- Email: [email protected]
Synthesis and characterization of photochromic star-like liquid crystal
Materials Letters 59 (2005) 2531 – 2534 www.elsevier.com/locate/matlet
Synthesis and characterization of photochromic star-like liquid crystal Jianqiang Liua,b,*, Qizhen Zhangc, Jingzhi Zhangc a
b
School of Physics and Microelectronics, Shandong University, Ji’nan 250100, PR China Key Laboratory of Education Ministry on Colloid and Interface Chemistry, Ji’nan 250100, PR China c School of Chemistry and Chemical Engineering, Shandong University, Ji’nan 250100, PR China Received 16 September 2004; accepted 17 March 2005 Available online 25 April 2005
Abstract A new photochromic star-like liquid crystal that is written by SiC4 and containing four butoxyazobenzene mesogens in its periphery has been synthesized and characterized by the spectroscopic methods and thermal analysis. SiC4 is nematic phase and its phase behavior is Cr138N147I145N118Cr. The quantum yield and photoisomerization of SiC4 in chloroform (CHCl3) and tetrahydrofuran (THF) have been studied by UV/Vis absorption spectra. The results indicate that the photochromism of SiC4 in CHCl3 and THF are in accordance with the first-order kinetics. The photochromism rate constants in CHCl3 and THF are 101 s1, they are 107 times larger than those of side-chain liquid crystalline polymers containing the same azobenzene moieties. These results show that the star-like structure does not significantly affect the photoisomerization activity of the azobenzene mesogen in its periphery. D 2005 Elsevier B.V. All rights reserved. Keywords: Star-like; Liquid crystal; Phase behavior; Photochromism
1. Introduction
2. Experiment
Photochromic materials have attracted considerable attention owing to their potential technological applications in reversible optic information storage, variable transmitting materials, optical switching and photonic devices [1– 6]. Liquid crystals are one of the most convenient materials to manipulate optical properties and recent advances in research dealing with the synthesis and study of optical properties of a series of photochromic LCs are considered [7 –12]. In this study, we carried out the molecular architecture of the photochromic LC and got the so-called star-like system composed of nematic fragments and four photochromic branches. The star-like compound functionalized with azobenzene group in its periphery and its photochemical reactions and phase behavior are briefly described here.
In this work, the following molecule design was carried out: starting with tetrachlorosilane (SiCl4) as the core molecule, the mesogenic monomer 4-butoxy-4¶hydroxyhexyloxy-azobenzene (BuA) reacted with SiCl4 directly and got a new star-like compound abbreviated to SiC4. The mesogenic monomer BuA is nematic liquid crystal and its phase behavior is Cr112N124I121N110Cr, it was prepared following [13]. The structure of SiC4 was shown in Fig. 1 and its preparation method is as follows: A mixture of 2.0 cm3 of dried pyridine, 15.0 cm3 of dried THF and 1.3 g (3.5 mmol) of BuA were put into a 50 cm3 Wolff bottle after a dry nitrogen inlet. 0.1 cm3 (0.8 mmol) of SiCl4 (prepared in our laboratory) diluted with 10 cm3 of dried THF was added in 0.5 h. The reaction solution was refluxed for 24 h. Then it was cooled to room temperature and filtrated. Purification was carried out by column chromatography (silica gel, THF/anhydrous ethyl alcohol = 3:1 (v/v) as an eluent) and recrystallization from THF/anhydrous ethanol. The SiC4 was obtained in 78%.
* Corresponding author. School of Physics and Microelectronics, Shandong University, Ji’nan 250100, PR China. Tel.: +86 531 8565947; fax: +86 531 8564886. E-mail address: [email protected] (J. Liu). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.03.039
2532
J. Liu et al. / Materials Letters 59 (2005) 2531 – 2534
The SiC4 was characterized by EA, IR, 1H NMR, POM, DSC and XRD. The photo irradiation 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.
