J. Phys. Chem. 1995,99, 17226-17234
17226
Reorientation Mechanism of Azobenzenes within the Trans
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Cis Photoisomerization
Zouheir Sekkat: Jonathan Wood: and Wolfgang Kn01l*~~~’ Max-Planck-Institutf i r Polymetforschung, Ackermannweg IO, 55128 Mainz, Germany, and Frontier Research Program, The Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-01, Japan Received: June 13, 1995; In Final Form: August 14, 1995@
We have recently shown, using waveguide spectroscopy, that irradiation with linearly polarized UV light (360 nm) induces a strong anisotropy in an azobenzene-containing polyglutamate film. Further irradiation with linearly polarized blue light (450 nm) suggested that the cis state aligns perpendicular to the initial UV light polarization. It this paper we can confirm unambiguously these results by using polarized UV-vis spectroscopy, since each isomer (e.g., trans and cis) may be assigned to a characteristic absorption band within the UV-vis spectrum. We also present a theoretical model that describes the photoisomerizationinduced reorientation of azobenzenes in a viscous environment. The equations of this phenomenological theory are solved by using the formalism of Legendre polynomials, and analytical solutions are derived. These solutions fully explain the experimental findings and give a good physical insight into the reorientation phenomenon that occurs within the photoisomerization reaction.
1. Introduction In the past few years azo-dye-containing polymeric films have been the subject of intensive investigations because of their possible application in the areas of optoelectronics, photonics, and optical signal processing.’-I0 The trans cis isomerization of azobenzene derivatives in bulk polymer films has been used for producing polarization holograms,2 optical channel waveg u i d e ~ writing-erasing ,~ optical memories,“ and second-order nonlinear optical effect^.^ The role of the reorientation of the azo-dye molecules within the isomerization reaction has been of particular interest. Indeed, when these photochromic materials are irradiated with linearly polarized light of the appropriate wavelength, the azo-dye molecules experience successive cycles of trans * cis isomerization and eventually align perpendicular to the irradiating light polarization. However, it is difficult to get a good physical insight into this reorientation process using such molecules (e.g., push-pull pseudostilbene type azobenzene molecules such as disperse red 1 (DRl))4 since both the trans and cis isomers exhibit their absorption maximum in the same visible region.43” Furthermore, the cis lifetime is relatively short (4-5 s in poly(methy1 methacrylates) (PMMA))? and so no information is available about the reorientation process in each isomerization step (e.g., the trans -t cis direct photoisomerization and the photoinduced and thermally activated back cis 4 trans reaction^).^ In contrast, azobenzene-containingpolymeric films are most appropriate materials for closely probing this reorientation process. The azobenzene trans and cis isomers present different absorption bands in the UV (around 360 nm) and the visible (around 450 nm) regions, and the lifetime of the azobenzene cis isomer is generally of the order of hours, depending on the polarity of the host material which may be a polymer or a solvent.” Indeed, using a polymer film with azobenzene side chain units, we have shown,’ using waveguide spectroscopy, that linearly polarized W light (360 nm) irradiation of this film induces a direct trans cis photoisomerization with the cis molecules aligned perpendicular to the polarization
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* To whom all correspondence should be addressed.
’ Max-Planck-Institut fur Polymerforschung. @
The Institute of Physical and Chemical Research. Abstract published in Advance ACS Abstrucrs, November 1, 1995.
of the irradiating W light. This result demonstrates that the azo molecules are already reoriented in the direct trans cis photoisomerization. Here we can confirm these experimental results using polarized UV-vis spectroscopy as the cis isomer may be clearly assigned to a characteristic absorption band in the visible region of the UV-vis spectrum.” Furthermore, we develop a theoretical model that fully reproduces the experimental findings and allows good physical insight into the reorientation mechanism of the azo molecules within the photoisomerization reaction.
