Infrared Imaging of Photochromic Contrast in Thiazolothiazole-Embedded Polymer Films

Abstract

The increasing demand for optical technologies with dynamic spectral control has driven interest in chromogenic materials, particularly for applications in tunable infrared metasurfaces. Phase-change materials such as vanadium dioxide and germanium–antimony–tellurium, for instance, have been widely used in the infrared regime. However, their reliance on thermal and electrical tuning introduces challenges such as high power consumption, limited emissivity tuning, and slow modulation speeds. Photochromic materials may offer an alternative approach to dynamic infrared metasurfaces, potentially overcoming these limitations through rapid, light-induced changes in their optical properties. This manuscript explores the potential of thiazolothiazole-embedded polymers, known for their reversible photochromic transitions and strong infrared absorption changes, for use in tunable infrared metasurfaces. The material exhibits low absorption and a strong photochromic contrast in the spectral range from 1500 ${\mathrm{cm}}^{-1}$ to 1700 ${\mathrm{cm}}^{-1}$, making it suitable for dynamic infrared light control. This manuscript reports on infrared imaging experiments demonstrating the photochromic contrast in thiazolothiazole-embedded polymer, and thereby provides compelling evidence for its potential applications in dynamic infrared metasurfaces.

1. Introduction

The growing demand for advanced optical technologies capable of the dynamic manipulation of spectral properties through external stimuli has spurred significant interest in chromogenic materials with tunable optical properties. These chromogenic materials can reversibly alter their optical properties in response to external stimuli, such as light, heat, electric or magnetic fields, mechanical stress, and chemical environments [1,2,3]. Applications for these chromogenic materials include high-density optical recording systems, tunable metasurfaces, tinted lenses, smart windows, memory devices, actuators, tunable filters, and imaging [4,5,6,7,8,9,10].

Among these applications, tunable optical metasurfaces are of particular interest. Metamaterials composed of engineered subwavelength structures interact strongly with light, allowing for precise control over its amplitude, phase, and polarization [11]. This enables the design and fabrication of compact optical devices, addressing the limitations of traditional bulky components [12,13]. Examples of metasurface applications include beam steering, planar lenses, holography, and narrowband and broadband filters [12,14,15,16].

In the infrared regime, phase-change materials like vanadium dioxide and germanium–antimony–tellurium have been utilized, where tunability is achieved through thermally and electrically induced metal-insulator phase transitions that result in significant changes in their optical properties [17,18]. However, these methods face several practical challenges, including high power consumption due to the need for continuous external power, a limited emissivity tuning range, and slow modulation speed [19]. In comparison with tuning using thermal and electrical stimuli, the optical modulation of metasurface components presents a direct and often rapid approach [20].

Such optical modulation can be achieved using photochromic materials, a class of chromogenic materials that exhibit reversible changes in their optical properties in response to optical stimuli [21,22]. These materials have been demonstrated to play a significant role in facilitating the development of tunable infrared metasurfaces through leveraging their light-induced changes in their optical properties [23]. The development of photochromic materials that exhibit strong and reversible changes in their optical properties in the infrared region could offer an alternative approach to achieving tunable infrared metasurfaces, potentially with advantages in terms of cost, fabrication, and power consumption.

Viologens represent an important class of photochromic materials [24]. Their properties can be enhanced by incorporating a thiazolo[5,4-d]thiazole (TTz) fused, conjugated bridge, an approach that has attracted increasing interest due to its strong fluorescence, solution-processability, and reversible photochromic transitions. Notably, dipyridinium TTz viologens exhibit high-contrast, rapid, and reversible photochromic changes when integrated into a polymer matrix. Upon exposure to radiation with energy exceeding 2.8 eV, they undergo a color transition from light yellow (${\mathrm{TTz}}^{2+}$) to purple (${\mathrm{TTz}}^{·+}$) and then to blue (${\mathrm{TTz}}^0$) due to two distinct photoinduced single-electron reductions. This reverse transition occurs through the reaction of the ${\mathrm{TTz}}^0$ state with molecular oxygen [25].

The optical properties of photochromic TTz-embedded in a polymer matrix have been reported in the visible, near-infrared, and infrared spectral ranges [25,26]. TTz-embedded polymers exhibit strong changes in the absorption bands in the infrared spectral range from 500 ${\mathrm{cm}}^{-1}$ to 1800 ${\mathrm{cm}}^{-1}$ due to the photochromic transition. In the narrow spectral range from 1500 ${\mathrm{cm}}^{-1}$ to 1700 ${\mathrm{cm}}^{-1}$, TTz-embedded polymers show low absorption and a strong photochromic contrast between its states before and after irradiation. These characteristics, specifically the photochromic contrast coupled with low absorption in this mid-infrared window, may be utilized for the development of dynamic metasurfaces. In this manuscript, the potential of TTz-embedded polymer as a promising material for dynamic infrared metasurfaces is investigated. Infrared imaging experiments demonstrating transmission contrast offer a strong initial validation of their effectiveness in the infrared spectral range.

