Atomic Force Microscopy Identifying Fuel Pyrolysis Products and Directing the Synthesis of Analytical Standards

Article abstract of DOI:10.1021/jacs.8b02525

Here we present a new method that integrates atomic force microscopy (AFM) with analytical tools such as high-performance liquid chromatography (HPLC) with diode-array ultraviolet-visible (UV) absorbance, and mass spectrometry (MS) along with synthetic ch

Full text of DOI:10.1021/jacs.8b02525

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Article
Atomic force microscopy identifying fuel pyrolysis products
and directing the synthesis of analytical standards
Shadi Fatayer, Nimesh Poddar, Sabela Quiroga, Fabian Schulz, Bruno Schuler, Subramanian Venkateswaran
Kalpathy, Gerhard Meyer, Dolores Pérez, Enrique Guitian, Diego Peña, Mary Julia Wornat, and Leo Gross
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b02525 • Publication Date (Web): 12 Jun 2018
Downloaded from http://pubs.acs.org on June 12, 2018
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Atomic force microscopy identifying fuel pyrolysis products and directing the synthesis of
analytical standards
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1Ψ
Shadi Fatayer, Nimesh B. Poddar, Sabela Quiroga, Fabian Schulz, Bruno Schuler, Subramanian V.
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3*
2*
Kalpathy, Gerhard Meyer, Dolores Pérez, Enrique Guitián, Diego Peña, Mary J. Wornat, Leo
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*
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Gross
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. IBM Research–Zurich, 8803 Rüschlikon, Switzerland
. Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803,
United States
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. Centro Singular de Investigación en Química Biolóxica e Materiais Moleculares
CiQUS), Departamento de Química Orgánica, Universidade de Santiago de Compostela. Santiago de
Compostela 15782, Spain
(
*
diego.pena@usc.es, *mjwornat@lsu.edu, *lgr@zurich.ibm.com
Ψ
Current address: Molecular Foundry, Lawrence Berkeley National Laboratory, California
4720, USA
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Abstract
Here we present a new method that integrates atomic force microscopy (AFM) with analytical
tools such as highꢀperformance liquid chromatography (HPLC) with diodeꢀarray ultravioletꢀvisible (UV)
absorbance and mass spectrometry (MS) along with synthetic chemistry. This allows the detection,
identification, and quantification of novel polycyclic aromatic hydrocarbons (PAH) in complex
molecular mixtures. This multiꢀdisciplinary methodology is employed to characterize the supercritical
pyrolysis products of nꢀdecane, a model fuel. The pyrolysis experiments result in a complex mixture of
both unsubstituted as well as highly methylated PAH. We demonstrate the AFMꢀdriven discovery of a
novel compound, benz[l]indeno[1,2,3ꢀcd]pyrene, with the chemical structure assignment serving as
input for the chemical synthesis of such molecule. The synthesis is verified by AFM and the synthesized
compound is used as a reference standard in analytical measurements, establishing the firstꢀever
unequivocal identification and quantification of this PAH as a fuel product. Moreover, the highꢀ
resolution AFM analysis detected several fiveꢀ to eightꢀring PAH, which represents novel fuel pyrolysis
and/or combustion products. This work provides a route to develop new analytical standards by
symbiotically using AFM, chemical synthesis, and modern analytical tools.
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Keywords: atomic force microscopy; COꢀfunctionalized tips; polycyclic aromatic hydrocarbons;
pyrolysis; synthetic chemistry
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1
. Introduction
The investigation of complex molecular mixtures relies on the use of standards for experimental
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comparison. Due to the limited number of molecular standards that are commercially available,
thorough studies require the synthesis of new reference compounds. However, choosing the molecules
to be synthesized for the characterization of a mixture with unknown components is challenging and
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often relies on the use of spectroscopic methods.
