Synthesis and optical properties of a library of multi-colored isomeric aryldibenzopyrylium halochromic cations

Article abstract of DOI:10.1016/j.tet.2020.131222

This report describes the synthesis of five new colorful 6-aryldibenzo[b,d]pyrylium cation salts, a largely unexplored structural unit. These rare compounds are benzannulated structural derivatives of the well-known flavylium cations found widespread in natural pigments. These new dyes are directly compared to three previously synthesized 6-aryldibenzo[b,d]pyrylium cation salts as well as eight colorful isomeric 9-aryldibenzo[b,e]pyrylium cation, or 9-arylxanthylium, salts. The 9-arylxanthylium unit is commonly found in the biologically important rhodamine and rosamine dyes, yet six of the analogs presented in this study were either previously unreported or not isolated. The visual and spectroscopic properties of all 16 compounds were analyzed as a function of the structural differences between the compounds. All compounds displayed reversible halochromism in organic solution, displaying bright colors under acidic conditions and becoming colorless under basic conditions.

Full text of DOI:10.1016/j.tet.2020.131222

Tetrahedron xxx (xxxx) xxx
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis and optical properties of a library of multi-colored isomeric
aryldibenzopyrylium halochromic cations
Samantha M. Meyer a, Eva M. Charlesworth-Seiler a, Joel G. Patrow a,
,
*
Jonathan P. Kitzrow a, Deidra L. Gerlach a, Eric W. Reinheimer b, Bart J. Dahl a
a Department of Chemistry, University of Wisconsin-Eau Claire, Eau Claire, WI, 54702, United States
b Rigaku Americas Corporation, 9009 New Trails Drive, The Woodlands, TX, 77381, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
This report describes the synthesis of five new colorful 6-aryldibenzo[b,d]pyrylium cation salts, a largely
unexplored structural unit. These rare compounds are benzannulated structural derivatives of the well-
known flavylium cations found widespread in natural pigments. These new dyes are directly compared
to three previously synthesized 6-aryldibenzo[b,d]pyrylium cation salts as well as eight colorful isomeric
9-aryldibenzo[b,e]pyrylium cation, or 9-arylxanthylium, salts. The 9-arylxanthylium unit is commonly
found in the biologically important rhodamine and rosamine dyes, yet six of the analogs presented in this
study were either previously unreported or not isolated. The visual and spectroscopic properties of all 16
compounds were analyzed as a function of the structural differences between the compounds. All
compounds displayed reversible halochromism in organic solution, displaying bright colors under acidic
conditions and becoming colorless under basic conditions.
Received 18 March 2020
Received in revised form
17 April 2020
Accepted 19 April 2020
Available online xxx
Keywords:
Dibenzopyrylium
Polycyclic aromatic hydrocarbons
Heterocycles
© 2020 Elsevier Ltd. All rights reserved.
Dyes
Halochromism
1. Introduction
[12e18]. Some less stabilized unsubstituted 9-arylxanthyliums
have been synthesized and their reactivity and physical proper-
Pyrylium is a well-known aromatic heterocycle containing an
oxonium cation (Fig. 1) which forms a wide variety of stable salts
with numerous counteranions [1]. Monobenzannulated pyrylium
cations, known as chromenylium or isochromenylium, are also
common (Fig. 1) [2]. Naturally-occurring pigments known as
anthocyanidins, found in blueberries, grapes, roses, cabbage and
other colorful vegetation, contain the 2-phenylchromenylium, or
flavylium, unit [3]. Isomeric dibenzannulated pyrylium cations,
known as dibenzo[b,d]pyrylium or dibenzo[b,e]pyrylium
(commonly named xanthylium) are also known. The 9-
arylxanthylium unit is commonly found in the ubiquitous biologi-
cally relevant rhodamine and rosamine-derived dyes and probes
[4e11]. In these cases the xanthylium ion is highly stabilized by
strongly electron donating amino substituents at the C3 and C6-
positions, causing the amino groups to carry the bulk of the posi-
tive charge. Stabilized 9-arylxanthylium containing compounds
also continue to find broad application as light harvesting materials
for solar cells, photocatalysis, and other related applications
ties have been studied [19e25]. In stark contrast to 9-
arylxanthyliums, the isomeric 6-aryldibenzo[b,d]pyrylium aro-
matic heterocycle subunit is comparatively uncommon (Fig. 1). This
is rather unexpected since 6-aryldibenzo[b,d]pyrylium salts have
the same carbon skeleton as important nitrogen-containing 6-
arylphenanthridinium dyes, such as ethidium bromide [26], and
are benzannulated analogs of the pervasive flavylium-containing
pigments found in nature [3,27].
To expand the library of reported 6-aryldibenzo[b,d]pyrylium
salts we had previously reported in a preliminary communication,
the synthesis and study of three such colorful salts (10e12 in
Scheme 1) [28]. Prior to this report only three 6-aryldibenzo[b,d]
pyrylium salts, including 10, had been prepared in the literature
with limited characterization and none reporting halochromism
[29e31]. We found that all three compounds 10e12 are brightly
colored in organic solution and displayed halochromic properties
through visual and spectroscopically measurable responses to
changes in acidity and basicity in organic solution [28]. The
structurally-similar naturally occurring flavylium containing pig-
ments are well known to be sensitive to pH and display hal-
ochromism with an optical response; the results of 10e12 mirrored
those of these known pigments [27]. Nucleophiles have been
* Corresponding author.
E-mail address: dahlbj@uwec.edu (B.J. Dahl).
https://doi.org/10.1016/j.tet.2020.131222
0040-4020/© 2020 Elsevier Ltd. All rights reserved.
Please cite this article as: S.M. Meyer et al., Synthesis and optical properties of a library of multi-colored isomeric aryldibenzopyrylium
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Fig. 1. Structures of different cationic aromatic pyrylium-based heterocycles and their derivates.
shown to attack flavylium cations at the C2 position [27] and are
suspected to attack the C6 position [30] of 6-aryldibenzo[b,d]
pyrylium cations, thus disrupting the conjugation in the molecules.
This preliminary communication indicated that compounds con-
taining the relatively unexplored 6-aryldibenzo[b,d]pyrylium sub-
unit are simple to synthesize and could show promise in dye
applications and as optical pH sensors [28].
researched due to their essential functions in dye and probe ap-
plications [4e18], simple analogs with no further substitution at
the xanthylium subunit itself are surprisingly rare [19e25]. Of these
simpler types of 9-arylxanthylium compounds, only a few have
been synthesized, isolated, and studied. This includes unsub-
stituted 19 and methoxy-containing 20 [19]. Other studies state the
in-situ generation of certain 9-arylxanthylium cations but do not
report full details on their preparation, nor the isolation or full
characterization of the compounds, including dimethylamino-
containing 21 [35] and biphenyl-containing 23 [36]. Prior to the
study described herein, compounds 22, 24e26 have not been re-
ported in any form and 21 and 23 have been reported in-situ but not
isolated or characterized fully [35,36]. We were able to synthesize,
isolate, and characterize 19e26, some in their protonated or semi-
protonated forms, for the purposes of making a direct comparison
to their 6-aryldibenzo[b,d]pyrylium 10e17 analogs. We have found
that, like their 6-aryldibenzo[b,d]pyrylium analogs, the structural
alterations in 19e26 have significant effect on modulating their
optical properties and that all compounds displayed halochromism
in organic solution.
