Synthesis and crystal structure determination of 2,6-di-tert-butyl-4-(2,4, 6-triphenylpyridinium-1-yl)phenolate and its corresponding perchlorate salt

Article abstract of DOI:10.1016/j.dyepig.2011.06.029

2,6-Di-tert-butyl-4-(2,4,6-triphenylpyridinium-1-yl)phenolate belongs to the class of negatively solvatochromic dyes commonly used for the empirical determination of solvent polarities. By way of exception, it crystallizes without any solvent of crystallization due to the presence of two bulky tert-butyl groups in the phenolate moiety. The synthesis of this hydrophobic betaine dye has been improved and an X-ray crystal structure analysis of this solventless zwitterionic dye and its protonated form (as a perchlorate salt) has been carried out.

Full text of DOI:10.1016/j.dyepig.2011.06.029

Dyes and Pigments 92 (2012) 1394e1399
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and crystal structure determination of 2,6-di-tert-butyl-4-(2,4,6-
triphenylpyridinium-1-yl)phenolate and its corresponding perchlorate salt
,
b
b
b
Sergey V. Shekhovtsov a , Irina V. Omelchenko , Viktoriya V. Dyakonenko , Oleg V. Shishkin ,
*
Rudolf Allmann c, Thomas Libor d, Christian Reichardt d, Nikolay O. Mchedlov-Petrossyan a
a Kharkov V. Karazin National University, Department of Physical Chemistry, Svoboda Square 4, Kharkov 61077, Ukraine
b National Academy of Science of Ukraine, Division of Functional Materials Chemistry, Institute for Single Crystals (SSI), Kharkov, Ukraine
c Philipps University, Department of Geo Sciences, Institute for Mineralogy, Petrology, and Crystallography, Marburg, Germany
d Philipps University, Department of Chemistry, Marburg, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
2,6-Di-tert-butyl-4-(2,4,6-triphenylpyridinium-1-yl)phenolate belongs to the class of negatively sol-
vatochromic dyes commonly used for the empirical determination of solvent polarities. By way of
exception, it crystallizes without any solvent of crystallization due to the presence of two bulky tert-butyl
groups in the phenolate moiety. The synthesis of this hydrophobic betaine dye has been improved and an
X-ray crystal structure analysis of this solventless zwitterionic dye and its protonated form (as
a perchlorate salt) has been carried out.
Received 4 May 2011
Received in revised form
21 June 2011
Accepted 23 June 2011
Available online 2 July 2011
Ó 2011 Published by Elsevier Ltd.
Keywords:
Betaine dyes
Solvatochromism
Zwitterion
Cation
X-ray structure analysis
1. Introduction
protonated, with disappearance of the solvatochromic visible
absorption band as a consequence. That is, pyridinium-N-phenolate
Pyridinium-N-phenolate betaine dyes belong to the most widely
used class of functional dyes [1] because of their pronounced nega-
tive solvatochromism (perichromism), i.e., hypsochromic shift of the
long-wavelength visible absorption band with increasing polarity of
the surrounding solvent (environment). The extreme negative
solvatochromism of the standard betaine dye 2,6-diphenyl-4-
(2,4,6-triphenylpyridinium-1-yl)phenolate (1, Fig. 1) has been used
to introduce an empirical scale of solvent polarity, ET(30) [2,3]; the
corresponding normalized scale is called ETN scale [2b].
The standard betaine dye 1 has been used to characterize the
polarity of traditional molecular solvents [2,3], and also that of ionic
liquids [4], various types of organized solutions [3b,3d,5], including
supramolecular systems [3b,6], as well as liquid/liquid, solid/liquid,
and solid/air interfaces [7]. In addition, the unique spectral sensi-
tivity of 1 has been used to examine the nature of microenviron-
ments in nano-dispersed systems [8]. Because of their basic
phenolate moiety, these betaine dyes can be easily and reversibly
betaine dyes are in fact combined solvatochromic and acid/base
indicators [5e,9].
Consequently, not only highly dipolar zwitterionic betaine dyes
Dꢀ, but also their corresponding protonated colorless cationic
species HDþ can participate in the formation of the desired
analytical signal. Therefore, it is of significance to shed light on the
changes in molecular structure connected with the protonation of
these betaine dyes. This is of particular interest for such investi-
gations in which the betaine indicator is embedded into thin
interfacial regions such as the Stern layer of micelles formed by
ionic surfactants.
