Photophysical probes for multiple-redox and multiple-excited-state interactions in molecular aggregates.

Photosynthesis takes place through a highly efficient series of energy, electron, and proton transfers initiated by absorption of one or more photons within the visible region of the solar spectrum. Because of the presence of multiple chromophores needed for effective light harvesting, extinction coefficients must be very high. The absorbing multiunit array is held within a rigidly arranged structure that facilitates each electron hop. A fully artificial, yet biomimetic, alternative to photosynthesis that produces fuels directly and efficiently from sunlight and simple low molecular weight molecules would change the world. Achieving this goal requires a detailed understanding of the mechanisms of the key steps of the complex chemical and photochemical processes taking place in natural photosynthesis. One of these mechanisms relies on light harvesting to initiate multiple-step sequences to obtain combustible molecules suitable for burning. In particular, we are devising and testing photophysical models with characteristics that facilitate multiple electron transfers within a single aggregate and are directly relevant to light harvesting. We focus on structural features that promote photoinduced electron transfer at high dye densities, placed for optimal solar utilization and catalysis. The reaction producing oxygen is further complicated by the need for four electrons to complete the sequence, even though the first initiation step is presumably absorption of a single photon. Therefore we explore steps that accumulate charge or have the potential to do so. We also emphasize the synthesis of model systems that probe the complexity of individual steps. This Account examines the factors that influence the efficiency of electron redistribution in multiple-dye, multiple-excited-state, and multiple-redox equivalent arrays. Such knowledge will allow us to optimize the efficiency of electron migration and may contribute to a better understanding of multiple-equivalent light harvesting events by which photosynthetic energy storage takes place.

Surface effects in water-soluble shell-core hybrid gold nanoparticles in oligonucleotide single strand recognition for sequence-specific bioactivation.

Highly monodisperse water-soluble shell-core hybrid gold nanoparticles were prepared by in situ reduction of gold salts in the presence of a bifunctional thiol, tiopronin. The resulting particles exhibited diameters of 20-60 nm, as characterized by scanning electron microscopy. The tiopronin coating was uniform, exposing a carboxylated monolayer surface that was further modified by coupling with an amino-maleimide linker. The polar heteroorganic shell rendered these materials water-soluble and, as a result, amenable to conditions for coupling with molecules of biological importance, for example, thiolated oligonucleotides. When brought into contact with a homogeneously dispersed oligonucleotide chain, the functionalized shell-core particle binds with a complementary oligonucleotide chain with high specificity. Binding could be qualitatively recognized by easily observed fluorescence differences. The largest particles (ca. 60 nm diameter) were unstable in buffered water, and further condensation ultimately led to aggregation and precipitation. In contrast, particles with diameters of 20-30 nm were stable in buffered water and were easily further functionalized with matched oligonucleotide double strands. This work thus constitutes a new route for the preparation of stable modified gold nanoparticles that can be easily further modified to deliver a metal core particle in aqueous media, as required for recognition, and manipulation, of specific biological sequences. Surface properties are key variables for these applications.

The effect of dye density on the efficiency of photosensitization of TiO2 films: light-harvesting by phenothiazine-labelled dendritic ruthenium complexes.

A family of dendritic tris-bipyridyl ruthenium coordination complexes incorporating two or four carboxylate groups for binding to a TiO(2) surface site and another dendritic linker between the metal complex and highly absorptive dyes were formulated as thin films on TiO(2) coated glass. The family included phenothiazine-substituted dendrons of increasing structural complexity and higher optical density. The dye-loaded films were characterized by steady-state emission and absorption measurements and by kinetic studies of luminescence and transient absorption. Upon photoexcitation of the bound dyes, rapid electron injection into the metal oxide film was the dominant observed process, producing oxidized dye that persisted for hundreds of milliseconds. Complex decay profiles for emission, transient absorption, and optical bleaching of the dendritic dyes point to highly heterogeneous behavior for the films, with observed persistence lifetimes related directly to structurally enhance electronic coupling between the metal oxide support and the dendritic dyes.

A perspective on organic chemistry: physical organic chemistry.

A physical organic chemist wants to understand the detailed sequence of bond makings and bond breakings by which new or well-known reactions take place, i.e., the reaction mechanism, and to identify any metastable intermediates involved. The ultimate goal of such studies is to predict, and hope to control, chemical reactivity by determining how molecular structure and the immediate local environment affect a reaction of probative interest. Physical organic chemistry, in turn, provides structural insight, upon which others significantly depend for making new materials and for predicting and understanding new chemical and biochemical processes. I was fortunate in my own research to be able to study photocatalysis and photoinduced electron transfer as unifying themes that underlined our most significant work.

