A novel oxindolyl-based radical for stimuli-responsive chromic materials

Abstract

Disclosed herein are a radical of formula Ia and / or a dimer of formula Ib, and a compound according to formula II. Also disclosed herein is a film comprising a radical of formula Ia and / or a dimer of formula Ib, and a polymeric material, wherein the radical of formula Ia and / or the dimer of formula Ib are distributed throughout the film. Also disclosed herein is an intermediate film comprising a compound according to formula II, and a polymeric material, wherein the compound of formula II are distributed throughout the film.

Description

[0001] A NOVEL OXINDOLYL-BASED RADICAL FOR STIMULI-RESPONSIVE CHROMIC MATERIALS

[0002] Field of Invention

[0003] The present invention generally relates to stimuli-responsive chromic materials and more particularly relates to radicals, dimers, and compounds for stimuli-responsive chromic materials.

[0004] Background

[0005] The listing or discussion of a prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

[0006] Self-cooling and self-tinting glass window based on thermochromism or mechanochromism is an excellent solution to cut the heat load in the buildings and thus reduce the air-conditioning cost. Stimuli-responsive chromic materials, which exhibit a distinct color change when exposed to an external stimulus, have attracted growing attention in the field of materials science as they allow the visualization of the effects caused by external stimuli. Various methods have been well developed to construct novel chromic materials with intriguing performance. Typically, dynamic covalent systems that generate metastable organic radicals hold great promise, when facilitated by the simple radical-radical coupling reaction with no additional catalysts or by-products being involved. Methyl radicals substituted with spindelocalizing units and carbonyl functionalities, namely dicyanomethyl radical, fluorenyl radical and benzofuranone (BF) radical (Fig. 1a), have been known to provide stable radicals with a high propensity towards o-dimerization, but low reactivity towards oxygen. Among these, dynamic aryl dicyanomethyl radical derivatives have achieved remarkable progress to afford chromic materials. Nevertheless, the lactone structure in BF may pose additional difficulty in further modifications and functionalizations of the structure due to its intrinsic reactivity in basic conditions.

[0007] Therefore, there exists a need for new species for stimuli-responsive chromic materials.

[0008] Summary of Invention

[0009] Aspects and embodiments of the invention are provided in the following numbered clauses. 1. A radical of formula la and / or a dimer of formula lb: wherein: each R1independently represents CN, halo, Ci to Ce alkyl, OR4a, NR4bR4° or C(=O)OR4d

[0010] R2represents CH2Ph, Ci to Ce alkyl, -CH=CH2;

[0011] R3represents p-fBuPh, p-CNPh, p-OR5aPh, p-NR5bR5cPh; each of R4ato R4dand R5ato R5cindependently represents Ci to Ce alkyl or a phenyl group; and a, b and c each represent 0 or 1 , provided that at least one of a to c is 1 . 2. The radical and / or the dimer according to Clause 1, wherein each R1independently represents CN, methyl, OMe or N(Me)2.

[0012] 3. The radical and / or the dimer according to Clause 1 or Clause 2, wherein R2represents CH2Ph.

[0013] 4. The radical and / or the dimer according to any one of the preceding clauses, wherein R3represents p-fBuPh.

[0014] 5. The radical and / or the dimer according to any one of the preceding clauses, wherein: a is 1 , b and c are 0; b is 1 , a and c are 0; c is 1 , a and b are 0; a and b are 1 and c is 0; a and c are 1 and b is 0; b and c are 1 and a is 0; and a, b and c are 1 .

[0015] 6. The radical and / or the dimer according to Clause 5, wherein a is 1 , b and c are 0.

[0016] 7. The radical and / or the dimer according to any one of the preceding clauses, wherein the radical of formula la is selected from the list:

[0017] and the dimer of formula lb that corresponds to the above compounds of formula la.

[0018] 8. The radical and / or the dimer according to any one of the preceding clauses, wherein: the dimer of formula lb dissociates to provide two radicals of formula la upon application of a stimulus and the radical of formula la undergoes dimerization to provide a dimer according to a compound of formula lb upon removal of the stimulus; or the radical of formula la undergoes dimerization to provide a dimer according to a compound of formula lb upon application of a stimulus and the dimer of formula lb dissociates to provide two radicals of formula la upon removal of the stimulus.

[0019] 9. The radical and / or the dimer according to Clause 8, wherein the stimulus is selected from one or more of the group consisting of a change in temperature and a mechanical force. 10. A compound according to formula II:

[0020] where R1to R3and a to c are as defined in any one of Clauses 1 to 6.

[0021] 11. The compound according to Clause 10, wherein the compound of formula II is selected

[0022] 5 from the list:

[0023]

[0024]

[0025] 12. A film comprising: a radical of formula la and / or a dimer of formula lb as described in any one of Clauses 1 to 9; and a polymeric material, wherein the radical of formula la and / or the dimer of formula lb are distributed throughout the film.

[0026] 13. An intermediate film comprising: a compound according to formula II as described in Clause 10 or Clause 11 ; and a polymeric material, wherein the compound of formula II is distributed throughout the film.

[0027] Drawings

[0028] Fig. 1 depicts an illustration of representative carbon-centred radicals (dicyanomethyl radical, fluorenyl radical and benzofuranone radical) for dynamic covalent chemistry (a) and the target molecules (oxindolyl-based radicals and their dimers) in the present disclosure (b).

[0029] Fig. 2 depicts synthetic routes of I2, 22, 32, and 42. a) 4-iodobenzonitile, 'PrMgCl-LiCI, THF, - 20 °C for 1 h, corresponding benzyl-6-(4-(tert-butyl)phenyl)indoline-2,3-dione (5) and stirring at room temperature; b) SnCh saturated in concentrated HCI, CH3COOH, 100 °C for 3 h; c) DDQ, DCM, RT; d) aryl bromide, THF, -78 °C, n-BuLi, corresponding benzyl-6-(4-(fert- butyl)phenyl)indoline-2, 3-dione (5) and stirring at room temperature. Bn: benzyl group, DDQ: 2,3-dichloro-5,6-dicyano-p-benzoquinone, DCM: dichloromethane.

[0030] Fig. 3 depicts top view (left) and side view (right) of the single crystal structures of I2 (a) and 22 (b) (O, N and the bond length of the covalent bonds between two monomers are labelled, H atoms are omitted for clarity), c)1H-NMR spectra of dimer I2 in CD2CI2 at variable temperatures.

[0031] Fig. 4 depicts variable-temperature (VT)-1H-NMR spectrum of 22 in CD2CI2. (500 MHz).

[0032] Fig. 5 depicts VT-1H-NMR spectrum of 32 in CD2CI2. (500 MHz).

[0033] Fig. 6 depicts VT-1H-NMR spectrum of 42 in CD2CI2. (500 MHz, indicate trace residual ethyl acetate and MeOH which are difficult to remove due to the low bond dissociation energy of the bond between monomeric units in 42, which tends to break under vacuum for a longer time).

[0034] Fig. 7 depicts temperature dependence of the UV-vis-NIR spectra of 42(a) in toluene and (b) in DMF. (The concentrations of the test solution were both 10’3M).

[0035] Fig. 8 depicts a) ultraviolet-visible-near infrared (UV-vis-NIR) absorption spectrum of dimer 42dissolved in DMF (1*10-3M) tested at different temperature (insert: images for the thermochromic behavior), b) Electron spin resonance (ESR) spectra of the powder of dimer 42 before and after grinding (insert: images for the mechanochromic behavior).

[0036] Fig. 9 depicts temperature dependence of the UV-vis-NIR spectra of (a) 12, (b) 22 and (c) 32 in DMF (IO3M).

[0037] Fig. 10 depicts variation of the absorption spectra against decreasing temperature of (a) I2, (b) 22, (c) 32and (d) 42in DMF (103M).

[0038] Fig. 11 depicts images before grinding (left) and after grinding (right) for the mechanochromic behavior of I2, 22 and 32 and 42.

[0039] Fig. 12 depicts temperature dependence of the ESR spectra of (a) 12, (b) 22 and (c) 32 in DMF.

[0040] Fig. 13 depicts van’t Hoff plots (In Keqvs T1) for (a) 12, (b) 22, (c) 32 and (d) 42 in DMF. Fig. 14 depicts spin density distribution maps for 1’ (a), 2* (b), 3’ (c) and 4* (d). (UB3LYP / 6- 31 G*, spin isosurface value = 0.04 electron au’3). Numerical values are the amount of spin on the central carbon atom of the OxRs.

[0041] Description

[0042] It has been surprisingly found that the present invention provides oxindolyl-based radicals (OxRs) that are valuable building blocks for organic radical-based dynamic covalent chemistry, which can be further applied in mechanochromic and thermochromic materials. Furthermore, the OxRs are stable under ambient environment, can tolerate harsh conditions, and can be easily modified for further functionalization with different substituents for various applications. As will be appreciated, the dynamic process of radical-radical coupling is based on reversible association / dissociation reaction of monoradicals so it is very simple and does not contain any catalyst and by-product. Additionally, the color change phenomenon comes from the radical and dimer species themselves, thus the present invention may find opportunities in low-cost ink-free printing and smart windows applications.

[0043] Thus, in a first aspect of the present invention, there is provided a radical of formula la and / or a dimer of formula lb:

[0044] wherein: each R1independently represents CN, halo, Ci to Ce alkyl, OR4a, NR4bR4cor C(=O)OR4dR2represents CH2Ph, Ci to Ce alkyl, -CH=CH2;

[0045] R3represents p-fBuPh, p-CNPh, p-OR5aPh, p-NR5bR5cPh; each of R4ato R4dand R5ato R5cindependently represents Ci to Ce alkyl or a phenyl group; and a, b and c each represent 0 or 1 , provided that at least one of a to c is 1 .

