Facile synthesis of stacked, heteronuclear porphyrin arrays with varied architectures

Electrochemical, spectroscopic, and (1)O2 sensitization characteristics of 10,10-dimethylbiladiene complexes of zinc and copper

The synthesis, electrochemistry, and photophysical characterization of a 10,10-dimethylbiladiene tetrapyrrole bearing ancillary pentafluorophenyl groups at the 5- and 15-meso positions (DMBil1) is presented. This nonmacrocyclic tetrapyrrole platform is robust and can serve as an excellent ligand scaffold for Zn²⁺ and Cu²⁺ centers. X-ray diffraction studies conducted for DMBil1 along with the corresponding Zn[DMBil1] and Cu[DMBil1] complexes show that this ligand scaffold binds a single metal ion within the tetrapyrrole core. Additionally, electrochemical experiments revealed that all three of the aforementioned compounds display an interesting redox chemistry as the DMBil1 framework can be both oxidized and reduced by two electrons. Spectroscopic and photophysical experiments carried out for DMBil1, Zn[DMBil1], and Cu[DMBil1] provide a basic picture of the electronic properties of these platforms. All three biladiene derivatives strongly absorb light in the visible region and are weakly emissive. The ability of these compounds to sensitize the formation of ¹O₂ at wavelengths longer than 500 nm was probed. Both the free base and Zn²⁺ 10,10-dimethylbiladiene architectures show modest efficiencies for ¹O₂ sensitization. The combination of structural, electrochemical, and photophysical data detailed herein provides a basis for the design of additional biladiene constructs for the activation of O₂ and other small molecules.

Factors Controlling the Spectroscopic Properties and Supramolecular Chemistry of an Electron Deficient 5,5-Dimethylphlorin Architecture

A new 5,5-dimethylphlorin derivative (3H(Phl^CF₃ )) was prepared and studied through a combination of redox, photophysical, and computational experiments. The phlorin macrocycle is significantly distorted from planarity compared to more traditional tetrapyrrole architectures and displays solvatochroism in the soret region of the UV-vis spectrum (∼370-420 nm). DFT calculations indicate that this solvatochromic behavior stems from the polarized nature of the frontier orbital (LUMO+1) that is most heavily involved in these transitions. Compound 3H(Phl^CF₃ ) also displays an intriguing supramolecular chemistry with certain anions; this phlorin can cooperatively hydrogen-bond two equivalents of fluoride to form 3H(Phl^CF₃ )·2F^- but does not bind larger halides such as Cl^- or Br^-. Analogous studies revealed that the phlorin can hydrogen-bond with carboxylate anions such as acetate to form 1:1 complexes such as 3H(Phl^CF₃ )·OAc^- . These supramolecular assemblies are robust and form even in relatively polar solvents such as MeCN. Hydrogen-bonding of fluoride and acetate anions to the phlorin N-H residues significantly attenuates the redox and photophysical properties of the phlorin. Moreover, The ability to independently vary the size and pK_a of a series of carboxylate hydrogen-bond acceptors has allowed us to probe how phlorin-anion association is controlled by the anion's size and/or basicity. These studies elucidate the physical properties and the electronic effects that shape the supramolecular chemistry displayed by the phlorin platform.

Synthesis, electrochemistry, and photophysics of a family of phlorin macrocycles that display cooperative fluoride binding

A homologous set of 5,5-dimethylphlorin macrocycles in which the identity of one aryl ring is systematically varied has been prepared. These derivatives contain ancillary pentafluorophenyl (3H(Phl(F))), mesityl (3H(Phl(Mes))), 2,6-bismethoxyphenyl (3H(Phl(OMe))), 4-nitrophenyl (3H(Phl(NO2))), or 4-tert-butylcarboxyphenyl (3H(Phl(CO2tBu))) groups at the 15-meso-position. These porphyrinoids were prepared in good yields (35-50%) and display unusual multielectron redox and photochemical properties. Each phlorin can be oxidized up to three times at modest potentials and can be reduced twice. The electron-donating and electron-releasing properties of the ancillary aryl substituent attenuate the potentials of these redox events; phlorins containing electron-donating aryl groups are easier to oxidize and harder to reduce, while the opposite trend is observed for phlorins containing electron-withdrawing functionalities. Phlorin substitution also has a pronounced effect on the observed photophysics, as introduction of electron-releasing aryl groups on the periphery of the macrocycle is manifest in larger emission quantum yields and longer fluorescence lifetimes. Each phlorin displays an intriguing supramolecular chemistry and can bind 2 equiv of fluoride. This binding is allosteric in nature, and the strength of halide binding correlates with the ability of the phlorin to stabilize the buildup of charge. Moreover, fluoride binding to generate complexes of the form 3H(Phl(R))·2F(-) modulates the redox potentials of the parent phlorin. As such, titration of phlorin with a source of fluoride represents a facile method to tune the ability of this class of porphyrinoid to absorb light and engage in redox chemistry.

