Self-assembly of oligomeric linear dipyrromethene metal complexes

Article abstract of DOI:10.1039/b820461f

BF₂ capped dipyrrin dimers were synthesized and have been used to terminate oligomerization to form a series of controlled length oligomers; the crystal structures of the metal complexes were investigated and correlations between the structures

Full text of DOI:10.1039/b820461f

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Self-assembly of oligomeric linear dipyrromethene metal complexesw
Qing Miao, Ji-Young Shin, Brian O. Patrick and David Dolphin*
Received (in Berkeley, CA, USA) 17th November 2008, Accepted 16th March 2009
First published as an Advance Article on the web 3rd April 2009
DOI: 10.1039/b820461f
BF2 capped dipyrrin dimers were synthesized and have been used
to terminate oligomerization to form a series of controlled length
oligomers; the crystal structures of the metal complexes were
investigated and correlations between the structures and optical
properties were established.
Recently, the study of molecular electronic/photonic wires has
become an active platform which involves the use of single or
small bundles of molecules as building blocks for energy
transportation and electronic applications.1,2 In addition,
molecular photonics and electronics have attracted much
interest due to the potential of storing vast amounts of
information in very small volumes.3
Dipyrromethenes (dipyrrins) are monoanionic divalent
Scheme 2
ligands that form neutral complexes with various metal ions
in self-assembly processes.4 Porphyrins can be considered as
the solid state, along with remarkable electron-transfer
cyclic bis-dipyrromethenes, and Wagner and Lindsey have
properties8,9 which offer many advantages for future studies.
introduced a molecular wire where a porphyrin array is linked
The protected ligands 7 and 9 were prepared as the primary
to a boron-dipyrromethene complex at one end of the
building blocks (Scheme 2) and when reacted with the dipyrrin
assembly.5 In this case, the boron-dipyrromethene acts as an
‘‘dimer’’, 2, oligomers of specific chain lengths were prepared.
optical input while the porphyrin array plays the role of the
The crystal structurez of 7 shows C2 symmetry, where a
transmission element of the molecular photonic device.
nitrogen-bound H-atom on the free-base dipyrromethene unit
Similarly, Weiss and his colleagues have designed a self-
is shared between N2 and N20 (Fig. 1, the shared proton is
assembled porphyrin photonic wire which performs a stepwise
marked as grey dots). Consequently, the pyrrole interior
energy transfer.6 In 2006, Maeda et al. employed dipyrrins as
C–N–C angles exhibit an average angle of 107.51, which is
scaffolds to form metal-coordinated dipyrrin polymers which
intermediate between the amino and imino values for this
exhibited spherical nanoarchitectures.7
delocalized aromatic system.
We have prepared a similar dipyrrin ‘‘dimer’’ 2 (Scheme 1)
As expected, ligands 7 and 9 are readily able to form unique
but instead of ‘‘uncontrolled’’ polymerization, we have been
metal complexes with various metals (Scheme 3). However,
able to control the oligomerization using a dipyrrin dimer
mixed coordination reactions of the mono-protected ligand 9
monoprotected as the BF2 complex. Boron-dipyrromethene
and dipyrrin ‘‘dimer’’ 2 produced a mixture, which contains
complexes have properties which combine high molar extinc-
self-assembled oligomers of different lengths (Scheme 4). The
tion coefficients and high fluorescence quantum yields, strong
formation of the self-assembled oligomers was confirmed by
chemical and photochemical stabilities in both solution and
MALDI-TOF mass spectrometry (Fig. 2). The structures of
the mono-metal complex 14 and di-metal complex 16 have
been defined by X-ray diffraction analysis (Fig. 3).z The two
crystals show similar metal–N bond lengths and inter-ligand
dihedral angles, and complex 16 contains an inversion center.
