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 S 1 → S 0 internal conversion to the excitation energy of the S 1 state, the energy gap, Δ E . 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 S 1 state. This then leads to an increase of the vibrational Franck−Condon factor governing the rate of internal conversion where f v is the Franck−Condon factor and α is a proportionality constant. As Δ E decreases, the rate of the S 1 → S 0 transition increases in competition with the rate of fluorescence, resulting in the suppression of fluorescence with an increasing MW of the asphaltene. Thus, the vibrational Franck−Condon factor, f v, has the desirable feature that it is not directly reliant on chemical composition but only the excitation energy of the chromophore that matches the requirement for applicability to asphaltene. This recognition revealed the possibility of a critical role of IC in the laser ionization process applied in mass spectroscopy. Here, photoexcitation leads through a vertical transition to a higher lying electronic state, which, upon internal conversion to the S 1 state, S n > 1 → S 1, generates some excess vibrational energy in the S 1 state. This IC is a fast process, with k (IC) ∼ 10 13 s −1, much faster than the S 1 → S 0 IC, with k ∼ 10 6 −10 9 s −1 . However, the excess vibrational energy has a marked accelerating effect on the latter rate, which becomes an exponential function of the excess vibrational energy. With an increasing size of the aromatic chromophore (MW), Δ E decreases and the excess vibrational energy, E v, increases, and as a result, both increase the rate of k (IC) S 1 → S 0 up to a level commensurate with k (S n > 1 → S 1 ). Increased rates for k (IC) S 1 → S 0 mean an increased loss of photons and a consequently lower ion yield in the MS. The effect occurs in parallel with MW, and at a critical MW, the ion yield may drop below the detection limit. Thus, it is suggested that the extremely low value found for the MW of asphaltene in most laser ionization mass spectrometry (LIMS) experiments is due to this inherent problem in the photophysics of asphaltene. Laser photo-ionization is a complicated process that has its limitations, and its non-critical use can lead to misleading results.

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