The role of a [2.2]paracyclophane moiety in piezofluorochromism of crystalline organoboron complexes

Abstract

Fluorescence (FL) properties of crystals of [2.2]paracyclophane-containing organoboron complexes (pCP-H and pCP-iPr) were investigated under high pressure using a diamond anvil cell to evaluate the effects of intramolecular π–π interactions in the [2.2]paracyclophane moiety on piezofluorochromism (PFC). Crystals of both pCP-H and pCP-iPr were found to display remarkable PFC with redshifts of more than 100 nm under high pressures up to ca. 8 GPa. However, the pressure-sensitivities of FL of pCP-H and pCP-iPr crystals differed. The results of X-ray crystallography studies under ambient and high pressure revealed that PFC of the pCP-H crystal mainly originates from intermolecular π–π interactions taking place in a π-stacked dimer. Density functional theory calculations also showed that intermolecular orbital interactions in the π-stacked dimer play an important role in the PFC of pCP-H. In contrast, it was found that pCP-iPr does not form a π-stacked dimer in the crystal state. Therefore, PFC of the pCP-iPr crystal is mainly controlled by intramolecular π–π interactions in the [2.2]paracyclophane moiety, making it less sensitive than pCP-H to pressure changes.

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Introduction

Crystals of many aromatic compounds undergo reversible changes in their fluorescence color upon application of an external stimulus, such as grinding and exposure to organic solvent vapor.^1,2 The fluorescence changes of these crystals are a consequence of changes that take place in their flexible crystal structures. Piezofluorochromism (PFC) is a term used to characterize changes in the fluorescence (FL) color of crystals that occur reversibly in response to isotropic pressure changes.^3–5 Since the late 1970s, many studies have shown that crystals of aromatic molecules^3–9 display PFC in association with drastic FL color changes taking place under extremely high, GPa range pressures applied by using a diamond anvil cell (DAC).^10–14 Because these FL color changes are reversed simply by removing the applied pressure, PFC has attracted the attention of not only physical organic chemists, having interests in the structural/electronic foundation of the process, but also scientists developing new types of pressure sensors. PFC of crystals of organic molecules can be roughly classified into two classes depending upon whether it is promoted by changes in intermolecular or intramolecular interactions. In the former route, application of high, GPa range pressure leads to a decrease in the distance between molecules in the crystal and a subsequent increase in intermolecular π–π interactions. Typically, reducing the distance between π-planes of molecules in a crystal leads to destabilization of the ground state, but it promotes stabilization of the photoexcited state via excimer formation that is accompanied by redshifted excimer FL.^15–24 In the intramolecular mechanism for PFC, high pressure induces an increase in π-conjugation in a molecule in the crystal as a consequence of a decrease in rotational freedom. For example, in aromatic molecules containing donor and acceptor moieties and that have intramolecular charge transfer (ICT) photoexcited states, applying high pressure can reduce the dihedral angle between the donor and acceptor moieties, resulting in stabilization of the photoexcited ICT state owing to more extensive intramolecular π-conjugation.^20,25 However, the operational efficiencies of the two mechanisms depend not only on the structure of the molecule but also molecular packing in the crystal. This feature makes it difficult to control PFC in organic molecular crystals.

The aim of this study was to gain information about the pressure-sensitivity of an intramolecular π–π interaction.²⁶ [2.2]Paracyclophane is a molecule composed of two benzene rings possessing an intramolecular π-stacked dimer fixed by the presence of two ethylene linkers at para-positions.^27–29 Strong intramolecular π–π interactions in these substances lead to a unique array of electronic and optical properties.^30–33 Based on the expectation that this type of intramolecular π–π interaction should enable [2.2]paracyclophane to display pressure-sensitivity and consequent PFC³⁴ in a crystal structure independent manner, in an earlier study,³⁵ we evaluated the PFC properties of a crystal of the [2.2]paracyclophane-containing organoboron complex with a para-tert-butylphenyl substituent (pCP-tBu, Chart 1). We anticipated that this substance would display pressure-sensitive fluorescence^36–38 owing solely to changes in intramolecular π–π interactions in the [2.2]paracyclophane moiety.³⁴ Specifically, we reasoned that the p-tert-butyl group on the phenyl ring in pCP-tBu would significantly reduce PFC caused by intermolecular π–π interactions. Indeed, the pCP-tBu crystal did exhibit remarkable PFC corresponding to a more than 150 nm shift in its emission maximum under high pressure applied in a DAC. However, in contrast to our expectation, the results of X-ray crystallographic analyses and theoretical calculations showed that PFC of the pCP-tBu crystal is caused by changes in intermolecular π–π interactions taking place in a π-stacked dimer formed in the crystal. Although X-ray crystallography data showed that the size of the [2.2]paracyclophane moiety in the pCP-tBu crystal decreases significantly under high pressure, it remained unclear how the intramolecular π–π interactions influence the PFC properties.

Chart 1 Graphical description of key features of this study. (top) Molecular structures of pCP-H, pCP-iPr, and pCP-tBu. (bottom) An illustration of PFC of the pCP-H and pCP-iPr crystals in a diamond anvil cell.

