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Synthesis, characterization, photophysical and electrochemical properties of fac-tricarbonyl(4,7-dichloro-1,10-phenanthroline)rhenium(I) complexes
Accepted Manuscript Synthesis, characterization, photophysical and electrochemical properties of fac-tricarbonyl(4,7-dichloro-1,10-phenanthroline)rhenium(I) complexes Márcia R. Gonçalves, Karina P.M. Frin PII: DOI: Reference:
S0277-5387(15)00248-X http://dx.doi.org/10.1016/j.poly.2015.05.007 POLY 11305
To appear in:
Polyhedron
Received Date: Accepted Date:
29 January 2015 4 May 2015
Please cite this article as: M.R. Gonçalves, K.P.M. Frin, Synthesis, characterization, photophysical and electrochemical properties of fac-tricarbonyl(4,7-dichloro-1,10-phenanthroline)rhenium(I) complexes, Polyhedron (2015), doi: http://dx.doi.org/10.1016/j.poly.2015.05.007
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Synthesis, characterization, photophysical and electrochemical properties of fac-tricarbonyl(4,7-dichloro-1,10phenanthroline)rhenium(I) complexes Márcia R. Gonçalves and Karina P. M. Frin* Federal University of ABC – UFABC, Av. dos Estados 5001 - Santo André – SP - Brazil, 09210-170, *E-mail: [email protected] Abstract The
fac-[Re(CO)3Cl(Cl2phen)]
and
fac-[Re(CO)3(py)(Cl2phen)]PF6
complexes were synthesized, purified and characterized by 1H NMR, UV-vis and IR spectroscopies and both photophysical and electrochemical behaviors were investigated. The electronic absorption spectra exhibit two main absorption bands: the higher energy band, which was assigned to IL, and the lower energy band, assigned to MLCT. The fac-[ReCl(Cl2phen)(CO)3] and fac[Re(py)(Cl2phen)(CO)3]PF6 complexes showed emission at room temperature in both CH3CN solution (λmax = 640 nm, φ = 0.0027; λmax = 590 nm, φ = 0.055) and rigid media (λmax =590 nm, λmax = 535 nm in PMMA) arising from the lowest lying metal to ligand charge transfer (3MLCTRe→Cl2phen) excited state. Additionally, the PVK emission band is almost completely quenched in the presence of 4% rhenium(I) complex. Both compounds showed metal-centered oxidation, Re(I)→(II), and ligand-based reduction, Cl2phen → Cl2phen*-, in CH2Cl2 solution. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of both complexes were estimated. The electrochemical and emission properties indicate a potential application of the rhenium(I) compounds in OLED devices.
1
Keywords: rhenium(I) tricarbonyl complexes; electron withdrawing polypyridyl ligand; photophysical properties; 3MLCT emission
Introduction A growing need for new sources of energy, arising from the prediction that fossil fuels would be completely dried up in the near future, has encouraged researchers to pursue potential alternatives to fulfill world energy needs. To meet this energy need, transition metal complexes, especially those with d6 electron configuration, have been receiving great attention in investigations related to solar energy conversion 1-2, photocatalysis devices such as LECs
6-8
and OLEDs
3-5
, and electroluminescent
9-12
, among others. The reasons for the
great interest in this class of compound are various and include absorption of radiation in a lower region of the spectrum than observed in organic counterparts; strong spin-orbit coupling, especially for the second and third transition metal, which eliminates the spin-forbidden T1→S0 radiative transition; excited states with intense redox activity allowing the occurrence of a series of electron transfer reactions and/or energy, among others
13-14
.
One of the attractive features of transition metals is their ability to form complexes with a variety of polypyridine ligands, NN, such as 1,10phenanthroline and its derivatives, which tune their ground and excited state properties. In particular, the rhenium(I) tricarbonyl polypyridyl complexes have been widely investigated due to their interesting triplet emissive properties
3, 14-
16
. Additionally, intense effort has been dedicated to better understand the effect
of the position and attachment of electron withdrawing/donating groups to ligands on the mechanistic events after excitation
15, 17-21
, which can contribute
to a deeper comprehension of their emissive properties. For instance, electron 2
withdrawing groups attached to polypyridyl ligands promote stabilization of 3
MLCTRe→NN excited state energy level (metal-to-ligand charge-transfer)15, 17, 19,
21-24
. On the other hand, electron donating groups, such as methyl, promote
destabilization of 3MLCTRe→NN excited state energy level and at the same time reduces the energy of ligand-centered excited state, 3ILNN polypyridyl complexes show a dominant
3
19, 21, 23
. The rhenium
MLCT emission, but some
3
IL
character could be also evident to a lesser or greater degree, depending on the coordinated NN ligand. Therefore, relative energy of these states plays an important role in determining the efficiency and mechanism of deactivation pathways after excitation. In this
work,
we
report the synthesis and characterization of fac-
[Re(CO)3(Cl2phen)Cl] and fac-[Re(CO)3(Cl2phen)(py)]+, where Cl2phen = 4,7dichloro-1,10-phenanthroline and py = pyridine, as well as their electrochemical properties. Additionally, their photophysical properties in fluid and rigid media were investigated with a strong motivation for understanding the electron withdrawing effect imparted by the two chlorine groups attached to 1,10phenanthroline at the 4 and 7 positions.
Experimental Procedures Materials. All solvents, from Aldrich, Synth or Merck, were reagent grade, except for those used in the photophysical and electrochemical measurements, where HPLC grade solvents from Aldrich, were employed. [ClRe(CO)5], 4,7dichloro-1,10-phenanthroline (Cl2phen), trifluoromethanesulfonic acid (tfms), pyridine (py), poly(methyl methacrylate) (PMMA, MW = 110,000 g mol-1), and 3
poly(9-vinylcarbazole) (PVK, MW = 25,000-50,000 g mol-1) from Aldrich were used as received. Synthesis The fac-[ReCl(CO)3(Cl2phen)] complex was synthesized with slight modification to the procedure previously described for fac-[ReCl(CO)3(5-Clphen)]
20
.
[ClRe(CO)5] (0.55 g, 1.5 mmol) and an excess of 4,7-dichloro-1,10phenanthroline, Cl2phen (0.73 g, 2.9 mmol), were mixed in 30 mL of xylene (Merck) and heated to reflux for 6 h. The resulting solid was separated by filtration before cooling to room temperature. Purification was achieved by recrystallization from CH2Cl by slow addition of n-pentane. Yield 57%. Anal. Calc. for ReC15H6Cl3N2O3: C, 32.51%; N, 4.82%; H, 1.01%. Found: C, 32.59%; N, 4.87%; H, 0.97%. 1H NMR (CD3CN, δ / ppm): 9.32 (d, 2H), 8.53 (s, 2H), 8.09 (d,2H). The fac-[Re(tfms)(CO)3(Cl2phen)] complex was synthesized as previously reported
25
for a similar compound by suspending fac-[ReCl(CO)3(Cl2phen)]
(0.30 g, 0.54 mmol) in 30 mL argon-saturated CH2Cl2, to which a 12-fold excess of trifluoromethanesulfonic acid (tfms) was added. The solution was stirred for one hour, and the intermediate fac-[Re(tfms)(CO)3(Cl2phen)] was obtained by slow addition of ethyl ether. Yield 91%. The fac-[Re(py)(CO)3(Cl2phen)]PF6 complex was synthesized with slight modification to the procedure reported for similar compounds
20, 26
. The fac-
[Re(tfms)(CO)3(Cl2phen)] complex (0.33 g, 0.49 mmol) and pyridine (py 80 µL, 1.06 mmol) were mixed in methanol (30 mL) and kept in reflux for 7 h. After cooling to room temperature, the final product was precipitated by the addition 4
of solid NH4PF6. The yellow solid was separated by filtration, washed with water to remove the excess NH4PF6, then washed with ethyl ether. Yield 25%. Anal. Calc. for C20H11N3O3F6PRe.H2O: C, 31.55%; N, 5.52%, H, 1.72%. Found: C, 31.57%; N, 5.44%; H, 1.52%.1H RMN (CD3CN, δ / ppm): 9.55 (d,2H), 8.25 (d,2H), 8.50 (s,2H), 8.38 (m,2H), 7.78 (m, 1H), 7.23 (m,2H). Preparation of polymer films. Doped films of poly(methyl methacrylate), PMMA, or poly(9-vinylcarbazole), PVK, were prepared following the procedure described elsewhere
20, 27
. An acetonitrile (HPLC) solution (5 mg:2.0 mL) of the
compound was added to a PMMA acetonitrile solution (250 mg:5.0 mL) and left to dry in the absence of humidity. For the PVK film, a dichloromethane (HPLC) solution (5 mg:2.0 mL) of the compound was added to a dichloromethane PVK solution (120 mg:5.0 mL) and left to dry. Methods.
