Structural and light driven molecular engineering in photochromic polymers

Journal Pre-proof Structural and light driven molecular engineering in photochromic polymers Adam Szukalski, Aleksandra Korbut, Ewelina Ortyl PII:

S0032-3861(20)30149-X

DOI:

https://doi.org/10.1016/j.polymer.2020.122311

Reference:

JPOL 122311

To appear in:

Polymer

Received Date: 4 September 2019 Revised Date:

19 February 2020

Accepted Date: 21 February 2020

Please cite this article as: Szukalski A, Korbut A, Ortyl E, Structural and light driven molecular engineering in photochromic polymers, Polymer (2020), doi: https://doi.org/10.1016/ j.polymer.2020.122311. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Adam Szukalski: Conceptualization, Formal analysis, Investigation, Data curation, Writing - Original daft, Visualization; Aleksandra Korbut: Formal analysis, Investigation, Resources, Data curation, Writing - Original draft; Ewelina Ortyl: Writing - Review & Editing.

Structural and light driven molecular engineering in photochromic polymers

Adam Szukalski a,*, Aleksandra Korbut b, Ewelina Ortylb

a

Faculty of Chemistry, Advanced Materials Engineering and Modelling Group, Wroclaw

University of Science and Technology, Wybrzeze Wyspianskiego 27, 50320 Wroclaw, Poland b

Faculty of Chemistry, Department of Polymer Engineering and Technology, Wroclaw

University of Science and Technology, Wybrzeze Wyspianskiego 27, 50320 Wroclaw, Poland. e-mail address: [email protected]; [email protected]; [email protected] *Corresponding Author: [email protected] (A. Szukalski)

Abstract Light driven properties and remote control modulation in organic materials is still a challenging subject. Optical data storage has revolutionized modern technology. According to increasing interest of more advanced and better-controlled systems composed of the organic materials, photochromic polymers take significant part of still desired objective. In this contribution, we demonstrate two novel single-component macromolecular photo-responsive systems, characterized by optical Kerr effect (OKE) experiment. Productive and fully reversible all-optical switching can be easily obtained with utilization of thin polymeric film. An employed 30 cycles of remote controlled light driven (de)activation refractive index anisotropy (∆n) demonstrate efficient optical modulation. Importantly, two copolymers structures intentionally varying slightly in the chemical construction characterize significantly different nonlinear optical (NLO) response. The output ∆n signal, enclosing kinetics, magnification, reversibility and stability can be easily controlled by applying molecular engineering concerning chemical architecture of the photo-responsive system. We present herein the photoisomerization studies and nonlinear optical spectroscopic analysis considering OKE phenomenon and obtained parameters, including photoinduced birefringence and 3rd order NLO susceptibility. Aforementioned approach allowed to construct organic-based device dedicated to the future utilization in opto-electronics and photonics as effective optical modulator or switch.

Keywords: photochromism, photo-responsive materials, polymers, refractive index anisotropy, optical Kerr effect, optical switching

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 [4-7]. 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 [8-12]. 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 photoresponsive 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 [17-19]. In the group of azobenzene-based polythiophenes the 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 [21-24], surface relief gratings (SRG) [25-28], 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 [39-41]. 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, photoresponsive 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 [48-51], and photochromic polymers, where the active part is chemically incorporated into the main or side chain [52-54]. Macromolecular frameworks, including also co-polymers, seem to be compact, singlecomponent and multifunctional architectures, which provide sufficient material's photoresponse, designed and remote controlled in optical manner. Nevertheless, among plenty of currently available photochromic polymers and copolymers, 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 [58-60]. 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 alloptical 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.

Experimental 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). 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. Synthesis route of the azo dyes The azo dyes: 4-[(E)-[4-[2-hydroxyethyl(methyl)amino]phenyl]azo]-N-(4-methylpyrimidin-2yl)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%. 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 Figure 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 Figure 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%. Synthesis route of the azo monomers The preparation procedure and characterization details of both monomers M-SMERm and MSMERe, 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 minutes. In the final step, the reaction mixture was poured into an excess amount of distilled water. The monomers were prepared with 95-96% yield. 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 MSMERm monomer was presented in Figure 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 1

H NMR spectrum of M-SMERe monomer was presented in Figure 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%. 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%.

