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Electric linear dichroism study of some Sudan dyes using electro-optic and spectroscopic methods
Journal of Molecular Liquids 109 (2004) 143–148
Electric linear dichroism study of some Sudan dyes using electro-optic and spectroscopic methods A. Ghanadzadeha,*, M.S. Zakerhamidia, H. Tajallib a
Department of Chemistry, Faculty of Science, Gilan University, Rasht, Iran b Center of Applied Physics, Tabriz University, Tabriz, Iran Received 6 November 2002; accepted 6 August 2003
Abstract Contrast ratios CR and order parameters S of Sudan black B and solutions containing two phenolic Sudan dyes (Sudan III and Sudan IV) in nematic solvents (E7 and E8) were investigated using polarized spectroscopy in a guest–host system based on homogeneous–homeotropic alignment. The orientation of the dye molecules was controlled using an electric field, and this enabled the contrast ratio of the dye to be obtained by electrically switching. The transient molecular reorientation and linear electric dichroism of the guest–host system (Sudan black B-E8) was also investigated in an electro-optical system using the rectangular-shaped pulsed electric field. Moreover, temperature dependence of the order parameter of Sudan black B in the nematic host was studied. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Sudan dyes; Contrast ratio; Order parameter; Guest–host interaction; Electro-optical effect
1. Introduction When a dichroic dye is dissolved as a guest in an ordered nematic liquid crystalline host, the dye molecules align with the long molecular axes along the nematic director. The orientation of the dye molecules can be switched along the nematic orientation by the application of an electric field. This phenomenon was first observed by Heilmeier, Castellano and Zanoni w1,2x and is known as guest–host interaction. Conventional guest–host cells use nematic liquid crystals with positive dielectric anisotropy and an initial planar (homogeneous) alignment. Many researchers have investigated various kinds of dyes in order to improve the dichroic ratio w3–6x. At present, mainly azo and anthraquinone dyes are investigated, because they have relatively high dichroic ratios, generally high stability and high solubility w7–11x. Oriented nematic liquid crystals are excellently suited as anisotropic solvents for polarized spectroscopy w12– 16x. The use of nematic liquid crystals in this manner provides a quick and simple method for spectroscopic *Corresponding author. Fax: q98-131-30912. E-mail address: [email protected] (A. Ghanadzadeh).
studies of the anisotropy of physical properties. Moreover, the optical polarization measurements made using nematic solvents give important information on the orientation of the solute molecules, the direction of transition moments and the polarizations of electronic absorption bands w17x. We have recently reported w11x the linear dichroism investigation of some Sudan dyes in nematic liquid crystals (E7 and MBBA) without using an electric field and showed that the solvation of the guest molecules in the liquid crystalline host could be affected by a specific anisotropic interaction related to the alignment by the liquid crystal property as well as solvent polarity. Moreover, we also concluded that the order parameters of the Sudan dyes depend on the size, shape, polarity, steric effect and aggregative properties of the substituents w11x. The chosen dyes have generally high solubility. Sudan black B has high stability and also the advantage of a high dichroic ratio because of its rod-like molecular shape. Therefore, this dye would be suitable for commercial applications because it is electrochemically and photochemically stable enough for LCD applications. Apart from Sudan black B, two of the azo dyes investigated in this work would not be suitable for commer-
0167-7322/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2003.08.001
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Fig. 1. Molecular structure of the Sudan dyes.
