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Structure–function relationship in optically and electronically active ISA materials
Synthetic Metals 147 (2004) 63–65
Structure–function relationship in optically and electronically active ISA materials Charl F.J. Faul∗ Max Planck Institute of Colloids and Interfaces, Research Campus Golm, 14424 Potsdam, Germany Received 30 April 2004; received in revised form 7 June 2004; accepted 7 June 2004
Abstract The generation of structures on nanometer dimensions is preferentially performed by the rules of supramolecular chemistry, rather than by a covalent atom-by-atom approach. Ionic self-assembly (ISA) provides a route for the production of highly organised nanostructured assemblies through electrostatic interactions between charged surfactants and oppositely charged oligoelectrolytic species in a cooperative process. By virtue of this facile ‘toolbox’ approach, it is possible to construct supramolecular architectures from a variety of functional tectons. This concept will be demonstrated at the hand of functional ISA systems. © 2004 Elsevier B.V. All rights reserved. Keywords: Self-assembly; Nanostructured materials; Functional materials; ISA; Supramolecular synthesis
1. Introduction The generation of structures on nanometer dimensions is preferentially performed by the rules of supramolecular chemistry, rather than by a covalent atom-by-atom approach [1,2]. Even so, the supramolecular chemistry approach has benefited from the large body of results available from many years of very successful covalent chemistry research investigating the structure–function relationship within such systems (as found, for example, in the field of liquid crystals [3]). Activities in the field of supramolecular chemistry and the production of functional nanoscaled materials [4] have furthermore also benefited greatly from the lessons learnt and examples gathered from biological systems [5]. It is within the general framework presented in Scheme 1, that the ionic self-assembly (ISA) approach will be presented [6]. This supramolecular approach, making use of ionic interactions as primary construction motif to assemble the tectons into nanostructured materials, has been generalized ∗
Tel.: +49 331 567 9545; fax: +49 331 567 9502. E-mail address: [email protected].
0379-6779/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2004.06.041
for oligoelectrolyte-surfactant materials. Concepts presented here aim to give a general overview of the advance made so far in understanding the relationship between structure, properties and function of such self-assembled materials. Attention will be given to the two classes of tectons used in the assembly process, that is, surfactants and the oligoelectrolytic tectons.
2. Results and discussion In our initial studies we established that binding between charged surfactants and oppositely charged azobenzene dye tectons takes place in a cooperative way [7]. Once this was established, we focused on the systematic variation of the two tectons involved in the binding process. 2.1. Tecton 1: surfactant Surfactants and their aggregation behaviour (i.e. formation of mesophases) have already been extensively investigated, and are well known [8]. The importance and influence of
64
C.F.J. Faul / Synthetic Metals 147 (2004) 63–65
Scheme 1. The structure–property-function relationship, and the route towards applications.
parameters such as alkyl tail volume and length, headgroup and counterion size, are known. In the case of ISA materials, the importance of sufficient van der Waals interactions (hydrophobic contrast) to ensure phase separation on a molecular scale was therefore expected. This was found to be an important factor in the formation of well-organised architectures, as well as the properties of the formed materials [9]. Scheme 2 presents a general scheme to show the influence of the structure of the surfactant (i.e. tail length and tail number) on the properties of the ISA materials. Short single-tailed surfactants present a borderline case between a simple counterion-exchange process and the formation of supramolecular materials. It was observed that an increase in tail length led to the formation of crystalline supramolecular materials (as already observed in the case of polyelectrolyte–surfactant complexes [10]). Further increases in tail length and number led to the formation of soft organized materials, that is, liquid-crystalline materials [9]. Initially, these only showed phase changes with temperature, indicating their thermotropic nature [11–13]. We recently [14] found that lyotropic phase properties can also induced by changing the structure of the surfactant tecton. By use of the classical branched surfactant sodium bis(2ethylhexyl) sulfosuccinate (AOT), lyotropic phases of ISA materials were found in dimethyl sulfoxide (DMSO) and 1methyl-2-pyrrolidone (NMP). This of course necessitates the further investigation of other branched surfactant materials,
Fig. 1. Transmission electron microscopy micrograph of the gel formed from the complexation of a charged trisamide and dihexadecyl phosphate surfactant (from N,N-dimethylformamide). Experimental details are given in Ref. [15].
