A New Series of Fluorescent Indicators for Super Acids

The photophysical properties of fluorescent Hammett acidity indicator derived from 3,4,5,6‐tetrahydrobis(pyrido[3,2‐ g ]indolo)[2,3‐ a :3′,2′‐ j ]acridine (1a), 6‐bis(pyrido[3,2‐ g ]indol‐2′‐yl)pyridine (1b) and their analogues have been investigated in sulfuric acid solutions by means of absorption, fluorimetry, relaxation dynamics and computational approach. These new indicators undergo a reversible protonation process in the Hammett acidity range of H 0  < 0, accompanied by a drastic increase of the bright blue‐green (1a) or yellow (1b) fluorescence intensity upon increasing the acidity. For 1a in H 2 SO 4 , the emission yield increases as large as 200 folds from pH  = −0.41 to the Hammett acidity range of −5.17, the results of which are rationalized by a much increase of the steric hindrance upon third protonation toward the central pyridinic site, together with their accompanied changes of electronic configuration from charge transfer to a delocalized ππ * character in the lowest lying excited state. The combination of 1a and 1b renders a wide and linear range of H 0 measurement from −1.2 to −5.1 detected by highly intensive fluorescence.