Unconventional Fluorescence Quenching in Naphthalimide-CappedCdSe/ZnS NanoparticlesJordi Aguilera-Sigalat,† Vania F. Pais,‡ A. Domenech-Carbo,§ Uwe Pischel,‡ Raquel E. Galian,*,†
and Julia Perez-Prieto*,†
†Instituto de Ciencia Molecular (ICMol), Universidad de Valencia, Catedratico Jose Beltran 2, 46980, Paterna, Valencia, Spain‡CIQSO - Centro de Investigacion en Química Sostenible and Departamento de Ingeniería Química, Química Física y QuímicaOrganica, Universidad de Huelva, Campus de El Carmen, s/n, 21071 Huelva, Spain§Departamento de Química Analítica, Universidad de Valencia, Dr. Moliner 50, 46100 Burjassot, Valencia, Spain
*S Supporting Information
ABSTRACT: Core−shell (CS) CdSe/ZnS quantum dots(QD) capped with ligands that possess a mercapto or anamino group and a naphthalimide (NI) as chromophore unit,linked by a short ethylene chain (CS@S−NI and CS@H2N−NI, respectively), have been synthesized and fully characterizedby infrared and nuclear magnetic resonance spectroscopies,high-resolution transmission electron microscopy, and voltam-metry as well as by steady-state absorption and emissionspectroscopies. The organic ligands HS−NI and H2N−NI actas bidentate ligands, thereby causing a drastic decrease in the QD emission. This was particularly evident in the case of CS@S−NI. This behavior has been compared with that of commercially available QDs with octadecylamine as the surface ligand and aQD capped with decanethiol ligands (CS@S−D). The interaction between the anchor groups and the QD surface brings aboutdifferent consequences for the radiative and nonradiative kinetics, depending on the nature of the anchor group. Our resultssuggest that the naphthalimide group “stabilizes” empty deep trap states due to the carbonyl group capacity to act as both a σ-donor and a π-acceptor toward cations. In addition, the thiolate group can induce the location of electron density at shallow trapstates close to the conduction band edge due to the alteration of the QD surface provoked by the thiolate binding.
1. INTRODUCTION
Semiconductor nanoparticles (quantum dots, QDs) exhibithigh fluorescence, which makes them attractive for biologicaland medical applications, among others.1,2 The semiconductorbulk material possesses defect states that originate fromimpurities, divacancies, or surface reactions as a result of theirsynthesis. The ratio between the defect states and the numberof atoms increases in the semiconductor nanoparticle due to itshigh surface-to-volume ratio. The efficiency of the QD band-edge luminescence decreases through the implication ofadditional transition levels caused by defect states in theforbidden band of the QD. Energy relaxation and recombina-tion dynamics in QDs strongly depend on surface passivation.Hence, in core−shell (CS) CdSe/ZnS QDs, the ZnS shell playsa crucial role in their emissive properties, enhancing thechemical stability and photostability of the core. However, theshell is far from perfect at the surface, and if tunneling ofcharges through the shell occurs, these defects could also serveas trap sites due to the presence of dangling bonds.3,4 Thefluorescence of CdSe/ZnS QDs exhibits multiexponentialdecays; this phenomenon has been attributed, in addition todifferences between the individual nanoparticles in the colloidalsolution, to a recombination of (i) delocalized carriers in theinternal core states and (ii) localized carriers at the heterointer-
face of the QDs. The localization of electrons or holes may begenerated at the heterointerface of the QDs due to interfaceroughness or defects.5 Furthermore, there are two likelylocations for the trapped charges to reside on the CdSe/ZnSQDs: (i) at the core/shell interface and (ii) on the shell surface.In this regard, the capping with organic ligands, which is
needed to produce stable colloidal solutions of QDs in organicsolvents and water, plays a key role for the photophysicalproperties of the QDs. The spherical QD nanoparticle canincorporate a considerable number of organic molecules(ligands) in its periphery.The role of the organic ligands as quenchers of the QD
fluorescence, as a result of either photoinduced transferimplicating the QD electron (e−)/hole (h+) or energy transferto the ligand, has been extensively studied.6−9 However, thenature of the group anchoring the ligand to the QD surfaceatoms is of special relevance for the QD emissive properties,since that group can increase the coordination number of thesurface atoms and, as a consequence, reduce dangling bondenergy states within the band gap, which otherwise may act as
Received: December 29, 2012Revised: March 16, 2013Published: March 20, 2013
nonradiative centers.3,4 Alternatively, the anchoring group caninfluence the tunneling of the electron of the photogeneratedexciton and its localization in trap states. For example, it hasbeen reported that certain electron-accepting organic ligands,such as pyridine, attached to the QD surface can trap thephotogenerated exciton and decrease the emission efficiency asa result of creating new surface or mid-band-gap states that arecharacterized by a fast nonradiative relaxation.10 However, QDcapping with electron-donating moieties can result in a nearlycomplete suppression of QD blinking.11−13 The drasticdecrease of blinking after addition of short-chain thiols (suchas β-mercaptoethanol) to streptavidin-coated CdSe/ZnS QDshas been attributed to electron donation from the thiol moietyto the surface electron traps.11 However, the thiol effect onCdSe/ZnS QD blinking dynamics can be more complex, eitherincreasing or decreasing the frequency of blinking.14 Reductionof the QD blinking has been related with an increase in theradiative rate constant accompanied by a reduction in thenonradiative rate constant.12 For example, propyl gallate hasbeen proven to influence the radiative and nonradiative excitedstate decay of CdSe/ZnS QDs, yielding highly luminescentQDs. Inversion of the excited energy level structure of the QDsand preferential thermal population of the bright exciton stateor the partial mixing of bright character into the dark excitonhave been suggested as plausible explanations for the increasedradiative rate constant induced by the organic ligand.Especially with respect to the understanding of unconven-
tional quenching, not involving electron or energy transfer, itseems highly desirable to further investigate the dependence ofthe photophysical properties of the CdSe/ZnS QDs on thenature of the ligand anchor group. Here we report on theoptical properties of CdSe/ZnS QDs capped with ligands whichpossess a mercapto or an amino group and a naphthalimide(NI) group as chromophore unit, linked by a short ethylenechain (CS@S−NI and CS@H2N−NI, respectively; Figure 1).The organic ligands HS−NI and H2N−NI (Figure 1) may actas bidentate ligands, causing a drastic decrease in the QDemission, in particular in the case of CS@S−NI. Theinteraction between the anchor groups and the QD surfacehad different consequences on the radiative and nonradiativekinetics, depending on the nature of the anchor group. Steady-state and time-resolved absorption and emission spectroscopiesas well as voltammetric (CV) data are in accordance with theinterpretation that the naphthalimide group “stabilizes” emptydeep trap states and that the sulfur anchor group induces thelocalization of electron density at shallow trap states close tothe conduction band edge. As a consequence, the QD−ligandinteraction through the thiol sulfur and the naphthalimideprovides two different channels of controlling the CdSe/ZnSfluorescence quenching.