The apparent ambiguity in the results observed from our previous study stimulated an investigation to assess in detail the role that the [2.2]paracyclophane moiety in these substances plays in determining the intensity and mechanistic nature of PFC. For this purpose, we prepared the parent pCP-H^39,40 (Chart 1) and its isopropyl derivative pCP-iPr and investigated the pressure-sensitivity of their FL properties in the crystalline state. As described below, the results of this study demonstrated that while both complexes display PFC, the mechanisms responsible for these processes differ. Specifically, the results indicate that pCP-H crystals exhibit “pressure-sensitive” PFC owing mainly to changes in intermolecular π–π interactions occurring in π-stacked dimers, whereas pCP-iPr crystals display “pressure-insensitive” PFC owing to intramolecular π–π interactions in the [2.2]paracyclophane moiety.

Results and discussion

Racemic mixtures of pCP-H and pCP-iPr were synthesized using previously described^35,39 three step sequences starting with 4-acetyl[2.2]paracyclophane (1) (Scheme 1). Detailed descriptions of the methods are given in the Experimental section.

Scheme 1 Synthesis of pCP-iPr. The insets are photographs of pCP-iPr under room light.

Photophysical properties of pCP-H and pCP-iPr in CH₂Cl₂ and in crystalline states at ambient pressure. Photophysical properties of pCP-H in CH₂Cl₂ (1 × 10⁻⁵ M) are given in Table 1. As has been reported earlier,³⁹ the absorption spectrum of pCP-H in CH₂Cl₂ consists of a broad band with a maximum (λ_AB,MAX) at 393 nm. Upon photoexcitation at 393 nm, a solution of pCP-H in CH₂Cl₂ displays yellow FL with a maximum (λ_FL,MAX) at 557 nm and a quantum yield (Φ_FL) of 0.05 (Fig. 1a). The FL decay profile of pCP-H in CH₂Cl₂, fitted using a single exponential, showed a FL lifetime (τ_FL) of 10.4 ns. pCP-iPr in CH₂Cl₂ has the same photophysical properties as does pCP-H (Fig. 1b and Table 1), showing that little difference exists between the electronic states of pCP-H and pCP-iPr.

Table 1 Photophysical properties of pCP-H and pCP-iPr in CH₂Cl₂ (1 × 10⁻⁵ M) and crystalline states

Substances	State	λAB,MAX [nm]	λFL,MAX^a [nm]	ΦFL^a	τFL^b^c [ns]
a Excited at λAB,MAX in CH2Cl2.b Excited at 371 nm. Detected at each λFL,MAX.c The percentage in the parentheses shows the ratio of each τFL.
pCP-H	In CH2Cl2	393	557	0.05	10.4 (100%)
	In crystal	—	537	0.11	5.9 (24%)
					14.6 (76%)
pCP-iPr	In CH2Cl2	398	546	0.05	10.8 (100%)
	In crystal	—	538	0.10	5.0 (30%)
					14.5 (70%)

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Fig. 1 UV-vis absorption and FL spectra (left), and FL decay curves (right) of (a) pCP-H and (b) pCP-iPr in CH₂Cl₂ (1 × 10⁻⁵ M) and in crystalline states. FL spectra in CH₂Cl₂ and crystalline states were obtained using excitation at absorption maxima. FL decay curves were obtained by excitation at 371 nm.

Under UV light, pCP-H and pCP-iPr crystals exhibit green FL emission with respective λ_FL,MAX values of 537 and 538 nm (Fig. 1, Table 1). Unlike the method used to analyse solution state data, FL decay profiles of the crystals were fitted using biexponential curves, which provide τ_FL values of 5.9 (24%) and 14.6 ns (76%) for the pCP-H crystal and 5.0 (30%) and 14.5 ns (70%) for the pCP-iPr crystal. Therefore, the findings indicate that pCP-H and pCP-iPr crystals have similar lowest excited singlet states and electronic structures under ambient pressure.

FL properties of pCP-H and pCP-iPr crystals under high pressure. FL measurements under high pressure, performed using a DAC, showed that both the pCP-H and pCP-iPr crystals display PFC. Details of the measurement conditions are given in the Experimental section. Upon stepwise compression from 0.1 MPa to 5.6 GPa, the λ_FL,MAX of the pCP-H crystal undergoes a remarkable redshift from 520 to 658 nm, along with a corresponding FL color change from green to dark red and a decrease in FL intensity (Fig. 2a). In the high pressure region above 5.6 GPa, changes in the FL intensity and λ_FL,MAX become less pronounced, and at 8.1 GPa, the λ_FL,MAX reaches a plateau at 664 nm. When the applied pressure is gradually reduced, the λ_FL,MAX returns from 664 nm at 8.1 GPa to 520 nm at 0.1 MPa. On this occasion, the pCP-H crystal color changed from yellow to red under high pressure, suggesting that the electronic structure of the ground state also changes (Fig. S5a).

Fig. 2 (top) FL spectra and (bottom) photographs of the (a) pCP-H and (b) pCP-iPr crystals under various pressures (λ_EX = 365 nm). The photographs at 0.1 MPa and under high pressure were taken using different crystals. FL spectra were recorded a few minutes after the compression or decompression procedure was performed. Crystal sizes are ca. 0.2 × 0.2 × 0.1 mm³. The asterisk stands for FL of a ruby chip.

