Pyrene Derivatives of 2,2′-Bipyridine as Models for Asphaltenes

Nov 9, 2007 - Vapor Pressure Osmometry (VPO) Measurements. The measurements were carried out in toluene at 75 °C following the method of Agrawala ...
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Energy & Fuels 2008, 22, 715–720

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Pyrene Derivatives of 2,2′-Bipyridine as Models for Asphaltenes: Synthesis, Characterization, and Supramolecular Organization† Xiaoli Tan,‡ Hicham Fenniri,*,§ and Murray R. Gray*,‡ Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, Alberta T6G 2G6, Canada, and National Institute for Nanotechnology and Department of Chemistry, UniVersity of Alberta, Edmonton, Alberta T6G 2M9, Canada ReceiVed July 10, 2007. ReVised Manuscript ReceiVed September 21, 2007

The behavior of 4,4′-bis-(2-pyren-1-yl-ethyl)-[2,2′]bipyridinyl (PBP) was studied as a model for petroleum asphaltenes with a bridged structure. PBP consists of two pyrene groups bridged by a bipyridyl spacer, and exhibits similar solubility and chromatographic properties to some fractions of asphaltenes. On the basis of nuclear magnetic resonance, steady state fluorescence, vapor pressure osmometry, solubility, and adsorption behavior studies, PBP gave self-association in solution. On the basis of these data and single crystal X-ray analysis, this behavior was attributed to π–π stacking interactions involving both pyrene rings and the bipyridine spacer. These results demonstrate that bridged aromatic species with up to four fused aromatic rings are capable of self-association in solution.

Introduction Asphaltenes are commonly described as complex mixtures of molecules consisting of planar polyaromatic cores containing 4–10 fused rings and polar functional groups.1 Attached to the aromatic core are naphthenic rings and short aliphatic side chains. As a solubility class, asphaltenes are operationally defined as the portion of crude oil insoluble in light n-alkanes, but soluble in aromatic solvents.2 Colloidal and polymeric models have been proposed to explain the associative behavior of asphaltenes using small angle neutron scattering (SANS),3,4 vapor pressure osmometry (VPO),5–7 isothermal titration calorimetry,8 and infrared spectroscopy.9 Because of the complex mixture of components in asphaltenes, the aggregation mechanisms are not well-understood. Several reports10–13 suggest that strong specific forces, such as polar and stacking interactions † Presented at the 8th International Conference on Petroleum Phase Behavior and Fouling. * Towhomcorrespondenceshouldbeaddressed.E-mail:hicham.fenniri@ nrc-cnrc.gc.ca. Phone: 1-780-641-1750. Fax: 1-780-641-1601 (H.F.). E-mail: [email protected]. Phone: 1-780-492-7965. Fax: 1-780-4922881 (M.R.G.). ‡ Department of Chemical and Materials Engineering. § National Institute for Nanotechnology and Department of Chemistry. (1) Yen T. F.; Chilingarian, G. V.; Eds. Asphaltene and Asphalts, 2. DeVelopments in Petroleum Science; Elsevier Science: Amsterdam, The Netherlands, 2000. (2) Mitchell, D. L.; Speight, J. G. Fuel 1973, 52, 149–152. (3) Gawrys, K. L.; Spiecker, P. M.; Kilpatrick, P. K. Pet. Sci. Technol. 2003, 21, 461–489. (4) Sheu, E. Y.; Liang, K. S.; Sinha, S. K.; Overfield, R. E. J. Colloid Interface Sci. 1992, 153, 399–410. (5) Andersen, S. I.; Keul, A.; Stenby, E. Pet. Sci. Technol. 1997, 15, 611–646. (6) Wiehe, I. A.; Liang, K. S. Fluid Phase Equilib. 1996, 117, 201– 210. (7) (a) Yarranton, H. W.; Alboudwarej, H.; Jakher, R Ind. Eng. Chem. Res. 2000, 39, 2916–2924. (b) Agrawala, M.; Yarranton, H. W. Ind. Eng. Chem. Res. 2001, 40, 4664–4672. (8) Merino-Garcia, D.; Andersen, S. I. Pet. Sci. Technol. 2003, 21, 507– 525. (9) Moschopedis, S. E.; Speight, J. Fuel 1976, 55, 187–192. (10) Gutiérrez, L. B.; Ranaudo, M. A.; Mendez, B.; Acevedo, S. Energy Fuels 2001, 15, 624–628.

