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Photocurrent response enhanced by spin-orbit coupling on ruthenium(II) complexes with heavy atom ligands
Dyes and Pigments 140 (2017) 346e353
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Photocurrent response enhanced by spin-orbit coupling on ruthenium(II) complexes with heavy atom ligands Cristian A.M. Salla a, Hugo C. Braga b, Renata da S. Heying b, Jefferson S. Martins c, d, Welber G. Quirino c, d, Cristiano Legnani c, d, Bernardo de Souza b, Adailton J. Bortoluzzi b, Hugo Gallardo b, Juliana Eccher a, Ivan H. Bechtold a, * polis 88040-900, SC Brazil Departamento de Física, Universidade Federal de Santa Catarina, Floriano polis 88040-900, SC Brazil Departamento de Química, Universidade Federal de Santa Catarina, Floriano Departamento de Física, Instituto de Ci^ encias Exatas, Universidade Federal de Juiz de Fora, Juiz de Fora, MG 36036-900, Brazil d Centro de Estudos em Materiais (CEM), Instituto de Ci^ encias Exatas, Universidade Federal de Juiz de Fora, Juiz de Fora, MG 36036-900, Brazil a
b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 December 2016 Received in revised form 19 January 2017 Accepted 21 January 2017 Available online 24 January 2017
This work presents a simple and efficient method for synthesis of ruthenium complexes with organic ligands derivative from 1,10-phenanthroline containing S and Se. An octahedral geometry of the ligands around the metallic center was determined by single-crystal X-ray diffraction. Processed in solution into a simple diode structure, the compounds with S and Se presented higher photocurrent response, probably due to increased spin-orbit coupling caused by the heavy S and Se atoms. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Ruthenium complex Spin-orbit coupling Photocurrent Organic electronics Diode
1. Introduction Ruthenium (II) complexes (RuII-C) have been extensively studied in recent years by several research groups. These compounds are employed in numerous applications as sensors [1,2], catalysts [3,4], therapeutic agents [5,6] and electro-optical devices [7e14], and arouse great interest due to their versatility, facility of synthesis and physicochemical stability [15,16]. The Metal-to-Ligand Charge Transfer excited state (MLCT) of Ru complexes determines their photophysical properties and allows the absorption and emission bands in the visible region [17]. These properties, combined with appropriate energy levels, make them a promising alternative for photovoltaic applications and as an electroluminescent active layer [18e20]. In addition, these materials can be processed by solution into thin films [14] and dispersed into host materials [21], which simplifies the manufacturing process and reduces production costs of devices.
* Corresponding author. E-mail address: [email protected] (I.H. Bechtold). http://dx.doi.org/10.1016/j.dyepig.2017.01.059 0143-7208/© 2017 Elsevier Ltd. All rights reserved.
The manipulation of the organic ligands allows easy adjustment of the RuII-C properties [22] and a broad range of functional organic ligands has been coordinated for the modular solubility [2], charge carrier transport [23], photophysical and electrochemical properties [24,25]. However, the synthesis of coordinated complexes with high absorption in the visible range and facilitated exciton dissociation for application in photovoltaic devices and higher electroluminescence efficiency in light emitting devices remains a challenge. A recent study of our group on the organic ligands derivative from 1,10-phenanthroline, [1,2,5]thiadiazolo[3,4-f][1,10] phenantroline (TDZP) and [1,2,5]selenadiazolo[3,4-f][1,10]phenantroline (PhenSe) showed an increased triplet excited state lifetime due to strong spin-orbit coupling resultant of the presence of heavy chalcogenides S and Se atoms, which contributed significantly to improve photocurrent [26]. This work presents the study of RuII-C coordinated to TDZP and PhenSe ligands in comparison to their parent molecule (Phen). A new simple method of synthesis is described with high yield. The final compounds had their structure determined by X-ray singlecrystal, also, photophysical, thermal and electrochemical properties were studied. Moreover, from a simple diode structure with the
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active layer processed by solution, the new RuII-C with S and Se heavy atoms displayed improved photocurrent and electroluminescence response. The charge carrier mobility was determined from the current/voltage characteristics, being in agreement with similar systems in the literature. 2. Experimental 1,10-Phenanthroline was obtained from Sigma-Aldrich (St. Louis, MO). All solvents and other chemicals of analytical grade were obtained from standard commercial suppliers. TDZP and PhenSe ligands were prepared according to a previous developed protocol [26]. Proton nuclear magnetic resonance spectra (1H NMR) were obtained at 300.13 MHz on a Bruker AC-300 NMR spectrometer. Chemical shifts are reported in ppm, referenced to the solvent peak of DMSO-d6 or tetramethylsilane (TMS) as the external reference. Data are reported as follows: chemical shift (d), multiplicity, coupling constant (J) in Hertz and integrated intensity. The following abbreviations were used to designate multiplicities: s (singlet), d (doublet), t (triplet), q (quartet), quint (quintet), sext (sextet) and m (multiplet). High resolution mass spectra were recorded on a Bruker microTOF-Q II ESI mass spectrometer equipped with an automatic syringe pump for sample injection. FT-IR spectra were recorded on a Perkin-Elmer model FTIR-2000 spectrophotometer, using KBr pellets, in the range of 4000e400 cm1. Thermogravimetric analysis (TGA) was carried out using a Shimadzu analyzer with the TGA-50 module. A Cary Bio 50 spectrophotometer was used for UV and visible absorption measurements both in solution and in film. The fluorescence spectra in solution and film were recorded on a Varian Cary Eclipse Fluorescence Spectrophotometer. Cyclic voltammetry (CV) measurements were carried out using a system of three electrodes: Vitreous Glassy Carbon as the working electrode, a platinum wire as auxiliary and an Ag/Ag þ reference electrode. Supporting electrolyte: 0.1 mol L1 n-Bu4NPF6. The crystallographic analysis of [Ru(LS)3](BF4)2$3H2O and [Ru(PhenSe)3](BF4)2$3H2O complexes were performed with a Kappa APEX II DUO diffractometer with graphite-monochromated Mo-Ka radiation. The temperature of the sample was set at 200(2) K using an Oxford cryostream 700 device. The frames were collected by a 4 and u scan procedure. Intensities were integrated with the Bruker SAINT software package [27]. All data were corrected for Lorentz and polarization effects. The data were also corrected for absorption effects using the multi-scan method (SADABS) [28]. The structures were solved by direct methods and refined by full-matrix least-squares methods using SHELXS and SHELXL97 programs [29]. All non-hydrogen atoms were refined anisotropically. H atoms attached to C atoms were placed at their idealized positions, with C-H distances and Ueq values taken from the default settings of the refinement program. For the complex [Ru(LSe)3](BF4)2$3H2O, water solvate were found disordered and in both structures hydrogen atoms of the water molecules were not located from Fourier difference map. Selected crystallographic data are presented in Table 1 and full crystallographic tables for these Ru(II) complexes have been deposited with the Cambridge Crystallographic Data Center as supplementary publication numbers CCDC 1516415-1516416. These data can be obtained free of charge from The Cambridge Crystallographic Data Center via www.ccdc. cam.ac.uk/data_request/cif. The OLED devices were prepared under ambient conditions on glass substrates coated with ITO (4e8 U/,) previously etched and cleaned in acetone and isopropyl alcohol in an ultrasonic bath. Prior to coating the organic materials, the substrates were treated by O2 plasma for 15 min. Poly(9-vinylcarbazole) (PVK) from Sigma-
