meso-Aryl-3-alkyl-2-oxachlorins

Metal Complexes of Porphyrinoids Containing Nonpyrrolic Heterocycles

The replacement of one or more pyrrolic building block(s) of a porphyrin by a nonpyrrolic heterocycle leads to the formation of so-called pyrrole-modified porphyrins (PMPs), porphyrinoids of broad structural variability. The wide range of coordination environments (type, number, charge, and architecture of the donor atoms) that the pyrrole-modified frameworks provide to the central metal ions, the frequent presence of donor atoms at their periphery, and their often observed nonplanarity or conformational flexibility distinguish the complexes of the PMPs clearly from those of the traditional square-planar, dianionic, N₄-coordinating (hydro)porphyrins. Their different coordination properties suggest their utilization in areas beyond which regular metalloporphyrins are suitable. Following a general introduction to the synthetic methodologies available to generate pyrrole-modified porphyrins, their general structure, history, coordination chemistry, and optical properties, this Review highlights the chemical, electronic (optical), and structural differences of specific classes of metalloporphyrinoids containing nonpyrrolic heterocycles. The focus is on macrocycles with similar "tetrapyrrolic" architectures as porphyrins, thusly excluding the majority of expanded porphyrins. We highlight the relevance and application of these metal complexes in biological and technical fields as chemosensors, catalysts, photochemotherapeutics, or imaging agents. This Review provides an introduction to the field of metallo-PMPs as well as a comprehensive snapshot of the current state of the art of their synthesis, structures, and properties. It also aims to provide encouragement for the further study of these intriguing and structurally versatile metalloporphyrinoids.

Oxazolochlorins 21. Most Efficient Access to meso-Tetraphenyl- and meso-Tetrakis(pentafluorophenyl)porpholactones, and Their Zinc(II) and Platinum(II) Complexes

meso-Phenyl- and meso-pentafluorophenyl-porpholactones, their metal complexes, as well as porphyrinoids directly derived from them are useful in a number of technical and biomedical applications, and more uses are expected to be discovered. About a dozen competing and complementary pathways toward their synthesis were reported. The suitability of the methods changes with the meso-aryl group and whether the free base or metal derivatives are sought. These circumstances make it hard for anyone outside of the field of synthetic porphyrin chemistry to ascertain which pathway is the best to produce which specific derivative. We report here on what we experimentally evaluated to be the most efficient pathways to generate the six key compounds from the commercially available porphyrins, meso-tetraphenylporphyrin (TPP) and meso-tetrakis(pentafluorophenyl)porphyrin (TFPP): free base meso-tetraphenylporpholactone (TPL) and meso-tetrakis(pentafluorophenyl)porpholactone (TFPL), and their platinum(II) and zinc(II) complexes TPLPt, TFPLPt, TPLZn, and TFPLZn, respectively. Detailed procedures are provided to make these intriguing molecules more readily available for their further study.

Porpholactone Chemistry: Shining New Light on an Old Cofactor

The emergence of porpholactone chemistry, discovered over 30 years ago, has significantly stimulated the development of biomimetic tetrapyrrole chemistry. It offers an opportunity, through modifications of non-pyrrolic building blocks, to clarify the relationship between chemical structure and excited-state properties, deciphering the structural code for the biological functions of life pigments. With intriguing photophysical properties in the red to near-infrared (NIR) regions, facile modulation of their electronic nature by fine-tuning chemical structures, and coordination ability with diverse metal ions, these novel porphyrinoids have favorable prospects in the fields of optical materials, bioimaging and therapy, and catalysis. In this Minireview, we summarize the brief history of porpholactone chemistry, and focus on the studies carried out in our group, particularly on the regioisomeric effect, NIR lanthanide luminescence, and metal catalysis. We outline the perspectives of these compounds in the construction of porpholactone-related biomedical applications and optical and energy materials, in order to inspire more interest and further advance bioinspired inorganic chemistry and lanthanide chemical biology.

