J. Phys. Chem. 1996, 100, 4135-4140
4135
Biphoton-Induced Refractive Index Change in 4-Amino-4′-nitroazobenzene/Polycarbonate H. Bach,† K. Anderle,‡ Th. Fuhrmann,‡ and J. H. Wendorff*,‡ Fachbereich Physikalische Chemie und Wissenschaftliches Zentrum fu¨ r Materialwissenschaften, Philipps UniVersitaet Marburg, Hans Meerwein-Strasse, D-35032 Marburg, Germany, and Laboratory of Physical Chemistry, Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092 Zuerich, Switzerland ReceiVed: July 25, 1995; In Final Form: NoVember 30, 1995X
Transient biphotonic optical storage in the guest-host system 4-amino-4′-nitroazobenzene/polycarbonate is accomplished by excitation of the azo dye molecules at 488 nm with an argon ion laser and subsequent writing at 633 nm with two interfering HeNe laser beams. The mechanism of the observed refractive index change is investigated by detailed kinetic studies. It is based on the formation of cis isomers of the azo dye by the argon ion laser. These are subsequently transformed back to the trans isomers by fast photoinduced cis-trans isomerization where the interference of the writing beams is constructive and by slow thermal cis-trans isomerization where the interference of the HeNe beams is destructive, respectively. The cis concentration produced by the exciting light is thus modulated with the HeNe beams, resulting in a modulation of the refractive index.
Introduction The application of azo dye/polymer systems for reversible optical data storage, both holographic and digital, has been the subject of numerous investigations in the last few years. Optical data storage has been demonstrated with several liquid-crystalline and amorphous polymers, having azo dyes either covalently attached1-5 or dissolved.6 The storage process has been elucidated in this early work. It is based on the photoinduced trans-cis-trans isomerization of the azo dye molecules by single-photon absorption. Irradiation with linearly polarized light leads to the formation of cis isomers and, to a certain extent, to a reorientation of the azo dye molecules via photoselective isomerization cycles. The result of both effects is a significant local variation of the refractive index which is sufficient for holographic recording. Recently, holographic data storage in a side-chain liquidcrystalline polyester and in a host-guest system utilizing unusual photochemistry of azo dyes was reported.7-9 Fei et al. investigated the azo dyes methyl orange (MO) and ethyl orange (EO) embedded in a poly(vinyl alcohol) (PVA) matrix.9 They induced a weak holographic grating by excitation of the sample with an argon ion laser operating at 514 nm and subsequent irradiation with two intersecting HeNe beams (633 nm). The delay time between excitation and writing was several seconds. The authors speculate that the storage process is based on the isomerization of the azo dye molecules by a two-photon process involving triplet-triplet transition from metastable triplet states with lifetimes on the order of magnitude of seconds. Figure 1 shows the energy-level scheme for the isomerization of an azo dye molecule.10 The additional triplet states T2 were proposed for the two-photon process by Fei et al.9 According to Figure 1, the recording of a hologram could be accomplished if the azo dye molecules are illuminated with radiation at frequencies λ1 (514 nm) and λ2 (633 nm). The λ1 radiation pumps the S0 f S1 transition. The molecules in the S1 state rapidly decay to the metastable T1 state. Some of the molecules in the T1 state certainly isomerize; the others can be pumped with λ2 radiation to the T2 state. The molecules pumped to the †
Swiss Federal Institute of Technology. Philipps Universitaet Marburg. X Abstract published in AdVance ACS Abstracts, February 1, 1996. ‡
0022-3654/96/20100-4135$12.00/0
Figure 1. Energy level scheme for the isomerization of azo dye molecules according to the data given in refs 9 and 10. The two additional triplet states T2 were proposed by Fei et al.9 but have not been detected spectroscopically.10
