Synthesis of Swallowtail-Substituted Multiporphyrin Rods

The availability of multiporphyrin arrays with defined architectures and good solubility in organic solvents is essential for a wide variety of physical studies. Herein the synthesis of linear multiporphyrin arrays (triads, tetrad, pentad) bearing solubilizing 7-tridecyl (swallowtail) groups is presented. The rodlike arrays are composed of zinc porphyrins at the termini and 1, 2, or 3 free base porphyrins at the core. The free base porphyrins in the tetrad and pentad are joined to each other via p-phenylene linkers whereas the zinc porphyrins in each array are attached to the core free base porphyrins via 1,4-diphenylethyne linkers. The arrays are designed for studies of interporphyrin electronic communication

Theoretical investigation of thiophene-linked porphyrin-perylene photosensitiser for bulk-heterojunction solar cells

<p>In this work, quantum chemical calculations were applied to investigate a new series of porphyrin-perylene conjugates for their potential use as light-harvesting compounds in bulk-heterojunction solar cells. Molecular design relies on the integration of a electron-donating porphyrin and a electron-accepting perylene units via a spacer containing 0 to 10 thiophene rings. Electronic properties of these push–pull systems were studied using density functional theory (DFT) and time-dependent DFT approaches. Results show that introduction of thiophene-based spacer with different number of the thiophene rings significantly affects the electronic and absorption properties of the molecules. According to its suitable energy gap, efficient charge transfer and appropriate absorption behaviour determined by the calculation, the derivative having a terthiophene linker should be as the optimal compound among all molecules studied herein.</p

Picolinic diamide pincers for transition metal complexation and their electrochemical properties

<div><p>A series of novel molecular pincers was successfully synthesised by a copper-free Sonogashira coupling methodology. Complexation of the pincers with Cu(II), Ni(II) and Co(II) was performed in the presence of triethylamine. The formation of the desirable pincer ligands and complexes was confirmed by nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy and mass spectrometry. Electrochemical properties of the complexes of the target pincer dimers investigated by means of cyclic voltammetry suggested that the pincer dimers should be able to serve as an electron donor for phenyl-C61-butyric acid methyl ester and as an electron acceptor for poly(3-hexylthiophene) in bulk heterojunction solar cells.</p></div

Alkylthio Unit as an α-Pyrrole Protecting Group for Use in Dipyrromethane Synthesis

The synthesis of porphyrin precursors requires the successive introduction of substituents at the pyrrole α- and α‘-positions (2- and 5-, respectively). An α-pyrrole substituent that serves as a temporary masking agent and is not deactivating would greatly facilitate such syntheses, particularly for β-(3,4)-unsubstituted pyrroles, but has heretofore not been available. A series of α-RS groups (R = Me, Et, n-decyl, Ph) have been investigated in this regard, including the determination of the kinetics of substitution at the pyrrolic 3-, 4-, and 5-positions and the application to dipyrromethane formation. The RS group was readily introduced into the pyrrole α-position by the reaction of 2-thiocyanatopyrrole (prepared from pyrrole, ammonium thiocyanate, and iodine) and the corresponding Grignard reagent RMgBr. Each 2-alkylthio group activated the pyrrole ring toward deuteration at the 3- or 5- (vs 4-) position. The dipyrromethane synthesis was carried out using a 2:1 ratio of 2-(RS)pyrrole/benzaldehyde with a catalytic amount of InCl3 at room temperature in the absence of any solvent. The α-RS group was removed by hydrodesulfurization using Raney nickel or nickel complexes. This stoichiometric synthesis using the α-RS-protected pyrrole is in contrast to the traditional synthesis that employs an aldehyde and 25−100 mol equiv of pyrrole. Six meso-substituted dipyrromethanes were prepared by the reaction of 2-(n-decylthio)pyrrole/aldehyde/InCl3 (2.2:1:0.2 ratio) followed by hydrodesulfurization. Other reactions of the 1,9-bis(RS)dipyrromethane include oxidation to give (i) the 1,9-bis(RS)dipyrrin or (ii) the 1,9-bis(RSO2)dipyrromethane, which underwent subsequent complexation with dibutyltin dichloride. In summary, under mild reaction conditions, the 2-alkylthio group is readily introduced to the pyrrole nucleus, directs electrophilic substitution to the 5-position, and is readily removed as required for elaboration of porphyrinic precursors

Alkylthio Unit as an α-Pyrrole Protecting Group for Use in Dipyrromethane Synthesis

The synthesis of porphyrin precursors requires the successive introduction of substituents at the pyrrole α- and α‘-positions (2- and 5-, respectively). An α-pyrrole substituent that serves as a temporary masking agent and is not deactivating would greatly facilitate such syntheses, particularly for β-(3,4)-unsubstituted pyrroles, but has heretofore not been available. A series of α-RS groups (R = Me, Et, n-decyl, Ph) have been investigated in this regard, including the determination of the kinetics of substitution at the pyrrolic 3-, 4-, and 5-positions and the application to dipyrromethane formation. The RS group was readily introduced into the pyrrole α-position by the reaction of 2-thiocyanatopyrrole (prepared from pyrrole, ammonium thiocyanate, and iodine) and the corresponding Grignard reagent RMgBr. Each 2-alkylthio group activated the pyrrole ring toward deuteration at the 3- or 5- (vs 4-) position. The dipyrromethane synthesis was carried out using a 2:1 ratio of 2-(RS)pyrrole/benzaldehyde with a catalytic amount of InCl3 at room temperature in the absence of any solvent. The α-RS group was removed by hydrodesulfurization using Raney nickel or nickel complexes. This stoichiometric synthesis using the α-RS-protected pyrrole is in contrast to the traditional synthesis that employs an aldehyde and 25−100 mol equiv of pyrrole. Six meso-substituted dipyrromethanes were prepared by the reaction of 2-(n-decylthio)pyrrole/aldehyde/InCl3 (2.2:1:0.2 ratio) followed by hydrodesulfurization. Other reactions of the 1,9-bis(RS)dipyrromethane include oxidation to give (i) the 1,9-bis(RS)dipyrrin or (ii) the 1,9-bis(RSO2)dipyrromethane, which underwent subsequent complexation with dibutyltin dichloride. In summary, under mild reaction conditions, the 2-alkylthio group is readily introduced to the pyrrole nucleus, directs electrophilic substitution to the 5-position, and is readily removed as required for elaboration of porphyrinic precursors

