Novel compounds with a viologen skeleton and N-heterocycles on the peripheries: Electrochemical and spectroscopic properties

Article abstract of DOI:10.1002/hlca.201000361

The present article deals with novel compounds comprising a redox-active group as core and a nucleobase in the peripheries, linked covalently via a spacer. The new derivatives 1,1′,1″-(benzene-1,3,5- triyltrimethanediyl)tris{1′-[3-(3,4-dihydro-5-methyl-2,

Full text of DOI:10.1002/hlca.201000361

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Novel Compounds with a Viologen Skeleton and N-Heterocycles on the
Peripheries: Electrochemical and Spectroscopic Properties
by Simona Asaftei*, Ana Maria Lepadatu, and Marius Ciobanu
Organic Materials Chemistry, Bioorganic Chemistry, Institute of Chemistry, University of Osnabrꢀck,
Barbarastrasse 7, D-49069 Osnabrꢀck (phone: þ 49-541-969 2802;
fax: þ 49-541-969 2370; sasaftei@uos.de)
The present article deals with novel compounds comprising a redox-active group as core and a
nucleobase in the peripheries, linked covalently via a spacer. The new derivatives 1,1’,1’’-(benzene-1,3,5-
triyltrimethanediyl)tris{1’-[3-(3,4-dihydro-5-methyl-2,4-dioxopyrimidin-1(2H)-yl)propyl]-4,4’-bipyridi-
nium} hexafluorophosphate (1), 1,1’,1’’-(benzene-1,3,5-triyltrimethanediyl)tris{1’-[2-(4-chloro-7H-pyrro-
1
lo[2,3-d]pyrimidine-7-yl)ethyl]-4,4’-bipyridinium} hexachloride (2a) ), and 1,1’,1’’-(benzene-1,3,5-triyl-
trimethanediyl)tris{1’-[2-(2-amino-4-chloro-7H-pyrrolo[2,3-d]pyrimidine-7-yl)ethyl]-4,4’-bipyridinium}
hexabromide (2b)1) were synthesized by nucleobase-anion alkylation and linked to the 4,4’-bipyridinium
core. UV and CV analyses of these compounds were performed and revealed significantly different
properties.
1. Introduction. – Chemical modifications play an important role in the creation of
new compounds with dual functionality improving their properties. These include their
hybridization abilities [1], specific recognition of biological targets [2][3], and their
ability of the formation of specific H-bonds [4][5], combined with electrochemically
properties [6]. These characteristics make such structures very attractive for several
applications in the field of light-harvesting, electrochromics [7][8], catalysis, cellular
imaging, drug delivery [9], or for the construction of complex multifunctional
supramolecular systems [10][11].
The electrochromic properties of 4,4’-bipyridinium salts are well-documented, and
they have received significant attention for several decades because of their wide range
of potential applications [12][13]. On the other hand, dendrimers containing electro-
active moieties are currently under investigation due to their useful functions, such as
ion sensing with signal amplification [14][15], charge pooling [16], or as antiviral
agents [17]. The present article deals with the preparation, characterization, as well as
electrochemical and spectroscopic properties of a new family of compounds containing
4,4’-bipyridinium units as redox-active cores functionalized on their periphery with an
N-heterocyclic nucleobase, also referred to as base. The aim was to couple a redox unit
with well-known electrochemical properties with biologically active head groups. Such
compounds were expected to exhibit interesting properties because the 4,4’-bipyridi-
nium units are known as electron acceptors [12], used in electrochemical processes
1
)
The numbering of the pyrrolo[2,3-d]pyrimidine system follows the IUPAC rules and is different
from that of the purine ring system.
ꢁ 2011 Verlag Helvetica Chimica Acta AG, Zꢀrich

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[18], and the nucleobases might act as electron donors, capable of fast electron-transfer
reactions. For comparison, we also investigated the properties of a ꢂtris-viologenꢃ
derivative 1,1’,1’’-(benzene-1,3,5-triyltrimethanediyl)tris(1’-methyl-4,4’-bipyridinium)
hexafluorophosphate (MV6þ), which contains three viologen units (Scheme).
Scheme. Synthesis of compounds MV6þ, 1, 2a, and 2b
i) hexamethyldisilazane and Me3SiCl, 21 h, reflux. ii) 3 equiv. of 1,3-dibromopropane (DBP) in DMF,
24 h, 808. iii) MeCN, 96 h, 808. iv) NaH, DMF, 72 h, 708.
2. Results and Discussion. – 2.1. Synthesis. All compounds, MV6þ, 1, 2a, and 2b, were
synthesized by nucleophilic substitution reaction of alkylated nucleobase derivatives

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with the corresponding ꢂredox matrixꢃ, 1,1’,1’’-(benzene-1,3,5-triyltrimethanediyl)-
tris(4-pyridin-4-ylpyridinium) trihexafluorophosphate (PV3þ; Scheme).
