Advances and opportunities in Group 15 porphyrin chemistry

The chemistry of Group 15 porphyrins has been established relatively well among the main-group porphyrins. Thus far phosphorus(III), phosphorus(V), arsenic(III), arsenic(V), antimony(III), antimony(V), and bismuth(III) porphyrins have been reported. Their unique axial-bonding ability, rich redox, and optical properties offer an advantage over other main-group or transition metal porphyrins. They could be excellent candidates for a variety of applications such as solar energy harvesting, molecular electronics, molecular catalysis, and biomedical applications. Despite these unique properties, the Group 15 porphyrins are not exploited at their fullest capacity. Recently, there has been some interest, where the richness of Group 15 porphyrin chemistry was explored for some of the above applications. In this context, this article summarizes recent advances in Group 15 porphyrin chemistry and attempts to unravel the tremendous opportunities of these remarkable porphyrins.

Hypervalent Phosphorus(V) Porphyrins with meso-Methoxyphenyl Substituents: Significance of the Number and Position of Methoxy Groups in Promoting Intramolecular Charge Transfer

To study the photophysical and redox properties as a function of meso-aryl units, a series of hypervalent phosphorus(V) porphyrins, PP(OMe)₂·PF₆, PMP(OMe)₂·PF₆, PDMP(OMe)₂·PF₆, P345TMP(OMe)₂·PF₆, and P246TMP(OMe)₂·PF₆, with phenyl (P), 4-methoxyphenyl (MP), 3,5-dimethoxyphenyl (DMP), 3,4,5-trimethoxyphenyl (345TMP), and 2,4,6-trimethoxyphenyl (246TMP) units, respectively, have been synthesized. The P(+5) in the cavity makes the porphyrin ring electron-poor, whereas the methoxy groups make the meso-phenyl rings electron-rich. The presence of electron-rich and electron-poor portions within the porphyrin molecule promoted an intramolecular charge transfer (ICT). Also, the study suggests that the ICT depends on the number and position of the methoxy groups. The ICT is more prominent in m-methoxy-substituted phosphorus(V) porphyrins (PDMP(OMe)₂.PF₆, P345TMP(OMe)₂·PF₆) and almost no ICT was found in no-methoxy, o-methoxy, and/or p-methoxy phosphorus(V) porphyrins (PP(OMe)₂·PF₆, PMP(OMe)₂·PF₆, P246TMP(OMe)₂·PF₆). Transient absorption studies indicate that the ICT takes place on the picosecond time scale. The most striking results come from P246TMP(OMe)₂·PF₆, where each phenyl ring carries three methoxy units, like the P345TMP(OMe)₂·PF₆, but it failed to induce the ICT process. Electrochemical studies and time-dependent density functional theory (TD-DFT) calculations were used to support the experimental results. This study extensively explores why and how slight variations in meso-aryl substitutions lead to intricate changes in the photophysical and redox properties of phosphorus(V) porphyrins.

Phosphorus(V) tetrapyrazinocorrolazines bearing axial aryloxy groups as pH-sensitive fluorophores and photosensitizers

Phosphorus(V) complexes of octaphenyltetrapyrazinocorrolazine bearing two aryloxy groups in the axial position, [TPyzCAP(OAr)₂] (2a-c, Ar = phenyl (2a), 4-dimethylaminophenyl (2b), and 4-hydroxyphenyl (2c)), were prepared using a one-pot procedure by consecutive treatment of the dihydroxidophosphorus(V) derivative, [TPyzCAP(OH)₂] (1), with SOCl₂ and then with the corresponding phenol ArOH. Complex 2a containing axial PhO groups is fluorescent in all studied solvents (toluene, CH₂Cl₂, THF, and DMSO, Φ_F ∼ 0.16-0.31) and is efficient to generate singlet oxygen (Φ_Δ = 0.55 (THF), 0.68 (toluene)). The introduction of NMe₂ and OH groups in the para-position of the axial ArO ligands strongly affects the fluorescence parameters and photosensitizing properties due to the appearance of the solvent-sensitive and pH-switchable effects of photoinduced electron transfer (PET). The PET effect of NMe₂ groups completely quenches the excited state of 2b in all solvents, but it is switched-OFF upon their protonation, and in the presence of acid traces, the fluorescence of 2b becomes bright and singlet oxygen generation is strongly enhanced. The PET effect of the OH group is increased upon its deprotonation and in the presence of base 2c as well as 1 becomes non-fluorescent. Specific solvation in THF and DMSO increases the ionic character of the OH bonds, and the fluorescence and photosensitizing properties of 1 and 2c are strongly decreased in these solvents. According to the results of DFT calculations performed using the B3LYP functional with the cc-pVDZ basis set and cyclic voltammetric studies, the molecular orbitals localized on aryloxy ligands are destabilized upon the introduction of OH and especially NMe₂ groups and their close position to the HOMO of corrolazine macrocycle (above HOMO in 2b and between HOMO and HOMO-1 in 2c) leads to the appearance of the PET effect.

