Synthesis and photophysical properties of iridium(III) bisterpyridine and its homologues: A family of complexes with a long-lived excited state

Article abstract of DOI:10.1021/ja9833669

A new synthetic procedure has been developed which makes possible the preparation of IrLL′³⁺ complexes (L, L′ = terpyridine derivative) in good yields. In a first step, IrLCl₃ is obtained under relatively mild conditions as an intermediate. Subsequent reaction with L′ (a few minutes in refluxing ethylene glycol) affords IrLL′³⁺. The electrochemical behavior and ground- and excited-state spectroscopic properties of four IrLL′³⁺ complexes in nitrile solvents are reported. The X-ray structure of one of these complexes is also described. The complexes have been designed keeping in mind their incorporation in linearly arranged multicomponent arrays, according to a templating strategy based on the assembly of tpy-type ligands by the Ir(III) center. The complexes feature a high-lying level for the luminescent excited state (>2.5 eV), with a satisfactory room-temperature luminescence intensity (φ_em ≈ 10^-2) and lifetime on the microsecond time scale. These favorable properties indicate that the Ir(III)-tpy center will not be the final recipient of the energy-harvesting processes in multipartite systems built around them. Temperature-dependent studies of the luminescence properties in the 95-298 K range indicate that the higher-lying levels of these complexes are not efficient pathways for deactivation of the luminescent states. For these reasons, it is concluded that the studied Ir-tpy-type complexes are well suited (i) to play the role of photoactive center and to gather photo- and electroactive units or (ii) to act as electron relays in linearly arranged multicomponent arrays.

Full text of DOI:10.1021/ja9833669

J. Am. Chem. Soc. 1999, 121, 5009-5016
5009
Synthesis and Photophysical Properties of Iridium(III) Bisterpyridine
and Its Homologues: a Family of Complexes with a Long-Lived
Excited State
Jean-Paul Collin,*,1a Isabelle M. Dixon,1a Jean-Pierre Sauvage,*,1a J. A. Gareth Williams,1a,†
Francesco Barigelletti,*,1b and Lucia Flamigni*,1b
Contribution from the Laboratoire de Chimie Organo-Mine´rale, CNRS UMR 7513, UniVersite´ Louis
Pasteur, Institut Le Bel, 4 rue Blaise Pascal, F-67070 Strasbourg, France, and Istituto FRAE-CNR,
Via P. Gobetti 101, I-40129 Bologna, Italy
ReceiVed September 21, 1998
Abstract: A new synthetic procedure has been developed which makes possible the preparation of IrLL′3+
complexes (L, L′ ) terpyridine derivative) in good yields. In a first step, IrLCl3 is obtained under relatively
mild conditions as an intermediate. Subsequent reaction with L′ (a few minutes in refluxing ethylene glycol)
affords IrLL′3+. The electrochemical behavior and ground- and excited-state spectroscopic properties of four
IrLL′3+ complexes in nitrile solvents are reported. The X-ray structure of one of these complexes is also
described. The complexes have been designed keeping in mind their incorporation in linearly arranged
multicomponent arrays, according to a templating strategy based on the assembly of tpy-type ligands by the
Ir(III) center. The complexes feature a high-lying level for the luminescent excited state (>2.5 eV), with a
satisfactory room-temperature luminescence intensity (φem ≈ 10-2) and lifetime on the microsecond time scale.
These favorable properties indicate that the Ir(III)-tpy center will not be the final recipient of the energy-
harvesting processes in multipartite systems built around them. Temperature-dependent studies of the
luminescence properties in the 95-298 K range indicate that the higher-lying levels of these complexes are
not efficient pathways for deactivation of the luminescent states. For these reasons, it is concluded that the
studied Ir-tpy-type complexes are well suited (i) to play the role of photoactive center and to gather photo-
and electroactive units or (ii) to act as electron relays in linearly arranged multicomponent arrays.
Introduction
a sequence of elementary electron-transfer events which result
in the production of the D+-P-A- species.
To study photoinduced charge separation in multicomponent
systems, two available approaches are based on the assembly
of the active components via covalent bonds2a-c or via weak
interactions.2d,e The former approach looks appealing because
it offers the opportunity to gain a convenient control of the
geometric and electronic factors, a theme treated by many
research groups.2,3 Various types of multicomponent arrays have
been prepared in the past, and a frequently employed scheme
is based on the assembly of a central photoactive core (P) and
donor (D) and acceptor (A) electroactive units located on
opposite sides.4 In such systems, and provided the implied
energetic requirements are fulfilled, light absorption by P starts
The template strategy is ideally suited to the building up of
large-scale assemblies. According to this approach, a transition
metal cation, for instance Ru(II),5 Pt(II),6 or others,7 can act as
a templating center via coordination of suitable fragments
appended to the photo- and electroactive units.7,8 For instance,
Ru(II) coordination of 2,2′:6′,2′′-terpyridine (tpy) derivative
fragments attached to porphyrins led to assemblies of the type
shown in Chart 1.8,9 The substitution at the 4′-position of the
tpy enables the construction of linearly arranged systems and
also has the advantage of avoiding the coexistence of geo-
metrical and stereoisomers (present in the case of asymmetrical
bidentate ligands). Furthermore, suitable aromatic or aliphatic
† Present address: Department of Chemistry, University of Durham,
South Road, Durham DH1 3LE, UK.
(1) (a) Universite´ Louis Pasteur, Laboratoire de Chimie Organo-Mine´rale.
(b) Istituto FRAE-CNR.
(5) (a) Odobel, F.; Sauvage, J.-P. New J. Chem. 1994, 18, 1139. (b)
Harriman, A.; Odobel, F.; Sauvage, J.-P. J. Am. Chem. Soc. 1995, 117,
9461.
(2) (a) Nishitani, S.; Kurata, N.; Sakata, Y.; Misumi, S.; Karen, A.;
Okada, T.; Mataga, N. J. Am. Chem. Soc. 1983, 105, 7771. (b) Moore, T.
A.; Gust, D.; Mathis, P.; Mialocq, J.-C.; Chachaty, C.; Bensasson, R. V.;
Land, E. J.; Doizi, D.; Liddell, P. A.; Lehmann, W. R.; Nemeth, G. A.;
Moore, A. L. Nature 1984, 307, 630. (c) Wasielewski, M. R.; Niemczyk,
M. P.; Svec, W. A.; Pewitt, E. B. J. Am. Chem. Soc. 1985, 107, 1080. (d)
Sessler, J. L.; Wang, B.; Harriman, A. J. Am. Chem. Soc. 1992, 114, 388.
(e) Deng, Y.; Roberts, J. A.; Peng, S.-M.; Chang, C. K.; Nocera, D. G.
Angew. Chem., Int. Ed. Engl. 1997, 36, 2124.
(3) (a) Balzani, V.; Juris, A.; Venturi, M.; Campagna, S.; Serroni, S.
Chem. ReV. 1996, 96, 759. (b) Scandola, F.; Indelli, M. T.; Chiorboli, C.;
Bignozzi, C. A. Top. Curr. Chem. 1990, 158, 63.
(4) (a) Gust, D.; Moore, T. Top. Curr. Chem. 1991, 159, 103. (b)
Wasielewski, M. R. Chem. ReV. 1992, 92, 365.
(6) Ziessel, R. J. Chem. Educ. 1997, 74, 673.
(7) (a) Danielson, E.; Elliott, C. M.; Merkert, J. W.; Meyer, T. J. J. Am.
Chem. Soc. 1987, 109, 2519. (b) Opperman, K. A.; Mecklenburg, S. L.;
Meyer, T. J. Inorg. Chem. 1994, 33, 5295. (c) Harriman, A.; Ziessel, R. J.
