Cause of Asphaltene Fluorescence Intensity Variation with Molecular Weight and Its Ramifications for Laser Ionization Mass Spectrometry

The molecular-weight (MW)-dependent fluorescence of asphaltene discovered over 20 years ago has been re-examined from a theoretical point of view. It is concluded here that the dependence is a result of the sensitivity of the jump between the non-intersecting potential energy surfaces involved in the S1 → S0 internal conversion to the excitation energy of the S1 state, the energy gap, ΔE. The excitation energy of organoaromatic molecules, in general, depends upon the size of the aromatic chromophore which, in turn, has been shown to increase with the MW of the gel-permeation-chromatographic (GPC)-separated fractions of asphaltene. The larger the chromophore size, the lower the excitation energy of the S1 state. This then leads to an increase of the vibrational Franck−Condon factor governing the rate of internal conversion k(IC)∼1013fv∼1013exp(−αΔE) where fv is the Franck−Condon factor and α is a proportionality constant. As ΔE decreases, the rate of the S1 → S0 transition increases in competition with the...