Reversible mechanofluorochromism of aniline-terminated phenylene ethynylenes

Alkyl chain length tunes the reversion temperature of mechanofluorochromic phenylene-ethynylenes that show reversible force-induced change of fluorescence from green to orange.


Introduction
This paper describes a series of dialkylaniline-based PEs that display reversible mechanouorochromism with alkyl chain length dependent properties. Mechanouorochromic (MC) materials, which show changes in the intensity and/or wavelengths of luminescence in response to mechanical force, have experienced extensive development in the last decade. 1 This response to mechanical force is oen reversible with application of light, heat, or exposure to solvent vapor (fuming). MC materials have shown promise for applications in pressure sensing, 2,3 including in biologically relevant applications and wearable technology. [4][5][6] Other applications of MC materials have centered on molecular electronics, optical data storage, secure encoding of information, and logic gates. [7][8][9][10][11] Development of MC responsive solids beyond small molecule assemblies into polymers 12 and functional materials is currently expanding the utility of these materials.
While mechanisms of MC transitions are diverse, the general phenomenon depends upon molecular assemblies converting between sufficiently dissimilar states, each with differing conformations and/or intermolecular interactions that yield unique optical properties. Including competitive non-covalent interactions that stabilize differing states, 13 such as weak aromatic interactions that prefer dissimilar crystal phases, 14 or hydrogen-bonded assemblies that can be disrupted with mechanical force, 15 is a common strategy for designing MC behavior. Structure-property relationships oen provide mechanistic insight for MC materials; these studies include how chromophore structure impacts the force-induced changes in luminescence efficiency, the magnitude and direction of spectral shi, 14,[16][17][18][19][20][21][22][23][24] as well as the sensitivity to mechanical force. [25][26][27] Though alkyl side chains are not traditionally considered key to the optoelectronics of conjugated materials, the concept of side chain engineering has become increasingly popular for the optimization of optoelectronic devices in recent years. 28,29 In addition, their structures can inuence MC behavior [30][31][32] through perturbations of the differences in energies between polymorphs. [33][34][35] In some thermally reversible MC materials, the lengths of alkyl substituents can tune the coldcrystallization temperature (T cc , or the heat-recovery temperature) for reversion from the force-induced metastable state; an example of this trend has been observed in cruciform divinylanthracenes with alkyl substituents. 36,37 The ability to tune sensitivity to external stimulus through straightforward structural modication is valuable for practical applications of MC molecular assemblies, as shown in examples of thermochromic devices. [38][39][40] As a broad category of chromophores, the optoelectronic properties of PEs can be highly sensitive to perturbation, making them attractive candidates for stimuli-responsive materials. PEs generally have small energy barriers for rotation, while the extent of coplanarity of rings along PEs has a large inuence on the energies of molecular orbitals and radiative transitions. 41,42 There have been numerous creative examples of controlling the coplanarity (or lack thereof) of PEs, and thereby their optical properties, through covalent bonding or non-covalent interactions. [43][44][45][46][47][48][49][50][51][52][53][54] Moreover, the propensity of solid PEs to aggregate and show bathochromic shis and quenched luminescence presents additional opportunities and challenges for rational design of chemical structure to optimize PE-based solid-state uorophores. 44,51,[55][56][57][58][59][60][61][62][63] Our group has reported a class of three-ring PEs in which uorinated aromatic side chains interact cofacially with non-uorinated terminal rings of the PE chromophore (ArF-ArH interactions), resulting in large inter-ring torsional angles of 63-88 ( Fig. 1) along the PE backbone. 64,65 This structural motif both interrupts conjugation along the PE and prevents intermolecular aggregation, yielding solids that have absorbance and uorescence spectra either hypsochromically shied from or similar to those in dilute solution. In contrast, solids of compounds lacking these side chain/main chain ArF-ArH interactions have planarized and/or aggregated PE chromophores, with absorbance and uorescence spectra bathochromically shied from solution. Our work has also enabled rational design of these solid-state properties: tuning the energy of the ArF-ArH interaction through the electronic effects of substituents, as well as integrating other competing non-covalent interactions such as hydrogen bonds, can dictate the solid-state packing structure and the consequent optical properties. This sensitivity of the balance of non-covalent interactions, molecular packing, and optical properties to small structural changes can also render these solids responsive to mechanical force. As an example, we reported the MC behavior of one such compound, an octyloxy-substituted PE with absorbance and uorescence that shied bathochromically (from blue to green emissive) upon application of mechanical force. 64 Herein we describe a series of seven 3-ring PE analogs, each with different lengths of alkyl chains on the nitrogen atoms of terminal aniline rings. These compounds show reversible, force-induced bathochromic shiing from green to orange emission. The objectives of this work are the following: (i) develop a series of PE-based MC materials with emission spectra red-shied relative to those we reported previously, (ii) determine how the length of the alkyl substituents impact the force-responsive spectroscopy and thermal reversibility of the MC transitions, (iii) clarify the nature of the polymorphs accessed before and aer MC transition of these materials, and (iv) determine the extent to which polymer lms can be viable hosts for these materials while retaining MC activity. These results are important for the development of new MC compounds that respond to different temperatures, improving our understanding of MC luminogens, and extending the viability of MC function into polymeric materials.

