A comprehensive analysis of charge transfer effects on donor-pyrene (bridge)-acceptor systems using different substituents

The alternant polycyclic aromatic hydrocarbon pyrene has photophysical properties that can be tuned with different donor and acceptor substituents. Recently, a D (donor)-Pyrene (bridge)-A (acceptor) system, DPA, with the electron donor N,N-dimethylaniline (DMA), and the electron acceptor trifluoromethylphenyl (TFM), was investigated by means of time-resolved spectroscopic measurements (J. Phys. Chem. Lett. 2021, 12, 2226-2231). DPA shows great promise for potential applications in organic electronic devices. In this work, we used the ab initio second-order algebraic diagrammatic construction method ADC(2) to investigate the excited-state properties of a series of analogous DPA systems, including the originally synthesized DPAs. The additionally investigated substituents were amino, fluorine, and methoxy as donors and nitrile and nitro groups as acceptors. The focus of this work was on characterizing the lowest excited singlet states regarding charge transfer (CT) and local excitation (LE) characters. For the DMA-pyrene-TFM system, the ADC(2) calculations show two initial electronic states relevant for interpreting the photodynamics. The bright S1 state is locally excited within the pyrene moiety, and an S2 state is localized ~0.5 eV above S1 and characterized as a donor to pyrene CT state. HOMO and LUMO energies were employed to assess the efficiency of the DPA compounds for organic photovoltaics (OPVs). HOMO-LUMO and optical gaps were used to estimate power conversion and light-harvesting efficiencies for practical applications in organic solar cells. Considering the systems using smaller D/A substituents, compounds with the strong acceptor NO2 substituent group show enhanced CT and promising properties for use in OPVs. Some of the other compounds with small substituents are also found to be competitive in this regard.

Effect of Substituent Groups on the Strength of Intramolecular Hydrogen Bonds in 2,4-Dihydroxybenzophenone UV Absorbers

2,4-Dihydroxybenzophenone is the most widely used molecule in the benzophenone group of UV absorbers. It is known that the UV absorption ability is dependent on the substituents. Numerous studies have shown that the strength of intramolecular hydrogen bonds is the main factor affecting this type of UV absorber. However, the effect of substituents on the formation and nature of the hydrogen bonds has not been well studied. In this work, the effect of the type of substituent and the substitution position on the absorption intensity of 2,4-dihydroxybenzophenone molecules is verified both experimentally and theoretically. The effect of substituents on the intramolecular hydrogen bonding of 2,4-dihydroxybenzophenone was investigated by DFT calculations. The results indicate that the addition of different substituents leads to various changes in the strength of the hydrogen bonding in 2,4-dihydroxybenzophenone. On the X-substitution site or the Y-substitution site, halogen groups and electron-absorbing groups such as -CN and -NO₂ increase the strength of the hydrogen bond, while electron-giving groups such as -N(CH₃)₂ and -OCH₃ decrease the strength of the bond. For the same substituent, the one at the Y site has a higher effect on hydrogen bonding than that at the X site. By NBO analysis, it was found that the substituents would cause charge redistribution of the individual atoms of 2,4-dihydroxybenzophenones, thus affecting the formation and strength of the hydrogen bonds. Moreover, when the substituent is at the Y substitution site, the oxygen atom of the carbonyl group is less able to absorb electrons and more charge is attracted to the oxygen atom of the hydroxyl group, resulting in a larger charge difference between the two oxygen atoms and an increase of bond energy. Finally, a multiple linear regression analysis of the NPA charge number of the atoms involved in the formation of the hydrogen-bonded chelated six-membered ring was performed with the energy of the hydrogen bond and the percentage of influencing factors estimated, which were found to jointly affect the strength of hydrogen bonding. The aim of this study is to provide theoretical guidance for the design of benzophenone-based UV absorbers that absorb UV light of specific wavelength bands.

Triel Bonds with Au Atoms as Electron Donors

The novel triel bonds of BX₃ (X=H, F, Cl, Br, and I) and C₅ H₅ B as electron acceptors and AuR₂ (R=Cl and CH₃ ) as an electron donor were explored. The triel bond is a primary driving force for most complexes, while the contribution from a halogen-chlorine interaction in BX₃ -AuCl₂ (X=Cl, Br, and I) and an iodine-Au interaction in BI₃ -Au(CH₃ )₃ is also very important. Interestingly, the positively charged Au atom of AuCl₂ can attractively bind with the holes of BX₃ and C₅ H₅ B. The interaction energy lies in the range of 1 and 80 kcal/mol, in the order X=F<H<Cl<Br<I. In most cases, the triel bond of C₅ H₅ B is stronger than the triel bond of BX₃ . In the formation of B-Au triel bond, electrostatic energy is not dominant, while polarization energy including orbital interaction has the largest contribution for the strongly bonded complexes and dispersion energy for the weak triel bond.

