Tying a knot between crown ethers and porphyrins

Porphyrins and crown ether hybrids have emerged as a promising class of molecules composed of elements of a tetrapyrrole macrocycle and an oligo(ethylene glycol) segment. These hybrid systems constitute a broad group of compounds, including crowned porphyrins, crownphyrins, and calixpyrrole-crown ether systems forming Pacman complexes with transition metals. Their unique nature accustoms them as excellent ligands and hosts capable of binding guest molecules/ions, but also to undergo unusual transformations, such as metal-induced expansion/contraction. Depending on the design of the particular hybrid, they present unique features involving intriguing redox chemistry, interesting optical properties, and reactivity towards transition metals. In this perspective article, the overview of both the early designs of porphyrin-crown ether hybrids, as well as the most recent advances in the synthesis and characterisation of this remarkable group of macrocyclic systems, are addressed. The discussion covers the strategies employed in synthesising these systems, including cyclisation reactions, self-assembly, and their remarkable reactivity. The potential applications of porphyrin-crown ether hybrids are also highlighted. Moreover, the discussion identifies the challenges associated with synthesising and characterising hybrids, outlining the possible future directions.

In Situ Generation of Red-to-NIR Emissive Radical Cations in the Stomach for Gastrointestinal Imaging

Red-to-near-infrared (NIR) fluorescent probes, with advantages such as high spatiotemporal resolution and in situ sensing abilities, are highly attractive for diagnosis of gastrointestinal diseases and targeted drug development. However, conventional red-to-NIR fluorophores with electron closed-shell structures require tedious synthetic procedures for preparation, and it is difficult to further decorate them with sensing groups. In this study, a series of easily prepared pyrroles with simple structures that can quickly be transformed into red-to-NIR emissive radical cations in acidic buffer solution and in vivo stomachs is developed. The in-situ-generated red-to-NIR emissive pyrrole radical cations in the stomach have excellent biocompatibility and stability and can be used not only for intravital gastrointestinal imaging with high spatiotemporal resolution, but also for dynamic monitoring of the gastric emptying process and assessment of anti-gastric-acid therapy. The acidity-induced generation of pyrrole radical cations is believed to provide a facile strategy for developing red-to-NIR fluorophores and studying gastrointestinal diseases.

Real time sensor for Fe3+, Al3+, Cu2+ & PPi through quadruple mechanistic pathways using a novel dipodal quinoline-based molecular probe

A quinoline-based small molecular probe, H₂L was designed, synthesized and characterized by different spectroscopic methods. It was utilized as a multi-responsive probe for the detection of Fe³⁺, Al³⁺, Cu²⁺ and PPi. It showed very selective instant turn-on fluorimetric response towards Fe³⁺and Al³⁺ with a detection limit in nanomolar range. Solutions of H₂L containing Fe³⁺ or Al³⁺ could sequentially sense PPi by a turn-off mechanism. Also, H₂L could determine the presence of Cu²⁺ very selectively among a series of other metal ions by a sharp change in colour. Detection of Cu²⁺ through colorimetry was further investigated by systematic UV-Vis studies and the potential of H₂L to act as a potential colorimetric sensor for Cu²⁺ was suitably established. Filter-paper strip experiments were conducted to demonstrate the practical utility of the proposed sensor. Potential applications of H₂L as a sensor for pH in the acidic range has also been explored.

Synthesis of Pyridine Containing Crowned Fused Expanded Porphyrins

New examples of nonaromatic fused expanded porphyrins containing both pyridine and crown ether moiety as a part of macrocyclic framework were synthesized by condensing pyridine based pentapyrrane with polyether based diol in CH 2 Cl 2 in the presence of one equivalent of BF 3 ·OEt 2 under inert conditions followed by oxidation with DDQ in open air. The condensation was expected to form pyridine containing crowned expanded porphyrins but resulted in the formation of fused crowned expanded porphyrins due to intramolecular fusion of two pyrrole "N"s with two adjacent inverted thiophene "C"s as revealed by X-ray crystallography obtained for one of the macrocycle. The HRMS and NMR study supported the formation of fused crowned pyridine containing expanded porphyrins, and the macrocycles showed simple well-resolved NMR spectra where all resonances were identified easily by 2D NMR spectroscopy. The macrocycles exhibited typical nonaromatic absorption features and showed one broad band with peak maxima at 535 nm and one or two shoulder bands in the higher energy region. The protonation studies resulted in clear colour change from purple to blue and absorption bands experienced bathochromic shifts with a broad band at 662 nm which was extended up to 800 nm. The electrochemical studies revealed that the macrocycles were easier to oxidize but difficult to reduce. DFT studies indicated that the macrocycle attains a very puckered and distorted 'U' shaped structure owing to the flexibility of the crown ether chain and TD-DFT studies corroborated experimental results. The preliminary studies indicated that the macrocycles could be used as colorimetric optical sensor for detection of Cu 2+ ion.

Crowned Macrocycles Containing Two Pyrrolo[1,2-a] Indoles Created By Intramolecular Fusion

Heterocyclic fused-ring systems are of utmost importance because of their presence in many natural products with biological activity. Pyrroloindoles are tricyclic heterocycles that are present in various bioactive and medicinally valuable compounds. Herein, we report the synthesis of phenylene-bridged bis-pyrrolo[1,2-a]indole crowned macrocycles 1-3 in which the pyrrolo[1,2-a]indole moieties were formed via intramolecular fusion. The macrocycles were thoroughly characterized by 1D and 2D NMR, HRMS and X-ray crystallographic studies. The X-ray structure revealed that the two pyrrolo[1,2-a]indole moieties were parallel to each other, and one pyrrolo[1,2-a]indole unit was deviated by an angle of 9.54° while the other pyrrolo[1,2-a]indole unit was deviated by an angle of 12.0° from the mean plane defined by 28 core atoms. The macrocycles 1-3 absorb in the visible region and readily undergo oxidations because of their electron rich nature. The macrocycles 1-3 upon treatment with trifluoroacetic acid (TFA) generates the corresponding cation radicals 1-3^.+ which were stable in the open air for a week. The cation radicals 1-3^.+ absorb strongly in the NIR region and the experimental observations on crowned macrocycles 1-3 were corroborated by DFT and TD-DFT studies.