Transient Absorption Measurements and Data Analysis

Femtosecond UV–vis–NIR transient absorption spectra were collected using a Helios transient absorption setup (Ultrafast Systems).⁵⁴ The excitation beam was generated by a short-pulse titanium–sapphire oscillator (Mai-Tai, Spectra Physics, 70 fs), followed by a high-energy titanium–sapphire regenerative amplifier (Spitfire Ace, Spectra Physics, 100 fs, 1 kHz). The train of pulses exciting the sample was reduced to a 0.5 kHz repetition rate using a chopper. The 803 nm beam was split to form two beams: (1) pump (λ_exc = 803 nm) and (2) white light continuum probe pulses generated in a sapphire plate (440–780 nm).⁵⁴ The remaining 803 nm photons in the probe pulse were filtered out by a BG 38 cutoff filter placed before the sample to avoid the additional excitation of the sample. The instrument response function was approximately 200 fs wide. The pump pulse energy was approximately 1 μJ at the sample position and this relatively high intensity did not introduce any unwanted nonlinear effects. All the experiments were performed at room temperature. The data were acquired in a ∼3 ns time window. For each delay time t, the ΔA(t) was calculated from recorded 1000 probe spectra (500 spectra with the pump and 500 spectra without the pump). The whole kinetics were collected twice, once with gradually increasing delay times between the pump and the probe and after that with gradually decreasing times, in order to ensure there were no systematic changes in the samples during the experiments. The 3 ns width of the experimental time window was smaller than the values of the slowest lifetimes obtained from fitting the experimental results. For this reason the nanosecond components reported below should be treated with caution as very approximate estimation of the slow charge recombination kinetics.

The results were corrected for the spectral chirp of the white-light continuum using SurfaceXplorer software (Ultrafast Systems). Glotaran software was used to perform global and target analyses.⁵⁵ Steady-state absorption spectra were collected using a Hitachi-1900 UV–vis spectrophotometer.