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Investigation of the Photoinduced Optical Anisotropy of Azo Dye Mesophases Ch. Hahn,† I. Spring,‡ C. Thunig,‡ G. Platz,‡ and A. Wokaun*,†,§ General Energy Research, Paul Scherrer Institut, CH-5232 Villigen, Switzerland, Physical Chemistry I, University of Bayreuth, D-95440 Bayreuth, Germany, and Department of Chemical Engineering and Industrial Chemistry, Swiss Federal Institute of Technology, CH-8092 Zu¨ rich, Switzerland Received April 27, 1998. In Final Form: September 14, 1998
The photoinduced optical anisotropy (POA) of novel aqueous azo dye systems which form chromonic mesophases upon addition of methanol or sodium chloride was investigated. The phases were characterized between crossed polarizers. For further characterization rheological and holographic transient relaxation experiments were carried out. Distinctive differences in the hologram formation and decay were found for phases which exhibit the same texture in polarization microscopy. The generated holographic gratings could be assigned to either photoinduced phase transitions or an alignment of optical axes. In the present study we concentrate on a comparison of phases with the same macroscopic birefringence; results are discussed in comparison with models in the literature.
Introduction Mesophases formed by surfactants in aqueous solutions as a result of their amphiphilic character are well-known and thoroughly investigated. Much less well-known are the mesophases formed by flat, polyaromatic compounds in solution. The term “chromonic” was introduced for these types of liquid crystalline phases found in systems of certain antiallergic/antiasthmatic drugs, especially of disodium chromoglycate.1 In further studies it was found that these mesophases are not unique for certain drugs but are also common for water-soluble dyes like azo and cyanine dyes.2 The dye and drug aggregates are very different from surfactant aggregates, since the intramolecular forces are probably due to interactions of σ and π electrons of adjacent molecules and not to the hydrophobic effect. That means these aggregates are much more ordered and rigid than their amphiphilic counterparts, thus enabling long range order effects even at low concentrations. By now it has been established that the mesophases consist of columnar or layered stacks,3 but many effects are still not fully understood. On the other hand, azo dyes are well characterized, since they are commercially available and have widespread uses. Apart from their industrial utilization in the dyeing process of textile fibers, these compounds lately received attention for possible application in optical devices due to their photochromic property. Azo compounds in general are photochromic since they undergo trans-cis isomer* To whom correspondence should be addressed. † Paul Scherrer Institute. ‡ University of Bayreuth. § Swiss Federal Institute of Technology. (1) Cox, J.; Woodward, G.; McCrone, W. J. Pharm. Sci. 1971, 60, 1458. Hartshorne, N.; Woodward, G. Mol. Cryst. Liq. Cryst. 1973, 23, 343. Attwood, T.; Lyndon, J. Mol. Cryst. Liq. Cryst. 1984, 108, 349 (2) Attwood, T.; Lyndon, J. Liq. Cryst. 1986, 1, 499. Sadler, D.; Shannon, M.; Tollin, P.; Young, D.; Edmondson, M.; Rainsford, P. Liq. Cryst. 1986, 1, 509. (3) Tiddy, G.; Mateer, D.; Ormerod, A.; Harrison, W.; Edwards, D. Langmuir 1995, 11, 390. Harrison, W.; Mateer, D.; Tiddy, G. Faraday Discuss. 1996, 104, 139.
Chart 1. Chemical Structure of Levafix Goldgelb (LeGo)
ization upon irradiation at an appropriate wavelength.4 Applications which originate from that property include real-time holography,5 spatial filtering,6 optical switching,7 use as optical probe molecules,8 and much more. All these applications are due to the fact that upon irradiation a time-dependent alteration of properties depending on a spatial coordinate is taking place which may also alter the guest-host interaction. Very promising is the utilization of azo dyes in liquid crystal cells that enables photoinduced reorientation and thus optical switching. In the present study we explored the photoinduced anisotropy (POA) that is exhibited by the chromonic phases of ternary azo dye systems in order to obtain further insight into the properties of these special phases as well as to explore the possibilities of an application in optical devices. We chose the azo dye Levafix Goldgelb (LeGo), with the structural formula shown in Chart 1. Upon addition of sodium chloride or methanol, certain mesophases are formed. Supramolecular fibrils which can be found in the ternary system LeGo/water/methanol have already been investigated by small-angle neutron scattering (SANS) and UV/vis spectroscopy.9 The absorption spectrum of (4) Ross, D.; Blanc, J. In Photochromism; Brown, G., Ed.; Wiley: New York, 1971. (5) Chen, A.; Brady, D. Opt. Lett. 1992, 17, 441. (6) Kato, J.; Yamaguchi, I. Opt. Lett. 1996, 21, 767. (7) Kawanishi, Y.; Suzuki, Y.; Sakuragi, M.; Kamezaki, H.; Ichimura, K. J. Photochem. Photobiol. A 1994, 80, 433. (8) Hahn, C.; Wokaun, A. Langmuir 1997, 13, 391. (9) Imae, T.; Gagel, I.; Thunig, C.; Platz, G.; Iwamoto, T.; Funayama, K. Langmuir 1998, 14, 2197.
