Rhodamine-Installed Polynorbornenes: Molecular Design, Structure, and Stimuli-Responsive Properties.

The synthesis of a number of tailored architectures of rhodamine dye-norbornene conjugate monomers and corresponding homopolymers derived from them is described. The impact of the monomer architecture on the mechanochromic, photochromic, and thermochromic properties of rhodamine-modified polynorbornenes is reported. Color changes were caused by the reversible interconversion between the "open" and "closed" spirolactam form of the covalently attached dye. Monomers were synthesized in two principle architectures that varied on: (1) the number of polymerizable norbornene groups tethered to a bifunctional rhodamine dye; (2) the presence of flexible methylene spacers between the dye and the polymerizable norbornene groups. Introduction of norbornene groups on each of the two hydroxy groups of a bifunctional rhodamine resulted in a cross-linked polymer that exhibited better mechanochromic, photochromic, and thermochromic properties compared to the corresponding polymer without cross-links, derived from the derivatization of bifunctional rhodamine with only one norbornene. The introduction of flexible methylene spacers between the two polymerizable norbornenes and the dye molecule resulted in a polymeric framework with rapidly reversible color-changing properties upon mechanical or photostimulation. The ideal monomer molecular structure, whereby (1) attaching norbornene on both sides of the rhodamine dye and (2) methylene spacers between the dye and norbornenes on both sides afforded the nonpareil polymer structure that was capable of thermoreversible mechanochromic and photochromic features, and irreversible thermochromic features. These new materials may find utility as multi-stimuli-responsive soft materials.

Introduction

Stimuli-responsive polymers containing dye molecules can be prepared by either blending dye molecules with a variety of polymers or by covalently attaching the dye molecule to the main or side chains of polymers.[1−22] In powder or film form, these dye-containing polymers show a color change when exposed to different stimuli, including pressure, load, stress, strain, light, heat, or pH. These stimuli-chromic polymers are candidates for the realization of sensors, stimulus-dependent catalysts, drug delivery vehicles, data storage devices, or optoelectronic devices.[23−34]

Many of the dyes installed within polymer chains are spiropyran derivatives. For example, mechanical force-induced covalent bond activation by covalently embedding bifunctional spiropyran molecules in poly(methyl acrylate) (PMA) and poly(methyl methacrylate) (PMMA) networks that produced a visible color change upon stretching the polymeric materials was demonstrated.[1] The change in color of spiropyran is due to the mechanical stress-induced opening of the spiropyran form of the dye. The bond scission converts the dye from its colorless ring-closed orthogonal spiropyran (SP) form to a colored ring-opened planar zwitterionic merocyanine (MC) form.[1−3,5,25,35] Several factors influence the mechanoactivation of spiropyrans within polymers, such as loading history, nature of the polymer matrix, mechanical properties, mobility of the chains linking architecture, and environmental conditions.[1−3,5,36−41] For instance, polymers with longer crosslinkers showed decreased activation stress indicating an improved load transfer to the mechanophore.[5] A corresponding trend was also observed for the photochromism of these materials.[15] Thus, macromolecular engineering is key in tuning molecular mobility and stimulus response in mechanochromic elastomers. Recently, rhodamine-based multicolor mechanochromic elastomers were reported.[10] Along with emission of three primary colors on stretching, these materials showed high-fluorescence quantum yields. The unique broad band spectral features of this elastomer were achieved by bond scission as well as bond bending within the rhodamine moieties.

Piezochromism is a static pressure or a load-triggered color-changing phenomenon.[8] In mechanochromic polymers, the mechanophore needs to be covalently embedded within polymer matrices as a chain center or cross-linking agent. Tension on polymer chains then results in mechanochromism. An increase in color change intensity may be observed upon an increase of the pressure applied.[8] For piezochromism to take place, the color-inducing molecule can be doped into the polymer by physical incorporation or chemically inserted as a pendant group. It does not have to be necessarily embedded into the main chain of the polymer backbone by covalent attachment. Thus, mechanochromic and piezochromic behaviors are not always coupled with each other. Reversible piezochromism is achieved when the isomerization energy barrier between the conversion between the SP and MC forms is low, resulting in sensitive responses to pressure.[42]