Tuning the pH of Activation of Fluorinated Hydrazone-Based Switches─A Pathway to Versatile 19F Magnetic Resonance Imaging Contrast Agents

Molecular switches have become an area of great interest in recent years. They are explored as high-density data storage and organic diodes in molecular electronics as well as chemosensors due to their ability to undergo a transition between well-defined structures under the action of external stimuli. One of the types of such switches is hydrazones. They work by changing the configuration from E to Z under the influence of pH or light. The change in configuration is accompanied by a change in the absorption band and changes in the nuclear magnetic resonance (NMR) spectrum. In this publication, the structure-property relationship of fluorinated hydrazone switches was established. A linear relationship between the Hammett substituent constants and the pH where the switching occurs was found. Introduction of strong electron-donating groups allowed obtaining a hydrazone switch of pK_a = 6 suitable for application in ¹⁹F MRI as contrast agents.

Structure-Activity Relationships of Pyrimidine Derivatives and their Biological Activity - A Review

BACKGROUND

Heterocycles play a major role in many fields of biochemical and physiological such as amino acids, DNA bases, vitamins, endogenous neurotransmitters, etc. Nitrogencontaining heterocyclic compounds play a vital role in medicinal chemistry and exhibit notable biological and pharmacological activities. In the past two decades, scientists focused more on the diverse biological activities of pyrimidine derivatives. Pyrimidine is a six-membered heterocyclic compound, and it is present naturally in nucleic acid components (uracil, thymine, and cytosine) and vitamin B1; it is a promising lead molecule for synthesising compounds with various substitutions to treat various diseases.

OBJECTIVE

We focused on the structure-activity relationship of pyrimidine derivatives and its various biological activities reported from 2010 to date.

RESULTS

From this review, we concluded that the position of substituents in the pyrimidine nucleus greatly influences biological activities. Thus, the pyrimidine nucleus showed anti-microbial, anticancer, anti-inflammatory, anti-tubercular, anti-convulsant, antihypertensive, anthelmintic, antidepressant, analgesic, anti-hyperglycemic activities, etc. Conclusion: This study provides an overview of the pyrimidine nucleus and its derivatives from 2010 to date. There is a future scope for identifying a lead molecule for the target biological activity.

Substituent-mediated quantum interference toward a giant single-molecule conductance variation

Quantum interference (QI) in single molecular junctions shows a promising perspective for realizing conceptual nanoelectronics. However, controlling and modulating the QI remains a big challenge. Herein, two-type substituents at different positions ofmeta-linked benzene, namely electron-donating methoxy (-OMe) and electron-withdrawing nitryl (-NO₂), are designed and synthesized to investigate the substituent effects on QI. The calculated transmission coefficientsT(E) indicates that -OMe and -NO₂could remove the antiresonance and destructive quantum interference (DQI)-induced transmission dips at position 2. -OMe could raise the antiresonance energy at position 4 while -NO₂groups removes the DQI features. For substituents at position 5, both of them are nonactive for tuning QI. The conductance measurements by scanning tunneling microscopy break junction show a good agreement with the theoretical prediction. More than two order of magnitude single-molecule conductance on/off ratio could be achieved at the different positions of -NO₂substituent groups at room temperature. The present work proves chemical substituents can be used for tuning QI features in single molecular junctions, which provides a feasible way toward realization of high-performance molecular devices.

Tetrel Bonds between Phenyltrifluorosilane and Dimethyl Sulfoxide: Influence of Basis Sets, Substitution and Competition

Ab initio calculations have been performed for the complexes of DMSO and phenyltrifluorosilane (PTS) and its derivatives with a substituent of NH₃, OCH₃, CH₃, OH, F, CHO, CN, NO₂, and SO₃H. It is necessary to use sufficiently flexible basis sets, such as aug’-cc-pVTZ, to get reliable results for the Si···O tetrel bonds. The tetrel bond in these complexes has been characterized in views of geometries, interaction energies, orbital interactions and topological parameters. The electron-donating group in PTS weakens this interaction and the electron-withdrawing group prominently strengthens it to the point where it exceeds that of the majority of hydrogen bonds. The largest interaction energy occurs in the p-HO₃S-PhSiF₃···DMSO complex, amounting to −122 kJ/mol. The strong Si···O tetrel bond depends to a large extent on the charge transfer from the O lone pair into the empty p orbital of Si, although it has a dominant electrostatic character. For the PTS derivatives of NH₂, OH, CHO and NO₂, the hydrogen bonded complex is favorable to the tetrel bonded complex for the NH₂ and OH derivatives, while the σ-hole interaction prefers the π-hole interaction for the CHO and NO₂ derivatives.

