AZOBENZENE POLYMERS FOR PHOTONIC APPLICATIONS
2.2. PHOTOORIENTATION
low-molecular-weight azobenzene derivatives) or main-chain and side-chain azobenzene-containing polyelectrolytes. The results on photoinduced orientation in supramolecular complexes based on either H-bonds or ionic interactions will also be discussed. The ionic complexes are formed by either polyelectrolytes or surfactants and azobenzenes (similar to ones used for LbL process). Complexes between azobenzenes and surfactants formed by ionic self-assembly (ISA) will be concentrated on, as they have shown the best results for photoorientation.
Section 2.3 is dedicated to the light-induced formation of relief structures in supramolecular materials. To the best of the authors’ knowledge, not a lot of results have been published up to date. The authors have tried to include all results in their review, starting with LbL films and continuing with ionic and H-bond complexes, both amorphous and liquid crystalline ones. Wherever possible, they have commented on different mechanisms of the relief formation in these materials. In particular, they have emphasized the role of the matrix for the light-driven mass transport. They have also tried to combine these very recent results with those well known from the literature on complex formation, materials properties, and light-driven mass transport, if they found it useful for understanding.
In summary, the authors derive the advantages and drawbacks of the new supramolecular materials for photoinduced orientation and SRG formation.
Also, the questions that were aroused only on the application of the supramole- cular systems for orientation and SRG formation are briefly discussed.
anisotropy and stability through self-assembly, and self-organization processes, in which molecules associate spontaneously into ordered aggregates as a result of noncovalent interactions or entropic factors, is becoming one of the primary frontiers of materials research. To achieve this goal, several strategies such as H-bonding (Macdonald and Whitesides, 1994), metal coordination (Lehn, 1990), charge-assisted H-bonding (Hosseini, 2003), and, more recently, ISA (Faul and Antonetti, 2003) have been investigated. A rather large number of these activities are directed to generate mesophases, where the mesogenic units are formed by intermolecular interactions.
The mutual order is encoded not only in the shape and chemical functionality of the objects involved but also in the strength and directionality of the secondary interactions used. In classical supramolecular chemistry, these interactions are usually hydrogen bridges or coordinative metal binding. However, molecular recognition and amphiphilic association should also be considered. Table 2.1 summarizes the most important interactions playing the main role in organization of organic supramolecular materials. Manipulation of structural and macroscopic order in films and bulk solids, however, remains a major challenge for all of these approaches.
The approach of deriving new photosensitive materials using secondary noncovalent interactions like H-bonding and ionic interactions between different units such as polymers and low-molecular-weight functionalized units has gained a great interest in the past decade. This approach allows the development of a new generation of smart materials and their application in optoelectronics, hologra- phy, data recording, and imaging systems. New materials may advantageously combine the properties of polymers (film forming) and low-molecular-weight components (photochromism, high optical activity, easy processing, etc.).
The simplest way to use secondary interactions in design of photosen- sitive material would be to introduce them between the polymer backbone and functional photosensitive groups. This approach would allow direct com- parison of photoorientation properties with already well-studied materials.
A general application of this idea between different high-molecular-weight and
TA B L E 2 . 1 . Interactions and Some of Their Properties
Interaction Strength (kJ/mol) Range Character
van der Waals B50 Short Nonselective, nondirectional
H bonding 5 65 Short Selective, directional
Coordination binding 50 200 Short directional
Fit interactions 10 100 Short Very selective
Amphiphilic 5 50 Short Nonselective
Ionic 50 250a Long Nonselective
Covalent 350 Short irreversible
aData are for organic media, dependent on solvent and ion solution.
Source: Faul and Antonetti 2003. Reproduced with permission from Wiley VCH.
2.2. PHOTOORIENTATION 55
low-molecular-weight tectonic unit is shown in Fig. 2.3. For the first time, this approach was realized for photosensitive polymer–azobenzene unit complexes using ionic interaction (Fig. 2.2d) (Meyer et al., 1991) and H-bonding (Fig. 2.4a) (Kato et al., 1996), where nonlinear optical properties and influence of UV-vis irradiation on a reversible isothermal nematic–isotropic phase transition have been investigated, respectively.
