Strong Inhibition of Singlet Oxygen Sensitization in Pyridylferrocene−Fluorinated Zinc Porphyrin Supramolecular Complexes

RAFT Synthesis and Characterization of Poly(Butyl-co-2-(N,N-Dimethylamino)Ethyl Acrylates)-block-Poly(Polyethylene Glycol Monomethyl Ether Acrylate) as a Photosensitizer Carrier for Photodynamic Therapy

Polymer micelles are promising drug delivery systems for highly hydrophobic photosensitizers in photodynamic therapy (PDT) applications. We previously developed pH-responsive polymer micelles consisting of poly(styrene-co-2-(N,N-dimethylamino)ethyl acrylate)-block-poly(polyethylene glycol monomethyl ether acrylate) (P(St-co-DMAEA)-b-PPEGA) for zinc phthalocyanine (ZnPc) delivery. In this study, poly(butyl-co-2-(N,N-dimethylamino)ethyl acrylates)-block-poly(polyethylene glycol monomethyl ether acrylate) (P(BA-co-DMAEA)-b-PPEGA) was synthesized via reversible addition and fragmentation chain transfer (RAFT) polymerization to explore the role of neutral hydrophobic units in photosensitizer delivery. The composition of DMAEA units in P(BA-co-DMAEA) was adjusted to 0.46, which is comparable to that of P(St-co-DMAEA)-b-PPEGA. The size distribution of the P(BA-co-DMAEA)-b-PPEGA micelles changed when the pH decreased from 7.4 to 5.0, indicating their pH-responsive ability. The photosensitizers, 5,10,15,20-tetrakis(pentafluorophenyl)chlorin (TFPC), 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (TFPP), protoporphyrin IX (PPIX), and ZnPc were examined as payloads for the P(BA-co-DMAEA)-b-PPEGA micelles. The encapsulation efficiency depended on the nature of the photosensitizer. TFPC-loaded P(BA-co-DMAEA)-b-PPEGA micelles exhibited higher photocytotoxicity than free TFPC in the MNNG-induced mutant of the rat murine RGM-1 gastric epithelial cell line (RGK-1), indicating their superiority for photosensitizer delivery. ZnPc-loaded P(BA-co-DMAEA)-b-PPEGA micelles also exhibited superior photocytotoxicity compared to free ZnPc. However, their photocytotoxicity was lower than that of P(St-co-DMAEA)-b-PPEGA. Therefore, neutral hydrophobic units, as well as pH-responsive units, must be designed for the encapsulation of photosensitizers.

First-row transition metal porphyrins for electrocatalytic hydrogen evolution — a SPP/JPP Young Investigator Award paper

A series of first-row transition metal complexes of tetrakis(pentafluorophenyl)porphyrin (1), denoted as 1-M (M [Formula: see text] Mn, Fe, Co, Ni, Cu, and Zn), were synthesized and examined as electrocatalysts for the hydrogen evolution reaction (HER). All these transition metal porphyrins were shown to be active for HER in acetonitrile using trifluoroacetic acid (TFA) as the proton source. The molecular nature and the stability of these metal porphyrins when functioning as HER catalysts were confirmed, and all catalysts gave Faradaic efficiency of >97% for H2 generation during bulk electrolysis. Importantly, by using 1-Cu, a remarkably high turnover frequency (TOF) of 48500 s[Formula: see text] 1-Cu the most efficient among this series of metal porphyrin catalysts. This TOF value also represents one of the highest values reported in the literature. In addition, electrochemical analysis demonstrated that catalytic HER mechanisms with these 1-M complexes are different. These results show that with the same porphyrin ligand, the change of metal ions will have significant impact on both catalytic efficiency and mechanism. This work for the first time provides direct comparison of electrocatalytic HER features of transition metal complexes of tetrakis(pentafluorophenyl)porphyrin under identical conditions, and will be valuable for future design and development of more efficient HER electrocatalysts of this series.