The well-known precursor PV3þ, consisting of a mesityl derivative linked to three
4,4’-bipyridine units, plays the role of the branching unit in the divergent synthesis of
dendrimers. PV3þ was prepared according to the procedure described in [19] by
reaction of 1,3,5-tris(bromomethyl)benzene [20][21] with an excess of 4,4’-bipyridine
in MeCN, followed by ion exchange with NH4PF6. The precursor PV3þ salt with three
peripheral N-atoms reacted further quantitatively with the (bromoalkyl)-pyrimidine
derivative and MeI. The reference compound, 1,1’,1’’-(benzene-1,3,5-triyltrimethane-
diyl)tris(1’-methyl-4,4’-bipyridinium) hexafluorophosphate (MV6þ), was obtained in
60% yield by treating the precursor PV3þ with an excess of MeI in MeCN (Scheme,
Route II). Compounds 1, 2a, and 2b were synthesized by a divergent procedure starting
from the core to the periphery. The reaction sequence was strongly determined by the
stability of the redox units in alkaline medium. It is known that the 4,4’-bipyridine units
are reduced irreversibly at pH of ca. 10. The periphery of the compounds 1, 2a, and 2b
consist of substituted thymine and pyrrolo[2,3-d]pyrimidine units, respectively. There-
fore, in the first reactions step, the nucleobases were alkylated, and, in last reaction
step, they were bound to the ꢂredox matrixꢃ PV3þ. Difficulties arose during alkylation of
thymine because of a lack in regioselectivity. Thymine can be alkylated at the N(1),
N(3), O(2), or O(4); moreover, internal cyclization reactions were reported by Nawrot
et al. [22]. The problem was solved by protection of the functional groups according to
the reaction of Vorbrꢀggen et al. [23], which involves the conversion of pyrimidine-2,4-
dione bases to their bis[trimethylsilyl]-ether derivatives by treatment with hexame-
thyldisilazane and Me3SiCl. The bis(trimethylsilyl) derivative was then alkylated [24]
by treatment with an excess of dibromoalkane in DMF to afford 1-(3-bromopropyl)-5-
methylpyrimidine-2,4(1H,3H)-dione (Scheme, Route I) in low yield (loss during the
deprotection or conversion of the raw material) [25]. 7-(2-Bromoethyl)-4-chloro-7H-
pyrrolo[2,3-d]pyrimidine and 7-(bromoethyl)-4-chloro-7H-pyrrolo[2,3-d]pyrimidin-2-
amine were prepared according to [26], by subsequent nucleobase-anion alkylation
(NaH, DMF) with a 100-fold excess of 1,2-dibromoethane (Scheme, Route III), besides
small amounts (10 – 15%) of the 9-vinyl derivatives, which were separated by column
chromatography. Finally, the corresponding alkylated heterocycles were covalently
coupled on the core by nucleophilic reaction in MeCN with a yield of more than 70%.
The structures of the novel compounds were confirmed by 1H- and 13C-NMR as well as
by UV spectroscopy, and by elemental analyses.
2.2. Spectroelectrochemical Investigations. The electrochemical behavior of 1, 2a,
and 2b was studied at glassy carbon microelectrodes in a MeCN/NaClO4 electrolyte at
298 K. For comparison, the electrochemical behavior of 1,1’,1’’-(benzene-1,3,5-
triyltrimethanediyl)tris(1’-methyl-4,4’-bipyridinium) hexafluorophosphate (MV6þ)
has also been investigated. The cyclic voltammetry (CV) curves obtained for
compounds 1, 2a, and 2b are shown in Fig. 1.
The cathodic region showed two reversible one-electron transfer processes, I and II,
for all compounds, being typical for viologen derivatives corresponding to the
formation of a radical cation (I) and a neutral species (II) [27]. The first reduction
was generally very fast, and the second was often coupled to precipitation processes due
to the uncharged nature of the fully reduced species, which is insoluble in highly polar

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Fig. 1. Cyclic voltammograms of compounds MV6þ, 1, 2a, and 2b (0.1 mm) in 0.1m NaClO4 in MeCN at a
glassy carbon electrode (area: 0.0680 cm2); scan rate v ¼ 1 Vsꢀ1
.
solvents [28]. The reduction processes of compound 1 can be directly compared with
those of the model compound MV6þ. A sharp positive shift of the cathodic-peak
potential was observed in the case of compound 2a and 2b, indicating a destabilization
of the redox matrix compared to compound 1 and model compound MV6þ, presumably
caused by folding of the pyrrolo[2,3-d]pyrimidine units around the 4,4’-bipyridinium
units. These results point to an extension of the p-electron system, which corroborated
the UV/VIS results for the chemically reduced compounds 2a and 2b. The reduction
potential of pyrrolo[2,3-d]pyrimidine units measured with a glassy carbon electrode in
1m TBABr/H2O was highly negative (ꢀ1.764 V; Fig. 2).
