Porphyrin Dendrimers Research Articles

Porphyrin-Based Molecular Architectures for Light Energy Conversion

We have constructed a series of supramolecular photovoltaic cells composed of multi-porphyrin arrays such as porphyrin-alkanethiolate monolayer protected-gold nanoparticles [H2PCnMPC: n is the number of methylene groups in the spacer], porphyrin dendrimers [D n P n ], and porphyrin-peptide oligomers [P(H2P) n ] and fullerene (C60) on nanostructured SnO2 electrodes (OTE/SnO2). These multi-porphyrin arrays form complexes with fullerene molecules and they form clusters in acetonitrile/toluene mixed solvent. The highly colored composite clusters are assembled as three-dimensional arrays onto nanostructured SnO2 films [denoted as OTE/SnO2/(multi-porphyrin array+C60) m ] using an electrophoretic deposition method. These highly organized molecular assembly films attain drastic enhancement of light energy conversion properties as compared to the non-organized reference system. The maximum power conversion efficiency (η) of OTE/SnO2/(H2PC15MPC+C60) m reaches 1.5%, which is 45 times higher than that of the reference system (0.035%).

Construction of a Rigid Zn Porphyrin−C60 Dyad within Dendritic Structure: Dendrimer Effect on Singlet Energy Transfer

A snowflake-shaped Zn porphyrin dendrimer with a C60 terminal was prepared. Covalent linkage of the C60 unit results in significant quenching of the Zn porphyrin fluorescence mainly due to energy transfer from the Zn porphyrin core to the C60 terminal. Comparison of the energy-transfer efficiency with similar dendrons indicates that the dendritic structure considerably delays the energy-transfer rate.

Nanoscale substrate recognition by porphyrin dendrimers with patched structures

Dendrimer technology has enabled us to build macromolecules with nanosized defined structures. By introducing unsymmetrical patched structures in dendrimers, sophisticated artificial receptors exhibiting nanoscale substrate recognition can be obtained. In this review article, our recent studies on molecular recognition by porphyrin dendrimers with patched structures are summarized. Three topics are presented: (1) oligopeptide-patched dendrimers as a nanoscale receptor of cytochrome c protein; (2) pocket dendrimers as a nanoscale receptor for bimolecular guest accommodation; and (3) energy transfer in unsymmetrical dendrimers. These dendrimers nicely mimic proteins and enzymes, and also act as photofunctional artificial receptors, in which porphyrin’s strong photoabsorption and intense fluorescence signals can respond sensitively to the substrate binding.

Enhanced Electron Transfer by Dendritic Architecture: Energy Transfer and Electron Transfer in Snowflake-Shaped Zn Porphyrin Dendrimers

[structure: see text] Photoinduced electron transfer was observed for the snowflake-shaped dendrimer with the Zn porphyrin core and anthraquinonyl terminals. Comparison of the electron-transfer efficiency of the dendrimer with the linear analogues indicates the advantage of the dendritic structure for the electron-transfer process.

Entangled Photon Absorption in an Organic Porphyrin Dendrimer

Two-photon absorption spectroscopy is an intensity dependent nonlinear effect related to the excitation of virtual intermediate states. The classical two-photon absorption has an extremely low efficiency which is quantified by its cross-section (delta approximately 10(-48) cm4 s at 800 nm). To overcome this limitation, we demonstrate a novel effect of the two-photon absorption method utilizing the high degree of quantum optical correlation between photon pairs created by the process of spontaneous parametric downconversion. A large entangled two-photon absorption cross-section (delta(e) approximately 10(-17) cm2 at 800 nm) was measured in an organic porphyrin dendrimer. We also discuss the nonmonotonic behavior of variation of the entangled two-photon absorption cross-section by controlling the entanglement time. This novel effect may open new avenues for ultrasensitive detection in chemical and biological systems. TPA spectroscopy has been considered as a powerful tool in physics, chemistry, and biology. The inherent nonlinear process of the classical TPA is distinguishable from the single photon absorption (SPA) linear process. Although the benefits of greater penetration depth and better control and reduction of scattering, the TPA spectroscopy has been restricted by the necessity of a high power optical source due to the low efficiency of the TPA effect. The use of entangled photons from a correlated source for the purpose of the two-photon effect is promising in this regard as one may obtain two-photon effects with very small numbers of photons.

