Wolfgang Lüttke

Theoretische und spektroskopische untersuchungen an indigo-farbstoffen — II: Das chromophore system der Indigo-Farbstoffe

Zusanunenfassung Durch schrittweise Verkleinerung des Indigo-Molekulgerustes wurde an Hand von LCAO-MO-Rechnungen (zunachst in Huckel-Naherung) die Gruppierung I als das chromophore Stammsystem der Indigo-Farbstoffe ermittelt, sodann die Abhangigkeit der Eigenschaften der Indigoide von der Natur des jeweiligen Heteroatoms X durch Verwendung von verschiedenen Heteroatom Parametern untersucht. Parr-Pariser-Rechnungen bestatigten schliesslich die Ergebnisse der einfachen Naherungen and gaben weiteren Aufschluss uber den Aufbau, die Elektronenverteilung and die Lichtabsorption des Grundchromophors 1. Hiernach wird die tiefe Farbe der Indigo-Farbstoffe im wesentlichen durch die drei folgenden Faktoren bestimmt: o (1) durch die spezielle Verknupfung der Atome im Grundgerust I, das mit seinem doppelt gekreuztkonjugierten System aus je zwei Elektronen-Donator-(X) and Akzeptor-Gruppen den Stammchromophor darstellt; {fx315-1} (2) durch die Konstellation der beiden Carbonyl-Gruppen gegeniiber der zentralen C=C-Bindung in diesem Grunderust, and (3) durch die Natur des Heteroatoms X (fur X = NH zudem durch die Bildung von zwischenmolekularen Wasserstoffbrucken).

Rotationsisomerie und konjugation im Vinylcyclopropan—eine Elektronenbeugungsuntersuchung des freien moleküls in der gasphase

Zusammenfassung Durch Elektronenbeugungsuntersuchungen an gasformigem Vinylcyclopropan werden die Strukturdaten und die Konformationen des Molekuls bestimmt. Es liegt bei Raumtemperatur in einem Gleichgewicht zwischen 75 ± 6%s-trans- und 25 ± 6% gauche-Form vor. Der Enthalpieunterschied zwischen den beiden Rotameren betragt ΔG293 = −1·1 ± 0·2 kcal/Mol.

The photochemistry of 3,5,3′,5′- tetrasubstituted stilbenes: Formation of an equilibrium mixture of isomeric 4,4′-dihydrophenanthrene derivatives

Abstract Tetramethoxystilbene undergoes reversible trans [rlhar2] cis photoisomerization (PI). The extent of PI and the fluorescence yield vary only slightly with the temperature. Photocyclization of the cis isomer leads to two 4,4′-dihydrophenanthrene-like products (DHPs), only one of which is converted into cis by visible light. Above −100 °C a thermal equilibrium exists between the two DHPs, DHP 1 [rhlar2] DHP 2 . UV irradiation in the presence of iodine results mainly in photocyclodehydrogenation to (OCH 3 ) 4− phenanthrene. Tetrafluoro- and tetra( t -butyl)-stilbene photoisomerize to an extent decreasing on cooling, with the flourescence yield increasing concurrently. Photocyclization of tetrafluorostilbene, though less efficient than with tetramethoxy, again leads to two coloured modifications behaving as observed with tetramethoxy. In tetramethylstilbene, the corresponding DHP undergoes a two-stage decay, providing indirect evidence for the presence of two modifications. In tetra( t -butyl)-stilbene photocyclization is very inefficient.

Laser dyes with intramolecular triplet quenching

Abstract Laser characteristics of molecules with intramolecular triplet—triplet-energy transfer are reported. An efficiency increase of up to 500% was observed.

Verbesserte CNDO/2-konformationsberechnungen konjugierter moleküle

Abstract Torsional potential curves E (α) calculated using the CNDO/2 method have given, to date, only poor representations of the experimental data for the position and energies of the maxima and minima. This deficiency is most likely because the degree of conjugation of the central bond, being dependent on the angle of torsion α, has not been compensated. From calculations of the torsional potential curves E (α) for butadiene, acrolein, glyoxal, styrene, benzaldehyde and biphenyl, it can be shown that the problem can be eliminated if the decrease of π bonding of the central bond caused by internal rotation is included in the input geometry. The empirical modification is very successful for hydrocarbons and gives satisfactory results for heteroanalogues.

Photostability of 4,4'-dihydroxythioindigo, a mimetic of indigo.

The photochemical properties of indigo, a widely used industrial dye, has attracted both experimentalists and theoreticians from the beginning. Especially the high photostability of indigo has been the subject of intensive research. Recently, it was proposed that after photoexcitation an intramolecular proton transfer followed by a nonradiative relaxation to the ground state promote photostability. In indigo the hydrogen bond and the proton transfer occur between the opposing hemiindigo parts. Here, we provide experimental and theoretical evidence that a hydrogen transfer within one hemiindigo or hemithioindigo part is sufficient to attain photostability. This concept can serve as an interesting strategy towards new photostable dyes for the visible part of the spectrum.

Die rotationsisomerie des bicyclopropyls.

Abstract The determination of the enthalpy Δ H and entropy Δ S of the isomerization bicyclopropy l( trans )⇌ bicyclopropyl( gauche ) in the liquid phase by the IR intensity method is described. It is assumed that the ratio of the integral absorption coefficients of the two reference bands at 1351 cm −1 ( gauche ) and 1291 cm −1 ( trans ), which both belong to the same type of vibration, is temperature independent. The two values Δ H = −160 ± 40 cal Mol −1 and Δ S = −0.4 ± 0.5 cal (Mol · Grad) −1 respectively. Δ S U = −1.8 ± 0.5 cal (Mol · Grad) −1 thus determined agree well with the corresponding results obtained from NMR and electron diffraction measurements. However, from the pair of reference bands at 695 cm −1 ( gauche ) and 1291 cm −1 ( trans ), which do not belong to the same type of vibration, strongly differing values for Δ H and Δ S result under the same assumption as above, which apparently is not applicable in this case. It is shown through these data that the “Fateley-Test” does not provide a suitable tool to decide whether the absorption coefficients of the reference bands are temperature independent or not. The reason for this insignificance is the relatively poor accuracy and reproducibility of measured IR band intensities obtainable up to now. The relative density of bicyclopropyl between −60°C and + 50°C was determined.

Laser dyes III: Concepts to increase the photostability of laser dyes☆

Abstract Two approaches of increasing the photostability of the laser dye POPOP are reported. Desensibilisation against singlet oxygen was accomplished either by substitution with electron acceptor groups or by linking the dye molecule with a suitable quencher unit, e.g. tertiary amines. To test the efficiency of the modifications of the dye molecules the rate of their continuous UV-bleaching (λ = 366 nm) was investigated.

Tetrakis (1-Methylcyclopropyl) Ethylen

Zusammenfassung Tetrakis(1-methylcyclopropyl)ethylene, a potential precursor for the synthesis of tetra-tert-butylethylene, has been prepared and spectral properties are described.

Gekreuzt-konjugierte polyene: V. Spektroskopische Untersuchungen zur Konformationsanalyse des Bi-(4,4-dimethyl-2,5-cyclohexadien-1-yliden)

Abstract IR, Raman, UV and fluorescence spectra of bi-(4,4-dimethyl-2,5-cyclohexadiene-1-ylidene) 1 are performed in order to elucidate the conformation of the title compound in the fundamental and first excited state. Comparison of the results with the data of biphenyl 2 shows a different type of behaviour: the shape of 1 in the fundamental state corresponds to that of 2 in the excited state and vice versa. This behaviour of 1 and 2 is explained by specific differences in their π-electron distribution.