Investigation of stepwise covalent synthesis on a surface yielding porphyrin-based multicomponent architectures

Porphyrins have been shown to be a viable medium for use in molecular-based information storage applications. The success of this application requires the construction of a stack of components ("electroactive surface/tether/charge-storage molecule/linker/electrolyte/top contact") that can withstand high-temperature conditions during fabrication (up to 400 degrees C) and operation (up to 140 degrees C). To identify suitable chemistry that enables in situ stepwise synthesis of covalently linked architectures on an electroactive surface, three sets of zinc porphyrins (22 altogether) have been prepared. In the set designed to form the base layer on a surface, each porphyrin incorporates a surface attachment group (triallyl tripod or vinyl monopod) and a distal functional group (e.g., pentafluorophenyl, amine, bromo, carboxy) for elaboration after surface attachment. A second set designed for in situ dyad construction incorporates a single functional group (alcohol, isothiocyanato) that is complementary to the functional group in the base porphyrins. A third set designed for in situ multad construction incorporates two identical functional groups (bromo, alcohol, active methylene, amine, isothiocyanato) in a trans configuration (5,15-positions in the porphyrin). Each porphyrin that bears a surface attachment group was found to form a good quality monolayer on Si(100) as evidenced by the voltammetric and vibrational signatures. One particularly successful chemistry identified for stepwise growth entailed reaction of a surface-tethered porphyrin-amine with a dianhydride (e.g., 3,3',4,4'-biphenyltetracarboxylic dianhydride), forming the monoimide/monoanhydride. Subsequent reaction with a diamine (e.g., 4,4'-methylene-bis(2,6-dimethylaniline)) gave the bis(imide) bearing a terminal amine. Repetition of this stepwise growth process afforded surface-bound oligo-imide architectures composed of alternating components without any reliance on protecting groups. Taken together, the ability to prepare covalently linked constructs on a surface without protecting groups in a stepwise manner augurs well for the systematic preparation of a wide variety of functional molecular devices.     

Diverse redox-active molecules bearing O-, S-, or Se-terminated tethers for attachment to silicon in studies of molecular information storage

A molecular approach to information storage employs redox-active molecules tethered to an electroactive surface. Attachment of the molecules to electroactive surfaces requires control over the nature of the tether (linker and surface attachment group). We have synthesized a collection of redox-active molecules bearing different linkers and surface anchor groups in free or protected form (hydroxy, mercapto, S-acetylthio, and Se-acetylseleno) for attachment to surfaces such as silicon, germanium, and gold. The molecules exhibit a number of cationic oxidation states, including one (ferrocene), two [zinc(II)porphyrin], three [cobalt(II)porphyrin], or four (lanthanide triple-decker sandwich compound). Electrochemical studies of monolayers of a variety of the redox-active molecules attached to Si(100) electrodes indicate that molecules exhibit a regular mode of attachment (via a Si-X bond, X = O, S, or Se), relatively homogeneous surface organization, and robust reversible electrochemical behavior. The acetyl protecting group undergoes cleavage during the surface deposition process, enabling attachment to silicon via thio or seleno groups without handling free thiols or selenols.     

Synthesis of porphyrins bearing hydrocarbon tethers and facile covalent attachment to si(100)

The use of redox-active molecules as the active storage elements in memory chips requires the ability to attach the molecules to an electroactive surface in a reliable and robust manner. To explore the use of porphyrins tethered to silicon via carbosilane linkages, 17 porphyrins have been synthesized. Fourteen porphyrins bear a tether at a single meso site, and three porphyrins bear functional groups at two beta sites for possible two-point attachment. Two high-temperature processing methods (400 degrees C under inert atmosphere) have been developed for rapid (minutes), facile covalent attachment to Si platforms. The high-temperature processing conditions afford attachment either by direct deposition of a dilute solution (1 microM-1 mM) of the porphyrin sample onto the Si substrate or sublimation of a neat sample onto the Si substrate. The availability of this diverse collection of porphyrins enables an in-depth examination of the effects of the tether (length, composition, terminal functional group, number of tethers) and steric bulk of nonlinking substituents on the information-storage properties of the porphyrin monolayers obtained upon attachment to silicon. Attachment proceeds readily with a wide variety of hydrocarbon tethers, including 2-(trimethylsilyl)ethynyl, vinyl, allyl, or 3-butenyl directly appended to the porphyrin and iodo, bromomethyl, 2-(trimethylsilyl)ethynyl, ethynyl, vinyl, or allyl appended to the 4-position of a meso-phenyl ring. No attachment occurs with substituents such as phenyl, p-tolyl, mesityl, or ethyl. Collectively, the studies show that the high-temperature attachment procedure (1) has broad scope encompassing diverse functional groups, (2) tolerates a variety of arene substituents, and (3) does not afford indiscriminate attachment. The high-temperature processing conditions are ideally suited for use in fabrication of hybrid molecular/semiconductor circuitry.     