[0046] The word “comprising” refers herein may be interpreted as requiring the features mentioned, but not limiting the presence of other features. Alternatively, the word “comprising” may also relate to the situation where only the components / features listed are intended to be present (e.g. the word “comprising” may be replaced by the phrases “consists of” or “consists essentially of”). It is explicitly contemplated that both the broader and narrower interpretations can be applied to all aspects and embodiments of the present invention. In other words, the word “comprising” and synonyms thereof may be replaced by the phrase “consisting of’ or the phrase “consists essentially of’ or synonyms thereof and vice versa.

[0047] The phrase, “consists essentially of’ and its pseudonyms may be interpreted herein to refer to a material where minor impurities may be present. For example, the material may be greater than or equal to 90% pure, such as greater than 95% pure, such as greater than 97% pure, such as greater than 99% pure, such as greater than 99.9% pure, such as greater than 99.99% pure, such as greater than 99.999% pure, such as 100% pure. As used herein, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a composition” includes mixtures of two or more such compositions, reference to “the catalyst” includes mixtures of two or more such catalysts, and the like.

[0048] The term “halo”, when used herein, includes references to fluoro, chloro, bromo and iodo.

[0049] Unless otherwise stated, the term “alkyl” refers to an unbranched or branched, saturated or unsaturated (so forming, for example, an alkenyl or alkynyl) hydrocarbyl radical, which may be substituted or unsubstituted (with, for example, one or more halo atoms).

[0050] In some embodiments that may be mentioned herein, each R1may independently represent CN, methyl, OMe or N(Me)2.

[0051] In some embodiments that may be mentioned herein, R2may represent CH2Ph.

[0052] In some embodiments that may be mentioned herein, R3may represent p-fBuPh.

[0053] In some embodiments that may be mentioned herein, a may be 1 , b and c may be 0, b may be 1 , a and c may be 0, c may be 1 , a and b may be 0, a and b may be 1 and c may be 0, a and c may be 1 and b may be 0, b and c may be 1 and a may be 0, and a, b and c may be 1.

[0054] In particular embodiments that may be mentioned herein, a may be 1 , b and c may be 0.

[0055] In some embodiments that may be mentioned herein, the radical of formula la may be selected from the list:

[0056] and the dimer of formula lb that corresponds to the above compounds of formula la.

[0057] In some embodiments that may be mentioned herein, the dimer of formula lb may dissociate to provide two radicals of formula la upon application of a stimulus and the radical of formula la may undergo dimerization to provide a dimer according to a compound of formula lb upon removal of the stimulus.

[0058] In some embodiments that may be mentioned herein, the radical of formula la may undergo dimerization to provide a dimer according to a compound of formula lb upon application of a stimulus and the dimer of formula lb may dissociate to provide two radicals of formula la upon removal of the stimulus. Without wishing to be bound by theory, the dissociation of the dimer of formula lb and dimerization of the radical of formula la are based on reversible association / dissociation reaction of monoradicals. This is a simple process which advantageously provide chromic properties.

[0059] In some embodiments that may be mentioned herein, the stimulus may be selected from one or more of the group consisting of a change in temperature and a mechanical force.

[0060] For example, the dissociation of the dimer of formula lb may be favoured at higher temperatures while the dimerization of the radical of formula la may be favoured at lower temperatures (see Examples 5 and 6 below). Thus, a change in temperature of the radical and the dimer of the first aspect of the present invention may be accompanied by a notable color change, showcasing the radical’s and the dimer’s thermochromic properties.

[0061] For example, the mechanical force may be grinding. Example 6 provides more details on the mechanochromic properties of the radical and the dimer of the first aspect of the present invention.

[0062] In a second aspect of the present invention, there is provided a compound according to formula II: where R1to R3and a to c are as defined hereinbefore.

[0063] In some embodiments that may be mentioned herein, the compound of formula II may be selected from the list:

[0064] Compounds of formula la, lb and II may contain double bonds and may thus exist as E (entgegeri) and Z (zusammeri) geometric isomers about each individual double bond. All such isomers and mixtures thereof are included within the scope of the invention.

[0065] Compounds of formula la, lb and II may contain one or more asymmetric carbon atoms and may therefore exhibit optical and / or diastereoisomerism. Diastereoisomers may be separated using conventional techniques, e.g. chromatography or fractional crystallisation. The various stereoisomers may be isolated by separation of a racemic or other mixture of the compounds using conventional, e.g. fractional crystallisation or HPLC, techniques. Alternatively, the desired optical isomers may be made by reaction of the appropriate optically active starting materials under conditions which will not cause racemisation or epimerisation (i.e. a ‘chiral pool’ method), by reaction of the appropriate starting material with a ‘chiral auxiliary’ which can subsequently be removed at a suitable stage, by derivatisation (i.e. a resolution, including a dynamic resolution), for example with a homochiral acid followed by separation of the diastereomeric derivatives by conventional means such as chromatography, or by reaction with an appropriate chiral reagent or chiral catalyst all under conditions known to the skilled person. All stereoisomers and mixtures thereof are included within the scope of the invention.

[0066] Without wishing to be bound by theory, it is believed that different R1group substituents may result in different bond dissociation energies. Therefore, it is believed that the disclosed compounds can be modified to cater for different application requirements through the use of different R1groups. In a third aspect of the present invention, there is provided a film comprising: a radical of formula la and / or a dimer of formula lb as described in the first aspect of the present invention; and a polymeric material, wherein the radical of formula la and / or the dimer of formula lb are distributed throughout the film.

[0067] In a fourth aspect of the present invention, there is provided an intermediate film comprising: a compound according to formula II as described in the second aspect of the present invention; and a polymeric material, wherein the compound of formula II is distributed throughout the film.

[0068] Films according to the current invention may be formed by creating a mixed solution of the compounds of formula la, lb and II with any suitable polymeric material. The choice of polymeric material is primarily aimed at obtaining a desired viscosity, and any polymeric material capable of forming a good film can be utilized in a film according to the current invention. An example of a polymeric material that may be used herein is polymethyl methacrylate (PMMA). Subsequently, the mixture can be employed to fabricate the film using conventional techniques in the art, such as spin-coating and the like.

[0069] As will be appreciated, the film may be made using isolated radicals of formula la, isolated dimers of formula lb or isolated intermediate compounds of formula II. Of course, a mixture of the radicals of formula la and dimers of formula Ila may be used. If the intermediate compounds of formula II are used, the film may be subjected to a suitable activation step before use (e g. through oxidation or other suitable activation step).

[0070] The total weight percent of the compounds of formula la, lb and II relative to the polymeric material in the film may be any suitable amount. For example, the total amount of the compounds of formula la, lb and II may be from 5 wt% to 75 wt% relative to the weight of the polymeric material. The concentration of the compounds of formula la, lb and II can be adjusted to accommodate different scenarios. Films with higher concentrations of the compounds of formula la, lb and II may exhibit more pronounced responses when subjected to external stimuli, which may be desirable under certain use cases, while lower concentrations may be desirable for other use cases. As will be appreciated, the chromic materials disclosed herein may serve as chromophores in windows including but not limited to smart windows. The materials disclosed herein may be directly applied as additives in the window glass molding process, or it may be made into films for window glasses. Further, when the ambient temperature increases, the glass window containing the materials disclosed herein may turn colored to cut the heat load in buildings, thus preventing elevation of temperature in the buildings, reducing air-conditioning cost, and provides energy-saving. When the ambient temperature decreases, the glass window may get back its transparency.

[0071] Further aspects and embodiments of the invention will now be discussed by reference to the following non-limiting examples.

[0072] Examples

[0073] Materials

[0074] All reagents were purchased from commercial sources without further purification. Anhydrous tetrahydrofuran (THF) was distilled from sodium benzophenone immediately prior to use. Benzyl-6-(4-(ferf-butyl)phenyl)indoline-2,3-dione (5) was prepared according to the literature (J. Wang et al., Angew. Chem. Int. Ed. 2017, 56, 14154-14158).

[0075] 1H and13C NMR spectra were recorded using 500 MHz Bruker spectrometer in solution of CDC (7.26 ppm for1H NMR, 77.16 ppm for13C NMR) and CD2CI2(5.32 ppm for1H NMR) with tetramethylsilane (TMS) as the internal standard. The chemical shift was recorded in ppm and the following abbreviations were used to explain the multiplicities: s = singlet, d = doublet, t = triplet, m = multiplet, br = broad peak.

[0076] High resolution atmospheric pressure chemical ionization (APCI) mass spectroscopy High resolution APCI mass spectra were recorded on a MicrOTOF-Q II instrument.

[0077] High resolution electrospray ionization (ESI) mass spectroscopy

[0078] High resolution ESI mass spectrum was recorded on an Agilent 6546 Q-TOF LCMS instrument.

[0079] Ultraviolet-visible-near infrared (UV-vis-NIR) spectroscopy

[0080] UV-vis-NIR absorption spectra were recorded on a Shimadzu UV-3600 spectrometer. X-ray crystallographic analysis

[0081] The single crystal was measured at low temperature (T = 100 K) or room temperature ((T = 293 K) on a four circles goniometer Kappa geometry Bruker AXS D8 Venture equipped with a Photon 100 CMOS active pixel sensor detector using a Copper monochromatized (A = 1.54178 A) X-ray radiation.

[0082] Electron spin resonance (ESR) spectroscopy

[0083] Continuous wave X-band ESR spectra were obtained with a JEOL (FA200) spectrometer.

[0084] Example 1. Preparation

[0085] Under N2 atmosphere, a solution of 4-iodobenzenitrile (585 mg, 2.6 mmol) dissolved in anhydrous THF (15 mL) was cooled at -20 °C and then / PrMgCI • LiCI (1.3 M in THF, 2.8 mmol, 2.2 mL) was added dropwise. After stirring for 1 h, the mixture was added to the solution of 1- benzyl-6-(4-(fert-butyl)phenyl)indoline-2, 3-dione (5) (750 mg, 2.1 mmol) in THF at -20 °C. The mixture was warmed up to room temperature slowly then stirred overnight. The reaction was quenched by H2O at 0 °C, extracted with DCM for three times. The organic layer was collected and dried over anhydrous sodium sulfate. After removing the solvent by rotary evaporation, the product was purified by column chromatography (silica gel, ethyl acetate / hexane = 2:7) and alcohol intermediate 1-OH was obtained as a light-yellow solid in 41 % yield (410 mg).