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Aldrich, at concentration of 10 mg/mL in trichloroethylene, was spin coated onto the ITO substrates at 2000 rpm during 30 s as hole transport layer (HTL). The Ruthenium complexes were deposited at 1500 rpm during 60 s from solutions previously filtered with 0.2 mm pores filters at concentrations of 8 mg/mL for Ru(Phen)3 and 5 mg/mL for Ru(TDZP)3 and Ru(PhenSe)3 in acetonitrile. The spincoated films were annealed in an oven at 90 C during 10 min for complete evaporation of the solvent. The thickness and morphological aspects of the films were determined with an AFM, Nanosurf FlexAFM, operating in tapping mode under ambient conditions with a scanning rate of 1.0 Hz and 512 pixels 512 pixels, following the procedure previously described [30]. The thickness of films of PVK and RuII-C were around 55 nm and 25 nm, respectively. An Aluminum layer of 80 nm evaporated at 2.7.107 Torr vacuum with rate of 1.0 Å/s was deposited as cathode electrode. The device structure was ITO/PVK/RuII-C/Al with 6.0 mm2 of active area. The current-voltage-photocurrent characteristics were measured with a Keithley 2400 source meter. Photocurrent was analyzed at zero external voltage using a Xenon lamp (Oriel, 150 W) and wavelengths were selected with a monochromator Cornerstone 260 ¼ m model 74125. The intensity of light at the sample position was measured with a photodiode to be 100 mW/cm2. A Newport model 1936-c registered the radiant power and spectrophotometer USB400 UVeVis with optical fiber of 1000 mm diameter to measure the electroluminescence. All measurements were carried out under ambient conditions and without encapsulation. 3. Results and discussion 3.1. Synthesis The ligands containing sulfur (TDZP) and selenium (PhenSe), were prepared according to a recently developed methodology [26]. The Ru (II)-C were prepared using an efficient methodology that is an adaptation of a synthesis from the literature to the preparation of Ru(Phen)3 [31]. To a two-neck round-bottom flask under an argon atmosphere, [Ru(DMSO)4Cl2] (161 mg, 0.33 mmol) was added to ethylene glycol solution (10 mL) containing the corresponding ligand (1 mmol). After 3 h of reflux, a solution of sodium tetrafluoroborate (5 mL) was added and the solution remained for 1 h at 10 C. Finally, the precipitate was filtered and washed with cold ethanol, chloroform and dried in vacuum. The tetrafluoroborate salt was used because it is more soluble, in general than PF6- or Cl-salts. It was also found to be particularly useful to the formation of smooth films. The complexes had their structures elucidates by a combination of fourier transform infrared spectroscopy (FT-IR), Proton nuclear magnetic resonance (1H NMR), high-resolution mass spectroscopy (HRMS) and single crystal X-ray diffraction (SXRD). (IR, NMR and HRMS spectra are included in the supplementary data). 3.1.1. [Ru(Phen)3](BF4)2 Ru(Phen)3 FT-IR (KBr): 3420, 3050, 1629, 1426, 1410, 1147, 1083, 1058, 1036, 844, 722 cm1; 1HNMR (300 MHz, DMSO-d6, 25 C, TMS): d ¼ 7.77 (dd, 3J (H,H) ¼ 8.2 Hz 3J (H,H) ¼ 5.2 Hz, 6 H; CH), 8.09 (dd, 3J (H,H) ¼ 5.2 Hz 4J (H,H) ¼ 1.0 Hz, 6 H; CH), 8.39 (s, 6 H; CH), 8.78 (dd, 3 J (H,H) ¼ 8.2 Hz 4J (H,H) ¼ 1.0 Hz, 6 H; CH) ppm; ESI HRMS: calcd for [C36H24N6Ru]þ 321.0552, found 321.0557. 3.1.2. [Ru(TDZP)3](BF4)2 Ru(TDZP)3 FT-IR (KBr): 3420, 3073, 1627, 1409, 1395, 1123, 1084, 1064, 1039, 819, 728 cm1; 1HNMR (300 MHz, DMSO-d6, 25 C, TMS): d ¼ 7.86 (dd, 3J (H,H) ¼ 8.1 Hz 3J (H,H) ¼ 5.4 Hz, 6 H; CH), 8.25 (d, 3J (H,H) ¼ 5.4 Hz, 6 H; CH), 9.18 (dd, 3J (H,H) ¼ 8.1 Hz, 6 H; CH) ppm; ESI HRMS: calcd for [C36H18N12RuS3]þ 407.9993, found 407.9990.
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Table 1 Crystal data and structure refinement for Ru(II) complexes.
Empirical formula Formula weight Temperature (K) Wavelength (Å) Crystal system Space group Unit cell dimensions (Å ando)
Volume (Å3) Z Density (calc) (Mg/m3) m (mm1) F (000) Crystal size (mm3) Theta range for data collection ( ) Index ranges Reflections collected Independent reflections Absorption correction Max. and min. transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2s(I)] R indices (all data) Largest diff. peak and hole (e.Å3)
[Ru(TDZP)3](BF4)2$3H2O
[Ru(PhenSe)3](BF4)2$3H2O
C36H24B2F8N12O3RuS3 1043.54 200 (2) 0.71073 Triclinic Pı a ¼ 11.8671(5) b ¼ 11.9181 (5) c ¼ 16.0412 (6) a ¼ 104.5760 (10) b ¼ 90.6910 (10) g ¼ 113.6170 (10) 1995.46 (14) 2 1.737 0.644 1044 0.20 0.20 x 0.08 1.322 to 28.783 16 h 16, 16 k 16, 21 l 21 24085 10326 (Rint ¼ 0.0215) Semi-empirical from equivalents 0.9503 and 0.8820 Full-matrix least-squares on F2 10326/0/586 1.056 R1 ¼ 0.0471, wR2 ¼ 0.1277 R1 ¼ 0.0615, wR2 ¼ 0.1377 1.989 and 0.849
C36H24B2F8N12O3RuSe3 1184.24 200 (2) 0.71073 Triclinic Pı 11.9498 (5) 11.9762 (5) 16.0832 (7) 104.5030 (10) 90.2700 (10) 113.3600 (10) 2031.66 (15) 2 1.936 3.166 1152 0.42 0.22 x 0.14 1.317 to 30.163 16 h 16, 16 k 16, 22 l 22 52714 11963 (Rint ¼ 0.0213) Semi-empirical from equivalents 0.6656 and 0.3498 Full-matrix least-squares on F2 11963/0/616 1.036 R1 ¼ 0.0448, wR2 ¼ 0.1130 R1 ¼ 0.0623, wR2 ¼ 0.1275 3.146 and 1.644
3.1.3. [Ru(PhenSe)3](BF4)2 Ru(PhenSe)3 FT-IR (KBr): 3415, 3075, 1627, 1450, 1409, 1342, 1126, 1084, 1062, 1036, 815, 728 cm1; 1HNMR (300 MHz, DMSO-d6, 25 C, TMS): d ¼ 7.83 (dd, 3J (H,H) ¼ 8.1 Hz 3J (H,H) ¼ 5.3 Hz, 6 H; CH), 8.23 (d, 3J (H,H) ¼ 5.3, 6 H; CH), 9.18 (d, 3J (H,H) ¼ 8.1, 6 H; CH) ppm; ESI HRMS: calcd for [C36H18N12RuSe3]þ 478.9167, found 478.9164. The FT-IR, 1H NMR and HRMS spectra confirmed the chemical structures of the synthesized compounds. The bands found in the infrared spectrum are quite similar for all the complexes. In the region between 1600 and 1400 cm1 are situated the C¼C and C¼N stretches, characteristic of aromatic systems [26]. Moreover, between 1200 and 900 cm1 a broadband is located, with vibrational resolution of four signals, regarding tetrafluoroborate ion. It is possible to observe all signals related to the hydrogens of the 1,10phenanthroline system in the 1H-NMR spectrum. Additionally, the insertion of the heterocycles containing chalcogenides in the molecule caused a downfield shift of the signals for the protons with loss of the coupling constant 4J, when compared to Ru(Phen)3 complex. The obtained results of HRMS are in good accordance with the calculated values.
Table 2 Selected bond lengths [Å] and angles [ ] for Ru(II) complexes.
