New Insight on Photoisomerization Kinetics of Photo-Switchable Thin Films Based on Azobenzene/Graphene Hybrid Additives in Polyethylene Oxide

In this work, we reported a new insight on the kinetics of photoisomerization and time evolution of hybrid thin films considering the azo-dye methyl red (MR) incorporated with graphene accommodated in polyethylene oxide (PEO). The kinetics of photoisomerization and time-evolution of hybrid thin films were investigated using UV-Vis s and FTIR spectroscopies, as well as appropriate models developed with new analytical methods. The existence of azo-dye MR in the complex is crucial for the resource action of the trans ↔ cis cycles through UV-illumination ↔ Visible-illumination relaxations. The results of the UV–Vis and the FTIR investigations prove the cyclical trans ↔ cis-states. Consequently, PEO-(MR-Graphene) hybrid composite thin films can be introduced as possible applicants for photochromic molecular switches, light-gated transistors, and molecular solar thermal energy storage media.


Introduction
Photo-switchable thin films can be isomerized between two metastable states through light-illumination. This type of thin-film has gained noticeable attention in many applications in physics, chemistry, and biology. The striking feature of photo-switchable thin films is that the two isomers have different physical and chemical properties. Recently, the azobenzene (AZO) has gained great momentum owing to its potential applications in photochromic molecular switches [1], light-gated transistors [2], and molecular solar thermal energy storage media [3][4][5][6]. AZO has two geometric forms, trans-state and cis-state. All AZO thin films are initially in the transisomerization state since it is thermally stable at room temperature [7,8]. The trans → cis isomerization may occur by exposing thin films to the UV-light illumination, while the reverse cis → trans isomerization can occur by either illuminating thin films with visible light (400-450 nm) or by thermal excitation in a dark environment [9,10]. The photoisomerization mechanisms of the trans↔cis isomers in the blended polymers play a key role in extracting the proper chemo physical properties of additive azo benzene [7,[10][11][12][13].
Polymers go under a photodegradation process via UV light-absorption (in particular) by backbone-carbonyl groups induced by photochemical reactions. Photodegradation of polymers usually

Characterization of PEO-(MR-Graphene) Hybrid Nanocomposite Thin Film
As previously reported [3,33], the chemical structure of the AZO-Graphene used in this work can be visualized, as shown in Figure 1a. The coupling agent forming the AZO-Graphene hybrid contains the AZO-nitrogen atom bonded covalently with the carbon atom that is also bonded with the graphene aromatic ring. The chemical formula of PEO is C 2n H 4n+2 O n+1, and that of MR is given by C 15 H 15 N 3 O 3 . The covalent linkage between methyl red and Graphene exterior superficial shells can be determined by FTIR patterns shown in Figure 1b. The vibrational bands for graphene located at 2015 cm −1 and 2161 cm −1 represent C=C and C=O. FTIR spectrum for PEO exhibited vibrational bands between 700-1000 cm −1 representing bending C-H. Vibrational bands between 1000-1400 cm -1 are assigned to C-O stretching vibration, a band at 1470 cm -1 that could be ascribed to the -CH 2 bending vibration, while a band appearing at the 2886 cm -1 could be ascribed to the symmetric and asymmetric C-H stretching modes of the CH 2 group. Moreover, the significant pronounced vibrational bonds that appeared in PEO-(MR-Graphene) hybrid nanocomposite thin films were found to be related to the aromatic rings, the azo-chromophores (-N=N-), the stretching bonds of C-N, and others such as C-H. The in-plane C-H vibrational peak was found at 1122 cm −1 , and the out-of-plane vibrational peaks were found between 1000-700 cm −1 . The aromatic bands were observed between 1600 and 1400 cm −1 . Moreover, the spectral peak observed at 1342 cm −1 was apportioned to the C-O stretching bond in PEO. According to W. Pang et al. study [3], the absorption due to the C-N bonding is unlike the C=C bonding, and its frequency-range is centered at 1342 cm −1 caused by the influence of resonance which upsurges the bond-order assigned to that particular ring in the chain and the dangling N-atom. The presence of C-N bonds designates the covalent bonding between MR to the graphene-outer surface. The peak observed at 3732 cm −1 was assigned to the O-H bond in PEO. However, the peak at 2861 cm −1 was dispensed to C-H stretching. Moreover, the peak found at 2114 cm −1 was assigned between 150 to 200 °C. Within this interval, the mass-loss is significant (80%), indicating the influence of the intermolecular/intramolecular bonding change when the nanocomposite is exposed to high temperatures. PEO's thermal stability-(MR-Graphene) hybrid nanocomposite is lower than that of PEO, as shown in Figure 1d. For PEO, the mass-loss versus temperature drops sharply at around 300 °C [34,35]. Interestingly, despite the slight negligible slope in the TGA relation below 150 °C, TGA thermograms confirm that the PEO-(MR-Graphene) hybrid composite was stable below 150 °C. Most of the practical optical applications are feasible below 150 °C.   Figure 1c shows PEO and PEO's absorbance spectra-(MR-Graphene) hybrid composite thin films as a function of wavelength. The PEO absorption spectra-(MR-Graphene) hybrid composite is characterized by a band transition. Namely, the π − π * in the range of 350-650 nm. However, PEO thin film does not contain any absorption band identified in this region [9]. Clearly, the absorbance of the PEO-(MR-Graphene) hybrid composite thin film exhibits π − π * transition in the (350-650) nm range with a maximum at 422.5 nm. This spectral behavior is believed to occur due to the presence of MR in the film nanocomposite giving the films the red-appearance color. Hence, the films show high absorbance for green, blue, and violet-colored light [7,[10][11][12][13].
Furthermore, PEO's thermal stability-(MR-Graphene) hybrid nanocomposites were investigated using the thermogravimetric analysis (TGA) technique at temperatures up to 400 • C, as shown in Figure 1d. The PEO-(MR-Graphene) hybrid nanocomposite shows a stable weight-loss starting at the ambient temperature T and extends to 150 • C, at which the mass-loss becomes 8% approximately. The mass-loss was considered as the loss of water/solvents adsorbed in the sample. The mass-loss curve versus temperature then sharply declined in the temperature interval ranging between 150 to 200 • C. Within this interval, the mass-loss is significant (80%), indicating the influence of the intermolecular/intramolecular bonding change when the nanocomposite is exposed to high temperatures. PEO's thermal stability-(MR-Graphene) hybrid nanocomposite is lower than that of PEO, as shown in Figure 1d. For PEO, the mass-loss versus temperature drops sharply at around 300 • C [34,35]. Interestingly, despite the slight negligible slope in the TGA relation below 150 • C, TGA thermograms confirm that the PEO-(MR-Graphene) hybrid composite was stable below 150 • C. Most of the practical optical applications are feasible below 150 • C.

Kinetics of Photoisomerization Processes
In this section, we investigated and reported the kinetics of PEO transformation-(MR-Graphene) hybrid nanocomposite thin films from the original trans-state to cis-state using UV-exposure. The reversed transformation to trans-state via visible-illumination was also discussed and interpreted. UV-light primarily illuminates the PEO-(MR-Graphene) hybrid nanocomposite thin films for a certain period of time. The films are then exposed to Blue-light illumination for another period. Figure 2a illustrates the absorbance spectra of the PEO-(MR-Graphene) hybrid nanocomposite thin films illuminated by UV-light for various periods. The major absorption peak of PEO-(MR-Graphene) at the initial trans-state in the visible range was found to be at 422.5 nm with an absorbance amplitude of 0.169%. The film was then exposed to UV light of 366 nm for 30, 60, 120, 240, and 480 s, respectively. Moreover, the films show a variant