In addition, we synthesized azide-based clickable fluorophores 6 (crystal structure obtained) and 7 by reacting (R)-1 and (±)-5 with 2-azidoethylamine. These were coupled to maleimidealkyne 8 via Cu-catalysed azide-alkyne cycloaddition (CuAAC) chemistry (Scheme 2). The resultant fluorophore maleimides F3 and F4 containing triazole linkers were thus also available for further protein conjugation. We were able to obtain a crystal structure of compound 9, the methyl ester of 5, and compared it with the reported solid-state structure of 1. For 1, the angles of C6-S1-C5 and C7-N3-C4 are 100.1° and 123.0°, respectively. In the case of compound 9, the angles of C7-N6-C5 and C8-N4-C4 are 122.1° and 117.4°. This suggests that the two nitrogen atoms in 9 have a more planar conformation compared to the combination of sulfur and nitrogen in fluorophore 1. Recently, it has been reported that secondary amines in plane with terephthalonitriles (TN) exhibit enhanced emission properties. Next, we studied the photophysical properties of the fluorophore maleimides F1-F4 (Table ).