Conjugated Molecules Based on Carboranes

　o-Carborane is an icosahedral compound containing 2 carbon atoms and 10 boron atoms, and is a unique molecule in which the organic component carbon and the inorganic component boron are combined at the “molecular level”. In recent years, it has become clear that π-conjugated molecules connected with o-carboranes exhibit interesting luminescent properties, and are attracting increasing attention as scaffolds for luminescent materials. We aim to elucidate the geometrical and electronic effects of carboranes on conjugated systems for creating novel optical materials.

Development of highly-efficient solid-state light-emitting materials and elucidation of photochemical processes

　Direct attachment of an aromatic ring to o-carborane results in luminescence resulting from charge transfer from the aromatic ring to the carborane unit. Using this luminescence, we have succeeded in obtaining a molecule that emits light with an efficiency of 99% or more in the solid state [1]. We are also working on the elucidation of the luminescence mechanism. Twisted-intramolecular charge transfer (TICT), in which carbon-carbon bonds and aromatic rings of o-carborane are oriented perpendicularly, emits light efficiently [2] and the specific elongation of carbon-carbon bonds before and after luminescence [3].

1) Naito, H.; Nishino, K.; Morisaki, Y.; Tanaka, K.; Chujo, Y. J. Mater. Chem. C. 2017, 5, 10047-10054.
2) Naito, H.; Nishino, K.; Morisaki, Y.; Tanaka, K.; Chujo, Y. Angew. Chem. Int. Ed. 2017, 56, 254-259.
3) Ochi, J.; Tanaka, K.; Chujo, Y. Dalton. Trans. 2021, 50, 1025-1033.

Emission color change in response to mechanical stimulation

　o-Carborane has a spherical skeleton and does not have strong intermolecular interaction sites. Therefore, o-carborane-modified conjugated molecules tend to form a loose packing structure in the solid state. Focusing on these characteristics unique to carborane, we are developing molecules that respond to mechanical stimuli such as rubbing and tapping by tightly packing the molecules and changing the emission color [4].

4) Mori, H.; Nishino, K.; Wada, K.; Morisaki, Y.; Tanaka, K.; Chujo, Y. Mater. Chem. Front. 2018, 2, 573-579.

Emission color change in response to temperature change

　Since the TICT behavior of o-carborane is related to the molecular motion of “bond rotation”, it is further promoted at high temperature. On the basis of this character, we have reported a molecule that can detect external temperature as a change in emission color [5 & 6]. Furthermore, by dispersing carborane in a polymer film, we have succeeded in developing a material that can record thermal history as a change in emission color [7].

5) Nishino, K.; Yamamoto, H.; Tanaka, K.; Chujo, Y. Org. Lett. 2016, 18, 4064-4067.
6) Nishino, K.; Tanaka, K.; Chujo, Y. Asian J. Org. Chem. 2019, 8, 2228-2232.
7) Wada, K.; Hashimoto, K.; Ochi, J.; Tanaka, K.; Chujo, Y. Aggregate 2021, 2, e93.

Solid-state light-emitting materials using unique intermolecular interactions

　The hydrogen atom located on the carbon of o-carborane has electron deficiency and is known to act as a hydrogen-bond donor. We found that by combining carborane and nitrogen rings, it is possible to control the overlapping of π-planes in the crystal by intermolecular interactions, and to design molecules with continuous thermochromic responses [8].

8) Ochi, J.; Tanaka, K.; Chujo, Y. Inorg. Chem. 2021, 60, 8990–8997.