Piezochromic pyrylium salts

Abstract

Piezochromic pyrylium salts and applications using the piezochromic salts. A composition, medium, surface, or coating comprising the piezochromic pyrylium salts produces a piezochromic indication in response to a pressure change in the composition or medium or at the surface or coating.

Description

PIEZOCHROMIC PYRYLIUM SALTSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Application No. 63 / 537,414, filed September 8, 2023, which is incorporated into this application in its entirety.STATEMENT OF GOVERNMENT SUPPORT

[0002] Aspects of this disclosure were made with government support under Award Number DE-SC0023248, awarded by the U.S. Department of Energy. The government has certain rights in the invention.BACKGROUND

[0003] Pyrylium salts represent a class of small organic molecules built on a six- membered cationic heterocyclic structure with one positively charged oxygen ion and a variety of counterions, such as ’Cl, CIO4, BF4, ’PF6, OTs,_N(Tf2) and -FeCi4 These compounds have versatile applications ranging from synthetic utility for construction of heterocyclic compounds, complex macrocycles or metallo-supramolecules to photopolymerization reactions, photosensitizers, lasers in various organic solvents, and nonlinear optical properties. The piezochromic properties of pyrylium salts have never been previously reported.

[0004] Piezochromic materials have attracted much attention because of their multicolor- switching behavior and their applications in the field of advanced photonics, such as optical recording, memory, and sensors. The ability for a material to exhibit piezochromic properties is due to a variety of mechanisms such as change in molecular aggregated state, molecular conformation, or alterations of chemical bonds with applied high pressure (HP). As a result, piezochromic materials isomerize, undergo structural phase changes, experience lattice defects and modifications in electronic energy levels, affecting peak shape and position of their electron absorption and emission spectra. Although many HP studies have reported piezochromic mechanisms of synthesized novel structural phases with new optical properties, knowledge of piezochromic materials lack universality. Moreover, in most cases, multicolor- switching materials show full reversibility upon hydrostatic pressure release. Therefore, HP synthesis of materials stable at ambient conditions with novel structural and optical properties remains a challenge.SUMMARY

[0005] In one aspect, disclosed is a piezochromic composition, medium, surface, or coating, comprising a piezochromic amount of a pyrylium salt having the structure:wherein R’-R3are independently hydrogen, hydroxyl, amino, alkyl, cycloalkyl, bicyclic cycloalkyl, alkenyl, cycloalkenyl, alkynyl, cycloalkynyl, aryl, heteroaryl, alkoxy, or halide; and wherein Y is an anion; wherein the composition, medium, surface, or coating produces a piezochromic indication in response to a pressure change in the composition or medium or at the surface or coating.

[0006] In another aspect, disclosed is a method for sensing a change in pressure in a composition or medium or on a surface or coating, comprising measuring a piezochromic response from a pyrylium salt, the prylium salt having the structure:wherein R’-R3are independently hydrogen, hydroxyl, amino, alkyl, cycloalkyl, bicyclic cycloalkyl, alkenyl, cycloalkenyl, alkynyl, cycloalkynyl, aryl, heteroaryl, alkoxy, or halide; and wherein Y is an anion.BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The foregoing summary, as well as the following description of the disclosure, is better understood when read in conjunction with the appended drawings. For the purpose of illustrating the disclosure, the drawings illustrate some, but not all, alternative embodiments.This disclosure is not limited to the precise arrangements and instrumentalities shown. The following figures, which are incorporated into and constitute part of the specification, assist in explaining the principles of the disclosure.

[0008] FIG. 1 shows images reflecting the compression cycle of 2,4,6-triphenylpyrylium tetrachloroferrate from 0-9 GPa pressure range, which shows a color change from yellow to orange to red to burgundy at increasing pressures.

[0009] FIG. 2 shows images from the decompression of 2,4,6-triphenylpyrylium tetrachloroferrate from 0-9 GPa pressure range, demonstrating reversibility of piezochromism by color change back to its original yellow color.

