Phosphorescence Enables Identification of Electronic State for Acridinium Salt in Solutions

Acridinium derivatives are an important class of photocatalysts, where the interaction between the catalyst and the environment is under-reported. Here we show that the Lewis acidic acridinium salt exhibits various degrees of interactions with different Lewis bases, including water (HOH), methanol (CH₃OH), tetrahydrofuran (THF, ROR), amines (R₃N), and tert-butoxide (RO^-) due to distinct physical properties stemming from different resonance forms. Interactions with water and methanol produce almost identical ¹H NMR spectra but lead to drastically different UV absorption and luminescence emission, particularly phosphorescence; interactions with CH₃OH/methanol and THF, which are differentiated by heat calorimetry titration, share the same luminescence spectra but show two different sets of ¹H NMR peaks. These distinct physical properties could only be revealed by a combination of NMR and molecular fluorescence/phosphorescence spectroscopic methods. The current report serves as an example of using phosphorescence spectroscopy as a complementary tool for identifying interactions between organic molecules.