Synthesis, electrochemical and optical properties of RuII- diphenylphenanthroline-ethynylpyrenephenanthroline systems

Article abstract of DOI:10.1002/ejic.200701097

The synthesis, electrochemical and optical spectroscopic properties of a Ru^II-diphenylphenanthroline complex decorated with ethynylpyrene (EP) appendages, 1, and those of its parent bromo-substituted complex, 2, are reported; complex 1 is [Ru(dpp)₂(phen-EP₂)](PF ₆)₂ and 2 is [Ru(dpp)₂(phen-Br ₂)](PF₆)₂, where dpp is 4,7-diphenyl-1,10- phenanthroline, phen-EP is 5,6-(1-ethynylpyrene)-1,10-phenanthroline and phen-Br₂ is 5,6-dibromo-1,10-phenanthroline. In CH₃CN solvent, both bromo- and EP-substituted complexes are redox active and metal-centred and ligand-centred processes are assigned with reference to the analogous steps for [Ru-(phen)₃]²⁺ and to steps for the EP appendages. The complexes exhibit strong absorption bands in the near-UV and visible regions due to ligand-centred (LC) and metal-to-ligand charge-transfer (MLCT) transition, respectively. For 1, the lowest-lying absorption band is slightly redshifted due to better delocalisation at the phen-EP ligand. At room temperature and in O₂-free solvent, complex 1 appeared to be practically nonluminescent, whereas 2 exhibited intense ³MLCT emission; quantum yields of f_em = 9 × 10^-4 and 6.1 × 10^-2 were found for 1 and 2, respectively and λ_exc = 462 nm in both cases. The lowest-lying excited states of 1 were assigned as ³LC states, which were localised on the EP moiety. These states are nonluminescent, which thus explains the weak room-temperature luminescence for this complex. At 77 K, 2 is emissive, as expected for ³MLCT levels; however, a moderately intense ³MLCT emission was also observed for 1 at this temperature. For the latter case, this outcome is tentatively explained in terms of local trapping of ³MLCT states at the 4,7-diphenylphenanthroline ligands free of EP appendages. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200701097

FULL PAPER
DOI: 10.1002/ejic.200701097
Synthesis, Electrochemical and Optical Properties of RuII–
Diphenylphenanthroline–Ethynylpyrenephenanthroline Systems
Christine Goze,[a] Cristiana Sabatini,[b] Andrea Barbieri,[b] Francesco Barigelletti,*[b] and
Raymond Ziessel*[a]
Keywords: Ruthenium / Pyrenes / Energy conversion / Electrochemistry / Photophysics
The synthesis, electrochemical and optical spectroscopic
properties of a RuII–diphenylphenanthroline complex decor-
ated with ethynylpyrene (EP) appendages, 1, and those of its
parent bromo-substituted complex, 2, are reported; complex
1 is [Ru(dpp)2(phen–EP2)](PF6)2 and 2 is [Ru(dpp)2(phen–
Br2)](PF6)2, where dpp is 4,7-diphenyl-1,10-phenanthroline,
phen–EP is 5,6-(1-ethynylpyrene)-1,10-phenanthroline and
phen–Br2 is 5,6-dibromo-1,10-phenanthroline. In CH3CN sol-
vent, both bromo- and EP-substituted complexes are redox
active and metal-centred and ligand-centred processes are
assigned with reference to the analogous steps for [Ru-
(phen)3]2+ and to steps for the EP appendages. The com-
plexes exhibit strong absorption bands in the near-UV and
visible regions due to ligand-centred (LC) and metal-to-li-
gand charge-transfer (MLCT) transition, respectively. For 1,
the lowest-lying absorption band is slightly redshifted due to
better delocalisation at the phen–EP ligand. At room tem-
perature and in O2-free solvent, complex 1 appeared to be
practically nonluminescent, whereas 2 exhibited intense
3MLCT emission; quantum yields of fem = 9ϫ10–4 and
6.1ϫ10–2 were found for 1 and 2, respectively and λexc
=
462 nm in both cases. The lowest-lying excited states of 1
3
were assigned as LC states, which were localised on the EP
moiety. These states are nonluminescent, which thus ex-
plains the weak room-temperature luminescence for this
complex. At 77 K, 2 is emissive, as expected for 3MLCT
levels; however, a moderately intense 3MLCT emission was
also observed for 1 at this temperature. For the latter case,
this outcome is tentatively explained in terms of local trap-
ping of 3MLCT states at the 4,7-diphenylphenanthroline li-
gands free of EP appendages.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
that upon light absorption (either by the transition-metal
chromophore or by the pyrene unit), the final population
of interacting metal-to-ligand charge-transfer and ligand-
centred triplets, 3MLCT and 3LC, respectively, takes
place.[6] In most cases, these are localised at the transition
metal and at the pyrene components, respectively, with the
3LC (pyrene) level lying only a few hundreds of wave-
Introduction
Within light energy conversion schemes, RuII–poly-
pyridine units are useful components of assemblies de-
signed for the photogeneration of energy and electron-
transfer processes.[1] A number of model molecular wires,
dyads, triads and dendritic multicomponent systems incor-
porate such units, and their optical absorption and emission
properties have been extensively investigated.[2,3] Remark-
ably, the luminescence lifetime of some RuII–polypyridine
emitters can be conveniently tuned as a consequence of ex-
cited state equilibration processes, wherein organic counter-
parts are involved.[4] Out of the various organic units em-
ployed, which are appended to the ligands of the first coor-
dination sphere, pyrene has proved to be quite effective in
the tuning process of RuII–tris(bichelating) emitters, namely
[Ru(bpy)3]2+, where bpy is 2,2Ј-bipyridine.[5] Reasons are
3
numbers below the MLCT level.