The development of H-bonded complexes is now considered. An influence of UV irradiation on optical properties of low-molecular-weight azobenzene-con- taining material (Fig. 2.4b) has been investigated (Aoki et al., 2000) on the basis of such interactions. The first observation of photoinduced optical anisotropy in H- bonded complexes of azobenzene dyes and copolymers (Fig. 2.4b) has been recently demonstrated (Medvedev et al., 2005). In this case, the induced aniso- tropy was stable, and the maximum dichroic ratio of 2 has been observed.
A kinetics of the induction of birefringence (maximum value of ca. 0.01) in one of these complexes is shown in Fig. 2.5. An influence of H-bonding on the mesomorphic and photoorientation properties was recently demonstrated (Cui and Zhao, 2004). In this approach, the amorphous azopyridine side-chain polymer was converted into liquid crystalline polymers through self-assembly with a series of commercially available, aliphatic, and aromatic carboxylic acids (Fig. 2.4d).
(a)
(d) (e)
(f) (g)
(b) (c)
Figure 2.3. Principles of molecular construction of LC polymers by self-assembly induced by hydrogen or ionic bonding.
The measurement of photoinduced birefringence revealed very different aniso- tropy. Considering the numerous compounds that could readily be complexed with this type of polymer, including chiral acids, phenols, and metals, the approach of using side-chain azopyridine polymers offers the possibility to produce a large number of new photoactive liquid crystalline materials without exhaustive synthetic efforts and with new properties to exploit. And really the approach has been exploited in recent works. The materials similar to that
Figure 2.4. Chemical structures of complexes with hydrogen bonds.
2.2. PHOTOORIENTATION 57
depicted in Fig. 2.4a have been reported (Gao et al., 2007b). However, the films of this H-bonded azobenzene polymer (Fig. 2.4f) exhibited dichroic ratio of only 1.2.
When in another investigation (Priimagi et al., 2007), commercially available azodye (Disperse Red 1) forming relatively week H-bond with the same poly- vinylpyridine (Fig. 2.4h) has been used, relatively high birefringence of 0.035 has been attained. Even higher value of birefringence (0.04, highest value for H-bonded complexes) has been obtained when the same azobenzene derivative was used in combination with polyvinylphenol using the opportunity of another H-bonding (Fig. 2.4h). All this improvement in photoinduced orientation (guest–
host system of the same dyes and polystyrene (PS) exhibited only birefringence of 0.005) was attributed to the prevention of dye aggregation due to formation of H-bonding [Priimagi et al., 2007). Rather different approach was realized earlier (Gao et al., 2007a), and it is based on intermolecular multiple H-bonding in azobenzene derivative forming an H-bonded main-chain azobenzene polymer (Fig. 2.4e). The latter material showed rather higher dichroic ratio of 5.5. On the contrary, mass transport properties of these last two polymers (discussed later) were vice versa. All these data cannot be analyzed, as different values (birefrin- gence and dichrosim) have been measured; however, it shows that by a variation of H-bond strength, structure of the azobenzene and polymer, significant modification of optical properties is possible.
In contrast to the H-bonded complexes, complexes based on ionic interactions have gained a much greater interest. Such complexes emerged as a multilayer
0 1000
0.0000 0.0004 0.0008 0.0012
3000
2000 4000
Time (s)
∆n
Figure 2.5. A rise of birefringence under laser irradiation in H-bonded azoben- zene polymer (Fig. 2.4: m = 39%, X = C, R = H) homeotropic film at room tem- perature. The dashed lines indicate the moment when the laser irradiation was switched off (P = 0.15 W/cm2). Source: Medvedev et al., 2005. Reprinted with permission from American Chemical Society.
assembly produced by an electrostatic LbL technique. LbL deposition was introduced in the early 1990s (Lvov et al., 1993) and has since gained a great popularity in the fabrication of thin films of different materials from simple ionic species to colloidal particles and biological macromolecules (Hammond, 2000;
Decher et al., 1998). These solid films consist of adjacent layers of oppositely charged polyelectrolytes and different other low molecular materials, nanoparti- cles, colloids, etc. In this case, ionic interaction is not only used to form a molecular complex but also results in layered structure at a molecular level. The films are manufactured by subsequent dipping of a substrate in solutions of these oppositely charged species (Fig. 2.6).
