Synthesis and structures of n-alkyl-1,13-dimethoxychromeno-[2,3,4-kl] acridinium salts: The missing azaoxa[4]helicenium

Article abstract of DOI:10.1002/chem.201400082

Helical structures are interesting due to their inherent chirality. Helicenium ions are triarylmethylium structures twisted into configurationally stable helicenes through the introduction of two heteroatom bridges between the three aryl substituents. Of the configurationally stable [4]helicenium ions, derivatives with sulfur, oxygen and nitrogen bridges have already been synthesised. However, one [4]helicenium ion has proven elusive, until now. We present herein the first synthesis of the 1,13-dimethoxychromeno[2,3,4-kl] acridinium (DMCA⁺) [4]helicenium ion. A series of six differently N-substituted DMCA⁺ ions as their hexafluorophosphate salts are reported. Their cation stability was evaluated and it was found that DMCA ⁺ is ideally suited as a phase-transfer catalyst with a pK _R+ of 13.0. The selectivity of nucleophilic addition to the central carbon atom of DMCA⁺ has been demonstrated with diastereotopic ratios of up to 1:10. The single-crystal structures of several of the DMCA⁺ salts were determined, and structural differences between N-aryl- and N-alkyl-substituted cations were observed. The results of a comparative study of the photophysics of the [4]helicenium ions are presented. DMCA⁺ is found to be a potent red-emitting dye with a fluorescence quantum yield of 20 % in apolar solvents and a fluorescence lifetime of 12ns. [4]Helicenium ions, including DMCA⁺, all suffer from solvent-induced quenching, which reduces the fluorescence quantum yields significantly (φ_fl<5 %) in polar solvents. A difference in photophysical properties is observed between N-aryl- and N-alkyl-substituted DMCA⁺, which has tentatively been attributed to a difference in molecular conformation. Last but not least! The last of the azaoxa[4]helicenium ions has been synthesised and studied in detail. It is found to be a potent red dye, a stable cation and a structure that can be perturbed depending on the nature of the substituent on the nitrogen atom (see figure).

Full text of DOI:10.1002/chem.201400082

DOI: 10.1002/chem.201400082
Full Paper
&
Helical Structures
Synthesis and Structures of N-Alkyl-1,13-dimethoxychromeno-
[2,3,4-kl]acridinium Salts: The Missing Azaoxa[4]helicenium
Thomas Just Sørensen, Anders Ø. Madsen, and Bo W. Laursen*[a]
Abstract: Helical structures are interesting due to their in-
herent chirality. Helicenium ions are triarylmethylium struc-
tures twisted into configurationally stable helicenes through
the introduction of two heteroatom bridges between the
three aryl substituents. Of the configurationally stable
[4]helicenium ions, derivatives with sulfur, oxygen and nitro-
gen bridges have already been synthesised. However, one
[4]helicenium ion has proven elusive, until now. We present
herein the first synthesis of the 1,13-dimethoxychromeno-
[2,3,4-kl]acridinium (DMCA+) [4]helicenium ion. A series of
six differently N-substituted DMCA+ ions as their hexafluoro-
phosphate salts are reported. Their cation stability was eval-
uated and it was found that DMCA+ is ideally suited as
a phase-transfer catalyst with a pKR+ of 13.0. The selectivity
of nucleophilic addition to the central carbon atom of
DMCA+ has been demonstrated with diastereotopic ratios
of up to 1:10. The single-crystal structures of several of the
DMCA+ salts were determined, and structural differences be-
tween N-aryl- and N-alkyl-substituted cations were observed.
The results of a comparative study of the photophysics of
the [4]helicenium ions are presented. DMCA+ is found to be
a potent red-emitting dye with a fluorescence quantum
yield of 20% in apolar solvents and a fluorescence lifetime
of 12 ns. [4]Helicenium ions, including DMCA+, all suffer
from solvent-induced quenching, which reduces the fluores-
cence quantum yields significantly (f <5%) in polar sol-
fl
vents. A difference in photophysical properties is observed
between N-aryl- and N-alkyl-substituted DMCA+, which has
tentatively been attributed to a difference in molecular con-
formation.
Introduction
The synthesis of carbo- and heterohelicenes has proven a chal-
lenge,[1] which has spurred on considerable efforts towards the
synthesis and resolution of various types of helical structures.[2]
[4]Helicene is the smallest helicene (Figure 1), with four con-
densed aromatic rings. The helical pitch of [4]helicene is open,
and interconversion between the enantiomeric forms occurs
rapidly under ambient conditions. If bulky substituents are in-
troduced into the helical pitch, stable [4]helicenes can be syn-
thesised.[3] Neutral, nitrogen-centred [4]helicenes have been
studied in detail,[4] and cationic, carbon-centred [4]helicenium
molecules, conformationally stabilised by methoxy groups in
the pitch (Figure 1), have recently gained considerable atten-
tion due to their facile synthesis and chiral resolution. [4]Heli-
cenium ions (Figure 1) have been explored as dyes,[5] chiral
platforms[6] and also for their unique chemistry.[7,8] [4]Heliceni-
um molecules are synthesised by nucleophilic aromatic substi-
tution (SNAr),[7e,9] and a similar synthetic pathway was also re-
cently used to synthesise the first [6]helicenium cations.[10] Al-
Figure 1. [4]Helicenes and helicenium ions.
though [6]helicenium ions have been prepared and reported
with aza/aza, aza/oxa and oxa/oxa bridges (X/Y in Figure 1),
[4]helicenium ions have proven more elusive as the selectivity
of the oxa-bridge formation is low.[9a,b] Nucleophilic insertion
and aza-bridge formation of primary amines occur stepwise
[a] Dr. T. J. Sørensen, Dr. A. Ø. Madsen, Prof. Dr. B. W. Laursen
Nano-Science Center and Department of Chemistry
University of Copenhagen, Universitetsparken 5
DK2100 København Ø (Denmark)
starting
from
tris(2,6-dimethoxyphenyl)methylium
(1+,
Scheme 1),[11] whereas the dealkylation of methoxy groups and
oxa-bridge formation is less well behaved. The aza/aza-bridged
[4]helicenium system (R2-DMQA+, Figure 1) was the first to be
synthesised, followed recently by the oxa/oxa-bridged system
Fax: (+45)35-32-02-14
E-mail: bwl@nano.ku.dk
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201400082.
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attack by hydroxy groups re-
leased by ether cleaving re-
agents.[7a,9a,b,d,11c,17] In this work,
DMCA+ was synthesised by the
second route, as presented in
Scheme 1.
Tris(2,6-dimethoxyphenyl)me-
thylium (1+) was synthesised as
described previously and isolat-
ed as its tetrafluoroborate
salt.[11b,14] Cation 1+ was treated
with primary amines in acetoni-
trile to afford N-alkyltetrame-
thoxyacridinium 2+ (TM-Acr, 1,8-
dimethoxy-9-(2,6-dimethoxy-
Scheme 1. Synthesis of N-alkyl-1,13-dimethoxychromeno[2,3,4-kl]acridinium (3+, DMCA+).
