Ionization of Cyclic Aromatic Amines by Free Electron Transfer: Products Are Governed by Molecule Flexibility

Luminescence feature of new 3,6-di(thiazolidin-5-one-2-yl)-carbazole derivative: synthesis, photophysical properties, density functional theory studies, and crystal shape effect

A new carbazole chromophore conjugated with substituted thiazolidine-4-one (CzPT) was synthesized by applying the Knoevenagel reaction between 3,6-diformyl-N-hexylcarbazole and ethyl 2-aceto-2-(5-oxo-3-phenylthiazolidin-2-ylidene)acetate. The chemical structure of the new derivative (CzPT) was elucidated by spectral studies. The CzPT absorption spectra in different solvents exhibited a red shift for λ_max by increasing solvent polarity. Bands at 430-474 nm appeared and were attributed to intramolecular charge transfer with high π-π* characteristics. CzPT fluorescence spectra exhibited a red shift after increasing the solvent polarity. To understand the Stokes' shift ( ∆ ν ¯ ) behaviour of the CzPT derivative referring to the polarity of solvents, Lippert-Mataga and linear solvation-energy relationship (LSER) models were employed in which the LSER exhibited respectable results compared with Lippert-Mataga (r² = 0.9707). Moreover, time-dependent density functional theory absorption spectra in hexane and dimethylformamide showed that λ_max had a major contribution in the highest occupied molecular orbital to lowest unoccupied molecular orbital transition in both solvents. In addition, the reduced uniformity of crystal features may lead to dislocation or anomalous arrangement of crystals with irregular spacing, which automatically enhances the optical properties of such crystals.

Direct synthesis of 2-oxo-acetamidines from methyl ketones, aromatic amines and DMF via copper-catalyzed C(sp3)-H amidination

A convenient method for the synthesis of 2-oxo-acetamidines from methyl ketones using aromatic amines and DMF as nitrogen sources is reported via copper-catalyzed C(sp3)-H amidination. Various methyl ketones react readily with aromatic amines and DMF, producing 2-oxo-acetamidines in yields of 47 to 92%. This protocol features the simultaneous formation of C-N and C[double bond, length as m-dash]N bonds using DMF and aromatic amines as two different nitrogen sources. It thus provides an efficient approach to construct acyclic amidines via three C(sp3)-H bond amidination. Based on the preliminary experiments, a plausible mechanism of this transformation is disclosed.

para-Aminothiophenol Radical Reaction-Functionalized Gold Nanoprobe for One-to-All Detection of Five Reactive Oxygen Species In Vivo

Five major reactive oxygen species (ROS) are generated in diseases including H₂O₂, ^•OH, O₂^•-, ROO^•, and ¹O₂. Simultaneous detection of the five ROS with a single probe is crucial for a comprehensive understanding of the development and progression of many diseases, such as cancer and inflammatory diseases. However, currently reported detection systems are limited by targeting one ROS with one probe. This one-to-one detection mode may fail to sufficiently unveil the diseased state. In this study, we achieved simultaneous detection of all the five ROS with one probe (i.e., one-to-all detection), by designing a novel para-aminothiophenol (PATP) and hemin-decorated gold (Au/PATP/Hemin) nanoprobe. The design is principled by our discovery that PATP can react with ^•OH, O₂^•-, ROO^•, and ¹O₂ by a radical oxidative coupling mechanism to form 4,4'-dimercaptoazobenzene (DMAB). The DMAB then elicited strong characteristic surface-enhanced Raman scattering (SERS) peaks at 1142, 1386, and 1432 cm^-1; which in turn enables direct detection of ^•OH, O₂^•-, ROO^•, and ¹O₂ and indirect detection of H₂O₂ by hemin-catalyzed fenton reaction to convert H₂O₂ into ^•OH. In two representative ROS-elevated mice models of tumors and allergic dermatitis, the Au/PATP/Hemin nanoprobe demonstrated its robust performance of monitoring tumor development and inflammation progression in a highly sensitive and quantitative manner.

Mechanism of the Dehydrogenative Phenothiazination of Phenols

The straightforward capture of oxidized phenothiazines with phenols under aerobic conditions represents a unique cross-dehydrogenative C-N bond-forming reaction in terms of operational simplicity. The mechanism of this cross-dehydrogenative N-arylation of phenothiazines with phenols has been the object of debate, particularly regarding the order in which the substrates are oxidized and their potentially radical or cationic nature. Understanding the selective reactivity of phenols for oxidized phenothiazines is one of the key objectives of this study. The reaction mechanism is investigated in detail by utilizing electron paramagnetic resonance spectroscopy, cyclic voltammetry, radical trap experiments, kinetic isotope effects, and solvent effects. Finally, the key reaction steps are calculated by using density functional theory (DFT) and broken-symmetry open-shell singlet DFT methods to unravel a unique biradical mechanism for the oxidative phenothiazination of phenols.

