Highly efficient one-pot synthesis of α-free pyrylium salts with tunable fluorescence emission via ring-expanding reaction of triarylcyclopentadienes

Article abstract of DOI:10.1016/j.tetlet.2013.03.113

A series of α-free pyrylium perchlorates were prepared by one-pot oxidation of substituted cyclopentadiene derivatives in the presence of perchloric acid. A tunable fluorescence with green to red emission in dichloromethane and green to orange emission in the solid state can be achieved by changing the aryl groups on 2- or 4-position of pyrylium ring. Furthermore, single crystal structures and quantum chemistry calculation were conducted to understand the relationship between structures and optical properties.

Full text of DOI:10.1016/j.tetlet.2013.03.113

Tetrahedron Letters 54 (2013) 2967–2971
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Highly efficient one-pot synthesis of
a-free pyrylium salts with tunable
fluorescence emission via ring-expanding reaction of triarylcyclopentadienes
⇑
⇑
Lijian Yang, Junwei Ye , Yuan Gao, Dai Deng, Weitao Gong, Yuan Lin, Guiling Ning
State Key Laboratory of Fine Chemicals and School of Chemical Engineering, Dalian University of Technology, Dalian 116012, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of a-free pyrylium perchlorates were prepared by one-pot oxidation of substituted cyclopenta-
Received 5 January 2013
Revised 11 March 2013
Accepted 25 March 2013
Available online 6 April 2013
diene derivatives in the presence of perchloric acid. A tunable fluorescence with green to red emission in
dichloromethane and green to orange emission in the solid state can be achieved by changing the aryl
groups on 2- or 4-position of pyrylium ring. Furthermore, single crystal structures and quantum chem-
istry calculation were conducted to understand the relationship between structures and optical
properties.
Keywords:
Ó 2013 Elsevier Ltd. All rights reserved.
a-Free pyrylium perchlorates
Fluorescence
Crystal structure
DFT calculation
Organic fluorescent materials have received considerable atten-
tion in both scientific research and practical aspects because of
their promising applications in electronic and photonic devices.1
For expansion of fluorescence-based applications, particularly in
fields of organic light-emitting diodes and molecular probes, con-
trolling emission properties of organic fluorophores has become a
hot research topic. So far, many approaches have been explored
to tune the emission wavelength of organic molecules in solution,
such as utilizing of exogenous additives2a–c and linking small mol-
ecules with macromolecule backbone.2d,e In contrast, only a few
studies have been reported to tune emission properties of lumines-
cent molecules in the solid state.3 As an important type of hetero-
cyclic luminescent compounds, pyrylium salts are used in many
fields such as organic intermediates,4a–c photosensitizers,4d,e Q-
switchers,4f laser dyes4g and organic luminophores.4h Recently,
many studies have been conducted to optimize the photophysical
and electrochemical behavior of pyrylium salts. For example, the
introduction of different electron-withdrawing or -donating sub-
stituents on pyrylium ring varied quantum efficiency and tuned
the emission properties.5 However, these studies are only limited
to 2,4,6-tri-substituted-pyrylium salts which are usually obtained
by a lengthy stepwise manner hinging on the formation and cycli-
zation of a 1,5-diketone.6 In our previous work, we developed a
high-efficiency one-pot synthesis of pyrylium salts with only one
compounds display unique reactivity to nucleophilic reagents.8
As a part of our continuous studies on -free pyrylium salts, we fo-
cus on the universality of our synthetic route and the influence of
substituents on the optical properties of -free pyrylium salts.
Herein, we report the synthesis of -free pyrylium perchlorates
a
a
a
with tunable fluorescence emission via ring-expanding reaction
of triarylcyclopentadienes with different aryl substituent groups.
The influence of substituents on the absorption and emission
behavior of products was examined by the variation of donor
strength and position on pyrylium ring. In addition, single crystal
structures and quantum chemistry calculation are conducted to
explain the relationship between structures and optical properties.
Cyclopentadiene derivatives were prepared according to the
published procedure.9 Pyrylium salts 2a–2g were synthesized
according to our previously reported synthetic route in high yields
(Scheme 1).7b,10 Their structures are well characterized by 1H NMR,
13C NMR, HRMS, MS, and X-ray single-crystal diffraction. The opti-
cal properties of 2a–2g were investigated by UV–vis absorption
spectroscopy (Fig. 1) and photoluminescence spectroscopy (Figs. 2
and 3) in dilute dichloromethane solution and in the solid state.