cial applications because they are phenols. Thus, they are not electrochemically or photochemically stable enough for LCD applications. However, from the scientific point of view, spectroscopic investigation of phenolic Sudan dyes is very important as they can interact via –OH group with the –CN of the chosen liquid crystals. However, in the course of the present study we report the results of the electro-optical behaviour of Sudan black B and two phenolic Sudan dyes (Sudan III and Sudan IV) using polarized spectroscopy in a guest–host system. The orientation of the dye molecules was controlled using an electric field, and this enabled the contrast ratio of the dye to be obtained by electrically switching. In this study, the dynamic behaviour and electric linear dichroism (ELD) of a thin layer of the guest–host system (Sudan black B-E8) was also investigated using a rectangular-shaped pulsed electric field. The ELD is one of the electro-optic methods that measure the change in the absorption of the incident light beams linearly polarized in parallel and perpendicular to the applied field direction. 2. Experimental 2.1. The guest–host materials Sudan dyes (Fig. 1) were obtained from BDH (proanalysis) and used without further purification as solutes
(guests). The eutectic nematic mixtures of E7 and E8 (nematic mixtures of cyanobiphenyl and terphenyl) with positive dielectric anisotropy (D´)0) were supplied by BDH Chemicals Ltd, and used as anisotropic hosts. The nematic–isotropic transition temperatures for E7 and E8 were obtained using a polarizing microscope (Reichert) equipped with a heating stage to be 333 and 343 K, respectively. The observed transition temperatures were in very good agreement with those reported in Ref. w18x. The dyes were dissolved separately in the liquid crystals with a concentration of 1%. 2.2. Liquid crystal cell preparation Indiumytin oxide (ITO) coated glass plates were used as transparent electrodes for the measurement of the polarized absorption as a function of electric field strength. The guest–host cells (Fig. 2) were made by sandwiching the solutions between two glass plates (2=1.2 cm2). The planar orientation (homogeneous alignment) of the guest and host molecules was achieved by surface treatment of the glass surfaces of the cells with polyvinyl alcohol (PVA) and additional rubbing process. This procedure gave a good homogeneous molecular orientation in a thin layer, which was controlled with the aid of the polarized microscope (Reichert) (Fig. 3). Mylar film (12.5 mm) was used as spacers.
Fig. 2. Cross-section of the experimental guest–host cell.
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displayed using a Thurlby Thandar storage oscilloscope (Scope Master SM 630). A pulse of voltage (1 s) was applied to the cell, and the corresponding optical signal was recorded on the oscilloscope. The pulse duration was adjusted sufficiently long to obtain the maximum orientation of the molecules at a given field strength. The higher the field, the shorter is the time required to reach the orientation equilibrium. The sample cell was mounted in an oven, which contained windows for the light beam. The temperature of the oven was measured using a copper constantan thermocouple in contact with the glass cell. 2.4. Measurement of contrast ratios
Fig. 3. Parallel-aligned film of the guest–host mixture (Sudan black B-E8) between glass plates that are coated by rubbed PVA at 22 8C; thickness 12.5 mm; crossed polarizers; magnification 200=.
The cell was filled with the chosen sample by capillary action. Copper wires were connected to the plates using a two-part silver-loaded epoxy (RS components 567604) and electrically conductive paint (RS components 555-156). 2.3. Measurements of the electric linear dichroism using a rectangular-shaped pulsed electric field The absorption changes due to the dichroism can be deduced from the transient recorded when the electric pulse is applied to the sample. The measurement of the dynamic behaviour and linear electric dichroism of a thin layer of the guest–host system (Sudan black B dye-E8) was performed with the following apparatus (Fig. 4). A Spectra physics HeyNe laser (Scientific and Cook, model SLHy2) emitting at a wavelength of 632.8 nm with a power of 5 mW was used as a light source. ITO coated glass plates were used as transparent electrodes for the measurement of the transmittance as a function of electric field strength. The polarizer was of commercial origin. The detection of the signal was achieved using a photomultiplier tube, type E. M. I. 9816B. The optical signal from the photomultiplier was
The polarized absorption spectra of the guest–host mixtures in the visible spectral region were measured using a Shimadzu UV–Vis double-beam spectrophotometer (Model UV-2100) equipped with polarizers. The temperature of the cells was regulated and controlled with an accuracy of "0.1 K. The polarized absorption spectra of the dissolved dyes were taken with the electric field off and then with the electric field on (30 V: saturated voltage). The ratio between maximum absorption in the on and off states (contrast ratio) was calculated using CRsAoff yAon. 3. Results and discussion 3.1. The linear electric dichroism using polarized spectroscopy Sudan dyes were dissolved separately in the host in a cell configured for planar alignment. The electrooptical effects of the dyes were measured using a parallel-aligned guest–host cell, and their contrast ratios (CRsAoff yAon) were obtained. The contrast ratios of the dyes were calculated from the absorption of light in the electric field on and electric field off states. Here Aoff is the absorbance at the maximum absorption wavelength in the off state and Aon is that in the on state. Since the direction of the transition dipole moment of the dye molecules are largely parallel to the long molecular axis, order parameters S of the dyes were calculated from the equation Ss(Ry1)y(Rq2). The experimental results are shown in Table 1. It should be noted that the value of S might differ from the order parameter for the molecular axis SM. This is the case
Fig. 4. Optical arrangement for the measurement of the dynamic behaviour of a thin guest–host cell.
