Electrochemical, in silico and time-resolved EPR behaviour of semiconductive π-conjugated poly(p-phenyleneethynylenevinylene)s (PPEV)

A series of poly(p-phenyleneethynylenevinylene)s (PPEV), synthesized by step-growth polymerization of 1,4-dialkoxybenzene and 9,9-bis(2'-ethylhexyl)fluorene-diethynyl monomers and featuring aryl-(E)-enyne repeating units were investigated by experimental and computational techniques with respect to their conjugative and conductive properties. HOMO-LUMO band gaps in the range 2.64-2.79 eV were determined by cyclic voltammetry and UV-vis spectroscopy. In agreement with these data, conductivity measurements performed on thin films drop cast from dichloromethane solutions exhibit values in the range 10-10 S cm depending on the identity of the repeat units and on the length of alkyl chains. A p-type doping of the materials was achieved through exposure of the films to iodine vapor with a relevant increase of conductivity up to 7-folds compared to the undoped polymers. The cationic hole resulting by electron transfer from one PPEV to the electron acceptor PCBM was observed in situ by TREPR spectroscopy. In addition, DFT calculations performed on well-defined oligomeric models of the PPEVs provide HOMO-LUMO band gaps in agreement with the experimental values and disclose the dependence on both the length and type of conformers in the polymeric chain. Plots of the calculated band gaps versus 1/n, with n the number of monomer units, show a linear trend and predict that the maximum values of conductivity of the materials are reached with n as low as >= 6-7, which correspond to the average degrees of polymerization achievable by the synthetic procedure.