Structural and light driven molecular engineering in photochromic polymers

Keywords

Photochromism

Photo-responsive materials

Polymers

Refractive index anisotropy

Optical Kerr effect

Optical switching

1. Introduction

Advanced optical materials have attracted a lot of attention recently, mainly due to their broad utilization in spectroscopy, photonics and opto-electronics [1,2]. Light-driven properties, such as polarization beam control, light amplification, harmonics of light generation or photoinduced refractive index anisotropy delivered by remote control fashion make materials engineering one of the pivotal field in the modern science [3]. Especially, organic materials play significant role in the current technologies. Giving an example, biomaterials are applied in various opto-electronic devices, wherein an optical gain, data transfer or storage or switching processes are based on photons, which paved the way to construct easily reconfigurable networks, light amplifiers, logic gates or finally, optical-based computer [[4], [5], [6], [7]]. Presently, one of the biggest challenge for the photonic devices is to up-grade their properties in order to provide stable signal over time and under various stimuli (thermal, optical, etc.). The idea assumes to use photons instead of electrons, which are much faster carriers to send and/or to collect the data in an effective way [[8], [9], [10], [11], [12]]. Photochromic polymers inscribe into multifunctional, organic materials, which found plenty of utilization in various fields. Hence, due to their increasing photochemical stability and thanks to the useful photo-responsive properties, they have been extensively explored since few decades [13,14]. Photochromic polymers were already used in biology, for drug delivery system, where serve as the photosensitive medicament carrier [15,16]. Then, reversible and photo-controlled separation system wettability in the form of micro-fluidics or membranes was constructed with the usage of photochromic polymers [[17], [18], [19]]. In the group of azobenzene-based polythiophenesthe photo- and thermochromism was observed [20]. Consequently, if consider only optical features, the following examples of application of photochromic polymers can be selected: optical switches [[21], [22], [23], [24]], surface relief gratings (SRG) [[25], [26], [27], [28]], optical data storage [29,30], dynamic holographic recording [31,32], or other, even more complex integrated optical devices [33].

All of the abovementioned applications of photochromic polymers were achieved due to their peculiar feature provided by, in the most cases, the azobenzene group [34]. Photochromism is a well-known phenomenon, studied before not only in polymers [34,35], which involves two photo-controlled and reversible molecular states characterizing absorption bands localized at various wavelengths [36,37]. Due to this feature and significant changes on the absorption cross-section value (coming from geometry reconfiguration), the following physical parameters can be easily modulated: dipole moment, phase change, refractive index [34]. If consider polymeric systems doped with Disperse Red 1 (DR1), as the example of efficient nonlinear chromophore, the photoinduced trans→cis→trans isomerizations yield was estimated at room temperature as follows: Φtrans→cis = 0.11 and Φcis→trans = 0.70, respectively [38]. One of the possible conformational transition mechanism responsible for the photochromism phenomenon in the various types of systems (both, guest:host, or photochromic polymers) is mentioned above trans-cis convertion. The trans(E) conformer is thermodynamically more stable than meta stable cis (Z) one and it can be converted to the latter mentioned by laser light irradiation, which is in resonance with its absorption band. In that way, the refractive index indicatrix is modulated and its shape is modified according to time revolution for its two components - parallel and perpendicular due to the linearly polarized laser mode direction [[39], [40], [41]]. The inverted reaction (cis→trans) can be photocontrolled by the intense light enclosed at the absorption region of meta stable conformer or based on the, so called, dark/thermal back transition, which is defined by k = 0.2 s−1, if consider again the DR1-based molecular system at room temperature [38]. Photoinduced trans-cis conformational changes in azobenzenes have been already reported in the literature, where various systems were investigated, i.e. solutions [42], spin-coated [43] or Langmuir-Blodgett [44] films. Azobenzene group is an example of the other, photo-responsive moieties characterizing efficient and reversible photochromic reaction. Amongst others, stilbene derivatives [45], spiropyran (merocyanine) [46] or furylfulgide (dihydrobenzofurane derivative) [47], could be distinguished. Currently, in order to implement photo-responsive fragment into the chemical structure, two main approaches are known and commonly employed. Namely, so called guest:host systems including specific, intermolecular interactions between matrix and dopants [[48], [49], [50], [51]], and photochromic polymers, where the active part is chemically incorporated into the main or side chain [[52], [53], [54]]. Macromolecular frameworks, including also co-polymers, seem to be compact, single-component and multifunctional architectures, which provide sufficient material's photo-response, designed and remote controlled in optical manner.

