电致荧光变色

&Conjugated Polymers
Electrofluorochromic Detection of Cyanide Anions Using
a Nanoporous Polymer Electrode and the Detection Mechanism
Guoqiang Ding,[a]TingTing Lin,[b]Rui Zhou,[a]Yuliang Dong,[a]Jianwei Xu,*[b]and Xuehong Lu*[a]
1.Introduction
Electrofluorochromism of a material refers to the reversible change in fluorescence emission induced by external electric potentials.The first report on electrochemically switchable in-trinsic fluorophores appeared in 2004.[1]A few types of electro-fluorochromic (EFC)materials were reported in the following few years,[2]but the utilization of the EFC functionalities of the materials in various fields was sparsely explored.These func-tional materials have only recently gained more attention owing to their potential applications in areas such as fluores-cence imaging,[3]optical memories,[4]and sensors.[5]Commonly studied EFC materials include molecular dyads,conjugated polymers (CPs),and intrinsically switchable fluorophores.[6]Among these materials,EFC CPs have attracted much interest as CPs exhibit efficient electronic delocalization and rapid transport of excitons along the p -conjugated chains.[7]It has been reported that compared with fluorosensors that use small molecules as receptors,CP-based fluorosensors exhibit
amplified sensitivity in response to very minor perturbations owing to the collective properties resulting from the large number of receptor units along the polymer chains.[8]It is thus expected that EFC CPs are also attractive candidates for sen-sors,as they may be switched to a critical redox state at which a significant change in fluorescence intensity may be induced by their interaction with a low concentration of analytes in sur-rounding environments.
Cyanide is extremely toxic in biological systems,[9]and the widespread industrial use of cyanide-containing chemicals has generated strong interest in the sensitive and reliable detec-tion of cyanide.A commonly used cyanide detection approach is the fluorometric method,which makes use of fluorescence changes in receptors that react or interact with cyanide.Cya-nide is highly nucleophilic,so its nucleophilic attack on elec-tron-deficient compounds such as benzothiadiazole,[10]acridini-um,[11]pyrylium salt,[12]oxazine,[13]triazene,[14]trifluoroacetophe-none,[15]dipyrrole carboxamide,[16]salicylaldehyde,[17]and squaraine [18]offers a common fluorometric approach toward cyanide detection.Enlightened by this fluorometric detection approach,we recently demonstrated EFC detection of cyanide for the first time using a carbazole-benzothiadiazole (BTD)-based EFC conjugated copolymer as the receptor.[5]Through adjustment of the electronic structure of the fluorophore to a critical quenching state by application of a suitable potential,sensitive (and selective)“turn-on”detection of cyanide at 1.0m m is achieved.
The free cyanide anion (CN À)in aqueous solutions is the most common form of cyanide pollutant.For more efficient detection of CN À,it is desirable for a CN Àsensor to work well in aqueous environments.[19]The carbazole-BTD-based EFC co-[a]G.Ding,R.Zhou,Y.Dong,Prof.X.Lu
School of Materials Science and Engineering Nanyang Technological University
50Nanyang Avenue,639798,Singapore Fax:(+65)6790-9081E-mail:asxhlu@.sg [b]Dr.T.Lin,Dr.J.Xu
Institute of Materials Research and Engineering 3Research Link,117602,Singapore Fax:(+65)6872-7528
E-mail:
jw-xu@.sg
Supporting information for this article is available on the WWW under /10.1002/chem.201403133.
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Full Paper
DOI:10.1002/chem.201403133
polymer could form radical cations and dications along the backbone in oxidized states,which may render the polymer chains somewhat hydrophilic and help to pull water as well as dissolved CNÀinto the polymer film to some extent.To explore the possibility of the EFC detection of cyanide in aqueous envi-ronments,in this work,we designed and synthesized a carba-zole-BTD-based fluorophore that can be polymerized electro-chemically to form a nanoporous EFC CP electrode.This would significantly shorten the distance for CNÀto diffuse into the oxidized polymer thin film,increasing the accessible interac-tion sites between the receptor units and CNÀ.In this article, the structure,morphology,and EFC properties of the nanopo-rous polymer electrode are reported.Its capability for CNÀde-tection is also demonstrated in electrolytes with different volume fractions of water.Thus,it is shown that sensitive and selective EFC detection of cyanide can be achieved with this nanoporous electrode in a predominantly aqueous medium. Furthermore,molecular simulation results are also presented to illustrate the mechanism of the EFC detection.
