Alexander Gorski

From purely organic to metallo-organic chiral magnetic materials

There is presently a widespread search for materials which present magnetic ordering and appropriate chiroptical properties which would allow the observation of magneto-chiral effects. In this paper, we reflect on the possible uses of organic materials to probe this phenomenon, either as pure compounds or as their coordination compounds with metal ions.

Magnetic circular dichroism of neutral and ionic forms of octaethylhemiporphycene

Abstract Magnetic circular dichroism (MCD) spectra are reported and analyzed for neutral, doubly protonated and doubly deprotonated forms of a recently synthesized porphyrin isomer, 2,3,6,7,11,12,16,17-octaethylhemiporphycene. MCD measurements have allowed the bands corresponding to the Q and Soret electronic transitions in this porphyrin isomer to be separated and assigned. In addition, several higher-lying electronic transitions were located. The absorption and MCD characteristics of the lowest excited electronic states agree well with predictions based on a perimeter model. Hemiporphycene turns out to be a negative-hard chromophore, i.e., the energy separation between two highest occupied molecular orbitals is smaller than the corresponding value for the two lowest unoccupied orbitals. As a consequence, the spectral properties of hemiporphycene are similar to those of another constitutional isomer of porphyrin, the previously studied porphycene. The latter isomer, however, displays a much stronger negative-hard chromophore character than does hemiporphycene, the subject of the present study. The experimental results reported here are corroborated by TD-B3LYP/6-31G** and semiempirical INDO/S calculations of excited electronic states.

The structure of the phototransformation product of monothiodibenzoylmethane

Infrared (IR) and electronic spectroscopy studies were carried out for monothiodibenzoylmethane isolated in rare gas matrices, low-temperature glasses, polymer films, KBr tablets (IR) and liquid solutions. This molecule is known to undergo light-induced conversion, but the nature of the product has long been a subject of discussion. Comparison of the experimental IR spectra with theoretical predictions (B3LYP/cc-pVDZ) obtained for various possible structures shows that the photoproduct corresponds to a non-chelated –SH exo-rotamer of the (Z)-enethiol tautomeric form of monothiodibenzoylmethane. This conclusion is consistent with the experimentally determined energy of the activation barrier for the reverse ground-state process that brings the molecule back to the initial enolic, intramolecularly hydrogen-bonded structure.

Molecular dynamics and density functional theory simulations of matrix deposition. II. Absolute site structure assignment for porphyrin in xenon

Molecular dynamics calculations reveal that the main trapping site for porphyrin embedded in a xenon matrix corresponds to a hexagonal cavity formed after removal of seven host atoms. Tautomerization involving two inner hydrogen atoms leads to two trans forms that interact differently with the matrix cage. Therefore, both electronic and infrared spectra are split into doublets. Comparison of the experimentally observed splitting patterns with the results of density functional theory calculations that explicitly include the nearest xenon atoms allows assigning each spectral feature to one of two different configurations of the chromophore inside the xenon cavity. The main factor responsible for the splittings is a distortion of the molecular skeleton from a squarelike towards rectangular geometry.

ELECTRONIC STATES OF ANTHANTHRENE. LINEAR AND MAGNETIC CIRCULAR DICHROISM, FLUORESCENCE ANISOTROPY, AND QUANTUM CHEMICAL CALCULATIONS

The pericondensed benzenoid hydrocarbon anthanthrene (dibenzo[def,mno{]chrysene, dibenzo[}cd,jk{]pyrene) was investigated by UV-VIS and IR linear dichroism (LD) spectroscopy in stretched polyethylene, by UV-VIS magnetic circular dichroism (MCD) spectroscopy, and by fluorescence polarization anisotropy measurements. The combined results led to a detailed experimental characterization of electronic transitions in the near-UV and visible regions, with determination of polarization directions. In particular, the results allowed an unambiguous assignment of the weak L} b { transition (corresponding to Clars }α {-band), in spite of overlap with the much stronger L} a { transition (Clars} p{-band). This assignment, which is at variance with a previous suggestion, was supported by a study of solvent and temperature shifts. The experimental results were compared with theoretical predictions using the LCOAO procedure. Excellent agreement with observed transitions was obtained, including transition moment directions and MCD signs. The theoretical analysis revealed a significant breakdown of the alternant pairing symmetry for anthanthrene, corresponding to Δ HOMO > Δ LUMO. Similar to benzo[}a{]pyrene, anthanthrene can be classified as a positive-hard MCD chromophore in Michls terminology.

Excited states of 4-dimethylaminopyridines: Magnetic circular dichroism and computational studiesThis paper is dedicated to Professor Jean Kossanyi on the event of his 70th birthday.

The ultraviolet transitions of 4-(dimethylamino)pyridine and its derivatives: 3-methyl-4-(dimethylamino)pyridine and 3,5-dimethyl-4-(dimethylamino)pyridine were investigated by electronic absorption and magnetic circular dichroism (MCD) spectroscopy, and by quantum chemical calculations. The ortho-methylation in the pyridine ring creates a steric hindrance to coplanarity. The resulting changes in the strength of the dimethylamino substitutent are reflected in the MCD spectra. Near-UV experimental data indicate a presence of three low-energy transitions, assigned to 1Lb, 1La and 1(n,pi*) excited states. These results are corroborated by TD-DFT and INDO/S calculations.