Abs
max
The maximum absorption wavelengths k
of these compounds
are in the range of 372–457 nm (Table 1), which are caused by
Abs
p–
p⁄ transition of molecular plane. As expected, the k
of 2a
max
and 2b shifted hypochromatically of 18 nm and 4 nm compared
with that of Br-substituted 2c owing to the stronger electron-with-
drawing effect of CF3 and F. Compared with 2c, the kAmbasx of 2d–2g is
bathochromically shifted of 67, 50, 30, and 65 nm, respectively,
which is due to the extending conjugation in the backbone and
the enhanced charge transfer from the pyrylium O-atom to the
electron donating naphthyl, biphenyl, and naphthyl–phenyl groups
substituent at
a position (named a-free pyrylium salts) from
polyphenyl–cyclopentadiene compounds in the presence of sil-
ver(I) perchlorate or perchloric acid.7 Much to our surprise, these
⇑
Corresponding authors. Tel.: +86 411 84986067; fax: +86 411 84986065.
E-mail addresses: junweiye@dlut.edu.cn (J. Ye), ninggl@dlut.edu.cn (G. Ning).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.03.113

2968
L. Yang et al. / Tetrahedron Letters 54 (2013) 2967–2971
Scheme 1. Synthetic route to a-free pyrylium perchlorates from cyclopentadiene derivatives.
Figure 1. Normalized UV–vis absorption spectra of 2a–2g in dichloromethane at
5 Â 10À5 M concentration.
Figure 3. (a) Crystal photographs of compounds 2a–2c, 2f, and 2g under the
irradiation of UV lamp (365 nm); (b) normalized PL spectra of 2a–2g in the solid
state.
introducing electron-donating substituents but is weakly influ-
enced by electron-acceptors. For instance, the maximum emitting
peaks of 2c, 2e, 2d, and 2g in dichloromethane solution are 538,
547, 595, and 680 nm, respectively. The PL quantum efficiency
(UFL) of 2a–2g in dilute dichloromethane solution ranged from
0.0125 to 0.90 (Table 1).
Then the fluorescence emission properties of 2a–2g in solid
state were measured (Fig. 3). The emission of 2a–2g is in the blue
to yellow region with the emission peaks from 469 to 580 nm (Ta-
ble 1). However, the shapes of PL spectra of 2d, 2f, and 2g are sim-
ilar, indicating that these emitting bands should be a local emission
from the pyrylium fragment. The photographs of crystals 2a, 2b,
2c, 2f, and 2g under illumination with UV light (kex = 365 nm) are
shown in Figure 3a. Pyrylium salts 2a–2c emit strong blue, strong
green, and weak yellow–green light, respectively, whereas 2f and
2g exhibit yellow emission. To investigate the properties in further
detail, the fluorescence quantum efficiency (UFL) in solid state was
examined as summarized in Table 1. The pyrylium salts 2f with
biphenyl in 4-position exhibit a weak UFL, owing to the hindered
rotation of single bond in 4-position which is unfavorable for quan-
tum efficiency. In comparison, other pyrylium salts have much
Figure 2. Normalized PL spectra of 2a–2g in dichloromethane at 5 Â 10À5
M
concentration.
on 2- or 4-position. Compound 2d possesses the longer wavelength
absorption than that of 2e–2f, which may be attributed to the
retardation of electronic delocalization in 2e–2f caused by the
more twist configurations between the pyrylium ring and a-linked
aryl groups. The photoluminescence (PL) spectra of 2a–2g in dilute
dichloromethane solution showed emitting peaks at 535–680 nm
with light region from green to red (Fig. 2). The fluorescence max-
imum wavelength of pyrylium salts shifts bathochromically with

L. Yang et al. / Tetrahedron Letters 54 (2013) 2967–2971
2969
Table 1
Thermal properties, optical properties, and energy levels of
a-free pyrylium perchlorates
Eogptc/eV
D
Ecgal/eV
Compound
Td/°C
kabsa/nm
kema/nm
kemb/nm
UFLa/%
UFLb/%
ELUMOd/eV
EHOMOe/eV
D
2a
2b
2c
2d
2e
2f
215
200
215
215
200
235
235
372
386
390
457
440
421
455
546
536
538
595
547
615
680
469
511
526
550
547
580
581
3.62
8.40
7.00
30.6
90.0
4.18
1.25
28
48
45
39
46
6.0
65
2.50
2.48
2.48
2.28
2.39
2.33
2.16
À4.33
À4.27
À4.31
À4.25
À4.27
À4.29
À4.26
À1.83
À1.79
À1.83
À1.98
À1.88
À1.96
À2.10
2.28
2.29
2.32
2.04
2.47
2.21
1.17
2g
a
b
c
Measured in dichloromethane solution (5 Â 10À5 M).
