Luminescent Au(III) organometallic complex of N-confused tetraphenylporphyrin

Article abstract of DOI:10.1039/b807922f

An organometallic Au(iii) complex of N-confused tetraphenylporphyrin has been synthesized and its electrochemical and photophysical properties investigated; unique emission is observed in solution at ambient temperature. The Royal Society of Chemistry.

Full text of DOI:10.1039/b807922f

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Luminescent Au(III) organometallic complex of N-confused
tetraphenylporphyrinw
Motoki Toganoh, Teppei Niino and Hiroyuki Furuta*
Received (in Austin, TX, USA) 9th May 2008, Accepted 8th June 2008
First published as an Advance Article on the web 23rd July 2008
DOI: 10.1039/b807922f
An organometallic Au(III) complex of N-confused tetraphenyl-
porphyrin has been synthesized and its electrochemical and
photophysical properties investigated; unique emission is
observed in solution at ambient temperature.
The luminescence of organometallic transition metal com-
plexes has been receiving considerable attention because of
its promising applications in organic light-emitting devices,
photocatalysis and bio-imaging.1 These complexes are often
accompanied by Csp2–metal or Csp–metal bonds, which are
important for improved emission efficiency. For example,
Ir(III)(N^C-ppy)3 (1) is an efficient triplet green emitter2 and
Au(III)(C^N^C-dpp)(CRCPh) (2)3 is a rare example of a
luminescent Au(III) complex at room temperature (Chart 1).
In this context, studies on the photochemistry of carbapor-
phyrin metal complexes would be fascinating because they
usually have Csp2–metal bonds and also have rigid structures,
which is also favorable for efficient emission.4 However, these
studies have yet to be undertaken fully due to lack of available
emissive compounds. Here, we report a new luminescent
Au(III) complex, N-confused tetraphenylporphyrin (NCTPP)5
Au(III) (3). This is not only the first example of an emissive
organometallic carbaporphyrinoid but is also a rare example
of an emissive Au(III) complex, even at room temperature.6
Note that standard porphyrin Au(III) complexes are emissive
only at lower temperatures.7
Scheme 1 The preparation of 3.
difficult, chlorinated N-fused tetraphenylporphyrin (5) and 21-
Cl-NCTPP were detected in the crude product.8,9 These unex-
pected side reactions could be avoided by prior bromination of 4
with N-bromosuccinimide (NBS).8 3 was obtained in 37% yield
upon treating 21-Br-NCTPP (6) with 3.3 equivalents of AuClꢀ
SMe2 in toluene under reflux for 11 h. The product was stable
under usual experimental conditions and could be isolated by
standard silica gel column chromatography. The 1H NMR, 13
C
Au(III) complex 3 was obtained through a reaction with AuClꢀ
SMe2 (Scheme 1). The reaction of AuClꢀSMe2 with NCTPP (4)
was not efficient, affording only a trace amount of 3 due to
competitive side reactions. While complete separation was
NMR and mass spectra of 3 are consistent with its structure. For
example, the signal attributable to the proton at the C3 position
is observed at d 9.43, but no signal due to NH protons is
detected. The 13C NMR spectrum resembles that of the NCTPP
Ag(III) complex,10 and the parent ion peak is observed at m/z =
809.19395 (calc. for MH+: 809.19795) in the mass spectrum
(ESI positive mode).
The structure of 3 was confirmed by X-ray crystallographic
analysis (Fig. 1).z The Au atom is placed in the center of the
macrocycle. The Csp2–Au bond length is 2.016(12) A, which is
shorter than those of 2 (2.073(7) and 2.071(7) A) and longer
than the Csp–Au bond length of 2 (1.979(7) A).3a The N–Au
bond lengths (average 2.044 A) are longer than that of
tetraphenylporphyrin Au(III) complex ([Au(TPP)]+[AuCl4]ꢁ,
average 2.017 A).11 The N-confused porphyrin plane deviates
from planarity, where the root mean-square deviation for the
24 heavy atoms is 0.252 A (Fig. 1(b)). Because no axial ligand
or counterion is found, unlike standard porphyrin Au(III)
complexes,7,12 3 forms an intermolecular stacking array in
the solid state (Fig. 2). The inter-plane distances (3.954 and
4.006 A) are significantly longer than those of common p–p
Chart 1 Luminescent organometallic transition metal complexes.
