Synthesis, optical spectroscopy and laser potential of pyrylium tosylates

Article abstract of DOI:10.1016/j.molstruc.2018.06.025

Safe and inexpensive methods for synthesis of a series of four substituted 2,4,6-triphenylyrylium tosylate salts with different substituents are reported. The synthesis methods use p-toluenesulfonic acid monohydrate instead of conventional acid catalysts

Full text of DOI:10.1016/j.molstruc.2018.06.025

Journal of Molecular Structure 1171 (2018) 458e465
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Synthesis, optical spectroscopy and laser potential of pyrylium
tosylates
Jung Jae Koh a, Christina Inbok Lee a, Mihaela Alexa Ciulei a, Haesook Han a,
,
, c
Pradip Kumar Bhowmik a *, Vladimir Kartazaev b, Swapan Kumar Gayen b
a Department of Chemistry and Biochemistry, University of Nevada Las Vegas, 4505 S. Maryland Parkway, Box 45003, Las Vegas, NV, 89154, USA
b Department of Physics, Center of Discovery and Innovation, The City College of New York, 160 Convent Avenue, New York, NY, 10031, USA
c Physics Program, The Graduate Center, City University of New York (CUNY), 365 5th Ave., New York, NY 10016, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Safe and inexpensive methods for synthesis of a series of four substituted 2,4,6-triphenylyrylium tosylate
salts with different substituents are reported. The synthesis methods use p-toluenesulfonic acid mon-
ohydrate instead of conventional acid catalysts including perchloric acid or boron trifluoride diethyl
etherate that pose explosion danger and difficult storage problems, respectively. The chemical structures
of these salts were established using FTIR, 1H and 13C NMR spectroscopic techniques and elemental
analysis. Thermogravimetric analysis (TGA) showed that these salts have good thermal stability, and
differential scanning calorimetry (DSC) analysis showed that they have lower melting transitions than
the corresponding tetrafluoroborate and perchlorate salts. Solutions of the salts in organic solvents (such
as acetonitrile and methanol) absorbed strongly in the blue-near-ultraviolet spectral range and emitted
efficiently in the blue-green spectral range depending on the substituents in 2- and 6-positions of phenyl
groups. The fluorescence quantum yields of the salts ranged from 0.33 to 0.56. Under intense ultrashort
laser pumping, an acetonitrile solution of the salt, 2,4,6-triphenylpyrylium tosylate, the salt with the
highest quantum yield, demonstrated stimulated emission and laser action with only nominal feedback.
© 2018 Elsevier B.V. All rights reserved.
Received 10 April 2018
Received in revised form
5 June 2018
Accepted 6 June 2018
Available online 14 June 2018
Keywords:
Pyrylium tosylates
Absorption spectra
Fluorescence
Quantum
Yields
Laser
Differential scanning calorimetry
1. Introduction
fluorescence and electron acceptor properties of these salts have
been explored to design sensors for cyanide ion [9], amines, amino
The interest in developing safe and inexpensive synthetic
methods for pyrylium salts derives from their many potential ap-
plications in photochemistry, photobiology and beyond. First, these
heterocyclic compounds are used as versatile precursors in a vari-
ety of synthetic applications including substituted furan, pyridine,
pyridinium salts, and betaine dyes [1e6]. Second, their electron
accepting properties make them suitable as sensitizers for photo-
induced electron transfer processes in chemical transformations
[7]. Third, their strong light emission properties are indicative of
potential applications as active materials for dye lasers operating in
the blue-green spectral region [7]. Furthermore, their two- and
three-photon absorption properties may be useful for diverse ap-
plications, such as, three-dimensional optical data storage, photo-
dynamic therapy, upconversion lasers, optical power limiting, as
well as micro- and nano-fabrications [8]. Recently, the absorption,
acids, nitric oxide [10], and proteins [11].
In general, perchloric acid and boron trifluoride diethyl etherate
are used as common acid catalysts to synthesize pyrylium salts of
diverse chemical structures with ClOꢀ4 or BF4ꢀ as counterions [12].
However, perchloric acid poses explosion danger, and boron tri-
fluoride diethyl etherate requires difficult handling, problematic
storage, and higher cost. To develop safer, inexpensive and efficient
synthesis routes for this important class of compounds, we
explored the use of p-toluenesulfonic acid (tosic acid) monohydrate
which is a solid, relatively safe to handle, and an inexpensive re-
agent. Although tosic acid has previously been used in many
diverse organic transformations [13e18], it has not yet been
explored for the synthesis of pyrylium salts of diverse architectures
with tosylate counterions. This work explored the use of tosic acid
for synthesis of pyrylium salts employing several synthetic path-
ways under different reaction conditions including solvent-free
green synthetic approach. Four 2,4,6-triarylpyrylium tosylates
were synthesized and their chemical, thermal, optical
* Corresponding author.