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Table 1 Dichroic ratios R and order parameters S measured for Sudan dyes in E8 solvent used in the guest–host experiments Compound
lmax (nm)
AoffyAon
Sdye
Sdyea
Sudan III Sudan IV Sudan black B
520 533 620
3.7 3.4 5.7
0.47 0.43 0.61
0.49 0.45 0.62
a
The data are taken from our previous work w11x.
when there is an angle between the transition moment and the long molecular axis. The contrast ratio CR directly relates to the order parameter for the transition moment, while the molecular structure is considered to relate to the order parameter for the molecular axis. As it can be seen from Table 1 that the contrast ratio values obtained in the present work are slightly smaller than the dichroic ratio values obtained in the previous report w11x. It should be noted that the dichroic ratios have been calculated from the absorption of light polarized parallel (A≤) and perpendicular (AH) to the liquid crystal alignment. As Aon (electric field direction dependence) and AH are not really the same quantities, the dichroic ratio and the contrast ratio values are different. The experimental results show that Sudan black B has a high dichroic ratio and order parameter with respect to the two other phenolic dyes. This is a consequence of the long molecular structure and the lack of lateral groups for this molecule. The high-order parameter of Sudan black B is due in part to the high polarizability of the molecule along the long molecular axis. The presence of the –OH group in the two Sudan dyes causes the direction of transition moment to deviate from the long molecular axis. However, the lower-order parameter of phenolic Sudan dyes compared to that of the Sudan black B could also be due in part to the interaction between the –OH group of the dyes and the –CN group of the liquid crystals. Fig. 5 shows the polarized absorption spectra of 1% nematic solution of Sudan black B and in different electric field strength. With zero a.c. electric field (Es 0), the liquid crystal molecules are in a uniform parallel orientation and the dye molecules were oriented with the long molecular axes parallel to the electric field vector of the polarized light (i.e. parallel to the nematic director). The intense band (Fig. 5a) became weak when the field was applied. In a voltage of 30 V (Fig. 5d), the voltage–absorption curve was saturated; in fact, a minimum absorption (Aon) of the dye was recorded. The similar trend was observed for the two phenolic dyes (Fig. 6). Figs. 5 and 6 indicate that A≤yAH is positive for the absorption band of the dyes when the transition corresponding to the visible band is polarized in the direction of the long molecular axis, and can be considered as
Fig. 5. Polarized absorption spectra of 1% Sudan black B in the oriented nematic mixture of E8 at different voltages (5 kHz); (a) 0; (b) 5; (c) 25; (d) 35 V, and at 22 8C.
parallel transitions (i.e. p–p*). In other words, during the p–p* transition a net displacement of charge occurs parallel to the molecular plane. Fig. 7 shows the voltage dependence of the transmittance of the polarized light measured at 620 nm (the maximum absorption wavelength) for Sudan black B dissolved in nematic solvents. As it can be seen from this figure, the degree of color switching is a function of field strength. Fig. 8 shows the guest order parameter in the liquid crystal E7 as a function of the temperature. For com-
Fig. 6. Polarized absorption spectra of 1% Sudan IV in the oriented nematic mixture of E8 at different voltages (5 kHz); (a) 0; (b) 5; (c) 25; (d) 35 V.
A. Ghanadzadeh et al. / Journal of Molecular Liquids 109 (2004) 143–148
Fig. 7. The voltage dependence of the transmittance of the polarized light measured at 620 nm for Sudan black B in the nematic mixtures (E7 and E8).