Fig. 2. Cyclic voltammogram of 7-(2-Bromoethyl)-4-chloro-7H-pyrrolo[2,3-d]pyrimidine (c ¼ 1 mm)
recorded with a glassy carbon electrode in 1m TBABr/H2O, scan rate v ¼ 0.2 Vsꢀ1
The half-wave potential (E1/2) and peak-separation data of the compounds
examined are collected in the Table.

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Table. Electrochemical Parameters(n in v/sꢀ1 and E in mV) for the Compounds MV6þ, 1, 2a, and 2b
v
Epc1
Epa1
E81
DE1
Epc2
Epa2
E82
DE2
MV6þ
ꢀ 0.1
ꢀ 0.3
ꢀ 0.6
ꢀ 1
ꢀ 444
ꢀ 435
ꢀ 437
ꢀ 427
ꢀ 378
ꢀ 371
ꢀ 378
ꢀ 376
ꢀ 411
ꢀ 403
ꢀ 407
ꢀ 401
66
64
59
51
ꢀ 876
ꢀ 876
ꢀ 886
ꢀ 879
ꢀ 823
ꢀ 811
ꢀ 818
ꢀ 828
ꢀ 849
843
53
65
68
51
ꢀ 852
ꢀ 853
1
ꢀ 0.1
ꢀ 0.3
ꢀ 0.6
ꢀ 1
ꢀ 461
ꢀ 422
ꢀ 427
ꢀ 422
ꢀ 349
ꢀ 327
ꢀ 332
ꢀ 330
ꢀ 405
ꢀ 374
ꢀ 379
ꢀ 376
112
95
95
ꢀ 886
ꢀ 898
ꢀ 898
ꢀ 901
ꢀ 779
ꢀ 796
ꢀ 789
ꢀ 798
ꢀ 832
ꢀ 847
ꢀ 843
ꢀ 850
107
102
109
103
92
2a
ꢀ 0.1
ꢀ 0.3
ꢀ 0.6
ꢀ 1
ꢀ 168
ꢀ 166
ꢀ 166
ꢀ 161
ꢀ 129
ꢀ 132
ꢀ 129
ꢀ 125
ꢀ 148
ꢀ 149
ꢀ 147
ꢀ 143
39
32
37
36
ꢀ 560
ꢀ 562
ꢀ 432
ꢀ 522
ꢀ 420
ꢀ 432
ꢀ 552
ꢀ 430
ꢀ 238
ꢀ 497
ꢀ 492
ꢀ 476
140
130
120
92
2b
ꢀ 0.1
ꢀ 0.3
ꢀ 0.6
ꢀ 1
ꢀ 193
ꢀ 190
ꢀ 186
ꢀ 181
ꢀ 151
ꢀ 154
ꢀ 154
ꢀ 151
ꢀ 172
ꢀ 172
ꢀ 170
ꢀ 166
39
36
32
30
ꢀ 583
ꢀ 598
ꢀ 583
ꢀ 576
ꢀ 449
ꢀ 442
ꢀ 452
ꢀ 447
ꢀ 516
ꢀ 520
ꢀ 472
ꢀ 511
134
156
131
129
All electrochemical parameters were determined using a glassy carbon working
electrode, vs. Ag/AgCl immersed in a solution of compounds MV6þ, 1, 2a, and 2b
(0.1 mm) in MeCN/0.1m NaClO4.
The first half-wave potential (E(1)1/2) shifts for compound 1 are comparable with
the reference MV6þ, whereas compounds 2a and 2b are ca. 260 MV more positive
compared to the reference. This finding indicated that the cation state was influenced
by Coulomb destabilization which emerged in the presence of aromatic pyrrolo[2,3-
d]pyrimidine system with functional groups such as Cl and NH2 with þ M effects on the
periphery. During the voltammetry experiments, a green/blue plume for compounds 2a
and 2b was observed in the vicinity of the electrode. The color of the electrolyzed
solution indicated the formation of the radical cation dimers, as supported by the UV/
VIS results for the chemically reduced compounds 2a and 2b. On the basis of the
parameters in the Table, dimer formation did not appear to inhibit the reversibility of
the first reductions wave. Peak-separation values indicate that this electrochemical
process is reversible for compounds 2a and 2b, independent of the scan rate and varies
from 40 to 60 mV, while that of compound 1 is strongly dependent of the scan rate
(Fig. 3).
At a moderate scan rate, an adsorbed one-electron reduced species on the electrode
surface was evident in the voltammograms for compound 1 (Fig. 3). At faster scan rates
(1 V sꢀ1), the voltammetric data still revealed that the first reduction was quite fast; the
largest DEp value measured in these experiments was 112 mV. The half-wave potentials
corresponding to the second electron uptake exhibited the same trend, shifted to the
less negative values for compound 1. The ratio of the anodic and cathodic peak heights
(ipa1/ipc1) provides information about the chemical reversibility of the first redox wave.
