A ruthenium tetrazole complex-based high efficiency near infrared light electrochemical cell

Article abstract of DOI:10.1039/c7cc02878d

We report on the exploitation of a new tetrazole-substituted 1,10-phenanthroline and a 2,2′-bipyridine (bpy) ancillary ligand modified with an electron-donating group in cationic ruthenium complexes. This complex, placed in between two electrodes without any polymer, demonstrates high efficiency near-infrared (NIR) electroluminescence (EL). The comparison between bpy and its methyl-substituted ancillary ligand shows that the cationic Ru tetrazolate complex containing methyl groups exhibits a red shift in the EL wavelength from 620 to 800 nm compared to [Ru(bpy)₃]²⁺ and an almost twofold reduction in the turn-on voltage, i.e., from 5 to 3 V, with respect to 5-tetrazole-1,10-phenanthroline. An external quantum efficiency of 0.95% for the dimethyl derivative is demonstrated, which is a remarkable result for non-doped NIR light electrochemical cells based on ruthenium polypyridyl.

Full text of DOI:10.1039/c7cc02878d

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
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A ruthenium tetrazole complex-based high efficiency
near infrared light electrochemical cell
Received 00th January 20xx,
Accepted 00th January 20xx
Hashem Shahroosvand a*, Saeid Abaspour a, Babak Pashaei a, b, Eros Radicchi b, c, Filippo
De Angelis c, d, Francesco Bonoccorosoe, f
DOI: 10.1039/x0xx00000x
www.rsc.org/
We report on the exploitation of a new tetrazole-substituted 1,10- with the energy level of anode and cathode electrodes and
phenanthroline and 2,2'-bipyridine (bpy) ancillary ligand the charge transport process remain issues to be solved to
a
modified by an electron-donating group in cationic ruthenium enhance the LEC performances.5 Ruthenium(II) polypyridyl
complexes. This complex, placed in between two electrodes complexes are suitable candidates for the light-emitting layer;6
without any polymer, demonstrates high efficiency near-infrared as they are in high demand, they are currently less expensive
(NIR) electroluminescence (EL). The comparison between bpy and than iridium cyclometalated complexes.7 However, most
its methyl-substituted ancillary ligand shows that the cationic Ru ruthenium-based LECs emit in the orange and red spectral
tetrazolate complex containing methyl groups exhibits a red shift regions.8 To overcome this limitation and tune the emission
in EL wavelength from 620 to 800 nm compared to [Ru(bpy)3]2+ wavelength, a main strategy is the substitution of different
and an almost twofold reduction of turn-on voltage, i.e., from 5 to functional groups including electron donor–acceptors on π-
3 V, with respect to 5-tetrazole-1,10-phenanthroline. We achieved conjugated ligands.9 Near-infrared (NIR) light-emitting sources
an external quantum efficiency value of 0.95% for the dimethyl are typically expensive due to the exploitation of multi layers
7
derivative, which is a remarkable result for non-doped NIR light costly material. In this context, LECs could serve as low-cost
alternatives in application areas where NIR luminescence has a
key role such as telecommunications,10 bio-imaging11 and
electrochemical cells based on ruthenium polypyridyl.
A major challenge for scientists designing new generations of wound healing.9,12 However, solid-state NIR LECs, i.e., having
light emitting diodes (LEDs) is finding novel materials that the EL peak wavelength in the NIR, are not fully investigated,
combine important aspects such as environmental being the research effort mostly focused on ionic transition-
friendliness, high efficiency and economical cost. 1,2 Solid-state metal complexes (iTMCs).3 The intrinsic difficulty of NIR-EL
lighting (SSL) classes, including light-emitting diodes (LEDs) and emission arises from the energy gap law that disfavors
organic light-emitting diodes (OLEDs), often reliably possess all radiative transition at lower emission energies, which causes
the above requirements.3 Among SSLs, light electrochemical these systems to exhibit EQEs <0.1%.6a,13 Intensive research
cells (LECs) are the simplest LED class, which provide some has been focused on modifying ligands coordination sphere to
exclusive properties not found in other classes. Some induce a red shift in emission wavelength and to improve the
remarkable advantages of LECs compared to OLEDs include the overall efficiency of NIR EL. Moreover, since the design of
following: (i) their figures of merit do not strongly depend on ligands around the metal cores is key in the electron transfer
the thickness of the layers, (ii) the difference of the work process and light-emitting performances,14 the introduction of
functions of the electrodes does not affect the external aromatic rings, containing multi nitrogen donor atoms such
quantum efficiency (EQE), (iii) the value of the turn-on voltage imydazolyl,15 triazolyl 16 or pyrazolyl 17 rings in the backbone of
is close to the optical band gap of the light-emitting layer, and polypyridyl ligand, allowed the modulation of the
(iv) LECs have a simple and inexpensive architecture due to the spectroscopic and redox properties through the tuning of the
reduction of the number of deposited layers.4 However, the HOMO and LUMO gap of the resulting complexes.18 However,
optimization of the emitter layer is not trivial and has several literature on the incorporation of tetrazole ring on polypyridyl
issues. The alignment of the HOMO and LUMO of the complex ligand is not exhaustive to date. Here, we propose new NIR
electroluminescent compounds, investigating the effect of
ancillary ligands. For this purpose, we designed and
synthesized
a series of novel ruthenium(II) complexes
a.
