Rational Syntheses of Cyclic Hexameric Porphyrin Arrays for Studies of Self-Assembling Light-Harvesting Systems

Two new cyclic hexameric arrays of porphyrins have been prepared in a rational, convergent manner. The porphyrins in each cyclic hexamer are joined by diphenylethyne linkers affording a wheel-like array with a diameter of ∼35 Å. One array is comprised of five zinc (Zn) porphyrins and one free base (Fb) porphyrin (cyclo-Zn5FbU) while the other is comprised of an alternating sequence of two Zn porphyrins and one Fb porphyrin (cyclo-Zn2FbZn2FbU). The prior synthesis employed a one-flask template-directed process and afforded alternating Zn and Fb porphyrins or all Zn porphyrins. More diverse metalation patterns are attractive for manipulating the flow of excited-state energy in the arrays. The rational synthesis of each array employed three Pd-mediated coupling reactions with four tetraarylporphyrin building blocks bearing diethynyl, diiodo, bromo/iodo, or iodo/ethynyl groups. The final ring closure yielding the cyclic hexamer was achieved by reaction of a porphyrin pentamer + porphyrin monomer or the joining of two porphyrin trimers. In the presence of a tripyridyl template, the yields of the 5 + 1 and 3 + 3 reactions ranged from 10 to 13%. The 5 + 1 reaction in the absence of the template proceeded in 3.5% yield, thereby establishing the structure-directed contribution to cyclic hexamer formation. The 3 + 3 route relied on successive ethyne + iodo/bromo coupling reactions. One template-directed route to cyclo-Zn2FbZn2FbU employed a magnesium porphyrin, affording cyclo-Zn2FbZn2MgU from which magnesium was selectively removed. The arrays exhibit absorption spectra that are nearly the sum of the spectra of the component parts, indicating weak electronic coupling. Fluorescence spectroscopy showed that the quantum yield of energy transfer in toluene at room temperature from the Zn porphyrins to the Fb porphyrin(s) was 60% in cyclo-Zn5FbU and 90% in cyclo-Zn2FbZn2FbU. Two dipyridyl-substituted porphyrins, a Zn tetraarylporphyrin and a Fb oxaporphyrin, have been synthesized for use as guests in the cyclic hexamers, affording self-assembled arrays for light-harvesting studies

Rational Syntheses of Cyclic Hexameric Porphyrin Arrays for Studies of Self-Assembling Light-Harvesting Systems

Two new cyclic hexameric arrays of porphyrins have been prepared in a rational, convergent manner. The porphyrins in each cyclic hexamer are joined by diphenylethyne linkers affording a wheel-like array with a diameter of ∼35 Å. One array is comprised of five zinc (Zn) porphyrins and one free base (Fb) porphyrin (cyclo-Zn5FbU) while the other is comprised of an alternating sequence of two Zn porphyrins and one Fb porphyrin (cyclo-Zn2FbZn2FbU). The prior synthesis employed a one-flask template-directed process and afforded alternating Zn and Fb porphyrins or all Zn porphyrins. More diverse metalation patterns are attractive for manipulating the flow of excited-state energy in the arrays. The rational synthesis of each array employed three Pd-mediated coupling reactions with four tetraarylporphyrin building blocks bearing diethynyl, diiodo, bromo/iodo, or iodo/ethynyl groups. The final ring closure yielding the cyclic hexamer was achieved by reaction of a porphyrin pentamer + porphyrin monomer or the joining of two porphyrin trimers. In the presence of a tripyridyl template, the yields of the 5 + 1 and 3 + 3 reactions ranged from 10 to 13%. The 5 + 1 reaction in the absence of the template proceeded in 3.5% yield, thereby establishing the structure-directed contribution to cyclic hexamer formation. The 3 + 3 route relied on successive ethyne + iodo/bromo coupling reactions. One template-directed route to cyclo-Zn2FbZn2FbU employed a magnesium porphyrin, affording cyclo-Zn2FbZn2MgU from which magnesium was selectively removed. The arrays exhibit absorption spectra that are nearly the sum of the spectra of the component parts, indicating weak electronic coupling. Fluorescence spectroscopy showed that the quantum yield of energy transfer in toluene at room temperature from the Zn porphyrins to the Fb porphyrin(s) was 60% in cyclo-Zn5FbU and 90% in cyclo-Zn2FbZn2FbU. Two dipyridyl-substituted porphyrins, a Zn tetraarylporphyrin and a Fb oxaporphyrin, have been synthesized for use as guests in the cyclic hexamers, affording self-assembled arrays for light-harvesting studies

Rational Syntheses of Cyclic Hexameric Porphyrin Arrays for Studies of Self-Assembling Light-Harvesting Systems

