Synthetic Chlorins, Possible Surrogates for Chlorophylls, Prepared by Derivatization of Porphyrins

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.

Porphyrins as Colorimetric and Photometric Biosensors in Modern Bioanalytical Systems

Advances in porphyrin chemistry have provided novel materials and exciting technologies for bioanalysis such as colorimetric sensor array (CSA), photo-electrochemical (PEC) biosensing, and nanocomposites as peroxidase mimetics for glucose detection. This review highlights selected recent advances in the construction of supramolecular assemblies based on the porphyrin macrocycle that provide recognition of various biologically important entities through the unique porphyrin properties associated with colorimetry, spectrophotometry, and photo-electrochemistry.

Site-Selective Modification of a Porpholactone-Selective Synthesis of 12,13- and 17,18-Dihydroporpholactones

The reaction of meso-tetrakis(pentafluorophenyl)porpholactone with azomethine ylides and nitrones affords pyrrolidine-fused and isoxazolidine-fused dihydroporpholactones that display, respectively, isobacteriochlorin- and chlorin-type UV–Vis spectra. These reactions are site-selective, yielding, respectively, 17,18- or 12,13-dihydroporpholactones. The crystal and molecular features of pyrrolidine-fused and isoxazolidine-fused dihydroporpholactones were unveiled from single-crystal X-ray diffraction studies.

Porphyrin Derivative Nanoformulations for Therapy and Antiparasitic Agents

Porphyrins and analogous macrocycles exhibit interesting photochemical, catalytic, and luminescence properties demonstrating high potential in the treatment of several diseases. Among them can be highlighted the possibility of application in photodynamic therapy and antimicrobial/antiparasitic PDT, for example, of malaria parasite. However, the low efficiency generally associated with their low solubility in water and bioavailability have precluded biomedical applications. Nanotechnology can provide efficient strategies to enhance bioavailability and incorporate targeted delivery properties to conventional pharmaceuticals, enhancing the effectiveness and reducing the toxicity, thus improving the adhesion to the treatment. In this way, those limitations can be overcome by using two main strategies: (1) Incorporation of hydrophilic substituents into the macrocycle ring while controlling the interaction with biological systems and (2) by including them in nanocarriers and delivery nanosystems. This review will focus on antiparasitic drugs based on porphyrin derivatives developed according to these two strategies, considering their vast and increasing applications befitting the multiple roles of these compounds in nature.

Cyclophane-Type Chlorin Dimers from Dynamic Covalent Chemistry of 2,18-Porphyrinyl Dicyanomethyl Diradicals

2,18-Bis(dicyanomethyl)-substituted Ni^II porphyrin 8 and Zn^II porphyrin 11 were prepared and subjected to oxidation with PbO₂ in CH₂ Cl₂ at 298 K to give cyclophane-type chlorin dimers (9)₂ and (12)₂ as a consequence of double recombination of biradicals 9 and 12, respectively. Dimer (9)₂ takes a syn-conformation of two distorted Ni^II chlorins but (12)₂ takes an anti-conformation of relatively planar Zn^II chlorins. At 298 K, dimer (9)₂ is stable and its ¹ H NMR spectrum is sharp but becomes broad at high temperature, while the ¹ H NMR spectrum of (12)₂ is considerably broad even at 298 K but becomes sharper at low temperature. These results indicate that the chlorin dimers dissociate to radical species, but the activation barrier of the dissociation of (12)₂ is much less than that of (9)₂ . The involvement of diradicals in dynamic covalent chemistry has been suggested by thermal scrambling of hetero dimer (16)₂ to give homo dimers (9)₂ and (15)₂ .

