Synthesis and characterization of new dual stimuli-responsive bisazobenzene derivatives

Abstract

New organophosphonates containing bisazobenzene moieties in the main chain were synthesized by solution polycondensation reaction of 4,4′-(4,4′-(biphenyl-4,4′-diylbis(oxy))bis(4,1-phenylene))bis(diazene-2,1-diyl)diphenol and phenylphosphonic dichloride/phenyl dichlorophosphate. FTIR and NMR spectroscopy were used to confirm the chemical structure of the new compounds, while thermal properties were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The optical properties of these compounds were evaluated in different solvents and in the presence or absence of several external stimuli by UV-vis and fluorescence spectroscopy. The photoresponsive trans–cis photoisomerization and cis–trans relaxation process of these compounds were investigated. The absorption and fluorescence properties were sensitive to chemical stimuli (acid/base treatments). In the presence of NaOH a strong bathochromic shift of the absorption band of the bisazobenzene derivatives was observed (deprotonation). The fluorescence signal was noticeably improved upon deprotonation of the polymer solution.

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Introduction

Organic multicomponent materials containing two or three chromophores (such as azobenzenes, stilbenes and spiropyrans) have attracted increasing attention due to their possible use as photoresponsive molecular switches,^1–3 data storage devices^4,5 and sensors.^6,7 Stimuli-response is a desired feature for many smart materials. A simple azobenzene system exists mainly as the trans isomer but, when photoexcited, it converts to the cis isomer. Instead, in multicomponent materials containing two or more azobenzene photochromic components, unexpected interactions such as energy transfer, photochemical quenching, steric interactions and dipole–dipole interactions can appear, thus leading to a mixture of three isomers trans,trans, trans,cis and cis,cis at the photostationary state.⁸ A detailed analysis of the photo-isomerization of a double-azobenzene system showed that the isomerization of the first azo-unit quenches the photo-isomerization of the second azo-unit.⁹ In 2014, J. Vapaavuori et al.¹⁰ investigated the differences in the photoresponse between monoazobenzene derivatives and bisazochromophores. They found that polymer–bisazobenzene complexes show high photoresponsive characteristics, more specifically in photo-orientation and all-optical surface patterning compared to analogous monoazobenzene derivatives.

The photoresponsive properties and kinetic behavior of a wide range of compounds containing two azobenzene units have been reported.^8,9,11,12 However, only a few studies address the sensitivity of these compounds to the chemical stimuli (acid/alkali).⁹ Recently, S. Subhas et al. have prepared and characterized a new crosslinker in which a central piperazine unit links two azobenzene chromophores.¹³ A wide range of bisazobenzene complexes possessing siloxane groups,¹⁴ banana-calamitic dimers and trimmers,¹⁵ polyhedral oligomeric silsesquioxane core,¹¹ and poly(amide-imide)s with two azobenzene chromophores¹⁶ has been reported. Many applications are possible for smart materials due to changes in the properties of these systems when applying different stimuli, such as light, magnetic field, heat and chemical agents.

Phosphorus-containing materials can be employed for a wide range of technological applications.¹⁷ For instance, they were largely used in industry due to their ability to bind metals.¹⁸ Organophosphonates exhibited interesting complexing properties and were used as dispersants, corrosion inhibitors, or to prevent deposit formation. In the scientific communities, the increasing demand of new high performance materials, based on polymers, brings out the need of satisfying very stringent requirements, such as flame retardancy and thermal stability, which are closely related to health and environment risks. Recently, the development of halogen-free materials, due to latest legislation, gave to phosphorylated polymers new opportunities to be used as flame retardants,^19–21 where phosphorus is known to be very efficient. Also, the incorporation of phosphorus in polymers could increase the solubility, thermal stability and fire retardant properties. While halogen based flame retardants are continuously banned, more efficient materials containing phosphorus, became highly appreciated in development of new high performance systems, non-harmful for human and environmental health and with proper flame resistance.^22,23

In this work a series of two systems containing bisazobenzene units was prepared starting from 4,4′-bis(4-hydroxyphenyldiazenyl-4-phenoxy)biphenyl (BAPB-Azo). The synthesized compounds were characterized by FTIR, NMR, TGA, DSC, UV-vis and fluorescence electronic spectroscopy. For these compounds, we found that the thermal stability depends on the chemical structure of the investigated compounds. The photo-induced trans–cis–trans isomerization of BAPB-Azo, BAPB-Azo-C₆H₅ and BAPB-Azo-OC₆H₅ in DMSO solution was discussed. The absorption and fluorescence properties exhibited strongly and reversible changes upon acid/base stimuli treatments, which mainly can be attributed to the deprotonation and protonation processes. The emission of BAPB-Azo is remarkable increased when the sample is titrated with a dilute solution of NaOH.