Photochromism is a light-triggered color-changing phenomenon. The isomeric forms of a photochromic compound may be interconverted by the absorption of light energy. Spiropyran and spiroxazine are the most commonly used photochromic compounds in polymers due to their ease of preparation, good reversibility, photostability, compatibility with polymers, and multi-stimuli-response capabilities.[16,43] Their ring-closed orthogonal SP forms change to the ring-opened planar zwitterionic MC form under UV irradiation. The photochromic effect is impacted by the polymer matrix and the method of including the photochromic compound.[15,44]

Thermochromism is another stimuli-chromic behavior, characterized by a change in color with temperature variation.[23,24] In spiropyran-class dyes, the colored MC form may be converted to the colorless SP form upon an increase in temperature.[23,24]

Chromic polymers that are responsive to more than two stimuli are rare,[45−47] though some spiropyran-based polymers exhibit this trait.[34,46,48,49] For example, a dihydroxylated rhodamine-based molecule (HO-Rh-OH) was embedded within polyurethane networks to create a polymer with reversible piezochromic and photochromic properties.[8] The creation of materials capable of responding to three different stimuli is desirable as they may exhibit a higher degree of sensitivity, possess expanded switching windows, can be responsive to multiple stimuli in the environment, or show responses to altered switching conditions, all for their potentially broader applicability.

Using the dihydroxy rhodamine derivative HO-Rh-OH, covalently linked to one or more norbornenes with or without spacers, we are presenting here a systematic study of the impact of the nature of the polymer produced upon polymerization and the way the dye is installed within the polymer matrix on its structure, thermal, mechanical, and multi-stimuli-responsive properties. Specifically, we investigated the number of covalent linkages to the dye and the length of spacers connecting the dye to the polymer. The rhodamine dye was chosen as a model dye due to its excellent optical properties, stability, high-fluorescence quantum yields, and proven stimuli responsiveness.[8] We will show to which extent the different polymers derived respond to mechanical force, UV light, and heat.

Monomer Syntheses

Attachment of norbornene onto the phenolic hydroxy group of rhodamine dye HO-Rh-OH molecule was achieved using an ester linkage using a hydroxybenzotriazole (HOBT)-catalyzed N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC) coupling strategy (Figure and Scheme S1). The reaction was stoichiometrically controlled, with the most nucleophilic phenolic alcohol preferentially forming the mono-adduct. Attachment of norbornene onto both hydroxy groups of HO-Rh-OH to yield Monomer 2 (Figure and Scheme S2) proceeded using the same coupling strategy but using a stoichiometric excess of 5-norbornene-2-carboxylic acid.

Figure 1

(a) Types of attachment of the rhodamine dye to norbornene—one sided (monofunctional, Monomer 1) and two sided (bifunctional, Monomer 2). (b) Introduction of 10 carbon methylene spacers between norbornene and the dye in Monomer 1 produced Monomer 3 (monofunctional), while the introduction of these spacers on both sides of dye in Monomer 2 produced Monomer 4 (bifunctional).

The methylene spacer groups were established by means of the reaction of the potassium salt of 5-norbornene-2-carboxylate with 11-bromoundecanoic acid, forming the ester-linked monomer NB-(CH2)10-COOH intermediate. One (for Monomer 3, Figure and Scheme S3) and two (for Monomer 4, Scheme S4) of this carboxylic acid were linked to rhodamine HO-Rh-OH as described above. All monomers showed the expected spectroscopic and analytical properties. Details are provided in the Supporting Information.

Polymer Syntheses

Ring-opening metathesis polymerization (ROMP) using a modified Grubbs second-generation catalyst was used to synthesize homopolymers, Polymers 1–4, from Monomers 1–4, respectively (Table ). As Monomers 1 and 3 had only one polymerizable norbornene moiety, both yielded noncrosslinked polymer architectures with and without a methylene spacer between the dye and polymer chain. Monomers 2 and 4 with two polymerizable norbornenes yielded partially cross-linked polymer architectures, again with and without a methylene spacer between the dye and polymer chain. The introduction of the 10 carbon methylene spacers was designed to help us understand the importance of the motional decoupling/coupling balance between the polynorbornene backbone and HO-Rh-OH dye when analyzing and comparing the mechanochromic, photochromic, and thermochromic properties of the corresponding polymers.

