Fluorescent and electroactive cyclic assemblies from perylene tetracarboxylic acid bisimide ligands and metal phosphane triflates

Article abstract of DOI:10.1002/1521-3765(20010216)7:4<894::AID-CHEM894>3.0.CO;2-N

Tetraaryloxy-substituted perylene tetracarboxylic acid bisimides with one or two 4-pyridyl receptor substituents at the imide functionality were synthesized and employed in transition metal directed self-assembly with Pd^II and Pt^II p

Full text of DOI:10.1002/1521-3765(20010216)7:4<894::AID-CHEM894>3.0.CO;2-N

FULL PAPER
Fluorescent and Electroactive Cyclic Assemblies from Perylene
Tetracarboxylic Acid Bisimide Ligands and Metal Phosphane Triflates
Frank Würthner,*[a] Armin Sautter,[a] Dietmar Schmid,[b] and Peter J. A. Weber[c]
Abstract: Tetraaryloxy-substituted per-
ylene tetracarboxylic acid bisimides with
one or two 4-pyridyl receptor substitu-
ents at the imide functionality were
synthesized and employed in transition
metal directed self-assembly with PdII
and PtII phosphane triflates. Upon mix-
ing of the components, quantitative for-
mation of functional molecular square-
type complexes containing four dye
molecules and model complexes of a
2:1 (perylene bisimide ligand:transition
metal ion) stoichiometry was observed.
The isolated metallosupramolecular
cyclic dye assemblies (molecular weight
(3a) ˆ 8172, Pt Pt corner diagonal ca.
3.4 nm). Studies of the optical absorp-
tion and fluorescence properties and the
electrochemistry and spectroelectro-
chemistry of both the perylene bisimide
ligands and the perylene bisimide metal
complexes show that PtII coordination
does not interfere with the optical and
electrochemical properties of the per-
ylene bisimide ligands; this gives squares
with high fluorescence quantum yields
(FF (3a) ˆ 0.88) and three fully rever-
sible redox couples. The latter could be
unambiguously related to quantitative
formation of perylene bisimide radical
‡
cations (E1/2 ˆ ‡0.93 V vs. Fc/Fc ), rad-
‡
ical anions (E1/2
and dianions (E1/2
ˆ
1.01 V vs. Fc/Fc ),
‡
ˆ
1.14 V vs. Fc/Fc );
these redox reactions change the charge
state of the cyclic assembly from ‡12 to
zero. In contrast, PdII coordination in-
fluenced the electrochemical properties
of the assembly because of an irrever-
sible palladium reduction at E1/2
ˆ
‡
1.15 V versus Fc/Fc . Finally, dynamic
ligand exchange processes between dif-
ferent metallosupramolecular assem-
blies were investigated by multinuclear
NMR and electrospray mass spectrom-
etry. These studies confirmed the rever-
sible nature of the pyridine PtII/PdII
coordination process.
1
squares were characterized by H and
31P {1H} NMR spectroscopy as well as
conventional electrospray ionization
(ESI) and ESI-FTICR mass spectrome-
try, which gave evidence for the struc-
ture and the high stability of these giant
Keywords: chromophores
voltammetry fluorescence
N ligands ´ supramolecular chemistry
´ cyclic
´
´
Introduction
The elucidation of the light-harvesting complex of purple
bacteria revealed cyclic arrangements of chromophores as a
fundamental structural feature for its functionality.[1] Therein,
an important aspect is the way that nature assembles a
considerable number of chlorophyll and b-carotene dyes by
purely noncovalent interactions between the dye molecules
and the proteins. This process may be exemplified by Figure 1,
where the circles represent the proteins and the rectangles the
Figure 1. Self-assembly of chromophores to cyclic structures by noncova-
lent interactions, as in the light-harvesting complex of purple bacteria
where n ˆ 8[1c] and n ˆ 9[1a] are the numbers of chlorophyll units organized
in the B800 ring.
[a] Dr. F. Würthner, A. Sautter
Abteilung Organische Chemie II, Universität Ulm
Albert-Einstein-Allee 11, 89081 Ulm (Germany)
Fax : (‡49)731-5022840
eight[1c] (or nine)[1a] chlorophyll dyes of the B800 unit (weakly
coupled dyes), respectively, in the crystallographically re-
solved examples (b-carotenes and a second ring of 16 or
18 chlorophyll units (strongly coupled dyes) were omitted for
simplicity). Indeed, this kind of structure formation is not only
elegant for economic reasons (only one protein has to be
encoded at the genetic level for a huge quaternary structure)
but results in an assembly of high symmetry that makes all
chromophores within the ring structurally and energetically
E-mail: frank.wuerthner@chemie.uni-ulm.de
[b] D. Schmid
Institut für Organische Chemie, Universität Tübingen
Auf der Morgenstelle 18, 72076 Tübingen (Germany)
[c] Dr. P. J. A. Weber
Fakultät für Biowissenschaften, Pharmazie & Psychologie
Institut für Biochemie, Universität Leipzig
Talstrasse 33, 04103 Leipzig (Germany)
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894±902
equal. This property seems to be important for the energy
transfer capabilities of these systems.
analyses, 1H and 31P {1H} NMR spectroscopy, and electrospray
mass spectrometry (ESI-MS).
Since the discovery of these natural light-harvesting sys-
tems, several groups have explored the possibilities for the
arrangement of dye molecules in high symmetry in a cyclic
fashion by self-assembly.[2±5] The concept of molecular squares
introduced by Fujita and Ogura[2a] proved to be especially
powerful in the preparation of macrocycles in quantitative
yields by a thermodynamically controlled self-assembly
process between linear bridging ligands and cis-coordinating
transition metal ions. Recently, cyclic porphyrin assemblies
and luminescent rhenium containing molecular squares were
reported,[3c, 4, 5] which show the rapidly increasing interest in
the introduction of functional properties in such systems.
