Structuring over many length scales is a design strategy widely used in Nature to create materials with unique functional properties. We here present a comprehensive analysis of an adult sea urchin spine, and in revealing a complex, hierarchical structure, show how Nature fabricates a material which diffracts as a single crystal of calcite and yet fractures as a glassy material. Each spine comprises a highly oriented array of Mg-calcite nanocrystals in which amorphous regions and macromolecules are embedded. It is postulated that this mesocrystalline structure forms via the crystallization of a dense array of amorphous calcium carbonate (ACC) precursor particles. A residual surface layer of ACC and/or macromolecules remains around the nanoparticle units which creates the mesocrystal structure and contributes to the conchoidal fracture behavior. Nature’s demonstration of how crystallization of an amorphous precursor phase can create a crystalline material with remarkable properties therefore provides inspiration for a novel approach to the design and synthesis of synthetic composite materials.

The principle of polymer-controlled crystallization of inorganic materials has been successfully transferred to functional aromatic organic dyes, in this instance 3,4,9,10-perylenetetracarboxylic acid potassium salt (PTCAPS), after its single-crystal structure was determined. The cationic double hydrophilic block copolymer poly(ethylene glycol)-block-branched-poly(ethyleneimine) (PEG-b-PEI) was used as the polymer additive to modify the crystallization of PTCAPS. Ultralong hierarchically structured PTCAPS microbelts with constant width and thickness of each individual belt have been fabricated. The belts are a mesocrystalline assembly of primary nanoparticles with high-energy anionic {001} faces stabilized by polymer complexation. Polarization microscopy, X-ray diffraction, optical absorption spectra, and fluorescence spectra indicate the favorable orientation of the 1D microbelts in the close-stacking direction and reveal a specific 1D superstructure fluorescence. Electrical conductivity measurements performed on a single nanobelt disclose in the doped state a remarkably high electronic conductivity and further demonstrate extended, wirelike pi-pi interactions along the [020] long axis of the belts. Together with the very large length of the belts and their organic-organic hybrid nanostructure, this makes these organic wires potentially interesting for the field of nano-/micro-optoelectronics.

General syntheses have been developed for meso-substituted porphyrins with one or two substituents in the 5,10-positions and no beta substituents. 5-Substituted porphyrins with only one meso substituent are easily prepared by an acid-catalyzed condensation of dipyrromethane, pyrrole-2-carbaldehyde, and an appropriate aldehyde using a "[2+1+1]" approach. Similarly, 5,10-disubstituted porphyrins are accessible by simple condensation of unsubstituted tripyrrane with pyrrole and various aldehydes using a "[3+1]" approach. The yields for these reactions are low to moderate and additional formation of either di- or monosubstituted porphyrins due to scrambling of the intermediates is observed. However, the reactions can be performed quite easily and the desired target compounds are easily removed due to large differences in solubility. A complementary and more selective synthesis involves the use of organolithium reagents for S(N)Ar reactions. Reaction of in situ generated porphyrin (porphine) with 1.1-8 equivalents of RLi gave the monosubstituted porphyrins, while reaction with 3-6 equivalents of RLi gave the 5,10-disubstituted porphyrins in yields ranging from 43 to 90 %. These hitherto almost inaccessible compounds complete the series of different homologues of A-, 5,15-A(2)-, 5,10-A(2)-, A(3)-, and A(4)-type porphyrins and allow an investigation of the gradual influence of type, number, and regiochemical arrangement of substituents on the properties of meso-substituted porphyrins. They also present important starting materials for the synthesis of ABCD porphyrins and are potential synthons for supramolecular materials requiring specific substituent orientations.

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