3. Results and discussion 3.1. Chemical material Elemental analysis of compound SiC4 was carried out with a Perkin-Elmer 240 C auto elementary analyser, calculated for SiO12C88H116N8: C 68.07, H 7.53, N 7.22 (%); found: C 67.94, H 7.42, N 7.11 (%); Infrared spectra were recorded on a Nicolet 5DX Fourier transform infrared photoacoustic spectral system (KBr/cm1): 2936, 2872 ( –CH2 –), 1602, 1580 (ph), 1498, 1473 (NfN), 1244.4 ( –OCH2); 1H NMR was carried out with a Japan Jeol FX90Q (90MHz, CDCl3, y ppm): 0.96 (t, 12H, – CH3), 1.12 – 1.92 (m, 48H, CH2), 3.60 – 4.08 (m, 24H, OCH2), 6.88, 7.00, 7.64, 7.88 (m, 32H, ph– H). This showed that the compound has the expectant structure as Fig. 1. 3.2. Characterization of LC phase Polarizing optical microscope with a heating stage was used to observe the optical texture of SiC4. Upon slow cooling from the isotropic phase, two and four black brush defects (Schlieren texture), typical of a nematic phase, are observed, as shown in Fig. 2 [14,15]. The transition temperatures and the associated enthalpies and entropies of SiC4 have been established for the heating and cooling cycles using the differential scanning calorimetry (DSC, Perkin-Elmer 7 series thermal analysis system) (see Table 1). In order to compare with the common liquid crystals and liquid crystalline polymers, the enthalpies and
O
O O
O
Si O
Fig. 2. Picture observed under a POM (cooling 138 -C, Shlieren texture).
entropies per mesogen of SiC4 were also given in the Table 1. The DSC data of SiC4 showed on heating two distinct endothermic transitions and the lower and the higher temperatures corresponding to melting and isotropization, respectively. Upon cooling, a modest degree of supercooling (16 K) was found for the latter transition so it showed broader liquid crystalline phase domain than in the heating process. The transition enthalpies per mesogen of SiC4 is about 1.6 kJImol1Imesogen1, it belongs to the statistic values 1.3 – 3.6 or 0.84 – 9.6 (kJImol1) of N – I phase transition enthalpies [16,17], so the DSC data hold out that the nematic phase was in existence for SiC4. Wide angle X-ray diffraction (D/max-gB diffractometer with Cu Ka radiation) studies on the mesophase have been carried out at ambient temperature. The resulting diffraction shows a slow peak in the little angle (2h < 10-) and a dispersion peak in the larger angle (2h = 19.8-) (see Fig. 3). The intense peak at 2h = 19.8- is due to the mesogen– mesogen spacing of 0.45 nm which is a value typically found in LC phases [16]. The dispersion peak in the larger 2h values is like the isotropic liquid; it shows that molecules array disorder in landscape orientation. The small intensity of the signal at low 2h values may suggest the presence of a low ordered system in the direction of longitudinal axis of the molecule. This is in agreement with the low transition enthalpy observed calorimetrically. Furthermore, there is strong intensity near the origin, so the XRD pattern can be
O
O
Table 1 DSC data for SiC4 Compound Conditions T/-C
O
N=N
DS/J mol1 K1
1
DS/J mol1 K1 mesogen1
DH/kJ mol mesogen1 SiC4
=
DH/kJ mol1
OC4H9
Fig. 1. The structure of star-like liquid crystal SiC4.
1st heating 1st cooling 2nd heating
138.26 147.43 102.68 6.38 25.67 1.60 144.8 118.35 6.10 79.7 1.53 19.93 138.25 147.43 98.0 6.35 24.5 1.63
260 15.04 65 3.76 14.6 210.56 3.65 52.64 240.64 15.04 60.16 3.76
J. Liu et al. / Materials Letters 59 (2005) 2531 – 2534
2500
2533
1 1.1
1
0.7
2 1.9
Counts
1600
3 2.9 4 3.6
0.5
900
5 4.1
A
400
6 5.0 0.3
100
7 7.2
0.0
8 ∞ (s) 0
10
20
30
40
2 /(º)
8
0.1
Fig. 3. WAXRD curve of the star-like liquid crystal SiC4.
300
350
400
450
500
550
λ (nm)
3.3. Photochromic behavior of SiC4 A series of different concentration solutions of SiC4 in CHCl3 and THF were prepared, they were scanned in the range of 300– 600 nm. Then we can find the maximum absorption spectra is 360 nm, so k max = 360 nm. Draw the absorbency of k max with the concentration and then get a line, the slope of the line is the molar absorption coefficient e of SiC4 and its E in CHCl3 and THF were 1.31 105 and 1.17 105 (dm3Imol1Icm1), respectively. They are about
Fig. 4. UV/Vis spectrum of SiC4 in chloroform and tetrahydrofuran.
