Fluorescent supramolecular polymers: Metal directed self-assembly of perylene bisimide building blocks

Article abstract of DOI:10.1021/ma047737j

Complezation properties of 2,2′:6′,2″-terpyridine (tpy) have been studied with, a series of first row transition metal ions by UV-vis, ¹H NMR, and isothermal titration calorimetry, and AH values for the tpy complexation processes have been dete

Full text of DOI:10.1021/ma047737j

Macromolecules 2005, 38, 1315-1325
1315
Fluorescent Supramolecular Polymers: Metal Directed Self-Assembly of
Perylene Bisimide Building Blocks
†
†
‡
†
Rainer Dobrawa, Marina Lysetska, Pablo Ballester, Matthias Gr u1 ne, and
Frank W u1 rthner*,†
Institut f u¨ r Organische Chemie, Universit a¨ t W u¨ rzburg, Am Hubland, D-97074 W u¨ rzburg, Germany,
and Instituci o´ Catalana de Recerca i Estudis Avan c¸ ats (ICREA) and Institut Catal a` d'Investigaci o´
Qu ı´ mica (ICIQ), Avgda. Pa ı¨ sos Catalans, s/n, 43007-Tarragona, Spain
Received November 3, 2004; Revised Manuscript Received December 2, 2004
ABSTRACT: Complexation properties of 2,2′:6′,2′′-terpyridine (tpy) have been studied with a series of
1
first row transition metal ions by UV-vis, H NMR, and isothermal titration calorimetry, and ∆H values
2
+
for the tpy complexation processes have been determined. These studies reveal that terpyridine-Zn
complex constitutes an ideal supramolecular building block characterized by thermodynamically stable
and kinetically labile coordination bonding. Thus, at room-temperature, perylene bisimide fluorophores
equipped with one or two terpyridine functionalities formed coordination dimers and polymers,
2
+
respectively, upon addition of Zn metal ions. The reversible formation of coordination dimers and
1
polymers has been established by H NMR and additionally by DOSY NMR and fluorescence anisotropy
measurements. The optical properties of dimeric and polymeric complexes have been investigated by
UV-vis and fluorescence spectroscopy which prove that the Zn2+ coordination to the terpyridine unit
does not effect the advantageous fluorescence properties of perylene bisimide moieties. Coordination
polymer strands can be visualized by atomic force microscopy (AFM), which also reveals the formation of
a monolayer film at higher concentration. The average polymer length has been determined by AFM to
1
1
5 repeat units, which correlates well with the value estimated by H NMR to >10 repeat units.
Introduction
formation of kinetically labile but nevertheless thermo-
dynamically stable bonds. Accordingly, it is not surpris-
ing that metal-ligand coordination polymers, which are
processable in solution, have gained considerable inter-
est in the past years.4 The properties of coordination
polymers can be widely varied due to the availability of
a multitude of metal ions and ligands, both having effect
on binding strength, reversibility, and solubility. A
ligand which has been of particular importance for the
construction of metallosupramolecular systems is 2,2′:
Photoactive organic compounds have gained signifi-
cant attention in the past decade as materials for
1
organic light emitting diodes (OLEDs) and plastic solar
-6
2
cells. For both applications, photoactive polymers, as
well as small-size functional molecules, have been
3
successfully applied. In most cases, device fabrication
involves either the spin-coating of soluble polymers or
the vacuum deposition of thin films of functional
molecules. Although both procedures are meanwhile
established, they have certain disadvantages. Polymeric
materials are difficult to purify after the polymerization
step, and defective linkages disrupting the conjugation
cannot be always avoided. Furthermore, solubility of
conjugated polymers is often low, thus sophisticated
soluble precursor polymers are necessary to make them
processable. On the other hand, the vacuum deposition
process requires extensive instrumentation, and grain
boundaries have detrimental effect on the device prop-
erties. The programmed supramolecular organization
6
′,2′′-terpyridine (tpy). In contrast to the related 2,2′-
bipyridine (bpy) ligand, which gives rise to the formation
of M(bpy)32 -type metal complexes with ∆/Λ chirality,
tpy forms defined octahedral coordination compounds
M(tpy)22 with a large number of transition metal ions.
Constable and Cargill Thompson have drawn attention
to this advantage and have developed the concept for
the utilization of ditopic tpy ligands as building blocks
+
+
7
for coordination oligomers and polymers. In the past
years, a large number of studies has been reported on
the construction of linear rods8
-10
and dendrimers
11
(
“self-assembly”) of photoactive components might open
2+
a new avenue to overcome these obstacles since small-
size molecules are accessible in high purity and their
supramolecular polymerization can be accomplished at
room temperature in a reversible and predictable man-
ner.
using the tpy complexes of kinetically inert Ru and
2+
Os as photochemically active units and examples of
coordination polymers built up from tpy ligands have
been introduced by several research groups.1
2-14
Although a large number of coordination polymer
structures have been reported, only few examples are
known where functional units such as dyes have been
Metal-ligand coordination seems to be particularly
attractive for this goal, as it combines defined, directed
interactions with high binding constants. In particular,
by proper choice of a metal-ligand combination, it is
possible to realize ideal conditions for self-assembly, i.e.,
incorporated into supramolecular coordination poly-
15-18
mers.
Recently, photoactive and mechanoresponsive
gels based on tpy-related bis(2,6-bis(1′-methylbenz-
imidazolyl)-pyridine coordination polymers have been
*
Author to whom correspondence should be addressed. E-
19
successfully prepared by Beck and Rowan. The adapt-
mail: wuerthner@chemie.uni-wuerzburg.de.
ability of these materials arises from labile bonding of
the ligand to the metal ion, a feature which is not given
by classical covalent chemical bonds.
†
Institut f u¨ r Organische Chemie.
Instituci o´ Catalana de Recerca i Estudis Avan c¸ ats and Institut
‡
Catal a` d’Investigaci o´ Qu ´ı mica.
1
0.1021/ma047737j CCC: $30.25 © 2005 American Chemical Society
Published on Web 01/25/2005

1
316 Dobrawa et al.
Macromolecules, Vol. 38, No. 4, 2005
1596.64); elemental analysis calcd (%) for C106 (1597.9):
In the past, perylene bisimide dyes, which exhibit
84 8 8
H N O
C 79.68, H 5.30, N 7.01; found C 79.48; H 5.31, N 6.96.
