Design, self-assembly, and photophysical properties of pentameric metallomacrocycles: [M5(N-hexyl[1,2-bis(2,2′:6′,2″- terpyridin-4-yl)]carbazole)5][M = Fe(II), Ru(II), and Zn(II)]

Article abstract of DOI:10.1039/b509662f

A novel family of metallopentacycles was constructed by the facile self-assembly of a bis(terpyridine)-carbazole monomer utilizing terpyridine-metal(II)-terpyridine connectivity; its photophysical properties were investigated. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b509662f

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Design, self-assembly, and photophysical properties of pentameric
metallomacrocycles: [M (N-hexyl[1,2-bis(2,29:69,20-terpyridin-4-
5
yl)]carbazole) ][M 5 Fe(II), Ru(II), and Zn(II)]{
5
a
a
b
c
Seok-Ho Hwang, Pingshan Wang, Charles N. Moorefield, Luis A. God ´ı nez, Juan Manr ´ı quez,
c
c
Erika Bustos and George R. Newkome*
ab
Received (in Columbia, MO, USA) 7th July 2005, Accepted 26th July 2005
First published as an Advance Article on the web 23rd August 2005
DOI: 10.1039/b509662f
A novel family of metallopentacycles was constructed by the
facile self-assembly of a bis(terpyridine)–carbazole monomer
utilizing terpyridine–metal(II)–terpyridine connectivity; its
photophysical properties were investigated.
The design of highly ordered supramolecular structures has
attracted considerable interest in that the self-assembly of
specifically tailored building blocks, possessing appropriate
directionality of binding loci underpin the generation of novel,
Scheme 1 Reagents and conditions: (a) 1-bromohexane, KOH, DMF,
reflux; (b) DMF, POCl ; (c) (i) 4.4 equiv. 2-acetylpyridine, NaOH, (ii)
3
utilitarian supramolecular complexes with two- and three-dimen-
1–3
NH OAc, AcOH, reflux.
4
sional nano- and macro-structures. Owing to their electronic
and steric versatility, aromatic N-heterocycles continue to play a
1
dialdehyde 2 (52%). This transformation was supported ( H and
4,5
prominent role as classical ligands in coordination compounds,
13
C NMR) by the appearance of peaks at 10.15 (–CHO) and
6–8
bridging ligands in binuclear derivatives, and as building blocks
for supramacromolecular assemblies. In addition to the ability of
these polyheteroaromatics to be connected by metal centers, they
provide the opportunity of p-back-bonding and thereby may affect
the delocalization and transport of electrons as well as the capture
1
91.72 (–CHO) ppm, respectively, and the 3,6-substitution pattern
was confirmed by the aromatic coupling constants. Dialdehyde 2
was treated with 3 equiv. of 2-acetylpyridine under basic
conditions for 24 h at 25 uC, followed by the addition of excess
9,10
4
NH OAc in AcOH. After refluxing for 12 h, the desired
of photons for light-energy conversion.
So far, many successful strategies have been developed for the
angular building block 3 (34%) was isolated and structurally
1
confirmed ( H NMR) by the peaks at 4.38 ppm (t) for NCH ,
2
11,12
construction of metallomacrocyclic structures with trigonal,
1
3–19
20–22
23
24
8.11 ppm (d) for 3-ArH, and 7.90 ppm (dd) for 4,40-tpyH (these
square,
the pentagonal shape has been reported
extent relative to its homologous counterparts.
We herein report the assembly and photophysical properties
hexagonal,
heptagonal, and octagonal shapes;
25,26
signals integrated in the expected 1 : 1 : 2 ratio). Also, a mass peak
+
albeit to a limited
(ESI-MS) at m/z 715.2 [M + H] was recorded.
2 2
Treatment of 3 with one equiv. of FeCl ?4H O in MeOH for
12 h at 25 uC (Scheme 2) gave the self-assembled pentameric
(
luminescence and photovoltaics) of a related family of metallo-
macrocycles based on a carbazole unit connecting two, rigidly
held, and appropriately directed (ca. 105u) terpyridine ligands. This
bis-ligand was then self-assembled to give metallopentacycles
utilizing terpyridine–metal(II)–terpyridine connectivity.
