Blue fluorescent zinc(II) complexes based on tunable imidazo[1,5-a]pyridines

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.

One-Dimensional and Two-Dimensional Zn(II) Coordination Polymers with Ditopic Imidazo[1,5-a]pyridine: A Structural and Computational Study

Zn(II) coordination polymers are being increasingly studied for their stability and properties. Similarly, there is a growing interest in imidazo[1,5-a]pyridine derivatives, which show great potential in luminescence and pharmaceutical applications. In this work, we successfully synthesized and crystallized three new coordination polymers, using Zn(II) as the metallic node, dicarboxylic acids of different length and nature as linkers, and a linear ditopic imidazo[1,5-a]pyridine derivative, to explore the role of this molecule as a propagator of the dimensionality of the structure or as an ancillary ligand. Our work demonstrates the structural capability of imidazo[1,5-a]pyridines in an unexplored domain for this family of ligands. Notably, we observed a pronounced ability of this heterocyclic scaffold to establish π···π interactions in the solid state. The supramolecular π-stacked assemblies were theoretically analyzed using DFT calculations based on model structures.

Construction of Imidazole-Fused-Ring Systems by Iron-Catalyzed C(sp3)-H Amination-Cyclization under Aerobic Conditions

An iron-catalyzed efficient C-H amination for the construction of imidazole-fused-ring systems was developed under aerobic conditions. Compared to previous studies, this work exhibited green features. The reaction was conducted in the green solvent anisole, with water as the only byproduct. Four C(sp3)-H bonds were cleaved and three C-N bonds were formed in this transformation. Imidazo[1,5-a]pyridine-, imidazo[5,1-b]oxazole-, imidazo[5,1-b]thiazole-, imidazo[1,5-a]pyrazine-, and imidazo[1,5-a]imidazole-related N-heterocycles were obtained in acceptable-to-excellent yield.

Novel Fluorescent Tetrahedral Zinc (II) Complexes Derived from 4-Phenyl-1-octyl-1H-imidazole Fused with Aryl-9H-Carbazole and Triarylamine Donor Units: Synthesis, Crystal Structures, and Photophysical Properties

We present here the design, synthesis, and photophysical properties of two novel fluorescent zinc (II) complexes, ZnCl2(ImL1)2 and ZnCl2(ImL2)2, containing 4-(1-octyl-1H-imidazol-4-yl)-N,N-diphenyl-[1,1-biphenyl]-4-yl)-4-amine ImL1 and 9-(4-(1-octyl-1H-imidazol-4-yl)-[1,1-biphenyl]-4-yl)-9H-carbazole ImL2 ligands. The newly synthesized free ligands and their zinc (II) complexes were characterized using several spectroscopic techniques; their structures were identified by single-crystal X-ray diffraction; and their photophysical properties have been studied in the context of their chemical structure. The ZnCl2(ImL1)2 and ZnCl2(ImL2)2 complexes showed good thermal stability at 341 °C and 365 °C, respectively. Photophysical properties, including UV-Vis absorption spectra in ethanol solution and photoluminescence (PL) in both solid state and ethanol solution, were determined. UV-Vis adsorption data indicated that both free ligands had similar UV-Vis absorption properties, while their Zn (II) complexes had distinctive absorption characteristics. The fluorescence spectra show that both ligands and their corresponding Zn (II) complexes emit violet to cyan luminescence in the solid state at room temperature, while in ethanol solution at the same temperature, they exhibit efficient photoluminescence properties in the UV-A emission spectral region. Because of these photophysical properties, the synthesized ligands and their cognate Zn (II) complexes can be used as scaffolds for the potential development of optoelectronic materials.

A Review: Blue Fluorescent Zinc (II) Complexes for OLEDs-A Last Five-Year Recap

Blue emissions in organic light-emitting diodes (OLEDs) are essential for their application in solid-state lighting and full-colour flat panel displays. On the other hand, high-power blue emitters are still uncommon, especially those that can achieve the Commission Internationale de l'Eclairage (CIE, X, Y) coordinates of (0.14, 0.08) in the National Television System Committee (NTSC) blue standard and have high external quantum efficiencies (EQE) of more than 5% because their molecular design presents an enormous challenge. Therefore, creating effective, stable, pure, and deep blue fluorescent materials is vital. Here, it is addressed how useful blue fluorescent Zn (II) complexes are for making organic light-emitting diodes (OLEDs). Utilizing Zn (II) complexes is appealing because of their favourable luminous characteristics, acceptance and mobility, and affordability. This mini-review article aims to provide an overview of Zn (II) complexes that emit blue fluorescent light and have been reported since 2018, while highlighting the unique qualities that make them appropriate OLED materials.