Materials and methods

Materials

4,4′-Bis(4-aminophenoxy)biphenyl, hydrochloric acid (HCl, 37%), sodium nitrite (NaNO₂), phenol, sodium hydroxide (NaOH), phenylphosphonic dichloride, phenyl dichlorophosphate, N,N-dimethylformamide (DMF), triethylamine (TEA), tetrahydrofuran (THF), N-methyl-2-pyrrolidinone (NMP), and dimethyl sulfoxide (DMSO) were purchased from Sigma-Aldrich and used without further purification.

Synthesis

Synthesis of 4,4′-bis(4-hydroxyphenyldiazenyl-4-phenoxy)biphenyl (BAPB-Azo). The diazoic monomer denoted as BAPB-Azo was synthesized by coupling reaction of diazonium salt of 4,4′-bis(4-aminophenoxy)biphenyl with phenol (Scheme 1). The synthetic procedure is described below: in an one-necked round-bottom flask equipped with magnetic stirrer, 4,4′-bis(4-aminophenoxy)biphenyl and a solution of HCl 1.5 M were introduced. After the temperature of the reaction mixture was established below 5 °C, an aqueous solution of NaNO₂ was added. In parallel, in a round-bottom flask, it was prepared an aqueous solution containing phenol, NaOH and sodium carbonate. The clear diazonium salt solution was carefully poured into aqueous phenolic solution. The resulting mixture was further stirred for 5 hours. Afterwards, the reaction mixture was filtered and the resulting yellow precipitate of BAPB-Azo was washed with distilled water. (Yield 72%) FTIR (KBr pellet, cm⁻¹): 3270 (ν_OH), 3066 (ν_C–H), 1591 (ν_C=C of aromatic rings), 1494 (ν_C=C of aromatic rings), 1402 (ν_N=N), 1257 (ν_C–O–C), 854 (δ_CH), 749 (δ_CH), 697 (δ_CH). ¹H-NMR (400 MHz, DMSO-d₆, δ, ppm): 6.91 (4H, d, 9 Hz, H-10), 7.20–7.23 (8H, d, 5.7 Hz, H-7, d, 6 Hz, H-2), 7.75–7.78 (8H, d, 6.2 Hz, H-11, d, 6 Hz, H-6), 8.06 (4H, d, 9 Hz, H-3), 10.30 (2H, s, OH) (Fig. S1†). ¹³C-NMR (100 MHz, DMSO-d₆, δ, ppm): 116.05 (C-10), 118.70 (C-7), 119.80 (C-2), 124.08 (C-3), 124.70 (C-6), 128.3 (C-11), 135.37 (C-8), 144.91 (C-9), 148.00 (C-5), 155.30 (C-4), 158.60 (C-1), 161.40 (C-12) (Fig. S2†).

Scheme 1 Synthetic route of the 4,4′-bis(4-hydroxyphenyldiazenyl-4-phenoxy)biphenyl (BAPB-Azo).

Synthesis of polymers

The polymers BAPB-Azo-C₆H₅ and BAPB-Azo-OC₆H₅ were synthesized by solution polycondensation of 4,4′-bis(4-hydroxyphenyldiazenyl-4-phenoxy)biphenyl (BAPB-Azo) and phenylphosphonic dichloride or phenyl dichlorophosphate, respectively (Scheme 2). A typical example of synthetic procedure for BAPB-Azo-C₆H₅ is described below: in a 25 mL round flask equipped with magnetic stirrer, nitrogen inlet/outlet and reflux condenser, 0.578 g BAPB-Azo, 6.41 mL N,N-dimethylformamide (DMF) and 0.139 mL triethylamine (TEA) were introduced. The mixture was stirred to complete solving of BAPB-Azo, and then 0.1418 mL phenylphosphonic dichloride was added dropwise. The reaction mixture was stirred for 20 hours at 60 °C. It was allowed to cool down, and poured into water to give a precipitate which was collected by filtration, washed with fresh water and dried in vacuum oven for 10 hours at 100 °C.

