Synthesis, structure, and spectral and electrochemical properties of new visible to NIR absorbing 3-pyrrolyl BODIPY derivatives

We report the synthesis, characterization, and studies of novel 3-pyrrolyl BODIPY-based Schiff base products 3-6 and 3-pyrrolyl BODIPY-based benzo[<i>d</i>]thiazol-2-yl derivatives 7-8. The Schiff base compounds 3-6 were synthesized <i>via</i> condensation of α-formyl 3-pyrrolyl BODIPY with various amine derivatives, while the Knoevenagel condensation products 7-8 were obtained by reacting α-formyl 3-pyrrolyl BODIPY with 2-(benzo[<i>d</i>]thiazol-2-yl) acetonitrile and bis(benzo[<i>d</i>]thiazol-2-yl) methane, respectively. The compounds were thoroughly characterized by using HR-MS, 1D and 2D NMR spectroscopy, and X-ray crystallography for two compounds. The crystal structures revealed distinct conformational differences between the Schiff base product 3 and the Knoevenagel condensation product 8. In compound 3, the appended pyrrole was oriented towards the BF₂-dipyrrin core and placed almost in the same plane, while in 8, the pyrrole was inverted, deviated from the BF₂-dipyrrin plane, and aligned with the bis(benzothiazolyl) moiety, which adopted a transoid configuration. The Schiff base compounds 3-6 exhibited absorption bands in the region of 610-665 nm, whereas the benzo[<i>d</i>]thiazol-2-yl derivatives (7 and 8) showed enhanced optical properties with a red shifted absorption band that extended into the NIR region. These structural modifications of the 3-pyrrolyl BODIPY chromophore enable precise tuning of their electronic, absorption, and emission properties from the red to the NIR region. Furthermore, we also synthesized a 3-pyrrolyl BODIPY-Re(I) complex 8-Re(I) using bis(benzo[<i>d</i>]thiazol-2-yl)-3-pyrrolyl BODIPY 8. In the 8-Re(I) complex, the bis(benzothiazolyl) moiety adopted a cisoid configuration with a bite angle of 41.60°, and the Re(I) center exhibited a distorted octahedral geometry with a boat-like conformation. In the 8-Re(I) complex, the Re(I) was coordinated to three axial CO ligands, two nitrogen atoms from the bis(benzothiazolyl) units, and one chloride atom. DFT and TD-DFT studies corroborated our experimental findings, providing deeper insights into these compounds' structural, photophysical, and electronic properties. Overall, this study demonstrates the versatility of 3-pyrrolyl BODIPY derivatives in tuning absorption properties from the visible to the NIR region and their potential in forming stable Re(I) chelates, highlighting their potential in the development of NIR fluorophores and coordination chemistry.