t-Octylphenoxy Ligand 3b. This compound was synthe-
sized and purified in the same way as that described for 3a
starting from the respective 1,6,7,12-tetra(4-1,1,3,3-tetra-
methylbutylphenoxy)perylene-3,4:6,10-tetracarboxylic acid
bisanhydride (1b) (400 mg, 0.33 mmol) and 4′-p-aminophenyl-
outstanding fluorescence and redox properties, have
been applied to construct numerous supramolecular
2
0
21
22,23
assemblies by π-π-stacking, hydrogen-bonding,
and metal-ligand interaction.24,25 Moreover, they have
been incorporated in classical polyimide main chain
polymers by the groups of M u¨ llen and Thelakkat.26
Here, we report on metal-directed self-assembly of
highly fluorescent perylene bisimides equipped with the
2,2′:6′,2′′-terpyridine (2) (320 mg, 1 mmol) to yield 3b (72.3
mg, 0.04 mmol, 12%) as a bright-red powder: mp > 350 °C;
1
H NMR (400 MHz, CDCl , TMS): δ ) 8.78 (s, 4H; H3′, H5′),
3
2
,2′:6′,2′′-terpyridine complexing unit27 to photoactive
8.72 (d, J ) 4.0 Hz, 4H; H6, H6′′), 8.66 (d, J ) 8.0 Hz, 4H; H3,
H3′′), 8.22 (s, 4H; HPery), 8.03 (d, J ) 8.5 Hz, 4H; H3′′′, H5′′′),
coordination polymers and present their spectroscopic
and structural properties. In particular, we will show
7
.86 (td, J ) 7.5, 2.0 Hz, 4H; H4, H4′′), 7.40 (d, 2H, J ) 5.5
2
+
Hz, 4H; H2′′′, H6′′′), 7.35 (dd, J ) 7.0, 5.0 Hz, 4H; H5, H5′′),
that coordination to Zn ions provides an ideal supra-
molecular bond which combines high thermodynamic
stability with fast exchange kinetics and no detrimental
effects on the photoluminescence of the perylene bis-
imide fluorophore.
7
1
.17 (d, J ) 8.5 Hz, 8H; HAr), 6.85 (d, J ) 8.5 Hz, 8H; HAr),
.71 (s, 8H; CH ), 1.34 (s, 24H; CH ); 0.75 (s, 36H; CH ); UV-
Cl ): λmax (ꢀ) ) 586 (54 800), 548 (33 800), 453 nm
2
3
3
vis (CH
18 100 M cm ); fluorescence (CH
quantum yield (CH Cl ): fl ) 0.95; MS (MALDI-TOF,
DHB ): m/z 1821.74 [M + H] (calcd for C122
1820.89); elemental analysis calcd (%) for C122
1840.3): C 79.62, H 6.46, N 6.09; found: C 79.89, H 6.42, N
.94.
2
2
-1
-1
(
2 2
Cl ): λmax ) 620 nm,
2
2
Φ
3
2
+
H
116 8 8
N O :
Experimental Section
116 8 8 2
H N O ‚H O
(
5
General. Solvents were purified and dried according to
standard procedures.28 Chromatography was performed with
silica gel 60 (0.035-0.070 mm) and basic alumina, which was
deactivated with 4 wt% of water. 4′-p-Aminophenyl-2,2′:6′,2′′-
terpyridine (2) was prepared from 4′-p-nitrophenyl-2,2′:6′,2′′-
Monotopic Ligands 5a,b. The respective mixtures24 of
mono- (4a,b) and bisanhydrides (1a,b) (contains 20% of
monoanhydride) were reacted with 4′-p-aminophenyl-2,2′:6′,2′′-
terpyridine (2) and purified accordingly, as described above.
Monotopic tert-butylphenoxy ligand 5a (353 mg, 0.26 mmol,
2
9
terpyridine by reduction with Pd/C and hydrazine hydrate
in ethanol. Perylene bisanhydrides applied in this work are
accessible by literature procedures.22,24,26 2,2′:6′,2′′-Terpyridine,
zinc trifluoromethane sulfonate (triflate, OTf) and the metal
4
0% referred to the amount of monoanhydride), blue powder:
1
mp > 350 °C; H NMR (400 MHz, CDCl
3
, TMS): δ ) 8.76 (s,
2
8
8
7
H; H3′, H5′), 8.72 (ddd, J ) 5.0, 2.0, 1.0 Hz, 2H; H6, H6′),
1
perchlorate hexahydrate salts were commercially available. H
.66 (dt, J((H,H) ) 8.0, 1.0 Hz, 2H; H3, H3′), 8.26 (s, 2H; HPery),
.25 (s, 2H; HPery), 8.04 (d, J ) 8.5 Hz, 2H; HAr), 7.87 (td, J )
.5, 2.0 Hz, 2H; H4, H4′), 7.41 (d, J ) 8.5 Hz, 2H; HAr), 7.35
NMR spectra were recorded on a 400 MHz spectrometer and
chemical shifts δ (ppm) calibrated against tetramethylsilane
(
TMS) as internal standard. MALDI-TOF-MS were measured
(
m, 2H; H5, H5′), 7.26-7.22 + CHCl
3
(m; HAr), 7.87-6.83 (m,
H; HAr), 4.12 (t, J ) 7.5 Hz, 2H; NCH ), 1.67 (m, 2H; CH ),
,43 (m, 2H; CH ), 1.30 (s, 18H; CH ), 1.26 (s, 18H; CH ), 0.94
); UV-vis (CHCl
using reflector mode. All fluorescence spectra were corrected.
Fluorescence quantum yields were determined by the optical
8
1
2
2
2
3
3
3
0
dilute method (A < 0.05) using N,N′-di(2,6-diisopropyl-
phenyl)-1,6,7,12-tetraphenoxyperylene-3,4:9,10-tetracarbox-
(
(
(
t, J ) 7.5 Hz, 3H; CH
3
3
): λmax (ꢀ) ) 587
-1
-1
52 100), 547 (31 000), 455 nm (17 700 M cm ); fluorescence
3
1
ylic acid bisimide (Φfl ) 0.96 in chloroform) as standard.
Fluorescence lifetimes were determined by using a 337 nm
nitrogen laser/dye laser system. Fluorescence decay curves
were evaluated with the software supplied with the instru-
ment. Fluorescence anisotropy measurements were performed
employing Glan-Thompson polarizers. Spectroscopy grade
solvents were used for UV-vis and fluorescence studies.
Details on DOSY NMR, fluorescence anisotropy titrations,
isothermal titration calorimetry (ITC), and AFM measure-
ments are given in the Supporting Information.