In designing compounds with specific optical properties, it is
understood that altering the functional group identity of the donor
or altering the conjugation pathway of donor-acceptor compounds
has significant effect on the optical properties of donor-acceptor
compounds with intramolecular charge transfer (ICT) [32e34]. In
this current study, we sought to explore how these alterations in
functional groups and conjugation pathways would affect the
properties of the still highly unexplored 6-aryldibenzo[b,d]pyry-
lium cations (Scheme 1) to provide a more extensive picture of the
potential of these compounds. To this end we have synthesized five
previously unreported colorful 6-aryldibenzo[b,d]pyrylium cations,
some isolated in their protonated or semi-protonated forms, con-
taining either shorter monophenyl or longer biphenyl spacer
conjugating units containing no donors, weak methoxy donors, or
stronger dimethylamino or diphenylamino donors (13e17). We
have found that these structural alterations indeed have significant
effect on modulating their optical properties and that all com-
pounds displayed halochromism in organic solution.
2. Results and discussion
2.1. Synthesis
With the large variety of 6-aryldibenzo[b,d]pyrylium 10e17
cations now in hand for this study (Scheme 1), we sought to
compare them to their expectedly more common 9-arylxanthylium
analogs 19e26 (Scheme 2) to determine what effect, if any, the
molecular structure of the dibenzopyrylium unit had on the optical
properties of the compounds. However, we discovered that while
more complex 9-arylxanthylium based compounds are well
In this current study, 6-aryldibenzo[b,d]pyrylium cations
(10e17) were prepared via addition of the appropriate arylmag-
nesium bromide (10e12, 14, 16) or aryllithium (13, 15, 17) to 6H-
benzo[c]chromen-6-one 9 (Scheme 1). All 9-arylxanthylium cat-
ions (19e26) were prepared similarly via addition of the appro-
priate arylmagnesium halide (19, 20, 23, 24) or aryllithium (21, 22,
25) to xanthone 18 (Scheme 2) The intermediate hemiketals 10a-
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Scheme 1. General Synthesis of Expanded Library of 6-aryldibenzo[b,d]pyrylium salts.
17a or phenylogous hemiketals 19a-26a were directly dehydrated
in solutions containing perchloric acid or fluoroboric acid resulting
in the desired pyrylium products. Preparation of 6-aryldibenzo[b,d]
pyrylium cations 10e12 have been described by us previously [28]
and their syntheses replicated for this study. 9-arylxanthylium
cations 19 [19] and 20 [19] have been previously reported and
characterized; their syntheses were repeated for this study. Aryl-
xanthylium 21 [35] has been reported to form via treatment of 21a
with sulfuric acid but had not yet been isolated (Scheme 2) or
characterized beyond in-situ UVeVis spectroscopy. Arylxanthylium
22 [36] has also been reported to be generated in-situ from its
xanthene precursor but was not isolated or characterized. Most of
the pyrylium products synthesized were easily precipitated from
diethyl ether solutions of their crude alcohol precursors via elimi-
nation with aqueous HClO4 (10e12, 14, 15, 19, 20, 23, 24) or aqueous
HBF4 (13, 22) with or without added acetic anhydride.
characterization. Using an alternative strategy, we found that
dimethylamino-containing xanthylium salt 21 could be isolated in
its unprotonated forms by careful addition of neat 1:1 tetra-
fluoroboric acid diethyl ether complex to CH2Cl2 solutions con-
taining the purified alcohol precursor 21a. The solid product was
then afforded by carefully layering diethyl ether on top of the so-
lutions and allowing for slow diffusion over several days.
Dimethylamino-containing salts 16 and 25, however, were still
found to be too sensitive, regardless of method and careful stoi-
chiometric addition of acid, and could ultimately only be precipi-
tated in their protonated forms, as evidenced by their yellow
coloring and corresponding spectral data, discussed below. Analo-
gous diphenylamino-containing salts 17 and 26 were engineered to
reduce the acid sensitivity of the amino group, compared to 16 and
25. However, spectroscopic evidence, discussed below, points to
these compounds being isolated in partially protonated forms,
regardless of method and careful stoichiometric addition of acid
used to form the pyrylium group. Despite this, diphenylamino 17
and 26 are brightly colored blue and green compounds, respec-
tively, in direct contrast to the yellow color of dimethylamino 16
and 25.
Strongly acidic environments are required for conversion of the
alcohol precursors (10a-17a, 19a-26a) into pyrylium structures.
Several of these pyrylium products, especially those containing
basic amino donor groups further removed from the pyrylium
subunit (16, 17, 21, 25, 26), were very sensitive to over-protonation
during this elimination step resulting in intractable isolation and
All aryl bromides used in the syntheses of 10e17 (Scheme 1) and
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Scheme 2. General Synthesis of Expanded Library of 9-aryldibenzo[b,e]pyrylium (xanthylium) salts.
19e26 (Scheme 2) are previously known and were either pur-
chased commercially (1e3, 5) or prepared via literature methods
using Suzuki-Miyaura or Buchwald-Hartwig coupling reactions (4
[37], 6 [38], 7 [39], 8, [40]) in acceptable yields. Lactone 9 (Scheme
1) was prepared with a Baeyer-Villiger oxidation of 9-fluorenone as
described by us previously [28] and xanthone 18 (Scheme 2) was
purchased commercially.
spacer unit connecting the donor to the pyrylium subunit (mono-
phenyl 10e13/19e22 vs biphenyl 14e17/23e26), and the presence
and identity of the donor unit itself (no donor, weaker methoxy
donor, or stronger amino donor). The colors displayed by the xan-
thylium salts are all indicative of a red shift in absorbance relative to
the analogous dibenzo[b,d]pyrylium salts. The colors of compounds
containing biphenyl spacers are also indicative of a red shift in
absorbance relative to the analogous compounds with monophenyl
spacers (except for 16 and 25). Those containing no donor unit are
yellow or yellow orange; those with weak donors are yellow, or-
ange, or red; those containing amino donors are purple, blue and
green. This demonstrates that colors of pyrylium salts can be
directly engineered by simple manipulation of molecular structure
resulting in a large array of possible colors.
Visual inspection of 16 and 25 provide evidence of protonation
at the amino group due to their yellow coloring compared to the
bright blues and purples of 12, 13, 21, and 22. The protonation of the
amino group results in significant attenuation in its ability donate
electron density into the oxonium atom of the pyrylium acceptor
compared to the other amine containing pyrylium salts. It is known
that the degree of pi orbital overlap is reduced due to the extra
torsions and therefore unfavorable geometries in oligophenyl
2.2. Visual characterization and comparison
Pyrylium salts 10e17 and 19e26 display a wide variety of bright
and attractive colors in solution (Figs. 2 and 3) upon visual in-
spection. The salts containing only unsubstituted aryl groups (10,
14, 19, and 23) appear yellow or yellow orange. The pyrylium salts
that contain donor groups have more vibrant colors. Those con-
taining weaker methoxy donors (11, 15, 20, and 24) appear yellow,
orange, or red. Those containing stronger amino donors (12, 13, 17,
21, 22, and 26) display bright purple, blue, and green colors.