Nevertheless, among plenty of currently available photochromic polymers and co-polymers, still well-optimized and individualized structures are sought after, in order to provide appropriate material to the dedicated utilization. One of the currently known challenges in that field is to design and synthesize material, which can face all the requirements, like higher flexibility or rigidity [27,55], low or high glass temperature guarantees thermal stability in defined environment [56,57] or efficient, fast and stable over time optical response [[58], [59], [60]]. In view of the future application, final product should characterize diversified properties. However, it is well known that by reducing the physicochemical factors, like: polymer mass (Mw) or its glass transition value (Tg), the higher modulation depth is feasible [61]. Moreover, material engineering focused on the (co-)polymer's chemical structure shape impacts to the intra- and intermolecular interactions, including specific and non-specific interplay between the chains, but also the solvents and/or low-molecular dopants used during sample processing. While the branched macromolecular structures are created, the well-fitted network is formed and molecular packing increases, which causes its and (if so) applied additives lower mobility. Such processes and their consequences are crucial if consider, i.e. the surface relief gratings generation, its kinetics and magnification of the achieved folding deepness, what is involved with light diffraction efficiency, consequently [28]. Analogous situation is observed if consider molecular ordering caused by external stimuli, e.g. thermal or optical. The higher modulation depth, especially by means of photons, which are in resonance with the active medium and penetrate sample's volume, is connected with an optical path dimension. Two regimes should be distinguished - Bragg's and Raman-Nath [62]. Since the sample thickness is thin, the SRG can be recorded faster and the applied laser light may be less powerful. However, by employing thicker medium, it is possible to increase generated grating relief and obtain more effective light diffraction or amplification (e.g. in distributed feedback lasers) [7,63,64]. Comprising with aforementioned parameters it is possible to achieve individualized chemical structure able to undergoes particular phenomena driven by intense and polarized light source.

In this work we present two newly synthesized photochromic co-polymers, which differ in the photoisomerizable side-chain construction. In this contribution, the molecular engineering of the photo-responsive materials was developed in order to correlate chemical structure and nonlinear optical (NLO) response coming from the system. Moreover, by applying optically passive side groups, the structure's flexibility increased in a controllable way, likewise glass transition temperature was reduced. The synthetic route was introduced, and the structures were identified by nuclear magnetic resonance (NMR) spectroscopy and defined with CHNS elemental analysis. Then, selected physicochemical (μ, α, β, etc.) and material's (Mw, Tg, etc.) parameters were calculated with the usage of quantum chemical insight (Gaussian 09, Materials Studio 8.0). Finally, thanks to the spectroscopic investigation of obtained co-polymers in the shape of thin films, it allowed to characterize kinetics and dimension of the photo-induced effects, both, linear ones (UV–Vis photoisomerization and ellipsometry) and 3rd order nonlinear optical phenomenon. The latter mentioned considers all-optical switching process observed in the typical pump-probe laser set-up, where the optical Kerr effect(OKE) was investigated. Photoinduced birefringence with its kinetics, then also second, nonlinear optical refractive index value as well as 3rd order NLO susceptibility parameter, were characterized. The achieved experimental results show a great potential for the photochromic polymers and their utilization in the opto-electronic devices construction, and also prove their main advantage. By incorporating the NLO active medium into the side chain of the macromolecular structure, it was possible to create compact, single-component and multifunctional organic system dedicated to the all-optical switching operation, useful in the electro-optic logic gates and re-configurable networks.

2. Experimental

2.1. Materials and methods

Sulfamerazine, 2-(methylphenylamino)ethanol, 2-(N-ethylanilino)ethanol, azobisisobutyronitrile (AIBN, 98%), γ-butyrolactone, DMSO‑d6, methacrylic anhydride, 4-(dimethylamino)pyridine, 4-methoxyphenol, N-isopropylacrylamide (NIPAM) were purchased from Aldrich and were used without further purification. Tetrahydrofurane, pyridine, sodium nitrite, anhydrous sodium acetate, concentrated hydrochloric acid, and glacial acetic acid were purchased from POCH company (Poland).

2.2. NMR spectroscopy and elemental analysis

Chemical structure of the all of synthesized products was characterized by nuclear magnetic resonance spectroscopy (1H NMR) and CHNS elemental analysis. The 1H NMR spectra were recorded on a NMR Bruker AvanceTM600 MHz spectrometer using DMSO‑d6 as solvent and tetramethylsilane as an internal standard. The CHNS analyses were performed on a FLASH 2000 ThermoScientific elemental analyser. Glass transition temperatures of the polymers were determined with a Mettler Toledo 821e DSC apparatus. The heating and cooling rate were set at 10 K/min. Average molecular weight of the polymers were determined by Gel Permeation Chromatography (GPC). The measurements were carried out using azopolymer solutions in DMF solvent with the addition of 5 mmol/L LiBr. Polystyrene standards were used as the reference.

2.3. Synthesis route of the azo dyes

The azo dyes: 4-[(E)-[4-[2-hydroxyethyl(methyl)amino]phenyl]azo]-N-(4-methylpyrimidin-2-yl)benzenesulfonamide (SMERm) and 4-[(E)-[4-[ethyl(2-hydroxyethyl)amino]phenyl]azo]- N-(4-methylpyrimidin-2-yl)benzenesulfonamide (SMERe) were synthesized according to the previously reported method [50], followed by coupling reaction of the diazonium salt of sulfamerazine with 2-(methylphenylamino)ethanol (SMERm) or 2-(N-ethylanilino)ethanol (SMERe). The yields of aforementioned reactions were in the range of 95–98%.

