Self-Condensation of Pyrene-4,5-dione: An Approach to Generate Functional Organic Fluorophores

Article abstract of DOI:10.1021/acs.orglett.9b03297

A new class of π-conjugated organic fluorophores containing a 6H-phenanthro[4,5-cde]pyreno[4′,5′:4,5]imidazo[1,2-a]azepin-6-one core was synthesized by a facile one-pot condensation method. The molecular structures and solid-state packing properties of th

Full text of DOI:10.1021/acs.orglett.9b03297

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Self-Condensation of Pyrene-4,5-dione: An Approach To Generate
Functional Organic Fluorophores
Farshid Shahrokhi and Yuming Zhao*
Department of Chemistry, Memorial University of Newfoundland, St. John’s, NL A1B 3X7, Canada
S
* Supporting Information
ABSTRACT: A new class of π-conjugated organic fluorophores
containing a 6H-phenanthro[4,5-cde]pyreno[4′,5′:4,5]imidazo[1,2-a]-
azepin-6-one core was synthesized by a facile one-pot condensation
method. The molecular structures and solid-state packing properties of
these compounds were investigated by X-ray single crystallographic
analysis. UV−vis absorption and fluorescence spectroscopic studies
disclosed interesting mechanofluorochromic properties in the solid
state and highly sensitive acidochromic behavior in the solution phase.
yrene derivatives are popular molecular building blocks in
the development of advanced organic optoelectronic
butyl substituted pyrene-4,5-diones 1b and 1c were prepared,3c
and they were subjected to the same self-condensation
conditions to afford analogous products with both improved
solubility and yields (Scheme 1). The molecular structures of
condensation products 2b and 2c were unambiguously
confirmed by X-ray single crystallography in addition to NMR
and MS analysis. The X-ray structure of 2b (Figure 1A) shows
P
materials, owing to the rich electronic and photophysical
properties of pyrene.1 Studies in this area have been
continuously motivated by the development of new function-
alized pyrenes, which serve as useful precursors for the
preparation of advanced π-conjugated materials.2 In recent
years, pyrene-4,5-dione3 has found growing application,
especially after the disclosure of an efficient large-scale synthesis
of pyrene-4,5-diones by Bodwell and co-workers.3c Syntheti-
cally, pyrene-4,5-dione can undergo various reactions to yield
more extended π-systems.4 Among them, condensation
reactions constitute a very appealing approach due to their
high efficiency and simplicity in operation.
In our recent study of pyrenoimidazole derivatives,4e an
interesting type of self-condensation reaction was discovered. As
shown in Scheme 1, a mixture of pyrene-4,5-dione (1a),
Scheme 1. Synthesis of Pyrene Derivatives 2a−c through Self-
Condensation of Pyrene-4,5-diones
Figure 1. ORTEP drawings (50% probability) of (A) 2b and (B) 2c as
well as the packing motifs of their enantiomeric pairs in the crystalline
state.
that the steric interactions between 6-one and pyrenyl 8-H make
the molecule adopt a nonplanar helical shape. Two types of
enantiomers with opposite helicity are observed in the crystal
structure. As shown in Figure 1A, enantiomeric pairs of 2b are
packed in a slip-stacked fashion. Density functional theory
(DFT) calculations revealed that the racemization energy
barriers of 2 are quite low (e.g., 6.46 kcal/mol for 2a, see Figure
S-16, Supporting Information), suggesting that individual
molecules of 2, in the gas phase or well dissolved in a solvent,
ammonium acetate, and glacial acetic acid was heated at 100 °C
for 10 h, resulting in the formation of 6H-phenanthro[4,5-
cde]pyreno[4′,5′:4,5]imidazo[1,2-a]azepin-6-one (2a) in 25%
yield. Although 2a shows relatively low solubility in common
organic solvents, its molecular structure could still be reasonably
elucidated based on 1H NMR and MS analyses (see the
Supporting Information). To address the solubility problem, t-
Received: September 19, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b03297
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Organic Letters
Letter
would undergo rapid configurational exchanges under standard
conditions.
