Fluorous Molecules for Dye-Sensitized Solar Cells: Synthesis andCharacterization of Fluorene-Bridged Donor/Acceptor Dyes withBulky Perfluoroalkoxy SubstituentsGabriela Marzari,† Javier Durantini,† Daniela Minudri,† Miguel Gervaldo,† Luis Otero,†
Fernando Fungo,*,† Gianluca Pozzi,*,‡ Marco Cavazzini,‡ Simonetta Orlandi,‡ and Silvio Quici‡
†Departamento de Química, Universidad Nacional de Río Cuarto, Agencia Postal No 3, X5804BYA, Río Cuarto, Argentina‡Istituto di Scienze e Tecnologie Molecolari del Consiglio Nazionale delle Ricerche, ISTM-CNR, via Golgi 19, 20133 Milano, Italy
ABSTRACT: A series of structurally related sensitizerscontaining fluorene as a π-conjugated central core have beensynthesized, characterized, and applied in the development ofdye-sensitized solar cells (DSSCs). The new electron donor(diphenylamine)−acceptor (cyanoacrylic acid) D-π-A molec-ular structures hold perfluoro-tert-butyl substituents in differ-ent positions. It is demonstrated that the fluorous substitutionpattern remarkably affects the behavior of dyes as photo-sensitizers in DSSCs, leading to an improvement in the powerconversion efficiencies with respect to analogous nonfluorousmolecules.
■ INTRODUCTION
The continuous growth of energy demand around the worldand the environmental pollution resulting in global warminghave led to a greater focus on research in renewable energysources over the past decades. Contrary to the fossil fuels, solarenergy is available profusely in most of the world’s regions; still,it can be used for direct electricity production by means ofphotovoltaic and photoelectrochemical cells. In this frame dye-sensitized solar cells (DSSCs) are currently the most efficientand stable excitonic photocells,1,2 offering several advantagesover traditional silicon-based devices, notably, simplicity infabrication and reduction in production costs. These uniquefeatures give the chance to develop DSSC technology even incountries with lower industrialization levels.The design of suitable photosensitizers remains one of the
most important research topics in DSSCs with regard forimproving their performance.3 To this end a number of light-absorbing compounds with different molecular structures havebeen proposed and tested,2 starting with Ru(II) polypyridylcomplexes, which are still now one of the most successful classof photosensitizers.4 However, the limited availability of Ru andpossible environmental problems could limit its extensiveapplication. Furthermore, these complexes are relativelyexpensive and hard to purify compared with organic sensitizersthat can be obtained at reasonable cost and high purity gradethrough well-established synthetic techniques. Organic dyespresent other potential advantages, including the huge diversityof molecular structures and high molar extinction coefficients,generally superior to those of Ru dyes (<20 000 M−1 cm−1).This allows using thinner spaced nanostructured oxidesemiconductor films without losing light harvesting efficiency,
a key factor in solid-state DSSC development. Anotherimportant aspect that is feasible to control with organic dyesis the chance to construct semitransparent and multicolor solarcells that can be used, for example, in power-producingwindows. DSSCs with dyes that are transparent over a region ofthe visible spectrum would allow part of visible light to enter abuilding while converting the adsorbed portion of the solarradiation into electricity. Thus, this kind of cells can be thetarget of a high-value market segment.5
Generally, metal-free organic photosensitizers possessmolecular structures comprised of a donor part (D) and anacceptor counterpart (A) bridged by a π-conjugated link-age.6−11 As a class of donors, triphenylamine (TPA) and itsderivatives have shown promising applications in the develop-ment of photovoltaic devices,12 whereas cyanoacetic acidmoiety is one of the most used units as an electron acceptor/anchoring group in the design of organic dyes for highlyefficient DSSCs.13 Irradiation of these dipolar moleculesgenerates photoinduced intramolecular charge transfer states,which are able to inject electrons to the TiO2 conduction band.The preferential orientation of the dye anchored to the TiO2
surface not only improves charge injection but also keeps thephotooxidized donor away from photoinjected electrons. Thisfact diminishes the deleterious back electron transfer process.Several dipolar metal-free dyes have been reported as sensitizersin DSSCs in recent years.14−16 For this kind of dye, theabsorption region responding to the solar spectrum is
Received: June 15, 2012Revised: August 22, 2012Published: September 21, 2012
determined by the nature of the D and A moieties combinedwith π-conjugated central cores (D-π-A). The presence of anextended π-conjugation core to bridge D and A units, which redshifts the absorption spectra of the dye, is helpful in order tomaximize the absorption of solar radiation. As a consequence ofthis molecular design strategy, organic sensitizers are oftencharacterized by prolonged rodlike structures with inherentaggregation tendency. These can result in self-quenchingbetween dye molecules in the excited state, one of the majorfactors for the low light to energy conversion efficiency shownby many organic dyes in DSSCs. Introduction of a nonplanararchitecture into the organic framework of the dye isinstrumental to overcome this drawback. On the basis of thisapproach, we have recently reported a series of spirobifluorenecore D-π-A sensitizers for DSSCs, with efficient binding to theTiO2 surface and suppression of aggregation phenomena,
making these dyes promising candidates for use in efficientenergy conversion devices.17
Structurally simpler D-π-A sensitizers featuring fluorene as aπ-conjugated central core have been also successfully used inDSSCs.18 This basic molecular setup can be further refined bythe insertion of a heteromatic ring (e.g., thiophene) betweenfluorene and cyanoacrylic acid (A).19,20 In this case theelectron-rich fluorene (or spirobifluorene) unit not only acts asa π-conjugated bridge but also as a secondary electron donor inaddition to the arylamine moiety (D) giving rise to a D-D-π-Amolecular configuration.6,21 This dual role enhances theelectron-donating ability of donors and facilitates intra-molecular charge transfer, increasing the performance of thesolar conversion devices.Highly fluorinated organic compounds, including fluorous
molecules that are characterized by the presence of extended
Chart 1. Fluorene-Bridged Donor/Acceptor Dyes
Scheme 1. Synthesis of Fluorene Dyes
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saturated fluorocarbon domains, have been already successfullyexplored in novel functional materials,22 in electronic andoptoelectronic applications,23 and in energy storage andconversion devices.24 In the photovoltaic field, highlyfluorinated molecules and polymers with improved stabilityand tailored highest-occupied molecular orbital (HOMO)−lowest-unoccupied molecular orbital (LUMO) energy statesresulting from the electron-withdrawing character of fluorinesubstituents have been recently proposed as active componentsof p−n heterojunctions.25 Morphology control at differentscales in electron acceptor−donor blends can be attainedthanks to the unique phase properties of fluorous molecules,leading to improved photovoltaic performance in bulkheterojunction polymer solar cells.26 In the case of DSSCs,highly fluorinated ionic additives have been used incombination with solid hole-transporters in order to facilitatecharge compensation,27 whereas the long-term thermal andlight soaking stability of the devices have been increased byusing a gel electrolyte based on a stable polyfluorinatedcopolymer.28 However, only two examples of fluorous dyessuitable for DSSCs, both containing transition metals, havebeen reported so far.29,30 Different from what was observed inthe case of dyes bearing single fluorine atoms directly attachedto the π-conjugated core,31 possible electron-withdrawingeffects associated with the introduction of saturated fluo-rocarbon domains had little or no impact on the photophysicaland photovoltaic performances of fluorous dyes.As a part of our ongoing research program on the application
of fluorous materials in solar cells,32 we here report thesynthesis and photophysical and electrochemical character-ization of a series of simple D-π-A dyes (Chart 1) with fluorenecore holding fluorophilic perfluoro-tert-butyl substituents indifferent positions, as well as their behavior as photosensitizersin DSSCs. It will be shown that the fluorous substitutionpattern affects the photosensitizers performance, leading toimproved power conversion efficiencies with respect tostructurally related nonfluorous dyes.
