Energy & Fuels 2009, 23, 1555–1562
1555
Cause of Asphaltene Fluorescence Intensity Variation with Molecular Weight and Its Ramifications for Laser Ionization Mass Spectrometry Otto P. Strausz,* Imre Safarik, and Elizabeth M. Lown Department of Chemistry, UniVersity of Alberta, Edmonton, Alberta T6G 2G2, Canada ReceiVed June 18, 2008. ReVised Manuscript ReceiVed January 14, 2009
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 f S0 internal conversion to the excitation energy of the S1 state, the energy gap, ∆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 S1 state. This then leads to an increase of the vibrational Franck-Condon factor governing the rate of internal conversion k(IC) ∼ 1013fv ∼ 1013 exp(-R∆E) where fv is the Franck-Condon factor and R is a proportionality constant. As ∆E decreases, the rate of the S1 f S0 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, fv, 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 S1 state, Sn > 1 f S1, generates some excess vibrational energy in the S1 state. This IC is a fast process, with k(IC) ∼ 1013 s-1, much faster than the S1 f S0 IC, with k ∼ 106-109 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), ∆E decreases and the excess vibrational energy, Ev, increases, and as a result, both increase the rate of k(IC) S1 f S0 up to a level commensurate with k(Sn > 1 f S1). Increased rates for k(IC) S1 f S0 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.
Introduction Asphaltene, a black amorphous solid material, is a regular component of crude oil. Its concentration varies from traces in very light oils to about 20% in extra heavy oils, such as the bitumens of the Western Canada Sedimentary Basin. Asphaltene is the highest molecular-weight (MW) component of crude oil, and its presence adversely impacts the viscosity and all phases of the recovery, transportation, and upgrading/refining technologies of the oil. Chemically, asphaltene is an extremely complex mixture of n-alkyl-, naphthenoaromatic, and sulfur aromatic compounds, featuring two or more aromatic chromophores interconnected by polymethylene and sulfide sulfur bridges. These molecules have a high propensity for supramolecular aggregation via aromatic stacking by the Guldberg Waage mass action law and, * To whom correspondence should be addressed. E-mail: opstrausz@ shaw.ca.
as has been recently discovered,1 in the presence of moisture for micelle formation. These processes add an extra dimension to the difficulties, arising from the polydispersity of chemical composition and MW distribution, encountered in the exploration of their chemical and physical properties. Detailed chemical studies established the presence of a host of structural elements in the asphaltene molecule, including the following: (i) aromatic-attached n-alkyl-, R-methyl-, R-ethyl-, and R-n-propyl-n-alkanes (C1-C31), (ii) polymethylene bridges connecting aromatic ring structures (C3-C25), (iii) n-alkylbenzenes and monomethyl-n-alkylbenzenes (C12-C24), (iv) alkylaromatic hydrocarbons of the structural formula CnH2n-x (x ) 6-14) (C9-C28), (v) 2-n-alkyl and 2,5-di-n-alkylthiophenes (1) (a) Yarranton, H. W.; Aboudwarej, H.; Jakher, R. Ind. Eng. Chem. Res. 2000, 39, 2916. (b) Andersen, S. I.; del Rio, J. M.; Khvostitchenko, D.; Shakir, S.; Lira-Galeana, C. Langmuir 2001, 1307. (c) Andersen, S. I.; Birdi, K. S. Energy Fuels 2000, 38. (d) Murgich, J.; Lira-Galeana, C.; Merino-Garcia, D.; Andersen, S. I. Langmuir 2002, 9080.
10.1021/ef800480y CCC: $40.75 2009 American Chemical Society Published on Web 02/20/2009
1556 Energy & Fuels, Vol. 23, 2009
Figure 1. Emission spectra of the whole asphaltene and its fractions 1-5 in benzene. Concentration ) 3.0 and 60 mg/L. Dotted curves ) intensity (right-hand scale). λexc ) 350 nm. W ) whole asphaltene. (- · - · -) Solvent blank.1,3
(C10-C28), (vi) 2-n-alkyl and 2,6-di-n-alkylthiolanes and thianes (C10-C29), (vii) 2-n-alkyl-, 4-n-alkyl-, and 2,4-di-n-alkylbenzothiophens (C10-C24), (viii) 1-n-alkyldibenzothiophenes (C12-C24), (ix) alkylaromatic sulfur compounds CnH2n-x (x ) 10-16, 20) (C10-C26), (x) 9-n-alkylfluorenes (C14-C26), and (xi) hosts of biological markers: hopanes, steranes, cheilanthanes, etc. Any one of these molecules can be attached to the asphaltene core at different sites, resulting in a multitude of different combinations. When it is remembered that there is also a large number of other molecules detected (not mentioned here), that the detected molecules represent only a fraction of the total constituents present, and also that the combination may give rise to large MW variations from several hundreds to several thousands grams per mole, taken together, this gives a sense of appreciation of the complexity of the asphaltene molecule. As if this were not enough, the chemistry is further complicated by the omnipresent supramolecular aggregation, obscuring the domains of covalency; for example, in MW measurements, it is difficult to distinguish the MW of the covalent from that of the supramolecular or micellar aggregates. In effect, this has been a contentious issue in the chemistry of asphaltene for the past several decades, as will be discussed later in this paper. Another and somewhat related current issue in asphaltene chemistry, which is the subject of the present paper, is related to the fluorescence of asphaltene. Over 20 years ago, it was reported from this laboratory2 that the fluorescence yield from the ultraviolet irradiation of asphaltene varies with the MW of the gel permeation chromatographic (GPC) fractions3 of asphaltene. The fluorescence spectra of the five GPC fractions 1 f 5 isolated, having vapor pressure osmometric (VPO) MW of 16 900, 13 700, 7100, 3400, and 1200 g/mol, respectively, are reproduced in Figure 1.1,3 The spectra were taken in dilute benzene solutions at concentrations of 3.0 mg/L (and 60 mg/L), where asphaltene aggregates formed either by mass action or micellization should be dissociated into their covalent constituents. [At the time of the publication of the fluorescence and absorption spectra (Figures 1 and 2), it was not known that the aromatic rings in the asphaltene molecule were so richly substituted with the compounds listed above and others that intramolecular exciplex formation would (2) Yokota, T.; Scriven, F.; Montgomery, D. S.; Strausz, O. P. Fuel 1986, 65, 1142–1150. (3) Ignasiak, T. M.; Kotlyar, L.; Samman, N.; Montgomery, D. S.; Strausz, O. P. Fuel 1983, 62, 363–369.
