Evaluation of Pyrazolyl-Indolizine Derivatives as Antimicrobial Agents: Synthesis, In vitro, In silico ADMET and Molecular Docking Studies

Herein, we report analogues of s-indacene by the synthesis of novel indolizine derivatives. Using chloroform as an appropriate solvent, sixteen derivatives of pyrazolyl-indolizine (4--19) were prepared by the reaction of 3-(dimethylamino)-1-(1H-pyrrol-2-yl)prop-2-en-1-one (1) with hydrazonoyl chloride derivatives (2) in the presence of triethylamine in good to excellent yields. We used NMR spectra, IR, mass spectrometry, as well as elemental analyses to prove the chemical structures and the purity of the synthesized compounds 4-19. Among all tested compounds 5, 9, 13 and 19 had a potent antimicrobial efficiency against Bacillus subtilis, Staphylococcus aureus, Pseudomonas aerginousea, Sallmonella typhemerium, and Candida albicans. Furthermore, a significant increase in lipid peroxidation (LPO) toward the Gram-negative bacteria, Pseudomonas aeruginosa when treated with compound 9 was observed, while compound 13 remarkably increased the cell membrane oxidation of Salmonella typhimurium. Additionally, we utilized docking studies and in silico methods to evaluate the drug-likeness, physicochemical properties, and ADMET profiles of the compounds. The results of the molecular docking simulation revealed that the synthesized compounds displayed decreased binding energy when interacting with the active sites of important enzymes, including Sterol 14-demethylase of C. albicans, Dihydropteroate synthase of S. aureus, LasR of P. aeruginosa, Glucosamine-6-phosphate synthase of S. typhimurium, and Gyrase B of B. subtilis.

Switchable Synthesis of Spirodihydroindolizines and Indolizines from Aurones and Pyridin-2-yl Active Methylene Compounds

A novel and flexible domino reaction of aurones with pyridin-2-yl active methylene compounds promoted by I2/BF3 has been developed to afford spirodihydroindolizines and indolizines in a controllable manner. When the reaction was performed in 1,2-dichloroethane at 80 °C, a variety of spirodihydroindolizines were obtained, whereas it almost exclusively provided a series of indolizines when the reaction was performed in a mixed solvent of 1,2-dichloroethane and N,N-dimethylformamide at a relatively higher temperature of 100 °C. Being metal-free, excellent product selectivity, high atom economy, good functional group tolerance, and feasibility for large-scale synthesis are the salient features of the developed methodology.

Acetic Acid-Catalyzed Regioselective C(sp2)-H Bond Functionalization of Indolizines: Concomitant Involvement of Synthetic and Theoretical Studies

An atom economical and environmentally benign protocol has been developed for the regioselective C(sp²)-H bond functionalization of indolizines. The acetic acid-catalyzed cross-coupling reaction proceeds under metal-free conditions, producing a wide range of synthetically useful indolizine derivatives. The present protocol showed good functional group tolerance and broad substrate scope in good to excellent yields. Quantum mechanical investigation using density functional theory (DFT) has played a crucial role in understanding that acetic acid is the key player in determining the actual pathway as the catalyst and its ultrafast nature. Different pathways involving inter- and intramolecular proton transfer, with or without acetic acid, were investigated. Calculated results revealed that a proton shuttle mechanism is involved for the least energetic, most favorable acetic acid-catalyzed pathway. Furthermore, regioselectivity has also been explained theoretically.

Photoinduced successive oxidative ring-opening and borylation of indolizines with NHC–boranes

A facile photoinduced successive oxidative ring-opening and borylation of indolizines with NHC–boranes via a one-pot method has been unveiled. This photo-promoted strategy enables the formation of unsaturated NHC–boryl carboxylates under transition metal-free and radical initiator-free conditions. A wide array of pyridine-containing NHC–boryl carboxylates were directly prepared in moderate to good yields. This work contributes to a better understanding of the reactivity and photo-behavior of both indolizines and NHC–boranes.

A facile photoinduced successive oxidative ring-opening and borylation of indolizines with NHC–boranes via a one-pot method has been unveiled. This photo-promoted strategy enables the formation of unsaturated NHC–boryl carboxylates under transition metal-free conditions.

