Mechanofluorochromic Properties of 1,4-Diphenylanthracene Derivatives with Hypsochromic Shift

Several types of 1,4-diphenylanthracene derivatives 1–4 were prepared, and their photophysical properties were observed in the solid and solution states. Interestingly, the CN-group-substituted 1,4-diphenylanthracene derivative 2 was found to exhibit a higher fluorescence quantum yield (ϕf = 0.71) in the solid state than in the solution state, probably due to the formation of an intermolecular Ar–CN⋯H–Ar hydrogen bond and antiparallel type locked packing structure in the solid state. Furthermore, for some derivatives, an increase in the fluorescence quantum yield was observed in the PMMA film (1 wt%) over both the solid state and the solution state. More interestingly, some of the 1,4-diphenylanthracene derivatives exhibited unusual mechanofluorochromic properties with a “hypsochromic shift” in luminous color depending on the substituents of the phenyl group, and with the derivatives having CF3, OMe, CN, and two F substituents (1d–1f, 2–4) showing a significant luminous color change with a “hypsochromic shift” after grinding. However, no change in the luminous color was observed for the derivatives having H, Me, and one F substituent (1a–1c), and especially for some of the CN-substituted derivatives, a reversible luminous color change with a “hypsochromic shift” was observed, probably due to the formation of an antiparallel type packing structure. These “hypsochromic” anthracene derivatives could probably be utilized as new mechanofluorochromic materials.

Recently, our research group reported that structurally simple 1,8-diphenylanthracene derivatives exhibit characteristic mechanofluorochromic properties with the normal "bathochromic shift" [62].Interestingly, the core anthracene itself does not exhibit any "bathochromic shift" [62].Interestingly, the core anthracene itself does not exhibit any mechanofluorochromic properties at room temperature, but the introduction of phenyl substituents causes it to exhibit mechanofluorochromic properties [65].In order to develop this structurally simple mechanofluorochromic material and to clarify the reason for the role of the substituted phenyl groups and their substitution positions (i.e., whether or not the substitution at the 1,8-positions of the phenyl group is essential), we synthesized 1,4-diphenylanthracene derivatives 1-4 and studied several mechanofluorochromic properties.For the mechanofluorochromic properties of the 1,4-diphenylanthracene derivatives 1a (R = H), 1b (R = Me), and 1c (R = F), the blue luminous color of the pristine sample did not change after grinding.Interestingly, however, "hypsochromic shift" of the luminous color changes (e.g., yellow-green to blue-green) was observed after grinding for derivatives 1e, 1f, and 2-4 having OMe, CF3, CN, and two F groups.Especially the 1,4-diphenylanthracene derivatives 1f (R = CN), 2 (3-CN), and 3 (3,5-CN2, R′ = CN) substituted with CN groups underwent repeated reversible luminous color changes with "hypsochromic shift" upon grinding, heating, and fuming with solvent vapors, although some of the derivatives could not be fully recovered.This unique "hypsochromic shift" with repeated reversible emission color changes is thought to be controlled by the hydrogen bonding between the Ar-CN and Ar-H groups (Ar-CN⋯H-Ar) and the formation of an antiparallel type packing structure.These unique "hypsochromic" anthracene derivatives could probably be applied as new mechanofluorochromic materials.
