时间分辨光谱在光催化和光反应动力学中的应用

Rate models and the determination of the species associated spectra of the intermediate states
The data evaluation on the basis of single spectral traces has a number of disadvantages: – Inferior signal to noise ratio – Effects of spectral diffusion, e.g. caused by solvation or vibrational cooling result in additional non-exponential contributions to the decay signal – Contributions of similar time constants cannot be clearly distinguished – No complete spectral information of intermediate states is obtained
UV/Vis absorption and fluorescence spectra of riboflavin tetraacetate (left). Difference spectrum of the S1 state of riboflavin tetraacetate (bottom) 1 ps after excitation at λ = 480 nm.
Time resolved spectroscopy in photocatalysis
UV/Vis absorption spectroscopy: More than just ε!
Normalized absorption of 4-(3′-butenyloxy)quinolone in trifluorotoluene at different concentrations.
Scheme of a state-of-the-art ultrafast pump-probe spectrometer
Spectroscopy on the ns to μs time scale
Simplified scheme of pump probe setup for the measurement of TA spectra on time scales from ns to hundreds of μs.
Jablonski diagram of the example chromophore
Jablonski diagram of the reductive quenching of the S1 state of riboflavin tetraacetate (RFTA). The rate 1kET is controlled by diffusion of RFTA and methoxybenzyl alcohol (MBA).
Diffusion fit function with an additional exponential decay according to Equation 16.35 applied on TA data of the reductive quenching of excited RFTA by MBA.
Costs of photocatalysis: The reaction quantum yield
Exponential fit applied to the transient signal of the stimulated emission. The main time range of hundreds of ps is augmented by a 1.5 ps component discussed below. Upper panel: experimental data and fit. Lower panel: residuum on an expanded scale.
Negative solvatochromism of eosin Y (2′,4′,5′,7′-tetrabromo-fluorescindianion, left) and positive solvatochromism of riboflavin tetraacetate (right). The empiric values of the polarity of the solvents on the Reichardt scale are 1.00 for H2O, 0.65 for ethanol (EtOH), 0.46 for acetonitrile (MeCN), and 0.44 for dimethyl sulfoxide (DMSO)
时间分辨光谱

1999年诺贝尔化学奖授予埃及出生的科学 家艾哈迈德· 泽维尔（AhmedH.Zewail），以 表彰他应用超短激光闪光成照技术观看到 分子中的原子在化学反应中如何运动，从 而有助于人们理解和预期重要的化学反应， 为整个化学及其相关科学带来了一场革命。
Spectroscopy on the fs to ps time scale
With regards to the investigation of photocatalytic processes,
especially the following questions are of specific interest: – Which are the intermediate steps of the photocatalytic reaction? – Is the catalytic reaction singlet or triplet chemistry? – How does diffusion contribute to the reaction dynamics? – Are intramolecular charge transfer processes involved? – Are catalyst-substrate-complexes formed and which impact has the complex formation on the electronic properties of a molecule?
Diffusion fit function (Equation 16.34, red line) and extended diffusion fit function (Equation 16.35, blue line) applied to the transient signal of the stimulated emission.
16.2 Time-Resolved spectroscopic methods from fs to μs to elucidate photocatalytic processes
The change of absorption of the excited state with respect to the ground state absorption can be either positive (ΔOD > 0) or negative (ΔOD < 0): Excited state absorption (d state > absorption of ground state Ground state bleach (GSB), ΔOD < 0 Absorption of excited state < absorption of ground state Stimulated emission (SE), ΔOD < 0 Stimulated emission amplifies probe light within the spectral region of the fluorescence Product absorption (PA), ΔOD > 0 Newly formed species will absorb in general and their absorption will differ from the absorption spectra of the reactants
Optical absorption spectra of ground state (a), excited singlet state (b) and lowest triplet state (c) of an example chromophore. Since the spectra ε(λ) describe the wavelength dependent absorption properties of each state they are named species associates spectra (SAS).