Direct Spectroscopic Evidence for Constituent HeteroatomsEnhancing Charge Recombination at a TiO2−Ruthenium DyeInterfaceKe Hu,† Holly A.Severin,‡ Bryan D.Koivisto,‡ KiyoshiC. D.Robson,‡ Eduardo Schott,∥
Ramiro Arratia-Perez,*,§ Gerald J.Meyer,*,† and Curtis P.Berlinguette*,‡,⊥
†Departments of Chemistry and Materials Science and Engineering, Johns Hopkins University,3400 North Charles Street, Baltimore,Maryland 21218,United States‡Department of Chemistry,University of Calgary,2500 University Drive NW,Calgary,Alberta,Canada T2N 1N4§Departamento de Ciencias Químicas, Relativistic Molecular Physics Group, Universidad Andres Bello, Republica 275, Santiago, Chile⊥Departments of Chemistry and Chemical& BiologicalEngineering,The University of British Columbia,2036 Main Mall,Vancouver,British Columbia,Canada V6T1Z1∥Laboratorio de BionanotecnologıFa,Universidad Bernardo O’Higgins,GeneralGana 1780,Santiago,Chile
*S Supporting Information
ABSTRACT:A seriesof three bis(tridentate) cycloruthenated sensitizerswith furyl,thiophene,or selenophene units attached to the cyclometalatedligand were designed to examine how chalcogen atomseffectinterfacialelectron transfer events that occur following the absorption of visible light bythe sensitizers when attached to mesoporous titania thin films immersed inCH3CN electrolytes.Spectroelectrochemistryestablished thattheRuIII/II
reduction potentials were confined to the 0.954−0.965 V vs NHE range forthe seriesand thatthe density ofTiO2 acceptorstateswere sensitizer-independent.Pulsed light excitation into the metal-to-ligand charge transferband of the sensitized thin films resulted in rapid excited state injection,kinj> 108 s−1.Charge recombination (RuIII/TiO2(e−) →RuII/TiO2) rate constants were insensitive to the identity of the cyclometalated compound,yet the open circuit photovoltageswere markedly lower for the compound containing selenophene.These lower photovoltages appear to be a manifestation of a 4-fold-larger,second-orderrate constantmeasured forthe reaction between triiodide and TiO2(e−) in the case thattheselenophene comprises the donorfragmentof the dye.Adductformation between oxidized iodide(s) and the selenophenemoiety of the sensitizer is implicated.
■INTRODUCTIONInterfacialprocessesat the titaniaphotoanodein a dye-sensitized solarcell (DSSC) aresensitiveto a myriad offactors.1−3The electron promoted by lightfrom a dye intotitania is ideally collected atthe transparentconducting glasssubstrate and returned to an oxidized dye by a redox mediatorthat is in intimate contact with the counter electrode.4 Theseinjected electrons,however,can also be intercepted by thesmall concentration of photo-oxidized dyes on the surface5−7orby the oxidized form ofthe redox shuttle (e.g.,I2, I3−).8,9Alower open-circuit voltage (Voc) is often attributed to the latterrecombination event,which can be suppressed by aliphaticsubstituents tethered to the dye molecules that serve to inhibitredox-activespeciesfrom reachingthesemiconductorsur-face.10,11The measuredphotovoltagesare alsointimatelyrelated to the regeneration ofthe photooxidized dyes by theelectrolyte.5
Resolving the recombination and regeneration processes thatmanifestas lowerphotovoltagesin operationaldevicesiscomplicated by the dynamic nature of the cell.Regeneration of
dyes by I−, for example, most likely involves the formation of anadductwith thedye.12−14A consequenceof this adductformation is that specific chemical components of the dye mayalterinterfacialelectron transferkinetics and,in turn,deviceefficiency. We recently elaborated on such a feature for a pair ofmetal-free triarylamine-based donor−π-bridge−acceptor mole-culesthatdiffered only in the identity oftwo heteroatoms,sulfur and oxygen,comprising the donor unit of the respectivedyes (Figure 1).5 Although this study demonstrated that sulfuratomscan havea measurableeffecton photovoltages,particularlyat thepowerpointor open circuitconditionswherecompetitiverecombinationpathwaysplaya moreprominent role,a seemingly contrarian observation was madeearlierby O’Regan and co-workers8 when they showed thatsulfuratomsin a commonruthenium-baseddyescaffold
enhanced recombination with I3− or I2. We conjecture that the
relative positions of the heteroatoms account for these apparentdisparities;namely,the organicdyespresented in Figure 1involve a heteroatom change within the donorunitthatispositioned away from the surface and spatially optimized forreaction with the electrolyte,whereas the ruthenium scaffoldpositions the sulfur atom closer to the titania surface and awayfrom the anionic NCS− ligands that are a more likely site forregeneration.8
We thereforeset out to reconciletheseexperimentaldifferences by examining how heteroatoms,carefully positionedwithinthe donorunitsof otherwisestructurallyrelatedruthenium-based dyes,impact both regeneration and recombi-nation.The unique seriesof bis(tridentate) cycloruthenateddyes used for this study contain terminalfuryl,thiophene,andselenophene substituents (Figure 1) that offer the opportunityto examine how interfacial electron transfer kinetics are affectedfor a seriesof metal-based dyeswith uniform opticalandelectrochemicalproperties.It wasfound thatchanging thechalcogen within the five-membered ringsof these terminalsubstituents did notimpactthe regeneration step,as we hadanticipatedat the outsetof the study.Notwithstanding,transientspectroscopicmethodsindicatethat the morepolarizable selenophene unitincreases the rate ofrecombina-tion with I3−, therebycompromising themeasured photo-voltages.We attribute these observations to the close proximityof the chalcogen atoms within the flanking five-membered ringsto the surface.
