A priori calculations of the free energy of formation from solution of polymorphic self-assembled monolayers.

Modern quantum chemical electronic structure methods typically applied to localized chemical bonding are developed to predict atomic structures and free energies for meso-tetraalkylporphyrin self-assembled monolayer (SAM) polymorph formation from organic solution on highly ordered pyrolytic graphite surfaces. Large polymorph-dependent dispersion-induced substrate-molecule interactions (e.g., -100 kcal mol(-1) to -150 kcal mol(-1) for tetratrisdecylporphyrin) are found to drive SAM formation, opposed nearly completely by large polymorph-dependent dispersion-induced solvent interactions (70-110 kcal mol(-1)) and entropy effects (25-40 kcal mol(-1) at 298 K) favoring dissolution. Dielectric continuum models of the solvent are used, facilitating consideration of many possible SAM polymorphs, along with quantum mechanical/molecular mechanical and dispersion-corrected density functional theory calculations. These predict and interpret newly measured and existing high-resolution scanning tunnelling microscopy images of SAM structure, rationalizing polymorph formation conditions. A wide range of molecular condensed matter properties at room temperature now appear suitable for prediction and analysis using electronic structure calculations.

Little exchange at the liquid/solid interface: defect-mediated equilibration of physisorbed porphyrin monolayers.

The transition from low to high density 2D surface structures of copper porphyrins at a liquid/solid interface requires specific defects at which nearly all exchange of physisorbed molecules with those dissolved in the supernatant occurs.

Atomic-Resolution Kinked Structure of an Alkylporphyrin on Highly Ordered Pyrolytic Graphite.

The atomic structure of the chains of an alkyl porphyrin (5,10,15,20-tetranonadecylporphyrin) self-assembled monolayer (SAM) at the solid/liquid interface of highly ordered pyrolytic graphite (HOPG) and 1-phenyloctane is resolved using calibrated scanning tunneling microscopy (STM), density functional theory (DFT) image simulations, and ONIOM-based geometry optimizations. While atomic structures are often readily determined for porphyrin SAMs, the determination of the structure of alkyl-chain connections has not previously been possible. A graphical calibration procedure is introduced, allowing accurate observation of SAM lattice parameters, and, of the many possible atomic structures modeled, only the lowest-energy structure obtained was found to predict the observed lattice parameters and image topography. Hydrogen atoms are shown to provide the conduit for the tunneling current through the alkyl chains.

Electrochemistry and spectroelectrochemistry of beta,beta'-fused quinoxalinoporphyrins and related extended bis-porphyrins with Co(III), Co(II), and Co(I) central metal ions.

A series of cobalt(II) and cobalt(III) porphyrins with fused quinoxaline rings at one or more beta,beta'-pyrrolic units of the macrocycle were synthesized and characterized as to their electrochemical properties in nonaqueous media. Their UV-visible spectra were also measured before and during oxidation or reduction in a thin-layer cell. The investigated quinoxalinoporphyrins are represented as (PQ)Co, (QPQ)CoCl, (PQ(2))CoCl, Co(P)-TA-(P)Co, and Co(PQ)-(QP)Co, where PQ = the dianion of 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)-quinoxalino[2,3-b']porphyrin, QPQ = the dianion of the corresponding linear bisquinoxalino[2,3-b':12,13-b'']porphyrin, PQ(2) = the dianion of the corresponding corner bisquinoxalino[2,3-b':7,8-b'']porphyrin, and (P)-TA-(P) = the tetraanion of the bis-porphyrin with 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrins fused at opposite ends of tetraazaanthracene. (P)Co and (P)CoCl were also characterized where P = the dianion of 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin. Each compound could be cycled between their Co(III), Co(II), and Co(I) forms under the application of a given oxidizing or reducing potential, although a one-electron reduction of the Co(II) quinoxalinoporphyrins led to a product with mixed Co(I) and porphyrin pi-anion radical character followed by generation of a pure Co(I) pi-anion radical species at more negative potentials. The effect of the position and number of quinoxaline groups on the redox potentials and mechanisms of each electron transfer were elucidated, and comparisons made to structurally similar compounds containing both redox active and redox inactive central metal ions. Surprisingly, the position and number of quinoxaline groups on the macrocycle has little or no effect on the redox potentials for the Co(II) --> Co(III) or Co(III) --> Co(II) processes, but this is not the case for other electron transfer reactions where significant differences are seen between the examined compounds. Significant interactions are also observed between the two porphyrin macrocycles of the laterally bridged dicobalt(II) bis-porphyrin dyad Co(P)-TA-(P)Co in its singly and doubly reduced form, but only weak interactions are seen between the two Co(PQ) units of the single bond biquinolalinyl-bridged dicobalt(II) bis-porphyrin dyad Co(PQ)-(QP)Co.

