Functional Dyes

Most of our research work is devoted to highly colored and often beautifully luminescent organic dyes that are investigated for a broad range of applications as functional dyes. Particularly, we deal with the following chromophore structures:

References:

R. Dobrawa, F. Würthner, J. Polym. Sci. Part A: Polym. Chem. 2005, 43, 4981–4995.
Metallosupramolecular Approach towards Functional Coordination Polymers

Biomolecular Recognition

DNA Recognition

Figure 3. Proposed model for the electrostatic crosslinking of dsDNA strands by a perylene bisimide dye.

.

Figure 4. Fluorescence microscope image of cells (L-fibroblasts) stained by a perylene bisimide dye.

Nanomaterials

A. Merocyanine Dye Nanorods

We apply self-assembly processes to obtain supramolecular polymers and nanomaterials. Highly ordered cylindrical merocyanine dye assemblies, which are formed by electrostatic interactions between the highly dipolar dyes (Figure 5) and further hierarchical growth (Figure 6) represent one current example.

Figure 5. Model for the supramolecular polymerization of merocyanines by dipolar aggregation.

Figure 6. Model for the hierarchical growth of merocyanine assemblies: six helical supramolecular polymers form rod-type H-aggregates which are packed in a hexagonal arrangement.

Figure 7. Left: Bis(merocyanine) building blocks 1 and 2. Right: AFM phase images of kinetically (a) and thermodynamically (b) self-assembled nanorods.

References:

A. Lohr, M. Lysetska, F. Würthner, Angew. Chem. Int. Ed. 2005, 44, 5071–5074.
Supramolecular Stereomutation in Kinetic and Thermodynamic Self-Assembly of Helical Merocyanine Nanorods

S. Yao, U. Beginn, T. Greß, M. Lysetska, F. Würthner, J. Am. Chem. Soc. 2004, 126, 8336–8348.
Supramolecular Polymerization and Gel Formation of Bis(Merocyanine) Dyes Driven by Dipolar Aggregation

F. Würthner, S. Yao, U. Beginn, Angew. Chem. Int. Ed. 2003, 42, 3247–3250.
Highly Ordered Merocyanine Dye Assemblies by Supramolecular Polymerization and Hierarchical Self-Organization

B. Light-Harvesting Chlorin Dye Nanorods

The light harvesting antennae in the chlorosomes of green phototrophic bacteria provide a fascinating example of self-organized functional assembly. These antennae consist of bacteriochlorophylls (mainly BChl c) and do not require any protein skeleton for their structural organization. Because of their high exciton diffusion length, these antennae are very efficient in light harvesting.

Figure 8. Cylindrical BChl dye aggregates found in the light-harvesting units of photosynthetic green bacteria.

Since the report on in vitro BChl c aggregates, there have been intensive investigations to mimic the chlorosome-type aggregates using synthetic metallochlorins as model systems. It turned out that in vitro aggregates of BChl c and also that of the best model systems, zinc chlorins, are prone to further macroscopic aggregation in solution leading to precipitation. Therefore, we develop new zinc chlorins, whose self-organized rod-like antennae are highly soluble. This property enables thorough spectroscopic investigation of the reversible formation of these important dye aggregates. In addition, AFM images provide the first microscopic evidence for cylindrical micelles.

Figure 9. Left: UV/Vis spectra of a highly soluble semi-synthetic zinc chlorin in heptane/ di-n-butylether at different temperatures (15 - 95 °C). The monomer (648 nm) and aggregate (742 nm) absorption bands are reversibly changed. Right: The aggregation process goes along with a color change from blue to green because of the large bathochromic shift.

Both BChl c and zinc chlorin assemblies are very efficient in harvesting blue and red light but they do not use the significant green part of the solar spectrum. In order to make the rods suitable for dye-sensitized solar cells it is inevitable to utilize maximally the sunlight available on earth´s surface. Therefore, we explore the possibility to attach a variety of additional antenna chromophores to zinc chlorins.

C. p/n Heterojunctions for Organic Solar Cells

Supramolecular chemistry offers the possibility to create nanoscopic p/n heterojunctions by co-self-organization of electron donor and acceptor chromophores. For such an application oligo(p-phenylene vinylene) (OPV) donors can be attached to bay-substituted perylene bisimide (PERY) acceptors by hydrogen bonding or by covalent bonds creating a p/n heterojunction. This work is done in close collaboration with the groups of Albert Schenning, René Janssen and Bert Meijer at TU Eindhoven.

Figure 10. Chemical structures of OPV-PERY-OPV arrays which are the building blocks for supramolecular p/n heterojunctions.

The donor-acceptor-donor arrays self-assemble into chiral stacks by π-π-interactions (Figure 11). On the basis of transient absorption spectroscopy, fluorescence quenching of OPV and PERY in the assembly can be related to the essential photoinduced electron transfer on subpicosecond time scale.

Figure 11. Left: Left-handed helical stacking model for an OPV-PERY-OPY complex from OPV (n=1) and PERY (M enantiomer). Right: Energy-minimized (CAChe 5.0 MM2 force field) structure of the PERY M enantiomer.

