An optical modulator on the pyrazolone-based bi-component system

Dyes and Pigments 172 (2020) 107805

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An optical modulator on the pyrazolone-based bi-component system a,⁎

Adam Szukalski , Beata Jędrzejewska

b,⁎⁎

c

, Przemysław Krawczyk , Agnieszka Bajorek

T

b

a

Faculty of Chemistry, Advanced Materials Engineering and Modelling Group, Wroclaw University of Science and Technology, Wybrzeże Wyspiańskiego 27, 50320, Wroclaw, Poland UTP University of Science and Technology, Faculty of Chemical Technology and Engineering, Seminaryjna 3, 85326, Bydgoszcz, Poland c Nicolaus Copernicus University, Collegium Medicum, Faculty of Pharmacy, Kurpińskiego 5, 85950, Bydgoszcz, Poland b

A R T I C LE I N FO

A B S T R A C T

Keywords: Pyrazolone Photoinduced birefringence All-optical switching Optical Kerr effect TD-DFT PCM model

A highly effective and reversible photo-controlled organic light modulator based on the pyrazolone ring and stilbene group has been introduced. Bi-component system results in initially isotropic optical active layer, featuring all-optical switching fully controlled by laser light towards refractive index anisotropy. Fully organicbased optical switch is demonstrated in a device in form of thin film, with photoinduced birefringence (Δn) estimated of about 5 × 10−4. Interestingly, productivity equal to 100% due to the optically induced Δn reversibility, classifies considered hybrid system as one of the best currently available among organic all-optical switchers. Such efficient light modulator based on the stilbene's flexibility feature, open new-generation alternatives of all-optical switches for opto-electronic and photonics applications.

1. Introduction Recently, tremendous attention is paid for seeking organic materials, which can be utilized in many commercial as well as scientific fields [1–5]. Considering photonics and spectroscopy, some of them should be highlighted. Light amplification phenomena induced in various mechanisms [6–13], nonlinear spectroscopy including multiphoton imaging [14–19] or higher orders of harmonics of light generation [20–24]. Among many others, an all-optical switching [25–29], as the example of optical control on the active material, is significant. In the very close future, it can lead to remarkable development of the complex photonics systems [30]. From this reason, only the most effective, fast and stable over time materials can play an important role. To construct the opto-electronic network based on many optically controlled logic switches, the most responsive and reversible materials are required. Azo-compounds are well-known in the literature thanks to their useful features [31–34]. Nevertheless, new groups, an alternative for them are well-seen, especially if there is a chance to find another or better solutions [35–37]. Therefore, further organic group is being considered as the valuable candidate to compete with azo- or azobenzene compounds [38–41]. Progressive potential coming from the organic compounds containing stilbene group was already proved in the literature [42]. Recent reports about TTF-based [43], thiophene [44] or pyrazoline derivatives

⁎

[45,46], amongst others, characterize efficient and significant refractive index anisotropy generation. Aforementioned low-molecular organic dyes were already considered as the alternative for wellexploited azo-compounds. The undertaken idea in given examples of research was to seek for various moieties being more and more efficient considering electron donors (D) and/or acceptors (A). By sustaining πelectron bridge and providing internal charge transfer (ICT) and thanks to applying stilbene group, it became possible not only to generate, but also to play with optically generated refractive index anisotropy as well as construct efficient all-optical modulators [30]. One of the candidates inscribes in abovementioned requirements is introduced in this paper newly synthesized pyrazolone derivative. The molecule based on the three most important presuppositions: (i) pyrazolone ring providing easily feasible internal charge transfer through the entire structure effectively and (ii) at the same moment being very productive electron acceptor moiety. Then lastly, (iii) double stilbene bonding providing high molecular flexibility and few possibly achieved conformational states. All together can provide effective, fast and reversible molecular switching fully controlled by light. At most, according to the best of our knowledge, it is the first announcement of the pyrazolone's nonlinear optical (NLO) features and its light-controlled refractive index anisotropy modulation.

Corresponding author. Corresponding author. E-mail addresses: [email protected] (A. Szukalski), [email protected] (B. Jędrzejewska).

⁎⁎

https://doi.org/10.1016/j.dyepig.2019.107805 Received 21 May 2019; Received in revised form 5 August 2019; Accepted 16 August 2019 Available online 16 August 2019 0143-7208/ © 2019 Elsevier Ltd. All rights reserved.

Dyes and Pigments 172 (2020) 107805

A. Szukalski, et al.

precipitate was filtered off under suction, washed with cooled ethanol and then with hot distilled water and dried. The residue was purified by crystallization from ethanol to give PYR-ππ-pAM compound as a dark red solid, which was shown in the inset of Fig. 1, where macroscopic photograph of the powder form of the dye was presented. Yield 54.8% (0.508 g); m.p. 169–171 °C (lit.161–162 °C [48]); Rf = 0.55. 1 H NMR (400 MHz, DMSO‑d6) δ (ppm) 2.24 (s, 3H, CH3), 3.06 (s, 6H, N-CH3), 6.80–6.82 (d, 3JH-H = 8.0 Hz, 2H, ArH), 7.13–7.17 (t, 1H, ArH), 7.39–7.43 (t, 2H, ArH), 7.48–7.52 (d, 3JH-H = 15.0 Hz, 1H, =CH), 7.54–7.56 (d, 3JH-H = 8.0 Hz, 2H, ArH), 7.58–7.61 (d, 3JH3 JH-H = 8.0 Hz, 2H, ArH), H = 12.0 Hz, 1H, =CH-), 7.92–7.94 (d, 8.18–8.22 and 8.21–8.25 (dd, 3JH-H = 15.0 Hz, 1H, =CH-); 13 C NMR (100 MHz, DMSO‑d6) δ (ppm) 13.0 (CH3); 40.1 (N(CH3)2); 112.4, 112.6, 118.1, 118.2, 124.4, 129.2, 129.3, 131.4, 131.8, 146.1, 147.9, 153.1 (CH); 120.8, 123.3, 139.1, 150.5, 153.0, 163.6 (C); IR (KBr) νmax (cm−1) 2916, 1671, 1622, 1578, 1544, 1497, 1371, 1319, 1233, 1170, 1037, 1021, 1001, 982, 809, 749, 690, 660.

