where B1 is a bridge member and A1 and A2 are independently of each other the radical of a monoazo, polyazo, metal complex azo, stilbene or anthraquinone dye, or where B1 and A1 are each as defined above and A2 is a phenyl or naphthyl radical substituted by a heterocyclic radical or by a benzoylamino or phenylamino radical, or where B1 is a direct bond and A1 and A2 are each the radical of a metal