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Photochromism of liquid crystalline dendrimer with azobenzene terminal groups in solution
Reactive & Functional Polymers 67 (2007) 416–421
REACTIVE & FUNCTIONAL POLYMERS www.elsevier.com/locate/react
Photochromism of liquid crystalline dendrimer with azobenzene terminal groups in solution Jianqiang Liu a,b,*, Peng Ni b, Depeng Qiu b, Wanguo Hou b, Qizhen Zhang c a
b
School of Physics and Microelectronics, Shandong University, Ji’nan 250100, Shandong Province, PR China Key Laboratory of Education Ministry on Colloid and Interface Chemistry, Ji’nan 250100, Shandong Province, PR China c School of Chemistry and Chemical Engineering, Shandong University, Ji’nan 250100, Shandong Province, PR China Received 13 September 2005; accepted 28 March 2006 Available online 1 March 2007
Abstract The photochromism of a liquid crystalline dendrimer (LCD) with 36 hexyloxyazobenzene terminal groups in solution was described in this paper. The molar absorption coefficient, quantum yield, photoisomerization, photo back-isomerization, thermal back-isomerization and activation energy of LCD in solution are studied by UV/Vis absorption spectra. The results indicate that the photochromism, photo and thermal back-isomerization of LCD in chloroform (CHCl3) and tetrahydrofuran (THF) solutions are in accordance with the first order kinetics. The photochromism rate constants of LCD are 101 s1, it is 107 times larger than that of side-chain liquid crystalline polymers (LCP) containing the same azobenzene moieties. These results indicate that the dendritic structure does not significantly affect the photoisomerization activity of the azobenzene unit in its periphery. The kt/kc of LCD is less than that of azobenzene unit shows that the LCD has better photo-reversibility. So the liquid crystalline dendrimer has potential applications. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Photochromism; Liquid crystalline dendrimer; Photo back-isomerization; Thermal back-isomerization; Activation energy
1. Introduction Photochromism is a fascinating phenomenon and photochromic materials have recently attracted intense interest owing to their potential technological applications such as various photooptical devices and data recording media [1–4]. Liquid crystals are promising materials for optical switching * Corresponding author. Address: School of Physics and Microelectronics, Shandong University, Ji’nan 250100, Shandong Province, PR China. Tel.: +86 531 88565947; fax: +86 531 8564886. E-mail address: [email protected] (J. Liu).
and image storage because of their high resolution and sensitivity. In recent years, some advances in research dealing with the synthesis and study of optical properties of a series of photochromic LCs are considered [5–8]. Over the past few years there has been intense interest in the unique properties exhibited by dendrimers with a regular treelike array of branching units and numerous dendritic structures have been synthesized and studied [9– 12]. Quite recently, dendrimers with different designed functionalities have become objects of particular academic and practical interest because of their unique superbranched architectures, high densities of peripheral functionalities, symmetrical
1381-5148/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2006.03.022
J. Liu et al. / Reactive & Functional Polymers 67 (2007) 416–421
O
O
O
O
O
O
O
shapes, and monodispersity. Novel dendrimers with electroactive, photoactive, and recognition elements at the core, within the branches, or at the periphery of the dendritic structure have recently been reported as potential functional materials [13–19]. We have successfully carried out the molecular design and got a series of liquid crystalline carbosilane dendrimers with 4, 12 and 36 4-hexyloxy-40 hydroxyhexyloxy-azobenzene (HeA) units in the periphery [20–22]. The structure of the LCD with 36 azobenzene terminal groups was shown in Fig. 1, and the dendritic compound containing mesogenic and photochromic groups was structurally and functionally integrated system. It is well-known that azobenzene-type compounds undergo an efficient and fully reversible photoisomerization reaction, and thereby the presence of the azo photochromic group provides its sensitivity to the light (see Fig. 2). The photochromism of LCD in solution are briefly studied by the UV absorption spectrum in this context.
O
Si
O
O O
O
O O
O O O
O
Si
O
Si
O O
O
O O
O
O
Si
O Si O
O O
Si
O O O O
Si
Si
Si
O
O Si O O
Si Si
Si
O
O
O
Si
O
O O
O O
O O
O
O O
Si
Si O O
O
O O O
O O
O
O O
Si
O
O
O O
O
O
O
O
O
O
= C6H13O
OC6H13
/visible light
OC6H13
O
O Eisomer
N=N
UV light
2. Experimental 2.1. Materials and characterization The synthesis of the LCD was described in Ref. [22]. Elemental analysis of LCD Si17O108C912H1284 N72 was carried out with a Perkin-Elmer 240C auto elementary analyser: calculated: C, 70.84, H, 8.37, N, 6.52 (%); found: C, 70.56, H, 8.29, N, 6.39 (%). Infrared spectra were recorded on a Nicolet 5DX Fourier transform infrared photoacoustic spectral system (KBr/cm1): 2936, 2868 (–CH2–), 1601, 1580, 1500, 1472 (ph, N@N), 1246 (–OCH2–); 1H NMR was carried out with a Japan Joel FX902 (90 MHz, CDCl3, d ppm): 0.68–0.88 (m, 172H, SiCH2, CH3), 1.20–1.88 (m, 608H, CH2), 3.64–4.04 (m, 216H, OCH2), 6.80–7.88 (m, 288H, ph-H). This showed that the compound have expectant structure as Fig. 1. Polarizing optical microscope (POM, Seagull XPID polarizing optical microscope with Mettler FP80 and FP82 hot stage and controller) was used to observe the optical texture of the LCD. Upon slow heating and cooling during its liquid crystalline phase domain, black brush defects (Schlieren texture) and threads, typical of nematic phase are observed. The transition temperatures and the associated enthalpies and entropies of the samples was examined by means of differential scanning calorimetry (DSC, Perkin–Elmer 7 series thermal analysis system). X-ray diffraction analysis (XRD, D/max-cB diffractometer with Cu Ka radiation, ˚ ) studied on the mesophase have been k = 1.54 A carried out at ambient temperature. The resulting diffraction showed a slow peak in the little angle (2h < 10°) and a dispersion peak in the larger angle (2h = 20.1°). To make a comprehensive view on the results of POM, DSC and XRD hereinbefore, we can make sure that the LCD shows nematic phase and its phase behaviour was Cr90N105I113N75Cr [22]. 2.2. Photochromism experiment
N=N
Fig. 1. The structure of LCD.
