Atomic-Level Insight into the Postsynthesis Band Gap Engineering of a Lewis Base Polymer Using Lewis Acid Tris(pentafluorophenyl)borane

In this report, we investigate the binding properties of the Lewis acid tris(pentafluorophenyl)borane with a Lewis base semiconducting polymer, PFPT, and the subsequent mechanism of band gap reduction. Experiments and quantum chemical calculations confirm that the formation of a Lewis acid adduct is energetically favorable (Delta G degrees < -0.2 eV), with preferential binding at the pyridyl nitrogen in the polymer backbone over other Lewis base sites. Upon adduct formation, ultraviolet photoelectron spectroscopy indicates only a slight decrease in the HOMO energy, implying that a larger reduction in the LUMO energy is primarily responsible for the observed optical band gap narrowing (Delta E-opt = 0.3 eV). Herein, we also provide the first spatially resolved picture of how Lewis acid adducts form in heterogeneous, disordered polymer/tris(pentafluorophenyl)borane thin films via one- and dimensional (2D) solid-state nuclear magnetic resonance. Notably, solid-state 1D B-11, C-13{H-1}, and C-13{F-19} cross-polarization magic-angle spinning (CP-MAS) NMR and 2D H-1{F-19} and H-1{H-1} correlation NMR analyses establish that BCF molecules are intercalated between branched C16H33 side chains with the boron atom facing toward the pyridyl nitrogen atoms of PFPT.