Electron-acceptor Substituents Research Articles

Development of Donor-acceptor-type Molecules and Their Application as Analytical Reagents

π-Extended donor-acceptor (D-A)-type molecules, which bear both electron-donor and electron-acceptor substituents on the backbone, exhibit unique optical properties, such as bathochromic shifts in absorption and emission, large Stokes shifts, solvatochromic behavior, and fluorescence quenching in polar solvents. These unique properties are attributed to intramolecular charge transfer (ICT) or twisted intramolecular charge transfer (TICT) in the ground and excited states. This review article introduces three types of D-A-type molecules that are used as detection reagents for (1) methanol, (2) amino acids during solid-phase peptide synthesis (SPPS), and (3) amines present in the biological environment. For methanol detection, D-A-type fluorophores with basic guanidine moieties were developed to differentiate between methanol (MeOH) and ethanol (EtOH) based on the small difference in their pKa values (ΔpKa=0.4). Selective protonation of the guanidine moiety in methanol disrupts the D-A structure, allowing emission in the resultant polar environment. Similarly, an acid-base reaction between the hydrogen chloride (HCl) salts of the D-A-type molecules and amines is applied to detect amines during SPPS. In this method, a colorless solution of an HCl salt of the D-A-type molecule is deprotonated by amines, forming a yellow solution. This is the first reported quantitative and non-destructive colorimetric method for detecting amines. Finally, a turn-on-type amine-labeling reagent was developed for the nucleophilic aromatic substitution (SNAr) reaction. This new reagent enables protein staining of living cells with a large Stokes shift and without solvent-polarity-dependent fluorescence quenching.

SOLVENT AND SUBSTITUENT NATURE EFFECT ON THE 2-NITROAZOBENZENES HYDROGENATION SELECTIVITY ON SKELETAL NICKEL

The elucidation of the reasons and character of the influence of nature substituent and solvent composition on the kinetic features and selectivity of liquid-phase catalytic hydrogenation of compounds containing several reactive capable groups is a theoretical and practically significant task. In the presented work the process of 2-nitroazobenzene derivatives hydrogenation with various substituents was investigated in order to obtain the corresponding benzotriazoles on a skeletal nickel catalyst in a neutral azeotropic aqueous solution of 2-propanol, and in the presence of acetic acid or sodium hydroxide. According to the data obtained, the solvent composition has a decisive influence on the selectivity of this process. The transformation of azo- and nitro groups, included in the initial compound, proceeds in parallel stages, and the introduction of acidic or basic additives leads to а change in the rate ratio of these stages. Appropriately, selectivity of 2-nitroazobenzene derivatives hydrogenation changes with respect to reaction products containing a triazole ring, in particular, with respect to N-oxides of substituted benzotriazoles, and also with respect to nitrohydrazo compounds. The greatest influence of substituent nature on the hydrogenation selectivity in a solvent with any composition is manifested for the group whose transformation rate in a given medium is higher. In the presence of acid the substituent variation is more strongly reflected in the hydrogenation rate of azo group, and in the basic medium – of nitro group. It was found that the change in the transformation rate of both reactive capable groups corresponds to the Hammett constant alteration. The electron acceptor substituent has a deactivating effect on the transformation rate of nitro as well azo group. To increase the selectivity of substituted 2-nitroazobenzenes hydrogenation with respect to compounds containing a triazole ring, it is recommended to introduce a base into the aqueous solution of 2-propanol.