Shortwave Infrared Absorptive and Emissive Pentamethine-Bridged Indolizine Cyanine Dyes

Shortwave infrared (SWIR)-emitting small molecules are desirable for biological imaging applications. In this study, four novel pentamethine indolizine cyanine dyes were synthesized with N,N-dimethylaniline-based substituents on the indolizine periphery at varied substitution sites. The dyes are studied via computational chemistry and optical spectroscopy both in solution and when encapsulated. Dramatic spectral shifts in the absorption and emission spectrum wavelengths with added donor groups are observed. Significant absorption and emission with an emissive quantum yield as high as 3.6% in the SWIR region is possible through the addition of multiple donor groups per indolizine.

Regioselective C-H dithiocarbamation of indolizines with tetraalkylthiuram disulfide under metal-free conditions

An efficient and straightforward metal-free regioselective C-H dithiocarbamation of indolizines with tetraalkylthiuram disulfide has been described. A series of indolizine-dithiocarbamate derivatives were easily accessed in moderate to good yields with a broad scope. In addition, imidazo[1,2-a]pyridines were also well tolerated to afford diverse imidazoheterocycle-dithiocarbamate products, which are expected to be utilized for drug discovery. Of note, the reaction could be readily scaled up, and shows its practical value in organic synthesis.

The Hagfeldt Donor and Use of Next-Generation Bulky Donor Designs in Dye-Sensitized Solar Cells

"The Hagfeldt donor" is a bulky triarylamine building block with four alkyl chains in a 3-dimensional arrangement that is used with organic dyes in dye-sensitized solar cells (DSCs) in over 140 publications. Many of the highest performing DSC devices in literature make use of this group due to exceptional TiO₂ surface protection properties, which slows recombination of electrons in TiO₂ with the electrolyte. Importantly, record-setting cobalt and copper redox shuttle-based DSCs require exceptional surface protection to slow a facile recombination of electrons to these positively charged redox shuttles. Several syntheses have emerged for the Hagfeldt donor due to the need for iterative aryl-halide cross- coupling reactions complicating a straightforward route. Six synthetic strategies found in literature are described along with the challenges of each route. A recent method that has been put forward in the literature as a scalable, regioisomerically pure route is highlighted.

Donor-Acceptor-Donor NIR II Emissive Rhodindolizine Dye Synthesized by C-H Bond Functionalization

A NIR II emissive dye was synthesized by the C-H bond functionalization of 1-methyl-2-phenylindolizine with 3,6-dibromoxanthene. The rhodindolizine (RhIndz) spirolactone product was nonfluorescent; however, upon opening of the lactone ring by the formation of the ethyl ester derivative, the fluorophore absorbs at 920 nm and emits at 1092 nm, which are both in the NIR II region. In addition, 4-cyanophenyl- (CNRhIndz) and 4-methoxyphenyl-substituted rhodindolizine (MeORhIndz) could also be prepared by the C-H activation reaction.

Near-Infrared-Absorbing Indolizine-Porphyrin Push-Pull Dye for Dye-Sensitized Solar Cells

Porphyrins are attractive chromophores for application in dye-sensitized solar cells (DSCs), as judicious tuning of donor-acceptor properties can enable excellent near-infrared (NIR) absorption and exceptional device performance. Here, we report a porphyrin-based dye (SM85) conjugated to the planar strong electron donor, indolizine, designed to extend absorption further into the NIR region by inducing π-π interactions such as head-to-tail dye aggregation. The optoelectronic consequences of indolizine incorporation in SM85 include raising the ground-state oxidation potential and broadening and red-shifting ultraviolet-visible-NIR absorptions, along with increased molar absorptivity when compared to the dye SM315. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations confirm the push-pull character of SM85, which features an overlap of frontier occupied and unoccupied orbitals. Steady-state spectrophotometric analyses reveal the presence of solution aggregates via absorption and emission spectroscopies. Aggregate modes were probed by DFT and TD-DFT analyses, and plausible models are presented. SM85-based DSC devices demonstrate a 5.7% power conversion efficiency (PCE) at full sun (7.4% PCE at 10% sun) with an exceptional improvement to the incident photon-to-current conversion onset at ∼850 nm. Current dynamics measurements, time-correlated single photon counting, and computational analyses are used to better understand device performances. This study puts forward a novel intramolecular charge-transfer porphyrin system with a dramatic shift into the NIR region, as is needed for nonprecious metal-based sensitizers, and provides an example of controlled, donor-acceptor-mediated aggregation as a complementary strategy to traditional donor-acceptor modifications to single-molecule π-systems in accessing enhancements in long wavelength light harvesting in molecular-based optoelectronic devices.

