presented in detail.

Figure 1