Results and discussion
Scheme 1 shows the series of seven molecules that we prepared and studied in this work. These three-ring PEs have the same general structural features: a central peruorobenzyl terephthalate ring anked by substituted arylacetylenes. We chose p-dialkylaniline terminal rings for this study because the strong electron donating effect of the amine substituent is expected to yield particularly strong cofacial stacking interactions with the peruorobenzene side chains, which dictate the twisted conformation of PEs from the perspective of electrostatic considerations. The correlation of traditional Hammett-type substituent effects with ArF-ArH interaction strength have been established in both experimental and theoretical studies comprising monosubstituted ArH rings. [65][66][67][68][69] The series of bis(dialkylaniline) PEs discussed here vary in the lengths of the N-alkyl chains, ranging from as short as methyl to as long as n-octadecyl.
The syntheses of these molecules, A-R, in which "R" designates the length of the alkyl substituents, was straightforward (Scheme 1). Dialkylation of 4-iodoaniline under basic conditions yielded 1-R, except for 1-1, which is commercially available. Sonogashira coupling with trimethylsilylacetylene and subsequent deprotection gave the terminal arylacetylenes 2-R. A  nal double Sonogashira coupling between each of 2-R and the previously described 2,5-diiodoterephthalate 3 64 yielded the target compounds A-R, each of which were puried by recrystallization. 64,65 A control compound bearing non-uorinated benzyl ester pendants and therefore lacking the ArF-ArH interaction, A-1F0, was analogously synthesized from non-uorinated 2,5-diiodoterephthalate precursor 3-F0. 29 Photophysical properties in solution and solids Photophysical properties of the target compounds in solution are similar throughout the series of compounds (Table 1 & Fig. 2). Absorbance and emission maxima of yellow to orange emitting chloroform solutions show lower energy optical spectra with alkyl chain lengths longer than methyl due to increased donor character of the terminal aniline substituents (Fig. S1 †). All compounds display positive solvatochromism, which is consistent with the donor-acceptor nature of these PE chromophores, with emission maxima varying up to 122 nm from hexane (l max ¼ 502 nm for A-4) to dichloromethane (DCM) solutions (l max ¼ 624 nm for A-4). Fluorescence is noticeably quenched in higher polarity solvents: for A-4 we observe a quantum yield of 0.72 and lifetime of 2.7 ns in hexane, which decrease to 0.23 and 1.7 ns in chloroform, and drop sharply to 0.03 and 0.4 ns in orange emitting DCM solution. This quenching increases with E T (30) values of the solvents, and indicates that non-radiative decay pathways become dominant in polar solvents (Fig. S2 †). While k r drops four-fold between hexane and ethyl acetate, k nr increases over thirty-fold. This coincidence of decreasing k r with increasing E T (30) values is consistent with the "positive solvatokinetic effect," 70 of donoracceptor type compounds, and demonstrates intramolecular charge transfer (ICT) in the excited state, in which an increase in dipole moment of the excited state relative to the ground state with solvent polarity enhances non-radiative decay. [71][72][73][74][75][76] Further, while quenched in pure tetrahydrofuran (THF) solution (F F ¼ 0.03 for A-4, 0.04 for A-18), these materials show aggregationinduced enhanced emission (AIEE) [77][78][79][80] in THF/water mixtures, which oen occurs in force responsive materials (Fig. 2). 13,20,81 While the emission intensity varies among the compounds, a representative sample of the series (A-2, A-4, A-8, A-18) consistently shows turn-on uorescence at or above 60% water fraction (f w ) with roughly a three-fold increase of intensity relative to pure THF solution and gradual hypsochromic shi in emission spectra with increasing water fraction (38 nm shi in l max from 0% to 90%, Fig. S3 †). We attribute the observed AIEE effect to a combination of reduction in the local polarity of nanoprecipitated uorophores and the prevention of cofacial interactions between PE chromophore units by ArF-ArH interactions (vide infra).