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), ~0.791.15 (6H, m, H-18). Complete 1H NMR spectrum of p(SMERm-NIPAM) copolymer was presented in Figure S5. Elemental analysis: found for C33H47N7O5S: C 57.90%, H 7.19%, N 14.93%, S 5.09%.

Scheme 1. Synthesis route of the azo copolymers. The azobenzene units were marked in the orange background, whereas applied different space moieties (methyl and ethyl groups) were schematically presented as X, which refers to the red and blue subtitles associated with the used acronyms of photochromic polymers, p(SMERm-NIPAM) and p(SMERe-NIPAM), respectively.

(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 Figure 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 (CH2=C) at ~5.65 ppm and ~6.00 ppm. Moreover, on the spectra were also recorded signals from two doublesubstituted 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 nonchromophoric co-monomer (N-isopropylacrylamide - NIPAM), the two novel azobenzenebased 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 methacrylate monomers 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]: %

=

, ,

∙ 100%

(1)

where H7, H8 and H18 (or H19 for p(SMERe-NIPAM)) are integrals of the signals at ~7.787.90 and ~0.7-1.15 ppm, respectively (Figure S5 and S6). Calculated final molar content of azo (%AZO) were 61.2 mol% for p(SMERm-NIPAM) and 45.2 mol% for p(SMEReNIPAM). 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 3-21g 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. 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. 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. 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)

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 ∆ describes the phase difference of the reflected p and s polarized light (∆=∆p-∆s). Experimentally estimated values of Ψ and ∆ 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)

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 (∆nr) can be calculated according to the equation (4): &"# = "#' − "# )*+* where "#' and "#

)*+* +,+.

+,+.

(4)

are the real part of the refractive index before and after laser light

illumination, measured at the photostationary state, respectively. Nonlinear spectroscopy Photoinduced birefringence was measured in a typical pump-probe laser set-up [71]. To characterize created optical anisotropy of refractive index (∆n), 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 Figure 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]: #01

.+#,/ #01

where, .'

#01

and .+#,/

#01

= .' 2 "3 4

56∆/789:;9 + < =>?@

A

(5)

denote probe laser intensity measured just before the sample and

behind the analyzer, respectively. Whereas .

BC

refers to the pump beam intensity inducing

NLO response. Photoinduced birefringence (∆n) is strictly connected with the phase change (∆ϕ), 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]: D,E = FG2 3 H

(6)

where D,E denotes absorption probability and H 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 (ℎJ) for molecular conformational transition (from the lower in energy trans state to the meta stable cis one) [73]: )L

K"2 MN F 2

(7)

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]: )L O *# PQ

F 2 MRRRRRN

K"2

(8)

where ℎJ S 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]: Δ"7.

BC

, < = "U 7.

BC

, < − "∥ 7.

BC

, <

(9)

where "U and "∥ represent the " 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 ("3 ), defined by equation (10) is generated [72]: Δ"7.

BC

, < = " − "' = "3 .

BC

(10)

where, "' 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]: WU = F' /"U

(11)

W∥ = F' /"∥

(12)

where WU and W∥ represent, so called, slow and fast axis for electromagnetic wave propagation

in the anisotropic medium, respectively, and F' denotes light velocity in the vacuum

environment. Based on the experimentally estimated values of the refractive index anisotropy, the third order nonlinear optical susceptibility (X

Y

) considering SI unit system, can be

evaluated according to the equation (13) [60, 72]: X

Y

=

Z/[ /\[ ]\ ^\ Y

(13)

where _' corresponds to the dielectric constant in vacuum. Results and discussion Physicochemical parameters calculation In order to provide physicochemical insight into the newly synthesized photochromic copolymers, the quantum chemical approach was applied (Table 1 and 2). Going into details, the basic properties derived from structure's geometry optimization were estimated. 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 (∆HF) 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]. Table 1. Physicochemical properties of the trans and cis isomers of azo-monomers provided by quantum chemical methods (Gaussian 09/RHF method). Monomer Properties µ (D) µ (Cm) ·10-30 Vm (cm3/mol) α0 (C2m2J-1) ·10-40 β0 (C3m3J-1) ·10-40 ∆HF (kJ/mol)

M-SMERm trans cis 8.96 2.01 29.9 6.71 379.64 326.47 51.5 47.2 17.1 6.47 82.12

M-SMERe trans cis 9.05 2.07 30.2 6.92 381.14 296.36 53.4 49.1 18.2 7.00 82.04

Basically, ∆HF 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 Vm parameter 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. Table 2. Physicochemical properties of the copolymers achieved by theoretical approach thanks to the Synthia module in Materials Studio software. Polymer Properties* Tg (°C)

αV (ppm/K) ρ (g/cm3)

Cp of solid (J·(mol·K)-1) κ (J·(K·m·s)-1) n (-) E (GPa) * estimated at 298 K

p(SMERm-NIPAM)

p(SMERe-NIPAM)

85.8 270.5 1.23 374.3 0.158 1.571 6.52

80.4 274.3 1.22 387.2 0.158 1.568 6.16

During computational studies performance, we assumed repeatable model of alternating copolymers, where all monomer units are linked in a head-to-tail fashion. 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 85.8oC and 80.4oC for methyl and ethyl-functionalized copolymers, respectively. Instead, Tg parameter measured experimentally (Table 3) gave even lower numbers, 58oC and 57oC, accordingly. If compare these values with the same parameter referring to the commercially available synthetic polymers, like poly(methyl metacrylate) (Tg ≈ 57oC) [77, 78] polycarbonate (Tg ≈ 150oC) [79], or poly(vinyl carbonate) (Tg ≈ 200oC) [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. 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 Figure 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

Table 3. Selected physicochemical and optical properties of the obtained photochromic copolymers. Description in the text. Polymer Properties Mw (g/mol) Mw/Mn Tg (oC) Azopolymer solution concentration Absorbance λmax (nm) Absorbance change, ∆A* (%) nr ∆nr d (nm) * ∆`% = `' − `a /`'

p(SMERm-NIPAM)

p(SMERe-NIPAM)

20000 3.1 58

15500 2.4 57

5% 436 27 1.556 0.024 428

10% 436 18 1.575 0.010 1123

5% 434 28 1.571 0.028 543

10% 438 20 1.585 0.011 1194

Figure 1. Changes in the UV-Vis absorption spectra during trans-cis photoisomerization induced by laser line (λ = 445 nm) (a) and cis-trans dark thermal relaxation (b) with their kinetics, (c) and (d), properly, of p(SMERm-NIPAM) thin film (5 wt.%).

typical spectral changes of absorption bands (including occurrence of two isozbestic points) during laser exposure, considering pSMERm-NIPAM copolymer, are visible in Figure 1(a). Analogous results for the second derivative, containing ethyl group, was placed in the ESI

file, in Figure S7. It is clearly shown that the azo-based chromophore moieties in the obtained copolymers undergo the quick trans-cis photoisomerization, resulting in a strong decrease in value of absorbance at ~436 nm, on about 27%. Absorbance changes (∆A) 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 four minutes 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 (Figure 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 hours. 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 [84-86]. 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-cis molecular transformations process kinetics can be evaluated following the equation (14) [87]: bc d

bc \

=H∙

−%e

+ 1−H ∙

−%3

(14)

where, `' , `+ , and `a 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. Figure 1(c) shows the second-order plot for the photoinduced trans-cis isomerization 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-trans isomerization was studied. Kinetics parameters of the thermal relaxation were calculated due to the equation (15): \c d

\c b

= HS ∙

%eS

+ 1−H ∙

%3S

(15)

where, %eS and %3S denote rate constants related to the cis-trans isomerization, and H S 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 Figure 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 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. Polymer

p(SMERm-NIPAM) Kinetic parameters Concentration of the 5 wt.% 10 wt.% azopolymer Photoinduced trans→cis transformation A∞/A0 0.73 0.82 α 0.44 0.86 k1 · 10−3 (s−1) 62.9 34.7 1-α 0.56 0.14 k2 · 10−3 (s−1) 13.9 2.47 Dark, thermal relaxation (cis→trans) α' 0.25 0.17 −3 −1 k1' · 10 (s ) 2.02 3.24 1-α' 0.75 0.83 k2' · 10−3 (s−1) 0.075 0.051

p(SMERe-NIPAM) 5 wt.%

10 wt.%

0.72 0.63 53.9 0.37 14.7

0.80 0.61 43.7 0.39 10.2

0.27 2.65 0.73 0.082

0.29 2.42 0.71 0.097

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 eq. (14) and (15)).