E-mail address: pradip.bhowmik@unlv.edu (P.K. Bhowmik).
https://doi.org/10.1016/j.molstruc.2018.06.025
0022-2860/© 2018 Elsevier B.V. All rights reserved.

J.J. Koh et al. / Journal of Molecular Structure 1171 (2018) 458e465
459
spectroscopic, and quantum electronic properties were studied.
The synthesized salts include: 2,4,6-triphenylpyrylium tosylate
(salt 1), 2,6-bis(4-methylphenyl)-4-phenylpyrylium tosylate (salt
2), 2,6-bis(4-bromophenyl)-4-phenylpyrylium tosylate (salt 3), and
2,6-bis(4-methoxyphenyl)-4-phenylpyrylium tosylate (salt 4). It is
also of significant interest to examine the physical properties of
these salts such as solubility in various organic solvents when
compared with those with ClOꢀ4 or BFꢀ4 as counterions. The latter
salts have limited solubility in organic solvents, but provide relative
ease of their isolation because of high crystallinity of inorganic ions.
The thermal properties and stability of the salts were deter-
mined using differential scanning calorimetry (DSC) and ther-
mogravimetric analysis (TGA), respectively. Their optical
spectroscopic properties (both in solution and in solid state) were
studied using UVeVisible absorption spectroscopy, fluorescence
spectroscopy, and time-resolved fluorescence spectroscopy tech-
niques. Introduction of electron donating substituents in para-
positions of 2- and 6-phenyl rings of pyrylium salts showed that
the substituents influence the optical spectroscopic properties of
the salts in solution as well as thermal properties.
scope C4334) coupled with a spectrometer (Imaging Spectrograph
G50is). The laser system generated 800-nm light pulses of 130 fs
full-width-at-half-maximum (FWHM) duration, at a repetition rate
of 1 kHz. The second harmonic of this laser output at 400-nm was
used to excite into the first absorption band of the salt solution in
acetonitrile contained in a quartz cuvette. The fluorescence from
the sample was collected by an optical fiber connected with the
streak camera spectrometer. Since typical dyes have fluorescence
lifetimes of a few nanoseconds, the streak camera was used in the
20-ns time frame with a corresponding temporal resolution of
approximately 200 ps. It can provide a temporal resolution of ~20
ps when used in the 1-ns time frame.
2.2. Synthesis of 2,4,6-triphenylpyrylium tosylate (salt 1)
The 2,4,6-triphenylpyrylium tosylate (salt 1) was prepared by
one-step method from both trans-chalcone [20], and 1,5-diketone
[21] by using tosic acid in accordance of the identical procedures
[22e24] that were used for the preparation of the corresponding
ClOꢀ4 and BFꢀ4 salts with modifications (Scheme 1). It was also
prepared by one-step method from benzaldehyde and acetophe-
none in four different methods as shown in Scheme 2.
2. Materials and methods
2.2.1. Trans-chalcone method (Scheme 1)
2.1. General information
This procedure is an improved modification of that described by
Dimroth [24]. The p-toluenesulfonic acid monohydrate (9.49 g,
49.9 mmol) was slowly added into a solution of trans-chalcone
(10.4 g, 49.9 mmol), acetophenone (3.00 g, 25.0 mmol), and 20 mL
of 1,2- dichloroethane while keeping the temperature at 50 ꢁC. The
mixture was heated to reflux on stirring for 24 h. After the reaction
was completed, the solution was concentrated by a rotary evapo-
rator. The concentrated solution was poured into diethyl ether, and
the crude product was collected by filtration and washed with
diethyl ether. The crude product was purified by dissolving in the
minimum amount of dichloromethane and precipitated in diethyl
ether, and the final product was dried under vacuum oven to yield
3.96 g (8.24 mmol, 33% yield) of dark yellow solid.
All chemicals including spectral grade solvents were purchased
from commercial suppliers and used without further purification.
The 1H and 13C NMR spectra were recorded using a Varian NMR
400 MHz spectrometer equipped with two radiofrequency (RF)
channels at room temperature. The NMR sample solutions were
prepared by dissolving 10 mg of respective compounds in DMSO‑d6
with tetramethylsilane as an internal standard. FTIR spectra were
recorded with Shimadzu spectrometer (IR Affinity-1) using KBr
pellets. Elemental analyses for the salts were obtained from the
Atlantic Microlab Inc., Norcross, GA.
The phase transition temperature of ionic compounds were
studied using TA differential scanning calorimetry (DSC) Q200 se-
ries in nitrogen at heating and cooling rates of 10 ꢁC/min. The
temperature axis of the DSC thermograms was calibrated with
reference standards of high purity of indium and tin. The thermal
stability of each compound was analyzed using TA TGA Q50 in ni-
trogen at a rate of 10 ꢁC/min operated in the temperature range
between 30 and 800 ꢁC.
