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Photosensitive crown ether–siloxane copolymers bearing azobenzene chromophores
European Polymer Journal 38 (2002) 2265–2270 www.elsevier.com/locate/europolj
Photosensitive crown ether–siloxane copolymers bearing azobenzene chromophores Rodinel Ardeleanu *, Anton Airinei, Gabriela Sacarescu, Liviu Sacarescu ‘‘Petru Poni’’ Institute of Macromolecular Chemistry, Aleea Grigore Ghica Voda 41A, 6600 Iasi, Romania Received 21 September 2001; received in revised form 1 April 2002; accepted 17 April 2002
Abstract This paper presents the synthesis procedure of new photosensitive siloxane–crown ether copolymers using the polycondensation reaction of an oligosiloxane with COCl end groups and bisazo derivatives of dibenzo-18-crown-6 polyether. The azodiphenol compounds were prepared via the coupling reaction of 4,40 -diamine-DB18C6 diazonium salt with phenol or 1-naphthol. The polymers were characterized by IR absorption and 1 H-NMR spectra as well as by thermal analysis and GPC. A study of the photoisomerization process as well as the thermal relaxation of azoaromatic chromophores in the polysiloxane–crown ether copolymer in dimethyl sulfoxide solution and of the corresponding complexes with KSCN was also presented. Ó 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction A wide variety of polymers and copolymers containing photochromic molecules such as azobenzene, have been prepared and their photofunctional properties have been investigated in solution, films or membranes [1–6]. If a photosensitive moiety, that either it is incorporated into a polymer chain or not, is combined with a crown ether, conformational changes in response to photoirradiation will occur as well as a change in the complexation ability. In the case of photorensponsive macrocycle systems, it is possible to control the ion-binding and transport processes by an on–off light switch, which would lead to the photoregulation of these processes [7–11]. The photocontrol of the cation binding could be made from light-mediated reversible isomerization of the azobenzene moiety. Thus, the photoisomerization of azobenzene moieties induces conformational changes in the polymer chain, which in turn will determine macroscopic modification of the physical and chemical properties of surroundings. In the trans conformation of the
*
Corresponding author. Tel.: +40-32-217454; fax: +40-32211299. E-mail address: [email protected] (R. Ardeleanu).
azobenzene chromophore the crown unit has weak coordination ability for metal cations. UV irradiation produces the trans–cis isomerization of the chromophore giving a supramolecular conformation with enhanced coordination ability towards metal cations. In our recent studies a series of siloxane copolymers of polyamide or polyester type containing crown ethers has been synthesized [12–14]. The aim of this work is the preparation and photochemical behaviour of some azocrowned polysiloxanes and their corresponding complexes with metal ions. These studies may be useful for molecular design and development of new sensitive cation-optical sensors.
2. Experimental section 2.1. Materials 4,40 -Diaminodibenzo-18-crown-6, a; x-bis(3-carboxypropyl)poly(dimethylsiloxane) (Mn ¼ 1500) were prepared as reported elsewhere [12,13]. Triethylamine was dried over KOH before distilling under nitrogen atmosphere. 1,2-Dichloroethane was kept and distilled over P2 O5 before use. All other reagents were purchased from commercial supplies and used as received.
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2.2. Measurements IR absorption spectra were run on a Carl Zeiss Jena Specord M80 spectrophotometer with KBr pellets. Electronic absorption spectra were obtained on a Carl Zeiss Jena Specord M42 spectrophotometer in DMSO solutions using 10 mm quartz cells fitted with poly(tetrafluoroethylene) stoppers. 1 H-NMR measurements were obtained with a JEOL C-80 HL spectrometer using DMSO-d6 as solvent and tetramethylsilane as an internal standard. Thermogravimetrical analysis (TGA) was made using a MOM derivatograph in air, at a heating rate of 12 °C/min. The viscosities were measured with a Ubbelohde suspended-level viscometer in DMF at 25 °C (c ¼ 0:5%). GPC measurements, in dimethylformamide, were conducted on a PL MD-950 apparatus equipped with an evaporative mass detector and two PL gel 5 lm columns using polystyrene as standard. A 350 W high-pressure mercury lamp fitted with a 365 nm glass filter was used for sample irradiation. The initial absorbance of the p–p absorption band was kept between 0.9 and 1.0. 2.3. Synthesis of a,x-bis[3-(chlorocarbonyl)propyl] poly(dimethylsiloxane) To a solution of a; x-bis(3-carboxypropyl)poly(dimethylsiloxane) (31.9 g) in anhydrous dichloroethane (50 ml), prepared under nitrogen, was added dropwise thionyl chloride (170 g, 10.5 ml). The reaction mixture was heated to reflux for 7 h. The solvent and thionyl chloride excess were removed by vacuum distillation. The product was obtained as a transparent, light viscous liquid in quantitative yield.
Elemental analysis calculated for: C32 H32 N4 O8 : C% ¼ 64:0; H% ¼ 5:33; N% ¼ 9:33. Found: C% ¼ 63:64%; H% ¼ 5:62; N% ¼ 9:81.
