Near-infrared emission from bis-PtII complexes of doubly N-confused calix[6]phyrins(1.1.1.1.1.1)

Article abstract of DOI:10.1002/anie.200801713

Light from confusion: A set of doubly N-confused calix[6]phyrins bearing two coordination sites (NNNC,NNNC or NNNN,NNCC) were synthesized. Their bis-Pt^II complexes exhibit near-infrared emission around 1000 nm, which could make these compounds ideal for biological applications. (Graph Presented).

Full text of DOI:10.1002/anie.200801713

Communications
DOI: 10.1002/anie.200801713
Calixphyrins
Near-Infrared Emission from Bis-PtII Complexes of Doubly
N-Confused Calix[6]phyrins(1.1.1.1.1.1)**
Dong-Hoon Won, Motoki Toganoh, Yosuke Terada, Susumu Fukatsu, Hidemitsu Uno, and
Hiroyuki Furuta*
Platinum(II) porphyrins display intense room-temperature
phosphorescence with significantly shorter lifetimes than
other types of phosphorescent organic compounds,[1–3] which
defies the fluorescence–phosphorescence distinction and
makes them useful for such applications as oxygen sensing
based on phosphorescence quenching[2] and light-emitting
devices (LEDs).[3] For potential use in biological systems,
however, the light-emitting PtII complexes should be designed
to cover the full visible and near-infrared range (i.e., 650–
1100 nm).[4]
Recently, we reported doubly N-confused hexa-
phyrin(1.1.1.1.1.1) (N2C-Hex) that are capable of forming
complexes with two metal ions in their rectangular coordina-
tion environment consisting of sixnitrogen and two carbon
atoms (NNNC, NNNC).[5] However, because of the synthetic
reason yet unclear and of susceptibility to oxidation, only one
type of bis-metal complex of dioxo-substituted hexaphyrin
(N2CO-Hex) bearing an NNNO,NNNO core has been
characterized; the emission bands due to the free ligand are
observed in the near-infrared (IR) region (> 1050 nm). In an
attempt to attain coordination with intact NNNC or NNCC
cores, which is expected to bring the emission band close to or
within the spectral window of interest, we examined the
synthesis of nonconjugated N-confused hexaphyrins and their
PtII metalation. The macrocycles contain two metal coordi-
nation cavities consisting of nitrogen and carbon atoms
similar to a conjugated one.
Herein, we report the synthesis of doubly N-confused
calix[6]phyrins bearing NNNC,NNNC or NNNN,NNCC
cores and their bis-PtII complexes, which exhibit phosphor-
escence around 1000 nm. These are the first confused
expanded porphyrinoids bearing two unsymmetrical coordi-
nation cavities that afford stable bis-metal complexes.
The title ligands (1–4; Scheme 1) were readily synthesized
by the condensation of N-confused tripyrrane[5a] with excess
cyclohexanone or acetone in the presence of acid catalyst and
subsequent oxidation.[6] According to the condensation
manner of N-confused tripyrrane (either head-to-tail or
head-to-head), two types of macrocycles with different
[*] D.-H. Won, Dr. M. Toganoh, Prof Dr. H. Furuta
Department of Chemistry and Biochemistry
Graduate School of Engineering
Kyushu University, Fukuoka 819-0395 (Japan)
Fax: (+81)92-802-2865
E-mail: hfuruta@cstf.kyushu-u.ac.jp
Y. Terada, Prof. Dr. S. Fukatsu
Graduate School of Arts and Sciences
The University of Tokyo, Tokyo 153-8902 (Japan)
Prof. Dr. H. Uno
Integrated Center for Science
Ehime University, Matsuyama 790-8577 (Japan)
[**] The present work is in part supported by PRESTO, JST, and a Grant-
in-Aid for the Global COE Program, “Science for Future Molecular
Systems” from the Ministry of Education, Culture, Sports, Science,
and Technology of Japan.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200801713.
Scheme 1. Synthesis of doubly N-confused calix[6]phyrins(1.1.1.1.1.1).
