Synthesis of azobenzene-containing polymers and investigation of their substituent-dependent isomerisation behaviour

Reactive & Functional Polymers 72 (2012) 242–251

Contents lists available at SciVerse ScienceDirect

Reactive & Functional Polymers journal homepage: www.elsevier.com/locate/react

Synthesis of azobenzene-containing polymers and investigation of their substituent-dependent isomerisation behaviour Ulrike Georgi a,c, Philipp Reichenbach b, Ulrich Oertel a, Lukas M. Eng b, Brigitte Voit a,c,⇑ a

Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Straße 6, 01069 Dresden, Germany Institut für Angewandte Photophysik, Technische Universität Dresden, 01062 Dresden, Germany c Chair of Organic Chemistry of Polymers, Department Chemistry and Food Chemistry, Technische Universität Dresden, 01062 Dresden, Germany b

a r t i c l e

i n f o

Article history: Received 29 November 2011 Received in revised form 26 January 2012 Accepted 14 February 2012 Available online 23 February 2012 Keywords: Azobenzene Isomerisation kinetics Thermal cis–trans-relaxation UV–Vis spectroscopy Polymer-analogous reaction

a b s t r a c t A variety of 4,40 -substituted azobenzenes has been synthesised and the kinetics of the thermal cis–transrelaxation of the substances was studied in detail in solution and embedded in a poly(methyl methacrylate) (PMMA) matrix by UV–Vis spectroscopy. Considerable differences in relaxation times were found for the various azobenzenes which interestingly could not be fully explained by simply comparing the substituents with regard to their electron-donating or -withdrawing nature. Any substituents, especially very polar ones, increase the thermal cis–trans-reaction rate. Some of the chromophores were covalently attached to a PMMA-copolymer and we found that this significantly slowed down the isomerisation kinetics compared to the embedding of low molar mass azobenzenes in a polymer matrix. But our study showed also that, even at room temperature, the thermal cis–trans-relaxation of 4,40 -substituted azobenzene chromophores can never be fully suppressed, but only slowed down. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Since their first synthesis in the middle of the 19th century [1,2], azobenzene (AB) and its derivatives (ABD) have received wide attention, especially in the field of functional material science. The diazo group which is lined by two phenyl rings is a strong chromophore; all azobenzene derivatives thus strongly absorb light in the UV and visible region. For unsubstituted azobenzene, the absorption maximum of the strong p–p-transition lies at 320 nm and the absorption of the weaker n–p-transition can be found around 450 nm (in THF solution). The band of the high energy r–r-transition is found at 230 nm. The planar trans-azobenzene has a dipole moment which is nearly zero. The trans-isomer can be converted to the corresponding non-planar cis-isomer by irradiation. Here the phenyl rings are twisted out of the plane for sterical reasons, thus partly destroying the conjugated system. Hence, the absorption at 320 nm strongly decreases while that at 450 nm increases significantly. Furthermore, the dipole moment radically increases from 0 to 3 Debye in the cis-isomer [3]. Upon irradiation of the trans-isomer with UV light, isomerisation of the NAN-double bond takes place and the configuration changes from trans to cis. As this process is photochemically and also thermally reversible, a photostationary state is reached after a certain irradiation time. The ratio of cis and trans isomers is dependent on the reaction ⇑ Corresponding author at: Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Straße 6, 01069 Dresden, Germany Tel.: +49 351 4658 590; fax: +49 351 4658565. E-mail address: [email protected] (B. Voit). 1381-5148/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2012.02.009

rates of the different isomerisation reactions, on the light intensity, on the quantum yields of photochemical cis–trans and trans–cis isomerisations and also on the extinction coefficients of the respective isomers at the irradiation wavelength. The rates of the thermal reactions are in turn dependent on the isomerisation mechanism which can take place either by inversion of a nitrogen centre where thermal activation of the azo-group leads to rehybridisation of one nitrogen atom from sp2- to sp-hybrid state, or via a rotation mechanism which is strongly favoured for derivatives with a strong dipole moment (Scheme 1). The appearance of these different mechanisms is strongly dependent on the polarity of the molecule and the surrounding medium [4–8]. More likely than the occurrence of just one pure mechanism during isomerisation, a mixed torsional–inversional transition state has been discussed in more recent works [9–14]. As their photoisomerisation process is excellently reversible even after numerous irradiation cycles and their thermal stability goes up to 350 °C, ABDs have been highly interesting for multiple applications and thus have been intensively studied for the last decades. One interesting point is for example the behaviour of amorphous high-Tg polymers containing ABDs in the side-chains: When irradiated with linearly polarised light, reorientation of the azobenzene groups takes place through a trans–cis–transisomerisation cycle. As a result, the azobenzene groups orient themselves perpendicular to the polarisation direction yielding materials then exhibiting birefringence [15,16]. Potential applications were especially sought in the fields of non-linear optical materials [17], volume holographic media [18–20], optical switching [21] and in data storage media [22,23].

243

U. Georgi et al. / Reactive & Functional Polymers 72 (2012) 242–251

N

N

rotational transition state N

N

N N

N

N

inversional transition state Scheme 1. Possible transition states for the thermal cis–trans-isomerisation of azobenzene.

During the last few years, the focus of potential application for ABDs has considerably widened and their use for biomedical applications is discussed intensely. Their integration in self-assembled supramolecular structures was for example studied by Tong and co-workers who synthesised a block copolymer with a short hydrophilic block and a long hydrophobic homo-ABD-block. This system forms micelles [24,25] and vesicles [24] in aqueous solution which disrupt upon irradiation with UV light and quickly reassemble upon irradiation with visible light. Similarly, photo-swellable and -disruptable vesicles and cylindrical micelles were formed by azobenzene-containing linear dendritic block copolymers, as described by del Barrio and co-workers [26]. Jochum and Theato synthesised ABD-containing acryl amide copolymers with a lower critical solution temperature (LCST) dependent on the amount of ABD in the chain and its isomerisation state [27]. They took advantage of their findings to synthesise amphiphilic block copolymers based on this system; these block copolymers assembled to micelles in aqueous medium, then could be disaggregated by photochemical isomerisation of the ABD units, and finally reassembled by their thermal relaxation [28]. The group of Gröhn investigated the formation of light- and pH-switchable supramolecular polyelectrolyte–dye nanoparticles by self-assembly through electrostatic interactions between macrocationic poly(amidoamine) dendrimers and the anionic azo dyes as well as the p–p-interactions between dye molecules [29,30]. Controlled light-induced release of active molecules encapsulated in light-responsive carrier systems is also an interesting option. To that effect, the encapsulation of eosin molecules in dendritic molecules, which is dependent on the isomerisation state of the ABD units attached to a dendrimeric structure, was examined by Puntoriero et al. [31]. Tamesue and co-workers created photoswitchable physical hydrogels formed from an ABD-side chain guest polymer and a cyclodextrine side chain host polymer which liquefies upon irradiation [32]. In a current review, Beharry and Woolley [33] nicely summarised the recent achievements to tag and photo-control biomolecules such as peptides, proteins, nucleic acids, and oligonucleotides with ABDs and then use them in biological systems, e.g. for photo-switchable ion channels [34] or to induce structural transitions in polypeptides (e.g. coil/a-helix-transition in a solution of modified poly(L-glutamic acid)) [35]. All these applications are immensely interesting due to their continuous reversibility, be it via a mostly photochemical pathway, or by taking advantage of thermal back-relaxation of the azobenzene derivatives into the trans-state. Interestingly, these applications could be fine-tuned for certain applications by variation of the substitution pattern on the phenyl rings. This may, on the one hand, help to customise the derivatives’ absorption maxima to a desired wavelength, while on the other hand be used to precisely

