Unusual supramolecular donor-acceptor complexes of bis(crown)stilbenes and bis(crown)azobenzene with viologen analogs

Article abstract of DOI:10.1007/s11172-008-0118-4

The results of the comprehensive study of novel supramolecular donor-acceptor complexes of bis(crown)stilbenes and bis(crown)azobenzene with viologen analogs are generalized. The original methodology for self-assembling of the organic donor-acceptor complexes possessing a very high thermodynamic stability is described. The hydrogen bonds between the peripheral fragments of the donor and acceptor play the key role in the self-assembling. The influence of different structural factors on the thermodynamic stability of the supramolecular donor-acceptor complexes and the efficiency of charge-transfer interactions between the donor and acceptor are discussed. The driving forces of the reaction leading to exotic trimolecular charge-transfer complexes are considered.

Full text of DOI:10.1007/s11172-008-0118-4

Russian Chemical Bulletin, International Edition, Vol. 57, No. 4, pp. 793—801, April, 2008
793
Unusual supramolecular donor—acceptor complexes of bis(crown)stilbenes
and bis(crown)azobenzene with viologen analogs
S. P. Gromov,★ A. I. Vedernikov, E. N. Ushakov, and M. V. Alfimov
Photochemistry Center, Russian Academy of Sciences,
7A ul. Novatorov, 119421 Moscow, Russian Federation.
Fax: +7 (495) 936 1255. Eꢀmail: gromov@photonics.ru
The results of the comprehensive study of novel supramolecular donor—acceptor complexes of
bis(crown)stilbenes and bis(crown)azobenzene with viologen analogs are generalized. The original
methodology for selfꢀassembling of the organic donor—acceptor complexes possessing a very high
thermodynamic stability is described. The hydrogen bonds between the peripheral fragments of
the donor and acceptor play the key role in the selfꢀassembling. The influence of different structural
factors on the thermodynamic stability of the supramolecular donor—acceptor complexes and
the efficiency of chargeꢀtransfer interactions between the donor and acceptor are discussed. The driving
forces of the reaction leading to exotic trimolecular chargeꢀtransfer complexes are considered.
Key words: crown ethers, stilbenes, azobenzenes, viologen analogs, selfꢀassembling, hydrogen
bonding, donor—acceptor complexes, chargeꢀtransfer complexes.
Introduction
Molecular systems consisting, as a rule, of two comꢀ
ponents, namely, donor (D) and acceptor (A), whose
interaction occurs mainly with a partial charge transfer,
are named chargeꢀtransfer complexes (CTC). Comꢀ
pounds containing strong electronꢀwithdrawing substituꢀ
ents (such as cyano and nitro groups) are used as accepꢀ
tors, whereas electronꢀrich compounds containing amiꢀ
no groups, alkoxy groups, and fragments with donating
sulfur atoms serve as donors.1,2
logK1 = 1.47
logK1 = 1.89
Donor—acceptor complexes (D—A complexes) play
an important role in many chemical and photochemical
reactions1,3 and in molecular selfꢀassembling processꢀ
es4,5 and are found in biological systems.6 Chargeꢀtransꢀ
fer complexes are of considerable interest as organic conꢀ
ductors7 and photoconductors,8 as well as materials with
nonlinear optical properties.9
two terminal ammonium groups were chosen as accepꢀ
tor components. As known,11 the ammonium cations
with 18ꢀcrownꢀ6 ethers form stable host—guest comꢀ
plexes through hydrogen bonds. Bis(crown)stilbenes and
bis(crown)azobenzene capable of ditopic complexation
with the diammonium compounds were chosen as donor
components.
The pyrene complexes with tetracyanoethylene and
tetracyanoquinodimethane are examples of organic charꢀ
geꢀtransfer complexes. However, these CTC have a subꢀ
stantial drawback: their stability constants are not high,1
and the CTC decompose easily to the starting compoꢀ
nents even upon a slight dilution of the solutions.
We suggested that the formation of hydrogen bonds
between the peripheral fragments of the donor and acꢀ
ceptor can provide both the very high thermodynamic
stability of the D—A complex and spatial preorganiꢀ
zation of the components for a more efficient interacꢀ
tion (Scheme 1).10 Analogs of viologens containing
Synthesis and structure of derivatives of the viologen
analogs, bis(crown)stilbenes, and bis(crown)azobenzene
Necessary for investigation ammonioalkyl derivatives
of the viologen analogs were synthesized by the quaterꢀ
nization reaction from the corresponding heterocyclic
bases in yields up to 72% (Scheme 2).10,12 A series of
bisquaternary salts 1—9 was obtained. They differ by
the length of the Nꢀammonioalkyl substituents or by
the acceptor ability of the central fragment.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 779—787, April, 2008.
1066ꢀ5285/08/5704ꢀ0793 © 2008 Springer Science+Business Media, Inc.