Highly Soluble Indigo Derivatives as Practical Diesel Absorption Markers

This work describes the practical production of novel indigo derivatives from commercially available and economically friendly indigo and investigation for their potential use as diesel markers. Introduction of solubilizing long alkyl chains into an indigo molecule via formation of arylimino moieties at its carbonyl sites and amidation at its amino groups greatly enhances the solubility in diesel and several common organic solvents. Effects of the number and position of the alkyl chains on the absorption behavior of the compounds are discussed. Because of their superior absorption in a region where the diesel cannot absorb, indigo N-arylimine and N-monoacyl-substituted indigo derivatives can serve as diesel absorption markers at detection wavelengths of 590 and 575 nm, respectively. UV–visible spectrophotometric analysis suggested that this target marker is stable in diesel for at least 3 months under ambient conditions. Furthermore, physical testing according to the American Society for Testing and Materials standards indicates that addition of these markers at a concentration of 5 ppm does not significantly affect the physical properties of the original diesel, thus confirming the applicability of these compounds for marking of commercial diesels

New Route to ABCD-Porphyrins via Bilanes

A new strategy for preparing porphyrins that bear up to four different meso-substituents (ABCD-porphyrins) relies on two key reactions. One key reaction entails a directed synthesis of a 1-protected 19-acylbilane by acid-catalyzed condensation at high concentration (0.5 M) of a 1-acyldipyrromethane and a 9-protected dipyrromethane-1-carbinol (derived from a 9-protected 1-acyldipyrromethane). Three protecting groups (X) were examined, including thiocyanato, ethylthio, and bromo, of which bromo proved most effective. The bilanes were obtained in 72−80% yield, fully characterized, and examined by 15N NMR spectroscopy. The second key reaction entails a one-flask transformation of the 1-protected 19-acylbilane under basic, metal-templating conditions to give the corresponding metalloporphyrin. The reaction parameters investigated for cyclization of the bilane include solvent, metal salt, base, concentration, temperature, atmosphere, and time. The best conditions entailed the 1-bromo-19-acylbilane at 100 mM in toluene containing DBU (10 mol equiv) and MgBr2 (3 mol equiv) at 115 °C exposed to air for 2 h, which afforded the magnesium porphyrin in 65% yield. The magnesium porphyrin is readily demetalated to give the free base porphyrin. A stepwise procedure (which entailed treatment of the 1-(ethylthio)-19-acylbilane to oxidation, metal complexation, desulfurization, carbonyl reduction, and acid-catalyzed condensation) was developed but was much less efficient than the one-flask process. The new route to ABCD-porphyrins retains the desirable features of the existing “2 + 2” (dipyrromethane + dipyrromethane-1,9-dicarbinol) method, such as absence of scrambling, yet has significant advantages. The advantages include the absence of acid in the porphyrin-forming step, the use of a metal template for cyclization, the ability to carry out the reaction at high concentration, the lack of a quinone oxidant, avoidance of use of dichloromethane, and the increased yield of macrocycle formation to give the target ABCD-metalloporphyrin

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 μC·cm-2) of the porphyrin dyads makes these constructs viable candidates for molecular-information-storage applications

Characterization of Porphyrin Surface Orientation in Monolayers on Au(111) and Si(100) Using Spectroscopically Labeled Molecules

The synthesis and surface IR characterization are reported for a series of porphyrins bearing vibrational spectroscopic labels that afford distinction of the two in-plane axes of the porphyrin ring. The labeled porphyrins include three different types of tethers for surface attachment, those containing either methylthio (−CH2S−) or benzylthio (−BzS−) for attachment to either Au or Si and a tripodal alkenyl group for attachment to Si. The spectroscopic labels are placed so as to enable distinction between the methyl group of a p-tolyl substituent that lies along the molecular axis distal to the tether versus the p-tolyl substituents that lie along the orthogonal in-plane axis lateral to the tether. The porphyrins include isotopically labeled species containing CD3 and species wherein a CF3 replaces CH3. The spectroscopically labeled molecules allow evaluation of both the tilt angle (θ) with respect to the surface normal and the rotation angle (φ) about the molecular axis. These two angles cannot be uniquely determined for typical porphyrins because the in-plane modes of the porphyrin are (nearly) degenerate, and unique vibrational signatures cannot be identified that define the two orthogonal in-plane axes. The surface IR studies suggest that all of the porphyrins on both Au and Si exhibit a distribution of tilt and rotation angles. The distribution of φ angles is (nearly) random about the molecular axis; the distribution of tilt angles is less broad, owing to steric interactions between the porphyrin substituents and the surface. The surface coverage affects the distribution of both the tilt and rotation angles. At lower surface coverage, the molecules exhibit larger tilt angles and rotation angles, that is, the porphyrin is more coplanar with the surface. The fact that all of the porphyrins, which bear structurally different types of tethers, on both Au and Si exhibit qualitatively similar surface orientation characteristics suggests that the adsorption geometry is primarily controlled by properties intrinsic to the porphyrin macrocycle rather than by properties of the tether and/or the surface

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 °C) and operation (up to 140 °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