Photofunctionalization of a pentamethylcyclopentadienyl ligand with the N-phenylcarbazolyl group to prepare a highly luminescent Tb3+ complex having a fast radiation rate

Article abstract of DOI:10.1021/om1003094

Lanthanide(III) bent-metallocene complexes with a novel photofunctionalized pentamethylcyclopentadienyl ligand having an N-phenylcarbazolyl group (Cp ^PhCar = η⁵-C₅Me₄CH ₂C₁₈H₁₂N), [Ln(Cp^PhCar) ₂I(THF)] (Ln = Tb (1), Gd (2)), were prepared and their molecular structures and luminescence properties were investigated. The f-f emission from the terbium metal center of 1 was efficiently sensitized by Cp^PhCar (ε = 0.88 × 10⁴ M^-1 cm^-1 at 331 nm, Φ = 0.67 at 330 nm excitation, k_r = 1.68 × 10³ s^-1). Additionally, the solid-state structure of potassium salts of C₅-ring (η⁵-C₅R₅, R = Me, H) derivatives was uncommonly characterized in Cp^PhCar.

Full text of DOI:10.1021/om1003094

2390 Organometallics 2010, 29, 2390–2393
DOI: 10.1021/om1003094
Photofunctionalization of a Pentamethylcyclopentadienyl Ligand with
the N-Phenylcarbazolyl Group To Prepare a Highly Luminescent
Tb3þ Complex Having a Fast Radiation Rate
Takeshi Yatabe,† Hidetaka Nakai,*,† Koichi Nozaki,*,‡ Tomoo Yamamura,§ and
Kiyoshi Isobe*,†
†Department of Chemistry, Graduate School of Natural Science & Technology, Kanazawa University,
Kakuma-machi, Kanazawa 920-1192, Japan, ‡Department of Chemistry, Graduate School of Science &
Engineering, University of Toyama, 3190 Gofuku, Toyama, Toyama 930-8555, Japan, and
§Institute for Materials Research, Tohoku University, Sendai, Miyagi 980-8577, Japan
Received April 16, 2010
Summary: Lanthanide(III) bent-metallocene complexes with a
novel photofunctionalized pentamethylcyclopentadienyl ligand
having an N-phenylcarbazolyl group (CpPhCar=η5-C5Me4CH2-
C18H12N), [Ln(CpPhCar)2I(THF)] (Ln=Tb (1), Gd (2)), were
prepared and their molecular structures and luminescence pro-
perties were investigated. The f-f emission from the terbium
metal center of 1 was efficiently sensitized by CpPhCar (ε=0.88 ꢀ
104 M-1 cm-1 at 331 nm, Φ=0.67 at 330 nm excitation, kr=
1.68ꢀ103 s-1). Additionally, the solid-state structure of potas-
sium salts of C5-ring (η5-C5R5, R = Me, H) derivatives was
isolation of the ligand.5 We believe that photofunctionalized
Cp* ligands could open a new field of organometallic chemis-
try, including developments of novel photocatalysts and lumi-
nous materials. We have now found that a new class of photo-
functionalized ligands, the N-phenylcarbazolyl-functionalized
Cp* ligand (CpPhCar = η5-C5Me4CH2C18H12N), serves as an
efficient photosensitizer for the terbium(III) ion. Thus, a
highly luminescent organolanthanide complex having a fast
radiation rate was successfully prepared by proper combina-
tion of the metal ion and the photofunctionalized Cp* ligand
(vide infra). Herein we report the synthesis, structure, and
photophysical properties of a terbium(III) complex with
CpPhCar, [Tb(CpPhCar)2I(THF)] (1). The gadolinium(III) com-
plex [Gd(CpPhCar)2I(THF)] (2) was also synthesized to gain
insight into the photosensitization of the N-phenylcarbazolyl
group. Furthermore, the solid-state polymeric structure of
the THF-solvated CpPhCar potassium salt, which is a key
precursor for the preparation of 1 and 2, is presented. Solid-
state structural information for the Cp* or Cp (η5-C5H5)
derivative alkali-metal salts is comparatively limited, even
though their salts are widely used for preparing organome-
tallic complexes.4a,6
uncommonly characterized in CpPhCar
.