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: ∆ 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 ∆ and Ψ parameters for p(SMERm-NIPAM) and p(SMERe-NIPAM) were acquired and are presented in Figure 2(a) and (b), respectively. When the laser beam was directed to the polymer film an immediate increase of Ψ parameter and a decrease of ∆ parameter, were observed. During further irradiation of the sample, the values of both parameters changed until reaching the photostationary state. After switching off the laser (in 300 s of measurement), partial return of both parameters to their initial values was observed. That relationship (similarly like during the UV-Vis measurements) can be explained with the fact that the thermal relaxation occurred at ambient conditions without any external stimuli. Collected data served to calculate real part of the refractive index (nr) and its change (∆nr). Depending on the copolymer and its concentration,

Figure 2. Change of the ellipsometric parameters ∆ and Ψ, experimentally defined for the p(SMERm-NIPAM) (a) and p(SMERe-NIPAM) (b) under laser beam (λ = 445 nm) irradiation.

change of the refractive index real part was estimated between 0.010 and 0.028. The most significant reversible changes of ∆nr were observed for both copolymers (in 5 wt.% concentration), and resulted in the refractive index change of 0.024 and 0.028 for p(SMERmNIPAM) and p(SMERe-NIPAM), accordingly. Contrary, the higher concentration of azo polymers (10 wt.%) cause smaller changes of the real part of refractive index. Constantly, the reason of such behavior is related with the limited free volume for polymer chains movement and their mobility during laser light illumination. The nr and ∆nr values for the all samples were gathered above, in the Table 3. Nonlinear spectroscopy In order to provide an overview towards nanoscale dimension, the polymer's morphology was schematized in Figure 3(a). From the left side, a typical spaghetti-shape architecture was presented. When the prospect is enlarged (the middle panel), few polymeric chains marked in a black color were shown.

Figure 3. Scheme of the molecular reorientation (from macro to nano scale) achieved in the photosensible polymeric materials (a), typical pump-probe experimental set-up dedicated for the 3rd order nonlinear optical measurements (when the pump laser is OFF and ON, on the left and right panel, respectively) (b), and linear correlation between photoinduced birefringence and pump beam fluence for two investigated compounds (c). Description in the text.

Then, two types of side groups were distinguished by various colors - in gray and orange, which correspond to the optically passive and active side chains, respectively. The first mentioned type serves as the spatial hindrance for the second mentioned - photo-responsive azobenzene units. Due to the applied materials engineering (structural insight) into chemical structure, it was possible to fully control the free volume available between the two next photoisomerizable units by implementing various moieties in the X position, visible in Figure 3(a) - panel on the right side. Methyl and ethyl groups, marked in the red and blue color (which corresponds to the copolymers acronyms), were applied. The first mentioned substituent (-CH3) in its shape is similar to the sphere, due to the sp3 hybridization of carbon atom and consequently, the tetrahedron atoms distribution. Whereas, the ethyl group is more similar to the ellipsoid and occupies approximately two times more space than the methyl moiety. Such dissimilarity between the investigated copolymers makes a significant structural difference, if consider molecular reorientation processes driven by light in the trans-cis conformational transformations. From the main chain polymer's perspective, the azobenzene chromophore units can be differently spatially stabilized/blocked by the sequence of optically passive side groups, if consider electrostatic interaction with covalently attached methyl or ethyl groups to the optically active lateral groups and formed in that way spatial hindrance, respectively. The experimental set-up dedicated to investigate the optical Kerr effect was presented in Figure 3(b), however it was already described in detail in the Methods section. Nevertheless, for better understanding, few things are worth to be distinguished. On the left panel, when the pump laser is in "OFF" state, there is no output signal, which could be acquired by the photodiode. In principle, the optically isotropic material is not able to change the laser light polarization distribution. Converse situation is observed, when the pump laser is in "ON" mode, and the photo-controlled molecular alignment process is launched. The NLO response is being created by pump laser light wavelength (active medium absorption crosssection value is dependent of the wavenumber), its intensity and peculiar systems feature provided by considered chemical architecture. The collected signal presented as the correlation photoinduced birefringence vs. pump beam intensity, was illustrated in Figure 3(c). Interestingly, the nonlinear optical response coming from the copolymer containing ethyl group as spatial separator/stabilizer provided on about two times higher ∆n signal for the same optical pumping than its equivalent with the incorporated methyl moiety. Giving an example, if consider Ipump ~35 mW/cm2, it is clear that p(SMERm-NIPAM) provides photoinduced birefringence on about 1.8 × 10-3, while the p(SMERe-NIPAM) gives ∆n ~5.2 ×