absorbance band in the middle of the visible region (380-625 nm) with a peak-blue-shifted and amplitude-decrease transferring the trans-state to a cis-state, as expected. The photoisomerization process is not a spontaneous instantaneous process. Instead, it occurs through steadily building up sub-stages. The film exposure to the UV-illumination reveals a noticeable reduction in the absorbance at~422.5 nm in the trans-isomerization stage and the presence of a dual peak at~470 nm created from the cis-isomerization state. A steady reduction in the π-π * electron transition band of the trans-state upon the increasing of UV-illumination time duration until a photostationary equilibrium state between the transand the cis-isomers was detected. The absorbance spectra of PEO-(MR-Graphene) thin films prior-to and in post-UV illumination revealed the existence of dual isosbestic figure-tips that appeared close to 380 and 625 nm, respectively. Moreover, the UV-illumination of PEO-(MR-Graphene) hybrid nanocomposite thin film leads to the transformation from trans-state to a cis-state. In other words, the absorbance peak at wavelengths ranging from 380 nm to 625 nm decreased as the period of illumination increased. The prolonged illumination does not lead to any changes in the absorption spectra, confirming that a quasi-state of photo-stationary phase occurs between the transand the cis-stats. The long-time isomerization (480 s of illumination) is needed until a photo-stationary equilibrium is achieved, compared with previous results reported [7,9,10]. The observed electronic transitions in the visible range indicated a suitably effective absorption of solar power with high-energy. The solar energy absorbed in the material is of prime necessity to achieve two-or three-multiples of energy more than the activation barrier-energy ∆E i (needed for complete isomerization of AZO-molecules) [36]. Based on the findings in the literature, the energy reserved/AZO-molecules (∆H) for cis-isomerization has to be less than ∆E i . In other words, ∆H = ∆E i − ∆E a , where, ∆E i is the activation barrier (energy needed to isomerize one AZO molecule), ∆E a is the thermal/optical activation barrier and efficiency of storing energy (the energy stored per the solar energy absorbed) is normally < 30% [36,37].
The kinetics of the trans-cis photoisomerization of PEO-(MR-Graphene) hybrid nanocomposite thin films were investigated using the first-order kinetics via calculating the rate of photoisomerization, as well as the thermal/optical activation barrier ∆E a for trans-cis photoisomerization [38,39]. The photoionization rate as a function of time (p) could be written as, where A 0 , A t and A ∞ are the absorbance at various time conducts, namely prior to the light exposure (initial trans-state), during radiation in time t, and post to the light exposure for a protracted time.
The average value of ln(A t − A ∞ /A 0 − A ∞ ) of PEO-(MR-Graphene) hybrid nanocomposite thin film is plotted against the period (t), as shown in Figure 2b. Obviously, ln( has two distinct linarites. The discontinuity occurs at t c ∼ 240 min, then the curve falls off with less steepness as a function of illumination time. The area's ratio under the absorption curve in the visible range for all UV-illumination periods for the cis-hybrid isomerization was calculated with respect to the trans-hybrid isomerization. It was found to be around 95.5% at t c for the PEO-(MR-Graphene) hybrid nanocomposite thin films. The rate constant (p) was found from the slope to be Moreover, the variations in p could be reasonably attributed to the disparity of ∆E a [37] and calculated as, where: τ 1/2 is the time needed to transfer half the trans-states into cis-states; τ 1/2 = ln 2/p, k is Boltzmann constant, h is Plank constant, and T is the temperature. Table 1 shows that τ 1 1/2 = 3.022 min for films illuminated for a time less than t c , while τ 1 1/2 = 70.692 min for those films kept under irradiation for time-periods longer than t c . Equation (2) indicates that ∆E a = 2.012 eV at 0 < t < t c and = 2.093 eV at (t ∞ > t > t c ), respectively.
