New Heteroleptic bis-phenanthroline copper(I) complexes with dipyridophenazine or imidazole fused phenanthroline ligands: Spectral, electrochemical, and quantum chemical studies

Article abstract of DOI:10.1021/ic2006343

Two new sterically challenged diimine ligands L₁ (2,9-dimesityl-2-(4′-bromophenyl)imidazo[4,5-f][1,10]phenanthroline) and L₂ (3,6-di-n-butyl-11-bromodipyrido[3,2-a:2′,3′-c] phenazine) have been synthesized with the aim to build original heteroleptic copper(I) complexes, following the HETPHEN concept developed by Schmittel and co-workers. The structure of L₁ is based on a phen-imidazole molecular core, derivatized by two highly bulky mesityl groups in positions 2 and 9 of the phenanthroline cavity, preventing the formation of a homoleptic species, while L₂ is a dppz derivative, bearing n-butyl chains in α positions of the chelating nitrogen atoms. The unambiguous formation of six novel heteroleptic copper(I) complexes based on L₁, L ₂, and complementary matching ligands (2,9-R₂-1,10- phenanthroline, with R = H, methyl, n-butyl or mesityl) has been evidenced, and the resulting compounds were fully characterized. The electronic absorption spectra of all complexes fits well with DFT calculations allowing the assignment of the main transitions. The characteristics of the emissive excited state were investigated in different solvents using time-resolved single photon counting and transient absorption spectroscopy. The complexes with ligand L₂, bearing a characteristic dppz moiety, exhibit a very low energy excited-state which mainly leads to fast nonradiative relaxation, whereas the emission lifetime is higher for those containing the bulky ligand L₁. For example, a luminescence quantum yield of about 3 × 10^-4 is obtained with a decay time of about 50 ns for C2 ([Cu^I(nBu-phen) (L₁)]⁺) with a weak influence of strong coordinating solvent on the luminescence properties. Overall, the spectral features are those expected for a highly constrained coordination cage. Yet, the complexes are stable in solution, partly due to the beneficial π stacking between mesityl groups and vicinal phenanthroline aromatic rings, as evidenced by the X-ray structure of complex C3 ([Cu^I(Mes-phen)(L₂)]⁺). Electrochemistry of the copper(I) complexes revealed reversible anodic behavior, corresponding to a copper(I) to copper(II) transition. The half wave potentials increase with the steric bulk at the level of the copper(I) ion, reaching a value as high as 1 V vs SCE, with the assistance of ligand induced electronic effects. L₁ and L₂ are further end-capped by a bromo functionality. A Suzuki cross-coupling reaction was directly performed on the complexes, in spite of the handicapping lability of copper(I)-phenanthroline complexes.

Full text of DOI:10.1021/ic2006343

ARTICLE
pubs.acs.org/IC
New Heteroleptic Bis-Phenanthroline Copper(I) Complexes with
Dipyridophenazine or Imidazole Fused Phenanthroline Ligands:
Spectral, Electrochemical, and Quantum Chemical Studies
Yann Pellegrin,† Martina Sandroni,† Errol Blart,† Aurꢀelien Planchat,† Michel Evain,*,§ Narayan C. Bera,‡
Megumi Kayanuma,‡ Michel Sliwa,*,|| Mateusz Rebarz,|| Olivier Poizat,|| Chantal Daniel,*,‡ and
Fabrice Odobel*,†
†CEISAM, Universitꢀe de Nantes, CNRS, 2 rue de la Houssiniꢁere, 44322 Nantes Cedex 3, France
‡Laboratoire de Chimie Quantique, Institut de Chimie, UMR 7177 CNRS-Universitꢀe de Strasbourg, 4 rue Blaise Pascal, CS 90032,
F-67081 Strasbourg Cedex, France
§IMN, Universitꢀe de Nantes, CNRS, 2 rue de la Houssiniꢁere, 44322 Nantes Cedex 3, France
Laboratoire de Spectrochimie Infrarouge et Raman, UMR 8516 CNRS-Universitꢀe Lille 1 Sciences et Technologies, 59655 Villeneuve
d’Ascq Cedex, France
S Supporting Information
b
ABSTRACT: Two new sterically challenged diimine ligands L1 (2,9-dimesityl-
2-(40-bromophenyl)imidazo[4,5-f][1,10]phenanthroline) and L2 (3,6-di-n-
butyl-11-bromodipyrido[3,2-a:20,30-c]phenazine) have been synthesized with the
aim to build original heteroleptic copper(I) complexes, following the HETPHEN
concept developed by Schmittel and co-workers. The structure of L1 is based on a
phen-imidazole molecular core, derivatized by two highly bulky mesityl groups in
positions 2 and 9 of the phenanthroline cavity, preventing the formation of a
homoleptic species, while L2 is a dppz derivative, bearing n-butyl chains in
α positions of the chelating nitrogen atoms. The unambiguous formation of six novel heteroleptic copper(I) complexes based on L1,
L2, and complementary matching ligands (2,9-R2-1,10-phenanthroline, with R = H, methyl, n-butyl or mesityl) has been evidenced,
and the resulting compounds were fully characterized. The electronic absorption spectra of all complexes fits well with DFT
calculations allowing the assignment of the main transitions. The characteristics of the emissive excited state were investigated in
different solvents using time-resolved single photon counting and transient absorption spectroscopy. The complexes with ligand L2,
bearing a characteristic dppz moiety, exhibit a very low energy excited-state which mainly leads to fast nonradiative relaxation,
whereas the emission lifetime is higher for those containing the bulky ligand L1. For example, a luminescence quantum yield of about
3 ꢁ 10ꢀ4 is obtained with a decay time of about 50 ns for C2 ([CuI(nBu-phen)(L1)]+) with a weak influence of strong coordinating
solvent on the luminescence properties. Overall, the spectral features are those expected for a highly constrained coordination cage.
Yet, the complexes are stable in solution, partly due to the beneficial π stacking between mesityl groups and vicinal phenanthroline
aromatic rings, as evidenced by the X-ray structure of complex C3 ([CuI(Mes-phen)(L2)]+). Electrochemistry of the copper(I)
complexes revealed reversible anodic behavior, corresponding to a copper(I) to copper(II) transition. The half wave potentials
increase with the steric bulk at the level of the copper(I) ion, reaching a value as high as 1 V vs SCE, with the assistance of ligand
induced electronic effects. L1 and L2 are further end-capped by a bromo functionality. A Suzuki cross-coupling reaction was directly
performed on the complexes, in spite of the handicapping lability of copper(I)ꢀphenanthroline complexes.
’ INTRODUCTION
mandatory to observe photoluminescence, since they prevent the
flattening distortion of the complex from the CuI preferred
tetrahedral geometry to the formal CuII square planar arrange-
ment, leading to a very efficient quenching of the excited state.
However, the rather low molar extinction coefficient of the MLCT
absorption band and the ligand scrambling around Cu(I), pre-
venting the isolation of a stable [CuI(imineA)(imineB)]+ hetero-
leptic complex, has put a dramatic curb on the development of
Copper(I)ꢀdiimine complexes ([CuI(diimine)2]+), where
the diimine ligand is α-substituted by bulky groups, have
attracted much attention since the discovery of their lumines-
cence properties in solution and in the solid state.1ꢀ5 They share
with rutheniumꢀpolypyridine complexes several appealing fea-
tures for solar energy conversion, in particular the existence of a
metal to ligand charge transfer transition (MLCT) around
460 nm, which could be formally described as [CuII(imine•ꢀ)-
(imine)]+.3ꢀ6 Importantly, the presence of sterically challenging
groups in the α position of the coordinating nitrogen atoms is
Received: March 28, 2011
Published: October 21, 2011
r
2011 American Chemical Society
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Chart 1. Structures of the Ligands and Complexes Described in This Study
copper-based photosystems. Indeed, the formation of a mere
dyad (e.g., with imineA as the electron acceptor and imineB as the
electron donor) was strictly impossible, in contrast with the nearly
matchless versatility of rutheniumꢀpolypyridine chemistry.7
However, the ability to prepare heteroleptic diimine copper(I)
complexes would be extremely useful for solar energy conversion
applications as it would be easier to accurately tune their photo-
physical and photoredox properties. This is well demonstrated
with the abundant use of heteroleptic ruthenium complexes for
dye-sensitized solar cells (DSSCs), in which an anchoring ligand
(such as 4,40-dicarboxy-2,20-bipyridine) is mixed with electron
releasing ligands (such as thiocyananto or a bpy substituted with
thienyl or oligophenylenethynylene chains) in order to reach the
suitable electronic properties to inject electrons in TiO2 and to
harvest the maximum number of photons in the solar spectrum.8
Second, the preparation of heteroleptic diimine copper(I) com-
plexes opens the route to the development of rodlike multicom-
ponent assemblies, which was not conveniently accessible with the
octahedral ruthenium trisbipyridine complex. In 1997, Schmittel
and co-workers broke through this barrier, with the HETPHEN
concept (where HETPHEN stands for HETeroleptic PHENan-
throline copper(I) complexes):9ꢀ12 by grafting very bulky sub-
stituents such as mesityl groups (where mesityl stands for 2,4,6-
trimethylphenyl) at the 2 and 9 positions of the phenanthroline
skeleton, the formation of the corresponding homoleptic com-
plex is prohibited. As a result, numerous impressive and carefully
designed copper(I)ꢀphenanthroline supramolecular architec-
tures were isolated and fully characterized.12ꢀ16 A very similar
approach has been undertaken using tert-butyl instead of mesityl
and proved to be very successful.17 However, in the HETPHEN
concept, the mesityl groups borne by phenA are involved in a
πꢀπ interaction with phenB, providing further stability to the
molecular scaffold.
At the convergence point of the HETPHEN concept and the
luminescence properties of sterically hindered copper(I) com-
plexes lies the possibility to design photosensitive dyads, capable
of sustaining photoinduced electron transfer, where phenA
behaves as an electron donor and phenB as an acceptor. In addition
to the obvious financial advantage of using copper instead of
ruthenium, the tetrahedral arrangement of the [CuI(phenA)-
(phenB)]+-based arrays is very attractive for the design of linear
photosensitive donorꢀacceptor polyads.
In this work, we describe the synthesis and characterizations of
two new, synthetically versatile ligands, L1 (2,9-dimesityl-2-
(40-bromophenyl)imidazo[4,5-f][1,10]phenanthroline) and L2
(3,6-di-n-butyl-11-bromodipyrido[3,2-a:20,30-c]phenazine), and
their related heteroleptic copper(I) complexes with phen (1,10-
phenanthroline), Me-phen (2,9-dimethyl-1,10-phenanthroline),
nBu-phen (2,9-di-n-butyl-1,10-phenanthroline), and Mes-phen
(2,9-dimesityl-1,10-phenanthroline) (Chart 1). The molecular
structures of L1 and L2 are inspired by well-known phenanthro-
line derivatives, phen-imidazole and dppz, respectively (where
phen-imidazole is 2-phenylimidazo[4,5-f][1,10]phenanthroline
and dppz is dipyrido[3,2-a:20,30-c]phenazine), with well docu-
mented electronic properties. In particular, dppz has been
extensively used in metal-based photosensitive polyads18ꢀ20
and DNA intercalating agents.21 On the other hand, phenꢀ
imidazole derivatives are electron donating in nature and bear a
labile proton. With a modest number of new ligands, several
original complexes can be synthesized, and the electronic effects
of the former on the overall properties of the latter can be
explored and constitute the core of this work. Additionally, let us
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ARTICLE
note that L1 and L2 possess a bromo unit, which could be
involved in a catalyzed cross-coupling reaction in order to
construct larger arrays by chemistry on the complex, a lead that
will also be presented in this contribution. The aim of the present
work is to provide a detailed analysis of the structural, electro-
chemical, and spectral properties of the newly synthesized
complexes combining experimental and theoretical studies.
TD-DFT calculation has been performed using a modified version of the
Gaussian 03 quantum chemistry software.30
Synthesis. 2,9-Dimesityl-1,10-phenanthroline-5,6-dioxolane (3).
In a Schlenk tube fitted with a water condenser, 2 (47 mg, 0.15 mmol),
mesitylboronic acid (60 mg, 0.37 mmol), and barium hydroxide (94 mg,
0.55 mmol) were suspended in 5 mL of a 9:1 (v:v) mixture of 1,2-
dimethoxyethane (DME) and water, respectively. The setup was
thoroughly degassed with argon, and palladium tetrakis triphenylpho-
sphine (20 mg, 0.02 mmol) was quickly added. The mixture was heated
to 115 °C and stirred for 16 h, under argon. It was then allowed to cool
down to room temperature, and water was added. The suspension was
extracted four times with dichloromethane. The organic layers were
gathered, dried on sodium sulfate, and evaporated under reduced pres-
sure to afford a brown oil. The latter was purified by chromatography on
silica gel, prepared in a 1:1 mixture of hexanes and dichloromethane. The
second, bright orange ring was collected, and the solvents were removed
by rotary evaporation, yielding 3. Yield: 63 mg (88%). 1H NMR (300
MHz, CDCl3, 25 °C): δ 8.31 (d, 2H, H4 and H7), 7.55 (d, 2H, H3 and H8),
6.92 (s, 4H, Hmes), 2.31 (s, 6H, CH3,mes), 2.12 (s, 12H, CH3,mes),
1.92 (s, 6H, CH3,dioxolane) ppm. ES-MS (m/z) 489.2 [M + H]+. Anal. for
’ EXPERIMENTAL SECTION
1
General. H NMR spectra were recorded on an AMX 400 MHz
1
Bruker spectrometer. Chemical shifts for H NMR spectra are refer-
enced relative to residual protium in the deuterated solvent (CDCl3 δ =
7.26 ppm). MALDI-TOF analyses were performed on an Applied
Biosystems Voyager DE-STR spectrometer in positive linear mode at
a 20 kV acceleration voltage with α-cyano 4-hydroxycinnamic acid
(CHCA) as the matrix.
Preparative thin-layer chromatography (preparative TLC) was per-
formed with a Merk Kieselgel 60PF254. Column chromatography was
carried out with a Merk 5735 Kieselgel 60F (0.040ꢀ0.063 mm mesh).
Air sensitive reactions were carried out under argon in dry solvents and
glassware. Chemicals were purchased from Aldrich and used as received.
Compounds 2,9-dichloro-1,10-phenanthroline-5,6-dioxolane (2), 2,9-
di-n-butyl-1,10-phenanthroline (nBu-phen), 2,9-di-n-butyl-1,10-phe-
nanthroline-5,6-dione, and Mes-phen were synthesized using literature
procedures.22
3.1/2DME 1/2CH2Cl2. Found (%): C, 74.01; H, 6.38; N, 4.61. Calcd:
3
C, 74.01; H, 6.65; N, 4.86.
2,9-Dimesityl-1,10-phenanthroline-5,6-dione (4). Compound 3
(163 mg, 0.33 mmol) was suspended in distilled water (5 mL). TFA
(10 mL) was then added dropwise, at room temperature, and the yellow
suspension eventually turned bright red. The mixture was then heated at
60 °C for 5 h. The solvents were evaporated, and the brown residue was
dissolved in 10 mL of dichloromethane, on top of which was added
10 mL of aqueous sodium hydrogenocarbonate (1.0 mol Lꢀ1). After 2 h
of vigorous stirring, the mixture turned into a turbid, green-yellow
emulsion. The latter was extracted four times with dichloromethane.
The organic layers were gathered, washed with brine, dried on sodium
sulfate, and evaporated under reduced pressure. The resulting yellow oil
was further dried on a vacuum line at room temperature, until a powder
eventually appeared. Yield: 130 mg (87%). 1H NMR (300 MHz, CDCl3,
25 °C): δ 8.53 (d, 2H, H4 and H7), 7.49 (d, 2H, H3 and H8), 6.92 (s, 4H,
Hmes), 2.30 (s, 6H, CH3,mes), 2.14 (s, 12H, CH3,mes) ppm. HR-MS
(m/z): 893.4067 [2 M + H]+.
UVꢀvisible spectra were recorded in analytically pure solvents, with a
UV 2501PC Shimadzu spectrophotometer. Luminescence excitation
spectra were recorded using a FluoroMax3 (Jobin Yvon Horiba), and
emission was corrected for the spectral sensitivity of the instrument.
Nanomicrosecond transient absorption experiments were performed
using a laser flash photolysis apparatus. Excitation pulses at 460 nm (4 ns,
1 mJ) were provided by a 10-Hz Nd:YAG laser coupled to an OPO
(Continuum Panther EX OPO pumped by a Surelite II). The probe light
was provided by a pulsed Xe lamp (XBO 150W/CR OFR, OSRAM).
Samples were contained in a quartz cell (10 ꢁ 10 mm2 section) at an
adjusted concentration (∼ 10ꢀ4 mol dmꢀ3) to get an OD value of about
1.0 at the pump excitation wavelength. The transmitted light was dis-
persed by a monochromator (Horiba Jobin-Yvon, iHR320) and analyzed
with a photomultiplier (R1477ꢀ06, Hamamatsu) coupled to a digital
oscilloscope (LeCroy 454, 500 MHz). The experiment was repeated
for different wavelengths of the monochromator, and transient spectra
were afterward reconstructed. Electrochemistry measurements were
performed with an Autolab PGSTAT 302N potentiostat in freshly
distilled dichloromethane, with a platinum disk working electrode, a
platinum foil counter electrode, and a saturated calomel reference elec-
trode (SCE). Tetra-n-butylammonium hexafluorophosphate in dichloro-
methane (0.1 mol Lꢀ1) was used as a supporting electrolyte. All potentials
are referenced vs SCE.
Computational Chemistry. The geometries of complexes C1 to C6 in
the 1A electronic ground state have been optimized at the density
functional theory (DFT) level using the PBE-D functional23 and TZP
basis sets24 for all of the atoms. These calculations are under a C1
symmetry constraint in a vacuum and have been performed using
ADF2010.02 quantum chemistry software.25 Although PBE-D repro-
duces the π-stacking structure, it is not good at describing the planarity
of the floppy system and tends to make L1 curved. Therefore, we add
several constraints to keep L1 planar. The theoretical absorption spectra
of complexes C3 and C6 have been calculated by means of the time-
dependent DFT (TD-DFT) method26 using the B3LYP functional27
without a solvent effect with the enlarged cc-pVDZ basis sets for the
hydrogen, bromide, and second-row atoms28 and the modified Ahlrichs
TZV basis set (7s, 6p, 5d) contracted to [6s, 3p, 3d] for the Cu atom.29
2,9-Dimesityl-1,10-phenanthroline-[a:b]imidazo-(40-bromophenyl)
(L1). Compound 4 (50 mg, 0.10 mmol), p-bromobenzaldehyde (20 mg,
0.11 mmol), and ammonium acetate (170 mg, 2.2 mmol) were dissolved
in glacial acetic acid (5 mL) and the mixture was refluxed overnight.
After concentration under reduced pressure, water was added to pre-
cipitate a beige solid. The latter was subjected to column chromato
graphy on silica, eluting with a gradient of methanol in dichloromethane
(0 to 1%). The main yellow ring was collected, and the solvent was
removed by rotary evaporation, to afford an off-white powder. Yield:
42mg(62%). 1H NMR (300 MHz, [D6] DMSO, 25°C): δ13.81 (s, broad,
1H, Himidazole), 8.97 (d, 2H, H4 and H7), 8.27 (d, 2H, H11 and H14), 7.82
(d, 2H, H12 and H13), 7.74 (d, 1H, H3), 7.68 (d, 1H, H8), 6.95 (s, 4H,
Hmes), 2.29 (s, 6H, CH3,mes), 2.03 (s, 12H, CH3,mes) ppm. HR-MS
(m/z) 611.1810 [M+H]+. Anal. for L1 1/2CH3COOH. Found (%): C,
3
71.57; H, 5.31; N, 8.38. Calcd: C, 71.14; H, 5.18; N, 8.73.
2,9-Bis-n-butyl-30-bromo-dipyrido[3,2-a:20,30-c]phenazine (L2). Com-
pound 5 (91 mg, 0.28 mmol) and 4-bromo-1,2-diaminobenzene (53 mg,
0.28 mmol) were dissolved in 30 mL of ethanol. The mixture was reflux-
ed overnight. After concentration under reduced pressure, the addition of
water yielded a brown powder, which was chromatographed on silica, with
dichloromethane as an eluent. Yield: 56 mg (62%). 1H NMR (300 MHz,
CDCl3, 25 °C): δ 9.50 (m, 2H, H4 and H7), 8.53 (d, 1H, H13), 8.20
(d, 1H, H11), 7.95 (dd, 1H, H12), 7.68 (d, 2H, H3 and H8), 3.28 (m, 4H,
CH2,butyl), 1.95 (m, 4H, CH2,butyl), 1.55 (m, 4H, CH2,butyl), 1.06 (t, 6H,
CH3,butyl).HR-MS(m/z) 473.1351 [M]+. Anal. forL2 1/2H2O. Found (%):
3
C, 64.72; H, 5.38; N, 11.35. Calcd: C, 64.73; H, 5.43; N, 11.61.
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0
0
General Procedure for the Synthesis of Copper(I) Complexes
C1ꢀC6. Cu(CH3CN)4PF6 was dissolved in distilled dichloromethane
and thoroughly degassed by a few vacuum/argon cycles. An argon
purged solution of mesityl bearing ligand phenA (L1 or Mes-phen; 1.0
equivalent) in dichloromethane was then syringed into the former
solution, and the mixture was stirred at room temperature. After 5
min, an argon purged solution of ligand phenB (1.0 equivalent, phenB =
phen, Me-phen, nBu-phen, or L2) in dichloromethane was syringed into
the yellow mixture. The latter immediately turned deep red and was left
to stir for half an hour at room temperature. The solvent was then
removed by rotary evaporation. The red residue was eventually purified
by chromatography on silica gel. Reactions were all performed at least
three times, and yields were all comprised between 75 and 85%. In what
follows, H nuclei belonging to phenB (nonmesityl derivatized) are
primed.
H3 and H8), 7.51 (d, 2H, H3 and H8 ), 6.20 (s, 4H, Hmes,arom), 2.31
(m, 4H, CH2,butyl), 1.91 (s, 6H, CH3,mes), 1.54 (s, 12H, CH3,mes), 1.16
(m, 4H, CH2,butyl), 0.81 (m, 4H, CH2,butyl), 0.67 (m, 6H, CH3,butyl).
HR-MS (m/z): 771.3489 [M ꢀ PF6]+. Anal. for C6 CH3OH H2O.
3
3
Found (%): C, 63.31; H, 6.08; N, 5.44. Calcd: C, 63.31; H, 6.04; N, 5.79.
Complex C7. In a Schlenk tube fitted with a water condenser, C2
(27 mg, 0.024 mmol) and p-methoxyphenyl boronic acid (19 mg, 0.13
mmol) were dissolved in 4 mL of a 9:1 (v/v) mixture of 1,2-dimethox-
yethane (DME) and water, respectively. Barium hydroxide (50 mg, 0.29
mmol) was then added. The setup was thoroughly degassed with argon,
and palladium tetrakis triphenylphosphine (3 mg, 0.003 mmol) was
quickly added. The mixture was heated to 110 °C and stirred for 16 h,
under argon. It was then allowed to cool down to room temperature, and
water was added. The orange suspension was extracted three times with
dichloromethane. The organic layers were gathered, dried on sodium
sulfate, and evaporated under reduced pressure. The product was puri-
fied by column chromatography on silica gel and eluted with a gradient
of methanol in dichloromethane (from 2% to 10%) to yield a red
powder. Yield: 9 mg, 34%. 1H NMR (300 MHz, CDCl3, 25 °C): δ 15.30
(s broad, 1H, Himidazole), 10.46 (s broad, 1H, H4 or H7), 9.40 (d, 1H, H7
C1: obtained with phenA = L1, phenB = Me-phen. 1H NMR (300 MHz,
CDCl3, 25 °C): δ 11.92 (s, 1H, Himidazole), 9.45 (d, 1H, H4 or H7), 9.36
0
0
(d, 1H, H7 or H4), 8.34 (d, 2H, H11 and H14), 8.22 (d, 2H, H4 and H7 ),
0
0
7.82 (d, 2H, H12 and H13), 7.79 0(s, 2H, H5 and H6 ), 7.72 (d, 2H, H3
0
and H8), 7.49 (d, 2H, H3 and H8 ), 6.11 (s, 2H, Hmes,arom), 6.02 (s, 2H,
0
0
H
mes,arom), 2.21 (s, 6H, CH3,methyl), 1.76 (s, 3H, CH3,mes), 1.73 (s, 3H,
or H4),0 8.88 (d, 2H, H11 and H14), 8.22 (d, 2H, H4 and H7 ), 7.77 (m,
0
CH3,mes), 1.63 (s, 6H, CH3,mes), 1.56 (s, 6H, CH3,mes). HR-MS (m/z):
6H, H5 , H6 , H30, H8, H12 and H13), 7.63 (d, 2H, H15 and H18), 7.46 (d,
0
881.2021 [M-PF6]+. Anal. for C1 CH3OH. Found (%): C, 58.73; H,
2H, H3 and H8 ), 7.00 (d, 2H, H16 and H17), 6.20 (s, 2H, Hmes,arom),
6.11 (s, 2H, Hmes,arom), 3.88 (s, 3H, OCH3), 2.35 (m, 4H, CH2,butyl),
1.90 (s, 3H, CH3,mes), 1.86 (s, 3H, CH3,mes), 1.57 (s, 6H, CH3,mes), 1.49
(s, 6H, CH3,mes), 1.18 (m, 4H, CH2,butyl), 0.82 (m, 4H, CH2,butyl), 0.62
(t, 6H, CH3,butyl). HR-MS (m/z): 993.4243 [M ꢀ PF6]+.
3
4.28; N, 7.91. Calcd: C, 58.90; H, 4.47; N, 7.93.
1
C2: obtained with phenA = L1, phenB = nBu-phen. H NMR (300
MHz, CDCl3, 25 °C): δ 11.97 (s, 1H, Himidazole), 9.37 (m, 2H, H4
0
0
and H7), 8.31 (d, 2H, H11 and H14), 8.27 (d, 2H, H4 and H7 ), 7.83 (s, 2H,
0
0
H5 and H6 ), 7.78 (d, 2H, H12 and H13), 7.65 (d, 2H, H3 and H8), 7.48
Complex C8. In a Schlenk tube fitted with a water condenser, C3
(25 mg, 0.022 mmol) and p-methoxyphenyl boronic acid (18 mg,
0.12 mmol) were dissolved in 2 mL of a 9:1 (v/v) mixture of 1,2-dimeth-
oxyethane (DME) and water, respectively. Barium hydroxide (39 mg,
0.23 mmol) was then added. The setup was thoroughly degassed with
argon, and palladium tetrakis triphenylphosphine (5 mg, 0.004 mmol)
was quickly added. The mixture was heated to 110 °C and stirred for
16 h, under argon. It was then allowed to cool down to room tempera-
ture, and water was added. The orange suspension was extracted three
times with dichloromethane. The organic layers were gathered, dried on
sodium sulfate, and evaporated under reduced pressure. The product
was purified by column chromatography on silica gel and prepared with
CH2Cl2/CH3OH (= 94:6) to yield a red powder.
0
0
(d, 2H, H3 and H8 ), 6.22 (s, 2H, Hmes,arom), 6.11 (s, 2H, Hmes,arom),
2.33 (m, 4H, CH2,butyl), 1.91 (s, 3H, CH3,mes), 1.85 (s, 3H, CH3,mes),
1.58 (s, 6H, CH3,mes), 1.49 (s, 6H, CH3,mes), 1.18 (m, 4H, CH2,butyl),
0.80 (m, 4H, CH2,butyl), 0.61 (t, 6H, CH3,butyl). HR-MS (m/z):
965.2970 [M ꢀ PF6]+. Anal. for C2 1/2CH3OH. Found (%): C,
3
61.21; H, 4.85; N, 7.28. Calcd: C, 61.20; H, 5.09; N, 7.26.
C3: obtained with phenA = Mes-phen, phenB = L2. H NMR (300
1
0
0
MHz, CDCl3, 25 °C): δ 9.56 (m, 2H, H4 and H7 ), 8.76 (d, 2H, H4 and
0
0
H7)0, 8.60 (d, 1H, H13 ), 8.28 (m, 3H, H5, H6 and H11 ),0 8.04 (dd, 1H,
0
H
H
12 ), 7.79 (d, 2H, H3 and H8), 7.71 (m, 2H, H3 and H8 ), 6.19 (s, 2H,
mes,arom), 6.17 (s, 2H, Hmes,arom), 2.38 (m, 4H, CH2,butyl), 1.72 (s, 6H,
CH3,mes), 1.62 (s, 6H, CH3,mes), 1.60 (s, 6H, CH3,mes), 1.24 (m, 4H,
CH2,butyl), 0.84 (m, 4H, CH2,butyl), 0.70 (m, 6H, CH3,butyl). HR-MS
Yield: 21 mg, 84%. 1H NMR (300 MHz, CDCl3, 25 °C): δ 9.59 (2d,
0
0
0
2H, H4 and H7 ), 8.76(d, 2H, H4 and H7), 8.55 (d, 1H, H13 ), 8.43 (d,
(m/z) 953.2801 [M ꢀ PF6]+. Anal. for C3 CH3OH. Found (%): C,
3
0
0
0
0
1H, H11 ), 8.28 (m, 3H, H5, H6 and H12 ), 07.86 (d, 02H, H14 and H17 ),
60.63; H, 4.86; N, 7.34. Calcd: C, 60.56; H, 5.08; N, 7.43.
0
7.79 (d, 02H, H3 and H8), 7.68 (2d, 2H, H3 and H8 ), 7.13 (d, 2H, H15
and H16 ), 6.19 (s, 4H, Hmes,arom), 3.94 (s, 3H, OCH3), 2.39 (m, 4H,
CH2,butyl), 1.74 (s, 6H, CH3,mes), 1.62 (s, 12H, CH3,mes), 1,25 (m, 4H,
CH2,butyl), 0.85 (m, 4H, CH2,butyl), 0.70 (m, 6H, CH3,butyl). HR-MS
(m/z): 979.4152 [M-PF6]+.
1
C4: obtained with phenA = L1, phenB = L2. H NMR (300 MHz,
0
0
0
0
CDCl3, 25 °C): δ 9.57 (d, 1H, H4 or H7 ), 9.55 (d, 1H, H7 or H4 ), 9.45
0
(d, 2H, H4 and H7), 8.64 (d, 1H, H13 ), 8.38 (d, 2H, H11 and H14), 8.31
0
0
(d, 1H, H11 ), 8.08 (dd, 1H, H120 ), 7.82 (0d, 2H, H3 and H8), 7.74 (d, 2H,
H
12 and H13), 7.65 (m, 2H, H3 and H8 ), 6.17 (s, 2H, Hmes,arom), 6.15
(s, 2H, Hmes,arom), 2.40 (m, 4H, CH2,butyl), 1.72 (s, 6H, CH3,mes), 1.62
(s, 6H, CH3,mes), 1.60 (s, 6H, CH3,mes), 1.25 (m, 4H, CH2,butyl), 0.84
(m, 4H, CH2,butyl), 0.64 (m, 6H, CH3,butyl). HR-MS (m/z): 1145.2266
Crystal Structure Data for C56H53BrCuN6 PF6 (ClCH3)0.629.
3
3
Structure data are as follows: Mw = 1130.2, colorless block, 0.4 ꢁ 0.2 ꢁ
0.15 mm3, triclinic, P1, a = 13.6027(6) Å, b = 14.2161(14) Å, c =
15.4400(5) Å, α = 73.945(6)°, β = 83.606(6)°, γ = 85.364(7)°, V =
2847.5(3) Å3, Z = 2, Dx = 1.318 g cmꢀ3, μ = 1.2 mmꢀ1. A total of 82 229
reflections were measured on a Nonius-Kappa CCD diffractometer
[M ꢀ PF6]+. Anal. for C4 2CH3OH. Found (%): C, 57.43; H, 4.56; N,
3
8.02. Calcd: C, 57.51; H, 4.75; N, 8.25.
C5: obtained with phenA = L1, phenB = Phen. Data for complex C5:
1H NMR (300 MHz, CDCl3, 25 °C): δ 9.36 (d, 2H, H4 and H7), 8.50
(graphite monochromator, λ= 0.71073 Å) up to a resolution of (sin θ/λ)max
=
=
0
0
0
0
(d, 2H, H2 and H9 ), 8.39 (d, 2H, H4 and H7 ), 8.32 (d, 2H, H11 and
0.66 Åꢀ1 at 100 K. A total of 13 612 reflections were unique (Rint
0
0
H14), 7.86 (s, 2H, H50 and H6 ), 7.82 (d, 2H, H12 and H13), 7.70 (m, 4H,
0.10). The structure was solved using the charge flipping algorithm31
implemented in the Superflip program32 and refined with the JANA2006
program33 against F2 forall reflections. A disorder intheL2 moiety, i.e., the
superposition of the two regioisomers, was correctly modeled using a rigid
body approach. However, the solvent molecules occupying the large void
(>200 Å) could not be determined; only two chloromethane solvent
molecules with a partial occupancy ratio were identified. Obviously,
the solvent molecules are disordered correlatively with the L2 moiety.
0
H3, H8, H3 and H8 ), 5.88 (s, 4H, Hmes,arom), 1.70 (s, 12H, CH3,mes),
1.53 (s, 6H, CH3,mes). HR-MS (m/z): 853.1721 [M-PF6]+. Anal. for
C5 CH3OH. Found (%): C, 58.15; H, 4.17; N, 7.84. Calcd.: C, 58.17;
3
H, 4.20; N, 8.14.
C6: obtained with phenA = Mes-phen, phenB = nBu-phen. 1H NMR
0
(300 MHz, CDCl3, 25 °C): δ 8.70 (d, 2H, H4 and H7), 8.31 (d, 2H, H4
0
0
0
and H7 ), 8.24 (s, 2H, H5 and H6), 7.85 (s, 2H, H5 and H6 ), 7.76 (d, 2H,
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Inorganic Chemistry
ARTICLE
a
Scheme 1. Synthesis of Ligands L1 and L2
a i: 2-nitropropane, Na2CO3, reflux under argon; yield: 91%. ii: dimethylethyleneglycol/H2O (10/1), Ba(OH)2, mesityl boronic acid, Pd(PPh3)4, reflux
under argon; yield: 88%. iii: TFA, H2O, CH2Cl2, R.T.; yield: 87%. iv: 4-bromobenzaldehyde, NH4OAc (20 eqv.), glacial acetic acid, reflux; yield: 62%.
v: 4-bromo-1,2-phenylenediamine, ethanol at reflux; yield: 62%.
Non-hydrogen atoms were refined with anisotropic displacement para-
meters. All H atoms were introduced in geometrically optimized
positions and refined with a riding model, except for methyl group H
atoms, the positions of wich were refined under constraints. Altogether,
674 parameters were refined. R1/wR2 [I g 2σ(I)] = 0.1124/0.2534. R1/
wR2 [all reflections] = 0.1625/0.2675, S = 3.25. Residual electron density
is rather large (between 4.2 and ꢀ4.9 eꢀÅꢀ3), due to the disorder and
the unsolved solvent void.
1H NMR confirmed the molecular structures of both L1 and L2. As
regards L1, it is worth noticing the presence of a broad, very unshielded
signal at 13.81 ppm, assigned to the slightly acidic proton borne by the
imidazole ring. Furthermore, a singlet is observed at 6.95 ppm, char-
acteristic of the four hydrogen nuclei from the two mesityl groups. The
signals corresponding to the phenanthroline moiety (H3, H4, H7, and
H8) deserve, however, a comment. In Figure S1 (Supporting In-
formation), H3 and H8 are not chemically equivalent, being displayed
as two separate doublets. The presence of an impurity, like an oxazole
derivative, traditionally obtained as a side product in Steck and Day’s
synthetic protocol,37 can be ruled out since HR-MS and microanalysis
are both in agreement with the proposed structure for L1. Such a peculiar
behavior could be explained by the asymmetry of the imidazole ring in
which one nitrogen is protonated and the other is free. Indeed, in the
presence of a strong acid (trifluoroacetic acid) or an organic base
(triethylamine), the two doublets actually merge, giving rise to the
spectrum of a fully C2v symmetric molecule (see Figure S1). Himidazole
might somehow induce some asymmetry in the system, resulting in
nonequivalent H3 and H8. The effect is milder on H4 and H7, yet the
corresponding doublet is slightly broader, with a full width at half-
maximum of 2.9 vs 2.3 Hz for H3 and H8. Despite this puzzling feature,
the spectra of L1 and L2 fall within the same range of chemical shifts as
their nonsubstituted counterparts, 2-(4-bromophenyl)imidazo[4,5-f]-
1,10-phenanthroline38,39 (see Figure S2, Supporting Information) and
11-bromodipyrido[3,2-a:20,30-c]phenazine,20 respectively, highlighting
the feeble electronic interaction between the phenanthroline core and
the bulky groups grafted on positions 2 and 9.
CCDC 818959 contains the supplementary crystallographic data for
this paper. These data can be obtained free ofcharge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Synthesis. The syntheses of ligands L1 and L2 require two very
similar building blocks: 2,9-dimesityl- and 2,9-di-n-butyl-1,10-phenan-
throline-5,6-dione, 4 and 5, respectively (Scheme 1). The synthesis of
the latter has been previously reported34 and consists of the classic
chemical oxidation of n-Bu-phen35 in a mixture of sulphuric and nitric acid
with sodium bromide. These rather harsh conditions require a cautious
“every second control” of the reaction medium (temperature, stirring
velocity, etc.). Under the same conditions, Mes-phen was irremediably
damaged, and no traces of 4 were ever detected. We therefore explored a
different approach, proposed by Sauvage and co-workers,22 relying on
Suzuki’s palladium catalyzed cross coupling reaction. Dione 1 was
obtained following literature procedures.22,36 First of all, 1 was protected
in the presence of 2-nitropropane in a basic aqueous medium; the result-
ing ketal was reacted with mesityl boronic acid, in the presence of barium
hydroxide and tetrakis triphenylphosphine palladium(0). The protec-
tion step appeared to be mandatory to avoid degradation of the phen-
dione core into a fluorenone derivative, under the Suzuki reaction con-
ditions. The bis-functionalized product 3 was obtained in 88% yield,
after purification by chromatography. Subsequent deprotection in TFA
afforded dione 4, which was reacted with 4-bromo-benzaldehyde in
acetic acid at reflux, in the presence of excess ammonium acetate,
yielding ligand L1.37
The copper(I) complexes were synthesized in distilled, argon-purged
dichloromethane (Scheme 2). In the HETPHEN concept, the idea is to
place highly bulky phenA (Mes-phen or parent L1) opposite to a less
sterically challenged phenanthroline ligand (phenB) in the presence of
copper(I) in solution. The steric bulkiness of mesityl pendant groups
prevents the formation of homoleptic species. Tetrakis(acetonitrile)
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Inorganic Chemistry
ARTICLE
Scheme 2. Synthesis of Copper(I) Complexes C1ꢀC5
copper(I) hexafluorophosphate [CuI(CH3CN)4]PF6 was first dis-
40
solved, and one equivalent of ligand L1 or Mes-phen was added, under
argon. The solution turned yellow at once, upon formation of the solvato
complexes [CuI(Mes-phen)(CH3CN)]+ or [CuI(L1)(CH3CN)]+.
Once the complementary ligand phenB (phenB = phen, Me-phen,
nBu-phen, or L2) was added to the mixture, the color quickly turned
deep red, evidencing the formation of a copper complex bearing two
phenanthroline ligands. Complexes [CuI(L1)(phenB)](PF6) (complex
C1 if phenB = Me-phen, complex C2 if phenB = nBu-phen, complex C4 if
phenB = L2 and complex C5 if phenB = phen) and [CuI(Mes-phen)-
(L2)](PF6) (complex C3) were chromatographed on silica gel and
characterized by 1H NMR, HR-MS, and elemental analysis, evidencing
the unambiguous formation of pure heteroleptic copper(I) complexes.
Complex C6 ([CuI(Mes-phen)(nBu-phen)](PF6) was synthesized too,
being a convenient and simple model of the coordination cages for
C1ꢀC5. 1H NMR spectra revealed again that some hydrogen nuclei are
nonequivalent, all the singlets describing mesityl aromatic and aliphatic
protons being for instance split into two signals. In presence of traces of
water, the spectrum of a fully symmetrical molecule was obtained, along
with the disappearance of the imidazole signal, supporting that Himidazole
is playing a crucial role in the symmetry of the complexes C1, C2, and
C4. In addition to this, an overall slight downfield shift for most signals
was observed, due to the presence of copper in the phenanthroline
cavity. On the other hand, πꢀπ interactions between the mesityl groups
of L1 (or Mes-phen) and the phenanthroline core of each ligand phenB
lead to the upfield shift of the aromatic mesityl H signal (see Figure S3,
Supporting Information).
Figure 1. Top: ORTEP view of complex C3. Ellipsoids are drawn at
the 50% probability level. Bottom: definitions of the rocking and
wagging angles.
afforded crystals that were suitable for a structure resolution by
X-ray diffraction at 100 K. The ORTEP view is given in Figure 1,
and the obtained structure is featured in the Supporting Informa-
tion (Figure S4, H atoms, two chloromethane solvent molecules,
ꢀ
and counterion PF6 have been removed for clarity). The
hypothesized geometry of the metal center is confirmed, namely,
a very distorted tetrahedral symmetry, with a copper(I) ion
surrounded by four coordinating nitrogen atoms. Relevant dis-
tances and angles are given in Table 1. The structure symmetry is
triclinic P1. Although a rather clear picture of the coordination
’ RESULTS
X-Ray Structure vs DFT Optimized Structures. Slow diffu-
sion of cyclohexane into a solution of C3 in dichloromethane
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Inorganic Chemistry
ARTICLE
Table 1. Selected Bond Lengths (in Å) and Bond Angles (in deg) of Complexes C1 to C6 (Numbering of the Atoms According to
the Structures Depicted in Figure 2)a
geometrical parameters
C1
C2
C3
C4
C5
C6
CuꢀNphenB
CuꢀN4
2.003
2.005
2.003
2.009
2.075
1.999
1.996
2.011
CuꢀN3
2.028
2.020
2.060
2.014
2.072
1.999
2.025
2.006
2.075
2.017
2.115
2.008
CuꢀNphenA
CuꢀN1
2.004
2.075
CuꢀN2
2.031
2.057
2.068
2.023
2.071
2.124
82.15
bite angles NphenBꢀCuꢀNphenA
bite angles NphenAꢀCuꢀNphenB
NphenBꢀCuꢀNphenA
N3ꢀCu-N4
N1ꢀCuꢀN2
N3ꢀCuꢀN1
N2ꢀCuꢀN3
N1ꢀCuꢀN4
N2ꢀCuꢀN4
83.35
82.65
82.24
83.27
82.36
81.45
82.26
81.88
82.32
82.49
82.12
81.96
81.98
124.55
123.38
97.31
131.84
109.10
127.58
127.58
124.87
100.42
142.04
121.34
124.52
96.75
133.08
109.35
123.85
131.24
124.32
98.68
95.53
144.47
147.18
120.47
119.68
144.67
120.10
143.09
122.21
a Bold cells: experimental corresponding values measured from the X-ray structure of complex C3 shown in Figure 1. PhenA and PhenB refer respectively
to mesityl and non-mesityl derivatized ligands.
sphere of copper(I) is given by the X-ray structure, the L2 moiety
is subjected to a very disordered environment, as evidenced in
Figure 1. Indeed, the bromine atom, which is end-capping the
phenazine appendix of L2, entails the existence of two regioi-
somers for C3, C3a and C3b. Unexpectedly though, both
molecules crystallize almost the same way, in identical crystal-
lographic positions at the level of the {Cu(Mes-Phen)(nBu-
Phen)} moiety; the structures of the isomers start to slightly
diverge along the phenazine spacer axis. The complex diffraction
pattern could nevertheless be well fitted considering a ratio of 3:7
for C3a/C3b, with a very satisfying R factor, vouching for the
accuracy of the structure. This rather peculiar behavior has been
previously observed for the complex [ReCl(CO)3(dppzBr)]2+,
where dppzBr is same as L2 but devoid of n-butyl chains.20
Indeed, the singly brominated dppz ligand appeared nonetheless
symmetrical, as if doubly substituted, in the structure. In our case,
we present in Figure 1 the overlay of the structures of both
regioisomers, which do not exactly coincide at the level of the
brominated end but perfectly match at the level of the copper
coordination cage. Let us add that part of the remaining disorder
afflicting the structure is due to the fact that solvent molecules for
C3a and C3b do not occupy the same crystallographic positions.
Figure 2. DFT/PBE-D optimized structures of complexes C1 to C6.
To sum up, CuꢀN bond lengths are typical of copper(I)
phenanthroline complexes,35,38,39,41,42 each ligand being coordi-
nated to copper(I) through two nonequivalent CuꢀN bonds
entails a strong deviation from the ideal tetrahedral geometry at
the copper(I) center toward a heavily distorted trigonal pyramid.2
(CuꢀN1 = 2.004 Å, CuꢀN2 = 2.124 Å, CuꢀN3 = 2.009 Å,
CuꢀN4 = 2.115 Å). NꢀCuꢀN bite angles are very similar
As a consequence, the other mesityl substituent (label A in
(81.98° and 81.45°), owing to the rigidity of both phenanthroline
derivatized ligands Mes-phen and L2. Most importantly, an
obvious π stacking is evidenced between just one mesityl sus-
btituent of ligand Mes-phen (label B in Figure 1) and the adjacent
L2 phenanthroline, with a typical distance of ca. 3.45 Å.38,39 This
Figure 1) is unable to interact with L2, being too remote from the
latter. This explains as well why the dihedral angles between the
mesityl groups and the phenanthroline plane of Mes-phen are
significantly different (64.11° for the more constrained mesityl
group vs 72.97° for the other). Overall, ligand Mes-phen has
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Inorganic Chemistry
ARTICLE
The calculated Cu-heteroleptic ligands’ bond angles are very close
to the experimental angles of 81.45° and 81.98°. The structures of
C2, C3, C4, and C6, which have n-Bu-phen ligands, are very
similar. They have two shorter CuꢀN bonds (2.00ꢀ2.02 Å) and
longer CuꢀN bonds (2.06ꢀ2.08 Å), and the angles of
N1ꢀCuꢀN4 are large (142ꢀ145°). On the other hand, in C1
and C5 complexes, which do not contain n-Bu-phen ligands, all
four CuꢀN bonds are very close (2.00ꢀ2.03 Å), and the
N1ꢀCuꢀN4 angles amount to 128° and 124°, respectively. These
differencesmay beattributedtothe steric hindrance of n-Bu and π-
stacking mesityl groups toward one side of the n-Bu-phen ligand.
Cartesian coordinates of these structures are shown in Table S1
(Supporting Information).
UVꢀVisible Absorbance Spectroscopy and TD-DFT Ab-
sorption Spectra. The electronic absorption spectra of com-
plexes C1ꢀC6 are given in Figure 3, and corresponding data are
gathered in Table 2. All complexes displayed a set of intense
bands in the UV region that were all assigned to ligand-centered
πꢀπ* transitions.
In the visible domain, all spectra are dominated by the
expected copper(I) to phen MLCT, around 460 nm, which
corresponds to the promotion of one electron from the copper-
(I) center toward a phen centered π* orbital, yielding a charge
separated state that can be formulated CuII-(phen•ꢀ). With the
exception of complex C5, the MLCT energy of complexes
C1ꢀC4 and C6 is only slightly dependent on the structure.
Nevertheless, the absorption maximum of C1 is slightly but
sensibly red-shifted compared to C2, C3, and C4. The reason lies
in the steric hindrance within the coordination cage around the
copper(I) cation. Indeed, it has been reported that the bulkier the
substituents, the more rigid the coordination cage, and the higher
the energy of the MLCT.42,43 There are exceptions to this simple
rule, in particular for [CuI(2,9-diphenyl-1,10-phenanthroline)2]+,
since π stacking of the phenyl substituents with the vicinal
phenanthroline may entail the D2dfD2 transition.2 For com-
plexes C1ꢀC4 and C6, alkyl chains are opposed to mesityl
groups, creating a rather bulky environment within the coordina-
tion sphere of copper(I); the fact that the methyl group is less
sterically demanding than the n-butyl chain could account for the
slight red-shift of the MLCT in C1, compared to C2ꢀC4 and
C6.43 This is even more blatant in the case of complex C5, where
the encumbered 2,9-dimesityl-phenanthroline type ligand L1 is
facing plain phenanthroline. Additionally, the MLCT of C5
exhibits a broad shoulder around 550 nm, which is a diagnostic
band for stronger D2 deformation, likely due to π stacking
between phenanthroline and the mesityl groups.2,3,41 This transition
is nevertheless rather weak when compared to other complexes,
especially those built on 2,9-diphenyl phenanthroline (R = phenyl).
For L1 (and Mes-phen), the presence of the mesityl groups greatly
increases the steric hindrance of the coordination cage, thus limiting
the extent of D2d to D2 deformation.
Figure 3. Electronic absorption spectra of complexes C1ꢀC6 in
dichloromethane. Inset: magnification of the 400ꢀ600 nm domain.
Table 2. Spectroscopic and Electrochemical Data for Com-
plexes C1ꢀC6 and Reference [CuI(Me-phen)2]+a
λMLCT/nm
(ε/L molꢀ1 cmꢀ1
E1/2 (V) vs SCE
(ΔE = Epa ꢀ Epc, mV)b
complex
)
C1
C2
C3
C4
C5
C6
468 (6300)
463 (5500)
465 (6300)
467 (5700)
476 (7850)
458 (4250)
455 (7950)c
0.87 (95)
0.94 (105)
ꢀ1.09 (95), 1.06 (85)
ꢀ1.03 (80), 1.05 (140)
0.71 (95)
0.99 (140)
[CuI(Me-phen)2]+
0.83 (110)
a All measurements performed in distilled dichloromethane at room
temperature. b Recorded in dichloromethane/TBAP 0.1 M with a plati-
num disk as the working electrode and SCE as a reference. c See ref 47.
“rocked and wagged away” from the ideal D2d symmetry. This is
in full agreement with the structures of previously published
complexes of the type [CuI(Mes-phen)(Phen0)]+,11,13,16,39
although in the latter case Phen0 did not bear any bulky
substituents at the 2 and 9 positions and was not substituted
with fused imidazole or phenazine.
The structures of complexes C1ꢀC6 have been scrutinized
using DFT/PBE-D/TZP calculations. Selected theoretical bond
lengths and bond angles of complexes C1ꢀC6 are reported in
Table 1 together with some experimental X-ray data of complex
C3 for comparison. The optimized structures are represented in
Figure 2.
The optimized geometries compare rather well with the X-ray
data reported in Table 1 for complex C3 and with the structures of
similar complexes of Cu(I). The four main CuꢀN coordination
bond distances vary between 2.00 and 2.08 Å, slightly shorter than
the experimental values which span between 2.004 and 2.124 Å.
Interestingly, the energies of the MLCT in complex C6 and
homoleptic [CuI(Me-phen)2]+ are almost identical, being blue-
shifted by 5 to 10 nm when compared to C1ꢀC4. The
coordination spheres of C1ꢀC4 and C6 are very similar
(which is supported by DFT calculations), and the rigidity of
the copperꢀphenanthroline core cannot therefore be invoked
here to rationalize this experimental fact. On the other hand, the
conjugation of L1 and/or L2 likely entails a stabilization of the
MLCT state; the effect remains however rather weak. Let us add
that heteroleptic copper complexes display usually a sensibly
lower energy MLCT than their homoleptic counterparts (about
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Inorganic Chemistry
ARTICLE
Table 3. TD-DFT Wavelengths (in nm) to the Low-Lying
Singlet Excited States of Complexes C3 and C6 and Asso-
ciated Oscillator Strengths Greater than 0.02a
complex
state
ΔE (nm)
f
C3
MLCTphenB
MLCTphenA
MLCTphenA
MLCTphenB
522
494
482
464
393
384
378
372
362
357
349
338
335
333
330
330
325
319
315
308
305
303
297
295
295
293
285
275
273
272
0.02
0.04
0.02
0.03
0.06
0.05
0.04
0.02
0.04
0.10
0.02
0.02
0.02
0.02
0.04
0.02
0.02
0.02
0.03
0.13
0.02
0.07
0.44
0.03
0.22
0.04
0.14
0.02
0.04
0.11
LLCTphenB/ILphenB
LLCTphenB/ILphenB
LLCTphenB
Figure 4. TD-DFT Theoretical Absorption Spectra of Complexes C3
and C6.
ILphenB
10 nm). The existence of a dipole taking its origin in the
asymmetry of complexes C1ꢀC5 could as well be held respon-
sible for stabilizing the MLCT, this dipole being of a lesser
intensity for C6. Additionally, the πꢀπ interaction between the
mesityl groups and the adjacent phenanthroline might as well
account for the observed red shift of the MLCT for C1ꢀC4 (and
marginally C6) compared to [CuI(nBu-phen)2]+.
A greater delocalization of the π electrons on conjugated L2
may explain the slight red-shift of the MLCT of complex C4
compared to C2 confirmed by theory (see Table 2). The energy
change here is very small though. Much larger shifts to longer
wavelengths have been observed for phenyl- or phenylalkynyl-
substituted phenanthrolines.2,44 But in the famous case of dppz
and complexes thereof, it has been demonstrated that the fused
phenazine and bipyridine moieties behave a lot like independent
units, from the spectroscopic and electrochemical points of
view.45,46 It is therefore not surprising that the MLCT energy
is only marginally affected by the presence of the phenazine
appendix, just like ruthenium complexes are.
LLCTphenB/ILphenB
ILphenB/LLCTphenB
MLCTphenA/ILphenB
ILphenA
LLCTphenB
LLCTphenB/ILphenA
MLCTphenA/LLCTphenB
LLCTphenB/ILphenA
LLCTphenB/ILphenB
ILphenB/LLCTphenB
ILphenB/LLCTphenB
LLCTphenB/MLCTphenB
LLCTphenA/LLCTphenB
MLCTphenB
MLCTphenB/LLCTphenB
LLCTphenB
MLCTphenB
MLCTphenB
The extinction coefficient of the MLCT depends as well on the
structure of the coordination sphere caging the copper(I) cation.
Since the latter remains roughly the same all along the series
C1ꢀC4 and C6, the values of ε are all held within a narrow
frame, ranging from 4250 to 6300 Mꢀ1 cmꢀ1, in good agreement
with previously published data.1,3,4 It is worth mentioning here
that poorer ε are not uncommon; values as low as 3000
ILphenB
MLCTphenA
ILphenB/LLCTphenA
ILphenB/ILphenA
C6
MLCTphenB/MLCTphenA
MLCTphenA
492
489
466
357
355
351
332
272
272
271
271
268
267
266
265
264
262
259
256
255
0.03
0.02
0.02
0.02
0.08
0.02
0.07
0.11
0.03
0.07
0.26
0.03
0.04
0.07
0.07
0.10
0.03
0.10
0.10
0.05
M
ꢀ1 cmꢀ1 were obtained for complex [CuI(2,9-diphenyl-1,10-
phenanthroline)2]+. This was mainly due to a counterproductive
effect of the electron delocalization on the transition dipole.2,47
As evoked above, mesityl groups are much more sterically
challenging than mere phenyl rings, and the torsion angle
between the phenanthroline and the mesityl planes dramatically
decreases the eventual interaction of their π systems. This is in
agreement with the similar spectral features measured for L1 and
its unsubstituted counterpart 2-(parabromo phenylimidazo)-
[4,5-f][1,10]phenanthroline. Yet, the extinction coefficient of
the MLCT is significantly smaller for C6 than for C1ꢀC4. This
might be due to feeble delocalization of the electrons on the
mesityl rings, lessening the transition dipole. This effect would
then be counterbalanced by the strong delocalization of the
electrons on the π systems of L1 or L2 for C1ꢀC5. Let us finish
by saying that the spectra of all complexes remain unaltered in
aerated dichloromethane or acetonitrile solutions for several
days, and even when exposed to air moisture. The stability of
these species could be the result of the favorable πꢀπ stacking
between mesityl aromatic groups and the vicinal phenanthroline
ligand phen.
MLCTphenA/MLCTphenB
MLCTphenB/LLCTphenB
MLCTphenA/ILphenA
MLCTphenA/ILphenA
MLCTphenA/ILphenA
MLCTphenA/ILphenA
MLCTphenA
LLCTphenB/ILphenB
ILphenB/LLCTphenB/MLCTphenB
MLCTphenB
MLCTphenA/ILphenB/ILphenA
MLCTphenA/ILphenA
LLCTphenB/ILphenA
LLphenB/MLCTphenA/ILphenA
MLCTphenB/LLCTphenB
MLCTphenA/ILphenA
MLCTphenA/ILphenA
MLCTphenA/ILphenA
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Table 3. Continued
complex
C4 where all ligands bear bulky substituents at positions 2 and 9
of the phenanthroline cores. The fact that methyl groups are less
sterically demanding than n-butyl chains likely accounts for the
100 mV shift toward more positive potentials from C1 to C2.
Moreover, complexes C3 and C4 built with ligand L2 show a
further increase in E1/2; steric effects cannot be put forward here
since the coordination spheres of C2, C3, and C4 are virtually the
same. Nevertheless, dppz is a well-known π-acceptor and exerts a
strong electron withdrawing power, which could be held respon-
sible for the anodic shift of the potentials for C3 and C4.
state
ΔE (nm)
f
MLCTphenA
254
253
253
251
247
0.03
0.06
0.02
0.05
0.03
MLCTphenB/LLCTphenB
LLCTphenB
MLCTphenB/LLCTphenB
MLCTphenB/LLCTphenB
a PhenA and PhenB refer respectively to mesityl and non-mesityl
derivatized ligands.
The cathodic behavior of our complexes has been investigated
within the limits imposed by the solvent used (dichloromethane)
under our conditions. For complexes C1, C2, and C5, no
reduction wave was observed, meaning that phenanthroline
based reductions (or Cu(0) formation) occur at potentials below
ꢀ1.6 V/SCE, a fact that is corroborated by the literature for
similar complexes.43,48 Conversely, the cathodic behavior of L2-
built complexes C3 and C4 revealed, for both, a well-defined one-
electron, reversible reduction wave at E1/2 = ꢀ1.09 and ꢀ1.03 V
vs SCE respectively. The shape of the wave, the value of the half
wave potentials, and the apparent insensitivity of the latter to the
complementary ligand (L1 or Mes-phen) are very reminiscent of
those of other metal based dppz complexes, in particular the
archetypal [RuII(bpy)2(dppz)]2+. We therefore assigned these
reduction waves to the addition of an extra electron on the
phenazine moiety. To further probe this assumption, we per-
formed cathodic spectroelectrochemistry on complex C3. Upon
electrolysis, several spectral changes are monitored (Figure 5); in
particular, a broad shoulder around 560 nm, extending to
620 nm, is rising while the L2 centered πꢀπ* transition at
275 nm collapses. These features are characteristic of (metal-
bound) dppz•ꢀ radical anion formation,46 corroborating the
initial assumption. Interestingly, the MLCT shows very little
change, the intensity of which being only slightly enhanced,
probably because of a spectral overlap with the 560 nm transition.
This weak electronic coupling between the phenazine moiety
and the “CuI(phen)2” core is once again reminiscent of similar
ruthenium complexes, and it seems therefore justified to
consider complexes C3 and, by extension, C4 as bichromo-
phoric systems. Additionally, several isosbestic points evidence
the formation of only one species during the electrolysis,
accounting for the stability of the reduced complex.
The TD-DFT/B3LYP absorption spectra of C3 and C6 are
presented in Figure 4. These two complexes contain a phenan-
throline substituted in 2,9 positions by butyl groups (abbreviated
phenB) and another phenanthroline substituted in 2,9 positions
by mesityl groups (abbreviated phenA). The comparison with the
above experimental spectra is not straightforward since the
computations are performed in a vacuum for the isolated
molecules. The TD-DFT solvent corrected absorption spectra
will be presented elsewhere in an article devoted to the theore-
tical study where the methodological points will be discussed.
The transition energies to the low-lying singlet excited states
together with the associated oscillator strengths are reported in
Table 3 for complexes C3 and C6.
The two complexes are characterized by an intense absorption
in the UV centered at 297 nm for complex C3 and at 271 nm for
complex C6. As mentioned above, this band corresponds to a
mixed MLCT/LLCT state for complex C3, while it is a mixture
of LLCT, IL, and MLCT transitions for complex C6. The
shoulder around 500 nm corresponds to MLCT states localized
on the phenA and phenB ligands in both complexes. The
theoretical spectra are characterized by a peak around 350 nm,
corresponding to the mixture of an IL state localized on the
phenB ligand and an LLCT state for complex C3. In complex C6,
this band is less intense, in agreement with experimental spectra,
and corresponds to a mixed MLCT/IL state localized on phenA.
From the TD-DFT results, we may observe a very high density of
states in this class of molecules, the nature and position of which
are very sensitive to the substituents.
The main excitations of some characteristic excited states and
related orbitals are shown in Table S2 and Figure S5 (Supporting
Information), respectively.
Photoluminescence Study. As mentioned in the Introduc-
tion, the main challenge to obtaining luminescent copper com-
plexes is to avoid or minimize the flattening distortion of the
complex from the CuI preferred tetrahedral geometry to the
formal CuII square planar arrangement in the MLCT excited
state.3,4 In the flattened relaxed excited state, the complex has a
smaller gap with the ground state, and the nonradiative decay is
faster. Moreover, a very efficient quenching of the excited state by
the solvent and exciplex formation can happen. Alto-
gether, these two effects shorten the lifetime and luminescence
quantum yield of the MLCT excited state. For example, while
[CuI(phen)2]+ shows no luminescence at room temperature, the
complex [CuI(Me-phen)2]+ containing methyl substituents in
positions 2 and 9 of the phenanthroline emits at 730 nm and
exhibits a luminescence quantum yield of about 4 ꢁ 10ꢀ4 with a
time decay of about 90 ns in dichloromethane at room
temperature.10 The best example of sterically blocked distortion
is given by the heteroleptic complex CuI(tert-butyl-phen)(Me-
phen)]+, in which the tert-butyl and methyl groups prevent the
flattening and give a significant luminescence quantum yield of
Electrochemistry. All six complexes were studied by cyclic
and square wave voltammetry in dry, argon purged dichloro-
methane. In all cases, a well-defined reversible oxidation wave
was monitored and naturally assigned to the removal of one
electron from the d orbitals of the copper(I) ion. The half-wave
potentials were highly dependent on the extent of the steric
hindrance at the [CuI(phenA)(phenB)]+ core.42 Indeed, the
latter undergoes a shift from a tetrahedral to a square planar
geometry when the oxidation number of the metal ion formally
changes from +I to +II, respectively. Since mesityl, n-butyl, or
methyl groups at the 2 and 9 positions of the phenanthroline
moiety prevent such a deformation of the coordination cage, the
oxidized form [CuII(phenA)(phenB)]2+ (where phenB = Me-
phen, nBu-phen or L2) is destabilized, and the potentials are
overall anodically shifted. To summarize, the larger the steric
hindrance, the higher the potential of the metal centered oxida-
tion. This is indeed exemplified in our case since E1/2 grows from
0.5 V for [CuI(phen)2]+ to 0.7 V for C5 and finally reaches values
between 0.85 and 1.05 V vs SCE for complexes C1, C2, C3, and
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ARTICLE
Figure 5. Evolution of the electronic absorption spectra of complex C3 upon cathodic electrolysis (in dichloromethane, with 0.1 M tetra-n-
butylammonium hexafluorophosphate as a supporting electrolyte and a platinum grid as the working electrode).
Figure 6. Emission spectra and excited-state time decay recorded in degassed dichloromethane for complexes C1ꢀC6 (excitation at 460 nm).
Table 4. Luminescence and Time Resolved Data for
Table 4. These measurements were recorded in dichloromethane
and acetonitrile, because these solvents have very different di-
electric constants and coordination abilities. All measurements
have been performed under the same conditions (absorbance at
the excitation wavelength and instrumental parameters) to enable
a straightforward and qualitative comparison of luminescence
quantum yields.
A general trend is a slight bathochromic shift of the maximum
emission wavelength and a shortening of the emission lifetime as
we shift from dichloromethane to acetonitrile (Table 4). This is
the consequence of the higher dielectric constant of acetonitrile,
which stabilizes the charge transfer state, and its stronger
coordinating ability, which increases exciplex quenching by
pentacoordination of Cu(II). Heteroleptic complex C6 was
prepared to assess the influence of the fused heterocycle moiety
capping the back of the phenanthroline. As the substituent size
increases, within the series C5, C1, and C2, the emission lifetime
of the MLCT naturally increases too, owing to restricted flatten-
ing of the excited state. The comparison of complex C2 with
reference complex C6 indicates that the fused imidazole ring has
little impact on the emission characteristic of the complex. On
the other hand, the ligand L2 bearing the phenazine spacer has a
Complexes C1ꢀC6a
emission and excited
state lifetime λMLCT/nm
(τ/ns (4 ns)
absorption maximum of MLCT
excited state in CH2Cl2/nm
complex
CH2Cl2
CH3CN
C1
C2
C3
C4
C5
C6
754 (31)
740 (52)
753 (21)
743 (30)
nd (e4)
nd (e4)
nd (e4)
738 (25)
540
550
630
620
550
570
∼ 800 (5)
> 800 (e4)
nd (e4)
730 (64)
a All measurements were performed under degassed conditions. nd =
not detected, very weak signal, and hardly distinguishable from the
instrumental noise.
1% and a lifetime of 730 ns in dichloromethane.17 The lumines-
cence spectra and the lifetime of MLCT excited states for
C1ꢀC6 are given in Figure 6, and the data are compiled in
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to assess this last point, we involved two of our bromo terminated
copper complexes in a palladium catalyzed cross-coupling Suzuki
reaction. The latter was chosen with regard to its high impact in
modern organic chemistry and great versatility, giving access to a
plethora of structures and new functionalities. As a proof of concept,
para-methoxy phenyl boronic acid was thus engaged in a Suzuki
coupling reaction with complexes C2 and C3 (Figure 7). A classical
procedure (catalytic Pd(PPh3)4 and Ba(OH)2 8H2O in a refluxing
3
4:1 mixture of DME/H2O overnight) appeared to be successful,
affording complexes C7 and C8 with yields of 34% and 84%,
respectively. The higher yield for C8 than C7 can be rationalized by
the greater activation of the CꢀBr bond, given the electron
withdrawing character of phenazine. It is important to notice that,
however, a small amount of homoleptic complex was formed
through the course of the reaction, which could not be removed
by chromatography owing to very similar mobilities on the
stationary phase, and prevented us from getting satisfactory
microanalysis.
Figure 7. Suzuki cross-coupling reaction performed on complexes C2
and C3, yielding C7 and C8, respectively.
profound impact on the emission properties of the resulting
copper(I) complexes C3 and C4, the latter being barely emissive
(Table 4).
Stability Assessment. The small, yet significant amount of
homoleptic complex formed during the Suzuki cross coupling
reactions unveiled a possible stability issue of our heteroleptic
systems. A series of tests was therefore undertaken, with the model
complex C6. When the latter is dissolved and heated in dichlor-
omethane (apolar, noncoordinating) or acetonitrile (polar, coor-
dinating), no [CuI(nBu-phen)2]+ was detected by 1H NMR, even
after several hours of heat (and light) stress, which tends to confirm
the stability of C6, and by extrapolation of any heteroleptic complex
with a related structure. However, in the presence of excess nBu-
phen, the heteroleptic complex spontaneously dissociates to yield the
homoleptic complex [CuI(n-Bu-phen)2]+. A quantitative analysis of
the 1H NMR data revealed that the homoleptic species is approxi-
mately 7 times more stable than C6 (Figures S7ꢀS9, Supportng
Information). The HETPHEN concept states that heteroleptic
copper complexes are stable, thanks to the bulkiness of mesityl
groups, but in the presence of an exogenous ligand, capable of
yielding homoleptic complexes (typically here, n-Bu-phen), a fast
ligand exchange occurs, owing to the well-known lability of such
species. Eventually, the more thermodynamically stable complex is
formed. In our case, the n-butyl groups are believed to enhance
the σ-donating ability of the phenanthroline chelate and there-
fore stabilize [CuI(nBu-phen)2]+ vs C6. The higher steric bulk in
C6 might account for a lesser stability too, compared to the less
sterically stringed homoleptic complex. Comparing the opti-
mized structure of [CuI(nBu-phen)2]+ and C6 using the DFT/
PBE-D method, [CuI(nBu-phen)2]+ has shorter CuꢀN bonds
(CuꢀN1, 2.021 Å; CuꢀN2, 2.021 Å; CuꢀN3, 2.023 Å; CuꢀN4,
2.023 Å) than those of C6 (Table 1) on average. [CuI(n-Bu-
phen)2]+ has an almost symmetric structure (N1ꢀCuꢀN2,
82.87°; N3ꢀCuꢀN4, 82.66°; N1ꢀCuꢀN3, 124.23°;
N1ꢀCuꢀN4, 124.34°; N2ꢀCuꢀN3, 124.34°; N2ꢀCuꢀN4,
124.23°) and from the viewpoint of coordination, this complex
is more stable than the π-stacking C6 complex. In addition to
this, when n-Bu-phen, Mes-phen, and [Cu(CH3CN)4]PF6 are all
mixed together in dichloromethane, a mixture of homo- and
heteroleptic complexes is obtained. C6 happens to be the major
species formed in this rather kinetically controlled experiment,
but should polar and coordinating acetonitrile or DME be used as
solvents instead of dichloromethane, that the trend would be
inverted (Figures S7ꢀS9). It is henceforth of paramount im-
portance to respect the order of introduction of the different
reactants, to avoid the formation of the undesired homoleptic
complex. Under the Suzuki reaction conditions, DME, H2O, and
Nanosecond transient absorption spectroscopy was used to
follow the relaxation of the triplet MLCT excited-state (Figure
S6, Supporting Information) for each complex. The transient
trace of the MLCT excited state in these complexes is red-shifted
relative to that of the other complexes (Figure S6), showing some
resemblance with the spectrum of the radical anion of the
complex C3 recorded by spectro-electrochemistry (Figure 5).
The photophysical properties of [RuII(bpy)2dppz]2+ were in-
vestigated in great detail,18,49 and it is well-accepted that the dppz
ligand can be considered a weakly coupled dyad in which the
pyrido and phenazine units behave quite independently. The
emission lifetime of the complex [RuII(bpy)2dppz]2+ is de-
creased relative to that of [RuII(bpy)3]2+ owing to the quenching
of the MLCT excited state by electron transfer to the phenazine
π-accepting unit. In addition, hydrogen bonding between the
dppz and a protic solvent is another cause of the excited-state
quenching of the MLCT excited state. With reference to these
studies on [RuII(bpy)2dppz]2+, and with the support of the
transient emission and absorption studies, the quenching of the
MLCT luminescence in C3 and C4 is likely to due to a
photoinduced electron transfer from Cu(I) to phenazine. The
charge separated state is however very short-lived, since recom-
bination occurs on the nanosecond time scale. This is never-
theless an encouraging feature, legitimating the possibility of
copper(I)ꢀphenanthroline based photochemistry.
Chemistry on the Complex. Thanks to the HETPHEN
concept, many intricate supramolecular structures were obtained
with minimal synthetic effort.9,13,15,39,50,51 It would be therefore
particularly appealing to know whether chemical alteration is
possible on such assemblies, without degrading the integrity of
the latter: copper(I) phenanthroline complexes are indeed well-
known to be kinetically labile, and their stability is at stake when they
are in the presence of any other potential ligand, like a mere Lewis
base. Encouragingly, a copper catalyzed Glaser homocoupling39 and
a Sonogashira cross-coupling50 were reported too, although in the
latter case a “kinetically locked” copper complex was used.52 “Click
chemistry” has been performed too on catenanes,53 assembled via
coordination of copper(I) by two phenanthroline cavities. Com-
plexes C1ꢀC4 are nevertheless more sterically challenged and
therefore potentially less stable than catenanes, and it is also
necessary to test the stability of heteroleptic copper complexes in
the presence of a different catalyst than Cun+ (n= 0, 1, or 2). In order
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ARTICLE
PPh3 are all potentially coordinating and could be involved in ligand
exchange reactions, leading eventually to the formation of the
homoleptic species. Nevertheless, only 5% of the latter was formed
under rather harsh, nonoptimized conditions (120 °C), asserting that
a careful choice of reactants and reaction conditions should allow for
performing chemistry on the complex.
’ ACKNOWLEDGMENT
The “Agence Nationale de la Recherche” (ANR) is gratefully
acknowledged for the financial support of this research through
the “Programme Blanc” entitled “HeteroCop” referenced ANR-
09-BLAN-0183-01. These studies were also supported by the
European program COST D35. F.O. warmly thanks Benoit
Colasson (Universitꢀe Paris Descartes) for fruitful and inspiring
discussions about the present work. The calculations were
carried out in part at the IDRIS and CINES computer centers
through a grant of computer time from the Conseil Scientifique.
’ CONCLUSIONS
This work stands within the frame of unveiling the synthesis of
copper(I) phenanthroline complexes and their physicochemical
properties. To achieve this goal, two bulky original ligands, L1 and
L2, were synthesized. L1 results from the fusion of a benzimidazole
skeleton with the bulky 2,9-dimesityl-1,10-phenanthroline. The
structure of L2 stems from the well-known dppz, bearing two n-
butyl chains in α positions of the chelating nitrogen atoms. Both
ligands provide a strong steric bulk at the copper center, leading to
the obtaining of heteroleptic complexes by combination of L1 and
L2 with adequate phenanthroline derivatives, thanks to the HET-
PHEN concept developed by Schmittel and co-workers. In spite of
the lability of phenanthroline ligands coordinated to copper(I), a
palladium catalyzed cross-coupling reaction has been successfully
performed, directly on the complex, with only marginal damage on
the starting molecules. With complex C3, the copper(I) cation is in a
distorted tetrahedral arrangement, due to π-stacking between
mesityl groups and the neighboring dppz ligand. Additionally, most
welcome stability is thus provided to the complex. From one ligand
to the other, the arrangement is basically linear, which is a
particularly important feature of copper(I)ꢀphenanthroline com-
plexes, paving the way to optimized acceptorꢀdonor photosensitive
devices. The new complexes were characterized by electrochemistry,
UVꢀvis absorption spectra, steady state and time-resolved lumi-
nescence spectroscopy, and pumpꢀprobe transient spectroscopy
and fully interpreted with the aid of DFT quantum chemical
calculations. The electron attracting character of the phenazine
unit in ligand L2 leads to fast nonradiative quenching of the
MLCT excited-state of the corresponding copper(I) complexes,
whereas for the mesityl bulky subtituents in ligand L1, a
luminescence quantum yield of about 3 ꢁ 10ꢀ4 is obtained,
with a decay time of about 50 ns for C2 measured. Therefore, the
latter complex has a potential use as a sensitizer for the design of
molecular arrays for photoinduced charge separation. Work is in
progress in our groups toward this goal.
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’ ASSOCIATED CONTENT
Supporting Information. 1H NMR spectra of the ligand
S
b
L1 and of complexes C1ꢀC6, X-ray structure of complex C3, the
representation and the nature of the orbitals involved in the
optical transitions for complexes C3 and C6, the transient
absorption spectra of complexes C1ꢀC6, the stability assess-
ment for complex C6, and the Cartesian coordinates of the
optimized structures of the complexes C1ꢀC6. This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
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Corresponding Author
*Tel.: +33 368 851314 (C.D.), +33 251125429 (F.O.). Fax:
+33 368851589 (C.D.), +33 251125402 (F.O.). E-mail: michel.
evain@univ-nantes.fr, c.daniel@chimie.u-strasbg.fr, Fabrice.Odobel@
univ-nantes.fr.
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