Ru1-N1 Ru1-N10 Ru1-N21 Ru1-N30 Ru1-N41 Ru1-N50 N1-Ru1-N10 N1-Ru1-N41 N21-Ru1-N1 N21-Ru1-N10 N21-Ru1-N41 N21-Ru1-N50 N30-Ru1-N1 N30-Ru1-N10 N30-Ru1-N21 N30-Ru1-N41 N30-Ru1-N50 N41-Ru1-N10 N50-Ru1-N1 N50-Ru1-N41 N50-Ru1-N10
[Ru(TDZP)3](BF4)2$3H2O
[Ru(PhenSe)3](BF4)2$3H2O
2.065 (3) 2.069 (3) 2.059 (2) 2.057 (3) 2.065 (2) 2.062 (3) 79.59 (10) 97.31 (10) 87.43 (10) 96.16 (10) 174.14 (10) 95.66 (10) 96.04 (11) 174.32 (10) 79.93 (10) 96.09 (10) 87.99 (10) 88.12 (10) 175.31 (10) 79.83 (10) 96.53 (10)
2.058 (3) 2.065 (3) 2.063 (3) 2.060 (3) 2.060 (3) 2.058 (3) 79.37 (11) 95.97 (11) 97.37 (11) 88.31 (11) 96.26 (11) 174.07 (11) 174.98 (11) 96.44 (11) 79.64 (10) 88.40 (11) 95.65 (10) 173.88 (10) 87.57 (11) 79.90 (11) 95.85 (10)
3.2. Crystalline structure of Ru(TDZP)3 and Ru(PhenSe)3 From the slow growth in dimethyl sulfoxide, prismatic red crystals of [Ru(TDZP)3](BF4)2$3H2O and [Ru(PhenSe)3](BF4)2$3H2O were obtained, which were used for structural characterization by single crystal X-ray diffraction analysis. Selected bond lengths and angles are listed in Table 2. These ruthenium complexes are isostructural, which in the triclinic crystal system, space group Pı. For both structures, the asymmetric unit consists of one cation complex, two BF4- counterion and three water molecules as solvate. In complexes [Ru(L)3]2(BF4)$3H2O, the discrete cationic molecule consists of an RuII center surrounded by three ligand units, in a sixcoordinate environment to form a propeller-shaped complex cation
Fig. 1. The molecular structure of the three complexes obtained with ChemDraw program is illustrated in Fig. 1S at the supplementary data. The length of the RueN bonds [ranging from 2.057(3) to 2.069(3) Å] and the ligand bite angles [ranging from 79.37(11) to 79.93(10) ] fall within the interval of values reported in the Cambridge Structural Database [32,33] for structures containing the [Ru(phen)3]2þ fragment. These values are in full agreement with a slightly distorted RuN6 octahedral geometry around the metallic center. The 5-membered chelating rings of RuII and the N atoms of tetracyclic ligands exhibit nearly perfect coplanarity; the N1 - C12 -
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Fig. 1. Cationic ruthenium species in the crystal structure of [Ru(TDZP)3] 2BF4$3H2O (a) and [Ru(PhenSe)3] 2BF4$3H2O (b) with heteroatom labeling. The ellipsoids are represented at a 40% probability level. Hydrogen atoms are omitted for clarity. The BF4 anion was omitted.
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C11 - N10 and N21 - C32 - C31 - N30 and N41 - C52 - C51 - N50; torsion angles are 2.1(4) , 2.7(4) and 2.2(4) for [Ru(TDZP)3]2þ and 1.9(4) , 3.1(4) and 2.4(4) for [Ru(PhenSe)3]2þ, respectively. The dihedral angles between mean planes of the two TDZP molecules are very similar in both ruthenium complexes. The angles between mean planes N1/N10 e N21/N30, N1/N10 e N41/N50 and N21/N30 e N41/N50 are 83.29(3) , 85.15(4) and 87.21(3) for [Ru(TDZP)3]2þ and 84.60(4) , 84.34(3) and 87.92(3) for [Ru(PhenSe)3]2þ, respectively. 3.3. Thermal behavior The thermal properties of the RuII-C were investigated by thermogravimetric analysis (TGA). The results are presented in Fig. 2S of the supplementary data, indicating that all the complexes are stable at room temperature, with the compounds decomposing only above 350 C. 3.4. Spectroscopy and electrochemistry UVeVis spectroscopy and emission spectra of the RuII-C obtained from solution in acetonitrile and spin coated films are presented in Fig. 2. In the UV region, the absorption spectra shows bands at 224 nm and 262 nm for the Ru(Phen); at 260 nm and two other at 297 nm and 318 nm for the Ru(TDZP)3; and band centered
Ru(Phen)3 Ru(PhenSe)3
1.0
Normalized Emission
Normalized Absorbance
1.0
Ru(TDZP)3
0.8
0.6
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(a) 0.0 200
250
300
350
400
450
500
550
600
650
0.0 700
λ (nm)
Ru(Phen)3 Ru(PhenSe)3
1.0
Normalized Emission
Normalized Absorbance
1.0
Ru(TDZP)3
0.8
0.6
0.8
0.6
0.4
0.4
0.2
0.2
0.0 200
(b) 250
300
350
400
450
500
550
600
650
0.0 700
λ (nm) Fig. 2. Optical absorbance (left) and emission (right) of RuII-C in solution (a) and film (b).
at 265 nm, 305 nm and 342 nm Ru(PhenSe)3. The bands of each compound with peaks in 262 nm, 318 nm and 342 nm are attributed to the p-p* transitions of the organic ligand [26]. As expected, a broad absorption band observed around 450 nm for all complexes and is associated to MLCT transitions [17]. Therefore, the optical band gap (Eg) is about the same for all complexes (~2.5 eV). Emission spectra in solution show only a large band in the orange-red region, associated to the MLCT transitions with maxima at 591 nm, 599 nm and 603 nm for Ru(Phen)3, Ru(TDZP)3 and Ru(PhenSe)3, respectively. The absorption and emission spectra in film are very similar to the solution, with a small red-shift, usually associated to molecular aggregations in the film structure. The quantum yields for the emission of the complexes were estimated using the Ru(bpy)3 as reference [34] and are shown in Table 3. The higher values for the Ru(PhenSe)3 and Ru(TDZP)3 complexes can be associated with an increased phosphorescence rate (kP) caused by the stronger spin-orbit coupling induced by the presence of the heavier chalcogen atoms in the organic ligands [26,35]. The value for Ru(Phen)3 agrees with the literature [36]. Cyclic voltammograms of Ru(PhenSe)3 and Ru(TDZP)3 were registered at scan rates of 100 mV s1. From the voltammograms (Fig. 3S of the supplementary data) the E1/2 can be calculated as 0.98 V and 1.40 V for Ru(PhenSe)3, and 1.04 V and 1.44 V for Ru(TDZP)3, versus Fc/Fcþ. In each case, the E1/2 potential was used to estimate the HOMO and LUMO energy levels [37]. The electrochemical band gap values shown in Table 3 are very close to the optical band gaps calculated from the absorption spectra. 3.5. Photovoltaic characterization In a previous work we demonstrated that the presence of heavy atoms in organic ligands promoted an increase of the excited state lifetime due to strong spin orbit coupling and triplet conversion, which contributed to improve the photovoltaic activity [26]. Therefore, a similar behavior could also be expected in the complexes containing the same ligands. Fig. 3 shows the photocurrent response of the ITO/PVK/RuII-C/AL devices irradiated selectively using a monochromator at zero external voltage. The photocurrent peak at 345 nm corresponds to the absorption peak of PVK, as previously reported [39], this peak is also present in the reference device only with PVK as active layer (ITO/PVK/Al). However, the response around 450 nm observed only for Ru(TDZP)3 and Ru(PhenSe)3 corresponds to the absorption band associated to MLCT transitions. The fact that no photocurrent was observed around 450 nm for Ru(Phen)3 corroborates with the stronger spin orbit coupling due to the presence of heavy atoms (S and Se) in these ligands. That is probably making the, in principle forbidden S3 0 MLCT transition, allowed, or at least with a relatively higher transition dipole moment and thus turning these complexes into better light absorbers in the visible region. 3.6. Electrical characterization Electroluminescent properties of the devices ITO/PVK/RuII-C/Al for Ru(PhenSe)3 and Ru(Phen)3 were also exploited. The energy level diagrams are illustrated in Fig. 4S of the supplementary data, where a small energetic barrier (around 0.4 eV for Ru(PhenSe)3 and 0.8 eV for Ru(Phen)3) from the Al to the LUMO of the compounds favors electron injection. On the other hand, a much larger barrier of about 1.0 eV has to be crossed for hole injection from ITO into the HOMO of the PVK. From the HOMO of PVK to the HOMO of RuII-C, the energetic barriers for hole injection are of 0.4 eV for Ru(PhenSe)3 and 0.3 eV for Ru(Phen)3. The PVK layer also acts as electron blocking layer in order to confine the recombination process inside the RuII-C. However, the recombination can occur close
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Table 3 Optical and electrochemical data. Compound
HOMO (eV)a
LUMO (eV)a
Electrochemical band gap (eV)
Optical band gapb
Quantum yield F
Ru(Phen)3 Ru (TDZP)3 Ru(PhenSe)3
6.1c 6.2 6.2
3.4c 3.8 3.8
2.7c 2.4 2.4
2.5 2.5 2.5
0.0280 0.0647 0.0974
a b c
Determined from peak potentials [37]. Calculated from the onset of the absorption spectra in solution. Extracted from Ref. [38].