2.7 – 3.8 times higher than that of BuA [23]. So the absorption of SiC4 is stronger than the mesogen’s for the more azobenzene number and the polarity of solvent can decrease the e of SiC4. The quantum yield was determined DAV using the formula u ¼ eLI , where DA represents the a t absorption at the irradiation wavelength, I a 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 cell. We can calculate the quantum yield u of SiC4 in CHCl3 and THF to be 0.127 and 0.152, respectively. The u of SiC4 is less than that of mesogen for the steric influence [23]. The solutions of SiC4 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. The intensity of the UV absorption bands decrease with irradiation time in the 360 nm region. At the same time, the shoulder at about 440 nm gradually more resolved giving rise at the photostationary state to an absorption band with a distinct maximum at 440 nm (see Fig. 4). The occurrence of two distinct isosbestic points at 425 and 320 nm, as well as the similarity of the UV spectra of the irradiated samples at the photo4.0
CHCl3 THF
3.5
ln(A0- A∞)/( At- A∞)
accorded with the characteristics of nematic and smectic A LC phases [18]. To make a comprehensive view on the results of POM, DSC and XRD hereinbefore, we can make sure that the SiC 4 is nematic LC and its phase behavior is Cr138N147I145N118Cr. The star-like liquid crystal’s phase behavior is the same as the mesogen monomer and they are all nematic phases. There is a viewpoint that the nature of chain ends and monomer dramatically affects the properties of the whole dendritic molecule [3,12,19,20]. The mesogenic unit shows nematic phase and it was grafted on the surface of the star-like molecule, so the SiC4 shows nematic phase too. In farther thinking, the properties of LC could be determined in relation to the molecular structure and intermolecular interaction [21,22]. Because of the existence of the rigid rod-like azobenzene moieties in the periphery of the star-like molecule, the distortion took place in the spherical molecule and its branches directed along the long axis of the rod-like moieties, so the whole molecule showed anisotropy. Clearly, attachment of the rod-like mesogenic units by the flexible hexamethyl spacer leads to cooperative deformation of the star-like molecule and formation of LC phase. But the formation of LC phase is so complex that need further study, it maybe related to the mesogen density as well as the spacer length. The melting point and clearing temperature of SiC4 are higher than the mesogenic monomer’s due to its greater anisotropy. The liquid crystal’s domain of SiC4 is broader than the monomer’s, because the deformations of its especial star-like structure need higher temperature and more energy.
3.0 2.5 2.0 1.5 1.0 0.5 0.0 0
1
2
3
4
5
6
7
8
t/s Fig. 5. Photoisomerization of SiC4 in chloroform and tetrahydrofuran.
2534
J. Liu et al. / Materials Letters 59 (2005) 2531 – 2534
stationary state with that of cis-azobenzene, indicates that only two absorbing species (trans and cis isomers) are present and no side reactions such as photocrosslinking or photodegradation occur. In all cases ln[(A 0 A V) / (A t A V)], where A 0 A t and A V are the absorbances at 360 nm at time zero, t and infinite, respectively, shows a linear dependence on the irradiation time (see Fig. 5). They are in accordance with the first-order kinetics and the slope is the photochromism rate constant k p. The k p of SiC4 in CHCl3 and THF were 0.567 and 0.514 (s1), respectively. They were close to those of BuA and 107 times larger than those of side-chain liquid crystalline polymers in the same solutions [23 – 25]. These results indicate that the star-like structure does not significantly affect the photoisomerization rate, probably due to the presence of the flexible hexamethyl spacers and the mesogens were partly decoupled from the core. At the same time, there were no chain entanglements and the bondage of main chain to the side chains in the starlike molecule, so the SiC4 has better photoresponse behavior than the common liquid crystalline polymers.
4. Conclusion A new photochromic star-like liquid crystal SiC4 was synthesized, its phase behavior is Cr138N147I145N118Cr and its liquid crystal domain is broader than the monomer’s. Its photochromism rate constants in CHCl3 and THF are 101 s1, they are much larger than those of side-chain liquid crystalline polymers containing the same azobenzene moieties. The terminal azobenzene fragments not only provide its sensitivity to the light but also perform a development of LC phases for their rigid rodlike form. Undoubtedly, the preparation of photoactive star-like liquid crystal presents an evident scientific and practical interest because the star-like molecule allows one to anticipate a fast optical response. This aspect should be interesting for the development of fast-acting photosensitive materials, which can be easily handled. So the starlike liquid crystal has potential applications and will become a new type photocontrollable switch and information functional material.
