Steric-Induced Bending and Energy Level Shifts Induced by Covalently Bonding Anthracene to Si(111)

Hybrid structures, formed by joining organic and inorganic semiconductors, can harness the unique photophysical properties of each material to achieve new functionality. In these hybrid systems, the interface between disparate materials must be carefully structured to dictate the properties of the whole. When exciton transfer between materials is the goal, the interface must be crafted to promote electronic coupling between the energy-donating and -accepting layers. Here, we study how covalent attachment of anthracene to a Si(111) surface influences the molecular and electronic structure of the molecule-solid interface. Using a combination of density functional theory (DFT) geometry optimizations and Born-Oppenheimer molecular dynamics (BOMD), we find anthracene molecules twist and distort after bonding to the silicon surface, lowering their symmetry and widening their HOMO-LUMO gap by approximately 160 meV. Electronic sum frequency generation (ESFG) measurements of anthracene-functionalized Si(111) surfaces identify anthracene-associated electronic resonances that are blue-shifted relative to the linear absorption features of anthracene in solution, supporting the predictions. These findings highlight the critical role of molecular distortion in tuning electronic properties at hybrid interfaces, offering insight into the design of functional organic/inorganic materials for optoelectronic applications.