C₃-Symmetric Positional Isomers of BODIPY Substituted Triazines: Synthesis and Excited State Properties

A series of meso- O-aryl functionalized BODIPY trimers positioned along the C₃-symmetric axis of triazine ring have newly been synthesized to probe the ground and excited state intramolecular type interactions between the BODIPY entities within the trimer. The developed synthetic strategy resulted in BODIPY trimers in good yields. The electron rich, meso- O-aryl functionalized BODIPYs revealed larger HOMO-LUMO gap and higher Stokes shift and fluorescence lifetimes compared to the traditional BODIPY derivatives having an aryl group attached at the meso position. The optical absorption, steady-state fluorescence, and electrochemical studies revealed weak, if any, intramolecular type interactions among the BODIPY entities within the trimer and the central triazine unit to be both photo- and redox-salient. The possibility of singlet-singlet energy migration among the BODIPY entities was investigated using time-resolved emission and femtosecond transient absorption studies. Excitation of a BODIPY entity in the trimers led to successful formation of ¹BODIPY*, which populated the ³BODIPY* via intersystem crossing. Among the three trimers, although very weak, only trimer 8 revealed excitation transfer to some extent. The present findings suggest that the meso- O-aryl functionalized BODIPYs due to their superior fluorescence properties are better probes to build light energy harvesting supramolecular oligomeric systems and for other applications such as sensing and imaging.