Distance Dependent Electron Transfer at TiO<sub>2</sub> Interfaces Sensitized with Phenylene Ethynylene Bridged Ru<sup>II</sup>–Isothiocyanate Compounds
Patrik
G. Johansson
Andrew Kopecky
Elena Galoppini
Gerald J. Meyer
10.1021/ja402193f.s002
https://acs.figshare.com/articles/media/Distance_Dependent_Electron_Transfer_at_TiO_sub_2_sub_Interfaces_Sensitized_with_Phenylene_Ethynylene_Bridged_Ru_sup_II_sup_Isothiocyanate_Compounds/2409655
Excess electrons
present in semiconductor nanocrystallites generate
a significant electric field, yet the role this field plays in molecular
charge transfer processes remains poorly understood. Three ruthenium
bipyridyl <i>cis</i>-Ru(bpy)(LL)(NCS)<sub>2</sub> compounds,
where LL is a 4-substituted bpy, with zero, one, or two phenylene
ethynylene bridge units, were anchored to mesoporous nanocrystalline
TiO<sub>2</sub> thin films to specifically quantify interfacial charge
transfer with chromophores designed to be set at variable distances
from the surface. Injection of electrons into TiO<sub>2</sub> resulted
in a blue shift of the metal-to-ligand charge transfer absorption
consistent with an underlying Stark effect. The electroabsorption
data were used to quantify the electric field experienced by the compounds
that decreased from 0.85 to 0.22 MV/cm as the number of OPE spacers
increased from 0 to 2. Charge recombination on the 10<sup>–8</sup>–10<sup>–5</sup> s time scale correlated with the magnitude
of the electric field with an apparent attenuation factor β
= 0.12 Å<sup>–1</sup>. Slow components to charge recombination
observed on the 10<sup>–4</sup>–10<sup>–1</sup> s time scale that were unaffected by temperature, irradiance, or
the bridge units present on the molecular sensitizer were attributed
to electron tunneling between TiO<sub>2</sub> acceptor states. The
photocurrent efficiencies of solar cells based on these compounds
decreased markedly when the bridge units were present on the sensitizer.
Iodine was found to form adducts with all three compounds, <i>K</i> = 1.8 ± 0.2 × 10<sup>4</sup> M<sup>–1</sup>, but only significantly lowered the excited state injection yield
for those that possessed the bridge units.
2013-06-05 00:00:00
TiO 2 Interfaces Sensitized
sensitizer
electron
distance Dependent Electron Transfer
compound
charge transfer processes
2. Charge recombination
mesoporous nanocrystalline TiO 2
OPE
MV
attenuation factor β
phenylene ethynylene bridge units
TiO 2 acceptor states
injection
bridge units
LL
time scale