10.1021/la980480w CCC: $15.00 © 1998 American Chemical Society Published on Web 10/24/1998
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LeGo is characterized by a broad band around 430 nm which is assigned to the lowest π f π* transition. Excitation of this transition leads to trans-cis isomerization, as usual for all azo compounds. The POA was investigated with an optical setup for forced Rayleigh scattering (FRS) that we used before.8,10 The method is based upon generation of an optical grating in the sample. After termination of the irradiation, the decay of the grating can easily be monitored with the help of a probe beam that is diffracted off the holographic pattern. Conclusions can be drawn on dynamic processes such as diffusion, lifetimes, or interaction of excited states. Experimental Section Materials and Sample Preparation. The dye Levafix Goldgelb was purchased from Dyestar (Hoechst). Purification was necessary in view of the technical grade quality of the material. Therefore, a 6 wt % solution of the crude dye in bidistilled water was filtered through a sintered glass funnel (borosilicate N5) and freeze dried. Fifteen grams of the product was stirred into 500 mL of ethanol p.a. (Merck) and refluxed for 1 h. After addition of 300 mL of bidistilled water, the solution was refluxed for another hour. Then, the solution was stored overnight at 5 °C. The orange precipitate was filtered and dried in high vacuum. Phase Diagrams. The samples were homogenized by heating the appropriate amounts of dye, water, methanol, and/or sodium chloride (100 °C). The phase diagrams were established by observing the mixtures under temperature-controlled conditions for several weeks. Rheological Experiments. In order to obtain further information on the phases, the rheological behavior of the samples was investigated with a Bohlin CS 10 stress-controlled rheometer. Forced Rayleigh Scattering. The method has been described in detail elsewhere.11 For this study an argon ion laser (Spectra Physics, Model 165, λ ) 514 nm) was used for excitation, and a He-Ne laser (Spectra Physics, λ ) 633 nm) for detection of the grating. The output of the argon ion laser is split into two beams of equal intensity by a cubic beam splitter, and the beams are crossed in a quartz cuvette (Hellma, 10 × 2 mm2). The fringe spacing could be varied by changing the angle of intersection, R. The length of the writing pulse was conveniently controlled by means of a Kerr cell (Gsa¨nger, LM 0202 P). Since the probing wavelength is far from an absorption band of the sample, the diffracted first-order signal is due to a pure phase grating. Because of the high absorption coefficients of the samples, the developed gratings corresponded to thin holograms; that is, constructive interference is occurring for all angles of detection, and usually several orders of diffraction may be observed. All measurements have been performed at room temperature if not noted otherwise. For determination of the diffusion coefficients, the data are evaluated according to the standard FRS theory.11,12 The diffracted intensity I(t) decays from its starting value according to (1)
I(t) ) (Ae-t/τ + B)2 + C2
(1)
Since the decay constant τ is influenced by both the relaxation time of the cis isomer and the diffusion coefficient D, FRS measurements are performed at different fringe spacings (typical values are 10-30 µm). Then, the relaxation rates τ-1 are plotted against the square of the grating wave vector q:
1 1 ) Dq2 + τ τdye
(2)
(10) Hahn, C.; Kaiser, S.; Wokaun, A. Tenside, Surfactants, Deterg. 1996, 33, 3. (11) Eichler, H.; Gu¨nther, P.; Pohl, D. Laser-Induced Dynamic Gratings; Springer Series in Optical Sciences 50; Springer: Berlin, 1986. (12) Terazima, M.; Okamoto, K.; Hirota, N. J. Chem. Phys. 1993, 97, 5188.