2.Experimental Section
2.1Materials
3-Bromo-9-octylcarbazole was synthesized according to the litera-ture.[20]4,7-Bis(6-bromo-9-octylcarbazol-3-yl)-2,1,3-benzothiadiazole (Br-C-BTD-C-Br)was synthesized according to our previously re-ported procedure.[5]The other chemicals for synthesis and electro-chemical characterization were used as purchased.
2.2Chemical and morphological characterization
1H and13C NMR spectra were recorded on a Bruker DRX400MHz NMR spectrometer at room temperature using deuterated chloro-form(CDCl3)as solvent and tetramethylsilane(TMS)as an internal standard,with operating frequencies of400.13and100.61MHz for1H and13C NMR,respectively.High-resolution mass spectrosco-py(HRMS)of the monomer was performed with a Finnigan LCQ Mass Spectrometer.Size exclusion chromatography(SEC)analyses were performed on a Waters2690system using THF as eluent and polystyrene standards.Absorption spectra of the monomer and electrodeposited copolymer were measured with an Agilent Cary 5000UV/Vis/NIR spectrophotometer.Photoluminescence(PL)spec-tra of the polymer were recorded with a Perkin–Elmer LS-55fluo-rescence spectrometer.The surface morphology of the electrode-posited polymer thin film was examined with a Jeol JSM-6340F field-emission scanning electron microscope(FESEM).
2.3Monomer synthesis
Br-C-BTD-C-Br(848.8mg, 1.00mmol),3-hexylthiophene-2-boronic acid pinacol ester(588.5mg,2.00mmol),Aliquat 336(0.15g),and toluene(8mL)were added to a50mL Schlenk tube.After dissolu-tion of all the compounds,Na2CO3aqueous solution(2m,6mL) was added,and the mixture was degassed.Pd(PPh3)4(11.6mg, 0.01mmol)was then added and the tube was sealed.The mixture was stirred and heated at reflux at1108C for24h.After the reac-tion,the organic layer was separated and the solvent was removed under reduced pressure.The crude product was purified by silica gel column chromatography using hexane/dichloromethane(DCM, v/v=3:2)as eluent.Finally,the monomer,HT-C-BTD-C-HT(HT and C represent3-hexylthiophene and9-octylcarbazole,respectively) was obtained as an orange solid(94%).1H NMR:d=8.72(s,2H), 8.24(s,2H),8.16(d,2H),7.93(s,2H),7.57(t,4H),7.47(d,2H),7.24 (d,2H),7.02(d,2H),4.39(t,4H),2.73(t,4H),1.96(m,4H),1.66(m, 4H),1.24-1.30(m,32H),0.88(t,6H),0.81ppm(t,6H);13C NMR: d=155.1,141.2,140.7,139.3,138.6,133.7,129.7,129.1,128.9, 128.4,128.0,127.8,126.1,123.6,123.5,122.0,121.8,109.3,109.1, 43.8,32.2,32.1,31.5,29.8,29.6,29.1,27.8,23.0,14ppm;HRMS (ESI)m/z calcd for C66H78N4S3:1022.5389;found:1022.5388.