Indeed, optical and electrochemical addressability in combi-
nation with guest inclusion should open up ways toward
nanoreactors, in which it might become possible to specifically
trigger the reactivity of the enclosed guest molecules.[6]
Herein, we present the synthesis and characterization of a
new class of molecular squares based on highly fluorescent
and electroactive perylene tetracarboxylic acid bisimides that
bear 4-pyridine receptor substituents.[7, 8] Coordination of
these ligands to cis-PdII and PtII metal ions results in the
formation of cyclic arrangements of four perylene bisimide
chromophores in close vicinity in a highly symmetric molec-
ular square-type structure of considerable size in the nano-
meter regime. For reasons of comparison and simplicity, we
also prepared model complexes that contain two monotopic
perylene bisimide ligands coordinated to one metal ion
corner. We investigated the fluorescence, electrochemical as
well as spectroelectrochemical properties of the perylene
bisimide ligands and the assemblies containing two and four
dyes. Finally, dynamic ligand exchange experiments in mixed
systems were evaluated to prove the reversibility of the
metal ± ligand coordination process, which is of fundamental
importance in supramolecular self-organization.
The 1H NMR spectra of all complexes show species of high
symmetry with only a single set of signals for both the dppp
moieties and the perylene bisimide ligands and characteristic
changes in the chemical shifts for the a- and b-pyridyl protons
(Ddꢀ0.3 ppm) with respect to the free ligands (Figure 2a).[9b]
All 31P {1H} NMR spectra show only one sharp singlet that is
shifted upfield by about Dd ˆ 10 ppm with respect to the
uncomplexed precursor complexes [M(dppp)][(OTf)2] (M ˆ
Pt2‡/Pd2‡) (Figure 2b).[9b] In addition, 195Pt-31P spin ± spin
coupling results in Pt satellites in the spectra of 3 and 7. The
a-pyridyl protons and the protons of the dppp ligand exhibit
rather broad 1H NMR signals both in the 2:1 model complexes
(7, 8) and the molecular squares (3a, b, 4). As no concen-
tration dependence but a distinct temperature dependence
could be observed, we attribute this line broadening to
conformational flexibility of the dppp unit and not to complex
dissociation. A temperature-dependent 1H NMR study of PdII
complex 8 revealed a coalescence temperature of around
300 K. Below that temperature, each of the broad singlets is
split into three signals. At higher temperatures, the broad
singlets sharpen notably. Similar observations have already
been reported for related metal dppp pyridine complex-
es.[9a, c] The respective X-ray crystal structures revealed that
the dppp phenyl rings and the complexing pyridine rings are
within the distance of p ± p interaction;[9] thus this restricts the
conformational flexibility of the dppp ligand and explains the
1
observed temperature-dependent broad H NMR signals of
the a-pyridyl and dppp protons.
ESI-MS allowed us to unambiguously prove the existence
of perylene bisimide platinum squares 3a and 3b. Addition-
ally, 3a was investigated by using an Electrospray Fourier
Transform Ion Cyclotron mass spectrometer (ESI-FTICR-
MS). This technique was used because of both its capability of
achieving extremely high mass resolution and the possibility
of measuring with a very high mass accuracy.[10] With this
method, signals that arise from the isotopic distribution of
multiply charged large molecules generated by ESI can be
resolved. From the isotopic pattern, the charge state can be
derived, and thus the molecular weight can be calculated. In
view of the high degree of symmetry of molecule 3a, it is
important to prove that the signal is not caused by a species
with a lower charge state but with the same m/z value.
In the overview spectrum (solution of 3a in acetone), the
main signals are the differently charged intact square species
[3a 3OTf]3‡, [3a 4OTf]4‡, [3a 5OTf]5‡, and [3a
6OTf]6‡ generated by the successive loss of triflate anions
(Figure 3). The very high resolution power of FTICR-MS
allows unambiguously the assignment of all these signals by
way of their characteristic peak separation of 1/3 mass unit for
the [3a 3OTf]3‡ species, 1/4 mass unit for the [3a 4OTf]4‡
species, 1/5 mass unit for the [3a 5OTf]5‡ species, and 1/6
mass unit for the [3a 6OTf]6‡ species. The experimental
masses and isotopic patterns closely match the calculated ones
and, with the exception of the signals that are marked with
asterisks in Figure 3, all other signals in the spectrum of 3a
could be assigned to fragments of 3a, for example, loss of one
perylene bisimide ligand L ([3a 1L]n‡, n ˆ 2, 3; m/z 3368.4,
Results and Discussion
Synthesis and characterization of perylene bisimide ligands
and perylene bisimide metal complexes: The ditopic perylene
bisimide ligands 2 were synthesized by condensation of the
corresponding perylene bisanhydrides 1[8] with 4-aminopyr-
idine in quinoline with catalytic amounts of zinc(ii)acetate.
The same reaction afforded the monotopic perylene bisimide
ligand 6 if a mixture of 1a and 5, obtained by partial
saponification of the corresponding N,N'-dibutyl perylene
bisimide (see Experimental Section), was allowed to react
(Scheme 1). The monotopic ligand 6 could be separated from
2a by chromatography.
The perylene bisimide metal complexes were prepared in
nearly quantitative yields by simply mixing the perylene
bisimide ligands with [M(dppp)][(OTf)2] (dppp ˆ 1,3-bis-(di-
phenylphosphano)propane;
OTf ˆ trifluoromethanesulfo-
nate) in dichloromethane at room temperature (Scheme 1).
Precipitation with diethylether afforded analytically pure
complexes 3, 4, 7, and 8 that were characterized by elemental
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Scheme 1. Synthesis of mono- and ditopic perylene bisimide ligands (2, 6) and formation of perylene bisimide metal squares (3, 4) and model complexes (7,
8): a) 4-aminopyridine, Zn(OAc)2, quinoline, 1808C, 16 h, argon, 59 ± 66%; b) [M(dppp)][(OTf)2], CH2Cl2, RT, 24 h, argon, 88 ± 94%; c) [M(dppp)]-
[(OTf)2], CH2Cl2, RT, 24 h, argon, 71 ± 89%.
Figure 3. ESI-FTICR-MS of perylene bisimide platinum square 3a
(solution in acetone).
Figure 2. a) 1H NMR spectra of 2a (top) and 3a (bottom); b) 31P {1H}
NMR spectra of [Pt(dppp)][(OTf)2] (top) and 3a (bottom).
not multiples of other possible charge states other than 1 ‡ or
3 ‡ and 1 ‡ , respectively, so that overlapping with other
2195.9), loss of one platinum phosphane corner C ([3a 1C]n‡
,
signals could be excluded. This was confirmed by measuring
the species [3a 5OTf]5‡ with a resolution of 600000
(FWHH) in the ultra high resolution mode.
n ˆ 2, 4; m/z 3484.3, 1667.6), and loss of both ([3a 1L 1C]2‡,
m/z 2915.9).