Ligand functionalization is a common approach to form
metal complexes that have a desired structure and function-
ality. To date, various photofunctionalized ligands have
been developed and successfully utilized as photosensitizers
for transition-metal and lanthanide complexes.1 Among
them, macrocyclic polyamines such as cyclam (1,4,8,11-tetra-
azacyclotetradecane) and tacn (1,4,7-triazacyclononane), which
are popular ligands in inorganic chemistry, are widely emp-
loyed as useful frameworks to construct adaptable and effi-
cient photofunctionalized ligands.2,3 Although the anionic
pentamethylcyclopentadienyl ligand (Cp* = η5-C5Me5) is
one of the most common and important ligands in organo-
metallic chemistry,4 photofunctionalized Cp* derivatives have
been hitherto unknown because of troublesome synthesis and
(5) Photofunctional Cp (η5-C5H5) ligands, instead of Cp*, are scar-
cely known: (a) Cicogna, F.; Colonna, M.; Houben, J. L.; Ingrosso, G.;
Marchetti, F. J. Organomet. Chem. 2000, 593-594, 251. (b) Carano, M.;
Cicogna, F.; D0Ambra, I.; Gaddi, B.; Ingrosso, G.; Marcaccio, M.; Paolucci,
D.; Paolucci, F.; Pinzino, C.; Roffia, S. Organometallics 2002, 21, 5583.
(c) Tse, S. K. S.; Guo, T.; Sung, H. H.-Y.; Williams, I. D.; Lin, Z.; Jia, G.
Organometallics 2009, 28, 5529.
*To whom correspondence should be addressed. E-mail: nakai@
cacheibm.s.kanazawa-u.ac.jp (H.N.); nozaki@sci.u-toyama.ac.jp (K.N.);
isobe@cacheibm.s.kanazawa-u.ac.jp (K.I.).
(1) (a) Photochemistry and Photophysics of Coordination Compounds I
and II; Balzani, V., Campagna, S., Eds.; Springer-Verlag: Berlin, Heidelberg,
(6) Evans, W. J.; Brady, J. C.; Fujimoto, C. H.; Giarikos, D. G.;
€
2007; Topics in Current Chemistry 280 and 281. (b) Eliseeva, S. V.; Bunzli,
Ziller, J. W. J. Organomet. Chem. 2002, 649, 252.
J.-C. G. Chem. Soc. Rev. 2010, 39, 189. (c) Armelao, L.; Quici, S.; Barigelletti,
F.; Accorsi, G.; Bottaro, G.; Cavazzini, M.; Tondello, E. Coord. Chem. Rev.
2010, 254, 487.
(2) (a) Prodi, L.; Maestri, M.; Ziessel, R.; Balzani, V. Inorg. Chem.
1991, 30, 3798. (b) Alexander, V. Chem. Rev. 1995, 95, 273. (c) DeRosa,
F.; Bu, X.; Ford, P. C. Inorg. Chem. 2005, 44, 4157. (d) Nonat, A.;
(7) Jutzi, P.; Heidemann, T.; Neumann, B.; Stammler, H. G. Synth-
esis 1992, 1096.
(8) Crystal data for KCpPhCar: C32H34KNO, M=487.72, pale yellow
block, T=123 ( 1 K, monoclinic, space group P21/c, a=9.1871(14) A,
b=28.378(4) A, c=10.3886(17) A, β=100.365(4)ꢀ, V=2664.2(7) A3,
Z=4, 27 744 reflections measured, 6041 unique (Rint=0.055), R1=0.0768
(I > 2σ(I)), wR2=0.2347 (all data), GOF=1.081. CCDC 772830. Crystal
ꢀ
Imbert, D.; Pecaut, J.; Giraud, M.; Mazzanti, M. Inorg. Chem. 2009, 48,
4207.
data for 1 2Et2O: C68H80IN2O3Tb, M=1259.22, colorless block, T=123 (
3
(3) (a) Nonat, A.; Gateau, C.; Fries, P. H.; Mazzanti, M. Chem. Eur.
J. 2006, 12, 7133. (b) Quici, S.; Marzanni, G.; Cavazzini, M.; Anelli, P. L.;
Botta, M.; Gianolio, E.; Accorsi, G.; Armaroli, N.; Barigelletti, F. Inorg.
Chem. 2002, 41, 2777. (c) Nasso, I.; Bedel, S.; Galaup, C.; Picard, C. Eur. J.
Inorg. Chem. 2008, 2064.
(4) (a) Harder, S. Coord. Chem. Rev. 1998, 176, 17. (b) Jutzi, P.;
Reumann, G. J. Chem. Soc., Dalton Trans. 2000, 2237. (c) Evans, W. J.;
Davis, B. L. Chem. Rev. 2002, 102, 2119. (d) Comprehensive Organome-
tallic Chemistry III; Crabtree, R. H., Mingos, D. M. P., Eds.; Elsevier:
Amsterdam, 2007; Vol. 4.
1 K, triclinic, space group P1, a = 8.6876(9) A, b = 17.4407(14) A, c =
20.9653(16) A, R = 66.995(4)ꢀ, β = 88.645(6)ꢀ, γ = 88.545(6)ꢀ, V =
=
2922.7(4) A3, Z = 2, 32 800 reflections measured, 13 080 unique (Rint
0.056), R1 = 0.0599 (I > 2σ(I)), wR2 = 0.1551 (all data), GOF = 1.109.
CCDC 772828. Crystal data for 2 2Et2O: C68H80IN2O3Gd, M=1257.55,
3
colorless block, T=123 ( 1 K, triclinic, space group P1, a=8.6986(13) A,
b=17.4586(18) A, c=21.033(2) A, R=67.005(7)ꢀ, β=88.451(12)ꢀ, γ=
88.659(12)ꢀ, V=2939.0(6) A3, Z=2, 31 615 reflections measured, 13 050
unique (Rint =0.076), R1=0.0678 (I > 2σ(I)), wR2=0.1719 (all data),
GOF=1.107. CCDC 772829.
r
pubs.acs.org/Organometallics
Published on Web 05/18/2010
2010 American Chemical Society

Communication
Organometallics, Vol. 29, No. 11, 2010 2391
Scheme 1. Preparation of the Potassium Salt KCpPhCar and
Lanthanide(III) Complexes 1 and 2
Figure 1. ORTEP drawing of a zigzag [K(CpPhCar)(THF)]¥ chain.
Hydrogen atoms are omitted for clarity, thermal ellipsoids are at
50% probability. Selected bond distances (A) and angles (deg):
K-ringcen(1) = 2.746, K-ringcen(2) = 2.760, K-O = 2.663(2),
K
C12 = 3.495(2); ringcen(1)-K-ringcen(2) = 141.68(4), K-
The N-phenylcarbazolyl-functionalized Cp* ligand CpPhCar
was synthesized by a method used to synthesize other types
of C5Me4R ligands, where R=NMe2, allyl, indenyl, dithia-
nyl,7 and was isolated as the potassium salt, [K(CpPhCar)-
(THF)] (KCpPhCar), in 25% overall yield from 3-bromo-
9-phenylcarbazole (Scheme 1). Pale yellow crystals suitable
for X-ray diffraction analysis were obtained from a saturated
THF solution of KCpPhCar at room temperature. An X-ray
diffraction study shows that the crystal structure of KCpPhCar
consists of polymeric zigzag chains of composition [K(CpPhCar)-
(THF)]¥, in which a weak interaction between the potassium
ion and the tethered N-phenylcarbazolyl substituent is obser-
3 3 3
ringcen(1)-K = 174.01(5). “ringcen” indicates the centroid of the
five ring-carbon atoms.
the angle of ringcen-metal-ringcen, 87.17(12)-91.6(5)ꢀ for
the angle of halide-metal-oxygen).9
Figure 3 (black) displays the UV-vis absorption spectrum
of 1 in THF at room temperature. The absorption bands at
296, 331, and 345 nm (ε =4.56ꢀ104, 0.88ꢀ 104, and 0.84ꢀ
104 M-1 cm-1, respectively) are assigned to the intramole-
cular π f π* transition of the N-phenylcarbazolyl moieties.10
Under the excitation of 1 at 330 nm, the luminescence spec-
trum characteristic of terbium ions was observed (Figure 3, red).