2. Materials and Methods Melting points were determined with a Boëthius apparatus (type PHMK 05, Germany) and are uncorrected. Flash column chromatography was performed using silica gel 60 Å (220–440 mesh) eluting with chloroform and n-hexane (1:6 v/v). Analytical thin layer chromatography (TLC) was carried out on silica gel plates with QF-254 indicator (Fluka) and visualized by UV. The 1H (400 MHz) and 13C (100 MHz) NMR spectra were recorded in perdeuterated dimethylsulfoxide (DMSO‑d6) with the use of a Bruker Ascend™ 400 NMR spectrometer. All chemical shifts are quoted in ppm, relative to the tetramethylsilane (TMS), using the residual solvent peak as a reference standard (DMSO‑d6: ~2.49 ppm 1H; ~39.5 ppm 13C). Coupling constants (J) were reported in Hertz units. The IR spectra (from KBr discs) were recorded in the range of 400–4500 cm−1 with the spectral resolution < 2 cm−1 on a Brüker Vector 22 FT-IR spectrophotometer. The UV–Vis and fluorescence spectra were recorded in solvents of different polarity on a Shimadzu UV–Vis Multispec-1501 spectrophotometer and Hitachi F-7100 fluorescence spectrophotometer, respectively. The concentration of the dye in solvents of different polarity was ca. 10−5 M and 10−6 M for absorption and fluorescence measurements, respectively. The fluorescence quantum yields for the dyes were calculated based on equation (1):

Φdye = Φref

2 Idye Aref ndye ⋅ 2 Iref Adye nref

2.2. Theoretical approach All geometrical parameters of the investigated molecules in their ground (SGS) and excited states (SCT) were calculated using density functional theory (DFT) approach implemented in Gaussian 09 program package [49] with TIGHT threshold option and PBE0/6–311++G(d,p) basis set. In order to verify that all the structures correspond to the minima on the potential energy surface, an analysis of Hessians was performed. The electronic properties were characterized by computations of the vertical absorption and emission spectra, which were obtained using the time-dependent density functional theory (TDDFT/ PBE0) [50] and by including the state-specific (SS) corrected linear response (cLR) approach [51]. All of the spectroscopic calculations were performed using several different functionals, namely standardhybrid PBE0 and B3LYP functionals [52,53], as well as long-range asymptotically corrected functionals such as CAM-B3LYP [54], HSEH1PBE [55,56], mPW1PBE and mPW3PBE [57]. The dipole moment and polarity of the charge-transfer state (CT) were evaluated by numerical differentiation of the excitation energies (E) in the presence of an electric field F of 0.001 a.u. strength:

(1)

where: Φref is the fluorescence quantum yield of reference (Rhodamine B in ethanol; Φref = 0.5 [47]) sample in ethanol, Adye and Aref are the absorbances of the dye and reference samples at the excitation wavelengths (A ≈ 0.1 at 496 nm), Idye and Iref are the integrated emission intensities for the dye and reference samples, ndye and nref are the refractive index values of the solvents used for the dye and the reference, respectively. The fluorescence lifetimes were measured using a singlephoton counting system (FLS920P Spectrometers). The apparatus utilizes a picosecond diode laser for the excitation generating pulses of about 81.5 ps at 466.6 nm. Its maximal average power is 5 mW. The fluorescence decays were fitted to two-exponential functions. The average lifetime, τav is calculated as τav = (Σiαiτi)/(Σiαi), where αi and τi are the amplitudes and lifetimes.

Δμgi − CT =

2.1. Synthesis route and basic characterization

Eg (+F i ) − Eg (−F i ) ECT (+F i ) − ECT (−F i ) − i −2F −2F i

(2)

where i stands for the Cartesian component of the dipole moment difference. The isotropic average polarizability (α ), polarizability anisotropy (Δα) and first-order hyperpolarizability (βvec) were determined based on the Gaussian 09 program and defined as:

The synthesis route of the 4-[3-(4-dimethylamino-phenyl)-allylidene]-5-methyl-2-phenyl-2,4-dihydro-pyrazol-3-one (in a short: PYRππ−pAM) is based on the well-known procedure described in literature before and has shown in Fig. 1 [48]. A mixture of 3-methyl-5-pyrazolone (0.55 g, 5.6 mmol, 1 equiv.), 4-(N,N-dimethyl)cinnamaldehyde (0.981 g, 5.6 mmol, 1 equiv.), anhydrous sodium acetate (0.46 g, 5.6 mmol, 1 equiv.) and acetic anhydride (2.65 mL, 28 mmol, 5 equiv.) was reflux for 4 h. Then, the mixture was cooled to room temperature, diluted with ethanol (5–10 mL) and kept at 0 °C for 20 min. The

α=

α xx + α yy + α zz

Δα =

(3)

3

2 2 2 (α xx − α yy )2 + (α xx − αzz )2 + (α yy − α zz )2 + 6(α xy ) + α xz + α yz

2 (4) Fig. 1. General route for the 4-[3-(4-dimethylamino-phenyl)-allylidene]-5-methyl2-phenyl-2,4-dihydro-pyrazol-3-one synthesis (its acronym PYR-ππ-pAM with marked significant regions: in red colour: pyrazolone ring, in blue: two conjugated πbonds creating stilbene groups in the final product, in green: 3rd order amine group); inset shows macroscopic photo of the synthesized product. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

2

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βvec =

∑ i = x , y, z

μi βi μ

(usually in the shape of thin film). When the pump beam is applied into set-up in a continuous way (without any signal modulations), only total (or so called - static) value of the photoinduced birefringence can be estimated (Δntotal). In order to understand mechanism of the observed photoinduced birefringence, it should be highlighted first that it depends on pump beam intensity in time Δn(Ipump,t). Following equation (6) including refractive index components should be introduced [61]:

(5)

() 1

where βi (i = x , y, z ) is given by βi = 3 ∑j = x , y, z (βijj + βjij + βjji ) . The density differences were obtained at the PBE0/6–311++G(d,p) level and are represented with a contour threshold of 0.02 a.u. In these graphs (presented in section 3.2), the blue (purple) zones indicate density decrease (increase) upon electronic transition. The charge transfer parameters, namely the charge-transfer distance (DCT) and the amount of transferred charge (qCT), have been determined following the Le Bahers' procedure [58]. The solvent effect on the linear and nonlinear optical properties has been taken into account using the Integral Equation Formalism for the Polarizable Continuum Model (IEF–PCM) [59,60].

