Intramolecular and Lateral Intermolecular Hole Transfer at the Sensitized TiO₂ Interface

Abstract

Characterization of the redox properties of TiO2 interfaces sensitized to visible light by a series of cyclometalated ruthenium polypyridyl compounds containing both a terpyridyl ligand with three carboxylic acid/carboxylate or methyl ester groups for surface binding and a tridentate cyclometalated ligand with a conjugated triarylamine (NAr3) donor group is described. Spectroelectrochemical studies revealed non-Nernstian behavior with nonideality factors of 1.37 ± 0.08 for the Ru(III/II) couple and 1.15 ± 0.09 for the NAr3(•+/0) couple. Pulsed light excitation of the sensitized thin films resulted in rapid excited-state injection (k(inj) > 10(8) s(-1)) and in some cases hole transfer to NAr3 [TiO2(e(-))/Ru(III)-NAr3 → TiO2(e(-))/Ru(II)-NAr3(•+)]. The rate constants for charge recombination [TiO2(e(-))/Ru(III)-NAr3 → TiO2/Ru(II)-NAr3 or TiO2(e(-))/Ru(II)-NAr3(•+) → TiO2/Ru(II)-NAr3] were insensitive to the identity of the cyclometalated compound, while the open-circuit photovoltage was significantly larger for the compound with the highest quantum yield for hole transfer, behavior attributed to a larger dipole moment change (Δμ = 7.7 D). Visible-light excitation under conditions where the Ru(III) centers were oxidized resulted in injection into TiO2 [TiO2/Ru(III)-NAr3 + hν → TiO2(e(-))/Ru(III)-NAr3(•+)] followed by rapid back interfacial electron transfer to another oxidized compound that had not undergone excited-state injection [TiO2(e(-))/Ru(III)-NAr3 → TiO2/Ru(II)-NAr3]. The net effect was the photogeneration of equal numbers of fully reduced and fully oxidized compounds. Lateral intermolecular hole hopping (TiO2/Ru(II)-NAr3 + TiO2/Ru(III)-NAr3(•+) → 2TiO2/Ru(III)-NAr3) was observed spectroscopically and was modeled by Monte Carlo simulations that revealed an effective hole hopping rate of (130 ns)(-1).

Lateral Intermolecular Self-Exchange Reactions for Hole and Energy Transport on Mesoporous Metal Oxide Thin Films

“…In retrospect, the term "conduction band" was unfortunate as it was unclear whether the redox active states in TiO 2 were conduction band electrons or electrons trapped in surface states (perhaps as localized Ti(III) states) residing energetically within the forbidden energy gap; however, a more recent study indicated that conduction band electrons were indeed involved. 46 In any event, the data provided evidence that the redox active states in TiO 2 were important to the Ru(dcbq 0/− ) self-exchange. The reduction of the coordinated dcbq ligand occurred at more positive potentials than does TiO 2 reduction such that electrons injected into TiO 2 from vibrationally hot excited states were trapped on ground-state sensitizers, producing long-lived states that recombined through selfexchange reactions that are described further below.…”

“…At TiO 2 interfaces, the electric field contributions appear to be most significant; however, the number of experimental studies remains quite limited and deserves further study. 46,53 Light-Driven Self-Exchange. There have been several occasions where lateral self-exchange reactions were initiated with light.…”

Kinetic pathway for interfacial electron transfer from a semiconductor to a molecule

“…A further complication is that observed rate constants may not unambiguously report on the interfacial electron transfer of interest 18 . The acute sensitivity of this reaction to the number of TiO 2 electrons present in the nanocrystallite 17 and the very weak driving-force dependence reported by most [19][20][21][22] , but not all 23 , has led many to conclude that observed rate constants report only on the diffusional encounters of the TiO 2 electrons with the molecular acceptors [24][25][26][27] . Clifford et al found that interfacial electron transfer was most easily understood when the physical location of the acceptor frontier molecular orbitals was taken into account 19 , which has been exploited to optimize interfacial electron transfer with highly doped degenerate semiconductors 28 .…”

Facile Synthesis of a Luminescent Material, PAN@{SiO2}n, Having a Simultaneous Binding Capacity of High and Low Oxidation States: HOMO and LUMO, Quantum-mechanical Descriptor of Break-through Capacity

“…S9 and S10) HOMO-LUMO becomes well separated. Thus, both the charge and hole are transferred as per Scheme S1a, 47,48 rather than Scheme S1b (suggests a high chance of charge recombination), 49 and suggests a longer life time of the ejected electron placed at the SiO2 moiety. The expected spectral lines for this kind of charge/hole transfer occurs at the UV-vis range (3.18 (392.6 nm), 3.59 (348 nm), 1.99 (835.2 nm), 1.73 (724.5 nm), 2.5 (483.9 nm), 1.74 (720.3 nm) eV) corresponding to their HOMO-LUMO band gap (Fig.…”