Figure 1. Phase diagram of the ternary system LeGo/H2O/ methanol (sb ) shear induced birefringent, L1 ) isotropic, L3m ) sb + elastic shear waves, qlc ) quasi liquid crystalline, LR ) stationary birefringent). where
q)
4π sin(R/2) λ
(3)
with λ being the excitation wavelength and R being the angle of intersection.
Results LeGo/H2O/Methanol. Phase Diagram. The phase diagram is shown in Figure 1. At low concentrations of dye in water there is only a single isotropic L1 phase, independent of the methanol concentration. With increasing amounts of dye and methanol, first a shearinduced birefringent phase (sb) is observed and then a L3m phase, which exhibits birefringent shear waves between cross polarizers that look rainbow-like. At about 4-9 wt % dye in water and 20-50 vol % methanol, there exists a highly viscous stationary birefringent phase (LR), which reveals a hard nematic Schlieren texture under polarization microscope. The nematic texture is herringbone-like structured. With increasing methanol concentration a second stationary birefringent phase (LR) is formed that exhibits a colored Schlieren texture. With polarization microscopy phase separation is detected. At high dye and methanol contents one reaches the existence region of an inhomogeneous phase which is denoted qlc/LR/L1. The abbreviation qlc means quasi liquid crystalline; it describes a phase that exhibits properties in between those of a pure crystal and those of a liquid crystal. The phase appears to be crystalline, but under high shear forces or at high temperatures a transition to the liquid crystalline state is observed. With FRS we investigated the phases which are formed by a 4 wt % aqueous dye solution upon addition of methanol. The exact sample compositions are indicated by rhombs in the phase diagram (Figure 1). The best strategy for FRS experiments on solutions with unknown dynamic properties is to vary the exciting pulse length from microseconds to seconds so that different kinds of decay mechanism will be detected. We experimented with different pulse lengths and frequencies on all phases and finally decided to perfom two series of experiments with the following parameters: (1) successive excitation with 500 ms irradiation and pauses of 5 s in order to detect a decay signal which is due to diffusion and has a relaxation constant also in the millisecond range and (2) successive excitation with 500 ms both for irradiation and pause in order to detect a signal which decays in the scale of minutes
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or longer. Excitation and detection energies were in the range of several hundred milliwatts per square centimeter. No FRS signal at all was detected for the isotropic L1 phases. This is probably due to fast reisomerization processes. The strong effect of pH in aqueous solutions on cis-trans isomerization of the NdN double bond is well-known in the literature;10,13 even in neutral solutions the reisomerization is accelerated considerably by protonation. In binary dye/water solutions we observed FRS signals after increasing the pH value, but since the addition of sodium hydroxide impacts the aggregation behavior, we did not pursue this strategy. Samples of the LR and the L3m phases exhibited an FRS response signal. The first conclusion is that aggregation in these mesophases stabilizes the cis isomer, which leads to a prolonged lifetime and thus enables the recording of the holographic grating. For some of the investigated samples it was possible to determine diffusion coefficients, while for others a complex detection signal was recorded which could not be evaluated within the framework of the conventional FRS theory. We start by comparing two examples from the L3m phase: one with 20 vol % and the other with 50 vol % methanol, in the following denoted L3m(20) and L3m(50), respectively. Although the optical texture between crossed polarization filters is shear-induced birefringent for both, the behavior in the FRS experiments is quite different. The difference in the rheological behavior is that L3m(20) shows properties like a Maxwell element whereas the viscosity of L3m(50) is too low to be investigated with the Bohlin rheometer. For the phase with the higher methanol content, FRS signals are detected which exhibit the usual exponential decay of the grating due to diffusional processes (see Figure 2). Variation of the angle R showed that for this case the lifetime of the cis isomer seems to be indefinite (with respect to the diffusion times, that is hundreds of milliseconds) and the decay is solely determined by diffusion. Evaluation of the data yields a diffusion coefficient of 1.9 × 10-11 m2 s-1. That is a reasonable value for dye aggregates in a solution of low viscosity, indicating that the aggregates are not very large. A simple calculation according to the Stokes-Einstein theory gives an aggregate diameter of about 15 nm. In contrast, the sample with the lower methanol content exhibits a continuous rise of the probe beam signal even after termination of the grating beams. Investigation of the sample with polarization microscopy revealed a birefringent grating (Figure 3). The time-dependent
