A photo- and thermo-responsive star-shaped diblock copolymer with a porphyrin core prepared via consecutive ATRPs

Abstract

A star-shaped and diblock copolymer poly(6-[4-(4-methoxyphenylazo)phenoxy]hexylmethacrylate)s-b-poly(N-isopropylacrylamide)s with a zinc-porphyrin core (PAzo-b-PNIPAM) is synthesized via consecutive atom transfer radical polymerizations (ATRPs). The synthesis involves the preparation of the intermediate poly(6-[4-(4-methoxyphenylazo)phenoxy]hexyl methacrylate) with a zinc-porphyrin core (por-PAzo) via ATRP, and then por-PAzo is used as a macroinitiator to polymerize N-isopropylacrylamide (NIPAM) via ATRP to gain PAzo-b-PNIPAM. The structures of the target products are characterized by FT-IR, ¹H NMR and UV-vis spectra, and the polydispersity index (PDI) indicates that the molecular weight distribution is narrow and the polymerization is well controlled. Furthermore, the photo- and thermo-responsive properties are also investigated in detail, which demonstrate that the star-shaped diblock copolymer PAzo-b-PNIPAM has a potential application in targeted photodynamic therapy and photo-electro informational storage.

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1. Introduction

With the rapid development of intelligent polymer materials, well-defined and functionalized polymers¹ with 3-dimensional structures such as dendrimers² and star polymers³ are very attractive. Especially, star copolymers⁴ and polymers⁵ have flexible architecture and can be easy to prepare with a high molecular weight with various functionalities. It is well known that the formation of the star polymers is mainly dependent on the core used for functionalized initiators. In this regards, porphyrin derivatives with multiple functional groups at their periphery are desirable cores for synthesizing star polymers.⁶ Additionally, porphyrin is a planar aromatic macrocycle consisting of four pyrrolic subunits linked by four methine bridges, which can bring about new excellent photochemical/physical characteristics and various applications, such as fluorescent imaging,⁷ photodynamic therapy,⁸ drug delivery⁹ and photocatalytic degradation.¹⁰

However, compared with other photosensitizing agents, most of porphyrins have poor solubility in water and tend to aggregate in aqueous solution, thus resulting in a loss of photochemical activity in biological environment.¹¹ To alleviate the above problems, one effective solution is to encapsulate the porphyrins into micelles¹² or immobilize the porphyrin onto the water-soluble polymer.¹³ It is well known that poly(N-isopropylacrylamide) (PNIPAM) as a thermo-responsive polymer has attracted much attention, the state of which could change from a clear solution to a turbid suspension at a lower critical solution temperature (LCST) of 32 °C.¹⁴ Furthermore, PNIPAM also shows well-established water-solubility and biological low toxicity. These outstanding properties encourage it to become a leading star in preparing functional polymers as stimuli-responsive chains.¹⁵

In our past work, we has synthesized several porphyrin-functional PNIPAM via ATRP, and also studied the photodynamic therapy¹⁶ and the third-order nonlinear optical properties.¹⁷ Indeed, the polymers retain the photoactivities of porphyrin, as well as possess water soluble and thermo-responsive property, but the strength of some functional performance, for example, fluorescence or the saturated absorption diminishes obviously owing to the introduction of the polymer chain which may reduce the conjugate or planar properties. In order to solve the problem, one very promising strategy is to construct conjugated molecules to functionalize porphyrin. Azobenzene, as one of conjugated molecules, can undergo trans/cis isomerization through a cycle of UV and visible light irradiation,¹⁸ which can be applied in optical switches,¹⁹ optical data storage²⁰ and liquid crystal displays.²¹ In addition, the azobenzene repeat units are embedded into the copolymer chain which can lead to significant photoresponsive behavior.²² Therefore, it is necessary to use the appropriate polymerization method to prepare porphyrin functional block copolymer with polyazobenzene chain segment. ATRP, as one of the most investigated controlled/“living” radical polymerization (CRP) techniques, has been successfully used to prepare block copolymers and dendrimers with a controlled architecture, narrower molecular weight distribution and high chain-end functionality.²³ However, to our knowledge, there are few related reports about porphyrin functional block copolymer with polyazobenzene chain segment.

