Rhodamine 6G-Based Molecular Ion-Active Switches of Optical and Fluorescent Properties

A series of rhodamine 6G derivatives with o-hydroxyarylimine substituents have been synthesized. The obtained compounds in acetonitrile have been found capable of the H+-induced transformation of the spirolactam form into the open one. The process has been accompanied by the formation of a new long-wave absorption maximums at 527-529 nm and fluorescence bands at 557-560 nm. The compound containing 2-hydroxy-4-dimethylaminophenyl substituent has exhibited switching of emission at 429 nm in the presence of Zn2+ cations. This fluorescence has been quenched selectively by the CN- anions. Copper(II) cations have caused the transformation of the spiro structure of rhodamine bearing 2-hydroxy-3-tert-butylbenzofuran group into the open form. The isomerization has been accompanied by chromogenic naked-eye effect: change in the solution color from pale yellow to pink and the appearance of fluorescence at 556 nm. The observed ion-induced spectral transformations can be used in the design of optical and fluorescent chemosensors.