The seven bands in the spectrum are assigned to the following
transitions: 5D4 f 7F6 (497 nm), 5D4 f 7F5 (547 nm), 5D4 f
7F4 (590 nm), 5D4 f 7F3 (620 nm), 5D4 f 7F2 (648 nm), 5D4 f
ved (K C12(carbazole) distance 3.495(2) A) (Figure 1).8 The
3 3 3
K-O(THF) distance is 2.663(2) A. The centroid of the five ring-
carbon atoms in the cyclopentadienyl group, ringcen, is 2.746 A
from the potassium ion on one side and 2.760 A from another
potassium ion on the other. The ringcen-K-ringcen and
K-ringcen-K angles are 141.68(4) and 174.01(5)ꢀ, respectively.
This is a rare example of a crystallographically characterized
THF-solvated potassium salt of Cp* or Cp derivatives.6
7F1 (662 nm), and D4 f F0 (679 nm).1b,c The absence of
broad luminescence arising from the ligand-centered tran-
sitions indicates that the ligand-centered excited state in the
CpPhCar ligand was almost completely quenched by the ter-
bium ion (Figure S1, Supporting Information). Furthermore,
5
7
the excitation spectrum monitored at 547 nm (5D4 f F5
7
The reaction of two equivalent KCpPhCar groups with TbI3
in THF at room temperature led to the formation of the
terbium(III) complex 1 in good yield (83%) as a white
powder (Scheme 1). The isolated complex is stable indefi-
nitely under a dry nitrogen atmosphere. Crystals of 1 suitable
for X-ray diffraction analysis were grown from a saturated
Et2O solution at -22 ꢀC. The solid-state molecular structure
of 1 is depicted in Figure 2.8 The structure is typical of a bent
metallocene species that contains two additional ligands. The
ligand arrangements around the formally eight-coordinate
Tb atom are pseudotetrahedral, with the two C5 rings of
CpPhCar, an oxygen atom of THF, and an iodide ligand.
The ringcen-Tb-ringcen and iodide-Tb-oxygen angles are
134.03(9) and 88.30(9)ꢀ, respectively. These values are in the
range of typical bis(pentamethylcyclopentadienyl)lanthanide
complexes with two other ligands (133(1)-137.22(11)ꢀ for
transition) was almost identical to the absorption spectrum
of 1 (Figure S2). These findings univocally indicate that
an efficient intramolecular energy transfer occurs from the
N-phenylcarbazolyl moieties to the terbium ion.
In the luminescent lanthanide complexes, the energy level
of the lowest triplet ligand-centered excited state is one of
the crucial factors determining the luminescence quantum
yields.1b,c To estimate the energy level of the lowest triplet
state in 1, the gadolinium(III) complex 2 having the same
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2392 Organometallics, Vol. 29, No. 11, 2010
Yatabe et al.
absorption spectrum of 2 in THF at room temperature is
also consistent with that of 1 (Figure S4): λmax = 296, 331,
and 345 nm (ε = 4.38 ꢀ 104, 0.83 ꢀ 104, and 0.81 ꢀ 104 M-1
cm-1, respectively). From the phosphorescence spectra of
the gadolinium(III) complex 2 in Me-THF at 77 K (Figure S5),
the energy level of the lowest excited state in the N-phenyl-
carbazolyl moieties was estimated as approximately 24 200 cm-1
.
Thus, the energy gap (ΔE) between the lowest ligand-centered
(24200 cm-1) and metal-centered (20 100 cm-1) levels in 1 is
calculated to be 4100 cm-1, much higher than that normally
placed as an upper limit (ΔE=1850 cm )
-1 12 for back energy
transfer from the terbium ion at room temperature: the
energy gap in 1 is large enough to cause no back energy
transfer to the ligand center. This result supports the efficient
and straightforward intramolecular energy transfer from the
CpPhCar ligand to the terbium ion in 1.
The luminescence quantum yield (Φ) of 1 was determined
to be 0.67 in THF using 9,10-diphenylanthracene in cyclo-
hexane as a reference (Φ = 0.97).13 The quantum yield is
comparable to those for recently reported highly luminescent
terbium complexes, which used macrocyclic polyamines
(0.43-0.78),3 pyridine derivatives (0.31-0.95),14 2-hydroxy-
isophthalamide derivatives (0.40-0.63),15 and others
(0.40-0.85)16 as ligands. The lifetime (τ) of 1 in THF at
room temperatue (400 μs) is considerably shorter than those
found in the highly luminescent terbium complexes discussed
above (∼3.16 ms)3,14-16 and is slightly longer than that found
in the organometallic terbium complex [Tb(η5-C5H4CH3)2-
Cl(THF)] (300 μs).17 Thus, the radiative rate constant (kr =
Φ/τ) of 1 is calculated to be kr =1.68ꢀ103 s-1, which is the
highest in the terbium complexes discussed above (∼1.59 ꢀ
103 s-1: mostly less than 0.90ꢀ103 s-1). Since it is known that
asymmetric environments of lanthanide(III) ions promote
f-f radiative transition and thereby provide higher quantum
yields,18 the asymmetric structure around the terbium ion of
Figure 2. ORTEP drawing of terbium(III) complex 1. Hydro-
gen atoms and cocrystallized solvent molecules are omitted for
clarity; thermal ellipsoids are at the 50% probability level. Selec-
ted bond distances (A) and angles (deg): Tb-ringcen(1)=2.391,
Tb-ringcen(2) = 2.404, Tb-I = 3.0308(4), Tb-O = 2.414(3);
ringcen(1)-Tb-ringcen(2) = 134.03(9), I-Tb-O = 88.30(9),
ringcen(1)-Tb-O = 104.98(12), ringcen(2)-Tb-O = 106.63(13),
ringcen(1)-Tb-I = 105.44(7), ringcen(2)-Tb-I = 108.01(7).
“ringcen” indicates the centroid of the five ring-carbon atoms.
(12) Latva, M.;Takalo, H.;Mukkala, V.-M.;Matachescu, C.;Rodrı
´
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Ubis, J. C.; Kankare, J. J. Lumin. 1997, 75, 149.
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2009, 11, 9850.
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Helv. Chim. Acta 2003, 86, 3402. (c) Comby, S.; Imbert, D.; Chauvin, A.-S.;
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N-phenylcarbazole derivatives (ca. 24 000 cm-1 11
are con-
)
siderably lower than that of the metal-centered state for the
gadolinium(III) ion (32 000 cm-1), the ligand-centered lumi-
nescence would not be quenched in the gadolinium(III)
complex 2. The solid-state molecular structure of 2 is depicted
in Figure S3.8 The unit cell constants show that the lantha-
nide complexes 1 and 2 are isomorphous. The ringcen-metal-
ringcen and iodide-metal-oxygen angles of 2 (133.85(10)
and 88.58(10)ꢀ, respectively) are consistent with those of
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Communication
Organometallics, Vol. 29, No. 11, 2010 2393
1 (vide ante, Figure 2) would contribute to the photophysical
properties of 1.
Acknowledgment. This work was financially supported
by a Grants-in-Aid for Scientific Research (KAKENHI) in
Priority Areas and “Photochromism (No. 471)” and by the
Grant-in-Aid “21750059” from the Ministry of Education,
Culture, Sports, Science and Technology (MEXT) of Japan,
and a “Sasakawa Scientific Research Grant” from the Japan
Science Society. We thank Prof. N. Kitamura (Hokkaido
University) for useful discussions and Prof. I. Sato and
Dr. K. Shirasaki (Laboratory for R-ray emitters, Institute
for Materials Research, Tohoku University) for their
cooperation in elemental analyses.
In conclusion, we have demonstrated the potentiality of
the photofunctionalized Cp* ligand CpPhCar to form a highly
luminescent lanthanide complex. The newly prepared ligand
was isolated as the potassium salt, and its solid-state struc-
ture was unequivocally characterized. The terbium complex
1 supported by the CpPhCar ligands shows the efficient photo-
sensitization of f-f luminescence via energy transfer from
the photosensitized N-phenylcarbazolyl group and the fairly
fast radiation rate. Thus, a new class of photofunctionalized
ligands, CpPhCar and related Cp* derivative ligands, could
provide a new strategy to construct novel photofunctional
organometallic compounds and open a new field of organo-
metallic photochemistry. Further work is currently in progress
on the basis of this approach.
Supporting Information Available: Text, table, and figures
containing experimental and characterization details of
KCpPhCar, 1, and 2 and a CIF file giving crystallographic details
of KCpPhCar, 1, and 2. This material is available free of charge via
the Internet at http://pubs.acs.org.