(DMCX+).[9a,b] In this work we have partially solved the synthet-
ic challenge of the selective oxygen ring closure in the first
synthesis of the aza/oxa-bridged derivative N-alkyl-1,13-
dimethoxychromeno[2,3,4-kl]acridinium[12] (3+, DMCA+).
Helicenium ions can be viewed as bridged triarylmethylium
dyes. Although the flexible parent compounds are non-fluores-
cent, the locked structures are potent fluorophores.[13] The
mono-bridged derivatives, xanthenium and acridinium, bearing
donor groups on the periphery, are the best known, that is,
the Rhodamine, Fluorescein and Acridine Orange dyes. Be-
cause of the cation stability of xanthenium (pKR+ =2.5) it has
found no applications,[14] whereas acridinium dyes are used as
chloride sensors.[15] Helicenium ions are triarylmethylium dyes
with two bridges. N,N’-Dialkyl-1,13-dimethoxyquinolino-
[2,3,4-kl]acridinium (DMQA+, X=Y=NꢀR in Figure 1) and 1,13-
dimethoxychromeno[2,3,4-kl]xanthenium (DMCX+, X=Y=O in
Figure 1) are red dyes, with poor-to-mediocre quantum
yields.[5,9b] Helicenium ions suffer from solvent-induced
quenching, which results in very low quantum yields in polar
solvents. This issue is alleviated if a third bridge is introduced.
The resulting triangulenium dyes are strong emitters with high
quantum yields, highly polarised transitions and long emission
lifetimes.[11b,c,16]
phenyl)-10-alkylacridinium), which was isolated by precipitation
with aqueous potassium hexafluorophosphate or sodium tetra-
fluoroborate in quantitative yields. Further purification by re-
crystallisation in methanol, or in the case of 2g chromatogra-
phy, yielded analytically pure 2·PF6/BF4 in good-to-excellent
yields.
Single ring closure of 2+ to form 3+ was achieved by heat-
ing 2+ in a mixture of acetic acid and 50% sulfuric acid. The
major byproduct was the fully ring-closed compound azadioxa-
triangulenium 4+. To isolate 3+, chromatography was per-
formed on the neutral centre adduct 3-H (Scheme 1, inset), ac-
quired by reaction of the crude product obtained above with
NaBH4 in DMSO. This allowed easy separation of 3-H from 4-H
by flash chromatography. Oxidation by using elemental io-
dine[9a,b] followed by ion exchange resulted in the isolation of
the hexafluorophosphate salt of 3+ in moderate-to-excellent
yields. The key steps with regards to the yield are the four pre-
cipitations required for the ion exchange. Here, significant
amounts of material can be lost. This is particularly problemat-
ic with the more lipophilic side-chains, octyl (2c) and 2-ethyl-
hexyl (2d), and the water-soluble methyl derivative (2a).
Unlike the two previously reported [4]helicenium systems
(DMQA+ and DMCX+), DMCA+ has no axis of symmetry
(Figure 1). As a consequence, the NMR spectrum cannot be
readily assigned. Through extensive NMR studies using COSY,
NOESY, TOCSY, HMBC and HSQC, the spectrum of 3b could be
fully assigned. The key correlations are shown in Figure 2.
Starting from the protons on the a-carbon atom on the N-sub-
stituent, NOESY enabled the assignment of the two protons
ortho to the aza bridge, and NOESY and TOCSY allowed the as-
signment of the proton that is in a ring system containing
a methoxy substituent, and which of the protons with NOE
from the proton on the a-carbon on the alkyl chain is on the
ring with the aza/oxa bridges. For the full assignment see the
Supporting Information. The protons on the aromatic ring with
two bridges are the least shielded, and the protons on the aro-
matic ring with two oxygen substituents are the most efficient-
ly shielded.
Herein we report on the synthesis of six different derivatives
of N-alkyl-1,13-dimethoxychromeno[2,3,4-kl]acridinium (3+,
DMCA+) and the product of addition of hydride to the central
carbon atom. We discuss the cation stability and single-crystal
structures of three different derivatives. Finally, the photophys-
ics of the three types of aza/oxa[4]helicenium ions are com-
pared, and the DMQA+ and DMCA+ dye systems are shown to
be potential fluorophores in apolar media.
Results and Discussion
Synthesis
Two synthetic pathways to helicenium ions have been report-
ed. A stepwise route, in which a xanthone-like structure is
formed and then treated with a carbon nucleophile to incorpo-
rate the third arm into the structure,[7e,10] or a direct route, in
which the molecular framework is initially formed and then
bridged by either external attack by primary amines or internal
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nating heteroatoms on which charge can be partially localised,
whereas the triangulenium ions have perfectly flat conjugation
pathways, but only three heteroatoms in the structure. The
cation stabilities of the aza/oxa[4]helicenium and aza/oxatrian-
gulenium ions are compiled in Table 1. A cursory inspection of
Table 1. Cation stabilities, measured as pKR+, of the [4]helicenium and tri-
angulenium ions. In all cases the propyl derivatives were used.
Parent compound
[4]helicenium
X
Y
Z
pKR+
O
N
N
O
O
N
–
–
–
6–8[a]
13.0[b]
19.1[c]
triangulenium
O
N
N
N
O
O
N
N
O
O
O
N
9.1[d]
14.5[e]
19.4[e]
23.7[e]
Figure 2. 1D NMR spectrum of 3b showing NOESY and TOCSY cross-correla-
tions.
[a] Values from ref. [9b]. [b] Slope of line=0.957 and corresponding R2 =
0.994. [c] Values from ref. [8]. [d] Values from ref. [14]. [e] Values from
ref. [11b].
Cation stability and centre adducts
The cation stability is measured by the pKR+ value, which re-
flects the pH at which the cation and carbinol are present in
equal quantities.[11b,18] For very stable carbocations, the pH
scale can be extended beyond aqueous media by using
DMSO/water mixtures containing tetramethylammonium hy-
droxide and the C scale.[11b] The pKR+ value is a measure of the
stability of the neutral OH centre adduct relative to that of the
cation. Figure 3 shows the results of the determination of the
cation stability of DMCA+: a value of 13.0 was found. The
value is inside the pH range of water, as can be observed as
decolouration occurs when DMCA+ is dissolved in concentrat-
ed sodium or potassium hydroxide, but determination of pKR+
was not possible in highly concentrated alkaline solutions and
the C scale had to be used.
the data shows that the number of nitrogen atoms in the mo-
lecular structure is decisive for cation stability. On closer in-
spection it can be seen that the more nitrogen bridges there
are in the [4]helicenium structure, the less the loss of conjuga-
tion (due to twisting) as compared with the stability of the cor-
responding triangulenium ion.
Although DMCX+ has a cation stability that is too low for it
to be considered a stable species in aqueous solution, DMCA+
will be the dominant species up to pHꢁ11 ([ROH]![R+]).
