Effects of five membered aromatic heterocycles at the meso-position on the electronic properties of 3-pyrrolyl BODIPY

The first examples of 3-pyrrolyl boron dipyrromethenes (3-pyrrolyl BODIPY) containing five membered aromatic heterocycles such as pyrrole, thiophene and furan in place of a six membered aryl group at the meso-position were synthesized under simple reaction conditions. The presence of a five membered aromatic heterocycle in place of a six membered aryl group at the meso-position significantly alters the electronic properties of 3-pyrrolyl BODIPY as reflected in their spectral, photophysical and electrochemical properties. The X-ray structure solved for meso-furyl 3-pyrrolyl BODIPY showed that the appended pyrrole is more deviated from the mean plane of the BODIPY core compared to meso-aryl 3-pyrrolyl BODIPY. However, the structure also revealed that the meso-furyl group is more in plane with the BODIPY core and participates in p-delocalization unlike meso-aryl 3-pyrrolyl BODIPY in which the meso-aryl group is almost in perpendicular orientation with the BODIPY core. The absorption studies indicated that the meso-heterocyclic 3-pyrrolyl BODIPYs showed bathochromic shifts with maximum shifts noted for mesofuryl 3-pyrrolyl BODIPY compared to meso-aryl BODIPY. The meso-heterocyclic 3-pyrrolyl BODIPYs were weakly fluorescent and stable under electrochemical reduction conditions.