Design and synthesis of new Ru-complexes as potential photo-sensitizers: Experimental and TD-DFT insights

Article abstract of DOI:10.1039/c6ra16458g

We report density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations on a novel organic ligand and a novel class of ruthenium complexes; cis-RuL₂X₂ with L = 2,2′-bipyridine-6,6′-bis ethyl ester phosphonate and phosphonic acid, X = Cl, CN or NCS. The calculations show that cis-configurations are more stable than the trans-counterparts. The DFT results have been used to help design such novel complexes for potential use as sensitizers. We demonstrate the opportunity to synthesize such complexes with high purity. The synthesis of these complexes relies on the preparation of the key intermediates cis-Ru(2,2′-bipyridine-6,6′-bisdiethyl ester phosphonate)Cl₂. These complexes were characterized by ¹H, ¹³C, and ³¹P NMR, elemental analysis and FTIR spectroscopy. The NCS complex shows the smallest optical band gap followed by the Cl and CN complexes, respectively, with the highest performance upon use as a sensitizer in dye-sensitized solar cells.

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Design and Synthesis of New Ru-Complexes as Potential Photo-sensitizers: Experimental and
TD-DFT insights
Walid Sharmoukh1, Walid M.I. Hassan2, Philippe C. Gros3 and Nageh K. Allam4,*
1Department of Inorganic Chemistry, National Research Centre, Dokki, Giza 12622, Egypt
2 Department of Chemistry, Faculty of Science, Cairo University, Giza 12613, Egypt
3Photosens, Université de Lorraine, UMR SRSMC, VandoeuvreꢀLèsꢀNancy, Nancy, France
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4Energy Materials Laboratory (EML), School of Sciences and Engineering, The American University in
Cairo, New Cairo 11835, Egypt. *Eꢀmail: nageh.allam@aucegypt.edu.
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Abstract
We report density functional theory (DFT) and timeꢀdependent density functional theory
(TDDFT) calculations on a novel organic ligand and a novel class of ruthenium complexes; cisꢀ
RuL2X2 with L = 2, 2`ꢀbipyridineꢀ6, 6`ꢀbis ethyl ester phosphonate and phosphonic acid, X = Cl,
CN or NCS. The calculations show that cisꢀconfigurations are more stable than the transꢀ
counterparts The DFT results have been used to help designing such novel complexes for potential
use as a sensitizer. We demonstrate the opportunity to synthesize such complexes with high purity.
The synthesis of these complexes relies on the preparation of the key intermediates cisꢀ
Ru(2,2`bipyridineꢀ6,6`ꢀbisdiethyl ester phosphonate)Cl2. These complexes were characterized by
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1H, C, and P NMR, elemental analysis and FTIR spectroscopy. The NCS complex shows the
smallest optical band gap followed by the Cl and CN complexes, respectively, with the highest
performance upon use as a sensitizer in dyeꢀsensitized solar cells.
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Keywords: DFT; optical properties; dipole moment; Ru complexes; NMR spectroscopy.
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1. Introduction
With the looming energy crisis and the depletion of petroleum and naturalꢀgas reserves,
various ways have been investigated to convert the sun light directly into chemical fuel or
electricity.1 However, the capital cost of such devices is still inconvenient for largeꢀscale
implementation. To this end, dyeꢀsensitized solar cells (DSSCs) have attracted considerable
attention due to their high quantum efficiency giving the opportunity of lowꢀcost conversion of
solar energy into electrical power.2 The ongoing research in DSSCs is mainly focusing on the
optimization of all cell components; namely, the photoactive material, the dye and the redox
electrolyte.3
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The development of optically active sensitizers over a wide region of the solar spectrum
is a hot topic nowadays. In particular, ruthenium (Ru)ꢀbased complexes have been extensively
used as sensitizers.4,5 Ruthenium metal has been particularly investigated for a number of
reasons:6 (1) it has octahedral geometry that enable the introduction of specific ligands in a
controlled manner; (2) it forms very inert bonds with imine nitrogen centers; (3) it possesses
various stable and accessible oxidation states. However, the main problem limiting the further
development of sensitizers is the fact that dyes with high absorption coefficients has narrow
bands and viceꢀversa. Consequently, the optimization of sensitizers is mainly based on “guessꢀ
andꢀcheck” procedures. A more systematic approach is required in order to find optimum
sensitizers with the required specifications. In this regard, density function theory (DFT)
calculations are considered ideal to solve the “guessꢀandꢀcheck” problem involved in the design
of DSSCs as well as solar hydrogen production systems. Specifically, it can be used to study the
changes in the optical and electronic structures of the sensitizers.
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Herein, we report detailed DFT calculations on new cisꢀ ruthenium complexes with
coordinated bipyridine bisphosphonate ligands. Our DFT findings were confirmed via the
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preparation and characterization of such sensitizers and their use in dyeꢀsensitized solar cell
devices.
