Real-time observation of intersystem crossing induced by charge recombination during bimolecular electron transfer reactions (original) (raw)
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Russian Journal of Physical Chemistry B - RUSS J PHYS CHEM B, 2007
The two-stage model of photoinduced electron transfer includes an explicit description of photoexcitation and nonradiative charge transfer. The model takes into account intramolecular reorganization and Coulomb interaction between the electron being transferred and medium polarization fluctuations. We studied the influence of intramolecular electron density redistribution on the rate of photoinduced electron transfer in donor-acceptor complexes. Excitation pulse frequency and donor dipole moment variations in the excited state changed the nonthermal and thermal electron transfer rates by 2–3 orders of magnitude.
Journal of Physical Chemistry A, 1998
The dynamics of the intermediate generated upon diffusional electron transfer (ET) quenching of 9,10dicyanoanthracene by electron donors of varying oxidation potential in acetonitrile has been investigated using several transient grating techniques. With most of the donor/acceptor pairs studied, the transient grating spectrum cannot be differentiated from those of the free ions. Exciplex fluorescence, with the same lifetime as that of the ion pair, is observed with all donors. To extract from the measured kinetics the rate constant of exciplex dissociation, k dis EX , and of back ET, k BET EX , within these exciplexes, three different schemes have been considered. The best agreement is obtained by assuming that charge recombination predominantly takes place within the exciplex. The obtained k BET EX values are substantially different from the BET rate constants deduced indirectly from the free-ion yields and with a donor-independent rate constant of separation. For each class of donors, k BET EX exhibits a logarithmic free energy dependence with a slope of about-2 eV-1. Moreover, k dis EX is not constant but increases continuously with diminishing donor's oxidation potential.
Photochromic Control of Photoinduced Electron Transfer. Molecular Double-Throw Switch
Journal of the American Chemical Society, 2005
A molecular double-throw switch that employs a photochromic moiety to direct photoinduced electron transfer from an excited state donor down either of two pathways has been prepared. The molecular triad consists of a free base porphyrin (P) linked to both a C60 electron acceptor and a dihydroindolizine (DHI) photochrome. Excitation of the porphyrin moiety of DHI-P-C60 results in photoinduced electron transfer with a time constant of 2.3 ns to give the DHI-P •+ -C60 •charge-separated state with a quantum yield of 82%. UV (366 nm) light photoisomerizes the DHI moiety to the betaine (BT) form, which has a higher reduction potential than DHI. Excitation of the porphyrin of BT-P-C 60 is followed by photoinduced electron transfer with a time constant of 56 ps to produce BT •--P •+ -C60 in 99% yield. Isomerization of BT-P-C60 back to DHI-P-C60 may be achieved with visible light, or thermally. Thus, photoinduced charge separation originating from the porphyrin is reversibly directed down either of two different pathways by photoisomerization of the dihydroindolizine. The switch may be cycled many times.
The Journal of Physical Chemistry A, 2006
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.
EPR-detected photoinduced electron transfer in three structurally related molecular triads
Applied Magnetic Resonance, 2006
A comparative electron paramagnetic resonance (EPR) study has been performed on a series of structurally related molecular triads which undergo photoinduced electron transfer and differ one from the other in terms of the acceptor or donor moieties. The molecular triads, C-P-C6o , TTF-P-C6o and C-P-PF, share the same free-base tetraarylporphyrin (P) as the primary electron donor, which after light excitation initiates the electron transfer process, but differ either in terms of the electron acceptor (fullerene derivative, C6o , versus fluorinated free-base porphyrin, PF), or in terms of the final electron donor (carotenoid polyene, C, versus tetrathiafulvalene, TTF). AII these molecular triads can be considered artificial photosynthetic reaction centers in their ability to mimic several key properties of the reaction center primary photochemistry. Photoinduced charge separation and recombination have been followed by time-resolved EPR in a glass of 2-methyltetrahydrofuran and in the nematic phase of the uniaxial liquid crystal E-7. All the triads undergo photoinduced electron transfer, with the generation of charge-separated states in both the low-dielectric environment of the 2-methyltetrahydrofuran glass and in anisotropic E-7 medium. Different photochemical pathways have been recognized depending on the specific donor and acceptor moieties constituting the molecular triads. In the presence of the tetrathiafulvalene electron donor singlet-and triplet-initiated electron transfer routes are concurrently active. Recombination to the low-lying carotenoid triplet state occurs in the carotene-based triads, while singlet recombination is the only active route for the TTF-P-C6o triad, where a low-lying triplet state is lacking. Long-lived charge separation has been observed in the case of TTF-P-C6o: about 8 p.s for the singlet-born radical pair in the glassy isotropic matrix and about 7 ~ts for the triplet-born radical pair in the nematic phase of E-7. For all the molecular triads, a weak exchange interaction (J ~ I G) between the electrons in the final spin-correlated radical pair has been evaluated by simulation of the EPR spectra, providing evidence for superexchange electronic interactions mediated by the tetraarylporphyrin bridge.
Journal of Photochemistry and Photobiology C: Photochemistry Reviews, 2006
This review presents some of the efforts that have been made over the last decades to produce systems in which photo-excitation leads to one or more intramolecular electron transfer events ultimately resulting in a charge-transfer (CT) excited state with a relatively long lifetime. This process is generally considered as a mimic of natural photosynthesis and is not only of relevance in relation to solar energy conversion but also in relation to perspectives such as molecular information storage, molecular electronics, and molecular photonics. A long-lived CT state in general may be considered as a weakly coupled radical (ion)pair and in this review we focus especially on the consequences of the eventual electron spin correlation in that radical (ion)pair. If substantial spin-spin interaction is still present, such as in compact dyads, CT states can be assigned pure singlet or triplet configurations (1 CT, 3 CT) and as we demonstrate this configuration has significant influence on the CT lifetime because charge recombination from 3 CT is spin forbidden. For small spin-spin interaction such as is typical for CT states in which the radical sites are further removed from each other-e.g., in triads, tetrads, etc.,-rapid interconversion of 1 CT and 3 CT becomes possible especially via a hyperfine interaction (HFI) driven mechanism. This HFI driven mechanism is strongly influenced by external magnetic fields, which allows sensitive detection of the actual spin-spin interaction via magnetic field effects on the electron transfer kinetics, as well as via time-resolved EPR and field-dependent CIDNP. Examples of such studies on artificial multichromophoric electron transfer systems are presented and the results are discussed.