tune the time-scale of thermal relaxation in different media. The latter technique would allow e.g. to tune re-association times or the period of enzyme activity after irradiation. Focusing on these possibilities, it is interesting that only some of the publications cited above refer to the thermal isomerisation behaviour, i.e. the stability of the cis-isomer of their derivative used. Some reports mention the time scales with which the thermal relaxation takes place [25,34,35], but as usually just one azobenzene derivative per publication is investigated, the possibility of property tuning by utilisation of different substitution pattern is not in focus. Other publications, in turn, completely lack information on the stability of cis-isomers [26,32]. For that reason, we present in this work the synthesis of a variety of ABDs with electron-withdrawing and -donating groups in 4- and 40 -position to the diazo-bond. Thus, it was possible to systematically study the effects of the different substituents on the thermal cis–trans-isomerisation. Furthermore, the reaction was investigated for chromophores embedded in different matrices, i.e. in THF solution, blended in poly(methyl methacrylate) (PMMA) and covalently bound to a PMMA-copolymer complementing uniquely substitution effects by matrix effects. All substances were thoroughly characterised by 1H NMR spectroscopy, 13C NMR spectroscopy, UV–Vis spectroscopy and thermoanalytic measurements. Furthermore, Raman spectra were measured in order to investigate the relation between substitution pattern, electronic properties of the substituents, the character of the N@N-bond and the thermal relaxation time [33,36]. 2. Experimental 2.1. Chemicals 4-(4-Dimethylaminophenylazo)phenol was purchased at TCI Europe. 4-(4-Hydroxyphenylazo)benzoic acid was synthesised according to literature [37]. Chloromethyl styrene (CMS) and methyl methacrylate (MMA) were purchased from Sigma Aldrich and passed over neutral aluminium oxide right before use to remove the inhibitor. All other chemicals were purchased from Sigma Aldrich and used as received. 2.2. Physico-chemical characterisation 1

H (500.13 MHz) and 13C (125.74 MHz) NMR spectra were recorded at 30 °C from solutions in CDCl3 on a Bruker DRX 500 spectrometer. The NMR spectra were referenced on the solvent signal (d(1H) = 7.26 ppm; d(13C) = 77.0 ppm). The determination of molar mass and molar mass distributions was carried out on a modular built SEC-system (Knauer, Germany) composed of a multi-angle laser light scattering (MALLS) detector

244

U. Georgi et al. / Reactive & Functional Polymers 72 (2012) 242–251

(DawnR-EOS, Wyatt Technologies, k = 632 nm) and a viscosity- and refractive index (RI) detector ETA-2020 (WEG Dr. Bures GmbH and Co. KG, Germany, k = 620 nm). The pump used was an HPLC-pump by Agilent Technologies (series 1200). The column applied was a PL MIXED-C with a pore size of 5 lm (Polymer Laboratories, Great Britain). The eluent was THF with a flow rate of 1.0 ml/min. Photochemical and thermal isomerisations were carried out in sealed cuvettes with 1.0 or 0.4 cm pathway (Hellma). The cuvettes were placed in a custom-made thermostated sample holder and held at 25 °C. By means of light guides, this sample holder was connected with a Cary 50 UV–Vis spectrophotometer (spectral bandwith 1.5 nm, driven by a special software) and with an optical bench with a 300 W high pressure HgXe lamp (Hamamatsu), an Oriel 77250 monochromator (Oriel) and a software controlled shutter. Thus, software-driven in situ measurements of thermal and photochemical reactions were carried out. Chromophore solutions were prepared from stock solutions by appropriate dilution. The chromophore concentration in the THF solutions was chosen between 2  105 M and 5  105 M for all substances, except for (CP3) and (CP5) were it was 5  104 M. The thin PMMA films containing with azobenzene derivatives were prepared by dip-coating on quartz glass from a chloroform solution containing 0.01 M azobenzene derivative and 1 wt% PMMA. The films of the azobenzene containing copolymers were dip-coated from a 1 wt% solution of the respective polymer in chloroform. The Raman spectra were recorded with RAMAN Imaging System WITEC alpha300R (532 nm laser, power 0.01 mW), 0.25 s integration time and 500–2000 accumulations. The samples were measured on glass slides. The spectra were filtered by S–G-method and were baseline corrected. As the samples were extremely sensitive to laser irradiation, the lowest laser power possible was applied. The Differential Scanning Calorimetry (DSC) measurements were carried out under nitrogen on a DSC Q 1000 of TA Instruments in the temperature range from 80 °C to 100 °C ((2)), 110 °C ((3) and (5)), 130 °C ((1)) or 150 °C (azobenzene-containing copolymers), according to the thermal stability of the different samples. The scan rate was ±10 K/min. A Heating–Cooling–Heating cycle was carried out to realise a uniform thermal history. The data was calculated from the second heating cycle. The Thermo-Gravimetric Analysis (TGA) was carried out under nitrogen on a TGA Q5000 of TA Instruments from room temperature to 800 °C with a heating rate of 10 K/min.

DSC: Tm = 122 °C. TGA: 260 °C (98.5%). 2.3.2. Synthesis of (4-ethoxyphenyl)phenyldiazene (2) (4-Ethoxyphenyl)phenyldiazene (2) was prepared in a similar manner to (1). Yield: 78%. 1 H NMR (500.13 MHz, CDCl3, ppm): 7.97 (d, 2H, 5), 7.94 (d, 2H, 8), 7.54 (t, 2H, 9), 7.47 (t, 1H, 10), 7.03 (d, 2H, 4), 4.12 (q, 2H, 2), 1.48 (t, 3H, 1). 13 C NMR (125.74 MHz, CDCl3, ppm): 161.47 (3), 152.78 (6), 146.88 (7), 130.24 (10), 128.96 (9), 124.72 (5), 122.51 (8), 114.64 (4), 63.74 (2), 14.69 (1). DSC: Tm = 76 °C. TGA: 202 °C (99.5%). 2.3.3. Synthesis of (4-ethoxyphenyl)-1-naphthalenyldiazene (3) 2.3.3.1. Synthesis of 4-(1-naphthylazo)phenol Fig. 1. A solution of 0.5 g 1-aminonaphthalene (3.5 mmol) in 2.4 ml concentrated HCl and 5 ml deionised water was cooled in an ice bath. 241 mg sodium nitrite (3.5 mmol) were dissolved in as little water as possible and slowly added to the naphthalene solution while keeping the temperature below 0 °C. The resulting brown diazonium salt solution was added drop-by-drop to a vigorously stirred ice-cooled solution of 494 mg phenol (5.2 mmol) and 3.5 g sodium carbonate (33 mmol) in 10 ml of water. The pH was to be kept basic. Some sodium chloride was added to the solution and the aqueous phase was extracted thrice with diethyl ether. The combined organic phases were washed twice with water, dried over MgSO4, and the solvent was evaporated under reduced pressure. The deep red product was purified by flash chromatography (first pure CHCl3 to remove main by-product, then with 1/1 v/v CHCl3/methanol to yield the product). Finally, the product fraction was once more dissolved in diethyl ether and extracted with water twice. The organic phases were dried over MgSO4 and the solvent was evaporated under reduced pressure. Yield: 50%. 1 H NMR (500.13 MHz, CDCl3, ppm): 8.90 (d, 1H, 12), 8.01 (d, 2H, 3), 7.96 (d, 1H, 8), 7.93 (d, 1H, 9), 7.78 (d, 1H, 6), 7.64 (t, 1H, 11), 7.58 (t, 1H, 10), 7.56 (t, 1H, 7), 6.98 (d, 2H, 2), 5.34 (OH). 13 C NMR (125.74 MHz, CDCl3, ppm): 158.27 (1), 147.89 (4&5), 134.30 (14), 131.14 (13), 130.70 (8), 127.90 (9), 126.63 (11), 126.38 (10), 125.68 (7), 125.28 (3), 123.47 (12), 115.87 (2), 111.76 (6).