Absorption
spectra
were
recorded
on
an
Agilent
8453
spectrophotometer. Infrared spectra were recorded on a Perkin Elmer FTIR Spectrum Two and an UATR Two accessory was used in measurements of the sample in the solid state. Proton nuclear magnetic resonance spectra (1H NMR) were obtained on a Bruker AC-200 (200 MHz) spectrometer at 298 K using CD3CN as a solvent. Residual CH3CN signals were employed as an internal standard. Emission spectra at room temperature were recorded with a Varian Cary Eclipse steady state spectrophotometer using a 1.00 cm optical length quartz cuvette for the fluid solution and a front face arrangement for the polymer films. Emission quantum yields of rhenium(I) compounds were determined as described
elsewhere20
using
fac-[ReCl(CO)3(phen)],
phen
=
1,10-
phenanthroline, as the standard (0.018 in CH3CN, 298 K)14 and keeping their absorbance at excitation wavelength between 0.1-0.2. Cyclic voltammetry 5
measurements were obtained in a µAutolab type III potentiostat/galvanostat (Autolab, The Netherlands) using glassy carbon as the working electrode, platinum as the counter electrode, and Ag/Ag+ as the reference electrode. All measurements were carried out in deaerated CH2Cl2 solution containing the complex (1 mM) and tetrabutylammonium hexafluorophosphate (0.1 M) as the supporting electrolyte at a scan rate of 100 mV s -1. Potentials were recorded vs. Ag/Ag+ and ferrocenium/ferrocene couple (Fc +/Fc) internal standard and were converted to Ered(onset) and Eox(onset) vs. the normal hydrogen electrode (NHE) using E1/2 (Fc +/Fc) = + 0.69 V vs. NHE in dichloromethane
28
. From the
onset, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels were estimated according to the equation
10, 29
EHOMO = - 4.5 - Eoxonset (- 4.5 V for NHE with respect to the
zero vacuum level) and ELUMO = - 4.5 – Eredonset.
Results and discussion IR spectra of Cl2phen and the Re(I) complexes are plotted in Fig. 1.
6
2000 1800 1600 1400 1200 1000
800
600
800
600
a
b
c 2000 1800 1600 1400 1200 1000 -1
ν (cm )
Fig.
1.
IR
spectra
of
(a)
Cl 2phen,
(b)
fac-[ReCl(CO)3(Cl 2phen)]
and
(c)
fac-
[Re(py)(CO)3(Cl2phen)]PF6 complexes
The bands in the higher energy region observed in both rhenium(I) complexes are the CO stretching vibrations and are characteristic of facial geometry. For fac-[ReCl(CO)3(Cl2phen)], Fig. 1b, the higher energy peak is observed at 2022 cm-1 and the lower one, poorly resolved, appeared as a broad band at 1883 cm1
. fac-[Re(py)(CO)3(Cl2phen)]PF6, Fig. 1c, the higher energy peak is found at
2030 cm-1 and the lower energy peak, also poorly resolved, appeared as a broad band at 1913 cm-1, and the intense IR band at 840 cm-1 is characteristic of PF6-
30
. Since there is a mirror plane through the axial carbonyl and the
rhenium(I) metal center, which separates the two equatorial carbonyls and the polypyridine ligand, it is expected for both compounds to exhibit three ν(CO)
7
bands defined in Cs symmetry with three vibrational modes: two A’ and one A’’ (Fig. 2). However, a wide band can be observed for both compounds, which can be ascribed to overlapping of the A’(2) and A’’ modes 31 . Additionally, the bands in the 1600-700 cm-1 region are ring vibrations of the coordinated ligand32 and are similar to those of the Cl2phen free ligand (Fig. 1a). 0/+ Cl +
L
O
Cl O
+
N
C C
N
C Cl
+
O
A'(1)
Re N
C
C +
O
Cl +
O
A'(2)
N
C
Re +
O
L
O
C
N
C
Cl +
L
Re +
0/+
0/+
N
C
O
Cl +
O A''
Fig. 2. Vibrational modes for Cs symmetry.
The shifts in peaks positions observed for fac-[Re(CO)3(Cl2phen)(py)]PF6 indicate the existence of a back donation competition between the py and CO moieties. The removal of electron density on the metal ion weakens the M-CO bond, which decreases the length of the CO bond and shifts these bands to higher frequencies. The 1H NMR spectral data for the ligands and complexes in CD3CN are listed in Table 1 (1H NMR spectra are depicted in Supporting Information, S1-S3) and the assignments have been made based on other rhenium(I) polypyridyl compounds 20, 33.
8
1
Table 1. H NMR spectral data for fac-[ReCl(CO)3(Cl 2phen)], fac-[Re(py)(CO)3(Cl2phen)] complexes and Cl2phen ligand in CD3CN (200 MHz).
Compound Py34 η θ
θ µ
N
Proton Hµ Hη Hθ
δ (ppm) 8.60 7.65 7.26
J (Hz)
Hα(d,2H) Hβ(d,2H) Hδ(s,2H)
9.04 7.87 8.40
4.7 4.9
Hα(d,2H) Hβ(d,2H) Hδ(s,2H)
9.32 8.09 8.53
5.5 5.6
Hα(d,2H) Hβ(d,2H) Hδ(s,2H) Hµ(m,2H) Hη(m,2H) Hθ(m,1H)
9.55 8.25 8.50 8,38 7.78 7.23
5.7 5.7
+
µ
Cl2phen β
Cl
α N δ δ
N α
Cl
β
fac-[ReCl(CO)3(Cl2phen)] β Cl
Cl
α N
OC
δ
Re OC
δ
N
CO
α
Cl
β
fac-[Re(py)(CO)3(Cl 2phen)]+ η θ
θ
µ
µ α
N
Cl
β
N
OC
δ
Re OC CO
δ
N α
Cl
β
In both rhenium(I) complexes, the aromatic proton signals were assigned based on the 1H NMR spectrum of the free ligands (Cl2phen and py) and similar Re(I) compounds20,
33-34
. The polypyridyl proton signals are shifted to a higher
frequency region after coordination to the metal center. On the other hand, the coordinated pyridine proton signals are upfield in comparison with the free ligand. The absorption spectra of fac-[ReCl(CO)3(Cl2phen)] and fac[Re(py)(CO)3(Cl2phen)]+, along with the free ligand, Cl2phen, in acetonitrile 9
solution are presented in Fig. 3 and their spectral parameters are listed in Table 2.