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Russ.Chem.Bull., Int.Ed., Vol. 57, No. 4, April, 2008
Gromov et al.
Scheme 1
In addition, we synthesized the model acceptors: anaꢀ
logs of compounds 1—9 bearing no ammonium groups in
the substituents at the N atoms and, hence, incapable of
hydrogen bonding. The acceptor ability of the central
fragment of the organic dication was also varied in this
series of compounds. The Xꢀray diffraction study demonꢀ
strated13 almost planar structure of their conjugated fragꢀ
ment (Fig. 1, a, b).
a
b
N(1A)
N(1)
N(1A)
N(1)
c
O(3)
O(6A)
O(2)
O(4)
O(1A)
O(5A)
O(5)
O(1)
O(2A)
O(4A)
O(6)
O(3A)
O(5A)
O(3A)
O(4A)
d
O(6A)
O(1)
O(2)
O(2A)
O(1A)
O(6)
O(4)
O(3)
O(5)
Fig. 1. Structures of the N,N´ꢀdiethylꢀ(Е)ꢀ1,2ꢀdi(4ꢀquinolyl)ethylene dication (a, b) and bis(18ꢀcrownꢀ6)stilbene 12 (c, d) in the front (a, c)
and side (b, d) projections.

Complexes of bis(crown) ethers and viologens
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 4, April, 2008
795
Scheme 2
Scheme 3
m = 0 (10), 1 (11), 2 (12)
ing interactions to occur in the CTC. Another specific
feature of tetrasubstituted stilbenes is their ability to exist
as two conformers (Scheme 4) with different distances
between the crownꢀether fragments, which allows a stilꢀ
bene molecule to be finely tuned to the effective length of
the acceptor components.
Scheme 4
Bis(crown)stilbenes were first synthesized according
to a described procedure.14 Unfortunately, the total yield
of the target compounds ranged from several percentage
to 31% and their purification was very laborꢀconsuming.
Therefore, we developed15 a simple and highly efficient
oneꢀpot method for the synthesis of bis(crown)stilbenes
10—12, which made it possible to obtain these comꢀ
pounds virtually in any necessary amounts (Scheme 3).
The same procedure was used to synthesize model stilꢀ
bene 13 containing four methoxy groups, which reproꢀ
duce the electronꢀdonor effect of two crownꢀether moiꢀ
eties. Stilbene 13 forms no complexes with compounds
containing ammonium groups.
To extend the series of donor components, we syntheꢀ
sized bis(18ꢀcrownꢀ6)azobenzene 14 using the twoꢀstep
The crystal and molecular structures of compounds
10—13 were studied15 by Xꢀray diffraction analysis and
1H and 13C NMR spectroscopy. As turned out, the stilꢀ
bene fragments of all molecules (see, e.g., Fig. 1, c and d)
are almost planar, which is important for efficient stackꢀ