Bioconjugatable synthetic chlorins rendered water-soluble with three PEG-12 groups via click chemistry

Chlorins provide many ideal features for use as red-region fluorophores but require molecular tailoring for solubilization in aqueous solution. A chlorin building-block bearing 18,18-dimethyl, 15-bromo and 10-[2,4,6-tris(propargyloxy)phenyl] substituents has been transformed via click chemistry with CH3(OCH2CH[Formula: see text]-N3 followed by Suzuki coupling with 3-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)propanoic acid, thereby installing a water-solubilization motif and a bioconjugatable handle, respectively. In toluene, [Formula: see text]-dimethylformamide (DMF) or water, the resulting facially encumbered free base chlorin exhibits characteristic chlorin absorption ([Formula: see text] [Formula: see text]412, 643 nm) and fluorescence ([Formula: see text] [Formula: see text]645 nm) spectra with only modest variation in fluorescence quantum yield ([Formula: see text] values (0.24, 0.25 and 0.19, respectively). The zinc chlorin derived therefrom exhibits similar spectral constancy ([Formula: see text] [Formula: see text]414 and 613 nm, [Formula: see text] [Formula: see text]616 nm) and [Formula: see text] 0.094, 0.10 and 0.086 in the three solvents. The results together indicate the viability of the molecular design and synthetic methodology to create red-region fluorophores for use in diverse applications.

Ligands with Two Monoanionic N,N-Binding Sites: Synthesis and Coordination Chemistry

Polytopic ligands have become ubiquitous in coordination chemistry because they grant access to a variety of mono- and polynuclear complexes of transition metals as well as rare-earth and main-group elements. Nitrogen-based ditopic ligands, in which two monoanionic N,N-binding sites are framed within one molecule, are of particular importance and are therefore the primary focus of this review. In detail, bis(amidine)s, bis(guanidine)s, bis(β-diimine)s, bis(aminotroponimine)s, bis(pyrrolimine)s, and miscellaneous bis(N,N-chelating) ligands are reviewed. In addition to the general synthetic protocols, the application of these ligands is discussed along with their coordination chemistry, the multifarious binding modes, and the ability of these ligands to bridge two (or more) metal(loids).

Investigation of the demetallation of 10-aryl substituted synthetic chlorins under acidic conditions

The acidic demetallation of a series of sparsely substituted Zn(II) chlorins is reported. The chlorins were functionalized in the 10-position with substituents ranging from strongly electron donating mesityl and p-methoxyphenyl to electron-withdrawing p-nitrophenyl and pentafluorophenyl groups. The demetallation kinetics were investigated using UV-Visible absorption spectroscopy. Demetallation was carried out by exposing the metallochlorins dissolved in CH₂Cl₂ to an excess of trifluoroacetic acid. Reasonable correlation was found between the Hammett constant of the 10-substituent and the rate constant of the loss of the metal ion. The largest differences were observed between the p-methoxyphenyl and p-nitrophenyl-substituted Zn(II) chlorins, undergoing loss of Zn(II) with pseudo first order rate constants of 0.0789 × 10^-3 and 3.70 × 10^-3 min^-1, respectively. Taken together, these data establish the dramatic influence even subtle changes can have in altering the electronic properties of chlorins, which in turn impacts metallochlorin function.

Regioselective reduction of 5-aryl-10,15,20-tris(pyridyl) porphyrin to 5-aryl-10,15,20-tris(pyridyl)dihydroporphyrin (chlorin)

Dihydroporphyrins or chlorins differ from porphyrins only by saturation of a peripheral double bond of the macrocycle. However, this small structural difference leads to a significant increase of the absorption band at approximately 650 nm, which makes them very interesting candidates for photodynamic therapy applications. The reduction of porphyrins bearing two, three or four pyridyl substituents with tin(II) chloride has been developed for the synthesis of dihydroporphyrins in yields of 15–73%. The reduction of 5-(aryl)-10,15,20-tris(2 or 4-pyridyl)porphyrin with tin(II) chloride dihydrate demonstrated good regioselectivity. Porphyrins with one meso-aryl bearing one electron-donating group (EDG) gave 5-aryl-10,15,20-tris(2- or 4-pyridyl)-17,18-dihydroporphyrins in 17–72% yield. Porphyrins with one meso-aryl bearing one or more electron-withdrawing groups (EWG) gave 5-aryl-10,15,20-tris(4-pyridyl)-17,18-dihydroporphyrins or 5-aryl-10,15,20-tris(4-pyridyl)-7,8-dihydroporphyrins in 15–21% yield and isobacteriochlorin. We have also proven the possibility of functionalizing these compounds to design new regioisomerically pure photosensitizers.