Two new cyclic hexameric arrays of porphyrins have been prepared in a rational, convergent manner. The porphyrins in each cyclic hexamer are joined by diphenylethyne linkers affording a wheel-like array with a diameter of ∼35 Å. One array is comprised of five zinc (Zn) porphyrins and one free base (Fb) porphyrin (cyclo-Zn5FbU) while the other is comprised of an alternating sequence of two Zn porphyrins and one Fb porphyrin (cyclo-Zn2FbZn2FbU). The prior synthesis employed a one-flask template-directed process and afforded alternating Zn and Fb porphyrins or all Zn porphyrins. More diverse metalation patterns are attractive for manipulating the flow of excited-state energy in the arrays. The rational synthesis of each array employed three Pd-mediated coupling reactions with four tetraarylporphyrin building blocks bearing diethynyl, diiodo, bromo/iodo, or iodo/ethynyl groups. The final ring closure yielding the cyclic hexamer was achieved by reaction of a porphyrin pentamer + porphyrin monomer or the joining of two porphyrin trimers. In the presence of a tripyridyl template, the yields of the 5 + 1 and 3 + 3 reactions ranged from 10 to 13%. The 5 + 1 reaction in the absence of the template proceeded in 3.5% yield, thereby establishing the structure-directed contribution to cyclic hexamer formation. The 3 + 3 route relied on successive ethyne + iodo/bromo coupling reactions. One template-directed route to cyclo-Zn2FbZn2FbU employed a magnesium porphyrin, affording cyclo-Zn2FbZn2MgU from which magnesium was selectively removed. The arrays exhibit absorption spectra that are nearly the sum of the spectra of the component parts, indicating weak electronic coupling. Fluorescence spectroscopy showed that the quantum yield of energy transfer in toluene at room temperature from the Zn porphyrins to the Fb porphyrin(s) was 60% in cyclo-Zn5FbU and 90% in cyclo-Zn2FbZn2FbU. Two dipyridyl-substituted porphyrins, a Zn tetraarylporphyrin and a Fb oxaporphyrin, have been synthesized for use as guests in the cyclic hexamers, affording self-assembled arrays for light-harvesting studies

Formation of Porphyrins in the Presence of Acid-Labile Metalloporphyrins: A New Route to Mixed-Metal Multiporphyrin Arrays

The ability to incorporate distinct metalloporphyrins at designated sites in multiporphyrin arrays is essential for diverse applications in materials and biomimetic chemistry. The synthesis of such mixed-metal arrays via acid catalyzed reactions has largely been restricted to metalloporphyrins of stability class II (e.g., Cu, Co, Ni) or I. We describe routes for the rational synthesis of mixed-metal arrays via acid-catalyzed condensations that are compatible with metalloporphyrins of stability class III (e.g., Zn) and IV (e.g., Mg). The routes are demonstrated for p-phenylene-linked arrays. The key finding is that several mild Lewis acids [InCl3, Sc(OTf)3, Yb(OTf)3, and Dy(OTf)3], which are known to catalyze the dipyrromethane + dipyrromethane−dicarbinol condensation in CH2Cl2 at room temperature without acidolysis, do not demetalate zinc or magnesium porphyrins under the same conditions. Rational routes to porphyrin dyads and triads employ reaction of a (porphyrin)−dipyrromethane and a (porphyrin)−dipyrromethane−dicarbinol. The porphyrin-forming reactions (six examples) proceed in yields of 18−28%. The metalation states of the arrays prepared in this manner include Zn-free base (ZnFb), MgFb, ZnFbMg, ZnFbZn, and ZnFbFb. Studies of the catalysis process indicate that the dipyrromethane + dipyrromethane−dicarbinol condensation is catalyzed by both the Lewis acid and a Brønsted acid derived in situ from the Lewis acid. Taken together, the ability to employ otherwise “acid-labile” metalloporphyrins as precursors in condensation procedures should broaden the scope of accessible mixed-metal multiporphyrin arrays and motivate further studies of the application of mild Lewis acid catalysts in porphyrin chemistry

A Self-Assembled Light-Harvesting Array of Seven Porphyrins in a Wheel and Spoke Architecture

A shape-persistent cyclic array of six zinc porphyrins provides an effective host for a dipyridyl-substituted free base porphyrin, yielding a self-assembled structure for studies of light harvesting. Energy transfer occurs essentially quantitatively from uncoordinated to pyridyl-coordinated zinc porphyrins in the cyclic array. Energy transfer from the coordinated zinc porphyrin to the guest free base porphyrin is less efficient (Φtrans ∼40%) and is attributed to a Förster through-space process

Structural Characterization of Modular Supramolecular Architectures in Solution

Structures of modular supramolecular architectures consisting of a hexameric, diphenylethyne-linked porphyrin macrocyclic array and the corresponding host−guest complex formed by inclusion of a tripyridyl guest molecule were characterized in solution using high-angle X-ray scattering. Scattering measurements made to 6 Å resolution coupled with pair distance function (PDF) analyses demonstrated that (1) the porphyrin architectures are not rigid but are distributed across a conformational ensemble with a mean diameter that is 1.5 Å shorter than the diameter of a symmetric, energy-minimized model structure, (2) the conformational envelope has limits of 3 Å positional dispersion and full rotational freedom for all six porphyrin groups, and (3) insertion of the tripyridyl guest molecule expands the diameter of the host conformer by 0.6 Å and decreases the configurational dispersion by approximately 2-fold. These results validate the molecular design, provide a new measure of conformational ensembles in solution that cannot be obtained by other techniques, and establish a structural basis for understanding the photophysical and guest−hosting functions of the hexameric porphyrin architectures in liquids