Nucleophilic Dearomatization of Activated Pyridines

Amongst nitrogen heterocycles of different ring sizes and oxidation statuses, dihydropyridines (DHP) occupy a prominent role due to their synthetic versatility and occurrence in medicinally relevant compounds. One of the most straightforward synthetic approaches to polysubstituted DHP derivatives is provided by nucleophilic dearomatization of readily assembled pyridines. In this article, we collect and summarize nucleophilic dearomatization reactions of - pyridines reported in the literature between 2010 and mid-2018, complementing and updating previous reviews published in the early 2010s dedicated to various aspects of pyridine chemistry. Since functionalization of the pyridine nitrogen, rendering a (transient) pyridinium ion, is usually required to render the pyridine nucleus sufficiently electrophilic to suffer the attack of a nucleophile, the material is organized according to the type of N-functionalization. A variety of nucleophilic species (organometallic reagents, enolates, heteroaromatics, umpoled aldehydes) can be productively engaged in pyridine dearomatization reactions, including catalytic asymmetric implementations, providing useful and efficient synthetic platforms to (enantioenriched) DHPs. Conversely, pyridine nitrogen functionalization can also lead to pyridinium ylides. These dipolar species can undergo a variety of dipolar cycloaddition reactions with electron-poor dipolarophiles, affording polycyclic frameworks and embedding a DHP moiety in their structures.

Development of type-I/type-II hybrid dye sensitizer with both pyridyl group and catechol unit as anchoring group for type-I/type-II dye-sensitized solar cell

A type-I/type-II hybrid dye sensitizer with a pyridyl group and a catechol unit as the anchoring group has been developed and its photovoltaic performance in dye-sensitized solar cells (DSSCs) is investigated. The sensitizer has the ability to adsorb on a TiO₂ electrode through both the coordination bond at Lewis acid sites and the bidentate binuclear bridging linkage at Brønsted acid sites on the TiO₂ surface, which makes it possible to inject an electron into the conduction band of the TiO₂ electrode by the intramolecular charge-transfer (ICT) excitation (type-I pathway) and by the photoexcitation of the dye-to-TiO₂ charge transfer (DTCT) band (type-II pathway). It was found that the type-I/type-II hybrid dye sensitizer adsorbed on TiO₂ film exhibits a broad photoabsorption band originating from ICT and DTCT characteristics. Here we reveal the photophysical and electrochemical properties of the type-I/type-II hybrid dye sensitizer bearing a pyridyl group and a catechol unit, along with its adsorption modes onto TiO₂ film, and its photovoltaic performance in type-I/type-II DSSC, based on optical (photoabsorption and fluorescence spectroscopy) and electrochemical measurements (cyclic voltammetry), density functional theory (DFT) calculation, FT-IR spectroscopy of the dyes adsorbed on TiO₂ film, photocurrent-voltage (I-V) curves, incident photon-to-current conversion efficiency (IPCE) spectra, and electrochemical impedance spectroscopy (EIS) for DSSC.

Panchromatic cross-conjugated π-bridge NIR dyes for DSCs

Four organic sensitizers incorporating a cross-conjugated cyclopenta[2,1-b:3,4-b']dithiophene (CPDT) π-bridge have been synthesized. As a result of molecular engineering, broad high energy bands and red shifted absorption maxima and onset is observed relative to a benchmark analogue (C218) using a non-cross-conjugated CPDT π-bridge. The use of a cross-conjugated bridge allows a new strategy for tuning dye energetics and introduction of increased absorption uniformity by adding additional high-energy absorption bands. These dyes show solar-to-electric conversion up to 800 nm with one derivative exceeding the performance of C218 under identical conditions.

Low-Recombination Thieno[3,4-b]thiophene-Based Photosensitizers for Dye-Sensitized Solar Cells with Panchromatic Photoresponses

We report four NIR photosensitizers employing a low-recombination donor and a thieno[3,4-b]thiophene (3,4-TT) π bridge for use in dye-sensitized solar cells. The inclusion of electron rich π spacers red-shifts the dye absorbance with solution absorption onsets reaching 700 nm. Dyes were found to have suitable energy levels for rapid electron transfers using cyclic voltammetry and UV/Vis-NIR absorption spectroscopy. Computationally optimized ground-state geometries show an increased torsional angle between π spacer and π bridge brought about by an added alkyl chain. This results in a widened optical band gap and increased oxidation potentials owing to a weakening of the electron-accepting ability of 3,4-TT for solution-state measurements. Interestingly in terms of device parameters, the alkylated π spacer had a nearly identical incident photon-to-current conversion efficiency (IPCE) curve onset when compared to a non-alkylated analogue, suggesting more similar dye geometries on the surface of TiO₂ . Elevated short-circuit current density (J_SC ) values and comparable open-circuit voltage (V_OC ) values were observed in the alkylated-π-spacer-dye-based devices with power conversion efficiencies up to 6.8 % observed with IPCE onsets exceeding 800 nm.

