DOI: 10.1002/chem.201402946
Communication
&
Sensors
Boron Diiminate with Aggregation-Induced Emission and
Crystallization-Induced Emission-Enhancement Characteristics
Ryousuke Yoshii, Amane Hirose, Kazuo Tanaka, and Yoshiki Chujo*[a]
the other hand, we have reported that boron ketoiminates
Abstract: Organoboron complexes having aggregation-
were AIE-active materials, and their AIE properties were strong-
and crystalization-induced emission properties are pre-
ly affected by the boron-chelating ring, which includes
sented. The series of boron diiminates were synthesized
a boron-nitrogen (BꢀN) bond.[10]
and the emission behaviors of the synthesized com-
pounds were investigated. Finally, it was found that signif-
icant enhancement of the emission depended on the crys-
tallinity in the solid states.
Solid-state fluorescent organic dyes have attracted a great deal
Scheme 1. Boron diketonate derivatives.
of attention because of their fundamental importance and ap-
plicability in various modern technologies, such as organic
light-emitting diodes,[1] photodynamic therapy,[2] and fluores-
Herein, we focus on boron diiminate derivatives, which are
cent sensors.[3] Although the emission of fluorescent organic
an analogue to boron diketonate, as a new class of AIE- and
dyes is usually quenched in solid state by aggregation-caused
quenching (ACQ), effect due to the formation of delocalized
excitons or excimers,[4] several molecules, which exhibit aggre-
gation-induced emission (AIE) property were first reported by
Tang and co-workers in 2001.[5] In addition, Tang et al. have
also presented some fluorescent organic dyes with crystalliza-
tion-induced emission-enhancement (CIEE) characteristics:[6]
the strong emission of p-conjugated system can be induced
efficiently only in the crystal states. This behavior is exactly op-
posite to the common ACQ effect of usual chromophores.
Thus, CIEE-active molecules are promised to be multistimuli-re-
sponsive materials, such as thermal and vapor sensors. Howev-
er, only limited number of CIEE-active organic compounds has
been discovered to date. Therefore, an exploration in new CIEE
chromophores is still of great interest.
CIEE-active organoboron complexes, because boron diiminates
have a boron-chelating ring containing two BꢀN bonds
(Scheme 1). Furthermore, comparing to boron diketonates and
boron ketoiminates, there is room in boron diiminates for func-
tionalization deriving from their two nitrogen atoms, which
can bond various substituents. Because the regulation of inter-
molecular interactions in solid state is very important to devel-
op a CIEE property, boron diiminates with high functionality
have a high potential as CIEE-active materials, although only
a few examples of the synthesis and the optical properties of
boron diiminate have been reported.[11]
Herein, we describe the efficient synthesis, the photoproper-
ties of boron diiminate derivatives, and the verification of their
AIE properties. They exhibited not only AIE characteristics but
also CIEE characteristics (in THF: quantum yield of photolumi-
nescence FPL <0.01, in the crystal states: FPL =0.11–0.23, in
the amorphous states: FPL =0.02), resulting from suppression
of molecular motions involving boron-chelating rings and dif-
ference of the packing structure in the solid states. In addition,
we demonstrate the reversible regulations of optical properties
by simple external stimuli, such as fuming/heating or heating/
cooling cycles.
Boron diketonate derivatives are an important class of orga-
noboron dyes due to their prominent photoproperties.[7] Fraser
and co-workers have found prominent fluorescence and room-
temperature phosphorescence from boron diketonates[8a,b] and
further intriguing reversible mechanochromic fluorescence
from boron avobenzone between solid and melt states.[8c] Pre-
viously, we have also embarked on a keynote aimed at prepar-
ing high-luminescent materials based on boron diketonate de-
rivatives.[9] However, in the solid state, the emissions of those
boron diketonate moieties usually decreased through the ACQ
effect caused by intermolecular p–p stacking interaction. On
To synthesize 1,3-diaryldiimine derivatives, the reaction of N-
phenylimine
1 and imidoyl chloride 2 was employed
(Scheme 2). As a result, the diimine derivatives were successful-
ly obtained. Finally, the desired boron diiminates (4a and b)
were prepared by boron complexation by using BF3·OEt2. It
was proposed that this synthetic strategy should be valid for
the preparation of boron diiminates with bulky groups at the
nitrogen atoms.
[a] R. Yoshii, A. Hirose, K. Tanaka, Prof. Y. Chujo
Department of Polymer Chemistry, Kyoto University
Katsura, Nishikyo-ku, Kyoto 615-8510, (Japan)
The optical properties of 4a and b were investigated by
UV/Vis absorption spectroscopy in acetonitrile (concentration
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402946.
Chem. Eur. J. 2014, 20, 8320 – 8324
8320
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