Development of Near-Infrared Emissive and Stimuli-Responsive π-Conjugated Materials Using Hypervalent Antimony Compounds

Recently, main-group elements have been introduced into π-conjugated scaffolds for functionalization. However, heavy main-group elements have not been studied much because of their difficult handling and high toxicity. Our group previously reported hypervalent compounds combined with heavy main-group elements and π-conjugated scaffolds to explore the use of the heavy elements. In our previous work on the hypervalent tin (Tanaka, K. et al. Chem. Eur. J. 2021, 27, 7561), the heavy main-group element has unique electronic contributions to π-conjugated scaffolds, like a narrower energy gap between HOMO and LUMO because of the three-center four-electron (3c-4e) bond. In addition, the hypervalent tin compounds were able to interact with Lewis bases such as DMSO by using the Lewis acidic hypervalent tin, and the coordination changed the absorption and emission colors. In other words, we have succeeded in reflecting the changes in the elements as the variations in the electronic state of the π-conjugated scaffolds. Therefore, further development of heavy main-group elements in hypervalent states is expected to find novel functionalities derived from the properties of the elements.
This research focuses on “Antimony (Sb)” in group 15, fifth period. Antimony has different oxidation numbers (+3 and +5), and various hypervalent antimony compounds have been reported. We newly synthesized hypervalent antimony compounds Sb and Sb5 with varying numbers of oxidation. Sb showed pseudo-trigonal bipyramidal geometry from their crystal structures, while Sb5 showed octahedral geometry. The absorption spectra of Sb and Sb5 implied significant differences in electronic states. According to density functional theory (DFT) calculation, these differences were caused by the electronic contribution of the hypervalent state. Furthermore, owing to the octahedral geometry, we revealed that Sb5 was able to avoid the aggregation-caused quenching (ACQ). P-Sb5, polymerized Sb5 with a bithiophene comonomer, showed high luminescence from the deep-red to the near-infrared region both in solution and film.
Next, focusing on the different optical properties of each oxidation number of the antimony, we tried to change their oxidation state by mechanochemical oxidation. In general, mechanochromism requires the crystallization of compounds, which makes it challenging to develop in materials. In addition, it is difficult to predict changes before and after mechanical stimulation, and new strategies are needed to design more designable mechanically stimuli-responsive materials. Therefore, we focused on mechanochemical reactions that do not require crystallization. Mechanochemical reactions can predict the optical properties of reactants and products, which can lead to high designability. Since trivalent antimony is easily oxidized to pentavalent antimony, we observed the change of the optical properties in response to mechanical stimuli by oxidation reaction in the solid state. As a result, we achieved different directions of the color changes with hypsochromic and bathochromic shifts based on the exact mechanism of the oxidation of the antimony. Furthermore, we were able to predict the color changes by quantum chemical calculation of the reactant and the product.
In summary, we synthesized novel hypervalent antimony compounds and developed unique materials derived from the properties of the hypervalent state of the heavy main-group elements. In the first topic, we succeeded in synthesizing near-infrared luminescent materials owing to the geometry of the elements. In the second topic, we have developed mechanically stimuli-responsive materials by utilizing the reactivity of the hypervalent antimony to change color in response to mechanical stimuli. The unique utilization of heavy main-group elements in hypervalent states, combined with π-conjugated scaffolds, demonstrates the potential to create a wide range of novel materials that have not been previously observed.