Quinoline-Annulated Porphyrins

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.

Direct [4 + 2] Cycloaddition to Isoquinoline-Fused Porphyrins for Near-Infrared Photodynamic Anticancer Agents

The synthesis of efficient porphyrin-based photosensitizers with intense near-infrared (NIR) absorption is in high demand for photodynamic therapy (PDT) but remains a challenging task. Herein we show the construction of a type of isoquinoline-fused porphyrins 3 and 4 with an impressive NIR-absorbing capacity. In light of the extraordinary singlet oxygen generation capabilities of 3 upon NIR irradiation, the representative nanoparticles (3a-NPs) assembled show excellent tumoricidal behavior with good biocompatibility in the phototherapeutic window (650-850 nm).

Quinoline-annulated porphyrin platinum complexes as NIR emitters

The platinum(II) complexes of known quinoline-annulated porphyrins were prepared and spectroscopically characterized. Their optical properties (UV-vis absorption and phosphorescence spectra and phosphorescence lifetimes) were recorded and contrasted against their 2,3-dioxoporphyrin precursor platinum(II) complex. The absorbance and emission spectra (in EtOH glass at 77 K) of the quinoline-annulated porphyrins fall within the NIR optical window of tissue, ranging, depending on the derivative, between [Formula: see text]950 and 1200 nm. The much red-shifted optical spectra, when compared to their non-quinoline-annulated precursors, are attributed to the [Formula: see text]-extension and conformational non-planarity that the annulation causes. The emission yields of the mono-quinoline-annulated derivatives are too low and their lifetimes too short to be practical emitters, but the bis-annulated derivative possesses a practical lifetime and emission yield, suggesting its further exploration, particularly since the methodology toward the solubilization of the quinoline-annulated porphyrins in biological media through derivatization is known.

An unusual [4 + 2] fusion strategy to forge meso-N/O-heteroarene-fused (quinoidal) porphyrins with intense near-infrared Q-bands

Divergent synthesis of meso-N/O-heteroarene-fused (quinoidal) porphyrins was established via rhodium-catalyzed β-C–H activation/annulation of quinoidal porphyrins with alkynes.

Here we present a divergent synthesis of brand-new types of meso-N/O-heteroarene-fused (quinoidal) porphyrins through Rh-catalyzed β-C–H activation/annulation of 5,15-dioxoporphyrins and dioxime derivatives with alkynes, in which the synthetic disconnections are difficult to access through the commonly used intramolecular cyclization strategy. Using the O-methyl oxime as a traceless oxidizing directing group, the meso-N-embedded pyridine-fused anti-quinoidal porphyrin 3 and pyridinium-fused cation 4 are formed with controllable chemoselectivity and complete anti-selectivity. Replacing the exocyclic oxime with a carbonyl group delivers the pyran-fused porphyrin 5, achieving structural conversion from a quinoidal conformation to a stable porphyrin macrocycle. Further oxidation of the expanded dimer 5ea gives the oxonium 6, which exhibits intense near-infrared (NIR) Q-bands up to 1300 nm. Theoretical studies demonstrate that the incorporation of a heteroatom at the meso-position enables more effective π-extension, resulting in a 22π aromatic (vs. 18π aromatic) character of pyran-fused porphyrins (syn/anti-5aa). Compared with the commercially available methylene blue (MB), syn-5al exhibits a better ability (Φ_Δ = 0.61) to sensitize singlet oxygen (¹O₂) when irradiated with a 680 nm laser beam, and has potential as a photodynamic therapy (PDT) photosensitizer in the body's therapeutic window (650–900 nm).

