Surface-attached Groups Research Articles

Vibrational structure analysis of cobalt fluoro-porphyrin surface coatings on gallium phosphide

Grazing angle attenuated total reflectance Fourier transform infrared (GATR–FTIR) spectroscopy is used to characterize chemically modified gallium phosphide (GaP) surfaces containing grafted cobalt(II) porphyrins with 3-fluorophenyl substituents installed at the meso-positions. In these hybrid constructs, porphyrin surface attachment is achieved using either a two-step method involving coordination of cobalt fluoro-porphyrin metal centers to nitrogen sites on an initially applied thin-film polypyridyl surface coating, or via a direct modification strategy using a cobalt fluoro-porphyrin precursor bearing a covalently bonded 4-vinylphenyl surface attachment group at a [Formula: see text]-position. Both surface-attachment chemistries leverage the UV-induced immobilization of alkenes but result in distinct structural connectivities of the grafted porphyrin units and their associated vibrational spectra. In particular, the in-plane deformation vibrational frequency of metalloporphyrin components in samples prepared via coordination to the polymeric interface is characterized by an eight wavenumber shift to higher frequencies compared to that measured on metalloporphyrin-modified surfaces prepared using the one-step attachment method. The more rigid ring structure in the polymeric architecture is consistent with coordination of porphyrin cobalt centers to pyridyl-nitrogen sites on the surface graft. These results demonstrate the use of GATR–FTIR spectroscopy as a sensitive tool for characterizing porphyrin-modified surfaces with absorption signals that are close to the detection limits of many common spectroscopic techniques.

Tetrapyrrolic Surface Coatings for Applications in Photoelectrosynthetic Fuel Production

Hybrid materials capable of linking light capture and conversion technologies with the ability to drive reductive chemical transformations are attractive as components in photoelectrosynthetic cells. [1] We have recently reported methods of applying molecular surface coatings composed of metalloporphyrin redox catalysts onto solid-state substrates that are either conductive or semi-conductive. [2-5] The metalloporphyrin catalysts used in this work are capable of activating electrochemical transformations including the conversion of protons to hydrogen and carbon dioxide to carbon monoxide. In one approach, metalloporphyrin precursors are prepared via a novel synthetic strategy to yield a macrocycle with a pendent 4-vinylphenyl surface attachment group at the beta-position of the porphyrin ring structure. [2] This modification allows use of a photo-induced immobilization chemistry to attach intact metalloporphyrins to a range of (semi)conducting surfaces. In addition, we have shown that initial application of thin-film polymer surface coatings can provide a molecular interface for assembling metalloporphyrin catalysts in a subsequent wet chemical treatment step. [3] In this presentation, spectroscopic characterization of these materials coupled with electrochemical analysis will be presented. These findings offer an improved understanding of the structure and function relationships governing this class of materials. A. M. Beiler, D. Khusnutdinova, S. I. Jacob, G. F. Moore, Ind. & Eng. Chem. Research , 55 , 5306-5314 (2016); DOI: 10.1021/acs.iecr.6b00478D. Khusnutdinova, A. M. Beiler, B. L. Wadsworth, S. I. Jacob, G. F. Moore, Chem. Sci., 8, 253-259 (2017); DOI: 10.1039/c6sc02664hA. M. Beiler, D. Khusnutdinova, B. L. Wadsworth, G. F. Moore, Inorg. Chem. , 56, 12178 (2017); DOI: 10.1021/acs.inorgchem.7b01509A. M. Beiler, D. Khusnutdinova, S. I. Jacob, G. F. Moore, ACS Appl. Mater. Interfaces , 8, 10038-10043 (2016); DOI: 10.1021/acsami.6b01557B. L. Wadsworth, A. M. Beiler, D. Khusnutdinova, S. I. Jacob, G. F. Moore, ACS Catal. 6, 8048-8057 (2016); DOI: 10.1021/acscatal.6b02194 Figure 1

Synthesis and characterization of a cobalt(II) tetrakis(3-fluorophenyl) porphyrin with a built-in 4-vinylphenyl surface attachment moiety

Metalloporphyrins serve important roles in biology and as components in emerging technological assemblies for energy conversion. In this report, we describe the synthesis and characterization of a novel cobalt(II) 5,10,15,20-tetrakis (3-fluorophenyl)porphyrin bearing a 4-vinylphenyl surface attachment group at a beta position on the macrocycle. Electrochemical measurements show the 3-fluorophenyl groups at the meso positions of the porphyrin perturb the reduction potentials of the complex to more positive values as compared to non-fluorinated analogs, thus allowing access to reduced cobalt porphyrin species at significantly less negative applied bias potentials. The complex, cobalt(II) 5,10,15,20-tetrakis(3-fluorophenyl)-2-(4-vinylphenyl)porphyrin, is abbreviated in this article as Gov-1 in honor of Govindjee for his pioneering investigations in the role of fluorine as a promoter of novel protein-substrate interactions and the inspirational role he continues to have in encouraging young investigators in the areas of natural and artificial photosynthesis.