Chemistry Department, University of Zanjan, Zanjan, Iran.
bComputational Laboratory of Hybrid/Organic Photovoltaics (CLHYO), CNR-ISTM,
via Elce di Sotto 8, I-06123 Perugia, Italy.
containing 5-tetrazole-1,10-phenanthroline (Tzphen) and 2,2′-
bipyridine (bpy), 4,4′-di-methyl-2,2′-bipyridine (dmbpy), or
1,10-phenanthroline (phen) as ancillary ligands, named as SA1-
SA3. The schematic of the synthesis procedure and the
structures of the ligands and their complexes [Ru(Tzphen)(N
N)2](ClO4)2, SA1-SA3,
cDipartimento di Chimica,Biologia e Biotecnologie, Università degli Studi di Perugia,
via Elce di Sotto 8, I-06123 Perugia, Italy.
dCompuNet, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy.
e Istituto Italiano di Tecnologia, Graphene Labs, 16163 Genova, Italy.
fDipartimento di Chimica e Chimica Industriale, Università degli Studi di Genova,
Via Dodecaneso 31, Genoa, 16146, Italy.† Electronic Supplementary Informaꢀon
(ESI) available: See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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Table 1. UV/Vis, PL and redox properties of SA1-SA3.aInDMF solutions (1×10−5M). bIn degassed DMF solutions at 298 K. c From CV
measurements, E1/2=1/2(Epa+Epc); 0.1 M acetonitrile/TBAP versus Ag/AgCl. d EHOMO= -(Eox(vs.Fc/FC+)+ 4.8 eV), ELUMO = E HOMO + E 0-0 eV,
E0-0 was calculated from the intersection of absorption and emission spectra in acetonitrile solution.
f
Complexes
Absorbance
λ (ε) a
Emission
λmax(Φ)b
Eox (V)c
Ered1/2 (V) c
HOMOd
LUMOe
E0-0 (
eV)
SA1
SA2
SA3
447 (3.55)
629(0.055)
+1.34
+1.28
1.36
-1.31, -1.52
-6.15
-6.16
-6.08
-5.74
- 3.89
- 3.97
- 3.79
- 3.14
2.26
2.19
2.29
2.60
460 (3.75)
456 (3.90)
451 (4.17)
659(0.067)
618(0.012)
621 (0.095)
-1.13, -1.51
-1.30, -1.45
-1.31i
2+
Ru(bpy)3
1.29 i
are shown in Fig. 1 (see Supplementary Information –ESI-for dependent density functional theory (TDDFT) calculations, see
the synthesis details and the characterization). The structures ESI, S3, Table S2. Calculations predict a spin-orbit coupling on
of the complexes are identified by FTIR spectroscopy, all the lowest energy band of prototype SA1 complex, leading to
displaying the characteristic band of the N-H of Tzphen at 2950 broadening and red-shifting of the lowest absorption band,
cm-1. Moreover, the removal of the CN band at 2200 cm-1 in with the appearance of the band tail, which is experimentally
phen(CN) indicates the formation of the Tzphen ligand.20 observed, see ESI.S3, Fig S8-S11. The λmax,abs of the MLCT
1
Analyzing the H NMR spectra, for all complexes, it is possible values in the series follow the expected trend of increasing
to observe the signals at approximately 8.8 ppm associated electron donation, following the sequence SA3 <SA1<SA2. The
with the CH of C6 in the Tzphen ligand (ESI.S2, Figs. S1, S3, S5). room temperature photoluminescence (PL) emission spectra
The 13C NMR spectra of SA1-3 show the tetrazole carbon (Ct) of the complexes display a broad and featureless band in DMF
resonance (i.e., chemical-shift value δ≈162 ppm) 21 (see ESI S2, solution, and the emission spectrum of complex SA2 is red
Figs S2, S4 and S6), a clear evidence of the presence of shifted by approximately 40 nm compared to SA1. According
tetrazole moiety in the complexes.
to DFT calculation, this result also demonstrates that the
presence of the electron-donating methyl groups on the
dmbpy ligands of complex SA2 destabilizes the HOMO of
complex SA2 compared to complexes SA1 and SA3 (ESI. S7 and
S8).