2.2.2. 1,5-Diketone method (Scheme 1)
1,3,5-Triphenyl-1,5-pentanedione was prepared first by
following a modified procedure of that described by Hirsch and
Bailey [21]. The p-toluenesulfonic acid monohydrate (1.45 g,
7.61 mmol) was slowly added to a mixture of 1,3,5-triphenyl-1,5-
pentanedione (1.00 g, 3.05 mmol) and triphenylmethanol (1.98 g.
7.61 mmol) in 20 mL of acetic anhydride. After the mixture was kept
at 100 ꢁC for 2 h, it was cooled to room temperature and yellow
precipitate was obtained by addition of 60 mL of water into the
flask. The yellow solid was collected and washed with water to
remove any residual acid from the reaction. The identical purifi-
cation procedure was followed as described in trans-chalcone
method (vide supra) to yield 1.30 g (2.71 mmol, 88% yield) of the
pure product [16].
The UVeVis absorption spectra of the salts dissolved in aceto-
nitrile were recorded using Varian Cary 50 Bio UVevisible spec-
trophotometer in quartz cuvettes at room temperature.
Photoluminescence spectra of the salts in solution were recorded
using a Perkin-Elmer LS-55 luminescence spectrometer with a
xenon lamp as a light source. Quantum yields were estimated using
the following relation:
ꢀ
ꢁ ꢀ
ꢁ
GradX
GradST
h2
h2
X
ФX ¼ ФST
(1)
ST
2.2.3. Method 1 (Scheme 2)
where the subscripts ST and
diphenylanthracene) and the unknown, respectively,
X
denote the standard (9,10-
This procedure is a modification of the method that described by
Moghimi et al. [22] for the corresponding perchlorate salt. The
POCl3 (23.9 g, 103 mmol) was added slowly to a solution of benz-
aldehyde (9.23 g, 87.0 mmol) and acetophenone (26.1 g, 218 mmol)
in an ice bath. Then the solution was stirred and kept at 60 ꢁC for
8 h. After cooling down, the solution was concentrated by a rotary
evaporator to remove excess POCl3, and the viscous solution was
dissolved in ethanol. The p-toluenesulfonic acid monohydrate
(20.7 g,109 mmol) was slowly added to the solution over a period of
5 min at room temperature and kept on stirring for 30 min at
identical temperature. The ethanol was removed under vacuum
Ф is the
fluorescence quantum yield, is the refractive index of the solvent,
h
and Grad is the gradient from the plot of integrated fluorescence
intensity vs. absorbance of the minimum of five solutions prepared
by serial dilution [19].
Time-evolution of fluorescence and pump-power dependence
of the fluorescence spectra and fluorescence lifetime of the salt
with the highest fluorescence quantum yield were measured using
a Ti: sapphire laser and regenerative amplifier system (Spectra-
Physics Spitfire) and a Streak Camera system (Hamamatsu streak

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J.J. Koh et al. / Journal of Molecular Structure 1171 (2018) 458e465
Scheme 1. Synthesis of 2,4,6-triphenylpyrylium tosylate.
Scheme 2. Synthesis of pyrylium tosylates 1e4.
resulting a black viscous solution, and yellow precipitate was ob-
tained followed by addition of diethyl ether. The precipitate was
collected and washed to remove excess p-toluenesulfonic acid
monohydrate. The identical purification procedure was followed as
described in trans-chalcone method (vide supra) to yield 22.6 g
(47.0 mmol, 54% yield) of the pure product 1. Similarly, salts 2e4
were prepared by using benzaldehyde and the corresponding
substituted acetophenones with the lower yields than that of 1 as
shown in Scheme 2.
precipitate was collected and washed with water to remove excess
p-toluenesulfonic acid monohydrate. The identical purification
procedure was followed as Route 1 to yield 1.04 g (2.17 mmol, 26%
yield) of the pure product.
2.2.5. Method 3 (Scheme 2)
The identical synthetic procedure was followed as in Method 2
using 5 mL of 1,2-dichloroethane instead of toluene to yield 1.10 g
(2.29 mmol, 28% yield) of the pure product [24].
2.2.4. Method 2 (Scheme 2)
2.2.6. Method 4 (Scheme 2)
This procedure is an improved modification of that described by
Bello and Kotra [23] for the corresponding tetrafluoroborate salt. A
mixture of benzaldehyde (0.883 g, 8.33 mmol), acetophenone
(2.00 g, 16.7 mmol), and p-toluenesulfonic acid monohydrate
(3.17 g, 16.7 mmol) in 5 mL toluene was heated to reflux for 24 h.
The reaction flask was cooled to room temperature and the black
viscous solution was poured into diethyl ether. The yellow
The identical synthetic procedure was followed as Method 2
without a solvent at 100 ꢁC to yield 1.13 g (2.35 mmol, 28% yield) of
the pure product [15]. IR (cmꢀ1): 3066, 2914,1624, 1500,1448,1199,
1122, 1033, 767.1H NMR (DMSO‑d6, 400 MHz, ppm)
d
¼ 9.18 (2H, s),
8.61 (6H, d, J ¼ 7.2 Hz), 7.89 (3H, t, J ¼ 7.6 Hz), 7.80e7.77 (6H, m),
7.48 (2H, d, J ¼ 6.4 Hz), 7.11 (2H, d, J ¼ 8.0 Hz), 2.28 (s, 3H). 13C NMR
(DMSO‑d6, 100 MHz, ppm)
d
¼ 170.5, 165.5, 146.3, 137.9, 135.6, 135.4,

J.J. Koh et al. / Journal of Molecular Structure 1171 (2018) 458e465
461
132.9, 130.5, 130.3, 130.2, 129.5, 129.2, 128.4, 125.9, 115.6, 21.2. Anal.
Calculated (Found) for C30H24O4S (%): C, 74.98 (74.87); H, 5.03
(5.16); S, 6.67 (6.54).
with acetophenone in 1:2 mol ratio in a one-step procedure using
tosic acid in four different methods. Method 1 using POCl3 gener-
ated in situ 2,4,6-triphenylpyrylium salt with chloride as a coun-
terion, and subsequently it was isolated as tosylate salt with the
addition of tosic acid. This method gave the highest yield (54%) of 1,
but comparable yields of 2e4 when compared with that perchlo-
rate of a pyrylium salt (29%) reported by Moghimi et al. [22]. These
results suggest that the yields of pyrylium salts are highly depen-
dent on the substituents present in 2-, 4- and 6-phenyl groups. Salts
2e4 were also prepared from the reaction of benzaldehyde and
corresponding substituted acetophenone in 1:2 mol ratio in a one-
step procedure with tosic acid in different methods and conditions.
We also explored the synthesis of pyrylium salts in the melts
without solvents [16]. The reaction conditions and yields are
compiled in Scheme 2. Trans-chalcone and 1,5-diketone methods
gave higher yields of pyrylium salts than the direct method, that is,
from the reactions of benzaldehyde or substituted benzaldehydes
with acetophenone or substituted acetophenones. The FTIR, 1H, and
13C NMR spectra of salts 1e4 along with their elemental analysis
data were consistent with the chemical structures of desired salts.
2.3. Synthesis of 2,6-bis(4-methylphenyl)-4-phenylpyrylium
tosylate (salt 2)
The identical synthetic procedures from Methods 2e4 were
followed by using 40-methylacetophenone instead of acetophenone
for the preparation of salt 2 [25]. Its yields from three different
methods were given in Scheme 2. IR (cmꢀ1): 3061, 2914,1620,1500,
1193, 1120, 1033, 773.1H NMR (DMSO‑d6, 400 MHz, ppm)
d
¼ 9.05
(2H, s), 8.57 (2H, d, J ¼ 7.2 Hz), 8.49 (4H, d, J ¼ 8.4 Hz), 7.87 (1H, t,
J ¼ 7.6 Hz), 7.78 (2H, t, J ¼ 8.0 Hz), 7.60 (4H, d, J ¼ 8.4 Hz), 7.48 (2H, d,
J ¼ 8.0 Hz), 7.11 (2H, d, J ¼ 8.0 Hz), 2.50 (6H, overlap with the solvent
residual peak), 2.28 (3H, s). 13C NMR (DMSO‑d6, 100 MHz, ppm)
d
¼ 170.2, 164.8, 146.6, 146.3, 137.9, 135.3, 133.0, 130.9, 130.3, 130.1,
129.1, 128.4, 126.9, 125.9, 114.7, 21.9, 21.2. Anal. Calculated (Found)
for C32H28O4S (%): C, 75.56 (75.28); H, 5.55 (5.62); S, 6.30 (6.09).
2.4. Synthesis of 2,6-bis(4-bromophenyl)-4-phenylpyrylium
tosylate (salt 3)
3.2. Thermal properties
The identical synthesis procedures from Methods 2e4 were
followed by using 40-bromoacetophenone instead of acetophenone
for the preparation of salt 3 [25]. Its yields from three different
methods were also given in Scheme 2. IR (cmꢀ1): 3061, 2914, 1622,
1489, 1215, 1120, 1033, 767, 489.1H NMR (DMSO‑d6, 400 MHz, ppm)
The thermal properties of salts 1e4 were studied using ther-
mogravimetric analysis (TGA) and differential scanning calorimetry
(DSC) measurements. The results are presented Table 1 and
compared with the corresponding tetrafluoroborates [25,27] and
perchlorates [26,28]. The melting transitions of the salts were lower
than those of pyrylium salts both containing perchlorates, and
tetrafluoroborates. This is expected, since bulky organic anion such
as tosylate lowers the melting point of organic salts than compared
to that done by inorganic counterions. Salt 4 did not show any
crystallization exotherm either in the first cooling cycle or in the
second, it had a tendency to form an amorphous phase. Addition-
ally, it showed a Tg at 72 ꢁC (Table 1 and Fig. S1).
