To understand the change in fluorescence intensity upon forming asphaltene dimers, we investigated the oscillator strength of the C22-X series. Based on the oscillator strength of the first excited state of monomer X, the 132 monomers can be divided into two groups: the "bright" (X's oscillator strength greater than 0.01) and the "dark" (the rest). The X monomers can also be classified based on its fluorescence energy compared to C22's: