STRUCTURAL EFFECTS ON THE THERMOCHROMIC PROPERTIES OF POLYTHIOPHENE DERIVATIVES

Temperature dependent optical absorption measurements have revealed im portant thermochromic effects in poly(3-dodecylthiophene) and poly[(3- octyloxy)-4-methylthiophene] between 25 and 150-degrees-C which are re lated to a conformational transition of the conjugated polymer from a co-planar structure to a non-planar form. This conformational transiti on is caused by an increase of the repulsive intrachain steric interac tions upon heating which then force the thiophene backbone to adopt a non-planar conformation. On the other hand, as observed with poly(3-oc tyloxythiophene), if the thermally induced disorder in the side chains does not lead to large steric interactions, the polymer can keep a hi ghly conjugated structure even at high temperatures whereas, if the st eric interactions are too strong (e.g. as in poly(3,3'-dihexyl-2,2'-bi thiophene), no co-planar conformation can take place.