In Figure 4, the comparison of the measured and calculated
spectra of a) the species [3a 6OTf]6‡ and b) [3a 5OTf]5‡
with a resolution of 60000 to 50000 (FWHH, full peak width
at half peak height) leaves no doubt about the results. In
addition, the 6 ‡ and 5 ‡ charge states of the two species are
Nonetheless, owing to the less stable Pd N bond, ESI-MS
characterization of the Pd square 4 failed and was difficult for
the Pd model complex 8 as thermal and collision dissociation
of the Pd complexes predominated in the spectrometer under
the applied ESI conditions.
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Fluorescent and Electroactive Cyclic Assemblies
894±902
Figure 6. Cyclic voltammogram of ditopic perylene bisimide ligand 2a in
CH2Cl2 (sweep rate 100 mVs 1).
systems and attributed to oxidation processes that involve the
nitrogen lone pairs.[8d]
Figure 4. Comparison of the calculated (top) and measured (bottom)
To assign the redox couples, 2a was studied by spectroelec-
trochemistry, and pronounced changes in the UV/Vis-NIR
absorption spectra were observed when a negative potential,
that was increased in a stepwise manner, was applied. In the
vicinity of the first reduction of 2a, the absorption bands of
the neutral species 2a diminish (585 nm, 544 nm, 454 nm),
while new bands in the NIR appear with a very intense
absorption at 792 nm and less intense absorptions at 977 nm
and 1085 nm, typical for radical anionic species (Figure 7a).
After a further increase of the potential towards the second
spectra a) of the [3a 6OTf]6‡ and b) [3a 5OTf]5‡ species.
Functional properties of perylene bisimide ligands: The
optical absorption and fluorescence properties of the ligands 2
and 6 and their relative fluorescence quantum yields FF were
investigated. For 2a, we also studied the electrochemical and
spectroelectrochemical properties. All perylene bisimide
ligands show an absorption maximum at 585 Æ 4 nm and an
emission maximum at 618 Æ 3 nm in dichloromethane; this
shows the negligible effect of the different phenoxy and imide
substituents in 2 and 6 on the absorption and emission
properties of the tetraphenoxyperylene bisimide chromo-
phore. The UV/Vis and fluorescence spectra of 2a are shown
in Figure 5. The relative fluorescence quantum yields FF are
very high and range between 0.88 Æ 0.05 (6) and 0.94 Æ 0.05
(2a) in chloroform.
.
reduction process, the absorption bands of 2a completely
disappear, while a broad absorption of tetraphenoxy perylene
bisimide dianionic species 2a2 appears at 678 nm (Figure 7b).
Thus, 2a is reduced by two one electron reductions via its
.
radical anionic states 2a to the dianionic species 2a2 .
Figure 7. Spectroelectrograms of ditopic perylene bisimide ligand 2a in
CH2Cl2. Stepwise increase of the applied potential to a) the first reduction
.
(radical anionic perylene bisimide species 2a ) and b) second reduction of
2a (dianionic perylene bisimide species 2a2 ).
Figure 5. UV/Vis absorption and fluorescence spectra of ditopic perylene
bisimide ligand 2a (ÐÐ) (c ˆ 3.5  10 5 m) and the corresponding perylene
bisimide platinum square 3a (- - - -) (c ˆ 1.9  10 6 m) in CH2Cl2
(lex ˆ 550 nm).
Functional properties of perylene bisimide metal complexes:
Upon PdII and PtII metal coordination, bathochromic shifts of
5 ± 7 nm in the UV/Vis and fluorescence spectra of the
perylene bisimide ligands 2 and 6 are observed (Figure 5).
For the complexes 3, 4, 7, and 8, no new optical transitions
related to the metal phosphane corners or metal perylene
bisimide charge-transfer bands could be observed in the
visible region. As there are no significant changes or new
bands in the spectral fine structure, strong excitonic coupling
of the complexed perylene bisimide dyes is unlikely.[11]
Relative fluorescence quantum yields of FF ˆ 0.88 Æ 0.05
(3a) and 0.86 Æ 0.05 (4) for the perylene bisimide metal
squares were determined. In our opinion, this maintenance of
The electrochemical properties of 2a were investigated by
cyclic voltammetry in dichloromethane (Figure 6). The cyclic
voltammogram (CV) of the ditopic perylene bisimide ligand
2a shows two reversible reductions at E1/2
E1/2
oxidation process is irreversible, and this causes adsorption of
2a on the platinum electrode. A similar irreversible oxidation
process was observed for other azaaromatic conjugated
ˆ
1.08 V and
‡
ˆ
1.23 V, versus ferrocene/ferrocenium (Fc/Fc ), but the
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the highly desired fluorescence properties of the perylene
bisimide ligands in the metal-coordinated state is due to an
electronic decoupling of the coordination sites of the metal-
coordinating 4-pyridine unit from the HOMO and LUMO of
the perylene bisimide chromophores.[7, 12] As a consequence,
the excitation energy is emitted as fluorescence and not
quenched by coupling to low-lying MLCT (metal-ligand-
charge-transfer) transitions.
The CV of the perylene bisimide platinum square 3a
(Figure 8) exhibits two reversible waves in the reductive cycle
‡
(E1/2
ˆ
1.01 Vand E1/2
ˆ
1.14 V vs. Fc/Fc ). Upon platinum
coordination, both reductions are shifted by about 70 to
90 mV with respect to the perylene bisimide ligand 2a.
Figure 9. Spectroelectrograms of perylene bisimide platinum square 3a in
CH2Cl2. Increase of the applied potential to a) the first reduction,
b) second reduction, and c) first oxidation process of 3a; this generates
radical anionic (a), dianionic (b), and radical cationic (c) perylene bisimide
species.
Figure 8. Cyclic voltammogram of perylene bisimide platinum square 3a
in CH2Cl2 (sweep rate 100 mVs 1).
