One-pot conversion of fluorophores to phosphorophores

Article abstract of DOI:10.1021/acs.orglett.6b02877

Facile, one-pot conversion of free base 5,10,15-tris(pentafluorophenyl)corrole, (H₃)tpfc, into the coinage metal complexes of 2,3,17,18-tetraiodo-5,10,15-tris(pentafluorophenyl)corrole, (I₄-tpfc)M (M = Cu, Ag, Au), is reported. The iodination/metalation procedures provide much higher yields and larger selectivity than both conceivable stepwise syntheses. Photophysical analysis shows that the gold(III) complex (I₄-tpfc)Au displays phosphorescence at room temperature and a substantial quantum yield for singlet oxygen formation.

Full text of DOI:10.1021/acs.orglett.6b02877

Letter
pubs.acs.org/OrgLett
One-Pot Conversion of Fluorophores to Phosphorophores
Matan Soll,†,§ Kolanu Sudhakar,†,§ Natalia Fridman,† Alexander Muller,‡ Beate Roder,‡
̈
̈
and Zeev Gross*,†,§
†Schulich Faculty of Chemistry, Technion - Israel Institute of Technology, Haifa 32000, Israel
‡AG Photobiophysik, Humboldt-Universitat zu Berlin MNF, Newtonstrass, 1512489 Berlin, Germany
̈
S
* Supporting Information
ABSTRACT: Facile, one-pot conversion of free base 5,10,15-tris(pentafluorophenyl)corrole, (H3)tpfc, into the coinage metal
complexes of 2,3,17,18-tetraiodo-5,10,15-tris(pentafluorophenyl)corrole, (I4-tpfc)M (M = Cu, Ag, Au), is reported. The
iodination/metalation procedures provide much higher yields and larger selectivity than both conceivable stepwise syntheses.
Photophysical analysis shows that the gold(III) complex (I4-tpfc)Au displays phosphorescence at room temperature and a
substantial quantum yield for singlet oxygen formation.
he development of chromophores that efficiently absorb
visible light, display long-lived excited states, and emit at the
that compound. The next attempt was to perform the iodination
reactions in pyridine as solvent, considering that the bromine/
pyridine complex is a very good brominating agent18,19 and
excisting evidence for formation of iodine/pyridine com-
plexes.19,20 Indeed, treatment of a pyridine solution of
(H3)tpfc by NIS yielded 2,3,17,18-tetraiodo-5,10,15-tris-
(pentafluorophenyl)corrole, (H3)(I4-tpfc), in a reasonable 40%
yield. Confirmation about the selective iodination of only the
directly connected pyrrole subunits was achieved by NMR
analyses (Figure S2) that disclosed the C2 symmetry axis therein;
and via X-ray crystallography of the corresponding metal
chelates. The successful synthesis of the iodinated free-base
corrole allowed for accessing the corresponding transition-metal
complexes, of which the choice was to focus on the group 11
triad. The Cu, Ag, and Au complexes of both (H3)tpfc and
(H3)(I4-tpfc) were prepared for obtaining a meaningful series of
compounds that might differentiate between the effects of the
iodine atoms and the chelated metal on the physical properties.
Three strategies were employed for the synthesis of the desired
(I4-tpfc)M complexes with M = Cu, Ag, and Au: (a) iodination of
(H3)tpfc, followed by metalation of (H3)(I4-tpfc); (b) metal-
ation of (H3)tpfc, followed by iodination of the (tpfc)M
complexes; and (c) a one-pot metalation/iodination procedure
applied on (H3)tpfc (Figure 1). The main drawback of the first
option is that the iodinated free-base corrole is obtained in 40%
yield only. Regarding the second approach, one severe limitation
is that insertion of gold into (H3)tpfc proceeds in low and highly
T
far-red end of the spectrum is of great importance for many
applications, including light-emitting diodes (LED),1,2 sensing of
oxygen and other gaseous molecules, bioimaging,3,4 and more.