Solvent induced control of energy transfer within Zn(II)-porphyrin dendrimers

Solvent induced optimization of energy transfer properties in a series of Zn(II)-porphyrin-appended dendrimers has been studied by means of exciton–exciton annihilation. Upon changing from a polar solvent (tetrahydrofuran) to a non-polar solvent (3-methyl-pentane), the annihilation energy transfer rates increase by 28–44%. This is related to a decrease of the hydrodynamic radius, which enhances the communication between the Zn(II)-porphyrin chromophores. As a consequence, the overall energy transfer efficiency is increased, thereby yielding complete annihilation between all the chromophores in the smallest generation dendrimer.

Relationship between Incoherent Excitation Energy Migration Processes and Molecular Structures in Zinc(II) Porphyrin Dendrimers

Multiporphyrin dendrimers are among the most promising architectures to mimic the oxygenic light-harvesting complex because of their structural similarities and synthetic convenience. The overall geometries of dendrimers are determined by the core structure, the type of dendron, and the number of generations of interior repeating units. The rigid core and bulky volume of exterior porphyrin units in multiporphyrin dendrimers give rise to well-ordered three-dimensional structures. As the number of generations of interior repeating units increases, however, the overall structures of dendrimers become disordered and randomized due to the flexibility of the repeating units. To reveal the relationship between molecular structure and processes of excitation-energy migration in multiporphyrin dendrimers, we calculated the molecular structure and measured the time-resolved transient absorption and fluorescence anisotropy decays for various hexaarylbenzene-anchored polyester zinc(II) porphyrin dendrimers along with three types of porphyrin dendrons as references. We found that the congested two-branched type dendrimers exhibit more efficient energy migration processes than one- or three-branched type dendrimers because of multiple energy migration pathways, and the three-dimensional packing efficiency of dendrimers strongly depends on the type of dendrons.

Multifunctional platinum porphyrin dendrimers as emitters in undoped phosphorescent based light emitting devices

The authors report on a class of platinum porphyrin based phosphorescent multifunctional first generation dendrimers (Pt-8Cn-TPP) incorporating a platinum porphyrin core as the emissive center and carbazole side groups as the hole, as well as energy transport fragments. Furthermore, the alkyl groups prevent aggregation and allow for the formation of transparent homogeneous films directly from solution. The intra- and intermolecular energy transfers by Förster and Dexter mechanisms from the carbazole side groups to the platinum porphyrin core have been investigated. Light emitting devices were fabricated and bright red electrophosphorescence was achieved from a single undoped emitting layer of Pt-8C4-TPP. The results indicate that platinum porphyrin dendrimers are very promising to make bright, red saturated and spin-coated phosphorescent devices with a simplified undoped emissive layer.

New Drug Delivery for Corneal Neovascularization Using Polyion Complex Micelles

To introduce photodynamic therapy (PDT) for corneal neovascularization (NV) by polyion complex (PIC) micelles, a novel drug delivery system. Development of specific drug delivery systems to corneal NV sites is an important part of next-generation photodynamic therapy. Nanocarriers consisting of PIC micelles bound by polyethylene glycol (PEG) shell exhibit good stability, high drug-loading capacity, and excellent potential for controlled drug release. Encapsulation of the photosensitizer dendrimer porphyrin (DP) into PIC micelles (DP-micelles) conveys adequate stability and increased photocytotoxicity without compromising photophysical properties of DP stability. We assessed the accumulation of DP-micelles and observed that the DP-micelles were incorporated into the corneal NV area. PIC micelles possess attractive features of selective accumulation at the corneal NV site. PDT using DP-micelles appears to be effective and safe for the treatment of corneal NV.