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Gromov et al.
ε•10–3/L mol–1 cm–1
synthesis according to a recently described modified proꢀ
cedure.16
The structure of compound 14 has not earlier been
studied by Xꢀray diffraction analysis. We found that the
crystals of this compound, unlike bis(crown)stilbenes,
contain simultaneously the both conformers.17
1.2
1
0.8
0.4
0
Selfꢀassembling, structure, and properties
of supramolecular donor—acceptor complexes
2
The starting components for the synthesis of D—A
complexes have absorption maxima only in the shortꢀ
wavelength spectral region. Upon mixing of their colorꢀ
less or yellowish solutions, a deep color appears with
a color change from greenishꢀbrown to redꢀviolet, indiꢀ
cating the interaction of the components to form the CTC
(Scheme 5).10,12,18 For instance, the absorption specꢀ
trum of the D—A complex between stilbene 12 and acꢀ
ceptor 2 contains the characteristic chargeꢀtransfer band
with a maximum at ~500 nm (Fig. 2).
3
400
500
600
λ/nm
Fig. 2. Absorption spectra of supramolecular CTC 2•12 (1) and
2•(12)2 (2) and the sum of the absorption spectra of acceptor 2 and
donor 12 (3) in MeCN.
crownꢀcontaining stilbene 12 with the metal ions. This
process is accompanied by the disappearance of the longꢀ
wavelength chargeꢀtransfer band.12 The most interesting
changes are observed in the fluorescence spectra. For
example, the initial CTC 2•12 does not fluoresce
(Fig. 3). However, the addition of barium or calcium
salts induces a strong fluorescence caused by the formaꢀ
tion of stilbene complexes with the metal ions (Scheꢀ
me 6).12 Since this reaction is ionꢀselective, supramolecꢀ
ular CTC can be used as fluorescent molecular sensors
for the determination of barium and calcium ions.10
Scheme 5
Scheme 6
M = Ba, Ca
We succeeded to synthesize the complex of bisꢀ
(crown)stilbene 11 with two calcium cations in the crysꢀ
talline form and to determine its structure. The data
obtained show that the compounds of this type are capaꢀ
ble of ditopic binding with both ammonium ions and
metal cations.19
The addition of barium or calcium ions to solutions of
the D—A complexes results in the displacement of the
acceptor component and formation of complexes of
The quantum chemical simulation confirmed the asꢀ
sumption that the acceptor and donor components in the

Complexes of bis(crown) ethers and viologens
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 4, April, 2008
797
I (arb. units)
3
2
1,12ꢀDiammoniododecane•12
1
0
1
2
3
CМ/CCTC
Fig. 3. Plots of the fluorescence intensity of supramolecular CTC
2•12 (2•10–6 mol L–1, MeCN) vs relative concentration of
the added salt: Mg(ClO4)2 (1), Ca(ClO4)2 (2), and Ba(ClO4)2 (3).
The observation wavelength was 375 nm.
supramolecular CTC are bound through six hydrogen
bonds between the ammonium groups of the substituents
at the N atoms and the oxygen atoms of the crownꢀether
fragments. The decrease in the number of methylene units
in the substituents at the N atoms shortens the distance
between the components, which can be accompanied by
an increase in the efficiency of the intermolecular charge
transfer.20
The NMR spectra make it possible to determine the
structures of all the supramolecular complexes syntheꢀ
sized. The changes in the chemical shifts in the 1H NMR
spectra of the series of the D—A complexes are shown
in Fig. 4. As should be expected, the interaction of
the ammonium groups with the crownꢀether moieties
results in the downfield shift of the signals of the majority
of their protons due to the electronꢀacceptor influence of
the ammonium ion. The protons of the double bonds and
aromatic protons exhibit a more complicated pattern
with the predomination of upfield shifts of the signals.
This can occur only if the protons of one component
appear in the shielding regions of another component,
i.e., for their mutual arrangement above each other.
A complete analysis of the values and directions of these
shifts suggests the specific orientation of the donor and
acceptor components relative to each other.12
Additional data on the mutual arrangement of the
components in the complex can be obtained from the
NOE spectra containing the signals that indicate the close
spatial arrangement of the chemically unbound protons.
In this case, these are the proton of the pyridine cycle and
the protons of the double bond and benzene ring. The
intermolecular NOE interactions determined for the seꢀ
ries of donor—acceptor complexes are shown in Fig. 5.
Their analysis and regularities of changes in the chemical
shifts show that the overlapping of the projections of
Fig. 4. Changes in the chemical shifts in the 1H NMR spectra for
selected D—A complexes.
the central fragments of the donor and acceptor is preꢀ
dominantly Xꢀlike.12 It is most likely that for complex
3•12 a considerably greater overlapping of the projecꢀ
tions of the conjugated fragments and their closer arꢀ
rangement in space are observed. All intermolecular cross
peaks are lowꢀintensity compared to the intramolecular
cross peaks, indicating relatively long distances between
the interacting protons of the donor and acceptor in the
complexes (3.5—5.0 Å).
In some cases, we succeeded to determine the strucꢀ
ture of the donor—acceptor complexes using Xꢀray difꢀ
fraction analysis. Steric correspondence is observed when
the viologen analog contains a double bond. The strucꢀ
ture of the complex of bis(crown)azobenzene 14 with the
ammoniopropyl derivatives of dipyridylethylene 3 is preꢀ

