INTERACTION BETWEEN ELECTROLYTE AND CHARGE-TRANSFER STATES - EVIDENCE FOR COMPLEX-FORMATION

The effects of tetraalkylammonium salts on intramolecular charge-transfer absorption and emission spectra in solution are measured. Addition of tetrabutylammonium hexafluorophosphate (TBAPF(6)) to methylene chloride solutions of the donor-spacer-aacceptor compound 2 reduces the intensity of the charge-transfer emission band and red shifts the emission maximum. The band maximum (cm(-1)) and width of the lowest-energy, charge-transfer absorption transition in 1 increase upon addition of salts. The observed shift and broadening are strongly dependent on the structure of the salt cation, being larger for tributylmethylammonium (TBMPF(6)) than for tetrabutylammonium (TBAPF(6)) salts, but are independent of the salt anion. The dependence of the absorption band shape of 1 on salt concentration is simulated using a model that includes (1) an equilibrium between monomeric and dimeric (aggregated) salt molecules and (2) an equilibrium between uncomplexed 1 and 1 complexed to monomeric salt molecules. The charge-transfer absorption spectra of the complex between 1 and TBMPF(6) and between 1 and TBAPF(6) are determined and are used to obtain estimates of the association constants for complex formation. The relevance of complex formation, between neutral charge-transfer states and added salts, to ongoing studies of electrolyte perturbation of reaction kinetics is discussed.