In this article, we for the first time design and prepare star-shaped diblock copolymer poly(6-[4-(4-methoxyphenylazo)phenoxy]hexylmethacrylate)-b-poly(N-isopropylacrylamide)s with zinc-porphyrin core (PAzo-b-PNIPAM) via consecutive atom transfer radical polymerizations (ATRPs). And their thermo-sensitivity, photo-isomerization and fluorescent characteristic are also investigated accurately. By taking the advantage of the poly-N-isopropylacrylamide and poly-azobenzene block, the PAzo-b-PNIPAM shows excellent thermo-responsive property and trans–cis photoisomerization reaction, as well as significant enhancement in fluorescence intensity compared with porphyrin in DMF solution.

2. Experimental section

2.1. Materials

N-Isopropylacrylamide (99%, Aldrich) was recrystallized twice from benzene/hexane (10 : 1, v/v) prior to use. 2-Bromopropionyl bromide (99%, Acros) and copper(I) bromide (CuBr, 99%, Aldrich) were available commercially and used without further purification. Tris(2-(dimethylamino)ethyl)amine (Me₆TREN) was synthesized according to the literature.²⁴ All other chemicals were purchased from Sinopharm Chemical Reagent Co. and were used as received.

2.2. Synthesis of star-shaped por-PAzo with zinc-porphyrin core via ATRP

The preparation procedures of 5,10,15,20-tetra(p-hydroxyphenyl)-porphine (THPP), 5,10,15,20-tetra(p-hydroxyphenyl)-zincporphyrin (Zn-THPP), 5,10,15,20-tetra(4-(2-bromopropoxy)phenyl)-zincporphyrin (ZnTHPP-Br) and 6-[4-(4-methoxyphenylazo)phenoxy]hexylmethacrylate (AzoMA) were depicted in the ESI data.†²⁵ por-PAzo was synthesized according to the following steps (Scheme 1): a mixture of CuBr (30 mg, 0.2 mmol) and N,N,N′,N′,N′-pentamethyldiethylenetriamine (PMDETA) (40 μL, 0.2 mmol) in anisole (0.8 mL) was placed on one side of an H-shaped glass ampoule and stirred at room temperature. AzoMA (790 mg, 2 mmol) and ZnTHPP-Br (44 mg, 0.04 mmol) in anisole (1.5 mL) were placed on the other side of the ampoule. Nitrogen was bubbled through both mixtures for 15 min to remove any oxygen. Three freeze–pump–thaw cycles were performed to degas the solution, then mixed and placed at 80 °C for 20 h. After the reaction, the original products was diluted and passed through a neutral Al₂O₃ column to remove the copper complex. After precipitation, the product was collected by filtration and dried in a vacuum oven overnight (141 mg, 70% yield). M_n = 9100, M_w/M_n = 1.08.

Scheme 1 Synthetic route of por-PAzo.

2.3. Synthesis of star-shaped copolymer PAzo-b-PNIPAM with zinc-porphyrin core via ATRP

NIPAM (448 mg, 4.00 mmol), CuBr (11 mg, 0.05 mmol) and Me₆TREN (12 μL, 0.05 mmol) were dissolved in DMF/water (2 : 1, v/v), followed by degassing via three freeze–pump–thaw cycles. Deoxygenated solution of por-PAzo (91 mg, M_n = 9100, 0.01 mmol) was then added into the reaction flask to start the polymerization (Scheme 2). The reaction was carried out for 72 h at 60 °C under nitrogen atmosphere, and then the reaction mixture was passed through an alumina column. At last, the resulting copolymer was purified by dialysis using a cellophane tube (MWCO, 10 000) in DMF to gain the PAzo-b-PNIPAM (229 mg, 45% yield). M_n = 37 000, M_w/M_n = 1.29.

Scheme 2 Synthetic route of PAzo-b-PNIPAM.