CHCl
3
): λem ) 622 nm, quantum yield (CHCl
3
): Φfl ) 0.89;
+
MS (MALDI-TOF, dithranol): m/z 1346.50 [M + H] (calcd
for C89
79 5 8
H N O : 1345.59); elemental analysis calcd (%) for
89 79 5 8
C H N O (1346.61): C 79.38, H 5.91, N 5.20; found C 79.18;
H 6.08, N 5.12. Monotopic tert-octylphenoxy ligand 5b: (10
mg, 0.06 mmol, 7% referred to the amount of monoanhydride),
1
bright-red powder: mp > 350 °C; H NMR (400 MHz, CDCl
3
,
TMS): δ ) 8.77 (s, 2H; H3′, H5′), 8.72 (d, J ) 4.0 Hz, 2H; H6,
H6′′), 8.66 (d, J ) 8.0 Hz, 2H; H3, H3′′), 8.19 (s, 2H; HPery),
8
.18 (s, 2H; HPery), 8.03 (d, J ) 8.5 Hz, 2H; H3′′′, H5′′′), 7.86
t-Butylphenoxy Ligand 3a. 1,6,7,12-Tetra(4-tert-butyl-
phenoxy)perylene-3,4:6,10-tetracarboxylic acid bisanhydride
(td, J ) 7.5, 2.0 Hz, 2H; H4, H4′′), 7.40 (d, 2H, J ) 5.5 Hz,
2H; H2′′′, H6′′′), 7.35 (dd, J ) 7.0, 5.0 Hz, 2H; H5, H5′′), 7.29-
7.25 (m, 8H; HAr), 6.89-6.85 (m, 8H; HAr), 4.11 (t, J ) 7.5 Hz,
(
2
1a)(0.70 g, 0.71 mmol) was reacted with 4′-p-aminophenyl-
,2′:6′,2′′-terpyridine (2) (0.58 g, 1.79 mmol) and anhydrous
2H; NCH
CH ); 1.43-1.33 (m, 26H; CH
0.79 (s, 18H; CH ), 0.75 (s, 18H; CH
(ꢀ) ) 590 (49 000), 549 (29 400), 454 nm (17 500 M cm-1);
fluorescence (CHCl ): λmax ) 620 nm, quantum yield (CHCl ):
fl ) 0.90; MS (MALDI-TOF, dithranol): m/z 1570.58 [M +
2
), 1.73 (s, 4H; CH
2
), 1.70 (s, 4H; CH
), 0.94 (t, J ) 7.0 Hz, 3H; CH
); UV-vis (CHCl
2
), 1.65 (m, 2H;
zinc acetate as catalyst (70 mg) in quinoline (20 mL) for 5 h
at 180 °C under argon. After being cooled to room temperature,
the mixture was poured on aqueous HCl (200 mL, 1 M); the
resulting precipitate was allowed to settle overnight and
isolated by filtration and subsequently washed with water and
methanol. The crude product was redissolved in a minimum
amount of dichloromethane and precipitated by addition of
methanol. Purification was achieved by repetitive column
chromatography on aluminum oxide (basic, activity II) with a
2
3
3
),
3
3
3
): λmax
-
1
3
3
Φ
+
H] (calcd for C105
H
111
N
5
O
8
: 1569.84); elemental analysis calcd
(%) for C105
C 79.72, H 7.13, N 4.24.
H N O (1571.0): C 80.27, H 7.12, N 4.46; found:
111 5 8
Complex 6a. To a solution of the monotopic ligand 5a (13.3
gradient from CHCl
3
to CHCl
3
/MeOH (90:10) to yield 3a (415
3 3
mg, 9.8 µmol) in CHCl -CH CN (80:20, 4 mL) a stock solution
mg, 37%) as a dark-blue microcrystalline powder. mp > 350
of zinc triflate (10 mM, 490 µL, 4.9 µmol) was added and the
1
°
C; H NMR (400 MHz, CDCl
3
, TMS): δ ) 8.78 (s, 4H; H3′,
solution was stirred for 1 h at room temperature. Exact 2:1
1
H5′), 8.73 (ddd, J ) 5.0, 2.0, 1.0 Hz, 4H; H6, H6′′), 8.67 (d, J
stoichiometry is confirmed by H NMR and, if necessary, is
)
8.0 Hz, 4H; H3, H3′′), 8.29 (s, 4H; HPery), 8.05 (d, J ) 8.5
adjusted until no residual signals of the uncomplexed ligand
is present. The solution was concentrated; the product was
precipitated by addition of more acetonitrile (10 mL) and
Hz, 4H; HAr), 7.87 (td, J ) 8.0, 2.0 Hz, 4H; H4, H4′′), 7.42 (d,
J ) 8.5 Hz, 4H; HAr), 7.35 (m, 4H; H5, H5′′), 7.24 (d, J ) 9.0
1
Hz, 4H; HAr), 6.87 (d, J ) 9.0 Hz, 4H; HAr), 1.27 (s, 36H; CH
UV-vis (CHCl
3
);
isolated quantitatively by centrifugation. H NMR (400 MHz,
3
): λmax (ꢀ) ) 590 (54 500), 549 (32 800), 458 nm
3 3
CDCl /CD CN (80:20), TMS): δ ) 9.00 (s, 4H; H3′, H5′), 8.72
-
1
-1
(
17 500 M cm ); fluorescence (CHCl
quantum yield (CHCl ): Φfl ) 0.91, lifetime (λex ) 520 nm,
em ) 645 nm): τ ) 6.1 ns (CHCl ), 4.8 ns (DMF); MS (MALDI-
3
):
λ
max ) 620 nm,
(d, J ) 8.5 Hz, 4H; H3, H3′′), 8.31 (d, J ) 8.5 Hz, 4H; HAr),
8.24-8.12 (12H; HPery, H4, H4′′), 7.80 (d, J ) 5.0 Hz, 4H; H6,
3
λ
3
3
H6′′), 7.65 (d, J ) 8.5 Hz, 4H; HAr), 7.44 + CHCl (m, 4H; H5,
32
+
TOF, dithranol ): m/z 1597.51 [M + H] (calcd for C106
H
84
N
8
O
8
:
H5′′), 7.28 (m, 16H; HAr), 6.87 (m, 16H; HAr), 4.10 (t, J ) 6.5

Macromolecules, Vol. 38, No. 4, 2005
Fluorescent Coordination Polymers 1317
2
+
2+
Figure 1. UV-vis titrations of tpy with (A) Zn and (B) Cu perchlorate hexahydrates in acetonitrile (0.5 mM tpy); black lines:
2
+
2+
<
0.5 equiv Cu , red lines: > 0.5 equiv Cu ; insets show absorption changes at characteristic wavelengths.
Hz, 4H; NCH
2H; CH ), 1.29 (s, 32H; CH
UV-vis (CHCl ): λmax (ꢀ) ) 589 (103 500), 548 (60 700), 457
2
), 1.66 (m, 4H; CH
2
), 1.41 (m, 4H; CH
2
), 1.31 (s,
nm (Figure 1A). Titration curves (Figure 1, insets) show
a linear increase and a sharp endpoint at a metal/ligand
3
3
3
), 0.95 (t, J ) 7.5 Hz, 6H; CH
3
);
3
2+
ratio of 0.5:1, indicating the formation of a M(tpy)2
-
1
-1
nm (34 000 M cm ); fluorescence (CHCl
quantum yield: Φfl ) 0.90 (CHCl ), 0.75 (DMF), lifetime (λex
520 nm, λem ) 645 nm): τ ) 6.3 ns (CHCl ), 5.5 ns (DMF);
3
): λmax ) 623 nm,
(
2:1) complex. Due to the lack of curvature in the
3
titration curve, no binding constants could be deter-
mined. Except for the Cu2 complex (Figure 1B), no
change of the UV-vis spectra could be observed when
more than 0.5 equiv of metal were added to the solution.