T2 state undergo the isomerization reaction before they have time to relax to lower energy states. In the ideal case for twophoton holography, most of the molecules reach the chemically active T2 state and do not isomerize directly from the lower T1 state. However, a detailed investigation of the energy levels and lifetimes of the excitation states of methyl red10 gives no indication for the population of higher triplet states. Biphotonic holographic recording in organic materials involving higher excited states was reported before. These require the simultaneous irradiation with UV or visible light to pump the molecules to a metastable state and with a near-infrared laser to produce triplet-triplet transitions.11-14 In this paper, we report results from detailed holographical and spectroscopical studies that yield information about the dynamical processes and the states of the azo dyes involved in holographic data storage by simultaneous and subsequent irradiation with laser light of the wavelengths 488 and 633 nm. The finding is that it is in principle possible to record holograms in certain azo dye/polymer systems by excitation with polarized light at 488 nm and subsequent writing with polarized light at 633 nm. However, the mechanism is not based on isomerization of azo dye molecules by a novel biphotonic storage process according to the energy-level scheme shown in Figure 1 but on the well-known photochemistry of azo dyes. Two experimental techniques were employed in this work. Holographic grating © 1996 American Chemical Society
4136 J. Phys. Chem., Vol. 100, No. 10, 1996 experiments were used to show the feasability of biphotonic optical data storage and to investigate the dynamical processes involved in it.15 Transient absorption spectroscopy was exploited to get direct insight into the isomerization reaction and to obtain information about the excited states of the azo dye molecules. Azo compounds can be roughly classified in two groups with different spectroscopical and photochemical characteristics: azobenzene-type molecules and molecules of the pseudo-stilbene type, e.g., push-pull (electron donor-acceptor) substituted azobenzenes.16 The mechanism of the photoinduced isomerization for azobenzene-type molecules is well characterized.16 There is evidence from experimental data and calculations that only singlet states are involved in the direct photoinduced isomerization of azobenzene-type molecules. This type of molecules seems, therefore, not to be suited for two-photon chemistry. The mechanism of the photoinduced isomerization of pseudostilbene-type molecules has not been elucidated so far, but triplet states probably take part in the isomerization process. They have been directly observed by transient absorption spectroscopy for some push-pull-substituted azobenzenes, and their lifetimes in viscous media are in the nanosecond-to-microsecond range.17 It is therefore conceivable that the isomerization reaction of this type of molecules might happen by a two-photon process involving metastable triplet states. The push-pull-substituted azo dyes methyl orange and ethyl orange investigated by Fei et al. belong to these pseudo-stilbenes. For our investigations, we chose the push-pull-substituted azo compound 4-amino-4′nitroazobenzene embedded in a matrix of polycarbonate. Experimental Section 4-Amino-4′-nitroazobenzene was synthesized and purified by the group of Prof. Boldt at TU Braunschweig, Germany. The polymer used was the commercially available polycarbonate Macrolon from Bayer AG. The samples for the holographic experiment were prepared by dissolving 0.3 mg of azo compound and 0.3 g of polycarbonate in 10 mL of dichloromethane. Part of the solvent was evaporated to yield a highly viscous solution. This viscous solution was poured into a small glass bowl previously washed with dichloromethane to give an optically transparent yellow film of several micrometers thickness after the solvent had evaporated. Residual solvent was removed by annealing the samples in vacuo at 40 °C for 2 h. Two different samples were used for the spectroscopical studies: (i) dye-doped polymer films on thin glass substrates prepared analogously to the samples for the holographic measurements, and (ii) a solution of 0.20 mg of the azo dye dissolved in 10 mL of dichloromethane, respectively. The experimental setup used for the biphotonic holographic measurements is shown in Figure 2. The sample was excited with an argon ion laser operating at 488 nm, and the power of the exciting light was 4 W/cm2. The holographic grating was induced by the interference of two HeNe laser beams (633 nm) under a given angle in the sample region. The angle was adjusted to obtain a thick grating with grating constant Λ ) 5 µm; the writing intensity was 80 mW/cm2 for each beam. The grating was probed by self-diffraction of one of the writing beams. The detector was placed in the diffraction order -1 to probe the grating without interruption of the writing process. A heatable sample holder was used to perform experiments at different temperatures. The apparatus for the transient absorption spectroscopy is shown in Figure 3. The spectrometer comprises a 75-W xenon
Bach et al.
Figure 2. Experimental setup for biphotonic holographic measurements. The solid lines indicate the light path and the dashed lines the electrical path.