The molecular structure of 2c (Figure 1B) shows similarity to
that of 2b, but the π-stacking of each enantiomeric pair in the
crystalline state appears to be more cofacial than 2b as a result of
less t-butyl groups present in 2c. Note that 2-t-butylpyrene-4,5-
dione (1c) is an unsymmetrical diketone, which in theory may
yield four different regioisomeric products through self-
condensation (see Figure S-17, Supporting Information). For
example, Emery et al. reported that the self-condensation of an
unsymmetric diketone under conditions similar to ours afforded
two regioisomers in nearly equal yields.5 In our reaction,
however, only one major product 2c was obtained from the self-
condensation of 1c. As DFT calculations indicated that the
structure of 2c is the least thermodynamically stable one among
other possible self-condensation products (Figure S-17,
Supporting Information), the regioselectivity of this reaction is
likely kinetically controlled rather than thermodynamically
controlled.
Figure 2. (A) ORTEP drawing (50% probability) and (B) crystal
packing diagram of compound 5. Hydrogen atoms are omitted for
clarity.
for the different reactivities of pyrene-4,5-dione versus
phenanthrene-9,10-dione can be made based on the reaction
mechanisms outlined in Scheme 3. Herein, two reaction
Scheme 3. Proposed Mechanisms for the Self-Condensation
Reactions of Phenanthrene-9,10-dione (3) and Pyrene-4,5-
dione (1a), Respectively
To gain a deeper insight into the reaction mechanism of the
above self-condensation reactions, two control reactions were
carried out. First, phenanthrene-9,10-dione (3) was subjected to
heating in the presence of ammonium acetate and glacial acetic
acid. This reaction only yielded phenanthroimidazole 4 as the
major product (Scheme 2). Product 4 was formed as a
Scheme 2. Control Condensation Reactions of
Phenanthrene-9,10-dione (3) and Pyrene-4,5-dione (1b)
pathways are illustrated for the self-condensation reactions of
3 and 1a, respectively. Phenanthrene-9,10-dione yields a diimine
intermediate IM-1 when treated with ammonium acetate and
glacial acetic acid. IM-1 then reacts with 3 through condensation
to form a spiral intermediate IM-2, which undergoes a ring
opening process through water nucleophilic attack to give
product 4.5−7 The rotational freedom at the biphenyl unit in 4
allows the carboxylic group to shun away from the imidazolyl
group in order to minimize steric crowding. DFT optimized
geometry of 4 shows that the distances between carboxylic
carbon and two imidazolyl nitrogen atoms are 3.53 and 5.45 Å,
respectively (Figure S-18, Supporting Information). Such a
structure disfavors intramolecular condensation between the
carboxylic and imidazolyl groups. Pyrene-4,5-dione 1a can also
form a diimine intermediate IM-3, followed by a spiral
intermediate IM-4. Ring opening of IM-4 leads to IM-5 in
which the carboxylic group and the imidazolyl unit are closely
positioned as a result of their direct linkage to the rigid
phenanthrene unit. The distances between carboxylic carbon
and two imidazolyl nitrogen atoms in the optimized geometry of
IM-5 (Figure S-19, Supporting Information) are 3.03 and 3.08
Å, which are much shorter than those in 4. The preorganization
of IM-5 thus enables the intramolecular condensation between
the carboxylic and imidazolyl groups, which eventually yields 2a
as a stable product.
precipitate with very poor solubility in common organic solvents
due to the presence of a free carboxylic group. This outcome
contrasts the cases of pyrene-4,5-diones but is consistent with
the observation reported by Lantos in 1975.6 In another control
reaction, a mixture of diones 1b and 3 was subjected to the same
condensation conditions. In this reaction, two soluble products
were isolated in nearly equal yields. The first product is 2b,
resulting from the self-condensation of 1b. The second product
turns out be a cross-condensation product 5 (Scheme 2). The
molecular structure of 5 was validated by X-ray structural
analysis (Figure 2A) in addition to NMR and MS. Similar to 2b/
c, enantiomeric pairs exist in the crystal structure of 5, and they
form ordered columnar supramolecular assemblies through π-
stacking (Figure 2B). In theory, cross-condensation of two
different diketones should yield two products. The fact that only
5 was formed suggests a high degree of regioselectivity, which
deserves more light to be shed upon.
The two control reactions underscore the indispensable role
of pyrene-4,5-dione in the formation of the seven-membered
azepinone ring during the condensation process. Rationalization
B
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Given the relatively good solubility of 2b in organic solvents, it
was further investigated by UV−vis absorption and fluorescence
spectroscopic analysis to probe the electronic properties of these
new pyrene-based π-systems. Figure 3A compares the UV−vis
Figure 3. (A) Normalized UV−vis absorption spectra of 2b measured
in different organic solvents. (B) Normalized fluorescence spectra of 2b
(in CCl4) measured at different concentrations (λex = 313 nm).