2.4. Structural analysis of the azo dyes

(SMERm) 1H NMR (DMSO‑d6 with 0.05% v/v TMS, 600 Hz): δH 11.80 (1H, s, H-4), 8.31 (1H, d, H-3), 8.07 (2H, d, H-6), 7.87 (2H, d, H-5), 7.78 (2H, d, H-7), 6.83–6.90 (3H, m, H-2 and H-8), 4.80 (1H, s, H-12), 3.54 (2H, t, H-11), 3.34 (2H, s, H-10), 2.49 (3H, t, H-9), 2.31 (3H, s, H-1). Complete 1H NMR spectrum of SMERm dye was presented in Electronic Supplementary Information (ESI file), in Fig. S1.

Elemental analysis: calcd for C21H28N6O3S: C 56.74%, H 6.35%, N 18.90%, S 7.21%. Found: C 55.59%, H 5.71%, N 18.79%, S 7.64%.

(SMERe): 1H NMR (DMSO‑d6 with 0.05% v/v TMS, 600 Hz): δH 11.80 (1H, s, H-4), 8.30 (1H, d, H-3), 8.07 (2H, d, H-6), 7.84 (2H, d, H-5), 7.77 (2H, d, H-7), 6.81–6.90 (3H, m, H-2 and H-8), 4.83 (1H, s, H-13), 3.58 (2H, t, H-12), 3.50 (2H, t, H-11), 2.48 (2H, t, H-9), 2.30 (2H, s, H-10), 1.89 (3H, s, H-1). Complete 1H NMR spectrum of SMERe dye was presented in Fig. S2.

Elemental analysis: calcd for C22H30N6O3S: C 57.62%, H 6.59%, N 18.33%, S 6.99%. Found: C 57.48%, H 6.04%, N 18.84%, S 7.20%.

2.5. Synthesis route of the azo monomers

The preparation procedure and characterization details of both monomers M-SMERm and M-SMERe, were described in our previous paper, recently [25]. Briefly, the dyes SMERm or SMERe (0.01 mol), 4-(dimethylamino)pyridine (0.12 g) and 4-methoxyphenol (0.006 g) were dissolved in 20 mL of anhydrous pyridine. Subsequently, methacrylic anhydride (0.013 mol), was gradually added dropwise. The reaction was performed at boiling point and under an inert atmosphere of argon for 15 min. In the final step, the reaction mixture was poured into an excess amount of distilled water. The monomers were prepared with 95–96% yield.

2.6. Structural analysis of the monomers

(M-SMERm): 1H NMR (DMSO‑d6 with 0.05% v/v TMS, 600 Hz): δH 12.00 (1H, s, H-4), 8.33 (1H,d, H-3), 8.12 (2H, d, H-6), 7.89 (2H, d, H-5), 7.81 (2H, d, H-7), 6.90–6.93 (3H, m, H-2 and H-8), 5.97 (1H, s, H-14), 5.65 (1H, t, H-13), 4.32 (2H, t, H-11), 3.83 (2H, s, H-10), 3.10 (3H, s, H-9), 2.33 (3H, s, H-1), 1.83 (3H, s, H-12). Complete 1H NMR spectrum of M-SMERm monomer was presented in Fig. S3.

Elemental analysis: calcd for C25H32N6O4S: C 58.57%, H 6.29%, N 16.39%, S 6.26%. Found: C 57.92%, H 5.95%, N 16.61%, S 6.57%.

(M-SMERe): 1H NMR (DMSO‑d6 with 0.05% v/v TMS, 600 Hz): δH 12.10 (1H, s, H-4), 8.58 (1H, d, H-3), 8.33 (2H, d, H-6), 7.89 (2H, d, H-5), 7.79 (2H, t, H-7), 6.89–6.93 (3H, m, H-2 and H-8), 6.02 (1H,s, H-15), 5.68 (1H, s, H-14), 4.32 (2H, t, H-12), 3.77 (2H, t, H-11), 3.52–3.56 (3H, m, H-9), 3.35 (2H, s, H-10), 2.33 (3H, s, H-1), 1.86 (3H,s, H-13). Complete 1H NMR spectrum of M-SMERe monomer was presented in Fig. S4.

Elemental analysis: calcd for C26H34N6O4S: C 59.30%, H 6.51%, N 15.96%, S 6.09%. Found: C 59.54%, H 6.02%, N 15.92%, S 6.11%.

2.7. Synthesis route of the azo copolymers

The azo copolymers (p(SMERm-NIPAM) and p(SMERe-NIPAM)) were synthesized following the typical procedure of the radical polymerization [66] and was presented in the Scheme 1. The azo monomer M-SMERm or M-SMERe (0.01 mol) and non-chromophoric monomer NIPAM (0.01 mol), were dissolved in a mixture solvent containing 27 mL THF and 3 mL distilled water (9:1 v/v ratio). The synthesis was initiated with AIBN (10% by weight towards monomers). The reaction mixture was purged with nitrogen and heated at 70 °C for 68 h. Afterwards, the content was poured into methanol. The precipitate was separated from methyl alcohol and dried at 60 °C. The yields of the obtained copolymers were in the range of 65–70%.