■ EXPERIMENTAL METHODS
Synthesis of Dyes. General Remarks. An overview of thesynthesis is depicted in Schemes 1 and 2. All commerciallyavailable chemicals were used as received without furtherpurification. Solvents were purified by standard methods anddried if necessary. 2,7-Dibromo-9,9-dibutyl-9H-fluorene 6,33
9 ,9 -b i s (3 -b romopropy l ) -2 ,7 -d i iodo -9H -fluorene , 3 4
(CF3)3CONa, and tosylate 1635 were prepared as described inthe literature. Reactions were monitored by thin layerchromatography (TLC) that was conducted on plates pre-coated with silica gel Si 60-F254 (Merck, Germany). Columnchromatography was carried out on silica gel SI 60 (Merck,Germany), mesh size 0.063−0.200 mm (normal) or 0.040−0.063 mm (flash). 1H NMR and 13C NMR were recorded on a
Bruker Avance 400 spectrometer (400 and 100.6 MHz,respectively); 19F NMR spectra were recorded on a BrukerAC 300 spectrometer (282 MHz). Electrospray ionization(ESI) mass spectra were obtained with an ICR-FTMS APEX II(Bruker Daltonics) mass spectrometer. Elemental analyses werecarried out by the Departmental Service of Microanalysis(University of Milano).
9,9-Bis{3-[1,1,1,3,3,3-hexafluoro-2-(trifluoromethyl)-propan-2-yloxy]propyl}-2,7-diiodo-9H-fluorene (7). A flame-dried Schlenk tube was charged with 9,9-bis(3-bromopropyl)-2,7-diiodo-9H-fluorene (1.25 g, 1.9 mmol), (CF3)3CONa (1.47g, 5.7 mmol), and dry dimethylformamide (DMF, 6.0 mL),evacuated, and backfilled with nitrogen. The reaction mixturewas stirred overnight at 110 °C, after which the resulting yellowsuspension was allowed to reach room temperature. H2O (30mL) and diethyl ether (Et2O, 40 mL) were added, and thelimpid liquid layers were separated. The aqueous phase wasextracted with Et2O (3 × 15 mL), and the combined organicphases were washed with H2O and brine and dried overMgSO4. After removal of the solvent at reduced pressure, theresidue was purified by column chromatography (silica gel,hexane) affording the title compound as an off-white solid (1.49g, 81% yield). 1H NMR (400 MHz, CDCl3): δ [ppm] = 7.70(dd, JHH = 8.0, 1.5 Hz, 2H), 7.64 (d, JHH = 1.4 Hz, 2H), 7.43 (d,JHH = 8.0 Hz, 2H), 3.71 (t, JHH = 6.0 Hz, 4H), 2.10−2.04 (m,4H), 0.99−0.89 (m, 4H). 13C NMR (100.6 MHz, CDCl3): δ[ppm] = 150.4, 139.8, 136.8, 131.9, 121.7, 120.3 (q, JCF = 292Hz), 93.5, 79.4 (m), 69.3, 54.3, 35.6, 24.2. 19F NMR (282 MHz,CDCl3): δ [ppm] = −71.2 (s). Anal. Calcd for C27H18F18I2O2:C 33.42, H 1.87. Found: C 33.48, H 1.91.
