Generation of Circularly Polarized Light of Highly Oriented Poly( P -Phenylene Vinylene)
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Generation of Circularly Polarized Light of Highly Oriented Poly(P-Phenylene Vinylene) A. Marletta1, D. Gonçalves, O. N. Oliveira Jr., R. M. Faria and F. E. G. Guimarães 1 Instituto de Física de São Carlos, Universidade de São Paulo, C.P. 369, 13560-970, São CarlosSP, Brazil. ABSTRACT Langmuir-Blodgett (LB) films of poly(p-phenylene vinylene) (PPV) were prepared using an amphiphilic precursor, where the counterion chloride was partially replaced by a long chain dodecylbenzenesulfonate ion. These LB-PPV films are highly ordered along the dipping direction as demonstrated by linear dichroism experiments using linearly polarized optical absorption and emission. The intensity ratio between emitted light parallel and perpendicular to the dipping direction was ca. 17, which is much higher than the dichroic ratio of 3.6 found in the polarized absorption experiment. These values indicate an efficient energy and/or charge transfer between low conjugated segments (disordered) and highly conjugated (ordered) ones after excitation. In addition, the LB films displayed large circular polarization with an asymmetry factor ge which varies from 0.6 to –0.2 when the sample temperature is increased from 30 to 300 K.
INTRODUCTION Electroluminescent polymers such as poly(p-phenylene vinylene) (PPV) have been used as an active component of light emitting diodes (LEDs) due to a significant progress in the synthesis and solution processibility of conjugated polymers [1]. The structural disorder, however, remains as one of the most relevant limitations for technological applications of these materials [2]. High quality films are required for most potential applications, such as polarized light emitting diodes (LEDs). Owing to the preferential orientation of the polymer chains, the external quantum efficiency is expected to be higher as compared to isotropic polymer [3-4]. In particular, doped and stretched-oriented PPV films show conductivity values 103 times larger in the direction parallel to the polymer chains than perpendicular to them [5]. This has prompted researchers to employ the Langmuir-Blodgett (LB) technique as a promising tool for the fabrication of ultrathin films with high degree of order, controllable thickness and low number of defects [6]. Highly anisotropic optical properties have been obtained for stretched layers [7], and for LEDs, which showed polarized emission with an intensity ratio of 3.1 between parallel and perpendicular emitted light to the stretching direction [8]. Such films can also be strongly birefringent with different refractive indexes taken at the preferential directions [9]. In the present work, we demonstrate for the first time that LB-PPV films can incorporate the ability to accomplish circular polarized luminescence. This can be seen by a strong anisotropy related to the transition involving the π-π* bands in the PPV films and the presence of circularly polarized emission, which the asymmetry factor between the left and right polarization is dependent on the sample temperature. While the anisotropy
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