U. Lemmer

Conformational effects in poly(p-phenylene vinylene)s revealed by low-temperature site-selective fluorescence

Low-temperature site-selective fluorescence (SSF) spectroscopy is employed to study morphological effects on the conformation of poly(p-phenylene vinylene) (PPV) and its phenyl-substituted, soluble derivative poly(phenylphenylenevinylene) (PPPV). Samples of PPV prepared as spin-coated thin films and stretch-aligned free-standing films, and samples of PPPV prepared as cast films and as blends with poly(methylmethacrylate) and polycarbonate have been studied. The results that the authors present are considered with the notion that each polymer sample consists of an array of ordered chain segments whose average length reflects the perfection of the local structure. The statistical distribution of the segment lengths is responsible for inhomogeneous broadening of the optical spectra (absorption and emission). The dominant electronic excitation created by photoexcitation across the pi - pi * energy gap is a singlet exciton that can execute a random walk among the chain segments. SSF spectroscopy allows the authors to distinguish the contributions to the apparent fluorescence Stokes shift that arise from energy relaxation through excitation migration (spectral diffusion) and from structural relaxation of the polymer chain (self-localization). The structural contribution to the Stokes shift approaches zero in well aligned PPV and reaches values of up to 500 cm-1 in highly disordered PPPV films. The SSF method also provides a means of assessing the extent of phase separation that occurs in PPPV blends.

Time resolved luminescence study of recombination processes in electroluminescent polymers

Picosecond luminescence studies on poly(p‐phenylene vinylene) (PPV) and related compounds in the temperature range of 10–300 K are reported. We identify a fast nonradiative and considerably slower radiative recombination channel. The fastest decay is observed in PPV. In the phenyl substituted derivative poly(phenyl‐p‐phenylenevinylene) a significant contribution from a slow component (∼1.3 ns) is found. Dilution of the polymer in a blend leads to an increase of the contribution of the slow component. We attribute the fast decay to luminescence quenching due to energy dependent trapping of mobile excitations whereas the slow component reflects the radiative recombination.

Microcavity effects in a spin‐coated polymer two‐layer system

We report on the design and the optical properties of a spin‐coated multilayer organic microcavity. Tri(stilbene)amine blended with polysulfone as the first layer and an oxadiazole derivative (BPBD) blended with polystyrene as the second layer are sandwiched between two planar mirrors. Enhancement of the luminescence and spectrally narrow emission are observed. By means of time resolved luminescence spectroscopy we show that the spontaneous emission rate is increased in the cavity.

Picosecond hopping relaxation in conjugated polymers

Abstract The energy relaxation of optical excitations in poly( p -phenyl-phenylenevinylene) (PPPV) and PPPV blended in polycarbonate (PC) is investigated by means of picosecond time-resolved luminescence spectroscopy. In PPPV a pronounced dependence on the photon energy of the excitation laser is found. For excitation into the tail states of the inhomogeneously broadened density of states distribution, the spectral shape of the luminescence remains almost constant with time. However, a transient red-shift is observed for excitation at higher energies. Experiments on the diluted system PPPV / PC allow us to distinguish between intra-and inter-chain processes. The experimental results can be explained with a hopping model.

Energy transfer in molecularly doped conjugated polymers

Abstract We present a detailed investigation of energy transfer of optical excitations in a molecularly doped conjugated polymer. We have incorporated dye molecules into a polymer blend system consisting of poly (phenylphenylene vinylene) (PPPV) and polycarbonate. Using time-resolved luminescence spectroscopy we observe efficient picosecond energy transfer from the PPPV chromophores to the dye 4-dicyanomethylene-2-methyl-6- p -dimethylaminostyryl-4H-pyran (DCM) molecules. Our results are interpreted in terms of Forster-type energy transfer.

Dynamics of photoexcitations in electric fields in poly(p-phenylenevinylene) diodes

Abstract We report on a study of the dissociation of neutral photoexcitations in electric fields in ITO/poly( p -phenylenevinylene) (PPV)/Al diodes. This process leads simultaneously to photoconductivity and field-induced quenching of the photoluminescence. By illuminating the samples through the different contacts we show that the high electric fields are restricted to a region close to the Al layer as characteristic for a Schottky diode. Time-resolved luminescence measurements show that the dissociation takes place on a picosecond time scale. The experimental data are compared with a Monte-Carlo simulation of the dynamics of photoexcitations created in a disorder-broadened density of states.

Time-resolved studies of two-photon absorption processes in poly(p-phenylenevinylene)s

Abstract Transient absorption spectroscopy and two-photon fluorescence techniques were used to study the nonresonant optical nonlinearities in poly ( p -phenylenevinylene) (PPV) and its phenyl substituted derivative (PPPV). Strong two-photon absorption with a coefficient β in the range of 5 cm/GW was found. The temporal behavior of the two-photon absorption process in PPV and PPPV, however, is significantly different, which is attributed to the different morphology of the polymer samples. The two-photon resonance lies energetically well above the first excited state in both materials.

Control of the Emission Properties of Conjugated Polymers: Trapping and Microcavity Effects

Abstract We report on two different approaches to modify the emission spectra of poly(phenyl-phenylenevinylene). By doping with different amounts of a laser dye, the fluorescence spectra shift to lower energies due to picosecond transfer of the excitations from conjugated segments of the polymer to the acceptor molecules. As an alternative approach we show that Fabry-Perot microcavities can be used to control and enhance the spontaneous emission of a conjugated polymer layer.

Dynamics of excitation transfer in dye doped Π-conjugated polymers

Abstract We present a time-resolved study of the transfer of optical excitations in conjugated polymers using luminescence spectroscopy with subpicosecond time resolution. In our experiments optical excitations are transferred from the initially excited polymers to admixtured dye molecules acting as acceptors. The temporal evolution of the emitted luminescence reveals transfer times of several picoseconds depending on the concentration of the dye molecules. From the dependence of the transfer time on the mean distance between excited polymer chromophores and acceptor dye molecules, we conclude the occurrence of a Forster-type energy transfer.

Time-resolved luminescence study of self-trapped exciton relaxation in ordered and disordered one-dimensional MX-chain systems

Abstract In a disordered MX-chain complex the self-trapped excitons exhibit nonexponential decay. The decay rate varies with energy across the luminescence band and depends on the photon energy of the excitation laser. This behavior is explained by the site selective generation of self-trapped excitons and their subsequent hopping relaxation.