Scheme 2 Synthesis of BAPB-Azo-C₆H₅ and BAPB-Azo-OC₆H₅.

BAPB-Azo-C₆H₅. FTIR (KBr pellet, cm⁻¹): 3059 (ν_C–H), 1597 (ν_C=C of aromatic rings), 1500 (ν_C=C of aromatic rings), 1472 (P–Ph), 1168 (P=O), 1153 and 931 (P–O–Ph), 1231 (ν_C–O–C), 850 (δ_CH), 750 (δ_CH), 692 (δ_CH). ¹H-NMR (400 MHz, DMSO-d₆, δ, ppm): 6.95 (4H, d, 8.7 Hz, H-10), 7.17–7.40 (11H, m, H-7, H-2, H-15, H-17), 7.70–7.84 (10H, m, H-11, H-6, H-16), 7.90 (4H, d, 9 Hz, H-3), 10.30 (2H, s, OH) (Fig. S3†). ¹³C-NMR (100 MHz, DMSO-d₆, δ, ppm): 115.87 (C-10), 118.64 (C-7), 119.76 (C-2), 120.42 (C-17), 124.08 (C-16), 124.08 (C-3), 124.60 (C-6), 128.30 (C-11), 129.1 (C-15), 135.34 (C-8), 145.17 (C-9), 147.60 (C-14), 145.17 (C-5), 155.30 (C-4), 157.18 (C-13), 158.62 (C-1), 160.67 (C-12) (Fig. S4†). ³¹P-NMR (162 MHz, DMSO-d6, δ, ppm): −12.07 (Fig. S5†).

BAPB-Azo-OC₆H₅. FTIR (KBr pellet, cm⁻¹): 3051 (ν_C–H), 1597 (ν_C=C of aromatic rings), 1500 (ν_C=C of aromatic rings), 1168 (P=O), 1149 and 932 (P–O–Ph), 1239 (ν_C–O–C), 850 (δ_CH), 760 (δ_CH), 691 (δ_CH). ¹H-NMR (400 MHz, DMSO-d₆, δ, ppm): 6.95 (4H, d, 7.2 Hz, H-10), 7.15–7.40 (11H, m, H-15, H-7, H-2, H-17), 7.65–7.85 (10H, H-11, H-6, H-16), 7.94 (4H, d, 7 Hz, H-3), 10.30 (2H, s, OH) (Fig. S6†). ¹³C-NMR (100 MHz, DMSO-d₆, δ, ppm): 115.85 (C-10), 118.63 (C-7), 119.85 (C-2), 120.30 (C-17), 123.95 (C-16), 124.05 (C-3), 124.56 (C-6), 128.28 (C-11), 129.60 (C-15), 135.33 (C-8), 145.16 (C-9), 147.93 (C-5), 154.97 (C-14), 155.29 (C-4), 157.72 (C-13), 158.60 (C-1), 160.66 (C-12) (Fig. S7†). ³¹P-NMR (162 MHz, DMSO-d6, δ, ppm): −11.9 (Fig. S8†).

Experimental equipment

Melting points of the monomers and intermediates were measured on a Melt-Temp II (Laboratory Devices).

Infrared spectra were measured using KBr pellets on a FTIR Bruker Vertex 70 spectrophotometer. Scans were recorded between 4000 and 500 cm⁻¹ at a resolution of 4 cm⁻¹.

The NMR spectra were recorded on a Bruker Avance DRX 400 MHz Spectrometer equipped with a 5 mm QNP direct detection probe and z-gradients. Spectra were recorded in DMSO-d₆ at room temperature.

The thermal stability of the samples was investigated by using the thermogravimetric analysis (TGA) (STA 449F1 Jupiter, Netzsch). The thermogravimetric (TG) and derivative thermogravimetric (DTG) curves were recorded in nitrogen in a temperature range of 25–750 °C and with a heating rate of 10 °C min⁻¹. The weight of sample was about 8 mg.

