Robert S. Loewe

Practical synthesis of perylene-monoimide building blocks that possess features appropriate for use in porphyrin-based light-harvesting arrays

Abstract Perylene-monoimide dyes with solubilizing aryloxy substituents at the perylene perimeter and a synthetic handle on the N-aryl group are valuable building blocks for incorporation as accessory pigments in porphyrin-based light-harvesting arrays. A family of such dyes has been prepared by reaction of 1,6,9-tris(4-tert-butylphenoxy)perylene-3,4-dicarboxylic anhydride with a set of 4-iodo/ethynyl anilines (with or without 2,6-diisopropyl substituents) in the presence of Zn(OAc)2·2H2O in imidazole/mesitylene at 130°C. The workup procedures throughout the synthesis have been streamlined for scale-up purposes, minimizing chromatography. Two bis(perylene)porphyrin building blocks were prepared in a rational manner and examined in Sonogashira and Glaser polymerizations. The two isopropyl groups on the N-aryl group and the three 4-tert-butylphenoxy groups at the perylene perimeter are essential for high solubility of the bis(perylene)porphyrins and corresponding oligomers in organic solvents.

Synthesis of perylene–porphyrin building blocks and rod-like oligomers for light-harvesting applications

We present the synthesis of four perylene–porphyrin building blocks for use in Glaser, Sonogashira, or Suzuki polymerizations. The building blocks bear synthetic handles (4-ethynylphenyl, 4-iodophenyl, bromo) at the trans (5,15) meso-positions of a zinc porphyrin and contain two or four perylene-monoimide dyes attached at the 3,5-positions of the non-linking meso-aryl rings of the porphyrin. Each perylene-monoimide bears three 4-tert-butylphenoxy substituents (at the 1-, 6-, and 9-positions) and two isopropyl groups (on the N-aryl unit) for increased solubility. In each case the intervening linker is a diarylethyne unit that bridges the N-imide position of the perylene and the meso-position of the porphyrin. The perylene–porphyrin building blocks were prepared by (1) reaction of a diperylene-dipyrromethane with an aldehyde yielding a trans-A2B2-porphyrin, (2) reaction of a diperylene-aldehyde with a dipyrromethane yielding a trans-A2B2-porphyrin, and (3) reaction of a diperylene-dipyrromethane with a dipyrromethane-dicarbinol yielding a trans-AB2C-porphyrin or ABCD-porphyrin. The building blocks were subjected to Glaser, Sonogashira, or Suzuki coupling conditions in an effort to prepare oligomers containing porphyrins joined via 4,4′-diphenylbutadiyne (dpb), 4,4′-diphenylethyne (dpe), or 1,4-phenylene linkers (p), respectively. Each porphyrin in the backbone bears two or four pendant perylene-monoimide dyes. The Glaser and Sonogashira reactions afforded a distribution of oligomers, whereas the Suzuki reaction was unsuccessful. The oligomers were soluble in solvents such as toluene, THF, or CHCl3 enabling routine handling. The use of perylenes results in (1) increased light-harvesting efficiency particularly in the green spectral region where porphyrins are relatively transparent and (2) greater solubility than is achieved with the use of porphyrins alone. The soluble perylene–porphyrin oligomers are attractive for use as light-harvesting materials in molecular-based solar cells.

Design and synthesis of light-harvesting rods for intrinsic rectification of the migration of excited-state energy and ground-state holes

We present the design of molecular materials for ultimate use in solid-state solar cells. The molecular materials are semi-rigid oligomeric rods of defined length with metalloporphyrins in the backbone and a carboxy group at one end for attachment to a surface. The rods are designed to absorb visible light, and then undergo excited-state energy transfer and ground-state hole transfer in opposite directions along the length of the rod. The rational synthesis of the multiporphyrin arrays relies on joining porphyrin building blocks in an efficient and controlled manner. Several porphyrin building blocks have been synthesized that bear bromophenyl, iodophenyl, trimethylsilylethynylphenyl and/or ethynylphenyl substituents for use in a copper-free Sonogashira reaction using Pd2(dba)3 and P(o-tol)3. Competition experiments performed on equimolar quantities of an iodo-porphyrin and a bromo-porphyrin with an ethynyl-porphyrin show iodo + ethyne coupling with a low amount (35 °C) or undetectable amount (22 °C) of bromo + ethyne coupling. Efficient coupling of bromo-porphyrins with ethynyl-porphyrins was achieved using the same copper-free Sonogashira reaction conditions at higher temperature (50 °C or 80 °C). These findings allow successive coupling reactions to be achieved using substrates bearing iodo and bromo synthetic handles. Thus, a porphyrin-based tetrad (or pentad) was synthesized with a final convergent coupling of a bromo-substituted dyad (or triad) and an ethynyl-substituted dyad. A porphyrin triad was prepared by sequential iodo + ethyne coupling reactions. The triad, tetrad, and pentad each are comprised of a terminal magnesium porphyrin bearing one carboxy group (for surface attachment) and two pentafluorophenyl groups; the remaining porphyrins in each array are present as the zinc chelate. Electrochemical characterization of benchmark porphyrins indicates the presence of the desired electrochemical gradient for hole hopping in the arrays. Static absorption data indicate that the arrays are weakly coupled, while static fluorescence data indicate that the excited-state energy flows in high yield to the terminal magnesium porphyrin. Time-resolved spectroscopic analysis leads to rate constants in THF of (9 ps)−1, (15 ps)−1, and (30 ps)−1 for ZnMg dyad 20, Zn2Mg triad 13, and Zn3Mg tetrad 15, respectively, and quantum efficiencies ≥99% for energy flow to the magnesium porphyrin in each case. These design and synthesis strategies should be useful for the construction of materials for molecular-based solar cells.

