Free-base Porphyrin Research Articles

Free-base porphyrins as CEST MRI contrast agents with highly upfield shifted labile protons.

CEST has become a preeminent technology for the rapid detection and grading of tumors, securing its widespread use in both laboratory and clinical research. However, many existing CEST MRI agents exhibit a sensitivity limitation due to small chemical shifts between their exchangeable protons and water. We propose a new group of CEST MRI agents, free-base porphyrins and chlorin, with large exchangeable proton chemical shifts from water for enhanced detection. To test these newly identified CEST agents, we acquired a series of Z-spectra at multiple pH values and saturation field strengths to determine their CEST properties. The data were analyzed using the quantifying exchange using saturation power method to quantify exchange rates. After identifying several promising candidates, a porphyrin solution was injected into tumor-bearing mice, and MR images were acquired to assess detection feasibility in vivo. Based on the Z-spectra, the inner nitrogen protons in free-base porphyrins and chlorin resonate from -8 to -13.5 ppm from water, far shifted from the majority of endogenous metabolites (0-4 ppm) and Nuclear Overhauser enhancements (-1 to -3.5 ppm) and far removed from the salicylates, imidazoles, and anthranillates (5-12 ppm). The exchange rates are sufficiently slow to intermediate (500-9000 s-1 ) to allow robust detection and were sensitive to substituents on the porphyrin ring. These results highlight the capabilities of free-base porphyrins and chlorin as highly upfield CEST MRI agents and provide a new scaffold that can be integrated into a variety of diagnostic or theranostic agents for biomedical applications.

Fabrication of Langmuir–Blodgett chiral films from cationic (L)-proline-porphyrin derivatives

Stable porphyrin films expressing specific chirality at a supramolecular level have been achieved by Langmuir–Blodgett (LB) deposition of two inherently chiral (L)-proline porphyrin derivatives. Spectroscopic measurements (UV-vis, CD and fluorescence) evidenced that the stereogenic center stored on the peripheral proline residue dictates the specific supramolecular organization of the macrocycles at the air/water interface, which is found to be significantly different from that observed in solution ([Formula: see text] EtOH/H2O 25/75, v/v solvent mixture). In the case of free-base porphyrin, the firmness as well as the homogeneity of the corresponding chiral LB films are not optimal for a number of layers below 10. For the zinc derivative, thermal annealing helps to make the films more oriented and produces an amplification of the chirality of the treated films. The results described are of relevance, for example, for the development of stereoselective sensors, where the fabrication of chiral surfaces with specific and reproducible stereochemistry represents a crucial issue.

Degradation of Calmagite by dichloride (5,10,15,20tetraphenylporphyrinato)antimony hexachloridoantimonate:[Sb(TPP)Cl2] SbCl6

In this paper we reported on the spectroscopic, physicochemical and the catalytic properties of dichloride (5,10,15,20-tetraphenylporphyrinato)antimony(V) hexachloridoantimonate(V) [Sb(TPP)Cl2] SbCl6(2). Optical spectroscopy data showed a red-shift of both absorption bands; the Soret and the Q, which was observed of the antimony complex compared with that free base porphyrin H2TPP. The optical gap Eg-op was calculated using the Tauc plot method suggesting a semi-conductor product. The [Sb(TPP)]+ and the [Sb(TPP)Cl2]+ fragments were determined using the mass spectrometry. The fluorescence investigation showed a high fluorescence quantum yield with a lifetime of (2) compared to the free base porphyrin.32% of the calmagite solution was removed (C0 = 40 mg/L, pH = 6, mass = 3 mg) using (1). However, using the system (2)/H2O2 (C0 = 40 mg/L, pH = 6, t = 240 mins), the yield value of decolourisation was found to be 94% at equilibrium.

Sn(IV) Multiporphyrin Arrays as Tunable Photoactive Systems.