Differential scanning calorimetry (DSC) measurements were carried out on a Mettler T28E calorimeter. Samples (3–5 mg) were heated from 25 to 250 °C with a heating rate of 10 °C min⁻¹ under nitrogen atmosphere. The glass transition temperature of the samples was obtained from the second heating run in the plot of heat flow versus temperature scan, which is assigned by the mid-point of the inflexion curve.

UV-visible spectral measurements were taken by using a Shimadzu 3600 UV-vis-NIR spectrophotometer. Fluorescence spectra were made by using a Perkin Elmer LS 55 fluorimeter. The photoisomerization investigations were performed by using a medium pressure mercury lamp.

Results and discussion

General characterization

A new diazoic monomer 4,4′-bis(4-hydroxyphenyldiazenyl-4-phenoxy)biphenyl was synthesized by coupling reaction of diazonium salt of 4,4′-bis(4-aminophenoxy)biphenyl with phenol. This new diazoic monomer was used to obtain organophosphonates containing bisazobenzene moieties in the main chain. To achieve this goal the diazoic monomer 4,4′-bis(4-hydroxyphenyldiazenyl-4-phenoxy)biphenyl and phenyl-phosphonic dichloride/phenyl dichlorophosphate were used (Scheme 2). The chemical structures of the new monomer BAPB-Azo, organophosphonates containing bisazobenzene moieties in the main chain (BAPB-Azo-C₆H₅/BAPB-Azo-OC₆H₅) were confirmed by FTIR and NMR spectroscopy.

FTIR spectrum of BAPB-Azo showed a broad band centered at 3270 cm⁻¹ corresponding to the stretching vibrations of OH phenolic bond; a band at 3066 cm⁻¹ attributed to the stretching vibrations of aromatic C–H; a strong doublet at 1591 cm⁻¹ and 1494 cm⁻¹ corresponding to the stretching vibrations of aromatic C=C; a strong band at 1257 cm⁻¹ associated with stretching vibrations of aromatic ether C–O–C. The absorption band characteristic of N=N bond was observed in the FTIR spectrum of monomer at 1402 cm⁻¹. The phenolic proton of BABP-Azo appeared in the ¹H-NMR spectrum at 10.3 ppm. The aromatic protons were found in the interval 8.06–6.91 ppm.

Fig. 1 shows the FTIR spectra of BAPB-Azo-C₆H₅ and BAPB-Azo-OC₆H₅. As can be observed from the FTIR spectra of these compounds, the most important absorption bands are associated to the C–H aromatic (3059–3051 cm⁻¹, stretching vibration), ether C–O–C aromatic (1231(9) stretching vibrations), P–O–Ph (931(2) cm⁻¹ and 1153(49) cm⁻¹, stretching vibrations), P=O (1168 cm⁻¹), P–Ph (1472 cm⁻¹). Aromatic C=C absorption bands were found at 1597 and 1500 cm⁻¹.

Fig. 1 FTIR spectra of the BAPB-Azo-C₆H₅ and BAPB-Azo-OC₆H₅.

¹H, ¹³C and ³¹P-NMR spectroscopy were used to confirm the chemical structure of the new organophosphonates containing bisazobenzene moieties in the main chain. ¹H and ¹³C-NMR spectra of the BAPB-Azo-C₆H₅ and BAPB-Azo-OC₆H₅ are shown in Fig. 2. The aromatic protons of the BAPB-Azo-C₆H₅ and BAPB-Azo-OC₆H₅ appear in a range of δ between 6.9 and 7.9 ppm. The ³¹P-NMR spectrum revealed only one signal at −12.07 and −11.9 ppm, supporting the proposed molecular structure of the BAPB-Azo-C₆H₅ and BAPB-Azo-OC₆H₅.

Fig. 2 ¹H (a) and ¹³C-NMR (b) spectra of the BAPB-Azo, BAPB-Azo-C₆H₅ and BAPB-Azo-OC₆H₅.

