Directly linked porphyrin arrays with tunable excitonic interactions

Physicochemical insights in non-covalent interaction of a newly designed triporphyrin with fullerenes C60 and C70 in solution

The present paper reports the synthesis of free-base (H(2)-1) and zinc-triporphyrins Zn-1, and their supramolecular complexes with C(60) and C(70) in toluene. While UV-Vis studies reveal ground state interaction between fullerenes and triporphyrins, steady state fluorescence measurements establish quenching of fluorescence of triporphyrins by fullerenes. Binding constants data evoke that Zn-1 may be employed as an efficient tweezers for C(70) in toluene. Time resolved emission studies establish relatively long-lived charge separated state for the C(70)/triporphyrin complexes. Molecular mechanics force field calculations in vacuo interpret the stability difference between C(60)/and C(70)/complexes of triporphyrin.

Long-range electron transfer in zinc-phthalocyanine-oligo(phenylene-ethynylene)-based donor-bridge-acceptor dyads

In the context of long-range electron transfer for solar energy conversion, we present the synthesis, photophysical, and computational characterization of two new zinc(II) phthalocyanine oligophenylene-ethynylene based donor-bride-acceptor dyads: ZnPc-OPE-AuP(+) and ZnPc-OPE-C(60). A gold(III) porphyrin and a fullerene has been used as electron accepting moieties, and the results have been compared to a previously reported dyad with a tin(IV) dichloride porphyrin as the electron acceptor (Fortage et al. Chem. Commun. 2007, 4629). The results for ZnPc-OPE-AuP(+) indicate a remarkably strong electronic coupling over a distance of more than 3 nm. The electronic coupling is manifested in both the absorption spectrum and an ultrafast rate for photoinduced electron transfer (k(PET) = 1.0 × 10(12) s(-1)). The charge-shifted state in ZnPc-OPE-AuP(+) recombines with a relatively low rate (k(BET) = 1.0 × 10(9) s(-1)). In contrast, the rate for charge transfer in the other dyad, ZnPc-OPE-C(60), is relatively slow (k(PET) = 1.1 × 10(9) s(-1)), while the recombination is very fast (k(BET) ≈ 5 × 10(10) s(-1)). TD-DFT calculations support the hypothesis that the long-lived charge-shifted state of ZnPc-OPE-AuP(+) is due to relaxation of the reduced gold porphyrin from a porphyrin ring based reduction to a gold centered reduction. This is in contrast to the faster recombination in the tin(IV) porphyrin based system (k(BET) = 1.2 × 10(10) s(-1)), where the excess electron is instead delocalized over the porphyrin ring.

Exciplex formation and energy transfer in a self-assembled metal-organic hybrid system

Exciting assemblies: A metal-organic self-assembly of pyrenebutyric acid (PBA), 1,10-phenanthroline (o-phen), and Mg(II) shows solid-state fluorescence originating from a 1:1 PBA-o-phen exciplex. This exciplex fluorescence is sensitized by another residual PBA chromophore through an excited-state energy-transfer process. The solvent polarity modulates the self-assembly and the corresponding exciplex as well as the energy transfer, resulting in tunable emission of the hybrid (see figure).

Free-base tetraarylporphyrin covalently linked to [60]fullerene through ethynylfluorene spacer

The synthesis, structural and electronic characterization of novel electroactive systems based on porphyrin-fullerene in which the chromophores are linked by an ethynylfluorene spacer unit is reported. Sonogashira couplings have been used in short and efficient sequences to give access to these new molecules on a practical scale. The absorption studies, voltamperometric measurements and theoretical calculations at DFT level reveal the push-pull behavior for these systems.

Peripherally fused porphyrins via the Scholl reaction: synthesis, self-assembly, and mesomorphism

Oxidative coupling of activated aryl groups attached to β-positions of the porphyrin ring provides convenient access to derivatives containing peripherally fused phenanthrene and benzo[g]chrysene units. Tetra(benzochryseno)porphyrin, reported here for the first time, contains a nonplanar, sterically locked π system and shows very intense electronic absorptions in the Q range of the electronic spectrum. Tetraphenanthroporphyrins show a tendency to aggregate in solution. In one case, a discrete dimer is formed, whose structure was investigated spectroscopically and theoretically. Derivatives bearing long alkyl chains are mesomorphic and exhibit columnar phases (tetraphenanthroporphyrins) and a monoclinic 3D phase (tetrabenzochrysenoporphyrin). The symmetry of column packing in the columnar phases is dependent on the number of alkyl chains per molecule. X-ray diffraction measurements show that, in spite of their nonplanarity, the aromatic cores in the mesophases are tightly stacked within the column. The corresponding stacking patterns were derived from the structure of the dimer, on the basis of geometrical analysis and molecular modeling.