Synthesis of thiol-derivatized porphyrin dimers and trimers for studies of architectural effects on multibit information storage

We present the rational design and synthesis of multiporphyrin arrays containing thiol-derivatized linkers for the purpose of multibit molecular information storage. Porphyrin dimers and trimers were synthesized by the Pd-mediated coupling of iodo-substituted and ethynyl-substituted porphyrin building blocks in 5-51% yields. Each porphyrin dimer bears one S-acetylthio group. The architecture of the trimers incorporates a trans-substituted porphyrin (central) bearing two S-acetylthio groups and two diphenylethyne-linked porphyrins (wings) in a trans geometry. The central porphyrin and the wing porphyrins bear distinct substituents and central metals, thereby affording different oxidation potentials. The S-acetylthio groups provide a means for attachment of the arrays to an electroactive surface. The dimers are designed for vertical orientation on an electroactive surface while the trimers are designed for horizontal orientation of the central porphyrin. Altogether seven different arrays were synthesized. Each array forms a self-assembled monolayer (SAM) on gold via in situ cleavage of the S-acetyl protecting group. The SAM of each array is electrochemically robust and exhibits multiple, reversible oxidation waves. In general, however, the trimeric arrays appear to form more highly ordered monolayers that exhibit sharper, better-defined redox features.     

Porphyrins bearing mono or tripodal benzylphosphonic acid tethers for attachment to oxide surfaces

The ability to attach redox-active molecules to oxide surfaces in controlled architectures (distance, orientation, packing density) is essential for the design of a variety of molecular-based information storage devices. We describe the synthesis of a series of redox-active molecules wherein each molecule bears a benzylphosphonic acid tether. The redox-active molecules include zinc porphyrins, a cobalt porphyrin, and a ferrocene-zinc porphyrin. An analogous tripodal tether has been prepared that is based on a tris[4-(dihydroxyphosphorylmethyl)phenyl]-derivatized methane. A zinc porphyrin is linked to the methane vertex by a 1,4-phenylene unit. The tripodal systems are designed to improve monolayer stability and ensure vertical orientation of the redox-active porphyrin on the electroactive surface. For comparison purposes, a zinc porphyrin bearing a hexylphosphonic acid tether also has been prepared. The synthetic approaches for introduction of the phosphonic acid group include derivatization of a bromoalkyl porphyrin or use of a dimethyl or diethyl phosphonate substituted precursor in a porphyrin-forming reaction. The latter approach makes use of dipyrromethane building blocks bearing mono or tripodal dialkyl phosphonate groups. The zinc porphyrin-tripodal compound bearing benzylphosphonic acid legs tethered to a SiO(2) surface (grown on doped Si) was electrically well-behaved and exhibited characteristic porphyrin oxidation/reduction waves. Collectively, a variety of porphyrinic molecules can now be prepared with tethers of different length, composition, and structure (mono or tripodal) for studies of molecular-based information storage on oxide surfaces.     

Synthesis and photophysical characterization of porphyrin, chlorin and bacteriochlorin molecules bearing tethers for surface attachment

The ability to tailor synthetic porphyrin, chlorin and bacteriochlorin molecules holds promise for diverse studies in artificial photosynthesis. Toward this goal, the synthesis and photophysical characterization of five tetrapyrrole compounds is described. Each compound bears a surface attachment group. One set contains three meso-substituted porphyrins that differ only in the nature of a surface-binding tether-isophthalic acid, ethynylisophthalic acid or cyanoacrylic acid. The other set includes a porphyrin, chlorin and bacteriochlorin each of which bears an ethynylisophthalic acid tether. The ester derivative of each compound was prepared for solution photophysical characterization studies. The photophysical studies include determination (in toluene or acetonitrile) of the electronic absorption and fluorescence spectra, fluorescence yield and lifetime of the lowest excited singlet state. The excited-state lifetimes range from 1 to 5.6 ns for the five compounds. The radiative rate constant for the excited-state decay was estimated from the photophysical data (fluorescence yield and excited-state lifetime) and from Strickler-Berg analysis of the absorption and fluorescence spectra. The synthesis and characterization of the tetrapyrrole compounds underpin their use as sensitizers in molecular-based solar cells.     