Thermal properties

The most important thermogravimetric characteristics of the BAPB-Azo and BAPB-Azo-C₆H₅/BAPB-Azo-OC₆H₅, obtained from the thermogram curves are listed in Table 1. The thermal degradation of BAPB-Azo occurs in two stages. The char yield measured at 700 °C was 54.25%. The BAPB-Azo-C₆H₅ and BAPB-Azo-OC₆H₅ show a degradation mechanism similar to BAPB-Azo monomer. The thermal stability of the studied organophosphonates containing bisazobenzene moieties in the main chain was estimated by using the temperature at which the thermal degradation starts (T_onset). The T_onset for BAPB-Azo-C₆H₅ and BAPB-Azo-OC₆H₅, respectively started at around 219 °C and could be attributed to the elimination of bounded water and solvent from the oligomer chain (Fig. S9†).^24,25

Table 1 Thermogravimetric parameters and glass transition temperatures (T_g) of the BAPB-Azo, BAPB-Azo-C₆H₅ and BAPB-Azo-OC₆H₅^a

Code	St	Tonset^b (°C)	Tpeak^c (°C)	Tendset^d (°C)	W^e (%)	R^f (%)	Tg (°C)
a St – stages of thermal degradation.b The temperature at which the thermal degradation starts at every stage.c The temperature at which the thermal degradation is maximum.d The temperature at which the thermal degradation process ends at every stage.e Mass change in every stage of samples decomposition.f Residual mass of the samples measured at 700 °C.
BAPB-Azo	I	264	285	299	24.35	54.25	17.3
	II	444	533	588	21.4
BAPB-Azo-C6H5	I	220	230	232	2.4	70.47	24.9
	II	245	253	264	6.12
	III	373	437	500	21.01
BAPB-Azo-OC6H5	I	219	223	307	2.55	69.46	20
	II	251	264	307	10.63
	III	430	469	630	17.36

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From the TGA data can be observed that the thermal stability of BAPB-Azo-OC₆H₅ is higher than that of BAPB-Azo-C₆H₅. The BAPB-Azo-OC₆H₅ shows an extensive structural conjugation leading to an increase in rigidity and also in the thermal resistance of the sample.²⁶ The thermal stability value of BAPB-Azo-C₆H₅ decreases, by 6 °C (T_onset = 245 °C), due to the destruction in conjugation. It also can be observed that the char yield measured at 700 °C in the case of BAPB-Azo-C₆H₅ is 70.47% and for BAPB-Azo-OC₆H₅ is 69.46%, while the BAPB-Azo exhibits 54.25%, with approximately 30% less than the studied organophosphonates containing bisazobenzene moieties in the main chain. According to these results, it can be assumed that the introduction of phosphorus, contributes to higher charring.²² The glass transition temperature (T_g) of BAPB-Azo was discussed compared to those of the studied organophosphonates containing bisazobenzene moieties in the main chain. This parameter have been estimated from the second heating DSC curves, which were listed in Table 1. The bisphenol BAPB-Azo, having a molecular weight of 578.62 g mol⁻¹, presented itself a glass transition at 17.3 °C, while no melting transition was detected by DSC when heating the sample up to 400 °C. The glassy behavior of BAPB-Azo may be attributed to the insertion of flexible ether linkages, between the rigid bisazobenzene cores, which facilitates the motion of the molecule. When comparing the T_g of BAPB-Azo to those of the BAPB-Azo-C₆H₅ and BAPB-Azo-OC₆H₅, respectively, a slightly increase of 3 °C was observed due to the introduction of phenoxy moieties into the macromolecular chain, while an additional increase of the glass transition temperature was observed for the BAPB-Azo-C₆H₅ based on phenylphosphonic-dichloride, as a consequence of the aromatic ratio increase into the structural unit.

Optical properties

The absorption spectral data of the investigated samples in tetrahydrofuran (THF), N-methyl-2-pyrrolidinone (NMP), N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) solutions are collected in Table 2. The absorption spectra of the BAPB-Azo sample in various solvents are shown in Fig. 3. The spectra of studied samples show in all solvents the specific transitions for aminoazobenzene derivatives, like one intense absorption band with a maximum at λ = 365 nm corresponding to the π–π* transition and a weak absorption located between 400 and 500 nm originating from the forbidden symmetry of n–π* transitions. The main absorbance band (π–π* transition) is easily red-shifted in the NMP solution compared to the wavelength values corresponding to the maximum in THF (see Table 2).