Sulfur rich electron donors - formation of singlet versus triplet radical ion pair states featuring different lifetimes in the same conjugate

An unprecedented family of novel electron-donor acceptor conjugates based on fullerenes as electron acceptors, on one hand, and triphenyl amines as electron donors, on the other hand, have been synthesized and characterized in a variety of solvents using steady state absorption/emission as well as transient absorption spectroscopy. These are unprecedented in terms of their outcome of radical ion pair formation, that is, the singlet versus triplet excited state. This was corroborated by femto/nanosecond pump probe experiments and by molecular orbital calculations. Not only has the donor strength of the triphenylamines been systematically altered by appending one or two sulfur rich dithiafulvenes, but the presence of the latter changed the nature of the radical ion pair state. Importantly, depending on the excitation wavelength, that is, either where the fullerenes or where the triphenylamines absorb, short-lived or long-lived radical ion pair states, respectively, are formed. The short-lived component with a lifetime as short as 6 ps has singlet character and stems from a fullerene singlet excited state precursor. In contrast, the long-lived component has a lifetime of up to 130 ns in THF, has triplet character, and evolves from a triplet excited state precursor. Key in forming more than three orders of magnitude longer lived radical ion pair states is the presence of sulfur atoms, which enhance spin-orbit coupling and, in turn, intersystem crossing. Independent confirmation for the singlet versus triplet character came from temperature dependent measurements with a focus on the radical ion pair state lifetimes. Here, activation barriers of 2.4 and 10.0 kJ mol^-1 for the singlet and triplet radical ion pair state, respectively, were established.

Oxidative coupling of anilines to azobenzenes using heterogeneous manganese oxide catalysts

Cryptomelane-type manganese oxide shows high efficiency for the NN formation reaction in the oxidative coupling of anilines to azobenzenes. The difference of the Hammett constants (Δσ) of two substituted groups determines the selectivity to unsymmetric azobenzenes in cross-coupling reactions, which are favored at Δσ < 0.32.

Ionic liquid effect: selective aniline oxidative coupling to azoxybenzene by TiO2

TiO₂ promotes the oxidative catalytic coupling of anilines to diazo compounds. In ionic liquids, azoxybenzene is formed in high selectivity, whereas in toluene azobenzene is formed almost exclusively.

Novel carbazole-based two-photon photosensitizer for efficient DNA photocleavage in anaerobic condition using near-infrared light

Novel carbazole derivatives are first reported as two-photon photosensitizers for DNA photodamage under near-infrared light exposure.

Oxidative dehydrogenative couplings of pyrazol-5-amines selectively forming azopyrroles

New oxidative dehydrogenative couplings of pyrazol-5-amines for the selective synthesis of azopyrrole derivatives have been described. The former reaction simultaneously installs C-I and N-N bonds through iodination and oxidation, whereas the latter involved a copper-catalyzed oxidative coupling process. The resulting iodo-substituted azopyrroles were employed by treatment with various terminal alkynes through Sonogashira cross-coupling leading to new azo compounds.

Type 1 Phototherapeutic Agents. 2. Cancer Cell Viability and ESR Studies of Tricyclic Diarylamines

Type 1 phototherapeutic agents based on diarylamines were assessed for free radical generation and evaluated in vitro for cell death efficacy in the U937 leukemia cancer cell line. All of the compounds were found to produce copious free radicals upon photoexcitation with UV-A and/or UV-B light, as determined by electron spin resonance (ESR) spectroscopy. Among the diarylamines, the most potent compounds were acridan (4) and 9-phenylacridan (5), with IC50 values of 0.68 μM and 0.17 μM, respectively.

Free electron transfer--relations between molecule dynamics and reaction kinetics

Electron transfer in non-polar media (alkanes, alkyl chlorides) exhibits some essential peculiarities. For instance the reaction of hetero-substituted aromatics with parent solvent radical cations results in the parallel formation of metastable donor radical cations and fragmentation products, in comparable amounts. The fragmentation products originate from a dissociative donor radical cation which decays extremely rapidly, i.e., in a few femtoseconds. This phenomenon is explained in terms of intramolecular dynamic motions which cause changes of the electron density (pi- and n-orbitals) in dependence on the deformation angle between the substituents and the aromatic ring. Hence femtosecond dynamics is reflected in the nanosecond time range and can be observed with real-time spectroscopy. Therefore, the process is named free electron transfer (FET) which corresponds to an unhindered electron jump occurring in the first approach of the reactants. This dynamic controlled process is compared with the classical electron transfer theories which are based on equilibrium kinetics. From the FET mechanism some new aspects for chemical reaction kinetics can be derived (critical review, 69 references).

