Corrole NH tautomers: spectral features and individual protonation

Recent advances in the functionalization of formyl and acroleyl appended corroles

The field of corrole systems, particularly those with functional groups at their peripheral positions, has experienced a surge of interest in recent years, driven by their exceptional optical and electronic properties, which hold significant promise for a range of applications. This timely review article mainly focuses on synthetic strategies of reaction of meso-triarylcorroles having formyl or acroleyl groups at peripheral positions, with specific emphasis on the influence of core-metal insertion, the quantity of reagent and meso-substituents. Corroles bearing formyl and acroleyl substituents have been exploited as synthons for preparing novel compounds with a magnificent bouquet of characteristics. Furthermore, the reactivity of these corroles derivatives with active methylene compounds and substituted pyrroles is highlighted. The detailed exploration of these functionalizations is helping to advance new developments in the field. Additionally, the review addresses the potential applications of corroles in chemosensing, catalysis, photovoltaics, and nonlinear optics. It also examines the systematic advancements in the optical properties of corroles, providing a thorough overview of their photophysical and redox characteristics. This will help researchers working in this area and promote exceptional future investigations.

Hydrazone-Functionalized trans-A2B-Corroles: Effective Synergy in Photodynamic Therapy of Lung Cancer

A synthetic route to trans-A2B-corroles combining the macrocyclic core with a hydrazone moiety, based on the reactivity of azoalkenes toward dipyrromethanes, has been established with the aim of developing a new class of photosensitizers for photodynamic therapy of lung cancer. The study of the photophysical properties of the novel macrocycles allowed the identification of photosensitizers with absorption within the phototherapeutic window and high singlet oxygen quantum yield. Relevant structure-photodynamic activity correlations were established by studying the new corroles-based photodynamic therapy (PDT) in human lung cancer cell lines (A549 and H1299). The methyl-hydrazone corroles were more active than phenyl-hydrazone corroles, with the N-Boc and N-Ts groups being key structural features to ensure high activity. The lead photosensitizers, with IC50 values below 100 nM and no cytotoxicity per se, were significantly more active than 5,10,15-triphenylcorrole, showing that the presence of the hydrazone functional group has a strong influence on PDT activity.

The Excited-State N-H Tautomerization Rate in Free-Base Corroles

Corrole is a tetrapyrrolic dye with a structure that resembles porphyrin, apart from a single missing carbon. The absence of this carbon results in the re-arrangement of the double bonds within the macrocycle, and the presence of three pyrrolic protons in the central cavity in its free-base form. These protons lead to the existence of two distinct tautomeric structures that exist in a dynamic equilibrium. Although the ground-state energies of the tautomers are similar, the excited states show a significant difference in energy which unbalances the equilibrium between the tautomers and results in rapid excited-state tautomerization, favouring one tautomeric species over the other. Although the excited-state tautomerization process has been known for a long time, very few studies have been performed on it, leaving many key aspects of the process poorly understood. Herein we show how ultrafast photoluminescence can be used to experimentally determine the rates of excited-state tautomerization and activation energies of three free-base corrole derivatives thus allowing us to completely describe the excited-state dynamics of the unusual excited state of free-base corrole and opening the door to the development of new materials that can exploit its unique characteristics.

Free-Base Corrole Anion

Free-base corroles have long been known to be acidic, readily undergoing deprotonation by mild bases and in polar solvents. The conjugate base, however, has not been structurally characterized until now. Presented here is a first crystal structure of a free-base corrole anion, derived from tris(p-cyanophenyl)corrole, as the tetrabuylammonium salt. The low-temperature (100 K) structure reveals localized hydrogens on a pair of opposite pyrrole nitrogens. DFT calculations identify such a structure as the global minimum but also point to two cis tautomers only 4-7 kcal/mol above the ground state. In terms of free energy, however, the cis tautomers are above or essentially flush with the trans-to-cis barrier so the cis tautomers are unlikely to exist or be observed as true intermediates. Thus, the hydrogen bond within each dipyrrin unit on either side of the molecular pseudo-C2 axis through C10 (i.e., between pyrrole rings A and B or between C and D) qualifies as or closely approaches a low-barrier hydrogen bond. Proton migration across the pseudo-C2 axis entails much higher activation energies >20 kcal/mol, reflecting the relative rigidity of the molecule along the C1-C19 pyrrole-pyrrole linkage.