The vast majority of compounds that fulfill the above
requirements are based on complexes with the most rare metals
such as platinum and iridium, thus limiting their utility. This is
also true for the first reported phosphorescent corrole
complexes, (tpfc)Ir, (tpfc)Au, and (tpfc)Rh, where tpfc stands
for the trianion of 5,10,15-tris(pentafluorophenyl)corrole, (H3)-
tpfc.5−9 More recently, we have decided to introduce the heavy
atom effect via covalent C−I bonds on the corrole skeleton rather
than relying on heavy and expensive metal ions.11 In line with
that, we have performed iodination on the aluminum, gallium,
and phosphorus complexes (M = Al, Ga, P(OH)2, in (tpfc)M),
which led to successful preparation of the corresponding
complexes of 2,3,17,18-tetraiodo-5,10,15-tris(pentafluoro-
phenyl)corrole (I4-tpfc)M and 2,3,17-trisiodo-5,10,15-tris-
(pentafluorophenyl)corrole (I3-tpfc)M.10 Spectroscopic exami-
nations indicated that this kind of complexes, and to some extent
those of octabrominated corroles, display prompt fluorescence,
phosphorescence, and delayed thermal fluorescence, all at room
temperature.10,12−17
To generalize the above approach, we tried to obtain iodinated
free base corrole by treating (H3)tpfc with N-iodosuccinimide
(NIS) or molecular iodine in a methanol solution, exactly as
previously performed for its corresponding metal complexes.10,16
NMR examination of the thus-obtained main product revealed it
to be nonaromatic (Figure S1). A more in-depth characterization
was, however, not performed because of the limited stability of
Received: September 23, 2016
© XXXX American Chemical Society
A
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Letter
Figure 2. Comparison of chemical yields and the identity of the main
product in the stepwise approaches and the one-pot procedure for the
synthesis of iodinated-corrole chelates of copper, silver, and gold.
Figure 1. Three approaches for the synthesis of group 11 metal
complexes of β-pyrrole-iodinated corroles.
approach, the selectivity to the tetraiodinated corrole is larger,
and the reaction conditions for metalation are much milder. This
suggests that the one-pot procedure starts with iodination
followed by formation of the more stable metal complexes. As an
outcome, this methodology ends up being more facile in terms of
reaction conditions and time, more selective to the tetraiodinated
complexes, and of higher overall yields 76% vs 36−64% for
copper, 45% vs 27% for Ag, and 53% vs 29% for Au.
variable yields.9a,b In addition, iodination might proceed with
different selectivity for each of the applied (tpfc)M complexes.
The first strategy proved to be quite beneficial regarding
reaction times and the respectable metalation yields. (H3)(I4-
tpfc) (1 equiv) was stirred in pyridine with Cu(OAc)2 (10
equiv), AgOAc (5 equiv), or Au(OAc)3 (2 equiv) at room
temperature under ambient conditions to yield (I4-tpfc)Cu, (I4-
tpfc)Ag, and (I4-tpfc)Au, respectively. For this two-step
procedure, the overall yields relative to (H3)tpfc were 36%,
26.8%, and 28.8%, respectively. For the other stepwise synthesis,
pyridine solutions of (H3)tpfc (1 equiv) were treated with either
Cu(OAc)2 (10 equiv, 2 h, rt) or Ag(OAc) (3 equiv, 12 h, reflux),
or Au(OAc)3 (2 equiv, 24 h, reflux) to yield (tpfc)Cu, (tpfc)Ag,
and (tpfc)Au in 91%, 60%, and 8% yield, respectively. Iodination
of the isolated (tpfc)M complexes was performed at rt for either
1 h (M = Cu and Au) or 12 h (M = Ag). The main product in the
case of copper was (I4-tpfc)Cu, while the tris-iodinated
complexes dominated for silver and gold yielding (I3-tpfc)Ag
and (I3-tpfc)Au. The overall yields relative to (H3)tpfc were
63.7%, 33%, and 4.16%, respectively. One main conclusion from
the comparison between the two kinds of two-step procedures is
that metal insertion proceeds under milder conditions and
provides larger chemical yields for (H3)(I4-tpfc) than for
(H3)tpfc, particularly in the case of gold. This is reminiscent of
observations regarding auration of other β-pyrrole-substituted
corroles.9b−d
Crystal structures obtained for the three (I4-tpfc)M complexes
(Figure 3) revealed that all are neutral, and each features a 4-
Figure 3. Drawing of the (I4-tpfc)M complexes with the numbering of
the substituted C atoms (a) and side views (with the meso-C6F5 groups
omitted for clarity) of the X-ray-determined molecular structures of (I4-
tpfc)Cu (b), (I4-tpfc)Ag (c), and (I4-tpfc)Au (d).
Based on the acquired knowledge, the in situ iodination/
metalation procedure was explored. Pleasingly, the results were
very good. Pyridine solutions composed of (H3)tpfc (1 equiv),
NIS (10 equiv), and either one of the metal salts Cu(OAc)2(10
equiv), AgOAc, or Au(OAc)3 (2 equiv in each case) yielded the
corresponding tetraiodinated metal complexes (I4-tpfc)Cu, (I4-
tpfc)Ag, and (I4-tpfc)Au in yields of 76%, 45%, and 53%,
respectively, within 1−6 h under ambient reaction conditions
(SI).
