The controlled and reversible interconversion between the free-base and the doubly dehydrogenated form of a 5,10,15,20-tetra(p-hydroxyphenyl)porphyrin molecule in an ordered array is demonstrated. This is achieved through voltage pulses by hydrogen transfer between the center of the porphyrin and the tip of a scanning tunneling microscope (STM). The local dehydrogenation leads to significant shifts in the energetic positions of the molecular orbitals. Density functional theory (DFT) calculations corroborate our conclusions and allow to gain more insight into the different energy level alignment before and after dehydrogenation. Due to the different conductance at a given voltage a clear distinction of both molecular species is possible, which also enables the application as a single-molecular switch.

The formation of hydrogen-bonded organic nano-structures and the role of the substrate lattice thereby were investigated by scanning tunneling microscopy. The self-organization of 5,10,15,20-tetra(p-hydroxyphenyl)porphyrin (H 2 THPP) molecules leads to two molecular arrangements on Au(111). One of these is characterized by pair-wise hydrogen bonding between hydroxyl groups and a low packing density which enables a rotation of individual molecules in the structure. A different interaction with stronger chain-like hydrogen bonding and additional interactions of phenyl groups was observed for the second structure. The influence of the substrate on the epitaxial behavior is demonstrated by the adsorption of H 2 THPP on the highly anisotropic Ag(110) substrate. There, several balances between the occupation of favorable adsorption positions and the number of hydrogen bonds per molecule were found. The molecules form molecular chains on Ag(110) and also assemble into two-dimensional periodic arrangements of differently sized close-packed blocks similar to the second type of supramolecular ordering found on Au(111). Dispersion corrected Density Functional Theory calculations were applied to understand the adsorption and complex epitaxy of these molecules. It is shown that the azimuthal orientation of the saddle-shape deformed molecule plays an important role not only for the intermolecular but also for the molecule-substrate interaction.

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The temperature dependent stepwise deprotonation of 5,10,15,20-tetra(p-hydroxyphenyl)porphyrin is investigated using photoelectron spectroscopy. An abundance of pyrrolic relative to iminic nitrogen and a decrease in the ratio of the amount of −NH− to −N= with increasing annealing temperature is found. In contrast to the molecules adsorbed on Au(111), on the more reactive Ag(110) surface, partial dissociation of the hydroxyl groups and subsequent diffusion and rebonding of hydrogen to the central nitrogen atoms resulting in a zwitterionic molecule was clearly observed. Moreover, partial C−H bond cleavage and the formation of new covalent bonds with adjacent molecules or the surface starts at a relatively high annealing temperature of 300 • C. This reaction is identified to occur at the carbon atoms of the pyrrole rings, which leads also to a shift in the N 1s signal and changes in the valence band of the molecules. Our results show that annealing can significantly alter the molecules which were deposited depending on the maximum temperature and the catalytic properties of the specific substrate. The thermal stability should be considered if a molecular monolayer is prepared from a multilayer by desorption, or if annealing is applied to enhance the self-assembly of molecular structures.

The bottom-up approach to replace existing devices by molecular-based systems is a subject that attracts permanently increasing interest. Molecular-based devices offer not only to miniaturize the device further, but also to benefit from advanced functionalities of deposited molecules. Furthermore, the molecules itself can be tailored to allow via their self-assembly the potential fabrication of devices with an application potential, which is still unforeseeable at this time. Herein, we review efforts to use discrete (metallo)porphyrins for the formation of (sub)monolayers by surface-confined polymerization, of monolayers formed by supramolecular recognition and of thin films formed by sublimation techniques. Selected physical properties of these systems are reported as well. The application potential of those ensembles of (metallo)porphyrins in materials science is discussed.

A b s t r a c t I n t h i s r e po r t 5 ,1 0, 15 ,2 0-t etr ak i s ( 4 -methoxyphenyl)porphyrin (H 2 TMPP) has been used as electrode material for an easy and direct detection of NO in cancer cells using different electrochemical techniques. Cyclic voltammograms of H 2 TMPP-modified GCE in aqueous electrolyte showed oxidation peaks at 0.5 and 0.73 Vand reduction peak at 0.64 Vand a small peak at 0.4 V which are attributed to the redox processes of the system. The anodic and cathodic peak currents were proportional to the square root of the scan rate indicating a diffusion controlled mechanism. The H 2 TMPP-modified GC electrode exhibits a good stability after several circles. The fabricated electrode system was used to probe the concentration change of NO in HeLa cells using chronoamperometry. The NO sensor registered an amperometric current sensitivity of 0.0138 nA/μL with a linear correlation coefficient of 0.99571 in the presence of extracellular NO released upon activation of HeLa cells. Via confocal laser scanning microscopy, it has been demonstrated that H 2 TMPP could be used as a fluorescent indicator for cell imaging.

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