To understand the relationship between the crystal packing and solid state photophysical properties (fluorescence quantum yield, fluorescence lifetime, and luminous color) of the 1,4-diphenylanthrcene derivatives, the single crystal structures of 1a (R = H, light-blue luminous color, LB), 1b (R = Me, light-blue luminous color, LB), 1c (R = F, light-blue luminous color, LB), 1d (R = OMe, yellow-green luminous color, YG), 1f (R = CN, yellow-green luminous color, YG), and 2 (3-CN, yellow-green luminous color, YG) were analyzed.Single crystals were obtained from dimethoxyethane (DME) solutions for 1d and 1f, from hexane solutions for 1a and 1c, from a CHCl 3 solution for 2, and from a CH 2 Cl 2 solution for 1b.For the bimolecular stacked crystal packing of 1a (R = H), 1b (R=Me), 1c (R = F), and 1d (R = OMe), the 1,4-diphenylanthracene rings are arranged in the same direction, called the "parallel orientation" (Figure 2).On the other hand, in the bimolecular stacked crystal packing of the CN-group substituted derivatives 1f (R = CN) and 2 (3-CN), the 1,4-diphenylanthracene rings are arranged in the "antiparallel orientation", and a stronger π-stacking is observed than in the "parallel orientation" (Figure 2).Additionally, Ar-CN• • • H-Ar or Ar-F• • • H-Ar hydrogen bonds were observed in the crystal packing of 1c (R = F), 1f (R = CN), and 2 (3-CN) with CN or F substituents (Figures S28, S32 and S34).To understand the reason for the formation of the "parallel" and "antiparallel" orientation packings of the 1,4-diphenylanthracene rings in the crystal structure, the electrostatic potential surfaces were calculated using DFT calculations according to the obtained crystal data (Figures S41 and S42).For the CN-group substituted derivatives 1f (R = CN) and 2 (3-CN), the positive charges are located at the edge of the substituted phenyl ring, as shown in Figures S41 and S42, and these positively charged edges are stacked with the center part of the phenyl ring of the other molecules to form a stable stacking bimolecular "antiparallel" packing.Additionally, in the crystal structure of the bimolecular packing of 1f (R = CN) and 2 (3-CN), many interactions, such as CH-π interactions between two 1,4-diphenylanthracen rings, were observed compared to the other non-CN substituted derivatives (1a-1d), as shown in Figure S36, and it may be showing that the "antiparallel" orientation is more stable than the "parallel" orientation in the case of electron withdrawing CN-group substituted derivatives.According to the crystal structure analysis and photophysical data of the 1,4diphenylanthracene derivatives, we can discuss the following: (1) Relation between solid-state luminous color, crystal packing, Hirshfeld surface analysis, and fluorescence lifetimes: As already mentioned, the luminous colors of the pristine samples of the 1,4-diphenylanthracene derivatives show light-blue (1a-1c, LB), green (1e, 4, G), and yellow-green (1d, 1f, 2, 3, YG) colors.In the case of the light-blue luminous derivatives 1a-1c, the emission is probably derived from the monomer of the 1,4diphenylanthracene rings since the crystal packing of 1a-1c showed only partial overlapping of the anthracene ring, as indicated by the purple and green colors in Figure 2a,c.On the other hand, in the case of the yellow-green luminous derivatives 1f (R = CN) and 2 (3-CN), the emission is probably derived from the excimer state of the 1,4-diphenylanthracene rings, and a greater overlapping of the anthracene rings compared to 1a-1c is observed, as shown in Figure 2e,f.In the case of the yellow-green luminous MeO-group substituted derivative 1d, the crystal packing is very similar (overlapping only one benzene ring) to the light-blue luminous derivatives 1a-1c, as shown in Figure 2a-d, suggesting that an intermolecular charge transfer may be operating (see Figures S39 and S40, DFT calculation) and showing a yellow-green luminous color.To further understand the contribution of the π-π stacking for the crystal packing, the Hirshfeld surface analysis according to the crystal data was performed (Figure 3).The contribution of π-π stacking for 1f (R = CN, contribution of the C-C interactions: 13.8%) and 2 (3-CN, contribution of the C-C interactions: 13.5%) is greater than that for 1a-1c (contribution of the C-C interactions: 6.4-9.8%),indicating that the yellow-green emission of 1f and 2 is due to the formation of the π-π stacking (excimer emission).The fluorescence lifetimes of the 1,4-diphenylanthracene derivatives for the pristine state also show the difference in the contribution of the π-π stacking interactions as follows.Short fluorescence lifetimes between 2.27-3.96ns were observed for the light-blue luminous samples 1a-1c; however, slightly longer fluorescence lifetimes were observed for the green luminous samples 1e (4.69 ns, R = CF 3 ) and 4 (5.22 ns, R ′ = F, 3,5-F 2 ) and for the yellow-green luminous samples 1f (R = CN), 2 (3-CN) and 3 (R ′ = CN, 3,5-CN 2 ), longer fluorescence lifetimes were observed between 7.39 and 109 ns (Table 1, Figure S20).These observed fluorescence lifetimes are increasing in the order of light-blue < green < yellow-green luminous color (the fluorescence wavelengths also increased in the same order).These results indicated that the contribution of the π-π stacking interactions is also increasing in the same order, which is in good agreement with the crystal data only except for 1d.