Synthesis of Cyclic Hexameric Porphyrin Arrays. Anchors for Surface Immobilization and Columnar Self-Assembly

To investigate new architectures for the self-assembly of multiporphyrin arrays, a one-flask synthesis of a shape-persistent cyclic hexameric array of porphyrins was exploited to prepare six derivatives bearing diverse pendant groups. The new arrays contain 6−12 carboxylic acid groups, 12 amidino groups, 6 thiol groups, or 6 thiol groups and 6 carboxylic acid groups in protected form (S-acetylthio, TMS-ethyl, TMS-ethoxycarbonyl). The arrays contain alternating Zn and free base (Fb) porphyrins or all Zn porphyrins. The one-flask synthesis entails a template-directed, Pd-mediated coupling of a p/p‘-substituted diethynyl Zn porphyrin and a m/m‘-substituted diiodo Fb porphyrin. The porphyrin building blocks (trans-A2B2, trans-AB2C) contain the protected pendant groups at nonlinking meso positions. A self-assembled monolayer (SAM) of a Zn3Fb3 cyclic hexamer containing one thiol group on each porphyrin was prepared on a gold electrode and the surface-immobilized architecture was examined electrochemically. Together, the work reported herein provides cyclic hexameric porphyrin arrays for studies of self-assembly in solution or on surfaces

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

A Tin-Complexation Strategy for Use with Diverse Acylation Methods in the Preparation of 1,9-Diacyldipyrromethanes

The acylation of dipyrromethanes to form 1,9-diacyldipyrromethanes is an essential step in the rational synthesis of porphyrins. Although several methods for acylation are available, purification is difficult because 1,9-diacyldipyrromethanes typically streak extensively upon chromatography and give amorphous powders upon attempted crystallization. A solution to this problem has been achieved by reacting the 1,9-diacyldipyrromethane with Bu2SnCl2 to give the corresponding dibutyl(5,10-dihydrodipyrrinato)tin(IV) complex. The reaction is selective for dipyrromethanes that bear acyl groups at both the 1- and 9-positions but otherwise is quite tolerant of diverse substituents. The diacyldipyrromethane−tin complexes are stable to air and water, are highly soluble in common organic solvents, crystallize readily, and chromatograph without streaking. Four methods (Friedel−Crafts, Grignard, Vilsmeier, benzoxathiolium salt) were examined for the direct 1,9-diacylation of a dipyrromethane or the 9-acylation of a 1-acyldipyrromethane. In each case, treatment of the crude reaction mixture with Bu2SnCl2 and TEA at room temperature enabled facile isolation of multigram quantities of the 1,9-diacyldipyrromethane−tin complex. The diacyldipyrromethane−tin complexes could be decomplexed with TFA in nearly quantitative yield. Alternatively, use of a diacyldipyrromethane−tin complex in a porphyrin-forming reaction (reduction with NaBH4, acid-catalyzed condensation with a dipyrromethane, DDQ oxidation) afforded the desired free base porphyrin in yield comparable to that obtained from the uncomplexed diacyldipyrromethane. The acylation/tin-complexation strategy has been applied to a bis(dipyrromethane) and a porphyrin-dipyrromethane. In summary, the tin-complexation strategy has broad scope, is compatible with diverse acylation methods, and greatly facilitates access to 1,9-diacyldipyrromethanes

Mechanisms, Pathways, and Dynamics of Excited-State Energy Flow in Self-Assembled Wheel-and-Spoke Light-Harvesting Architectures

Static and time-resolved optical measurements are reported for two cyclic hexameric porphyrin arrays and their self-assembled complexes with guest chromophores. The hexameric hosts contain zinc porphyrins and 0 or 3 free base (Fb) porphyrins (denoted Zn6 or Zn3Fb3, respectively). The guests are a tripyridyl arene (TP) and a dipyridyl-substituted free base porphyrin (DPFb), each of which coordinates to zinc porphyrins of a host via pyridyl−zinc dative bonding. Each architecture is designed to have an overall gradient of excited-state energies that affords excitation funneling within the host and ultimately to the guest. Collectively, the studies delineate the various pathways, mechanisms, and rate constants of energy flow among the weakly coupled constituents of the host−guest complexes. The pathways include downhill unidirectional energy transfer between adjacent chromophores, bidirectional energy migration between identical chromophores, and energy transfer between nonadjacent chromophores. The energy transfer to the lowest-energy chromophore(s) within the backbone of a hexameric host (Fb porphyrins in Zn3Fb3 or pyridyl-coordinated zinc porphyrins in Zn6·TP and Zn6·DPFb) proceeds primarily via a through-bond mechanism; the transfer is rapid (∼40 ps depending on the array) and essentially quantitative (≥98%). The energy transfer from a pyridyl-coordinated zinc porphyrin of the host to the Fb porphyrin guest in the Zn6·DPFb complex is almost exclusively Förster through-space in nature; this process is much slower (∼1 ns) and has a lower yield (65%). These studies highlight the utility of cyclic architectures for efficient light harvesting and energy transfer to a designated trapping site