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 (CBZ) and boron dipyrromethene (BODIPY) chromophores in the dyes, BODIPY(CBZ)2PO3H2 (8) and BODIPY(Tol)2PO3H2 (7), are described. The oxide-bound dyes have been explored as light absorbers in dye-sensitized photoelectrosynthesis cell (DSPEC) applications. The BODIPY-CBZ phosphonate ester (6) 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)2Br (4) in 0.1 M [nBu4N][PF6] 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 TiO2, a reversible one-electron oxidation appears for 7 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 SnO2/TiO2 electrodes provide direct evidence for excited-state electron injection into the conduction band of TiO2 following 590 nm excitation. A longer lifetime for 8+ compared to 7+ is consistent with extensive intramolecular charge separation between the CBZ and BODIPY units on the surface. Photoelectrochemical studies on 8 on a SnO2/TiO2 photoanode resulted in sustained photocurrents with current maxima of ∼200 μA/cm2 with hydroquinone added as a reductant under 1 sun (AM1.5 100 mW·cm–2) illumination at pH 4.5 in 0.1 M acetate buffer and 0.4 M LiClO4. On mixed SnO2/TiO2 electrode surfaces, with the added catalyst [Ru(Mebimpy)((4,4′-(OH)2PO-CH2)2bpy)(OH2)]2+ and chromophores 7 and 8, addition of 0.1 M benzyl alcohol resulted in sustained photocurrents of 12 and 35 μA/cm2, consistent with oxidation to benzaldehyde.