Along these lines of investigation, we prepared the new
phen–EP2 ligand and two complexes: a bichromophore,
[Ru(dpp2)2(phen–EP2)]2+ (1) and its parent disubstituted di-
bromophenanthroline species (2), where dpp is 4,7-di-
phenyl-1,10-phenanthroline and EP is ethynylpyrene
(Schemes 1 and 2). In 1, the Ru-based component is a hetero-
leptic system containing dpp and 1,10-phenanthroline li-
gands (phen), and the latter contains two EP appendages.
These are designed to influence the photophysics of the Ru–
dpp core; for the reference complex [Ru(dpp)3]2+, the room-
temperature luminescence quantum yield, fem, ≈ 0.4,[7–9] is
six times higher than that of [Ru(bpy)3]2+, fem, ≈ 0.06.[10]
We examined the excited-state dynamics of this complex
[a] Laboratoire de Chimie Moléculaire, École de Chimie, Po-
lymères, Matériaux (ECPM), Université Louis Pasteur (ULP),
25 rue Becquerel, 67087 Strasbourg Cedex 02, France
E-mail: ziessel@chimie.u-strasbg.fr
3
[b] Istituto per la Sintesi Organica e la Fotoreattività, Consiglio
Nazionale delle Ricerche (ISOF-CNR),
and found that the lowest-lying excited state is of LC na-
ture and based on the EP moiety. However, it does not act
Via P. Gobetti 101, 40129 Bologna, Italy
E-mail: franz@isof.cnr.it
as an “energy reservoir” for the higher-lying 3MLCT
Eur. J. Inorg. Chem. 2008, 1293–1299
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FULL PAPER
3
level(s); instead, the weak MLCT emission that can be de- vs. SCE and is attributed to RuIIǞRuIII oxidation. This
tected, both at room temperature (fem ≈ 10–3) and at 77 K, process is facilitated by 140 mV relative to that of [Ru-
is likely due to excitation trapping effects at the Ru(dpp)- (phen)3]2+[11], which results from a balance of effects from
based moiety of [Ru(dpp)2(phen–EP2)]2+ complex 1.
the dibromophenanthroline and diphenylphenanthroline li-
gands; similar effects were previously observed in tetrasub-
stituted RuII–phenanthroline complexes.[12,13] For pyrene-
grafted complex 1, this process is irreversible on the electro-
chemical timescale (E1/2 = +1.35 V vs. SCE), presumably
because of some overlap between the Ru- and pyrene-based
oxidation steps given that oxidation of 1-ethynylpyrene is
usually irreversible and occurs around +1.45 V vs. SCE.[14]
For both complexes, the subsequent reduction waves are li-
gand-based, as usually found for octahedral ruthenium(II)
complexes.[10] Comparison with the case of [Ru(phen)3]
(E1/2 = –1.41 V vs. SCE; Table 1) suggests that the first re-
duction potential in complexes 1 and 2 (E1/2 = –1.11 and
–1.23 V vs. SCE, respectively) can be attributed to the re-
duction of the 4,7-disubstituted-1,10-phenanthroline li-
gand. The 180 and 300 mV positive shift is likely due to the
σ-electron-withdrawing effect of the bromo and ethynylpy-
rene substituents, respectively. For complex 1, the higher
degree of conjugation in the phenanthroline–ethynylpyrene
ligand is also likely to favour a more-pronounced positive
shift. Consistent with this, the fact that the first reduction
in 1 takes place at a potential that is more positive than
that of 2 might be taken as an indication that the LUMO
of 1 is localised on the ethynyl-containing ligand.
Results and Discussion
Synthesis
The phen–EP2 ligand was conveniently prepared as
sketched in Scheme 1 by using a palladium cross-coupling
reaction between 1-ethynylpyrene and 5,6-dibromo-1,10-
phenanthroline. Unfortunately, we were unable to prepare
the ruthenium(II) complex of this ligand by a classical pro-
cedure, because phen–EP2 proved to be poorly soluble in
the required solvent. The problem was circumvented by the
synthesis of pivotal starting complex 2, which bears a di-
substituted dibromophenanthroline framework (Scheme 2).
This compound was obtained by coordination of 5,6-di-
bromo-1,10-phenanthroline to [Ru(dpp)2Cl2]. This complex
could then react smoothly in the employed solvent by using
low-valent palladium(0) and 1-ethynylpyrene (2 equiv.) to
provide desired pyrene substituted complex 1 in 57% yield.
Table 1. Electrochemical data for the Ru complexes.[a]
Scheme 1. Reagents and conditions: (i) 1-ethynylpyrene, C6H6/
iPr2NH, Pd(PPh3)4 (6 mol-%), 60 °C.
Eap or E1/2, V (∆E, mV)
2
+1.26 (70)
+1.35 (irr)[b]
+1.40 (60)
–1.23 (70), –1.40 (70)
–1.57 (70)
–1.11 (60), –1.39 (60),
–1.60 (70), –1.75 (80)[c]
–1.41 (60), –1.54 (60),
–1.84 (70)
1
2+[d]
Ru(phen)3
[a] Potentials determined by cyclic voltammetry at room tempera-
ture in 0.1  TBAPF6/CH3CN solution, complex concentration ca.
3ϫ10–3 . All potentials (Ϯ15 mV) are reported in volts vs. SCE
as reference electrode with the use of Fc+/Fc (+0.38 V, ∆E =
60 mV) as internal standard. Scan rate 200 mVs–1. [b] Eap (anodic
peak potential) corresponds to an irreversible electrode process. [c]
Double current intensity due to the reduction of the two ethynylpy-
rene appendages. [d] Ref.[11]
Scheme 2. Reagents and conditions: (i) 1-ethynylpyrene, CH3CN/
C6H6 (1:1), iPr2NH, Pd(PPh3)4 (6 mol-%), 60 °C.
The two complexes were unambiguously characterised by
NMR, FAB+ and FTIR spectroscopy and elemental analy-
sis (see Experimental Section), and all data were consistent
with the proposed structures. In particular, a weak band
attributed to the CϵC stretching vibration is observed at
2181cm–1 for the phen–EP2 ligand and at 2180 cm–1 for
complex 1 in the FTIR spectra.
The second reduction potential in both complexes 1 and
2 is very close to the first reduction potential of the [Ru-
(phen)3]2+ complex,[11] whereas the third reduction poten-
tial is close to the second reduction wave of [Ru(phen)3]2+
(Table 1). For complex 1, the additional cathodic potential
(E1/2 = –1.75 V vs. SCE) is assigned to the quasireversible
reduction of both ethynylpyrene subunits. This last re-
duction step occurs with a twofold increase in the current
intensity relative to that of the lowest cathodic reduction
Electrochemistry
Electrochemical data are gathered in Table 1. The single- step due to close reduction of both pyrene residues. Such
oxidation potential in complex 2 occurs at E1/2 = +1.26 V behaviour has also been observed for related complexes.[14]
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Eur. J. Inorg. Chem. 2008, 1293–1299

RuII–Diphenylphenanthroline–Ethynylpyrenephenanthroline Systems
Optical Spectroscopy
dition, as suggested by the absorption tail of the phen–EP2
ligand (Figure 1, inset), 1LC transitions localised at the eth-
ynylpyrene fragments or intraligand ethynylpyreneǞphen
CT (1ILCT) transitions could be present in this range.[17]
A summary of the luminescence properties for complexes
1 and 2 and the phen–EP2 ligand is given in Table 2; λexc
was 462 nm for the complexes and 415 nm for phen–EP2.
Figures 2 and 3 show the room temperature and 77 K lumi-
nescence spectra, respectively. For complex 2 at room tem-
perature, the emission properties are typical of 3RuLCT
emitters: λem = 620 nm, fem = 6.1ϫ10–2, τem = 0.9 µs and
The ground-state absorption spectra of 1, the reference
complex without the EP appendages [Ru(dpp)2(phen–
Br2)2+ (2)] and of the phen–EP2 ligand are displayed in Fig-
ure 1; spectroscopic data are collected in Table 2. In the re-
gion 270–280 nm, complex 1 shows ligand-based absorp-
tion features[10] similar to those of 2, apart from a slight
redshift of the peak. This is an expected result[12,13] given
that the first coordination sphere of the two complexes is
composed of the same number of (derivatised) phen chro-
mophores, and the redshift of the absorption peak observed
for 1 is ascribable to electronic delocalisation effects by the
EP appendages. In the region 300–400 nm, 1 shows absorp-
tion characteristics not exhibited by 2. As suggested by
comparison with the absorption profile of phen–EP2 (Fig-
ure 1, inset), the absorption peak for 1 with λmax = 364 nm
and ε = 49200 –1 cm–1 is ascribable to ligand-localised
transitions at the EP moiety. With regard to the range 400–
radiative rate constant, kr = 6.8ϫ104 s–1, with kr = fem
/
[10,12,13]
τem
.