■EXPERIMENTALPreparation of Compounds.All reagents were purchased
from Aldrich and used without further purification,except forRuCl3·3H2O (PressureChemicalCompany)and trimethyl-4,4′,4″-tricarboxylate-2,2′:6′,2″-terpyridine (L4;Helios Chem-icalCompany,Switzerland).Purification by column chroma-tography was carried out using silica (Silicycle: Ultrapure FlashSilica).Analyticalthin-layerchromatography(TLC) wasperformed on aluminum-backed sheetsprecoated with silica60 F254 adsorbent(0.25 mm thick;Merck,Germany) andvisualized under UV light.Routine1H and13C NMR spectrawere recorded at 400 and 100 MHz,respectively,on a BrukerAV 400 instrumentatambienttemperatures.Chemicalshifts(δ) are reported in parts per million (ppm) from low- to high-field and referencedto residualnondeuteratedsolvent.Standardabbreviationsindicatingmultiplicityare usedasfollows:s = singlet;d = doublet;t = triplet; m = multiplet.All proton assignmentscorrespond to the generic molecularschemesthatareprovided (Figure2). Organicprecursorsselenophene-2-carbaldehyde,15 1-(2-(3-bromophenyl)-2-
oxoethyl)pyridinium iodide,16 (E)-1-(pyridin-2-yl)-3-(thio-phen-2-yl)prop-2-en-1-one (P2),17 2-(furan-2-yl)-4,4,5,5-tetra-methyl-1,3,2-dioxaborolane (P7),18 and 4,4,5,5-tetramethyl-2-(thiophen-2-yl)-1,3,2-dioxaborolane (P8)19 were prepared aspreviously reported.
Figure 1. (a) Series of ruthenium chromophores Ru−X (X = O,S,Se) used in this study.(b) Previously documented dyes used to probe interfacialelectron transfer (X = O,S).5,8
6-(3-Bromophenyl)-4-(furan-2-yl)-2,2′-bipyridine(P4). Amixtureof (E)-3-(furan-2-yl)-1-(pyridin-2-yl)prop-2-en-1-one(P1) (1.74 g,8.79 mmol),ammonium acetate (17.8 g,228mmol),and 1-(2-(3-bromophenyl)-2-oxoethyl)pyridinium io-dide (3.55 g,8.79 mmol) and formamide (30 mL) was slowlybrought to 120 °C under a dinitrogen atmosphere with stirring.The reaction was left at 120 °C overnight and then cooled toroom temperature.The precipitate wasisolated by vacuumfiltration and washed with EtOH.The solid was solubilized inDCM, dried with MgSO4, filtered,and concentrated beforebeingpurifiedby columnchromatography(SiO2: DCM/EtOAC, 9:1;Rf = 0.83)to yield theproductas a darkorangey-brown oil(1.87 g,56.4%).1H NMR (400 MHz,CDCl3): δ = 8.70 (d,1H,3J = 5.5 Hz, Ha), 8.60 (d,1H,3J = 7.9Hz,Hd), 8.58 (d,1H,4J = 1.4 Hz,He), 8.33 (t,1H,4J = 1.8 Hz,Hj), 8.06 (d,1H,3J = 8.9 Hz, Hk), 7.95 (d, 1H,4J = 1.4 Hz, Hi),7.84 (td,1H,3J = 7.7 Hz,4J = 1.8 Hz,Hc), 7.56−7.53 (m,2H,Hh, Hm), 7.53 (t,1H,3J = 7.9 Hz,Hl), 7.32 (ddd,1H,3J = 7.4Hz,3J = 4.8 Hz,4J = 1.1 Hz,Hb), 7.04 (d,1H,3J = 3.4 Hz,Hf),6.54 (dd,1H,3J = 1.8 Hz,3J = 3.4 Hz,Hg). 13C NMR (100MHz, CDCl3): δ = 156.5,156.1,155.6,151.9,149.3,143.9,141.5,139.6,137.1,132.2,130.4,130.3,125.7,124.2,123.2,121.6,114.6,114.3,112.4,109.4.HRMS (EI): m/z = 376.0193[(M)+] (calcd for C20H13N2OBr+: m/z = 376.0211).
