Excimer of Perylene in a Xerogel Matrix Shows Both Solution‐ and Solid‐Like Dynamics

Abstract

Fluorescence quenching in solid‐state applications using polyaromatic hydrocarbons (PAHs) in optoelectronics can be mitigated by embedding them in a xerogel matrix. This study demonstrates a cost‐effective method to reduce perylene's aggregation‐caused quenching (ACQ) by coassembling it with a low‐molecular‐weight gelator made from cholesteryl chloroformate and pentaerythritol. For the first time, the optical properties of perylene in a xerogel are explored through comparative studies of perylene in powder, solution, and xerogel at various concentrations using steady‐state and time‐resolved emission spectroscopy. Time‐resolved area‐normalized emission spectra (TRANES) show a single‐step interconversion between monomers and excimers in both perylene powder and solution. Perylene powder exhibits aggregate‐induced excimer emission peaking at 600 nm, while concentrated perylene solution displays dynamic excimer emission at 540 nm with a single isoemissive point at 523 nm. In the xerogel matrix, perylene shows emissions from both monomeric and excimer states. The exciton dynamics at different concentrations reveal emission at 570 nm, the Y‐state, a partially relaxed excitonic state, which then relaxes into the excimer (E) state emitting at 595 nm. These behaviors suggest a promising way to tune optical properties for PAHs‐based optoelectronic materials. A cholesteryl‐based host matrix also shows potential for maintaining solid‐state emission in organic optoelectronic applications.

Perylene when dissolved in solution or embedded in a xerogel matrix shows different emissive states. Typical monomer emission can be observed in the bluer wavelength range with vibronic structure. This species evolves into a greenish species with weak vibronic features. Further, this species gives rise to a relatively broad, featureless emission spectrum at redder wavelengths. However, in the solid state, only the red excimer emits.

1. Introduction

Widespread research in the field of organic fluorophores is primarily driven by their broad applicability in advanced technologies [1, 2], including organic solar cells [3], organic light‐emitting diodes (OLEDs) [4], organic lasers [5], organic field effect transistors (OFETs) [6], memory devices [2], chemosensors [7], and bioprobes [8], etc. In this context, polyaromatic hydrocarbons (PAHs) such as perylene, pyrene, and anthracene have been extensively studied owing to their vivid colors, strong absorption, and emission characteristics, as well as excellent photothermal and photochemical stability [9], which make them promising materials for light harvesting devices [1]. Among them, perylene crystals are widely studied due to their high absorption coefficient, charge carrier mobility, and quantum yield [10] for photo‐functional organic devices [11, 12, 13, 14]. Perylene crystallizes in two polymorphic forms (α and β‐phase). α‐phase exists as a dimer, while β‐phase exists as a monomer [15]. This dimeric crystal structure (α‐phase) is responsible for changes in the excited‐state photophysical dynamics [16].

For practical applications, these fluorophores are often used in condensed phases, such as molecular aggregates, crystals, or solid‐state films, where their photophysical behavior is critically influenced by aggregation [17]. While the emission from conventional organic fluorophores in isolated molecules is strong, it gets significantly reduced or completely quenched in a closely‐packed ensemble or aggregated state. This reduction in emission is known as aggregation‐caused quenching (ACQ), which arises from the formation of detrimental species [18]. ACQ primarily originates from strong π–π interactions present in planar aromatic cores of these molecules, enhancing nonradiative relaxation pathways [17, 19], which severely limits their applicability in solid‐state where emission is necessary.

Therefore, it is meaningful yet challenging to design fluorophores that exhibit strong emission both in solution and in aggregated states. Although various strategies have been explored over the years to mitigate ACQ, only a few have succeeded due to the inherent pronounced π–π stacking tendency of PAHs [17]. A major breakthrough came in 2001, when Tang and coworkers introduced the concept of aggregation‐induced emission (AIE) in silole‐based systems, which counteracts ACQ by restricted intramolecular motions (RIM) inhibiting nonradiative decay pathways in aggregated state [20, 21]. Another promising direction involves mechanochromic fluorescence (MCF) materials whose photophysical properties change in response to mechanical stimuli [22]. However, these strategies often require chemical or physical modification of the dyes [20]. Alternatively, strategies like the metal–organic framework (MOF), metallacages, and the coassembly approach for PAHs have been used to spatially separate the fluorophores by molecular barriers, but these are fluorophore‐specific strategies [23, 24, 25]. To address these problems, noncovalent encapsulation of dyes within single‐walled carbon nanotubes (SWCNTs) [26] or within a supramolecular host matrix to preserve solid‐state fluorescence could potentially serve as an ideal solution [24, 27, 28, 29].

