Iodinated aluminum(III) corroles with long-lived triplet excited states

Article abstract of DOI:10.1021/ja202692b

The first reported iodination of a corrole leads to selective functionalization of the four C-H bonds on one pole of the macrocycle. An aluminum(III) complex of the tetraiodinated corrole, which exhibits red fluorescence, possesses a long-lived triplet ex

Full text of DOI:10.1021/ja202692b

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Iodinated Aluminum(III) Corroles with Long-Lived Triplet
Excited States
Jenya Vestfrid,† Mark Botoshansky,† Joshua H. Palmer,‡ Alec C. Durrell,‡ Harry B. Gray,*,‡ and Zeev Gross*,†
†Schulich Faculty of Chemistry, Technion - Israel Institute of Technology, Haifa 32000, Israel
‡Beckman Institute, California Institute of Technology, Pasadena, California 91125, United States
S Supporting Information
b
Chart 1. Structures of Corroles and Corresponding Metal
ABSTRACT: The first reported iodination of a corrole
leads to selective functionalization of the four CÀH bonds
on one pole of the macrocycle. An aluminum(III) complex
of the tetraiodinated corrole, which exhibits red fluores-
cence, possesses a long-lived triplet excited state.
Complexes Whose Photophysical Properties Have Been
Published Previouslya
mong the most prominent features of porphyrinoids are
A
their photophysical properties: intense visible absorption
and (often) emission in the visible or near-IR. Understanding the
variables that affect intersystem crossing (ISC) as well as emis-
sion wavelengths and lifetimes opens the way for the design of
compounds tailored to particular applications. For example, a
low ISC yield is important for photodynamic detection (PDD),1
fluorescence-based imaging,2 and dye-sensitized solar cell
(DSSC) efficiency;3 hence, free-base porphyrinoids or the cor-
responding chelates with light post-transition metals are optimal.
On the other hand, long-lived triplet excited states are necessary
for singlet oxygen production for applications such as photo-
dynamic therapy (PDT).4 These states are generated through
efficient ISC that can be achieved by the inclusion of heavy metals
or heavy-atom ring substituents.
a All of the metal complexes have axial ligands, which have been omitted
for clarity.
temperatures, although we should note that hitherto long-lived,
room-temperature corrole luminescence has been reported only
for the six-coordinate iridium(III) complexes 1-Ir and 1b-Ir
having two trimethylamine or pyridine molecules as axial
ligands.14 Both complexes display phosphorescence at room
temperature, with 1b-Ir exibiting a somewhat longer triplet
lifetime than 1-Ir. Triplet emission from other metallocorroles
has been observed only at low temperatures,15 as for example in
the case of partially brominated germanium corroles.16
The photophysical properties of metallocorroles have not
been investigated as extensively as those of metalloporphyrins,
although much progress has been made toward understanding
the singlet excited states of gallium and aluminum derivatives,
among others.5 The aluminum(III) complex of 5,10,15-tris-
(pentafluorophenyl)corrole [1-Al in Chart 1; an analogue of
magnesium(II) in porphyrins and chlorophylls] is brightly
fluorescent, with a quantum yield (0.76) exceeding those of all
other porphyrinoids.6 This finding has been attributed to a
combination of the electron-withdrawing meso-C6F5 substitu-
ents, a light metal, and the planarity of the macrocycle.
H3tpfc (1-H3 in Chart 1) and its metal complexes undergo
facile and selective electrophilic substitution,7 a property that has
been exploited for the development of corrole-based fluorescent
probes.8 The gallium(III) complex of an amphiphilic corrole
(2-Ga in Chart 1) has been used for imaging serum,9 cells,8,10
organs, and whole animals (mouse models).11 One inherent
limitation of biological fluorescence imaging is interference from
cellular autofluorescence; this problem can be overcome by the
use of fluorescence lifetime imaging (FLIM)12 and/or two
photon excitation,13 but these techniques require very specialized
instrumentation. A simpler solution involves the use of chromo-
phores that possess longer luminescence lifetimes at ambient
We set out to prepare a new metallocorrole with a long-lived
triplet excited state. The compound was designed according to
the following criteria: (a) the metal of choice should be
aluminum(III), whose corrole complexes have the highest emis-
sion quantum yields on record; (b) the heavy-atom substituents
required for promoting ISC should be located on the corrole
periphery, since the central metal is a relatively light element; (c)
the compound should be dipolar and amenable to further
chemical modification, in order to broaden the scope of potential
applications. The methodology we picked was iodination of 1-Al
because (a) iodine is the heaviest atom that can be readily
incorporated in the ring, (b) iodination is likely to be much
more selective than bromination,17 and (c) CÀI bonds are good
candidates for further chemical manipulation. Our expectations
were fulfilled: iodination did not proceed further than four
iodides per corrole, all four CÀI bonds were formed on one
pole of the molecule, and most importantly, a long-lived triplet
state was produced upon electronic excitation of the complex.
While molecular iodine reacted with 1-Al, thin-layer chroma-
tography revealed the formation of multiple products, and full
Received: April 4, 2011
Published: July 27, 2011
r
2011 American Chemical Society
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|

