Aluminium complexes of N2O2-type dipyrrins: The first hetero-multinuclear complexes of metallo-dipyrrins with high fluorescence quantum yields

Article abstract of DOI:10.1039/b820141b

The aluminium complexes of N₂O₂-dipyrrins were synthesized and the absorption/fluorescence spectral changes caused by the complex formation with ZnCl₂, ZnBr₂ and Zn(OAc) ₂ were examined. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b820141b

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Aluminium complexes of N2O2-type dipyrrins: the first
hetero-multinuclear complexes of metallo-dipyrrins with high
fluorescence quantum yieldsw
Chusaku Ikeda, Satoko Ueda and Tatsuya Nabeshima*
Received (in Cambridge, UK) 11th November 2008, Accepted 16th February 2009
First published as an Advance Article on the web 16th March 2009
DOI: 10.1039/b820141b
The aluminium complexes of N2O2-dipyrrins were synthesized
and the absorption/fluorescence spectral changes caused by the
complex formation with ZnCl2, ZnBr2 and Zn(OAc)2 were
examined.
Burgess reported a boron complex of an N2O2-type dipyrrin
that is highly fluorescent (FF = 0.41 in CHCl3).13 However,
the chelating ability of the N2O2-type dipyrrin complexes has
not hitherto been reported. We now chose aluminium as the
central metal ion because (i) aluminium is ubiquitous and
cheap and some aluminium complexes are highly fluorescent,
(ii) the N2O2 binding site may serve as a planar equatorial unit
for octahedral coordination geometry around aluminium, and
(iii) oxygen atoms in the aluminium complexes are suitable for
subsequent chelation because the oxygen atoms of the Al–O
bonds are more negatively charged than those of the B–O
bond.14
Boron dipyrrins such as BODIPY1 (difluoroboradiaza-s-
indacene) have been attracting considerable interest in many
research areas due to their intense absorption and fluorescence
in the visible region and their excellent photostability.2 These
complexes have found applications in biological labeling,3
laser dyes,4 paint or ink compositions, and electroluminescent
devices.5
In this communication, we describe the synthesis and char-
acterization of aluminium complexes of N2O2-type dipyrrins.
The aluminium complexes further chelate to zinc(II) and an
enhanced fluorescence was observed for the Al(III)–Zn(II)
halide complexes. To the best of our knowledge, this is the
first example of hetero-multinuclear complexes based on the
dipyrrin framework.
Since control of the fluorescence wavelength is important
for practical applications, considerable efforts have been
devoted to the modification of the BODIPY framework.6–9
The synthesis of the fluorescent dipyrrin complexes with
zinc(II),9a indium(III)10 and gallium(III)10 has also been
reported, though their emission intensities are weaker than
that of the BODIPY dye.
The acid-catalyzed condensation of 2-(2-methoxyphenyl)-
pyrrole and arylaldehyde in methylene chloride and sub-
sequent oxidation with DDQ afforded the dipyrrin precursors
2a and 2b in 78 and 59% yield, respectively. Deprotections of
the phenol moieties with BBr3 quantitatively afforded the
N2O2 dipyrrins 1a and 1b. Aluminium complexes 1aAl and
1bAl were quantitatively prepared from 1a and 1b by the
reaction with Al(OiPr)3 in chloroform–methanol. The boron
complex 1bB was also prepared by treatment with B(OMe)3.
Interestingly, the optical properties of the aluminium com-
plexes drastically changed upon the addition of zinc(II) salts
such as ZnCl2 and ZnBr2. Strong interactions between the
aluminium complexes with the zinc(II) salts were supported by
the absorption spectral changes. The addition of ZnCl2 to
1aAl in toluene–methanol (99 : 1) decreased the absorption at
If fluorescent dipyrrin complexes have a coordination ability
toward other metal ions, changes in the absorption/
fluorescence wavelength upon subsequent complexation and
fine-tuning of the optical properties by changing the metal ions
or the counter ions are expected. Additionally, complexation
between the dipyrrin metal complexes may afford multinuclear
dipyrrin dimers. The formation of such molecular assemblies
is fascinating as a novel method to afford supramolecular
dipyrrin architectures.11
We have focused on studying the coordination ability of the
dipyrrins bearing 2-hydroxyphenyl groups at the pyrrole-a
positions (shown in Scheme 1) because complexation at the
N2O2-type tetradentate binding site would afford a metallo-
ligand as a bidentate ligand through its oxygen atoms. The
combination of metals or counter anions may change the
absorption/fluorescence wavelength of the metalloligands by
unique coordination at the O2-type chelate site.
