Influence of Acid on Ring-Opening, Fluorescence, Aggregation, Electrochemical Oxidation, and C–C Bond Coupling of Spiropyran Derivatives
Ishana Kathuria, Sweety Rani, Rajiv K. Srivastava, Arvind Kumar, Satish Kumar
Abstract
Two benzothiazolinic spiropyran derivatives differently substituted at phenolic segment were synthesized to study the effect of acid on their ring-opening and electrochemical oxidation properties. Upon protonation, conventional spiropyrans undergo C spiro –O bond breakage, resulting in the formation of their ring-open protonated cisoid merocyanine form, which rearranges itself to its stable transoid merocyanine. Benzothiazolinic spiropyrans have also displayed pH-controlled photochromism, with the spiro moiety being opened by trifluoroacetic acid and reverted to spiro form on neutralization with triethylamine. It was demonstrated that the increase in the concentration of trifluoroacetic acid (TFA) influences the rate of ring-opening permitting its identification by 1 H NMR, 2D NMR, UV Visible spectroscopy, and single-crystal X-ray crystallography. The aggregation-induced emission characteristics of benzothiazolinic spiropyrans were also explored using fluorescence and DLS measurements. Moreover, the thia-indoline segment of spiropyran undergoes electrochemical oxidation followed by C–C coupling due to the presence of electron-rich nitrogen and sulfur atoms. The influence of acid on electrochemical oxidation and substituent was also investigated. Further, the density functional theory (DFT) based studies were performed to study the electrochemical oxidation and acidochromism in benzothiazolinic spiropyrans. Time-dependent DFT estimated variables also provided additional information regarding the acidochromism process.