involving heteroatoms and aromatic moieties, drive asphaltene aggregation while weaker nonspecific dispersion forces dominate asphaltene precipitation. Although the nature of intermolecular interactions that drive asphaltene aggregation is generally accepted, there is considerable debate over the molecular arrangement of the heteroatom, alkyl, and aromatic moieties within the molecular framework. Two different models have been proposed in the literature to depict the aromatic ring structure of representative asphaltenes with masses in the range 500 to 1000 Da. One is the “pericondensed” model of asphaltenes,14–16 which consists of a large, highly condensed aromatic core with peripheral aliphatic chains, as inferred from X-ray diffraction and fluorescence depolarization experiments.17–21 Interactions of pericondensed monomers would likely form densely packed aggregates through stacking interactions of the aromatic cores.20,21 Alternatively, the “archipelago” model was proposed in which clusters of polycondensed groups, consisting of one to seven aromatic rings bridged by short aliphatic spacers, possibly containing polar heteroatom bridges.16,22–24 The archipelago model is supported (11) Spiecker, M. P.; Gawrys, K. L.; Kilpatrick, P. K. J. Colloid Interface Sci. 2003, 267, 178–193. (12) Buckley, J. S. Fuel Sci. Technol. Int. 1996, 14, 55–74. (13) Buckley, J. S. Energy Fuels 1999, 13, 328–332. (14) Groenzin, H.; Mullins, O. C. Energy Fuels 2000, 14, 677–684. (15) Rogel, E.; Carbognani, L. Energy Fuels 2003, 17, 378–386. (16) Murgich, J. Mol. Simul. 2003, 29, 451–461. (17) Groenzin, H.; Mullins, O. C.; Eser, S.; Mathews, J.; Yang, M.-G.; Jones, D. Energy Fuels 2003, 17, 498–503. (18) Ruiz-Morales, Y.; Mullins, O. C. Energy Fuels 2007, 21, 256– 265. (19) Yen, T. F.; Erdman, J. G.; Pollack, S. S. Anal. Chem. 1961, 33, 1587–1594. (20) Murgich, J.; Abanero, J. A.; Strausz, O. P. Energy Fuels 1999, 13, 278–286. (21) Sharma, A.; Groenzin, H.; Tomita, A.; Mullins, O. C. Energy Fuels 2002, 16, 490–496. (22) Sheremata, J. M.; Gray, M. R.; Dettman, H. D.; McCaffrey, W. C. Energy Fuels 2004, 18, 1377–1384. (23) Gray, M. R. Energy Fuels 2003, 17, 1566–1569. (24) Calemma, V.; Iwanski, P.; Nali, M.; Scotti, R.; Montanari, L. Energy Fuels 1995, 9, 225–230.

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by chemical and thermal degradation studies which concluded that the extent of aromatic condensation in asphaltenes is low.25 The studies of crude oils by affinity separation on palladium followed by FT-ICR MS26,27suggested that the aromatic fractions contains polycylic aromatic sulfur heterocycles which are connected with other aromatic groups by bridges. Molecular simulation studies on a proposed archipelago structure suggested that the presence of long aliphatic bridges gave asphaltenes the capacity to fold into complex three-dimensional globular structures with similar core structure.16 One effective method to understand the molecular and selfassociation behaviors of asphaltenes is to synthesize and study model molecules that match the chemical functional groups and physical characteristics of this solubility class. Some polynuclear aromatic hydrocarbons using pyrenyl and hexabenzocoronene derivatives have recently been synthesized and investigated.28,29 Investigations of these model compounds suggested the importance of the large alkylated polynuclear aromatic hydrocarbons and oxygen-containing polar chains for self-association. In this paper, a bipyridyl derivative, 4,4′-bis-(2-pyren-1-yl-ethyl)[2,2′]bipyridinyl (PBP) containing polynuclear aromatic hydrocarbons, aliphatic chains, and nitrogen heterocycles, was synthesized as a minimalist archipelago model of asphaltenes. The solubility of PBP at room temperature was measured, and adsorption behaviors on silica gel and alumina were studied by column chromatography. The associative properties of PBP in solution, the liquid state, and the solid state were determined over a range of temperatures using nuclear magnetic resonance (NMR) spectroscopy, steady-state fluorescence spectroscopy, vapor-pressure osmometry (VPO), single crystal X-ray diffraction, and thermogravimetric analysis.

