Ionic Self Assembly and Low Conversion Temperature ( P -Phenylene Vinylene)
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Ionic Self Assembly and Low Conversion Temperature (P-Phenylene Vinylene) A. Marletta1, F. A. Castro, C. A. M. Borges., 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 We developed an alternative approach to produce self-assembled (SA) thin films of poly(p-phenylene vinylene) (PPV) by controlled substitution of the chloride counterion of the precursor poly(xylylidene tetrahydrothiophenium chloride) (PTHT) by a long chain dodecylbenzenesulfonate (DBS) anion. The main advantage of this novel procedure is that thermal conversion through the elimination of DBS may be performed at considerably lower temperatures (80-100 oC) in few minutes. In addition it provides PPV films with better optical properties and low incorporation of structural defects, like carbonyl groups. An important feature of these SA-PPV films is the well resolved vibronic structures in the photoluminescence and absorption spectra. We observe a dramatic improvement in quantum efficiency of PPV when the conversion temperature is decreased from 230 to 80 oC. This effect may be explained by the decrease of extrinsic defect incorporations (carbonyl groups) detected by infrared (IR) measurements. This improvement in the optical properties at low temperatures may help us to understand basic phenomena, such as the nature of excitons in PPVs.
INTRODUCTION Self-assembly (SA) films of poly(p-phenylene vinylene) (PPV) were prepared by dipping quartz substrates alternately in solution of PPV polymer precursor poly(xylylidene tetrahydrothiophenium chloride) (PTHT) and in solution of sodium salt of dodecylbenzenesulfonic acid (DBS). The advantage of introducing DBS lies in the possibility of converting films with a high conjugation length at 115 oC within only 3 min [1]. The exchange of the Cl- by DBS- counterions in solution is governed by their different basicity. In general, sulfonium groups, such as tetrahydrothiophenium, are stable only when they are counter-balanced by counter-ions which are conjugated bases of strong acids [2]. Incorporation of DBS was originally conceived to improve processibility of a PPV-precursor to fabricate multilayer Langmuir-Blodgett (LB) films [4-5]. These PPV-LB films feature additional characteristics such as strongly ordered PPV chains, as shown by the polarization-dependent optical measurements and the high definition of luminescence peaks [4]. In the present work, we used the DBS in SA multilayers to produce PPV films with less defects compared to films converted at high temperatures. This is due to the absence of carbonyl-related defects, which leads to higher quantum yields in the luminescence processes [3]. Such flexibility in film fabrication and efficient low temperature conversion to PPV offer an advantageous alternative to the standard conversion procedures for PPV, which may have important implications for device applications.
C9.1.1
PTHT+DBS
cDBS = 1 M
(b)
0.2 M 0,02
CDBS = 1 M
0,6
4 mM 2 mM 0,01
Absorbance
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