Interdigited π-stacked low band gap polymers through Friedel Crafts post functionalization of Suzuki coupled phenylene-alt-thiophene copolymers

Friedel Crafts post functionalization, a new molecular engineering approach, is experimented on structures of two semiconducting polymers (i.e., 1,4-dihexyloxy and dioctyloxy-phenylene-alt-thiophene copolymers) obtained through Suzuki cross coupling reaction to increase molecular weight and lowering of band gap. This unique post treatment demonstrates two and half fold increase of molecular weight and significant reduction of band gap (1.8–1.5 eV); the pre-functionalized value is above 2 eV. AFM and SEM micrograph before and after post functionalization show a clearer, smoother, more compact and interconnected biphasic nanostructure for post functionalized polymers in solid state. Significant red shifting of absorption onset (Δλabsonset ≈ 85–210 nm) and emission peak maxima (Δλem.max ≈ 175 nm) at solid state is attributed to the formation of more rigid planar interdigited π-stack 3D structure. Exfoliation of polymers layers to single polymer wire in polar solvent is clearly demonstrated by comparing the photophysical properties in solid and solution.

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► Friedel Craft post functionalization as a new approach to increase molecular weight of low molecular semi-conducting polymers is described.
► Friedel Craft post functionalization also significantly reduced the band gap resulting materials.
► Probable mechanism for the formation of interdigited π-stacked planar 3D structure in solid state is described.
► Possible effect of polar solvent on π-stacked planar 3D structure is evaluated.
► Microstructure and morphology of the polymers before and after post functionalization are compared to correlate the optical properties.