Figure 3. Experimental setup for the transient absorption spectroscopy. The solid lines indicate the light path and the dashed lines the electrical path.
lamp, a monochromator to select the probe wavelength, the sample, a second monochromator to cut off the light of the argon ion laser, and a photomultiplier. The signal of the photomultiplier was processed with a lock-in amplifier. The sample was excited by an argon ion laser operating at 488 nm through an optical fiber with unpolarized light of intensity 20 mW/cm2. The control of the experiment and the acquisition of the data were performed with a personal computer in both experiments. Results (1) Grating Experiments. All holographic experiments were performed in the following way: First, the intensity I0 of the probe beamsi.e., one of the writing beamsswas determined. Second, the shutter of the HeNe laser was opened and closed in cycles of 10 min. The sample was not excited by the argon ion laser in the first cycle in order to find out whether photochemical processes occur by irradiation at 633 nm. In the following cycles, the sample was excited with the argon ion laser before or while writing with the HeNe beams, respectively. The diffraction efficiency was calculated by dividing the online monitored intensity I diffracted in the order -1 by the intensity I0. If not stated otherwise, the experiments were performed at room temperature. Figure 4 shows a typical holographic growth curve for consecutive excitation and writing. In the first cycle, when the sample was not excited with the argon ion laser, the diffraction efficiency increased rapidly to 10-6 due to enhanced background radiation and slowly to a steady value of 1.2 × 10-6. In the second cycle, the sample was excited with the argon ion laser (488 nm, 4 W/cm2) for 6 s. After a delay time of 0.06 s, the sample was irradiated with the two HeNe beams. The diffraction efficiency now increased rapidly to a peak and decayed afterwards to the steady value already obtained in the first cycle without excitation of the sample. The same behavior was
Biphoton-Induced Refractive Index Changes
Figure 4. Holographic growth curves for writing (633 nm) and for consecutive excitation (488 nm) and writing (633 nm) in a 4-amino4′-nitroazobenzene-doped polycarbonate film. The intensity of the writing light was 80 mW/cm2 for each beam. The intensity of the argon ion laser was 4 W/cm2, and the sample was excited for 6 s.
observed in the following cycles. In all experiments, the diffraction efficiency decreased immediately to zero after closing the HeNe shutter because the decay of the holographic grating could not be monitored with the setup shown in Figure 2. In order to investigate the stability of the induced grating, the setup was modified slightly so that one of the two HeNe beams was always probing the sample. The diffraction efficiency decreased to zero approximately exponentially within 10 min in this experiment when the writing process was terminated. From these basic experimental results, the following conclusions can be drawn: (i) The possibility of inducing a weak holographic grating by irradiating solely at 633 nm shows that photochemical processes can take place at this wavelength. (ii) The excitation of the sample at 488 nm and subsequent writing at 633 nm result in a rapid growth of the diffraction efficiency to a peak followed by a decay to the value already obtained without excitation. This can be explained with an excited species formed by irradiation at 488 nm which subsequently reacts in a red-light process. (iii) The induced gratings are not stable, indicating a reversible process. The diffraction efficiency decays to zero within 10 min when the writing beams are turned off. Three parameters were varied in order to obtain information about the dynamical processes involved in the biphotonic holographic data storage: (i) The delay time between excitation and writing was varied to get information about the lifetimes of the excited species formed by excitation at 488 nm. (ii) The temperature was varied in order to find out whether thermally activated processes play a role. (iii) In a further experiment, the sample was irradiated with the argon ion laser and the HeNe beams at the same time. The exciting light was switched off after 5 min, and the evolution of the diffraction efficiency was monitored for a further 10 min. (i) Figure 5 shows the maximum diffraction efficiency after switching on the writing light (“peak height”) vs delay time between excitation and writing. The peak height drops rapidly for small delay times and slowly afterwards. The peak disappears for delay times longer then approximately 150 s, and the growth curve resembles the one obtained without excitation. On a molecular level, this means that the excited species formed by irradiation at 488 nm has only a certain lifetime and relaxes back to the ground state. After approximately 150 s, no excited molecules are left which could react with the red light to give a peak. It is remarkable that even for delay times in the range of 100 s, a peak behavior is observed. This delay time is in
J. Phys. Chem., Vol. 100, No. 10, 1996 4137
Figure 5. Peak height (maximum diffraction efficiency) as a function of delay time between excitation (488 nm) and writing (633 nm).