parison, the host order parameter values were obtained using Vuks’ method w19,20x, and are also shown in Fig. 8. The character of the changes of the guest order parameter with rising temperature is similar to that of the host. The dichroic dye dissolved in the nematic solvent aligns, on average, parallel to the nematic director. However, with an increase in temperature the thermal fluctuation of dye molecules increases and therefore the order parameter decreases. It had previously been found that the presence of a dichroic dye in a liquid crystal can change the nematic–isotropic transition temperature, TNI w21x. However, it was observed that the addition of Sudan black B (1% wyw) did not have much effect on the nematic–isotropic transition temperature of E7 and E8. 3.2. The electric linear dichroism using a pulsed electric field The optical response of the guest–host system to a short duration applied field (f1 s) is transient in nature w22x. Fig. 9 shows a schematic optical transient due to the variation of the transmittance resulting from the application of a rectangular-shaped electrical pulse. Three distinct regions of optical response are observed, a ‘rise region’ in which the transmittance is increasing and has a value of DIR (t) at time t following the application of the electric field, a ‘steady-state region’ in which the transmittance remains constant (DIsDIo) and a ‘decay period’ during which the transmittance decreases and has a value of DID (t) at time t after the electric field has been removed. Since the electric dichroism is positive (i.e. Aoff) Aon), the intensity of the light that is transmitted by the optical cell increases in response to the applied electric
147
Fig. 8. Temperature dependence of the order parameter of Sudan black B (guest) in E7 (host). The calculated order parameter (using Vuks’ method) for E7 is included for comparison.
field. Therefore, the rise transient corresponds to an increase in the intensity of light transmitted and the decay curve is characterized by a return to the field-off light level. The optical transient for the Sudan black B in the nematic host was recorded and the electric dichroism was determined as a function of the electric field strength. The transients were obtained using homogeneously aligned samples (i.e. parallel with respect to the windows of the optical cell). Fig. 10 shows the voltage dependence of the transmittance I≤ of the polarized light measured at 632.8 nm for the dye dissolved in the nematic solvent (E8). It can be seen that transmittance I≤ measured for the planar cell increases as the voltage increases. This is because surface layers of the liquid crystal become thin as the voltage is increased. The dichroism data obtained using a pulsed electric field method are in a good agreement with the spectroscopic method. The steady-state reduced dichroism w22x, DAyAs (A≤yAH)yA, of the Sudan black B-E8 solution at a wavelength of ls632.8 nm was obtained to be 1.7. This value is close to 1.8 obtained from the spectroscopic method.
Fig. 9. A schematic optical transient due to the variation of the transmittance resulting from the application of a rectangular-shaped electrical pulse.
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References
Fig. 10. The voltage dependence of the nematic solution of the Sudan black B. Applied field pulse was rectangular and of 1-s duration.
4. Conclusion Sudan black B has the highest dichroic ratio R and the highest-order parameters S. This is a consequence of the long molecular structure and the lack of lateral groups for this molecule, so that the transition moment vector of this dye may be considered to be parallel to the long molecular axis. The high-order parameter of Sudan black B can be due in part to the high polarizability of the molecule along the long molecular axis. Therefore, this dye would be suitable for commercial applications. We found that Sudan black B-E8 mixture was easily reoriented by the applied electric field. The reduced dichroism for Sudan black B-E8 mixture obtained from both methods is in very good agreement.
