Photoinduced electron transfer in photozyme systems

Photozymes are novel water-soluble polymers usually constructed by copolymerization of a mixture of water-soluble and water-insoluble comonomers, some of which contain chromophores capable of absorbing light and transmitting the excitation energy by means of the antenna effect to selected traps. The interactions between the hydrophobic and hydrophilic groups in the polymer with water cause the formation of hypercoiled pseudomicellar conformations of the polymer coil, leading to hydrophobic regions or pockets in the interior of the macromolecular coil. If the water contains hydrophobic organic molecules, they will locate preferentially in these hydrophobic polymer microdomains, and in the presence of light they can be photochemically transformed into useful products with high efficiency and selectivity. This paper reviews some recent results on photochemical reactions initiated by photoinduced electron transfer in these novel systems, and their possible commercial applications to pollution abatement, and solar production of hydrogen from water.     

Studies of the antenna effect in polymer molecules 25. Photozymes with rose bengal chromophores

Several copolymers of poly(sodium styrenesulfonate-styrene-vinylbenzylchloride) and poly(sodium styrenesulfonate-2-vinylnaphthalene- vinylbenzylchloride) containing various amounts of rose bengal chromophores attached to the polymer chain were synthesized. The copolymers are soluble in water and can solubilize large hydrophobic compounds. They are efficient genarators of singlet oxygen, and act as photosensitizers in the oxidation of singlet oxygen acceptors which are dissolved in the water phase and solubilized in the hydrophobic polymeric microdomains.     

Polymer catalysts for important photoelectron transfer reactions

Photozymes are novel water-soluble polymers made by the copolymerization of mixtures of hydrophobic and hydrophilic monomers, some of which contain chromophores capable of absorbing light and transmitting the excitation energy to selected traps by means of the antenna effect. The interactions between these groups and water force the polymer to adopt a hypercoiled conformation with hydrophobic pockets similar to those in the catalytic sites of natural enzymes. Hydrophobic organic compounds in the water solution will seek out and localize themselves in these regions, where they are subjected to electronic energy transfer from the light-excited antenna chromophores. The chemical reactions which occur are often different and more specific than in the case of photoreactions in common organic solvents. In a number of cases the reactions appear to proceed by an electron transfer mechanism. This paper summarizes recent results on the dechlorination of chlorinated aromatic and aliphatic compounds, and laser studies of multiphoton processes in aromatic compounds such as 9-(acetoxymethyl)phenanthrene (AMP).     

Photoinduced intermolecular and intramolecular actions between eosin and porphyrin

Two dyads of eosin and porphyrin linked with a semi-rigid (-CH2phCH2-) or flexible (-(CH2)4-) bridge and their reference model compounds were synthesized and characterized The intermoleccular interaction and intramolecular photoinduced singlet energy transfer and electron transfer were studied by their absorp tion spectra,fluorescence emission,excitation spectra and fluorescence lifetime The model compounds,ethyl ester of eosm (EoEt) and porphyrin (PorEt),could form complexes in the ground state.When the eosin moieties in dyads were excited,they could transfer some singlet energy to the porphyrins; in the meantime,they could also ndsce electron transfer between two chromophores.Exciting the porphyrin moieties in dyads could induce electron transfer from eosin moieties to porphyrin moieties.The efficiencies (EnT,ET) and rate constants (kEnT,kET) were related to the polarity of solvents and mutual orientation of the two chromophores in dyads.     

Design of three-dimensional, millimeter-scale models for molecular folding.

This communication describes the fabrication of three-dimensional structures of organic polymers using principles of design inspired by protein folding. The structures consist of rigid polyhedral components with dimensions of a few millimeters ("microdomains"), representing alpha-helical and beta-sheet secondary structures, connected with flexible linkers representing loops or turns. These structures were fabricated from polyurethane using photolithographic and soft lithographic techniques. The surfaces of the microdomains were patterned into hydrophobic and hydrophilic regions, and a hydrophobic photocurable liquid (serving both as lubricant and adhesive) was selectively precipitated onto the hydrophobic areas. The unfolded structures were suspended in water and agitated by tumbling. Self-assembly occurred through coalescence of the thin films of hydrophobic liquid, and was caused by minimization of the free energy of the interface between the liquid adhesive and the water. The self-assembled structures were locked in place by curing the adhesive with UV light. These results demonstrate the use of concepts abstracted from the study of proteins-including attractive hydrophobic interactions, shape complementarity, and conformational constraint-in the self-assembly of complex, three-dimensional structures on the millimeter scale.     

Flourescence quenching in aqueous solution of Amphiphilic copolymers: a kinetic model for downward curvature in a stern—volmer-type plot

A kinetic model is presented for downward curvature in a Stern—Volmer-type plot for flourescence quenching of flourophores in hydrophobic microdomains of amphiphilic copolymers in aqueous media. In the extreme case where quenchers were rigorously associated with the microdomains, the association constant and the aggregation number of the chromophores in the microdomains could be estimated.     