Figure 4. (A) Photographic images of 2b (1.49 × 10−5 M) dissolved or
suspended in different organic solvents (λem values are indicated). (B)
Photographic images of 2b in crystalline form. (C) Photographic
images of 2b in powdery form. All samples were placed under the
irradiation with a UV lamp (365 nm). (D) Fluorescence spectra of 2b
(1.49 × 10−5 M) measured in various solvents (λex = 313 nm). (E)
Normalized fluorescence spectra of 2b measured in crystalline and
powdery forms (λex = 350 nm).
spectra of 2b measured in different organic solvents. The
absorption profiles of 2b show similar features in most of the
solvents examined, except in methanol and chloroform. In
methanol, the lowest-energy π → π* absorption band of 2b is
considerably blueshifted to 383 nm, while in chloroform it
shows the most pronounced redshift (λmax = 400 nm). The
solvatochromic effects observed here do not follow a clear trend
in correlation with solvent polarity. More likely, solubility and/
or aggregation effects play some critical roles in the electronic
absorption properties of 2b.
The emission properties of 2b in the solution phase were
studied by fluorescence spectroscopy. Carbon tetrachloride was
used as the solvent, considering the good solubility it affords.
Figure 3B shows the fluorescence spectra of 2b measured at
different concentrations. At relatively high concentration (10−4
M), the spectrum features only one broad band peaking at 520
nm. When the concentration is reduced by 1 order of magnitude
(10−5 M), this emission band is slightly blueshifted to 505 nm. In
the meantime, a new shoulder band emerges in the high-energy
region (ca. 380−440 nm), and the relative intensity of this new
band continues to increase under further diluted conditions
(10−6 M). The concentration-dependent fluorescence behavior
can be attributed to the formation of excimers in solution.8 At
high concentration, excimers dominate the emission process to
give the long-wavelength band at 520 nm. At low concentration,
however, the emission of individual molecules of 2b (i.e.,
monomers) becomes more significant in the range of 380−400
nm. As such, the fluorescence profile reflects the contributions of
both excimers and monomers.
When 2b was dissolved in different organic solvents at
concentration of 10−5 M, the solutions were found to emit
different luminescent colors upon long-wavelength UV light
irradiation (see Figure 4A). For solvents with good solubility,
such as carbon tetrachloride, p-xylene, benzene, THF, and ethyl
acetate, the solutions of 2b give greenish luminescence. For
solvents with moderate solubility (chloroform, acetone,
acetonitrile, and DMSO), the luminescent color is yellowish.
For solvents with poor solubility such as methanol and ethanol,
the luminescence is very weak. However, in hexane, the
luminescent color is blue. The luminescent colors show a
good correlation with the maximum emission wavelengths (λem)
observed in their fluorescence spectra (Figure 4D). It was
further noticed that compound 2b in the solid state exhibits
mechanofluorochromic behavior. Crystals of 2b yield blue-
colored luminescence. Subjecting the crystals to mechanical
grinding resulted in amorphous powder, which gave green-
colored luminescence (Figure 4B,C). Solid-state fluorescence
spectroscopic analysis showed that crystals of 2b emit a broad
peak around 508 nm, while the powder of 2b (amorphous state)
yields two emission bands at 441 and 508 nm, respectively
(Figure 4E). The mechanofluorochromic properties of 2b can
be attributed to the emission of excimers and monomers. In
crystalline state, 2b molecules are closely stacked to facilitate the
formation of excimers when they are electronically excited. As
such, the emission profile of 2b only features the long-
wavelength excimer band, which yields blue-colored lumines-
cent light. In amorphous solid state, 2b molecules are randomly
and disorderly packed, so they emit both long-wavelength
(excimer) and short-wavelength (monomer) light. The
combined emission bands thus constitute the observed green-
colored luminescence.
The solutions of 2b were also found to show acidochromic
fluorescence behavior. As shown in Figure 5, the fluorescence
intensity of 2b grows steadily with increasing addition of a strong
organic acid, trifluoroacetic acid (TFA). The fluorescence
enhancement reached saturation after ca. 3100 mol equiv of
TFA was added, while the λem was observed to redshift in a
Figure 5. Fluorescence spectra of 2b (1.49 × 10−4 M in CCl4) in
response to the addition of TFA from 0 to 3.10 × 103 molar equivalents.
