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.

Poly­(p-phenylene vinylene) (PPV) is known to be a typical π-conjugated polymer and used as a commodity platform for constructing optoelectronic organic devices because of superior optical and material properties. PPV derivatives generally show a large degree of light absorption and intense emission in the visible region; meanwhile, very few examples have been reported to offer near-infrared (NIR) absorptive and emissive PPV derivatives. In this study, we designed and synthesized a novel PPV derivative, named BAz-PPV, containing boron-fused NN double bond units in the main chain. BAz-PPV showed intense NIR absorption and emission (λabs = 702 nm, λPL = 760 nm, and ΦPL = 2.0% in diluted toluene). This polymer had a narrow energy band gap because of not only extension of main chain π-conjugation over 50 monomer units but also stabilization of the energy level of the lowest unoccupied molecular orbital. Moreover, the polymer shows high stability toward photodegradation and sufficient carrier-transport ability for the applications in organic semiconducting devices. Advantages of the introduction of the NN double bond to main-chain conjugation followed by boron coordination for the development of NIR materials are demonstrated in this manuscript.

We report highly‐efficient and solid‐state excimer emission based on the acridine–o‐carborane dyad possessing the ethynyl spacer. The previous pyrene‐modified o‐carborane showed excimer emission only at 77 K in the crystalline state, meanwhile, the current acridine‐modified molecule presented excimer emission with high efficiency (ΦPL = 0.23) in the crystalline state at room temperature. From single‐crystal X‐ray crystallography, it was indicated that two acridine moieties were stacked and the third acridine molecule was out of alignment. Such a packing mode could restrict exciton splitting over the columnar packing structure. Moreover, molecular interactions through the nitrogen atom in the acridine moiety and the hydrogen atom in the o‐carborane unit contribute to suppression of molecular motions. As a consequence, an improved emission efficiency was obtained. This study demonstrates that the ethynyl–o‐carborane skeleton should work as the excimer‐inducible component in the solid state.

The dynamic and reversible changes of coordination numbers between five and six in solution and solid states, based on hypervalent tin(IV)-fused azobenzene (TAz) complexes, are reported. It was found that the TAz complexes showed deep-red emission owing to the hypervalent bond composed of an electron-donating three-center four-electron (3c-4e) bond and an electron-accepting nitrogen-tin (N-Sn) coordination. Furthermore, hypsochromic shifts in optical spectra were observed in Lewis basic solvents because of alteration of the coordination number from five to six. In particular, vapochromic luminescence was induced by attachment of dimethyl sulfoxide (DMSO) vapor to the coordination point at the tin atom accompanied with a crystal-crystal phase transition. Additionally, the color-change mechanism and degree of binding constants were well explained by theoretical calculation. To the best of our knowledge, this is the first example of vapochromic luminescence by using stable and variable coordination numbers of hypervalent bonds.

What promoted you to investigate this topic/problem? We achieved practical optical resolution of planar chiral 4,12-disub-stituted and 4,7,12,15-tetrasubstituted [2.2]paracyclophane compounds in 2012 and 2014, respectively.W eh ave prepared various conjugated compounds consisting of the di-or tetrasubstituted planar chiral [2.2]paracyclophane units. These compounds emit intense circularly polarized luminescence (CPL) with high photolumi-nescence (PL) quantum efficiencyinsolution, whereas the PL quantum efficiencyd ecreases dramatically in the solid state. We tried to overcome the problem of aggregation-caused quenching and obtain CPL-emissive thin films by taking advantage of ad endritic structure. What is the most significant result of this study? We could solve the above-mentioned problem completely;n amely, we achieved intense CPL from the dendrimer thin films with high PL quantum efficiencies. The dendrons sufficiently isolate the planar chiral [2.2]paracyclophane-containing conjugated core unit; this suppresses the aggregation-caused quenching of the core. In addition, the dendrons show al ight-harvesting effect and provide film-forming ability.T hus, the dendrimer films exhibited large molar extinction coefficients, high PL quantum efficiencies, intense PL and CPL, and large CPL dissymmetry factors. We realized gram-scale optical resolution and transformations of 4,7,12,15-tetrasub-stituted [2.2]paracyclophanes in this study. What other topics are you working on at the moment? We have been interested in "chirality". In particular,w eh ave fo-cused on the central chirality of the phosphorus atom in addition to the planar chirality of [2.2]paracyclophane. It is well known that the trivalent phosphorus atom in phosphine can act as ac hiral center because of the high inversion energy,u nlike that of nitrogen atoms. From this viewpoint, we have focused on P-chiral phos-phines and synthesized optically active P-chiral polymers, oligo-mers, and cyclic compounds using P-stereogenic bisphosphines as chiral building blocks. Recently,apractical synthetic route to P-ste-reogenic diphosphacrowns (crown ether derivatives consisting of chiral phosphorus atoms in the ring skeletons) was developed. Syntheses of various P-stereogenic phosphacrowns, investigations on the host-guest chemistry,a nd attempts for application to the field of materials chemistry are underway. Invited for the cover of this issue is YasuhiroM orisaki at Kwansei Gakuin University together with the group of Yoshiki Chujo at Kyoto University.T he image depicts the circularly polarizedl uminescence of the dendrimer films described in their Full Paper.R ead the full text of the article at