Whereas DMQA+ and the triangulenium ions with two or
three nitrogen bridges are too stable to form the correspond-
ing carbinols in aqueous solution, DMCA+ has a cation stability
such that centre adducts can be formed in a polar phase in
the presence of a high concentration of nucleophiles and then
subsequently be transferred, as a neutral species, to a less
polar phase. Thus, the stability of the DMCA+ helicenium ion is
in a very attractive range for application as a chiral phase-
transfer catalyst.[19]
The thermodynamic stability of a carbocation is to a large
extent determined by the ability of the molecular structure to
delocalise charge in a conjugated system. The [4]helicenium
ions have a twisted conjugation pathway and four electron-do-
To investigate the selectivity of the DMCA framework as
a potential chiral catalyst, we scrutinised the hydride centre ad-
ducts, formed as intermediates in the synthesis of all the deriv-
atives. Furthermore, we chose to investigate the methyl and
cyanomethyl adducts of N-propyl-DMCA+ (3b). Addition of
a nucleophile or hydride to DMCA+ potentially leads to the
formation of two diastereomers (see Scheme 2). Lacour and
co-workers investigated the diastereoselective addition of nu-
cleophiles (hydride and organolithium) to unsymmetrical
DMQA+ helicenium systems having two different peripheral
substituents on the two nitrogen atoms in the otherwise sym-
metrical helical framework.[7a] They assigned the selectivity to
Figure 3. Determination of the cation stability of DMCA+. The concentra-
tions were determined by absorption; Pr-DMCA·PF6 was used in the experi-
ments.
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Table 2. Selectivity of different derivatives of DMCA+ in reactions with
anionic nucleophiles.[a]
Entry
Side-chain
Adduct
d.r.
1
2
3
4
5
6
7
8
propyl
propyl
methyl
propyl
-CH3
-CH2CN
-H
-H
-H
-H
-H
-H
20:80
17:83
17:83
9:91
octyl
9:91
ethylhexyl
phenyl
naphthyl
17:83
25:75
20:80
[a] The diastereomeric ratio was determined from the integration of the
1H NMR spectra of the hydrogen atom(s) added to the central carbon
atom, see the Supporting Information for examples.
Scheme 2. Structural implications of nucleophilic attack on DMCA+.
(see below). The diastereoselectivity is largest in the simple
alkyl systems, which indicates that steric bulk is more impor-
tant for the selectivity than the nature of the nitrogen atom. In
fact, trigonal-planar nitrogen atoms with aryl substituents
reduce the selectivity.
a difference in the structure around the nitrogen atoms, which
is dependent on the nature of their substituents, that is, an N-
alkyl substituent results in a pyramidal nitrogen[16f,g] whereas
an N-aryl substituent gives a trigonal-planar nitrogen.[16a] If the
bulk of the aryl substituent is increased by using mesityl in-
stead of phenyl, the diastereoselectivity can be increased fur-
ther. Attack of a nucleophile on the [4]helicenium structure
can proceed by addition at either the Re or Si face of the struc-
ture (note the terminology is reversed in DMCA+ and DMQA+,
see the Supporting Information). For a given helicity (M/P),
attack at a given face must result in planarisation of a given
arm of the helical structure. For DMCA+ nucleophilic addition
at the Re face of the M enantiomer will result in planarisation
of the xanthene system, whereas addition at the Si face will
result in a planar acridine system (see Scheme 2). If planarisa-
tion of either system is energetically favoured, there will be
a marked difference in the thermodynamic stability of the two
diastereomers. The kinetic selectivity is expected to be gov-
erned by the accessibility of a face, for which the bulk of the
substituent on the nitrogen atom will be the critical factor. The
work of Lacour and co-workers indicates that kinetic factors,
that is, steric bulk, are more important for the diastereoselec-
tivity in the reduction of DMQA+ to H-DMQA than for the ad-
dition of organolithium reagents.[7a]
In DMCA+, one side of the molecular structure is open, as
the oxygen heteroatom carries no substituent. Based on the
results of Lacour and co-workers,[7a] we expected that we
would be able to obtain a much higher diastereoselectivity.
This assumption is based on the large difference in donor
strength between oxygen and nitrogen, which should result in
a clear preference for creating a planar acridine system. As it
turned out, the data clearly shows that planarisation of the
xanthene system is favoured over planarisation of the acridine
system, even with aryl substituents on the acridine nitrogen.
The diastereomeric ratios of the centre adducts formed by
nucleophilic attack on DMCA+ are shown in Table 2. The data
show that the values of d.r. range from 25:75 to 9:91, with the
major isomer being the species with a planar xanthene system
Scheme 2 also includes the option that the nucleophilic
attack is accompanied by inversion of the helicene (isomerisa-
tion). However, inversion is not expected to take place as the
energy barrier was determined to be around 140 kJmolꢀ1 [3]
.
Furthermore, the computational study showed that although
the barrier to racemisation is reduced upon formation of the
centre adduct, this effect is far too small to make this pathway
favourable at ambient temperatures.
Molecular structure
We were fortunate that several compounds crystallised, either
by recrystallisation or by slow evaporation of solvent from
CH2Cl2/methanol, CH2Cl2/ethanol or CH2Cl2/heptane mixtures.
Compound 2d·BF4 crystallised as a racemate with some disor-
der in the side-chain, whereas, of the title compounds, N-
methyl-DMCA·PF6 (3a·PF6), N-propyl-DMCA·PF6 (3b·PF6) and N-
phenyl-DMCA·PF6 (3e·PF6) crystallised neatly, allowing their
single-crystal structures to be directly determined. In all cases,
both the M and P isomers were found in the unit cell. Figure 4
shows a single isomer of each of the compounds. Little varia-
tion as a function of the side-chain is immediately apparent;
the helical structures are close to identical irrespective of the
nature of the side chain, and all compounds display a signifi-
cant helical pitch.
However, significant differences between the three DMCA+
derivatives can be found on closer inspection. Table 3 shows
selected crystallographic information, including a measure of
the helical pitch and the local structure around the nitrogen
atom measured by the out-of-plane displacement (N OOP, for
more detailed information and definitions see the Supporting
Information). A large difference can be seen between the heli-
cal pitches of the alkyl- and aryl-substituted DMCA+ structures,
with the pitch of the former being 44.2/44.18 (3a/3b) and that
of the latter reduced to 41.48 (3e). The origin of the shallower
pitch is not clear, as the nature of the nitrogen atom appears
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Figure 4. Molecular structures of N-methyl-DMCA·PF6 (3a·PF6; top left), N-
propyl-DMCA·PF6 (3b·PF6; top right), N-phenyl-DMCA·PF6 (3e·PF6; bottom
left), and N-propyl-DMCA·PF6 (3b·PF6; side view, bottom right) determined
by single-crystal diffraction analysis. The structures are shown with ellipsoids
drawn at the 50% probability level.
Figure 5. Molecular structures of N-propyl-13b-methyl-DMCA (Me-3b) deter-
mined by single-crystal diffraction analysis. The structures are shown with el-
lipsoids drawn at the 50% probability level.
cantly different, with different helical pitches and different
pitch symmetries (see Table 3 and Table S1 in the Supporting
Information). These differences can only be due to packing or
crystal effects and highlight the necessity of further studies if
the substituent-induced structural perturbations in the [4]heli-
cenium ions are to be elucidated. The key point is that the
major diastereomer formed is the one in which the xanthene
system is planar (M,R or P,S). We hope to investigate and docu-
ment this point further in future work.
Table 3. Selected crystallographic data.
