Synthesis and photophysical properties of covalently liked boron dipyrromethene dyads

Abstract

Two covalently linked boron dipyrromethene (BODIPY) dyads containing meso-phenyl BODIPY and meso-furyl BODIPY units connected via meso-meso and meso-α positions were synthesized by a Pd(0) coupling reaction. The dyads are freely soluble in common organic solvents and their structures were confirmed by HR-MS, 1D and 2D NMR techniques. Absorption studies indicate that the meso-aryl BODIPY and the meso-furyl BODIPY absorb in two different regions and that the meso-furyl BODIPY absorbs at lower energy compared to the meso-aryl BODIPY. The steady state fluorescence studies carried out by exciting the meso-aryl BODIPY unit clearly indicated an efficient singlet–singlet energy transfer from the meso-aryl BODIPY unit to meso-furyl BODIPY unit in both dyads. Furthermore the meso-α linked BODIPY dyad (φ_f = 0.41) is more fluorescent than its corresponding BODIPY monomers whereas the meso-meso linked BODIPY dyad (φ_f = 0.017) is weakly fluorescent. This unexpected observation was tentatively attributed to the restricted rotation of the BODIPY units in the meso-α linked dyad resulting in enhancement of radiative transitions. The time-resolved fluorescence study also indicated that meso-α linked BODIPY dyad is more fluorescent with singlet state lifetime of 3.7 ns. The DFT studies carried out on dyads are in agreement with the experimental observations.