First report of trans-A2B-corrole derived from a lapachone derivative: photophysical, TD-DFT and photobiological assays

In this work, the synthesis, characterization and photophysical assays of a new trans-A₂B-corrole derivative from the naturally occurring quinone are described. β-Lapachone is a naturally occurring quinoidal compound that provides highly fluorescent heterocyclic compounds such as lapimidazoles. The new trans-A₂B-corrole compound was obtained from the reaction between 2,3,4,5,6-(pentafluorophenyl)dipyrromethane and the lapimidazole bearing an aldehyde group. The dyad was characterized by high-resolution mass spectrometry (HRMS), NMR spectroscopy (¹H, COSY 2D, HMBC, ¹⁹F), FT-IR, UV-vis, steady-state and time-resolved fluorescence, electrochemical studies (CV), TD-DFT analysis and photobiological experiments, in which includes aggregation, stability in solution, photostability and partition coefficients assays. Finally, ROS generation assays were performed using 1,3-diphenylisobenzofuran (DPBF) method and the presented compound showed significant photostability and singlet oxygen production.

The role of solvents and concentrations in the properties of oxime bearing A2B corroles

A comprehensive study on the electronic spectral, photophysical and acid-base properties of phenyl- and methyl-oxime corrole derivatives and of triphenylcorrole (model corrole) has been performed, aiming to shed light on the existing species in the ground and excited states. Solvents and corrole concentration are found to govern the properties of the studied compounds and are determinants of their applicability in in vivo studies. In THF, the neutral corrole has two tautomeric forms (T1 and T2). In DMSO, the deprotonated form shows a characteristic long-wavelength Q band slightly shifted to blue when compared with the T1 tautomer and a higher fluorescence quantum yield. In ACN, with the increase of the corrole concentration formation of an aggregate due to homoconjugation (with dimer characteristics) is observed, and pioneeringly reported using UV-Vis and fluorescence studies and confirmed by carrying out titrations with TFA. The effect of the oxime group on the pK values of a corrole is found to influence the formation of a homoconjugate, namely by precluding its formation (at higher concentrations) when compared with the model corrole. TDDFT electronic quantum calculations support the experimental observations, namely the existence of tautomers and deprotonated species, with their respective electronic spectral features, further allowed proposing a structure for the homoconjugate complex in ACN. The characteristics of the oxime-corroles, namely a pK of ∼ 5, absorption and emission at ca. 650 nm and solvent dependent properties, make them good candidates for their use in biological systems either as probes, sensors, or as new sensitizers for photodynamic therapy.

Hallmarks of anticancer and antimicrobial activities of corroles

Corroles provide a remarkable opportunity for the development of cancer theranostic agents among other porphyrinoids. While most transition metal corrole complexes are only therapeutic, post-transition metallocorroles also find their applications in bioimaging. Moreover, corroles exhibit excellent photo-physicochemical properties, which can be harnessed for antitumor and antimicrobial interventions. Nevertheless, these intriguing, yet distinct properties of corroles, have not attained sufficient momentum in cancer research. The current review provides a comprehensive summary of various cancer-relevant features of corroles ranging from their structural and photophysical properties, chelation, protein/corrole interactions, to DNA intercalation. Another aspect of the paper deals with the studies of corroles conducted in vitro and in vivo with an emphasis on medical imaging (optical and magnetic resonance), photo/sonodynamic therapies, and photodynamic inactivation. Special attention is also given to a most recent finding that shows the development of pH-responsive phosphorus corrole as a potent antitumor drug for organelle selective antitumor cytotoxicity in preclinical studies. Another biomedical application of corroles is also highlighted, signifying the application of water-soluble and completely lipophilic corroles in the photodynamic inactivation of microorganisms. We strongly believe that future studies will offer a greater possibility of utilizing advanced corroles for selective tumor targeting and antitumor cytotoxicity. In the line with future developments, an ideal pipeline is envisioned on grounds of cancer targeting nanoparticle systems upon decoration with tumor-specific ligands. Hence, we envision that a bright future lies ahead of corrole anticancer research and therapeutics.

Effects of Substituents on the Photophysical/Photobiological Properties of Mono-Substituted Corroles

The trans-A₂B-corrole series was prepared starting with 5-(pentafluorophenyl)dipyrromethene, which was then reacted with respective aryl-substituted aldehyde by Gryko synthesis. It was further characterized by HRMS and electrochemical methods. In addition, we investigated experimental photophysical properties (absorption, emission by steady-state and time-resolved fluorescence) in several solvents and TDDFT calculations, aggregation, photostability and reactive oxygen species generation (ROS), which are relevant when selecting photosensitizers used in photodynamic therapy and many other photo-applications. In addition, we also evaluated the biomolecule-binding properties with CT-DNA and HSA by spectroscopy, viscometry and molecular docking calculations assays.

