Phosphorescence Enables Identification of Electronic State for Acridinium Salt in Solutions.

Xiaolong Zhang,Fan Liao,Guoqing Zhang,Jiajun Du,Hui Miao,Xuepeng Zhang,Hao Su

doi:10.1021/acs.jpclett.1c03584

Abstract

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.

Highlights

Understanding intermolecular interactions between Lewis acid and base pairs is of fundamental importance in predicting non-covalent bonding and chemical reactivity
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.[1]
Nicewicz and co-workers developed ADs as a general photocatalyst for catalytic alkene anti-Markovnikov hydrofunctionalization reactions.[3]. They have 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.[4,5,6,7,8]

Summary

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Representation for molecular structures and Photoluminescence of ADBr and its interaction with dichloromethane, tetrahydrofuran, triethylamine. We 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 1H-NMR and luminescence.[13] by applying the same method, we reason that a combination of NMR and luminescence can be used to reveal the electronic states of AD photocatalysts in the solution state. 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.[14,15,16,17] 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

MeOH THF

Molar Ratio