3. Results and discussion

In order to prepare a well-defined block polymer, the synthesis of precursors at a narrow molecular weight distribution is necessary. It is well known that ATRP could prepare block copolymers and dendrimers with a controlled architecture and narrower molecular weight distribution. So in this work, por-PAzo and PAzo-b-PNIPAM were prepared respectively by ATRP. By controlling the feed ratios of NIPAM to por-PAzo macroinitiator, series of block copolymers PAzo-b-PNIPAM with different molecular weight were obtained. Detailed experimental conditions and physical data of these block copolymers are given in Table 1.

Table 1 Polymerization data of por-PAzo and PAzo-b-PNIPAM

Monomer	[M]0/[I]0^a	Time^d	Conv.^e (%)	Mn(theor)^f	Mn(NMR)^g	Mn(GPC)^h	Mw/Mn^i	PAzoMA^j (wt%)	LCST^k (°C)
a Feed molar ratio of the monomer [M]0 to initiator [I]0.b [ZnTHPP-Br] : [PMDETA] : [Cu(I)Br] = 1 : 9 : 9, [AzoMA]0 = 2.0 M.c [por-PAzo] : [Me6TREN] : [Cu(I)Br] = 1 : 10 : 10.d Polymerization time in hours.e Determined from ^1H NMR spectrum of the reaction mixture in DMSO-d6.f Mn(theor) = Mmonomer [monomer]0conv./100[initiator]0 + Minitiator.g Molecular weight determined by ^1H NMR.h Number-average molecular weight determined by GPC.i Polydispersity determined by GPC.j Weight fraction of the PAzoMA in the star-shaped block copolymers.k Measured by turbidimetry using UV-vis spectrophotometer.
por-(PAzo4)5	50^b	16	37	8500	9100	9200	1.08	90.0	—
(PAzo4)5-b-(PNIPAM4)40	200^c	48	72	25 400	26 000	28 000	1.25	11.0	—
(PAzo4)5-b-(PNIPAM4)62	400^c	72	60	36 200	37 000	39 500	1.29	6.6	23.0
(PAzo4)5-b-(PNIPAM4)82	600^c	72	55	45 000	46 000	47 900	1.32	5.7	25.5
(PAzo4)5-b-(PNIPAM4)122	800^c	96	58	62 900	64 000	65 900	1.34	3.9	27.0
(PAzo4)5-b-(PNIPAM4)157	1000^c	96	62	79 000	80 000	81 700	1.39	3.0	29.5

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The obtained por-PAzo and PAzo-b-PNIPAM are characterized by FT-IR, GPC and ¹H NMR techniques. The FT-IR absorption spectra of the por-PAzo and PAzo-b-PNIPAM are shown in Fig. S1,† respectively. Fig. 1A shows the ¹H NMR spectra of por-PAzo, the corresponding peaks of protons attribute to porphine ring appear at 8.77 and 8.11, as well as the major peaks attributed to AzoMA moiety of por-PAzo can be detected at 7.91, 7.22, 4.07, 1.52 and 1.03. Fig. 1B shows the ¹H NMR spectra of PAzo-b-PAM, comparing to por-PAzo some new peaks can be detected at 6.25, 5.53, 4.07, 1.75 and 1.07 ppm which are attributed to the protons for repeated units of NIPAM. Above all, these results of ¹H NMR and FT-IR indicate that star-shaped por-PAzo and PAzo-b-PNIPAM are successfully prepared via consecutive ATRPs. In addition, the molecular weight of macroinitiator por-PAzo is obtained by ¹H NMR, so the molecular weight of PAzo-b-PNIPAM can be acquired by calculating the integral area of peaks to methylene (g) of por-PAzo and methyne (i) of PAzo-b-PNIPAM (Table 1). Furthermore, the M_n(GPC) of polymers are higher than its M_n(NMR), it is because that the molecular weights obtained from GPC using polystyrene standards are a little high.^26a And the M_n(GPC) of PAzo-b-PNIPAM are smaller than its actual molecular weight, a plausible reason for the difference is that a small amount of initiator loses its activity in the process of the polymerization.

Fig. 1 ¹H NMR spectra of por-PAzo (A) and PAzo-b-PNIPAM (B).