Table 1

Structures and Schematic Representations of Polymers 1–4 Derived from the Ring-Opening Metathesis Polymerization (ROMP) of Monomers 1–4 Using a Modified Grubbs Second-Generation Catalyst in Dichloromethane (CH2Cl2) or Tetrahydrofuran (THF); Termination of the Polymerization by Addition of Ethyl Vinyl Ether (EVE). Polymer 1: Monofunctional without Spacer; Polymer 2: Bifunctional without Spacer; Polymer 3: Monofunctional with Spacer on One Side; and Polymer 4: Bifunctional with Spacer on Both Sides

The structure and composition of Polymers 1–4 were characterized by 1H NMR, gel permeation chromatography (GPC), and attenuated total reflection infrared (ATR-IR) spectroscopy (Table , Figures S15–S21). The broad peak between 5.29 and 5.5 ppm was indicative of the norbornene olefinic backbone. The targeted molecular weights of the synthesized polymers were 20 kDa. Noncrosslinked polymers Polymer 1 (Mn = 14.5 kDa, ĐM = 1.3) and Polymer 3 (Mn = 16.4 kDa, ĐM = 1.2) were synthesized in under 1 h. These narrow dispersities are in agreement with the narrow molecular weight distribution generally observed for polynorbornenes. Polymer 2 and Polymer 4 were also synthesized in 1 to <1 h. The molecular weights of cross-linked polymers, Polymer 2 and Polymer 4, could not be determined by GPC.

Table 2

Composition and Thermal Properties of Polymersa,b

Polymers	Mn (kDa)	ĐM	Tg (°C)	Td (°C)
P(NB-Rh-OH), Polymer 1	14.5	1.3	201.2	333.2
P(NB-Rh-NB), Polymer 2	N/D	N/D	N/D	380.0
P(NB-(CH2)10-Rh-OH), Polymer 3	16.4	1.2	116.1	321.2
P(NB-(CH2)10-Rh-(CH2)10-NB), Polymer 4	N/D	N/D	95.5	371.6

All transition temperatures were observed from differential scanning calorimetry (DSC) first cooling scan. Mn (number average molecular weight) and ĐM (Dispersity) were determined by GPC (gel permeation chromatography) with THF as eluent and calibrated using polystyrene (PS) standards. Theoretical Mn was calculated based on 100% initiation of the catalyst and 100% conversion of monomer to polymer.

Polymers 2 and 4 were partly cross-linked and insoluble in most of the solvents. Thus, Mn and ĐM were not determined (N/D). Gel fractions of Polymers 2 and 4 were carried out by taking weight difference before and after immersion in THF for 7 days. The gel fraction of Polymer 2 was ∼40%; of Polymer 4 ∼30–35%.

The different polymer architectures obtained upon polymerization of the four monomers are shown in Table . We chose polynorbornenes over polyacrylate and methacrylate backbones due to the ability to tailor the glass-transition temperature, presence of cross-links, and mechanical properties. The impact of the architecture on structure, thermal properties, preparation of films, and multistimuli responsive properties of rhodamine-installed polynorbornenes was explored.

Results and Discussion

Synthesis of Monomers and Polymers

We prepared four different norbornene-dye monomers (Monomers 1–4; Figure ); then polymerized them into four polymers using comparable conditions (Polymers 1–4; Table ). In the first structural variation of the monomer, we tethered one or two norbornenes to one or both of the hydroxyl groups of the HO-Rh-OH rhodamine dye framework. This led to one-sided (monofunctional, Monomer 1) and two-sided (bifunctional, Monomer 2) attachment of polymerizable moieties. In the second structural monomer variation, 10 carbon methylene spacer groups between the one (Monomer 3) and two (Monomer 4) polymerizable monomers and the rhodamine dye moieties were introduced.

