Kinetics of Thermal Cis-Trans Isomerization. III

Fate of photoexcited trans-aminostilbenes

“…Trans-stilbene adopts a planar structure in the ground state, and torsions about the phenyl-vinylene CÀ ÀC and the central vinylene C¼C bonds encounter a barrier of $4 and $43 kcal/mol, respectively [29,30]. Photoexcitation of trans-stilbene ( 1 t*) significantly weakens the bond order of the C¼C bond such that the torsion barrier is largely diminished to 3.5 kcal/mol, which facilitates the trans !…”

The Photochemistry of Stilbenoid Compounds and Their Role in Materials Technology

“…The photocyclodimerization of (E)-stilbene (1) was discovered as early as the beginning of the twentieth century by Ciamician and Silver,"221 who rank among the pioneers of photochemistry. The structure of the main product 35 These excimers, which correspond to flat energy minima,[1251 transfer to the minima D* of doubly excited singlet state for a pericyclic geometry of the intermediate ("pericyclic minimum") and are rapidly deactivated to the ground state of the products.11261 The geometry at the pericyclic minimum causes the energy of the D* state to dip below that of the degenerate S, state of these excimers within a narrowly defined local region. The avoided crossing of the energy surfaces probably results in a small energy barrier in the reaction profile.…”

Spectroscopic and Theoretical Identification of Two Thermal Isomerization Pathways for Bistable Chiral Overcrowded Alkenes

“…In theory, it is possible that the thermal isomerization of the metastable state follows an alternative and competing pathway other than thermal helix inversion (THI). Structurally similar stilbene switches are able to undergo thermal E – Z isomerization (TEZI) from cis to trans , although the activation energy for this process usually exceeds 150 kJ mol −1 . For some overcrowded alkenes though, this barrier has been observed to be significantly lower due to the steric strain in the minimum energy configurations, thus forcing the double bond far from planarity.…”

“…Structurally similar stilbene switchesa re able to undergo thermal E-Z isomerization (TEZI) from cis to trans,a lthought he activation energy for this process usually exceeds 150 kJ mol À1 . [49,50] For some overcrowdeda lkenes though, this barrierh as been observed to be significantly lower due to the steric strain in the minimume nergy configurations, thus forcing the double bond far from planarity.A sa ne xample, bis-fluorenylidenes exhibit activation energiesf or the TEZI of 105 kJ mol À1 . [51,52] For second-generation motors, in order to positively identify the outcome of the thermal isomerization of the photochemically generated metastable state the lower half has to be desymmetrized.…”