The synthesis of the standards itself is often
expensive and time consuming. As an example, the number of possible polycyclic aromatic
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hydrocarbons (PAH) isomers increases significantly with increasing molecular weight. Therefore, given
a molecular formula, the number of unique PAH structures is large for molecules containing a
combination of fiveꢀ and sixꢀmembered rings. For instance, the C H benzenoid PAH, with only sixꢀ
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membered rings, display 12 isomers. For the same molecular formula, but with two fiveꢀmembered rings
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instead, there are a total of 266 isomers. In the absence of a reference standard, other analytical methods
8,10
can be employed for determining the structure of unknown PAH,
but some of these methods are
limited by low concentration and need extensive purification of individual PAH in complex molecular
matrices.
PAH are ubiquitous pollutants formed during the pyrolysis and/or combustion of a wideꢀvariety
of fuels and/or fuel components. The formation and subsequent growth of PAH is of practical concern
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for four principal reasons: (1) PAH are known precursors to soot,
which poses serious health
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hazards
and is a major contributor to global climate change;
(2) PAH are found in the extracts of
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solid deposits from catalytic hydrocracking of petroleum, suggesting that PAH could be precursors to
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these deposits, which can clog process piping and cause catalyst fouling,
downtime; (3) Some PAH are known to be potent carcinogens and/or mutagens;
resulting in process
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and (4) PAH are
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precursors to carbonaceous solids,
which can clog fuel lines and injection nozzles and foul heat
transfer surfaces, thereby seriously jeopardizing the safe operation of future highꢀspeed aircraft.
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In the context of the preꢀcombustion environments
of future highꢀspeed aircrafts, better
understanding of pyrolytic reactions that lead to PAH formation and growth would facilitate the design
of fuel systems that are not prone to solids formation. Therefore, we conducted supercritical pyrolysis
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experiments
of the model fuel nꢀdecane, an nꢀalkane component of jet fuels.
An nꢀalkane
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fuel is chosen because, under supercritical pyrolysis conditions, these jetꢀfuel components exhibit a high
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propensity for solids formation.
To thoroughly understand PAH formation and growth reactions,
particular emphasis is placed on the isomerꢀspecific analysis of the PAH products by highꢀperformance
liquid chromatography (HPLC) with diodeꢀarray ultravioletꢀvisible (UV) absorbance and mass
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spectrometric (MS) detection. Previous HPLC/UV/MS analyses
of the supercritical nꢀdecane
pyrolysis products led to the identification of over 400 threeꢀ to nineꢀring PAH, many of which contain
one or more methyl functionalities. The quantification of the PAH products of supercritical nꢀdecane
pyrolysis reveals that the yields of higherꢀringꢀnumber PAH are about an order of magnitude lower than
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their lowerꢀringꢀnumber counterparts.
In addition to the low yields of higherꢀringꢀnumber PAH, nꢀ
decane produces many alkylated PAH, which makes the identification and quantification of PAH
mixtures of > 5 rings an enormous undertaking.
Recently, highꢀresolution atomic force microscopy (AFM) with COꢀfunctionalized tips showed
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to be capable of identifying individual molecules
and thus, molecular mixtures can be studied
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moleculeꢀbyꢀmolecule.
With this approach, details about the mechanisms of onꢀsurface organic
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reactions
and the molecular architecture and composition of complex molecular mixtures
have
been obtained. Due to the singleꢀmolecule sensitivity of AFM, the detection of molecules present in
miniscule amounts is viable. Important for our study of fuel pyrolysis products, fiveꢀ and sixꢀmembered
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carbon rings as well as various aliphatic groups can be differentiated and identified by AFM.
.
Here we demonstrate a method that employs atomicꢀresolution lowꢀtemperature AFM to identify
reference standards. AFM is integrated with HPLC/UV/MS and synthetic chemistry to identify
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previously unidentified PAH products of fuel pyrolysis in two different steps. First, to identify unknown
PAH within the PAHꢀproduct fractions of supercritical nꢀdecane pyrolysis, providing candidates for the
targeted synthesis of new reference standards. Second, to confirm the molecular structure of the
synthesized standard. With this approach, first, a novel sevenꢀring PAH, benz[l]indeno[1,2,3ꢀcd]pyrene,
is identified by AFM. Subsequently, it is synthesized and then also confirmed by AFM. The synthesized
standard of benz[l]indeno[1,2,3ꢀcd]pyrene is analyzed by HPLC/UV, providing the UV absorbance
spectrum, a fingerprint property of PAH, allowing the identification and quantification of this novel
sevenꢀring PAH in an isomerꢀspecific manner as a product of supercritical nꢀdecane pyrolysis.
