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 pi-conjugation at the bibenzothiophene moiety in the o-carborane-fused compound were obtained, whereas robust pi-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 pi-conjugation was restrictedly developed within the bibenzothiophene moiety in frontier orbitals of the o-carborane-fused compound. In contrast, pi-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 pi-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.