In an earlier study,³⁵ we observed that the energy at the λ_FL,MAX (E_FL,MAX = hc/λ_FL,MAX) of the pCP-tBu crystal changes in multiple stages as the pressure is increased (compression) and decreased (decompression) and that the compression and decompression processes have different profiles. Specifically, the pCP-tBu crystal exhibits PFC with hysteresis. A plot of E_FL,MAX for crystalline pCP-H vs. pressure (P), given in Fig. 3, shows that the E_FL,MAX of the pCP-H crystal arises and disappears in three stages (Stages i–iii) and that the stepwise changes in E_FL,MAX are also found at the same location during both the compression and decompression stages. This observation suggests that the PFC of the pCP-H crystal is different from that of the pCP-tBu crystal in that it is microscopically reversible. In stage i (0.1 MPa–0.7 GPa) and stage ii (0.7–5.0 GPa), the E_FL,MAX of the pCP-H crystal decreases steeply with respective slopes of −15.6 × 10⁻² and −8.85 × 10⁻² eV GPa⁻¹ (Table 2). In contrast, the slope of the plot in stage iii (5.0–8.1 GPa) of −1.09 × 10⁻² eV GPa⁻¹ is smaller by about one tenth of that in stage ii. One possible reason for the significant flattening in stage iii is likely because the intermolecular distance reaches its shortest limit at ca. 5 GPa. An alternative explanation is a phase transition, but it is currently impossible to determine whether this is the case.

Fig. 3 Plots of energies at FL maxima (E_FL,MAX) vs. applied pressure (P) for the pCP-H (red) and pCP-iPr (blue) crystals. The fitted lines are drawn using the points in the compression processes.

Table 2 Slopes of the E_FL,MAX vs. P plots of the pCP-H and pCP-iPr crystals during the compression process

Crystal	Slope [eV GPa^−1]
	Stage i	Stage ii	Stage iii
pCP-H	−15.6 × 10^−2	−8.85 × 10^−2	−1.09 × 10^−2
pCP-iPr	−15.0 × 10^−2	−4.18 × 10^−2	—

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In the initial phase of the compression (0.1 MPa–1.0 GPa) of the pCP-iPr crystal, the intensity at the λ_FL,MAX undergoes only a small decrease (Fig. 2b). At higher pressures, the λ_FL,MAX exhibits a red shift from 518 nm at 0.1 GPa to 620 nm at 7.6 GPa and a corresponding FL color change from green to red with changes in the pCP-iPr crystal color from yellow to red (Fig. 2b and Fig. S5b). Moreover, during decompression from 7.6 GPa to 0.1 MPa, the FL of the pCP-iPr crystal returns to almost the original state at 0.1 MPa. This finding reveals that the pCP-iPr crystal also displays PFC. The pressure-induced shift of the E_FL,MAX of the pCP-iPr crystal takes place in two stages and is microscopically reversible (Fig. 3). In stage i (0.1 MPa–1.0 GPa), the slope of the plot of E_FL,MAX vs. P is −15.0 × 10⁻² eV GPa⁻¹, a value that is almost the same as that for stage i compression of the pCP-H crystal. However, the slope of the plot for the pCP-iPr crystal in stage ii is −4.18 × 10⁻² eV GPa⁻¹, which is about half of that of compression of the pCP-H crystal in stage ii. Therefore, the pCP-H crystal displays “pressure-sensitive” PFC, while pCP-iPr exhibits only a “pressure-insensitive” PFC. It is also noteworthy that the E_FL,MAX change of the pCP-iPr crystal is linear in the 1–8 GPa region. Although the measurements above 8 GPa are not possible owing to equipment limitations, we anticipate that the pCP-iPr crystal would display PFC at even higher pressures.

Crystal structures of pCP-H and pCP-iPr at ambient and high pressures. To elucidate the mechanism which induces the difference in the pressure-sensitivity of the FL in stage ii, we next assessed the structural changes that occur in pCP-H and pCP-iPr crystals when exposed to a high pressure of 3.1 GPa. X-ray crystallographic analysis at 0.1 MPa revealed that the pCP-H crystal is racemic and belongs to a trigonal system with the R3̄ space group. The crystal is composed of a π-stacked dimer of the S_p- and R_p-enantiomers of pCP-H, and back sides of the π-planes of the benzene and dihydrodioxaborinine rings in each enantiomer are engaged in strong intermolecular π–π interactions (Fig. 4a).

Fig. 4 (a and b) (left) The front views at 0.1 MPa and (right) the side views with overlay of the conformations at 0.1 MPa (shown in translucent) and 3.1 GPa (shown in solid) of the π-stacked dimers of the pCP-H and pCP-iPr crystals. (c and d) The visualization of the pressure-induced changes of the (top) intramolecular π–π interactions by the NCI plots and (bottom) intermolecular π–π interactions by the Hirshfeld surface analyses of the pCP-H and pCP-iPr crystals. The red-highlighted spots on the Hirshfeld surface show C–H⋯π and C–H⋯F interactions.

