Spectroscopic and ¹O₂ Sensitization Characteristics of a Series of Isomeric Re(bpy)(CO)₃Cl Complexes Bearing Pendant BODIPY Chromophores

Two new Re(I)bipyridyltricarbonyl chloride complexes, Re(BB3)(CO)₃Cl and Re(BB4)(CO)₃Cl, featuring BODIPY groups appended to the 5,5'- or 6,6'-positions of the bipyridine ligand, respectively, were synthesized as structurally isomeric compliments to a previously reported 4,4'-substituted homologue, Re(BB2)(CO)₃Cl. X-ray crystal structures of the compounds show that the 4,4'-, 5,5'-, and 6,6'-substitution patterns place the BODIPY groups at progressively shorter distances of 9.43, 8.39, and 5.56 Å, respectively, from the complexes' Re centers. The photophysical properties of the isomeric complexes were investigated to ascertain the manner in which the heavy rhenium atom might induce intersystem crossing of the pendant BODIPY moieties positioned at progressively shorter through-space distances. Electronic absorption spectroscopy revealed that the three metal complexes retain the strong visible absorption features characteristic of the bpyBODIPY (BB2-BB4) ligands; however, the fluorescence of the parent borondipyrromethane appended ligands is attenuated by more than an order of magnitude in Re(BB2)(CO)₃Cl and Re(BB3)(CO)₃Cl and by more than two orders of magnitude in Re(BB4)(CO)₃Cl. Furthermore, phosphorescence from Re(BB4)(CO)₃Cl is observed under a nitrogen atmosphere, consistent with highly efficient ISC to the triplet-excited state. Singlet oxygen sensitization studies confirm that all three complexes produce singlet oxygen with quantum yields that increase as the distance of the BODIPY groups to the heavy rhenium center is decreased. The trends observed across the series of rhenium complexes with respect to emission and ¹O₂ sensitization properties can be rationalized in terms of the varied distal separation between the metal center and BODIPY groups in each system.