Synthesis of a 5-Carboxy Indole-Based Spiropyran Fluorophore: Thermal, Electrochemical, Photophysical and Bovine Serum Albumin Interaction Investigations

In this study, we synthesized a spiropyran containing an electron-withdrawing carboxyl group in good yield by condensation of an aromatic aldehyde with enamine indole. The spiropyran absorbed at the ultraviolet region with a maximum at approximately 300 nm, demonstrating slight solvatochromism (~3 nm). A fluorescent emission around 360 nm was observed with a higher solvatochromic effect (~12 nm), indicating higher electronic delocalization in the excited state. The photoreversibility of the open and closed forms of spiropyran excited at 300 nm and 365 nm was not observed, indicating that the absence of the nitro group plays a fundamental role in this equilibrium. Theoretical calculations were also applied for better understanding the photophysics of these compounds. Electrochemical characterization revealed the values of the HOMO and LUMO energy levels at −1.89 eV (electron affinity) and −5.61 eV (ionization potential), respectively. Thermogravimetric analysis showed excellent thermal stability of the spiropyran, with 5% weight loss at approximately 250 °C. Finally, the photophysical features were used to explore the interaction of spiropyran with bovine serum albumin in a phosphate buffer solution, where a significant suppression mechanism was observed.