(B) Plot of maximum emission intensity against equivalents of TFA.
C
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Letter
moderate degree, from 513 to 530 nm, during the TFA titration.
The relative fluorescence quantum yield (ϕF) of 2b was
determined to be 0.101 in CCl4, and it was increased to 0.340
after addition of excess TFA (3100 equiv).
To understand the detailed interacting modes between 2b and
TFA in the solution phase, 1H NMR titration experiments were
performed, and the results are given in Figure 6. The aromatic
ochromic, and acidochromic fluorescence properties were
observed for this new class of pyrene-based organic
fluorophores, and the origins of these fluorescence behaviors
can be attributed to excimer/monomer equilibria. Our findings
open a new avenue for the development of pyrene-based
luminescent materials with potential applications in advanced
molecular optoelectronics.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.9b03297.
Detailed experimental procedures, characterization, and
DFT calculation results (PDF)
Accession Codes
CCDC 1946909−1946911 and 1952458 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: yuming@mun.ca.
ORCID
Farshid Shahrokhi: 0000-0001-8222-5817
Yuming Zhao: 0000-0002-1300-9309
Notes
1
Figure 6. H NMR spectra of 2b (6.41 × 10−3 M in CD2Cl2) in
response to the addition of TFA from 0 to 9.21 mol equiv. Signals of
protons at 15 and 17 positions are highlighted.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
proton signals exhibit varied degrees of shift with TFA addition,
while the most significant changes can be noted in the downfield
region of the spectrum. Herein, the two doublets at 9.63 and
9.17 ppm are assigned to the aromatic protons at 17 and 15
positions (referred to as H17 and H15) according to DFT
calculations (Figure S-21, Supporting Information). Upon
addition of TFA, the imidazolyl nitrogen atom (N16) is
protonated since it is the most basic site in the molecule. The
proposed protonation mode is also corroborated by the single-
crystal structure of the complex of [2b·(TFA)2] (shown in
Figure S-23A, Supporting Information). Eventually, the
protonation of 2b with TFA leads to a substantial degree of
upfield shift of H17 (from 9.63 to 8.95 ppm), while the signal of
H15 is only moderately shifted from 9.17 to 9.06 ppm. In the
crystal structure of protonated 2b, the newly formed
imidazolium N−H group shows a much closer distance to H17
than to H15. This accounts for that observation that the H17
signal is the most shifted after protonation. The X-ray packing
diagram (Figure S-23B, Supporting Information) indicates that
complexation of 2b with TFA leads to highly ordered
supramolecular assemblies. It is therefore reasonable to propose
that addition of TFA to the solution of 2b could also enhance
intermolecular interactions, which facilitate excimer formation
and increase fluorescence emission.
We thank NSERC and Memorial University for funding support.
Dr. Michael Ferguson at University of Alberta and Dr. Michael
Katz at Memorial University are acknowledged for assistance in
the X-ray single crystallographic analysis.
REFERENCES
(1) (a) Figueira-Duarte, T. M.; Mu
■
̈
llen, K. Pyrene-based materials for
organic electronics. Chem. Rev. 2011, 111, 7260−7314. (b) Li, J.; Chen,
S.; Wang, Z.; Zhang, Q. Pyrene-fused acenes and azaacenes: synthesis
and applications. Chem. Rec. 2016, 16, 1518−1530.
(2) (a) Mateo-Alonso, A. Pyrene-fused pyrazaacenes: from small
molecules to nanoribbons. Chem. Soc. Rev. 2014, 43, 6311−6324.
(b) Casas-Solvas, J. M.; Howgego, J. D.; Davis, A. P. Synthesis of
substituted pyrenes by indirect methods. Org. Biomol. Chem. 2014, 12,
̈
̈
212−232. (c) Zophel, L.; Enkelmann, V.; Rieger, R.; Mullen, K. Saddle
shaped hexaaryl[a,c,fg,j,l,op]tetracenes from 4,5,9,10-tetrafunctional-
ized pyrenes. Org. Lett. 2011, 13, 4506−4509. (d) Feng, X.; Hu, J. Y.;
Redshaw, C.; Yamato, T. Functionalization of pyrene to prepare
luminescent materials-typical examples of synthetic methodology.
Chem. - Eur. J. 2016, 22, 11898−11916.