w1x G.H. Heilmeier, L.A. Zanoni, Appl. Phys. Lett. 13 (1968) 91. w2x G.H. Heilmeier, J.A. Castellano, L.A. Zanoni, Mol. Liq. Cryst. Liq. 8 (1969) 239. w3x P. Chingdvang, S. Bualek, O. Phaovibui, Mol. Cryst. Liq. Cryst. 132 (1986) 131. w4x H. Seki, Y. Shibata, Mol. Cryst. Liq. Cryst. 138 (1986) 349. w5x M.A. Osman, L. Pietrone, T.J. Scheffer, H.R. Zeller, J. Chem. Phys. 74 (1981) 5377. w6x S. Imazeki, A. Mukoh, T. Yoneyama, M. Kaneko, Mol. Cryst. Liq. Cryst. 154 (1987) 79. w7x H. Seki, C. Shishido, S. Yasui, T. Vchida, Jap. J. Appl. Phys. 21 (1982) 191. w8x A.V. Ivashcheneko, Mol. Cryst. Liq. Cryst., Part A 150 (1987) 1. w9x S. Imazeki, A. Mukoh, Mol. Cryst. Liq. Cryst. 225 (1993) 197. w10x D. Bauman, H. Moryson, J. Mol. Struct. 404 (1997) 113. w11x A. Ghanadzadeh, H. Ghanadzadeh, G. Ghasmi, J. Mol. Liq. 88 (2000) 299. w12x H. Iwanaga, K. Naito, F. Effenberger, Liq. Cryst. 27 (2000) 115. w13x M. Bagheri, A. Entezami, A. Ghanadzadeh, J. Mol. Liq. 94 (2001) 249. w14x A. Ghanadzadeh, R. Sariri, K. Bahraima, Spectrochim. Acta, Part A 57 (2001) 155. w15x A. Ghanadzadeh, M.A. Zanjanchi, Spectrochim. Acta, Part A 57 (2001) 1865. w16x A. Ghanadzadeh, M.A. Zanjanchi, R. Tirbandpay, J. Mol. Struct. 616 (1–3) (2002) 167. w17x J. Michl, E.W. Thulstrup, Spectroscopy of Polarised Spectroscopy, VCH Book, 1995. w18x A. Ghanadzadeh, M.S. Beevers, J. Mol. Liq. 94 (2) (2001) 97. w19x M.F. Vuks, Optics Spectrosc. 20 (1966) 361. w20x A. Ghanadzadeh, H. Ghanadzadeh, N. Khoshroo, J. Mol. Liq. 94 (1) (2001) 37. w21x D. Bauman, T. Martynski, J. Jadzyn, Acta Phys. Pol. A 85 (1994) 553. w22x E. Fredericq, C. Houssier, Electric Dichroism and Electric Birefringence, Clarendon, Oxford, 1972.
Electric linear dichroism study of some Sudan dyes using electro-optic and spectroscopic methods A. Ghanadzadeha,*, M.S. Zakerhamidia, H. Tajallib a
Department of Chemistry, Faculty of Science, Gilan University, Rasht, Iran b Center of Applied Physics, Tabriz University, Tabriz, Iran Received 6 November 2002; accepted 6 August 2003
Abstract Contrast ratios CR and order parameters S of Sudan black B and solutions containing two phenolic Sudan dyes (Sudan III and Sudan IV) in nematic solvents (E7 and E8) were investigated using polarized spectroscopy in a guest–host system based on homogeneous–homeotropic alignment. The orientation of the dye molecules was controlled using an electric field, and this enabled the contrast ratio of the dye to be obtained by electrically switching. The transient molecular reorientation and linear electric dichroism of the guest–host system (Sudan black B-E8) was also investigated in an electro-optical system using the rectangular-shaped pulsed electric field. Moreover, temperature dependence of the order parameter of Sudan black B in the nematic host was studied. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Sudan dyes; Contrast ratio; Order parameter; Guest–host interaction; Electro-optical effect
1. Introduction When a dichroic dye is dissolved as a guest in an ordered nematic liquid crystalline host, the dye molecules align with the long molecular axes along the nematic director. The orientation of the dye molecules can be switched along the nematic orientation by the application of an electric field. This phenomenon was first observed by Heilmeier, Castellano and Zanoni w1,2x and is known as guest–host interaction. Conventional guest–host cells use nematic liquid crystals with positive dielectric anisotropy and an initial planar (homogeneous) alignment. Many researchers have investigated various kinds of dyes in order to improve the dichroic ratio w3–6x. At present, mainly azo and anthraquinone dyes are investigated, because they have relatively high dichroic ratios, generally high stability and high solubility w7–11x. Oriented nematic liquid crystals are excellently suited as anisotropic solvents for polarized spectroscopy w12– 16x. The use of nematic liquid crystals in this manner provides a quick and simple method for spectroscopic *Corresponding author. Fax: q98-131-30912. E-mail address: [email protected] (A. Ghanadzadeh).
studies of the anisotropy of physical properties. Moreover, the optical polarization measurements made using nematic solvents give important information on the orientation of the solute molecules, the direction of transition moments and the polarizations of electronic absorption bands w17x. We have recently reported w11x the linear dichroism investigation of some Sudan dyes in nematic liquid crystals (E7 and MBBA) without using an electric field and showed that the solvation of the guest molecules in the liquid crystalline host could be affected by a specific anisotropic interaction related to the alignment by the liquid crystal property as well as solvent polarity. Moreover, we also concluded that the order parameters of the Sudan dyes depend on the size, shape, polarity, steric effect and aggregative properties of the substituents w11x. The chosen dyes have generally high solubility. Sudan black B has high stability and also the advantage of a high dichroic ratio because of its rod-like molecular shape. Therefore, this dye would be suitable for commercial applications because it is electrochemically and photochemically stable enough for LCD applications. Apart from Sudan black B, two of the azo dyes investigated in this work would not be suitable for commer-
0167-7322/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2003.08.001
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Fig. 1. Molecular structure of the Sudan dyes.
