Phosphorescence Enables Identification of Electronic State for Acridinium Salt in Solutions

Understanding intermolecular interactions between Lewis acid and base pairs is of fundamental importance in predicting non-covalent bonding and chemical reactivity. Here we show that an acridinium derivative, a Lewis acid, exhibits various degrees of interactions with Lewis bases of increasing nucleophilicity, including water (HOH), methanol (CH3OH), tetrahydrofuran (ROR), amines (R3N) and t-butoxide (RO-). Each interaction appears to result in a different type of solution state: solvation (e.g., water), coordination (e.g., tetrahydrofuran), chemical bonding (e.g., triethylamine), and radicals (e.g., t-butoxide). The solvated and coordinated acridinium molecules exhibit almost identical 1H-NMR spectra, but possess drastically different UV absorption and luminescence emission, particularly phosphorescence; on the other hand, coordinated and chemically bonded acridinium species which are differentiated by heat calorimetry titration, share the same luminescence spectra but show two different sets of 1H-NMR peaks. These distinct solution states could only be revealed by a combination of NMR and molecular fluorescence/phosphorescence spectroscopic methods, which could provide important clues in the mechanistic understanding of many important processes such as photo-redox reactions. The current report serves as an example of using phosphorescence spectroscopy as a complementary tool for characterizing electronic structures for interaction between organic molecules.