Figure 6. Holographic growth curves for consecutive excitation (488 nm) and writing (633 nm) at different temperatures. (a) 35 °C, (b) 50°C, (c) 75 °C.
agreement with observations for similar systems, for which lifetimes of 300 s have been measured.8 It lies in the time range of the rate constants for the cis-trans isomerization of azobenzenes and not for a triplet decay (for the final assignment of the transient species, see the Discussion section). (ii) Figure 6 shows the growth curves at different temperatures and a constant delay time of 0.06 s. At higher temperatures, the peak becomes smaller, and the peak width decreases. At 75 °C, the peak disappears completely. The lifetime of the excited speciesscorrelated to the peak heightsand the processes responsible for the peak shape obviously depend on temperature. (iii) Two-photon processes normally take place by simultaneous irradiation with both light sources. Figure 7 shows the hologram growth curves of an experiment, where the sample was irradiated simultaneously with the exciting light and the two writing beams. The exciting light source was switched off after 5 min, and the sample was irradiated with the writing beams for further 10 min. At the beginning, the diffraction efficiency immediately surged to a steady valuesapproximately similar to the steady value obtained in the first cycle where the sample was not excited. Just at that moment when the shutter of the argon ion laser was closed, the peak behavior was obtained. Excitation of the sample at 488 nm is obviously prerequisite but not sufficient for the peak behavior. Simultaneous irradiation with the exciting light source and the writing beams suppresses the peak assigned to a two-photon process.
4138 J. Phys. Chem., Vol. 100, No. 10, 1996
Figure 7. Holographic growth curves for simultaneous irradiation with the exciting light (488 nm) and the writing beams (633 nm). The argon ion laser was switched off after 5 min, and the sample was irradiated with the HeNe laser for a further 10 min.
This is noteworthy, because this behavior is not in accordance with the model of a simple two-photon process. (2) Transient Spectroscopy. In the holographic experiments, we obtained information about the dynamical processes involved in biphotonic optical data storage. This was achieved by probing the changes of the index of refraction occurring in the sample due to photochemical processes. However, we did not gain direct information on the isomerization reaction and the excited states of the azo dye molecules. These can be obtained by the means of transient absorption spectroscopy. If the isomerization reaction takes place by a two-photon process involving triplet states of both isomers with lifetimes on the order of magnitude of seconds according to Figure 1, a growth of absorption in the red spectral range is expected after excitation of the molecules at 488 nm. When the exciting light source is switched off, a decay of the transient absorption with rate constants characteristic for the excited species should occur. The transient spectroscopical studies were performed in the following way: (i) The absorption of the trans isomer of the azo dyeswhich is the stable form in the thermal equilibriumswas measured. (ii) The sample was irradiated with an argon ion laser at 488 nm to adjust the photochemical equilibrium between trans and cis isomer. (iii) After turning the exciting light off, the relaxation of the transient absorption was monitored. Figure 8 shows the absorption of the trans isomer, the time evolution of the absorption during excitation, and the relaxation of the transient absorption of a solution of the compound in dichloromethane at the detection wavelength 600 nm. The absorption increases due to excitation and relaxes exponentially back to the start value with a rate constant of 76 s, obtained by fitting with a monoexponential. Corresponding experiments were performed at different detection wavelength between 320 and 650 nm in 10-nm steps. Figure 9 shows the transient absorption spectrum in comparison with the absorption spectrum of the trans isomer. The transient absorption is slightly higher in the spectral range from 520 to 640 nm, which is indeed in agreement with the energy level scheme in Figure 1. The transient absorption in the investigated spectral range may consist of the absorption of the trans and the cis isomers in photochemical equilibrium after excitation and triplet-triplet absorption according to Figure 1. The former is expected for shorter wavelengths; the latter is expected for the red spectral range. The relaxation of the transient absorption was monoexponential in the whole spectral
Bach et al.
Figure 8. Absorption of the trans isomer at 600 nm, time evolution of the absorption during excitation (488-nm argon ion laser, 20 mW/ cm2), and relaxation of the transient absorption of a solution of 4-amino4′-nitroazobenzene (0.2 mg) in 10 mL of dichloromethane (similar behavior is observed in polycarbonate).