Polymers 2020, 12, x FOR PEER REVIEW 6 of 15 where: / is the time needed to transfer half the trans-states into cis-states; / = ln 2 / , is Boltzmann constant, ℎ is Plank constant, and is the temperature. Table 1 shows that / = 3.022 min for films illuminated for a time less than , while / = 70.692 min for those films kept under irradiation for time-periods longer than . Equation (2) indicates that Δ = 2.012 eV 0 < < = 2.093 eV at ( > > ), respectively. Beyond , the isomerization energy barrier is higher than that below, and the process proceeds even slower. The calculations agree well with the observed red-shift for the band - * in the absorption spectrum beyond where the cis-isomer contents have exceeded the trans-isomer contents in the PEO-(MR-Graphene) hybrid nanocomposite thin film. Therefore, the separation between the two neighboring trans-isomers gets longer, and the intermolecular interactions reduce accordingly. In this scenario, the transformation from the trans-to-cis photoisomerization is limited to  Beyond t c , the isomerization energy barrier is higher than that below, and the process proceeds even slower. The calculations agree well with the observed red-shift for the band n-π * in the absorption spectrum beyond t c where the cis-isomer contents have exceeded the trans-isomer contents in the PEO-(MR-Graphene) hybrid nanocomposite thin film. Therefore, the separation between the two neighboring trans-isomers gets longer, and the intermolecular interactions reduce accordingly. In this scenario, the transformation from the trans-to-cis photoisomerization is limited to its steric effect, and consequently, the reaction barriers are primarily dominated by the electronic configuration of the −N = N− group [3]. Based on the related findings in the literature [3,33], one may plot the chemical structure of AZO-Graphene used in this work, as shown in Figure 2c. The chemical structure alters as the period of UV-illumination increases leading to enhanced transformation from trans-states to cis-states. Figure 2d,e shows the cis-isomerization of the AZO-Graphene hybrid composite prior to and beyond the t c reaching the 95.5%.
Another evidence for the kinematic transformation from trans-state to cis-state for the PEO-(MR-Graphene) hybrid composite thin film is revealed by analyzing the FTIR spectrum. Figure 3 shows the FTIR spectra for the transand cis-states in the spectral range of 500-4000 cm −1 . It was observed that the spectral peak for the O-H bond found at 3732 cm −1 in the trans-state had disappeared in the cis-state. This indicated that the free radicals generated by the films' UV irradiation had broken the weak O-H bonds in a photoreaction process. Moreover, the transmittance spectra as a function of frequency for cisand trans-MR showed nearly similar trends with small portions of the bands that were shifted to lower energies. This reflection confirmed that such interactions had a slight influence on N=N bands at frequencies between 1400-1600 cm −1 . The evaluated intensity in the infra-red region of N=N band for the cis-MR was enormously high compared to that for the trans-MR, confirming the enhanced photoisomerization process of the PEO-(MR-Graphene) hybrid nanocomposite thin film.
Polymers 2020, 12, x FOR PEER REVIEW 7 of 14 transformation from trans-states to cis-states. Figure 2d,e shows the cis-isomerization of the AZO-Graphene hybrid composite prior to and beyond the reaching the 95.5%. Another evidence for the kinematic transformation from trans-state to cis-state for the PEO-(MR-Graphene) hybrid composite thin film is revealed by analyzing the FTIR spectrum. Figure 3 shows the FTIR spectra for the trans-and cis-states in the spectral range of 500-4000 cm −1 . It was observed that the spectral peak for the O-H bond found at 3732 cm −1 in the trans-state had disappeared in the cis-state. This indicated that the free radicals generated by the films' UV irradiation had broken the weak O-H bonds in a photoreaction process. Moreover, the transmittance spectra as a function of frequency for cis-and trans-MR showed nearly similar trends with small portions of the bands that were shifted to lower energies. This reflection confirmed that such interactions had a slight influence on N=N bands at frequencies between 1400-1600 cm −1 . The evaluated intensity in the infra-red region of N=N band for the cis-MR was enormously high compared to that for the trans-MR, confirming the enhanced photoisomerization process of the PEO-(MR-Graphene) hybrid nanocomposite thin film. Furthermore, Figure 2a indicates that each absorbance peak of the PEO-(MR-Graphene) hybrid nanocomposite thin film contains two single absorption peaks (two frequency bands). As the UVillumination duration increased, the absorption amplitude decreased, indicating that the transformation from trans-state to cis-state had occurred accordingly. Increasing the duration of the Furthermore, Figure 2a indicates that each absorbance peak of the PEO-(MR-Graphene) hybrid nanocomposite thin film contains two single absorption peaks (two frequency bands). As the UV-illumination duration increased, the absorption amplitude decreased, indicating that the transformation from trans-state to cis-state had occurred accordingly. Increasing the duration of the UV-light exposure leads to developing new configured dual-shapes in the absorption peaks shown in Figure 2a, which indicates the development of an impeded dual-frequency bands forming two distinct crests instead of one main absorption peak. Additional evidence for the existence of dual overlapped sub-peaks was revealed from the clear shoulders that appeared mainly at the lower and higher frequency spectrum segments. Namely, at 470 nm and 422.5 nm, respectively.