5
-1
ε (10 L.mol .cm )
4
4
-1
3
2
1
0 200
250
300
350
400
450
500
λ (nm)
Fig. 3. Electronic spectra of fac-[ReCl(CO)3(Cl 2phen)] (___), fac-[Re(CO)3(Cl 2phen)(py)]+ (---) and Cl2phen (-.-.-.) in acetonitrile. Table 2. Spectral parameters of rhenium(I) complexes and free ligand in acetonitrile.
Compounds
λmax (nm) (ε / 104.L.mol-1.cm-1 )
Cl2phen
205(2.71); 241(3.38); 266(3.58)
fac-[ReCl(CO)3(Cl2phen)]
213(4.59); 269(3.32); 384(0.48)
fac-[Re(py)(CO)3(Cl 2phen)]+
213(4.97); 241(2.40); 259(2.80); 278(2.93); 334(0.65)a
a
shoulder
The intense high-energy absorption bands at 200–300 nm for both complexes are assigned to polypyridyl intraligand transitions ILπ- π*. The lower energy band is ascribed to a metal-to-ligand charge transfer (MLCTRe→Cl2phen) transition by analogy with a variety of similar diimine rhenium(I) complexes
15, 19, 35-39
. The
electronic spectrum of fac-[ReCl(CO)3(Cl2phen)] is similar to the parent fac[ReCl(CO)3(phen)] compound except for a small bathochromic shift of the low
10
energy band caused by the MLCT stabilization promoted by the two electron withdrawing chloro attached to the phen. In the fac-[Re(py)(CO)3(Cl2phen)]+ complex, an absorption band in the 250–270 nm region is also observed, which, when compared to the spectrum of the analogue complex with chloride, can also be attributed to a transition located in pyridine, ILpy. In addition, in the 320 nm region, a contribution after coordination with pyridine (MLCTRe→py) can be observed due to its extended π system. The electronic spectrum of fac-[Re(py)(CO)3(Cl2phen)]+ is similar to the parent (fac[Re(py)(CO)3(ph2phen)]+)
19
and both exhibited a small tail extension in the low
energy region when compared with the analogue non-substituted phen complex. The photophysical properties were investigated and the emission spectra of fac[ReCl(CO)3(Cl2phen)]
and
fac-[Re(py)(CO)3(Cl2phen)]+
compounds
under
excitation at 350 nm in acetonitrile and in PMMA are presented in Fig. 4.
11
Intensity
a
b
500
550
600
650
700
750
800
λ (nm)
___
Fig. 4. Room temperature emission spectra of fac-[ReCl(CO)3(Cl2phen)] ( ) and fac+ [Re(py)(CO)3(Cl2phen)] (---) in acetonitrile (a) and in PMMA (b).
The fac-[ReL(CO)3(Cl2phen)] complexes exhibits characteristic broad and structureless emission band in both fluid solution and rigid media, arising from the 3MLCTRe→Cl2phen excited state. Emission maxima are dependent on the medium rigidity as well as the coordinated ligand. The spectra of both complexes in a PMMA film are blue shifted in comparison to the fluid CH3CN solution due to the 3MLCT excited state being very sensitive to the rigidity of the medium, known as the rigidochromic effect 40. Changing the polypyridinic ligands and using electron-withdrawing substituents mainly affects the 3MLCT excited state energy. The emission maxima for the fac-[Re(CO)3(Cl2phen)L] complexes are 640 and 590 nm for L = Cl and py, respectively, while for the corresponding non-substituted phenanthroline 12
compounds λmax = 600 nm
24
and λmax = 567 nm
19
in the same solvent
conditions. Thus, the hydrogen replacement with chloro at the 4 and 7 positions results in a bathochromic shift of the emission maxima compared with the nonsubstituted compound due to a more efficient 3MLCT excited state stabilization by the two electron-withdrawing chloro groups. Moreover, when comparing the electron-withdrawing ability of substituents on the diimine ligand, such as the phenyl in fac-[Re(CO)3(ph2phen)(py)]+ (λmax = 576 nm)
19
, which exhibits the
same trend in emission maxima, it is possible to conclude that the 3MLCT excited state energy in fac-[Re(CO)3(Cl2phen)(py)]+ is more stabilized relative to the other two parent rhenium complexes. The emission quantum yields were determined to be 0.0027 and 0.055 (in CH3CN)
for
fac-[ReCl(CO)3(Cl2phen)]
and
fac-[Re(py)(CO)3(Cl2phen)]+,
respectively, which are much lower than the data reported for other rhenium(I) compounds
14, 19
, showing that the main path of excited state deactivation is by
non-radiative decay. The lower emission quantum yield for the former compound is consistent with the energy gap law; the non-radiative decay decrease as the energy of the MLCT increases due to a higher vibrational overlap between the ground state and the excited state41-43. Similarly, the emission quantum yield for the fac-[Re(CO)3(phen)L] complexes at the same solvent are 0.018 and 0.18 for L = Cl14 and py19, respectively. Additionally, when comparing with the non-substituted phen complexes, the lower emission quantum yields along with a more efficient 3MLCT excited state stabilization observed for fac-[Re(CO)3(Cl2phen)L]+ are also in agreement with the energy gap law.
13
In order to test the energy transfer efficiency for applications in light emitting device such as OLEDs, fac-[Re(CO)3Cl(Cl2phen)] was also incorporated on a PVK film (Fig. 5) because the PVK host architecture has been already reported 13, 27
. However, it is important to note that the dopant/host interaction observed
in the photophysical investigation does not necessarily correlate with
Intensity
electroluminescent behavior in a device.
400
500
600
700
800
λ (nm)
Fig. 5. Room temperature emission spectra of 0% (…..) and 4% (___) w/w of fac[ReCl(CO)3(Cl 2phen)] in PVK film, λexc = 300 nm.
It can be observed that the PVK emission maximum at 420 nm almost entirely disappeared as a 4% concentration of dopant was added to the film, while the characteristic rhenium(I) complex emission at 600 nm remains. This behavior has also been observed for other Re(I) complexes
10, 27, 44
and was associated
with quenching in the host emission, consistent with Förster energy transfer where there is a spectral overlap of the host (PVK) emission band with the dopant 1MLCT absorption band. This is followed by a fast intersystem crossing to the 3MLCT excited state and subsequent emission from this state. However, 14
the Dexter energy transfer mechanism has been discussed for a PVK film doped with Iridium(III) complex The
electrochemical
45
.
behavior
fac-[Re(py)(CO)3(Cl2phen)]+
of
complexes
fac-[ReCl(CO)3(Cl2phen)] was
investigated
with
and cyclic
voltammetry experiments, shown in Fig. 6 and Table 3.
0,06
0,04
I (mA)
0,02
0,00
-0,02
-0,04
-0,06 -2
-1
0
1
2
E (V vs NHE)
Fig. 6. Cyclic voltammograms of fac-[ReCl(CO)3(Cl2phen)] (___) and fac+ [Re(py)(CO)3(Cl 2phen)] (-----) measured in CH2Cl2 vs. NHE at a scan rate of 100 mV/s.