To date, only a few complexes of N2O2-type dipyrrins have
been reported. Bloom reported the nickel, cobalt and zinc
complexes of an N2O2-type dipyrrin, though the emission
properties of the complexes were not provided.12 Recently,
Gradate School of Pure and Applied Sciences, University of Tsukuba,
Tsukuba, Ibaraki, 305-8577, Japan.
E-mail: nabesima@chem.tsukuba.ac.jp; Fax: +81-29-853-4507
w Electronic supplementary information (ESI) available: Synthetic
procedure and spectral data for 1aAl, 1bAl, and their zinc(II) salt
adducts. CCDC 708972 and 708973. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/b820141b
Scheme 1
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Fig. 3 Absorption and fluorescence spectra of 1aAl (0.9 mM) in the
absence (--) and presence (—) of excess ZnCl2 in toluene–methanol
(99 : 1). The samples were excited at 580 or 560 nm, respectively.
It is noteworthy that 1aAlꢄZnCl2 showed much sharper
absorption and fluorescence spectral bands than 1aAl
(Fig. 3). The full widths at half-maximum height (fwhm)
values of the absorption and fluorescence band of 1aAl are
970 and 1120 cmꢃ1, but the corresponding values are 600 and
750 cmꢃ1 in the presence of an excess amount of zinc(II)
chloride. Additionally, the Stokes shift decreased from 560
to 360 cmꢃ1 on zinc(II) complexation.
Fig. 1 Absorption spectral changes of 1aAl upon addition of ZnCl2.
The inset shows the binding isotherm recorded at 607 nm with the
calculated 1 : 1 binding curve (solid line): [1aAl] = 8.5 mM, 0 r
[Zn2+]/[1aAl] r 2.4, toluene–methanol (99 : 1).
627 nm with a concomitant increase in the absorption at
607 nm as shown in Fig. 1.
The 1 : 1 stoichiometry was suggested from the titration
curve and the binding constant was evaluated to be (6.1 ꢁ 1.1)
ꢂ 106 Mꢃ1 from nonlinear least-squares regression analysis.
Indeed, deep purple microcrystals of the 1 : 1 complex,
1aAlꢄZnCl2, were obtained in 58% yield from a 1 : 1 mixture
of 1aAl and ZnCl2. Similarly, the ZnBr2 complex of 1aAl and
the ZnCl2 complex of 1bAl were obtained in 37 and 98%
yields, respectively.
Also notable is the enhanced fluorescence of the zinc(II)
complexes. The fluorescence quantum yield of 1aAl increased
from 0.23 to 0.55 upon binding to zinc(II) chloride (Table 1).
As shown in Table 1, 1aAlꢄZnBr2 also showed the same
intense fluorescence as 1aAlꢄZnCl2. The enhanced emission
intensities of the Al(III)–Zn(II) halide complexes are probably
due to the increased rigidity of the dipyrrin skeleton caused by
the chelation, which reduced the loss of energy via radiation-
less thermal vibrations. As expected, 1bAl and 1bAlꢄZnCl2
showed larger fluorescence quantum yields than 1aAl and
1aAlꢄZnCl2, respectively, probably due to the restricted rota-
tion of the mesityl group.9a In contrast, the boron complex
1bB showed no interaction with the zinc(II) salts. The distorted
structure of the N2O2 binding site caused by the tetrahedral
boron atom and lower negative charge of the oxygen atoms
may suppress the coordination to the zinc(II) salts.13
An X-ray crystallographic analysis revealed the structure of
1aAlꢄZnCl2 (Fig. 2).z The aluminium ion sits in the dipyrrin
N2O2 site and has an octahedral coordination environment
formed by the N2O2 donor atoms in the equatorial plane, and
two water oxygen atoms in the axial position. The pyrrole
rings and the phenol rings are nearly coplanar to afford the
planar N2O2 site. The zinc(II) ion is coordinated with the two
m-phenoxo oxygen atoms (O1 and O2) and two chloride
atoms. The tetrahedral structure of the zinc is slightly distorted
with O–Zn–Cl, Cl–Zn–Cl and O–Zn–O bond angles of
112.01,15 123.61 and 74.81, respectively.