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2. Computational and Experimental Methods and Materials
2.1 Density Functional Theory and Time-dependent Density Functional Theory
The molecular geometry for all complexes and ligands was fully optimized using 6ꢀ31+G
mixed basis sets for C, H, N, O, P, Cl and S atoms and effective core potential
LANL2/LANL2DZ basis set 7ꢀ10 for Ru atom with the density functional (DFT) level. The DFT
calculations were carried out using the hybrid threeꢀparameter density functional method
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abbreviated as B3LYP, which includes Beck’s 3ꢀparameter gradient exchange correction
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function (B3)
and the Lee, Yang and Parr correlation functional 12,13. The B3LYP method
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typically provides energetic better than Hartree–Fock method
and can reproduce better
geometrical parameters comparable to the experimental values than any other method 15. No
symmetry constrains were implemented during all the geometry optimization procedures. All
possible configurations for the complexes were calculated and the one shown here is the lowest
in energy or the global minimum for each complex. All the geometry optimization, vibrational
frequency calculations were done using the Gaussian 09 software package 16. The optimized
structures were visualized using Chemcraft version 1.6 packages 17. The bond dissociation
energy of AꢀB bond is calculated by subtracting the total energy of the optimized geometry for
AB molecule from single point energy of its constituents A and B at the same geometry, which is
more accurate than substring it from optimized energy of isolated molecules as this would
include some part of possible conformation relaxation energy changes. Ionization potential and
electron affinity of cation and anion total energy subtracted from neutral total energy at
optimized geometry without further optimization as electron movements are assumed to be faster
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than nuclear ones. Timeꢀdependent DFT calculations were done on optimized geometry using
number of excited states equal to 10. Exciton binding energies were estimated by the difference
between LUMO and HOMO in DFT and in TDꢀDFT calculation. The electronic absorption
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spectra were created using convolution with Gaussian function using Gauss View Software
using UVꢀVis peak HalfꢀWidth at half height of 0.1 eV.
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2.2 Experimental Materials and Synthesis
All start materials are high grade chemicals and solvents that were purchased from Sigma
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Aldrich. The solvents were distilled before use. H ,13C NMR and 31PNMR spectra were
recorded using 5 mm tube on a Bruker ACꢀ250 (250.133 and 62.896 MHz, respectively) or
Varian Gemini 2000 (199.976 and 50.289 MHz, respectively) and were referenced to
tetramethylsilane (TMS) and triphenylphosphine.
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• Synthesis and Characterization of 2,2`- bipyridine N,N`-Dioxide
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2, 2`ꢀ bipyridine (4g, 25.61 mmol) was dissolved in 50 ml chloroform. mCPBA mꢀ
chloroperbenzoic acid (22.1 g, 64.03 mmol) dissolved in 200 ml chloroform was added slowly (4
h) to this solution at zero temperature. After the completion of addition, the solution was stirred
for another 2 days. The reaction was filtered off and quenched by the addition of methanol for
one day. The precipitation filtered and dried at room temperature; (Yield 92 %)
1H NMR (200 MHz, D2O, ppm): 8.31ꢀ 8.27(d, 2H), 7.67ꢀ7.54(m, 6 H);
13CNMR (100 MHz, D2O, ppm) 139.69, 137.60, 129.23, 126.75, 126.31
• Synthesis and Characterization of 6,6`- dicyano 2,2`- bipyridine
2,2`ꢀ bipyridine N,N`ꢀDioxide (6 g, 31.88 mmol) and potassium cyanide (12.3 g, 189 mmol)
were dissolved in 100 ml water. Methylene chloride 40 ml and benzoyl chloride (15.69g, 111.6
mmol) were added slowly upon three hours at zero temperature. After completion of addition,
the mixture was stirred at 0 °C for about four hours. The solution was filtered and quenched by
ethanol for one day. The precipitation filtered and dried at room temperature to Yield 75%.
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1H NMR (200 MHz, CDCl3, ppm): 8.74ꢀ8.69(dd, 2H), 8.06ꢀ 7.98(t, 2H), 7.79 – 7.75(dd, 2H);
13CNMR (100 MHz, CDCl3, ppm) 153.83, 138.78, 131.48, 129.01, 123.88, 116.24; EA found: C,
69.95; H, 2.83; N, 27.39%. Theoretical: 69.90; H, 2.93; N, 27.17%).
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• Synthesis and Characterization of 6, 6`- bis(ethoxycarbonyl) 2,2`- bipyridine
6,6`ꢀ dicyano 2,2`ꢀ bipyridine (5g, 24.25 mmol) was dissolved in 80 ml ethanol and 34 ml
of sulfuric acid was added slowly to solution. The reaction mixture was heated to reflux for one
day. The solution poured over 100 g of ice and stirred for 2 h, then extracted with methylene
chloride. The solution was washed with brine three times. After that the solution dried over
magnesium sulfate anhydrous and reduced to yellow white solid (85%).