Measured in the solid state.
Bandgaps calculated from absorption edge:
D .
Eogpt = 1240/konset
d
e
The LUMO energy levels were calculated from Ered values of CV curves: ELUMO = À(Ered + 4.8) eV.
The HOMO energy levels were calculated using the equation:
D
Eg = EHOMO À ELUMO
.
higher UFL values. Especially, 2g with naphthyl–phenyl on 2-posi-
tion exhibits a highest UFL value of 65%.
cules. As shown in Figures. 4b, 6b and 7b, the building blocks of
these structures are not monomers, but rather the dimers based
To understand the relationship between molecular structures
and optical properties, the crystal structures of 2a–2c and 2g were
determined by single-crystal X-ray diffraction analysis. A summary
of the crystallographic data and structure refinements is tabulated
in Table S1 while the selected bond lengths and angles are listed in
Table S2. The thermal ellipsoid plots and packing diagrams of 2a–
2c and 2g are shown in Figures. 4–7. The asymmetric unit contains
an independent pyrylium cation (py+) and a ClO4À anion. There are
on the C–HÁ Á Á
p interactions between adjacent two molecules. Fur-
thermore, the dimers adopt a stair-like packing mode or a kind of
distorted J-type stacking style to form molecular columns. These
results suggested the latent relationship between emission proper-
ties and crystal structures. Firstly, multiple C–HÁ Á Á
p interactions in
2a–2c and 2g effectively reduce the vibration and rotation of mol-
ecules. Secondly, the distorted J-type stacking in 2a, 2c, and 2g
effectively avoids the strong intermolecular face-to-face interac-
tion (H-aggregation) which could lead to the formation of an exci-
mer complex and non-radiative relaxation. Thirdly, the large
distance between vertical dimers in 2a (7.73 Å), 2c (6.98 Å), and
C–HÁ Á Á
p
and
pÁ Á Áp interactions between py+ cations. The distance
and the angle of these interactions are tabulated in Table S3. Sim-
ilar crystal packing structure was observed from compounds 2a,
2c, and 2g indicating the generality of these pyrylium salt mole-
2g (10.41 Å) makes
pÁ Á Áp interaction impossible. Therefore, these
Figure 4. ORTEP drawing with 30% probability thermal ellipsoids (a) and packing arrangements (b and c) of 2a.
Figure 5. ORTEP drawing with 30% probability thermal ellipsoids (a) and packing arrangements (b and c) of 2b.

2970
L. Yang et al. / Tetrahedron Letters 54 (2013) 2967–2971
Figure 6. ORTEP drawing with 30% probability thermal ellipsoids (a) and packing arrangements (b and c) of 2c.
Figure 7. ORTEP drawing with 30% probability thermal ellipsoids (a) and packing arrangements (b and c) of 2g.
three points may be used to explain the much higher UFL values of
these compounds in the solid state compared to those in solution.
To further understand the correlation between structures
and optical properties, quantum chemistry computation was
conducted. The geometries of the molecules have been optimized
using DFT calculations with the GAUSSIAN 09 program.11 The torsion
angles collected in DFT calculations and single crystals of 2a–2c
and 2g are tabulated in Table S4, which can be used to explain
the blue-shift of emission in the solid state. As shown in Table
S4, there are almost no changes of torsion angles between pyryli-
um ring and aryl groups on 4- and 5-positions. However, the
torsion angles between aryl groups on 2-position and pyrylium
ring in crystal 2a–2c and 2g are 12, 27, 10, and 9°, respectively,
which are much larger than those in single molecular state
(1–2°). Due to the more twisted configuration in the solid state,
the maximum emission wavelength occurs with blue-shift com-
pared to that in solution. For 2g, the blue-shift is 99 nm because
the terminal naphthyl in 2-position is twisted out of the pyrylium
ring by 67.9°, which greatly decreases the degree of
Electrochemical studies (Fig. S22) were carried out to ascertain the
redox behavior of 2a–2g. The
Eogpt and energy values of HOMO and
LUMO of 2a–2g are shown in Table 1. The HOMO energy values of
2a–2g are from À2.10 to À1.79 eV and the
Eogpt values determined
from the absorption edge are from 2.16 to 2.50 eV which are sim-
ilar to the
Egcal (1.17–2.47 eV). Compound 2g has the lowest Eogpt
and -conjugation of the
Ecgal which is attributed to the better
p-conjugation.
