6-Aryldibenzo[b,d]pyrylium salts: Synthesis and characterization of a reversible pH-driven optical and spectroscopic response

Article abstract of DOI:10.1016/j.tetlet.2012.09.054

This preliminary report describes the synthesis of three 6-aryldibenzo[b,d]pyrylium salts, a relatively unexplored structural unit. The optical and spectroscopic properties of the 6-aryldibenzo[b,d]pyrylium salts were examined as a function of pH. All thr

Full text of DOI:10.1016/j.tetlet.2012.09.054

Tetrahedron Letters 53 (2012) 6433–6435
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
6-Aryldibenzo[b,d]pyrylium salts: synthesis and characterization
of a reversible pH-driven optical and spectroscopic response
⇑
Erin E. Prust, Erik J. Carlson, Bart J. Dahl
University of Wisconsin-Eau Claire, 105 Garfield Ave., Department of Chemistry, Eau Claire, WI 54702, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
This preliminary report describes the synthesis of three 6-aryldibenzo[b,d]pyrylium salts, a relatively
unexplored structural unit. The optical and spectroscopic properties of the 6-aryldibenzo[b,d]pyrylium
salts were examined as a function of pH. All three compounds displayed bright colors under acidic con-
ditions and were colorless under basic conditions. The halochromism of the molecules was shown to be
reversible.
Received 27 August 2012
Revised 11 September 2012
Accepted 13 September 2012
Available online 21 September 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Dibenzo pyrylium
Polycyclic aromatic hydrocarbon
Heterocycle
Dye
Halochromism
Small positively charged polyaromatic hydrocarbons containing
oxygen are well-known synthetic and naturally-occurring
substances. Numerous preparations of compounds containing
9-aryldibenzo[b,e]pyrylium, or 9-arylxanthylium, aromatic hetero-
cycle subunit have been reported (Fig. 1).1 Important compounds
that contain this subunit include various rhodamine and rosamine
dyes, typically used as cell and tissue labels.2–7 In these cases the
xanthylium ion is highly stabilized by amino substituents at the
C3 and C6 positions, causing the amino groups to carry the majority
of the positive charge. Stabilized 9-arylxanthylium containing com-
pounds have also been recently found to be organic photosensitiz-
ers in dye sensitized solar cell applications.8,9 Additionally, less
stabilized unsubstituted 9-arylxanthyliums have been synthesized
and their reactivity and physical properties have been studied.10–16
While 9-arylxanthylium salts are quite well known, compounds
containing the isomeric 6-aryldibenzo[b,d]pyrylium aromatic
heterocycle subunit are surprisingly rare (Fig. 1). To the author’s
knowledge, only three 6-aryldibenzo[b,d]pyrylium salts have been
prepared to date.17–19 No practical applications of these compounds
were discussed in these early reports; however, further exploration
of compounds containing this subunit could ultimately result in a
new class of synthetic dyes much like their ubiquitous xanthylium
analogs. Additionally, 6-aryldibenzo[b,d]pyrylium salts have the
same carbon skeleton as nitrogen-containing 6-arylphenanthridinium
dyes, such as ethidium bromide,20 and are benzannulated analogs
of flavylium-containing pigments,21 such as anthocyanins.
We sought to expand the library of reported 6-aryl-
dibenzo[b,d]pyrylium salts and explore their spectroscopic re-
sponses to changes in pH. Nucleophiles have been shown to
attack flavylium cations at the C2 position21 and are suspected to
attack the C6 position18 of 6-aryldibenzo[b,d]pyrylium cations.
Given this, changes in pH have the potential to reversibly disrupt
intramolecular charge transfer (ICT) in 6-aryldibenzo[b,d]pyrylium
compounds containing a para electron donating unit. This could
ultimately lead to applications in pH-sensitive, or halochromic,
materials. To this end, perchlorate salts (1–3) have been synthe-
sized and their spectroscopic character in both acidic and basic
media has been explored.
Compound 1 is known but has been limited to characterization
by melting point and elemental analysis.18 Previous unsuccessful
attempts were made to synthesize 2 by peroxidation and acid-pro-
moted ring-expansion of 9-p-methoxyphenylfluorene: in this case
the authors reported a brightly colored orange solution but could
not afford a precipitate.18 In this current study, all perchlorate salts
(1–3) were prepared via Grignard addition of the appropriate aryl-
magnesium halide to 6H-benzo[c]chromen-6-one (Scheme 1). 6H-
Benzo[c]chromen-6-one was synthesized using a Baeyer–Villiger
oxidation of 9-fluorenone. Dehydration in a cold, stirring, ethereal
solution of the intermediate hemiketal with a mixture of acetic
anhydride and 70% perchloric acid was paramount for obtaining
the desired products as solid precipitates. Numerous other meth-
ods were attempted that led to the products oiling out of solution.
⇑
Corresponding author. Tel.: +1 715 836 4179; fax: +1 715 836 4979.
E-mail address: dahlbj@uwec.edu (B.J. Dahl).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.09.054