On the other hand, the 2 : 1 complex, (1aAl)2ꢄZn(OAc)2, was
obtained in 67% yield from a mixture of 1aAl and zinc(II)
acetate (instead of zinc(II) chloride). The fluorescence quantum
yield of the 2 : 1 complex in solution was not determined due
to dissociation under the dilute conditions (B1 mM).
Table 1 Optical properties of the N2O2-dipyrrin complexes
labs/nm
lflu/nm
FF
1aAl
1bAl
1bB
624
625
626
607
607
608
650
647
645
620
620
619
0.23
0.72
0.72
0.55
0.56
0.83
Fig. 2 Crystal structure of 1aAlꢄZnCl2. Thermal ellipsoids are plotted
at the 50% probability level. Hydrogen atoms and non-coordinating
solvent molecules are omitted for clarity. Selected bond lengths (A):
Zn1–O1 2.0339(10), Zn1–O2 2.0296(10), Zn1–Cl1 2.2303(5), Zn1–Cl2
2.1815(4), Al1–O1 1.8720(10), Al1–O2 1.8701(10), Al1–N1 1.9363(12),
Al1–N2 1.9364(12).
a
1aAlꢄZnCl2a
1aAlꢄZnBr2a
1bAlꢄZnCl2
a
Recorded in the presence of an excess amount of zinc(II) salts to
prevent dissociation.
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Chem. Commun., 2009, 2544–2546 | 2545

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This work was supported by Grants-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science, and Technology, Japan.
Notes and references
z Crystal data for [1aAlꢄZnCl2(H2O)2]ꢄ3Et2O: C39H51AlCl2N2O7Zn,
M = 823.07, monoclinic, space group P21/n (no. 14), a = 10.7486(3),
b = 24.8464(7), c = 15.2711(4) A, b = 98.9556(11)1, U = 4028.64(19)
A3, T = 120 K, Z = 4, 37 895 reflections measured, 9161 unique
(Rint = 0.0258). R1 = 0.0307 (I 4 2s(I)), wR2 = 0.0881 (all data),
GOF(F2) = 1.066.16
Crystal data for [(1bAl)2ꢄZn(OAc)2(MeOH)2]ꢄMeOH: C126H110Al4Cl6-
ꢀ
N8O24Zn2, M = 2571.58, triclinic, space group P1 (no. 2), a =
11.2352(7), b = 14.0083(10), c = 19.4195(13) A, a = 70.594(2), b =
89.998(2), g = 87.1542(18)1, U = 2878.7(3) A3, T = 120 K, Z = 2,
28 385 reflections measured, 12999 unique (Rint = 0.1202). R1 = 0.0777
(I 4 2s(I)), wR2 = 0.2200 (all data), GOF(F2) = 1.057.16
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plotted at 50% probability level: (a) and (b) show the top and side
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unit. Selected bond lengths (A): Zn1–O1 2.071(4), Zn1–O2 2.143(4),
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However, different optical properties of the 2 : 1 complex
compared to that of the 1 : 1 complex were suggested from the
absorption spectral pattern in which two strong absorption
bands at 568 and 601 nm were observed (see Fig. S1w). X-Ray
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bridged by a zinc(II) ion (Fig. 4).z The trinuclear dimeric
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Al1–Zn1 and Al1*–Zn1. The m-phenoxo oxygen atoms
(O1 and O2) of the dipyrrin coordinate to Zn1 at the inversion
center to give octahedral geometry. Since the dimeric complex
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complex with Zn(OAc)2 could be applied to the construction
of dynamic dipyrrin assemblies.
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In conclusion, novel aluminium complexes of an N2O2-type
dipyrrin were synthesized and their complexes with ZnCl2,
ZnBr2 and Zn(OAc)2 were characterized. The fluorescence
wavelength and the intensity of the aluminium complexes
drastically changed upon Zn complex formation. The results
obtained in this study can be applied not only to tunable
fluorescence materials, but also to metallosupramolecules with
cation-responding properties. We are currently studying other
multinuclear complexes based on N2O2-type dipyrrins which
possess interesting optical and magnetic properties.
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¨
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This journal is The Royal Society of Chemistry 2009
2546 | Chem. Commun., 2009, 2544–2546