The comparison between the one-pot synthesis vs the two
stepwise approaches is summarized in Figure 2. The first notable
aspects are that prior metalation of (H3)tpfc by silver and gold
requires long heating and that the sequential iodination leads to
the tris- rather than the tetraiodinated complexes. In the opposite
coordinate metal ion,21 consistent with expectations for the
formal d8 + 3 oxidation state therein. There are, however, distinct
differences in the degree of planarity of the macrocycle: large
deformation in the case of copper, somewhat less so for silver,
while the gold complex is almost perfectly planar. This is
manifested by the angles measured between the plane defined by
the 4 N atoms and the I−C−C−I units of the two iodinated
pyrrole moieties, as well as by the angles between the two latter
(Table 1). The reason for this remarkable differences has been
previously deduced by Ghosh et al. to be due to an increase in the
redox-innocence of (corrolato)M complexes upon moving from
Cu to Ag and finally to Au.9c
All of the examined iodinated corroles exhibited typical near-
UV Soret band and far-visible Q bands (Figure S22 and Table
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DOI: 10.1021/acs.orglett.6b02877
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Letter
Table 1. Selected Structural Data for Demonstrating the Remarkable Differences between the Gold and the Silver/Copper
Complexes
deviation from the N4 plane (Å)
entry
I−C2−C3−I plane
I−C17−C18−I plane
dihedral angle (Å) between I−C−C−I planes
(I4-tpfc)Cu
(I4-tpfc)Ag
(I4-tpfc)Au
19.60
14.86
3.03
20.35
22.56
3.96
34.15
32.19
6.57
S2), with significant red shifts relative to (H3)tpfc and the
(tpfc)M complexes: 8, 11, 8, and 6 nm for the Soret band and 58,
24, 17, and 19 nm for the lowest energy Q-band of (H3)(I4-tpfc),
(I4-tpfc)Cu, (I4-tpfc)Ag, and (I4-tpfc)Au, respectively. Shifts in
the UV−vis spectra of porphyrins and corroles are not easy to
analyze because they are sensitive to both electronic and
structural variations.17,22 Nevertheless, the red shifts obtained for
the almost perfectly planar (I4-tpfc)Au is reminiscent of
nondistorted octabrominated metallocorroles where the
HOMO−LUMO gap was found to decrease due the addition
of electron withdrawing groups on the macrocycle periph-
ery.10,12−17,23,24 The major expected effect of replacing C−H by
C−I bonds is reduced fluorescence intensity because of several
aspects: (a) an increase of nonradiative decay rates due to the
weaker C−I bonds; (b) additional nonradiative decay channels if
the macrocycle becomes more distorted; and (c) an increased
intersystem probability due to the heavy atom effect.10,11 Indeed,
all of the iodinated corroles displayed heavily quenched
fluorescence intensity relative to (H3)tpfc (Figures S23 and
S24(b)). On the other hand, only the gold(III) complexes
(tpfc)Au and (I4-tpfc)Au displayed phosphorescence (vide infra)
at room temperature. This is clearly related to their much larger
structural rigidity relative to the corresponding copper and silver
corroles.8,9 We may hence conclude that macrocycle deforma-
tion must be prevented for turning fluorophores into
phosphorophores via heavy atom substitution. This conclusion
is further supported by the recently reported phosphorescence of
the aluminum, gallium, and phosphorus chelates of iodinated
corroles, in all of which the macrocycle is remarkably
planar.10,16,17
Figure 4. Phosphorescence detected under anaerobic conditions and 0.1
ms post excitation for: (a) (tpfc)Au with excitation at 420 nm; (b) of the
same (I4-tpfc)Au sample, but by excitation of the Soret band (421 nm,
red line) and the Q-band (581 nm, blue line); (c) of (I4-tpfc)Au under
anaerobic conditions (blue line) and after exposure to O2 (red line).
The most straightforward indication for long-lived photo-
excited states is found by recording the emission spectrum long
after the excited singlet states decayed (<1−5 ns). Parts a−c of
Figures 4 display the emission spectra of (tpfc)Au and (I4-
tpfc)Au, obtained after a 100 μs postexcitation delay. This is
clearly indicative of a decay from long-lived triplet states, a
conclusion that was further supported by the large Stoke shift for
(I4-tpfc)Au (λmax(abs) = 583 nm, λmax(emission) = 787 nm) and the
effective quenching under an oxygen atmosphere. Interestingly,
much more intense phosphorescence is detectable when (I4-
tpfc)Au is exited at the Q-band maximum of 583 nm rather than
at the Soret maximum of 421 nm, despite of the 5-fold smaller
optical density of the former.
