Synthesis and fluorescence properties of DMCX+ - A stable oxygen-bridged [4]helicenium dye

Article abstract of DOI:10.1016/j.tetlet.2012.11.092

A one-step synthesis of the stable [4]helicenium dye, 1,13-dimethoxy- chromeno[2,3,4-kl]xanthenium hexafluorophosphate (DMCX⁺) from the readily available tris(2,6-dimethoxyphenyl)carbenium ion is reported. The crystal structure, the chemical stability, and dye properties of the DMCX ⁺ helicenium system are described.

Full text of DOI:10.1016/j.tetlet.2012.11.092

Tetrahedron Letters xxx (2012) xxx–xxx
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and fluorescence properties of DMCX+—a stable oxygen-bridged
[4]helicenium dye
⇑
Thomas Just Sørensen, Anders Ø. Madsen, Bo W. Laursen
Department of Chemistry Nano-Science Center, University of Copenhagen, Universitetsparken 5, DK-2100 Kbenhavn , Denmark
a r t i c l e i n f o
a b s t r a c t
Article history:
A one-step synthesis of the stable [4]helicenium dye, 1,13-dimethoxy-chromeno[2,3,4-kl]xanthenium
hexafluorophosphate (DMCX+) from the readily available tris(2,6-dimethoxyphenyl)carbenium ion is
reported. The crystal structure, the chemical stability, and dye properties of the DMCX+ helicenium sys-
tem are described.
Received 10 September 2012
Revised 12 November 2012
Accepted 22 November 2012
Available online xxxx
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Helicenes
Carbenium ions
Fluorescent dyes
Cation stability
Helicenium
Synthesis
Cationic triphenylmethane dyes have been known and used for
more than a century.1,2 The dyes are potent colorants, but it is the
hetero-bridged triphenylmethylium dyes that have received the
most attention. Bridging two of the triphenylmethylium arms, with
either oxygen or nitrogen, forms the intensely studied xanthenium
and acridinium dyes.3–6 Bridging of all the arms results in the for-
mation of a triangulenium dye:7–12 these are promising fluoro-
phores that have been studied in supramolecular assemblies,13–22
as materials,23,24 for their photophysical properties,25–30 and as
phase-transfer catalysts.31
12
2
O
O
11
10
13
1
3
4
C+
C+
9 X
Y 5
N
N
R
R
8
6
7
[4]helicene
[4]helicenium
DMQA+
Figure 1. Structures of [4]helicene and [4]helicenium ions.
If only two heteroatom-bridges are introduced in the trip-
henylmethylium system, [4]helicenium structures are obtained.
The helical framework is inherently chiral, which makes the
[4]helicenium compounds interesting chiral equivalents of the
bridged triphenylmethylium dyes.32,33 The [4]helicenium ions
can also be considered as a new charged subclass of the heteroh-
elicenes. A phosphorus [4]heterohelicene has been reported,34
and research involving heterohelicenes appears frequently.35–45
The [4]helicenium dyes have the general structure shown in
Figure 1. The parent [4]helicene and [4]helicenium ions are not
locked structures and the enantiomers cannot be resolved.46–48
Addition of methoxy groups at the 1 and 13 positions gives rise
to a substantial isomerization barrier and stable left and right
handed isomers.32 While the nitrogen-bridged [4]helicenium dyes
(X = Y = NR, DMQA+) have been studied extensively,49–55 only a
single paper from 2010 by Lacour and co-workers reports a five-
step synthesis and resolution of the oxygen-bridged [4]helicenium
derivative; 1,13-dimethoxy-chromeno[2,3,4-kl]xanthenium (X =
Y = O, DMCX+).56 In a theoretical study, we showed that the ob-
served racemization barrier of the DMCX+ ion may be strongly
influenced by the formation of leuco-adducts.32 The formation of
leuco-adducts will in turn depend on the cation stability (the
pKR+ value). Similarly leuco-adducts will influence the optical
properties. For these reasons, studies of the cation stability and
optical properties of DMCX+ salts with non-nucleophilic counteri-
ons are of importance if we are to understand the properties of
the new helicenium dyes. In this Letter, we report a simplified
one-step synthesis, the single crystal structure, studies of the cat-
ion stability, and the fluorescent properties of DMCXÁPF6.
DMCX+ can be envisaged as an intermediate in the synthesis of
trioxatriangulenium (TOTA+) from tris(2,6-dimethoxyphenyl)car-
benium derivative 1 as shown in Scheme 1, which takes place in
⇑
Corresponding author. Tel.: +45 35321881; fax: +45 35320214.
E-mail address: bwl@nano.ku.dk (B.W. Laursen).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.11.092
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2
T. J. Sørensen et al. / Tetrahedron Letters xxx (2012) xxx–xxx
one step from 1 using ether-cleaving reagents. The low yield is
similar to the overall yield of the previous reported five-step
synthesis.