[0010] FIGs. 3A and 3B show Raman (FIG. 3A) and Far-Infrared spectra (FIG. 3B) before the compression-decompression cycle with 2,4,6-triphenylpyrylium tetrachloroferrate.

[0011] FIGs. 3C and 3D show show Raman (FIG. 3C) and Far-Infrared spectra (FIG. 3D) after the compression-decompression cycle with 2,4,6-triphenylpyrylium tetrachloroferrate.

[0012] FIG. 4 shows a Raman spectrum of 2,4,6-triphenylpyrylium tetrachloroferrate at ambient pressure.

[0013] FIG. 5 shows a Raman spectrum of 2,4,6-triphenylpyrylium tetrachloroferrate at selected pressure points in 0-9 GPa pressure range during decompression.

[0014] FIG. 6 shows a Far-Infrared spectrum of 2,4,6-triphenylpyrylium tetrachloroferrate at selected pressure points in 0-9 GPa pressure range during decompression.

[0015] FIG. 7 shows a Raman spectrum of 2,4,6-triphenylpyrylium tetrachloroferrate at ambient pressure a) before compression and b) after decompression from 9 GPa.

[0016] FIGs. 8A-8C show XRD patterns of pyrylium FeCU’: (FIG. 8A) during the compression; vertical bars indicate peak positions of previously reported orthorhombic crystal structure of pyrylium FeCU’; (FIG. 8B) during the decompression step and (FIG. 8C) a comparison of XRD spectra before and after the compression-decompression cycle.DETAILED DESCRIPTION

[0017] This disclosure demonstrates that pyrylium salts, such as 2,4,6-triphenylpyrylium tetrachloroferrate (Py-FeCL), are subjected to pressure, e.g., 0-9 GPa pressure range, multi- color switching is observed. This color change evolution is reversible upon pressure decrease. When the samples are recovered, they exhibit similar chemicaFstructural properties as the material prior to compression. Without being bound by theory, to unveil the mechanism responsible for Py-FcCU piezochromic behavior, Raman and Far-Infrared (Far-IR)spectroscopies combined with powder X-ray Diffraction technique (pXRD) were performed during a compression / decompression cycle. Experimental findings suggest pressure-induced electron transfer between the pyrylium cation and the -FeCU anion is the mechanism of Py- FeCU color-switching behavior. The observed piezochromism ofPy-FeCI4 indicates that this organic-inorganic salt and structurally related salts can be utilized for designing high pressure sensing technologies with the ability to detect pressure changes in extreme environments, among many other applications.I. Definitions

[0018] “Alkyl” refers to a branched or unbranched saturated hydrocarbon group of 1 to 24 carbon atoms, such as methyl, ethyl, / / -propyl, isopropyl, ra-butyl, isobutyl, s-butyl, Ebutyl, ra-pentyl, isopentyl, s-pentyl, neopentyl, hexyl, heptyl, octyl, nonyl, decyl, dodecyl, tetradecyl, hexadecyl, eicosyl, tetracosyl, and the like. The alkyl group can be cyclic or acyclic. The alkyl group can also be substituted or unsubstituted. For example, the alkyl group can be substituted with one or more groups including, but not limited to, alkyl, cycloalkyl, alkoxy, amino, ether, halide, hydroxy, nitro, silyl, sulfo-oxo, or thiol, as described herein. “Alkyl” can be a Ci alkyl, C1-C2 alkyl, C1-C3 alkyl, C1-C4 alkyl, C1-C5 alkyl, C1-C6 alkyl, C1-C7 alkyl, C1-C8 alkyl, C1-C9 alkyl, C1-C10 alkyl, and the like up to and including a C1-C24 alkyl. “Heteroalkyl” refers to an alkyl group in which one or more of the hydrogen atoms bonded to carbon are substituted with a heteroatom including but not limited to O, S, or N(R)2, in which each R can independently be hydrogen or a non-hydrogen substituent.