The peak height ratio was calculated after base-line correction from voltammograms,
and varied from 0.9 to 1.2 for all compounds at scan rates from 300 mV– 1 V sꢀ1. This
indicated relatively good chemical reversibility. The kinetics of the electron-transfer

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Fig. 3. Cyclic voltammograms of compounds MV6þ, 1, 2a, and 2b (0.1 mm) in 0.1m NaClO4 in MeCN at a
glassy carbon electrode (area: 0.0680 cm2), scan rates v ¼ 0.1, 0.3, 0.6, and 1 V sꢀ1
reaction appeared to be fast, although very marked absorbed effects were observed. As
was the case for the first reduction process, these effects were most pronounced for
redox matrix with thymine units in the periphery (i.e., 1). Overall, the trend observed in
the E1/2 values revealed that the reduction process was thermodynamically favored,
independently of the heterocycle at the periphery. The electrode response of
compounds 1 and 2a, and 2b in the CV experiment could thus be assigned to a quasi-
reversible case of the heterogeneous electron transfers. Further investigation of the
electron transfer mechanism has not been performed because of a strong absorption of
the reduced species on the electrode minigrid used as working electrode.
2.3. UV/VIS Spectroscopy. Compounds 1, 2a, and 2b contain three types of
chromophoric units: thymine, pyrrolo[2,3-d]pyrimidine units, 4,4’-bipyridine, and
benzene. The absorption spectra of the alkylated thymine and pyrrolo[2,3-d]pyrimidine
units in an EtOH solution at 298 K are shown in Fig. 4, and the spectra of compounds
MV3þ, 1, 2a, and 2b in MeCN at 298 K in the oxidated states are also displayed for
comparison.
The spectrum was dominated by a strong absorption of the 4,4’-bipyridine units
(MV6þ: lmax ¼ 256 nm, e ¼ 43556 mꢀ1 cmꢀ1) with some contribution of thymine (1;
lmax ¼ 266 nm, e ¼ 59460 mꢀ1 cmꢀ1); 4-chloropyrrolo[2,3-d]pyrimidine (2a; lmax
¼
265 nm, e ¼ 83550 mꢀ1 cmꢀ1), and 4-chloropyrrolo[2,3-d]pyrimidin-2-amine units (2b;
lmax ¼ 235 nm, e ¼ 73200 mꢀ1 cmꢀ1, lmax ¼ 262 nm, e ¼ 62700 mꢀ1 cmꢀ1). The absorbance
of benzene was negligible at ca. 260 nm (lmax ¼ 255 nm, e ¼ 250 mꢀ1 cmꢀ1) [29]. The
intensity of the absorption spectrum in the UV region increased by the presence of an
aromatic ring and the auxochromic groups with þ M effects on the periphery. The
result suggests that there are interactions between the chromophoric units within the
dendrimers, as confirmed by the observation that the dendrimers exhibit a broad and
weak absorption peak in the visible region (2a: lmax ¼ 384 nm, e ¼ 83550 mꢀ1 cmꢀ1; 2b:
lmax ¼ 475 nm, e ¼ 7070 mꢀ1 cmꢀ1), which could not be seen in the spectrum of
compound 1, as well as of the alkylated N-heterocyclic units. The absorbance of
alkylated heterocyclic units in EtOH is shown in the inset of Fig. 4; an absorbance
maximum was present for thymine (a) at lmax ¼ 271 nm (e ¼ 10310 mꢀ1 cmꢀ1), for 4-

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Fig. 4. Adsorption spectra of MV6þ (black line), 1 (medium grey line), 2a (light grey line), and 2b (dark
grey line) in oxidated state, 0.1 m m in 0.1m NaClO4 in MeCN. Inset: Adsorption spectra of 1-(3-
bromopropyl)-5-methylpyrimidine-2,4(1H,3H)-dione (a), 7-(2-bromoethyl)-4-chloro-7H-pyrrolo[2,3-
d]pyrimidine (b) and 7-(bromoethyl)-4-chloro-7H-pyrrolo[2,3-d]pyrimidin-2-amine (c) in EtOH
solution.
chloropyrrolo[2,3-d]pyrimidine (b) at lmax ¼ 227 nm (e ¼ 27690ømꢀ1 cmꢀ1) and lmax
¼
277 nm (e ¼ 21980 mꢀ1 cmꢀ1), and for 4-chloropyrrolo[2,3-d]pyrimidin-2-amine- (c) at
lmax ¼ 236 nm (e ¼ 36170 mꢀ1 cmꢀ1) and lmax ¼ 263 nm (e ¼ 8350 mꢀ1 cmꢀ1). In the case
of c, a pronounced bathochromic shift was also observed in the long-wavelength range
at lmax ¼ 316 nm (e ¼ 7230 mꢀ1 cmꢀ1).