O
O
O
O
O
Deep-orange feather-like crystals of DMCXÁPF6 were grown
from dichloromethane/ethanol allowing the structure of the helical
DMCX+ ion to be studied in detail by a single crystal X-ray analysis
(CCDC 842400). The structure is shown in Figure 2. The space
group is C 2/c and both the M- and P-enantiomers were present
in symmetry equivalent sites. The bottom aromatic ring is in-plane
with the central carbon and the two oxygen bridges. The two phe-
nyl arms are symmetrically twisted out of the molecular plane in
opposite directions creating a helical pitch of 2.7 Å. A detailed com-
parison of bond lengths showed that DMCX+ was essentially iso-
structural to DMQA+, except when considering the helical pitch.
We recently calculated the helical pitch of DMQA+ and DMCA+ as
49° and 43.0°, respectively; the values determined in the crystals
are lower at 42–44° for DMQA+ and at 39° for DMCX+.32,49 Although
the absolute value predicted by the calculations is too high by 4–7°
the difference between DMQA+ and DMCA+ is well-predicted. The
values found indicate that the electron-donating ability of the
1,13-methoxy groups is more strongly required in the DMCX+ sys-
tem, resulting in a lower helical pitch. This is to be expected when
comparing the donor strength of oxygen and nitrogen.4
In our synthesis and that reported by Lacour et al., the triiodide
salt of DMCX+ is formed. To investigate the photophysics of DMCX+
it is vital that no triiodide remains. This can be controlled by exam-
ining the absorption spectrum of DMCX+ as shown in Figure 3. A
dip in the absorption is present around 300 nm, where triiodide
absorbs. If any triiodide remains it can be seen in this window in
the DMCX+ absorption. Inspection of the absorption spectrum of
DMCX+ shows that it is a yellow–red dye with a weak absorption
in the yellow and a stronger absorption in the blue–green part of
the spectrum. The absorption bands at 375 and 450 nm correspond
to absorptions in xanthenium systems.27 A contribution to the high
energy band derives from the absorption of the chromenylium
unit.58 The two red absorptions must be due to transitions local-
ized over the entire conjugated system. It should be stated that
these findings are in direct conflict with the circular dichroism
and absorption spectrum reported in the 2010 paper by Lacour
and co-workers, where only UV transitions were observed.56 This
O
O
O
O
O
O
O
O
1
2
TOTA+
DMCX+
Scheme 1. Reaction of 1 in pyridine hydrochloride at 180 °C.
molten pyridine hydrochloride at elevated temperatures.7 How-
ever, this was not the case as it was found that cleavage of the
methyl ethers was substantially faster than bridge formation. The
cationic triphenylmethylium dyes (e.g. 1) can be readily observed
by MALDI-TOF mass spectrometry.10,57 However, when monitoring
the reaction in Scheme 1, DMCX+ was not observed at any point.
When the reaction was run to 50% conversion and the reaction
mixture purified, no trace of DMCX+ was observed. The tempera-
ture effect was investigated by running the reaction in a mixture
of pyridine and pyridinium hydrochloride between 100 and
180 °C, and in pure pyridine hydrochloride between 180 and
220 °C, in 20 °C intervals. These studies showed that dealkylation
and bridge formation in pyridine hydrochloride were not a viable
route to DMCX+.
Martin and Smith reported that refluxing 1 in 1 M HCl(aq) affor-
ded the tetramethoxy-xanthenol 3 in quantitative yield (Scheme
2).7 Thus, other ether-cleaving reagents can be used to make
bridged triphenylmethylium dyes starting from 1. It is evident that
a combination of ether-cleaving reagent and heat is required for
the process to occur. Taking these facts into account, a series of
experimental conditions were tested. Various concentrations of
sulfuric, hydrobromic, and hydrochloric acid in water and acetic
acid were used at temperatures from 80 to 130 °C. Using sulfuric
acid, the starting material was converted into unidentified com-
pounds without a trace of DMCX+ being formed. By using hydro-
chloric acid the xanthenium ion was formed quantitatively, as
reported by Martin and Smith, but only a trace of the DMCX prod-
uct was observed, which could not be isolated. It was evident from
MALDI-TOF that the ethers of 1 were cleaved, but DMCX+ was not
formed to any appreciable extent. Starting from isolated 3 allowed
for the formation of DMCX+ in a limited yield <5% by using 50% sul-
furic acid in glacial acetic acid (Scheme 2).
discrepancy is assigned to some undesired side reaction leading
À
to the formation of
a
leuco-adduct in the I3 salt first
reported.32,33,59
It was found that the balance between dealkylation and bridge
formation was best in aqueous hydrobromic acid, with the highest
yield being obtained after 120 min at 120 °C. Using these reaction
À
conditions, the title compound was isolated as its PF6 salt in a
yield of 135 mg (15%), with an additional 10% recovered material
as the fully ring-closed TOTA+. Thus, DMCX+ can be obtained in
O
O
O
O
O
O
1M HCl
36% HBr
Δ
Δ
1
O
O
O
OH
50% H2SO4
CH3COOH
O
O
O
O
O
O
Figure 2. Crystal structure of DMCXÁPF6 (CCDC 842400), both enantiomers arÀe
shown. The packing is a slipped stack with the next DMCX+ unit lying over the PF6
of this layer. Hydrogen atoms are not shown for clarity. Thermal ellipsoids are
shown at the 50% probability level.