Acknowledgments The authors thank Professor Wanguo HOU’s helpful advice and the National Natural Science Foundation of China (No. 29874020) for financial support. References [1] A. Archut, G.C. Azzellini, V. Balzani, L. De Cola, F. Vo¨gtle, J. Am. Chem. 120 (1998) 12187. [2] S. Kawata, Y. Kawata, Chem. Rev. 100 (2000) 1777. [3] L. Tajber, A. Kocot, J.K. Vij, K. Merkel, J. Zalewska-Rejdak, G.H. Mehl, R. Elsa¨sser, J.W. Goodby, M. Veith, Macromolecules 35 (2002) 8601. [4] S. Kurihara, S. Nomiyama, T. Nonaka, Chem. Mater. 12 (2002) 9. [5] S. Yasuhiko, U. Hisayuki, U. Toshiki, S. Yoshikawa, K. Moriwaki, D. Nagahama, H. Nakano, Opt. Mater. 21 (2003) 249. [6] K. Sandhya, Yesodhaa, K. Chennakattu, Prog. Polym. Sci. 29 (2004) 45. [7] A. Adronov, P. Malenfant, J. Fre´chet, Chem. Mater. 12 (2000) 1463. [8] N.I. Boiko, X.-M. Zhu, A.Y. Bobrovsky, V.P. Shibaev, Chem. Mater. 3 (2001) 1447. [9] A. Bobrovsky, A. Pakhomov, X. Zhu, N. Boiko, V. Shibaev, J. Stumpe, J. Phys. Chem., B 106 (2002) 540. [10] A. Miniewicz, A. Gniewek, J. Parka, Opt. Mater. 21 (2002) 605. [11] V. Shibaev, A. Bobrovsky, N. Boiko, Prog. Polym. Sci. 28 (2003) 729. [12] K. Merkel, A. Kocot, J.K. Vij, G.H. Mehl, T. Meyer, J. Chem. Phys. 121 (2004) 5012. [13] Q.Z. Zhang, J.Q. Liu, X. Yin, J. Zhang, Acta Chimi. Sin. 60 (2002) 2232. [14] D. Demus, Textures of Liquid Crystals, Verleg Chemie Weinheim, New York, 1978. [15] R.J. Twieg, V. Chu, C. Nguyen, C.M. Dannels, C. Viney, Liquid Cryst. 20 (1996) 287. [16] G.W. Gray, J.W.G. Goodby, Smetic Liquid Crystals, Leonard Hill, Glasgow, 1984, p. 21. [17] D. Marzotko, D. Demus, Liquid Crystal Proceedings of the International Conference, Bangalore, 1973, p. 189. [18] A. De Vries, Mol. Cryst. Liq. Cryst. 131 (1985) 125. [19] K.L. Wooley, C.J. Hawker, J.M. Pochan, J.M.J. Fre´chet, Macromolecules 26 (1993) 1514. [20] K. Lorenz, D. Ho¨lter, B. Stu¨hn, H. Frey, Adv. Mater. 8 (1996) 414. [21] W.H. de Jeu, W.A.P. Claassen, J. Chem. Phys. 67 (1977) 3705. [22] J.W. van Dijk, W.H. de Jeu, J. Chem. Phys. 79 (1983) 3888. [23] Q.Z. Zhang, J.Q. Liu, X. Yin, J. Zhang, Y. Ji, X. Zhao, G. Li, Acta Chimi. Sin. 61 (2003) 1108. [24] Q.Z. Zhang, J.Z. Zhang, Y. Wang, Acta Polym. Sin. 1 (1996) 121. [25] J.Z. Zhang, Q.Z. Zhang, Acta Chimi. Sin. 55 (1997) 930.