Δn (Ipump, t ) = n⊥ (Ipump, t ) − n|| (Ipump, t )

(6)

where n⊥ and n|| denote perpendicular to each other two components of refractive index parameter rendered in their spatial distribution (refractive index indicatrix). Since the pump beam is used, their values are not equal anymore (initial sphere becomes ellipse at the end) and two competitive directions with different n values are generated. Such a difference responsible for the optical anisotropy is called photoinduced birefringence and it is related with phase changes (Δϕ) and optical path (d) and can be monitored in OKE set-up by probe beam (λprobe) [61]:

2.3. Nonlinear spectroscopy Optical Kerr effect (OKE) is located in the 3rd order nonlinear optical effects and described by that formalism. Applying typical pumpprobe experimental set-up it is feasible to estimate the second, nonlinear refractive index value as the consequence of the optically induced anisotropy of refractive index indicatrix [61]. Experimental setup in details was already described elsewhere in the literature [30], nevertheless for the clarity of this article few important issues are presented (Fig. 5(a)) and discussed also here. Preparation description of PYR-ππ-pAM dye embedded in polystyrene (PS) matrix in the shape of thin film and its absorption spectra is available in ESI file, S6 and Fig. S1, respectively. For the needs of spectroscopic measurements thin polymeric film doped with nonlinear chromophore was prepared. In our studies the drop-casting technique was implemented in order to achieve well prepared and optically homogenous sample. However, in the case of thin films obtained using spin-coating fabrication technique, the sample thickness would be decreased from few microns (like in our case) below 1 μm. From theory [61], it is known that photoinduced birefringence (based on the phase change) is related with the optical path, which consists of the photo-responsive component(s). In the case of thinner films, than investigated herein, phase change would be smaller, however theoretical description of the considered phenomenon (eq. (8)) takes into account the crucial parameter, which is sample thickness. The more important issue is to have similar thickness values if compare different compounds or systems. Then, observed phenomena should characterize as similar sample shape as it is possible to control and provide. On the same spectra (Fig. S1), characteristic wavelengths were marked. In the case of pump-probe experimental set-up, two laser lines are required to characterize material's 3rd order optical nonlinearity. In order to provide sufficient energy for the molecular photoalignment, one of them should be inscribed in the absorption resonance range of the investigated compound (in our case between 400 and 600 nm). To generate photoinduced birefringence numerous of the molecular transitions take place. One of the possible considered mechanism, which allows to reach optical anisotropy, are multiple transcis-trans photoisomerization. Their kinetics or effectiveness, can be fully controlled by the external electric field coming from, so called, pump beam (λpump) [42–46]. Therefore, if consider its optical features, at the end of the whole process (which takes usually from ms up to minutes or even hours [62]) initially isotropic system becomes anisotropic. However, the second laser line used during the experiment, which is so called - probe/reference beam (λprobe or λref, properly), should be out of the investigated nonlinear chromophore's absorption range. In this way using reference laser beam it is possible to control (but not interact at the same moment) in situ all of the kinetics of processes which take place during the experiment and are induced by the pump beam. Moreover, in order to observe optical anisotropy inducement, crossedpolarizer system was applied (cf. Fig. 5(a)). Therefore, two laser lines used for the OKE characterization intersect on the sample's surface

Δn (Ipump, t ) =

λprobe Δϕ (Ipump, t ) (7)

2πd

In order to correlate aforementioned parameters and experimental set-up technical details, namely initial and transmitted probe beam probe , respectively) dependent on time, equation intensities (I0probe and Itrans (8) was introduced [61]: probe (t ) = I0probe sin2 ⎛⎜ Itrans ⎝

πd Δn (Ipump) ⎞ λprobe

⎟

⎠

(8)

In that way macroscopic data directly measured during the experiment allow to estimate nonlinear optical parameters, like photoinduced birefringence (Δn), 3rd order nonlinear optical susceptibility (χ(3)) or second, nonlinear refractive index value (n2). Moreover, by playing with experimental set-up configurations and oscilloscope settings it is feasible to extract dynamic components of the OKE experiment. Namely, it is dynamic part of Δn (Δndyn), which directly indicates Kerr constant (B) value. The latter one is the next NLO parameter characterizes Kerr media in general, also in liquid form [42–44,61]. Furthermore, since the external mechanical signal modulator is working with variable frequency values (Fig. 5(a)), molecular conformational transitions kinetics can be measured and analyzed. Consequently, AC mode in oscilloscope setting has to be used. In such a configuration of the OKE set-up, it is possible to characterize how many and how efficient trans-cis-trans transformations are optically induced in the investigated material. Then lastly, these transitions (as mentioned before) are responsible for generating total molecular alignment (optical anisotropy). Mechanism of such transitions was already described in details in literature [42–46]. However, it is worth to mention that two thermodynamic states can be distinguished. Lower in energy and thermodynamically stable - trans one, and higher in energy, metastable cis one [62]. Active molecules present in the investigated system would always tend to reach the lowest possible energetic state. Thus, after pump laser line irradiation (independently of the used dynamic or static set-up configurations) majority of the molecular population goes back to their thermodynamic equilibrium in darkness conditions. In other words, when the molecules are not treated by the pump light, optically induced anisotropy of the refractive index decreases rapidly (diminish). Whole system goes back to the initial - optically isotropic conditions. One of the considered and known difficulty for the thermal relaxation reversibility process is i.e. steric hindrance caused by other molecules or matrix chains current distribution. Thus, static mode of the OKE set-up's configuration allows to declare what is the total photoinduced birefringence value in the examined system. Whereas, dynamic set-up configuration enables to characterize all of these conformational transformations, which create macroscopic optical anisotropy. 3

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3. Results and discussion

Table 1 Basic photophysical data of PYR-ππ-pAM dye.

3.1. Synthesis and basic spectroscopic features The pyrazolone dye was synthesized by refluxing 3-methyl-5-pyrazolone and 4-(N,N-dimethyl)cinnamaldehyde in acetic anhydride in the presence of anhydrous sodium acetate. The purity of the compound and its structure was proved by NMR and IR analysis. For example, the formation of oxazolone backbone is confirmed by two strong bands observed in the IR spectra at ca. 1660–1680 cm−1 and 1500–1610 cm−1 due to the C=O and C=C double bonds, respectively and the signals of the olefinic proton in the 1H NMR spectra at ca. 7.48–8.25 ppm. The 1H and 13C NMR as well as IR spectra are presented in Electronic Supporting Information (ESI) file. Chemical structure of the investigated nonlinear chromophore represents typical push-pull construction. From one side aromatic ring, instead from the opposite 3rd order amine group (marked in green colour in Fig. 1) located in para position according to the other aromatic ring, create terminal moieties rich with free electrons providing negative charge to the central part of the molecule. Then, two π-conjugated double bonds (blue colour, Fig. 1) sustain π-electron bridge, which provides internal charge transfer through the all parts of the molecule. Double π-bond bridge is located between pyrazolone and one of the aromatic rings. The double stilbene bonding is the most labile part of the investigated structure. Moreover, it is responsible for every kind of conformational transformations (i.e. trans-cis-trans) or partial molecular rotation of terminal aromatic ring expanded by amine group. All of these, can be induced by strong electric field (in our case coming from pump laser line). Then lastly, in the middle part of the dye pyrazolone ring is located (marked in the red colour in Fig. 1), which especially with its oxygen atom serves as the strong electron acceptor unit, which makes the PYR-ππ-pAM compound highly effective nonlinear chromophore. The electronic absorption and fluorescence spectra of the PYR-ππpAM in 9 solvents with varying polarity are presented in Fig. 2. The

