Efficient Synthesis of Light-Harvesting Arrays Composed of Eight Porphyrins and One Phthalocyanine

Effective light-harvesting arrays require multiple photoactive energy donors that funnel energy to an energy acceptor. Porphyrins and phthalocyanines are attractive components for light-harvesting arrays due to their strong absorption in the blue and red regions, respectively, and because energy transfer can occur from porphyrin to phthalocyanine regardless of their respective metalation states. Star-shaped light-harvesting arrays comprised of eight peripheral porphyrins and one core phthalocyanine have been prepared by a streamlined synthesis involving minimal reliance on protecting groups, a high degree of convergence, and facile chromatographic purification. The synthesis involves three distinct stages of complementary chemistries (porphyrin formation, Pd-mediated porphyrin dimer formation, phthalocyanine formation). Statistical reaction of p-iodobenzaldehyde, a phthalonitrile-linked benzaldehyde, and 5-mesityldipyrromethane afforded the desired trans-iodo/phthalonitrile-substituted porphyrin, which underwent Pd-mediated coupling with a monoethynyl porphyrin to give the porphyrin dimer bearing a phthalonitrile unit. Reaction of the dimer in 1-pentanol in the presence of MgCl2 and DBU for 48 h at 145 °C afforded the all-magnesium (porphyrin)8−phthalocyanine nonamer (MgP)8MgPc in 5.0% yield. The same reaction with lithium pentoxide in 1-pentanol for 2 h at 145 °C gave the all-free base nonamer (H2P)8H2Pc in 34% yield. The all-zinc nonamer (ZnP)8ZnPc was prepared by addition of zinc acetate at the end of the reaction. Similar treatment of a monomeric porphyrin−phthalonitrile afforded the pentameric (ZnP)4ZnPc in 58% yield. The (MgP)8MgPc was also obtained by magnesium insertion of (H2P)8H2Pc. The three nonamers were readily purified and are soluble in solvents such as toluene, THF, and CH2Cl2. Each nonamer absorbs strongly across the solar spectrum and exhibits efficient energy transfer from the porphyrins to the phthalocyanine

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

Synthesis and Properties of Star-Shaped Multiporphyrin−Phthalocyanine Light-Harvesting Arrays

Light-harvesting arrays containing four porphyrins covalently linked to a phthalocyanine in a star-shaped architecture have been synthesized. Cyclotetramerization of an ethyne-linked porphyrin−phthalonitrile in 1-pentanol in the presence of MgCl2 and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) afforded the all-magnesium porphyrin−phthalocyanine pentad in 45% yield. Similar reaction using Zn(OAc)2·2H2O afforded the all-zinc porphyrin−phthalocyanine pentad in 15% yield. Arrays with different metals (free base, Mg, Zn) in the porphyrin and phthalocyanine macrocycles have been prepared by selective demetalation and metalation steps. This approach provides rapid and convergent access to multiporphyrin−phthalocyanine arrays in diverse metalation states. The arrays are reasonably soluble in organic solvents such as toluene, THF, and CH2Cl2. The arrays exhibit strong absorption in the blue and red regions. Time-resolved and static optical measurements indicate that intramolecular singlet-excited-state energy transfer from the porphyrin to the phthalocyanine moiety is extremely rapid (picoseconds) and efficient. Ground-state electronic communication among the porphyrins is indicated by rapid hole/electron hopping among the metalloporphyrins in the arrays as detected by EPR measurements on the singly oxidized pentads. These physical measurements indicate that the porphyrin−phthalocyanine pentads possess favorable characteristics for light harvesting and other photonics applications

Synthesis of Thiol-Derivatized Europium Porphyrinic Triple-Decker Sandwich Complexes for Multibit Molecular Information Storage

The storage of multiple bits of information at the molecular level requires molecules with a large number of distinct oxidation states. Lanthanide triple-decker sandwich molecules employing porphyrins and phthalocyanines afford four cationic states and are very attractive for molecular information storage applications. Five triple-decker building blocks have been prepared of the type (phthalocyanine)Eu(phthalocyanine)Eu(porphyrin), each bearing one iodo, one ethyne, or one iodo and one ethyne group attached to the porphyrin unit. Two triple-decker building blocks with different oxidation potentials were derivatized with an S-acetylthiophenyl unit for attachment to an electroactive surface. To explore the preparation of arrays comprised of triple deckers, which may lead to the storage of a larger number of bits, two types of dyads of triple deckers were prepared. An ethyne-linked dyad of triple deckers bearing one S-acetylthiophenyl unit was prepared via repetitive Sonogashira couplings, and a butadiyne-linked dyad was prepared via a modified Glaser coupling. The triple deckers were characterized by absorption spectroscopy, laser-desorption mass spectrometry, and 1H NMR spectroscopy. The thiol-derivatized triple deckers form self-assembled monolayers (SAMs) on gold via in situ cleavage of the thiol protecting group. The SAM of each array is electrochemically robust and exhibits three well-resolved, reversible oxidation waves. These electrochemical characteristics indicate that these types of molecules are well suited for storing multiple bits of information

Synthesis and Characterization of Bis(<i>S</i>-acetylthio)-Derivatized Europium Triple-Decker Monomers and Oligomers

