Absorption and Emission Dichroism of Polyethylene Films with Molecularly Dispersed Push-Pull Terthiophenes Dedicated to Professor Gerhard Wegner on the occasion of his 65 th birthday Andrea Pucci, 1,2 Nicola Tirelli, 3 Giacomo Ruggeri, 1,2 Francesco Ciardelli* 1,4 1 Department of Chemistry and Industrial Chemistry, University of Pisa, Via Risorgimento 35, 56126 Pisa, Italy E-mail: fciard@dcci.unipi.it 2 INSTM, Pisa Research Unit, Via Risorgimento 35, 56126 Pisa, Italy 3 School of Pharmacy and Centre of Molecular Materials, University of Manchester, Oxford Road, M13 9PL, Manchester, UK 4 INFM PolyLab, Via Risorgimento 35, 56126 Pisa, Italy Received: April 29, 2004; Revised: July 21, 2004; Accepted: July 22, 2004; DOI: 10.1002/macp.200400171 Keywords: absorption and fluorescence dichroism; chromophore dye dispersions; orientation; polyethylene (PE); push-pull terthiophenes Introduction In recent years, considerable efforts have been devoted to the development of new efficient linear polarizers, e.g. by recycling reflected or scattered radiation, [1] or directly generating polarized light using dichroic photolumines- cence [2–4] for applications as optical components in optical instruments and liquid-crystal displays. [5,6] A commonly accepted point is that high polarizing efficiency is obtained by orienting anisotropic and strongly absorbing dyes through tensile action (according to a pseudo-affine deformation scheme) [1,7] on ultradrawable polyolefins, such as Ultra High Molecular Weight Poly- (Ethylene)(UHMWPE), [7,8] LinearLowdensityPE(LLDPE) and High Density PE (HDPE). [9] We have recently studied absorption polarizers based on guest terthiophene chromo- phores dispersed in oriented polyethylene (PE) host matri- ces. [10–14] These chromophores were chosen due to the linear and easy-to-functionalize structure; moreover, the slight non- coplanarity of the aromatic rings in the solid state (torsion angles of 171–1748 between 2 molecules of thiophene) [15] helps in keeping low intermolecular interactions based on p-stacking and, at the same time, does not reduce the p-orbital overlap, responsible for the conjugation. [16] Moreover these Summary: Substituted terthiophenes act as excellent chro- mophores for linear polarizers based on oriented polyethy- lene thin films. The dispersibility in the polymer matrix exerts a determining role on the optical performances. The modu- lation of the crystallization of the dye by attaching to the chromophoric moiety different linear or branched alkyl chains and by varying the polyethylene disordered phase (from UHMWPE to LLDPE) allows to obtain different extent of molecular dispersion. Scanning electron microscopy (SEM) and x-ray microanalysis help in determining the de- tailed morphology of the binary blend films. After uniaxial stretching, the films from molecularly dispersed dyes show excellent dichroic properties in absorption with dichroic ratios up to 60. On the contrary, the anisotropy in emission of the same films is lower than for the corresponding films from more aggregated dyes owing to the more effective depolar- ization phenomena. Absorption spectra with parallel (08) and perpendicular (908) polarization to the drawing direction of a 0.8 wt.% V2/ UHMWPE film (draw ratio ¼ 30) (left side) and drawing extent dependence of the dichroic ratio (R) of Y3/ and V2/ UHMWPE films (inset). Macromol. Chem. Phys. 2005, 206, 102–111 DOI: 10.1002/macp.200400171 ß 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 102 Full Paper