A Spiropyran-Doped Poly(methyl methacrylate) Matrix for Sensor Applications

Abstract

In this paper, the relaxation isomerization properties of a spiropyran-doped poly(methyl methacrylate) (PMMA) UV light-responsive dynamic material are presented. The polar liquid (acetonitrile) and solid (PMMA matrix) medium may contain two merocyanine isoforms of the selected spiropyran. A complex equilibrium of the colored and colorless spiropyran isoforms resulted in specific transformations of the absorption spectra during the relaxation process. Bands at 522 nm and 580 nm characterized the absorption of the fabricated dynamic material in the ground (non-activated) state in the visible range. The presence of colored and colorless isoforms of spiropyran incorporated into the PMMA matrix using Raman spectroscopy was revealed. The photosensitive polymer samples were irradiated with a UV LED (365 nm and 390 nm) under various temperatures (0–30 °C). Spectral changes within the relaxation process demonstrated their complex dynamics. Surprisingly, the photodegradation of the dynamic material was estimated at about 25% (using 390 nm) and 7% (using 365 nm) after eight photoswitch cycles for one isoform, and no fatigue after an additional one (using 390 nm) was discovered. The presented light-responsive material is of interest for UV sensors and new hybrid material design.

1. Introduction

In recent years, stimuli-responsive polymer materials have seen remarkable progress and development. These polymers are designed to react to specific external stimuli, such as light, temperature, pH, humidity, electrical or magnetic fields, pressure, or mechanical force, and undergo reversible changes in their physical or chemical properties [1,2,3,4,5]. Stimuli-responsive materials (often called smart or dynamic materials) are widely used in many fields, such as sensors, shape memory and thermochromic materials, optical devices, security documents [2], and advanced biomedical applications including drug delivery, tissue engineering, biosensing, and diagnostics [1].

Among stimuli-responsive polymers, light-responsive materials and systems are being rapidly developed for sensor applications [6,7,8,9,10]. These high-tech materials are often based on various photoswitchable molecules: azobenzenes, spirooxazines, chromenes, spiropyrans, diarylethenes, fulgides, polycyclic aromatic compounds, and others [11]. Spiropyrans are a very popular class of photochromic spiro compounds that undergo reversible structural transformations followed by color changes (absorption spectrum) when exposed to UV radiation. The possibility of observing two spiropyran isomers—ring-closed (SPs) and ring-opened merocyanines (MCs)—makes spiropyrans unique among the mentioned photoswitchers with vastly different physical and chemical properties [5]. Figure 1 illustrates the typical process of a spiropyran’s transformation from the SP to the MC and reverse under heating (T) or illumination with visible light (VIS). The SP and MC states differ significantly in their structure, with SP occupying less volume than MC, SP being optically transparent in the visible region, and MC having a deep purple color due to the strong absorption of visible light in that specific spectral range [5]. The formation of an MC significantly depends on the nature and position of substituent groups in the benzopyran ring and the properties of the surrounding medium [2,12]. A difference was also found in the forward and reverse isomerization rates of SPs and MCs [13].

Experimental [14,15,16] and simulation [17] data indicate the presence of several merocyanine isoforms (at least more than one) in polar solvents, even without irradiation with ultraviolet radiation. The possibility of tunable reverse photochromism in polymer materials using commercially available dyes has been reported. Taking application requirements into account, the proper selection of the photochromic compound and surrounding medium may allow for tuning photochromic phenomena dynamics (decoloration times from seconds to hours) [15]. The resulting properties of the colored state are based on the equilibrium between various MC forms, in particular the non-protonated MC isoform with low-energy visible-light absorption and the protonated MC isoform with high-energy visible-light absorption [14,15]. The results of calculations indicate a complex excited-state dynamic behavior with at least four different transformation pathways, and the ring-opening process can also occur in the excited state [17].

Despite the widespread use of spiropyrans as photochromic compounds, their main problems are low stability and high degradability, which have motivated numerous experiments on their incorporation into a polymer matrix [5]. Various approaches to produce photoswitcher-bound polymers have been developed, and the polymerization of photochromic molecule-based monomers and grafting on pre-formed polymer chains have mainly been employed for these purposes [5]. The free radical polymerization method results in the covalent incorporation of spiropyran molecules into the chain [18]. The advantages of the covalent anchoring of the spiropyran units into the polymer matrix are high photostability and photofatigue resistance without dye aggregation or photobleaching [19,20]. Many polymer matrices are available for the production of photosensitive materials: various polymethacrylates, polyvinyl acetate (PVA), cellulose acetate (CA), polystyrene (PS), and others [12,16,21]. PMMA is widely used in sensor design and fiber optics technology [22,23]. A PMMA matrix doped with spiropyrans is one of the most prominent photochromic optical sensor materials due to the synergy of the high transparency and thermoplasticity of polar polymers and the pronounced isomerization of the filling photochromic compounds [2,24].