Herein, we adopted an innovative approach for controlling and optimizing the photophysical properties of perylene in solid‐state. We embedded perylene in a supramolecular cholesterol based robust gelator host matrix (Scheme 1). The xerogel matrix is found to effectively modulate intermolecular packing and excited‐state interactions, leading to altered excimer behavior and reduced ACQ without chemical modification or functionalization of the fluorophore [24].

SCHEME 1.

Schematic representation of xerogel synthesis process.

In a steady‐state study, a broad and structureless emission spectrum is one of the most characterstic features of an excited‐state dimer known as excimer, proposed by Förster and Birks in pyrene solution [30, 31], which occurs due to coplanar sandwiched dimeric structure of α‐pyrene. Excimers can adopt different spatial configurations, typically sandwich‐like arrangements (they can be fluorescent at low temperature) [32] or head‐to‐tail or slipped arrangements (usually fluorescent at room temperature) [33]. The extended emission spectrum of an excimer can improve the efficiency of OLEDs [34]. or they can act as trap states, thereby hindering exciton diffusion [35]. Recent studies suggest that excimer formation can also influence crucial processes such as singlet fission and symmetry‐breaking charge separation (SB‐CS) [36, 37]. Hence, a detailed understanding of excimer dynamics is essential for the development of advanced functional materials [27, 28, 29, 30, 31].

Previous studies on perylene have primarily focused on its excited‐state dynamics in crystalline and aggregated systems, particularly the formation of excimers through a precursor Y‐state in α‐phase perylene crystals at low temperatures [15]. Several approaches have been employed to modulate perylene emission, including incorporation into polymer matrices, gels, and nanoaggregates. For example, perylene dispersed in polymer matrices exhibits concentration‐dependent monomer‐to‐excimer transitions [14], while gel‐based systems have demonstrated reversible switching between monomer and excimer emission through hydrophobic aggregated state [38]. Furthermore, studies on the nanoaggregate of perylene derivatives revealed the effect of structural modification on exciton‐excimer dynamics and diffusion length of exciton [9].

This work represents one of the first systematic investigations of the solid‐state photophysics of perylene while preserving its emission through confinement within a supramolecular xerogel network, revealing how host–guest interactions regulate its excimer behavior. A comparative investigation of perylene in different physical forms, including powder, solution, and xerogels at various concentrations, is carried out using steady‐state and time‐resolved emission spectroscopy. These investigations reveal distinct photophysical behaviors arising from changes in molecular environment and aggregated states, providing insights into the excited‐state dynamics of perylene in confined xerogel matrices. We observed excimer formation in the xerogel via a shallow/partially trapped yellow emission (Y) at approximately 570 nm, which subsequently relaxes to a deep, trapped excimer state (E) appearing at around 595 nm [9, 14, 15, 39, 40]. In concentrated solution, an emission band at ∼540 nm is observed in the excited‐state, which is directly assigned to dynamic excimer (E‐state), whereas perylene powder shows aggregation induced excimer emission at 600 nm [41, 42, 43]. This observation of excimer‐driven luminescence with characteristic perylene emission colors in the xerogel highlights its potential for use as a molecular probe and in the development of optoelectronic devices [38].

2. Results and Discussion

2.1. Steady‐State Spectra Show Excimers

Figure 1a presents the steady‐state spectra normalized at the most intense peak of a 10 µM perylene solution in dichloromethane (DCM). The shaded blue region represents the absorption spectrum, which shows well‐resolved vibronic transitions between 350–445 nm, with an absorption maximum at 437 nm. This 0–0 vibronic transition corresponds to the long‐axis polarized ¹L_a band of perylene [40]. The excitation spectrum ( λ _em = 470 nm) of the dilute perylene solution (blue line) closely resembles its absorption spectrum, confirming its monomeric nature [7]. The emission spectrum of dilute solution upon excitation at λ _ex = 410 nm (red line) displays narrow and well‐resolved vibronic peaks. The υ _0–0 transition occurs at 440 nm, υ _0–1 at 470 nm, υ _0–2 at 502 nm, and υ _0–3 at 537 nm. The emission spectrum of dilute perylene comprises a mirror image of the S₀–S₁ transition of absorption. As the electronic transitions do not significantly alter the nuclear geometry, they result in the similar spacing between the vibrational energy levels of both ground and excited states, resulting in corresponding vibronic features in absorption and emission spectra [44].