2,3-Dicyano-5,6-dichlorobenzoquinone-Mediated and Selective C-O and C-C Cross-Couplings of Phenols and Porphyrins

A selective C-O cross-coupling reaction between porphyrins and phenols has been developed through 2,3-dicyano-5,6-dichlorobenzoquinone (DDQ)/Sc(OTf)₃ oxidation, efficiently delivering meso-etherified porphyrins in good yields (≤93%). The radical complex process was proposed and calculated as the rationalized mechanism to block the homocoupling process. In addition, the switchable selective C-C cross-coupling reaction was achieved by using bulky electron-rich phenols and naphthols under DDQ oxidation conditions.

Use of the Nascent Isocyclic Ring to Anchor Assembly of the Full Skeleton of Model Chlorophylls

The chlorophyll skeleton contains a chlorin macrocycle and an annulated fifth (or isocyclic) ring bearing 13¹-oxo and 13²-carbomethoxy substituents. The isocyclic ring has traditionally been constructed by annulation of an intact tetrapyrrole macrocycle. Here, a complementary route employs reaction of a gem-dimethyl-substituted dihydrodipyrrin-carboxaldehyde (AD half) and a dipyrromethane bearing a 3-methoxy-1,3-dioxopropyl group (BC half). A McMurry-like reaction of a 2-(2-nitro-5-oxohexyl)pyrrole was employed to construct the second pyrrole ring in one of three BC halves, whereas the other two were prepared by known routes. An AD half and a BC half were joined by Knoevenagel condensation at room temperature, affording the AD,BC-substituted 2-methoxycarbonyl-2-propenone. The subsequent reaction of three AD, BC-propenones (mixture of Z,E-isomers) in CH₃CN containing InCl₃ and In(OTf)₃ at 80 °C afforded the chlorophyll skeleton as the chloroindium(III) chelate; the reaction proceeds via Nazarov cyclization (to form the isocyclic ring), S_EAr (to construct the macrocycle), and 2e^-,2H⁺ oxidation (to give the aromatic chromophore). The absorption spectra of the complexes closely resemble that of chlorophyll a. The present work exploits the nascent isocyclic ring as an anchor for directed assembly of the AD and BC halves, forming both the chlorin macrocycle and the isocyclic ring in a single-flask transformation.

Current Advances in the Synthesis of Valuable Dipyrromethane Scaffolds: Classic and New Methods

This review presents the most recent developments on the synthesis of dipyrromethanes, covering classical synthetic strategies, using acid catalyzed condensation of pyrroles and aldehydes or ketones, and recent breakthroughs which allow the synthesis of these type of heterocycles with new substitution patterns.

Ring-Fused Diphenylchlorins as Potent Photosensitizers for Photodynamic Therapy Applications: In Vitro Tumor Cell Biology and in Vivo Chick Embryo Chorioallantoic Membrane Studies

Ring-fused diphenylchlorins as potent low-dose photosensitizers for photodynamic therapy of bladder carcinoma and esophageal adenocarcinoma are described. All studied molecules were very active against HT1376 urinary bladder carcinoma and OE19 esophageal adenocarcinoma cell lines, showing IC50 values below 50 nM. The in vivo evaluation of the more promising photosensitizer, using an OE19 tumor/chick embryo chorioallantoic membrane model, showed a tumor weight regression of 33% with a single photodynamic therapy treatment with the photosensitizer dose as low as 37 ng/embryo.