Tan et al. Scheme 1. Synthesis of 4,4′-Bis-(2-pyren-1-yl-ethyl)-[2,2′]bipyridinyl (PBP)

Scheme 2. Chemical Structure of PBP with Assignment of Carbon Atoms

Materials and Methods Unless otherwise noted, all chemicals and starting materials were obtained commercially and used without further purification. 4,4′Bis-(2-pyren-1-yl-ethyl)-[2,2′]bipyridinyl (PBP) and 4-methyl-4′[2-(1-pyrenyl)ethyl]-2,2′-bipyridine (PB) were synthesized according to literature procedures.30,31 Column chromatography was performed on silica gel 60 and aluminum oxide 90 (Merck). Organic solvents were of reagent or spectra grade and were used as supplied. 1D and 2D 1H and 13C NMR were carried out on a Varian Inova 500 spectrometer, using the residual protons of the solvent (CDCl3) as an internal standard at δ 7.26 ppm. The mass spectra were obtained on a Mariner Biospectrometry Workstation, an orthogonal time-of-flight instrument with an ESI source. Preparation of 4,4′-Bis-(2-pyren-1-yl-ethyl)-[2,2′]bipyridinyl (PBP). 1-Pyrenylmethanol was reacted with SOCl2 in CH2Cl2 at 0 °C to give 1-(chloromethyl)pyrene in 85% yield. This compound was then reacted with 4,4′-dimethyl-2,2′-bipyridine in freshly distilled THF under argon atmosphere at -10 °C to give PBP in 15% yield (Scheme 1). The crude product (a mixture of PBP, PB, and starting materials) was purified by repeated dissolution in toluene and precipitation with 40 parts n-heptane, following the (25) Strausz, O. P.; Mojelsky, T. W.; Lown, E. M. Fuel 1992, 71, 1355– 1363. (26) Müller, H.; Andersson, J. T.; Schrader, W. Anal. Chem. 2005, 77, 2536–2543. (27) Panda, S. K.; Schrader, W.; Al-Hajji, A.; Andersson, J. T. Energy Fuels 2007, 21, 1071–1077. (28) Akbarzadeh, K.; Bressler, D. C.; Wang, J.; Gawrys, K. L.; Gray, M. R.; Kilpatrick, P. K.; Yarraton, H. W. Energy Fuels 2005, 19, 1268– 1271. (29) Rakotondradany, F.; Fenniri, H.; Rahimi, P.; Gawrys, K. L.; Kilpatrick, P. K.; Gray, M. R. Energy Fuels 2006, 20, 2439–2447. (30) Bair, K. W.; Tuttle, R. L.; Knick, V. C.; Cory, M.; McKee, D. D. J. Med. Chem. 1990, 33, 2385–2393. (31) McClenghan, N. D.; Barigelletti, F.; Maubert, B.; Campagna, S. Chem. Commun. 2002, 602–603.

ASTM-D2007-80 method for separation of asphaltenes from crude oil. Three to four cycles of dissolution and precipitation were required to purify the PBP. The structure (Scheme 2) was confirmed by 1H NMR, 13C NMR, and mass spectrometry. The spectral assignments were decided by 1D and 2D 1H and 13C NMR analysis and coupling constants. The Mp was 235.50 °C. 1H NMR (500 MHz, CDCl ) δ (ppm): 8.60–8.59 (C1H, d, 2H), 3 8.43 (C4H, s, 2H), 8.34–8.32 (C21H, d, 2H), 8.21–8.19 (C15H, d, 2H), 8.19–8.18 (C13H, d, 2H), 8.16–8.14 (C20H, d, 2H), 8.12–8.10 (C10H, d, 2H), 8.04 (C17H, C18H, 4H), 8.02–7.99 (C14H, t, 2H), 7.85–7.83 (C11H, d, 2H), 7.13–7.12 (C2H, d, 2H), 3.75 (C7H, t, 4H), 3.27 (C6H, t, 4H). 13C NMR (125 MHz, CDCl ) δ (ppm): 151.9 (C3, C5), 149.5 3 (C1), 134.8 (C22, C23), 131.3 (C12, C16), 130.8 (C8), 130.0 (C19), 128.5 (C9), 127.5 (C13, C15), 127.3 (C17, C18, C20), 126.7 (C14), 124.9 (C10, C11), 124.0 (C2), 122.9 (C21), 121.2 (C4), 37.5 (C6), 34.4 (C7). Positive ESI-MS: expected mass, 612.3; observed, 613.3 ((M + H+)/z, 100%), 614.3 ((M + 2H+)/z, 42%). 1H NMR Titration and Variable-Temperature Experiments. Titration and variable-temperature 1H NMR experiments were carried out on Varian Inova four-channel 500 or 800 MHz spectrometers. 1H NMR titration experiments of PBP and PB in CDCl3 were carried out at 27 °C. Variable temperature experiments were performed at temperatures ranging from -60 to +60 °C at 8 × 10-4 M in CDCl3. Fluorescence Spectroscopy. Steady-state fluorescence spectra of PBP and PB were recorded at room temperature in a 1 cm path length cell on a Photon Technology International (PTI) MP1 spectrofluorometer by front-face illumination with excitation and emission slits of 1∼2 nm.