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Chemie
coordination modes (NNNC,NNNC or NNNN,NNCC) were
formed. Because the two macrocycles have the same molec-
ular symmetry, their H NMR spectra were very similar. For
The explicit structural assignment, especially on the
configurations of the confused pyrrole rings for the two
isomers, came from the X-ray single-crystal diffraction
analyses of free bases 3 and 4 and their PtII complexes
(Figure 2).[9] For 3 and 4, the arrangement of the confused
1
example, in the spectra of 1 (and 2) in CDCl3, one singlet
signal ascribable to NH protons was seen at d = 13.53
(13.58) ppm and two singlet signals due to a-CH and b-CH
protons of the confused pyrrole ring were observed at d =
10.28 (9.94) and 6.03 (5.83) ppm, respectively. Therefore,
conclusive assignment of the structures for 1 and 2 as shown in
Scheme 1 was based on X-ray single-crystal analysis (see
below) together with the theoretical calculations on each
tautomeric form.[7] The new macrocycles are very stable and
the a positions of the confused pyrrole rings remain intact in
solution under air, which is in a marked contrast to the
conjugated doubly N-confused hexaphyrin(1.1.1.1.1.1), which
affords the dioxo derivative readily.[5a]
The corresponding PtII complexes were prepared in good
yields by treating 1–4 with [PtCl2(NCPh)2] in benzonitrile at
1308C.[1c,8] For comparison, a PtII complexof calix[4]phyrin
(5)[6] was also synthesized (Figure 1). The coordination of two
Figure 2. X-ray crystal structures of 3 and 3-Pt-Pt: a) top view and
b) side view of 3; c) top view and d) perspective view of 3-Pt-Pt.
(Pentafluorophenyl and methyl groups are omitted for clarity in (c)
and (d).)
pyrrole rings was verified to be NNNC,NNNC for 3 and
NNNN,NNCC for 4. Both macrocycles adopt a rooflike (L-
shaped) conformation bending along the axis defined by the
two sp3-hybridized meso-carbon atoms. The roof angles of 3
and 4 are 103.88 and 104.78, respectively. In contrast, 3-Pt-Pt
and 4-Pt-Pt display ruffled molecular structures. The distances
between two Pt metals are 3.86 and 3.88 Š, respectively.
Direct interactions between the Pt metals are, thus, not
expected.
The absorption spectra of 3 and 4 are slightly different,
showing split absorption maxima at 505 and 530 nm for 3, but
just one broad peak for 4 in toluene. The corresponding bis-
PtII complexes, however, show a similar absorption profile
(Figure 3).
Figure 1. Bis-PtII complexes of doubly N-confused calix[6]phyrins (1–4)
and PtII complex for calix[4]phyrin (5).
No emission was detected from free ligands 1–4 and
calix[4]phyrin (5), whereas their PtII complexes were found to
display light-emitting properties in the near-IR region at
room temperature as shown in Figure 3c,d and Table 1.
Emission bands of PtII calix[n]phyrins (n = 4 and 6) in toluene
were observed around 850 and 1000 nm, respectively, showing
large Stokes shifts relative to those of PtII porphyrins
(ca. 3100 cmÀ1).[1] When the solution was deoxygenated by
argon bubbling, the emission intensities and the quantum
efficiencies of PtII complexes were found to increase while the
1270 nm emission band due to singlet oxygen was diminished,
indicating the relaxation pathway involving energy transfer
down to the oxygen molecule from the higher-lying excited
triplet state of the PtII complexes. Among the bis-PtII
complexes, 4-Pt-Pt exhibits much stronger emission and
PtII metals in the cavities of calix[6]phyrins was indicated by
the absence of NH protons and the a-CH protons of the
confused pyrroles in the 1H NMR spectra. Interestingly, in the
reactions of 1 and 4, mono-PtII complexes were separated as
side products; however, similar mono-PtII complexes were not
isolated in the reactions of 2 and 3. The metalation site of
mono-Pt complex 1-Pt was determined by the characteristic
1H NMR signals, one for NH proton and the other for a-CH
proton of the confused pyrrole ring. In the case of mono-Pt
complexof 4 (4-Pt), the Pt metal is captured only in the
NNCC core, as judged from the 1H NMR spectrum, in which
the signal of two a-CH protons of the confused pyrrole rings
disappears while the signal of NH protons remains.
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Figure 3. Absorption (solid line) and emission (doted line,
lex =610 nm) spectra of a) 3, b) 4, c) 3-Pt-Pt, and d) 4-Pt-Pt in toluene
at 298 K.
Figure 4. Kohn–Sham orbitals of a) LUMO and b) HOMO of 3-Pt-Pt,
and c) LUMO and d) HOMO of 4-Pt-Pt.
Table 1: Photophysical properties for PtII calix[n]phyrins (n=4 and 6).
unsymmetrical derivatives. The d orbitals of the two metal
atoms in the core contribute equally to the HOMO of 3-Pt-Pt,
whereas only one of the two metal atoms corresponding to the
NNCC core contributes the d orbital to the HOMO of 4-Pt-
Pt. Importantly, almost no d-orbital contribution from the
metal atoms to the LUMO is observed for either complex.
Accordingly, symmetry considerations mean that 3-Pt-Pt has
only a vanishingly small transition dipole moment for the
HOMO–LUMO transition, whereas 4-Pt-Pt has a significant
transition dipole moment, which is expected to allow the
quick release of energy during the relaxation processes.