[0086] 1H-NMR (500 MHz, CDCI3, <5 ppm): 7.65 (m, 2H) 7.55 (dt, 2H, J = 8.75, 1.56Hz), 7.46-7.44 (dt, 2H, J = 8.1 , 2.15 Hz), 7.40-7.38 (m, 2H), 7.36-7.29 (m, 5H), 7.27 (m, 2H), 7.04 (s, 1 H), 5.07 (d, 1H, J= 15.6 Hz), 4.89 (d, 1 H, J= 15.6 Hz), 3.30 (br, 1H), 1.35 (s, 9H).13C-NMR (125 MHz, CDC , 6 ppm): 176.81 , 151.26, 145.32, 143.93, 143.18, 137.40, 135.13, 132.52, 131.29, 129.08, 128.11 , 127.38, 126.83, 126.35, 125.93, 125.15, 122.80, 118.53, 112.25, 118.53, 112.25, 108.82, 77.69, 44.30, 34.64, 31.33. HRMS analysis (ESI) calcd for C32H29N2O2 (M+H)+: m / z: 473.2224, found: 473.2222 (error: 0.3 ppm).

[0087] Example 2. General procedure (GP) for the preparation of alcohol intermediates Under argon atmosphere, a solution of aryl bromide (2.27 mmol) in dry THF (50 mL) was cooled to -78 °C and then n-BuLi (2 M in n-hexane, 1.25 mL, 2.50 mmol) was added slowly. The solution was stirred at -78 °C for 1 h and then 1-benzyl-6-(4-(ferf-butyl)phenyl)indoline- 2, 3-dione (5) (923 mg, 2.5 mmol) was added. The mixture was slowly warmed to room temperature and stirred overnight. The reaction was quenched by NH4CI aqueous solution and extracted by DCM for three times. The combined organic solution was washed with sodium chloride aqueous solution and dried over anhydrous sodium sulfate. After removal of the solvent, the residue was purified by flash column chromatography, and the alcohol intermediate was obtained.

[0088] 2-OH

[0089] This compound was prepared as described in the GP for alcohol intermediates, with 4- bromotoluene (388 mg, 2.27 mmol), purification of the crude product by column chromatography (silica gel, ethyl acetate / hexane = 2:7) gave the 2-OH as a white solid with 48% yield (503 mg).

[0090] 1H-NMR (500 MHz, CDCI3, <5 ppm): 7.46-7.45 (m, 2H), 7.42-7.40 (m, 2H), 7.38 (m, 1 H), 7.36- 7.34 (m, 6H), 7.32-7.29 (m, 1 H), 7.28-7.26 (br, 1 H), 7.20 (d, 2H, J = 7.7 Hz), 6.99 (d, 1 H, J = 1.2 Hz), 5.12 (d, 1 H, J = 15.7 Hz), 4.90 (d, 1 H, J = 15.7 Hz), 2.37 (s, 3H), 1.37 (s, 9H).13C- NMR (125 MHz, CDCI3, 6 ppm): 177.94, 150.91, 143.20, 143.12, 138.24, 137.80, 137.26, 135.50, 130.29, 129.43, 128.94, 127.82, 127.32, 126.83, 125.82, 125.33, 125.12, 122.43, 108.49, 77.82, 44.10, 34.59, 31.34, 21.17. HRMS analysis (ESI) calcd for C32H32NO2 (M+H)+: m / z: 462.2428, found: 462.2421 (error: 1.5 ppm).

[0091] 3-OH

[0092] This compound was prepared as described in the GP for alcohol intermediates, with 4- bromoanisole (425 mg, 2.27 mmol), purification of the crude product by column chromatography (silica gel, ethyl acetate / hexane = 1:3) gave the 3-OH as a white solid with 58% yield (542 mg).

[0093] 1H-NMR (500 MHz, CDCI3, 6 ppm):7.45-7.43 (m, 2H), 7.41-7.38 (m, 4H), 7.36-7.33 (m, 5H), 7.29- 7.25 (m, 2H), 6.97 (s, 1 H,), 6.90-6.88 (m, 2H), 5.08 (d, 1 H, J = 15.7 Hz), 4.87 (d, 1 H, J = 15.7 Hz), 3.80 (s, 3H), 1.35 (s, 9H).13C-NMR (125 MHz, CDCI3, 5 ppm): 177.94, 150.91 , 143.20, 143.12, 138.24, 137.80, 137.26, 135.50, 130.29, 129.43, 128.94, 127.82, 126.82, 125.82, 125.33, 125.12, 122.43, 108.49, 77.82, 44.10, 34.59, 31.34. HRMS analysis (ESI) calcd for C32H3iNNaC>3 (M+Na)+: m / z: 500.2196, found: 500.2189 (error: 1.4 ppm).

[0094] 4-OH

[0095] This compound was prepared as described in the GP for alcohol intermediates, with 4-bromo- N,N-dimethylaniline (452 mg, 2.27 mmol), purification of the crude product by column chromatography (silica gel, ethyl acetate / hexane = 1:3) gave the 4-OH as a white solid with 42% yield (468 mg).

[0096] 1H-NMR (500 MHz, CDCk, <5 ppm): 7.45-7.43 (m, 2H), 7.40 (t, 2H, J = 2.1 Hz), 7.38-7.37 (m, 2H), 7.36 (t, 1 H, J = 2.6 Hz), 7.34-7.31 (m, 4H), 7.29-7.25 (m, 2H), 6.94 (s, 1H), 6.74 (d, 2H, J = 8.35 Hz), 5.08 (d, 1 H, J = 15.8 Hz), 4.86 (d, 1 H, J = 15.7 Hz), 3.20 (br, 1 H), 2.95 (s, 6H), 1.35 (s, 9H).13C-NMR (125 MHz, CDCh, 6 ppm): 178.10, 150.81 , 143.15, 142.87, 137.92, 135.62, 130.33, 128.90, 127.72, 127.28, 126.82, 126.59, 125.80, 125.13, 122.24, 112.59, 108.39, 77.59, 44.01 , 40.57, 34.59, 31 .35. HRMS analysis (ESI) calcd for C33H35N2O2 (M+H)+: m / z: 491.2693, found: 491.2691 (error: 0.4 ppm).

[0097] Example 3. General procedure (GP) for the preparation of mono-hydro precursors

[0098] The alcohol intermediate (0.65 mmol) was dissolved in acetic acid (5 mL) and the solution was degassed by bubbling through N2 for 10 minutes. Then, 0.5 mL of concentrated hydrochloric solution saturated by Sn&2 was added and the mixture was stirred for 3 hours at 100 °C. After cooling to room temperature, the mixture was washed with water and extracted with DCM. The organic layer was washed with NaHCOs aqueous solution and extracted with DCM for three times. The combined organic solution was dried over anhydrous sodium sulfate. After removal of the solvent, the residue was purified by column chromatography to give monohydro precursors as white solid.

[0099] 1-H

[0100] This compound was prepared as described in the GP for mono-hydro precursors, with 1-OH (307 mg, 0.65 mmol). The crude product was purified by column chromatography (silica gel, ethyl acetate / hexane = 1 :3) to afford 1-H as a white solid (125 mg, 42%).

[0101] 1H-NMR (500 MHz, CDCI3, <5 ppm): 7.68 (t, 1 H, J= 1.9 Hz), 7.66 (t, 1 H, J = 1.8 Hz), 7.47- 7.45 (m, 2H), 7.43-7.40 (m, 4H), 7.34-7.32 (m, 4H), 7.30-7.18 (m, 2H), 7.19 (dd, 1 H, J = 7.8, 1.1 Hz), 7.02 (d, 1 H, J = 1.5 Hz), 5.02 (d, 1 H, J= 15.6 Hz), 4.94 (d, 1 H, J = 15.7 Hz), 4.80 (s, 1H), 1.36 (s, 9H).13C-NMR (125 MHz, CDCI3, 5 ppm): 175.02, 150.97, 144.16, 142.45, 142.03, 137.73, 135.58, 132.76, 129.37, 128.96, 127.90, 127.40, 126.80, 125.96, 125.89, 125.31 , 121.98, 118.65, 111.70, 108.30, 51.72, 44.21 , 34.61 , 31.34. HRMS analysis (APCI) calcd for C32H29N2O (M+H)+: m / z: 457.2274, found: 457.2290 (error: -3.5 ppm).

[0102] 2-H

[0103] This compound was prepared as described in the GP for mono-hydro precursors, with 2-OH (300 mg, 0.65 mmol). The crude product was purified by column chromatography (silica gel, DCM / hexane = 3:1) to afford 2-H as a white solid (232 mg, 80%).

[0104] 1H-NMR (500 MHz, CDCI3, <5 ppm):7.46-7.41 (m, 4H), 7.36-7.31 (m, 4H), 7.29-7.26 (m, 1H), 7.24-7.21 (m, 2H), 7.19-7.15 (m, 4H), 6.98 (d, 1 H, J = 1.5 Hz), 5.04 (d, 1 H, J = 15.7 Hz), 4.94 (d, 1 H, J = 15.6 Hz), 4.71 (s, 1 H), 2.35 (s, 3H), 1.36 (s, 9H).13C-NMR (125 MHz, CDCI3, 6 ppm): 176.91 , 151.02, 144.50, 142.04, 138.46, 137.74, 136.34, 134.12, 130.08, 129.22, 128.72, 128.24, 128.05, 127.77, 127.18, 126.17, 125.60, 122.03, 108.27, 51.98, 44.39, 34.95, 31.73, 21.53. HRMS analysis (APCI) calcd for C32H32NO (M+H)+: m / z: 446.2478, found: 446.2489 (error: -2.4 ppm).

