Representation for molecular structures and Photoluminescence of ADBr and its interaction with dichloromethane, tetrahydrofuran, triethylamine. KEYWORDS: Phosphorescence, Lewis acid, Lewis base, Bimolecular interaction, Acridinium Acridinium salts (ADs) are an emerging class of organic photocatalysts, that are less expensive and more sustainable compared to their transition-metal complex counterparts, for a myriad of chemical reactions such as photooxidations, C-H functionalization, crosscoupling and cross-dehydrogenative reactions. In 2004, Fukuzumi et al. pioneered in using ADs as an electron-transfer photocatalyst for efficient oxygenation reactions of anthracenes and olefins by taking advantage of the long-lived charge-separated state of the AD catalysts upon photoexcitation. More recently, Nicewicz and co-workers developed ADs as a general photocatalyst for catalytic alkene anti-Markovnikov hydrofunctionalization reactions. In addition, they have also achieved site-selective arene C-H amination and arene C-H fluorination via ADs-based photoredox catalysis, which could be of vital significance for pharmaceutical synthesis. In addition to the role as a photocatalyst separately, ADs have also been shown to work synergistically with other reagents in a dualcatalyst manner to activate "traditionally tough" reactions. For instance, Wu et al. have utilized ADs in combination with a cobalt catalyst to engineer the Heck-type coupling between unactivated aliphatic acids and terminal alkenes. Rueping et al. have also integrated ADs with nickel catalysts for remote, site-specific arylation of ketones from easily accessible tertiary alcohols through C-C bond-cleaving cross-coupling. One of the most predominant environmental parameters for catalysts performance is the solvation, i.e., a change in solvent usually leads to altered reaction efficiency or main product, which is also true for AD catalysts. As stated by Alexanian et al., the C-H azidation in presence of ADs in 1,2-dichloroethane, 2,2,2-trifluoroethanol and hexafluoroisopropanol gave azidocyclooctane with yields of 30%, 50% and 70%, respectively, while maintaining the same condition for everything else. Similarly, Wu et al. described that the crosscoupling reaction between 3,3-dimethylbutanoic acid and styrene showed a high yield of >80% in toluene; however, low and mediocre yields of 12% and 61% were achieved in acetonitrile and (trifluoromethyl)benzene respectively. Nonetheless, most published reports focus mainly on screening the scope of substrates-products with respect to catalyst structures in reaction methodologies, with a lesser emphasis on the in-depth investigation on the role of solvent. One of the reasons for the lack of enthusiasm is perhaps due to the complexity in solvent-assisted organic reactions in general, and the fact that there are only limited choices for solvents. As a result, the majority of proposed mechanisms to date were based on first-principles calculations. The special property about ADs is that they are extremely Lewis acidic due to a combination of an extended -conjugation, and the presence of a positive charge, which lowers the * orbital. We thus expect that a small change in the electronic potential energy in the solvent could lead to a significant change in their interaction with ADs. Previously, we have shown that such Lewis acid-base pairing, revealed by single-crystal X-ray diffraction, could cause substantial changes in both 1 H-NMR and luminescence. Herein, by applying the same method, we reason that a combination of NMR and luminescence can also be used to reveal the electronic states of AD photocatalysts in the solution state. Using an acridinium bromide derivative (ADBr) as the core structure, we found that with increased nucleophilicity of the Lewis base, ADBr could in fact exhibit four drastically different electronic states, i.e., solvation, coordination, chemical bonding and radical pair (Figure , ADBr). Compared to fluorescence spectroscopy, the phosphorescence spectrometry demonstrated its advantage in exploiting slow decay dynamics of the triplet excited state and cutting off interfering sources commonly seen for steady-state measurements. The multiple electronic states of ADs-Lewis base interactions revealed by abovementioned methods would have great implications in understanding a wide variety of reaction mechanisms concerning ADs photocatalysts. As shown in Figure , the 1 H-NMR spectra of ADBr in three different deuterated solvents H2O-d2, MeOH-d4 and THF-d8 (for THF-d8, a mixed solvent with 86% DMSO-d6