Entry
3a·PF6
3b·PF6
propyl
3e·PF6
phenyl
Me-3b
propyl
methyl
1
2
3
4
5
6
isomer(s)
Z
space group
pitch[a] [8]
calcd[a]
N OOP[b]
P/M
4
P21/c
44.16
46.09
0.015
P/M
2
¯
P1
44.10
–
0.267
P/M
2
¯
P1
41.41
–
0.031
M,R
4
P21
49.01/50.57
–
0.116/0.131
Photophysics
[a] See ref. [3] for details. [b] Nitrogen out-of-plane displacement, as de-
fined in ref. [7a].
As we had all three aza/oxa[4]helicenium ions in hand, we per-
formed a comparative study of their photophysical properties
under identical conditions. The data are compiled in Table 4.
We have previously published the data for DMCX+,[9b] and the
photophysics of DMQA+ has also been examined in detail by
Lacour and Vauthey and their co-workers.[5] Excitation spectra
and spectra showing the solvatochromism of DMCA+ can be
found in the Supporting Information.
Figure 6 shows the absorption spectra of DMCX+, DMCA+
and DMQA+. The first absorption band of the [4]helicenium
dyes is redshifted from 580 to 588 to 616 nm, as the number
of nitrogen atoms in the molecules increases. Similarly, the
molar absorptivity increases from 3100 to 9000 to
18100mꢀ1 cmꢀ1. The main absorption band involves the entire
conjugated system and it is evident that the oxygen donors
are not as directly involved as the nitrogen donors. The second
band at around 550 nm primarily involves the transfer of elec-
tron density from the methoxy-substituted aromatic rings;[5]
these are of a similar energy for all three species irrespective of
the nature of the electron donors. The band at around 450 nm
is strong in all spectra and corresponds to a local transition in
the xanthenium and acridinium systems. As xanthenium is
a better chromophore than acridinium, the band is strongest
not to be related to the shallowness of the pitch. In N-methyl-
DMCA+ (3a) and N-phenyl-DMCA+ (3e), the nitrogen atom is
strictly trigonal planar, whereas in N-propyl-DMCA+ (3b) it is
pyramidal (entry 6, Table 3). If the symmetry of the pitch is
measured (see Table S1 in the Supporting Information), it can
be concluded that the aryl substituent causes a flattening of
the acridinium system compared with the xanthenium system,
which results in the smaller pitch observed in the crystal struc-
ture of 3e. Note that the planar-trigonal nitrogen in N-methyl-
DMCA+ (3a) does not lead to a flat acridinium system.
A single isomer (R,M) of the centre adduct formed by the ad-
dition of methyllithium to N-propyl-DMCA·PF6 (methyl-3b)
crystallised in the chromatographic eluent. The NMR-scale reac-
tion provided three large crystals suitable for single-crystal X-
ray diffraction analysis; unfortunately, only a single data set of
sufficiently good quality to determine the structure was ob-
tained. With a d.r. ratio of 20:80, we concluded that the crystal
could only consist of one of the enantiomers of the major dia-
stereomeric couple. Four molecules are present in the unit cell
occupying two sites (see Figure 5). The structures are signifi-
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[b]
Table 4. Photophysical properties of DMCX·PF6,[a] N-propyl-DMCA·PF6 and N,N’-dipropyl-DMQA·BF4 in various solvents.
[4]Helicenium ion
Pr2DMQA+
Solvent
lmax
e[c]
lfl
Stokes’ shift
f [d]
[%]
tfl [ns]
tav,ampl
fl
[nm]
[cmꢀ1 mꢀ1
]
[nm]
[cmꢀ1
]
(amplitude contribution [%])
[ns]
CH2Cl2
MeCN
H2O/DMSO[e]
616
615
618
18100
17500
15000
650
661
666
850
1130
1170
33
8
2
11.4
3.5 (79) 7.2 (21)
1.5 (67) 9.4 (33)
11.4
4.6
4.1
PrDMCA+
PhDMCA+
DMCX+
CH2Cl2
MeCN
MeOH
588
584
583
9000
8400
8800
623
630
628
960
1250
1230
22
6
5
12.0
4.2 (99) 13 (1)
3.7 (99) 14 (1)
12
4.3
3.9
CH2Cl2
MeCN
MeOH
599
592
591
9400
8800
10600
630
633
634
820
1090
1150
18
5
3
10.2
3.4 (99) 11 (1)
3.1 (99) 12 (1)
10.2
3.5
3.2
CH2Cl2
MeCN
50
570
3100
2900
613
615
930
1280
2
0.05
2.3
--[f]
2.3
--[f]
[a] See ref. [9b]. [b] See ref. [8]. [c] An experimental error of 10% is assumed. [d] A minimum error of 5% is assumed. Determined by using Cresyl Violet as
reference (Ffl. =0.54, MeOH).[20] [e] 95% water with 5% DMSO. [f] Could not be determined due to the very low intensity.
Figure 7. Emission spectra of the [4]helicenium ions in acetonitrile following
Figure 6. Absorption spectra of the [4]helicenium ions in acetonitrile. Con-
excitation at 545 (DMCX), 550 (DMCA) and 448 nm (DMQA). Concentrations
are kept below 10ꢀ5 m.
centrations are kept below 10ꢀ4 m.
in DMCX+. The bands at higher energies arise from transitions
localised in the chromeno and quinolino substructures.
enced by the solvent, and particularly by the solvent polarity.
The decay kinetics become multi-exponential in polar solvents,
which suggests multiple populations or conformations present
in solution. The exact mechanism of the solvent-induced
quenching is still not known.
Figure 7 shows the emission spectra of the [4]helicenium
dyes. As for the absorption maximum, the emission maximum
is redshifted from 613 to 623 to 650 nm as the number of ni-
trogen donors in the structures increases. The Stokes’ shift is
identical at 35 nm (820–960 cm.1) for the three dyes. The emis-
sion maximum of DMCA+ is not solvent-dependent, but in-
creases with solvent polarity for DMQA+. The fluorescence
quantum yield is highly solvent-dependent for all the [4]helice-
nium ions (Table 4). If the values determined in CH2Cl2 are com-
To study the effect of the nitrogen substituent, the photo-
physical properties of N-propyl-DMCA+ (3b) and N-phenyl-
DMCA+ (3e) were studied. Note that the solid-state structures
of the two compounds are significantly different. The nitrogen
donor in N-phenyl-DMCA+ is fully sp2-hybridised, whereas it
has a greater degree of sp3 character in N-propyl-DMCA+. This
should be evident in the spectra. The introduction of a phenyl
group instead of a propyl group is followed by a 10 nm red-
shift of the absorption and emission, which confirms our as-
sumption. A slight increase in molar absorptivity and a shorten-
ing of the fluorescence lifetime indicate that N-phenyl-DMCA+
is a marginally stronger chromophore. Similar effects were ob-
served to have a large impact on the dye properties of phenyl-
substituted triazatriangulenium (TATA+).[16a] However, in this
case the effect on the quantum yield is small and should be
treated with caution.
pared, DMQA+ is the most emissive with f =33%, followed
fl
by DMCA+ with f =22% and DMCX+ with f =2%. The fluo-
fl
fl
rescence lifetimes are long in all systems, as expected for
medium-strength oscillators.[21]
If the solvatochromism of the [4]helicenium ions is com-
pared, minor shifts in band positions and widths are found.