Solvent

abs λmax

εmax

FWHMabs

δ1PA

PL λmax

ΔνSS

ΦPL

fos

TMP MCH Et2O Bu2O CHCl3 THF MeAc MeCN DMF DMSO

460 462 467 468 496 481 483 488 501 515

– – 5.60 4.23 5.23 5.31 5.13 4.38 4.06 4.22

3651.6 3664.7 4298.1 4174.6 4434.7 4548.4 4753.2 4858.8 4697.7 4617.5

– – 2.14 1.61 2.00 2.03 1.96 1.67 1.55 1.61

754 751 755 741 753 736 726 687 688 685

8477 8329 8168 7872 6881 7203 6930 5936 5425 4819

0.039 0.024 0.020 0.020 0.014 0.016 0.018 0.022 0.031 0.044

– – 1.05 0.76 1.10 1.11 1.16 1.02 0.92 0.94

abs PL ; nm) , PL maxima (λmax ; nm) , shift (Δí; cm−1), maximum Absorption (λmax 4 −1 molar absorption coefficient (εmax; 10 M cm−1), full width at half maximum (FWHM; cm−1), one-photon absorption cross-section (δ1PA; Å2), fluorescence quantum yield (ΦPL; %) and fos oscillator strength. a

spectral properties of the compound are summarized in Table 1, where all the solvents used are listed in the increasing order of their relative permittivity. The physical properties and parameters of different solvent functions of the employed solvents are collected in Table S2 in ESI file. The main broad absorption band is localized in the range of 400 nm–600 nm and may be attributed to the π-π* electronic transition of the charge transfer character. The second band with lower intensity is blue shifted and positioned around 265 nm. The latter one remains unchanged in solvents of different polarity and may be assigned to the n-π* electronic transition on the carbonyl group. The strong CT in PYRππ-pAM is reflected in a protonation effect on the absorption spectrum. The significant density reorganization upon excitation was prevented after the addition of gaseous HCl into the measurement cell (Fig. 3) and formation of the 4-NMe2H+ cation. The cation has weak electron-acceptor properties, because the lone pair of electrons on this nitrogen is no longer involved in conjugation. As a result, the UV–Vis absorption

Fig. 2. Normalized absorption (a) and fluorescence (PL) (b) spectra of PYR-ππ-pAM in solvents with different polarity. Possible resonance structures contributing to the ground and excited-state structures (c). 4

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Table 2 Estimated from eq. (9), coefficients (y0, aSP, bSdP, cSA, and dSB), their standard errors and correlation coefficients (R2) for the multiple linear regression anaabs Pl lysis of ν˜max , ν˜max and ΔνSS of PYR-ππ-pAM in ten various solvents as a function of the Catalán four-parameter solvent scale. y

abs ν˜max

Pl ν˜max

ΔνSS

y0 aSP

24590 ± 283 -(4482 ± 438) (39.9%)a -(1018 ± 87) (9.1%) -(5701 ± 1434) (50.7%) -(40 ± 109) (0.3%) 0.993

14478 ± 1566 -(2220 ± 2420) (15.9%) 769 ± 479 (5.5%) 10531 ± 7926 (75.2%) 483 ± 603 (3.4%) 0.613

10112 ± 1547 -(2262 ± 2389) (10.9%) -(1787 ± 473) (8.6%) -(16232 ± 7827) (78.0%) -(523 ± 595) (2.5%) 0.923

bSdP cSA dSB R2 a

Contribution percentages of the SP, SdP, SA and SB polarity parameters according to the data of multiple linear regressions. Fig. 3. Comparison of the electronic absorption spectra of the parent compound (PYR-ππ-Ph), PYR-ππ-pAM derivative, and its HCl salt (PYR-ππ-pAM + HCl gas).

and in Fig. S3 in ESI file. The average fluorescence lifetimes and quantum yields were applied to calculate the radiative kr and non-radiative kn-r rate constants (see Table S1 in ESI file). It was found that the non-radiative transition rate is approximately four orders faster than the radiative one. These measurements confirm that the deactivation of the first excited state is dominated by the non-radiative processes. Such phenomenon is well documented for oxazolone derivatives as resulting from geometrical isomerization from the lowest energy isomer (the E-isomer) to the less stable Z-isomer taking place via a triplet state [66–68]. The interactions of solvents with the dye were analyzed by means of the multiple regression analyses using solvent polarity parameter sets of the Catalán including polarizability (SP), dipolarity (SdP), acidity (SA) and polarizability basicity (SB) [69–71], according to eq. (9). The best fitting correlation results are summarized in Table 2.

maxima is shifted to slightly higher energies when compared with the free base and the unsubstituted congener, i.e. 5-methyl-2-phenyl-4-(3phenyl-allylidene)-2,4-dihydro-pyrazol-3-one (PYR-ππ-Ph). Analogous results were obtained by studying the effect of hydrochloric acid concentration on the position and intensity of the absorption spectra of PYR-ππ-pAM. As illustrated in Fig. S2 in ESI file, the acidification of the acetonitrile solution of PYR-ππ-pAM by adding 0.1 M HCl to the mother solution decreases absorption at 488 nm with a concomitant increase of the absorption at 350 nm. The isosbestic point at 398 nm indicates transition between two species, neutral molecule and monocation. The CT character of the π-π* electronic transition was also evidenced by theoretical calculationsm, which results are discussed in the next paragraph. Taking into account the influence of the solvent polarity on the UV–Vis absorption spectra of the dye, the maximum band is red-shifted from ca. 460 nm in TMP to 515 nm in DMSO, indicating again CT character of the transition due to the presence of N,N-dimethylaminobenzoyl group. The possible resonance structures are shown in Fig. 1(c). Other spectral effects indicative of the dye electronic structure change upon increasing solvent polarity are abatement of intensity, and broadening of the long-wavelength absorption band [63,64]. The molar absorption coefficient (ε) of PYR-ππ-pAM varies between 5.6 × 104 M−1cm−1 in Et2O and 4.1 × 104 M−1cm−1 in DMF, whilst the bandwidth changes from 3650 cm−1 to 4860 cm−1 as the polarity of the solvent increases in spite of the stronger solute-solvent interactions in more polar medium. On the other hand, the dye displays very low emission in all tested solvents with fluorescence quantum yields below 0.05%, which means that the compound is inherently non-fluorescent. The low fluorescence quantum yield results from an effective non-radiative deactivation paths, which dominate in flexible compounds [65]. The introduction of the additional methine unit into 4-(4-(dimethylamino)benzylidene)-3methyl-1-phenyl-1H-pyrazol-5(4H)-one (PYR-π-pAM) decreases fluorescence intensity by one order of magnitude. The fluorescence quantum yield of PYR-ππ-pAM equals ca. 0.024% in MCH, whereas for PYR-πpAM is ca. 0.2%. The fluorescence lifetimes of the dye in MCH, THF and DMSO were also determined. The PYR-ππ-pAM has two-exponential decays with a fast relaxation mechanism having a lifetime of several dozen picoseconds followed by a slower one on the nanosecond time scale. However, the amplitudes of the longer lifetime are not very high and slightly increase with increasing polarity of the environment. Thus, the average lifetimes are substantially longer in the stronger polar solvents, whereas the fast decay lifetimes decrease in that conditions as shown in Table S1

y = y0 + aSP SP + bSdP SdP + cSA SA + dSB SB

(9)