N=N
417
Zisomer
Fig. 2. Reversible photoisomerization of LCD.
The photoirradiation was carried out by a 200W high-pressure Hg–Xe lamp (Oriel) equipped with a glass filter and HP8451A UV–Vis spectrophotometer for ultraviolet irradiation [23]. A series of different concentration solutions of LCD in chloroform (CHCl3) and tetrahydrofuran (THF) were prepared, they were scanned in the range of 300–600 nm. Then we can find the maximum absorption spectra is
418
J. Liu et al. / Reactive & Functional Polymers 67 (2007) 416–421
360 nm, so kmax = 360 nm. The absorptions were recorded at different time intervals until spectral variation was no longer evident. The solutions of LCD in CHCl3 and THF in 1 cm path-length quartz cuvette were irradiated by ultraviolet light 360 nm at room temperature. The concentration of the solutions was adjusted to have an absorbance of about 0–1 at 360 nm. The intensity of UV irradiation were determined actinometrically and were equal to 1.89 107 J s1 mol1 (kir = 360 nm) [24,25]. The 1.70 107, 3.40 107, 5.10 107, 6.80 7 10 and 8.50 107 M LCD solutions were prepared in CHCl3 and THF using the isolated LCD prepared as described earlier. The solutions were immediately subjected to UV–Vis absorption measurement and absorbance at kmax = 360 nm of each solution was recorded with time. The plots between absorbance and molarity of the solution were then constructed in order to obtain the molar absorption coefficients e of the compound in each solvent. In the case of solutions of the compound, the process of E–Z isomerization is photochemically and thermally reversible; that is, under the action of visible light and annealing, back Z–E isomerization takes place (also see Fig. 2). The solutions of LCD were irradiated by the 360 nm UV light for enough time (5 min) and then immediately subjected to the 470 nm visible light or annealed in different temperature. Absorbance at different time was recorded to study the process of its photo and thermal back-isomerization.
3. Results and discussion 3.1. Molar absorption coefficients e of LCD The UV absorption of LCD at various concentrations in various solvents was measured. Draw the absorbency of kmax with the corresponding concentration, all the graphs gave straight line. The slope of the line is the molar absorption coefficient e of LCD and its e in CHCl3 and THF were 1.13 106 and 1.06 106 (L mol1 cm1), respectively. They are about 35 times higher than that of HeA [24]. So the absorption of LCD is stronger than the mesogenic units for the more azobenzene number it has. At the same time the polarity of solvent can decrease the e of LCD. Those molar absorption coefficients obtained from the slopes of the graphs were summarized in Table 1. 3.2. Quantum yield u of LCD Using the given formula u = DA V/(e L Ia t), where DA represents the absorption at the irradiation wavelength, Ia the irradiation intensity, e the molar absorption coefficient at the irradiation wavelength, t the time of irradiation, V the volume and L the length of the cuvette, we can calculate the quantum yield u of LCD in CHCl3 and THF were 0.0148 and 0.0150, respectively. The u of LCD is less than that of HeA for the steric influence [21,22]. The photochemical quantum yield varies depending on the medium conditions and the polar-
Table 1 e, u, kp, kt, kc, kt/kc, kh and EA of LCD and mesogenic unit HeA Sample
Solvent
e 104/(L mol1 cm1)
u
kp/(s1)
kt/(s1)
kc/(s1)
kt/kc
kh 106 (s1)/T(K)
EA 104/(J mol1)
LCD
CHCl3
113
0.0148
0.416
0.128
0.0308
4.16
2.77
THF
106
0.0150
0.346
0.0177
0.00328
5.40
1564/286 2110/298 2660/303 3080/308 2.187/284 146/298 170/303 180/308
HeA
CHCl3
3.27
0.528
0.514
0.0559
0.0117
4.78
THF
3.75
0.450
0.417
0.0324
0.00625
5.18
694.4/289 1130/298 2090/303 2410/308 7.5/289 119/298 137/303 163/308
1.21
6.03
2.44
J. Liu et al. / Reactive & Functional Polymers 67 (2007) 416–421
3.3. Photoisomerization of LCD The solutions of LCD in CHCl3 and THF were irradiated by ultraviolet light 360 nm at room temperature. The absorptions were recorded at different time intervals until spectral variation was no longer evident. As follows from Fig. 3, it leads to marked spectral changes in the case of solution irradiation: one may observe a dramatic decrease in the optical density in the spectral region corresponding to the p–p* transition with the maximum at 360 nm, whereas in the region of the n-p* transition (near 450 nm), a slight increase in absorbance is observed. The occurrence of two distinct isobestic points at 426 and 324 nm as well as the similarity of the UV spectra of the irradiated samples at the photostationary state with that of Z-azobenzene indicate that only two absorbing species (E and Z isomers) are present and no side reactions such as photocrosslinking or photodegradation occur. Furthermore, the presence of two isobestic points (at 324 and 426 nm) during reversible isomerization processes clearly indicated the effective and reversible conversion of the azo unit. It also indicated that the isomerization was not accompanied by degradation, which would have resulted in a shift at two isobestic points. In all cases ln [(A0 A1)/(At A1)], where A0, At and A1, are the absorbances at 360 nm at zero
