Evidence of Two Distinct Mechanisms Driving Photoinduced Matter

Jan 20, 2011 - G. Pawlik , A. Miniewicz , A. Sobolewska , A. C. Mitus. EPL (Europhysics Letters) ... Anna Sobolewska , Stanislaw Bartkiewicz. Applied ...
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Evidence of Two Distinct Mechanisms Driving Photoinduced Matter Motion in Thin Films Containing Azobenzene Derivatives F. Fabbri, D. Garrot, K. Lahlil, J. P. Boilot, Y. Lassailly, and J. Peretti* Laboratoire de Physique de la Matiere Condensee, Ecole Polytechnique, CNRS, 91128 Palaiseau, France

bS Supporting Information ABSTRACT: Photoinduced matter motion in thin films containing azobenzene derivatives grafted to a polymer backbone is investigated by means of near-field probe microscopy. We evidence the existence of two different photomechanical processes which produce mass transport. One is governed by the light intensity pattern and the other by the light polarization pattern. The intensity-driven mechanism is found to critically depend on the polymer matrix while the polarizationdriven mechanism occurs with almost the same efficiency in different materials. Depending on the relationship between the polarization and intensity patterns, the two processes may either compete or cooperate giving rise to a nontrivial directional mass transport process.

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olymer-like materials containing azobenzene units exhibit a variety of spectacular photomechanical responses.1 In particular, matter motion over distances much larger than the molecule size and polymer chain length is photoinduced upon illumination in the absorption band of the chromophores and gives rise to the formation of surface reliefs.2,3 This occurs in thin films containing azobenzene derivatives such as Dispersed Red 1 (DR1), which exhibit electron-donor electron-acceptor substituents allowing efficient reversible photoisomerization of the photochromic units between trans and cis states. It is often assumed that the photomechanical response of the material is closely related to the repeated photoisomerization of the molecules. This is supported by the fact that similar phenomena are observed in other materials4-6 which also show reversible photoinduced conformational changes at the molecular level. However, the complete microscopic mechanism scheme is still not identified in spite of many theoretical efforts.7-15 The main difficulty in the interpretation of the observed mass migration is the strong dependence of the matter motion efficiency on the light polarization. Most of the proposed theoretical models provide a dependence on the light polarization, which is in qualitative agreement with the experimental data. However, these models predict different directions of the matter motion. In order to identify the relevant description, it is thus necessary to determine the direction of the mass transport, that is, to establish the spatial relationship between the optical pattern and the photoinduced relief pattern.16 Up to now, the investigation of photoinduced mass motion in azo-polymer films was mainly based on the study of surface relief grating (SRG) formation upon illumination with an interference pattern. The kinetics of the SRG growth was monitored by measuring the diffraction efficiency in different polarization configurations. These techniques r 2011 American Chemical Society

give access to the photoinduced change in the material refractive index and indirectly to the amplitude of the surface relief,17,18 but the information on the spatial relationship between the optical pattern and the topographical pattern is lost. In this paper, we report on the study of the SRG formation based on the use of local-probe microscopy, which allows to simultaneously measure the optical pattern and the photoinduced topography.19 The polarization configuration of the interfering beams is tuned in order to vary the balance between polarization and intensity patterns. We evidence the existence of two distinct directional mass transport mechanisms, one governed by the light intensity distribution and the other by the polarization distribution. The polarization-driven mechanism has a similar efficiency in the different materials that we have studied, while the efficiency of the intensity driven mechanism strongly depends on the nature of the matrix. This allows us to single out the two mechanisms and to determine their respective directionality and efficiency. On the basis of these results, we discuss the relevance of theoretical descriptions proposed up to now. The experiments are performed on 200 nm thick photochromic films spin-coated on a glass substrate. We study materials based on two different matrices to which DR1 molecules are grafted. One, labeled Si-DR1, is a sol-gel silica material, whose synthesis is described elsewhere.20 The other, labeled PMMADR1, is an organic polymer prepared from commercial poly[(methyl methacrylate)-co-(Disperse Red 1 methacrylate)] (SigmaAldrich 570435), by dissolution of 25 mg of powder into 100 mL of dichloromethane. Both materials have a similar dye Received: November 4, 2010 Published: January 20, 2011 1363