According to the authors, LbL films, which are self-assembled layered structures, could be considered as solid-state supramolecular complexes formed by ionic interactions. Azobenzene-containing materials also found their place with this popular technique and could be divided from chemical point of view to azobenzene-containing polyelectrolytes (Aldea et al., 2007; He et al., 2004; Wang et al., 2004a,b, 1998; Zucolotto et al., 2004b, 2002; Wu et al., 2001; Lee et al., 2000;
Balasubramanian et al., 1998; Lvov et al., 1997), with azobenzene moieties in main- or side chain, and low-molecular-weight azobenzene derivatives (Zucolotto et al., 2003; dos Santos et al., 2002; Shinbo et al., 2002; Ziegler et al., 2002; Dragan et al., 2001; He et al., 2000; Advincula et al., 1999; Saremi and Tieke, 1998); some of them are conventional ionic azodyes.
The requirement and restriction to azodye in this method is a presence of at least two charged groups to be able to bind to oppositely charged sites in the polyelectolyte chains from two different adjacent layers (Fig. 2.6). As second nonactive component, usual polyelectrolytes such as PDADMAC, PAH, PEI,
A B A−B
1 2 3
Figure 2.6. Schematic illustration of the preparation of alternate polycation–
polyanion multilayer assemblies onto a negatively charged substrate.
2.2. PHOTOORIENTATION 59
PVS, or PSS or rather exotic ones like chitosan, carragenan, cyclodextrine, or dendritic polyelectrolytes have been employed. LbL layers with low-molecular- weight azobenzene species could be considered as analogue of typical materials used for photoorientation and photoinduced mass transport, namely, azobenzene side-chain polymers. In this case, covalent bonding of azobenzene moieties and polymer is replaced with ionic interactions (weak or strong depending on the type of polyelectolyte) (compare Fig. 2.2b and d). The above-mentioned restriction to the structure of azodye is also of crucial importance to resulting photochemical properties. Namely this causes a restriction on the freedom of movement of the azobenzene species, which seems to be of particular importance for the photo- induced mass transport. Depending on the number of charged groups and polyelectrolyte type, these restrictions would be more or less pronounced.
Photoisomerization has been investigated for some of these solid-state complexes. As generally LbL procedure is rather tedious and time consuming (even using robotic technique available nowadays), from the practical viewpoint it would be attractive only in case of rather thin films. Figure 2.7 displays low molecular azobenzene derivatives, some of them commercially available, which were used to study photoinduced orientation. Both negatively and positively charged dyes have been employed.
Relatively high dichroic ratio up to 2.3 due to geometry of the molecule (high aspect ratio of Direct Red 80) has been achieved (Advincula et al., 2003). To manufacture these films with PDADMAC of ca. 100-nm thickness, 100 layers were necessary. Less effective photoorientation of spin-coated and LbL films with other polyelectrolyte (PAH, PEI) was attributed to specific formation of J-aggregates in LbL films. In the LbL films produced from chitosan and Sunset Yellow (dos Santos et al., 2002), spontaneous birefringence of 0.04 for the film of 300-nm thickness was observed; however, the value was not affected by the light.
This fact underlines the importance of selection of dye and polyelectrolyte.