6-(3-Bromophenyl)-4-(thiophen-2-yl)-2,2′-bipyridine(P5).A mixture of(E)-1-(pyridin-2-yl)-3-(thiophen-2-yl)prop-2-en-1-one(P2) (2.00 g,9.29 mmol),1-(2-(3-bromophenyl)-2-oxoethyl)pyridinium iodide(3.75 g, 9.29 mmol),andformamide (25 mL) was stirred and slowly heated to 120 °Cundera N2 atmosphereand left undertheseconditionsovernight.The solution was cooled to room temperature,andthesolventremoved in vacuo.The remaining solidsweretrituratedwith EtOH and air-dried.The darksolidwassolubilized in DCM and dried with MgSO4, and the solventswere removed by rotovap.The product was purified by columnchromatography (SiO2: DCM/EtOAc,9:1; Rf = 0.95) to yieldan oilthat solidified upon standing; 1.67 g (45.7%).1H NMR(400 MHz,CDCl3): δ 8.70 (dd,1H,3J = 4.7 Hz,4J = 0.8 Hz,Ha), 8.61 (m, 2H, He, Hd), 8.33 (t, 1H,4J = 1.8 Hz, Hj), 8.06 (d,1H,3J = 7.8 Hz,Hk), 7.87 (d,1H,4J = 1.6 Hz,Hi), 7.85 (t,1H,3J = 7.8 Hz,4J = 1.8 Hz,Hc), 7.69 (dd,1H,3J = 3.7 Hz,4J = 1.0Hz,Hh), 7.56 (dq,1H,3J = 7.9 Hz,4J = 0.9 Hz,Hm), 7.43 (dd,1H,3J = 5.1 Hz,4J = 0.9 Hz,Hf), 7.37 (t,1H,3J = 7.9 Hz,Hl),7.33 (dd,1H,3J = 4.8 Hz,4J = 1.1 Hz,Hb), 7.16 (dd,1H,3J =5.0 Hz,3J = 3.7 Hz,Hg). 13C NMR (100 MHz,CDCl3): δ156.8,156.0,155.9,149.3,143.7,141.8,141.5,131.2,132.3,130.5,130.4,128.6,127.4,125.9,125.8,124.3,123.3,121.7,116.9,116.4.HRMS (EI):m/z = 393.9945 [(M)+] (calcd forC20H13N2SBr:m/z = 393.9962).
6-(3-Bromophenyl)-4-(selenophen-2-yl)-2,2′-bipyridine(P6).A stirring mixture of(E)-1-(pyridin-2-yl)-3-(selenophen-2-yl)prop-2-en-1-one(P3) (1.20g, 4.58mmol),1-(2-(3-bromophenyl)-2-oxoethyl)pyridinium iodide(1.85g, 4.58mmol), ammonium acetate (9.20 g, 119 mmol), and formamide(25 mL) was brought to 120 °C under a N2 atmosphere andleft overnight.The dark brown reaction mixture was allowed tocool to room temperature.The resulting solid was removed by
pyridine(L3H).6-(3-Bromophenyl)-4-(selenophen-2-yl)-2,2′-bipyridine (P3) (642 mg,1.46 mmol) and 4,4,5,5-tetramethyl-2-(selenophen-2-yl)-1,3,2-dioxaborolane(P9) (756 mg,2.94mmol)were solubilized in THF/water(9:1,125 mL)andsparged with N2 for 10 min.K2CO3 (2.02 g,14.6 mmol) andPd(PPh3)4 (236 mg,0.204 mmol) were then added,and thereaction was allowed to reflux undernitrogen for14 h.Thereaction mixture wasthen cooled to room temperature andwashed with brine.The product was extracted with Et2O. Theorganic layer was dried with MgSO4 and filtered,and then thesolventwasremovedin vacuo.Purificationby columnchromatography [SiO2: DCM/EtOAc (9:1); Rf = 0.34] yielded516 mg (72.1%) ofthe product as an orange solid.1H NMR(400 MHz,CDCl3): δ = 8.72 (ddd,1H,3J = 4.8 Hz,4J = 1.7Hz,5J = 0.9 Hz, Ha), 8.64 (d, 1H,3J = 8.0 Hz, Hd), 8.58 (d,1H,4J = 1.5 Hz,He), 8.36 (t,1H,3J = 1.7 Hz,Hj), 8.10 (d,1H,3J =5.6 Hz,Hh), 8.04 (d,1H,3J = 7.8 Hz,Hk), 7.98 (d,1H,3J = 5.5Hz,Hp), 7.90−7.81 (m,3H,Hc, Hf, Hi), 7.63 (d,1H,4J = 7.7Hz, Hm), 7.59 (d, 1H,3J = 3.7 Hz, Hn), 7.50 (t, 1H,3J = 7.7 Hz,Hl), 7.39 (dd, 1H,3J = 5.5 Hz,4J = 3.9 Hz, Hg), 7.36 (dd, 1H,3J= 5.4 Hz,4J = 3.8 Hz,5J = 1.0 Hz,Ho), 7.33 (ddd,1H,3J = 7.4Hz, 4J = 4.8 Hz,5J = 1.0 Hz,Hb). 13C NMR (100 MHz,CDCl3): δ = 157.0,156.6,156.1,150.9,149.2,148.2,145.3,140.1,137.1,137.0,132.7,131.0,130.8,130.5,129.4,128.1,127.4,126.4,125.8,125.3,124.1,121.6,117.5,116.6.HRMS(EI): m/z = 491.9639 [(M)+] (calcd for C24H16N2
80Se2+: m/z= 491.9644).
General Preparation of Methyl Ester Complexes (Ru−XMe). To a MeOH/H2O/THF solution (5:1:1,v/v/v,210 mL)containing 0.40 mmolof the ligand (e.g.,L1HL3H) wasadded 0.40 mmolof Ru(L4)Cl3 and N-ethylmorpholine (0.5mL).Following a 16 h reflux,AgNO3 (1.20 mmol) was addedto the reaction mixture,and then the mixture was allowed to
reflux for an additional 1.5 h.The mixture was then cooled andpreabsorbed on silica,and the solvent was removed in vacuo.The productwaspurifiedby chromatographictechniques(detailsspecified below).The desired fraction wascollectedand isolated to yield a dark red fine solid.
