Frustrated element-blocks: A new platform for constructing unique stimuli-responsive luminescent materials

Abstract

An element-block, which is the heteroatom-containing functional minimum unit, is a versatile building block for constructing functional polymers. In this focus review, we explain the new concept for developing stimuli-responsive luminochromic materials based on "the frustrated" element-block. When the element-blocks which show relatively larger degree of structural relaxation in the excited state are structurally restricted, we regarded that these molecules are in "the frustrated state". Recently, we discovered unique stimuli-responsive luminochromic behaviors from "the frustrated element-blocks". As a representative example, stimuli-responsive properties of "the frustrated boron clusters" are initially illustrated. The series of luminochromic properties, such as mechano-and thermochromism are described. Next, we designed the fused complexes for suppressing intramolecular motions to obtain "frustrated boron complexes". Not only constant emission in solution and solid but also mechanochromic luminescent materials have been obtained. Finally, "the frustrated state" by the polymerization is explained. Unique luminochromic behaviors, such as thermochromism in solution, mechanochromism in solid and vapochromism were observed from the polymers. Mainly from our recent works, stimuli-responsive luminochromic behaviors of "the frustrated element-block"containing polymers and their mechanisms are illustrated.

Solid-state excimer emission of o-carborane derivatives and applications for stimuli-responsive luminescent materials

“…Consequently, various solid-state luminescent materials with unique stimuli-responsiveness have been developed by utilizing o -carborane as “an element-block”, which is the functional building block containing inorganic elements for constructing advanced luminescent materials, such as chemical and environmental sensors. 10–14 When linked with an aromatic ring at the carbon atom in the cluster, the o -carborane unit can work as an electron-acceptor group and form robust electron-donor–acceptor interactions. 15 As a result, the intramolecular charge transfer (CT) process followed by intense emission can occur.…”

Selective modulation of energy levels of frontier orbitals in solid-state luminescent boron-fused azomethine polymers with orthogonal orientations to the main chains

“…We have recently proposed the concept of “element-blocks” which are functional structural units containing heteroatoms. 9 By the combination of element-blocks, we can create “element-block polymers” having unique functions originating from the loaded heteroatoms. 9 We have discovered that boron-fused azobenzene (BAz) 10 and BAm complexes 11 can work as AIE- and CIE-active element-blocks.…”

“…9 By the combination of element-blocks, we can create “element-block polymers” having unique functions originating from the loaded heteroatoms. 9 We have discovered that boron-fused azobenzene (BAz) 10 and BAm complexes 11 can work as AIE- and CIE-active element-blocks. Mechanistic studies revealed that molecular motions triggered in the excited state originating from their potential flexible and bendable structures should be a key factor for expressing AIE and CIE behaviors.…”

Design for a Pure-Blue-Emissive Polymer Film through the Selective Perturbation of the Energy Level of the Highest Occupied Molecular Orbital in a Boron Complex

“…Minimum functional units in heteroatom-containing materials are called “element-block”. − We accomplished the transformation of luminescent boron element blocks showing concentration quenching to AIE-active dyes . By weakening the dative bonds between boron and ligand, molecular motions would be facilitated in solution, resulting in emission annihilation.…”