Clean coupling of unfunctionalized porphyrins at surfaces to give highly oriented organometallic oligomers

The direct coupling of complex, functional organic molecules at a surface is one of the outstanding challenges in the road map to future molecular devices. Equally demanding is to meet this challenge without recourse to additional functionalization of the molecular building blocks and via clean surface reactions that leave no surface contamination. Here, we demonstrate the directional coupling of unfunctionalized porphyrin molecules--large aromatic multifunctional building blocks--on a single crystal copper surface, which generates highly oriented one-dimensional organometallic macromolecular nanostructures (wires) in a reaction which generates gaseous hydrogen as the only byproduct. In situ scanning tunneling microscopy and temperature programmed desorption, supported by theoretical modeling, reveal that the process is driven by C-H bond scission and the incorporation of copper atoms in between the organic components to form a very stable organocopper oligomer comprising organometallic edge-to-edge porphyrin-Cu-porphyrin connections on the surface that are unprecedented in solution chemistry. The hydrogen generated during the reaction leaves the surface and, therefore, produces no surface contamination. A remarkable feature of the wires is their stability at high temperatures (up to 670 K) and their preference for 1D growth along a prescribed crystallographic direction of the surface. The on-surface formation of directional organometallic wires that link highly functional porphyrin cores via direct C-Cu-C bonds in a single-step synthesis is a new development in surface-based molecular systems and provides a versatile approach to create functional organic nanostructures at surfaces.

A vanadium porphyrin with temperature-dependent phase transformation: synthesis, crystal structures, supramolecular motifs and properties

A vanadium porphyrin, V(O)TMeOPP (1; TMeOPP=5, 10, 15, 20-tetrakis(4-methoxyphenyl)-21 H, 23H-porphyrin), has been synthesized by solvothermal reactions and characterized by single-crystal X-ray diffractions at room temperature and low temperature to reveal two different structures 1R and 1L, respectively. Both 1R and 1L crystallized in the orthorhombic system, but their space groups were different: Pbca and Pca2(1) for 1R and 1L, respectively. The cell parameters of a, b, and c were different and the cell volume of 1R was larger than that of 1L by circa 200 Å(3). 1R and 1L were characteristic of an isolated motif with a five-coordinate vanadium(IV) ion and a saddle-distorted nonplanar porphyrin macrocycle. Molecules of 1R were interconnected through hydrogen-bonding interactions to yield a 3D framework; whilst for the low-temperature phase 1L, there were more hydrogen-bonding interactions that link the molecules to construct a more-complex 3D supramolecular network. In a solution of acetone, the title compound exhibited purple and green colors at room temperature and low temperature, respectively, which is unprecedented for vanadium porphyrins. The spectral data of UV/Vis, FT-IR, and MALDI-TOF-MS of 1R and 1L are reported together with the electrochemical data.

Incorporation of porphyrin acetylides into duplexes of the simplified nucleic acid GNA

A porphyrin-acetylide-modified GNA (glycol nucleic acid) phosphoramidite building block was synthesized in an economical fashion starting from (S)-glycidyl-4,4'- dimethoxytrityl ether in just 4 steps with an overall yield of 48%. The porphyrin acetylide nucleotide was incorporated into GNA duplexes opposite ethylene glycol abasic sites and the duplexes were analyzed by UV-melting, UV-vis, fluorescence spectroscopy, and circular dichroism. The modified GNA duplexes display lower thermal stabilities, however, the stabilities of the duplexes can be modulated by the incorporation of Zn(2+) (further destabilization) or Ni(2+) (stabilization relative to the uncomplexed porphyrin). Uncomplexed as well as Ni(2+)-coordinated porphyrins intercalate into the GNA duplex whereas Zn(2+)-coordinated porphyrins are most likely located outside the base stack. Adjacent porphyrins in opposite strands of GNA duplexes show an electronic interaction with each other which might be exploited in the future for the design of photoelectrical devices.