to the PVK/RuII-C interface, reducing the emitted energy. The morphology of the films was evaluated with AFM, being the root mean square roughness (RRMS) of 9.0 nm and 6.0 nm for Ru(Phen)3 and Ru(PhenSe)3, respectively (see images in Fig. 5S of the supplementary data). Fig. 4 shows the current density and radiation power versus voltage. The turn-on voltages are 7.5 V and 7.3 V for the devices containing Ru(Phen)3 and Ru(PhenSe)3, respectively, which is
18
ITO/PVK/Al ITO/PVK/Ru(Phen)3/Al
16
ITO/PVK/Ru(PhenSe)3/Al ITO/PVK/Ru(TDZP)3/Al
Photocurrent (nA/cm2)
14 12 10 8 6 4 2 0 300
350
400
450
500
550
λ (nm)
600
650
700
consistent with the similarity of the energy level values of both materials. However, the current density for Ru(PhenSe)3 increases faster with the applied voltage, indicating that the charge transport is more efficient in this material, probably due to the stronger dispersion interactions induced by the high polarizability of the selenium atom. The inset of Fig. 4 shows the EL spectra of both devices, where a broadband of red-orange color is the result of emission from the triplet MLCT state. The narrow band observed for the Ru(PhenSe)3 containing device is probably again due to the increased intermolecular interactions resulting in a tighter nuclear configuration in the solid state, or due to the longer excited lifetime. A device with ITO/Ru(PhenSe)3/Al was elaborated in order to investigate the charge carrier mobility of the Ru(PhenSe)3 complex. In this case, as the work functions of the ITO (4.8 eV) and the Al (4.2 eV) are very similar and close to the LUMO level of the complex (3.8 eV), the charge carriers are predominantly electrons. Fig. 5 shows the log-log plot of the current density as a function of the voltage, where two conduction regimes separated by a threshold voltage of 4.8 V are clearly observed. For low voltages, the slope equal to 1.1 indicates an Ohmic regime, followed by a space charge limited current (SCLC) regime on higher voltages. In the SCLC region, a slope of 4.4 corresponds to the trap limited-SCLC regime. In a trap-free semiconducting material the current density would increase quadratically with the applied voltage. The charge mobility was obtained directly from the experimental J-V curve by fitting the trap-limited SCLC regime with a theoretical model published previously from the present authors of this article [40,41]. This model considers an electric field dependent pffiffi €ssler [42], mobility of the form mðEÞ ¼ m0 eg E as suggested by Ba
Fig. 3. Photocurrent measurements of the diode structures vs. wavelengths at zero external voltage. ITO/PVK/Al was also measured for comparison. 2
10
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Ru(PheSe)3 550
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6
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4
ITO/PVK/Ru(PhenSe)3/Al
2
1
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SCLC
-10 2 cm /Vs
μ0 = 1.90 x 10
-3
γ = 3.37 x 10
(cm/V)1/2 4.8 V
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-1
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-4
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Voltage (V) Fig. 4. Current density as a function of the applied voltage for Ru(Phen)3 and Ru(PhenSe)3. In the inset is the normalized EL at a voltage of 17 V for Ru(Phen)3 and 10 V for Ru(PhenSe)3.
1
10 Voltage (V)
Fig. 5. Log-log plot of the current density as a function of the voltage for Ru(PhenSe)3. The red solid line represents the theoretical fitting. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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where m0 is the carrier mobility at zero field and g the field dependence of the mobility. From the fitting, m0 and g parameters were extracted and used in the mobility equation to determine the charge mobility as a function of applied voltage. The m0 and g values found were of 1.90 1010 cm2/V.s and 3.37 103 (cm/V)1/2 respectively, where the maximum mobility was of 1.24 107 cm2/ V.s for an applied voltage of þ9.0 V. In the literature, there are not mobility values for similar RuII-C, but this value is consistent with ruthenium polymeric systems [43]. 4. Conclusion This work presents a simple and efficient methodology for synthesis of ruthenium complexes with organic ligands derivative from 1,10-phenanthroline containing the chalcogens Sulfur and Selenium atoms. The complexes exhibited good thermal and electrochemical stability and absorption and emission in the visible spectrum. The presence of the heavier atoms in the organic ligand promoted photocurrent response in the visible light range. Investigations of the electro-optical properties in simple diode structure showed that the Ru(PhenSe)3 can be exploited as active layer in electroluminescent devices. The peculiar properties of the ligands containing sulfur and selenium seem to be strongly correlated to an increased spin-orbit coupling in the ligands, along with the central ruthenium atom. Acknowledgements The authors are grateful to the CNPq, CAPES, INEO, INCT-Cat alise and FAPESC-Pronex for support of this work. The authors are also grateful to CEBIME for the HRMS analysis. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2017.01.059. References [1] Zhang Y, Liu Z, Yang K, Zhang Y, Xu Y, Li H, et al. A ruthenium(II) complex as turn-on Cu(II) luminescent sensor based on oxidative cyclization mechanism and its application in vivo. Sci Rep 2015;5:8172. http://dx.doi.org/10.1038/ srep08172. [2] Hara D, Komatsu H, Son A, Nishimoto S, Tanabe K. Water-soluble phosphorescent ruthenium complex with a fluorescent coumarin unit for ratiometric sensing of oxygen levels in living cells. Bioconjug Chem 2015;26:645e9. http://dx.doi.org/10.1021/acs.bioconjchem.5b00093. [3] Schulze M, Kunz V, Frischmann PD, Würthner F. A supramolecular ruthenium macrocycle with high catalytic activity for water oxidation that mechanistically mimics photosystem II. Nat Chem 2016;8:576e83. http://dx.doi.org/ 10.1038/nchem.2503. [4] Zhang LL, Gao Y, Liu Z, Ding X, Yu Z, Sun LC. A trinuclear ruthenium complex as a highly efficient molecular catalyst for water oxidation. Dalton Trans 2016;45:3814e9. http://dx.doi.org/10.1039/C5DT04233J. [5] Zeng L, Chen Y, Liu J, Huang H, Guan R, Ji L, et al. Ruthenium(II) complexes with 2-Phenylimidazo[4,5-f][1,10]phenanthroline derivatives that strongly combat cisplatin-resistant tumor cells. Sci Rep 2016;6:19449. http:// dx.doi.org/10.1038/srep19449. [6] Dickerson M, Howerton B, Bae Y, Glazer EC. Light-sensitive ruthenium complex-loaded cross-linked polymeric nanoassemblies for the treatment of cancer. J Mater Chem B 2016;4:394e408. http://dx.doi.org/10.1039/ C5TB01613D. [7] Murali MG, Wang X, Wang Q, Valiyaveettil S. Design and synthesis of new ruthenium complex for dye-sensitized solar cells. RSC Adv 2016;6:57872e9. http://dx.doi.org/10.1039/C6RA10881D. [8] Wang P, Zakeeruddin SM, Moser JE, Nazeeruddin MK, Sekiguchi T, Gr€ atzel M. A stable quasi-solid-state dye-sensitized solar cell with an amphiphilic ruthenium sensitizer and polymer gel electrolyte. Nat Mater 2003;2:402e7. http://dx.doi.org/10.1038/nmat904. [9] Cao K, Lu J, Cui J, Shen Y, Chen W, Alemu G, et al. Highly efficient light harvesting ruthenium sensitizers for dye-sensitized solar cells featuring triphenylamine donor antennas. J Mater Chem A 2014;2:4945e53. http://dx.doi.org/ 10.1039/C3TA15134D.