The energetics of the electronic states of the ions are not
strongly perturbed by the solvent. Table 4 includes the quan-
tum yields determined for DMCX+, DMCA+ and DMQA+ in dif-
ferent solvents. It is evident that the kinetics governing the de-
population of the excited states in the dyes are strongly influ-
Chem. Eur. J. 2014, 20, 6391 – 6400
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6396
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graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Conclusion
We have developed an efficient route for the synthesis of N-
alkyl-1,13-dimethoxychromeno[2,3,4-kl]acridinium
(DMCA+),
and have tested it by synthesising six different derivatives. The
cation stability of the DMCA+ structure was determined to be
pKR+ =13.0. As a consequence, DMCA+ ions are stable in
aqueous media, but can be forced to the neutral carbinol form
at high pH. The chiral nature of the [4]helicenium structure
was investigated through the formation of adducts at the cen-
tral carbon atom. This resulted in diastereomeric molecules
that could be differentiated by NMR spectroscopy, and the dia-
stereomeric ratios were found to range between 25:75 and
9:91 depending on the N-substituent. The mechanism for the
stereoselectivity has been discussed.
Synthesis
General procedure for the preparation of 10-substituted 1,8-di-
methoxy-9-(2,6-dimethoxyphenyl)acridinium
hexafluorophos-
phate (2): Tris(2,6-dimethoxyphenyl)carbenium tetrafluoroborate
(1; 1 g, 2 mmol) was dissolved in acetonitrile (50 mL) and amine
(4 equiv) was added. The reaction mixture was stirred for 2 h after
which the reaction mixture was poured into 0.2m aqueous KPF6
(400 mL). The crude product was collected by filtration and recrys-
tallised from methanol (100 mLgꢀ1).
1,8-Dimethoxy-9-(2,6-dimethoxyphenyl)-10-methylacridinium
hexafluorophosphate (2a): Yield: 5.5 g (98%); dark-red powder.
1H NMR (500 MHz, DMSO): d=8.26 (dd, J=9.1, 8.0 Hz, 2H), 8.15 (d,
J=8.7 Hz, 2H), 7.44 (t, J=8.4 Hz, 1H), 7.21 (d, J=7.9 Hz, 2H), 6.81
(d, J=8.4 Hz, 2H), 4.72 (s, 3H), 3.53 (s, 3H), 3.52 ppm (s, 3H);
13C NMR (126 MHz, DMSO): d=159.69, 155.64, 155.28, 141.87,
139.55, 129.24, 119.10, 119.05, 110.25, 106.72, 103.69, 57.10, 55.81,
40.65 ppm; UV/Vis (MeCN): l [log(e/mꢀ1 cmꢀ1)]=530 sh [3.60], 497
[3.70], 467 sh [3.60], 397 [3.78], 356 [3.40], 337 [3.18], 286 [4.9],
246 nm [4.29]; elemental analysis calcd (%) for C24H24NO4PF6: C
53.84, H 4.52, N 2.62; found: C 53.81, H 4.45, N 2.52.
The single-crystal structures and photophysical properties of
DMCA+ were studied. In the solid-state, the nitrogen substitu-
ent changes the local structure around the nitrogen atom, and
the absorption and emission bands are redshifted by 10 nm
when an N-alkyl substituent is replaced by an N-aryl substitu-
ent. DMCA+ was shown to be a red dye with a moderate
quantum yield in apolar solvents and a poor quantum yield in
polar solvents. The reason for the solvent-induced quenching
is a subject for further study.
1,8-Dimethoxy-9-(2,6-dimethoxyphenyl)-10-propylacridinium
hexafluorophosphate (2b): Yield: 5.8 g (64%); dark-red needles.
1H NMR (500 MHz, DMSO): d=8.27 (t, J=8.6 Hz, 2H), 8.12 (d, J=
9.2 Hz, 2H), 7.43 (t, J=8.4 Hz, 1H), 7.22 (d, J=8.0 Hz, 2H), 6.81 (d,
J=8.4 Hz, 2H), 5.15 (m, 2H), 3.53 (s, 6H), 3.52 (s, 6H), 2.11 (m, 2H),
1.22 ppm (t, J=7.3 Hz, 3H); 13C NMR (126 MHz, DMSO): d=159.92,
155.98, 155.25, 140.96, 140.02, 129.26, 119.12, 119.06, 109.75,
106.82, 103.67, 57.11, 55.85, 53.15, 21.16, 10.64 ppm; UV/Vis
(MeCN): l [log(e/mꢀ1 cmꢀ1)]=530 sh [3.60], 497 [3.70], 467 sh
[3.60], 397 [3.78], 356 [3.40], 337 [3.18], 286 [4.9], 246 nm [4.29]; el-
emental analysis calcd (%) for C26H28NO4PF6: C 55.42, H 5.01, N
2.49;found: C 55.44, H 4.98, N 2.36.
Experimental Section
General
Chemicals and solvents were used as received. The synthesis of
tris(2,6-dimethoxyphenyl)carbenium tetrafluoroborate (1) was
adapted from refs. [8,11b,14]. Detailed information on the experi-
mental procedures involved in the determination of pKR+ values
can be found in ref. [11b]. Absorption spectra were recorded on
a Perkin–Elmer Lambda 1050 spectrophotometer in cuvettes with
a pathway of 1 cm. Fluorescence excitation and emission spectra
were recorded in
a conventional L-shaped configuration on
a Horiba Fluorolog 3 spectrofluorimeter equipped with an ana-
logue TC-SPC accessory.
1,8-Dimethoxy-9-(2,6-dimethoxyphenyl)-10-octylacridinium tet-
rafluoroborate (2c): Yield: 900 mg (80%); red powder. 1H NMR
(500 MHz, DMSO): d=8.28 (dd, J=9.1, 8.0 Hz, 2H), 8.09 (d, J=
9.2 Hz, 2H), 7.43 (t, J=8.4 Hz, 1H), 7.22 (d, J=8.0 Hz, 2H), 6.81 (d,
J=8.4 Hz, 2H), 5.18 (m, 2H), 3.53 (s, 6H), 3.52 (s, 6H), 2.06 (m, 2H),
1.66 (dt, J=15.3, 7.7 Hz, 2H), 1.42 (dt, J=14.6, 6.8 Hz, 2H), 1.34 (m,
2H), 1.29 (m, 4H), 0.88 ppm (t, J=6.9 Hz, 3H); 13C NMR (126 MHz,
DMSO): d=159.94, 155.96, 155.25, 140.92, 140.06, 129.26, 119.14,
119.07, 109.66, 106.82, 103.68, 57.11, 55.83, 51.91, 31.20, 28.68,
28.66, 27.75, 25.85, 22.07, 13.96 ppm; UV/Vis (MeCN): l [log
(e/mꢀ1 cmꢀ1)]=530 sh [3.60], 497 [3.70], 467 sh [3.60], 397 [3.78],
356 [3.40], 337 [3.18], 286 [4.9], 246 ppm [4.29]; elemental analysis
calcd (%) for C26H28NO4BF4: C 64.70, H 6.66, N 2.43; found: C 64.67,
H 6.69, N 2.36.