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1H NMR (200 MHz, CDCl3, ppm) 8.79 – 8.74(dd, 2H), 8.16ꢀ 8.12(dd, 2H), 8.02ꢀ 7.94)(t, 2H),
4.55 – 4.44(q, 4H), 1.50ꢀ 1.43(t, 6H);
13CNMR (100 MHz, CDCl3, ppm) 163.31, 153.63, 146.01, 136.18, 123.57, 122.84, 60.04 ppm.
EA found, C, 64.22; H, 5.37; N, 9.54%. Theoretical; C, 63.99; H, 5.37; N, 9.33%)
• Synthesis and Characterization of 6, 6`-Bis (hydroxymethyl)-2, 2`-bipyrine
6.6`ꢀDiꢀfunctionalized bipyridine was prepared from 6,6`ꢀdibromoꢀ2,2`ꢀbipyridine via lithiumꢀ
bromide exchange, followed by addition of suitable electrophiles such as N,Nꢀdimethyl formamide, to
give dialdehyde (19). The 6, 6`ꢀbis (hydroxymethyl)ꢀ2, 2`ꢀbipyrine was prepared following the
procedure reported in literature (20)
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1H NMR (400 MHz,CDCl3): 8,63ꢀ8,31(d,2H), 7,86ꢀ7.79(t,2H), 7,27ꢀ7.24(d, 2H), 4.83(s, 4H),
3.98(bs,2H) ppm;
13CNMR (100 MHz,CDCl3): 160.14, 150.23, 138.69, 120.90, 119.04, 62.33 ppm;
Anal, Calcd for C12H10N2O2, C, 66.65; H, 5.59; N, 12.96. Found; C, 66.46; H, 5.69; N, 13.18.
• Synthesis and Characterization of 6,6`-Bis(bromomethyl)-2,2`-bipyrine:-
6,6`ꢀbis(hydroxymethyl)ꢀ2,2`ꢀbipyridine (6.39 mmol, 1,3g) was dissolved in a mixture of 48%
HBr (30 ml) aqueous solution and concentrated sulfuric acid (10.5 ml). The resulting solution was
refluxed for 8h and then allowed to cool to room temperature. Then, 75 ml of water was added. The
pH was adjusted to neutral with NaOH solution. The resulting precipitate was filtered, and washed
with water. The product was dissolved in chloroform (60 ml) and filtered. The solution was dried over
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anhydrous magnesium sulfate and evaporated under vacuum to dryness. Product was isolated as a
white powder; (Yield 88%)
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1H NMR (400 MHz, CDCl3): δ 4.60(4H, s, CH2); 7.38(2H, d, J= 5Hz, aryl H on C5 and C5); 8.45(2H,
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s, aryl H on C3 andC3); 8.68 (2H, d, J= 5Hz, aryl H on C6 and C6) ppm.
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13C NMR (100 MHz, CDCl3): δ 165.20, 155.45, 137.91, 123.55, 120.50, 34.1ppm.
Anal, Calcd for C12H10N2Br2 :ꢀ C, 42.1; N, 8.18; H, 2.92. Found: C, 42.36; N, 7.86; H, 2.82.
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• Synthesis and Characterization of 6,6`-Bis(diethylmethy phosphonate)-2,2`-
bipyridine
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A chloroform (15 ml) solution of 6, 6`ꢀBis (bromomethyl)ꢀ2, 2`ꢀbipyrine (0.3g, 8.77mmol) and
2.91g (17.7 mmol) of triethyl phosphate was refluxed for 5 h under nitrogen. The excess phosphate
was removed under high vacuum, and then the crude product was obtained as yellow–white powder;
Yield 85%
1H NMR (400 MHz, CDCl3): δ 1.30 (12H), 3.5(4H, CH2P), 4.10(8H, OCH2), 7.35ꢀ7.38(2H), 8.34ꢀ
8.37(2H), 8.62(2H) ppm.
13CNMR (100 MHz, CDCl3): δ 155.58, 152.08 (d, J=7.9 Hz), 137.27(d, J=2.2 Hz), 124.16(d, J=4.6
Hz), 119.03(d, J= 3 Hz), 62.15(d, J=6.8 Hz), 37.33, 35.99 and 16.31(d, J= 6Hz) ppm.
Elemental anal, Calcd for, C20H30N2O6P2: C, 52.63; N, 6.14; H. 6.63. Found: C, 52.5; N, 5.93; H, 6.7.
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• Synthesis and Characterization of Ru [(6, 6`-(CH2PO3Et2)2bpy)]2Cl2.
A solution of 6,6`ꢀBis(diethylmethy phosphonate)ꢀ2,2`ꢀbipyridine (295 mg, 0.636mmol)
and LiCl(286 mg, 6.8 mmol), and Ru(DMSO)4Cl2 (205 mg, 0.423 mmol) in dry DMF (15 ml)
was refluxed for 7 h under nitrogen in the dark at (160ꢀ 170 0C). After the solution was cooled
to room temperature, methylene chloride was added and the precipitate was filtered and washed
with methylene chloride. Finally the precipitate was washed with diethyl Esther (three times 10
ml). After the compound was dried in a vacuum, and then the crude product was obtained as
brownꢀwhite (yield 85 %). the complex was used as such in the next step.