generation and decay of this LR phase could be followed in the microscope and was restricted to the grating areas. Further examination revealed that the mesophase was uniaxial. In order to decouple the isomerization from the phase transition, polarization filters were added to the setup in the optical path before and after the sample; the pump beams and the probe beam were polarized perpendicular with respect to the optical table (termed “vertical” in Figure 4). The results are presented in Figure 4; the arrows indicate the start and shut-off times of the grating beams. The horizontally polarized portion of the diffracted probe signal is solely due to polarization rotation by the induced anisotropic phase and therefore does not show the small peak that arises from a grating due to isomerization. Only the vertically polarized portion of the FRS signal reflects the isomerization. The graphs clearly indicate that the phase transition proceeds in a two-step mechanism. The inducing step is the photoisomerization to the cis isomer. The decay of the first transient could be due to either reisomerization or diffusion (or a combination of both). But since a very-well-defined birefringent grating is observed by polarization microscopy, diffusional processes can be excluded for the present case. That means that only isomerization back to the thermally stable trans isomer is relevant for the decay. This is no contradiction of the angle-dependent measurements cited above where we concluded that the lifetime of the cis isomer is indefinite in relation to the measured diffusional times, since for these experiments the decay happened in a time range of hundreds of milliseconds, whilst the time scale for the holographic experiments lies in the range of several minutes. Now we proceed to the stationary birefringent LR phases. Two samples were investigated, one with 30 vol % methanol and the other with 70 vol %, denoted LR(30) and LR(70). Both exhibit nematic Schlieren, but the existence regions are separated in the phase diagram (see Figure 1). They differ in their rheological behavior in that the sample with the lower content of methanol shows rheological properties like those of a Maxwell element while the other mixture is thixotropic and exhibits a yield stress value. The system with lower methanol content exhibits a monoexponential decay subsequent to irradiation that yields a diffusion coefficient of 1.4 × 10-11 m2 s-1, which is very similar to the value obtained for the L3m phase. For the other LR phase a determination of the diffusion coefficient is not possible: instead, after illumination a holographic grating is detected that remains constant for the duration of weeks. We also performed polarizationdependent measurements of the probe beam as described above. The result is seen in Figure 5. In contrast to the hologram formation in the L3m phase, there is an oscillatory rise of the signal until it reaches a constant value. The oscillations have a much longer period than the 1 Hz frequency of the pulses. Thermokinetic instabilities, which occur in strongly absorbing media during or after irradiation, are well described in the literature.14,15 Simple photochemical reactions like isomerization reactions may lead to bistabilities, multiple steady states, or oscillatory behavior. The holographic grating can also be observed with polarization microscopy; it consists of an oriented LR phase (an exemplary micrograph is shown in Figure 6). Additional information is gained from an FRS experiment which is performed on LR(70) after vigorous shaking. Now
(13) Lovrien, R.; Pesheck, P.; Tisel, W. J. Am. Chem. Soc. 1974, 96, 244.
(14) Nitzan, A.; Ortoleva, P.; Ross, J. J. Chem. Phys. 1973, 59, 241. (15) Nitzan, A.; Ross, J. J. Chem. Phys. 1974, 60, 3134.
Figure 2. Typical FRS signal of the L3m sample with 50 vol % methanol. A fit of the data using eq 1 is included as the solid line.
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Figure 3. Polarization micrograph of the irradiated L3m phase: clearly the induced birefringent grating is seen (the white bar represents 100 µm). This micrograph is also reproduced in color on the cover of this issue.
Figure 4. FRS signal of the L3m sample with 20 vol % methanol: the arrows indicate the start and end of the irradiation (pulse duration 500 ms, 1 Hz).
Figure 5. FRS signal of LR(70) with varying polarization of the probe beam (the dotted lines indicate start and stop of irradiation).
the thixotropic solution is of low viscosity, and a “normal” FRS signal can be detected. Evaluation yields a diffusion coefficient of 1.6 × 10-13 m2 s-1. This value is smaller by two orders of magnitude than the one obtained for the other LR phase. Accordingly, the aggregates of the LR phase with the higher methanol content are considerably larger.