Synthesis of "Porphyrin-linker-Thiol" molecules with diverse linkers for studies of molecular-based information storage

The attachment of redox-active molecules such as porphyrins to an electroactive surface provides an attractive approach for electrically addressable molecular-based information storage. Porphyrins are readily attached to a gold surface via thiol linkers. The rate of electron transfer between the electroactive surface and the porphyrin is one of the key factors that dictates suitability for molecular-based memory storage. This rate depends on the type and length of the linker connecting the thiol unit to the porphyrin. We have developed different routes for the preparation of thiol-derivatized porphyrins with eight different linkers. Two sets of linkers explore the effects of linker length and conjugation, with one set comprising phenylethyne units and one set comprising alkyl units. One electron-deficient linker has four fluorine atoms attached directly to a thiophenyl unit. To facilitate the synthesis of the porphyrins, convenient routes have been developed to a wide range of aldehydes possessing a protected S-acetylthio group. An efficient synthesis of 1-(S-acetylthio)-4-iodobenzene also has been developed. A set of porphyrins, each bearing one S-acetyl-derivatized linker at one meso position and mesityl moieties at the three remaining meso positions, has been synthesized. Altogether seven new aldehydes, eight free base porphyrins and eight zinc porphyrins have been prepared. The zinc porphyrins bearing the different linkers all form self-assembled monolayers (SAMs) on gold via in situ cleavage of the S-acetyl protecting group. The SAM of each porphyrin is electrochemically robust and exhibits two reversible oxidation waves.     

Porphyrins bearing arylphosphonic acid tethers for attachment to oxide surfaces

Synthetic molecules bearing phosphonic acid groups can be readily attached to oxide surfaces. As part of a program in molecular-based information storage, we have developed routes for the synthesis of diverse porphyrinic compounds bearing phenylphosphonic acid tethers. The routes enable (1) incorporation of masked phosphonic acid groups in precursors for use in the rational synthesis of porphyrinic compounds and (2) derivatization of porphyrins with masked phosphonic acid groups. The precursors include dipyrromethanes, monoacyldipyrromethanes, and diacyldipyrromethanes. The tert-butyl group has been used to mask the dihydroxyphosphoryl substituent. The di-tert-butyloxyphosphoryl unit is stable to the range of conditions employed in syntheses of porphyrins and multiporphyrin arrays yet can be deprotected under mild conditions (TMS-Cl/TEA or TMS-Br/TEA in refluxing CHCl(3)) that do not cause demetalation of zinc or magnesium porphyrins. The porphyrinic compounds that have been prepared include (1) A(3)B-, trans-AB(2)C-, and ABCD-porphyrins that bear a single phenylphosphonic acid group, (2) a trans-A(2)B(2)-porphyrin bearing two phenylphosphonic acid groups, (3) a chlorin that bears a single phenylphosphonic acid group, and (4) a porphyrin dyad bearing a single phenylphosphonic acid group. For selected porphyrin-phosphonic acids, the electrochemical characteristics have been investigated for molecules tethered to SiO(2) surfaces grown on doped Si. The voltammetric behavior indicates that the porphyrin-phosphonic acids form robust, electrically well-behaved monolayers on the oxide surface.     

Porphyrin dyads bearing carbon tethers for studies of high-density molecular charge storage on silicon surfaces