After series of purifications, the M_n values of the por-PAzo and PAzo-b-PNIPAM are determined by GPC using polystyrene as the standard. The results are presented in Table 1 and GPC traces are shown in Fig. 2. It can be seen that there is no tailing at either side, which suggests that the absence of any small molecule such as initiator, monomer or other byproducts residues. Moreover, the polydispersity indexes (PDI) of por-PAzo and PAzo-b-PNIPAM are relatively narrow, which demonstrate that the structure of prepared polymers are regular and accurate relatively.

Fig. 2 GPC traces of por-PAzo and PAzo-b-PNIPAM.

To investigate the thermo-responsive property of PAzo-b-PNIPAM, the LCST is examined by the optical transmittance of PAzo-b-PNIPAM aqueous solution using UV-vis spectroscopy. Fig. 3 represents the curves of transmittance with the increase of temperature, revealing the temperature dependence of optical transmittance at 500 nm for PAzo-b-PNIPAM aqueous solution. During the first step, it can be seen that the optical transmittance shows little change along with the temperature increasing, the result indicates that the hydrogen bonding interaction between the amido of PAzo-b-PNIPAM and water molecules is a main driving force for good solubilisation to hydrophobic core (por-PAzo) when the temperature is lower than LCST. However, when the temperature further increases, the strong association interaction of hydrophobic isopropyl may inhibit the formation of hydrogen bonds between the amido of PAzo-b-PNIPAM and water molecules, then a repulsive force from the increased hydrophobicity of micelles may cause large aggregates and consequent decrease in optical transmittance. In addition, it can be seen that the LCST values of PAzo-b-PNIPAM rise gradually with the increasing of molecular weight (Table 1), it is because that the molecular weight of hydrophobic core (por-PAzo) remain unchanged with the increasing of molecular weight of PAzo-b-PNIPAM, while the hydrophilic PNIPAM chains get longer and longer, so the solubilization effect of hydrophilic PNIPAM chains increase gradually as the growing of the PAzo-b-PNIPAM molecular weight which leads to the cloudy point temperature increase,^26a,b but the LCST values are lower than poly(N-isopropylacrylamide)s homopolymer.²⁶ On the contrary, the PAzo-b-PNIPAM with the lower M_n(NMR) of 26 000 cannot show the thermo-responsive property owing to the critical effect of the hydrophobic grouping.

Fig. 3 Temperature dependence of optical transmittance at wavelength of 500 nm obtained for aqueous solutions of PAzo-b-PNIPAM (M_n = 37 000) (2 mg mL⁻¹).

The repetitive cycle experiment of the thermo-responsive property for PAzo-b-PNIPAM is tested. The PAzo-b-PNIPAM aqueous solution takes on clear and turbidity dispersions respectively during a reversible cooling and heating cycles (Fig. 4). The solution is equilibrated for 15 min to each measuring temperature which is chosen at above and below LCST. The fluctuating value of transmittance exhibits a certain degree of cyclical characteristic of phase transition. To some extent, the phase transition of PAzo-b-PNIPAM is reversible, which demonstrates that the thermo-responsive property of PAzo-b-PNIPAM has good repeatable performance.

Fig. 4 Recycling experiments of thermo-responsive property for PAzo-b-PNIPAM aqueous solution with the concentration of 2 mg mL⁻¹ (M_n = 37 000) (15 min for each temperature, four heating/cooling cycles between 10 °C and 40 °C).

The UV-vis spectra of ZnTHPP-Br, por-PAzo and PAzo-b-PNIPAM are shown in Fig. 5. The characteristic B-band of ZnTHPP-Br is located at 413 nm and Q-bands at 548–589 nm respectively, which are in accord with those observed for similar metalloporphyrins.²⁷ Moreover, the UV-vis spectra of por-PAzo shows the strong absorption bands of azobenzene group at 360 nm, as well as the absorption spectra of zinc-porphyrin group at 424 nm and 545–590 nm, but the Soret-bands exhibit a bathochromic shift relative to the bands of ZnTHPP-Br. It may be because that the organic conjugated azobenzene arms take the place of the electron-withdrawing group (–Br), making the highest orbital energy levels rise and the lowest empty orbital lower, so the strongest absorption bands shift towards the longer wavelength side. Furthermore, we find another interesting phenomenon from the UV-vis spectra of PAzo-b-PNIPAM, the Soret-bands exhibit a hypochromatic shift comparing with por-PAzo, which is the result of the formidable steric hindrance from segmented copolymer.

