Reversible fluorescence redox switching in activated carbonyl-functionalized fluorophores through the conversion of α-keto esters and α-hydroxy esters

Abstract

Activated carbonyl groups containing α-keto esters (KE) attached with three different fluorophore units, ethyl-2-(4-(diphenylamino)phenyl)-2-oxoacetate (TPA-KE), ethyl-2-oxo-2-(4-(1,2,2-triphenylvinyl)phenyl) acetate (TPE-KE), and ethyl-2-oxo-2-(perylen-3-yl)acetate (PY-KE), were synthesized and their fluorescence properties in solution as well as solid state were studied. The compounds displayed fluorophore unit-dependent tunable solid-state fluorescence. TPA-KE showed green fluorescence (λ_max = 491 and 527 nm) and TPE-KE (λ_max = 459 nm) showed blue fluorescence, while PY-KE (λ_max = 659 nm) exhibited weak orange fluorescence in the solid state. In contrast, PY-KE showed strong fluorescence in solution and the other two compounds did not show significant fluorescence. The planar pyrene formed π-overlapped arrangements in the solid state, whereas twisted TPA and TPE units prevented π-overlapping that led to strong solid-state fluorescence. The reduction of α-keto esters using reducing agents produced the corresponding hydroxy ester (HE) derivatives (TPA-HE, TPE-HE and PY-HE) that caused fluorescence modulation in solution and solid state. After conversion, TPA-HE showed weak emission in the solid state and TPE-HE exhibited only slight variations in the emission wavelength. TPA-HE and PY-HE showed relatively strong fluorescence in solution compared to the solid state. Interestingly, oxidation of the HE derivatives reverted them to the KE derivatives and switched their fluorescence properties. Therefore, all three compounds exhibited reversible dual-state fluorescence switching due to a reversible α-keto ester–α-hydroxy ester conversion.