Synthesis of Highly Twisted, Nonplanar Aromatic Macrocycles Enabled by an Axially Chiral 4,5-Diphenylphenanthrene Building Block

The synthesis, structures, and properties
of highly twisted, nonplanar
aromatic macrocycles are described. These macrocycles with an approximately
90° twist angle were synthesized by an effective synthetic approach
through a quadruple Suzuki–Miyaura coupling of 4,5-bisarylphenanthrene,
a novel axially chiral nonplanar building block. By varying the cross-coupling
partner as the spacer, a family of twisted macrocycles was synthesized,
allowing for a systematic study of the effect of the spacer on macrocycle
shape and photophysical properties. For example, a unique macrocyclic
aggregation-induced emission (AIE) emitter with double tetraphenylethylene
units as the spacers was readily synthesized. Furthermore, attributed
to its conformationally restricted twisted structure, a 3,6-disubstituted-1,8-naphthalimide-incorporated
macrocycle showed remarkable solvatofluorochromism with high fluorescence
quantum yields. The excellent conformational stability of these macrocycles
further enabled complete enantiomeric resolution and characterization.
The racemization barrier of macrocycle was determined experimentally
and supported by DFT calculations.