Figure 5. Time-resolved 1O2 signal intensities in toluene. Fits of the data
were conducted following the standard biexponential model for singlet
oxygen kinetics.26 The goodness of the fit is indicated by the reduced χ2-
test. The concentrations were adjusted for an optical density of 0.241
0.003 at 405 nm (excitation wavelength of this measurement) in order to
estimate a relative singlet oxygen quantum yield.
The effective emission quenching under aerobic conditions
suggests that these complexes could activate triplet oxygen. This
was checked for the full series by looking at time-dependent
changes at 1270 15 nm, the characteristic λmax of singlet oxygen
phosphorescence. The data presented in Figure 5 and Table 2
clearly show that only (H3)tpfc, (tpfc)Au, H3(I4-tpfc), and (I4-
additional exponential decay in the standard biexponential
singlet oxygen decay model.25 (H3)tpfc and H3(I4-tpfc)
displayed similar singlet oxygen formation quantum yields, 0.7
0.1 relative to 5,10,15,20-tetraphenylporphyrin, while (tpfc)
Au and (I4-tpfc)Au displayed quantum yields of 0.4 0.1 and 1.5
0.2, respectively. (I4-tpfc)Au also displayed the longest triplet
1
tpfc)Au catalyze O2 formation. This is consistent with the
state lifetime in the presence of oxygen, 1.2
0.1 μs, which
absence of any detectable long-lived triplet-exited states for the
rest of the series. For the (H3)(I4-tpfc) signal fit, a strong
photosensitizer phosphorescence had to be taken into account
(approximately a factor of 2 in relative amplitude) via an
apparently accounts for its high quantum yield. Although
(H3)tpfc and (H3)(I4-tpfc) exhibited reasonable singlet oxygen
formation yields, they also bleached quite fast. In contrast, both
(tpfc)Au and (I4-tpfc)Au did exhibit very high photostability.
C
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a
Table 2. Photophysical Properties of All of the Corrole Series
entry
TΔ (1O2 meas) (μs)
TT (1O2 meas) (μs)
TT (LFPL meas) (μs)
ΦΔ/ΦΔ(TPP)
ΦFl/ΦFl(TPP)
1.2 0.1
<0.01
(H3)tpfc
22
25
27
27
1
1
1
1
0.3 0.1
0.3 0.1
0.5 0.1
1.2 0.1
0.2 0.1
0.3 0.1
0.4 0.1
1.2 0.1
0.7 0.1
0.7 0.1
0.4 0.1
1.5 0.2
<0.01
(H3)(I4-tpfc)
(tpfc)Au
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
(I4-tpfc)Au
(tpfc)Cu
(I4-tpfc)Cu
(tpfc)Ag
<0.01
<0.01
(I4-tpfc)Ag
(H2)tpp (ref)
<0.01
24
1
0.3 0.1
0.3 0.1
1.0 0.1
1.0 0.1
a
ΦΔ is singlet oxygen quantum yield, ΦFl is fluorescence quantum yield, τT is triplet lifetime, and τΔ is singlet oxygen lifetime.
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Appl. Mater. Interfaces 2016, 8, 18935−18942.
We introduce a very efficient and facile one-pot synthesis of
group 11 metals with tetraiodinated corroles. By doing so, the
intense fluorophore (H3)tpfc may be converted into novel
phosphorophores. Both C−H/C−I substitution on the macro-
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coordination core increase the singlet to triplet ISC probability,
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b02877.
1
Experimental section; H, 19F NMR, HRMS, absorption,
and emission spectra (PDF)
Crystallographic data (I4-tpfc)Cu (CIF)
Crystallographic data (I4-tpfc)Ag (CIF)
Crystallographic data (I4-tpfc)Au (CIF)
Crystallographic data (tpfc)Au (CIF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: chr10zg@tx.technion.ac.il.
Author Contributions
§M.S. and K.S. contributed equally.
Notes
The authors declare no competing financial interest.
(21) Crystallography data deposited at the Cambridge Crystallo-
graphic Centre: (tpfc)Au, CCDC-1486464; (I4-tpfc)Cu, CCDC-
1486466; (I4-tpfc)Ag, CCDC-1500821; (I4-tpfc)Au, CCDC-1500828.
(22) Liu, H.; Lai, T.; Yeung, L.; Chang, C. Org. Lett. 2003, 5, 617−620.
(23) Schechter, A.; Stanevsky, M.; Mahammed, A.; Gross, Z. Inorg.
Chem. 2012, 51, 22−24.
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(25) Preuß, A.; Saltsman, I.; Mahammed, A.; Pfitzner, M.; Goldberg, I.;
ACKNOWLEDGMENTS
■
This research was supported by a grant to Z.G. by the Pazy
Foundation. A.M. thanks the land Berlin for financial support
from the “Elsa-Neumann-Stipendium”.
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