1H NMR (400 MHz, D2O): δ 1.20(t, J= 6.8Hz, 12H, CH3), 1.22(t, J=7.2 Hz, 12H, CH3), 3.9(m, J
= 7.2 Hz, 8H, CH2), 3.8(m, J= 7.2 Hz, 8H, CH2), 3.4(d, J= 20.8 Hz, 8H, PCH2), 7.96(dd, J= 7.2
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Hz, 4HAnd J=2.4 Hz, 4H, H3,H4), 7.54(dd, J= 1.6 Hz And, J= 5.2 Hz, 2H, Hs) And J= 2, J= 7.2,
2H, H5’) ppm.
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13CNMR (100 MHz, D2O): δ 155.625, 155.6, 155.4, 155.33, 138.561, 124.727, 124.689, (d, J=
3.8 Hz), 120.573(d, J= 3 Hz), 61.527, 61.467 (d, J= 6 Hz, CH2), 37.71, 37.05, 36.458 (t, CH2P)
and 16.082, 16.021(d, J= 6.1 Hz) ppm.
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31PNMR (400 MHz, D2O): 28.167 ppm
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Elemental anal: Calcd for (C40H60Cl2N4O12P4Ru), C32H44Cl2N4O12P4Ru), theoretical; C, 39.5, H,
4.56, N, 5.67. Found, C, 41.0, H, 4.77, N, 6.1.
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1H NMR (400 MHz, CD3OD): δ 1.15(t, J= 7.2Hz, 12H, CH3), 1.13(t, J= 6.8 Hz, 12H, CH3),
3.8(d, J=7.2 Hz, 8H, CH2), 3.86(m, J=7.2Hz, 8H, CH2), 3.3(d, J= 23.2Hz, 8H, PCH2), 7.41(dd,
J= 8 Hz, 4H), 7.8(dd, J= 7.6 Hz and J= 8 Hz, 4H), 8.09(dd, J= 8 Hz, 4H) ppm.
13CNMR (100 MHz, CD3OD): δ 158.45, (d, J= 9.1 Hz), 155.49, 139.799, 126.35, (d, J= 6.8 Hz),
119.73, 61.72(d, J=6.1Hz, CH2)38.40, 37.12, 17.18, (d, J= 7.6 Hz) ppm.
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• Synthesis and Characterization of Ru[(6,6`-(CH2PO3H2)2bpy)]2Cl2.
A solution of Ru [(6, 6`ꢀ(CH2PO3Et2)2bpy)]2Cl2 (2.18 g, 0.212 mmol) in 15 ml of 18%
HCl was refluxed for 15 h. after that, the solvent was evaporated on a rotary evaporator. The
resulting yellowꢀbrown was dissolved in a minimal amount of water and evaporated. Then dry in
high vacuum to obtain a title compound in 90 % yield.
1H NMR (400 MHz, D2O): δ 2.71(PꢀOH, 8H), 3.59( d, J= 20.8 Hz, CH2ꢀP, 8H), 7.80(dd, J= 68
Hz, 4H), 8.311( dd, J= 68 Hz, 8H), notice: when we were using D2O as a solvent , we found two
peaks was overlap in one peak (H3,H4), and for methanol we found three peaks for pyridine ring.
1H NMR (400 MHz, CD3OD): δ 8.39(dd, J=7.6 Hz, 4H), 8.27(4H) and 7.78(dd, J=8 Hz, 4H),
3.64, (d, J= 21.6 Hz, CH2ꢀP, 8H) and 2.63( PꢀOH, 8H) ppm.
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13CNMR (100 MHz, D2O): δ 153.04, 146.718, 143.686, 128.176, 122.028, 36.292 and 35.041
ppm.
31PNMR (400 MHz, D2O): Two peaks, at 7.76 ppm and 25.718 ppm (reference PPh3).
Elemental anal, Calcd. For (C24H28Cl2N4O12P4Ru.H2O), theoretical C, 33.82, H, 3.44, N, 6.38.
Found, C, 32.54, H, 3.29, N, 6.15.
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• Synthesis and Characterization of Ru[(6,6`-(CH2PO3H2)2bpy)]2(CN)2.
A solution of Ru [(6, 6’ꢀ(CH2PO3H2)2bpy)]2Cl2,(50mg, 0.058 mmol) and KCN (75.5mg, 1.0
mmol) were dissolved in a mixture of water 10 ml and methanol 10 ml in a roundꢀ bottomed
flask 150 ml. And then the solution was purged with nitrogen. The flask was then covered with
aluminum foil and heated at reflux for 15h in dark. After the solution cooled at room temperature,
a grade acetone was added to the crude reaction mixture and the precipitate was filtered off and
washed with acetone. The result of complexes is to yield 80% brown white. To purify this
complex, dissolve in a minimum amount of water and purified by column chromatography using
silicaꢀgel (LH20) as stationary phase and water as eluent.
