Shpolskii Spectra of trans-1,2-Di(2'-pyridyl)ethylene. Identification and Evidence for Almost Planar Structure and .pi. Type HMO for the Trapped Rotamer

Abstract

The resolved absorption, fluorescence excitation, and fluorescence spectra of trans-stilbene (t-St) and trans-1,2-di(2'-pyridyl)ethylene (Di2PE) in polycrystalline n-Cs and n-Cg matrices at 5 K as well as their geometry in the crystal and potential energy curves along the Ca(ehylefic)-aTyl torsional coordinate have been investigated giving us the opportunity to compare two diarylethylene systems: one in which the Ha(e*ylefic)-Haryl repulsion is on and one in which this interaction is off. In the case of t-St, some lines previously not observed in matrices were assigned to transitions involving two quanta of the C,-phenyl torsion mode (137) of a, symmetry in CZh. We can conclude that in the n-alkane lattice, the t-St molecule tends to be twisted around the C,-phenyl bonds and that its symmetry is either CU, or Ci, as in the pure crystal. The vibronic structures of the sharp line excitation and fluorescence spectra of Di2PE are almost identical to those of t-St except for a slightly lower activity of the in-plane Ca=C,y-Phenyl bending mode (~2 5 ) and Ca=Ca, stretching mode (~7 ) .These results lead us to the conclusion that only one rotamer of Di2PE is trapped in n-C6 and in n-C8. It is characterized by (i) a first electronic transition of marked mc* character with no clear evidence for a contribution from the nonbonding electrons localized on the intracyclic nitrogen atoms and (ii) a conformation similar to that of t-St, with C2h or Ci symmetry. Ah41 computation of the ground-state potential energy curves along the torsional coordinate (Ca-wl) suggest, in the case of t-St, the existence of a very shallow potential barrier to planarity (0.12 kcaYmol) with an equilibrium value for the torsional angle smaller than 20" (Ci symmetry).In the case of Di2PE, these calculations confirm the existence of two minima in the potential curve. The rotamer in which the H3-Ha. repulsion is relieved is found to be the most stable, in agreement with the X-ray diffraction results. Compared to the other rotamer, it is characterized by a more shallow potential around the equilibrium and a smaller barrier to planarity. It has been identified with the rotamer trapped in the n-alkane matrices.

Photoelectron Spectra, Electronic Structures, and Conformational Properties of (E)-Stilbene, Styrylthiophenes, and (Thienylethenyl)pyridines

“…With regard to 2‐vinylpyridine ( 14 ), one would expect a preference for the synperiplanar ( sp ) conformer 2b , because of weaker H‐‐‐H interactions than in 2a . For trans ‐1,2‐bis(2‐pyridyl)ethene ( 19 ) an essentially planar structure with both pyridine rings in sp orientations was determined by X‐ray crystal structure analysis 27…”

Conformational Distribution oftrans-Stilbene in Solution Investigated by Liquid Crystal NMR Spectroscopy and Compared within VacuoTheoretical Predictions

“…The resolved absorption, fluorescence excitation and fluorescence spectra of t-St in n-C 6 and n-C 8 matrices at 5 K led the authors to assess that the t-St molecule tends to be, at most, just slightly twisted, so that its symmetry is either C 2h or C i , as in the pure crystal. 15 When, on the contrary, we pass to consider the results in vapor phase, we discover that they are quite controversial. Optical absorption, 16 two-color stimulated emission spectroscopy, 17 pure rotational coherence effects on picosecond fluorescence polarization experiments 18 and several other fluorescence studies 19−23 seem to lead to a C i or a C 2h planar structure for the isolated molecule.…”

Centrosymmetric and Non-centrosymmetric Packing of Aligned Molecular Fibers in the Solid State Self Assemblies of Cyclodextrin-based Rotaxanes

“…One of the componentsw as tentatively assigned as compound 16 based on 2D NMR spectroscopy and mass spectrometric studies. Thes olid states tructureso ft he rotaxanes 3 and 4 were examined usingX -ray crystallography.Crystals of each of ther otaxanes 3 and 4 were grownbyslow evaporation of methanol-d 4 /D 2 Os olvent over a period of severalweeks.Ineachcasethe asymmetric unito ft he unit cell containst wo independent moleculeso ft he rotaxane 3 or 4 together with numerous solventm olecules.W hen therea re substituentsonthe 3-and3 0 -positions of astilbene, three low-energy rotamers arepossible, as showninFigure2 [43][44][45][46].B otho ft he independent moleculeso ft he dichloride 4 show the anti,anti -conformation. Oneo f thoseinthe asymmetric unit of thedifluoride 3 shows disorder betweent he anti,syn-a nd syn , syn -conformations, whilet he otherhas the anti,anti -orientation.…”