Mono-, Bis-, and Tris-Chelate Zn(II) Complexes with Imidazo[1,5-a]pyridine: Luminescence and Structural Dependence

New mono-, bis-, and tris-chelate Zn(II) complexes have been synthesized starting from different Zn(II) salts and employing a fluorescent 1,3-substituted-imidazo[1,5-a]pyridine as a chelating ligand. The products have been characterized by single-crystal X-ray diffraction; mass spectrometry; and vibrational spectroscopy. The optical properties have been investigated to compare the performances of mono-, bis-, and tris-chelate forms. The collected data (in the solid state and in solution) elucidate an important modification of the ligand conformation upon metal coordination; which is responsible for a notable increase in the optical performance. An intense modification of the emission quantum yield along the series in the solid state is observed comparing mono-, bis-, and tris-chelate adducts; independently from the anionic ligand introduced by ionic exchange.

Imidazo[1,5-a]pyridine-Based Fluorescent Probes: A Photophysical Investigation in Liposome Models

Imidazo[1,5-a]pyridine is a stable scaffold, widely used for the development of emissive compounds in many application fields (e.g., optoelectronics, coordination chemistry, sensors, chemical biology). Their compact shape along with remarkable photophysical properties make them suitable candidates as cell membrane probes. The study of the membrane dynamics, hydration, and fluidity is of importance to monitor the cellular health and to explore crucial biochemical pathways. In this context, five imidazo[1,5-a]pyridine-based fluorophores were synthesized according to a one-pot cyclization between an aromatic ketone and benzaldehyde in the presence of ammonium acetate and acetic acid. The photophysical features of prepared compounds were investigated in several organic solvents and probes 2–4 exhibited the greatest solvatochromic behavior, resulting in a higher suitability as membrane probes. Their interaction with liposomes as artificial membrane model was tested showing a successful intercalation of the probes in the lipid bilayer. Kinetic experiments were carried out and the lipidic phase influence on the photophysical features was evaluated through temperature-dependent experiments. The results herein reported encourage further investigations on the use of imidazo[1,5-a]pyridine scaffold as fluorescent membrane probes.

Anomeric Stereoauxiliary Cleavage of the C−N Bond of d ‐Glucosamine for the Preparation of Imidazo[1,5‐a]pyridines

The targeted cleavage of the C−N bonds of alkyl primary amines in sustainable compounds of biomass according to a metal‐free pathway and the conjunction of nitrogen in the synthesis of imidazo[1,5‐a]pyridines are still highly challenging. Despite tremendous progress in the synthesis of imidazo[1,5‐a]pyridines over the past decade, many of them can still not be efficiently prepared. Herein, we report an anomeric stereoauxiliary approach for the synthesis of a wide range of imidazo[1,5‐a]pyridines after cleaving the C−N bond of d‐glucosamine (α‐2° amine) from biobased resources. This new approach expands the scope of readily accessible imidazo[1,5‐a]pyridines relative to existing state‐of‐the‐art methods. A key strategic advantage of this approach is that the α‐anomer of d‐glucosamine enables C−N bond cleavage via a seven‐membered ring transition state. By using this novel method, a series of imidazo[1,5‐a]pyridine derivatives (>80 examples) was synthesized from pyridine ketones (including para‐dipyridine ketone) and aldehydes (including para‐dialdehyde). Imidazo[1,5‐a]pyridine derivatives containing diverse important deuterated C(sp²)−H and C(sp³)−H bonds were also efficiently achieved.

Crystal engineering of aurophilic supramolecular architectures and coordination polymers based on butterfly-like Copper-dicyanoaurate complexes: vapochromism, P-T behaviour and multi-metallic cocrystal formation

Using the equilibrium properties of CuII in the presence of the chelating ligand and the characteristics of the dicyanoaurate anion, we were able to obtain a family of 10 bimetallic...

Dipyridylmethane Ethers as Ligands for Luminescent Ir Complexes

This work reports two new cationic heteroleptic cyclometalated iridium complexes, containing ether derivatives of di(pyridin-2-yl)methanol. The new ligands are based on dipyridin-2-ylmethane and are designed to obtain ether-based intermediates with extended electronic conjugation by insertion of π system such as phenyl, allyl and ethynyl. Different synthetic strategies were employed to introduce these units, as molecular wires, between the dipyridin-2-ylmethane chelating portion and the terminal N-containing functional group, such as amine and carbamide. The corresponding complexes show luminescence in the blue region of the spectrum, lifetimes between 0.6 and 2.1 μs, high quantum yield and good electrochemical behavior. The computational description (DFT) of the electronic structure highlights the key role of the conjugated π systems on optical and electrochemical properties of the final products.

