Charge density flow as a driving force of distortion in excited protonated azaaromatics

The INDO method was used to calculate electronic charge densities in the ground and lowest excited singlet and triplet states of neutral and protonated 2,3- and 1,4-diaza derivatives of naphthalene and phenanthrene, and benzo (a) phenazine. Experiniental changes of pK_a upon excitation can be correlated with the values of electron density flow into the heterocy- clic ring. Electron-density increase turns out to be a major factor which causes distortion in some excited protonated species.     

EXCITED STATES OF SIX-MEMBERED N-HETEROCYCLES. FLUORESCENCE, PHOSPHORESCENCE AND ACID-BASE EQUILIBRIA OF FIVE MONO- AND DIAZA-PHENANTHRENES IN THE LOWEST EXCITED (τ,τ) STATES

Abstract— Acid-base equilibria of five azaphenanthrenes (1-and 4-aza; 1,10-, 1,7-, and 4,7-diaza) in the two lowest excited (τ,τ) states have been determined by Förster's cycle. Pure electronic levels of the free bases and their conjugate acids have been located from the absorption, fluorescence and phosphorescence spectra. In diazaphenanthrenes the increase of the first pK_a and the decrease of the second pK_a under excitation has been observed, and an explanation is offered. In many cases the reactivity of the molecule excited to the lowest triplet state differs considerably from that in the ground state. The Pariser-Pam-Pople (PPP) calculations of the free bases have been performed. From the theoretical results, the changes of the net charge on nitrogen atoms under excitation and the magnitude of the S-T splitting in the L_a and L_b states have been discussed and compared with experiment.     

Excited-state intramolecular proton transfer reaction modulated by low-frequency vibrations: an effect of an electron-donating substituent on the dually fluorescent bis-benzoxazole

Excited-state intramolecular proton transfer (ESIPT) reaction has been studied in a molecule showing dual fluorescence, the 2,5-bis(2-benzoxazolyl)-4-methoxyphenol (BBMP), and its isotopomers, where the methoxy, and alternatively, the OH group has been deuterated. Attention is focused on the influence of electron donating OCH(3) substituent on fast excited state reaction. Comparison between the resonance-enhanced multiphoton ionization spectrum and the laser-induced excitation of the primary and phototautomeric emissions has been done. The geometry, electron density distribution, vibrational structure as well as the potential energy profiles in the S(0) and S(1) states of four possible rotameric forms of BBMP were calculated with application of the density functional theory (DFT). It allowed identifying the most probable conformer and assessing the role of low-frequency motions for the ESIPT efficiency.     

Changes in surface composition of Fe-Si-Cr-K oxide alkylation catalyst: the marked role of potassium ferrites

The influence of gas-phase methylation of phenol on the state of the Fe-Si-Cr-K oxide catalyst surface was investigated by thermodesorption of K atoms and ions. Changes in potassium desorption energies, as determined from Arrhenius-like plots, varied from 3.07 eV to 1.21 eV for the atoms and from 2.59 eV to 2.89 eV for ion desorption from the active and deactivated samples, respectively. The results were discussed in terms of transformation of the catalyst surface and formation of β-ferrite.     

ZnFe2O4 as a new catalyst in theC-methylation of phenol

This paper reports the investigation ofC-alkylation of phenol with methanol over zinc ferrite as a catalyst. The reactions were carried out in gas phase at atmospheric pressure in dependence of increasing temperature. The total selectivity towardsortho-cresol and 2,6-xylenol over 300°C surpassed 90%. ZnFe₂O₄ was obtained by oxidative precipitation method from solution of iron (II) sulphate and zinc sulphate at 60°C. Based on the results, the best equation describing the oxidation reaction rate was found. The obtained products have been investigated using X-ray fluorescence spectrometry and X-ray diffraction analysis.     

Excited states of six-membered aza-aromatic rings. IX. 1,4- and 5,12-diazatriphenylenes

The two isomeric diazatriphenylenes, 1,4-DAT and 5,12-DAT, and the corresponding singly and doubly protonated forms were investigated in the ground state, phosphorescent state and fluorescent state by the methods of electronic spectroscopy, ESR spectroscopy and quantum chemistry. The following properties were measured: (π, π¹) excitation energies, acid-base equilibrium constants, quantum yields of fluorescence, lifetimes and zero-field splitting parameters of the triplet states, for the bases and the conjugate acids. The results of semi-empirical calculations performed using the PPL LCI SCF method based on monoexcited configurations were compared with experimental data. The calculated electron densities on nitrogen were correlated with pK's for the two equilibria and the three electronic states. The unusually large increase of pK (by about six units) of the triplet 1,4-DAT, has been found and interpreted. A very good correlation of zero-field splitting parameter D^{$\text{1}\text{3}$} versus singlet-triplet splitting was obtained.     

Catalytic alkylation of 2-naphthol with C1−C3 alcohols

The vapour phase alkylation of 2-naphthol with C₁?C₃ alcohols in the presence of an iron catalyst that contains Cr-, Si- and K-oxides was investigated. 2-Naphthol was methylated in a temperature range 520–645 K giving first 1-methyl-2-naphthol and then 1,3-dimethyl-2-naphthol with total selectivity of ortho-alkylation over 98%. In the cases of ethanol and n-propanol, derivatives of 2-naphthol alkylated in the 1-position were obtained with high yield and selectivity. The reactions were carried out in a continuous process at atmospheric pressure.     

Hydrothermal preparation of CuO-ZnAl<Subscript>2</Subscript>O<Subscript>4</Subscript> catalyst for phenol <Emphasis Type="Italic">ortho</Emphasis>-alkylation with methanol: effect of the calcination temperature on the catalytic performance

The effect of heat-treatment on 10 wt% CuO-ZnAl₂O₄ catalytic activity in methylation of phenol and the degree of interaction of CuO active phase with support spinel phase were investigated. The CuO-ZnAl₂O₄ sample was subjected to heat-treatment up to 1000°C. The thermal products were characterized by X-ray diffraction (XRD) analysis, nitrogen adsorption-desorption at -196°C and temperature-programmed desorption (TPD-MS) of CO₂. Additionally, the reducibility of copper phases was investigated by temperature-programmed reduction (TPR). XRD patterns of the fresh catalyst sample (calcined at 600°C) indicated the presence of a mixture of poorly crystallized CuO and ZnAl₂O₄ spinel phase. The presence of two reducible copper species has been found on fresh CuO-ZnAl₂O₄ catalyst by TPR analysis. After subsequent calcinations in air at elevated temperatures some CuO disappeared with appearance of CuAl₂O₄ phase. The catalytic results revealed that the CuO addition to ZnAl₂O₄ increases the activity in ortho-methylation of phenol. Subsequent heat-treatment up to 900°C causes partial deactivation of copper centers, which is the result of transformation of CuO to the inactive CuAl₂O₄ phase.     

Intramolecular single and double proton transfer in benzoxazole derivatives

The “double” derivatives of benzoxazole, bis-2,5-(2-benzoxazolyl)hydroquinone (II) and bis-3,6-(2-benzoxazolyl)-pyrocatochol (III), have been investigated. In (II), only one proton is transferred in the S₁ state. Primary and tautomeric forms exist in a rapidly established equilibrium. In (III), two tautomers were detected. One is generated in the S₁ state by a double proton transfer without a potential barrier, while the other, generated by a single proton transfer, is already present in trace amounts in the S₀ state.