d
¼ 9.18 (2H, s), 8.61 (2H, d, J ¼ 7.6 Hz), 8.52 (4H, d, J ¼ 8.8 Hz), 7.98
(4H, d, J ¼ 8.4 Hz), 7.88 (1H, t, J ¼ 7.2 Hz), 7.77 (2H, t, J ¼ 8.0 Hz), 7.49
(2H, d, J ¼ 8.0 Hz), 7.11 (2H, d, J ¼ 8.0 Hz), 2.28 (3H, s). 13C NMR
(DMSO‑d6, 100 MHz, ppm)
d
¼ 169.6, 165.6, 146.0, 138.0, 135.8,
133.3, 132.7, 131.0, 130.6, 130.2, 129.9, 128.6, 128.4, 125.9, 115.9, 21.2.
Anal. Calculated (Found) for C30H22Br2O4S (%): C, 56.44 (56.42); H,
3.47 (3.51); S, 5.02 (4.88).
2.5. Synthesis of 2,6-bis(4-methoxyphenyl)-4-phenylpyrylium
tosylate (salt 4)
The identical synthetic procedure from Method 3 was followed
by using 40-methoxyacetophenone instead of acetophenone for the
synthesis of salt 4 [25,26]. At the end of the reaction, the solution
was concentrated under vacuum. The minimum amount of aceto-
nitrile was added to the flask to dissolve the dark viscous solution,
and dark orange solid was precipitated out by addition of ice. The
crude product was collected and recrystallized from chloroform to
yield 537 mg (11%, 0.99 mmol). IR (cmꢀ1): 3061, 2937, 1622, 1602,
1491, 1263, 1120, 1031, 773.1H NMR (DMSO‑d6, 400 MHz, ppm)
3.3. Solubility in organic solvents
In general, highly rigid materials that consist of aromatic rings
have limited solubility [29,30], but all of the pyrylium salts herein
except the salt 3 had relatively good solubility in common organic
solvents. They were dissolved both in low polarity solvents such as
chloroform, dichloromethane, and THF as well as high polarity
solvents such as methanol, acetonitrile, and DMSO.
d
¼ 8.88 (2H, s), 8.55 (6H, m), 7.82 (1H, t, J ¼ 7.2 Hz), 7.76 (2H, t,
Table 1
J ¼ 7.2 Hz), 7.49 (2H, d, J ¼ 8.0 Hz), 7.31 (4H, d, J ¼ 9.2 Hz), 7.11 (2H, d,
Thermal properties of 1e4 as determined by DSC and TGA.
J ¼ 8.0 Hz), 3.97 (6H, s), 2.28 (3H, s). 13C NMR (DMSO‑d6, 100 MHz,
a
b
c
Salt
Tg (ꢁC) Tm (ꢁC)
Td (ꢁC)
ppm)
d
¼ 169.3, 165.2, 163.6, 146.2, 137.9, 134.9, 133.1, 131.4, 130.1,
1, 1BF4, 1ClO4
2, 2BF4
3, 3BF4
222 (215)d, 255 [25], 273 [28]
316
130.0, 128.4, 125.9, 121.8, 115.8, 113.1, 56.5, 21.2. Anal. Calculated
(Found) for C32H28O6S (%): C, 71.09 (70.52); H, 5.22 (5.25); S, 5.93
(5.79).
219e (decomp.), 311 [25] and 268e269 [27] 281
268e (decomp.), 356 [25]
293
291
4, 4ClO4
(72)
218f (209), 274e275 [26]
a
Tg is the glass transition temperature.
Tm is the melting transition as determined by DSC obtained at heating and
3. Results and discussion
b
cooling rates of 10 ꢁC/min in nitrogen.
c
3.1. Synthesis
Td is the decomposition temperature at which 5% weight loss of salt occurred
obtained at a heating rate of 10 ꢁC/min in nitrogen.
d
Value in the parentheses obtained from the second heating cycle of DSC
Salt 1 was synthesized from both trans-chalcone [20] and 1,5-
diketone [21] in a one-step procedure using tosic acid as shown
in Scheme 1 in relatively high yields of 33 and 88%, respectively. In
Scheme 2, the salt 1 was directly synthesized from benzaldehyde
thermogram.
e
Obtained from the first heating cycle of DSC thermogram.
Overlapped endotherms (crystal-crystal transitions) prior to melting transition
f
in the first heating cycle only.

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J.J. Koh et al. / Journal of Molecular Structure 1171 (2018) 458e465
3.4. Optical spectroscopic properties
of 19000e40000 Mꢀ1cmꢀ1 in acetonitrile as calculated from the
respective Beer-Lambert law plots (Table 2, Fig. S2-S5), which are in
good agreement with the results of corresponding pyrylium per-
chlorates [32]. It is to be noted here that the recorded excitation
spectra of these salts were similar to their UVevisible absorption
spectra that provided the compelling evidence for the photo-
luminescence properties of the salts not the impurities in aceto-
nitrile. The fluorescence spectra of salts 1e4 in CH3CN are shown in
Fig. 2, which also indicate the respective excitation wavelengths.