The addition of excess metal ion should cause the
formation of a M(tpy)2 complex (1:1 complex) with
solvent molecules or counterions as additional ligands
for the case of reversible complexation. However, this
reversibility cannot be assessed from UV-vis titrations
as the changes in the spectra originate from the com-
plexation of one tpy molecule with one metal ion,
)
3
+
3
2
+
MS (MALDI-TOF, DCTB ): m/z (%) 2904.05 (100) [M - OTf] ,
+
+
2
1
3
755.15 (10) [M - 2OTf + H] , 1554.52 (10) [5a + Zn + OTf] ,
+
345.59 (20) [5a + H] (calcd for C180
158 6 10 22 2
H F N O S Zn:
056.81).
+
Coordination Polymer 8a. To a solution of the ditopic
3 3
ligand 3a (19.6 mg, 12.3 µmol) in CHCl -CH CN (80:20, 4 mL)
a stock solution of zinc triflate (10 mM, 1.23 mL, 12.3 µmol)
was added and the solution was stirred for 1 h at room
1
temperature. The exact 1:1 stoichiometry was checked by H
NMR and, if necessary, was adjusted until no residual signals
of uncomplexed ligand was observed. The solution was con-
centrated, the product was precipitated by addition of more
2+
independent of the type of complex, i.e., M(tpy) or
M(tpy)22 complex.
+
acetonitrile (10 mL) and isolated quantitatively by centrifuga-
1
tion. H NMR (400 MHz, CDCl
3
/CD
3
CN (80:20), TMS): δ )
In contrast to all other metal ions used in this study,
9
4
7
.03 (br, 4H; H3′), 8.75 (br, 4H), 8.4-8.1 (br, 8H), 7.82 (br,
H), 7.66 (br, 4H), 7.43 (br, 4H, overlapped with CHCl signal),
.31 (br, 8H; HAr), 6.92 (br, 8H; HAr), 1.31 (br, 36 H; CH ); UV-
2+
the respective titration with Cu (Figure 1B) reveals
3
an additional process. Up to a molar ratio of 0.5 (Figure
3
1
B, black lines), the curve resembles the one shown in
vis (DMF, ꢀ value calculated per perylene unit): λmax (ꢀ) ) 575
45 500), 537 (28 600), 446 nm (14 750 M-1 cm-1); fluorescence
DMF): λmax ) 613 nm, quantum yield (DMF): Φfl ) 0.61,
Figure 1A. However, a sharp change of the absorption
(
(
spectrum at Cu2 /tpy ratios > 0.5 were observed,
+
2+
lifetime (DMF, λex ) 520 nm, λem ) 645 nm): τ ) 4.9 ns.
indicating reversibility of the Cu(tpy)2 species (Figure
1
B, red lines) and suggesting that the binding mode of
2+
2+
Results and Discussion
the tpy units is different in Cu(tpy) and Cu(tpy)2
complexes.
Studies on tpy Complexation. Because selection
of a properly suited metal ion is crucial to obtain a long
polymeric chain by reversible supramolecular polymer-
ization of terpyridine-functionalized perylene bisimide
building blocks, our first attention was directed to
finding a suitable metal ion. Accordingly, the complex-
ation of several first row transition metal ions with the
parent terpyridine ligand was studied by UV-vis and
Further insight into the structure of the complex
species formed during the titration was obtained from
1
H NMR experiments, which were carried out using
iron(II) and zinc(II) perchlorate hexahydrate salts in d3-
acetonitrile. Due to the perpendicular arrangement of
the two aromatic tpy units in a M(tpy)22 complex, the
tpy proton signals undergo pronounced changes as a
+
1
33
H NMR titration, as well as isothermal titration
result of magnetic shielding; thus, the complexes
M(tpy)22 and M(tpy) can be distinguished by 1
+
2+
H
calorimetry (ITC). The metal(II) perchlorate hexa-
2
+
2+
2+
2+
2+
hydrate salts of Fe , Co , Ni , Cu , and Zn in
acetonitrile were used as a consistent set of compounds.
For all metal ions, distinct changes in tpy absorption
was observed upon complexation and clear isosbestic
points were found (Figure 1 and Figures S1-S3 in
Supporting Information), indicating that only a single
equilibrium between two species, namely free and
complexed tpy, occurs during the titration. Whereas iron
and cobalt complexes show intense absorption also in
the visible region (λ > 400 nm) due to metal-ligand
NMR, which is not possible by UV-vis and ITC. In
NMR studies, no dissociation was found for the
Fe(tpy)22 complex even after addition of an excess
amount of Fe2 ; rather, the signals of uncomplexed tpy
+
+
2+
ligand and the resulting Fe(tpy)2 complex were ob-
served. This behavior points at a high kinetic barrier
for the iron(II) complex. In contrast, titration of tpy
solutions with both zinc perchlorate and zinc tri-
fluoromethane sulfonate (triflate, OTf) led to a third set
of signals after exceeding the ratio of 0.5, which can be
attributed to the formation of a 1:1 complex of the type
2+
charge-transfer absorption, the complexes of Ni and
Cu only absorb weakly in this area and the Zn2+
2
+
Zn(tpy) with acetonitrile molecules or the counterions
2+
complex does not absorb at wavelengths higher than 350
as additional ligands (cf. Figure S4 in Supporting

1318 Dobrawa et al.
Macromolecules, Vol. 38, No. 4, 2005
2
+
Figure 2. (A) ITC experiment with tpy and zinc(II) perchlorate hexahydrate in acetonitrile (0.5 mM). Experiments with Fe
,
2
+
2+
Co , and Ni provided similar titration curves only differing in absolute ∆H values; (B) titration with copper(II) perchlorate
hexahydrate in acetonitrile (0.5 mM).
Table 1. Enthalpies and Lower Limits for the Binding
Information for the titration curve). These results also
Constants Determined for a Series of M(tpy)2(ClO4)2
confirm fast exchange kinetics (reversibility). Although
the coordination of Zn2+ to tpy is reversible, the ex-
change is slow on the NMR time scale; thus, separate
proton NMR signals of the repective species were
detected instead of averaged signals. Nevertheless,
equilibrium has been reached within less than a minute.
The results obtained from our NMR experiments in
acetonitrile are consistent with kinetic data determined
Complexes by ITCa
M2 f M(tpy)2+
+
M(tpy)2+ f M(tpy)22+
0
-1
K1/M-1
0
-1
K2/M-1
∆H 1/kcal mol
∆H 2/kcal mol
2+
8
8
8
8
8
10
10
10
8
Fe
Co
-19.1
-14.7
-16.0
-22.2
-14.5
>10
>10
>10
>10
>10
-19.1
-14.7
-16.0
-13.0
-14.5
>10
>10
>10
≈10
>10
2
+
2
+
Ni
2
+
Cu
2
+
10
Zn
3
4
by Hogg and Wilkins for the reaction of various
a
Errors are ( 0.5 kcal mol-1 for ∆H0.