Redox-active molecules that afford high charge density upon attachment to an electroactive surface are of interest for use in molecular-based information-storage applications. One strategy for increasing charge density is to covalently link a second redox center to the first in an architecture that uses the vertical dimension in essentially the same molecular footprint. Toward this end, a set of four new porphyrin dyads have been prepared and characterized. Each dyad consists of two zinc porphyrins, an intervening linker (p-phenylene or 4,4'-diphenylethyne), and a surface attachment group (ethynyl or triallyl group). The porphyrin dyads were attached to an electroactive Si(100) surface and interrogated via electrochemical and FTIR techniques. The charge density obtainable for the ethynyl-functionalized porphyrin dyads is approximately double that observed for an analogously functionalized monomer, whereas that for the triallyl-functionalized dyads is at most 40% larger. These results indicate that the molecular footprint of the former dyads is similar to that of a monomer while that of the latter dyads is larger. For both the ethynyl- and triallyl-functionalized porphyrin dyads, higher charge densities (smaller molecular footprints) are obtained for the molecules containing the 4,4'-diphenylethyne versus the p-phenylene linker. This feature is attributed to the enhanced torsional flexibility of the former linker compared with that of the latter, which affords better packed monolayers. The FTIR studies indicate that the adsorption geometry of all the dyads is qualitatively similar and similar to that of monomers. However, the dyads containing the 4,4'-diphenylethyne linker sit somewhat more upright on the surface than those containing the p-phenylene linker, generally consistent with the smaller molecular footprint for the former dyads. Collectively, the high surface charge density (34-58 muC.cm(-)(2)) of the porphyrin dyads makes these constructs viable candidates for molecular-information-storage applications.     

Synthesis of thiol-derivatized ferrocene-porphyrins for studies of multibit information storage

One approach toward storage of multiple bits of information at the molecular level requires the construction of molecular architectures comprised of multiple redox-active units. Four new ferrocene-porphyrins have been synthesized to investigate questions concerning (1) the scope of redox-active molecules that can be employed in molecular information-storage schemes and (2) writing/reading rates as well as retention of charge in redox-active units located at different sites in a molecular architecture. Three of the ferrocene-porphyrins have linkers of different lengths between the ferrocene and porphyrin. The fourth ferrocene-porphyrin has two ferrocenes positioned at the lateral sites on the porphyrin. The latter architecture is designed to provide a shorter distance between the electroactive surface and the ferrocene while maintaining an upright orientation of the porphyrin. Each ferrocene-porphyrin affords three cationic oxidation states (ferrocene monocation, porphyrin monocation, porphyrin dication) in addition to the neutral state, thereby affording the capability of storing two bits of information. Each ferrocene-porphyrin bears an S-acetyl or S-(N-ethyl)carbamoyl-protected thiol moiety, thereby avoiding handling of free thiols. Each ferrocene-porphyrin forms a self-assembled monolayer (SAM) on gold via in situ cleavage of the thiol protecting group. The SAM of each array is electrochemically robust and exhibits three well-resolved, reversible oxidation waves.     

A compact all-carbon tripodal tether affords high coverage of porphyrins on silicon surfaces

[Structure: See text] Redox-active molecules designed to give high charge density on electroactive surfaces are essential for applications in molecular information storage. To achieve a small molecular footprint and thereby high surface charge density, a compound consisting of a triallyl tripod attached via a p-phenylene unit to a porphyrin (1) has been synthesized. The zinc chelate of 1 (Zn-1) was attached to Si(100). Electrochemical measurements indicate that the molecular footprint (75 A) in the monolayer is only approximately 50% larger than the minimum achievable, indicating high surface coverage. IR spectroscopy indicates that the bands due to the nu(C=C) (1638 cm(-1)) and gamma(CH) (915 cm(-1)) vibrations present in the solid sample (KBr pellet) are absent from the spectra of the monolayers of Zn-1, consistent with saturation of the double bond in each of the three legs of the tripod upon the hydrosilylation process accompanying attachment. Comparison of the relative intensities of the in-plane (998 cm(-1)) versus out-of-plane (797 cm(-1)) porphyrin modes indicates the average tilt angle (alpha) of the porphyrin ring with respect to the surface normal is approximately 46 degrees , a value also observed for analogous porphyrins tethered to Si(100) via monopodal carbon linkers. Accordingly, the higher packing densities afforded by the compact tripodal linker are not due to a more upright orientation on the surface. The charge-retention half-lives (t1/2) for the first oxidation state of the Zn-1 monolayers increase from 10 to 50 s at low surface coverage (1-5 x 10(-11) mol.cm(-2)) to near 200 s at saturation coverage (approximately 2 x 10(-10) mol.cm(-2)). Taken together, the high surface charge density (despite the lack of upright orientation) of the triallyl-tripodal porphyrin makes this construct a viable candidate for molecular information storage applications.     

