A New Solution to an Old Problem: Synthesis of Unsubstituted Poly(<i>para</i>-phenylene)

Unsubstituted and structurally well-defined
poly­(para-phenylene) (PPP) has been long-desired
as an organic semiconductor
prototype of conjugated polymers. To date, several attempts to synthesize
unsubstituted, pristine, high-molecular-weight PPP have failed. Here
we solved this synthetic problem by a versatile precursor route. Suzuki
polymerization of kinked disubstituted 1,4-dimethoxycyclohexadienylene
monomers yielded a well-soluble, nonaromatic precursor polymer. Its
solubility allowed processing by spin-coating into nanometer-thick
films. Subsequent additive-free thermal treatment induced aromatization
and led to exclusively para-connected, highly fluorescent PPP with
a length of about 75 phenylene units.