[0019] “Cycloalkyl” refers to a non-aromatic carbon-based ring composed of at least three carbon atoms. Examples of cycloalkyl groups include, but are not limited to, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, norbomyl, and the like. “Heterocycloalkyl” is a non-aromatic carbon-based ring type of cycloalkyl group, where at least one of the carbon atoms of the ring is replaced with a heteroatom such as, but not limited to, nitrogen, oxygen, sulfur, or phosphorus. Representative heterocycloalkyl groups include, but are not limited to, pyrrolidinyl, pyrazolinyl, pyrazolidinyl, imidazolinyl, imidazolidinyl, piperidinyl, piperazinyl, oxazolidinyl, isoxazolidinyl, morpholinyl, thiazolidinyl, isothiazolidinyl, and tetrahydrofuryl. The cycloalkyl group and heterocycloalkyl group can be substituted or unsubstituted. The cycloalkyl group and heterocycloalkyl group can be substituted with one or more groups including, but not limited to, alkyl, cycloalkyl, alkoxy, amino, ether, halide, hydroxy, nitro, silyl, sulfo-oxo, or thiol.

[0020] “Bicyclic cycloalkyl” or “bicyclic heterocycloalkyl” refers to a compound in which two or more cycloalkyl or heterocycloalkyl groups are fused together. Non-limiting examples of bicyclic cycloalkyl groups include without limitation (lr,4r)- bicyclo [2. 1.1] hexane, (ls,4s)-bicyclo[2.2.1]heptane, (lR,6S)-bicyclo[4.2.0]octane, adamantane, and the like. Non- limiting examples of bicyclic heterocycloalkyl groups include without limitation any of the foregoing groups in which at least one of the carbon atoms is replaced with a heteroatom such as nitrogen, oxygen, sulfur, or phosphorus.

[0021] “Alkenyl” refers to a hydrocarbon having from 2 to 24 carbons with a structural formula containing at least one carbon-carbon double bond. Asymmetric structures such as ( A1A2)C=C( A3A4) are intended to include both the E and Z isomers. The alkenyl group can be substituted with one or more groups including alkyl, cycloalkyl, alkoxy, alkenyl, cycloalkenyl, alkynyl, cycloalkynyl, aryl, heteroaryl, aldehyde, amino, carboxylic acid, ester, ether, halide, hydroxy, ketone, azide, nitro, silyl, sulfo-oxo, or thiol, among others.

[0022] “Cycloalkenyl” refers to a non-aromatic carbon-based ring composed of at least three carbon atoms and containing at least one carbon-carbon double bound, i.e., C=C. Examples of cycloalkenyl groups include cyclopropenyl, cyclobutenyl, cyclopentenyl, cyclopentadienyl, cyclohexenyl, cyclohexadienyl, norbornenyl, among others. The term “heterocycloalkenyl” is a type of cycloalkenyl group and is included within the meaning of the term “cycloalkenyl,” where at least one of the carbon atoms of the ring is replaced with a heteroatom such as nitrogen, oxygen, sulfur, or phosphorus. The cycloalkenyl group and heterocycloalkenyl group can be substituted or unsubstituted. The cycloalkenyl group and heterocycloalkenyl group can be substituted with one or more groups including alkyl, cycloalkyl, alkoxy, alkenyl, cycloalkenyl, alkynyl, cycloalkynyl, aryl, heteroaryl, aldehyde, amino, carboxylic acid, ester, ether, halide, hydroxy, ketone, azide, nitro, silyl, sulfo-oxo, or thiol, among others.