Vibrational analysis, frontier orbitals, optical and structural properties of 1, 2-dithiete and di-thiolene derivatives: A quantum mechanical study

The organic chemistry of sulfur-containing compounds has attracted much research due to its importance in biology, biochemistry, and various industries. 1, 2-Dithiete is a cyclic and unsaturated organo-sulfuric compound. This compound can be converted to di-thiolene (its valence isomer). The structure, electronic properties, and energy of two valence isomers, i.e. di-thiolene and 1,2-dithiete (III) were studied using density functional theory (DFT). Results showed that di-thiolene has two conformers (s-trans (I), s-cis (II)) and the s-trans is more stable. The electron donor and acceptor substituents on the carbon atoms increase the polarity difference between these two valence isomers. The dipole moment and polarizability of di-thiolenes were also greater than those of 1, 2-dithietes. Conversion of 1,2-dithiete to di-thiolene, structural properties, polarity, and polarizability of some derivatives of these two valence isomers were examined by quantum mechanical method at the level of B3LYP theory and 6-311++g (d, p) bases sets using Gaussian09 computational program. This conversion occurs through the s-cis conformation pathway and is an exothermic reaction. In the cases of ten derivatives of C2S2R2 (R = CH3, CF3, CN, OCH3, CH2CH3, t-Bu, Ph, Cyclopropyl, Cyclobutyl, and Cyclopentyl) the s-cis conformers are so unstable that they are not obtained as local structures. In general, s-trans di-thiolenes are more stable than their cyclic valence isomers (1,2-dithietes). Of course, the degree of stability depends on substituent identity.

Hexagonal 2D covalent organic frameworks from nonpolar and symmetric electron-accepting substituents for electron transport layers in near-infrared PeLEDs

High-throughput screening and NAMD calculations indicate that nonpolar and symmetric electron-absorbing substituents in 2D Tp-DAAQ can suppress e–h nonradiative recombination.

Preparation of hyperbranched polyglycerol groups modified by sulfonic acid tags supported on magnetized kappa-carrageenan: A sustainable solid acid for the synthesis of 1-amidoalkyl-2-naphthols

In this study, the magnetization of kappa-carrageenan (Carr) and its modification by hyperbranched polyglycerol-sulfonic acid tags (PGly-SO3H) was found to be a good strategy to fabricate a bio-inspired solid acid as a heterogeneous nanocatalyst for the synthesis of 1-amidoalkyl-2-naphthols from aldehydes, 2-naphthol, and acetamide. The catalyst was produced by embedding Fe3O4 nanoparticles into carrageenan shell and modifying the as-produced Fe3O4@Carr with PGly tags. Then, the Fe3O4@Carr was modified by chlorosulfonic acid (Cl-SO3H) to obtain the Fe3O4@Carr-PGly-SO3H nanoparticles. The morphology and structure of Fe3O4@Carr-PGly-SO3H nanoparticles were studied by FT–IR, XRD, FE-SEM, EDX-mapping, TEM, VSM, and TGA. Aromatic aldehydes with electron-donor/ electron-acceptor substituents, heterocyclic aldehydes, and enolizable aldehyde gave the corresponding 1-amidoalkyl-2-naphthols with 85–96 % isolated yields within 15–35 min, under solvent-free conditions. The acidic content of Fe3O4@Carr-pPGly-SO3H nanoparticles was found to be 3.5 mmol H+/g, which determined by titration. As shown by TEM images, the Fe3O4 core facilitates the efficient magnetic separation. The use of a benign support material, ease of catalyst preparation and recovery, heterogeneous nature (on the basis of hot-filtration experiment), solvent-free conditions, and reusability of the nanoparticles (5 times) are some of the green aspects of the present study.

Synthesis of New Structural Analogues of Natural Integrastatins with a Basic Epoxybenzo[7,8]oxocine Skeleton: Combined Experimental and Computational Study