Figure 4a-f shows the major peaks of PEO-(MR-Graphene) hybrid nanocomposite thin films for all durations of the UV-light exposure in the visible range of the spectrum as fitted to pair Gaussian crests [9]. Figure 4a describes the major peak of PEO-(MR-Graphene) hybrid nanocomposite thin films in the starting trans-state fitted to the dual Gaussian crests. The absorption spectra exhibited two bands (high-and low-frequency) with maxima starting at around 417.89 and 471.13 nm and line widths of 53.58 and 120.26 nm, respectively. The absorbance spectrum changed in its configured shape as the UV-illumination exposer time increased, as shown in Figure 4b-f showing the transformation from the trans-state to the cis-state. A detailed quantitative analysis for the lower-and higher-frequency bands was performed to obtain a more in-depth insight into the influence of the UV-illumination time. A closer look at Figure 4 shows the specific detailed structure for each peak specifying the amplitude and the area variation underneath the absorbance curve as plotted in Figure 5a,b during the transformation from transto cis-isomerization. Each sub-band's single configuration behavior varies with the UV-light exposure duration as generated through the systematic deviation from the line-width in the related Gaussian function. As can be seen from Figure 5a,b, the area and the amplitude of overall Polymers 2020, 12, 2954 9 of 15 absorbance, low-frequency, high-frequency bands decrease continuously with time till the t c is reached. However, beyond the t c , the amplitude and area under the main overall absorbance curve for the low-, high-frequency bands become constant, confirming the achievement of 95.5% proportion of the cis-hybrid in AZO-Graphene hybrid nanocomposite. Moreover, the high-frequency sub-band has suffered from a blue-shifting process, which indicates a bathochromic change occurred due to the light absorbed by MR. This is considered as an H-bond donor source or a dissociated-intermediate H-bond donor source [40,41]. Consequently, the overall absorption band is anticipated to occur due to the H-bond's mutual interaction among the PEO and the azo-nitrogen contents through MR molecules. The blue-shifting for both sub-bands (low-and high-frequency bands) exhibit similar trends indicating that H-bonds have a fundamental influence on MR molecules' various sites. Moreover, the high-frequency band demonstrates steady bathochromic shifts during the UV-illumination process, demonstrating two unambiguous symmetries at the H-bonds' backgrounds accompanied by MR transformation. The shift in all bands' amplitude ensures that the time-dependent photoisomerization process is achieved by more than one step [42]. This is because the photoisomerization process is a complex response to the four-level model of transand cisisomerization as demonstrated by Sekkat Z. et al. [23] and Lee G.T. et al. [43], as shown in Figure 6. The four-levels model describing the transand the cisisomerization has been investigated thoroughly by Al-Bataineh Q.M. [9]. As is well known, the ground state of AZO-band is a singlet (S0) while the S1 and S2 are first and second singlet excited states, respectively. The S1 state can be generated by either direct excited transition from S0 to S1 or via intersystem crossing between S1 and S2 states (i.e., relaxation of the S2 state down to S1) seen in Figure 6. The states S1 and S2 generated via trans-AZO excitations are different from those of cis-AZO excited states in their energy and conformations.   As understood from azobenzene's behavior, it experiences a revocable photoisomerization process, i.e., a transformation from cisto trans-isomerization. This process is usually associated with enormous quantum yields with no significant reactions in the process [44]. Figure 7a illustrates the absorption varieties of the PEO-(MR-Graphene) hybrid films at cis-state for numerous visible blue-light illumination durations. Beyond 480 s of UV-light exposure, the films appeared to show a cis-isomerization case (lowest solid yellow line). The PEO's main absorption peak-(MR-Graphene) hybrid film in the visible region originated at 423 nm with a peak-amplitude of 0.134. The hybrid film was further exposed to the blue-visible light for 90, 180, 360, 720, and 1440 s, respectively. Figure 7a shows that the film gains various absorption bands in the intermedium portion of the visible region (380-625 nm) associated with a red-shift behavior in the crest value transporting back the film from cisto the trans-isomerization case as predicted. We noticed that the process of transforming the material from cisto trans-state is not a sudden spontaneous nor instantaneous transformation. However, it has been transferred via a sequence of phases. Interestingly, the kinematic process of transferring the cis-to-trans state demonstrates a kind of deformation from the first-order kinetics, as seen in Figure 7bEach hybrid form, the departure from the first-order kinetics is quite obvious. It may be assigned to the intermolecular interactions between AZO and Graphene and the modifications of AZO's electronic structure due to the novel steric structure of the hybrid. It is clear that ln(A ∞ − A t /A ∞ − A 0 ) − t has one discrete segment which behaves linearly. The rate constant κ could be extracted from the slope. The obtained value of p 1 = 2.621 × 10 −3 s −1 .