since the use of lyotropic phases might provide a more facile route for the processing of functional materials. 2.2. Tecton 2: oligoelectrolyte The initial aim was to make use of shape-defined and rigid oligoelectrolytes for the formation of nanostructured ISA materials. Investigations were performed with charged azobenzene dyes [9], which not only possess nonsymmetric molecular architecture, but the possibility for intramolecular interactions (azo ⇔ hydrazone tautomerism) as well. Further investigations were then focused on the use of more symmetric and rigid oligoelectrolytic tectons. Two triangular motifs, one of an inorganic nature [11] and one of organic nature [15], exhibiting the capability for H-bonding as secondary functionality, were employed. Both these systems displayed thermotropic behaviour after complexation. The secondary intermolecular functionality (in the case of the triamide system) led to the formation of 3D gel networks (see Fig. 1), which adds a whole range of new possibilities for the production of functional materials. Symmetric, double-charged cationic and anionic perylene derivatives were also synthesized and used for the formation of thermotropic liquid-crystalline material [13,14]. These thermally stable materials exhibited reversible phase
Scheme 2. The influence of surfactant structure on the materials properties of the formed ISA materials.
C.F.J. Faul / Synthetic Metals 147 (2004) 63–65
Fig. 2. Schematic representation of the alignment of the material at the LC – isotropic phase transition front (PTF) – with permission from Ref. [14].
behaviour. Shearing the material by hand showed promise, but a dichroic ratio of approximately 6 was not high enough to be used in any potential applications. 2.3. Functional material: perylenediimide derivates as an example As mentioned before, the use of branched surfactants led to the formation of amphotropic materials, that is, exhibiting both thermotropic and lyotropic behaviour. This was found for the combination of AOT and a cationic perylenediimide derivate. Several standard techniques (magnetic fields, electric fields, rubbed surface, etc.) were used to try and align both phases, but the dichroic ratio could not be increased above 6. The material could be aligned by making use of the lyotropic phase’s transition to an isotropic phase (at 28 wt.% solution in DMSO at approximately 100 ◦ C). The material was kept at in the isotropic state, which led to the slow evaporation of the DMSO and a consequent phase transition. Through this process, a phase transition front (PTF) was developed, where highly aligned material was formed. Dichroic ratios of 18 were regularly achieved in this way. Further characterization was performed using null elipsometry, providing an in-detail picture of the orientation of the chromophoric tectons, as shown in Fig. 2.
3. Conclusions This brief overview of the formation of nanostructured materials making use of the ionic self-assembly strategy has highlighted the (structure–property)-function relationship in the development of functional materials. The use of oppositely charged tectons not only provides the possibility for facile synthesis of self-organising materials, but also the chance to use this method as a toolbox for the production of functional materials. As our understanding and experience in
65
the use of ISA broadens, the possibilities to introduce further functionalities are obvious. This has already led to the use and development of a variety of functional surfactant tectons, including unsaturated [16], pyrrole-containing as well as ferrocenyl surfactant systems. Further tectons under investigation include the synthesis and use of oligoaniline species as well as further functional inorganic species. These new materials will hopefully add a variety of new functions, which should expand the applicability of the ISA approach even more. The use of biological tectons [17] is a further theme under development. Here the materials do not possess shaperigidity, which adds a further challenge to find tectons with suitable functionalities to ensure the formation of nanostructured materials. However, the range of functions accessible through such biological materials makes this a very attractive field for further research. Acknowledgements The following people are gratefully acknowledged for their participation and contribution to the ISA research field: Dr. Ying Guan, Dr. Franck Camerel, Danielle Franke, Byram H. Ozer, Michael Bojdys, Dr. Zhixiang Wei, Dr. Tierui Zhang, Dr. habil. Joachim Stumpe, Yuriy Zakrevskyy, Carmen Remde, Irina Shekova and Ingrid Zenke. Prof. Markus Antonietti is thanked for continuing support. The MPG and the ESF (SONS-SISAM project) is acknowledged for financial support.