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This method provides a powerful tool for directing the synthesis of target molecules and for the
structural confirmation of synthesized molecules, thereby providing a reference standard for subsequent
analysis. It is important to note that this method is not limited to a particular system but can be applied
to a wide range of molecular mixtures such as those found in nature, obtained from chemical synthesis,
and derived from fuel pyrolysis and/or combustion.
2
. Experimental section
2.1 Pyrolysis experiments and HPLC/UV/MS analysis
The supercritical pyrolysis experiments of the model fuel nꢀdecane (99.5% pure; critical
temperature, 344.5 °C; critical pressure, 20.7 atm) are conducted in an isothermal, isobaric silicaꢀlined
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stainlessꢀsteel flow reactor described elsewhere.
For the present study, the PAHꢀproduct mixture is
obtained from supercritical nꢀdecaneꢀpyrolysis experiments performed at 570 °C, 94.6 atm, and 133 sec,
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respectively, conditions of high PAH production and onset of solids formation.
The PAH products of supercritical nꢀdecane pyrolysis are comprised of a complex mixture of
unsubstituted PAH and their alkylated derivatives, which requires the application of a twoꢀdimensional
HPLC separation technique to achieve sufficient resolution for PAHꢀproduct identification and
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quantification. This HPLC separation procedure was established by Wise et al. and further developed
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by Bagley and Wornat
for the analysis of the PAH products of supercritical nꢀalkaneꢀfuel pyrolysis.
The PAH products are first fractionated by normalꢀphase HPLC into 15 individual fractions by isomer
group and/or number of πꢀbonds. Therefore, an individual PAHꢀproduct fraction contains unsubstituted
PAH of a particular isomer group, along with their corresponding alkylated derivatives. In the second
dimension of separation, each PAHꢀproduct fraction is analyzed by reversedꢀphase HPLC with diodeꢀ
array ultravioletꢀvisible (UV) absorbance and mass spectrometric (MS) detection for the isomerꢀspecific
identification of the PAH products. The quantification of these products is obtained from extensive
calibration of the UV absorbance detector with PAH reference standards.
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As demonstrated in our previous studies,
supercritical nꢀdecane pyrolysis produces PAH
products containing 2 to 10 aromatic rings, which are fraction collected based on the number of πꢀ
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bonds.
For the scope of the present study, the two PAHꢀproduct fractions that are analyzed by AFM
are: (1) Fraction P, comprised of PAH of > 5 rings (i.e. of aromatic carbon number > 20), obtained from
32,33
the preꢀfractionation step of our moreꢀinvolved normalꢀphase HPLC fractionation procedure,
and (2)
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Fraction F (labelled as Fraction 11),
primarily composed of sevenꢀring PAH of the isomer group
C H (or PAH with 13 πꢀbonds) and their alkylated derivatives, obtained from the moreꢀinvolved
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normalꢀphase HPLC fractionation procedure.
.2 AFM measurements
We performed AFM experiments by using a homeꢀbuilt lowꢀtemperature (T ≈ 5 K), ultrahigh
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vacuum (p ≈ 1 × 10 mbar) combined scanning tunneling microscopy (STM) and AFM setup. We
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employed a qPlus sensor operated in the frequencyꢀmodulation mode (resonance frequency f ≈ 30
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kHz, spring constant k ≈ 1800 N/m, quality factor Q ≈ 14,000, and oscillation amplitude A = 0.5 Å). We
applied the bias voltage V to the sample. AFM measurements were acquired in constantꢀheight mode at
V = 0 V. As substrate we used a Cu(111) single crystal partly covered by bilayer (100)ꢀoriented NaCl
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islands. We deposited a low coverage (approximately 0.02 nm ) of CO on the surface for tip
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preparation by admitting CO into the UHV chamber at a sample temperature of T ≈ 10 K. The PAH
fractions were separately prepared by spreading a small amount (on the order of 1 mg) of the solid, dry
material on a piece of Si wafer. Then, the wafer was flashꢀheated by resistive heating in front of the cold
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sample. For the microscope tip, we used a 25 ꢁmꢀthick PtIr wire, shortened and sharpened with a
focused ion beam. Thereafter, we prepared a clean and sharp Cu tip by repeated indentations into the Cu
surface. Finally, a COꢀfunctionalized tip was created by pickingꢀup a single CO molecule from the
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surface.