Size-Tunable Targeting-Triggered Nanophotosensitizers Based on Self-Assembly of a Phthalocyanine-Biotin Conjugate for Photodynamic Therapy

Self-assembled phototheranostic nanomaterials used for photodynamic therapy (PDT) have attracted increasing attention owing to their several advantages. Herein, we developed a novel strategy for size-tunable self-assembled nanophotosensitizers for PDT through a simple method. A series of switchable self-assembled nanophotosensitizers (NanoPc90, NanoPc40, NanoPc20, and NanoPc10) of different particle sizes were readily prepared based on an amphiphilic silicon(IV) phthalocyanine (SiPc)-biotin conjugate by regulating the amount of the Cremophor EL surfactant used. The photoactivities, including fluorescence and reactive oxygen species (ROS), of the self-assemblies could be regulated by the particle size. The self-assemblies could be specifically disassembled by tumor-overexpressing biotin receptors, leading to the recovery of quenched photoactivities. Demonstrated by the competitive assay, the self-assemblies were able to enter HepG2 cells through a biotin-receptor-mediated pathway, followed by biotin-receptor-triggered fluorescence recovery at the cellular level. Moreover, the particle size could also affect the in vitro and in vivo PDT effects and tumor targeting. The photocytotoxicity of NanoPc20 against HepG2 cells was more potent compared to that of NanoPc90 because of its strong intracellular fluorescence, higher intracellular ROS generation, and different subcellular localization. In addition, NanoPc20 showed higher in vivo tumor targeting and photodynamic therapeutic efficacy than NanoPc90. This work would provide a valuable reference for the development of self-assembled nanophotosensitizers for cancer diagnosis and therapy.

Porpholactone Chemistry: An Emerging Approach to Bioinspired Photosensitizers with Tunable Near-Infrared Photophysical Properties

Chlorophylls, known as the key building blocks of natural light-harvesting antennae, are essential to utilize solar energy from visible to near-infrared (NIR) region during the photosynthesis process. The fundamental studies for the relationship between structure and photophysical properties of chlorophylls disclosed the importance of β-peripheral modification and thus boosted the fast growth of NIR absorbing/emissive porphyrinoids via altering the extent of π-conjugation and the degree of distortion from the planarity of macrocycle. Despite the tremendous progress made in various porphyrin-based synthetic models, it still remains a challenge to precisely modulate photophysical properties through fine-tuning of β-peripheral structures in the way natural chlorophylls do. With this in mind, we initiated a program and focused on meso-C₆F₅-substituted porpholactone (F₂₀TPPL), in which one β-pyrrolic double bond was replaced by a lactone moiety, as an attractive platform to construct the bioinspired library of NIR porphyrinoids. In this Account, we summarize our recent contributions to the bioinspired design, synthesis, photophysical characterization, and applications of porpholactones and their derivatives. We have developed a general, convenient method to directly prepare porpholactones in large scale up to gram, which forms the chemical basis of porpholactone chemistry. By modulation of the saturation level and in particular regioisomerization of β-dilactone moieties, a synthetic library constituted by a series of porpholactones and their derivatives has been established. Thanks to the electron-withdrawing nature of lactone moiety, derivation of the saturation levels gives help to build stable models for chlorin, bacteriochlorin, and tunichlorin. It is worth noting that regioisomerization of dilactone moieties mimics the relative orientation of β-substituents in natural chlorophylls and hemes, which was considered as the key factor to tune NIR absorption and reactivity. Porpholactones can illustrate the capability of fine-tuning photophysical properties including the excited triplet states by subtle alteration of β-peripheral structures in the presence of transition metals and lanthanides (Ln). Furthermore, they can serve as efficient photosensitizers for singlet oxygen and NIR Ln, showing potential applications in cell imaging and photocytotoxicity studies. The high luminescence, tunable structures, high cellular uptake, and intense NIR absorption render them as promising and competitive candidates for theranostics in vitro and in vivo. Therefore, extending the studies of "porpholactone chemistry" not only tests the fundamental understanding of the structure-function relationship that governs NIR photophysical properties of natural tetrapyrrole cofactors such as chlorophylls but also provides the guiding principles for the bioinspired design of NIR luminescent molecular probes with various applications. Taken together, as a new synthetic porphyrin derivative, porpholactone chemistry shines light on synthetic porphyrin, bioinorganic, and lanthanide chemistry.