7-Bromo-9,9-dibutyl-9H-fluorene-2-carbaldehyde (8). Asolution of n-butyl lithium in hexane (1.6 M, 3.44 mL, 5.5mmol) was added dropwise over 15 min to a stirred solution offluorene 6 (2.18 g, 5 mmol) in dry tetrahydrofuran (25 mL) at−78 °C under nitrogen. The resulting mixture was stirred foran additional hour at this temperature, and then dry DMF (0.58mL, 7.5 mmol) was added dropwise over 5 min. The finalreaction mixture was allowed to attain room temperature andstirred for 2 h, followed by quenching with 5% aq HCl (5 mL)and extraction of the aqueous phase with Et2O (3 × 10 mL).The combined organic layers were washed with H2O and brineand dried over MgSO4. The solvent was evaporated underreduced pressure, and the residue was purified by flash columnchromatography (silica gel, petroleum ether/CH2Cl2 7/3)affording the title compound as a clear oil that slowly solidifiedon standing (1.51 g, 78% yield). 1H NMR (400 MHz, CDCl3):δ [ppm] = 10.06 (s, 1H), 7.87−7.79 (m, 3H), 7.64 (d, JHH =8.0 Hz, 1H), 7.53−7.49 (m, 2H), 2.08−1.91 (m, 4H), 1.14−1.01 (m, 4H), 0.67 (t, JHH = 7.4 Hz, 6H), 0.61−0.48 (m, 4H).13C NMR (100.6 MHz, CDCl3): δ [ppm] = 192.2, 154.2, 151.1,146.3, 138.5, 135.6, 130.6, 130.4, 126.4, 123.1, 122.2, 120.1,
Scheme 2. Synthesis of the Fluorous Diarylamine Precursor
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55.5, 39.9, 25.8, 22.9, 13.7. Anal. Calcd for C22H25BrO: C68.57, H 6.54. Found: C 68.53, H 6.55.9,9-Bis{3-[1,1,1,3,3,3-hexafluoro-2-(trifluoromethyl)-
propan-2-yloxy]propyl}-7-iodo-9H-fluorene-2-carbaldehyde(9). The title compound was obtained in 45% yield fromfluorene 7 following the procedure described above forcompound 8. The crude product was purified by flash columnchromatography (silica gel, hexane/Et2O 3/1). 1H NMR (400MHz, CDCl3): δ [ppm] = 10.02 (s, 1H), 7.91 (dd, JHH = 8.0,1.1 Hz, 1H), 7.85−7.83 (m, 2H), 7.77 (dd, JHH = 8.0, 1.4 Hz,1H), 7.73 (d, JHH = 1.1 Hz, 1H), 7.54 (d, JHH = 8.0 Hz, 1H),3.70 (t, JHH = 6.0 Hz, 4H), 2.27−2.07 (m, 4H), 1.01−0.85 (m,4H). 13C NMR (100.6 MHz, CDCl3): δ [ppm] = 191.6, 152.2,148.9, 146.2, 139.2, 137.1, 136.2, 132.3, 130.8, 123.0, 122.7,120.2 (q, JCF = 292 Hz), 120.5, 95.2, 69.2, 54.3, 35.5, 24.2. 19FNMR (282 MHz, CDCl3): δ [ppm] = −71.2 (s). Anal. Calcdfor C28H19F18IO3: C 38.55, H 2.20. Found: C 38.50, H 2.22.9,9-Dibutyl-7-(diphenylamino)-9H-fluorene-2-carbalde-
hyde (10). A flame-dried Schlenk tube was charged withfluorene 8 (154 mg, 0.4 mmol), diphenylamine (102 mg, 0.6mmol), Cs2CO3 (292 mg, 0.9 mmol), and Pd(OAc)2 (2.3 mg,0.001 mmol), evacuated, and backfilled with nitrogen. Dry,degassed toluene (2.0 mL) and PtBu3 (1 M in toluene, 22 μL,0.02 mmol) were subsequently added under nitrogen. TheSchlenk tube was sealed, and the reaction mixture was stirred at110 °C for 10 h. After cooling to room temperature thereaction mixture was diluted with CH2Cl2 and treated withsaturated aqueous NH4Cl. The organic phase was separated,washed with H2O and brine, and dried over MgSO4. Thesolvent was evaporated under reduced pressure, and the residuewas purified by flash column chromatography (silica gel,CH2Cl2/hexane 3/2) affording the title compound as a yellowfoam (153 mg, 81% yield). 1H NMR (400 MHz, CDCl3): δ[ppm] = 10.02 (s, 1H), 7.83−7.80 (m, 2H), 7.71 (d, JHH = 8.1Hz, 1H), 7.61 (d, JHH = 8.3 Hz, 1H), 7.31−7.24 (m, 4H),7.15−7.02 (m, 8H), 2.00−1.80 (m, 4H), 1.13−1.01 (m, 4H),0.69 (t, JHH = 7.3 Hz, 6H), 0.64−0.58 (m, 4H). 13C NMR(100.6 MHz, CDCl3): δ [ppm] = 192.2, 153.6, 151.3, 148.9,147.6, 147.5, 134.5, 134.0, 130.8, 129.3, 124.4, 123.1, 122.8,122.7, 121.6, 119.1, 118.2, 55.0, 39.7, 26.0, 22.9, 13.8. Anal.Calcd for C34H35NO: C 86.22, H 7.45, N 2.96. Found: C 86.24,H 7.51, N 2.927-[Bis(4-methoxyphenyl)amino]-9,9-dibutyl-9H-fluorene-
2-carbaldehyde (11). The title compound was obtained in 61%yield from fluorene 8 and bis(4-methoxyphenyl)aminefollowing the procedure described above for compound 10.The crude product was purified by flash column chromatog-raphy (silica gel, petroleum ether/CH2Cl2 1/1).
propan-2-yloxy]ethoxy]phenyl] amino}-9,9-dibutyl-9H-fluo-rene-2-carbaldehyde (12). The title compound was obtainedin 80% yield from fluorene 8 and amine 15 (see below)following the procedure described above for compound 10.
7-(Diphenylamino)-9,9-bis{3-[1,1,1,3,3,3-hexafluoro-2-(trifluoromethyl)propan-2-yloxy] propyl}-9H-fluorene-2-car-baldehyde (13). The title compound was obtained in 73%yield from fluorene 9 and diphenylamine following theprocedure described above for compound 10. The crudeproduct was purified by flash column chromatography (silicagel, hexane/MTBE 85/15). 1H NMR (400 MHz, CDCl3): δ[ppm] = 10.01 (s, 1H), 7.86 (dd, JHH = 7.8, 1.3 Hz, 1H), 7.79(br s, 1H), 7.73 (d, JHH = 7.8 Hz, 1H), 7.62 (d, JHH = 8.3 Hz,1H), 7.30−7.24 (m, 4H), 7.14−7.04 (m, 8H), 3.70 (t, JHH = 6.4Hz, 4H), 2.13−1.88 (m, 4H), 1.10−0.88 (m, 4H). 13C NMR(100.6 MHz, CDCl3): δ [ppm] = 192.7, 151.6, 149.5, 149.1,147.4, 134.7, 133.4, 131.2, 129.4, 124.7, 123.6, 122.8, 122.7,120.2 (q, JCF = 293 Hz), 122.0, 119.4, 116.9, 111.3, 79.7 (m),69.5, 53.9, 35.5, 24.4. 19F NMR (282 MHz, CDCl3): δ [ppm] =−71.2 (s). Anal. Calcd for C40H29F18NO3: C 52.58, H 3.20, N1.53. Found: C 52.56, H 3.24, N 1.50.