Due to their redox properties, the 4,4’-bipyridinium units can be examined by
spectroelectrochemical properties, because they change their color depending on their
redox status. The monomeric viologens exhibit absorption maxima in the range of
300 – 750 nm. Benzyl-substituted viologens have typical absorption maxima at 401 (e ¼
28900 mꢀ1cmꢀ1) and 608 nm (e ¼ 11700 mꢀ1cmꢀ1), phenyl-substituted viologens typi-
cally show absorption maxima at 446 (e ¼ 29000 mꢀ1 cmꢀ1) and 714 nm (e ¼ 15600 mꢀ1
cmꢀ1), and Me-substituted viologens show absorption maxima at 400 (e ¼ 18860 mꢀ1
cmꢀ1) and 600 nm (e ¼ 7084 mꢀ1 cmꢀ1). In case of dimers, the maxima are shifted to ca.
370 and 550 nm. In addition, in the long-wavelength range at ca. 900 nm, a new band
appears which seems to be due to charge-transfer (CT) complexes of the mentioned
compounds [30].
Fig. 5 shows UV/VIS spectra of 1, 2a, and 2b (0.1 m m) in 0.1m NaClO4 in MeCN
after chemical reduction, in a three-electrode system vs. Ag/AgCl at several potentials.
All compounds showed the typical absortion maxima as other viologenated dendrimers
at 400 (e(1) ¼ 8450, e(2a) ¼ 47860, e(2b) ¼ 13934 mꢀ1 cmꢀ1) and 600 nm ((e(1) ¼ 4460,
e(2a) ¼ 46045, e(2b) ¼ 10990 mꢀ1 cmꢀ1). We observed the same behavior as in the
cyclovoltammetry experiments for 2a and 2b which exhibit a maximum concentration of

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Fig. 5. Absorption spectra of 1, 2a, and 2b in the reduced state, 0.1 mm in 0.1m NaClO4 in MeCN at several
potentials

Helvetica Chimica Acta – Vol. 94 (2011)
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radical cation species at ꢀ 150 mV for 2a and at ꢀ 250 mV for 2b, respectively.
Maximum concentration of radical cation species is found at ꢀ 450 mV for compound 1
and reference MV6þ. The colors of the reduced solutions were deep green for the first,
deep purple for the second reduction step for compound 2a, and deep blue for the first
and deep violet for the second reduction step for compound 2b, with absorbance peaks
at 400, 595, and 950 nm. This indicates dimer formation [30]. Such an absorption tail
can be assigned to a CT transition from electron-donor units on the periphery to the
4,4’-bipyridinium electron-acceptor core.
In conclusion, a series of electroactive, functionalized dendrimers with interesting
properties were prepared by using a simple synthetic procedure. The 4,4’-bipyridinium
units undergo two distinct and fast one-electron transfer processes at ca. ꢀ 150 mVand
ꢀ 250 mV, respectively. This might be useful in photochemical energy-conversion
schemes as well as for information processing.
Experimental Part
General. All chemicals were purchased from Merck (D-Hohenbrunn), SigmaꢀAldrich, or from
Fluka. Solvents were of laboratory grade. TLC: aluminium sheets, silica gel 60 F254, 0.2-mm layer (Merck,
Germany). M.p.: Advanced SMP3; uncorrected. UV Spectra: 8453 UV-visible spectrophotometer
1
(Agilent, Germany); lmax in nm (e in mꢀ1 cmꢀ1). NMR Spectra: Bruker AMX-500 spectrometer; H:
500.13, 13C: 125.7 MHz; chemical shifts d in ppm rel. to Me4Si as internal standard for 1H and 13C.
Elemental analyses: VarioMICRO cube.
Electrochemistry and Spectroelectrochemistry. MeCN and NaClO4 (puriss., electrochem. grade) were
purchased from SigmaꢀAldrich and Acros Organics for cyclic voltammetry and spectroelectrochemical
studies. CVs were measured under Ar with the potentiostat PGSTAT 302N from AUTOLAB, controlled
by a PC running under GPES from Windows, version 4.9 (ECO Chemie B. V.); a glassy carbon electrode
(GCE) from Metrohm (Germany) with an active electrode surface of A ¼ 0.07 cm2 was used for CV. The
electrode surface was polished with Al2O3. The reference electrode was Ag/AgCl/KCl (3 m in H2O), and
the counter electrode was a Pt wire.
The spectroelectrochemical cell used for measurements of the dendrimers in soln. was a H-Type
spectroelectrochemical bulk electrolysis cell. The reference electrode was Ag/AgCl immersed in an
electrolyte vessel filled with LiCl (2 m in EtOH), separated from the cell by a glass frit, and the counter
electrode was a Pt foil. The working electrode was 0.039 g of graphitized carbon felt GFA-5 of ca.
0.021 m2 BET area from SGL carbon, the electrochemically active area of which is not known.
Absorbance changes were recorded with a Hewlett-Packard 8453 spectrophotometer.
The UV/VIS spectra of the dendrimers in their oxidized and reduced state have been recorded
spectroelectrochemically in MeCN/NaLiO4 (0.1m), and the corresponding lmax values and extinction
coefficients (e) are reported. The reference for UV/VIS spectra of dendrimers was the pure solvent/
electrolyte.