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Photocurrent response enhanced by spin-orbit coupling on ruthenium(II) complexes with heavy atom ligands Cristian A.M. Salla a, Hugo C. Braga b, Renata da S. Heying b, Jefferson S. Martins c, d, Welber G. Quirino c, d, Cristiano Legnani c, d, Bernardo de Souza b, Adailton J. Bortoluzzi b, Hugo Gallardo b, Juliana Eccher a, Ivan H. Bechtold a, * polis 88040-900, SC Brazil Departamento de Física, Universidade Federal de Santa Catarina, Floriano polis 88040-900, SC Brazil Departamento de Química, Universidade Federal de Santa Catarina, Floriano Departamento de Física, Instituto de Ci^ encias Exatas, Universidade Federal de Juiz de Fora, Juiz de Fora, MG 36036-900, Brazil d Centro de Estudos em Materiais (CEM), Instituto de Ci^ encias Exatas, Universidade Federal de Juiz de Fora, Juiz de Fora, MG 36036-900, Brazil a
b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 December 2016 Received in revised form 19 January 2017 Accepted 21 January 2017 Available online 24 January 2017
This work presents a simple and efficient method for synthesis of ruthenium complexes with organic ligands derivative from 1,10-phenanthroline containing S and Se. An octahedral geometry of the ligands around the metallic center was determined by single-crystal X-ray diffraction. Processed in solution into a simple diode structure, the compounds with S and Se presented higher photocurrent response, probably due to increased spin-orbit coupling caused by the heavy S and Se atoms. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Ruthenium complex Spin-orbit coupling Photocurrent Organic electronics Diode
1. Introduction Ruthenium (II) complexes (RuII-C) have been extensively studied in recent years by several research groups. These compounds are employed in numerous applications as sensors [1,2], catalysts [3,4], therapeutic agents [5,6] and electro-optical devices [7e14], and arouse great interest due to their versatility, facility of synthesis and physicochemical stability [15,16]. The Metal-to-Ligand Charge Transfer excited state (MLCT) of Ru complexes determines their photophysical properties and allows the absorption and emission bands in the visible region [17]. These properties, combined with appropriate energy levels, make them a promising alternative for photovoltaic applications and as an electroluminescent active layer [18e20]. In addition, these materials can be processed by solution into thin films [14] and dispersed into host materials [21], which simplifies the manufacturing process and reduces production costs of devices.
* Corresponding author. E-mail address: [email protected] (I.H. Bechtold). http://dx.doi.org/10.1016/j.dyepig.2017.01.059 0143-7208/© 2017 Elsevier Ltd. All rights reserved.
The manipulation of the organic ligands allows easy adjustment of the RuII-C properties [22] and a broad range of functional organic ligands has been coordinated for the modular solubility [2], charge carrier transport [23], photophysical and electrochemical properties [24,25]. However, the synthesis of coordinated complexes with high absorption in the visible range and facilitated exciton dissociation for application in photovoltaic devices and higher electroluminescence efficiency in light emitting devices remains a challenge. A recent study of our group on the organic ligands derivative from 1,10-phenanthroline, [1,2,5]thiadiazolo[3,4-f][1,10] phenantroline (TDZP) and [1,2,5]selenadiazolo[3,4-f][1,10]phenantroline (PhenSe) showed an increased triplet excited state lifetime due to strong spin-orbit coupling resultant of the presence of heavy chalcogenides S and Se atoms, which contributed significantly to improve photocurrent [26]. This work presents the study of RuII-C coordinated to TDZP and PhenSe ligands in comparison to their parent molecule (Phen). A new simple method of synthesis is described with high yield. The final compounds had their structure determined by X-ray singlecrystal, also, photophysical, thermal and electrochemical properties were studied. Moreover, from a simple diode structure with the
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active layer processed by solution, the new RuII-C with S and Se heavy atoms displayed improved photocurrent and electroluminescence response. The charge carrier mobility was determined from the current/voltage characteristics, being in agreement with similar systems in the literature. 2. Experimental 1,10-Phenanthroline was obtained from Sigma-Aldrich (St. Louis, MO). All solvents and other chemicals of analytical grade were obtained from standard commercial suppliers. TDZP and PhenSe ligands were prepared according to a previous developed protocol [26]. Proton nuclear magnetic resonance spectra (1H NMR) were obtained at 300.13 MHz on a Bruker AC-300 NMR spectrometer. Chemical shifts are reported in ppm, referenced to the solvent peak of DMSO-d6 or tetramethylsilane (TMS) as the external reference. Data are reported as follows: chemical shift (d), multiplicity, coupling constant (J) in Hertz and integrated intensity. The following abbreviations were used to designate multiplicities: s (singlet), d (doublet), t (triplet), q (quartet), quint (quintet), sext (sextet) and m (multiplet). High resolution mass spectra were recorded on a Bruker microTOF-Q II ESI mass spectrometer equipped with an automatic syringe pump for sample injection. FT-IR spectra were recorded on a Perkin-Elmer model FTIR-2000 spectrophotometer, using KBr pellets, in the range of 4000e400 cm1. Thermogravimetric analysis (TGA) was carried out using a Shimadzu analyzer with the TGA-50 module. A Cary Bio 50 spectrophotometer was used for UV and visible absorption measurements both in solution and in film. The fluorescence spectra in solution and film were recorded on a Varian Cary Eclipse Fluorescence Spectrophotometer. Cyclic voltammetry (CV) measurements were carried out using a system of three electrodes: Vitreous Glassy Carbon as the working electrode, a platinum wire as auxiliary and an Ag/Ag þ reference electrode. Supporting electrolyte: 0.1 mol L1 n-Bu4NPF6. The crystallographic analysis of [Ru(LS)3](BF4)2$3H2O and [Ru(PhenSe)3](BF4)2$3H2O complexes were performed with a Kappa APEX II DUO diffractometer with graphite-monochromated Mo-Ka radiation. The temperature of the sample was set at 200(2) K using an Oxford cryostream 700 device. The frames were collected by a 4 and u scan procedure. Intensities were integrated with the Bruker SAINT software package [27]. All data were corrected for Lorentz and polarization effects. The data were also corrected for absorption effects using the multi-scan method (SADABS) [28]. The structures were solved by direct methods and refined by full-matrix least-squares methods using SHELXS and SHELXL97 programs [29]. All non-hydrogen atoms were refined anisotropically. H atoms attached to C atoms were placed at their idealized positions, with C-H distances and Ueq values taken from the default settings of the refinement program. For the complex [Ru(LSe)3](BF4)2$3H2O, water solvate were found disordered and in both structures hydrogen atoms of the water molecules were not located from Fourier difference map. Selected crystallographic data are presented in Table 1 and full crystallographic tables for these Ru(II) complexes have been deposited with the Cambridge Crystallographic Data Center as supplementary publication numbers CCDC 1516415-1516416. These data can be obtained free of charge from The Cambridge Crystallographic Data Center via www.ccdc. cam.ac.uk/data_request/cif. The OLED devices were prepared under ambient conditions on glass substrates coated with ITO (4e8 U/,) previously etched and cleaned in acetone and isopropyl alcohol in an ultrasonic bath. Prior to coating the organic materials, the substrates were treated by O2 plasma for 15 min. Poly(9-vinylcarbazole) (PVK) from Sigma-