2.4Electropolymerization,cyclic voltammetry,and spectroe-lectrochemical characterization
The electrochemical polymerization was performed in a three-elec-trode cell using an Autolab PGSTAT302potentiostat.The cell con-sisted of a beaker filled with0.1m tetrabutylammonium hexafluor-ophosphate(TBAPF6)/polypropylene carbonate(PC)electrolyte (with a small amount of chloroform to dissolve the monomer fully),indium tin oxide(ITO)-coated glass(7W/sq)as the working electrode,platinum(Pt)foil as the counter electrode,and silver wire as the reference electrode.The monomer was dissolved in the electrolyte at a concentration of0.5m m,and then electropolymer-ized through the cyclic voltammetry(CV)method.The CV scans were conducted between0and1.1V for50cycles at a scan rate of100mV sÀ1.Cyclic voltammetry of the electrodeposited polymer film was conducted in a three-electrode electrochemical cell using an electrolyte of0.1m lithium perchlorate(LiClO4)/acetonitrile (ACN)as well as0.1m LiClO4dissolved in ACN/H2O mixed solvents (with different volume ratios),Pt foil as counter electrode,Ag wire as reference,and the same Autolab PGSTAT302potentiostat.The setup for spectroelectrochemical characterization was similar to that for the CV examinations except that a quartz cuvette and Pt wire were used to replace the beaker and Pt foil,respectively.Ab-sorption and photoluminescence(PL)spectra were recorded with the same Agilent Cary5000UV/Vis/NIR spectrophotometer and Perkin–Elmer LS-55fluorescence spectrometer as used in the chemical characterization,respectively,under varying potentials provided by the same potentiostat as used for the electrodeposi-tion.For both electrochemical and spectroelectrochemical charac-terization,the pseudoreference silver wire was calibrated versus Fc/Fc+by dissolving ferrocene in the electrolyte solution.
2.5Simulation
Geometry optimizations for the CNÀanion and the neutral and oxi-dized states of HT-C-BTD-C-HT were performed by using the DFT electronic structure program DMol3.[21]Perdew–Wang generalized-gradient approximation functionals(GGA-PW91)[22]and the density functional semi-core pseudopotentials(dspp)were chosen.A double numerical plus polarization(DNP)basis set with a global or-bital cutoff of4.0 was adopted.The convergence criteria of the energy,maximum force,maximum displacement,and self-consis-tent-field(SCF)density were set at10À5Hartrees,0.002Ha À1, 0.005 ,and10À6,respectively.The complexes of HT-C-BTD-C-HT and CNÀwere optimized by using the same method and criteria as for the isolated species.The binding energy D E was calculated as the total energy of the complex minus the sum of those of the two isolated species.The solvation effect was included by using the conductor-like screening model(COSMO).[23]The energies of the HOMO and LUMO were calculated for the optimized structure. Isosurfaces of the HOMO and LUMO were plotted with isovalues Æ0.03(positive in green and negative in orange).
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3.Results and Discussion
3.1Structure and morphology of the copolymer
The synthesis route used in this study is shown in Scheme 1.The monomer HT-C-BTD-C-HT is obtained through a Suzuki coupling reaction,and the subsequent electropolymerization
of HT-C-BTD-C-HT produces a conjugated copolymer,P EFC .In this copolymer,C-BTD-C moieties are redox-switchable fluoro-phores,whereas the HT units mainly play the role of fluoro-phore separators.The alkyl chains on the carbazole and HT rings help reduce the chance of interchain p –p stacking in the solid state.The structure of the monomer was verified by 1
H NMR spectroscopy and HRMS.In the 1H NMR spectrum (Fig-ure S1,Supporting Information),the peak at 4.38ppm corre-sponds to the H atoms that are bonded to the N atom in car-bazole (N ÀCH 2),and the peak at 2.73ppm corresponds to the H atoms from the CH 2unit that is bonded directly to the thio-phene ring (C ÀCH 2).The ratio of integration values of the two peaks is 1:1,which coincides with the value calculated on the basis of the molecular structure of the monomer,proving that the monomer was synthesized successfully.In addition,the HRMS results (cf.Section 2.3)further confirm the successful synthesis of the monomer.This compound shows good solu-bility in common organic solvents such as chloroform,DCM,and toluene,but its solubility in PC and ACN is not very good.Thus,a small amount of chloroform was added to 0.1m TBAPF 6/PC to dissolve the monomer completely for electro-chemical polymerization.