The standard betaine dye 1 always crystallizes with some
solvent of crystallization and is not available in an anhydrous form.
An anhydrous betaine dye was particularly necessary for the
determination of the ground- and excited-state dipole moment of
such zwitterionic betaine dyes [10,11]. For this reason, the two 2,6-
phenyl groups of
1 were replaced by two sterically more
demanding tert-butyl groups, in order to prevent hydrogen-
bonding of the phenolate oxygen atom with the hydrogen-bond
donor (HBD) solvents commonly used for the recrystallization of
these betaine dyes.
* Corresponding author.
E-mail address: sergey080179@mail.ru (S.V. Shekhovtsov).
0143-7208/$ e see front matter Ó 2011 Published by Elsevier Ltd.
doi:10.1016/j.dyepig.2011.06.029

S.V. Shekhovtsov et al. / Dyes and Pigments 92 (2012) 1394e1399
1395
Table 1
O
OH
N
Torsion angles in 3 and 2 in Å.
ClO4
R
R
(H3C)3C
H5C6
C(CH3)3
Torsion angles
3
2
C(3)eN(1)eC(24)eC(29)
C(2)eC(1)eC(6)eC(11)
N(1)eC(3)eC(18)eC(19)
119.1 (3)
152.2 (3)
135.0 (3)
124.8 (3)
122.7 (3)
164.2 (3)
125.1 (3)
135.7 (3)
H5C6
N
C6H5
C6H5
N(1)eC(4)eC(12)eC(13)
C6H5
: R=C6H5
: R=C(CH3)3
C6H5
3
2,6-di-tert-butyl-1,4-benzoquinone-4-monoxime 5. Palladium-
catalyzed reduction of 5 with dihydrogen at normal pressure
yielded the oxygen-sensitive 4-aminophenol 6, which was imme-
diately reacted with 2,4,6-triphenylpyrylium perchlorate to give
N-(4-hydroxyphenyl)pyridinium salt 3. Deprotonation of 3 with
sodium methanolate in methanol gave betaine dye 2 as glittering
black-green crystals, without any solvent of crystallization.
Both the organic cation of 3 and the betaine dye 2 contain bulky
phenyl and 2,6-di-tert-butyl-4-hydroxyphenyl groups which are
rotated with respect to the plane of the pyridinium ring due to their
mutual repulsion. Thereafter, the vicinal aryl-substituted at N(1),
C(3), and C(4) are much more twisted out of plane than the phenyl
group at C(1) (Table 1). However, this repulsion is counteracted by
1
2
Fig. 1. Molecular structure of pyridinium-N-phenolate betaine dyes 1,
perchlorate salt 3.
2 and the
This 2,6-di-tert-butyl-4-(2,4,6-triphenylpyridinium-1-yl)pheno-
late (2) crystallizes without any solvent of crystallization, and is now
even soluble in nonpolar solvents such as cyclohexane, but no longer
soluble in water. Because of its solubility in apolar solvents (i.e.,
solvents without own permanent dipole moment), it has been used
to determine its ground-state permanent dipole moment (mg
¼
14.8 ꢀ 1.2 D in 1,4-dioxane [10,11]). For the excited-state dipole
moment of 2 (me ¼ 6.2 ꢀ 0.3 D in 1,4-dioxane) changes its direction
[11,12].
the tendency to coplanarity due to
pep conjugation between all
aromatic rings which results in the occurrence of many shortened
intramolecular H…H, C…H, and C…C contacts. The presence of two
vicinal tert-butyl groups next to the hydroxyl group in 3 and the
phenolate oxygen in 2 causes many shortened weak intramolecular
contacts with these oxygen atoms. Most of these contacts are
probably attractive (Table 2).
Because perchlorate 3, contrary to zwitterion 2, crystallizes with
one molecule of water, it is appropriate to compare the X-ray
structure data of 3 with that obtained for the protonated form of
standard dye 1 by Ratajczak et al. [16]. In the crystal lattice of
perchlorate 3 the water molecule is located rather far away from its
phenolic OH group, without intermolecular H-bond formation. This
confirms the steric shielding of the OH group by the two volumi-
nous tert-butyl groups of 3. This is in contrast to the protonated
form of 1 (isolated as nitrate), for which intermolecular H-bonds
between its OH group and the oxygen of the water of crystallization
are registered (OeH$$$O bond length ¼ 1.67 Å [16]) (Fig. 1).