Chem. Soc., Chem. Commun. 1996, 1707.
(8) Harriman, A.; Sauvage, J.-P. Chem. Soc. ReV. 1996, 41.
(9) (a) Flamigni, L.; Barigelletti, F.; Armaroli, N.; Collin, J.-P.; Sauvage,
J.-P.; Williams, J. A. G. Chem., Eur. J. 1998, 4, 1744. (b) Flamigni, L.;
Barigelletti, F.; Armaroli, N.; Ventura, B.; Collin, J.-P.; Sauvage, J.-P.;
Williams, J. A. G. submitted to Inorg. Chem. (c) Flamigni, L.; Armaroli,
N.; Barigelletti, F.; Balzani, V.; Collin, J.-P.; Dalbavie, J.-O.; Heitz, V.;
Sauvage, J.-P. J. Phys. Chem. B 1997, 101, 5936. (d) Collin, J.-P.; Dalbavie,
J.-O.; Heitz, V.; Sauvage, J.-P.; Flamigni, L.; Armaroli, N.; Balzani, V.;
Barigelletti, F.; Montanari, I. Bull. Soc. Chim. Fr. 1996, 133, 749.
10.1021/ja9833669 CCC: $18.00 © 1999 American Chemical Society
Published on Web 05/14/1999

5010 J. Am. Chem. Soc., Vol. 121, No. 21, 1999
Collin et al.
Chart 1
Chart 2
spacers10,11 can be inserted between the templating center and
the active units, leading to systems with a varied intercomponent
distance.
In one of these arrays,9 the Ru-based core was found to be
extensively involved in energy-transfer processes, mainly
because of the presence of low-lying Ru f tpy charge-transfer
levels, located at 2 eV.9a,b Indeed, energy transfer from the
3
primary singlet porphyrin excited state to the MLCT excited
Chart 3
state localized on the complex was found to be competitive with
the desired electron-transfer step. There are cases in which the
templating center is wished not to take part in the energy-transfer
events within the array. In such cases, the metal center should
not possess easily accessible excited states so that its role, in
addition to a structural one, will likely be restricted to that of
an electroactive unit.
An additional pitfall of arrays based on a Ru(tpy)22+ core is
the fact that, in this chromophore, the lowest-lying excited state
undergoes a fast deactivation, its lifetime being shorter than 1
ns.12 One undesired consequence is that long-distance inter-
component steps may be scarcely competitive toward the
intrinsic deactivation of the templating unit.9c Concerning the
possible use of the longer-lived Os-based analogues, limitations
arise from the low energy level of their MLCT luminescent
2+
states (around 1.6 eV12). On this basis, Os(tpy)2 is bound to
play a dominant, parasitic role as an energy sinker with respect
to the processes taking place in the assembly.
Consideration of our previous attempts9 has led us to start
the design of multicomponent systems based on Ir(III)-tpy-type
templating centers,13-15 where the convenient geometric proper-
ties brought about by the tpy ligand11,12 are combined with the
high energy content of the excited states of the Ir(III) centers
(see below)14,15 and a fair ability to accept electrons. Chart 2
(10) Barigelletti, F.; Flamigni, L.; Collin, J.-P.; Sauvage, J.-P. J. Chem.
Soc., Chem. Commun. 1997, 333.
(11) A list of representative papers follows: (a) Boyde, S.; Strouse, G.
F.; Jones, W. E., Jr.; Meyer, T. J. J. Am. Chem. Soc. 1990, 112, 7395. (b)
Scandola, F.; Argazzi, R.; Bignozzi, C. A.; Indelli, M. T. J. Photochem.
Photobiol. A: Chem. 1994, 82, 191. (c) Show, J. R.; Webb, R. T.; Schmehel,
R. H. J. Am. Chem. Soc. 1990, 112, 7395. (d) De Cola, L.; Balzani, V.;
Barigelletti, F.; Flamigni, L.; Belser, P. Von Zelewsky, A., Franck, M.;
Vo¨gtle, F. Inorg. Chem. 1993, 32, 5228. (e) Vo¨gtle, F.; Franck, M.; Nieger,
M.; Belser, P.; Von Zelewsky, A.; Balzani, V.; Barigelletti, F.; De Cola,
L.; Flamigni, L. Angew. Chem. Int. Ed. 1993, 32, 1643. (f) Benniston, A.
C.; Grosshenny, V.; Harrimann, A.; Ziessel, R Angew. Chem., Int. Ed. Engl.
1994, 33, 1884. (g) Benniston, A. C.; Goulle, V.; Harriman, A.; Lehn, J.-
M.; Marczinke, B. J. Phys. Chem. 1994, 98, 7798. (h) Ho, P. K.-K.; Cheung,
K.-K.; Che, C.-M. J. Chem. Soc., Chem. Commun. 1996, 1197. (i) Vogler,
L. M.; Brewer, K. J. Inorg. Chem. 1996, 35, 818. (j) Bolger, J.; Gordon,
A.; Ishow, E.; Launay, J.-P. J. Chem. Soc., Chem. Commun. 1995, 1799.
(k) Wa¨rnmark, K.; Thomas, J. A.; Heyke, O.; Lehn, J.-M. J. Chem. Soc.,
Chem. Commun. 1996, 701.
(12) Sauvage, J.-P.; Collin, J.-P.; Chambron, J.-C.; Guillerez, S.; Coudret,
C.; Balzani, V.; Barigelletti, F.; De Cola, L.; Flamigni, L. Chem. ReV. 1994,
94, 993 and references therein.
(13) (a) Ayala, N. P.; Flynn, C. M., Jr.; Sacksteder, L.; Demas, J. N.;
DeGraff, B. A. J. Am. Chem. Soc. 1990, 112, 3837. (b) Mamo, A.; Stefio,
I.; Parisi, M. F.; Credi, A.; Venturi, M.; Di Pietro, C.; Campagna, S. Inorg.
Chem. 1997, 36, 5947. For Ir(III) complexes of bidentate ligands, see ref
14. Ir(III) complexes have also been employed for photoinitiated energy
collection in polymetallic species; see ref 15.
3+
summarizes two possible uses of the Ir(tpy)2 core, (a) as
assembling metal and electron relay or (b) as photoactive center
and electron relay.
In this report, we describe the synthesis, the electrochemical
behavior, and the spectroscopic and photophysical properties
of complexes 1-4 (Chart 3) and the X-ray structure of 1. We
performed a detailed investigation of their photophysical proper-
ties, particularly with respect to the possible role of high-lying,
(14) (a) Watts, R. J.; Crosby, G. A. J. Am. Chem. Soc. 1971, 93, 3184.
(b) Flynn, C. M., Jr.; Demas, J. N. J. Am. Chem. Soc. 1974, 96, 1959. (c)
Schmid, B.; Garces, F. O.; Watts, R. J. Inorg. Chem. 1994, 33, 9. (d)
Ohsawa, Y.; Sprouse, S.; King, K. A.; DeArmond, M. K.; Hanck, K. W.;
Watts, R. J. J. Phys. Chem. 1987, 91, 1047. (e) Didier, P.; Otmans, I.;
Kirsch-De Mesmaeker, A.; Watts, R. J. Inorg. Chem. 1993, 32, 5239.
(15) (a) Molnar, S. M.; Nallas, G.; Bridgewater, J. S. Brewer, K. J. J.
