The application of a UHPLC system to study the formation of various chemical species by compounds undergoing efficient self-aggregation and to determine the homodimerization constants (KDM) with values in the high range of 106–1010 M−1

Abstract

This work demonstrates a new concept for the use of UHPLC methodology for identification of the species formed by a self-aggregating compound depending on its concentration and solvent used, as well as to determine very large homodimerization constants (K_DM = 10⁶–10¹⁰ M⁻¹). It is impossible to obtain such data with traditional UV-VIS and NMR measurements in compounds that undergo easy self-aggregation when their K_DM values are very large (≥10⁷ M⁻¹). The application of the UHPLC method in tandem with a UV-VIS photodiode spectrophotometer as a detector, as well as an emission detector allowed us to perform measurements at extremely low dye concentrations (down to 10⁻⁸ M in the absorption measurement and down to 10⁻¹⁰ M in the fluorescence measurement). Using the well-known probe 7-aminocoumarin (C120) as a model system, we separated the monomer (M) and dimer (DM) species, and determined their concentrations and individual absorption spectra. The position of the long wavelength band in the monomer absorption spectrum agreed very well with theoretically calculated values of vertical excitation energy to the S₁ state of the C120 monomer. To the best of our knowledge, this is the first report on the very efficient self-aggregation of C120 in solution, with K_DM = 1.5 × 10⁹ M⁻¹ in polar ACN and K_DM = 9 × 10⁹ M⁻¹ in more weakly interacting 1-chlorobutane (ChB).