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Aldrich, at concentration of 10 mg/mL in trichloroethylene, was spin coated onto the ITO substrates at 2000 rpm during 30 s as hole transport layer (HTL). The Ruthenium complexes were deposited at 1500 rpm during 60 s from solutions previously filtered with 0.2 mm pores filters at concentrations of 8 mg/mL for Ru(Phen)3 and 5 mg/mL for Ru(TDZP)3 and Ru(PhenSe)3 in acetonitrile. The spincoated films were annealed in an oven at 90 C during 10 min for complete evaporation of the solvent. The thickness and morphological aspects of the films were determined with an AFM, Nanosurf FlexAFM, operating in tapping mode under ambient conditions with a scanning rate of 1.0 Hz and 512 pixels 512 pixels, following the procedure previously described [30]. The thickness of films of PVK and RuII-C were around 55 nm and 25 nm, respectively. An Aluminum layer of 80 nm evaporated at 2.7.107 Torr vacuum with rate of 1.0 Å/s was deposited as cathode electrode. The device structure was ITO/PVK/RuII-C/Al with 6.0 mm2 of active area. The current-voltage-photocurrent characteristics were measured with a Keithley 2400 source meter. Photocurrent was analyzed at zero external voltage using a Xenon lamp (Oriel, 150 W) and wavelengths were selected with a monochromator Cornerstone 260 ¼ m model 74125. The intensity of light at the sample position was measured with a photodiode to be 100 mW/cm2. A Newport model 1936-c registered the radiant power and spectrophotometer USB400 UVeVis with optical fiber of 1000 mm diameter to measure the electroluminescence. All measurements were carried out under ambient conditions and without encapsulation. 3. Results and discussion 3.1. Synthesis The ligands containing sulfur (TDZP) and selenium (PhenSe), were prepared according to a recently developed methodology [26]. The Ru (II)-C were prepared using an efficient methodology that is an adaptation of a synthesis from the literature to the preparation of Ru(Phen)3 [31]. To a two-neck round-bottom flask under an argon atmosphere, [Ru(DMSO)4Cl2] (161 mg, 0.33 mmol) was added to ethylene glycol solution (10 mL) containing the corresponding ligand (1 mmol). After 3 h of reflux, a solution of sodium tetrafluoroborate (5 mL) was added and the solution remained for 1 h at 10 C. Finally, the precipitate was filtered and washed with cold ethanol, chloroform and dried in vacuum. The tetrafluoroborate salt was used because it is more soluble, in general than PF6- or Cl-salts. It was also found to be particularly useful to the formation of smooth films. The complexes had their structures elucidates by a combination of fourier transform infrared spectroscopy (FT-IR), Proton nuclear magnetic resonance (1H NMR), high-resolution mass spectroscopy (HRMS) and single crystal X-ray diffraction (SXRD). (IR, NMR and HRMS spectra are included in the supplementary data). 3.1.1. [Ru(Phen)3](BF4)2 Ru(Phen)3 FT-IR (KBr): 3420, 3050, 1629, 1426, 1410, 1147, 1083, 1058, 1036, 844, 722 cm1; 1HNMR (300 MHz, DMSO-d6, 25 C, TMS): d ¼ 7.77 (dd, 3J (H,H) ¼ 8.2 Hz 3J (H,H) ¼ 5.2 Hz, 6 H; CH), 8.09 (dd, 3J (H,H) ¼ 5.2 Hz 4J (H,H) ¼ 1.0 Hz, 6 H; CH), 8.39 (s, 6 H; CH), 8.78 (dd, 3 J (H,H) ¼ 8.2 Hz 4J (H,H) ¼ 1.0 Hz, 6 H; CH) ppm; ESI HRMS: calcd for [C36H24N6Ru]þ 321.0552, found 321.0557. 3.1.2. [Ru(TDZP)3](BF4)2 Ru(TDZP)3 FT-IR (KBr): 3420, 3073, 1627, 1409, 1395, 1123, 1084, 1064, 1039, 819, 728 cm1; 1HNMR (300 MHz, DMSO-d6, 25 C, TMS): d ¼ 7.86 (dd, 3J (H,H) ¼ 8.1 Hz 3J (H,H) ¼ 5.4 Hz, 6 H; CH), 8.25 (d, 3J (H,H) ¼ 5.4 Hz, 6 H; CH), 9.18 (dd, 3J (H,H) ¼ 8.1 Hz, 6 H; CH) ppm; ESI HRMS: calcd for [C36H18N12RuS3]þ 407.9993, found 407.9990.
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Table 1 Crystal data and structure refinement for Ru(II) complexes.
Empirical formula Formula weight Temperature (K) Wavelength (Å) Crystal system Space group Unit cell dimensions (Å ando)
Volume (Å3) Z Density (calc) (Mg/m3) m (mm1) F (000) Crystal size (mm3) Theta range for data collection ( ) Index ranges Reflections collected Independent reflections Absorption correction Max. and min. transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2s(I)] R indices (all data) Largest diff. peak and hole (e.Å3)
[Ru(TDZP)3](BF4)2$3H2O
[Ru(PhenSe)3](BF4)2$3H2O
C36H24B2F8N12O3RuS3 1043.54 200 (2) 0.71073 Triclinic Pı a ¼ 11.8671(5) b ¼ 11.9181 (5) c ¼ 16.0412 (6) a ¼ 104.5760 (10) b ¼ 90.6910 (10) g ¼ 113.6170 (10) 1995.46 (14) 2 1.737 0.644 1044 0.20 0.20 x 0.08 1.322 to 28.783 16 h 16, 16 k 16, 21 l 21 24085 10326 (Rint ¼ 0.0215) Semi-empirical from equivalents 0.9503 and 0.8820 Full-matrix least-squares on F2 10326/0/586 1.056 R1 ¼ 0.0471, wR2 ¼ 0.1277 R1 ¼ 0.0615, wR2 ¼ 0.1377 1.989 and 0.849
C36H24B2F8N12O3RuSe3 1184.24 200 (2) 0.71073 Triclinic Pı 11.9498 (5) 11.9762 (5) 16.0832 (7) 104.5030 (10) 90.2700 (10) 113.3600 (10) 2031.66 (15) 2 1.936 3.166 1152 0.42 0.22 x 0.14 1.317 to 30.163 16 h 16, 16 k 16, 22 l 22 52714 11963 (Rint ¼ 0.0213) Semi-empirical from equivalents 0.6656 and 0.3498 Full-matrix least-squares on F2 11963/0/616 1.036 R1 ¼ 0.0448, wR2 ¼ 0.1130 R1 ¼ 0.0623, wR2 ¼ 0.1275 3.146 and 1.644
3.1.3. [Ru(PhenSe)3](BF4)2 Ru(PhenSe)3 FT-IR (KBr): 3415, 3075, 1627, 1450, 1409, 1342, 1126, 1084, 1062, 1036, 815, 728 cm1; 1HNMR (300 MHz, DMSO-d6, 25 C, TMS): d ¼ 7.83 (dd, 3J (H,H) ¼ 8.1 Hz 3J (H,H) ¼ 5.3 Hz, 6 H; CH), 8.23 (d, 3J (H,H) ¼ 5.3, 6 H; CH), 9.18 (d, 3J (H,H) ¼ 8.1, 6 H; CH) ppm; ESI HRMS: calcd for [C36H18N12RuSe3]þ 478.9167, found 478.9164. The FT-IR, 1H NMR and HRMS spectra confirmed the chemical structures of the synthesized compounds. The bands found in the infrared spectrum are quite similar for all the complexes. In the region between 1600 and 1400 cm1 are situated the C¼C and C¼N stretches, characteristic of aromatic systems [26]. Moreover, between 1200 and 900 cm1 a broadband is located, with vibrational resolution of four signals, regarding tetrafluoroborate ion. It is possible to observe all signals related to the hydrogens of the 1,10phenanthroline system in the 1H-NMR spectrum. Additionally, the insertion of the heterocycles containing chalcogenides in the molecule caused a downfield shift of the signals for the protons with loss of the coupling constant 4J, when compared to Ru(Phen)3 complex. The obtained results of HRMS are in good accordance with the calculated values.
Table 2 Selected bond lengths [Å] and angles [ ] for Ru(II) complexes.