Am. Chem. Soc. 1994, 116, 5206. (b) Nallas, G. N. A.; Jones, S. W.; Brewer,
K. J. Inorg. Chem. 1996, 35, 6980.

Iridium(III) Bisterpyridine and Its Homologues
J. Am. Chem. Soc., Vol. 121, No. 21, 1999 5011
mixture was evaporated to dryness, dissolved in CH2Cl2, and chro-
matographed on alumina (hexane/ether 100/0 to 0/100, then CH2Cl2/
MeOH 99/1) to give pure Ir(tButpy)Cl3 in a 43% yield (88 mg) as an
orange-red solid.
but thermally accessible, excited states. Given that, in these
arrays, the distance (and the electronic coupling) between the
opposite components will be varied by using 1,4-phenylene
spacers, the coordinated ligands of 1-4 feature a varying
number of aryl substituents.
1H NMR (200 MHz, CD2Cl2): δ 9.45 (ddd, 2H, H6 + H6′′, 3J ) 5.7
4
5
Hz, J ) 1.5 Hz, J ) 0.7 Hz); 8.26-8.22 (m, 4H, H3′ + H5′ + H3 +
3
4
H3′′); 8.04 (ddd, 2H, H4 + H4′′, J ) 7.9 Hz, J ) 1.7 Hz); 7.75 (ddd,
3
4
4
Experimental Section
2H, H5 + H5′′, J ) 7.3 Hz, J ) 1.5 Hz); 7.65 (t, 1H, Hp, J ) 1.7
Hz); 7.56 (d, 2H, Ho, J ) 1.7 Hz), 1.43 (s, 18H, CH3). FAB+-MS:
4
m/z 719 ([M]+, C29H31N3IrCl3 requires 720).
General Procedures. The following chemicals were obtained
commercially and were used without further purification: KPF6
(Janssen), NH4PF6 (Aldrich), 2-acetylpyridine (Lancaster), ammonium
acetate (Prolabo), acetamide (Acros), 2,2′:6′,2′′-terpyridine (Aldrich),
IrCl3 (Johnson Matthey), and IrCl3‚4H2O (Pressure Chemical). Thin-
layer chromatography was performed using plastic sheets coated with
silica or neutral alumina (Macherey-Nagel). Column chromatography
was carried out on silica gel 60 (Merck, 63-200 µm) or neutral alumina
90 (Merck, 63-200 µm).
Ir(tButpy)2(PF6)3 (2). Ir(tButpy)Cl3 (100 mg, 0.14 mmol) and
tButpy (60 mg, 0.14 mmol) were heated to 140 °C in degassed ethylene
glycol (40 mL), under argon and in the dark, for 2 h 30. The solvent
was evaporated under reduced pressure, and the crude mixture was
dissolved in MeCN. After precipitation with an aqueous solution of
KPF6, the yellow solid was recrystallized twice from CH2Cl2/THF to
give 2 in a 40% yield (82 mg).
1H NMR (200 MHz, acetone-d6): δ 9.52 (s, 2H, H3′ + H5′); 9.12
(d, 2H, H6 + H6′′, 3J ) 7.4 Hz); 8.32 (ddd, 2H, H4 + H4′′, 3J ) 7.5 Hz,
4J ) 1.2 Hz); 8.25 (dd, 2H, H3 + H3′′, 3J ) 6.3 Hz, 4J ) 1.1 Hz); 8.10
(d, 2H, Ho, 4J ) 1.7 Hz); 7.86 (t, 1H, Hp, 4J ) 1.8 Hz); 7.65 (ddd, 2H,
Starting Organic Compounds. 3,5-Di-tert-butylbenzaldehyde,16 4′-
tolyl-2,2′:6′,2′′-terpyridine (ttpy),17 and 4′-methyl-2,2′:6′,2′′-terpyridine
(Metpy)18 were prepared according to literature procedures.
4′-(3,5-Di-tert-butylphenyl)-2,2′:6′,2′′-terpyridine (tButpy). 2-Acetyl-
pyridine (5 g, 0.04 mol), 3,5-di-tert-butylbenzaldehyde16 (4.5 g, 0.02
mol), ammonium acetate (23 g, 0.3 mol), and acetamide (35 g, 0.6
mol) were heated to 160 °C for 2 h. After the mixture was cooled to
100 °C, NaOH (20 g in 40 mL of H2O) was added very slowly, and
this mixture was heated to 120 °C for a further 2 h without stirring.
The two phases were decanted, and the black supernatant was taken
into dichloromethane and washed with water. Chromatography on
alumina (hexane/ether from 100/0 to 97/3) gave tButpy in 27% yield
(2.32 g).
3
4
H5 + H5′′, J ) 6.9 Hz, J ) 1.2 Hz); 1.50 (s, 36 H, tBu). ES+-MS:
m/z 1325 ([M - PF6]+, C58H62N6IrP2F12 requires 1325).
Ir(ttpy)2(PF6)3 (3) and Ir(ttpy)(Metpy)(PF6)3 (4). IrCl3‚4H2O (120
mg, 0.34 mmol) in solution in EtOH (2.5 mL) was added to a solution
of ttpy17 (110 mg, 0.34 mmol) in hot EtOH (5 mL). Immediate
precipitation of a red-brown solid was observed. Heating was continued
for a further 5 h, and the solid was isolated by filtration and washed
with EtOH, toluene, and Et2O. This solid was assumed to be Ir(ttpy)-
Cl3, and a portion of the product (100 mg, 0.16 mmol) was taken into
ethylene glycol (5 mL). Metpy18 (40 mg, 0.16 mmol) was added to
the resulting suspension. Upon heating to 180 °C, the solid rapidly
dissolved, giving a dark green solution, which turned red after a further
1 h at this temperature. The crude mixture was then purified by column
chromatography (SiO2, from CH3CN to 70% CH3CN/29% H2O/1%
saturated KNO3 solution). After elution of small quantities of a number
of highly-colored compounds, Ir(ttpy)2(NO3)3 (3′) was obtained (20
mg, 12%), followed by Ir(ttpy)(Metpy)(NO3)3 (4′) (20 mg, 13%). 3
and 4 could be separately isolated through anion exchange using
NH4PF6.
1H NMR (200 MHz, CD2Cl2): δ 8.74 (m, 2H, H6); 8.72 (s, 2H, H3′
3
4
+ H5′); 8.69 (m, 2H, H3); 7.91 (ddd, 2H, H4, J ) 7.6 Hz, J ) 1.7
Hz); 7.67 (d, 2H, Ho, 4J ) 1.7 Hz); 7.58 (t, 1H, Hp, 4J ) 1.7 Hz); 7.37
(ddd, 2H, H5, J ) 6 Hz, J ) 7.6 Hz, J ) 1.2 Hz); 1.43 (s, 18H,
CH3). EI+-MS: m/z 421.4 (M+, C29H31N3 requires 421.58); 406.4 ([M
- CH3]+); 350.3 ([M - CH3 - t-Bu]+).
3
3
4
Ir(tpy)Cl3. 2,2′:6′,2′′-Terpyridine (tpy, 66 mg, 0.28 mmol) and IrCl3
(104 mg, 0.28 mmol) were heated at 160 °C in degassed ethylene glycol
(5 mL), under argon and in the dark, for 15 min, during which a red
precipitate formed. It was filtered off after cooling to room temperature
and was washed with EtOH, H2O, and Et2O to give pure Ir(tpy)Cl3 in
a 40% yield (60 mg) as an orange-red solid.
1
Data for 3′. H NMR (300 MHz, D2O/CD3CN): δ 9.11 (4H, s,
H3′); 8.72 (4H, d, H6, J ) 8.2 Hz); 8.19 (4H, dd, H5, coupling constants
too similar to allow exact values); 8.08 (4H, d, Ho, J ) 8.3 Hz); 7.68
(4H, d, H3, J ) 5.3 Hz); 7.57 (4H, d, Hm, J ) 8 Hz); 7.47 (4H, dd, H4,
coupling constants too similar to allow exact values); 2.49 (6H, s, CH3).