References [1] J.M. Lehn, Supramolecular Chemistry: Concepts and Perspectives, VCH, Weinheim, 1995. [2] D.N. Reinhoudt, M. Crego-Calama, Science 295 (2002) 2403. [3] C. Tschierske, J. Mater. Chem. 11 (2001) 2647. [4] O. Ikkala, G. ten Brinke, Science 295 (2002) 2407. [5] J.T. Davis, Angew. Chem. -Int. Ed. 43 (2004) 668. [6] C.F.J. Faul, M. Antonietti, Adv. Mater. 15 (2003) 673. [7] C.F.J. Faul, M. Antonietti, Chem. Eur. J. 8 (2002) 2764. [8] G.J.T. Tiddy, Phys. Rep. -Rev. Sect. Phys. Lett. 57 (1980) 2. [9] Y. Guan, M. Antonietti, C.F.J. Faul, Langmuir 18 (2002) 5939. [10] A.F. Thunemann, S. General, Langmuir 16 (2000) 9634. [11] F. Camerel, M. Antonietti, C.F.J. Faul, Chem. Eur. J. 9 (2003) 2160. [12] F. Camerel, P. Strauch, M. Antonietti, C.F.J. Faul, Chem. Eur. J. 9 (2003) 3764. [13] Y. Guan, Y. Zakrevskyy, J. Stumpe, M. Antonietti, C.F.J. Faul, Chem. Commun. (2003) 894. [14] Y. Zakrevskyy, C.F.J. Faul, Y. Guan, J. Stumpe, Adv. Funct. Mater. 14 (2004) 835. [15] F. Camerel, C.F.J. Faul, Chem. Commun. (2003) 1958. [16] D. Ganeva, C.F.J. Faul, C. G¨otz, R.D. Sanderson, Macromolecules 36 (2003) 2862. [17] S. General, M. Antonietti, Angew. Chem. -Int. Ed. 41 (2002) 2957.
Structure–function relationship in optically and electronically active ISA materials Charl F.J. Faul∗ Max Planck Institute of Colloids and Interfaces, Research Campus Golm, 14424 Potsdam, Germany Received 30 April 2004; received in revised form 7 June 2004; accepted 7 June 2004
Abstract The generation of structures on nanometer dimensions is preferentially performed by the rules of supramolecular chemistry, rather than by a covalent atom-by-atom approach. Ionic self-assembly (ISA) provides a route for the production of highly organised nanostructured assemblies through electrostatic interactions between charged surfactants and oppositely charged oligoelectrolytic species in a cooperative process. By virtue of this facile ‘toolbox’ approach, it is possible to construct supramolecular architectures from a variety of functional tectons. This concept will be demonstrated at the hand of functional ISA systems. © 2004 Elsevier B.V. All rights reserved. Keywords: Self-assembly; Nanostructured materials; Functional materials; ISA; Supramolecular synthesis
1. Introduction The generation of structures on nanometer dimensions is preferentially performed by the rules of supramolecular chemistry, rather than by a covalent atom-by-atom approach [1,2]. Even so, the supramolecular chemistry approach has benefited from the large body of results available from many years of very successful covalent chemistry research investigating the structure–function relationship within such systems (as found, for example, in the field of liquid crystals [3]). Activities in the field of supramolecular chemistry and the production of functional nanoscaled materials [4] have furthermore also benefited greatly from the lessons learnt and examples gathered from biological systems [5]. It is within the general framework presented in Scheme 1, that the ionic self-assembly (ISA) approach will be presented [6]. This supramolecular approach, making use of ionic interactions as primary construction motif to assemble the tectons into nanostructured materials, has been generalized ∗
Tel.: +49 331 567 9545; fax: +49 331 567 9502. E-mail address: [email protected].