As powders or drop-cast lms, compounds A-R are yellow with green uorescence, with absorbance maxima ranging from 400-424 nm and emission maxima from 510-550 nm aer heating to 100-220 C to remove solvent and allow relaxation toward a more thermodynamically stable phase. This initial annealing has only modest effects on the absorbance and uorescence of drop-cast lms, which have uniform green emission. The rapidly deposited lms resulting from spin casting, however, are green emissive, yellow/orange emissive, or both. Subsequent thermal annealing hypsochromically shis this emission to uniform green color, similar to those observed in drop-cast lms. While emission maxima blue-shi somewhat with alkyl chains longer than methyl and ethyl (up to 40 nm shi between A-1 and A-18), minor changes in the position of thin lm absorbance and emission spectra between compounds do not correlate with increasing alkyl chain length.  The luminescence of these lms is most akin to compounds dissolved in toluene, and the spectra are hypsochromically shied relative to chloroform solutions by 15-50 nm in absorbance l max and 25-50 nm in emission l max . As determined in our previous work, this lack of a signicant red-shi for this class of PE solids indicates solid-state packing environments in which the conjugated chromophores lack two features common in other PE-based materials: (i) highly coplanar aryl rings along the PE backbones and (ii) intermolecular aggregation of PE chromophores. The combination of intramolecular and intermolecular ArF-ArH interactions between the peruorobenzyl pendants on the central terephthalate ring and the terminal rings of the PE backbone both enforce these twisted conformations and prevent aggregation between the PE chromophores. For reference, compounds we reported previously that show the twisted, non-aggregated arrangement in crystal structures showed modest solution-to-solid hypsochromic shis relative to chloroform solutions either in absorbance and/ or emission (15-25 nm in absorbance l max and 5-20 nm in emission l max ). In contrast, analogs that showed coplanar and/ or aggregated PE chromophores in their crystal structures showed signicant bathochromic shis as solids (14-42 nm in absorbance l max and 60-128 nm in emission l max ). 65 Only A-1F0, which lacks uorinated pendants, demonstrates this bathochromic shi in recrystallized and annealed solids, and never shows the hypsochromically shied green emission.

Crystallography
Crystal structures obtained for compounds A-2, A-4, A-8, and A-1F0 and provide key insight for the stimuli-responsive behavior of this series of molecules. As with the annealed lms, single crystals of these compounds appeared as yellow needles with green uorescence. Emission spectra of the single crystals resemble those of the lms, as seen in Fig. 3 with only modest shis between lm and crystal emission l max (533 vs. 530 nm in A-2, 535 vs. 556 nm in A-4, 530 vs. 526 nm in A-8, and 597 vs. 625 nm in A-1F0). Similar correlations between excitation spectra and onset wavelengths exist.
The green emitting crystals of A-2, A-4, and A-8 have twisted crystal structures (Fig. 5) similar to the previously reported A-1, with ArF-ArH cofacial interactions as the central features. These interactions occur both intramolecularly and intermolecularly, resulting in ArF-ArH stacks that propagate along innite columns with centroid-centroid distances of 3.6-4.0Å. These ArF-ArH interactions, combined with the tendency of the ester functional groups to remain coplanar with the central phenylene ring, result in heavily twisted PE backbones. The inter-ring torsional angles along the PE chromophores are between 63.5 and 88.2 . The alkyl chains in structures of A-2, A-4, and A-8 extend in a roughly perpendicular trajectory from planes dened by the aniline rings of the PE backbones, which likely maintains close packing between neighboring stacks of the twisted PEs, as opposed to the possibility of alkyl chains extending parallel to the aniline rings, which may disrupt close packing.