Up-Converted Emission in a Series of Phenylazomethine Dendrimers with a Porphyrin Core

The nonlinear optical and time-resolved properties of a series of phenylazomethine-porphyrin dendrimers are reported. The linear optical properties were also investigated, and the efficiency of the energy transfer process was obtained. Measurements were also carried out with the basic building-block molecules. The process of frequency up-converted emission was observed in these porphyrin dendrimers. The mechanism for this effect is investigated and related to the process of "hot-band" absorption in the phenylazomethine-porphyrin system. Time-resolved measurements also suggested efficient intramolecular vibrational energy redistribution in these systems. These properties suggest that the porphyrin dendrimers may also have applications in light harvesting of low-frequency photons, as well as in sensors.

Energy transfer and conformational dynamics in Zn–porphyrin dendrimers

The energy transfer within a series of Zn–porphyrin appended dendrimers was studied by means of time-resolved fluorescence anisotropy. We show that the energy transfer process between the Zn–porphyrin units in the dendrimers is limited to a maximum of four porphyrin units. At 200 K, the energy transfer process takes place on a 100-ps time scale, and can be modeled by Förster theory. Our results at room temperature further show that the porphyrin units are very mobile within the dendrimer, exhibiting rotational dynamics similar to that of a monomeric building block.

Supramolecular Nanocarrier of Anionic Dendrimer Porphyrins with Cationic Block Copolymers Modified with Polyethylene Glycol to Enhance Intracellular Photodynamic Efficacy

Photo finish: A polymeric micelle system is formed in aqueous media by electrostatic assembly of an anionic dendrimeric porphyrin and a poly(ethylene glycol)–poly(L-lysine) block copolymer (see picture). The micelles exhibit high photocytotoxicity and stability. The hydrodynamic size (ca. 60 nm) makes this polymeric micelle system suitable for intravenous administration in photodynamic tumor therapy.

Supramolecular Nanocarrier of Anionic Dendrimer Porphyrins with Cationic Block Copolymers Modified with Polyethylene Glycol to Enhance Intracellular Photodynamic Efficacy

Supramolecular Nanocarrier of Anionic Dendrimer Porphyrins with Cationic Block Copolymers Modified with Polyethylene Glycol to Enhance Intracellular Photodynamic Efficacy

Supramolecular Photovoltaic Cells Based on Composite Molecular Nanoclusters: Dendritic Porphyrin and C60, Porphyrin Dimer and C60, and Porphyrin−C60Dyad

Novel organic solar cells have been prepared using molecular clusters of porphyrin dendrimer (donor) and fullerene (acceptor) dye units assembled on SnO2 electrodes. The molecular clusters of porphyrin with dendritic structure and fullerene exhibit controlled size and shape in contrast with the reference systems (a porphyrin dimer and a porphyrin−fullerene dyad) without dendritic structure in TEM images, which show rather irregular and smaller clusters. The composite molecular nanoclusters of dendritic porphyrin and fullerene prepared in acetonitrile/toluene mixed solvent absorb light over entire spectrum of visible light. The comparison of photoelectrochemical properties of composite molecular cluster of porphyrin and fullerene with that of molecular cluster of porphyrin−C60 dyad with covalent linkage shows the importance of interpenetrating structure in each network to transport hole and electron efficiently. Furthermore, organic photovoltaic cells using clusters of supramolecular complexes of V-shaped ...

Supramolecular Photovoltaic Cells Using Porphyrin Dendrimers and Fullerene

Organic photovoltaic cells made using supramolecular complexes of porphyrin dendrimers with fullerenes demonstrate remarkable enhancement in the photoelectrochemical performance as well as broader photoresponse in the visible and near‐infrared regions relative to the reference system. This demonstrates that molecular assemblies of C60 and porphyrins with dendritic structures are effective in photoenergy conversion systems (see Figure).

Measurement of muscle microvascular oxygen pressures: compartmentalization of phosphorescent probe.