Figure 1a shows cyclic voltammograms for the electropoly-merization of HT-C-BTD-C-HT.In the first cycle,the oxidation peak appearing at around 0.95V should correspond to the oxi-dation of HT-C-BTD-C-HT at the 5(a )-position of the HT units.In the subsequent cycles,increased current densities indicate the progress of the oxidative polymerization,in which the mono-mers are coupled to form p -conjugated copolymers.Decreases in the onsets of the oxidation and reduction current densities with increasing cycle number reveals the extension of the ef-fective conjugation length in the polymer backbone.[2b]From the inset of Figure 1a,it is observed that a shoulder at about 0.85V emerges from the second cycle.This shoulder
corre-
Scheme 1.Synthesis route leading to the monomer and
copolymer.
Figure 1.a)Consecutive cyclic voltammograms for the electropolymeriza-tion;the inset shows the curves of the first three cycles.1H NMR spectra of b)monomer and c)polymer;the asterisk indicates CDCl 3.
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sponds to the radical cations of the 3,6-carbazole units [24]in the newly formed oligomer/polymer,and thus,further confirms the successful coupling of the monomers.
Owing to the presence of alkyl side chains on the carbazole and HT units,the electrodeposited copolymer P EFC can be dis-solved in organic solvents such as chloroform,DCM,and tolu-ene,making NMR analysis possible.In comparison with the sharp peaks in the 1H NMR spectrum of the monomer (Fig-ure 1b),the proton peaks of P EFC are much broader (Figure 1c),supporting the successful coupling between monomer units.Moreover,for the monomer,the peaks corresponding to the proton at the 5(a )-position of HT are well-defined doublets at around 7.24ppm (circled in Figure 1b),whereas they become a very weak shoulder for the electrodeposited film (circled in Figure 1c).This directly verifies that linkages are created at the 5(a )-position between the HT rings.However,it is also worth noting that the peak at 7.02ppm (squared in Figure 1c)corre-sponding to the proton at the 4(b )-position of HT is also much weaker than the corresponding peak for the monomer (squared in Figure 1b).This indicates that a –b or b –b coupling probably occurs,rather than a –a coupling exclusively,in this electrochemical polymerization.[25]SEC measurement of P EFC gives the apparent number-average molecular weight M n =7,000Da,with a polydispersity index of 1.38.This reveals that a polymer chain contains about seven repeating units on aver-age.
Figure 2shows the UV/Vis absorption spectra of the mono-mer (solution)and copolymer (film and solution).The absorp-tion band at 315nm for the film and that at 310nm for both the polymer and monomer solutions correspond to carbazole segments,whereas the bands at 450nm for the film and 440nm for the solutions can probably be attributed to the p –p *transition of the C-BTD-C moiety.[26]The bands of the film (at 315and 450nm)exhibit small redshifts in comparison with those of the polymer solution (at 310and 440nm)owing to the interchain p –p stacking caused by the denser packing of polymer chains in the solid state.The band at 365nm for the polymer is very strong,which may result from HT-HT moieties
on the backbone.The very weak band at 350nm for the mo-nomer solution may arise from the interactions between HT and carbazole units.[5]However,this band cannot be observed for the polymer solution and film,as it may be overlapped with the strong neighboring band from the HT-HT moieties.The UV/Vis results further verify the structure of P EFC and pro-vide a basis for EFC studies,which will be discussed in the next section.
Figure 3shows the surface morphology of the electrodepos-ited P EFC film.It is observed that the film consists of uniformly distributed granules of about 150–250nm,which are separat-ed by narrow gaps and composed of even smaller particles.Such a nanoporous morphology provides a large contact area between the polymer and the electrolyte,facilitating the inser-tion and extraction of ions into/from the polymer film during the redox process.Furthermore,the nanoporous morphology also shortens the distance for the electrolyte to diffuse into the polymer.
