Photoinduced Electron Transfer Dynamics of Cyclometalated
Ruthenium (II)–Naphthalenediimide Dyad at NiO Photocathode
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Abstract
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<sup>∧</sup>C<sup>∧</sup>N)(N<sup>∧</sup>N<sup>∧</sup>N)
(Ru[II]) chromophore and a naphthalenediimide (NDI) acceptor, where
N<sup>∧</sup>C<sup>∧</sup>N represents 2,2′-(4,6-dimethyl-phenylene)-bispyridine
and N<sup>∧</sup>N<sup>∧</sup>N represents 2,2′,6′,6″-terpyridine
ligand. When the dyad is dissolved in a CH<sub>3</sub>CN solution,
electron transfer to form the Ru(III)–NDI<sup>–</sup> occurs with a rate constant <i>k</i><sub>f</sub> = 1.1
× 10<sup>10</sup> s<sup>–1</sup> (τ<sub>f</sub> =
91 ps), and electron–hole pair recombines to regenerate ground
state with a rate constant <i>k</i><sub>b</sub> = 4.1 ×
10<sup>9</sup> s<sup>–1</sup> (τ<sub>b</sub> = 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<sup>+</sup>)–Ru(II)–NDI<sup>–</sup>] with a rate constant <i>k</i><sub>f</sub> = 9.1 ×
10<sup>11</sup> s<sup>–1</sup> (τ<sub>f</sub> = 1.1 ps).
The recovery of the ground state occurs with a multiexponential rate
constant <i>k</i><sub>b</sub> = 2.3 × 10<sup>9</sup> s<sup>–1</sup> (τ<sub>b</sub> = 426 ps). The charge
recombination rate constant is slightly slower than a reference cyclometalated
ruthenium compound (O25) with no NDI group (τ<sub>b</sub> =
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