NIR at 809 and 908 nm along with a very broad band at
1225 nm (Figure 9c). Although we cannot compare these
changes to 2a, because of its irreversible oxidation, the
accompanying changes in the intensities of the perylene
bisimide UV/Vis bands let us believe that the perylene
bisimide radical cationic species are generated.
Two important conclusions are drawn from these spectro-
electrochemical studies: first, we note that the platinum
phosphane corner units are electrochemically inert within the
applied potential range between 1.3 and ‡0.9 V. They act
exclusively as structural building blocks that provide a 908
angle for perylene bisimide ligand coordination. Second, from
the complete disappearance of the absorption bands of the
neutral and radical anionic species in the spectroelectrochem-
ical experiments we conclude that all four perylene bisimide
ligands are independently oxidized and reduced in the square,
and this allows us to reversibly switch the charge state of the
cyclic perylene bisimide-platinum framework from 0 to ‡12
in steps of four electrons.[13]
Notably, it is possible to reversibly oxidize 3a at a potential of
E1/2 ˆ ‡0.93 V, whereas 2a was irreversibly oxidized and
adsorbed onto the platinum electrode. This is probably a
consequence of the pyridine platinum coordination, which
blocks the nitrogen lone pairs in a sense that they are no
longer susceptible to adsorption and facile oxidation. The
perylene platinum 2:1 complex 7 shows a similar CV with two
reversible reductions (E1/2
ˆ
1.15 V and E1/2
ˆ
1.29 V vs.
‡
Fc/Fc ) and one reversible oxidation wave (E1/2 ˆ ‡0.82 V).
Most interestingly, within the applied potential range, the
platinum phosphane corners are not involved in the redox
processes.
Further insight into the nature of the redox processes was
given by spectroelectrochemical studies of perylene bisimide
platinum square 3a, which could be compared with the
experiments of the corresponding perylene bisimide ligand
2a. An increase in the potential in a stepwise manner
towards the first reduction potential of 3a resulted in a
decrease of the UV/Vis absorption bands of 3a, whereas new
transitions in the NIR region appeared with a maximum
absorption at 791 nm and weaker transitions at 976 and
1082 nm (Figure 9a). Based on identical changes for the
ligand 2a (Figure 7), these new transitions are unambiguously
assigned to perylene bisimide radical anionic species. When
the potential is increased to the second reduction potential of
3a, the UV/Vis-NIR absorption bands of the perylene
bisimide radical anionic species completely disappear, while
a single broad absorption at 679 nm arises (Figure 9b). Again,
the same changes were observed for ligand 2a (Figure 7) and
therefore assigned to the perylene bisimide dianionic species.
When a positive potential is applied, new bands appear in the
The cyclic voltammograms of the palladium complexes 4
and 8 proved to be more complex, since an irreversible
palladium reduction is involved. Both the CVs of 4 and 8
display a reversible oxidation at E1/2 ˆ ‡0.93 V and E1/2
ˆ
‡
‡0.84 V (vs. Fc/Fc ), respectively, due to the formation of
perylene bisimide radical cationic species. The reductions of
both complexes show irreversible processes, for example, in
the CV of 8 a cathodic peak at 1.15 V appears with no peak
in the back scan, followed by the two perylene bisimide
reductions (Figure 10). Successive scans resulted in negatively
shifted potentials in the respective CVs. We assign the wave at
1.15 V to an irreversible palladium reduction, which might
be followed by changes in the coordination geometry of the
metal ion. The same occurs in the CV of 4, but the process is
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Fluorescent and Electroactive Cyclic Assemblies
894±902
that were employed, and this gives preference to the square
species containing only ligands of the same type.[9b, 14, 15] Our
bridging ligands 2a and 2b have the same length and differ
only in their alkyl groups at the phenoxy substituents.
Accordingly, we considered these ligands well-suited to study
exchange processes by ESI-MS and multinuclear NMR
spectroscopy.
In a first experiment, we mixed the two perylene bisimide
platinum squares 3a and 3b in a 1:1 stoichiometry in
deuterated chloroform. As shown in Figure 12, the resulting
Figure 10. Cyclic voltammogram of perylene bisimide palladium model
complex 8 in CH2Cl2. The arrow indicates an irreversible palladium
reduction.
overlaid by a perylene bisimide reduction in this case, which
leads to a significantly more intense first reduction wave.
Therefore, the accessible reversible potential range of the
perylene bisimide palladium complexes is limited from 1 to
about ‡1 V, which only allows us reversibly to switch the
charge state of the framework of square 4 between ‡8 and
‡12.
Dynamic ligand exchange processes: The essentially quanti-
tative formation of the molecular square-type complexes is
only conceivable by self-assembly with reversible intermolec-
ular coordination interactions. Exchange processes between
competing topologies (cyclic vs. linear oligomers) allow self-
repairing until the system finally reaches its thermodynami-
cally most stable state under the given experimental con-
ditions. According to a substantial proportion of published
work, the rigidity of the building blocks, the linear bridging
ligands, the inherent 908 angle of the adjacent coordination
sites in the cis-protected square planar metal ions, and a low
activation energy for the breaking of the metal pyridine bond
are considered to be important factors needed to drive the
assembly process to molecular squares to completion. Never-
theless, besides the consistently high yields, little proof of the
mechanism of reversible self-assembly is given in the liter-
ature of molecular squares so far. If, however, dynamic ligand
exchange processes could be observed in solutions of two
different squares that result in an equilibrium of different
ªmixedº square species (as depicted in Figure 11), there are
strong arguments for the reversibility of the bond-formation-
bond-breaking equilibrium.
Figure 12. Exchange experiment of perylene bisimide platinum squares 3a
1
and 3b. H NMR spectra (left) of the perylene bisimide core protons and
31P {1H} NMR spectra (right). Compound 3a in CDCl3 (a), 3b in CDCl3 (b),
and 1:1 mixture of 3a and 3b in CDCl3 (c).
Interestingly, such dynamic exchange experiments could
rarely be observed for the significant number of squares
described in the literature so far. A possible explanation might
be the typically different length of the diazaaromatic ligands
1H and 31P {1H} NMR spectra do not show a simple super-
position of the spectra of the two pure squares but reveal a
splitting of the signals (no coupling) without line broadening.