Cyclic-voltammetry measurements on bis-PtII calix[6]-
phyrins revealed the two reversible oxidation steps and the
two reversible reductions peaks (Table 2). The redoxpoten-
Compound labs [nm] lem[a] [nm] FPL
t
[c] [ns] Stokes
shift [cmÀ1
[b]
]
1-Pt-Pt
2-Pt-Pt
3-Pt-Pt
4-Pt-Pt
5-Pt
649, 725 1010
657, 722 1029
649, 716 986
654, 713 1003
6.8”10À4 29.6
1.2”10À3 20.1
1.8”10À3 39.2
2.7”10À3 25.5
3892
4132
3824
4055
6605
446, 546 854, 891[d] 1.3”10À2 1040
[a] lex =610 nm (A=0.1) in toluene. [b] Photoluminescence quantum
yield; IR1040 (FPL =0.012)[10] used as a standard. [c] Measured in
CH2Cl2. [ d]lex =590 nm.
efficient quantum yields, whereas 1-Pt-Pt shows lower quan-
tum efficiency. Substitution of cyclohexyl groups at the meso-
position of bis-PtII complexes resulted in a red shift by around
25 nm of the emission peak relative to those of the ligand with
methyl groups. The configuration of the cores was also found
to affect the emission properties. In fact, the emission peaks of
the bis-PtII complexes (2-Pt-Pt or 4-Pt-Pt) bearing two
different coordination environments were red-shifted by
around 20 nm relative to those of the symmetric counterparts
(1-Pt-Pt or 3-Pt-Pt).
Table 2: Cyclic voltammetry [E/V vs. ferrocene/ferrocenium] of bis-PtII
complexes in CH2Cl2 containing 0.1m TBAPF6 at a Pt working electrode.
[a]
Compound
Eox,2
Eox,1
Ered,1
Ered,2
DEꢀ1=2
1-Pt-Pt
2-Pt-Pt
3-Pt-Pt
4-Pt-Pt
0.89
0.88
0.87
0.90
0.56
0.53
0.55
0.53
À1.09
À1.10
À1.12
À1.12[b]
À1.35
À1.37
À1.32
À1.32[b]
1.65
1.63
1.67
1.65
Interestingly, configurational changes were even manifest
in the dynamic emission properties, and a clear correlation
was observed between decay lifetimes and the different
classes of cores. Consistently longer lifetimes were obtained
for compounds with symmetric cores (1-Pt-Pt and 3-Pt-Pt;
NNNC,NNNC) than for their unsymmetrical counterparts.
The switch of substituents from cyclohexyl to methyl groups
also brought an extension of lifetime. The emission lifetimes
of bis-PtII complexes are much shorter than that of mono-
metallic 5-Pt, which is probably due to the presence of two
heavy atoms and the small energy gaps involved in the energy
relaxation in the bis-PtII complexes.
[a] Potential difference between the first oxidation and first reduction
processes. [b] Pseudoreversible.
tials and consequently the electrochemical HOMO–LUMO
band gaps for all bis-PtII complexes show nearly the same
values of approximately 1.65 V, which indicates that the redox
processes take place at the p-ligand moieties and that the
effect of the coordination environment is modest.
In summary, we have synthesized new types of doubly N-
confused calix[6]phyrins (1–4) and their PtII complexes. The
bis-PtII calix[6]phyrins displayed phosphorescence as opposed
to fluorescence in the near-IR region around 1000 nm with
shorter-than-expected lifetimes at room temperature. The bis-
PtII calix[6]phyrins as well as monometallic PtII calix[4]phyrin,
Figure 4 shows the Kohn–Sham orbitals of 3-Pt-Pt and 4-
Pt-Pt calculated at the B3LYP/631 LAN level[11] in an attempt
to give an account for the short lifetimes as observed for the
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properties that can be tailored at wavelengths longer than
those of conventional PtII complexes, hold promise as a new
class of IR luminescent dye.
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Keywords: calix[n]phyrins · luminescence · phosphorescence ·
.
platinum
[7] See the Supporting Information.
´
´
[8] M. Ste˛pien, L. Latos-Graz˙ynski, Chem. Eur. J. 2001, 7, 5113.
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19.3562(6), c = 10.2527(3) Š, V= 5285.7(3) Š3, Z = 4, T=
150(2) K, R = 0.0634 (I > 2s(I)), Rw = 0.1768, GOF = 1.096; 3-
Pt-Pt, dark purple block, 2(C58H22F20N6Pt2)·2(C7H16), Mr =
1673.18, monoclinic, space group P21 (No. 4), a = 12.1513(11),
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Angew. Chem. Int. Ed. 2008, 47, 5438 –5441ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5441