1-(3-Bromopropyl)-5-methylpyrimidine-2,4(1H,3H)-dione [25]. To a soln. of thymine (2.24 g,
17.6 mmol) in hexamethyldisilazane (HMDS; 11.4 ml, 54 mmol) under Ar was added a cat. amount of
trimethylchlorosilane (1.08 ml, 8.54 mmol), and the mixture was stirred for 21 h under reflux. Excess
HMDS was then removed under reduced pressure to afford crude O-silylated thymine. The crude
material was taken up in DMF (10 ml), 1,3-dibromopropane (4.6 ml) was added, and the mixture was
stirred at 808 for 24 h. H2O (150 ml) was then added, and, after stirring for 10 min, the mixture was
filtered, and the aq. filtrate was extracted with CH2Cl2. The combined extracts were dried (MgSO4),
filtered, and concentrated under reduced pressure. Crystallization of the residue from abs. EtOH gave
white crystals of 1-(3-bromopropyl)-5-methylpyrimidine-2,4(1H,3H)-dione (1.5 g, 6.07 mmol, 34.4%).
M.p. 1408. 1H-NMR ((D6)DMSO): 11.17 (s, HꢀN(3)); 7.48 (s, HꢀC(6)); 3.73 (t, J ¼ 7.0, HꢀC(9)); 3.51 (t,
J ¼ 6.5, HꢀC(7)), 2.13 (q, J ¼ 6.7, HꢀC(8)); 1.74 (s, HꢀC(5)). 13C-NMR ((D6)DMSO): 164.19 (C(4));

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Helvetica Chimica Acta – Vol. 94 (2011)
151.17 (C(2)); 140.38 (C(6)); 110.46 (C(5)); 47.26 (C(7)); 31.29 (C(8)); 29.76 (C(9)); 12.60 (MeꢀC(5)).
Anal. calc. for C8H11BrN2O2 (247.09): C 38.89, H 4.49, N 11.34; found: C 39.32, H 4.73, N 11.47.
1,1’,1’’-(Benzene-1,3,5-triyltrimethanediyl)tris{1’-[3-(3,4-dihydro-5-methyl-2,4-dioxopyrimidin-
1(2H)-yl)propyl]-4,4’-bipyridinium} Hexafluorophosphate (1). A soln. of 1,1’,1’’-(benzene-1,3,5-triyltri-
methanediyl)tris[4-(pyridin-4-yl)pyridinium] trihexafluorophosphate (214 mg, 0.20 mmol) in MeCN
(10 ml) were added to a stirred soln. of 1-(3-bromopropyl)-5-methyl-1H-pyrimidine-2,4-dione (1,570 g,
1.61 mmol) in MeCN (45 ml). The mixture was refluxed for 4 d at 858. The cooled mixture was added
dropwise to 30 ml of a stirred tetrabutylammonium chloride soln. (Bu4NCl; 5% in MeCN). The yellowish
precipitate was filtered off and washed three times with MeCN (10 ml each) and three times with CH2Cl2
(10 ml each). After drying for 24 h in vacuo, the resulting powder was dissolved in H2O (20 ml) and
precipitated with a NH4PF6 soln. (10% in H2O). The residue was washed several times with H2O and
dried in vacuo for 24 h to yield 285 mg of 1 (0.14 mmol, 70%). 1H-NMR (CD3CN): 9.34 (s, HꢀN(3)); 8.98
(d, J ¼ 6.5, HꢀC(5’’,3’’)); 8.93 (d, J ¼ 7.0, HꢀC(5’,3’)); 8.43 – 8.34 (m, HꢀC(2’,6’,2’’,6’’); 7.68 (s,
HꢀC(12,14,16)); 7.24 (s, HꢀC(6)); 5.86 (s, HꢀC(10)); 4.68 (t, J ¼ 7.2, HꢀC(9)); 3.80 (t, J ¼ 6.2,
HꢀC(7)); 2.39 (q, J ¼ 6.7, HꢀC(8)); 1.85 (s, HꢀC(5)). 13C-NMR (CD3CN): 165.4 (C(4)); 152.7 (C(4’’));
151.7 (C(4’’)); 151.1 (C(2)); 146.8 (C(3’,5’,3’’,5’’)); 141.7 (C(6)); 136.1 (C(13,15,11)); 132.8 (C(12,14,16));
128.5 (C(2’,6’,2’’,6’’)); 111,6 (C(5)); 64.8 (C(10)); 60.4 (C(9)); 45.3 (C(7)); 31.4 (C(8)); 12.4 (MeꢀC(5)).