2.8
CHCl3 THF
2.4
ln(A0- A ∞ )/( At- A∞ )
ity of solvent can increase the u of LCD. The differences in the reaction quantum yields can be referred to the different tendency of the formation of a highly polar state [26]. The result indicates that equilibrium of photoisomerization depends upon concentration and polarity of the solvent used.
419
2.0 1.6 1.2 0.8 0.4 0
1
2
3
4
5
6
7
8
t/s Fig. 4. Photoisomerization of LCD in chloroform and tetrahydrofuran solutions.
time, t and infinite, respectively, shows a linear dependence on the irradiation time (see Fig. 4). They are in accordance with the first order kinetics and the slope is the photochromism rate constant kp kp of LCD in CHCl3 and THF were 0.416 and 0.346 (s1), respectively. They were very similar and of the same order of magnitude as those observed for the low molecular weight HeA or other azobenzenes [24,27] (see Table 1), at the same time they were 107 times larger than those of side-chain liquid crystalline polymers (LCP) containing the same azobenzene moieties in the corresponding solutions [28,29]. These results indicate that the dendritic structure does not significantly affect the photoisomerization rate, probably due to the presence of the flexible hexamethyl spacers. Because there were no chain entanglements and the bondage of main chain to the side chains in the dendritic molecule, the LCD has better photochromism and photoresponse behaviour than the common LCP. 3.4. Photo and thermal back-isomerization of LCD
Fig. 3. UV/Vis spectrum of LCD in tetrahydrofuran solution.
At first, the kinetics of the photo back-isomerization was studied. In the formula ln [(Ae A1)/ (Ae At)]=(A0 A1)/(Ae A1) kt t, A0 is the absorbance at kmax = 360 nm at zero time, and A1 is the absorbance after 5 min irradiation at kmax = 360 nm (t = 1) that can be regarded as the absorbance of the photostationary state. Then At and Ae are the absorbances at 470 nm at t time and infinite, respectively, which Ae is the absorbance of the equilibrium state of photo back-isomerization. kt is rate constant of the E–Z isomers reaction and kc is rate constant of the Z–E isomers reaction during the
420
J. Liu et al. / Reactive & Functional Polymers 67 (2007) 416–421 1.2
a
0.24
CHCl3 THF
0.8
ln(A0-A ∞)/(A 0-A t )
ln(Ae- A ∞ )/( Ae- At)
1.0
0.28
0.6 0.4 0.2
298K 303K 308K
0.20 0.16 0.12 0.08
0.0 0
1
2
3
4
5
6
7
8
0.04 200
t/s Fig. 5. Photo back-isomerization for LCD in chloroform and tetrahydrofuran solutions.
400
600
800
1000
1200
t/s
b
-5.5 -6.0 -6.5 CHCl3 THF
-7.0
lnk h
photo back-isomerization under the irradiation at k = 470 nm visible light. The ratio of kt/kc reflects the degree of photo back-isomerization, the value is more close to one means it is better to realize photo-reversibility. To calculate the rate of this process, the values of ln [(Ae A1)/(Ae At)] were plotted versus the time. We get the rate constant kt from the slope of the line (see Fig. 5). Then following the term of the equilibrium state of photo backisomerization: kc=[(A0 Ae)/(Ae A1)] kt, we can get the rate constant kc. They are in accordance with the first order kinetics and the values of kt/kc in CHCl3 and THF are 4.16 and 5.40, respectively (see Table 1). They are all less than the kt/kc of azobenzene unit in the same solvent, it shows that the LCD has better photo-reversibility. So the liquid crystalline dendrimer has potential applications and will become a new type photocontrollable switch and information functional material. Then the kinetics of the thermal back-isomerization was studied at different temperatures and the value of the activation energy (EA) of the process was calculated. To this end, the solution of LCD was irradiated with UV light for 5 min and annealed in dark at different temperatures. Fig. 6a shows the temperature dependence of the Z/E isomerization rate. To calculate the rate constant kh of this process, the values of ln [(A0 A1)/(A0 At)] were plotted versus the time, where A0 is the initial optical density, At and A1, stand for the values of optical density at the time t and at the steady state, respectively. The slope of the graphs is the rate constant kh of the thermal back-isomerization. So, Table 1 presents the rate constants of the process. As is well seen, the Z/E isomerization rate strongly
-7.5 -8.0 -8.5 -9.0 3.24
3.26
3.28
3.30 3
3.32
3.34
3.36
-1
1/T×10 (K ) Fig. 6. (a) Thermal back-isomerization for LCD in tetrahydrofuran solution at different temperatures and (b) Arrhenius plot for temperature dependence of rate constant of thermal Z/E process.