In contrast, at room temperature, complex 1 is
only weakly luminescent in O2-free solvent with λem
=
607 nm, fem = 9.0ϫ10–4, τem = 4.5 µs and kr = 200 s–1. The
3
kr value for 2 is typical of MLCT emitters, whereas for 1
it is rather indicative of a 3LC emission.[18] It is notable that
in air-equilibrated solvent, 1 is not luminescent within the
limits of detection (Table 2).
1
550 nm, where MLCT bands typically dominate in the ab-
sorption spectra of RuII–polypyridine complexes,[10] the in-
tensity of the band is found rather high for 1 (ε457
=
53700 –1 cm–1) relative to those of 2 and [Ru(phen)3]2+[15]
(ε443 = 27800 –1 cm–1 and ε447 = 18400 –1 cm–1, respec-
tively). This could be due to known effects related to a large
charge displacement, because the RuǞLCT event is likely
to somehow involve the distant EP fragments.[16] In ad-
Figure 2. Luminescence spectra at room temperature of complexes
1 (full line) and 2 (dashed line) for isoabsorbing and degassed
CH3CN solutions; λexc = 462 nm.
Upon excitation at 355 nm, the room-temperature differ-
ence transient absorption (TA) spectra (Figure 4) for times-
cales up to 100 µs are observed for 1 and phen–EP2. The
TA spectrum of the ligand (Figure 4, top) is to be attributed
Figure 1. Ground-state absorption spectra in CH3CN solutions of
complexes 1 (full line) and 2 (dashed line). The inset shows the
absorbance profile of the phen–EP2 ligand in CH2Cl2.
3
to the lowest-energy LC (3ππ) level. After a 0.7 µs delay, it
Table 2. Absorption and luminescence properties.[a]
Absorption[b]
Emission[c] at 295 K
Emission[c] at 77 K
λem, nm τem, µs
λmax, nm
λem, nm
fem
τem, µs
kr[d], s–1
(ε, –1 cm–1)
1
457 (53700)
443 (27800)
415[h]
607
(–)[f]
620
9ϫ10–4
(–)[f]
4.5[e]
(–)[f]
0.9
200
590
7.9
2
6.1ϫ10–2
(1.2ϫ10–2)
6.5ϫ10–2
6.8ϫ104
5.4ϫ107
610
5.6
(0.18)
1.2ϫ10–3[i]
486
3.8ϫ10–3
[g]
phen–EP2
480
[a] CH3CN and CH2Cl2 solvent for the complexes and the free ligand, respectively. [b] Lowest-energy band. [c] Degassed samples, within
brackets results for air-equilibrated cases; for the luminescence spectra, λexc = 462 and 415 nm for the complexes and the free ligand,
respectively; for the luminescence lifetimes, λexc = 465 and 373 nm, respectively. [d] From kr = fem/τem; for 1, given the weak luminescence
intensity the drawn kr value may be subject to a large uncertainty. [e] From TA experiments τTA = 57 µs; λexc = 355 nm, see text. [f] Not
detected. [g] Luminescence properties are due to fluorescence unless otherwise stated. [h] Not determined due to low solubility. [i] From
3
TA experiments, the LC level exhibits τTA = 28 µs; λexc = 355 nm, see text.
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FULL PAPER
and/or phen fragments. The spectrum of phen–EP2 decayed
3
with τ = 28 µs, as observed at 500 nm, whereas for the LC
level of neat pyrene τ = 100 µs;[18] for both cases the data
was recorded in O2-free CH2Cl2.
With regard to the TA spectrum of 1 (Figure 4, bottom;
degassed AN solvent), after a 1 µs delay, the ground-state
bleaching was found at 360 nm, which corresponds to the
ground-state absorption peak of the EP component in this
complex (Figure 1). In addition, broad absorption features
were found at λ Ͼ 450 nm (peak at 690 nm), with a modest
bump for some profiles at ca. 480 nm, which qualitatively
matches previous observations for RuII–bpy systems con-
taining pyrenylethylene subunits.[20] The TA spectrum of 1
decayed with τ = 57 µs as observed at 500 nm. All of this is
consistent with a 3LC nature for the lowest-lying excited
state of this complex.[20] The presence of this level affects
the luminescence behaviour of 1, as described in the follow-
ing.
Figure 3. Luminescence spectra at 77 K of complexes 1 (full line)
and 2 (dashed line) for roughly isoabsorbing and degassed CH3CN
solutions; λexc = 462 nm.
For bichromophores based on [Ru(LL)3]2+ and pyrene
exhibits ground-state bleaching, which peaks at 380 nm, (pyr) moieties, [Ru(LL)3]2+–pyrn (cases have been systemati-
and therefore the spectrum matches the absorption profile cally explored with n in the range 1 to 6),[21] the energy gap
3
3
appearing in the inset of Figure 1. Strong absorption fea-
∆
between Ru and lower-lying pyr levels was found to
TT
tures are registered for λ Ͼ 450 nm, and positive peaks are be a few hundred wavenumbers.[5,6,19,22,23] As a conse-
observed at 500 and 610 nm. The TA profile of phen–EP2 quence, at room temperature (295 K), thermal equilibration
differs somehow from that of neat pyrene, which shows pos- between these levels can be establish according to Equa-
itive peaks at 415 and 515 nm.[19] This suggests that for tion (1)
phen–EP2, the involved excited state is not fully localised at
the pyrene unit, but rather it is spread to include the ethynyl
(1)
where NRu/Npyr denotes the population ratio of the
3
3RuLCT and pyr states and ∆TT is the energy gap between
them; kBT = 205.9 cm–1 (295 K). On this basis, the lumines-
cence properties of such bichromophores can be under-
stood with the help of Equations (2) and (3).[21]
(2)
(3)
According to the above Equations, the overall lumines-
cence output fem is regarded to originate from the balance
of two localised contributions, (1 – α) and α, for the excited
3Ru* and 3pyr* states, respectively. The forward (f) and
backward (b) energy-transfer steps are defined with refer-
ence to the employed excitation wavelength, λexc = 355 nm,
so that predominant excitation of the pyr-containing EP
unit takes place. Nevertheless, it should be noted that from
1
3
3
the initially populated pyr* level, both the pyr* and Ru*
levels are independently accessed.[6] Thermal equilibration
is affected by the temperature, and below we discuss sepa-
rately (i) room temperature and (ii) 77 K luminescence re-
sults, while having in mind that ethynylpyrene (EP) is an
electronically more extended system than pyr.
Figure 4. TA spectra at room temperature of the phen–EP2 ligand
(top: degassed CH2Cl2 solvent; delays: 0.7, 11, 14, 20, 57 µs) and
complex 1 (bottom: degassed CH3CN solvent; delays: 1, 12, 20, 45,
100 µs); λexc = 355 nm. The insets show the temporal decay of the
band at 500 nm.
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RuII–Diphenylphenanthroline–Ethynylpyrenephenanthroline Systems
3
At room temperature, a pyr-like level is typically non- are populated upon light absorption. Under these circum-
emitting. This is due to its low radiative rate constant stances, low-temperature single-ligand electron trapping
(kr Ͻ103 s–1).[18] For bichromophores of the type [Ru- may result in different emission properties, depending on
(LL)3]2+–pyrn, a common observation is that only a 3MLCT which ligand, LLa or LLb, is bearing the excess electron. In
3
Ru-based emission is present, and it typically exhibits kr ≈ particular, whereas MǟL CT states localised at the phen–
105 s–1.[10] For such bichromophores, the 3pyr level only EP2 ligand could undergo deactivation by low-lying practi-
plays the role of “energy-reservoir”, provided its energy po- cally nonluminescent 3pyr-like levels localised on the EP
3
sition is very close but slightly lower than that of the Ru- moieties, the weak MLCT luminescence observed at 77 K
based emitter.[5,6,19,22,23] In this case, upon effective thermal for 1 (λem = 590 nm, τem = 7.9 µs) might be ascribable to
3
3
3
equilibration between the MLCT and pyr levels, the Ru- 3MǟL CT states localised (entrapped) on one of the dpp
based luminescence is found to feature (a) the same inten- ligands.
sity of the component complex alone and (b) a longer life-
time depending on α [Equation (2)].[21] For the present
bichromophore 1, constituted by a [Ru(dpp)3]2+ core and
Experimental Section
EP units (more electronically delocalised than pyr), the
3Ru-based luminescence intensity registered at room tem-
1
General Methods: The H and 13C NMR spectra were recorded at
perature in O2-free solvent, is weaker by two orders of mag- room temperature with a Bruker AC 200 MHz, a Bruker Avance
nitude than that for complex 2 (fem = 9ϫ10–4 and
400 MHz or a Bruker Avance 300 MHz spectrometer with the use
of perdeuterated solvents as internal standard: δH in ppm relative
6.1ϫ10–2, respectively). This finding suggests that in 1,
3
3
to residual proton in the solvents (not 100% deuteriated); δC in
ppm relative to the solvent. FTIR spectra were recorded as KBr
pellets. Absorption spectra were recorded in CH2Cl2 or CH3CN
with a UVIKON 933 absorption spectrometer. Fast atom bom-
bardment (FAB, positive mode) mass spectra were recorded with a
ZAB-HF-VB-analytical apparatus with m-nitrobenzyl alcohol (m-
NBA) as matrix. Electrospray (ES) mass spectra were recorded
with a 1100 MSD Hewlett Packard spectrometer. Electrochemical
thermal redistribution between the Ru* and EP* excited
levels [Equations (1–3)], is not effective (it is largely dis-
3
placed in favour of EP*), which might be a consequence
of an energy gap that is too large between these levels
(∆TT ϾϾkBT). Consistent with this, for air-equilibrated sol-
vent, no luminescence is detected for 1 (Table 2), which in-
3
dicates that deactivation of the long-lived lowest-lying LC
level (57 µs, see above) by O2 diffusional quenching success- studies employed cyclic voltammetry with a conventional three-
electrode system by using a BAS CV-50W voltammetric analyser
equipped with a Pt disk (2 mm2) working electrode and a silver
wire counter-electrode. Ferrocene was used as an internal standard
and was calibrated against a saturated calomel reference electrode
(SCE) separated from the electrolysis cell by a glass frit presoaked
with electrolyte solution. Solutions contained the electroactive sub-
strate in deoxygenated and anhydrous dichloromethane or acetoni-
trile containing doubly recrystallised tetra-n-butylammonium hexa-
fluorophosphate (0.1 ) as supporting electrolyte. The quoted half-
wave potentials were reproducible within ≈ 15 mV.
