Synthesis and optoelectronic properties of bis-BODIPY-meso-phenyleneethynylene meso-phenyleneethynylene dimers

Three different bis-BODIPY-meso-phenyleneethynylene dimers were prepared in a straightforward manner following a sequence Liebeskind-Srogl- Stille cross-couplings, which allowed us to control the architecture of the final products. The phenyleneethynylene (PE) bridged bis-meso-BODIPYs, indistinctly of the ortho, meta or para substitution exhibit in dichloromethane, a main excitonic (S0-S1 electronic transition) peak at 503 +/- 1 nm, similar to that of the monomer meso-phenyl BODIPY, evidencing the poor electronic communication between the two BODIPYs. This is supported by theoretical study where modest energy splitting of the S0-S1 electronic transition was found and by voltammetry where voltammograms of para and meta dimers show similar redox process to that of the monomer. The fluorescence spectra are also quite similar, but the ortho dimer presents two emission bands. The fluorescence quantum yield (phi) is much lower for para (0.6 %) and meta (2.9 %) dimers with respect to the monomer (4.3 %) and to the ortho homologous (15.1 %). This is likely due to non-radiative losses derived from the free rotation of the two BODIPYs in meso that is restricted in the ortho substitution. Intramolecular charge transfer (ICT) through photoinduced symmetry breaking can also occur in this latter dimer according to the Weller equation using the voltammetric oxidation/reduction potentials, and to the fluorescence quenching in polar solvents. Noncovalent interactions studies indicate significant it-it interactions in the excited state that could promote excimer formation. For which, the two emission bands in this compound could be also due to a monomer-like ICT state and excimers.