Unexpected Photoluminescence Enhancement upon Epoxidation of Coronene: A DFT/MRCI Study

Due to their pronounced fluorescence in the near-UV and visible spectral regions, polycyclic aromatic hydrocarbons (PAHs) are widely regarded as model photoluminescent systems. The attachment of heteroatoms or functional groups generally reduces fluorescence intensity, primarily due to structural distortions and the emergence of additional non-radiative relaxation pathways. However, the photoluminescent response critically depends on the type, position, and number of substituents.

This work focuses on quantifying the photoluminescent contribution of an epoxide group attached to peripheral sites of coronene. Owing to the high symmetry of coronene (D6h), two distinct edge positions were identified and selected for functionalization. Geometry optimization and the calculation of absorption and emission edge wavelengths for both pristine and functionalized structures were performed using the density functional theory multireference configuration interaction method (DFT/MRCI).

For pristine coronene, the calculated absorption and emission edges are 300 nm and 397 nm, respectively, in good agreement with experimental data. When the epoxide bridges two carbon atoms within a single benzene ring, the absorption edge undergoes a redshift, while the emission wavelength is slightly blueshifted—a trend consistent with typical substituent effects. In contrast, epoxidation across two adjacent benzene rings induces a pronounced bathochromic shift in both absorption and emission bands and, unexpectedly, an increase in the oscillator strength of the S1 → S0 transition.

These findings underscore that the substitution position is a critical factor governing the optical properties of PAHs. They further suggest that precise control over functionalization sites can not only mitigate fluorescence quenching but also enable deliberate tuning—and potentially even enhancement—of photoluminescence efficiency in functionalized carbon-based materials.