J. Phys. Chem. 1994,98, 7562-7565
7562
Photokinetics in Photochromic Polymers Studied by Holographic Recording V. Weiss'tt and V. A. Krongauz.4 Physics of Complex Systems and Organic Chemistry Departments, The Weizmann Institute of Science, Rehovot, Israel 76100 Received: December 9, 1993; In Final Form: March 22, 1994'
Photokinetic processes in photochromic polymers are studied by holographic recording. A semiempirical model for elucidating the photochemical and thermal transformations between the colorless and colored forms of photochromic compounds is presented. In this model, holographic recording in the visible range by bleaching of precolored material is emphasized. The model calculations are compared with the experimental exposure responses of holographic gratings recorded in spirooxazine containing polymer films. The results exhibit significant differences in the holographic exposure response with variations in the exposure configurations, suggesting that the thermal and photochemical transformations occur between colorless spirooxazine and at least two stereoisomeric forms of colored merocyanine dye.
Introduction
represented by the following simplified chemical equation,
Photochromic materials need no chemical or thermal development so they can be exploited for in situ and for real-time optical recording. Moreover, the recorded information can either be permanently stored or be erased for repeated recordings. Therefore, photochromic materials have potential for applications in high density storage and retrieval and optical data processing. Several organic and inorganic photochromic media were investigated in the past as recording materials for h~lography.l-~ Unfortunately, their exposure sensitivitieswerevery low, typically over 1 J/cmZ, so that they are not practical for real-time applications. The exposure sensitivity of photochromic systems depends on the photochemical and thermal reaction kinetics, which in turn depend on spectral absorption cross section, quantum yield, wavelength of excitation, temperature, concentration, and solvent of the photochromic compound.- A recent investigation on realtime holographic recording in photochromic spirooxazine and spiropyran doped polymer thin films revealed that the temporal holographic response also critically depended on the optical recording configurations? In one configuration, an increased sensitivity of about 250 mJ/cmz was obtained for the spirooxazine containing samples? with a diffraction efficiency (DE) of about 0.45% at 2000 Ip/mm. In this paper, photokinetic processes in photochromic polymers are studied by holographic recording. We present a photokinetic model for elucidating the photochemical and thermal transformations between the colorless and colored forms of photochromic compounds. In this model, holographic recording in the visible range by bleaching of precolored material is emphasized. The model predictions for three different exposure configurations are compared to the experimental DE exposure responses of holographic gratings recorded in spirooxazine containing polymer films. Photokinetic Model The photochemical and thermal conversions of photochromic dyes, such as spirooxazine and spiropyran derivatives, may be
A (Spimoxazlne)
B (Merocyanine)
where A and B are the colorless and colored stable forms, RAand Re are their photochemical rate constants, AA and AB are their excitation wavelengths and RT is the thermal rate constant of B to A. In solid polymers, the thermal conversion from A to B may be neglected.M B in eq 1 may be a mixture of several stereoisomers"J"13 of the merocyanine (MC) form and their aggregatea.7.14 For a comprehensivequantitative analysis, it would be necessary to consider the kinetics of all processes in eq 1, including transitions through excited and metastable statesw-8.1"12 and equilibration between stable isomersand aggregates. This would involve solving all reaction rate equations for such a system, which contain several mutually dependent variables; these dependenciesare difficult to determine for most photochromic systems, especially for spirocompounds.6~~However, a semiempirical approach, describing a closed system of one independent rate equation7with the overall rate constants indicated in eq 1, may be sufficient. It is assumed that a pseudo-first-orderprocess with linear intensity dependence adequately describes the present photochromic system.' These are reasonable assumptions, because, first, relatively low optical densities are involved (OD C l), and second, low light powers are applied, so that nonlinear effects from multiphoton absorption can be neglected. The overall change in concentration of B with time, can then simply be given by the following rate equation -dB/dt = RBB - RAA
+ RTB = RBB - RAA, + RAB + RTB
(2) where A0 is the total concentration of all the dye species, satisfying A0 = A ( t ) + B(t). Integration of eq 2 with respect to B and t yields a general solution given by
Physics of Complex Systems Department. t Organic Chemistry Department. a Abstract
published in Advance ACS Absrructs, June 1, 1994.