Model Pyrene DeriVatiVes of 2,2′-Bipyridine

Figure 1. Concentration-dependent 1H NMR chemical shifts of protons of PBP in CDCl3.

Vapor Pressure Osmometry (VPO) Measurements. The measurements were carried out in toluene at 75 °C following the method of Agrawala and Yarranton,7 using a Knauer K-7000 osmometer. Benzyl (M ) 210.23) was used as a calibration standard. A total of 6 different stock solutions of PBP at concentrations ranging from 1.8 to 5 g/L were prepared in toluene. A 30 µL portion of solution was injected into the osmometer chamber. After temperature equilibration, the voltage was recorded in millivolts. Five independent measurements were made at each concentration, and the experiments were duplicated. Single Crystal X-ray Diffraction. Crystals of PBP were grown by slow diffusion of diethyl ether into a toluene solution of PBP. Diffraction data were measured at -80 °C on a Bruker PLATFORM/ SMART 1000 CCD equipped with a graphite-monochromated Mo KR (λ ) 0.71073 Å) radiation. Thermogravimetric Analysis (TGA). TGA was performed on a Q500 thermogravimetric analyzer (TA Instruments, USA). About 10 mg of PBP was placed on the cleaned platinum TGA pan. The flow rate of the nitrogen for pyrolysis was fixed at 45 mL/min. The experiments were started at room temperature. Nitrogen was flushed through the TGA tube, and the temperature was ramped to 800 °C at a rate of 5 °C/min. The weight loss versus temperature was recorded. Replicate experiments gave less than 1% variation in overall weight loss at a given temperature.

Results and Discussion Solubility and Adsorption Behaviors. Asphaltenes show strong adsorption on silica gel and alumina during separation of crude oils by column chromatography. Therefore, the asphaltenes are precipitated from solution prior to separation of the saturate, aromatic, and resin chromatographic fractions.32,33 During purification of PBP, we attempted to use column chromatography on silica gel and alumina to separate PBP from the byproducts. Similarly to asphaltenes, the crude product could not be eluted from silica gel or alumina even with a mixture of chloroform and methanol (3:1 v/v). This initial observation emphasizes the similarity between this model compound and the fractions of asphaltenes. Furthermore, the successful separation of PBP in pure form from a mixture of products by the ASTM-D2007-80 method corroborated the hypothesis that PBP matches asphaltenes physical properties. NMR Studies. 1H NMR spectra of PBP at concentrations varying from 0.06 mM (36.72 mg/L) to 10 mM (6.12 g/L) in CDCl3 are shown in Figure 1, and the dependence of chemical (32) Castillo, J.; Fernández, A.; Ranaudo, M. A.; Acevedo, S. Pet. Sci. Technol. 2001, 19, 75–106. (33) Selucky, M. L.; Kim, S. S.; Skinner, F.; Strausz, O. P. Chemistry of Asphaltenes; Advances in Chemistry Series, 195; ACS: Washington, D.C., 1981; pp 83–118..

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Figure 2. Concentration-dependent 1H NMR chemical shifts of C4H of PBP and PB in CDCl3.