Scheme 1
Department of Chemistry, University of British Columbia,
2036 Main Mall, Vancouver, BC, Canada V6T 1Z1.
E-mail: david.dolphin@ubc.ca; Fax: +1-604-822-9678;
Tel: +1-604-822-4571
w Electronic supplementary information (ESI) available: Experimental
details; crystal data; 1H NMR, 13C NMR and optical spectra. CCDC
684023–684029 and 709603. For ESI and crystallographic data in CIF
or other electronic format, see DOI: 10.1039/b820461f
Fig. 1 Crystal structure of 7. The C2 symmetry axis is denoted by the
gray line. Thermal ellipsoids are scaled to the 50% probability level.
ꢀc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 2541–2543 | 2541

View Article Online
Table 1 Selected crystal data
Inter-ligand
dihedral angle/1
Bending
angle/1
Metal–N bond
length/A
a
Compound
10
11
12
13
14
15
58.4
70.5
54.5
78.5
84.0
84.7
20.5
8.4
19.2
2.0
3.4
3.4
1.92–1.93
1.95–1.99
1.89–1.90
1.96–1.97
1.96–1.98
1.97–1.98
Scheme 3
a
Bending angle (y) as shown in Fig. 4.
Scheme 4
Fig. 4 Crystal structures of 10 and 11 (a and b); H-atoms have been
omitted for clarity. Thermal ellipsoids are scaled to the 50%
probability level. The bending angle is shown as y.
as shown by the different dihedral angles of the inter-ligand
planes (Table 1). The structures of complexes 10 and 12 show
C2 symmetry, while the others, 11, 13–15, are non-symmetric
molecules (Fig. 4) (see ESIw). Interestingly, the two single
ligand units of 10 and 12 are bent with 20.51 and 19.21 angles,
as shown in Fig. 4. Compared with other metal complexes
having the same ligand unit 9, the CuII complex 11 shows
significant differences in metal–N bond lengths, resulting from
the Jahn–Teller effect. Because of steric hindrance between the
a-methyl groups, the dihedral angles of metal complexes of 9
were larger than those of the metal complexes of 7, which are
represented by the different coordination geometries and
electron distributions in the two NiII complexes 12 and 13.
The relatively shorter N–Ni bond lengths in complex 12
(1.89–1.90 A), compared with those in complex 13
(1.96–1.97 A), were considered as the effect of the lower steric
hindrance owing to the absence of a-substituents and the
reduced radius of the d8 low-spin state central metal ion.
The difference in the coordination and electron distribution
is further confirmed by NMR spectroscopy. Complex 12,
having a smaller dihedral angle, showed sharp signals in its
1H NMR spectrum, which proves it is a low-spin, diamagnetic
structure. On the other hand, the 1H NMR spectrum of 13
exhibited broad signals over a large range, correlating with a
high-spin, paramagnetic structure (see ESIw).
Fig. 2 MALDI-TOF spectrum of the crude reaction mixture which
contains an oligomeric mixture of 14, 16–19 and excess ligand 9.
Fig. 3 Crystal structures of 14 (a) and 16 (b); H-atoms have been
omitted for clarity. Thermal ellipsoids are scaled to the 50% prob-
ability level.
While the optical spectra, especially fluorescence, have been
thoroughly studied for the boron complexes of dipyrro-
methenes,8,9 those of simple dipyrromethenes and their metal
complexes have not been extensively studied. A. I. V’yugin
et al.10 examined solvent effects and showed that lmax is
determined by the polarization of the p system, which in turn
is governed by electronic and steric factors of the metal ions.
We have reported11 that hyperconjugation of peripheral alkyl
In order to better understand the role of the central metal
and the steric effects of the ligand, studies on the crystal
structures and spectral properties of various monomeric metal
complexes have been undertaken.
All the metal complexes 10–15 show nearly linear confor-
mations, in which the two dipyrromethene units linked to the
central dicationic metal exhibit distorted tetrahedral structures
ꢀc
This journal is The Royal Society of Chemistry 2009
2542 | Chem. Commun., 2009, 2541–2543

View Article Online
metal coordination. Each of these factors affect step-wise self-
assembly into similar arrays and related research is currently
in progress.