LeGo/H2O/NaCl. The phase diagram of this system is very similar to the one described above (see Figure 7). At low dye and salt concentrations an isotropic L1 phase is found which changes to a shear-induced birefringent (SB) phase upon addition of dye. At higher salt concentrations a quasi liquid crystalline (qlc) phase and a stationary birefringent (LR) phase are formed. The symbol K denotes the crystalline state. From a content of about 20 wt % NaCl and above, mainly inhomogeneous phases are found that are composed of LR and qlc phases. For this system we concentrated on the shear-induced birefringent phase region and the transition region to the quasi liquid crystalline state. We observed that all investigated samples are very sensitive to irradiation at 514 nm. Successive pulses of 500 ms duration were applied with the frequency 1 Hz. We found that the size of the signal is not determined by the pulse energy but by the total number of pulses. According to the results, the samples can be grouped in two parts: (a) Samples with low content of dye (up to 4 wt %) show an instantaneous decay of the FRS signal after shut-off of the grating beams (see Figure 8, top). The decay is nonexponential and thus cannot be evaluated to yield diffusion coefficients. (b) Samples with high dye content (5 wt % and higher) and the sample 4 wt % LeGo, 7 wt % NaCl exhibit decaying FRS signals after switching off the illumination, but soon the intensity starts to rise again and levels off at a constant value (see Figure 8, bottom). The samples only differ in the slopes of the increase and decay and in the size of the detected signal. The induced grating exists for hours, weeks, or even months, depending on the sample. In general the lifetime is prolonged with increasing dye content. In contrast to the case for the low dye content samples where the total number of pulses does not exert a strong impact on the signal, here a certain minimum number is necessary to induce the process that is responsible for the second rise of the signal. In the area of illumination a turbid spot can be seen by the eye that adheres to the glass wall even if the cuvette is put upside down: the low-viscosity solution
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Figure 6. Polarization micrograph of irradiated LR(70). In the nondirected colored Schlieren texture, a grating is observed that is constituted from a LR phase with unidirectional axes (the white bar represents 100 µm).
Figure 7. Phase diagram of the ternary system LeGo/H2O/ NaCl at T ) 25 °C (SB ) shear induced birefringent, L1 ) isotropic, qlc ) quasi liquid crystalline, LR ) stationary birefringent, K ) crystals).
is just flowing around the spot. With the polarization microscope a grating can be identified that consists of quasi liquid crystals in the otherwise shear birefringent phase (Figure 9, top). The phase looks finer structured and more strongly ordered than the one obtained by simple addition of sodium chloride, but the optical axes are not unidirectional. The enlargement (Figure 9, bottom) reveals that the long axes of the rodlike crystals have two preferential orientations, tilted by 45° with respect to the direction of the polarization (nearly parallel to the edge of the figure). If the signal is detected with the additional polarization filters in the optical path, it is clearly seen that the first peak (a) arises from isomerization, and only later on does the phase transition give rise to birefringence (Figure 10), just as was found for the L3m phase in the methanol system. The process is very dependent on the temperature, as can be seen in Figure 11: the diffracted intensity decreases with increasing temperature for both processes, the isomerization and the phase transition. If a cuvette with an already generated grating is cooled to 16 °C, the sample solidifies, and the diffracted signal vanishes due to scattering from the crystallites; however, after heating to
Figure 8. FRS signals of 3 wt % LeGo, 8 wt % NaCl (top) and 4 wt % LeGo, 7 wt % NaCl (bottom) in dependence on the pulse number (the dotted lines and arows indicate start and stop of the different irradiation times).
20 °C, the signal is restored completely. That means that the aggregation forces of the photoinduced qlc state are much stronger, since these assemblies continue to exist while the “normal” crystals undergo transition back to the isotropic state. Discussion To discuss the underlying mechanism of the photoinduced phase transition, we have a look at the models
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Figure 9. Polarization micrographs of photoinduced quasi liquid crystalline grating in the sb sample 4 wt % LeGo, 7 wt % NaCl (the white bar represents 100 µm).
