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Four monocarborane anion-based compounds connected to a pyrene moiety were synthesized by combining two monocarborane anion scaffolds and two counter cations. The effects of the cage structure and the counter cation on the optical properties were investigated from UV-vis absorption and photoluminescence (PL) measurements. The PL spectra in the solid state were unique for each compound, derived from the different shapes of the carborane cage and counter cation. These results suggest great potential for designing a monocarborane anion-based chromophore.

We synthesized new binuclear boron complexes based on pyrazine with ortho and para substitution patterns. It was demonstrated that the para-linked complexes possess a significantly narrow energy gap between highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO), leading to their far-red to near-infrared emission properties. Meanwhile, the ortho-substituted complex showed orange emission. Considering the HOMO and LUMO distributions of pyrazine, the boron complexation to the nitrogen atoms would stabilize its LUMO more efficiently than its HOMO because a nodal plane in the HOMO passes through the two nitrogen atoms. The theoretical study suggests that the para-substitution would not significantly perturb such a characteristic HOMO distribution originating from pyrazine in stark contrast to the ortho-substituted one. As a result, the HOMO-LUMO gap of the para-linked complex is dramatically narrower than that of the ortho-linked one.

We synthesized two types of the regioisomers fused by a phenylnaphthalene ring with variable connection points to the o-carborane scaffold. In this paper, we describe their photoluminescence (PL) properties and detailed photochemical mechanisms. According to the series of optical measurements, interestingly, they showed different PL characters in terms of wavelength and the dual-emission character despite that they have the common aromatic unit. Variable-temperature PL measurements and quantum chemical calculations suggested that the substitution position of aryl groups to o-carborane plays an important role in determining the energy barrier to the intramolecular charge-transfer (ICT) state at the S1 state. Finally, it is revealed that the relative position of the C–C bond of o-carborane and the aryl center should be responsible for the photophysical events of aryl-o-carboranes.

Element-blocks, the minimum heteroatom-containing functional units, are versatile building blocks for constructing functional polymers. In this focus review, we explain the new concept of developing stimuli-responsive luminochromic materials based on frustrated element-blocks. Herein, we termed “frustrated” element-blocks as those that show a relatively larger degree of structural relaxation in the excited state but are structurally restricted. As a result, frustrated element-blocks can have unique stimuli-responsive luminochromic behaviors. As a representative example, the stimuli-responsive properties of frustrated boron clusters are initially illustrated, and a series of their luminochromic properties, such as mechano- and thermochromism, are described. Next, we designed fused complexes for suppressing intramolecular motion to obtain frustrated boron complexes that display constant emission in both the solution phase and solid-state to produce mechanochromic luminescent materials. Finally, generating the frustrated state by polymerization is explained. These polymers display unique luminochromic behaviors, such as thermochromism in solution, mechanochromism in the solid-state and vapochromism. Finally, the stimuli-responsive luminochromic behaviors of frustrated element-block-containing polymers and their mechanisms are illustrated, mainly by examples from our recent works.