Strausz et al.
Figure 2. Absorption spectra of the whole asphaltene and its fractions 2-5 in benzene. Concentration ) 19.2 mg/L. [Dotted line (W)] ) whole asphaltene.2 (Inset) Concentration ) 60 mg/L.
have been effectively prevented by steric interference.] The emission from fraction 5 was checked for phosphorescence, but it was barely detectable. Nonetheless, strictly speaking, the observed emissions are the sums of fluorescence and phosphorescence in undefined proportions that may vary from molecule to molecule and from fraction to fraction. As is evident from the spectra, the fluorescence intensity in the spectral range of 360-620 nm exhibits a progressively decreasing trend with an increasing MW of the fractions. The integrated fluorescence yield at λ(excitation) ) 350 nm and concentration ) 3.0 mg/L could be estimated to be about 2, 6, 11, 23, and 58% of the total for fractions 1-5, respectively. This is in contrast to the trend in the absorptivity of the fractions2,4 (Figure 2), where the absorbance increases with an increasing MW. As suggested by its black color, asphaltene absorbs the entire visible range and indeed into the near-infrared up to 1000 nm and beyond.5 The only departure from this trend is apparent in the case of fraction 5. The reason for this exception is that fraction 5 is chemically different from the rest of the fractions and, in reality, does not belong to the asphaltene; it appears there only as a result of an equilibrium equipartitioning6 between the solution phase and the precipitated asphaltene during the course of the separation of asphaltene from the oil. It should be noted that the absorption spectrum of fraction 1 was identical to that of fraction 2 and therefore is not shown in Figure 2. Now, the total amounts of aromatic carbon in fractions 1-5, as determined7 by 13C nuclear magnetic resonance (NMR) spectroscopy, were 28, 27, 31, 33, and 38% by weight, and therefore, the increase in absorbance cannot be due to increasing aromatic carbon contents in the fractions. Quite the contrary, the increase in absorbance per aromatic carbon is even greater than it appears from the spectra, and this can be taken as a clear manifestation of the increasing size of the aromatic chromophore with an increasing MW of the fraction. Thus, it can be concluded (4) (a) Strausz, O. P.; Lown, E. M. The Chemistry of Alberta Oil Sands, Bitumen and HeaVy Oils; Alberta Energy Research Institute: Calgary, Alberta, Canada, 2003 (available at www.aeri.ab.ca). (b) Frakman, Z.; Ignasiak, T. M.; Montgomery, D. S.; Strausz, O. P. AOSTRA J. Res. 1988, 4, 171–179. (5) Badre, S.; Goncalves, C. C.; Norinaga, K.; Gustavson, G.; Mullins, O. C. Fuel 2006, 85, 1–11. (6) Strausz, O. P.; Torres, M.; Lown, E. M.; Safarik, I.; Murgich, J. Energy Fuels 2006, 20, 2013–2021. (7) Cyr, N.; McIntyre, D. D.; Toth, G.; Strausz, O. P. Fuel 1987, 66, 1709–1714.
Asphaltene Fluorescence Intensity Variation with MW
Energy & Fuels, Vol. 23, 2009 1557
that, the larger the aromatic sheet, the lower the fluorescence yield upon UV excitation. Additional support for this conclusion comes from the observation that the maximum of the fluorescence envelope shifts gradually to longer wavelengths with an increasing MW of the fraction (Figure 1), reflecting the wellknown characteristic bathochromic shifts in the onsets of the absorption spectra of aromatic molecules with increasing molecular size. It also follows that the difference in fluorescence intensity between fractions 1 and 4 is larger than it appears from the spectra because the spectra are not corrected for the progressively increased amount of light absorbed in going from fraction 4 to 1 (Figure 2). Even fraction 5, which fluoresces much stronger than fractions 1-3, has smaller absorbances and thus absorbs less light than fractions 1-3. In the following, the causes and consequences of these spectral characteristics of asphaltene will be examined with reference to the photophysics of asphaltene and the laser ionization mass spectrometry (LIMS) of asphaltene and aromatic molecules in general. Section 1 will deal with the photophysics of asphaltene, and section 2 will deal with the ramifications of asphaltene photophysics to LIMS. Discussion 1. Asphaltene Photophysics. When irradiated in their first absorption band in the UV or visible region, aromatic molecules in general are promoted from their ground electronic singlet state (S0) to their lowest (first) electronically excited singlet state (S1) (1) S0 + hν f S1 In non-photoreactive systems, this state can undergo one of three competing alternative unimolecular steps: internal conversion (IC), a nonradiative transition to the ground electronic state, fluorescence (F), a radiative transition to the ground electronic state, and intersystem crossing (ISC) to the lower-lying first excited triplet state (IC)
S1 98 S0
(2)
(F)
S1 98 S0 + hν
(3)
(ISC)
S1 98 T1
(4)
The competition among these three steps determines the fate of the S1 state. Sometimes, as in the present instance, it is possible to gain highly useful information concerning the competition between steps 2 and 4 and their ramifications with respect to various photophysical events from simple spectroscopic studies. Internal ConVersion. The maximum value for the rate of this step, k(IC), is ∼1013 s-1, but there is a sharp distinction between transitions from a higher than the first excited electronic level to the first excited level, Sn > 1 f S1, and from the first excited level to the ground state, S1 f S0. In the former case, the transition occurs through the crossing of potential energy surfaces, where the transitions are fully allowed and facile, whereas in the latter case, the S1 and S0 surfaces do not cross and the transition occurs through a jump related to quantum mechanical tunneling phenomena. For these processes, the rates are very inefficient and vary with the molecular properties of the absorbing chromophore.8 They are often limited by the (8) Turro, N. J. Modern Molecular Photochemistry; The Benjamin/ Cummings Publishing Co.: Menlo Park, CA, 1978.
