Systematic Control of Structural and Photophysical Properties of π-Extended Mono- and Bis-BODIPY Derivatives

A series of π-extended mono- and bis-BODIPY (BODIPY=boron-dipyrromethene) derivatives, namely, benzo[a]phenanthrene-fused BODIPY (Phena-Mono-BDP), benzo[a]anthracene-fused BODIPY (Ant-Mono-BDP), and dibenz[a,h]anthracene (DBA)-bridged bis-BODIPY (Ant-Bis-BDP), were designed and synthesized to examine their structural, electrochemical, and photophysical properties. Single-crystal X-ray crystallographic analyses demonstrated the planar configuration of Ant-Mono-BDP, in contrast to that of nonplanar Phena-Mono-BDP, whereas Ant-Bis-BDP has a DBA-centered planar configuration and two terminal nonplanar units of BODIPYs. The oxidation and reduction potentials agree with the estimated energies obtained through DFT calculations. The localized HOMO and LUMO states suggested the intramolecular charge-transfer characteristics in these BODIPY derivatives. The absorption spectra of these compounds extended up to the near-IR region. Strong redshifted trends of fluorescence spectra were observed in Ant-Bis-BDP with increasing solvent polarity, as supported by the differences in dipole moments estimated from Lippert-Mataga plots. To evaluate the excited-state dynamics of these molecules, the fluorescence quantum yield (Φ_FL ) of Ant-Bis-BDP dramatically increased in the range from 0.05 to 0.86, with decreasing solvent polarity. Finally, the efficient two-photon absorption cross section of Ant-Bis-BDP (ca. 1200 GM at λ=1000 nm) was also obtained by considering the large π-extended structure (acceptor-donor-acceptor type).