A series of four arrays made of a central Sn(IV) porphyrin as scaffold axially connected, via carboxylate functions, to two free-base porphyrins has been prepared and fully characterized. Three arrays in the series feature the same free-base unit and alternative tin-porphyrin macrocycles, and one consists of a second type of free-base and one chosen metallo-porphyrin. A thorough photophysical investigation has been performed on all arrays by means of time-resolved emission and absorption techniques. Specific focus has been given at identifying how structural modifications of the free-base and tin-porphyrin partners and/or variation of the solvent polarity can effectively translate into distinct photophysical behaviors. In particular, for systems SnTPP(Fb)2 (1) and SnOEP(Fb)2 (2), an ultrafast energy transfer process from the excited Sn(IV) porphyrin to the free-base unit occurs with unitary efficiency. For derivative SnTPP(FbR)2 (3), the change of solvent from dichloromethane to toluene is accompanied by a neat change in the intercomponent quenching mechanism, from photoinduced electron transfer to energy transfer, upon excitation of the Sn(IV) porphyrin unit. Finally, for array SnTpFP(Fb)2 (4), an ultrafast electron transfer quenching of both chromophores is detected in all solvents. This work provides a general outline, accompanied by clear experimental support, on possible ways to achieve a systematic fine-tuning of the quenching mechanism (from energy to electron transfer) of Sn(IV) multiporphyrin arrays.

Extreme Enhancement of Single-Molecule Fluorescence from Porphyrins Induced by Gold Nanodimer Antennas.

Porphyrins are typically weak emitters, which presents challenges to their optical detection by single-molecule fluorescence microscopy. In this contribution, we explore the enhancement effect of gold nanodimer antennas on the fluorescence of porphyrins in order to enable their single-molecule optical detection. Four meso-substituted free-base porphyrins were evaluated: two cationic, one neutral, and one anionic porphyrin. The gold nanodimer antennas are able to enhance the emission from these porphyrins by a factor of 105-106 increase in the maximum detected photon rates. This extreme enhancement is due to the combination of an antenna effect on the excitation rate that is estimated to be above 104-fold and an emission efficiency that corresponds to an increase of 2-10 times in the porphyrin's fluorescence quantum yield.

β-Octaalkoxyporphyrins: Versatile fluorometric sensors towards nitrated explosives

Real time detection of explosive residues is important to mitigate increasing security threats. Therefore, systematic studies are essential to optimize the performance of sensors. In this work, we have explored β-octamethoxyporphyrin and β-octabutoxyporphyrin to evaluate the effect of alkoxy groups in solution and in vapor phase sensing of nitrated explosives. Our systematic studies revealed a marked difference in sensitivity of these free-base porphyrins in solution state and vapor phase sensing of nitrated explosives simply by modulation of alkyl chain lengths. Alkoxyporphyrins exhibit very good sensitivity towards not only nitro aromatics but also alkyl nitro explosive taggants compared to β-octaethylporphyrin. Therefore, alkoxyporphyrins may act as versatile fluorescence turn-off based chemical sensors for nitrated explosives.

Sensitized Photochemical CO2 Reduction by Hetero-Pacman Compounds Linking a ReI Tricarbonyl with a Porphyrin Unit.

The hetero-Pacman architecture places two different metal coordination sites in close proximity, which can support efficient energy and/or electron transfer and allow for cooperative activation of small molecules. Here, the synthesis of dyads consisting of a porphyrin unit as photosensitizer and a rhenium unit as catalytically active site, which are held together by the rigid xanthene backbone, is presented. Mononuclear [(NN)Re(CO)3 (Cl)] complexes for CO2 reduction in which NN represents a bidentate diimine ligand (e.g., bipyridine or phenanthroline) lack light absorption in the visible region, resulting in poor photocatalysis upon illumination with visible light. To improve their visible-light absorption, we have focused on the incorporation of a strongly absorbing free base or zinc porphyrin unit. Resulting photocatalytic experiments showed a strong dependence of the catalytic performance on both the type of photosensitizer and the excitation wavelengths. Most notably, the intramolecular hetero-Pacman system containing a zinc porphyrin unit showed much better catalytic activity in the visible region (excitation wavelengths >450 nm) than the free base porphyrin version or the corresponding mononuclear rhenium compound or an intermolecular system comprised of a 1:1 mixture of the mononuclear analogues.