Chemically, carbazole has been shown to be easily functiona-
2
7
lized at its 3-, 6- or 9-positions. Therefore, synthesis of the key
building block 3 (Scheme 1) began with commercially available
9H-carbazole, which was N-alkylated with 1-bromohexane using
KOH to give 1 (71%) that was subsequently treated with 3 equiv.
of DMF and POCl
3
(Vilsmeier reagent) to form the desired
a
Department of Polymer Science, The University of Akron, Akron,
OH 44325, USA. E-mail: newkome@uakron.edu;
http://www.dendrimers.com
Maurice Morton Institute of Polymer Science, The University of Akron,
b
Akron, OH 44325, USA
Centro de Investigaci o´ n y Desarrollo Tecnol o´ gico en Electroqu ´ı mica,
c
Quer e´ taro, 76700, M e´ xico
{
Electronic supplementary information (ESI) available: Detailed experi-
mental procedures with analytical data and solar cell device fabrication
procedure. See http://dx.doi.org/10.1039/b509662f
2+
Scheme 2 Reagents and conditions: (a) (i) M , reflux; (ii) NH
4 6
PF –
MeOH.
4
672 | Chem. Commun., 2005, 4672–4674
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1
10] (4). The H NMR spectrum of 4 revealed
complex [Fe (3)
5
5
(PF
6
)
the metal–ligand charge-transfer (MLCT) transitions, which are
derived from the promotion of an electron from the metal [Fe(II)
or Ru(II)]-centered d-orbitals to unfilled ligand-centered p*
two sharp doublets at 8.56 (3-ArH) and 8.08 (2-ArH) ppm. These
sharp peaks imply the presence of a single homogenous
environment for all such groups; this is in contrast to the
broadened or multiple signals expected for linear oligomers. The
observed upfield shift for the doublet at 7.31 ppm (6,60-tpyHs;
Dd 5 21.49 ppm) and downfield shift for the singlet at 9.39 ppm
29
orbitals, appeared at 573 and 504 nm respectively. In the case
of the Zn(II) metallopentacycle 6, the absorption bands at
l
max 5 240, 285, 316, 398 nm originating from intra-ligand charge
1
transfer ( ILCT) are observed without MLCT peaks; the MLCT
30
(
39,59-tpyHs; Dd 5 0.46 ppm) when compared to the absorptions
of Zn(II) to terpyridine can be excluded.
for the uncomplexed starting material, confirm the symmetry
associated with macrocyclization. Lastly, the pentagonal motif was
established (ESI-MS) by the definitive signals for multiply-charged
entities ranging from the +5 to the +10 charge states derived from
Although the Ru(II) complexes of bipyridinyl ligands exhibit
3
characteristic red luminescence from the MLCT state, in the case
of the Ru(II) complex with terpyridinyl ligands, this state is
3
quenched by a low-lying MC state and room temperature
31
emission is not observed. Whereas the Ru(II) metallopentacycle
28
2
2
the loss of both PF5 and PF6
.
The self-assembled Ru(II) counterpart, [Ru (3) (PF ) ] (5), was
5
employing the carbazole moiety, which is a well known material
for fluorescence, does not show fluorescence at 25 uC, the Zn(II)
pentameric metallomacrocycle does show a strong yellow emission
when excited with UV light (400 nm). The fluorescence of the
Zn(II) pentamers in MeCN solution is shown in Fig. 1(C).