cial applications because they are phenols. Thus, they are not electrochemically or photochemically stable enough for LCD applications. However, from the scientific point of view, spectroscopic investigation of phenolic Sudan dyes is very important as they can interact via –OH group with the –CN of the chosen liquid crystals. However, in the course of the present study we report the results of the electro-optical behaviour of Sudan black B and two phenolic Sudan dyes (Sudan III and Sudan IV) using polarized spectroscopy in a guest–host system. The orientation of the dye molecules was controlled using an electric field, and this enabled the contrast ratio of the dye to be obtained by electrically switching. In this study, the dynamic behaviour and electric linear dichroism (ELD) of a thin layer of the guest–host system (Sudan black B-E8) was also investigated using a rectangular-shaped pulsed electric field. The ELD is one of the electro-optic methods that measure the change in the absorption of the incident light beams linearly polarized in parallel and perpendicular to the applied field direction. 2. Experimental 2.1. The guest–host materials Sudan dyes (Fig. 1) were obtained from BDH (proanalysis) and used without further purification as solutes
(guests). The eutectic nematic mixtures of E7 and E8 (nematic mixtures of cyanobiphenyl and terphenyl) with positive dielectric anisotropy (D´)0) were supplied by BDH Chemicals Ltd, and used as anisotropic hosts. The nematic–isotropic transition temperatures for E7 and E8 were obtained using a polarizing microscope (Reichert) equipped with a heating stage to be 333 and 343 K, respectively. The observed transition temperatures were in very good agreement with those reported in Ref. w18x. The dyes were dissolved separately in the liquid crystals with a concentration of 1%. 2.2. Liquid crystal cell preparation Indiumytin oxide (ITO) coated glass plates were used as transparent electrodes for the measurement of the polarized absorption as a function of electric field strength. The guest–host cells (Fig. 2) were made by sandwiching the solutions between two glass plates (2=1.2 cm2). The planar orientation (homogeneous alignment) of the guest and host molecules was achieved by surface treatment of the glass surfaces of the cells with polyvinyl alcohol (PVA) and additional rubbing process. This procedure gave a good homogeneous molecular orientation in a thin layer, which was controlled with the aid of the polarized microscope (Reichert) (Fig. 3). Mylar film (12.5 mm) was used as spacers.
Fig. 2. Cross-section of the experimental guest–host cell.
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displayed using a Thurlby Thandar storage oscilloscope (Scope Master SM 630). A pulse of voltage (1 s) was applied to the cell, and the corresponding optical signal was recorded on the oscilloscope. The pulse duration was adjusted sufficiently long to obtain the maximum orientation of the molecules at a given field strength. The higher the field, the shorter is the time required to reach the orientation equilibrium. The sample cell was mounted in an oven, which contained windows for the light beam. The temperature of the oven was measured using a copper constantan thermocouple in contact with the glass cell. 2.4. Measurement of contrast ratios
Fig. 3. Parallel-aligned film of the guest–host mixture (Sudan black B-E8) between glass plates that are coated by rubbed PVA at 22 8C; thickness 12.5 mm; crossed polarizers; magnification 200=.
