The solid-state E/Z-photoisomerization of 1,2-dibenzoylethene

The E/Z-photoisomerization of trans-1,2-dibenzoylethene (DBE) in the confinement of its crystal lattice proceeds readily, but not as a single crystal to single crystal process which was claimed previously by others. This model for the Z-->E isomerization at the 11-12 double bond of the retinal moiety in the crystal-like confinement of rhodopsin was investigated in view of the fact that the precise geometric features are crucial for a better understanding of the postulated twist mechanism. Atomic force microscopy (AFM) monitored long-range anisotropic molecular movements if trans-DBE was photoisomerized, but cis-DBE was unreactive even at the extreme sensitivity of AFM. The crystal lattices of both isomers cannot accommodate a rotational mechanism but at best the twist mechanism with the large groups not leaving their planes. The unidirectional solid-state photochemistry derives from the crystal packing of cis-DBE which exhibits severe 3D-interlocking. Thus, trans-DBE molecules are not formed in the cis-lattice, because their moving away would be prohibited. Conversely, photochemically formed cis-DBE molecules escape the foreign trans-DBE lattice easily along its glide planes, as is experimentally observed by AFM. These findings are reminiscent of the escape of 11-trans-retinal from the rhodopsin array in the vision cascade.     

Atomic force microscopy and solid state photolyses: phase rebuilding

Atomic force microscopy (AFM) was used for the study of the mechanism of phase rebuilding in photodimerizations in crystals and of photoreactions on polymer surfaces. The AFM features that are found upon photochemical reactions in the surface regions indicate far-reaching (up to 100 nm) molecular transport which are well directed in space and depend on the crystal face. Thus, not only proximity considerations (topochemistry) but more importantly phase-rebuilding mechanisms are crucial for solid state photoreactivity and this depends on the bulk crystal structure. 2-benzylidenecyclopentanone (d=4.123 Å) and trans-stilbene (d=5.720 Å) are not reactive, because no phase-rebuilding mechanism is available, while anthracene (d=6.038 Å) does form a photodimer. The phase-rebuilding mechanisms on two natural faces of antharacene are analysed and interpreted on the molecular level. The formation of three different photoproducts from 2,5-dibenzylidenecyclopentanone shows two different phase-rebulding mechanisms on the morphologically dominant face and there are extraordinarily wide molecular rotations of the highly skewed (46°, 131°) reactants in the crystal. Photolyses of polymeric foils of styrene-isopropyl-acrylate copolymer and polystyrene give rise to very nanostructures in the surface region that can be imaged by AFM.     

Quantitative reaction cascades of ninhydrin in the solid state

Crystalline ninhydrin (1) undergoes waste-free solid-state cascade reactions with dimedone, L-proline, three o-phenylenediamines, o-mercaptoaniline, two ureas, three thioureas, and methyl 3-aminocrotonate. The yields are quantitative and give pure crystalline products without workup just by milling stoichiometric mixtures of the crystalline reagents. The structures of the new and the previously obtained products with lower yields from solutions are established or confirmed by spectroscopic data and density functional calculations at the B3LYP/6-31G* level. The success of 3- and 4-cascade reactions in the crystal without melting is unusual and of unmatched atom economy. They are mechanistically investigated with atomic force microscopy techniques (AFM) on six different faces of 1 when o-phenylenediamine was the reagent (substitution, elimination, cyclization, elimination) and interpreted on the basis of known crystal structure data. Strict correlations to the crystal packings are observed. The characteristic surface features grow to microm heights in some cases at distances of 0.5 mm from the contact edge of the reacting crystals. The waste-free and easy syntheses of highly functionalized (C=O; O-H; C=N) heterocycles or of a tetraketone are also of interest for synthetic use.     

Collisional depolarization of excited114Cd 5s5p 3 P 1 atoms by collisions with molecular gases

Depolarization of excited¹¹⁴Cd 5s5p ³ P ₁ atoms induced by collisions with various molecular gases (N₂, H₂, D₂, CO, CO₂, CH₄, C₂H₆, C₂H₄) has been investigated using polarized fluorescence spectroscopy. After pulsed optical excitation of the Cd 5³ P ₁ level with appropriately polarized light the temporal behaviour of Zeeman quantum beats has been observed showing the influence of collisional destruction of orientation and alignment. By analyzing the signal curves at different molecular gas pressures the corresponding depolarization cross sections for¹¹⁴Cd atoms in the 5³ P ₁ state have been obtained. With regard to a test of a nuclear spin decoupling model for the collisions the cross sections were compared with previously measured hyperfine structure transfer cross sections of¹¹³Cd 5s5p ³ P ₁ atoms.     

Understanding the conformational dependence of spin-spin coupling constants: through-bond and through-space J(31P,31P) coupling in tetraphosphane-1,4-diides [M(L)x]2[P4R4

The characteristic dependence of J(31P,31P) spin-spin coupling constants of alkali metal tetraphosphane-1,4-diides on structure and composition has been analyzed by density functional methods. The computations confirm that the structure of the contact ion pairs is conserved in solution. Calculations on model systems M2P4H4, on naked P4H4(2-) anions, and on models including point charges, show that the role of the cations is mainly structural and to a smaller extent electrostatic. Three of the four J(P,P) coupling constants depend characteristically on the conformation of the anion, which in turn is determined by the substituents R and by cation-anion interactions. Several couplings exhibit a large through-space component and are thus strongly dependent on the relative orientation of nonbonding electron pairs on the phosphorus atoms involved. This is shown by visualization of coupling pathways using the recently introduced coupling energy density (CED), in combination with the electron localization function (ELF).