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Figure 1. (a) Schematic of the experimental setup. (b) Polarization configuration of the interfering beams in their respective wave plane. The angles j and -j are defined with respect to s polarization direction (y-axis). (c) Ix, Iy, and Iz intensity patterns calculated for j = 20. (d) Near-field optical image of the interference pattern measured when sweeping j from 0 to 90 by 10 steps. The dashed line at x = 0 shows the chosen reference on x-axis. (e) Transmitted light intensity profiles. For each j value, the full profile scale extends from 0 to 4.

concentration of 1 molecule/nm3 and exhibit almost identical optical absorption spectra. Figure 1a shows a schematic of our experimental setup. The sample is illuminated through the glass substrate by an interference pattern. The transmitted light intensity pattern and the photoinduced deformation are imaged in situ and in real time by the tapered optical fiber tip of a scanning near-field optical microscope equipped with shear-force detection of the surface topography.21 The light wavelength λ = 473 nm is close to the maximum of the dye visible absorption band. The incidence angle θ = 16.5 yields a spatial interference period Λ = λ/(2 sin θ) = 830 nm along the x-axis. Each beam power density is 1 mW/ mm2. The two beams are linearly polarized along directions tilted by opposite angles j and -j from the y-axis (Figure 1b). In this geometry, the interference pattern is the superimposition of three components Ix ¼ 4I0 cos2 θ sin2 j sin2 ðπx=ΛÞ Iy ¼ 4I0 cos2 j cos2 ðπx=ΛÞ Iz ¼ 4I0 sin2 θ sin2 j cos2 ðπx=ΛÞ I0 being the transmitted intensity when illuminating with a single beam. Note that the Ix component of the interference pattern is spatially shifted by Λ/2 with respect to Iy and Iz components (Figure 1c). Thus, in the general case, the interference pattern is the combination of a polarization interference pattern and an intensity interference pattern. Their relative contribution is controlled by setting the angle j. Figure 1d shows the experimental optical image of the interference pattern when varying the angle j from 0 (s polarization) to 90 (p polarization) by 10 steps. The intensity interference pattern shifts by Λ/2 when going from s to p polarization. By convention, we take as

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reference along the x-axis the position of the light intensity maximum when j = 0. It is indicated (modulo Λ) by the dashed line at x = 0. For each j value the transmitted light intensity profile is plotted in Figure 1e. For j = 0, a pure intensity interference pattern is produced (Ix = 0 and Iz = 0). The corresponding measured light intensity profile exhibits a contrast between bright and dark fringes very close to 100%. Pure polarization interference pattern is obtained for j = 47.5, as shown by the almost constant light intensity along the intensity profile recorded for j = 50. For j = 90, Iy is zero but because of the Λ/2 shift between Ix and Iz, the interference pattern is a combination of a polarization pattern and an intensity pattern exhibiting a contrast of 0.82. Figure 2 shows near-field images of the photodeformation kinetics for the two materials during illumination in three polarization configurations: j = 0, 45, and 90. With a tip scanning frequency of 1.15 Hz, 256-line images provide the evolution of the photodeformation over the first 200 s of exposure. The optical images shown in Figure 2a-c exhibit the same characteristic features as discussed above for the three j values. From the topography images, Figure 2e-g for Si-DR1 and Figure 2i-k for PMMA-DR1, we have extracted the kinetics of the SRG formation, i.e., the variation of the SRG amplitude A as a function of time (Figures 2h and 2l). We define A as the difference between the surface profile height Z at positions x = 0 and x = Λ/2: A = Z(0) - Z(Λ/2). With this definition the sign of A gives the direction of the mass transport: from x = 0 towards x = Λ/2 when A < 0 and from x = Λ/2 towards x = 0 when A > 0. These results raise two main comments. The first one concerns the efficiency of the photoinduced mass transport. For a polarization interference pattern (j = 45), matter migration efficiency is similar in both materials. In contrast, when an intensity interference pattern is present (j = 0, 90), the deformation efficiency strongly depends on the nature of the material and it is smaller in Si-DR1 than in PMMA-DR1. It is striking for j = 0. Indeed, beyond the matrix photoexpansion which is completed within the first few seconds of exposure,19 no deformation occurs in Si-DR1, while an SRG of negative amplitude forms in PMMADR1.22 We here unambiguously show that significant matter migration may occur with s-polarized interference. This suggests the existence of two distinct matter migration mechanisms: one governed by the intensity pattern and the other by the polarization pattern. The second comment concerns the directionality of the photoinduced matter migration. With a polarization interference pattern, matter moves from Ix-illuminated areas toward Iy- and Iz-illuminated areas, yielding an SRG with positive amplitude. With an intensity interference pattern, mass transport takes place from bright toward dark fringes, giving SRGs of opposite amplitudes for s and p polarizations, due to the intensity pattern shift. Thus, the efficiency and direction of the mass transport depend on the relative amplitude and position of the polarization and intensity patterns. This dependency is specific in each matrix as is clearly evidenced when performing SRG growth experiments for intermediate j values. In Figure 3 we plot the experimental values of the initial SRG growth rate F as a function of j. We define F as the average slope of A(t) over the first 20 s beyond the photoexpansion. In Si-DR1 (Figure 3a), FSi increases from zero at j = 0, reaches a maximum at j = 45, and decreases for higher j values. This variation is almost symmetric relative to j = 45. It appears that the intensity interference pattern does not provide any contribution to the mass transport and maximum efficiency is 1364