Notably, the number of intermolecular aromatic interactions between neighboring PEs decreases with alkyl chain length. The structure for A-1 shows C-H/p and C-F/p interactions between neighboring ArF rings and terminal anilines to terephthalates (2.9Å closest contacts). These interactions are mostly conserved in the structure for A-2, though the number of close contacts between stacks is reduced (3.0Å closest contacts). Packing in A-4 changes signicantly; these C-H/p and C-F/p interactions are lost as neighboring PEs show a 7Å pitch displacement relative to A-1 and A-2 ( Fig. 5 and S4 †). As the alkyl chains occupy a greater volume in the structure, the ArF rings and anilines are farther away from neighboring terephthalates. Only F/F interactions are present at 2.9Å in A-4. A dramatic change is seen in A-8, where there are no close contacts between neighboring chromophores, and a greater portion of the   having twisted PE backbones without aggregation. This relative similarity in uorescence in all cases where crystal structures could be obtained supports the conclusion that molecular packing in thin lms favors the twisted, nonaggregated motif seen in crystal structures. Powder X-ray diffraction (PXRD) studies of drop-cast lms also supports this conclusion. In contrast, an orange emitting crystal of A-1F0 (Fig. 4) shows aggregated PE chromophores with coplanar arenes.

Reversible mechanouorochromism
These seven compounds all show easily noticeable bathochromic shis in both color (yellow to orange, 37-50 nm shi in absorbance l max ) and uorescence (green to orange, 50-62 nm shi in emission l max ) in response to mechanical force ( Fig. 6 and S5, S6 †). This bathochromic shi is consistent with planarization/aggregation of at least some of the PE chromophores. 56,57 Though this transition can be achieved to some degree with hydrostatic pressure (15 000 psi), powders and lms are far more sensitive to shear force such as rubbing with a spatula. The extent of the bathochromic shi in optical spectra is similar across the compounds with modest differences primarily observed in the emission of the annealed lms. This response is fully reversible upon thermal annealing or with solvent vapor annealing and does not fatigue over multiple cycles of heating and grinding for any of the target compounds ( Fig. 6 and S7, S8 †). The lms become increasingly waxy with decreased melting points with longer alkyl chains (282 C for A-1 to 76 C for A-18), and are more easily deformed by rubbing.
The persistence of metastable orange emission from ground lms varies greatly across the series. For compounds with alkyl chains longer than propyl, the orange emission of ground lms relaxes to yellow at room temperature within minutes. Aer grinding, the original uorescence of A-4 mostly recovers at room temperature within 15 minutes. The ground phase becomes increasingly transient for A-6 and A-8 where the orange color may be seen on the timescale of 1 s, and is not observable whatsoever for A-18 at room temperature, but only momentarily ca. À80 C by cooling over a dry ice/acetone bath. Therefore, the recorded emission maxima for sheared lms of A-6 and A-8 do not reach ca. 600 nm, as do those of A-1 through A-4, instead showing smaller bathochromic shis to 550 nm. An interesting feature of this transient response is that lms of A-8 recover their green emission immediately aer grinding, and may be sheared continuously with no loss of MC behavior, with a single lm maintaining its activity over the course of at least a year.
Differential scanning calorimetry (DSC) of these molecules also demonstrates the effect of alkyl chain length on recovery from the metastable polymorph created upon grinding. Aer thermal annealing, no transitions are visible for these compounds between ambient temperature and their melting points. Ground powders, however, display broad exothermic transitions spanning roughly 15-30 C that are not visible in subsequent heating cycles (Fig. 7). Such transitions of metastable polymorphs are oen indicative of cold-crystallization from an amorphous to crystalline morphology, suggesting crystalline-to-amorphous MC transitions upon grinding. 33,36,[82][83][84][85][86][87][88][89][90][91][92][93][94][95][96][97] The T cc ranges decrease with alkyl chain length from A-1 to A-4, with peak maxima of 57 C for A-1 to 32 C for A-4 ( Table 2). For those molecules with longer alkyl chains, the transient nature of the ground phase at room temperature yields featureless thermograms (Fig. S9 †) with the exception of A-18, which shows an endotherm at 50-60 C in the rst heat for both annealed and ground powders, and then multiple reversible endothermic transitions in the second and third heating cycles (Fig. S10 †). Such transitions have been observed in our lab in related compounds with long alkyl chains, and will be the subject of future study.
