NaI-Mediated Electrochemical Cyclization-Desulfurization for the Synthesis of N-Substituted 2-Aminobenzimidazoles.

An electrochemical method for the synthesis of N-substituted 2-aminobenzimidazoles through a NaI-mediated desulfurization-cyclization process is reported. This electrosynthesis method utilizes cost-effective NaI as both a mediator and an electrolyte in a catalytic amount (0.2 equiv), replacing traditional oxidizing reagents. N-Substituted o-phenylenediamines and isothiocyanates undergo a thiourea formation/cyclization/desulfurization process to provide N-substituted 2-aminobenzimidazoles (55 examples, up to 98% yield) in a single reaction vessel. Importantly, this electrochemical methodology is applicable to gram-scale synthesis, maintaining reaction efficiency.

(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.

Electrocatalytic Hydrogen Evolution of Immobilized Copper Complex on Carbonaceous Materials: From Neutral Water to Seawater.

Electrochemical hydrogen evolution reaction (HER) is an appealing strategy to utilize renewable electricity to produce green H2. Moreover, use of neutral-pH electrolyte such as water and seawater for the HER has long been desired for eco-friendly energy production that aligns with net zero emission goal. Herein, new heterogeneous catalysts were developed by dispersing an HER-active copper complex containing N4-Schiff base macrocycle (CuL) on carbonaceous materials, i. e. multi-walled carbon nanotube (CNT) and graphene oxide (GO), via non-covalent interaction and investigated their HER performance. It was found that CuL/GO exhibited higher HER activity than CuL/CNT, possibly due to its significantly larger amount of CuL immobilized onto GO. In addition, CuL/GO showed satisfactory HER performance in a neutral (pH 7) NaCl electrolyte solution. Notably, the performances of CuL/GO were boosted up when performed in natural seawater sample with the faradaic efficiency of 70 % and 3 times higher amount of H2 at -0.6 V vs reversible hydrogen electrode (RHE), in comparison to the HER in a NaCl electrolyte. Furthermore, it possessed a low overpotential of 139 mV at -10 mA/cm2. This demonstrated the potential use of CuL/GO as an effective HER catalyst in seawater for further sustainable development.

Molecularly dispersed nickel complexes on N-doped graphene for electrochemical CO2 reduction.

In this work, new hybrid catalysts based on molecularly dispersed nickel complexes on N-doped graphene were developed for electrochemical CO2 reduction (ECR). Nickel(II) complexes (1-Ni, 2-Ni), and a new crystal structure ([2-Ni]Me), featuring N4-Schiff base macrocycles, were synthesized and investigated for their potential in ECR. Cyclic voltammetry (CV) in NBu4PF6/CH3CN solution demonstrated that the nickel complexes bearing N-H groups (1-Ni and 2-Ni) showed a substantial current enhancement in the presence of CO2, while the absence of N-H groups ([2-Ni]Me) resulted in an almost unchanged voltammogram. This indicated the necessity of the N-H functionality towards ECR in aprotic media. All three nickel complexes were successfully immobilized on nitrogen-doped graphene (NG) via non-covalent interactions. All three Ni@NG catalysts exhibited satisfactory CO2-to-CO reduction in aqueous NaHCO3 solution with the faradaic efficiency (FE) of 60-80% at the overpotential of 0.56 V vs. RHE. The ECR activity of [2-Ni]Me@NG also suggested that the N-H moiety from the ligand is less important in the heterogeneous aqueous system owing to viable hydrogen-bond formation and proton donors from water and bicarbonate ions. This finding could pave the way for understanding the effects of modifying the ligand framework at the N-H position toward fine tuning the reactivity of hybrid catalysts through molecular-level modulation.

A water soluble tri-cationic porphyrin-EDTA conjugate induces apoptosis in human neuroendocrine tumor cell lines.

In this study, a completely water soluble tri-cationic porphyrin-EDTA conjugate was synthesized. We present data demonstrating the tumoristatic effects of the novel fully water soluble cationic porphyrin TMPy(3)PhenEDTA-P-Cl(4) in the dark, in the medullary thyroid carcinoma cell lines MTC-SK and SHER-I and weaker effects in the small intestinal neuroendocrine tumor cell line KRJ-I. In addition, cytotoxic effects were also studied in normal human fibroblasts that represent normal tissue and the results are compared to the tumor cell lines.

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.

Natural rubber/reduced-graphene oxide composite materials: Morphological and oil adsorption properties for treatment of oil spills.

A green sorbent material was fabricated through the simple addition of reduced graphene oxide (rGO) to natural rubber (NR) latex. The effect of rGO content in the NR foam on petroleum oil adsorption was investigated. The addition of rGO in NR increased the petroleum oil adsorption capacity of the resulting NR/rGO (NRG) composite foam (12-21 g g-1) with respect to those of the pure NR foam (8-15 g g-1) and a commercial sorbent (6-7 g g-1). The adsorption capacity was optimal for 0.5 phr rGO (NRG-0.5). Further, the environmental conditions (temperature and waves) affected the oil adsorption capacity of the sorbent materials. The adsorption kinetics of the sorbent materials for crude AXL oil was best described with pseudo-second-order kinetics. The interparticle diffusion model revealed three steps whereas the adsorption isotherms approximated the Langmuir isotherms. Moreover, the oil adsorption mechanisms of the NR and NRG sorbent materials were compared to that of a commercial sorbent. The high elasticity of the NRG-0.5 composite foam improved not only the oil adsorption capacity but also the reusability of the sorbent material. The presence of rGO increased the strength of the NRG-0.5 compared to that of pure NR, which resulted in a high-performance and reusable material with an oil removal efficiency higher than 70% after 30 uses.

