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ABSTRACT: Ultrafast photochemical processes can occur in parallel with the relaxation of the optically populated excited state toward equilibrium. The latter involves both intra- and intermolecular modes, namely vibrational and solvent coordinates, and takes place on timescales ranging from a few tens of femtoseconds to up to hundreds of picoseconds, depending on the system. As a consequence, the reaction dynamics can substantially differ from those usually measured with slower photoinduced processes occurring from equilibrated excited states. For example, the decay of the excited-state population may become strongly nonexponential and depend on the excitation wavelength, contrary to the Kasha and Vavilov rules. In this article, we first give a brief account of our current understanding of vibrational and solvent relaxation processes. We then present an overview of important classes of ultrafast photochemical reactions, namely electron and proton transfer as well as isomerization, and illustrate with several examples how nonequilibrium effects can affect their dynamics. Expected final online publication date for the Annual Review of Physical Chemistry Volume 64 is March 31, 2013. Please see http://www.annualreviews.org/catalog/pubdates.aspx for revised estimates.
Annual Review of Physical Chemistry 01/2013; · 14.13 Impact Factor
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Marina Fedoseeva,
Jakob Grilj,
Oksana Kel,
Marius Koch,
Romain Letrun,
Vesna Markovic,
Irina Petkova,
Sabine Richert,
Arnulf Rosspeintner,
Peter S Sherin,
Diego Villamaina, Bernhard Lang,
Eric Vauthey
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ABSTRACT: The activities of our research group in the field of photoinduced electron transfer reactions are discussed and illustrated by several examples.
CHIMIA International Journal for Chemistry 01/2011; 65(5):350-2. · 1.21 Impact Factor
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Angewandte Chemie International Edition 11/2008; 47(47):9044-8. · 13.45 Impact Factor
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ABSTRACT: Ultrafast infrared transient absorption spectroscopy is used to study the photoinduced bimolecular electron transfer reaction between perylene in the first singlet excited state and 1,4-dicyanobenzene in acetonitrile and dichloromethane. Following vibrational marker modes on both donor and acceptor sides in real time provides direct insight into the structural dynamics during the reaction. A band narrowing on a time scale of a few tens of picoseconds observed on the antisymmetric CN stretching vibration of the dicyanobenzene radical anion indicates that a substantial part of the excess energy is channeled into vibrational modes of the product, despite the fact that the reaction is weakly exergonic. An additional narrowing of the same band on a time scale of several hundreds of picoseconds observed in acetonitrile only is interpreted as a signature of the dissociation of the geminate ion pairs into free ions.
The Journal of Physical Chemistry A 01/2007; 110(51):13676-80. · 2.95 Impact Factor
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ABSTRACT: The ultrafast ground state recovery (GSR) dynamics of the radical cation of perylene, Pe(*+), generated upon bimolecular photoinduced electron transfer in acetonitrile, has been investigated using pump-pump-probe spectroscopy. With 1,4-dicyanobenzene as electron acceptor, the free ion yield is substantial and the GSR dynamics of Pe(*+) was found to depend on the time delay between the first and second pump pulses, Deltat(12), i.e., on the "age" of the ion. At short Deltat(12), the GSR dynamics is biphasic, and at Deltat(12) larger than about 500 ps, it becomes exponential with a time constant around 3 ps. With trans-1,2-dicyanoethylene as acceptor, the free ion yield is essentially zero and the GSR dynamics of Pe(*+) remains biphasic independently of Deltat(12). The change of dynamics observed with 1,4-dicyanobenzene is ascribed to the transition from paired to free solvated ion, because in the pair, the excited ion has an additional decay channel to the ground state, i.e., charge recombination followed by charge separation. The rate constants deduced from the analysis of these GSR dynamics are all fully consistent with this hypothesis.
The Journal of Physical Chemistry A 07/2006; 110(24):7547-53. · 2.95 Impact Factor
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ABSTRACT: The dynamic Stokes shift of coumarin 153 has been measured in two room-temperature ionic liquids, 1-(3-cyanopropyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and 1-propyl-3-methylimidazolium tetrafluoroborate, using the fluorescence up-conversion technique with a 230 fs instrumental response function. A component of about 10-15% of the total solvation shift is found to take place on an ultrafast time scale < 10 ps. The amplitude of this component is substantially less than assumed previously by other authors. The origin of the difference in findings could be partly due to chromophore-internal conformational changes on the ultrafast time scale, superimposed to solvation-relaxation, or due to conformational changes of the chromophore ground state in polar and apolar environments. First three-pulse photon-echo peak-shift experiments on indocyanine green in room-temperature ionic liquids and in ethanol indicate a difference in the inertial component of the early solvent relaxation of <100 fs.
The Journal of Physical Chemistry A 06/2006; 110(22):7028-34. · 2.95 Impact Factor
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ABSTRACT: The charge recombination dynamics of the ion pairs formed upon electron-transfer quenching of perylene by tetracyanoethylene in acetonitrile has been investigated using ultrafast fluorescence upconversion, transient absorption, and transient grating techniques. For this donor/acceptor pair, charge separation is highly exergonic (ΔGCS= −2.2 eV), but charge recombination is weakly exergonic (ΔGCR = −0.6 eV). It was found that for more than 90% of the ion pair population, charge recombination is ultrafast and occurs in less than 10 ps. This decay component could not be observed in a previous investigation with a lower time resolution. The results indicate that the primary quenching product is a contact ion pair and not a solvent-separated ion pair as generally assumed for highly exergonic electron-transfer quenching processes. A possible explanation for this apparent divergence is that the contact ion pair is initially formed in an electronic excited state. Only a very minor fraction of the ion pair population undergoes the slow charge recombination predicted by Marcus theory for weakly exergonic charge-transfer processes (normal region).
12/2003;