2.8. Structural analysis of copolymers

(p(SMERm-NIPAM)): 1H NMR (DMSO‑d6 with 0.05% v/v TMS, 600 Hz): δH ~8.21 (2H, s, H-5), ~8.02 (2H, d, H-6), ~7.78–7.90 (4H, m, H-7 and H-8), ~6.82–6.90 (1H, m, H-2), ~6.69 (1H, m, H-15), ~4.31 (2H, m, H-11), ~4.18 (2H, m, H-10), ~4.10 (3H, m, H-17 and H-14), ~3.07 (3H, m, H-9), ~2.49 (3H, m, H-1), ~2.32 (2H, m, H-13), ~1.82 (3H, m, H-12), ~0.79–1.15 (6H, m, H-18). Complete 1H NMR spectrum of p(SMERm-NIPAM) copolymer was presented in Fig. S5.

Elemental analysis: found for C33H47N7O5S: C 57.90%, H 7.19%, N 14.93%, S 5.09%.

(p(SMERe-NIPAM)): 1H NMR (DMSO‑d6, with 0.05% v/v TMS, 600 Hz): δH ~11.90 (1H, s, H-4), ~8.29 (2H, s, H-5), ~8.09 (2H, d, H-6), ~7.77–7.82 (4H, m, H-7 and H-8), ~6.81–6.90 (1H, m, H-2), ~6.65 (1H, m, H-16), ~4.21 (2H, m, H-12), ~4.08 (2H, m, H-11), ~3.80 (3H, m, H-18 and H-15), ~3.68 (3H, m, H-9), ~3.55 (2H, m, H-10), ~2.51 (3H, m, H-1), ~2.29 (2H, m, H-14), ~1.92 (3H, m, H-13), ~0.70–1.17 (6H, m, H-19). Complete 1H NMR spectrum of p(SMERe-NIPAM) copolymer was presented in Fig. S6.

Elemental analysis: found for C34H49N7O5S: C 57.86%, H 7.34%, N 13.62%, S 4.56%.

All products were precisely characterized by available techniques, including 1H NMR and elemental analysis. These methods were used to confirm the structure shape and purity of synthesized dyes, monomers and copolymers. Elemental analysis enabled to define elemental composition of the obtained materials. Both of the photochromic monomers showed the vinyl proton signals characteristic for protons at C atoms in a double bond (CH2C) at ~5.65 ppm and ~6.00 ppm. Moreover, on the spectra were also recorded signals from two double-substituted benzene rings (~7.70–8.20 ppm), pyrimidine ring (~6.80–6.90 ppm) and methyl and ethyl group at nitrogen atom (~2.30 ppm and ~3.35 ppm, respectively). As a result of the radical polymerization of the photochromic monomer (M-SMERm or M-SMERe) and non-chromophoric co-monomer (N-isopropylacrylamide - NIPAM), the two novel azobenzene-based copolymers were obtained. 1H NMR spectra of the copolymers contain broad multiplets from protons of methylene groups of the non-chromophoric mer at ~0.80–1.20 ppm and characteristic signals from pyrimidine and benzene ring and methyl and ethyl group at nitrogen atom. However, no signals from protons at carbon atoms in both, chromophore and non-chromophoric methacrylatemonomers at double bond were observed. It may be further confirmation that monomers were reacted completely. The content of azo part in copolymer was estimated on the integrals calculated by 1H NMR spectra using following equation (1) [65]:(1) $%_{A Z O} = \frac{\frac{(H_{7}, H_{8})}{4}}{\frac{(H_{7}, H_{8})}{4} + (\frac{H_{18}}{6})} \cdot 100 %$ where H7, H8 and H18 (or H19 for p(SMERe-NIPAM)) are integrals of the signals at ~7.78–7.90 and ~0.7–1.15 ppm, respectively (Figs. S5 and S6). Calculated final molar content of azo (%AZO) were 61.2 mol% for p(SMERm-NIPAM) and 45.2 mol% for p(SMERe-NIPAM).

2.9. Quantum chemical calculations

The calculations were carried out using resources provided by Wroclaw Centre for Networking and Supercomputing (Gaussian software [67]). Geometry of repeating units occurring in copolymers presented in this work was optimized using RHF method and 3–21 g basis set. For the first hyperpolarizability of azobenzene-containing molecules calculations was used the same method and basis set. That combination gave reliable results in our previous works [25,68,69]. The computational chemistry methods were used also to predict selected material's parameters, like thermal, mechanical and structural properties of the obtained photochromic copolymers.

2.10. UV–vis spectroscopy

The absorption spectra were recorded on HITACHI U-1900 Spectrophotometer. The measurements were performed before and after irradiation with laser beam at 445 nm wavelength, for various exposure time. The measurements were carried out for the thin films prepared by spin-coating technique.

