Enhancement of Luminescence Efficiencies by Thermal Rearrangement from ortho- to meta-Carborane in Bis-Carborane-Substituted Acenes

In a previous study we synthesized a series of bis-o-carborane-modified acenes and revealed various unique luminescence properties, such as highly efficient solid-state emission and luminochromism. Here we report further enhancement of the luminescence efficiencies of bis-o-carborane-modified acenes by thermally induced structural rearrangement from ortho-to meta-carborane. We found that the thermal rearrangement of the anthracene derivative 1 occurred close to the melting point and gave luminescent products. From a detailed investigation of the product obtained from 1, the transformation of the molecular structure was proven by X-ray crystallography. In addition, the analytical data indicated that rearrangement caused by heating at 300 degrees C occurred at only one of the two o-carborane units. In accord with the results of theoretical investigations of the transition states of 1 by quantum chemical calculations, we propose that the thermal rearrangement could proceed because of a low activation energy in the first rearrangement step due to the presence of the anthracene. On the basis of optical measurements, enhancements of not only luminescence efficiencies but also aggregation-induced emission were observed. Furthermore, we have demonstrated that this strategy for enhancing emission properties is applicable ot the corresponding tetracene derivative, resulting in a deep-red luminescent dye.