Potential Tuning of Nanoarchitectures Based on Phthalocyanine Nanopillars: Construction of Effective Photocurrent Generation Systems

Surface Coupling of Octaethylporphyrin with Silicon Tetrachloride

The surface assembly of 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP) using silicon tetrachloride as a coupling agent was investigated using atomic force microscopy (AFM). Nanopatterned films of Si-OEP were prepared by protocols of colloidal lithography to evaluate the morphology, thickness, and molecular orientation for samples prepared on Si(111). The natural self-stacking of porphyrins can pose a challenge for molecular patterning. When making films on surfaces, porphyrins will self-associate to form co-planar configurations of random stacks of molecules. There is a tendency for the flat molecules to orient spontaneously in a side-on arrangement that is mediated by physisorption to the substrate as well as by π-π interactions between macrocycles to form a layered arrangement of packed molecules, analogous to a stack of coins. When silicon tetrachloride is introduced to the reaction vessel, the coupling between the surface and porphyrins is mediated through covalent Si-O bonding. For these studies, surface structures of Si-OEP were formed that are connected with a Si-O-Si motif to a silicon atom coordinated to the center of the porphyrin macrocycles. Protocols of colloidal lithography were used as a tool to prepare surface structures and films of Si-OEP to facilitate surface characterizations. Conceptually, by arranging the macrocycles of porphyrins with defined orientation, local AFM surface measurements can be enabled to help address mechanistic questions about how molecules self-assemble and bind to substrates.

Molecular Mechanism of Uptake of Cationic Photoantimicrobial Phthalocyanine across Bacterial Membranes Revealed by Molecular Dynamics Simulations

Phthalocyanines are aromatic macrocyclic compounds, which are structurally related to porphyrins. In clinical practice, phthalocyanines are used in fluorescence imaging and photodynamic therapy of cancer and noncancer lesions. Certain forms of the substituted polycationic metallophthalocyanines have been previously shown to be active in photodynamic inactivation of both Gram-negative and Gram-positive bacteria; one of them is zinc octakis(cholinyl)phthalocyanine (ZnPcChol⁸⁺). However, the molecular details of how these compounds translocate across bacterial membranes still remain unclear. In the present work, we have developed a coarse-grained (CG) molecular model of ZnPcChol⁸⁺ within the framework of the popular MARTINI CG force field. The obtained model was used to probe the solvation behavior of phthalocyanine molecules, which agreed with experimental results. Subsequently, it was used to investigate the molecular details of interactions between phthalocyanines and membranes of various compositions. The results demonstrate that ZnPcChol⁸⁺ has high affinity to both the inner and the outer model membranes of Gram-negative bacteria, although this species does not show noticeable affinity to the 1-palmitoyl-2-oleoyl- sn-glycero-3-phosphatidylcholine membrane. Furthermore, we found out that the process of ZnPcChol⁸⁺ penetration toward the center of the outer bacterial membrane is energetically favorable and leads to its overall disturbance and formation of the aqueous pore. Such intramembrane localization of ZnPcChol⁸⁺ suggests their twofold cytotoxic effect on bacterial cells: (1) via induction of lipid peroxidation by enhanced production of reactive oxygen species (i.e., photodynamic toxicity); (2) via rendering the bacterial membrane more permeable for additional Pc molecules as well as other compounds. We also found that the kinetics of penetration depends on the presence of phospholipid defects in the lipopolysaccharide leaflet of the outer membrane and the type of counterions, which stabilize it. Thus, the results of our simulations provide a detailed molecular view of ZnPcChol⁸⁺ "self-promoted uptake", the pathway previously proposed for some small molecules crossing the outer bacterial membrane.

Wisely Designed Phthalocyanine Derivative for Convenient Molecular Fabrication on a Substrate

An axial-substituted silicon phthalocyanine derivative, SiPc(OR)₂ (R = C₄H₉), that is soluble in organic solvent is conveniently synthesized. This silicon phthalocyanine derivative reacts with a hydroxyl group on a substrate and then with another phthalocyanine derivative under mild conditions. The accumulation number of the phthalocyanine molecules on the substrates is easily controlled by the immersion time. On the basis of AFM (atomic force microscopy) images, the surface of the phthalocyanine-modified glass substrate has uneven structures on the nanometer scale. ITO electrodes modified with the composition of the phthalocyanine derivative and PCBM show stable cathodic photocurrent generation upon light irradiation.