2. EXPERIMENTAL SECTIONMaterials. All reagents were commercially available and
used as received: decanethiol, 1,8-naphthalic anhydride, DL-dithiothreitol, cysteamine (Sigma-Aldrich); ethylenediamine,NH4Cl (Fluka); chloroform, ethyl acetate, ethanol, acetonitrile(Scharlau); Na2SO4 (Panreac). Core−shell QDs capped with along-chain primary amine were purchased from OceanNanoTech. Solvents were of reagent grade and used withoutfurther purification. HS−NI and H2N−NI are knowncompounds.15,16 The synthetic procedure of HS−NI wasmodified (see below). The NMR spectroscopic data matchedthe reported ones.
Synthesis of N-(2-Aminoethyl)-1,8-naphthalimide (H2N−NI). H2N−NI was synthesized according to a reportedprocedure (see Figure S1 in the Supporting Information).16
Synthesis of N-(2-Mercaptoethyl)-1,8-naphthalimide (HS−NI). 1,8-Naphthalic anhydride (396 mg, 2.0 mmol) was placedin a round-bottomed flask with a magnetic stirrer and refluxcondenser, and 15 mL of absolute ethanol and cysteamine (193mg, 2.5 mmol) were added. The solution was heated to refluxwith constant stirring for 6 h. After cooling to roomtemperature, 30 mL of water was added and the precipitatewas filtered. The solid was washed with water, 1:1 ethanol/water, ethanol, and vacuum-dried to yield 546 mg of a 90:10disulfide/thiol mixture. The mixture was dissolved in 20 mL ofCHCl3, and DL-dithiothreitol (660 mg, 4.28 mmol) was added.The suspension was warmed to 37 °C for 3 days. After coolingto room temperature 30 mL of water was added, and thecompound was extracted with ethyl acetate (2 × 15 mL). Theorganic phase was washed with brine (2 × 15 mL) andsaturated NH4Cl solution (2 × 15 mL) and dried overanhydrous Na2SO4. Removal of the solvent resulted in a solidwhich was recrystallized from (1:1) chloroform/acetonitrile.After vacuum drying, 246 mg of a slightly yellowish solid wasobtained (48% yield over the two steps). The 1H NMRspectrum of the herein prepared sample is identical to reporteddata (Figure S2).15 1H NMR (400 MHz, CDCl3): δ 8.61 (d, J =7.6 Hz, 2H), 8.23 (d, J = 8.4 Hz, 2H), 7.77 (t, J = 7.8 Hz, 2H),4.38 (t, J = 7.6 Hz, 2H), 2.92−2.86 (m, 2H), 1.53 (t, J = 8.6 Hz,1H) ppm.
Synthesis of CdSe/ZnS QDs Capped with HS−D Thiolate(CS@S−D). For the ligand exchange procedure, 2 mL of thecommercial QD and 0.296 mL of HS-D (molar ratio betweenQD:ligand is 1:5000) were added in a flask and heated to refluxin 40 mL of chloroform for 48 h, under N2 flow in the absenceof light. Then, the mixture was cooled to room temperature.For the purification, the nanocrystals were precipitated fromMeOH several times. Finally, CS@S−D was dissolved intoluene.
Synthesis of CdSe/ZnS QDs Capped with HS−NI or H2N−NI (CS@2HN−NI and (CS@S−NI). For the ligand exchangeprocedure, 2 mL of the commercial QD and 9.4 mg (3.65 ×10−5 mol) of HS−NI or 8.8 mg (3.65 × 10−5 mol) of H2N−NI(QD:ligand molar ratio of 1:500) were added in a flask andheated to reflux in 40 mL of chloroform for 48 h, under N2 flowin the absence of light. Then, the mixture was cooled down toroom temperature. For the purification, the nanocrystals wereprecipitated three times from dioxane. Finally, the QDs weredissolved in 3 mL of toluene.