[0023] “Alkynyl” means a hydrocarbon group of 2 to 24 carbon atoms with a structural formula containing at least one carbon-carbon triple bond. The alkynyl group can be unsubstituted or substituted with one or more groups including alkyl, cycloalkyl, alkoxy, alkenyl, cycloalkenyl, alkynyl, cycloalkynyl, aryl, heteroaryl, aldehyde, amino, carboxylic acid, ester, ether, halide, hydroxy, ketone, azide, nitro, silyl, sulfo-oxo, or thiol, among others.

[0024] “Cycloalkynyl” refers to a non-aromatic carbon-based ring composed of at least seven carbon atoms and containing at least one carbon-carbon triple bound. Examples of cycloalkynyl groups include cycloheptynyl, cyclooctynyl, cyclononynyl, and the like. Theterm “heterocycloalkynyl” is a type of cycloalkenyl group and is included within the meaning of the term “cycloalkynyl,” where at least one of the carbon atoms of the ring is replaced with a heteroatom such as, but not limited to, nitrogen, oxygen, sulfur, or phosphorus. The cycloalkynyl group and heterocycloalkynyl group can be substituted or unsubstituted. The cycloalkynyl group and heterocycloalkynyl group can be substituted with one or more groups including alkyl, cycloalkyl, alkoxy, alkenyl, cycloalkenyl, alkynyl, cycloalkynyl, aryl, heteroaryl, aldehyde, amino, carboxylic acid, ester, ether, halide, hydroxy, ketone, azide, nitro, silyl, sulfo-oxo, or thiol, among others.

[0025] “Aryl” refers to a group that contains any carbon-based aromatic group including, but not limited to, benzene, naphthalene, phenyl, biphenyl, anthracene, and the like. The aryl group can be substituted or unsubstituted. The aryl group can be substituted with one or more groups including, but not limited to, alkyl, cycloalkyl, alkoxy, alkenyl, cycloalkenyl, alkynyl, cycloalkynyl, aryl, heteroaryl, aldehyde, — NH2, carboxylic acid, ester, ether, halide, hydroxy, ketone, azide, nitro, silyl, sulfo-oxo, or thiol as described herein. In addition, the aryl group can be a single ring structure or comprise multiple ring structures that are either fused ring structures or attached via one or more bridging groups such as a carbon-carbon bond. For example, aryl can include biaryl in which two aryl groups are bound together via a fused ring structure, as in naphthalene, or are attached via one or more carbon-carbon bonds, as in biphenyl.

[0026] “Heteroaryl” refers to an aromatic group that has at least one heteroatom incorporated within the ring of the aromatic group. Examples of heteroatoms include, but are not limited to, nitrogen, oxygen, sulfur, and phosphorus, where N-oxides, sulfur oxides, and dioxides are permissible heteroatom substitutions. The heteroaryl group can be substituted or unsubstituted. The heteroaryl group can be substituted with one or more groups including, but not limited to, alkyl, cycloalkyl, alkoxy, amino, ether, halide, hydroxy, nitro, silyl, sulfo-oxo, or thiol as described herein. Heteroaryl groups can be monocyclic, or alternatively fused ring systems. Heteroaryl groups include, but are not limited to, furyl, imidazolyl, pyrimidinyl, tetrazolyl, thienyl, pyridinyl, pyrrolyl, A-methylpyrrolyl, quinolinyl, isoquinolinyl, pyrazolyl, triazolyl, thiazolyl, oxazolyl, isoxazolyl, oxadiazolyl, thiadiazolyl, isothiazolyl, pyridazinyl, pyrazinyl, benzofuranyl, benzodioxolyl, benzothiophenyl, indolyl, indazolyl, benzimidazolyl, imidazopyridinyl, pyrazolopyridinyl, and pyrazolopyrimidinyl. Further non-limiting examples of heteroaryl groups include, but are not limited to, pyridinyl, pyridazinyl, pyrimidinyl, pyrazinyl, thiophenyl, pyrazolyl, imidazolyl, benzo[i / ]oxazolyl, benzo[i / ]thiazolyl, quinolinyl, quinazolinyl, indazolyl, imidazo[l,2-b]pyridazinyl,imidazo[l,2-a]pyrazinyl, benzo[c][l,2,5]thiadiazolyl, benzo[c][l,2,5]oxadiazolyl, and pyrido [2,3 -b]pyrazinyl.