Salts 1e3 emitted in the blue-green spectral region with emission
peaks in the 450e478 nm range when excited at various wave-
lengths of light, and these values indicate that the variation of the
substituents in 2- and 6-phenyl rings had an effect on the light
emission properties. However, salt 4 showed a significant bath-
ochromic shift and emitted green light with lmax at 536 nm when
excited at 375 nm (Fig. 2). Fig. 3 also displays the fluorescence of
1e4 in acetonitrile when excited at hand-held UV lamp (365 nm).
The excited state energies of these salts in acetonitrile as calculated
from the intersections of absorption spectra and emission spectra
(Figs. S6-S9) were found to be 2.89 eV, 2.66 eV, 2.78 eV, and 2.46 eV,
respectively. The excited state energies for the two corresponding
perchlorates in acetonitrile are reported to be 2.83 eV and 2.70 eV
[33]. Their wavelengths of light emission were hypsochromically
shifted in acetonitrile when compared with those of the corre-
sponding perchlorate salts; and they showed Stokes shift of 2530,
2390, 2668 and 2520 cmꢀ1, respectively. The peak emission wave-
lengths of the corresponding perchlorates in acetonitrile are
466 nm, 490 nm, and 546 nm and their Stokes shifts are 3232, 2956,
and 3052 cmꢀ1, respectively [32]. The emission spectral range of
each salt solution had a full-width-at-half-maximum (FWHM) in
the 2448-2829 cmꢀ1 range which resembled that of Coumarin laser
dyes that operate in the blue-green spectral range. The single
emission peak and the magnitude of FWHM are indicative of a
single chromophore being responsible for the fluorescence. The
Among the various solvents, acetonitrile (CH3CN) was chosen
for investigating the optical spectroscopic properties since salt 3
showed very limited solubility in the other organic solvents.
The room-temperature optical absorption spectra of all four
pyrylium salts dissolved in CH3CN with the concentration of
4 ꢂ 10ꢀ6 M are displayed in Fig. 1. Solutions of all four salts absor-
bed strongly in the blue-near ultraviolet spectral range, except for
salt 4 which absorbed in the blue-green in addition to blue-near-UV
range. Each salt solution exhibits three principal absorption peaks
in the above-mentioned spectral range. The peak absorption
wavelengths and corresponding molar absorptivity, along with
other optical spectroscopic properties such as photoluminescence
spectra and quantum yields in acetonitrile (CH3CN) are summa-
rized in Table 2. According to the previously reported studies of
2,4,6-triphenylpyrylium perchlorate, two absorption peaks at
417 nm and 369 nm are observed in CH2Cl2 [7,32]. The absorption
spectrum of salt 1 in CH3CN (Fig. 1) is similar in appearance to that
of the 2,4,6-triphenylpyrylium perchlorate in CH2Cl2, but its peak
absorption (lmax) exhibited hypsochromic shifts caused by the
increased solvent polarity. Similarly, salts 2e4 also showed hyp-
sochromic shifts in acetonitrile when compared with those in
CH2Cl2 [32]. In salt 2 small bathochromic shifts of 12 nm and 6 nm
in the shorter wavelengths and large bathochromic shift of 25 nm
in the longer wavelength were observed when compared with
those in salt 1. Similarly, salt 3 showed small bathochromic shifts of
12 nm and 11 nm in the shorter wavelengths and large bath-
ochromic shift of 16 nm in the longer wavelength. Salt 4 contains
two methoxy groups (strong donor strength) bearing in 2- and 6-
phenyl rings and showed significant bathochromic shifts of
44 nm and 21 nm at the shorter wavelength and 66 nm at the
longer wavelength since the 2,6-diphenylpyrylium substructure is
responsible for the absorption at the longer wavelength. These
results confirmed that the strong electron-donating substituent
such as methoxy group caused significant changes in the longer
wavelength absorption in the 2,4,6-triphenylpyrylium cation,
because the primary chromophore is only available in the cation.
These salts have relatively large absorption coefficients in the range
quantum yields (ФF) of salts 1e4 were determined against diphe-
nylanthracene (ФF) in cyclohexane and were found to be in the
range 0.27e0.56. Salt 3 had the lowest quantum yield, as expected,
due to heavy atom effect [34]. The corresponding perchlorate salts
are reported [32] to have quantum yields of 0.60e0.82. The optical
spectroscopic properties of the pyrylium salts are greatly affected
by chemical structure modifications with electron donating sub-
stituents in its primary chromophore, which is the cationic moiety.
These results suggested that the optical properties of these pyry-
lium salts can be tuned with the variation of organic counterions
such as tosylate.