To determine whether the phosphorescent probe Oxyphor R2 (a palladium porphyrin dendrimer) becomes extravasated within normotensive skeletal muscle, R2 perfusion and washout studies were performed using a perfused rat hindlimb preparation. Phosphorescence signals were monitored in tibialis anterior muscles after 35 min of R2 blood perfusion and across a subsequent washout period that included vasodilation (sodium nitroprusside, SNP, approximately 3 x 10(-2) M). Two responses were evident: Group 1 (n = 4)--Inflowing blood pressure and vascular conductance remained stable close to initial values and subsequently a marked vasodilation was evident with SNP (vascular conductance; R2 blood perfusion, 0.096 +/- 0.005; washout, pre-SNP, 0.085 +/- 0.005, post-SNP, 0.110 +/- 0.005 mL/min/mmHg, p <.05, for pre- vs. post-SNP). Baseline phosphorescence signals could be monitored up to 99 +/- 36 s post-SNP when the phosphorescence signal disappeared. For these muscles, palladium content was undetectable. Group 2 (n = 3)--Inflowing blood pressure increased 112% and vascular conductance fell approximately 50%. These hindlimbs were unresponsive to SNP, phosphorescence signal was undiminished by washout and SNP, and muscles became edematous. These results suggest that in normotensive muscle (i.e., Group 1 above), extravasation of phosphorescent probe R2 over 35 min of perfusion is insufficient to yield a detectable phosphorescence signal in skeletal muscle.

Polyion complex micelles entrapping cationic dendrimer porphyrin: effective photosensitizer for photodynamic therapy of cancer

Photosensitizers play a crucial role in the photodynamic therapy (PDT) of cancer. In this study, a third-generation aryl ether dendrimer porphyrin with 32 primary amine groups on the periphery, [NH 2CH 2CH 2NHCO] 32DPZn, and pH-sensitive, polyion complex micelles (PIC) composed of the porphyrin dendrimer and PEG- b-poly(aspartic acid), were evaluated as new photosensitizers (PSs) for PDT in the Lewis Lung Carcinoma (LLC) cell line. The preliminary photophysical characteristics of [NH 2CH 2CH 2NHCO] 32DPZn and the corresponding micelles were investigated. Electrostatic assembly resulted in a red-shift of the Soret peak of the porphyrin core and the enhanced fluorescence. Compared to the dendrimer porphyrin [NH 2CH 2CH 2NHCO] 32DPZn, relatively low cellular uptake of dendrimer porphyrin [NH 2CH 2CH 2NHCO] 32DPZn incorporated in the PIC micelle was observed, yet the latter exhibited enhanced photodynamic efficacy on the LLC cell line. Importantly, the use of PIC micelles as a delivery system reduced the dark toxicity of the cationic dendrimer porphyrin, probably due to the biocompatible PEG shell of the micelles.

Characteristics of Organic Transformations in a Confined Dendritic Core: Studies on the AIBN‐Initiated Reaction of Dendrimer Cobalt(II) Porphyrins with Alkynes.

Cobalt(II) complexes of poly(aryl ester) dendrimer porphyrins [(m-[Gn]TPP)Co(II)] (generation number n=0-4), in the presence of azobisisobutyronitrile (AIBN) at 60 degrees C, underwent alkenylation with several alkynes at the metal center. A complete inhibition of double-bond migration (secondary transformation) was observed for [(m-[Gn]TPP)Co(II)] (n=3 and 4), which gave [(m-[Gn]TPP)Co(III)-C(=CH(2))R] (n=3 and 4) exclusively. Overall reaction rates for [(m-[Gn]TPP)Co(II)] (n=0-3) were hardly dependent on the size of the dendritic substituents, while a notable retardation was observed for the largest dendrimer, [(m-[G4]TPP)Co(II)]. Mechanistic studies on double-bond migration with pure [(m-[Gn]TPP)Co(III)-C(=CH(2))Bu] (n=0-4) demonstrated that the secondary transformation involves participation of [(m-[Gn]TPP)Co(III)H] (n=0-4), derived from [(m-[Gn]TPP)Co(II)] and AIBN, rather than [(m-[Gn]TPP)Co(II)] alone. Crossover experiments using [(m-[Gn]TPP)Co(III)-C(=CH(2))Bu] (n=2-4), in combination with nondendritic [(m-[G0]TPP)Co(II)] and AIBN, indicated a high level of steric protection of the active center by a robust [G4]-dendritic cage, as suggested by a (1)H NMR pulse relaxation time profile of m-[G4]TPPH(2).