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 and investigation of tetraphenyltetrabenzoporphyrins for electrocatalytic reduction of carbon dioxide.

We report the synthesis and electrochemical properties of freebase tetraphenyltetrabenzoporphyrin and its complexes of Zn(ii), Co(ii), Ni(ii), Cu(ii) and Sn(iv) towards electrochemical reduction of carbon dioxide (CO2). Based on cyclic voltammetry, it is shown that central metals significantly affect the electrocatalytic performance in the reduction of CO2 in terms of reduction potential and catalytic current enhancement. At an applied potential of -1.90 V vs. an Ag/AgCl quasi reference electrode for 20 h, the electrocatalytic reduction of CO2 realized by Zn(ii)- and Cu(ii)-tetraphenyltetrabenzoporphyrins to carbon monoxide resulted in faradaic efficiencies of around 48% and 33%, respectively.

Photoelectrocatalytic Synthesis of Hydrogen Peroxide by Molecular Copper-Porphyrin Supported on Titanium Dioxide Nanotubes.

We report on a self-assembled system comprising a molecular copper-porphyrin photoelectrocatalyst, 5-(4-carboxy-phenyl)-10,15,20-triphenylporphyrinatocopper(II) (CuTPP-COOH), covalently bound to self-organized, anodic titania nanotube arrays (TiO2 NTs) for photoelectrochemical reduction of oxygen. Visible light irradiation of the porphyrin-covered TiO2 NTs under cathodic polarization up to -0.3 V vs. Normal hydrogen electrode (NHE) photocatalytically produces H2O2 in pH neutral electrolyte, at room temperature and without need of sacrificial electron donors. The formation of H2O2 upon irradiation is proven and quantified by direct colorimetric detection using 4-nitrophenyl boronic acid (p-NPBA) as a reactant. This simple approach for the attachment of a small molecular catalyst to TiO2 NTs may ultimately allow for the preparation of a low-cost H2O2 evolving cathode for efficient photoelectrochemical energy storage under ambient conditions.

Structural control of the photodynamics of boron-dipyrrin complexes.

Boron-dipyrrin chromophores containing a 5-aryl group with or without internal steric hindrance toward aryl rotation have been synthesized and then characterized via X-ray diffraction, static and time-resolved optical spectroscopy, and theory. Compounds with a 5-phenyl or 5-(4-tert-butylphenyl) group show low fluorescence yields (approximately 0.06) and short excited-singlet-state lifetimes (approximately 500 ps), and decay primarily (>90%) by nonradiative internal conversion to the ground state. In contrast, sterically hindered analogues having an o-tolyl or mesityl group at the 5-position exhibit high fluorescence yields (approximately 0.9) and long excited-state lifetimes (approximately 6 ns). The X-ray structures indicate that the phenyl or 4-tert-butylphenyl ring lies at an angle of approximately 60 degrees with respect to the dipyrrin framework whereas the angle is approximately 80 degrees for mesityl or o-tolyl groups. The calculated potential energy surface for the phenyl-substituted complex indicates that the excited state has a second, lower energy minimum in which the nonhindered aryl ring rotates closer to the mean plane of the dipyrrin, which itself undergoes some distortion. This relaxed, distorted excited-state conformation has low radiative probability as well as a reduced energy gap from the ground state supporting a favorable vibrational overlap factor for nonradiative deactivation. Such a distorted conformation is energetically inaccessible in a complex bearing the sterically hindered o-tolyl or mesityl group at the 5-position, leading to a high radiative probability involving conformations at or near the initial Franck-Condon form of the excited state. These combined results demonstrate the critical role of aryl-ring rotation in governing the excited-state dynamics of this class of widely used dyes.

Rapid and simple colorimetric method for the quantification of AI-2 produced from Salmonella Typhimurium.

The aim of this study was to evaluate the feasibility of Fe(III) ion reduction for the simple and rapid quantification of autoinducer-2 (AI-2) produced from bacteria using Salmonella Typhimurium as a model. Since the molecular structure of AI-2 is somewhat similar to ascorbic acid it was expected that AI-2 would also act as a reducing agent and reduce Fe(III) ions in the presence of 1,10-phenanthroline to form the colored [(o-phen)3 Fe(II)]SO4 ferroin complex that could be quantified colorimetrically. In support of this, colony rinses and cell free supernatants from cultures of all tested AI-2 producing strains, but not the AI-2 negative Sinorhizobium meliloti, formed a colored complex with a λmax of 510nm. The OD510 values of these culture supernatants or colony rinses were in broad agreement with the % activity observed in the same samples using the standard Vibrio harveyi bioluminescence assay for AI-2 detection, and with previously reported results. This methodology could potentially be developed as an alternative method for the simple and rapid quantification of AI-2 levels produced in bacterial cultures.

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).

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 degrees 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.

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.

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.

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.

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.