FAB+-MS: m/z 963.5 ([M - NO3]+, C44H34N8O6Ir requires 963).
1H NMR (400 MHz, DMSO-d6): δ 9.23 (dd, 2H, H6, 3J ) 5.4 Hz,
3
3
4J ) 0.7 Hz); 8.78 (d, 2H, H3, J ) 8 Hz); 8.74 (d, 2H, H3′ + H5′, J
) 8 Hz); 8.34-8.20 (m, 3H, H4 + H4′); 7.98 (ddd, 2H, H5, 3J ) 6 Hz,
4J ) 0.7 Hz).
1
Data for 4′. H NMR (300 MHz, D2O): δ 9.04 (2H, s, H3′ (ttpy));
Ir(tpy)2(PF6)3 (1). Ir(tpy)Cl3 (40 mg, 0.075 mmol) and tpy (17.5
mg, 0.075 mmol) were heated to reflux in degassed ethylene glycol (3
mL), under argon and in the dark, for 15 mn. After evaporation of the
solvent and precipitation with an aqueous solution of KPF6, crude
Ir(tpy)23+ was purified by crystallization in acetone/MeOH/toluene and
MeCN/benzene and by column chromatography (Al2O3, acetone/water
from 100/0 to 95/5) to give 1 in a 61% yield (50 mg). Pale yellow
single crystals were obtained by slow diffusion from a 1:3 (v/v) acetone/
benzene mixture.
8.70 (2H, s, H3′ (Metpy)); 8.61 (2H, d, H6 (ttpy), J ) 8.2 Hz); 8.49
(2H, d, H6 (Metpy), J ) 8.3 Hz); 8.10 and 8.08 (4H, two overlapping
dd, H5 (ttpy) and H5 (Metpy)); 7.95 (2H, d, Ho (ttpy), J ) 8 Hz); 7.64
(2H, d, H3 (ttpy), J ) 10.4 Hz); 7.59 (2H, d, H3 (Metpy), J ) 8.6 Hz);
7.47 (2H, d, Hm (ttpy), J ) 8.2 Hz); 7.37 and 7.35 (4H, two overlapping
dd, H4 (ttpy) and H4 (Metpy)). FAB+-MS: m/z 887 ([M - NO3]+,
C38H30N8O6Ir requires 887). ES+-MS: m/z 380.4 ([M - 3NO3]2+
,
[C38H30N6Ir]2+ requires 381.5).
Equipment and Methods. 1H NMR spectra were acquired on either
a Bruker WP200 SY, a Bruker AC300, or a Bruker AM400 spectrom-
eter, using the deuterated solvent as the lock and residual solvent as
the internal reference. Mass spectra were obtained by using a VG ZAB-
HF spectrometer (FAB), a Fisons VG Platform (ES), or a Fisons VG
Trio 2000 (EI) spectrometer. Cyclic voltammetry experiments were
performed using an EG&G 273A potentiostat, a Pt working electrode,
a Pt counter-electrode, a saturated calomel electrode (SCE), 0.1 M Bu4-
NPF6 as supporting electrolyte, and MeCN and DMF as solvents. Details
concerning the crystal structure are given in Table 2. For all computa-
tions, the MolEN package was used.19 Absorption spectra were recorded
with a Perkin-Elmer Lambda 9 spectrophotometer in dilute (∼10-5 M)
acetonitrile solution. Luminescence experiments were performed (i) in
1H NMR (200 MHz, acetone-d6): δ 9.21 (d, 4H, H6, 3J ) 8.1 Hz);
3
3
8.94 (t, 2H, H4′, J ) 8.2 Hz); 8.93 (d, 4H, H3′ + H5′, J ) 8.1 Hz);
3
4
3
8.36 (ddd, 4H, H4, J ) 7.9 Hz, J ) 1.5 Hz); 8.11 (dd, 4H, H3, J )
4
3
4
5.7 Hz, J ) 0.8 Hz); 7.62 (ddd, 4H, H5, J ) 8 Hz, J5-3 ) 1.5 Hz).
FAB+-MS: m/z 949 ([M - PF6]+, C30H22N6IrP2F12 requires 949).
Ir(tButpy)Cl3. tButpy (120 mg, 0.28 mmol) was dissolved in
refluxing absolute ethanol (100 mL). A solution of IrCl3‚4H2O (100
mg, 0.27 mmol) in absolute ethanol (30 mL) was then added dropwise,
and this mixture was heated to reflux in the dark for 3 h. The crude
(16) Newman, M. S.; Lee, L. F. J. Org. Chem. 1972, 26, 4468.
(17) (a) Collin, J.-P.; Guillerez, S.; Sauvage, J.-P.; Barigelletti, F.; De
Cola, L.; Flamigni, L.; Balzani, V. Inorg. Chem. 1991, 30, 4230. (b) Spahni,
W.; Calzaferri, G. HelV. Chim. Acta 1984, 67, 450.
(18) Collin, J.-P.; Harriman, A.; Heitz, V.; Odobel, F.; Sauvage, J.-P. J.
Am. Chem. Soc. 1994, 116, 5679.
(19) Fair, C. K. In MolEN, An interactiVe intelligent system for crystal
structure analysis; Nonius: Delft, The Netherlands, 1990.

5012 J. Am. Chem. Soc., Vol. 121, No. 21, 1999
Collin et al.
air-equilibrated or deaerated acetonitrile solution at room temperature
in 1-cm cuvettes, (ii) in butyronitrile rigid matrix at 77 K (liquid nitrogen
temperature) by using samples contained in capillary tubes immersed
in a quartz finger Dewar, and (iii) in butyronitrile solvent in the
temperature interval between 293 and 90 K. For the latter case, we
employed a Thor Research cryostat and temperature controller. The
cryostat was home-modified by substituting the original sample holder
with a cell holder for hosting sealed quartz cells. The actual temperature
within the cells was obtained through calibration with the help of an
external thermocouple, which enabled comparison of the temperatures
indicated on the controller with true temperatures monitored by the
thermocouple; the uncertainty on the measured temperature was 2 K.
Deaerated samples were prepared at a vacuum line through pump-
freeze-thaw repeated cycles and stored in sealed quartz cells.
Luminescence spectra were obtained from a Spex Fluorolog II
spectrofluorometer equipped with a Hamamatsu R928 phototube.
Uncorrected luminescence band maxima are used throughout the text
unless otherwise stated. Luminescence quantum yields, φ, were
obtained as already published20 and were computed by following the
method described by Demas and Crosby.21 The experimental uncertainty
in the band maximum for absorption and luminescence spectra is 2
nm; for luminescence quantum yields it is 20%. Luminescence lifetimes
were obtained with an IBH single-photon-counting apparatus (N2 lamp,
excitation at 337 nm). The uncertainty on the evaluated lifetimes is
8%.
Table 1. Electrochemical Data of Complexes 1-4
Ereda (V vs SCE) Eredb (V vs SCE)
1
2
3
4
-0.77
-0.93
-0.92
-0.76
-0.77*
-0.81
-0.81
-0.91
-0.92*
-0.91
-0.89
-0.74
insoluble
-0.81
-0.97
-1.04
a MeCN, Bu4NPF6 0.1 M. b DMF, Bu4NPF6 0.1 M. All processes
were irreversible, except those indicated with an asterisk.