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.

Control of Aromaticity and cis-/trans-Isomeric Structure of Non-Planar Hexaphyrin(2.1.2.1.2.1) and Metal Complexes

Vinylene-bridged hexaphyrin(2.1.2.1.2.1) was synthesized from dipyrrolyl diphenylethenes by acid-catalyzed condensation reactions. Freebase hexaphyrin(2.1.2.1.2.1) forms a distorted structure with non-aromatic characteristics. The aromaticity and molecular configuration of non-planar hexaphyrin(2.1.2.1.2.1) can be controlled by insertion of metal ions. Freebase and zinc complexes show a distorted structure without macrocyclic aromaticity, whereas copper complexes show a figure-of-eight structure with macrocyclic aromaticity. It is the first example of aromaticity conversion of a distorted expanded porphyrin involving vinylene bridges.

Origins of the Electronic Modulations of Bacterio- and Isobacteriodilactone Regioisomers

Advances in the utilization of porphyrinoids for photomedicine, catalysis, and artificial photosynthesis require a fundamental understanding of the relationships between their molecular connectivity and resulting electronic structures. Herein, we analyze how the replacement of two pyrrolic C_β═C_β bonds of a porphyrin by two lactone (O═C-O) moieties modulates the ground-state thermodynamic stability and electronic structure of the resulting five possible pyrrole-modified porphyrin isomers. We made these determinations based on density functional theory (DFT) and time-dependent DFT computations of the optical spectra of all regioisomers. We also analyzed the computed magnetically induced currents of their aromatic π-systems. All regioisomers adopt the tautomeric state that maximizes aromaticity, whether or not transannular steric strains are incurred. In all isomers, the O═C_β-O_β bonds were found to support a macrocycle diatropic ring current. We attributed this to the delocalization of nonbonding electrons from the ring oxa- and oxo-atoms into the macrocycle. As a consequence of this delocalization, the dilactone regioisomers are as-or even more-aromatic than their hydroporphyrin congeners. The electronic structures follow different trends for the bacteriochlorin- and isobacteriochlorin-type isomers. The presence of either oxo- or oxa-oxygens conjugated with the macrocyclic π-system was found to be the minimal structural requirement for the regioisomers to exhibit distinct electronic properties. Our computational methods and mechanistic insights provide a basis for the systematic exploration of the physicochemical properties of porphyrinoids as a function of the number, relative orientation, and degree of macrocycle-π-conjugation of β-substituents, in general, and for dilactone-based porphyrinic chromophores, in particular.

Total synthesis campaigns toward chlorophylls and related natural hydroporphyrins - diverse macrocycles, unrealized opportunities

Covering: up to 2018 Chlorophylls, bacteriochlorophylls and related hydroporphyrins constitute invaluable natural products but have largely remained outside the scope of viable syntheses. The campaign toward chlorophyll a by Woodward and coworkers is a deservedly celebrated landmark in organic synthesis yet the route entailed 49 steps, relied on semisynthetic replenishment of advanced intermediates, and then pointed to (but did not implement) uncertain literature procedures for the final transformations. Indeed, the full synthesis at any scale of any (bacterio)chlorophylls - conversion of small-molecule starting materials to the product - has never been accomplished. Herein, the reported syntheses of (±)-bonellin dimethyl ester (0.93 mg) and tolyporphin A O,O-diacetate (0.38 mg), as well as the never-fully traversed route to chlorophyll a, have been evaluated in a quantitative manner. Bonellin and tolyporphin A are naturally occurring chlorin and bacteriochlorin macrocycles, respectively, that lack the characteristic fifth ring of (bacterio)chlorophylls. A practical assessment is provided by the cumulative reaction mass efficiency (cRME) of the entire synthetic process. The cRME for the route to chlorophyll a would be 4.3 × 10-9 (230 kg of all reactants and reagents in total would yield 1.0 mg of chlorophyll a), whereas that for (±)-bonellin dimethyl ester or tolyporphin A O,O-diacetate is approximately 6.4 × 10-4 or 3.6 × 10-5, respectively. Comparison of the three syntheses reveals insights for designing hydroporphyrin syntheses. Development of syntheses with cRME > 10-5 (if not 10-4), as required to obtain 10 mg quantities of hydroporphyrin for diverse physicochemical, biochemical and medicinal chemistry studies, necessitates significant further advances in tetrapyrrole chemistry.