vibrational Franck-Condon factor (fv), which is a very sensitive function of the gap ∆E(S1-S0) between the zero-point vibrational levels of the states undergoing internal conversion and has the form k(IC) ∼ 1013fv ∼ 1013 exp(-R∆E) (5) where fv is the Franck-Condon factor and R is a proportionality constant. The energy gap law can be attributed to the changes in the Franck-Condon overlaps of the nuclear wave functions, which become increasingly unfavorable with increasing energy separation.8 In addition, because the excitation energies of the S1 states of aromatic molecules tend to decrease with an increasing size of condensation, the values of k(IC) increase accordingly (eq 5). Experimentally, internal conversion from the lowest excited singlet state is usually negligibly slow compared to fluorescence or intersystem crossing for nonphotoreactive, relatively rigid molecules if ∆E(S1 f S0) is larger than 50-60 kcal/mol.8 For example, at ∆E ∼ 100 kcal/mol, fv ∼ 10-8, so that k(IC) ∼ 105 s-1, and even for ∆E ∼ 50-60 kcal/mol, fv ∼ 10-5 and k(IC) is only 108 s-1. The corresponding k(IC) values for small, rigid aromatic molecules usually lie in the 105-108 s-1 range. However, as was mentioned before, for higher excited states, IC to the S1 state is usually much faster, of the order of 1013 s-1, and fluorescence, with few exceptions (one of which is azulene, vi), occurs only from the S1 state. Quantum yield values have been reported for the S1 f S0 IC step (cf. Murow, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry, 2nd ed.; Marcel Dekker: New York, 1993). For small, rigid aromatics in liquid solutions (naphthalene, anthracene, fluorene, triphenylene, acenaphthene, chrysene, etc.), the values are low (e0.10); however, for coronene, the value is 0.21, and for the linear acenes, the values become significantly higher (0.31 for tetracene and 0.76 for pentacene). Also, some small aromatics have unusually high values, e.g., 0.22 for 9-phenylanthracene, 0.24 for indole, 0.30 for m-terphenyl, and 0.5 for acridine. In the case of large polyaromatic molecules absorbing light in the 450-760 nm region, where the S1-S0 energy gap can be as low as 38 kcal/mol, the quantum yields for the IC step should be quite high according to the energy gap law (eq 5). Fluorescence. In liquid solutions, the rates of fluorescence from common aromatic molecules8 lie in the range k(F) ∼ 106-108 s-1. Aside from electronic and symmetry effects, the geometric shape of the molecule is known to influence the values of both k(F) and k(IC): rigidity increases k(F) and decreases k(IC). When irradiated in their first absorption band, most aromatic molecules exhibit fluorescence. Intersystem Crossing. The rate for this step in most aromatic molecules is comparable to that of k(F) and has values in the range 106-108 s-1. The efficiency of ISC is governed by the electronic coupling, spin-orbit coupling, and energy gap between S1 and the triplet state to which crossing occurs. Unlike IC, ISC and F are, in general, sensitive to the heavy atom effect, which may increase the rate of ISC in the extreme by up to 3 or 4 orders of magnitude and decrease the rate of fluorescence. Sulfur exhibits a modest heavy atom effect causing a good order of magnitude increase in the rate of k(ISC) (Table 1). This may have significance in some aspects of asphaltene photophysics because of the high concentration of sulfur aromatics in asphaltene. Photophysics of Asphaltene. As was described above, the GPC-separated fractions of asphaltene (from the large Athabasca deposit) exhibited a gradual decrease in the fluorescence intensity with an increasing VPO MW of the fractions despite their increasing absorptivity with an increasing VPO MW. This
1558 Energy & Fuels, Vol. 23, 2009 Table 1. Reported Rate Constants for the Fluorescence and Intersystem Crossing of Fluorene and Its Heteroaromatic Derivativesa
Strausz et al.
Ia ) [S][k(IC) + k(F) + k(ISC)] ) [S]K
(6)
where K ) [k(IC) + k(F) + k(ISC)] Ia/K ) [S]
(7)
rate of fluorescence ) [S]k(F) ) (Ia/K)k(F)
(8)
where Ia is the light intensity absorbed. Under pulsed conditions dS/dt ) -KS dS/S ) -
a Nijegorodov, N.; Luhanga, P. V. C.; Nkoma, J. S.; Winkoun, D. P. Spectrochim. Acta, Part A 2006, 64, 1-5.