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1H NMR (400 MHz, CD3OD): δ 2.15(PꢀOH, 8H), 3.14(d, J= 19.6Hz, CH2ꢀP, 8H), 7.31(dd, J=
7.6 Hz, 4H), 7.81(dd, J= 7.6 Hz and J= 8 Hz, 4H), 7.88(dd, J= 8 Hz, 4H) ppm.
13CNMR (100 MHz, CD3OD) δ 171.19 (CN), 158.6(d, J= 6.9Hz), 154.86, 138.19, 124.49(d,
J=3.8), 119.55, 40.51, 39.33 ppm.
31PNMR (400 MHz, CD3OD), one peak at 25.23 ppm ref.pph3.
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IR (KBr) (CN) at 2052Cmꢀ1.
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• Synthesis and Characterization of [Ru[(6,6`-(CH2PO3H2)2bpy)]2(NCS)2.
A solution of Ru [(6, 6’ꢀ(CH2PO3H2)2bpy)]2Cl2,(50mg, 0.058 mmol) and KSCN
(22.25mg , 2.32 mmol), were dissolved in a mixture of water 10 ml and methanol 10 ml in a
roundꢀ bottomed flask 150 ml, And then the solution was purged with nitrogen. The flask was
then covered with aluminum foil and heated at reflux for 15h in the dark. After the solution
cooled at room temperature, a grade acetone was added to the crude reaction mixture and the
precipitate was filtered off and washed with acetone. The result of complexes is to yield 85%
brown.
1H NMR (400 MHz, CD3OD): δ 8.38(d, J=8Hz, 1H), 8.26(t, J=8Hz, 1H), 7.78(d, J=8Hz, 1H),
3.57(d, J=20.8Hz, 2H) ppm.
1H NMR (400 MHz, D2O): δ 8.15(broad peak, 2H), 7.67(broad peak, 1H), 3.49(d, J=42Hz, 2H)
ppm.
13CNMR (400 MHz, D2O): δ 154.0, 153.9, 146.95, 146.93, 143.3, 127.92, 127.87, 121.57, 36.79
and 35.58 ppm
31PNMR (400 MHz, CD3OD): one peak at 25.093 ppm ref.pph3
IR (KBr) (NCS) at 2113 cmꢀ1.
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• Fabrication and Testing of the DSSCs.
The DSSC devices were fabricated and tested as detailed in our previous work21.
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3. Results and Discussion
The optimized geometry of the cisꢀ and transꢀconfigurations of the synthetized sensitizers
is shown in Figures 1 and 2, respectively with their energetic parameters are summarized in
Table 1. Our calculations showed that cisꢀconfigurations are more stable than the transꢀ
counterparts. The P=O bond length was calculated to be 1.49 Å in all complexes. The RuꢀN bond
(connected to the organic part) length was found to be similar in all complexes with a range from
2.15 Å to 2.20 Å. Furthermore, the difference between the cisꢀ and transꢀ configurations is less
than 1%, indicating that the main change in bond lengths is around the Cl, CN and NCS ligands.
In the cisꢀconfigurations, the ClꢀRuꢀCl, NCꢀRuꢀCN and SCNꢀRuꢀNCS bond angles are 86.9°,
93.7° and 91.1°, respectively. On the other hand, in the transꢀconfigurations, the ClꢀRuꢀCl, NCꢀ
RuꢀCN and SCNꢀRuꢀNCS bond angles are 176.4°, 179.5° and 179.3°, respectively. In NCS
complexes, the NꢀCꢀS bond angle is 177.8° and 179.5° for the cisꢀ and transꢀconfigurations,
respectively. In case of the CN complexes, only hydrogen bonds are formed between the N of the
CN group and H of phosphate group. Note that this interaction does not exist in the other two
complexes. The calculations showed that two hydrogen bonds with average length of 1.77 Å are
observed in the transꢀconfiguration while only one hydrogen bond with 1.73 Å is formed in case
of the cisꢀconfiguration due to geometrical orientation of the phosphate groups.
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The RuꢀCl, RuꢀCN and RuꢀNCS bond lengths are 2.18 Å, 1.98 Å and 2.11 Å and 2.49 Å,
2.05 Å and 2.06 Å for cisꢀ and transꢀconfigurations, respectively. It is worth mentioning that the
longer bond lengths in case of RuꢀCl may be the reason for the greater total energy difference
ꢁET between cisꢀ and transꢀconfiguration of this complex relative to the others. The energy of
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the highest occupied molecular orbital (EHOMO) and the lowest unoccupied molecular orbital
(ELUMO) for the complexes under study are summarized in Table 1. The energy gap (ꢁE) between
ELUMO and EHOMO is inversely proportional to the reactivity of the complex. The reactivity as
function of ꢁE is in the order Cl > NSC > CN and NSC > Cl > CN for transꢀ and cisꢀ
configurations, respectively. The dipole moment of the cisꢀconfiguration is larger than that of the
transꢀcounterpart. The bond dissociation energy between Ru metal and ligand has been
calculated. The RuꢀCl, RuꢀNCS and RuꢀCN bond energies are 75.70, 96.19 and 118.89, and
97.63, 115.19 and 151.99 kcal/mol for transꢀ and cisꢀconfigurations, respectively. These numbers
clearly show that the strength of the inorganic ligands is in the order CN > NCS > Cl. It further
explains the greater stability of cisꢀ over transꢀconfiguration by at least 20%. On the other hand,
the average RuꢀN bond (where N is in the organic ligand) dissociation energies do not show that
greater variance where they are 27.11, 32.89 and 35.09, and 34.13, 35.97 and 35.78 kcal/mol for
Cl, NSC and CN complexes with transꢀ and cisꢀconfigurations, respectively. The NBO charges
on Ru are 0.13, ꢀ.017 and ꢀ0.32 for NCS, Cl and CN complexes, respectively. The most positive
charge is observed on P atom, which is almost 2.4 in all complexes. However, the most negative
charge is observed on adjacent O atom in the P=O with a value of about ꢀ1.1 in all complexes.