Characteristics of organic transformations in a confined dendritic core: studies on the AIBN-initiated reaction of dendrimer cobalt(II) porphyrins with alkynes.

Cobalt(II) complexes of poly(aryl ester) dendrimer porphyrins [(m-[Gn]TPP)Co(II)] (generation number n=0-4), in the presence of azobisisobutyronitrile (AIBN) at 60 degrees C, underwent alkenylation with several alkynes at the metal center. A complete inhibition of double-bond migration (secondary transformation) was observed for [(m-[Gn]TPP)Co(II)] (n=3 and 4), which gave [(m-[Gn]TPP)Co(III)-C(=CH(2))R] (n=3 and 4) exclusively. Overall reaction rates for [(m-[Gn]TPP)Co(II)] (n=0-3) were hardly dependent on the size of the dendritic substituents, while a notable retardation was observed for the largest dendrimer, [(m-[G4]TPP)Co(II)]. Mechanistic studies on double-bond migration with pure [(m-[Gn]TPP)Co(III)-C(=CH(2))Bu] (n=0-4) demonstrated that the secondary transformation involves participation of [(m-[Gn]TPP)Co(III)H] (n=0-4), derived from [(m-[Gn]TPP)Co(II)] and AIBN, rather than [(m-[Gn]TPP)Co(II)] alone. Crossover experiments using [(m-[Gn]TPP)Co(III)-C(=CH(2))Bu] (n=2-4), in combination with nondendritic [(m-[G0]TPP)Co(II)] and AIBN, indicated a high level of steric protection of the active center by a robust [G4]-dendritic cage, as suggested by a (1)H NMR pulse relaxation time profile of m-[G4]TPPH(2).

Porphyrin and tetrabenzoporphyrin dendrimers: tunable membrane-impermeable fluorescent pH nanosensors.

The pH dependencies of the UV-vis and fluorescent spectra of new water-soluble dendritic porphyrins and tetrabenzoporphyrins were studied. Because of extended pi-conjugation and nonplanar distortion, the absorption and the emission bands of tetraaryltetrabenzoporphyrins (Ar(4)TBP) are red-shifted and do not overlap with those of regular tetraarylporphyrins (Ar(4)P). When encapsulated inside dendrimers with hydrophilic outer layers, Ar(4)Ps and Ar(4)TBPs become water soluble and can serve as pH indicators, with pK's adjustable by the peripheral charges on the dendrimers. Two new dendritic porphyrins, Gen 4 polyglutamic porphyrin dendrimer H(2)P-Glu(4)OH (1) with 64 peripheral carboxylates and Gen 1 poly(ester amide) Newkome-type tetrabenzoporphyrin dendrimer H(2)TBP-Nw(1)OH (2) with 36 peripheral carboxylates, were synthesized and characterized. The pK's of the encapsulated porphyrins (pK(H)()2(P)(-)(Glu)()4(OH) = 6.2 and pK(H)()2(TBP)(-)(Nw)()1(OH) = 6.3) were found to be strongly influenced by the dendrimers, revealing significant electrostatic shielding of the cores by the peripheral charges. The titration curves obtained by differential excitation using the mixtures of the dendrimers were shown to be identical to those determined for the dendrimers individually. Due to their peripheral carboxylates and nanometric molecular size, porphyrin dendrimers cannot penetrate through phospholipid membranes. Dendrimer 1 was captured inside phospholipid liposomes, which were suspended in a solution containing dendrimer 2. No response from 1 was detected upon pH changes in the bulk solution, while the response from 2 was predictably strong. When proton channels were created in the liposome walls, both compounds responded equally to the bulk pH changes. These results suggest that porphyrin dendrimers can be used as fluorescent pH indicators for proton gradient measurements.