7-{Bis[4-[2-[1,1,1,3,3,3-hexafluoro-2-(trifluoromethyl)-propan-2-yloxy]ethoxy]phenyl] amino}-9,9-bis{3-[1,1,1,3,3,3-hexafluoro-2-(trifluoromethyl)propan-2-yloxy]propyl}-9H-fluorene-2-carbaldehyde (14). The title compound wasobtained in 49% yield from fluorene 9 and amine 15 (seebelow) following the procedure described above for compound10. The crude product was purified by flash columnchromatography (silica gel, hexane/AcOEt 85/15). 1H NMR(400 MHz, CDCl3): δ [ppm] = 9.99 (s, 1H), 7.84 (dd, JHH =7.8, 1.1 Hz, 1H), 7.77 (br s, 1H), 7.69 (d, JHH = 7.8 Hz, 1H),7.55 (d, JHH = 8.4 Hz, 1H), 7.07 (d, JHH = 8.9 Hz, 4H), 6.94(dd, JHH = 8.4, 2.1 Hz, 1H), 6.89 (d, JHH = 2.1 Hz, 1H), 6.85 (d,JHH = 8.9 Hz, 4H), 4.36 (t, JHH = 4.5 Hz, 4H), 4.20 (t, JHH = 4.5Hz, 4H), 3.74−3.66 (m, 4H) 2.09−1.91 (m, 4H), 1.06−0.88(m, 4H). 13C NMR (100.6 MHz, CDCl3): δ [ppm] = 191.8,155.0, 151.7, 150.3, 149.0, 147.8, 141.1, 134.5, 132.0, 131.3,126.7, 122.6, 121.9, 120.3 (q, JCF = 290 Hz), 120.2, 119.1,115.8, 114.2, 79.6 (m), 69.6, 68.3, 66.6, 53.9, 35.6, 24.5. 19FNMR (282 MHz, CDCl3): δ [ppm] = −71.2 (s), −71.3 (s).Anal. Calcd for C52H35F36NO7: C 42.49, H 2.40, N 0.95.Found: C 42.38, H 2.24, N 0.88.
(E)-2-Cyano-3-[9,9-dibutyl-7-(diphenylamino)-9H-fluoren-2-yl]acrylic acid (1). A solution of aldehyde 10 (237 mg, 0.5mmol), cyanoacetic acid (85 mg, 1 mmol), and ammoniumacetate (15 mg, 0.2 mmol) in AcOH (10 mL) was stirred underreflux for 6 h. The solvent was evaporated under reducedpressure, and the residue was taken up in CH2Cl2, washedextensively with H2O, and dried over MgSO4. After removal ofthe solvent at reduced pressure, the crude product was furtherpurified by column chromatography (silica gel, CH2Cl2, andthen CH2Cl2/MeOH 95/5 and CH2Cl2/MeOH 9/1) affordingthe title compound as a dark-red solid (254 mg, 94% yield).).
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24.8. 19F NMR (282 MHz, DMSO-d6): δ [ppm] = −72.6 (s).UV−vis (ClCH2CH2Cl): λmax (log ε) = 440 (4.17) nm. Anal.Calcd for C43H30F18N2O4: C 52.66, H 3.08, N 2.86. Found: C52.63, H 3.31, N 2.81. MS (ESI−): m/z calcd. forC43H30F18N2O4: 980.2. Found 979.5 [M−H]−.
( E ) - 3 - { 7 - [ B i s [ 4 - [ 2 - [ 1 , 1 , 1 , 3 , 3 , 3 - h e x a fluo r o - 2 -(trifluoromethyl)propan-2-yloxy]ethoxy] phenyl]amino]-9,9-bis[3-[1,1,1,3,3,3-hexafluoro-2-(trifluoromethyl)propan-2-yloxy] propyl]-9H-fluoren-2-yl}-2-cyanoacrylic acid (5). Thetitle compound was obtained in 91% yield from aldehyde 14following the procedure described above for compound 1. 1HNMR (400 MHz, CDCl3): δ [ppm] = 8.32 (s, 1H), 8.01−7.96(m, 2H), 7.67 (d, JHH = 8.0, Hz, 1H), 7.55 (d, JHH = 8.5 Hz,1H), 7.07 (d, JHH = 8.8 Hz, 4H), 6.95−6.84 (m, 5H), 4.35 (br s,4H), 4.20 (t, JHH = 4.5 Hz, 4H), 3.72 (t, JHH = 5.7 Hz, 4H),2.10−1.89 (m, 4H), 1.11−0.93 (m, 4H). 13C NMR (100.6MHz, DMSO-d6/CDCl3 2/1 v/v): δ [ppm] = 164.4, 155.0,151.5, 150.3, 149.7, 149.0, 144.5, 140.9, 132.5, 131.0, 130.2,126.7, 124.6, 122.1, 120.3 (q, JCF = 293 Hz), 120.1, 119.8,119.1, 116.1, 114.5, 70.4, 69.5, 66.8, 53.8, 35.0, 24.7. 19F NMR(282 MHz, CDCl3): δ [ppm] = −71.2 (s), −71.3 (s). UV−vis(ClCH2CH2Cl): λmax (log ε) = 475 (4.65) nm. Anal. Calcd forC55H36F36N2O8: C 42.98, H 2.36, N 1.82. Found: C 42.77, H2.30, N 1.74. MS (ESI+): m/z calcd. for C55H36F36N2O8:1536.2. Found 1537.2 [M+H]+.
9,9-Dibutyl-7-(diphenylamino)-9H-fluorene-2-carboxylicacid (19). Aldehyde 10 (189 mg, 0.4 mmol) was dissolved inacetone (20 mL), and the solution was cooled to 0 °C. Sulfamicacid (58 mg, 0.6 mmol) and H2O (4 mL) were added undervigorous stirring, followed by 80% NaOCl (46 mg, 0.4 mmol).After stirring for 2 h at 0 °C the mixture was brought to roomtemperature and further stirred for 4 h. CH2Cl2 (30 mL) andsolid Na2SO3 (50 mg) were added. The organic phase wasseparated, washed with H2O and brine, and dried over MgSO4.Solvents were evaporated under reduced pressure, and theresidue was purified by flash column chromatography (silica gel,hexane/AcOEt 3/2) affording the title compound as a paleyellow solid (155 mg, 79% yield). 1H NMR (400 MHz,CD2Cl2): δ [ppm] = 8.14 (dd, JHH = 8.1, 1.1 Hz, 1H), 8.06 (s,1H), 7.69 (d, JHH = 8.1 Hz, 1H) 7.64 (d, JHH = 8.1 Hz, 1H)7.31−7.27 (m, 5H), 7.17−7.14 (m, 4H), 7.10−7.05 (m, 3H),2.00−1.84 (m, 4H), 1.14−1.04 (m, 4H), 0.72 (t, JHH = 7.5 Hz,6H), 0.69−0.61 (m, 4H). 13C NMR (100.6 MHz, CD2Cl2): δ[ppm] = 172.2, 153.5, 150.7, 148.6, 147.7, 146.7, 134.4, 129.8,129.1, 126.6, 124.4, 124.3, 123.0, 122.9, 121.5, 118.8, 118.5,55.1, 39.8, 26.0, 22.9, 13.9. UV−vis (ClCH2CH2Cl): λmax (logε) = 381 (4.23) nm. Anal. Calcd for C34H35F18NO2: C 83.40, H7.20, N 2.86. Found: C 83.11, H 7.37, N 2.79. MS (ESI−): m/zcalcd. for C43H30F18N2O4: 489.3. Found 488.3 [M−H]−.