2.3. Synthesis If no figure is given for a specific synthesis procedure the numbering of the NMR signals refers to Schemes 2–4 for the small molecule azobenzene derivatives, P(MMA-co-CMS) and the azobenzene-containing copolymers, respectively. 2.3.1. Synthesis of [4-(4-ethoxyphenylazo)phenyl]dimethylamine (1) 0.5 g 4-(4-Dimethylaminophenylazo)phenol (2.1 mmol), 0.23 ml bromoethane (338 mg, 3 mmol), 0.7 g Cs2CO3 (2.1 mmol) and ca. 0.1 g potassium iodide were dissolved in 15 ml dry DMF. The solution was stirred at room temperature for 5 days. The solvent was evaporated, THF was added to the residue and the insoluble solids were filtered off. The THF solution was precipitated into water, the solid was filtered off and thoroughly dried at 50 °C in vacuo. Yield: 96%. 1 H NMR (500.13 MHz, CDCl3, ppm): 7.86 (d, 2H, 5), 7.84 (d, 2H, 8), 6.98 (d, 2H, 4), 6.78 (d, 2H, 9), 4.12 (q, 2H, 2), 3.08 (s, 6H, 11), 1.46 (t, 3H, 1). 13 C NMR (125.74 MHz, CDCl3, ppm): 160.29 (3), 152.00 (10), 147.32 (6), 143.81 (7), 124.52 (8), 123.83 (5), 114.63 (4), 111.72 (9), 63.73 (2), 40.38 (11), 14.80 (1).

2.3.3.2. Etherification of 4-(1-naphthylazo)phenol with bromoethane. The synthesis of (4-ethoxyphenyl)-1-naphthalenyldiazene (3) was carried out similarly to the synthesis of (1). Yield: 98%. 1 H NMR (500.13 MHz, CDCl3, ppm): 8.96 (d, 1H, 14), 8.08 (d, 2H, 10), 7.97 (d, 1H, 11), 7.94 (d, 1H, 5), 7.82 (d, 1H, 8), 7.67 (t, 1H, 13), 7.59 (m, 2H, 9&12), 7.07 (d, 2H, 4), 4.15 (q, 2H, 2), 1.49 (t, 3H, 1). 13 C NMR (125.74 MHz, CDCl3, ppm): 161.56 (3), 147.92 (6), 147.60 (7), 134.29 (16), 131.17 (15), 130.50 (10), 127.85 (11), 126.54 (13), 126.31 (12), 125.66 (9), 125.04 (5), 123.52 (14), 114.73 (4), 111.65 (8), 63.82 (2), 14.73 (1). DSC: Tm = 84 °C. TGA: 87 °C (1.5%), 252 °C (96.5%). 2.3.4. Synthesis of 4-(4-ethoxyphenylazo)benzoic acid (4): [38] 0.5 g 4-(4-Hydroxyphenylazo)benzoic acid (2.1 mmol), 0.63 ml bromoethane (0.915 g, 8.4 mmol), 0.580 g potassium carbonate (4.2 mmol) and ca. 0.1 g potassium iodide were refluxed for 8 h in 15 ml DMSO at 140 °C. The mixture was cooled down to room temperature, whereupon it slightly solidified, and then it was poured into 15 ml deionised water. The orange–brown precipitate was

U. Georgi et al. / Reactive & Functional Polymers 72 (2012) 242–251

245

Scheme 2. Overview over the different azobenzene derivatives synthesised. The numbering refers to the NMR spectroscopic signal assignment in Section 2.

Scheme 3. RAFT copolymerisation of MMA and CMS: The reaction was carried out at 80 °C for 6 h in anisole (200 vol% relative to monomers), [MMA + CMS]/[CTA]/ [AIBN] = 200/1/0.05. The numbering refers to the NMR spectroscopic signal assignment in Section 2.

Scheme 4. Attachment to polymer matrix: etherification of azobenzene derivatives with P(MMA-co-CMS). The numbering refers to the NMR spectroscopic signal assignment in Section 2.

filtered off and dissolved in a solution of 0.220 g potassium hydroxide (3.9 mmol) in 13 ml water and 30 ml ethanol. The solution was refluxed at 78 °C for 12 h. The red solution was cooled down to room temperature and ca. 4.5 ml 1 M HClaq were added until a yellow solid precipitated out of the solution. This solid was filtered off and dried at 45 °C in vacuo. To remove residual bromoethane, the product was precipitated twice from THF/methanol in water.

Yield: 77%. 1 H NMR (500.13 MHz, DMSO-d6, ppm): 13.14 (br, 1H, COOH), 8.12 (d, 2H, 9), 7.92 (d, 2H, 5),7.89 (d, 2H, 8), 7.12 (d, 2H, 4), 4.15 (q, 2H, 2), 1.37 (t, 3H, 1). 13 C NMR (125.74 MHz, DMSO-d6, ppm): 166.69 (11), 161.86 (3), 154.44 (10), 146.08 (6), 132.11 (7), 130.51 (5), 124.95 (8), 122.15 (4), 115.06 (9), 63.71 (2), 14.46 (1).

246

U. Georgi et al. / Reactive & Functional Polymers 72 (2012) 242–251 1

H NMR (500.13 MHz, CDCl3, ppm): 7.87 (d, 2H, 12), 7.85 (d, 2H, 9), 7.65 (d, 2H, 8), 7.46 (br, s, 1H, NH), 6.77 (d, 2H, 13), 3.72 (s, 3H, 1), 3.09 (s, 6H, 15), 2.48 (m, 4H, 3&5), 2.09 (quin., 2H, 4). 13 C NMR (125.74 MHz, CDCl3, ppm): 173.72 (6); 170.79 (2); 152.21 (14); 143.63 (10); 139.55 (11); 124.69 (12); 122.93 (9); 119.77 (8); 111.53 (13); 51.55 (1); 45.82 (15); 40.25 (5); 33.05 (3); 20.72 (4).

Fig. 1. 4-(1-Naphthylazo)phenol.

