Two-photon photodynamic therapy by water-soluble self-assembled conjugated porphyrins.

Studies on two-photon absorption (2PA) photodynamic therapy (PDT) by using three water-soluble porphyrin self-assemblies consisting of ethynylene-linked conjugated bis (imidazolylporphyrin) are reviewed. 2PA cross-section values in water were obtained by an open aperture Z-scan measurement, and values were extremely large compared with those of monomeric porphyrins such as hematoporphyrin. These compounds were found to generate singlet oxygen efficiently upon one- as well as two-photon absorption as demonstrated by the time-resolved luminescence measurement at the characteristic band of singlet oxygen at 1270 nm and by using its scavenger. Photocytotoxicities for HeLa cancer cells were examined and found to be as high as those of hematoporphyrin, demonstrating that these compounds are potential candidates for 2PA-photodynamic therapy agents.

Strong two-photon absorption and its saturation of a self-organized dimer of an ethynylene-linked porphyrin tandem.

The two-photon absorption properties of a self-organized dimer of a free-base and zinc(II) porphyrins tandem linked with an ethynylene group and terminated by imidazolyl and phenylethynyl groups were investigated. The self-organized dimer was found to exhibit strong two-photon absorption and furthermore the saturation of the two-photon absorption owing to the intense transition.

Synthesis and self-organization of fluorene-conjugated bisimidazolylporphyrin and its optical properties.

A conjugated-bisimidazolylporphyrin bridged by bis(ethynylfluorene) was synthesized and organized into linear polymer through self-coordination having mean molecular weights, M(w) and M(n), of ~2.1 × 105 Da and ~1.6 × 105 Da, respectively. A large two-photon absorption cross section value of 3.4 × 105 GM (per dimer unit) was observed. This value was comparable to that of the previously reported self-assembled linear polymer consisting of butadiyne-bridged imidazolylporphyrins. The two-photon absorption properties could be controlled by tuning the wavelength and absorption intensity of the one-photon absorption.

Fullerene- and pyromellitdiimide-appended tripodal ligands embedded in light-harvesting porphyrin macrorings.

Three new tripyridyl tripodal ligands appended with either fullerene or pyromellitdiimide moieties, named C(60)-s-Tripod, C(60)-l-Tripod, and PI-Tripod, were synthesized and introduced into a porphyrin macroring N-(1-Zn)(3) (where 1-Zn = trisporphyrinatozinc(II)). From UV-vis absorption and fluorescence titration data, the binding constants of C(60)-s-Tripod, C(60)-l-Tripod, and PI-Tripod with N-(1-Zn)(3) in benzonitrile were estimated to be 3 × 10(8), 1 × 10(7), and 2 × 10(7) M(-1), respectively. These large binding constants denote multiple interactions of the ligands to N-(1-Zn)(3). The binding constants of the longer ligand (C(60)-l-Tripod) and the pyromellitdiimide ligand (PI-Tripod) are almost the same as those without the fullerene or pyromellitdiimide groups, indicating that they interact via three pyridyl groups to the porphyrinatozinc(II) coordination. In contrast, the larger binding constants and the almost complete fluorescence quenching in the case of the shorter ligand (C(60)-s-Tripod) indicate that the interaction with N-(1-Zn)(3) is via two pyridyl groups to the porphyrinatozinc(II) coordination and a π-π interaction of the fullerene to the porphyrin(s). The fluorescence of N-(1-Zn)(3) was quenched by up to 80% by the interaction of C(60)-l-Tripod. The nanosecond transient absorption spectra showed only the excited triplet peak of the fullerene on selective excitation of the macrocyclic porphyrins, indicating that energy transfer from the excited N-(1-Zn)(3) group to the fullerenyl moiety occurs in the C(60)-l-Tripod/N-(1-Zn)(3) composite. In the case of PI-Tripod, the fluorescence of N-(1-Zn)(3) was quenched by 45%. It seems that the fluorescence quenching probably originates from electron transfer from the excited N-(1-Zn)(3) group to the pyromellitdiimide moiety.

Supramolecular organization of light-harvesting porphyrin macrorings.