The fact that sharp signals in the 1H and 31P {1H} NMR spectra
still predominate and no endgroups could be detected rules
out the formation of long chain oligomeric or polymeric
mixtures. Figure 12 shows the signals of the perylene core
protons and the 31P {1H} signals of the pure squares 3a and 3b
and the mixture. If we assume that the limited resolution of
the spectrometer allows us only to differentiate between
corners with identical ligands and corners with unequal
ligands we would expect four singlets for the perylene core
1
protons in the H NMR spectrum and four singlets in the 31
P
{1H} NMR spectrum. For an ideal statistical mixture, we
would expect an integral ratio of 1:1 for corners of identical
Figure 11. Illustration of a ligand exchange equilibrium in solution after
mixing two molecular squares with different ditopic ligands.
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F. Würthner et al.
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ligands to corners with unequal ligands for each of the two
perylene bisimide ligands.[16] Figure 12 shows that this is
exactly the case. We observe an integral ratio of 1:1 and a
splitting of each set of perylene core protons and of each set of
the phosphane 31P signals into two signals of equal intensity;
this gives evidence of the postulated exchange process, which
rapidly equilibrated after mixing the two species. Further-
more, the chemical shifts are in accordance with this notion
because two of the four signals remain at the positions of the
respective pure square species.
In a second experiment, the equilibrium was directly
characterized by ESI-MS. The mass spectra were interpreted
by the comparison of the experimental masses and charge
states with the masses of all possible fragments that might
form from the different ligands 2a and 2b and the platinum
corners. Signals of the intact squares A, B, and F (for
assignment of A ± F refer to Figure 11) minus three triflates
(z ˆ 3) as well as signals of the squares C and D and/or E (we
cannot differentiate between D and E as they exhibit the same
masses) minus one platinum corner minus three triflates were
detected (Table 1). These signals unambiguously prove the
existence of all squares A, B, C, D, E, and F in the equilibrium,
exactly as depicted in Figure 11. Smaller fragments with
different perylene bisimide ligands could also be detected, but
it is not possible to assign them to a distinct species as they
could have originated from different squares.
Furthermore, we showed that dynamic ligand exchange
processes take place in solution at room temperature to yield
defined mixed molecular squares containing statistical
amounts of ligands 2a and 2b.
Experimental Section
Materials and methods: Solvents and reagents were purchased from Merck
unless otherwise stated and purified and dried according to standard
procedures.[17] N,N'-Dibutyl-1,6,7,12-tetra(4-tert-butylphenoxy)perylene-
3,4:9,10-tetracarboxylic acid bisimide, 1,6,7,12-tetra(4-tert-butylphenoxy)-
perylene-3,4:9,10-tetracarboxylic acid bisanhydride 1a, 1,6,7,12-tetra(4-
(1,1,3,3-tetramethylbutyl)phenoxy)perylene-3,4:9,10-tetracarboxylic acid
bisanhydride 1b,[8] [Pd(dppp)][(OTf)2] ´ 2H2O (dppp ˆ 1,3-bis-(diphenyl-
phosphano)propane; OTfˆ trifluoromethanesulfonate),[9b] and [Pt(dppp)]-
[(OTf)2] ´ 2H2O[9b] were synthesized according to the literature. Column
chromatography was performed on silica gel (Merck Silica 60, particle size
0.063 ± 0.2 mm). UV/Vis spectra were recorded on
a Perkin-Elmer
Lambda 40P spectrometer, and fluorescence spectra were recorded on a
Perkin-Elmer LS50B fluorometer. Fluorescence quantum yields were
determined relative to N,N'-di(2,6-diisopropylphenyl)-1,6,7,12-tetraphen-
oxyperylene-3,4:9,10-tetracarboxylic acid bisimide (FR ˆ 0.96, CHCl3)[8a, 8e]
by the optically dilute method[18] (A ꢁ 0.04) in chloroform. NMR spectra
were recorded on a Bruker DRX400 and a Bruker AMX500 spectrometer
with TMS as internal standard. Electrospray mass spectra were taken on a
Perkin-Elmer Sciex single quadrupol API100 spectrometer. ESI-FTICR-
MS measurements were carried out with a passively shielded 4.7Tesla
APEXII-ESI-FT-ICR mass spectrometer from Bruker Daltonik (Bremen,
Germany). A saturated solution of 3a in acetone was used. For mass
calculation, data acquisition, processing, and apodization, the mass
spectrometry software XMASS version 5.0.7 (Bruker Daltonik) was used.
The number of data points was 512 K for acquisition and 1m for processing
within a mass range of 500 ± 4000 Da. To increase the signal-to-noise ratio,
100 scans were accumulated. For the external four-point calibration, an ES
Tuning Mix from Hewlett Packard (Waldbronn, Germany) was used. The
calculated m/z values corresponded to the average masses, and the
experimental (found) m/z values corresponded to the respective peak with
the highest intensity.
Table 1. Mixed species detected by ESI-MS in the exchange experiment of
3a and 3b (dioxane/acetone). The resolution of the spectrometer allowed
only the assignment of species with a maximum charge state of z ˆ 3.
Unresolved signals with a presumably higher charge state were not
evaluated.
m/z
z
n(Pt corners)
n(2a)
n(2b)
m/z (calcd)
2801.1
2876.9
2574.9
2348.1
2422.7
2043.9
2461.9
2687.5
2005.0
3
3
3
3
3
3
2
2
2
4
4
4
3
3
3
3
2
2
1
0
4
3
2
1
1
1
1
3
4
0
1
2
2
1
2
1
2799.4
2874.2
2575.0
2347.9
2422.7
2043.6
2459.0
2687.0
2006.2
Electrochemistry: Cyclic voltammetry was performed with an EG&G
PAR273 potentiostat in a three-electrode single compartment cell with
dichloromethane as solvent (5 mL). Working electrode: platinum disk;
counterelectrode: platinum wire; reference electrode: Ag/AgCl. All
‡
potentials were internally referenced to the Fc/Fc couple. The solutions
were purged with argon prior to use. The supporting electrolyte was 0.1m
[Bu4N][PF6] (Fluka), which was recrystallized twice from ethanol/water
and dried in a high vacuum. The experimental setup for spectroelectro-
chemistry has been described by Salbeck.[19]
Exchange experiments: For NMR experiments, solutions of the squares 3a
and 3b were prepared in CDCl3 at a concentration of 5mm. The solutions
were mixed 1:1 by volume and kept overnight at room temperature before
the measurement.
