3HF dyes (Fig. 1) show this property. In the parent 3HF molecule, the two bands in fluorescence emission are commonly observed because of the excited-state intramolecular proton transfer (ESIPT) reaction [1]. They reflect the presence of four states, the normal ground state (N), the normal excited state (N*), the tautomer excited state (T*), and the tautomer ground state (T)(see Scheme 1). The solvent-dependent dual emission can be observed in other dyes exhibiting ESIPT reaction, but 3HCs-3HFs are the unique case in which its appearance is not connected with conformational isomerizations and breakage of intramolecular hydrogen bonds. In addition, in some cases the 3-OH group can dissociate yielding a ground-state anionic form (A) and an excited-state dissociated form (A*) that may be detected in absorption/excitation and emission spectra, respectively. The electronic transitions between correspondent forms are represented by highly intensive bands separated on the wavelength scale (Fig. 2). It is remarkable that all these forms behave as distinct species that interact differently with the environment and are differently sensitive to a variety of interactions. Moreover, the interactions with the environment may produce a perturbation in the equilibrium between the ground-state and excited-state forms, which can dramatically influence the relative band intensities. A breakthrough in the probe design was the attachment at 4! position of 3HF of the electron donor dialkylamino group, which made these dyes strongly solvatochromic. Further improvement of their spectroscopic properties (shift of excitation spectra to longer wavelengths, increase of fluorescence quantum yield, and optimization of two-band response) was provided by additional chemical modifications [2–4].