Excited-State Electron Transfer from Ruthenium-Polypyridyl Compounds to Anatase TiO₂ Nanocrystallites: Evidence for a Stark Effect

Abstract

Photophysical studies were performed with [Ru(dtb)(2)(dcb)](PF(6))(2) and cis-Ru(dcb)(dnb)(NCS)(2,) where dtb is 4,4'-(C(CH(3))(3))(2)-2,2'-bipyridine, dcb is 4,4'-(COOH)(2)-2,2'-bipyridine, and dnb is 4,4'-(CH(3)(CH(2))(8))(2)-2,2'-bipyridine), anchored to anatase TiO(2) particles (∼15 nm in diameter) interconnected in a mesoporous, thin film (∼10 μm thick) immersed in Li(+)-containing acetonitrile electrolytes. Pulsed-laser excitation resulted in rapid, nonquantitative excited-state injection into TiO(2) with a rate constant that could not be time-resolved, k(inj) > 10(8) s(-1), to yield an interfacial charge-separated state. Return of this state to ground-state products displayed observation-wavelength-dependent kinetics due to charge recombination and a second process. The second process occurred in parallel and was assigned to a transient Stark effect created by the electric field originating from the electrons in TiO(2) on ruthenium sensitizers that had not undergone excited-state injection. The kinetics for this processes were well modeled by a stretched exponential function. The impact of this field on the metal-to-ligand charge transfer excited-state of Ru(dtb)(2)(dcb)(2+) or the oxidized form of cis-Ru(dcb)(dnb)(NCS)(2) were also investigated. Unambiguous identification of a Stark effect on the excited-state sensitizers was accomplished through fluence-dependent measurements. The possible influence of the electric field on the oxidized sensitizers was at best speculative. The unique relative orientation of the electric field and sensitizer afforded by the nanocrystal geometry resulted in unidirectional shifts in the absorption and photoluminescence spectra of the Ru(II) coordination compounds. On the basis of the magnitude of the shift, it was estimated that a transient field as large as 2.7 MV/cm was generated upon excited-state injection of electrons in TiO(2) at concentrations relevant to an operational dye-sensitized solar cell.

Ultrafast dynamics of the indoline dye D149 on electrodeposited ZnO and sintered ZrO2 and TiO2 thin films

“…We find evidence for the transient build-up of a local Stark effect, [28][29][30] where the electric field exerted by the D149 + radical cations and injected electrons influences the ground-state absorption spectrum of D149. In addition, undulatory structure in the transient spectra can be traced back to injected electrons modifying the optical response of the thin film.…”

“…This leads us to the conclusion that these spectral dynamics are likely dominated by the ''local Stark effect'' proposed by Cappel et al 28 and Meyer and co-workers. 29,30 Further support comes from the study of Cappel et al 28 They showed that PIA spectra of D149 on TiO 2 were dominated by the firstorder Stark effect: for the limiting case of aligned D149 molecules and an electric field acting normal to the surface, the change in absorbance (DOD) should be proportional to the first derivative of the steady-state absorption spectrum of D149. 28 The scaled derivative of the inverted steady-state absorption spectrum is depicted in Fig.…”

Spectroscopy and Photophysics in Cyclometalated RuII–Bis(bipyridyl) Complexes

“…The widespread investigation of Ru II -polypyridyl complexes over the past 30 years largely has roots in their impressive photophysical and redox properties, which make them amenable for adaptation to a myriad of technologies including dye-sensitized solar cells (DSSCs), [1][2][3][4][5][6][7] solar fuels photochemistry, [8] light-emitting electrochemical cells, [9] photoluminescence sensors, [10] biophotonics, [11,12] photochromics, [13][14][15][16][17][18] water-oxidation catalysis, [19] low-power photon upconversion, [20][21][22][23] as well as fundamental studies in photoinduced electron and energy transfer. [24][25][26][27][28][29] Interestingly, the related class of Ru II molecules that incorporate one or more cyclometalating ligands into their structure were only sparingly investigated over the same time frame.…”

Comparative study on photovoltaic properties of imidazole-based dyes containing varying electron acceptors in dye-sensitized solar cells

“…The ruthenium dyes produce overall photo-electrical conversion efficiency of higher than 10% due to their broad photoresponse area [3][4][5]. However, the manufacturing costs and environmental issues of the ruthenium sensitizers limit their large-scale application.…”