The colors visualized from the pyrylium cation solutions are
shown to be highly dependent on several factors: (1) the structure
of the pyrylium subunit (dibenzo[b,d]pyrylium 10e17 or dibenzo
[b,e]pyrylium/xanthylium 19e26), the length and structure of the
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Fig. 2. Photographs of 6-aryldibenzo[b,d]pyrylium salts 10e17 in solution. Top: Vials containing 2 mL solutions of 10e17, 1.0 mM in CH3CN. Middle: Vials from the top after addition
of 40 L of TEA. Bottom: Vials from the middle after addition of 120 L of TFA.
m
m
Fig. 3. Photographs of 9-aryldibenzo[b,e]pyrylium salts 19e26 in solution. Top: Vials containing 2 mL solutions of 19e26, 1.0 mM in CH3CN. Middle: Vials from the top after
addition of 80 L of TEA. Bottom: Vials from the middle after addition of 200 L of TFA.
m
m
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containing donor-acceptor compounds relative to similar mono-
phenyl donor-acceptor compounds [34]. Since the dimethylamino
groups of 16 and 25 are further removed from and less conjugated
to the strongly electron withdrawing pyrylium unit by a biphenyl
spacer, instead of a monophenyl spacer (12 and 21), they are more
basic and readily protonated by the strong acids required for
pyrylium formation. In order to attempt to attenuate the stronger
basicity of the dimethyl amino group in 16 and 25, 17 and 26 with
diphenylamino donor units and biphenyl spacers were synthesized.
The diphenylamino group allows for further delocalization of the
amine lone pair which reduces its basicity. Diphenylamino-donor
containing pyrylium salts with monophenyl spacers (13 and 22)
were also synthesized for direct comparison. The diphenylamino
group does reduce the basicity of the amino group as is evidenced
by the bright blue (17) and green (26) color of their solutions.
However, based on UVevis and NMR spectral data, discussed
below, we believe these compounds are still partially protonated.
We have previously demonstrated the halochromism of pyry-
lium salts 10e12 in organic solution. This feature is now seen to be
generally extended to a wide array of benzannulated pyrylium salts,
as all compounds in this study (10e17 and 19e26) display highly
visual halochromism (Figs. 2 and 3). These compounds are only
soluble in organic solution and thus halochromism was demon-
strated with the organic soluble acids and bases, trifluoroacetic acid
(TFA) and triethylamine (TEA). Compounds 10e17 and 19e26 when
dissolved 1.0 mM in CH3CN could all be made colorless directly
upon addition of excess TEA. Their colors could then return directly
upon excess addition of TFA. The highly acid sensitive salts 16 and
25 both began yellow before TEA addition and then returned to
yellow after TFA addition. However, 25 did display fleeting light
green yellow coloring during the addition process.
properties, crystals of dibenzo[b,d]pyrylium 10 and xanthylium 23
were grown that were suitable for X-ray crystallography. At the
time of this writing, crystals of 14 or 19 with sufficient quality for X-
ray analysis were not achieved after numerous attempts. Regard-
less, the X-ray crystallographic data (see supplementary data)
support our hypothesis: the average aryl-aryl dihedral between the
aryl spacer and dibenzo[b,d]pyrylium subunit of 10 was found to be
36(1)ꢀ whereas the analogous aryl-xanthylium dihedral for 23 was
56.2(4)ꢀ, with each dihedral listed with the estimated standard
deviation (esd) in parentheses. Therefore, there is a significant
structural variation between the two isomers with aryl-aryl di-
hedrals differing by approximately 20ꢀ in the crystalline state. The
structure in the crystalline state of dibenzo[b,d]pyrylium subunit is
more favorable for extended conjugation when compared to that of
the isomeric xanthylium.
The UVevis spectra (Fig. 4 and Table 1) expectedly show large
bathochromic shifts for those compounds containing amino-
donors (unprotonated) compared to the weaker methoxy donors
or those containing no donor groups due to stronger intramolecular
charge transfer (ICT). The monophenyl and analogous biphenyl
spacer containing compounds can also be directly compared. The
donor-acceptor biphenyl spacer containing compounds were ex-
pected to be red-shifted due to the longer conjugation pathway but
also have lower molar absorptivity due to less effective conjugation
inherent in cumulative aryl-aryl torsions, relative to the analogous
monophenyl spacer compounds [34]. This effect is the clearest
when comparing the methoxy-containing compounds 11 (mono-
phenyl) and 15 (biphenyl) or 20 (monophenyl) and 24 (biphenyl).
The amino containing compounds are more susceptible to pro-
tonation in the biphenyl-spacer containing compounds and a direct
comparison to their monophenyl analogs is less useful. However, it
is clear that the absorption is highly dependent on the identity of
the spacer unit in the amino-containing compounds.
2.3. Spectroscopic characterization and comparison
It is evident from the yellow color upon visual inspection and
lack of ICT band in the UVevis spectra that 16 and 25 are proton-
ated at the dimethylamino unit. However, the degree of proton-
ation of the diphenylamino unit on 17 and 26 appears to be reduced
relative to 16 and 25. There is clearly donor-acceptor character in 17
and 26 as evidenced by visual inspection of their color (blue and
green). In their UVevis spectra, compounds 17 and 26 both have a
measurable ICT band, but it is greatly attenuated relative to
monophenyl-spacer analogs 13 and 22. Conversely, comparison of
the ICT band for monophenyl and biphenyl methoxy substituted 11/
20 and 15/24 shows a less pronounced attenuation of absorption. It
is also apparent from 1H NMR data (see supplementary data) of 17
and 26 that the phenyl protons on the diphenylamino unit are
broadened. These broad peaks in the 1H NMR data suggests that
proton-transfer equilibration is occurring at the amine moiety.
In addition to UVevis data, observed 13C NMR chemical shifts of
the pyrylium C6 atoms in 10e17 and the pyrylium C9 atoms in
19e26 illuminates the degree of donor unit delocalization into the
oxonium acceptor (Table 2 and supplementary data). Chemical
shifts of pyrylium compounds containing monophenyl spacers
(10e13 and 19e22) vary considerably depending on the identity of
the substituent on the aryl group, with the amino donor sub-
stituents causing the greatest upfield shift. Conversely, those con-
taining biphenyl spacers (14e17 and 23e26) have similar chemical
shifts, most approaching that of unsubstituted monophenyl spacer
containing 10 and 19, regardless of donor identity. Once again, this
illustrates that the donor unit held further away from to the oxo-
nium acceptor and subject to cumulative torsions, is less delo-
calized, consistent with UVevis data. A direct comparison of
monophenyl spacer containing dibenzo[b,d]pyrylium compounds
10e13 with monophenyl spacer containing xanthylium compounds
19e22 can also be done. The observed 13C chemical shifts of
All pyrylium salts were also probed by UVevis spectroscopy
(Fig. 4 and Table 1) and the trends observed by visual inspection are
observed spectroscopically as well. In comparing the dibenzo[b,d]
pyrylium 10e17 containing salts with the analogous xanthylium
19e26 salts we see clear evidence of a bathochromic shift for the
xanthylium compounds, which is weaker in those compounds
containing only unsubstituted aryl units and more pronounced is
those with donor substituents. This can be at least partially
attributed to the longer distance between the donor unit and the
oxonium cation of the pyrylium unit in the xanthylium containing
compounds compared to the dibenzo[b,d]pyrylium compounds.
Comparison of 16 and 25 show only a small difference in highest
wavelength absorbed, providing evidence of protonation, because
the protonation of the amino unit would not allow for appreciable
donation into the oxonium acceptor. Another trend that can be
observed is that the molar absorptivity of the unprotonated donor-
containing dibenzo[b,d]pyrylium compounds (11e13, 15) is larger
than the analogous xanthylium compounds (20e22, 24) (Table 1).
