Molekulare Gymnastik : Rotationen, Schwingungen und chemische Reaktionen

Bei jeder chemischen Reaktion werden Bindungen gebrochen und andere neu geknüpft. Dabei ändert sich die Anordnung und eventuell Anzahl der Atome im Molekül. Voraussetzung hierfür sind Bewegungen der beteiligten Atome und Moleküle. Um chemische Umwandlungen in "Echtzeit" zu studieren, müssen Untersuchungen im Zeitbereich der Schwingungs- und Rotationsdynamik durchgeführt werden. Dazu nutzen Wissenschaftler des Instituts für Physikalische und Theoretische Chemie die Möglichkeiten der modernen Ultrakurzzeit-Lasertechnik

Ultrafast photoinduced electron transfer in coumarin 343 sensitized TiO2-colloidal solution

Photoinduced electron transfer from organic dye molecules to semiconductor nanoparticles is the first and most important reaction step for the mechanism in the so called “wet solar cells” [1]. The time scale between the photoexcitation of the dye and the electron injection into the conduction band of the semiconductor colloid varies from a few tens of femtoseconds to nanoseconds, depending on the specific electron transfer parameters of the system, e.g., electronic coupling or free energy values of donor and acceptor molecules [2–10]. We show that visible pump/ white light probe is a very efficient tool to investigate the electron injection reaction allowing to observe simultaneously the relaxation of the excited dye, the injection process of the electron, the cooling of the injected electron and the charge recombination reaction

Beyond Vibrationally Mediated Electron Transfer: Coherent Phenomena Induced by Ultrafast Charge Separation

Wave packet propagation succeeding electron transfer (ET) from alizarin dye molecules into the nanocrystalline TiO2 semiconductor has been studied by ultrafast transient absorption spectroscopy. Due to the ultrafast time scale of the ET reaction of about 6 fs the system shows substantial differences to molecular ET systems. We show that the ET process is not mediated by molecular vibrations and therefore classical ET theories lose their applicability. Here the ET reaction itself prepares a vibrational wave packet and not the electromagnetic excitation by the laser pulse. Furthermore, the generation of phonons during polaron formation in the TiO2 lattice is observed in real time for this system. The presented investigations enable an unambiguous assignment of the involved photoinduced mechanisms and can contribute to a corresponding extension of molecular ET theories to ultrafast ET systems like alizarin/TiO2.Comment: This work was supported by the German Research Foundation (DFG) (Hu 1006/6-1, WA 1850/6-1) and European Union projects FDML-Raman (FP7 ERC StG, contract no. 259158) and ENCOMOLE-2i (Horizon 2020, ERC CoG no. 646669

Discrimination between FRET and non-FRET quenching in a photochromic CdSe quantum dot/dithienylethene dye system

A photochromic Förster resonance energy transfer (FRET) system was employed to disentangle the fluorescence quenching mechanisms in quantum dot/photochromic dye hybrids. In the off-state of the dye the main quenching mechanism is FRET whereas the moderate quenching in the on-state is due to non-FRET pathways opened up upon assembly

Low temperature FTIR spectroscopy provides new insights in the pH-dependent proton pathway of proteorhodopsin

AbstractIn the presented study the low pH photocycle of proteorhodopsin is extensively investigated by means of low temperature FTIR spectroscopy. Besides the already well-known characteristics of the all-trans and 13-cis retinal vibrations the 77K difference spectrum at pH 5.1 shows an additional negative signal at 1744cm−1 which is interpreted as indicator for the L state. The subsequent photocycle steps are investigated at temperatures higher than 200K. The combination of visible and FTIR spectroscopy enabled us to observe that the deprotonation of the Schiff base is linked to the protonation of an Asp or Glu side chain — the new proton acceptor under acidic conditions. The difference spectra of the late intermediates are characterized by large amide I changes and two further bands ((−)1751cm−1/(+)1725cm-1) in the spectral region of the Asp/Glu ν(C=O) vibrations. The band position of the negative signature points to a transient deprotonation of Asp-97. In addition, the pH dependence of the acidic photocycle was investigated. The difference spectra at pH 5.5 show distinct differences connected to changes in the protonation state of key residues. Based on our data we propose a three-state model that explains the complex pH dependence of PR

