The electron transfer (ET) quenching dynamics of excited perylene (Pe), cyanoperylene (PeCN), methanolperylene (PeOH), and methylperylene (PeMe) in N,N-dimethylaniline (DMA) has been investigated using ultrafast fluorescence up-conversion. Measurements of the rotational dynamics of PeCN and PeMe in nonpolar and polar inert solvents using optically heterodyned polarization spectroscopy are also presented. The fluorescence decay in DMA is strongly nonexponential and about 10 times faster with PeCN than with the other electron acceptors. The quenching dynamics has been analyzed with a model distinguishing three types of donor molecules surrounding the acceptor: those with optimal orientation for ET and those requiring orientational or translational diffusion prior to ET. According to this model, which can account for the whole fluorescence decay, the faster quenching dynamics of PeCN is not due to a larger ET rate constant, but to a larger number of donor molecules, typically three to four, with an optimal orientation. This is explained by the effect of dipole-dipole interaction between PeCN and the donor molecules, which favors mutual orientations with a large electronic coupling. With the other acceptors, this interaction is either not present or does not lead to ET active geometries. The occurrence of this interaction is substantiated by the rotational dynamics measurements.
The excited-state dynamics of the radical cations of perylene (PE •+ ), tetracene (TE •+ ), and thianthrene (TH •+ ), as well as the radical anions of anthraquinone (AQ •-) and tetracenequinone (TQ •-), formed by γ irradiation in low-temperature matrices (PE •+ , TH •+ , AQ •-, and TQ •-) or by oxidation in sulfuric acid (PE •+ , TE •+ , and TH •+ ) have been investigated using ultrafast pump-probe spectroscopy. The longest ground-state recovery time measured was 100 ps. The excited-state lifetime of PE •+ is substantially longer in low-temperature matrices than in H 2 SO 4 , where the effects of perdeuteration and of temperature on the ground-state recovery dynamics indicate that internal conversion is not the major decay channel of PE •+ *. The data suggest that both PE •+ * and TE •+ * decay mainly through an intermolecular quenching process, most probably a reversible charge transfer reaction. Contrarily to AQ •-*, TQ •-* exhibits an emission in the visible which, according to theoretical calculations, occurs from an upper excited state.
A study of the dynamics of ground-state recovery of the perylene
radical cation (Pe•+), of perylene
radical
anion (Pe•-), and of anthraquinone radical
anion (AQ•-) is reported. In boric
acid glass, the excited-state
lifetime of Pe•+ is 35 ± 3 ps, while in
concentrated sulfuric acid, it is smaller than 15 ps, the time
resolution
of the experimental setup. The excited-state lifetime of
Pe•+, Pe•-, and
AQ•- generated by photoinduced
intermolecular electron-transfer reaction in MeCN is shorter than 15
ps. In the case of Pe•-, the
uncomplete
ground-state recovery is ascribed to the occurrence of electron
photoejection. The free ion yield in the
intermolecular electron-transfer reaction between
9,10-dicyanoanthracene (DCA) and two electron
acceptors
was measured in a two-pulse experiment, where the second pulse excited
the ensuing DCA•-. This
excitation
has no influence on the magnitude of the free ion yield, indicating a
short excited-state lifetime of
DCA•-*
relative to the time scale of back electron transfer and ionic
dissociation. A red emission, ascribed to the
fluorescence of protonated Pe, was detected in boric acid glass and
sulfuric acid. No fluorescence that could
be clearly ascribed to Pe•+* could be
observed.
The solvation dynamics of an organic dye, IR140, in methanol, ethanol, and in a series of six alkanenitriles
has been investigated using the transient grating technique. In all solvents, the dynamics exhibit ultrafast,
almost solvent-independent, components ascribed to inertial solvation, and a slower viscosity-dependent
component, due to diffusive solvation. The relative amplitudes of these components depend on both the solvent
and on the wavelength at which the experiment is performed. The contribution of inertial motion increases
with decreasing size of the solvent molecules and with decreasing wavelength. It appears that diffusive motion
is associated with a loose solvent shell, while inertial motion dominates when the solvation layer is dense.
The effect of excess excitation energy on the rotational dynamics of rhodamine 6G in the ground and the first singlet excited state has been investigated in series of n-alcohols and alkanenitriles using the picosecond polarization grating technique. In nitriles, the reorientation times are the same for excitation at the S1 ← S0 and S2 ← S0 transitions, and no state dependence could be detected. In alcohols, the rotational dynamics of rhodamine 6G in the excited state is about 25% faster when formed with 1.15 eV excess excitation energy. This effect is ascribed to a decrease of the hydrodynamic volume due to dissociation of solute/solvent hydrogen bond following intramolecular vibrational redistribution. An accompanying perturbation of the solvent shell structure caused by the fast local temperature jump is not excluded
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