The electrochemical responses of the metallopentacycles were
characterized by cyclic voltammetry (CV) experiments using
0.2 mM solutions of the relevant compound dissolved in DMF
with 0.1 M Bu NBF at 25 uC. Whereas pentamers 4 and 5
5
6 10
prepared by treatment of a MeOH solution of monomer 3 with
one equiv. of [Ru(DMSO) (Cl) ] over 36 h at 50 uC. This method
gave the desired pentameric complex possessing chloride counter-
4
2
2
ions, which, after chromatography and counterion exchange (Cl
2
1
6
to PF ), afforded the pure macrocycle 5 (overall ca. 35%). The H
NMR spectrum of 5 exhibited a similar pattern to that of 4,
including the diagnostic shifts for the doublet at 7.54 ppm
(
6,60-tpyHs; Dd 5 21.26 ppm), and the singlet at 9.24 ppm
4
4
(
39,59-tpyHs; Dd 5 0.31 ppm) in accord with ring-formation. The
showed metal related redox waves for Fe(III)|Fe(II) and
+
ESI-MS of 5 gave signals for the multiply-charged entities ranging
from the +5 to the +8 charge states.
Ru(III)|Ru(II) pentamers at 0.49 and 0.57 V vs. Fc |Fc respectively,
the electrochemical response of the Zn(III)|Zn(II) couple in
The [Zn
5
(3) (PF
5
6
)
10] complex (6) was generated by the reaction
pentamer 6 was not observed due to the completely filled d-shell
32
of a 1 : 1 mixture of ligand 3 with Zn(BF
4
)
2
?8H O in MeCN for
2
of the Zn(II) complex.
2
4 h at 80 uC affording a light yellow solid (35%). The absence of
Cathodic scans of the pentamers 4–6 however, exhibited a set of
reversible reduction waves that were followed by large current
reduction peaks at more negative potentials, reflecting complex
processes probably associated with decomposition. The first two
reduction processes for 4 (Table 1) are related to metal reduction
1
extraneous peaks in the H NMR spectrum excluded the presence
of starting materials, intermediates, and oligomeric materials. The
diagnostic shifts of the doublets at 7.93 ppm (6,60-tpyHs;
Dd 5 20.87 ppm), and the singlet at 9.17 ppm (39,59-tpyHs;
Dd 5 0.24 ppm) along with definitive ESI-MS data all support the
structural assignment. COSY and HETCOR spectra of the
bis(terpyridine)s and the corresponding self-assembled metalloma-
crocycles verified the peak assignments as well as the coupling
patterns.
22
events, and the only distinguishable peak in 5 is related to the
monoelectronic reduction of one of the coordinated terpyridine
2
2
ligands. In the case of 6, two closely positioned reversible waves
are observed before current induced irreversible events take place.
Based on the half-wave potential difference associated with these
signals, terpyridine and metal-related processes are suggested as an
To elucidate and support the composition of the metallopenta-
cycles, their UV-vis absorption spectra were measured (MeCN).
From the absorption spectra of 4 and 5 in Fig. 1, each showed
the lowest energy ligand-centered p–p* transitions of the
terpyridine moieties at y310 and y390 (shoulder) nm. While
2
1,22
explanation of the CV response,
terpyridine associated events.
instead of sequential
21
From the CV responses at different scan rates (20–100 mV s ),
values of the diffusion coefficients (D) of 4–6 were obtained
3
3
(
Table 1) using the Randles–Sevcik equation. These values are
nearly equal, further supporting the proposed structure of the three
metallopentacycles.
Due to their light absorption properties, these materials were
also studied as sensitizer materials for solar cell devices.
Photovoltaic performance experiments using dye-covered nano-
crystalline TiO electrodes (prepared by dipping the semiconductor
2
Table 1 Electrochemical data of metallopentacycles, 4–6, (0.2 mM)
obtained in 0.1 M Bu NBF in DMF at 25 uC using a glassy carbon
4 4
21
working electrode; scan rate 5 100 mV s
+
E
1/2(vs. Fc |Fc)/V
2
28
2 21
Complex L|L
M(I)|M(0) M(II)|M(I) M(III)|M(II) D/10 cm s
4
5
6
—
21.843
21.791
21.837
—
—
21.733
—
21.929
0.492
0.574
—
2.83
2.1
3.1
Fig. 1 UV-vis absorption (solid line) and emission (dashed line) spectra
for metallopentacycles 4 (A); 5 (B); 6 (C).