Porphyrin architectures bearing functionalized xanthene spacers

A modular synthetic strategy for the construction of cofacial porphyrin architectures bearing hydrogen-bond synthons on a xanthene platform is presented. The convergent approach is based on a xanthene aldehyde-ester building block that is easily obtainable on a multigram scale with minimal purification. Treatment of this xanthene derivative with a variety of aryl aldehydes and pyrrole under standard Lindsey conditions affords a family of meso-substituted porphyrins bearing a single functionalized xanthene spacer. Direct modification of the hydrogen-bond synthon after macrocyclization proceeds smoothly to furnish porphyrin systems with a variety of cofacial functionalities (e.g., carboxylic acid, ester, amide). Porphyrins bearing two trans-functionalized xanthene spacers are prepared by the MacDonald [2 + 2] condensation of the xanthene aldehyde-ester with readily available 5-aryl-substituted dipyrromethanes such as 5-mesityldipyrromethane to afford the pure alpha,alpha- and alpha,beta-porphyrin atropisomers after chromatographic separation. The versatility of this synthetic method offers intriguing opportunities for the use of these and related templates for the study of proton-coupled activation of small molecules.

New molecular arrays based on a tin(IV) porphyrin scaffold

Two new porphyrin arrays-a hexamer and a nonamer-have been synthesized and characterized by elemental analysis as well as mass, (1)H NMR, and UV-vis spectroscopic methods. The scheme of construction of these arrays employs a synthetic protocol involving sequential "organic" and "inorganic" reactions conducted, respectively, at the peripheral meso-phenyl ring and the central tin(IV) ion of the porphyrin scaffold. The architecture of the hexamer is such that it is based on a covalently linked tin(IV) porphyrin dimer, with each of the two tin(IV) centers trans-axially ligated to two free-base porphyrins, while the higher homologue features a tin(IV) porphyrin trimer as the basal unit, with its central metalloid ions having two free-base porphyrins as axial ligands. This extended, "axial-bonding"-type architecture of the new arrays has been investigated by the (1)H NMR method, which reveals characteristic ring-current-induced shifts and coupling patterns for the resonances due to protons of the axial free-base porphyrin subunits. The presence of any ring-ring (basal-basal, basal-axial, or axial-axial) interaction in these arrays is not obvious from their UV-vis and redox potential data, which are close to those of the corresponding constituent monomeric species. On the other hand, their singlet-state activities are quite different from those of the precursor reference compounds as probed by steady-state fluorescence. The results of the detailed investigations carried out on these hybrid, "bichromophoric" arrays have been interpreted in terms of the occurrence of intraarray, interchromophore energy- and electron-transfer reactions.

New methodologies for the preparation of porphodimethenes and their conversion to trans-porphyrins with functionalized naphthyl spacers

The MacDonald [2 + 2]-type condensation of readily available 5-aryl-substituted dipyrromethanes with acenaphthenequinone leads to the trans-syn- and anti-porphodimethenes, which in turn can be converted to the alpha,alpha- and alpha,beta-porphyrin atropisomers, respectively. Treatment of the metalated or unmetalated porphodimethenes with KOH or NaOMe in THF followed by protonation with HCl results in a ring opening of the acenaphthenone and formation of the trans-8-carboxynaphthylporphyrins or their esters (NaOMe) after oxidation. Alternatively, the porphyrin formation can be accomplished by reaction of the porphodimethenes with acids in the presence of water or methanol. Reaction with NaBH(4) in a THF--methanol mixture yields the corresponding dialcohols in nearly quantitative yields. Sixteen different building blocks were prepared in order to evaluate the generality of this new synthetic approach, with Ar = 2,4,6-Me(3)C(6)H(2); 2,6-Cl(2)C(6)H(3); 2,6-F(2)C(6)H(3); 3,4-tBu(2)C(6)H(3); 3,4,5-(MeO)(3)C(6)H(2); 4-BrC(6)H(4); 4-MeC(6)H(4); and 4-MeOOCC(6)H(4) at the meso positions. The synthesized porphodimethenes and porphyrins have been fully characterized, and the X-ray structure analyses of three representative derivatives are presented.

MacDonald [2 + 2]-type condensation with vicinal diketones: synthesis and properties of novel spiro-tricyclic porphodimethenes

[structure: see text] Acid-catalyzed [2 + 2] condensation reactions of polycyclic aromatic vicinal diketones including aceanthrenequinone, phenathrenequinone, and pyrene-4,5-dione with 5-mesityldipyrromethanes are outlined, and this methodology provides a flexible entry to spiro-tricyclic porphodimethenes. The porphodimethene products have been fully characterized, including X-ray structure analyses and fluorescence spectroscopy. In the case of the phenanthrenone-substituted macrocycle, the two spiro-locks can be ring-opened to form a trans-bis(2'-hydroxymethylbiphenyl)-substituted porphyrin.