[0105] 3-H

[0106] This compound was prepared as described in the GP for mono-hydro precursors, with 3-OH (310 mg, 0.65 mmol). The crude product was purified by column chromatography (silica gel, DCM / hexane = 3:1) to afford 3-H as a white solid (275 mg, 92%).

[0107] 1H-NMR (500 MHz, CDCh, 5 ppm): 7.46-7.41 (m, 4H), 7.36-7.31 (m, 4H), 7.28-7.26 (m, 1H), 7.24-7.22 (m, 1 H), 7.21-7.17 (m, 3H), 6.98 (s, 1H), 6.90 (m, 2H), 5.03 (d, 1 H, J = 15.6 Hz), 4.94 (d, 1 H, J = 15.7 Hz), 4.69 (s, 1 H), 3.81 (s, 3H), 1.36 (s, 9H).13C-NMR (125 MHz, CDCh, 6 ppm): 176.79, 159.27, 150.78, 144.21 , 141.81, 138.19, 136.08, 129.65, 128.98, 128.90, 128.02, 127.80, 127.52, 126.92, 125.92, 125.36, 121.78, 114.58, 108.03, 55.46, 51.28, 44.13, 34.70, 31.48. HRMS analysis (APCI) calcd for C32H32NO2 (M+H)+: m / z: 462.2428, found: 446.2442 (error: -3.2 ppm).

[0108] 4-H

[0109] This compound was prepared as described in the GP for mono-hydro precursors, with 4-OH (319 mg, 0.65 mmol). The crude product was purified by column chromatography (silica gel, ethyl acetate / hexane = 1 :3) to afford 4-H as a white solid (204 mg, 66%).

[0110] 1H-NMR (500 MHz, CDCh, <5 ppm): 7.45-7.40 (m, 4H), 7.36-7.30 (m, 4H), 7.28-7.24 (m, 1H), 7.22 (d, 2H, J = 1.1 Hz) 7.13 (d, 2H, J = 8.35 Hz), 6.96 (s, 1H), 6.74 (br, 2H), 5.03 (d, 1 H, J = 15.7 Hz), 4.93 (d, 1 H, J = 15.7 Hz), 4.65 (s, 1 H), 2.94 (s, 6H), 1.35 (s, 9H).13C-NMR (125 MHz, CDCh, <5 ppm): 177.06, 150.56, 144.10, 141.43, 138.17, 136.08, 129.09, 128.82, 128.36, 127.61, 127.40, 126.79, 125.77, 125.24, 121.56, 113.10, 107.79, 51.13, 43.96, 40.66, 34.57, 31.36. HRMS analysis (APCI) calcd for C33H35N2O (M+H)+: m / z: 475.2744, found: 475.2748 (error: -0.9 ppm).

[0111] Example 4. General procedure (GP) for preparation of dimers

[0112] In a two-neck flask, mono-hydro precursor (0.2 mmol) was dissolved in dichloromethane (15 mL) under N2 atmosphere, 35 mg of 2,3-dichloro-5,6-dicyano-1 ,4-benzoquinone (DDQ) (0.15 mmol) was added to the solution and then the mixture was stirred at room temperature. Progress of the reaction was monitored by thin layer chromatography (TLC) until completion. Two drops of triethylamine were added to quench the reaction and the organic layer was washed with water, extracted with DCM for three times. The combined organic solution was dried over anhydrous sodium sulfate and the solvent was removed by rotary evaporation. The crude product was purified by column chromatography.

[0113] This compound was prepared as described in the GP for dimers, with 1-H (91 mg, 0.2 mmol), purification of the crude product by column chromatography (silica gel, ethyl acetate / hexane = 1 :3) gave the I2 as a white solid with 75% yield (68 mg).

[0114] 1H-NMR (500 MHz, CD2CI2, 223 K, 6 ppm): 7.66 (d, 2H, J = 8.25 Hz), 7.57 (d, 2H, J = 8.1 Hz), 7.52 (d, 1 H, J = 8.8 Hz), 7.47-7.41 (m, 6H), 7.38-7.32 (m, 6H), 7.28-7.18 (m, 5H), 6.97 (t, 1 H, J = 7.4 Hz), 6.88-6.82 (m, 5H), 6.63-6.59 (m, 3H), 6.08 (d, 1H, J = 7.95 Hz), 5.09 (d, 1 H, J = 16.35 Hz), 5.06 (d, 1 H, J = 15.85 Hz), 5.00 (d, 1 H, J = 16.35 Hz), 4.94 (d, 1 H, J = 15.8 Hz), 1.26 (d, 18H, J = 2.2 Hz). HRMS analysis (APCI) calcd for C64H55N4O2 (M+H)+: m / z: 911.4320, found: 911.4303 (error: 1.8 ppm).

[0115] 2g

[0116] This compound was prepared as described in the GP for dimers, with 2-H (90 mg, 0.2 mmol), purification of the crude product by column chromatography (silica gel, DCM / hexane = 3:1) gave the 22as a white solid with 82% yield (73 mg).

[0117] 1H-NMR (500 MHz, CD2CI2, 213 K, 5 ppm): 7.45- 7.33 (m, 11 H), 7.26-7.18 (m, 5H), 7.12-7.10 (m, 4H), 7.02 (d, 2H, J = 8.4 Hz), 6.97 (t, 1H, J = 7.4 Hz), 6.86 (t, 2H, J = 7.6 Hz), 6.81-6.77 (m, 3H), 6.71 (d, 1H, J = 7.95 Hz), 6.57 (d, 2H, J = 7.5 Hz), 6.13 (d, 1 H, J = 7.85 Hz), 5.11 (d, 1H, J = 16.4 Hz), 5.03 (d, 1 H, J = 15.95 Hz), 4.85 (d, 1 H, J = 16.4 Hz), 4.29 (d, 1H, J = 15.9 Hz), 2.32 (d, 6H, J = 13.6 Hz), 1.26 (s, 18H). HRMS analysis (APCI) calcd for C64H6IN2O2(M+H)+: m / z: 889.4728, found: 889.4718 (error: 1.1 ppm).

[0118] This compound was prepared as described in the GP for dimers, with 3-H (92 mg, 0.2 mmol), purification of the crude product by column chromatography (silica gel, DCM / hexane = 4:1) gave the 32as a white solid with 50% yield (46 mg).

[0119] 1H-NMR (500 MHz, CD2CI2, 213 K, 6 ppm): 7.45 (m, 2H), 7.42-7.33 (m, 9H), 7.25-7.13 (m, 7H), 6.97 (t, 1 H, J = 7.5 Hz), 6.87-6.78 (m, 7H), 6.75- 6.71 (m, 3H), 6.56 (d, 2H, J = 7.6 Hz), 6.10 (d, 1 H, J = 7.9 Hz), 5.10 (d, 1H, J = 16.35 Hz), 5.04 (d, 1 H, J = 16 Hz), 4.88 (d, 1 H, J = 16.45 Hz), 4.30 (d, 1 H, J = 15.8 Hz), 3.78 (d, 6H, J = 16.8 Hz), 1.26 (s, 18H). HRMS analysis (APCI) calcd for C64HBIN2O4(M+H)+: m / z: 921.4646, found: 926.4619 (error: 0.7 ppm).

[0120] This compound was prepared as described in the GP for dimers, with 4-H (95 mg, 0.2 mmol), purification of the crude product by column chromatography (silica gel, ethyl acetate / hexane

[0121] = 1 :3) gave the 42 as a white solid with 72% yield (68 mg).

[0122] 1H-NMR (500 MHz, CD2CI2, 223 K, 6 ppm): 7.45-7.34 (m, 12H), 7.26-7.17 (m, 5H), 7.07 (d, 1H, J = 8.25 Hz), 6.96 (t, 1 H, J = 7.45 Hz), 6.86 (t, 2H, J = 7.55 Hz), 6.80-6.72 (m, 4H), 6.63- 6.53 (m, 6H), 6.10 (d, 1 H, J = 7.8 Hz), 5.10 (d, 1 H, J = 16.35 Hz), 5.02 (d, 1 H, J = 15.85 Hz), 4.86 (d, 1 H, J = 16.35 Hz), 4.28 (d, 1 H, J = 15.9 Hz), 2.92 (d, 12H, J = 15.35 Hz), 1.27 (s, 18H). HRMS analysis (APCI) calcd for CeaHeyN^ (M+H)+: m / z: 947.5259, found: 947.5267 (error: -0.9 ppm).

[0123] Example 5. Synthesis and characterization of oxindolyl-based radicals (OxRs)

[0124] Taking advantage of the dynamic covalent nature, herein, we focused on preparing new radical species as promising candidates for constructing stimuli-responsive chromic materials. BF-based radical derivatives have been widely used to design radical based polymer materials with encouraging thermochromic, photochromic and mechanochromic properties. However, the lactone structure in BF may pose additional difficulty in further modifications and functionalizations of the structure due to its intrinsic reactivity in basic conditions. Bearing these in mind, we proposed replacing the lactone moiety with lactam subunit to synthesize oxindolyl-based radical (OxR), upon which high stability coupled with superior tolerance towards basic conditions, are guaranteed (J. Wang et al., Angew. Chem. Int. Ed. 2017, 56, 14154-14158). Since OxR is an analogue of BF radical and highly stable towards oxygen, it could ensure the stability of the carbon-centered radical. The advent of OxR has opened a new door to the discovery of new sensitive probes in preparing materials with reversible chromic properties.

