Phase-transfer catalyst-induced changes in the absorption and fluorescencebehavior of some electron donor-acceptor molecules

The quaternary ammonium salts find extensive use in a variety of chemical a pplications as surfactants and phase-transfer catalysts. Even though the in teraction of the surfactants with various systems has been studied rather e xtensively, very Little information is available on how the phase-transfer catalysts (PTC) interact with the dipolar systems in nonpolar media and how /whether these solubilizing agents affect the properties of the dissolved m olecules. In this paper, the interaction of several tetraalkylammonium salt s, commonly used as PTC, with a number of electron donor-acceptor (EDA) sys tems has been studied by absorption and fluorescence spectroscopy. The phas e-transfer catalysts have been found to affect both the absorption and fluo rescence properties of the EDA systems. The spectral changes of the EDA mol ecules induced by the tetraalkylammonium salts suggest the formation of an 1:1 complex between the two in nonpolar media. An electrostatic interaction between the phase-transfer catalysts (which exist as ion pairs in nonpolar media) and the dipolar molecules is shown to be the driving force for the formation of the complex. The dependence of the formation constant of the c omplex on the polarity of the media suggests a charge-transfer nature of th e complex. It is shown that the anionic component of the salts serves as a source of electron to the positive end of the dipolar molecule, while the t etraalkylammonium cation, besides helping solubilization of its anionic cou nterpart in the nonpolar media, serves neutralizing the negative charge at the acceptor end of the EDA molecule. In effect, a cooperative influence of the cationic and anionic components of the PTC enhances the charge separat ion within the dipolar fluorophores. On the basis of the PTC-induced change s in the photophysical behavior of the EDA molecules, a possible structure for the 1:1 complex has been proposed. It has been concluded that a phase-t ransfer catalyst should not be treated as an innocuous substance that merel y helps transfer of a polar substance from a polar to a nonpolar environmen t. Instead, it is demonstrated that the association of a PTC with a dipolar species can significantly change various properties of the latter.