M(tpy)2 complexes with excess tpy in water (t1/2 for
2
+
2+
2+
Fe(tpy)2 , Ni(tpy)2 , and Co(tpy)2 are 8400, 610, and
6
0 min respectively, compared to t1/2 < 0.1 min for the
completed at a 0.5 molar ratio of Zn2+/tpy. This titration
provides evidence that the transformation of the 2:1
complex to the 1:1 complex (the existence of the latter
2
+
2+
Zn and Cu complexes). The intermediate ligand
2
+
exchange rate of Co(tpy)2 complexes has been inves-
3
5
36
1
tigated by Lehn et al. and Constable et al. by
utilizing this for the construction of dynamic combina-
has been proven by H NMR) does not give any
calorimetric signal, indicating that the only process
yielding reaction heat is the complexation of the free
tpy unit with metal ion. Similar observation has been
2+
3+
torial libraries. Oxidation of the Co(tpy)2 to Co(tpy)2
fixed the combinatorial library since Co3+ undergoes
nearly irreversible, i.e., kinetically inert, complexation
made for Fe2 , Co , Ni , and Zn
+
2+
2+
2+
with varying
1
4
2+
with tpy. Schubert and co-workers found from vis-
cosimetry experiments with tpy-substituted poly(ethyl-
absolute ∆H values (Table 1). Cu is the only system
in this series to deviate from this behavior, as already
observed by UV-vis titration. Figure 2B shows the ITC
titration results of the copper(II) perchlorate hexa-
hydrate/tpy system in acetonitrile. Two calorimetrically
distinguished processes can be found for Cu(II) in clear
contrast to the previously investigated metal ions. These
observations suggest that in the case of Cu(tpy)22 only
one tpy ligand acts as a tridentate ligand, whereas the
second tpy is merely attached by two of the three
pyridine units as known for 2,2′-bipyridine (bpy) com-
plexes.37
2
+
2+
ene oxide) that Fe and Ni complexes are kinetically
inert, whereas those of Co2+, Cu , and Cd were found
2+
2+
to be of labile nature.
Isothermal Titration Calorimetry (ITC) with
Terpyridine Complexes. The advantage of ITC method
for the determination of the complexation behavior is
that the titration experiment can be monitored in both
directions, i.e., addition of metal salt to tpy solution and
vice versa. Thus, the heat of complexation can also be
determined if tpy is added to a solution of the metal
salt, a process which cannot be monitored by NMR or
UV-vis spectroscopy. Figure 2A shows a typical ITC
titration experiment with zinc perchlorate hexahydrate
+
For all titrations by ITC, the ∆H° values and the
stoichiometries could be determined exactly due to
clearly defined start and end values of the isotherm. For
the same reason, it was not possible to derive exact
2
+
and tpy. As expected, the Zn(tpy)2
formation is

Macromolecules, Vol. 38, No. 4, 2005
Fluorescent Coordination Polymers 1319
Scheme 1
Chart 1
n-butyl bisimide.24 The mixture of products 3 and 5 was
separated by repetitive column chromatography on basic
alumina of activity II. Due to strong adsorption of the
tpy units to alumina, the isolated yields of the products
are only moderate. Despite extended π systems, all
compounds 3a,b and 5a,b exhibit good solubility in
halogenated organic solvents due to the bulky phenoxy
substituents bearing tert-butyl or 1,1,3,3-tetramethyl-
binding constants (or ∆G° values) and only a lower limit
8
-1
10
-1
can be given (between >10 M and >10
M ).
The data obtained from our titration experiments
2
+
20
suggest that the tpy/Zn interaction is very promising
for the construction of high-molecular-weight lumines-
cent coordination polymers from terpyridine-function-
alized perylene bisimides. First, due to the closed-shell
butyl (tert-octyl) groups.
Dimer Complex Formation With Zinc Triflate.
Complexation studies were first performed using the
monotopic model compound 5, which forms the dimer
complex 6 upon addition of zinc triflate (Scheme 2). The
solvent system for this complexation has to be well
balanced to provide enough polarity to solubilize the
metal salt and the ionic [Zn(tpy)2(OTf)2] unit but still
keep the extended aromatic system of the perylene
bisimide in solution. Mixtures of chloroform-methanol
were applied successfully in most complexation experi-
ments, but in some cases, chloroform-acetonitrile (80/
20) was the better choice.
10
2+
(
d ) electronic configuration of Zn , no MLCT states
arise upon coordination (UV-vis studies) which might
quench the fluorescence of the perylene bisimide chro-
mophore. Second, the pronounced ligand exchange rate
2+
of Zn /tpy coordination (“reversibility”) is favorable for
the self-healing process in supramolecular self-assembly
and offers the possibility for the formation of well-
defined polymeric structures and their further organiza-
tion in 2D and 3D solid-state materials (see AFM
studies). Despite its reversible nature, the binding
The complexation reaction of monotopic ligands 5 with
8
-1
1
constant of >10 M is pretty high compared to other
Zn(OTf)2 was monitored by H NMR. Figure 3 shows
the characteristic 1H NMR signals in the aromatic
noncovalent forces applied for supramolecular polym-
3
8
erization, and therefore, the reaction of a ditopic tpy
2
+
ligand with Zn at a 1:1 stoichiometry is expected to
lead to high-molecular-weight polymers even in dilute
solution. Third, the octahedral coordination of Zn2+ ions
will cause formation of rigid linear polymers of ditopic
building blocks 3a,b (in contrast to macrocyclization
which often competes with supramolecular polymeri-
zation). Because of this unique combination of favorable
properties, the supramolecular polymerization of tpy-
equipped perylene bisimides 3 has been investigated
Scheme 2
2
+
with Zn salts.
Synthesis. Ditopic terpyridine-functionalized perylene
bisimide ligands 3a,b were prepared by condensation
of 4′-p-aminophenyl-2,2′:6′,2′′-terpyridine (2) with the
22,26
respective bisanhydrides 1a,b
acetate as catalyst in isolated yields of 37% (for 3a) and
2% (for 3b). As the monotopic ligands 5 are valuable
in quinoline with zinc
1
model compounds for complexation studies, the conden-
sation reaction of 2 was carried out with a mixture of
the bisanhydride 1 and imide anhydride 4, the latter is
accessible via partial saponification of the corresponding

1320 Dobrawa et al.
Macromolecules, Vol. 38, No. 4, 2005
1
Figure 3. H NMR spectra at different ligand/metal ion ratios
for the monotopic ligand 5a and zinc triflate in CDCl
3
-CD
3
CN
(
(
(
80:20, 5 mM) from free 5a (upper spectrum) to dimer 6a
1
Figure 4. H NMR spectra at different ligand/metal ion ratios
for the ditopic ligand 3a and zinc triflate in CDCl -CD CN
middle) and the opened form 7a (bottom). The marked signal
*) corresponds to proton H6 adjacent to the lateral pyridine
3
3
(
80:20; concentrated of 3a: 5 mM) from free 3a (upper
nitrogen atoms.
spectrum) to polymer 8a (middle, 1:1) and the fragmented
dicomplexed form 9a (bottom).
Scheme 3
dimer 6a could be characterized by MALDI-TOF mass
spectrometry revealing the mass of the dimer cation
with one triflate counterion (Supporting Information,
Figure S5).
