Hula-twist Within the Confinement of Molecular Crystals¶

Abstract

E/Z‐photoisomerizations within molecular crystals are varied. Existing cases are summarized. They require crystal lattices that allow for long‐range molecular movements in the nontopotactic solid‐state mechanism. Reactivity and directionality can be foreseen on the basis of the crystal packing. The reacting crystal changes continuously by phase rebuilding, phase transformation and disintegration. Two possibilities for the chemical mechanism exist: (1) highly space‐demanding (cooperative) double‐bond rotations; and (2) space‐conserving hula‐twist (HT) motions while the substituents move within their planes and only one C—H unit undergoes out‐of‐plane translocation. If internal rotation cannot be reasonably modeled within the crystal lattice, HT remains the only choice, as in the case of trans‐1,2‐dibenzoylethene. Direct experimental proof is still lacking because the differences in the conformational outcome could not be assessed in the studied examples. Density functional theory calculations of cis‐1,2‐dibenzoylethene revealed very low differences in energy content of the helical (s‐cis, s‐cis)‐ and the almost orthogonal (s‐cis, s‐trans)‐cis‐conformers. The almost orthogonal (s‐cis, s‐trans)‐cis‐conformer that is found in the pure crystal is very similar to the calculated counterpart. It is suggested that more favorable initial conformers might be obtained by proper vinylic substitution. The stereochemical outcome of highly space‐demanding thermal vinylic‐bond rotations followed by cyclizations of conjugated bisallenes to give bismethylene cyclobutenes excludes the alternative HT mechanism (double‐bond isomerization) in the present cases. But space‐conserving HT might be a mechanistic alternative in less‐substituted cases under photoexcitation. The stereochemical consequences are discussed.

Photoinduced Curling of Organic Molecular Crystal Nanowires

“…This class of 9substituted-anthracene-1,3-butadiene derivatives is known to undergo a E!Z photoisomerization in solution, [19] and we confirmed that the E form could photoisomerize into the Z form in dilute solution. [20][21][22] Even though it is unimolecular, the E!Z photoreaction does not proceed to 100 % completion. Figure 5 shows a view of the crystal structures of both isomers.…”

“…This E!Z photoisomerization probably proceeds through a hulatwist or bicycle pedal mechanism, similar to other solid-state E!Z photoisomerizations. [20][21][22] Even though it is unimolecular, the E!Z photoreaction does not proceed to 100 % completion. 1 H NMR spectra of light-exposed (E)-DMAAM nanowires show that only 40-80 % of the molecules are converted to (Z)-DMAAM after a 1 s pulse of light (Supporting Information).…”