Luminescence functionalization by composite of TICT process of aryl-modified carborane anions and countercations Takumi Yanagihara and Kazuo Tanaka Kyoyo University, Japan Introduction In recent years, ortho -carborane ( o C ; the upper figure) has attracted a great deal of attention as a scaffolding framework for luminescent materials. [1] When aromatic compounds (Ar), such as pyrene, anthracene, are connected at the carbon atom in the carborane unit, intense emission can be observed from the twisted intramolecular charge transfer (TICT) in the solid state by o C working as an electron-accepting unit. By using this unique TICT emission, it is expected to be an excellent light-emitting functional material. By the way, the family of carboranes involves various structures other than o C with different number of vertices and carbon atoms. Utilization of its diverse skeletons may lead to further functionalization of light-emitting materials.In this research, we paid particular attention to nido -carborane anion ( n CA ), which is obtained by deboronation from o C with strong nucleophiles (the upper figure). By elucidating the photophysical process of Ar-substituted n CA ( Ar- n CA ), we aimed to develop further functionality of fluorescent materials using carborane. [2] Results and Discussion We synthesized Ar- n CA and succeeded in introducing five kinds of counter cations (NMe 4 + , NEt 4 + , NBu 4 + , PPh 4 + , and K + ).Photoluminescence spectra of Ar- n CA in various solvents (1.0×10 –5 M) showed two peaks: One is from locally excited (LE) emission around 400 nm, the other is from TICT emission around 500–550 nm. In the TICT process, π-plane rotates from perpendicular to parallel orientation along the C–C bond in the carborane unit in the excited state. At this process, n CA acts as an electron-donating unit. It is surprising that luminescence species changed by counter cations in nonpolar solvents (chloroform and toluene). When the counter cation was NBu 4 + , Ar- n CA showed TICT emission, whereas it was PPh 4 + , Ar- n CA showed LE emission. This drastic change should be stemmed from the difference in the interaction between Ar- n CA and counter cations. NBu 4 + should interact with Ar unit of Ar- n CA , while PPh 4 + might interact with the carborane unit. Therefore, in the case of PPh 4 + , the electron-donating ability might be weakened through charge cancellation, followed by the LE emission. This electrostatic interaction might be explained by the HSAB theory. The peculiar TICT process described above also occurs in the solid state (the lower-left figure). Ar- n CA is extremely useful as a stimuli-responsive material because the degree of rotation of the π-plane can be controlled according to the external environment of the molecule, such as temperature and crystallinity. When a pristine crystalline sample was ground until it became amorphous, the emission wavelength shifted to the longer wavelength region. The result of the variable temperature PL spectra indicates that the amorphization allowed the rotation of the π-plane. The ground sample can be annealed back to its original crystalline state. Furthermore, this unique stimuli-responsivity can be controlled by counter cations (the lower-right figure). From these results, we believe that Ar- n CA works as an excellent anion-based emitter.
References 1. Ochi, J.; Tanaka, K.; Chujo, Y. Angew. Chem. Int. Ed. 2020 , 59 , 9841–9855. 2. Yanagihara, T.; Tanaka, K. Adv. Opt. Mater. 2023 , 2300492.
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