depends on the temperature. Using the Arrhenius formula ln kh = EA/(R T) + B, we can calculate the EA of the thermal Z/E isomerization, the rate constants are plotted against the reciprocal temperature (Fig. 6b). EA is calculated from the slope of this plot and listed in Table 1, and it is found to be about 19.5 kcal/mol. The value of the activation energy is typical for azobenzene derivatives [30]. In conclusion, the thermal back-isomerization of LCD in different solvents is in accordance with the first order kinetics. The rate and the most maximum absorption (Amax) of thermal back-isomerization depend on the temperature of the process, so we can say the higher temperature is beneficial to the stabilization of E isomerization. At the same time, the reaction rate and the Amax of thermal backisomerization in CHCl3 are both far higher than those in THF, so the less polarity of solvent is beneficial to the stabilization of E isomerization, too.
J. Liu et al. / Reactive & Functional Polymers 67 (2007) 416–421
4. Conclusion Photoresponsive LCD is different from the traditional LCP such as main chain LCP and side chain LCP for its special structure. It has become a novel multifunctional material. Hence, in this work, the kinetics of E/Z and Z/E isomerization of the LCD was studied. These results indicate that the dendritic structure does not significantly affect the photoisomerization activity of the azobenzene unit in its periphery and the LCD has better photo-reversibility. In the same time, the rate constants of the thermal back-isomerization of LCD in different solvents were far small than those of photochromism and photo back-isomerization of LCD in same solvent, so the effect of thermal back-isomerization can be ignored in the common request. Undoubtedly, the LCD has potential applications and the study of photoactive LCD presents an evident scientific and practical interest because the dendritic molecules allows one to anticipate a fast optical response and the rearrangement of their branched structure under the action of external fields and, in particular, light irradiation. This aspect should be interesting for the development of fast-acting photosensitive materials, which can be easily handled. Further work in this direction will be focused on a comparative analysis of the kinetic features of photoisomerization and other photochemical behaviors for them of various generations and functional branch density. We also hope that photochromic dendritic molecules will have applications in transport systems based on the reversible perturbation of their ability to encapsulate small molecules [31,32]. There are attempts to use dendrimers in the targeted delivery of drugs and act as carriers or vectors. So the functional dendrimer must have the splendent future. Acknowledgment The authors thank the National Natural Science Foundation of China (No. 29874020) for financial support. References [1] Y. Yokoyama, Chem. Rev. 100 (5) (2000) 1717–1740. [2] S. Kawata, Y. Kawata, Chem. Rev. 100 (5) (2000) 1777– 1788.
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[3] S. Yasuhiko, U. Hisayuki, U. Toshiki, Opt. Mater. 21 (2002) 249–254. [4] K. Sandhya, Yesodhaa, K. Chennakattu, Prog. Polym. Sci. 29 (2004) 45–74. [5] T. Ikeda, O. Tsutsumi, Science 268 (5219) (1995) 1873– 1875. [6] S. Kurihara, S. Nomiyama, T. Nonaka, Chem. Mater. 12 (1) (2000) 9–12. [7] K. Ichimura, Chem. Rev. 100 (5) (2000) 1847–1874. [8] V. Shibaev, A. Bobrovsky, N. Boiko, Prog. Polym. Sci. 28 (5) (2003) 729–836. [9] D.A. Tomalia, A.M. Naylor, W.A. Goddard III, Angew. Chem. Int. Ed. Engl. 29 (2) (1990) 138–175. [10] K.L. Wooley, C.J. Hawker, J.M. Pochan, J.M.J. Fre´chet, Macromolecules 26 (1993) 1514–1519. [11] M.K. Ronald, M. Mahua, M.H. Steven, et al., Proc. Natl. Acad. Sci. USA 93 (1996) 10012–10017. [12] R. Jacques, C. Bogdan, Adv. Polym. Sci. 142 (1999) 179– 228. [13] L. Klaus, H. Dirk, S. Bernd, M. Rolf, F. Holger, Adv. Mater. 