Metal substituted metalloporphyrins as efficient photosensitizers for enhanced solar energy conversion

The electron transfer dynamics of tetra(4-carboxyphenyl)porphyrin with different central metal ions Fe(III), Cu(II), Co(II),Ni(II), Zn(II) adsorbed on ZnO nanorods have been investigated by using photophysical and electrochemical studies. Spectroscopic studies confirmed that metalloporphyrins anchored onto ZnO through the carboxylic group. XPS studies indicated that replacement of free base porphyrin with different metals resulted in change in symmetry with uniform distribution of electron density of nitrogen atom of pyrrole ring which gives one broad peak of N1S state. Absorption studies revealed the bathochromic shift of soret band in ZnO-TCPPM nanocomposites as compare to free base porphyrin and metalloporphyrins owing to the electronic coupling between ZnO and TCPPM which stabilize the π* orbital. The photoluminescence properties of metalloporphyrins are shown to be highly dependent on central metal atom. Steady state and PL lifetime measurements studies shows the electron transfer in ZnO-TCPPM occurred feasibly for TCPPZn sensitizer as Zn(II)porphyrin have long lived excited state as compare to other metalloporphyrins. Improved photovoltaic properties of the metalloporphyrins are originated from electron injection driving force from dye to ZnO. Introduction of metal in porphyrin cavity increase the electron transfer process. Incase of TCPPCo, TCPPNi fast recombination exists between metal cation and photogenerated electron that inhibits the photocurrent generation. J–V Characteristics revealed there is 4.5 fold increases in efficiency for ZnO-TCPPM device as compare to device composed of free base porphyrin.

Evolution of electronic and vibronic transitions in metal(II) meso-tetra(4-pyridyl)porphyrins.

The changing of the electronic and vibronic states due to the insertion of Zn(II), Cu(II), Ni(II) or Co(II) ions in the meso-tetrakis(4-pyridyl)porphyrin ring center is investigated. The combination of absorption, photoluminescence, Raman and infrared spectroscopies with second-derivative-based spectral deconvolution analysis reveals that the structuration of both B- and Q-bands is very sensitive to the decorating ion. Similar to free base porphyrins, metal(II) meso-tetra(4-pyridyl)porphyrins also present their Q-band constituted of multiple electronic transitions, where the central ion plays an important role in the selection of vibration modes that mediate the vibronic transitions. Our novel results will expand and reinterpret current assignments for metal(II) meso-tetra(4-pyridyl)porphyrins vibrational modes available in the literature.

Incremental Introduction of Organocatalytic Activity into Conformationally Engineered Porphyrins

To study the correlation of macrocycle nonplanarity and catalytic activity of free base porphyrins in detail, a series of six tetraphenylporphyrins with graded degree of β‐ethyl substitution (“H2EtxTPPs” 1–6; x = 0, 2, 4, 6, 8) was applied in organocatalyzed reactions. The macrocycles display incrementally increasing nonplanarity due to repulsive peri‐interactions. This creates an out‐of‐plane vector and better accessibility of the core amine and imine groups as the number of alkyl substituents increases. Following such a molecular engineering approach, the inner core system could be used to activate small molecules as a result of significant saddle distortion. The potential organocatalyst “H2EtxTPPs” were used in benchmark sulfa‐Michael reactions and we found a distinct relationship between nonplanarity and conversion. These observations were attributed to the combined effect of enhanced basicity and increased H‐bonding potential that could facilitate bifunctional organocatalysis. Ultimately, density functional theory (DFT) calculations were performed on 1–6 to monitor some electronic properties of the title compounds.

Carbon‐bridged Oligo(phenylenevinylene)s as Light‐harvesting Antenna for Porphyrins

The efficacy of carbon-bridged oligo(phenylenevinylenes)s (COPVs) as light-harvesting antenna for porphyrins is demonstrated using a series of 5,15-di-COPVn-substituted free-base and zinc porphyrins, COPVn-MP-COPVn (n=1-3, M=H2 , Zn). These molecules were synthesized by Suzuki-Miyaura cross-coupling reactions of COPVn-Bpin and Br-H2 P-Br. The absorption spectra of these compounds in solution show a significant expansion of the Soret band region together with a bathochromic shift of the Q band, suggesting a significant interaction between these chromophores in the ground state. The photoluminescence quantum yield of the porphyrin-COPV conjugates is enhanced up to four times relative to the parent porphyrins. Theoretical calculations also indicated interactions between these chromophores in the HOMO, which suggests that the light-harvesting ability stems from the expansion of the π-electron-conjugation system.