The assignment of the structures from the AFM images is based on previous works resolving
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molecules atomically by AFM with COꢀfunctionalized tips,
taking into account the known
For example, we distinguish
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effect of distortions due to the tilting of the CO molecule at the tip.
fiveꢀ and sixꢀmembered rings by the number of bond intersections and corners and by the respective
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positions and enclosed angles of adjacent moieties. Methyl and other aliphatic substituents are
assigned by their characteristic contrast that was obtained by measuring synthesized standards by AFM.
The presence of substitutional heteroatoms, such as nitrogen, cannot be decisively dismissed solely
37,54
based on the AFM results shown here.
However, its presence amongst the PAH products can be
ruled out because there is no source of nitrogen, either in the fuel nꢀdecane and/or added to the reaction
system, in the supercritical nꢀdecane pyrolysis experiments.
We emphasize that quantification of the abundance of compounds with the AFM is challenging,
because the acquisition time is on the order of 10 min for an atomically resolved image and limits the
number of molecules that can feasibly be imaged within a sample to a few hundred. A statistical analysis
of AFM data and its comparison with mass spectrometry indicated the statistical significance of the
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sampling by AFM ; however, the numbers of molecules to be imaged for their quantification in
complex mixtures by AFM would have to be increased by several orders of magnitude.
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Because of the preparation method employed, also the maximum size of molecules that can be
studied is limited. Typically, molecules of 1000 Da molecular weight or more fragment when they are
39
sublimed . The molecules investigated here are of molecular weights below 500 Da and thus
fragmentation in the sublimation process is negligible. Moreover, they are mostly planar which is
advantageous to obtain high resolution images with COꢀtip AFM. Even considering the challenges of
AFM resolving nonꢀplanar, larger and more fragile molecules, there are strategies for investigating nonꢀ
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planar molecules
, it is also possible to investigate larger and more fragile molecules using
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electrospray ionization deposition
. We envision the methodology shown here being jointly used
with these strategies. For a more detailed discussion of the pitfalls and challenges of AFM for structure
63
elucidation we refer the reader to a recent review.
3
. Results and Discussion
By employing AFM, we first measure the supercritical nꢀdecane pyrolysis product Fraction P.
This relatively broad PAHꢀproduct fraction is analyzed by AFM to obtain an overview of the PAH
produced during supercritical nꢀdecane pyrolysis. AFM results showing individual PAH contained in
Fraction P are displayed in Figure 1a. Based on the AFM measurements, a PAH structure is assigned to
each AFM image. The resolved and assigned molecules in Figure 1b are: fiveꢀring PAH P1 and P2; sixꢀ
ring PAH P3, P4, and P5; sevenꢀring PAH P6 and P7; and eightꢀring PAH P8. Of these eight PAH
products shown in Figure 1, five are unsubstituted PAH (P1, P2, P3, P6, and P7) and three are
methylated PAH (P4, P5, and P8). Additionally, these eight PAH fall under three main structural
classes: benzenoid PAH (P3, P4, and P5), fluoranthene benzologues (P1, P6, P7, and P8), and a
phenalene benzologue (P2). These findings are consistent with previous HPLC/UV/MS analysis of the
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supercritical nꢀdecane pyrolysis products.
Of the eight PAH shown in Figure 1, four (P1, P3, P4, and P5) were reported in previous
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HPLC/UV/MS analysis of the supercritical nꢀdecane pyrolysis products.