The optical parameters have been theoretically studied for the cisꢀconformer only and the
main characteristics are summarized in Table 2. The calculation of ionization potential (IP) and
electron affinity (EA) was done by subtracting the total energy of the ionic complex from neutral
one at the same geometry. It is worth mentioning that the IP and EA seem to be shifted by about
1 eV relative to the EHOMO and ELUMO. In addition, timeꢀdependent density functional theory
(TDꢀDFT) calculations were performed to estimate the optical electronic absorption spectra and
exciton binding energies, with the detailed data represented in Table 3. The NCS complex shows
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the smallest optical band gap, the highest oscillator strength and the lowest exciton binging
energy followed by the Cl and CN complexes, respectively, indicating a better performance for
use as a sensitizer in solar cells application. The electronic absorption spectra are shown for the
three complexes in Figure 3. The CN and NCS have three bands at 385 nm, 455nm and 505nm,
and 461nm, 610nm and 671nm, respectively. On the other hand, the Cl complex spectra have
almost one main band at about 530 nm and other bands seem to be overlapped underneath and
appearing as shoulders. As a general trend, the HOMO and HOMOꢀ1 orbitals are either п orbital
on CN and NCS or pꢀorbital on the Cl along with dꢀorbital form Ru atom, see Figure 4. The
LUMO and LUMO+1 are п* orbital localized on one of the two bipyridine ring. The LUMO+2
and LUMO+4 are also п* orbital on both of the two bipyridine ring. Furthermore, all the
calculated electronic absorption spectra involve transitions from dꢀ orbital on metal along with
orbitals on Cl, CN or NCS to orbitals on bipyridine rings.
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Based on the promising DFT results, herein, we show the possibility to synthesize such
complexes. However, we will limit our discussion to the cisꢀRu complexes as an example. The
investigated complexes are of the general formula cisꢀRuL2X2 with L = 2,2`ꢀbipyridineꢀ 6,6`ꢀ
bis(diethyl methyl phosphonate) and 2,2`ꢀbipyridineꢀ 6,6`ꢀ bis(methylphosphoric acid). The
preparation of this type of complexes usually involves two steps.(22,23) The first complex consists
of refluxing two equivalent of the bidentate ligand L with 1 equivalent of ruthenium trichloride in
DMF to yield a cis—RuL2Cl2. In the second step, the diethyl phosphate was hydrolysed to
phosphonic acid, and then the chloride ligands are substituted by potassium thiocyanate or
potassium cyanide. This is achieved by heating in a mixture of methanol or ethanol and water to
prepare complex that contains cyanide and thiocyanate. The preparation of complexes with
different positions (such as 4, and 5) has been previously patented by Grätzel et al.(24) However,
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no preparation and characterization of ligand and complexes were reported in the open literature.
Herein, we report on the synthesis of new ligand and new complexes, which are shown in Schemes
1 and 2.
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Starting from the phosphonated bipyridine as a ligand, these complexes were prepared
using Ru(DMSO)4Cl2 and RuCl3 in water to get highly pure complexes. This could be clearly seen
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1
6
from the HNMR spectrum of the crude reaction mixture that indicated a lower amount of
7
impurities when Ru (DMSO)4Cl2 was used. This is probably due to the use of a ruthenium (II)
precursor with weekly binding ligands (DMSO). A purification step was necessary to remove trisꢀ
bipyridine ruthenium complex that was formed during the reaction. The resulting complexes were
purified and crystallized using column chromatography. The diethyl ester phosphonate groups
were fully hydrolyzed by heating the complexes in hydrochloric acid solution. Chloride ligand
exchange with thiocyanate or cyanide was subsequently performed in water with an excess of
monodenate ligands. These complexes were found to be soluble in methanol and water with a
larger solubility at basic pH.