known in literature up to now. The phenomenon of photoinduced optical anisotropy (POA) is well-known in the literature, starting with Weigert and Nakashima’s publication in 1929.16 They described the effect of polarized light on solid dye solutions. Since then, different mechanisms have been applied to explain POA. They may roughly be grouped into photochemical and photophysical mechanisms. Teitel explained his observation of induced birefringence in dye films by a short-term localized heating17 that leads to a decrease of viscosity and accordingly an increase in rotational diffusion of the molecule. A rough calculation can be made to get a hint on the temperature increase in the sample due to laser heating (16) Weigert, F.; Nakashima, M. Z. Phys. Chem. 1929, 34, 258. (17) Teitel, A. Naturwissenschaften 1957, 44, 370.
in our experiments. The following equation is used to determine the temperature increase ∆T: ∆T ) E/CpVF (with E ) energy, Cp ) heat capacity, V ) volume in which irradiation is adsorbed, and F ) density). Since Cp and F have not yet been determined for the dye solutions, the values for pure water are used as an approximation. The calculation yields a value of less than 1 °C temperature increase for a typical pulse energy of 20 mJ absorbed in a volume of 5 × 10-3 cm3. This implies that laser heating is not likely to be the source of the investigated phenomena. Additionally, from FRS experiments11 it is known that heat diffusion takes place in the range of microseconds or less, which differs by several orders of magnitude from the time scales in which the POA is observed. The most important reason to exclude laser heating as the origin of the phase transitions is the fact that the induced phases
Photoinduced Optical Anisotropy of Azo Dye Mesophases
Figure 10. FRS signals of the sample 4 wt % LeGo, 7 wt % NaCl with varying polarization of the probe beam (the dotted lines indicate start and stop of irradiation).
Figure 11. FRS signals of the sample 4 wt % LeGo, 7 wt % NaCl in dependence on the temperature (the dotted lines indicate start and stop of irradiation).
always exhibit a higher degree of order and are found in the phase diagrams at lower temperatures. Other authors consider a long-lasting metastable state of the molecule in explaining the POA observed in solid sugar solutions of acridine dyes.18 An example of the second group, the photochemical processes inducing anisotropy, is the photo-oxidation of dyes.19 In the case of azo dyes, trans-cis isomerization is well-known to be responsible for changing the alignment of liquid crystals and thus inducing a phase transition to the isotropic state in doped polymer systems.7 The reason is that upon irradiation the molecule changes its geometrical shape from an elongated, rodlike and thus mesogenic form into a bent and therefore nonmesogenic form. Additionally, the isomerization may be followed by a reorientational process. One reason is that the cis isomer bends into a direction which is perpendicular to the propagation of the light wave as well as perpendicular to the direction defined by the polarization of the light. This is followed by a collective response of the neighboring molecules which results in a partial reorientation of the director.20 Another model is based upon the preferential orientation of the trans molecules after undergoing cycles of excitation and relaxation: the angular-dependent photoreaction leads to an increase of molecules with axes perpendicular to the incident polarization.7,21 Furthermore, a mechanism called angular hole burning or selective angular excitation (18) Cherdyntsev, S.; Vasserman, I. Zh. Eksp. Teor. Fiz. 1948, 18, 360. (19) Albrecht, A.; Simpson, W. J. Am. Chem. Soc. 1955, 77, 4454. (20) Wendorff, J.; Eich, M. Mol. Cryst. Liq. Cryst. 1989, 169, 133. (21) Makushenko, A.; Neporent, B.; Stolbova, O. Opt. Spectrosc. 1971, 31, 295. Makushenko, A.; Neporent, B.; Stolbova, O. Opt. Spectrosc. 1971, 31, 397.