AbstractIn this work, the cyclization reactivity of various 3-acetyl-2-methylpyridines (including 3-acetyl-2-methylquinoline) containing both electron donor and acceptor substituents with salicylaldehyde into epoxybenzooxocino[4,3-b]pyridine derivatives was studied. The reactions were carried out in mild (under room temperature or reflux in 2-propanol) and harsh (in a sealed glass ampoule) conditions. It was shown that 3-acetyl-2-methylpyridines with an aryl substituent in the 4-position do not react with salicylaldehyde either under normal convection heating conditions or under more severe conditions. This effect was explained by the steric hindrance of the substituents using quantum chemical calculations. It was found that electron donor substituents in 3-acetyl-2-methylpyridines significantly facilitate cyclization in epoxybenzooxocino[4,3-b]pyridines. The presence of electron acceptor substituents (NO2 group for example) in the 5-position of pyridine prevents cyclization under normal conditions, but gives a rather high conversion to oxocinopyridines under more specific conditions. This effect is quantum-chemically explained by the decrease in the basicity of pyridine. Pyridines with two pairs of methyl groups in ortho-positions to the acetyl group are capable to form mixtures of regioisomeric epoxybenzooxocinopyridines. Further, epoxybenzooxocinopyridines with methyl and acetyl groups can form a mixture of diastereomeric bisoxocins under more specific conditions. All 17 initial pyridines were studied quantum-chemically in order to understand what features of their structure and properties affect the success of the cyclization reaction and the yield of the target product. The pyridine molecules were calculated by the DFT RB3LYP/6-311++G(d,p) method taking into account the alcohol solvent within the CPCM model using Gaussian-2016 program. It was shown that the absence of steric hindrances in the form of bulky substituents in 4-position of pyridines is the main factor affecting the success of the cyclization reaction. Also, the yield of the target product is affected by the CH-acidity of the methyl group in 2-position, which, in turn, is affected by electron-donating and electron-withdrawing substituents in the 5- and 6-positions.

Influence of electron donor and acceptor substituents on tuning the nonlinear optical properties of zinc phthalocyanine derivatives.

Four phthalocyanine derivatives with different electron donor and acceptor substituents (B1, B2, B3, and B4) were synthesized by a solid-phase melting method. The influence of substituent type on the nonlinear optical properties of the materials was investigated in detail. In the case of similar conjugated structures, B3, which has amino electron-donor groups, presents high intramolecular charge transfer, a low energy gap (2.05 eV), and good nonlinear optical properties. Compared with B3, B4 has a larger π-conjugated structure and its energy gap is 0.04 eV smaller. Moreover, B4 has stronger reverse saturation absorption (7 × 10-12 m/W) and optical limiting performance. The four phthalocyanine derivatives exhibit third harmonic generation (THG) characteristics. Furthermore, the third harmonic strengths of B1, B2, B3, and B4 are 4 times, 9 times, 11 times, and 21 times that of SiO2, respectively. So, B4 has the best application potential in laser protection and frequency conversion.

Indolocarbazoles with Sterically Unrestricted Electron-Accepting Anchors Showcasing Aggregation-Induced Thermally Activated Delayed Mechanoluminescence for Host-Free Organic Light-Emitting Diodes.

We investigated the effects of sterically nonrestricted electron-accepting substituents of three isomeric indolocarbazole derivatives on their aggregation-induced emission enhancement, mechanochromic luminescence and thermally activated delayed fluorescence. The compounds are potentially efficient emitters for host-free organic light-emitting diodes. The films of indolocarbazole derivatives exhibit emissions with wavelengths of fluorescence intensity maxima from 483 to 500 nm and photoluminescence quantum yields from 31 to 58%. The ionization potentials of the solid samples, measured by photoelectron emission spectrometry, are in the narrow range of 5.78-5.99 eV. The electron affinities of the solid samples are in the range of 2.99-3.19 eV. The layers of the derivatives show diverse charge-transporting properties with maximum hole mobility reaching 10-4 cm2/Vs at high electric fields. An organic light-emitting diode with a light-emitting layer of neat compound shows a turn-on voltage of 4.1 V, a maximum brightness of 24,800 cd/m2, a maximum current efficiency of 12.5 cd/A and an external quantum efficiency of ca. 4.8%. When the compounds are used as hosts, green electroluminescent devices with an external quantum efficiency of ca. 11% are obtained. The linking topology of the isomeric derivatives of indolo[2,3-a]carbazole and indolo[3,2-b]carbazole and the electron-accepting anchors influences their properties differently, such as aggregation-induced emission enhancement, mechanochromic luminescence, thermally activated delayed fluorescence, charge-transporting, and electroluminescent properties. The derivative indolo[3,2-b]carbazole displays good light-emitting properties, while the derivatives of indolo[2,3-a]carbazole show good hosting properties, which make them useful for application in electroluminescent devices.