Porphyrin-based supramolecular nanostructures have been produced by the self-assembly of porphyrin macrorings with three benzoic acid groups (Acid-R) on each side of the rings through cooperative carboxyl-carboxyl hydrogen bonds. Structures of the organized Acid-R were analyzed by AFM, and two clear distribution peaks were observed at 3 and 27 nm in the height-distribution histogram. From the overall assessment, the higher objects are considered to be one-dimensional structures standing vertically on the mica substrate. The height corresponds to an 11-mer of a unit Acid-R. Light-harvesting functions were examined by using fluorescence titration, whereby an energy-acceptor molecule (Tripod 2) was employed that strongly interacted with Acid-R units (association constant: 2.0×10(8) M(-1) ), specifically from the inner pore. The titration results showed that the apparent stoichiometry [Tripod 2]/[Acid-R] was <0.5, and that the value was concentration dependent. Titration results reasonably account for the scheme in which Tripod 2 only interacts with each terminal in the organized Acid-R. The number of organization was fitted to a 10-mer of Acid-R in a 6.8×10(-7) M solution, and was consistent with that estimated from the AFM results. In the composites of organized Acid-R/Tripod 2, a singlet excitation energy transfer occurred among the Acid-R units, and to Tripod 2. The energy-transfer rate constants were estimated by using the decamer model, which employed kinetic parameters obtained from steady-state and time-resolved fluorescence experiments.

Strong two-photon and three-photon absorptions in the antiparallel dimer of a porphyrin-phthalocyanine tandem.

Imidazolyl-appended phthalocyaninatozinc(II) containing an ethynylporphyrin linkage was stacked by complementary coordination into an antiparallel dimer that displayed strong nonlinear optical properties involving two-photon and three-photon absorptions.

Design of two-photon absorbing materials for molecular optical memory and photodynamic therapy.

Molecular design of two-photon absorption materials toward three dimensional high-density optical memory and highly selective photodynamic therapy at deep tissue sites is introduced. Functionality such as photochromism was attached to highly efficient 2PA materials. In the case of PDT, solubilization of the 2PA agents in water without any aggregation is important.

Single supramolecular porphyrin wires bridging gold nanoparticles.

Gold nanoparticles functionalized with imidazolylporphyrinatozinc(II)s were bridged by successive imidazole-to-zinc coordination of bidirectional porphyrinatozinc(II) units.

Energy transfer followed by electron transfer in a porphyrin macrocycle and central acceptor ligand: a model for a photosynthetic composite of the light-harvesting complex and reaction center.

A system that models a photosynthetic composite of the light-harvesting complex and reaction center is reported in which light energy collected by cyclic antenna porphyrins is transferred to a central energy-acceptor porphyrin, followed by photoinduced electron transfer to a fullerene positioned above the ring plane. Pyridyl tripodal ligands appended with bis(phenylethynyl)porphyrinatozinc(II) (ZnP-Tripod) and additional fulleropyrrolidine moieties (C(60)-ZnP-Tripod) were synthesized as the reaction center units. The tripodal ligand was strongly accommodated by the light-harvesting porphyrin macrocycle N-(1-Zn)(3) (1-Zn = trisporphyrinatozinc(II)) by using three-point coordination of pyridyl to uncoordinated porphyrinatozinc sites to afford a stable 1:1 composite. The binding constants for ZnP-Tripod and C(60)-ZnP-Tripod in benzonitrile were estimated from steady-state fluorescence titrations to be 1.4x10(7) and 1.6x10(7) M(-1), respectively. The steady-state fluorescence titration, fluorescence lifetime, and transient absorption studies revealed that in both composites the excitation energy collected by the nine porphyrins of N-(1-Zn)(3) was efficiently transferred to a ZnP moiety by means of a through-space mechanism with a quantum yield of approximately 90%. Furthermore, in the composite with C(60)-ZnP-Tripod, the converged energy at the ZnP moiety induced electron transfer to the C(60) moiety, which afforded the stable charge-separated state (Phi(CS)>90%).

Energy transfer among light-harvesting macrorings incorporated into a bilayer membrane.