We believe this to be impacted by the structural differences be-
tween the aryl spacers on the two isomeric units. It would be
assumed that the conjugated aryl spacers could achieve better pi-
orbital overlap in the dibenzo[b,d]pyrylium compounds due to
less steric demand of what is effectively a mono-ortho substituted
aryl-aryl bond between the spacer and the dibenzo[b,d]pyrylium
acceptor subunit. The xanthylium compounds should have larger
dihedral angles between the aryl spacer units and the di-ortho
substituted xanthylium acceptor subunit. This would result in
more effective conjugation for the compounds containing the
dibenzo[b,d]pyrylium unit between the donor and oxonium
acceptor.
To further probe the effect of structure on the observed optical
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Fig. 4. UVevis spectra for: (A) 10e17 30.0 mM in CH3CN; (B) 19e26 30.0 mM in CH3CN; (C) samples from (A) after addition of 1 drop of TEA; (D) samples from (B) after addition of 1
drop of TEA.
Table 1
UVevis absorption data for 10e17 and 19e26 30.0 mM in CH3CN.
compd
lowest E abs lmax [nm] (ε [Mꢁ1cmꢁ1])
compd
lowest E abs lmax [nm] (ε [Mꢁ1cmꢁ1])
10
11
12
13
14
15
16
17
440 (483)
19
20
21
22
23
24
25
26
447 (3618)
496 (7878)
660 (12741)
660 (14510)
484 (6057)
498 (2159)
454 (3133)
633 (3394)
452 (9912)
549 (17607)
552 (24095)
444 (1100)
476 (3204)
445 (658)
583 (760)
donor unit has more effect on ICT in dibenzo[b,d]pyrylium com-
pounds compared to isomeric xanthylium compounds.
Finally, the halochromism of these pyrylium compounds is also
readily observed in the UVevis data (Fig. 4). UVevis spectra of
pyrylium compounds dissolved in CH3CN are shown in Fig. 4A and
B. A drop of TEA was added to the samples from 4A to 4B and the
subsequent UVevis spectra are seen in 4C and 4D. In all cases the
ICT bands have diminished and a general hypsochromic shift is
observed. This is consistent with the visual observations shown in
Figs. 2 and 3.
Table 2
Observed 13C NMR chemical shifts (
d) of C6 for 10e17 and C9 for 19e26. All were
dissolved in CDCl3 with added CF3CO2D except* 17, 21, and 26 which were dissolved
in CD2Cl2 with no added CF3CO2D, due to proton sensitivity.
compd
d
(ppm)
compd
d (ppm)
10
11
12
13
14
15
16
17*
184.9
182.4
178.5
177.2
183.8
183.7
184.2
183.4
19
20
21*
22
23
24
25
26*
175.4
174.6
168.4
170.8
175.2
174.9
175.2
175.3
3. Conclusions
A library of isomeric dibenzopyrylium salts containing either 6-
aryldibenzo[b,d]pyrylium or 9-aryldibenzo[b,e]pyrylium (9-
arylxanthylium) subunits were synthesized and their optical
properties were directly compared. Prior to this study, compounds
containing the 6-aryldibenzo[b,d]pyrylium subunit were quite rare
and poorly studied, despite the related flavylium cation being a
very common naturally-occurring colorant. Compounds containing
10e13 at C6 vary by 7.7 ppm, whereas the observed 13C chemical
shifts of 19e22 at C9 vary only by 4.6 ppm (not accounting for data
from 21, which is not directly comparable since it was collected
dissolved in CD2Cl2 with no added TFA due to acid sensitivity). This
is again in agreement with UVevis data which showed that the
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the 9-arylxanthylium cation with no further substitution and the
xanthylium unit are also surprisingly rare, despite their substituted
analogs being commonly featured in popular rhodamine and ros-
amine dyes. In this study, eleven new dibenzopyrylium compounds
have been synthesized, isolated and characterized (13e17, 21e26).
These were all compared with known pyrylium compounds 10e12,
and 19e20. We found that all dibenzopyrylium salts synthesized
for this study displayed a variety of attractive bright colors that
were directly dependent on 1) the isomeric structure of the
dibenzopyrylium subunit, 2) the presence and identity of a donor
unit on the appended aryl moiety, 3) the use of either a mono-
phenyl or biphenyl spacer between the donor unit and pyrylium
acceptor, and 4) if present, further protonation at the amino donor
group. All compounds studied demonstrated reversible hal-
ochromism in organic solution with color being visible in acidic
environments and absent in basic environments. This study reveals
that the optical properties of dibenzopyrylium salts can be directly
engineered by simple modular synthetic manipulation of molecular
structure resulting in an impressive array of possible colors.
concentrated under reduced pressure. The crude product was dis-
solved in 10 mL of diethyl ether in a round bottom flask and cooled
to 0 ꢀC with an ice bath. While stirring rapidly the solution, 3 mL of
acetic anhydride and 10 drops of 48% aq. HBF4 was added. The re-
action was allowed to stir for 1 h and filtered to yield a dark green/
black solid that was not purified further (994 mg, 42%). mp
213e215 ꢀC. 1H NMR (400 MHz, CDCl3/CF3CO2D):
d 8.66 (d,
J ¼ 8.3 Hz,1H), 8.61 (d, J ¼ 8.3 Hz,1H), 8.51 (d, J ¼ 8.3 Hz,1H), 8.33 (t,
J ¼ 7.3 Hz, 1H), 8.19 (d, J ¼ 9.3 Hz, 2H), 7.99-7.93 (m, 3H), 7.85-7.80
(m, 1H), 7.54 (t, J ¼ 7.7 Hz, 4H), 7.44 (t, J ¼ 7.3 Hz, 2H), 7.36 (d,
J ¼ 7.8 Hz, 4H), 7.16 d, J ¼ 9.0 Hz, 2H). 13C NMR (100 MHz, CDCl3/
CF3CO2D): d 177.2, 157.3, 150.2,143.2,140.6,137.1, 136.9, 134.6,133.6,
131.1, 130.5, 129.2, 128.4, 127.0 123.52, 123.47, 121.3, 119.3, 119.1,
118.7, 118.5. IR (ATR) nmax: 1580, 1552, 1046 cmꢁ1 HRMS (ESI) m/z
.
calcd for C31H22NO [Mþ] 424.1701, found 424.1691.
4.3. 6-([1,10-Biphenyl]-4-yl)benzo[c]chromen-5-ium perchlorate
(14)
Magnesium (0.190 g, 7.50 mmol) was placed into an oven-dried
round bottom flask and flushed with Ar for 30 min while stirring. 4-
Bromobiphenyl 5 (1.75 g, 7.50 mmol), anhydrous THF (10 mL), and a
crystal of I2 were then added and the reaction stirred for 1.5 h under
4. Experimental section
4.1. General comments
Ar. During this time 6H-benzo[c]chromen-6-one
9 (0.981 g,
Reactants 4 [37], 6 [38], 7 [39], 8 [40], and 9 [28] were prepared
via reported literature methods. All other reactants, reagents, and
solvents were purchased commercially and used without further
purification. All 1H and 13C NMR spectra were recorded on a Bruker
Avance II 400 (100) MHz instrument. Chemical shifts are reported
in parts per million (ppm) and referenced to the appropriate re-
sidual solvent peak. All IR spectra were recorded with a Thermo
Scientific Nicolet I5S FT-IR spectrometer with ATR. UVeVis spec-
troscopy was performed with a Cary 50 spectrophotometer using
1 cm quartz cells. High resolution mass spectrometry was per-
formed with an Agilent 6210 Time of Flight LC/MS. Purification by
column chromatography was performed using a Biotage Isolera
One flash chromatography system and Biotage SNAP KP-Sil car-
tridges. Melting points were obtained with a Mel-Temp electro-
thermal melting point apparatus. Diffraction data was collected
with a Rigaku XtaLAB Mini II benchtop X-ray diffractometer. Crys-
tallographic data (excluding structure factors) for the structures in
this paper have been deposited with the Cambridge Crystallo-
graphic Data Centre as supplementary publication 1989521 and
1989522 CCDC. Copies of the data can be obtained, free of charge,
on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK,
(fax: þ44-(0)1223e336033 or e-mail: deposit@ccdc.cam.ac.Uk).
