Page 8 - Shimadzu Journal vol.6 Issue1

Material Science

o-Carborane-Based AIE-Inducible “Element-Blocks” and
Extremely-Efficient Solid-State Emissive Materials
As the next example of the AIE-inducible “element-block”, unique optical
properties of organoboron clusters called as o-carboranes are introduced.
o-Carboranes are icosahedral clusters consisting of 10 boron and 2 carbon
atoms with three-center two-electron bonds. Since the three-dimensional
electron delocalization in the carborane cage contributes to improving
thermal and chemical stabilities, o-carborane derivatives have been used for
heat-resistant materials. In addition, owing to the intrinsic reactivity of
boron under radiation irradiation, o-carborane derivatives have been
applied as the positron sources in positron electron tomography treatments.
In 2009, we firstly found that o-carborane can work as a key unit for
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presenting AIE. The alternating polymers were prepared with o-carborane
and p-phenyleneethynylene sequences, and their optical characteristics
Fig. 2 (a) Typical luminescent behavior of conventional emissive dyes and were examined. In summary, the typical AIE behaviors were observed from
AIE-active molecules in the solution and solid states. (b) Schematic model the o-carborane-containing main chain-type polymers. Significant emission
of transformation of ACQ luminogen to AIE-active molecule by chemical was hardly observed from the THF solution of the polymers having
modification. Reprinted with permission from ref 5. Copyright 2013
Wiley-VCH Verlag GmbH & Co. KGaA. electron-donating К-conjugated linkers. On the other hand, by adding the
water content in the solution for the formation of aggregation, emission
intensity was enhanced. Finally, similarly to the previous example having AIE
Film-Type Sensors Based on Oxidation-Induced Emission properties, in the mixed solvent of THF/H2O = 1/99, bright emission was
Properties obtained. From these experiments, it was proposed that the o-carborane
polymers can work as an AIE-active material.
To extend the application of the stimuli-responsive AIE-active materials to To comprehend the AIE mechanism in the o-carborane-containing
the film-type chemical sensors, conjugated polymers composed of boron polymers, the series of mechanistic studies with o-carboranes were
diiminate were designed. The thin films with the AIE-active copolymers performed. Accordingly, it was clarified that the intramolecular charge
composed of fluorene comonomers and the sulfide-substituted boron transfer should occur by the photo-irradiation because of strong
diiminates were synthesized, and the changes in the optical properties by electron-accepting properties of o-carborane and electron-rich linkers. In
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the oxidation were monitored. the excited state, the electrons should be delocalized involving the C−C
Slight emission was observed from the film sample before the oxidation bond in the o-carborane unit. By the structural vibration at the o-carborane
by H2O2 which can transform sulfide to sulfoxide. By soaking the film moiety, excitation energy was consumed, leading to the quenching of the
sample of the polymer in the solution containing H2O2, the increase of the emission. In the solid state, most of molecular motions should be
yellow color emission of the film was detected. From the quantitative suppressed, resulting in the strong emissions. Thus, AIE behaviors should be
analyses for quantum yields, the emission intensity was finally obtained from the polymers. Based on this mechanism, we aimed to
elevated 3-times larger than that of the pristine film. By the modulate AIE properties by altering environmental factors.
oxidation, the electron-acceptability of boron diiminate should be
enhanced. As a result, it was proposed that the intramolecular
charge transfer should be also encouraged, leading to the emission Stimuli-Responsive AIE Regulation with o-Carborane Materials
enhancement from the charge transfer state through the polymer The modulation of emissive properties of the o-carborane-containing
main-chain. In other words, oxidation-induced AIE can be realized polymeric materials by environmental alteration was accomplished based
with the sulfide-modified organoboron polymers. It can be expected on hydrogel matrices. The cross-linking reagent composed of
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that the AIE-active polymeric materials should be a versatile platform o-carborane was synthesized (Fig. 4a). By using this cross-linker, the
as a biosensor for longitudinal monitoring of the bio-related translucent hydrogels consisting of poly(Ѝ-glutamic acid) were prepared.
reactions. Especially, H2O2 is known to be one of reactive oxygen By the gel-shrinkage from the change of ionic strength, it was found that
species and is endogenously produced in energy metabolism. the luminescence intensity from the modified hydrogels was drastically
Therefore, our materials could be valid for sensing generation of changed. Weak emission was obtained in the swollen state. The bright
reactive oxygen species and oxidative stresses caused by these emission was observed by drying the hydrogel. Moreover, these changes
reactive molecules. in the emission intensity were reversibly repeated many times between the
swollen and dried states. According to the AIE mechanism of the
o-carborane unit, these behaviors can be explained by the fact that the
molecular motion at the o-carborane cross-linkers should be suppressed in
the dried state. Then, the significant emission can be observed from the
material. In the swollen state, the molecular motion should be recovered,
leading to the annihilation of the emission.
We also prepared the o-carborane derivative and found multi-chromism
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behaviors (Fig. 4b). For example, we synthesized
di-o-carborane-substituted oligoacenes, and their structures as well as
emission behaviors were investigated in detail. Accordingly, from
recrystallization in CH2Cl2, C6H6, and CHCl3, solvent-incorporated crystals
were afforded. Each crystal exhibited different emission colors
(yellowish-orange, orange, and red, respectively). Additionally, the
emission colors can be tuned by heating, vapor fuming and external
Fig. 3 (a) Schematic illustration of film-type sensors for H2O2 based on mechanical stimuli. In summary, we can expect the o-carborane
sulfide-substituted boron diiminate conjugated polymers with
oxidation-induced AIE properties. (b) Other applications of boron “element-block” should be not only AIE-inducible unit but also a
ketoiminates and diiminates as luminescent chromism materials. Reprinted functional unit with multi-chromism behaviors.
with permission from ref. 6. Copyright 2014 American Chemical Society.

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