Ralf M. Peetz

Spectroscopic characterization of p-phenylene vinylene (PV) oligomers. Part I: A homologous series of 2,5-diheptyloxy substituted PV-oligomers

Abstract A homologous series of metathetically synthesized, monodisperse all- trans oligomers of 2,5-diheptyloxy- p -divinylbenzene 1 (DHepO-OPV n s, with n the number of aromatic rings) is characterized for n =1–7. Experiments were carried out in solution at ambient temperature, under matrix-isolated conditions (dispersed in a 2-methyltetrahydrofuran (2MeTHF) glass) at low temperatures (10 and 80 K) and as spin coated thin layers on quartz substrates at various temperatures. The investigations involve absorption and emission as well as site-selective fluorescence (SSF) spectroscopy. DHepO-OPV n s ( n ⩾2) form nanoaggregates and the tendency of the formation increases with increasing concentration and chain length and decreases with increasing temperature. Absorption spectra of isolated chains as well as aggregated species are simulated and agree nicely with the experimental data. Furthermore, the absorption spectra of the aggregates can be correlated with the spectra of the corresponding spin coated oligomer on a quartz substrate. The singlet (S 1 ←S 0 ) transition energies decrease with increasing conjugation length and converge for the longer oligomers towards a limiting value. The Huang–Rhys parameter S associated with the S 1 →S 0 electronic transition and its evolution with conjugation length has been determined via analysis of the vibronic fine structure. At low temperatures, the values determined can be attributed to aggregate formation. At ambient temperature, S decreases with increasing chain length. Low-temperature SSF spectroscopy is employed to study morphological effects on the conformation of DHepO-OPV3. A vanishing Stokes shift is recorded for the resonantly excited fluorescence spectra.

Mechanistic studies on the photodegradation of 2,5-dialkyloxyl-substituted para-phenylenevinylene oligomers by singlet oxygen

Time-dependent UV/visible absorption changes are observed on photolysis of aerated solutions of monodisperse, vinyl end-capped 2,5-diheptyloxyl substituted PV-oligomers (OPV, n = 1, 2, 3). In all cases, photolysis occurs in at least two distinct stages. Evidence for the intermediacy of singlet oxygen comes from pulse radiolysis and time-resolved luminescence studies, which confirm formation of the OPV triplet state and its quenching by molecular oxygen to produce singlet oxygen. The subsequent reaction with oligomers is observed directly by time-resolved phosphorescence and indirectly through product identification. The results strongly suggest that the reaction between singlet oxygen and OPV proceeds via cycloaddition, leading to bond scission and aldehyde group formation. The reactivity of vinylene linkages is greater than of vinyl end groups. Consequently, the photostability decreases as the chain length is extended. Implications on the oxidative degradation of poly(p-phenylenevinylene)s in optoelectronic devices, and of competing singlet oxygen and superoxide radical anion degradation pathways, are discussed.

Ring substituted PV oligomers synthesized via olefin metathesis; Fluorescence properties of monodisperse products

In this contribution first, a short review is presented on the metathesis reaction as a direct route for the synthesis of unsubstituted and ring substituted all-trans PV oligomers with valuable properties. The second part deals with unpublished data concerning the photoluminescence behaviour of monodisperse PV-oligomers with different substituents.

Diheptyloxy PV oligomers in solution : Photoreactivity

(2,5-Diheptyloxy-p-phenylene vinylene) oligomers exhibit a distinct photoreactivity in oxygen equilibrated solution. Using chloroform as solvent instead of cyclohexane leads to a dramatic increase in photolysis rate.