The cell was filled with the chosen sample by capillary action. Copper wires were connected to the plates using a two-part silver-loaded epoxy (RS components 567604) and electrically conductive paint (RS components 555-156). 2.3. Measurements of the electric linear dichroism using a rectangular-shaped pulsed electric field The absorption changes due to the dichroism can be deduced from the transient recorded when the electric pulse is applied to the sample. The measurement of the dynamic behaviour and linear electric dichroism of a thin layer of the guest–host system (Sudan black B dye-E8) was performed with the following apparatus (Fig. 4). A Spectra physics HeyNe laser (Scientific and Cook, model SLHy2) emitting at a wavelength of 632.8 nm with a power of 5 mW was used as a light source. ITO coated glass plates were used as transparent electrodes for the measurement of the transmittance as a function of electric field strength. The polarizer was of commercial origin. The detection of the signal was achieved using a photomultiplier tube, type E. M. I. 9816B. The optical signal from the photomultiplier was
The polarized absorption spectra of the guest–host mixtures in the visible spectral region were measured using a Shimadzu UV–Vis double-beam spectrophotometer (Model UV-2100) equipped with polarizers. The temperature of the cells was regulated and controlled with an accuracy of "0.1 K. The polarized absorption spectra of the dissolved dyes were taken with the electric field off and then with the electric field on (30 V: saturated voltage). The ratio between maximum absorption in the on and off states (contrast ratio) was calculated using CRsAoff yAon. 3. Results and discussion 3.1. The linear electric dichroism using polarized spectroscopy Sudan dyes were dissolved separately in the host in a cell configured for planar alignment. The electrooptical effects of the dyes were measured using a parallel-aligned guest–host cell, and their contrast ratios (CRsAoff yAon) were obtained. The contrast ratios of the dyes were calculated from the absorption of light in the electric field on and electric field off states. Here Aoff is the absorbance at the maximum absorption wavelength in the off state and Aon is that in the on state. Since the direction of the transition dipole moment of the dye molecules are largely parallel to the long molecular axis, order parameters S of the dyes were calculated from the equation Ss(Ry1)y(Rq2). The experimental results are shown in Table 1. It should be noted that the value of S might differ from the order parameter for the molecular axis SM. This is the case
Fig. 4. Optical arrangement for the measurement of the dynamic behaviour of a thin guest–host cell.
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Table 1 Dichroic ratios R and order parameters S measured for Sudan dyes in E8 solvent used in the guest–host experiments Compound
lmax (nm)
AoffyAon
Sdye
Sdyea
Sudan III Sudan IV Sudan black B
520 533 620
3.7 3.4 5.7
0.47 0.43 0.61
0.49 0.45 0.62
a
The data are taken from our previous work w11x.
when there is an angle between the transition moment and the long molecular axis. The contrast ratio CR directly relates to the order parameter for the transition moment, while the molecular structure is considered to relate to the order parameter for the molecular axis. As it can be seen from Table 1 that the contrast ratio values obtained in the present work are slightly smaller than the dichroic ratio values obtained in the previous report w11x. It should be noted that the dichroic ratios have been calculated from the absorption of light polarized parallel (A≤) and perpendicular (AH) to the liquid crystal alignment. As Aon (electric field direction dependence) and AH are not really the same quantities, the dichroic ratio and the contrast ratio values are different. The experimental results show that Sudan black B has a high dichroic ratio and order parameter with respect to the two other phenolic dyes. This is a consequence of the long molecular structure and the lack of lateral groups for this molecule. The high-order parameter of Sudan black B is due in part to the high polarizability of the molecule along the long molecular axis. The presence of the –OH group in the two Sudan dyes causes the direction of transition moment to deviate from the long molecular axis. However, the lower-order parameter of phenolic Sudan dyes compared to that of the Sudan black B could also be due in part to the interaction between the –OH group of the dyes and the –CN group of the liquid crystals. Fig. 5 shows the polarized absorption spectra of 1% nematic solution of Sudan black B and in different electric field strength. With zero a.c. electric field (Es 0), the liquid crystal molecules are in a uniform parallel orientation and the dye molecules were oriented with the long molecular axes parallel to the electric field vector of the polarized light (i.e. parallel to the nematic director). The intense band (Fig. 5a) became weak when the field was applied. In a voltage of 30 V (Fig. 5d), the voltage–absorption curve was saturated; in fact, a minimum absorption (Aon) of the dye was recorded. The similar trend was observed for the two phenolic dyes (Fig. 6). Figs. 5 and 6 indicate that A≤yAH is positive for the absorption band of the dyes when the transition corresponding to the visible band is polarized in the direction of the long molecular axis, and can be considered as
Fig. 5. Polarized absorption spectra of 1% Sudan black B in the oriented nematic mixture of E8 at different voltages (5 kHz); (a) 0; (b) 5; (c) 25; (d) 35 V, and at 22 8C.