(2) Relation between fluorescence quantum yield, fluorescence lifetime, and crystal packing of 1f (R=CN) and 2 (3-CN): The pristine sample of 2 (3-CN) exhibited a relatively high fluorescence quantum yield (ϕ f = 0.71) compared to the other 1,4-diphenylanthracene derivatives (ϕ f = 0.15-0.44,Table 1), as already described.To consider the reason for the higher fluorescence quantum yield of 2 (3-CN), we calculated the rate constant k f for the fluorescence and the rate constant k nr for non-radiative decay (Table S1) and compared k f and k nr to that of 1f (R = CN) and 2 (3-CN).The rate constants k f for the fluorescence of 1f (k f = 0.008 ns −1 ) and 2 (k f = 0.006 ns −1 ) were almost the same, but the rate constant k nr for the non-radiative decay of 2 (k nr = 0.003 ns −1 ) was about seven times lower than that of 1f (k nr = 0.022 ns −1 ), and the difference in k nr caused the fluorescence quantum yield of 2 (ϕ f = 0.71) to be higher than that of 1f (ϕ f = 0.26) in the pristine solid state.The difference in k nr between 1f and 2 may be due to the difference in the crystal packing structure.Due to the substitution of a CN group at the 3-position (meta-position) of the phenyl group in the derivative

Photophysical Properties of 1,4-Diphenylanthracene Derivatives in Solution and in PMMA Films
To compare the photophysical properties of the 1,4-diphenylanthracene derivatives in the solid and solution states, we measured their UV spectra, fluorescence spectra,

Photophysical Properties of 1,4-Diphenylanthracene Derivatives in Solution and in PMMA Films
To compare the photophysical properties of the 1,4-diphenylanthracene derivatives in the solid and solution states, we measured their UV spectra, fluorescence spectra, fluorescence quantum yields, and fluorescence lifetimes of the 1,4-diphenylanthracene derivatives in chloroform solutions (Table 1, Figures S14-S16 and S21).In the case of 1,4diphenylanthracene derivatives 1a-1c, 1e, 2, and 4, a blue luminous color (421-443 nm) was observed in the CHCl 3 solution.However, for derivatives 1d (R = OMe), 1f (R = CN), and 3 (3,5-CN 2 ), which have electron-donating and electron-withdrawing groups, a light-blue luminous color (454-458 nm) was observed in the CHCl 3 solution (Figures S15 and S16).These derivatives may be causing a charge-transfer interaction in the solution state, but only a small solvatochromic effect on the fluorescence spectra was observed in different solvents (Figure S17).For all the 1,4-diphenylanthracene derivatives, the fluorescence quantum yields were observed with a moderate efficiency around ϕ f = 0.41-0.56,and for almost all derivatives, the CHCl 3 solution state showed higher fluorescence quantum yields than the pristine solid state.Additionally, the fluorescence lifetimes were observed around τ = 2.57-3.36ns for all the derivatives, probably indicating a monomer emission in the solution state.Interestingly, only derivative 2 (3-CN) showed a higher fluorescence quantum yield (ϕ f = 0.71) in the pristine solid state than in the solution state (ϕ f = 0.46 in CHCl 3 ).
In order to develop new fluorescent materials, it is important that the newly synthesized fluorescent materials exhibit high fluorescence properties even in the polymersupported solid state.Therefore, we measured the photophysical properties of the 1,4diphenylanthracene derivatives in PMMA (polymethyl methacrylate).The desired 1,4diphenylanthracene derivative-containing PMMA film (1 wt%) was prepared by casting on a glass plate using a CHCl 3 solution.For most of the PMMA films obtained, a blue or light-blue luminous color was observed, and the fluorescence spectra was similar to that of the CHCl 3 solution but was significantly different from that of the pristine solid (Figures S18 and S19).These observations indicated that the blue emission of the 1,4diphenylanthracene derivatives in the PMMA films is derived from the monomer emission, which is a significant difference from the solid-state emission (Figure S18).The fluorescence quantum yields of the PMMA films (1 wt%) of the 1,4-diphenylanthracene derivatives show relatively high efficiencies around ϕ f = 0.62-0.71.Interestingly, the fluorescence quantum yields in the PMMA films are increased compared to the solid and solution states, except for 2 (3-CN).Additionally, the fluorescent lifetime of the PMMA film (1 wt%) of the 1,4-diphenylanthracene derivatives exhibited a slightly longer lifetime (τ = 3.45-4.16ns) than the solution state (Tables S1 and S2, Figure S22).Based on these photophysical measurements, the rate constant k f for fluorescence of the PMMA films (k f = 0.1425-0.2058)was found to be similar to that of the corresponding solution state (Tables S1 and S2).However, the rate constants k nr for non-radiative decay of the PMMA film (k nr = 0.0837-0.0936)was found to be much lower than for the solution and solid states (Tables S1 and S2).The lower k nr value showed the inhibition of the molecular motion in the PMMA film and showed an increasing fluorescence quantum yield in the PMMA films than in the solution state.In the pristine solid state of the 1,4-diphenylanthracene derivatives, the aggregation-caused quenching (ACQ) effect that may have occurred and the fluorescence quantum yields were reduced, except for derivative 2 (3-CN).
