Search

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 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.

Because of its unique electronic properties, o-carborane has attracted attention as a scaffold for constructing solid-state luminescent materials. Here, we report stimulus-responsive luminochromic materials based on polycyclic aromatic hydrocarbon (PAH), such as anthracene and pyrene, modified with the ethynyl-o-carborane unit. Initially, luminochromism originating from switching of different emission mechanisms is described between intramolecular charge transfer (ICT) emission and excimer emission triggered by mechanical stimuli, heat, and vapor annealing regarding the single o-carborane-substituted anthracene. Next, the luminescence properties of two ethynyl-o-carboranes at both ends of PAHs are presented. In particular, multi-step mechanochromic luminescence of the pyrene derivative is explained. Stepwisely changes triggered by weak stimuli that can induce cracking sites, followed by luminochromic behavior and by grinding treatment, which causes phase transition from crystal to amorphous, leading to luminescent mechanism changes from ICT emission to excimer emission.

It is still challenging to realize a dual-emission system, in which two luminescent bands simultaneously appear by photoexcitation, in solid with organic dyes due to the difficulty in regulation of electronic properties in the excited state and concentration quenching. o-Carborane is known to be a versatile platform for constructing solid-state emitters since the sphere boron cluster is favorable for suppressing intermolecular interactions and subsequently concentration quenching. Here, we show solid-state dual-emissive o-carborane derivatives. We prepared 4 types of o-carborane derivatives and found dual-emission behaviors both in solution and solid states. By regulating the rotation at the o-carborane unit with the intramolecular C[cage]H⋅⋅⋅O interaction, the dual-emission intensity ratios were changed. Finally, it was demonstrated that the overall photoluminescence spectra can be estimated using the binding energy of intramolecular interactions.

We designed the triad molecule, bis-o-carborane-substituted bis(thienylethynyl)benzene, as a filler for realizing thermochromic luminescent behaviors based on conventional polymer films, such as polystyrene. From the optical measurements, it was found that the triad can show solid-state emission and dual-luminescent properties with variable intensity ratios depending on media. From the mechanistic studies including the experiments with the methyl-substituted model compound, it was revealed that dual emission should be originated from the locally excited and twisted intramolecular charge transfer states, and the latter emission band is significantly enhanced in the solid states. We prepared amorphous films containing variable concentrations of the triad with the spin-coating method and investigated optical properties. It was found that intensity ratios were drastically changed by altering the concentration of the triad. By increasing the proportion of the triad, aggregation occurred, and emission color was apparently varied through the changes in intensity ratios of the dual emission property. Based on the aggregation-induced luminochromic property of the triad, thermochromic luminescence was finally realized by heating the amorphous films. The rational design for obtaining thermochromic luminescent amorphous films is illustrated in this paper.

o ‐Carborane, a cluster compound containing boron and adjacent carbon atoms, displays intriguing luminescent properties. Recently, compounds containing o ‐carborane units were found to show suppressed aggregation‐induced quenching and intense solid‐state emission; they also show potential for the development of stimuli‐responsive luminochromic materials. In this Minireview, we introduce three kinds of fundamental photochemical properties: aggregation‐induced emission, twisted intramolecular charge transfer in crystals, and environment‐sensitive excimer formation in solids. Based on these properties, several types of luminochromism, such as thermos‐, vapo‐, and mechanochromism, have been discovered. Based mainly on results from recent studies, we illustrate these mechanisms as well as unique luminescent behaviors of o ‐carborane derivatives.

The time‐dependent emission enhancement (TDEE) phenomena of the 1‐(o‐carboran‐1‐yl)ethynylpyrene dyad were reported. It was found that the emission intensity from the dyad increased in tetrahydrofuran (THF), acetone and dichloromethane with increasing incubation time. From the mechanistic studies, it was suggested that agglomeration of the dyad gradually proceeded in these media, followed by expression of excimer luminescence. Additionally, it was shown that the rates of TDEE of the dyad were sensitively accelerated in the presence of a trace amount of water. Based on this fact, a detection system for water contents in acetone was constructed. Before and after incubation for 96 h at room temperature, time courses of changes in optical properties were monitored. Finally, water contents in acetone can be estimated by the degrees of TDEE and emission color changes in the range from 0.1 wt % to 2.0 wt % and from 2.0 wt % to 20 wt %, respectively.