Page 7 - Shimadzu Journal vol.6 Issue1

Material Science

Development of Solid-State Emissive Materials Based on Aggregation-Induced

Emission-Inducible Organoboron “Element-Blocks”

Kazuo Tanaka and Yoshiki Chujo, Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University

Aggregation-Induced Emission as a Promising Solution commodity fluorescent organoboron dyes to the AIE-active
for Overcoming Aggregation Caused Quenching molecules is presented. Based on this result, the conjugated
polymers with AIE properties were obtained. The optical properties
Emissive organic materials are versatile for the application to advanced
organic devices such as paper displays, flexible photovoltaic cells, and of conjugated polymers involving boron element are illustrated.
printing electronic circuits. In particular, organoboron complexes including Moreover, the applications of these AIE-active polymers for the
polymers have gathered much attention as a key building block for the film-type sensors are mentioned. Next, as another instance of the
next-generation of opto-electronic devices because of their AIE-inducible “element-block” composed of organoboron molecules,
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opto-electronic functions. Most of organoboron molecules are composed the AIE behaviors observed in the carborane materials are
of four-coordinated boron and can show a variety of optical properties. mentioned. The material design and unique solid-state emission of
the carborane derivatives are demonstrated.
Indeed, so far, various types of luminescent materials involving the
four-coordinate boron have been prepared. As one of typical examples,
boron dipyrromethene (BODIPY) derivatives, which possess superior
optical properties such as large light absorption and emission ability,
sharp spectra, and high photo-stability, are used for a wide variety of
applications in material science as well as biotechnology. Thus,
optically-functional materials such as solid-state emitters, light absorbers,
light-harvesting antennae and white-light emitting materials can be
produced with BODIPY derivatives. Furthermore, we recently reported the
BODIPY derivative-containing conjugated polymers can work not only as
efficient emitters in deep-red and near infrared regions but also as an
efficient electron-carrier materials. It should be mentioned that higher
electron-carrier ability from the film samples composed of the BODIPY
derivative-containing conjugated polymers was obtained than those of Fig. 1 Chemical structures of AIE-inducible organoboron “element-blocks”.
Alq3 crystal which is commonly used as an electron-carrier material in the
conventional optical devices. This result means that these polymers should
be promised to be the key materials in modern organic opto-electronic Transformation of Conventional Fluorescent Dyes to
devices with printing methods. AIE-Active Molecules
When organoboron luminescent dyes are actually utilized in the devices, Boron diketonates are one of simple and stable organoboron complexes,
we often have one critical problem to be overcome. In general, bright and so far, various optical materials have been developed based on boron
emission can be observed from the diluted solution state. On the other diketonates. Therefore, boron diketonates are regarded as versatile
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hand, most of emission properties are spoiled in the condensed state such “element-blocks” for constructing functional emissive materials. However,
in the film and powder. These behaviors are called as concentration emission efficiencies were often crucially lowered in the solid state
quenching or aggregation-caused quenching (ACQ). To receive the because of ACQ. Particularly, organoboron conjugated polymers suffered
highly-efficient emission from the devices, ACQ should be avoided. On from ACQ when these materials have been used as an emissive unit in the
the contrary, it has been reported that some of organic compounds optical devices. In order to overcome ACQ and to realize AIE-active
presented stronger emission only in the aggregation states. Tang et al. materials, we designed and synthesized the new ligands based on boron
firstly showed unique behaviors with tetraphenyl-substituted silole diketonate. 5
compounds in 2001. Only when the compounds were aggregated in the It was presumed that by replacing oxygen to nitrogen, the flexibility of the
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poor solvent, the bright emission can be obtained. This phenomenon is complex should be enhanced. As a result, excitation energy can be
called aggregation-induced emission (AIE). In order to overcome ACQ and consumed in the solution state, resulting in annihilation. In the solid state,
to receive bright emission from the solid samples, the mechanism of AIE molecular tumbling should be restricted. Moreover, the distortion around
behaviors and AIE-active materials have been significantly focused. nitrogen could contribute to disturbing undesired intermolecular
Therefore, many researchers devoted their effort to explore new interaction. Therefore, we expected strong emission can be observed from
AIE-active units for obtaining highly-efficient materials. the solid sample. To confirm the validity of this idea, boron ketoiminates
and diiminates were synthesized, and their optical properties were
“Element-Block Polymers”, New Strategy for Material Design examined. In summary, significant AIE behaviors were observed from
these complexes. For instance, in THF, emission was hardly observed from
We have recently proposed a new concept of "element-block" consisting boron ketoiminate. In contrast, bright emission was observed from the
of nanobuilding units or clusters of heteroatoms for the material design. 3 samples in the mixed solvent system of THF / H2O (1/9 (v/v), Їagg =
In this review, we describe recent advances on the development of 0.30–0.76). Furthermore, in the crystalline state of boron diiminate,
AIE-inducible organoboron "element-blocks". Simply by introducing further enhancement to emission intensity was observed by the phase
“element-blocks” composed of organoboron complexes in the transition from amorphous to crystalline states. These data meant that
conjugated system and connecting with other functional units, the series new boron complexes have crystallization-induced emission enhancement
of functional emissive materials can be readily obtained. Additionally, by (CIEE) as well as AIE properties. Based on these solid-state emission
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the combination with stimuli-responsivity of the materials, emissive properties, various types of luminescent chromism were observed such as
chemical sensors can be constructed. Initially, the transformation of vapochromism6 and mechanoflurochromism . 7

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