The classical method used for ruthenium(II) and rhodium(III)
terpyridine complexes starts with the preparation of M(tpy)Cl3
(M ) Ru(II) or Rh(III)).27 This stepwise method also has the
advantage of allowing the preparation of asymmetrical systems,
provided the complexes are kinetically inert. As one would
expect, the coordination of the first terpyridine is easier than
the introduction of the second one, the latter requiring the
dechlorination of the M(tpy)Cl3 precursor. With iridium(III),
the classical procedures (e.g., with AgBF4 in refluxing acetone,
which has been used with Ir(I)28) were unsuccessful, probably
due to the great inertness of such complexes.
Depending on the nature of the terpyridine ligand, the reaction
conditions have to be adapted. The formation of Ir(tpy)Cl3
required the reaction of IrCl3 and tpy in ethylene glycol at 160
°C, for solubility reasons. On the other hand, Ir(tButpy)Cl3
and Ir(ttpy)Cl3 were prepared by reacting IrCl3‚4H2O and
tButpy or ttpy in refluxing ethanol, for 3 and 5 h, respectively.
Ir(tButpy)Cl3 was soluble in common solvents and could,
therefore, be purified by chromatography, unlike Ir(tpy)Cl3 and
Ir(ttpy)Cl3.
Transient absorption spectra were acquired by a pump and probe
method based on a 35-ps Nd:YAG laser (Continuum PY62-10) and an
OMA detector system described elsewhere.9c Excitation was performed
at 355 nm with 2.5 mJ/pulse; the absorbance of the samples at the
excitation wavelength was 0.9. The lifetimes of absorbing states were
determined by a laser flash photolysis apparatus with 20 ns resolution,22a,b
excitation at 355 nm, and 2 mJ/pulse. The uncertainty on the evaluated
lifetimes is 10%.
Complexes 1-4 were obtained by reacting Ir(L)Cl3 (L ) tpy
derivative) with the second terpyridine ligand in degassed
ethylene glycol under argon, at various temperatures (varying
from 140 to 196 °C) and for relatively short periods of time
(from 15 min to 2.5 h). The preparation of 1 required refluxing
ethylene glycol for 15 min, whereas the preparation of 2 was
achieved after 2.5 h at 140 °C. 3 and 4 were obtained by reaction
of Ir(ttpy)Cl3 and Metpy at 180 °C for 1 h, 3 being the
scrambled product. One has to keep in mind that the higher the
temperature and the longer the reaction time, the higher the
probability of preparing orthometalated complexes and other
byproducts.13b For our purpose, we tried to minimize these two
parameters in order to avoid orthometalation.29 All four
complexes were easily purified, either by crystallization (1 and
2) or by column chromatography on alumina (1) or on silica (3
and 4). 1 is pale yellow, whereas 2, 3, and 4 are yellow. 1 shows
blue luminescence on TLC plates under 365 nm irradiation,
while 2, 3, and 4 show bright green luminescence under similar
conditions. The purity of the complexes (impurity level esti-
mated lower than 1%) was carefully cheked by thin-layer
Results and Discussion
Syntheses. To the best of our knowledge, the synthesis of
iridium(III) bisterpyridine-type complexes has only been de-
scribed once in the literature, by Ayala and colleagues.13a This
procedure is based on a previously published synthesis of
Ir(bpy)33+ (bpy ) 2,2′-bipyridine),14b which entails melting the
inorganic precursor at 220 °C for 30 min and then heating it at
230 °C in the presence of the ligand for 6 h. The harsh reaction
conditions required and the tremendously difficult purification
led us to investigate a new synthetic pathway.
Our first attempts starting from labile Ir(I) complexes
([Ir(COD)Cl]2, COD ) 1,5-cyclooctadiene, or [Ir2(µ-Cl)2-
(COE)4],23 COE ) cyclooctene), inspired by results from
Mønsted et al.24 and from Suzuki et al.,25 led us to the desired
complexes but in poor yields (13% for 2). The method described
by Sullivan and Meyer26 for the synthesis of Ir(bpy)33+ seemed
milder than that of Flynn and Demas,14b but in the perspective
of using terpyridine ligands bearing donor or acceptor groups
of the porphyrinic type, we decided to explore a simpler
experimental procedure.
1
chromatography, high-resolution H NMR spectroscopy, and
mass spectrometry.
Electrochemistry. Cyclic voltammetry data are reported in
Table 1. The reduction potentials we report here are analogous
(20) Hammarstro¨m, L.; Barigelletti, F.; Flamigni, L.; Indelli, M. T.;
Armaroli, A.; Calogero, G.; Guardigli, M.; Sour, A.; Collin, J.-P., Sauvage,
J.-P. J. Phys. Chem. A 1997, 101, 9061.
(21) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.
(22) (a) Flamigni, L. J. Phys. Chem. 1992, 96, 3331. (b) Flamigni, L. J.
Chem. Soc., Faraday Trans. 1994, 90, 2331.
to those of Ir(bpy)3 published by Kahl et al.30 Two one-
3+
electron reduction waves are observed and are ligand-centered.31
(27) Sullivan, B. P.; Calvert, J. M.; Meyer, T. J. Inorg. Chem. 1980, 19,
1404.
(23) Herde, J. L.; Lambert, J. C.; Senoff, C. V. Inorg. Synth. 1974, 15,
18.
(24) Mønsted, L.; Mønsted, O.; Nord, G.; Simonsen, K. Acta Chem.
Scand. 1993, 47, 439.
(28) Mueting, A. M.; Alexander, B. D.; Boyle, P. D.; Casalnuovo, A.
L.; Ito, L. N.; Johnson, B. J.; Pignolet, L. H. Inorg. Synth. 1992, 29, 279.
(29) Nord, G.; Hazell, A. C.; Hazell, R. G.; Farver, O. Inorg. Chem.
1983, 22, 3429.
(25) Suzuki, T.; Rude, M.; Simonsen, K.; Morooka, M.; Tanaka, H.;
Ohba, S.; Galsbøl, F.; Fujita, J. Bull. Chem. Soc. Jpn. 1994, 67, 1013.
(26) Sullivan, B. P.; Meyer, T. J. J. Chem. Soc., Chem. Commun. 1984,
403.
(30) Kahl, J. L.; Hanck, K. W.; DeArmond, K. J. Phys. Chem. 1978,
82, 540.
(31) Coombe, V. T.; Heath, G. A.; MacKenzie, A. J.; Yellowlees, L. J.
Inorg. Chem. 1984, 23, 3423.

Iridium(III) Bisterpyridine and Its Homologues
J. Am. Chem. Soc., Vol. 121, No. 21, 1999 5013
Table 2. X-ray Experimental Data of Complex 1
formula C30H22N6Ir‚3PF6
molecular weight
1093.64
crystal system
space group
monoclinic
P121/c1
a (Å)
b (Å)
19.2566(2)
16.3846(2)
c (Å)
11.2328(1)
98.349(8)
3506.5(2)
â (deg)
V (Å3)
Z
color
4
yellow
crystal dimens (mm)
0.20 × 0.15 × 0.10
D
calc (g cm-3
)
2.07
F000
2112
4.055
173
µ (mm-1
)
temp (K)
wavelength (Å)
radiation
diffractometer
scan mode
0.710 73
Mo KR graphite-monochromated
KappaCCD
φ scans
Figure 1. ORTEP representation of the X-ray structure of 1. The
solvent molecules, PF6 anions, and hydrogens have been omitted for
clarity. Thermal ellipsoids are scaled to enclose 30% of the electronic
density. Selected distances (Å) and angles (deg): Ir-N(1), 2.054; Ir-
N(2), 1.979; Ir-N(3), 2.056; Ir-N(4), 2.058; Ir-N(5), 1.978; Ir-N(6),
2.056. N(1)-Ir-N(2), 80.3; N(1)-Ir-N(3), 160.2; N(2)-Ir-N(3),
80.0; N(1)-Ir-N(4), 92.4; N(1)-Ir-N(5), 100.4; N(1)-Ir-N(6), 91.4;
N(2)-Ir-N(4), 97.9; N(2)-Ir-N(5), 177.8; N(2)-Ir-N(6), 101.7.
hkl limits
0.13/0.22/ -24.24
2.5/29.55
34 171
θ limits (deg)
no. of data measd
no. of data with I > 3σ(I)
weighting scheme
no. of variables
R
7436
4Fo /(σ2(Fo ) + 0.0064Fo )
2
2
4
595
0.034
0.052
1.115
0.798
Rw
GOF
largest peak in final difference
The oxidation of Ir(III) into Ir(IV) was not observed, probably
taking place out of the potential range we used (from -1.4 to
+2.4 V vs SCE, scan rate 200 mV/s). All waves were
irreversible, likely due to adsorption of the reduced species,
except for complex 2 in DMF.