parallel transitions (i.e. p–p*). In other words, during the p–p* transition a net displacement of charge occurs parallel to the molecular plane. Fig. 7 shows the voltage dependence of the transmittance of the polarized light measured at 620 nm (the maximum absorption wavelength) for Sudan black B dissolved in nematic solvents. As it can be seen from this figure, the degree of color switching is a function of field strength. Fig. 8 shows the guest order parameter in the liquid crystal E7 as a function of the temperature. For com-
Fig. 6. Polarized absorption spectra of 1% Sudan IV in the oriented nematic mixture of E8 at different voltages (5 kHz); (a) 0; (b) 5; (c) 25; (d) 35 V.
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Fig. 7. The voltage dependence of the transmittance of the polarized light measured at 620 nm for Sudan black B in the nematic mixtures (E7 and E8).
parison, the host order parameter values were obtained using Vuks’ method w19,20x, and are also shown in Fig. 8. The character of the changes of the guest order parameter with rising temperature is similar to that of the host. The dichroic dye dissolved in the nematic solvent aligns, on average, parallel to the nematic director. However, with an increase in temperature the thermal fluctuation of dye molecules increases and therefore the order parameter decreases. It had previously been found that the presence of a dichroic dye in a liquid crystal can change the nematic–isotropic transition temperature, TNI w21x. However, it was observed that the addition of Sudan black B (1% wyw) did not have much effect on the nematic–isotropic transition temperature of E7 and E8. 3.2. The electric linear dichroism using a pulsed electric field The optical response of the guest–host system to a short duration applied field (f1 s) is transient in nature w22x. Fig. 9 shows a schematic optical transient due to the variation of the transmittance resulting from the application of a rectangular-shaped electrical pulse. Three distinct regions of optical response are observed, a ‘rise region’ in which the transmittance is increasing and has a value of DIR (t) at time t following the application of the electric field, a ‘steady-state region’ in which the transmittance remains constant (DIsDIo) and a ‘decay period’ during which the transmittance decreases and has a value of DID (t) at time t after the electric field has been removed. Since the electric dichroism is positive (i.e. Aoff) Aon), the intensity of the light that is transmitted by the optical cell increases in response to the applied electric
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Fig. 8. Temperature dependence of the order parameter of Sudan black B (guest) in E7 (host). The calculated order parameter (using Vuks’ method) for E7 is included for comparison.
field. Therefore, the rise transient corresponds to an increase in the intensity of light transmitted and the decay curve is characterized by a return to the field-off light level. The optical transient for the Sudan black B in the nematic host was recorded and the electric dichroism was determined as a function of the electric field strength. The transients were obtained using homogeneously aligned samples (i.e. parallel with respect to the windows of the optical cell). Fig. 10 shows the voltage dependence of the transmittance I≤ of the polarized light measured at 632.8 nm for the dye dissolved in the nematic solvent (E8). It can be seen that transmittance I≤ measured for the planar cell increases as the voltage increases. This is because surface layers of the liquid crystal become thin as the voltage is increased. The dichroism data obtained using a pulsed electric field method are in a good agreement with the spectroscopic method. The steady-state reduced dichroism w22x, DAyAs (A≤yAH)yA, of the Sudan black B-E8 solution at a wavelength of ls632.8 nm was obtained to be 1.7. This value is close to 1.8 obtained from the spectroscopic method.
Fig. 9. A schematic optical transient due to the variation of the transmittance resulting from the application of a rectangular-shaped electrical pulse.
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References
Fig. 10. The voltage dependence of the nematic solution of the Sudan black B. Applied field pulse was rectangular and of 1-s duration.
4. Conclusion Sudan black B has the highest dichroic ratio R and the highest-order parameters S. This is a consequence of the long molecular structure and the lack of lateral groups for this molecule, so that the transition moment vector of this dye may be considered to be parallel to the long molecular axis. The high-order parameter of Sudan black B can be due in part to the high polarizability of the molecule along the long molecular axis. Therefore, this dye would be suitable for commercial applications. We found that Sudan black B-E8 mixture was easily reoriented by the applied electric field. The reduced dichroism for Sudan black B-E8 mixture obtained from both methods is in very good agreement.
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