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Russ.Chem.Bull., Int.Ed., Vol. 57, No. 4, April, 2008
Gromov et al.
sented in Fig. 6. The central fragments of the donor and
acceptor components in this complex are characterized
by the planar geometry.17
The cyclic voltammograms of all the compounds studꢀ
ied exhibit many peaks corresponding to several stepped
redox transitions,21 the first of which are reversible in the
most part of cases. It follows from the electrochemical
measurements of the reduction potentials that the acceptꢀ
ing ability of the free tetracations increases in the followꢀ
ing order: dipyridylethylenes < diazapyrenes < violoꢀ
gens. However, the influence of the acceptor component
of the CTC on the first oxidation potential of donor 12
increases in an opposite order: viologens ≤ diazapyꢀ
renes < dipyridylethylenes. It is most likely that the optiꢀ
mum geometric correspondence between the central fragꢀ
ments of the donor and acceptor for the dipyridylethylene
derivatives and the planar structure of the acceptor in
the case of the diazapyrene derivatives favor a more effiꢀ
cient chargeꢀtransfer interaction in the D—A complex.
The structures of the supramolecular CTC adsorbed
on the silver electrode surface were studied using specꢀ
troscopy of surfaceꢀenhanced Raman scattering.
The supramolecular CTC synthesized by us are charꢀ
acterized by a very high thermodynamic stability and do
not decompose to individual components even upon
a strong dilution of the solution. According to the data of
spectrophotometric measurements, the logarithm of
the stability constant (logK) for the D—A complex inꢀ
volving stilbene 12 and dipyridylethylene derivative 1 in
MeCN reaches 9.42 (Table 1). The logK value is 8.59 for
the complex of compound 12 with 1,12ꢀdiammoniododeꢀ
cane containing no donor—acceptor interactions. Comꢀ
paring these values, one can conclude that the donor—
acceptor interaction makes a much smaller contribution
to the stability of the supramolecular CTC than hydrogen
bonding.12,24
This conclusion is confirmed by the results of studyꢀ
ing the complex formation of stilbene 12 with the violoꢀ
Fig. 5. Intermolecular NOE interactions for selected D—A comꢀ
plexes.
O(4A)
O(5A)
N(2A)
N(2)
O(3´A)
O(3´)
N(3A)
O(2)
O(3A)
O(6A)
N(1)
O(1)
O(3)
N(3)
O(1A)
O(2A)
O(6)
N(1A)
O(4)
O(5)
Fig. 6. Structure of bimolecular complexes 3•14. The perchlorate anions and the most part of hydrogen atoms are not shown. Hydrogen bonds
are shown by dashed lines.

Complexes of bis(crown) ethers and viologens
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 4, April, 2008
799
Table 1. Stability constants of the bimolecꢀ
ular D—A complexes of bis(crown)stilbene
12 with viologen analogs*
Scheme 7
Viologen analog
logK1
1
2
3
4
9.42
9.08
8.67
8.66
9.15
8.59
7
1,12ꢀDiammoniododecane
* Spectrophotometric titration, MeCN,
22 °C.
gen analogs containing no ammonium groups and, hence,
incapable of hydrogen bonding. It turned out that for
the complexes with the model acceptors logK < 2 (for
instance, for the dipyridylethylene derivative logK = 1.13).
Thus, the high stability of the supramolecular CTC is
achieved almost completely due to hydrogen bonds.12
The stabilization effect due to hydrogen bonds is so
high that allows one to obtain exotic trimolecular chargeꢀ
transfer complexes (Scheme 7). Upon the addition of
a large amount of stilbene 12 to a solution of complex
2•12 we observed a considerable increase in the intensity
and the bathochromic shift of the longꢀwavelength
chargeꢀtransfer absorption band (see Fig. 2).18
observed. This can occur only if the protons of acceptor 2
fall on the shielding region of simultaneously two donor
molecules. Therefore, all the three components of
the formed trimolecular complex are arranged one above
another to form a sandwich structure in which the accepꢀ
tor component 2 is localized between two molecules
of compound 12. Similar effects were also observed
for other donor—acceptor trimolecular complexes based
on bis(18ꢀcrownꢀ6)stilbene and bis(18ꢀcrownꢀ6)azoꢀ
benzene.
The NMR study suggests a possible structure of
the trimolecular complex. When stilbene 12 is added to
a solution of bimolecular complex 2•12, rather large
upfield shift of the signals of the aromatic protons and the
protons of the double bond in the acceptor component is
In one case, we succeeded to determine24 the strucꢀ
ture of the trimolecular complex using Xꢀray diffraction
analysis (Fig. 7). The results obtained confirmed an asꢀ
O(10A)
O(12A)
O(11A)
O(6A)
O(1WA)
N(1A)
O(9A)
O(7A)
O(1A)
O(5A)
O(2A)
O(4A)
O(8A)
N(2A)
O(3)
N(2)
O(3A)
O(8)
O(4)
O(2)
O(1W)
O(9)
O(1)
O(6)
N(1)
O(7)
O(12)
O(5)
O(10)
O(11)
Fig. 7. Structure of trimolecular complex 5•(12)2. The perchlorate anions and the most part of hydrogen atoms are not shown. Hydrogen
bonds are shown by dashed lines.