The fast development of research into photoresponsive materials calls for even more intensive work targeted at obtaining dynamic materials with the following required properties: photo- and thermostability, photoreversibility or irreversibility, controlled photodegradation, multi-responsibility, rapid detection, and photosensitivity at specific wavelengths [25,26]. For this purpose, various photoswitchable hybrid materials should be investigated. The development of hybrid materials (quantum dots (QDs), nanoparticle (NP)-doped photochromic polymers) is also a major challenge that has the potential for realization. QDs are nanostructural semiconductor materials that have attracted the attention of researchers with their unique properties, such as their broad absorption range, spectral purity, photochemical stability, and controllable dimensions, allowing light emission to be tuned within the entire visible range (the so-called quantum size effect) [27]. Polymer nanocomposites with embedded QDs allowing for high quantum yields have demonstrated significant advantages in experimental studies [28,29]. Moreover, nanomaterials with NPs have applications in the development of biological and chemical sensors, the design of miniaturized devices and microchips, and analytical metrological approaches [30,31].

A wide range of dynamic materials is becoming of increasing interest in terms of their applications as optical sensors [32,33,34,35,36,37]. Designing light-responsive optical fibers for optoelectronic applications (photoswitchers, optical data storage, holographic recordings, 3D imagining, UV irradiation sensors, active optical fibers—all sensitive to environmental conditions) using such materials has the advantages of enhanced sensitivity and fast response time to light [2,38,39]. Such outstanding properties can be achieved with a high ratio between the fiber surface and its volume.

2. Materials and Methods

Chemicals—monomer methyl methacrylate (MMA), benzoyl peroxide (BP), spiropyran (1′,3′-Dihydro-8-methoxy-1′,3′,3′-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2′-(2H)-indole])—were supplied by Sigma-Aldrich at 99% purity and were used without further purification. The monomer stabilizer agent was removed before the polymerization process.

Free radical polymerization [22] was performed in 24 h (temperature 60 °C to 80 °C). Three 12 mm outer diameter PMMA rods were fabricated (sample C: pure polymer; samples A and B: spiropyran concentrations of 1.53 and 0.62 mM, respectively). No visible polymerization defects were observed in the fabricated samples. The specimens were cut and polished into 6.0 mm thick rod discs (Figure 2). The absorbance was measured using a halogen Stellarnet SL1 lamp and Stellarnet Green Wave spectrometer in the range of 300 to 800 nm and at 0.5 nm resolution. Raman spectroscopy was performed with a Lambram 800 HR spectrometer equipped with a Nd-YAG laser of 532 nm wavelength (the laser power was 3 mW). All measurements were taken in the dark. The specimens were exposed to UV LEDs (with emission peaks at 365 nm and 390 nm) at irradiance of 59 W/m² for 2 min. Temperature was controlled using a 44.7 W Peltier element TEC1-127050 (Hirschau, Germany) and STC-1000 Thermostat controller Haswill Electronics (Xuzhou, China).

3. Results and Discussion

Some of the spectrophotometric and kinetic properties of the spiropyran (Figure 1) with substituents (methoxy and nitro groups) in the benzopyran ring have been studied previously [40]. The absorbance spectra of the acetonitrile solution of the spiropyran (concentration, 3.8 μM) were recorded in a polar solvent at 20 °C (Figure 3) to compare the influence of the polymer matrix on the profile of the spectral absorption and the kinetics of the changes in the SP→MC isomerization process. As mentioned above, polar solvents have a stimulant effect on the opening of the SP ring, even in the dark [20]. Thermal processes in polar solvents lead to an equilibrium of the SP (colorless) and several of the MC (colored) isomers in the ground state [41], which depends primarily on the solvent polarity, the type of spiro compound, and its concentration [2]. The solution’s bright purple color, as well as its absorption spectrum recorded at room temperature before UV stimulation (Figure 3, *—spectrum), reveals the presence of a colored MC isoform attributed to the band at 580 nm, along with a short-wavelength band at 355 nm attributed to the colorless SP isoform. This finding is in full agreement with the known behavior of spiro compounds in polar solvents [40]. Illumination with UV light traditionally results in a single MC absorption band in the visible range centered at 550–600 nm, the exact location of which depends on relative contributions within the equilibrium of the MC isoforms and the subsequent decoloration process also followed by the single absorption band [5]. The other experimental data provide an equilibrium between the protonated and non-protonated MC forms, resulting in the band centered at 528 nm [15]. The color of the selected spiropyran solution after exposure to UV irradiation (390 nm) for 2 min becomes deep purple, with an inhomogeneously broadened absorption band centered at 568 nm (Figure 3). The time dynamics of the subsequent phototransformations (Figure 3) clearly show the presence of more than one absorbing center. The decomposition of the mentioned band using two Gaussian curve approximations provides two bands at 524 and 580 nm. The presence of the mentioned bands corresponds to the high-energy protonated MC and low-energy non-protonated MC isoforms in the visible light absorption spectra, respectively [14,15]. The presence of more than one MC isomer in polar solutions of spiropyrans has also been shown [42]. Non-polar solvents often lead to the formation of MC aggregates (mainly dimers), which result in two well-resolved bands in the absorption spectrum in the range of 500–600 nm (the short- and long-wavelength bands are attributed to dimers and monomers, respectively) [5,25,43]. In our experiments, the polar solvent acetonitrile led to the presence of two colored MC isomers [44]. After the cessation of UV irradiation, the decoloration rate for each of the mentioned MC forms was different (Figure 3). So, while the long-wave band dominates in the beginning, the short-wave band becomes dominant in the absorption spectra after just 1 min in the dark. Thus, the selected spiropyran in polar solvent shows a more complicated decoloration process and is consistent with the results [17].

Exposure of the SP to UV irradiation resulted in the cleavage of chromene’s moiety C–O bond, leading to the MC isoform increasing the dipole moment of the photochrome from ~7.5 D (SP) to ~13.6 D (MC) [45]. Reverse isomerization occurs spontaneously but can also be stimulated by heating or visible light exposure [46]. MC isoforms, being of an ionic nature, can interact effectively with a polar polymer matrix [12]. PMMA chains can stabilize one or more MC isoforms. MC, via dipolar or ionic interactions, leads to an equilibrium between SP and MC isoforms in the same matrix even at ambient conditions or in the dark [47].

Due to significant structural transformations during spiropyran photoisomerization, the polymer matrix significantly affects the SP → MC (higher) and reverse (lower) switching rate in polar PMMA [12].

To obtain a new photoswitching material, the selected spiropyran was incorporated into the PMMA matrix utilizing the free radical polymerization procedure. Two samples of different concentrations and a pure polymer reference sample were prepared. Typical absorption spectra in the visible range for sample A (Figure 2, Section 2) before and after 2 min of UV irradiation are presented in Figure 4.

The as-prepared sample A (before UV irradiation) has a yellow color (Figure 2), and its absorption spectrum clearly shows two bands at 522 and 580 nm (after decomposition with two Gaussian curves), with more pronounced absorption at a low-wavelength peak. Most likely, the PMMA polymer chains have stabilized two MC isoforms even before UV stimulation. A similar result was reported for another spiro compound [12]. Two colored MC isoforms in the polymer matrix were also reported [16]. One can compare the absorption spectra of the spiropyran in the two polar mediums: acetonitrile solution (Figure 3) and PMMA matrix (Figure 4). During the isomerization processes in both media, absorption bands with maxima at 522/524 nm and 580 nm are present. The ground state of the spiropyran in acetonitrile is characterized by only one band at 580 nm in the visible range, attributed to the non-protonated MC isoform (Figure 3, *—spectrum), while in the PMMA, both bands are present, indicating the presence of both non-protonated and protonated MC isoforms (Figure 4, *—spectrum). One can assume a higher mobility of the molecules in the liquid medium. The interactions between MC isomers usually attributed to the second absorption peak are not relevant to our case because they play only a negligible role in the polar polymer matrix [48], and after a significantly long relaxation time, only the one peak attributed to MC stacks (H-aggregation) can be observed [5].