2.3.5. Synthesis of (4-ethoxyphenyl)-(4-nitrophenyl)diazene (5) 2.3.5.1. Synthesis of 4-(4-Nitrophenylazo)phenol Fig. 2. To an icecooled solution of 5 g 4-nitroaniline (36.2 mmol) in 100 ml methanol, 25 ml water and 12.5 ml concentrated HCl, a highly concentrated solution of 2.5 g sodium nitrite in water was added slowly while keeping the temperature below 0 °C. The solution was stirred for an hour and then added, drop by drop, to an ice-cooled solution of 3.4 g phenol in 50 ml acetone and 25 ml water. After the reaction, the solution was neutralised with 16 wt% aqueous NaOH solution to pH = 8. After another 2 h of stirring, 40 ml of 10 wt% aqueous HCl were added and the resulting precipitate was filtered off and recrystallised from a methanol/ethanolmixture. Yield: 20%. 1 H NMR (500.13 MHz, CDCl3, ppm): 8.37 (d, 2H, 7), 7.99 (d, 2H, 3), 7.95 (d, 2H, 6), 6.99 (d, 2H, 2), 5.26 (OH). 13 C NMR (125.74 MHz, CDCl3, ppm): 152.42 (1), 155.98 (5), 147.20 (8), 125.83 (3), 124.71 (7), 123.14 (6), 116.06 (2). 2.3.5.2. Etherification of 4-(4-nitrophenylazo)phenol with bromoethane. The synthesis of (4-Ethoxyphenyl)-(4-nitrophenyl)diazene (5) was carried out similarly to the synthesis of (1). Yield: 61%. 1 H NMR (500.13 MHz, CDCl3, ppm): 8.36 (d, 2H, 9), 7.97 (2d, 4H, 5&8), 7.03 (d, 2H, 4), 4.16 (q, 2H, 2), 1.48 (t, 3H, 1). 13 C NMR (125.74 MHz, CDCl3, ppm): 162.74 (3), 156.08 (7), 148.23 (10), 146.83 (6), 125.61 (5), 124.67 (9), 123.06 (8), 114.91 (4), 64.01 (2), 14.69 (1). DSC: Tm = 76 °C. TGA: 242 °C (99.5%). 2.3.6. Amidation of 4-(4-dimethylaminophenylazo)aniline (6) 0.62 g 4-(4-Dimethylaminophenylazo)aniline (2.6 mmol), 0.39 ml triethylamine (0.287 g, 2.8 mmol) and about 100 mg dimethylaminopyridine (DMAP) were dissolved in 10 ml acetone. 0.39 ml Glutaric acid monomethyl ester chloride (0.467 g, 2.8 mmol) was added and the solution was refluxed over night. The mixture was then cooled to room temperature; the precipitate was filtered off and thoroughly washed with acetone until it was colourless. The solvent of the filtrate was removed under reduced pressure and the solid was recrystallised twice from acetone. Yield: 69%.

N HO

1

4 2

N

NO2 5

8 6

7

3

Fig. 2. 4-(4-Nitrophenylazo)phenol.

2.3.7. Amidation of 4-(4-nitrophenylazo)aniline (Disperse Orange 3) (7) (7) was obtained by amidation of Disperse Orange 3 with Glutaric acid monomethyl ester chloride in a similar way as (6). Yield: 21%. 1 H NMR (500.13 MHz, CDCl3, ppm): 8.38 (d, 2H, 13), 8.02 (d, 2H, 12), 7.99 (d, 2H, 9), 7.75 (d, 2H, 8), 7.66 (br, s, 1H, NH), 3.73 (s, 3H, 1), 2.51 (m, 4H, 3&5), 2.11 (quin., 2H, 4). 13 C NMR (125.74 MHz, CDCl3, ppm): 173.65 (6); 171.53 (2); 155.94 (14); 148.28 (10); 142.98 (11); 124.57 (9); 124.46 (13); 123.10 (12); 119.71 (8); 51.46 (1); 36.22 (5); 33.09 (3); 20.62 (4). 2.3.8. RAFT copolymerisation of methyl methacrylate and 4-chloromethylstyrene ((8) and (9)) In a typical reaction, 8 ml (7.52 g, 75.1 mmol) MMA, 1.2 ml (1.27 g, 8.3 mmol) CMS, 95.7 mg (0.43 mmol) CPBD and 5.2 mg (0.03 mmol) AIBN were dissolved in 18 ml of anisole in a Schlenk tube with a magnetic stir bar. The Schlenk tube was sealed with a rubber septum and degassed three times by freeze–pump– thaw-cycles. The polymerisation was carried out under nitrogen and started in a preheated oil bath at 80 °C. After 6 h, the reaction was stopped by freezing the solution in liquid nitrogen. The mixture was diluted with THF, precipitated into n-hexane and dried at 45 °C in vacuo overnight. 1 H NMR (500.13 MHz, CDCl3, ppm): 7.89 (f), 7.51 (h), 7.35 (g), 7.23 (m), 7.02 (o), 4.55 (CH2Cl), 3.60 (OCH3), 2.45 (OCH3), 2.75– 2.30 (CHbackbone), 2.2–1.6 (CH2backbone), 1.5–0.5 (c, CH3backbone). 13 C NMR (125.74 MHz, CDCl3, ppm): 225.1/224.8 (d), 179–175 (C@O), 147.3/145.7/143.6 (i), 144.5 (e), 137.5/137.1/135.3 (p), 132.3 (h), 130–127 (h, o, m), 126.4 (f), 125.1/124.8 (a), 55–50 (CH2backbone), 51.6 (OCH3), 46.3–45.6 (CH2Cl), 45.5–44 (Cquat,backbone, b), 41–35 (CHbackbone), 30.5–30 (b, c), 25.8–25.5 (d), 22–15 (CH3backbone). For molar masses and composition of the polymers (8) and (9) see Table 1. 2.3.9. Introduction of the azobenzene moiety to the polymer (CP1–3, CP5) In a typical reaction, 0.7 g of P(MMA-co-CMS) (20% CMS content, 1.3 mmol Cl) and 1.1 eq of the corresponding azobenzene were dissolved in dry DMF and 0.534 g Cs2CO3 (1.6 mmol) were added. The solution was stirred at 40 °C under nitrogen for 5–10 d. The solid Cs2CO3 was filtered off and the solvent was evaporated under reduced pressure. The residue was taken up in THF and precipitated in a methanol/water-mixture (v/v = 2/1). The resulting polymer was dried in vacuo at 45 °C. (CP1): 1 H NMR (500.13 MHz, CDCl3, ppm): 8.0–7.6 (3, 6), 7.5–6.8 (Harom,styrene, 2), 6.8–6.5 (7), 5.06 (CH2O), 3.7–3.3 (OCH3), 3.06 (9), 3.1–2.8 (OCH3), 2.8–2.2 (CHbackbone), 2.2–1.3 (CH2backbone), 1.3–0.5 (CH3backbone). 13 C NMR (125.74 MHz, CDCl3, ppm): 179–175 (C@O), 160.0 (1), 152.1 (8), 147.6 (4), 147.5–144.5 (i), 143.7 (5), 138–134 (p), 130– 124 (o, m), 124.5 (6), 123,8 (3), 115.0 (2), 111.6 (7), 70.5–69.5 (CH2O), 55–50 (CH2backbone), 51.7 (OCH3), 51.0 (OCH3), 47–44 (Cquat,backbone), 40.3 (9), 30.3 (b), 25.8 (a), 22–16 ((CH3backbone). DSC: Tg = 116 °C;

247

U. Georgi et al. / Reactive & Functional Polymers 72 (2012) 242–251 Table 1 Results of RAFT polymerisation; molar masses determined by SEC with MALLS-detector and linear fitting process.

(8) (9) a

½nnMMA  CMS target

½nnMMA  CMS NMR

Conversion (%)

Mn,th.a (g/mol)

Mn,SEC (g/mol)

Mw,SEC (g/mol)

PDI

90/10 95/5

80/20 88/12

29 36

6200 7400

8900 10,300

10,000 11,700

1.12 1.14

Mn,th. = [M]0/[CTA]0  yield  MMonomer + MCTA.

