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

Excited-state intramolecular proton transfer (ESIPT) in 3-hydroxy avone dyes allows us to record, in addition to common spectroscopic parameters, the positions of absorption (nu(abs)) and emission (nu(N*)) maxima, two new parameters: the position of the emission maximum of the ESIPT product T* state (nu(T*)) and the intensity ratio of the two emission bands (I-N*/I-T*). An attempt was made to find a correlation between these parameters and physicochemical characteristics of microenvironment: polarity f(epsilon), electronic polarizability f(n) and H-bond donor ability. A detailed spectroscopic study of 4'-diethylamino-3-hydroxy avone in a set of 21 representative solvents demonstrates that the Stokes shift of the N* band (nu(abs)-nu(N*)) correlates strongly with the Lippert function L = f(epsilon)-(n), and this correlation does not depend on the effects of intermolecular H-bonding, while the correlation of log(I-N*/I-T*) with polarity f(epsilon) 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 nu(abs), nu(n*), nu(T*) and log(I-N*/I-T*) to provide a simultaneous estimation of three microenvironment characteristics: f(epsilon), 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-hydroxy avone family for studying complex microheterogeneous systems in physical chemistry, colloid chemistry and the biological sciences.