For the mechanofluorochromic properties of the 1,8-diphenylanthracene derivatives previously reported, some of the derivatives recovered to their original fluorescent color after grinding within 5 min.at room temperature, but the CN-group substituted derivatives did not recover to their original emission color at room temperature [62].However, this mechanofluorochromism of the 1,4-diphenylanthracene derivatives was not observed as a self-recovering phenomenon within 5 min.at room temperature in the luminous color changed derivatives 1d-1f and 2-4 (Figures S1 and S2).
For the yellow-green (YG) emitting CN-substituted derivatives 1f, 2, and 3, the fluorescence lifetimes (τ) of the ground samples became shorter than those of the pristine samples (Table 1), along with a "hypsochromic shift" of the luminous color.These results indicated that the π-π stacking structure dissociates after grinding, exhibiting a "hypsochromic shift" in the luminous color and a shortening of the fluorescence lifetimes.
To further understand the mechanofluorochromism of the 1,4-diphenylanthracene derivatives, the UV and IR spectra were observed in the solid state for the pristine and ground samples.In the UV spectra, a blue shift (hypsochromic shift) and decreasing of the longer wavelength peaks were observed for the derivatives 1d-1f, 2-4 (Figures 5b and S13), which showed a "hypsochromic shift" in the fluorescence spectra after grinding, indicating a decrease in the contribution of the π-stacking structure after grinding.Furthermore, infrared (IR) spectra of the CN-substituted 1,4-diphenylanthracene derivatives 1f (R = CN) and 2 (3-CN) before and after grinding were observed, and the CN stretching band of the 1f and 2 shifted to the higher wavenumber side from the pristine to ground one, indicating that the hydrogen bond (Ar-CN• • • H-Ar) strength decreased after grinding (Figures 6, S10 and S11).
For the mechanofluorochromic properties of the 1,8-diphenylanthracene derivatives previously reported, some of the derivatives recovered to their original fluorescent color after grinding within 5 min.at room temperature, but the CN-group substituted derivatives did not recover to their original emission color at room temperature [62].However, this mechanofluorochromism of the 1,4-diphenylanthracene derivatives was not observed as a self-recovering phenomenon within 5 min.at room temperature in the luminous color changed derivatives 1d-1f and 2-4 (Figures S1 and S2).
For the yellow-green (YG) emi ing CN-substituted derivatives 1f, 2, and 3, the fluorescence lifetimes (τ) of the ground samples became shorter than those of the pristine samples (Table 1), along with a "hypsochromic shift" of the luminous color.These results indicated that the π-π stacking structure dissociates after grinding, exhibiting a "hypsochromic shift" in the luminous color and a shortening of the fluorescence lifetimes.To further understand the mechanofluorochromism of the 1,4-diphenylanthracene derivatives, the UV and IR spectra were observed in the solid state for the pristine and ground samples.In the UV spectra, a blue shift (hypsochromic shift) and decreasing of the longer wavelength peaks were observed for the derivatives 1d-1f, 2-4 (Figures 5b and  S13), which showed a "hypsochromic shift" in the fluorescence spectra after grinding, indicating a decrease in the contribution of the π-stacking structure after grinding.Further-

Vapochromic and Thermochromic Properties of 1,4-Diphenylanthracene Derivatives
To understand the reversibility of the luminous color of the 1,4-diphenylanthracene derivatives, fluorescence images and fluorescence spectra of the ground samples were observed after fuming with the CH 2 Cl 2 vapor.The blue-green luminescent ground samples of derivatives 1d (R = OMe), 1e (R = CF 3 ), and 4 (3,5-F 2 , R ′ = F) showed no recovery of the luminous color (recovery to green or yellow-green, bathochromic shift) after fuming with the CH 2 Cl 2 vapor for 20 h (Figures S3 and S4).Interestingly, for the CN-group-substituted 1,4-diphenylanthracene derivatives 1f (R = CN), 2 (3-CN), and 3 (3,5-CN 2 , R ′ = CN), the yellow-green luminous color of the pristine sample turned to the green or blue-green luminous color ("hypsochromic shift") with grinding, and the original yellow-green luminous color was recovered by fuming with solvent vapor (Figures 7, S3 and S4).Additionally, repeated grinding-fuming cycles were also observed, but in some cases, a decrease in the fluorescence intensity and decreasing of the color-switching ability were observed (Figures 8, S5 and S6).To understand the reason for the decrease in the fluorescence intensity and decreasing of the color-switching ability, we measured fluorescence spectra and 1 H NMR spectra of the derivatives 1a (R = H) and 2 (3-CN) after UV irradiation in the solid state (365 nm, 5 min.).In the fluorescence spectra of 1a, no change in fluorescence intensity was observed after 365 nm UV irradiation.On the other hand, a slight decrease in fluorescence intensity was observed for derivative 2, but no spectral change was observed in the 1 H NMR spectrum after dissolving the irradiated samples (Figures S43 and S44).