2.5. Synthesis of 4,40 -bis(4-hydroxy-1-naphthylazo)dibenzo-18-crown-6 (CEAz2) 0.342 g (0.877 mmol) of bis(4-amino)dibenzo-18crown-6 were dissolved in water (7 ml) which contained concentrated HCl (0.41 ml). This reaction mixture was cooled in an ice bath and then 1.5 M NaNO2 solution (2.03 mol) was added maintaining the temperature below 5 °C. Separately, a solution consisting of distilled water (10 ml), 1-naphthol (0.286 g, 1.98 mmol), 10% NaOH (0.8 ml), and Na2 CO3 (1.06 g) was prepared and cooled to 0 °C. The diazonium salt solution was added dropwise, under continuous stirring to the last solution prepared, maintaining the reaction temperature below 0 °C. Then, the stirring was continued at room temperature for 2 h. The resulting suspension was filtered, washed with water and finally dried in vacuum. Yield: 0.529 g (86.2%) of a red solid, m:p: > 340 °C. IR (KBr, m cm1 ): 770 (naphthalene); 1130; 1235–1260 (C–O); 1420 (C@C); 1420, 1560, 1600 (C@C); 1580 (Ar– N@N–); 2880–2960 (CH2 , CH3 ); 3080 (Car–H); 1 H-NMR (DMSO, d ppm): 3.9–4.1 (m, 16H, Ha;b , –O–CH2 –CH2 –O); 6.66–8.25 (m, 18Har, Hc;d;e;...i arom). Elemental analysis calculated for C40 H36 N4 O8 : C% ¼ 68:57; H% ¼ 5:14; N% ¼ 8:0. Found: C% ¼ 68:22; H% ¼ 4:95; N% ¼ 8:36;
2.6. Synthesis of photosensitive polyester PSAz1 2.4. Synthesis of 4,40 -bis(4-hydroxyphenylazo)dibenzo18-crown-6 (CEAz1) 4,40 -Diaminodibenzo-18-crown-6 (0.725 g, 1.86 mmol) was added to a water solution (10 ml) of HCl (0.93 ml). The solution was cooled in an ice bath and then a 1.5 M NaNO2 solution (2.6 ml) was added. The reaction mixture was poured dropwise over a solution consisting of distilled water (12.5 ml), NaOH (0.224 g), Na2 CO3 (1.19 g), dimethylformamide (DMF, 2.5 ml) and phenol (0.399 g, 4.25 mmol) with rapid stirring and cooled to 0 °C. The resulting yellowish suspension was stirred bellow 0 °C for 2 h, filtered, washed with water and dried in a vacuum. Yield: 0.977 g (g ¼ 87:6%) of a yellow solid powder, m:p: ¼ 238–240 °C. IR (KBr, m, cm1 ): 3300–3500 (O–H); 2880, 2960 (CH2 , CH3 ); 1600, 1513, 1453 (C@C); 1260, 1120 (C–O). 1 H-NMR (DMSO; d ppm): 3.9–4.1 (d, 16Ha;b; –O– CH2 –CH2 –O–); 6.70–6.85, (d, 4Hg arom); 7.03 (s, 2He arom); 7.27 (s, 2Hc arom); 7.32–7.42 (d, 2Hd arom) 7.50–7.65 (d, 4Hf arom).
To an anhydrous DMF solution (20 ml) of 4,40 -bis(4hydroxyphenylazo)dibenzo-18-crown-6 (0.9 g, 1.5 mmol) was added anhydrous triethylamine (0.42 ml). Then, a dry THF solution (20 ml) containing a; x-bis[3-(chlorocarbonyl)propyl]poly(dimethylsiloxane) (2.43 g, 1.5 mmol) was added dropwise. The mixture was stirred at room temperature for 25 h. After the filtration of triethylamine hydrochloride, the reaction mixture was poured dropwise in distilled water. The yellow precipitate obtained in this way was filtered, washed with methanol and dried in vacuum. Yield: 2.76 g (83%) of product. IR (KBr, m cm1 ): 805, 1265 (Si–CH3 ), 1070 (Si–O–Si); 1720 (C@O); 1458, 1513, 1600 (C@Car); 1130, 1260 (–C–O–C–); 2880, 3080 (C–H). 1 H-NMR (DMSO, d ppm): 0.01 (s, 102 H, –O– Si(CH3 )2 ); 0.5 (s, 12H, –O–Si(CH3 )2 –CH2 –); 0.88 (m, 4H, BSi–CH2 –); 1.55 (m, 4H, BSi–CH2 –CH2 –); 2.20 (t, 4H, BSi–CH2 –CH2 –CH2 – 3.88 (m, 16H, –O–CH2 – CH2 –O–); 6.70–7.65 (m, 14Har, Hcg arom).
R. Ardeleanu et al. / European Polymer Journal 38 (2002) 2265–2270
Elemental analysis calculated: C% ¼ 41:9; H% ¼ 7:0; N% ¼ 2:6; Si% ¼ 23:4. Found: C% ¼ 41:3; H% ¼ 7:12; N% ¼ 2:6; Si% ¼ 24:1. 2.7. Synthesis of photosensitive polyester PSAz2 4,40 -Bis(4-hydroxynaphthylazo)dibenzo-18-crown-6 (1.05 g, 1.5 mmol) was dissolved in anhydrous DMF (20 ml) and anhydrous triethylamine (0.42 ml) was added. Then, a solution of a; x-bis[3-(chlorocarbonyl)propyl]poly(dimethylsiloxane) (2.43 g, 1.5 mmol) in dry tetrahydrofuran (20 ml) was poured, dropwise, under stirring. The mixture was stirred at room temperature for 25 h and was treated in a similar manner to that described in PSAz1 synthesis. Yield: 2.67 g (77%) of redcherry solid product. IR (KBr, m cm1 ): 805, 1265 (Si–CH3 ); 1070 (Si–O–Si); 1130, 1260 (–C–O); 1458, 1513, 1600 (C–Car); 1720 (C@O), 2880, 3080 (C–H).