Characterization. UV−vis spectra of the samples wererecorded with a UV−vis spectrophotometer (Agilent 8453E).The average diameter value of the nanoparticles was estimatedfollowing the procedure published by Peng et al.17 Steady-statefluorescence spectra were measured on a spectrofluorometerPTI, equipped with a lamp power supply (LPS-220B), motordriver (MD-5020), and Brytebox PTI, and working at roomtemperature. The fluorescence quantum yield was calculated byfollowing the procedure of Resch-Genger et al.18 The emissionlifetime measurements were done by time-correlated single-photon-counting (Edinburgh Instruments FLS 920) using apicosecond pulsed UV-LED (EPL 445, λ = 442.2 nm) as
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excitation source. Deconvolution analysis of the kinetic tracesyielded the luminescence lifetimes. The instrument responsefunction was recorded with a light-scattering Ludox solution.For multiexponential decays the amplitude-averaged emissionlifetime τav was stated. In this equation, τi are the decay timesand αi represent the amplitudes of the components. Theirrelationship with the fractional amplitude (ai), intensity ( f i),and quantum yield (Φi) of each component is also presented:
αα
=∑ =
aii
in
i1
∑ ∑τ τ= == =
a awith 1i
n
i ii
n
iav1 1
ττ
ττ
=∑
==
fa
aa
ii
in
i i
i ii
1 av
ττ
Φ = Φ = Φfa
i ii i
fav
f
Laser flash photolysis (LFP) studies were performed with apulsed Nd:YAG laser, using 355 nm as excitation source. Thepulse width was ca. 10 ns, and the energy was ca. 15 mJ/pulse.A xenon lamp was employed as the detecting light source. Thephotomultiplier-amplified output signal was transferred to apersonal computer. The optical density of the QD samples (3mL) was adjusted to ca. 0.3 at 355 nm, and they were bubbledwith N2 for 10 min prior to the measurements. IRmeasurements were performed on a Bruker Equinox 55/IR-Scope II equipped with a multiple reflection unit for attenuatedtotal reflectance (ATR) measurements on solids. Usually 1−2drops of QD solutions were placed on the sample holder, andafter drying, spectra were collected. 1H NMR spectra wererecorded on Bruker Avance DPX400 spectrometer equippedwith a QNP 1H/13C/19F/31P probe. Standard Bruker softwarewas used for acquisition and processing routines. Chemicalshifts are given in ppm and internally referenced to TMS.Images of the QDs were obtained by high-resolution tunnelingmicroscopy (HRTEM) on a FEI Tecnai G2 F20 at anaccelerating voltage of 200 kV. Samples were prepared bydropping the colloidal solution on a Lacey Formvar/carbon-coated copper grid. The digital analysis of the HRTEMmicrographs was done by using digital Micrograph TM 1.80.70for GMS by Gatan. Images were treated with the ImageJsoftware. Voltammetric measurements were carried out in aconventional three-electrode cell using BAS 660I equipment.Glassy carbon (GCE) and gold working electrodes werecomplemented with a Pt-wire auxiliary electrode and a Pt diskpseudoreference electrode. The potentials are stated relative tothe ferrocenium/ferrocene couple (0.1 mM). These measure-ments were performed with ca. 5 μM solutions of the differentQD systems in 50/50 (v/v) toluene/MeCN (0.10 MBu4NPF6) which were deaerated by bubbling with Ar. Blankexperiments were performed with 1.0 mM solutions of thedifferent capping reagents in the same electrolyte.
3. RESULTS AND DISCUSSIONSynthesis of the QDs. Commercial (Ocean Nanotech)
core−shell (CS) CdSe/ZnS QDs (CS518: λem = 518 nm andCS524: λem = 524 nm at λex = 400 nm; the only specification bythe supplier is that octadecylamine is the surface ligand) wereused as precursors of the naphthalimide-capped QDs (CS@S−
NI and CS@H2N−NI, Figure 1B). Typically, a chloroformsolution of the QDs and the naphthalimide ligand, [ligand]/
[QD] = 500, was heated to reflux under a nitrogen atmospherefor 48 h in the dark (see Experimental Section). The reactionmixture was cooled to room temperature, and the nanocrystalswere precipitated from dioxane. Finally, the QD was dissolvedin toluene, and they remained stable for more than 3 months.High-resolution transmission electron microscopy (HRTEM)images showed the QDs maintained the size and thecrystallinity of the commercial QD (see Figure S3 for CS@S−NI 3.7 ± 0.5 nm and CS518 3.7 ± 0.4 nm). For the sake ofcomparison, QDs capped with decanethiol (CS@S-D) werealso synthesized using the same methodology, except that theywere precipitated from methanol (for detailed characterizationdata of the QDs see Table S1).A comparison between the 1H NMR spectrum of CS@S−NI
and that of the free ligand (HS−NI), both in deuteratedtoluene (Figure S4 in Supporting Information), evidenced theattachment of the ligand to the QD surface. As expected, thespectrum showed considerable broadening of the ligand signalsbut also evidenced a considerable shift of its aliphatic andaromatic protons to a lower field and partial disappearance ofthe CH2−S signal. These facts suggest that the ligand could actas a bidentate ligand, involving not only the mercapto but alsothe naphthalimide group, the latter presumably through thecarbonyl group (Figure 1B).The infrared (IR) spectrum of CS@S−NI was also registered
to corroborate the interaction between the naphthalimidegroup and the QD surface. Figure 2 shows the comparisonbetween the strong absorption bands of νCO (1620−1720cm−1) of CS@S−NI and HS−NI. The IR spectrum of HS−NIshows the characteristic features of naphthalimides:19,20 fourbands in the 1600−1800 cm−1 region. The bands at 1695 and1657 cm−1 are assigned to the asymmetric and symmetric C
Figure 1. (A) Ligand structures and (B) proposed binding modes ofnaphthalimide ligands to the CdSe/ZnS QDs used in this report.
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O stretching modes, respectively, while the other two bands at1624 and 1603 cm−1 originate from aromatic ring vibrations(ν(Ar)). The changes of the CO bands were smaller in thecase of CS@H2N−NI, suggesting a less effective interactionbetween the naphthalimide group and the nanoparticle surface(see comparison between H2N−NI and CS@H2N−NI inFigure S5). The presence of a strong, broad band at ca. 1550cm−1 for CS@H2N−NI (not detected for the free ligand) canbe attributed to the NH2 scissoring mode of H2N−NI bound tothe shell.21
Photophysical Characterization of the QDs. Here weonly discuss the data related to the QDs arising from CS518(those arising from CS524 showed similar results). The UV−vis absorption spectrum of the QDs showed that the exchangeof the CS518 ligands by H2N−NI, HS−NI, and HS−D (Figure1) led to a red-shift of the exciton peak by ca. 4 nm. An averagenumber of 57 ligands per QD was estimated for CS@S−NIusing the molar absorption coefficient of the ligand (3535M−1 cm−1 at 355 nm). Figure 3 shows the comparison betweenthe spectra of CS@S−NI, CS518, and the free ligand.
The fluorescence spectrum of CS@S−NI was recorded at λex= 400 nm, where the ligand does not absorb. It should be notedthat the QDs showed no deep trap emission, which wouldmanifest at longer wavelengths than observed for the excitonemission. For the sake of comparison, Figure 4 shows theemission spectra of optically matched solutions of CS@S−NI,CS@S−D, CS@H2N−NI, and CS518. The thiol-capped QDs,CS@S−NI and CS@S−D, exhibited a slightly red-shifted
emission (by 2 and 1 nm, respectively) compared with that ofCS518. The QD emission quantum yields (Φf) followed theorder CS@S−NI (0.06) < CS@S−D (0.08) < CS@H2N−NI(0.18) < CS518 (0.45). Consequently, the reduced emission ofthe thiol-capped QDs appeared to be mainly related to theattachment of the organic ligand through the sulfur atom, butapparently the naphthalimide moiety also contributed to thedecrease of the luminescence of the naphthalimide-cappedQDs. It has to be taken into account that the thiolate group canbind to the surface of CdSe/ZnS QDs and give rise to anenhancement or a drastic decrease of the QD fluorescence,depending on the QD ligand and conditions used for the ligandexchange.22−25
Photoluminescence excitation (PLE) spectroscopy has beenused by Bawendi and co-workers for mapping the electronicstates of CdSe QDs.26,27 The comparison between theexcitation spectra (from 500 to 400 nm, monitored at theemission maximum of the QD) of CS@S−NI, CS@S−D, CS@NH2−NI, and CS518 normalized at 446 nm showed that theligand exchange did not produce a significant shift in the energyof the electronic states (Figure 5).
At high excitation photon energies, the PLE signal of the QDdecreased drastically compared with the absorption signal. Ithas been postulated that high excitation photon energies createholes with higher energy than that of the core−shell barrier.Such holes become more exposed to the shell surface defectsand are likely to be lost through nonradiative pathways.28
Figure 2. IR spectrum of CS@S−NI (red) and HS−NI (black) in the1570−1720 cm−1 region. The spectra have been scaled to the intensityof the ν(CO) band at ∼1700 cm−1.
Figure 3. Absorption spectra of the commercial CS518 (1 × 10−5 M,black), CS@S−NI (1 × 10−5 M, blue), and HS−NI (5.5 × 10−5 M,red) in deaerated toluene.
Figure 4. Comparative fluorescence spectra (λex = 400 nm, Abs400 =0.1) of deaerated toluene solutions of CS524 (black), CS@S−NI(red), CS@H2N−NI (blue), and CS@S−D (green).
Figure 5. Comparative excitation spectra (λem at 516 nm) of deaeratedtoluene solutions of CS518 (black), CS@S−NI (red), CS@H2N−NI(blue), and CS@S−D (green). The spectra were normalized at 446nm.
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In addition, the PLE spectra of CS@S−NI and CS@H2N−NI registered at the wavelength interval between 300 and 350
nm, where the naphthalimide also absorbs, evidenced the
occurrence of fluorescence resonance energy transfer (FRET)
from the naphthalimide moiety to the QD. The upper limit for
the FRET efficiency, estimated from the naphthalimide
emission quenching, was 0.56 for the case of CS@S−NI (seeSupporting Information).29
Moreover, we investigated the fluorescence lifetime of theQDs to gain further insight into the effect of naphthalimidegroup on the emissive properties of the QDs when they areexcited at a wavelength (λex = 446 nm) where thenaphthalimide does not absorb (see Table 1 and Table S2).
Table 1. Average Fluorescence Lifetime (τav) and Quantum Yield (Φf), Radiative (kr), and Nonradiative (knr) Rate Constants ofDeaerated Toluene Solutions of CS518, CS@S−NI, CS@H2N−NI, and CS@S−D in the Presence and in the Absence ofDodecylamine (DDA)a
CS518 0.45 24.7 1.82 2.23DDA 0 h 0.43 24.8 1.74 2.30DDA 72 h 0.45 23.3 1.93 6 2.36 6
CS@S−NI 0.06 15.8 0.38 5.94DDA 0 h 0.031 18.6 0.17 5.21DDA 72 h 0.074 22.2 0.33 −13 4.17 −30
CS@H2N−NI 0.18 21.9 0.82 3.73DDA 0 h 0.165 22.6 0.73 3.69DDA 72 h 0.217 22.5 0.96 17 3.47 −7
CS@S−D 0.08 38.8 0.21 2.37DDA 0 h 0.038 38.3 0.10 2.51DDA 72 h 0.091 38.4 0.24 14 2.37 0
aThe samples were excited at λex = 446 nm (where the naphthalimide does not absorb), and the decay was registered at λem = 525 nm for all QDsexcept for CS@S−NI (λem = 529 nm).