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Scheme 1. Selective Iodination of the Aluminum(III) Cor-
role 1-Al (L = Pyridine)
Figure 2. Emission spectra (λex = 514 nm) of 4-Al (top) in pyridine
(blue dotted curve) and benzene (red solid curve) at room temperature
and (bottom) in toluene in the presence of excess imidazole at room
temperature (blue dotted curve) and 77 K (red solid curve).
Analysis of X-ray quality crystals revealed that 4-Al (Figure 1)
retains a planar macrocyclic framework. The deviations of aluminum
with respect to the plane defined by the four pyrrole nitrogen atoms
are virtually nil (0.003 Å for 1-Al and 0.007 Å for 4-Al).23 As
expected, the presence of iodides increases the AlÀNpyrrole bond
lengths by 0.016À0.022 Å. There are two pyridine(py) ligands in
axial coordination sites that are nearly parallel to each other, with a
twist angle of 16.75°. This angle is somewhat larger than those
reported for other bispyridine metallocorrole complexes (2.3À
10.8°).25 The average AlÀNpy bond length in 4-Al is slightly shorter
than that in 1-Al (2.176 vs 2.208 Å, respectively).23 This bond
shortening is consistent with the expected electronic effect of the
iodides: the Lewis acidity of the aluminum center is enhanced by
removal of electron density from the pyrrole nitrogen atoms, as con-
firmed by a determination of the affinity of 4-Al for pyridine in toluene
solutions. The equilibrium constant for the equilibrium involving
the mono- and bispyridine-coordinated forms of 4-Al is 508 MÀ1,24
which is nearly 4 times larger than that of 1-Al (135 MÀ1).5
Preliminary investigations of the luminescence properties of
4-Al revealed both short- and long-lived components that are
highly sensitive to electronic effects attributable to axial ligation.
The primary emission bands are centered at 612 and 669 nm in
benzene, where the majority of the complex is five-coordinate,
and at 645 and 700 nm in pyridine, where the complex is mostly
six-coordinate (Figure 2 top). The quantum yield of 0.1% as well
as the independence of the intensities of these bands with respect
to both temperature (over the range À20 to +50 °C) and the
presence of oxygen is consistent with prompt fluorescence25 from
an excited singlet state that is rapidly quenched by ISC. The triplet
state thus obtained is only slightly emissive (phosphorescence at
long wavelengths, >850 nm), although it could still be observed at
room temperature and even more so at 77 K (Figure 2 bottom).26
Transient absorption spectroscopy was used to determine the
lifetimeof this state, which was found to be 92 and 1 μs in degassed
and aerobic toluene, respectively.
Figure 1. Top view of the molecular structure of 4-Al. The AlÀN bond
lengths are 1.8821À1.9123 and 2.1732À2.1787 Å for the equatorial and
axial ligands, respectively.
consumption of 1-Al required many days. With either of the organic
reagents N-iodosuccinimide (NIS) or 1,3-diiodo-5,5-dimethyl-
hydantoin (DIH),18 the reaction was complete after 1 h, but NIS
provided better results. Only one major product was obtained: 4-
Al, with four new CÀI bonds (Scheme 1).19 Traces of the triiodo
species 3-Al were eliminated by recrystallization. Acetonitrile
was the only solvent that gave the desired product in reasonable
yield and selectivity, and acidic conditions previously reported
for the iodination of aromatic compounds20 promoted decom-
position. We also ran the optimized procedure on the free base
1-H3, but no pure product could be isolated from the reaction
mixture.
The identification of 4-Al was based on a combination of
NMR, UVÀvis, and mass spectrometry and was further con-
firmed by X-ray crystallography. The 1H NMR spectrum (Figure
S1a in the Supporting Information) displays two β-pyrrole CÀH
resonances (2H each), suggesting that the molecule retains a C2
axis. The large J coupling constant (4.5 Hz) indicates that only
the directly bound pyrrole subunits were modified, consistent
with known reactivity patterns of triarylcorroles.21 The spectrum
also reveals two bound pyridine molecules, which display broad
peaks because of the dynamic equilibrium between the five- and
six-coordinate species (as reported for 1-Al).5 At low tempera-
ture, this process is slowed, and the shifted resonances for
coordinated pyridines are sharpened (Figure S1bÀd). The
UVÀvis spectrum (Figure S2) of 4-Al displays maxima that are
slightly red-shifted (by 14À22 nm) relative to 1-Al, as is often
observed upon halogenation of corroles or porphyrins that retain
planarity.22 A molar absorptivity of ∼105 MÀ1 cmÀ1 is main-
tained upon iodination.6
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dx.doi.org/10.1021/ja202692b |J. Am. Chem. Soc. 2011, 133, 12899–12901

