Mariangela Castriciano

The Relationship between Nanoscale Architecture and Function in Photovoltaic Multichromophoric Arrays as Visualized by Kelvin Probe Force Microscopy

The physicochemical properties of organic (multi)component films for optoelectronic applications depend on both the mesoscopic and nanoscale architectures within the semiconducting material. Two main classes of semiconducting materials are commonly used: polymers and (liquid) crystals of small aromatic molecules. Whereas polymers (e.g., polyphenylenevinylenes and polythiophenes) are easy to process in solution in thin and uniform layers, small molecules can form highly defined (liquid) crystals featuring high charge mobilities. Herein, we combine the two material types by employing structurally well-defined polyisocyanopeptide polymers as scaffolds to precisely arrange thousands of electron-accepting molecules, namely, perylenebis(dicarboximides) (PDIs), in defined chromophoric wires with lengths of hundreds of nanometers. The polymer backbone enforces high control over the spatial location of PDI dyes, favoring both enhanced exciton and charge transfer. When blended with an electron-donor system such as regioregular poly(3-hexylthiophene), this polymeric PDI shows a relative improvement in charge generation and diffusion with respect to monomeric, aggregated PDI. In order to correlate this enhanced behavior with respect to the architecture, atomic force microscopy investigations on the mixtures were carried out. These studies revealed that the two polymers form interpenetrated bundles having a nanophase-segregated character and featuring a high density of contact points between the two different phases. In order to visualize the relationship between the architecture and the photovoltaic efficiency, Kelvin probe force microscopy measurements were carried out on submonolayer-thick films. This technique allowed for the first time the direct visualization of the photovoltaic activity occurring in such a nanoscale phase-segregated ultrathin film with true nanoscale spatial resolution, thus making possible a study of the correlation between function and architecture with nanoscale resolution.

Micellar aggregates of platinum(II) complexes containing porphyrins

Abstract The complex [Pt(Cy2dim)Me]4(TpyP)(CF3SO3)4 (1) (Cy2dim=dicyclohexyldiimine; TpyP=5,10,15,20-tetrakis(4-pyridyl)-21H,23H-porphine) has been synthesized and characterized with a number of spectroscopic techniques. The solubilization of 1 in anionic (sodium dodecyl sulfate, SDS) and neutral (Triton X-100, TX-100) aqueous surfactant solutions has revealed two different modes of interaction: (i) aggregation in the hydrophobic region of SDS micelles; and (ii) aggregation in the hydrophobic region with partial intercalation in the solvent accessible area of TX-100 micelles. In the case of SDS, the formation of premicellar aggregates has been evidenced by a surfactant concentration range two orders of magnitude lower than the critical micellar concentration (cmc). The metalated Mn(III)TpyP derivative of complex 1 exhibits a moderate peroxidase activity in agreement with the aggregation models proposed for the micellar solutions.

Nucleation effects in the aggregation of water-soluble porphyrin aqueous solutions

Water soluble porphyrins bearing charged groups can be fostered to aggregate by simply screening the repulsive interactions between side-groups with the same charge. We have extensively studied the aggregation of synthetic porphyrins, which on changing the ionic strength give mesoscopic aggregates having a fractal behavior. The resulting structures can be modulated on changing the medium properties. Here we report on a kinetic study of such aggregation behavior on varying the salt concentration, the temperature and the history of the samples, showing the importance of the nucleation step in determining the growth of the aggregates.

Geometrical configuration of monomethyl–platinum(II) complexes driven by the size of entering nitrogen ligands

The reaction of the monoalkyl complex trans-[Pt(DMSO)2Cl(CH3)] with a large variety of heterocyclic nitrogen bases L, in chloroform solution, leads to the formation of uncharged complexes of the type [Pt(DMSO)(L)Cl(CH3)], containing four different groups coordinated to the metal center. Only two out of the three different possible isomers were detected in solution. These two trans(C,N) and cis(C,N) species can be unambiguously identified through 1H NMR spectroscopy. For the trans(C,N) isomers, average values of 2JPtH=75±4 Hz and 3JPtH=36±4 Hz have been observed for the coordinated methyl and DMSO ligands, respectively. In the case of the cis(C,N) isomers, these values increase to 2JPtH=83±2 Hz, and decrease to 3JPtH=26±3 Hz due to the mutual exchange of ligands in trans position to CH3 and DMSO. In the case of bulky asymmetric ligands, such as quinoline, 2-quinolinecarboxaldehyde, 2-methylquinoline, 5-aminoquinoline, 2-phenylpyridine and 2-chloropyridine, slow rotation of the hindered group around the PtN bond makes the coordinated DMSO ligand prochiral. NMR experiments have shown that the first reaction product is the trans(C,N) isomer as a consequence of the very fast removal of one DMSO ligand by the nitrogen bases from the starting complex trans-[Pt(DMSO)2Cl(CH3)]. This trans kinetic product undergoes a geometrical conversion into the more stable cis(C,N) isomer through the intermediacy of fast exchanging aqua-species. The rate of isomerization and the relative stability of the two isomers depends essentially on the rate of aquation and on the steric congestion imposed by the new L ligand on the metal.

Hexakis (pyridyl-functionalised porphyrinato)benzene as a building block for the construction of multi-chromophoric arrays

A new hexakis porphyrinato benzene compound has been synthesised and studied by different spectroscopic techniques. It consists of 6 porphyrin moieties which are covalently attached to one central benzene core. Each porphyrin contains 3 pyridyl functions, making a total of 18 potential coordination sites. The structure is a multi-porphyrin array in its own right but can be extended further by means of metal-directed self-assembly. This was demonstrated with the help of a platinum diimine complex substituted with two dodecyl-tails. The pyridine-functionalised hexa-porphyrin was found to bind only 6 instead of the expected 18 platinum complexes, which is ascribed to steric hindrance. Electron microscopy studies reveal that both the pyridine-functionalised hexaporphyrin and its metal-coordination product form well-defined ring structures of micrometer size on solid supports.

The role of counter-anions in the kinetics and chirality of porphyrin J-aggregates.

Kinetics of the growth of TPPS4 porphyrin J-aggregates slow down in the order H2SO4 > HCl > HBr > HNO3 > HClO4, in agreement with the Hofmeister series. The rate constants and the extent of chirality correlate with the structure-making or breaking abilities of the different anions with respect to the hydrogen bonding network of the solvent.

Multichannel near-field nanoscopy of circular and linear dichroism at the solid-state

We investigate the optical response of individual porphyrin (TPPS3) nanoaggregates anchored onto a glass substrate by using a specific configuration of Polarization-Modulation Scanning Near-Field Optical Microscopy (PM-SNOM). Subdiffraction spatial resolution and sensitivity to the chiroptical properties of the material is reported. By demodulating the transmitted signal at the first and the second harmonics, the response of the nanoggregates to circular and linear polarization is simultaneously acquired in the same scan. In order to evaluate the relevant dichroic coefficients, we analyze a sample comprising several tens of individual nanoaggregates by using a model based on the Mueller matrix formalism. Circular dichroism in the nanoaggregates is demonstrated to stem from their molecular structure, whereas linear dichroism occurs as a consequence of the strongly anisotropic shape of the deposited nanoaggregates.