Swallowtail Porphyrins: Synthesis, Characterization and Incorporation into Porphyrin Dyads

Meso-tetra(dioxanyl)porphyrins: Neutral, low molecular weight, and chiral porphyrins with solubility in aqueous solutions

The synthesis of the low-molecular weight, meso-tetra(dioxan-2-yl)porphyrin with considerable solubility in aqueous solution is described. The key intermediate dioxan-2-carbaldehyde is accessible in either racemic or in stereo-pure forms from commercially available starting materials in three steps. Using 4 × 1 or 2 + 2-type syntheses provide the porphyrin in modest yields. While the racemic aldehyde created an intractable mixture of diastereomers, the enantiopure aldehyde created a single enantiomer of the target porphyrin. The porphyrin was spectroscopically characterized. As its free base or zinc complex, it showed excellent solubility properties in organic and aqueous solvents, though free water-solubility was not achieved. The work expands on the availability of chiral porphyrins and neutral porphyrins with considerable solubility in aqueous solution.

Exquisite chemistries of meso-pentafluorophenyl and meso-(2,6-dichlorophenyl) dipyrromethanes

meso-Aryl dipyrromethanes are important building blocks to create various practical molecules, and the electron-withdrawing effect of the meso-aryls can play an important role in improving/controlling their reactivities in further reactions: meso-Pentafluorophenyl and meso-(2,6-dichlorophenyl) substituted dipyrromethanes towards the associating dipyrrins and other derivatives have been investigated, in which coordination with different transition metals followed by the dissociation of the metal coordination by interruption of proton and halide ions have been verified. Furthermore, simple DDQ-oxidations of the dipyrromethanes have afforded DDQ-dipyrrin adducts which were delivered to distinctly advanced molecular rearrangements in the presence of acid and base. Vinylene bisdipyrrin compounds, obtained in the reactions of DDQ-dipyrrin adducts with a Lewis base triethylamine, have produced expanded porphyrinoid bis-nickel complexes during the metalation procedure with nickel acetate, while a cobalt metalation of the vinylene bisdipyrrin compounds resulted in an isolation of diamagnetic octahedral cobalt(III) complex. Moreover, meso-pentafluorophenyl and meso-(2,6-dichlorophenyl) dipyrromethanes have been useful platforms to accomplish size-controlled series of meso-aryl-substituted expanded porphyrins and easy separation/purification of selective meso-aryl expanded porphyrins, in which advanced metal complexes and various fascinating properties and reactivities admitting moderate functional molecules have arisen.

Charge Transfer and Spin Dynamics in a Zinc Porphyrin Donor Covalently Linked to One or Two Naphthalenediimide Acceptors

Quantum coherence effects on charge transfer and spin dynamics in a system having two degenerate electron acceptors are studied using a zinc 5,10,15-tri(n-pentyl)-20-phenylporphyrin (ZnP) electron donor covalently linked to either one or two naphthalene-1,8:4,5-bis(dicarboximide) (NDI) electron acceptors using an anthracene (An) spacer, ZnP-An-NDI (1) and ZnP-An-NDI₂ (2), respectively. Following photoexcitation of 1 and 2 in toluene at 295 K, femtosecond transient absorption spectroscopy shows that the electron transfer (ET) rate constant for 2 is about three times larger than that of 1, which can be accounted for by the statistical nature of incoherent ET as well as the electron couplings for the charge separation reactions. In contrast, the rate constant for charge recombination (CR) of 1 is about 25% faster than that of 2. Using femtosecond transient infrared spectroscopy and theoretical analysis, we find that the electron on NDI₂^•- in 2 localizes onto one of the two NDIs prior to CR, thus precluding electronically coherent CR from NDI₂^•-. Conversely, CR in both 1 and 2 is spin coherent as indicated by the observation of a resonance in the ^3*ZnP yield following CR as a function of applied magnetic field, giving spin-spin exchange interaction energies of 2J = 210 and 236 mT, respectively, where the line width of the resonance for 2 is greater than 1. These data show that while CR is a spin-coherent process, incoherent hopping of the electron between the two NDIs in 2, consistent with the lack of delocalization noted above, results in greater spin decoherence in 2 relative to 1.