Rational design of an "all-in-one" phototheranostic

We report here porphodilactol derivatives and their corresponding metal complexes. These systems show promise as "all-in-one" phototheranostics and are predicated on a design strategy that involves controlling the relationship between intersystem crossing (ISC) and photothermal conversion efficiency following photoexcitation. The requisite balance was achieved by tuning the aromaticity of these porphyrinoid derivatives and forming complexes with one of two lanthanide cations, namely Gd3+ and Lu3+. The net result led to a metalloporphodilactol system, Gd-trans-2, with seemingly optimal ISC efficiency, photothermal conversion efficiency and fluorescence properties, as well as good chemical stability. Encapsulation of Gd-trans-2 within mesoporous silica nanoparticles (MSN) allowed its evaluation for tumour diagnosis and therapy. It was found to be effective as an "all-in-one" phototheranostic that allowed for NIR fluorescence/photoacoustic dual-modal imaging while providing an excellent combined PTT/PDT therapeutic efficacy in vitro and in vivo in 4T1-tumour-bearing mice.

Azulichlorins and Benzocarbachlorins Derived Therefrom

Acid-catalyzed condensation of azulidipyrrane aldehydes with a dihydrodipyrrin carbaldehyde afforded the first examples of azulichlorins. These macrocyclic products were isolated in a monoprotonated form, and the free bases proved to be somewhat unstable. The monocations were strongly diatropic, and proton NMR spectroscopy showed the internal C-H at ca. -2 ppm. Addition of TFA gave the related dications, but these exhibited significantly reduced aromatic ring currents. Reaction of an azulichlorin with tert-butyl hydroperoxide and KOH in dichloromethane/methanol gave a benzocarbachlorin and two related aldehydes. The UV-vis spectrum for the benzocarbachlorin showed a split Soret band at 414 and 430 nm, together with a strong chlorin-like absorption at 684 nm. The proton NMR spectrum indicated that the carbachlorin is strongly aromatic and the internal C-H was observed at -4.64 ppm. Addition of TFA afforded a C-protonated dication with a significantly increased diatropic ring current. The proton NMR spectrum, NICS calculations, and AICD plots indicated that the system favors a 22π electron delocalization pathway that runs through the fused benzo unit. Addition of TFA to the benzocarbachlorin aldehydes primarily led to the formation of monocations, and the generation of C-protonated dications was no longer favored due to the presence of electron-withdrawing formyl moieties.

Synthesis and Properties of Carbaporphyrin and Carbachlorin Dimethyl Esters Derived from Cyclopentanedialdehydes

Norbornenes with two ester substituents were prepared by Diels-Alder cycloadditions of cyclopentadiene with dimethyl fumarate and dimethyl 1,1-ethylenedicarboxylate. Oxidation with potassium permanganate gave good yields of related diols that were oxidatively ring-opened to afford cyclopentane dialdehydes. MacDonald-type "3 + 1" condensations with a tripyrrane, followed by oxidation with DDQ in refluxing toluene, gave carbaporphyrin or carbachlorin products in good yields. The macrocyclic products were highly diatropic and produced porphyrin-like UV-vis spectra. The carbaporphyrin was converted into silver(III) and gold(III) organometallic derivatives. Reaction with methyl iodide in the presence of potassium carbonate gave mono- and dialkylation products, and treatment of the former with Ni(OAc)₂ or Pd(OAc)₂ afforded nickel(II) and palladium(II) complexes. The free base carbaporphyrin and carbachlorin, and the nickel and palladium complexes, were characterized by X-ray crystallography. The carbachlorin also reacted with silver(I) acetate to give a silver(III) derivative. Carbaporphyrins and carbachlorins underwent deuterium exchange at the meso-positions with deuteriated TFA, and this observation indicates that protonation is occurring at the bridging carbons. The new route to carbaporphyrins and carbachlorins has enabled detailed studies on the properties of these systems and provides the foundations for future investigations.

