Peter Osswald

Impact of Molecular Flexibility on Binding Strength and Self-Sorting of Chiral π-Surfaces

In this work, we have explored for the first time the influence of conformational flexibility of π-core on chiral self-sorting properties of perylene bisimides (PBIs) that are currently one of the most prominent classes of functional dyes. For this purpose, two series of chiral macrocyclic PBIs 3a-c and 4a-c comprising oligoethylene glycol bridges of different lengths at the 1,7 bay positions were synthesized and their atropo-enantiomers (P and M enantiomers) were resolved. Single crystal analysis of atropo-enantiomerically pure (P)-3a not only confirmed the structural integrity of the ethylene glycol bridged macrocycle but also illustrated the formation of π-stacked dimers with left-handed supramolecular helicity. Our detailed studies with the series of highly soluble chiral PBIs 4a-c by 1- and 2-D (1)H NMR techniques, and temperature- and concentration-dependent UV/vis absorption and circular dichroism (CD) spectroscopy revealed that in π-π-stacking dimerization of these PBIs chiral self-recognition (i.e., PP and MM homodimer formation) prevails over self-discrimination (i.e., PM heterodimer formation). Our studies clearly showed that with increasing conformational flexibility of PBI cores imparted by longer bridging units, the binding strength for the dimerization process increases, however, the efficiency for chiral self-recognition decreases. These results are rationalized in terms of an induced-fit mechanism facilitating more planarized π-scaffolds of PBIs containing longer bridging units upon π-π-stacking.

Photophysical study of bay substituted perylenediimides

A detailed investigation of the photophysical properties of a series of perylenediimide systems bearing three different types of bay substituents is presented. Single photon timing and femtosecond transient absorption experiments reveal that the dynamics of interconversion between two conformational arrangements of these substituents occurs with a time constant of 550 ps. In addition, charge transfer from the electron-rich units attached to the bay area of the electron-poor perylenediimide core is observed. This process leads to a fast non-radiative deactivation of the locally excited state of the perylenediimide in polar solvents. When the experimental results of the investigated systems are compared we observe a shift from a conformational dynamics towards competitive excited state processes involving charge transfer in the -meta and -para substituted perylenediimide chromophore.

DABCO‐Mediated Self‐Assembly of Zinc Porphyrin–Perylene Bisimide Monodisperse Multichromophoric Nanoparticles

The light-harvesting systems (LHS) of photosynthetic plants and bacteria have inspired the design of a multitude of artificial dye assemblies to mimic the functional properties of natural LHS. The primary focus of most of those studies was on the incorporation of a multitude of dyes in a defined molecular framework and the elucidation of energytransfer efficiencies between the constituent dyes. The dendrimer concept has been shown to be very successful in this regard and molecular systems incorporating up to 32 dye units have been achieved. Metalloporphyrins and perylene bisimides (PBIs) are amongst the most frequently applied chromophores for artificial dye assemblies. While in natural LHS the protein matrix controls the position and orientation of the dye manifold in a rather precise manner, most artificial constructs, however, have to be considered as conformationally ill-defined with more or less randomly oriented and fluctuating transition dipole moments. Herein, we introduce a small model system composed of four metalloporphyrins that are tethered to a PBI dye core by quite a long spacer unit, respectively. Our spectroscopic and microscopic studies reveal that this conformationally illdefined pentachromophoric system can be transformed to a well-defined three-dimensional nanoobject by noncovalent binding of a small-molecule additive that, in a sense, mimics the role of the protein matrix. Our approach is based on the well-established interaction of zinc porphyrins and the ditopic 1,4-diazabicycloACHTUNGTRENNUNG[2.2.2]octane (DABCO) ligand, which exhibits high binding constants. Thus, we have synthesized the tetra(zinc porphyrin)-functionalized PBI building block 4 according to the route depicted in Scheme 1. The esterification of tetra(3-hydroxyphenoxy) PBI 1 with free-base porphyrin carboxylic acid 2 (for the synthesis of 1 and 2, see the Supporting Information) using dicyclohexylcarbodiimide (DCC) and 4-(N-dimethylamino)pyridinium toluene sulfonate (DPTS) as coupling reagents in dichloromethane at room temperature provided the free-base porphyrin–PBI conjugate 3 in 33 % yield. Subsequent metallation of 3 in dichloromethane with a saturated solution of zinc acetate in methanol afforded the target pentachromophoric system 4 in pure form after silica-gel column chromatography with chloroform as eluent in 57 % yield. Compound 4 was properly characterized by H NMR spectroscopy, MALDI-TOF mass spectrometry, and elemental analysis (for details, see the Supporting Information). The optical properties of 4 were investigated by UV/Vis spectroscopy in chloroform. The absorption spectrum of 4 is dominated by the intense Soret band of the zinc porphyrin (420–430 nm), whereas in the Q-band region of the porphyrin (500–650 nm) the absorptions of both chromophores overlap (Figure 1). A comparison of the UV/Vis spectrum of 4 with that of the parent zinc tetraphenylporphyrin (ZnTPP) reveals a good agreement for the Soret band (423 nm), indicating that no ground-state interaction between the zinc porphyrin units and the PBI of 4 is present. By contrast, in the Q-band region significant spectral differences are observed that can be attributed to the PBI chromophore. The difference spectrum (see Figure 1, inset) of 4 and four ZnTPP moieties (the spectrum of ZnTPP is magnified by a factor of four, since 4 contains four ZnTPP unites) shows the characteristic spectral features of aryloxy-substituted PBIs with an absorption maximum at 569 nm, which is in good agreement with the values reported for such PBIs. Accordingly, the longest wavelength transition in the absorption spectrum of 4 (Figure 1, inset) can be attributed to [a] Dr. P. Osswald, Dr. C.-C. You, Dr. V. Stepanenko, Prof. Dr. F. W rthner Universit t W rzburg, Institut f r Organische Chemie and Rçntgen Research Center for Complex Material Systems Am Hubland, 97074 W rzburg (Germany) Fax: (+49) 931-31-84756 E-mail : wuerthner@chemie.uni-wuerzburg.de [**] DABCO=1,4-diazabicyclo ACHTUNGTRENNUNG[2.2.2]octane Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200902944.

Synthetic Routes to Core-fluorinated Perylene Bisimide Dyes and their Properties

Numerous core-fluorinated perylene bisimide (PBI) dyes with various substituents at the imide positions have been synthesized by different methods. Core-difluorinated PBIs 4a-f are obtained by imidization of difluoro-substituted perylene bisanhydride 1 with appropriate primary amines or, alternatively, by nucleophilic halogen exchange reactions (Halex process) of the corresponding dibromosubstituted PBIs 2a-d,f with potassium fluoride. Core-tetrafluorinated PBIs 5a-c could also be synthesized by halogen exchange reactions of the respective tetrachlorinated PBIs 3a-c. In particular, core-fluorinated perylene bisimide pigments 4h, 5h containing hydrogen atoms in the imide positions could be obtained for the first time by deprotection of α-methylbenzyl-substituted precursors. Compared with core-unsubstituted perylene bisimides, these fluorinated dyes display hypsochromically shifted absorption and fluorescence spectra, and they exhibit fluorescence quantum yields up to unity, enabling bright yellow emission. The electrochemical properties of these electron-poor perylene bisimides have been studied. Furthermore, the packing features of a tetrafluorinated PBI derivative in the solid state have been discussed. Graphical Abstract Synthetic Routes to Core-fluorinated Perylene Bisimide Dyes and their Properties