In addition, heating through these exothermic transitions of A-1 to A-4 recovers most of the uorescence of the annealed lms, while heating to higher temperatures below the melting points completely recovers the original emission spectrum. Fig. 7 shows the visual response of the photoluminescence of lms of these compounds at increasing temperatures and the gradual recovery process over a temperature range. Emission spectra show the same behavior; emission l max of A-1 recovers from 605 nm to 567 nm when heated to 100 C for 15 minutes aer grinding, and heating at 220 C results in complete reversion of the l max to 550 nm. Similar results are seen in A-2, A-3, and A-4.
Furthermore, Fig. 7 shows a gradient of recovered emission during thermal recovery where compounds with longer alkyl chain lengths show hypsochromically-shied emission relative to those with shorter alkyl chain lengths at a given temperature. For example, in the 50 C image in Fig. 7, A-4 emission is the most blue-shied and A-1 emission is the most red-shied. This gradient in recovered emission is reproducible when heating at a xed temperature. Annealing at 100 C, A-4 achieves full recovery to emission l max of 545 nm, A-3 recovers to 552 nm, A-2 557 nm, and A-1 to 567 nm; this behavior persists over 5 heatgrinding cycles (Fig. S8 †). In conjunction with decreasing T cc , the temperatures required for post-grinding thermochromic recovery decreases with alkyl chain length. Other families of mechanochromic compounds have shown a similar structureproperty relationship. 36,37,85,86,89,90,96,98 Structural analysis of MC transition The results of PXRD experiments are consistent with a reversible crystalline-to-amorphous transition upon grinding, which explains the nature of MC behavior in this system. Patterns were collected for lms over four sequential steps (i) pristine (ii) ground (iii) thermally annealed (iv) solvent vapor annealed (fumed) aer thermal annealing. X-ray diffraction of pristine and annealed lms agree with simulated powder patterns from single crystal structures for A-1, A-2, A-4, A-8, and A-1F0 with strong diffraction peaks at 2q ¼ 15.7, 14.9, 13.7, 12.8, and 24.8 , respectively. However, grinding of the lms results in signicantly reduced peak intensity or featureless PXRD patterns (Fig. 8). The intensities of the crystalline peaks recover aer thermal and solvent annealing, which in conjunction with the exothermic transitions observed in DSC during thermal recovery, make evident that mechanical force causes a crystalline-to-amorphous transition. Subsequent heating through the T cc range of the ground lms shows some restoration of the crystalline patterns. The peaks of annealed and fumed lms are signicantly less intense than in the pristine lm due to disruption of the crystal structure and reduction of lm thickness during the grinding process, but comparison with the patterns of mechanically ground lms demonstrate clear amorphous-to-crystalline recovery with annealing or vapor fuming.