2.11. Thin films preparation

Copolymers p(SMERm-NIPAM) and p(SMERe-NIPAM) (5 wt% and 10 wt%, respectively) were dissolved in THF and filtered through the syringe filter. Thin azopolymers films were prepared by spin-coating technique using Laurell's WS-400-B-6NPP-LITE spin coater. The spin-up speed was set at 1000 rpm for 30 s. After deposition the films were dried at 50 °C for 24 h. The thickness of the films prepared from 5 wt% solutions of azopolymer is 428 and 543 nm for p(SMERm-NIPAM) and p(SMERe-NIPAM), respectively. Higher concentration of copolymers (10 wt%) resulted in obtaining more than twice as thick layers: 1123 and 1194 nm, respectively.

2.12. Ellipsometry

Ellipsometric measurements were performed with EL X–02C Ellipsometer, DRE-Dr, Ellipsometerbau Gmbh (Germany) operating at an incident angle of 70° and using linearly polarized laser beam at 632 nm and ca. 3 mW power. Before UV–Vis and ellipsometric measurements, the samples were stored in the dark at room temperature overnight to ensure that all of the azobenzene units were in the trans configuration. The polymer thin films were irradiated with laser beam at 445 nm wavelength. All of the measurements were carried out at the room temperature.

Ellipsometric experiment allows to define light polarization changes, which are involved with Fresnel coefficients and are described by the following equation [70]:(2) $r_{p} / r_{s} = t g (Ψ) e x p (i Δ)$ where rp and rs are reflection coefficients for light polarization parallel and perpendicular to the plane of incidence, Ψ determines amplitude of the polarized light and Δ describes the phase difference of the reflected p and s polarized light (Δ = Δp-Δs). Experimentally estimated values of Ψ and Δ parameters, which describe the real and imaginary part of the refractive index(n), properly, enable to calculate complex n parameter value. The total refractive index coefficient is expressed by the equation (3):(3) $n = n_{r} + i k$ where nr is defined as the real part of refractive index and ik describes imaginary part of n (k is extinction coefficient). The refractive index change (Δnr) can be calculated according to the equation (4):(4) $Δ n_{r} = n_{r}^{0} - n_{r}^{p h o t o s t a t .}$ where $n_{r}^{0}$ and $n_{r}^{p h o t o s t a t .}$ are the real part of the refractive index before and after laser light illumination, measured at the photostationary state, respectively.

2.13. Nonlinear spectroscopy

Photoinduced birefringence was measured in a typical pump-probe laser set-up [71]. To characterize created optical anisotropy of refractive index (Δn), the sample was placed in the cross-polarizer system, which allows to identify NLO signal by photodiode, placed behind the analyzer (set-up is shown in the next section, in the Fig. 3(b)). One of the linearly polarized laser lines, which is called probe or reference beam (λref), is out of the absorption resonance if consider azobenzene groups embedded in the polymer and serves only to monitor the sample and photoinduced material's changes. Initially, when the sample still sustains in the optical isotropic condition, no output signal is collected by the photodiode. While, the second, linearly polarized pump laser line (λpump) is “ON” and is absorbed by the active molecules (chromophores embedded in the side polymer chains), the optical anisotropy is generated in the function of optical path (d - sample thickness), time (t) and light intensity (eq. (5)) [59,60,72]:(5) $I_{t r a n s}^{r e f} (t) = I_{0}^{r e f} s i n^{2} (\frac{π d Δ n (I_{p u m p} (t))}{λ_{r e f}})$ where, $I_{0}^{r e f}$ and $I_{t r a n s}^{r e f} (t)$ denote probe laser intensity measured just before the sample and behind the analyzer, respectively. Whereas $I_{p u m p}$ refers to the pump beam intensity inducing NLO response. Photoinduced birefringence (Δn) is strictly connected with the phase change (Δϕ), which is involved with the long-term molecular photoordering and multiple short-term conformational transformations of the azobenzene chromophore groups [59,60,72]. From the very first moment, when the pump laser light is applied, the initially isotropically oriented photo-responsive molecules (molecular fragments) absorb the light according to the equation (6) [72]:(6) $P_{a b s} = c o s^{2} α$ where $P_{a b s}$ denotes absorption probability and $α$ constitutes the angle between the laser light polarization direction and the axis defining chromphore's main fragment orientation. Based on that process, irradiated population of azobenzenes sensitive for such laser treatment consumes supplied photons ( $h ν$ ) for molecular conformational transition (from the lower in energy trans state to the meta stable cis one) [73]:(7) $t r a n s \overset{h ν}{\to} c i s$