8 (5) (1996) 414–416. [14] O.A. Matthews, A.N. Shipway, J.F. Stoddart, Prog. Polym. Sci. 23 (1) (1998) 1–56. [15] N. Boiko, X. Zhu, A. Bobrovsky, V. Shibaev, Chem. Mater. 13 (5) (2001) 1447–1452. [16] S.M. Grayson, J.M.J. Fre´chet, Chem. Rev. 101 (12) (2001) 3819–3868. [17] S. Ghosh, A.K. Banthia, Tetrahedron Lett. 42 (3) (2001) 501–503. [18] A.Y. Bobrovsky, A.A. Pakhomov, X.M. Zhu, et al., J. Phys. Chem. B 106 (3) (2002) 540–546. [19] L.X. Liao, D.M. Junge, D.V. McGrath, Macromolecules 35 (2) (2002) 319–322. [20] J.Q. Liu, Q.Z. Zhang, J.Z. Zhang, Chin. Chem. Lett. 15 (12) (2004) 1423–1426. [21] Q.Z. Zhang, J.Q. Liu, et al., Chem. J. Chin. Univ. 24 (10) (2003) 1887–1890. [22] Q.Z. Zhang, J.Q. Liu, et al., Chem. J. Chin. Univ. 24 (9) (2003) 1704–1707. [23] Z. Chen, J.Z. Zhang, S.L. Ding, et al., Chin. Sci. Bull. 43 (1988) 37–41. [24] Q.Z. Zhang, J.Q. Liu, et al., Chem. J. Chin. Univ. 25 (7) (2004) 1368–1372. [25] J.Q. Liu, Q.Z. Zhang, et al., J. Funct. Mater. 35 (suppl.) (2004) 172–174. [26] H. Wu¨nscher, G. Haucke, et al., J. Photochem. Photobiol. A 151 (2002) 75–82. [27] S. Seiji, N. Takahiro, N. Yoshihiro, O. Toshiyuki, J. Am. Chem. Soc. 102 (18) (1980) 5860–5865. [28] Q.Z. Zhang, J.Z. Zhang, Y. Wang, Acta Polymerica. Sinica (1) (1996) 121–125. [29] J.Z. Zhang, Q.Z. Zhang, Acta Chim. Sinica 55 (1997) 930– 936. [30] J.P. Otruba III, R.G. Weiss, J. Org. Chem. 48 (1983) 3448– 3453. [31] A. Archut, G.C. Azzellini, V. Balzani, et al., J. Am. Chem. Soc. 120 (47) (1998) 12187–12191. [32] G.M. Dykes, D.K. Smith, G.J. Seeley, Angew. Chem. Int. Ed. Engl. 41 (17) (2002) 3254–3257.
REACTIVE & FUNCTIONAL POLYMERS www.elsevier.com/locate/react
Photochromism of liquid crystalline dendrimer with azobenzene terminal groups in solution Jianqiang Liu a,b,*, Peng Ni b, Depeng Qiu b, Wanguo Hou b, Qizhen Zhang c a
b
School of Physics and Microelectronics, Shandong University, Ji’nan 250100, Shandong Province, PR China Key Laboratory of Education Ministry on Colloid and Interface Chemistry, Ji’nan 250100, Shandong Province, PR China c School of Chemistry and Chemical Engineering, Shandong University, Ji’nan 250100, Shandong Province, PR China Received 13 September 2005; accepted 28 March 2006 Available online 1 March 2007
Abstract The photochromism of a liquid crystalline dendrimer (LCD) with 36 hexyloxyazobenzene terminal groups in solution was described in this paper. The molar absorption coefficient, quantum yield, photoisomerization, photo back-isomerization, thermal back-isomerization and activation energy of LCD in solution are studied by UV/Vis absorption spectra. The results indicate that the photochromism, photo and thermal back-isomerization of LCD in chloroform (CHCl3) and tetrahydrofuran (THF) solutions are in accordance with the first order kinetics. The photochromism rate constants of LCD are 101 s1, it is 107 times larger than that of side-chain liquid crystalline polymers (LCP) containing the same azobenzene moieties. These results indicate that the dendritic structure does not significantly affect the photoisomerization activity of the azobenzene unit in its periphery. The kt/kc of LCD is less than that of azobenzene unit shows that the LCD has better photo-reversibility. So the liquid crystalline dendrimer has potential applications. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Photochromism; Liquid crystalline dendrimer; Photo back-isomerization; Thermal back-isomerization; Activation energy
1. Introduction Photochromism is a fascinating phenomenon and photochromic materials have recently attracted intense interest owing to their potential technological applications such as various photooptical devices and data recording media [1–4]. Liquid crystals are promising materials for optical switching * Corresponding author. Address: School of Physics and Microelectronics, Shandong University, Ji’nan 250100, Shandong Province, PR China. Tel.: +86 531 88565947; fax: +86 531 8564886. E-mail address: [email protected] (J. Liu).