GeneralPreparation ofCarboxylic Acid Complexes(Ru−XH). A solution containing 0.13 mmolof estermetalcomplexes(Ru−XMe) in DMF/H 2O/NEt3 (3:1:1,v/v/v,25mL) was refluxed for 4 h.The solution was then cooled,andthe solvent was removed in vacuo.The product was trituratedwith DCM and collected via vacuum filtration to yield a darkred fine solid.
PhysicalMethods.Elementalanalysis (EA),electrosprayionization massspectrometry (ESI-MS),matrix-assisted laserdesorption/ionization mass spectrometry (MALDI-TOF),andelectron impact (EI) mass spectrometry data were collected atthe Chemistry Instrumentation Facility ofthe University ofCalgary.Electrochemicalmeasurementson dyesin solutionwere performed under anaerobic conditions with a PrincetonApplied Research VersaStat 3 potentiostat using dry solvents,aglassy carbon working electrode,a platinum counter electrode,a silverpseudoreferenceelectrode,and a0.1 M NBu4BF4supportingelectrolyte.Electronicspectroscopicdatawerecollected on MeCN solutionsusinga Cary5000 UV−visspectrophotometer (Varian).
Sensitized TiO2 SubstratePreparation.Mesoporousnanocrystalline TiO2 thin filmswere deposited onto micro-scope glass slides (for spectroscopic study) or fluorine-dopedtin oxide (FTO) substrate (for electrochemistry) as previouslydescribed.20 The filmswerethenimmersedin methanolsolution containing ∼1 × 10−4 M dye molecules and 1 equivof tetrabutylammonium hydroxide for∼20 min to achieve asurface coverage of∼2 × 10−8 mol/cm2. The sensitized filmswere then washed with neatmethanoland acetonitrile.Thefilms were diagonally positioned in a standard 1 cm2 quartz
cuvette with electrolyte and purged with argon gas for at least30 min prior to experimentation.
Spectroelectrochemistry. Steady-state UV−vis absorptionspectra were obtained on a Varian Cary 50 spectrophotometeratroom temperature.A potentiostat(BAS modelCV-50W)was employed for measurements in a standard three-electrodeconfiguration with a sensitized TiO2/FTO working electrode,aplatinum disk counterelectrode,and aAg/AgClreferenceelectrode (BioanalyticalScientific Instruments,Inc.) in 0.5 MLiClO4/CH3CN solution.All potentials were reported versusthenormalhydrogen electrode(NHE). The ferrocenium/ferrocene half-wave potentialwas measured atroom temper-ature before and aftereach experimentand wasused asanexternalstandardto calibratethe referenceelectrode.Conversion constantof −640 mV from NHE to Fc+/Fc wasused in acetonitrile at 25 °C.21
Transient Absorption Spectroscopy.Nanosecond tran-sientabsorptionmeasurementswereacquiredwith anapparatus similar to that which has been previously described.20
Briefly,sampleswereexcitedby a frequency-doubledQ-switched,pulsedNd:YAG laser(QuantelUSA (BigSky)BrilliantB; 532nm,5−6ns full width athalf-maximum(fwhm),1 Hz,∼10 mm in diameter) directed 45° to the filmsurface.A 150 W xenon arc lamp served as the probe beam(Applied Photophysics) thatwas aligned orthogonally to theexcitation light.Detection was achieved with a monochromator(Spex 1702/04) optically coupled to an R928 photomultipliertube (Hamamatsu).Typically,30 laser pulses were averaged ateach observation wavelength for fullspectra generation.Singlewavelength kineticmeasurementwasacquired by 100−200laser averages.Kinetic data fitting and spectralmodeling wasperformed in Origin 8,and least-squareserrorminimizationwasaccomplished using the Levenberg−Marquardtiterationmethod.
Open Circuit Photovoltage Measurements.SensitizedTiO2/FTO substrate was sandwiched against a platinized FTOcounter electrode with a vinylfilm (Warps,8 milVinyl-Pane)spacer. Electrolyte containing 0.5 M LiI and 0.05 M I2 dissolvedin CH3CN wasemployed.Steady-state lightexcitation wasachieved with the514.5 nm lineof an InnovaAr+ laser(Coherent). The laser line was expanded by a Thorlabs BE 10Xbeam expander.Alteration ofthe incident light irradiance wasachieved byusinga combination ofneutraldensityfilters(Newport). The cell area illuminated was 2.2 cm2. Photovoltagewas measuredwith a potentiostat(BASi Epsilon-EC,Bioanalytical).Light-soaking wasperformed to mitigate anyhysteresis from the lowestto the highestirradiance and thenback and forth untilthe open circuitphotovoltage stabilized(<5 mV) at each irradiance prior to data acquisition.The light-soaking process typically took two cycles.