0022-3654/94/2098-7562$04.50/0
0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 31, 1994 7563
Photokinetics in Photochromic Polymers concentration of B at t = 0. Equation 3 represents the time dependency of B, exposed simultaneously to all photochemical and thermal processes, as indicated in eqs 1 and 2. The photochemical processes are characterized by their respective rate constants, which for species i, is given by Ri = ZiFici~,(Xi/Nhc)
(4)
where Zi is the light intensity (in joules cm-2 s-1) at wavelength Xi, Fi is a photokinetic factor, which corrects for absorption, ti is the decadic molar absorption coefficient [or cross section in cmz/ mol], 4i is the quantum yield, N is Avogadro’s number, h is Planck’s constant, and c the speed of light. For some exposure configurations,not all the processes will be active (Zi, Fi = 0 ) ,and eq 3 can be simplified, as will be shown below. The photokinetic factor ineq4isnotaconstant,as theabsorbanceis timedependent. However, as will be described in more detail below, its time dependency can either be neglected or corrected for by normalization. When B is exposed to an interference pattern between two plane waves at Xg, a cosinusoidal spatial modulation of concentration in the xdirection, B(x) will be formed and thereby recorded as the holographic grating. Temporal holographicrecording will therefore consist of a time dependent spatial modulation B(x,t), as
+
B(x,t) = Bo(?) B,(t) cos(2ax/d)
being formed by thehomogeneousUVbeam. Thetimedependent modulation amplitude and average absorbance can be characterized by B(t),and B(r)dn, which are theconcentrationsobtained in the regions of maximum and minimum intensity of the interference pattern. By taking B(0) = BS and substituting eq 8 in eu 3, in the region of maximum intensity, we obtain
(5)
where Bo(t) and Bl(t) are the average and the peak of the (time dependent) spatial modulation in concentration [in mol/cm3], respectively, and d is the grating period. We take it to be that our photochromic gratings consist essentially of a spatial modulation in absorbance; we believe this to be a valid assumption, because the dispersive contribution was found to be negligibly small at the readout wavelength with spiropyran containing films.15 The time dependent modulation then becomes
where f = Cz/(R,, + RT).In the regions of minimum intensity, for two recording beams of equal intensity, we have Zdn= 0 and therefore Rg = 0 so that B(t)min = BS. The time dependent modulation depth, which is twice the modulation amplitude, and, similarly, the modulation average, can now be determined by 2B1( t ) = B(t)min - B(t),x
2Bo(t) = B(t)min + B(t),x (10)
In configuration (b), an initial coloration by the homogenous and incoherent excitation beam at XA = 364 nm (from a UV lamp) is again carried out to reach thesteady-stateconcentration Bw The initial exposure is then being followed by a separate exposure to the two coherent recording beams at Xg = 514 nm (from an argon ion laser). At this stage of the exposure, the holographic interference pattern, created by the two coherent beams, is recorded by bleaching of the colored MCs. By taking B(0) = BS and substituting eq 8 in eq 3, in the region of maximum intensity, we get B(t)-
= B, exp[-(RB
DE(?)= exp(-2ao(t)l/cos 0) sinh2(a,(r)L/2 cos e) (7) where ao(t) and al(t) are the (time dependent) average and amplitude of the absorbance modulation, respectively, and are defined by eqs 5 and 6; L is the grating thickness, and 8 is the readout angle.16 Let us now consider the holographicresponse for three specific exposure configurations(a), (b), and (c); first, we have to derive Bo(r) and Bl(t), which will determine the corresponding DE(?). In configuration (a),the photochromic films are initially exposed to a homogeneous and incoherent excitation beam at XA = 364 nm (from a UV lamp), so that the films become colored. An exposure sufficient to reach a ready-state concentration BuEis applied, which for configuration ( a ) is given by @s 2 and 3, as
The initial exposure is then being followed by a simultaneous exposure to two coherent recording beams at AB = 5 14 nm (from an argon ion laser) and to the above UV excitation beam. At this stage of the exposure, the holographic interference pattern, created by the two coherent beams, are recorded by bleaching of the colored MC molecules, while, at the same time, new MCs are
(1 1)
In the regions of minimum intensity, for two recording beams of equal intensity, we have I d n = 0 and therefore Rg = 0 so that B(t),,,j,, = B, exp[-RTt]
(12)
The modulation average and modulation depth for configuration (b), according eq 10, now become 2Bo(t), Bl(t) = 2B, eXp[-R,t](l
where CB is the absorption coefficient at the readout wavelength. The readout radiation can in principle also lead to photochemical interactions; therefore, in order to minimize destructive readout, the powers have to be significantly lower than those of the recording beams? The temporal diffraction efficiency of the thick amplitude gratings can now be given as
+ RT)rl
f eXp[-Rg?]]
