Influence of surface protonation on the sensitization efficiency of porphyrin-derivatized TiO ₂

The influence of electrolyte pH on the sensitization efficiency of porphyrin-derivatized TiO ₂ was studied by photoelectrochemical and transient absorption measurements. The porphyrins 5-(4-carboxyphenyl)-10,15,20- trimesitylporphinatozinc(II) (1), 5-(4-carboxyphenyl)-10,15,20- trimesitylporphine (2), 5-(4-carboxyphenyl)-10,15,20- trimesitylporphinatoplatinum(II) (3), and 5-(4-dihydroxyphosphorylphenyl)-10,15, 20-trimesitylporphinatozinc(II) (4) were anchored to low-surface-area and nanocrystalline TiO ₂ films. The TiO ₂ conduction band edge potential (E _CB) shifted 59 ± 2 mV/pH, from -0.43 V vs Ag/AgCl _(aq) at pH 12 to +0.16 V vs Ag/AgCl _(aq) at pH 2. Excited-state potentials (E _1/2(P ^+/*)) of 1-4 ranged from -1.58 to -0.91 V vs Ag/AgCl _(aq), well negative of E _CB. Despite the thermodynamic favorability of electron injection, a 10-fold increase in sensitized photocurrent was measured for 1-4 upon acidification of the electrolyte from pH 10 to 4. Transient absorption data revealed that sensitization of nanocrystalline TiO ₂ by 1-4 depended on pH in an identical manner. A mechanism is proposed wherein protonation of a surface site is required for charge compensation of injected electrons. Thus, the magnitude of sensitized photocurrent is determined by surface protonation-deprotonation equilibria.