Synthesis and photophysical properties of covalently linked boron dipyrromethene dyads

Two covalently linked boron dipyrromethene (BODIPY) dyads containing meso-phenyl BODIPY and meso-furyl BODIPY units connected via meso-meso and meso-alpha 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-alpha. linked BODIPY dyad (phi(f)=0.41) is more fluorescent than its corresponding BODIPY monomers whereas the meso-meso linked BODIPY dyad (phi(f)=0.017) is weakly fluorescent. This unexpected observation was tentatively attributed to the restricted rotation of the BODIPY units in the meso-alpha linked dyad resulting in enhancement of radiative transitions. The time-resolved fluorescence study also indicated that meso-alpha 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. (C) 2011 Elsevier Ltd. All rights reserved.