Crystallography
Single crystals were selected by using a polarising microscope. Dif-
fraction data were collected on a Nonius–KappaCCD diffractometer
with graphite-monochromated Mo Ka radiation. The temperature
was maintained at 122.4(5) K by using an Oxford Cryosystems low-
temperature liquid nitrogen cooling device. Data reduction was
performed by using the Nonius COLLECT suite of programs.[22] The
structures were obtained by direct methods using SHELXS-97 and
refined by full-matrix least-squares methods against F2 using the
SHELXL-97 program.[23] Positions and anisotropic displacement pa-
rameters were refined for all non-hydrogen atoms. Hydrogen
atoms were located based on difference maps and their positions
and isotropic displacement parameters were refined. Given the use
of Mo radiation, the absolute configuration of the chiral crystal
Me-3b could not be determined. Details about the data collection,
structure determination and structures are given the Supporting
Information.
1,8-Dimethoxy-9-(2,6-dimethoxyphenyl)-10-(2-ethylhexyl)-
acridinium tetrafluoroborate (2d): Yield: 2.1 g (93%); orange nee-
dles. 1H NMR (500 MHz, CDCl3): d=8.38 (dd, J=180.7, 171.9 Hz,
2H), 7.94 (d, J=9.1 Hz, 2H), 7.37 (t, J=8.4 Hz, 1H), 7.02 (d, J=
8.0 Hz, 2H), 6.66 (d, J=8.4 Hz, 2H), 5.22 (dd, J=7.8, 1.0 Hz, 2H),
3.57 (s, 6H), 3.56 (s, 6H), 2.26 (m, 1H), 1.54 (m, 2H), 1.45–1.5 (m,
6H), 0.94 (t, J=7.4 Hz, 3H), 0.81 ppm (t, J=7.2 Hz, 3H); 13C NMR
(126 MHz, CDCl3): d=160.89, 157.44, 142.45, 140.12, 140.09, 129.66,
CCDC- 922657 (2d·BF4), -922660 (3a·PF6), -922658 (3b·PF6), -922659
(3e·PF6) and -922661 (Me-3b) contain the supplementary crystallo-
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6397
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120.29, 119.75, 109.92, 106.61, 103.56, 57.08, 56.19, 55.38, 39.73,
30.60, 28.80, 24.24, 23.03, 14.05, 11.18 ppm; UV/Vis (MeCN): l [log
(e/mꢀ1 cmꢀ1)]=534 sh [3.56], 500 [3.64], 468 sh [3.54], 400 [3.72],
356 [3.25], 337 [3.11], 286 [4.9], 246 nm [4.18]; elemental analysis
calcd (%) for C26H28NO4BF4: C 64.70, H 6.66, N 2.43; found: C 64.76,
H 6.75, N 2.35.
1H), 4.44 (s, 3H), 3.84 (s, 3H), 3.80 ppm (s, 3H); 13C NMR (126 MHz,
DMSO): d=159.49, 158.92, 155.15, 149.94, 143.71, 143.14, 139.63,
138.88, 137.69, 137.37, 116.31, 113.84, 110.87, 110.20, 108.82,
108.65, 108.27, 106.39, 104.30, 56.23, 38.53 ppm; UV/Vis (MeCN):
l [log(e/mꢀ1 cmꢀ1)]=591 [3.92], 557 [3.91], 435 sh [3.91], 360
+
[4.13], 297 nm [4.71]; HRMS (ESI-TOF): m/z calcd for C22H18NO3
:
344.1287; found: 344.1281. elemental analysis calcd (%) for
C22H18NO3I3: C 36.44, H 2.50, N 1.93; found: C 36.45, H 2.29, N 1.78.
1,8-Dimethoxy-9-(2,6-dimethoxyphenyl)-10-phenylacridinium
hexafluorophosphate (2e): Acridinium salt 2e was prepared as de-
scribed in the literature.[24]
N-Propyl-1,13-dimethoxychromeno[2,3,4-kl]acridinium
hexa-
fluorophosphate (3b): Yield: 300 mg (92%); dark-red pow-
der.1H NMR (500 MHz, DMSO): d=8.33 (dd, J=8.8, 8.1 Hz, 1H), 8.18
(dd, J=9.0, 8.0 Hz, 1H), 8.13 (d, J=8.9 Hz, 1H), 7.91 (t, J=8.4 Hz,
1H), 7.88 (d, J=9.1 Hz, 1H), 7.73 (d, J=7.9 Hz, 1H), 7.32 (dd, J=
8.4, 0.8 Hz, 1H), 7.20 (d, J=8.0 Hz, 1H), 7.11 (d, J=7.8 Hz, 1H), 4.97
(m, 1H), 4.81 (m, 1H), 3.83 (s, 3H), 3.79 (s, 3H), 2.02 (d, J=2.7 Hz,
2H), 1.17 ppm (s, 3H); 13C NMR (126 MHz, DMSO): d=159.83,
158.90, 155.15, 150.17, 143.30, 142.93, 139.82, 138.11, 137.67,
137.57, 116.42, 113.80, 110.88, 109.93, 108.82, 108.39, 108.37,
106.34, 104.08, 56.22, 51.41, 40.01, 39.93, 39.84, 39.76, 39.67, 39.51,
39.34, 39.17, 39.01, 20.60, 10.77 ppm; UV/Vis (MeCN): l [log
(e/mꢀ1 cmꢀ1)]=584 [3.92], 550 [3.91], 437 [4.07], 361 [3.81], 340 sh
[3.78], 324 [3.95], 298 [4.80], 256 nm [4.26]; HRMS (ESI-TOF): m/z
calcd for C24H22NO3+: 372.1594; found: 372.1587; elemental analy-
sis calcd (%) for C24H22NO3PF6: C 55.71, H 4.29, N 2.71; found C,
55.65, H 4.16, N 2.51.
1,8-Dimethoxy-9-(2,6-dimethoxyphenyl)-10-(1-naphthyl)acridini-
um tetrafluoroborate (2 f): Purified by chromatography with
1
EtOAc as eluent. Yield: 430 mg (45%); dark-brown powder. H NMR
(500 MHz, CDCl3): d=8.33 (d, J=8.4 Hz, 1H), 8.16 (d, J=8.3 Hz,
1H), 7.89 (dd, J=8.3, 7.4 Hz, 1H), 7.83 (dd, J=9.0, 8.0 Hz, 2H), 7.68
(d, J=8.2 Hz, 1H), 7.65 (m, 1H), 7.44 (m, 2H), 7.02 (d, J=7.9 Hz,
2H), 6.76–6,66 (m, 5H), 3.69 (s, 3H), 3.66 (s, 3H), 3.62 ppm (s, 6H);
13C NMR (126 MHz, CDCl3): d=160.98, 160.44, 155.92, 155.77,
142.96, 140.50, 135.17, 134.65, 132.39, 130.20, 129.53, 129.49,
128.43, 128.40, 126.87, 126.63, 121.08, 119.93, 119.11, 110.68,
107.02, 103.76, 103.67, 57.32, 56.38, 56.30 ppm; UV/Vis (MeCN):
l [log(e/mꢀ1 cmꢀ1)]=542 sh [3.59], 506 [3.67], 476 sh [3.59], 406
[3.68], 355 [3.46], 336 [3.35], 286 [4.9], 247 nm [4.27]; elemental
analysis calcd (%) for C33H28NO4BF4: C 61.21, H 4.36, N 2.16; found:
C 61.20, H 4.39, N 2.19.