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The NMR spectra of the complexes were measured in D2O and CD3OD solutions to
achieve sufficiently large concentrations, compatible with fast spectrum recording and high
1
signalꢀtoꢀnoise ratio. The H NMR spectra of all complexes show six sharp and wellꢀresolved
signals in the aromatic region, corresponding to the six magnetically inequivalent protons of the
bipyridine (Figure 5). Using D2O as a solvent, the downfield shifted protons resonance peaks can
be assigned to the protons H3 and H4 that are merged in one peak. On the other hand, upon using
CD3OD as solvent, H3 and H4 peaks were found to be very close to each other. The assignment is
based on the assumption that deshielding of the protons can be due to an induced magnetic field
created by the ring current on bipyridine aromatic moieties. This deshielding is only significant
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at short distance and therefore affects only protons that are close to the bipyridine. In this case,
we found that the complexes are cisꢀoctahedral, with two equivalent bipyridine ligands. In
complex (I) in Scheme 2, the two 6, 6’ꢀ methylene phosphonate ethyl ester groups (CH2PO3Et)
are inequivalent and they give rise to four multiplets. Therefore, the observed patterns are in
agreement with the cis geometry of the complexes. The results were also confirmed via 13C and
31P NMR spectra, which were recorded in D2O and CD3OD as solvents, see the supporting
information. The Trans geometry would have resulted in simpler NMR due to the higher
symmetry and would have yielded only three peaks in the aromatic region due to the four
magnetically equivalent pyridine units.
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Infrared (IR) spectra were recorded in KBr pellets, and the vCN bands of complex (III)
and (IV) were located, respectively at 2052 cmꢀ1 and 2113 cmꢀ1, see the supporting information
for more details. It was demonstrated that the band appears around 2110 cmꢀ1 can be assigned to
Nꢀbound thiocyante, (25,26). Note that it is shifted to lower energy for the Sꢀbound complex to
2052 cmꢀ1.
To validate the results, dyeꢀsensitized solar cell devices were fabricated using the
synthesized complexes as sensitizers to TiO2 nanotubes under AM 1.5 illumination (Figure 6 and
Table 4), as detailed in our previous report. Note that the power conversion efficiency (PCE)
increases in the order –SCN > ꢀCN > ꢀCl, resulting in PCE of 1.91, 2.38 and 2.9%, respectively.
Note that those efficiencies are at least twice that reported for the same complexes with the
substituents in the 4ꢀ and 5ꢀ positions 27, confirming the importance of the position of the
functional group in determining the performance of the Ruꢀcomplexes.
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Conclusion
DFT and TDDFT calculations were performed on cisꢀ[Ru(bipyridine)2 (phosphonic
acid)2]X2 where X= Cl, CN complexes. Due to geometrical orientation of the phosphate groups,
two hydrogen bonds with average length of 1.77 Å were observed in the transꢀconfiguration with
only one hydrogen bond with 1.73 Å is formed in case of the cisꢀconfiguration. The order of
reactivity as function of ꢁE was found to be in the order Cl > NSC > CN and NSC > Cl > CN for
transꢀ and cisꢀ configurations, respectively. The dipole moment of the cisꢀconfiguration is larger
than that of the transꢀcounterpart. The NCS complex shows the smallest optical band gap, the
highest oscillator strength and the lowest exciton binging energy followed by the Cl and CN
complexes, respectively, indicating a better performance for use as a sensitizer in solar cells
application. The CN and NCS have three bands at 385 nm, 455 nm and 505 nm, and 461 nm, 610
nm and 671 nm, respectively. These complexes were found to be soluble in methanol and water
with a larger solubility at basic pH. The cisꢀconfiguration of such novel ruthenium complexes was
prepared and characterized by several spectroscopic methods. Upon their use as photosensitizers in
DSSC devices, the power conversion efficiency (PCE) increases in the order –SCN > ꢀCN > ꢀCl,
resulting in PCE of 1.91, 2.38 and 2.9%, respectively. We hope that our work will open a new
route toward the synthesis and use of optically active Ruꢀcomplexes based on phosphonate
functional groups compared to carboxylic counterparts.
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Acknowledgement
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27
The Authors acknowledge the financial support from The Science and Technology Development
Fund (STDF) provided by the Egyptian Government (Grant # 5415) and the IRD provided by the
French Government (Grant # 005ꢀ2013)
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Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L.
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Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V.
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Coreil, and Melisa A. Hackney. J.Org.Chem.1982, 47, 4116ꢀ 4120.
24. Nazeeruddin, M. K.; Liska, P.; Vlachopoulos, N.; Gratzel, M. J. Am. Chem. Soc. 1993,
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Chem. 1994, 33, 5741ꢀ5749.
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26. Gratzel, M.; Kohle, O.; Nazeeruddin, M. K.; Pechy, P.; Rotzinger, F. P.; Ruile, S.;
Zakeeruddin, S. M. In PCT Int. Appl.; Ecole Polytechnique Federale de Lausanne,
Switz.: WO 95 29, 924, Nov. 1995; 52 pp.
27. Katja Neuthe, Florian Bittner, Frank Stiemke, Benjamin Ziem, Juan Du, Monika Zellner,
Michael Wark, Thomas Schubert, Rainer Haag; Dyes and Pigments 104 (2014) 24ꢀ33.