Synthesis and characterization of novel photoactive polymer poly(vinyl alcohol)‐graft‐poly(vinyl naphthalene)

A copolymer of poly(vinyl naphthalene) grafted onto poly(vinyl alcohol) has been synthesized with nitroxide‐mediated controlled radical polymerization. By separating the processes of the generation of grafting sites and polymerization, we can avoid the formation of the homopolymer. Because of its architecture, the polymer is soluble in water, despite the high content of hydrophobic groups. The naphthalene chromophores tend to aggregate, forming hydrophobic microdomains in an aqueous solution. Those aggregates exist in a very constrained environment that leads to extraordinarily large redshifts of both the absorption and emission of the polymer. The polymer acts as an efficient photosensitizer in photoinduced electron transfer. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2675–2683, 2006     

Toward transparent molecular wires: electron and energy transfer in transition metal derivatized conducting polymers

A great deal of research has concentrated on long range electron and energy transport in transition metal-based systems, including molecular donor-acceptor assemblies, electron and energy transfer cascades, dendrimers, and derivatized polymer systems. In an effort to improve efficiencies for electron and energy transport over large distances, several groups have now turned to conjugated systems. Several challenges exist to incorporating conducting materials/polymers in the study of photoinduced electron and energy transfer: solubility and processibility of the materials, thermal stability and limitations on direct spectroscopic characterization due to band gap absorptions. We have prepared a new series of conducting materials that provides for direct incorporation of chromophores and electrophores within the backbone of a conducting polymer. Energy transfer dynamics between conducting polymer bridges and porphyrin or metal-to-ligand charge transfer (MLCT) chromophores can be controlled through intermolecular interactions in solid vs solution samples. We have also developed a methodology to incorporate transmissive benzothiophene-type polymers such as polyisothianaphthene (PITN) within a copolymer assembly. These new materials are now being used to investigate long range electronic coupling and have potential applications that range from artificial photosynthesis to light emitting diodes.     

Singlet oxygen generation via two-photon excited FRET

A new approach to two-photon excited photodynamic therapy has been developed. A dendritic array of eight donor chromophores capable of two-photon absorption (TPA) was covalently attached to a central porphyrin acceptor. Steady-state fluorescence measurements demonstrated that the donor chromophores transfer excited-state energy to the porphyrin with 97% efficiency. Two-photon excitation of the donor chromophores at 780 nm resulted in a dramatic increase in porphyrin fluorescence relative to a porphyrin model compound. Enhanced singlet oxygen luminescence was observed from oxygen-saturated solutions of the target compound under two-photon excitation conditions.     

Mechanisms, pathways, and dynamics of excited-state energy flow in self-assembled wheel-and-spoke light-harvesting architectures

Static and time-resolved optical measurements are reported for two cyclic hexameric porphyrin arrays and their self-assembled complexes with guest chromophores. The hexameric hosts contain zinc porphyrins and 0 or 3 free base (Fb) porphyrins (denoted Zn(6) or Zn(3)Fb(3), respectively). The guests are a tripyridyl arene (TP) and a dipyridyl-substituted free base porphyrin (DPFb), each of which coordinates to zinc porphyrins of a host via pyridyl-zinc dative bonding. Each architecture is designed to have an overall gradient of excited-state energies that affords excitation funneling within the host and ultimately to the guest. Collectively, the studies delineate the various pathways, mechanisms, and rate constants of energy flow among the weakly coupled constituents of the host-guest complexes. The pathways include downhill unidirectional energy transfer between adjacent chromophores, bidirectional energy migration between identical chromophores, and energy transfer between nonadjacent chromophores. The energy transfer to the lowest-energy chromophore(s) within the backbone of a hexameric host (Fb porphyrins in Zn(3)Fb(3) or pyridyl-coordinated zinc porphyrins in Zn(6)*TP and Zn(6)*DPFb) proceeds primarily via a through-bond mechanism; the transfer is rapid (approximately 40 ps depending on the array) and essentially quantitative (>or=98%). The energy transfer from a pyridyl-coordinated zinc porphyrin of the host to the Fb porphyrin guest in the Zn(6)*DPFb complex is almost exclusively F?rster through-space in nature; this process is much slower ( approximately 1 ns) and has a lower yield (65%). These studies highlight the utility of cyclic architectures for efficient light harvesting and energy transfer to a designated trapping site.     