Covalently Linked Bis(Amido-Corroles): Inter- and Intramolecular Hydrogen-Bond-Driven Supramolecular Assembly

Four bis-corroles linked by diamide bridges were synthesized through peptide-type coupling of a trans-A₂ B-corrole acid with aliphatic and aromatic diamines. In the solid state, the hydrogen-bond pattern in these bis-corroles is strongly affected by the type of solvent used in the crystallization process. Although intramolecular hydrogen bonds play a decisive role, they are supported by intermolecular hydrogen bonds and weak N-H⋅⋅⋅π interactions between molecules of toluene and the corrole cores. In an analogy to mono(amido-corroles), both in crystalline state and in solutions, the aliphatic or aromatic bridge is located directly above the corrole ring. When either ethylenediamine or 2,3-diaminonaphthalene are used as linkers, incorporation of polar solvents into the crystalline lattice causes a roughly parallel orientation of the corrole rings. At the same time, both NHCO⋅⋅⋅NH corrole hydrogen bonds are intramolecular. In contrast, solvation in toluene causes a distortion with one of the hydrogen bonds being intermolecular. Interestingly, intramolecular hydrogen bonds are always formed between the -NHCO- functionality located further from the benzene ring present at the position 10-meso. In solution, the hydrogen-bonds pattern of the bis(amido-corroles) is strongly affected by the type of the solvent. Compared with toluene (strongly high-field shifted signals), DMSO and pyridine disrupt self-assembly, whereas hexafluoroisopropanol strengthens intramolecular hydrogen bonds.

Introduction of an isoxazoline unit to the β-position of porphyrin via regioselective 1,3-dipolar cycloaddition reaction

Isoxazoline-linked porphyrins have been synthesized by a regioselective 1,3-dipolar cycloaddition reaction between vinylporphyrin 2 and nitrile oxides. The steric interaction directed the reaction trajectory, in which only the product with a link between the 5-position of the isoxazoline and the β-position of porphyrin was observed. The isoxazoline-porphyrins 3a,b have been characterized by absorption, emission, 1H NMR and mass spectra. Later, the crystal structure of 3a was obtained and confirmed the basic features of the NMR-derived structure. Furthermore, a pair of enantiomers of 3a presented in the crystal, which formed a dimeric complex through intermolecular coordination between the Zn2+ center and the carbonyl group of the second molecule.

Photoisomerization of Enediynyl Linker Leads to Slipped Cofacial Hydroporphyrin Dyads with Strong Through-Bond and Through-Space Electronic Interactions

Photoisomerization of 3,4-di(methoxycarbonyl)-enediyne linker in hydroporphyrin (chlorin or bacteriochlorin) dyads leads to thermally stable cis isomers, where macrocycles adopt a slipped cofacial mutual geometry with an edge-to-edge distance of ∼3.6 Å (determined by density functional theory (DFT) calculations). Absorption spectra exhibit a significant splitting of the long-wavelength Q_y band, which indicates a strong electronic coupling with a strength of V = ∼477 cm^-1 that increases to 725 cm^-1 upon metalation of hydroporphyrins. Each dyad features a broad, structureless emission band, with large Stokes shift, which is indicative of excimer formation. DFT calculations for dyads show both strong through-bond electronic coupling and through-space electronic interactions, due to the overlap of π-orbitals. Overall, geometry, electronic structure, strength of electronic interactions, and optical properties of reported dyads closely resemble those observed for photosynthetic special pairs. Dyads reported here represent a novel type of photoactive arrays with various modes of electronic interactions between chromophores. Combining through-bond and through-space coupling appears to be a viable strategy to engineer novel optical and photochemical properties in organic conjugated materials.

