[2,2](5,8)Picenophanedienes: Syntheses, Structural Analyses, Molecular Dynamics, and Reversible Intramolecular Structure Conversion

This
study presents an important and efficient synthetic approach
to 5,8-dibromo-2,11-di-tert-butylpicene (3), with multigram scale, which was then converted to a new series
of picenophanes (6–10). The tub-shaped
[2,2]­(5,8)­picenophanediene 8 with two cis-ethylene linkers was explored using X-ray crystallography. The
tub-to-tub inversion proceed through the successive bending of the
linkers and the barrier for isopropyl-substituted derivative 10 was experimentally estimated to be 18.7 kcal/mol. Picenophanes
with a large π-system and semi-rigid structure exhibited anomalous
photophysical properties. The ethano-bridged picenophane shows the
weak exciton delocalization while the cis-ethylene-bridged
picenophane exhibits dual emission rendered by the weakly delocalized
exciton and excimer. With the aid of the ultrafast time-resolved emission
spectroscopy, the mechanism of the excimer formation is resolved,
showing a unique behavior of two-state reversible reaction with fast
structural deformation whose lifetime is around 20 ps at 298 K. This
work demonstrates that the slight difference in the bridge of tub-shaped
picenophanes renders distinct photophysical behavior, revealing the
potential of harnessing inter-moiety reaction in the picenophane systems.