Strain-Induced Stereoselective Formation of Blue-Emitting Cyclostilbenes

We
describe the synthesis of two conjugated macrocycles that are
formed from the end-to-end linking of stilbenes. We have named these
macrocycles cyclo­stilbenes. The two cyclo­stilbene isomers
created in this study differ in the configuration of the double bond
in their subunits. These macrocycles are formed selectively through
a stepwise reductive elimination from a tetra­platinum precursor
and subsequent photo­isomerization. Single-crystal X-ray diffraction
reveals the formation of channel architectures in the solid state
that can be filled with guest molecules. The cyclo­stilbene macrocycles
emit blue light with fluorescence quantum yields that are high (>50%)
and have photoluminescence lifetimes of ∼0.8–1.5 ns.
The breadth and large Stokes shift in fluorescence emission, along
with broad excited-state absorption, result from strong electronic–vibronic
coupling in the strained structures of the cyclo­stilbenes.