(e Å-3
)
X-ray Structure of Complex 1. Pale yellow single crystals
of 1 were obtained by slow diffusion of benzene into an acetone
solution of the complex (1:3 (v/v) acetone/benzene). The
ORTEP representation of 1 is shown in Figure 1. By comparison
with the most relevant X-ray structures of Ir(III) complexes in
the literature (Ir(bipy)3(ClO4)‚2.33H2O32 and Ir(2,3,5,6-tetrakis-
(2-pyridyl)pyrazine)Cl333), one sees that the Ir-N distances and
the N-Ir-N angles found in 1 are very close to the previously
reported data. Ir-N average bond lengths are 1.980 Å for the
central pyridine rings and 2.056 Å for the peripheral cycles.
The angle between the two ortho substituents on the central
pyridine ring is 102.5°, which reflects a relatively distorted
coordination octahedron around Ir(III).
Figure 2. Room-temperature absorption spectra of acetonitrile solutions
of 1-4.
Spectroscopic and Photophysical Properties. The ground-
state absorption spectra of the investigated complexes are
displayed in Figure 2. Table 3 reports the absorption maxima
and intensities. The energy onset and the resolved profiles
characterized by peaks with extinction coefficients of the order
of 104-105 M-1 cm-1 are, as expected for predominantly ligand-
centered transitions (1LC), consistent with previous reports of
Ir(III) complexes of bpy and tpy ligands.13-15 The absorption
intensities are slightly different in the series, presumably due
to the different number of aromatic rings present in the
complexes (Table 3). This seems to be in agreement with the
fact that the least intense spectrum in the series is that for 1,
where the coordinated ligand is unsubstituted tpy. A similar line
of reasoning can likely be used for the energy onset: the onset
for 1 corresponds to a higher energy level than for the other
cases, which presumably may be ascribed to the smaller
electronic delocalization expected for the case of tpy with respect
to a 4′-aryl-2,2′:6′,2′′-terpyridine.34
Table 3. Ground-State Absorption Maxima and Intensitiesa
λ
max/nm (ꢀ/M-1 cm-1
)
1
2
3
4
210 (21 400); 221 (18 500); 251 (27 600); 277 (22 200); 313
(11 400); 325 (11 800); 336 (8500); 352 (5800); 374 (1300)
211 (77 000); 251 (55 000); 277 (43 300); 304 (42 200); 320
(39 500); 346 (31 400); 372 (23 800)
205 (77 400); 251 (62 300); 278 (45 900); 305 (46 200); 321
(43 000); 347 (36 800); 373 (29 000)
209 (48 000); 251 (52 000); 279 (43 200); 310 (31 900); 322
(29 500); 340 (23 300); 354 (18 000); 372 (13 200)
a Acetonitrile solvent.
Another feature of the spectral profiles of Figure 2 is the
absorption tail on the low-energy side, which is apparent for
all complexes but less pronounced for 1. In principle, this tail
could be due to (i) contribution from low-energy metal-to-ligand
charge transfer (1MLCT) transitions, (ii) formally forbidden
(34) (a) Damrauer, N. H.; Boussie, T. R.; Devenney, M.; McCusker, J.
K. J. Am. Chem. Soc. 1997, 119, 8253. (b) Damrauer, N. H.; Weldon, B.
T.; McCusker, J. K. J. Phys. Chem. A 1998, 102, 3382.
(32) Hazell, A. C.; Hazell, R. G. Acta Crystallogr. 1984, C40, 806.
(33) Vogler, L. M.; Scott, B.; Brewer, K. J. Inorg. Chem. 1993, 32, 898.

5014 J. Am. Chem. Soc., Vol. 121, No. 21, 1999
Collin et al.
2+
2+
Figure 4. Luminescence spectra of Zn(tpy)2 and Zn(ttpy)2 in
butyronitrile solutions at 77 K.
Figure 3. Room-temperature luminescence spectra of air-equilibrated
isoabsorbing acetonitrile solutions of 1-4; λexc was 350 nm.
Table 4. Luminescence Properties and Photophysical Parameters
293 Ka
77 Kb
λc (nm) τd (µs)
10-4 × kr 10-5 × knr
transitions leading to direct population of triplet levels (S0 f
T), because of the heavy atom effect,35 (iii) population of metal-
centered (1MC) levels, or (iv) conjugation effects due to the
aryl group borne by the 4′-position of the tpy nucleus. Assuming
an IrIII/IrIV oxidation potential value of +2.4 V vs SCE or
greater, and given that the tpy ligand reduction occurs at about
-0.8 V vs SCE, we expect the 1MLCT excited state to have an
energy content of roughly 3.2 eV, which corresponds to
absorption around 387 nm or below.
λc (nm) τd (µs) 102 × φe
(s-1
)
(s-1
)
1
2
3
4
458 1.0 (1.2)
506 1.6 (6.8)
506 2.4 (9.5)
506 2.3 (5.8)
2.5
2.2
2.9
2.6
2.5
1.4
1.2
1.1
9.8
6.1
4.0
4.2
458
486
494
490
26
39
39
37
a Acetonitrile solvent. b Butyronitrile solvent. c Band maximum for
the highest-energy peak, uncorrected spectra. d Air-equilibrated samples,
values in parentheses for deaerated solvent. e Quantum yields obtained
from corrected spectra.
Figure 3 shows room-temperature luminescence spectra for
air-equilibrated acetonitrile solutions of the complexes, the
excitation wavelength being λexc ) 350 nm in all cases. The
spectra are vibrationally resolved, and their temporal decay
occurs with lifetime values on the microsecond time scale (see
below). On the basis of these observations, the luminescence is
tentatively ascribed to 3LC excited states, as had been previously
reported for Ir(tpy)2(CF3SO3)3,13a even if some MLCT character
might contribute to the emission. By comparison of the spectral
profiles, one sees that the highest energy band maximum in the
series is that for 1, with λ ) 458 nm, while for 2, 3, and 4, λ
) 506 nm. This result is in agreement with the absorption data
and indicates that the luminescent level of 1 is higher in energy
by ∼2000 cm-1 with respect to the other complexes.
To better understand the nature of the luminescent levels of
the studied complexes, we have obtained the 77 K luminescence
2+
spectra for butyronitrile solutions of Zn(tpy)22+ and Zn(ttpy)2
(Figure 4).
The luminescence profiles in both cases, expected to conform
to pure LC emissions from tpy and ttpy ligands,13a were closely
similar to that exhibited by 1 but somewhat different from the
spectra exhibited by 2-4. This seems to indicate that the
luminescence of the complexes containing an aryl tpy ligand is
not of pure LC nature and might involve excited states of MLCT
nature.