Ru1-N1 Ru1-N10 Ru1-N21 Ru1-N30 Ru1-N41 Ru1-N50 N1-Ru1-N10 N1-Ru1-N41 N21-Ru1-N1 N21-Ru1-N10 N21-Ru1-N41 N21-Ru1-N50 N30-Ru1-N1 N30-Ru1-N10 N30-Ru1-N21 N30-Ru1-N41 N30-Ru1-N50 N41-Ru1-N10 N50-Ru1-N1 N50-Ru1-N41 N50-Ru1-N10
[Ru(TDZP)3](BF4)2$3H2O
[Ru(PhenSe)3](BF4)2$3H2O
2.065 (3) 2.069 (3) 2.059 (2) 2.057 (3) 2.065 (2) 2.062 (3) 79.59 (10) 97.31 (10) 87.43 (10) 96.16 (10) 174.14 (10) 95.66 (10) 96.04 (11) 174.32 (10) 79.93 (10) 96.09 (10) 87.99 (10) 88.12 (10) 175.31 (10) 79.83 (10) 96.53 (10)
2.058 (3) 2.065 (3) 2.063 (3) 2.060 (3) 2.060 (3) 2.058 (3) 79.37 (11) 95.97 (11) 97.37 (11) 88.31 (11) 96.26 (11) 174.07 (11) 174.98 (11) 96.44 (11) 79.64 (10) 88.40 (11) 95.65 (10) 173.88 (10) 87.57 (11) 79.90 (11) 95.85 (10)
3.2. Crystalline structure of Ru(TDZP)3 and Ru(PhenSe)3 From the slow growth in dimethyl sulfoxide, prismatic red crystals of [Ru(TDZP)3](BF4)2$3H2O and [Ru(PhenSe)3](BF4)2$3H2O were obtained, which were used for structural characterization by single crystal X-ray diffraction analysis. Selected bond lengths and angles are listed in Table 2. These ruthenium complexes are isostructural, which in the triclinic crystal system, space group Pı. For both structures, the asymmetric unit consists of one cation complex, two BF4- counterion and three water molecules as solvate. In complexes [Ru(L)3]2(BF4)$3H2O, the discrete cationic molecule consists of an RuII center surrounded by three ligand units, in a sixcoordinate environment to form a propeller-shaped complex cation
Fig. 1. The molecular structure of the three complexes obtained with ChemDraw program is illustrated in Fig. 1S at the supplementary data. The length of the RueN bonds [ranging from 2.057(3) to 2.069(3) Å] and the ligand bite angles [ranging from 79.37(11) to 79.93(10) ] fall within the interval of values reported in the Cambridge Structural Database [32,33] for structures containing the [Ru(phen)3]2þ fragment. These values are in full agreement with a slightly distorted RuN6 octahedral geometry around the metallic center. The 5-membered chelating rings of RuII and the N atoms of tetracyclic ligands exhibit nearly perfect coplanarity; the N1 - C12 -
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Fig. 1. Cationic ruthenium species in the crystal structure of [Ru(TDZP)3] 2BF4$3H2O (a) and [Ru(PhenSe)3] 2BF4$3H2O (b) with heteroatom labeling. The ellipsoids are represented at a 40% probability level. Hydrogen atoms are omitted for clarity. The BF4 anion was omitted.
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C11 - N10 and N21 - C32 - C31 - N30 and N41 - C52 - C51 - N50; torsion angles are 2.1(4) , 2.7(4) and 2.2(4) for [Ru(TDZP)3]2þ and 1.9(4) , 3.1(4) and 2.4(4) for [Ru(PhenSe)3]2þ, respectively. The dihedral angles between mean planes of the two TDZP molecules are very similar in both ruthenium complexes. The angles between mean planes N1/N10 e N21/N30, N1/N10 e N41/N50 and N21/N30 e N41/N50 are 83.29(3) , 85.15(4) and 87.21(3) for [Ru(TDZP)3]2þ and 84.60(4) , 84.34(3) and 87.92(3) for [Ru(PhenSe)3]2þ, respectively. 3.3. Thermal behavior The thermal properties of the RuII-C were investigated by thermogravimetric analysis (TGA). The results are presented in Fig. 2S of the supplementary data, indicating that all the complexes are stable at room temperature, with the compounds decomposing only above 350 C. 3.4. Spectroscopy and electrochemistry UVeVis spectroscopy and emission spectra of the RuII-C obtained from solution in acetonitrile and spin coated films are presented in Fig. 2. In the UV region, the absorption spectra shows bands at 224 nm and 262 nm for the Ru(Phen); at 260 nm and two other at 297 nm and 318 nm for the Ru(TDZP)3; and band centered
Ru(Phen)3 Ru(PhenSe)3
1.0
Normalized Emission
Normalized Absorbance
1.0
Ru(TDZP)3
0.8
0.6
0.4
0.8
0.6
0.4
0.2
0.2
(a) 0.0 200
250
300
350
400
450
500
550
600
650
0.0 700
λ (nm)
Ru(Phen)3 Ru(PhenSe)3
1.0
Normalized Emission
Normalized Absorbance
1.0
Ru(TDZP)3
0.8
0.6
0.8
0.6
0.4
0.4
0.2
0.2
0.0 200
(b) 250
300
350
400
450
500
550
600
650
0.0 700
λ (nm) Fig. 2. Optical absorbance (left) and emission (right) of RuII-C in solution (a) and film (b).
at 265 nm, 305 nm and 342 nm Ru(PhenSe)3. The bands of each compound with peaks in 262 nm, 318 nm and 342 nm are attributed to the p-p* transitions of the organic ligand [26]. As expected, a broad absorption band observed around 450 nm for all complexes and is associated to MLCT transitions [17]. Therefore, the optical band gap (Eg) is about the same for all complexes (~2.5 eV). Emission spectra in solution show only a large band in the orange-red region, associated to the MLCT transitions with maxima at 591 nm, 599 nm and 603 nm for Ru(Phen)3, Ru(TDZP)3 and Ru(PhenSe)3, respectively. The absorption and emission spectra in film are very similar to the solution, with a small red-shift, usually associated to molecular aggregations in the film structure. The quantum yields for the emission of the complexes were estimated using the Ru(bpy)3 as reference [34] and are shown in Table 3. The higher values for the Ru(PhenSe)3 and Ru(TDZP)3 complexes can be associated with an increased phosphorescence rate (kP) caused by the stronger spin-orbit coupling induced by the presence of the heavier chalcogen atoms in the organic ligands [26,35]. The value for Ru(Phen)3 agrees with the literature [36]. Cyclic voltammograms of Ru(PhenSe)3 and Ru(TDZP)3 were registered at scan rates of 100 mV s1. From the voltammograms (Fig. 3S of the supplementary data) the E1/2 can be calculated as 0.98 V and 1.40 V for Ru(PhenSe)3, and 1.04 V and 1.44 V for Ru(TDZP)3, versus Fc/Fcþ. In each case, the E1/2 potential was used to estimate the HOMO and LUMO energy levels [37]. The electrochemical band gap values shown in Table 3 are very close to the optical band gaps calculated from the absorption spectra. 3.5. Photovoltaic characterization In a previous work we demonstrated that the presence of heavy atoms in organic ligands promoted an increase of the excited state lifetime due to strong spin orbit coupling and triplet conversion, which contributed to improve the photovoltaic activity [26]. Therefore, a similar behavior could also be expected in the complexes containing the same ligands. Fig. 3 shows the photocurrent response of the ITO/PVK/RuII-C/AL devices irradiated selectively using a monochromator at zero external voltage. The photocurrent peak at 345 nm corresponds to the absorption peak of PVK, as previously reported [39], this peak is also present in the reference device only with PVK as active layer (ITO/PVK/Al). However, the response around 450 nm observed only for Ru(TDZP)3 and Ru(PhenSe)3 corresponds to the absorption band associated to MLCT transitions. The fact that no photocurrent was observed around 450 nm for Ru(Phen)3 corroborates with the stronger spin orbit coupling due to the presence of heavy atoms (S and Se) in these ligands. That is probably making the, in principle forbidden S3 0 MLCT transition, allowed, or at least with a relatively higher transition dipole moment and thus turning these complexes into better light absorbers in the visible region. 3.6. Electrical characterization Electroluminescent properties of the devices ITO/PVK/RuII-C/Al for Ru(PhenSe)3 and Ru(Phen)3 were also exploited. The energy level diagrams are illustrated in Fig. 4S of the supplementary data, where a small energetic barrier (around 0.4 eV for Ru(PhenSe)3 and 0.8 eV for Ru(Phen)3) from the Al to the LUMO of the compounds favors electron injection. On the other hand, a much larger barrier of about 1.0 eV has to be crossed for hole injection from ITO into the HOMO of the PVK. From the HOMO of PVK to the HOMO of RuII-C, the energetic barriers for hole injection are of 0.4 eV for Ru(PhenSe)3 and 0.3 eV for Ru(Phen)3. The PVK layer also acts as electron blocking layer in order to confine the recombination process inside the RuII-C. However, the recombination can occur close
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Table 3 Optical and electrochemical data. Compound
HOMO (eV)a
LUMO (eV)a
Electrochemical band gap (eV)
Optical band gapb
Quantum yield F
Ru(Phen)3 Ru (TDZP)3 Ru(PhenSe)3
6.1c 6.2 6.2
3.4c 3.8 3.8
2.7c 2.4 2.4
2.5 2.5 2.5
0.0280 0.0647 0.0974
a b c
Determined from peak potentials [37]. Calculated from the onset of the absorption spectra in solution. Extracted from Ref. [38].