To compare the data of multiple linear regressions, they have been transformed into contribution percentages of different polarity parameters. According to the results, the PYR-ππ-pAM has same performances in the absorption, fluorescence and Stokes shift in the employed solvents. In other words, solvent's acidity, polarizability and dipolarity show important role in solvation of the solute as indicated by the percentages of the polarity parameters. Furthermore, the basicity term, characterized by the lowest values and high standard errors in all cases, is negligible due to lack of a suitable hydrogen bond donor moiety of the tested molecule. Consequently, the regression excluding d yield better R2 of 0.994 and 0.636 for absorption and fluorescence, and a little worse standard error equals 0.862 for Stokes shift (see Table S3 in ESI file). Since absorption is shorter time process, as compared to orientation of solute and solvent molecules, the negative sign of Catalán solvent polarity parameters in absorption process demonstrates that the ground state's energy level increases through these parameters. Thus, the absorption band position undergoes a bathochromic shift with increasing solvent polarizability (SP), dipolarity (SdP), acidity (SA) and basicity (SB). Additionally, the smaller errors are observed in the case of solvent polarizability (SP) and dipolarity (SdP) indicating that the absorption arises from polarized π-π* transition in the NMe2 substituted molecule and that the chromophore presents a solvatochromism dominated by non-specific interactions. However, due to presence of the pyrazolone carbonyl moiety and dimethylamino group in the dye molecule suitable for hydrogen-bonding, solvent acidity also shows a crucial role in solvatochromism of the PYR-ππ-pAM. The multiple linear regression analysis of fluorescence in the Catalán scale results in a modest correlation (R2 = 0.613). Analysis of the data revealed that an increasing polarizability SP causes a bathochromic shift (negative value for a) whilst an increasing dipolarity SdP 5

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PYR-ππ-pAM isomers to environmental changes, as well as to those occurring during photo-excitation to the first singlet excited state (SCT). In particular, they are mostly visible within the double C=C bond bridge. The changes in this fragment of the molecule lead to the elongation of the trans conformer with increasing solvent polarity and photo-excitation becomes the factor maximizing this dependence. The lower in energy conformer (E) in the excited state is less planar than in the ground state (SGS). Also, in the case of the excited state, this planarity decreases with increasing dielectric permittivity of the medium, which is in inverse relation to SGS. Interestingly, for the cis conformer the same observation is consistent only for the ground state. However, photo-excitation favours the reduction of the dihedral angles in the π-electron bridge. At the same time, the increasing solvent polarity promotes a further reduction of these angles and a significant approach of the pyrazolone ring surface to the aromatic ring. Unfortunately, it has not been possible to optimize the CT state of the cis isomer in Bu2O, Et2O and THF. Although, the full optimization of the transition states (TS) has not been performed, the estimated barrier of trans→cis isomerization amounts to 95.19 kcal/mol. According to the relaxed scan of PES along the two torsion angles (Fig. 4(b)–(c)), it results mainly from the change in the ΘC8 = C9-C10 = C11 angle, i.e. from 174.2° to 24.2° during the forming of the TS structure. In the case of ΘC5-C8=C9-C10 angle, the change is from −10.4° to 169.6°. 3D projection of the discussed potential energy surface scan has also shown in ESI file (Fig. S5). The calculated values of dipole moments and their vectors for both conformers, can be very helpful in understanding the mechanism of the photoinduced molecular transformations for PYR-ππ-pAM molecules embedded in the polymer matrix. The dipole moment values of both conformers increase as a function of the medium polarity (Figs. 5(a) and S22). However, the trans conformer is characterized by a higher value of the ground state dipole moment (μGS) relative to the cis one trans − cis : 2.59 D → 4.50 D from vacuum to water). In the case of (ΔμGS

causes a hypsochromic shift (positive value for b). However, the significant standard error for estimated coefficient determining the influence of solvent polarizability diminished influence of this parameter on PYR-ππ-pAM fluorescence. Nonetheless, the overall shift of the fluorescence maximum is hypsochromic in more polar solvents due to a high contribution of SdP, SA and SB in the regression. In other words, positive sign of Catalán solvent polarity parameter in fluorescence process indicates the excited state's energy level's increase via these parameters, and evidently the stabilization of excited state decreases by increasing solvent polarity. As shown in Fig. 2(b), the fluorescence band shows a blue-shift of about 89 nm on changing the solvent from non-polar one like MCH to polar i.e. DMSO. The decrease of the Stokes shift of the dye in the Catalán solvent scale is mainly a result of the solvent dipolarity and acidity. The contribution of solvent polarizability and basicity to the variation of the ΔνSS are negligible despite the same trend due to the high standard deviations. Following the trends of the experimental absorption and emission data, the Stokes shifts are high and solvent dependent. The larger values are obtained for less polar solvents. Generally, the values of Stokes shift change from 8477 cm−1 in TMP to 4819 cm−1 in DMSO. 3.2. Computational details 3.2.1. Geometry optimization and electrochemical properties The optimized structures of the most stable trans and cis conformers of PYR-ππ-pAM molecule are presented in Fig. 4(a). There are several local minima on the Potential Energy Surface (PES) (listed in Tables S5–S8; atom labelling in Fig. S4 in ESI), however in this study a detailed analysis of linear and nonlinear optical properties is presented only for the minima characterizing the lowest energy of trans (E) and cis (Z) conformers. The meta stable cis conformer is 6.96 kcal/mol higher in energy than the trans one. The analysis of the structural parameters revealed a high sensitivity of bond lengths and dihedral angles of both

Fig. 4. Optimized structures of the trans and cis isomers (a) and potential energy surface scan for the trans (E)→ cis (Z) conformational transformation in two different structural regions (b) and (c) for the PYR-ππ-pAM molecule in the ground state. 6

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Fig. 5. Electrochemical properties of the PYR-ππ-pAM trans (E) and cis (Z) conformers: dipole moment 3D projection; acronyms: GP - gas phase, GS - ground state, CT - charge transfer (a), HOMO → LUMO maps (b), density difference (c) and MEP (d).