Attempted syntheses of N-confused hydroporphyrins through modified Lindsey routes

Unlike N-confused porphyrins which are well-known and extensively studied tetrapyrroles, N-confused hydroporphyrins are almost unknown, largely because so far they have resisted attempts at rational synthesis. Here, we report our efforts towards the total synthesis of N-confused hydroporphyrins. We have prepared N-confused building blocks analogous to the non-N-confused substrates in the Lindsey synthesis of sparsely substituted chlorins. We have systematically flipped the A, B and C pyrrole rings in the dipyrrolic precursors of the target N-confused macrocycles, preparing in total an N-confused “Western half” (tetrahydrodipyrrin) and two N-confused “Eastern halves” (brominated formyldipyrromethanes). These were subjected to a range of cyclization conditions. While we successfully isolated and identified three macrocyclic products, none of these proved to be the desired N-confused hydroporphyrin.

Isomers of β,β-Dinitro-5,10,15,20-tetraphenylporphy-rin Derivatives: Valuable Starting Materials for Further Transformations

The synthesis, chromatographic isolation, and structure elucidation of β,β-substituted isomers of dinitro-5,10,15,20-tetraphenylporphyrin complexes are described. meso-Tetraphenyl-porphyrin chelates (Cu^II, Ni^II, Co^II) upon reaction wit e.g., itric acid (yellow HNO₃, d = 1.52, diluted to 25⁻50%) in CHCl₃ formed a mixture of nitro-derivatives with combined yields of ca 80%. This nitration (under optimized conditions: 25⁻30% HNO₃, 30⁻40 min, r.t.) can be carried out selectively to give mainly β,β-dinitro-compounds in yields of up to 73%. From the above mixtures of five possible regioisomers that can be formed, usually two or three of them were isolated, for which the structures were assigned on the basis of ¹H NMR spectra including COSY and NOESY measure-ments. These types of products are attractive starting materials for synthesis of potential anticancer PDT agents with unique structures, being practically unavailable by any other alternative method.

Helix-Sense-Selective Polymerization of Phenylacetylenes Having a Porphyrin and a Zinc-Porphyrin Group: One-Handed Helical Arrangement of Porphyrin Pendants

Newly synthesized two kinds of achiral phenylacetylenes having a free-base- or a zinc-porphyrin (1 and Zn1, respectively) were polymerized by using a chiral rhodium catalyst system, Rh⁺(nbd)[(η⁶-C₆H₅)B⁻(C₆H₅)₃] catalyst and (R)-(+)- or (S)-(⁻)-1-phenylethylamine ((R)- or (S)-PEA, respectively) cocatalyst. Poly(1) and poly(Zn1) in THF showed a Cotton signal at the absorption region of the porphyrin and the main chain in the circular dichroism (CD) spectra. This result suggests that poly(1) and poly(Zn1) exist in a conformation with an excess of one-handed helix sense and the porphyrin moiety arranged in chiral helical fashion. The one-handed helical structure of poly(1) could be sustained in a mixture of THF/HMPA (10/2, v/v) due to stabilizing by stacking effect of porphyrin moieties along the main chain. This is the first example about helix-sense-selective polymerization by using Rh⁺(nbd)[(η⁶-C₆H₅)B⁻(C₆H₅)₃] catalyst. Additionally, poly(Zn1) showed about 10 times larger CD intensity in comparison with poly(1). This result suggests the regularity of arrangement of the porphyrin in poly(Zn1) is higher compared with poly(1). Spatial arrangement of porphyrins was achieved by utilizing a one-handed helical poly(phenylacetylenes) as a template.

Aromaticity versus regioisomeric effect of β-substituents in porphyrinoids

Maximizing the regioisomeric effect of β-substituents on photophysical properties of porphyrinoids through disruption of TT-conjugation and reducing the aromaticity.

Annulated bacteriochlorins for near-infrared photophysical studies

Bacteriochlorins with phenaleno or benzo annulation absorb at 913 or 1033 nm and exhibit excited-state lifetimes of 150 or 7 ps, suggesting applications in photoacoustic imaging.

