Electron-Withdrawing BODIPY Dyes As Visible Light Absorber/Sensitizers on Semiconductor Oxide Surfaces

The synthesis, characterization, 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 nm 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 NHE, reveal reversible oxidations at 1.19 and 1.41 V and a reversible reduction at -0.59 V. On 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/T...