10-3. It means that by providing more free volume easily accessible by photoisomerizable chromophore group (which is changing its shape during transformation), the long-term photoalignment process achieves higher efficiency, noticeable as the bigger number of ∆n parameter. Intriguingly, for the system with methyl group applied, the photoinduced NLO response can be still modulated in the higher pump intensities (up to ca. 70 mW/cm2), which expands the operating range for this material. Increase of pump beam intensity more than 40 mW/cm2 and 70 mW/cm2 for p(SMERe-NIPAM) and p(SMERm-NIPAM) reaches the optical anisotropy saturation level. Based on the estimated value of photoinduced birefringence for both copolymers, the nonlinear refractive index parameter (n2) and 3rd order NLO susceptibility (χ(3)) were calculated according to the eq. (10) and (13), respectively. Methylbased copolymer characterizes n2 parameter value equal to (1.5±0.1) × 10-3 m2/W, whereas for p(SMERe-NIPAM) n2 = (5.0±0.3) × 10-3 m2/W, which is more than three times higher result. If consider 3rd order nonlinear optical susceptibility, the situation is similar. Namely, ethylbased copolymer characterizes χ(3) value equal to 3.9 × 10-5 m2/V2, which is more than three times higher if compare the same parameter estimated for p(SMERm-NIPAM) (1.2 × 10-5 m2/V2). Kinetics of the photoinduced birefringence by applying various values of the pump beam intensity for the two investigated systems (p(SMERm-NIPAM) and p(SMERe-NIPAM)) is presented in the Figure 4(a) and (b), respectively. Interestingly, for the Ipump equal to about 33 mW/cm2, methyl-based photochromic polymer reaches almost three times lower ∆n value (≈ 1.8 × 10-3) than its ethyl-based equivalent (∆n ≈ 5.2 × 10-3). It is related with the free volume available for the side-chains functionalized with the azobenzene units. Since ethylfunctionalized chromophoric side-groups make greater spatial hindrance between the next lateral structures, it is allowed to obtain more efficient molecular photoordering during pump laser treatment. However, the kinetics of the obtained molecular reorientation characterizes higher value for the methyl-based photochromic copolymer. The NLO response observed in the photochromic polymers can be reliable described utilizing biexponential kinetics equation for the experimental points approximation [91-96]. Such approach is related with the chemical structure of macromolecules with photo-responsive moieties incorporated in the side-chains. The first coefficient (usually smaller) is related with the lateral-structure movements related with the azobenzene units transformations, whereas the second one (usually higher) is due to the main polymeric chain reorientation, which needs more time in order to achieve

Figure 4. Photoinduced molecular reorientation process dynamics under laser light irradiation (λ = 405 nm) with the inset presenting complete kinetics of ∆n build-up and decay under dark thermal relaxation environment for p(SMERm-NIPAM) (a) and p(SMERe-NIPAM) (b), respectively. Multiple signal response, when the pumping light is in ON and OFF states, and the following switching cycles marked in black, dash lines are presented additionally as the inset, where the high signal stability and repeatability is observed for p(SMERm-NIPAM) (c) and p(SMERe-NIPAM) (d), accordingly. photostationary arrangement. The time-constant values correlated with the ∆n signal increase e 3 of p(SMERm-NIPAM) were estimated as fg/^ = 5.0±0.1 s and fg/^ =42.0±1.2 s, respectively.