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Figures
(a)
(b)
(c)
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Figure 1. The optimized geometry of cisꢀRu Complexes, bond lengths are shown in angstrom.
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Figure 2. The optimized geometry of transꢀRu Complexes, bond lengths are shown in angstrom.
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Wavelength, nm
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4
Figure 3. The theoretical electronic absorption spectra of the studied complexes
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HOMO
LUMO
(a)
(b)
(c)
Figure 4. The HOMO and LUMO plots for (a) Cl, (b) NCS and (c) CN complexes
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Scheme 1: Reaction and conditions: i) PꢀCPBA, CHCl3 at 0 oC 2 d ii) KCN, H2O, PhCOCl,
DCM, 12 h iii) EtOH, H2SO4, reflux,12 h iv) NaOH, EtOH, Reflux 12 h v)NaBH4, DCM, r.t 6 h
vi) HBr, H2SO4, reflux, 8 h vii) P(OEt)2, CHCl3, reflux 5 h.
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O
P
O
N
N
N
O O
P
O
O
Cl
Cl
DMF
O
Ru(DMSO)4Cl2
N
+
N
O
P
Ru
N
O
reflux
O
P
O
O
O
O
P
P
O
O
O
O
( I )
O
P
OH
O
P
OH
N
N
N
N
N
N
OH
OH
HO P
O
OH
OH
HO P
O
HCl /H2O
reflux
Cl
Cl
CN
CN
KCN /MeOH
reflux
Ru
N
Ru
N
OH
OH
P
O
O
P
HO
HO
P
OH
OH
P
OH
OH
O
O
( II )
( III )
O
P
OH
N
N
N
OH
OH
HO P
O
NCS
Ru
OH
P
O
NCS
P
HO
OH
OH
N
O
( IV )
1
2
3
4
Scheme 2: Stepwise synthesis of the Ruꢀbased complexes
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Figure 5. 1HNMR spectra of complex I and II recorded in D2O andCD3OD
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Figure 6. J−V characteristics of the solar cell devices measured under the irradiance of AM 1.5G
full sunlight (100 mW cm‑2) with a cell active area of 0.4 cm2
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Tables
Table 1: The total energy ET, difference in total energy ꢁET in Hartree, ELUMO, EHOMO, ꢁE in eV
and dipole moment ꢂ (Debye) for complexes.
Configuration
Complex
ET,(a.u.)
Trans
NCS
Cis
NCS
Cl
CN
ꢀ3698.63057783
Cl
CN
ꢀ3698.63603973
ꢀ5062.31702835
ꢀ4495.03286616
ꢀ5062.3544477
ꢀ4495.04200812
ꢁET(kcal/mol) 23.4810
5.7367
ꢀ2.951
ꢀ5.834
2.883
4.02
3.4274
ꢀ2.957
ꢀ6.446
3.489
3.59
0.0
0.0
0.0
ELUMO, eV
EHOMO, eV
ꢁE, eV
ꢀ2.358
ꢀ5.065
2.706
4.39
ꢀ2.207
ꢀ4.983
2.776
12.89
ꢀ2.9219
ꢀ5.3938
2.472
ꢀ2.856
ꢀ5.897
3.041
15.93
ꢂ, D
16.30
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Table 2: The Ionization potential IE, Electron Affinity EA, energy of the first excited singlet
state S1, its Oscillator strength and Exciton binding energy for cisꢀcomplexes
Cis Complexes
IP, eV
Cl
NCS
6.510
CN
7.019
5.901
EA, eV
TDꢀDFT S1
f
ꢀ1.126
1.9864
0.0075
0.7896
ꢀ1.1792
1.8205
0.0171
0.6515
ꢀ1.704
2.2186
0.0017
0.8224
Eexciton
4
5
6
Table 3: The TDDFT results for singlet excited states with the highest oscillator strength.
Compound State No.
Main
Coefficient f
λ, nm
Configuration
H →L
Hꢀ1→L
Hꢀ2→L
H →L
Cl
S1
S3
S4
S1
S4
S6
S8
S1
S2
S8
S10
0.61083
0.55092
0.57511
0.68420
0.57914
0.60695
0.42140
0.67866
0.63450
0.41559
0.41978
0.0075
624.2
558.9
530.2
558.8
499.9
454.6
387.9
681.0
618.3
471.6
461.5
0.0284
0.0521
0.0017
0.0219
0.0142
0.0184
0.0171
0.0161
0.0081
0.0131
CN
Hꢀ2→L
Hꢀ1→L+1
H →L+4
H →L
H →L+1
H →L+2
H →L+2
NCS
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12
Table 4: Comparison between photovoltaic parameters of the DSSCs prepared using different
complexes.
Dye
Cl
Jsc [mA cmꢀ2]
3.78
Voc [V]
0.67
FF
η [%]
1.91
2.38
2.9
0.75
0.76
0.77
CN
SCN
5.47
6.27
0.57
0.60
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Graphical Abstract