feature of asphaltene fluorescence, in conjunction with the shift of the position of the maximum in the fluorescence envelope to longer wavelengths with increasing VPO MW, provides a beautiful validation of the role of the Franck-Condon effect and the energy gap law in governing the rate of IC in the aromatic chromophores of asphaltene and is a convincing demonstration of the role played by IC in the photophysics of asphaltene. The competition among steps 2-4, which is important for governing the fluorescence yield, changes with the size of the aromatic chromophore. For small, rigid molecules (fraction 5), the competition is between k(F) and k(ISC). However, with an increasing MW, as indicated by the bathochromic shift in the fluorescence maximum, the competition is, by and large, between k(F) plus k(ISC) and k(IC). This is principally due to the decrease in ∆E, the energy gap between the zero-point vibrational levels of the states undergoing internal conversion in the expression for the Franck-Condon overlap (eq 5). The UV-vis spectra of aromatic chromophores is attributed to π* r π (S1 r S0) transitions. The spectra of linear acenes consist of three π* r π transition bands (E1, E2, and B), whose energy of highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) separation and the position of λ(max) for each absorption band depends upon the number of condensed aromatic rings, and in general, λ(max) increases with the number of condensed aromatic rings. The onset of absorption in the linear acene series also increases with the number of rings and as does the short-wavelength tail end of their emission spectra (benzene, 265 and 280 nm; naphthalene, 290 and 320 nm; anthracene, 380 and 390 nm; tetracene, 475 and 485 nm; pentacene, 580 and 590 nm for absorption and emission, respectively). These spectral characteristics are substantially affected by the mode of condensation in nonlinearly condensed aromatic chromophores and substitution. As is obvious from Figure 1, the fluoresecence spectra are all continua, practically without any significant features, and it is not possible to make any assignments except for the bands due to porphyrins.2,4,9,10 Finally, it should be noted that the true value of k(F), kO(F), cannot be determined from fluorescence measurements alone. It is obtained from the integrated absorption spectrum or the combination of the values of k(F) with the quantum yields of fluorescence. The relationship between the rate of fluorescence and the inherent, spectral kO(F) value can be seen from the simple photostationary kinetic treatment of the processes involved. Under photostationary state conditions (9) Evdokimov, I. N.; Losev, A. P. Energy Fuels 2008, 22, 2470–2473. (10) Becker, R. S. Theory and Interpretation of Fluorescence and Phosphorescence; Wiley Interscience: New York, 1969.
∫ Kdt
(9) (10)
If the conditions are set so that at t ) 0 the concentration is [S]0 and at t > 0 the concentration is [S] then, after integration, we obtain ln([S]/[S]0) ) -Kt
(11)
and [S] ) [S]0e-Kt (12) Thus, in fluorescence measurements, the relevant rate coefficient is the sum of the rate coefficients of all three reactions contributing to the decay of the S1 state. Asphaltene is known to contain multichromophore molecules, in which two (or more) aromatic centers are interconnected by an aliphatic structural element (a polymethylene, sulfide, naphthenic, etc.) bridge.11,12 In such structures and under favorable conditions, the electronic excitation energy may be transferred intramolecularly from the absorbing chromophore (the donor) to an acceptor chromophore Such intramolecular electronic
excitation transfer processes may proceed at very high speeds,13 of the order of 1011 s-1, which is faster than the IC, resulting in a very fast fluorescence decay. Nonetheless, the acceptor (S1) will decay at a normal rate, and under steady irradiation, the only footprint left is the spectrum of the bathochromically shifted fluorescence of the acceptor chromophore. (This process may generate additional amounts of excited molecules in the lower S1 excitation region that can more readily undergo IC.) One example of such a process is the solution-phase photolysis of the naphthalene-(CH2)-anthracene molecule, where irradiation in the naphthalene spectrum leads to naphthalene fluorescence, with a decay rate of the order of 1011 s-1, and an anthracene fluorescence, with the normal decay rate of ∼108 s-1.13 Therefore, the overall fluorescence plus phosphorescence yield was not significantly affected by the intramolecular energy transfer. Summary. Over 20 years ago, it was reported that fluorescence intensities emerging from the liquid solutions of GPC-separated fractions of Athabasca asphaltene upon ultraviolet excitation exhibit progressively declining intensities with increasing MW of the fractions. In the interim period, no explanation for the observed trend was advanced. Here, the cause responsible for the observed trend is elucidated in terms of the basic laws of photophysics, namely, the effects of the Franck-Condon factor (11) Mojelsky, T. W.; Ignasiak, T. M.; Frakman, Z.; McIntyre, D. D.; Lown, E. M.; Montgomery, D. S.; Strausz, O. P. Energy Fuels 1992, 6, 83–96. (12) Peng, P.; Morales-Izquierdo, A.; Hogg, A.; Struasz, O. P. Energy Fuels 1997, 11, 1171–1187, and references therein. (13) (a) Speiser, Sh.; Hasegaura, M.; Enomoto, Sh.; Hoshi, T.; Igarashi, K.; Nishimura, Y.; Yasuda, T.; Yamazaki, T. J. Lumin. 2003, 102-103, 278–282. (b) Hasegaura, M.; Enomoto, Sh.; Hoshi, T.; Igarashi, K.; Yamazaki, T.; Nishimura, Y.; Speiser, Sh. J. Phys. Chem. B 2002, 106, 4925–4932.