Effect of Donor Strength and Bulk on Thieno[3,4-b]-pyrazine-Based Panchromatic Dyes in Dye-Sensitized Solar Cells

Near-infrared-absorbing organic dyes are critically needed in dye-sensitized solar cells (DSCs). Thieno[3,4-b]pyrazine (TPz) based dyes can access the NIR spectral region and show power conversion efficiencies (PCEs) of up to 8.1 % with sunlight being converted at wavelengths up to 800 nm for 17.6 mA cm^-2 of photocurrent in a co-sensitized DSC device. Precisely controlling dye excited-state energies is critical for good performances in NIR DSCs. Strategies to control TPz dye energetics with stronger donor groups and TPz substituent choice are evaluated here. Additionally, donor size influence versus dye loading on TPz dyes is analyzed with respect to the TiO₂ surface protection designed to prevent recombination of electrons in TiO₂ with the redox shuttle. Importantly, the dyes evaluated were demonstrated to work well with low Li⁺ concentration electrolytes, with iodine and cobalt redox shuttle systems, and efficiently as part of co-sensitized devices.

Synthesis, optical and electrochemical properties, and photovoltaic performance of a panchromatic and near-infrared (D)2–π–A type BODIPY dye with pyridyl group or cyanoacrylic acid

(D)₂–π–A type BODIPY dyes bearing a pyridyl group or cyanoacrylic acid group and two diphenylamine–thienylcarbazole moieties which possess near-infrared adsorption ability as well as panchromatic adsorption ability, have been developed.

Synthesis and optical and electrochemical properties of julolidine-structured pyrido[3,4-b]indole dye

The julolidine-structured pyrido[3,4-b]indole dye ET-1 possesses the ability to act as a calorimetric and fluorescent sensor for Brønsted and Lewis acids.

Organic sensitizers featuring thiophene derivative based donors with improved stability and photovoltaic performance

Thiophene derivatives, including thieno[3,2-b][1]benzothiophene (TBT), benzo[b]thiophene (BT), 2-phenylthieno[3,2-b]thiophene (PTT) and 2-phenylthiophene (PT), have been introduced as donor for construction triarylamine organic dyes.

Impact of the molecular structure and adsorption mode of D-π-A dye sensitizers with a pyridyl group in dye-sensitized solar cells on the adsorption equilibrium constant for dye-adsorption on TiO2 surface

D-π-A dyes NI-4 bearing a pyridyl group, YNI-1 bearing two pyridyl groups and YNI-2 bearing two thienylpyridyl groups as the anchoring group on the TiO₂ surface have been developed as dye sensitizers for dye-sensitized solar cells (DSSCs), where NI-4 and YNI-2 can adsorb onto the TiO₂ electrode through the formation of the coordinate bond between the pyridyl group of the dye and the Lewis acid site (exposed Tiⁿ⁺ cations) on the TiO₂ surface, but YNI-1 is predominantly adsorbed on the TiO₂ electrode through the formation of the hydrogen bond between the pyridyl group of the dye and the Brønsted acid sites (surface-bound hydroxyl groups, Ti-OH) on the TiO₂ surface. The difference in the dye-adsorption mode among the three dyes on the TiO₂ surface has been investigated from the adsorption equilibrium constant (K_ad) based on the Langmuir adsorption isotherms. It was found that the K_ad values of YNI-1 and YNI-2 are higher than that of NI-4, and more interestingly, the K_ad value of YNI-2 is higher than that of YNI-1. This work demonstrates that that for the D-π-A dye sensitizers with the pyridyl group as the anchoring group to the TiO₂ surface the number of pyridyl groups and the dye-adsorption mode on the TiO₂ electrode as well as the molecular structure of the dye sensitizer affect the K_ad value for the adsorption of the dye to the TiO₂ electrode, that is, resulting in a difference in the K_ad value among the D-π-A dye sensitizers NI-4, YNI-1 and YNI-2.