This work was supported by the Natural Sciences and
Engineering Research Council (NSERC) of Canada. We
thank the Mass Spectroscopy lab of the Department of
Chemistry, University of British Columbia.
Notes and references
z Crystal data for 7: C26H21BN4F2, M = 438.28, monoclinic, a =
6.2217(8), b = 30.104(4), c = 11.954(2) A, b = 96.072(9)1, V =
2226.4(6) A3, T = 298 K, space group C2/c (no. 15), Z = 4, 11 946
reflections measured, 2611 unique (Rint = 0.037). R1 = 0.044(I 4
Scheme 5
2.00s(I)), wR2
C52H40B2N8F4Cuꢁ2CH2Cl2,
=
0.122(all data). Crystal data for 10:
M
=
1107.93, orthorhombic, a =
22.128(5), b = 8.569(2), c = 27.299(6) A, V = 5176(2) A3, T =
173 K, space group Pbcn (no. 60), Z = 4, 14 088 reflections measured,
3365 unique (Rint = 0.122). R1 = 0.054(I 4 2.00s(I)), wR2 =
0.136(all data). Crystal data for 11: C56H48B2N8F4Cuꢁ2CH2Cl2,
M = 1164.04, monoclinic, a = 18.142(2), b = 20.146(3), c =
15.483(3) A, b = 90.191(7)1, V = 5659(2) A3, T = 173 K, space
group P21/c (no. 14), Z = 4, 32 007 reflections measured, 7184 unique
(Rint = 0.130). R1 = 0.070(I 4 2.00s(I)), wR2 = 0.209(all data).
Crystal data for 12: C52H40B2N8F4Niꢁ2CH2Cl2, M = 1103.10, ortho-
rhombic, a = 22.249(2), b = 8.5705(7), c = 27.197(2) A, V =
5186.1(7) A3, T = 173 K, space group Pbcn (no. 60), Z = 4, 15 765
reflections measured, 3328 unique (Rint
=
0.083). R1
=
0.072(I 42.00s(I)), wR2 = 0.121(all data). Crystal data for 13:
C56H48B2N8F4Niꢁ2CHCl3, M = 1228.09, triclinic, a = 10.840(1),
b = 14.501(2), c = 19.830(2) A, a = 109.761(6)1, b = 104.839(6)1,
3
ꢀ
g = 93.027(6)1, V = 2802.2(5) A , T = 173 K, space group P1 (no. 2),
Z = 2, 30 194 reflections measured, 9772 unique (Rint = 0.045). R1 =
0.075(I 4 2.00s(I)), wR2 = 0.230(all data). Crystal data for 14:
C56H48B2N8F4Coꢁ2CH2Cl2, M = 1159.43, triclinic, a = 10.669(1),
b = 14.432(2), c = 19.145(2) A, a = 70.375(4)1, b = 78.215(4)1, g =
86.892(4)1, V = 2717.8(5) A , T = 173 K, space group P1 (no. 2), Z =
2, 58 453 reflections measured, 10 121 unique (Rint = 0.062). R1 =
0.061(I 4 2.00s(I)), wR2 = 0.169(all data). Crystal data for 15:
Fig. 5 Optical spectra of ligand 9 and complexes 14, 16 and 17 in
CH2Cl2.
3
ꢀ
groups results in bathochromic shifts and Motekaitis–Martell
MO theory allows for the calculation of dihedral angles for
metal complexes.
C56H48B2N8F4Znꢁ2CH2Cl2,
M
=
1165.87, triclinic,
a
=
In this study we have prepared two sets of reference
compounds (20 and 21, and 22–33, Scheme 5) to provide the
electronic spectra of individual metal-dipyrrins and boron-
dipyrrins. For the same metal, an increase in the inter-ligand
dihedral angles results in a bathochromic shift (compare 22,
23; 28, 29; 24, 25 and 30, 31, see ESIw). The bathochromic shift
also occurs with a cyano group on the meso-aryl instead of a
methyl group. This is particularly obvious in NiII complexes
where the distorted square-planar structures exhibit a large
bathochromic shift (14 nm, compare 24 with 30). By contrast,
the two distorted tetrahedral NiII complexes of a-methyl
dipyrromethenes (compare 25 with 31) show only a relatively
small bathochromic shift (2 nm).