The synthetic schemes and detailed synthetic procedures and spectra data of Monomer 1 (NB-Rh-OH), Monomer 2 (NB-Rh-NB), Monomer 3 (NB-(CH2)10-Rh-OH), and Monomer 4 (NB-(CH2)10-Rh-(CH2)10-NB) can be found in Sections 2.1–2.4, respectively, of the Supporting Information. 1H NMR spectra and peak assignments of the Monomers 1–4 are presented in Figures S1–S5, respectively. HO-Rh-OH is known to undergo a chemical transformation from a twisted spirolactam to a planarized ring-opened amide in an acidic environment. Ultraviolet–visible (UV–vis) absorbance spectra of Monomers 1–4 before and after the addition of trifluoroacetic acid (TFA) are given in Figures S7–S10. The emergence of a peak at 535 nm on the addition of TFA confirms the isomerization of HO-Rh-OH implanted in the monomers. Ring-opening metathesis polymerization (ROMP) using a modified second-generation Grubbs catalyst was used to polymerize Monomers 1–4 to synthesize Polymers 1–4, respectively. The chemical structures of the polymers synthesized are listed in Table . The synthetic routes to afford Polymers 1–4 can be found in Schemes S5–S8, respectively, and in Sections 3.1–3.4 in the Supporting Information. Solvent swollen gels of partly cross-linked Polymers 2 and 4 were analyzed by 1H NMR to show the conversion of the respective monomers to polymers. 1H NMR spectra and peak assignments of Polymers 1–4 are presented in Figures S15–S18, respectively. Attenuated total reflection-infrared (ATR-IR) of cross-linked Polymers 2 and 4 are presented in Figures S19 and S20, respectively. The structure and composition of Polymers 1–4 were characterized by 1H NMR, GPC, and ATR-IR spectroscopy (Table , S15–S21).

Thermal Properties

Decomposition temperatures of monomers (Figure S11) and homopolymers (Figure S22) were determined by thermogravimetric analysis (TGA). The monomers had lower decomposition temperatures (∼230 °C) than their respective homopolymers (∼350 °C) due to the difference in overall molecular weights. The homopolymers were stable up to 380 °C when heated under nitrogen at a rate of 10 °C/min.

The thermal decomposition temperatures at which 5 wt % loss of Polymers 1 through 4 occurred are presented in Table . Uncrosslinked samples, Polymers 1 and 3, showed a lower decomposition temperature compared to corresponding cross-linked networks, Polymers 2 and 4.

The glass transition temperature (Tg) for the amorphous polynorbornene backbone in Polymers 1–4 was determined by differential scanning calorimetry (DSC). All of the samples were heated from −30 to 275 °C at a rate of 10 °C/min under a nitrogen atmosphere to eliminate the thermal history, cooled to −30 °C, and then finally heated to 275 °C. The first cooling cycle was used to obtain data displayed in Figure for Polymers 1–4 (see Figure S12 for DSC of Monomers 1–4). Based on DSC thermograms, Polymer 1 presented the highest Tg of the four samples at 201.2 °C due to lack of spacers connecting rhodamine to the polymer backbone, while Polymer 3 presented a Tg at 116.1 °C due to the presence of spacers on one side of the rhodamine molecule connecting to the polynorbornene backbone, which imparted some flexibility and mobility to the polymer chains. Polymer 2 did not show any Tg due to the lack of spacers between bifunctional rhodamine and polynorbornene chains as well as the presence of cross-linked network structure leading to decreased flexibility and restricted chain mobility, while Polymer 4 presented the lowest Tg at 95.5 °C due to the presence of spacers wherein the flexibility of the spacers overcome mobility restrictions due to the network structure.

Figure 2

DSC thermograms of the first cooling cycle exhibiting the thermal transitions of Polymers 1–4. Polymers 1, 3, and 4 showed glass transition temperatures, whereas Polymer 2 did not show any thermal transition.

Mechanochromic Properties

Polymer samples in the powder form were loaded into a porcelain mortar, and the powders were abraided using a porcelain pestle and the change in color was noted for each polymer (Figure ). Notably, Polymer 1 showed a much slighter change on abrasion as compared to its methylene-spacer-modified analogue Polymer 3, which changed from gray to light pink. The differences between the non-spacer-modified and spacer-modified cross-linked analogues, Polymer 2 from white to red and Polymer 4 from white to dark pink/purple, are more amplified compared to Polymers 1–3.