and image storage because of their high resolution and sensitivity. In recent years, some advances in research dealing with the synthesis and study of optical properties of a series of photochromic LCs are considered [5–8]. Over the past few years there has been intense interest in the unique properties exhibited by dendrimers with a regular treelike array of branching units and numerous dendritic structures have been synthesized and studied [9– 12]. Quite recently, dendrimers with different designed functionalities have become objects of particular academic and practical interest because of their unique superbranched architectures, high densities of peripheral functionalities, symmetrical
1381-5148/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2006.03.022
J. Liu et al. / Reactive & Functional Polymers 67 (2007) 416–421
O
O
O
O
O
O
O
shapes, and monodispersity. Novel dendrimers with electroactive, photoactive, and recognition elements at the core, within the branches, or at the periphery of the dendritic structure have recently been reported as potential functional materials [13–19]. We have successfully carried out the molecular design and got a series of liquid crystalline carbosilane dendrimers with 4, 12 and 36 4-hexyloxy-40 hydroxyhexyloxy-azobenzene (HeA) units in the periphery [20–22]. The structure of the LCD with 36 azobenzene terminal groups was shown in Fig. 1, and the dendritic compound containing mesogenic and photochromic groups was structurally and functionally integrated system. It is well-known that azobenzene-type compounds undergo an efficient and fully reversible photoisomerization reaction, and thereby the presence of the azo photochromic group provides its sensitivity to the light (see Fig. 2). The photochromism of LCD in solution are briefly studied by the UV absorption spectrum in this context.
O
Si
O
O O
O
O O
O O O
O
Si
O
Si
O O
O
O O
O
O
Si
O Si O
O O
Si
O O O O
Si
Si
Si
O
O Si O O
Si Si
Si
O
O
O
Si
O
O O
O O
O O
O
O O
Si
Si O O
O
O O O
O O
O
O O
Si
O
O
O O
O
O
O
O
O
O
= C6H13O
OC6H13
/visible light
OC6H13
O
O Eisomer
N=N
UV light
2. Experimental 2.1. Materials and characterization The synthesis of the LCD was described in Ref. [22]. Elemental analysis of LCD Si17O108C912H1284 N72 was carried out with a Perkin-Elmer 240C auto elementary analyser: calculated: C, 70.84, H, 8.37, N, 6.52 (%); found: C, 70.56, H, 8.29, N, 6.39 (%). Infrared spectra were recorded on a Nicolet 5DX Fourier transform infrared photoacoustic spectral system (KBr/cm1): 2936, 2868 (–CH2–), 1601, 1580, 1500, 1472 (ph, N@N), 1246 (–OCH2–); 1H NMR was carried out with a Japan Joel FX902 (90 MHz, CDCl3, d ppm): 0.68–0.88 (m, 172H, SiCH2, CH3), 1.20–1.88 (m, 608H, CH2), 3.64–4.04 (m, 216H, OCH2), 6.80–7.88 (m, 288H, ph-H). This showed that the compound have expectant structure as Fig. 1. Polarizing optical microscope (POM, Seagull XPID polarizing optical microscope with Mettler FP80 and FP82 hot stage and controller) was used to observe the optical texture of the LCD. Upon slow heating and cooling during its liquid crystalline phase domain, black brush defects (Schlieren texture) and threads, typical of nematic phase are observed. The transition temperatures and the associated enthalpies and entropies of the samples was examined by means of differential scanning calorimetry (DSC, Perkin–Elmer 7 series thermal analysis system). X-ray diffraction analysis (XRD, D/max-cB diffractometer with Cu Ka radiation, ˚ ) studied on the mesophase have been k = 1.54 A carried out at ambient temperature. The resulting diffraction showed a slow peak in the little angle (2h < 10°) and a dispersion peak in the larger angle (2h = 20.1°). To make a comprehensive view on the results of POM, DSC and XRD hereinbefore, we can make sure that the LCD shows nematic phase and its phase behaviour was Cr90N105I113N75Cr [22]. 2.2. Photochromism experiment
N=N
Fig. 1. The structure of LCD.
N=N
417
Zisomer
Fig. 2. Reversible photoisomerization of LCD.
The photoirradiation was carried out by a 200W high-pressure Hg–Xe lamp (Oriel) equipped with a glass filter and HP8451A UV–Vis spectrophotometer for ultraviolet irradiation [23]. A series of different concentration solutions of LCD in chloroform (CHCl3) and tetrahydrofuran (THF) were prepared, they were scanned in the range of 300–600 nm. Then we can find the maximum absorption spectra is
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360 nm, so kmax = 360 nm. The absorptions were recorded at different time intervals until spectral variation was no longer evident. The solutions of LCD in CHCl3 and THF in 1 cm path-length quartz cuvette were irradiated by ultraviolet light 360 nm at room temperature. The concentration of the solutions was adjusted to have an absorbance of about 0–1 at 360 nm. The intensity of UV irradiation were determined actinometrically and were equal to 1.89 107 J s1 mol1 (kir = 360 nm) [24,25]. The 1.70 107, 3.40 107, 5.10 107, 6.80 7 10 and 8.50 107 M LCD solutions were prepared in CHCl3 and THF using the isolated LCD prepared as described earlier. The solutions were immediately subjected to UV–Vis absorption measurement and absorbance at kmax = 360 nm of each solution was recorded with time. The plots between absorbance and molarity of the solution were then constructed in order to obtain the molar absorption coefficients e of the compound in each solvent. In the case of solutions of the compound, the process of E–Z isomerization is photochemically and thermally reversible; that is, under the action of visible light and annealing, back Z–E isomerization takes place (also see Fig. 2). The solutions of LCD were irradiated by the 360 nm UV light for enough time (5 min) and then immediately subjected to the 470 nm visible light or annealed in different temperature. Absorbance at different time was recorded to study the process of its photo and thermal back-isomerization.
