Multiparametric probing of intermolecular interactions with fluorescent dye exhibiting excited state intramolecular proton transfer

Excited-state intramolecular proton transfer (ESIPT) in 3-hydroxyflavone dyes allows us to record, in addition to common spectroscopic parameters, the positions of absorption (νabs) and emission (νN*) maxima, two new parameters: the position of the emission maximum of the ESIPT product T* state (νT*) and the intensity ratio of the two emission bands (IN*/IT*). An attempt was made to find a correlation between these parameters and physicochemical characteristics of microenvironment: polarity f(ε), electronic polarizability f(n) and H-bond donor ability. A detailed spectroscopic study of 4′-diethylamino-3-hydroxyflavone in a set of 21 representative solvents demonstrates that the Stokes shift of the N* band (νabs − νN*) correlates strongly with the Lippert function L = f(ε) − f(n), and this correlation does not depend on the effects of intermolecular H-bonding, while the correlation of log(IN*/IT*) with polarity f(ε) can be represented by linear functions that are different for protic and aprotic environments. Cross-correlation analysis of the spectroscopic parameters provides criteria to distinguish specific (H-bonding and other) from universal probe interactions with the environment. We suggest an algorithm, which uses four spectroscopic parameters νabs, νN*, νT* and log(IN*/IT*) to provide a simultaneous estimation of three microenvironment characteristics: f(ε), f(n) and H-bond donor ability. An application of this algorithm in the studies of binary solvent mixtures, reverse micelles and binding sites of proteins demonstrates the power of this approach and suggests a unique possibility to develop a new generation of fluorescence probes and labels in the 3-hydroxyflavone family for studying complex microheterogeneous systems in physical chemistry, colloid chemistry and the biological sciences.