fully competes against the forward transfer [Equation (3)].
On passing from room temperature to 77 K, a blueshift
of the emission peak is registered for 2 and λem moves from
620 to 610 nm. This is an expected behaviour for CT emit-
ters upon going from fluid to frozen polar solvents.[10] At
77 K, 1 is also luminescent, with λmax = 590 nm and a de-
tected luminescence intensity that is ca. 0.1 times that of 2
(Figure 3). On the basis of the emission profiles (Figure 3)
and lifetimes (τ = 7.9 and 5.6 µs for 1 and 2, respectively;
3
Table 1), the 77 K results indicate a MLCT nature for the
Materials: 1-Ethynylpyrene,[14] cis-[Ru(4,7-diphenyl-1,10-phenan-
throline)2Cl2],[26] [Pd(PPh3)4][27] and 5,6-dibromo-1,10-phenan-
throline[28] were prepared and purified according to literature pro-
cedures. Diisopropylamine and acetonitrile were dried with suitable
reagents and distilled under an atmosphere of argon immediately
prior to use. CH2Cl2 and iPr2NH were distilled from P2O5 and
KOH, respectively.
emission in both cases. At low temperatures, the NRu/Npyr
ratio is expected to be much smaller than that at room tem-
perature, and the observed 3MLCT nature for the 77 K
emission of 1 cannot be explained in terms of thermal equil-
ibration [Equations (1–3)], which is already not effective at
room temperature.
An explanation for the low temperature observations
might rely on the heteroleptic nature of complex 1 as op-
posed to the homoleptic cases. Actually, for the equivalent
ligands of a homoleptic system and according to an interli-
5,6-Bis(1-ethynylpyrene)-1,10-phenanthroline
(phen–EP2):
A
Schlenk flask equipped with a septum, a Teflon-coated magnetic
stirring-bar and an argon inlet was charged with 5,6-dibromo-1,10-
phenanthroline (100 mg, 0.30 mmol) and 1-ethynylpyrene (81 mg,
gand hopping model for the ligand-localised description of 0.36 mmol) in argon-degassed benzene/iPr2NH (20:2 mL), and fi-
3
the MǟL CT event,[10] the observed emission properties
nally Pd(PPh3)4 (21 mg, 6% mol) was added as a solid. The solu-
tion was heated at 60 °C, until complete consumption of the start-
are obviously independent of the individual ligand bearing
the 3MLCT excitation (i.e., the excess electron).[10,24] This is
ing material was determined by TLC. After cooling, the precipitate
was filtered and washed with water (2ϫ20 mL) and diethyl ether
(2ϫ10 mL). The analytically pure compound was recovered with-
out any additional treatment (141 mg, 75%). Because of its insolu-
also the case at 77 K, where interligand hopping of the ex-
cess electron of the luminescent 3MLCT state is severely
limited (a hopping barrier of the order of 700–900 cm–1
bility, NMR spectra were not obtained. IR (KBr): ν = 2912 (m),
˜
may be evaluated),[25] but the emission is in any circum-
stance centred on one of the equivalent ligands. In contrast,
2850 (m), 2181 (m), 1619 (s), 1595 (s), 1432 (s), 1261 (m), 1096 (m),
876 (m), 844 (m), 716 (m) cm–1. MS (ESI, CH2Cl2): m/z = 629.2 [M
+ H]+. C48H24N2 (628.73): calcd. C 91.70, H 3.85, N 4.46; found C
for a heteroleptic system of the type [Ru(LLa)2(LLb)]2+
,
1MLCT states involving the individual LLa or LLb ligands 91.51, H 3.57, N 4.82.
Eur. J. Inorg. Chem. 2008, 1293–1299
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1297

C. Goze, C. Sabatini, A. Barbieri, F. Barigelletti, R. Ziessel
[Ruthenium(II)(4,7-diphenyl-1,10-phenanthroline)2(5,6-dibromo- tained with Uvikon 933 or Perkin–Elmer Lambda 45 spectrome-
FULL PAPER
1,10-phenanthroline)](PF6)2 (2): In a Schlenk flask containing a
stirred ethanol/water (10:1 mL) solution of cis-[Ru(4,7-diphenyl-
phenanthroline)2]Cl2 (0.1 g, 0.13 mmol) was added the 5,6-dibro-
mophenanthroline ligand (0.05 g, 0.15 mmol). The mixture was
heated for 16 h at 80 °C until complete consumption of the starting
material was observed. After cooling, the solution was filtered, po-
tassium hexafluorophosphate in water was added, and the solution
was evaporated. The crude precipitate was washed with water (2ϫ)
and diethyl ether (1ϫ), and it was then chromatographed on a col-
umn packed with alumina (MeOH/CH2Cl2, gradient 0:100 to 2:98).
The fractions containing the pure complex were evaporated to dry-
ness and recrystallised by slow evaporation of CH2Cl2 from a mix-
ture of CH2Cl2/hexanes(approximately 80:20). Recrystallisation
gave 120 mg (66%) of analytically pure product.1H NMR
(300 MHz, [D6]acetone, 20 °C): δ = 9.02 (dd, 3J = 8.7 Hz, 4J =
ters; the ligand was poorly soluble and only its absorbance profile
was obtained. The luminescence spectra of O2-free or air-equili-
brated solutions at room temperature (absorbance Ͻ 0.15 at the
excitation wavelength) and at 77 K were measured with a Spex
Fluorolog II spectrofluorometer with an excitation wavelength of
415 and 462 nm for the ligand and the complexes, respectively. De-
gassing of the samples was accomplished by argon bubbling or
freeze–pump cycling in a vacuum line. Corrected luminescence
spectra were obtained by using a correction curve for the phototube
response provided by the manufacturer. Luminescence quantum
efficiencies (fem) were evaluated by comparing wavelength-inte-
grated intensities (I) with reference to [Ru(bpy)3]Cl2 (fr = 0.028,
air-equilibrated water)[29] and by using Equation (4):[30]
3
4
3
1.1 Hz, 2 H), 8.65 (dd, J = 5.3 Hz, J = 1.0 Hz, 2 H), 8.61 (d, J
3
= 5.5 Hz, 2 H), 8.57 (d, J = 5.5 Hz, 2 H), 8.33 (s, 4 H), 7.98 (dd,
3
3
3J = 8.7 Hz, J = 5.3 Hz, 2 H), 7.79 (d, J = 5.6 Hz, 2 H), 7.75 (d,
3J = 5.6 Hz, 2 H), 7.65–7.63 (m, 20 H) ppm. 13C{1H} NMR
(75 MHz, [D6]acetone, 20 °C): δ = 154.9, 153.9, 153.7, 150.2, 150.1,
149.65, 149.60, 149.1, 138.4, 136.65, 136.63, 132.0, 130.8, 130.7,
130.6, 130.04, 129.93, 129.87, 128.6, 127.3, 127.2, 127.1 ppm.
(4)
where A and η are the absorbance value (Ͻ0.15) at the employed
excitation wavelength and the refractive index of the solvent,
respectively. Band maxima and relative luminescence intensities are
obtained with uncertainty of 2 nm and 20%, respectively. The lumi-
nescence lifetimes of the complexes were obtained with an IBH
5000F single-photon equipment by using nanoled excitation
sources at 373 and 465 nm, for the ligand and complexes, respec-
tively. Analysis of the luminescence decay profiles against time was
accomplished by using software provided by the manufacturers. Es-
timated errors are 10% on lifetimes, and the working temperature
was either 295Ϯ2 K (1-cm square optical cells employed) or 77 K
(with samples contained in capillary tubes immersed in liquid nitro-
gen). Transient absorption (TA) spectra for degassed solutions were
observed in the microsecond time domain by using a Proteus nano-
second flash photolysis apparatus by Ultrafast Systems LLC.[31]
The excitation from a Continuum Surelite II Nd:YAG laser was at
355 nm (5-ns pulse duration, 5 mJ per pulse). The probe light
source was a Spectra Physics 69907 150-W continuous wave Xe arc
lamp. Light signals were passed through a Chromex/Bruker 250IS
monochromator and collected with a high-speed Silicon DET210
Thorlabs detector. After signal amplification by a Femto DHPVA-
200 variable-gain wideband voltage amplifier and registration at a
Tektronix TDS 3032 B digital oscilloscope, treatment of the signals
was performed with the help of acquisition software by Proteus; to
extract lifetimes, the temporal decay of the TA band at 500 nm was
monitored.
