X‐ray photoelectron spectroscopy (XPS) has been used to characterize poly(3,4‐ethylene dioxythiophene)–poly(styrene sulfonate) (PEDT/PSS), one of the most common electrically conducting organic polymers. A correlation has been established between the composition, morphology, and polymerization mechanism, on the one hand, and the electric conductivity of PEDT/PSS, on the other hand. XPS has been used to identify interfacial reactions occurring at the polymer‐on‐ITO and polymer‐on‐glass interfaces, as well as chemical changes within the polymer blend induced by electrical stress and exposure to ultraviolet light. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2561–2583, 2003

Individual multichromophoric dendrimer molecules, bearing eight perylenecarboximide chromophores at the rim, immobilized in a thin polyvinylbutyral (PVB) film were studied by far-field fluorescence microscopy. Fluorescence intensity trajectories as a function of time (transients), spectra, and decay traces were recorded separately or simultaneously. For comparison, similar measurements have been performed on a model compound containing one perylenecarboximide chromophore. Collective on/off jumps of the fluorescence intensity were observed for single dendrimer molecules, resembling previously reported collective jumps for the emission of single light-harvesting antenna systems. Spectra and decays of both non-interacting and dimer-like interacting chromophoric sites could be distinguished within an individual dendrimer. Transitions between the different spectral forms and decay times, observed for a single molecule, underline the dynamic character of the interactions among the chromophores. Evidence for a stepwise bleaching process of the multichromophoric system was found. Furthermore, the single-molecule data incontestably prove the assumptions stated in the ensemble model.

T he current renaissance of the use of the fluorescence resonance energy-transfer (FRET) (1) process between two weakly coupled dipoles, one acting as donor and the other acting as acceptor, results from various elegant and successful studies of distance changes in individual biomolecules by using singlemolecule spectroscopy (SMS). This type of study is often referred to as single-pair Förster resonance ET (2-4). Recent advances in fluctuation microscopy allow the observation of changes in the ET efficiency in single-pair Förster resonance ET systems at the millisecond-to-nanosecond time scale (5, 6). It has been suggested that, in the latter time scale, thermally excited conformational dynamics of (bio)macromolecules will become accessible (5). Thus far, FRET measurements at these time scales have overlooked competitive Förster-type ET pathways, which might complicate data analysis and interpretation of the FRET data. The FRET process involves nonradiative transfer from a donor to an acceptor. For molecules within the weak coupling limit, Förster derived an expression for the rate constant of dipole-dipole-induced ET (7). This relation shows that the ET efficiency scales with the sixth power of the distance between the chromophores:Here, R 0 is the distance at which the efficiency equals 50%, i.e., the distance at which an equal probability exists for the excited chromophore to relax to the ground state via emission of a photon or to undergo ET. It includes the spectral properties and the relative orientation (in terms of the orientation factor 2 ) of donor and acceptor transition dipoles. If the distances and orientations of the chromophores are kept constant, the ET efficiency is determined by the spectral overlap of the corresponding transitions, i.e., the overlap of the emission spectrum of the donor and the absorption spectrum of the acceptor. Therefore, FRET can also occur between identical molecules if the Stokes shift is small enough to allow for a sufficient overlap of the emission and absorption spectrum of the chromophores. This so-called homo-transfer or energy hopping represents a key mechanism for energy transport in some light-harvesting complexes (8).However, Förster-type resonance ET is not restricted to the nonradiative transfer of energy from a donor in the excited state to a ground-state acceptor. Transfer processes that are allowed within the Förster formalism are those for which there are no changes in electron spin in the acceptor transition. Therefore, even ET processes between donor and acceptor molecules with different spin multiplicity likewise are possible. Hence, the transfer of excitation energy from a chromophore residing in the first excited singlet (S) state to another chromophore residing either in the triplet (T) or in an excited singlet state are possible and competitive ET pathways.In the present contribution, we demonstrate that, because of the specific excitation conditions that are applied in SMS, indeed several Förster-type ET pathways are prevalent in single bichromopho...