Excited-State Photodynamics of Perylene−Porphyrin Dyads. 5. Tuning Light-Harvesting Characteristics via Perylene Substituents, Connection Motif, and Three-Dimensional Architecture†

Seven perylene-porphyrin dyads were examined with the goal of identifying those most suitable for components of light-harvesting systems. The ideal dyad should exhibit strong absorption by the perylene in the green, undergo rapid and efficient excited-state energy transfer from perylene to porphyrin, and avoid electron-transfer quenching of the porphyrin excited state by the perylene in the medium of interest. Four dyads have different perylenes at the p-position of the meso-aryl group on the zinc porphyrin. The most suitable perylene identified in that set was then incorporated at the m- or o-position of the zinc porphyrin, affording two other dyads. An analogue of the o-substituted architecture was prepared in which the zinc porphyrin was replaced with the free base porphyrin. The perylene in each dyad is a monoimide derivative; the perylenes differ in attachment of the linker (either via a diphenylethyne linker at the N-imide or an ethynylphenyl linker at the C9 position) and the number (0-3) of 4-tert-butylphenoxy groups (which increase solubility and slightly alter the electrochemical potentials). In the p-linked dyad, the monophenoxy perylene with an N-imide diphenylethyne linker is superior in providing rapid and essentially quantitative energy transfer from excited perylene to zinc porphyrin with minimal electron-transfer quenching in both toluene and benzonitrile. The dyads with the same perylene at the m- or o-position exhibited similar results except for one case, the o-linked dyad bearing the zinc porphyrin in benzonitrile, where significant excited-state quenching is observed; this phenomenon is facilitated by close spatial approach of the perylene and porphyrin and the associated thermodynamic/kinetic enhancement of the electron-transfer process. Such quenching does not occur with the free base porphyrin because electron transfer is thermodynamically unfavorable even in the polar medium. The p-linked dyad containing a zinc porphyrin attached to a bis(4-tert-butylphenoxy)perylene via an ethynylphenyl linker at the C9 position exhibits ultrafast and quantitative energy transfer in toluene; the same dyad in benzonitrile exhibits ultrafast (<0.5 ps) perylene-to-porphyrin energy transfer, rapid (∼5 ps) porphyrin-to-perylene electron transfer, and fast (∼25 ps) charge recombination to the ground state. Collectively, this study has identified suitable perylene-porphyrin constructs for use in light-harvesting applications.

Solution STM images of porphyrins on HOPG reveal that subtle differences in molecular structure dramatically alter packing geometry

Solution STM images are reported for free-base octaethylporphyrin (H2OEP) and the Cu(II) and Zn(II) chelates (CuOEP and ZnOEP) on HOPG in 1,2-dichlorobenzene solution under ambient conditions. H2OEP and CuOEP each form a quasi-square lattice, whereas ZnOEP forms a quasi-hexagonal lattice on the surface. The binary mixture of any two of the porphyrins forms a well-ordered two-layer structure on the HOPG surface, with one species occupying the bridge sites of the array of the other species. All of the mixed adlayers exhibit a quasi-hexagonal lattice with a higher packing density than the single-component adlayers. Collectively, these observations indicate that the structure of the adlayers is controlled by a complex interplay of substrate-molecule and molecule-molecule interactions.

Synthesis of perylene-porphyrin dyads for light-harvesting studies

The spectral coverage of porphyrin-based light-harvesting arrays can be enhanced through the use of suitable accessory pigments. Perylene-monoimide dyes can serve as valuable accessory pigments with porphyrins. To investigate the choice of perylene-monoimide and the effects of molecular architecture on light-harvesting efficacy, five perylene-porphyrin dyads were prepared. Each dyad employs a diphenylethyne linker that bridges the perylene N-imide site and the porphyrin meso-position. Three dyads incorporate a mono-phenoxy perylene at the o-, m-, or p-position of the meso-aryl group on the porphyrin. The two remaining dyads incorporate a perylene-monoimide (bearing zero or three phenoxy substituents) at the p-position of the meso-aryl group on the porphyrin. The introduction of phenoxy groups on the perylenes increases the solubility, a key requirement for use in light-harvesting arrays. The long-wavelength absorption band of the perylene shifts from 506 nm to 532 or 533 nm upon substitution with one or three phenoxy groups, respectively. The synthesis of the dyads entails Pd-mediated coupling of a bromo-perylene and an ethynyl porphyrin, or the mixed-aldehyde condensation with a perylene-aldehyde, mesitaldehyde, and pyrrole. Five perylene-monoimide dyes bearing an ethyne or bromo substituent at the p-position of the N-aryl unit were developed for this modular chemistry. Each perylene-porphyrin dyad exhibits efficient energy transfer from the excited perylene to the ground-state porphyrin.