The Selective Monobromination of a Highly Sterically Encumbered Corrole: Structural and Spectroscopic Properties of Fe(Cl)(2-Bromo-5,10,15-tris(triphenyl)phenyl corrole)

The corrole ligand serves as a versatile tri-anionic, macrocyclic platform on which to model biological catalytic systems, as well as to effect mechanistically challenging chemical transformations. Here in we describe the synthesis, structure, and characterization of an isomerically pure corrole ligand, selectively mono-brominated at the β-carbon position adjacent to the corrole C-C bond (2-C) and produced in relatively high yields, as well as its iron chloride complex. Analysis of the iron metalated complex by cyclic voltammetry shows that the bromine being present on the ligand resulted in anodic shifts of +93 and +63 mV for first oxidation and first reduction of the complex respectively. The Mossbauer spectrum of the iron metalated complex shows negligible change relative to the non-brominated analog, indicating the presence of the halide substituent predominantly effects the orbitals of the ligand rather than the metal.

NH Tautomerism of N-Confused Porphyrin: Solvent/Substituent Effects and Isomerization Mechanism

The effects of substituents and solvents on the NH tautomerism of N-confused porphyrin (2) were investigated. The structures, electronic states, and aromaticity of NH tautomers (2-2H and 2-3H) were studied by absorption and nuclear magnetic resonance (¹H, ¹³C, and ¹⁵N) spectroscopies, single-crystal X-ray diffraction analysis, and theoretical calculations. The relative stability of the tautomers is highly affected by solvents, with the 3H-type tautomer being more stable in nonpolar solvents, while the 2H-type tautomer being highly stabilized in polar solvents with high donor numbers such as N,N-dimethylformamide (DMF), pyridine, and acetone. Electron-withdrawing groups on the meso-aryl substituents as well as the methyl group at the ortho position also stabilize the 2H-type tautomer. Kinetically, the tautomerism rate is significantly influenced by solvent and concentration, and a particularly large activation entropy (ΔS^⧧) is obtained in pyridine. The first-order deuterium isotope effect on the reaction rates of NH tautomerism (k_H/k_D) is determined to be 2.4 at 298 K. On the basis of kinetic data, the mechanism of isomerization is identified as an intramolecular process, including the rotation of the confused pyrrole in pyridine/chloroform and DMF/chloroform mixed solvent systems, and as a pyridine-mediated process in pyridine alone.

Corrole basicity in the excited states: Insights on structure–property relationships

Steady-state fluorescence measurements and quantum-chemical DFT geometry optimizations are applied to extend the structure–property relationships between the free-base corrole macrocycle conformation and its basicity to the lowest excited S[Formula: see text] and T[Formula: see text] states. Direct basicity estimation in the lowest excited S[Formula: see text] state is demonstrated by means of fluorescence quantum yield measurements. The long wavelength T1 tautomer is found to retain its basicity in the S[Formula: see text] state, whereas the short wavelength T2 tautomer shows a noticeable decrease in basicity in the S[Formula: see text] state, which is related to the in-plane tilting of the pyrrole ring to be protonated. The conformational changes upon going from the ground to the lowest excited T[Formula: see text] state and the influence of the meso-aryl substitution pattern on the overall degree of distortions and tilting of the pyrrole ring to be protonated are also discussed from the point of view of macrocycle basicity.

Photophysical and electrochemical properties of two trans-A2B-corroles: differences between phenyl or pyrenyl groups at the meso-10 position

The photophysical properties of two meso–trans-A₂B-type corroles containing phenyl or pyrenyl units were reported in this study. TD-DFT calculations and electrochemical analysis were conducted to better understand the corrole molecular structures.

A program for automatically predicting supramolecular aggregates and its application to urea and porphin

Not only the molecular structure but also the presence or absence of aggregates determines many properties of organic materials. Theoretical investigation of such aggregates requires the prediction of a suitable set of diverse structures. Here, we present the open-source program EnergyScan for the unbiased prediction of geometrically diverse sets of small aggregates. Its bottom-up approach is complementary to existing ones by performing a detailed scan of an aggregate's potential energy surface, from which diverse local energy minima are selected. We crossvalidate this approach by predicting both literature-known and heretofore unreported geometries of the urea dimer. We also predict a diverse set of dimers of the less intensely studied case of porphin, which we investigate further using quantum chemistry. For several dimers, we find strong deviations from a reference absorption spectrum, which we explain using computed transition densities. This proof of principle clearly shows that EnergyScan successfully predicts aggregates exhibiting large structural and spectral diversity. © 2018 Wiley Periodicals, Inc.