Rational Design and Synthesis of OEP and TPP Centered Phosphorus(V) Porphyrin-Naphthalene Conjugates: Triplet Formation via Rapid Charge Recombination

Six new "axial-bonding" type "phosphorus(V) porphyrin-naphthalene" conjugates have been prepared consisting of octaethylporphyrinatophosphorus(V) (POEP⁺)/tetraphenylporphyrinatophosphorus(V) (PTPP⁺) and naphthalene (NP). The distance between the porphyrin and NP was systematically varied using polyether bridges. The unique structural topology of the octaethylporphyrinatophosphorus(V) (POEP⁺) and tetraphenylporphyrinatophosphorus(V) (PTPP⁺) enabled construction of mono- and disubstituted phosphorus(V) porphyrin-naphthalene conjugates, respectively. The steady-state and transient spectral properties were investigated as a function of redox properties, distance, and molecular topology. Strong electronic interactions between the phosphorus(V) porphyrin and NP in directly bound conjugates were observed. The established energy diagrams predicted reductive electron transfer involving singlet excited phosphorus(V) porphyrin and NP to generate high-energy (∼1.83-2.11 eV) charge-separated states (POEP/PTPP)^•-(NP)^•+. Femtosecond transient absorption spectral studies revealed rapid deactivation of singlet excited phosphorus(V) porphyrin due to charge separation wherein the estimated forward rate constants were in the range of 10⁹-10¹⁰ s^-1 and were dependent on the distance between the NP and porphyrins units, as well as the redox potentials of the type of the phosphorus(V) porphyrin. Additionally, due to high exothermicity and low-lying triplet states, the charge recombination process was found to be rapid, leading to populating the triplet states of phosphorus(V) porphyrins.

First entry into nonmetal-centred porphycenes: synthesis of a phosphorus(v) complex of octaethylporphycene

The first synthesis and structural characterization of a phosphorus(v) complex of porphycene, a constitutional isomer of porphyrin, are reported.

Additive effect of heparin on the photoinactivation of Escherichia coli using tricationic P-porphyrins

Polycationic porphyrins have received substantial attention in developing singlet oxygen-sensitizers for biological use such as in the photoinactivation of bacteria and photodynamic therapy (PDT) of tumor cells because they have strong binding affinities for DNA and proteins. However, these strong cellular interactions can retard elimination of the drug after PDT. Therefore, the studies on the interactions of porphyrins with other molecules present much interest, in order to modulate the sensitizers' activity or even remove them from the human body after PDT. Here, we studied the additive effect of heparin on the photoinactivation by polycationic porphyrins using Escherichia coli as a model cell. Tricationic P-porphyrin sensitizers substituted with an N-alkylpyridinium group (alkyl = pentyl (1a), hexyl (1b), and heptyl (1c)) or N-hexylammonium (1d) as the axial ligand were used. Additionally, dicationic Sb-porphyrin substituted with an N-hexylpyridinium group (1e) was prepared. We studied the additive effect of heparin on the photoinactivation of E. coli by 1a-1e. The bactericidal activities were evaluated using the half-life (T_1/2 in min) of E. coli and the minimum effective concentrations ([P]) of the porphyrin sensitizers. In the absence of heparin, the [P] values were determined to be 0.4-0.5 μM for 1a-1c and 2.0 μM for 1d-1e. The bactericidal activity of 1a-1c was completely retarded by the addition of heparin (1.0 μM). However, the addition of heparin (1.0 μM) could not completely retard the bactericidal activity of 1d-1e whose [P] values were relatively large. It is suggested that tricationic 1a-1c adsorbed onto the anionic heparin through electrostatic interactions. The adsorption of 1 on heparin disturbs the uptake of 1 into E. coli cells. Thus, the addition of heparin was found to be a useful method for retarding photoinactivation.