It should also be noted that the geometry of the zwitterionic dye
2 does not undergo substantial changes during the protonation of 2
to give 3 (compare Figs. 2 and 3). The length of the CeO bond in 3
In order to evaluate the steric shielding of the phenolate part of
this di-tert-butyl-substituted betaine dye, an X-ray diffraction
study of the starting perchlorate 3 and betaine dye 2 has been
carried out; X-ray crystal structure determinations of other
pyridinium-N-phenolate betaine dyes have been reported
[13ae13d]. In contrast to 2, the N-(4-hydroxyphenyl)pyridinium
salt 3 crystallizes with one water molecule per unit cell.
Becauseof theusefulness of the hydrophobic solvent-freebetaine
dye 2, its synthesis has been improved (Scheme 1 and Experimental
section). The negative solvatochromism of 2 is with lmax ¼ 918 (in
cyclohexane) and 582 nm (in formamide) / Δ
l
¼ ꢁ336 nm [2a,14]
nearly as large as that of the standard betaine dye 1. The ET(2) values
(in kcal/mol) correlate linearly with ET(1) [ ¼ ET(30)] values:
ET(2) ¼ 0.781$ET(30) þ 4.94 (n ¼ 47; r ¼ 0.943) [14].
2. Results and discussion
An improved version of the four-step synthesis of betaine dye 3,
first given in reference [2a], is described in Scheme 1: Nitrosation of
commercially available 2,6-di-tert-butylphenol 4 gave the tautomeric
and
2
is typical for ortho-substituted phenols and the
O
OH
OH
(H3C)3C
C(CH3)3 + NaNO /HCl (H3C)3C
C(CH3)3
(H3C)3C
C(CH3)3
2
+ 2H2 (Pd/C)
- H2O
- NaCl
- H2O
N
5
NH2
6
HO
OH
4
O
C6H5
O
C6H5 ClO4
(H3C)3C
ClO4
C(CH3)3
(H3C)3C
C6H5
C(CH3)3
C6H5
+
C6H5
+ CH3ONa
6
- CH3OH
- NaClO4
C6H5
N
C6H5
N
- H2O
C6H5
3
C6H5
2
Scheme 1. Synthesis of betaine dye 2 and its perchlorate salt 3 [2a,14].

1396
S.V. Shekhovtsov et al. / Dyes and Pigments 92 (2012) 1394e1399
Table 2
It is noteworthy that the base strength of 2 in solution
Shortened intramolecular contacts in 3 and 2 in Åa.
dramatically exceeds that of 1. For instance, their protonation
constants differ by 1.7 logarithmic units measured in methanol [18]
and by 1.8 units in a benzene/ethanol/water mixture (mass ratio
47:47:6) [18]; in microemulsions based on cationic and anionic
surfactants, the corresponding differences are 1.5 and 2.3 loga-
rithmic units, respectively [9b]. Such large differences are also
observed for other 2,6-di-tert-butylphenols [19].
An additional factor in accordance with such effects is the
shortening of the bond lengths C(27)eC(28) and C(26)eC(27) on
going from 2 to 3 (Table 3). Such bond shortenings are generally
observed for substituted phenolate ions (mean bond length 1.438 Å
[17]), but with two bulky ortho-substituents the steric shielding of
the phenolate moiety increases drastically. As result, both the
proton transfer from the phenolic OH group to a solvent molecule
and the stabilizing solvation of the corresponding phenolate ion
should be hindered. In the interaction of betaine dyes with
lanthanide/dipivaloylmethane complexes in acetone, dye 2 also
appears as a stronger base as compared with 1 [20].
The bonds between the aromatic rings in 2, i.e., C(1)eC(6), C(3)e
C(18), C(4)eC(12), and N(1)eC(24), are slightly elongated as
compared to the corresponding bonds in 3 and to the mean bond
lengths of 1.487 and 1.431 Å [21], respectively (Table 3). The
interplanar angles between the aromatic rings in 2 and 3 are very
close to each other. Only a small decrease of the angle between the
pyridinium and the phenolate ring at C(1) is observed in 2 as
compared to 3, i.e., 152.2(3)ꢂ and 164.2(3)ꢂ, respectively (Table 1).