Increasing alkyl chain length leads to important differences between the pristine patterns. Compounds A-1, A-2, and A-1F0 show greater crystallinity relative to samples with longer alkyl chain lengths. We see greater amorphous character (semicrystalline material) in the pristine patterns of A-4, A-6, A-8, and an almost entirely amorphous pattern in A-18 (Fig. S11 †). The increase in the amorphous nature of the lms presumably originates from the weakening of intermolecular interactions  across adjacent planes of PEs, particularly in structures for A-4 and A-8. Control compound A-1F0 also shows a reversible crystalline-to-amorphous transition with grinding. However, there is no change in the luminescence of the solids with grinding. The bathochromically shied emission in both the crystalline and amorphous phases of A-1F0, relative to the uorinated compounds, is consistent with our model for the MC transition in this system, in which initially twisted and nonaggregated PEs become increasingly planarized and aggregated upon application of mechanical force. 33 Comparing the experimental and calculated patterns, it is possible to identify reection planes and corresponding dspacings from the crystal structures that are present in lms. Importantly, a plane of reections lying between columns of twisted PEs gives rise to the most intense diffraction peak 2q ¼ 15.7, 14.9, 13.7, and 12.8 in A-1, A-2, A-4, and A-8, respectively. This peak is seen in all pristine patterns except for A-18 (Fig. S11 †) and is the only peak that is recovered with signicant intensity among all samples in post-grinding annealed lms. This conserved peak corresponds to the well-ordered nature of twisted PE columns formed by innite ArF-ArH stacks (Fig. 8). The relative intensity of this peak decreases with alkyl chain length in conjunction with the decreasing crystallinity of lm samples, which coincides with general trends in material properties for these compounds (vide infra). The increasingly amorphous nature of the lms is observed in the broad amorphous peak from 2q ¼ 11-15 appearing in pristine samples of A-6 and A-8, and the highly amorphous pattern of A-18. This loss of diffraction peaks indicates that increasing alkyl chain length decreases crystallinity of powder samples to the extent that, while pristine lms always display green emission corresponding to twisted, non-aggregated PEs, the long-range order of parallel columns of innitely stacked ArF-ArH units is lost. In agreement with photophysical data, these stacks of twisted PEs are disrupted with grinding and restored with annealing/ fuming. Additionally, this peak shis to smaller 2q (corresponding to greater d-spacing) with increasing alkyl chain length, from 5.6Å in A-1 to 6.9Å in A-8 (Fig. 8), which is consistent with looser molecular packing with longer chains that occupy more volume and result in elongated interchromophore distances. A peak at 2q ¼ 14.9 , corresponding to plane of reections lying between parallel columns of coplanar, aggregated PEs is also present in the A-1F0 diffraction pattern. It should be noted that in the case of A-1F0 the peak at 2q ¼ 24.8 , corresponding to coplanar backbone, which contributes to the bathochromically-shied emission in solids, is the most intense peak aer recovery, whereas the peak 2q ¼ 14.9 is minimally recovered. The fact that the peak corresponding to ordered columns of PEs is signicantly recovered in all cases from A-1 through A-8, but not in A-1F0, suggests the ArF-ArH interactions are essential for such structural restoration.
The crystal structures of A-1, A-2, A-4, and A-8, conrmed with PXRD data, provide an explanation for the decrease in T cc with increasing alkyl chain length. As alkyl chain length increases, intermolecular aromatic-p interactions are reduced (vide supra), and alkyl chains, with propensity for disorder, take up greater volume fractions in the structures, resulting in greater d-spacing between columns of PEs. These trends agree with the increasingly amorphous character of pristine lms ( Fig. 8 and S11 †), as well as the observation of decreasing material rigidity among compounds with longer alkyl chains, the powders of which are more easily deformed with grinding. Further, these structural parameters coincide with decreasing melting points as alkyl chain length increases (282, 256, 149, and 107 C for A-1, A-2, A-4, and A-8 respectively). We propose the decrease in T cc may originate from a complementary weakening of interactions in the amorphous phase and inhibition of close packing, which results in easier deformation of the supramolecular structures and more facile thermal restoration of the crystalline phase with increasing chain length. This is consistent with reported examples of mechano-uorochromic crystalline-to-amorphous transitions where T cc decreases with increasing alkyl chain length due to weaker intermolecular interactions between chromophores. 85,89,90,98 Polymer hosts In the interest of developing MC functional coatings and sensors, crystalline molecular assemblies are typically not practical for device fabrication due to their poor mechanical properties. To address this challenge, MC polymers have been reported, 12,99-101 some with quantitation of mechanical stress. 102 Aside from preparation of homopolymers and supramolecular polymers, other reports have demonstrated the utility of MC luminogens incorporated into polymer matrices. 26,103,104 Considering the potential for incorporating the side-chain controlled PE design into functional polymers, we have assessed the MC behavior of the target compounds doped into various polymethacrylate hosts that differ in glass transition temperature (T g ). We found that poly(methyl methacrylate) (PMMA) lms doped with 20% (w/w) A-R and drop-cast from chloroform showed reliable MC responses in all cases with spectral shis comparable to the pure PE lms. Intriguingly, orange emission aer grinding in lms doped with A-18 was persistent at room temperature, which cannot be observed in pure A-18 lms.
