Journal of Physical Organic Chemistry最新文献

Methyl pyropheophorbide-a, one of the chlorophyll-a derivatives, was reacted with thallium(III) trifluoroacetate in tetrahydrofuran at room temperature to yield the corresponding Tl(III) complex with trifluoroacetate as an axial ligand. The anionic trifluoroacetate ligand was readily exchanged for chloride by treatment with brine. The resulting five-coordinated complex with Cl⁻ was fully characterized by ¹H NMR and visible absorption spectroscopies as well as mass spectrometry. Notably, the meso-protons of trifluoroacetate and chloride complexes coupled strongly with the stable Tl isotopes (the nuclear spin quantum numbers = 1/2) through four bonds to give doublet peaks (the coupling constants = 40–58 Hz). Their ¹H NMR spectra in deuterated chloroform indicated that the Tl(III) chlorins with an axial ligand were epimeric mixtures around the chiral central metal. Molecular model calculation proposed their absolute configurations at the asymmetric Tl(III) atom. The α-ligated complexes were estimated to be preferable over the β-ligated epimers. The epimeric ratios varying from 1.6 to 2.0 were dependent on the axial ligands. The obtained Tl(III) complex in dichloromethane efficiently absorbed violet and red lights but hardly emitted red-to-far-red light due to the heavy atom effect.

UV–Vis and X-ray crystallographic measurements, together with the analysis of the potential energy surfaces, demonstrated a critical role of halogen-bonded (HaB) complexes in the electron transfer (ET) reactions between diiodine and aromatic amines. Specifically, UV–Vis spectral studies showed that the addition of I₂ to solutions of tetramethyl-p-phenylenediamine (TMPD) or various N,N-dimethylanilines resulted in the immediate formation of HaB complexes. In systems with TMPD, this was followed by rapid ET—even at −85°C—resulting in the formation of persistent TMPD^+• cation radicals (which were crystallized as salts with iodide counterions). The formation of HaB complexes upon mixing I₂ with dimethylanilines was followed by the generation of I₃⁻, indicating the occurrence of redox processes. Subsequent chemical reactions led to the crystallization of triiodide salts of protonated m-methoxy-N,N-dimethylaniline, p-iodo-N,N-dimethylaniline (from the reaction with N,N-dimethylamine), or, surprisingly, 4,4′-bis(dimethylamino)benzhydrylium cations (from the reaction of I₂ with p-bromo-N,N-dimethylaniline). Most notably, the rates of the redox processes in all these systems were many orders of magnitude faster than those predicted by the classical Marcus (outer-sphere) ET theory. These high rates were accounted for by considering the donor–acceptor electronic coupling in the HaB precursor complexes, which drastically lowers the activation barriers for inner-sphere ET. These results confirm that HaB complexes are vital intermediates in the ET reactions of halogenated electrophiles, and their role should be taken into account when analyzing potential ET steps in chemical transformations involving such molecules.

In this study, synthesis, characterization, photophysical properties, and DFT calculations of a new tetraaryl-1,4-dihydropyrrolo[3,2-b]pyrrole (TAPP) derivative molecule of A-π-D-π-A type containing benzoxazole as an acceptor group at 1,4-positions have been carried out. The target A-D-A type molecule was synthesized using 4-(benzo[d]oxazol-2-yl)aniline by the method known in the literature, and it was aimed to obtain the classical TAPP structure in an iron-catalyzed environment. No product was obtained with the first method; therefore, the reaction conditions were optimized. The reaction time and temperature were varied in a controlled manner; the optimum conditions were determined with a reaction time of 7 h at 60°C. The photophysical behavior of the synthesized benzoxazole-substituted TAPP derivative was investigated by UV–vis and fluorescence spectroscopy depending on different solvent polarities. In the absorption spectra, the maximum absorption band observed around 330 nm in low polarity solvents shifted to 340 nm in polar solvents. Similarly, fluorescence emissions systematically red-shifted from hexane to DCM (470–550 nm), and quantum yields decreased significantly (0.41 in hexane and 0.03 in DCM). This bathochromic shift and decrease in intensity indicate that the molecule has high polarity in the excited state whereas the intrinsic charge transfer (ICT) and twisted ICT (TICT) states stabilize with increasing solvent polarity. At the same time, nonluminescent transition channels such as loss of planarity, oxygen-induced quenching, and internal conversion in the solvent-excited state also decrease the fluorescence intensity. The findings reveal that this TAPP derivative forms a strong donor-acceptor system and exhibits high sensitivity to solvent environment.

The structure of active centers of the KOH/NMP and KOBu^t/NMP superbase systems was studied using quantum chemical modeling with the combined B2PLYP-D2/6-311+G**//B3LYP/6-31+G* approach. It was shown that the closest solvation shell of the cation in both KOH and KOBu^t includes five N-methylpyrrolidone molecules. The structure of KOH·5NMP and KOBu^t·5NMP pentasolvate complexes was obtained and described. These complexes were then compared with similar dimethyl sulfoxide-based pentasolvate complexes previously studied, namely, KOH·5DMSO and KOBu^t·5DMSO. The effect of a water molecule on the structural and energy characteristics of superbase complexes with N-methylpyrrolidone was evaluated.

A series of biologically active phenothiazine derivatives were studied as inhibitors of the free-radical chain oxidation of isopropyl alcohol by atmospheric oxygen. It was found that these compounds exhibit antioxidant properties, effectively suppressing the oxidation process. The key rate constants quantitatively characterizing the efficiency of their antioxidant action were determined. Mathematical modeling was used to analyze the proposed oxidation mechanism, identifying the critical stages that determine the inhibitory efficacy of phenothiazines. It was shown that the inhibitor regeneration reaction plays a significant role, leading to a high inhibition capacity. The application of mathematical modeling allowed the proposal of an oxidation mechanism that provides a comprehensive explanation for all currently available experimental and literature data.