Again, for derivatives 1d (R = OMe), 1e (R = CF 3 ), and 4 (3,5-F 2 , R ′ = F), no recovery to the original luminous color (pristine state) by heating the ground sample was observed (Figures S3 and S4).However, for the CN-group-substituted 1,4-diphenylanthracene derivatives 1f (R = CN), 2 (3-CN), and 3 (3,5-CN 2 , R ′ = CN), a similar recovery of the luminous color of the ground sample to the luminous color of the pristine sample by heating was also observed (Figures 7, S3, S4, and S7-S9).The green luminous ground sample of 1f (R = CN) turned yellow-green when heated at 80 • C for 5 min., a recovery of the green luminous color was observed upon regrinding (Figures 7 and 8a), and the repeated cycles of grinding-heating are shown in Figure 8c.Although repeated grindingheating cycles were observed, in some cases, a decrease in the fluorescence intensity and decreasing of the color-switching ability were observed.ures 8, S5, and S6).To understand the reason for the decrease in the fluorescence intensity and decreasing of the color-switching ability, we measured fluorescence spectra and 1 H NMR spectra of the derivatives 1a (R=H) and 2 (3-CN) after UV irradiation in the solid state (365 nm, 5 min.).In the fluorescence spectra of 1a, no change in fluorescence intensity was observed after 365 nm UV irradiation.On the other hand, a slight decrease in fluorescence intensity was observed for derivative 2, but no spectral change was observed in the 1 H NMR spectrum after dissolving the irradiated samples (Figures S43 and S44).Again, for derivatives 1d (R = OMe), 1e (R = CF3), and 4 (3,5-F2, R′ = F), no recovery to the original luminous color (pristine state) by heating the ground sample was observed (Figures S3 and S4).However, for the CN-group-substituted 1,4-diphenylanthracene derivatives 1f (R = CN), 2 (3-CN), and 3 (3,5-CN2, R′ = CN), a similar recovery of the luminous color of the ground sample to the luminous color of the pristine sample by heating was also observed (Figures 7, S3, S4, and S7-S9).The green luminous ground sample of 1f (R = CN) turned yellow-green when heated at 80 °C for 5 min., a recovery of the green luminous color was observed upon regrinding (Figures 7 and 8a), and the repeated cycles of grinding-heating are shown in Figure 8c.Although repeated grinding-heating cycles were observed, in some cases, a decrease in the fluorescence intensity and decreasing of the color-switching ability were observed.To further understand the grinding-heating process of the CN-substituted 1,4-diphenylanthracene derivative 1f (R = CN), differential scanning calorimetry (DSC) was conducted to observe the phase transition between the ground and ground-heated samples.For the yellow-green luminous pristine sample of 1f, only the endothermic peak of melting (281.6 °C, 285.9 °C) was observed (Figure 9a).On the other hand, a broad exothermic peak was observed around 89.1 °C in the green luminous ground sample, which may have caused the transition from the green to yellow-green luminous state.Similar exothermic peaks were also observed in the ground samples for derivative 2 with a substituted CN group (Figure 9b).The DSC analysis of the other 1,4-diphenylanthracene derivatives 1a-1e was examined, but only endothermic melting peaks were observed (Figure S12).
