The compartmental ligand 2,6‐bis[2‐(N‐ethyl)thiopheneiminomethyl]‐4‐chlorophenol (HL) undergoes Cu(ClO4)2 induced partial hydrolysis to yield a monoamine ligand 4‐chloro‐2‐formyl‐6‐[2‐(thiophen‐2‐yl)ethyliminomethyl]phenol (HL′). On further reaction with Cu(ClO4)2, HL′ produces a dinuclear complex, [Cu2(L′)2(ClO4)2] (2), which has been characterized in solution by UV/Vis, ATR, NMR, and ESI‐MS spectral techniques and in the solid state by X‐ray single‐crystal structure analysis. The in situ generated 2 is highly fluorescent. On reaction with sodium acetate it transforms to a fluorescence inactive tetranuclear species, [Cu4(L)2(OAc)4(O)] (3), and thereby acts as an excellent chemodosimeter for selective detection of the acetate anion via fluorescence “turn‐off” signalling. The direct reaction of HL and copper(II) acetate dihydrate yielded another fluorescence inactive tetranuclear species, [Cu4(L)2(OAc)4(O)] (1), which is a diastereoisomer of 3 as is evident from X‐ray single‐crystal structural analyses.