Figure 3. Temperature-dependent 1H NMR chemical shifts of C4H of PBP in CDCl3 (8 × 10-4M).

shifts of C4H on the concentration for PBP and PB34 are shown in Figure 2. As the concentrations of PBP and PB increased, the aromatic and aliphatic protons underwent a significant upfield shift suggesting that the aromatic and pyridinic rings are engaged in strong face-to-face π–π stacking, as previously observed.35 The larger upfield chemical shift observed for PBP suggests a stronger propensity to aggregate than PB at the same concentration. To verify these results, we carried out variable temperature 1H NMR of PBP in CDCl3. We observed a larger upfield chemical shift (Figure 3) as we lowered the temperature, indicating more favorable π–π stacking at low temperature. Association of asphaltenes in solutions has been the subject of a number of studies. More and more proof by various experimental techniques shows the onset of asphaltene aggregation at concentrations close to or below 10 mg/L,36–39 as well as (34) The detailed properties of PB will be published elsewhere. (35) (a) Watson, M. D.; Jackel, F.; Severin, N.; Rabe, J. P.; Müllen, K. J. Am. Chem. Soc. 2004, 126, 1402–1407. (b) Fechtenkötter, A.; Saalwächter, K.; Harbison, M. A.; Müllen, K.; Spiess, H. W. Angew. Chem., Int. Ed. 1999, 38, 3039–3042. (c) Shetty, A. S.; Fischer, P. R.; Storck, K. F.; Bohn, P. W.; Moore, J. S. J. Am. Chem. Soc. 1996, 118, 9409–9414. (d) Shetty, A. S.; Zhang, J.; Moore, J. S. J. Am. Chem. Soc. 1996, 118, 1019–1027. (e) Kastler, M.; Pisula, W.; Wasserfallen, D.; Pakula, T.; Müllen, K. J. Am. Chem. Soc. 2005, 127, 4286–4296. (f) Prince, R. B.; Saven, J. G.; Wolynes, P. G.; Moore, J. S. J. Am. Chem. Soc. 1999, 121, 3114–3121. (36) Goncalves, S.; Castillo, J.; Fernandez, A.; Hung, J. Fuel 2004, 83, 1823–1828. (37) Sheu, E. Y. Energy Fuels 2002, 16, 74–82. (38) Ghosh, A. K. Fuel 2005, 84, 153–157. (39) Liao, Z.; Zhou, H.; Graciaa, A.; Chrostowska, A.; Creux, P.; Geng, A. Energy Fuels 2005, 19, 180–186.

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Figure 4. Steady-state fluorescence spectra of pyrene, PB, and PBP (λex) 347 nm) in CHCl3.

Tan et al.

Figure 6. Concentration-dependent fluorescence spectra of PBP in CHCl3.

Figure 7. Apparent molecular weight of PBP in toluene at 75 °C measure by vapor pressure osmometry. Figure 5. Normalized excitation spectra of PBP (6 × 106 mol/L) in CHCl3, monitored at 377 nm (monomer, solid line) and at 475 nm (excimer, dashed line).

massive aggregation at concentrations of 50–150 mg/L.40 The stacking of the aromatic regions of the asphaltenes molecules also contributes noticeably to the formation of the aggregates.41 PBP showed aggregation at concentrations below 40 mg/L, but the onset of aggregation could not be detected by NMR measurement because satisfactory spectra could not be obtained below 6 × 10-5 M, even on an 800 MHz NMR spectrometer. Steady-State Fluorescence Spectroscopy. The electronic spectrum provided a sensitive probe for the supramolecular aggregation states.35e,f Typical fluorescence emission spectra of PBP, PB, and pyrene in chloroform (λexc ) 347 nm) are shown in Figure 4. It is noticeable that emission spectra of the three compounds above are in the emission range of asphaltenes.17,18 Consistent with prior studies,42 no excimer emission was observed for pyrene at 1.2 × 10-5 M, whereas PBP and PB showed excimer emission at the same total molar concentration of pyrene moieties. The existence of PBP aggregates in chloroform above 6 × 10-6 M (3.7 mg/L) was confirmed by acquiring its monomer (λem ) 377 nm) and excimer (λem ) 475 nm) excitation spectra (Figure 5). The red-shift (∆λ ) 3 nm) observed in the excitation spectra43 at λmax indicated that the ground-state PBP does aggregate in solution. Similarly, excitation spectra of PB also showed evidence for aggregates at concentrations above 2 × 10-5 M. There are two possibilities (40) Badre, S.; Goncalves, C. C.; Norinaga, K.; Gustavson, G.; Mullins, O. C. Fuel 2006, 85, 1–11. (41) Murgich, J. Pet. Sci. Technol. 2002, 20, 983–997. (42) Venkataramana, G.; Sankararaman, S. Org. Lett. 2006, 8, 2739– 2742. (43) Winnik, F. M. Chem. ReV. 1993, 93, 587–614.