Room-temperature luminescence data, together with derived
photophysical parameters and some luminescence results ob-
tained at 77 K, are gathered in Table 4. From these it can be
seen that (i) the energy stored within the excited state is 2.7 eV
for 1 and 2.5 eV for 2-4, (ii) for all complexes, the room-
temperature luminescence quantum yields and lifetimes obtained
in air-equilibrated solvent are φ ≈ 10-2 and 1-2 µs, respec-
tively,38 and (iii) at 77 K and with respect to room temperature,
(a) the luminescence lifetime increases by ca. 1 order of
magnitude in each case and (b) the highest energy band
maximum is unchanged for 1 while, by contrast, it undergoes
a slight hypsochromic shift for 2-4.
To further characterize the excited states of the complexes,
the transient absorption spectra were measured. The spectra, as
detected at the end of a 35-ps laser pulse for the different
complexes in acetonitrile solutions in comparable experimental
conditions, are shown in Figure 5. Compounds 2-4 display the
same spectral profile with a broad band around 650-700 nm,
while complex 1 has a much weaker transition toward the UV
side of the spectrum.
It can be noted that the room-temperature luminescence
profiles of 2-4 closely overlap each other and that they are
less vibrationally resolved than the spectrum of 1. The difference
in energy and shape between the luminescence spectra for 1
and the other complexes can likely be ascribed to the predomi-
nant LC nature of the excited states. Actually, (i) for 1 the LC
excited state is to be restricted on the tpy frame, while, on the
contrary, for 2-4 a larger ligand framework is available and
(ii) for the latter complexes, the dihedral angle between the
proximate rings of the tolyl and terpy fragments is ∼30° in the
solid state.36 In solution, however, thermal distribution of
conformers might result in less resolved luminescence spectra
(Figure 3). Other explanations for the difference in energy and
spectral shape between complex 1 on one hand and complexes
2-4 on the other hand could be based on the presence of
energetically accessible excited states of MLCT nature, which
are known to exhibit unresolved luminescence bands (the MC
excited states can reasonably be located above 3.5 eV and,
therefore, can be excluded as a deactivation pathway).37a,b
The decay of the transients in deaerated acetonitrile solutions
was a single exponential with lifetimes of 1.1 µs for 1, 6 µs for
(37) (a) Roundhill, D. M. Photochemistry and Photophysics of Metal
Complexes; Plenum: New York, 1994. (b) Horva´th, O.; Stevenson, K. L.
Charge Transfer Photochemistry of Coordination Compounds; VCH:
Weinheim, 1993. (c) Demas, J. N.; Harris, E. W.; McBride, R. P. J. Am.
Chem. Soc. 1977, 99, 3547.
(35) (a) Pankuch, B. J.; Lacky, D. E.; Crosby, G. A. J. Phys. Chem.
1980, 84, 2061. (b) Kober, E. M.; Marshall, J. L.; Dressick, W. J.; Sullivan,
B. P.; Caspar, J. V.; Meyer, T. J. Inorg. Chem. 1985, 24, 2755.
(36) (a) Kim, Y.; Lieber, C. M. Inorg. Chem. 1989, 28, 3990. (b) Helms,
H.; Heiler, D.; McLendon, G. J. Am. Chem. Soc. 1991, 113, 4325.
(38) Ir(tpy)2(CF3SO3)3 was reported to exhibit τem ) 70 ns; see ref 13a.

Iridium(III) Bisterpyridine and Its Homologues
J. Am. Chem. Soc., Vol. 121, No. 21, 1999 5015
2, 9.5 µs for 3, and 6.6 µs for 4, in good agreement with the
luminescence lifetimes of the excited state (Table 4). The effect
of air on the lifetimes of the transients is in full agreement with
the luminescence data. Therefore, the observed bands are
assigned to a triplet excited state of the complexes, also
responsible for their luminescence.
The excited states of the complexes are quenched by oxygen
with a rate which can be derived from the data of Table 4. This
is of the order of 108 M-1 s-1, in agreement with previously
reported values,13a and somehow smaller than the quenching
constants of other polypyridine transition metal complexes.37a,c
The poor spectral overlap between the donor’s emission and
acceptor’s absorption, and the unfavorable driving force for
electron transfer to dioxygen, could be the reason for such
inefficient quenching.
Figure 5. Absorption changes at the end of a 35 ps laser pulse (355
nm, 2.5 mJ) detected in acetonitrile solutions of 1-4. Absorbance of
the solutions at the excitation wavelength was 0.9.
The effect of counterion on the photophysical properties of
-
the complexes has been considered for compound 3, with NO3
as the anion. All the photophysical properties detailed above
-
for compound 3 with PF6 anion were maintained, within
-
experimental uncertainty, by the corresponding NO3 salt.
Temperature Dependence of Luminescence. Following an
approach employed in previous studies of luminescent Ru- and
Os-tpy-type complexes,17a,20,39,40 we further investigated the
possible role of the interplay of excited states involved in the
luminescence of complexes 1-4.
In general terms, the room-temperature photophysical be-
havior of a luminophore is governed by the electronic properties
of its lowest-lying excited state; however, it may be affected
by higher-lying energy levels. For instance, for the largely
studied Ru(II)-polypyridine complexes, the luminescence level
3
3
is of MLCT nature, but thermally accessible MC levels play
an important role.20,40 This is because the potential energy curve
for MC states is strongly displaced with respect to that of the
ground state, which results in their effective coupling and, in
turn, in fast deactivation pathways for the MLCT state.
Temperature-dependent studies of the luminescence may
provide the energy gap, ∆E, between the luminescent and
thermally accessible higher-lying levels, together with the related
kinetic parameter, A, in terms of Arrhenius-like equations (eq
2), where ko is a low-temperature limiting value.40,41
Figure 6. Luminescence spectra of butyronitrile solutions of 1 (top
panel) and 3 (bottom panel) in the 95-180 K range.
sharper in degassed solvent (Table 4). Given that the energy
level of the emissive state of 1 is some 2000 cm-1 higher in
energy than the corresponding levels of the other complexes, it
may well be that the energy layout of higher-lying states plays
a different role in the two cases.
To get a picture of the possible interplay of excited states in
the two cases, we have performed temperature-dependent studies
on the luminescence properties of complexes 1 and 3. In Figures
6 and 7 are collected luminescence spectra and luminescence
lifetimes of ∼10-5 M deaerated butyronitrile solutions of the
two complexes, as registered against the temperature.
-∆E
RT
1/τ ) A exp
+ ko
(2)
(
)
In the present cases, the luminescent level is probably of
predominant 3LC nature, as discussed above, and the lumines-
cence lifetime is τ g 1 µs. It may be noted that the lifetime of
complex 1 is significantly shorter than that of 2, 3, and 4 (ca.
1 vs. 2 µs in air-equilibrated solvent), the difference being
From Figure 6, it can be seen that the energy position of 3 is
affected by the temperature, showing a bathochromic shift (by
∼600 cm-1) on going from glassy to fluid solvent, i.e., in the
110-150 K interval, which includes the glass-to-fluid transition
temperature of the solvent (Tg ≈ 125 K).12,40 This behavior
usually indicates that the luminescent excited state is coupled
with nuclear rearrangements which are affected by the state of
the solvent. By contrast, no energy variation is registered for
the spectrum of 1. One may further notice that, for both cases,
the spectral shape is not affected by the temperature nor by the
rigidity of the solvent. An explanation for these observations
could be based on the role of torsional rearrangements, expected
for 3 but not for 1, between the plane of the tolyl ring and that
of the terpyridine fragment. Actually, it is known that, for the
ground state of a biphenyl-like fragment, this dihedral angle is
(39) Collin, J.-P.; Guillerez, S.; Sauvage, J.-P.; Barigelletti, F.; De Cola,
L.; Flamigni, L.; Balzani, V. Inorg. Chem. 1992, 31, 4112.