3.2Electrochemical and electrofluorochromic properties For the investigation of the possibility of using the nanoporous P EFC film for EFC detection of cyanide in aqueous or partially aqueous solutions,cyclic voltammetry was first conducted in electrolytes of 0.1m LiClO 4in ACN/H 2O mixed solvents with dif-ferent volume ratios of ACN/H 2O (v/v =1:0,1:1,1:2,0:1,denot-ed as R ,R 1:1,R 1:2,and R 0:1,respectively).Cyclic voltammograms of the electrodeposited P EFC film on ITO glass in R at different scan rates are shown in Figure 4a.At a low scan rate (10mV s À1),P EFC shows a clear oxidation peak and a weak shoulder at 0.88and 0.76V,respectively,as well as a reduction peak at 0.60V.For CPs containing N-alkyl substituted 3,6-car-bazole,two pairs of redox peaks corresponding to radical cat-ions and dications usually appear in their CV curves.[24]For P EFC ,only the redox peaks corresponding to dications are obvious.The first oxidation peak corresponding to the formation of rad-ical cations appears as a weak shoulder,and its corresponding reduction peak is probably too weak and therefore covered by the dication one.The small gap between the two peaks
(
Figure 2.Absorption spectra of the electrodeposited polymer film,polymer solution,and monomer solution.Chloroform was used as solvent for the two
solutions.
Figure 3.SEM images with different magnifications showing the surface morphology of the electropolymerized P EFC film.
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%0.12V)indicates a relatively easy transition from the radical cation to dication state,which could be attributed to the elec-tron-acceptor BTD,which facilitates electron removal from electron-rich carbazole,and thus,reduces the oxidation poten-tial.[27]CV curves of the P EFC film in R 1:2(as well as R 1:1,see Fig-ure S2)show similar redox peaks,but the oxidation potentials are significantly higher (Figure 4d)owing to the reduced swel-ling of P EFC in partially aqueous electrolytes.The anodic and cathodic peak current densities obtained from the CV curves are plotted against the scan rate and square root of the scan rate,respectively,as shown in Figure 4b,c for R and 4e,f for R 1:2.Nearly linear relationships between the peak current den-sities and scan rate,as well as nonlinear relationships between the peak current densities and square root of the scan rate,are obtained,indicating that the redox reaction of the polymer is close to a non-diffusion-controlled process [28]even in largely aqueous electrolyte.This may be attributed to the nanoporous morphology of the P EFC film,which makes the diffusion dis-tance inside the film fairly small,and hence,facilitates the quick motion of the ions across the film.However,in the fully aqueous electrolyte (R 0:1),P EFC cannot be oxidized at potentials below 1.5V (Figure S3),but the application of a potential above 1.5V would lead to over-oxidation and subsequent deg-radation of P EFC .Thus,the largely aqueous electrolyte R 1:2was chosen for cyanide detection studies,which will be discussed in the next section.
To demonstrate the EFC properties of the polymer,PL spec-tra of P EFC were recorded under a series of potentials.In the neutral state,P EFC shows a fluorescence peak at 580nm (the curve with the highest intensity in Figure 5a).Under positive potentials,the intensity of this peak starts to decrease at 0.4V
(Figure 5a),and drops to nearly zero at 0.7V.Thus,the oxida-tive quenching offers a high contrast ratio (I f /I f0)of 19.2(I f and I f0denote the PL intensities at 0.7V and in the neutral state,re-spectively).This fluorescence quenching is induced by electro-chemical oxidation,upon which the photoexcited polymer chains undergo nonradiative exciton decay.[2b]Indeed,the CV results (Figure 4a)show that the oxidation becomes obvious when the potential is over the onset oxidation potential at around 0.6V.The quenched fluorescence can be recovered under negative potentials (Figure 5b),and the quenching and recovery processes are reversible and repeatable (Figure 5c).However,under dynamic switching conditions,the contrast ratio (9.5)is lower (Figure 5c)than that under constant poten-tials (Figure 5a),which may be attributed to the relatively slow redox process.Similar EFC phenomena can be observed when using R 1:1or R 1:2as the electrolyte.3.3EFC detection of cyanide
The setup designed for EFC detection of cyanide consists of a three-electrode spectroelectrochemical cell filled with elec-trolyte R 1:2,which contains 67vol %water.The as-electrode-posited P EFC film (on ITO glass)is used as the detection elec-trode and immersed into the electrolyte,and Pt wire and Ag wire are used as the counter and reference electrodes,respec-tively.In addition,a spectrophotometer is used to record the PL intensities and a potentiostat is employed to provide vari-ous potentials.The detection procedure was as follows.At zero potential,the stabilized PL intensity was recorded as I 0when the system reached equilibrium.At a certain potential,the ratio of the corresponding stabilized PL intensity (I )to I
Figure 4.Cyclic voltammograms of the P EFC film in electrolytes R (a)and R 1:2(d)at different scan rates (10,25,50,75,and 100mV s À1).Plots of peak current density against scan rate (b and e)and scan rate 1/2(c and f)obtained from (a)and (d),respectively.