[0125] As depicted in Fig. 2, the synthesis of the OxRs are quite straightforward, which started with 1-benzyl-6-(4-(tert-butyl)phenyl)indoline-2, 3-dione (5) prepared following a reported procedure (J. Wang et al., Angew. Chem. Int. Ed. 2017, 56, 14154-14158). The regioselective nucleophilic addition of different substituents to the carbonyl group at the 3-position in 5 gave the corresponding intermediate alcohols in 41%, 48%, 50% and 42% yields. Due to the potential reactivity of the cyano group with n-BuLi, 4-cyanomagnesium iodide was used for preparation of 1-OH, while the other three intermediate alcohols were obtained using the corresponding organolithium reagents. These intermediate alcohols were easily reduced into mono-hydro precursors by saturated SnCh in concentrated HCI, contributing to a 42% yield for 1-H, 80% for 2-H, 92% for 3-H and 66% for 4-H. Finally, the mono-hydro precursors were oxidized with 2,3-dichloro-5,6-dicyano-1 ,4-benzoquinone (DDQ) to produce the corresponding monoradicals, which would dimerize spontaneously to get the target dimers I2, 22, 32, and 42 in 75%, 82%, 50% and 72% yields, respectively. These dimer compounds were obtained as white powders and can transform into the corresponding monomeric radicals reversibly under specific conditions, as will be elucidated below.

[0126] The structures of dimers I2 to 42 were confirmed by low-temperature1H-NMR spectrum and high-resolution mass spectra. In addition, single crystals of 12and 22suitable for X-ray crystallographic analysis were obtained from slow solvent evaporation of their DCM solutions under ambient environment (Deposition numbers 2087052 (for I2) and 2087051 (for 22) contain the supplementary crystallographic data for the present disclosure. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.). However, the single crystal of 32and 42were not available due to the weaker bond dissociation energy (BDE) of the dynamic C-C bonds in 32and 42. The molecular structures and crystallography data of 12and 22 are illustrated in Figs. 3a-b and Tables 1 and 2, which revealed that the C(sp3)-C(sp3) bonds formed by radical association both adopt a gauche conformation, with the para-substituted phenyl-C(sp3)-C(sp3)-para-substituted phenyl dihedral angles being 35° for I2 and 44° for 22, respectively. The bond analysis demonstrated that the C-C bond between monomers were significantly longer (1.59 A for I2 and 1.60 A for 22, respectively) than normal C(sp3)-C(sp3) bond (1.54 A), suggesting that the bond energy of these C-C bonds was weaker than normal C(sp3)-C(sp3) bond, further confirming the dynamic nature of these bonds.

[0127] Table 1. X-ray crystallographic data for 12.

[0128] Identification code 12

[0129] Empirical formula C64H54N4O2

[0130] Formula weight 911.11

[0131] Temperature / K 99.98

[0132] Crystal system orthorhombic

[0133] Space group Pbca

[0134]

[0135] Table 2. X-ray crystallographic data for 22.

[0136] Temperature-dependent1H-NMR spectral features of dimers 12to 42can be observed, which was a common phenomenon for a diverse class of dimer-monomer equilibriums (Figs. 3c and

[0137] 4-6) (B. Adinarayana et al., Angew. Chem. Int. Ed. 2020, 59, 4320-4323; and B. Adinarayana et al., Chem. Sci. 2019, 10, 6007-6012). Taking dimer 12as a typical example, the variabletemperature1H-NMR spectrum showed broad signals at room temperature due to the existence of radical species. With decreasing temperature, the signals become well-defined and increasingly sharper, which can be ascribed to the equilibrium between the dimer 12and its corresponding monomeric radical 1*. That is, the radical species were prone to dimerize at decreased temperature. The combined findings suggested that, at room temperature, the C- C bond between monomers were partially broken and a substantial amount of radical 1* existed, while the equilibrium strongly favors the formation of dimer 12at lower temperature.

[0138] Example 6. Thermochromic and mechanochromic properties

[0139] Theoretical calculations

[0140] Density functional theory (DFT) calculations were employed with Gaussian 09 package (Gaussian 09, Revision D.01 , M. J. Frisch etal., Gaussian, Inc., Wallingford CT, 2009) utilizing the UB3LYP level of theory for monomers (1*, 2’, 3*, 4’) and B3LYP level of theory for dimers (12, 22, 32, 42) under Pople basis set 6-31 G(d,p) (A. D. Becke, J. Chem. Phys. 1993, 98, 5648; C. Lee, W. Yang & R. G. Parr, Phys. Rev. B: Condens. Matter 1988, 37, 785; T. Yanai, D. Tew & N.A. Handy, Chem. Phys. Lett. 2004, 393, 51-57; R. Ditchfield, W. J. Hehre & J. A. Pople, J. Chem. Phys. 1971 , 54, 724; W. J. Hehre, R. Ditchfield & J. A. Pople, J. Chem. Phys. 1972, 56, 2257; and P. C. Hariharan & J. A. Pople, Theoretica chimica acta. 1973, 28, 213-222). TD- DFT calculations of monomers and dimers were conducted under same level of theory and basis set with PCM model as solvation of DMF.

[0141] Results and discussion

[0142] The solvent effect on the stability and delocalization of aryl dicyanomethyl radicals has been well documented, specifically, captodative radicals are indeed stabilized by polar solvents (J. P. Peterson et al., J. Org. Chem. 2017, 82, 6497-6501). Before investigated the thermochromic behavior of the OxRs in solution, we focused on identifying a better solvent for the experiments. To leverage on solvent effect, the UV-vis-NIR absorption spectra of dimer 42 in different solvents were recorded (Fig. 7). It is evident that the equilibrium of the 42-DMF solution shifts more towards dissociation compared to that of 42-toluene. The intensity of the characteristic absorption peaks increased more significantly in DMF, which further demonstrated that the OxR was more stable in polar solvent. At room temperature, the UV- vis-NIR absorption spectra of dimers 12-42in DMF solution only showed strong absorption between 250-400 nm which correspond to the typical absorption of dimers. Upon increasing the temperature, some new absorption peaks appeared at long-wavelength region (Figs. 8a and 9). Time-dependent (TD) DFT (E. K. U. Gross & W. Kohn, Adv. Quant. Chem. 1990, 21, 255-291 ; and E. Runge & E. K. U. Gross, Phys. Rev. Lett. 1984, 52, 997-1000) calculation results (Tables 3-10) indicated that these new peaks were ascribed to the formation of radicals. The increased intensity of the absorption peaks suggested that the amount of dissociated radical increased with increased temperature. Notably, the color of 42 in DMF changed from colorless to dark green with elevated temperature (inset of Fig. 8a), demonstrating the thermochromic behavior of the synthesized compounds. Upon cooling to 298K, the solution color returned to its original state (inset of Fig. 8a) without any notable change in the UV-vis- NIR absorption spectra (Fig. 10) compared to the initial measurement. Therefore, at a lower temperature, the equilibrium favors the formation of dimers that is colorless in solution. After exposure to high temperature, the equilibrium favors the dissociation back to monomers with a dark green color. This indicates the reversibility of the equilibrium between the dissociation and association reactions. Because of such color change phenomenon, oxindolyl-based radical derivatives may be applied as thermochromic materials.

[0143] Table 3. Selected TD-DFT (RB3LYP / 6-31G(d,p)) wavelength, oscillator strength and contributions of major electronic transitions of 12. Wavelength Osc. Symmetry Major contributions

[0144] (nm) Strength

[0145] 348 0.0177 Singlet-A HOMO->LUMO (95%)

[0146] 340 0.0426 Singlet-A H-1->LUMO (94%)

[0147] 314 0.0058 Singlet-A HOMO->L+1 (96%)

[0148] 309 0.0024 Singlet-A H-1->L+1 (95%)

[0149] 301 0.0574 Singlet-A H-2->LUMO (92%)

[0150] 299 0.0786 Singlet-A H-3->LUMO (89%)

[0151] 292 0.3722 Singlet-A HOMO->L+2 (82%)

[0152] 284 0.2762 Singlet-A H-1->L+3 (29%), HOMO->L+3 (55%)

[0153] 282 0.0272 Singlet-A H-1->L+2 (94%)

[0154] 280 0.0961 Singlet-A H-1->L+3 (53%), HOMO->L+3 (37%)

[0155] 278 0.1 Singlet-A H-10->LUMO (15%), H-4->LUMO (64%)

[0156] 275 0.0011 Singlet-A H-2->L+1 (95%)

[0157] 273 0.0122 Singlet-A H-3->L+1 (94%)

[0158] 271 0.0366 Singlet-A H-5->LUMO (72%)

[0159] 269 0.0476 Singlet-A HOMO->L+4 (83%)

[0160] 267 0.0402 Singlet-A H-10->LUMO (23%), H-8->LUMO (22%),

[0161] H-4->LUMO (19%)

[0162] 266 0.3233 Singlet-A H-2->L+2 (43%), H-1->L+4 (12%)

[0163] 265 0.0681 Singlet-A H-1->L+4 (45%)

[0164] 264 0.0044 Singlet-A H-7->LUMO (78%)

[0165] 263 0.0079 Singlet-A H-6->LUMO (79%)

[0166] Table 4. Selected TD-DFT (UB3LYP / 6-31G(d,p)) wavelength, oscillator strength and contributions of major electronic transitions of T.