Benzimidazole Derivatives as Energetic Materials: A Theoretical Study

The explosive properties and stability of benzimidazole compounds are studied to determine the influence of substituents and their position. The results obtained reveal the conjugation of substituents as one of the crucial factors for the thermal stability of these compounds. We also found that two -CH₃ substituents increase the thermal stability of the parent compound, while nitro groups decrease it. Moreover, the study clearly exhibits that the combination of an -NO₂ substituent with -CH₃ does not change the stability of the benzimidazole. On the other hand, nitro groups increase the chemical stability and explosive properties of the compounds under investigation, but their sensitivity could not fully satisfy the requirements of their safety and increase their toxicity. The main results of the study indicate that high thermal and chemical stability, low toxicity and sensitivity, and good explosive properties could be achieved by the precise combination of nitro, -CH₃, and triazole ring substituents. These findings are very important for the design of new, effective, and non-sensitive explosives.

The Effects of Different Catalysts, Substituted Aromatic Aldehydes on One-Pot Three-Component Biginelli Reaction

AIM AND OBJECTIVE

The Biginelli reaction, first reported in 1893, is one great example of the important multicomponent reactions reported from 1893. Under the same conditions, the influence of the common catalysts on the yield of the Biginelli reaction was investigated.

MATERIALS AND METHOD

To a round-bottom flask equipped with a spherical condenser were added 1,3- dicarbonyl compound (1.0 eq), urea (1.45 eq), aromatic aldehyde (1.0 eq), catalyst and methanol. The mixture was heated at reflux for 16 h. After cooling off, the mixture was filtered and washed with cold methanol to give DHPMs. Reaction solution was further purified by recrystallization with petroleum ether and ethyl acetate. Six catalytic systems, different 1,3-dicarbonyl compounds and different substituted aromatic aldehydes with varied substitutions are described for the Biginelli reaction. An analysis was also performed to study the factors that affect the yield of the reaction.

RESULTS

When 1,3-dicarbonyl compound was ethyl acetoacetate, the CuCl/ conc.H2SO4 system gave the highest yield (90.5%). While when acetoacetamide was used, the yields of DHPMs in presence of PTSA/conc. HCl, conc. HCl or FeCl3•6H2O were all over 90%. Nine DHPMs with different substituents were obtained.

CONCLUSION

The Lewis acid or mixed catalyst had no significant advantage over a single protonic acid as catalyst. Conc. HCl as the catalyst was found to be the most effective condition for the preparation of DHPMs. The aromatic aldehyde with weak electron-withdrawing substituent such as Br resulted in the best yield.

Coinage-Metal Bond between [1.1.1]Propellane and M2/MCl/MCH3 (M = Cu, Ag, and Au): Cooperativity and Substituents

A coinage-metal bond has been predicted and characterized in the complexes of [1.1.1]propellane (P) and M₂/MCl/MCH₃ (M = Cu, Ag, and Au). The interaction energy varies between −16 and −47 kcal/mol, indicating that the bridgehead carbon atom of P has a good affinity for the coinage atom. The coinage-metal bond becomes stronger in the Ag < Cu < Au sequence. Relative to M₂, both MCl and MCH₃ engage in a stronger coinage-metal bond, both -Cl and -CH₃ groups showing an electron-withdrawing property. The formation of coinage-metal bonding is mainly attributed to the donation orbital interactions from the occupied C-C orbital into the empty metal orbitals and a back-donation from the occupied d orbital of metal into the empty C-C anti-bonding orbital. In most complexes, the coinage-metal bond is dominated by electrostatic interaction, with moderate contribution of polarization. When P binds simultaneously with two coinage donors, negative cooperativity is found. Moreover, this cooperativity is prominent for the stronger coinage-metal bond.