A comparison of the X-ray crystallographic data of the pCP-H crystal at different pressures enables an analysis of pressure-induced structural changes that lead to the PFC. While proceeding from 0.1 MPa to 3.1 GPa, the unit cell of the pCP-H crystal is compressed isotropically to 91% of its original volume, a phenomenon that is associated with a reduction in the distance between the two molecules in the π-stacked dimer (Fig. S7). However, the crystal system and the space group are not changed by the pressure increase, indicating that the pCP-H crystal does not undergo a phase transition in the range of 0.1 MPa to 3.1 GPa. Although the pCP-H crystal is compressed under high pressure, shrinkage of the [2.2]paracyclophane moiety does not take place. This conclusion comes from the observation that the intramolecular π-stacked distance (D_INTRA) between centroids of two benzene rings in the [2.2]paracyclophane moieties is 2.96 Å at both 0.1 MPa and 3.1 GPa (Fig. 4a). Non-covalent interaction (NCI) plots, which enable visualization of intra- and intermolecular interactions,^41–43 also show that no remarkable changes occur in proceeding from 0.1 MPa to 3.1 GPa (Fig. 4c). These observations suggest that the intramolecular π–π interaction of the [2.2]paracyclophane moiety in the pCP-H crystal is not pressure-sensitive and, therefore, that this interaction is not responsible for the observed PFC. The intermolecular π-stacked distance (D_INTER) decreased from 3.72 Å at 0.1 MPa to 3.50 Å at 3.1 GPa. The changes in the D_INTER was subjected to Hirshfeld surface analysis,^44–46 which clearly shows that the enhancement of intermolecular π–π interactions at high pressure (Fig. 4c) is the main reason for the PFC of the pCP-H crystal.

An analogous X-ray crystallographic analysis of the pCP-iPr crystal gave results that are contrastingly different from those obtained from studies with the pCP-H crystal. First, the pCP-iPr crystal at 0.1 MPa belongs to the orthorhombic crystal system with the P2₁2₁2₁ space group. The pCP-iPr crystal is homochiral owing to spontaneous resolution and an S_p crystal was subjected to the analysis (Fig. 4b). Because of this feature, the pCP-iPr crystal does not form a π-stacked dimer with a strong intermolecular orbital overlap like that present in the pCP-H crystal. Molecules of pCP-iPr in the crystal are translationally oriented and have a relationship slipped by 40° with regard to each other. The consequence of this spatial alignment is that the front side of the [2.2]paracyclophane moiety in one molecule faces the back side of the isopropylphenyl moiety in the other molecule (Fig. S6b). Importantly, the intermolecular π–π interaction between molecules in the pCP-iPr crystal is small owing to the reduced intermolecular overlap between the π-planes. A pCP-iPr chiral crystal was subjected to X-ray crystallographic analysis at 3.1 GPa. The data show that significant shrinkage of the unit cell occurs (16.4%) and no phase transition takes place (Fig. S8). In accord with the shrinkage, the D_INTRA decreases from 2.97 Å at 0.1 MPa to 2.90 Å at 3.1 GPa (Fig. 4b). The NCI plot shows that the interaction region in the [2.2]paracyclophane moiety is expanded (Fig. 4d), suggesting that enhancement of the intramolecular π–π interaction occurs in the pCP-iPr crystal under high pressure. The pressure increase also promotes a decrease in the distance between pCP-iPr molecules in the crystal (D_INTER from 4.68 Å at 0.1 MPa to 4.17 Å at 3.1 GPa). However, the intermolecular overlap of the π-planes is not large enough to create sufficiently strong intermolecular π–π interactions for it to be the main factor for PFC of the pCP-iPr crystal. This conclusion suggests that the origin of PFC of the pCP-iPr crystal is a pressure-induced change in the intramolecular π–π interaction in the [2.2]paracyclophane moiety.

Next, we assessed the pressure-sensitivity of the [2.2]paracyclophane moiety. Pressure-induced changes of the crystal structure of the pCP-iPr crystal, which showed the simple pressure-sensitivity of the FL over a wide pressure range above 1 GPa, were examined by using density functional theory (DFT) calculations. The results of these calculations, performed using Quantum ESPRESSO ver. 6.3 program package,^47,48 well match the experimentally observed decrease occurring in the unit cell axis length at 3.1 GPa (Fig. S9a). They also indicate that a continuous contraction of the unit cell of the pCP-iPr crystal occurs in the pressure range of 0.1 MPa to 7.8 GPa and, in fact, the D_INTRA value is found to decrease linearly as the pressure increases (Fig. S9b). Although the calculated D_INTRA value is about 0.07–0.09 Å larger than the value obtained from X-ray crystallographic analysis, its variations strongly suggest that intramolecular π–π interactions are enhanced by applying pressure at least up to 7.8 GPa and, thus, contribute to PFC of the pCP-iPr crystal.

Frontier molecular orbitals in pCP-H and pCP-iPr crystals. DFT calculations using the atomic coordinates derived from the X-ray crystallographic analysis were employed to explore frontier molecular orbitals of the crystals. The HOMO of the monomer in the pCP-H crystal at 0.1 MPa was found to be mainly localized on the [2.2]paracyclophane moiety (Fig. 5a). On the other hand, the LUMO is delocalized throughout the entire molecule, except for the overlapping benzene rings of the [2.2]paracyclophane moiety. Energies of the HOMO and LUMO (E_H and E_L) of the monomer in this crystal at 0.1 MPa were calculated to be −7.92 and −0.89 eV, respectively. At 3.1 GPa, the distribution and energies of the HOMO and LUMO of the monomer in the pCP-H crystal are nearly the same as the values calculated for the crystal at low pressure, which is in accord with the small structural changes found using X-ray crystallographic analysis. Therefore, application of pressure does not impact the electronic states of the monomer in the pCP-H crystal.

