The invention relates to optical devices comprising a substrate and at least one semiconductive polymer supported by said substrate, wherein said semiconductive polymer is a copolymer in which one of the repeat units is a group of formula (I) or a homopolymer in which the repeat unit is a group of formula (I): ##STR1##

wherein: A and B are the same or different and each comprises wholly or partially an aryl moiety or a heteroaryl moiety, said moiety in A being fused to the bond a-b and said moiety in B being fused to the bond c-d, and X is a linking unit, X being such that there is a torsion angle of at least 5 between the bond a-b and the bond c-d about the bond b-d. The invention also relates to semiconductive polymers, monomers for preparing same, and methods for preparing random statistical conjugated polymers.