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1 H-NMR (DMSO, d ppm): 0.01 (s, 102H, –O– Si(CH3 )2 ); 0.5 (s, 12H, –O–Si(CH3 )2 –CH2 –); 0.88 (m, 4H, O–Si–CH2 –CH2 –CH2 ); 1.55 (m, 4H, O–Si–CH2 – CH2 –CH2 ); 2.20 (t, 4H, O–Si–CH2 –CH2 –CH2 ); 3.88 (m, 16H, –O–Si–CH2 –CH2 –O); 6.66–8.25 (m, 18H, Har, Hc;d arom).
3. Results and discussion The photosensitive monomers containing the azobenzene chromophore and the dibenzo-18-crown-6 moiety were prepared by the synthetic procedure according to the reaction (Scheme 1). Phenol or a-naphthol was coupled with the diazonium salt of 4,40 -diaminodibenzo-18-crown-6 in the presence of triethylamine to yield CEAz1 and CEAz2, respectively. During azo coupling reaction the temperature was maintained within 0–5 °C and pH ¼ 8. The crowned azobenzene siloxane polymers were obtained by solution polycondensation reaction of
Scheme 1.
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Scheme 2.
a;x-bis(3-chloroformylpropyl)polydimethylsiloxane and azodiphenols CEAz1 and CEAz2 as illustrated in (Scheme 2). Azodiphenols, CEAz1 and CEAz2, have a good solubility in polar solvents, while the siloxane oligomer is soluble in chlorinated or ethereal solvents. In these conditions, the DMF–THF mixture was chosen as solvent in the synthesis of the azocrowned siloxane polyesters. The polycondensation reaction takes place at room temperature and the separation of the reaction products was achieved by the dilution of the reaction mixture with distilled water. Siloxane–crown ether photosensitive copolymers PSAz1 and PSAz2 contain segments with different chemical structures. In consequence the IR and 1 H-NMR spectra displayed very specific absorption bands and protons signals respectively. From the protons signals ratio assigned to the polyethereal cycle (3.28 ppm) and those of the methyl groups (0.01 ppm) in the 1 H-NMR spectrum of PSAz1, it is possible to calculate the number of the silicon atoms from the siloxane chain. Through this method the combination ratio of the two monomers was estimated. The intrinsic viscosities of the two polymers determined in DMF are 0.27 (PSAz1) and 0.23 dL/g (PSAz2), respectively. The number–average molecular weights, determined by GPC, of the copolymers were 8200 and 6585 g/mol with a narrow molecular weight distribution, Mw =Mn ¼ 1:231 respectively 1.375. The thermal stability is one of the most important properties, which was taken into account in the copolymers synthesis. Both azodiphenols ECAz1, ECAz2 and copolymers PSAz1 and PSAz2 were analysed (Fig. 1). The incorporation of siloxane segments in the main polymer chain lead to a slight improvement of the thermal properties only in the case of PSAz1 copolymer. Azobenzene derivatives are known to undergo photoisomerization from the thermodynamically more stable trans to the cis configuration when irradiated with
Fig. 1. TGA Analysis of copolymers and azocrowned ethers (- - -) PSAz1; ( ) PSAz2; (––) ECAz1; (---) ECAz2.
UV light, while reverse process, cis–trans isomerization occurs through a thermally path or upon irradiation with visible light (k > 430 nm) [15–17]. The photoisomerization of azobenzene chromophore results in a temporary change of the geometrical shape and polarity. Thus, the trans configuration is planar with zero dipole moment, while in the cis-configuration the phenyl rings are not coplanar and the resulting dipole moment is 3.0 D [1,4]. Reversible trans–cis isomerization reactions in these azocrowned siloxane polymers can be readily monitored by electronic absorption spectra. The absorption band at about 374 nm of the siloxane polyester PAz1 originates from the p–p transition of trans-azobenzene unit. The small shoulder at about 440 nm can be assigned to n–p transition [4,15]. In order to study the photoisomerization of azobenzene chromophore, the DMSO solutions of PSAz1 were irradiated with 365 nm UV light. In Fig. 2
R. Ardeleanu et al. / European Polymer Journal 38 (2002) 2265–2270
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Fig. 2. Changes in electronic absorption spectra of PSAz-1 upon UV irradiation (0, 5, 15, 25, 60, 90, 180, 240, 300 s).