Highly Regioselective Synthesis of 3,6‐Diaryl‐2‐(het)arylpyridines by aza‐Diels‐Alder Reactions between 6‐H‐1,2,4‐Triazine Dienes and 2‐Amino‐4‐aryloxazole‐Based Dienophiles

AbstractA solvent‐free reaction between C6‐unsubstituted 5‐aryl‐1,2,4‐triazines and 2‐amino‐4‐aryloxazoles at high temperature was studied. The reaction proceeds via inverse electron demand aza‐Diels‐Alder reaction between the oxazole moiety as dienophile and the 1,2,4‐triazine moiety as diene with the highly regioselective formation of 3,6‐diarylpyridines in up to 60 % isolated yield. The structure of one of the key‐products was proven by the X‐ray diffraction analysis, and the mechanism of the transformation was proposed. The reaction proceeds well for both 6‐aryl and 6‐hetaryl‐1,2,4‐triazines in the presence of the electron acceptor substituent at the position 3. In the case of the donor substituent in position 3, the pyridine yields are significantly reduced. In the case of 5,6‐nonsubstituted 1,2,4‐triazines, the triazine cycle is degraded without product formation. The high regioselectivity of the reaction as well as solvent‐free conditions are valuable advantages of the present synthetic approach.

Density Functional Theory Study of the Secondorder Nonlinear Optical Properties of Novel Fluorenone Derivatives

Based on the molecular structure of novel fluorenone derivative named FO52, a series of new molecules have been designed by extending its π-conjugated bridge and introducing electron donor or acceptor substituents. The electronic transition and second-order non-linear optical response properties of these fluorenone derivatives were theoretically studied in detail by using the density functional theory computational methods. The results showed that the non-linear optical response of the molecule FO52 can be improved by introducing five-membered heterocycles into its skeleton structure. In addition, the introduction of strong substituents results in significant enhancement of the first hyperpolarizability of molecular nonlinear optical properties. These fluorenone derivatives could be treated as excellent candidates for nonlinear optical materials due to the narrow energy gap of its frontier molecular orbitals, distinct intramolecular charge transfer character and large first hyperpolarizabilities

Synthesis of 3-(Pyridin-2-yl)quinazolin-2,4(1H,3H)-diones via Annulation of Anthranilic Esters with N-pyridyl Ureas.

A new route for the synthesis of quinazolin-2,4(1H,3H)-diones and thieno [2,3-d]pyrimidine-2,4(1H,3H)-diones substituted by pyridyl/quinolinyl moiety in position 3 has been developed. The proposed method concluded in an annulation of substituted anthranilic esters or 2-aminothiophene-3-carboxylates with 1,1-dimethyl-3-(pyridin-2-yl) ureas. The process consists of the formation of N-aryl-N'-pyridyl ureas followed by their cyclocondensation into the corresponding fused heterocycles. The reaction does not require the use of metal catalysts and proceeds with moderate to good yields (up to 89%). The scope of the method is more than 30 examples, including compounds with both electron-withdrawing and electron-donating groups, as well as diverse functionalities. At the same time, strong electron-acceptor substituents in the pyridine ring of the starting ureas reduce the product yield or even prevent the cyclocondensation step. The reaction can be easily scaled to gram quantities.

Effects of electron-accepting substituents on the fluorescence of oxygen-bridged triarylamine

New electron-accepting oxygen-bridged triphenylamines were synthesized with nitro and dimesitylboryl substituents. Using optical and electrochemical characterization, the effect of electron-withdrawing substituents on quasi-planar triarylamine skeletons was clarified. Compared with the unbridged nitro-substituted triphenylamine reference, the oxygen-bridged structure featured a smaller optical gap, shifting the absorption and fluorescence wavelengths to longer wavelengths. Structural relaxation in the electronically excited state was suppressed, and the Stokes shift was reduced. Replacing the nitro substituent with a dimesitylboryl group resulted in an increase in the fluorescence quantum yield due to the suppression of the vibronic coupling.