Optical and electrochemical properties of shell-core dendrimers: ruthenium coordination complexes capped with sized phenothiazine-substituted bipyridines.

A family of ruthenium coordination compounds capped by 4,4'-dimethylbipyridine ligands bearing 0, 1, 2, and 3 generations of dendritic phenothiazine moieties exhibit size-dependent electrochemical properties and structure-dependent absorption and luminescence spectra. Emission quantum yields, photoluminescence decay kinetics, and transient absorption spectra varied roughly with dendrimeric size and complexity on timescales from subnanoseconds to milliseconds. All complexes exhibit microsecond-lived lowest lying metal-to-ligand charge transfer excited states. Luminescence spectra were broadened compared with the unsubstituted parent, and quantum yields of emission for the large dendritic clusters were reduced by factors of two to five. Although the observed decay kinetics were complex, one transient decayed rapidly on a nanosecond scale, and a second intermediate showed no decay over a time scale of several tens of nanoseconds. Transient absorption measurements showed ground-state bleaching in regions of high ground-state absorptivity by the metal-to-ligand charge-transfer transient, as well as transient absorption shifts characteristic of isolated phenothiazine and ruthenium bipyridyl moieties.

Intramolecular charge transfer in 1:1 Cu(II)/pyrenylcyclam dendrimer complexes.

Two newly synthesized pyrenylcyclam dendrons (1 and 2) exhibit a new emission band centered at 450 nm when coordinated with copper triflate. Observed fluorescence shifts induced by coordinative metalation indicate an unusual intramolecular charge transfer from a pyrenyl excited state to the coordinated metal ion that competes with pyrene excimer formation. This interaction likely proceeds by photoexcitation of pi-complex of the appended arene, followed by intramolecular charge transfer within the dendritic 1:1 cyclam/metal complex, effecting reduction of the bound Cu(II) metal ion. The appended dendritic groups not only decrease the equilibrium binding constant with Cu(II) but also participate in a new excited-state pathway as an alternative to energy-dissipative excimer formation.

Synthesis of TiO2 photocatalysts in supercritical CO2 via a non-hydrolytic route.

Nanoscaled TiO2 powders with narrow size dispersion were prepared in supercritical carbon dioxide via non-hydrolytic acylation/deacylation of titanium alkoxide precursors with or without tris-fluorination. The microstructures of these powders were characterized by spectroscopic (FTIR, TGA, and XRD), microscopic (SEM or TEM), and surface area (BET) measurements. Photocatalytic oxidation of 1-octanol on these calcined TiO2 powders and on commercial T805 TiO2 suspended in aerated supercritical carbon dioxide revealed relative reactivity controlled by the powder microstructures. Calcined TiO2 prepared from titanium(IV) isopropoxide and trifluoroacetic anhydride was effectively dispersed in aerated supercritical carbon dioxide under stirring and exhibited high photocatalytic oxidation activity.

Electrochemical charging of a fullerene-functionalized self-assembled monolayer on au(111).

A fullerene derivative 10 with a terminal thiol group dissolves easily in common organic solvents and forms a densely packed self-assembled monolayer on gold surfaces. The functionalization of C(60) is based on the 1,3-dipolar cycloaddition of the azomethine ylide generated in situ from the corresponding aldehyde and N-methylglycine. The monolayers were characterized by grazing angle reflectance FTIR spectroscopy, scan tunneling microscopy, and cyclic voltammetry. The cyclic voltammogram of a SAM of 10 showed two well-resolved reversible cathodic waves corresponding to the first two one-electron reductions of the fullerene fragment.

The influence of core size on electronic coupling in shell-core nanoparticles: gold clusters capped with pyrenoxylalkylthiolate.

The mean size of the metallic core of monolayer-protected gold clusters densely capped with 9-(1-pyrenoxyl)octane-1-thiol or 1-dodecanethiol can be adjusted by controlling the thiol/HAuCl4 ratio during synthesis. Upon reduction of tetrachloroaurate by NaBH4, lower mean metal core diameters were attained in those composites prepared from mixtures with higher thiol/Au ratios. The efficiency of electronic coupling between the core metal cluster and pyrenyl groups appended as an outer shell in this size-graded family was studied by absorption and fluorescence spectroscopy. The observed emission intensity from the bound fluorophores is independent of the gold core size.

Metal-core--organic shell dendrimers as unimolecular micelles.