Amphiphilic bis(imidazolyl)tris(porphyrinatozinc) complexes having omega-carboxyalkyl meso substituents were prepared. The barrel-type porphyrin macroring organized through their complementary coordination was incorporated into a liposomal bilayer membrane with orientation normal to the surface. Under the conditions concentrated in the membrane, introduction of a fullerotripyridyl ligand into the cavity of the macroring quenched fluorescence not only from the host ring itself but also from other surrounding macrorings. Energy transfer among antenna macrorings in the membrane mimics the assembly of a bacterial light-harvesting antenna system.

Synthesis and two-photon absorption enhancement of porphyrin macrocycles.

Improving the nonlinear optical properties of organic materials through transition to macromolecular architectures of highly conjugated chromophores has been shown to be a viable strategy for generating materials suitable for TPA applications. In this study we display a simple and elegant method by which to synthesize macrocycles of porphyrin dimers. Two-photon absorption studies show 2 orders of magnitude enhancement of cross-section of the material, giving a maximum delta(2) of 10(6) GM.

Construction of giant porphyrin macrorings self-assembled from thiophenylene-linked bisporphyrins for light-harvesting antennae.

As a model of bacterial photosynthetic light-harvesting antenna, a large number of porphyrin units were organized into barrel-shaped macrorings. Two imidazolylporphyrinatozinc(II) molecules were linked through either unsubstituted thiophenes or 3,4-dioctylthiophenes 1 a and 1 b, respectively. These structures were spontaneously organized by complementary coordination of the imidazolyl to zinc and produced a series of self-assembled fluorescent polygonal macrorings under high dilution conditions. The ring size increased compared with previous m-phenylene examples. The size distribution was also controlled by the presence of octyl substituents. A wide distribution of macrorings from 7- to >15-mer was obtained from 1 a, whereas macrorings ranging from 7- to 11-mer with a maximum population focused at the 8-mer were formed with 1 b. The size distribution was governed by competition between entropy-favored, smaller-ring formation and the enthalpy-favored, less-strained larger macroring. The UV/Vis spectra showed a gradual redshift for the larger rings reflecting an increase in the transition dipole interactions.

Stepwise elongation effect on the two-photon absorption of self-assembled butadiyne porphyrins.

Butadiyne-porphyrin dimer arrays, which were generated by complementary coordination of the central zinc atom to imidazolyl followed by metathesis, were elongated stepwise and their 2PA properties were explored.

Recent advances in two-photon photodynamic therapy.

Photodynamic therapy (PDT) is a treatment for tumors and accepted in several countries in the world. Introduction of two-photon absortion (2PA) into PDT allows spatially selective treatment of cancers. Possibilities and limitations of the use of two-photon excitation in PDT are discussed, and many literatures in this area are reviewed. The conclusions are that 2PA-PDT has an advantage for higher selectivity than one-photon absorption PDT, and femtosecond pulsed laser is more suitable for 2PA-PDT than pico- and nanosecond pulses. However, most of photosensitizers used in the past studies had low 2PA cross section values less than 50 GM, and resulted in a low PDT efficiency under two-photon irradiation conditions. To realize 2PA-PDT, much larger 2PA cross sections must be required.

Tandem cofacial stacks of porphyrin-phthalocyanine dyads through complementary coordination.

A novel straightforward methodology to organize discrete heterogeneous stacks of porphyrin and phthalocyanine employed an imidazolyl-to-zinc complementary coordination protocol for a Zn(II) phthalocyanine that contains an imidazolyl terminal with an ethynylporphyrin as a coplanar spacer. Structural elucidation was performed by means of size-exclusion chromatography, spectral titration, and NMR spectroscopy. The association constants for the complementary coordination of the heterogeneous slipped-cofacial tetrads reached extremely high values, in the order of 10(14) M(-1). Close contact of the porphyrin and phthalocyanine planes led to a strong shielding of the cofacial protons, which were split due to the slipped-cofacial heterogeneous environment. In variable-temperature NMR spectroscopy, the split signals remained in the aromatic region, a result suggesting structural robustness. Addition of trifluoroacetic acid dissociated the coordination structure to unify the split signals. The stacked tetrads showed unique electronic structures, such as strong exciton coupling and charge-transfer properties between the porphyrin and phthalocyanine units, which were modulated by the peripheral substituents of the phthalocyanine subunit and by the solvent. Interconversion between the coordination tetrad and the corresponding dyad was observed upon addition of an axial ligand.