However, the reverse transition (cis→trans) is observed. It takes place due to the dark, thermal relaxation processes, defined as thermo-physical transformation. Another way to provide reverse conformational transition is to use wavelength in resonance of the cis molecules [59,73]:(8) $c i s \overset{h ν^{'} o r k T}{\to} t r a n s$ where $h ν^{'}$ refers to the photons being absorbed by meta stable cis conformers, and k and T denote thermodynamic constant value and environmental temperature, respectively. Consequently, after multiple molecular photo-ordering processes involving chromophore groups, the obtained optical anisotropy of the refractive index can be described as [60]:(9) $Δ n (I_{p u m p}, t) = n_{⊥} (I_{p u m p}, t) - n_{∥} (I_{p u m p}, t)$ where $n_{⊥}$ and $n_{∥}$ represent the $n$ components oriented perpendicular and parallel due to the inducing laser polarization direction. The total value of refractive index can be modulated by intense and polarized light being in resonance with the active material. As a consequence the second, nonlinear refractive index coefficient ( $n_{2}$ ), defined by Equation (10) is generated [72]:(10) $Δ n (I_{p u m p}, t) = n - n_{0} = n_{2} I_{p u m p}$ where, $n_{0}$ represents linear value of refractive index, correlated with wavelength. In such a way, the indicatrix is modified from sphere to the ellipsoid, and created optical birefringence allows electromagnetic wave to pass through the active material with different kinetics, according to the following Equations (11), (12) [60]:(11) $V_{⊥} = c_{0} / n_{⊥}$ (12) $V_{∥} = c_{0} / n_{∥}$ where $V_{⊥}$ and $V_{∥}$ represent, so called, slow and fast axis for electromagnetic wave propagation in the anisotropic medium, respectively, and $c_{0}$ denotes light velocity in the vacuum environment. Based on the experimentally estimated values of the refractive index anisotropy, the third order nonlinear optical susceptibility ( $χ^{(3)}$ ) considering SI unit system, can be evaluated according to the equation (13) [60,72]:(13) $χ^{(3)} = \frac{4 n_{2} n_{0}^{2} ε_{0} c_{0}}{3}$ where $ε_{0}$ corresponds to the dielectric constant in vacuum.

3. Results and discussion

3.1. Physicochemical parameters calculation

In order to provide physicochemical insight into the newly synthesized photochromic copolymers, the quantum chemical approach was applied (Table 1, Table 2). Going into details, the basic properties derived from structure's geometry optimization were estimated.

Table 1. Physicochemical properties of the trans and cis isomers of azo-monomers provided by quantum chemical methods (Gaussian 09/RHF method).

Properties	Monomer
	M-SMERm		M-SMERe
	trans	cis	trans	cis
μ (D)	8.96	2.01	9.05	2.07
μ (Cm) ∙10−30	29.9	6.71	30.2	6.92
Vm (cm3/mol)	379.64	326.47	381.14	296.36
α0 (C2m2J−1) ∙10−40	51.5	47.2	53.4	49.1
β0 (C3m3J−1) ∙10−40	17.1	6.47	18.2	7.00
ΔHF (kJ/mol)	82.12		82.04

Table 2. Physicochemical properties of the copolymers achieved by theoretical approach thanks to the Synthia module in Materials Studio software.

Propertiesa	Polymer
Propertiesa	p(SMERm-NIPAM)	p(SMERe-NIPAM)
Tg (°C)	85.8	80.4
αV (ppm/K)	270.5	274.3
ρ (g/cm3)	1.23	1.22
Cp of solid (J∙(mol∙K)−1)	374.3	387.2
κ (J∙(K∙m∙s)−1)	0.158	0.158
n (−)	1.571	1.568
E (GPa)	6.52	6.16

a: Estimated at 298 K.

Hence, in both cases, the dipole moment (μ) values are significantly higher for the trans isomers if compare with their cis equivalents. Considering the first mentioned conformers, μ parameter was evaluated to be around 9 Debyes for M-SMERm and M-SMERe compounds, respectively. While, the cis conformers characterize dipole moment values around 2 D, for both monomers, adequately. The similar tendency was observed and reflects in the other analyzed parameters, like polarizability (α0) and first hyperpolarizability (β0) values. The value of potential energy difference (ΔHF) between trans and cis isomers was around 82 kJ/mol for both monomers and was similar to the other azo-based compounds sulfonamide, which are described in the literature [68,74].

Basically, ΔHF increases with increasing length of the aliphatic spacer between nitrogen atom and methacrylic group in monomer [74]. In our case distance between them is the same for both monomers. Slight structural difference between obtained photochromic polymers provided significant change in their molar volumes (Vm), if compare both, pairs of conformers (trans and cis) as well as pairs of derivatives (M-SMERm and M-SMERe), respectively. The Vmparameter is always higher for thermodynamically more stable trans forms of about 16% and 29% for methyl and ethyl-functionalized copolymers, respectively. Interestingly, when stress out the molar volumes of the cis forms for both of the considered intermediates, the one containing ethyl moiety contributes much less space than its methyl derivative. Hence, the molecular ordering based on the trans-cis transformations can provide higher efficiency for M-SMERe monomers (forthcoming pSMERe-NIPAM copolymer) than M-SMERm one (future pSMER-mNIPAM macrostructure), respectively. Considered behavior is strictly involved with molecular packing of chromophore and non-chromophoric side-chain spacers influencing on the free volume as well as the molar volume values. Furthermore, using Synthia module in Materials Studio software, the basic material's properties of the obtained copolymers were also computed (Table 2). The calculations were performed for the room temperature (298 K) conditions, whereas the assumed molecular weight value of copolymers was set at 10 000 Da. Such approach allowed to deliver deep insight into materials properties and correlate them with slightly changed chemical structure. However, majority of the analyzed parameters were similar, if consider both derivatives (pSMERm-NIPAM and pSMERe-NIPAM), and their trans and cis conformers. For instance, glass transition temperature (Tg), coefficient of volumetric thermal expansion (αV), density (ρ), thermal conductivity (κ) or Young's modulus (E) vary on about 5%, approximately.