Tautomeric Equilibrium in 1-Benzamidoisoquinoline Derivatives.

In this study, the tautomeric equilibrium of a sequence of 1-benzamidoisoquinoline derivatives was investigated with the tools of NMR spectroscopy and computational chemistry. The equilibrium between different tautomers in these systems could be controlled via the substitution effect, and the relative content of the amide form varied from 74% for the strong electron-donating NMe2 substituent to 38% for the strong electron-accepting NO2 group in the phenyl ring. In contrast to the previously investigated 2-phenacylquinoline derivatives, the most stable and thus most abundant tautomer in the 1-benzamidoisoquinoline series except the two most electron-accepting substituents was an amide. The intramolecular hydrogen bond present in the enol tautomer competed with the intermolecular hydrogen bonds created with the solvent molecules and thus was not a sufficient factor to favor this tautomer in the mixture. Although routinely computational studies of tautomeric equilibrium are performed within the continuum solvent models, it is proven here that the inclusion of the explicit solvent is mandatory in order to reproduce the experimental tendencies observed for this type of system, facilitating strong intermolecular hydrogen bonds.

Solvent-Controlled Photoswitching of Azobenzene: An Excited State Shuttle.

Light controlled excited state trans-cis isomerization process is a key to the development of conversion of light energy to mechanical motion at the molecular level. Considerable efforts have been made in tuning the isomerization process with electron donor and acceptor substituents by altering the excited state reaction coordinate. Here, we report a two novel push-pull series of para-diethylamino (DEA) and diphenylamino (DPA) substituted (E)-4'-((4-(diethylamino)phenyl)diazenyl)-N,N-diphenyl-[1,1'-biphenyl]-4-amine (1) and (E)-4'-((4-(diphenylamino)phenyl)diazenyl)-N,N-diphenyl-[1,1'-biphenyl]-4-amine (2). Compound 1 and 2 undergoes both photochemical and photophysical excited state deactivation pathways which can be controlled by varying the solvent polarity. These structural motifs of 1 and 2 would undergo torsional motions upon excitation to exhibit either trans → cis photoisomerization or to form a twisted intramolecular charge transfer state and both the process originates from the same excited state and competes with each other. Thus, alternations in the surrounding environment such as solvent polarity, solution viscosity, and protonation were employed to understand the preferential excited state deactivation pathway and thereby these systems could be employed as a new class of azobenzene-based luminescent photochromic molecules. For instance, in nonpolar solvent, toluene photoisomerization is preferred over TICT, but polar solvent, ethanol preferentially stabilizes the TICT state by virtue of N-C rotation which renders the energy barrier unfavourable for photoisomerization. The photostationary state of 1 in toluene is predominantly the Z isomer, whereas in ethanol E isomer is nearly two-fold higher than the Z isomer. These feature sets up a new approach towards the construction of multinary molecular switches and subsequent development in diverse fields.

Influence of crystal structure and composition on optical and electronic properties of pyridinium-based bismuth iodide complexes.

This study investigates the impacts of structure and composition on the optical and electronic properties of a series of pyridinium-based bismuth iodide complexes. Organic substrates with various functional groups, such as 4-aminopyridine (4-Ampy), 4-methylpyridine (4-Mepy), 4-dimethylaminopyridine (4-Dmapy), and 4-pyridinecarbonitrile (4-CNpy) with different electron-donating and electron-withdrawing groups at the para position of the pyridine ring were employed. Crystallographic analysis reveals various bismuth iodide structures, including 1D chains and discrete 0D motifs. The optical band gap of these materials, identified via diffuse reflectance spectroscopy (DRS) and verified with density functional theory (DFT) calculations, is influenced by the crystal packing and stabilising interactions. Through a comprehensive analysis, including Hirshfeld surface (HS) and void assessment, the study underscores the influence of noncovalent intermolecular interactions on crystal packing. Spectroscopic evaluations provide insights into electronic interactions, elucidating the role of electron donor and acceptor substituents within the lattice. Thermogravimetric differential thermal analysis (TG-DTA) indicates structural stability up to 250 °C. Linear sweep voltammetry (LSV) reveals significant conductivity in the range of 10-20 mS per pixel at 298.15 K. X-ray absorption spectroscopy (XAS) at the Bi L3 edge indicates a similar oxidation state and electronic environment across all samples, underscoring the role of bismuth centres surrounded by iodides.

