A quantitative model of charge injection by ruthenium chromophores connecting femtosecond to continuous irradiance conditions

Abstract

A kinetic framework for the ultrafast photophysics of tris(2,2-bipyridine)ruthenium(II) phosphonated and methyl-phosphonated derivatives is used as a basis for modeling charge injection by ruthenium dyes into a semiconductor substrate. By including the effects of light scattering, dye diffusion, and adsorption kinetics during sample preparation and the optical response of oxidized dyes, quantitative agreement with multiple transient absorption datasets is achieved on timescales spanning femtoseconds to nanoseconds. In particular, quantitative agreement with important spectroscopic handles—the decay of an excited state absorption signal component associated with charge injection in the UV region of the spectrum and the dynamical redshift of a ∼500 nm isosbestic point—validates our kinetic model. Pseudo-first-order rate coefficients for charge injection are estimated in this work, with an order of magnitude ranging from 1011 to 1012 s−1. The model makes the minimalist assumption that all excited states of a particular dye have the same charge injection coefficient, an assumption that would benefit from additional theoretical and experimental exploration. We have adapted this kinetic model to predict charge injection under continuous solar irradiation and find that as many as 68 electron transfer events per dye per second take place, significantly more than prior estimates in the literature.

Water oxidation by a dye-catalyst diad in natural sunlight: timing and coordination of excitations and reactions across timescales of picoseconds to hours

“… 5 In that work, the Ru II –Ru III (OH 2 ) diad was used as the starting state, and spectroscopic measurements were used to identify species present under irradiation in yellow light at 100 mW cm −2 for 600 s, which mainly excites the dye directly into its triplet state. 13 The species observed by resonance Raman spectroscopy were assigned to Ru II –Ru II (OH 2 ), Ru III –Ru III (OH 2 ) and Ru II –Ru IV (O), and with additional rather intense features in the spectra potentially arising from a peroxide species. This suggests that the diad only executed one catalytic cycle under those experimental conditions, because the Ru II –Ru II (OH 2 ) resting state should have been rapidly oxidized to start a new cycle.…”

“…Photoexcitations and charge injection under solar ux are sporadic due to its diffuse nature. 12,13 The only stage in the water oxidation cycle that does not involve a chemical reaction is the rst one, where Ru II -Ru II (OH 2 ) is converted to Ru II -Ru III (OH 2 ). Fig.…”

“…The kinetics of photophysical processes including charge injection that have been extracted using simulations of transient absorption data to represent the dye, and have been reported in full elsewhere. [11][12][13] Briey, the experimental ultrafast transient dye photophysics for a family of Ru polypyridyl dyes in methanol or acetonitrile solutions have been reproduced using detailed kinetics models, and the kinetics have been used to predict excitations under steady state solar irradiation. Using the molecular schemes, the photophysics for these dyes attached to Zr and Ti oxide surfaces have been determined, including the charge injection rate coefficients.…”

“…and therefore we focus on a well-studied model diad system couples a Ru-based dye with a Ru-based catalyst (Fig. 1), where detailed mechanistic data are available for both the dye photophysics [11][12][13][14][15] and the catalytic redox cycle. 5,[16][17][18][19][20][21] This catalyst is not efficient for water oxidation especially at low pH, 21 indeed superior catalysts are known, 3,22 however it and its close analogs are the only ones for which there is sufficient information available to construct a physically and chemically detailed, predictive model.…”