Among all the applications of conjugated ions, layer-by-layer self-assembly is most attractive recently. Through the attractive electrostatic interaction between the chains, pairs of oppositely charged polyelectrolytes in aqueous solution are well-known to form complexes. The process, which is extremely simple, is depicted in Figure 1.2.1
for the case of polyanionpolycation deposition on a positively charged surface.
Figure 1.2.1 The process of layer-by-layer adsorption
Strong electrostatic attraction occurs between a charged surface and an oppositely charged molecule in solution. In principle, the absorption of molecules carrying more than one equal charge allows for charge reversal on the surface, which has two important consequences: (i) repulsion of equally charged molecules and thus self-regulation of the adsorption and restriction to a single layer, and (ii) the ability of an oppositely charged molecule to be adsorbed in a second step on top of the first one.
Cyclic repetition of both adsorption steps leads to the formation of multilayer structures.
The incorporation of proteins in multilayers films may lead to the application of polyelectrolyte multilayers as biosensors25 or in biotechnology26,27, the latter may even provide the base for new developments in multistep chemical catalysis. A crucial point in this type of application will be the control of transport in multilayer films.28,29 Multilayer microcapsules may have biomedical applications as well. Multilayer films can also be fabricated on colloids, which may have implications for photovoltaics.30
Other possible applications include the incorporation of dye molecules to tailor the optical properties of polyion films.31-37 In the case of rodlike amphiphiles carrying hydrophilic head groups at both ends and a central diacetylene group, multilayer systems may also have high inplane order, because the topochemical polymerization of the diacetylene group only works in a single crystal.38
The use of multilayers as gas separation membranes is a currently developing technology.39 As of today, the most advanced development of polyion-based films is probably their potential for the fabrication of light-emitting diodes.40-49 This interest was kindled by the demonstration that a water-soluble polyelectrolyte precursor of the intractable electroluminescent polymer PPV can be incorporated into polyelectrolyte films and subsequently thermally converted to PPV.40 The nuetron reflectivity scan shows that the multilayer structure of such films, which contain hydrophobic PPV layers after elimination, remains intact.
Conjugated polymers have been incorporated as active materials into several kinds of electronic, optical, magnetic and miscellaneous devices. The development and utilization of conjugated polymers as the active elements of thin-film electronic and optical devices continue to be a highly pursued area of research. These materials are generally manipulated into thin films via simple spin-casting techniques. However, it is becoming increasingly more apparent that more control over the molecular and supramolecular organizations of these materials is needed to fully exploit their novel optical and electrical properties. For example, multilayer thin films comprised of separate hole and electron transport layers are currently being considered for use in
light-emitting diodes based on conjugated polymers.50,51
Figure 1.2.2 Schematic representation of the structures of the polymer interlayers
The ultimate realization of this particular approach would be the ability to manipulate conjugated polymers into layers with molecular dimensions in a highly controlled manner. The layer-by-layer processing technique, which is a viable means to manipulate polymers into multilayer thin films, provides molecular-level control over the thickness and architecture of multilayer thin films and is readily extended to a wide variety of polymers including many different electroactive polymers (Figure 1.2.2). In 1994, M. F. Rubner et al. first reported the manipulation of conjugated polymers via this technique.52,53 In his group work, conducting polymers, poly(thiophene-3-acetate)
Insulating Cation:
NH CH C O
H3N
CH2 CH2 NH2
PEI
PVP CH2 CH
NH CH2 CH
N N CH3 PVI
CH2 CH2 NH3 CH2
PAH
CH2 CH NH3 PEI
PAMA CH2 CH
CH2 NC2H5 H5C2
CH2
CH2 C CH3
C O O CH2 CH2 NCH3 H3C
CH3
Semionducting Cation:
S S
Pre-(1,4-PV 1,4-NV) CH CH2 S
PPV precursor O
O N
N
(+) PPP
Dye Cation:
Ru
N N
OCO(CH2)10OCO
N N N
N
RuP 2+
CH2 CH OSO3-
PVS Insulating Anion:
PSS CH2
SO3- CH2 CH2 C
CH3
COO- CH2 CH
COO-
PMA PAA
C C
-HOOC COOH-
N O
N O
H H R
Polyimide precursor
Conducting Cation:
N N
H H
PPY PAN N
N N
N
Semiconducting Anion:
-SO3(CH2)3O
O(CH2)3SO3-
(-) PPP
S CH2COO-
PTAA
Conducting Anion:
N N
SO3-
H H
SPAn
(PTAA), sulfonated polyaniline (SPAN) and polypyrrol and light-emitting polymer, PPV precursor were successfully fabricated into multilayer thin films with non-conjugated polyelectrolyte via self-assembly. Then besides Rubner’s group, a number of other groups have utilized this approach to fabricate light-emitting devices from both PPV precursor materials and non-conjugated polyelectrolyte54,46,48 or fully conjugated polyions, such as PPV/SPAN55 and Bu-PHPyV/SPAN.45 In 1998, M.F.
Rubner et al. reported the first sequentially adsorbed multilayer devices in which both the polycation and polyanion layers are based on the same emitting polymer, PPP.6 Recently, M.F. Rubner et al. have fabricated light emitting devices based on sequentially adsorption of an electrochemiluminescent active Ru(bpy)32+ polyester and various polyanion including a small molecule Ru(II) dye, sulfonated poly(p-phenylene) (SPPP), sulfonated polystyrene (PSS), poly(methacrylic acid) (PMA) and poly (acrylic acid) (PAA) and external device efficiencies in the range of 1-3% have been achieved.56,59 Now the self-assembly multilayers fabricated from conjugated polymers have been used to modify the electrode as the hole or electronic injection or transporting layer in the device. P.K.H. Ho et al. interposed a well-defined, continuous and ultrathin polymer layer at the interface between the indium tin oxide electrode layers and the emissive polymer.60 They fabricated separate devices with a conducting polymer interlayer of self-doped PAni, a semiconducting polymer interlayer containing PPV or an insulating polymer interlayer of saturated main chain polymers. The results show that interlayer significantly alters charge injection and enhances the device electroluminescence efficiency. M. F. Rubner has recently studied the Forster energy
transferred between PPV and PPP monolayers using the sequential adsorption process to adjust the layer number between those two layers.61