Figure 6. Fractional amplitudes (A), lifetimes (B), and fractional quantum yields (C) of the three components (short-, medium-, and long-lived) ofCS@S−D, CS@S−NI, CS@H2N−NI, and CS518 in the absence and in the presence of dodecylamine.
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The decay of the emission was registered at λem = 525 nm forall QDs except for CS@S−NI (λem = 529 nm). Time-resolvedfluorescence experiments showed that the average fluorescencelifetime (τav) of CS518 was ca. 25 ns, and those of the othersfollowed the order CS@S−D > CS@H2N−NI > CS@S−NI(values of ca. 38, 22, and 16 ns, respectively). Taking intoaccount the QD emission quantum yield (Φf) and theexpression Φf = kr/(kr + knr), where kr and knr refer to theradiative and nonradiative rate constants, respectively, thevariation of the kr and knr values with the capping ligand naturewas analyzed.Both CS@S−NI and CS@S−D exhibited kr values much
smaller than that of CS518, indicating that the sulfur atomsslow down the radiative process. This result is in accordancewith the previous finding of smaller radiative constants forthiol-capped CdSe/ZnS QDs as compared to primary-amine-capped QDs.6 This can be related with the reported capacity ofthe thiolate anchor group to donate electrons to the QD surfaceelectron traps, thereby making them incapable of acceptingelectrons from the QD.11
Interestingly, CS@S−NI showed a much higher nonradiativerate constant compared to that of the CS518 precursor. Thiscannot be attributed to the sulfur atoms at the QD surface,since knr of CS@S−D was very similar to that of CS518, but itshould be ascribed to the naphthalimide moiety. In accordance,CS@H2N−NI exhibited a greater knr than CS518, though itsvalue was considerably smaller than that of CS@S−NI. Theinvolvement of the naphthalimide moiety in electron-transferprocesses should be ruled out, since its LUMO is at ca. 0.6 eVhigher energy than that of the QD conduction band (seevoltammetric measurements below). Therefore, the enhancednonradiative QD emission decay may be attributed to an activetrapping surface state on the QD/naphthalimide interface,30
making the nonradiative processes more competitive.The complex emission of the QDs was fitted with a sum of
three exponential decays: a short (1.15−3.66 ns), a medium(10.06−18.21 ns), and a long (57.76−73.99 ns) component;see data in Table S2 as well as the fitting for CS518 and CS@S−NI in Figures S6 and S7, respectively, and the fitting detailsin the Supporting Information. In addition, Figure 6 shows thechanges of the lifetime (τ1, τ2, and τ3), the fractional emissionamplitude (a1, a2, and a3), and quantum yield (Φ1, Φ2, Φ3) ofthe three (short, medium, long) components induced by ligandexchange of CS518 by NH2−NI, HS−NI, and HS−D (seeExperimental Section for the relation between these variables).Noticeably, important differences or similarities were appre-ciated for the three fractional amplitudes for the different QDs.Thus, the relative contribution of the three emissioncomponents were similar for CS@H2N−NI and CS518,being the medium component strongly dominant (Figure6A). However, both the short and medium components had animportant contribution (a1 and a2 > 20%) in CS@S−NIfluorescence, while the medium and long components weredominant for CS@S−NI (a2 and a3 > 20%). With respect tothe lifetimes, the medium component lifetime (τ2) decreased inCS@S−NI and CS@H2N−NI compared with that of CS518(Figure 6B), while the lifetime of the long component (τ3) ofCS@S-D was the largest. Finally, the fractional quantum yieldof each component of the prepared QDs was compared withthat of CS518, whose medium and long components were thelargest. Thus, (i) the quantum yield of the three componentsdecreased drastically in CS@S−D, (ii) Φ2 was even smaller inCS@S−NI than in CS@S−D, while CS@S−NI exhibited a
larger Φ1, and (iii) CS@H2N−NI showed an intermediatebehavior, with a smaller decrease of Φ2 and Φ3, but presentingthe highest value for Φ1. It should be noted that besides thediscussed dynamic quenching, also the occurrence of staticquenching of the QD emission by the ligand is supposed toproceed due to the preorganized nature of the ligands on theQD surface.Laser flash photolysis (LFP, Nd:YAG, 355 nm, 10 ns pulse,
15 mJ/pulse) studies of the QDs were also performed, aimingto further reveal the role of naphthalimide in the excited statebehavior of the herein investigated QDs. Time-resolvedtransient spectra of deaerated solutions of CS518 showed astrong bleaching of the QD absorption assigned to the CdSe1s3/21se transition (see Figure 7). However, excitation of CS@
S−NI gave rise to an absorption band with a maximum at 470nm, which can be ascribed to the naphthalimide triplet excitedstate,31 and a broad absorption from 520 to 700 nm (Figure 7).The latter could be attributed to a combination of surface-charge trapping and excited-state absorption;32 the naphthali-mide group played a key role in both its formation andconsiderable long lifetime (ca. 70 ns). The same species wasdetected when the measurements were performed with CS518/H2N−NI (1:100 molar ratio) mixtures. However, in the case ofCS@S−D the transient was absent.It should be noted that a time-dependent buildup of a small
transient feature between 400 and 450 nm (yellow spectrum inFigure 7) was detected in the LFP of CS@S−NI; this mayindeed be tentatively ascribed to the radical anion. A possibleexplanation is the homo-electron-transfer between excited- andground-state naphthalimides which is favored by their closemutual interaction on the quantum dot surface. Thisphenomenon has been discussed for bisnaphthalimidedyads.31 The time scale (70 ns after the pulse) of theappearance of the radical anion feature makes it in principle lesscredible that this transient has its origin in excited singletnaphthalimides, which are known to be much shorter lived (<1ns).31 However, the corresponding homo-electron-transferinvolving excited triplet states of the ligand was estimated tobe thermodynamically less favored than for the singlet state.31
The radical anions are not expected to derive from electrontransfer from the excited quantum dot to the naphthalimidesbecause the time scale of the appearance of the signal is longerthan the time scale of the quantum dot emission quenching.