Figure 3

Photographs showing the mechanochromic behavior of Polymers 1–4: (a) Comparing the mechanochromic behavior of Polymers 1–4 as observed on abrading them in a porcelain mortar with a porcelain pestle. (b) Reversibility of mechanochromism by abrading Polymers 1–4 on a paper and restoring the original color by heating.

Typically, reversible color change in rhodamines originates from the isomerization of the twisted spirolactam (SP) in the ring-closed form to a planarized zwitterionic structure in the ring-open state. However, when present in a polymer chain, some spirolactam rings of the rhodamine molecule will open and some will remain closed upon application of the stimulus. This will impact the color intensity associated with ring-opening/closing observed on the application of the stimulus. We included images of the color change associated with the application of different stimuli with the understanding that it is a completely qualitative analysis. However, to quantify color intensity differences before and after the application of a stimulus, abrasion in this case, for each polymer containing rhodamine, ImageJ histogram software was used and the data analyzed. As seen in Figure , Polymer 2 showed better mechanochromic features due to the lack of spacers as compared to Polymer 4.

Figure 4

Color intensities of Polymers 1–4 before and after abrasion treatment analyzed by ImageJ software. The raw histogram data used to analyze color intensity changes before and after abrasion of Polymers 1–4 is shown in the Supporting Information file (ESI).

To test the reversibility of the mechanochromic behavior, the powder samples of Polymers 1–4 were abraided on paper using a porcelain pestle. The resulting colors and the time taken for the reversal of the abrasion-induced color changes upon heat treatment at 100 °C were noted. Polymer 2, for example, was not completely restored to its original color after 15 min heat treatment, while Polymer 4 showed full reversibility and its original color was fully restored within 1 min. A similar trend was observed in the comparison of Polymers 1 and 3 (Figure S24). These observations can be attributed to the molecular structure of Polymer 4, wherein the SP ring is covalently confined within a cross-linked network structure with sufficient coupling to the polymer backbone due to 10 carbon methylene spacer that enables reversible mechanochromic behavior from the SP to MC form. Furthermore, the covalently conjugated SP ring should be present in a conformation in Polymer 4 such that the reversible SP to MC form under mechanical stress is feasible. The methylene spacers in Polymer 4 rendered the polymer flexible due to which it was easy and fast for the ring-opened dye to switch back to its original color on heating, which was not observed in the case of Polymer 2 that lacked methylene spacers. Based on these results, we concluded that Polymer 2 presented better mechanochromic features due to the lack of methylene spacers compared to Polymer 4. However, Polymer 2 showed partly reversible mechanochromism upon heat treatment, while Polymer 4 showed complete reversible mechanochromism upon heat treatment. In sharp contrast, the color change of free pristine HO-Rh-OH dye obtained upon abrasion is irreversible (Figure S13).

Piezochromic Properties

While analyzing the piezochromic properties of Polymers 1–4, a pressure of 500 MPa was used to treat all of the four polymers as the ring-opening spirolactam unit was maximized at 500 MPa pressure as observed in previous literature.[8] Powders of each polymer were placed into a pellet pressing die, which was in turn placed into a Carver press and 500 MPa of pressure was applied. The polymer film thus formed was carefully removed and the color change upon film formation was recorded (Figure S27 Supporting Information). Under these conditions, Polymer 1 did not show any change in color and the film was very brittle. Polymers 2–4 showed piezochromic properties on the edges of the film due to their architectures that favored the ring-opening reaction of the spirolactam upon application of pressure. The cross-linked Polymer 2 exhibited a red color, noncrosslinked Polymer 3 showed a light pink color, and cross-linked Polymer 4 showed a purple color. These results were qualitatively in agreement with the mechanochromic properties exhibited by Polymers 1–4 upon abrasion. Similar to the abrasion treatment, the piezochromic results were also quantified using ImageJ software by analyzing the before and after images of polymer samples upon application of 500 MPa pressure. As seen in Figure , both Polymers 2 and 4 showed similar changes in color intensities before and after the application of 500 MPa.