This is accompanied by an elongation of the C(1)eC(6) bond in both
cases. The interplanar angle between the pyridinium and the
phenolate ring in 2 (122.7(3)ꢂ) is large enough to nearly cut off the
Contact
3
2
O(1).H(31B)
O(1).H(35A)
O(1).H(36A)
O(1).C(35)
H(29).H(37C)
H(29).H(37B)
H(25).H(33C)
H(25).H(33B)
H(2).H(7)
2.46
2.27
2.41
2.905
2.08
2.26
2.15
2.21
2.24
2.24
2.71
2.66
2.74
2.69
2.88
2.37
2.35
2.31
2.17
2.10
2.06
2.12
1.97
2.02
2.62
2.63
2.62
2.59
2.73
2.79
H(5).H(11)
C(7).H(2)
C(2).H(7)
C(5).H(11)
C(11).H(5)
H(5).C(13)
H(13).C(5)
H(2).C(19)
H(19).C(2)
C(23).C(24)
C(17).C(24)
C(18).C(25)
2.77
2.81
2.978
3.073
3.112
3.00
2.95
2.99
a
Sum of the van-der-Waals radii [15]: O.H 2.46 Å, O.C 3.00 Å, H.H 2.34 Å,
C.H 2.88 Å, C.C 3.42 Å.
corresponding phenolate ions, respectively: the C(27)eO(1)
phenolate bond of 2 (1.282(4) Å) is considerably shorter than the
analogous bond in cation 3 (1.371(4) Å). The first bond length is
close to the typical values for ortho, ortho-disubstituted phenolate
ions, i.e., 1.299 Å [17], and the second one is close to the mean value
found for ortho, ortho-disubstituted phenols, i.e., 1.374 [17].
Fig. 2. Perspective view of the cation of 3 with atom numbering. All H- atoms except
Fig. 3. Perspective view of betaine dye 2 with atom numbering. All H- atoms are
that of the hydroxy group are omitted for clarity.
omitted for clarity.

S.V. Shekhovtsov et al. / Dyes and Pigments 92 (2012) 1394e1399
1397
Table 3
with that of other ortho, ortho-disubstituted phenolates and
phenols, respectively.
Selected bond lengths for 3 and 2.
The elongation of the C(26)eC(27) and C(27)eC(28) bonds on
going from 3 to 2 is in accord with the much stronger basicity of
betaine dye 2 in solution, as compared to that of other pyridinium-
N-phenolate betaine dyes. The large interplanar angle between the
Bond
3
2
O(1)eC(27)
N(1)eC(24)
C(3)eC(18)
C(4)eC(12)
C(1)eC(6)
C(1)eC(2)
C(2)eC(3)
C(3)eN(1)
N(1)eC(4)
C(4)eC(5)
1.371 (4)
1.463 (4)
1.475 (4)
1.480 (4)
1.482 (4)
1.381 (4)
1.363 (4)
1.377 (4)
1.371 (4)
1.370 (4)
1.384 (4)
1.372 (4)
1.379 (4)
1.407 (4)
1.402 (4)
1.387 (4)
1.373 (4)
1.282 (4)
1.482 (4)
1.493 (4)
1.509 (4)
1.500 (4)
1.399 (4)
1.387 (4)
1.383 (4)
1.360 (4)
1.390 (4)
1.393 (4)
1.365 (4)
1.376 (4)
1.445 (4)
1.472 (4)
1.377 (4)
1.383 (4)
pyridinium and phenolate ring in 2 prevents
p
ep-conjugation
between the two
this betaine dye.
