Franz Neufingerl

Beiträge zur Chemie der Pyrrolpigmente, 18. Mitt.: Pyrromethenone—Partialstrukturen von Gallenpigmenten: Struktur und Eigenschaften in Lösung

Depending on solvent polarity and concentration a monomer—dimer equilibrium for pyrromethenones was deduced by means of absorption and fluorescence spectroscopy and measurement of a colligative property. The monomer could be characterized as asyn-(Z) form with a dihedral angle at the methine single bond of about fourty degrees using the lanthanide shift technique and nuclearOverhauser effect studies. Two types of dimer geometry with differing bond strengths were found by comparingPPP calculation results with the measured absorption spectra and the liquid crystal induced circular dichroism.The (Z) isomers can be transformed to the corresponding (E) isomers by means of a photoreaction. The latter is thermally stable but rather sensitive against oxygen, light and protons, an effect which varies strongly with the substitution pattern of the pyrrole fragment thus enabling isolation in certain cases only.

Beiträge zur Chemie der Pyrrolpigmente, 26. Mitt.: Zur anaeroben Photochemie von Gallenpigmenten: Die Lumineszenz von Gallenpigment-Partialstruktursystemen und integralen Farbstoffen

The luminescence properties of about twentyfive model compounds representing the three main partial structures of bile pigments (namely pyrromethenes, pyrromethenones and methylenepyrrolylmethylenepyrrolinones) as well as tripyrrines and the geometrical isomers of aetiobiliverdin-IV-γ are reported. In these compounds there is usually only a very faint or even no fluorescence and phosphorescene as the pathway of deexcitation. Vibrational internal conversion and photoisomerizations at the exocyclic double bonds are the main anaerobic paths of radiationless decay of the excited states.

Beiträge zur Chemie der Pyrrolpigmente, 27. Mitt.

Pyrromethenones are isomerized into their diastereomers at the exocyclic double bond by light, whereas dipyrromethenes and methylenepyrrolylmethyllenepyrrolinones as well as integral bile pigments containing these two partial structures (biliviolines, biliverdines) are not. The reason for this dichotomy was shown by investigation of model compounds to be neither the thermodynamically and kinetically instable product nor the availability of low states of lone pair character. Instead the very efficient radiationless decay of the excited singlet proceeds via a desactivation process being a tunneling between excited and ground state potential surfaces. Calculations according to this theory ofFormosinho affords quantitative results which are in excellent agreement with experimental data. Another desactivation mode comes from the crossing of the potential surfaces of the ground state of one tautomeric form with those of the excited state of another, leading to a proton transfer process. The molecular basis of these processes is shown to be inter—as well as intramolecular hydrogen bonding.

Beiträge zur Chemie der Pyrrolpigmente, 13. Mitt. Die thermische Stabilität der geometrischen Isomeren von Arylmethylidenpyrrolinonen

From kinetic studies and the investigation of several model compounds it has been deduced that the interconversion of (E)- and (Z)-arylmethylidenepyrrolinones proceeds via a thermal accessible dipolar (singlet) state thus being analogous to the many known thermal (Z)—(E)-isomerizations. There is a pronounced dependence of this reaction on the solvent and the substitution of the aryl part of the molecule; the equilibrium position between (Z)- and (E)-isomers is determined by the substituent in position “4” of the pyrrolinone ring.

Beiträge zur Chemie der Pyrrolpigmente, 11. Mitt.: Die Lichtabsorption geometrisch isomerer Arylmethyliden-pyrrolinone —Konformationsanalytische Studien

By investigation of the geometrical isomers of several substituted arylmethylidene-pyrrolinones using lanthanide induced shift experiments in connection with a computer program, the ring current model,PPP—SCF—LCAO—MO—CI calculations and UV spectroscopy the following conclusions could be drawn: The light absorptions ofZ- andE-isomers vary only in the case of different dihedral angles at the arylmethine bond. The dihedral angles in theE-isomers are determined by the substituent in 4-position of the lactam ring. Hydrogen makes no difference whereas methyl leads to a more twistedE-structure which therefore shows a marked hypsochromic shift of the intense long wavelength absorption band, as compared to the correspondingZ-isomer.By investigation of the geometrical isomers of several substituted arylmethylidene-pyrrolinones using lanthanide induced shift experiments in connection with a computer program, the ring current model,PPP—SCF—LCAO—MO—CI calculations and UV spectroscopy the following conclusions could be drawn: The light absorptions ofZ- andE-isomers vary only in the case of different dihedral angles at the arylmethine bond. The dihedral angles in theE-isomers are determined by the substituent in 4-position of the lactam ring. Hydrogen makes no difference whereas methyl leads to a more twistedE-structure which therefore shows a marked hypsochromic shift of the intense long wavelength absorption band, as compared to the correspondingZ-isomer.

Beiträge zur Chemie der Pyrrolpigmente, 29. Mitt.: Zur anaeroben Photochemie von Gallenpigmenten: Zum Mechanismus der photochemischen Isomerisierung an exocyclischen Doppelbindungen

Studies of heavy atom effects as well as sensitization and quenching experiments with several model compounds suggested that in pyrromethenones and biladienes-ac the photoisomerization at exocyclic double bonds proceeds predominantly via the singlet path on direct irradiation. The energy of the triplet state of pyrromethenones and arylmethylenepyrrolinones was estimated by sensitization experiments to be in the region from 140 to 160 kJ/mol. On sensitization the reaction follows the triplet path. With bilatrienes-abc the situation is complicated by the fact that in homogenous phase there is only unidirectional isomerization but there is evidence that the direct process proceeds also via the singlet path. These experiments are supported by calculations using theFormosinho theory of radiationless deexcitation.

Zur photochemischen (Z)⇆(E)-Isomerisierung von Gallenpigment-Partialstrukturen

The photochemical (Z)⇆(E) isomerization of the exocyclic double bond is only possible in pyrromethenones (partial structure1) but not in2–4. From spectral investigations, chelation and methylation of certain positions within these structures it can be shown that it is most probably a proton transfer reaction which competes successfully with the isomerization.

Zum Mechanismus der photochemischen (Z)⇆(E)-Isomeriseierung von Gallenpigment-Partialstrukturen

The photochemical isomerization (Z)⇆(E) at the exocyclic double bond of pyrromethenones is shown to proceed via a singlet excited state using sensitization and quenching experiments as well as measuring quantum yields of isomerization, fluorescence and phosphorescence.