4.6 Luminescent Properties and Mechano-Responsive Luminescence
117
of 1ac (Figs. 4.19 and 4.20). The HOMO-LUMO energy gap in 1ac is by 0.47 eV
larger than that in 1ch. The HOMO in both crystals in located on the gold atoms
(Fig. 4.20).
As shown in Fig. 4.13, 1 ground exhibited an emission spectrum with weak intensity
(Φ em = 0.28) and an emission maximum at λ em,max = 539 nm (Fig. 4.13, blue solid
line), which is similar to that of 1ch (λ em,max = 542 nm). Moreover, the emission
decay of 1 ground (τ av = 5.30 μs) is comparable to that of 1ch (Fig. 4.17 and Table 4.3).
These similarities in the photophysical properties of 1ch and 1 ground are inconsistent
with the fact that the PXRD pattern of 1 ground is reasonably matched to the simulated
pattern of 1ac rather than 1ch. This may indicate that 1 ground contains a small amount
of the original 1ch phase within a major crystalline domain consisting of 1ac. This
contamination of a small amount of 1ch could be responsible for the overall green
emission of 1 ground , followed by energy transfer from the main 1ac domain [6e]. The
1 ground showed no further changes over time, after ceasing the grinding, indicating
that there was no further crystal-to-crystal seeding event from the non-altered minor
domain of 1ch to 1ac (Fig. 4.22).
4.7 Summary
Gold complex 1, which contains a biphenyl moiety, afforded chiral crystals of 1ch
and centrosymmetric (achiral) crystals of 1ac, both of which exhibit distinct emission
properties. The single-crystal and powder XRD analyses revealed that chiral crystals
of 1ch transformed into achiral crystals similar to 1ac upon grinding. The change of
emission and optical properties of 1ch upon grinding is correlated to the crystalline
structural changes. This is the first example of mechano-induced chiral-crystal-toachiral-crystal (CCAC) transition under concomitantly changing luminescent properties. These results indicate that the dynamic alternation of chirality of crystalline
phases may be a promising key strategy to design universal mechano-responsive
functional materials.
4.8 Experimental Section
4.8.1 General
All commercially available reagents and solvents are of reagent grade and were used
without further purification unless otherwise noted. Solvents for the synthesis were
purchased from commercial suppliers, degassed by three freeze-pump-thaw cycles
and further dried over molecular sieves (4 Å). NMR spectra were recorded on a JEOL
JNM-ECX400P or JNM-ECS400 spectrometer (
1 H: 400 MHz;
13 C: 99.5 MHz) using
tetramethylsilane and CDCl 3 as internal standards, respectively. Emission spectra
117
of 1ac (Figs. 4.19 and 4.20). The HOMO-LUMO energy gap in 1ac is by 0.47 eV
larger than that in 1ch. The HOMO in both crystals in located on the gold atoms
(Fig. 4.20).
As shown in Fig. 4.13, 1 ground exhibited an emission spectrum with weak intensity
(Φ em = 0.28) and an emission maximum at λ em,max = 539 nm (Fig. 4.13, blue solid
line), which is similar to that of 1ch (λ em,max = 542 nm). Moreover, the emission
decay of 1 ground (τ av = 5.30 μs) is comparable to that of 1ch (Fig. 4.17 and Table 4.3).
These similarities in the photophysical properties of 1ch and 1 ground are inconsistent
with the fact that the PXRD pattern of 1 ground is reasonably matched to the simulated
pattern of 1ac rather than 1ch. This may indicate that 1 ground contains a small amount
of the original 1ch phase within a major crystalline domain consisting of 1ac. This
contamination of a small amount of 1ch could be responsible for the overall green
emission of 1 ground , followed by energy transfer from the main 1ac domain [6e]. The
1 ground showed no further changes over time, after ceasing the grinding, indicating
that there was no further crystal-to-crystal seeding event from the non-altered minor
domain of 1ch to 1ac (Fig. 4.22).
4.7 Summary
Gold complex 1, which contains a biphenyl moiety, afforded chiral crystals of 1ch
and centrosymmetric (achiral) crystals of 1ac, both of which exhibit distinct emission
properties. The single-crystal and powder XRD analyses revealed that chiral crystals
of 1ch transformed into achiral crystals similar to 1ac upon grinding. The change of
emission and optical properties of 1ch upon grinding is correlated to the crystalline
structural changes. This is the first example of mechano-induced chiral-crystal-toachiral-crystal (CCAC) transition under concomitantly changing luminescent properties. These results indicate that the dynamic alternation of chirality of crystalline
phases may be a promising key strategy to design universal mechano-responsive
functional materials.
4.8 Experimental Section
4.8.1 General
All commercially available reagents and solvents are of reagent grade and were used
without further purification unless otherwise noted. Solvents for the synthesis were
purchased from commercial suppliers, degassed by three freeze-pump-thaw cycles
and further dried over molecular sieves (4 Å). NMR spectra were recorded on a JEOL
JNM-ECX400P or JNM-ECS400 spectrometer (
1 H: 400 MHz;
13 C: 99.5 MHz) using
tetramethylsilane and CDCl 3 as internal standards, respectively. Emission spectra