Synthesis and properties of covalently linked phenyl bridged 3-pyrrolyl BODIPY-BODIPY dyads

Two different covalently linked phenyl-bridged 3-pyrrolyl BODIPY-BODIPY dyads were synthesized through a series of synthetic steps and thoroughly characterized and studied using HR-MS, 1D and 2D NMR spectroscopy, X-ray crystallography, absorption, steady state and time-resolved fluorescence, cyclic voltammetry, and DFT/TD-DFT techniques. The X-ray structure obtained for one of the dyads showed that both the BODIPY and 3-pyrrolyl BODIPY units were oriented almost perpendicular to each other and the bridging p-phenyl ring was oriented at an angle of 60.34 degrees relative to the BF2-dipyrrin core of the BODIPY unit resulting in the tilted structure of the dyad. Both dyads show two well separated absorption bands corresponding to BODIPY and 3-pyrrolyl BODIPY units. In these dyads, the BODIPY unit absorbs at higher energy and acts as an energy donor whereas the 3-pyrrolyl BODIPY unit absorbs at lower energy and acts as an energy acceptor. Steady-state and time-resolved fluorescence studies indicated the possibility of intramolecular singlet-singlet energy transfer from BODIPY to 3-pyrrolyl BODIPY units with good efficiency upon selective excitation of the BODIPY unit in both 3-pyrrolyl BODIPY-BODIPY dyads. DFT and TD-DFT studies were consistent with the experimental observations. Two different covalently linked 3-pyrrolyl BODIPY-BODIPY dyads were synthesized and their fluorescence studies indicated a possibility of singlet-singlet energy transfer from BODIPY to 3-pyrrolyl BODIPY unit.