Disclosed are cyanine dyes having meso-substituents in the polymethine chain linking the heterocyclic ring systems. The dyes are of formula (1): 1 in which groups R3 and R4 are attached to the Z1 ring structure and groups R5 and R6 are attached to the Z2 ring structure, and nnull1, 2 or 3; Z1 and Z2 independently represent the atoms necessary to complete one ring, or two fused ring aromatic or heteroaromatic systems, each ring having five or six atoms selected from carbon atoms and optionally no more than two atoms selected from oxygen, nitrogen and sulphur; X and Y are the same or different and are selected from oxygen, sulphur, nullCHnullCHnull and the group: 2 at least one of groups R1, R2, R3, R4, R5, R6 (and R8 and R9 if present) is the group nullEnullF where E is a spacer group and F is a target bonding group; one of groups R7 is selected from nullCN, nullCl, nullF, nullCF3 and nullC(O)R10 wherein R10 is selected from H, C1-C6 alkyl and aryl. Preferred group R7 is meso-substituted nullCN, which confers an unexpected hypsochromic shift of approximately 40 nm in the emission spectrum, when compared with the corresponding unsubstituted analogue. The dyes are useful for labelling and detecting a variety of target molecules.