Synthesis of thiol-derivatized europium porphyrinic triple-decker sandwich complexes for multibit molecular information storage

The storage of multiple bits of information at the molecular level requires molecules with a large number of distinct oxidation states. Lanthanide triple-decker sandwich molecules employing porphyrins and phthalocyanines afford four cationic states and are very attractive for molecular information storage applications. Five triple-decker building blocks have been prepared of the type (phthalocyanine)Eu(phthalocyanine)Eu(porphyrin), each bearing one iodo, one ethyne, or one iodo and one ethyne group attached to the porphyrin unit. Two triple-decker building blocks with different oxidation potentials were derivatized with an S-acetylthiophenyl unit for attachment to an electroactive surface. To explore the preparation of arrays comprised of triple deckers, which may lead to the storage of a larger number of bits, two types of dyads of triple deckers were prepared. An ethyne-linked dyad of triple deckers bearing one S-acetylthiophenyl unit was prepared via repetitive Sonogashira couplings, and a butadiyne-linked dyad was prepared via a modified Glaser coupling. The triple deckers were characterized by absorption spectroscopy, laser-desorption mass spectrometry, and (1)H NMR spectroscopy. The thiol-derivatized triple deckers form self-assembled monolayers (SAMs) on gold via in situ cleavage of the thiol protecting group. The SAM of each array is electrochemically robust and exhibits three well-resolved, reversible oxidation waves. These electrochemical characteristics indicate that these types of molecules are well suited for storing multiple bits of information.     

Triple-decker sandwich compounds bearing compact triallyl tripods for molecular information storage applications

The design of redox-active molecules that afford multistate operation and high charge density is essential for molecular information storage applications. Triple-decker sandwich compounds composed of two lanthanide metal ions and three porphyrinic ligands exhibit a large number of oxidation states within a relatively narrow electrochemical window. High charge density requires a small footprint upon tethering triple deckers to an electroactive surface. All triple deckers examined to date for information storage have been tethered via the terminal ligand and have exhibited large footprints (approximately 670 A2). Five new homonuclear (Eu or Ce) triple deckers have been prepared (via statistical or rational methods) to examine the effect of tether attachment site on molecular footprint. Three triple deckers are tethered via the terminal ligand (porphyrin) or central ligand (porphyrin or imidazophthalocyanine), whereas two triple deckers each bear two tethers, one at each terminal ligand. The tether is a compact triallyl tripod. Monolayers of the triple deckers on Si(100) were examined by electrochemical and FTIR techniques. Each triple decker exhibited the expected four resolved voltammetric waves, owing to formation of the mono-, di-, tri-, and tetracations. The electrochemical studies of surface coverage (gamma, obtained by integrating the voltammetric waves) reveal that coverages approaching 10(-10) mol cm(-2), corresponding to a molecular footprint of approximately 170 A2, are readily achieved for all five of the triple deckers. The surface coverage observed for the tripodal functionalized triple deckers is approximately 4-fold higher than that obtained for monopodal-functionalized triple deckers (carbon, oxygen, or sulfur anchor atoms) attached to either Si(100) or Au(111). The fact that similar, relatively high, surface coverages can be achieved regardless of the location (or number) of the tripodal tether indicates that the tripodal functionalization, rather than the location of the tether, is the primary determinant of the packing density.     

Diverse redox-active molecules bearing identical thiol-terminated tripodal tethers for studies of molecular information storage

To examine the effects of molecular structure on charge storage in self-assembled monolayers (SAMs), a family of redox-active molecules has been prepared wherein each molecule bears a tether composed of a tripodal linker with three protected thiol groups for surface attachment. The redox-active molecules include ferrocene, zinc porphyrin, ferrocene-zinc porphyrin, magnesium phthalocyanine, and triple-decker lanthanide sandwich coordination compounds. The tripodal tether is based on a tris[4-(S-acetylthiomethyl)phenyl]-derivatized methane. Each redox-active unit is linked to the methane vertex by a 4,4'-diphenylethyne unit. The electrochemical characteristics of each compound were examined in solution and in SAMs on Au. Redox-kinetic measurements were also performed on the SAMs (with the exception of the magnesium phthalocyanine) to probe (1) the rate of electron transfer in the presence of an applied potential and (2) the rate of charge dissipation after the applied potential is disconnected. The electrochemical studies of the SAMs indicate that the tripodal tether provides a more robust anchor to the Au surface than does a tether with a single site of attachment. However, the electron-transfer and charge-dissipation characteristics of the two tethers are generally similar. These results suggest that the tripodal tether offers superior stability characteristics without sacrificing electrochemical performance.     