Fig. 5 Energy levels and distributions (isovalue = 0.02) of frontier orbitals of monomers and π-stacked dimers of (a) pCP-H and (b) pCP-iPr in crystal. The frontier orbitals are calculated by DFT (ωB97XD/6-31G**) using the molecular coordinates determined by X-ray crystallographic analyses.

Calculations on the π-stacked dimer in the pCP-H crystal at 0.1 MPa show that E_H is 0.19 eV larger and E_L is 0.03 eV smaller than those of the monomer, showing a 0.22 eV decrease of HOMO–LUMO transition energy. Because the LUMO and LUMO+1 of the π-stacked dimer have orbital distributions which correspond to a simple combination of those of the monomer LUMOs, the magnitude of the energy difference between the LUMO and LUMO+1 (ΔE_L+1–L) should be a good indicator of the magnitude of the intermolecular π–π interactions. Indeed, ΔE_L+1–L is calculated to increase from 0.15 eV at 0.1 MPa to 0.23 eV at 3.1 GPa, indicating that the intermolecular π–π interaction is enhanced under high pressure.

In contrast to those of the monomer in the pCP-H crystal, the HOMO and LUMO properties of the monomer in the pCP-iPr crystal display a remarkable response to the applied pressure (Fig. 5b). For example, E_H of the monomer in the pCP-iPr crystal is −7.95 eV at 0.1 MPa and −7.62 eV at 3.1 GPa, a large increase (0.33 eV) that originates from the enhancement of the intramolecular π–π interactions promoted by the decrease of D_INTRA. On the other hand, E_L shows a small increase (0.08 eV) from −0.83 eV at 0.1 MPa to −0.75 eV at 3.1 GPa, because of the lower contribution of the LUMO of the overlapped benzene ring orbitals in the [2.2]paracyclophane moiety. As a result, the HOMO–LUMO transition energy decreases by 0.25 eV.

The calculated frontier molecular orbitals of the π-stacked dimer in the pCP-iPr crystal are not greatly different from those of the monomer at both 0.1 MPa and 3.1 GPa. This finding indicates that, in contrast to the pCP-H crystal, the intermolecular π–π interaction in the pCP-iPr crystal cannot be the key factor for the PFC. This is caused by the weakness of intermolecular orbital interactions, reflected by small ΔE_L+1–L values of 0.02 and 0.04 eV resulting from the large (>4 Å) D_INTER.

Excited singlet states in pCP-H and pCP-iPr crystals. To determine the FL domains that contribute to the PFC, time-dependent (TD-) DFT calculations were performed to determine the lowest excited singlet states (E_S1) of molecules in the crystals. The results show that E_S1 of the monomer in the pCP-H crystal is 3.72 eV at 0.1 MPa and 3.83 eV at 3.1 GPa (Fig. 6a). The 0.11 eV increase of E_S1 differs from the experimentally observed decrease of E_FL,MAX, indicating that conformational changes in the monomer cannot be a major factor for the PFC of the pCP-H crystal. In sharp contrast, E_S1 of the π-stacked dimer in the pCP-H crystal was calculated to decrease from 3.51 eV at 0.1 MPa to 3.48 eV at 3.1 GPa. Although a large difference exists between the calculated and observed shifts of E_FL,MAX (ca. −0.4 eV at 3.3 GPa), the fact that E_S1 decreases in response to the applied pressure is qualitatively consistent with the PFC. Therefore, the intermolecular π–π interaction is suggested to be a key factor for the “pressure-sensitive” PFC of the pCP-H crystal.

Fig. 6 Relative energy diagrams of the lowest excited singlet states (S₁) and ground states (S₀) of monomers and π-stacked dimers of (a) pCP-H and (b) pCP-iPr crystals. ΔE_S1 refers the changes in E_S1 promoted by compression.

As indicated by analysis of the frontier molecular orbitals, E_S1 of the monomer in the pCP-iPr crystal displays a large decrease from 3.82 eV at 0.1 MPa to 3.44 eV at 3.1 GPa (Fig. 6b), a magnitude (0.38 eV) that is large enough to explain the observed decrease of E_FL,MAX (ca. −0.2 eV at 3.3 GPa). On the other hand, calculations on the π-stacked dimer in the pCP-iPr crystal show that E_S1 has almost the same value as that of the monomer at both 0.1 MPa and 3.1 GPa. This result shows that the pressure sensitivity of the intermolecular π–π interaction in the pCP-iPr crystal only slightly influences PFC, suggesting that the enhancement of the intramolecular π–π interaction in the [2.2]paracyclophane moiety is the main contributor to the “pressure-insensitive” PFC of the pCP-iPr crystal.