the UV–VIS absorption spectra of PSAz1 are shown prior and subsequent to UV exposure. Correspondingly, shape, intensity and the position of absorption band were modified. The UV irradiation causes a decrease of the p–p absorption band intensity until a photostationary state is reached within 10 min, when approximately 65– 70% of the azobenzene moieties occupy the higher energy cis configuration. The position of the p–p absorption band of PSAz1 showed a small hypsochromic shift during irradiation. Upon photoisomerization isosbestic points at 293 and 445 nm are also observed, confirming that two absorbing species (trans and cis form of azobenzene chromophore) are present in solution. The azocrowned siloxane polyester containing naphthalene groups exhibit an absorption band at 414 nm and a shoulder at about 452 nm. During 405 nm light irradiation, the absorbance at 414 nm is very little modified, after 150 nm of irradiation the ratio A=A0 becomes 0.93. Irradiation with polychromatic light results in the photodecoloration of this sample. In the case of low molecular weight compounds, ECAz1 and ECAz2, the UV irradiation produces a very fast recovery process almost instantaneously to the initial state. Kinetic photoisomerization data can be expressed by a first-order rate equation lnðA0 A1 =At A1 Þ ¼ kt, where A0 , At and A/ are the absorbance values at times zero, t and /, respectively and k is the rate constant of trans–cis photoisomerization process. The plot of lnðA0 A/ Þ=ðAt A/ Þ versus irradiation time for trans– cis isomerization of azobenzene chromophore is exhibited in Fig. 3 for PSAz1. As shown in Fig. 4, the isomerization process can be reversed upon heating at 70 °C. The absorbance of the PSAz1 sample at photostationary state steadily increases up to the starting value before irradiation, confirming thus the photochromic properties of this siloxane copolymer (Fig. 4). Thermal cis–trans isomerization was also investigated kinetically and the cis isomers can reversibly convert to trans isomers following a first-order kinetics (Fig. 5). The thermal recovery requires more higher times (about 100 min) suggesting that in this process the
Fig. 3. First-order plots for trans–cis photoisomerization in DMSO solutions of azocrowned siloxane polymers. (1––PSAz1; 2––PSAz1–KSCN ¼ 1:1; 3––PSAz1–KSCN ¼ 2:1).
Fig. 4. Variation of electronic absorption spectra of PSAz1 on thermal recovery (2, 8, 15, 23, 31, 40, 57 min).
Fig. 5. Kinetics of thermal cis–trans isomerization of azocrowned siloxane polymers. (1: PSAz1; 2: PSAz1–KSCN ¼ 1:2; 3: PSAz1–KSCN ¼ 1:1).
azobenzene moieties can isomerize freely taking into account the fact that the process kinetics is described by a straight line.
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The isomerization process of the azobenzene moieties may be affected on the cation complexing properties of the crown ether unit. The photoinduced changes in the behaviour of PSAz1 complexed with KSCN were also followed. Two complexes having the ratio PSAz1: KSCN of 1:1 and 1:2 were obtained. The first order rate constant for thermal isomerization was determined spectrophotometrically by the increase in the band intensity corresponding to trans isomer. The incorporation of metal cation into the crown ether cavity leads to the increase of the constant rate of the trans–cis photoisomerization process. Examination of these data reveals the following order of the constant rate: PSAz1 < PSAz1–KSCN ð1 : 1Þ < PSAz1–KSCN ð1 : 2Þ (Fig. 3) The photoisomerization rate of PSAz1 is lower than of the complexed polyesters due to the conformational reorganization of the polyether macrocycle and to increase of lipophilicity by the participation of the heteroatom in the complexation between crown ether and alkali metal cation. It may be remarked that the complex formation favours a higher mobility of the siloxane units in the polymer chain. References [1] Kumar GS, Neckers DC. Chem Rev 1989;89:1915. [2] Smets G. Adv Polym Sci 1983;50:18.
[3] Kinoshita T. Prog Polym Sci 1995;20:527. [4] Kumar GS. Azo functional polymers. Lancaster: Technomic; 1992. [5] Xie S, Natansolm A, Rochon P. Chem Mater 1993;5: 403. [6] Shinkai S, Miyazaki K, Manabe O. J Chem Soc Perkin Trans 1987;1:449. [7] Tokuhisa H, Yokoyama M, Kimura K. Bull Chem Soc Jpn 1996;69:2123. [8] Inoue Y, Gokel GW. Cation binding by macrocycles. New York: Marcel Dekker, Inc.; 1990. [9] Shinkai S, Yoshioka A, Nakayama H, Manabe O. J Chem Soc Perkin Trans 1990;2:1905. [10] Balzani V, de Cola L, Prodi L, Scandola F. Pure Appl Chem 1990;62:1457. [11] Shinkai S, Kinda H, Ishihara M, Manabe O. J Polym Sci, Polym Chem Ed 1983;21:3525. [12] Ardeleanu R, Voiculescu N, Marcu M, Sacarescu L, Sacarescu G. J Macromol Sci Pure Appl Chem 1997; A34:525. [13] Ardeleanu R, Voiculescu N, Marcu M, Roman G, Buchidau C, Sacarescu L, et al. Macromol Rapid Commun 1997;18:739. [14] Ardeleanu R, Airinei A, Sacarescu L, Sacarescu G, Voiculescu N, Roman G. Proc Int Conf Mater Sci Eng Brasov 1999;4:473. [15] Rau H. In: Rabek JF, editor. Photochemistry and photophysics, vol. 2. Boca Raton, FL: CRC Press; 1990. [16] Fischer E. J Am Chem Soc 1960;82:3249. [17] Shen YQ, Rau H. Makromol Chem 1991;192:945.