TGA: 202 °C (3.8%), 240 °C (3.3%), 318 °C (29.4%), 426 °C (44.8%). (CP2): 1 H NMR (500.13 MHz, CDCl3, ppm): 8.0–7.7 (3, 6), 7.49 (7), 7.43 (8), 7.02 (2) 7.4–6.7 (Harom,styrene), 5.08 (CH2O), 3.8–3.3 (OCH3), 3.1– 2.8 (OCH3), 2.8–2.2 (CHbackbone), 2.2–1.3 (CH2backbone), 1.3–0.5 (CH3backbone). 13 C NMR (125.74 MHz, CDCl3, ppm): 179–175 (C@O), 161.2 (1), 152.7 (5), 147.6 (4), 147.5–144.5 (i), 137–134 (p), 130.3 (8), 129.0 (7), 130–125 (o, m), 124.7 (3), 122,5 (6), 115.1 (2), 70.5–69.5 (CH2O), 55–50 (CH2backbone), 51.7 (OCH3), 50.9 (OCH3), 47–44 (Cquat,backbone), 30.3 (b), 25.8 (a), 22–16 ((CH3backbone). DSC: Tg = 105 °C. TGA: 203 °C (3.6%), 250 °C (4.2%), 313&333 °C (39.1%), 420 °C (38.8%). (CP3): 1 H NMR (500.13 MHz, CDCl3, ppm): 8.90 (12), 8.03 (3), 7.95 (8), 7.92 (9), 7.78 (6), 7.63 (11), 7.57 (7, 11), 7.10 (2), 7.5–6.8 (Harom,styrene), 5.11 (CH2O), 3.8–3.4 (OCH3), 3.1–2.8 (OCH3), 2.8–2.3 (CHbackbone), 2.2–1.3 (CH2backbone), 1.3–0.5 (CH3backbone). 13 C NMR (125.74 MHz, CDCl3, ppm): 179–175 (C@O), 161.1 (1), 147.7 (4, 5), 147.5–144 (i), 137–133 (p), 134.17 (14), 131.05 (8), 130.5 (13), 127.8 (9), 129–124 (o, m), 126.5 (11), 126.2 (10), 125.5 (7), 124.9 (3), 123.4 (12), 115.1 (2), 111.6 (6), 70.5–69.5 (CH2O), 55–50 (CH2backbone), 51.6 (OCH3), 50.9 (OCH3), 46.5–44 (Cquat,backbone), 37.5 (CHbackbone), 30.3 (b), 25.8 (a), 22–16 (CH3backbone). DSC: Tg = 106 °C. TGA: 205 °C (4.5%), 238 °C (4.4%), 303 °C (16.2%), 415 °C (53.9%). (CP5): 1 H NMR (500.13 MHz, CDCl3, ppm): 8.36 (7), 7.98 (3, 6), 7.07 (2), 7.5–6.8 (Harom,styrene), 5.11 (CH2O), 3.8–3.3 (OCH3), 3.1–2.8 (OCH3), 2.8–2.2 (CHbackbone), 2.2–1.3 (CH2backbone), 1.3–0.5 (CH3backbone). 13 C NMR (125.74 MHz, CDCl3, ppm): 179–175 (C@O), 162.3 (1), 155.9 (5), 148.2 (8), 146.9 (4), 147.5–143 (i), 136–132 (p), 130–126 (o, m), 125.5 (3), 124.6 (7), 123.0 (6), 115.3 (2), 70.5–69.5 (CH2O), 55–50 (CH2backbone), 51.6 (OCH3), 50.9 (OCH3), 46.5–44 (Cquat,backbone), 37.5 (CHbackbone), 30.3 (b), 25.8 (a), 22–16 (CH3backbone). DSC: Tg = 112 °C. TGA: 103 °C (2.6%), 238 °C (4.7%), 339 °C (44.3%), 400 °C (36.5%).

3. Results and discussion The goal of this work was the systematic investigation of the influence of azobenzene substituents on the one hand, and matrix effects on the other hand, on the kinetics of the thermal cis–trans-isomerisation. Therefore, a series of azobenzenes with electron-withdrawing and -donating groups was synthesised and the photochemical trans–cis-isomerisation as well as the thermal cis–trans-isomerisation in solution (solvent THF) and physically mixed into a polymer matrix (PMMA) was investigated. Furthermore, the chromophores were covalently attached to a PMMAcopolymer to find out if this leads to a change in thermal stability of the cis-isomer.

3.1. Synthesis of the azobenzene molecules and the azobenzene containing polymers All azobenzene chromophores listed in Scheme 2 were, if not purchased, synthesised via the classical azo-coupling approach and subsequent etherification (substances (1)–(5) or amidation (substances (6)–(7)). The chromophores (1)–(3) and (5) were furthermore investigated when covalently attached to a polymer matrix. Therefore, a copolymer of methyl methacrylate (MMA) and chloromethylstyrene (CMS) was synthesised via Reversible Addition Fragmentation Chain Transfer polymerisation (RAFT polymerisation Scheme 3). Two different polymers were prepared, one being composed of 20% CMS as calculated from the 1H NMR spectrum (Fig. 3), the other one containing 12% CMS. The results are shown in Table 1. These ratios in the polymer were obtained when 10 n/n% and 5 n/n% CMS, respectively, were applied in the original polymerisation mixture. The favoured integration of the styrenic monomer can be explained by the different properties of the monomers (styrene: electron-donating substituent; MMA: electron-accepting substituent) resulting in a statistically azeotropic copolymerisation. At lower concentrations of the styrenic monomer in the polymerisation mixture it is built into the polymer chain preferentially [39]. The molar masses Mn obtained by SEC measurements with a Multi Angle Laser Light Scattering Detector (MALLS) somewhat differ from the expected theoretical molar mass Mn,th., which is obtained by relating the amount of monomer to that of chain transfer agent (CTA) and taking into account the yield. This deviation is very likely a result of not ideal initiator efficiency. Nevertheless, the polydispersity of the obtained polymers was satisfyingly narrow (1.10–1.15). In a post-polymerisation step, the phenol derivatives of chromophores (1) to (3) and (5) were attached to the P(MMAco-CMS) backbone by caesium carbonate mediated etherification at 45 °C in DMF as solvent (Scheme 4). The reactions proceeded smoothly to conversions >99% of the benzyl chloride groups (see Fig. 3). The results of this reaction step are summarised in Table 2. The rise in molar mass of the polymers corresponds rather well with the theoretical calculation based on the molar mass of the MMA–CMScopolymers assuming 100% conversion. The slightly higher as expected increase in molar mass, which can still be observed, could be due to chain coupling: The NMR measurements indicated that the dithioester group is unstable under the extremely basic reaction conditions. Therefore, very probably sulphide groups are formed. Those are able to form disulphide bridges between two chains thus increasing the overall molar mass. This theory is supported by the PDI results from GPC measurements using a MALLS detector as the polydispersities of the azobenzene-containing polymers were broader (1.20–1.36) than the starting material (see Fig. 4). 3.2. Investigation of the thermal cis–trans-isomerisation In order to investigate the rate of their thermal cis–trans-isomerisation the synthesised ABDs were irradiated as prepared (having seen day light) with monochromatic light at the absorption

248

U. Georgi et al. / Reactive & Functional Polymers 72 (2012) 242–251

(CP1)

P(MMA-co -CMS) (8)

-N(CH3)2

-OCH3

-OCH3 CH2Cl

8.0

7.0

6.0

5.0

-CH2O-

4.0

3.0

2.0

1.0

0.0

8.0

Chemical Shift [ppm]

7.0

6.0

5.0

4.0

3.0

2.0

1.0

0.0

Chemical Shift [ppm]

Fig. 3. 1H NMR spectra of P(MMA-co-CMS) (8) (left) and (CP1) (right). As the signal around 4.6 ppm vanishes completely, the spectra indicate a degree of modification of the chloromethylene group close to 100%. Analogous results were obtained for the other derivatives.

Table 2 Results of etherification of P(MMA-co-CMS) with azobenzene chromohores; molar masses determined by SEC with MALLS-detector and linear fitting process.