Immediately after UV exposure, a single absorption band in the visible spectrum range at 600 nm is observed (Figure 4). The spectrum obtained after 5 min in the dark at 20 °C shows an obvious and inhomogeneously broadened absorption band splitting into two maxima with respect to the time of the decoloration process. From that moment on, very specific behaviors of the spectra are observed: the long-wavelength band initially centered at 615 nm undergoes a hypsochromic shift to 580 nm, while the band at 550 nm downshifts to 522 nm (as decomposed with two Gaussian curves). This suggests that the appearance of the non-linear absorption spectra changes because of varying equilibrium conditions resulting from additional isomerization processes due to the UV irradiation of the protonated MC Z/E-isomers [12,49,50]. The reverse switching rate is different for the mentioned isoforms and is much higher for the one initially centered at 615 nm because of the different contributions of both components in terms of intensity with respect to the time. It should be noted that the relaxation time in the dark from the deep-purple-colored MC to the yellow-colored initial non-activated state is very long, taking up to 24 h. Therefore, the long relaxation time [15] is attributed to the steric hindrance and the small free volume present between the polymer chains of the matrix for the covalently incorporated spiropyran spatial transformations [20], especially in comparison with associations between non-covalent dyes [5]. Thus, after sufficiently long decoloration times, two peaks at 522 and 580 nm can be observed again.

The complex behavior of the decoloration dynamics of the photosensitive polymer necessitated the use of Raman spectroscopy for vibrational analysis of the samples without UV irradiation (Figure 5).

The Raman spectrum of the pure PMMA was obtained from an open access database [51], while the spectrum of the spiropyran-doped PMMA was measured by the authors. The independently recorded spectra (Figure 5) are in very good agreement with each other and demonstrate the same characteristic vibrational bands of PMMA. Very similar relative intensities of the bands can be seen in both spectra. The observed spectra are in fairly good agreement with those available in the literature [52,53]. The most interesting part of the spectra is presented in the inset. Obviously, the bands at 1602 and 1640 cm⁻¹ are presented in spectrum 2 but are missing in spectrum 1 (Figure 5). According to the assignments of the Raman spectra of the pure PMMA, there are no characteristic bands in that range [52,53,54]. The observed peaks correlate with those for the spiropyrans being analyzed after comparing the calculated spectra and those presented in the literature [55,56]. The very specific marker vibrational band at 1640 cm⁻¹ corresponds to the SP isoform and is assigned to the C=C stretching vibration of the pyran ring. The vibrational band at 1602 cm⁻¹ corresponds to the MC isoform and is assigned to the C–C stretching vibration. The presented features of the vibrational spectrum confirm the presence of both colorless SP and colored MC isoforms stabilized by the polymer chain in the samples. The Raman measurement results do not indicate the presence of several MC isoforms.

It is known that the MC → SP decoloration reaction accelerates with increasing temperature [12,57], and it is a very important property for practical applications of polymer materials. The MC → SP decoloration dynamics for the tested photosensitive materials (samples A and B) under temperatures of 0, 10, 20, and 30 °C using a 390 nm UV LED are presented in Figure 6a,b.

Because of the non-linear decoloration dynamic of the samples as well as the much more pronounced intensity changes in the absorption spectra of the long-wavelength band, it was decided to follow the absorbance values at 580 nm related to the initial band position (before UV irradiation). The relative absorbance difference was chosen as the decoloration parameter ∆ [58] in consideration of the recorded absorption spectra with respect to the undoped polymer reference sample C (Figure 2):

$$∆=\frac{A_0-A}{A},$$

(1)

where A₀ and A are the initial (immediately after UV irradiation) and final (after 30 min in the dark) absorbances of the samples at 580 nm, respectively.

A significant effect of the temperature on the reverse switching rate for both samples A and B was observed (Figure 6). Thereby, very slow and mutually similar MC → SP changes were observed for the samples of both concentrations at 0 °C. Decoloration occurred at 10, 20, and 30 °C for sample B, showing faster dynamics when compared with sample A which had a higher spiropyran concentration. The most noticeable difference in rates is observed at the highest studied temperatures of 20 and 30 °C. It is likely that the amplitude of thermal vibrations influences the MC structure to change faster. Despite the observed differences, the general character of decoloration temperature dependencies for both concentrations is similar. Using a 365 nm UV LED shows the same character of the decoloration dynamics with respect to the temperature (results are not presented).

The effective interaction between ionic MC and polar PMMA provides rather long thermal-decoloration times. The glass transition temperature T_g of pure PMMA is estimated at about 100 °C and slightly increases with spiropyran doping [12]. Temperatures in our experiments are much lower than glass transition T_g or T_m melting temperatures (usually T_g < T_m). One can note that, among other polar matrices, PMMA’s comparatively high T_g results in a lower molecular mobility that also affects the rate of the thermal relaxation process.

The variations in the absorption of the samples are not linear; this may suggest that the thermal decoloration mechanism of the fabricated dynamic material is not a first-order process. There are two colored MC isoforms (protonated and not-protonated MC, as previously mentioned), and our results correlate fairly well with those in the literature [16].