Photosensitive crown ether–siloxane copolymers bearing azobenzene chromophores Rodinel Ardeleanu *, Anton Airinei, Gabriela Sacarescu, Liviu Sacarescu ‘‘Petru Poni’’ Institute of Macromolecular Chemistry, Aleea Grigore Ghica Voda 41A, 6600 Iasi, Romania Received 21 September 2001; received in revised form 1 April 2002; accepted 17 April 2002
Abstract This paper presents the synthesis procedure of new photosensitive siloxane–crown ether copolymers using the polycondensation reaction of an oligosiloxane with COCl end groups and bisazo derivatives of dibenzo-18-crown-6 polyether. The azodiphenol compounds were prepared via the coupling reaction of 4,40 -diamine-DB18C6 diazonium salt with phenol or 1-naphthol. The polymers were characterized by IR absorption and 1 H-NMR spectra as well as by thermal analysis and GPC. A study of the photoisomerization process as well as the thermal relaxation of azoaromatic chromophores in the polysiloxane–crown ether copolymer in dimethyl sulfoxide solution and of the corresponding complexes with KSCN was also presented. Ó 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction A wide variety of polymers and copolymers containing photochromic molecules such as azobenzene, have been prepared and their photofunctional properties have been investigated in solution, films or membranes [1–6]. If a photosensitive moiety, that either it is incorporated into a polymer chain or not, is combined with a crown ether, conformational changes in response to photoirradiation will occur as well as a change in the complexation ability. In the case of photorensponsive macrocycle systems, it is possible to control the ion-binding and transport processes by an on–off light switch, which would lead to the photoregulation of these processes [7–11]. The photocontrol of the cation binding could be made from light-mediated reversible isomerization of the azobenzene moiety. Thus, the photoisomerization of azobenzene moieties induces conformational changes in the polymer chain, which in turn will determine macroscopic modification of the physical and chemical properties of surroundings. In the trans conformation of the
*
Corresponding author. Tel.: +40-32-217454; fax: +40-32211299. E-mail address: [email protected] (R. Ardeleanu).
azobenzene chromophore the crown unit has weak coordination ability for metal cations. UV irradiation produces the trans–cis isomerization of the chromophore giving a supramolecular conformation with enhanced coordination ability towards metal cations. In our recent studies a series of siloxane copolymers of polyamide or polyester type containing crown ethers has been synthesized [12–14]. The aim of this work is the preparation and photochemical behaviour of some azocrowned polysiloxanes and their corresponding complexes with metal ions. These studies may be useful for molecular design and development of new sensitive cation-optical sensors.
2. Experimental section 2.1. Materials 4,40 -Diaminodibenzo-18-crown-6, a; x-bis(3-carboxypropyl)poly(dimethylsiloxane) (Mn ¼ 1500) were prepared as reported elsewhere [12,13]. Triethylamine was dried over KOH before distilling under nitrogen atmosphere. 1,2-Dichloroethane was kept and distilled over P2 O5 before use. All other reagents were purchased from commercial supplies and used as received.
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2.2. Measurements IR absorption spectra were run on a Carl Zeiss Jena Specord M80 spectrophotometer with KBr pellets. Electronic absorption spectra were obtained on a Carl Zeiss Jena Specord M42 spectrophotometer in DMSO solutions using 10 mm quartz cells fitted with poly(tetrafluoroethylene) stoppers. 1 H-NMR measurements were obtained with a JEOL C-80 HL spectrometer using DMSO-d6 as solvent and tetramethylsilane as an internal standard. Thermogravimetrical analysis (TGA) was made using a MOM derivatograph in air, at a heating rate of 12 °C/min. The viscosities were measured with a Ubbelohde suspended-level viscometer in DMF at 25 °C (c ¼ 0:5%). GPC measurements, in dimethylformamide, were conducted on a PL MD-950 apparatus equipped with an evaporative mass detector and two PL gel 5 lm columns using polystyrene as standard. A 350 W high-pressure mercury lamp fitted with a 365 nm glass filter was used for sample irradiation. The initial absorbance of the p–p absorption band was kept between 0.9 and 1.0. 2.3. Synthesis of a,x-bis[3-(chlorocarbonyl)propyl] poly(dimethylsiloxane) To a solution of a; x-bis(3-carboxypropyl)poly(dimethylsiloxane) (31.9 g) in anhydrous dichloroethane (50 ml), prepared under nitrogen, was added dropwise thionyl chloride (170 g, 10.5 ml). The reaction mixture was heated to reflux for 7 h. The solvent and thionyl chloride excess were removed by vacuum distillation. The product was obtained as a transparent, light viscous liquid in quantitative yield.
Elemental analysis calculated for: C32 H32 N4 O8 : C% ¼ 64:0; H% ¼ 5:33; N% ¼ 9:33. Found: C% ¼ 63:64%; H% ¼ 5:62; N% ¼ 9:81.
