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 H2SO4, 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 * 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.