A reference standard of P6
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is available to us and the UV absorbance maxima of P2 are published; however, neither of these two
PAH were previously identified as products of supercritical nꢀdecane pyrolysis. It could be that P2 and
P6 are present in amounts below the detection limit (0.3 ng of material) of our HPLC instrument or their
chromatographic signal is being masked by one or more coꢀeluting higherꢀyield PAH products. Studies
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report P6 as a pyrolysis product,
but not from fuel pyrolysis experiments. Unlike P2 and P6, a
reference standard and/or UV spectrum of P8 is unavailable. Presumably, the high sensitivity of AFM
due to singleꢀmolecule detection enabled us to detect P2, P6, and P8, which were previously not
reported as fuel pyrolysis and/or combustion products.
Figure 1. Fraction P of the products of nꢀdecane pyrolysis at 570 °C, 94.6 atm, and 133 sec measured by
AFM: (a) Constantꢀheight AFM measurements with a COꢀfunctionalized tip. Scale bars are 5 Å. P1, P2,
P3 and P5 were measured on Cu(111); P4, P6, P7, and P8 were measured on bilayer NaCl on Cu(111).
(
[
(
b) Proposed chemical structures: benzo[b]fluoranthene (P1), 7Hꢀbenzo[hi]chrysene (P2), benzoꢀ
ghi]perylene (P3), 1ꢀmethylbenzo[ghi]perylene (P4), 1ꢀmethylnaphtho[2,1ꢀa]pyrene (P5), rubicene
P6) benz[l]indeno[1,2,3ꢀcd]pyrene (P7), and 7,10ꢀdimethylꢀ1,2ꢀdihydrocyclopenta[5,6]acenaphtho[1,2ꢀ
a]pyrene (P8).
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Notably, we also detected with AFM a sevenꢀring C H PAH benz[l]indeno[1,2,3ꢀcd]pyrene
26 14
(
P7, in Figure 1), whose synthesis and/or identification as a fuel product has previously not been
reported in the literature. Therefore, based on the AFM detection of P7, we undertook the synthesis of
this novel molecule to facilitate its unequivocal identification and quantification in nꢀdecane’s products
by HPLC/UV/MS.
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Figure 2. Synthesis of benz[l]indeno[1,2,3ꢀcd]pyrene (P7): (a) Retrosynthetic analysis, (b) Synthetic
route to P7, and (c) AFM image of the synthesized compound on Cu(111), confirming the successful
preparation of P7. Scale bar is 5 Å.
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Compound P7 was prepared in eight steps based on the retrosynthetic analysis shown in Figure
a. The synthesis was initiated by a DielsꢀAlder cycloaddition of aryne 1, generated by fluorideꢀinduced
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decomposition of triflate 5, with furan 2 followed by acid treatment to obtain 1ꢀtriphenylenol 6 in 82%
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yield (Figure 2b). Transformation of compound 6 into the corresponding triflate, followed by Pdꢀ
catalyzed Sonogashira coupling and baseꢀinduced desilylation afforded terminal alkyne 7 in 61% yield.
Again, a Pdꢀcatalyzed Sonogashira coupling between alkyne 7 and bromide 4 led to compound 8, which
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was treated with PtCl to induce a cycloisomerization to obtain triflate 9. Finally, Pdꢀcatalyzed
2
cyclization of compound 9 afforded P7 in 50% yield (see supporting information for details). AFM
measurements unequivocally confirmed the structure of the synthesized standard as benz[l]indeno[1,2,3ꢀ
cd]pyrene (P7, Figure 2c), which is then used as reference standard for HPLC/UV/MS analysis.
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HPLC Elution Time (min)
Figure 3. Reversedꢀphase HPLC chromatogram of Fraction F of the products of nꢀdecane pyrolysis at
70 °C, 94.6 atm, and 133 sec. The 11 black structures, which include four unsubstituted C H
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benzenoid PAH, five singlyꢀmethylated PAH, and two doublyꢀmethylated (or ethylated) PAH are
33
reported in literature. The red structure corresponds to the newly identified PAH P7.