1-{2-[1,1,1,3,3,3-Hexafluoro-2-(trifluoromethyl)propan-2-yloxy]ethoxy}-4-iodobenzene (17). A mixture of 4-iodophenol(2.20 g, 10 mmol), tosylate 16 (4.35 g, 10 mmol), K2CO3 (4.14g, 30 mmol), and KI (0.08 g, 0.5 mmol) in acetone (30 mL)was stirred under reflux overnight. The mixture was cooled toroom temperature and treated with CH2Cl2 (80 mL) and H2O(30 mL). The organic phase was washed with 10% aq NaOHand brine and dried over MgSO4. After removal of the solventat reduced pressure, the crude product was purified by columnchromatography (silica gel, hexane/CH2Cl2 9/1) affording thetitle compound as a white solid (3.61 g, 75% yield). 1H NMR(400 MHz, CDCl3): δ [ppm] = 7.56 (d, JHH = 9.0 Hz, 2H),6.68 (d, JHH = 9.0 Hz, 2H), 4.33 (t, JHH = 4.5 Hz, 2H), 4.16 (t,JHH = 4.5 Hz, 2H). 13C NMR (100.6 MHz, CDCl3): δ [ppm] =
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158.3, 138.3, 120.3 (q, JCF = 292 Hz), 117.1, 83.6, 79.9 (m),68.0, 66.3. 19F NMR (282 MHz, CDCl3): δ [ppm] = −70.8 (s).Anal. Calcd for C12H8F9IO2: C 29.90, H 1.67. Found: C 29.78,H 1.71.t e r t - Bu t y l - b i s { 4 - [ 2 - [ 1 , 1 , 1 , 3 , 3 , 3 -h e xafluo ro - 2 -
(trifluoromethyl)propan-2-yloxy]ethoxy] phenyl}carbamate(18). A flame-dried Schlenk tube was charged with iodide 17(482 mg, 1.0 mmol), CuI (9.6 mg, 0.050 mmol), tert-butylcarbamate (56 mg, 0.48 mmol), and K3PO4 (430 mg, 2.03mmol), evacuated, and backfilled with nitrogen. N,N-Dimethylethylenediamine (22 μL, 0.20 mmol) and dry toluene(1.0 mL) were added under nitrogen. The Schlenk tube wassealed, and the reaction mixture was stirred at 110 °C for 23 h.The resulting pale brown suspension was allowed to reachroom temperature and filtered through a short pad of Celite,eluting with 10 mL of Et2O. The filtrate was concentrated, andthe residue was purified by flash column chromatography (silicagel, hexane/MTBE 4/1) affording the title compound as awhite foam (346 mg, 84% yield). 1H NMR (400 MHz, CDCl3):δ [ppm] = 7.13 (d, JHH = 8.9 Hz, 2H), 6.68 (d, JHH = 8.9 Hz,2H), 4.33 (t, JHH = 4.5 Hz, 2H), 4.16 (t, JHH = 4.5 Hz, 2H),1.44 (s, 9H). 13C NMR (100.6 MHz, CDCl3): δ [ppm] =156.0, 154.1, 136.9, 130.0, 120.3 (q, JCF = 292 Hz), 114.8, 80.9,79.5 (m), 68.1, 66.5, 28.2. 19F NMR (282 MHz, CDCl3): δ[ppm] = −71.1 (s). Anal. Calcd for C29H25F18NO6: C 42.19, H3.05, N 1.70. Found: C 42.41, H 3.00, N 1.73.Bis{4-[2-[1,1,1,3,3,3-hexafluoro-2-(trifluoromethyl)propan-
2-yloxy] ethoxy] phenyl} amine (15). To a stirred solution ofcarbamate 18 (1.03 g, 1.25 mmol) in dry tetrahydrofuran (6mL) cooled to 10 °C, trifluoroacetic acid (6 mL) was addeddropwise. The solution was stirred 6 h at room temperatureafter which the volatiles were eliminated under reducedpressure. The residue was dissolved in CH2Cl2 and the solutionwas washed with 10% aq NaOH, H2O, and brine and dried overMgSO4. The solvent was evaporated under reduced pressureaffording the title product as a red-brown oil that slowlysolidified on standing (860 mg, 95% yield). 1H NMR (400MHz, CD2Cl2): δ [ppm] = 6.96 (d, JHH = 8.6 Hz, 4H), 6.85 (d,JHH = 8.6 Hz, 2H), 4.35 (t, JHH = 4.5 Hz, 2H), 4.18 (t, JHH = 4.5Hz, 2H). 13C NMR (100.6 MHz, CD2Cl2): δ [ppm] = 172.3,157.8, 139.8 (q, JCF = 290 Hz) 138.6, 135.2, 99.1 (m), 88.0,86.5. 19F NMR (282 MHz, CD2Cl2): δ [ppm] = −71.1 (s).Anal. Calcd for C24H17F18NO4: C 39.74, H 2.36, N 1.93.Found: C 39.71, H 2.42, N 1.88.Spectroscopic and Electrochemical Characterization
of Dyes. Absorption and fluorescence spectra were recordedon a Shimadzu UV-2401PC spectrometer and on a SpexFluoroMax fluorometer, respectively. Both Et2O and 1,2-dichloroethane (DCE) were investigated as solvents. Spectrawere recorded using 1-cm path length quartz cells. Electro-chemical studies were carried out using a Pt disk workingelectrode in DCE deoxygenated solution (nitrogen bubbling)with 0.10 M tetra-n-butylammonium hexafluorophosphate(TBAPF6) as the supporting electrolyte. Concentrations were1 mM for dyes 1, 4, and 19 and 0.5 mM for 3 and 5. Because ofthe low solubility of 5 dye in DCE, it was dissolved in a 3:1:1mixture of DCE:benzene:acetonitrile. A silver wire quasirefer-ence electrode was used. The Pt working electrode was cleanedbetween experiments by polishing with 0.3 mL of alumina pastefollowed by solvent rinses. After each voltammetric experiment,ferrocene (Fc/Fc+= 0.70 V vs NHE) was added as an internalstandard, and the potential axis was calibrated against thereference electrode.36,37
DSSC Preparation. A fluorine-doped SnO2 conductingglass (FTO) was first cleaned with a neutral cleaner and thenwashed with deionized water, acetone, and isopropyl alcohol,sequentially. The conducting surface of the FTO wassubmerged in a solution of titanium chloride tetrahydrofurancomplex for 20 min at 70 °C and then calcined at 450 °C for 30min. TiO2 pastes were coated onto the treated conducting glassby using the doctor blade technique. A first layer of colloidalTiO2 suspension (Ti-Nanoxide HT/SP Solaronix) was spreadusing scotch tape as a spacer, heated at 80 °C for 30 min, andthen a second layer of colloidal TiO2 suspension (DyeSolWER4−0) was spread. The TiO2 layers were calcined at 450°C for 30 min. The TiO2 layer was again submerged in asolution of titanium chloride tetrahydrofuran complex for 20min at 70 °C and finally calcined at 450 °C for 30 min. Thethicknesses of the films were about 20 μm. Dye-coating of theTiO2 films was carried out by soaking the films in 10−4 M dyeabsolute ethanol solution, immediately after the high-temper-ature annealing and while it was still warm (∼50 °C). TheDSSCs were assembled using a platinized conducting glass anda 25 μm hot melt spacer. Redox electrolyte [0.6 M methylpropyl imidazolium iodide, 0.03 M I2 (99.9%) and 0.5 M 4-tert-butylpyridine in acetonitrile/valeronitrile (85:15)] was intro-duced through a hole drilled in the counter electrode that wassealed afterward.