Synthesis and characterization of photochromic star-like liquid crystal Jianqiang Liua,b,*, Qizhen Zhangc, Jingzhi Zhangc a
b
School of Physics and Microelectronics, Shandong University, Ji’nan 250100, PR China Key Laboratory of Education Ministry on Colloid and Interface Chemistry, Ji’nan 250100, PR China c School of Chemistry and Chemical Engineering, Shandong University, Ji’nan 250100, PR China Received 16 September 2004; accepted 17 March 2005 Available online 25 April 2005
Abstract A new photochromic star-like liquid crystal that is written by SiC4 and containing four butoxyazobenzene mesogens in its periphery has been synthesized and characterized by the spectroscopic methods and thermal analysis. SiC4 is nematic phase and its phase behavior is Cr138N147I145N118Cr. The quantum yield and photoisomerization of SiC4 in chloroform (CHCl3) and tetrahydrofuran (THF) have been studied by UV/Vis absorption spectra. The results indicate that the photochromism of SiC4 in CHCl3 and THF are in accordance with the first-order kinetics. The photochromism rate constants in CHCl3 and THF are 101 s1, they are 107 times larger than those of side-chain liquid crystalline polymers containing the same azobenzene moieties. These results show that the star-like structure does not significantly affect the photoisomerization activity of the azobenzene mesogen in its periphery. D 2005 Elsevier B.V. All rights reserved. Keywords: Star-like; Liquid crystal; Phase behavior; Photochromism
1. Introduction
2. Experiment
Photochromic materials have attracted considerable attention owing to their potential technological applications in reversible optic information storage, variable transmitting materials, optical switching and photonic devices [1– 6]. Liquid crystals are one of the most convenient materials to manipulate optical properties and recent advances in research dealing with the synthesis and study of optical properties of a series of photochromic LCs are considered [7 –12]. In this study, we carried out the molecular architecture of the photochromic LC and got the so-called star-like system composed of nematic fragments and four photochromic branches. The star-like compound functionalized with azobenzene group in its periphery and its photochemical reactions and phase behavior are briefly described here.
In this work, the following molecule design was carried out: starting with tetrachlorosilane (SiCl4) as the core molecule, the mesogenic monomer 4-butoxy-4¶hydroxyhexyloxy-azobenzene (BuA) reacted with SiCl4 directly and got a new star-like compound abbreviated to SiC4. The mesogenic monomer BuA is nematic liquid crystal and its phase behavior is Cr112N124I121N110Cr, it was prepared following [13]. The structure of SiC4 was shown in Fig. 1 and its preparation method is as follows: A mixture of 2.0 cm3 of dried pyridine, 15.0 cm3 of dried THF and 1.3 g (3.5 mmol) of BuA were put into a 50 cm3 Wolff bottle after a dry nitrogen inlet. 0.1 cm3 (0.8 mmol) of SiCl4 (prepared in our laboratory) diluted with 10 cm3 of dried THF was added in 0.5 h. The reaction solution was refluxed for 24 h. Then it was cooled to room temperature and filtrated. Purification was carried out by column chromatography (silica gel, THF/anhydrous ethyl alcohol = 3:1 (v/v) as an eluent) and recrystallization from THF/anhydrous ethanol. The SiC4 was obtained in 78%.
* Corresponding author. School of Physics and Microelectronics, Shandong University, Ji’nan 250100, PR China. Tel.: +86 531 8565947; fax: +86 531 8564886. E-mail address: [email protected] (J. Liu). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.03.039
2532
J. Liu et al. / Materials Letters 59 (2005) 2531 – 2534
The SiC4 was characterized by EA, IR, 1H NMR, POM, DSC and XRD. The photo irradiation 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.