2. Experimental Section In this study, we used the same material that we had investigated previously,’ i.e., poly(5-(2-(4-(4-decyloxyphenylazo)phenoxy)ethyl)-L-glutamate). The molecular structure of this material, henceforth referred to as the azo-polyglutamate, is given in Figure la. In these molecules the main chain winds to an a-helix that is stabilized by hydrogen bonds, thus forming a rigid rod with the side chains pointing to the outside. This is a type of so-called “hairy rod” polymers which are well-suited for constructing multilayer structures by the Lmgmuir-BlodgettKuhn t e ~ h n i q u e . ’ ~Synthetic *’~ and characterization details are described e1~ewhere.l~ Azobenzene derivatives have two geometric isomers, the “trans” and the “cis” forms (Figure lb). The photoisomerization reaction begins by elevating molecules to electronically excited states after which nonradiative decay brings them back to the ground state in either the “cis” or “trans” forms. The ratio of cisltrans states is dependent on the quantum yield of the appropriate photoisomerizationreaction (e.g., c#Jtc and c#Jct for the direct trans 4 cis and reverse cis trans photoisomerization reactions, respectively (see Figure IC)). As the azobenzene trans isomer is generally more stable than the azobenzene cis isomer (the energy barrier at room temperature is about 50 Idlmol-I), molecules in the cis form may relax back to the trans form by one of two mechanisms: (1) a spontaneous thermal backreaction or (2) a reverse cis * trans photoisomerization cycle. As the complete cis trans thermal back-reaction generally requires several hours at room temperature, the cis isomer can be considered as stable on a time scale of minutes. Spectro;=)
;=)
0022-365419512099-17226$09.00/0 0 1995 American Chemical Society
Reorientation Mechanism of Azobenzenes
Trans
J. Phys. Chem., Vol. 99, No. 47, 1995 17227
Cis
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Wavelength, h I nm
Trans Figure 1. (a, top) Chemical structure of poly(5-(2-(4-(4-decyloxypheny1azo)phenoxy)ethyl)-L-glutamate), referred to in the text as azopolyglutamate. (b, middle) Trans cis isomerization of azobenzenes. (c, bottom) Simplified model of the molecular states. ut and uc are the cross sections for absorption of one photon by a molecule in the trans or the cis state, respectively. yo is the thermal relaxation rate,
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and actand Qk are the quantum yields of photoisomerization.
scopic characterization of azobenzene type molecules is performed by observing a low-energy band in the visible region of the absorption spectrum and a high-energy band in the UV region." Samples for the UV-vis experiments were prepared by spincasting the azo-polyglutamate from chloroform solution onto cleaned quartz glass substrates. These substrates were cleaned ultrasonically by successive immersion and sonication in pure ethanol and diluted solution of Helmanex (an alkaline detergent) for 5 min at 60 "C before rinsing with copious amounts of ultrapure water (Milli-Q). Further immersion and sonication in a fresh Helmanex solution were followed by a final ultrasonication in ultrapure water (Milli-Q) for 5 min at 60 "C. Following this cleaning procedure, the glass substrates were dried in an oven at 80 OC for 30 min before being spun-cast with the azo-polyglutamate solution. Two substrates were not coated with the polymer, although they were subject to the cleaning procedure stated above, and were kept for use as reference samples in the W - v i s spectroscopic experiments. The samples were then stored in a sealed vessel prior to the dichroism experiments. Photoisomerization of the azobenzene units was induced by irradiation with UV light (360 nm) for the trans cis reaction and with visible light (blue, 450 nm) for the cis 4 trans backisomerization. The irradiating light source was a high-pressure mercury lamp (Oriel, 200 W) with a glass filter for UV light and an interference filter for blue light. At these wavelengths the lamp power was adjusted to 2 mW/cm2for unpolarized light, and for UV linearly polarized light irradiation the intensity was reduced by a factor of 10. A Hewlett-Packard UV-vis diode array spectrometer (Model 8452A) was used to record linearly polarized absorption spectra of the reference and azo-polyglutamate substrates. A self-built sample holder enabled us interchange the reference and azo-polyglutamate substrates without altering sample position or orientation. Dichroism measurements were performed by irradiating the samples with a linearly polarized UV light and immediately recording absorption spectra with the probe light polarization parallel (Absll) and perpendicular (Absl) to the initial UV polarization. Ambient red light conditions were employed to avoid the influence of the room light on the isomerization reaction.
0.2 0.0
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Wavelength, h I nm
Figure 2. UV-vis absorption spectra of the azo-polyglutamate film (a, top) before ( I ) and after (2 (5 min), 3 (10 min), 4 (15 min), 5 (25 min), and 6 (35 min)) linearly polarized UV (360 nm) irradiation and (b, bottom), under 10 min unpolarized UV irradiation ( I ) and after ( 2 (20 s), 3 (40s), 4 (80 s), and 5 (120 s)) unpolarized blue (450 nm)
irradiation. The probe light was linearly polarized and oriented parallel to the exciting UV irradiation. The inset is an expanded view of the cis absorption.
3. Results and Discussions 3.1. Photoinduced Dichroism in the Azo-polyglutamate
Films. The absorption spectra in Figure 2 were recorded with the linearly polarized probe light oriented parallel to the irradiating W light polarization. The insets show an expanded view of the cis absorption band in the visible region. Figure 2a shows the absorption spectra of the azo-polyglutamate layer, before and after various amounts of polarized UV irradiation. The change in spectral shape and the existence of the isosbestic points about 300 and 420 nm clearly demonstrate the trans cis photoisomerization reaction. The isosbestic points do not occur exactly at one wavelength, which is probably due to small differences in the alignment of the sample. As stated previously, the photoinduced or thermal cis * trans back-isomerization can bring the azo molecules back to the initial state. In Figure 2b the photostationary state was reached by irradiating the sample with the UV light for 10 min before commencing a series of irradiations with unpolarized blue light and immediately recording the absorption spectrum after each irradiation. The recovery of the trans spectra and the existence of the isosbestic points clearly demonstrate the blue lightinduced cis 4 trans back-isomerization. It can be clearly seen that, after only 20 s of blue light irradiation, an appreciable
+
17228 J. Phys. Chem., Vol. 99, No. 47, 1995
Sekkat et al. 2.0 1
l A
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Time, t I s
Figure 3. (a, top) Real time kinetics of the thermally activated cis
-
trans back-reaction. (b, bottom) A logarithmic plot of this kinetic with an inset that shows an expanded view of the first points of the figure. The y axis of this figure is defined as (Abs - Absw)/(Abso- Abs"), where Abs is the absorbance of the sample and Abso and Abs" are the absorbances at the time t = 0 and at the steady state of the relaxation, respectively (see (a)). Squares indicate the experimental points, and the solid lines indicate theoretical fits (monoexponential for the inset and double exponential for the full figure). The result of the monoexponential fit is given by the cis lifetime in the inset. The initial state corresponds to the photostationary state reached under UV irradiation.
amount of the azo molecules have already been switched back into the trans state. The thermal cis trans back-reaction was studied in detail by irradiating the azo-polyglutamate sample with UV light until the photostationary state was attained and, subsequently, recording the absorbance of the sample at various intervals (a reading every 30 s for the first half hour and subsequently every 5 min). The sample was kept in a dark environment during these experiments. The analysis wavelength was 360 nm, which corresponds to the maximum absorption wavelength of the trans-azobenzene, The real time dependence of the absorbance of the sample is shown in Figure 3a. The thermal relaxation of the cis form is not of first order as shown by the logarithmic plot of the absorption evolution in Figure 3b; nevertheless, a monoexponential decay may be fitted to the data acquired over the first few hours (see inset), with a rate constant of U(3.5 h), showing that the thermal cis 4 trans back-reaction is relatively slow for the azobenzene in the azo-polyglutamate polymer. This is typical of azobenzene type molecules." The non-monoexponential thermal back-reaction kinetics reflects a distribution of free sites in the azo-polyglutamate polymer, which leads to steric hindrance of the azo molecules, as have been observed =+
7-7-,
',\
- _ _ - _ - - - - - _ -__
-_
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----___ 550
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Wavelength, h I nm
Figure 4. (a, top) W - v i s absorption spectra of the azo-polyglutamate layer (I) and after 35 min (2, 3) of linearly polarized UV (360 nm) irradiation. The probe light was also linearly polarized, and spectra were obtained for both parallel, Absll, (3) and perpendicular, Absl, ( 2 ) orientations. Identical spectra were obtained for both Absi1 and Absl prior to UV irradiation. For reasons of clarity, therefore, only Absl, ( I ) , is shown. (b, bottom) Spectra ( I ) obtained after unpolarized UV (360 nm) irradiation for 20 min; further blue (450nm) irradiation for 2 min leads to spectra (2, 3). Note that the dichroism has been significantly erased in comparison to (a). The insets are as defined for Figure 2 .
for other azo-dye-containing polymeric materials.4*' The analysis of this relaxation shows that after 10 min relaxation, only 5% of the initial spectra prior to UV irradiation was restored, demonstrating that the thermally activated cis trans back-reaction is very slow at room temperature. In other words, the cis state is stable within this time period. A similar result was obtained while using the waveguide spectroscopy technique.' Figure 4a shows the dichroism observed in the azo-polyglutamate layer. These spectra were obtained after 35 min irradiation with linearly polarized W light. It is clear that the absorption Absl, is higher than the absorption, Absll. Identical spectra were recorded for both Absi1 and Absl prior to W irradiation (only Absl shown), demonstrating that the sample was optically in-plane isotropic at that time. These findings are true for the trans absorption band in the W region around 360 nm and also for the cis absorption band in the visible region around 450 nm (see the inset in Figure 4a). This clearly shows that both the trans and cis azo molecules are preferentially distributed perpendicular to the initial UV polarization and unambiguously demonstrates that the cis isomer aligns perpen-+
Reorientation Mechanism of Azobenzenes
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dicular to the initial W polarization within the trans cis isomerization. Irradiation with unpolarized blue light does not erase this dichroism, because only the cis isomer has significant absorption in the blue region around 450 nm, and consequently, the trans molecules cannot be exited and reoriented. The inplane isotropy in both the trans and cis molecular distributions can be restored only after successive unpolarized UV and blue light irradiations (see Figure 4b). However, the initial spectrum of a freshly prepared sample prior to irradiation is not restored by this procedure because a net out-of-plane orientation of the azo molecules remains. The absorption band that appears blueshifted in spectra 1 and 1 in Figures 2b and 4b, respectively, is due to the cis absorption as previously observed in solution.' It is worth mentioning that the spectra recorded with different light polarizations do not show the same isosbestic point because of the anisotropy that exists in the sample. Heating the azo-polyglutamate sample at 80 "C for 30 min and 14.5 h failed to erase the W irradiation-induced dichroism in the sample. At this temperature the sample is still at least 120 "C below the side chain isotropization temperature (T > 200 "C) of the p01ymer.I~ Molecular movement in polymeric material is generally governed by the difference between the operating temperature T and the glass transition temperature (T,) of the polymer;I6 in other words, the smaller the difference T - Tg is, the greater the molecular movement. In Figure 5a we have plotted the absorbance of linearly polarized probe light (at 360 and 450 nm) at various qngles, Y, between the polarizations of probe and UV lights. Sinusoidal behavior is clearly shown and unambiguously demonstrates the photoselection in both trans and cis molecular distributions. As in Figure 4a, the highest absorption for both trans and cis distrubitons is observed when the probe and irradiation beams have perpendicular polarizations. The small drift in this absorption data results from the amount of time required to perform this experiment (e.g., 25 min). During this time there is a small, but noticable, amount of cis * trans thermal recovery. We have also measured the UV-induced dichroism for different UV irradiation times for both cis and trans molecular distributions. This was performed by irradiating the sample with linearly polarized UV light for a defined period of time, recording the absorption spectrum and subsequently calculating the anisotropy AAA and the order paramater SA
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at wavelength (360 and 450 nm for trans and cis molecular distributions, respectively). Parts b and c of Figure 5 show these anisotropy and order parameter at various W irradiation times, respectively, and demonstrate that the dichroism increases with increasing irradiation times until a maximum value is reached; further irradiation progressively produces less dichroism. This will be further discussed in the next section. 3.2. Theoretical Study of the Reorientation Process of the Azo Molecules within the Trans Cis Photoisomerization. The light polarization sensitivity of this photoisomerization reaction originates from the anisotropic shape of the azobenzene molecules. Indeed, considering that the transition dipole moment is along the long principal axis of the molecule, the probability of an azo unit for absorbing a photon and subsequently isomerizing is proportional to the cosine square of the angle between the transition dipole moment and the polarization direction of the exciting light. The molecules aligned parallel to the polarization of the exciting light, therefore, clearly have
Figure 5. (a, top) Dependence of the absorbance of linearly polarized probe light at 360 nm (m) and 450 nm (O), on the angle Y between the probe and the UV irradiation light polarization. This behavior is
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fitted with a law in cos2 Y with an amplitude that decays in accordance with the cis trans thermal back-reaction. The theoretical fits are indicated by dotted curves. Evolution of (b) the anisotropy AAA and (c) the order parameter SA defined in relation 1 for the wavelengths 360 nm (m) and 450 nm (O),with the time of W (360 nm) irradiation.