2.5. Synthesis of 4,40 -bis(4-hydroxy-1-naphthylazo)dibenzo-18-crown-6 (CEAz2) 0.342 g (0.877 mmol) of bis(4-amino)dibenzo-18crown-6 were dissolved in water (7 ml) which contained concentrated HCl (0.41 ml). This reaction mixture was cooled in an ice bath and then 1.5 M NaNO2 solution (2.03 mol) was added maintaining the temperature below 5 °C. Separately, a solution consisting of distilled water (10 ml), 1-naphthol (0.286 g, 1.98 mmol), 10% NaOH (0.8 ml), and Na2 CO3 (1.06 g) was prepared and cooled to 0 °C. The diazonium salt solution was added dropwise, under continuous stirring to the last solution prepared, maintaining the reaction temperature below 0 °C. Then, the stirring was continued at room temperature for 2 h. The resulting suspension was filtered, washed with water and finally dried in vacuum. Yield: 0.529 g (86.2%) of a red solid, m:p: > 340 °C. IR (KBr, m cm1 ): 770 (naphthalene); 1130; 1235–1260 (C–O); 1420 (C@C); 1420, 1560, 1600 (C@C); 1580 (Ar– N@N–); 2880–2960 (CH2 , CH3 ); 3080 (Car–H); 1 H-NMR (DMSO, d ppm): 3.9–4.1 (m, 16H, Ha;b , –O–CH2 –CH2 –O); 6.66–8.25 (m, 18Har, Hc;d;e;...i arom). Elemental analysis calculated for C40 H36 N4 O8 : C% ¼ 68:57; H% ¼ 5:14; N% ¼ 8:0. Found: C% ¼ 68:22; H% ¼ 4:95; N% ¼ 8:36;
2.6. Synthesis of photosensitive polyester PSAz1 2.4. Synthesis of 4,40 -bis(4-hydroxyphenylazo)dibenzo18-crown-6 (CEAz1) 4,40 -Diaminodibenzo-18-crown-6 (0.725 g, 1.86 mmol) was added to a water solution (10 ml) of HCl (0.93 ml). The solution was cooled in an ice bath and then a 1.5 M NaNO2 solution (2.6 ml) was added. The reaction mixture was poured dropwise over a solution consisting of distilled water (12.5 ml), NaOH (0.224 g), Na2 CO3 (1.19 g), dimethylformamide (DMF, 2.5 ml) and phenol (0.399 g, 4.25 mmol) with rapid stirring and cooled to 0 °C. The resulting yellowish suspension was stirred bellow 0 °C for 2 h, filtered, washed with water and dried in a vacuum. Yield: 0.977 g (g ¼ 87:6%) of a yellow solid powder, m:p: ¼ 238–240 °C. IR (KBr, m, cm1 ): 3300–3500 (O–H); 2880, 2960 (CH2 , CH3 ); 1600, 1513, 1453 (C@C); 1260, 1120 (C–O). 1 H-NMR (DMSO; d ppm): 3.9–4.1 (d, 16Ha;b; –O– CH2 –CH2 –O–); 6.70–6.85, (d, 4Hg arom); 7.03 (s, 2He arom); 7.27 (s, 2Hc arom); 7.32–7.42 (d, 2Hd arom) 7.50–7.65 (d, 4Hf arom).
To an anhydrous DMF solution (20 ml) of 4,40 -bis(4hydroxyphenylazo)dibenzo-18-crown-6 (0.9 g, 1.5 mmol) was added anhydrous triethylamine (0.42 ml). Then, a dry THF solution (20 ml) containing a; x-bis[3-(chlorocarbonyl)propyl]poly(dimethylsiloxane) (2.43 g, 1.5 mmol) was added dropwise. The mixture was stirred at room temperature for 25 h. After the filtration of triethylamine hydrochloride, the reaction mixture was poured dropwise in distilled water. The yellow precipitate obtained in this way was filtered, washed with methanol and dried in vacuum. Yield: 2.76 g (83%) of product. IR (KBr, m cm1 ): 805, 1265 (Si–CH3 ), 1070 (Si–O–Si); 1720 (C@O); 1458, 1513, 1600 (C@Car); 1130, 1260 (–C–O–C–); 2880, 3080 (C–H). 1 H-NMR (DMSO, d ppm): 0.01 (s, 102 H, –O– Si(CH3 )2 ); 0.5 (s, 12H, –O–Si(CH3 )2 –CH2 –); 0.88 (m, 4H, BSi–CH2 –); 1.55 (m, 4H, BSi–CH2 –CH2 –); 2.20 (t, 4H, BSi–CH2 –CH2 –CH2 – 3.88 (m, 16H, –O–CH2 – CH2 –O–); 6.70–7.65 (m, 14Har, Hcg arom).
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Elemental analysis calculated: C% ¼ 41:9; H% ¼ 7:0; N% ¼ 2:6; Si% ¼ 23:4. Found: C% ¼ 41:3; H% ¼ 7:12; N% ¼ 2:6; Si% ¼ 24:1. 2.7. Synthesis of photosensitive polyester PSAz2 4,40 -Bis(4-hydroxynaphthylazo)dibenzo-18-crown-6 (1.05 g, 1.5 mmol) was dissolved in anhydrous DMF (20 ml) and anhydrous triethylamine (0.42 ml) was added. Then, a solution of a; x-bis[3-(chlorocarbonyl)propyl]poly(dimethylsiloxane) (2.43 g, 1.5 mmol) in dry tetrahydrofuran (20 ml) was poured, dropwise, under stirring. The mixture was stirred at room temperature for 25 h and was treated in a similar manner to that described in PSAz1 synthesis. Yield: 2.67 g (77%) of redcherry solid product. IR (KBr, m cm1 ): 805, 1265 (Si–CH3 ); 1070 (Si–O–Si); 1130, 1260 (–C–O); 1458, 1513, 1600 (C–Car); 1720 (C@O), 2880, 3080 (C–H).