5.00 mmol) was dissolved in 10 mL of anhydrous THF in a separate
oven dried round bottom flask. The flask was cooled to 0 ꢀC with an
ice bath and sparged for 30 min with Ar. The arylmagnesium bro-
mide solution was transferred to the solution containing 9 at 0 ꢀC
via cannula under Ar pressure. The cooling bath was removed, and
the reaction was stirred for 18 h. 50 mL of sat. NH4Cl was added to
the reaction and subsequently extracted 3x with 25 mL of Et2O. The
combined organics were dried with Na2SO4, filtered, and concen-
trated under reduced pressure. The crude product was dissolved in
25 mL of diethyl ether and cooled to 0 ꢀC in an ice bath and stirred
very rapidly. A chilled (0 ꢀC) solution of 70% perchloric acid in acetic
anhydride (1:3 by volume) was slowly added drop-wise to the
stirring ethereal solution until a colored precipitate formed. The
crude product was filtered and recrystallized in glacial acetic acid to
yield orange-yellow needles (1.36 g, 63%). mp 234e236 ꢀC. 1H NMR
(400 MHz, CDCl3/CF3CO2D):
d
8.93 (d, J ¼ 8.3 Hz, 1H), 8.87 (d,
J ¼ 8.3 Hz, 1H), 8.79 (d, J ¼ 8.3 Hz, 1H), 8.61 (t, J ¼ 8.3 Hz, 1H), 8.36
(d, J ¼ 8.5 Hz, 2H), 8.29 (d, J ¼ 8.5 Hz, 1H), 8.20-8.04 (m, 5H), 7.80-
7.77 (m, 2H), 7.61-7.52 (m, 3H). 13C NMR (100 MHz, CDCl3/
CF3CO2D):
d 183.8, 151.5, 150.0, 143.9, 139.6, 138.1, 136.2, 134.6,
134.2, 132.0, 130.8, 129.8, 129.4, 128.6, 127.9, 127.6, 123.1, 123.9,
121.5, 120.6, 119.9. IR (ATR) nmax: 1597, 1542, 1069 cmꢁ1. HRMS (ESI)
m/z calcd for C25H17O [Mþ] 333.1279, found 333.1276.
4.2. 6-(4-(Diphenylamino)phenyl)benzo[c]chromen-5-ium
tetrafluoroborate (13)
4.4. 6-(40-Methoxy-[1,10-biphenyl]-4-yl)benzo[c]chromen-5-ium
perchlorate (15)
4-bromotriphenylamine 4 (1.50 g, 4.63 mmol) was dissolved in
20 mL of anhydrous THF in an oven dried round bottom flask. The
solution was cooled to ꢁ78 ꢀC in a dry ice/acetone bath and sparged
with Ar for 30 min. 1.6 M n-BuLi in hexane (3.48 mL, 5.57 mmol)
was added dropwise via syringe and the reaction was stirred for
1 h at ꢁ78 ꢀC. During this time 6H-benzo[c]chromen-6-one 9
(0.910 g, 4.63 mmol) was dissolved in 20 mL of anhydrous THF in a
separate oven dried round bottom flask. The solution was cooled
to ꢁ78 ꢀC in a dry ice/acetone bath and sparged with Ar for 30 min.
The aryllithium solution was transferred to the solution containing
9 at ꢁ78 ꢀC via cannula under Ar pressure. The cooling bath was
removed, and the reaction was allowed to stir for 2 h. The reaction
was poured into 150 mL of sat. NH4Cl and extracted 3x with 75 mL
of EtOAc. The combined organics were washed 2x with 70 mL of
H2O and 2x with 70 mL of brine, dried with MgSO4, and
4-bromo-40-methoxybiphenyl 6 (0.671 g, 2.55 mmol) was dis-
solved in 10 mL of anhydrous THF in an oven dried round bottom
flask. The solution was cooled to ꢁ78 ꢀC in a dry ice/acetone bath
and sparged with Ar for 45 min. 1.6 M n-BuLi in hexane (1.70 mL,
2.72 mmol) was added dropwise via syringe and the reaction was
stirred for 1.5 h at ꢁ78 ꢀC. During this time 6H-benzo[c]chromen-6-
one 9 (0.500 g, 2.55 mmol) was dissolved in 10 mL of anhydrous
THF in a separate oven dried round bottom flask. The solution was
cooled to ꢁ78 ꢀC in a dry ice/acetone bath and sparged with Ar for
30 min. The aryllithium solution was transferred to the solution
containing 9 at ꢁ78 ꢀC via cannula under Ar pressure. The cooling
bath was removed, and the reaction was allowed to stir for 2 h.
50 mL of sat. NH4Cl was added to the reaction and subsequently
Please cite this article as: S.M. Meyer et al., Synthesis and optical properties of a library of multi-colored isomeric aryldibenzopyrylium
halochromic cations, Tetrahedron, https://doi.org/10.1016/j.tet.2020.131222

S.M. Meyer et al. / Tetrahedron xxx (xxxx) xxx
9
extracted 3x with 25 mL of Et2O. The combined organics were dried
with MgSO4, filtered, and concentrated under reduced pressure.
The crude product was dissolved in 25 mL of diethyl ether and
cooled to 0 ꢀC in an ice bath and stirred very rapidly. A chilled (0 ꢀC)
solution of 70% perchloric acid in acetic anhydride (1:3 by volume)
was slowly added drop-wise to the stirring ethereal solution until a
colored precipitate formed. The crude product was filtered and
recrystallized in glacial acetic acid to yield a red-orange powder
(0.508 g, 43%). mp 270e271 ꢀC. 1H NMR (400 MHz, CDCl3/
dry ice/acetone bath and sparged with Ar for 20 min. 2.5 M n-BuLi
in hexane (1.60 mL, 4.00 mmol) was added dropwise via syringe
and the reaction was stirred for 15 min at ꢁ78 ꢀC. The cooling bath
was removed, and the reaction stirred for 10 min. The reaction was
cooled back to ꢁ78 ꢀC and stirred another 15 min. During this time
6H-benzo[c]chromen-6-one 9 (713 mg, 3.63 mmol) was dissolved
in 10 mL of anhydrous THF in a separate oven dried round bottom
flask. The solution was cooled to ꢁ78 ꢀC in a dry ice/acetone bath
and sparged with Ar for 10 min. The aryllithium solution was
transferred to the solution containing 9 at ꢁ78 ꢀC via cannula under
Ar pressure. The cooling bath was removed, and the reaction stirred
for 10 min. The reaction was cooled back to ꢁ78 ꢀC and stirred
another 15 min. The reaction was quenched by the addition of
20 mL of sat. NH4Cl over 10 min at ꢁ78 ꢀC. The cooling bath was
removed, and the reaction was allowed to warm to rt with stirring.