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COMMUNICATION
We have introduced iodination as a new methodology for
preparing metallocorroles with unique photophysical properties.
The process is facile and leads to selective functionalization of
four of the eight peripheral carbon atoms. The tetraiodinated
aluminum(III) corrole 4-Al, which has been fully characterized
by NMR spectroscopy and X-ray crystallography, possesses both
short- and long-lived excited states. The reaction chemistry of the
long-lived triplet state of 4-Al is currently under investigation in
our laboratories, with particular emphasis on work with biologi-
cally relevant ligands. Syntheses of derivatives of this unique
corrole for in-depth photochemical and photophysical studies
also are underway.
(12) Hwang, J. Y.; Agadjanian, H.; Medina-Kauwe, L. K.; Gross, Z.;
Gray, H. B.; Sorasaenee, K.; Farkas, D. L. Proc. SPIE 2008, 6859,
68590G.
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A. G.; Koronyo, Y.; Medina-Kauwe, L. K.; Gross, Z.; Gray, H. B.; Farkas,
D. L. Biomed. Opt. Express 2011, 2, 356.
(14) Palmer, J. H.; Durrell, A. C.; Gross, Z.; Winkler, J. R.; Gray,
H. B. J. Am. Chem. Soc. 2010, 132, 9230.
(15) Ventura, B.; Degli, E. A.; Koszarna, B.; Gryko, D. T.; Flamigni, L.
New J. Chem. 2005, 29, 1559.
(16) Nardis, S.; Mandoj, F.; Paolesse, R.; Fronczek, F. R.; Smith,
K. M.; Prodi, L.; Montalti, M.; Battistini, G. Eur. J. Inorg. Chem.
2007, 2345.
(17) (a) Golubkov, G.; Bendix, J.; Gray, H. B.; Mahammed, A.;
Goldberg, I.; DiBilio, A. J.; Gross, Z. Angew. Chem. 2001, 113, 2190.
(b) Aviv-Harel, I.; Gross, Z. Coord. Chem. Rev. 2011, 255, 717. (c) Du,
R. B.; Liu, C.; Shen, D. M.; Chen, Q. Y. Synlett 2009, 2701.
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’ ASSOCIATED CONTENT
S
Supporting Information. Crystallographic data for 4-Al
b
(CIF), materials and methods, and Figures S1ÀS4. This material
is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Authors
hbgray@caltech.edu; chr10zg@tx.technion.ac.il
(19) Synthesis of 4-Al: A sample of 1-Al (20 mg, 20 μmol) and
excess NIS (45 mg, 200 μmol) were stirred in CH3CN (5 mL) at room
temperature for 1 h. The resulting solution was washed with water, dried
over Na2SO4, and evaporated. The green-purple product was purified by
chromatography (CH2Cl2/EtOAc/pyridine = 100:15:1), which af-
’ ACKNOWLEDGMENT
1
This research was supported by the U.S.ÀIsrael Binational
forded pure 4-Al (20 mg, 67% yield). H NMR (400 MHz, C6D6):
δ 8.4 (d, J = 4.5 Hz, 2H), 8.6 (d, J = 4.5 Hz, 2H). 1H NMR (600 MHz,
toluene-d8, 209 K): δ 2.1 (unresolved t, 4H), 4.0 (t, 4H), 4.6 (t, 2H), 8.6
(d, J = 4.0 Hz, 2H), 8.8 (d, J = 4.0 Hz, 2H). 19F NMR (376 MHz, C6D6):
δ À137.84 (unresolved td, 6F, ortho), À153.19 (t, 3F, para), À162.08
(td, 2F, meta), À162.83 (td, 4F, meta). MS (MALDIÀTOF LDÀ): m/z
1323 ([M]À, 100%). UVÀvis (toluene) λmax [ε (MÀ1 cmÀ1)]: 410
[33138], 433 [116392], 596 [30159].
Science Foundation (BSF) to Z.G. and H.B.G.
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