p-systems and confirms the zwitterionic nature of
4. Experimental section
C(5)eC(1)
General: melting points (not corrected): Kofler Mikroheiztisch
(Reichert) e IR spectra: Interferometer IFS 88 (Bruker) with KBr
discs. e 1H and 13C NMR spectra: Spectrometers ARX-200 and AC-
300 (Bruker). e Mass spectra: MAT 711 (Varian) with field desorp-
tion (FD). e UV/Visible spectra: UV/Visible/NIR spectrophotometer
U-3410 (Hitachi) and DK-2 (Beckman), with 1.00-cm Suprasil
(Hellma) quartz cells. e Elemental analyses: CHN-Automat Rapid
(Heraeus) at the Analytik-Servicelabor of the Department of
Chemistry, Marburg, and Microanalytical Laboratory Bernhardt,
Mülheim/Ruhr. e Solvents: Solvents for synthetic work were puri-
fied according to usual standard methods. Solvents for UV/Visible
spectroscopic measurements were used in the highest quality
commercially available (analytical or spectroscopical grade) and
were additional dried and purified by means of molecular sieves
and, if necessary, by filtration through a column of basic aluminium
oxide (B-Super I) in order to remove traces of acids. The solvents
must be free of water and, in addition, absolutely acid-free because
the betaine dye is easily protonated at the phenolate oxygen atom,
with disappearance of the long-wavelength solvatochromic visible
absorption band as consequence.
C(24)eC(25)
C(25)eC(26)
C(26)eC(27)
C(27)eC(28)
C(28)eC(29)
C(29)eC(24)
Table 4
Crystallographic data parameters of data collection, structure solution, and refine-
ment for crystal structure of perchlorate.
3
2
Crystal data
Empirical formula
Mr, g/mol
Crystal system
Space group
a, Å
[C37H38NO]ClO4∙H2O
630.15
monoclinic
P21/n
10.019 (3)
21.087 (6)
16.026 (4)
96.58 (2)
3364
C37N37NO
511.7
monoclinic
P21/n
15.796 (5)
15.002 (4)
12.760 (4)
94.33(2)
3015
b, Å
c, Å
ꢂ
b
,
V, Å3
Z
4
4
Data collection was performed with an automatic four-circles
Siemens P3/PC for 3, and an STOE diffractometer for 2 (graphite
Calculated density, Vc/mg mꢁ3
1.244
0.160
293
1.566
0.157
298
m
(MoK
a
), mmꢁ1
monochromator, MoKa radiation, q/2q e scans). Both structures
T, K
were solved by direct methods. Structure of 2 was refined using the
program system MULTAN [22]. All non-hydrogen atoms were
refined in anisotropic approximation, all H- atoms were refined
isotropically. Structure of 3 was refined on F2 by full-matrix least
squares procedure with the SHELX-97 program package [23]. All
non-hydrogen atoms were refined in anisotropic approximation.
H- atom positions were located geometrically and refined using the
riding-model with Uiso ¼ nUeq of the carrier atom (n ¼ 1.5 for
H- atoms of hydroxyl and methyl groups and 1.2 for the others
H- atoms). Bond lengths OeH and distances H.H in the water
Data collection
F(000)
Independent Reflections
1336
5878
0.033
50
1332
4092
0.045
50
Rint
ꢂ
2
qmax,
Refinement
Reflections with F > 4
s(F)
2764
0.057
0.167
0.86
2677
0.043
0.153
0.97
R1 [F2 > 2 (F2)]
s
wR2 (all data)
S (goodness-of-fit)
molecule were constrained to 0.880(1)
respectively.
Å and 1.410(5) Å,
conjugation between the p-systems of both rings, thus confirming
the zwitterionic nature of this betaine dye (Table 4).
Additional material comprising atomic coordinates, anisotropic
thermal parameters, full bond lengths and angles have been
deposited with the Cambridge Crystallographic Data Centre as
supplementary publication no. CCDC 808911 (2) and CCDC 808908
(3). These data can be obtained free of charge via www.ccdc.cam.ac.
uk/data_request/cif.
3. Conclusions
The solvatochromic di-tert-butyl-substituted betaine dye 2
crystallizes unexpectedly without any solvent (e.g., water) of
crystallization, in contrast to all other betaine dyes known [2,13],
due to steric shielding of the usually H-bond accepting phenolate
oxygen atom. This made it possible to determine experimentally
the dipole moment of 2 as representative of all other betaine dyes.
The perchlorate 3 (as protonated form of 2) crystallizes with one
water molecule per unit cell, however, the water molecule is
located far away from the phenolic OH group without forming an
intermolecular H2O/HeO bond, thus also confirming the steric
shielding of the OH group in 3.
