Bifunctional photo- and ionochromic hybrids of indolyl(thienyl) diarylethenes and rhodamine

Abstract

Bifunctional hybrids of a new type containing photochromic indolyl(thienyl) diarylethenes with a pyrroldione bridge directly linked by a dimethylene spacer to an ionochromic rhodamine moiety were synthesized. The synthesized hybrids in toluene exhibit absorption at 450–455 nm and fluorescence at 560 nm. Irradiation with UV light leads to the formation of ring-closed non-fluorescent diarylethene isomeric fragments. Under visible light (or gradually in the dark conditions), reverse isomerization to the initial forms occurs. The spectral and kinetic characteristics of the processes have been studied. In the presence of H+ cation, a striking naked eye effect is observed due to the opening of the rhodamine-spirolactam ring and appearance of a characteristic pink-crimson coloration at 561 nm along with intense long-wavelength emission at 586 nm. This process is completely reversible upon exposure to triethylamine. The mechanism of this transformation was confirmed by additional 1H and 13C NMR experiments. The M06-2X/Def2-TZVP method and AIM analysis were successfully used to explain observed light-induced isomerizations as well as proton-induced transformations. Thus, reversible switching of the fluorescent properties of hybrid molecules can be operated by light irradiation and proton.