Whereas, if consider p(SMERe-NIPAM) system and its ∆n inducement, they were e 3 defined by fg/^ =9.5±0.1 s and fg/^ = 68.6±0.7 s, accordingly (the values come from the NLO

signal induced by the pump laser treatment intensity around 33 mW/cm2 in both cases). Observed differences are related with the molecular reorientation availability. Azobenzene units tend to re-order in a way to be oriented perpendicularly to the pump laser light polarization direction, what can be realized up to the moment, when no more molecular rearrangement is possible. Methyl groups attached by covalent bonding to the chromophoric

group do not provide any of artificial free volume, so the photo-responsive lateral parts will follow photoordering process less effective and relatively faster than in the case, when polymeric structure provides the possibility to improve molecular reorientation by additional space among the macromolecular architecture. Furthermore, in the case of methyl-based photochromic polymer the shorter exposure time of pump laser is needed to achieve photostationary state, which is equal to about 100 s. While, if consider p(SMERe-NIPAM) system, even after 250 seconds it seems that saturation level of the generated photoinduced birefringence has still not reached. A complete ∆n signal's light-driven inducement and thermodynamically controlled molecular isotropic re-ordering for both photochromic polymers, was shown in the insets of the Figure 4(a) and (b). The remnant optical birefringence after the first cycle of molecular ordering and re-alignment was marked by the black dash lines. The acquired ∆n reversibility is remarkable higher than observed in, i.e. azobenzene-functionalized

poly(esterimide)

[91],

or

poly[(4-nitronaphthyl)[4-[[2-

(methacryloyloxy)ethyl]ethylamino]phenyl]diazene] [92], or disperse red 19-functionalized polyoxazolidone [93], where even 80% of the generated optical anisotropy remained. The p(SMERe-NIPAM) system characterizes higher reversibility than p(SMERmNIPAM) on about 10%, which is again related with the molecular reorientation availability among the photochromic polymer volume. Importantly, the first mentioned photochromic polymer characterizes reversibility defined as more than 50% after the first cycle, although further switching cycles bring even more imposing results for both of the materials (∆n reversibility around 100%, Figure 4(c) and (d)). Interestingly, the reverse process, thermal molecular re-ordering is defined by similar time-constant parameters for both investigated copolymers. If consider p(SMERm-NIPAM) e 3 e and p(SMERe-NIPAM) one can see f60^ =6.9±0.2 s and f60^ = 124.0±12.2 s, and f60^ = 3 6.5±0.1 s and f60^ = 118.9±5.2 s, respectively. It is clear that relaxation processes controlled

only by thermodynamic stimulus are slower than these, which are controlled by light [91-93]. However, after few cycles of the molecular re-ordering induced by laser light being in resonance with active material, the photochromic polymers can provide fully reversible (close to 100%), stable and repeatable output NLO response (Figure 4(c) and (d)). It is worth to mention that both systems reach high stability after few "ON/OFF" cycles and provide significant output signal modulation. Moreover, the slight structural modification in the considered photochromic polymers brought significant difference in the all-optical switching properties. The p(SMERe-NIPAM) efficiency in NLO response is more than two times higher

