12 White-Light Emissive Materials Based on Supramolecular …
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units were introduced at both ends of each component. In fluorescence titrations
of 1 with either 2 or 3, the emission of the donor was quenched when excited at
364 nm, which is consistent with the maximum wavelength of 1, whereas the emission
intensity of these acceptors increased simultaneously. High Stern-Volmer constants
suggest efficient energy transfer in the solution through UPy-based hydrogen interactions between the components. Supramolecular copolymers containing all three
RGB components were prepared by simultaneous incorporation of 2 and 3 into the
chains of 1. Fine-tuning of the component ratio allowed for a partial energy transfer
(Fig. 12.2d), resulting in the appearance of WLE at a certain ratio (1/2/3 = 59:33:8)
as shown in Fig. 12.2e. Interestingly, WLE could not been achieved using the mixture
of oligo(fluorene), oligo(phenylene vinylene), and perylene bisimide chromophores
that lacked the UPy units.
Similar approach using self-complementary hydrogen bonding was also reported
by Bassani et al. (Tseng et al. 2011). Three kinds of π-conjugated fluorene-,
bis(fluorenyl)benzothiadiazole-, and bis(thiophenyl)benzothiadiazole-derivatives
with biuret groups at the terminal groups were synthesized to emit red (R), green (G),
and blue (B), respectively. It is interesting to note that highly emissive RGB organic
nanospheres formed spontaneously when dropcast from dilute THF solutions onto
a substrate, where energy transfer process occurred in the tricomponent spherical
aggregates.
The investigation of co-crystal or multicomponent crystals has been interested
from the standpoint of advanced optical and electrical materials (Yan and Evans
2014). Ono and Hisaeda reported three-, four-, and five-component co-crystals with
flexile color tuning capability involving WLE (Ono and Hisaeda 2019). Toward this
end, supramolecular host was prepared by complexation of N,N-dipyrid-3-yl-1,4,5,8naphthalenediimide (NDI) with two tris(pentafluorophenyl)borane (TPFB) linked
by boron–nitrogen dative bonds as Lewis acid–base pairs, which spontaneously
formed upon mixing the components (Fig. 12.3a). Initially, three-component crystals with 14 representative aromatic guests (4–17) were evaluated by single-crystal
X-ray structure analysis, where various intermolecular interactions such as C–H···F
hydrogen bonding interactions, π–π stacking, charge-transfer interactions, and inclusion phenomena were of significance for crystal formation, although co-crystals
cannot be prepared with C18.
The photophysical properties of the three-component crystals C4–C17
(Fig. 12.3a), as well as guest-free crystal (GF), were investigated by means of UV-vis
diffuse-reflectance spectroscopy measurements with an integrating sphere. Although
GF and C4 showed similar absorption spectra, C5–C17 showed a new absorption
band in the visible region from 400 to 600 nm, which was dependent on the guest
molecules included. Those co-crystals (C4–C17) also showed unique photoluminescence characters upon excitation at 330–380 nm, which reflected the nature of
the guest molecules. It was found that color variation over the range of electrondeficient guests such as difluorobenzene (C5–C7), from visible-to-NIR luminescence, was achieved. The emission occurred with high quantum yields of 16.7% for
C9, 18.9% for C10, 28.3% for C11, 17.3% for C13, 26.2% for C14, and 10.8% for
C15, in the crystalline state. This result is attributable to the crystal structure and the
413
units were introduced at both ends of each component. In fluorescence titrations
of 1 with either 2 or 3, the emission of the donor was quenched when excited at
364 nm, which is consistent with the maximum wavelength of 1, whereas the emission
intensity of these acceptors increased simultaneously. High Stern-Volmer constants
suggest efficient energy transfer in the solution through UPy-based hydrogen interactions between the components. Supramolecular copolymers containing all three
RGB components were prepared by simultaneous incorporation of 2 and 3 into the
chains of 1. Fine-tuning of the component ratio allowed for a partial energy transfer
(Fig. 12.2d), resulting in the appearance of WLE at a certain ratio (1/2/3 = 59:33:8)
as shown in Fig. 12.2e. Interestingly, WLE could not been achieved using the mixture
of oligo(fluorene), oligo(phenylene vinylene), and perylene bisimide chromophores
that lacked the UPy units.
Similar approach using self-complementary hydrogen bonding was also reported
by Bassani et al. (Tseng et al. 2011). Three kinds of π-conjugated fluorene-,
bis(fluorenyl)benzothiadiazole-, and bis(thiophenyl)benzothiadiazole-derivatives
with biuret groups at the terminal groups were synthesized to emit red (R), green (G),
and blue (B), respectively. It is interesting to note that highly emissive RGB organic
nanospheres formed spontaneously when dropcast from dilute THF solutions onto
a substrate, where energy transfer process occurred in the tricomponent spherical
aggregates.
The investigation of co-crystal or multicomponent crystals has been interested
from the standpoint of advanced optical and electrical materials (Yan and Evans
2014). Ono and Hisaeda reported three-, four-, and five-component co-crystals with
flexile color tuning capability involving WLE (Ono and Hisaeda 2019). Toward this
end, supramolecular host was prepared by complexation of N,N-dipyrid-3-yl-1,4,5,8naphthalenediimide (NDI) with two tris(pentafluorophenyl)borane (TPFB) linked
by boron–nitrogen dative bonds as Lewis acid–base pairs, which spontaneously
formed upon mixing the components (Fig. 12.3a). Initially, three-component crystals with 14 representative aromatic guests (4–17) were evaluated by single-crystal
X-ray structure analysis, where various intermolecular interactions such as C–H···F
hydrogen bonding interactions, π–π stacking, charge-transfer interactions, and inclusion phenomena were of significance for crystal formation, although co-crystals
cannot be prepared with C18.
The photophysical properties of the three-component crystals C4–C17
(Fig. 12.3a), as well as guest-free crystal (GF), were investigated by means of UV-vis
diffuse-reflectance spectroscopy measurements with an integrating sphere. Although
GF and C4 showed similar absorption spectra, C5–C17 showed a new absorption
band in the visible region from 400 to 600 nm, which was dependent on the guest
molecules included. Those co-crystals (C4–C17) also showed unique photoluminescence characters upon excitation at 330–380 nm, which reflected the nature of
the guest molecules. It was found that color variation over the range of electrondeficient guests such as difluorobenzene (C5–C7), from visible-to-NIR luminescence, was achieved. The emission occurred with high quantum yields of 16.7% for
C9, 18.9% for C10, 28.3% for C11, 17.3% for C13, 26.2% for C14, and 10.8% for
C15, in the crystalline state. This result is attributable to the crystal structure and the