Table 2 The absorption maxima, λ_max (nm), for the investigated compounds in different media

Samples	λmax (nm)
	THF	NMP	DMF	DMSO
BAPB-Azo	250; 359	367	364	366
BAPB-Azo-C6H5	257; 357	364	362	364
BAPB-Azo-OC6H5	258; 357	365	362	364

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Fig. 3 UV-vis absorption spectra of the BAPB-Azo sample in different media.

Photoisomerization of investigated bisazobenzene derivatives

The photoresponsive properties of the investigated new monomer and organophosphonates containing bisazobenzene moieties in the main chain were studied by UV-vis spectroscopy in solution.

The absorption spectral changes when irradiating a BAPB-Azo sample in DMSO solution are shown in Fig. 4. By irradiating the BAPB-Azo sample in DMSO solutions with UV light (using 334 or 365 nm light filters), the cis isomeric form was more evident after approximately 120 minutes of the exposure of sample solution at UV light due to electronic structure of the chromophores having two conjugated azobenzene units. Instead, after irradiating the same sample with polychromatic light, the cis isomeric form was more evident after 480 s of irradiations as depicted in Fig. 4.

Fig. 4 Absorption spectra of the BAPB-Azo sample in a DMSO solution, recorded for various irradiation times. Arrows indicate changes upon irradiation.

It is known that the isomerization that occurs with visible light rather than UV light irradiation is preferred for biological applications.¹³ For compounds containing bisazobenzene units the existence of the trans–trans, trans–cis and cis–cis isomeric states¹³ and isomerization of the first azo-unit quenches the photo-isomerization of the second azo-unit⁸ was also demonstrated.

As shown in Fig. 4, the photobehavior of investigated BAPB-Azo upon irradiation with a polychromatic light source consist in a decreases in intensity of the main absorption band corresponding to the π–π* transition and the shoulder attributed to the n–π* transition (specific to cis form) become more evidently upon 360 s of irradiation (see Fig. 4 inset). The BAPB-Azo-C₆H₅ and BAPB-Azo-OC₆H₅ samples show similar behavior. Upon 480 s irradiation of BAPB-Azo sample in a DMSO solution by using a polychromatic light source, the isomerization from trans to a cis isomeric form occurs, but not the complete disappearance of the π–π* electronic transition because the cis form returns faster back to the trans form at the room temperature and due to high instability of the cis isomer of these type of azobenzene derivatives.²⁷

In the Fig. 5 it was illustrate the thermal reverse cis–trans isomerization process, when the sample was then kept in dark, at the room temperature and its absorption spectra were recorded at different time intervals. It is found that the cis–trans isomerization (Fig. 5) of the BAPB-Azo sample is slow when compared to the trans–cis photoisomerization. The cis form of this sample could be 98% converted back in trans form, after were kept it 24 hours in the dark.

Fig. 5 Changes in the absorption spectra of the BAPB-Azo in a DMSO solution during the cis–trans thermal relaxation (at the room temperature).

The response of the studied bisazobenzene derivatives to chemical stimuli (acid/base)

Stimuli-responsive polymeric materials attract a greater interest in the recent years due to their numerous applications (e.g. scanning probe writing, switchable wettability, sensors and actuators).^28,29

The absorption spectral behavior of these bisazobenzene derivatives can be changed in basic or acid environments. We monitored the absorption changes when the BAPB-Azo sample is titrated with diluted NaOH and then with HCl solution. The UV-vis spectra of this sample in the presence of different amounts of diluted NaOH solution were showed in Fig. 6.

Fig. 6 Changes in the UV-vis spectra of BAPB-Azo (a) and BAPB-Azo-C₆H₅ (b) in DMSO solution, before (initial) and after titration with a dilute solution of NaOH (c = 1 N). Arrows indicate changes in UV-vis spectra.

With increasing of the concentration of aqueous solution of sodium hydroxide (NaOH), the absorption maxima (initially at 366 nm) gradually decreases in intensity and at the same time a new strong absorption band appears in the visible region with the maximum at 490 nm, slightly shifted to a shorter wavelength (481 nm) and with an progressive increasing intensity (as shown in Fig. 6). This new absorbance band (from 490 nm) was assigned to the deprotonated BAPB-Azo sample form. In Fig. 6 it can also be observed that at 100 μL NaOH added into 2.5 mL of BAPB-Azo in DMSO, the band at λ_max = 366 nm disappears and a new band located between 400 and 550 nm appears. These changes correspond to the deprotonation of investigated bisazobenzene derivatives and also were observed detectable color changes from colorless of neutral form to yellow-orange which correspond to deprotonated form of the BAPB-Azo sample.