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Gromov et al.
Table 2. Stability constants of the trimoꢀ
lecular D—A—D complexes of bisꢀ
(crown)stilbene 12 with viologen anaꢀ
logs 1—5 and 7*
inverse electron transfer reaction is 2 times slower in this
case. This is possibly caused by the delocalization of
the excited electron state of the symmetric D—A—D
complex due to the adiabatic interaction of two equivaꢀ
lent states with charge transfer. The results obtained need
additional theoretical interpretation.25
Viologen analog
logK2
1
2
3
4
5
7
2.73
3.20
2.18
2.54
3.80
1.73
Conclusion
Thus, we showed that the ammonioalkyl derivatives
of the viologen analogs form stable 1 : 1 complexes with
bis(crown)stilbenes and bis(crown)azobenzene due to
ditopic binding involving hydrogen bonds. The chargeꢀ
transfer complexes under study are promising examples
of supramolecular systems capable of inducing a strong
optical response upon the interaction with metal cations.
The ammonioalkyl derivatives of the viologen analogs
can also form stable 2 : 1 complexes with the biscrownꢀ
containing donor compounds. These complexes have
a threeꢀdeck sandwich structure in which the acceptor
component is localized between two donor molecules.
The regularities found for the formation of the supramoꢀ
lecular D—A complexes of the new type can be used for
the design of optical sensors, organic semiconductors,
and conductors in molecular electronics and of photoꢀ
sensitive architecture in nanotechnology.
* Spectrophotometric titration, MeCN,
22 °C.
sumption about the coordination of the ammonium groups
of acceptor component 5 with the crownꢀether fragments
of two molecules of stilbene 12. The central parts of the
acceptor and donor components in complex 5•(12)2 are
almost planar, indicating a low degree of tension in this
structure and a possibility of efficient stacking interacꢀ
tions.
We measured the stability constants of the trimolecuꢀ
lar donor—acceptor complexes (Table 2). It turned out
that the values of the constants depend on the length of
the ammonioalkyl substituent, the accepting ability of
the central fragment of the molecule, and steric correꢀ
spondence of the acceptor component to the size of
the gap between two molecules of crownꢀcontaining stilꢀ
bene.12 Analyzing the data obtained, we concluded that
the main driving force of the reaction affording the trimoꢀ
lecular CTC (see Scheme 7) is steric strain in the precurꢀ
sor bimolecular complex.
The data on the dynamics of the excited electron state
of bimolecular CTC 2•12 obtained by femtosecond specꢀ
troscopy are presented in Fig. 8. The excitation of the
complex by the visible light results directly in the lowest
singlet excited electronꢀtransfer state. After the very fast
internal vibrational relaxation, the excited electron state is
deactivated via the inverse electron transfer with a time
constant of 540 fs.25
This work was financially supported by the Division
of Chemistry and Materials Science of the Russian Acadꢀ
emy of Sciences (Program No. 2), the Russian Foundaꢀ
tion for Basic Research (Project Nos 06ꢀ03ꢀ32434,
06ꢀ03ꢀ33162, and 08ꢀ03ꢀ00577), the Ministry of Educaꢀ
tion and Science of the Russian Federation, and the INTAS
and CRDF Foundations.
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