Computational Methods. The Gaussian 03 computationalpackage22 was usedto perform ground-stategeometryoptimization calculationsemploying Becke’sthree-parameterhybrid exchange functionaland the Lee−Yang−Parr nonlocalcorrelation functional B3LYP23−25and LANL2DZ basis set26,27
with an effective core potentialfor Ru,and a 6-31G* basis setwas used for Se,S,C, N, O, and H atoms.28Time-dependentdensityfunctionaltheory(TDDFT) calculationswerealsoperformed using thismethodology,and the first60 singletexcitedstateswerecalculated.Calculationsby the first-principlesmethod wereused to obtain accurateexcitationenergies and oscillator strengths.We modeled the solvent with
approach provided accessto the seriesof tridentate cyclo-metalating ligands,L1H−L3H,on a relatively large scale usingestablished procedures.Pro-ligands P4−P6 were each synthe-sized upon a KroYhnke condensation of 1-(2-(3-bromophenyl)-2-oxoethyl)pyridinium iodide16 with P1−P3,respectively,toyield the substituted phenylbipyridine derivativesthatwerefurther reacted with Suzukireagents P7−P9 to furnish L1H−L3H in high yields (Scheme 1).The syntheses ofcomplexesRu−XMe (X = O, S, Se) followed apreviouslydescribedprotocol30−33involving the reaction of Ru(L4)Cl3 with L1H−L3H, respectively.The resultantchromatographicallypuremethylester versions of the complexes were acquired in yieldsranging from 52 to 78% (Scheme 2).The isomer containingthe substituentparato the anioniccarbon wasisolated inexclusivity in allcases.
The structuralidentities ofall ligands and complexes wereconfirmed bya combination ofNMR spectroscopy,massspectrometry,or elementalanalysis.The1H NMR spectra forRu−XMe revealthat the chalcogen heterocycle resonances (Hg,Hh, Ho, Hp) shift progressively downfield for X = O,S,and Se,respectively.The relativeelectron-donatingcharacterisattributed to lesser orbitaloverlap and elongation ofthe XC bonds with the largerchalcogenatoms,resultinginheterocycles possessing more olefinic character.
The RuIII/RuII reduction potentials for Ru−XMemeasured bycyclic voltammetry in MeCN (Table 1) indicate only a slightsensitivity to terminal substituents. The furyl rings appear to actas weakly electron-donating substituents (corroborated by the
NMR data);thethiopheneand selenopheneunitshaveanominaleffecton the Ru(III) reduction potentials.UV−visabsorption spectrarecorded on the complexesdissolved inMeCN also reveala lack ofsensitivity to the identity ofthechalcogenides (Figure 3).The presence ofthe five-memberedrings benefit light absorption,as evidenced by higher extinctioncoefficientsof the majorabsorptionbandsrelativeto aruthenium complex not containing the substituents.TD-DFTcalculations performed on ground-state optimized structures ofthe respective dyes indicate four metal-to-ligand charge transfertransitionsare responsible forthe absorption bandsin thevisible region (Figure 3).
InterfacialCharge Recombination in the Absence ofIodide.The respective dyeswere immobilized on TiO2 tointerrogatetheelectron transferprocessesat theinterface.Nanosecond transientabsorption spectroscopy wasused toquantifythe interfacialelectrontransferreactionsfromTiO2(e−) to the oxidized sensitizers(RuIII-X/TiO2(e−) →
Table 1.Reduction Potentials of Ru−X (X = O,S,Se) inSolution and on Mesoporous TiO2
E1/2, Ru(III)/Ru(II) (V vs NHE)sample Ru−XMe
a Ru−X/TiO2bRu−O 1.014 0.965Ru−S 1.024 0.950Ru−Se 1.027 0.9541c 1.028 0.950
aData collected using 0.1 M NBu4BF4 MeCN solutions at 100 mV/sand referencedto a [Fc]/[Fc]+ internalstandard,followedbyconversion to NHE;[Fc]/[Fc+] = +640 mV vsNHE in MeCN.Datacollected using 0.5 M LiClO4 as theelectrolytein MeCNsolutions.cBenchmark complex: [Ru(pbpy)(L4)](NO3).31
RuII-X/TiO2). Absorption differencespectraareshown inFigure 4 for this reaction after pulsed 532 nm laser excitation in0.5 M LiClO4/CH3CN.
The immediateappearanceof the oxidized dyespectraindicated that the electron injection to TiO2 acceptor states wasultrafast (kinj> 108 s−1). Normalizable spectra at all delay timesand maintenance ofsharp isosbestic points at∼400 and 720nm confirmed one charge-separated state:RuIII−X/TiO2(e−).Figure5 showssinglewavelength absorption changesthatcorrespond to charge recombination between TiO2(e−) and theoxidized sensitizer.A probe wavelength of 583 nm was selectedbecause itallowed forobservation ofthe oxidized sensitizerwithoutcomplications thatarise from the shift ofthe groundstateabsorptioninducedby the injectedTiO2(e−)s34
(SupportingInformationFigureS2). The kineticswerenonexponentialbut weresatisfactorilymodeledby Kohl-rausch−Williams−Watts(KWW) function,35,36eq 1,with acommon β value of0.16.Average rate constants(kcr) werecalculated as the first moment by using eq 2. The rate constantswerewithinexperimentalerror the samefor all thesecompounds and,hence,are summarized as one,kcr = (2.5 ±0.6) × 102 s−1.
Δ = − βA ktAbs exp[ ( ) ] (1)
Figure 3.ExperimentalUV−vis absorption spectrum overlaid with calculated transitions represented by verticalbars (only the transitions withcontributions >30% are shown). Details of calculated transitions (theoretical wavelength in nm, oscillator strength, % contribution to transition): (a)Ru−OMe; λ1, HOMO → LUMO (714,0.024,93%); λ2, HOMO → LUMO+1 (587,0.009,64%); λ3,HOMO-1 → LUMO and HOMO → LUMO+1 (513,0.172,51% and 31%);λ4, HOMO → LUMO+5 and HOMO-2 → LUMO+2 (416,0.199,46% and 30%).(b) Ru−SMe; λ1, HOMO →LUMO (690,0.026,94%); λ2, HOMO → LUMO+1 (575,0019,53%); λ3, HOMO-1 → LUMO and HOMO → LUMO+1 (502,0.179,39% and38%); λ4,HOMO-1 → LUMO+3 and HOMO → LUMO+5 (420,0.216,42% and 41%).(c) Ru−SeMe; λ1,HOMO → LUMO (691,0.026,94%);λ2, HOMO → LUMO+1 (575, 0.019, 54%); λ3, HOMO-1 → LUMO and HOMO → LUMO+1 (503, 0.193, 41% and 39%); λ4, HOMO → LUMO+3 and HOMO → LUMO+5 (421,0.248,50% and 40%).
Figure 4. Absorption difference spectra measured at the indicated delay times after pulsed 532 nm excitation (laser fluence, 0.5 mJ/cm2) of (a) Ru−O/TiO2 thin film; (b) Ru−S/TiO2 thin film; and (c) Ru−Se/TiO2 thin film immersed in 0.5 M LiClO4/CH3CN.(black squares,45 ns; red circles,100 ns; blue triangles, 500 ns; magenta reverse triangles, 5 μs; green diamonds, 50 μs; blue solid traces, ground state absorption spectra with y-axis onthe right; orange solid curves,simulations based on the difference spectra of RuIII-X/TiO2 and RuII-X/TiO2 by spectroelectrochemistry)
Figure 5. Absorption changes measured after pulsed laser excitation ofthe sensitized film in 0.5 M LiClO4/CH3CN. (λexc, 532 nm; probe, 583nm; laser fluence,0.5 mJ/cm2; ground state absorption at 532 nm,≈0.22).Overlaid in yellow are the best fits to KWW kinetic model.
(2)Sensitizer Regeneration by Iodide.The regeneration of
RuIII−X/TiO2(e−) by iodide was investigated with nanosecondtransientabsorption.A pulsed 532 nm laserwasused togeneratethe interfacialchargeseparatedstate,RuIII−X/TiO2(e−), in an electrolytesolutioncontainingspecificquantitiesof iodide.The time required forregeneration byiodide was tracked by monitoring the spectralchanges at 583nm (Supporting Information Figure S1),where it representedonly recovery ofthe ground state sensitizermoleculesasafunction of time.The sensitizer regeneration kinetics have beenprevious modeled by single-exponential,37biexponential5,38orKWW39functions,but none of them were able to satisfactorilymodelthe data herein.Triexponentialfunction (eq 3) wasfound to be adequate,and an average observed regenerationrate constant(kobs) can be obtained by eq 4.The observedregeneration rate constants,kobs, were linearwith respecttoiodide concentration (Figure 6),and the linear fitting ofthe
data indicates that the second-order rate constants (kreg) for dyeregeneration are effectively the same (Table 2).
Δ = + − + − +−
t A A k t A k t A
k t
Abs( ) exp( ) exp( )exp( )
0 1 1 2 2 3
3 (3)
= + ++ +k
A k A k A kA k A k A k
/ / // / /obs1 1 2 2 3 3
1 12
2 22
3 32 (4)
InterfacialCharge Recombination in the Presence ofIodide. The open-circuit voltages (Voc) for the sensitized filmsweremeasuredas a functionof incidentsteady-statemonochromatic 514.5 nm lightirradiance in the presence of0.5 M LiI and 0.05 M I2 in CH3CN electrolyte (Figure 7).TheVoc valuesshowed alinearresponsewith respectto thelogarithmicincidentlight flux describedby the diodeequation.40−42A 59 mV increase ofVoc perdecade increase
of the incident light irradiance was also predicted if the idealityfactor was 1.Indeed,the solar cell that contained Ru−Se/TiO2under our experimental condition showed a Vocresponse of 59mV/dec oflogarithmic irradiance with reproducibility.Solarcells incorporating Ru−O/TiO2 or Ru−S/TiO2 showed slopesof 81 mV/decor 75 mV/dec,respectively.The slopescorresponded to ideality factors of 1.37 or 1.27.
The strikinglylow photovoltagesfor the selenophenylderivativeprompted usto measureanydifferencesin theacceptor states ofTiO2 for the series ofsensitized substrates,butnone were found,according to the congruentdensity ofstatesfor the seriesdetermined by monitoring the spectralchangesof the TiO2(e−) as a function ofapplied potential(Supporting Information Figure S2,S3).The disappearance ofphotochemicallygenerated I3
− wasmonitored bytransientabsorption at 375 nm (Figure 8),where I3
Figure 7.Plot of open circuit voltage as a function of incident steadystate monochromatic light (λexc, 514.5 nm) for Ru−O/TiO2, Ru−S/TiO2, and Ru−Se/TiO2 measured in solar cells containing 0.5 M LiIand 0.05 M I2 dissolved in CH3CN.The inset shows the ground stateabsorption spectra ofthe three sensitizers anchored on TiO2/FTOsubstrate for the measurement.