3.5. Time-resolved and intensity-dependent emission
Steady-state fluorescence measurements revealed that salt 1
has the highest quantum yield of 0.56 among the four, and hence
was further explored using time-resolved and pump intensity-
dependent measurements to assess its potential as an active ma-
terial for a dye laser [35]. The time-evolution of fluorescence of an
acetonitrile solution of the salt (concentration 2 ꢂ 10ꢀ3 M) is shown
in Fig. 4(b) for different pump intensity levels. The corresponding
emission spectra are presented in Fig. 4(a). The solution was con-
tained in a 10 mm ꢂ 10 mm x 30 mm rectangular quartz cuvette
and was excited using the 400 nm, 130 fs, 1 kHz repetition rate
pulses from the Ti: sapphire laser system mentioned in Section 2.
The average beam power was maintained at 10 mW, but the spot
size and hence the intensity were changed using different focusing
conditions. The salient features of the fluorescence decay curves
and the corresponding emission spectra are as follows. When the
pump beam was focused to a 2 mm diameter spot (average pump
intensity of 0.032 W/cm2) and the emission was collected from the
Fig. 1. UVevisible spectra of the solutions of salts 1e4 in CH3CN (concentration of
4 ꢂ 10ꢀ6 M).

J.J. Koh et al. / Journal of Molecular Structure 1171 (2018) 458e465
463
Table 2
Optical spectroscopic properties of salts 1e4.
Entry
salt 1
salt 2
salt 3
salt 4
Peak absorption Wavelength, lmax (nm)
275, 354, 404
å275 ¼ 19000
å354 ¼ 34000
å404 ¼ 26000
450
2.89 (23310)
2530
2677
287, 360, 429
å287 ¼ 24000
å360 ¼ 36000
å429 ¼ 29000
478
2.73 (22026)
2390
2814
287, 365, 420
å287 ¼ 21000
å365 ¼ 40000
å420 ¼ 32000
473
2.78 (22422)
2668
2829
319, 375, 470
å319 ¼ 28000
å375 ¼ 37000
å470 ¼ 34000
536
2.46 (19881)
2520
2448
Molar absorptivity (Mꢀ1cmꢀ1
)
Peak Emission Wavelength (nm)a
b
Excited State Energy (eV)b (cmꢀ1
)
c
Stokes shift (cmꢀ1
)
FWHM (cmꢀ1
)
d
^
OF
0.56
0.47
0.27
0.33
a
All measurements are for salt solutions in CH3CN.
b
c
Intersection of the absorption and emission spectra (Fig. S6-S9) is used as estimate of excited state energy in eV (cmꢀ1).
Difference between emission maxima and the largest wavelength absorption maxima.
d
Quantum yield was calculated against diphenyl anthracene as standard (ФF ¼ 0.9) [19,31].
linewidth were obtained (not shown in the figure) for much lower
concentrations (10ꢀ5 M and 10ꢀ6 M) solutions, as well. As the pump
intensity was increased (spot size 0.2 mm, average intensity 3.2 W/
cm2) the lifetime shortened 3.5 0.1 ns and the linewidth nar-
rowed 71 1 nm, for similar collection geometry (Curve 2 in both
figures). The longer fluorescence lifetimes are indicative of spon-
taneous emission, while shorter lifetimes are consistent with
amplified spontaneous emission (ASE) or stimulated emission.
When more intense transverse pumping using a cylindrical lens
was employed generating a narrow excited stripe (0.02 mm width
and 5 mm length) across the cuvette, stimulated emission was
observed from the two sides of the cuvette at a direction perpen-
dicular to the pump beam. This emission had substantial direc-
tionality, a much reduced lifetime of 615 ps and a significantly
narrow linewidth of 21.5 0.5 nm, as seen in curve 3 of Fig. 4(b) and
Fig. 4(a), respectively. The directionality, lifetime shortening and
linewidth reduction are indications of stimulated emission [35,36].
What was even more interesting, when the orientation of the
cuvette was adjusted so that a fraction of the emission reflected
from its side walls could pass through the excited region, the output
from the side resembled a symmetric pulse of FWHM width
247 5 ps [Fig. 4(b) Curve 4] and linewidth 7.5 0.5 nm [Fig. 4(b)
Curve 4] indicative of laser action. The onset of stimulated emission
and laser action were observed with the higher-concentration so-
lution (concentration 2 ꢂ 10ꢀ3 M), while with the lower-
concentration solution (concentrations 1 ꢂ 10ꢀ4 M or lower) only
the spontaneous emission was observed. A detailed systematic
Fig. 2. Emission spectra of salts 1e4 recorded in CH3CN at various excitation
wavelengths.
front surface at an angle with respect to the incident direction, the
fluorescence decayed as a single exponential with a fluorescence
lifetime of 4.6 0.1 ns (Curve 1 in Fig. 4(b)) and the FWHM line-
width was 77 1 nm (Curve 1 in Fig. 4(a)). Similar lifetime and
Fig. 3. Salts 1e4 in CH3CN under hand-held UV lamp (365 nm) exhibiting fluorescence.