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was defined as the normalized intensity.The values of I /I 0ob-tained at various potentials were plotted against the potential,E ,as shown by the black squares (&)in Figure 6a.Then,the detection electrode was inserted into another cell filled with the corresponding electrolyte containing a certain concentra-tion of TBACN,and then the stabilized PL intensities were re-corded using the same series of potentials,giving another I /I 0ÀE curve.Three I /I 0ÀE curves corresponding to cyanide con-centrations of 10À6,10À5,and 10À4m ,respectively,were ob-tained,as shown in Figure 6a.
From Figure 6a,it is observed that the normalized intensity of P EFC in R 1:2decreases with increasing positive potential owing to oxidative quenching.However,with R 1:2+TBACN,the normalized intensities are higher than those obtained from R 1:2
only at each positive potential,indicating that the oxidative quenching is significantly weakened in the presence of TBACN.At potentials above 0.6V,TBACN concentrations as low as 1m m could induce a clear reduction in normalized intensity.The weakened PL intensity probably results from the partial re-duction caused by the interaction between nucleophilic CN Àand electron-deficient BTD,which will be illustrated in more detail in the next section.It is also observed that the normal-ized intensity is higher for electrolytes with higher TBACN con-centrations under the same positive potential.Hence,these I /I 0ÀE curves corresponding to different cyanide concentrations can be used as calibration curves to estimate the CN Àconcen-trations in the analytes to be tested.
The results presented above indicate that CN Àcould diffuse well into the polymer film in the largely aqueous electrolyte R 1:2.It is believed that the nanoporous morphology of the P EFC film shortens the distance for the CN Àanions in H 2O/ACN dif-fusing into the film,and the formation of radical cations and dications in oxidized states may bring some hydrophilicity to the polymer chains,and thus,benefit the diffusion of H 2O.Therefore,the concentration of CN Àin an aqueous analyte can be determined by mixing the analyte with a small amount of reference electrolyte R and using the mixed solution as
the
Figure 5.PL intensity (l exc =450nm)change of the P EFC film in R under a)positive and then b)negative potentials;c)PL intensity–time profile for switching between +0.7V and À0.8V with 80s per
cycle.
Figure 6.Normalized PL intensities (l exc =450nm,l monitor =580nm)of the P EFC film as a function of potential,E .a)The curves were obtained using electrolytes R 1:2and R 1:2+TBACN.The TBACN concentrations are 10À6,10À5,and 10À4m .b)The curves were obtained using R 1:2and R 1:2+TBACN
(10À5m ),and R 1:2+seven types of other TBA-based anions (F À,Cl À,Br À,I À,AcO À,NO 3À,and HSO 4À;10À5m each),with and without TBACN (10À5m ).
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electrolyte in EFC tests.The amount of ACN required is rela-tively small,so the detection limit would not be reduced sig-nificantly.
