Porphyrin Crown Ether Research Articles

Tying a knot between crown ethers and porphyrins.

Porphyrins and crown ether hybrids have emerged as a promising class of molecules composed of elements of a tetrapyrrole macrocycle and an oligo(ethylene glycol) segment. These hybrid systems constitute a broad group of compounds, including crowned porphyrins, crownphyrins, and calixpyrrole-crown ether systems forming Pacman complexes with transition metals. Their unique nature accustoms them as excellent ligands and hosts capable of binding guest molecules/ions, but also to undergo unusual transformations, such as metal-induced expansion/contraction. Depending on the design of the particular hybrid, they present unique features involving intriguing redox chemistry, interesting optical properties, and reactivity towards transition metals. In this perspective article, the overview of both the early designs of porphyrin-crown ether hybrids, as well as the most recent advances in the synthesis and characterisation of this remarkable group of macrocyclic systems, are addressed. The discussion covers the strategies employed in synthesising these systems, including cyclisation reactions, self-assembly, and their remarkable reactivity. The potential applications of porphyrin-crown ether hybrids are also highlighted. Moreover, the discussion identifies the challenges associated with synthesising and characterising hybrids, outlining the possible future directions.

Synthesis and characterization of a porphyrin-crown ether conjugate as a potential intermediate for drug delivery application

The development of synthetic strategies for functional building units plays a central role in supramolecular chemistry. Both porphyrin and crown ethers have attracted the attention of researchers worldwide owing to their unique properties. It is envisioned that the integration of the two molecules will result in hybrid materials with potential applications in many fields. In the present study, a new porphyrin derivative 3 appended with four 18-crown-6 (18C6) ether moieties was synthesized through the Suzuki-Miyaura coupling of boronic ester porphyrin 1 and 4-bromobenzo-18-crown-6 2 in 80% yield. Porphyrin 3 was fully characterized by 1H/[Formula: see text]C NMR spectroscopy and high resolution mass spectrometry. The tendency of the 18C6 to form host-guest complexes with ammonium cations was exploited to assemble cation responsive hybrid material of porphyrin 3 and ammonium immobilized mesoporous silica nanoparticles (MSNs). Furthermore, the potential application of the 3/MSNs conjugate as a cation-responsive drug delivery vehicle was investigated in solution by UV-vis and fluorescence spectroscopies.

Porphyrins with exocyclic rings. Part 25: synthesis of porphyrins with a fused cyclic ether subunit from tetrahydro-4 H-pyan-4-one

A novel porphyrin system with a fused dihydropyran ring has been synthesized from commercially available 4-oxotetrahydropyran. The cyclic ketone reacted with oximes derived from acetoacetate esters in the presence of zinc dust, sodium propionate, and propionic acid at 150–155 °C to give good yields of 5-oxatetrahydroindoles. A low yield of a benzo-fused analog was also obtained using 4-oxochromanone as the starting reagent. Reaction of the b-annelated pyrroles with N-chlorosuccinimide gave the 7-chloro-derivatives and these underwent an acid catalyzed condensation with α-unsubstituted pyrroles to give dipyrrolic products linked by a dihydropyran moiety. Hydrogenolysis of a dipyrrole dibenzyl ester gave the corresponding dicarboxylic acid and this was converted into a dialdehyde by treatment with TFA–CH(OMe) 3. MacDonald ‘2+2’ condensation with two different dipyrrylmethanes under optimized conditions afforded the porphyrin ethers in 30–36% yield. This approach provides a pilot study for the synthesis of porphyrin crown ether structures. In addition, insightful results on the conformations of dihydropyran-linked dipyrroles were obtained by careful examination of their proton NMR spectra in CDCl 3 and DMSO- d 6.

Light harvesting porphyrin-crown ether conjugates: toward artificial photosynthetic systems

In this paper we report on our ongoing progress in the preparation of artificial photosynthetic systems through the preparation of light harvesting multi-porphyrins. The synthesis of these antennae is described herein and the energy transfer capabilities of these devices demonstrated. A terminal porphyrin/crown ether conjugate has been maintained in each case to ensure a coordination site for the complexation of an ammonium/ C 60 derivative, which could be chosen as the electron acceptor partner for the preparation of artificial photosynthetic systems.

Supramolecular Complexes Obtained from Porphyrin—Crown Ether Conjugates and a Fullerene Derivative Bearing an Ammonium Unit.