DSSC Characterization. Photocurrent action spectra wereobtained by illumination through the back contact of the TiO2electrode of the DSSCs with monochromatic light obtainedfrom a 75-W high-pressure Xe lamp (Photon TechnologyInstrument, PTI) and a computer-controlled PTI high intensitygrating monochromator. The incident light intensities atdifferent wavelengths were measured with a Coherent Laser-Mate Q radiometer. The photocurrent and voltage curves weremeasured using a SF 150 Sciencetech solar simulator equippedwith an AM 1.5 G filter. Three series of at least three cells weretested for each dye to validate the obtained trends in DSSCperformance. The reported data are average values.
■ RESULTS AND DISCUSSIONDesign and Synthesis of Fluorene Dyes. In a previous
paper we first demonstrated the beneficial influence ofperfluoroalkyl substituents CnF2n+1 introduced at the peripheryof unsymmetrical Zn-phthalocyanine photosensitizers.29 In-deed, we observed improved incident photon to currentefficiency (IPCE) values and a clear antiaggregation effectdetermined by bulky fluorous groups tethered to the corephthalocyanine structure by means of appropriate electron-richspacers that counterbalance the strong electron-withdrawingaction of CnF2n+1. Experimental data also suggested that thefluorous surroundings might shield the titania surface and dyeitself preventing back recombination with the redox electrolyte.These findings were confirmed by the results of a parallel studyon fluorous Ru(II) polypyridyl complexes due to and Lu andco-workers.30
Because of its outstanding properties and ease offunctionalization, the fluorene core was selected in order toevaluate the viability of the fluorous approach to purely organicD-π-A photosensitizers, which is so far unexplored. Structurallyrelated fluorous and nonfluorous fluorene dyes 1−5 were thussynthesized starting from 7,7′-dihalogenated fluorenes 6 and 7,according to the pathway depicted in Scheme 1. Fluorene 7bearing fluorous branched ponytails at the 9,9 positions waseasily obtained by etherification of 9,9-bis(3-bromopropyl)-2,7-
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diiodo-9H-fluorene with preformed sodium perfluoro-tert-butoxide. Pd-catalyzed C−N coupling reaction of aldehydes 8and 9 (prepared by monolithiation of 6 and 7, respectively,followed by quenching with DMF and water) with theappropriate diarylamine afforded aldehydes 10−14 in moderateto good yields (50−80%). Fluorous diarylamine 15 required forthe synthesis of aldehydes 12 and 14 was prepared in threesteps (Scheme 2) starting from the branched fluorous tosylate16 introduced by Rabai and co-workers.35 A slight modificationof Buchwald’s methodology for the Cu-catalyzed C−Ncoupling reaction of aryl halides with tert-butyl carbamate wasapplied to the fluorous derivative 17,38 leading to the Boc-protected intermediate 18 from which amine 15 was readilyreleased by acid cleavage. This straightforward procedure wasfound to work pretty well with fluorous p-iodoarenes providinga convenient access to a variety of fluorous diarylamines.39
The target fluorene dyes 1−5 were finally obtained byreaction of aldehydes 10−14 with cyanoacetic acid in refluxingacetic acid in the presence of ammonium acetate. A sixthfluorene-based D-π-A compound (dye 19, an analog of 1 whereA is a simple carboxylic group directly linked to the fluorenecore, see Chart 1) was prepared by mild oxidation of thecarbonyl group of 10 in order to verify the influence of thenature of A on the performance of these dyes.Spectroscopic and Electrochemical Characterization
of the Dyes. Figure 1 shows the absorption and emission
spectra of the new fluorene dyes recorded in DCE solution.Two maxima can be observed in the absorption spectrum of allthese dyes. One peak, corresponding to the π−π* electronictransition of the fluorene core, is in the UV zone (∼305 nm),and the maximum of this band is similar for all studiedmolecular structures. On the other hand, a second broad peakassigned to intramolecular charge transfer (ICT) dominates thevisible region, the maximum absorption wavelength of which isdependent on the dye molecular nature.The ICT characteristic is confirmed by the strong reverse
solvatochromism observed when the absorption maxima are
compared in different polarity media (DCE and diethyl ether,Table 1).11,17,40−42 Moreover, the charge transfer character of
these dyes is supported by the red shift generally observed inthe fluorescence maximum upon increasing the solvent polarity(fluorescence solvatochromism, Table 1). Conversely, when thedyes are anchored on the porous TiO2 film, the light absorptionmaxima of the ICT bands show a blue shift (Table 1) comparedto the corresponding maxima observed in solution. This blueshift is attributed to the deprotonation of carboxylic acid uponadsorption onto the TiO2 surface,43,44 and the resultingcarboxylate-TiO2 unit is a weaker electron acceptor comparedto the carboxylic acid.45