FIGURE 1.

Steady‐state spectra of perylene in solution by exciting at 410 nm and λ _em = 470 nm. (a) Absorption spectrum (blue‐shaded region), excitation spectrum (blue line), and emission spectrum (red‐shaded region) of 10 µM solution. Inset shows the molecular structure of perylene with long (a‐axis) and short (b‐axis). (b) Truncated absorption spectrum (blue line) at around 446 nm, excitation spectrum (purple‐shaded region) and emission spectrum (red‐shaded region) of 14 mM concentrated solution.

Figure 1b shows the steady‐state spectra of a concentrated perylene solution. A concentrated solution (14 mM) of perylene was prepared in DCM. The absorption spectrum (blue line) is redshifted and broad as compared to the dilute solution (absorption spectrum is truncated around 446 nm due to detector saturation at high absorbance). In contrast, the excitation spectrum (λ _em at 470 nm, shaded purple region) remains similar to that of the dilute solution, indicating that excitation still occurs through monomeric absorption. However, the emission spectrum of concentrated solution (λ _ex = 410 nm, red‐shaded region) is broad and redshifted. It does not exhibit the mirror image of excitation spectrum like the dilute solution, because all excitations are not relaxing via monomer emission; some relax through nonradiative pathways or excimer/aggregate states. The overlap between first vibronic emission peak (440 nm) with the excitation peak, is resulting in a decrease in intensity due to self‐absorption [44]. The broad emission band spanning from 550 to 750 nm is attributed to dynamic excimer formation [7, 41].

The typical features of excimer evolution such as, spectral broadening and increased lifetime are also examined. With increasing concentration of PAHs, monomer fluorescence was quenched and a broad, lower‐energy emission emerges, corresponding to excimer or aggregate formation driven by strong π–π interactions, which are usually lower in intensity [15, 45]. The quenching rate and excimer formation follow a diffusion‐controlled process in solution [46]. This fluorescence quenching limits the application of PAHs in fabrication of optoelectronic devices [17]. Therefore, we synthesized a strong low‐molecular weight gelator in a single step using easily available materials such as cholesteryl chloroformate and pentaerythritol (detailed synthesis and characterization are provided in Section S1) [24]. We have significantly reduced the ACQ of perylene in xerogel compared to its concentrated solution, solid powder and thin film, as shown in Figure 2a. This enhancement can be attributed to the 3D intervention of fluorophores within the supramolecular xerogel matrix, as previously reported [17, 20, 47]. The estimated absolute photoluminescence quantum yields of perylene in different states are provided in Table S1.

FIGURE 2.

Steady‐state spectra of perylene in different states: (a) Comparison of emission intensity of perylene in different states, 10 µM solution (red line), 10 mM perylene in xerogel (black line), powder (yellow line), 23 mM perylene in xerogel (blue line), 14 mM solution (dark cyan line), and thin film (green dash line). Excitation spectra (purple‐shaded region) and emission spectra (red‐shaded region) of (b) powder λ _em = 595 nm, (c) 10 mM perylene in xerogel, λ _em = 470 nm, and (d) 23 mM perylene in xerogel, λ _em = 470 nm. All emission spectra measurements were carried out using 410 nm excitation.

We embedded perylene at concentrations of 10 and 23 mM in the gelator matrix. As shown in Figure 2a, the emission intensity of 10 mM perylene in xerogel (black line) is significantly enhanced compared to that in the concentrated solution (dark cyan line), perylene powder (yellow line), and film (dashed green line). However, the 10 µM dilute solution (red line) exhibits strong monomer emission, free from aggregation, quenching, and reabsorption pathways present at higher concentrations [10]. Upon increasing the perylene concentration to 23 mM (blue line), the xerogel retains the vibronic features similar to those observed in solution and still exhibits higher emission intensity than the concentrated solution. However, the excimer intensity of 23 mM perylene in xerogel is reduced compared to that in the powder, probably due to the strong nondissociative nature of static excimer in powder [18].