[0027] “Alkoxy” refers to an alkyl or cycloalkyl group bonded to a disclosed salt through an ether linkage, i.e., — OR where R is alkyl or cycloalkyl.

[0028] “Halide” refers to F, Cl, Br, or I. “Haloalkyl,” “haloalkenyl,” and the like refer to compounds or groups which include at least one halide substituent at any position.

[0029] II. Piezochromic Pyrylium Salts

[0030] In one aspect, the piezochromic pyrylium salts have the structure:wherein R’-R3are independently hydrogen, hydroxyl, amino, alkyl, cycloalkyl, bicyclic cycloalkyl, alkenyl, cycloalkenyl, alkynyl, cycloalkynyl, aryl, heteroaryl, alkoxy, or halide; and wherein Y is an anion;

[0031] In one aspect, Y is OTs, BF4, CF3SO3, N(SO2CF3)2, CIO4, FeCi4, or an anion having the structure:

[0032] In one aspect, R1-R3are independently hydrogen, alkyl, alkoxy, or halide. The alkoxy can be C1-C4 alkoxy, for example, — OCH3. In a further aspect, the halide is F, Cl, or Br. In one specific aspect, R'-R3are each hydrogen. In a further specific aspect, Y is FcCU.

[0033] Specific non-limiting examples of the piezochromic salts include the following:III. Methods of Making the Pyrylium Salts

[0034] The pyrylium salts can be made through a variety of methods, including for example the route shown in Scheme 1. An initial pyrylium salt (Py-A) for example can be made via a diketone method. The pyrylium salt (Py-A) can then be converted to the pseudobase with the treatment of a reactant such as an acetate salt in a suitable solvent. The pseudobase can in turn be converted to an intermediate salt (Py-Q) by treatment with an appropriate acid in a suitable solvent. A final salt (Py-Y) can be formed in a suitable solvent that includes a “Y” precursor, for instance in the case of FeCI4 FeCI3 can be used under reflux conditions.IV. Applications of the Pyrylium Salts

[0035] The piezochromic pyrylium salts are useful in a variety of applications. In one aspect, for example, the salt can be present in a composition or medium, or on a surface or in / on a coating in a piezochromic amount, such that the composition, medium, surface, or coating produces a piezochromic indication in response to a pressure change in the composition or medium or at the surface or coating. In one aspect, the piezochromic indication is a detectable color change in the composition or medium or at the surface or coating. The detectable color change can in some aspects be visually detectable by the human eye.

[0036] Similarly, the pyrylium salts can be used to detect pressure. For example, a method for sensing a change in pressure in a composition, medium, or on a surface or in / on a coating can comprise measuring a piezochromic response from a disclosed pyrylium salt. As discussed above, the piezochromic response can be a color change, such as a color change detectable by the human eye.

[0037] As will be appreciated, the compositions, mediums, surfaces, coatings, and methods for detecting pressure changes, can be applied in a number of ways. For instance, the pyrylium salts can be used in pressure sensors. When subjected to mechanical pressure, the change in the salt’s optical properties, such as color, can be used to detect and measure pressure variations. These properties are useful for instance in industrial and scientific instruments. Similarly, the piezochromic pyrylium salts can be incorporated into smart materials that change color or appearance when pressure is applied. These materials have applications in design and aesthetics, as well as in engineering for stress visualization.

