title: pH-dependent electron transfer from re-bipyridyl complexes to metal oxide nanocrystalline thin films.
abstract: Photoinduced interfacial electron transfer (ET) from molecular adsorbates to semiconductor nanoparticles has been a subject of intense recent interest. Unlike intramolecular ET, the existence of a quasicontinuum of electronic states in the solid leads to a dependence of ET rate on the density of accepting states in the semiconductor, which varies with the position of the adsorbate excited-state oxidation potential relative to the conduction band edge. For metal oxide semiconductors, their conduction band edge position varies with the pH of the solution, leading to pH-dependent interfacial ET rates in these materials. In this work we examine this dependence in Re(L(P))(CO)3Cl (or ReC1P) [L(P) = 2,2'-bipyridine-4,4'-bis-CH2PO(OH)2] and Re(L(A))(CO)3Cl (or ReC1A) [L(A) = 2,2'-bipyridine-4,4'-bis-CH2COOH] sensitized TiO2 and ReC1P sensitized SnO2 nanocrystalline thin films using femtosecond transient IR spectroscopy. ET rates are measured as a function of pH by monitoring the CO stretching modes of the adsorbates and mid-IR absorption of the injected electrons. The injection rate to TiO2 was found to decrease by 1000-fold from pH 0-9, while it reduced by only a factor of a few to SnO2 over a similar pH range. Comparison with the theoretical predictions based on Marcus' theory of nonadiabatic interfacial ET suggests that the observed pH-dependent ET rate can be qualitatively accounted for by considering the change of density of electron-accepting states caused by the pH-dependent conduction band edge position.
authors.affiliation: Department of Chemistry, Emory University, Atlanta, Georgia 30322, USA.
authors.forename: Chunxing
authors.initials: C
authors.lastname: She
authors.forename: Neil A
authors.initials: NA
authors.lastname: Anderson
authors.forename: Jianchang
authors.initials: J
authors.lastname: Guo
authors.forename: Fang
authors.initials: F
authors.lastname: Liu
authors.forename: Wan-Hee
authors.initials: WH
authors.lastname: Goh
authors.forename: Dai-Tao
authors.initials: DT
authors.lastname: Chen
authors.forename: Debra L
authors.initials: DL
authors.lastname: Mohler
authors.forename: Zhong-Qun
authors.initials: ZQ
authors.lastname: Tian
authors.forename: Joseph T
authors.initials: JT
authors.lastname: Hupp
authors.forename: Tianquan
authors.initials: T
authors.lastname: Lian