Polymer Formation from Ditopic Ligands 3a,b
with Zinc Triflate. In analogy to dimer complex
formation of monotopic ligand 5, ditopic ligand 3a,b
should afford extended coordination polymers when
mixed with exactly 1 equiv of Zn2 . To confirm the
formation of coordination polymers, a similar titration
as in the case of ligand 5 was performed with the ditopic
+
1
ligand 3a. As can be seen from Figure 4, the H NMR
titration resembles the situation observed for the forma-
tion of complex 6, and the characteristic proton chemical
shifts of different species are observed in the same
regions. The formation of a coordination polymer can
be concluded from the following observations: Upon
addition of zinc triflate, broadening is observed for the
first upcoming set of signals, which becomes the only
set of signals present in the spectrum when 1:1 stoichi-
ometry is reached. As the solution remains clear and
no precipitation or further aggregation can be observed,
this signal broadening can be attributed to the forma-
1
region during the titration at different ligand/metal
ratios. The formation of dimer 6a is clearly indicated
by the disappearance of the original set of signals which
belong to the uncomplexed ligand 5a and the rise of a
second set of signals, exemplified by the drastic high-
field shift of protons H6,6′′ at the outer pyridine units
tion of coordination polymer 8a. Comparison of the H
NMR spectra of polymer 8a (Figure 4, 1:1) and the
model dimer 6a (Figure 3, 2:1) shows the identical set
of signals, but in the former case, they are significantly
broadened. Again, reversible complexation is observed
when excess Zn2 is added to the coordination polymer
solution, which results in the shortening of the polymer
strands to form a distribution of oligomeric and mono-
meric fragments such as 9a with two monocomplexed
+
(marked with * in Figure 3) from 8.74 to 7.80 ppm,
which is mainly due to the aromatic shielding effect of
the neighboring second tpy unit. At a ratio of 2:1, only
1
2+
the dimer 6a is present in solution and the H NMR
Zn units at the ends.
signals can be clearly assigned. Once this ratio is
The polymer length at a 1:1 ratio can be estimated
from the 1H NMR signals of the end groups to a
minimum of 10 repeat units corresponding to a chain
length of 30 nm and a minimum molecular weight of
∼16 600 g/mol. Since the coordination polymer is of (A-
B)n type, its chain length depends critically on realiza-
tion of an exact 1:1 stoichiometry, and even small
deviations cause a drastic decrease in polymer length.22
This fact, together with the very hygroscopic nature of
the zinc triflate is the reason that all coordination
compounds have not been prepared in a classical
2
+
exceeded by addition of more Zn ions, the dimer
signals decrease in intensity again and a third set of
signals evolves, demonstrating again reversible Zn2
+
-
terpyridine bonding. The new formed species may be
assigned to a 1:1 complex 7a with the additional
coordination sites of the metal center saturated either
by acetonitrile molecules or triflate counterions (see
Scheme 3). Also here, as in the case of unsubstituted
tpy, the exchange kinetics is slow on the NMR time
scale, but equilibrium has been reached within a few
seconds after addition of zinc triflate. Despite its lability,
1
synthetic manner but by a titration method using H

Macromolecules, Vol. 38, No. 4, 2005
Fluorescent Coordination Polymers 1321
1
Figure 5. Aromatic region of the H DOSY NMR spectra of monomer 3b (A), dimer 6b (B), polymer 8b (C), and the fragmented
2
species 9b (D) in chloroform-methanol (80:20; concentration 2.5 mM). The diffusion coefficients, D (in m /s), are plotted in a
logarithmic scale against the chemical shift, δ.
NMR to ensure the precise 1:1 stoichiometry. Despite
several attempts, mass spectrometric characterization
of the coordination polymers by MALDI-TOF MS was
not successful, as only fragments showing the ligand
with one Zn ion could be observed due to fragmenta-
tion during the ionization process.
(diffusion ordered spectroscopy)39 NMR experiments
were carried out. This two-dimensional NMR technique
1
correlates the H NMR signals with the diffusion
coefficient of the respective species in solution and has
recently been successfully introduced for the character-
ization of supramolecular coordination polymers.4
The DOSY spectra were recorded in chloroform-
methanol (80:20) first for the monomeric ligand 3b
alone, then after addition of 1 equiv of Zn2 providing
coordination polymer 8b, and subsequently after addi-
tion of 2 equiv more of Zn2 , which should lead to
fragmentation of polymer 8b to low-molecular-weight
species such as 9b. Figure 5 shows the aromatic region
of the DOSY NMR spectra for monomer 3b (A), polymer
2
+
0,41
Although the initial solubility of both coordination
polymers 8a,b is good in chloroform/methanol, as well
as chloroform/acetonitrile mixtures, differences are
observed for long-term solubility. Whereas polymer 8a
with the tert-butylphenoxy residues at the perylene
bisimide core slowly starts to precipitate within 1 day
after preparation, polymer 8b is solubilized sufficiently
by the bulky tert-octylphenoxy residues to form stable,
clear solutions for weeks. Both coordination polymers
are also highly soluble over long periods of time in DMF,
+
+
1
8b (C), and fragments (D) with the corresponding H
NMR spectrum plotted against the logarithm of the
diffusion coefficient. As the monomer 3b is itself a large
molecule with a molecular weight of M ) 1822 g/mol, it
appears at a significantly smaller diffusion coefficient
(log D ) -9.45) in comparison to the chloroform
molecule (log D ) -8.66). The diffusion coefficient of
the coordination polymer 8b is in the range of log D )
-10.45, 1 order of magnitude smaller than that of the
monomer 3b. The strong decrease of diffusion coefficient
upon addition of one equivalent of Zn2 ion to monomer
3b clearly indicates the formation of an extended, high-
molecular-weight polymer structure with the corre-
1
and H NMR spectra recorded in deuterated N,N-
dimethylformamide (DMF) reveal that the coordination
remains unchanged. Several attempts have been made
to assess the polymer length by gel permeation chro-
matography (GPC), but using different solvents under
varying conditions, only the signal corresponding to the
molecular mass of the monomer has been found. This
observation can be explained in terms of the reversible
nature of the complex bond. Due to the nonequilibrium
conditions during a GPC run, the constant dilution and
shear forces might cause fragmentation of the coordina-
tion polymers. A further explanation for the exclusive
detection of monomers may be that the charged polymer
chains adsorb to the stationary phase.
+
1
sponding H NMR signals characteristic for the com-
plexed form of the tpy unit. Upon addition of more than
1 equiv of Zn2 to monomer 3b, the diffusion coefficient
increases to a value in the range of log D ) -9.5 to -9.7,
which is slightly smaller than that of the monomer,
+
DOSY NMR Studies. To further substantiate the
1
formation of oligomeric and polymeric species, H DOSY

1322 Dobrawa et al.
Macromolecules, Vol. 38, No. 4, 2005
Figure 6. Spectral changes upon addition of zinc triflate to monotopic (A, 5a) and ditopic (B, 3a) perylene bisimide ligands in
2
+
3
CHCl -MeOH (60:40); insets show apparent absorption coefficient (at 325 nm) as a function of the Zn /ligand ratio.