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Table 2 Photovoltaic performances and surface coverage values of
ITO/TiO /pentamer/KI–I electrolyte/graphite dye-sensitized cells
measured using a polychromatic 1.5 mW cm light source
3 G. F. Swiegers and T. J. Malefetse, Coord. Chem. Rev., 2002, 225,
91.
4 S. Schmatloch and U. S. Schubert, Chem. Unserer Zeit., 2003, 37,
2
2
2
2
1
80.
W. R. McWhinnie and J. D. Miller, Adv. Inorg. Chem. Radiochem.,
969, 11, 135.
D. J. Berg, J. M. Boncella and R. A. Anderson, Organometallics, 2002,
1, 4622.
a
22
b
c
d
211e
22
Complex jsc /mA cm
V
oc /mV ff (%) g (%) C/10
/mol cm
5
6
1
4
5
6
a
90.4
300.4
137.6
250
290
269
32.6
26.3
29.5
0.49
1.53
0.73
3.03
5.49
6.22
2
7
8
9
W. Kaim, Coord. Chem. Rev., 2002, 230, 127.
b
Short-circuit photocurrent density. Open-circuit photovoltage.
W. Kaim, Angew. Chem., Int. Ed. Engl., 1983, 22, 171.
S. Serroni, S. Campagna, F. Puntoriero, C. Di Pietro,
N. D. McClenaghan and F. Loiseau, Chem. Soc. Rev., 2001, 30,
c
d
Fill factor. Photoconversion efficiency of the cell. Surface
e
coverage.
367.
10 J.-P. Launay, Chem. Soc. Rev., 2001, 30, 386.
substrate in a 0.2 mM MeCN solution of each one of the
11 S.-H. Hwang, C. N. Moorefield, F. R. Fronczek, O. Lukoyanova,
L. Echegoyen and G. R. Newkome, Chem. Commun., 2005, 713.
12 S. J. Lee, A. Hu and W. Lin, J. Am. Chem. Soc., 2002, 124, 12948.
34
metallopentacycles) properly fitted in a solar cell device, were
2
2
conducted using a polychromatic (1.5 mW cm ) incident light
source and an electrolyte containing 0.3 M KI + 0.015 M I
dissolved in a 4 to 1 ratio of propylene and ethylene carbonate.
The surface coverage (C) of each metallopentacycle on the TiO
electrode was calculated using UV-vis spectroscopy experiments as
1
3 F. A. Cotton, C. Lin and C. A. Murillo, J. Am. Chem. Soc., 2001, 123,
670.
2
2
14 F. A. Cotton, L. M. Daniels, C. Lin, C. A. Murillo and S.-Y. Yu,
J. Chem. Soc., Dalton Trans., 2001, 502.
5 H. Gang, G. Dong, D. Chun-ying, M. Hong and M. Qing-jin, New J.
2
1
Chem., 2002, 26, 1371.
6 S. J. Lee and W. Lin, J. Am. Chem. Soc., 2002, 124, 4554.
3
5
reported by Gr a¨ tzel et al. Notably, the values obtained (Table 2)
are fairly close to each other, thus supporting the similar structure
and dimensions of compounds 4–6. Discharge experiments
conducted with these devices also allowed the calculation of some
1
17 X. Liu, C. L. Stern and C. A. Mirkin, Organometallics, 2002, 21, 1017.
1
8 J. J. Pak, J. Greaves, D. J. McCord and K. J. Shea, Organometallics,
002, 21, 3552.
9 S.-S. Sun, J. A. Anspach and A. J. Lees, Inorg. Chem., 2002, 41,
862.
2
1
3
4
of the values in Table 2. Whereas the fill factor (ff) of the three
1
electrodes studied here remains similar, the Ru(II) metallopenta-
20 G. R. Newkome, T. J. Cho, C. N. Moorefield, G. R. Baker,
M. J. Saunders, R. Cush and P. S. Russo, Angew. Chem., Int. Ed.,
cycle showed the best results for the short circuit photocurrent (jsc
)
1999, 38, 3717.
as well as the open circuit photopotential (Voc). Also, total
photoconversion efficiency of the cell (g) using electromagnetic
radiation spanning the visible region of the spectrum exhibited the
highest value in the case of the Ru(II) pentamer 5.