(13)
Finally, in configurarion (c), the photochromic film is first simultaneously exposed to the homogeneous UV excitation beam and to the twocoherent holographic recording beams; an exposure sufficient to reach steady-state concentrations is applied, which will be characterized by B- and BuEin the regions of maximum and minimum intensity of the interference pattern, respectively. B,, is given according eq 3 by
This initial simultaneousexposure to the excitation and recording beams is then immediately followed by an additional exposure to the recording beams only. At this stage of the exposure, the initially recorded holographic grating, is enhanced by a further bleaching of the colored MCs. By taking B(0) = Bm and ZA = 0 so that RA = 0 and by substituting eq 14 in eq 3, in the region of maximum intensity, we obtain
In the regions of minimum intensity, for two recording beams of equal intensity, we have I- = 0 and therefore Rg = 0 so that B(t)dnis given by eq 12. The modulation average and modulation depth for configuration (c), according eq 10, now become
Results and Discussion In order toverify our proposed model, we performed holographic recording experiments with polymethylmethacrylate (PMMA)
7564 The Journal of Physical Chemistry, Vol. 98, No. 31, I99
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Figure 1. Real-timeholographicgrowth curves of gratings recorded with two Ar-laser beams (514 nm) in spirooxazine/PMMA films for three exposure configurations (a), (b), and (c); readout was at 633 nm: configuration (a) initial exposure to the UV (364 nm) excitation beam of 5 mW/cm2 and then simultaneous exposure to the UV beam and to the two recording Ar-laser beams each of 9 mW/cm2;configuration (b) initial exposure to the UV beam and then separately to the recording beams; configuration (c) additionalexposureto the recordingbeams only, immediately following the simultaneous exposure to all the beams.
films containing 10 wt % of compound A (see eq 1). The details of film preparation and of the holographic setup are described el~ewhere.~ The holographicexperimentsincluded three different exposure configurations? as described above in the theoretical section. The temporal diffraction efficiency (DE) (holographic growth) was measured in real-time at 633 nm by a He-Ne laser, incident at sufficiently low power (0.5 mW/cm*), so as to minimize destructive readout.9 The results are presented in Figure 1; as is evident, significantly different holographic growth rates are obtained for the three exposure configurations. In order to calculate the theoretical temporal holographic response, we first need to determine the thermal and photochemical rate constants. The thermal rate constant was determined by measuring the color decay of our precolored films in a diode array spectrophotometer (HP8452A). The measured data could be fit by a two exponential decay with rate constants of about 5 X 10-3 and 1 X 10-3 s-1. The photochemical rate constants are determined by fitting to our experimental holographic growth data; independent values were calculated according eq 4, which contains additional parameters that have to be determined. These include the absorption coefficients, which were obtained from the absorption spectra, yielding ~ ~ ( 3 nm) 64 = 4.92 X 106, ee(514 nm) = 2.50 X 106, and ~ ( 6 3 nm) 3 = 3.33 X 106cmZ/mol. Thequantum yield of coloration was determined by measuring the coloration kinetics in the spectrophotometer with the UV lamp attached to it. It was found to be 4~ 0.9 [Einsteins/mol], which is similar to the value reported for compound A in solution.* We did not determine the quantum yield of decoloration, which is difficult to measure by spectroscopy.68 Nevertheless, a rough estimate can be done, by exploiting the fact that it is usually about one order of magnitude lower than the corresponding coloration quantum yield.6 The photokinetic factor F in eq 4 corrects for absorbance, which, in our case, may be treated as a constant, because of the followingreasons.17 First, neither of the two photochromic states A and B absorbs at the irradiation wavelength of the other state; second, the absorbance of B is almost identical at the irradiation and readout wavelengths. In addition, when the samples are irradiated by UV light only or with mixed irradiations, the changes in absorbance of A are negligibly small. Therefore, by normalization of the calculated temporal holographic response, we correct for F. Furthermore, normalization of the experimental temporal
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Figure 2. Calculated and experimental normalized DE and calculated color decay as a functionof exposure time of holographicgratingsrecorded in spirooxazine containingPMMA films, for three exposure configurations (a), (b), and (c), each shown in its corresponding Figure 2a-c:. The calculated rate constants are shown on top of each figure.
