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AbstractLight controlled on- and off-switching of intramolecular energy and charge transfer is demonstrated in donor-switch-acceptor (D-S-A) supermolecules. The supermolecules are designed based on our experience on donor-bridge-acceptor (D-B-A) systems. Switching function is achieved by incorporating optically bistable photochromic fulgides and diarylethenes and proved via fs transient absorption. For both transfer processes, the transfer function is suppressed in the closed form of the switching sub-unit.

Optically-detected magnetic resonance (ODMR)-experiments at zero field were performed on the lowest triplet state of anthracene single crystals monitoring the delayed fluorescence (DF) as well as the phosphorescence (P) at 1.2 K. At low RF-power levels the P-ODMR and DF-ODMR signals are almost identical and can be attributed to at leat two X-traps with the following ZFS-parameters: D1= 0.06967(3), |E1| = 0.00793(3), D2= 0.06940(3) and |E2| = 0.00808(3). At RF-levels exceeding the threshold of the trap signals by about 5 orders of magnitude, DF-ODMR signals from excitons are observed. Their ZFS-parameters show clearly, that the motion is mainly between translationally non-equivalent oriented anthracene molecules [D*= −0.004256(1) cm −1and E*= 0.03311(1) cm −1]. ODMR-linewidth analysis for optical excitation directly into one of the two Davydov components of the first triplet exciton band indicate, that radiationless interbranch transitions between both components can populate k≠ 0 states in the lower band with band energies higher than kT. The thermalization time within this band can be estimated to be about 10 −7s.

The electron spin relaxation rates T1e−1and T2e−1of the radical anion salt (2,5-dimethyl-DCNQI)2Li are measured temperature- and angular dependent, respectively, at ν= 9.8 GHz. The temperature dependence can be explained by a smooth transition from motional narrowing to spin exchange. A more pronounced transition at 60 K leads to spin pairing. With further decreasing temperature the correlation time of the spin motion becomes longer to reach at 35 K τ= ωL−1= 10−10s. Below that temperature T1e−1becomes smaller than T2e−1which increases: at 4.5 K T1e−1is 0.67 μs−1(T1e= 1500 ns) and T2e−1= 15 μs−1(T2e= 67 ns). In the temperature range 300 – 35 K T1eis shorter than T2e, which demands an anisotropic fluctuating field with a spectral density: J(ω= ωL) > J(ω= 0). The angular dependencies of T1e−1and T2e−1cannot be explained by the symmetry of dipolar spin-spin interaction along the DCNQI stacks and the anisotropy of the g-factor, solely. We have to assume contributions from adjacent stacks, as well.

Traps of low concentration in molecular crystals are used as probes for studying energy transfer processes. β-halogenated naphthalene molecules in naphthalene crystals are a good system to study both the geometrical and the electronic structure of the traps. Direct trap to trap energy transfer already at relatively low concentrations is observed. Triplet exciton transport is temperature dependent, the mobility increases with decreasing temperature. This is the result of an analysis of the temperature dependence of sensitized delayed fluorescence in naphthalene-anthracene mixed crystals. Many different exciton interaction processes at high excitation densities have been studied. All attempts to observe exciton molecules or condensation of excitons in molecular crystals have failed so far.

AbstractExcitonic properties of the excited triplet state in single crystals of diphenylpolyenes all-trans(1, 4-diphenyl)-1, 3-butadiene (DPB) and all-trans(1, 6-diphenyl)-1, 3, 5-hexatriene (DPH) are derived from T1←S0photoexcitation spectra at low temperatures.The 0-0-transition is split into 2 (DPH monoclinic) and 4 (DPB orthorhombic) Davydov components in zero-field. Measurements with an applied external magnetic field made accessible the respective fine-structure components and their assignment to the principal axes of the fine-structure tensor. In the case of DPB orthorhombic the determination of the excitonic D* and E* fine-structure parameters was possible due to the small linewidths occuring in the spectra.The intensity ratios of the Davydov components were evaluated for different polarizations of the excitation light with respect to the crystal axes. The conclusion is drawn that in both DPH and DPB the transition moment is oriented mainly perpendicular to the molecular plane and that the total symmetry of the T1-state is Au.The vibrational structure in the photoexcitation spectra was assigned by using selectively deuterated crystal samples for comparison. Since the vibrations involving the phenyl endgroup are absent in the spectra of DPH and carry only very weak intensity in the spectra of DPB it is concluded that the T1-excitation energy is localized mainly on the polyene chain.