Orientation of thin films of conjugated systems by different techniques

Abstract

Oriented thin films of good optical quality are necessary for applications of conjugated systems in non linear optical devices. In this paper, we focus on the preparation and spectroscopic characterization of thin films of two different polymers, a polydiacetylene, poly[9-(N-carbazolyl)-2,4-hexadyine-1-ol-1, ethylurethane] (pCNEU) and a polyphenylene–ethynylene, the (11-undecanol) benzoate containing polymer (pPEn). pCNEU gives the best orientation by epitaxial growth on polytetrafluoroethylene (PTFE) layers aligned on glass. On the contrary pPEn is best oriented on glass substrates by rubbing technique.

Introduction

The application of conjugated systems in most of optoelectronic devices requires the preparation of thin films of good optical quality. To this aim, several techniques such as Langmuir–Blodgett (LB), spin-coating or organic molecular beam epitaxy (OMBE) are used [1]. Most of them, however, are limited to a strict range of molecules and substrates. With the purpose of extending the variety of molecules to be studied, we have focused our interest in the preparation of thin films on glass because of its optical transparency in a wide spectral range and chemically modifiable surface. Since, it is well-known also that in highly oriented samples nonlinear optical properties or electroluminescence can be enhanced along the orientation direction, we have prepared and characterized by spectroscopic techniques and electron microscopy a series of thin films of polydiacetylenes (PDAs) and phenylene–ethynylene derivatives (PPEs) aligned on glass directly by rubbing [2] or indirectly by epitaxial growth on oriented PTFE layers [3]. Both techniques have been used for the orientation of many conjugated systems. Among all the compounds studied, we will report here, only the results on two compounds selected for the particularly good orientation achieved. In details, one of these compounds, pCNEU, whose relevant nonlinear optical properties have been already measured for samples epitaxially grown on potassium acid phthalate (KAP) [4], has been oriented by epitaxial technique on PTFE. The other one, the pPEn which has been prepared for the realization of organic LEDs [5], has been oriented by the rubbing technique.