Flash Photolysis Studies on Photocatalytic Systems.
Light can initiate chemical reactions in molecules. These reactions often follow a path different from thermal reactions, and can yield different products. Photons of visible or UV light carry enough energy to induce reactions that are endothermic or have to overcome a large barrier. The sun provides visible and UV light as a environment friendly and renewable energy source. Photoreactions can employ this energy not only as a storage for chemical energy, but also for chemical synthesis of compounds that are otherwise difficult to obtain.
In normal photochemical reactions the reactant is also the chromophore, i.e. it absorbs the light that induces the reaction. In a photocatalytic reaction, the functions of reactant and chromophore are separated (see scheme below): The chromophore CH is excited by a photon and perfoms a first reaction step, e.g. abstracting an electron from the substrate molecule S. Then the oxidized S undergoes the desired reaction. Finally, the initial state of the chromophore is reestablished by a thermal reaction with a second reactant R. In this reaction sequence the chromophore acts like a catalyst, and the photon is consumed like a chemical reagent.
Figure: Left: Schematic view of a photocatalytic reaction: The photocatalyst C absorbs a photon and is excited to the excited state C*. This undergoes electron transfer with a substrate molecule S. The oxidized substrate radical reacts thermally to the desired product P. The photocatalyst is thermally reoxidized by compound R. Right: Accumulation of the catalyst (FMNH radical) in the absence of the reactant R (molecular oxygen).
In the DFG Graduiertenkolleg (Research Training Group)
GRK 1626 several research teams from organic, inorganic, and physical chemistry cooperate in the design and understanding of new photochemical systems. Our part in this is the study of the reaction mechanisms. We do this by transient absorption measurements following a short laser flash. In this way we follow the concentration time profiles of the reaction intermediates in the time range from 50 ns to 10 ms. We use a special and unique experimental setup built around a streak camera (see below). This enables us to measure simultaneously the concentration time profiles (kinetics) and the spectra of the intermediates.
Method: Transient absorption spectroscopy with a streak camera.
The transient spectral data form a matrix A whose columns represent wavelengths and the rows represent time. Simple pump-probe experiments measure a single data point of the matrix A for each excitation cycle. The efficiency can be improved by measuring a complete time profile for each excitation cycle at one fixed wavelength, which corresponds to the measurement of a column of the matrix A. In order to obtain the full matrix A this measurement must be repeated at different wavelengths, keeping all other parameters of the apparatus constant. Alternatively, the whole transient absorption spectrum can be measured for fixed delay times with a short white light pulse, a monochromator, and an optical multichannel detector. This corresponds to the measurement of a row of the matrix A. The time needed for the determination of the complete matrix A will be similar to that for the column-wise measurements. We measure the complete data matrix A for each excitation cycle by a double multiplex method, using the combination of a continuous white probe light, a monochromator, and a streak camera. This results in a multiplex factor of more than 100, i.e., the same information can be obtained with 1% of the material needed by the conventional techniques.
The sample is excited by a light pulse which is generated either directly by a Nd:YAG laser (532 nm or 355 nm), or by an optical parametric oscillator (OPO) pumped by the third harmonic of the Nd:YAG. The pulse duration is typically 8 ns, and the energy typi-cally 10 mJ. The probe light is generated by a pulsed xenon flash lamp which produces a white light pulse with very flat intensity profile of ca. 2 ms duration. The lenses used in the original setup for the probe light path were replaced by toroidal mirrors. This makes the light path achromatic. An additional aperture and shutter in the probe light path re-duces the white light falling on the sample outside of the time window of the measure-ment. After passing through the sample the probe light is dispersed by a spectrograph and then temporally analyzed by the streak camera. The latter consists of two parts: the streak tube which produces an image on a phosphor screen, and a CCD camera which digitizes this image.
The example above shows results for the LOV1-C57G protein domain of the green alga Chlamydomonas reinhardtii. In this domain, the photoactive cysteine C57 of the wild type is replaced by glycine. This introduces a "short-cut" into the photocycle which now proceeds from the triplet state of the flavin back to the ground state with a time constant of ca. 30 µs. The left side of the figure shows a false color representation of the two-dimensional spectral data. The fluorescence of flavin is clearly visible as the narrow blue horizontal stripe. The bleaching of the flavin ground state appears as a green vertical stripe centered at 450 nm. Red colored stripes on both sides indicate transient absorption by the triplet state. A global fit to these data sets reveals two lifetime components. One is short with fitted decay times between 40 - 60 ns. In the 20 µs time window 40 ns correspond to a single pixel, hence the decay of this component is not resolved. The corresponding spectrum, shown in right part of the figure, is negative throughout the whole wavelength range with peaks at 495 and 520 nm. It is assigned to the fluorescence. The second component is fitted by a monoexponential decay with a lifetime of 30 µs. It is due to the decay of the thiplet state (peak at 715 nm) back to the ground state (negative peak at 445 nm).
T. Langenbacher, D. Immeln, B. Dick, T. Kottke, Microsecond Light-induced Proton Transfer to Flavin in the Blue Light Sensor Plant Cryptochrome, J. Amer. Chem. Soc. 2009, 131, 14274-14280.