D
D
D
D
D
p
4-(naphthyl)phenyl substituent. Figure 8 shows the optimized
geometries and the orbital distributions of HOMO and LUMO
energy levels of these compounds. The electron clouds of the
HOMOs and LUMOs are dominated by the different parts of
Figure 8. Calculated molecular orbital amplitude plots of HOMO and LUMO levels.

L. Yang et al. / Tetrahedron Letters 54 (2013) 2967–2971
2971
2. (a) Joshi, H. S.; Jamshidi, R.; Tor, Y. Angew. Chem., Int. Ed. 1999, 38, 2721–2725;
(b) Bolduc, A.; Dong, Y. M.; Guerinz, A.; Skene, W. G. Phys. Chem. Chem. Phys.
2012, 14, 6946–6956; (c) Lee, H. S.; Kim, H. G.; Kang, J. G. Photochem. Photobiol.
Sci. 2011, 10, 1338–1345; (d) Adachi, C.; Nagai, K.; Tamoto, N. Appl. Phys. Lett.
1995, 20, 2679–2681; (e) Renak, M. L.; Bartholomew, G. P.; Wang, S. J.; Ricatto,
P. J.; Lachicotte, R. J.; Bazan, G. C. J. Am. Chem. Soc. 1999, 121, 7787–7799.
3. (a) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem. 1998, 110, 416–443;
(b) Yuji, M.; Norimitsu, T.; Mikiji, M.; Yasuchika, H. Chem. Commun. 2005,
1839–1841; (c) Bhaumik, C.; Maity, D.; Das, S.; Baitalik, S. RSC Adv. 2012, 2,
2581–2594; (d) Fei, Z.; Kocher, N.; Mohrschladt, C. J.; Ihmels, H.; Stalke, D.
Angew. Chem., Int. Ed. 2003, 42, 783–787; (e) Yoshida, K.; Ooyama, Y.; Miyazaki,
H.; Watanabe, S. J. Chem. Soc., Perkin Trans. 2 2001, 708–714.
4. (a) Laine, P.; Bedioui, F.; Ochsenbein, P.; Marvaud, V. J. Am. Chem. Soc. 2002, 124,
1364–1377; (b) Koya, P. R.; Tetsuro, K.; Ryota, S.; Yuki, Y.; Shoko, K.; Masayuki,
N.; Hiroki, O.; Hiroshi, N. Chem. Commun. 2011, 2330–2332; (c) Ba, F.; Poul, P.
L.; Guen, F. R.; Cabon, N.; Caro, B. Tetrahedron Lett. 2010, 51, 605–608; (d)
Miranda, M. A.; Marın, M. L.; Amatb, A. M.; Arques, A.; Segui, S. Appl. Catal. B
Environ. 2002, 35, 167–174; (e) Branchi, B.; Bietti, M.; Ercolani, G.; Izquierdo, M.
A.; Miranda, M. A.; Stella, L. J. Org. Chem. 2004, 69, 8874–8885; (f) Miranda, M.
A.; Garcia, H. Chem. Rev. 1994, 1063; (g) Gavrilyuk, S.; Polyutov, S.; Jha, P. C.;
Rinkevicius, Z.; Agren, H.; Gelmukhanov, F. J. Phys. Chem. A 2007, 111, 11961–
11975; (h) Pigliucci, A.; Nikolov, P.; Rehaman, A.; Gagliardi, L.; Cramer, C. J.;
Vauthey, E. J. Phys. Chem. A 2006, 110, 9988–9994.
molecules except 2d, showing the intramolecular charge transfer
of these compounds.
