Arene Substitution Design for Controlled Conformational Changes of Dibenzocycloocta-1,5-dienes

We
report that an agile eight-membered cycloalkane can be stabilized
by fusing a benzene ring on each side, substituted with proper functional
groups. The conformational change of dibenzocycloocta-1,5-diene (DBCOD),
a rigid–flexible–rigid organic moiety, from its Boat
to Chair conformation requires an activation energy of 42 kJ/mol,
which is substantially lower than those of existing submolecular shape-changing
units. Experimental data corroborated by theoretical calculations
demonstrate that intramolecular hydrogen bonding can stabilize Boat,
whereas electron repulsive interaction from opposing ester substituents
favors Chair. Intramolecular hydrogen bonding formed by 1,10-diamide
substitution stabilizes Boat, spiking the temperature at which Boat
and Chair can readily interchange from −60 to 60 °C. Concomitantly
this intramolecular attraction raises the energy barrier from 42 kJ/mol
for unsubstituted DBCOD to 68 kJ/mol for diamide-substituted DBCOD.
Remarkably, this value falls within the range of the activation energy
of highly efficient enzyme-catalyzed biological reactions. With shape
changes once considered only possible with high energy, our work reveals
a potential pathway exemplified by a specific submolecular structure
to achieve low-energy-driven shape changes for the first time. The
intrinsic cycle stability and high-energy output systems that would
incur damage under high-energy stimuli could particularly benefit
from this new kind of low-energy-driven shape-changing mechanism.
This work has laid the basis to construct systems for low-energy-driven
stimuli-responsive applications, hitherto a challenge to overcome.