The title dicyano compound was synthesized via cyanation and it self-assembles in nonpolar solvents giving red-shifted and broad absorption maxima just as the bacteriochlorophylls which are encountered in the light-harvesting organelles of early photosynthetic bacteria. In the crystal, stacks are formed through a hierarchic combination of pi-stacking and a CN-Zn electrostatic interaction. Push-pull 15-N,N-dialkylamino-5-cyano congeners could be obtained in high yields using a solvent- and catalyst-free direct amination of meso-bromoporphyrins. Importantly, the fluorescence of the self-assembled species due to the very orderly manner in which the chromophores are arranged is not entirely quenched and has a surprisingly long lifetime of over 1 ns. This lends hope of using the trapped energy in biomimetic hybrid solar cells.
Single-walled nanotubes (SWNTs) have outstanding electrical, optical, and mechanical properties, such as an extreme tensile strength and a very high Young's modulus. Applications exploiting the unusual properties of carbon nanotubes have become an active research area and include chemically modified tips for force microscopy, nanocircuits in tomorrow's computers, nanotube-reinforced composite materials, and field-emission displays-to name just a few.[1] In materials science, this field has developed in parallel with the availability of larger quantities of purified material. Kretschmer's fullerene research [2] in the 1980s bridged the gap from a peak in the mass spectra to filling vials with purified fullerenes, which were thus made available to chemists.[3] Fullerenes then stimulated the study of tubular carbon allotropes, which rapidly became the domain of solid-state physics. SWNTs since their discovery [4] have also been of interest to chemists. However, chemical functionalization has lagged behind materials characterization primarily due to the heterogeneity of the SWNT samples. Recently, through a controlled catalytic growth initiated by metal nanoparticles, [5] larger amounts of nanotubes with a better reproducible quality have been obtained and several synthetic methods have been reported [6] on both their covalent [7] and noncovalent [8] derivatization.As a result of their extended p-surfaces as-prepared SWNTs tend to aggregate/bundle and so are insoluble and hard to purify/separate from the catalyst nanoparticles. They are in general not amenable to chemical reactions that require a homogeneous phase. It would be highly desirable to be able to chemically derivatize carbon allotropes and solubilize/disperse these in a common organic solvent and in this way purify them from the catalyst. Furthermore, after solubilization conventional purification methods, such as density-gradient centrifugation or size-exclusion chromatography, could be applied in order to obtain homogeneous (in size and chirality) nanotube preparations. Subsequent defunctionalization by either thermal or chemical means could then restore the original but now highly purified nanotubes.With this aim in mind, we found suitable reaction conditions for performing a high-yield polyacylation of as-prepared SWNTs. In a similar manner to olefin diacylation reactions, a standard method for the preparation of pyrylium salts, [9] we could polyacylate the SWNTs. However, at room temperature with a variety of Friedel-Crafts (FC) catalysts and in various solvents, the SWNTs failed to react. After reading a recent report of the successful aroylation of multi-walled nanotubes (MWNTs) catalyzed by polyphosphoric acid at 130°C for 80 h, [10] we decided to use forcing conditions employing acyl chlorides in high-temperature reactions. Optimal results were obtained by using either nitrobenzene as a FC solvent at 180°C for three hours and aluminum chloride as a molar reactant, or, even more conveniently, an AlCl 3 /NaCl melt [11]
Studying the relaxation pathways of porphyrins and related structures upon light absorption is crucial to understand the fundamental processes of light harvesting in biosystems and many applications. Herein, we show by means of transient absorption studies, following Q‐ and Soret‐band excitation, and ab initio calculations on meso‐tetraphenylporphyrinato magnesium(II) (MgTPP) and meso‐tetraphenylporphyrinato cadmium(II) (CdTPP) that electronic relaxation following Soret‐band excitation of porphyrins with a heavy central atom is mediated by a hitherto disregarded dark state. This accounts for an increased rate of internal conversion. The dark state originates from an orbital localized at the central nitrogen atoms and its energy continuously decreases along the series from magnesium to zinc to cadmium to below 2.75 eV for CdTPP dissolved in tetrahydrofuran. Furthermore, we are able to directly trace fast intersystem crossing in the cadmium derivative, which takes place within (110±20) ps.
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