1. A compound of formula (I) or (II) wherein Z 1 -Z 4 and Z 1′ -Z 4′ are the same or different and each is R 1 , R 2 , R 3 , R 4 , R 2′ , R 3′ , and R 4′ , are the same or different and each is (CR 6 R 7 ) n — or (CR 8 R 9 ) m —X—(CR 10 R 11 ) m′ —, R 1′ is (CR 6 R 7 ) n R 12 or —(CR 8 R 9 ) m —X—(CR 10 R 11 ) m′ —R 12 , R 5′ , R 6 , R 7 , R 8 , R 9 , R 10 , R 11 , and R 12 are the same or different and each is selected from hydrogen, alkyl, alkenyl, cycloalkyl, alkoxy, aryl, aryloxy, heteroaryl, hydroxyalkyl, thioalkyl, thioalkoxy, thioaryl, and an amino acid-containing group, wherein each group other than hydrogen is optionally substituted, X is O, S, or NR 13 , R 13 is hydrogen, alkyl, alkenyl, cycloalkyl, aryl, or amino-acid containing group, wherein each group other than hydrogen is optionally substituted, n is an integer selected from 5-7, and m and are the same or different and each is an integer selected from 0-6, such that 4<(m+m′)≦6; and wherein the compound optionally further comprises a biomolecule (i) that is covalently linked, either directly or through a linker in the compound of formula (I), to a carbon or nitrogen atom of one or more of R 1 -R 4 , or (ii) that is covalently linked, either directly or through a linker in the compound of formula (II), to carbon or nitrogen atom of one or more of R 1′ -R 5′ or a nitrogen atom of one or both of Z 1′ or Z 4′ .