Figure 2b shows the excitation spectrum (purple‐shaded region) recorded at λ _em = 595 nm and emission (red‐shaded region) spectrum recorded at λ _ex = 410 nm of perylene solid. Perylene powder exhibited broad structureless excitation and emission spectra having two emission peaks at 565 and 595 nm, which are ascribed to the static excimer formation from the excitation of preassociated dimer in the ground state [7, 19]. Coulombic coupling due to molecular aggregations can influence the excitation spectrum [48]. Figure 2c depicts the steady‐state spectra of 10 mM perylene in xerogel, excitation (purple‐shaded region) and emission (red‐shaded region) spectra. Emission spectrum of xerogel has preserved the vibronic features of monomer along with excimer band (red‐shaded region) at ∼540–750 nm. Figure 2d shows the excitation and emission spectra of 23 mM perylene in xerogel. Emission spectrum (red‐shaded region) is broad and redshifted compared to solution due to excimer emission in the range of 560–800 nm. In both cases excitation spectra (purple‐shaded region, λ _em = 470 nm) resemble that of dilute solution confirming dynamic excimer formation [7].

It is very well discribed in literature that α‐perylene crystals are known to emit from dimeric self‐trapped (ST) excitons [15] due to strong exciton–phonon interactions [9] localized in a specific region of a molecular or crystal lattice [13]. These dimeric ST excitons, known as excimers, emit broad, redshifted luminescence [9, 40]. The emission spectra of perylene in the range of 460–800 nm may contain contributions of monomeric, Y‐state or dimeric ST due to spectral overlap. Emission of these states is sensitive to temperature and lattice environments. It is not feasible to assign different spectral features associated to specific excited‐state species solely from steady‐state spectra, hence, we performed time‐resolved emission studies to understand the excited‐state dynamics and involved species.

2.2. Time‐Resolved Emission Spectroscopic Studies

To further explore the excimer dynamics of perylene in different forms as previously discussed, we performed time‐resolved emission measurements using time‐correlated single photon counting (TCSPC) technique. TCSPC has extensively been used in understanding excited‐state dynamics such as solvation [49], charge‐carrier dynamics [50], excimer formation [27, 47], and singlet‐exciton fission [51]. Wavelength‐dependent emission decays were recorded upon excitation at 410 nm and monomer decay (blue‐end) along with the concomitant rise times of excimer (red‐end) were extracted by fitting the decays with multiexponential functions. The calculated instrument response function was ∼170 ps, as shown in Figure S1a. Figure S1b shows emission decays of 10 µM perylene solution recorded at 437 and 700 nm. The lifetimes were calculated by fitting the decays with a mono‐exponential function and were found to be approximately 4.85 ns, fitting parameters are provided in Table S2.

Figure 3 shows emission decay profiles of perylene in the solid state, concentrated solution, and xerogel containing 10 and 23 mM of perylene. We recorded the emission decays corresponding to both the monomer decay (green lines) and the concomitant rise of excimer (red lines) of perylene in all of the above discussed samples. For perylene powder, the decay was measured at 490 nm for monomer and at 700 nm for the excimer rise and subsequent decay (Figure 3a). The monomer and the excimer emission kinetics in powder perylene were analyzed using a triexponential fitting function [15], yielding a fastest decay and concomitant rise time of 0.71 ns, respectively. However, in concentrated solution, the monomer decay (430 nm) and concomitant excimer (700 nm) rise time were found to be longer, approximately 1.65 ns, by fitting using a biexponential function as shown in Figure 3b. A slower time component can be explained as the excimer formation in solution is a diffusion‐assisted mechanism [31, 52].

FIGURE 3.

Emission decay curves of perylene in different states at λ _ex = 410 nm: (a) Normalized emission decays at λ _em = 490 nm (green) and concomitant excimer rising at λ _em = 700 nm (red) in powder, (b) λ _em = 430 nm (green) and concomitant excimer rising at λ _em = 700 nm (red) in 14 mM solution, (c) λ _em = 445 nm (green) and concomitant excimer rising at λ_em = 700 nm (red) in 10 mM perylene in xerogel, and (d) λ _em = 440 nm (green) and concomitant excimer rising at λ _em = 700 nm (red) in 23 mM perylene in xerogel. Decays were best fitted by multiexponential functions. Fitted lines of decays are represented by straight solid lines. Fitting parameters are provided in Tables S3–S6, respectively.

