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  • Mechanism of Exciplex Decay: The Quantum Yields and the Rate Constants of Radical Ion Formation from Exciplexes with Partial Charge Transfer
    D.N. Dogadkin, E.V. Dolotova, I.V. Soboleva, M.G. Kuzmin, V.F. Plyusnin, I.P. Pozdnyakov, V.P. Grivin, E. Vauthey, P. Brodard and O. Nicolet
    High Energy Chemistry, 38 (6) , 2004, p392-400
    DOI:10.1023/B:HIEC.0000048237.12132.26 | unige:3249 | Abstract | Article PDF (141k) | Article PDF
The dynamics of exciplex and radical ion formation was studied in donor–acceptor systems with G * et > –0.1 eV. It was shown that the quenching of excited singlet states of aromatic molecules by electron donors in polar solvents led to the formation of radical ions via exciplex dissociation resulting to complete charge separation. Intersystem crossing and internal conversion into the ground state (back electron transfer) compete with this process. The quantum yields and the rate constants of the radical ion formation were measured.
The electron transfer quenching dynamics of excited perylene and cyanoperylene in various donating solvents has been investigated by using ultrafast fluorescence up-conversion and multiplex transient grating. The strongly nonexponential fluorescence decays have been analyzed by using the orientational model described in the first article of this series (J. Phys. Chem. A 2003, 107, 5375). It appears that the solvent dependence of the quenching dynamics is strongly connected to the number of surrounding donor molecules enabling ultrafast electron transfer. This number depends mainly on the driving force for electron transfer, on steric interactions, and on the occurrence of dipole−dipole interactions with the acceptor. The quenching product is an exciplex with a strong charge-transfer character. The complicated wavelength dependence of the fluorescence dynamics in the exciplex region, as well as the spectral dynamics observed in the transient grating data, is attributed to dipolar solvation, which leads to an increase of the charge-transfer character of the exciplex. The strong donor dependence of the exciplex lifetime is very similar to that reported earlier for the charge recombination time of geminate ion pairs in acetonitrile, and can be rationalized in terms of different intramolecular reorganization energies and electronic coupling constants.
A model of nonequilibrium charge recombination from an excited adiabatic state of a donor-acceptor complex induced by the nonadiabatic interaction operator is considered. The decay of the excited state population prepared by a short laser pulse is shown to be highly nonexponential. The influence of the excitation pulse carrier frequency on the ultrafast charge recombination dynamics of excited donor-acceptor complexes is explored. The charge recombination rate constant is found to decrease with increasing excitation frequency. The variation of the excitation pulse carrier frequency within the charge transfer absorption band of the complex can alter the effective charge recombination rate by up to a factor 2. The magnitude of this spectral effect decreases strongly with increasing electronic coupling.
The influence of the excitation pulse carrier frequency on the ultrafast charge recombination dynamics of excited donor-acceptor complexes has been explored both theoretically and experimentally. The theoretical description involves the explicit treatment of both the optical formation of the nuclear wave packet on the excited free energy surface and its ensuing dynamics. The wave packet motion and the electronic transition are described within the framework of the stochastic point-transition approach. It is shown that the variation of the pulse carrier frequency within the absorption band can significantly change the effective charge recombination dynamics. The mechanism of this phenomenon is analyzed and a semiquantitative interpretation is suggested. The role of the vibrational coherence in the recombination dynamics is discussed. An experimental investigation of the ultrafast charge recombination dynamics of two donor-acceptor complexes in valeronitrile also is presented. The decays of the excited state population were found to be highly nonexponential, the degree of non-exponentiality depending on the excitation frequency. For one complex, the charge recombination dynamics was found to slow down upon increasing the excitation frequency, while the opposite behavior was observed with the other complex. These experimental observations follow qualitatively the predictions of the simulations.
The fluorescence dynamics of perylene in the presence of tetracyanoethylene in acetonitrile was studied experimentally and theoretically, taking into consideration that the quenching is carried out by remote electron transfer in the Marcus inverted region. The initial stage was understood as a convolution of the pumping pulse with the system response accounting for the fastest (kinetic) electron transfer accompanied by vibrational relaxation. The subsequent development of the process was analyzed with differential encounter theory using different models of transfer rates distinguished by their mean square values. The single channel transfer having a bell-shaped rate with a maximum shifted far from the contact produces the ground state ion pair. It was recognized as inappropriate for fitting the quenching kinetics at moderate and long times equally well. A good fit was reached when an additional near contact quenching is switched on, to account for the parallel electron transfer to the electronically excited state of the same pair. The concentration dependence of the fluorescence quantum yield is well fitted using the same rates of distant transfer as for quenching kinetics while the contact approximation applied to the same data was shown to be inadequate.
An investigation of the ultrafast excited state dynamics of triporphyrin and hexaporphyrin arrays consisting of covalently linked Zn tetraphenylporphine (ZnTPP) and free base tetraphenylporphine (FbTPP) units is reported. The interchromophoric distance in the hexamers is of the order of 13 Å, while it varies from 26 to 70 Å in the trimers. These arrays exhibit several features that differ substantially from those of the monomeric units:  a broadening of the Soret band, a shortening of the S2 lifetime of the ZnTPP chromophores, and additional ultrafast decay components of the S1 fluorescence. In the hexaporphyrin arrays, most of these features are attributed to the presence of excitonic states that result from the strong coupling between the Bx,y transition dipoles. The time constants for S1 energy transfer between ZnTPP chromophores as well as between ZnTPP and FbTPP moieties, deduced from anisotropic and isotropic time-resolved fluorescence measurements, were found to be of the order of a few tens of picoseconds. Moreover, back energy transfer from the FbTPP to ZnTPP units is also observed. At high to moderate excitation intensity, S1−S1 annihilation becomes an important decay mechanism of the excited state population of the hexaporphyrins. In the triporphyrins, the differences relative to the monomer are ascribed to the interaction with the phenylacetylene linkers, which lifts the degeneracy of the S2 states. S2 and S1 energy transfer were found to take place in the triporphyrin with the shortest linker only. In the other triporphyrins, an efficient energy transfer from the linker to the porphyrin units was observed.
  • Femtosecond times-resolved studies on bimolecular electron transfer processes
    S. Pagès, B. Lang and E. Vauthey
    in "Femtochemistry and Femtobiology" M. Martin and J. T. Hynes Eds., Elsevier, 2004, p319
  • Influence of solute-solvent interactions on the quenching dynamics of perylene derivatives in an electron donating solvent
    A. Morandeira, A. Fürstenberg and E. Vauthey
    in "Femtochemistry and Femtobiology" M. Martin and J. T. Hynes Eds., Elsevier, 2004, p327
  • Effect of excitation wavelength on the charge recombination dynamics of excited donor acceptor complexes
    O. Nicolet, A.I. Ivanov and E. Vauthey
    in "Femtochemistry and Femtobiology" M. Martin and J. T. Hynes Eds., Elsevier, 2004, p331
  • Ultrafast Spectroscopic Investigation of the Charge Recombination Dynamics of Ion Pairs Formed upon Highly Exergonic Bimolecular Electron-Transfer Quenching: Looking for the Normal Region
    S. Pagès, B. Lang and E. Vauthey
    Journal of Physical Chemistry A, 108 (4) , 2004, p549-555
    DOI:10.1021/jp036796g | unige:3517 | Abstract | Article HTML | Article PDF
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).



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Eric Vauthey

Physical Chemistry Department - Sciences II - University of Geneva
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