0379-6779/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2004.06.041
for oligoelectrolyte-surfactant materials. Concepts presented here aim to give a general overview of the advance made so far in understanding the relationship between structure, properties and function of such self-assembled materials. Attention will be given to the two classes of tectons used in the assembly process, that is, surfactants and the oligoelectrolytic tectons.
2. Results and discussion In our initial studies we established that binding between charged surfactants and oppositely charged azobenzene dye tectons takes place in a cooperative way [7]. Once this was established, we focused on the systematic variation of the two tectons involved in the binding process. 2.1. Tecton 1: surfactant Surfactants and their aggregation behaviour (i.e. formation of mesophases) have already been extensively investigated, and are well known [8]. The importance and influence of
64
C.F.J. Faul / Synthetic Metals 147 (2004) 63–65
Scheme 1. The structure–property-function relationship, and the route towards applications.
parameters such as alkyl tail volume and length, headgroup and counterion size, are known. In the case of ISA materials, the importance of sufficient van der Waals interactions (hydrophobic contrast) to ensure phase separation on a molecular scale was therefore expected. This was found to be an important factor in the formation of well-organised architectures, as well as the properties of the formed materials [9]. Scheme 2 presents a general scheme to show the influence of the structure of the surfactant (i.e. tail length and tail number) on the properties of the ISA materials. Short single-tailed surfactants present a borderline case between a simple counterion-exchange process and the formation of supramolecular materials. It was observed that an increase in tail length led to the formation of crystalline supramolecular materials (as already observed in the case of polyelectrolyte–surfactant complexes [10]). Further increases in tail length and number led to the formation of soft organized materials, that is, liquid-crystalline materials [9]. Initially, these only showed phase changes with temperature, indicating their thermotropic nature [11–13]. We recently [14] found that lyotropic phase properties can also induced by changing the structure of the surfactant tecton. By use of the classical branched surfactant sodium bis(2ethylhexyl) sulfosuccinate (AOT), lyotropic phases of ISA materials were found in dimethyl sulfoxide (DMSO) and 1methyl-2-pyrrolidone (NMP). This of course necessitates the further investigation of other branched surfactant materials,
Fig. 1. Transmission electron microscopy micrograph of the gel formed from the complexation of a charged trisamide and dihexadecyl phosphate surfactant (from N,N-dimethylformamide). Experimental details are given in Ref. [15].
since the use of lyotropic phases might provide a more facile route for the processing of functional materials. 2.2. Tecton 2: oligoelectrolyte The initial aim was to make use of shape-defined and rigid oligoelectrolytes for the formation of nanostructured ISA materials. Investigations were performed with charged azobenzene dyes [9], which not only possess nonsymmetric molecular architecture, but the possibility for intramolecular interactions (azo ⇔ hydrazone tautomerism) as well. Further investigations were then focused on the use of more symmetric and rigid oligoelectrolytic tectons. Two triangular motifs, one of an inorganic nature [11] and one of organic nature [15], exhibiting the capability for H-bonding as secondary functionality, were employed. Both these systems displayed thermotropic behaviour after complexation. The secondary intermolecular functionality (in the case of the triamide system) led to the formation of 3D gel networks (see Fig. 1), which adds a whole range of new possibilities for the production of functional materials. Symmetric, double-charged cationic and anionic perylene derivatives were also synthesized and used for the formation of thermotropic liquid-crystalline material [13,14]. These thermally stable materials exhibited reversible phase
Scheme 2. The influence of surfactant structure on the materials properties of the formed ISA materials.
C.F.J. Faul / Synthetic Metals 147 (2004) 63–65
Fig. 2. Schematic representation of the alignment of the material at the LC – isotropic phase transition front (PTF) – with permission from Ref. [14].
behaviour. Shearing the material by hand showed promise, but a dichroic ratio of approximately 6 was not high enough to be used in any potential applications. 2.3. Functional material: perylenediimide derivates as an example As mentioned before, the use of branched surfactants led to the formation of amphotropic materials, that is, exhibiting both thermotropic and lyotropic behaviour. This was found for the combination of AOT and a cationic perylenediimide derivate. Several standard techniques (magnetic fields, electric fields, rubbed surface, etc.) were used to try and align both phases, but the dichroic ratio could not be increased above 6. The material could be aligned by making use of the lyotropic phase’s transition to an isotropic phase (at 28 wt.% solution in DMSO at approximately 100 ◦ C). The material was kept at in the isotropic state, which led to the slow evaporation of the DMSO and a consequent phase transition. Through this process, a phase transition front (PTF) was developed, where highly aligned material was formed. Dichroic ratios of 18 were regularly achieved in this way. Further characterization was performed using null elipsometry, providing an in-detail picture of the orientation of the chromophoric tectons, as shown in Fig. 2.