Figure 5

Color intensities of Polymers 2–4 before and after the application of 500 MPa pressure analyzed by ImageJ software. Polymer 1 did not show any piezochromic response.

Photochromic Properties

The photochromic properties of Polymers 1–4 were tested using UV light (365 nm). The powder samples of polymers were placed on a white sheet of paper and exposed to UV light for 10 min and the color changes were recorded (Figure ). Under these conditions, Polymer 1 showed a change in color to faint orangish-red color, Polymer 2 showed a change in color to reddish color, Polymer 3 showed a change in color to faint orangish red, and Polymer 4 showed the most change in color intensity before and after UV treatment and turned to a reddish-purple color. Quantitative analysis was carried out using ImageJ software and the results can be seen in Figure . It was concluded that Polymer 4 showed better photochromic behavior due to the presence of methylene spacers on both sides of rhodamine. In comparison, pristine HO-Rh-OH dye showed a faint photochromic color change from white to very light pink (λmax ∼ 550 nm) (Figure S13). Furthermore, upon heat treatment (100 °C), Polymer 4 showed complete conversion to the original closed form of the spirolactam ring. In contrast, except Polymer 4, other polymers as well as pristine HO-Rh-OH did not present complete reversibility to the closed spirolactam ring upon heat treatment.

Figure 6

(a) Photographs of photochromic behavior of Polymers 1–4. (b) UV–vis absorbance of solid powder Polymers 1–4 after irradiation by 365 nm UV light for 10 min. (c) UV–vis absorbance of solid powder Polymers 1–4 before irradiation by 365 nm UV light.

Figure 7

Color intensities of Polymers 1–4 before and after UV irradiation of Polymers 1–4 analyzed by ImageJ software histograms.

The UV–vis absorption spectra of polymer films were recorded. The absorbance band feature around in the 500–550 nm indicates that the color likely arises from spirolactam ring opening (absorbance of a free ring-opened form of the dye HO-Rh-OH is ∼550 nm), whereby the shifted peaks arise from a conformational modulation of the ring-opened form within the steric constraints of the polymer. A red shift in the absorption peaks was observed from ∼500–530 nm in the order of Polymer 1 < Polymer 3 < Polymer 2 < Polymer 4. Cross-linking in the presence of the two methylene spacers helped to induce the ring-opening reaction of spirolactam ring in more rhodamine molecules within Polymer 4 than in the other polymers. In general, the photochromic behavior was also impacted by cross-linking and the presence of methylene spacers in polymers. Due to the most intense reddish-purple photochromic color change and very good reversibility upon heat treatment, we find that Polymer 4 had the most optimal architecture for the reversible photochromic activity among the polymers tested here.

Thermochromic Properties

Colorless powders of Polymers 1–4 were individually sandwiched between Kapton films and compression molded by ramping the temperature from 25 °C to a temperature where a color change was observed at a ramp of 5 °C and maintained for 15 min at each temperature. Polymer 1 did not show any significant color change on increasing the temperature and ultimately degraded into black char. Polymer 2 showed a color change to brown at 330 °C but did not form a film (Figure . Polymer 3 did not show any significant color change on increasing the temperature and ultimately degraded into black char. In sharp contrast to Polymer 2, Polymer 4 presented a change to red color at a lower temperature of 215 °C and was also able to form a film at this temperature. On quantifying the thermochromic results using ImageJ software, it was further concluded that Polymer 4 showed better thermochromic behavior due to the presence of methylene spacers on both sides of rhodamine as compared to Polymer 2 (Figure ). The UV–vis absorbance of the Polymer 4 film at 215 °C showed an absorption peak at 500 nm that indicated the presence of the opened form of spirolactam molecule (Figure ). The thermochromic behavior of Polymer 4 was irreversible and maintained the red color of the opened spirolactam form. In comparison to Polymers 1–4, pristine dye HO-Rh-OH showed an irreversible color change to dark pink at 215 °C (Figure S13). Furthermore, Monomers 1–4 do not show thermochromic behavior (Figure S14).

