The absorption spectrum of 1-hydroxy 2a exhibited two major bands at 400 nm (moderate) and 420-500 nm (broad, weak) under neutral conditions (Figure ). When 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) was titrated with 2a, the absorption at 450 nm gradually increased and became saturated with the addition of 1.0 equivalent of DBU. In contrast, no chromic changes were observed when excess trifluoroacetic acid (TFA) was added. The emission spectrum of 2a shows yellow-green fluorescence at 537 nm (ex = 381 nm) under neutral conditions (Figure ). When DBU was added to the solution of 2a, the fluorescence was dramatically strengthened at almost the same wavelength. Reversible photophysical changes in 2a with DBU were observed upon the addition of acid. The hyperchromic property of 2a is induced by deprotonation of the phenolic proton. It was clearly proved that almost the same absorption spectra was observed for methyl ether 4 both in the absence or presence of DBU or TFA (Figure ). Interestingly, the chromic properties of 9-hydroxy derivative 3a were different from those of 1-hydroxy derivative 2a. Although 3a exhibits major bands at 400 nm (moderate) and 410-530 nm (broad, weak) under neutral and acidic conditions, respectively, the absorption at longer wavelengths is red-shifted to 420-630 nm upon titration with DBU. The bathochromic shift was saturated with the addition of 30 equivalents of DBU (Figure ). The colour of the solution turned red under basic conditions. The emission of 3a was observed at 571 nm under neutral conditions (ex = 381 nm). Upon the addition of DBU (30 equiv.), ‡ emission at longer wavelength (715 nm) was observed albeit weak (Figure ).