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Supramolecular complexes obtained from porphyrin–crown ether conjugates and a fullerene derivative bearing an ammonium unit

A methanofullerene derivative with an ammonium subunit ( 1 ) has been prepared and its ability to form a supramolecular complex with a porphyrin–crown ether conjugate evidenced by NMR, UV–vis, electrospray mass spectrometry (ES-MS) and luminescence experiments. Interestingly, in addition to the ammonium–crown ether recognition, intramolecular stacking of the fullerene moiety and the porphyrin subunit has been evidenced. Due to this additional recognition element, the association constant for the supramolecular complex is increased by two orders of magnitude when compared to the K a values found for the complexation of 1 with benzo-18-crown-6. Finally, non-covalent systems resulting from the association of cation 1 with porphyrin derivatives bearing two crown ether subunits have been investigated. Intramolecular C 60–porphyrin interactions have also been evidenced within these supramolecular complexes. As a result, the 2:1 complexes are very stable as shown by the ES-MS studies.

Self-assembly of a C60 derivative bearing an ammonium unit with porphyrin-crown ether conjugates

Our recent progresses in the synthesis and the study of supramolecular porphyrin-fullerene conjugates are reported. In particular, we show that π-stacking of the two chromophores can have a dramatic effect on recognition interactions in non-covalent C 60-porphyrin ensembles. Specifically, a supramolecular complex has been obtained from a porphyrin-crown ether conjugate and a methanofullerene derivative bearing an ammonium function. In addition to the ammonium-crown ether interaction, intramolecular stacking of the fullerene moiety and the porphyrin subunit has been evidenced in the supramolecular system. Due to this additional recognition element, the association constant ( K a ) for the complex is increased by two orders of magnitude when compared to K a values previously found for complexation of the fullerene-ammonium derivative with other crown ethers. Finally, non-covalent systems resulting from the association of the fullerene-ammonium derivative with porphyrins bearing two crown-ether subunits have been investigated. Intramolecular C 60-porphyrin interactions have also been observed within these supramolecular arrays. As a result, the 2:1 complexes are very stable.

A supramolecular cup-and-ball C60-porphyrin conjugate system.

In addition to the ammonium-crown ether recognition, pi-stacking interactions between the C60 sphere and the porphyrin moiety have been evidenced in a supramolecular complex obtained from a porphyrin-crown ether conjugate and a fullerene derivative bearing an ammonium unit.

Porphyrin derivatives as water-soluble receptors for peptides†

The porphyrin–crown ether derivatives described here exhibit affinities to unprotected peptides of hitherto unknown magnitude in aqueous solution; the optical signal built-in to porphyrin receptors makes these applicable also to sensor techniques; the results can also form the basis of new catalytic systems, e.g. for the catalytic oxidation of peptides (such as tyrosinase models).

Time‐resolved EPR studies of covalently linked porphyrin—crown ether—quinones, dissolved in liquid crystals

AbstractTime‐resolved EPR spectra are reported for photoexcited porphyrin—quinones with butylene and cis‐ or trans‐ cyclohexylene spacers between porphyrin donors and crown ether—quinone acceptors. In the nematic phases of liquid crystals, polarized triplet EPR spectra of the porphyrin moiety could be observed at early delay times (0.1–0.6 μs) after laser flash irradiation. At later delay times (1–2 μs) the triplet EPR spectra of the charge‐separated radical pair state, created by intramolecular electron transfer from the porphyrin donor to the crown ether quinone acceptor, could be detected. The similarity of the zero‐field splitting parameters D of the cyclohexylene‐ and butylene‐linked compounds shows that the radical pair of the flexible butylene‐linked compound is in a stretched conformation. However, longer laser irradiation of the species with the flexible bridge results in the formation of a novel triplet radical pair species with a shorter porphyrin—quinone separation indicating a photoinduced backfolding of the quinone over the porphyrin plane. Comparison of the cis‐ and trans‐cyclohexylene‐linked compounds reveals a distortion of the cyclohexylene spacer in the cis compound.

Photoinduced electron transfer in zinc porphyrin-crown ether supramolecule with Eu3+ electron acceptor in crown void

Photoinduced electron transfer (ET) reactions in the zinc porphyrin-crown ether (ZnPCE) supramolecule, in which one crown ether moiety containing Eu3+ as electron acceptor is convalently linked to zinc porphyrin (ZnP), were studied by flash photolysis. In methanol solutions, highly efficient charge separation occurs via intramolecular ET from3ZnP to Eu3+ encapsulated in the crown ether void k1=(3±1)×103s−1) and intramolecular ET from 3ZnP to uncomplexed Eu3+ (k2=(2.5±0.5)×103s-1). Intermolecular ET from Eu2+ escaped from the crown ether void to ZnP.+ (kr=(4.3±0.7)×108 M−1s−1) seems to be the main pathway of charge recombination.