Figure 8

(a) Photographs showing the thermochromic behavior of Polymers 2 and 4. (b) UV–vis absorbance of the Polymer 4 film at 25 and 215 °C. UV–vis absorbance of the Polymer 4 film at 215 °C showed a peak at 500 nm.

Figure 9

Color intensities of Polymers 2 and 4 before and after heating at 330 and 215 °C, respectively, using ImageJ software.

The presence of flexible methylene spacers is essential for the thermochromic feature of Polymer 4. These spacers help to link the spirolactam ring with the polynorbornene backbone while still endowing the dye molecule and the polymer chains with some mobility, which also enables the processing of the powdered samples into compression molded films.

Impact of Molecular Engineering of the Monomer on the Polymer Structure and Properties

To enable multiresponsive behavior in polynorbornene-based rhodamine derivatives, two different structural elements need to be molecularly engineered in the monomer-bearing rhodamine. These include the following: (1) presence of spacer(s) between rhodamine and norbornene, and (2) two norbornenes attached at either end of HO-Rh-OH. These two structural elements of the monomer manifest into polymers containing rhodamine with or without spacers between rhodamine and norbornene as well as noncrosslinked or cross-linked/network systems. These structural elements in the monomers directly show marked differences in the architecture and stimuli-responsive properties of polymers.

Monomers 1–4 show a very faint color change with UV compared to Polymers 1–4, and this may be due to aggregation-induced quenching that is more common in small molecules compared with polymers.[50−52] At room temperature, HO-Rh-OH and Monomers 1–2 exist as solids while Monomers 3 and 4 exist as viscous liquids or waxy solids. Moreover, HO-Rh-OH and Monomers 1–4 lack mechanical integrity compared to Polymers 1–4. Monomers 1–4 show negligible color change upon abrasion, application of pressure, and increase in temperature. Hence, there is a need to synthesize and use the polymeric version of these dye molecules for stimuli-responsive applications.

The stimuli-responsive properties of Polymers 1–4 are not only distinct from one another but also different from HO-Rh-OH (Figure . Polymers 2 and 4 presented more reliable color change due to the ring opening of SP molecules upon application of mechanical force, pressure, UV light, and temperature compared to Polymers 1 and 3. More importantly, Polymer 4 showed reversible stimuli-responsive behavior upon heat treatment, compared to Polymers 1–3 and pristine rhodamine molecules. We attribute this thermoreversible stimuli-responsive behavior of cross-linked Polymer 4 to the presence of methylene spacers on both sides of rhodamine, which allows for some mobility of polynorbornene chains as well as the decoupling of the polymer chain dynamics from that of rhodamine molecules as compared to Polymer 2. The cross-linked structure is integral for the mechano and piezochromic behavior of Polymer 4. However, the decoupling of the polymer chain dynamics from rhodamine molecules allows the stimulus to ring open the SP, while chain mobility enhances the opened SP ring to find its complementary unit to ring close upon heat treatment.

Figure 10

Polymer 4 that exhibited mechanochromic, photochromic, and thermochromic behavior, followed by ring-opening mechanism of rhodamine dye on mechanical, UV, and heat stimuli. Blue lines in the structure are shown to highlight 10 carbon methylene spacers present in Polymer 4.

Conclusions

The architectures of rhodamine dye HO-Rh-OH incorporated monomers were designed by (1) varying the length of methylene spacers and (2) number and point of attachment of polymerizable norbornene in monomers. The molecular structure of Monomer 4 with 10 carbon methylene spacers terminated by polymerizable norbornene on both the sides of the dihydroxy bifunctional dye HO-Rh-OH was essential in producing reversible stimuli-responsive features in Polymer 4. This molecular structure of Monomer 4, compared to other monomers, afforded optimal balance of covalently conjugating the rhodamine dye with the polynorbornene backbone, which is important for reversible ring-opening and closure of the spirolactam ring, while still retaining some flexibility and mobility to undergo reversible and fast ring-opening and closure even in the cross-linked polymeric state. Furthermore, covalently embedding the dye molecule within a polymeric framework, for example in Polymer 4, not only provided reversible stimuli-responsive properties but also enhanced processability and film-forming properties owing to the thermal and mechanical attributes of a macromolecular network structure compared to Polymer 2. The stimuli-responsive behavior of Polymer 4 is in sharp contrast to the pristine rhodamine dye, HO-Rh-OH, which was not covalently linked to polynorbornene backbone and was unable to undergo reversible ring-opening and closure with the application of various stimuli. Thus, simply tuning the structure at a molecular level of the dye-containing monomer could amplify and improve the stimuli-responsive properties of the dye-containing polymer at a macroscopic level for creation of sensory devices.