Conclusions

In this study, we demonstrated that a correlation exists between the PFC properties and crystal structures of pCP-H and pCP-iPr. Both the pCP-H and pCP-iPr crystals display remarkable PFC, reflected in a redshift greater than 100 nm under high pressure applied by using the DAC. However, the pressure sensitivity and mechanisms of PFC of these crystals differ. The PFC of the pCP-H crystal has about a two-fold higher pressure sensitivity than that of the pCP-iPr crystal. X-ray crystallographic analyses show that pCP-H forms a π-stacked dimer possessing a remarkably large intermolecular π–π interaction. The results of X-ray crystallographic analysis under high pressure and (TD-) DFT calculations show that PFC of the pCP-H crystal originates from an enhancement of intermolecular π–π interactions promoted by an increase in applied pressure. On the other hand, the pCP-iPr crystal does not form a π-stacked dimer like the pCP-H crystal does. Therefore, intermolecular π–π interactions in this crystal are weak even under high pressure. In contrast, the pCP-iPr crystal displays an enhancement of intramolecular π–π interactions caused by shrinkage of the [2.2]paracyclophane moiety under high pressure, which is suggested to be the major factor of the PFC of the pCP-iPr crystal.

The results of this investigation demonstrate that alterations in intramolecular π–π interactions in the [2.2]paracyclophane moiety can play a novel role in the design of new pressure sensors. Our findings suggest that PFC caused by the presence of the [2.2]paracyclophane moiety would be significantly useful because it would convey high linearity over a wide range of applied pressures. It is also noteworthy that the pressure sensitivity of the [2.2]paracyclophane moiety should not be influenced by the presence of other molecular moieties, and thus, it should be possible to control the characteristics of the PFC by introducing the [2.2]paracyclophane moiety into a wide variety of luminophores. Therefore, it should be possible to incorporate the [2.2]paracyclophane moiety in the design of pressure-sensitive materials. In contrast to initial expectations, this study also revealed that the pressure sensitivity of the [2.2]paracyclophane moiety strongly depends on the structure of the crystal, especially on chirality in the three-dimensionally asymmetric [2.2]paracyclophane moiety. The results obtained from this study have provided valuable insight into not only the mechanism of PFC of crystalline organic substances but also the structural and electronic properties of the [2.2]paracyclophane ring system.

Experimental section

General

Melting points (mp) were measured using a Yanaco MP-500 apparatus and are reported uncorrected. ¹H and ¹³C NMR spectra were recorded on a Bruker AVANCE NEO 400 spectrometer, operating at 400 and 100 MHz for ¹H and ¹³C NMR, respectively. Chemical shifts (δ_ppm) were reported in ppm with respect to tetramethylsilane (TMS, 0.00 ppm in ¹H NMR) and CDCl₃ (77.0 ppm in ¹³C NMR) as the internal standard. Atmospheric pressure solids analysis probe (ASAP) mass spectroscopy (MS) was carried out using a Shimadzu LCMS-2020 and LabSolutions LCMS software. Fourier transform infrared (FT-IR) spectra were recorded on a JASCO FT/IR-8300 spectrophotometer with the attenuated total reflection (ATR) method. Elemental analyses were performed at the Advanced Science Research Center at Kanazawa University or the Analytical Center, Graduate School of Science, Osaka Metropolitan University. UV-vis absorption spectra were recorded on a JASCO V-570 spectrophotometer. FL spectra at ambient pressure were recorded on a JASCO FP-8500 spectrophotometer. τ_FL values were determined using a HORIBA Jobin Yvon FluoroCube lifetime spectrofluorometer equipped with a HORIBA NanoLED-370 (excitation wavelength, λ_EX = 370 nm) as an excitation light source and analyzed using DAS6 FL decay analysis software. Φ_FL values were recorded on a Hamamatsu Photonics C9920-02 absolute photoluminescence quantum yield measurement system using the integrating sphere method.

Preparation of organic substances

Dry tetrahydrofuran (THF) was prepared by using a GlassContour solvent dispensing system. Dry toluene was purified by pre-drying over CaH₂ and distillation from Na. Solvents of spectroscopic grade were used for spectroscopic analyses.

Synthetic procedures

Synthesis of 1-(4-isopropylphenyl)-3-(4-[2.2]paracyclopha-nyl)propane-1,3-dione (3-iPr). To a suspension of NaH (60% dispersion in mineral oil, 1.59 g, 40.0 mmol) in THF (50 mL) was added 4-acetyl[2.2]paracyclophane (1, 2.50 g, 10.00 mmol) at room temperature under an argon atmosphere. To this mixture, a solution of 4-isopropylbenzoate (2-iPr, 2.67 g, 15.0 mmol) in THF (25 mL) was added dropwise (1–2 drops per s). The mixture was stirred at reflux for 18 h and then cooled to room temperature. After quenching by adding 10% aqueous HCl (250 mL), the aqueous layer was extracted with ethyl acetate (80 mL × 3). The combined organic layers were washed with brine (150 mL), dried over anhydrous Na₂SO₄ and filtered. The filtrate was concentrated by evaporation under reduced pressure. Purification of the crude material by silica-gel column chromatography (ethyl acetate/n-hexane = 1/19) followed by recrystallization from ethyl acetate afforded pure 3-iPr as pale orange blocks (2.00 g, 5.03 mmol) in a 50% yield. The corresponding 1,3-diketo-tautomer was not observed by ¹H NMR analysis in CDCl₃. 3-iPr: pale orange blocks; mp 103–104 °C (ethyl acetate); ¹H NMR (CDCl₃, 400 MHz) δ_ppm 1.29 (d, J = 6.9 Hz, 6H), 2.92–3.22 (m, 1H + 7H), 3.91 (m, 1H), 6.45–6.47 (m, 2H), 6.54–6.68 (m, 5H), 6.91 (d, J = 1.8 Hz, 1H), 7.34 (AA′XX′, J = 8.3 Hz, 2H), 7.90 (AA′XX′, J = 8.3 Hz, 2H), 16.87 (s, 1H); ¹³C NMR (CDCl₃, 100 MHz) δ_ppm 23.7 (2C), 34.2, 35.2, 35.3, 35.5, 35.7, 96.5, 126.8 (2C), 127.2 (2C), 131.7, 132.4, 132.5, 132.6, 132.7, 133.2, 135.8, 136.4, 136.9, 139.3, 139.8, 140.0, 140.3, 153.8, 184.3, 189.0; IR (ATR, neat) ν/cm⁻¹ 2963, 2928, 1608, 1593 (C=O), 1547, 1505, 1448, 1301, 1284, 1228, 1190, 1043, 901, 859, 851, 829, 800, 716, 707, 664; MS (ASAP) m/z 397 ([M + H]⁺); Anal. Calcd for C₂₈H₂₈O₂: C, 84.81; H, 7.12; N, 0.00. Found: C, 84.87; H, 7.10; N, 0.00.