6434
E. E. Prust et al. / Tetrahedron Letters 53 (2012) 6433–6435
Figure 1. Structures of some positively charged heterocyclic aromatic compounds
with commonly accepted numbering of atoms.
All three compounds were obtained as brightly colored solids
(1 = yellow, 2 = orange, 3 = metallic black/purple).
Naturally-occurring flavylium containing pigments, such as
anthocyanins, are known to be sensitive to pH with an instanta-
neous halochromic response.21 All three 6-aryldibenzo[b,d]pyryli-
um-containing compounds (1–3) are brightly colored in organic
solution (Fig. 2) and upon addition of excess triethylamine (TEA),
the solutions instantly become colorless. The colors of all three
compounds return immediately upon addition of excess trifluoro-
acetic acid (TFA). Visual inspection indicated that the reversibility
of the color changes was consistent through several cycles of acid/
base additions.
Figure 2. Photographs of 1–3. Top (A): from left to right, compound 1 (0.56 mM), 2
(0.64 mM), and 3 (0.46 mM) in 10 mL of CH3CN. Middle (B): solutions in (A) after
addition of 10
lL (0.072 mmol) of TEA to 1–2 and 100
lL (0.720 mmol) of TEA to 3.
Bottom (C): solutions in (B) after addition of 200
l
L (2.60 mmol) of TFA.
Interestingly, compound 3 required more than ten times the
volume of TEA, a weak base, to become completely colorless rela-
tive to compounds 1 and 2. This is indicative of effective resonance
stabilization of the pyrylium unit by the dimethylamino group in 3.
The extra stabilization of the dimethylamino group is also readily
apparent by the nonlinear downfield trend in 13C NMR chemical
shifts at the C6 positions for all three compounds (see Supplemen-
tary data for full spectral details). Unsubstituted pyrylium 1 reso-
nates at 184.9 ppm. Substitution with the p-methoxy unit
decreases the C6 chemical shift to 182.4 ppm for 2, a 2.5 ppm dif-
ference, whereas substitution with the p-dimethylamino unit de-
creases the C6 chemical shift to 178.5 ppm, a 6.4 ppm difference.
The pH-induced states of the 6-aryldibenzo[b,d]pyrylium salts
were further analyzed by UV–vis spectroscopy (Fig. 3). Solutions
of compounds 2 and 3 in acetonitrile both exhibit intramolecular
charge transfer (ICT) bands in the visible region at 445 nm
= 20,478 MÀ1 cmÀ1) and 548 nm ( = 7251 MÀ1 cmÀ1). The lower
(e
e
energy HOMO–LUMO band gap for 3 is once again more evident of
the expected stabilization to the pyrylium unit by the dimethyl-
amino group. Upon addition of excess triethylamine, none of the
molecules absorb in the visible, thereby effectively disrupting con-
jugation between the electron donating groups and the pyrylium
moiety.
Numerous 9-arylxanthylium based compounds, such as rhoda-
mine and rosamine, are very useful fluorescent dyes.2–7 Compound
1, containing a tetrachloroferrate counterion, was originally re-
ported in 1909 and was initially indicated to be nonfluorescent.17
We have also found that acetonitrile solutions of 1–3 are not visibly
Scheme 1. The synthesis of 6-aryldibenzo[b,d]pyrylium perchlorate salts.

E. E. Prust et al. / Tetrahedron Letters 53 (2012) 6433–6435
6435
In summary, three colorful 6-aryldibenzo[b,d]pyrylium perchlo-
rate salts (1–3) have been prepared in two steps from 9-fluorenone.
Compounds
2 and 3 were previously unknown. All three
compounds are brightly colored in solution and are optically and
spectroscopically responsive to changes in pH. Compounds 1 and
2 were shown to be fluorescent, which could be completely
diminished upon addition of TEA. These preliminary studies
indicate that compounds containing the relatively unexplored
6-aryldibenzo[b,d]pyrylium subunit are simple to synthesize and
could show promise in dye applications and as optical pH sensors.
Future studies are planned to synthesize and examine other
6-aryldibenzo[b,d]pyrylium derivatives and to further explore their
reactivity and dye properties.
Acknowledgments
Acknowledgment is made to the donors of the American Chem-
ical Society Petroleum Research Fund (ACS PRF 51259-UNI1) for
support of this research. We also thank the National Science Foun-
dation (CHE-0850701) and the Student Blugold Commitment Dif-
ferential Tuition funds through the University of Wisconsin-Eau
Claire Faculty/Student Research Collaboration Grants program for
financial support.
Supplementary data
Supplementary data (details of spectroscopic studies, synthetic
procedures, and characterization data for all new compounds)
associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.tetlet.2012.09.054.
Figure 3. UV–vis spectra of 1–3. Top (A): 1–3 in CH3CN at the indicated
concentrations. Bottom (B): 1–3 in CH3CN at the indicated concentrations after
addition of 1 drop of TEA.
References and notes
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Figure 4. Emission spectra of 1 (solid line), 0.52 mM in CH3CN and 2 (dashed line),
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luminescent upon UV irradiation. However, 1 and 2 were indeed
determined to be fluorescent upon closer inspection with spectro-
fluorometric analysis (Fig. 4). Compound 1 emits at 526 nm (exci-
tation wavelength = 427 nm) and compound 2 emits with lower
intensity at 515 nm (excitation wavelength 445 nm). Compounds
1 and 2 exhibit no detectable fluorescence upon addition of trieth-
ylamine. Compound 3 exhibited no detectable fluorescence in ace-
tonitrile at any wavelength.