Figure 8. Absorption changes measured after pulsed laser excitation ofthe sensitized films in 0.4 M LiI + 0.1 M LiClO4/CH3CN (λexc, 532nm; probe,375 nm; laserfluence,2.8 mJ/cm2; groundstateabsorptionsat532 nm,≈0.26).The sensitized filmswere electro-chemically biased to −240 mV vs NHE. Overlaid in yellow are the bestfits to the KWW kinetic model.
(ε ∼ 19 000 M−1 cm−1).43 Triiodidewasassumed to beconsumed by TiO2(e−) by one-electron reduction to form 1equivof diiodide(I2•−) and I−.14,44This recombinationreactionoccurredin the millisecond-to-secondtimescalewith kinetics that were best modeled by the KWW function (eq1). Note thatto increasethe recombination rate,excesselectronswereintroduced into theTiO2 thin film with apotentiostat.The negative applied potentialcorresponded to∼20−25 electronsperTiO2 nanocrystallite on the basisofspectroelectrochemicaldata shown in Supporting InformationFigure S3.The average recombination rate constants (kcr′ ) werecalculatedusingeq 2. The rateconstantsand β′ aresummarized in Table3. To ensurethatthe resultswere
generalto many TiO2(e−) concentrations,the recombinationstudieswerecompletedat threedifferentbiasconditions(Supporting Information Figure S4).■DISCUSSIONThe most common ruthenium-based dyes in the literature arederivatives of N3 and therefore contain a chalcogen atom in thedonor portion of the molecule.2,4It has been asserted that themost likely site of adduct formation with the electrolyte speciesis at or near the isothiocyanato ligands.45This claim resonateswith our previous analysis of organic dyes that show faster ratesof dye regeneration when sulfur atoms reside within the donorunitinstead ofoxygen atoms.5 These collective observationssupport the notion that the sulfur atoms of N3 help to mediatedye regeneration by the electrolyte.However,there exists nodirectspectroscopicprooffor thisclaim in theliteraturebecausethe directmeasurementof dye−electrolyteinter-actions,in which theheteroatomsin the donorunitsofruthenium dyes are systematically modified,has notyetbeendemonstrated.A studythatmodifiesthechalcogen oftheisothiocyanato ligand isnotonly a synthetically challengingendeavor,it would also lead to disparate frontiermolecularorbitalenergies that would preclude an accurate evaluation ofrelative dye generation and charge recombination kinetics.
The three ruthenium dyes investigated in this study sidestepsaid challengesbecause each memberof the seriescontainsnearlyenergeticallydegeneratefrontiermolecularorbitals,despitedifferingin the identityof the chalcogenthatcompromisesthe donorportion ofthe molecule.It is overthe donor fragment of the molecule that the HOMO resides (aswellas the singly occupied molecular orbitalof the RuIII−X,which is more relevantto the regeneration step;SupportingInformation Figure S5) and thusis the mostlikely site ofadductformation with the electrolyte.The opticalpropertiesfor the three complexes are effectively superimposable,and theHOMO energies are almostthe same for the series,both insolution and on titania.These featurestherefore enable theunprecedented opportunity to measure charge-transfer kineticsfor a series of ruthenium dyes, with different atoms in the donorunit,whereabsorptivitiesand redoxparametersare heldeffectively at parity.
A key structuralfeature enabling this particular study is thereplacementof the isothiocyanatoligandswith a cyclo-metalatingligand.Becausethe arylringof this chelatinganionic ligand acts as a strong donor,the HOMO is delocalizedover the metal,the aryl ring,and terminal substituents attachedto the arylring.The Ru(III) reduction potentialis typicallysensitive to substituents attached to the arylringparticularlythose para to the carbanion but the electrochemicalproper-ties of Ru−X indicate that the identity of the chalcogen in thefive-membered aromatic rings does not significantly affect theelectron density at the metalcenter.(The five-membered ringattached to the less-electron-rich pyridine ring of the chelatingligand compromises the HOMO-2 leveland is therefore notexpected to influence the metal-based reduction potentials.)Opticalprofiles were also found to be the same for the Ru Xseries,with the bandsin the visibleregion arisingfromtransitionsfrom themetal−aryl−substituentportion ofthemolecule to the tridentate ligand bearing the anchoring groups.This scenario satisfies a key aspect ofsensitization in that thedonorunitresidesaway from the semiconductorsurface,afeature thatbenefitsdye regeneration,and the excited-stateelectron density resides between the surface and the rutheniumcenter to enable facile electron injection.
Attachment of each of the dyes to anatase mesoporous titanialeadsto minorbutexpected changesin opticaland electro-chemicalproperties,reflecting the slight differences in electrondonating character of the anchoring ligand upon absorption tothe surface.The ground state absorption spectra forRu−X/TiO2 recorded in MeCN had the same response to lithiumcations and were not affected by changes in the concentrationof iodide or triiodidewithin experimentaluncertainty(Supporting Information Figures S6−S8). Quantitative electroninjection into the sensitized thin films was observed for eachmember of the series at rates of >108 s−1, and the rates of dyeregeneration by the TiO2(e−) were found to be the same forthe series.Spectroelectrochemistryalsorevealedthattheelectrochemicalreduction ofthe titania was insensitive to theidentity ofthe surface-anchored dye molecules(SupportingInformation Figure S2).These collective features set the stageforstudying the interaction ofRuIII-X/TiO2 with the iodideelectrolyte without needing to account for differences in lightabsorption and energy levels of the dye−semiconductor.
