A. Grupp

Hyperfine sublevel correlation (hyscore) spectroscopy: a 2D ESR investigation of the squaric acid radical

Abstract We propose a two-dimensional pulsed ESR experiment and demonstrate its capabilities in correlating hyperfine sublevels belonging to the same electron spin.

Transient ESR observation of triplet-soliton pairs in a conjugated polymer single crystal

Abstract We have observed directly by pulsed ESR the formation and decay of a triplet state in a polydiacetylene single crystal after photoexcitation at 337 nm. The unusual spin polarization properties of the observed triplet suggest the formation of a tripletsoliton pair.

Pulsed EPR on the photoexcited triplet state of C60 fullerene

Abstract We report on the observation of two different triplet states in photoexcited C 60 fullerene molecules. The following fine structure parameters were determined from spectral fits to the triplet powder spectrum: | D 1 | = 0.0117 cm −1 (12.5 mT), | E 1 | = 0.0014 cm −1 (1.5 mT) and | D 2 | = 0.0047 cm −1 (5 mT), | E 2 | = 0.0 cm −1 . Below 10 K mainly triplet 1 was observed, whereas triplet 2 appeared only with increasing temperature. It is thermally activated with an activation energy Δ E ≈ 17 meV. Due to motional narrowing the parameters D 1 and E 1 decrease slightly at higher temperatures. Depopulation kinetics as well as spin-lattice relaxation rates were determined.

Design, synthesis and assembly of new thiophene-based molecular functional units with controlled properties

Abstract Individual aspects in the field of ‘molecular electronics’ are addressed using new materials which are based on conjugated thiophenes as key building blocks. Among then, functionalized polythiophenes represent prototype compounds in which the three-dimensional network of ‘molecular wires’ can interact with the specific functional groups. Corresponding structurally defined oligothiophenes serve as model compounds and allow the correlation of various properties with structure. In addition, novel properties and superior processibility of these oligomers lead to interesting applications. Light emitting diodes and metal/organic/metal nanostructures are constructed by evaporation of thin oligomer films. A clear dependence of electroluminescence and electrical transport properties on chain length is found. Finally, the supramolecular assembly of functionalized (oligo)thiophenes is achieved by oriented adsorption on single crystal surfaces and self-organization in highly ordered LB films.

Zero-field-splitting in the lowest triplet state of C60

Abstract Zero-field-splitting parameters D and E for the fullerene C 60 in its Jahn-Teller distorted triplet state were calculated and compared to recent experimental results. The E value vanishes due to symmetry, while we obtained a non-zero D value of D = −0.009 cm −1 in the D 5d distorted lowest triplet state. The absolute value of D is in good agreement with experimental data, which suggests that the sign of D is negative.

Pulsed-EPR on the photoexcited triplet state of C60 in fluid solution: electron transfer from end-capped quaterthiophene and C60-radical anion formation

Abstract The photoexcited triplet state of C 60 in fluid toluene solution was investigated by pulsed-EPR spectroscopy in a temperature range T = 200–300 K. The time evolution of the triplet EPR signal is found to be determined by the spin-lattice relaxation time T 1 and the triplet decay. In the presence of the electron donor end-capped quaterthiophene the C 60 -triplet decay is quenched by electron transfer reaction. The temperature dependence of the triplet quenching rate k q suggests the formation of an exciplex intermediate. Moreover we observe the formation of the C 60 -radical anion following electron transfer. The kinetics and polarization of the C 60 - EPR signal are discussed.

Photo-induced electron-transfer processes in (anthracene–) quinquethiophene–fullerene diads and triads

We have investigated novel model systems for photo-induced intramolecular electron-transfer processes. The molecules presented here are of the structure T5–C60 or A–T5–C60, i.e., fullerene C60 is used as acceptor, and quinquethiophene (T5) or anthracene (A) can act as a donor. From theoretical considerations we learn that a charge transfer should occur after excitation of the quinquethiophene or anthracene part, whereby an energy transfer is still not excluded. Optical absorption spectra show the electronic independence of the fullerene from the substituent and hint at isolated molecules in solutions. After laser-flash excitation of frozen solutions of the molecules, we used pulsed electron spin resonance (ESR) to detect the radical signals of the charge-separated state. In both the diad and the triad, we observe at low temperatures a free induction decay (FID) and a broad spin-echo spectrum with the same lifetime of 1–2 μs. The signals can be assigned to the C60− anion (FID) and the T5+cation (spin echo)...

The structure of the soliton in trans-polyacetylene: A pulse endor investigation

Abstract Detailed ENDOR, TRIPLE resonance and ENDOR Multiple-Quantum spectroscopy on trans-polyacetylene below 20 K reveal a soliton halfwidth at half maximum of w=12 CH-units and a negative-to-positive spin density ratio of ug=0.42.

ESR and ENDOR of triplet states in the charge-transfer crystal naphthalene-tetracyanobenzene

Abstract The ENDOR spectrum of localized triplet states (X-traps) in napthalene-tetracyanobenzene crystals at 4.2 J has been analyzed. From the symmetry of the spin density distribution on the donor and acceptor, it is concluded that the chargetransfer state is distributed over one donor and two acceptors. Between 130 and 300 K, the ESR spectrum or mobile triplet excitons is measured.

Electron paramagnetic resonance lineshape analysis of the photoexcited triplet state of C60 in frozen solution. Exchange narrowing and dynamic Jahn–Teller effect

The EPR lineshape of the photoinduced triplet state of C60 in frozen toluene solution was studied by pulsed EPR spectroscopy. Lineshape calculations of the triplet spectra were performed including dynamical exchange effects. The observed spectra in a glassy matrix are compatible with zero field splitting parameters ‖D1‖ = 0.0114 cm−1 (12.2 mT) and ‖E1‖ = 0.0005 cm−1 (0.5 mT). The temperature dependence of the powder lineshape was simulated using a dynamical exchange model, where the triplet principal axis jumps between all equivalent sites allowed by the D5d symmetry for the lowest excited triplet state. The determined exchange rate turned out to be only weakly temperature dependent and suggests that the dynamic process is due to tunneling between Jahn–Teller distorted states rather than to real molecular jumps. In addition we have observed a different triplet state with zero field splitting parameters ‖D2‖ = 0.0100 cm−1 (10.6 mT), ‖E2‖ = 0.0015 cm−1 (1.6 mT) after annealing of the matrix. We attribute th...