Post-synthetic methods for functionalization of imidazole-fused porphyrins

Several methods for the post-synthetic modification of imidazo[4,5-[Formula: see text]]porphyrins are reported. First, a synthetic approach to the isomeric difunctionalized porphyrins, containing two [Formula: see text]-fused 2-aryl-1[Formula: see text]-imidazole cycles at adjacent or opposite pyrrole rings of the macrocycle is developed. The core chemistry of this synthetic route is the transformation of 2-aryl-1[Formula: see text]-imidazo[4,5-[Formula: see text]]porphyrins into corresponding imidazodioxochlorins followed by Debus–Radziszewski condensation with aromatic aldehyde. Next, 2-(4-bromophenyl)-1[Formula: see text]-imidazo[4,5-[Formula: see text]]-5,10,15,20-tetramesitylporphyrin was transformed into useful carboxy- and phosphonato-substituted precursors for material chemistry according to palladium-catalyzed C–C and C–P bond forming reactions.

Embedding heteroatoms: an effective approach to create porphyrin-based functional materials

Incorporation of planarized heteroatom(s) onto the porphyrin periphery is an effective approach to create porphyrin-based functional materials. In the last three decades, such an "embedding heteroatom" strategy has been actively explored in order to realize attractive electronic, optical, and electrochemical properties. This review aims to cover a variety of synthetic methodologies that have been developed for the construction of heteroatom-embedded porphyrins. Moreover, we also summarize their structure-property relationships as well as possible applications in various research fields including artificial photosynthesis, molecular engineering, organic electronics, and bioimaging.

In vivo photoacoustic tumor tomography using a quinoline-annulated porphyrin as NIR molecular contrast agent

The synthesis and photophysical properties of a tetra-PEG-modified and freely water-soluble quinoline-annulated porphyrin are described. We previously demonstrated the ability of quinoline-annulated porphyrins to act as an in vitro NIR photoacoustic imaging (PAI) contrast agent. The solubility of the quinoline-annulated porphyrin derivative in serum now allowed the assessment of the efficacy of the PEGylated derivative as an in vivo NIR contrast agent for the PAI of an implanted tumor in a mouse model. A multi-fold contrast enhancement when compared to the benchmark dye ICG could be shown, a finding that could be traced to its photophysical properties (short triplet lifetimes, low fluorescence and singlet oxygen sensitization quantum yields). A NIR excitation wavelength of 790 nm could be used, fully taking advantage of the optical window of tissue. Rapid renal clearance of the dye was observed. Its straight-forward synthesis, optical properties with the possibility for further optical fine-tuning, nontoxicity, favorable elimination rates, and contrast enhancement make this a promising PAI contrast agent. The ability to conjugate the PAI chromophore with a fluorescent tag using a facile and general conjugation strategy was also demonstrated.

meso-N-Pyrrole as a Versatile Substituent Influencing the Optical Properties of Porphyrin

A meso-N-pyrrole porphyrin converts into a π-extended porphyrin forming an indolizine-3-one motif. The indolizine-3-one frame opens a lactam subunit preserving a six-membered, heterocyclic structure fused with the main macrocycle. The optical properties of formed derivatives follow the structural modifications giving the absorbance and emission eventually modulated by the NH-centered modifications of the fused unit.

Quinoline-annulated chlorins and chlorin-analogs

The OsO4-mediated dihydroxylation of quinoline-annulated porphyrin generates a quinoline-annulated dihydroxychlorin in a regioselective fashion. Its dihydroxypyrroline moiety, located at the opposite of the annulated pyrrole, is susceptible to the same functional group interconversions we previously demonstrated for non-annulated dihydroxychlorins: oxidations to the corresponding dione, lactone, and dialkoxymorpholine derivatives. The quinoline-annulated chlorin and derivatives are all characterized by absorption spectra that are much broadened and between 130 and 220 nm red-shifted compared to their non-annulated analogs. Absorbance maxima in the NIR up to well above 800 nm were recorded. We attribute the bathochromic shift to their extended [Formula: see text]-systems and inferred non-planarity, highlighting that quinoline-annulation is a particularly effective and simple strategy to red-shift the absorption spectra of chlorins and chlorin analogs.

Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds: Synthetic Routes, Properties, and Applications

Two-dimensionally extended, polycyclic heteroaromatic molecules (heterocyclic nanographenes) are a highly versatile class of organic materials, applicable as functional chromophores and organic semiconductors. In this Review, we discuss the rich chemistry of large heteroaromatics, focusing on their synthesis, electronic properties, and applications in materials science. This Review summarizes the historical development and current state of the art in this rapidly expanding field of research, which has become one of the key exploration areas of modern heterocyclic chemistry.

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.

Mono- and Bisquinoline-Annulated Porphyrins from Porphyrin β,β'-Dione Oximes

An acid-induced reaction of meso-tetraphenyl-2-hydroxyimino-3-oxoporphyrin leads, with concomitant loss of water, to a formal electrophilic aromatic substitution of the ortho-position of the phenyl group adjacent to the oxime, forming a quinoline moiety. Owing in part to the presence of a π-extended chromophore, the resulting meso-triphenylmonoquinoline-annulated porphyrin (λmax = 750 nm) possesses a much altered optical spectrum from that of the starting oxime (λmax = 667 nm). An oxidative DDQ-induced ring-closure process is also possible, generating the corresponding meso-triphenylmonoquinoline-annulated porphyrin quinoline N-oxide, possessing a slightly shifted and sharpened UV-vis spectrum (λmax = 737 nm). The connectivity of the chromophores was conclusively shown by NMR spectroscopy. Both ketone functionalities in meso-tetraphenyl-2,3-dioxoporphyrin can be converted, via the oxime and using the acid- or oxidant-induced reaction pathways, either in one step or in a stepwise fashion, to bisquinoline-annulated porphyrin (λmax = 775 nm) and its N-oxide (λmax = 779 nm), respectively. This process is complementary to a previously established pathway toward bisquinoline-annulated porphyrins. Their zinc(II), nickel(II), and palladium(II) complexes are also described. Several examples of the quinoline-annulated porphyrins were crystallographically characterized, proving their connectivity and showing their conformations that are extremely distorted from planarity. The work presents a full account on the synthesis, structure, and spectroscopic properties of these classes of NIR-absorbing dyes.

The evaluation of NIR-absorbing porphyrin derivatives as contrast agents in photoacoustic imaging

Six free base tetrapyrrolic chromophores, three quinoline-annulated porphyrins and three morpholinobacteriochlorins, that absorb light in the near-IR range and possess, in comparison to regular porphyrins, unusually low fluorescence emission and (1)O2 quantum yields were tested with respect to their efficacy as novel molecular photo-acoustic imaging contrast agents in a tissue phantom, providing an up to ∼2.5-fold contrast enhancement over that of the benchmark contrast agent ICG. The testing protocol compares the photoacoustic signal output strength upon absorption of approximately the same light energy. Some relationships between photophysical parameters of the dyes and the resulting photoacoustic signal strength could be derived.

Synthesis of extended porphyrins by connection of meso-aryl groups with β-pyrrolic positions

This review focuses on synthetic strategies leading to extended porphyrins, but is limited to examples in which at least one β-pyrrolic carbon is linked to a meso-aryl substituent. Friedel–Crafts reactions are covered first because historically they were the first reactions employed to link β-pyrrolic carbons with meso-aryl groups, thus creating new six-membered fused rings. Other reactions inserting heteroatoms follow. Direct cyclizations between an ortho-carbon and a β-pyrrolic position are presented next. Finally, new highly extended chromophores are described. These compounds are generally obtained in two consecutive reactions. First a Suzuki coupling is used to introduce an aryl group in one or two meso positions. Then the new fused rings are generated by oxidative cyclizations.

Diazo compounds in the functionalization of porphyrin macrocycles

This highlight presents an overview on the functionalization of porphyrin macrocycles using diazo compounds.

Triphenylene-fused porphyrins

Triphenylene has been successfully fused to the porphyrin periphery through a convenient oxidative ring-closure reaction. Bistriphenylene-fused porphyrins and a dibenzo[fg,op]tetracene-fused porphyrin have also been obtained using a similar approach. These π-extended porphyrins exhibited broadened and bathochromic shifted UV-vis absorptions.