[0167] Wavelength Osc. Symmetry Major contributions

[0168] (nm) Strength

[0169] 811 0.0504 2.027-A HOMO(B)->LUMO(B) (95%)

[0170] 582 0.2575 2.071-A H-1(B)->LUMO(B) (88%)

[0171] 517 0.0039 2.057-A H-7(B)->LUMO(B) (31 %), H-5(B)->LUMO(B) (21%), H-

[0172] 2(B)->LUMO(B) (35%)

[0173] 455 0.0004 2.044-A H-7(B)->LUMO(B) (10%), H-5(B)->LUMO(B) (14%), H-

[0174] 2(B)->LUMO(B) (60%) 451 0.0385 2.153-A HOMO(A)->LUMO(A) (17%), H-6(B)->LUMO(B)

[0175] (18%), H-5(B)->LUMO(B) (12%), H-3(B)->LUMO(B) (32%)

[0176] 448 0.0343 2.116-A HOMO(A)->LUMO(A) (11%), H-5(B)->LUMO(B)

[0177] (12%), H-3(B)->LUMO(B) (59%)

[0178] 437 0.0001 2.034-A H-4(B)->LUMO(B) (88%)

[0179] 423 0.0066 2.096-A H-7(B)->LUMO(B) (42%), H-6(B)->LUMO(B) (46%)

[0180] 397 0.5628 2.207-A HOMO(A)->LUMO(A) (58%), H-5(B)->LUMO(B) (18%)

[0181] 373 0.0588 2.595-A HOMO(A)->L+1(A) (31%), H-8(B)->LUMO(B) (27%)

[0182] 341 0.0621 2.820-A H-8(B)->LUMO(B) (36%)

[0183] 335 0.0131 2.563-A HOMO(A)->L+1(A) (18%), H-9(B)->LUMO(B) (44%)

[0184] 331 0.008 3.402-A H-3(A)->L+4(A) (17%), H-4(B)->L+6(B) (10%), H-

[0185] 2(B)->L+4(B) (18%)

[0186] 329 0.0338 3.181-A H-1(A)->LUMO(A) (16%), H-9(B)->LUMO(B) (14%),

[0187] HOMO(B)->L+1 (B) (19%)

[0188] 319 0.0054 2.187-A HOMO(A)->L+2(A) (85%)

[0189] 314 0.0557 2.341-A H-1(A)->LUMO(A) (10%), HOMO(A)->L+1(A) (15%),

[0190] HOMO(A)->L+3(A) (31%), H-9(B)->LUMO(B) (12%)

[0191] 306 0.0201 3.120-A H-1(A)->L+3(A) (14%)

[0192] 301 0.0903 2.959-A HOMO(A)->L+3(A) (16%), H-9(B)->LUMO(B) (10%),

[0193] HOMO(B)->L+1 (B) (13%)

[0194] 297 0.216 2.533-A H-1(A)->LUMO(A) (48%), HOMO(B)->L+1 (B) (16%)

[0195] 291 0.0125 2.747-A HOMO(A)->L+3(A) (26%), HOMO(A)->L+5(A) (15%),

[0196] HOMO(B)->L+1 (B) (11 %)

[0197] Table 5. Selected TD-DFT (RB3LYP / 6-31G(d,p)) wavelength, oscillator strength and contributions of major electronic transitions of 22.

[0198] Wavelength Osc. Symmetry Major contributions

[0199] (nm) Strength

[0200] 306 0.1203 Singlet-A HOMO->LUMO (89%)

[0201] 300 0.1501 Singlet-A H-1->LUMO (87%)

[0202] 291 0.1762 Singlet-A HOMO->L+1 (87%)

[0203] 283 0.0849 Singlet-A H-1->L+1 (93%)

[0204] 276 0.2765 Singlet-A H-3->LUMO (10%), H-2->LUMO (66%) 274 0.1597 Singlet-A H-4->LUMO (10%), H-3->LUMO (30%), H-2->LUMO (19%)

[0205] 270 0.1278 Singlet-A H-4->LUMO (46%), HOMO->L+2 (15%)

[0206] 267 0.2595 Singlet-A H-3->LUMO (28%), HOMO->L+2 (58%)

[0207] 264 0.2753 Singlet-A H-4->LUMO (23%), H-1->L+2 (52%)

[0208] 261 0.1327 Singlet-A H-3->LUMO (11 %), H-2->L+1 (64%)

[0209] 258 0.1003 Singlet-A H-5->LUMO (71%)

[0210] 257 0.003 Singlet-A H-4->L+1 (15%), HOMO->L+4 (16%)

[0211] 255 0.0571 Singlet-A H-4->L+1 (30%), H-3->L+1 (14%)

[0212] 255 0.0712 Singlet-A H-4->L+1 (29%), H-3->L+1 (35%)

[0213] 254 0.0308 Singlet-A H-10->LUMO (24%), H-10->L+1 (10%), H-1->L+11 (10%)

[0214] 253 0.3149 Singlet-A H-3->L+1 (10%), HOMO->L+3 (18%), HOMO->L+4 (42%)

[0215] 251 0.0412 Singlet-A H-6->LUMO (16%), H-1->L+4 (18%), HOMO->L+3

[0216] (27%)

[0217] 250 0.0166 Singlet-A H-6->LUMO (37%)

[0218] 249 0.0583 Singlet-A H-1->L+4 (23%), HOMO->L+6 (13%), HOMO->L+7 (19%)

[0219] 248 0.019 Singlet-A H-1->L+4 (14%), HOMO->L+3 (13%), HOMO->L+5 (22%), HOMO->L+6 (27%)

[0220] Table 6. Selected TD-DFT (UB3LYP / 6-31G(d,p)) wavelength, oscillator strength and contributions of major electronic transitions of 2*.

[0221] Wavelength Osc. Symmetry Major contributions

[0222] (nm) Strength

[0223] 750 0.0629 2.027-A HOMO(B)->LUMO(B) (95%)

[0224] 546 0.2195 2.081-A H-1(B)->LUMO(B) (85%)

[0225] 494 0.0068 2.063-A H-7(B)->LUMO(B) (46%), H-4(B)->LUMO(B) (14%),

[0226] H-2(B)->LUMO(B) (31 %)

[0227] 449 0.0017 2.058-A H-6(B)->LUMO(B) (11%), H-5(B)->LUMO(B) (19%), H-

[0228] 4(B)->LUMO(B) (57%)

[0229] 431 0.014 2.143-A H-7(B)->LUMO(B) (12%), H-6(B)->LUMO(B) (61%)

[0230] 415 0.003 2.040-A H-7(B)->LUMO(B) (25%), H-2(B)->LUMO(B) (63%)

[0231] 409 0.0018 2.074-A H-3(B)->LUMO(B) (87%)

[0232] 397 0 2.032-A H-5(B)->LUMO(B) (75%) 379 0.561 2.407-A HOMO(A)->LUMO(A) (70%)

[0233] 350 0.0403 2.338-A HOMO(A)->L+1(A) (27%), H-8(B)->LUMO(B) (46%)

[0234] 338 0.1285 3.062-A H-2(A)->LUMO(A) (11%), H-1 (B)->L+1 (B) (11 %)

[0235] 330 0 3.474-A H-3(A)->L+2(A) (18%), H-5(B)->L+4(B) (13%), H- 2(B)->L+2(B) (20%)

[0236] 326 0.0447 2.677-A HOMO(A)->L+1 (A) (15%), H-9(B)->LUMO(B) (28%), H-8(B)->LUMO(B) (11 %)

[0237] 318 0.0357 2.505-A HOMO(A)->L+1 (A) (20%), HOMO(A)->L+2(A) (15%), HOMO(A)->L+6(A) (10%), H-9(B)->LUMO(B) (12%)

[0238] 312 0.002 2.258-A HOMO(A)->L+3(A) (64%)

[0239] 307 0.0266 3.005-A H-1(A)->L+1(A) (15%), H-8(B)->LUMO(B) (19%)

[0240] 301 0.0601 2.948-A H-1(A)->LUMO(A) (23%), HOMO(A)->L+1 (A) (10%), H-9(B)->LUMO(B) (26%), HOMO(B)->L+1(B) (14%)

[0241] 296 0.0267 2.563-A HOMO(A)->L+2(A) (10%), HOMO(A)->L+4(A) (38%), HOMO(A)->L+5(A) (22%)

[0242] 295 0.0439 2.918-A H-1 (A)-> LUM O(A) (14%), HOMO(A)->L+6(A) (20%)

[0243] 291 0.0049 2.385-A HOMO(A)->L+2(A) (55%)

[0244] Table 7 Selected TD-DFT (RB3LYP / 6-31G(d,p)) wavelength, oscillator strength and contributions of major electronic transitions of 32.

[0245] Wavelength Osc. Symmetry Major contributions (nm) Strength

[0246] 304 0.1399 Singlet-A HOMO->LUMO (84%)

[0247] 299 0.2138 Singlet-A H-1->LUMO (81%), HOMO->LUMO (10%)

[0248] 294 0.1305 Singlet-A H-2->LUMO (91%)

[0249] 290 0.159 Singlet-A HOMO->L+1 (86%)

[0250] 285 0.1067 Singlet-A H-1->L+1 (92%)

[0251] 282 0.0591 Singlet-A H-3->LUMO (50%), H-2->L+1 (36%)

[0252] 279 0.1307 Singlet-A H-3->LUMO (33%), H-2->L+1 (56%)

[0253] 273 0.1243 Singlet-A H-4->LUMO (25%), H-3->L+1 (15%), HOMO->L+2 (17%)

[0254] 269 0.0793 Singlet-A H-5->LUMO (15%), H-3->L+1 (16%), H-1->L+2 (30%)

[0255] 266 0.2465 Singlet-A H-4->LUMO (28%), HOMO->L+2 (58%)

[0256] 266 0.0084 Singlet-A H-4->LUMO (13%), H-3->L+1 (59%), HOMO->L+2 (12%)

[0257] 262 0.0205 Singlet-A H-2->L+2 (75%) 261 0.4365 Singlet-A H-5->LUMO (42%), H-1->L+2 (40%)

[0258] 257 0.0101 Singlet-A H-4->LUMO (12%), H-4->L+1 (27%), HOMO->L+3

[0259] (12%)

[0260] 255 0.0923 Singlet-A H-9->L+1 (13%), H-4->L+1 (27%)

[0261] 254 0.0314 Singlet-A H-10->LUMO (15%), H-5->L+1 (10%), H-3->L+2

[0262] (12%)

[0263] 253 0.0679 Singlet-A H-10->LUMO (13%), H-5->L+1 (12%), H-3->L+2

[0264] (22%)

[0265] 253 0.2235 Singlet-A H-5->L+1 (41%), H-4->L+1 (15%), H-3->L+2 (10%)

[0266] 251 0.167 Singlet-A H-3->L+2 (12%), HOMO->L+3 (37%)

[0267] 250 0.026 Singlet-A H-7->LUMO (11 %), H-2->L+3 (10%), HOMO->L+4

[0268] (14%)

[0269] Table 8. Selected TD-DFT (UB3LYP / 6-31G(d,p)) wavelength, oscillator strength and contributions of major electronic transitions of 3*.

