Kenta Nishino

Solid-State Emission of the Anthracene-o-Carborane Dyad from the Twisted-Intramolecular Charge Transfer in the Crystalline State

The emission process of the o-carborane dyad with anthracene originating from the twisted intramolecular charge transfer (TICT) state in the crystalline state is described. The anthracene-o-carborane dyad was synthesized and its optical properties were investigated. Initially, the dyad had aggregation- and crystallization-induced emission enhancement (AIEE and CIEE) properties via the intramolecular charge transfer (ICT) state. Interestingly, the dyad presented the dual-emissions assigned to both locally excited (LE) and ICT states in solution. From the mechanistic studies and computer calculations, it was indicated that the emission band from the ICT should be attributable to the TICT emission. Surprisingly, even in the crystalline state, the TICT emission was observed. It was proposed from that the compact sphere shape of o-carborane would allow for rotation even in the condensed state.

Development of Solid-State Emissive Materials Based on Multifunctional o-Carborane-Pyrene Dyads.

The molecular design based on o-carborane dyads is described for preparing multifunctional luminescent molecules such as dual emissions, aggregation and crystallization-induced emission enhancements, and luminescent color changes. The pyrene-substituted o-carborane dyads were synthesized via the insertion reaction between decaborane and 1-ethynylpyrene in the presence of Lewis base in a good yield. Finally, extremely bright luminescent compounds with solid-state emission properties (ΦPL > 0.99) were obtained.

Highly-efficient solid-state emissions of anthracene–o-carborane dyads with various substituents and their thermochromic luminescence properties

This manuscript describes the synthesis and optical properties of a series of anthracenyl-o-carborane dyads exhibiting highly-efficient luminescence with various types of substituent groups at the adjacent carbon atom of o-carborane. The restricted rotational motion of the anthracene moiety and the ideal orientation for intramolecular charge transfer from the anthracene moiety to the carborane cluster resulted in yellow and orange emissions (λem = 563 nm and 604 nm) with approximately 100% absolute fluorescence quantum efficiencies in the crystalline state of the methyl- and trimethylsilyl (TMS)-substituted dyads, respectively. In additional, clear thermochromic luminescence properties were also observed. Computer calculations were carried out to investigate the influence of the substituent effect on emission efficiency.

Electron-donating abilities and luminescence properties of tolane-substituted nido-carboranes

The luminescence properties of nido-carborane (7,8-dicarba-nido-carborate anion)-substituted tolane derivatives were investigated. It was shown that the nido-carborane unit acted as an electron-donating group in all derivatives, and their emission colors can be tuned by the substituents at the tolane moiety. According to theoretical investigation, it was proposed that frontier orbitals were distinctly separated by the twisted structure of the methyl substituents at the nido-carborane unit. Thus, in the absorption spectra, drastic changes were obtained because the HOMO–LUMO transition was forbidden. These structural and electrical substituent effects should serve as good guidelines for designing optoelectronic materials based on nido-carboranes.

Enhancement of Aggregation-Induced Emission by Introducing Multiple o-Carborane Substitutions into Triphenylamine

The enhancement of aggregation-induced emission (AIE) is presented on the basis of the strategy for improving solid-state luminescence by employing multiple o-carborane substituents. We synthesized the modified triphenylamines with various numbers of o-carborane units and compared their optical properties. From the optical measurements, the emission bands from the twisted intramolecular charge transfer (TICT) state were obtained from the modified triphenylamines. It was notable that emission efficiencies of the multi-substituted triphenylamines including two or three o-carborane units were enhanced 6- to 8-fold compared to those of the mono-substituted triphenylamine. According to mechanistic studies, it was proposed that the single o-carborane substitution can load the AIE property via the TICT mechanism. It was revealed that the additional o-carborane units contribute to improving solid-state emission by suppressing aggregation-caused quenching (ACQ). Subsequently, intense AIEs were obtained. This paper presents a new role of the o-carborane substituent in the enhancement of AIEs.

Modulation of luminescence chromic behaviors and environment-responsive intensity changes by substituents in bis-o-carborane-substituted conjugated molecules

Two types of multi-functional emissive bis-o-carborane-substituted 1,4-bis(phenylethynyl)benzene molecules were synthesized, and their optical properties were investigated in detail. The pristine o-carborane-substituted molecule CBH simultaneously exhibited dual emission from the locally excited (LE) and twisted intramolecular charge transfer (TICT) states in solution. Originating from changes in the intensity ratios between both emission bands, clear solvatochromic and thermochromic behaviors were observed. Surprisingly, TICT emission was observed even in the solid state. Aggregation- and crystallization-induced emission enhancement (AIEE and CIEE, respectively) were also presented by CBH. These solid-state emission enhancements could be derived from the suppression of aggregation-caused quenching (ACQ) by the bulky cage structure and the spherical shape of o-carborane. Next, we also synthesized the methyl-substituted derivative (CBMe) and found environment-resistant highly-efficient emission in both the solution and solid states. Finally, CBMe presented mechanochromic luminescence in the solid state. The substituent effects on the optical properties are discussed.

Dual emission via remote control of molecular rotation of o-carborane in the excited state by the distant substituents in tolane-modified dyads

Dual-emissive properties and their environment responsiveness are reported based on the tolane-o-carborane dyad having various substituents at the tolane moiety. A series of o-carborane dyads were synthesized and characterized. It was shown that their luminescent colors were tuned by the substituents at the tolane moiety. Some derivatives with strong electron-donating substituents such as –OMe, –(OMe)3 and –NMe2 simultaneously showed dual emission bands from the locally-excited (LE) state in the parallel conformation and an intramolecular charge transfer (ICT) state in the twisted one in the octane solution. Furthermore, intensity ratios between two emission bands were varied by heating. These results mean that the electronic properties of the o-carborane dyad system can be modulated by the degree of electron donating properties at the substituents via electronic conjugation.

Comparison of luminescent properties of helicene-like bibenzothiophenes with o-carborane and 5,6-dicarba-nido-decaborane

This article describes comparison of the anchoring effect on electronic properties of the helicene-like bibenzothiophene between o-carborane and 5,6-dicarba-nido-decaborane. The o-carborane and nido-decaborane-fused bibenzothiophenes were simultaneously obtained in the same reaction and successfully isolated. Initially, the X-ray single crystal analysis revealed that the helicene-like distorted structure was realized in the nido-decaborane-fused bibenzothiophene. From optical measurements in the solution state, distinct different characteristics depending on the type of anchors were observed. It was summarized that the absorption and luminescent properties originated from weak π-conjugation at the bibenzothiophene moiety in the o-carborane-fused compound were obtained, whereas robust π-conjugation and significant emission from the intramolecular charge transfer state were detected from the nido-decaborane-fused compound. These data can be explained by the theoretical results that π-conjugation was restrictedly developed within the bibenzothiophene moiety in frontier orbitals of the o-carborane-fused compound. In contrast, π-conjugation can be constructed even through the distorted bibenzothiophene because of the nido-decaborane unit. Moreover, the intramolecular charge transfer state should be realized because of electronic interaction involving the nido-decaborane unit in the excited state. Furthermore, it was demonstrated that the nido-decaborane-fused compound possessed solid-state emission and mechanochromic luminescent properties. The π-conjugation on the distorted structure supported by the nido-decaborane anchor should play a significant role in suppressing aggregation-caused quenching followed by presenting solid-state emission with stimuli responsiveness.