Some indications of the influence of ionic azobenzene derivative structure (Fig. 2.7) and polyelectrolyte can be deduced from a number of azobenzene derivative studied (Ziegler et al., 2002). All studied LbL films (24 layers) exhibited only out-of-plane orientation. Only for anionic-derivative Z1 (indicated in Fig. 2.7) and only with PDADMAC, dichroic ratio of 2 was observed. In the layers of Z1, but with PEI as polyelectrolyte, photoorientation and photoisome- rization are totally restricted by strong aggregation. The high value of dichroism of PDADMAC/Z1 layers should be caused by the hexamethylene spacer of Z1, which results in higher mobility necessary for the orientation process. Cationic Z2–Z5 were all manufactured into LbL films using the same polyelectrolyte PSS, and maximum value of 1.4 (dichroic ratio) was found.
Instead of low-molecular-weight azobenzene derivatives, some azobenzene- containing polyelectrolytes were also employed (Aldea et al., 2007; He et al., 2004;
Wang et al., 2004a,b, 1998; Zucolotto et al., 2004b, 2002,; Wu et al., 2001; Lee et al., 2000; ; Balasubramanian et al., 1998; Lvov et al., 1997). By chemical structure, azobenzene-containing polyelectrolytes can be divided into a group with azoben- zene moiety situated in the main chain and a group with side-chain azobenzene
(Fig. 2.8). Side-chain polymers are more typical, and commercially available polymer PAZO (Aldrich) was already used for LbL assembly in 1997 (Lvov et al., 1997).
Both PAZO and PS-119 (Fig. 2.8) could be considered as strong polyelec- trolytes being in salt form. It is important that charged group be connected to azobenzene moiety, and therefore one can expect the ionic interactions responsible for the formation of LbL structure to restrict azobenzene motion. Birefringence value of 0.09 for the 200-nm film (dos Santos et al., 2006) was achieved for structures of PS-119 with cationic dendrimers. Interestingly, for higher dendrimer generation (with higher charge density) stronger absorption of PS-119 has been observed. That fact led to the lower values of birefringence, which is related to restriction of chromophore molecular motion. For PS-119/PAH architecture Figure 2.7. Chemical formula of azodyes used for the preparation of LbL multi- layers and supramolecular complexes.
2.2. PHOTOORIENTATION 61
(Zucolotto et al., 2004b), the structure and charge density of cationic polyelec- trolyte PAH can be governed by pH during LbL deposition, and it changes the LbL films properties. By increasing the pH from acidic to basic, it was possible to increase the bilayer thickness by the factor of 2.4 because of the loose conforma- tion of PAH in the LbL structure. The authors compared the films with similar amount of azobenzene (the same optical absorption) prepared at different pH and came to the conclusion that an increase in the pH led to a slowing of kinetics of the induction of photoorientation by the factor of 4. For another similar polyelec- trolyte PAZO (Fig. 2.8), the data on photoinduced orientation in LbL structures are not available. In contrast to LbL structures, the results on ionic complex of PAZO with PEI (Stumpe et al., 2006a) showed that dichroism was lower in this case compared with the neat PAZO films (Stumpe et al., 2006b; Goldenberg et al., 2005), approximately proportional to a decrease in azobenzene loading. It seems that there is no restriction of azobenzene motion by ionic interactions with oppositely charged polyelectrolyte in this complex. This demonstrates that the layered structure of the LbL films should be responsible for this restriction.
Two other azobenzene-containing polyelectrolytes (Fig. 2.8) are copolymers.
The important difference is that ionizable group (COOH) is attached not to the azobenzene but to another unit on the polymer chain. Therefore, one could expect Figure 2.8. Chemical formula of azobenzene-containing polyelectrolytes used for the preparation of LbL multilayers and supramolecular complexes.
that ionic interactions, necessary for the LbL organization, will have lower impact on the properties of azobenzene moiety. However, some of these copolymers (PBANT-AC, PBACT-AC, PAA-AZ) exhibited only low dichotic ratio (up to 1.3) (Wang et al., 2004a,b). Other LbL structures with PAA-AN, MA-co-DR13 showed birefringence value up to 0.068 (Zucolotto et al., 2002; Lee et al., 2000).