to the PVK/RuII-C interface, reducing the emitted energy. The morphology of the films was evaluated with AFM, being the root mean square roughness (RRMS) of 9.0 nm and 6.0 nm for Ru(Phen)3 and Ru(PhenSe)3, respectively (see images in Fig. 5S of the supplementary data). Fig. 4 shows the current density and radiation power versus voltage. The turn-on voltages are 7.5 V and 7.3 V for the devices containing Ru(Phen)3 and Ru(PhenSe)3, respectively, which is
18
ITO/PVK/Al ITO/PVK/Ru(Phen)3/Al
16
ITO/PVK/Ru(PhenSe)3/Al ITO/PVK/Ru(TDZP)3/Al
Photocurrent (nA/cm2)
14 12 10 8 6 4 2 0 300
350
400
450
500
550
λ (nm)
600
650
700
consistent with the similarity of the energy level values of both materials. However, the current density for Ru(PhenSe)3 increases faster with the applied voltage, indicating that the charge transport is more efficient in this material, probably due to the stronger dispersion interactions induced by the high polarizability of the selenium atom. The inset of Fig. 4 shows the EL spectra of both devices, where a broadband of red-orange color is the result of emission from the triplet MLCT state. The narrow band observed for the Ru(PhenSe)3 containing device is probably again due to the increased intermolecular interactions resulting in a tighter nuclear configuration in the solid state, or due to the longer excited lifetime. A device with ITO/Ru(PhenSe)3/Al was elaborated in order to investigate the charge carrier mobility of the Ru(PhenSe)3 complex. In this case, as the work functions of the ITO (4.8 eV) and the Al (4.2 eV) are very similar and close to the LUMO level of the complex (3.8 eV), the charge carriers are predominantly electrons. Fig. 5 shows the log-log plot of the current density as a function of the voltage, where two conduction regimes separated by a threshold voltage of 4.8 V are clearly observed. For low voltages, the slope equal to 1.1 indicates an Ohmic regime, followed by a space charge limited current (SCLC) regime on higher voltages. In the SCLC region, a slope of 4.4 corresponds to the trap limited-SCLC regime. In a trap-free semiconducting material the current density would increase quadratically with the applied voltage. The charge mobility was obtained directly from the experimental J-V curve by fitting the trap-limited SCLC regime with a theoretical model published previously from the present authors of this article [40,41]. This model considers an electric field dependent pffiffi €ssler [42], mobility of the form mðEÞ ¼ m0 eg E as suggested by Ba
Fig. 3. Photocurrent measurements of the diode structures vs. wavelengths at zero external voltage. ITO/PVK/Al was also measured for comparison. 2
10
Current Density (mA/cm2)
0.8
18
0.6
16 14
0.4
12
0.2
10
0.0 500
8
Ru(PhenSe)3 Theoretical 2 Current Density (mA/cm )
1.0
20
Normalized EL
22
Ru(Phe)3
Ru(PheSe)3 550
600
650
700
750
800
Wavelength (nm)
6
ITO/PVK/Ru(Phen)3/Al
4
ITO/PVK/Ru(PhenSe)3/Al
2
1
10
SCLC
-10 2 cm /Vs
μ0 = 1.90 x 10
-3
γ = 3.37 x 10
(cm/V)1/2 4.8 V
0
10
Ohmic
-1
10
0 -6
-4
-2
0
2
4
6
8
10
12
14
16
18
Voltage (V) Fig. 4. Current density as a function of the applied voltage for Ru(Phen)3 and Ru(PhenSe)3. In the inset is the normalized EL at a voltage of 17 V for Ru(Phen)3 and 10 V for Ru(PhenSe)3.
1
10 Voltage (V)
Fig. 5. Log-log plot of the current density as a function of the voltage for Ru(PhenSe)3. The red solid line represents the theoretical fitting. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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where m0 is the carrier mobility at zero field and g the field dependence of the mobility. From the fitting, m0 and g parameters were extracted and used in the mobility equation to determine the charge mobility as a function of applied voltage. The m0 and g values found were of 1.90 1010 cm2/V.s and 3.37 103 (cm/V)1/2 respectively, where the maximum mobility was of 1.24 107 cm2/ V.s for an applied voltage of þ9.0 V. In the literature, there are not mobility values for similar RuII-C, but this value is consistent with ruthenium polymeric systems [43]. 4. Conclusion This work presents a simple and efficient methodology for synthesis of ruthenium complexes with organic ligands derivative from 1,10-phenanthroline containing the chalcogens Sulfur and Selenium atoms. The complexes exhibited good thermal and electrochemical stability and absorption and emission in the visible spectrum. The presence of the heavier atoms in the organic ligand promoted photocurrent response in the visible light range. Investigations of the electro-optical properties in simple diode structure showed that the Ru(PhenSe)3 can be exploited as active layer in electroluminescent devices. The peculiar properties of the ligands containing sulfur and selenium seem to be strongly correlated to an increased spin-orbit coupling in the ligands, along with the central ruthenium atom. Acknowledgements The authors are grateful to the CNPq, CAPES, INEO, INCT-Cat alise and FAPESC-Pronex for support of this work. The authors are also grateful to CEBIME for the HRMS analysis. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2017.01.059. References [1] Zhang Y, Liu Z, Yang K, Zhang Y, Xu Y, Li H, et al. A ruthenium(II) complex as turn-on Cu(II) luminescent sensor based on oxidative cyclization mechanism and its application in vivo. Sci Rep 2015;5:8172. http://dx.doi.org/10.1038/ srep08172. [2] Hara D, Komatsu H, Son A, Nishimoto S, Tanabe K. Water-soluble phosphorescent ruthenium complex with a fluorescent coumarin unit for ratiometric sensing of oxygen levels in living cells. Bioconjug Chem 2015;26:645e9. http://dx.doi.org/10.1021/acs.bioconjchem.5b00093. [3] Schulze M, Kunz V, Frischmann PD, Würthner F. A supramolecular ruthenium macrocycle with high catalytic activity for water oxidation that mechanistically mimics photosystem II. Nat Chem 2016;8:576e83. http://dx.doi.org/ 10.1038/nchem.2503. [4] Zhang LL, Gao Y, Liu Z, Ding X, Yu Z, Sun LC. A trinuclear ruthenium complex as a highly efficient molecular catalyst for water oxidation. Dalton Trans 2016;45:3814e9. http://dx.doi.org/10.1039/C5DT04233J. [5] Zeng L, Chen Y, Liu J, Huang H, Guan R, Ji L, et al. Ruthenium(II) complexes with 2-Phenylimidazo[4,5-f][1,10]phenanthroline derivatives that strongly combat cisplatin-resistant tumor cells. Sci Rep 2016;6:19449. http:// dx.doi.org/10.1038/srep19449. [6] Dickerson M, Howerton B, Bae Y, Glazer EC. Light-sensitive ruthenium complex-loaded cross-linked polymeric nanoassemblies for the treatment of cancer. J Mater Chem B 2016;4:394e408. http://dx.doi.org/10.1039/ C5TB01613D. [7] Murali MG, Wang X, Wang Q, Valiyaveettil S. Design and synthesis of new ruthenium complex for dye-sensitized solar cells. RSC Adv 2016;6:57872e9. http://dx.doi.org/10.1039/C6RA10881D. [8] Wang P, Zakeeruddin SM, Moser JE, Nazeeruddin MK, Sekiguchi T, Gr€ atzel M. A stable quasi-solid-state dye-sensitized solar cell with an amphiphilic ruthenium sensitizer and polymer gel electrolyte. Nat Mater 2003;2:402e7. http://dx.doi.org/10.1038/nmat904. [9] Cao K, Lu J, Cui J, Shen Y, Chen W, Alemu G, et al. Highly efficient light harvesting ruthenium sensitizers for dye-sensitized solar cells featuring triphenylamine donor antennas. J Mater Chem A 2014;2:4945e53. http://dx.doi.org/ 10.1039/C3TA15134D.
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