Figure 2. UV-Visible region of absorption and PL (λexc=405 nm)
spectra of Ru(Tzphen) complexes (SA1-SA3) in DMF solution at
10-5 M. The star shows the second harmonic of laser radiation.
Figure 1..a) Chemical structures of Tzphen ligand and SA1-SA3
complexes The synthesis procedure is summarized: ( i) NaClO,
tetrabutyl ammonium hydrogen sulfate (TBAHS), pH=8.5, 18
°C, 2h (ii) KCN, H2O, 4h (iii) NaN3, NH4Cl, DMF, 140 °C, 48h (iv)
H2O,HCl, pH=3.5 (v) RuCl3.xH2O, 2mol N^N, 5 h reflux in DMF,
140°C (vi) Tzphen, 5 h reflux, 140°C .
To determine the LEC performance of the as-synthesized
emitters, it is key to investigate the oxidation-reduction
properties. In this context, the electrochemical behaviours of
SA1−SA3 were investigated by cyclic voltammetry (CV) (Figure
3). The nature of the ancillary ligands affects both the
potentials and the quasi-reversibility of the redox processes.
All compounds display one oxidation half-wave in the positive
region due to the oxidation/reduction (Ox/Red) of Ru(II) to
Ru(III) and three reduction processes in the negative potential
region due to the Ox/Red of the ligands. The inset of Figure 3
also shows the oxidation wave of SA2 on the platinum
electrode at various scan rates ranging from 0.00 to 300 mV
s−1. Moreover, there is a linear correlation between the anodic
current and υ½, suggesting that the kinetics of the overall
process is controlled by mass transport. The formal half-wave
potential values of the quasi-reversible processes are reported
in Table 1.
Characterizations by elemental analysis and electrospray
ionization mass spectrometry (ESI-mass) also confirm the
synthesis of the complexes. Specific data are given in the
Supplementary Information. The UV–Vis absorption and
photoluminescence (PL) spectra of the complexes in N, N
dimethyl formamide (DMF) solutions are shown in Figure 2
and summarized in Table 1. Complexes SA1–SA3 show intense
intra-ligand absorption bands in the UV region and less intense
metal-to-ligand charge-transfer (MLCT) absorption bands at
≈450nm with a tail extending up to 530 nm.22 The latter bands
are originated from excitation of an electron from the
ruthenium-based t2g HOMO to the low-lying unoccupied π*
anti-bonding orbital of the ligands. This is confirmed by time
2 | J. Name., 2012, 00, 1-3
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Table 2. Electroluminescence Spectral Data of ITO/[Ru(N^N)2(Tzphen)]2+ / Ga:In Devices.aCIE(x, y): Commission Internationale de
L’Eclairage.b Maximum current density (mA cm−2).Turn-on voltage (V).c Maximum luminance, Lmax, (cd m−2). d Luminous efficiency, Leff, (cdA-
1
)
e External quantum efficiency (%) at 6 V.
b
c
Complexes
λmax
(EL)[nm]
690
CIEa
[x, y]
FWHM
[nm]
155
Jmax
V turn on
[V]
Lmax
[cd m-2
]
Leffd [cd A-1]
EQEe(%)
[mA cm-2]
450 at 6 (V)
475 at 4 (V)
185 at 4(V)
SA1
SA2
[0.652,0.315]
[0.662, 0.316]
[0.693, 0.308]
5.0
375 at 6 (V)
360 at 4 (V)
1800 at 4 (V)
0.85 at 6 (V)
0.75 at 4 (V)
-
0.55
0.95
1.2
725, 800
632
153
137
3.0
2+
Ru(bpy)3
2.3
Figure 4. EL spectra of Ru SA1 and SA2 and [Ru(bpy)3]2+ as
reference. Inset: the photograph of LEC emission based on
SA1.
Figure 3. Cyclic voltammograms of complexes SA1-SA3 acetonitrile
solution under N2 atmosphere. Inset: Cyclic voltammograms of SA2
on platinum electrode at various scan rates; (a) 50, (b) 100, (c) 150,
(d) 200, (e) 250, and (f) 300 mV s−1
The current over time for the two devices is almost the same
and shows good stability (ESI. Fig. S13). Moreover, the LEC
based on complex SA2 with substituted bpy has a significantly
lower turn-on voltage than SA1 with bpy, at 3 and 5 V,
respectively,27 reaching luminance values up 360 cd/m2 in the
NIR region.
It is useful to note the cyclic voltammetry of SA1-3 is very
sensitive to the nature of solvent which the polarity of solvent
will effect on the redox behaviour.23 All three complexes in
DMF solvent were shown irreversible redox properties
However, as shown in figure 3, the CV of SA1-3 in acetonitrile
solvent was shown quasi-reversible half-wave potentials.
The electronic nature of the dmbpy ligand (R =CH3) influences
the π* acceptor energy levels in SA2. The introducing electron-
donating methyl groups destabilize the bpy-π* orbitals,
decreasing the extent of dπ−π* back-bonding from Ru(II) to the
dmbpy ligand. This decreased back-bonding destabilizes the
6
dπ core, diminishing the Ru3+/2+ redox potentials with respect
to SA1, which is containing bpy ancillary ligand. This result
suggests a low turn-on voltage due to the low driving force for
ion mobility in a LEC. In the following, we describe a simple
LEC formed by the [Ru(N^N)2(Tzphen)] complex and two
electrodes, i.e., without PEDOT: PSS or any doped-polymeric
24
layers. Moreover, the working devices were obtained using
low melting point alloy (Ga:In) cathode contacts, which
potentially allows the avoidance of vacuum evaporation
Figure 5. Current density and maximum luminance versus
applied voltage for Ru-based LEC devices (SA1 and SA2).
techniques. Upon application of
a bias < 3V to the
Currently, the highest demonstrated EQE value of NIR EL for a
LEC with an anode (ITO)/ Ru polypyridyl complex/cathode (Au)
ITO/SA2/Ga:In device, light emission and current density
dramatically increase over time. However, all our attempts to
obtain a suitably compact layer of SA3 failed because of the
aggregation of SA3 under the surface. 25 The EL spectra have a
broad line shape, which are similar to those of the PL spectra
obtained from the complexes in solution, showing only a red
shift of ≈100 nm in wavelength maxima (Figure 2). The device
based on SA2 shows two broad bands at ≈725 and ≈ 800nm,
which are the lowest-energy emissions observed so far in LEC
devices based on ruthenium tetrazole complexes.26
28
configuration is 0.03%,
electron-assisted polymers such as PEDOT: PSS allows to
while the addition of hole-or
29
achieve an EQE value of 2.06%. The latter being the current
record high for such devices. The EQE value of 0.95% for SA2
represents the current state of the art for a NIR LEC based on
the ruthenium polypyridyl family with a configuration of
anode(ITO)/Ru polypyridyl complex/cathode (Ga:In) (see ESI
Table S3.). 30 The remarkable EL performances, i.e., low turn on
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16. (a) W. R. Browne, R. Hage, J. G. Vos, Coord. Chem. Rev. 2006, 250, 1653.
(b) M. H. Klingele, S. Brooker, Coord. Chem. Rev. 2003, 241, 119. (c) S. Fanni,
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voltage and high EQE, demonstrate the influence of the
electron-donating group in the ancillary ligand.
In conclusion, this manuscript reports the synthesis of novel Ru
complexes and their exploitation in NIR LECs, achieving EQE up
to 0.95%. The influence of the electron-donating group in
tuning the EL wavelength as well as the turn-on voltage of the
devices is clearly observed, as the turn-on voltage is reduced
from ≈ 5 V to ≈3 V with respect to non-dimethyl bpy derivate.
The simple design of the proposed device is remarkable,
avoiding the use of PEDOT: PSS or other hole-electron
transport layer. In addition, the possibility to use liquid Ga:In
cathode is particular interesting for the commercial application
such as printing and injection methods for cathode deposition.
18. L.-L. Wu, C.-H. Yang, I.-W. Sun, S.-Y. Chu, P.-C. Kao, H.-H. Huang,
Organometallics 2007, 26, 2017.
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Chem. 2007, 46, 2541.
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Soc., 2002, 124, 12210. (b) M. Alterman, A. Hallberg, J. Org. Chem., 2000, 65,
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Soc., 2003, 125, 9983. (d) D. P. Matthews, J. E. Green, A. J. Shuker. J. Comb.
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Acknowledgments. Authors wish thank university of Zanjan for
financial supports.
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