A compound of Formula 1: wherein R 1 is selected from C 1-20 alkyl, C 2-20 alkenyl, C 2-20 alkynyl, and a 5 or 6 membered aryl or heteroaryl, each of which is optionally substituted with one or more substituents independently selected from halo, CN, NO 2, C R 2, OR 2, OS 2 R 2, NR 2 R 3, SR 2, aryl and heteroaryl; R 2 and R 3 are each independently selected from hydrogen and C 1-6 alkyl; Each R 4 is independently selected from hydrogen, halo, CN, NO 2, C R 2, OR 2, OS 2 R 2, NR 2 R 3, SR 2, optionally substituted C 1-20 alkyl, optionally substituted C 2-20 alkenyl, optionally substituted C 2-20 alkynyl, optionally substituted 5 or 6 membered aryl or heteroaryl, and optionally two R 4 substituents from adjacent ring atoms are joined together to form an optionally substituted 5 or 6 membered aryl or heteroaryl ring; wherein R 2 and R 3 are each independently defined as above; T is a tether group provided by a linear chain of 1 to 3 atoms independently selected from C, N, O, S, Si and B, each of which is optionally substituted with one or more substituents independently selected from hydrogen, halo, CN, NO 2, =O, C R 2, OR 2, OS 2 R 2, NR 2 R 3, SR 2, optionally substituted C 1-10 alkyl, optionally substituted C 2-10 alkenyl, optionally substituted C 2-10 alkynyl and optionally substituted 5 or 6 membered aryl or heteroaryl, and optionally fused with a 3-6 membered carbocycle or heterocycle; and wherein R 2 and R 3 are each independently defined as above, A is a monocyclic or bicyclic heterocyclic ring comprising a 5 or 6 membered ring, containing a donor atom D and a ring atom A 1, and optionally having 1 or 2 additional ring heteroatoms selected from O, S and N, wherein the heterocyclic ring is optionally substituted with 1 to 3 substituents independently selected from halo, CN, NO 2, C R 2, OR 2, OS 2 R 2, NR 2 R 3, SR 2, optionally substituted C 1-20 alkyl, optionally substituted C 2-20 alkenyl, optionally substituted C 2-20 alkynyl, and optionally substituted 5 or 6 membered aryl or heteroaryl; wherein R 2 and R 3 are each independently defined as above; A 1 is selected from C, N and CR 7; wherein R 7 is selected from hydrogen, halo, CN, NO 2, C R 2, OR 2, OS 2 R 2, NR 2 R 3, SR 2, optionally substituted C 1-20 alkyl, optionally substituted C 2-20 alkenyl, optionally substituted C 2-20 alkynyl, optionally substituted 5 or 6 membered aryl or heteroaryl; R 2 and R 3 are each independently defined as above; and D is a donor atom selected from N, O, S, P, Se and Te. R 1 is selected from C 1-20 alkyl, C 2-20 alkenyl, C 2-20 alkynyl, and a 5 or 6 membered aryl or heteroaryl, each of which is optionally substituted with one or more substituents independently selected from halo, CN, NO 2, C R 2, OR 2, OS 2 R 2, NR 2 R 3, SR 2, aryl and heteroaryl; R 2 and R 3 are each independently selected from hydrogen and C 1-6 alkyl; Each R 4 is independently selected from hydrogen, halo, CN, NO 2, C R 2, OR 2, OS 2 R 2, NR 2 R 3, SR 2, optionally substituted C 1-20 alkyl, optionally substituted C 2-20 alkenyl, optionally substituted C 2-20 alkynyl, optionally substituted 5 or 6 membered aryl or heteroaryl, and optionally two R 4 substituents from adjacent ring atoms are joined together to form an optionally substituted 5 or 6 membered aryl or heteroaryl ring; wherein R 2 and R 3 are each independently defined as above; T is a tether group provided by a linear chain of 1 to 3 atoms independently selected from C, N, O, S, Si and B, each of which is optionally substituted with one or more substituents independently selected from hydrogen, halo, CN, NO 2, =O, C R 2, OR 2, OS 2 R 2, NR 2 R 3, SR 2, optionally substituted C 1-10 alkyl, optionally substituted C 2-10 alkenyl, optionally substituted C 2-10 alkynyl and optionally substituted 5 or 6 membered aryl or heteroaryl, and optionally fused with a 3-6 membered carbocycle or heterocycle; and wherein R 2 and R 3 are each independently defined as above, A is a monocyclic or bicyclic heterocyclic ring comprising a 5 or 6 membered ring, containing a donor atom D and a ring atom A 1, and optionally having 1 or 2 additional ring heteroatoms selected from O, S and N, wherein the heterocyclic ring is optionally substituted with 1 to 3 substituents independently selected from halo, CN, NO 2, C R 2, OR 2, OS 2 R 2, NR 2 R 3, SR 2, optionally substituted C 1-20 alkyl, optionally substituted C 2-20 alkenyl, optionally substituted C 2-20 alkynyl, and optionally substituted 5 or 6 membered aryl or heteroaryl; wherein R 2 and R 3 are each independently defined as above; A 1 is selected from C, N and CR 7; wherein R 7 is selected from hydrogen, halo, CN, NO 2, C R 2, OR 2, OS 2 R 2, NR 2 R 3, SR 2, optionally substituted C 1-20 alkyl, optionally substituted C 2-20 alkenyl, optionally substituted C 2-20 alkynyl, optionally substituted 5 or 6 membered aryl or heteroaryl; R 2 and R 3 are each independently defined as above; and D is a donor atom selected from N, O, S, P, Se and Te.