Swallowtail porphyrins: synthesis, characterization and incorporation into porphyrin dyads

The incorporation of symmetrically branched tridecyl ("swallowtail") substituents at the meso positions of porphyrins results in highly soluble building blocks. Synthetic routes have been investigated to obtain porphyrin building blocks bearing 1-4 swallowtail groups. Porphyrin dyads have been synthesized in which the zinc or free base (Fb) porphyrins are joined by a 4,4'-diphenylethyne linker and bear swallowtail (or n-pentyl) groups at the nonlinking meso positions. The swallowtail-substituted Zn(2)- and ZnFb-dyads are readily soluble in common organic solvents. Static absorption and fluorescence spectra and electrochemical data show that the presence of the swallowtail groups slightly raises the energy level of the filled a(2u)(pi) HOMO. EPR studies of the pi-cation radicals of the swallowtail porphyrins indicate that the torsional angle between the proton on the alkyl carbon and p-orbital on the meso carbon of the porphyrin is different from that of a porphyrin bearing linear pentyl groups. Regardless, the swallowtail substituents do not significantly affect the photophysical properties of the porphyrins or the electronic interactions between the porphyrins in the dyads. In particular, time-resolved spectroscopic studies indicate that facile excited-state energy transfer occurs in the ZnFb dyad, and EPR studies of the monocation radical of the Zn(2)-dyad show that interporphyrin ground-state hole transfer is rapid.     

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.     

Picolinic diamide pincers for transition metal complexation and their electrochemical properties

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.     

Regiospecifically alpha-13C-labeled porphyrins for studies of ground-state hole transfer in multiporphyrin arrays

Insight into the electronic communication between the individual constituents of multicomponent molecular architectures is essential for the rational design of molecular electronic and/or photonic devices. To clock the ground-state hole/electron-transfer process in oxidized multiporphyrin architectures, a p-diphenylethyne-linked zinc porphyrin dyad was prepared wherein one porphyrin bears two (13)C atoms and the other porphyrin is unlabeled. The (13)C atoms are located at the 1- and 9-positions (alpha-carbons symmetrically disposed to the position of linker attachment), which are sites of electron/spin density in the a(1u) HOMO of the porphyrin. The (13)C labels were introduced by reaction of KS(13)CN with allyl bromide to give the allyl isothiocyanate, which upon Trofimov pyrrole synthesis followed by methylation gave 2-(methylthio)pyrrole-2-(13)C. Reaction of the latter with paraformaldehyde followed by hydrodesulfurization gave dipyrromethane-1,9-(13)C, which upon condensation with a dipyrromethane-1,9-dicarbinol bearing three pentafluorophenyl groups gave the tris(pentafluorophenyl)porphyrin bearing (13)C labels at the 1,9-positions and an unsubstituted meso (5-) position. Zinc insertion, bromination at the 5-position, and Suzuki coupling with an unlabeled porphyrin bearing a suitably functionalized diphenylethyne linker gave the regiospecifically labeled zinc porphyrin dyad. Examination of the monocation of the isotopically labeled dyad via electron paramagnetic resonance (EPR) spectroscopy (and comparison with the monocations of benchmark monomers, where hole transfer cannot occur) showed that the hole transfer between porphyrin constituents of the dyad is slow (<10(6) s(-1)) on the EPR time scale at room temperature. The slow rate stems from the a(1u) HOMO of the electron-deficient porphyrins, which has a node at the site of linker connection. In contrast, analogous dyads of electron-rich porphyrins (wherein the HOMO is a(2u) and has a lobe at the site of linker connection) studied previously exhibit rates of hole transfer that are fast (>5 x 10(7) s(-1)) on the EPR time scale at room temperature.     

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.     

(Metallo)porphyrins as Potent Phototoxic Anti‐Cancer Agents after Irradiation with Red Light

Novel photoactive (metallo)porphyrins were synthesised and characterised. When irradiated with light at a wavelength greater than 600 nm, these porphyrins act as photosensitisers and show high cytotoxicity towards two different human cancer cell lines with IC₅₀ values down to 0.4 μM . A paramagnetic copper(II) porphyrin is the first photosensitiser to display excellent phototoxicity, explained by the electron paramagnetic resonance (EPR) spin trapping of hydroxy radicals and experimentally confirmed by the discovery of elevated levels of reactive oxygen species (ROS) inside A2780 cells after irradiation with red light. This finding indicates that paramagnetic compounds should be considered for photodynamic therapy (PDT). Furthermore, an additive effect of cisplatin and a zinc porphyrin, both at subtherapeutic concentrations of 0.22 μm, was observed.     

Alkylthio unit as an alpha-pyrrole protecting group for use in dipyrromethane synthesis

The synthesis of porphyrin precursors requires the successive introduction of substituents at the pyrrole alpha- and alpha'-positions (2- and 5-, respectively). An alpha-pyrrole substituent that serves as a temporary masking agent and is not deactivating would greatly facilitate such syntheses, particularly for beta-(3,4)-unsubstituted pyrroles, but has heretofore not been available. A series of alpha-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 alpha-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 alpha-RS group was removed by hydrodesulfurization using Raney nickel or nickel complexes. This stoichiometric synthesis using the alpha-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.     

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.     

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.     

Meso-13C-labeled porphyrins for studies of ground-state hole transfer in multiporphyrin arrays

Understanding electronic communication among interacting chromophores provides the foundation for a variety of applications. The ground-state electronic communication in diphenylethyne-linked zinc-porphyrin dyads has been investigated by a novel molecular design strategy that entails introduction of a 13C-atom (*) at specific sites of the porphyrins where there is substantial electron density in the relevant frontier (highest occupied) molecular orbital. The site of 13C substitution is at a meso-position, either the site of attachment of the linker (proximal, "P") or the site trans to the linker (distal, "D"). The substituents (R) at the non-linking meso-positions are mesityl, tridec-7-yl ("swallowtail"), or p-tolyl groups. Altogether five isotopically labeled porphyrin dyads have been prepared. The hole/electron-transfer properties of one-electron oxidized dyads have been examined by electron paramagnetic resonance (EPR) spectroscopy. The introduction of the meso-13C label provides a "clock" (via the hyperfine interactions) that allows investigation of a time scale for hole transfer that is 3-4 times shorter than that provided by the natural abundance 14N nuclei of the pyrrole nitrogen atoms. The EPR studies indicate that the hole transfer, which has been previously shown to be fast on the time scale of the 14N hyperfine clock ( approximately 220 ns), remains fast on the time scale of the 13C hyperfine clock ( approximately 50 ns).