The relationship between the ICT electronic transitions andthe molecular structure is clearly observed trough a comparativeanalysis of the absorption spectra of the different dyes in polarmedia (e.g., DCE, Table 1). The absorption maximum of 2 is40 nm red-shifted with respect to the same transition in 1 (λmax= 442 nm, Table 1). This effect can be ascribed to the presenceof the two methoxy electron-donor groups in 2, which increasethe electronic density in the donor phenylamine moieties,stabilizing the positive charge density photogenerated in theICT electronic transition. In agreement with this, ICTabsorptions for dyes 3 and 5 show a similar (37 and 33 nm,respectively) red shift with respect to the maximum wavelengthobserved for 1, thus showing that the remote perfluoroalkoxyand methoxy groups affects the ICT transition in a similar way.The electronic interaction between the electron-withdrawingtrifluoromethyl groups and the chromophore center is onlymarginal, because these elements are connected through anonconjugated chain. Finally, dye 4 shows ICT transition at awavelength similar to 1, due to the fact that 4 lacks theelectron-donor substitution in the phenylamine groups. On theother hand, dye 19 lacks both methoxy (donor) and cyano(acceptor) groups, which is reflected in a strong hypsochromicshift (around 100 nm) in the ITC absorption with respect tothe same band for dye 2. As already mentioned, thefluorescence observed in the dyes is originated from ICTstates, thus a similar correlation between photoluminescencewavelength maximum and molecular structure can be deduced(see Table 1).The electrochemical properties of the dyes were investigated
by using cyclic voltammetry in DCE solution containingTBAPF6 as support electrolyte and a Pt working electrode.Oxidation potentials (Eox) were calculated using the expression(Ep forward + Ep backward)/2. As it can be seen from Figure2, all the dyes present a quasireversible oxidation process, which
Figure 1. Normalized absorption (solid line) and emission (dash line)spectra of studied dyes in DCE solution. The emission spectra weremeasured by excitation at the maximum of the respective ICT bands.The emission spectrum of dye 2 was obtained in Et2O solution.
Table 1. Absorption and Emission Characteristics of theStudied Dyes
aThe emission spectra were measured by excitation at the maximum ofthe respective ICT bands. bNo emission was observed in DCE for thisdye, probably due to aggregation and self-quenching.
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is attributed to removal of one electron from the diphenylamine groups.46
The ground state oxidation potentials (Figure 3) rangingfrom 0.92 to 1.21 V vs NHE are more positive than the I−/I3
−
redox couple (0.4 V vs NHE),47 ensuring that there is enoughdriving force for the regeneration of the oxidized sensitizer dye.Differences in the oxidation potentials are directly related to thedye structures and provide useful indications on the effectexerted by the various D and A groups attached to the fluorenecore. Thus, 19 and 1 present similar oxidation potential values,indicating that changing the nature of the A group from asimple COOH to a cyanoacrylic acid moiety has almost noinfluence on this parameter. In the case of dyes featuring thesame A group, 2 and 3 show lower oxidation potentials than theparent molecule 1. This is because in both 2 and 3 the diphenylamine unit bears electron donating alkoxy groups, whichstabilize the amine radical cation.46 The anodic 50 mV shiftobserved for 3 (respect to 2) reflects a residual inductive effectof the O-shielded trifluoromethyl units, which however doesnot offset the electron donating capacity of the ethyleneoxygroup.29 On the other hand 4 exhibits a 40 mV higher redox
potential than 1, showing that the introduction of electron-withdrawing perfluoro-tert-butoxy units in the propyl sidechains slightly alters the electrochemical properties. Dye 5features all the fluorous domains independently present in 3and 4, which produce opposite effects, the electron donatingone being superior. As a result the oxidation potential decreasesrespect to the model molecule 1.The excited-state oxidation potential E(S+/S*) can be
extracted from the redox potential of the ground state E(S+/S) and the zero−zero excitation energy E0−0 according to theequation E(S+/S*) = E(S+/S) − E0−0, in which E0−0 is obtainedfrom the intersection point of absorption and emission spectra.The excited-state oxidation potentials of the dyes (see Figure 3)are notably more negative than the potential of the TiO2conduction band edge (−0.5 V vs NHE),48,49 providingthermodynamic driving force for electron injection.
Calculation Analysis of Dye Structure. To analyze thespatial configuration and characteristic features of the electronicstructure of the synthesized dyes, molecular geometries andfrontier molecular orbitals were obtained through AM1calculations.50 Figure 4 shows that the HOMOs of all dyes
are localized mostly on the diphenylamino groups, whereas theLUMOs are localized on the cyanoacrylic (1−5) or carboxylic(19) units, regardless of the presence or absence ofperfluoroalkoxy (3−5) or methoxy (2) substituents. Thus,the electron distributions are located mainly on the donor unitsin the ground state and move on the acceptor units close to theanchoring groups upon photoexcitation, which favors the
Figure 2. Cyclic voltammetry of fluorene dyes at Pt electrode. Fordetails see the Experimental section.
Figure 3. Energy level diagram of the fluorene dyes. The figure showsthe oxidation potentials in the ground and excited states. The redoxpotential of the iodine couple and the lower edge of the conductionband (CB) of TiO2 are introduced for comparison.
Figure 4. Geometric optimization (AM1 semiempirical calculations atHyperChem software) of the dye structures. HOMO−LUMO frontiermolecular orbitals.
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electron injection from the dye molecules to the conductionband edge of TiO2.Photovoltaic Devices Characterization. Photoelectric
effects generated by the excitation of the D-π-A dyes adsorbedon TiO2 are evidenced by the incident-photon to currentefficiency (IPCE) spectra depicted in Figure 5. A goodagreement between the photocurrent action spectra and thelight absorption of the adsorbed dyes (see Figure 1 and Table1) is observed.