Transmembrane nanopores from porphyrin supramolecules.

A porphyrin-based nanopore was constructed through a bilayer lipid membrane (BLM). The macrocyclic porphyrin (the inner diameter = ca. 2.1 nm) having six carboxylic acid groups directed up and down was synthesized by self-assembly of three trisporphyrins, metathesis linking, and subsequent hydrolysis. The porphyrin was incorporated into soybean lecithin based BLM, and the ion currents through the nanopore were observed under 500 mM KCl, LiCl, CaCl2, and tetraalkylammonium chloride solutions, indicating formation of a large pore through BLM. Blocking of the ion current was achieved by addition of the polycationic fourth-generation PAMAM dendrimer.

Light-harvesting supramolecular porphyrin macrocycle accommodating a fullerene-tripodal ligand.

Trisporphyrinatozinc(II) (1-Zn) with imidazolyl groups at both ends of the porphyrin self-assembles exclusively into a light-harvesting cyclic trimer (N-(1-Zn)(3)) through complementary coordination of imidazolyl to zinc(II). Because only the two terminal porphyrins in 1-Zn are employed in ring formation, macrocycle N-(1-Zn)(3) leaves three uncoordinated porphyrinatozinc(II) groups as a scaffold that can accommodate ligands into the central pore. A pyridyl tripodal ligand with an appended fullerene connected through an amide linkage (C(60)-Tripod) was synthesized by coupling tripodal ligand 3 with pyrrolidine-modified fullerene, and this ligand was incorporated into N-(1-Zn)(3). The binding constant for C(60)-Tripod in benzonitrile reached the order of 10(8) M(-1). This value is ten times larger than those of pyridyl tetrapodal ligand 2 and tripodal ligand 3. This behavior suggests that the fullerene moiety contributes to enhance the binding of C(60)-Tripod in N-(1-Zn)(3). The fluorescence of N-(1-Zn)(3) was almost completely quenched (approximately 97 %) by complexation with C(60)-Tripod, without any indication of the formation of charge-separated species or a triplet excited state of either porphyrin or fullerene in the transient absorption spectra. These observations are explained by the idea that the fullerene moiety of C(60)-Tripod is in direct contact with the porphyrin planes of N-(1-Zn)(3) through fullerene-porphyrin pi-pi interactions. Thus, C(60)-Tripod is accommodated in N-(1-Zn)(3) with a pi-pi interaction and two pyridyl coordinations. The cooperative interaction achieves a sufficiently high affinity for quantitative and specific introduction of one equivalent of tripodal guest into the antenna ring, even under dilute conditions ( approximately 10(-7) M) in polar solvents such as benzonitrile. Additionally, complete fluorescence quenching of N-(1-Zn)(3) when accommodating C(60)-Tripod demonstrates that all of the excitation energy collected by the nine porphyrins migrates rapidly over the macrocycle and then converges efficiently on the fullerene moiety by electron transfer.

A photochromic porphyrin-perinaphthothioindigo conjugate and its two-photon absorption properties.

A porphyrin-perinaphthothioindigo conjugate having two-photon absorption cross-sections of approximately 2000 GM and approximately 700 GM for trans- and cis-isomers, respectively, was synthesized and exhibited clear photochromic behavior upon one-photon and two-photon excitation.

Ligand-induced interconversion of a coordination-organized porphyrin dimer: a potential fluorescence-based molecular memory monitor.

A novel interconversion system between less-fluorescent stacked (S) dimer and fluorescent extended (E) dimer of the monoimidazolylbisporphyrinatozinc complex was investigated. The addition of pyridine induces transformation from the S to the E dimer, whereas the addition of acetic acid and subsequent heating reverses the transformation. The interconversion rate is controlled by ligand concentration and thermal treatment. The system can be applied to repeatedly readable molecular memory by highly sensitive fluorescence detection.

Fine-tuning of a ferrocene[porphyrin]ITO redox cascade for efficient sequential electron transfer commenced by an S2 photoexcited special-pair mimic.

A systematic series of ferrocene/porphyrin redox cascade architectures was assembled through a slipped-cofacial porphyrin dimer on ITO electrode in optimizing the anodic photocurrent generation to perform the highest quantum yield compared to reported values on ITO electrodes.