[0038] The salts can be also used in other materials science and engineering applications, for instance to map stress distribution in structures, helping to identify areas of high stress and potential failure. In civil engineering, the pyrylium salts can be used for structural health monitoring of bridges, buildings, and other infrastructure. Changes in color or optical properties can indicate structural stresses or damage. The salts are also useful in high- pressure studies in geology and materials science. The salts can act as indicators of pressure changes under extreme conditions. In the field of biomechanics, the pyrylium salts cab be applied to study pressure distribution in biological tissues, aiding in understanding the mechanics of joints, bones, and soft tissues. The salts can also be used in tamper-evident packaging or security labels. In electronic devices and microelectronics, the salts can be used to map pressure distribution on touchscreens or control panels, enhancing user interfaces and enabling new interaction methods. There are also aesthetic applications. Artists and designers can use the piezochromic materials to create interactive artworks and designs that change appearance in response to touch or pressure, adding an element of dynamic interactivity. In addition, the piezochromic salts can serve as educational tools in science and engineering to demonstrate principles of pressure and stress.EXAMPLES

[0039] The following examples further illustrate this disclosure. The scope of the disclosure and claims is not limited by the scope of the following examples.I. Synthesis of Py-FeCI4

[0040] Py-FeCI4 was prepared according to Scheme 2.Scheme 2

[0041] The method used for makingPy-FeCI4 is safer and less expensive that conventional methods and does not require using acids as condensing agents. Pyrylium tosylate was prepared by a two-step procedure via the diketone method. It was converted to the pseudobase with the treatment of sodium acetate in ethanol on heating for 30 min. The pseudobase was then converted to PyCl with the treatment of concentrated HCl / ethanol on heating for 1 h. Finally, PyCl was converted to the PyFeCU on heating to reflux in acetonitrile with FeCh. Ferric chloride (395 mg, 1.147 mmol) and 2,4,6-triphenylpyrylium chloride (186 mg, 1. 147mmol) were dissolved in 25 mL acetonitrile and heated to reflux for 24 h. The solvent was removed under reduced pressure and the yellow solid was washed with ether to furnish 530 mg PyFeCU (91% yield). Mp: 220-225°C.II. Piezochromic Testing

[0042] The synthesized bright yellowPy-FeCI4 powder was loaded into a symmetric diamond anvil cell (DAC) to generate high pressures. A 250 pm thick stainless-steel gasket with 100-150 pm wide hole drilled in the center of 50 pm indentation was used to confine the sample in a DAC. To consistently measure pressure, a ruby sphere was centered into thedrilled opening then packed with solidPy-FeCI4 . Once a DAC was loaded, the yellow starting material was pressurized from 0-9 GPa with 0.5 GPa increment. FIG. 1 shows progressive color switching behavior ofPy-FeCI4 with pressure increase. From ambient up to 2 GPa, a gradual change of color from yellow to orange was observed.

[0043] Increase of pressure (up to 7 GPa) induced color transformation to red which eventually became burgundy. In the pressure range between 7-9 GPa, the sample turned and stayed black. During the decompression, the gradual return ofPy-FeCI4 , color was detected (FIG. 2) suggesting a reversibility of its piezochromism.

[0044] In order to elucidate the main mechanism ofPy-FeCI4 color-switching behavior, Raman and Far-IR spectroscopies were carried out. With a DAC, Raman spectra of compressed samples at room temperature were measured in the backscattering configuration using a HR800 spectrometer equipped with a liquid nitrogen-cooled CCD detector (Horiba Jobinlvon) with spectral and spatial resolutions of 0.25 cm and 5 pm, respectively. A 532 nm solid-state KPT-crystal laser (LASOS) was used to excite the DAC samples. No pressure- transmitting media such as methanol / ethanol was used in Raman measurements asPy-FeCI4 undergoes dissociation reactions with this type of solvents. Far-IR measurements were performed at the 02B1-1beamline at the Canadian Light Source (CLS). Hydrostatic pressure was generated with potassium bromide (KBr) as a pressure-transmitting medium during Far- IR spectroscopic analysis within 0-9 GPa pressure.

