Thermal E/ Z Isomerization in First Generation Molecular Motors.

Determination of a thermal E/ Z isomerization barrier of first generation molecular motors is reported. Stable ( E)-1a directly converts to stable ( Z)-1c without photochemical E/ Z isomerization. The activation Gibbs energy of the isomerization was determined to be 123 kJ mol-1 by circular dichroism spectral changes. Density functional theory calculations show that ( Z)-1c is ∼11.4 kJ mol-1 more stable than ( E)-1a.