Syntheses, photophysical properties, and photocytotoxicities of tetrakis(fluorophenyl)porphyrin derivatives bearing 2-hydroxyethylthio groups

Porphyrin derivatives for photodynamic therapy are frequently modified with hydrophilic groups to improve their water solubility; however, such hydrophilic groups not only improve the solubility but also affect the photodynamic behavior of the compound. The suitable number and pattern of the hydrophilic substituents depend on the nature of the hydrophilic groups. In this article, we explore the optimum architecture for 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (TFPP) derivatives bearing 2-hydroxyethylthio substituents. All five derivatives, namely mono-, cis-bis-, trans-bis-, tris- and tetrakis-substituted TFPP, were successfully synthesized by the nucleophilic aromatic substitution of TFPP with 2-hydroxyethanethiol, separated, and subsequently identified using ESI-TOF mass spectrometry and (1)H and (19)F NMR spectroscopies. The hydrophilicity of the compounds increased with an increase in the number of 2-hydroxyethylthio groups. The singlet oxygen and hydroxyl radical generation efficiencies were estimated using chemical probes following photoirradiation (λ>500nm). trans-Bissubstituted TFPP exhibited the highest efficiency for both singlet oxygen and hydroxyl radical generation. The photocytotoxicities of the photosensitizers were evaluated in HeLa cells following photoirradiation (λ>500nm, 16Jcm(-2)), and increased with an increase in number of 2-hydroxyethylthio groups. In the case of 2-hydroxyethylthio-substituted TFPPs, the fully substituted TFPP was the most efficient architecture plausibly because of the result of the hydrophilicity of the compound rather than a greater efficiency in the generation of reactive oxygen species.

Photocatalytic oxygenation of 10-methyl-9,10-dihydroacridine by O₂ with manganese porphyrins

Photocatalytic oxygenation of 10-methyl-9,10-dihydroacridine (AcrH₂) by dioxygen (O₂) with a manganese porphyrin [(P)Mn^III: 5,10,15,20-tetrakis-(2,4,6-trimethylphenyl)porphinatomanganese(III) hydroxide [(TMP)Mn^III(OH)] (1) or 5,10,15,20-tetrakis(pentafluorophenyl)porphyrinatomanganese(III) acetate [(TPFPP)Mn^III(CH₃COO)] (2)] occurred to yield 10-methyl-(9,10H)-acridone (Acr=O) in an oxygen-saturated benzonitrile (PhCN) solution under visible light irradiation. The photocatalytic reactivity of (P)Mn^III in the presence of O₂ is in proportion to concentrations of AcrH₂ or O₂ with the maximum turnover numbers of 17 and 6 for 1 and 2, respectively. The quantum yield with 1 was determined to be 0.14%. Deuterium kinetic isotope effects (KIEs) were observed with KIE = 22 for 1 and KIE = 6 for 2, indicating that hydrogen-atom transfer from AcrH₂ is involved in the rate-determining step of the photocatalytic reaction. Femtosecond transient absorption measurements are consistent with photoexcitation of (P)Mn^III, resulting in intersystem crossing from a tripquintet excited state to a tripseptet excited state. A mechanism is proposed where the tripseptet excited state reacts with O₂ to produce a putative (P)Mn^IV superoxo complex. Hydrogen-atom transfer from AcrH₂ to (P)Mn^IV(O₂^•–) generating a hydroperoxo complex (P)Mn^IV(OOH) and AcrH^• is likely the rate-determining step, in competition with back electron transfer to regenerate the ground state (P)Mn^III and O₂. The subsequent reductive O–O bond cleavage by AcrH^• may occur rapidly inside of the reaction cage to produce (P)Mn^V(O) and AcrH(OH), followed by the oxidation of AcrH(OH) by (P)Mn^V(O) to yield Acr=O with regeneration of (P)Mn^III.

The synthesis of sugar-decorated hydrophilic porphyrins

The synthesis of porphyrins conjugated with sugar moieties is described. meso-Aminophenyl-substituted and β-amino-substituted porphyrin derivatives reacted with benzyl protected glucuronic acid leading to gluco-conjugated hybrids, which after reductive deprotection of OH groups ( H2, 10% Pd/C ) gave the desired target products of increased hydrophilicity. Alternatively, this type of similar conjugates were obtained through SNAr reaction of meso-tetrakis(pentafluorophenyl)porphyrin with aminomethyl sacharides. The substitution took place selectively in para-position of meso-perfluorophenyl rings, thus giving rise to one, two, or three times substituted products carrying N-linked glucoside residues.