2. Experiment

Dipyridinium TTz was synthesized through refluxing dithiooxamide and 4-pyridinecarboxaldehyde in dimethylformamide at 153 $·\mathrm{C}$ for 8 h. The resulting dipyridyl TTz was then treated with 3-bromopropyl trimethylammonium bromide in dimethylformamide to alkylate the pyridine rings, enhancing its water-solubility. Further details on the synthesis of dipyridinium TTz can be found in our previous publications [25,27]. To fabricate the TTz-embedded polymer hydrogel samples, 3.4 w% dipyridinium TTz was dissolved in a polyvinyl alcohol solution, followed by the addition of borax, as described in previous studies [27]. The viscous polymer/TTz mixture was blade-coated onto a mylar sheet (0.1 mm, 4 mil polyethylene terephthalate) and dried in ambient air for 24 h. After drying, the resulting polymer film was peeled from the mylar sheet to yield a free-standing film with a nominal thickness of 100 µm.

The TTz-embedded polymer sample was investigated using a Hyperion 3000 microscope (Bruker Inc.) in combination with a Vertex 70 Fourier-transform infrared spectrometer (Bruker Inc., Billerica, MA, USA). The Bruker Hyperion 3000 microscope utilizes a confocal beam path and allows for data acquisition in both transmission and reflection modes. The microscope is equipped with a broadband LED in the visible spectral range, while a silicon carbide globar serves as the light source in the infrared spectral range. The instrument features two infrared detectors: a single-element mercury cadmium telluride (MCT) detector and a 64 × 64 (4096 pixels) MCT focal plane array (FPA). The MCT FPA allows for the acquisition of hyperspectral infrared images in the spectral range from 900 ${\mathrm{cm}}^{-1}$ to 4000 ${\mathrm{cm}}^{-1}$. A shadow mask was employed to facilitate the spatially controlled exposure of the TTz-embedded polymer films using the microscope’s broadband white-light source. The patterned mask depicted in the inset of Figure 1a was fabricated using transparency sheets printed with the desired pattern. The pattern was composed of transparent and opaque regions to allow for the white-light exposure of the TTz-embedded polymer sample in desired areas.

Figure 1 illustrates the optical arrangement for visible-light exposure and imaging in transmission mode using the microscope. As depicted in Figure 1a, the shadow mask was placed in the collimated illumination path of the microscope before the condenser lens. The vertical position of the condenser was adjusted to ensure that the shadow mask plane and the sample plane were optically conjugate. This configuration enabled the condenser lens to project a 6× reduced image of the mask pattern on the TTz-embedded polymer sample with high spatial fidelity. After 10 s exposure, the white light source was turned off and the condenser lens was re-adjusted to restore the microscope to its confocal imaging configuration. A KRS-5 optical filter with a cut-on wavelength of 700 nm was introduced in the source side beam path before the sample. With the optical filter in place, unintended photochromic changes induced by the broadband visible light source during visible image acquisition were prevented. In this arrangement, the MCT FPA was employed to acquire infrared transmission images, while a commercial CCD camera array was used to record the photochromic contrast in the visible images, as shown in Figure 1b. The images were obtained at room temperature in ambient oxygen at four different x-y positions to capture the entire photochromic pattern using a high-accuracy, motorized x-y sample translation stage.

3. Results and Discussion

In order to ensure the acquisition of infrared images with observable photochromically induced changes, the identification of spectral bands that exhibit adequate dielectric contrast between the ${\mathrm{TTz}}^{2+}$- and ${\mathrm{TTz}}^0$-states of the TTz-embedded polymer is required. For imaging in the transmission configuration, it is also necessary to identify spectral regions that are sufficiently transparent. Spectroscopic ellipsometry measurements of the TTz-embedded polymer in the infrared spectral range before and after the photochromic transition revealed significant absorption in the spectral range from 500 ${\mathrm{cm}}^{-1}$ to 1800 ${\mathrm{cm}}^{-1}$ using the imaginary part of the complex dielectric function, where ${\epsilon }_2$ is typically above 0.1. Only in the narrow spectral window from 1500 ${\mathrm{cm}}^{-1}$ to 1700 ${\mathrm{cm}}^{-1}$ did the TTz-embedded polymer exhibit significantly lower absorption. In this spectral range, there was also a notable change of 0.05 in ${\epsilon }_2$ between its states before and after irradiation with a 405 nm diode laser source [26].

In order to further narrow the spectral band, which could provide sufficient contrast for transmission infrared imaging, unpolarized transmission measurements were performed on the TTz-embedded polymer before ($T^{{\mathrm{TTz}}^{2+}}$) and after ($T^{{\mathrm{TTz}}^0}$) the photochromic transition in the spectral range from 1300 ${\mathrm{cm}}^{-1}$–1900 ${\mathrm{cm}}^{-1}$. The transmission difference spectrum ($T^{{\mathrm{TTz}}^{2+}}-T^{{\mathrm{TTz}}^0}$) of these unpolarized transmission measurements is shown in Figure 2.

