Electron-Withdrawing Boron Dipyrromethene Dyes As Visible Light Absorber/Sensitizers on Semiconductor Oxide Surfaces.

The synthesis, characterization, and electrochemical and photophysical properties of the phosphonate-derivatized carbazole () and boron dipyrromethene () chromophores in the dyes, BODIPY(CBZ)POH () and BODIPY(Tol)POH (), are described. The oxide-bound dyes have been explored as light absorbers in dye-sensitized photoelectrosynthesis cell (DSPEC) applications. The BODIPY-CBZ phosphonate ester () features a broad, intense UV-visible absorption spectrum with absorptions at 297 and 650 nm that arise from mixed transitions at the CBZ and BODIPY units. Electrochemical measurements on BODIPY(CBZ)Br () in 0.1 M [nBuN][PF] in dichloromethane, vs normal hydrogen electrode (NHE), reveal reversible oxidations at 1.19 and 1.41 V and a reversible reduction at -0.59 V. On indium tin oxide (ITO) and TiO, a reversible one-electron oxidation appears for at 0.86 and 0.90 V vs NHE in dichloromethane, respectively, which demonstrates the redox stability on metal oxide surfaces. The results of nanosecond transient absorption measurements on SnO/TiO electrodes provide direct evidence for excited-state electron injection into the conduction band of TiO following 590 nm excitation. A longer lifetime for compared to is consistent with extensive intramolecular charge separation between the CBZ and BODIPY units on the surface. Photoelectrochemical studies on on a SnO/TiO photoanode resulted in sustained photocurrents with current maxima of ∼200 μA/cm with hydroquinone added as a reductant under 1 sun (AM1.5 100 mW·cm) illumination at pH 4.5 in 0.1 M acetate buffer and 0.4 M LiClO. On mixed SnO/TiO electrode surfaces, with the added catalyst [Ru(Mebimpy)((4,4'-(OH)PO-CH)bpy)(OH)] and chromophores and , addition of 0.1 M benzyl alcohol resulted in sustained photocurrents of 12 and 35 μA/cm, consistent with oxidation to benzaldehyde.