4.1. Synthesis of betaine 2 and its phenolate salt 3 [2a,14]
4.1.1. 2,6-Di-tert-butylcyclohexa-2,5-dien-1,4-dione-4-oxime (5)
Following the procedure of Kharasch et al. [24], to a solution of
2,6-di-tert-butylphenol (4) (from SigmaeAldrich) in ethanol and
conc. hydrochloric acid was added an aqueous solution of sodium
nitrite at ꢁ5 ꢂC, to give, after work up and twofold recrystallization
from cyclohexane, oxime 5 (81%) as yellow plates with m.p.
220e221 ꢂC (ref. [19] 221e222 ꢂC). e IR (KBr): v/cmꢁ1 ¼ 3320 (OH),
Protonation of 2 to give 3 only marginally changes the geometry
of the molecule, the bond lengths of which are in good agreement
1610 (C]O), 1008 (NeO).e 1H NMR (CD3COCD3):
d/ppm ¼ 1.21
(s, 9H, CMe3), 1.22 (s, 9H, CMe3), 6.93 (d, 4J ¼ 2.4 Hz, 1H, 3-H), 7.52

1398
S.V. Shekhovtsov et al. / Dyes and Pigments 92 (2012) 1394e1399
(d, 4J ¼ 2.5 Hz, 1H, 5-H), 12.1 (s, 1H, OH). e 13C NMR (CD3COCD3):
520 (940), and 296 (30 000) in 1,4-dioxane. Visible absorption
maxima of 2 for further 27 solvents of different polarity can be
found in ref. [2a]. e C37H37NO (511.7): calcd. 86.85, H 7.29, N 2.74;
found C 87.09, H 7.23, N 2.81.
d
/ppm ¼ 29.7 and 29.8 (CH3), 35.7 and 36.4 (CMe3), 117.9 (C-3),
131.4 (C-5), 150.0 (C-2), 150.7 (C]NOH), 152.3 (C-6), 187.8 (C]O).
e MS (FD): m/z (%) ¼ 235 (100) [Mþ]. e C14H21NO2 (235.3): calcd. C
71.47, H 9.00, N 5.95; found C 71.57, H 8.89, N 5.93.
Acknowledgments
4.1.2. 4-Amino-2,6-di-tert-butylphenol (6)
A suspension of the palladium catalyst (10 cg/g Pd on charcoal,
ca. 200 mg) in a solution of 5 (4.71 g, 21.3 mmol) in anhydrous
ethanol (150 mL) was reduced with dihydrogen at room tempera-
ture and normal pressure in a hydrogenation apparatus until the
necessary amount of dihydrogen was absorbed. The catalyst was
filtered off with a reversed fritted glass filter under nitrogen and
washed with ethanol. From the combined ethanolic solutions the
solvent was removed by distillation and the residue was triturated
with petroleum ether (b.p. 40e60 ꢂC). The solid was collected by
filtration, washed with petroleum ether, and dried in vacuo.
Reducing the mother liquor to a smaller volume gave a second crop
of product. Combined yield of 6 (3.55 g; 80%) was obtained as rosa-
coloured crystals of m.p. 110e112 ꢂC (ref. [25] 108e112 ꢂC; ref. [26]
114e116 ꢂC), which easily oxidise on contact with air to a dark red
product and should either be stored under nitrogen or immediately
S.V.S and N.O.M.-P. acknowledge the support via grant
0110U001454 from the Ukrainian Ministry of Education and
Science. C. R. thanks the Fonds der Chemischen Industrie, Frankfurt
(Main), for financial support.
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converted into the pyridinium salt 3. e IR (KBr):
n
/cmꢁ1 ¼ 3325
(NH2), 1602 (NH2). e 1H NMR (CDCl3):
d
/ppm ¼ 1.40 (s, 18H, CH3),
3.21 (2H, NH2), 4.62 (s, 1H, OH), 6.56 (s, 2H, 3-H, 5-H). e 13C NMR
(CDCl3):
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d
/ppm ¼ 30.2 (CH3), 34.3 (CMe3), 112.5 (C-3, C-5), 137.3
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(C-2, C-6), 138.4 (CeNH2), 146.6 (CeOH). e MS (FD): m/z (%) ¼ 221
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C 75.97, H 10.47, N 6.33; found C 76.13, H 10.68, N 6.32.
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aminophenol
6 (5.0 g, 22.6 mmol), 2,4,6-triphenylpyrylium
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