Thermal Behavior of 2,2,3,3,5,5,6,6,8,8,9,9,11,11,12,12-Hexadecamethyl-2,3,5,6,8,9,11,12-octasila-1,4,7,10(2,5)-tetrathiophenacyclododecaphane in the Crystalline State

Disilanylene-bonded
thiophenophanes constitute a new
class of σ–π-conjugated
macrocycles with potential for unique solid-state behaviors. Herein,
we report the synthesis of 2,2,3,3,5,5,6,6,8,8,9,9,11,11,12,12-hexadecamethyl-2,3,5,6,8,9,11,12-octasila-1,4,7,10(2,5)-tetrathiophenacyclododecaphane, c-Th4, via Pd-catalyzed cyclization.
Single-crystal X-ray diffraction revealed a chair-like square geometry
in which all thiophene sulfur atoms are oriented outward, and CH−π
interactions assemble the molecules into columns along the crystallographic a-axis. Unlike the previously reported 1,4-phenylene analogue c-Ph4, which exhibits a thermosalient
phase transition, c-Th4 undergoes no structural phase transition up to its melting point.
Variable-temperature X-ray diffraction and differential scanning calorimetry
confirmed little structural deformation and few thermal events or
phase transitions with an increase in temperature, respectively. Comparative
analysis indicates that the narrower bond angle of the 2,5-thienylene
spacer restricts intramolecular motion and suppresses thermal responsiveness.
These findings highlight the critical role of intramolecular free
volume in the design of stimuli-responsive organosilicon macrocycles.
In addition, we have added a brief clarification on how the narrower
2,5-thienylene bond angle reduces conformational freedom in c-Th4.