Both rhenium(I) compounds showed a reduction process associated with the polypyridine ligand (Cl2phen → Cl2phen•-) while the anodic wave is associated with a Re(I)-Re(II) oxidation process. For fac-[ReCl(CO)3(Cl2phen)] is also observed an oxidation peak at -0.73 V vs. NHE suggesting a decomposition as previously reported for similar rhenium(I) complexes
46-50
. It is remarkable that
the difference between the oxidation and reduction potentials is much larger in fac-[Re(py)(CO)3(Cl2phen)]+
than
in
the
parent
compound
fac-
[ReCl(CO)3(Cl2phen)]. This behavior is due to the poor σ-donating and good π-
15
accepting ability of the py ligand, which renders the Re(I) metal center destitute of electron density and thus increases its oxidation potential51. To provide insight into an electroluminescent device application of these two compounds, the HOMO and the LUMO energy levels were estimated (Table 3). The HOMO of a M-L bond is determined mainly by the orbitals of the metal unit, while the LUMO is controlled mainly by the orbitals of the ligand unit. Table 3. Electrochemical data and calculated HOMO and LUMO energy levels Eoxonset
Compound
vs Eredonset
vs HOMO
LUMO
NHE (V)
NHE (V)
(eV)
(eV)
fac-[Re(CO)3(Cl2phen)Cl]
+1.54
-1.00
-6.04
-3.60
fac-[Re(CO)3(Cl2phen)(py)]+
+1.63
-0.84
-6.13
-3.66
The HOMO and LUMO energy values are similar to the data reported by Iha and co-workers
52
for fac-[ClRe(CO)3(bpy)] using PVK as the emissive layer in
an ITO/PEDOT:PSS/PVK/butyl-PBD/Al OLED architecture. Because the energy of the HOMO and LUMO of the emitter layer matrix material has to fit between the adjacent layers13 in a successful emitting layer, the authors showed that rhenium(I) compounds act as electron traps in PVK-based devices. In addition, the triplet-excited state of the polymer matrix must be at a higher energy than the triplet excited state of the emitting species (dopant); otherwise, the dopant emission would be quenched. Although the low emission quantum yield of fac[ReCl(CO)3(bpy)] at solution and room temperature41, ɸem = 0.005, Iha and coworkers52 showed that the luminous efficiency of organic light-emitting diodes based on PVK was improved by adding fac-[ReCl(CO)3(bpy)] complex to the 16
emitter layer matrix. Consequently, fac-[Re(CO)3(Cl2phen)(L)] complexes can be used as electron traps in PVK based devices although their low emission quantum yields.
Conclusions Novel Re(I) complexes with substituted electron withdrawing Cl groups on 1,10phenanthroline, fac-[ReCl(CO)3(Cl2phen)] and
fac-[Re(py)(CO)3(Cl2phen)]+,
were synthesized, characterized and their photophysical and electrochemical properties have been investigated. Both compounds displayed predominant 3
MLCT emission in solution and rigid media and the hydrogen replacement with
chloro at the 4 and 7 positions of 1,10-phenanthroline results in a bathochromic shift of the emission maxima compared to the non-substituted species due to a more efficient 3MLCT excited state stabilization by the two electron-withdrawing chloro
groups.
Photoluminescence
of
a
PVK
film
doped
with
fac-
[ReCl(CO)3(Cl2phen)] showed an efficient energy transfer from polymer host to the dopant. The ancillary py ligand shifted the oxidation potential in the positive direction by removing electron density from Re(I) in comparison to fac[ReCl(CO)3(Cl2phen)]. The electrochemical and emission properties of fac[Re(CO)3(Cl2phen)L] complexes indicate a potential application of rhenium(I) compounds in OLED devices.
Acknowledgements The authors would like to acknowledge financial support from Fundacao de Amparo a Pesquisa de Sao Paulo (Grant 2011/23408-0).
17
Supporting Information Available: 1H NMR spectra of Cl2phen, fac[ReCl(CO)3(Clphen)] and fac-[Re(CO)3(Cl2phen)(py)]+ compounds and cyclic voltammograms of complexes measured in CH2Cl2 vs. Ag/Ag+.
References 1. Alstrum-Acevedo, J. H.; Brennaman, M. K.; Meyer, T. J., Inorg. Chem. 44 (2005) 68026827. 2. Kalyanasundaram, K.; Graetzel, M., Curr. Opin. Biotechnol. 21 (2010) 298-310. 3. Takeda, H.; Koike, K.; Morimoto, T.; Inumaru, H.; Ishitani, O., Adv. Inorg. Chem. 63 (2011) 137-186. 4. Tamaki, Y.; Koike, K.; Morimoto, T.; Ishitani, O., J. Catal. 304 (2013) 22-28. 5. Asatani, T.; Nakagawa, Y.; Funada, Y.; Sawa, S.; Takeda, H.; Morimoto, T.; Koike, K.; Ishitani, O., Inorg. Chem. 53 (2014) 7170-7180. 6. Rudmann, H.; Rubner, M. F., J. Appl. Phys. 90 (2001) 4338-4345. 7. Fan, F.-R. F.; Bard, A. J., J. Phys. Chem. B 107 (2003) 1781-1787. 8. Zanoni, K. P. S.; Sanematsu, M. S.; Murakami Iha, N. Y., Inorg. Chem. Commun. 43 (2014) 162-164. 9. Sasabe, H.; Kido, J., J. Mater. Chem. C 1 (2013) 1699-1707. 10. Liu, X.; Xia, H.; Gao, W.; Wu, Q.; Fan, X.; Mu, Y.; Ma, C., J. Mater. Chem. C 22 (2012) 3485-3492. 11. Si, Z.; Li, J.; Li, B.; Zhao, F.; Liu, S.; Li, W., Inorg. Chem. 46 (2007) 6155-6163. 12. Yu, T.; Tsang, D. P.-K.; Au, V. K.-M.; Lam, W. H.; Chan, M.-Y.; Yam, V. W.-W., Chemistry – A European Journal 19 (2013) 13418-13427. 13. Yersin, H., Triplet Emitters for OLED Applications. Mechanisms of Exciton Trapping and Control of Emission Properties. In Transition Metal and Rare Earth Compounds, Springer Berlin Heidelberg: 2004; Vol. 241, pp 1-26. 14. Evans, R. C.; Douglas, P.; Winscom, C. J., Coord. Chem. Rev. 250 (2006) 2093-2126. 15. Polo, A. S.; Itokazu, M. K.; Frin, K. M.; de Toledo Patrocínio, A. O.; Murakami Iha, N. Y., Coord. Chem. Rev. 250 (2006) 1669-1680. 16. Sun, S. S.; Lees, A. J., Coord. Chem. Rev. 230 (2002) 171-192. 17. Sacksteder, L.; Zipp, A. P.; Brown, E. A.; Streich, J.; Demas, J. N.; DeGraff, B. A., Inorg. Chem. 29 (1990) 4335-4340. 18. Zipp, A. P.; Sacksteder, L.; Streich, J.; Cook, A.; Demas, J. N.; DeGraff, B. A., Inorg. Chem. 32 (1993) 5629-5632. 19. Wallace, L.; Rillema, D. P., Inorg. Chem. 32 (1993) 3836-3843. 20. Frin, K. P. M.; Murakami Iha, N. Y., Inorg. Chim. Acta 376 (2011) 531-537. 21. Patrocinio, A. O. T.; Brennaman, M. K.; Meyer, T. J.; Murakami Iha, N. Y., J. Phys. Chem. A 114 (2010) 12129-12137. 22. Sacksteder, L.; Lee, M.; Demas, J. N.; DeGraff, B. A., J. Am. Chem. Soc. 115 (1993) 82308238. 23. Wallace, L.; Jackman, D. C.; Rillema, D. P.; Merkert, J. W., Inorg. Chem. 34 (1995) 52105214. 24. Frin, K. P. M.; Zanoni, K. P. S.; Murakami Iha, N. Y., Inorg. Chem. Commun. 20 (2012) 105-107. 25. Frin, K. M.; Iha, N. Y. M., J. Braz. Chem. Soc. 17 (2006) 1664-1671.