Fig. 5 UV-vis spectrum of ZnTHPP-Br, por-PAzo (M_n = 9100) and PAzo-b-PNIPAM (M_n = 37 000) in CH₂Cl₂ (2 × 10⁻⁴ mol L⁻¹).

The trans–cis isomerization of por-PAzo and PAzo-b-PNIPAM are recorded after UV exposure, and the change of the UV-vis absorption spectra under different times irradiation with UV-vis light are shown in Fig. 6. The por-PAzo and PAzo-b-PNIPAM exhibit one strong and one weak absorption bands, which are related to the π–π* and n–π* transition bands of the trans- and cis-azobenzene respectively. The intensity of the π–π* transition band at about 360 nm decrease, whereas the n–π* transition band at about 470 nm gradually increase with prolonged UV irradiation time. The photostationary state of por-PAzo is realized after 30 min, while the photostationary state of PAzo-b-PNIPAM is realized after about 60 min, which need more time to complete the trans–cis isomerization comparing with por-Pazo. It is because that the introduction of poly(N-isopropylacrylamide)s chain segment changes the spatial structure of copolymer, which obstructs the steps of the trans–cis isomerization of PAzo-b-PNIPAM (Fig. 6B). In the dark or being irradiated under visible light for about 20 h, the trans-azobenzene groups of por-PAzo and PAzo-b-PNIPAM are recovered completely since the trans form corresponds to an energy lower.

Fig. 6 Spectral changes of por-PAzo (A) (M_n = 9100) with the concentration of 2 × 10⁻⁴ mol L⁻¹ in DMF under UV irradiation (365 nm, 8 W), irradiation times: (a) 0 min, (b) 10 min, (c) 20 min and (d) 30 min; spectral changes of PAzo-b-PNIPAM (B) (M_n = 37 000) with the concentration of 2 × 10⁻⁴ mol L⁻¹ in DMF under UV irradiation, irradiation times: (e) 0 min, (f) 20 min, (g) 40 min and (h) 60 min.

The fluorescence properties of por-PAzo and PAzo-b-PNIPAM with different molecular are shown in Fig. 7, ZnTHPP, ZnTHPP-Br and poly(N-isopropylacrylamide) with zinc-porphyrin (ZnTHPP-PAM)¹⁷ are used as counterparts with the concentration of 2 × 10⁻⁴ mol L⁻¹ in DMF. It is obvious that the fluorescence intensity of por-PAzo much stronger than ZnTHPP, the reasons may be explained as following: firstly, the azobenzene group of por-PAzo has strong absorption between 330 and 390 nm overlapping partly with excitation spectrum of ZnTHPP (380 nm), so the por-PAzo gets much excitation energy which is beneficial to enhance fluorescence intensity. Secondly, the introduction of azobenzene can induce a great enhancement of π-electronic conjugate. However, in some degree the fluorescence intensity of PAzo-b-PNIPAM is weaker than por-PAzo because of the formidable steric hindrance from segmented copolymer that makes the π–π stacking interactions decline. Fortunately, the fluorescence intensity of PAzo-b-PNIPAM enhances significantly comparing with ZnTHPP-PAM (Fig. 7A). Furthermore, the effect of molecular weight on the fluorescence intensity of PAzo-b-PNIPAM is also studied. As shown in Fig. 7B, the fluorescence intensity of PAzo-b-PNIPAM slowly decays with the increasing of molecular weight, but no significant fluorescence quenching occurs after the polymerization reaction, which suggests that PAzo-b-PNIPAM is capable of applications in fluorescence materials by controlling molecular weight reasonably.