Conclusion
For ESI-MS studies, solutions of approximately the same concentration of
the squares 3a and 3b were prepared in dioxane and mixed 1:1 by volume.
The resulting solution was diluted with acetone (1:1 by volume) and stored
at room temperature overnight in a sealed vial. ESI mass spectra were
recorded from this solution.
The synthesis of ditopic perylene bisimide ligands 2 enabled
us to introduce functional properties into Pd and Pt molecular
squares 3. Remarkably, the optical and electrochemical
properties of the perylene bisimide ligands are completely
conserved in the metal-assembled state. Fluorescence quench-
ing by the metal ions is negligible, and the complexes exhibit
multiple reversible redox couples due to the formation of
perylene bisimide radical cations, radical anions, and dianions.
N,N'-Di(4-pyridyl)-1,6,7,12-tetra(4-tert-butylphenoxy)perylene-3,4:9,10-tetra-
carboxylic acid bisimide (2a): 1,6,7,12-Tetra(4-tert-butylphenoxy)perylene-
3,4:9,10-tetracarboxylic acid bisanhydride 1a (296 mg, 0.3 mmol), 4-amino-
pyridine (Fluka, 94 mg, 1.0 mmol), and zinc(ii)acetate (Fluka, 30 mg,
0.16 mmol) were stirred under argon at 1808C for 16 h in quinoline
(10 mL). HCl (2n, 50 mL) was added, and the precipitate was collected,
washed with water and methanol, and dried. Column chromatography
(CH2Cl2/MeOH 98:2) afforded 2a (225 mg, 66%). M.p. > 3008C; 1H NMR
‡
In the potential range of 1.3 to ‡0.95 V versus Fc/Fc , the
PtII phosphane corners act purely as structural building
blocks that allow undisturbed switching of the perylene
bisimide ligandsꢁ redox states. However, the reversible
potential range of the Pd square is limited to 1.1 V versus
3
(500 MHz, CDCl3, 258C, TMS): d ˆ 8.77 (d, J(H,H) ˆ 5.6 Hz, 4H; Ha-py),
8.23 (s, 4H), 7.24 (m, 12H; Har‡Hb-py), 6.84 (d, 3J(H,H) ˆ 8.7 Hz, 8H; Har),
1.27 (s, 36H; HtBu); UV/Vis (CH2Cl2): lmax (e) ˆ 585 (46100), 545 (28000),
455 (17000), 268 nm (44900 mol 1 dm3 cm 1); fluorescence (CH2Cl2):
‡
Fc/Fc when PdII undergoes an irreversible reduction process.
900
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0704-0900 $ 17.50+.50/0
Chem. Eur. J. 2001, 7, No. 4

Fluorescent and Electroactive Cyclic Assemblies
894±902
lmax ˆ 618 nm; fluorescence quantum yield (CHCl3): FF ˆ 0.94; FAB-MS
(m-NBA, m-nitrobenzyl alcohol): m/z (%): calcd 1137.3; found 1137.6;
elemental analysis calcd (%) for C74H64N4O8 (1137.3): C 78.15, H 5.67, N
4.93; found C 77.90, H 5.68, N 4.94.
(214400), 551 (131700), 458 nm (71000 mol 1 dm3 cm 1); fluorescence
(CH2Cl2): lmax ˆ 628 nm; ESI-MS (acetone): m/z (%): calcd for [M
3OTf]3‡
: 2874.2; found 2874.5; elemental analysis calcd (%) for
C476H488N16O56F24P8Pt4S8 ´ 4H2O (9141.8): C 62.54, H 5.47, N 2.45, S 2.81;
found C 62.57, H 5.42, N 2.33, S 2.74.
N,N'-Di(4-pyridyl)-1,6,7,12-tetra(4-(1,1,3,3-tetramethylbutyl)phenoxy)per-
ylene-3,4:9,10-tetracarboxylic acid bisimide (2b): The compound 1,6,7,12-
tetra(4-(1,1,3,3-tetramethylbutyl)phenoxy)perylene-3,4:9,10-tetracarbox-
ylic acid bisanhydride 1b (363 mg, 0.3 mmol), 4-aminopyridine (94 mg,
1.0 mmol), and zinc(ii)acetate (30 mg, 0.16 mmol) were allowed to react
and worked up in the same way as described for 2a to give 2b (240 mg,
59%). M.p. > 3008C; 1H NMR (400 MHz, CDCl3, 258C, TMS): d ˆ 8.77 (d,
3J(H,H) ˆ 6.2 Hz, 4H; Ha-py), 8.17 (s, 4H), 7.28 (d, 3J(H,H) ˆ 8.7 Hz, 8H;
Perylene bisimide palladium square (4): [Pd(dppp)][(OTf)2] ´ 2H2O
(128.0 mg, 0.15 mmol) and ditopic perylene 2a (170.6 mg, 0.15 mmol) were
allowed to react and worked up in the same way as described for 3a to give
1
a violet solid (280 mg, 94%). M.p. 3048C (decomp); H NMR (500 MHz,
CDCl3, 258C, TMS): d ˆ 9.09 (brs, 16H; Ha-py), 8.12 (s, 16H), 7.66 (m, 32H;
Har(dppp)), 7.40 ± 7.30 (m, 48H; Har(dppp)), 7.20 (d, 3J(H,H) ˆ 8.7 Hz, 32H; Har),
7.04 (d, 3J(H,H) ˆ 5.6 Hz, 16H; Hb-py), 6.77 (d, 3J(H,H) ˆ 8.7 Hz, 32H; Har),
3.20 (brs, 16H; HP CH2), 2.23 (brs, 8H; HCH2), 1.24 (s, 144H; HtBu); 31P {1H}
NMR (202 MHz, CDCl3, 258C, 85% H3PO4): d ˆ 6.66 (s); UV/Vis
(CH2Cl2): lmax (e) ˆ 590 (218000), 549 (136000), 458 nm (74000
mol 1 dm3 cm 1); fluorescence (CH2Cl2): lmax ˆ 624 nm; fluorescence quan-
tum yield (CHCl3): FF ˆ 0.86; elemental analysis calcd (%) for
3
3
Har), 7.23 (d, J(H,H) ˆ 6.2 Hz, 4H; Hb-py), 6.88 (d, J(H,H) ˆ 8.7 Hz, 8H;
Har), 1.71 (s, 8H; HCH2), 1.34 (s, 24H; HCH3), 0.75 (s, 36H; HtBu); UV/Vis
(CH2Cl2): lmax (e) ˆ 588 (48800), 547 (28800), 454 (17700), 268 nm
(46200 mol 1 dm3 cm 1); fluorescence (CH2Cl2): lmax ˆ 621 nm; MALDI-
TOF-MS (dithranol): m/z (%): calcd 1361.8; found 1362.7; elemental
analysis calcd (%) for C90H96N4O8 (1361.8): C 79.38, H 7.11, N 4.11; found C
79.16, H 7.10, N 3.97.