Evanescent-field spectroscopy using structured optical fibers: detection of charge-transfer at the porphyrin-silica interface.

The fabrication of porphyrin thin films derived from dichloro[5,10,15,20-tetra(heptyl)porphyrinato]tin(IV) [Cl-Sn(THP)-Cl] in the holes of photonic crystal fibers over 90 cm in length is described. Evanescent field spectroscopy (EFS) is used to investigate the interfacial properties of the films, with the high surface optical intensity and the long path length combining to produce significant absorption. By comparison with results obtained for similar films formed from Cl-Sn(THP)-Cl inside fused-silica cuvettes and on glass slides, the film is shown to be chemisorbed as a surface Si-O-Sn(THP)-X (X = Cl or OH) species. In addition to the usual porphyrin Q and Soret bands, new absorptions in the in-fiber films are observed by EFS at 445 nm and between 660-930 nm. The 660-930 nm band is interpreted as a porphyrin to silicon charge-transfer transition and postulated to arise following chemisorption at mechanical-strain induced defect sites on the silica surface. Such defect sites are caused by the optical fiber production process and are less prevalent on other glass surfaces. EFS within optical fibers therefore offers new ways for understanding interface phenomena such as surface adsorbates on glass. Such understanding will benefit all devices that exploit interface phenomena, both in optical fibers and other integrated waveguide forms. They may be directly exploited to create ultrasensitive molecular detectors and could yield novel photonic devices.

Androgynous porphyrins. Silver(II) quinoxalinoporphyrins act as both good electron donors and acceptors.

The metal-centered and macrocycle-centered electron-transfer oxidations and reductions of silver(II) porphyrins were characterized in nonaqueous media by electrochemistry, UV-vis spectroelectrochemistry, EPR spectroscopy, and DFT calculations. The investigated compounds are {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrinato}silver(II), {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b']porphyrinato}silver(II), {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b':7,8-b'']porphyrinato}silver(II), and {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b':12,13-b'']porphyrinato}silver(II). The first one-electron oxidation and first one-electron reduction both occur at the metal center to produce stable compounds with Ag(III) or Ag(I) metal oxidation states, irrespective of the type of porphyrin ligand. The electrochemical HOMO-LUMO gap, determined by the difference in the first oxidation and first reduction potentials, decreases by introduction of quinoxaline groups fused to the Ag(II) porphyrin macrocycle. This provides a unique androgynous character to Ag(II) quinoxalinoporphyrins that enables them to act as both good electron donors and good electron acceptors, something not previously observed in other metalloporphyrin complexes. The second one-electron oxidation and second one-electron reduction of the compounds both occur at the porphyrin macrocycle to produce Ag(III) porphyrin pi-radical cations and Ag(I) porphyrin pi-radical anions, respectively. The macrocycle-centered oxidation potentials of each quinoxalinoporphyrin are shifted in a negative direction, while the macrocycle-centered reduction potentials are shifted in a positive direction as compared to the same electrode reactions of the porphyrin without the fused quinoxaline ring(s). Both potential shifts are due to a stabilization of the radical cations and radical anions by pi-extension of the porphyrin macrocycle after fusion of one or two quinoxaline moieties at the beta-pyrrolic positions of the macrocycle. Introduction of quinoxaline groups fused to the Ag(II) porphyrin macrocycle provides a unique androgynous character to Ag(II) quinoxalinoporphyrins that enables them to act as both good electron donors and good electron acceptors.