During computational studies performance, we assumed repeatable model of alternating copolymers, where all monomer units are linked in a head-to-tailfashion. In turn, as a result of the radical polymerization random copolymers were obtained. In principle, the physicochemical properties of the polymers are highly influenced by many factors, like molecular weight, tacticity, synthesis method and the degree of polymerization, amongst others [75,76]. Giving an example, the glass transition temperature (Tg) was estimated theoretically at 85.8 °C and 80.4 °C for methyl and ethyl-functionalized copolymers, respectively. Instead, Tg parameter measured experimentally (Table 3) gave even lower numbers, 58 °C and 57 °C, accordingly. If compare these values with the same parameter referring to the commercially available synthetic polymers, like poly(methyl metacrylate) (Tg ≈ 57 °C) [77,78] polycarbonate (Tg ≈ 150 °C) [79], or poly(vinyl carbonate) (Tg ≈ 200 °C) [80,81], it seems that in our case they are much lower. Consequently, it influences straightforwardly to the polymers rheological features, including their flexibility [61,74]. Nevertheless, PMMA, PC or PVK represent only optically passive materials (usually, thanks to their high transparency and molecular stability upon various stimuli, like laser pulses [82]), while just synthesized copolymers are photo-responsive, which makes them multifunctional and optically active macromolecules.

Table 3. Selected physicochemical and optical properties of the obtained photochromic copolymers. Description in the text.

Properties	Polymer
Properties	p(SMERm-NIPAM)		p(SMERe-NIPAM)
Mw (g/mol)	20000		15500
Mw/Mn	3.1		2.4
Tg (oC)	58		57
Azopolymer solution concentration	5%	10%	5%	10%
Absorbance λmax (nm)	436	436	434	438
Absorbance change, ΔAa (%)	27	18	28	20
nr	1.556	1.575	1.571	1.585
Δnr	0.024	0.010	0.028	0.011
d (nm)	428	1123	543	1194

a

$Δ A_{%} = (A_{0} - A_{\infty}) / A_{0}$

3.2. Photoisomerization investigation

The photoinduced trans-cis and reverse, dark thermal cis-trans isomerization of the azo polymers in thin films were investigated. Beforehand the spectroscopic measurements, all of the samples due to their optical sensitivity, were stored in the dark environment at room temperature overnight. It was necessary to ensure that all of the azobenzene units remain in the lower in energy, molecular trans configuration. Afterwards, the reversible trans-cis photoisomerization of the synthesized copolymers was examined using UV–Vis spectroscopy. The absorption spectra were recorded by few different approaches: (i) at the beginning, before any of light illumination, then subsequently (ii) after various exposure time illumination of laser light (λ = 445 nm) and finally, (iii) during thermal relaxation accomplished in complete darkness conditions. The spectral changes due to the absorption band position and intensity, likewise aforementioned molecular transformation kinetics for thin films prepared from two different polymer concentrations of the initial solutions (5% wt. and 10% wt.), were analyzed (Table 3 and Fig. 1). Then, the maximum value of absorption band (λmax) position of the considered azo polymers was observed in the blue region of spectrum, around 434–438 nm. The exact values of λmax and basic properties of copolymers were gathered in Table 3. The typical spectral changes of absorption bands (including occurrence of two isozbestic points) during laser exposure, considering pSMERm-NIPAM copolymer, are visible in Fig. 1(a). Analogous results for the second derivative, containing ethyl group, was placed in the ESI file, in Fig. S7. It is clearly shown that the azo-based chromophore moieties in the obtained copolymers undergo the quick trans-cisphotoisomerization, resulting in a strong decrease in value of absorbance at ~436 nm, on about 27%. Absorbance changes (ΔA) were defined as a difference in absorbance value of the initial state (A0) and after 20 min of laser irradiation (A∞). A photostationary state for both copolymers was reached after 4 min of laser light illumination. After that time no significant spectral changes were observed. The higher change of absorbance value was observed for 5% wt. sample in comparison with 10 wt% one of the considered photochromic copolymers. After switching off light source, the dark thermal relaxation was conducted during which chemical structure was slowly returned to the initial (trans) state and, consequently, the value of absorbance of considered form increased (Fig. 1(b)).