Investigating the Photoisomerization Cycles
Exposing the material in the trans-isomerization state to UV-light reveals the conversion to the cis-isomerization state. The new cis-isomer state could be converted back to the trans-isomerization state by either a thermal or optical relaxation process. The PEO-(MR-Graphene) hybrid films are treated with the optimum UV-illumination conditions for trans to cis-state and then converted back to trans-state via blue-light relaxation process several times. Figure 8 shows a periodically repeated photoisomerization by optical relaxation process, confirming reliable hysteresis cycles for PEO-(MR-Graphene) hybrid nanocomposite thin films with no significant characteristic-loss. This fact confirms that the PEO-(MR-graphene) hybrid composite thin films may be considered candidates for many applications such as photochromic molecular switches, light-gated transistors, and molecular solar

Investigating the Photoisomerization Cycles
Exposing the material in the trans-isomerization state to UV-light reveals the conversion to the cis-isomerization state. The new cis-isomer state could be converted back to the trans-isomerization state by either a thermal or optical relaxation process. The PEO-(MR-Graphene) hybrid films are treated with the optimum UV-illumination conditions for trans to cis-state and then converted back to trans-state via blue-light relaxation process several times. Figure 8 shows a periodically repeated photoisomerization by optical relaxation process, confirming reliable hysteresis cycles for PEO-(MR-Graphene) hybrid nanocomposite thin films with no significant characteristic-loss. This fact confirms that the PEO-(MR-graphene) hybrid composite thin films may be considered candidates for many applications such as photochromic molecular switches, light-gated transistors, and molecular solar thermal energy storage media.

Atomic Force Microscope (AFM) Studies
Atomic Force Microscope (AFM) measurements were performed to investigate the morphological features of the PEO-(MR-Graphene) hybrid nanocomposite thin films. Figure 9 shows the AFM before (a) and after curing (b). In both cases, films exhibit amorphous nature. Before UV curing, films' surfaces appeared to be inhomogeneous compared with the surfaces of films after curing. Exposing films to UV curing induces strong film polymerization, yielding highly homogenous surfaces-the random distribution of the inserted MR-Graphene into PEO-host results in extremely inhomogeneous surfaces. However, illuminating films with UV light triggers MR-Graphene components of the composite to rearrange, occupying ordered sites throughout the PEO matrix. The relaxation of different composite components occurred to satisfy the minimum energy requirements and smoothen the surfaces greatly.

Conclusions
The fundamental mechanisms of the kinetics of photoisomerization of the PEO-(MR-Graphene) hybrid nanocomposite thin films are explored, evaluated, and investigated thoroughly. Deliberately, we explore and provide a new insight into the photoisomerization kinetics and time evolution for a hybrid thin-film based on Azo dye methyl red (MR) incorporated with graphene hosted in polyethylene oxide. The kinetics of photoisomerization and time evolution of the hybrid thin film was examined via the UV-Vis and the FTIR spectroscopic techniques and by using specific analytical models. The existence of the Azo Dye MR in the amalgamated composites is crucial for the effectual acts of the trans ↔ cis cyclic isomerization via UV-illumination ↔ Visible light-relaxation. Moreover, the UV-Vis and FTIR investigations confirm the hysteresis cycles of trans-cis-states. In conclusion, the PEO-(MR-Graphene) hybrid nanocomposite thin films are proven to be potential candidates for many applications. These applications include photochromic molecular switches, light-gated transistors, in addition to molecular solar thermal energy storage media.