β-Fluorinated porpholactones and metal complexes: synthesis, characterization and some spectroscopic studies

We describe the synthesis of β-fluorinated porpholactones by oxidation of the fluorinated CC bond of the pyrrolic subunit in porphyrin using the “RuCl₃ + Oxone®” protocol.

The Breaking and Mending of meso-Tetraarylporphyrins: Transmuting the Pyrrolic Building Blocks

Naturally occurring porphyrins and hydroporphyrins vary with respect to their ring substituents and oxidation states, but their tetrapyrrolic frameworks remain fully preserved across all kingdoms of life; there are no naturally occurring porphyrin-like macrocycles known that contain nonpyrrolic building blocks. However, the study of porphyrin analogues in which one or two pyrroles were replaced with nonpyrrolic building blocks might shed light on the correlation between structural modulation and ground and excited state optical properties of the "pigments of life", unlocking their mechanisms of function. Also, porphyrinoids with strong absorbance and emission spectra in the NIR are sought after in technical (e.g., light-harvesting) and biomedical (e.g., imaging and photochemotherapy) applications. These porphyrin analogues, the so-called pyrrole-modified porphyrins (PMPs), are synthetically accessible using total syntheses. Alternatively-and most handily-they can also be formed by conversion of synthetic porphyrins. Guided by older reports of the fortuitous modifications of porphyrins into PMPs, our research program generalized the so-dubbed "Breaking and Mending of Porphyrins" approach toward PMPs. This method to convert a pyrrole in meso-tetraarylporphyrins to a nonpyrrolic building block with high precision relies on a number of distinct steps. Step 1: The porphyrin is functionalized in a way that activates one or two peripheral double bonds toward breakage; in all cases surveyed here, this step is an osmium tetroxide-mediated dihydroxylation to generate dihydroxychlorin and tetrahydroxybacteriochlorins. Step 2: The activated, dihydroxylated β,β'-bond is "broken". Step 3: The functional groups resulting from the ring-cleavage reactions are utilized in subsequent "mending" steps to form the PMPs, that themselves may be subject to further modifications, Step 4. Thus, PMPs in which a pyrrole was degraded to an imine linkage, contracted to a four-membered ring, or expanded by oxygen, sulfur, carbon, or nitrogen atoms to form six-membered building blocks have become accessible. This approach also allowed the replacement of a single β-carbon atom by a nitrogen or oxygen atom. Depending on the ring size, conformation, conformational flexibility, the oxidation state of the pyrrole replacements, or the presence of substituents that π-extend the chromophores, the PMPs possess porphyrin- or hydroporphyrin-like optical spectra, or they show altogether unique electronic properties. Some PMP classes allow the fine-tuning of their absorption range; others exhibit panchromatic absorption spectra from the UV to the NIR. Several PMPs take up persistent chiral helimeric conformations that could be resolved. This Account summarizes the scopes of the "Breaking and Mending" methodology with a special focus on laying out the structural diversity of PMPs accessible from meso-tetraarylporphyrins and highlighting their optical properties, with the aim of encouraging their further study and application.

Porphyrin Macrocycle Modification: Pyrrole Ring-Contracted or -Expanded Porphyrinoids

In recent years, several synthetic strategies aiming at the peripheral functionalization of porphyrins were developed. Particularly interesting are those involving the modification of β-pyrrolic positions leading to pyrrole-modified porphyrins containing four-, five-, six- or seven-membered heterocycles. Azeteoporphyrins, porpholactones and morpholinoporphyrins are representative examples of such porphyrinoids. These porphyrin derivatives have recently gained an increasing interest due to their potential application in PDT, as multimodal imaging contrast agents, NIR-absorbing dyes, optical sensors for oxygen, cyanide, hypochlorite and pH, and in catalysis.