Experimental Section

Materials

Materials were used as received: 2-(4-diethylamino-2-hydroxybenzoyl)benzoic acid (98%, TCI America), resorcinol (1,3-benzene diol, 99%, Sigma-Aldrich), trifluoroacetic acid (TFA, 99%, Sigma-Aldrich), ethanolamine (98%, Sigma-Aldrich), anhydrous ethanol (99.5%, Sigma-Aldrich), 5-norbornene-2-carboxylic acid (NBCOOH, mixture of endo- and exo-isomers, 98%, Sigma-Aldrich), N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC, 98%, Proteochem), hydroxybenzotriazole (HOBt, 97%, Sigma-Aldrich), 11-bromoundecanoic acid (99%, Sigma-Aldrich), potassium carbonate (K2CO3, 99%, Sigma-Aldrich), anhydrous dimethylformamide (DMF, 99%, Acros Organics), dry methylene chloride (CH2Cl2, DCM, 99.8%, Acros Organics), anhydrous tetrahydrofuran (THF, 99.8%, Acros Organics), triethylamine (99.5%, Sigma-Aldrich), pyridine (99.8%, Acros Organics), second-generation Grubbs catalyst ((H2IMes)(pyr)2(Cl)2RuCHPh (mG2), 98%, AK Scientific), and ethyl vinyl ether (EVE, 99%, Acros Organics). Rhodamine HO-Rh-OH was synthesized using a literature protocol.[8]

Materials Characterization

Characterization of Monomers and Polymers

1H NMR spectroscopy was performed on a Bruker DMX 400 MHz NMR spectrometer in dimethyl sulfoxide (DMSO)-d6, CDCl3, CD2Cl2, and C4D8O (THF-d8) at room temperature. Mass spectra were obtained on a Micromass Quattro-II triple quadruple mass spectrometer equipped with an electrospray ionization (ESI) mode source. Gel permeation chromatography (GPC) of polymers was performed using a Waters 1515 instrument, coupled with a PL-ELS1000 evaporative light scattering (ELS) detector and a Waters 2487 dual-wavelength absorbance UV–vis detector, using tetrahydrofuran (THF) or dimethylacetamide as eluents, and polystyrene (PS) standards for constructing a conventional calibration curve. The number average molecular weight (Mn) and the dispersity (ĐM) of polymers were reported. ATR-IR studies were carried out on a Nicolet Magna 560 FT/IR Specac Quest–single reflection diamond crystal ATR instrument, at 32 scans, with a resolution of 4 cm–1.

Thermogravimetric analysis (TGA) was performed using a Q500 TGA analyzer (TGAQ-500, TA Instrument, New Castle). About 10 mg of monomers/polymers were placed in a platinum pan and heated from room temperature to 700 °C at a rate of 10 °C min–1 in nitrogen to determine the degradation temperatures of monomers and their polymers.

Differential scanning calorimetry data (DSC) of polymers was obtained using a TA-2920 instrument (Q-100 series), calibrated with an indium standard; 5–10 mg of sample quantities were tested at scanning rates of 10 °C/min; phase transition temperatures were determined during the first cooling cycle and the data was analyzed using Universal Analysis software. UV–vis spectra of the monomers and polymers dissolved in DCM or THF were recorded using 1 × 1 cm2 quartz cells using a Cary 50 (Varian) instrument. Piezochromic experiments were performed on a Carver press at 500 MPa pressure and changes in the color of the polymers upon application of pressure were recorded. The before and after color intensities for all polymer samples on the application of different stimuli were digitally recorded and analyzed using ImageJ histogram software with three trials. The color intensities were then averaged and plotted for each polymer based on the stimuli given.