β-Substituted ferrocenyl porphyrins: synthesis, structure, and properties

β-Substituted ferrocenyl porphyrins were designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction. The UV-vis absorption, emission, and cyclic voltammetric results indicate strong electronic communication between ferrocene and porphyrin. The porphyrin 4a is non-emissive in nature, while 4b and 4c show reduced fluorescence quantum yield. The single crystal X-ray structure of 4b is reported, which shows extensive C-H-π interactions.

Supramolecular assemblies for electron transfer

Recent advances in electron transfer chemistry of supramolecular electron donor-acceptor assemblies – containing porphyrins and metalloporphyrins – in connection with ICPP-3 are reviewed briefly by focusing on aspects of electron and energy transfer reactions.

An extremely long-lived charge-separated state of zinc tetraphenylporphyrin coordinated with pyridylnaphthalene-diimide

The nitrogen of pyridylnaphthalenediimide ( PyNIm ) coordinates to the metal center of zinc tetraphenylporphyrin ( ZnTPP ) to form a donor-acceptor complex: ZnTPP-PyNIm . Formation of the ZnTPP-PyNIm complex was probed by UV-vis, fluorescence and NMR spectra. The fluorescence of ZnTPP is strongly quenched and the fluorescence lifetime is shortened significantly in the complex. The transient absorption spectrum of the charge-separated state ( ZnTPP •+- PyNIm •-) is successfully detected by laser flash photolysis measurements of the ZnTPP-PyNIm system in benzonitrile. The charge-separated state of the complex produced by the photoinduced electron transfer has the longest lifetime, (450 μs) at 288 K, ever reported for donor-acceptor systems linked covalently or non-covalently in solution. However, when benzonitrile is replaced by benzene, the triplet excited state (3 ZnTPP* ), rather than the charge-separated state, is formed upon laser excitation of the ZnTPP-PyNIm complex, due to the lower energy of (3 ZnTPP* ) compared to the charge-separated state in benzene.

In vitro heavy-atom effect of palladium(II) and platinum(II) complexes of pyrrolidine-fused chlorin in photodynamic therapy

Introduction of a heavy atom into photosensitizers generally facilitates intersystem crossing and improves the quantum yield (Phi(Delta)) of singlet oxygen ((1)O(2)), which is a key species in photodynamic therapy (PDT). However, little information is available about the physiological importance of this heavy-atom effect. The aim of this study is to examine the heavy-atom effect in simple metallochlorins in vitro at the cellular level. 1,3-Dipolar cycloaddition of azomethine ylide to 5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato palladium(II) and platinum(II) afforded metallochlorins 4b and 4c in yields of 17.1 and 12.9%, respectively. The Phi(Delta) values increased in the order of 4a (0.28) < 4b (0.89) < 4c (0.92) in C(6)D(6). The photocytotoxicity of 4a, 4b, and 4c was evaluated in HeLa cells at a light dose of 16 J x cm(-2) with lambda > 500 nm and increased in the order of 4a < 4b < 4c at the concentration of 0.5 microM. The photocytotoxicity of 4b and 4c was significantly inhibited by addition of sodium azide, but not D-mannitol, suggesting that (1)O(2) is the major species causing cell death. Our results clearly indicate that 4b and 4c act as efficient (1)O(2) generators due to the heavy-atom effect in a cellular microenvironment as well as in nonphysiological media.

