Imidazo[1,2-a]pyridine and Imidazo[1,5-a]pyridine: Electron Donor Groups in the Design of D-π-A Dyes

This work investigates the electron-donating capabilities of two 10-π electron nitrogen bridgehead bicyclic [5,6]-fused ring systems, imidazo[1,2-a]pyridine and imidazo[1,5-a]pyridine rings. Eight compounds with varying positions of electron-withdrawing moieties (TCF or DCI) coupled to the imidazopyridine ring were synthesized and studied. DCI-containing compounds (Ib-IVb) exhibited a purely dipolar nature with broad absorption bands, weak fluorescence, large Stokes shifts, and strong solvatochromism. In contrast, TCF-containing compounds (Ia-IVa) demonstrated diverse properties. Imidazo[1,2-a]pyridine derivatives Ia and IIa were purely dipolar, while imidazo[1,5-a]pyridine derivatives IIIa and IVa displayed a cyanine-like character with intense absorption and higher quantum yields of emission. The observed gradual red shift in optical properties with changing electron-donor groups (IIb < Ib < IIIb < IVb) and (IIa < Ia < IIIa < IVa) underscores the stronger electron-donor character of imidazo[1,5-a]pyridine compared to that of imidazo[1,2-a]pyridine. Furthermore, crystalline powders of imidazo[1,2-a]pyridine derivatives exhibited fluorescence despite minimal emission in solution. Two compounds (Ib and IVa) were successfully formulated into nanoparticles for potential in vivo imaging applications in zebrafish embryos.

Imidazopyridine Family: Versatile and Promising Heterocyclic Skeletons for Different Applications

In recent years, there has been increasing attention focused on various products belonging to the imidazopyridine family; this class of heterocyclic compounds shows unique chemical structure, versatile optical properties, and diverse biological attributes. The broad family of imidazopyridines encompasses different heterocycles, each with its own specific properties and distinct characteristics, making all of them promising for various application fields. In general, this useful category of aromatic heterocycles holds significant promise across various research domains, spanning from material science to pharmaceuticals. The various cores belonging to the imidazopyridine family exhibit unique properties, such as serving as emitters in imaging, ligands for transition metals, showing reversible electrochemical properties, and demonstrating biological activity. Recently, numerous noteworthy advancements have emerged in different technological fields, including optoelectronic devices, sensors, energy conversion, medical applications, and shining emitters for imaging and microscopy. This review intends to provide a state-of-the-art overview of this framework from 1955 to the present day, unveiling different aspects of various applications. This extensive literature survey may guide chemists and researchers in the quest for novel imidazopyridine compounds with enhanced properties and efficiency in different uses.

Real-time fluorescence imaging of anthocyanins complexed with diphenylboric acid 2-aminoethyl inside B16-F10 melanoma cells

Anthocyanins (AN), natural compounds daily consumed by humans, have outstanding therapeutical potential if administered topically in melanoma pathology. However, the search for efficient therapy development is still in progress, owing to the lack of complete understanding of the AN intracellular path, once they are uptaken by the cells. This target is constrained by the need for an imaging strategy that would enable their intracellular detection and localization in-situ. In this light, diphenylboric acid 2-aminoethyl (DPBA), a non-fluorescent reagent, was here successfully used to form fluorescent complexes with AN. The AN used are the cyanidin aglycon as a free standard molecule (CY), and the glycosylated compounds, extracted and purified from chokeberry fruits (AE). In solution, it was observed that the fluorescence emission increased by 39% (CY@DPBA), and by 34% (AE@DPBA), which concludes that AN form fluorescent complexes with DPBA (CY@DPBA and AE@DPBA). In addition, using NMR (nuclear magnetic resonance) spectroscopy, and HRMS (high-resolution mass spectrometry) analysis, the structure of the CY@DPBA complex was efficiently elucidated. In-vitro experiments showed that the complexes formed after the treatment proved to be non-toxic on B16-F10 cells. The sub-cellular visualization of all AN was monitored by fluorescence microscopy and flow cytometry, demonstrating detectable signals of the non-metabolized CY and glycosylated CY inside melanoma cells. This study reports that the use of DPBA to image AN intracellularly is a sensitive, non-invasive and successful method that can extend its application in broad fields like drug development or metabolism-associated mechanisms.

Driving the Emission Towards Blue by Controlling the HOMO-LUMO Energy Gap in BF2 -Functionalized 2-(Imidazo[1,5-a]pyridin-3-yl)phenols

Several boron compounds with 2-(imidazo[1,5-a]pyridin-3-yl)phenols, differentiated by the nature of the substituent (R) in the para position of the hydroxy group, have been synthesized and thoroughly characterized both in solution (1 H, 13 C, 11 B, 19 F NMR) and in the solid state (X-ray). All derivatives displayed attractive photophysical properties like very high Stokes shift, high fluorescence quantum yields and a good photostability in solution. Time-Dependent Density Functional Theory (TD-DFT) calculations allowed to define the main electronic transitions as intra ligand transitions (1 ILT), which was corroborated by the Natural Transition Orbitals (NTOs) shapes. The HOMO-LUMO energy gap was correlated to the electronic properties of the substituent R on the phenolic ring, as quantified by its σp Hammett constant.

Imidazo[1,5-a]pyridin-3-ylidenes as π-accepting carbene ligands: substituent effects on properties of N-heterocyclic carbenes

Imidazo[1,5-a]pyridine-derived NHCs gained strong π-accepting character due to their structural features.

Copper-catalyzed aerobic oxidative amination of C(sp3)–H bonds: synthesis of imidazo[1,5-a]pyridines

Copper-catalyzed synthesis of imidazo[1,5-a]pyridine-1-carboxylates through oxidative amination of C(sp³)–H bonds under mild aerobic conditions with broad substrate scope has been achieved.

Fluorinative hydrolysis of phosphorothioic acid esters with a binaphthyl group through axis-to-center chirality transfer leading to the formation of P-chiral phosphorothioic monofluoridic acid salts

Asymmetric synthesis of P-chiral phosphorothioic monofluoridic acid ammonium salts was achieved via axis-to-center chirality transfer reactions by using phosphorothioic acid O-esters with a binaphthyl group, and the absolute stereochemistry of the salts was determined by X-ray analyses and by comparison of their CD spectra.