holographic response corrects for the losses from reflections and absorption and compensates for variations in the merocyanine dye concentration, as a result of variations in film thickness and fluctuations in Ar laser and UV lamp intensity. Figure 2a-2c show the calculated and experimental DE-time response and the calculated color decay in spirooxazinecontaining PMMA films for the three exposure configurations (a), (b), and (c). The calculated DE responses were calculated according eq 7 with constants of L = 8 Mm and 0 = 37.7O and the corresponding rate constants, as indicated on top of each figure. Furthermore, the modulation average and modulation depth that were derived in the theoretical part for each of the configurationsare substituted in eq 7. Specifically, the response of configuration (a) was calculated after successive substitutions of eq 6, 10,9, and 8 into 7. The response of configuration (b) was calculated after successive substitutions of eqs 6, 13, and 8 into 7. Finally, the response of configuration (c) was calculated after successive
O
Photokinetics in Photochromic Polymers
substitutionsofeqs6,16,and8into7. Asevident,goodagreement is obtained between theory and experiment, indicating that holographic recording in photochromic polymer films may be suitably simulated by our model. As shown in Figure 2, the calculated results in configuration (c) were obtained with thermal and photochemical decoloration (bleaching) rate constants that were significantly higher than in configurations a and b. Specifically, the photokinetic rate constants RB increased from about 0.06 s-l in configurations a and b, to about 0.10 s-1 in configuration (c). The corresponding quantum yields which were calculated according eq 4, increased from about 0.09 to 0.16 Einsteins/mol. The existence of two thermal and photochemical decoloration rate constants indicates that the simplified reaction scheme in eq 1is not sufficient to explain the observed photochromic responses for all exposure configurations. A more suitable reaction scheme may be given by
where B1 and B2 are now two forms of the colored MC molecules, satisfying the relation B = B1 B2; RBIand R Bor~ R T and ~ RT~ are the respective photochemical or thermal rate constants for the transformations from BI to A and from BZto B1, and kT1 is the thermal rate constant for the transformation from BI to Bz. The reaction scheme in eq 17 suggests a consecutive conversion between A, B1, and B2. However, a parallel conversion between A and the two MC forms may also be considered. The results in Figure 2 can now be interpreted in terms of the relative populations of B1 and B2. In configurations (a) and (b), the photochromic film is initially exposed to the UV excitation beam alone, so that at steady state the more stable colored form B2 will be dominantly populated. A following exposure to the visible recording beams causes decoloration to A, with the rate determiningstep being the transformation from B2 to B1 (and/or B2 to A in a parallel scheme). With configuration (c), however, the initial exposure to all beams yields a mixture of all the forms, with a significant population of B2. Therefore, the following exposure to the recording beams causes decoloration at a significantly increased rate, being mainly determined by the transformation from Bl to A. The identity of the two MC forms B1and BZcannot be derived with certainty from this investigation. We may nevertheless suggest the most probably possibility by eliminating other alternatives. First, the influence of free volume distribution in the polymer matrix7 must be discarded as an explanation of the photokinetic effects observed by us. Such local changes in the polymer matrix may influence the photochromic kinetics by steric hindrance;yet, changesin the exposure conditions will not change the relative MC concentrations at different sites within the matrix, because these do not equilibrate with each other. Second, aggregate formation does not seem to play an important role in our experiments, as we did not observe any detectable spectral shifts7J4during coloration and thermal color decay. Furthermore, the differences in photochemical decoloration rates would have been much more pronounced between MC aggregates and m0n0mers.l~ Finally, we also exclude thermal heating effects during simultaneousirradiations for explaining the experimental evidence, because, first, relatively low powers were used, and, second, good thermal exchange exists between our thin films and the substrate. We therefore believe that B1 and B2 are stereoisomeric MC forms of different thermal and photochemical stability.’-13 This work again shows that holographic recording is an important tool for the investigation of photochemical/photophysical processes in solid matrices, because it has certain advantages over conventional optical recording or spectros-
+
The Journal of Physical Chemistry, Vol. 98, No. 31, 1994 7565 copy.18J9 First, at very low exposures, we actually probe the square of the concentration changes and the rate constants, as can be shown9.18 by simplification of eq 7; this fact makes the holographic method significantly more sensitive,especially during the initial phases of the photoinduced processes. For comparison, in conventional optical recording or spectroscopy, we monitor optical density (OD), which is directly proportional to concentration. Second, with holographic recording, the light induced changes are probed directly, as we monitor the DE, which is proportional to the modulation amplitude (see eq 7). With the conventional methods, on the other hand, the measured OD data also contain background from nonreactive species and optical noise. Also, in order to determine small concentration changes, we have to subtract two OD values of the same magnitude, which increases the experimental error. Therefore, signal-to-noise decreases with increasing initial OD, which becomes of significance, when monitoring color decay in photobleaching reactions.