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1 H-NMR (DMSO, d ppm): 0.01 (s, 102H, –O– Si(CH3 )2 ); 0.5 (s, 12H, –O–Si(CH3 )2 –CH2 –); 0.88 (m, 4H, O–Si–CH2 –CH2 –CH2 ); 1.55 (m, 4H, O–Si–CH2 – CH2 –CH2 ); 2.20 (t, 4H, O–Si–CH2 –CH2 –CH2 ); 3.88 (m, 16H, –O–Si–CH2 –CH2 –O); 6.66–8.25 (m, 18H, Har, Hc;d arom).
3. Results and discussion The photosensitive monomers containing the azobenzene chromophore and the dibenzo-18-crown-6 moiety were prepared by the synthetic procedure according to the reaction (Scheme 1). Phenol or a-naphthol was coupled with the diazonium salt of 4,40 -diaminodibenzo-18-crown-6 in the presence of triethylamine to yield CEAz1 and CEAz2, respectively. During azo coupling reaction the temperature was maintained within 0–5 °C and pH ¼ 8. The crowned azobenzene siloxane polymers were obtained by solution polycondensation reaction of
Scheme 1.
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Scheme 2.
a;x-bis(3-chloroformylpropyl)polydimethylsiloxane and azodiphenols CEAz1 and CEAz2 as illustrated in (Scheme 2). Azodiphenols, CEAz1 and CEAz2, have a good solubility in polar solvents, while the siloxane oligomer is soluble in chlorinated or ethereal solvents. In these conditions, the DMF–THF mixture was chosen as solvent in the synthesis of the azocrowned siloxane polyesters. The polycondensation reaction takes place at room temperature and the separation of the reaction products was achieved by the dilution of the reaction mixture with distilled water. Siloxane–crown ether photosensitive copolymers PSAz1 and PSAz2 contain segments with different chemical structures. In consequence the IR and 1 H-NMR spectra displayed very specific absorption bands and protons signals respectively. From the protons signals ratio assigned to the polyethereal cycle (3.28 ppm) and those of the methyl groups (0.01 ppm) in the 1 H-NMR spectrum of PSAz1, it is possible to calculate the number of the silicon atoms from the siloxane chain. Through this method the combination ratio of the two monomers was estimated. The intrinsic viscosities of the two polymers determined in DMF are 0.27 (PSAz1) and 0.23 dL/g (PSAz2), respectively. The number–average molecular weights, determined by GPC, of the copolymers were 8200 and 6585 g/mol with a narrow molecular weight distribution, Mw =Mn ¼ 1:231 respectively 1.375. The thermal stability is one of the most important properties, which was taken into account in the copolymers synthesis. Both azodiphenols ECAz1, ECAz2 and copolymers PSAz1 and PSAz2 were analysed (Fig. 1). The incorporation of siloxane segments in the main polymer chain lead to a slight improvement of the thermal properties only in the case of PSAz1 copolymer. Azobenzene derivatives are known to undergo photoisomerization from the thermodynamically more stable trans to the cis configuration when irradiated with
Fig. 1. TGA Analysis of copolymers and azocrowned ethers (- - -) PSAz1; ( ) PSAz2; (––) ECAz1; (---) ECAz2.
UV light, while reverse process, cis–trans isomerization occurs through a thermally path or upon irradiation with visible light (k > 430 nm) [15–17]. The photoisomerization of azobenzene chromophore results in a temporary change of the geometrical shape and polarity. Thus, the trans configuration is planar with zero dipole moment, while in the cis-configuration the phenyl rings are not coplanar and the resulting dipole moment is 3.0 D [1,4]. Reversible trans–cis isomerization reactions in these azocrowned siloxane polymers can be readily monitored by electronic absorption spectra. The absorption band at about 374 nm of the siloxane polyester PAz1 originates from the p–p transition of trans-azobenzene unit. The small shoulder at about 440 nm can be assigned to n–p transition [4,15]. In order to study the photoisomerization of azobenzene chromophore, the DMSO solutions of PSAz1 were irradiated with 365 nm UV light. In Fig. 2
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Fig. 2. Changes in electronic absorption spectra of PSAz-1 upon UV irradiation (0, 5, 15, 25, 60, 90, 180, 240, 300 s).
