Functionalized Oligothiophenes for Optoelectronic Applications: 3 ,4 ,3 ,4 -Tetra[(methoxycarbonyl)methyl]-2,2 :5 ,2 :5 ,2 :5 ,2 -quinquithiophene and Related Polymers

We present the optical properties and LED performance of single-layer devices of a tetra-ester ofR-quinquithiophene (TET5) and of two related polymers (PTET5 and PDET3) and discuss them inconnection with the crystal structure features of this oligomer. The solution photoluminescence quantumyield (PLQY) of TET5 is smaller than that of the corresponding unsubstituted oligomer, while its valuein the solid state is appreciable. Molecular packing, consisting of a sequence of molecular stacks linkedby relatively strong polar hydrogen-bond-like interactions, favors PL quenching and hence accounts forthe limited quantum efficiency of LED devices built by a single-layer film displaying substantial order(èext g 5â10-3%). Films of the corresponding PTET5 polymer are amorphous, morphologicallyhomogeneous, and behave differently, with LED devices showing over 500 cd/m2 at 15 V. The probablereason for the difference between TET5 and PTET5 is to be found in molecular aggregation and orientationwith respect to the substrate, implying that PLQY in this class of materials is substantially influenced byself-assembly. This is confirmed by the poor efficiency of the PDET3 polymer, the films of which aresubstantially more ordered than those of PTET5.