Perylene Bisimide based Liquid Crystals and Organogels

The planar π-systems of perylene bisimide dyes cause not only π-π-stacking of the molecules in the crystalline state but also in solution. Perylene bisimides bearing solubilizing substituents allow elucidation of concentration and solvent effects on aggregation processes by UV/Vis and fluorescence spectroscopy. For the parent perylene bisimides, triple hydrogen-bonding to melamines can be used to cross-link these dye aggregates. This leads to interesting supramolecular organogels.

Figure 12a. Structures of perylene bisimides 1a and 1b

Figure 12b. The luminescence of perylene bisimide 1a in toluene is concentration dependent. The concentrations from left to right are: 10^-6, 10^-5, 10^-4, 10^-3 and 10^-2 M.

Figure 12c. Concentration dependent UV/Vis spectra of perylene bisimide 1b.

In the solid state the same perylene bisimide dyes form liquid-crystalline (LC) phases over a broad temperature range. It is assumed that the π-systems pack in a similar way as observed for the dye aggregates in solution despite additional packing constraints that arise in the bulk of the LC phase.

Figure 13. Schematic illustration of chromophore arrangements at different concentrations.

LC textures can be visualized by polarization or fluorescence microscopy. Figure 14 shows textures of a perylene bisimide.

Figure 14. Example for LC textures of a perylene bisimide observed by polarization microscopy.

Optoelectronic Materials

A. Organic Semiconductors

π-conjugated compounds represent a new class of materials for electronics and photonics with a wide variety of attractive properties. These are similar and in many respects complementary to those of conventional inorganic semiconductors. For instance, π-conjugated molecules may harvest light with extremely high cross section, and optically excited molecules can emit photons with nearly 100 % quantum efficiency. Although electrical transport in organic materials usually suffers from their amorphous or polycrystalline nature, high charge carrier mobility can be reached if high structural order can be established as demonstrated, e.g., for organic single crystals. Moreover, organic materials provide novel and advantageous technical solutions for emerging device technologies. This is the reason why organic semiconductors have already found commercial applications, e.g. as photoconductors in laser printers and xerography and as active layers in organic light-emitting displays.
Our group makes efforts to understand and to engineer the solid state organization of perylene bisimide dyes which are among the most promising n-type semiconductors for organic field-effect transistors and solar cells. The charge carrier mobilities of our compounds are investigated at Delft Technical University. For the development of hybrid and organic solar cells we collaborate with several groups from industry and academia.

References:

Z. Chen, M. G. Debije, T. Debaerdemaeker, P. Osswald, F. Würthner, ChemPhysChem 2004, 5, 137–140.
Tetrachloro-substituted Perylene Bisimide Dyes as Promising n-Type Organic Semiconductors: Studies on Structural, Electrochemical and Charge Transport Properties

B. Organic Photovoltaics

In the recent years, the vibrant research field of organic photovoltaics (OPV) received growing interest, as it may contribute to meet the global energy demand by renewable green energy sources. Thereby, the most common organic solar cell design consists of interpenetrating networks of organic n-type semiconductors as electron acceptor and organic p-type semiconductors as electron donor moiety (Figure 15).

Figure 15. Schematic representation of the applied cell design.

The active layer in such bulk heterojunction (BHJ) OPV cells can either be manufactured by vapor-deposition or solution processed techniques, the latter representing a promising method for large scale production with low manufacturing costs. Whilst in the solution processed BHJ OPV cells the commonly used organic n‑type semiconductors are soluble fullerenes like [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), a huge variety of organic p-type semiconductors are employed. The solution processed BHJ OPV cells can be classified in two classes, those based on p-type semiconducting polymers like poly(3-hexylthiophene) (P3HT) and those based on small molecular p-type organic semiconductors (Figure 16).

Figure 16. Illustration of different organic p-type and n-type semiconductors.

Our research is focused on such semiconducting small molecules. Thereby, we tackle all challenges that address the synthesis, characterization and investigation of optimized p-type materials in regard to their optical and electrical properties in solution and solid state along with their arrangement and packing properties in the solid state and in blends with PCBM to understand and improve the device morphology. The two functional dye classes that are employed in our solar cells are merocyanine and squaraine dyes. Both groups are featuring sharp and intense absorption bands that allow the efficient exploitation of the solar radiation combined with ideal properties for tandem solar cells. In addition, they are easily modified and offer a huge structural variety. Merocyanines, for example, are bearing the great advantage, that the absorption can be adjusted to almost every wavelength of the visible spectrum (Figure 17 left). Squaraines, on the other hand, feature absorption in the far red, up to the NIR spectral region. Therefore, they are promising materials for the realization of transparent devices as demanded for application on window glass.

Figure 17. left. UV/Vis diagram of various merocyanine dyes; right: JV diagram of solar cell devices containing different squaraine dyes.

For the investigation of our materials we have fruitful collaborations with the Meerholz group in Cologne and BASF S.E.