1,1’,1’’-(Benzene-1,3,5-triyltrimethanediyl)tris{1’-[2-(4-chloro-7H-pyrrolo[2,3-d]pyrimidin-7-yl)eth-
yl]-4,4’-bipyridinium} Hexachloride (2a). A soln. of 1,1’,1’’-(benzene-1,3,5-triyltrimethanediyl)tris[4-
(pyridin-4-yl)pyridinium] trihexafluorophosphate (81 mg, 0.08 mmol) in MeCN (16 ml) were added in
portions of 1 ml within 4 h to a stirred soln. of 7-(2-bromoethyl)-4-chloro-7H-pyrrolo[2,3-d]pyrimidine
(104 mg, 0.4 mmol) in MeCN (10 ml). The mixture was refluxed for three d at 708. The cooled mixture
was added dropwise to 15 ml of a stirred Bu4NCl soln. (5% in MeCN). The yellowish precipitate was
filtered off and washed three times with MeCN (10 ml each) and three times with CH2Cl2 (10 ml each).
After drying for 24 h in vacuo, 87 mg of 2a (0.067 mmol, 84%) were obtained as a brown powder.
1H-NMR (D2O): 10.0 (d, J ¼ 6.5, HꢀC(6’’,2’’)); 9.45 (d, J ¼ 6.0, HꢀC(6’,2’)); 9.15 (d, J ¼ 6.5, HꢀC(5’’,3’’));
8,95 (d, J ¼ 7.0, HꢀC(5’,3’)); 8.22 (s, HꢀC(12,14,16)); 8.02 (s, HꢀC(2)); 7.85 (d, J ¼ 3.0, HꢀC(6)); 7.10 (d,
J ¼ 4.0, HꢀC(5)); 5.99 (s, HꢀC(10)); 4.89 (t, J ¼ 11.5, HꢀC(8)); 4.06 (t, J ¼ 11.5, HꢀC(9)). 13C-NMR
(D2O): 153.8 (C(4’)); 151.5 (C(4)); 150.9 (C(4’’)); 150.1 (C(6’’,2’’)); 149.3 (C(7a)); 146.0 (C(6’,2’)); 144.6
(C(5’’,3’’)); 135.4 (C(11,13,15)); 134.6 (C(12,14,16)); 130.6 (C(6)); 127.2 (C(5’,3’)); 121.8 (C(2)); 110.1
(C(4a)); 98.0 (C(5)); 62.8 (C(10)); 46.3 (C(8)); 31.5 (C(9)).
1,1’,1’’-(Benzene-1,3,5-triyltrimethanediyl)tris{1’-[2-(2-amino-4-chloro-7H-pyrrolo[2,3-d]pyrimidin-
7-yl)ethyl]-4,4’-bipyridinium} Hexabromide (2b). A soln. of 1,1’,1’’-(benzene-1,3,5-triyltrimethanediyl)-
tris[4-(pyridin-4-yl)pyridinium] trihexafluorophosphate (81 mg, 0.08 mmol) in MeCN (16 ml) were
added dropwise in portions of 1 ml within 4 h to a soln. of 7-(bromomethyl)-4-chloro-7H-pyrrolo[2,3-
d]pyrimidin-2-amine (110 mg, 0.4 mmol) in MeCN (16 ml). The mixture was refluxed for 3 h at 708. The
cooled mixture was filtered, and washed three times with MeCN and Et2O, resp., and dried. The mother
liquor was evaporated, and the residue was dried. The combined solids were resuspended in H2O (15 ml)
and precipitated with 6 ml of a NH4PF6 soln. (10% in H2O). The residue was washed several times with
H2O and dried in vacuo for 48 h. Subsequently, anion-exchange chromatography was applied with 103 mg
of a Brꢀ/PFꢀ6 mixed salt of the crude product and a 5% Bu4NBr soln. (8 ml) to afford 96 mg of 2b
(0.058 mmol, 72%). Pink powder. 1H-NMR (D2O): 9.85 (d, J ¼ 5.0, HꢀC(6’’,2’’)); 9.36 (d, J ¼ 6.0,
HꢀC(6’,2’)); 8.91 (d, J ¼ 6.0, HꢀC(5’’,3’’)); 8.81 (d, J ¼ 5.0, HꢀC(5’,3’)); 7.78 (s, HꢀC(12,14,16)); 7.58 (d,
J ¼ 3.5, HꢀC(10)); 6.78 (d, J ¼ 3.5, HꢀC(11)); 6.15 (s, HꢀC(10)); 4.64 (t, J ¼ 11.0, HꢀC(8)); 3.98 (t, J ¼
11.0, HꢀC(9)). 13C-NMR (D2O): 157.1 (C(2)); 153.1 (C(4’)); 153.9 (C(7a)); 150.6 (C(4’’)); 149.5
(C(4)); 144.9 (C(6’’,2’’)); 144.4 (C(6’,2’)); 136.1 (C(11,13,15,)); 131,3 (C(12,14,16)); 126.9 (C(5’’,3’’));
123.8 (C(5’,3’)); 116.7 (C (4a)); 98.2 (C(6)); 97.8 (C(5)); 62.6 (C(10)); 58.9 (C(8)); 45.1 (C(9)).
REFERENCES
[1] J. E. Betancourt, J. M. Rivera, Org. Lett. 2008, 10, 2287.
