Conformational Restrictions in meso -(2-Thiazolyl)-BODIPYs: Large Stokes Shift and pH-Dependent Optical Properties

The design, synthesis, and characterization of a series of thiazole-appended boron-dipyrromethene (BODIPY) derivatives 1-4 is reported. The fluorescence emission colors of 1-4 were fine-tuned by modulating the dihedral angle between BODIPY and thiazole by judiciously varying the number of methyl substituents on both the BODIPY and thiazole moieties. Compounds 1 and 2 with fewer methyl substituents on the BODIPY core showed a large Stokes shift (≈110 nm) relative to derivatives 3 and 4 with more methyl substituents. In addition, the optical properties of 1 and 2 are sensitive to the pH of the medium. The experimentally observed results were corroborated with theoretical calculations.