To understand in detail the mechanofluorochromic properties of the 1,4-diphenylanthracene derivatives, the powder X-ray diffraction pa erns (PXRD) of the pristine, ground, ground-fumed, and ground-heated samples of derivative 1f (R = CN) were observed.The "yellow-green" luminous pristine sample of 1f showed a sharp and strong peak in the PXRD measurement (Figure 10a), but no diffraction peak was observed in the ground "green luminous" sample (Figure 10b).After heating at 80 °C for 5 min., the ground sample recovered its "yellow-green luminous color" and showed small crystallization peaks (Figure 10c).Similar crystallization of the ground sample was also observed when fuming with CH2Cl2 vapor for 2 h (Figure 10d).These observations, like those in the previously reported paper, show that this mechanofluorochromism may be operating under a phase transition from the crystalline state to the amorphous state  To further understand the grinding-heating process of the CN-substituted 1,4diphenylanthracene derivative 1f (R = CN), differential scanning calorimetry (DSC) was conducted to observe the phase transition between the ground and ground-heated samples.For the yellow-green luminous pristine sample of 1f, only the endothermic peak of melting (281.6 • C, 285.9 • C) was observed (Figure 9a).On the other hand, a broad exothermic peak was observed around 89.1 • C in the green luminous ground sample, which may have caused the transition from the green to yellow-green luminous state.Similar exothermic peaks were also observed in the ground samples for derivative 2 with a substituted CN group (Figure 9b).The DSC analysis of the other 1,4-diphenylanthracene derivatives 1a-1e was examined, but only endothermic melting peaks were observed (Figure S12).
Although it is difficult to further understand the critical mechanism of this mechanofluorochromism, it can be classified as follows: (Type-A) the 1,4-diphenylanthracene derivatives 1a-1c show a blue luminous color in the pristine state, and the crystal packing showed a parallel-type orientation.Only a weaker π-π stacking was observed, and its blue luminous color probably comes from monomer emission.After grinding the pristine sample, it does not change the original luminous color (Scheme 2a); (Type-B) the derivatives 1d, 1e, and 4 show green and yellow-green luminous colors in the pristine state, and   Furthermore, the IR spectra of the ground-heated and ground-fumed samples showed a recovery of the wavenumber of the CN stretching bands from the ground state in derivatives 1f and 2 (a shorter wavenumber shift from the ground sample was observed; Figures 6, S10 and S11), and only the CN-substituted 1,4-diphenylanthracene derivatives showed a reversible emission color change, indicating that the hydrogen bond between the Ar-CN group and the Ar-H hydrogen atom (Ar-CN⋯H-Ar) is important for the controllable reversible mechanofluorochromism in the CN-substituted 1,4-diphenylanthracene derivatives.
Although it is difficult to further understand the critical mechanism of this mechanofluorochromism, it can be classified as follows: (Type-A) the 1,4-diphenylanthracene derivatives 1a-1c show a blue luminous color in the pristine state, and the crystal packing showed a parallel-type orientation.Only a weaker π-π stacking was observed, and its blue luminous color probably comes from monomer emission.After grinding the pristine sample, it does not change the original luminous color (Scheme 2a); (Type-B) the derivatives 1d, 1e, and 4 show green and yellow-green luminous colors in the pristine state, and Furthermore, the IR spectra of the ground-heated and ground-fumed samples showed a recovery of the wavenumber of the CN stretching bands from the ground state in derivatives 1f and 2 (a shorter wavenumber shift from the ground sample was observed; Figures 6, S10 and S11), and only the CN-substituted 1,4-diphenylanthracene derivatives showed a reversible emission color change, indicating that the hydrogen bond between the Ar-CN group and the Ar-H hydrogen atom (Ar-CN• • • H-Ar) is important for the controllable reversible mechanofluorochromism in the CN-substituted 1,4-diphenylanthracene derivatives.
Although it is difficult to further understand the critical mechanism of this mechanofluorochromism, it can be classified as follows: (Type-A) the 1,4-diphenylanthracene derivatives 1a-1c show a blue luminous color in the pristine state, and the crystal packing showed a parallel-type orientation.Only a weaker π-π stacking was observed, and its blue luminous color probably comes from monomer emission.After grinding the pristine sample, it does not change the original luminous color (Scheme 2a); (Type-B) the derivatives 1d, 1e, and 4 show green and yellow-green luminous colors in the pristine state, and the crystal packing of 1d showed a parallel-type orientation.Only weak π-π stacking was observed, and its green and yellow-green luminous color probably comes from the partial excimer emission or charge-transfer interactions.After grinding the pristine sample, it changes its original luminous color to a bluish color, but its original luminous color does not recover by heating and fuming of the ground samples (Scheme 2b); (Type-C) the derivatives 1f, 2, and 3, which having CN-groups, show a yellow-green luminous color in the pristine state, and the crystal packing showed an antiparallel-type orientation.A stronger π-π stacking was observed, and its yellow-green luminous color probably comes from the excimer emission.After grinding the pristine sample, its original luminous color changes to the shorter wavelength side (yellow-green to green or yellow-green to blue-green), and its original luminous color was repeatedly recovered by heating and fuming of the ground samples.Thus, only some of the CN-substituted 1,4-diphenylanthracene derivatives exhibited a repetitive reversible mechanofluorochromism with a "hypsochromic" luminous color change upon grinding, heating, and fuming with solvent vapors, which may be due to the stronger π-π stacking attributed to the antiparallel crystal packing structure (Scheme 2c).The mechanofluorochromic properties of other phenylanthracence derivatives, such as 1,5-diphenylanthracence and fused phenylanthracene derivatives [111] are currently under investigation.
