ESIPT-rhodol derivatives with enhanced Stokes shift: Synthesis, photophysical properties, viscosity sensitivity and DFT studies

Abstract

Three rhodol derivatives, namely spirolactamized phenanthrene-imidazole, diphenyl-imidazole and benzothiazole substituted N, N-diethylamine rhodols were designed, synthesized and characterized. Their photophysical properties were studied in spirocyclic and open form from non-polar to polar solvents. The spirocyclic form exhibited larger Stokes shifts (50–260 nm) due to presence of ESIPT process whereas, open form showed small Stokes shift (10–40 nm) due to the lack of protons. Positive solvatochromism was observed for both the spirocyclic and open forms which is well supported by the linear (Lippert-Mataga and Mac-Rae) and multi-linear (Kamlet-Taft and Catalan parameters) analysis. Solvent polarizability (dSP) is the major factor responsible for red shift in absorption/emission and larger Stokes shift for spirocyclic form. Polarity graphs and charge transfer descriptors are in good relation with Generalized Mulliken-Hush (GMH) parameters. From the solvatochromic data we observed that open forms show good TICT characteristics as compared to their respective spirocyclic forms. Open form is highly sensitive to viscosity as compared to spirocyclic form in a mixture of polar-protic solvents (EtOH:PEG 400). The experimental results are well correlated theoretically using Density Functional Theory (DFT) computations.