Figure 7. Transient absorption spectra of deaerated toluene solutionsof CS518 0.05 μs (black), 0.07 μs (red), and 0.12 μs (dark blue) afterthe laser pulse and of CS@S−NI 0.05 μs (green), 0.07 μs (yellow),and 0.12 μs (light blue) after the laser pulse. Inset: kinetic decay traceof the transient absorption of CS@S−NI registered at 620 nm.
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For comparison, the laser excitation of the ligand HS−NI at355 nm showed not only the absorption band ascribed to theNI triplet excited state (at longer time scales) but also that ofthe naphthalimide radical anion at 420 nm (at a short timescale), resulting from electron transfer from the thiol group(see Figure S8). The dominant formation of the radical anionin the case of the free ligand is contrasted by a rather weaktransient absorption of the quantum dot conjugate. Wepresume that electron transfer from the surface-bound thiol isquite hindered.Addition of Primary Amines to the QDs. The addition
of compounds with a less electron-donating anchor group thanmercapto could counteract the electron donation of the thiolateor/and remove the naphthalimide group from the QD surface.Therefore, we studied the effect of primary amines, such asdodecylamine (DDA) and phenylethylamine (PEA), on thephotophysical properties of the QDs, with and withoutnaphthalimide capping ligands.The QD emission changes caused by the addition of primary
amines ([amine]/[QD] = 100) were registered at differenttimes of standing (from 0 to up to 72 h). The amines producedno detectable variations of CS518 emission, while in the case ofthe other QDs, the quenching of the emission registeredimmediately after the addition of the amine was not onlyrecovered but even enhanced after 72 h of the addition(enhancement between 14 and 23%, Table 1). Figure 8compares the results for the different naphthalimide-cappedQDs.
With regard to the nonradiative rate constants, the amineaddition caused a 30% and 7% decrease in the case of CS@S−NI and CS@H2N−NI, respectively, but there was a 6% increaseand no effect on the CS518 and QD@S−D rate constants,respectively. Moreover, the amine addition increased theradiative rate constant in all cases, except for CS@S−NI.Figure 6 shows the changes of the lifetime, fractional
emission amplitude, and quantum yield of the three emissioncomponents induced by the amine addition to the correspond-ing QD. Note that the average lifetime tends to hide the realchanges in the components (Table S2). In all cases, it caused anincrease of the lifetime of the short and the long components,while no significant effect was detected on the mediumcomponent lifetime. With regard to the fractional quantumyield of the three components (Φ1, Φ2, Φ3), the amine addition
caused (i) a drastic increase of all Φ1, (ii) no variation of Φ2,and (iii) a decrease of Φ3 in CS518, but an increase of thisvalue for CS@S−NI and CS@S−D and less pronounced forCS@H2N−NI.Remarkably, while amine addition did not induce any change
of the shape of the PLE spectrum of CS@S−NI, it did so in thecase of CS@H2N−NI (Figure S9). Thus, DDA addition toCS@H2N−NI prevented FRET from the naphthalimide ligandto the QD core; this was indicative of ligand exchange betweenthe amine ligands, moving the naphthalimide moiety far apartfrom the QD surface. Reduction of FRET after amine additionalso occurred in the case of CS@S−NI, but to a lesser degree,which is reasonable when taking into account that the sulfuranchor group of HS−NI would remain attached to the QDsurface, while H2N−NI is bound less strongly and consequentlyit is easier to remove.The effect of the amine on the transient absorption spectrum
of CS@S−NI was also studied. The amine addition reduced theformation of the transient absorption, but it had only a smalleffect on its kinetics (not shown).Also, amine addition induced changes in the IR spectra of the
naphthalimide-capped QDs; the intensity of the symmetricCO band increased and the attachment of the amine to theQD surface of CS@S−NI was detected (Figure S5).
Voltammetric Measurements. The electrochemistry ofQDs is receiving considerable attention.33−36 The anodic andcathodic peaks of a QD voltammogram are related with theelectron transfer mediated through its valence band (h1) andconduction band (e1) edges.