for molecules to form excimers at such a low concentration: (a) intermolecular π–π interactions could bring two molecules of PBP close enough to result in excimer emission and (b) intramolecular association of the pyrenyl groups in the case of PBP could also lead to excimer emission. To differentiate intermolecular versus intramolecular interactions,44 concentration-dependent emission measurements were carried out.45 In effect, the ratio of monomer/excimer emission in the case of PBP decreased as the concentration of PBP increased (Figure 6), strongly suggesting that excimer emission results from intermolecular interaction.45 Furthermore, the fact that PB, which cannot form intramolecular excimers, showed aggregation near the same concentration range further confirmed the intermolecular nature of the excimer emissions observed for both PB and PBP. In agreement with this notion, the onset of aggregation of PBP in toluene monitored by state–state fluorescence was higher than that observed in CHCl3 (1.5 × 10-5 M versus 6 × 10-6 M), likely the result of enhanced solute–solvent π–π stacking interactions in toluene.46 Vapor Pressure Osmometry Studies. The apparent molecular weight of PBP in toluene at 75 °C was 1105 ( 165 Da and remained constant at concentrations above 1.8 g/L (Figure 7), suggesting that PBP is most likely a mixture of dimers and monomers in toluene at 75 °C. The results from NMR and steady-state fluorescence spectra show that both pyridine and pyrene groups take part in the aggregation. Comparison (44) Lakowicz, J. R.; Ed. Principles of Fluorescence Spectroscopy, 3rd ed.; Springer Science: New York, 2006. (45) Cho, D. W.; Fujitsuka, M.; Choi, K. H.; Park, M. J.; Yoon, U. C.; Majima, T. J. Phys. Chem. B 2006, 110, 4576–4582. (46) (a) Sanders, G. M.; van Dijk, M.; van Veldhuizen, A.; van der Plas, H. C.; Hofstra, U.; Schaafsma, T. J. J. Org. Chem. 1988, 53, 5272– 5281. (b) Bryant, J. A.; Knobler, C. B.; Cram, D. J. J. Am. Chem. Soc. 1990, 112, 1254–1255.

Model Pyrene DeriVatiVes of 2,2′-Bipyridine

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Figure 8. Perspective view of the 4,4′-bis(2-(4-pyrenyl)ethyl)-2,2′bipyridine molecule showing the atom labelling scheme.

Figure 10. Thermogravimetry analysis of PBP. The heating rate ) 5 °C/min.

Figure 9. Illustration of the crystal packing of PBP. The view direction is parallel to the crystal b axis (top) and a axis (bottom).

with 1,10-dipyrenyl decane, which did not aggregate in solution,28 suggests either that the heterocyclic moiety of PBP plays a significant role in the aggregation process or that long alkyl bridges are unfavorable for aggregation. Vapor pressure osmometry analysis of asphaltenes association showed that the apparent molecular weights for Athabasca pentane-insoluble and heptane-insoluble asphaltenes in toluene increased linearly for concentrations below 5 g/L and then reached a limiting value at ∼20 g/L. This behavior was to the formation of aggregates of two to six monomers.7 In contrast with the data of asphaltenes, PBP showed no increase in aggregation with concentration. These results indicate that the extent of aggregation for asphaltenes is larger than PBP in toluene, which could in part be due to polydispersity of asphaltenes mixtures or to a structural differences with the model compounds. Pericondensed model compounds of asphaltenes (C6-HBC) also showed a predominance of dimeric aggregates in toluene,29 in agreement with theoretical predictions of pericondensed model aggregates for asphaltenes.16 The data for the association of model compounds suggests, therefore, that both archipelago and pericondensed models may account for some aspects of asphaltenes physical behavior in solution but that these pure compounds give less extensive association. Single Crystal X-ray Diffraction. PBP crystallized in the monoclinic crystal class with unit cell parameters of a ) 4.6343(11) Å, b ) 14.097(3) Å, c ) 23.290(5), β ) 95.506(4)°, V ) 1514.5(6) Å3, Z ) 2, and Fc ) 1.344 g/cm3. Both the pyrene and pyridyl units gave near-parallel orientations, indicating that PBP is unstrained (Figure 8). The crystal packing of PBP is shown in Figure 9. Each unit cell contains two molecules of PBP with the pyrene and pyridyl moieties stacked in a faceto-face orientation. The interplanar separation in the dimer was