3. Results and discussion 3.1. Chemical material Elemental analysis of compound SiC4 was carried out with a Perkin-Elmer 240 C auto elementary analyser, calculated for SiO12C88H116N8: C 68.07, H 7.53, N 7.22 (%); found: C 67.94, H 7.42, N 7.11 (%); Infrared spectra were recorded on a Nicolet 5DX Fourier transform infrared photoacoustic spectral system (KBr/cm1): 2936, 2872 ( –CH2 –), 1602, 1580 (ph), 1498, 1473 (NfN), 1244.4 ( –OCH2); 1H NMR was carried out with a Japan Jeol FX90Q (90MHz, CDCl3, y ppm): 0.96 (t, 12H, – CH3), 1.12 – 1.92 (m, 48H, CH2), 3.60 – 4.08 (m, 24H, OCH2), 6.88, 7.00, 7.64, 7.88 (m, 32H, ph– H). This showed that the compound has the expectant structure as Fig. 1. 3.2. Characterization of LC phase Polarizing optical microscope with a heating stage was used to observe the optical texture of SiC4. Upon slow cooling from the isotropic phase, two and four black brush defects (Schlieren texture), typical of a nematic phase, are observed, as shown in Fig. 2 [14,15]. The transition temperatures and the associated enthalpies and entropies of SiC4 have been established for the heating and cooling cycles using the differential scanning calorimetry (DSC, Perkin-Elmer 7 series thermal analysis system) (see Table 1). In order to compare with the common liquid crystals and liquid crystalline polymers, the enthalpies and
O
O O
O
Si O
Fig. 2. Picture observed under a POM (cooling 138 -C, Shlieren texture).
entropies per mesogen of SiC4 were also given in the Table 1. The DSC data of SiC4 showed on heating two distinct endothermic transitions and the lower and the higher temperatures corresponding to melting and isotropization, respectively. Upon cooling, a modest degree of supercooling (16 K) was found for the latter transition so it showed broader liquid crystalline phase domain than in the heating process. The transition enthalpies per mesogen of SiC4 is about 1.6 kJImol1Imesogen1, it belongs to the statistic values 1.3 – 3.6 or 0.84 – 9.6 (kJImol1) of N – I phase transition enthalpies [16,17], so the DSC data hold out that the nematic phase was in existence for SiC4. Wide angle X-ray diffraction (D/max-gB diffractometer with Cu Ka radiation) studies on the mesophase have been carried out at ambient temperature. The resulting diffraction shows a slow peak in the little angle (2h < 10-) and a dispersion peak in the larger angle (2h = 19.8-) (see Fig. 3). The intense peak at 2h = 19.8- is due to the mesogen– mesogen spacing of 0.45 nm which is a value typically found in LC phases [16]. The dispersion peak in the larger 2h values is like the isotropic liquid; it shows that molecules array disorder in landscape orientation. The small intensity of the signal at low 2h values may suggest the presence of a low ordered system in the direction of longitudinal axis of the molecule. This is in agreement with the low transition enthalpy observed calorimetrically. Furthermore, there is strong intensity near the origin, so the XRD pattern can be
O
O
Table 1 DSC data for SiC4 Compound Conditions T/-C
O
N=N
DS/J mol1 K1
1
DS/J mol1 K1 mesogen1
DH/kJ mol mesogen1 SiC4
=
DH/kJ mol1
OC4H9
Fig. 1. The structure of star-like liquid crystal SiC4.
1st heating 1st cooling 2nd heating
138.26 147.43 102.68 6.38 25.67 1.60 144.8 118.35 6.10 79.7 1.53 19.93 138.25 147.43 98.0 6.35 24.5 1.63
260 15.04 65 3.76 14.6 210.56 3.65 52.64 240.64 15.04 60.16 3.76
J. Liu et al. / Materials Letters 59 (2005) 2531 – 2534
2500
2533
1 1.1
1
0.7
2 1.9
Counts
1600
3 2.9 4 3.6
0.5
900
5 4.1
A
400
6 5.0 0.3
100
7 7.2
0.0
8 ∞ (s) 0
10
20
30
40
2 /(º)
8
0.1
Fig. 3. WAXRD curve of the star-like liquid crystal SiC4.
300
350
400
450
500
550
λ (nm)
3.3. Photochromic behavior of SiC4 A series of different concentration solutions of SiC4 in CHCl3 and THF were prepared, they were scanned in the range of 300– 600 nm. Then we can find the maximum absorption spectra is 360 nm, so k max = 360 nm. Draw the absorbency of k max with the concentration and then get a line, the slope of the line is the molar absorption coefficient e of SiC4 and its E in CHCl3 and THF were 1.31 105 and 1.17 105 (dm3Imol1Icm1), respectively. They are about
Fig. 4. UV/Vis spectrum of SiC4 in chloroform and tetrahydrofuran.
