Single-molecule redox blinking of perylene diimide derivatives in water

Dynamic developments in ultrasensitive and superresolution fluorescence microscopy call for improved fluorescence markers with increased photostability and new functionalities. We used single-molecule spectroscopy to study water-soluble perylene dicarboximide fluorophores (PDI), which were immobilized in aqueous buffer by attaching the fluorophore to DNA. Under these conditions bright fluorescence, comparable to that of single-molecule compatible organic fluorophores, is observed with homogeneous spectral and fluorescence decay time distributions. We additionally show how the fluorescence of the PDI can be controlled through photoinduced electron-transfer reactions by using different concentrations of reductants and oxidants, yielding either blinking or stable emission. We explain these properties by the redox potentials of PDI and the recently introduced ROXS (reducing and oxidizing system) concept. Finally, we evaluate how this fluorescence control of PDIs can be used for superresolution "Blink-Microscopy" in aqueous or organic media and more generally for single-molecule spectroscopy.

Tuning the packing density of 2D supramolecular self-assemblies at the solid-liquid interface using variable temperature

The two-dimensional (2D) crystal engineering of molecular architectures on surfaces requires controlling various parameters related respectively to the substrate, the chemical structure of the molecules, and the environmental conditions. We investigate here the influence of temperature on the self-assembly of hexakis(n-dodecyl)-peri-hexabenzocoronene (HBC-C(12)) adsorbed on gold using scanning tunneling microscopy (STM) at the liquid/solid interface. We show that the packing density of 2D self-assembled HBC-C(12) can be precisely tuned by adjusting the substrate temperature. Increasing the temperature progressively over the 20-50 degrees C range induces three irreversible phase transitions and a 3-fold increase of the packing density from 0.111 to 0.356 molecule/nm(2). High-resolution STM images reveal that this 2D packing density increase arises from the stepwise desorption of the n-dodecyl chains from the gold surface. Such temperature-controlled irreversible phase transitions are thus a versatile tool that can then be used to adjust the packing density of highly ordered functional materials in view of applications in organic electronic devices.

Charge-Transfer Controlled Crystallization of a Model Oligomer for Donor-Acceptor-Polythiophenes

A model donor-acceptor oligomer consisting of benzodithiophene-diketone and thiophene has been investigated with regard to its molecular packing and opto-electronic properties. The crystal structure suggests that the packing is dominated by charge-transfer interactions between the electron-rich part of the molecule and the electron-poor part. A series of observations corroborate this assumption, among them are a charge-transfer band in the film absorption spectra and exceptionally low π-π distances. A detailed analysis of the energetic levels of the present system reveals that only the HOMO level of the acceptor is shifted by conjugation to the donor. The results can contribute to the development of improved materials for organic electronics.

Macromolecular scaffolding: the relationship between nanoscale architecture and function in multichromophoric arrays for organic electronics

The optimization of the electronic properties of molecular materials based on optically or electrically active organic building blocks requires a fine-tuning of their self-assembly properties at surfaces. Such a fine-tuning can be obtained on a scale up to 10 nm by mastering principles of supramolecular chemistry, i.e., by using suitably designed molecules interacting via pre-programmed noncovalent forces. The control and fine-tuning on a greater length scale is more difficult and challenging. This Research News highlights recent results we obtained on a new class of macromolecules that possess a very rigid backbone and side chains that point away from this backbone. Each side chain contains an organic semiconducting moiety, whose position and electronic interaction with neighboring moieties are dictated by the central macromolecular scaffold. A combined experimental and theoretical approach has made it possible to unravel the physical and chemical properties of this system across multiple length scales. The (opto)electronic properties of the new functional architectures have been explored by constructing prototypes of field-effect transistors and solar cells, thereby providing direct insight into the relationship between architecture and function.

Development of molecular and solid catalysts for the direct low-temperature oxidation of methane to methanol

The direct low-temperature oxidation of methane to methanol is demonstrated on a highly active homogeneous molecular catalyst system and on heterogeneous molecular catalysts based on polymeric materials possessing ligand motifs within the material structure. The N-(2-methylpropyl)-4,5-diazacarbazolyl-dichloro-platinum(II) complex reaches significantly higher activity compared to the well-known Periana system and allows first conclusions on electronic and structural requirements for high catalytic activity in this reaction. Interestingly, comparable activities could be achieved utilizing a platinum modified poly(benzimidazole) material, which demonstrates for the first time a solid catalyst with superior activity compared to the Periana system. Although the material shows platinum leaching, improved activity and altered electronic properties, compared to the conventional Periana system, support the proposed conclusions on structure-activity relationships. In comparison, platinum modified triazine-based catalysts show lower catalytic activity, but rather stable platinum coordination even after several catalytic cycles. Based on these systems, further development of improved solid catalysts for the direct low-temperature oxidation of methane to methanol is feasible.