(CP1) (CP2) (CP3) (CP5) a

From polymer

Mn,theo.a (g/mol)

Mn,SEC (g/mol)

Mw,SEC (g/mol)

PDI

(8) (8) (8) (9)

14,000 13,200 14,100 13,500

16,000 16,200 17,400 19,200

21,700 20,400 21,300 23,000

1.36 1.26 1.22 1.20

Mn,th. calculated on the basis of degree of polymerisation and CMS-content of (8) and (9).

The irradiation only leads to a photostationary state of the cis- and trans-form of the chromophore and not to an overall conversion into the cis-form. It is not exactly known how much of each isomer is present in the mixture, but, in a first approximation, the time required to reach the thermal equilibrium is supposed to give significant insight in the dynamics of the thermal back-reaction. The results obtained are summarised in Table 3. Please note, that the kinetics of the photochemical trans–cis-reaction is not the focus of this study and thus, it is not included in the following evaluations.

Normalized MALLS signal

1.00 (8) (CP1) (CP2) (CP3)

0.75

0.50

0.25

0.00

5

6

7

8

9

Elution Time [min] Fig. 4. The GPC traces of P(MMA-co-CMS) (8) (solid line), (CP1) (dashed line), (CP2) (dotted line) and (CP3) (dash-dotted line). The broadening of the distribution after postpolymerisation modification as well as the increase in molecular weight (shift to lower elution times) is well visible.

maximum until a photostationary state was reached. The samples were then allowed to thermally relax at room temperature. The thermal equilibrium thus reached is defined by the equilibrium constant of the thermal cis–trans- and trans–cis-isomerisations. The absorbance measured in the thermal equilibrium was higher than that of the initial solution which is a representation of the photostationary state due to synthesis and sample preparation. Hence, the amount of trans-isomer could be maximised. With this thermal equilibrium, a defined starting point with regard to the UV absorption of the chromophore was provided for the study. Based on this maximum value at room temperature, the sample was re-irradiated and, after the photostationary state was reached, the thermal relaxation process was monitored by measuring spectra at regular intervals, until the thermal equilibrium was reached again. In order to compare the rates of the different derivatives, the time required to recover 90% of the absorption in the thermal equilibrium was determined (see Scheme 5). These measurements were carried out in a diluted THF solution, in an ABD-containing PMMA matrix and for the ABD-functionalised polymers.

3.2.1. Substituent effects Upon variation of the azobenzene-substituents in positions 4 and 40 with a variety of electron-withdrawing to electron-donating substituents, the position of the chromophore’s absorption maximum in UV–Vis shifts as anticipated. Furthermore, the reaction rate of the thermal cis–trans-isomerisation is influenced considerably. Next to the unsubstituted AB which showed the most prolonged relaxation time, the azobenzene derivatives (2) and (4) ex-

Scheme 5. Graphical outline of the determination of t90%.

249

U. Georgi et al. / Reactive & Functional Polymers 72 (2012) 242–251 Table 3 Overview of AB-derivatives and their rates of thermal cis–trans-isomerisation.

(AB) (1) (2) (3) (4) (5) (6) (7)

kmax (nm)a

Colour

Substituent effectsb

t90%c (in THF)

t90%c (in PMMA)

t90%c (polymer boundd)

Type

322 408 349 383 351 378 415 386

Light orange Yellow Yellow–orange Dark red Yellow Orange Orange–red Orange

– +M (NMe2) – – I (COOH) I (NO2) +M (NMe2) M (NO2)

>16 h (ca. 8.5%) 2.7 h >25 h (ca. 60%) 0.6 h 32 h 1.1 h 0.02 h 1.2 h

>20 h (ca. 25%) 4.5 h >21.2 h (ca. 39%) 4.7 h >20 h (ca. 40%) 3.7 h 1.2 h 9.2 h

– 6.5 h >20.8 h (ca. 42%) 14.6 h – 11 h – –

Azobenzene Aminoazobenzene Azobenzene Aminoazobenzene Azobenzene Aminoazobenzene Pseudo-stilbene Aminoazobenzene

a

In THF. OCH2CH3 (substances (1)–(5)) and Amide (substances (6) and (7)): +M. c Time after which 90% of k0max initial intensity is regained during thermal cis–trans-isomerisation; please note: in some cases the 90% value was not reached in the course of the study and other % are listed in the table. d Values for corresponding copolymers CP1, CP2, CP3, CP5. b

hibit by far the lowest thermal cis–trans-reaction rate. These ABDs are unsubstituted in 4-position or bear substituents with only very small influence on the electronic structure of the p-system (ether groups with weak + M effect, carboxylic acid with very weak – I effect) [40,41]. These substances are typical azobenzene type chromophores, meaning that they display a low intensity n–p-band in the visible region and a high intensity p–p-band in the UV (see exemplary Fig. 5). However, in comparison with the unsubstituted AB, molecules (2) and (4) still react considerably faster: For AB, 8.5% of k0max initial absorption is regained after 16 h, whereas (2) and (4) regain this amount already after 3.4 h and 1.7 h, respectively. These results are in agreement with suggestions in literature that upon substitution of the phenyl rings the isomerisation rate is accelerated, regardless of the substituent type [10,42]. It was furthermore stated that an increase in the molecule’s dipolar moment (or a change in the resonance structure) lowers the activation barrier for thermal relaxation [10,13,33]. This is consistent with our results, as the rate of thermal relaxation is increased notably for ABD bearing very polar substituents, thus displaying an increased dipole; in contrast, the rate is low for molecules containing no (AB) or only slightly polar groups ((2) and (4)). The substances (1), (3), (5) and (7) display intermediate reaction rates being located well above those of ABD (2) and (4). These derivatives bear more polar groups in comparison to the ABDs discussed above and therefore show faster thermal relaxation. The UV–Vis spectra show that the n–p-band and the p–p-band partly overlap in the violet or the near visible UV (see exemplary Fig. 6).

The AB-derivative (6) exhibits by far the fastest back-isomerisation. Although the reaction is as expected considerably slowed down in the PMMA matrix, it is even then still the fastest one in comparison with the other derivatives. This is because the molecule shows pseudo-stilbene type characteristics, meaning that the absorption spectra of the two isomers overlap strongly and only differ in their extinction coefficients (see Fig. 7). Therefore, irradiation with 415 nm light evokes both the forward (trans–cis) and the backward (cis–trans) reaction, the backward reaction being additionally supported by thermal excitation. Hence, practically seen, smaller amounts of cis-isomer than for azobenzene- or aminoazobenzene-type molecules should be present in the mixture in the photostationary state, again reducing t90% observed. The pseudo-stilbenes’ thermal cis–trans-isomerisation typically takes place in the order of seconds to milliseconds [43], which corresponds to the results obtained for compound (6) as well. Furthermore (6) exhibits the typical colour for pseudo-stilbenes, a deep orange-red. These properties are usually caused by strong push– pull-substituents (e.g. electron-withdrawing nitro- and electrondonating dimethylamino-groups in 4- and 40 -position to the azogroup, respectively). Interestingly, (6) does not bear the typical push–pull substitution pattern but only holds a strong electron donor (dimethylamino) and a weak one (amide = protected amine) in 4 and 40 , respectively. A similar substitution is present in (1) which carries also a dimethylamino group as strongly activating group and an ether group as a weakly activating one [40,41]. Interestingly, in contrast to molecule (6) the ABD (1) shows an immensely increased thermal stability of the cis-isomer in solution (t90% = 135

0.8

1.0

0.6

Absorption

Absorbance

0.8 0.6 0.4

0.4

0.2 0.2 0.0 225

0.0 300

375

450

525

350

400

450

500

550

Wavelength [nm]

Wavelength [nm] Fig. 5. UV–Vis spectrum of AB in methanol solution in thermal equilibrium (solid line) and in the photostationary state after irradiation at 320 nm (dotted line). The well separated n–p-band at 450 nm and the p–p-band at 320 nm can be seen.