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Figure 2. (a-c) Interference pattern images measured under illumination with polarization angles j = 0, 45, and 90, respectively. (d) Transmitted light intensity It/I0 versus time measured in the brightest fringes. (e-g) Real-time topographical images of the photodeformation in Si-DR1 film for the three polarizations. (h) Time evolution of the SRG amplitude A. The inset shows a zoom over the first 20 s. (i-l) Same as (a-h) for PMMA-DR1.

Ey ¼ - 2 cos j cosðπx=ΛÞE0 Ez ¼ 2 sin θg sin j cosðπx=ΛÞE0

)

)

)

)

where E0 accounts for the time dependence of the field and θg is the propagation angle in the glass substrate. It is convenient to define E = Exx, the field component along the direction of the grating vector uG, and E^ = Eyy þ Ezz, the field component perpendicular to uG. These two components are spatially shifted by Λ/2 and their respective amplitudes are E0 = 2 cos θg sin j E0 and E0^ = 2(cos2 j þ sin2 θg sin2 j)1/2E0. An intensity interference pattern is present when |E0| 6¼ |E0^|. Complementarily, the amplitude of the polarization interference pattern is equal to the smallest of the values of |E0| and |E0^|. The polarization interference pattern is thus characterized by the quantity qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ηpol ¼ minfcos θg sin j, cos2 j þ sin2 j sin2 θg g In Figure 3a, we plot ηpol as a function of j. For 0 e j e 47.5, ηpol = cos θg sin j increases like sin j. For 47.5 e j e 90, ηpol = (cos2 j þ sin2 j sin2 θg)1/2 decreases with cos j but does not fully vanish and takes the value sin θg at j = 90.

)

Ex ¼ 2 cos θg sin j sinðπx=ΛÞE0

It clearly appears that the variation of FSi is very well correlated to the one of ηpol. Thus, in Si-DR1 the mass transport is fully governed by the polarization pattern and is directed from areas excited with E component toward areas excited with E^ component. This is coherent with previous observations of alternating mass motion induced by modulating the light polarization.16 In the case of PMMA-DR1 (Figure 3b) the variation of the SRG growth rate FPMMA versus j no longer follows the one of ηpol. For j = 0, as already mentioned, an SRG of negative amplitude forms, which reveals a mass transport driven by a pure intensity interference pattern directed from bright toward dark fringes. Then, when j increases, the mass transport direction reverses. This is strikingly evidenced by the experiments presented in Figure 4. Initial illumination with j = 0 gives rise to a negative amplitude SRG. Then, when rotating j to 10 (Figure 4a), a very small change in the growth kinetics is observed, while when rotating j to 20 (Figure 4b) the direction of the mass transport reverses and gives rise to an SRG shifted by Λ/2. For low j values the polarization- and intensity-driven mechanisms compete and produce mass transport in opposite directions. Thus, adjusting the value of j allows to select the prevalent mechanism and to choose the mass transport direction. For large j values, the situation is completely different. Bright fringes coincide with E pattern and thus intensity and polarization patterns produce mass transport in the same direction. As a consequence, when 47.5 e j e 90 the SRG growth rate )

obtained for a pure polarization pattern. Let us consider the components of the incoming electric field at the film-substrate interface

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Figure 4. Topography images recorded during SRG formation in PMMADR1. Corresponding kinetics are plotted. At time t = 0, the light polarization is set at j = 0. At t = 130 s, j is rotated to 10 (a) and 20 (b).