10.7063(11), b = 14.4785(12), c = 19.1263(18) A, a = 70.425(4)1,
b = 78.184(4)1, g = 86.994(4)1, V = 6037(3) A3, T = 173 K, space
ꢀ
group P1 (no. 2), Z = 2, 21 133 reflections measured, 18 068 unique
(Rint = 0.046). R1 = 0.066(I 4 2.00s(I)), wR2 = 0.175(all data).
Crystal data for 16: C84H72B2N12F4Co2ꢁ2CHCl3, M = 1703.75,
monoclinic, a = 13.4104(15), b = 26.078(2), c = 12.7437(14) A,
b = 115.666(5)1, V = 4017.0(7) A3, T = 173 K, space group P21/c
(no. 14), Z = 2, 30 263 reflections measured, 7078 unique (Rint
=
0.054). R1 = 0.059(I 4 2.00s(I)), wR2 = 0.173(all data). CCDC
684023–684029 and 709603. For crystallographic data in CIF or other
electronic format, see DOI: 10.1039/b820461f
1 J. M. Tour, Acc. Chem. Res., 2000, 33, 791–804.
2 N. Aratani, H. S. Cho, T. K. Ahn, S. Cho, D. Kim, H. Sumi and
A. Osuka, J. Am. Chem. Soc., 2003, 125, 9668–9681.
3 J. M. Tour, A. M. Rawlett, M. Kozaki, Y. Yao, R. C. Jagessar,
S. M. Dirk, D. W. Price, M. A. Reed, C.-W. Zhou, J. Chen,
W. Wang and I. Campbell, Chem.–Eur. J., 2001, 7, 5118–5134.
4 T. E. Wood and A. Thompson, Chem. Rev., 2007, 107, 1831–1861.
5 R. W. Wagner and J. S. Lindsey, J. Am. Chem. Soc., 1994, 116,
9759–9760.
6 M. Koepf, A. Trabolsi, M. Elhabiri, J. A. Wytko, D. Paul,
A. M. Albrecht-Gary and J. Weiss, Org. Lett., 2005, 7, 1279–1282.
7 H. Maeda, M. Hasegawa, T. Hashimoto, T. Kakimoto, S. Nishio
and T. Nakanishi, J. Am. Chem. Soc., 2006, 128, 10024–10025.
8 M. Wada, S. Ito, H. Uno, T. Murashima, N. Ono, T. Urano and
Y. Urano, Tetrahedron Lett., 2001, 42, 6711–6713.
The strongest UV-Vis absorption of ligand 9, in CH2Cl2,
exhibits a sharp band at 514 nm as a result of boron com-
plexation, but this band remains only as a shoulder upon
metallation (Fig. 5). The electronic absorption spectra of
complexes 14, 16 and 17 show a hypsochromic shift with
increasing length and increasing number of metal ions.
The optical spectra of all the metal complexes 10–17
approximate the sum of the individual boron-dipyrrins and
metal-dipyrrins, which suggests a minimal overlap between the
p-systems (see ESIw).
9 W. Qin, M. Baruah, M. Van der Auweraer, F. C. De Schryver and
N. Boens, J. Phys. Chem. A, 2005, 109, 7371–7384.
10 G. B. Guseva, E. V. Antina, M. B. Berezin and A. I. V’yugin, Russ.
J. Gen. Chem., 2004, 74, 1282–1285.
Dipyrrins allow for great flexibility in the constitution and
conformation of the linking groups (and their substitution
patterns), peripheral constituents, terminal substituents and
11 C. Bruckner, V. Karunaratne, S. J. Rettig and D. Dolphin, Can. J.
¨
Chem., 1996, 74, 2182–2193.
ꢀc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 2541–2543 | 2543