18
26. Patrocinio, A. O. T.; Frin, K. P. M.; Murakami Iha, N. Y., Inorg. Chem. 52 (2013) 58895896. 27. Mizoguchi, S. K.; Patrocínio, A. O. T.; Murakami Iha, N. Y., Synth. Met. 159 (2009) 23152317. 28. Moore, G. F.; Blakemore, J. D.; Milot, R. L.; Hull, J. F.; Song, H.-e.; Cai, L.; Schmuttenmaer, C. A.; Crabtree, R. H.; Brudvig, G. W., Energy Environ. Sci 4 (2011) 2389-2392. 29. Allen J. Bard, L. R. F., Electrochemical Methods: Fundamentals and Applications. 2nd ed.; John Wiley and Sons, Inc: New York, 2001. 30. Hansen, P. W.; Jensen, P. W., Spectrochimica Acta Part A: Molecular Spectroscopy 50 (1994) 169-183. 31. Dattelbaum, D. M.; Martin, R. L.; Schoonover, J. R.; Meyer, T. J., J. Phys. Chem. A 108 (2004) 3518-3526. 32. Itokazu, M. K.; Polo, A. S.; de Faria, D. L. A.; Bignozzi, C. A.; Iha, N. Y. M., Inorg. Chim. Acta 313 (2001) 149-155. 33. Frin, K. P. M.; Itokazu, M. K.; Iha, N. Y. M., Inorg. Chim. Acta 363 (2010) 294-300. 34. Abraham, R. J.; Reid, M., Journal of the Chemical Societ , Perkin Transactions 2 0 (2002) 1081-1091. 35. Sacksteder, L.; Zipp, A. P.; Brown, E. A.; Streich, J.; Demas, J. N.; DeGraff, B. A., Inorg. Chem. 29 (1990) 4335-4340. 36. Van Wallendael, S.; Shaver, R. J.; Rillema, D. P.; Yoblinski, B. J.; Stathis, M.; Guarr, T. F., Inorg. Chem. 29 (1990) 1761-1767. 37. Tao, T.; Qian, H.-F.; Huang, W.; You, X.-Z., Organometallics 33 (2014) 5120-5128. 38. Wrighton, M.; Morse, D. L., J. Am. Chem. Soc. 96 (1974) 998-1003. 39. Yam, V. W. W.; Yang, Y.; Zhang, J.; Chu, B. W. K.; Zhu, N., Organometallics 20 (2001) 4911-4918. 40. Chen, P.; Meyer, T. J., Chem. Rev. 98 (1998) 1439-1477. 41. Caspar, J. V.; Meyer, T. J., J. Phys. Chem. 87 (1983) 952-957. 42. Caspar, J. V.; Kober, E. M.; Sullivan, B. P.; Meyer, T. J., J. Am. Chem. Soc. 104 (1982) 630-632. 43. Kalyanasundaram, K., Photochemistry of polypyridine and porphyrin complexes. Academic Press: 1992. 44. Li, F.; Zhang, M.; Cheng, G.; Feng, J.; Zhao, Y.; Ma, Y.; Shiyong, L.; Shen, J., Appl. Phys. Lett. 84 (2004) 148-150. 45. Noh, Y.-Y.; Lee, C.-L.; Kim, J.-J.; Yase, K., The Journal of Chemical Physics 118 (2003) 2853-2864. 46. Morse, D. L.; Wrighton, M. S., J. Organomet. Chem. 125 (1977) 71-77. 47. Luong, J. C.; Nadjo, L.; Wrighton, M. S., J. Am. Chem. Soc. 100 (1978) 5790-5795. 48. Christensen, P.; Hamnett, A.; Muir, A. V. G.; Timney, J. A., J. Chem. Soc., Dalton Trans. (1992) 1455-1463. 49. Yam, V. W. W.; Lau, V. C. Y.; Wu, L. X., J. Chem. Soc., Dalton Trans. (1998) 1461-1468. 50. Ko, C.-C.; Siu, J. W.-K.; Cheung, A. W.-Y.; Yiu, S.-M., Organometallics 30 (2011) 27012711. 51. Ranjan, S.; Lin, S.-Y.; Hwang, K.-C.; Chi, Y.; Ching, W.-L.; Liu, C.-S.; Tao, Y.-T.; Chien, C.H.; Peng, S.-M.; Lee, G.-H., Inorg. Chem. 42 (2003) 1248-1255. 52. Mizoguchi, S. K.; Santos, G.; Andrade, A. M.; Fonseca, F. J.; Pereira, L.; Murakami Iha, N. Y., Synth. Met. 161 (2011) 1972-1975.
19
S0277-5387(15)00248-X http://dx.doi.org/10.1016/j.poly.2015.05.007 POLY 11305
To appear in:
Polyhedron
Received Date: Accepted Date:
29 January 2015 4 May 2015
Please cite this article as: M.R. Gonçalves, K.P.M. Frin, Synthesis, characterization, photophysical and electrochemical properties of fac-tricarbonyl(4,7-dichloro-1,10-phenanthroline)rhenium(I) complexes, Polyhedron (2015), doi: http://dx.doi.org/10.1016/j.poly.2015.05.007
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Synthesis, characterization, photophysical and electrochemical properties of fac-tricarbonyl(4,7-dichloro-1,10phenanthroline)rhenium(I) complexes Márcia R. Gonçalves and Karina P. M. Frin* Federal University of ABC – UFABC, Av. dos Estados 5001 - Santo André – SP - Brazil, 09210-170, *E-mail: [email protected] Abstract The
fac-[Re(CO)3Cl(Cl2phen)]
and
fac-[Re(CO)3(py)(Cl2phen)]PF6
complexes were synthesized, purified and characterized by 1H NMR, UV-vis and IR spectroscopies and both photophysical and electrochemical behaviors were investigated. The electronic absorption spectra exhibit two main absorption bands: the higher energy band, which was assigned to IL, and the lower energy band, assigned to MLCT. The fac-[ReCl(Cl2phen)(CO)3] and fac[Re(py)(Cl2phen)(CO)3]PF6 complexes showed emission at room temperature in both CH3CN solution (λmax = 640 nm, φ = 0.0027; λmax = 590 nm, φ = 0.055) and rigid media (λmax =590 nm, λmax = 535 nm in PMMA) arising from the lowest lying metal to ligand charge transfer (3MLCTRe→Cl2phen) excited state. Additionally, the PVK emission band is almost completely quenched in the presence of 4% rhenium(I) complex. Both compounds showed metal-centered oxidation, Re(I)→(II), and ligand-based reduction, Cl2phen → Cl2phen*-, in CH2Cl2 solution. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of both complexes were estimated. The electrochemical and emission properties indicate a potential application of the rhenium(I) compounds in OLED devices.
1
Keywords: rhenium(I) tricarbonyl complexes; electron withdrawing polypyridyl ligand; photophysical properties; 3MLCT emission
Introduction A growing need for new sources of energy, arising from the prediction that fossil fuels would be completely dried up in the near future, has encouraged researchers to pursue potential alternatives to fulfill world energy needs. To meet this energy need, transition metal complexes, especially those with d6 electron configuration, have been receiving great attention in investigations related to solar energy conversion 1-2, photocatalysis devices such as LECs
6-8
and OLEDs
3-5
, and electroluminescent
9-12
, among others. The reasons for the
great interest in this class of compound are various and include absorption of radiation in a lower region of the spectrum than observed in organic counterparts; strong spin-orbit coupling, especially for the second and third transition metal, which eliminates the spin-forbidden T1→S0 radiative transition; excited states with intense redox activity allowing the occurrence of a series of electron transfer reactions and/or energy, among others
13-14
.