the UV–VIS absorption spectra of PSAz1 are shown prior and subsequent to UV exposure. Correspondingly, shape, intensity and the position of absorption band were modified. The UV irradiation causes a decrease of the p–p absorption band intensity until a photostationary state is reached within 10 min, when approximately 65– 70% of the azobenzene moieties occupy the higher energy cis configuration. The position of the p–p absorption band of PSAz1 showed a small hypsochromic shift during irradiation. Upon photoisomerization isosbestic points at 293 and 445 nm are also observed, confirming that two absorbing species (trans and cis form of azobenzene chromophore) are present in solution. The azocrowned siloxane polyester containing naphthalene groups exhibit an absorption band at 414 nm and a shoulder at about 452 nm. During 405 nm light irradiation, the absorbance at 414 nm is very little modified, after 150 nm of irradiation the ratio A=A0 becomes 0.93. Irradiation with polychromatic light results in the photodecoloration of this sample. In the case of low molecular weight compounds, ECAz1 and ECAz2, the UV irradiation produces a very fast recovery process almost instantaneously to the initial state. Kinetic photoisomerization data can be expressed by a first-order rate equation lnðA0 A1 =At A1 Þ ¼ kt, where A0 , At and A/ are the absorbance values at times zero, t and /, respectively and k is the rate constant of trans–cis photoisomerization process. The plot of lnðA0 A/ Þ=ðAt A/ Þ versus irradiation time for trans– cis isomerization of azobenzene chromophore is exhibited in Fig. 3 for PSAz1. As shown in Fig. 4, the isomerization process can be reversed upon heating at 70 °C. The absorbance of the PSAz1 sample at photostationary state steadily increases up to the starting value before irradiation, confirming thus the photochromic properties of this siloxane copolymer (Fig. 4). Thermal cis–trans isomerization was also investigated kinetically and the cis isomers can reversibly convert to trans isomers following a first-order kinetics (Fig. 5). The thermal recovery requires more higher times (about 100 min) suggesting that in this process the
Fig. 3. First-order plots for trans–cis photoisomerization in DMSO solutions of azocrowned siloxane polymers. (1––PSAz1; 2––PSAz1–KSCN ¼ 1:1; 3––PSAz1–KSCN ¼ 2:1).
Fig. 4. Variation of electronic absorption spectra of PSAz1 on thermal recovery (2, 8, 15, 23, 31, 40, 57 min).
Fig. 5. Kinetics of thermal cis–trans isomerization of azocrowned siloxane polymers. (1: PSAz1; 2: PSAz1–KSCN ¼ 1:2; 3: PSAz1–KSCN ¼ 1:1).
azobenzene moieties can isomerize freely taking into account the fact that the process kinetics is described by a straight line.
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The isomerization process of the azobenzene moieties may be affected on the cation complexing properties of the crown ether unit. The photoinduced changes in the behaviour of PSAz1 complexed with KSCN were also followed. Two complexes having the ratio PSAz1: KSCN of 1:1 and 1:2 were obtained. The first order rate constant for thermal isomerization was determined spectrophotometrically by the increase in the band intensity corresponding to trans isomer. The incorporation of metal cation into the crown ether cavity leads to the increase of the constant rate of the trans–cis photoisomerization process. Examination of these data reveals the following order of the constant rate: PSAz1 < PSAz1–KSCN ð1 : 1Þ < PSAz1–KSCN ð1 : 2Þ (Fig. 3) The photoisomerization rate of PSAz1 is lower than of the complexed polyesters due to the conformational reorganization of the polyether macrocycle and to increase of lipophilicity by the participation of the heteroatom in the complexation between crown ether and alkali metal cation. It may be remarked that the complex formation favours a higher mobility of the siloxane units in the polymer chain. References [1] Kumar GS, Neckers DC. Chem Rev 1989;89:1915. [2] Smets G. Adv Polym Sci 1983;50:18.
[3] Kinoshita T. Prog Polym Sci 1995;20:527. [4] Kumar GS. Azo functional polymers. Lancaster: Technomic; 1992. [5] Xie S, Natansolm A, Rochon P. Chem Mater 1993;5: 403. [6] Shinkai S, Miyazaki K, Manabe O. J Chem Soc Perkin Trans 1987;1:449. [7] Tokuhisa H, Yokoyama M, Kimura K. Bull Chem Soc Jpn 1996;69:2123. [8] Inoue Y, Gokel GW. Cation binding by macrocycles. New York: Marcel Dekker, Inc.; 1990. [9] Shinkai S, Yoshioka A, Nakayama H, Manabe O. J Chem Soc Perkin Trans 1990;2:1905. [10] Balzani V, de Cola L, Prodi L, Scandola F. Pure Appl Chem 1990;62:1457. [11] Shinkai S, Kinda H, Ishihara M, Manabe O. J Polym Sci, Polym Chem Ed 1983;21:3525. [12] Ardeleanu R, Voiculescu N, Marcu M, Sacarescu L, Sacarescu G. J Macromol Sci Pure Appl Chem 1997; A34:525. [13] Ardeleanu R, Voiculescu N, Marcu M, Roman G, Buchidau C, Sacarescu L, et al. Macromol Rapid Commun 1997;18:739. [14] Ardeleanu R, Airinei A, Sacarescu L, Sacarescu G, Voiculescu N, Roman G. Proc Int Conf Mater Sci Eng Brasov 1999;4:473. [15] Rau H. In: Rabek JF, editor. Photochemistry and photophysics, vol. 2. Boca Raton, FL: CRC Press; 1990. [16] Fischer E. J Am Chem Soc 1960;82:3249. [17] Shen YQ, Rau H. Makromol Chem 1991;192:945.