The IPCE spectra reflect the differences observed in theabsorption spectra for all the fluorene dyes and their shortcircuit photocurrent generation capability. Regarding thewavelength region where the dyes are active, a relativecorrelation between photocurrent action spectra and thewavelength absorption maximum at TiO2 adsorbed state ismaintained (see Table 1). The clearest case is dye 19, whereIPCE follows the reduced light harvesting capability of the dye.In the case of dye 2, the presence of methoxy groups attachedto the amine moiety seems to have a detrimental role on theIPCE spectrum, in agreement with what is observed for related(iso)truxene dyes.51,52 This deleterious effect is not fullyunderstood until now, but fully similar results wereexperimentally observed in independents laboratories. In thepresent case, the incorporation of perfluoroalkoxy substituentsclearly reverts the deleterious effect of methoxy groupsimproving dye performance, and this is a useful clue for futureimprovements in dye molecular design.The photovoltaic parameters of the DSSCs constructed with
the new fluorene dyes are summarized in Table 2, and thecorresponding photocurrent−voltage curves are shown inFigure 6.
As it is clearly seen from these data, dye 19, which holds acarboxylic instead of a cyanoacrylic acid as electron acceptorand anchoring group, gives the lower energy conversionefficiency in this set of compounds. In comparison, dye 1,characterized by the same D group and fluorene core of 19,shows efficiency four times higher. From Figure 6 it is alsoevident that molecule 19 is the least panchromatic of all dyestested. As consequence, the light harvesting capacity of the 19photosensitized electrodes is low, producing low energyconversion efficiency. Moreover, the presence of the electronwithdrawing cyano group in 1 favors the photoinducedintramolecular electron transfer, producing higher electrondensity near TiO2 nanoparticulated surface, increasing theelectron-injection probability and global photocurrent gener-ation process.On the other hand, the introduction of typical electron-
donating substituents such as methoxy on the diphenylaminogroup (2) results in a reduced energy conversion efficiency ofthe DSSC in comparison with what is observed with dye 1,even in the presence of a typical coadsorbent additive such asstearic acid (Table 2).53 Regardless of the similar (or higher) fillfactor and open circuit photovoltage obtained with 2, the lowershort circuit photocurrent drives to have lower efficiencies. Onthe contrary, the presence of a bulky fluorous domain in theelectron-donating ether moiety on the diphenylamino group(3) clearly increases the dye capacity for photocurrentgeneration, and the concomitant energy conversion efficiency.Taking into account that the electrochemical characteristics andfrontier molecular orbitals calculations evidence a limitedelectronic communication between the perfluoroalkoxy groupsand the conjugated fluorene core, we propose that theirbeneficial effect is mainly due to the antiaggregation propertiesthat such bulky groups confer to the D-π-A dye, together withthe possible shielding of titania surface, that avoids deleteriousback electron transfer to the iodine electrolyte. This idea isreinforced by the fact that the photoelectrochemical active
Figure 5. Photocurrent action spectra of fluorene dye-containingDSSCs.
aDSSCs with 2 and stearic acid used as a coadsorbent (see ref 53)were also assembled and tested for comparison purposes. Photovoltaicparameters were not affected.
Figure 6. Photocurrent−voltage curves for fluorene dyes containingDSSCs.
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centers and the anchoring groups in 2 and 3 are the same.Moreover, it is illustrative to analyze the characteristics andbehavior of dye 4, which shares the same D-π-A photoactivecore with 1, but with perfluoroalkoxy substituents in the 9,9positions of the fluorene group. In this case, the DSSCperformance is less effective than that observed with cellssensitized by 1 and 3, evidenced by a low short circuitphotocurrent. As mentioned earlier, the bulky perfluoroalkoxysubstituents groups were introduced with the objective ofpreventing dye aggregation and self-quenching betweenfluorene dyes, but in the case of 4 the steric hindrance coulddiminish dye loading over the TiO2 surface, making DSSC lessefficient. Nevertheless, all the cells exhibit similar lightabsorption in the visible range (electrode absorbance at λ =450 nm ∼2, except for 19). This suggests that the lateralsubstitution in the studied D-π-A dyes could affect otheradsorption characteristics (such as the orientation and mutualinteraction of dye molecules) over the titanium oxide surfacebut not the loading itself. In this regard, it is worth noting thatdye 5, the molecular structure of which combines the mainfeatures of dyes 3 and 4, holding the same photoactive core andperfluoroalkoxy substitution in both D and fluorene sides,shows a DSSC energy conversion yield higher than 4 but lowerthan 3. This last results confirms that, from a molecularstructure point of view, the introduction of perfluoroalkoxysubstituents in the tail end of a D-π-A photosensitizer dye isdesirable in order to improve the energy conversion efficiencyof DSSCs.
■ CONCLUSIONSThe insertion of fluorous domains in the molecular structure ofsuitable photosensitizers has recently emerged as a promisingtool for the development of DSSCs.22,28,29 This approach hasbeen applied here for the first time to purely organic dyes, andthe influence of perfluoroalkoxy substituents on the efficiencyof fluorene centered D-π-A photosensitizers has been thusdemonstrated. In particular, the introduction of such bulkygroups in the tail D end of molecular structure (dye 3) drives toobtain higher energy conversion efficiency due to thegeneration of larger photocurrent densities. We propose thatthis is due to the antiaggregation properties of perfluoroalkoxysubstituents and the possible shielding of the titania surface,which avoids back electron transfer to the electrolyte. Theresults obtained reinforce our original proposition that properlydesigned fluorous substitution remarkably affects the behaviorof dyes as photosensitizers in DSSCs, leading to an improve-ment in the power conversion efficiencies with respect toanalogous nonfluorous molecules.
■ ACKNOWLEDGMENTSThe authors are grateful to the CNR-Regione Lombardia“Mind in Italy” project for financial support. Subventions fromSecretaria de Ciencia y Tecnica, Universidad Nacional de Rio
Cuarto (Secyt-UNRC), Consejo Nacional de InvestigacionesCientificas y Tecnicas (CONICET), and Agencia Nacional dePromocion Cientifica y Tecnologica (ANPCYT) of Argentinaare also acknowledged.
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