Methylammonium groups at the solid walls of nanometer-sized, water-filled monolayer gaps as binding sites for a tetraanionic porphyrin

Short- and long-distance porphyrin heterodimers in bulk aqueous solution and nanowells

Short-distance porphyrin heterodimers may be used for very efficient titrations in water measuring fluorescence quenching in 1:1 molecular complexes between diamagnetic and paramagnetic metalloporphyrins. Radical dimers produce strong charge transfer bands near 900 nm. Long-distance heterodimers, e.g. chlorophyll-ubiquinone, play a central role in the charge separation process of biological photosynthesis. Attempts to model such dimers in form-stable nanowells are discussed and compared with corresponding molecular complexes and covalent redox systems. Heterodimers may, of course, not only be made of two porphyrins. Other acceptor molecules, which fit exactly in the nanowells, e.g. large quinones may also be applied.

Synthesis, electrochemical, photovoltaic, and photo-physical properties on the lanthanide(III) complexes of acetylacetonate and meso-tetraalkyltetrabenzoporphyrin with electric-field induction

Lanthanide(III) complexes with acetylacetonate and meso-tetraalkyltetrabenzoporphyrin (TATBP) having the general formula Ln ( TATBP )acac (where Ln = Tb , Dy , Ho , Er , Tm , Yb ; A = C 12 H 25; Hacac = acetylacetone) are reported. These compexes have been studied by elemental analyses, ultraviolet visible spectra, infrared spectra, molar conductance, 1 H NMR spectra, cyclic voltammetry, surface photovoltage spectroscopy (SPS), and luminescence spectroscopy. The infrared spectral bands of the ligand and complexes were assigned. In dimethylformamide (DMF), 0.1 M tetrabutylammonium perchlorate (TBAP), the synthesized TATBP exhibit two one-electron reversible redox reactions, and Ln(TATBP)acac shows three redox reactions respectively, within the accessible potential window of the solvent. The absorption bands of the complexes appear in the range 431-433 (Soret band), 578-580 (Q band) and 627-631 (Q band) nm. The photovoltaic properties and charge transfer process of these compounds were investigated by surface photovoltage spectroscopy (SPS) and electric-field-induced surface photovoltage spectroscopy (EFISPS) techniques, which reveal that the ligand TATBP and the complexes Er(TATBP)acac are p-type semiconductors. The spectral bands of TATBP correspond to π → π* transitions. Quantum yields of the S 1 → S 0 fluorescence are in the region 0.25-0.27 and the fluorescence lifetimes are in the region 0.014-0.022 ms at room temperature. The phosphorescence bands of the complex at 77 K appears 714 nm.

Light-induced electron transfer over distances of 5, 10, and 15 Å within water-filled yoctowells

A small series of variable-depth yoctowell cavities with 'functional' walls on aminated silica particles and gold electrodes has been established. The dimensions of the gaps formed were 2.2 nm in diameter with varying 'functional' depths of 5, 10, and 15 Å, depending on the length of bolaphiles applied and the position of the positive rim; these gaps were prepared through a Michael addition of the incorporated ene-amide groups. Using this construct and electrostatic interactions between the positive rim and anionic quinones as a means of immobilization, a porphyrin-quinone dyad system has been prepared. The distance between the donor and acceptor was changed systematically in aqueous solution, whilst maintaining a similar environment in each case. Upon photoexcitation of the porphyrin, efficient electron transfer occurs between the porphyrin and quinone units in a distance-dependent manner on the nanosecond timescale.

Rigid lipid membranes and nanometer clefts: motifs for the creation of molecular landscapes

