Near-IR phosphorescence of iridium(III) corroles at ambient temperature

Article abstract of DOI:10.1021/ja101647t

The photophysical properties of Ir(III) corroles differ from those of phosphorescent porphyrin complexes, cyclometalated and polyimine Ir(III) compounds, and other luminescent metallocorroles. Ir(III) corrole phosphorescence is observed at ambient tempera

Full text of DOI:10.1021/ja101647t

Published on Web 06/22/2010
Near-IR Phosphorescence of Iridium(III) Corroles at Ambient Temperature
Joshua H. Palmer,† Alec C. Durrell,† Zeev Gross,*,‡ Jay R. Winkler,† and Harry B. Gray*,†
Beckman Institute, California Institute of Technology, Pasadena, California 91125, and Schulich Department of
Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel
Received March 2, 2010; E-mail: chr10zg@tx.technion.ac.il (Z.G.); hbgray@caltech.edu (H.B.G.)
Abstract: The photophysical properties of Ir(III) corroles differ
from those of phosphorescent porphyrin complexes, cyclometal-
ated and polyimine Ir(III) compounds, and other luminescent
metallocorroles. Ir(III) corrole phosphorescence is observed at
ambient temperature at wavelengths much longer (>800 nm) than
those of most Ir(III) phosphors. The solvatochromic behavior of
Ir(III)-corrole Soret and Q absorption bands suggests that the
lowest singlet excited states (S2 and S1) are substantially more
polar than the ground state.
Figure 1. Details of synthesis and characterization (NMR, MS, XRD) of
the three corroles are reported in the Supporting Information.
Porphyrin complexes displaying phosphorescence at ambient
temperatures have been employed for photodynamic therapy,1
oxygen detection,2 and organic light-emitting diodes.3 A great deal
of research has been done on d8 (mainly PtII, PdII, and AuIII)
complexes,4 which emit at relatively long wavelengths (>600 nm)
with lifetimes in the microsecond range. In sharp contrast, d6
metalloporphyrins have scarcely been investigated, although room
temperature phosphorescence of ruthenium(II) porphyrins has been
reported.5
Metallocorroles have shown promise as therapeutic agents,6 with
biodistribution and bioavailability profiles as well as cellular uptake
and intracellular locations7 determined for fluorescent gallium(III)
derivatives.8 Although progress has been made, much work remains
before we can claim to have developed optimized compounds for
optical examination of biological systems.9 It would be beneficial
to have agents that emit with microsecond lifetimes beyond 700
nm, as the most common obstacles to efficient biological
imagingstissue absorbance and intrinsic fluorescencescould thus
be circumvented.
Figure 2. Emission spectra of Ir(III) corroles in degassed toluene solution
(λex ) 496.5 nm): (a) 298 K; (b) 77 K.
that the phosphorescence in each case is attributable to a transition
3
from a corrole πfπ* triplet state that likely has partial MLCT
character.
Here we report the photophysical properties of iridium(III)
corroles,10 which differ significantly from those of cyclometalated
and polyimine Ir(III) compounds,11 other luminescent metallocor-
roles,12 and free-base corroles.13 Iridium(III) corrole phosphores-
cence is observed at ambient temperature at wavelengths much
longer (>800 nm) than those of most other luminescent Ir(III)
complexes.11 Our investigations focused on the three corroles shown
in Figure 1.
Emission spectra were recorded in toluene at 298 and 77 K
(Figure 2). The spectra display two intense features separated by
approximately 1400 cm-1. It is likely that a ring-based vibration is
excited in the transition to the lower energy component.14
At low temperature, the higher energy emission maximum of
each Ir(III) corrole blue shifts by 10 nm (a rigidochromic effect
indicating that the transition involves charge transfer)15 (Figure 2b).
We conclude from emission band shapes and vibronic splittings
taken together with results from electronic structure calculations16
Luminescence quantum yields and lifetimes in degassed and
aerated toluene solutions at room temperature (and lifetimes at 77
K) are set out in Table 1. 1-Ir(tma)2 and 1b-Ir(tma)2 have relatively
short lifetimes and low quantum yields, but 1-Ir(py)2 exhibits a
much higher luminescence quantum yield (1.2%) and a longer
lifetime. We also measured luminescence lifetimes in methanol
solutions (Table S1, Supporting Information). These lifetimes are
similar to those in toluene at 77 K, but they are substantially shorter
in the more polar solvent at room temperature.
UV-vis absorption spectra (Figure 3) of the Ir(III) corroles
exhibit split Soret (S0fS2) and Q (S0fS1) bands. The splitting of
the Soret bands is 1400-1700 cm-1 in all cases; the Q bands are
split by roughly 1800 cm-1 in the spectra of 1-Ir(tma)2and 1b-
Ir(tma)2 but only by 1000 cm-1 in the spectrum of 1-Ir(py)2.
The effect of solvent polarizability on Ir(III)-corrole spectra was
investigated to probe the extent of charge transfer in initially formed
electronic excited states. UV-vis spectra were obtained in a variety
of solvents: Soret band maxima were plotted against polarizability
f (Figure 4), defined as f(n) ) (n2 - 1)/(2n2 + 1),17 where n is the
refractive index of the solvent (the ground states are relatively
† California Institute of Technology.
‡ Technion-Israel Institute of Technology.
9
9230 J. AM. CHEM. SOC. 2010, 132, 9230–9231
10.1021/ja101647t  2010 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Photophysical Data for Ir(III) Corroles in Toluene
ligand. We also have shown that substitutions on the corrole
framework can tune the redox and photophysical properties as well
as the solubility behavior of these molecules over a very wide range.
We intend to employ these and related Ir(III) corroles in experiments
that require tunable near-IR phosphors.
Solutionsa
1-Ir(tma)2
1b-Ir(tma)2
1-Ir(py)2
b
Φph
3.3 × 10-4
788/0.220
0.170
786/2.77
1.5 × 103
4.54 × 106
3.9 × 10-3
795/1.19
0.760
786/4.72
3.28 × 103
8.4 × 105
1.2 × 10-2
792/4.91
0.380
793/7.69
2.44 × 103
2.0 × 105
λAr (nm)/τAr (µs)
c
τair (µs)
Acknowledgment. This work was supported by the NSF Center
for Chemical Innovation (CCI Powering the Planet, Grants CHE-
0802907 and CHE-0947829), the US-Israel BSF, CCSER (Gordon
and Betty Moore Foundation), and the Arnold and Mabel Beckman
Foundation.
λ77K (nm)/τ77K (µs)
kr (s-1
)
)
knr (s-1
a At 298 K unless noted otherwise. b Luminescence quantum yields
were standardized against free-base tetraphenylporphyrin (Φf ) 0.13 in
toluene solution at 298 K). c Measured under atmospheric conditions.
Supporting Information Available: Additional experimental details.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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Figure 3. UV-vis spectra of Ir(III) corroles in toluene solution at 298 K.
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Our work has established that Ir(III) corroles phosphoresce in
the near-infrared region at ambient temperature with lifetimes and
quantum yields that depend strongly on the nature of the axial
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