Fig. 7 Fluorescent spectra of ZnTHPP, ZnTHPP-Br, ZnTHPP-PAM (M_n = 8300), por-PAzo (M_n = 9100) and PAzo-b-PNIPAM (M_n = 37 000) with the concentration of 2 × 10⁻⁴ mol L⁻¹ in DMF (λ_Em = 507 nm) (A). Fluorescent intensity changes of PAzo-b-PNIPAM with different molecular weight with the concentration of 1.5 × 10⁻⁴ mol L⁻¹ in DMF (λ_Em = 507 nm) (B).

The effect of different concentration on the fluorescence intensity of PAzo-b-PNIPAM is studied, the fluorescent spectra of PAzo-b-PNIPAM does not exhibit any changes other than a corresponding increase in intensity as the concentration of PAzo-b-PNIPAM gradually increasing from 0.1 × 10⁻⁴ mol L⁻¹ to 2 × 10⁻⁴ mol L⁻¹ (Fig. 8A). Furthermore, as shown in Fig. 8B, the fluorescence intensity has a linear increase along with the concentration of PAzo-b-PNIPAM, which demonstrates that the PAzo-b-PNIPAM can well dissolve in water without the lower fluorescence intensity caused by the aggregation.

Fig. 8 Concentration-dependent fluorescence spectra of PAzo-b-PNIPAM (M_n = 37 000) in aqueous solution (λ_Em = 507 nm) (A). The corresponding emission intensity at 507 nm along the change of concentration (B).

The temperature-dependant fluorescence spectra of PAzo-b-PNIPAM in aqueous solution is also investigated and the corresponding result is shown in Fig. 9. The emission maximum of PAzo-b-PNIPAM is at 507 nm around with excitation wavelength at 380 nm, and the fluorescence intensity of PAzo-b-PNIPAM increases significantly as the temperature rises from 19 to 23 °C (Fig. 9A). Fig. 9B represents the curves of the fluorescence intensity of PAzo-b-PNIPAM with the increase of temperature, when the temperature is below 20 °C, the fluorescence intensity shows little change along with the temperature increasing. However, when the temperature subsequently rises, the fluorescence intensity of PAzo-b-PNIPAM gradually increases, and a sharp increase of the fluorescence intensity happens as the temperature being up to LCST. Clearly, the change of fluorescence intensity is the results of the phase transformation of PAzo-b-PNIPAM. When the system temperature approaches the LCST of PAzo-b-PNIPAM, the thermo-sensitive chain turns to a micelle-like structure from random coils, in the micelle-like area, the concentration of the photo-responsive core is relatively higher,²⁸ so the fluorescence intensity increases significantly. When the temperature continues to rise, an obvious decline of fluorescence intensity happens from 26 to 30 °C, it is because that the active center (por-PAzo) is embedded by PNIPAM chains, which impedes the absorbing of exciting light and fluorescence emission. After the PAzo-b-PNIPAM reaches phase stable state, the fluorescence intensity almost remains unchanged as the temperature rise.

Fig. 9 Temperature-dependent fluorescence spectra of PAzo-b-PNIPAM (M_n = 37 000) in aqueous solution (2 × 10⁻⁴ mol L⁻¹) (λ_Em = 507 nm) (A). The corresponding emission intensity at 507 nm along the temperature change (B).

4. Conclusions

In summary, series of novel star-shaped diblock PAzo-b-PNIPAM are synthesized successfully via the consecutive ATRPs. The PDI of PAzo-b-PNIPAM indicate that the molecular weight distribution are narrow and the polymerization are well controlled. The thermo-responsive and photo-responsive properties of PAzo-b-PNIPAM are investigated and the results show that the star-shaped diblock copolymer has typical thermo-responsive property and trans–cis photoisomerization reaction under sufficient UV irradiation. Importantly, PAzo-b-PNIPAM has significant enhancement in fluorescence intensity compared with porphyrin, furthermore, the fluorescence intensity shows a corresponding response to the change of molecular weight, concentration and temperature.

Acknowledgements

We are grateful to Jilin science & technology department (20090326, 20070508, 20140204017GX), Science and Technology Bureau of Changchun City project (2008280, 2013060) Foundation for Strategical Research for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra10464a

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