Furthermore, as the concentration of perylene increases in the xerogel matrix, excimer formation becomes faster due to availability of perylene molecules in closer vicinity. As shown in Figure 3c,d, the monomer decay (∼440–445 nm) and concomitant excimer (700 nm) rise time was obtained to be 1.34 and 1.05 ns for the 10 mM xerogel and 23 mM xerogel, using a biexponential fitting function, respectively. This indicates that the excimer rise time becomes faster as perylene molecules are in closer proximity, which allows them to interact more efficiently.

2.3. Time‐Resolved Area‐Normalized Emission Spectra (TRANES) Show Monomer‐Excimer Dynamics

Further, for the investigation of spectral evolution due to excited‐state dynamics of perylene in different forms, we constructed the time‐resolved emission spectra (TRES) by combining the steady‐state intensity and wavelength‐dependent emission decays ranging from 420 to 800 nm (details in Section S2). TRES maps are shown in Figure S2. TRES could be a powerful method if only a single emissive species/entity is present (the spectral shape is unchanged over the course of time). It cannot provide the information about presence of multiple emissive processes or states having different spectral properties. For this purpose, we have constructed the TRANES by dividing every spectrum with its integral intensity. TRANES can distinguish between two interconverting emissive species, which are independent of their origin or the underlying excited‐state dynamics. TRANES maps are shown in Figure S3 [53, 54].

Figure 4 depicts the TRANES slices of perylene in different forms. The detailed analysis of TRANES construction is described in Section S3. Figure 4a shows TRANES slices from an average time of 0.16–0.2 ns (green lines) to 50–60 ns (red lines) for the powder with an isoemissive point at 555 nm. The slices reveal monomer emission in the 490–555 nm region (green lines) that decays over time, accompanied by a concurrent rise in the 580–800 nm region (red lines), peaking at ∼600 nm. This feature is attributed to the static excimer or dimeric ST excitons [9]. Figure 4b shows the TRANES slices for saturated solution ranging from 0.16–0.2 to 50–60 ns. TRANES slices exhibit the vibronic structure in the initial time due to contribution of monomer (425–523 nm, green lines) emission and a broad emission in the longer time (red lines) due to an excimer formation (523–750 nm range) peaking at 540 nm. This interconversion occurs in a single step with an isoemissive point at 523 nm [41, 43].

FIGURE 4.

Time‐resolved area‐normalized emission spectra (TRANES) slices of perylene in various states from average time range of 0.16–0.2 ns (green lines) to 50–60 ns (red lines), showing interconverting species over time in (a) powder with isoemissive point at 555 nm. (b) 14 mM solution with isoemissive point at 523 nm. (c) 10 mM perylene in xerogel with isoemissive point at 530 nm. (d) 23 mM perylene in xerogel with isoemissive point at 527 nm.

Figure 4c,d show the TRANES slices of perylene in xerogel at concentrations of 10 and 23 mM, respectively. The slices were plotted from 0.16–0.2 to 50–60 ns. Similarly, Figure 4c,d show the vibronic features in the TRANES slices of perylene in xerogel due to monomer emission (green lines), which become broad at longer times due to excimer emission (red lines). Furthermore, in Figure 4d, we observed a distinct vibronic peak in TRANES slices during intermediate times at 570 nm corresponding to the emission from a partially relaxed/shallow trapped, Y‐state, which further decays to a fully relaxed/deep trapped E‐state in the range of 595–800 nm. Furthermore, steady‐state emission spectra exhibited temperature dependent changes and difference spectrum obtained by subtracting the high‐temperature emission spectrum from the low‐temperature spectrum exhibited a distinct peak centered at approximately 570 nm (black line), which coincides with the Y‐state emission, as shown in Figure S4.

In the case of 23 mM perylene in xerogel (Figure 4d), progressive redshift in the TRANES slices at longer delay times provides clear evidence of Y‐to‐E relaxation pathway. The Y‐state contribution is more pronounced at 23 mM as compared to 10 mM perylene in xerogel, which can be attributed to differences in molecular packing and the influence of the xerogel microenvironment on perylene aggregation. Moreover, the absence of a second isoemissive point indicates that Y‐ and E‐states are not two independent emissive species; rather, the Y‐state represents a partially relaxed excimer that subsequently evolves into the fully relaxed E‐state. Such transient Y‐state emissions have been frequently reported in perylene crystals and thin films, particularly at low temperatures [15, 55]. The exciton dynamics of perylene in xerogel probably resemble those observed in the α‐phase of perylene crystal due to excimer emission [15].

