Abstract
Conjugated polymers containing boron and gallium diiminate complexes were prepared with various electron-donating comonomers via pre- and post-complexation methods, respectively. From a comparison of emission quantum yields between solution and film states, it was seen that all polymers containing group 13 elements possessed an aggregation-induced emission property. Additionally, the frontier orbital energies and the optical and electrochemical properties of the polymers can be tuned by altering a central element at the complex moieties as well as by changing a comonomer unit. In particular, it was demonstrated that the gallium atom can contribute to stabilizing the energy levels of the lowest unoccupied molecular orbitals, resulting in narrow band gaps of the conjugated polymers. This study presents the potential of gallium not only for preparing solid-state emissive conjugated polymers but also for fabricating low-band gap materials by employing the conjugated ligand.
Highlights
The introduction of heteroatoms into conjugated polymers is one of the valid strategies for obtaining the unique properties originating from each element
The results indicated that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) were mainly located at the comonomer unit and the diiminate complex moiety in all compounds, respectively
It was strongly suggested that the red shifts observed in the absorption and photoluminescence spectra should be derived from the enhanced stabilization effect of the LUMO levels by the alteration of the central element from boron to gallium
Summary
The introduction of heteroatoms into conjugated polymers is one of the valid strategies for obtaining the unique properties originating from each element. Owing to the emissive properties and stability of boron-containing “element-blocks”, various kinds of boron-containing conjugated molecules including polymers possessing useful luminescent properties such as near-infrared emission [11,12], a sharp intense spectrum [13], light-harvesting antenae [14,15,16], and aggregation-induced emission (AIE) [17,18], which can be obtained only in the condensed state of luminescent dyes without aggregation-caused quenching (ACQ), have been reported [19,20,21,22] These materials are expected to be a key material for realizing advanced optical devices. From the mechanical studies including theoretical approaches, it was proposed that stabilization of the energy levels of molecular orbitals should be induced not by the charge transfer process but by the intrinsic property of gallium This is, to the best of our knowledge, the first study to experimentally present the feasibility of gallium for AIE-active conjugated polymers and for low-band gap materials by employing the conjugated ligand
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