Asphaltene Fluorescence Intensity Variation with MW
and the energy gap law on the internal conversion step competing with fluorescence. First we consider all of the experimental facts: (1) Absorbance (in the λexc ) 280-500 nm region) increases with MW. This means that the number of aromatic carbons increases, the size of the aromatic chromophores increases, or both. (2) 13C NMR measurements show that the number of aromatic carbons slightly decreases with an increasing MW. That leaves the aromatic chromophore size increase as the only cause for the increase in absorbance with the MW of the GPC fraction. (3) λmax of the fluorescence envelope shifts to longer wavelengths with an increasing MW of the fractions. This feature is in agreement with the increasing size of the aromatic chromophores. (4) λexc in the fluorescence measurements was 350 nm, which is above the absorption limits of benzene, naphthalene, phenanthrene, fluorene, benzo- and dibenzothiophene, etc., i.e., one-, two-, and most three-ring aromatics. Therefore, these types of compounds had no role in the experiments reported.2 Now, chemically nonreactive molecules at low concentrations may undergo either of three transitions in their S1 states, with rates k(IC), k(F), or k(ISC), The rate k(IC) can be very fast, much faster than either k(F) or k(ISC), but is often limited by the Franck-Condon factor “fv” as a consequence of the poor overlap between the vibrational wave functions of the states involved in the transition, and the rate can be expressed as (5) k(IC) ∼ 1013fv ∼ 1013 exp(-R∆E) where fv is the Franck-Condon factor, R is a proportionality constant, and ∆E is the energy gap between the zero-point vibrational levels of the S1 and S0 states. It is also an experimentally and theoretically well-established fact that the energy gap exhibits, in general, a decreasing trend with an increasing size of the aromatic chromophore. This is evident from the gradual increase in the wavelength of the onset and maximum of absorption and the position of the maximum in the fluorescence envelope with the increasing size of the aromatic chromophore. The critical value of the energy gap, ∆E, that is, the S0 f S1 excitation energy, where k(IC) becomes significantly competitive with k(F) and k(ISC), is around 60 kcal/ mol, corresponding to λ ) 477 nm. Thus, at ∆E ∼ 60 kcal/ mol, the fluorescence begins to fade. Therefore, in general, colored polyaromatic compounds absorbing light above λ ) 500 nm have high values for k(IC), and their fluorescence intensity is gradually reduced with increasing molecular size and therefore with progressively lower ∆E values, as has been experimentally observed. Hence, the spectra seen in Figures 1 and 2 may be taken as a beautiful validation of the Franck-Condon energy gap law to the photophysics of asphaltene. A practically important corollary of the latter is related to the process of laser ionization (LI) employed in the mass spectroscopy of asphaltene, as will be discussed in the following section. 2. Possible Roles of the Franck-Condon Factor and the Energy Gap Laws in LIMS of Asphaltene. The MWdependent suppression of fluorescence by IC in asphaltene solutions suggested the possibility of a similar role by IC in the gas-phase photo-ionization process of asphaltene. This would not only lower the ion yield and thus the detection sensitivity in LIMS, but it would also render the sensitivity MW-dependent and could ultimately lead to the arrest of photo-ionization altogether at some point. IC from higher excited singlet states (Sn > 1 f S1) is one of the fastest steps in photophysics and photochemistry and could potentially compete with any other process. In the solution phase, it involves the stepwise energy cascade from the excited singlet state reached in the photon absorption of organic molecules from state to state by the
Energy & Fuels, Vol. 23, 2009 1559
crossing of the potential energy surfaces and results in the conversion of the electron excitation energy to the vibrational excitation energy of the nuclei. There are different types of crossings, and some permit transition from one surface to another, while some others would restrict it. The types of crossings that occur in Sn > 1 f S1 transitions favor transitions, and those encountered in the S1 f S0 transitions restrict it. This is the reason why fluorescence and intersystem crossings can favorably compete with S1 f S0 type IC, but in general, they cannot compete with Sn > 1 f S1 type IC. The high excitation energies needed in LIMS applications are generally produced in multiphoton processes because the photon energies produced in the commonly used nanosecond pulsed lasers (e110 kcal/mol) are below the ionization thresholds of most organic molecules. Therefore, at least two photons or more, depending upon the ionization threshold of the molecule to be analyzed, the energy of the photons, and compliance with other reaction parameters, are needed to effect ionization. The photon absorption may proceed via two different mechanisms, a linear, i.e., sequential, or a nonlinear, i.e., simultaneous, two-photon absorption. The critically energized molecules with excitation energies equal to or somewhat higher than the ionization threshold have very short but finite lifetimes of the order of the IC lifetimes for the Sn > 1 f S1 transitions in organic molecules (10-13 s-1). However, it would appear that the possible role that IC transitions might play in the dynamics of the ionization of critically energized organic molecules in LIMS applications has not been extensively considered. The impact of the IC on the fluorescence yield of asphaltene, as discussed in section 1, points to the possibility that IC might play a decisive role in the overall dynamics of electron ejection from the critically excited asphaltene molecule as well. Any of the excited states of the absorber lying in the appropriate energy range can be populated by either an energetic single-photon or a less energetic multiple-photon excitation. However, the twophoton excitation step offers the advantage of using light of wavelengths lying in the experimentally more accessible ultraviolet, instead of the less accessible vacuum ultraviolet, region of the spectrum. The mode of laser excitation, as would be surmised (single or multiple photon, single or multicolor, and order of application), however, as will be shown below, may have a major impact on the mechanism and yield of ion production. As has been discussed in the literature,14,15 multiphoton processes occur to a significant extent only when the density of photons in the exciting light beam is sufficiently high. The light intensity required depends, among other parameters, upon the number of photons needed to effect ionization (the order of the reaction and the lifetime of the intermediate excited electronic state(s) of the absorber molecules) and on the laser pulse width. Nanosecond pulses [most often used in laser desorption ionization MS (LDIMS) and matrix-assisted LDIMS (MALDIMS)] with intensities in the range of 106-1011 W/cm typically result in ionization efficiencies in the range of 10-5 to several percent.14 However, at the very high light intensities achievable in femtosecond laser pulse excitation, Coulombic field intensities comparable to the electric field strength inside a molecule, of the order of E ) -87 V/nm, can be realized; this is sufficient to bring about field ionization by the tunneling mechanism.14 (14) Burlingame, A. L.; Boyed, R. K.; Gastrell, S. Anal. Chem. 1998, 70, 647. (15) Aronbjerg, J.; Johnsen, M.; Ogilby, P. R. Spectrum 2008, 21, 20– 27.
1560 Energy & Fuels, Vol. 23, 2009
Strausz et al.
