Rational Designs of AIE-Active Molecules and Luminochromic Materials Based on Group 13 Element-Containing Element-Blocks
There are numerous numbers of boron complexes having luminescent properties originating from rigid π-conjugated systems. However, similarly to other commodity chromophores, emissive boron complexes often suffer from aggregation-caused quenching (ACQ) in the applications as a luminescent film or an optical device. This ACQ problem is still a critical issue to be solved for expanding applicability of emissive boron complexes as advanced organic devices, environment sensors, and bioprobes. Recently, the class of boron complexes possessing aggregation-induced emission (AIE) properties has been discovered, and in particular, their environment-responsive luminescent characters have attracted attention as a key phenomenon to construct not only solid-state luminescent dyes but also stimuli-responsive luminochromic materials. In this review, AIE-active boron complexes are introduced mainly from our studies. Initially, the series of o-carboranes which are the boron cluster compound are introduced. Originating from intrinsic electron deficiency at the carbon atoms in the cluster, o-carborane works as a strong electron-accepting unit in the conjugated system. As a result, intense emission with the intramolecular charge transfer character can be observed. It should be noted that these emission properties can be preserved even in the condensed state by disturbing intermolecular interaction because of steric hindrance. Based on these facts, various types of solid-state luminescent materials have been developed with the modified o-carboranes. The mechanism and optical properties are demonstrated. Next, AIE-active boron complexes with β-diketonate derivatives are demonstrated. Transformation from conventional ACQ-presenting luminescent dyes to AIE-active molecules is presented. By using the key structures such as boron ketoiminate and diiminate, luminescent polymers were obtained. As an application, the film-type sensors for bio-significant molecules are shown. Finally, rational design for AIE-active molecules with theoretical calculations from the scratch is illustrated. Based on flexible boron complexes that show large degree of structural relaxation in the excited state, the AIE behaviors were able to be realized. The results are explained in this review.
KeywordsBoron complex Carborane Solid-state luminescence Luminochromism Element-block
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