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

Abstract

The ultrafast photoinjection and subsequent relaxation steps of the indoline dye D149 were investigated in detail for a mesoporous electrodeposited ZnO thin film and compared with experiments on sintered TiO 2 and ZrO 2 thin films, all in contact with air, using pump-supercontinuum probe (PSCP) transient absorption spectroscopy in the range 370-770 nm. D149 efficiently injects electrons into the ZnO surface with time constants from r70 fs (time-resolution-limited) up to 250 fs, without the presence of slower components. Subsequent spectral dynamics with a time constant of 20 ps and no accompanying change in the oscillator strength are assigned to a transient Stark shift of the electronic absorption spectrum of D149 molecules in the electronic ground state due to the local electric field exerted by the D149 + radical cations and conduction band electrons in ZnO. This interpretation is consistent with the shape of the relaxed PSCP spectrum at long times, which resembles the first derivative of the inverted steady-state absorption spectrum of D149. In addition, steady-state difference absorption spectra of D149 + in solution from spectroelectrochemistry display a bleach band with distinctly different position, because no first-order Stark effect is present in that case. Interference features in the PSCP spectra probably arise from a change of the refractive index of ZnO caused by the injected electrons. The 20 ps component in the PSCP spectra is likely a manifestation of the transition from an initially formed bound D149 + -electron complex to isolated D149 + and mobile electrons in the ZnO conduction band (which changes the external electric field experienced by D149) and possibly also reorientational motion of D149 molecules in response to the electric field. We identify additional spectral dynamics on a similar timescale, arising from vibrational relaxation of D149 + by interactions with ZnO. TiO 2 exhibits similar dynamics to ZnO. In the case of ZrO 2 , electron injection accesses trap states, which exhibit a substantial probability for charge recombination. No Stark shift is observed in this case. In addition, the spectroelectrochemical experiments for D149 + in dichloromethane and acetonitrile, which cover the spectral range up to 2000 nm, provide for the first time access to its complete D 0 -D 1 absorption band, with the peak located at 1250 and 1055 nm, respectively. Good agreement is obtained with results from DFT/TDDFT calculations of the D149 + spectrum employing the MPW1K functional.

Photoinduced ultrafast dynamics of the triphenylamine-based organic sensitizer D35 on TiO2, ZrO2 and in acetonitrile

“…We will show that the influence of the local Stark effect on the transient spectra is quite drastic, similar to the previous study for D149-ZnO. 23 Therefore, this finding has considerable consequences for the interpretation of transient spectral changes in any pump-probe experiment on dye-sensitized mesoporous semiconductor oxide surfaces.…”

“…23,[25][26][27] For the experiments in acetonitrile, a flow cell (1 mm path length) with a 200 mm thick quartz entrance window was used. Thin films were mounted on an x/y translation stage and moved in a plane perpendicular to the probe light path to expose a fresh spot for every laser shot.…”

“…The details of the setup were described previously: we used an optically transparent thin-layer electrochemical (OTTLE) cell with 200 mm path length, 28 connected to a potentiostat (Metrohm Autolab PGSTAT 101). 23 The cell consists of a Pt-disk working electrode, a Pt-gauze auxiliary electrode and an Ag-wire as a pseudoreference electrode. A Varian Cary 5000 spectrometer was employed to record absorption spectra over the spectral range 200-2000 nm.…”

“…Previously, we found very good agreement between the calculated and experimental electronic transitions for the indoline radical cation D149 + , when the MPW1K functional was employed. 23 In the current study, we used the same approach for both the D35 + radical cation and neutral D35, and results for the first 20 (cation) and 5 (neutral) electronic transitions are included in Fig. 2 as red columns.…”

“…One exception is a previous study of the indoline dye D149 attached to electrodeposited ZnO thin films, where the local Stark effect was characterized on timescales from femto-to several hundred picoseconds. 23 A transient build-up of the Stark shift with a time constant of ca. 20 ps was observed, which was assigned to the break-up and separation of initially formed D149 + -electron complexes, and possibly also reorientational motion of ground-state D149 in the local electric field.…”