20 mL of water was added, and the mixture was subsequently
extracted 3x with 50 mL of EtOAc. The combined organics were
washed with brine, dried with Na2SO4, filtered, and concentrated
under reduced pressure to yield a crude oily solid. The crude
product was purified with column chromatography under gradient
elution (10:90 CH2Cl2:hexane / 80:20 CH2Cl2:hexane) to yield 17a
as a light-yellow powder (750 mg, 40%) but not characterized
further. 300 mg (0.580 mmol) of 17a was dissolved in 20 mL of
CH2Cl2 in a graduated cylinder and 0.1 mL of a neat 1:1 HBF4 diethyl
ether complex and 0.5 mL of Ac2O was carefully added resulting in a
blue solution. Diethyl ether (30 mL) was layered on top of the so-
lution and was allowed to sit for 48 h as a ppt formed. The ppt was
filtered to achieve a greenish/black solid (273 mg, 80%). The prod-
uct was acid sensitive and was therefore analyzed via NMR spec-
troscopy dissolved in CD2Cl2 as a solvent with no added CF3CO2D,
which reduced its solubility. mp 226e229 ꢀC. 1H NMR (400 MHz,
CF3CO2D):
d
8.90 (d, J ¼ 8.5 Hz, 1H), 8.86 (d, J ¼ 8.5 Hz, 1H), 8.76 (dd,
J ¼ 8.3 Hz, J ¼ 1.5 Hz, 1H), 8.59 (t, J ¼ 7.9 Hz, 1H), 8.36 (d, J ¼ 8.5 Hz,
2H), 8.27 (dd, J ¼ 8.4 Hz, J ¼ 1.1 Hz, 1H), 8.17 (t, J ¼ 8.3 Hz, 1H), 8.12
(td, J ¼ 7.9 Hz, J ¼ 1.5 Hz, 1H), 8.07-8.01 (m, 3H), 7.77 (d, J ¼ 9.0 Hz,
2H), 7.12 (d, J ¼ 8.8 Hz, 2H), 3.95 (s, 3H). 13C NMR (100 MHz, CDCl3/
CF3CO2D):
d 183.7, 160.6, 151.6, 150.2, 143.9, 139.7, 136.2, 134.9,
134.3,132.1, 131.2, 131.0, 129.2,128.2, 127.3, 124.1, 124.0, 121.5, 120.5,
119.8, 115.3, 55.9. IR (ATR) nmax: 1592, 1483, 1078 cmꢁ1. HRMS (ESI)
m/z calcd for C26H19O2 [Mþ] 363.1385, found 363.1373.
4.5. 6-(4’-(Dimethylamino)-[1,10-biphenyl]-4-yl)benzo[c]chromen-
5-ium tetrafluoroborate (16)
Magnesium (0.156 g, 6.40 mmol) was placed into an oven-dried
round bottom flask and flushed with Ar for 30 min while stirring.
40-Bromo-4-dimethylaminobiphenyl 7 (1.45 g, 5.23 mmol), anhy-
drous THF (25 mL), and 2 drops of 1,2-dibromoethane were then
added. The reaction was heated to reflux and stirred for 16 h under
Ar. During this time 6H-benzo[c]chromen-6-one
9 (0.802 g,
4.09 mmol) was dissolved in 35 mL of anhydrous THF in a separate
oven dried round bottom flask. The flask was cooled to 0 ꢀC with an
ice bath and sparged for 30 min with Ar. The arylmagnesium bro-
mide solution was transferred to the solution containing 9 at 0 ꢀC
via cannula under Ar pressure. The cooling bath was removed, and
the reaction was stirred for 18 h. 25 mL of sat. NH4Cl was added to
the reaction and subsequently extracted 4x with 25 mL of Et2O. The
combined organics were washed 2x with 75 mL of H2O, dried with
CaCl2, filtered, and concentrated under reduced pressure. The crude
intermediate was purified via column chromatography using
gradient elution (hexane /20% EtOAc in hexane) but not charac-
terized further. The purified intermediate was dissolved in 10 mL of
diethyl ether and cooled to 0 ꢀC in an ice bath and stirred rapidly.
While stirring rapidly the solution, 3 mL of acetic anhydride and
48% aq. HBF4 was added until precipitation, likely protonated at the
amino group. The reaction was allowed to stir for 1 h and filtered to
yield an orange-yellow solid (0.228 g, 12%). The product was only
able to be isolated in its protonated form was therefore analyzed via
NMR spectroscopy dissolved in CD3Cl as a solvent with added
CF3CO2D, which improved its solubility. mp 238e239 ꢀC. 1H NMR
CD2Cl2):
d
8.92 (d, J ¼ 8.6 Hz, 1H), 8.84 (d, J ¼ 8.3 Hz, 1H), 8.78 (d,
J ¼ 8.1 Hz,1H), 8.60 (t, J ¼ 7.7 Hz,1H), 8.36 (d, J ¼ 8.6 Hz, 2H), 8.27 (d,
J ¼ 8.3 Hz, 1H), 8.17 (t, J ¼ 7.8 Hz, 1H), 8.15-8.02 (m, 4H), 7.73 (br,
2H), 7.36 (br, 5H) 7.20 (br, 7H). 13C NMR (100 MHz, CD2Cl2):
d 183.4,
151.9, 144.1, 139.8, 136.5, 135.2,135.1, 132.6, 131.2, 130.2, 129.1, 128.2,
127.5, 126.3, 124.7, 124.6, 124.1, 122.1, 121.0, 120.3. IR (ATR) nmax
:
1583, 1485, 1050 cmꢁ1 HRMS (ESI) m/z calcd for C37H26NO [Mþ]
.
500.2014, found 500.2023.
4.7. 9-(4-(dimethylamino)phenyl)xanthylium tetrafluoroborate
(21)
4-Bromo-N,N-dimethylaniline 3 (1.00 g, 4.98 mmol) was dis-
solved in 30 mL of anhydrous THF in an oven dried round bottom
flask. The solution was cooled to ꢁ78 ꢀC in a dry ice/acetone bath
and sparged with Ar for 30 min. 1.6 M n-BuLi (3.75 mL, 6.00 mmol)
was added dropwise and the reaction was stirred for 1 h at ꢁ78 ꢀC.
During this time xanthone 18 (981 mg, 5.00 mmol) was dissolved in
50 mL of anhydrous THF in a separate oven dried round bottom
flask. The solution was cooled to ꢁ78 ꢀC in a dry ice/acetone bath
and sparged with Ar for 30 min. The aryllithium solution was
transferred to the solution containing 18 at ꢁ78 ꢀC via cannula
under Ar pressure. The cooling bath was removed, and the reaction
was stirred for 4 h. The reaction was quenched with 75 mL sat.
NH4Cl. The mixture was subsequently extracted 3x with 50 mL of
EtOAc. The combined organics were washed with brine, dried with
Na2SO4, filtered, and concentrated under reduced pressure to yield
a highly acid-sensitive crude yellow/brown solid. The crude prod-
uct was purified via column chromatography (5% EtOAc in hexane
/50% EtOAc in hexane, both solvents containing 1% triethylamine)
to obtain 21a as a yellow powder (560 mg, 35%) but not charac-
terized further. 215 mg (0.677 mmol) of 21a was dissolved in 12 mL
of CH2Cl2 in a graduated cylinder and 0.1 mL of a neat 1:1 HBF4
diethyl ether complex was carefully added resulting in a blue
(400 MHz, CDCl3/CF3CO2D):
d
8.95 (d, J ¼ 8.8 Hz, 1H), 8.89 (d,
J ¼ 8.8 Hz,1H), 8.80 (d, J ¼ 7.8 Hz,1H), 8.65 (t, J ¼ 7.8 Hz,1H), 8.38 (d,
J ¼ 7.8 Hz, 2H), 8.34 (d, J ¼ 8.8 Hz, 1H), 8.23-8.17 (m, 2H), 8.13-8.09
(m, 3H), 7.89 (d, J ¼ 8.1 Hz, 2H), 7.76 (d, J ¼ 8.3 Hz, 2H), 3.45 (s, 6H).
