Photo- and ionochromic N-acylated 2-(aminomethylene)benzo[b]thiophene-3(2H)-ones containing a terpyridine receptor

New photo- and ionochromic N-acylated 2-(aminomethylene)benzo[b]thiophen-3(2H)-ones containing a terpyridine receptor have been synthesized. They absorb in the visible region of the spectrum at 423–426 nm and exhibit emission at 463–468 nm. Their irradiation in acetonitrile with light of 436 nm leads to a multi-stage rearrangement, including Z/E isomerization about the CC bond, N → O migration of the acyl group and the formation of non-emissive O-acylated E-isomers, which were isolated preparatively. Their structure was established unambiguously by IR, ¹H, ¹³C NMR spectroscopy and HR mass spectrometry. The reverse thermal reaction with O → N migration of the acyl group is catalyzed by acid catalysts. N-Acylated compounds in acetonitrile selectively form non-fluorescent complexes with Fe²⁺ cations, which is accompanied by the naked-eye effect with changing the color from yellow to maroon. A successful selective interaction with AcO⁻ led to the restoration of the initial absorption and emission properties. Density functional theory (DFT) calculations at the M06-2X/def2-TZVP level with the PCM solvation method were used to explain the observed photoisomerizations and ionochromic transformations. The obtained compounds represent multifunctional on-off-on molecular switches of optical and fluorescent properties upon sequential exposure to light and H⁺ or sequential addition of Fe²⁺ and AcO⁻ ions. A combinatorial logic gate was designed based on the basis of these switching properties.