2
1 G. R. Newkome, T. J. Cho, C. N. Moorefield, R. Cush, P. S. Russo,
L. A. God ´ı nez, M. J. Saunders and P. Mohapatra, Chem. Eur. J., 2002,
8, 2946.
2 G. R. Newkome, T. J. Cho, C. N. Moorefield, P. P. Mohapatra and
2
L. A. God ´ı nez, Chem. Eur. J., 2004, 10, 1493.
In summary, we have demonstrated the formation and
characterization of a series of unique, self-assembled, metallopen-
tacycles. Employing terpyridine–metal(II)–terpyridine connectivity,
these complexes are stable and irreversible under the reaction
conditions. The structures of these pentagonal architectures were
2
3 B. Grossmann, J. Heinze, E. Herdtweck, H. Noth, M. Schwenk,
W. Wachter and B. Weber, Angew. Chem., Int. Ed. Engl., 1997, 36, 387.
24 P. L. Jones, K. J. Byrom, J. C. Jeffey, J. A. McCleverty and M. D. Ward,
Chem. Commun., 1997, 1361.
25 H. Jiang and W. Lin, J. Am. Chem. Soc., 2003, 25, 8084.
26 B. Hasenknopf, J.-M. Lehn, B. O. Kneisel, G. Baum and D. Fenske,
1
13
characterized by means of H and C NMR, UV-vis spectro-
scopy, and mass spectroscopy. Preliminary studies of photoelec-
trochemical performance for the metallopentacycles demonstrated
their potential for solar cell development. Further experiments are
currently ongoing to investigate the electroluminescence behavior
using LED devices.
Angew. Chem., Int. Ed. Engl., 1996, 35, 1838.
27 J. A. Joule, Adv. Heterocycl. Chem., 1984, 35, 83.
28 E. C. Constable, C. E. Housecroft, M. Neuburger, A. G. Schneider and
M. Zehnder, J. Chem. Soc., Dalton Trans., 1997, 2427.
9 M. Beley, J.-P. Collin, J.-P. Sauvage, F. Heisel and A. Miech, J. Chem.
2
Soc., Dalton Trans., 1991, 3157.
30 X.-Y. Wang, A. D. Guerzo and R. H. Schmehl, Chem. Commun., 2002,
2344.
The authors gratefully thank the National Science Foundation
31 N. W. Alcock, P. R. Barker, J. M. Haider, M. J. Hannon, C. L. Painting,
(DMR-041780, INT-0405242), the Air Force Office of Scientific
Z. Pikramenou, E. A. Plummer, K. Rissanen and P. Saarenketo,
J. Chem. Soc., Dalton Trans., 2000, 1447.
2 G. D. Storrier, S. B. Colbran and D. C. Craig, J. Chem. Soc., Dalton
Research (F49620-02-1-0428,02), the Ohio Board of Regents and
the Mexican Council for Science and Technology (CONACyT),
for financial support.
3
Trans., 1998, 1351.
3 A. J. Bard and L. R. Faulkner, Electrochemical Methods: Fundamentals
3
and Applications, John Wiley & Sons, New York, 2nd edn, 2001.
4 E. Bustos, J. Manr ´ı quez, L. Echegoyen and L. A. God ´ı nez, Chem.
Commun., 2005, 1613.
3
Notes and references
1
2
B. J. Holliday and C. A. Mirkin, Angew. Chem., Int. Ed., 2001, 40, 2022.
S. Leininger, B. Olenyuk and P. J. Stang, Chem. Rev., 2000, 100,
35 P. Bonh oˆ te, E. Gogniat, S. Tingry, C. Barb e´ , N. Vlachopoulos,
F. Lenzmann, P. Comte and M. Gr a¨ tzel, J. Phys. Chem. B, 1998, 102,
1498.
853.
4
674 | Chem. Commun., 2005, 4672–4674
This journal is ß The Royal Society of Chemistry 2005