Ultrathin films of poly(2,5-dicyano-p-phenylene-vinylene)-co-(p-phenylene-vinylene) DCN-PPV/PPV: A Langmuir and Langmuir-Blodgett films study

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Highlights

  • DCN and PPV formed a new monolayer at the air–water interface.

  • New devices could be obtained by transferring the films to solid supports.

  • Organization of polymers influenced luminescence properties.

  • Materials have potential for application in optoelectronic devices.

Abstract

Manipulation of polymeric films at the molecular level can be appropriate to improve the properties of optoelectronic devices. In this paper, the copolymer poly(2,5-dicyano-p-phenylene-vinylene)-co-(p-phenylene-vinylene) (DCN-PPV/PPV) was spread on the air-water interface forming Langmuir films. The monolayers were investigated with surface pressure-area isotherms, and polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS). The monomolecular films were then transferred to solid supports through the Langmuir-Blodgett (LB) technique, and characterized by PM-IRRAS and atomic force microscopy, with the viability of the film as an optical device investigated with fluorescence spectroscopy. The immobilization of this copolymer on solid supports as LB films provided a molecular architecture with control of their molecular properties, which may have applications for the production of optoelectronic devices.

Introduction

The control of molecular architectures has been proven as a key feature for the development of organic devices with efficiency, as it is the case for polymer light-emitting diodes [1], [2], [3] for which many types of polymers are now employed. One of the first polymers to be investigated for this purpose was poly(p-phenylene vinylene) (PPV) and its derivatives [4], [5], because of their electrical, mechanical, and optical properties. In this sense, many PPVs have been still used as active layers in light emitting diodes and solar cells with good response by using proper substituents [6], [7].

In this sense, many strategies have been developed to produce based-polymer devices in order to enhance optical and electronic properties. One of these strategies has been the immobilization of these polymers in solid matrices as ultrathin films, as in layer-by-layer films [8], [9], and as Langmuir-Blodgett (LB) films [10], [11], [12], [13], [14]. They are ways to control the structuring of materials in solid supports with deposition of layers structured at the molecular level, which may be used to control surface roughness, thickness, and to provide a layer compatible with a given type of molecule. Also, the thermal stability of the luminescent properties can be improved by manipulating the molecular architecture.

Particularly, films produced with the LB technique are based on the transfer of monolayers formed at the air-water interface to solid supports that intercept vertically the surface of the water. With high control of the surface pressure and of the dipping rate, it is possible to produce films with control of their thickness. LB films of synthetic polymers have been produced either for pure polymers at the air-water interface [15], or for mixtures of polymers and lipids [12]. These strategies have been used to improve the spreading coefficient of such polymers, usually with low capability to form true Langmuir monolayers. In order to circumvent this problem, some strategies have been employed, such as alkylation of polymers [16] or mixture with lipids [12] and other polymers [17].

Another way to produce Langmuir films avoiding alkylation or mixtures is by changing the chemical structure of the backbone. Particularly, the cyano group is a candidate to integrate the backbone of conjugated polymers, in special, for PPVs. It has been shown that the solid state photoluminescence efficiency is considerably higher in comparison with PPV prepared with the sulfonium ion as precursor [18]. Because of the high electron affinity that allows the use of more stable metal electrodes for electron injection, the application in organic light electrodevices is desired [19]. The cyano group can occupy different relative positions in the PPV chain, being important the fact that the introduction of this group in the vinylene unit reduces the LUMO energy without significantly change of the absorption [20]. However, the influence in the backbone promotes a weak steric interaction between adjacent units, which in some cases may cause a blue-shift effect, maintaining their electron transport properties and keeping emissions efficient at longer wavelengths in the visible spectra [21], [22]. In this sense, they have been synthetized in alternative ways to reach better operation lifetimes in order to better manipulate their photoluminescent (PL) and electroluminescent (EL) properties [22], [23]. One specific application reported for DCN-PPV is the fact that perchlorate reduces significantly possible quenching effects by allowing ion exchange of formed bromide with the nonquenching perchloride anion [22], which can be have implications on the properties of this polymer when immobilized on solid supports.

With these ideas in mind, this work aims to investigate the copolymer poly(2,5-dicyano-p-phenylene-vinylene)-co-(p-phenylene-vinylene) (DCN-PPV/PPV) as Langmuir and Langmuir-Blodgett films. Although many manuscripts have been reported on PPV LB films in alternative ways [12], [15], [24], no report have been found in the literature of DCN-PPV/PPV as Langmuir and LB films, which makes this strategy a novelty and a way to reach devices with better performances. Despite of the fact that the LB technique does not allow batch-production of devices so far, its use should be justified as an important tool for molecular control and investigation of the film properties. Consequently we can take advantage of the use of the LB technique if we envisage that the properties obtained with such studies be better exploited for the fabrication of optical devices such as active layers of efficient light-emitting diodes, whose features must be understand in detail.

Section snippets

Synthesis and characterization

The copolymer DCN-PPV/PPV (4) was synthesized via the well-known Wessling route [25], using 2,5-dicyano-4-bis(tetrahydrothiopheniomethyl)xylene dichloride (1) and 4-bis(tetrahydrothiopheniomethyl)xylene dichloride (2) monomers prepared using p-xylene (Aldrich, 99%; CAS number 106-42-3; linear formula C6H4(CH3)2; molecular weight 106.17), α,α′-Dichloro-p-xylene (Aldrich, 98%; CAS number 623-25-6; linear Formula C6H4(CH2Cl)2; molecular Weight 175.06), thiophene (Aldrich, 99%, CAS Number 110-02-1;

Langmuir films

Fig. 2 shows the surface pressure-area isotherms for DCN-PPV/PPV. The onset area is about 34 Å2, where the liquid-expanded state is attained. Sequential compression leads to other physical states featuring two regions of low elasticity, in which the surface pressure values increases continuously, but with a lower rate in relation to the rest of the isotherm, until the collapse of the monolayer is attained. The first region occurs at the range area of 22–15 Å2 (surface pressure around 18 mN m−1) and

Conclusions

In this present work, we showed that DCN-PPV/PPV polymers can form stable Langmuir films at the air-water interface, which can be successfully transferred to solid supports as LB films. Molecular-level interactions at the surface determine the properties of the films, as investigated with PM-IRRAS spectroscopy. Also, the LB films exhibited optical (photoluminescence) properties that could be associated to the homogenous morphology as observed with AFM. In terms of technological applications,

Acknowledgments

The authors gratefully acknowledge CAPES–Rede NanoBioMed, CAPES-INCT (Eletrônica Orgânica), FAPESP (07/50742-2 and 13/10213-1); and CNPq.

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