Control of Porphyrin Planarity and Aggregation by Covalent Capping: Bissilyloxy Porphyrin Silanes

Porphyrins are cornerstone functional materials that are useful in a wide variety of settings, ranging from molecular electronics to biology and medicine. Their applications are often hindered, however, by poor solubilities that result from their extended, solvophobic aromatic surfaces. Attempts to counteract this problem by functionalizing their peripheries have been met with only limited success. Here, we demonstrate a versatile strategy to tune the physical and electronic properties of porphyrins using an axial functionalization approach. Porphyrin silanes (PorSils) and bissilyloxy PorSils (SOPS) are prepared from porphyrins by operationally simple κ⁴N-silylation protocols, introducing bulky silyloxy "caps" that are central and perpendicular to the planar porphyrin. While porphyrins typically form either J- or H-aggregates, SOPS do not self-associate in the same manner: the silyloxy axial substituents dramatically improve the solubility by inhibiting aggregation. Moreover, axial porphyrin functionalization offers convenient handles through which optical, electronic, and structural properties of the porphyrin core can be modulated. We observe that the identity of the silyloxy substituent impacts the degree of planarity of the porphyrin in the solid state as well as the redox potentials.

A General Approach to Biocompatible Branched Fluorous Tags for Increased Solubility in Perfluorocarbon Solvents

A modular, cost-effective route to a library of branched fluorous tags with two short, biocompatible, fluorinated chains (C6F13) is reported. These branched fluorous tags provide high fluorous content without the use of long-chain linear perfluorocarbons, which have rising health concerns due to their bioaccumulation. By attaching these tags to a porphyrin, it is demonstrated that high solubility can be achieved in fluorous solvents that are readily cleared from mammals. This work enhances the biocompatibility of perfluorocarbon nanoemulsions for photodynamic therapy.

Modifications of Porphyrins and Hydroporphyrins for Their Solubilization in Aqueous Media

The increasing popularity of porphyrins and hydroporphyrins for use in a variety of biomedical (photodynamic therapy, fluorescence tagging and imaging, photoacoustic imaging) and technical (chemosensing, catalysis, light harvesting) applications is also associated with the growing number of methodologies that enable their solubilization in aqueous media. Natively, the vast majority of synthetic porphyrinic compounds are not water-soluble. Moreover, any water-solubility imposes several restrictions on the synthetic chemist on when to install solubilizing groups in the synthetic sequence, and how to isolate and purify these compounds. This review summarizes the chemical modifications to render synthetic porphyrins water-soluble, with a focus on the work disclosed since 2000. Where available, practical data such as solubility, indicators for the degree of aggregation, and special notes for the practitioner are listed. We hope that this review will guide synthetic chemists through the many strategies known to make porphyrins and hydroporphyrins water soluble.

Synthesis and photophysical characterization of bacteriochlorins equipped with integral swallowtail substituents

The two pyrroline units of bacteriochlorins can now bear gem-dialkyl or diaryl groups (L), which project above and below the macrocycle plane, whereas dimethyl groups generally have been accessible previously.

Structural engineering of porphyrin-based small molecules as donors for efficient organic solar cells

Porphyrin-based small molecules as donors have long been ignored in bulky heterojunction organic solar cells due to their unfavorable aggregation and the low charge mobility. With the aim of striking a delicate balance between molecular design, morphology, interfacial layer and device fabrication to maximize the power conversion efficiency (PCE) of organic solar cells, three comparable porphyrin-based small molecules with an acceptor-donor-acceptor configuration have been developed for use as donor materials in solution processed small molecule bulk heterojunction organic solar cells. In these molecules, electron-deficient 3-ethylrhodanine is introduced into the electron-rich porphyrin core through 5,15-bis(phenylethynyl) linkers. Structural engineering with 10,20-bis(2-hexylnonyl) aliphatic peripheral substituent on the porphyrin core, instead of the aromatic substituents such as 10,20-bis[3,5-di(dodecyloxyl)phenyl], and 10,20-bis(4-dodecyloxylphenyl), can simultaneously facilitate stronger intermolecular π-π stacking and higher charge transfer mobility in the film, leading to a maximum PCE of 7.70% in a conventional device. The inverted devices have also been demonstrated to have long-term ambient stability and a comparable PCE of 7.55%.