[0270] Wavelength Osc. Symmetry Major contributions

[0271] (nm) Strength

[0272] 745 0.0965 2.026-A HOMO(B)->LUMO(B) (93%)

[0273] 551 0.237 2.071-A H-1(B)->LUMO(B) (86%)

[0274] 483 0.0028 2.062-A H-7(B)->LUMO(B) (45%), H-6(B)->LUMO(B) (14%), H-3(B)->LUMO(B) (14%), H-2(B)->LUMO(B) (21%)

[0275] 451 0.0198 2.109-A H-3(B)->LUMO(B) (20%), H-2(B)->LUMO(B) (57%)

[0276] 421 0.0019 2.060-A H-7(B)->LUMO(B) (14%), H-6(B)->LUMO(B) (73%)

[0277] 405 0.0049 2.039-A H-7(B)->LUMO(B) (30%), H-3(B)->LUMO(B) (53%)

[0278] 401 0.0013 2.075-A H-4(B)->LUMO(B) (96%)

[0279] 389 0.0003 2.030-A H-5(B)->LUMO(B) (92%)

[0280] 383 0.5771 2.327-A HOMO(A)->LUMO(A) (69%)

[0281] 351 0.0175 2.372-A HOMO(A)->L+1 (A) (21%), H-8(B)->LUMO(B) (52%)

[0282] 344 0.1421 2.880-A HOMO(A)->L+1(A) (19%), HOMO(A)->L+2(A) (10%), HOMO(B)->L+1(B) (10%)

[0283] 335 0.0222 2.504-A HOMO(A)->L+2(A) (59%)

[0284] 330 0.002 3.434-A H-5(A)->L+5(A) (12%), H-3(A)->L+3(A) (20%), H- 5(B)->L+5(B) (12%), H-3(B)->L+2(B) (17%)

[0285] 329 0.036 2.676-A HOMO(A)->L+1(A) (16%), HOMO(A)->L+2(A) (12%), H-9(B)->LUMO(B) (12%), H-8(B)->LUMO(B) (14%) 318 0.0178 2.580-A HOMO(A)->L+1(A) (19%), HOMO(A)->L+6(A) (10%), H-9(B)->LUMO(B) (22%)

[0286] 309 0.0118 3.144- A H-1(A)->L+1(A) (14%), H-8(B)->LUMO(B) (14%)

[0287] 302 0.0273 2.766-A H-1(A)->LUMO(A) (14%), HOMO(A)->L+4(A) (40%)

[0288] 300 0.037 2.651-A H-1(A)->LUMO(A) (13%), HOMO(A)->L+3(A) (21 %), H-9(B)->LUMO(B) (20%)

[0289] 297 0.0412 2.694-A HOMO(A)->L+4(A) (11%), HOMO(A)->L+6(A) (23%), H-9(B)->LUMO(B) (10%)

[0290] 296 0.0195 2.270-A HOMO(A)->L+3(A) (69%), H-9(B)->LUMO(B) (14%)

[0291] Table 9. Selected TD-DFT (RB3LYP / 6-31G(d,p)) wavelength, oscillator strength and contributions of major electronic transitions of 42.

[0292] Wavelength Osc. Symmetry Major contributions (nm) Strength

[0293] 348 0.1001 Singlet-A HOMO->LUMO (96%)

[0294] 337 0.1631 Singlet-A H-1->LUMO (84%), HOMO->L+1 (14%)

[0295] 331 0.0228 Singlet-A H-1->LUMO (14%), HOMO->L+1 (80%)

[0296] 320 0.0344 Singlet-A H-1->L+1 (94%)

[0297] 302 0.0737 Singlet-A HOMO->L+2 (89%)

[0298] 301 0.124 Singlet-A H-2->LUMO (84%)

[0299] 294 0.1146 Singlet-A H-3->LUMO (66%), H-1->L+2 (17%)

[0300] 292 0.0943 Singlet-A H-3->LUMO (19%), H-1->L+2 (71%)

[0301] 289 0.2048 Singlet-A H-2->L+1 (83%)

[0302] 284 0.0394 Singlet-A HOMO->L+3 (30%), HOMO->L+4 (51%)

[0303] 281 0.0473 Singlet-A H-3->L+1 (86%)

[0304] 280 0.0355 Singlet-A HOMO->L+3 (25%), HOMO->L+4 (30%), HOMO->L+8

[0305] (11%)

[0306] 277 0.0064 Singlet-A H-1->L+3 (10%), H-1->L+4 (12%), HOMO->L+11 (31 %)

[0307] 275 0.065 Singlet-A H-1->L+3 (17%), HOMO->L+5 (54%)

[0308] 275 0.0389 Singlet-A H-1->L+4 (44%), HOMO->L+5 (12%), HOMO->L+11

[0309] (19%)

[0310] 274 0.0128 Singlet-A HOMO->L+3 (23%), HOMO->L+5 (12%), HOMO->L+6 (40%)

[0311] 273 0.0489 Singlet-A H-1->L+3 (13%), H-1->L+4 (18%), H-1->L+6 (11%), HOMO->L+6 (24%)

[0312] 270 0.0522 Singlet-A HOMO->L+7 (46%) 270 0.0743 Singlet-A H-5->LUMO (21%), H-3->L+2 (12%), H-2->L+2 (14%)

[0313] 269 0.0246 Singlet-A HOMO->L+7 (21%), HOMO->L+8 (45%)

[0314] Table 10. Selected TD-DFT (UB3LYP / 6-31G(d,p)) wavelength, oscillator strength and contributions of major electronic transitions of 4*.

[0315] Wavelength Osc. Symmetry Major contributions

[0316] (nm) Strength

[0317] 777 0.2761 2.025-A HOMO(B)->LUMO(B) (93%)

[0318] 588 0.1491 2.037-A H-1(B)->LUMO(B) (93%)

[0319] 471 0.0197 2.141-A H-2(B)->LUMO(B) (72%)

[0320] 461 0.0075 2.066-A H-7(B)->LUMO(B) (41%), H-3(B)->LUMO(B) (31%)

[0321] 422 0.001 2.052-A H-7(B)->LUMO(B) (17%), H-6(B)->LUMO(B) (73%)

[0322] 395 0.6254 2.181-A HOMO(A)->LUMO(A) (71 %), H-2(B)->LUMO(B) (13%)

[0323] 386 0.0045 2.043-A H-7(B)->LUMO(B) (26%), H-3(B)->LUMO(B) (61%)

[0324] 380 0.0046 2.078-A H-4(B)->LUMO(B) (97%)

[0325] 368 0.0017 2.029-A H-6(B)->LUMO(B) (13%), H-5(B)->LUMO(B) (79%)

[0326] 358 0.0782 2.404-A HOMO(A)->L+1 (A) (12%), HOMO(A)->L+2(A) (42%),

[0327] H- 8(B)->LUMO(B) (10%)

[0328] 356 0.0811 2.829-A HOMO(A)->L+3(A) (11 %), HOMO(A)->L+6(A) (11 %),

[0329] H- 8(B)->LUMO(B) (18%), HOMO(B)->L+1(B) (14%)

[0330] 355 0.0472 2.347-A HOMO(A)->L+3(A) (44%), H-8(B)->LUMO(B) (26%)

[0331] 339 0.0242 2.589-A HOMO(A)->L+2(A) (14%), H-8(B)->LUMO(B) (24%)

[0332] 330 0 3.457-A H-6(A)->L+4(A) (14%), H-4(A)->L+1(A) (17%), H-

[0333] 3(B)->L+2(B) (20%)

[0334] 323 0.0034 2.635-A HOMO(A)->L+2(A) (16%), HOMO(A)->L+3(A) (10%),

[0335] HOMO(A)->L+6(A) (10%)

[0336] 318 0.0055 2.511-A HOMO(A)->L+1 (A) (56%)

[0337] 315 0.0122 3.005-A H-1(A)-> LUM O(A) (18%), HOMO(A)->L+6(A) (12%)

[0338] 312 0.0063 2.340-A HOMO(A)->L+4(A) (29%), HOMO(A)->L+5(A) (38%)

[0339] 308 0.0084 2.875-A HOMO(A)->L+4(A) (17%), HOMO(A)->L+6(A) (13%)

[0340] 307 0.0023 2.296-A HOMO(A)->L+4(A) (37%), HOMO(A)->L+5(A) (27%),

[0341] HOMO(A)->L+6(A) (12%) Mechanochromic phenomenon was also observed in dimer 42, the color of the powder changed from white to dark green upon grinding (Fig. 11 and inset of Fig. 8b). According to the UV-vis-NIR absorption spectroscopic analysis and TD-DFT calculations mentioned above, the green color stemmed from oxindolyl-based radicals, which show typical absorption in the visible light region. Additionally, the color of the powder could be immediately recovered by adding solvent onto the powder and then drying. These results indicated that the C-C bond was flexible for recovery, and recombination was restricted in the solid state due to constrained molecular flexibility. We measured the ESR spectrum of 42 in the solid state before and after grinding. As shown in Fig. 8b, the ESR signal increased significantly after grinding, suggesting an increased concentration of monoradical species. These results revealed that the dimer 42 could dissociate into monomeric radical species after grinding, further indicating that the dynamic C-C bond could be broken when external force was applied. It may be important to tune the BDE of the dynamic o-bond and the spin delocalization to control the chromic properties.