Figure 7. Fluorescence quenching (A) of monotopic perylene bisimide ligand 5a by addition of iron(II) perchlorate hexahydrate
and fluorescence quantum yields (B) as a function of iron(II)/5a ratio (triangles) and zinc(II)/5a ratio (circles). All measurements
-
5
3
in CHCl -MeOH (60:40), 10 M.
indicating fragmentation of the coordination polymer to
low-molecular-weight oligomers. For comparison, also
the DOSY result of the dimer model compound 6b is
depicted in Figure 5B. The diffusion coefficient of the
model complex, which exhibits nearly twice the volume
of the uncomplexed ditopic ligand 3b, is virtually
identical to that of ligand 3b. This indicates that a
significantly larger number of units is necessary to
cause a 10-fold decreased diffusion coefficient as found
for the polymer 8b.
pyridine units in an all-cis conformation. The dimeric
compex of 5a does not show any significant change in
the perylene bisimide absorption band compared to the
uncomplexed form (see Figure 6A), while the polymer
exhibits a small increase in the absorption coefficient
and a small red-shift of the absorption maximum from
591 to 594 nm.
Fluorescence Properties. All tpy-substituted per-
ylene bisimide compounds exhibit intense red fluores-
cence (λmax ) 620 nm) with fluorescence quantum yields
around Φfl ) 0.9 in halogenated solvents such as
chloroform or dichloromethane. The effect of metal
complexation on the fluorescence properties of the
perylene bisimide fluorophore unit was investigated by
steady-state fluorescence spectroscopy. Figure 7A de-
picts the fluorescence titration of monotopic perylene
ligand 5a with both zinc(II) triflate and iron(II) per-
chlorate hexahydrate. The fluorescence of 5a is drasti-
cally quenched upon addition of Fe2 ions (Figure 7A),
which is in accord with the observed decrease of the
fluorescence quantum yield from >0.9 for the uncom-
plexed ligand 5a to <0.1 for the iron(II) complex (Figure
7B, triangles). In contrast, complexation with zinc(II)
(Figure 7B, circles) has virtually no effect on the
fluorescence quantum yield of perylene bisimide unit.
The fluorescence quantum yields in DMF are reduced
by ∼20% independent of that, whether the perylene
bisimide ligand is complexed or free (Table 2, compare
6a and 3a). This moderate reduction of quantum yield
is apparently due to the solvent polarity. Likewise,
fluorescence lifetimes are almost identical for uncom-
plexed ligand, dimer complex, and polymer species.
These results imply that no electronic interactions take
UV-Vis Absorption Properties. All uncomplexed
monotopic (5a,b) and ditopic (3a,b) ligands show the
characteristic absorption bands of the tetraphenoxy-
substituted perylene bisimide chromophore between 500
-
1
-1
and 650 nm (S0fS1, ꢀ ) 50 000-55 000 L mol cm
)
and 400-500 nm (S0fS2), whereas at wavelengths
below 350 nm, both the perylene bisimide and the
terpyridine units absorb. UV-vis titration experiments
of the monotopic and ditopic building blocks were
performed to assess the ground state interaction of the
perylene bisimide chromophore with the terpyridine
unit; the latter should act purely as the structure
determining part. Figure 6 shows the UV-vis titration
of the mono- and ditopic tert-butylphenoxy-substituted
ligands 5a and 3a with zinc triflate. As expected from
the studies with the parent terpyridine ligand, com-
plexation does not have any significant influence on the
absorption of the perylene bisimide chromophore. How-
ever, an increase in absorbance between 300 and 350
nm and a decrease at wavelengths below 300 nm was
observed, corresponding to the change in tpy absorption
+
(
cf. Figure 1A). These changes can be assigned to the
complexation of the tpy unit which fixes the three

Macromolecules, Vol. 38, No. 4, 2005
Fluorescent Coordination Polymers 1323
Table 2. Emission Properties of Uncomplexed Ligands
fluorescence transition dipole moments, which are
located along the N-N axis of the perylene bisimide
unit, potential energy transfer within the perylene
bisimide units along the strands will not decrease
anisotropy. As expected, the anisotropy decreases again
upon exceeding the 1:1 stoichiometry due to the frag-
mentation of the polymer into oligomeric units. The
value saturates at r ) 0.091, indicating that these
fragments are significantly smaller than the polymer
but still larger than the monomeric unit. These results
are consistent with that of the DOSY NMR experiments.
A similar titration experiment with ligand 3a showed
the same characteristics, only differing in the absolute
r values, which were r ) 0.048 for the monomer, r )
and Zn Complexesa
2+
CHCl3
DMF
Φfl
τ/ns
Φfl
τ/ns
monotopic ligand 5a
dimer 6a
ditopic ligand 3a
polymer 8a
0.89
0.90
0.92
-
-
-
6.3
6.1
-
0.74
0.75
0.61
5.5
4.8
4.9
-
a
Errors for quantum yields are (0.04, errors for lifetimes are
0.2 ns except for 8a: (0.4 ns.
(
0
.096 for the polymer (8a), and r ) 0.062 for the
2+
oligomeric fragments (at Zn /monomer ratios > 1.7).
The smaller values for 8a may be explained in terms of
the sterically less demanding tert-butyl groups com-
pared to the very bulky tert-octyl residues in 8b. The
smaller steric demand lowers the molecular volume of
8a and, consequently, increases the diffusion coefficient.
Atomic Force Microscopy (AFM) of the Coordi-
nation Polymers. The aim of atomic force microscopy
experiments was to determine the shape and length of
the single polymer strands, as well as their two-
dimensional organization on a surface. The uncom-
plexed ligand molecules 3a, which were deposited via
spin-coating of DMF solution onto freshly cleaved mica,
appeared as isolated dots (Figure 9A). The height of
these globular objects was found to be quite homoge-
neous with a mean value of 1.6 ( 0.1 nm. Due to the
tip broadening effect, a rather high value of 9.2 ( 1.2
nm was obtained for the mean diameter of the particles.
Samples of the coordination polymers 8a,b were pre-
pared by spin-coating of DMF solutions of different
concentration on both mica and HOPG (highly ordered
pyrolytic graphite) substrates. AFM images of the
polymers 8a,b (Figure 9B, F-H) clearly show the
fibrous appearance of the polymers on both types of
substrates. Mean heights of the strands reveal the same
value of 1.6 ( 0.1 nm for both polymers 8a,b. However,
in comparison to the very uniform heights, the apparent
length of the rods shows a broad distribution. For both
polymers, long filaments up to 400 nm in length, as well
as very small globular objects, similar to the ligand
molecules, can be observed. The apparent length of the
polymers measured by AFM revealed a mean value of
Figure 8. Fluorescence anisotropy titration of monomer 3b
with zinc triflate in chloroform-methanol (60:40, [3b] ) 2.5
-
5
×
10 M, λex ) 550 nm, λem ) 610 nm, 20.0 ( 0.05 °C), error
bars are (0.005.
place between Zn(tpy)2 coordination sites and the
perylene bisimide unit.