Ultrafast Spectroscopic and Matrix Isolation Studies of p-Biphenylyl, o-Biphenylyl, and 1-Naphthylnitrenium Cations

p-biphenylyl, o-biphenylyl, and 1-naphthyl azides were deposited in argon at low temperature in the presence and absence of HCl. In the absence of HCl, the known electronic and vibrational spectra of the corresponding triplet nitrenes, azirines, and didehydroazepines were observed, whereas in the presence of HCl, photolysis of these azides produces new electronic spectra assigned to the corresponding nitrenium cations. For p-biphenylyl azide the resulting spectrum of the nitrenium ion is very similar to the previously observed solution-phase spectrum of this species. The vibrational spectrum of this cation was recorded for the first time. Spectroscopic evidence for the previously unknown o-biphenylylnitrenium cation and 1-naphthylnitrenium cation are provided. The spectra of p- and o-biphenylylnitrenium cations and 1-naphthylnitrenium cation are well reproduced by CASSCF and CASPT2 calculations. The same nitrenium cations were detected in solution by femtosecond time-resolved laser flash photolysis of the appropriate azides in 88% formic acid. The transient spectra of the nitrenium cations recorded in solution are in good agreement with the spectra obtained in HCl matrices. The rates of formation of these cations equal the rates of decay of the singlet nitrenes in 88% formic acid and are as follows: p-biphenylyl (taugrowth = 11.5 ps), o-biphenylyl (taugrowth = 7.7 ps), and 1-naphthylnitrenium cations (taugrowth = 8.4 ps). The decay lifetimes of p- and o-biphenylylnitrenium cations are 50 and 27 ns, respectively. The decay lifetimes of 1-naphthylnitrenium cation is 860 ps in 88% formic acid.

Ionization of Aromatic Sulfides in Nonpolar Media: Free vs Reaction-Controlled Electron Transfer

The primary products of the bimolecular free electron transfer (FET) from aromatic sulfides (PhSCH2Ph, PhSCHPh2, PhSCPh3) to n-butyl chloride radical cations are two radical cation conformers: a dissociative and a metastable one. In analogy with formerly studied donor systems, this result seems to reflect femtosecond oscillations in the ground state of the sulfides such as torsion motions around the Ar-S bond. This motion is accompanied by a marked electron fluctuation within the HOMO (or the n) orbitals. The FET products observed in the nanosecond time scale such as the metastable sulfide radical cations (Ar-S-CR3*+), the dissociation products R3C+; and R3C*, and their (experimentally) nondetectable counterparts Ar-S* as well as Ar-S+ can be understood with the simplified assumption of two extreme conformations, namely a planar and a twisted donor molecule. Using mediator radical cations (benzene, butylbenzene, biphenyl), the stepwise reduction of the free energy of the electron transfer from -DeltaH = 2.5 to <or=0.5 eV changes the dissociation pattern of the sulfide radical cations toward a uniform product such as the metastable sulfide radical cation. From that, we tentatively interpret the mechanism of the studied electron transfer as diffusion-controlled (collision-determined FET) at higher -DeltaH values and as reaction-controlled at -DeltaH below 0.5 eV.

Femtodynamics Reflected in Nanoseconds: Bimolecular Free Electron Transfer in Nonpolar Media

The (free) electron transfer (FET) from electron donor molecules to parent solvent radical cations of alkanes and alkyl chlorides exhibits mechanistic peculiarities that are conditioned by the low polarity of these solvents. Because of the negligible solvation of ions in such systems and the almost complete lack of an activation barrier, the electron jump takes place at the very first encounter of the reactants and, as such, in extremely short times of <or=10(-15) s. Molecular oscillations (deformation, bending) occurring within the femtosecond time domain result directly in significant changes of the pi- or n-electron distribution in the HOMO ground state of the donor molecule, thereby generating a distribution of conformers. This is considered to be a rationale for a possible generation of different product radical cations in the free electron transfer, which exhibit different spin and charge distribution and, consequently different stability. Experimentally, the latter has been verified for aromatic donors, substituted with mobile, i.e., not rigidly fixed heteroatom-centered groups (various phenol type compounds, thiophenols, aromatic amines, benzyltrimethylsilanes etc.). The individually characteristic product distribution could be visualized and quantified by time-resolved spectroscopy in the nanosecond time domain. On the basis of a manifold of experimental data and supported by quantum-chemical calculations, the free electron transfer phenomenon is analyzed and discussed in detail in this summarizing report. The results presented here stand also for a, seemingly paradox, situation in which the products of a diffusion-controlled bimolecular reaction are governed by femtosecond events.