The photoreversibility, along with the photofatigue resistance, of photosensitive polymer materials was also investigated. Sample A was tested following repetitive cycles of coloration/decoloration in the dark. Irradiations with UV light for 2 min were performed using 365 nm and 390 nm UV LEDs, with subsequent thermal relaxation at 20 °C and evaluated via spectrophotometry. The decoloration parameter ∆ (as used before, see Formula (1)) was applied within each cycle to demonstrate the differences between the initial and final absorbance values at 522 nm and 580 nm [58]. The results for sample A are presented in Figure 7.

Using two LEDs for the sample excitation with wavelengths of 365 nm and 390 nm led to unexpected results, as demonstrated by the significantly different dependencies in Figure 7. According to [5], the photoswitching abilities of the spiropyran immobilized in the PMMA matrix vary from significant fatigue after only several isomerization cycles up to an induced degradation of about 21% after ten switching cycles, and in some cases, even more stable samples were obtained. Our results show different thermal relaxation dynamics in the dark after irradiation with 365 or 390 nm wavelengths at both 522 and 580 nm. Irradiation of sample A at 390 nm led to slight changes in the shape of the absorption bands during the first four cycles. Although subsequent cycles reveal more pronounced spectral shape changes, the photochromic effect could be detected at least up to the eighth cycle, wherein the initial photoswitching abilities of the tested sample partly degrade. Our photosensitive material demonstrates two MC isomers attributed to the absorption peaks at 522 nm and 580 nm, with different degradation dynamics. The long wavelength attributed to the MC isomer degrades by about 25% within eight cycles, and it is comparable with previously reported data [5]. Moreover, the short wavelength attributed to the MC isomer shows less pronounced but more stable and repeatable photoswitching behavior when compared with the long-wavelength one (Figure 7). It can be assumed that excitation at 390 nm provides repetitive switching, with only one of two MC isoforms being differently stabilized by the polymer chains of PMMA. Surprisingly, excitation at 365 nm does not significantly affect the absorption spectra’s shape even after eight cycles, in contrast to the previous case. Some minor changes in the spectra’s shape can be observed due to slight photofatigue. The estimated photodegradation of the sample at 580 nm is about 7%. The sample switching abilities at 522 nm decrease up to the fourth cycle and remain stable thereafter.

To figure out the possible structural changes in the photosensitive polymer material, Raman measurements were conducted for the sample (type A) using different degrees of photochromic degradation (390 nm UV LED used). The vibrational spectra of all the samples were almost identical (Figure 5, spectrum 2). No visible structural changes (appearing or disappearing bands; changes in relative intensity of the bands) were observed.

4. Conclusions

Polar PMMA results in the stabilization of several SP and MC isoforms of the selected spiropyran being covalently incorporated into the polymer chain, activated by both UV irradiation and non-activated states. The absorption spectra of the fabricated dynamic material are the result of the complex equilibrium between SP and protonated and non-protonated MC isoforms, reflecting the complex nature of isomerization dynamics. Polar acetonitrile solvent results in the presence of the only non-protonated MC isoforms in the ground state.

The investigated photosensitive PMMA material is identified in the absorption spectrum by two bands at 522 nm and 580 nm attributed to the MC isoforms in the ground state with non-linear thermal relaxation dynamics after activation via UV irradiation in the dark.

The fabricated material shows reversible transformations of its photochromic properties in the range of 0–30 °C. Two samples of different spiropyran concentrations demonstrate similar dynamics of the relaxation transformations. Different relaxation rates for the mentioned MC isomers were observed. Those rates accelerate with temperature within a relaxation period of 30 min, differing by about 8 and 12 times (samples A and B, respectively) at 0 and 30 °C.

Surprisingly, the same photosensitive material may exhibit different photodegradation dynamics under UV excitation at different wavelengths. The acceptable photodegradation of the photochromic properties of the material at 580 nm is up to 25% after eight photoswitching cycles under 390 nm UV irradiation, while under 365 nm UV irradiation, photodegradation amounts to only 7%, wherein the shape of the absorption spectra remains unchanged.

The vibrational spectra of the samples with different degrees of photodegradation show no structural changes in the photosensitive material.

The properties of the featured functional material demonstrate that commercially available spiropyrans can be utilized to create dynamic polymers with multiple photochromic responses and exhibiting different isomerization kinetics.

The presented spiropyran-doped PMMA material demonstrates uniquely complex photochromic properties, with potential applications in UV optical sensor and fiber technologies, information storage devices, and imaging.