Fine-Tuning of β-Substitution to Modulate the Lowest Triplet Excited States: A Bioinspired Approach to Design Phosphorescent Metalloporphyrinoids

Learning nature's approach to modulate photophysical properties of NIR porphyrinoids by fine-tuning β-substituents including the number and position, in a manner similar to naturally occurring chlorophylls, has the potential to circumvent the disadvantages of traditional "extended π-conjugation" strategy such as stability, molecular size, solubility, and undesirable π-π stacking. Here we show that such subtle structural changes in Pt(II) or Pd(II) cis/trans-porphodilactones (termed by cis/trans-Pt/Pd) influence photophysical properties of the lowest triplet excited states including phosphorescence, Stokes shifts, and even photosensitization ability in triplet-triplet annihilation reactions with rubrene. Prominently, the overall upconversion capability (η, η = ε·Φ(UC)) of Pd or Pt trans-complex is 10(4) times higher than that of cis-analogue. Nanosecond time-resolved infrared (TR-IR) spectroscopy experiments showed larger frequency shift of ν(C═O) bands (ca. 10 cm(-1)) of cis-complexes than those of trans-complexes in the triplet excited states. These spectral features, combining with TD-DFT calculations, suggest the strong electronic coupling between the lactone moieties and the main porphyrin chromophores and thus the importance of precisely positioning β-substituents by mimicking chlorophylls, as an alternative to "extended π-conjugation", in designing NIR active porphyrinoids.

Effects of substituents on synthetic analogs of chlorophylls. Part 4: How formyl group location dictates the spectral properties of chlorophylls b, d and f

Photosynthetic organisms are adapted to light characteristics in their habitat in part via the spectral characteristics of the associated chlorophyll pigments, which differ in the position of a formyl group around the chlorin macrocycle (chlorophylls b, d, f) or no formyl group (chlorophyll a). To probe the origin of this spectral tuning, the photophysical and electronic structural properties of a new set of synthetic chlorins are reported. The zinc and free base chlorins have a formyl group at either the 2- or 3-position. The four compounds have fluorescence yields in the range 0.19-0.28 and singlet excited-state lifetimes of ca 4 ns for zinc chelates and ca 8 ns for the free base forms. The photophysical properties of the 2- and 3-formyl zinc chlorins are similar to those observed previously for 13-formyl or 3,13-diformyl chlorins, but differ markedly from those for 7-formyl analogs. Molecular-orbital characteristics obtained from density functional theory (DFT) calculations were used as input to spectral simulations employing the four-orbital model. The analysis has uncovered the key changes in electronic structure engendered by the presence/location of a formyl group at various macrocycle positions, which is relevant to understanding the distinct spectral properties of the natural chlorophylls a, b, d and f.

PEGylated meso-arylporpholactone metal complexes as optical cyanide sensors in water

A number of water-soluble metal complexes of PEGylatedmeso-fluorophenylporpholactones display a specific optical response upon addition of cyanide.

MS/MS fragmentation behavior study of meso-phenylporphyrinoids containing nonpyrrolic heterocycles and meso-thienyl-substituted porphyrins

Free base and cobalt(II) complexes of six meso-tetraphenylporphyrinoids containing nonpyrrolic heterocycles and of three meso-thienylporphyrins were investigated using electrospray ionization tandem mass spectrometry (ESI-MS/MS). Their fragmentation was studied in a quadrupole ion trap as a function of the porphyrinoid macrocycle structure and compared with the fragmentation behavior of the benchmark compound meso-tetraphenylporphyrin. In situ oxidation of the neutral cobalt(II) complexes under ESI conditions produced singly charged cobalt(III) porphyrinoid ions; the free bases were ionized by protonation. For the porphyrinoids with an intact porphyrin core, the major fragmentation pathways observed were the losses of the meso-substituent (for meso-phenyl groups) and characteristic fragmentations of one or more meso-substituents (for the meso-thienyl group). Complex fragmentation pathways were observed for porphyrinoids with modifications to the porphyrin core but chemically reasonable structures could be assigned to most fragments, thus delineating general patterns for the behavior of pyrrole-modified porphyrins under CID conditions. ᅟ