N-Octyl-1,13-dimethoxychromeno[2,3,4-kl]acridinium
hexa-
General procedure for the preparation of N-substituted 1,13-
fluorophosphate (3c): Yield: 310 mg (30%); dark-red powder.
1H NMR (500 MHz, CDCl3): d=8.24 (dd, J=8.7, 8.2 Hz, 1H), 8.17
(dd, J=9.0, 8.1 Hz, 1H), 7.79 (m, 1H), 7.79 (t, J=8.3 Hz, 1H), 7.59 (t,
J=9.1 Hz, 2H), 7.22 (dd, J=8.4, 0.9 Hz, 1H), 7.04 (d, J=8.0 Hz, 1H),
6.89 (d, J=7.8 Hz, 1H), 4.94 (m, 1H), 4.78 (m, 1H), 3.89 (s, 3H), 3.84
(s, 3H), 2.15 (m, 2H), 1.71 (m, 2H), 1.38 (m, 2H), 1.32 (m, 6H),
0.91 ppm (s, 3H); 13C NMR (126 MHz, DMSO): d=159.84, 158.88,
155.14, 150.17, 143.25, 142.88, 139.85, 138.05, 137.67, 137.61,
116.43, 113.83, 110.87, 109.86, 108.81, 108.39, 108.30, 106.35,
104.08, 56.22, 49.96, 31.17, 28.69, 28.65, 27.18, 25.96, 22.05,
13.95 ppm; UV/Vis (MeCN): l [log(e/mꢀ1 cmꢀ1)]=584 [3.92], 550
[3.91], 437 [4.07], 361 [3.81], 340 sh [3.78], 324 [3.95], 298 [4.80],
256 nm [4.26]; HRMS (ESI-TOF): m/z calcd for C29H32NO3+: 442.2377;
found: 442.2379.
dimethoxychromeno[2,3,4-kl]acridinium
hexafluorophosphate
(3): 10-Substituted 1,8-dimethoxy-9-(2,6-dimethoxyphenyl)acridini-
um hexafluorophosphate (2; 1 mmol) was dissolved in acetic acid
(50 mL) and 50% sulfuric acid (25 mL) was added. The reaction
mixture was heated at reflux until the stating material peak in the
MALDI-TOF mass spectrum had disappeared (24–36 h). The hot re-
action mixture was poured into 0.2m aqueous KPF6 (400 mL). The
crude product was collected by filtration and was directly washed
off the filter by using DMSO (50 mL). NaBH4 (250 mg) was then
added to the DMSO solution. After 10 min, the reaction mixture
was poured into water (200 mL) and the colourless neutral product
was extracted with diethyl ether (3ꢁ150 mL). The combined organ-
ic extracts were pooled and washed with water (3ꢁ200 mL), dried
over MgSO4 and the solvent was removed. Column chromatogra-
phy on silica 60 (0.063–0.200 mm) with 5 vol% diethyl ether in n-
heptane gave the reduced product 3-H as a white crystalline mate-
rial. The pure hydride adduct was taken up in diethyl ether (50 mL)
and I2 (1 g in 100 mL diethyl ether) was added. The mixture was
N-(rac-2-Ethylhexyl)-1,13-dimethoxychromeno[2,3,4-kl]acridini-
um hexafluorophosphate (3d): Yield: 180 mg (22%); dark-red
1
powder. H NMR (500 MHz, DMSO): d=8.34 (t, J=8.4 Hz, 1H), 8.22
ꢀ
stirred for 1 h and then the I3 salt ꢀof the product was collected
(d, J=7.1 Hz, 1H), 8.18 (dd, J=9.0, 8.1 Hz, 1H), 7.91 (t, J=8.4 Hz,
1H), 7.88 (m, 1H), 7.74 (d, J=8.0 Hz, 1H), 7.33 (dd, J=8.4, 0.8 Hz,
1H), 7.20 (d, J=8.0 Hz, 1H), 7.11 (d, J=8.5 Hz, 1H), 5.05 (m, 2H),
3.83 (s, 3H), 3.80 (s, 1.5H), 3.79 (s, 1.5H), 1.99 (brs, 1H), 1.54–0.92
(m, 8H), 0.91–0.45 ppm (m, 6H); 13C NMR (126 MHz, DMSO): d=
158.90, 155.21, 155.19, 150.18, 150.13, 143.55, 143.42, 143.30,
139.69, 137.82, 137.41, 116.56, 116.53, 110.90, 110.87, 110.69,
108.84, 108.45, 106.40, 106.38, 104.08, 56.35, 56.28, 56.20, 51.63,
38.08, 29.15, 27.31, 23.01, 22.00, 13.25, 10.37 ppm; UV/Vis (MeCN):
l [log(e/mꢀ1 cmꢀ1)]=584 [3.92], 550 [3.91], 437 [4.07], 361 [3.81],
340 sh [3.78], 324 [3.95], 298 [4.80], 256 nm [4.26]; HRMS (ESI-TOF):
m/z calcd for C29H32NO3+: 442.2377; found: 442.2371; elemental
and taken up in acetonitrile. The PF6 salt was precipitated by the
addition of 0.2m aqueous KPF6 (600 mL) and collected by filtration.
The precipitation was repeated twice. After the final precipitation,
the product purity was determined by UV/Vis spectroscopy. If the
ꢀ
spectrum showed any sign of I3 absorption (ca. 300 nm), the
crude product was taken up in CH2Cl2 and washed with 0.2m
aqueous KPF6 (4ꢁ200 mL) and water (2ꢁ200 mL). The CH2Cl2
phase was dried over MgSO4 and concentrated in vacuo. The prod-
uct was precipitated with n-heptane and collected by filtration.
N-Methyl-1,13-dimethoxychromeno[2,3,4-kl]acridinium triiodide
(3a): Prepared as the triiodide salt. Yield: 1.1 g (40%); dark-red
powder. 1H NMR (500 MHz, DMSO): d=8.33 (dd, J=8.8, 8.1 Hz,
1H), 8.19 (dd, J=9.0, 8.0 Hz, 1H), 8.11 (m, 1H), 7.91 (t, J=8.4 Hz,
1H), 7.87 (d, J=9.0 Hz, 1H), 7.73 (dd, J=8.0, 0.5 Hz, 1H), 7.33 (dd,
J=8.4, 0.9 Hz, 1H), 7.21 (d, J=7.9 Hz, 1H), 7.12 (dd, J=8.4, 0.7 Hz,
analysis calcd (%) for C29H32NO3PF6·1= CH2Cl2: C 56.24, H 5.28, N
2
2.22; found: C 56.37, H 5.22, N 2.11.
N-Phenyl-1,13-dimethoxychromeno[2,3,4-kl]acridinium
hexa-
fluorophosphate (3e): Yield: 460 mg (75%); dark-red powder.
