Lewis acids and bases (ligands) used for activation or stabilization of organometallic complexes offer underexplored possibilities for tuning the electronic and optical properties of organic semiconducting materials. For instance, binding of B(C6F5)3 to nitrogen atoms in the backbone of narrow bandgap conjugated polymers leads to a decrease of the optical transition energies.  Experimental and DFT studies indicate that coordination of the polymer to the electron deficient boron results in a lowering of both HOMO and LUMO levels.  However, the effect is more pronounced for the LUMO, which resides predominantly near the electron poor fragment in the polymer backbone structure.  This differential influence over the HOMO and the LUMO is responsible for the observed decrease in bandgap energy. In another demonstration, a new fullerene structure was obtained from the reaction of a Lewis basic N-heterocyclic carbene (NHC) and the Lewis acidic C60. The molecular features of the zwitterionic adduct can be described as a molecular gyroscope with the fullerene cage acting as the rotator that spins about one distinct axis given by its C�C single bond linkage with the imidazolium heterocycle stator. A detailed structural analysis of the compound carried out by means of single-crystal X-ray diffraction (XRD) revealed significant differences in the packing motifs of solvent-free and solvent-containing crystals. Variable temperature single-crystal XRD experiments (80 K = T = 480 K) carried out to investigate the rotational dynamics of the fullerene revealed atomic displacement parameters consistent with fast rotation of the highly symmetric fullerene in the solid state, whereas the imidazolium unit remains in a fixed position and therefore represents the stator. DFT calculations and semiempirical methods were applied to get insight into the profile of the rotational potential of the fullerene unit, particularly considering interactions with the neighboring molecules in the crystal lattice.

Modulation of the optical properties and electronic structures of organic semiconductors via Lewis acid/base interactions

248th National Meeting (2014)

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