Hydrophilic bioconjugatable trans-AB-porphyrins and peptide conjugates

Porphyrins bearing a single bioconjugatable group and a single water-solubilization motif in a trans-AB-architecture (with no other substituents) provide a compact design of value for studies in diverse disciplines. Established synthetic methods have been employed to prepare four new free base porphyrins and one Mn ( III ) chelate. The hydrophilic motif includes 4-N-methylpyridinium, 2,4,6-tris(carboxymethoxy)phenyl, 2,6-bis(phosphonomethoxy)phenyl, and carboxy; the bioconjugatable unit includes carboxy, maleimido, and N-hydroxysuccinimido (NHS) ester. Bioconjugation experiments with a protected porphyrin-diphosphate or unprotected porphyrin-diphosphonate were examined in organic solution or water, respectively. Both approaches were employed to conjugate to the ε-amino group of Lys11 in AcKPV- NH 2, a tripeptide fragment [ Ac -α-MSH(11-13)- NH 2] of melanocyte stimulating hormone, yielding porphyrin-peptide conjugates.

Progress Towards Synthetic Chlorins with Graded Polarity, Conjugatable Substituents, and Wavelength Tunability

Advances in chlorin synthetic chemistry now enable the de novo preparation of diverse chlorin-containing molecular architectures. Five distinct molecular designs have been explored here, including hydrophobic bioconjugatable (oxo)chlorins; a hydrophilic bioconjugatable chlorin; a trans-ethynyl/iodochlorin building block; a set of chlorins bearing electron-rich (methoxy, dimethylamino, methylthio) groups at the 3-position; and a set of ten 3,13-disubstituted chlorins chiefly bearing groups with extended π-moieties. Altogether 23 new chlorins (17 targets, 6 intermediates) have been prepared. The challenge associated with molecular designs that encompass the combination of "hydrophilic, bioconjugatable and wavelength-tunable" chiefly resides in the nature of the hydrophilic unit.

Synthetic transformations of porphyrins – Advances 2004-2007

Recent developments in the synthesis and transformation of porphyrins and their derivatives are presented. In connection with the Fifth International Conference on Porphyrins and Phthalocyanines (ICPP-5) a survey of current method developments and reactivity studies is made. The review focuses on synthetic advances in porphyrin chemistry. A brief survey of important developments covers selectively the literature from 2004 to late 2007.

Imine-substituted dipyrromethanes in the synthesis of porphyrins bearing one or two meso substituents

5,15-substituted porphyrins are valuable compounds in bioorganic and materials chemistry. A new synthesis has been developed that employs 1,9-diformylation of a dipyrromethane, conversion of the diformyldipyrromethane to the bis(imino) derivative, and reaction of the bis(imino)dipyrromethane + a dipyrromethane to give the zinc-porphyrin bearing trans-AB-substituents. 1,9-diformylation was achieved via Vilsmeier reaction. Imination was achieved by treatment of the 1,9-diformyldipyrromethane with excess amine under neat conditions at room temperature. The porphyrin-forming reaction was carried out over 2 h in refluxing ethanol containing zinc acetate exposed to air. Oxidation of the intermediate porphyrinogen occurs aerobically. A complex composed of two bis(imino)dipyrromethanes and two zinc atoms was observed to form reversibly during the course of the reaction. A set of zinc-porphyrins with trans-AB-, A 2-, or A-substituents has been prepared in yields of ~30% (without detectable scrambling) with straightforward purification. The reaction is applicable to A/B substituent combinations of aryl/aryl, aryl/alkyl, and aryl/H.

