Intense fluorescence of air-stable sodium diarylformamidinate: effective control with a novel electron-withdrawing 4-cyano-3,5-bis(trifluoromethyl)phenyl group

Abstract

We herein present the 4-cyano-3,5-bis(trifluoromethyl)phenyl group as a novel and potent monoaromatic acceptor unit. The CF₃ and CN substitution effects were evaluated through the absorption and emission properties of readily accessible sodium N,N′-diarylformamidinate salts endowed with electron-withdrawing monoaromatic substituents on the nitrogen atoms. Among the salts examined, the amidinates substituted with 4-cyano-3,5-bis(trifluoromethyl)phenyl were remarkably stable at ambient temperature under air and room light, remaining unchanged for over a year. The maxima of absorption and emission wavelengths aligned well with the DFT calculation results. The CF₃ and CN substituents function as σ- and π-electron-withdrawing groups, respectively, reducing the LUMO levels, resulting in photoluminescence of the amidinates under visible light. The intense blue-to-green fluorescence of the organic anions originates from the singlet locally excited (¹LE) states, with photoluminescence quantum yields (Φ) of up to 91%. The air tolerability of the present amidinate salts enabled investigation of the solid-state emission properties and the propensity of the cation and solvent effects, indicating that the more “naked” anions emitted stronger fluorescence.