The impact of solvents on the singlet and triplet states of selected fluorine corroles – absorption, fluorescence, and optoacoustic studies

This paper examines the influence of aprotic solvents on the spectroscopic properties as well as the energy deactivation of two free-base corrole dyes substituted with C₆F₅ and/or 4-NO₂C₆H₄ groups.

Phosphorescence of free base corroles

The phosphorescence features of free base corroles have been analysed, completing the picture on their excited state photophysical properties.

Femtosecond to Microsecond Dynamics of Soret-Band Excited Corroles

We present a comprehensive photophysical investigation on a series of three corroles (TTC, P-TTC, Ge-TTC dissolved in toluene), employing femtosecond and nanosecond transient absorption spectroscopy (TAS) measurements. Systematic analyses of the TAS data determined the rates and corresponding time constants of photophysical processes: internal conversion (τ_IC) in the 898-525 fs range, vibrational relaxation (τ_{_VR}) in the 7.44-13.6 ps range, intersystem crossing (τ_ISC ) in the 033-1.09 ns range and triplet lifetime (τ__triplet) in the 0.8-3.5μs range. The estimated triplet quantum yields (Φ_Triplet) were in the 0.42 - 0.61 range. Comparatively, GeTTC displayed faster τ_IC and higher(Φ_Triplet). Additionally, the time-dependent density functional theory (DFT) calculations were performed for the three molecules. The HOMO/LUMO energy levels and the oscillator strengths of various transitions were determined and presented.

New meso-substituted corroles possessing pentafluorophenyl groups – synthesis and spectroscopic characterization

In this paper, the synthesis and spectroscopic characterization of new free-base corroles substituted with different peripheral groups are reported.

Adventures in corrole features by electrospray ionization mass spectrometry studies

In this short review the importance of electrospray mass spectrometry in corrole chemistry is highlighted.

A spectroscopic and theoretical investigation of a free-base meso-trithienylcorrole

The unique optical properties of free-base meso-tris(5-methylthien-2-yl)corrole were compared to those of the widely investigated meso-triphenyl-substituted analogue. A combination of spectroscopic and computational experiments was undertaken to elucidate the relationship between structural features of the neutral, mono-anionic and mono-cationic forms of the corroles and their corresponding optical properties. A general bathochromic shift was measured for the thienyl-substituted corrole. The experimental spectra are supported by excited state calculations. A systematic series of ground state minimizations were performed to determine energy minima for the flexible and solvent-sensitive molecules. Trithienylcorrole was found to have a more nonplanar macrocycle in conjunction with a high degree of π-overlap with the meso-substituents. Both structural features contribute to their bathochromically shifted optical spectra. The configurational character of the thienyl-substituted corrole is shown to have a larger degree of molecular orbital mixing and doubly excited character, which suggest a more complex electronic structure that does not fully adhere to the Gouterman four-orbital model. The reactivity of the thienyl groups, particularly with respect to their ability to be (electro)-polymerized, combined with the tight coupling of the meso-thienyl groups with the corrole chromophore elucidated in this work, recommends the meso-thienylcorroles as building blocks in, for instance, organic semiconductor devices.

Decorating metal oxide surfaces with fluorescent chlorosulfonated corroles

We have prepared 2,17-bis(chlorosulfonyl)-5,10,15-tris(pentafluorophenyl)corrole (1), 2,17-bis(chlorosulfonyl)-5,10,15-tris(pentafluorophenyl)corrolatoaluminum(III) (1-Al), and 2,17-bis(chlorosulfonyl)-5,10,15-tris(pentafluorophenyl)corrolatogallium(III) (1-Ga). The metal complexes 1-Al and 1-Ga were isolated and characterized by electronic absorption and NMR spectroscopies, as well as by mass spectrometry. Relative emission quantum yields for 1, 1-Al, and 1-Ga, determined in toluene, are 0.094, 0.127, and 0.099, respectively. Reactions between 1, 1-Al, and 1-Ga and TiO2 nanoparticles (NPs) result in corrole-TiO2 NP conjugates. The functionalized NP surfaces were investigated by solid-state Fourier transform infrared and X-ray photoelectron spectroscopies and by confocal fluorescence imaging. The fluorescence images for 1-Al-TiO2 and 1-Ga-TiO2 suggest a promising application of these NP conjugates as contrast agents for noninvasive optical imaging.