Table 1. Fluorescence properties of luciferin and 2′-2 stretched racemic free acids of analogues. All compounds measured in a Tecan Infinite spectrometer with at 50 μM in TEM buffer (pH 7.8) (+/–5 nm). For quantum yields, absorptions measured at 50 μM (pH 7.8) of 100 μl, which are all less than 0.1 AU to avoid inner filter effects. Eosin Y dye used as a standard (490 nm excitation and 0.67 QY value) in basic ethanol¹⁶ .

Compound	Excitation λ max (nm)	Emission λ max (nm)	Fluorescence quantum yield
1	336	538	0.50
4	365	600	0.27
5	372	650, 505	0.17