The broad emission spectrum of perylene at room temperature, spanning 440–520 nm, can be primarily attributed to the radiative decay of monomeric free excitons. At higher concentrations, however, additional emissive pathways become evident. In concentrated solution, a gradual rise in emission intensity around ∼540 nm is observed with time, characteristic of excimer formation. Similarly, in the powder state, a delayed emission feature emerges near ∼600 nm, which is assigned to aggregate‐induced static excimer emission, exhibiting a single step interconversion of monomer to excimer. In xerogel matrix, the emissive behavior of perylene is different as compared to that in powder and solution. Here, an intermediate band emerges at ∼570 nm, corresponding to a transiently trapped Y‐state. This state further relaxes into a deep trap, called E‐state, giving rise to an emission centered at ∼595 nm. The excited‐state dynamics of perylene in different states as well as in xerogel agrees well with previous studies [9, 26, 40, 41, 43]. Scheme 2 illustrates the excimer dynamics of perylene in various states.

SCHEME 2.

Illustration of E‐(excimer) state of perylene in various states: partially relaxed emission (Y‐state) at 570 nm in xerogel and aggregate induced E‐emission at 600 nm in both powder and xerogel.

3. Conclusions

This study aims to preserve the solid‐state fluorescence of perylene by embedding it in a supramolecular gelator‐based host matrix where, the intertwining of dye molecules with three dimensional gelator fibers inhibit the π–π stacking. Additionally, vibronic peaks were preserved in xerogel, indicating retention of a solution‐like state. Furthermore, TRES and TRANES analysis revealed the intricate exciton dynamics of perylene in the xerogel.

Perylene powder and concentrated solution exhibit single‐step monomer to excimer conversion, yielding aggregate‐induced (∼600 nm) and dynamic (∼540 nm) excimers, respectively. In xerogel, a shallow trapped/partially relaxed Y‐state (∼570 nm) relaxes to a deep relaxed E‐state (∼595 nm). Thus, by tuning the photophysical properties of perylene in xerogel, we highlighted its potential application in the fabrication of optoelectronic devices such as OLEDs and display of electronic devices with enhanced fluorescence brightness and photostability.

4. Experimental Section

4.1. Materials

Perylene (M.W. = 252.31 g/mol) and DCM, were purchased from Sigma‐Aldrich and spectrochem and used without further purification. Perylene solutions were prepared by dissolving the powder in DCM to obtain a concentrated stock solution, which was subsequently diluted to the desired concentrations. To make the perylene xerogel (1 wt%), gelator and perylene (concentration in Section S1) were taken together and dissolved in a 1:1 hexane/ethanol mixture by heating. Cooling the hot solution to room temperature resulted in formation of a translucent gel. Obtained cogel was then flash frozen using liquid nitrogen and dried under vacuum to produce the xerogel. Further details about sample preparation and characterization are reported elsewhere [24].

4.2. Steady‐State Measurements

A Shimadzu UV‐2600i UV–vis Spectrophotometer was used to record steady‐state absorption data for both solution and solid‐state samples. Steady‐state fluorescence measurements were performed using a Horiba Fluoromax‐plus spectrofluorometer.

4.3. Time‐Resolved Measurements

Time‐resolved measurements were conducted with a Delta Flex‐01‐DD/HORIBA TCSPC system. Excitation was carried out using a 410 nm diode LED, and the excited‐state lifetime decays were fitted using the following multiexponential function

$$I\left(t\right)=\sum a_i{e}^{\raisebox{1ex}{$‐t$}\!\left/ \!\raisebox{-1ex}{${\tau }_i$}\right.}$$

where t is time and $a_i$ is the amplitude of the i ^th component having a lifetime ${\tau }_i$. All solution spectra were recorded using 600 μL of the samples in a quartz cuvette with a 10 × 2 mm path length. For solid‐state measurements, samples were sandwiched between clear quartz glass microscope slides and coverslips. The glass slides and coverslips were cleaned by ultrasonication in acetone and ethanol for 30 min each, then wiped by methanol and dried.

Funding

This study was supported by the Science and Engineering Research Board (EEQ/2022/000475).

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supplementary Material

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.