Fabian Nolde

The Relationship between Nanoscale Architecture and Function in Photovoltaic Multichromophoric Arrays as Visualized by Kelvin Probe Force Microscopy

The physicochemical properties of organic (multi)component films for optoelectronic applications depend on both the mesoscopic and nanoscale architectures within the semiconducting material. Two main classes of semiconducting materials are commonly used: polymers and (liquid) crystals of small aromatic molecules. Whereas polymers (e.g., polyphenylenevinylenes and polythiophenes) are easy to process in solution in thin and uniform layers, small molecules can form highly defined (liquid) crystals featuring high charge mobilities. Herein, we combine the two material types by employing structurally well-defined polyisocyanopeptide polymers as scaffolds to precisely arrange thousands of electron-accepting molecules, namely, perylenebis(dicarboximides) (PDIs), in defined chromophoric wires with lengths of hundreds of nanometers. The polymer backbone enforces high control over the spatial location of PDI dyes, favoring both enhanced exciton and charge transfer. When blended with an electron-donor system such as regioregular poly(3-hexylthiophene), this polymeric PDI shows a relative improvement in charge generation and diffusion with respect to monomeric, aggregated PDI. In order to correlate this enhanced behavior with respect to the architecture, atomic force microscopy investigations on the mixtures were carried out. These studies revealed that the two polymers form interpenetrated bundles having a nanophase-segregated character and featuring a high density of contact points between the two different phases. In order to visualize the relationship between the architecture and the photovoltaic efficiency, Kelvin probe force microscopy measurements were carried out on submonolayer-thick films. This technique allowed for the first time the direct visualization of the photovoltaic activity occurring in such a nanoscale phase-segregated ultrathin film with true nanoscale spatial resolution, thus making possible a study of the correlation between function and architecture with nanoscale resolution.

Self-assembly of discotic molecules into mesoscopic crystals by solvent-vapour annealing

Solvent vapour annealing (SVA) is used to control the reorganization of ultrathin films of three different polycyclic aromatic hydrocarbons self-assembled on solid surfaces. To this end, two perylene-bis(dicarboximide) (PDI) derivatives exposing branched side alkyl chains with different length and a dodecyl substituted hexa-peri-hexabenzocoronene (HBC) have been used, in view of their n- and p-type semiconducting nature. For all the three molecules, nanoscopic crystals grown from solution by spin-coating and drop-casting undergo reorganization into sub-millimetric fibers, domes and needles, proving the general applicability of the SVA method. Moreover, such an approach exhibits a mass transport of the molecules on surfaces over hundreds of microns. The self-healing of the films through SVA treatment leads to a decrease of the structural defects and an increase in the lateral size of the self-assembled domains, ultimately providing an improved 3D conjugation. Therefore the SVA can be considered an important strategy with potential to enhance the performance of macroscopic organic electronic devices.

Intramolecular electronic excitation energy transfer in donor/acceptor dyads studied by time and frequency resolved single molecule spectroscopy

Electronic excitation energy transfer has been studied by single molecule spectroscopy in donor/acceptor dyads composed of a perylenediimide donor and a terrylenediimide acceptor linked by oligo(phenylene) bridges of two different lengths. For the shorter bridge (three phenylene units) energy is transferred almost quantitatively from the donor to the acceptor, while for the longer bridge (seven phenylene units) energy transfer is less efficient as indicated by the occurrence of donor and acceptor emission. To determine energy transfer rates and efficiencies at the single molecule level, several methods have been employed. These comprise time-correlated single photon counting techniques at room temperature and optical linewidth measurements at low temperature (1.4 K). For both types of measurement we obtain broad distributions of the rate constants of energy transfer. These distributions are simulated in the framework of Forster theory by properly taking into account static disorder and the flexibility of the dyads, as both effects can substantially contribute to the distributions of energy transfer times. The rate constants of energy transfer obtained from the calculated distributions are smaller on average than those extracted from the experimental distributions, whereby the discrepancy is larger for the shorter bridge. Furthermore, by plotting the experimentally determined transfer rates against the individual spectral overlaps, approximately linear dependencies are found being indicative of a Forster-type contribution to the energy transfer. For a given single molecule such a linear dependence could be followed by spectral diffusion induced fluctuations of the spectral overlap. The discrepancies between measured energy transfer rates and rates calculated by Forster theory are briefly discussed in light of recent results of quantum chemical calculations, which indicate that a bridge-mediated contribution is mainly responsible for the deviations from Forster theory. The availability of the inhomogeneous distributions of donor and acceptor electronic transition frequencies allows for comparing the energy transfer process at liquid helium and room temperature for the same set of molecules via simple simulations. It is found that on average the energy transfer is by a factor of approximately 3 faster at room temperature, which is due to an increase of spectral overlap.