[0045] FIG. 3 displays Raman spectra (800-1800 cm1) as well as Far-IR spectra of Py- FeCU at selected pressure points. A complete Py-FeCI4 Raman spectrum at ambient conditions is presented in FIG. 4. Within the Py-FeCI4 Raman spectrum, four characteristic peaks can be recognized: at —335 cm-1corresponding to the Fe-Cl stretching motion of -FeCl4anion, peaks around 950 and -1000 cm-1(aromatic ring breathing), as well as at -1590 cm-1(benzene ring stretching) representing the vibrational modes of 2,4,6-triphenylpyrylium cation. Due to the proximity of two characteristic peaks located at -335 and -1000 cm-1with other spectral features (-300 and -1015 cm-1), their HP behavior cannot be examined (peaks merging prevents their own distinguishing). Therefore, two intense single peaks (954 and 1596 cm-1) were followed for spectral analysis ofPy-FeCI4 chemical and structural transformations responsible for its piezochromic behavior within the 0-9 GPa pressure range (see FIGs. 3A and 3B). For investigating the HP evolution of vibrational modes of the -FeCI4 anion and capture its contribution intoPy-FeCI4 piezochromic behavior, the peak at 393 cm-1(Fe-Cl stretching), in far-IR range, was studied (see FIGs. 3C and 3D).

[0046] As depicted in FIGs. 3B and 3D, all peaks demonstrate similar wavenumber shift-pressure dependence. Two abrupt shifts between 2-3 GPa and 7-8 GPa pressure ranges were observed. This spectroscopic behavior corresponds to the color switching behavior seen in FIG. 1. Upon decompression, all peaks within Raman and Far-IR spectra returned to their original positions demonstrating full reversibility of chemicaPl / ys-tFruecCtuI4ral transformations (See FIG. 5, FIG. 6, and FIG. 7). The abrupt Raman shift of characteristic spectral peaks upon compression can be indication of either structural phase or electronic structure transitions. To obtain more insights about piezochroPmyi-cF meCeIc4hanism, additional pXRD measurements at HP were conducted.

[0047] Powdered Py-FeC wI4as loaded into DACs and XRD patterns at HP were collected via monochromatic X-rays at 16.5 keV (0.0751 A) energy at the 16 BM-D beamline of the High Pressure Collaborative Access Team (HP-CAT) facilities at the Advanced Photon Source (APS). FIG. 8A shows XRD patterns of at sPeyle-cFteeCdI p4ressures within 0-9 GPa. The Py-FcCU XRD pattern at ambient pressure corresponds to a reported orthorhombic crystal structure of Py- wFeitChI t4he Pbcm space group and the following lattice parameters: a = 10.949 A, b = 17. 190 A, c = 12.099 A, and (3 = 90°. Upon compression, all XRD peaks undergo a gradual shift towards higher 20 indicating a reduction of the unit cell volume. However, no formation of new XRD peaks in 0-9 GPa is detected demonstrating the absence of any Py-FeCI4 structural transformations (See FIG. 8A). During decompression from 9 GPa to ambient pressure, Py-FECI4 peaks gradually return to their original positions as depicted in FIG. 8B. XRD patterns of Py- pFreiCorI4 and post compression are similar (see FIG. 8C) showing the complete structural reversibilty of within 0P-y9-F GePCaI4 pressure range.