Porphyrin–crown ether macrotricyclic co-receptors for bipyridinium cations

In order to employ non-covalent interactions to position methyl viologen close to a photoactive centre (in this case a porphyrin) a dibenzo-crown ether, containing meta-substituted aromatic rings and tetraethylene glycol ether chains, was strapped across a porphyrin to produce a macrotricyclic host. This host was observed to complex methyl viologen (paraquat)(Ka 50 and 250 dm3 mol–1 and ΔG°–2.3 and –2.9 kcal mol–1 at 298 K and 263 K, respectively) and [Pt(bpy)(NH3)2](PF6)2(Ka 1350 dm3 mol–1, ΔG°–4.3 kcal mol–1 at 298 K) by an induced fit mechanism, as a result of rotational barriers associated with the diaryl amide linkage. Efforts to improve the complexing ability of these types of hosts involved a shortening of the linking ether chains and reduction of the amide group linking the crown ether and porphyrin moieties. Of these two directions the reduced host was most successful, complexing all the bipyridinium dications examined, although ligand exchange reactions complicated the binding of the platinum complex (for paraquat Ka 480 dm3 mol–1, ΔG°–3.6 kcal mol–1 at 298 K; for diquat Ka 80 dm3 mol–1, ΔG°–2.6 kcal mol–1 at 298 K). In contrast, the host containing shortened ether chains was observed to have similar complexing abilities to its predecessor, as a result of comparable solution conformations (for paraquat Ka 50 dm3 mol–1, ΔG°–2.3 kcal mol–1 at 298 K; for [Pt(bpy)(NH3)2](PF6)2Ka 365 dm3 mol–1, ΔG°–2.9 kcal mol–1 at 298 K). However, in this case binding by the platinum complex within the constricted cavity of the host resulted in deformation of the porphyrin and an increased activation barrier to complexation.

Porphyrin–crown ether based macrocyclic receptors for bipyridinium cations

Design modifications to potential receptors for the bipyridinium cations paraquat, diquat and [Pt(bpy)(NH3)2]2+, in which a porphyrin is appended by benzo crown ethers in two different configurations, are reported. A porphyrin strapped by a single dibenzo crown ether 4, and a porphyrin-based molecular tweezer7, in which a porphyrin is surmounted by two benzo-15-crown-5 units, are described. The macrobicyclic receptor 4 failed to complex either the paraquat or diquat dications to any significant extent, yet formed an inclusion complex with [Pt(bpy)(NH3)2]2+(Ka 155 dm–3 mol–1, ΔG°–2.9 kcal mol–1) with an orthogonal orientation of the guest relative to the porphyrin plane. This lack of paraquat and diquat complexation may be due to either an incorrect number or geometry of ether oxygen atoms for electrostatic interactions, and/or unfavourable orientations of the crown aromatic rings due to their increased conformational mobility within 4 when compared to the macrotricyclic analogue 1 reported previously by us. The complexation of [Pt(bpy)(NH3)2]2(PF6)2 by 4 was accompanied by a conformational change within the crown ether subunit, and a deformation of the porphyrin nucleus indicating that the ether strap is stretched to accommodate [Pt(bpy)(NH3)2]2+. This suggests that (i) decreasing the number of ether chains from two to one only partially removes the steric effects from the ether chain on the crown aromatic rings, or (ii) the amide bond is still dominant in influencing the solution conformations of the crown aromatic rings. The crystal structure of an isomer 4 showed the ether chain twisted around the porphyrin, as a result of atropisomerisation during the crystallisation process.The crown ether subunit of 9 was found to be conformationally solvent dependent. Two conformational extremes were identifiable; in acetone the crown aromatic rings are essentially coplanar with the meso-phenyl plane, whereas in chloroform the crown aromatic rings are nearly orthogonal to the meso-phenyl ring. The tweezer was observed to bind paraquat, diquat, and [Pt(bpy)(NH3]2]2+ in acetone solution with association constants and free energies of complexation of 32, 20, 140 dm3 mol–1, and –2.1, –1.8, –2.9 kcal mol–1, respectively. The stronger complexation of [Pt(bpy)(NH3)2]2+ presumably reflects the additional stabilisation of the inclusion complex by hydrogen bonding between the ammine ligands of [Pt(bpy)(NH3)2]2+ and the oxygen atoms of the crown tweezer. The inclusion geometry of these guests within the pincers of 7 is consistent with the aromatic rings of the guest sandwiched between the aromatic benzo-crown rings as predicted. An analogous tweezer molecule 8 lacking the crown ether chains did not bind any of the guests. Although 7 was observed to complex all three guests, the smaller association constants and free energies of complexation compared to 1 at 298 K reflect the lesser degree of preorganisation present in the crown-ethers of the complexing subunit of 7. Thus, although the removal of the bridging ether chains allows an increase in the conformational mobility of the crown aromatic rings the nett result is a decrease in the level of pre-organisation, and a reduction in the receptor binding strength.