if consider both, signal amplitude (9V) and modulation range (4V) (inset of the Figure 4(d)). While p(SMERm-NIPAM) characterizes signal amplitude on about 4 volts and output voltage modulation in the range ca. 1.5V (Figure 4(c) inset). After achieving photostationary state by the photo-responsive materials, the acquired next 20 switching operations (pump beam ON and OFF), prove their high stability after multiple modulations. It simply means that by playing with chemical architecture of photochromic polymer, it is possible to control its spectroscopic attributes considering NLO response kinetics, amplitude and operational range. In order to investigate deeply the photoinduced trans-cis transformations in the photoresponsive side-chains, the dynamic OKE signal was analyzed (Figure 5(a) and (b)). Azobenzene spatial transformations were investigated with the applied external pump signal modulation in the range of 10-100 Hz. The maximum amplitude collected for both systems was correlated with 10 Hz modulation and it was equal to about 60 mV and 115 mV for p(SMERm-NIPAM) and p(SMERe-NIPAM), respectively. Observed tendency is consistent with aforementioned long-term molecular ordering kinetics for the considered photochromic polymers and proves the mechanism, responsible for acquired various signals. For the ethylbased polymer achieved amplitude is almost two times higher than for methyl-based equivalent. Such behavior can be connected only with the greater spatial hindrance provided by ethyl moiety in p(SMERe-NIPAM) macromolecular architecture. The latter photoresponsive side-chain structure (the part connected behind the azobenzene unit constitutes terminal group) is exactly the same for the both investigated systems and from theory cannot provide different effects (please see Figure 3(a)). The complete kinetics of the induced photoisomerizations was gathered in the insets of Figure 5.

Figure

5.

Dynamic

signal

modulation,

reflecting

trans-cis-trans

photoinduced

transformations in the function of applied frequency, in the range of 10-100 Hz (insets) for p(SMERm-NIPAM) (a) and p(SMERe-NIPAM) (b), respectively. Ipump ≈ 33 mW/cm2.

The signal increase corresponds to the trans-cis conformational change, while decay is related with thermal, dark cis-trans transformation. A careful analysis on the time-constant values defining dynamic ∆n changes was provided with applied pump beam intensity ≈ 33 mW/cm2 and frequency modulation at 10 Hz. In here, in order to approximate the experimental results a monoexponential function was implemented (Figure S8) [92, 97]. If consider methyl-based photochromic polymer the following parameters were obtained: fg/^ = 33±1 ms and f60^ = 62±2 ms. Whereas, the p(SMERe-NIPAM) trans-cis conformational changes refer to the fg/^ = 65±1 ms and f60^ = 97±3 ms. In the case of the analyzed kinetics, the closest azobenzene environment is crucial, because it influences straightforwardly for the molecular movements. The higher values of the time-constants for the ethyl-based polymer are reasonable, because implemented moiety is almost two times heavier than methyl one, and may decreases structure's flexibility of that region. However, from the same reason, more rigid base serves as stabilizer and provides more space needed to perform spatial molecular reorientation.

Conclusions In this contribution two novel photochromic polymers were introduced. Based on the previous findings in the literature [91-93], and also thanks to the materials engineering approach, by applying differently structured photo-responsive side-chains in the investigated copolymers, the macrostructure's abilities concerning light driven refractive index anisotropy, was developed in this contribution. We discovered that slight structural changes can affect on the important differences in the light-driven output NLO response. The signal kinetics, its magnification, reversibility and efficiency, are strictly connected with the carefully designed macrostructure architecture. We found that by applying methyl group in the photo-responsive side-chain it was possible to generate optical birefringence faster, however signal amplitude and its reversibility is limited and lower in comparison with the second derivative co-polymer. Consequently, the ethyl-based photochromic polymer characterizes higher ∆n signal amplitude and reversibility. Besides, we found that aforementioned slight structural change can influence on the azobenzene fragment so far to influence on the trans-cis photoinduced transformations kinetics. Nevertheless, the most advantage coming from performed experiments is based on the ease possibility to design and control output NLO signal from the single-component organic system, where chemical structure determines future, spectroscopic features. Thanks to the implemented acrylate-based non-chromophoric side-chains, obtained

copolymers characterize low glass transitions temperature, which make them more flexible and adapt better advanced for the processing operations. To summarize, all of the obtained both, theoretical and experimental results prove great achievements in the molecular engineering, which paved the way to design and fully control smart optical single-component materials. Such invented photochromic polymers can find many applications in holography or photonics, as remote-controlled optical modulators or data storage. Acknowledgements The research leading to these results has received funding from the statutory funds of Faculty of Chemistry at Wroclaw University of Science and Technology.

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1. Molecular engineering influences on the light-driven NLO output. 2. Novel photo-responsive macromolecular systems were demonstrated. 3. Obtained systems were characterized by the optical Kerr effect (OKE) experiment.

Declaration of interests X The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐ The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: