Rodlike fluorescent π-conjugated 3,3′-bipyridazine ligand: Optical, electronic, and complexation properties

Article abstract of DOI:10.1021/ic901925w

We report on the design and synthesis of a new quadrupolar π-conjugated 3,3′-bipyridazine D-A-D ligand. Its electronic and optical properties were investigated. Besides high fluorescence and pronounced solvatochromism, it exhibits an inherent electroactivity exploited to build an organic green light emitting device. Moreover, the ability of this ligand to complex metallic centers (Cu^I, Ni^II, Pt^II, and Ir^III) was also investigated to access different geometries and to tune their electronic and optical properties. These preliminary results open up the synthesis of heavy-metal complexes to obtain phosphorescent emitters.

Full text of DOI:10.1021/ic901925w

Inorg. Chem. 2010, 49, 3991–4001 3991
DOI: 10.1021/ic901925w
Rodlike Fluorescent π-Conjugated 3,30-Bipyridazine Ligand: Optical, Electronic,
and Complexation Properties
†
†
,†
Frederic Lincker, David Kreher, Andre-Jean Attias,* Jaekwon Do,‡ Eunkyoung Kim,‡ Philippe Hapiot,§
ꢀ ꢀ
€
ꢀ
^
X
X
ˆ
Noella Lemaıtre, Bernard Geffroy, Gilles Ulrich, and Raymond Ziessel
†
ꢀ
ꢁ
Universite Pierre et Marie Curie - UMR CNRS 7610 - Laboratoire de Chimie des Polymeres 4,
place Jussieu - Case Courrier 185 F 75252 Paris CEDEX 05, France, ‡Department of Chemical and
Biomolecular Engineering, Yonsei University, 262 Seongsanno, Seodaemun-gu, Seoul 120-749, Korea,
§
ꢀ
Universite de Rennes 1, Sciences Chimiques de Rennes, CNRS UMR 6226 Campus de Beaulieu F 35042 Rennes
ꢀ
CEDEX, France, CEA INES - DRT/LITEN/DTS/LCP Savoie Technolac, 50 av. du Lac Leman,
BP332 F 73377 Le Bourget du Lac CEDEX, France, ^CEA-LITEN, LPICM, Ecole Polytechnique F 91128
Palaiseau, France, and XLaboratoire de Chimie Organique et Spectroscopies Avancees, CNRS, ECPM,
ꢀ
UdS 25 rue Becquerel F 67087 Strasbourg CEDEX, France
Received September 30, 2009
We report on the design and synthesis of a new quadrupolar π-conjugated 3,30-bipyridazine D-A-D ligand. Its
electronic and optical properties were investigated. Besides high fluorescence and pronounced solvatochromism, it
exhibits an inherent electroactivity exploited to build an organic green light emitting device. Moreover, the ability of this
ligand to complex metallic centers (CuI, NiII, PtII, and IrIII) was also investigated to access different geometries and to
tune their electronic and optical properties. These preliminary results open up the synthesis of heavy-metal complexes
to obtain phosphorescent emitters.
Introduction
(phen), 2,20,60,200-terpyridines (tpy), and their derivatives. Be-
sides these systems bearing two or more N-heteroatom rings,
bisdiazine ligands are of special interest because they repres-
ent chelating ligands leading to complexes (Ru(II) complexes
particularly) with higher photostabilities toward metal-
ligand photodissociation, compared to bipyridine analogues.5
This is due to the strong “back-bonding” (good π- acceptor
and σ-donor ability) toward π-electron rich metal fragments,
and to a substantially lower lowest unoccupied mole-
cular orbital (LUMO) energy level by comparison with that
of many of the previous bipyridine derivatives.5a,6 Despite
these interesting features, until now, little attention has
Throughout the past few decades, the development of new
ligands and related coordination compounds has been an
increasing field of research because of their potential applicat-
ions ranging from self-assembly and supramolecular chemis-
try,1 to solar energy conversion and storage,2,3 and more recently
to phosphorescent organic light-emitting diodes (PHOLED).4
Among the great variety of ligands, the most common
chelators are 2,20-bipyridines (bpy), 1.10-phenanthrolines
*To whom correspondence should beaddressed. Phone: þ33(0)1 442753 02.
Fax: þ33 (0)1 44 27 70 89. E-mail: andre-jean.attias@upmc.fr.
(1) (a) Lehn, J.-M. In Supramolecular Chemistry: Concepts and Perspectives;
VCH: Weinheim, Germany, 1995. (b) Lehn, J.-M. Chem. Soc. Rev. 2007, 36,
151–160.
(2) (a) Deisenhofer, J.; Epp, O.; Miki, K.; Huber, R.; Michel, H. Nature
1986, 318, 618–624. (b) Hoertz, P. G.; Staniszewski, A.; Marton, A.; Higgins,
G. T.; Incarvito, C. D.; Rheingold, A. L.; Meyer, G. J. J. Am. Chem. Soc. 2006,
128(25), 8234–8245. (c) Meylemans, H. A.; Lei, C.-F.; Damrauer, N. H. Inorg.
Chem. 2008, 47, 4060–4076. (d) Fukuzumi, S. Eur. J. Inorg. Chem. 2008,
1351–1362.
(3) (a) O’Reagan, B.; Gratzel, M. Nature 1991, 353, 737–740. (b) Jang,
S.-R.; Yum, S.-R.; Klein, C.; Kim, K.-J.; Wagner, P.; Officer, D.; Gratzel, M.;
Nazeeruddin, M. K. J. Phys. Chem. C 2009, 113, 1998–2003. (c) Grabulosa, A.;
Beley, M.; Gros, P. C.; Cazzanti, S.; Caramori, S.; Bignozzi, C. A. Inorg. Chem.
2009, 48, 8030–8036. (d) Subbaiyan, N. K.; Wijesinghe, C. A.; D'Souza, F.
(4) (a) Baldo, M. A.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson,
M. E.; Forrest, S. R. Nature 1998, 395, 151–154. (b) O'Brien, D. F.; Baldo,
M. A.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 1999, 74, 442–444.
(c) Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.; Forrest, S. R.
Appl. Phys. Lett. 1999, 75, 4–6. (d) Tung, Y.-L.; Wu, P.-C.; Liu, C.-S.; Chi, Y.;
Yu, J.-K.; Hu, Y.-H.; Chou, P.-T.; Peng, S.-M.; Lee, G.-H.; Tao, Y.; Carty, A. J.;
Shu, C.-F.; Wu, F.-I. Organometallics 2004, 23, 3745–3748. (e) Lee, S.-J.; Park,
J.-S.; Song, M.; Shin, I. A.; Kim, Y.-I.; Lee, J. W.; Kang, J.-W.; Gal, Y.-S.; Kang,
S.; Lee, J. Y.; Jung, S.-Y.; Kim, H.-S.; Chae, M.-Y.; Jin, S.-H. Adv. Funct. Mater.
2009, 19, 2205–2212.
€
€
(5) (a) Ernst, S.; Kaim, W. Inorg. Chim. Acta 1986, 114, 123–125. (b) D€urr,
H.; Schwarz, R.; Andreis, C.; Willner, I. J. Am. Chem. Soc. 1993, 115, 12362–
12365. (c) Berger, S.; Klein, A.; Kaim, W.; Fiedler J. Organomet. Chem. 1986,
302, 211–215.
€
J. Am. Chem. Soc. 2009, 131, 14646–14647. (e) Gratzel, M.; Durrant, J. R. In
Series on Photoconversion of Solar Energy; Archer, M. D., Nozik, A. J., Eds.;
World Scientific: Singapore, 2009; Vol. 3, pp 503-538.
(6) Ernst, S.; Kaim, W. Angew. Chem. 1985, 97, 431–433.
r
2010 American Chemical Society
Published on Web 04/02/2010
pubs.acs.org/IC

3992 Inorganic Chemistry, Vol. 49, No. 9, 2010
Lincker et al.
Chart 1. Structures of (a) Previous 6,60-(Disubstituted)-3,30-bipyridine,
and (b) Novel 3,30-Bipyridazine Ligand L
core with the more electron withdrawing 3,30-bipyridazine
core (Chart 1b), expecting that the two added nitrogen atoms
should take part in metal binding.
More precisely, we report on the design and synthesis of a
new centrosymmetric π-conjugated 6,60-distyryl-3,30-bipyri-
dazine based ligand L (Chart 1b) with extended conjugation
length, end-capped with terminal dibutylamino groups on
both ends. Thus we created a quadrupolar donor-acceptor-
donor (D-A-D) motif with an expected symmetrical charge-
transfer from the external electron-donor amino groups of
the molecule to the central 3,30-bipyridazine core acting as an
electron-acceptor moiety. We present the electrochemical
and photophysical (absorption and emission) properties of
L, as well as the use of this new compound in a green OLED.
Moreover, the ability of this ligand to complex metallic cen-
ters (CuI, NiII, PtII, and IrIII) was also investigated (Chart 2)
to access different geometries and to tune their electronic and
optical properties.
been given to complexes bearing bisdiazine ligands, more
particularly 3,30-bipyridazine.7 Besides oligo(pyridazine)
structures,8 more surprisingly, to our knowledge, there is
no work related to extended π-conjugated 3,30-bipyridazine
ligand. The potential interest of this design is the expected
tuning of the photophysical and electronic properties by
modulating the conjugation length and the nature of end-
capping groups.
Previously, we developed a new class of multifunctional
extended π-conjugated symmetric and unsymmetric 6,60-(di-
substituted)-3,30-bipyridine based chromophores (Chart 1a).9
Depending on the π-conjugated bridge and by varying the
nature of the acceptor(A)/donor(D) end-capping groups,
we were then able to tune the polymorphism,9a,b electroche-
mical and photoluminescent characteristics, and second- as
well as third order non linear optical (NLO) properties of this
class of chromophores.9c-f As an application, lasing proper-
ties were demonstrated, and a blue-OLED was fabricated.9i
Unfortunately, these materials are not suitable for metal-ion
binding, preventing their use as metal-ion sensing fluoro-
phores or as ligand for the formation of coordination com-
plexes.
Experimental Section
General Considerations. 1H NMR and 13C NMR spectra were
recorded at 250 and 63 MHz respectively. Proton chemical shifts
(δ) are reported in parts per million (ppm) downfield from
tetramethylsilane (SiMe4). Quantum Chemistry Calculations
were performed using the Gaussian 03W package.10 Gas phase
geometries and electronic energies were calculated by full opti-
mization without imposed symmetry of the conformations using
the B3LYP density functional,11 with the 6-31G* basis set,12
starting from preliminary optimizations performed with semi-
empirical AM1 methods. The Onsager radius10b a was then
estimated from the radius of the equivalent sphere with a
molecular volume defined as the volume inside a contour of
3
˚
0.001 electrons/bohr density þ 0.5 A as recommended for the
Onsager solvent reaction field model.10c Cyclic Voltammetry
experiments were performed with a three-electrode setup using a
platinum counter-electrode and a reference electrode. The re-
ference electrode was an aqueous saturated calomel electrode
with a salt bridge containing the supporting electrolyte. The
working electrode was either disks of glassy carbon (L 0.8 mm,
Tokai Corp.), gold (L 1 mm), or platinum (L 1 mm). The
working electrodes were carefully polished before each set of
voltammograms with 1 μm diamond paste and cleansed in an
ultrasonic bath with dichloromethane. The electrochemical
instrumentation consisted of a PAR Model 175 Universal
programmer and of a home-built potentiostat equipped with a
positive feedback compensation device. The data were acquired
with a 310 Nicolet oscilloscope. The potential values were
In this context, here we pursued a new design strategy for
fluorescent ion responsive molecules capable of metal-ion
binding. More specifically, we defined a new class of
π-conjugated ligands by replacing the 3,30-bipyridine acceptor
€
(7) (a) Kropf, M.; Durr, H.; Collet, C. Synthesis 1996, 5, 609–613.
(b) Kropf, M.; Joselevich, E.; D€urr, H.; Willner, I. J. Am. Chem. Soc. 1996,
118, 655–665. (c) Kropf, M.; Van Loyen, D.; Schwarz, O.; D€urr, H. J. Phys.
Chem. A 1998, 102, 5499–5505. (d) Schwarz, O.; Van Loyen, D.; Jockush, S.;
Turro, N. J.; D€urr, H. J. Photochem. Photobiol. A: Chem. 2000, 132, 91–98.
(e) Gardner, J. S.; Strommen, D. P.; Szulbinski, W. S.; Su, H.; Kincaid, J. R.
J. Phys. Chem. A 2003, 107, 351–357.
(8) (a) Baxter, P. N. W.; Lehn, J.-M.; Fisher, J.; Youinou, M.-T. Angew.
Chem., Int. Ed. 1994, 33, 2284–2287. (b) Baxter, P. N. W.; Lehn, J.-M.; Baum,
G.; Fenske, D. Chem.;Eur. J. 2000, 6, 4510–4517. (c) Marquis, A.; Kintzinger,
J.-P.; Graff, R.; Baxter, P. N. W.; Lehn, J.-M. Angew. Chem., Int. Ed. 2002, 41,
2760–2764. (d) Bouffard, J.; Eaton, R. F.; M€uller, P.; Swager, T. M. J. Org. Chem.
2007, 72, 10166–10169.
(10) (a) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery Jr., J. A.; Vreven, T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,
A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Ragha-
vachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.;
Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Challacombe, M. Gill, P. M. W.; Johnson, B.;
Chen, W.; Wong, M. W.; Gonzalez, C. Pople, J. A. Gaussian 03, Revision
B.04; Gaussian, Inc.: Pittsburgh, PA, 2003. (b) Onsager, L. J. Am. Chem. Soc.
1936, 58, 1486–1493. (c) Wong, M. W.; Wiberg, K. B.; Frisch., M. J. J. Am.
Chem. Soc. 1992, 114, 1645–1652.
(9) (a) Attias, A.-J.; Cavalli, C.; Bloch, B.; Guillou, N.; Noel, C. Chem.
Mater. 1999, 11, 2057–2068. (b) Attias, A.-J.; Hapiot, P.; Wintgens, V.; Valat, P.
Chem. Mater. 2000, 12, 461–471. (c) Lemaitre, N.; Attias, A.-J.; Ledoux, I.;
ꢀ
Zyss, J. Chem. Mater. 2001, 13, 1420–1427. (d) Cherioux, F.; Attias, A.-J.;
Maillotte, H. Adv. Funct. Mater. 2002, 12, 203–208. (e) Chen, Q.; Sargent, E. H.;
Leclerc, N.; Attias, A.-J. Appl. Phys. Lett. 2003, 82, 4420–4422. (f) Chen, Q.;
Sargent, E. H.; Leclerc, N.; Attias, A.-J. Appl. Opt. 2003, 42, 7235–7241.
(g) Leclerc, N.; Serieys, I.; Attias, A.-J. Tetrahedron Lett. 2003, 44, 5879–5882.
(h) Leclerc, N.; Galmiche, L.; Attias, A.-J. Tetrahedron. Lett. 2003, 44, 5883–
5887. (i) Leclerc, N.; Sanaur, S.; Galmiche, L.; Mathevet, F.; Attias, A.-J.; Fave,
J.-L.; Roussel, J.; Hapiot, P.; Lemaitre, N.; Geffroy, B. Chem. Mater. 2005, 17,
502–513.
(11) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.
(12) Hariharan, P. C.; Pople, J. A. Chem. Phys. Lett. 1972, 16, 217–219.

Article
Inorganic Chemistry, Vol. 49, No. 9, 2010 3993
Chart 2. Complexes Structures
internally calibrated against the ferrocene/ferricinium couple (Eꢀ
=0.405 V vs SCE) for each experiment. All the cyclic voltam-
metry experiments were carried out at 20 ꢀC using a cell
equipped with a jacket allowing circulation of water from the
thermostat. Oxygen was removed from all solutions by bubbling
argon. The solvent was a toluene/acetonitrile mixture (50/50 v/v),
and the supporting electrolyte was tetrabutylammonium tetra-
determined using the equation
!
2
ꢀ
ꢁ
ꢀ
ꢁ
Iref
I
n
Aref
A
φ ¼ φref
nref
where φ is the relative quantum yield of sample, I is the
integrated intensity of fluorescence spectrum of sample, A is
the intensity of absorbance spectrum of sample, and n is the
refractive index. The subscript ref refers to the value of the
reference fluorescence material of known quantum yield.
Luminescence quantum yields in the 650-800 nm region
were measured at room temperature in Argon deaerated solu-
fluoroborate (Fluka, Puriss) at a concentration of 0.1 mol L-1
.
3
Acetonitrile was from Merck, (Uvasol quality less than 0.01%
of water) and toluene from Prolabo (R.P. Normapur). The
UV-visible absorption spectra were obtained in different solvents
and solvent mixtures with Avaspec spectroscopy (S:Avaspec-
2048, D:Avalight-DHS). Appropriate concentrations (absorb-
ance below 0.2 au) were chosen to avoid inner filter effects. The
fluorescence emission spectra in the 450-800 nm region were
obtained from the same sample used for absorption measurements,
using a Luminescence spectrometer ‘LS55’ (PerkinElmer). To
minimize the atmospheric H2O and O2 contamination in the
sample, argon gas was purged through the solution before
tion relative to Cresyl violet in ethanol (φF = 0.51; λEX
=
578 nm),14 and tetramethoxy-bis-isoindolodipyrromethene-di-
fluoroborate in dichloromethane (φF =0.41; λEx =660 nm).15
Quantum yields were corrected for the change in refractive index
between the chosen solvent and dichloromethane solution.
Luminescence studies for the platinum and iridium complexes
were made with a Jobin-Yvon Fluoromax 4 spectrometer and
the emission spectra were automatically corrected from the
detector response. As given by the constructor the correction
is less accurate above 850 nm. The configuration of the OLED
measurements. The scan speed was fixed at 500 nm min-1
.
Molar absorption coefficients (εmax) were determined using
the Beer-Lambert law. Fluorescence quantum yields were
determined in different solvents by using 2,4,6-triphenylpyry-
lium tetrafluoroborate (in dichloromethane under argon, φref
=
0.58) as a reference.13 The quantum yield of the sample was
(14) Olmsted, J., III J. Phys. Chem. 1979, 83, 2581–2584.
(15) Ulrich, G.; Goeb, S.; De Nicola, A.; Retailleau, P.; Ziessel, R.
Synlett. 2007, 10, 1517–1520.
(13) Melhuish, W. H. J. Phys. Chem. 1961, 65, 229–235.

3994 Inorganic Chemistry, Vol. 49, No. 9, 2010
Lincker et al.
Figure 1. Device structure, energy level diagram of the OLED, and chemical structure of its components.
18
device, depicted in Figure 1 was ITO/CuPc/NPB/(DPVBi:L
(ꢀ wt %))/Alq3/LiF/Al. The chemical structure and the high-
est occupied molecular orbital (HOMO) and the LUMO en-
ergy levels of the diode components are also presented in
Figure 1.16 CuPc is copper phthalocyanine as a buffer layer,
R-NPB is N,N0-bis(naphthalen-1-yl)-N,N0-bis(phenyl)benzidine
as a hole transporting layer, DPVBi is 4,40-bis(2,2-diphenyl-
vinyl)-1,10-biphenyl as a host, Alq3 is tris(8-hydroxyquinolato)
aluminum as an electron transporting layer. The EL device was
fabricated as follow. ITO-coated glass substrates (with a sheet
resistance of 10 Ω/sq) were first cleaned by successive ultrasonic
treatments in detergent, deionized water, and ethanol. After that
chemical cleaning, the substrates were treated by irradiation in a
UV/Ozone chamber for 30 mn. The organic materials (from
Aldrich and Syntec) and the cathode LiF/Al were sequentially
deposited by sublimation under high vacuum (<10-6 Torr) at a
rate of 0.2-0.3 nm s-1. The entire device is fabricated in the
same run without breaking the vacuum. The active area of the
devices defined by the overlap of the ITO anode and the metallic
cathode was 0.3 cm2. The emissive layer of (DPVBi:L) was made
by co-evaporation of the two products from different sources.
The I-V-L characteristics of the diodes were measured with a
regulated power supply (ACT 100 Fontaine) combined with a
multimeter (I-V) and a 1 cm2 area silicon calibrated photodiode
(Hamamatsu). The spectral emissions (Commission Internationale
de l’Eclairage (CIE) coordinates) were recorded with a spectro-
photometer (SpectraScan PR650). All the measurements were
performed under ambient conditions. All reagents were pur-
chased from commercial suppliers and used without further
purification. Flash chromatography was carried out with silica
gel (Merck, Si 60, 40-63 μm). Thin layer chromatography
(TLC) was performed with use of aluminum silica gel sheets
(60 F254, Merck). [Pt(DMSO)2Cl2],17 [Ir(C∧N)2Cl]2 were
synthesized according to the literature procedures and yielded
satisfactory mass and 1H NMR spectra.
Ligand Synthesis. 4,40-(1E,10E)-2,20-(3,30-bipyridazine-6,60-
diyl)bis(ethene-2,1-diyl)bis(N,N-dibutylaniline) (L). A solution of
6,60-dimethyl-3,30-bipyridazine (4 g, 21.48 mmol), 4-(dibutylamino)-
benzaldehyde (10.5 g, 45.00 mmol), and catalytic amount of
4-methylbenzenesulfonic acid (20 mg, 0.12 mmol) was stirred under
N2 at reflux. After 12 h, another portion of 4-methylbenzenesul-
fonic acid (20 mg, 0.12 mmol) was then added, and the mixture was
stirred at reflux for an additional 12 h. The reaction mixture was
then cooled to room temperature, and solvent was removed under
vacuum. The organic layer was washed with water, dried over
MgSO4, and evaporated to dryness. Column chromatography
(SiO2, Et2O/Hexane =3/97) and recrystallization in EtOH gave
L as an orange solid with 25% yield. 1H NMR (250 MHz, CDCl3):
δ 0.99 (t, 12H, 3J(H,H)=6.9 Hz), 1.44 (qt, 8H, 3J(H,H)=6.9 Hz,
3J(H,H) = 6.9 Hz), 1.61 (tt, 8H, 3J(H,H) = 6.9 Hz), 3.32 (t,
3
8H, 3J(H,H)=6.9 Hz), 6.66 (d, 4H, J(H,H)=9.3 Hz), 7.17 (d,
2H, 3J(H,H)=16.1 Hz), 7.50 (d, 4H, 3J(H,H)=9.3 Hz), 7.68 (dd,
2H, 3J(H,H)=9.3 Hz), 7.71 (d, 2H, 3J(H,H)=16.1 Hz), 8.70 (d,
2H, 3J(H,H)=9.3 Hz); 13C{1H} NMR (100.6 MHz, CDCl3): δ:
14.0, 20.4, 26.0, 50.7, 110.7, 118.7, 123.3, 13.9, 124.0, 129.3, 136.3,
148.7, 154.0, 158.3; Elem anal. Found (%): C, 77,94; H, 8.61;
N, 13.45. calcd for C40H52N6: C, 77.88; H, 8.50; N, 13.52. mp =
214.5 ꢀC.
Complexes Synthesis. Complex [L2Cu]PF6. A suspension of
the free bipyridazine based ligand L (171 mg, 0.277 mmol) in
anhydrous DMF (10 mL) was stirred under N2 at 60 ꢀC. After
30 min, a solution of Cu(MeCN)4PF6 (52 mg, 0.139 mmol) in
anhydrous DMF (2 mL) was then added, and the mixture was
stirred at reflux for 12 h. The reaction mixture was then cooled to
room temperature, and solvent was removed under vacuum.
The crude product was taken up in acetone (5 mL) and was
(16) Hosokawa, C.; Higashi, H.; Nakamura, H.; Kusumoto, T. Appl.
Phys. Lett. 1995, 67, 3853–3855.
(17) Price, J. H.; Birk, J. P.; Wayland, B. B. Inorg. Chem. 1978, 17,
2245–2250.
(18) Sprouse, S.; King, K. A.; Spellane, P. J.; Watts, R. J. J. Am. Chem.
Soc. 1984, 106, 6647–6653.

Article
Inorganic Chemistry, Vol. 49, No. 9, 2010 3995
Scheme 1. Synthetic Route of L
precipitate in hexane (100 mL). Several precipitations with
acetone/hexane (v/v=1/20) gave [L2Cu]PF6 as a dark purple
solid in 95% yield. Elem anal. found (%): C, 66.41; H, 7.00; N,
11.32. calcd. for C80H104N12PF6Cu: C, 66.62; H, 7.27; N, 11.65; P,
2.15; F, 7.90; Cu, 4.41. MS ESIþ m/z: calcd for C80H104N12Cuþ
1295.78 Da found 1295.9 Da.
Complex [L3Ni](NO3)2. A suspension of the free bipyridazine
based ligand L (110 mg, 0.178 mmol) in anhydrous DMF (8 mL)
was stirred under N2 at 60 ꢀC. After 30 min, a solution of Ni-
(NO3)2 6H2O (17.3 mg, 0.059 mmol) in anhydrous DMF (3 mL)
3
was then added, and the mixture was stirred at reflux for 12 h.
The reaction mixture was then cooled to room temperature, and
solvent was removed under vacuum. The crude product was
taken up in acetone (5 mL) and was precipitated in diethylether
(100 mL). Several precipitations with acetone/diethylether (v/v=
1/20) gave [L3Ni](NO3)2 as a dark purple solid in 90% yield.
Elem anal. found (%): C, 69.39; H, 7.52; N, 13.27. calcd. for
C120H156N20O6Ni 2H2O: C, 69.65; H, 7.79; N, 13.54. MS ESIþ
3
m/z: calcd for C120H156N18Ni2þ 954.11 Da found 954.3 Da.
Complex [LPtCl2]. A Schlenk flask was charged with the
appropriate [Pt(DMSO)2Cl2] precursor (94 mg, 0.223 mmol),
ligand L(110 mg, 0.178 mmol), and tetrahydrofuran (THF, 15 mL).
Thesolutionwasdegassedfor15minwithargonandheatedat60ꢀC
during 2 h. During the course of the reaction the color of the solution
progressively turned dark purple and after a short time thin layer
chromatography revealed the absence of free ligand. After cooling
the solution the solvent was rotary evaporated, and the residue was
purified by column chromatography, on flash silica using a mixture
of CH2Cl2/methanol as mobile phase (gradient from 0.2 to 1.0%
methanol). Recrystallization was ensured by slow diffusion of die-
thylether in a dichloromethane solution. Complex [LPtCl2] was
obtained as a dark blue solid (145 mg, 92%). 1H NMR (400.1 MHz,
d2-dichloromethane): δ 7.97 (d, 3J=8 Hz, 2H), 7.89 (d, 3J=8 Hz,
2H), 7.59 (d, 3J=16 Hz, 2H), 7.38 (d, 3J=8 Hz, 4H), 6.81 (d, 3J=
16 Hz, 2H), 6.56 (d, 3J=8 Hz, 4H), 3.26 (d, 3J=7.5 Hz, 8H), 1.57 (m,
Figure 2. Optimized B3LYP/6-31G* geometry for L analogue (N(Ethyl)2
terminal groups). (a) Numbers are the adjacent dihedral angles; (b) HOMO
and (c) LUMO molecular orbitals.
7.64 (d, 3J=8.8 Hz, 2H), 7.22 (d, 3J=15.8 Hz, 2H), 7.49 (d, 3J=
8.0 Hz, 4H), 7.18 (d, 3J=8 Hz, 4H), 7.03 (d, 3J=8.8 Hz, 4H), 6.63
(d, 3J=15.8 Hz, 2H), 6.42 (d, 3J=8.8 Hz, 4H), 3.21 (t, 3J=7.5 Hz,
8H), 2.44 (s, 6H), 1.55 (m, 8H), 1.34 (m, 8H), 0.97 (t, 3J=7.5 Hz,
12H); 13C{1H} NMR (100.6 MHz, d2-dichloromethane): δ 159.9,
153.6, 149.5, 139.0, 135.0, 131.8, 130.1, 128.8, 127.1, 125.9, 125.7,
122.2, 116.4, 111.1, 102.8, 86.9, 50.6, 29.4, 21.1, 20.2, 13.7 ; ES-MS m/z
(nature of the peak, relative intensity): 1043.3 ([MþH]þ, 100),
1042.3 ([MþH]þ, 100), 1041.3 ([MþH]þ, 60) ; Anal. Calcd for
C58H66N6Pt: C, 66.84; H, 6.38; N, 8.06. Found: C, 66.52; H,
6.21; N, 7.84 ; UV-vis (CH2Cl2) λ(nm), ε(M-1 cm-1): 267 (31200),
330 (18100), 498 (sh, 34700), 556 (38800).
Complex [LIr(C∧N)2]BF4. A Schlenk flask was charged with
L (66 mg, 0.107 mmol), and the μ-chloro-Ir-dimer [Ir(C∧N)2Cl]2
(57 mg, 0.053 mmol), and dichloromethane (10 mL). The solu-
tion was degassed for 10 min with argon and heated at 50 ꢀC.
TLC was used to follow the progress of the reaction. During the
course of the reaction the color of the solution progressively
turned deep-red. After one night the reaction was quenched with
water and the solution evaporated to dryness. The residue was
dissolved in the minimum N,N-dimethylformamide (DMF, 5 mL)
and dropwise added via a pipette filled with Celite to an aqueous
solution (10 mL) containing NaBF4 (500 mg). During the ad-
dition a voluminous precipitate formed. After 1 h stirring at
room tempertaure (rt), the suspension was filtered over paper
and washed with water (3ꢀ100 mL). Natural drying was ensured
under the fumehood. Purification was performed by column
chromatography, on alumina deactivated with 6% water using a
mixture of CH2Cl2/methanol as mobile phase (gradient from 1
to 1.5% methanol). Recrystallization was ensured by slow
diffusion of diethylether in a dichloromethane solution. The
[LIr(C∧N)2]BF4 complex was obtained as a dark violet solid
3
8H), 1.37 (m, 8H), 0.97 (t, J = 7.5 Hz, 12H) ; 13C{1H} NMR
(100.6 MHz, d2-dichloromethane): δ 160.1, 153.8, 149.3, 135.2,
130.5, 128.8, 125.7, 122.5, 116.4, 111.4, 52.6, 29.4, 20.3, 13.7 ; ES-
MS m/z (nature of the peak, relative intensity): 884.3 ([MþH]þ,
100), 883.3 ([MþH]þ, 90), 881.2 ([MþH]þ, 70); Anal. Calcd for
C40H52N6PtCl2: C, 54.42; H, 5.94; N, 9.52. Found: C, 54.19; H, 5.67;
N, 9.32; UV-vis (CH2Cl2) λ (nm), ε (M-1 cm-1): 255 (12500), 330
(20200), 488 (32300), 608 (48000).
Complex [LPt(-ꢁ-Ph)2]. A Schlenk flask was charged with
[LPtCl2] (65 mg, 0.074 mmol), and tolylacetylene (36 mg,
0.309 mmol), and a mixture of dichloromethane/triethylamine
(10/3 mL). The solution was vigorously degassed for 30 min with
argon. Then CuI (1.4 mg, 0.0074 mmol) was added under argon
as a solid. During the course of the reaction the color of the
solution progressively turned violet. After 24 h the reaction was
quenched with water, and the solution evaporated to dryness.
Purification was ensured by column chromatography on flash
silica using a mixture of CH2Cl2/methanol as mobile phase
(gradient from 0.2 to 0.5% methanol). Recrystallization was
ensured by slow evaporation of dichloromethane from a di-
chloromethane/cyclohexane solution. Complex [LPt(-ꢁ-Ph)2]
1
(89 mg, 69%). H NMR (400.1 MHz, d2-dichloromethane): δ
8.61 (d, 3J=9.0 Hz, 2H), 7.93 (d, 3J=8.2 Hz, 2H), 7.76 (m, 4H),
7.71 (d, 3J=9.2 Hz, 2H), 7.65 (d, 3J=5.6 Hz, 2H), 7.30 (d, 3J=
9.2 Hz, 4H), 7.27 (d, 3J=16 Hz, 2H),), 7.14 (t, 3J=7.2 Hz, 2H),
1
was obtained as a dark violet solid (60 mg, 77%). H NMR
(400.1 MHz, d2-dichloromethane): δ 7.82 (d, 3J=8.8 Hz, 2H),

3996 Inorganic Chemistry, Vol. 49, No. 9, 2010
Lincker et al.
Table 1. Maximum Absorption and Emission Energies, Quantum Yield of Ligand L in Solution in Several Solvent and Solvent Parameter Values
absorption
emission
λabs
εmax
λem
Stokes shift
(eV)
polarity
Δf22,23
refraction index
quantum yield
solvent
n-hexane
toluene
diethyl ether
chloroform
ethyl acetate
dichloromethane
2-butanone
ethanol
(nm)
(eV)
(L mol-1 cm-1
)
(nm)
(eV)
ε22,23
n22,23
Φ (%)
422
436
430
454
436
453
443
456
443
2.935
2.844
2.880
2.731
2.844
2.737
2.799
2.719
2.799
64700
64700
70200
72150
67850
71250
71200
72600
73650
461
495
504
533
539
557
579
602
605
2.689
2.505
2.455
2.324
2.300
2.226
2.139
2.059
2.049
0.246
0.339
0.425
0.407
0.544
0.511
0.659
0.659
0.750
1.88
2.38
4.20
4.80
6.02
8.93
18.11
24.55
35.94
0.000411
0.012311
0.163353
0.149038
0.200506
0.218511
0.271627
0.289263
0.30613
1.37
1.50
1.35
1.45
1.37
1.42
1.38
1.36
1.34
73
43
97
80
85
18
13
32
acetonitrile
The trans stereochemistry of L was clearly established,
based on the coupling constants (15.6 Hz) in the 1H NMR
spectra of the protons on the olefinic carbons.
Quantum Chemical Calculations. To gain better insight
into the geometry and electronic properties of the molecule
L, quantum-chemical calculations were performed at the
B3LYP/6-31G* level in the density functional theory
(DFT) formalism. The optimized ground-state geometry
of the analogue of L (terminal butyl chains have been
replaced by ethyl groups) is reported in Figure 2a. First
of all, this molecule adopts an almost rigid planar
structure, the dihedral angle between the two pyridazine
rings being smaller than 1.2ꢀ, while the pyridazine
and phenyl rings are in the same plane (pyridazine-vinyl
and phenyl-vinyl twist angles between 1.2 and 1.4ꢀ and
2.0-2.6ꢀ, respectively). Consequently, this elongated
˚
planar molecule (22.84 A between the two final N(Et)2
groups) is centrosymmetric and has no dipolar moment.
It should be noted that this molecule is more planar
than its 3,30-bipyridine based analogue, for which the
dihedral angle between the pyridine rings is 34.6ꢀ corres-
ponding to a twisted conformation in the ground state.
Second, the bond-length alternation (BLA) para-
meter,20 defined as the difference between single and
Figure 3. Normalized absorption spectra of L (0.6 ꢀ 10-5 M) in several
solvents at 25 ꢀC. Toluene (g), diethylether (O), dichloromethane (4),
chloroform (0), ethylacetate (f), 2-butanone (b), acetonitrile (2), and
ethanol (9).
6.98 (m, 4H), 6.68 (d, 3J=16 Hz, 2H), 6.61 (d, 3J=9.0 Hz, 4H),
6.39 (d, 3J=7.2 Hz, 2H), 3.32 (t, 3J=7.6 Hz, 8H), 1.60 (m, 8H),
1.38 (m, 8H), 0.97 (t, 3J=7.5 Hz, 12H); 13C{1H} NMR (100.6 MHz,
d2-Dichloromethane): δ 168.2, 161.3, 154.2, 149.9, 149.2, 143.7,
139.8, 137.9, 131.7, 129.9, 129.7, 128.0, 125.9, 125.5.3, 124.1,
123.0, 122.0, 121.9, 119.6, 115.7, 111.4, 50.7, 29.4, 20.3, 13;72 ;
ES-MS m/z (nature of the peak, relative intensity): 1117.3 ([M]þ,
100); Anal. Calcd for C62H68N8IrBF4: C, 61.83; H, 5.69; N, 9.30.
Found: C, 61.47; H, 5.34; N, 9.13 ; UV-vis (CH2Cl2) λ (nm),
ε (M-1 cm-1): 253 (41800), 314 (28900), 575 (60800).
˚
double bonds on the vinyl bridge, is 0.106 A. Since this
value is close to the polyene limit, this high degree of
bond length alternation is indicative of a low contribu-
tion of the charge-separation resonance form to the
ground-state configuration of L.20 Third, the HOMOs
and LUMOs exhibit a specific pattern (Figure 2b,c). The
HOMO orbital is mainly localized at the terminal rings,
whereas a reversed pattern is observed for the LUMO
which is mostly located at the central pyridazine rings.
According to the electron localization in HOMO and
LUMO, the one-photon transition from the ground state
(S0) to the lowest excited state (S1) should contribute to a
polar property of (S1), explaining the observed solvato-
chromism in one-photon excitation experiments, despite
the centrosymmetric geometry in the ground state S0, as
reported with other D-A-D systems.21
Results and Discussion
Ligand Synthesis. The symmetrical quadrupolar mole-
cule L was conveniently prepared via a Knoevenagel
condensation, under acidic conditions, as outlined in
Scheme 1. 6,60-Dimethyl-3,30-bipyridazine (1) was synthe-
sized by adapting the homocoupling of halopyridines medi-
ated by nickel-phosphine complexes,9a,19 with 3-chloro-6-
methylpyridazine as starting material. Thus L was ob-
tained in a one-pot reaction by reacting one mole of (1)
with two moles of the 4-(dibutylamino)benzaldehyde,
and isolated in 25% yield as a bright-orange powder.
The structures and purity of all the compounds were
confirmed by using 1H and 13C NMR spectroscopy.
(20) Marder, S. R.; Perry, J. W.; Bourhill, G.; Gorman, C. B.; Tiemann,
B. G.; Mansour, K. Science 1993, 261, 186–189.
(21) (a) Marcotte, N.; Fery-Forgues, S. J. Chem. Soc., Perkin Trans. 2
2000, 1711–1716. (b) Detert, H.; Sugiono, E.; Kruse, G. J. Phys. Org. Chem.
2002, 15, 638–641. (c) Woo, H. Y.; Liu, B.; Kohler, B.; Korystov, D.;
Mikhailovsky, A.; Bazan, G. C. J. Am. Chem. Soc. 2005, 127, 14721–14729.
(d) Amthor, S.; Lambert, C.; Du€mmler, S.; Fischer, I.; Schelter, J. J. Phys. Chem.
A 2006, 110, 5204–5214. (e) Terenziani, F.; Painelli, A.; Katan, C.; Charlot, M.;
Blanchard-Desce, M. J. Am. Chem. Soc. 2006, 128, 15742–15755.
(19) Tiecco, M.; Testaferri, L.; Tingoli, M.; Chianelli, D.; Montanucci,
M. Synthesis 1984, 9, 736–737.

Article
Inorganic Chemistry, Vol. 49, No. 9, 2010 3997
Figure 5. Plot of Δν versus Δf, [(ε - 1)/(2ε þ 1) - (n2 - 1)/(2n2 þ 1)] for
L in different solvents.
weakly red-shifted, up to 33 nm, ranging from 422 nm in
n-hexane to 456 nm in ethanol.
While the absorption spectra of molecule L are only
slightly solvent-dependent, it is obvious that the photo-
luminescence (PL) spectra show a pronounced batho-
chromic solvatochromism (Figure 4). Solution of L in
n-hexane shows a fluorescence band exhibiting a well
resolved vibrational structure with the maximum emis-
sion wavelength (λem) at 461 nm. When increasing solu-
bility and solvent polarity a loss of the vibronic structure
is observed, a progressive red-shift of the emission wave-
length (λem∼ 605 nm in acetonitrile) occurs, and the
fluorescence quantum yield reaches a maximum and then
decreases; the maximum value (φF=0.97) is obtained in
chloroform.
Accordingly, the Stokes shifts (Δν = νabs - νem) sig-
nificantly increase with increasing solvent polarity, ran-
ging from 2438 cm-1 in triethylamine to 6045 cm-1 in
acetonitrile. Such behavior, generally observed for dipo-
lar compounds, indicates marked intramolecular charge
transfer (ICT) in the excited state, inducing consequently
a large dipole moment, stabilized in polar solvents. In the
case of centrosymmetric quadrupolar molecules such as
L, this behavior is attributed to symmetry breaking in the
excited-state because of polar solvation.21 The gain in the
molecular dipole moments induced by the photoexcitation
can be calculated by using the Lippert-Mataga relat-
ionship:24-26
Figure 4. Fluorescence emission (a) absolute and (b) normalized spectra
of L (1.0 ꢀ 10-6 M) in several solvents at 25 ꢀC. Toluene (g), diethylether
(O), dichloromethane (4), chloroform (0), ethylacetate (f), 2-butanone
(b), acetonitrile (2), and ethanol (9).
Optical Spectroscopies. All optical measurements
(i.e., absorption and emission spectra) have been per-
formed in solution (∼ 10-6 M) in a wide range of solvent
polarity (from ε = 1.88 for n-hexane to ε = 35.94 for
acetonitrile), and the results are summarized in Table 1
and Figures 3-4.
As shown in Figure 3, L exhibits a strong intense broad
absorption band, which indicates a highly π-conjugated
system. The spectral shape of the peak and the calculated
molar absorption coefficient, εmax, remained relatively
constant (∼ 70000 L mol-1 cm-1) with solvent (Table 1).
This absorption is attributed to a π-π* transition. As the
solvent polarity increased, the absorption maxima were
abs
max
ems
max
Δν ¼ ν - ν
!
2
n2 - 1
2ðμe - μgÞ
ε - 1
¼
-
þ Const:
hca3
2ε þ 1 2n2 þ 1
2
2ðμe - μgÞ
¼
Δf þ Const
ð1Þ
(22) (a) Laurence, C.; Nicolet, P.; Dalati, M. T.; Abboud, J.-L. M.;
Notario., R. J. Phys. Chem. 1994, 98, 5807–5816. (b) McClellan., A. L. Tables
of Experimental Dipole Moments; W. H. Freeman: San Francisco, 1963;
Vol. 1. (c) McClellan., A. L. Tables of Experimental Dipole Moments; Rahara
Enterprises: El Cerrito, CA, 1974; Vol. 2. (d) McClellan., A. L. Tables of
Experimental Dipole Moments; Rahara Enterprises: El Cerrito, CA, 1986;
Vol. 3.
hca3
where μe and μg refer to the dipole moments of excited and
ground states, respectively, h is Planck’s constant, c is the
velocity of light, a is the radius of the spherical Onsager
(23) (a) Marcus, Y. J. Solution Chem. 1991, 20, 929–944. (b) Reichardt, C.
Solvents and Solvent Effects in Organic Chemistry, 2nd ed.; VCH: Weinheim,
1988; Chapter 7, p 339. (c) Reichardt, C. In Solvents and Solvent Effects in
Organic Chemistry; VCH: Weinheim, Germany, 2004; Chapter 6-7.
(24) Lippert, E. Z. Naturforsch. A 1955, 10, 541–545.
(25) Mataga, N.; Kaifu, Y.; Koizumi, M. Bull. Chem. Soc. Jpn. 1956, 29,
465–470.
(26) Lippert, E.; Moll, F. Z. Elektrochemie 1954, 718–724.

3998 Inorganic Chemistry, Vol. 49, No. 9, 2010
Lincker et al.
Figure 6. Cyclic voltammetry of L at a 1.1 mmol L-1 concentration in a toluene/acetonitrile mixture (50/50 v/v) containing 0.1 mol L-1 NBu4BF4 as a
supporting electrolyte on a 1 mm diameter disk glassy carbon electrode: (a) reduction and (b) oxidation voltammograms. Scan rate = 0.2 s-1; T = 20 ꢀC,
E in V versus SCE.
cavity10b,c which fits the molecule, and ε and n are the
solvent dielectric constant and refractive index, and Δf is
Lippert’s function, respectively. As observed in Figure 5, a
linear plot is obtained for compound L. On the basis of
these data, the dipole moment difference between the
ground and excited state, Δμ=με - μg, could be calculated
from the slope (10337 cm-1) of the plot in Figure 5. A
change of 19.3 D in dipole moment is deduced, using the
˚
Onsager’s cavity radius (ao) estimatedas7.18Abychemical
modeling (see Experimental Section).10b,c Such a change
confirms that the species formed in the excited state have
different charge localization to that in the ground state, this
result being consistent with a stabilization of highly polar
emitting excited states by polar solvents.
Electrochemistry. In view of the potential application
of compound L in optoelectronics, it is important to con-
sider their respective energy gap -ΔEg, relative ionization
Figure 7. Electroluminescent spectra of the OLED at 30 mA cm-2 as a
function of the percentage of dopant.
potential (IP), and electron affinity (EA), parameters
which characterize the facility for injecting holes and
electrons, respectively, in the material. These values can
From this reversible voltammogram, the formal potential
be estimated from the first oxidation and reduction poten-
tials in solution (Eoxꢀ and Eredꢀ), which are associated to
the HOMO and LUMO energy levels, respectively.27 For
this purpose, the electrochemical behavior of L was
investigated by cyclic voltammetry (CV), to measure the
redox potentials, and thus to estimate IP, EA, and the
energy gap (that can be derived from their electrochemi-
cal gap ΔEg,elec= Eoxꢀ - Eredꢀ). Moreover, electrochemi-
cal measurements allow rapid determinations of the
chemical stabilities (lifetimes) of the corresponding gen-
erated ionic species. The electrochemical oxidation and
reduction of L were examined in toluene/acetonitrile
mixture (50/50 v/v). This media allows a good solubility
(in the millimolar range in most cases) of the studied
compound as required for electrochemical experiments.
Typical voltammograms of the reduction and oxidation
of L are displayed in Figures 6 a and b respectively. In
reduction (Figure 6a), L presents one well-defined rever-
sible process. Considering that the reduction process
was reversible at any investigated scan rate (down to
0.1 V s-1), this results show a good chemical stability of
the radical-anion (lifetime higher than 10 s) (Figure 6a).
E0 was immediately derived as the half-sum between
=
red
the forward and the reverse scan peak potentials (E0
red
-1.68 V vs SCE). In oxidation, only one irreversible
process was observed. This peak remains irreversible even
when the scan rate was increased up to 100 V s-1
(Figure 6b), indicating that the lifetime of the produced
radical cation is shorter than a few milliseconds in our
experimental conditions. It is noticeable that the radical-
cation has a much lower chemical stability than the
corresponding radical-anion although in the presence of
the two donating N(Bu)2 groups. The Eꢀox value was
approximated by the corresponding peak potential (Eox=
0.69 V vs SCE), thus taking the previous EꢀRed values,
leads to an electrochemical gap range about 2.37 eV.
Concerning the HOMO and LUMO energy levels, to a
first approximation, the oxidation potential can be re-
lated to the ionization potential and thus to the HOMO
energy level, according to the following equation: HOMO=
(Eox þ 4.75).27 The calculation shows that L possesses an
ionization potential ranging in the 5.4 eV. A similar
relationship can be made for the reduction potential,
the electron affinity and LUMO energies, leading to an
estimated high electron affinity (∼3.1 eV). This value is
higher than those previously determined on 6,60-distyryl-3,
(27) Janietz, S.; Bradley, D. D. C.; Grell, M.; Giebeler, C.; Inbasekaran,
M.; Woo, E. P. Appl. Phys. Lett. 1998, 73, 2453–2355.

Article
Inorganic Chemistry, Vol. 49, No. 9, 2010 3999
Table 2. Electroluminescent Characteristics of the Devices Obtained at 30 mA cm-2 As a Function of the Doping Percentage
efficiencies
CIE
dopant in DPVBi (wt %)
λ
em (nm)
Us (V)
%
cd A-1
lm W-1
x
y
452
5.3
4.8
5.3
5.0
5.1
4.6
5.3
4.8
3.6
3.9
4.1
3.9
4.2
4
4
1.2
3.0
3.2
3.6
3.6
3.6
3.7
3.6
0.155
0.230
0.245
0.272
0.285
0.297
0.306
0.315
0.130
0.438
0.503
0.549
0.570
0.576
0.583
0.582
0.4%
0.6%
0.8%
1.2%
1.8%
2%
448/500/528
450/500/527
448/502/529
504/529
508/528
508/529
512/529
10.7
12.1
12.7
13.7
13.4
14.1
12.3
4.1
3.6
2.3%
30-bipyridine derivatives,9a-c and indicates that electrons
could be easily injected in L.
Organic Light-Emitting Diode. The basic structure of
the OLED device, presented in Figure 1 is ITO/CuPc/
NPB/DPVBi: compound L (ꢀ wt %)/Alq3/LiF/Al whereꢀ
is the dopant ratio. The DPVBi is known to be a highly
efficient blue emitter in multilayer OLED.28 It has been used
as well as an efficient matrix for white OLEDs with fluo-
rescent dopant.29 The compound L was selected to be tested
in OLED because of a good fluorescent yield (70%) in solid
(1% dispersion in PMMA thin film) in the green (emission
at 528 nm). The percentage of dopant L was varied from
0.4% to 2.3% in DPVBi to determine its influence of the
diode performances. The EL spectra of the devices are
presented in Figure 7, and their electroluminescence (EL)
performances and CIE coordinates are reported in Table 2.
At low percentage of dopant (<1%), the EL spectrum of
the OLED comprises a small peak at 450 ( 4 nm, char-
acteristic of the DPVBi emission and a peak around 505 nm
with a shoulder at 528 nm, from the dopant emission. The
small blue emission from DPVBi and the good quantum
efficiencies (around 4%) of the doped devices evidence an
efficient energy transfer from the host DPVBi to compound
L. This energy transfer is likely favorable because of the
energy levels of DPVBi and L (LUMO/HOMO of 2.8 eV/
5.9 eV and 3.1 eV/4.75 eV respectively). Above 1% of
dopant, the peak at 450 nm is negligible, and the emission
comes only from L. The best performances at 30 mA cm-2
are obtained for the device at 2% of compound L and
with a luminous efficiency of 14.1 cd A-1, a power efficiency
of 3.7 lm W-1 and an external quantum efficiency of 4.1%.
The device emits green light with CIE coordinates x=0.306
and y= 0.583. According to the device structure without
enhanced light extraction system (Figure 1), an external
quantum efficiency of 4% is close to the theoretical maxi-
mum value of 5% for OLEDs based on small fluorescent
molecules like for the L material. The EL results show that
DPVBi can be used as an efficient host for making green
OLEDs.30
Figure 8. Normalized absorption spectra of the free bipyridazine based
ligand L and related complexes [L2Cu]PF6 and [L3Ni](NO3)2 in acetone
(1 ꢀ 10-5 mol L-1) at 25 ꢀC.
yields upon dimethylformamide reflux treatment of free
bipyridazine based ligand L with tetrakis(acetonitrile)
copper(I) hexafluorophosphate (2:1) and nickel(II) nitrate
hexahydrate (3:1), respectively. [L2Cu]PF6 and [L3Ni](NO3)2
FAB/Mass spectra are in accordance with the postulated
structures of the Ni(II) and Cu(I) complexes.
On the other hand, to reach other geometries, platinum
(II) and iridium(III) complexes were as well routinely
prepared from adequate metallic precursors such as
[Pt(DMSO)2Cl2], μ-di-Cl-[Ir(C∧N)2Cl]2 (C∧N account
for orthometalated 2-phenylpyridine). Replacement of
the chloro ligands by specific alkyne derivatives, in the
[LPtCl2] complex, was also promoted by copper(I) cata-
lysis under anaerobic conditions. In the iridium complex a
single isomer trans-(N) was isolated as unambiguously
assigned by NMR spectroscopy.
Upon chelating with metal ions, linear optical proper-
ties are drastically changed as shown in Figures 8-11.
Regarding the [L2Cu]PF6 and [L3Ni](NO3)2 complexes,
an important red-shift of the intense ILCT (intraligand
charge transfer) band is observed (Δλmax=49 and 55 nm,
respectively, Figure 8) and no luminescence is observed in
both cases.
Complexations. To test the ability of compound L to
form transition metal complexes, first the tetrahedral
complex [L2Cu]PF6 and octahedral complex [L3Ni](NO3)2
(Chart 2) were synthesized. They were obtained in high
On the other hand, upon chelating of Pt or Ir centers,
complexes exhibit completely different optical spectra as
shown in Figures 9-11. First, the yellow color of the free
ligand is replaced by an intense blue and dark violet color,
respectively, complexation of ligand L with Pt inducing a
bathochromic shift of the lowest absorption to 608 nm.
This absorption is likely assigned to a MLCT as previously
(28) Shaheen, S. E.; Jabbour, G. E.; Morrell, M. M.; Kawabe, Y.;
Kippelen, B.; Peyghambarian, N.; Nabor, M.-F.; Schlaf, R.; Mash, E. A.;
Armstrong, N. R. J. Appl. Phys. 1998, 84, 2324–2327.
(29) Geffroy, B.; Lemaitre, N.; Lavigne, J.; Denis, C.; Maisse, P.;
Raimond, P. Nonlinear Opt., Quantum Opt. 2007, 37, 9–19.
(30) (a) Tang, C. W.; VanSlycke, S. A.; Chen, C. H. J. Appl. Phys. 1989,
65, 3610. (b) Liu, T. H.; Iou, C. Y.; Wen, S. W.; Chen, C. H. Thin Solid Films
2003, 441, 223–227.

4000 Inorganic Chemistry, Vol. 49, No. 9, 2010
Lincker et al.
Figure 9. Absorption (1 ꢀ 10-5 M), emission (2 ꢀ 10-6 M, λex = 550 nm)
and excitation spectra (λem = 826 nm) of [LPtCl2] in dichloromethane
at 25 ꢀC.
Figure 11. Absorption (1 ꢀ 10-5 M), emission (2 ꢀ 10-6 M, λex =550nm)
and excitation (λem = 754 nm) spectra of [LIr(C∧N)2]BF4 in dichlorome-
thane at 25 ꢀC.
degassed conditions and falls to 0.08 in air. Clearly, the
oxygen dependence of the quantum yield is in favor of
phosphorescence from a MLCT excited state. For the
[LPtCl2] complex the emission quantum yield is too weak
to be measured precisely. Unfortunately, we were unable
to measure accurately the lifetime of the platinum comp-
lexes with our experimental setup because of the weak
emission intensity. However, in both platinum complexes
the excitation spectrum reveals a perfect match with the
absorption spectra which is a proof for the contribution
of all absorption in the luminescence of the complexes
(Figures 9 and 10).
The Ir complex reveals similar bathochromic shifts and
a strong absorption at 608 nm which is also attributed to a
dual absorption of the MLCT and ligand centered (LC)
transitions.33 In this case an emission quantum yield of
0.36 was determined at 816 nm whereas this yield drops
to 0.08 in non-degassed conditions. The excitation spec-
trum overlaps the absorption spectra (Figure 11). For
the [LIr(C∧N)2]þ complex an excited state lifetime of
190 ( 35 ns has been determined which is in favor for a
phosphorescence from an MLCT manifold as previously
found for related Ir-complexes.33 For all three complexes
the same generic spectral features are retained in dichloro-
methane in which the Beer-Lambert law is followed.
Further characterization of these excited states is cur-
rently in progress and falls outside of the scope of this
contribution.
In addition, the electrochemical data of these com-
plexes are summarized in Table 3. All the complexes show
one irreversible oxidation peak around þ0.76/þ0.79 V
versus SCE likely because of the irreversible oxidation of
the dibutylamino residues of ligand L (found at þ0.70 V
vs SCE in dichloromethane). For the Ir complex an
additional quasi reversible oxidation process was found
at þ1.32 V in keeping with the oxidation of the Ir center as
found in related complexes.34
Figure 10. Absorption (1 ꢀ 10-5 M), emission (1 ꢀ 10-6 M, λex =550nm)
and excitation spectra (λem = 776 nm) of [LPt(-ꢁ-Ph)2] in dichloro-
methane at 25 ꢀC.
observed for related derivatives.31 Interestingly, by sub-
stitution of the two chloro ligands by tolylacetylene
residues such as in the Pt-ꢁ- case, the less energetic
absorption is an envelope including at least two major
absorptions safely assigned to a MLCT and a LLCT band
clearly observed on the red side of the band (Figure 9),32
this latter band being absent in the corresponding
[LPtCl2] complex. Clearly the signature of the tolyl-
acetylene fragments is also found in the intense π-π*
absorption at 267 nm which is absent in the precursor
absorption spectra. Second, both platinum complexes are
weakly emissive in the near IR regime at 744 and 766 nm
(Figures 9 and 10). In both cases the positions of the
absorption bands is not dependent on the concentra-
tion in the 10-5 to 10-7 M range which exclude metal-
metal aggregation or strong π-π interactions. For
[LPt(-ꢁ-Ph)2] the emission quantum yield is 0.19 in
For each complex two reversible reduction processes
were found at a potential less cathodic versus the free
(31) Shikhova, E.; Danilov, E. O.; Kinayyigit, S.; Pomestchenko, I. E.;
Tregubov, A. D.; Camerel, F.; Retailleau, P.; Ziessel, R.; Castellano, F. N.
Inorg. Chem. 1997, 36, 3038–3048.
€
(33) Colombo, M. G.; Hauser, A.; Gudel, H. U. Top. Curr. Chem. 1994,
(32) Clark, M. L.; Diring, S.; Retailleau, P.; McMillin, D. R.; Ziessel, R.
Chem.;Eur. J. 2008, 14, 7168–7179.
171, 143–171.
(34) Lowry, M. S.; Bernhard, S. Chem.;Eur. J. 2006, 12, 7970–7977.

Article
Inorganic Chemistry, Vol. 49, No. 9, 2010 4001
Table 3. Cyclic Voltammetry Data at 20 ꢀCa
groups. This yellow and highly fluorescent compound (green
emitter) exhibits a pronounced intramolecular charge trans-
fer excited state of push-pull type, the acceptor being
represented by the central 3,30-bipyridazine core. The com-
pound is subjected to a pronounced solvatochromism which
has been studied in some detail. Its emission properties were
exploited to build an organic light emitting device (OLED) by
doping into a fluorescent DPVBi matrix. The best device
provides a power efficiency of 3.7 lm W-1 and an external
efficiency of 4.1% with a doping ratio of 2% of L into
DPVBi. Complexation of the ligand with Cu(I) or Ni(II) salts
provide respectively tetrahedral and octahedral geometries in
which the fluorescence of the ligand is quenched. Complexa-
tion of ligand L to square planar Pt(II) or octahedral Ir(III)
complexes is also feasible providing robust complexes weakly
luminescent in the near-infrared. The rich electroactivity of
these latter complexes is due to the presence of surrounding
ligands (σ-ethynyl moieties for Pt and orthometalated C∧N
framents for Ir). The versatility of the chemistry disclosed
here opens interesting perspectives for modifications of the
donor/acceptor character of the terminal substituents of the
molecule and consequently for the tuning of the electronic
properties. The solvatochromism in emission and fluorescence
quenching upon complexation with the first series of transition
metals would also be auspicious for the design of integrated
chemical sensors. In the same way, the design of new ligands
and the application of them in the synthesis of heavy-metal
complexes to obtain phosphorescent emitters (e.g., ruthenium
complexes) as sensitizers are currently in progress.
Eap (ox, soln) (V),
ΔE (mV)
E0’(red, soln) (V),
ΔE (mV)
[LPtCl2]
[LPt(-ꢁ-Ph)2]
þ0.76 (irr.)
þ0.79 (irr.)
þ0.77 (irr.)
þ1.32 (90)
-0.87 (60), -1.46 (70)
-1.00 (70), -1.57 (80)
-1.12 (60), -1.68 (80)
[LIr(C∧N)2]BF4
a Potentials determined by cyclic voltammetry in anhydrous and
deoxygenated CH2Cl2 solution, containing 0.1 M TBAPF6, [electro-
chemical window from þ1.6 to -2.2 V], at a solute concentration of
about 1.0 mM, using a scan rate of 200 mV s-1 unless otherwise
indicated. Potentials were standardized versus ferrocene (Fc) as internal
reference and converted to the SCE scale assuming that E1/2 (Fc/Fcþ)=
þ0.38 V (ΔEp=60 mV) versus SCE. For reversible processes E1/2
=
(Epa þ Epc)/2. Error in half-wave potentials is (10 mV. For irreversible
processes the peak potentials (Eap) are quoted. All reversible redox steps
result from one-electron processes except otherwise quoted.
ligand L when studied under the same conditions (-1.66 V
vs SCE). The redox wave separation is 590, 570, and 560 mV
for [LPtCl2], [LPt(-ꢁ-Ph)2], and [LIr(C∧N)2]þ, respect-
ively. For the Pt complexes the relative insensitivity of
the reduction potentials toward the nature of the acetylide
or chloro ligand suggests that these reductions are mainly
localized on the ligand L with some mixing with the Pt(II)
metal orbitals.35,36 These data are consistent with data
found with related [Pt(tBu3tpy)X]þ complexes.37
The cyclic voltammetry data for the Ir complex is in
keeping with related orthometalated complexes where the
reduction occurs at a more negative potential versus the
Pt complexes. Likewise the first reduction is likely as-
signed to the reduction of the coordinated ligand L
whereas the second reduction is attributed to the ortho-
metalated C∧N ligand in light of redox properties deter-
mined in related complexes.38
Acknowledgment. We gratefully acknowledge help-
ful technical support for mass spectrometry from
Pr. Lange and from Pr. Rammondec, both from the
IRCOF Institute (Rouen); many thanks are due as well
to Pr. Le Bozec from O2’s Group (Rennes University) for
very fruitful scientific discussion and help concerning
Conclusions
We succeeded in the preparation of a new π-conjugated
ꢀ
complexation aspects. Dr. Stephane Diring is acknowl-
3,30-bipyridazine ligand carrying two dibutylamino donor
edged for the first synthesis of a small sample of the platinum
complexes. Raymond Ziessel (Strasbourg) warmly thanks
Johnson Matthey PLC for a loan of precious metal salts.
(35) Hill, M. G.; Bailey, J. A.; Miskowski, V. M.; Gray, H. B. Inorg.
Chem. 1996, 35, 4585–4590.
(36) Crites, D. K.; Cunningham, C. T.; McMillin, D. R. Inorg. Chim. Acta
1998, 273, 346–353.
(37) Rachford, A. A.; Goeb, S.; Ziessel, R.; Castellano, F. N. Inorg.
Chem. 2008, 47, 4348–4355.
(38) Slinker, J. D.; Gorodetsky, A. A.; Lowry, M. S.; Wang, J.; Parker, S.;
Rohl, R.; Berhard, S.; Malliaras, G. G. J. Am. Chem. Soc. 2004, 126,
2763–2767.
Note Added after ASAP Publication. An acknowl-
edgment sentence was added along with several other
text changes throughout this paper which was published
ASAP April 2, 2010; the version containing the changes
reposted on April 9, 2010.