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.
Phosphorescence Enables Identification of Electronic State for Acridinium Salt in Solutions
we provide detail synthetic procedure and some photophysical data in this part.