Molecules 2024, 29, x FOR PEER REVIEW 13 of 21 the crystal packing of 1d showed a parallel-type orientation.Only weak π-π stacking was observed, and its green and yellow-green luminous color probably comes from the partial excimer emission or charge-transfer interactions.After grinding the pristine sample, it changes its original luminous color to a bluish color, but its original luminous color does not recover by heating and fuming of the ground samples (Scheme 2b); (Type-C) the derivatives 1f, 2, and 3, which having CN-groups, show a yellow-green luminous color in the pristine state, and the crystal packing showed an antiparallel-type orientation.A stronger π-π stacking was observed, and its yellow-green luminous color probably comes from the excimer emission.After grinding the pristine sample, its original luminous color changes to the shorter wavelength side (yellow-green to green or yellow-green to bluegreen), and its original luminous color was repeatedly recovered by heating and fuming of the ground samples.Thus, only some of the CN-substituted 1,4-diphenylanthracene derivatives exhibited a repetitive reversible mechanofluorochromism with a "hypsochromic" luminous color change upon grinding, heating, and fuming with solvent vapors, which may be due to the stronger π-π stacking a ributed to the antiparallel crystal packing structure (Scheme 2c).The mechanofluorochromic properties of other phenylanthracence derivatives, such as 1,5-diphenylanthracence and fused phenylanthracene derivatives [111] are currently under investigation.

General Methods
The 1 H and 13 C NMR spectra were recorded using Bruker (Billerica, MA, USA) Avance III ( 1 H: 500 and 13 C: 125 MHz) spectrometers.The FAB-mass spectra were recorded by a JEOL (Tokyo, Japan) JMS-700 mass spectrometer.The absorption spectra (solid and

General Methods
The 1 H and 13 C NMR spectra were recorded using Bruker (Billerica, MA, USA) Avance III ( 1 H: 500 and 13 C: 125 MHz) spectrometers.The FAB-mass spectra were recorded by a JEOL (Tokyo, Japan) JMS-700 mass spectrometer.The absorption spectra (solid and solution samples) were recorded by a Jasco (Tokyo, Japan) V-750 UV-Visible spectrometer.The fluorescence spectra for the solution samples were recorded by a Jasco (Tokyo, Japan) FP-8300 spectrometer, and the fluorescence spectra were calibrated using rhodamine B as the reference.The fluorescence spectra for the solid samples were recorded by a Flame-S, Ocean Optics (Orlando, FL, USA) fiber-optic spectrometer (λ ex = 365 nm).The absolute PL quantum yields (Φ) were determined by a Quantaurus-QY (Hamamatsu Photonics, Hamamatsu, Japan) instrument.The emission lifetimes were measured by a Quantaurus-Tau (Hamamatsu Photonics) instrument.The DSC thermograms of the solid samples were recorded by a TA Instruments (New Castle, Delaware, USA) DSC 2920 modulated DSC and were recorded at the constant heating rate of 10 • C/min.The infrared spectra were recorded using a Jasco (Tokyo, Japan) FTIR 4600 spectrometer equipped with an attenuated total reflection (ATR) unit.All the solvents and reagents were purified according to standard procedures.3,5-dicyanophenylboronic acid [65] and 1,4-dibromoanthracene [112][113][114] were prepared according to published procedure.

Computational Methods
Density functional theory (DFT) with the B3LYP functional at the 6-31G* basis set level was used to calculate the electro cloud distribution of the HOMO and LUMO in a vacuum.For the calculation from the obtained single crystal structures, two molecules, which were stacked on each other, were located, and the non-hydrogen atom coordinates were fixed.Only the hydrogen atoms were optimized to obtain the conformation for the DFT calculation.All of the calculations were performed using Gaussian 16W C.01 [115].