One of the attractive features of transition metals is their ability to form complexes with a variety of polypyridine ligands, NN, such as 1,10phenanthroline and its derivatives, which tune their ground and excited state properties. In particular, the rhenium(I) tricarbonyl polypyridyl complexes have been widely investigated due to their interesting triplet emissive properties
3, 14-
16
. Additionally, intense effort has been dedicated to better understand the effect
of the position and attachment of electron withdrawing/donating groups to ligands on the mechanistic events after excitation
15, 17-21
, which can contribute
to a deeper comprehension of their emissive properties. For instance, electron 2
withdrawing groups attached to polypyridyl ligands promote stabilization of 3
MLCTRe→NN excited state energy level (metal-to-ligand charge-transfer)15, 17, 19,
21-24
. On the other hand, electron donating groups, such as methyl, promote
destabilization of 3MLCTRe→NN excited state energy level and at the same time reduces the energy of ligand-centered excited state, 3ILNN polypyridyl complexes show a dominant
3
19, 21, 23
. The rhenium
MLCT emission, but some
3
IL
character could be also evident to a lesser or greater degree, depending on the coordinated NN ligand. Therefore, relative energy of these states plays an important role in determining the efficiency and mechanism of deactivation pathways after excitation. In this
work,
we
report the synthesis and characterization of fac-
[Re(CO)3(Cl2phen)Cl] and fac-[Re(CO)3(Cl2phen)(py)]+, where Cl2phen = 4,7dichloro-1,10-phenanthroline and py = pyridine, as well as their electrochemical properties. Additionally, their photophysical properties in fluid and rigid media were investigated with a strong motivation for understanding the electron withdrawing effect imparted by the two chlorine groups attached to 1,10phenanthroline at the 4 and 7 positions.
Experimental Procedures Materials. All solvents, from Aldrich, Synth or Merck, were reagent grade, except for those used in the photophysical and electrochemical measurements, where HPLC grade solvents from Aldrich, were employed. [ClRe(CO)5], 4,7dichloro-1,10-phenanthroline (Cl2phen), trifluoromethanesulfonic acid (tfms), pyridine (py), poly(methyl methacrylate) (PMMA, MW = 110,000 g mol-1), and 3
poly(9-vinylcarbazole) (PVK, MW = 25,000-50,000 g mol-1) from Aldrich were used as received. Synthesis The fac-[ReCl(CO)3(Cl2phen)] complex was synthesized with slight modification to the procedure previously described for fac-[ReCl(CO)3(5-Clphen)]
20
.
[ClRe(CO)5] (0.55 g, 1.5 mmol) and an excess of 4,7-dichloro-1,10phenanthroline, Cl2phen (0.73 g, 2.9 mmol), were mixed in 30 mL of xylene (Merck) and heated to reflux for 6 h. The resulting solid was separated by filtration before cooling to room temperature. Purification was achieved by recrystallization from CH2Cl by slow addition of n-pentane. Yield 57%. Anal. Calc. for ReC15H6Cl3N2O3: C, 32.51%; N, 4.82%; H, 1.01%. Found: C, 32.59%; N, 4.87%; H, 0.97%. 1H NMR (CD3CN, δ / ppm): 9.32 (d, 2H), 8.53 (s, 2H), 8.09 (d,2H). The fac-[Re(tfms)(CO)3(Cl2phen)] complex was synthesized as previously reported
25
for a similar compound by suspending fac-[ReCl(CO)3(Cl2phen)]
(0.30 g, 0.54 mmol) in 30 mL argon-saturated CH2Cl2, to which a 12-fold excess of trifluoromethanesulfonic acid (tfms) was added. The solution was stirred for one hour, and the intermediate fac-[Re(tfms)(CO)3(Cl2phen)] was obtained by slow addition of ethyl ether. Yield 91%. The fac-[Re(py)(CO)3(Cl2phen)]PF6 complex was synthesized with slight modification to the procedure reported for similar compounds
20, 26
. The fac-
[Re(tfms)(CO)3(Cl2phen)] complex (0.33 g, 0.49 mmol) and pyridine (py 80 µL, 1.06 mmol) were mixed in methanol (30 mL) and kept in reflux for 7 h. After cooling to room temperature, the final product was precipitated by the addition 4
of solid NH4PF6. The yellow solid was separated by filtration, washed with water to remove the excess NH4PF6, then washed with ethyl ether. Yield 25%. Anal. Calc. for C20H11N3O3F6PRe.H2O: C, 31.55%; N, 5.52%, H, 1.72%. Found: C, 31.57%; N, 5.44%; H, 1.52%.1H RMN (CD3CN, δ / ppm): 9.55 (d,2H), 8.25 (d,2H), 8.50 (s,2H), 8.38 (m,2H), 7.78 (m, 1H), 7.23 (m,2H). Preparation of polymer films. Doped films of poly(methyl methacrylate), PMMA, or poly(9-vinylcarbazole), PVK, were prepared following the procedure described elsewhere
20, 27
. An acetonitrile (HPLC) solution (5 mg:2.0 mL) of the
compound was added to a PMMA acetonitrile solution (250 mg:5.0 mL) and left to dry in the absence of humidity. For the PVK film, a dichloromethane (HPLC) solution (5 mg:2.0 mL) of the compound was added to a dichloromethane PVK solution (120 mg:5.0 mL) and left to dry. Methods.
Absorption
spectra
were
recorded
on
an
Agilent
8453
spectrophotometer. Infrared spectra were recorded on a Perkin Elmer FTIR Spectrum Two and an UATR Two accessory was used in measurements of the sample in the solid state. Proton nuclear magnetic resonance spectra (1H NMR) were obtained on a Bruker AC-200 (200 MHz) spectrometer at 298 K using CD3CN as a solvent. Residual CH3CN signals were employed as an internal standard. Emission spectra at room temperature were recorded with a Varian Cary Eclipse steady state spectrophotometer using a 1.00 cm optical length quartz cuvette for the fluid solution and a front face arrangement for the polymer films. Emission quantum yields of rhenium(I) compounds were determined as described
elsewhere20
using
fac-[ReCl(CO)3(phen)],
phen
=
1,10-
phenanthroline, as the standard (0.018 in CH3CN, 298 K)14 and keeping their absorbance at excitation wavelength between 0.1-0.2. Cyclic voltammetry 5
measurements were obtained in a µAutolab type III potentiostat/galvanostat (Autolab, The Netherlands) using glassy carbon as the working electrode, platinum as the counter electrode, and Ag/Ag+ as the reference electrode. All measurements were carried out in deaerated CH2Cl2 solution containing the complex (1 mM) and tetrabutylammonium hexafluorophosphate (0.1 M) as the supporting electrolyte at a scan rate of 100 mV s -1. Potentials were recorded vs. Ag/Ag+ and ferrocenium/ferrocene couple (Fc +/Fc) internal standard and were converted to Ered(onset) and Eox(onset) vs. the normal hydrogen electrode (NHE) using E1/2 (Fc +/Fc) = + 0.69 V vs. NHE in dichloromethane
28
. From the
onset, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels were estimated according to the equation
10, 29
EHOMO = - 4.5 - Eoxonset (- 4.5 V for NHE with respect to the
zero vacuum level) and ELUMO = - 4.5 – Eredonset.
Results and discussion IR spectra of Cl2phen and the Re(I) complexes are plotted in Fig. 1.
6
2000 1800 1600 1400 1200 1000
800
600
800
600
a
b
c 2000 1800 1600 1400 1200 1000 -1
ν (cm )
Fig.
1.