(40) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.;
von Zelewsky, A. Coord. Chem. ReV. 1988, 84, 85.
(41) (a) Meyer, T. J. Pure Appl. Chem. 1986, 58, 1193. (b) Meyer, T. J.
Acc. Chem. Res. 1989, 22, 163. (c) Macatangay, A.; Zheng, G. Y.; Rillema,
D. P.; Jackman, D. C.; Merkert, J. W. Inorg. Chem. 1996, 35, 6823. (d)
Islam, A.; Ikeda, N.; Nozaki, K.; Ohno, T. Chem. Phys. Lett. 1996, 263,
209. (e) Wallace, L.; Jackman, D. C.; Rillema, D. P.; Merkert, J. W. Inorg.
Chem. 1995, 34, 5210. (f) Hughes, H. P.; Vos, J. G. Inorg. Chem. 1995,
34, 4001. (g) Maruszewski, K.; Kincaid, J. R. Inorg. Chem. 1995, 34, 2002.
(h) Maruszewski, K.; Bajdor, K.; Strommen, D. P.; Kincaid, J. R. J. Phys.
Chem. 1995, 99, 6286. (i) Sykora, M.; Kincaid, J. R. Inorg. Chem. 1995,
34, 5852. (j) Wang, R.; Vos, J. G.; Schmehl, R. H.; Hage, R. J. Am. Chem.
Soc. 1992, 114, 1964. (k) Barigelletti, F.; De Cola, L.; Juris, A. Gazz. Chim.
Ital. 1990, 120, 545. (l) Lumpkin, R. S.; Kober, E. M.; Worl, L. A.; Murtaza,
Z.; Meyer, T. J. J. Phys. Chem. 1990, 94, 239. (m) Ryu, C. K.; Schmehl,
R. H J. Phys. Chem. 1989, 93, 7966. (n) Kawanishi, Y.; Kitamura, N.;
Tazuke, S. Inorg. Chem. 1989, 28, 2968.

5016 J. Am. Chem. Soc., Vol. 121, No. 21, 1999
Collin et al.
exponential factor A is of the order of 1012 s-1 40,41
.
By contrast,
for both 1 and 3, A , 1012 s-1 (Table 5). This result suggests
that the thermally accessible energy level is not responsible for
fast deactivation paths. As discussed above, for 1 and probably
also for 3, the luminescent level is of predominant 3LC nature,
and the fact that thermal population of an upper-lying level,
which might be of MLCT nature, does not cause a substantial
enhancement of the decay rate suggests that this latter level is
not an efficient doorway for radiationless deactivation.
Conclusion
Many examples of multicomponent systems containing poly-
pyridine complexes of the Ru(II), Os(II), Rh(III), and Ir(III)
families are known.3,11 As mentioned above, from the viewpoint
n+
of the control of the geometric properties, the use of M(tpy)2
Figure 7. Luminescence lifetimes of butyronitrile solutions of 1 and
3 in the 95-298 K interval. The full lines are the result of a fitting
procedure according to eq 3 of text.
templating units offers remarkable advantages because the
introduction of substituents at the 4′-terpyridine position allows
the development of linearly arranged arrays.3,6,7,10,12,42 In view
of photoinduced electron transfer, drawbacks are predicted upon
Table 5. Kinetic Parameters for Excited-State Decaya
A (s-1
)
∆E (cm-1
)
k0 (s-1
)
B (s-1
)
TB (K)
2+
2+
the use of templating units of the Ru(tpy)2 and Os(tpy)2
1
3
7.6 × 109
1790
1040
4.2 × 104
1.7 × 105
197
215
types: the former chromophore exhibits a short-lived MLCT
excited state (<1 ns), and the latter can store excitation energy
at a low energy level (∼1.6 eV).12 In this respect, both units
are unsuited to play a templating role for the assembly of linearly
arranged arrays because they represent an energy trap interposed
between the other two active units. By contrast, for both types
of systems shown in Chart 2, the series of the presently
investigated Ir(III) complexes offers a good combination of
advantages, given that (i) a synthetic procedure is now available
which makes such complexes relatively accessible, in particular
asymmetrical systems due to the two-step synthesis described
here, (ii) their excited states levels are located above 2.5 eV,
and (iii) their decay occurs on the microsecond time scale even
in air-equilibrated solutions. For the multicomponent arrays we
plan to develop, these points represent remarkable properties
in order for intercomponent processes to be competitive with
respect to the intrinsic deactivation at the templating center.
2.2 × 107
2.1 × 104
3.9 × 104
a Butyronitrile solvent, degassed samples unless otherwise specified.
Parameters obtained from least-squares nonlinear fitting of eq 3 of the
text to the experimental points of Figure 7.
around 30°, while for the corresponding lowest-lying excited
state, the two fragments are close to planarity.36
In Figure 7 are reported the 1/τ vs 1/T experimental points,
together with the results of a fitting procedure based on the
following equation,12,40
1
τ
-∆E
RT
B
) A exp
+
+ k0 (3)
(
)
1 + exp[C(1/T - 1/TB)]
where A and ∆E are the preexponential factor and the energy
barrier, respectively, for an Arrhenius-type process, B is the
increase in radiationless rate occurring at a certain temperature,
TB, C is an empirical parameter, and k0 is a low-temperature
limiting rate. Results of the numerical analyses based on eq 3
are collected in Table 5.
Acknowledgment. Discussions with Prof. V. Balzani are
acknowledged. This research was supported by the CNRS and
the TMR Research Network, Contract No. FMRX-CT98-0226,
at FRAE-CNR. We thank the French Ministry of Education,
Research and Technology (ID) and the Royal Society for a
fellowship under the European Science Exchange Program
(J.A.G.W.). We thank Andre´ De Cian, Nathalie Kyritsakas, and
Jean Fischer for the X-ray structure. We gratefully acknowledge
Johnson Matthey PLC for a generous loan of IrCl3.
Some interesting points are the following.
(i) In both cases, the lifetime values are not affected by the
glass-to-fluid transition of the solvent (occurring at Tg ≈ 125
K), whereas they are affected by a process taking place at higher
temperature (TB g 200 K) and resulting in an increase in the
nonradiative rate, B. This process appears to be allowed by a
certain degree of fluidity of the solvent and is, therefore, likely
to be due to some type of physical interaction. Self-association
processes should be excluded, on the basis both of the low
concentration employed (∼10-5 M) and of the 3+ charge of
the species. In conclusion, some interaction with the solvent is
likely taking place, the nature of which we are unable to identify
better at the moment.
Supporting Information Available: X-ray data of 1 (PDF).
These data are also available in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
JA9833669
(42) (a) Constable, E. C.; Cargill Thompson, A. M. W. J. Chem. Soc.,
Dalton Trans. 1992, 3467. (b) Constable, E. C.; Cargill Thompson, A. M.
W.; Harveson, P.; Macko, L.; Zehnder, M. Chem. Eur. J. 1995, 1, 360. (c)
Grosshenny, V.; Harriman, A.; Hissler, M.; Ziessel, R. Platinum Metals
ReV. 1996, 40, 72.
(ii) For the case of the well-documented 3MLCT-3MC
coupling occurring in Ru(II)-polypyridine emitters, the pre-