In summary, a new class of
a-free pyrylium perchlorates was
synthesized by one-step oxidation of cyclopentadiene derivatives
in the presence of perchloric acid. A tunable fluorescence in solu-
tion or in the solid state can be achieved by changing the aryl
groups on 2- or 4-position of pyrylium ring. These compounds
exhibited maximum emission at 535–680 nm in solution (CH2Cl2)
and at 469–581 nm in the solid state. Single crystal X-ray diffrac-
tion analysis shows that multiple C–HÁ Á Á
p interactions in 2a–2c
and 2g lead to the much higher UFL values in solid states. Electro-
chemical studies showed the HOMO energy values of 2a–2g were
from À2.10 to À1.79 eV and the
D
Eogpt values were from 2.16 to
2.50 eV. This new one-pot synthesis route of pyrylium salts may
have great value for synthesis chemistry and materials chemistry.
These compounds may be promising candidates as organic lumi-
nescent materials.
5. (a) Tripathi, S.; Simalty, M.; Kossanyi, J. Tetrahedron Lett. 1985, 26, 1995; (b)
Lampre, I.; Markovitsi, D.; Fiorini, C.; Charra, F.; Veber, M. J. Phys. Chem. 1996,
100, 10701; (c) Vollmer, F.; Rettig, W.; Birckner, E.; Haucke, G.; Czemey, P. J.
Inform. Rec. Mater. 1994, 21, 497.
6. (a) Balaban, A. T.; Dinculescu, A.; Dorofeenko, G. N.; Fischer, G. W.; Koblik, A. V.;
Mezheritskii, V. V.; Schroth, W. Pyrylium Salts: Synthesis, Reactions and Physical
Properties; Academic Press: New York, 1982; (b) Kharchenko, V. G.;
Pchelintseve, N. V.; Markova, L. I.; Fedotova, O. V. Chem. Heterocycl. Compd.
2000, 36, 1007–1025.
7. (a) Ning, G. L.; Li, X. C.; Megumu, M.; Gong, W. T.; Masahiko, M.; Tadao, K. J. Org.
Chem. 2004, 69, 1432–1434; (b) Gong, W. T.; Ning, G. L.; Li, X. C.; Wang, L.; Lin,
Y. J. Org. Chem. 2005, 70, 5768–5770.
8. (a) Xu, T.; Gong, W. T.; Ye, J. W.; Lin, Y.; Ning, G. L. Organometallics 2010, 29,
6744–6748; (b) Zhang, X. D.; Ye, J. W.; Wang, S. N.; Gong, W. T.; Lin, Y.; Ning, G.
L. Org. Lett. 2011, 13, 3608–3611.
Acknowledgments
This work was supported by the National Natural Science Foun-
dation of China (51003009 and 20772014), the Specialized Re-
search Fund for the Doctoral Program of Higher Education
(200801411121), and the Fundamental Research Funds for the
Central Universities of China (DUT11LK16).
Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.03.
113.
9. (a) Hirsch, S. S.; Bailey, W. J. J. Org. Chem. 1978, 43, 4090–4094; (b) Yang, L. J.;
Ye, J. W.; Xu, L. F.; Yang, X. Y.; Gong, W. T.; Lin, Y.; Ning, G. L. RSC Adv. 2012, 2,
11529–11535.
10. Synthetic procedure for compounds 2a–2g: a mixture of triarylcyclopentadiene
derivative (0.62 mmol) and perchloric acid (70%, 1 mL) in 15 mL toluene/
acetonitrile (3/1) was stirred at room temperature. After 1–3 days, 30 mL
diethyl ether was poured in this mixture and the precipitate was generated.
The flitered precipitate was washed with diethyl ether (3 Â 15 mL) and dried to
give crude product. The resulting solid was recrystallized from acetic acid for
purification.
References and notes
1. (a) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913–915; (b) Tang, C.
W.; VanSlyke, S. A.; Chen, C. H. J. Appl. Phys. 1989, 65, 3611–3615; (c) Shang, Y.;
Wen, Y.; Li, S.; Du, S.; He, X.; Cai, L.; Li, Y.; Yang, L.; Gao, H.; Song, Y. J. Am. Chem.
Soc. 2007, 129, 11674–11675; (d) Grimsdale, A. C.; Chan, K. L.; Martin, R. E.;
Jokisz, P. G.; Holmes, A. B. Chem. Rev. 2009, 109, 897–1091.
11. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B., et al GAUSSIAN 03. Revision D.01;
Gaussian: Wallingford, CT, 2004.