We report the synthesis of monomers, dimers, trimers, and oligomers of triple-decker (TD) complexes bearing S-acetylthio groups at the termini:  AcS−(TD)n−SAc. Each TD was of type (Pc)Eu(Pc)Eu(Por), where H2Pc = tetra-tert-butylphthalocyanine and H2Por is a meso-tetraarylporphyrin bearing functional groups at the 4-aryl position such as ethynyl, TMS−ethynyl, TIPS−ethynyl, or iodo. The TD arrays were prepared by Sonogashira- and Glaser-type coupling reactions, affording 1,4-diphenylethyne or 1,4-diphenylbutadiyne linkers joining the TDs. Each TD array exhibited high solubility in organic solvents such as CHCl3 or CH2Cl2. Self-assembled monolayers (SAMs) of all the TDs were prepared on Au substrates and investigated via a variety of electrochemical techniques aimed at determining redox potentials, rates of electron transfer under applied potential, and rates of charge retention in the absence of applied potential. The electrochemical measurements were accompanied by ellipsometric studies aimed at determining SAM thickness and, hence, the orientation of the complexes with respect to the surface plane. All of the TD SAMs exhibit robust, reversible voltammetry yielding four well-resolved waves in the potential range of 0 to +1.6 V (corresponding to the mono-, di-, tri-, and tetracations). The electron-transfer rates for the various oxidation states of all of the TD SAMS are similar and in the 104−105 s-1 range. The charge-dissipation rates (measured in terms of a charge-retention half-life) are also similar and are in the 10−60 s range. These rates are influenced by both the packing density of the molecules and the orientation of the molecules on the surface. The full body of data supports the view that all of the dithio-derivatized TD complexes assume a similar geometry on the surface. In particular, the complexes are oriented with their linkers/macrocycle planes generally parallel with the surface, unlike monothio-derivatized analogues, which are in a more perpendicular geometry. The parallel geometry of the dithio-derivatized TDs is qualitatively consistent with covalent attachment to Au via both thiols

Synthesis of Thiol-Derivatized Europium Porphyrinic Triple-Decker Sandwich Complexes for Multibit Molecular Information Storage

The storage of multiple bits of information at the molecular level requires molecules with a large number of distinct oxidation states. Lanthanide triple-decker sandwich molecules employing porphyrins and phthalocyanines afford four cationic states and are very attractive for molecular information storage applications. Five triple-decker building blocks have been prepared of the type (phthalocyanine)Eu(phthalocyanine)Eu(porphyrin), each bearing one iodo, one ethyne, or one iodo and one ethyne group attached to the porphyrin unit. Two triple-decker building blocks with different oxidation potentials were derivatized with an S-acetylthiophenyl unit for attachment to an electroactive surface. To explore the preparation of arrays comprised of triple deckers, which may lead to the storage of a larger number of bits, two types of dyads of triple deckers were prepared. An ethyne-linked dyad of triple deckers bearing one S-acetylthiophenyl unit was prepared via repetitive Sonogashira couplings, and a butadiyne-linked dyad was prepared via a modified Glaser coupling. The triple deckers were characterized by absorption spectroscopy, laser-desorption mass spectrometry, and 1H NMR spectroscopy. The thiol-derivatized triple deckers form self-assembled monolayers (SAMs) on gold via in situ cleavage of the thiol protecting group. The SAM of each array is electrochemically robust and exhibits three well-resolved, reversible oxidation waves. These electrochemical characteristics indicate that these types of molecules are well suited for storing multiple bits of information

Template-Directed Synthesis, Excited-State Photodynamics, and Electronic Communication in a Hexameric Wheel of Porphyrins

To investigate new architectures for molecular photonics applications, a shape-persistent cyclic hexameric architecture (cyclo-Zn3Fb3U-p/m) has been prepared that is comprised of three free base (Fb) porphyrins and three zinc porphyrins linked at the meso-positions via diphenylethyne units. The synthesis involves the Pd-mediated coupling of a p/p-substituted diethynyl Zn porphyrin and a m/m-substituted diiodo Fb porphyrin, forming p/m-substituted diphenylethyne linkages. The isolated yield of cyclo-Zn3Fb3U-p/m is 5.3% in the presence of a tripyridyl template. The array has C3v symmetry, 108 atoms in the shortest path, and a face-to-face distance of ∼35 Å across the cavity. The excited-state lifetime of the Zn porphyrin in cyclo-Zn3Fb3U-p/m is 17 ps, giving a rate of energy transfer to each adjacent Fb porphyrin of ktrans = (34 ps)-1 and a quantum efficiency of Φtrans = 99.2%. This rate is comparable to that in a dimer (ZnFbU-p/m) having an identical linker, but slower than that of a p/p-linked ZnFb dimer, which has ktrans = (24 ps)-1. At ambient temperatures, the hole/electron hopping rate in [cyclo-Zn6U-p/m]+ is comparable to or faster than the EPR time scale (∼4 MHz). The hole/electron hopping rate in [cyclo-Zn6U-p/m]+ appears to be more than 2-fold larger than for [Zn2U-p/m]+; [Zn2U-p/m]+ has a rate at least 10-fold slower than for the p/p-linked dimer [Zn2U]+. Both excited-state energy transfer and ground-state hole/electron hopping proceed via through-bond mechanisms mediated by the diphenylethyne linker. The origin of the slightly slower energy-transfer rate, and substantially slower ground-state hole/electron hopping rate, in the p/m-linked arrays versus the p/p-linked analogues, is attributed primarily to the larger electron density of the frontier molecular orbitals at the p- versus m-position of the phenyl ring in the diphenylethyne linker. Collectively, these results indicate that the site of attachment of the porphyrin to the linker could be used to direct energy and/or hole/electron flow in a controlled manner among porphyrins in diverse 3-dimensional (linear, cyclic, tubular) architectures