Materials that undergo singlet fission, an energy conversion process that divides a high energy spin-singlet state into a pair of low-energy triplet excitations, can enhance schemes for solar energy harvesting and photocatalysis. However, meeting these goals necessitates forming junctions that facilitate energy transfer from singlet fission-capable organic molecules to inorganic semiconductors, such as silicon. To build such interfaces, we have adopted a covalent tethering strategy wherein organic molecules are directly bonded to silicon to enhance their electronic coupling. Using a series of spectroscopic measurements, we find the electronic structure of anthracene molecules becomes highly perturbed upon bonding to Si(111) via their 9 positions. Electronic sum frequency generation spectra show the absorption onset of anthracene’s S<sub>0</sub> to S<sub>1</sub> transition is shifted to higher energy with respect to monomeric anthracene while photoluminescence spectra indicate emission from this state is shifted to lower energy relative to anthracene monomers. These observations are consistent with formation of a 2D exciton band structure that results from nearest-neighbor electronic coupling between surface-bound anthracene molecules. Using a Holstein-type Hamiltonian, we reproduce these spectral shifts and find the intermolecular coupling between surface-bound anthracene molecules is ~50 meV. Interestingly, this coupling is ~5-10× larger than computed estimates for the coulombic coupling between surface-bound anthracene molecules, suggesting coupling between these molecules is additionally mediated by partial mixing of anthracene orbitals with those of the silicon surface. This conclusion is supported by electronic structure calculations, which show formation of anthracene electronic states that spatially extend into the silicon surface. Our results indicate the direct bonding of organic molecules to inorganic surfaces can indeed create hybrid states that span the organic-inorganic interface. Such states have potential to serve as doorways for facilitating energy exchange between organic and inorganic materials, which can enable new light-harvesting devices that use singlet fission to enhance their performance.

Building electronically coupled organic: Inorganic junctions for photon conversion

ACS Spring 2022

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