Synthetic routes to meso-patterned porphyrins

Synthetic meso-substituted porphyrins offer significant attractions compared with naturally occurring beta-substituted porphyrins. The attractions include the rectilinear arrangement of the four meso substituents and potential synthetic amenability from pyrrole and simple acyl reactants, thereby avoiding the cumbersome syntheses of beta-substituted pyrroles. In practice, however, the classical methods for the synthesis of meso-substituted porphyrins were characterized by high-temperature reactions, limited scope of substituents, and statistical mixtures accompanied by laborious chromatography if porphyrins bearing two different types of substituents were sought. Such methods left unrealized the tremendous utility of meso-substituted porphyrins across the enormously broad field of porphyrin science, which touches pure chemistry; energy, life and materials sciences; and medicine. This Account surveys a set of strategies, developed over a generation, that provide rational access to porphyrins bearing up to four distinct meso substituents. A "2 + 2" route employs a dipyrromethane-1,9-dicarbinol and a dipyrromethane (bearing ABC- and D-substituents, respectively) in a two-step, one-flask process of acid-catalyzed condensation followed by oxidation at room temperature to form the free base "ABCD-porphyrin." A "bilane" route relies on the acid-catalyzed reaction of a 1-acyldipyrromethane (CD substituents) and a 9-bromodipyrromethane-1-carbinol (AB substituents) to form the corresponding 19-acyl-1-bromobilane. Reaction of the latter compound in the presence of MgBr(2), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), and toluene at reflux exposed to air affords the corresponding magnesium(II) porphyrin. The two routes are complementary, both in scope and in implementation. A suite of methods also affords trans-A(2)B(2)-porphyrins by reaction of a dipyrromethane and an aldehyde, self-condensation of a dipyrromethane-1-carbinol, or self-condensation of a 1-acyldipyrromethane. These new routes are also useful for preparing sparsely substituted porphyrins, which bear fewer than four meso substituents (e.g., trans-AB-porphyrins, A-porphyrins). Because of their compact size and the ability to incorporate hydrophilic or amphipathic groups, such molecules are ideal for biological applications. The success of these new synthetic strategies has relied on a number of advances including (1) the development of simple yet efficient routes to dipyrromethanes, acyldipyrromethanes, and dipyrromethane-carbinols, (2) the identification of acid catalysts and reaction conditions for condensations of pyrromethane species without accompanying acidolysis (which underlies scrambling and formation of a mixture of porphyrin products), (3) the development of analytical methods to rapidly screen for scrambling and to characterize the distribution of oligopyrromethanes and macrocycles, (4) selection and refinement of synthetic methods to increase yields and to limit or avoid use of chromatography, thereby achieving scalability to multigram levels, and (5) exploitation of discoveries concerning the fundamental chemistry of pyrrolic species. With these developments, the prior era of porphyrin synthesis has been supplanted with rational routes that proceed under very mild conditions and afford a single porphyrin bearing up to four distinct meso substituents. The meso substituents encompass a very wide range of molecular complexity. The resulting porphyrins can serve as building blocks in the construction of model systems, as components of molecular materials, and as surrogates for naturally occurring tetrapyrrole macrocycles.

Chlorin-bacteriochlorin energy-transfer dyads as prototypes for near-infrared molecular imaging probes: controlling charge-transfer and fluorescence properties in polar media

The photophysical properties of two energy-transfer dyads that are potential candidates for near-infrared (NIR) imaging probes are investigated as a function of solvent polarity. The dyads (FbC-FbB and ZnC-FbB) contain either a free base (Fb) or zinc (Zn) chlorin (C) as the energy donor and a free base bacteriochlorin (B) as the energy acceptor. The dyads were studied in toluene, chlorobenzene, 1,2-dichlorobenzene, acetone, acetonitrile and dimethylsulfoxide (DMSO). In both dyads, energy transfer from the chlorin to bacteriochlorin occurs with a rate constant of approximately (5-10 ps)(-1) and a yield of >99% in nonpolar and polar media. In toluene, the fluorescence yields (Phif=0.19) and singlet excited-state lifetimes (tau approximately 5.5 ns) are comparable to those of the benchmark bacteriochlorin. The fluorescence yield and excited-state lifetime decrease as the solvent polarity increases, with quenching by intramolecular electron (or hole) transfer being greater for FbC-FbB than for ZnC-FbB in a given solvent. For example, the Phif and tau values for FbC-FbB in acetone are 0.055 and 1.5 ns and in DMSO are 0.019 and 0.28 ns, whereas those for ZnC-FbB in acetone are 0.12 and 4.5 ns and in DMSO are 0.072 and 2.4 ns. The difference in fluorescence properties of the two dyads in a given polar solvent is due to the relative energies of the lowest energy charge-transfer states, as assessed by ground-state redox potentials and supported by molecular-orbital energies derived from density functional theory calculations. Controlling the extent of excited-state quenching in polar media will allow the favorable photophysical properties of the chlorin-bacteriochlorin dyads to be exploited in vivo. These properties include very large Stokes shifts (85 nm for FbC-FbB, 110 nm for ZnC-FbB) between the red-region absorption of the chlorin and the NIR fluorescence of the bacteriochlorin (lambdaf=760 nm), long bacteriochlorin excited-state lifetime (approximately 5.5 ns), and narrow (<or=20 nm) absorption and fluorescence bands. The latter will facilitate selective excitation/detection and multiprobe applications using both intensity- and lifetime-imaging techniques.