[0048] Without being bound by any theory, it can be suggested that the mechanism of Py- FeC14 piezochromic behavior is related to HP-induced electronic transformations associated with electron transfer between organic-inorganic components. Under high external pressure, the molecules in the lattice may be forced to move closer. The inter-molecular contacts between the electron-rich FeCI4 and the electron-deficient pyrylium moiety may be shortened to favorable distances to induce electron transfer from FeCI4’ to pyrylium ion. This may lead to a metastable state with two radical sites per molecule, one delocalized over the pyrylium moiety, the other over the FeCI4 groups. There could also be intermolecular radical-radical interactions, which, together with planar configuration of the pyrylium moiety, help to stabilize the radical state. The reverse electron transfer may be slow and unable to keep pace with the recovery of the intra- and intermolecular parameters upon decompression. Although the molecular and crystal structure is completely recovered after removal of pressure, the radical state may remain in a certain proportion. The back transferprocess may continue upon further standing at ambient pressure to finally regenerate the original state. Electron transfer may lead to new absorption bands, and the gradual shifts of the bands during the compression / decompression process could be attributed to the modulation effects of pressure on intermolecular interactions and molecular conformation. The piezochromic properties of "P mya-yFe bCeI4 similar to those of viologen-carboxylate zwitterion molecules where electron transfer occurs between an electron rich carboxylate ion and the electron deficient viologen moiety.

[0049] The following additional analogs of Py-FeCI4 were observed to show piezochromic behavior.

[0050] Features and advantages of this disclosure are apparent from the detailed specification, and the claims cover all such features and advantages. Numerous variations will occur to those skilled in the art, and any variations equivalent to those described in this disclosure fall within the scope of this disclosure. Those skilled in the art will appreciate that the conception upon which this disclosure is based may be used as a basis for designing other compositions and methods for carrying out the several purposes of this disclosure. As a result, the claims should not be considered as limited by the description or examples.

Claims

CLAIMSWhat is claimed is:

1. A piezochromic composition, medium, surface, or coating, comprising a piezochromic amount of a pyrylium salt having the structure:wherein R1-R3are independently hydrogen, hydroxyl, amino, alkyl, cycloalkyl, bicyclic cycloalkyl, alkenyl, cycloalkenyl, alkynyl, cycloalkynyl, aryl, heteroaryl, alkoxy, or halide; and wherein Y is an anion; wherein the composition, medium, surface, or coating produces a piezochromic indication in response to a pressure change in the composition or medium or at the surface or coating.

2. The composition, medium, surface, or coating of claim 1, wherein the piezochromic indication is a detectable color change in the composition or medium or at the surface or coating.

3. The composition, medium, surface, or coating of claim 1, wherein Y is OTs, BF4, CF3SO3, N(SO2CF3)2, CIO4, FeC14, or an anion having the structure:

4. The composition, medium, surface, or coating of claim 1, wherein R1-R3are independently hydrogen, alkyl, alkoxy, or halide.

5. The composition, medium, surface, or coating of claim 4, wherein alkoxy is C1-C4 alkoxy.

6. The composition, medium, surface, or coating of claim 5, wherein alkoxy is — OCH3.

7. The composition, medium, surface, or coating of claim 4, wherein halide is F, Cl, or Br.

8. The composition, medium, surface, or coating of claim 1, wherein RhR3are each hydrogen.

9. The composition, medium, surface, or coating of claim 8, wherein Y is FeCI410. A method for sensing a change in pressure in a composition or medium, or on a surface or coating, comprising measuring a piezochromic response from a pyrylium salt in the composition or medium, or on the surface or coating, the prylium salt having the structure:wherein R'-R3are independently hydrogen, hydroxyl, amino, alkyl, cycloalkyl, bicyclic cycloalkyl, alkenyl, cycloalkenyl, alkynyl, cycloalkynyl, aryl, heteroaryl, alkoxy, or halide; and wherein Y is an anion.

11. The method of claim 10, wherein Y is OTs, BF4, CF3SO3, N(SO2CF3)2, CIO4, FeCI4 or an anion having the structure:

12. The method of claim 10, wherein R'-R3are independently hydrogen, alkyl, alkoxy, or halide.

13. The method of claim 12, wherein alkoxy is C1-C4 alkoxy.

14. The method of claim 13, wherein alkyoxy is — OCH3.

15. The method of claim 12, wherein halide is F, Cl, or Br.

16. The method of claim 10, wherein R3-R3are each hydrogen.

17. The method of claim 16, wherein Y is FeCI4

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