Chem. Eur. J. 2014, 20, 6391 – 6400
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6398
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
1H NMR (500 MHz, CDCl3): d=8.00 (dd, J=8.7, 8.1 Hz, 1H), 7.91
(dd, J=8.9, 8.0 Hz, 1H), 7.87–7.78 (m, 4H), 7.58 (dd, J=8.9, 1.3 Hz,
1H), 7.57 (dd, J=8.0, 0.7 Hz, 1H), 7.50 (dt, J=3.2, 1.3 Hz, 1H), 7.24
(d, J=0.9 Hz, 1H), 7.05 (d, J=7.8 Hz, 1H), 6.96 (d, J=8.1 Hz, 2H),
6.74 (dd, J=8.9, 0.6 Hz, 1H), 3.94 (s, 3H), 3.90 ppm (s, 3H);
13C NMR (126 MHz, CDCl3): d=160.66, 159.99, 156.73, 151.34,
146.45, 145.06, 140.47, 139.79, 138.57, 137.32, 132.48, 131.65,
131.57, 128.89, 128.13, 116.58, 114.33, 112.29, 110.68, 109.72,
109.68, 109.14, 106.46, 104.84, 56.93, 56.80 ppm; UV/Vis (MeCN):
l [log(e/mꢀ1 cmꢀ1)]=592 [3.92], 555 [3.91], 442 [4.07], 361 [3.81],
341 sh [3.85], 329 [3.95], 299 [4.73], 257 nm [4.26]; HRMS (ESI-TOF):
m/z calcd for C27H20NO3+: 406.1438; found: 406.1432; elemental
Production Sciences (grant no. #10-093546) and the Danish Na-
tional Research Foundation under the Danish–Chinese Centre
for Self-Assembled Molecular Electronic Nanosystems are
gratefully acknowledged for financial support.
Keywords: carbocations · fluorescence · helical structures ·
organic dyes · X-ray diffraction
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Commun. 2009, 0, 6652–6654; b) L. Severa, D. Koval, P. Novotna, M.
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analysis calcd (%) for C27H20NO3PF6·1= CH2Cl2: C 57.15, H 3.61, N
4
2.54; found: C 57.10, H 3.45, N 2.48.
N-(1-Naphthyl)-1,13-dimethoxychromeno[2,3,4-kl]acridinium
hexafluorophosphate (3 f): Yield: 100 mg (53%); dark-red powder.
1
Major: H NMR (500 MHz, DMSO): d=8.44 (d, J=8.1 Hz, 1H), 8.30
(d, J=8.3 Hz, 1H), 8.05 (m, 1H), 7.98 (m, 2H), 7.93 (m, 1H), 7.87
(dd, J=8.7, 8.2 Hz, 1H), 7.72 (dd, J=14.1, 7.5 Hz, 2H), 7.48 (dd, J=
11.2, 4.1 Hz, 1H), 7.41 (d, J=8.4 Hz, 1H), 7.27 (d, J=8.4 Hz, 1H),
7.19 (t, J=8.2 Hz, 2H), 6.67 (d, J=8.8 Hz, 1H), 6.45 (d, J=8.9 Hz,
1H), 3.91 (s, 3H), 3.87 ppm (s, 3H); 13C NMR (126 MHz, DMSO): d=
160.13, 159.40, 155.72, 150.33, 145.58, 144.49, 140.06, 139.62,
138.46, 137.89, 134.41, 133.99, 131.63, 129.16, 128.81, 128.64,
127.78, 127.12, 126.82, 121.86, 116.65, 113.84, 111.18, 110.53,
109.03, 109.03, 108.48, 106.63, 104.44, 56.41, 56.38 ppm; UV/Vis
(MeCN): l [log(e/mꢀ1 cmꢀ1]=592 [3.92], 555 [3.91], 442 [4.07], 361
[3.81], 341 sh [3.85], 329 [3.95], 299 [4.73], 257 nm [4.26]. HRMS
(ESI-TOF): m/z calcd for C31H22NO3+: 456.1600; found: 456.1616.
ˇ
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N-Propyl-1,13-dimethoxy-13b-methylchromeno[2,3,4-kl]acridine
(Me-3b): Acridine Me-3b was prepared as described in ref. [7a].
Major diastereomer: 1H NMR (500 MHz, CDCl3): d=7.19 (t, J=
8.2 Hz, 1H), 7.10 (td, J=8.1, 1.9 Hz, 2H), 6.79 (dd, J=8.3, 0.8 Hz,
1H), 6.73 (d, J=8.2 Hz, 1H), 6.68 (m, 2H), 6.59 (dd, J=8.2, 1.1 Hz,
1H), 6.51 (dd, J=8.1, 0.8 Hz, 1H), 3.97 (pd (quintet of doublets),
J=14.1, 6.2 Hz, 2H), 3.81 (s, 3H), 3.34 (s, 3H), 1.99 (m, 2H), 1.69 (s,
3H), 1.08 ppm (t, J=7.4 Hz, 3H). Minor diastereomer: 1H NMR
(500 MHz, CDCl3): d=7.19 (m, 1H), 7.07 (m, 2H), 6.91 (dt, J=10.1,
5.1 Hz, 1H), 6.73 (m, 1H), 6.68 (d, J=0.4 Hz, 2H), 6.55 (d, J=7.9 Hz,
1H), 6.51 (m, 1H), 3.97 (pd, J=14.1, 6.2 Hz, 2H), 3.78 (s, 3H), 3.39
(s, 3H), 1.84 (s, 3H), 1.80 (dd, J=15.5, 7.8 Hz, 2H), 1.05 ppm (t, J=
7.5 Hz, 3H).
N-Propyl-13b-cyanomethyl-1,13-dimethoxychromeno[2,3,4-kl]-
acridine (NCCH2-3b): Acridine NCCH2-3b was synthesised as de-
scribed in ref. [8]. Major diastereomer: 1H NMR (500 MHz, DMSO):
d=7.34 (dd, J=16.8, 8.4 Hz, 1H), 7.21 (m, 2H), 6.97 (d, J=8.1 Hz,
1H), 6.92 (d, J=8.0 Hz, 1H), 6.77 (m, 3H), 6.64 (m, 1H), 4.02 (m,
2H), 3.78 (m, 4H), 3.36 (m, 3H), 3.22 (d, J=16.7 Hz, 1H), 1.05 ppm
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1
(d, J=7.3 Hz, 3H). Minor diastereomer: H NMR (500 MHz, DMSO):
d=7.45 (t, J=8.2 Hz, 1H), 7.22 (s, 1H), 7.12 (s, 1H), 7.05 (d, J=
8.1 Hz, 1H), 6.88 (d, J=8.3 Hz, 1H), 6.77 (m, 3H), 6.64 (m, 1H), 4.02
(m, 2H), 3.76 (s, 3H), 3.69 (d, J=17.1 Hz, 1H), 3.39 (s, 3H), 2.88 (d,
J=16.7 Hz, 1H), 1.00 ppm (s, 3H).
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Chem. Eur. J. 2014, 20, 6391 – 6400
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Received: January 8, 2014
Published online on April 15, 2014
Chem. Eur. J. 2014, 20, 6391 – 6400
www.chemeurj.org
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