In order to observe the reversibility of the deprotonation process, in the solution of completely deprotonated form of the BAPB-Azo sample ((i) + 100 μL NaOH, see Fig. 6a) were added various amounts of dilute solution of HCl and their effects on the absorption spectral profile were investigated. Thus, it was observed that the main absorption band from 366 nm increases in intensity and the new band from 490 nm decreases, with gradually increases of HCl concentration (see Fig. 7). Moreover, for 100 μL HCl added in system the intensity of the main absorption band (from 366 nm) is comparable to those of the initial sample solution (98.44% of intensity at 366 nm was recovered when 100 μL HCl were added), indicating that the deprotonation/protonation processes are reversible for these samples (the absorption spectrum revert to initial form (see Fig. 7)). The BAPB-Azo-C₆H₅ and BAPB-Azo-OC₆H₅ samples showed similar response to chemical stimuli. Instead, for the BAPB-Azo-C₆H₅ sample (see Fig. 6b), the intensity of the absorption band from 490 nm at the same content of NaOH from the system is smaller. The base/acid treatments of these stimuli-responsive materials can be repeated. After two base/acid treating cycles, (C(1) + 100 μL NaOH + 100 μL HCl(C2)) similar spectral changes were observed for the BAPB-Azo sample in DMSO solution and the absorption intensities were recovered 89.92% compared to the initial solution (C(1)). The BAPB-Azo sample in a pure DMSO solution is almost non-emissive in its neutral form, but in a basic environment, when different amounts of NaOH were added in the system, the fluorescence signal was intensified (see Fig. 8). Thus, a broad and almost structureless fluorescence band with a maximum at 412 nm, (λ_exc = 365 nm) was evidentiated after the titration with dilute sodium hydroxide solutions.

Fig. 7 Absorbance spectra of the BAPB-Azo in DMSO solution, before and after the addition of 100 μL NaOH, and recovered by the addition of various amounts of HCl dilute solution (c = 1 N). Arrows indicate changes in UV-vis spectra.

Fig. 8 Changes in the emission spectra of the BAPB-Azo in DMSO, under neutral (initial(i)) and basic conditions (titrations with a dilute solution of sodium hydroxide).

Fig. 8 shows the fluorescence spectra of BAPB-Azo in DMSO solution before and after addition of different amounts of NaOH. This enhancement of the fluorescence signal can be explained by the effects of the structural and/or conformational changes that were intensified by the deprotonation of the investigated compounds.³⁰ The increasing the amount of NaOH, determines the deprotonation of the sample and the intermolecular space being occupied by the sodium ions which consequently led a molecular rearrangement. These aspects can explain the fluorescence enhancement of this system.

Conclusion

New organophosphonates containing bisazobenzene moieties in the main chain were synthesized and characterized in this paper by the first time. The obtained oligomers were characterized by FTIR, NMR, thermal stability and optical properties. The studied bisazobenzene derivatives showed a remarkable and reversible response to both optical and chemical stimuli. These spectral changes were assigned to the conformational transitions from the system induced by the rotation of –N=N– bonds (the trans–cis–trans isomerization upon light irradiation) and to the deprotonation/reprotonation processes. The deprotonation/reprotonation processes (acid–base treatments) were reversible and induced the absorption turn-on/off character of these compounds. The emission properties of these compounds were highly sensitive to the pH of the medium. The fluorescence signal was significantly enhanced while increasing of the pH values of the medium.

Acknowledgements

The authors acknowledge the financial support of CNCSIS–UEFISCSU, Project Number PN-II-RU-TE-0123 no. 28/29.04.2013.

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Footnote

† Electronic supplementary information (ESI) available: NMR spectra and the TG curves of the BAPB-Azo, BAPB-Azo-C₆H₅, BAPB-Azo-OC₆H₅. See DOI: 10.1039/c6ra07803f

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