C
412H360N16O56F24P8Pd4S8 ´ 8H2O (7961.4): C 62.16, H 4.76, N 2.81, S 3.22;
found C 62.37, H 4.61, N 2.81, S 3.29.
Perylene bisimide platinum (2:1-complex 7): [Pt(dppp)][(OTf)2] ´ 2H2O
(38.0 mg, 0.041 mmol) and monotopic perylene 6 (89.3 mg, 0.08 mmol)
were allowed to react in CH2Cl2 (20 mL) and worked up as described for 3a
to give a violet solid (90 mg, 71%); M.p. > 3008C; 1H NMR (500 MHz,
CDCl3, 258C, TMS): d ˆ 9.14 (brs, 4H; Ha-py), 8.20 (s, 4H), 8.15 (s, 4H),
7.69 (brs, 8H; Har(dppp)), 7.38 (brs, 8H; Har(dppp)), 7.34 (brs, 8H; Har(dppp)),
7.25 ± 7.19 (m, 16H; Har), 7.11 (brs, 16H; Hb-py), 6.82 ± 6.79 (m, 16H; Har),
4.11 (t, 4H; HBu), 3.29 (brs, 4H; HP CH2), 2.20 (brs, 2H; HCH2), 1.65 (m, 4H;
HBu), 1.39 (m, 4H; HBu), 1.29 (s, 36H; HtBu), 1.24 (s, 36H; HtBu), 0.93 (t, 6H;
N-(4-Pyridyl)-N'-butyl-1,6,7,12-tetra(4-tert-butylphenoxy)perylene-3,4:9,10-
tetracarboxylic acid bisimide (6): Partial saponification of N,N'-dibutyl-
1,6,7,12-tetra(4-tert-butylphenoxy)perylene-3,4:9,10-tetracarboxylic acid
bisimide (7.67 g, 7 mmol) with KOH (150 g, 267 mmol) in isopropyl alcohol
(1000 mL) and H2O (100 mL) under argon by stirring at reflux for 15 h,
followed by acidic workup, and thorough washing and drying, yielded a
mixture (5.8 g) of perylene bisanhydride 1a and perylene mono butylimide
monoanhydride 5 in a ratio of about 6:4. This mixture (1.48 g) was reacted
in the same way as described for 2a with 4-aminopyridine (0.47 g,
5.0 mmol) and zinc(ii)acetate (0.18 g, 0.8 mmol) in quinoline (30 mL) at
1808C for 16 h. After workup and chromatography on a Merck-Lobar-C
(LiChroprep Si60) column (CH2Cl2/MeOH 98:2), 2a (0.52 g) and 6 (0.38 g)
were isolated. Compound 6: m.p. > 3008C; 1H NMR (500 MHz, CDCl3,
258C, TMS): d ˆ 8.77 (d, 3J(H,H) ˆ 5.9 Hz, 2H; Ha-py), 8.24 (s, 2H), 8.23 (s,
2H), 7.26 ± 7.21 (m, 10H; Har‡Hb-py), 6.84 (m, 8H; Har), 4.12 (t, 2H), 1.66
(m, 2H), 1.39 (m, 2H), 1.29 (s, 18H; HtBu), 1.27 (s, 18H; HtBu), 0.94 (t, 3H);
UV/Vis (CH2Cl2): lmax (e) ˆ 581 (45200), 542 (27800), 453 (17200), 286 nm
(45200 mol 1 dm3 cm 1); fluorescence (CH2Cl2): lmax ˆ 615 nm; fluores-
cence quantum yield (CHCl3): FF ˆ 0.88; MALDI-TOF-MS (dithranol):
m/z (%): calcd 1116.4; found 1116.4; elemental analysis calcd (%) for
C73H69N3O8 (1116.4): C 78.54, H 6.23, N 3.76; found C 78.32, H 6.25, N 3.43.
H
Bu); 31P {1H} NMR (202 MHz, CDCl3, 258C, 85% H3PO4): d ˆ 15.14 (s);
UV/Vis (CH2Cl2): lmax (e) ˆ 586 (99000), 547 (61000), 455 (35000), 285 nm
(98000 mol 1 dm3 cm 1); fluorescence (CH2Cl2): lmax ˆ 619 nm; ESI-MS
(acetone): m/z (%): calcd for [M 2OTf]2‡
: 1420.1; found 1420.0;
elemental analysis calcd (%) for C175H164N6O22F6P2PtS2 ´ 2H2O (3174.4):
C 66.21, H 5.33, N 2.65, S 2.02; found C 66.32, H 5.36, N 2.60, S 2.07.