)

interference pattern results from the superimposition of two intensity interference subsets, |E |2 = |Ex|2 and |E^|2 = |Ey|2 þ |Ez|2, the resulting displacement rate along uG is expected to be proportional to ηint ¼ cos2 θg sin2 j - ðsin2 θg sin2 j þ cos2 jÞ=2

Figure 3. (a,b) SRG growth rate F (filled squares) as a function of the polarization angle j. The solid gray line represents the calculated variation of η = ηpol þ β ηint for β = 0 (a) and β = 0.75 (b). (c) Variation of Fint = FPMMA - FSi (empty squares) as a function of j. The solid gray line represents the calculated variation of β ηint for β = 0.75.

remains constant (Figure 3b): the decrease in the efficiency of the polarization-driven mechanism is compensated by the increase in the efficiency of the intensity-driven mechanism. If we assume that the polarization-driven mechanism has a similar efficiency in both materials, we can single out the contribution Fint of the intensity-driven process to the SRG formation by subtracting the growth rate measured in Si-DR1 from the one measured in PMMA-DR1: Fint = FPMMA - FSi (Figure 3c). For small j values, Fint is negative. It changes sign for j = 35, then increases and reaches its maximum value for j = 90. Note that the variation of Fint versus j is not symmetrical with respect to j = 45 as would be expected for a mechanism fully governed by the intensity pattern. This indicates that the intensity-driven mechanism is anisotropic. Let us consider that the matter motion is proportional to the azobenzene molecule photoisomerization rate and is directed along the azobenzene molecule dipole. Since the molecule absorption cross section is strongly anisotropic, it is clear that the efficiency of displacement will depend on the orientation of the electric field with respect to uG. As shown in the Supporting Information, averaging over the molecule orientation, the displacement along uG is more efficient by about a factor of 2 when the electric field is parallel to uG than when it is perpendicular to uG. Then, since the intensity

The negative sign between the two terms in the expression of ηint is due to the fact that the two intensity interference subsets produce mass transport in opposite direction since they are spatially shifted by Λ/2. In Figure 3c, we plot the variation of ηint as a function of j. This very simple phenomenological description of the intensitydriven mass transport fits with the experimental variation of Fint. The assumed directionality of the mass motion along the molecule dipole allows to account for the nonsymmetrical variation of Fint with respect to j = 45. When taking into account both the polarization- and intensity-driven mechanisms, the resulting mass transport efficiency can be described by the quantity η = ηpol þ βηint where β is an adjustable parameter characterizing the respective efficiency of the two mechanisms. In Figure 3b, we find a very convincing qualitative agreement between FPMMA and η. In conclusion, we evidence the existence of two distinct mechanisms which produce photoinduced directional mass migration in azo-polymer materials. The first mechanism is governed by the polarization pattern. Several models are proposed to describe polarization-dependent mass transport.7,10,15 One of these models, which attributes the microscopic origin of the matter motion to the electromagnetic force,15 predicts a mass transport direction in agreement with our experimental results. The second mechanism is governed by the intensity pattern. For a given polarization, matter motion takes place from bright toward dark fringes. The photoinduced deformation process is well described by a simple phenomenological model inspired from theoretical approaches which interpret photoinduced matter migration in terms of directional motion of individual entities (molecules, chains, ...).9,23 As shown here, depending on the illumination conditions, the competition or cooperation between the polarization-driven and intensity-driven mechanisms results in a nontrivial photomechanical response. A realistic description 1366

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The Journal of Physical Chemistry B of photoinduced deformation phenomena in any azo-polymer should thus take into account both types of mechanism. Let us moreover emphasize that the balance between these mechanisms depends on the nature of the matrix. Indeed, in sol-gel silica network, the intensity-driven process is completely inhibited while in PMMA-DR1 both mechanisms have similar efficiencies. We show in the Supporting Information that intermediate behaviors can be obtained with other polymer materials.

’ ASSOCIATED CONTENT

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(21) Bertrand, P.; Conin, L.; Hermann, C.; Lampel, G.; Peretti, J.; Safarov, V. I. J. Appl. Phys. 1998, 83, 6834–6836. (22) The dim dark line observed at the top of each ridge in images g and i of Figure 2 is a persistence of the photoexpansion pattern before it is fully erased by the photoinduced matter migration. It is thus only observed in illumination configuations which, on one hand, provide a large light intensity contrast and, on another hand, induce matter transport. (23) Juan, M. L.; Plain, J.; Bachelot, R.; Royer, P.; Gray, S. K.; Wiederrecht, G. P. Appl. Phys. Lett. 2008, 93, 153304.

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Supporting Information. Results obtained on another photoactive material and details on the model used to interpret the data of Figure 3c. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work has been partially funded by the EC (LIMM project IST-2001-35503). ’ REFERENCES (1) Barrett, C. J.; Mamiya, J. I.; Yager, K. G.; Ikeda, T. Soft Matter 2007, 3, 1249–1261. (2) Rochon, P.; Batalla, E.; Natansohn, A. Appl. Phys. Lett. 1995, 66, 136–138. (3) Kim, D. Y.; Tripathy, S. K.; Li, L.; Kumar, J. Appl. Phys. Lett. 1995, 66, 1166–1168. (4) Asatryan, K. E.; Galstian, T.; Vallee, R. Phys. Rev. Lett. 2005, 94, 087401. (5) Kikuchi, A.; Harada, Y.; Yagi, M.; Ubukata, T.; Yokoyama, Y.; Abe, J. Chem. Commun. 2010, 46, 2262–2264. (6) Trunov, M. L.; Lytvyn, P. M.; Dyachyns'ka, O. M. Appl. Phys. Lett. 2010, 97, 031905. (7) Barrett, C. J.; Rochon, P. L.; Natansohn, A. L. J. Chem. Phys. 1998, 109, 1505–1516. (8) Kumar, J.; Li, L.; Jiang, X. L.; Kim, D. Y.; Lee, T. S.; Tripathy, S. Appl. Phys. Lett. 1998, 72, 2096–2098. (9) Lefin, P.; Fiorini, C.; Nunzi, J. M. Pure And Appl. Opt. 1998, 7, 71–82. (10) Pedersen, T. G.; Johansen, P. M.; Holme, N. C. R.; Ramanujam, P. S.; Hvilsted, S. Phys. Rev. Lett. 1998, 80, 89–92. (11) Bian, S. P.; Williams, J. M.; Kim, D. Y.; Li, L. A.; Balasubramanian, S.; Kumar, J.; Tripathy, S. J. Appl. Phys. 1999, 86, 4498–4508. (12) Bublitz, D.; Fleck, B.; Wenke, L. Appl. Phys. B: Lasers Opt. 2001, 72, 931–936. (13) Geue, T. M.; Saphiannikova, M. G.; Henneberg, O.; Pietsch, U.; Rochon, P. L.; Natansohn, A. L. Phys. Rev. E 2002, 65, 052801. (14) Bellini, B.; Ackermann, J.; Klein, H.; Grave, C.; Dumas, P.; Safarov, V. J. Phys. Condens. Matter 2006, 18, S1817–S1835. (15) Yang, K.; Yang, S. Z.; Kumar, J. Phys. Rev. B 2006, 73, 165204. (16) Fabbri, F.; Lassailly, Y.; Lahlil, K.; Boilot, J. P.; Peretti, J. Appl. Phys. Lett. 2010, 96, 081908. (17) Lagugne Labarthet, T.; Buffeteau, F.; C., S. J. Phys. Chem. B 1999, 103, 6690–6699. (18) Sobolewska, A.; Bartkiewicz, S.; Miniewicz, A.; Schab-Balcerzak, E. J. Phys. Chem. B 2010, 114, 9751–9760. (19) Garrot, D.; Lassailly, Y.; Lahlil, K.; Boilot, J. P.; Peretti, J. Appl. Phys. Lett. 2009, 94, 033303. (20) Landraud, N.; Peretti, J.; Chaput, F.; Lampel, G.; Boilot, J. P.; Lahlil, K.; Safarov, V. I. Appl. Phys. Lett. 2001, 79, 4562–4564. 1367

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