Photoinduced Electron Transfer Dynamics of Cyclometalated Ruthenium (II)–Naphthalenediimide Dyad at NiO Photocathode

Both forward and backward electron transfer kinetics at the sensitizer/NiO interface is critical for p-type dye-sensitized photocathodic device. In this article, we report the photoinduced electron transfer kinetics of a Ru­(II) chromophore–acceptor dyad sensitized NiO photocathode. The dyad (O26) is based on a cyclometalated Ru­(N^∧C^∧N)­(N^∧N^∧N) (Ru­[II]) chromophore and a naphthalenediimide (NDI) acceptor, where N^∧C^∧N represents 2,2′-(4,6-dimethyl-phenylene)-bispyridine and N^∧N^∧N represents 2,2′,6′,6″-terpyridine ligand. When the dyad is dissolved in a CH₃CN solution, electron transfer to form the Ru­(III)–NDI^– occurs with a rate constant k_f = 1.1 × 10¹⁰ s^–1 (τ_f = 91 ps), and electron–hole pair recombines to regenerate ground state with a rate constant k_b = 4.1 × 10⁹ s^–1 (τ_b = 241 ps). When the dyad is adsorbed on a NiO film by covalent attachment through the carboxylic acid group, hole injection takes place first within our instrument response time (∼180 fs) followed by the subsequent electron shift onto the NDI to produce the interfacial charge-separated state [NiO­(h⁺)–Ru­(II)–NDI^–] with a rate constant k_f = 9.1 × 10¹¹ s^–1 (τ_f = 1.1 ps). The recovery of the ground state occurs with a multiexponential rate constant k_b = 2.3 × 10⁹ s^–1 (τ_b = 426 ps). The charge recombination rate constant is slightly slower than a reference cyclometalated ruthenium compound (O25) with no NDI group (τ_b = 371 ps). The fast formation of interfacial charge separated state is a result of ultrafast hole injection resulting in the reduced form of sensitizer, which provides a larger driving force for NDI reduction. The kinetic study suggests that Ru­(II) chromophore–acceptor dyads are promising sensitizers for the NiO photocathode devices.