Department of Chemistry and Biochemistry, Graduate School of
Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka
819-0395, Japan. E-mail: hfuruta@cstf.kyushu-u.ac.jp
Fax: +81 92-802-2865; Tel: +81 92-802-2865
w Electronic supplementary information (ESI) available: Details on
synthesis and properties of 3 and on calculation of 3 and 4. CCDC
reference number 679312. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/b807922f
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Fig. 3 Absorption spectra of 3 and 4 in CH2Cl2.
atom. Although the LUMO energy level of 3 is similar to that
of 4, the HOMO energy level of 3 is significantly lower than
that of 4. In the HOMO of 3, a large coefficient on the carbon
atom and small coefficients on the three nitrogen atoms are
observed in the NNNC core. As a result, moderate interaction
between the metal d-orbital and the porphyrin p-orbital is
expected, which may result in stabilization of the HOMO
energy level. In contrast, only small coefficients on the two
internal nitrogen atoms are found in the LUMO of 3, and thus
the interaction would be trivial.
Fig. 1 Crystal structure of 3: (a) top view, (b) side view. Solvent
molecules are omitted and the phenyl groups are removed in (b) for
clarity. Thermal ellipsoids are shown at the 30% probability level.
stacked porphyrin arrays (3.4–3.6 A)13 because of steric
repulsion imposed by the meso-phenyl groups. The inter-
molecular Au–Au distances are long (5.132 and 5.060 A)
and no metal–metal interactions are found in the solid state.
The absorption spectra of 3 and 4 in CH2Cl2 are shown in
Fig. 3. In the absorption spectrum of 3, the Soret band is
observed at 436 nm, and the Q-bands are observed at 513, 549
and 636 nm. The remarkable blue shift of the Q-bands from 3
to 4 is caused by lowering of the HOMO energy level (vide
infra). In the electrochemical measurements of 3, the first
oxidation potential (Eox) is found at +0.53 V (reversible, vs.
Fc/Fc+) and the first reduction potential (Ered) is found at
–1.54 V (irreversible). The relatively large |Eox ꢁ Ered| value of
3 (2.07 V) is consistent with the blue shift seen in the absorp-
tion spectrum.14
Interestingly, 3 shows a distinct emission at ambient tem-
perature, rare example among Au(III) complexes.15 The unique
emission profile of 3 in CH2Cl2 is shown in Fig. 5, and the
photophysical properties of 3 in various solvents are listed in
Table 1. Emission peaks are observed at 650, 707 and 789 nm
in CH2Cl2, where the intensity of the last one is the largest.
The emission quantum yield is 2.3 ꢃ 10ꢁ4, which is compar-
able to those of 2 and its derivatives.3 Despite the unique
emission profile, almost no solvent effect was found among the
various solvents we investigated (toluene, THF and MeOH).
While the emission mechanism is totally unknown at present,
multiple pathways, like the cases of standard porphyrin Au(III)
complexes,7 are inferred from our preliminary photophysical
measurements. The emission peaks corresponding to lem1 and
lem2 could be explained by fluorescence from the S1 state,
because they resemble those of free-base NCTPP (4).16,17 The
large Stokes shift for lem3 (3049 cmꢁ1) and the existence of a
heavy atom in 3 imply phosphorescence from the T1 state, but
The low-lying HOMO energy level of 3 is supported by a
theoretical study. The calculated Kohn-Sham orbitals of 3 and
4 are shown in Fig. 4. The calculation was achieved by the
B3LYP method, with the SDD basis set for Au and the
6-31G** basis set for the other atoms (denoted as B3LYP/
631S). Basically, the HOMO and LUMO of 3 are mainly
composed of porphyrin p-orbitals and resemble those of 4,
except for a small contribution of the d-orbitals of the Au
Fig. 2 Intermolecular stacking diagram of 3 in the solid state. The
Fig. 4 Kohn-Sham orbitals and their energy levels for 3 and 4,
viewpoints are different in (a) and (b).
calculated at B3LYP/631S level.