the highest probability of being exited and isomerized. Consequently, when starting from isotropically distributed trans isomers, linearly polarized W light bums a hole in this angular distribution due to the selectivity of the molecular excitation (angular hole buming). Furthermore, as the principal axis of the azo unit is reoriented due to the isomerization reaction, the principal axis of the cis state will be tilted with respect to the direction of initial W polarization (angular redistribution). Both angular hole-buming and angular redistribution lead to an anisotropic distribution of the azo molecules with the photo-
17230 J. Phys. Chem., Vol. 99, No. 47, 1995
Sekkat et al.
isomerization reaction. The isotropy can be restored by rotational diffusion that can be described by the rotational Brownian motion resulting from thermal agitation. For modeling the photoisomerization and photoinduced reorientation of azobenzene molecules, we use equations similar to the ones we used in the case of push-pull azobenzene derivatives? The model developed in the following section assumes that only the trans isomer absorbs significanlty in the UV region at 360 nm. This is in very good agreement with the experimental physical case since the absorbance of the cis form at 360 nm is in fact very weak and is at least 14 times lower than the absorbance of the trans form at the same wavelength. This is unambiguously shown in ref 1, which reports both the cis and trans spectra; such a direct comparison cannot be made from the spectra shown in this paper since the photostationary state was never reached in any of these spectra. The evolution of the angular distribution results from the angular hole burning, angular redistribution, and rotational diffusion. If we assume that the trans and cis molecules are anisometrically shaped and that they may be characterized by uniaxial molecular polarizability tensors (cigar-shaped molecules), then the evolution of the angular distribution has the general form
LISQ( Q’-Q)
nc(Q’) dQ’
+ D,R2n,(Q,t)
3
I-I: I
Figure 6. Black lines indicate the laboratory axes (1, 2, 3), and the gray lines indicate the molecular long principal axes for both trans (52) and cis (52’) isomers. The irradiation1 light beam may be linearly polarized along the 3-axis, and also circularly polarized in the { 1,2)plane, or unpolarized propagating in the direction of the 3-axis.