Conclusions A photokinetic model for elucidating the photochemical and thermal transformationsbetween the colorless and colored forms of photochromic compounds has been presented, emphasizing holographic recording in the visible range by bleaching of precolored material. The predictions for three different recording configurations agreed well with the experimental diffraction efficiency versus exposure responses of holographic gratings recorded in spirooxazine containing polymer films, indicating the validity of our model. Significant differenes in the exposure sensitivities and holographic growth rates were found with variations in the exposure configurations, indicatingdifferent photochemical decay kinetics. The results suggest that the thermal and photochemical transformations occur between colorless spirooxazine and at least two stereoisomericforms of colored MC. This work also discusses the advantages of holographic recording over conventional spectroscopy for the investigation of photokinetic mechanisms, especially with photobleaching of colored species. Acknowledgment. The authors wish to thank Prof. A. Friesem for a critical review of this work. This research was partly supported by Holo’or Ltd., Kiryat Weizmann, Nes Ziona, Israel.
References and Notes (1) Mikealiane, A. L.;Axenchikov, A. P.; Bobrinev, V. I.;Gulaniane, E. H.; Shatun, V. V. IEEE J. Quant. El. 1968, QE-4,757. (2) k i n s k y , M.; Miler, M. Opt. Com. 1970, 1, 417; 1972, 5, 104. (3) Friesem. A. A.: Walker. J. L.
[email protected], 9. 201. (4) Tomlink, W. J.; Chandross, E. A.:Fori, R.L.;Pryde,C. A.; Lamola, A. A. Appl. Opt. 1972, 11, 533. (5) Kirby, C. J. G.; Bennion, I. IEE Proc. 1986, 133.98. 16) Bertelson. R. C. In Photochromism: Brown. G. H.. Ed.: Wilev InteGience: 1971; Chapter 111. (7) Diirr, H.; Bouas-Laurant, H. Photochromism, Molecules and Systems; Elsevier: 1990; Chapter 2, 8, 10, and 21. (8) Crano, J. C.; Kwak, W. S.;Welch, C. N. In Applied Photochromic Polymer Systems; McArdle, C. B., Ed.;Blackie: Glasgow and London, 1992; Chapter 2. (9) Weiss, V.;Friesem, A. A.; Krongauz, V. A. Opt. Lett. 1993,18(13), 1089. (10) Schiele, C.; Amold, G. Tetrahedron Lett. 1967, 13, 1191. (1 1) Kellmann, A.; Tfibel, F.; Dubcsf R.; Levoir, P.; Aubard, J.; Pottier, E.; Guglielmetti, R. J. Photochem. Photobiol. A: Chem. 1989, 49,63. (12) Bohne, C.; Fan, M. G.; Li, C. J.; Liang, Y. C.; Lusztyk, J.; Scaiano, J. C. J. Photochem. Photobiol. A: Chem. 1992.66.79. (13) Maeda, S.;Mitsuhashi, K.; Osano, Y. T.; Nakanura, S.; Ito, M. In Chemistry of Functional Dyes, Vol. 2; Proceedings of the 2nd International Symposium on Chemical Functional Dyes, Kobe, 1992;Ymhida, Z., Shirota, Y., Eds.; Mita Rm:Japan, 1993; p 352. (14) Hashida, T.;Hibno, J.; Suzuki, M.; Kishimoto, Y. in ref 13; p 345. (1 5) Wciss, V. Thesis Ph.D. Scientific Councel;The Weizmann Institute of Science: Rehovot, Israel, 1994. (16) Kogelnik, H. Bell. Sys. Tech. J . 1969, 48, 2909. (17) Gauglitz. G. in ref 7, Chapter 25. (18) Briuchle, C.; Burland, D. M. Angew. Chem., Int. Ed. Engl. 1983, 22,582. (19) Gehrtz, M.; Pinsl, J.; BrHuchle, C. Appl. Phys. B 1987, 13,61.