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‘‘Science for Future Molecular Systems’’ from the Ministry of
Education, Culture, Sports, Science and Technology of Japan.
We thank HORIBA Ltd. for lifetime measurements.
Notes and references
z Crystal data for 3, violet prism, C44H27AuN4ꢀCH2Cl2, Mw = 893.59,
monoclinic, space group P21/c (no. 14), a = 8.9794(12), b = 15.034(2),
c = 25.640(3) A, b = 91.826(3)1, V = 3459.5(8) A3, Z = 4, T =
243 K, 18 755 reflections measured, 6252 unique data (Rint = 0.1045),
R = 0.0782 (I 4 2s(I)), Rw = 0.1872 (all data), GOF on F2 = 1.019
(all data).w
Fig. 5 Emission spectrum of 3 in CH2Cl2 at ambient temperature.
Table 1 Photophysical properties of 3
1 (a) W.-Y. Wong, Dalton Trans., 2007, 4495; (b) A. Vogler and H.
Kunkely, Coord. Chem. Rev., 2004, 248, 273.
Soreta Q1
Q2
lem1 lem2
lem3 Fem
a
a
a
Q3
a
a
a
b
2 K. A. King, P. J. Spellane and R. J. Watts, J. Am. Chem. Soc.,
1985, 107, 1431.
Solvent
3 (a) V. W.-W. Yam, K. M.-C. Wong, L.-L. Hung and N. Zhu,
Angew. Chem., Int. Ed., 2005, 44, 3107; (b) C. W. Chan, W. T.
Wong and C. M. Che, Inorg. Chem., 1994, 33, 1266.
Toluene 438
515
515
513
510
b
551 639
549 636
549 636
shc 636
654
650
646
651
701
707
705
693
790
789
784
794
2.3
2.9
2.2
2.6
CH2Cl2
THF
MeOH
a
435
435
432
4 (a) T. D. Lash, Synlett, 1999, 279; (b) M. Ste˛pien
Grazynski, Acc. Chem. Res., 2005, 38, 88.
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´
and L. Latos-
´
˙
Wavelength in nm. Emission quantum yields (ꢃ10ꢁ4) based on
relative intensity to tetraphenylporphyrin.16 c sh = shoulder.
´
˙
and T. G"owiak, Angew. Chem., Int. Ed. Engl., 1994, 33, 779.
6 T. D. Lash, D. A. Colby and L. F. Szczepura, Inorg. Chem., 2004,
43, 5258.
the lifetimes obtained so far are quite short,18 and a detailed
study is essential for further discussion.
7 (a) A. Antipas, D. Dolphin, M. Gouterman and E. C. Johnson,
J. Am. Chem. Soc., 1978, 100, 7705; (b) M. P. Eng, T. Ljungdahl, J.
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2005, 109, 1776; (c) J. Andre
asson, J. Martensson and B. Albinsson, J. Phys. Chem. A,
asson, G. Kodis, S. Lin, A. L. Moore,
Emission from 3 might be facilitated by electron donation
from the ligand to the metal center. In contrast to the rich
luminescent properties of Au(I) complexes, Au(III) complexes
have rarely been observed to emit.15 Au(III) has a d8 electronic
configuration and usually forms square-planar complexes.
Such planar Au(III) complexes have low-energy ligand field
d–d transitions due to the electron-poor nature of the Au(III)
center, and these transitions are assumed to be a key factor in
quenching the luminescence of Au(III) complexes. To impair
this factor, one suggested strategy is the introduction of an
electron-donating ligand, such as an alkynyl group, to the
Au(III) center, such as in 2.3 In the case of 3, electron donation
from the confused pyrrole to the Au(III) atom through the
´
T. A. Moore, D. Gust, J. Martensson and B. Albinsson, Photo-
chem. Photobiol., 2002, 76, 4700.