Amphiphilic lipids associate in water spontaneously to form micelles, vesicles, monolayers, or biological membranes. These aggregates are soft and their shape can be changed easily. They behave like complex fluids because they are merely held together by weak, nondirected forces. The most important characteristic of these monolayers is their ability to dissolve hydrophobic molecules in the form of freely movable monomers. The fluid molecular layers are not suitable to anchor the components of chain reactions. However, if the alkyl chains are replaced by rigid skeletons or if the head groups are connected through intermolecular interactions, the aggregates become rigid and their fluid solvent character is lost. The construction of chiral surfaces by synkinesis (synthesis of noncovalent compounds) and of enzyme-type surface clefts of defined size can now be carried out by using rigid lipid membranes. Monolayers and nanometer pores on solid substrates attain sharp edges, and upright nanometer columns on smooth surfaces no longer dissipate. Five examples illustrate the advantages of using rigid molecular assemblies: 1) Cationic domains of rigid edge amphiphiles in fluid membranes act as manipulable ion channels. 2) Spherical micelles, micellar helical fibers, and vesicular tubes can be dried and stored as stable material. Molecular landscapes form on smooth surfaces. 3) alpha,omega-Diamide bolaamphiphiles form rigid nanometer-thick walls on smooth surfaces and these barriers cannot be penetrated by amines. Around steroids and porphyrins, they form rigid nanometer clefts whose walls and water-filled centers can be functionalized. 4) The structure of rigid oligophenylene- and quinone monolayers on electrodes can be changed drastically and reversibly by changing the potential. 5) 10(10) Porphyrin cones on a 1-cm2 gold electrode can be controlled individually by AFM- and STM-tips and investigated by electrochemical, photochemical, and mechanical means. In summary, rigid monolayers and bilayers allow the formation of a great variety of membrane structures that cannot be obtained from classical fluid alkyl amphiphiles.

Tetrachelate Porphyrin Chromophores for Metal Oxide Semiconductor Sensitization: Effect of the Spacer Length and Anchoring Group Position

Four Zn(II)-tetra(carboxyphenyl)porphyrins in solution and bound to metal oxide (TiO2, ZnO, and ZrO2) nanoparticle films were studied to determine the effect of the spacer length and anchoring group position (para or meta) on their binding geometry and photoelectrochemical and photophysical properties. The properties of three types of anchoring groups (COOH and COONHEt3) for four Zn(II)-porphyrins (Zn(II)-5,10,15,20-tetra(4-carboxyphenyl)porphyrin (p-ZnTCPP), Zn(II)-5,10,15,20-tetra(3-carboxyphenyl)porphyrin (m-ZnTCPP), Zn(II)-5,10,15,20-tetra(3-(4-carboxyphenyl)phenyl)porphyrin (m-ZnTCP2P), and Zn(II)-5,10,15,20-tetra(3-ethynyl(4-carboxyphenyl)phenyl)porphyrin (m-ZnTC(PEP)P)) were compared. In m-ZnTCPP, m-ZnTCP2P, and m-ZnTC(PEP)P the four anchoring groups are in the meta position on the meso-phenyl rings of the porphyrin macrocycle, thus favoring a planar binding mode to the metal oxide surfaces. The three meta-substituted porphyrin salts have rigid spacer units of increasing length (phenyl (P), biphenyl (P2), and diphenylethynyl (PEP)) between the porphyrin ring and the carboxy anchoring groups, thus raising the macrocycle from the metal oxide surface. All porphyrins studied here, when bound to TiO2 and ZnO, exhibited quenching of the fluorescence emission, consistent with electron injection into the conduction band of the semiconductor. Steady-state UV-vis and fluorescence studies of p-ZnTCPP on insulating ZrO2 showed evidence of aggregation and exciton coupling. This was not observed in any of the meta-substituted porphyrins. The photoelectrochemical properties (IPCE, Voc, and Isc) of the porphyrins bound to TiO2 films in solar cells have been measured and rationalized with respect to the sensitizer binding geometry and distance from the surface.

Hydrophobic and hydrophilic yoctowells

Rigid molecular monolayers made of alpha,omega-diamido lipids form yoctoliter-sized gaps ("yoctowells", 1 yL = 10(-24) L or 1 nm3) around porphyrin islands on smooth surfaces. Their hydrophobic walls adsorb cyclic edge amphiphiles, e.g., trans-1,2-cyclohexanediol, cellobiose, and tyrosine, which fill-up the wells slowly and irreversibly by a process called "kinetic trapping". Wells with oligoethylene or oligoamide walls are effective as 3D-crown ethers or oligoamide barrels for reversible "thermodynamic trapping" of amines or amides. Three porphyrins A, B, and C were sorted as stacks within the yoctowells, and a methylammonium ring was established at their rim to fixate a fourth molecule D at a longer distance. Yoctowells are easy to prepare, characterize, and modify and provide simple models of biological modules.