37 The position of these peaks and,consequently, the band gap energy depend mainly on the QDsize, and they are usually only slightly affected by the nature ofthe capping ligand.37,38 The herein used CdSe QDs areprotected by the ZnS shell, and we have not detected sizechanges after the ligand exchange. Hence, no significantchanges in the position of the anodic and cathodic peaksbetween the QDs may be expected.Voltammetric measurements of the QD solutions were
performed in order to correlate electrochemical parameterswith the band gap and the band edge positions of the QDs. Tocomplete the view of such electrochemistry, Figures S10 andS11 have been included in the Supporting Information toemphasize two relevant features in our work: (i) thedifferentiation between capping-localized and QD-centeredelectrochemical processes and (ii) the influence of the cappingand the amine in the voltammetric response of QDs. Blankexperiments of solutions of the different capping reagents in thesame electrolyte were also performed.To rationalize the voltammetric features of QD solutions, it
is pertinent to discern the signals due to QDs and thoseassociated with the electrolyte and, eventually, to theelectroactive capping. Figure 9 compares the cyclic voltam-metric response of HS−NI, DDA, and CS@S−NI/DDA, all in1/1 toluene/MeCN (v/v) solutions containing 0.10 MBu4NPF6 as the supporting electrolyte. The recording of thevoltammograms of HS−NI and DDA has been extended tomore negative potentials in order to show the electrolytesignals.39,40 The most relevant features for such systems areamine/thiol-localized irreversible oxidations at positive poten-tials and an essentially reversible couple at −1.81 V (HS−NIand CS@S−NI) or −1.83 V (CS@S−NI/DDA) vs Fc+/Fc.These couples, which are similar to those recorded for thecorresponding H2N−NI systems, can be attributed to thereduction of the carbonyl units. The CO signals, however,
Figure 8. Fluorescence spectra (λex = 400 nm) of deaerated toluenesolutions of CS@Th (black), CS@S−NI/DDA (0 and 72 h, red andyellow, respectively), CS@H2N−NI (blue), CS@H2N−NI/DDA (0and 72 h, green and orange, respectively); DDA/QD molar ratio of100:1.
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are considerably diminished in the naphthalimide-capped QDsand are accompanied by additional features which can beattributed to “genuine” QDs signals. Such signals, attributableto the CdSe/Se0 (oxidation) and CdSe/Cd0 (reduction)couples, respectively, can be more clearly seen using square-wave voltammetry (SWV); see Figure 10 where both negative-going (a) and positive-going (b) potential scans are shown.Interestingly, SWV can also give information on the surface
defects, and this would be valuable to determine the effect ofthe QD capping on such surface states. Thus, the mostremarkable feature of the SWV of the QDs is that the extremesignals which appear at ca. +0.2 and −2.2 V vs Fc+/Fc are
accompanied by intermediate signals (Figure 10). Such signalshave been interpreted in terms of charge transfer processesinvolving defect states of the QDs.41,42
On comparing the SWV for the studied systems, one canobserve that both the extreme oxidation and reductionpotentials of the QDs appear to be essentially identical for allthe studied QDs and QD/DDA systems. It is pertinent to notethat QD oxidation processes occur at less positive potentialsthan those where the oxidation of the amine capping takesplace. Hence, there is no opportunity for a significant influence,via redox mediation, of the capping in the QD oxidation.Remarkably, although the QD reduction takes place atpotentials similar to those at which reduction of HS−NI andH2N−NI occurs, it appears that there is no significantmodification of the QD reduction signals in the presence ofthe capping ligands.Table 2 summarizes the peak potentials vs Fc+/Fc,
electrochemical band gap, and band structure parameterscalculated relative to the vacuum scale taken as E°(Fc+/Fc) =+0.560 V vs SHE (standard hydrogen electrode) in MeCN43
and E°SHE = −4.60 eV, using usual approximations.44−46 Noticethat the position of the VB (valence band) and CB (conductionband) edges is shifted possibly because a toluene/MeCNmixture rather than pure MeCN has been used as a solvent.Additionally, possible problems dealing with referencepotentials have been discussed by several authors.47−49
Interestingly, the intermediate signals appearing in the SWVbecome more or less pronounced depending on the cappingligand. This can be seen in Figure 11, where the SWV ofCS518, CS@H2N−NI, and CS@H2N−NI/DDA are com-pared. In fact, the separation of the intermediate QD-localizedvoltammetric peaks relative to the extreme QD peaks agreeswell with the position of the different trap energy levels fordifferent types of defect sites reported in the literature42
(illustrated in Figure 11).These features can be rationalized assuming that QD capping
results in different empty energy levels associated with specificligand−QD interactions so that, depending on the capping, oneor another type of defect site is electrochemically “visible”.Intermediate oxidation processes would correspond to electronrelease from such defect sites of the QDs mediated by thecapping ligand, whereas intermediate reduction would corre-spond to the electron transfer to empty energy levels of the QDdefect sites. Table 3 summarizes the peak potential separationsrelative to the CB edge obtained from voltammetric experi-ments and their attribution to different trap energy levels fordifferent types of defect sites reported in the literature.42 Therelative intensity of such intermediate voltammetric signalsvaries from one QD system to another, while their potentialsremain essentially identical in all cases.Our results suggest that the position of the VB and CB edges
and the electrochemical band gap calculated from the potentialsof the extreme QD-centered signals are only slightly influencedby the nature of the capping.The presence of oxygen vacancies was clearly detected for all
of the QDs, while Se vacancy and Se divalent vacancy were not“visible” for CS@S−D, suggesting a considerable electrondensity in its shallow trap states. In addition, the two Se/Cddivacancies were only slightly visible in the case of the QDscapped with naphthalimide ligands, suggesting a low electrondensity in their deep trap states. The addition of the amine tothe QDs “activated” the Se vacancy and the Se divalent vacancy
Figure 9. Cyclic voltammograms at glassy carbon electrode (GCE) for(a) 6 μM CS@S−NI/DDA, (b) 1 mM DDA, and (c) 1 mM HS−NI,in 0.10 M Bu4NPF6 solution in 1/1 toluene/MeCN (v/v). Potentialscan rate was 50 mV/s. The dotted line separates the region of largelynegative potentials where electrolyte signals appear.