Crystallographic Analysis
Crystals suitable for X-ray crystallography were grown from the solutions of 1a (R = H), 1c (R = F) in hexane solution that were stored in a test tube and left to stand for several days.For compounds 1d (R = OMe) and 1f (R = CN), the crystals were obtained from DME solution.For compounds 1b (R = Me) and 2 (3-CN), the crystals were obtained from CH 2 Cl 2 and CHCl 3 solutions, respectively.For the crystallographic data collection of 1a (R = H) and 2 (3-CN), Rigaku XtaLAB mini with graphite-monochromated Mo-Kα radiation was used.Calculations were performed using the Olex2, version 1.3.0crystallographic software packages [116].The crystal structures were solved by a direct method using Shelxt Version 2014/2015 [117,118].The structure refinements were performed by a full-matrix least squares method using Shelxl Version 2014/2015 [119,120].For the crystallographic data collection of 1b (R = Me), 1c (R = F), 1d (R = OMe), and 1f (R = CN), a Bruker D8 VENTURE equipped with a rotating anode of Mo-Kα or Cu-Kα radiation and a PHOTON III detector was used.Calculations were performed using the Bruker APEX3, 2019 [121] software package.Details of the data are summarized in Tables S3 and S4.All nonhydrogen atoms were anisotropically refined.All the hydrogen atoms were placed in idealized positions and were included in the structure factor calculations but were not refined.CCDC-2314555-2314560 contains the supplementary crystallographic data for this paper.These data can be obtained free of charge from The Cambridge Crystallographic Data Centre.

PXRD Analysis
Powder X-ray diffraction (PXRD) patterns of the samples were recorded using a Rigaku Smartlab X-ray diffractometer with CuK α radiation (1.5406 Å) at a voltage of 40 kV and a current of 30 mA, and the data were collected with a step size of 0.01 • over a 2θ range of 5-50 • .

Conclusions
In this study, several types of 1,4-diphenylanthracene derivatives 1-4 were prepared, and their photophysical properties were observed in the solid and solution states.The CN-group substituted 1,4-diphenylanthracene derivative 2 exhibited a relatively high fluorescence quantum yield (Φ = 0.71) in the solid state than that in the solution state.Interestingly, for some derivatives, an increase in the fluorescence quantum yield was observed in the PMMA film (1 wt%) over both the solid state and the solution state.More interestingly, some of the 1,4-diphenylanthracene derivatives exhibited mechanofluorochromic properties with "hypsochromic shift" of the luminous color depending on the substituents of the phenyl group.Especially for some CN-substituted derivatives, a reversible luminous color change with a "hypsochromic shift" was observed, probably due to the formation of the antiparallel-type packing structure.These "hypsochromic" anthracene derivatives could probably be applied as new mechanofluorochromic materials.

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and the intermolecular hydrogen bond (Ar-CN• • • H-Ar) formed by the CN group, the crystal packing structure of 2 forms a tight, lock-and-key type packing compared to the loose packing structure of 1f (Figures2e,f and S37).The tight packing structure of 2 is considered to suppress the molecular vibration in the solid state compared to the packing structure of 1f and exhibits a very low k nr (0.003 ns −1 for 2) and a high fluorescence quantum yield (ϕ f = 0.71).

Figure 5 .
Figure 5. Fluorescence and UV-vis spectra of 1,4-diphenylanthracene derivative 1f (R=CN) in the solid state: (a) fluorescence spectra of the pristine and ground samples (λ ex = 365 nm), (b) UV-vis absorption spectra of the pristine and ground samples.
cating a decrease in the contribution of the π-stacking structure after grinding.Furthermore, infrared (IR) spectra of the CN-substituted 1,4-diphenylanthracene derivatives 1f (R = CN) and 2 (3-CN) before and after grinding were observed, and the CN stretching band of the 1f and 2 shifted to the higher wavenumber side from the pristine to ground one, indicating that the hydrogen bond (Ar-CN⋯H-Ar) strength decreased after grinding (Figures 6, S10 and S11).

Scheme 2 .
Scheme 2. Schematic diagram of different types of mechanofluorochromic properties of 1,4diphenylanthracene derivatives: (a) Type-A, non-mechanofluorochromic phenomenon for derivatives 1a-1c; (b) Type-B shows mechanofluorochromic phenomenon but non-reversible change by fuming and heating for derivatives 1d, 1e, and 4; (c) Type-C shows mechanofluorochromic phenomenon repeatedly changing by fuming and heating for the CN-substituted derivatives 1f, 2, and 3.