Bioconjugatable porphyrins bearing a compact swallowtail motif for water solubility

A broad range of applications requires access to water-soluble, bioconjugatable porphyrins. Branched alkyl groups attached at the branching site to the porphyrin meso position are known to impart high organic solubility. Such "swallowtail" motifs bearing a polar group (hydroxy, dihydroxyphosphoryl, dihydroxyphosphoryloxy) at the terminus of each branch have now been incorporated at a meso site in trans-AB-porphyrins. The incorporation of the swallowtail motif relies on rational synthetic methods whereby a 1,9-bis(N-propylimino)dipyrromethane (bearing a bioconjugatable tether at the 5-position) is condensed with a dipyrromethane (bearing a protected 1,5-dihydroxypent-3-yl unit at the 5-position). The two hydroxy groups in the swallowtail motif of each of the resulting zinc porphyrins can be transformed to the corresponding diphosphate or diphosphonate product. A 4-(carboxymethyloxy)phenyl group provides the bioconjugatable tether. The six such porphyrins reported here are highly water-soluble (> or =20 mM at room temperature in water at pH 7) as determined by visual inspection, UV-vis absorption spectroscopy, or 1H NMR spectroscopy. Covalent attachment was carried out in aqueous solution with the unprotected porphyrin diphosphonate and a monoclonal antibody against the T-cell receptor CD3epsilon. The resulting conjugate performed comparably to a commercially available fluorescein isothiocyanate-labeled antibody with Jurkat cells in flow cytometry and fluorescence microscopy assays. Taken together, this work enables preparation of useful quantities of water-soluble, bioconjugatable porphyrins in a compact architecture for applications in the life sciences.

Energy and Electron Transfer in Polyacetylene-Linked Zinc-Porphyrin-[60]Fullerene Molecular Wires

The synthesis and electrochemical and photophysical studies of a series of alkyne-linked zinc-porphyrin-[60]fullerene dyads are described. These dyads represent a new class of fully conjugated donor-acceptor systems. An alkynyl-fullerene synthon was synthesized by a nucleophilic addition reaction, and was then oxidatively coupled with a series of alkynyl tetra-aryl zinc-porphyrins with 1-3 alkyne units. Cyclic and differential pulse voltammetry studies confirmed that the porphyrin and fullerene are electronically coupled and that the degree of electronic interaction decreases with increasing length of the alkyne bridge. In toluene, energy transfer from the excited zinc-porphyrin singlet to the fullerene moiety occurs, affording fullerene triplet quantum yields of greater than 90 %. These dyads exhibit very rapid photoinduced electron transfer in tetrahydrofuran (THF) and benzonitrile (PhCN), which is consistent with normal Marcus behavior. Slower rates for charge recombination in THF versus PhCN clearly indicate that charge-recombination events are occurring in the Marcus inverted region. Exceptionally small attenuation factors (beta) of 0.06+/-0.005 A(-1) demonstrate that the triple bond is an effective mediator of electronic interaction in zinc-porphyrin-alkyne-fullerene molecular wires.