(3) (a) Young, E. R.; Funk, R. L. A practical synthesis of pyrene-4, 5-
dione. J. Org. Chem. 1998, 63, 9995−9996. (b) Hu, J.; Zhang, D.;
Harris, F. W. Ruthenium(III) chloride catalyzed oxidation of pyrene
and 2,7-disubstitued pyrenes: an efficient, one-step synthesis of pyrene-
4,5-diones and pyrene-4,5,9,10-tetraones. J. Org. Chem. 2005, 70, 707−
708. (c) Walsh, J. C.; Williams, K. L. M.; Lungerich, D.; Bodwell, G. J.
Synthesis of pyrene-4,5-dione on a 15 g scale. Eur. J. Org. Chem. 2016,
2016, 5933−5936.
In summary, we have discovered a straightforward synthetic
method to construct polycyclic aromatic structures based on
6H-phenanthro[4,5-cde]pyreno[4′,5′:4,5]imidazo[1,2-a]-
azepin-6-one. Interesting solvatofluorochromic, mechanofluor-
(4) (a) Prakash Rao, H. S.; Vijjapu, S. Synthesis and photochromic
properties of benzofuran−phenanthrene and benzofuran−pyrene
D
DOI: 10.1021/acs.orglett.9b03297
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
hybrids. RSC Adv. 2014, 4, 25747−25758. (b) Fan, Q.; Liu, Y.; Xiao,
M.; Tan, H.; Wang, Y.; Su, W.; Yu, D.; Yang, R.; Zhu, W. Donor-
acceptor copolymers based on benzo[1,2-b:4,5-b’]dithiophene and
pyrene-fused phenazine for high-performance polymer solar cells. Org.
Electron. 2014, 15, 3375−3383. (c) Gu, P.-Y.; Zhang, J.; Long, G.;
Wang, Z.; Zhang, Q. Solution-processable thiadiazoloquinoxaline-
based donor−acceptor small molecules for thin-film transistors. J.
Mater. Chem. C 2016, 4, 3809−3814. (d) Liu, Y.; Bai, Q.; Li, J.; Zhang,
S.; Zhang, C.; Lu, F.; Yang, B.; Lu, P. Efficient pyrene-imidazole
derivatives for organic light-emitting diodes. RSC Adv. 2016, 6, 17239−
17245. (e) Tabasi, Z. A.; Younes, E. A.; Walsh, J. C.; Thompson, D. W.;
Bodwell, G. J.; Zhao, Y. Pyrenoimidazolyl-benzaldehyde fluorophores:
synthesis, properties, and sensing function for fluoride anions. ACS
Omega 2018, 3, 16387−16397. (f) Karthik, S.; Ajantha, J.; Nagaraja, C.;
Easwaramoorthi, S.; Gandhi, T. Synthesis and photophysics of
extended π-conjugated systems of substituted 10-aryl-pyrenoimida-
zoles. Org. Biomol. Chem. 2016, 14, 10255−10266. (g) Mardanya, S.;
Karmakar, S.; Mondal, D.; Baitalik, S. An imidazolyl-pyreno-imidazole
conjugate as a cyanide sensor and a set−reset memorized sequential
logic device. Dalton Trans. 2015, 44, 15994−16012. (h) Jadhav, T.;
Dhokale, B.; Mobin, S. M.; Misra, R. Aggregation induced emission and
mechanochromism in pyrenoimidazoles. J. Mater. Chem. C 2015, 3,
9981−9988.
(5) Emery, F. S.; Maria do Carmo, F.; de Simone, C. A.; Malta, V. R.;
́
da Silva Junior, E. N.; Pinto, A. V. Complex diazaazulenones from the
reaction of ortho-naphthoquinones with ammonium acetate. Synlett
2010, 2010, 1931−1934.
(6) Lantos, I. Reaction of phenanthrenequinone with ammonium
acetate. J. Org. Chem. 1975, 40, 1641−1642.
̈
(7) Biegger, P.; Schaffroth, M.; Brodner, K.; Tverskoy, O.; Rominger,
F.; Bunz, U. H. Bisalkynylated 3,6-diiminocyclohexa-1,4-diene-1,4-
diamine. Chem. Commun. 2015, 51, 14844−14847.
̈
(8) (a) Forster, T. Excimers. Angew. Chem., Int. Ed. Engl. 1969, 8,
333−343. (b) Winnik, F. M. Photophysics of preassociated pyrenes in
aqueous polymer solutions and in other organized media. Chem. Rev.
1993, 93, 587−614.
E
DOI: 10.1021/acs.orglett.9b03297
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