Crystallographic elucidations of indium(III) porphyrin conformations, morphology and aggregation behaviour: Comparative optical study of free base porphyrins and their indium(III) derivatives at varying pH

The supramolecular structural architectures of 5, 10, 15, 20-Tetrakis-(4-methoxyphenylporphyrinato) indium (III) chloride (I) and 5, 10, 15, 20-Tetrakis-(4-methylphenylporphyrinato) indium (III) chloride (II) have been determined from their three dimensional X-ray diffraction data. Complex I crystallizes in the orthorhombic space group, Pbca with a = 23.0083 (4) Å, b = 15.2447 (3) Å, c = 23.3737 (4) Å, α = β = γ = 90°, V = 8198.5 (3) Å3 and Z = 8, whereas complex II crystallizes in the triclinic space group, P-1 with a = 12.0737 (5) Å, b = 12.1000 (5) Å, c = 14.1682 (6) Å, α = 88.560 (3)°, β = 78.730 (3)°, γ = 78.280 (3)° V = 1987.40 (14) Å3 and Z = 2. The geometries around In+3 ions in both the complexes are square pyramidal giving dome shaped architectures to the metalloporphyrins. In their respective crystal lattices, the monomeric units of complex I and II are packed in J-type (head-to-tail) supramolecular aggregation. These monomers are linked into their three dimensional supramolecular networks by various non covalent forces. Further the packing of monomers of each complex in their respective unit cells leaves significant void volumes (porosities). The optical absorption spectra of complex I and II and their free base analogues i.e. 5, 10, 15, 20-Tetrakis-(4-methoxyphenyl) porphyrin, H2TMP and 5, 10, 15, 20-Tetrakis-(4-methylphenyl) porphyrin, H2TTP have also been investigated in CH2Cl2 at different ionic strengths of the media. The investigations revealed that both the free bases exist as simple monomers, j-aggregates and dianionic monomers at neutral (pH = 7), acidic (pH = 4 and lower) and basic (pH = 10) media conditions. Also at high ionic strength of the media, the free bases undergo charge transfer electronic transitions. The two In3+ porphyrin exists as simple monomers at pH = 7. However at pH = 5 and lower, the In3+ ions in complex II is remarkably easily replaced by the protons of the acid to generate the corresponding protonated free base monomers which simultaneously aggregate in J-type stacking mode. In contrast to complex II that exhibits low stability in acidic media, the complex I remain intact in such media reflecting extremely high stability of the complex. The insertion of In3+ ions into the porphyrinic cores of free bases causes bathochromic shift followed by increase in the intensities of absorption bands. The appearance of two set of doublets for ortho phenyl protons in the 1H NMR spectra of each complex is attributed to the magnetic inequivalence of their ortho phenyl protons caused by the pulling of In+3 ions above the mean porphyrin planes.

Investigation of ultrafast energy transfer mechanism in BODIPY–Porphyrin dyad system

Novel β-fused BODIPY-Porphyrin compounds that contain free base porphyrin (TPP2BDP) and its Ni(II) complex (NiTPP2BDP) were synthesized to investigate intramolecular energy transfer mechanisms of β-fused BODIPY-porphyrin dyads and effect of the unfilled d shell metal ion on energy transfer mechanism. Fluorescence spectra of compounds exhibited that BODIPY emission was diminished upon excitation of the BODIPY unit because of the energy transfer from BODIPY to porphyrin unit. Femtosecond pump-probe spectroscopy measurements revealed that energy transfer of investigated compounds are faster than previous studies in literature. Rapid energy transfer (about 500 fs) from BODIPY to porphyrin was observed for both compounds when BODIPY unit is excited due to direct linkage of BODIPY to porphyrin unit. Intersystem crossing mechanism was also observed for the compound that contains free base porphyrin (TPP2BDP), whereas d-d transition was observed for the compound that contains metalloporphyrin (NiTPP2BDP) due to unfilled d orbital of Ni(II) ion.