Comparison of electron-transfer rates for metal- versus ring-centered redox processes of porphyrins in monolayers on Au(111)

The standard electron-transfer rate constants ( k ( 0 )) are measured for redox processes of Fe versus Zn porphyrins in monolayers on Au(111); the former undergoes a metal-centered redox process (conversion between Fe (III) and Fe (II) oxidation states) whereas the latter undergoes a ring-centered redox process (conversion between the neutral porphyrin and the pi-cation radical). Each porphyrin contains three meso-mesityl groups and a benzyl thiol for surface attachment. Under identical solvent (propylene carbonate)/electrolyte (1.0 M Bu 4NCl) conditions, the Zn (II) center has a coordinated Cl (-) ion when the porphyrin is in either the neutral or oxidized state. In the case of the Fe porphyrin, two species are observed a low-potential form ( E l (0) approximately -0.6 V) wherein the metal center has a coordinated Cl (-) ion when it is in either the Fe (II) or Fe (III) state and a high-potential form ( E h (0) approximately +0.2 V) wherein the metal center undergoes ligand exchange upon conversion from the Fe (III) to Fe (II) states. The k ( 0 ) values observed for all of the porphyrins depend on surface concentration, with higher concentrations resulting in slower rates, consistent with previous studies on porphyrin monolayers. The k ( 0 ) values for the ring-centered redox process (Zn chelate) are 10-40 times larger than those for the metal-centered process (Fe chelate); the k ( 0 ) values for the two forms of the Fe porphyrin differ by a factor of 2-4 (depending on surface concentration), the Cl (-) exchanging form generally exhibiting a faster rate. The faster rates for the ring- versus metal-centered redox process are attributed to the participating molecular orbitals and their proximity to the surface (given that the porphyrins are relatively upright on the surface): a pi molecular orbital that has significant electron density at the meso-carbon atoms (one of which is the site of attachment of the linker to the surface anchoring thiol) versus a d-orbital that is relatively well localized on the metal center.     

Investigation of tightly coupled porphyrin arrays comprised of identical monomers for multibit information storage

Our prior designs for molecular-based information storage devices have employed multiple redox-active units organized in weakly coupled, covalently linked arrays. To explore a simpler design, we report here the synthesis of porphyrin arrays where porphyrins with identical oxidation potentials are directly linked to one another instead of joined via a molecular linker. Oxidative coupling with AgPF(6) of zinc(II)-5,15-bis(4-tert-butylphenyl)-10-phenylporphyrin, obtained by a rational synthesis, afforded the expected dimer joined by a meso-meso linkage and an unexpected trimer joined by meso-meso linkages. For attachment to an electroactive surface we synthesized a meso-linked porphyrin dimer with a thiol-linker in one of the meso positions. The S-acetyl protecting group was used to avoid handling free thiol groups. Coupling of zinc(II)-5,10,15-tris(3, 5-di-tert-butylphenyl)porphyrin ("upper half") and zinc(II)-5-[4-(S-acetylthio)phenyl]-10,20-bis(3, 5-di-tert-butylphenyl)porphyrin ("lower half") afforded three different meso-linked dimers with the desired dimer as the main product. Electrochemical examination of the meso-linked dimer in solution shows that the first two oxidation potentials of the array differ by approximately 0.15 V and straddle the value exhibited by the monomeric constituents. The third and fourth oxidation potentials of the array are also split although to a lesser extent ( approximately 0.08 V) than the first and second. For the meso-linked trimer, the first three oxidation waves are also split; however, these waves are severely overlapped. The electrochemical behavior of the dimers and trimer is indicative of strong electronic interactions among the porphyrins. The thiol-derivatized meso-linked dimers form self-assembled monolayers (SAMs) on gold via in situ cleavage of the S-acetylthio protecting group. The porphyrin SAM exhibits four well-resolved oxidation waves. Regardless, the meso-meso linkage is relatively unstable upon formation of the pi-cation radical(s). This characteristic indicates that the structural motif is of limited utility for molecular information storage elements.     

