Anion-dependent fluorescence in bis(anilinoethynyl)pyridine derivatives: switchable ON-OFF and OFF-ON responses

Abstract

Urea and sulfonamide derivatives of 1 exhibit ON-OFF and OFF-ON switchable fluorescent and colorimetric responses upon protonation. The magnitude of the fluorescence event is dictated by the anion, resulting in a rare, fully organic "turn-on" fluorescent sensor for chloride.

Fig. 1

Structures of 2,6-bis(2-anilinoethynyl)pyridine receptor core 1 and urea (2,3a,b) and sulfonamide (4a,b) derivatives.

Fig. 2

UV-Vis spectra of 1 and 2a as both protonated and neutral receptors ([Host] and [Host•H⁺] = 12 μM in CHCl₃).

Fig. 3

Normalized emission spectra of both neutral and protonated electron-rich receptors ([Host] and [Host•H⁺] = 12 μM in CHCl₃; excitation 1: 360 nm, 2,3a,b: 343 nm; 4a: 338 nm).

Fig. 4. Emission spectra of electron-poor receptors both neutral and protonated with TFA or HCl ([Host] and [Host•H⁺] = 12 μM in CHCl₃; excitation 3b: 360 nm, 4b: 365 nm)

Fig. 5

Colorless solutions of neutral compounds in CHCl₃ turn yellow upon protonation (top); fluorescence (excitation 365 nm) is quenched in electron-rich systems 3a, 4a and turned “on” in electron poor systems 3b, 4b (bottom).

Fig. 6

Calculated frontier molecular orbitals for neutral ureas 3a,b and sulfonamides 4a,b. Non-overlapping HOMO and LUMO orbitals in the electron-acceptor substituted systems results in charge transfer quenching of the neutral receptors.