‘
where Nt and Nc are the molecular densities at the trans and cis states, respectively, and N is the total molecular density. Considering that the molecular distribution is axially symmetric (the symmetry axis is defined by the direction of the irradiating light polarization), it is obvious that the Legendre polynomials are the eigenfunctions for the system.I8 The trans and cis populations can then be written in the following form:
ZC
Pc(S2’-S2) and Q(S2’-S2) are the probabilities that a molecule will rotate in the “redistribution” processes of trans cis.optical transition and cis trans thermal recovery, respectively. The “angular hole burning” is represented by a probability proportional to cos2 8. The last terms on the right-hand side of eqs 2 describe the rotational diffusion due to Brownian motion. This is a Smoluchowski equation for the rotational diffusion characterized by a constant of diffusion Dc (4) for the cis (trans) c~nfiguration.’~ R is the rotational operator; fl * (4, - dl)/ (dl 20;) represents the normalized trans molecular anisotropy, and 8 = (dl 2dl)/3 denotes the trans isotropic absorption cross section. dl and dl are the absorption cross sections in the parallel and perpendicular directions of the trans long molecular axis, respectively. e = 1 corresponds to linearly polarized light along the symmetry axis of the system, and e = -‘/2 corresponds to an unpolarized light or to a circularly polarized light in the plane perpendicular to this symmetry axis. I is the intensity of the irradiating light expressed in flux of photons per square centimeter. nt(S2) dS2 and n,(S2) dS2 are the number of trans and cis molecules whose representative moment of transition along the long axis is present in the elementary solid angle dS2 around the direction Q(O,q), respectively. The normalizations are
+
+
As the cis and trans populations change with the irradiating light
intensity, it is convenient to define nonnormalized expansion coefficients T,,and C,,for Legendre polynomial P,,(cos e).These are given by T,, = Kn,(O)P,,(cos 6) sin 8 d e
The redistribution processes Pc(S2’-S2) and Q(S2’-S2) only depend on the rotation angle, x, between the directions S2 and S2’ (see Figure 6). We can, therefore, expand them in terms of Legendre polynomials P,,(cos x):
with these definitions To = N,; Co= N,;
+
Nt N, = N
JJPtc(Q’-Q)
dQ’ = 1
J@(Q’-Q)
dQ’ = 1 (3)
pi = Qo = 1
(7)
The general formalism given in eqs 2 can be simplified when Legendre formalism is used, and the variations of the cis and trans populations are given by the variations of their expansion parameters T,,and C,,, respectively. One obtains the following system of equations:
Reorientation Mechanism of Azobenzenes
J. Phys. Chem., Vol. 99, No. 47,1995 17231
where yo = 1 h c ; Kn+
(n = (2n
+ l)(n + 2) + 1)(2n + 3);
Kn
z, = 8z
2n2 + 2 n - 1 . = (2n - 1)(2n 3)' n(n - 1) K,- (8) (2n - 1)(2n 1)
+
1
I
+
I
I
150
200
-
100
50
Normalized Irradiation Intensity
This system (eqs 8) shows that the temporal behavior of the nth order is related to the temporal behavior of the (n - 2)th
+
and ( n 2)th orders and demonstrates that the irradiating light only creates unpolar molecular alignment that leads to a macroscopic anisotropy. To solve this system analytically, we neglect the expansion parameters above the third o r d e r t h e fourth Legendre polynomial moment is only a small correction to the second Legendre polynomial moment that gives the anis~tropy'~~'~-and we assume that the rates of both the cis 4 trans thermal reaction and the diffusion in the cis and trans forms are very small (70x Dt x D, x 0). This corresponds very well to the experimental conditions observed. Furthermore, the diffusion time is considerably higher than the minute time scale since the UV-induced dichroism is quasi-permanent, and even heating at 80 "C for more than 14 h does not erase this dichroism. If the irradiating W light is turned at the time t = 0, we obtain
N T2(t)= -[exp(-A,t) 5
- exp(-Aot)]
-
y
- 1 ) exp(-iof)]
with
C2(t)= -PFT2(t) and Co(t)= N - To(t)
(9)
where
A, = (1 + 1 .23d)Zt and io = (1 - 0.65er')Zt The boundary conditions are T2(t=0) = C2(t=O) = T2(t=m) = C2(t=-) = 0, Co(t=O) = 0, and To(t=O) = N . These analytical solutions show, through simple expressions, that the molecular alignment and the population change evolve with two time constants that mainly depend on the irradiating light intensity I and the quantum yield &, of the direct trans * cis photoisomerization. It can be seen that high-intensity irradiation for a short time and low-intensity irradiation for a long time result in the same eventual effect; the factor that governs the evolution is It, where t represents the time. Parts a and b of Figure 7 show the time evolution simulation of the cis and trans populations (Nc (or CO)and Nt (or TO))and alignment (C2 and T2) under linearly polarized W light irradiation, respectively. The samples were assumed to be isotropic initially, and
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