Toward Rational Design of Polarity Probes by Noncovalent Functionalization of Phosphorene

Noncovalent functionalization of semiconductors is an effective way to modify their electronic properties. In this paper, we determine factors that sensitize the band gap of phosphorene, a representative two-dimensional (2D) material, to the permittivity of the solvent. This enables an ab initio design approach for polarity sensing devices. Charge transfer (CT) to certain cationic conjugated acceptor substrates is highly sensitive to the solvation at the surface. This provides regulation of their acceptor properties by the solvent, which turns out to have a large effect on the material’s band gap. As a result, we predict that highly electropositive cationic adsorbates, such as aromatic viologens, have the potential for polarity sensing across a wide range of permittivities. Time-dependent density-functional theory (TD-DFT) calculations of their absorption spectra on phosphorene using hybrid functionals show solvent-sensitive excitations inside the near-infrared (NIR) region, as is desirable for high penetration bioimaging. We compare our results to a representative set of acceptor molecules, including well-established electron acceptors like tetracyano­quinodimethane (TCNQ), that do not exhibit significant polarity response.