Perylene bisimide palladium (2:1-complex 8): [Pd(dppp)][(OTf)2] ´ 2H2O
(16.3 mg, 0.02 mmol) and monotopic perylene 6 (42.5 mg, 0.038 mmol) in
CH2Cl2 (5 mL) were allowed to react and worked up as described for 3a to
yield of violet solid (52 mg, 89%); M.p. 2858C (decomp); 1H NMR
(500 MHz, CDCl3, 258C, TMS): d ˆ 9.10 (brs, 4H; Ha-py), 8.20 (s, 4H), 8.15
(s, 4H), 7.65 (brs, 8H; Har(dppp)), 7.39 ± 7.35 (m, 12H; Har(dppp)), 7.21 (m, 16H;
Har), 7.05 (brs, 4H; Hb-py), 6.80 (m, 16H; Har), 4.11 (t, 4H; HBu), 3.20 (brs,
4H; HP CH2), 2.22 (brs, 2H; HCH2), 1.65 (m, 4H; HBu), 1.39 (m, 4H; HBu),
1.29 (s, 36H; HtBu), 1.24 (s, 36H; HtBu), 0.93 (t, 6H; HBu); 31P {1H} NMR
(202 MHz, CDCl3, 258C, 85% H3PO4): d ˆ 6.61 (s); UV/Vis (CH2Cl2): lmax
(e) ˆ 584 (99000), 545 (61000), 455 (35000), 283 (98000), 267 nm
(122000 mol 1 dm3 cm 1); fluorescence (CH2Cl2): lmax ˆ 618 nm; ESI-MS
Perylene bisimide platinum square (3a): [Pt(dppp)][(OTf)2] ´ 2H2O
(141.3 mg, 0.15 mmol) and ditopic perylene 2a (170.6 mg, 0.15 mmol) were
stirred under argon at room temperature for 24 h in CH2Cl2 (30 mL). After
filtration, the solution was concentrated to 5 mL, and diethylether was
added to precipitate 3a. The solid was collected by centrifugation, washed
twice with diethylether (5 mL), and dried in vacuo at 508C to afford a violet
solid (274 mg, 88%). M.p. > 3008C; 1H NMR (500 MHz, CDCl3, 258C,
TMS): d ˆ 9.13 (brs, 16H; Ha-py), 8.12 (s, 16H), 7.69 (brs, 32H; Har(dppp)),
(acetone): m/z (%): calcd for (M 2OTf)2‡
: 1376.8; found 1375.7;
elemental analysis calcd (%) for C175H164N6O22F6P2PdS2 ´ 2H2O (3085.8):
C 68.12, H 5.49, N 2.72, S 2.08; found C 68.31, H 5.45, N 2.71, S 2.10.
3
7.40 (m, 32H; Har(dppp)), 7.32 (m, 16H; Har(dppp)), 7.20 (d, J(H,H) ˆ 8.4 Hz,
32H; Har), 7.09 (d, 3J(H,H) ˆ 5.9 Hz, 16H; Hb-py), 6.77 (d, 3J(H,H) ˆ 8.4 Hz,
32H; Har), 3.29 (brs, 16H; HP CH2), 2.20 (brs, 8H; HCH2), 1.24 (s, 144H;
H
tBu); 31P {1H} NMR (202 MHz, CDCl3, 258C, 85 % H3PO4): d ˆ 15.11
Acknowledgements
(s); UV/Vis (CH2Cl2): lmax (e) ˆ 591 (217000), 550 (135000), 459 nm
(72000 mol 1 dm3 cm 1); fluorescence (CH2Cl2): lmax ˆ 625 nm; fluores-
cence quantum yield (CHCl3): FF ˆ 0.88; ESI-FTICR-MS (acetone): m/z
(%): calcd for [M 2OTf]2‡: 3936.969; found 3936.906; calcd for [M
3OTf]3‡: 2574.957; found 2574.779; calcd for [M 4OTf]4‡: 1893.950;
found 1894.056; calcd for [M 5OTf]5‡: 1485.346; found 1485.256; calcd
for [M 6OTf]6‡: 1212.944; found 1212.716; elemental analysis calcd (%)
for C412H360N16O56F24P8Pt4S8 ´ 8H2O (8316.4): C 59.50, H 4.56, N 2.69, S
3.08; found C 59.29, H 4.59, N 2.54, S 3.03.
This work was supported by Fonds der Chemischen Industrie and BMBF
(Liebig grant for F.W.), DFG (Habilitation grant for F.W.), and the Ulmer
Universitätsgesellschaft (startup grant). We gratefully acknowledge BASF
AG and Degussa-Hüls AG for the donation of chemicals and Prof. P.
Bäuerle (Ulm) and Prof. G. Jung (Tübingen) for helpful discussions.
Perylene bisimide platinum square (3b): [Pt(dppp)][(OTf)2] ´ 2H2O
(47.1 mg, 0.05 mmol) and ditopic perylene 2b (68.1 mg, 0.05 mmol) were
allowed to react and worked up in the same way as described for 3a to give
a violet solid (104 mg, 91%). M.p. > 3008C; 1H NMR (500 MHz, CDCl3,
258C, TMS): d ˆ 9.13 (brs, 16H; Ha-py), 8.04 (s, 16H), 7.67 (brs, 32H;
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Chem. Eur. J. 2001, 7, No. 4
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0704-0901 $ 17.50+.50/0
901

F. Würthner et al.
FULL PAPER
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213, 338 ± 344.
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18, 758 ± 769; b) P. J. Stang, D. H. Cao, S. Saito, A. M. Arif, J. Am.
Chem. Soc. 1995, 117, 6273 ± 6283; c) E. Iengo, B. Milani, E.
Zangrando, S. Geremia, E. Alessio, Angew. Chem. 2000, 112, 1138 ±
1141; Angew. Chem. Int. Ed. 2000, 39, 1096 ± 1099.
[15] An interesting experiment has been reported by Stang and co-workers
on an assembly process that involves trans-Pt phosphane species and
results in the formation of a statistical mixture of molecular squares: J.
Manna, C. J. Kuehl, J. A. Whiteford, P. J. Stang, D. C. Muddiman, S. A.
Hofstadler, R. D. Smith, J. Am. Chem. Soc. 1997, 119, 11611 ± 11619.
[16] The expected probabilities for the squares in the equilibrium are A
(6.25%), B (6.25%), C (25%), D (12.5%), E (25%), and F (25%) if a
completely independent incorporation of the ligands 2a and 2b is
assumed.
[17] D. D. Perrin, W. L. F. Armarego, Purification of Laboratory Chem-
icals, 2nd ed., Pergamon, Oxford, 1980.
[18] J. N. Demas, G. A. Crosby, J. Phys. Chem. 1971, 75, 991 ± 1024.
[19] J. Salbeck, J. Electroanal. Chem. 1992, 340, 169.
[10] a) A. G. Marshall, C. L. Hendrickson, G. S. Jackson, Mass Spectrom.
Rev. 1998, 17, 1 ± 35; b) I. J. Amster, J. Mass Spectrom. 1996, 31, 1325 ±
1337.
Received: June 29, 2000 [F2576]
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