Non-aqueous electrochemistry of rhodamine B acylhydrazone

Rhodamine B hydrazide and rhodamine B acylhydrazone are spirocyclic compounds that provide a versatile framework for developing chemical and electrochemical sensors with turn-on optical response. Herein, we explore how a simple conversion of rhodamine B hydrazide into rhodamine B acylhydrazone can change its electrochemical behavior. Non-aqueous electrochemistry of rhodamine B acylhydrazone is studied using cyclic voltammetry, in situ electron spin resonance spectroscopy, bulk electrolysis, spectrophotometry, and liquid chromatography/mass spectrometry. Cyclic voltammograms of the dye demonstrate two reversible electrooxidation waves peaked at 0.54 and 0.69 V (vs. Fc+/Fc). In situ electron spin resonance spectroscopy data suggest that the electrooxidation mechanism of rhodamine B acylhydrazone involves several free radical intermediates. Bulk electrolysis of rhodamine B acylhydrazone generates a variety of products in a potential-dependent manner. Although rhodamine B acylhydrazone and rhodamine B hydrazide have similar molecular structures, they exhibit differing electrochemical behavior. In particular, protection of the hydrazide moiety by the propan-2-ylidene substituent in rhodamine B acylhydrazone molecules drastically increases the reversibility of electrooxidation. The obtained results can be used for developing new sensors with desired electrochemical properties based on the derivatives of rhodamine B hydrazide.