3. Results and discussion 3.1. Molar absorption coefficients e of LCD The UV absorption of LCD at various concentrations in various solvents was measured. Draw the absorbency of kmax with the corresponding concentration, all the graphs gave straight line. The slope of the line is the molar absorption coefficient e of LCD and its e in CHCl3 and THF were 1.13 106 and 1.06 106 (L mol1 cm1), respectively. They are about 35 times higher than that of HeA [24]. So the absorption of LCD is stronger than the mesogenic units for the more azobenzene number it has. At the same time the polarity of solvent can decrease the e of LCD. Those molar absorption coefficients obtained from the slopes of the graphs were summarized in Table 1. 3.2. Quantum yield u of LCD Using the given formula u = DA V/(e L Ia t), where DA represents the absorption at the irradiation wavelength, Ia the irradiation intensity, e the molar absorption coefficient at the irradiation wavelength, t the time of irradiation, V the volume and L the length of the cuvette, we can calculate the quantum yield u of LCD in CHCl3 and THF were 0.0148 and 0.0150, respectively. The u of LCD is less than that of HeA for the steric influence [21,22]. The photochemical quantum yield varies depending on the medium conditions and the polar-
Table 1 e, u, kp, kt, kc, kt/kc, kh and EA of LCD and mesogenic unit HeA Sample
Solvent
e 104/(L mol1 cm1)
u
kp/(s1)
kt/(s1)
kc/(s1)
kt/kc
kh 106 (s1)/T(K)
EA 104/(J mol1)
LCD
CHCl3
113
0.0148
0.416
0.128
0.0308
4.16
2.77
THF
106
0.0150
0.346
0.0177
0.00328
5.40
1564/286 2110/298 2660/303 3080/308 2.187/284 146/298 170/303 180/308
HeA
CHCl3
3.27
0.528
0.514
0.0559
0.0117
4.78
THF
3.75
0.450
0.417
0.0324
0.00625
5.18
694.4/289 1130/298 2090/303 2410/308 7.5/289 119/298 137/303 163/308
1.21
6.03
2.44
J. Liu et al. / Reactive & Functional Polymers 67 (2007) 416–421
3.3. Photoisomerization of LCD The solutions of LCD in CHCl3 and THF were irradiated by ultraviolet light 360 nm at room temperature. The absorptions were recorded at different time intervals until spectral variation was no longer evident. As follows from Fig. 3, it leads to marked spectral changes in the case of solution irradiation: one may observe a dramatic decrease in the optical density in the spectral region corresponding to the p–p* transition with the maximum at 360 nm, whereas in the region of the n-p* transition (near 450 nm), a slight increase in absorbance is observed. The occurrence of two distinct isobestic points at 426 and 324 nm as well as the similarity of the UV spectra of the irradiated samples at the photostationary state with that of Z-azobenzene indicate that only two absorbing species (E and Z isomers) are present and no side reactions such as photocrosslinking or photodegradation occur. Furthermore, the presence of two isobestic points (at 324 and 426 nm) during reversible isomerization processes clearly indicated the effective and reversible conversion of the azo unit. It also indicated that the isomerization was not accompanied by degradation, which would have resulted in a shift at two isobestic points. In all cases ln [(A0 A1)/(At A1)], where A0, At and A1, are the absorbances at 360 nm at zero
2.8
CHCl3 THF
2.4
ln(A0- A ∞ )/( At- A∞ )
ity of solvent can increase the u of LCD. The differences in the reaction quantum yields can be referred to the different tendency of the formation of a highly polar state [26]. The result indicates that equilibrium of photoisomerization depends upon concentration and polarity of the solvent used.
419
2.0 1.6 1.2 0.8 0.4 0
1
2
3
4
5
6
7
8
t/s Fig. 4. Photoisomerization of LCD in chloroform and tetrahydrofuran solutions.
time, t and infinite, respectively, shows a linear dependence on the irradiation time (see Fig. 4). They are in accordance with the first order kinetics and the slope is the photochromism rate constant kp kp of LCD in CHCl3 and THF were 0.416 and 0.346 (s1), respectively. They were very similar and of the same order of magnitude as those observed for the low molecular weight HeA or other azobenzenes [24,27] (see Table 1), at the same time they were 107 times larger than those of side-chain liquid crystalline polymers (LCP) containing the same azobenzene moieties in the corresponding solutions [28,29]. These results indicate that the dendritic structure does not significantly affect the photoisomerization rate, probably due to the presence of the flexible hexamethyl spacers. Because there were no chain entanglements and the bondage of main chain to the side chains in the dendritic molecule, the LCD has better photochromism and photoresponse behaviour than the common LCP. 3.4. Photo and thermal back-isomerization of LCD
Fig. 3. UV/Vis spectrum of LCD in tetrahydrofuran solution.
