2.3 Emission Color Changes Induced by Solvent Addition …
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This indicates that 3/solvent all have crystalline nature, even though the powders of
3/solvent are prepared simply by adding a solvent to amorphous 3 ground and letting
the solvent evaporate under ambient conditions. Careful inspection of the powder
XRD patterns of the eleven 3/solvent samples confirmed that the peak positions
are different from each other (Fig. 2.6). This observation suggests that the crystal
structures of the resulting 3/solvent are different from each other and depend on the
solvent added to induce the phase transition of 3 ground . NMR spectroscopy indicates
that all 3/solvent contain solvent molecules inside the crystalline lattices (Fig. 2.6).
These results imply that the formation of different crystalline structures of 3/solvent
and solvent inclusion is responsible for their varied emission properties.
When other solvents not listed in the previous paragraph were added to 3 ground ,
no detectable emission color changes were observed in the resulting powders. We
also added 25 other different volatile compounds to 3 ground , but no emission color
change was observed; the yellow emission mostly remained. XRD measurements
of the resulting powders imply that the addition of these solvents does not induce
distinctive changes in molecular arrangement of 3. For example, the XRD patterns of
the powders obtained after treatment with toluene exhibit several peaks with very low
intensity (grey line in Fig. 2.6). These weak diffraction peaks indicate that toluene
only induces partial crystallization of 3. Such diffraction patterns with low intensity were also obtained after 3 ground was treated with benzene, trichloroethene, and
THF. Meanwhile, the powders of 3 obtained after treatment with hexane or MeOH
exhibited diffraction patterns with almost no peaks that were similar to that of 3 ground
(Fig. 2.19). We suppose that these differences in the powder XRD patterns are caused
by the different solubility of 3 in these solvents: the solubility of 3 in hexane or MeOH
is too low to recover a crystalline phase. In contrast, 3 partially dissolves in other
solvents (benzene, toluene, THF, or trichloroethene) to recover partial crystalline
phase. From the observation, we speculate that the above solvents that do not induce
emission color changes of 3 cannot form the corresponding solvated crystals with 3,
so 3 ground remains after solvent addition.
Mechanical stimulation can eliminate solvent molecules from 3/solvent to allow
reuse of 3 for solvent detection, which is more convenient than other reactivation
processes. Grinding eleven samples of 3/solvent using a spatula or ball milling regenerates yellow-emitting powder 3 ground irrespective of the type of solvent in 3/solvent
(Figs. 2.17 and 2.18). The powder XRD patterns of 3 ground obtained after grinding of 3/solvent are featureless, indicating that all 3 ground sample are amorphous
(Fig. 2.22). These amorphous phases of 3 ground do not include solvent molecules,
as shown by NMR spectroscopy and thermogravimetric analyses (TGA, Fig. 2.24),
indicating solvent release is induced by mechanical stimulation. The resulting 3 ground
again exhibited various emission color changes upon addition of specific solvents to
form 3/solvent. Thus, sequential emission color changes through multiple mechanical grinding/solvent addition processes is possible. Figure 2.7a presents the emission
color changes of a single sample. Initially, 3 ground shows an emission color change
from yellow to green upon CH 2 Cl 2 addition. Yellow emission can then be recovered
by mechanical stimulation (reactivation), which changes to blue upon addition of
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This indicates that 3/solvent all have crystalline nature, even though the powders of
3/solvent are prepared simply by adding a solvent to amorphous 3 ground and letting
the solvent evaporate under ambient conditions. Careful inspection of the powder
XRD patterns of the eleven 3/solvent samples confirmed that the peak positions
are different from each other (Fig. 2.6). This observation suggests that the crystal
structures of the resulting 3/solvent are different from each other and depend on the
solvent added to induce the phase transition of 3 ground . NMR spectroscopy indicates
that all 3/solvent contain solvent molecules inside the crystalline lattices (Fig. 2.6).
These results imply that the formation of different crystalline structures of 3/solvent
and solvent inclusion is responsible for their varied emission properties.
When other solvents not listed in the previous paragraph were added to 3 ground ,
no detectable emission color changes were observed in the resulting powders. We
also added 25 other different volatile compounds to 3 ground , but no emission color
change was observed; the yellow emission mostly remained. XRD measurements
of the resulting powders imply that the addition of these solvents does not induce
distinctive changes in molecular arrangement of 3. For example, the XRD patterns of
the powders obtained after treatment with toluene exhibit several peaks with very low
intensity (grey line in Fig. 2.6). These weak diffraction peaks indicate that toluene
only induces partial crystallization of 3. Such diffraction patterns with low intensity were also obtained after 3 ground was treated with benzene, trichloroethene, and
THF. Meanwhile, the powders of 3 obtained after treatment with hexane or MeOH
exhibited diffraction patterns with almost no peaks that were similar to that of 3 ground
(Fig. 2.19). We suppose that these differences in the powder XRD patterns are caused
by the different solubility of 3 in these solvents: the solubility of 3 in hexane or MeOH
is too low to recover a crystalline phase. In contrast, 3 partially dissolves in other
solvents (benzene, toluene, THF, or trichloroethene) to recover partial crystalline
phase. From the observation, we speculate that the above solvents that do not induce
emission color changes of 3 cannot form the corresponding solvated crystals with 3,
so 3 ground remains after solvent addition.
Mechanical stimulation can eliminate solvent molecules from 3/solvent to allow
reuse of 3 for solvent detection, which is more convenient than other reactivation
processes. Grinding eleven samples of 3/solvent using a spatula or ball milling regenerates yellow-emitting powder 3 ground irrespective of the type of solvent in 3/solvent
(Figs. 2.17 and 2.18). The powder XRD patterns of 3 ground obtained after grinding of 3/solvent are featureless, indicating that all 3 ground sample are amorphous
(Fig. 2.22). These amorphous phases of 3 ground do not include solvent molecules,
as shown by NMR spectroscopy and thermogravimetric analyses (TGA, Fig. 2.24),
indicating solvent release is induced by mechanical stimulation. The resulting 3 ground
again exhibited various emission color changes upon addition of specific solvents to
form 3/solvent. Thus, sequential emission color changes through multiple mechanical grinding/solvent addition processes is possible. Figure 2.7a presents the emission
color changes of a single sample. Initially, 3 ground shows an emission color change
from yellow to green upon CH 2 Cl 2 addition. Yellow emission can then be recovered
by mechanical stimulation (reactivation), which changes to blue upon addition of