Ultrafast Photoinduced Electron Transfer in Directly Linked Porphyrin−Ferrocene Dyads

The ultrafast electron transfer occurring upon Soret excitation of three new porphyrin-ferrocene (XP-Fc) dyads has been studied by femtosecond up-conversion and pump-probe techniques. In the XP-Fc dyads (XP-Fcs) designed in this study, the ferrocene moiety is covalently bonded to the meso positions of 3,5-di-tert-butylphenyl zinc porphyrin (BPZnP-Fc), pentafluorophenyl zinc porphyrin (FPZnP-Fc), and 3,5-di-tert-butylphenyl free-base porphyrin (BPH2P-Fc). Charge separation and recombination in the XP-Fcs were confirmed by transient absorption spectra, and the lifetimes of the charge-separated states were estimated from the decay rate of the porphyrin radical anion band to be approximately 20 ps. The charge-separation rates of the XP-Fcs were found to be >10(13) s-1 from the S2 state and 6.3x10(12) s-1 from the S1 state. Charge separation from the S2 state was particularly efficient for BPZnP-Fc, whereas the main reaction pathway was from the S1 state for BPH2P-Fc. Charge separation from the S2 and S1 states occurred at virtually the same rate in benzene and tetrahydrofuran and was much faster than their solvation times. Analysis of these results using semiquantum Marcus theory indicates that the magnitude of the electronic-tunneling matrix element is rather large and far outside the range of nonadiabatic approximation. The pump-probe data show the presence of vibrational coherence during the reactions, suggesting that wavepacket dynamics on the adiabatic potential energy surface might regulate the ultrafast reactions.

Persistent Electron-Transfer State of a π-Complex of Acridinium Ion Inserted between Porphyrin Rings of Cofacial Bisporphyrins

A free-base cofacial bisporphyrin, H(4)DPOx, forms a pi-complex with acridinium ion (AcH(+)) by pi-pi interaction in benzonitrile (PhCN). Formation of the H(4)DPOx-AcH(+) pi-complex was probed by UV-vis and NMR spectroscopy. The binding constant between AcH(+) and H(4)DPOx is determined as 9.7 x 10(4) M(-)(1). Photoinduced electron transfer (ET) from the H(4)DPOx to the AcH(+) moiety occurred efficiently in the pi-complex to form the ET state (H(4)DPOx(*)(+)-AcH(*)). The ET state is successfully detected by laser flash photolysis. The lifetime of the ET state is 18 mus in PhCN at 298 K, and the quantum yield of the ET state is 90%. The temperature dependence of the ET state lifetime has been examined in the range from 273 to 353 K. The ET state lifetime exhibited a large temperature dependence, and the linear plot of ln(k(BET)T(1/2)) vs T(-)()(1), in accordance with the Marcus equation, affords the ET reorganization energy (0.54 eV). As a result, a remarkably long-lived ET state has been attained at low temperature, and virtually no decay of the ET state was observed at 77 K. Such an extremely long-lived ET state is indeed detected by steady-state UV-vis absorption spectroscopy.

Novel spectral and electrochemical characteristics of triphenylamine-bound zinc porphyrins and their intramolecular energy and electron transfer

Porphine bearing triphenylamine (TPA) pendant groups and their zinc complexes, zinc meso-tetra-p-(di-p-phenylamino)phenylporphyrin (ZnTDPAPP) and zinc meso-tetra-p-(di-p-tolylamino)phenylporphyrin (ZnTDTAPP) are synthesized and their spectral and electrochemical characteristics are studied. Zinc meso-tetraphenylporphyrin (ZnTPP) and zinc meso-tetra-p-aminophenylporphyrin (ZnTAPP) are also used as reference complexes. The B and Q bands of ZnTDPAPP and ZnTDTAPP are located at higher wavelengths and the bandwidths become broader compared with those of ZnTPP and ZnTAPP, indicating the peripheral TPA affects the electronic configuration of zinc porphyrins. Upon excitation in CH2Cl2 at room temperature, the compounds exhibit intramolecular singlet energy transfer from the TPA to the porphyrin core, and emission from the porphyrins are observed. Both ZnTDPAPP and ZnTDTAPP are easier to be oxidized and harder to be reduced than ZnTPP, in agreement with the strong electron-donating effect of the TPA groups. Extra waves corresponding to the oxidation of TPA substituents are also observed. The cation radical ZnTDTAPP+* exhibits an absorption spectrum very different from the typical spectra for porphyrin cation radicals. The NIR absorption band at 1296 nm indicates the electron transfer occurs intramolecularly. The above results evince the ability of TPA to modulate the electronic structure of zinc porphyrins.