To check whether the cyanide anion would be detected selec-tively using this system,we added seven other types of tet-rabutylammonium (TBA)-based anions including fluoride (F À),chloride (Cl À),bromide (Br À),
iodide (I À),acetate (AcO À),ni-trate (NO 3À),and bisulfate (HSO 4À)(10À5m each)into R 1:2together.As shown in Figure 6b,
the changes in PL intensity upon oxidative quenching are almost the same for the electrolytes with (*)and without (&)these anions.This confirms that CN Àrather than the TBA cation dominates the partial recovery of the PL intensity during oxidative quenching.More importantly,it shows that
the cyanide–P EFC interaction is very specific,as P EFC is not sensi-tive to the other anions.The electrolyte of R 1:2
containing TBACN (10À5m )and seven other types of anions (10À5m each)was used to further verify the selectivity for CN À
.The results
show that the normalized PL intensities of P EFC obtained using the electrolyte R 1:2containing TBACN (10À5m )are not distur-bed by the presence of the other seven types of anions,fur-ther confirming the selectivity of this detection system for
CN À
.
3.4M echanism of cyanide detection
It is believed that the effective EFC detection of cyanide comes from the energetically favorable noncovalent interaction be-tween the nucleophilic anion (CN À)and electropositive aromat-ic ring (BTD),which is a reversible process.[5]
The selective de-tection of CN Àover other anions such as F Àmay be attributed to the strong nucleophilicity and triple bond of CN À.The triple bond may provide an electron-rich p system for noncovalent interaction with the electron-deficient p system of BTD,lead-ing to charge transfer between CN Àand BTD (p À–p +interac-tion),and hence,weakening the oxidative quenching.
For verification of the postulated underlying mechanism,one repeating unit of the copolymer,HT-C-BTD-C-HT,was mod-eled theoretically as a surrogate for P EFC ,and optimized by density functional theory (DFT)calculations.Figure 7shows the calculated HOMOs and LUMOs of the model compound (MC)during its interactions with CN Àin different redox states.It is observed that the CN À–BTD interaction in the neutral state is not obvious.However,in the oxidized states,strong interac-tions occur between CN Àand BTD,revealed by DFT calcula-tions of radical cation (and dication)–CN Àinteractions (as shown in Figure 7).It is also seen that further oxidation (dicat-ion)does not lead to significantly stronger interactions than
for the radical cation.This is probably because the electron de-ficiency of p +
(BTD)is sufficient for the radical cation to inter-act with nucleophilic p À(CN À).The p +–p Àinteraction between a radical cation and a CN Àanion is illustrated in Figure 8.On the basis of the DFT calculation results shown in Table 1,it is found that the polymer–CN Àinteraction is energy favora-ble for each medium,vacuum,ACN,and H 2O,respectively,that is,E neutral p -cyanide <E radical cation-cyanide <E dication-cyanide .These results confirm the interaction between the polymer and CN À.In addi-tion,it is also observed that the free energy of the polymer–
CN Àinteraction complex increases with the oxidation state of the polymer,indicating that a stronger CN À–p interaction occurs for p in a higher oxidized state.This is consistent with the experimental results that higher oxidation potentials induce stronger weakening effects to oxidative quenching.Fur-thermore,for the electrolytes of the mixed solvent ACN/H 2O (v/v =1:1and 1:2),the dielectric constants are within the range of those of the two pure solvents.Thus,the CN À–p interaction
in these electrolytes is anticipated to result in a similar increase
in free energy for higher oxidation states of p .For further confirmation of the nature of the CN À–p interac-tion,cyclic voltammetry of the P EFC film was conducted in the electrolytes containing different concentrations of CN À.As
il-Figure 7.Spatial distributions of the calculated HOMOs and LUMOs of model compound HT-C-BTD-C-HT,which in-teracts with CN À
in the neutral,radical cation,and dication states (in vacuum).Note:all alkyl side chains are re-placed by methyl groups for better schematic
representation.
Figure 8.Schematic illustration of the p +–p Àinteraction between a radical
cation and a CN Àanion.Hydrogen atoms are hidden and the unlabeled
balls represent carbon atoms.
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