Normalꢀphase HPLC analysis of the standard of this newly synthesized C H PAH P7 reveals
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that it elutes in PAHꢀproduct Fraction F of nꢀdecane pyrolysis, which mainly contains PAH with 13 πꢀ
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bonds, i.e. C H PAH and their alkylated derivatives.
By employing the reversedꢀphase HPLC
method, which is used to analyze the Fraction F, we determined that P7 has a HPLC elution time of
6.1 min. This elution time corresponds to the single largest peak in the reversedꢀphase HPLC
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9
chromatogram of Fraction F, displayed in Figure 3. Previous HPLC analysis of Fraction F led to
33
identification of 11 PAH (black structures in Figure 3). However, due to the previous lack of a
reference standard and/or UV spectrum that matched with the chromatographic peak eluting at 96.1 min
in Fraction F, this HPLC peak was left unassigned. The mass spectrum of this unidentified PAH, shown
in Figure 4a, eluting at the same time as P7, portrays the primary ion at 326, which corresponds to the
molecular weight of a C H PAH (same as P7). In Figure 4b, we present the UV spectrum of the
26
14
unknown C H PAH product of supercritical nꢀdecane pyrolysis (in black), along with that of the
2
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14
synthesized standard of benz[l]indeno[1,2,3ꢀcd]pyrene (P7, in red). Figure 4b displays the excellent
agreement between the UV spectra of the product/standard pair, establishing the unequivocal
identification of benz[l]indeno[1,2,3ꢀcd]pyrene (P7) as a product of supercritical nꢀdecane pyrolysis.
The quantification of P7 reveals that for nꢀdecane pyrolysis experiments conducted at 570 °C, 94.6 atm,
and 133 sec, this newly identified molecule accounts for 5.9% of the mass of Fraction F, which is
equivalent to 0.267 ꢁg of P7 per g of nꢀdecane fed to the reactor. Therefore, by employing AFM in
conjunction with HPLC/UV/MS and synthetic chemistry, we established the exact structural identity of
the novel sevenꢀring C H PAH benz[l]indeno[1,2,3ꢀcd]pyrene.
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Figure 4. (a) Mass spectrum of the supercritical nꢀdecane pyrolysis product, eluting at 96.1 min in the
HPLC chromatogram in Figure 3; and (b) UV absorbance spectrum (in black) of the supercritical nꢀ
decane pyrolysis product, eluting at 96.1 min in the HPLC chromatogram in Figure 3, along with the UV
absorbance spectrum (in red) of the reference standard of P7.
Because our HPLC analysis revealed that benz[l]indeno[1,2,3ꢀcd]pyrene (P7) elutes in Fraction
F of the products of nꢀdecane pyrolysis, we further analyzed this PAHꢀproduct fraction by AFM to
search for the presence of additional unknown PAH. The AFM images of six molecules measured in
Fraction F, labelled F1 to F6, along with tentatively assigned chemical structures are shown in Figure 5.
The AFM measurements of Fraction F indicate that all of the imaged molecules have side groups
attached to the PAH core and, expectedly, the most common side groups are methyl. Most of the
molecules are sevenꢀring PAH, except for F5 and F6, which contain eight and six rings, respectively.
Each of these six PAH detected by AFM have an aromatic core that contains 13 πꢀbonds, which is
3
2,33
consistent with our previous findings
bonds. Three of these six molecules (F1, F4, and F5) have an aromatic core that corresponds to a C H
14
that showed that Fraction F primarily contains PAH with 13 πꢀ
2
6
PAH. The aromatic cores of F2 and F3 correspond to C H PAH and that of F6 corresponds to a
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C H PAH. Although the aromatic cores of F2, F3, and F6 constitute a slightly larger molecular
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formula than C H they still contain 13 πꢀbonds, consistent with them eluting in Fraction F.