13C NMR (100 MHz, CDCl3/CF3CO2D):
d 184.2, 152.0, 147.7, 144.8,
142.5, 141.7, 140.4, 136.2, 135.5, 134.0, 132.5, 131.6, 130.3, 129.2,
129.1,124.2,124.1, 121.6,121.2,120.8,120.0, 47.9. IR (ATR) nmax: 1597,
1544,1071 cmꢁ1. HRMS (ESI) m/z calcd for C27H22NO [Mþ] 376.1701,
found 376.1687.
4.6. 6-(4’-(Diphenylamino)-[1,10-biphenyl]-4-yl)benzo[c]chromen-
5-ium tetrafluoroborate (17)
40-Bromo-N,N-diphenyl-[1,10-biphenyl]-4-amine
8
(1.60 g,
4.00 mmol) was dissolved in 25 mL of anhydrous THF in an oven
dried round bottom flask. The solution was cooled to ꢁ78 ꢀC in a
Please cite this article as: S.M. Meyer et al., Synthesis and optical properties of a library of multi-colored isomeric aryldibenzopyrylium
halochromic cations, Tetrahedron, https://doi.org/10.1016/j.tet.2020.131222

10
S.M. Meyer et al. / Tetrahedron xxx (xxxx) xxx
solution. Diethyl ether (35 mL) was layered on top of the solution
and was allowed to sit for 48 h as a ppt formed. The ppt was filtered
to achieve a red/purple solid (109 mg, 42%). The product was acid
sensitive and was therefore analyzed via NMR spectroscopy dis-
solved in CD2Cl2 as a solvent with no added CF3CO2D, which
reduced its solubility. mp 156e158 ꢀC. 1H NMR (400 MHz, CD2Cl2):
purified via column chromatography using gradient elution (5%
EtOAc in hexane /50% EtOAc in hexane) but not characterized
further. The purified intermediate was dissolved in 10 mL of diethyl
ether and 70% aq. HClO4 was added dropwise at rt until a colored
precipitate formed. The reaction was allowed to stir for 1 h and
filtered to yield an orange solid. The solid was recrystallized in
glacial acetic acid to yield small orange needles (1.73 g, 51%). mp
d
8.32 (dd, J ¼ 8.6 Hz, 1.2 Hz, 2H), 8.23 (t, J ¼ 8.0 Hz, 2H), 8.05 (d,
J ¼ 8.3 Hz, 2H), 7.84-7.80 (m, 4H), 7.14 (d, J ¼ 9.3 Hz, 2H), 3.35 (s,
6H). 13C NMR (100 MHz, CD2Cl2):
168.4, 156.9, 156.3, 140.7, 138.0,
266e267 ꢀC. 1H NMR (400 MHz, CDCl3/CF3CO2D):
d 8.55 (t,
d
J ¼ 8.8 Hz, 2H), 8.42 (d, J ¼ 8.3 Hz, 2H), 8.32 (dd, J ¼ 8.7 Hz, J ¼ 1.6,
2H), 8.05 (d, J ¼ 8.3 Hz, 2H), 7.99 (t, J ¼ 8.8 Hz, 2H), 7.81-7.76 (m,
4H), 7.59 (t, J ¼ 7.6 Hz, 2H), 7.52 (t, J ¼ 7.3 Hz, 1H). 13C NMR
133.3, 128.4, 122.7. 121.3, 119.9, 114.1, 41.1. IR (ATR) nmax: 1595, 1465,
1050 cmꢁ1 HRMS (ESI) m/z calcd for C21H18NO [Mþ] 300.1388,
.
found 300.1401.
(100 MHz, CDCl3/CF3CO2D): d 175.2, 158.5, 146.4, 144.3, 138.8, 132.0,
131.6, 129.6, 129.4, 129.2, 128.1, 127.4, 123.6, 120.1. IR (ATR) nmax
:
1599, 1578, 1088 cmꢁ1 HRMS (ESI) m/z calcd for C25H17O [Mþ]
.
4.8. 9-(4-(diphenylamino)phenyl)xanthylium tetrafluoroborate
(22)
333.1279, found 333.1270.
4.10. 9-(40methoxy-[1,10-biphenyl]-4-yl)xanthylium perchlorate
(24)
4-Bromotriphenylamine 4 (2.48 g, 7.64 mmol) was dissolved in
30 mL of anhydrous THF in an oven dried round bottom flask. The
solution was cooled to ꢁ78 ꢀC in a dry ice/acetone bath and sparged
with Ar for 30 min. 1.6 M n-BuLi in hexane (5.80 mL, 9.28 mmol)
was added dropwise via syringe and the reaction was stirred for
1 h at ꢁ78 ꢀC. During this time xanthone 18 (1.50 g, 7.64 mmol) was
dissolved in 40 mL of anhydrous THF in a separate oven dried round
bottom flask. The solution was cooled to ꢁ78 ꢀC in a dry ice/acetone
bath and sparged with Ar for 30 min. The aryllithium solution was
transferred to the solution containing 18 at ꢁ78 ꢀC via cannula
under Ar pressure. The cooling bath was removed, and the reaction
was allowed to stir for 2 h. The reaction was poured into 150 mL of
sat. NH4Cl and extracted 3x with 75 mL of EtOAc. The combined
organics were washed 2x with 70 mL of H2O and 2x with 70 mL of
brine, dried with MgSO4, and concentrated under reduced pressure.
The crude product was dissolved in 10 mL of diethyl ether in a
round bottom flask and cooled to 0 ꢀC with an ice bath. While
stirring rapidly the solution, 3 mL of acetic anhydride and 10 drops
of 48% aq. HBF4 was added. The reaction was allowed to stir for 1 h
and filtered to yield a blue-green solid that was not purified further
(1.68 g g, 43%). mp 272e273 ꢀC. 1H NMR (400 MHz, CDCl3/
Magnesium (0.132 g, 5.42 mmol) was placed into an oven-dried
round bottom flask and flushed with Ar for 30 min while stirring. 4-
Bromo-40-methoxybiphenyl 6 (1.43 g, 5.42 mmol), anhydrous THF
(8 mL), and 2 drops of 1,2-dibromoethane were then added. The
reaction was heated to reflux, stirred for 30 min under Ar, and
allowed to cool back to rt over 1 h. During this time xanthone 18
(0.532 g, 2.71 mmol) was dissolved in 16 mL of anhydrous THF in a
separate oven dried round bottom flask. The flask was cooled to
0
ꢀC with an ice bath and sparged for 30 min with Ar. The aryl-
magnesium bromide solution was transferred to the solution con-
taining 18 at 0 ꢀC via cannula under Ar pressure. The cooling bath
was removed, and the reaction was stirred for 2 h. 50 mL of sat.
NaHCO3 was added to the reaction and subsequently extracted 3x
with 100 mL of CH2Cl2. The combined organics were washed dried
with Na2SO4, filtered, and concentrated under reduced pressure.