electrons from the -N(CH3)2 donor group towards the pyrazolone ring, which fulfils the role of the electron acceptor, is observed. This indicates that the lowest-lying excited state can be assigned as a π-π* transition mixed with an ICT process. Isomerization does not change the distribution of the both frontier molecular orbitals. There is also no significant change in the value of the energy separation between HOMO → LUMO orbitals (EGAP). Namely, in the vacuum environment the cis − trans difference is equal to 0.07 eV. Although, the EGAP is slightly ΔEGAP decreased in the function of solvent polarity (Table S9). The investigated conformers are characterized by a low value of the chemical hardness (η) and should be treated as soft molecules with very high reactivity. Moreover, the calculated electronegativity (χ), which is greater than 4.0 eV for both conformers, indicates an easy formation of covalent bonds during various chemical processes.

dipole moment value in the CT state (μCT), the trans isomer is characterized by a higher μCT value only in GP and in highly polar media. In the weakly polar solvents, from TMP to THF, a higher value of μCT is observed for the cis isomer. The cis isomer has also more polar CT state cis − trans (ΔμGS → CT > 3D). For both conformers, μCT and ΔμGS → CT increase as a function of the medium polarity, with the exception of the gas phase, where there is a slight decrease in both quantities. Interestingly, the vector orientation is similar in both conformers (Fig. 5(a)), the presented observation indicates that the trans → cis transitions occur with a substantial dipole moment change. The charge transfer excitation for both conformers of the PYR-ππ-pAM molecule corresponds basically to the HOMO → LUMO transition (Fig. 5(b)). For the trans isomer, HOMO electrons are delocalized on the entire surface of the molecule, while for LUMO ones they disappear on the aromatic ring. The transfer of 7

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Fig. 6. Schematically shown pump-probe experimental set-up for estimating 3rd order optical nonlinearity; P - polarizer, A - analyzer, S - sample, F - neutral density filter, PD - photodiode, λ/2 - half-wave plate (a), photoinduced birefringence kinetics with its external optical pumping (increase) and thermodynamic relaxation (decay in darkness conditions) (b), linear correlation between estimated Δn value and various pump beam intensity (c) and multiple and reversible inducement of refractive index anisotropy under pump beam illumination (d). Ipump for (b) and (d) was set to be about 160 mW/cm2; blue and black background represents experimental conditions, when the pump beam is ON and OFF, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

by slightly smaller solubility in weak and medium polar solvents and a higher one in strongly polar solvents. For both molecules, the ΔGsolv decreases with increasing solvent polarity, while rapidly growing in water. This indicates the low solubility of the PYR-ππ-pAM molecule in water. To predict reactive sites for nucleophilic (positive, blue regions) and electrophilic (negative, red and yellow regions) attack of the investigated conformers, the Molecular Electrostatic Potential (MEP) surfaces were calculated. According to Fig. 5(d), the most negative site is located on the oxygen atom. The region for the electrophilic attack (V (r) = −0.030 a.u.) is also the nitrogen atom N15 bonded by a double bond with the carbon atom C16 in the pyrazolone ring. Also, negative electrostatic potential zones are observed around the benzene ring connected to the pyrazolone ring. However, the maximum positive region is localized in the space created by the angle of the nitrogen atom and methyl groups of the electron donor substituent, indicating a possible site for the nucleophilic attack. The value of V(r) decreases to +0.040 for hydrogen atoms of the nearby aromatic ring and the methyl group attached to the pyrazolone ring. The trans → cis isomerization does not change the specific zones, where the compound can exhibit intermolecular interactions.

Spectral properties considered below comprising one strong absorption and emission band, corresponding to the HOMO → LUMO photoexcitation (π-π* transitions). However, non-negligible contributions from the other orbitals may occur. In order to determine the nature of these electronic states, the density variation upon photo-excitation (Δρ(r)), computed for the first electronic transitions, is graphically depicted in Fig. 5(c) and Table S4. For trans isomer the plots of Δρ(r) show that the density depletion zones (blue) depend on the solvent polarity. In the gas phase and very polar solvents they are mostly delocalized on the aromatic ring and to a small extent on the -N(CH3)2 part. Starting from THF, these regions are located mainly on the electron donating group. In turn, in all solvents the regions of density increment (purple) are mostly localized on the carbon bridge and pyrazolone ring. This is reflected in the DCT parameter (Table S10), the size of which exceeds 3 Å, with the exception of Bu2O and Et2O, where these values are equal to 0.747 Å and 0.979 Å, respectively. Moreover, the qCT decreases with increasing polarity of the medium, from 0.886 e in vacuum to 0.496 e in water. In the case of cis isomer, the presence of a solvent does not affect the occurrence of depletion zones. The charge transfer distance increases with the environment polarity, and slightly higher qCT decreases from 0.659 e in vacuum to 0.5 e in water. For both conformers, the DCT indicates the CT character and confirms the contributions from HOMO → LUMO transition, although minor contributions from the other orbitals should be expected. Based on the free energy of solvation (ΔGsolv, Table S11), the cis isomer is characterized

3.2.2. Theoretical insight to the linear and nonlinear spectral properties The polarizability and hyperpolarizability of molecules irradiated with intense laser light giving the suitable electric field is the subject of 8