Toward efficient functionalization of polystyrene backbone through ketene chemistry: Synthesis, characterization, and DFT study

AbstractIn this study, polystyrene was functionalized with Meldrum's acid toward the introduction of the ketenes (CCO) system to its backbone for producing a dramatically reactive intermediate. Meldrum's acid, as a ketene source, was reacted by poly(styrene‐co‐p‐chloromethyl styrene) through a simple nucleophilic reaction to synthesize poly(styrene‐co‐styryl Meldrum's acid). Then, the pendant Meldrum's acid under thermal treatment converted to ketene intermediate resulting in highly reactive polystyrenes derivatives, which rapidly reacted by nucleophilic reagents to afford ultimate organic building blocks. The polystyrene derivatives were characterized using elemental analysis, FT‐IR, high‐resolution solid‐state NMR, thermogravimetric analysis (TGA), and differential thermogravimetric analysis (DTG). To clarify the evolutionary mechanisms of polystyrene products, density functional theory (DFT) method B3LYP with the 6–311++G(2d,p) basis set was used. We studied the preparation of polystyrene model compounds through Meldrum's acid thermolysis and nucleophilic substitution. The kinetic and thermodynamic parameters in all reactions and the structural and electronic properties of all molecules were calculated. These data exhibited that based on Gibbs Free energy values, the structure of syndiotactic polystyrene is more stable than that of isotactic polystyrene. Furthermore, it was found that the presence of an electron donor or acceptor substituent on the polystyrene structure affects the electronic bandgap.

STUDY OF THE OZONE REACTION OF BENZYL ACETATE AND ITS DERIVATIVES IN THE LIQUID PHASE

Purpose. Investigation of the kinetic regularities of the oxidation reaction of benzyl acetate and its derivatives by ozone in the liquid phase to determine the effect of alcohol on the ozonation of methylbenzenes by the methyl group.
 Methodоlogy. A 20 ml glass vessel equipped with a magnetic stirrer was loaded sequentially with a calculated amount of toluene or benzyl acetate of sulfuric acid and 10 ml of acetic anhydride. After dissolution, the reaction components were loaded into a thermostatic glass column. The column was connected to a reflux condenser, the resulting solution was cooled to the required temperature and ozone-containing gas was passed through it. Oxidation was carried out to a given degree of substrate conversion. The process was controlled by gas-liquid chromatography. The ozone concentration in the gas phase was determined spectrophotometrically on an SF-46 spectrophotometer. The concentration of toluene and its derivatives in the solution was determined by gas-liquid chromatography on the chromatograph with a flame-ionization detector on the column with a length of 1 m and a diameter of 3.5 mm, filled with a carrier – INERTON-Super (0.16–0.2 mm), with a fixed phase “FFAP” applied to it in the amount of 5% of the mass of the carrier under the following conditions: evaporator temperature is between 503–533 K; carrier gas velocity (nitrogen) – 2.0 L•h-1; hydrogen velocity – 1.8 L•h-1; air velocity – 18 L•h-1. Thermostating was carried out in the temperature range of 363–448 K, the temperature rise was carried out by the program - 8оC/min.
 Findings. The kinetic parameters of the ozonation reaction of benzylacetate and its derivatives in the liquid phase were studied. The rate constants of ozone reaction with benzyl acetate at different temperatures and the influence of the structure of substituents on its reactivity were determined. It is shown that the presence of electron-acceptor substituents increases the selectivity of oxidation along the side chain to 54%, and electron-donor – to a decrease of up to 8%. The kinetics of the reaction of ozone with benzyl acetate at a temperature of 293 K is described by a second-order equation that assumes a non-chain oxidation mechanism. At elevated temperatures, the chain consumption of ozone becomes noticeable due to its reaction with the products of destruction of the aromatic ring.
 Originality. For the first time, the kinetics of the oxidation reaction of benzyl acetate and its substituted ozone in the liquid phase, which significantly affects the equilibrium concentration of alcohols in the oxidation of the corresponding methylbenzene. It was found that depending on the structure of acylated benzyl alcohol, the oxidation selectivity along the side chain is from 8 to 54%. The reaction rate constants are determined and the mechanism of ozonation of benzyl acetate is proposed, which will allow optimizing the parameters for the development of technological bases for the production of benzyl alcohol and its derivatives by ozonation of methylbenzenes in the liquid phase.
 Practical value. The results are the basis for the development of low-temperature method for the synthesis of benzyl alcohols used in the production of active pharmaceutical ingredients.