Photophysics of New Water-Soluble Terrylenediimide Derivatives and Applications in Biology

The photophysical properties of three new water-soluble terrylenediimide (WS-TDI) derivatives are investigated and their utilization in biological experiments is demonstrated. Each of these dyes can be excited in the far red region of the visible spectrum, making them good candidates for in-vivo studies. Single-molecule techniques characterize their photophysics that is, the number of emitted photons, blinking characteristics and survival times until photobleaching takes place. All three dyes exhibit bright fluorescence, as well as an extremely high resistance against photodegradation compared to other well-known fluorophores. Due to their different characteristics the three new WS-TDI derivatives are suitable for specialized biological applications. WS-TDI dodecyl forms non-fluorescent aggregates in water which can be disrupted in a hydrophobic environment leading to a monomeric fluorescent form. Due to its high lipophilicity WS-TDI dodecyl anchors efficiently in lipid bilayers with its alkyl chain and hence can be ideally used to image membranes and membrane-containing compartments in living cells. In contrast, the positively charged WS-TDI pyridoxy is a new type of chromophore in the WS-TDI family. It is fully solubilized in water forming fluorescent monomers and is successfully used to label the envelope of herpes simplex viruses. Finally, it is shown that a WS-TDI derivative functionalized with N-hydroxysuccinimide ester moiety (WS-TDI/NHS ester) provides a versatile reactive dye molecule for the specific labelling of amino groups in biomolecules such as DNA.

Three-dimensional orientational colocalization of individual donor-acceptor pairs

We report on the determination of the three-dimensional orientation of the donor and acceptor transition dipoles in individual fluorescence resonance energy transfer (FRET) pairs by means of scanning optical microscopy with annular illumination. Knowledge of the mutual orientation of the donor and acceptor dipole is mandatory for reliable distance determination based on FRET efficiency measurements. In our model system perylenediimide as the donor and terryelenediimide as the acceptor are coupled via a stiff p-terphenyl linker. The absorption dipoles of the donor and acceptor are selectively addressed by the 488 nm and 647 line of an Ar/Kr mixed gas laser, respectively. A clear deviation from collinearity is observed with a distribution of misalignment angles peaked around 22 degrees.

Flexibility of phenylene oligomers revealed by single molecule spectroscopy.

The rigidity of a p-phenylene oligomer (p-terphenyl) has been investigated by single molecule confocal fluorescence microscopy. Two different rylene diimide dyes attached to the terminal positions of the oligomer allowed for wavelength selective excitation of the two chromophores. In combination with polarization modulation the spatial orientation of the transition dipoles of both end groups could be determined independently. We have analyzed 597 single molecules in two different polymer hosts, polymethylmethacrylate and Zeonex. On average we find a 22 degrees deviation from the linear gas phase geometry (T = 0 K), indicating a rather high flexibility of the p-phenylene oligomer independent of the matrix. To substantiate our experimental results, we have performed quantum chemical calculations at the density functional theory level for the molecular geometry and the electronic excitations. Our findings are in agreement with former experiments on the persistence length of poly(p-phenylenes).

Energy Transfer at the Single-Molecule Level: Synthesis of a Donor–Acceptor Dyad from Perylene and Terrylene Diimides

In 2004, we reported single-pair fluorescence resonance energy transfer (spFRET), based on a perylene diimide (PDI) and terrylene diimide (TDI) dyad (1) that was bridged by a rigid substituted para-terphenyl spacer. Since then, several further single-molecule-level investigations on this specific compound have been performed. Herein, we focus on the synthesis of this dyad and the different approaches that can be employed. An optimized reaction pathway was chosen, considering the solubilities, reactivities, and accessibilities of the building blocks for each individual reaction whilst still using established synthetic techniques, including imidization, Suzuki coupling, and cyclization reactions. The key differentiating consideration in this approach to the synthesis of dyad 1 is the introduction of functional groups in a nonsymmetrical manner onto either the perylene diimide or the terrylene diimide by using imidization reactions. Combined with well-defined purification conditions, this modified approach allows dyad 1 to be obtained in reasonable quantities in good yield.

Quantification of the Singlet-Singlet Annihilation Times of Individual Bichromophoric Molecules by Photon Coincidence Measurements

Singlet-singlet annihilation (SSA) times in individual bichromophoric molecules have been quantified by time-resolved photon coincidence measurements. An analytical expression has been derived to obtain the SSA times from the coincidence histograms. The results have been confirmed by Monte Carlo simulations. SSA was found to be about three times faster than the fluorescence lifetime of the chromophores. Considering the spectral overlap for SSA and for energy transfer from an excited to a ground state chromophore, we conclude that in the weak coupling limit for any arrangement of the two chromophores both processes occur on similar time scales.