Blue TADF Emitters Based on Indenocarbazole Derivatives with High Photoluminescence and Electroluminescence Efficiencies

A series of blue thermally activated delayed fluorescence (TADF) emitters were designed and synthesized using 2,4,6-triphenyl-1,3,5-triazine as the acceptor unit and indenocarbazole derivatives as the electron-donating moiety. In contrast with other six-membered heterocycles, like phenothiazine, phenoxazine, and dihydroacridine, where the TADF efficiency is affected by the presence of different conformers, indenocarbazole derivatives do not show this effect. Therefore, InCz23FlTz, InCz23DPhTz, InCz23DMeTz, and InCz34DPhTz allow the investigation of the effect of different substituents and substitution positions on TADF properties, without the influence of different conformations. We have demonstrated that the substituted position on the carbazole and different substituents in the same position have clear influence on the donor character of indenocarbazole derivatives. Also, the color purity of blue emission and excited states could be adjusted by substituents and substituted position, and thus excellent blue emitters can be obtained. Besides, the four compounds show relatively small TADF contribution under optical excitation; however, excellent performances are obtained in the electroluminescent devices, especially with InCz34DPhTz, which shows a maximum external quantum efficiency of around 26%. In the end, we find an effective way to design high-efficiency blue TADF materials and deeply study the relation between the structure and property in indenocarbazole derivatives.

Probing the Effects of the Number and Positions of -OCH3 and -CN Substituents on Color Tuning of Ir(III) Complex Derivatives through a Joint Computational and Experimental Study

We performed a joint theoretical and experimental study on sixteen Ir(III) complexes bearing a similar molecular platform of bis(2-phenylbenzothiozolato-N,C^2' ) iridium(III) (acetylacetonate) by grafting -OCH₃ group and/or -CN group on different positions of the C-related arene moiety of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>C</mml:mi> <mml:mspace/> <mml:mover><mml:mspace/> <mml:mo>^</mml:mo></mml:mover> <mml:mspace/> <mml:mi>N</mml:mi></mml:mrow> </mml:math> ligand (C-ring). Our results reveal that the introduction of -CN renders an overall drop in the FMO energy levels while a reverse increase is observed for -OCH₃ . The ortho- and para-sites of the C-ring are more effective substitution positions to modulate the HOMO energy level due to the fact that the electronic density of HOMO mainly locates at them while the meta-site would induce a stronger impact on LUMO since the electronic density of LUMO mainly distributes over the position. Utilizing the synergistic effects of the substituents and the substituted positions, a wide color-tuning range from 479 nm to 637 nm was achieved, which covers nearly the whole window of visible spectrum. In particular, the tri-substituted Ir35mo4cn complex (λ_em max =637 nm) may be a potential candidate for high efficiency red OLEDs materials due to its greatly enhanced absorption processes, relatively higher ³ MLCT (%), lower ΔE_S1-T1 , enlarged separation between ³ MLCT/π-π* and ³ MC d-d states, and good hole and particle-transporting performances. Finally, six representative complexes were synthesized and their spectra were determined, which confirm the reliability of our computational strategy.

Tetrel Bond between 6-OTX₃-Fulvene and NH₃: Substituents and Aromaticity

Carbon bonding is a weak interaction, particularly when a neutral molecule acts as an electron donor. Thus, there is an interesting question of how to enhance carbon bonding. In this paper, we found that the –OCH₃ group at the exocyclic carbon of fulvene can form a moderate carbon bond with NH₃ with an interaction energy of about −10 kJ/mol. The –OSiH₃ group engages in a stronger tetrel bond than does the –OGeH₃ group, while a reverse result is found for both –OSiF₃ and –OGeF₃ groups. The abnormal order in the former is mainly due to the stronger orbital interaction in the –OSiH₃ complex, which has a larger deformation energy. The cyano groups adjoined to the fulvene ring not only cause a change in the interaction type, from vdW interactions in the unsubstituted system of –OCF₃ to carbon bonding, but also greatly strengthen tetrel bonding. The formation of tetrel bonding has an enhancing effect on the aromaticity of the fulvene ring.

Regulating the Optoelectronic Properties of Nickel Dithiolene by the Substituents: A Theoretical Study

Dithiolene-based complexes show great potential to be applied as materials for organic optoelectronic devices. In this study, we theoretically designed a series of complexes based on nickel dithiolene and its substituted derivatives, the optoelectronic properties of which were comparatively studied by density functional theory (DFT)/time-dependent density functional theory (TD-DFT). The results show that the charge injection property of nickel dithiolene complexes can be significantly improved with introduction of electron-withdrawing groups. The charge transportation property of nickel dithiolene depends on the conjugation degree of the system. The energy gaps between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are determined by the substituents, which makes the maximum absorption wavelength red-shift from the visible to the near-infrared (NIR) region. The electron density difference graph shows that the electron transition from the ground state to the first excited state is assigned to π-π* transition mainly from HOMO to LUMO. The regularity of substituent effect revealed by us in this study will shed light on the application of nickel dithiolenes as potential optoelectronic materials.

