Jani Kallioinen

Photoinduced Ultrafast Dynamics of Ru(dcbpy)2(NCS)2-Sensitized Nanocrystalline TiO2 Films: The Influence of Sample Preparation and Experimental Conditions

In most of the previous ultrafast electron injection studies of Ru(dcbpy)2(NCS)2-sensitized nanocrystalline TiO2 films, experimental conditions and sample preparation have been different from study to study and no studies of how the differences affect the observed dynamics have been reported. In the present paper, we have investigated the influence of such modifications. Pump photon density, environment of the sensitized film (solvent and air), and parameters of the film preparation (crystallinity and quality of the film) were varied in a systematic way and the obtained dynamics were compared to that of a well-defined reference sample: Ru(dcbpy)2(NCS)2-TiO2 in acetonitrile. In some cases, the induced changes in the dynamics were uncorrelated to the electron injection process. High pump photon density (not in the linear response region) and exposure of the sensitized film to air altered the picosecond-time-scale kinetics considerably, and the changes were attributed mostly to degradation of the dye. In other cases, changes in the measured kinetics were related to the electron injection processes: reducing the firing temperature of the nanocrystalline film or making the film via electron beam evaporation (EBE) resulted in a decrease of the overall crystallinity of the film, and the electron injection slowed. In the sensitized EBE films, in addition to an increased contribution of triplet excited-state electron injection, a new electron transfer (ET) process with a time constant of 200 fs was observed.

Photochemical reactivity of halogen-containing ruthenium–dcbpy (dcbpy = 4,4′-dicarboxylic acid-2,2′-bipyridine) compounds, trans(Br)-[Ru(dcbpy)(CO)2Br2] and trans(I)-[Ru(dcbpy)(CO)2I2]

The ruthenium mono(dcbpy) (dcbpy = 4,4′-dicarboxylic acid-2,2′-bipyridine) complexes, trans(Br)-[Ru(dcbpy)(CO)2Br2] and trans(I)-[Ru(dcbpy)(CO)2I2], have been synthesized and structurally characterized. Both compounds show strong photochemical activity. Under illumination in acetonitrile the colour of the dye solutions changes and infrared spectra indicate an irreversible change, with two CO stretching bands disappearing from the spectra with concomitant appearance of a new CO stretching band, an indication of a loss of a CO ligand. Changes in the proton NMR spectra suggest that the release of the CO ligand is followed by reorganization of the halogen ligands and attachment of one solvent molecule leading to formation of the cis(X)-Ru(dcbpy)(CO)(CH3CN)X2] (X = Br, I) isomer. In the visible spectra, a new absorption band appears under illumination at 510 nm allowing observation of the kinetics of the reaction. The quantum yields of the reactions are 0.68 and 0.34 for the bromine and iodine complexes, respectively. The trans(I)-[Ru(dcbpy)(CO)2I2] compound shows a temperature dependent luminescence spectrum in the temperature range 77 to 116 K, with an activation energy of 850 cm−1. We assign this dependence to a thermal population of a secondary triplet size, slightly above the emitting state, that is capable of initiating the photochemical reaction and/or a non-radiative relaxation to the ground state. The redox properties of the starting materials and the photoreaction products were studied with cyclic voltammetry and the results are discussed with reference to the reaction mechanisms.

A dye-sensitized solar cell driven electrochromic device

A new dye-sensitized solar cell driven electrochromic device has been fabricated. The device consists of an electrochromic display and a solar cell in a single nanocrystalline film. The optimization of the electrochromic and the solar cell functions was carried out. An applied potential of 1.0 V was required for coloring and the best solar energy conversion efficiency 1.1% was achieved. The efficiency may be compared to an efficiency of 4.6% obtained in a similar dye-sensitized solar cell without the display property. Coloring and bleaching times of the device were less than one second and a transmittance change from 38.7% (bleached) to 15.9% (colored) at best was achieved. The optimization of the electrochromic property of the device lead to decreasing efficiency of the solar cell and vice versa.