FTIR (KBr): ν = 3437 (m), 3058 (m), 2919 (m), 1622 (m), 1594
˜
(m), 1444 (m), 1417 (s), 1116 (m), 1096 (m), 836 (s), 765 (m), 702
(m) cm–1. UV/Vis (CH3CN): λ (ε, –1 cm–1) = 443 (27800), 273
(142300) nm. MS: (ESI, CH3CN): m/z = 1249.2 [M – PF6]+.
C60H38Br2F12N6P2Ru (1393.80): calcd. C 51.70, H 2.75, N 6.03;
found C 51.45, H 2.39, N 5.71.
[Ruthenium(II)(4,7-diphenyl-1,10-phenanthroline)2(5,6-{1-ethynyl-
pyrene}-1,10-phenanthroline)](PF6)2 (1): In a Schlenk flask contain-
ing a stirred degassed acetonitrile/benzene solution (1.5:1.5 mL) of
[ruthenium(II)(4,7-diphenyl-1,10-phenanthroline)2(5,6-dibromo-
1,10-phenan-throline)](PF6)2 (30 mg, 0.021 mmol) was sequentially
added [Pd(PPh3)4] (2 mg, 6 mol-%), diisopropylamine (0.5 mL) and
1-ethynylpyrene (12 mg, 0.054 mmol). The mixture was heated un-
der an atmosphere of argon for 16 h until the complete consump-
tion of the starting material was observed. The solution was cooled
to room temperature and potassium hexafluorophosphate in water
was added, and the solvent was evaporated. The crude precipitate
was washed with water (2ϫ) and diethyl ether (1ϫ), and it was
chromatographed on a column packed with silica gel (acetonitrile/
water/aqueous saturated KNO3, 85:15:0 to 85:15:0.2). After anionic
exchange, the analytically pure compound was obtained after
recrystallisation from dichloromethane/hexane (20 mg, 57%). 1H
3
NMR (300 MHz, [D6]acetone, 20 °C): δ = 8.84 (d, J = 9.0 Hz, 2
3
H), 8.76 (d, J = 5.5 Hz, 2 H), 8.72–8.69 (m, 4 H), 8.61 (d, 3J =
8.1 Hz, 2 H), 8.40 (d, 3J = 8.1 Hz, 2 H), 8.38 (s, 4 H), 8.34–8.24
Acknowledgments
3
4
(m, 8 H), 8.08 (dd, J = 8.5 Hz, J = 5.2 Hz, 2 H), 8.03–7.91 (m, 4
H), 7.87 (d, 3J = 5.6 Hz, 2 H), 7.84 (d, 3J = 5.5 Hz, 2 H), 7.70–
7.73 (m, 20 H), 7.60 (d, 3J = 9.2 Hz, 2 H) ppm. 13C{1H} NMR
(75 MHz, [D6]acetone, 20 °C): δ = 152.8, 149.3, 148.8, 148.1, 135.7,
131.2, 130.70, 130.66, 129.89, 129.85, 129.7, 129.34, 129.26,
129.17, 129.0, 127.2, 126.7, 126.4, 126.2, 126.1, 124.9, 115.7, 90.2
The authors thank CNRS and IST/ILO (Contract 2001-33057) for
funding and acknowledge support from Consiglio Nazionale delle
Ricerche, Italy (Project MACOL PM.P04.010). Professor F. N.
Castellano from the University of Bowling Green State University
in the USA is also acknowledged for providing us with a sample
of 5,6-dibromo-1,10-phenanthroline.
(CCethynyl) ppm. FTIR (KBr): ν = 3436 (m), 3143 (m), 2922 (m),
˜
2853 (m), 2513 (m), 2180 (m), 1620 (s), 1595 (s), 1428 (s), 1186 (m),
1120 (m), 837 (s), 703 (m) cm–1. UV/Vis (CH3CN): λ (ε, –1 cm–1)
= 457 (53700), 435 (52000), 361 (50000), 277 (155300), 232 (122900) [1] J. H. Alstrum-Acevedo, . K. Brennaman, T. J. Meyer, Inorg.
PF6]+.
Chem. 2005, 44, 6802–6827.
nm. MS: (ESI, CH3CN): m/z
=
1539.2 [M
–
[2] V. Balzani, A. Juris, M. Venturi, S. Campagna, S. Serroni,
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C96H56F12N6P2Ru (1684.53): calcd. C 68.45, H 3.35, N 4.99; found
C 68.22, H 3.09, N 4.75.
Optical Spectroscopy: UV/Vis absorption spectra of the ligand and
of the complexes in CH2Cl2 and CH3CN, respectively, were ob-
1298
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Eur. J. Inorg. Chem. 2008, 1293–1299

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Received: October 11, 2007
Published Online: January 15, 2008
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Eur. J. Inorg. Chem. 2008, 1293–1299
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