A combined QTAIM/IRI topological analysis of the effect of axial/equatorial positions of NH2 and CN substituents in the [(PY5Me2)MoO]+ complex

By means of the Interaction Region Indicator (IRI) and Quantum Theory of Atoms in Molecules (QTAIM), the influence exerted by NH2 (amino) and CN (cyano) as electron donor and electron acceptor substituent groups, respectively, located at para–positions of axial and equatorial pyridine rings of derivatized complexes coming from the [(PY5Me2)MoO]+ complex during the hydrogen molecular release in the gas phase was analyzed. In any case, a H–H covalent bond is forming at the transition state, with a strengthening of the electron density of 5.5% when the substituent group involved is NH2 at the para–position of the axial pyridine ring. However, there was no difference between NH2 and CN when these substituent groups are located at the para–positions of the equatorial pyridine rings. The topological properties of electron densities from the QTAIM are not perturbed by the electron donor and electron acceptor nature of the substituents, even when these substituent groups are located at the axial or equatorial pyridine rings of the Mo–based complex.

Ultrafast Excited State Dynamics of Forward and Reverse trans-cis Photoisomerization of Red-Light-Absorbing Indigo Derivatives.

Femtosecond time-resolved transient absorption (TRTA) spectroscopy was carried out to investigate the ultrafast excited state dynamics of both trans → cis and trans ← cis photoisomerization of red-light-absorbing indigo derivatives. For N,N'-bis(tert-butyloxycarbonylmethyl)indigo (tBOMI), the excited state lifetime of the trans-form was measured to be 41 ps while that of the cis-form was as short as 730 fs in acetonitrile (Acn). The excited state lifetime of trans-N,N'-dimethylindigo (DMI) in Acn was also measured to be as short as 10 ps. These values are much shorter than those of the blue-light-absorbing trans-forms of indigo derivatives such as N,N'-diacetylindigo (DAI) and thioindigo (ThI). The chromophore of indigo is composed of two pairs of electron donor and acceptor substituents conjugated in the shape of a letter "H" (so-called "H-chromophore"), although DFT and TDDFT calculations suggest that the charge transfer (CT) character is not very significant. Nevertheless, when a weak CT within the H-chromophore is promoted, the absorption band shifts to longer wavelengths and the excited state lifetime shortens. For the photoisomerization of DAI and ThI, a relatively long excited state lifetime is required for the photoisomerization, while for tBOMI and DMI, a vibrationally hot ground state that overcomes the energy barrier in the ground state is produced by rapid nonradiative decay through conical intersection. In the case of cis-tBOMI, the repulsion between the two adjacent negatively charged carbonyl groups and the weakening of the central C═C double bond in the S1 state twist the molecule, shorten the excited state lifetime, and increase the quantum yield of the trans ← cis photoisomerization.

Tuning the antiaromatic character and charge transport of pentalene-based antiaromatic compounds by substitution

Substituting electron-donating and electron-accepting substituents on antiaromatic pentalene cores can significantly tune the optical absorption, energy levels, antiaromaticity, and transistor switch-on behavior.