Preparation of α-Zirconium Phosphate Nanosheet Stacked Solid Films with High Transparency and Intercalating with Cationic Free-base Porphyrins

In this paper, a method is presented for the preparation of a transparent thin solid film by stacking α-zirconium phosphate nanosheets (αZrPNSs), and intercalating with cationic free-base porphyrin...

Insight into conformationally-dependent binding of 1-n-alkyl-3-methylimidazolium cations to porphyrin molecules using quantum mechanical calculations.

The first step in the biodegradation of imidazolium-based ionic liquids involves the insertion of the -OH group into the alkyl side chain, and it is believed to be triggered by cytochrome P450. However, at present, there is a lack of fundamental understanding of why the hydroxylation process is observed only for longer alkyl chain analogues. As the initial step of the hydroxylation reaction involves the ionic liquid binding to Fe-porphyrin (FeP) - the catalytic center of cytochrome P450, the orientation of ionic liquids presented to FeP is expected to play a crucial role in eventual hydroxylation of the alkyl side chain. In order to elucidate the chain-length dependent binding preferences exhibited by the homologous series of 1-n-alkyl-3-methylimidazolium (n = 2, 4, 6, 8, and 10) [Cnmim]+ cations, a quantum mechanical treatment of the cations in the presence of free base porphyrin (FBP) and FeP is carried out at the B3LYP-D2 and M06 levels. The binding energy of different complexes with FBP and FeP is investigated by considering three vastly different starting relative orientations of the cations with respect to FBP and FeP: tail down, tail up, and interplanar. Our calculations of binding energies reveal that the cation orientations initiated from the tail down conformations (alkyl chain facing the porphyrin molecules) are progressively destabilized as the alkyl chain length increases. The decomposition of the binding energies into various energetic contributions shows that the interaction energy between the cations and porphyrin molecules varies with the cation geometries presented to porphyrin molecules and is the primary determinant of the magnitude of the binding energies. We further demonstrate that the propensity of the cation-FeP complexes to acquire an electron, the next step in the hydroxylation reaction cycle upon substrate binding, is favored independent of the cations and conformations, suggesting that this step is not the reason for the low biodegradability of short alkyl chain bearing cations. Furthermore, the weaker binding of the ionic liquid to FeP is anticipated to facilitate dioxygen binding to FeP, the step following the electron transfer reaction. Overall, the results of the present calculations indicate that the destabilization of the tail down conformations relative to the other two conformations correlates with the experimental results of the chain length-dependent biodegradation of imidazolium-based ionic liquids.

Electrocatalytic reduction of dioxygen by Mn(iii) meso-tetra(N-methylpyridinium-4-yl)porphyrin in universal buffer.

The electrochemical characterization of manganese(iii) meso-tetra(N-methylpyridinium-4-yl)porphyrin pentachloride ([Mn(TMPyP)Cl][Cl]4) via cyclic voltammetry (CV) and UV-vis spectroelectrochemistry (UV-vis SEC) was performed across the entire pH domain in aqueous buffered conditions. Assessment of the homogeneous electrocatalytic efficiency for the oxygen reduction reaction (ORR) from pH 3 to 6 using rotating-ring disk electrode experiments (RRDE) found it to be selective for water (82 to 93%). The observed efficiency for water is in contrast to previous reports on electrocatalytic ORR activity by Mn porphyrins in aqueous systems, which identified H2O2 as the primary product using indirect RDE methods only. The results described here are consistent with recent reports on the electrocatalytic behavior of Mn porphyrins under nonaqueous conditions, where the similar selectivity for water was also determined by RRDE methods. At pH 1, UV-vis SEC experiments also revealed that decomposition was occurring; free-base porphyrin was observed after the application of reducing potentials.

Direct synthesis of dilithium tetraphenylporphyrin: facile reaction of a free-base porphyrin with vapor-deposited lithium.

A solvent-free dilithium porphyrin was synthesized by direct reaction of free-base meso-tetraphenylporphyrin with elemental lithium in ultra-high vacuum. The reaction product dilithium tetraphenylporphyrin was studied by temperature-programmed desorption mass spectrometry (TPD-MS) and hard X-ray photoelectron spectroscopy (HAXPES). The solid-state reaction is thermodynamically favored, according to density functional theory (DFT) calculations.