[2] F. Puntoriero, P. Ceroni, V. Balzani, G. Bergamini, F. Vçgtle, J. Am. Chem. Soc. 2007, 129, 10714.

Helvetica Chimica Acta – Vol. 94 (2011)
1101
[3] M. Gingras, J.-M. Raimundo, Y. M. Chabre, Angew. Chem. 2007, 119, 1028; M. Gingras, J.-M.
Raimundo, Y. M. Chabre, Angew. Chem., Int. Ed. 2007, 46, 1010.
[4] B. M. Rosen, C. J. Wilson, D. A. Wilson, M. Peterca, M. I. Imam, V. Percec, Chem. Rev. 2009, 109,
6275.
[5] N. Sakai, Y. Kamikawa, M. Nishii, T. Matsuoka, T. Kato, S. Matile, J. Am. Chem. Soc. 2006, 128, 2218.
[6] Y. Ochi, M. Suzuki, T. Imaoka, M. Murata, H. Nishihara, Y. Einaga, K. Yamamoto, J. Am. Chem.
Soc. 2010, 132, 5061.
[7] F. Vçgtle, S. Gestermann, R. Hesse, H. Schwierz, B. Windisch, Prog. Polym. Sci. 2000, 25, 987.
[8] V. Percec, M. Glodde, T. K. Bera, Y. Miura, I. Shiyanovskaya, K. D. Singer, V. S. K. Balagurusamy,
P. A. Heiney, I. Schell, A. Rapp, H.-W. Spiess, D. Hudson, H. Duan, Nature 2002, 417, 384.
[9] B. Helms, E. W. Meijer, Science 2006, 313, 929.
[10] V. Percec, D. A. Wilson, P. Leowanawat, C. J. Wilson, A. D. Hughes, M. S. Kauscher, D. A. Hammer,
D. H. Levine, A. J. Kim, F. S. Bates, K. P. Davis, T. P. Lodge, M. L. Klein, R. H. DeVane, E. Aqad,
B. M. Rosen, A. O. Argintaru, M. J. Sienkowska, K. Rissanen, S. Nummelin, J. Ropponen, Science
2010, 328, 1009.
[11] D. Astruc, E. Boisselier, C. Ornelas, Chem. Rev. 2010, 110, 1857.
[12] P. M. S. Monk, ꢂThe Viologensꢃ, John Wiley & Sons, Chichester, 1998.
[13] M. Mçller, S. Asaftei, D. Corr, M. Ryan, L. Walder, Adv. Mater. 2004, 16, 1558.
[14] V. Balzani, S. Ceroni, S. Gestermann, C. Kauffmann, M. Gorka, F. Vçgtle, Chem. Commun. 2000,
853.
[15] M.-H. Xu, J. Lin, Q.-S. Hu, L. Pu, J. Am. Chem. Soc. 2002, 124, 14239.
[16] C. M. Ronconi, J. F. Stoddart, V. Balzani, M. Baroncini, P. Ceroni, C. Giansante, M. Venturi,
Chem.ꢀEur. J. 2008, 14, 8365.
[17] S. Asaftei, E. De Clercq, J. Med. Chem. 2010, 53, 3480.
[18] V. Balzani, A. Credi, F. M. Raymo, J. F. Stoddart, Angew. Chem., Int. Edn. 2000, 39, 3348.
ˇ
´
[19] D. B. Amabilino, P. R. Ashton, M. Belohradsky, F. M. Raymo, J. F. Stoddart, J. Chem. Soc., Chem.
Comm. 1995, 751.
[20] W. P. Cochrane, P. L. Pauson, T. S. Stevens, J. Chem. Soc. C 1968, 630.
[21] F. Vçgtle, M. Zuber, R. G. Lichtenthaler, Chem. Ber. 1973, 106, 717.
[22] B. Nawrot, O. Michalak, S. Olejniczak, M. W. Wieczorek, T. Lis, W. J. Stec, Tetrahedron 2001, 57,
3979.
[23] H. Vorbrꢀggen, K. Krolikiewicz, B. Bennua, Chem. Ber. 1981, 114, 1234.
[24] O. F. Schall, G. W. Gokel, J. Am. Chem. Soc. 1994, 116, 6089.
[25] N. Baret, J.-P. Dulcere, J. Rodriguez, J.-M. Pons, R. Faure, Eur. J. Org. Chem. 2000, 1507.
[26] S. Asaftei, M. Reichelt, H. Reuter, H. Rosemeyer, Helv. Chim. Acta 2009, 92, 1944.
[27] C. L. Bird, A. T. Kuhn, Chem. Soc. Rev. 1981, 10, 49.
[28] A. Mirzoian, A. E. Kaifer, Chem.ꢀEur. J. 1997, 3, 1052.
[29] I. B. Berlman, ꢂ Handbook of Fluorescence Spectra of Aromatic Moleculesꢃ, Academic Press,
London, 1965.
[30] E. M. Kosower, J. L. Cotter, J. Am. Chem. Soc. 1964, 86, 5524.
Received September 23, 2010