Fig. 6. UV–Vis spectrum of (1) in methanol solution in thermal equilibrium in the dark (solid line) and in the photostationary state after irradiation at 405 nm (dotted line). It can be seen that the n–p-band around 450 nm and the p–p-band at 405 nm slightly overlap.

250

U. Georgi et al. / Reactive & Functional Polymers 72 (2012) 242–251 1.0

1.0

Normalized Absorbance

0.8

Absorption

0.4 0.3

0.6 0.4 0.2 0.0

0.2

400

500

600

Wavelength [nm]

0.1 0.0 300

400

500

600

Normalized Absorption [a.u.]

0.5

0.8

0.6

0.4

0.2 325

700

(3) in THF (3) in PMMA (CP3) in THF (CP3) film

350

Wavelength [nm] Fig. 7. Absorption spectra of compound (6) in THF in thermal equilibrium in the dark (solid line) and in photostationary state after irradiation with 405 nm (dotted line). The spectra are practically identical except for the absorption coefficient (see normalised absorption spectra, inset).

times higher) and still a rather strong increase in PMMA matrix (t90% = 4 times higher). This seems to lead to the conclusion that the amide group is not forcibly only electron donating, but is furthermore capable of deactivating the aromatic systems up to an amount high enough to induce pseudo-stilbene characteristics in molecule (6). It also indicates that the transition state for the thermal relaxation is better stabilised by the substituents present in (6) which allows the cis–trans-reaction to proceed more easily. In order to better understand these different results, Raman spectra were recorded and the N@N-vibrations as well as the CAN-vibrations were identified. Those values can be related to the force constants of the respective bonds, which are dependent on the bond order. It is sometimes stated that this order directly influences the rate of thermal cis–trans-isomerisation [33]. The bond order is dependent on the bond’s electron density. Biswas and Umapathy proposed that these properties depend on the nature of the ABD’s substituents [36]. They investigated 4-nitroazobenzene (NAB), 4-(dimethylamino)-azobenzene) (DAB) and 4-nitro-40 -(dimethylamino)-azobenzene (NDAB) theoretically and by IR and Raman spectroscopy. They stated that electron density and bond order finding expression in N@N’s and CAN’s vibrational frequencies are strongly dependent on the nature of the substituents influencing the delocalisation of the electrons in the system. Therefore, Raman investigations were expected to reveal similar results for the relation between vibrational frequency, bond order and thermal stability of the cis-isomers. The measured Raman vibrations for the substances synthesised in this work as well as the ones described in [36] are summarised in Table 4.

375

400

The N@N-vibrations for the NO2-bearing derivatives (5) and (7) correspond well with those of NAB; the same applies to the ABDs with NMe2 ((1) and (6)). Nevertheless, this is not true for the CAN-vibrations which, for AB and its derivatives (1)–(7), are all found within a range of 8 cm1. Finally, no correlation could be found between the electronic nature of the substituent and Raman vibrations observed. 3.2.2. Matrix effects In diluted THF-solution the ABD molecules exist mostly isolated in a very fluid environment. The chromophores are thus highly mobile and the isomerisation reactions are largely unaffected. Due to the highly enlarged viscosity in solid thin films of PMMA and therefore an importantly decreased mobility of the ABDs in this matrix, the relaxation time increases crucially. The t90% observed increases from two- up to tenfold for most derivatives, for (6) even a 60-fold increase was observed, but still showing by far the fastest rate of all derivatives. When the chromophores were covalently attached to a PMMA-block copolymer, the relaxation time tripled for most ABDs ((CP2), (CP3) and (CP5)). The only exception was copolymer (CP1) where the relaxation time only slightly augmented. A reason for this behaviour could not be found. Another interesting observation was made for the naphthyl derivatives (3) and (CP3). For this system, a hypsochromic shift appeared in the absorption maximum of the naphthyl derivative when mixed into PMMA (Dk = 7 nm, Fig. 8). Such a deviation of kmax was not observed for the other derivatives, and also not when (3) was covalently bound to a polymer ((CP3)), although for all measured solid thin films a considerable broadening of the absorption

N@N (2) (cm1)

N@N (3) (cm1)

CAN (cm1)

1443 1451 1446 1447 1456 1458 1447 1458 1462 1442 1423

– 1411 1422 1420 1409 1406 1427 1408 – – –

– – – – – – 1402 – – – –

1316 1317 1314 1312 1312 1309 1315 1314 1272 1300 1313

Normalized1 [a.u.]

(AB) (1) (2) (3) (4) (5) (6) (7) NAB [36] DAB [36] NDAB [36]

N@N (1) (cm1)

450

Fig. 8. Absorption spectra of (3) and (CP3) in THF-solution and in thin polymer film.

(5) in THF (5) in PMMA (CP5) in THF (CP5) thin film

1.0 Table 4 Raman vibrations of NAN-double bond and CAN-single bond for AB-derivatives.

425

Wavelength [nm]

0.8

0.6

0.4

0.2 325

350

375

400

425

450

Wavelength [nm] Fig. 9. Absorption spectra of (5) and (CP5) in THF-solution and in thin polymer film.

U. Georgi et al. / Reactive & Functional Polymers 72 (2012) 242–251

band was detected (see UV–Vis spectra of ABD (5) in Fig. 9, the other substances show comparable behaviour). This hypsochromic shift could be caused by a change in environmental conditions and could, in this case, be explained by aggregation of the chromophores. This should be facilitated for the naphthyl derivative as the p-electron-system here is significantly enlarged in comparison to the other chromophores. As p–p-stacking results in stabilisation of the lone electron pairs in the non-bonding n-orbitals as well as in the anti-bonding p-orbitals, the absorption wavelength of the p–p-transition is changed [44]. This effect cannot be observed when the chromophores are isolated in a highly diluted solution and can also be prevented in thin films/bulk when covalent bonding to the polymer chain limits the possibility of diffusion and thus prevents the chromophores from aggregating. Photochemical studies on well-defined naphthyl azo derivatives in solution and matrix [45,46] or polymer-bound [47,48] are rare and the observed indication for aggregation when mixed in a PMMA matrix has not been described so far. 4. Conclusions We have synthesised a variety of 4,40 -substituted azobenzene derivatives and investigated their isomerisation behaviour by UV–Vis spectroscopy in solution and embedded in PMMA. Furthermore, some of the synthesised chromophores were covalently bound to a methyl methacrylate-chloromethyl styrene-copolymer through etherification. This allowed to study uniquely how substituents as well as matrix incorporation (physically mixed or covalently bound) influence the isomerisation kinetics in a highly comprehensive and systematic way. Special interest was taken in the observation of the thermal cis–trans-relaxation and its dependence on the kind of substituents (electron donating or withdrawing). Considerable differences in relaxation time were found for the substances which interestingly could not be fully explained by simply comparing the substituents electron donating or withdrawing. Also, no correlation was found between the Raman vibrations of the N@N- or CAN-bond and the relaxation time. However, we can conclude that any, and especially polar substituents increase the thermal cis–trans-reaction rate. In solution, the relaxation time varied strongly from very fast (seconds to minutes, 4-amino40 -amido-substituted ABD) to extremely slow (hours to days, 4-ethoxy- substituted ABD). As expected, the relaxation time increased considerably when the chromophore was mixed into PMMA, and was once more decelerated significantly when the ABDs were chemically attached to the polymer matrix. Nevertheless, the study exhibited that even at room temperature the thermal cis–trans-relaxation of 4,40 -substituted azobenzene chromophores can never be fully suppressed, only slowed down, and may significantly limit their use in some optical technologies where long-term stability of the cis-isomer is needed. Acknowledgements We would like to thank Bettina Pilch for technical support with the UV–Vis measurements, Dr. Hartmut Komber for help with the NMR measurements, Dr. Dieter Fischer for recording the Raman spectra and Liane Häußler for thermoanalytic measurements. We greatly acknowledge Schwerpunktprogramm (SPP) 1327 of Deutsche Forschungsgemeinschaft (DFG) for financial support.