Amphiphilic diblock star polymer catalysts via atom transfer radical polymerization

Diblock star polymers were synthesized via atom transfer radical polymerization from a palladium porphyrin macroinitiator. The arms of the star polymers had an amphiphilic design, with the central Pd-porphyrin surrounded by a relatively hydrophobic block of poly(butyl acrylate) and terminated by a hydrophilic block of poly(oligoethyleneglycol monomethylether monomethacrylate). The size of both the interior and exterior blocks of the polymer arms were tuned over a wide range of molecular weights with the exterior block used to solubilize the stars in polar media. The star polymers showed enhanced reactivity in the oxidation of 2-furaldehyde relative to a small molecule porphyrin, suggesting that the polymer backbone aids with catalytic turnover. Oxygen diffusion studies indicate that the polymer backbone shields the porphyrin excited state from oxygen quenching. Shielding is independent of molecular weight and polymer composition, but it is not pronounced enough to retard the rate of singlet oxygen generation under preparative photooxidation conditions. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4939–4951, 2006     

Ultrafast aggregate-to-aggregate energy transfer within self-assembled light-harvesting columns of zinc phthalocyanine tetrakis(perylenediimide)

Light harvesting in photosynthetic antenna proteins involves a series of highly efficient ultrafast energy transfers between spectroscopically different populations of chlorophylls. Several strategies have recently been employed to mimic this natural energy transfer process, including polymers, dendrimers, and oligomeric porphyrin arrays linked by covalent bonds or by self-assembly. In all of these systems, excitation energy transfer occurs from one molecule to another, while very few of them involve energy transfer from one very strongly interacting chromophore aggregate to another such aggregate. Here we report the synthesis and characterization of a covalent zinc phthalocyanine-2,3,9,10,16,17,23,24-octacarboxytetraimide in which all four imide nitrogen atoms are substituted with N-octyl-N'-(4-aminophenyl)-1,7(3',5'di-tert-butylphenoxy)perylene-3,4:9,10-bis(dicarboximide) (ZnPcIm4-PDI4). The individual molecules self-assemble into stacked heptamers in solution as evidenced by small-angle X-ray scattering and form long fibrous structures in the solid as evidenced by TEM. The ZnPcIm4 and PDI molecules both stack in register with the same components in an adjacent covalent building block. Ultrafast energy transfer occurs with tau = 1.3 ps from the aggregated peripheral PDI chromophores to the core ZnPcIm4 chromophore aggregate. Exciton hopping between the ZnPcIm4 chromophores occurs with tau = 160 fs.     

Association of surfactants and polymers studied by luminescence techniques

Light emission spectroscopy has unique possibilities for the study of central issues of surfactants and associating polymers. With the help of luminescent probes, information may be obtained on matters such as molecular association, microstructure, and molecular dynamics; this constitutes an important contribution to the understanding and control of macroscopic properties, as well as biological function and technical applications. Important aspects of these systems considered in this review are: formation of micelles and hydrophobic microdomains; aggregation numbers of surfactants; shape of molecular aggregates; size of droplets in water or in oil in microemulsions; formation and stability of vesicles; intra- vs. intermolecular association in polymers; conformational changes in polymers; polymer–surfactant association; surfactant organization in adsorbed layers; kinetic aspects regarding the formation and disintegration of self-assembly structures; residence times of molecules in microdomains and migration of active molecules.     

Hydrogen‐Bonding Induced Cooperative Effect on the Energy Transfer in Helical Polynorbornenes Appended with Porphyrin‐Containing Amidic Alanine Linkers

Polynorbornenes appended with porphyrins containing a range of different linkers are synthesized. The use of bisamidic chiral alanine linkers between the pending porphyrins and the polymeric backbone has been shown to bring the adjacent porphyrin chromophores to more suitable orientation for exciton coupling owing to hydrogen bonding between the adjacent linkers. The hydrogen bonding between the adjacent pendants in these polymers may induce a cooperative effect and therefore render single‐handed helical structures for these polymers. Such a cooperative effect is reflected in the enhancement of FRET efficiencies between zinc–porphyrin and free base porphyrin in random copolymers.     

TTF-porphyrin dyads as novel photoinduced electron transfer systems

A TTF-linked porphyrin dyad and its zinc complex have been synthesized as novel photosystems with a redox-active pendant. The two chromophores of these dyads are not interactive in the absorption spectra, but the fluorescence of the porphyrin chromophore is dramatically quenched by intramolecular electron transfer from the TTF pendant.     