It should be noted that spin-coated films (Lee et al., 2000) showed the same results, and it means that there is no advantage in tedious LbL assembly in this case. Also, the authors (Zucolotto et al., 2002) mentioned much longer time necessary to induce maximum value of orientation in LbL systems, and this was attributed to the influence of ionic and H-bonding interactions. In addition, the drier was the LbL film, the more sluggish was the kinetics of induction of photoorientation. In addition to all the above-mentioned side-chain polyelectrolytes, some azobenzene- containing polyelectrolytes with chromophore in main chain have been investi- gated (Fig. 2.8) (Jung et al., 2003; Hong et al., 1999). Unfortunately, only small value of dichroic ratio of about 1.4 was achieved in this case.
In contrast to LbL structures, ionic supramolecular complexes (schematically depicted in Fig. 2.2d) can be first formed in solution and then build into films. The complex formation can be proved by FT-IR and NMR spectra (Zhang et al., 2008; Kulikovska et al., 2007; Xiao et al., 2007; Lin et al., 2003, 2002). In this case, the azobenzene units, although bound with ionic interactions to polymer chains, are devoid of confinement of layered structure. The first observation of photo- orientation was recently demonstrated in such polyelectrolyte–azobenzene com- plex of azodye and PEI (Kulikovska et al., 2007; Stumpe et al., 2006a). In the investigated materials, only temporally stable photoinduced anisotropy was found contrary to the glassy polymers. Figure 2.9 displays the induction and relaxation of optical anisotropy in a film of the material on the basis of PEI and the dye Alizarin Yellow GG (see Fig. 2.7). Irradiation of the film with 488-nm light gives rise to the orthogonal polarization component of the transmitted probe beam (633 nm) absent in the incident beam, indicating the induction of optical anisotropy. After the switch-off of the actinic light, the induced anisotropy relaxes, whereas the relaxation is well described by a two-exponential decay function. Alternation of the polarization of the actinic light allows switching off of the induced molecular orientation, as demonstrated in the Fig. 2.9b. The observed instability of the light-induced orientation of chromophores is caused by the ionic nature of the azobenzene–polyelectrolyte materials. Unlike as in the functiona- lized polymers (Fukuda et al., 2006; Kulikovska et al., 2002), in this case polymer chains provide for a spatial distribution of charges that counteracts the orientation effect of polarized light, returning the ionic chromophores to their initial positions.
Stable light-induced anisotropy in ionic complexes of poly(vinyl-N-alkylpyr- idinium) with Methyl Orange has been recently obtained (Zhang et al., 2008; Xiao et al., 2007). Both complexes have chemical structure (see Fig. 2.10a) similar to the complexes reported earlier (Lin et al., 2003, 2002). However, supposedly due to different synthetic procedure, they form smectic A (SmA) liquid crystalline phases.
Induction of birefringence in one of these complexes is shown in Fig. 2.11.
2.2. PHOTOORIENTATION 63
Orthogonal polarization component
On Off
(a)
(b)
0 10 20 30 40 50
Time (min)
IrradiationIrradiation
1.8 1.6
0.2
Intensities of probe beam (V)Intensities of probe beam (V)
0.0
1.4 1.3 1.2 1.1 0.2 0.1 0.0
140 145 150
Time (min)
State 1 State 2 State 1 State 2 State 1 State 2 State 1 Intensity of the transmitted probe beam(a.u.)
Fit to the orthogonal polarization component
Figure 2.9. Optical anisotropy induced by the light in a film of the compound Alizarin Yellow GG-PEI: kinetics of the parallel (dashed black line) and orthogo- nal (solid gray line) polarization components of the transmitted probe beam.
(a) Induction and relaxation, solid black line presents two-exponential decay fit to the data, and (b) switching by alternation of polarization of the irradiating beam.Source: Kulikovska et al., 2007. Reproduced with permission from Amer- ican Chemical Society.