Modifications of Porphyrins and Hydroporphyrins for Their Solubilization in Aqueous Media

The increasing popularity of porphyrins and hydroporphyrins for use in a variety of biomedical (photodynamic therapy, fluorescence tagging and imaging, photoacoustic imaging) and technical (chemosensing, catalysis, light harvesting) applications is also associated with the growing number of methodologies that enable their solubilization in aqueous media. Natively, the vast majority of synthetic porphyrinic compounds are not water-soluble. Moreover, any water-solubility imposes several restrictions on the synthetic chemist on when to install solubilizing groups in the synthetic sequence, and how to isolate and purify these compounds. This review summarizes the chemical modifications to render synthetic porphyrins water-soluble, with a focus on the work disclosed since 2000. Where available, practical data such as solubility, indicators for the degree of aggregation, and special notes for the practitioner are listed. We hope that this review will guide synthetic chemists through the many strategies known to make porphyrins and hydroporphyrins water soluble.

Phosphorus complexes of porphyrinoid macrocycles

Phosphorus complexes of porphyrinoids such as porphyrins, corroles, [Formula: see text]-fused porphyrins, [Formula: see text]-fused expanded porphyrins and expanded porphyrins can be prepared readily by treating the porphyrinoid with phosphorylating agents such as PX3, PX5and POX3(X [Formula: see text] Cl or Br) under standard reaction conditions. In phosphorus porphyrinoids, the phosphorus is generally high valent [Formula: see text]5 oxidation state and the coordination number is varied from four to six. The insertion of P(V) into porphyrinoids alters the structure and electronic properties of the macrocycle significantly. Specially, the phosphorus binding mode is different from one porphyrinoid to another. The oxophilicity nature of P(V) in P(V) porphyrinoids is very useful to change variety of axial ligands and also helps to synthesize multi-porphyrin arrays by axial bonding approach. This review summarizes the phosphorus complexes of five different porphyrinoids and discusses their structure and electronic properties as well as their applications in the synthesis of more elaborate P(V) porphyrinoid based architectures.

pH-Response Optimization of Amino-Substituted Tetraphenylporphyrin Derivatives as pH-Activatable Photosensitizers

Amino-substituted tetraphenylporphyrin derivatives have been designed as pH-activatable photosensitizers for photodynamic cancer therapy. The photophysical processes of the monoamino-substituted derivative N1 and nonsubstituted derivative N0 have been studied. The quantum yields of the fluorescence and photosensitization of singlet oxygen by N1 were very low in the neutral condition (OFF state), but these quantum yields were recovered by adding acid (ON state). These changes were not observed for N0; therefore, N1 is expected to be applicable as a pH-activatable photosensitizer. The ON/OFF switching mechanism of N1 has also been clarified. To optimize the pH response, tri- and tetraamino-substituted derivatives (N3 and N4) have also been explored. The pH response intensified as the number of amino groups increased. Furthermore, the ON/OFF switching ratio of N3 was 100, which is quite high.

DNA/HSA interaction and nuclease activity of an iron(III) amphiphilic sulfonated corrole

The DNA binding of amphiphilic iron(III) 2,17-bis(sulfonato)-5,10,15-tris(pentafluorophenyl)corrole complex (Fe-SC) was studied using spectroscopic methods and viscosity measurements. Its nuclease-like activity was examined by using pBR322 DNA as a target. The interaction of Fe-SC with human serum albumin (HSA) in vitro was also examined using multispectroscopic techniques. Experimental results revealed that Fe-SC binds to ct-DNA via an outside binding mode with a binding constant of 1.25 × 10(4) M(-1). This iron corrole also displays good activity during oxidative DNA cleavage by hydrogen peroxide or tert-butyl hydroperoxide oxidants, and high-valent (oxo)iron(V,VI) corrole intermediates may play an important role in DNA cleavage. Fe-SC exhibits much stronger binding affinity to site II than site I of HSA, indicating a selective binding tendency to HSA site II. The HSA conformational change induced by Fe-SC was confirmed by UV/Vis and CD spectroscopy.