(a) Structures of the coumarin derivatives AB21, AB22, AB23 and AB24. Numbering of the coumarin core structure is shown on AB21. (b) Coumarin derivatives displaying similar absorbance properties (50 μM, 1 cm cuvette, 1X PBS Buffer, pH 7.4).(c) Fluorescence emission of the coumarin derivatives showing quenched fluorescence of AB21 and AB22 (10 μM, 1X PBS Buffer, pH 7.4, λex= 392 nm (AB22/24), 397 nm (AB21) and 402 nm (AB23)).