[0342] Example 7. Effect of the substituents on BDE

[0343] Estimation of BDE

[0344] According to previous study (T. Kobashi, D. Sakamaki & S. Seki, Angew. Chem. Int. Ed. 2016, 55, 8634-8638; and K. Imato et al., Angew. Chem. Int. Ed. 2015, 54, 6168-6172), the enthalpy and entropy changes for the dissociation of the dimers can be estimated by the temperature dependence of the dissociation equilibrium constant through van’t Hoff equation as shown below:

[0345] Keq= DR]2 / [D] where [R] and [D] are the concentrations of the radical and dimer species, respectively. The concentrations of the radical species in the solutions were calculated by comparing the double integral of the ESR spectra with a standard solution with known concentration. The standard solution used here is 2,2-diphenyl-1-picrylhydrazyl (DPPH), a stable commercialized free- radical molecule, in DMF.

[0346] Results and discussion

[0347] The mechanochromic effect on dimers I2, 22, and 32 was less pronounced than that on dimer 42, likely due to differences in the BDE of the dynamic C-C bond between monomeric units (T. Kosuge et al., Macromolecules 2016, 49, 5903-5911). To specifically investigate substituent effect on the OxRs, we calculated the BDE of the dynamic covalent bond between monomeric units in all dimers based on variable-temperature (VT) ESR data. As shown in Figs. 12 and 13, the thermodynamic parameters were evaluated using van’t Hoff plots generated from VT ESR spectra. Similar to the results of VT UV-vis-NIR study, the intensity of ESR signal increased significantly with elevated temperature, clearly demonstrating the increased concentration of radical species in the solution. The calculated thermodynamic parameters of dimers I2, 22, 32, and 42 are summarized in Table 11 , the value of AHdiss corresponds to the BDE for the C-C bond between monomeric units. The BDE of the dynamic covalent o-bond decreased in the following order: I2 > 22 > 32 > 42 (-CN > -Me > -OMe > -NMe2).

[0348] Table 11. Experimental dimerization parameters (tested in DMF as solvent) and DFT calculated stabilization energies of the dimers.

[0349] The results in Table 11 show that BDE could be tuned from 98.4 kJ mol'1to 148.9 kJ mol'1. Thus, materials with different substituents may be applied in different conditions. For example, when the chromic phenomenon needs to occur under a lower temperature, compounds with electron-donating groups may be used; the thermochromic effect can be triggered at a lower temperature because of the weaker bonding. On the contrary, when the chromic material needs to be able to tolerate higher temperatures, compounds with electron-withdrawing groups may be suitable.

[0350] This finding is consistent with the aforementioned VT UV-vis-NIR absorption and ESR results. The stabilization energies of the dimers were also calculated by DFT calculations at the B3LYP / 6-31G(d,p) level of the theory (Table 12). As expected, 42 exhibited the lowest stabilization energy among the four dimers, making it easier to dissociate. Therefore, it was presumed that compounds with a greater electron donating effect exhibit weaker binding, exactly like in the case of 42.

[0351] Table 12. Summary of energy of local minimum states of dimers and monomers. E (H, F) E+ZPE (H, F) G (H, F)

[0352] 12-2839.871075 -2839.931970 -2840.034985

[0353] 1« -1419.934873 -1419.964914 -1420.029858

[0354] 22-2733.963438 -2734.024425 -2734.127829

[0355] 2* -1366.981130 -1367.011225 -1367.076353

[0356] 32-2884.363837 -2884.426424 -2884.532369

[0357] 3« -1442.182419 -1442.213217 -1442.279044

[0358] 42-2923.186491 -2923.252723 -2923.363748

[0359] 4* -1461.592329 -1461.625089 -1461.693799

[0360] The BDE mainly depends on the stability of the monoradicals formed through dissociation reaction (D. Sakamaki, S. Ghosh & S. Seki, Mater. Chem. Front. 2019, 3, 2270-2282; and T. Xu et al., J. Mater. Chem. C 2023, 11, 7957-79Q ). Specifically, an unstable monoradical is more prone to dimerize and remain in dimer form, resulting in a strong linkage between monomeric units. Otherwise, it would maintain its monomer form, resulting in a weak bond in the dimer form. As for the as-synthesized OxRs, the acylamino group, with its electronwithdrawing characteristics, facilitated the adhesion of OxR core to an electron-donating group (phenyl ring with an R group, see Figs. 1 and 2), creating a donor-acceptor (D-A) effect in the structure that helps stabilize the radical species. Consequently, monoradical 1*, with its electron-withdrawing -CN group, was the most unstable among the four monoradicals. This further confirmed that the dynamic C-C o-bond in 12 is the strongest. The spin density distribution was further calculated to get more insights into the BDE differences among the dimers 22, 32, and 42 with electron donating groups (Fig. 14). It was found that the spin density on the central carbon atom increased in the following order: 4* < 3* < 2’, which could be ascribed to the highest electron-donor ability of -NMe2compared to -OMe and -Me groups. The small amount of spin on the central carbon atom in r is due to the delocalization of spin onto the -CN group. These results indicate that the stronger electron-donor group (phenyl ring with an R group; see Figs. 1b and 2) in OxRs decreased the spin density on the bridgehead carbon atom to a larger extent, consequently weakening the bridging C-C bond in the dimeric form. This further demonstrates that the introduction of different substituents can effectively tune the strength of dynamic covalent bonds based on the dissociation / association reactions of radicals. It provides significant insight for designing new rr-building blocks with tunable mechanochromic and thermochromic behavior.

[0361] Conclusion Herein, the theromochromic and mechanochromic properties of the OxR derivatives based on homolytic cleavage of central carbon-carbon covalent bonds and the stability of the radical species generated are reported. In the present disclosure, a series of OxRs with different substituents (phenyl ring with an R group, R = -CN, -Me, -OMe and -NMez) were designed and synthesized in consideration of the captodative effect, and their theromo- and mechano- responsive behavior were demonstrated (Fig. 1 b). The tert-butyl phenyl group at the 6-position (Fig. 1 b) was employed for protection, as the radical could easily delocalize to this position and undergo an irreversible dimerization reaction (J. Wang etal., Angew. Chem. Int. Ed. 2017, 56, 14154-14158). The benzyl group (also indicated in Fig. 1 b) was used to protect the reactive nitrogen site, which can be readily altered for subsequent functionalization and modification. The BDE of the dynamic covalent bonds between monomers were calculated through variable temperature ESR measurements. DFT calculation and experimental results both suggested that the OxRs with electron-donating substituents lead to a weaker binding, while electronwithdrawing substituents helps to afford stronger bonds, resulting in a captodative effect additionally contributed by the cleaved monoradical.

[0362] In conclusion, we have successfully synthesized four carbon-centered OxRs with different substituents. These monoradicals can dimerize and dissociate reversibly when subjected to external stimuli such as grinding, changes in temperature, and solvent polarity. The structures of the dimer compounds were confirmed by low-temperature1H-NMR and single crystal X-ray diffraction analysis. VT UV-vis-NIR absorption studies and ESR measurements indicated that an equilibrium exists between the dimers and the corresponding monomers. This equilibrium favors radical formation at higher temperatures, after grinding, or with the addition of polar solvents, accompanied by a notable color change. Additionally, BDE calculations demonstrated that compounds with higher electron-donation capability exhibit weaker binding in the dimer and easier dissociation to monoradicals. In contrast, OxR cores with strong electron-withdrawing groups favor the formation of dimers with strong linkages between monomeric units. The present disclosure demonstrates that OxRs are valuable building blocks for organic radical-based dynamic covalent chemistry and can be further applied in mechanochromic and thermochromic materials. It also offers significant insights for designing new TT-bui I d i ng blocks to explore more tunable mechanochromic and thermochromic materials in this field.

Claims

Claims1. A radical of formula la and / or a dimer of formula lb:wherein: each R1independently represents CN, halo, Ci to Ce alkyl, OR4a, NR4bR4cor C(=O)OR4dR2represents CH2PI1, Ci to Ce alkyl, -CH=CH2;R3represents p-tBuPh, p-CNPh, p-OR5aPh, p-NR5bR5cPh; each of R4ato R4dand R5ato R5cindependently represents Ci to Ce alkyl or a phenyl group; and a, b and c each represent 0 or 1 , provided that at least one of a to c is 1 .

2. The radical and / or the dimer according to Claim 1, wherein each R1independently represents CN, methyl, OMe or N(Me)2.

3. The radical and / or the dimer according to Claim 1 or Claim 2, wherein R2represents CH2Ph.

4. The radical and / or the dimer according to any one of the preceding claims, wherein R3represents p-fBuPh.

5. The radical and / or the dimer according to any one of the preceding claims, wherein: a is 1 , b and c are 0; b is 1 , a and c are 0; c is 1 , a and b are 0; a and b are 1 and c is 0; a and c are 1 and b is 0; b and c are 1 and a is 0; and a, b and c are 1 .

6. The radical and / or the dimer according to Claim 5, wherein a is 1 , b and c are 0.

7. The radical and / or the dimer according to any one of the preceding claims, wherein the radical of formula la is selected from the list:and the dimer of formula lb that corresponds to the above compounds of formula la.

8. The radical and / or the dimer according to any one of the preceding claims, wherein: the dimer of formula lb dissociates to provide two radicals of formula la upon application of a stimulus and the radical of formula la undergoes dimerization to provide a dimer according to a compound of formula lb upon removal of the stimulus; or the radical of formula la undergoes dimerization to provide a dimer according to a compound of formula lb upon application of a stimulus and the dimer of formula lb dissociates to provide two radicals of formula la upon removal of the stimulus.

9. The radical and / or the dimer according to Claim 8, wherein the stimulus is selected from one or more of the group consisting of a change in temperature and a mechanical force.

10. A compound according to formula II:where R1to R3and a to c are as defined in any one of Claims 1 to 6.

11. The compound according to Claim 10, wherein the compound of formula II is selected from the list:

12. A film comprising: a radical of formula la and / or a dimer of formula lb as described in any one of Claims 1 to 9; and a polymeric material, wherein the radical of formula la and / or the dimer of formula lb are distributed throughout the film.

13. An intermediate film comprising: a compound according to formula II as described in Claim 10 or Claim 11; and a polymeric material, wherein the compound of formula II is distributed throughout the film.

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