Simultaneous two-photon absorption processes are mediated via so-called virtual states of the absorber, which can nonetheless be described as a sum over all of the real eigenstates, j, of the molecule.15 The two-photon absorption cross-section, δ, in terms of one-photon transition moments, µ, between the real states of the molecule, can be expressed by the equation δ(ω)R
|µ0j|2µjf|2
∑ ω (ω - ω)
2
j
j
+ Γj2
substituents, etc., will behave identically and proceed with identical ion yields. One of the representatives of the smallest condensed aromatic molecules is the blue solid azulene molecule, an isomer of the more common naphthalene molecule:
g(2ω)
where the unit for δ is the Go¨pert-Mayer (GM), with 1 GM ) 10-50 cm4 s. The µs are one-photon transition moments between the real states of the molecule. ω in the equation denotes the frequency of the laser light; g(2ω) is a band shape function; and ωj and Γj are the eigenfrequency and line width of the jth state, respectively. The equation features two key parameters for the simultaneous two-photon absorption cross-section, namely, a quadratic light intensity dependence and the (ωj ω) detuning term. When the two-photon laser frequency corresponding to half the transition energy approaches the onset of the one-photon absorption band, the two-photon absorption exhibits a dramatic increase. This increase is the consequence of the decrease in the value of the detuning term in the denominator of the equation, (ωj - ω), which is the difference between the laser frequency and the transition frequency to the jth one-photon state. This is the phenomenon of resonanceenhanced multiphoton ionization (REMPI).15 LI and REMPI MS of asphaltene has been the subject of longstanding, controversial debates in the literature.16-21 The central issue of the debate is the question whether the MW of asphaltene exceeds a narrow, low-MW range of about 300 to somewhat over 1000 amu or whether it extends far beyond that range into the thousands. We propose here a clarification of the problem, based on our experimental observations on the fluorescence of asphaltene as described in section 1 and some recent, relevant information from the literature22 concerning the LIMS determination of asphaltene MW. In the gas-phase photo-ionization processes taking place upon laser excitation in the mass spectrometer, any number of the numerous low-lying electronic states accessible with energies up to or somewhat exceeding the ionization potential of the molecule may be involved. Also, some of the electronic states (superexcited states, double-excited states, and Rydberg states) may be highly unstable with unusual properties. Electronically excited molecules usually carry vibrational excitation superimposed on the electronic excitation as well. In some cases, electronic transitions may be subject to a “propensity rule”, requiring the preservation of the vibrational quantum state by restricting the transition probability to ∆V ∼ 0. For these and related reasons, the photo-ionization mechanisms of condensed aromatic molecules are highly complicated, and in the context of the LIMS of asphaltene, it cannot be expected that all molecules, regardless of their chemical composition, size,
Its photophysics are somewhat irregular inasmuch as its fluorescent state is not the S1 state but the S2 state. In an elegant, detailed study22 published recently on the time-dependent photoionization of azulene and the competition between ionization and relaxation in highly excited states, it was shown that the phenomena observed in the study may be quite generic in polycyclic aromatic molecules. A detailed consideration of all of the results and their interpretations lies outside the scope of the present paper, and only the relevant salient features of this study will be discussed here, as follows: (1) In the λ ) 266 nm irradiation, azulene is promoted to its S4 electronic state hν
S0 98 S4 (V ) 0) + Ev (0.26 eV)
(14)
266 nm
having an excitation energy of 4.40 eV, followed by the reactions hν
S4 (V ) 0) + Ev (0.26 eV) 98 D1 (V ) 0) + 0.82 eV |w0 266 nm
(15) IC
98 S2 (V ) 0) + Ev (1.1 eV)
(16)
hν
S2 (V ) 0) + Ev (1.1 eV) 98 D0 (V ) 0) + 1.92 eV |w0 266 nm
(17) where Ev represents excess vibrational energy, D0 and D1 are the ground and first electronically excited states of the cation, and | w 0 indicates that the ∆V ) 0 propensity rule is observed and the reaction is allowed. (2) The rate of the IC transition (16) is very rapid, and the population created in S4 has relaxed to S2 on a 120 fs or lower time scale, which is commensurate with the time scale for the photo-ionization of S4 (15); thus, D1 is detectable only in the first 120 fs of the laser pulse. (3) The lower limit for k(IC)16 can be estimated to have a value of ∼1/(120 × 10-15) s ) 0.8 × 1013 s-1. (4) In turn, the rate for the S2 f S*0 IC (reaction 18) IC
(16) Mullins, O. C.; Martinez-Haya, B.; Marshall, A. G. Energy Fuels 2008, 22, 1765–1773. (17) Hortal, A. R.; Hurtado, P.; Martinez-Haya, B.; Mullins, O. C. Energy Fuels 2007, 21, 2863–2868, and references therein. (18) Asphaltenes, HeaVy Oils, and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, G., Eds.; Springer: New York, 2007. (19) Pomerantz, A. E.; Hammond, M. R.; Morrow, A. L.; Mullins, O. C.; Zare, R. N. J. Am. Chem. Soc. 2008, 130, 7216–7217. (20) Herod, A. A.; Kandioty, R.; Bartle, K. D. Fuel 2006, 85, 1950– 1951, and references therein. (21) Herod, A. A.; Bartle, K. D.; Kandioty, R. Energy Fuels 2007, 21, 2176–2203. (22) Blanchet, V.; Raffael, K.; Turri, G.; Chatel, B.; Girard, B.; Garcia, I. A.; Wilkinson, I.; Whitaker, B. J. Chem. Phys. 2008, 128, 164318.