Figure 9. Transient absorption spectrum after excitation at 488 nm and absorption spectrum of the trans isomer of 4-amino-4′-nitroazobenzene (0.2 mg) in 10 mL of dichloromethane (a similar spectrum is obtained in polycarbonate). The inset shows a magnification of the red spectral range (550-650 nm).
range, and the two cases should therefore be distinguishable by determining the relaxation times. The relaxation times were obtained by fitting a single-exponential function to the decay curve. Figure 10 shows the relaxation times in the whole spectral range. Of course, a substantial spreading of the values at the different wavelengths is observed, but there is no significant change in the value at any wavelength. We conclude that the transient absorption, even the transient absorption at wavelength above 600 nm, is caused by the formation of a single transient species. This transient species has to be assigned to the cis isomer of the azo dye which relaxes thermally back to the trans form following a first-order rate law in solution after the exciting light source is turned off. The relaxation time for the thermal cis-trans isomerization was calculated by averaging all the relaxation times at the different wavelengths to be 109 ( 6 s. In order to have complete comparability with the holographic experiments, the absorption spectrum of the trans isomer and the transient absorption spectrum were also determined in a polycarbonate matrix. Similar spectra were obtained with the important difference that a bathochromic shift of about 20 nm was observed, reflecting the strong influence of the polarity of
Biphoton-Induced Refractive Index Changes
J. Phys. Chem., Vol. 100, No. 10, 1996 4139 further simplification, the rate law of the isomerization in a polymeric matrix is approximated to be first order. Thus, the isomerization reaction follows the following reaction scheme: A
[trans] y\ z [cis] B,K
Figure 10. Relaxation times of the transient absorption.
the solvent on the position of the absorption band of pseudostilbenes.16 Here, the thermal cis-trans isomerization can approximately be described by two single exponentials. The relaxation times τ1 and τ2 obtained by fitting two singleexponential functions to the decay curves at all wavelengths and subsequent averaging were τ1 ) 160 ( 9 s and τ2 ) 13 ( 1 s. They may be assigned to molecules that have sufficient free volume for the isomerization and molecules in a strained geometry, respectively.18 Discussion The important result of the transient spectroscopical studies to be emphasized is that the increase of the absorption in the red spectral range after excitation at 488 nm can be explained without the assumption of any triplet-triplet-transition as the formation of cis isomers. Because the cis isomer of the azo compound absorbs at wavelengths between 600 and 650 nm, it is reasonable to assume that in our holographic experiment, the cis-trans isomerization is also induced photochemically at 633 nm. The assignment of the transient species to the cis isomer of 4-amino-4′-nitroazobenzene is mainly based on the measured relaxation times, which are in good agreement with tabled values for the rate constants of the thermal cis-trans isomerization. The rate constants were reported to be 2.75 s-1 in acetone and 0.004 s-1 in benzene, strongly dependent on the polarity of the solvent.17 In this work, we obtained rate constants of 0.0091 s-1 in dichloromethane and 0.0063 s-1 in polycarbonate (at 25 °C). The latter is substantially faster than the thermal isomerization of unsubstituted azobenzene in the same matrix (3.5 × 10-4 at 70 °C),19 as expected for donor-acceptor-substituted azobenzenes. However, an identification of the transient species with a triplet state can be excluded, because triplet states with lifetimes on the order of magnitude of 100 s have never been observed to our knowledge (typical values are about k ) 108 s-1).20 The interpretation corresponds to results obtained recently by Ramanujam et al.,21 who found a similar transient absorption behavior in azo polymers and also assigned the biphotonic process to the reisomerization reaction. With the results of transient absorption spectroscopy, it is possible to develop a model that explains all the holographic growth curves obtained in the holographic experiments. The refractive index grating is essentially caused by (i) a modulation of the concentration of cis and trans isomers and by (ii) a modulation of the orientational distribution of the dyes. It turns out that a detailed description of the growth curves can be obtained by considering only the concentration profile. As a
[trans] and [cis] are the concentrations of the isomers and A, B, and K are the rate constants of the photochemical trans-cis isomerization, the photochemical cis-trans isomerization, and the thermal cis-trans isomerization, respectively. The rate constants A and B depend on the intensity I of the light (A, B ∝ I). It shall be assumed that only the writing processes (at 632 nm) are considered, whereas the exciting light at 488 nm provides the start configuration. In the holographic experiment, the incident intensity I of the writing light is modulated in the x direction. The x direction is defined perpendicular to the interference fringes in the sample.
[
(2πx Λ )]
Ix ) 2I 1 + cos
(1)
The differential equation describing the time evolution of the cis concentration can be easily solved, and the following expression is obtained:
Ax × Ax + Bx + K ([cis]t)0 + [trans]t)0)(1 - exp(-(Ax + Bx + K)t)) (2)
[cis]t ) [cis]t)0 exp(-(Ax + Bx + K)t) +
Of course, an additional thermal grating because of heating effects would appear in the rate constant B, but we believe the photochemical process to be the main pathway for the reisomerization. Indeed, for this interpretation, there are hints from other experiments.21 Nevertheless, the interpretation of grating modulation would be the same. Equation 2 allows the simulation of the holographic growth curves. If the sample is irradiated for a short time with the exciting laser, cis isomers are formed in the whole sample region. The concentration of the cis isomers after excitation is [cis]t)0. After termination of the excitation process and a delay time of a few seconds, an intensity modulation of red light in the x direction is produced by the interfering HeNe beams, leading to a change in the population distribution between the two isomers. The new photochemical equilibrium at the positions of constructive interference lies on the side of the trans isomers. At the places where interference of the HeNe beams is destructive, no photochemical processes occur and the cis isomers are transformed slowly to the trans isomers by thermal isomerization. Figure 11 shows a simulation of the time evolution of the cis concentration in the x direction according to eq 2. The holographic diffraction efficiency η issif we assume that the hologram formation is essentially caused by the refractive index modulationsproportional to the square of the modulation amplitude n (η ∝ n2) for small values of η (η < 0.01%).15,22 It is reasonable to assume that the modulation amplitude n is approximately proportional to the difference between the cis concentration at the positions where interference is destructive (Ix ) 0) and the positions where interference is constructive Ix ) maximum).
∆[cis]t ) [cis]Ix)0 - [cis]Ix)max
(3)
Figure 12 shows the simulation of the holographic growth curve η(t). The experimental curves being corrected by a value of about 10-6 due to background light resemble the simulated
4140 J. Phys. Chem., Vol. 100, No. 10, 1996
Figure 11. Simulation of the time evolution of the cis concentration in the x direction (perpendicular to the interference fringes in the sample) according to eq 2. The parameters were choosen as follows: [cis]t)0 > [trans]t)0, B > A > K, I ) 0.5, Λ ) 1.
Bach et al. ing to an energy level scheme shown in Figure 1 but on the well-known photochemistry of azo dye molecules. By excitation at 488 nm, cis isomers are formed in the whole sample. These cis isomers are photochemically (and to a lesser extent thermally) transformed back to trans isomers by irradiation at 633 nm where the interference of the HeNe beams is constructive and thermally where the interference of the writing beams is destructive, respectively. The rates of the photoinduced cistrans isomerization at 633 nm and of the thermal cis-trans isomerization differ strongly. Thus, we observe a transient modulation of the cis concentration resulting in a significant modulation of the refractive index. Our results might influence the experimental setups generally used to investigate optical data storage in azo dye systems. So far, it was assumed that nondestructive readout is achieved by probing holograms at 633 nm. If an isomerization reaction of the azo dye 4-amino-4′-nitroazobenzene in polycarbonate is feasible by irradiation at 633 nm, also other systems might show this effect. Photochemistry at 633 nm especially could play a role in push-pull-substituted azo dyes embedded in highly polar polymers where the absorption band is strongly shifted to the red. Acknowledgment. This work has been financially supported by the BMFT (Bundesministerium fuer Forschung und Technologie). We thank the group of Prof. Boldt, TU Braunschweig, for providing the azo compounds. References and Notes
Figure 12. Simulation of the holographic growth curve η ∝ (∆[cis]t)2. ∆[cis]t was calculated according to eq 3. The parameters were the same as in Figure 11.
holographic growth curves. For growing delay times between excitation and the writing process, the cis concentration, which can be modulated by the HeNe laser beams, becomes smaller because of thermal cis-trans isomerization. Thus, the peak becomes smaller. At higher temperatures, the thermal cis-trans isomerization is accelerated. The peak becomes therefore smaller, and the peak width narrows. If the sample is irradiated simultaneously with both light sources, the peak is suppressed because the cis concentration remains almost constant in the whole sample. This is evident because the irradiation at 488 nm dominates the photostationary equilibrium. Due to its weaker influence, the writing intensity is not sufficient for a strong modulation. Summary We conclude from these results that it is possible to store transient holographic gratings in the guest-host system 4-amino4′-nitroazobenzene/polycarbonate by excitation of the sample at 488 nm and subsequent writing at 633 nm. The mechanism is not based on isomerization of the azo dye molecules by a two-photon process involving triplet-triplet absorption accord-
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