Zn Coordination and the Identity of the Halide Ancillary Ligand Dramatically Influence the Excited-State Dynamics and Bimolecular Reactions of 2,3-Di(pyridin-2-yl)benzo[g]quinoxaline

The optical properties of coordination complexes with ligands containing nitrogen heterocycles have been extensively studied for decades. One subclass of these materials, metal complexes utilizing substituted pyrazines and quinoxalines as ligands, has been employed in a variety of photochemical applications ranging from photodynamic therapy to organic light-emitting diodes. A vast majority of this work focuses on characterization of the metal-to-ligand charge-transfer states in these metal complexes; however, literature reports rarely investigate the photophysics of the parent pyrazine or quinoxaline ligand or perform control experiments utilizing metal complexes that lack low-lying charge-transfer (CT) states in order to determine how metal-atom coordination influences the photophysical properties of the ligand. With this in mind, we examined the steady-state and time-resolved photophysics of 2,3-di(pyridin-2-yl)benzo[g]quinoxaline (dpb) and explored how the coordination of ZnX₂ (X = Cl^-, Br^-, I^-) affects the photophysical properties of dpb. In dpb, we find that the dominant mode of deactivation from the singlet excited state is intersystem crossing (ISC). Coordination of ZnX₂ perturbs the relative energies of the ππ* and nπ* excited states of dpb, leading to drastically different rates of ISC as well as radiative and nonradiative decay in the [Zn(dpb)X₂] complexes compared to dpb. These differences in the rates change the dominant singlet-excited-state decay pathway from ISC in dpb to a mixture of ISC and fluorescence in [Zn(dpb)Cl₂] and [Zn(dpb)Br₂] and to nonradiative decay in [Zn(dpb)I₂]. Coordination of ZnX₂ and the choice of the halide ligand also have profound effects on the rate constants for excited-state bimolecular reactions, including triplet-triplet annihilation and oxygen quenching. These results demonstrate that metal coordination, even in complexes lacking low-lying CT states, and the choice of the ancillary ligand can dramatically alter the photophysical properties of chromophores containing nitrogen heterocycles.

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.

Microwave-Assisted Synthesis, Optical and Theoretical Characterization of Novel 2-(imidazo[1,5-a]pyridine-1-yl)pyridinium Salts

In the last few years, imidazo[1,5-a]pyridine scaffolds and derivatives have attracted growing attention due to their unique chemical structure and optical behaviors. In this work, a series of pyridylimidazo[1,5-a]pyridine derivatives and their corresponding pyridinium salts were synthesized and their optical properties investigated to evaluate the effect of the quaternization on the optical features both in solution and polymeric matrix. A critical analysis based on the spectroscopic data, chemical structures along with density functional theory calculation is reported to address the best strategies to prevent aggregation and optimize the photophysical properties. The obtained results describe the relationship between chemical structure and optical behaviors, highlighting the role of pendant pyridine. Finally, the presence of a positive charge is fundamental to avoid any possible aggregation process in polymeric films.

Steric crowding of a series of pyridine based ligands influencing the photophysical properties of Zn(II) complexes

The combination of α-acetamidocinnamic acid (HACA) and different N, N,N and N,N,N pyridines (dPy) leads to crowded Zn(ii) metal centers. The increasing bulkiness competes with the chelation enhanced effect (CHEF) in the resulting quantum yields.

Diversifying the luminescence of phenanthro-diimine ligands in zinc complexes

Strongly blue fluorescent 1-phenyl-2-(pyridin-2-yl)-1H-phenanthro[9,10-d]imidazole (L1) is a facile block for the construction of multichromophore organic molecules, and simultaneously serves as a chelating diimine ligand.

Imidazo[1,5-a]pyridine derivatives: useful, luminescent and versatile scaffolds for different applications

In the last few years, imidazo[1,5-a]pyridine nuclei and derivatives have attracted growing attention due to their unique chemical structure and versatility, optical behaviours, and biological properties.

Methoxy-substituted copper complexes as possible redox mediators in dye-sensitized solar cells

Methoxy-substituted aromatic diimines and corresponding homoleptic copper(i) and copper(ii) complexes as possible redox mediators in dye-sensitized solar cells.