S2 98 S*0
(18)
(where the asterisk signifies vibrational excitation) is much slower and follows an exponential energy gap law as a function of the vibrational energy in the S2 state. (5) Log k(IC)18 increases with excess vibrational energy in S2 and is a good linear function of the excess vibrational energy as measured in the vibrational energy range of 0-1.10 eV, where log k18 varies between 8.4 and 10.3 s-1. (6) The photo-ionization follows a two-color (wavelength), three-photon pathway, even if energetically allowed in a
Asphaltene Fluorescence Intensity Variation with MW Scheme 1
stepwise absorption of two photons, and involves one or possibly more highly unstable doubly excited states (Scheme 1), which can undergo on an ultrafast time scale to IC to a common set of Rydberg states or can auto-ionize if their origins lie above the ionization potential (IP), where DES* defines an unstable, highly reactive double excited state, R1 and R2 are Rydberg states, and D0 and D1 are the ground and first electronically excited states of the cation; all of these state are vibrationally excited as well. (7) Photolysis of the Rydberg states leads to photo-ionization. (8) The intensity of the photoelectron spectrum is proportional to the population in S2. Now, considering the data in 2-5 we can see that k(IC)16 for the S4 f S2 transition (the analogue of the Sn > 1 f S1 transition in most polycyclic aromatic molecules) is independent of the vibrational energy and about 2-4 orders of magnitude larger than k(IC) for the S2 f S0 transition (the analogue of the S1 f S0 transition), which is a function of the excess vibrational energy in S2, ∼1.3 × 1013 s-1 versus 108.4-1010.3 s-1,22 in line with the known general trend for k(IC)n > 1 f 1 and k(IC)1 f 0. Following the conclusions drawn from that work that “these processes may be quite general in aromatic systems in which doubly excited states below the IP are likely to be ubiquitous”, we will now examine how to apply the knowledge gained from the work described in ref 22 to the LIMS of asphaltene and what kind of prediction it will lead us to. First, we examine the question of how the value of k(IC)16 would change in going from azulene to a large polycyclic aromatic hydrocarbon. In the large molecule, as discussed in section 1, the S1sS0 energy gap would be lower and that would, according to eq 5, cause an increase in k(IC) of the S1 f S0 reaction. Employing the same light source, the excess vibrational energy would be higher as a consequence of the lower S1 state energy and that would cause a further increase in the value of k(IC) for the S1 f S0 reaction. At λ ) 266 nm irradiation, the excess vibrational energy in azulene is 1.1 eV and k(IC) at this energy level is 1010.3 s-1. If S1 in the large molecule is lower by 1.1 eV than S2 in azulene then, according to the linear functional dependence of log k(IC) for the S1 f S0 step (Figure 5 in ref 14), the value of k(IC) for the S1 f S0 step in the large molecule would be greater than 1012 s-1 and, thus, large molecules with 3.56 eV (S2 in azulene) - 1.1 eV ) 2.46 eV or λ (onset of light absorption) g 500 nm would have the capacity for very rapid IC. This means that the S1 population would decay faster, thus reducing its population and the intensity of the resulting photoelectron spectrum in proportion to the population in S1 (cf. 8) along with the yield of ionization. In other words, the ion signal intensity from the large molecule, under identical experimental conditions, would be significantly lower than from azulene. Thus far in this estimation, we only considered the effect in the increase of the gas-phase energy gap law. However, the 1.1 eV decrease in the S1-S0 excitation energy would also increase the rate of the
Energy & Fuels, Vol. 23, 2009 1561
zero-excess vibrational energy, i.e., the solution phase component of the energy gap law, and therefore, the actual rate could substantially exceed the one obtained above (1012 s-1). If this argument holds, then it is needless to say that, at an appropriately low value of the S1-S0 excitation energy, the ion signal in the mass spectrometer could fall below the detection limit and the high-MW molecule becomes invisible. Also, in the case of a complex mixture of condensed aromatic molecules, such as asphaltene, the integrity of the concentration determination would become compromised by the built-in bias against increasing MW components. The foregoing discussion leads us to the conclusion that in the gas phase there are two superpositioned exponential energy gap laws governing the magnitude of the rate of the S1 f S0 type intersystem crossing. In addition to the energy gap law expressing the effect of the S1-S0 electronic energy difference (as was discussed in section 1) operative in the liquid phase, superpositioned is the energy gap law describing the effect of the excess vibrational energy in the S1 state. In the case of the azulene molecule, the gas phase k(IC) for the S2 f S0 reaction with zero-excess vibrational energy in S2 has a value22 of 2.5 × 108 s1, which is of the same order of magnitude as would be expected for the solution-phase reaction. Indeed, the value for the latter has been reported to be 7 × 108 s-1, in reasonably good agreement with expectations. It also follows that, unlike in the solution phase, where k(IC) for the Sn > 1 f S1 reaction is orders of magnitude higher than k(IC) for the S1 f S0 reaction, the two rates may become commensurate in the gas phase, depending upon the amount of excess vibrational energy involved. Returning now to the LIMS of asphaltene, we recall here that, as has been shown in Section 1, increasing the VPO MW of asphaltene means increasing the physical dimension of the principal aromatic chromophore in the asphaltene molecule. This in turn means a lower S1-S0 excitation energy and higher excess vibrational energy in S1, giving rise to higher photon loss, owing to an increased S1 f S0 IC. Evidently, in the case of azulene, photon loss is a more serious problem in the 266 nm two-photon irradiation experiment than in the 266 nm + 2 × 400 nm threephoton experiments. In most LIMS of asphaltene, two-photon irradiations were used and the example of azulene would seem to suggest that better sensitivity could perhaps be achieved with appropriate three-photon irradiation. Thus, our proposal as outlined above provides a simple logical reason why covalent petroleum asphaltene molecules appear in LIMS and LI-Fourier transform MS to yield such unexpectedly low values for the upper limit of asphaltene MW. The question then arises, how can higher masses, in the several thousands, appear in some LIMS measurements of asphaltene. These masses are likely due to asphaltene clusters. The predominant mode of cluster formation in a moisture-free solution of asphaltene is aromatic stacking arising from the faceto-face interactions of their molecular π orbitals. These exciplexes are stable in their ground states and should be even more stable in their S1 states. Also, their stability may depend upon the size of their aromatic chromophores, i.e., their VPO MW, and their stability would be expected to increase with increasing MW. Moreover, their degree of association would not necessarily be restricted to two, but their number may be in the low multiples. If the basic features of their ionization mechanisms are not altered fundamentally, they will appear in the LIMS as high-MW covalent molecules. The preceding discussions by no means are specific for the asphaltene molecule. This is evident from the results of a study
