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K. Tanaka et al.
in the crystal than the boron complex. In addition, environment-sensitive luminescent character regarding vapochromic properties was discovered. By exposing the
crystal powder of gallium diiminate to vapor of relatively smaller sizes of volatile
organic compounds (VOCs), emission color was changed from blue to green, blue
color emission was maintained in the presence of larger sizes of VOC vapors. From
the mechanistic studies, the crystal–crystal transition was induced by smaller sized
VOCs, resulting in luminescent chromism. Because of larger atomic size of gallium
than that of boron, pore structures should be created in the crystalline surfaces. As a
consequence, structural alterations are able to be induced when VOCs are inserted.
This material is expected to be a sensor for detecting air pollutants.
9.5 Rational Design for AIE-Active Molecules
Except for the above examples, it is shown that AIE-active molecules are versatile
for stimuli-responsive luminescent materials for the applications to organic optoelectronic devices and bioprobes (Tang 2015). Therefore, development of new AIE-active
molecules is of great significance for improving these properties in the products. So
far, most of the AIE-active molecules seem to have multiple phenyl rings to induce
emission annihilation in solution as well as to suppress ACQ. Recently, it was demonstrated that new AIE-active molecules are able to be designed from scratch through
computer calculation based on “flexible boron complexes” which show relatively
larger degree of structural relaxation in the excited state (Gon et al. 2019c). In this
section, this protocol for predicting AIE-active dyes and the brilliant new dye with
the highly planar structure is illustrated.
According to the mechanism on AIE behaviors, flexible conjugated boron
complexes can have possibility to show AIE properties. In the solution state, vigorous
molecular motions occur, resulting in emission annihilation, while emission can be
observed in aggregation (Fig. 9.26) (Yamaguchi et al. 2017). On the basis of this
assumption, new boron complexes were explored by computer calculations. As a
result, the optimized structures in the ground and excited states were estimated with
density functional theory (DFT) and time-dependent DFT calculations, respectively.
By comparing both conformations, the boron complex which showed relatively large
difference was selected and synthesized.
BPI was proposed to be a potential candidate from the comparisons (Fig. 9.27)
(Yamaguchi et al. 2017). In the ground state, the planar conformation was obtained,
meanwhile the bent conformation was suggested as the most stable structure in the
excited state. To fix the conformation, FBPI was also designed and synthesized as a
control compound. From the optical measurements, the clear AIE and CIE characters
were observed only from BPI. From the width of the Stokes shift, it was supported
that large structural relaxation should proceed in the excited state of BPI. In the
case of FBPI, intense emission with small Stokes shift was obtained only from the
solution, and critical ACQ was observed similarly to other conventional organic dyes.
This result indicates that the AIE-active molecule can be theoretically predicted.
K. Tanaka et al.
in the crystal than the boron complex. In addition, environment-sensitive luminescent character regarding vapochromic properties was discovered. By exposing the
crystal powder of gallium diiminate to vapor of relatively smaller sizes of volatile
organic compounds (VOCs), emission color was changed from blue to green, blue
color emission was maintained in the presence of larger sizes of VOC vapors. From
the mechanistic studies, the crystal–crystal transition was induced by smaller sized
VOCs, resulting in luminescent chromism. Because of larger atomic size of gallium
than that of boron, pore structures should be created in the crystalline surfaces. As a
consequence, structural alterations are able to be induced when VOCs are inserted.
This material is expected to be a sensor for detecting air pollutants.
9.5 Rational Design for AIE-Active Molecules
Except for the above examples, it is shown that AIE-active molecules are versatile
for stimuli-responsive luminescent materials for the applications to organic optoelectronic devices and bioprobes (Tang 2015). Therefore, development of new AIE-active
molecules is of great significance for improving these properties in the products. So
far, most of the AIE-active molecules seem to have multiple phenyl rings to induce
emission annihilation in solution as well as to suppress ACQ. Recently, it was demonstrated that new AIE-active molecules are able to be designed from scratch through
computer calculation based on “flexible boron complexes” which show relatively
larger degree of structural relaxation in the excited state (Gon et al. 2019c). In this
section, this protocol for predicting AIE-active dyes and the brilliant new dye with
the highly planar structure is illustrated.
According to the mechanism on AIE behaviors, flexible conjugated boron
complexes can have possibility to show AIE properties. In the solution state, vigorous
molecular motions occur, resulting in emission annihilation, while emission can be
observed in aggregation (Fig. 9.26) (Yamaguchi et al. 2017). On the basis of this
assumption, new boron complexes were explored by computer calculations. As a
result, the optimized structures in the ground and excited states were estimated with
density functional theory (DFT) and time-dependent DFT calculations, respectively.
By comparing both conformations, the boron complex which showed relatively large
difference was selected and synthesized.
BPI was proposed to be a potential candidate from the comparisons (Fig. 9.27)
(Yamaguchi et al. 2017). In the ground state, the planar conformation was obtained,
meanwhile the bent conformation was suggested as the most stable structure in the
excited state. To fix the conformation, FBPI was also designed and synthesized as a
control compound. From the optical measurements, the clear AIE and CIE characters
were observed only from BPI. From the width of the Stokes shift, it was supported
that large structural relaxation should proceed in the excited state of BPI. In the
case of FBPI, intense emission with small Stokes shift was obtained only from the
solution, and critical ACQ was observed similarly to other conventional organic dyes.
This result indicates that the AIE-active molecule can be theoretically predicted.