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Figure 5. Fraction F of the products of nꢀdecane pyrolysis at 570 °C, 94.6 atm, and 133 sec measured by
AFM: (a) AFM measurements. Scale bars are 5 Å. F4 and F5 were measured on bilayer NaCl, all the
other molecules were measured on Cu(111). (b) The PAH cores of the proposed chemical structures are
benz[l]indeno[1,2,3ꢀcd]pyrene (F1), benz[4,5]indeno[2,1ꢀa]pyrene (F2), fluoreno[2,3ꢀe]pyrene (F3),
benz[a]anthanthrene (F4), 4,5ꢀdihydrodibenzo[b,ghi]cyclopenta[lm]perylene (F5), and dibenzo[b,k]ꢀ
chrysene (F6).
F1 is a substituted derivative of benz[l]indeno[1,2,3ꢀcd]pyrene (P7), whose two side groups
could not be identified. The bright appearance on the bottom rightꢀhand side of the AFM image of F1, in
Figure 5a, indicates a nonꢀplanar, bulky sideꢀgroup at this position. Assignment of these groups might
become possible in the future when more AFM characteristics for fingerprinting of molecular moieties
4
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will have been measured.
Of these six PAH shown in Figure 5, UV spectra and/or reference
standards are available for benz[l]indeno[1,2,3ꢀcd]pyrene, dibenzo[b,ghi]perylene, and
dibenzo[b,k]chrysene, which correspond to the aromatic cores of F1, F5, and F6, respectively, but for
none of their substituted counterparts. The reference standards or UV spectra of F2, F3, and F4 and/or
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their unsubstituted counterparts are unavailable to us. Therefore, it is certainly plausible that some of the
unidentified peaks in the HPLC chromatogram of Fraction F, in Figure 3, could correspond to one or
more of these novel PAH detected by AFM. The joint usage of STM orbital imaging with AFM can
further aid in structure determination of unknown PAH in fuelꢀproduct mixtures, an example being F4
(more details in the supporting information). In this case, the benz[a]anthanthrene motif is properly
identified by comparing the experimentally obtained lowest unoccupied molecular orbital (LUMO)
image with the calculated LUMO via density functional theory. This is the first time any of these six
PAH have been reported as fuel pyrolysis and/or combustion products.
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. Conclusion
This study shows that an AFMꢀbased strategy, in conjunction with synthetic chemistry and analytical
tools, is advantageous for investigating molecular ensembles. We showcase the detection, identification,
synthesis, and quantification of a novel sevenꢀring PAH product, benz[l]indeno[1,2,3ꢀcd]pyrene, of
supercritical nꢀdecane pyrolysis. Aided by the AFM capability of probing single molecules, even at
lower concentrations than 1 ppm, novel molecules within fuel pyrolysis products were reported. The
demonstrated multiꢀdisciplinary approach directs the production of chemical standards in a costꢀ
effective and timeꢀsaving manner and should generally be applicable in fields where a diverse range of
PAH are being formed.
5
. Acknowledgments
We thank Rolf Allenspach for comments on the manuscript. We are also grateful to Dr. Arthur
Lafleur and Ms. Elaine Plummer, of the Massachusetts Institute of Technology; Dr. John Fetzer,
FETZPAHS; and Dr. Werner Schmidt, PAHꢀForschung, for providing reference standards and UV
spectra of PAH. The authors acknowledge financial support by the European Research Council
(Advanced grant ‘CEMAS’ – agreement no. 291194 and Consolidator grant ‘AMSEL’ – agreement no.
6
82144), EU projects ‘PAMS’ (Contract no. 610446) and Initial training network ‘ACRITAS’ (Contract
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no. 317348), the Air Force Office of Scientific Research (Grant FA9550ꢀ13ꢀ1ꢀ0172), the Spanish
Agencia Estatal de Investigación (MAT2016ꢀ78293ꢀC6ꢀ3ꢀR and CTQ2016ꢀ78157ꢀR), the Xunta de
Galicia (Centro singular de investigación de Galicia accreditation 2016ꢀ2019, ED431G/09), and the
European Regional Development Fund (ERDF).
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Supplementary Material: Additional AFM measurements, chemical structures, and synthetic procedures.
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