Influence of Surface Protonation on the Sensitization Efficiency of Porphyrin-Derivatized TiO₂

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.