IR
spectra
of
(a)
Cl 2phen,
(b)
fac-[ReCl(CO)3(Cl 2phen)]
and
(c)
fac-
[Re(py)(CO)3(Cl2phen)]PF6 complexes
The bands in the higher energy region observed in both rhenium(I) complexes are the CO stretching vibrations and are characteristic of facial geometry. For fac-[ReCl(CO)3(Cl2phen)], Fig. 1b, the higher energy peak is observed at 2022 cm-1 and the lower one, poorly resolved, appeared as a broad band at 1883 cm1
. fac-[Re(py)(CO)3(Cl2phen)]PF6, Fig. 1c, the higher energy peak is found at
2030 cm-1 and the lower energy peak, also poorly resolved, appeared as a broad band at 1913 cm-1, and the intense IR band at 840 cm-1 is characteristic of PF6-
30
. Since there is a mirror plane through the axial carbonyl and the
rhenium(I) metal center, which separates the two equatorial carbonyls and the polypyridine ligand, it is expected for both compounds to exhibit three ν(CO)
7
bands defined in Cs symmetry with three vibrational modes: two A’ and one A’’ (Fig. 2). However, a wide band can be observed for both compounds, which can be ascribed to overlapping of the A’(2) and A’’ modes 31 . Additionally, the bands in the 1600-700 cm-1 region are ring vibrations of the coordinated ligand32 and are similar to those of the Cl2phen free ligand (Fig. 1a). 0/+ Cl +
L
O
Cl O
+
N
C C
N
C Cl
+
O
A'(1)
Re N
C
C +
O
Cl +
O
A'(2)
N
C
Re +
O
L
O
C
N
C
Cl +
L
Re +
0/+
0/+
N
C
O
Cl +
O A''
Fig. 2. Vibrational modes for Cs symmetry.
The shifts in peaks positions observed for fac-[Re(CO)3(Cl2phen)(py)]PF6 indicate the existence of a back donation competition between the py and CO moieties. The removal of electron density on the metal ion weakens the M-CO bond, which decreases the length of the CO bond and shifts these bands to higher frequencies. The 1H NMR spectral data for the ligands and complexes in CD3CN are listed in Table 1 (1H NMR spectra are depicted in Supporting Information, S1-S3) and the assignments have been made based on other rhenium(I) polypyridyl compounds 20, 33.
8
1
Table 1. H NMR spectral data for fac-[ReCl(CO)3(Cl 2phen)], fac-[Re(py)(CO)3(Cl2phen)] complexes and Cl2phen ligand in CD3CN (200 MHz).
Compound Py34 η θ
θ µ
N
Proton Hµ Hη Hθ
δ (ppm) 8.60 7.65 7.26
J (Hz)
Hα(d,2H) Hβ(d,2H) Hδ(s,2H)
9.04 7.87 8.40
4.7 4.9
Hα(d,2H) Hβ(d,2H) Hδ(s,2H)
9.32 8.09 8.53
5.5 5.6
Hα(d,2H) Hβ(d,2H) Hδ(s,2H) Hµ(m,2H) Hη(m,2H) Hθ(m,1H)
9.55 8.25 8.50 8,38 7.78 7.23
5.7 5.7
+
µ
Cl2phen β
Cl
α N δ δ
N α
Cl
β
fac-[ReCl(CO)3(Cl2phen)] β Cl
Cl
α N
OC
δ
Re OC
δ
N
CO
α
Cl
β
fac-[Re(py)(CO)3(Cl 2phen)]+ η θ
θ
µ
µ α
N
Cl
β
N
OC
δ
Re OC CO
δ
N α
Cl
β
In both rhenium(I) complexes, the aromatic proton signals were assigned based on the 1H NMR spectrum of the free ligands (Cl2phen and py) and similar Re(I) compounds20,
33-34
. The polypyridyl proton signals are shifted to a higher
frequency region after coordination to the metal center. On the other hand, the coordinated pyridine proton signals are upfield in comparison with the free ligand. The absorption spectra of fac-[ReCl(CO)3(Cl2phen)] and fac[Re(py)(CO)3(Cl2phen)]+, along with the free ligand, Cl2phen, in acetonitrile 9
solution are presented in Fig. 3 and their spectral parameters are listed in Table 2.
5
-1
ε (10 L.mol .cm )
4
4
-1
3
2
1
0 200
250
300
350
400
450
500
λ (nm)
Fig. 3. Electronic spectra of fac-[ReCl(CO)3(Cl 2phen)] (___), fac-[Re(CO)3(Cl 2phen)(py)]+ (---) and Cl2phen (-.-.-.) in acetonitrile. Table 2. Spectral parameters of rhenium(I) complexes and free ligand in acetonitrile.
Compounds
λmax (nm) (ε / 104.L.mol-1.cm-1 )
Cl2phen
205(2.71); 241(3.38); 266(3.58)
fac-[ReCl(CO)3(Cl2phen)]
213(4.59); 269(3.32); 384(0.48)
fac-[Re(py)(CO)3(Cl 2phen)]+
213(4.97); 241(2.40); 259(2.80); 278(2.93); 334(0.65)a
a
shoulder
The intense high-energy absorption bands at 200–300 nm for both complexes are assigned to polypyridyl intraligand transitions ILπ- π*. The lower energy band is ascribed to a metal-to-ligand charge transfer (MLCTRe→Cl2phen) transition by analogy with a variety of similar diimine rhenium(I) complexes
15, 19, 35-39
. The
electronic spectrum of fac-[ReCl(CO)3(Cl2phen)] is similar to the parent fac[ReCl(CO)3(phen)] compound except for a small bathochromic shift of the low
10
energy band caused by the MLCT stabilization promoted by the two electron withdrawing chloro attached to the phen. In the fac-[Re(py)(CO)3(Cl2phen)]+ complex, an absorption band in the 250–270 nm region is also observed, which, when compared to the spectrum of the analogue complex with chloride, can also be attributed to a transition located in pyridine, ILpy. In addition, in the 320 nm region, a contribution after coordination with pyridine (MLCTRe→py) can be observed due to its extended π system. The electronic spectrum of fac-[Re(py)(CO)3(Cl2phen)]+ is similar to the parent (fac[Re(py)(CO)3(ph2phen)]+)
19
and both exhibited a small tail extension in the low
energy region when compared with the analogue non-substituted phen complex. The photophysical properties were investigated and the emission spectra of fac[ReCl(CO)3(Cl2phen)]
and
fac-[Re(py)(CO)3(Cl2phen)]+
compounds
under
excitation at 350 nm in acetonitrile and in PMMA are presented in Fig. 4.
11
Intensity
a
b
500
550
600
650
700
750
800
λ (nm)
___
Fig. 4. Room temperature emission spectra of fac-[ReCl(CO)3(Cl2phen)] ( ) and fac+ [Re(py)(CO)3(Cl2phen)] (---) in acetonitrile (a) and in PMMA (b).