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inducement, it shows very high process reversibility efficiency. Correlation between photoinduced birefringence and pump beam intensity was presented in Fig. 6(c). Since the relation between aforementioned parameters is linear, investigated material can be called Kerr medium, which behaves accordingly due to the general formalism of the 3rd order NLO effects [61]. In other words, the active molecules are effective and photoresponsive, but more importantly material's degradation is not observed. Furthermore, based on this correlation the second, nonlinear refractive index value was estimated to be about (4.1 ± 0.2) × 10−6 cm2/mW. Comparing similar in structure lowmolecular NLO dyes embedded in polymeric matrices, suchlike values of n2 parameter, can be found in the literature. For example, in the case of thiophene and pyrazoline derivatives embedded in PMMA solid films, the nonlinear refractive index value was estimated around 1.6 × 10−7 [44] and 3.9 × 10−7 cm2/mW [46], respectively. For the first mentioned system dye content was set as 2% w/w and for the latter one at 3% w/w, what can be the reason of the one order of magnitude difference between these three compared systems. Namely, in the case of PYR-ππ-pAM dye and its high absorption abilities, it was set to be 0.5% w/w (please see S6). Also, DCNP compound embedded in PMMA matrix (2% w/w), well-known nonlinear chromophore, characterizes n2 parameter value estimated experimentally around 6.6 × 10−7 cm2/mW [45], which complies with the results presented in this manuscript. As the inset of Fig. 6(c) several experimental curves of the photoinduced birefringence was shown. Since the pump beam power increases, the nonlinear optical response from the investigated system is higher as well. Signal amplitude values of the birefringence inducement were taken to create Δn = f(Ipump) correlation. Subsequently, multiple molecular photoalignment (optical anisotropy generation) and misalignment (back to the optically isotropic state) was shown in Fig. 6(d). It is clearly visible that the effectiveness (NLO response) increases even after few cycles of the photo-controlled alignment and misalignment of the active PYR-ππ-pAM molecules. Static (or so called total) Δn value varies from the first cycle from about 4.0 × 10−4 up to 5.5 × 10−4 in the fifth cycle of complete molecular photo-ordering. It means that the system efficiency increased on about 45% (1st → 5th cycle), and pump laser beam treatment does not affect any of undesirable photodegradation processes. Remnant birefringence, from the first cycle up to the last measured one increased to about 23% due to its currently reached total Δn value. Such behaviour can be related with the molecular steric hindrance (coming from both, guest and host ones), which prevents total initial arrangement reversibility. Another reason can be found as not enough darkness relaxation time in order to achieve again the lowest possible thermodynamic energetic state of the entire molecular population. From the first cycle, where only 16% of molecular population was not effectively converted back to the initial arrangement, this population increases together with the number of cycles (up to the 23% after the 5th cycle). It means that the remnant birefringence value increases twice less after multiple photoordering (and seems to be stabilized) than the total effectiveness of molecular photoalignment (static Δn level). Such correlation based on the experimental data was shown in ESI in Fig. S7. Interestingly, for the better comparison with available literature data cited in this manuscript, the Authors performed additional experiment for 2% sample (w/ w ratio between used dye and solid-state matrix). Multiple pump laser beam treatment with the set intensity at ~240 mW/cm2, after the second finite cycle brought as the result complete molecular reorientation (δ = 100%). The system high efficiency with defined value of productivity equal to 100% due to the optically induced Δn reversibility, classifies PYR-ππ-pAM/PS hybrid system as one of the best currently available among organic all-optical switchers (please, see Fig. S8). Obtained value of the photoinduced birefringence for the investigated PYR-ππ-pAM polymeric system is equal to 5.5 × 10−4, which corresponds to the previously mentioned literature data [44–46]. In comparison with azobenzene derivatives (i.e. S3 or CPND5),

many research in terms of understanding various linear and nonlinear optical properties. In particular, these studies include the interrelationship of nonlinear properties with the electronic structure to design new multifunctional molecules. The calculated values of NLO parameters for both trans and cis conformers in different solvents are collected in Table S18. For both trans and cis conformers, the α , Δα and βvec values increase monotonously with the environmental polarity. The trans isomer is characterized by higher polarizability and first-order hyperpolarizability values. Simultaneously, the ΔNLOtrans − cis difference increases with the solvent polarity. Taking into account the applied functionals, the CAM-B3LYP underestimates NLO parameters relative to the PBE0, and Δα is from 10 a.u. in GP to 27.5 a.u. in water. For the first-order hyperpolarizability, the Δβvec is from 1000 a.u. to 4000 a.u. The HSEH1PBE slightly overestimates these values, on average 5 a.u. and 450 a.u. for α and βvec, respectively. It is worth mentioning that these differences also increase as a function of solvent polarity. 3.3. Photoinduced birefringence Experimental set-up used for the optical Kerr effect investigation is presented in Fig. 6(a) and already described in details in Materials and Methods section (2.3. Nonlinear spectroscopy). In Fig. 6(b) complete dynamics of the photoinduced birefringence and thermal relaxation process was presented. Static Δn generation in this case characterizes time constant in order of tens of seconds (τinc = 61 s), which considering similar low-molecular organic dyes embedded in polymeric solid-state matrix complies with the literature data [42–46]. However, thermal dark relaxation (Δn decay) seems to be about two times faster than photoinduced birefringence (τdec = 29 s). It is particular, cause typically such a relaxation processes characterize higher time constant than induced by light (or other external stimuli, i.e. electric field) meta stable molecular alignment. Thermal relaxation phenomena are spontaneous, forced only by lowering Gibbs free energy and tends to reach again thermodynamic equilibrium by whole system (molecular population re-arrangement). Nevertheless, available literature data also presents such behaviour of the investigated low-molecular nonlinear chromophores embedded in the solid-state matrix. For example, thiophene derivative compound characterizes time constant values for the photo-controlled molecular ordering and thermal darkness molecular misalignment phenomena to be about 36.0 and 17.9 s, properly [44]. In the case of pyrazoline derivatives abovementioned constants are different and strictly depends on the chemical structure. For example, in the case of relatively extended structure with additional electron-acceptor moiety substituted in the middle of molecular plane (close to the stilbene group) arise and decay time constants are equal to 26.2 and 78.5 s, respectively [45]. Although, in the case of less extended (simple in structure, smaller molecules, although still containing pyrazole ring) time constants describing molecular photo-ordering and thermodynamic relaxation processes are equal to 6.2 and 4.0 s for DCNP and 3.4 and 1.2 s for PY-pNO2, respectively [46]. Time constants directly describe photoinduced birefringence kinetics, which is crucial if consider future possible application of the such working photo-responsive materials. The systems characterizing low values of the τinc and τdec can be utilized as the all-optical switches. However, the materials with higher time constant values can serve as the optical sensors or (in the utmost case, like photochromic polymers or azo-functionalized nanoparticles in the liquid crystals [72]) optical data storage. However, two significant issues should be pointed out here. Δn inducement is not completed in the case of investigated system, because characteristic plateau range was not achieved (Fig. 6(b)). It means that the active molecules still can be alternated more efficient to the optical anisotropy state. From two reasons it was not observed: available and used time scale is not enough to observe complete conversion and/or pump beam intensity was too low. Unfortunately, experimental set-up limitations did not allow to investigate the material till the signal's saturation. From the other hand, since thermal relaxation process is faster than Δn 9