Synthesis of 1-(4-isopropylphenyl)-3-(4-[2.2]paracyclopha-nyl)propane-1,3-dionatoboron difluoride (pCP-iPr). To a solution of 3-iPr (1.19 g, 3.00 mmol) in toluene (66 mL) was added a boron trifluoride–diethyl ether complex (0.57 mL, 4.5 mmol) under an argon atmosphere. After the mixture was stirred at room temperature for 12 h, volatile materials were removed under reduced pressure. Recrystallization of the crude material from ethyl acetate afforded pure pCP-iPr (0.667 g, 1.50 mmol) as yellow powder in a 50% yield. pCP-iPr: yellow powder; mp 181–182 °C (ethyl acetate); ¹H NMR (CDCl₃, 400 MHz) δ_ppm 1.30 (d, J = 6.9 Hz, 6H), 2.98–3.24 (m, 1H + 7H), 3.98 (m, 1H), 6.47 (dd, J = 7.7, 1.6 Hz, 1H), 6.54–6.66 (m, 4H), 6.77 (dd, J = 7.8, 1.8 Hz, 1H), 6.81 (s, 1H), 7.08 (d, J = 1.8 Hz, 1H), 7.41 (AA′XX′, J = 8.4 Hz, 2H), 8.06 (AA′XX′, J = 8.4 Hz, 2H); ¹³C NMR (CDCl₃, 100 MHz) δ_ppm 23.5 (2C), 34.5, 35.0, 35.1, 35.6, 36.6, 96.3, 127.3 (2C), 129.1 (2C), 129.7, 132.1, 132.2, 132.7, 132.8, 132.2, 134.1, 137.2, 138.3, 139.2, 139.7, 140.7, 143.1, 157.3, 181.9, 185.0; IR (ATR, neat) ν/cm⁻¹ 2965, 2935, 1604, 1537, 1519, 1499, 1474, 1457, 1427, 1374, 1362, 1306, 1286, 1250, 1188, 1159, 1146, 1120, 1095, 1071, 1056, 1038, 1011, 976, 863, 823, 811, 797, 719, 698, 678, 669; MS (ASAP) m/z 445 ([M + H]⁺); Anal. Calcd for C₂₈H₂₇BF₂O₂: C, 75.69; H, 6.13; N, 0.00. Found: C, 75.35; H, 6.45; N, 0.16.

FL measurement of crystals under high pressure

FL spectra of crystals under high pressure were recorded on a Hamamatsu Photonics PMA-11 spectrophotometer with a Panasonic UJ-30 UV-LED system as an excitation light source (λ_EX = 365 nm). Crystals were placed in a hole (diameter: 300 μm) of an SUS301 steel gasket equipped with DAC for applying isotropic pressure. Kerosene was used as a pressure-transmitting medium.^17,35,49 Hydrocarbons, including petroleum ether and kerosene, have been known to maintain good hydrostaticity up to 10 GPa.⁵⁰ The applied pressure was determined by using the ruby FL method.⁵¹ Photographs of crystals in DAC were taken on a FL microscope using irradiation by a 365-nm UV light or white LED light.

X-ray crystallographic analysis

General. Single crystals for X-ray crystallographic analyses were obtained by slow vapor diffusion of n-hexane into toluene solution for pCP-H or ethyl acetate solution for pCP-iPr. The X-ray diffraction data were collected at 293 K on a Rigaku XtaLAB Synergy-DW diffractometer with a Rigaku HyPix-6000 area detector by using Mo Kα radiation (λ = 0.71075 Å). The high-pressure measurements were performed using a sample holder equipped with the DAC and kerosene as the pressure medium. The inner pressure in the DAC was determined using the unit-cell parameters of NaCl.⁵² The structures were resolved by using the intrinsic phasing method and a SHELXT program⁵³ and were refined by using the least squares method of the squared amplitudes of the structure factor using the SHELXL program.⁵⁴ For treatment of the ambient-pressure measurement data, non-hydrogen atoms were refined using an anisotropic temperature factor, while for the high-pressure measurement data, non-hydrogen atoms were refined using an isotropic temperature factor because of the limited opening angle of the DAC. The hydrogen atoms for both ambient- and high-pressure measurement data were fixed at the positions calculated by the riding model. The Olex2 Version 1.3 program⁵⁵ was used for all analyses.