251

References [1] E. Mitscherlich, Ann. Phys. Chem. XXXII (1834) 224. [2] A. Noble III, Just. Liebigs Ann. Chem. 98 (1856) 253–256. [3] H. Fliegl, A. Köhn, C. Hättig, R. Ahlrichs, J. Am. Chem. Soc. 125 (2003) 9821– 9827. [4] H. Rau, E. Lueddecke, J. Am. Chem. Soc. 104 (1982) 1616–1620. [5] T. Asano, T. Okada, J. Org. Chem. 49 (1984) 4387–4391. [6] T. Asano, H. Furuta, H.J. Hofmann, R. Cimiraglia, Y. Tsuno, M. Fujio, J. Org. Chem. 58 (1993) 4418–4423. [7] P.D. Wildes, J.G. Pacifici, G. Irick, D.G. Whitten, J. Am. Chem. Soc. 93 (1971) 2004–2008. [8] S. Kobayashi, H. Yokoyama, H. Kamei, Chem. Phys. Lett. 138 (1987) 333–338. [9] A. Cembran, F. Bernardi, M. Garavelli, L. Gagliardi, G. Orlandi, J. Am. Chem. Soc. 126 (2004) 3234–3243. [10] J. Dokic, M. Gothe, J. Wirth, M.V. Peters, J. Schwarz, S. Hecht, P. Saalfrank, J. Phys. Chem. A 113 (2009) 6763–6773. [11] G. Tiberio, L. Muccioli, R. Berardi, C. Zannoni, ChemPhysChem 11 (2010) 1018– 1028. [12] C.R. Crecca, A.E. Roitberg, J. Phys. Chem. A 110 (2006) 8188–8203. [13] H.M.D. Bandara, T.R. Friss, M.M. Enriquez, W. Isley, C. Incarvito, H.A. Frank, J. Gascon, S.C. Burdette, J. Org. Chem. 75 (2010) 4817–4827. [14] Y. Ootani, K. Satoh, A. Nakayama, T. Noro, T. Taketsugu, J. Chem. Phys. 131 (2009). 194306-194301–194306-194310. [15] A. Natansohn, P. Rochon, J. Gosselin, S. Xie, Macromolecules 25 (1992) 2268– 2273. [16] T. Buffeteau, M. Pezolet, Macromolecules 31 (1998) 2631–2635. [17] S.K. Yesodha, C.K. Sadashiva Pillai, N. Tsutsumi, Prog. Polym. Sci. 29 (2004) 45– 74. [18] M. Eich, J.H. Wendorff, B. Reck, H. Ringsdorf, Makromol. Chem., Rapid Commun. 8 (1987) 59–63. [19] M. Häckel, L. Kador, D. Kropp, H.W. Schmidt, Adv. Mater. (Weinheim, Ger.) 19 (2007) 227–231. [20] S. Hvilsted, C. Sanchez, R. Alcala, J. Mater. Chem. 19 (2009) 6641–6648. [21] I. Willner, S. Rubin, A. Riklin, J. Am. Chem. Soc. 113 (1991) 3321–3325. [22] T. Ikeda, O. Tsutsumi, Science 268 (1995) 1873–1875. [23] M.-J. Lee, D.-H. Jung, Y.-K. Han, Mol. Cryst. Liq. Cryst. 444 (2006) 41–50. [24] G. Wang, X. Tong, Y. Zhao, Macromolecules 37 (2004) 8911–8917. [25] X. Tong, G. Wang, A. Soldera, Y. Zhao, J. Phys. Chem. B 109 (2005) 20281– 20287. [26] J. del Barrio, L. Oriol, C. Sanchez, J.L. Serrano, A. Di Cicco, P. Keller, M.-H. Li, J. Am. Chem. Soc. 132 (2010) 3762–3769. [27] F.D. Jochum, P. Theato, Polymer 50 (2009) 3079–3085. [28] F.D. Jochum, P. Theato, Chem. Commun. (Cambridge, UK) 46 (2011) 6717– 6719. [29] I. Willerich, F. Gröhn, Angew. Chem., Int. Ed. 49 (2010) 8104–8108. [30] I. Willerich, T. Schindler, F. Gröhn, J. Phys. Chem. B 115 (2011) 9710–9719. [31] F. Puntoriero, P. Ceroni, V. Balzani, G. Bergamini, F. Vögtle, J. Am. Chem. Soc. 129 (2007) 10714–10719. [32] S. Tamesue, Y. Takashima, H. Yamaguchi, S. Shinkai, A. Harada, Angew. Chem., Int. Ed. 49 (2010) 7461–7464. [33] A.A. Beharry, G.A. Woolley, Chem. Soc. Rev. 40 (2011) 4422–4437. [34] M. Banghart, K. Borges, E. Isacoff, D. Trauner, R.H. Kramer, Nat. Neurosci. 7 (2004) 1381–1386. [35] O. Pieroni, A. Fissi, N. Angelini, F. Lenci, Acc. Chem. Res. 34 (2000) 9–17. [36] N. Biswas, S. Umapathy, J. Phys. Chem. A 104 (2000) 2734–2745. [37] U. Oertel, H. Mart, H. Komber, F. Böhme, Opt. Mater. 32 (2009) 54–61. [38] D. Chen, H. Liu, T. Kobayashi, H. Yu, J. Mater. Chem. 20 (2010) 3610–3614. [39] H.-G. Elias, Macromolecules, first ed., Wiley-VCH Verlag GmbH& Co. KGaA, 2005. [40] J. March, Advanced Organic Chemistry – Reactions, Mechanisms, and Structure, third revised ed., John Wiley & Sons, 1985. [41] H.G.O. Becker, W. Berger, G. Domschke, Organikum, 22nd ed., Wiley-VCH, 2004. [42] N. Nishimura, T. Sueyoshi, H. Yamanaka, E. Imai, S. Yamamoto, S. Hasegawa, Bull. Chem. Soc. Jpn. 49 (1976) 1381–1387. [43] C. Barrett, A. Natansohn, P. Rochon, Chem. Mater. 7 (1995) 899–903. [44] S. Altmaier, Modifizierung und Funktionalisierung geordneter mesostrukturierter Materialien, Dissertation 2003, Fachbereich Chemie, Universität Hannover, Hannover. [45] S.Y. Grebenkin, B.V. Bol’shakov, Chem. Phys. 234 (1998) 239–248. [46] S.Y. Grebenkin, B.V. Bol’shakov, J. Photochem. Photobiol. A: Chem. 122 (1999) 205–209. [47] L. Angiolini, D. Caretti, C. Carlini, J. Polym. Sci., Part A: Polym. Chem. 32 (1994) 1159–1168. [48] S.Y. Grebenkin, B.V. Bol’shakov, J. Polym. Sci., Part B: Polym. Phys. 37 (1999) 1753–1761.