There were no observable ground state absorption changeswhen either iodide or triiodide was titrated into the reactionvessel containing Ru−X/TiO2 (Supporting Information FiguresS7, S8). The generation of the interfacial charge-separated state,RuIII−X/TiO2(e−), in an electrolyte solution containing specificquantities ofiodide revealed regeneration rate constants,kobs,thatwere within experimentalerrorthe same (Figure 6 andTable 2);however,the reaction ofRuIII−X/TiO2(e−) withtriiodide to form I2
•− and I− revealed a clear sensitivity to X.This reactionwas found to occur with an averagerecombination rate constant(kcr′ ) of 1.94 s−1 for RuIII−O/TiO2(e−) and 2.18 s−1for RuIII−S/TiO2(e−), but was 10.8 s−1
for thereaction ofRuIII−Se/TiO2(e−) with triiodide.Thisfinding suggeststhatselenium playsa role in mediating theinteraction of I3
− with the semiconductor surface,a feature thatis corroborated by the distinctively lowerphotovoltagesforRu−Se/TiO2 relative to the other members of the series.
This collective data indicates thatmodifying the chalcogenatom in the series of dyes does not impact the regeneration ofthe oxidized dye by the electrolyte.This finding is not alignedwith our examinationof organicdyesthatshoweddye
Table 3. Rate Constants for Charge Recombination betweenTiO2(e−) and I3− for Ru−X/TiO2a
regeneration wassensitive to atomsin the donorunit.5 Weattribute these differences in measurements to the chalcogenhybridization ofRu−X/TiO2 relative to those in the organicdyes presented in Figure 1.Another factor to consider is thatRu−X/TiO2 contains a chalcogen atom that is not attached tothe arylring,thereby offering a second binding site not withinthe donor fragment of the dye that could preclude resolution ofthe regeneration step.Future studies willaddress this issue byexcluding substituentsattached to the pyridine ring ofthechelating ligand.
The data clearly shows that the chalcogen atoms do effect theinteraction between the surface and the electrolyte,a findingthat resonates with an earlier study by O’Regan and co-workersindicating thatthe higheraffinity foriodine to bind to sp3-hybridizedchalcogen atomsleadsto fasterrecombinationrates.8 Although we had notanticipated thatpositioning thechalcogens within the donor unit would affect recombination,the molecular structures of Ru−X do allow for the chalcogen toreside close to the semiconductor surface.This proximity couldtherefore enhance adduct formation close to the semiconductorsurface,therebyenablingmorefacilerecombinationwithTiO2(e−) and compromising the photovoltages.This findingcallsattention to theneed to position atomscapableofinteracting with the oxidized iodide species in the electrolyteaway from the surfaces.■CONCLUSIONThe Ru−X cycloruthenated sensitizers reported here provideda means to interrogate the influence ofchalcogen atoms oninterfacialelectron transferevents thatoccurfollowing light-induced electron injection into titania.It was spectroscopicallydetermined in titration experiments that the reaction betweentriiodide and TiO2(e−) was ∼5-fold faster in the case ofRu−Se/TiO2 relative to the dyes containing furyland thiophenesubstituents.This featureand the correspondinglylowermeasured photovoltage is ascribed to the heterocycle fosteringadduct formation with oxidized iodide near the semiconductorsurface.Differences in reaction rates for dye regeneration werenot observed for the series,despite a variance in atom identityin the donor fragment of the molecule.We conjecture that thecongruency in dye regeneration rates for the series is due to thebasicities of the heterocycles’being too similar or the presenceof the heterocycle notattached to the arylring precludingresolution of regeneration at the donor unit.■ASSOCIATED CONTENT*S Supporting InformationAdditionalspectroscopicdataand DFT calculationsofmolecular orbitals.This materialis available free ofcharge viathe Internet at http://pubs.acs.org.■AUTHOR INFORMATIONCorresponding Authors*E-mail: [email protected].*E-mail: [email protected]*E-mail: [email protected] AddressG.J.M.:The Universityof North Carolinaat ChapelHill,Chemistry Department,CB 3290,ChapelHill, NC 27599-3290.NotesThe authors declare no competing financialinterest.
■ACKNOWLEDGMENTSG.J.M.and K.H.acknowledge supportby a grantfrom theDivision of Chemical Sciences,Office of Basic Energy Sciences,Office ofEnergy Research,U.S.Departmentof Energy (DE-FG02-96ER14662).K.H. acknowledgesa Johns HopkinsUniversity Harry and Cleio GreerFellowship.The Canadianauthorsare gratefulto CanadianNaturalScienceandEngineeringResearchCouncil,CanadianFoundationforInnovation,AlbertaIngenuity,and theCanadaSchoolofEnergy and Environment(CSEE) forsupport.E.S.and R.A.thankFONDECYT 1110758,1130707,RC120001de laIniciativaCientı FficaMileniodel Ministeriode Economı Fa,Fomento y Turismo.
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