Structure and conformation of photosynthetic pigments and related compounds. 15. Conformational analysis of chlorophyll derivatives – implications for hydroporphyrinsin vivo

Controlling the function of chlorophylls depends in part on their 3D conformation. The NSD program presents a powerful tool to identify the distortion modes in phytochlorins.

Diazachlorin and diazabacteriochlorin for one- and two-photon photodynamic therapy

Diazachlorin and diazabacteriochlorin have been prepared through reduction of diazaporphyrin and their in vitro and in vivo activity in photodynamic therapy has been investigated.

Syntheses of Chalcone-Type Chlorophyll Derivatives Possessing a Bacteriochlorin, Chlorin or Porphyrin π-System and Their Optical Properties

C3-(Trans-2-arylethenyl)carbonylated chlorophyll derivatives possessing a bacteriochlorin or chlorin π-system were synthesized by cross-aldol (Claisen-Schmidt) condensation of methyl pyrobacteriopheophorbide-a or 3-acetyl-3-devinyl-pyropheophorbide-a bearing the C3-acetyl group with p-(un)substituted benzaldehydes under basic conditions. The corresponding porphyrin-type chlorophyll derivatives were prepared by the oxidation (17,18-didehydrogenation) of the chlorin-type. Their Qy absorption and fluorescence emission maxima in dichloromethane correlated well with Hammett substituent constants of the p-substituents. Several electron-withdrawing p-substituents suppressed the emission due to photoinduced electron transfer quenching in a molecule. The substitution sensitivities for their maxima and fluorescence quantum yields decreased in the order of bacteriochlorin-, chlorin- and porphyrin-type derivatives.

Photosensitizers with Aggregation-Induced Emission: Materials and Biomedical Applications

Photodynamic therapy is arising as a noninvasive treatment modality for cancer and other diseases. One of the key factors to determine the therapeutic function is the efficiency of photosensitizers (PSs). Opposed to traditional PSs, which show quenched fluorescence and reduced singlet oxygen production in the aggregate state, PSs with aggregation-induced emission (AIE) exhibit enhanced fluorescence and strong photosensitization ability in nanoparticles. Here, the design principles of AIE PSs and their biomedical applications are discussed in detail, starting with a summary of traditional PSs, followed by a comparison between traditional and AIE PSs to highlight the various design strategies and unique features of the latter. Subsequently, the applications of AIE PSs in photodynamic cancer cell ablation, bacteria killing, and image-guided therapy are discussed using charged AIE PSs, AIE PS molecular probes, and AIE PS nanoparticles as examples. These studies have demonstrated the great potential of AIE PSs as effective theranostic agents to treat tumor or bacterial infection. This review hopefully will spur more research interest in AIE PSs for future translational research.

Chemotaxis-Instructed Intracellular Staphylococcus aureus Infection Detection by a Targeting and Self-Assembly Signal-Enhanced Photoacoustic Probe

Intracellular invasion and the survival of Staphylococcus aureus in phagocytic cells has been regarded as one of the mechanisms that leads to the treatment failure of S. aureus infection and potential antibiotic resistance. The detection of infected phagocytic cells plays an important role in guiding antibiotic treatment and in reducing drug resistance. The development of a sensitive and specific imaging probe to visualize the intracellular bacteria is quite challenging. In this work, we report a photoacoustic agent (MPC) that is able to detect intracellular S. aureus infection through a dynamic process, including (i) active targeting and internalization into macrophage cells, (ii) specific molecular tailoring by caspase-1 in infected macrophage cells, and (iii) enhancement of the photoacoustic (PA) signal owing to molecular self-assembly. The PA signal per area of the "stimuli-induced assembly" agent (MPC) increases more than 2-fold over that of the active targeting control agent (MP_SC). Finally, based on this approach, the average PA signal in the infected site is enhanced by approximately 2.6-fold over that of the control site. We envision that this PA contrast agent may provide a new approach for the selective and sensitive diagnosis of an intracellular bacterial infection.