Figure 10. Square-wave voltammograms at GCE of 6 μM solution ofCS518 in 1/1 (v/v) toluene/MeCN (0.10 M Bu4NPF6) mixture.Potential scan initiated at (a) +0.80 V in the negative direction and (b)−2.7 V in the positive direction. Potential step increment 4 mV;square wave amplitude 25 mV; frequency 5 Hz. Extreme QD signalsare marked by solid arrows while intermediate QD-localized ones aremarked by dotted arrows.
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of CS@H2N−NI, CS@S−NI, and CS@S−D, while it had noeffect on the charge density in the Se/Cd divacancy of the QDs.To explain the less competitive recombination of the
delocalized charges in the thiolate-capped QDs, it has to betaken into account that the binding of thiolates to the ZnSsurface can cause a significant alteration of the surface, movingZn cations above the ZnS surface.23 Consequently, the binding
of thiolates to the ZnS shell of CdSe/ZnS QDs could increasethe electron density close to the CdSe CB edge. On the otherhand, the “stabilization” effect of empty trap states by thenaphthalimide anchor group could be due to the carbonylgroup capacity to act as both a σ-donor and a π-acceptor; oneof the lone pairs of the oxygen of the carbonyl group wouldcombine with the lowest unoccupied molecular orbital of theZn cation, resulting in a σ-bond between the carbonyl and theZn, and the stability of the interaction would increase by π-donation from the filled Zn d orbitals to the CO π* orbital.In addition, the steric effect of the ligand could also play a rolein “stabilizing” the empty state.
4. CONCLUSION
Here we showed how the use of time-resolved spectroscopiesand voltammetry can be applied as important tools for gaininginsight into the mechanism responsible for the effect of anchorgroups, such as thiolates and imides, on the emission of CdSe/ZnS QDs. We demonstrated that ligands with a mercaptogroup and a naphthalimide chromophore unit can act asbidentate ligands, causing a drastic decrease in the CdSe/ZnSQD emission. Interestingly, the thiolate anchor group decreasesthe radiative recombination of the delocalized charges while thecarbonyl group of the naphthalimide moiety increases thenonradiative recombination of the localized charges. Thus, thetwo anchor groups, sulfur and naphthalimide, provide twodifferent channels for controlling emission properties of CdSe/ZnS QDs.
Table 2. Peak Potentials (in mV vs Fc+/Fc), Apparent Electrochemical Band Gap (eV), and Band Structure Parametersa
aFrom SWVs at GCE. Potential step increment 4 mV; square-wave amplitude 25 mV; frequency 5 Hz. bError ±20 mV. cError ±0.04 eV.
Figure 11. Square-wave voltammograms for (from left to right)CS518, CS@H2N−NI, and CS@H2N−NI/DDA, superimposed to adiagram of energies for the conduction and valence bands and defectstates.42 Voltammograms obtained under the same conditions than inFigure 9. The most weak signals are not shown.
Table 3. Comparison of Potential Separation from the CB Edge Obtained by Voltammetric Experiments in the Present Reportand the Trap Energy Levels Reported in the Literature for the Different Types of Defect Sites42
system Se divalent vacancy Se vacancy O vacancies O vacancies Se/Cd divacancy
CS518 a a 0.62 0.93 1.40/1.90CS518/DDA a a 0.64 0.92 1.40/1.90CS518/PEA a a 0.66 0.94 1.44/1.90CS@H2N−NI a a 0.65 0.92 aCS@H2N−NI/DDA 0.13 0.24 0.66 0.94 aCS@H2N−NI/PEA 0.15 0.25 0.64 0.94 aCS@S−NI a a 0.65 0.94 aCS@S−NI/DDA 0.15 0.25 0.64 0.94 aCS@S−NI/PEA 0.13 0.23 0.62 0.96 aCS@S-D 0.66 0.94 1.45/1.90CS@S-D/DDA 0.12 0.24 0.64 0.96 1.40/1.90
aOnly weak signals were observed.
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■ ASSOCIATED CONTENT*S Supporting InformationEstimation of FRET efficiency from the decrease of thenaphthalimide emission; characterization data of the QDs;average lifetime, and lifetimes and fractional amplitudes of thefluorescence decay components of CS518, CS@S−NI, CS@H2N−NI, and CS@S−D; LUMO energy of the systems; 1HNMR of N-(2-aminoethyl)-1,8-naphthalimide; N-(2-mercap-toethyl)-1,8-naphthalimide and CS@S−NI; TEM images ofCS518, CS@S−Th, CS@H2N−NI, and CS@S-D; IR spectra ofCS@S−NI, CS@S−NI + DDA, HS−NI, CS@H2N−NI, andCS@H2N−NI + DDA; emission decay kinetics of CS518 andCS@S−NI; transient absorption spectra of NI−SH; normalizedexcitation spectra of CS@S−NI and CS@H2N−NI; square-wave voltammograms of CS@S−NI and CS@S−NI + PEA.This material is available free of charge via the Internet athttp://pubs.acs.org.
■ AUTHOR INFORMATIONCorresponding Author*E-mail [email protected]; Fax +34 963543576; Tel +34963543050.NotesThe authors declare no competing financial interest.
■ ACKNOWLEDGMENTSWe thank MICINN (Project CTQ2008-06777-C02-01,CTQ2008-06777-C02-02, CTQ2011-27758, and CTQ2011-28390, contract for J.A.S.), FGUV (contract for R.E.G.) and theJunta de Andalucıa (Excellence Project FQM-3685) forfinancial support.
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