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obtained Δn parameter for the investigated compound is about one order of magnitude lower [73]. 3rd order nonlinear optical susceptibility value (1.1 × 10−7 m2/V2 estimated for PYR-ππ-pAM) is about three to five orders of magnitude higher in comparison with the previously mentioned thiophene and pyrazoline derivatives [44–46]. Such high value of the χ(3) parameter can be related with the higher negative charge transfer delocalization through the investigated molecule in comparison with the less expanded chemical structures (in general) and also less substituted main heteroatomic ring (in particular) of the compared before other nonlinear chromophores. In order to estimate dynamic component of the photoinduced birefringence (Δndyn) external mechanical signal modulator was applied (cf. Fig. 6(a)). Therefore, to observe the optical response of the system, AC mode in oscilloscope was used as well, in order to characterize kinetics of the molecular transformations caused by short pump beam laser pulses. The results of that part were presented in ESI file, in Fig. S9(a). In this case, signal increase is connected with all available trans → cis transformations which need photons to reach higher in energy meta stable cis conformational state. Then, when laser pulse diminishes, all of the molecules which were excited to the cis state, go back to the initial, lower in energy trans conformational state (cis → trans). Such transition is shown as the signal decay in Fig. S9(a). Multiple of abovementioned conformational transformations, as it was stated before, in fine allow to generate total optical anisotropy through investigated material. Such kind of transitions take place in the time regime of milliseconds, which is consistent with the literature data [42–46]. Time constant values of trans → cis and cis → trans transformations with applied signal modulation at 10 Hz in our system are very similar to each other and they were estimated at around 11.1 and 8.6 ms, respectively. Available in the literature data shows that these values can vary from less than 0.5 ms [44] up to few of milliseconds [42,43,45,46]. However, signal modulation frequency was about one order of magnitude higher in compared literature data than in our investigations. Namely, it was 200 and 150 Hz, respectively, whereas only 10 Hz was used in our case. Such big difference with signal modulation frequency can also affect time constants related to the analyzed processes. Numerous of the conformational transformations were acquired and presented in Fig. S9(b). It proofs high stability and repeatability of Δn dynamic component of the investigated system. Also, in this case, the Authors have provided similar experimental data for additional sample, containing 2% of the used nonlinear chromophore. To see details, please find ESI (Fig. S10). Moreover, Kerr constant value has been estimated for PYR-ππ-pAM polymeric system [74]. It was defined on the level of 3.5 × 10−3 m/V2, which - as in the case of χ(3) - is about three orders of magnitude higher in comparison with available literature data [42–46]. More extended chemical dye's structure, and therefore lower free volume understood as the space between solid state matrix and active matter can be the reason of such B value. Additionally, all experimental results cited in the discussion section, come from the system where poly(methyl methacrylate) (PMMA) served as the matrix. Whereas in our system polystyrene was used. Optical features of the used and compared polymers are similar [75], although their chain shape, in particular - their complexity, is totally different. PMMA characterizes much more branched structure than polystyrene. However, PS contains aromatic rings in its structure. All of these structural details undoubtedly influent to all of the molecular motion kinetics and specific and non-specific interactions between all participators in guest:host systems. Such investigation could be a topic of the future projects.

discussed in the manuscript. Quantum chemical calculations allowed to identify the most stable trans and cis conformers and to determine the energy barrier for the transition between them. In addition, a high agreement between theoretically determined optical properties and experimental data was obtained. This dependence allowed to state that the maximum of absorption for the cis conformer is shifted towards longer wavelengths, while the trans isomer is characterized by a red shift in the λPL position only in weakly polar solutions with respect to cis isomer. Although the cis conformer has a more polar CT state, the trans isomer is characterized by higher polarizability and first-order hyperpolarizability values. In fine, precious all-optical switching features of the PYR-ππ-pAM/PS bi-component system were acquired and analyzed. All the results decisively indicate investigated organic system as the effective, reversible and sensitive nonlinear optical light modulator based on the pyrazolone ring and stilbene bonding. It was proven that even after few cycles of photo-controlled molecular re-alignment, NLO response is still on the very satisfied level or even higher in obtained intensity (in this case, Δn value) than after one cycle considering the ratio between acquired signal and observed background level. Such behaviour is an evident for PYR-ππ-pAM powerful optical stability, which is desired for the organic materials dedicated for the various photonic applications. Stilbene derivatives based on the pyrazolone ring constitute an alternative for already well-exploited azobenzene derivatives in the context of their utilization in optically-controlled refractive index anisotropy phenomena. Acknowledgement This work was financed by statutory funds of Faculty of Chemistry at Wroclaw University of Science and Technology (Wrocław, Poland). This research was supported by the Computational Grant No. 249, PCSS (Poznan, Poland) and in part by PL-Grid Infrastructure. The Authors would like to thank Professor Piotr Cysewski for assistance with the theoretical calculations and helpful discussion. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.dyepig.2019.107805. References [1] Chudakov DM, Matz MV, Lukyanov S, Lukyanov KA. Fluorescent proteins and their applications in imaging living cells and tissues. Physiol Rev 2010;90:1103–63. [2] Hanczyc P, Samoć M, Norden B. Multi-photon absorption in amyloid protein fibres. Nat Photonics 2013;7:969–72. [3] Chida T, Kawabe Y. Transient grating formation in azo-doped polymer and its application to DNA-based tunable dye laser. Opt Mater 2014;36:778–81. [4] Galisteo-Lopez JF, Ibisate M, Sapienza R, Froufe-Perez LS, Blanco A, Lopez C. Selfassembled photonic structures. Adv Mater 2011;23:30–69. [5] Zhang H, Feng G, Wang S, Yang C, Yin J, Zhou S. Coherent random lasing from liquid waveguide gain channels with biological scatters. Appl Phys Lett 2014;105:253702. [6] Costela A, García O, Cerdán L, García-Moreno I, Sastre R. Amplified spontaneous emission and optical gain measurements from pyrromethene 567-doped polymer waveguides and quasi-waveguides. Opt Express 2008;16:7023–36. [7] Wiersma DS, van Albada MP, Lagendijk A. Random lasers. Nature 1995;373:203–4. [8] Herrmann J, Wilhelmi B. Mirrorless laser action by randomly distributed feedback in amplifying disordered media with scattering centers. Appl Phys B 1998;66:305–12. [9] Noginov MA. Solid-state random laser. New York: Springer; 2005. [10] Costela A, Garcia-Moreno I, Cerdan L, Martin V, Garcia O, Sastre R. Dye-doped POSS solutions: random nanomaterials for laser emission. Adv Mater 2009;21:4163–6. [11] Cao H. Review on latest developments in random lasers with coherent feedback. J Phys A Math Gen 2005;38:10497–535. [12] Rau I, Szukalski A, Sznitko L, Miniewicz A, Bartkiewicz S, Kajzar F, Sahraoui B, Mysliwiec J. Amplified spontaneous emission of Rhodamine 6G embedded in pure deoxyribonucleic acid. Appl Phys Lett 2012;101:171113. [13] Sznitko L, Szukalski A, Cyprych K, Karpinski P, Miniewicz A, Mysliwiec J. Surface roughness induced random lasing in bio-polymeric dye doped film. Chem Phys Lett 2013;576:31–4. [14] Pikas DJ, Kirkpatrick SM, Tewksbury E, Brott LL, Naik RR, Stone MO. Nonlinear

4. Conclusions Highly effective and reversible photo-controlled organic light modulator based on stilbene group has been introduced. Active molecules come from newly synthesized pyrazolone derivative. Its synthesis route with basic materials and optical features were presented and 10

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