3. Conclusions This brief overview of the formation of nanostructured materials making use of the ionic self-assembly strategy has highlighted the (structure–property)-function relationship in the development of functional materials. The use of oppositely charged tectons not only provides the possibility for facile synthesis of self-organising materials, but also the chance to use this method as a toolbox for the production of functional materials. As our understanding and experience in
65
the use of ISA broadens, the possibilities to introduce further functionalities are obvious. This has already led to the use and development of a variety of functional surfactant tectons, including unsaturated [16], pyrrole-containing as well as ferrocenyl surfactant systems. Further tectons under investigation include the synthesis and use of oligoaniline species as well as further functional inorganic species. These new materials will hopefully add a variety of new functions, which should expand the applicability of the ISA approach even more. The use of biological tectons [17] is a further theme under development. Here the materials do not possess shaperigidity, which adds a further challenge to find tectons with suitable functionalities to ensure the formation of nanostructured materials. However, the range of functions accessible through such biological materials makes this a very attractive field for further research. Acknowledgements The following people are gratefully acknowledged for their participation and contribution to the ISA research field: Dr. Ying Guan, Dr. Franck Camerel, Danielle Franke, Byram H. Ozer, Michael Bojdys, Dr. Zhixiang Wei, Dr. Tierui Zhang, Dr. habil. Joachim Stumpe, Yuriy Zakrevskyy, Carmen Remde, Irina Shekova and Ingrid Zenke. Prof. Markus Antonietti is thanked for continuing support. The MPG and the ESF (SONS-SISAM project) is acknowledged for financial support.
References [1] J.M. Lehn, Supramolecular Chemistry: Concepts and Perspectives, VCH, Weinheim, 1995. [2] D.N. Reinhoudt, M. Crego-Calama, Science 295 (2002) 2403. [3] C. Tschierske, J. Mater. Chem. 11 (2001) 2647. [4] O. Ikkala, G. ten Brinke, Science 295 (2002) 2407. [5] J.T. Davis, Angew. Chem. -Int. Ed. 43 (2004) 668. [6] C.F.J. Faul, M. Antonietti, Adv. Mater. 15 (2003) 673. [7] C.F.J. Faul, M. Antonietti, Chem. Eur. J. 8 (2002) 2764. [8] G.J.T. Tiddy, Phys. Rep. -Rev. Sect. Phys. Lett. 57 (1980) 2. [9] Y. Guan, M. Antonietti, C.F.J. Faul, Langmuir 18 (2002) 5939. [10] A.F. Thunemann, S. General, Langmuir 16 (2000) 9634. [11] F. Camerel, M. Antonietti, C.F.J. Faul, Chem. Eur. J. 9 (2003) 2160. [12] F. Camerel, P. Strauch, M. Antonietti, C.F.J. Faul, Chem. Eur. J. 9 (2003) 3764. [13] Y. Guan, Y. Zakrevskyy, J. Stumpe, M. Antonietti, C.F.J. Faul, Chem. Commun. (2003) 894. [14] Y. Zakrevskyy, C.F.J. Faul, Y. Guan, J. Stumpe, Adv. Funct. Mater. 14 (2004) 835. [15] F. Camerel, C.F.J. Faul, Chem. Commun. (2003) 1958. [16] D. Ganeva, C.F.J. Faul, C. G¨otz, R.D. Sanderson, Macromolecules 36 (2003) 2862. [17] S. General, M. Antonietti, Angew. Chem. -Int. Ed. 41 (2002) 2957.