1562 Energy & Fuels, Vol. 23, 2009
reported23 over a decade and a half ago, in which the tryptophan molecule and its low-MW peptides (condensates) up to a MW of
2500 g/mol were investigated in LI and electron impact MS for their molecular ion yields as a function of the MW of the molecule. In this series of compounds, the chromophore remained the same, while the MW varied from 200 to 2500 g/mol. It was found that for all three modes of excitation used (single photon, two photon, and electron impact) the molecular ion yield progressively fell off as the MW increased. The ion yield functions were different for the three modes of excitation: the fastest decline appeared for the case of single-photon excitation, and the slowest decline appeared for two-photon excitation. The yield curves could have been affected by fragmentation of the molecular ions, but nonetheless, the suppressing effect of MW on the appearance of the parent molecular ion is clear. The possible mechanisms proposed for the molecular ion falloff with increasing MW of the molecules include the following:23 (1) “in larger molecules ionization occurs through the intermediate formation of charge pairs which can ionize in a unimolecular-like fashion due to an energy fluctuation” and (2) “a more probable route is, however, for the charge pairs to recombine”. Thus, in large molecules, “the initially-formed state is a charge pair interacting by a screened Coulomb potential where the molecules in effect act as a solvent for their own excited chromophores. The qualitative experimental implication of the detailed theoretical treatment was that the ionization efficiency of the charge pair (Rydberg excitation) will decline with increasing size, as is the case with asphaltene and tryptophan peptides, and increase with excess energy. The bias against higher MW could be ameliorated and the critical mass for loss of detection extended by increasing the excitation energy and shortening the laser pulse width”.23 In light of the foregoing discussion, it is self-evident that the LIMS method for MW and MW distribution measurements suffers from a progressively severe bias against higher MW species, ultimately leading to the complete loss of detection beyond a critical MW. This necessitates a thorough revision of all LIMS measurement data on asphaltene MW and MW distribution and, at the same time, the urgent development of new methodologies for the MW determination of asphaltene and other compounds. With respect to the latter, the laser ionization protocol (number of photons, wavelengths of irradiation, sequence of irradiation, etc.) should be explored to minimize photon loss and increase the sensitivity of detection.
Strausz et al.
Section 1 of this paper dealt with the suppressing effect of IC on the fluorescence emerging from the UV irradiation of asphaltene solutions. The suppression exhibited an increasing trend with increasing MW of the asphaltene fractions and the aromatic chromophores. This effect was in good qualitative agreement with the predictions of the vibrational Franck-Condon factor (FCF) and the energy gap law (EGL). IC may originate from any level of the electronic manifold of the chromophore, subject to conservation and symmetry rules of quantum mechanics. There is, however, a sharp difference between those originating from the first excited singlet state, S1, and any of the higher singlet excited states, Sn > 1. The former occurs as a quantum jump between two noncrossing (matching) S1 and S0 electronic surfaces, and its probability is governed by the FCF and EGL. The latter occurs at surface crossings near the V ) 0 vibrational level of the Sn > 1 state with the S1 state, Sn > 1 f S1, and it is very fast, with a rate of 1013 s-1. In gas-phase photo-ionization, the excitation is a vertical process and electronic excitation is accompanied by some vibrational excitation. The excess vibrational excitation superimposed on the electronic transition features two characteristic attributes important for the final outcome of the photo-ionization process: (1) the influence of excess vibrational energy on the rate of the S1 f S0 IC playing a prominent role in the photophysics of ionization in the form of an exponential energy gap law and (2) the imposition of a quasi-selection rule to preserve the vibrational quantum state via a ∆V ∼ 0 “propensity” rule. (1a) The exponents of the electronic excitation energy gap and the vibrational energy gap are additive, and log k(S1 f S0) is a linear function of the vibrational energy in S1. At zeroexcess vibrational energy, the value of k(S1 f S0) is about the rate of IC occurring in the solution phase, as determined by the electronic energy gap law. As consequence, the value of k(S1 f S0) in the gas phase may become commensurate with that of k(Sn > 1 f S1), unlike in the solution phase, where k(Sn > 1 f S1) . k(S1 f S0). This can then open a channel to the efficient loss of excitation energy. (2a) The propensity rule is a covert selection rule that may raise the effective value of IP, D0, D1, etc. to D0 + Ev or D1 + Ev, etc. The work described in ref 22 stresses that the final outcome of gas-phase photo-ionization is determined not only by the total energy of the photons used for excitation but also the mode of delivery, number of photons and colors, as well as the order of application. The important conclusion with respect to asphaltene LIMS is that, employing a fixed wavelength of irradiation, the outcome of photo-ionization will depend upon the value of the electronic energy gap, which determines the value of the vibrational excitation as well
Summary and Conclusions
E(hν) ) ∆Ee + ∆Ev
The central phenomenon that is the focus of this paper is internal conversion, a directly non-observable photophysical phenomenon. As a consequence, the detection and quantitative estimation of IC are both subject to considerable uncertainty. More often than not, IC occurs in competition with other, directly observable photophysical (fluorescence, phosphorescence, ISC, and photo-ionization) and photochemical phenomena. Detection and quantitative estimation may then become possible from the effects observed on the competing phenomena. Under favorable conditions, IC can be a very fast process, which may successfully compete with the fastest photophysical process.
because, with increasing MW, the size of the aromatic chromophore increases, leading to a decrease in ∆Ee and thus an increase in ∆Ev. This means that, in the process illustrated in Scheme 1 (for azulene, but it may be generic for polyaromatic molecules22), the IC step S1 f S0 becomes faster and faster with an increasing MW. Then, beyond a certain MW value, essentially all of the S1 state will be consumed in S1 f S0 IC and the molecule will not be detectable in LIMS. Throughout the MW spectrum, the method will become biased against increasing MW and will report a distorted MW distribution. Employing multicolor, multiphoton irradiation may ameliorate the problem.
(23) Schlag, E. W.; Grotemeyer, J.; Levine, R. D. J. Chem. Phys. Lett. 1992, 190, 521–527.
EF800480Y
