Synthesis of Imine-Bridged Phenylenepyridine Ladder Polymers. Optical Band Gap Widening through Intramolecular Charge Transfer in Planar Polymers

The syntheses of planar (ladder) poly(phenylenepyridine)s [(PPhPy)s] are described using Pd-catalyzed cross couplings of aryldistannanes and aryl dihalides. Imine bridges are utilized to effect the planarization of the rigid-rod polymer. In one set of (PPhPy)s, the phenyl rings bear the nitrogen portion of the bridging imines while the pyridine bears the carbon portion of the bridging imine. A second type of (PPhPy)s has the reverse imine-bridging mode. Surprisingly, the study here indicates that construction of alternating donor/acceptor repeat units for inducement of intramolecular charge-transfer resulted in an optical band widening; a result opposite to that obtained in nonplanar polymers possessing alternating donor/acceptor repeat units.