While maintaining the same conditions for preparation, MC response of these lms can be maintained in other polymer hosts ( Fig. 9 & S12 †), though the optical shi in emission l max is considerably reduced. Comparison of MC properties for methacrylate lms doped at 20% (w/w) with A-4 across PMMA (T g ¼ 99 C), poly(butyl methacrylate) (PBMA, T g ¼ 65 C), poly (isobutyl methacrylate) (PIBMA, T g ¼ 20 C), and poly(hexyl methacrylate) (PHMA, T g ¼ À5 C) hosts showed a dependence of the optical shi, sensitivity, and reversibility on the host. Annealed PMMA lms showed both green and orange emitting domains. This polymorphism persisted in PBMA and PIBMA lms, though green emission dominated in the PIBMA lm, and PHMA host yielded uniform green emission. Sensitivity to mechanical force was notably higher in PHMA lms than the other hosts, and the MC transition could be achieved without breaking or deforming the lm. The annealed lms showed a gradual hypsochromic shi in emission with decreasing T g of polymer host. At the same time, the MC bathochromic shi in emission decreased (Fig. 9). Furthermore, green emission in the PMMA and PBMA lms could not be recovered by annealing; only the lower T g hosts PIBMA and PHMA showed reversible MC response over multiple heating-grinding cycles, though the optical shis in emission maxima are muted relative to A-4 lms. This variable response across the hosts indicates that thermal recovery from planarized, aggregated assemblies to the twisted, non-aggregated state depends on the relative mobility of assemblies. In a rigid host such as PMMA with high T g , the original uorescence may not be recovered aer grinding even though A-4 normally recovers its green uorescence within minutes at room temperature. In contrast, hosts well above the T g at ambient temperature such as PHMA allows for facile Fig. 9 Emission spectra of A-4 in various polymethacrylate hosts: annealed (left) and ground (center), l ex ¼ 430 nm. Plot of emission maximum over five heat-grinding cycles of A-4 and A-4 20% (w/w) in PIBMA, PHMA (right). molecular rearrangement, which in turn diminishes the extent of the bathochromic shi. These results not only demonstrate the MC capability of these PEs in polymer scaffolds, but reveal a simple means for tuning MC behavior.

Conclusions
Seven structurally analogous three-ring PEs with dialkylaniline terminal substituents show MC behavior that is fully reversible upon thermal or solvent vapor annealing. Mechanical force causes a crystalline-to-amorphous transition as shown by DSC and PXRD, during which the uorescence bathochromically shis from green to orange as PE chromophores rearrange from twisted, non-aggregated assemblies as seen in single crystal structures to increasingly planarized and/or aggregated states in the amorphous phase. The MC behavior is lost in the absence of the ArF-ArH interaction, which dictates the optical properties and crystal packing of these molecules. The transition is fully reversible with heat or solvent vapor annealing, and the temperature required for thermal recovery of the crystalline phase from the ground phase decreases as the lengths of alkyl chains on the aniline substituents increase. This trend coincides with weakening intermolecular interactions in the crystal structures, allowing for greater molecular mobility of the chromophores. The MC properties of these compounds translate to polymer hosts, and can be tuned simply by changing the environment of the PE through choice of the polymer. Varying the length of alkyl substituents can be an effective method for tuning thermal recovery and material properties in small molecule MC luminogens that show crystalline-to-amorphous transitions.
Overall, this work demonstrates that this class of PEs shows robust, reversible MC behavior with structurally tunable thermochromism, elucidates details of their MC behavior, and shows how the thermal recovery depends on crystal packing, as well as how important features of the crystalline assembly in thin lms respond to mechanical grinding and annealing/ fuming. This knowledge will guide rational design of future MC molecular assemblies, especially in using non-covalent control to direct crystal packing in related stimuli-responsive systems.

Conflicts of interest
The authors declare no conicts of interest.