The synthesis and characterization of nanoparticle-cored dendrimers (NCDs), consisting of a metal core capped by arylpolyethers terminated with ester or carboxylate groups, are reported. These NCDs, comprising nanometer-sized gold clusters at the core and organic dendrons radially connected to the gold core by gold-sulfur bonds, were analyzed by TEM, TGA, UV, IR, and NMR spectroscopies. The density of the branching units connected to the core decreased from 1.90/nm(2) for a first-generation NCD (Au-G1(CO(2)Me)) to 0.80/nm(2) for a fourth-generation NCD (Au-G4(CO(2)Me)). Although the ester-terminated NCDs were stable and resisted aggregation, they were easily hydrolyzed to the corresponding water-soluble sodium salts. Aqueous solutions of (Au-Gn(CO(2)Na)) exhibited micellar properties. Since these NCDs possess a relatively unpassivated metal core and an organic aryl ether shell with micellar and dendritic properties, they are expected to have important potential applications in catalysis.

Novel photoswitchable receptors: synthesis and cation-induced self-assembly into dimeric complexes leading to stereospecific [2+2]-photocycloaddition of styryl dyes containing a 15-crown-5 ether unit.

Styryl dyes 4a-e containing a 15-crown-5 ether unit and a quinoline residue with a sulfonatoalkyl or sulfonatobenzyl N-substituent were synthesized. The relationship between the photochemical behavior of these dyes and their aggregates derived from complexation with Mg(2+) in MeCN was studied using (1)H NMR and absorption spectroscopy. The E-isomers of 4a-e were shown to form highly stable dimeric (2:2) complexes with Mg(2+). Upon irradiation with visible light, the dimeric complexes undergo two competing photoreactions, viz., geometric E --> Z isomerization, resulting in an anion-capped 1:1 complex of the Z-isomer with Mg(2+) and stereospecific syn-head-to-tail [2+2]-cycloaddition, affording a single isomer of bis-crown-containing cyclobutane. The N-substituent in the dye has a dramatic effect on the photochemical behavior of the dimeric complex. Molecular dynamics and semiempirical quantum-chemical calculations were carried out to interpret the observed photocycloaddition in the dimer. Conformational equilibria for the dimer of (E)-4b were analyzed using (1)H NMR spectroscopy.

Nanoparticle-cored dendrimers: synthesis and characterization.

The synthesis and characterization of a group of new dendrimers-namely, nanoparticle-cored dendrimers (NCDs)-are described. These materials were obtained by the reduction of hydrogen tetrachloroaurate phase-transferred into toluene in the presence of Fréchet-type polyaryl ether dendritic disulfide wedges of generation 1-5. These materials, possessing nanometer-sized gold clusters at the core and dendritic wedges radially connected to the core by Au-S bonds, were analyzed by TEM and TGA, and by UV, IR, and NMR spectroscopies. The number of branching units connected to the core decreased with the generation of the dendritic wedge, and this number changed from 2.18/nm(2) for Au-G-2 to 0.27/nm(2) for Au-G-5. This result suggests that, in the higher-generation NCDs, a large fraction of the surface area of the metal cluster is not passivated and is therefore available for catalytic activity.

A dendrimer-based electron antenna: paired electron-transfer reactions in dendrimers with a 4,4'-bipyridine core and naphthalene peripheral groups.

Paired electron transfers (ET) induced by the absorption of two photons by synthetic dendrimers are observed in first-, second-, and third-generation dendrimers comprised of a viologen-like core and an array of naphthalene peripheral groups. Flash photolysis and transient absorption techniques show that the yield of photoinduced double ET depends on laser intensity in the two largest dendrimers, NBV2(+2) and NBV3(+2). Their photochemical behavior thus requires an unusual multiphoton kinetic scheme. These dendrimers constitute the first synthetic models capable of multiple electron redox events deriving from a defined molecular architecture, thus mimicking natural light-collecting antenna systems.

Intramolecular Excited State Electronic Coupling Along an alpha-Helical Peptide.

Several families of peptides composed of alternating L-alanine (Ala) and alpha-aminoisobutyric acid (Aib) residues with an appended N,N-dimethylanilino and/or 2-naphthalenyl group exist in MeOH and CDCl(3) as alpha-helices. Steady state and time-resolved fluorescence measurements show that the distance and dihedral angle between the appended donor and acceptor and the alignment of the vectors for intramolecular charge transfer interaction (from donor to acceptor) with or against that of the helical dipole moment significantly influence the efficiency of photoinduced electronic coupling and, hence, of intramolecular fluorescence quenching.