Subnaphthalocyanines as Electron Acceptors in Polymer Solar Cells: Improving Device Performance by Modifying Peripheral and Axial Substituents

A new class of subnaphthalocyanines bearing various peripheral and axial substituents have been synthesized for use as electron acceptors in solution-processed bulk-heterojunction polymer solar cells. The resulting solar cells exhibit modest photovoltaic performance with contributions from both the polymer donor and subnaphthalocyanine acceptor to the photocurrent.

Promoting and Tuning Porosity of Flexible Ether-Linked Phthalazinone-Based Covalent Triazine Frameworks Utilizing Substitution Effect for Effective CO2 Capture

Five porous ether-linked phthalazinone-based covalent triazine frameworks (PHCTFs) were successfully constructed via ionothermal polymerizations from flexible dicyano monomers containing asymmetric, twisted, and N-heterocyclic phthalazinone structure. All the building blocks could be easily prepared by simple and low-cost aromatic nucleophilic substitution reactions, showing the large-scale application potential of thermal stable phthalazinone structure in constructing porous materials. Generally, the flexible building blocks are avoided to prevent the networks from collapsing in constructing high surface area porous materials. Our experimental results revealed that the introduction of the substituents can effectively decrease the probability of the network interpenetration from the longer struts and the intermolecular/intramolecular intercalation from the increased degree of conformation freedom in the flexible ether-linkage, the BET surface areas of PHCTFs increasing from 676 to 1270 m² g^-1. Meanwhile, the effects of introducing different sizes (methyl or phenyl group) and amounts (one or two) of substituents on the porosities of the target polymer networks were also investigated in detail. The high CO₂ adsorption capacity of 10.3 wt % (273 K, 1 bar) can be ascribed to the strong affinity of the electron-rich N,O-containing networks with CO₂. Excitingly, PHCTF-5 demonstrates the high CO₂/N₂ selectivity up to 138 (273 K, 1 bar), according to the ideal adsorbed solution theory (IAST) for the higher proportion of V_micro accompanied the electron-rich heteroatoms characteristic. Such high CO₂ adsorption capacity and good separation properties are superior to those of many other microporous organic polymers. These properties along with easily up-scalable synthesis make porous PHCTFs promising candidates applied in gas sorption and separation field.

Isolation and characterization of quinine from Polygonatum verticillatum: A new marker approach to identify substitution and adulteration

Polygonatum verticillatum (Mahameda) is an important ingredient of Ashtawarga and Ayurvedic formulations. Nowadays, it comes under the category of endangered plants due to large scale and indiscriminate collection of wild material. To overcome the scarcity, substitutes of Mahameda are also commonly used in market. These additives are different from the authentic plant by Ayurvedic and pharmacological theory of drug action, thereby resulting in substitution/adulteration. Substitution is a critical issue in isolation and quantification of the therapeutically active ingredients that can be used as markers in the identification of substitution/adulteration. Methanolic extract of the rhizomes of P. verticillatum was subjected to preliminary phytochemical screening for the detection of phytoconstituents, followed by column chromatography for isolation of the marker. The column was first eluted with pure hexane, and polarity of the solvent was gradually increased. A total of 1180 fractions were collected and pooled on the basis of thin-layer chromatography profile. The single compound was isolated and confirmed by chemical test, melting point, spectral analysis, and comparison with literature. Phytochemical screening of the extract shows the presence of alkaloids, flavonoids, carbohydrates, terpenoids, and phenolics. A pure white crystalline powder was isolated by column chromatography which was characterized as (6-methoxyquinolin-4-yl-8-vinylquinuclidin-2-yl) methanol, i.e. Quinine. The isolated compound, Quinine, was identified as a novel compound in Mahameda as it has not been reported in the genus Polygonatum, till now. It can be used as a marker for the identification of substitution/adulteration and standardization of P. verticillatum.

Dioxane dibromide mediated bromination of substituted coumarins under solvent-free conditions

An efficient solvent-free protocol for regioselective bromination of substituted coumarins has been developed by using dioxane dibromide as the solid brominating agent. The efficacy of the solvent-free protocol has been established. The effects of the electronic nature and location of the substituents on the outcome of the reaction have been rationalized with a proposed mechanism.