N-Ligand-Enabled Aromatic Nucleophilic Amination of 1,2-Diaryl-o-Carboranes with (R2 N)2 Mg for Selective Synthesis of 4-R2 N-o-Carboranes and 2-R2 N-m-Carboranes

The nucleophilic aromatic BH substitution reaction of carboranes is uncommon, compared to the electrophilic one. This work reported a pyridine-enabled transition-metal-free regioselective nucleophilic aromatic cage B(4)-H amination of 1,2-diaryl-o-carboranes with magnesium bisamides, giving a series of B(4)-aminated o-carboranes. DFT calculations showcased a stepwise B-N formation/B-H cleavage process, in which Mg-H formation/cage closure is the rate-determining step. Unprecedentedly, in the presence of 4,4'-di-tert-butyl-2,2'-dipyridyl (dtbpy), a tandem B(4)-amination/cage isomerization reaction of o-carboranes was discovered for the facile preparation of B(2)-aminated m-carboranes. Control experiments indicated that magnesium complex, bidentate ligand (dtbpy) and reaction temperature were crucial in the cage isomerization process. This direct nucleophilic aromatic cage B-H amination reaction offers an alternative strategy for selective amination of o- and m-carboranes.

Conformation-Dependent Electron Donation of Nido-Carborane Substituents and Its Influence on Phosphorescence of Tris(2,2′-bipyridyl)ruthenium(II) Complex

In this work, we report the synthesis of the nido-carborane-substituted ruthenium complexes and the substituent effects of nido-carboranes on the optical properties. Initially, from the optical measurements, it is shown that deep-red phosphorescence was obtained from the synthesized molecule, and the phosphorescent quantum yields were significantly improved by loading onto a polyethylene glycol film. This result represents that nido-carborane can work as a strong electron donor and should be an effective unit for enhancing the solid-state phosphorescence of ruthenium complexes. Further, it is suggested that the electron-donating properties of the nido-carborane units and subsequently the optical properties can be tuned by controlling the conformation of the nido-carborane units with the steric substituents. We demonstrate in this study the potential of nido-carborane as a building block for constructing optical materials as well as fundamental information regarding electronic interactions with π-conjugated systems.

Manipulation of electronic and structural effects on the solid-state emission of multiple linked anthracenyl-o-carborane dyads

A direct boost of emission efficiency is presented based on the strategy of linking multiple weak AIE-active anthracenyl-o-carborane dyads together for improving solid-state luminescence.

Experimental proof for emission annihilation through bond elongation at the carbon-carbon bond in o-carborane with fused biphenyl-substituted compounds

Because of their unique luminescence properties, such as aggregation-induced emission (AIE), intense solid-state luminescence and stimuli-responsive luminochromism, aryl-substituted o-carboranes have attracted attention as a platform for developing functional optoelectronic materials. However, there still remains one fundamental issue with the detailed mechanism of solution quenching in AIE behaviors. Aryl-modified o-carboranes with AIE properties exhibit intense emission not in solution but in the solid state. According to quantum calculations and many experimental results, the elongation at the carbon-carbon bond in o-carborane in the excited state, followed by nonradiative decay, has been proposed as a main path for emission annihilation in solution. However, intramolecular rotation would simultaneously occur, and there is a possibility that emission annihilation could be induced by the combination of both bond elongation and rotation. In this study, we designed two types of biphenyl-substituted o-carboranes having fused structures at the neighbor carbon and boron atoms for fixing molecular conformation. In these molecules, bond elongation is allowed, while rotation would be prohibited. From the series of optical measurements and theoretical investigations, we proved that emission annihilation can occur through bond elongation in the absence of rotation. Moreover, we show that bond elongation could be suppressed by introducing a bulky substituent at the adjacent carbon, and emission color tuning was achieved. This is the first example, to the best of our knowledge, to prove that excitation decay can proceed only through bond elongation without electronic perturbation caused by rotation.

Recent Progress in the Development of Solid-State Luminescent o-Carboranes with Stimuli Responsivity

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.

Aggregation-Induced Fluorescence of Carbazole and o-Carborane Based Organic Fluorophore

Carbazole based fluorophores 9-butyl-3,6-bis-(phenylethynyl)-9H-carbazole (1) and 9-butyl-3,6-bis-(2-phenyl-o-carborane)-9H-carbazole (2) were synthesized via Sonogashira type cross-coupling reaction and followed by insertion with decaborane. Compound 1 exhibited far more intense fluorescence than 2 in THF solution, while in solid state 2 exerted stronger fluorescence than 1. The fluorescence quenching behavior of 2 in THF solution could be attributed to the intramolecular charge transfer of donor-acceptor system in 2, which was confirmed by electrochemical experiments and DFT calculations. The fluorescence enhancement of 2 in solid state can be ascribed to aggregational induced packing which was evidenced by crystallographic study. In addition, compound 2 showed typical aggregation induced emission (AIE) behavior.

Theoretical insights into the effect of ligands on platinum(ii) complexes with a bidentate bis(o-carborane) ligand structure

Carboranes feature a wealth of unique structures and properties in phosphorescent transition-metal complexes (PTMCs). Herein, we identify the influence between the electronic structure in carboranes and the main ligand based on the density functional theory (DFT) and time-dependent density functional theory (TD-DFT), which affects the phosphorescence properties of carborane-containing Pt compounds. Furthermore, the mechanism, including singlet-triplet splitting energies ΔE(Sn - T1), transition dipole moment for S0 - Sn transitions, the zero-field splitting (ZFS), the radiative decay rate constant (kr), the Huang-Rhys factor (S), and the spin-orbit coupling (SOC) matrix elements <T1|HSOC|Sn> have been carefully investigated. The results presented here reveal the functional action 1,1'-bis(o-carborane) contributes to the emission process owing to the manipulation of main ligand dtb-bpy and complex 1a shows promising prospects for achieving highly efficient phosphorescence via engineering the conjugation of the main ligand dtb-bpy.