2.7 – 3.8 times higher than that of BuA [23]. So the absorption of SiC4 is stronger than the mesogen’s for the more azobenzene number and the polarity of solvent can decrease the e of SiC4. The quantum yield was determined DAV using the formula u ¼ eLI , where DA represents the a t absorption at the irradiation wavelength, I a 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 cell. We can calculate the quantum yield u of SiC4 in CHCl3 and THF to be 0.127 and 0.152, respectively. The u of SiC4 is less than that of mesogen for the steric influence [23]. The solutions of SiC4 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. The intensity of the UV absorption bands decrease with irradiation time in the 360 nm region. At the same time, the shoulder at about 440 nm gradually more resolved giving rise at the photostationary state to an absorption band with a distinct maximum at 440 nm (see Fig. 4). The occurrence of two distinct isosbestic points at 425 and 320 nm, as well as the similarity of the UV spectra of the irradiated samples at the photo4.0
CHCl3 THF
3.5
ln(A0- A∞)/( At- A∞)
accorded with the characteristics of nematic and smectic A LC phases [18]. To make a comprehensive view on the results of POM, DSC and XRD hereinbefore, we can make sure that the SiC 4 is nematic LC and its phase behavior is Cr138N147I145N118Cr. The star-like liquid crystal’s phase behavior is the same as the mesogen monomer and they are all nematic phases. There is a viewpoint that the nature of chain ends and monomer dramatically affects the properties of the whole dendritic molecule [3,12,19,20]. The mesogenic unit shows nematic phase and it was grafted on the surface of the star-like molecule, so the SiC4 shows nematic phase too. In farther thinking, the properties of LC could be determined in relation to the molecular structure and intermolecular interaction [21,22]. Because of the existence of the rigid rod-like azobenzene moieties in the periphery of the star-like molecule, the distortion took place in the spherical molecule and its branches directed along the long axis of the rod-like moieties, so the whole molecule showed anisotropy. Clearly, attachment of the rod-like mesogenic units by the flexible hexamethyl spacer leads to cooperative deformation of the star-like molecule and formation of LC phase. But the formation of LC phase is so complex that need further study, it maybe related to the mesogen density as well as the spacer length. The melting point and clearing temperature of SiC4 are higher than the mesogenic monomer’s due to its greater anisotropy. The liquid crystal’s domain of SiC4 is broader than the monomer’s, because the deformations of its especial star-like structure need higher temperature and more energy.
3.0 2.5 2.0 1.5 1.0 0.5 0.0 0
1
2
3
4
5
6
7
8
t/s Fig. 5. Photoisomerization of SiC4 in chloroform and tetrahydrofuran.
2534
J. Liu et al. / Materials Letters 59 (2005) 2531 – 2534
stationary state with that of cis-azobenzene, indicates that only two absorbing species (trans and cis isomers) are present and no side reactions such as photocrosslinking or photodegradation occur. In all cases ln[(A 0 A V) / (A t A V)], where A 0 A t and A V are the absorbances at 360 nm at time zero, t and infinite, respectively, shows a linear dependence on the irradiation time (see Fig. 5). They are in accordance with the first-order kinetics and the slope is the photochromism rate constant k p. The k p of SiC4 in CHCl3 and THF were 0.567 and 0.514 (s1), respectively. They were close to those of BuA and 107 times larger than those of side-chain liquid crystalline polymers in the same solutions [23 – 25]. These results indicate that the star-like structure does not significantly affect the photoisomerization rate, probably due to the presence of the flexible hexamethyl spacers and the mesogens were partly decoupled from the core. At the same time, there were no chain entanglements and the bondage of main chain to the side chains in the starlike molecule, so the SiC4 has better photoresponse behavior than the common liquid crystalline polymers.
4. Conclusion A new photochromic star-like liquid crystal SiC4 was synthesized, its phase behavior is Cr138N147I145N118Cr and its liquid crystal domain is broader than the monomer’s. Its photochromism rate constants in CHCl3 and THF are 101 s1, they are much larger than those of side-chain liquid crystalline polymers containing the same azobenzene moieties. The terminal azobenzene fragments not only provide its sensitivity to the light but also perform a development of LC phases for their rigid rodlike form. Undoubtedly, the preparation of photoactive star-like liquid crystal presents an evident scientific and practical interest because the star-like molecule allows one to anticipate a fast optical response. This aspect should be interesting for the development of fast-acting photosensitive materials, which can be easily handled. So the starlike liquid crystal has potential applications and will become a new type photocontrollable switch and information functional material.
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