Giant porphyrin disks: control of their self-assembly at liquid-solid interfaces through metal-ligand interactions

The synthesis and self-assembly behaviour of porphyrin dodecamers 1H(2) and Zn-1, which consist of twelve porphyrins that are covalently attached to a central aromatic core, is described. According to STM, 1D and 2D NMR studies, and molecular modelling calculations, the porphyrin dodecamers have a yo-yo-shaped structure. Their large pi surface, in combination with their disk-like shape, allows them to form self-assembled structures, which in the case of Zn-1 can be tuned by adding bidentate ligands. The self-assembly of the molecules at the liquid-solid interface of 1-phenyloctane with highly oriented pyrolytic graphite or Au(111) was imaged by using STM. The porphyrin disks in the self-assembled arrays have an edge-on orientation on the surface. The addition of bidentate axial ligands to the Zn-1 molecules in the arrays allows their intermolecular distance to be precisely controlled.     

卟啉自组装膜与分子信息存储

基于卟啉化合物良好的光电性能,结合自组装膜技术,对其信息存储进行了研究.卟啉分子单体的结构、自组装膜表面卟啉的空间定位以及自组装膜基底材料的选择等对卟啉的信息存储产生着重要影响.目前,卟啉自组装膜的信息存储研究已由单点存储向超高密度多点存储发展.     

Metal-molecule interactions upon deposition of copper overlayers on reactively functionalized porphyrin monolayers on Si(100)

The interaction of evaporated Cu deposited on a series of porphyrins in monolayers covalently attached to Si(100) substrates was investigated using cyclic voltammetry and FTIR spectroscopy. Each porphyrin contains a triallyl tripod attached to the porphyrin via a p-phenylene unit. The tripod anchors the porphyrin to the Si(100) substrate via hydrosilylation of the allyl groups. Two of the porphyrins are Zn chelates that possess meso p-cyanophenyl substituentsone, ZnP-CND, contains a single group opposite (distal) to the tripodal surface anchor, whereas the other, ZnP-CNL, contains two groups orthogonal (lateral) to the surface anchor. A third Zn porphyrin, ZnP, containing nonreactive p-tolyl groups at all three nonanchoring meso positions, was examined for comparison. The fourth porphyrin, FbP-HD, is a metal-free species (free base) that contains nonreactive phenyl (distal) and p-tolyl groups (lateral) at the three nonanchoring meso positions. The fifth porphyrin, CuP-HD, is the Cu chelate of FbP-HD, and serves as a reference complex for evaluating the effects of Cu metal deposition onto FbP-HD. The studies indicate that all of the porphyrin monolayers are robust under the conditions of Cu deposition, experiencing no noticeable degradation. In addition, the Cu metal does not penetrate through the monolayer to form electrically conductive filaments. For the ZnP-CND monolayers, the deposited Cu quantitatively reacts/complexes with the distal cyano group. In contrast, for the ZnP-CNL monolayers no reaction/complexation of the lateral cyano groups is observed. For the FbP-HD monolayers, Cu deposition results in quantitative insertion of Cu into the free base porphyrin. Collectively, the studies demonstrate that porphyrin monolayers are amenable to direct deposition of Cu overlayers and that functionalization of the porphyrins can be used to mediate the attributes of the metal-molecule junction.     

Stepwise formation and characterization of covalently linked multiporphyrin-imide architectures on Si(100)

A major challenge in molecular electronics and related fields entails the fabrication of elaborate molecular architectures on electroactive surfaces to yield hybrid molecular/semiconductor systems. A method has been developed for the stepwise synthesis of oligomers of porphyrins linked covalently via imide units. A triallyl-porphyrin bearing an amino group serves as the base unit on Si(100), and the alternating use of a dianhydride (3,3',4,4'-biphenyltetracarboxylic dianhydride) and a porphyrin-diamine for reaction enables the rapid and simple buildup of oligomers composed of 2-5 porphyrins. The properties of these porphyrin "multad" films on Si(100) were interrogated using a variety of techniques. The charge densities of the redox-active porphyrin oligomers were determined via electrochemical methods. The stepwise growth was evaluated in detail via Fourier transform infrared (FTIR) spectroscopy and by selected X-ray photoelectron spectroscopic (XPS) studies. The morphology was probed via AFM methods. Finally, the thickness was evaluated by using a combination of ellipsometry and AFM height profiling, accompanied by selected XPS studies. Collectively, these studies demonstrate that high charge density, ultrathin, multiporphyrin films of relatively well-controlled thickness can be grown in a stepwise fashion using the imide-forming reaction. The increased charge densities afforded by the porphyrin multads may prove important for the fabrication of molecular-based information-storage devices. This bottom-up process for construction of surface-tethered molecular architectures complements the top-down lithographic approach for construction of functional devices with nanoscale dimensions.