3.47 Å, close to the optimum calculated distance (3.4 Å)47 for systems held by face-to-face stacking interactions. Thermogravimetric Studies. Two mass transitions were observed (Figure 10): a gradual 3% mass reduction occurred between 150 and 240 °C, likely due to evaporation; then, an 83% reduction occurred between 240 and 465 °C due to a combination of evaporation and cracking. The most rapid loss in mass was at 440 °C, from the slope of the weight-loss curve. The same temperature range dominated the weight-loss curves of petroleum asphaltenes.48–50 For example, the maximum rates of weight loss were observed for Cold Lake bitumen asphaltenes48 at 480 °C; Saskachewan heavy oil asphaltenes49 at 450 °C; Daqing curde oil asphaltenes50 at ca. 481 °C; and Garzan and Raman crude oil asphaltenes51 at 468 and 458 °C, respectively. Thermal breakage of one bridge in PBP would release two possible fragments: a pyrene group or PB. Both fragments would vaporize rapidly at temperatures over 350 °C. A pyrolysis residue of 14% was recorded at 500 °C, which is similar to the microcarbon residue content by ASTM methods. This yield of pyrolysis residue was lower than petroleum asphaltenes at 500 °C, which range from 35% to 70%.23,52,53 Several factors could account for this discrepancy. First, the molecular weight of pure PBP is lower than the range of values in petroleum asphaltenes, so that all fragments of bond breakage would be more volatile. Second, the lack of side chains in PBP could limit cross-linking reactions that promote coke formation.23 Third, we cannot exclude the possibility that the bridged structure may not reflect the dominant structure in the petroleum asphaltenes. Conclusions PBP matched solubility and adsorption behaviors of asphaltenes. 1H NMR, steady-state fluorescence, and VPO experiments showed that PBP self-associates in CHCl3 and forms dimers in toluene. The onset of aggregation for PBP in solution was in (47) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525–5534. (48) Khulbe, K. C.; Sachdev, A. K.; Mann, R. S.; Davis, S. TGA studies of asphaltenes derived from Cold-Lake (Canada) Bitumen. Fuel Process. Technol. 1984, 8, 259–266. (49) Ambalae, A.; Mahinpey, N.; Freitag, N. Energy Fuels 2006, 20, 560–565. (50) Dong, X.-G.; Lei, Q.-F.; Fang, W.-J.; Yu, Q.-S. Thermochim. Acta 2005, 427, 149–153. (51) Karacan, O.; Kok, M. V. Energy Fuels 1997, 11, 385–391. (52) Calemman, V.; Rausa, R. J. Anal. Appl. Pyrolysis 1997, 40–41, 568–584. (53) Japanwala, S.; Chung, K. H.; Dettman, H. D.; Gray, M. R. Energy Fuels 2002, 16, 477–484.

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the same concentration range of asphaltenes with an average molecular weight of 500–700 Da. However, the extent of aggregation for PBP was smaller than asphaltenes in solution, possibly because of the absence of intramolecular interactions and hydrogen bonding ability.16 On the basis of the solid-state structure of PBP, NMR, and fluorescence data, we propose that dimer formation in solution is most likely enforced via π–π interactions, with participation of both the pyrene and the pyridine groups. These results show that bridged aromatic structures can give significant aggregation.

Tan et al. Acknowledgment. The authors are grateful for funding from Alberta Ingenuity and the Natural Sciences and Engineering Research Council and for support from the National Institute for Nanotechnology and the University of Alberta. This research was undertaken, in part, thanks to funding from the Canada Research Chairs Program. Supporting Information Available: X-ray crystallographic file for PBP (CIF). This material is available free of charge via the Internet at http://pubs.acs.org. EF700395G