The cis→trans process was carried out for 120 min. In this time no complete return of absorbance to the initial value was observed, only a partial increase was recorded. Hence, the longer time is needed to achieve the initial state. Therefore, the relaxation process was conducted for 24 h. After this time a complete return of absorbance value to the initial value was achieved. The kinetic parameters of the synthesized copolymers are comparable with results for thin films of the similar materials [69,83], however the kinetic behavior in polymer solution are more often reported in literature [[84], [85], [86]].

Afterwards, the kinetics of trans-cis photoisomerization and thermal relaxation process, was investigated. The experimental data were analyzed according to the second-order kinetics expression. Hereof, the photoinduced trans-cismolecular transformations process kinetics can be evaluated following the equation (14) [87]:(14) $\frac{A_{\infty} - A_{t}}{A_{\infty} - A_{0}} = α \cdot e x p (- k_{1} t) + (1 - α) \cdot e x p (- k_{2} t)$ where, $A_{0}$ , $A_{t}$ , and $A_{\infty}$ represent the absorbance of the trans form corresponding to the time 0 (initial one), t and photostationary state, respectively. α coefficient describes the fraction of the fast photoisomerization stage in total conversion of the system. Then, k1 and k2 are the rate constants of the trans-cis conformational transformation.

Fig. 1(c) shows the second-order plot for the photoinduced trans-cisisomerization of the considered materials in the two concentration regimes, each. The photoisomerization rate constants were higher for the samples containing 5 wt% of azo copolymer. Indeed, the lower concentration (due to the second sample with 10 wt%) of the azo polymer gives more freedom necessary for the molecular movements of azobenzene groups during photoinduced molecular reorientation. The mobility of azobenzene derivatives in the side-polymer chains strongly depends on the free volume available around the chromophore's unit [88,89]. Moreover, entanglement of polymer chain and the chromphore/non chromophore block ratio will play important roles in mobility as well. Hence, the considered trans-cis photoisomerization process in copolymers is faster than in the homopolymers. This is in agreement with the previous studies of azobenzene polymers [25,90].

Subsequently, kinetic behavior of the copolymers during so called dark cis-transisomerization was studied. Kinetics parameters of the thermal relaxation were calculated due to the equation (15):(15) $\frac{A_{0} - A_{t}}{A_{0} - A_{\infty}} = α \cdot e x p (k_{1}^{'} t) + (1 - α) \cdot e x p (k_{2}^{'} t)$

where, $k_{1}^{'}$ and $k_{2}^{'}$ denote rate constants related to the cis-trans isomerization, and $α^{'}$ describes the fraction in total conversion of system. The rate constants for thermal relaxation are significantly smaller than for the aforementioned photoinduced isomerization process. It is related with the process environment causing thermal relaxation, which was carried out in the complete darkness condition without any external stimuli, i.e. laser line being in resonance with the cis isomer band. Comparison of the thermal relaxation kinetics for the both derivatives in two different concentrations, is presented in Fig. 1(d). Intriguingly, for the reverse molecular reorientation, the ethyl-based photochromic polymer characterizes higher rate constant values than its methyl equivalent. It may results from the less transversed molecules population by the photoinduced process (trans-cis). In such a case reverse transition is feasible faster in order to achieve again the thermodynamic equilibrum state. The trans-cis-trans isomerization constants and other kinetic parameters are listed in Table 4. Both processes were well determined with the second-order kinetics (due to the eqs. (14), (15)).

Table 4. Kinetic data for the trans-cis photoisomerization and cis-trans back transition, thermal relaxation in the darkness condition, determined by UV–Vis measurements.

Kinetic parameters	Polymer
Kinetic parameters	p(SMERm-NIPAM)		p(SMERe-NIPAM)
Concentration of the azopolymer	5 wt%	10 wt%	5 wt%	10 wt%
Photoinduced trans→cis transformation
A∞/A0	0.73	0.82	0.72	0.80
α	0.44	0.86	0.63	0.61
k1 ∙ 10−3 (s−1)	62.9	34.7	53.9	43.7
1-α	0.56	0.14	0.37	0.39
k2 ∙ 10−3 (s−1)	13.9	2.47	14.7	10.2
Dark, thermal relaxation (cis→trans)
α′	0.25	0.17	0.27	0.29
k1’ ∙ 10−3 (s−1)	2.02	3.24	2.65	2.42
1-α′	0.75	0.83	0.73	0.71
k2’ ∙ 10−3 (s−1)	0.075	0.051	0.082	0.097

3.3. Ellipsometry

Photochromic properties of the synthesized copolymers were also investigated using Ellipsometry technique. Ellipsometry is a non-destructive method, which enables to determine two parameters: Δ and Ψ, which describe amplitude and phase shift of the polarized light, respectively. Therefore, it is possible to estimate variations of the refractive index occurring during laser light illumination. Measurements were carried out using thin polymer films, while and after periodic illumination with laser light. Subsequently, significant changes of Δ and Ψ parameters for p(SMERm-NIPAM) and p(SMERe-NIPAM) were acquired and are presented in Fig. 2(a) and (b), respectively.