Counting of labelled tyrosine molecules in hydrophobic yoctolitre wells filled with water

Time-dependent radioactivity and solid-state 13C-NMR measurements of tyrosine entrapped in water-filled yoctolitre (10(-24) L) wells with hydrophobic walls are reported; the results indicate that such wells induce the formation of quasi solid tyrosine if they are brought in contact with 0.1 M solutions of this edge amphiphile.

Slow Motion, Trapping, and Sorting of Water- and Chloroform-Soluble Porphyrins in Nanowells

A two-step self-assembly procedure on smooth, aminated silica particles established holey monolayers. At first, single, flat-lying porphyrin tetraamides (A) were bound covalently, followed by the build-up of a rigid monolayer made of diamido bolaamphiphiles (bolas) around the porphyrin islands. "Nanowells" around porphyrin (A) bottoms with a uniform diameter of 2.2 nm and varying depths of 0.6, 1.0, or 1.5 nm depending on the length of the applied bolas were thus obtained. Oligoethylene headgroups solubilized the particles in water, ethanol, and chloroform/ethanol, and two hydrogen bond chains between the secondary amide groups prevented swelling of the monolayer. Manganese(III) porphyrinates (B) migrated from the bulk solution to the bottom of the form-stable nanowells with a speed of about 1 pm/s and were trapped there above porphyrin (A). After isolation of the (A,B) particles by centrifugation or ultrafiltration, the particles were suspended in a chloroform solution of a chlorin (C), which was also fixated irreversibly on the bottom of the nanowells. The nanowells thus contained three different porphyrins A,B,C in a noncovalent stack. The reverse sequence A,C,B was built-up correspondingly, first in chloroform/ethanol, and then in water. The "sorting" of A,B,C and A,C,B systems was characterized by visible spectra, sequence-dependent fluorescence quenching, and cyclic voltammetry of the top component. The molecular sorting method is the first of its kind and should be generally useful for the production of noncovalent reaction systems on any smooth surface.

Irreversible or reversible self-assembly procedures yield robust zirconium (iv)-porpyrinphosphonate cones or μm-long fibers of monomolecular thickness

Stepwise, irreversible self-assembly of porphyrinphosphonates by zirconium(IV) produces cones of 20 nm height and similar widths. Side-on growth cannot be prevented. Reversible fiber growth without metal ions gave micrometer long fibers on mica of 2 nm width, because charge repulsion allowed only for end-on growth.

Nanowells on silica particles in water containing long-distance porphyrin heterodimers

Smooth and nonswelling spherical silica particles with a diameter of 100 nm and an aminopropyl coating are soluble in water at pH 11, coagulate quickly at pH 3, and redissolve at pH 9. Electron microscopy as well as visible spectra of covalently attached porphyrins indicate the aggregation state of the particles. Long-chain alpha,omega-dicarboxylic acids with a terminal oligoethyleneglycol (=OEG)-amide group were attached in a second self-assembly step to the remaining amine groups around the porphyrins. Form-stable 2-nm wells were thus obtained and were characterized by fluorescence quenching experiments using the bottom porphyrin as a target. The one-dimensional diffusion of fitting quencher molecules along the 2-nm pathway took several minutes. Porphyrins with a diameter above 2 nm could not enter the form-stable gaps at all. Added tyrosine stuck irreversibly to the walls of the nanowells and prevented the entrance of quencher molecules, the OEG-headgroups fixated 2,6-diaminoanthraquinone. A ring of methylammonium groups was then fixed at the walls of the wells at a distance of 5 or 10 A with respect to the bottom porphyrin. 2,6-Disulfonatoanthraquinone was attached only loosely to this ring, but the exactly fitting manganese(III) meso-(tetraphenyl-4-sulfonato)porphyrinate (Mn(III) TPPS) was tightly bound. Transient fluorescence experiments showed a fast decay time of 0.2 ns for the bottom porphyrin, when the Mn(III) TPPS was fixated at a distance of 5 A. Two different dyes have thus been immobilized at a defined subnanometer distance in an aqueous medium.