Composition comprising a solid organic support material to which either directly or via a bridging group, are covalently linked fluorescent host chromophores and fluorescent guest chromophores, wherein the fluorescence emission spectrum of the host chromophore overlaps with the absorption spectrum of the guest chromophore, wherein the host chromophore is selected from the group consisting of benzo[4,5]imidazo[2,11-a]isoindol-11-ones.