138744-79-3Relevant articles and documents
Intermolecular π electron interactions made visible. Correlation of ground- and excited-state interactions with specific photoreactivities of isomorphously crystallized isoelectronic compounds
Ortmann, Ingo,Werner, Stefan,Krüger, Carl,Mohr, Siegfried,Schaffner, Kurt
, p. 5048 - 5054 (2007/10/02)
Solid-state UV irradiation of 4-acetoxy-6-styryl-α-pyrone 1b yields [2π + 2π] dimers of different constitutions (2b, 26%; 3b, 30%) at 69% conversion of 1b. This dichotomy is a consequence of the crystal topology of 1b, which contains two topochemically relevant reaction centers, each composed of two monomers at a center of inversion. In contrast, the isomorphously crystallized isoelectronic 4-acetoxy-6-styryl-l,5-oxazinone 4 is photostable. The electronic properties of 1b and 4 in the ground and excited states correlate with this difference in photoreactivity. Pyrone 1b exhibits monomer and excimer fluorescence emission in dilute solution and in the crystal, respectively, whereas the oxazinone 4 under both conditions shows solely monomer fluorescence. Furthermore, the electron deformation density (EDD) of 1b, determined by the X-X method (the resolution is such that the lone at all heteroatoms of 4 and the localization of the styryl C=C double bond are distinctly visible), shows that the main axes of the nonspheric elongation of positive EDD contour lines are turned from right angles to the molecular plane in order to avoid repulsive π-π interactions. This indicates intermolecular π-π electron interaction between the two reacting double bonds of neighboring molecules of 1b. The corresponding contour lines of the photostable oxazinone 4 do not show any such distortion. Multipole expansions of the atoms in the crystal structure analyses are in agreement with the results obtained from standard structural refinement.
