3.4 Studies of Crystal Structure Alterations by Mechanical Stimulation
89
by holding the samples under vacuum at 65 °C for 1 h (Fig. 3.12). Both (rac)-1G
and (S)-1G remained crystalline after removal of CH 2 Cl 2 , and their emission colors
did not change (Figs. 3.11 and 3.12). These results indicate that inclusion/desorption
of CH 2 Cl 2 do not correspond to the alterations of the crystal structures and the
mechanochromic properties.
3.5 Luminescent Mechanochromism of (rac)-1G and (S)-1G
Emission spectroscopy measurements of pristine (rac)-1G and (S)-1G and their
ground phases were performed. The emission spectrum of (rac)-1G is broad with a
maximum emission wavelength λ em,max of 546 nm upon excitation at 365 nm (light
green solid line in Fig. 3.4a). Upon grinding, the emission of (rac)-1G shows a clear
red shift. The ground powder (rac)-1G ground exhibited a broad emission spectrum
with λ em,max at 664 nm (orange solid line in Fig. 3.4a). The emission spectrum of
(S)-1G (green solid line in Fig. 3.4a) is different from that of (rac)-1G. For example,
λ em,max of (S)-1G is 517 nm, which is shorter than that of (rac)-1G. In contrast, the
emission spectrum of (S)-1G ground (λ em,max = 664 nm) (red solid line in Fig. 3.4a)
is almost the same as that of (rac)-1G ground .
Excitation spectra of the samples were measured. The excitation spectrum of
(rac)-1G contained a broad band ranging from 350 to 480 nm with a maximum
excitation wavelength λ ex,max of 454 nm (λ em = 546 nm, light green solid line in
Fig. 3.4b). The excitation spectrum of (rac)-1G ground also possessed a similar broad
band with λ ex,max = 449 nm (λ em = 546 nm, orange solid line in Fig. 3.4b). In the
case of (S)-1G, similar excitation spectral changes were observed. The excitation
spectrum of (S)-1G obtained at 517 nm contained a broad band spanning 350–500 nm
with λ ex,max = 458 nm (green solid line in Fig. 3.4b). Excitation of (S)-1G ground at
664 nm produced a similar broad excitation band with λ ex,max = 445 nm. This result
indicates that the excitation spectra of this system are not strongly influenced by
distinct molecular packing arrangements.
Emission lifetimes and quantum yields Φ em of both crystals and their ground
powders were measured. (rac)-1G showed a low Φ em of 0.08 with an average emission lifetime τ av of 5.34 μs, while the ground powder (rac)-1G ground displayed an
increased Φ em of 0.21. τ av of (rac)-1G ground is 5.10 μs, which is slightly shorter
than that of (rac)-1G (Table 3.1). In the case of (S)-1G, Φ em was also low (0.02).
After grinding, Φ em of (S)-1G ground increased to 0.13. Meanwhile, τ av of (S)-1G and
(S)-1G ground were almost the same at 5.34 and 5.36 μs, respectively. The photoluminescent intensities of (rac)-1 and (S)-1 in CH 2 Cl 2 were very weak, so we could not
measure their emission properties. This indicates that the high emission intensities
of solid-state (rac)-1 and (S)-1 strongly depend on their aggregated structures [9].
The relationships between the emission properties and crystal structures of each
phase are now discussed. Dimer units constructed from two molecules of the same
enantiomer, such as S-S dimers, are observed in both (rac)-1G and (S)-1G. The
fundamental structures of dimers found in (rac)-1G and (S)-1G crystals are very
89
by holding the samples under vacuum at 65 °C for 1 h (Fig. 3.12). Both (rac)-1G
and (S)-1G remained crystalline after removal of CH 2 Cl 2 , and their emission colors
did not change (Figs. 3.11 and 3.12). These results indicate that inclusion/desorption
of CH 2 Cl 2 do not correspond to the alterations of the crystal structures and the
mechanochromic properties.
3.5 Luminescent Mechanochromism of (rac)-1G and (S)-1G
Emission spectroscopy measurements of pristine (rac)-1G and (S)-1G and their
ground phases were performed. The emission spectrum of (rac)-1G is broad with a
maximum emission wavelength λ em,max of 546 nm upon excitation at 365 nm (light
green solid line in Fig. 3.4a). Upon grinding, the emission of (rac)-1G shows a clear
red shift. The ground powder (rac)-1G ground exhibited a broad emission spectrum
with λ em,max at 664 nm (orange solid line in Fig. 3.4a). The emission spectrum of
(S)-1G (green solid line in Fig. 3.4a) is different from that of (rac)-1G. For example,
λ em,max of (S)-1G is 517 nm, which is shorter than that of (rac)-1G. In contrast, the
emission spectrum of (S)-1G ground (λ em,max = 664 nm) (red solid line in Fig. 3.4a)
is almost the same as that of (rac)-1G ground .
Excitation spectra of the samples were measured. The excitation spectrum of
(rac)-1G contained a broad band ranging from 350 to 480 nm with a maximum
excitation wavelength λ ex,max of 454 nm (λ em = 546 nm, light green solid line in
Fig. 3.4b). The excitation spectrum of (rac)-1G ground also possessed a similar broad
band with λ ex,max = 449 nm (λ em = 546 nm, orange solid line in Fig. 3.4b). In the
case of (S)-1G, similar excitation spectral changes were observed. The excitation
spectrum of (S)-1G obtained at 517 nm contained a broad band spanning 350–500 nm
with λ ex,max = 458 nm (green solid line in Fig. 3.4b). Excitation of (S)-1G ground at
664 nm produced a similar broad excitation band with λ ex,max = 445 nm. This result
indicates that the excitation spectra of this system are not strongly influenced by
distinct molecular packing arrangements.
Emission lifetimes and quantum yields Φ em of both crystals and their ground
powders were measured. (rac)-1G showed a low Φ em of 0.08 with an average emission lifetime τ av of 5.34 μs, while the ground powder (rac)-1G ground displayed an
increased Φ em of 0.21. τ av of (rac)-1G ground is 5.10 μs, which is slightly shorter
than that of (rac)-1G (Table 3.1). In the case of (S)-1G, Φ em was also low (0.02).
After grinding, Φ em of (S)-1G ground increased to 0.13. Meanwhile, τ av of (S)-1G and
(S)-1G ground were almost the same at 5.34 and 5.36 μs, respectively. The photoluminescent intensities of (rac)-1 and (S)-1 in CH 2 Cl 2 were very weak, so we could not
measure their emission properties. This indicates that the high emission intensities
of solid-state (rac)-1 and (S)-1 strongly depend on their aggregated structures [9].
The relationships between the emission properties and crystal structures of each
phase are now discussed. Dimer units constructed from two molecules of the same
enantiomer, such as S-S dimers, are observed in both (rac)-1G and (S)-1G. The
fundamental structures of dimers found in (rac)-1G and (S)-1G crystals are very