The fac-[ReL(CO)3(Cl2phen)] complexes exhibits characteristic broad and structureless emission band in both fluid solution and rigid media, arising from the 3MLCTRe→Cl2phen excited state. Emission maxima are dependent on the medium rigidity as well as the coordinated ligand. The spectra of both complexes in a PMMA film are blue shifted in comparison to the fluid CH3CN solution due to the 3MLCT excited state being very sensitive to the rigidity of the medium, known as the rigidochromic effect 40. Changing the polypyridinic ligands and using electron-withdrawing substituents mainly affects the 3MLCT excited state energy. The emission maxima for the fac-[Re(CO)3(Cl2phen)L] complexes are 640 and 590 nm for L = Cl and py, respectively, while for the corresponding non-substituted phenanthroline 12
compounds λmax = 600 nm
24
and λmax = 567 nm
19
in the same solvent
conditions. Thus, the hydrogen replacement with chloro at the 4 and 7 positions results in a bathochromic shift of the emission maxima compared with the nonsubstituted compound due to a more efficient 3MLCT excited state stabilization by the two electron-withdrawing chloro groups. Moreover, when comparing the electron-withdrawing ability of substituents on the diimine ligand, such as the phenyl in fac-[Re(CO)3(ph2phen)(py)]+ (λmax = 576 nm)
19
, which exhibits the
same trend in emission maxima, it is possible to conclude that the 3MLCT excited state energy in fac-[Re(CO)3(Cl2phen)(py)]+ is more stabilized relative to the other two parent rhenium complexes. The emission quantum yields were determined to be 0.0027 and 0.055 (in CH3CN)
for
fac-[ReCl(CO)3(Cl2phen)]
and
fac-[Re(py)(CO)3(Cl2phen)]+,
respectively, which are much lower than the data reported for other rhenium(I) compounds
14, 19
, showing that the main path of excited state deactivation is by
non-radiative decay. The lower emission quantum yield for the former compound is consistent with the energy gap law; the non-radiative decay decrease as the energy of the MLCT increases due to a higher vibrational overlap between the ground state and the excited state41-43. Similarly, the emission quantum yield for the fac-[Re(CO)3(phen)L] complexes at the same solvent are 0.018 and 0.18 for L = Cl14 and py19, respectively. Additionally, when comparing with the non-substituted phen complexes, the lower emission quantum yields along with a more efficient 3MLCT excited state stabilization observed for fac-[Re(CO)3(Cl2phen)L]+ are also in agreement with the energy gap law.
13
In order to test the energy transfer efficiency for applications in light emitting device such as OLEDs, fac-[Re(CO)3Cl(Cl2phen)] was also incorporated on a PVK film (Fig. 5) because the PVK host architecture has been already reported 13, 27
. However, it is important to note that the dopant/host interaction observed
in the photophysical investigation does not necessarily correlate with
Intensity
electroluminescent behavior in a device.
400
500
600
700
800
λ (nm)
Fig. 5. Room temperature emission spectra of 0% (…..) and 4% (___) w/w of fac[ReCl(CO)3(Cl 2phen)] in PVK film, λexc = 300 nm.
It can be observed that the PVK emission maximum at 420 nm almost entirely disappeared as a 4% concentration of dopant was added to the film, while the characteristic rhenium(I) complex emission at 600 nm remains. This behavior has also been observed for other Re(I) complexes
10, 27, 44
and was associated
with quenching in the host emission, consistent with Förster energy transfer where there is a spectral overlap of the host (PVK) emission band with the dopant 1MLCT absorption band. This is followed by a fast intersystem crossing to the 3MLCT excited state and subsequent emission from this state. However, 14
the Dexter energy transfer mechanism has been discussed for a PVK film doped with Iridium(III) complex The
electrochemical
45
.
behavior
fac-[Re(py)(CO)3(Cl2phen)]+
of
complexes
fac-[ReCl(CO)3(Cl2phen)] was
investigated
with
and cyclic
voltammetry experiments, shown in Fig. 6 and Table 3.
0,06
0,04
I (mA)
0,02
0,00
-0,02
-0,04
-0,06 -2
-1
0
1
2
E (V vs NHE)
Fig. 6. Cyclic voltammograms of fac-[ReCl(CO)3(Cl2phen)] (___) and fac+ [Re(py)(CO)3(Cl 2phen)] (-----) measured in CH2Cl2 vs. NHE at a scan rate of 100 mV/s.
Both rhenium(I) compounds showed a reduction process associated with the polypyridine ligand (Cl2phen → Cl2phen•-) while the anodic wave is associated with a Re(I)-Re(II) oxidation process. For fac-[ReCl(CO)3(Cl2phen)] is also observed an oxidation peak at -0.73 V vs. NHE suggesting a decomposition as previously reported for similar rhenium(I) complexes
46-50
. It is remarkable that
the difference between the oxidation and reduction potentials is much larger in fac-[Re(py)(CO)3(Cl2phen)]+
than
in
the
parent
compound
fac-
[ReCl(CO)3(Cl2phen)]. This behavior is due to the poor σ-donating and good π-
15
accepting ability of the py ligand, which renders the Re(I) metal center destitute of electron density and thus increases its oxidation potential51. To provide insight into an electroluminescent device application of these two compounds, the HOMO and the LUMO energy levels were estimated (Table 3). The HOMO of a M-L bond is determined mainly by the orbitals of the metal unit, while the LUMO is controlled mainly by the orbitals of the ligand unit. Table 3. Electrochemical data and calculated HOMO and LUMO energy levels Eoxonset
Compound
vs Eredonset
vs HOMO
LUMO
NHE (V)
NHE (V)
(eV)
(eV)
fac-[Re(CO)3(Cl2phen)Cl]
+1.54
-1.00
-6.04
-3.60
fac-[Re(CO)3(Cl2phen)(py)]+
+1.63
-0.84
-6.13
-3.66
The HOMO and LUMO energy values are similar to the data reported by Iha and co-workers
52
for fac-[ClRe(CO)3(bpy)] using PVK as the emissive layer in
an ITO/PEDOT:PSS/PVK/butyl-PBD/Al OLED architecture. Because the energy of the HOMO and LUMO of the emitter layer matrix material has to fit between the adjacent layers13 in a successful emitting layer, the authors showed that rhenium(I) compounds act as electron traps in PVK-based devices. In addition, the triplet-excited state of the polymer matrix must be at a higher energy than the triplet excited state of the emitting species (dopant); otherwise, the dopant emission would be quenched. Although the low emission quantum yield of fac[ReCl(CO)3(bpy)] at solution and room temperature41, ɸem = 0.005, Iha and coworkers52 showed that the luminous efficiency of organic light-emitting diodes based on PVK was improved by adding fac-[ReCl(CO)3(bpy)] complex to the 16
emitter layer matrix. Consequently, fac-[Re(CO)3(Cl2phen)(L)] complexes can be used as electron traps in PVK based devices although their low emission quantum yields.
Conclusions Novel Re(I) complexes with substituted electron withdrawing Cl groups on 1,10phenanthroline, fac-[ReCl(CO)3(Cl2phen)] and
fac-[Re(py)(CO)3(Cl2phen)]+,
were synthesized, characterized and their photophysical and electrochemical properties have been investigated. Both compounds displayed predominant 3
MLCT emission in solution and rigid media and the hydrogen replacement with
chloro at the 4 and 7 positions of 1,10-phenanthroline results in a bathochromic shift of the emission maxima compared to the non-substituted species due to a more efficient 3MLCT excited state stabilization by the two electron-withdrawing chloro
groups.
Photoluminescence
of
a
PVK
film
doped
with
fac-
[ReCl(CO)3(Cl2phen)] showed an efficient energy transfer from polymer host to the dopant. The ancillary py ligand shifted the oxidation potential in the positive direction by removing electron density from Re(I) in comparison to fac[ReCl(CO)3(Cl2phen)]. The electrochemical and emission properties of fac[Re(CO)3(Cl2phen)L] complexes indicate a potential application of rhenium(I) compounds in OLED devices.
Acknowledgements The authors would like to acknowledge financial support from Fundacao de Amparo a Pesquisa de Sao Paulo (Grant 2011/23408-0).
17
Supporting Information Available: 1H NMR spectra of Cl2phen, fac[ReCl(CO)3(Clphen)] and fac-[Re(CO)3(Cl2phen)(py)]+ compounds and cyclic voltammograms of complexes measured in CH2Cl2 vs. Ag/Ag+.
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