Exploration of minimum energy conical intersection structures of small polycyclic aromatic hydrocarbons: toward an understanding of the size dependence of fluorescence quantum yields.

Minimum energy conical intersection (MECI) geometries were searched for polycyclic aromatic hydrocarbons (PAHs) consisting of up to 26 atoms. The energy barriers to the obtained MECIs showed a correlation with their fluorescence quantum yields. This provides a theoretical rationale for the size dependence of the fluorescence quantum yields seen in these PAHs.