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also influences the orientational distribution.22 In good approximation it can be considered that the polarizability tensor of azo molecules only has one component along the long molecular axis, the z-axis. The probability for a molecule to be pumped to the excited state is proportional to Rz2 cos2 θ, where Rz is the polarizability tensor element and θ is the angle between the pumping light electric field and the molecular axis. Accordingly there is a preferential isomerization of molecules with axes parallel to the electric field of the incident light. In the present study we observed POA either due to phase transitions from nonstationary birefringent to stationary birefringent phases or due to an alignment of the optical axis in a nematic phase. In the first class of cases, a separation of isomerization and phase transition which led to birefringence was found. This is not true for the nematic LR(70) phase. Therefore both mechanisms should be discussed separately. POA in the Nematic Phase. We start with the alignment process of the LR(70) phase of the system LeGo/ H2O/methanol. The nematic phase is composed of domains, each having its own optical axis. In the original sample the distribution is random, but after irradiation a unique optical director is established. Palto and coworkers developed a model to explain a similar observation in nematic Langmuir-Blodgett films.23 They showed that upon isomerization a redistribution of the local oscillators occurs, which leads to a generation of centers of recrystallization. The newly crystallized domains then exhibit a common direction of the local optical axes. This explanation seems to be applicable to the present case, especially since the detected hologram is induced by the irradiation but continues to develop after termination of the grating beams. This is in congruence with the assumption of the induction of recrystallization centers by the laser light, while the process of crystallization takes much longer but is restricted to the illuminated area. The long lifetime of the hologram shows that the orientation of the domains remains preserved. The lack of ability to preserve the structure in the LR phase with the lower methanol content may explain why here only the usual exponential decay due to reisomerization was detected after termination of irradiation. From the rheological experiments we know that the solution is of low viscosity in contrast to the thixotropc nature of the other LR sample; thus, the orientation of the optical axes of the domains is rapidly randomized by a high rotational diffusion coefficient. This is supported by the findings that after vigorous shaking of the thixotropic LR(70), which destroyed the supramolecular structure and led to low viscosity, an instantaneous decay of the grating was found as well. A hint to the microscopic structure is given by the diffusion coefficient of the aggregates, which for LR(70) is two orders of magnitude smaller than that for the sample LR(30). That means that the aggregates are also larger by orders of magnitude, thus leading upon relaxation to the supramolecular structure exhibiting thixotropic properties. Also, the microstructures of the neighboring L3m and LR should be similar, since the samples L3m(50) and LR(30) exhibit a similar FRS response signal and yield diffusion coefficients of the same order of magnitude. This is affirmed by the small coexistence region of both phases in the phase diagram. POA in Samples without Stationary Birefringence. The mechanism must be different for the samples (22) Sekkat, Z.; Dumont, M. Appl. Phys. B 1992, 54, 486. (23) Palto, S.; Shtykov, N.; Khavrichev, V.; Yudin, S. Mol. Mater. 1992, 1, 3.
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where a photoinduced phase transition was found, rather than only an alignment of the optical axis. Although the lifetimes are different for the induced qlc and LR phases, the inducing step for both is the isomerization to the cis form and subsequent relaxation back to the trans isomer. The possibility of a physical process which includes the heat generation upon relaxation can be excluded, since higher pulse energy did not result in an increase of the observed signal. Since the optical axes after irradiation are not unidirectional for the qlc grating, it cannot be a mechanism based only on the angular-selective excitation either. Possibly, a torque acts on the cis isomer due to substituents, solvent cage, or surroundings and reorients it by an angle R before isomerization back to the trans state occurs. The next hint is the time scale: the isomerization and relaxation cycle is much faster and finished before the restructuring of the phase occurs. We suggest that the isomerization induces the phase transition by the relaxation of the excitation energy, which may be used to achieve a higher degree of order. If, after a certain number of pulses, a sufficient number of molecules is newly aligned, the recrystallization of the remaining aggregates is expected to follow. It is plausible that, for the present case as well, the possibility to preserve a preferred alignment will play an important role. Conclusions First experiments were described on a novel dye system that forms complex mesophases upon addition of methanol
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or sodium chloride. We demonstrated that valuable hints on the microscopic structure of a phase and on the mechanism of phase transitions can be found with the help of transient holographic gratings. The observed photoinduced anisotropy in a nematic phase was assigned to an alignment of optical axes. Originally, the distribution is random, but upon irradiation a redistribution of the local oscillators occurs, which leads to centers of recrystallization. A mechanism called angular-selective excitation is discussed for this redistribution. The POA observed in phases without stationary birefringence is assigned to a phase transition. Here also, the angular hole-burning is the initiating step. But since the induced quasi liquid crystalline phase does not show an unique optical director, but a preferential orientation tilted by 45°, a second step that influences the crystallization has to follow. A necessary condition for POA in both investigated phases is the ability to preserve the structure; otherwise, a randomization of the local axes is caused by a high rotational diffusion coefficient. From this behavior, conclusions can be drawn on the microscopical aggregation. Ongoing work includes transmission freeze microscopy and X-ray diffraction, which should help to clarify the nature of the aggregates and to establish a model for the phase diagrams. LA980480W