464
J.J. Koh et al. / Journal of Molecular Structure 1171 (2018) 458e465
electronic characterization of the salt as a laser dye will require
determining optimal concentration, design and incorporation of an
adequate optical cavity with a wavelength tuning element, and
careful measurements of lasing threshold, laser linewidth, output
power, tuning range, slope efficiency and other laser parameters, all
of which are planned to be pursued in a future study.
4. Conclusions
This study described various methods for the synthesis of
several 2,4,6-triphenylpyrylium tosylate salts via dehydrocycliza-
tion reaction using p-toluenesulfonic acid, which provides signifi-
cantly safe reaction condition than the conventional acids as
condensing agents. The chemical structures were fully character-
ized from their FTIR, 1H, 13C NMR spectra and elemental analysis.
They have good solubility in various common organic solvents, and
their thermal stabilities were at the temperatures ca. 300 ꢁC. Given
the organic salts, their thermal stabilities are considered to be
excellent. Salt 4 showed an amorphous phase as evident from
thermal analyses with DSC measurements. Due to the presence of
chromophores, they exhibited photoluminescence properties in
solution. Salts 1e3 emitted intense light in the blue range
(450e478 nm) and 4 emitted intense light in the green range
(536 nm). In an exploratory experiment, salt 1 with the highest
quantum yield demonstrated stimulated emission and free-
running laser action with only nominal feedback from cuvette
walls, which is indicative of the potential of the salt as the active
material for an efficient dye laser operating in the blue-green
spectral range. The optical spectroscopic properties of pyrylium
salts were dependent on the chemical structures of not only cations
but also anions. Their properties can be further tuned by structural
modification to suit their multitude applications. Additionally, they
can be useful for the synthesis of various heterocyclic molecules. To
sum up, the synthesis of these pyrylium salts using safer and
inexpensive routes open up possibility of their wide use as light
emitters, sensitizers, lasers, and other nonlinear optical and bio-
photonic applications.
Fig. 4. Pump intensity dependence of (a) fluorescence spectra, and (b) fluorescence
lifetime of a 2.0 ꢂ 10ꢀ3 M solution of 2, 4, 6-triphenylpyrylium tosylate (salt 1) in
acetonitrile at room temperature. Curve 1 and Curve 2 in both Fig. 4(a) and Fig. 4(b)
were recorded for longitudinal pumping into the cuvette containing the dye solution.
The spot size and average intensity of the pump beam were 2.0 mm and 0.032 W/cm2
(for Curve 1); 0.2 mm and 3.20 W/cm2 (for Curve 2). Curve 3 and Curve 4 in both
Fig. 4(a) and (b) were recorded for transverse pumping of the dye solution in the
cuvette using a cylindrical lens. The beam was focused to a 0.02 mm ꢂ 5 mm line and
the average intensity was 4.02 W/cm2. Curve 3 (in both figures) was recorded without
making any effort to feed the reflected fluorescence from cuvette walls back through
the pumped volume of the dye, while Curve 4 (in both figures) was obtained by
adjusting the orientation of the cuvette so that a significant fraction of the reflections
go back and forth through the pumped volume. Since the solution had high gain even
this small feedback led to laser oscillation, as evident from shortest linewidth and
pulse width, as well as, symmetric line shape and pulse shape of Curve 4 in Fig. 4(a)
and (b), respectively.
Supplementary information
DSC thermograms of 4, Beer-Lambert law plots and absorption
and emission spectra of 1e4 are available.
Acknowledgments
The work at UNLV is in part supported by the NSF under Grant
No. 0447416 (NSF EPSCoR RING-TRUE III), NSFSmall Business
Innovation Research (SBIR) Award (Grant OII-0610753), NSF-STTR
Phase I Grant No. IIP-0740289, and NASA GRC Contract No.
NNX10CD25P. The work at CCNY was supported in part by the NSF
CREST Center for the Interface Design and Engineered Assembly of
Low Dimensional systems (IDEALS), under Grant No HRD-1547830.
We thankfully acknowledge the anonymous reviewers for their
suggestions to improve on the manuscript.
concentration-dependent study is required to determine optimal
concentration for efficient laser action. Both the center wavelength
of stimulated emission and laser spectra coincided with the peak of
the fluorescence spectrum, where the gain is maximum. The laser
action was obtained with reflection from the cuvette side walls
(approximately 8% from each surface), which is an indication of the
high optical gain of the salt 1 solution. The laser linewidth was
rather broad and multimodal since no wavelength selection
element was used in the cavity. The results on laser action pre-
sented here culminate from an exploratory experiment, which
demonstrates the potential of the salt as a high-gain laser dye in the
blue-green spectral region. However, the detailed quantum
Appendix A. Supplementary data
Supplementary data related to this article can be found at
https://doi.org/10.1016/j.molstruc.2018.06.025.
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