3
2
Scheme 2. Synthesis of DMCX+ (2).
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T. J. Sørensen et al. / Tetrahedron Letters xxx (2012) xxx–xxx
3
the solutions with or without iodide and water. Also, there was
no difference when comparing the two counterions. As heating to
150 °C can be considered as being harsh condition for an organic
molecule, and that no reaction was observed in the first 2 h, it
can be concluded that DMCX+ is stable in DMSO even at elevated
temperatures.
Wavenumber (cm-1)
50000
40000
40000
30000
20000
Determination of the cation stability or pKR+ of DMCX+ was at-
tempted.8,10 It was immediately apparent that the pKR+ was lower
for DMCX+ than for TOTA+ (pKR+ = 9.05).27,7 PBS, phosphate, citrate,
and borax buffers were used together with pure water in an
attempt to determine the pKR+ of DMCX+. All the buffers either re-
acted with or precipitated the compound. For these reasons, only
an approximate value for the cation stability of DMCX+ (pKR+
ꢀ6–8) could be determined. This suggests that the formation of
leuco-adducts is likely to occur in many solvent systems. In con-
trast, the formation of leuco-adducts does not take place with
the more stable DMQA+ helicenium cation (pKR+ ꢁ19).49 A leuco-
adduct of DMCX+ is the likely source of the absorption spectra
reported by Lacour and co-workers and for the relative low racemi-
zation barrier determined in the same paper.56,32
In conclusion, we have simplified the synthesis of 1,13-dime-
thoxy-chromeno[2,3,4-kl]xanthenium and confirmed the molecu-
lar helical structure by determining the single crystal structure.
We have shown that the molecule is chemically stable under high
temperature conditions as both the triiodide and hexafluorophos-
phate salts. We estimated the pKR+ of DMCX+ to be in the range
of 6 to 8, emphasizing the importance of considering the possibility
of the formation of leuco-adducts, which may influence both the
spectral properties and the racemization barriers. Furthermore,
we have shown that DMCX+, when in its cationic state, is a potent
dye with emission in the red.
30000
20000
10000
0
200
300
400
500
λ (nm)
600
700
800
Wavenumber (cm-1
25000 22500 20000 17500 15000 12500
)
3
3
2
1
0
Excitation spectrum (em 650 nm)
Emission spectrum (ex 550 nm)
Asorption spectrum
2
1
0
500
550
600
650
700
750
800
λ (nm)
Figure 3. Top: absorption spectrum of DMCXÁPF6 in acetonitrile. Bottom: absorp-
tion, excitation, and emission spectra of DMCXÁPF6 in dichloromethane.
Acknowledgments
The Danish Research Council for Independent Research/Natural
Sciences (instrument Grant #09-066663), The Danish Research
Council for Independent Research/Technology and Production Sci-
ences (Grant #10-093546), and The Danish National Research
Foundation under the Danish-Chinese Centre for Self-Assembled
Molecular Electronic Nanosystems are gratefully acknowledged
for financial support.
We proceeded to investigate the fluorescence emission of
DMCX+. The emission obeys the mirror image rule and a 0–0 tran-
sition energy of 597 nm (16,750 cmÀ1) can be determined. The
excitation spectra and absorption spectrum were identical to the
low energy transitions. The quantum yield of fluorescence (afl)
was determined against cresyl violet and was found to be low
and highly solvent dependent, see Table 1. The quantum yield in
dichloromethane was only 1.5% and dropped as the solvent polar-
Supplementary data
ity increased. The excited state lifetime (
mined in dichloromethane and was found to be 2.3 ns, resulting
in a radiative lifetime (
0) of 153 ns. This long radiative lifetime ex-
plains the low quantum yield, as quenching by oxygen and others,
even very inefficient, quenching pathways can compete with the
slow rate of emission.
sfl) could only be deter-
Supplementary data (the synthetic procedure, crystallographic
details, NMR spectra, quantum yield determinations, TC-SPC data,
and example pKR+ measurements and stability tests) associated
with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.tetlet.2012.11.092.
s
The chemical stability of the DMCX+ cation was evaluated by
heating a solution of the compound as the triiodide and hexa-
fluorophosphate salt in dry DMSO-d6 to 150 °C for 2 and 72 h. No
reaction was observed after 2 h, in contrast to the observation
made by Lacour and co-workers,56 thus water and iodine were
added to a series of solutions and left for an additional 72 h. After
these long reaction times all the solutions showed 20–25% conver-
sion of DMCX+ into TOTA+; no difference was observed between
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Private communication.
Please cite this article in press as: Sørensen, T. J.; et al. Tetrahedron Lett. (2012), http://dx.doi.org/10.1016/j.tetlet.2012.11.092