At first, the kinetics of the photo back-isomerization was studied. In the formula ln [(Ae A1)/ (Ae At)]=(A0 A1)/(Ae A1) kt t, A0 is the absorbance at kmax = 360 nm at zero time, and A1 is the absorbance after 5 min irradiation at kmax = 360 nm (t = 1) that can be regarded as the absorbance of the photostationary state. Then At and Ae are the absorbances at 470 nm at t time and infinite, respectively, which Ae is the absorbance of the equilibrium state of photo back-isomerization. kt is rate constant of the E–Z isomers reaction and kc is rate constant of the Z–E isomers reaction during the
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a
0.24
CHCl3 THF
0.8
ln(A0-A ∞)/(A 0-A t )
ln(Ae- A ∞ )/( Ae- At)
1.0
0.28
0.6 0.4 0.2
298K 303K 308K
0.20 0.16 0.12 0.08
0.0 0
1
2
3
4
5
6
7
8
0.04 200
t/s Fig. 5. Photo back-isomerization for LCD in chloroform and tetrahydrofuran solutions.
400
600
800
1000
1200
t/s
b
-5.5 -6.0 -6.5 CHCl3 THF
-7.0
lnk h
photo back-isomerization under the irradiation at k = 470 nm visible light. The ratio of kt/kc reflects the degree of photo back-isomerization, the value is more close to one means it is better to realize photo-reversibility. To calculate the rate of this process, the values of ln [(Ae A1)/(Ae At)] were plotted versus the time. We get the rate constant kt from the slope of the line (see Fig. 5). Then following the term of the equilibrium state of photo backisomerization: kc=[(A0 Ae)/(Ae A1)] kt, we can get the rate constant kc. They are in accordance with the first order kinetics and the values of kt/kc in CHCl3 and THF are 4.16 and 5.40, respectively (see Table 1). They are all less than the kt/kc of azobenzene unit in the same solvent, it shows that the LCD has better photo-reversibility. So the liquid crystalline dendrimer has potential applications and will become a new type photocontrollable switch and information functional material. Then the kinetics of the thermal back-isomerization was studied at different temperatures and the value of the activation energy (EA) of the process was calculated. To this end, the solution of LCD was irradiated with UV light for 5 min and annealed in dark at different temperatures. Fig. 6a shows the temperature dependence of the Z/E isomerization rate. To calculate the rate constant kh of this process, the values of ln [(A0 A1)/(A0 At)] were plotted versus the time, where A0 is the initial optical density, At and A1, stand for the values of optical density at the time t and at the steady state, respectively. The slope of the graphs is the rate constant kh of the thermal back-isomerization. So, Table 1 presents the rate constants of the process. As is well seen, the Z/E isomerization rate strongly
-7.5 -8.0 -8.5 -9.0 3.24
3.26
3.28
3.30 3
3.32
3.34
3.36
-1
1/T×10 (K ) Fig. 6. (a) Thermal back-isomerization for LCD in tetrahydrofuran solution at different temperatures and (b) Arrhenius plot for temperature dependence of rate constant of thermal Z/E process.
depends on the temperature. Using the Arrhenius formula ln kh = EA/(R T) + B, we can calculate the EA of the thermal Z/E isomerization, the rate constants are plotted against the reciprocal temperature (Fig. 6b). EA is calculated from the slope of this plot and listed in Table 1, and it is found to be about 19.5 kcal/mol. The value of the activation energy is typical for azobenzene derivatives [30]. In conclusion, the thermal back-isomerization of LCD in different solvents is in accordance with the first order kinetics. The rate and the most maximum absorption (Amax) of thermal back-isomerization depend on the temperature of the process, so we can say the higher temperature is beneficial to the stabilization of E isomerization. At the same time, the reaction rate and the Amax of thermal backisomerization in CHCl3 are both far higher than those in THF, so the less polarity of solvent is beneficial to the stabilization of E isomerization, too.
J. Liu et al. / Reactive & Functional Polymers 67 (2007) 416–421
4. Conclusion Photoresponsive LCD is different from the traditional LCP such as main chain LCP and side chain LCP for its special structure. It has become a novel multifunctional material. Hence, in this work, the kinetics of E/Z and Z/E isomerization of the LCD was studied. These results indicate that the dendritic structure does not significantly affect the photoisomerization activity of the azobenzene unit in its periphery and the LCD has better photo-reversibility. In the same time, the rate constants of the thermal back-isomerization of LCD in different solvents were far small than those of photochromism and photo back-isomerization of LCD in same solvent, so the effect of thermal back-isomerization can be ignored in the common request. Undoubtedly, the LCD has potential applications and the study of photoactive LCD presents an evident scientific and practical interest because the dendritic molecules allows one to anticipate a fast optical response and the rearrangement of their branched structure under the action of external fields and, in particular, light irradiation. This aspect should be interesting for the development of fast-acting photosensitive materials, which can be easily handled. Further work in this direction will be focused on a comparative analysis of the kinetic features of photoisomerization and other photochemical behaviors for them of various generations and functional branch density. We also hope that photochromic dendritic molecules will have applications in transport systems based on the reversible perturbation of their ability to encapsulate small molecules [31,32]. There are attempts to use dendrimers in the targeted delivery of drugs and act as carriers or vectors. So the functional dendrimer must have the splendent future. Acknowledgment The authors thank the National Natural Science Foundation of China (No. 29874020) for financial support. References [1] Y. Yokoyama, Chem. Rev. 100 (5) (2000) 1717–1740. [2] S. Kawata, Y. Kawata, Chem. Rev. 100 (5) (2000) 1777– 1788.
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