Beyond vibrationally mediated electron transfer: interfacial charge injection on a sub-10-fs time scale

The electron transfer (ET) from organic dye molecules to semiconductor-colloidal systems is characterized by a special energetic situation with a charge transfer reaction from a system of discrete donor levels to a continuum of acceptor states. If these systems show a strong electronic coupling they are amongst the fastest known ET systems with transfer times of less than 10 fs. In the first part a detailed discussion of the direct observation of an ET reaction with a time constant of about 6 fs will be given, with an accompanying argumentation concerning possible artifacts or other interfering signal contributions. In a second part we will try to give a simple picture for the scenario of such superfast ET reactions and one main focus will be the discussion of electronic dephasing and its consequences for the ET reaction. We show that the actual ET process can be understood as a kind of dispersion process of the initially located electron into the colloid representing a real motion of charge density from the alizarin to the colloid

Ultrafast coherent oscillations reveal a reactive mode in the ring-opening reaction of fulgides

The ultrafast ring-opening reaction of photochromic fulgides proceeds via conical intersections to the ground state isomers involving activation barriers in the excited state. The coherent oscillations observed in the femtosecond transient absorption signal of a methyl-substituted indolylfulgide were analysed in the framework of vibrational wavepackets to expose a dominant low-frequency mode at similar to 80 cm(-1). The quantum chemical calculations in the relaxed excited state geometry of this fulgide revealed that the experimentally observed vibrational normal mode has a dominant contribution to the relevant ring-opening reactive coordinate

Ultrafast photoinduced electron transfer in coumarin 343 sensitized TiO2-colloidal solution

Photoinduced electron transfer from organic dye molecules to semiconductor nanoparticles is the first and most important reaction step for the mechanism in the so called “wet solar cells” [1]. The time scale between the photoexcitation of the dye and the electron injection into the conduction band of the semiconductor colloid varies from a few tens of femtoseconds to nanoseconds, depending on the specific electron transfer parameters of the system, e.g., electronic coupling or free energy values of donor and acceptor molecules [2–10]. We show that visible pump/ white light probe is a very efficient tool to investigate the electron injection reaction allowing to observe simultaneously the relaxation of the excited dye, the injection process of the electron, the cooling of the injected electron and the charge recombination reaction

Mechanistic Elucidation of the Hula-Twist Photoreaction in Hemithioindigo

The Hula-Twist (HT) photoreaction represents a fundamen- tal photochemical pathway for bond isomerizations and is defined by the coupled motion of a double bond and an adjacent single bond. This photoreaction has been suggested as the defining motion for a plethora of light-responsive chromophores such as retinal within opsins, coumaric acid within photoactive yellow protein, or vitamin D precursors, and stilbenes in solution. However, due to the fleeting character of HT photoproducts a direct experimental observation of this coupled molecular motion was severely hampered until recently. To solve this dilemma, the Dube group has designed a molecular framework able to deliver unambiguous experimental evidence of the HT photoreaction. Using sterically crowded atropisomeric hemithioindigo (HTI) the HT photoproducts are rendered thermally stable and can be observed directly after their formation. However, following the ultrafast excited state process of the HT photoreaction itself has not been achieved so far and thus crucial information for an elementary understanding is still missing. In this work, we present the first ultrafast spectroscopy study of the HT photoreaction in HTI and probe the competition between different excited state processes. Together with extensive excited state calculations a detailed mechanistic picture is developed explaining the significant solvent effects on the HT photoreaction and revealing the intricate interplay between productive isomerizations and unproductive twisted intramolecular charge transfer (TICT) processes. With this study essential insights are thus gained into the mechanism of complex multibond rotations in the excited state, which will be of primary importance for further developments in this field