The crude intermediate was purified via column chromatography
using gradient elution (5% EtOAc in hexane /50% EtOAc in hexane)
but not characterized further. The purified intermediate was dis-
solved in 10 mL of diethyl ether and 70% aq. HClO4 was added
dropwise at rt until a colored precipitate formed. The reaction was
allowed to stir for 30 min and filtered to yield a red solid. The solid
was recrystallized in glacial acetic acid to yield small red needles
(0.595 g, 47%). mp 230e231 ꢀC. 1H NMR (400 MHz, CDCl3/
CF3CO2D):
d
8.40e8.44 (m, 4H), 8.17 (d, J ¼ 8.8 Hz, 2H), 7.89 (t,
J ¼ 7.7 Hz, 2H), 7.67, (d, J ¼ 8.8 Hz, 2H), 7.53-7.49 (m, 4H), 7.39-7.35
(m, 6H), 7.28 (d, J ¼ 8.6, Hz, 2H). 13C NMR (100 MHz, CDCl3/
CF3CO2D):
d 170.8, 157.2, 154.7, 144.3, 141.8, 136.0, 132.3, 130.4,
128.6,127.4, 127.1,122.6,122.5,119.7 118.5. IR (ATR) nmax: 1597,1567,
1064 cmꢁ1 HRMS (ESI) m/z calcd for C31H22NO [Mþ] 424.1701,
CF3CO2D):
d
8.51 (ddd, J ¼ 8.6 Hz, J ¼ 7.0 Hz, J ¼ 1.2 Hz, 2H), 8.37 (d,
.
found 424.1692.
J ¼ 8.8 Hz, 2H) 8.33 (dd, J ¼ 8.7 Hz, J ¼ 1.3 Hz, 2H), 8.01-7.94 (m, 4H),
7.78 (d, J ¼ 8.1 Hz, 2H), 7.73 (d, J ¼ 8.8 Hz, 2H), 7.10 (d, J ¼ 8.8 Hz, 2H)
3.94 (s, 3H). 13C NMR (100 MHz, CDCl3/CF3CO2D):
d 174.9, 160.2,
4.9. 9-([1,10-biphenyl]-4-yl)xanthylium perchlorate (23)
158.3, 145.7, 144.0, 132.0, 131.9, 131.4, 129.4, 128.7, 128.6, 127.3,
123.5,120.0,114.8, 55.6. IR (ATR) nmax: 1596, 1578, 1083 cmꢁ1. HRMS
(ESI) m/z calcd for C26H19O2 [Mþ] 363.1385, found 363.1382.
Magnesium (0.389 g, 16.2 mmol) was placed into an oven-dried
round bottom flask and flushed with Ar for 30 min while stirring. 4-
Bromobiphenyl 5 (3.73 g, 16.0 mmol), anhydrous THF (20 mL), and
2 drops of 1,2-dibromoethane were then added. The reaction was
heated to reflux, stirred for 30 min under Ar, and allowed to cool
back to rt over 1 h. During this time xanthone 18 (1.54 g, 7.85 mmol)
was dissolved in 50 mL of anhydrous THF in a separate oven dried
round bottom flask. The flask was cooled to 0 ꢀC with an ice bath
and sparged for 30 min with Ar. The arylmagnesium bromide so-
lution was transferred to the solution containing 18 at 0 ꢀC via
cannula under Ar pressure. The cooling bath was removed, and the
reaction was stirred for 2 h. 50 mL of sat. NaHCO3 was added to the
reaction and subsequently extracted 3x with 100 mL of CH2Cl2. The
combined organics were washed dried with Na2SO4, filtered, and
concentrated under reduced pressure. The crude intermediate was
4.11. 9-(4’-(dimethylamino)-[1,10-biphenyl]-4-yl)xanthylium
perchlorate (25)
40-Bromo-4-dimethylaminobiphenyl 7 (1.70 g, 6.14 mmol) was
dissolved in 50 mL of anhydrous THF in an oven dried round bottom
flask. The solution was cooled to ꢁ78 ꢀC in a dry ice/acetone bath
and sparged with Ar for 30 min. 1.6 M n-BuLi in hexane (5.00 mL,
8.00 mmol) was added dropwise via syringe and the reaction was
stirred for 30 min at ꢁ78 ꢀC. During this time xanthone 18 (1.21 mg,
6.14 mmol) was dissolved in 30 mL of anhydrous THF in a separate
oven dried round bottom flask. The solution was cooled to ꢁ78 ꢀ
C
in a dry ice/acetone bath and sparged with Ar for 30 min. The
Please cite this article as: S.M. Meyer et al., Synthesis and optical properties of a library of multi-colored isomeric aryldibenzopyrylium
halochromic cations, Tetrahedron, https://doi.org/10.1016/j.tet.2020.131222

S.M. Meyer et al. / Tetrahedron xxx (xxxx) xxx
11
aryllithium solution was transferred to the solution containing
Declaration of competing interest
18 at ꢁ78 ꢀC via cannula under Ar pressure and stirred for 2 h. The
cooling bath was removed, and the reaction stirred while warming
to rt. The reaction was quenched by the addition of 150 mL of sat.
NH4Cl. The mixture was subsequently extracted 3x with 75 mL of
EtOAc. The combined organics were washed with brine, dried with
Na2SO4, filtered, and concentrated under reduced pressure to yield
a crude solid. The purified intermediate was dissolved in 10 mL of
diethyl ether and 70% aq. HClO4 was added dropwise at rt until a
colored precipitate formed. The reaction was allowed to stir for
30 min and filtered to yield a yellow/brown. The solid was recrys-
tallized in glacial acetic acid and acetic anhydride to yield a yellow
powder (0.789 g, 27%). The product was only able to be isolated in
its protonated form was therefore analyzed via NMR spectroscopy
dissolved in CDCl3 as a solvent with added CF3CO2D, which
improved its solubility. mp 277e278 ꢀC. 1H NMR (400 MHz, CDCl3/
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Acknowledgements
Acknowledgment is made to the Research Corporation for Sci-
ence Advancement Cottrell College Science Award (ID: 22480) and
to the donors of the American Chemical Society Petroleum
Research Fund (ACS PRF 51259-UNI1) for support of this research.
We are also grateful for the Student Blugold Commitment Differ-
ential Tuition (BCDT) funds through the University of Wisconsin-
Eau Claire Faculty/Student Research Collaboration Grants program
and the Wisconsin Louis Stokes Alliance for Minority Participation
(WiscAMP) for financial support.
CF3CO2D):
d
8.59 (t, J ¼ 7.8 Hz, 2H), 8.42 (d, J ¼ 8.8 Hz, 2H), 8.33 (d,
J ¼ 8.6 Hz, 2H), 8.07-7.98 (m, 6H), 7.83 (d, J ¼ 8.1 Hz, 2H), 7.77 (d,
J ¼ 8.6 Hz, 2H), 3.49 (s, 6H). 13C NMR (100 MHz, CDCl3/CF3CO2D):
Appendix A. Supplementary data
d
175.2, 158.7, 144.8, 143.8, 142.4, 141.9, 132.1, 131.7, 130.5, 130.2,
130.0, 128.5, 123.8, 121.1, 120.1, 48.0. IR (ATR) nmax: 1598, 1579,
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tet.2020.131222.
1087 cmꢁ1 HRMS (ESI) m/z calcd for C27H22NO [Mþ] 376.1701,
.
found 376.1692.
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Please cite this article as: S.M. Meyer et al., Synthesis and optical properties of a library of multi-colored isomeric aryldibenzopyrylium
halochromic cations, Tetrahedron, https://doi.org/10.1016/j.tet.2020.131222