Effect of the Porous Texture of Activated Carbons on the Electrochemical Properties of Molecule-Grafted Carbon Products in Organic Media

Two commercial activated carbons, different from their texture, were grafted with electroactive molecules and tested for determining what texture is well-suited for the grafting. Microporous and mesoporous carbons, having approximately the same BET surface area, were selected. The electroactive molecule consists in a naphthalimide compound having an amine as surface attachment group. The present work was divided in two parts. In a first part, the modified carbons were characterized by thermal gravimetric analysis, X-ray photoelectron spectroscopy, elemental chemical analysis and nitrogen gas adsorption measurements have been used for studying the impact of the grafting on the textural properties of carbons. In a second part, the electrochemical properties of the modified carbons were studied in propylene carbonate +1 M Bu4NBF4. Results show that the grafting ruins the performances of the microporous carbon, while the mesoporous carbon appears well-suited for the grafting, showing a good compromise between electrolyte-accessibility and ionic transportation.

Structural Optimization of Photoswitch Ligands for Surface Attachment of α-Chymotrypsin and Regulation of Its Surface Binding

A modified gold surface that allows photoregulated binding of alpha-chymotrypsin has previously been reported. Here the development of this surface is reported, through the synthesis of a series of trifluoromethyl ketones and alpha-keto esters containing the azobenzene group and a surface attachment group as photoswitch inhibitors of alpha-chymotrypsin. All of the compounds are inhibitors of the enzyme, with activity that can be modulated by photoisomerization. The best photoswitch shows a reversible change in IC(50) inhibition constant of >5.3 times on photoisomerization. The trifluoromethyl ketone 1 exhibited excellent photoswitching and was attached to a gold surface in a two-step procedure involving an azide-alkyne cycloaddition. The resulting modified surface bound alpha-chymotrypsin to a degree that could be modulated by UV/Vis irradiation, showing "slow-tight" enzyme binding as observed for inhibitors in solution.

Synthesis of porphyrins for metal deposition studies in molecular information storage applications

Porphyrins may serve as the active charge-storage medium in memory chips. A series of 14 porphyrins (trans-A2B2, trans-AB2C, or A4 type; free base, copper or zinc chelate) has been synthesized for investigation of metal deposition on porphyrin monolayers. Each trans-AB2C porphyrin is equipped with a surface attachment group and a distal functional group (or flanking functional groups). The surface attachment groups include S-acetylthiomethyl, hydroxymethyl, TMS-ethynyl, allyl, and triallylmethyl; the functional groups include amino, cyano, dipyrrin-5-yl, formyl, and nitro; all of which are attached to the p-position of a porphyrin meso-phenyl group. Two non-redox-active triallylmethyl-substituted arenes were also prepared for use as control compounds.

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.

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.

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.

Porphyrin architectures tailored for studies of molecular information storage.

A molecular approach to information storage employs redox-active molecules tethered to an electroactive surface. Zinc porphyrins tethered to Au(111) or Si(100) provide a benchmark for studies of information storage. Three sets of porphyrins have been synthesized for studies of the interplay of molecular design and charge-storage properties: (1) A set of porphyrins is described for probing the effect of surface attachment atom on electron-transfer kinetics. Each porphyrin bears a meso-CH2X group for surface attachment where X = OH, SAc, or SeAc. (2) A set of porphyrins is described for studying the effect of surface-charge density in monolayers. Each porphyrin bears a benzyl alcohol for surface attachment and three nonlinking meso substituents of a controlled degree of bulkiness. (3) A set of porphyrins is described that enables investigation of on-chip patterning of the electrolyte. Each porphyrin bears a formyl group distal to the surface attachment group for subsequent derivatization with a molecular entity that comprises the electrolyte. Taken together, this collection of molecules enables a variety of studies to elucidate design issues in molecular-based information storage.

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.

Electrochemical Control of the Second Harmonic Generation Property of Self-Assembled Monolayers Containing a trans-Ferrocenyl-Nitrophenyl Ethylene Group on Gold

The second harmonic (SH) intensity at gold electrodes modified with self-assembled monolayers (SAMs) of two novel molecules, which have trans-ferrocenyl-nitrophenyl ethylene and thiol groups as the second harmonic generation and surface-attached groups, respectively, was found to be strongly dependent on the oxidation state of the ferrocene group. The SH intensity increased when the ferrocene moiety of the SAMs was oxidized electrochemically to the ferricenium cation, and it decreased and returned to the original value when the ferricenium cation of the ferrocene moiety was reduced to neutral ferrocene. These changes were reversible and were repeated many times. The molecular orientation change in each SAM upon reduction/oxidation of the ferrocene moiety was investigated by in situ Fourier transform infrared reflection−absorption spectroscopy, and the hyperpolarizabilities of these molecules were calculated using a computational electronic structure model. The origin of the SH intensity change is discussed on the basis of these results.