A compound of Formula D-S 1 -A-S 2 -B 1, Formula wherein: A represents -Ar 1 -(FL-Ar 2) n - and comprises from 1 to 8 FL groups; Ar 1 and Ar 2 in each occurrence are independently selected from the group comprising Ar a and a bond; Ar a represents a diradical comprising 1 aromatic, heteroaromatic or FL moiety, or 2, 3, 4 or 5 aromatic, heteroaromatic and/or FL moieties mutually connected by a single bond; n is an integer from 1 to 8; FL is a fluorene moiety of the structure incorporated into the chain through covalent bonds at C-2 and C-7; the R groups of each FL moiety are identical and are selected from the group consisting of straight chain or branched achiral C 1- C 14 alkyl, C 1 -C 14 haloalkyl, C 1 -C 14 fluoroalkyl, C 2 -C 14 alkenyl group, optionally wherein 1, 2, 3, 4 or 5 CH 2 groups are replaced by an oxygen provided that no acetal, ketal, peroxide or vinyl ether is present in the R group; D represents a cross linkable group or, when B 1 represents a hydrogen, D represents -B 2 -S 3 -B 3 -S 1a -A-S 2a -B 1a, -S 3 (B 2)-B 3 -S 1a -A-S 2a -B 1a, -S 3 (B 2)(B 3)-S 1a -A-S 2a -B 1a, -S 3 (B 2)(B 3), or a cross linkable group wherein the dash at the left-hand end of the chain represents the point of attachment to S 1; B 1 represents a cross linkable group or a hydrogen atom; B 1a represents a cross linkable group or a hydrogen atom; B 2 and B 3 each represents a cross linkable group; S 1, S 2, S 1a and S 2a are flexible linker groups, which in each occurrence are independently selected from straight chain or branched achiral C 5 -C 14 alkyl groups, optionally wherein 1, 2, 3, 4 or 5 methylene groups are substituted for an oxygen atom, provided that no peroxide, ketal or acetal is present, that is connected to A through either a bond or an ether, ester, carbonate, thioether, amine or amide linkage and that is connected through either a bond or an ether, ester, carbonate, thioether, amine or amide linkage to D, B 1, B 2, B 3 or S 3 as determined by the nature of D; and S 3 is a spacer group. A represents -Ar 1 -(FL-Ar 2) n - and comprises from 1 to 8 FL groups; Ar 1 and Ar 2 in each occurrence are independently selected from the group comprising Ar a and a bond; Ar a represents a diradical comprising 1 aromatic, heteroaromatic or FL moiety, or 2, 3, 4 or 5 aromatic, heteroaromatic and/or FL moieties mutually connected by a single bond; n is an integer from 1 to 8; FL is a fluorene moiety of the structure incorporated into the chain through covalent bonds at C-2 and C-7; the R groups of each FL moiety are identical and are selected from the group consisting of straight chain or branched achiral C 1- C 14 alkyl, C 1 -C 14 haloalkyl, C 1 -C 14 fluoroalkyl, C 2 -C 14 alkenyl group, optionally wherein 1, 2, 3, 4 or 5 CH 2 groups are replaced by an oxygen provided that no acetal, ketal, peroxide or vinyl ether is present in the R group; D represents a cross linkable group or, when B 1 represents a hydrogen, D represents -B 2 -S 3 -B 3 -S 1a -A-S 2a -B 1a, -S 3 (B 2)-B 3 -S 1a -A-S 2a -B 1a, -S 3 (B 2)(B 3)-S 1a -A-S 2a -B 1a, -S 3 (B 2)(B 3), or a cross linkable group wherein the dash at the left-hand end of the chain represents the point of attachment to S 1; B 1 represents a cross linkable group or a hydrogen atom; B 1a represents a cross linkable group or a hydrogen atom; B 2 and B 3 each represents a cross linkable group; S 1, S 2, S 1a and S 2a are flexible linker groups, which in each occurrence are independently selected from straight chain or branched achiral C 5 -C 14 alkyl groups, optionally wherein 1, 2, 3, 4 or 5 methylene groups are substituted for an oxygen atom, provided that no peroxide, ketal or acetal is present, that is connected to A through either a bond or an ether, ester, carbonate, thioether, amine or amide linkage and that is connected through either a bond or an ether, ester, carbonate, thioether, amine or amide linkage to D, B 1, B 2, B 3 or S 3 as determined by the nature of D; and S 3 is a spacer group.