Fluorescence Anisotropy Titration. The signifi-
cant change in size and shape of the species upon
2+
addition of Zn ions into a solution of perylene bisimide
ligands due to the formation of coordination polymer
from monomer and fragmentation of polymer to oligo-
2
+
mers on addition of excess Zn has pronounced effect
1
on the diffusion properties ( H DOSY NMR experi-
ments). With the intense fluorescence of the coordina-
tion polymer 8a,b another method to obtain structural
and photophysical information becomes available, i.e.,
fluorescence anisotropy measurements. The values of
fluorescence anisotropy, r, can range from -0.2 to +0.4.
In solvents of low viscosity at room temperature, the
anisotropy is decreased by rotational diffusion, making
it suitable to study changes in size of fluorescent
systems. The maximum value can be reached in the
absence of rotational diffusion and other depolarization
effects, such as energy transfer, and if a parallel
orientation of the absorption and emission transition
5
0.1 ( 5.3 nm. Molecular modeling of the ligand and
the coordination polymer (Figure 10) reveals a length
of 3 nm for each repeat unit. Comparison of the
measured mean length with the calculated model sug-
gests that a single rod is formed from approximately
15 repeat units corresponding to a molecular weight of
4
2
dipole moments is given.
Figure 8 shows a plot of the fluorescence anisotropy,
r, of the perylene ligand 3b against the Zn2+/3b ratio.
∼
25 000. The diameter of the polymer strand deter-
The chromophore was excited in the S0-S1 transition
at 550 nm, and the fluorescence was detected at 610
nm to ensure the highest possible anisotropy value. The
titration was performed in a chloroform-methanol
mixture to ensure comparability with the UV-vis and
fluorescence titration experiments. For the monomer 3b,
an anisotropy value of r ) 0.045 was observed which
increased nearly linearly to a maximum of r ) 0.133 at
mined from molecular modeling is 1-2 nm and depends
on the conformation of the phenoxy residues. This is in
very good agreement with the measured height of 1.6
nm, suggesting that no lateral aggregation of the
strands takes place. Other information which can be
extracted from molecular modeling studies is the rela-
tive flexibility of the polymer strand. Although the
M(tpy)2 unit facilitates a very rigid linkage, the two
single bonds of the phenyl rings connecting perylene
bisimide and tpy provide some flexibility which has also
been found in AFM micrographs.
a 1:1 ratio of Zn2+/3b. These results are in accordance
with the formation of coordination polymer since sig-
nificantly slower rotational diffusion of polymer lead to
higher anisotropy because of its enlarged size. Due to
the fact that in the rigid coordination polymer all
chromophores are aligned in the direction of their
Investigation of the polymer samples prepared using
solutions of higher concentration (1 mM) reveals the

1324 Dobrawa et al.
Macromolecules, Vol. 38, No. 4, 2005
Figure 9. (A) AFM image of ligand 3a on mica and the corresponding coordination polymer 8a prepared from dilute (B, 0.1 mM)
and concentrated (C, 1 mM) DMF solution on mica; (D) high-resolution AFM image with assembled polymeric chains. A cross-
section along the red line is presented in (E); (F) coordination polymer 8a (0.1 mM) on HOPG; coordination polymer 8b (0.05 mM)
on mica (G) and HOPG (H). In all AFM images, the scale bar corresponds to 250 nm, the z data scale is 5 nm. All samples were
prepared by spin-coating.
Figure 10. Molecular modeling (Fujitsu Quantum CAChe 5.2, MM3 force field) and schematic representation of coordination
polymer 8a; three repeat units are shown, perylene bisimide units are represented in red, Zn(tpy) units in blue.
2
formation of a monolayer over large areas in the
micrometer range for both types of substrates (Figure
coordination polymer, being a polycation, is expected to
be very strong, which explains the uniform coverage
with a monolayer film. Formation of such monolayers
is highly desirable for any kind of (opto)electronic device.
As shown by our study, it seems possible to achieve well-
ordered 2D monolayers from metallosupramolecular 1D
polymeric chains formed in solution. Notably, such
polymeric structures of ionic compounds cannot be
realized by vacuum sublimation techniques.
9
C and D). Also in this case, analysis of the apparent
height measured from the AFM images revealed a
constant value of 1.5 ( 0.1 nm (Figure 9D, E). From
the AFM images, it is evident that the neighboring
strands tend to interact with each other, leading to two-
dimensional self-assembly. Because of the spreading of
the polymer samples in the surface plane, probably
caused by the spin-coating process, defects in the
cylinders’ packing are observed, enabling the visualiza-
tion of single rods. Lateral dimensions of the single rods
were measured to be approximately 4 nm. Definitely,
at higher concentration, the polymer tends to aggregate
by contacting terminally and laterally to form a network
of closely packed rods. As the surface of freshly cleaved
mica bears negative charge, the interaction with the
Conclusion
Starting with studies on the complexation behavior
of 2,2′:6′,2′′-terpyridine with first row transition metal
ions and the characterization of the thermodynamically
stable (high binding constant) but kinetically labile (fast
ligand exchange) coordination of Zn2 ions, metallo-
supramolecular self-assembly of functional perylene
+

Macromolecules, Vol. 38, No. 4, 2005
Fluorescent Coordination Polymers 1325
bisimide dyes to polymeric structures has been realized.
Perylene bisimide chromophores were equipped with tpy
ligands by condensation of the respective anhydrides
with 4′-p-aminophenyl-2,2′:6′,2′′-terpyridine (2) to form
monotopic (5a,b) and ditopic (3a,b) building blocks. The
complex formation was investigated by the preparation
of a dimer complex (6a,b) using two monotopic units.
On the basis of these results, highly soluble coordination
polymers (8a,b) have been prepared from Zn2 com-
plexation of ditopic building blocks. For both dimer and
polymer, reversible coordination was observed depend-
(8) Sauvage, J.-P.; Collin, J.-P.; Chambron, J.-C.; Guillerez, S.;
Coudret, C.; Balzani, V.; Barigelletti, F.; De Cola, L.;
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9
charged substrates. This observation and preliminary
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using the multilayer assembly technique introduced by
4
4
Decher render this self-assembled functional system
as an interesting alternative to traditional main chain
(
26
perylene bisimide polymers for application in artificial
light harvesting systems or organic light emitting diode
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Acknowledgment. Financial support by the
Deutsche Forschungsgemeinschaft (DFG grant project
WU 317/3-1), and DAAD “Acciones Integradas Hispano-
Alemanas” is gratefully acknowledged. We are thankful
to Shimadzu Germany for the measurement of the
MALDI-TOF MS of compound 6a. The authors also
thank L. De Cola, M. Rehahn, and R. Ziessel for helpful
discussions on terpyridine metal complexes and H.
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DHB ) 2,5-dihydroxybenzoic acid, DCTB ) trans-2-(3-4-tert-
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Supporting Information Available: Experimental de-
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1
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(
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2
+
2+
2+
1
React. Mech. 2001, 3, 1-23.
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MA047737J