Crystal data for pCP-H at 0.1 MPa. Yellow block, trigonal, R3̄, a = b = 27.579(4) Å, c = 13.7469(15) Å, V = 9055(3) ų, Z = 18, ρ_calcd = 1.328 g cm⁻³, T = 293 K, 2θ_max = 57.31°, 10 588 reflections measured, 4745 unique reflections, R_int = 0.0652, 271 parameters, R₁ = 0.0765 (I > 2σ(I)), wR₂ = 0.2433 (all data), CCDC-2241676.

Crystal data for pCP-H at 3.1 GPa. Yellow block, trigonal, R3̄, a = b = 26.750(4) Å, c = 13.298(17) Å, V = 8241(11) ų, Z = 18, ρ_calcd = 1.573 g cm⁻³, T = 293 K, 2θ_max = 50.696°, 5626 reflections measured, 1603 unique reflections, R_int = 0.0913, 271 parameters, R₁ = 0.1341 (I > 2σ(I)), wR₂ = 0.4339 (all data), CCDC-2241677.

Crystal data for pCP-iPr at 0.1 MPa. Yellow block, orthorhombic, P2₁2₁2₁, a = 10.7074(6) Å, b = 14.6344(8) Å, c = 14.8516(8) Å, V = 2327.2(2) ų, Z = 4, ρ_calcd = 1.268 g cm⁻³, T = 293 K, 2θ_max = 74.866°, 15 966 reflections measured, 4754 unique reflections, R_int = 0.0289, 300 parameters, R₁ = 0.0435 (I > 2σ(I)), wR₂ = 0.1182 (all data), CCDC-2241678.

Crystal data for pCP-iPr at 3.1 GPa. Yellow block, orthorhombic, P2₁2₁2₁, a = 9.9470(13) Å, b = 14.3749(17) Å, c = 13.61(4) Å, V = 1946(6) ų, Z = 4, ρ_calcd = 1.517 g cm⁻³, T = 293 K, 2θ_max = 41.594°, 6130 reflections measured, 523 unique reflections, R_int = 0.0843, 133 parameters, R₁ = 0.0820 (I > 2σ(I)), wR₂ = 0.2184 (all data), CCDC-2241679.

Computational methods

Molecular orbital distributions and energies of singlet states in crystals were calculated by using DFT or TD-DFT with the ωB97XD functional and the 6-31G** basis set using the Gaussian 09W program.⁵⁶ The crystal coordinates derived from the X-ray crystallographic analyses were used for these calculations without geometry optimization. The non-covalent interaction plots were analysed using the Multiwfn program^57–59 and visualized using the VMD v1.9.3 program.⁶⁰ The Hirshfeld surface analyses were conducted using the CrystalExplorer v17.5 program.⁶¹ The geometry optimizations of the crystal structures under high pressure were carried out by using the PWSCF program in the Quantum ESPRESSO ver. 6.3 program package^47,48 with the van der Waals density functional (vdW-DF) for the non-local correlation.^62,63 The projector augmented wave (PAW) method was employed to describe the inner-shell electrons of each atom as pseudopotentials with respect to the valence electrons. The energy cut-off for the plane wave basis set and the charge density were 49 and 325 Ry, respectively. Convergence tolerance for the self-consistent field (SCF) calculation was set to the estimated total energy error of less than 1.0 × 10⁻⁶ Ry. For the structural optimization, the convergence tolerance of the total energy, the force, and the pressure on the unit cell were set to 1.0 × 10⁻⁴ a.u., 1.0 × 10⁻³ a.u., and 0.05 GPa, respectively.

Author contributions

T. Ogaki and H. Ikeda designed this research project. S. Irii, T. Ogaki, and Y. Matsui analysed the experimental data. S. Irii and S. Yamamoto synthesized molecules. S. Irii measured photophysical properties under ambient pressure. H. Miyashita, K. Nobori, and H. Iida carried out the high-pressure FL measurements under the supervision of Y. Ozawa and M. Abe. H. Sato carried out the X-ray crystallographic analyses under ambient and high pressures. S. Irii conducted the theoretical calculations. The manuscript was prepared by contribution of all authors. All authors approved the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: ¹H and ¹³C NMR spectra, photophysical data, crystal structures, and DFT calculation results. See DOI: https://doi.org/10.1039/d5tc03195h.

CCDC 2241676–2241679 contain the supplementary crystallographic data for this paper.^64a–d

Acknowledgements

This study was partially supported by JSPS KAKENHI Grants (no. JP24H01092, JP22K14667, JP22H05377, JP20K15264, JP21H04564, JP22K05069, JP22K05147, JP16H06514, JP21K19029, JP17H06375, JP17H06371, JP18H01967, and JP21H05494); by grants from the Konica Minolta Science and Technology Foundation, Tobe Maki Foundation (25-JA-016), the Hattori Hokokai Foundation, Mayekawa Houonkai Foundation (A3-24006), and the 2024 Osaka Metropolitan University Strategic Research Promotion Project (Young Researcher); and by JST Grants of the establishment of university fellowships towards the creation of science technology innovation (no. JPMJFS2138) and of SPRING (no. JPMJSP2139).

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