The unpolarized transmission-difference spectrum in the spectral range from 1300 ${\mathrm{cm}}^{-1}$ to 1900 ${\mathrm{cm}}^{-1}$ is dominated by two strong and narrow absorption lines centered around 1500 ${\mathrm{cm}}^{-1}$ and 1610 ${\mathrm{cm}}^{-1}$. For the demonstration of the infrared transmission imaging of photochromically induced changes in the TTz-embedded polymer, the strongest absorption line, located at 1610 ${\mathrm{cm}}^{-1}$, was selected. For the false-color infrared transmission images shown in Figure 3, the area of the absorption line located at 1610 ${\mathrm{cm}}^{-1}$ was integrated for the full-width half-maximum of the peak, i.e., from 1600 ${\mathrm{cm}}^{-1}$ to 1618 ${\mathrm{cm}}^{-1}$, as indicated by the shaded area in Figure 2.

Figure 3 presents the imaging results for both visible and infrared spectral ranges, captured with a commercial CCD camera array and a 64 × 64 pixel MCT FPA, respectively. The left panel shows a transmission image of the TTz-embedded polymer in the visible spectral range, where the background in the ${\mathrm{TTz}}^{2+}$ state contrasts with the recorded pattern in the ${\mathrm{TTz}}^0$ state.

The right panel of Figure 3 depicts a false-color representation of the narrowband infrared transmission contrast. The image was generated through the integration of the hyperspectral imaging data of TTz-embedded polymer over the full-width half-maximum of the peak located at 1610 ${\mathrm{cm}}^{-1}$, as indicated by the shaded area in Figure 2. It can be seen that the difference in the dielectric function between the ${\mathrm{TTz}}^{2+}$ and ${\mathrm{TTz}}^0$ states of the TTz-embedded polymer in the spectral range from 1600 ${\mathrm{cm}}^{-1}$–1618 ${\mathrm{cm}}^{-1}$ results in a notable contrast in the infrared transmission image. As shown in Figure 2, a 13% change in transmission can be observed at 1610 ${\mathrm{cm}}^{-1}$. This change in transmission can be projected as a spatial pattern, as demonstrated in Figure 3. The spatial resolution of the projected pattern achieved with the mask used here is approximately 50 µm. These results highlight the material’s ability to record spatial information, suggesting its applicability in tunable optical components. The ability to induce these optical changes through light exposure, without requiring high-power thermal or electrical control, positions TTz-embedded polymers as promising candidates for dynamic infrared metasurfaces.

However, a potential limitation of the TTz-embedded polymer is that it shows strong asymmetry between the time required to trigger the photochromic transition to ${\mathrm{TTz}}^0$ state and the time needed to revert to ${\mathrm{TTz}}^{2+}$ state under standard ambient conditions. This behavior arises from the strong dependence of the reaction kinetics on the surrounding environment [25]. Although advantageous for oxygen-sensing applications [25], this characteristic constrains the material’s applicability in dynamic metasurface systems.

4. Conclusions

This study explores the tunable optical properties of TTz-embedded polymer films for potential applications in infrared metasurfaces. The hyperspectral infrared imaging results reveal a significant photochromically induced transmission contrast in the mid-infrared range, demonstrating the material’s infrared optical modulation capabilities. Compared to conventional phase-change materials, TTz-embedded polymers could offer an alternative approach that does not require high power consumption or complex thermal control.

The TTz-embedded polymer exhibits low absorption and a dielectric contrast in the 1500 ${\mathrm{cm}}^{-1}$ to 1700 ${\mathrm{cm}}^{-1}$ spectral range, making it suitable for dynamic infrared light control. Unpolarized transmission measurements were conducted to further narrow down the spectral bands with sufficient dielectric contrast before and after irradiation, along with adequate transparency. The unpolarized transmission measurements reveal that the spectral range between 1500 ${\mathrm{cm}}^{-1}$ and 1700 ${\mathrm{cm}}^{-1}$ is dominated by two prominent and narrow absorption peaks centered at approximately 1500 ${\mathrm{cm}}^{-1}$ and 1610 ${\mathrm{cm}}^{-1}$. To maximize the photochromically induced transmission contrast during the infrared imaging experiment, the strongest absorption line, located at 1610 ${\mathrm{cm}}^{-1}$, was selected. The difference in dielectric function between the ${\mathrm{TTz}}^{2+}$ and ${\mathrm{TTz}}^0$ states of the TTz-embedded polymer in the spectral range from 1600 ${\mathrm{cm}}^{-1}$ to 1618 ${\mathrm{cm}}^{-1}$ produces a significant contrast in the infrared transmission image. These results highlight the material’s ability to record spatial information, suggesting its applicability in tunable optical components.

The ability to induce optical changes through light exposure, without requiring high-power thermal or electrical control, positions TTz-embedded polymers as promising candidates for dynamic infrared metasurfaces. The development of photochromic materials that exhibit strong and reversible changes in their optical properties in the infrared region could offer an alternative approach to achieving tunable infrared metasurfaces, potentially with advantages in terms of cost, fabrication, and power consumption.