8 (a) H. Furuta, T. Ishizuka, A. Osuka and T. Ogawa, J. Am. Chem.
Soc., 1999, 121, 2945; (b) H. Furuta, T. Ishizuka, A. Osuka and T.
Ogawa, J. Am. Chem. Soc., 2000, 122, 5748.
9 (a) A. M"odzianowska, L. Latos-Grazyn
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ski, L. Szterenberg and M.
˙
, Inorg. Chem., 2007, 46, 6950; (b) M. Toganoh, T. Ishizuka
Ste˛pien
´
and H. Furuta, Chem. Commun., 2004, 2464; (c) M. Toganoh, S.
Ikeda and H. Furuta, Chem. Commun., 2005, 4589; (d) M.
Toganoh, S. Ikeda and H. Furuta, Inorg. Chem., 2007, 46, 10003.
10 H. Furuta, T. Ogawa, Y. Uwatoko and K. Araki, Inorg. Chem.,
1999, 38, 267.
11 A. M. Shachter, E. B. Fleischer and R. C. Haltiwanger, Acta
Crystallogr., Sect. C: Cryst. Struct. Commun., 1987, C43, 1876.
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5525.
C
sp2–Au(III) bond might contribute to the relatively electron
rich Au(III) center, which results in a raising of the energy of
the d–d transitions high enough to inhibit emission quenching.
In summary, we have synthesized an NCTPP Au(III) com-
plex, 3, and revealed its structure by X-ray crystallographic
analysis. It displays a distinct emission in solution at ambient
temperature, where no solvent effect is observed. This rare
example of a luminescent Au(III) complex might be attributed
to facile electron donation from the confused pyrrole to the
Au(III) atom through the Csp2–Au(III) bond, implying new
potential for the confusion approach. Aided by the rich
chemistry of porphyrin Au complexes,19 the further develop-
ment of N-confused porphyrin Au complexes can be expected,
and a detailed study of their unique photophysical properties
is now under way.
14 While exact potentials for 4 cannot be measured, probably due to
NH tautomerism, |Eox ꢁ Ered| is estimated to be 1.7B1.8 V from
those of related compounds: (a) H. Furuta, T. Ishizuka, A. Osuka,
H. Dejima, H. Nakagawa and Y. Ishikawa, J. Am. Chem. Soc.,
2001, 123, 6207; (b) M. Toganoh and H. Furuta, Chem. Lett., 2005,
34, 1034.
15 A. Vogler and H. Kunkely, Coord. Chem. Rev., 2001, 219–221, 489.
16 (a) J. P. Belair, C. J. Ziegler, C. S. Rajesh and D. A. Modarelli,
J. Phys. Chem. A, 2002, 106, 6445; (b) J. L. Shaw, S. A. Garrison,
E. A. Aleman, C. J. Ziegler and D. A. Modarelli, J. Org. Chem.,
´
2004, 69, 7423.
17 Preliminary lifetime measurements for 3 afforded two lifetimes
(1.8 ns (18%) and 7.1 ns (82%)) in toluene at 650 nm.
18 Similar lifetimes but a different ratio from that measured at 650 nm
are observed for 3 at 790 nm (1.7 ns (56%) and 7.0 ns (44%)).
19 (a) C. Y. Zhou, P. W. H. Chan and C. M. Che, Org. Lett., 2006, 8,
325; (b) R. W. Y. Sun, W. Y. Yu, H. Sun and C. M. Che,
ChemBioChem, 2004, 5, 1293; (c) L. Flamigni, F. Barigelletti, N.
Armaroli, J.-P. Collin, I. M. Dixon, J.-P. Sauvage and J. A. G.
Williams, Coord. Chem. Rev., 1999, 190–192, 671.
The present work was supported by a Grant-in-Aid for
Scientific Research (19750036) and the Global COE Program
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This journal is The Royal Society of Chemistry 2008
4072 | Chem. Commun., 2008, 4070–4072