Photocatalytic Oxygenation by Water-Soluble Metalloporphyrins as a Pathway to Functionalized Polycycles

Photocatalytic processes are present in natural biochemical pathways as well as in the organic synthetic ones. This minireview will cover the field of photocatalysis that uses both the free-base and specially metallated porphyrins as catalysts. While free-base porphyrins are valuable sensitizers to output singlet oxygen, metalloporphyrins are even more adjustable as photocatalysts because of their coordination capacity, generating a wider range of oxidation reactions. They can be applied in autooxidation reactions, hydroxylations, or direct oxygen transfer producing epoxides. This review will mainly focus on how manganese and some iron porphyrins can be utilized for the functionalization of compounds that have a polycyclic skeleton in their structure. These kinds of compounds are notoriously taxing to obtain and difficult to further functionalize by conventional organic synthetic methods. We have focused on photocatalytic oxygenation reactions in mild conditions with the use of water-soluble porphyrins, as this has been proven to be a good tool for these transformations. In the photocatalytic reactions of some polycyclic heteroaromatic compounds, new polycyclic epoxides, enediones, ketones, alcohols, and/or hydroperoxides are yielded, depending on the catalyst applied. The application of anionic and cationic Mn(III) porphyrins under different reaction parameters results in different reaction pathways generating a vast number of photocatalytic products. Recently, Co and Ni complexes have been also photophysically investigated and confirmed as potential photocatalysts for the functionalization of organic substrates.

Differential quenching of the angular momentum of the B and Q bands of a porphyrin as a result of extended ring π-conjugation

A novel porphyrin, whose [Formula: see text]-system has been extended via the presence of two additional carbon–carbon triple bonds on opposite meso-positions and by fusion of a single naphthalene unit simultaneously bridging the third meso-position and the [Formula: see text]-carbon of one of the pyrroles, has been synthesized in good yield. Absorption, magnetic circular dichroism, emission, and theoretical spectra are reported for the fused and unfused trans-naphthalene free base and zinc porphyrins. The fusing of one of the naphthalene moieties results in significant changes to the absorption spectrum and, very unusually, the bridged meso-[Formula: see text]-pyrrole fusion results in quenching of the MCD Faraday pseudo-A term in the porphyrin’s B band (S2). This unique effect was interpreted as resulting from the origin of the electronic structure of the second excited state (the B state). The [Formula: see text] and [Formula: see text] polarizations are completely mixed by the electronic effects of the non-symmetric extended conjugation of the [Formula: see text] ring. Analysis of the origin of the MCD signal indicates that the presence of this novel mixed polarization leads to negligible angular momentum in the important B state. To our knowledge, this is the first report in which the magnetic moment in a porphyrin’s intensely absorbing B band has been quenched while the angular momentum in the Q band, the first excited state, remains as normal. This implies that the photophysical properties of the B state are likely very different than those of the Q state, which has novel and significant implications for applications, especially in non-linear spectroscopy.

Molecular Engineering of Free-Base Porphyrins as Ligands-The N-H⋅⋅⋅X Binding Motif in Tetrapyrroles.

The core N−H units of planar porphyrins are often inaccessible to forming hydrogen‐bonding complexes with acceptor molecules. This is due to the fact that the amine moieties are “shielded” by the macrocyclic system, impeding the formation of intermolecular H‐bonds. However, methods exist to modulate the tetrapyrrole conformations and to reshape the vector of N−H orientation outwards, thus increasing their availability and reactivity. Strategies include the use of porpho(di)methenes and phlorins (calixphyrins), as well as saddle‐distorted porphyrins. The former form cavities due to interruption of the aromatic system. The latter are highly basic systems and capable of binding anions and neutral molecules via N−H⋅⋅⋅X‐type H‐bonds. This Review discusses the role of porphyrin(oid) ligands in various coordination‐type complexes, means to access the core for hydrogen bonding, the concept of conformational control, and emerging applications, such as organocatalysis and sensors.