Synthesis of thermo‐responsive 4‐arm star‐shaped porphyrin‐centered poly(N,N‐diethylacrylamide) via reversible addition‐fragmentation chain transfer radical polymerization

Tetrafunctional porphyrins‐containing trithiocarbonate groups were synthesized by an ordinary esterification method. This tetrafunctional porphyrin (TPP‐CTA) could be used as a chain transfer agent in a controlled reversible addition‐fragmentation chain transfer (RAFT) radical polymerization to prepare well‐defined 4‐arm star‐shaped polymers. N,N‐Diethylacrylamide was polymerized using TPP‐CTA in 1,4‐dioxane. Poly(N,N‐diethylacrylamide) (PDEA) is known to be a thermo‐responsive polymer, and exhibits a lower critical solution temperature (LCST) in water. The star‐shaped PDEA polymer (TPP‐PDEA) was therefore also thermo‐responsive, as expected. The LCST of this polymer depended on its concentration in water, as confirmed by turbidity, dynamic light scattering (DLS), static light scattering (SLS), and ¹H NMR measurements. The porphyrin cores were compartmentalized in PDEA shells in aqueous media. Below the LCST, the fluorescence intensity of TPP‐PDEA was about six times larger than that of a water‐soluble low molecular weight porphyrin compound (TSPP), whose fluorescence intensity was independent of temperature. Above the LCST, the fluorescence intensity of TPP‐PDEA decreased, while the intensity was about three times higher than that of TSPP. These observations suggested that interpolymer aggregation occurred due to the hydrophobic interactions of the dehydrated PDEA arm chains above the LCST, with self‐quenching of the porphyrin moieties arising from these interactions. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2009     

Polymerized micelles with compartments

Multicompartment micelles with distinct hydrophobic microdomains having different properties have been realized via micellar polymers bearing hydrocarbon and fluorocarbon fragments. In aqueous solution, such polymers can self-organize into coexisting hydrocarbon and fluorocarbon microdomains that selectively solubilize different hydrophobes. The most efficient approach uses block copolymer polysoaps, but simpler designs may be possible.     

Solvent effects on competitive intramolecular electron transfer and energy transfer in a covalently linked porphyrin-phthalocyanine heterodimer

Photophysical properties of a porphyrin-phthalocyanine heterodimer covalently linked with a dipentoxy chain have been studied.Absorption spectra show that there is weak exciton coupling between the two chromophores in the ground state.Fluorescence spectra show that intramolecular energy transfer from porphyrin to phthalocyanine moiety occurs in competition with electron transfer.The efficiency of these two processes depends upon the mutual orientation of the two chromophores.The effect of solvent polarity on the intramolecular processes is also discussed.     

Bridge-dependent electron transfer in porphyrin-based donor-bridge-acceptor systems.

Photoinduced electron transfer in donor-bridge-acceptor systems with zinc porphyrin (or its pyridine complex) as the donor and gold(III) porphyrin as the acceptor has been studied. The porphyrin moieties were covalently linked with geometrically similar bridging chromophores which vary only in electronic structure. Three of the bridges are fully conjugated pi-systems and in a fourth, the conjugation is broken. For systems with this bridge, the quenching rate of the singlet excited state of the donor was independent of solvent and corresponded to the rate of singlet energy transfer expected for a F?rster mechanism. In contrast, systems with a pi-conjugated bridging chromophore show a solvent-dependent quenching rate that suggests electron transfer in the Marcus normal region. This is supported by picosecond transient absorption measurements, which showed formation of the zinc porphyrin radical cation only in systems with pi-conjugated bridging chromophores. On the basis of the Marcus and Rehm-Weller equations, an electronic coupling of 5-20 cm(-)(1) between the donor and acceptor is estimated for these systems. The largest coupling is found for the systems with the smallest energy gap between the donor and bridge singlet excited states. This is in good agreement with the coupling calculated with quantum mechanical methods, as is the prediction of an almost zero coupling in the systems with a nonconjugated bridging chromophore.     

Aggregation Behavior of Mono-endcapped Hydrophobically Modified Poly(sodium acrylate)s in Aqueous Solution

Titration microcalorimetry and steady-state fluorescence spectroscopy have been used to study the aggregation of mono-endcapped hydrophobically modified poly(sodium acrylate)s in aqueous solution. Polymers with molecular weights varying between 800 and 31,700 were synthesized by radical polymerization using an initiator and chain transfer agent. The resulting polymers form hydrophobic microdomains in aqueous solutions. The following conditions were applied: no salt and pH 5 and 9, respectively; 1 M sodium citrate and pH 9. At pH 5 the critical aggregation concentration (CAC, the concentration at which microdomains are formed) increases with increasing molecular weight of the polymers. The concentration range for aggregation is about 0.2-2.4 mM. At pH 9 the carboxylic acid groups are deprotonated and electrostatic repulsions are introduced; therefore the concentration for aggregation rises to about 80 mM. Interestingly, in case of polymers having M(n)<1400 the CAC decreases with increasing molecular weight due to a counterion-concentration gradient toward the hydrophobic microdomain. Near the microdomain the counterion binding is increased, reducing the electrostatic repulsions and allowing for lower aggregation concentrations. In the presence of 1 M sodium citrate this anomalous trend is suppressed to a large extent; since the overall counterion binding is increased and the CAC is lower. The concentration for aggregation is then in the same range as at pH 5 in the absence of salt. Copyright 2000 Academic Press.