Article pubs.acs.org/JPCC
Unconventional Fluorescence Quenching in Naphthalimide-Capped CdSe/ZnS Nanoparticles Jordi Aguilera-Sigalat,† Vânia F. Pais,‡ A. Doménech-Carbó,§ Uwe Pischel,‡ Raquel E. Galian,*,† and Julia Pérez-Prieto*,† †
Instituto de Ciencia Molecular (ICMol), Universidad de Valencia, Catedrático José Beltrán 2, 46980, Paterna, Valencia, Spain CIQSO - Centro de Investigación en Química Sostenible and Departamento de Ingeniería Química, Química Física y Química Orgánica, 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 an amino 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 characterized by infrared and nuclear magnetic resonance spectroscopies, high-resolution transmission electron microscopy, and voltammetry as well as by steady-state absorption and emission spectroscopies. The organic ligands HS−NI and H2N−NI act as 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 a QD capped with decanethiol ligands (CS@S−D). The interaction between the anchor groups and the QD surface brings about different consequences for the radiative and nonradiative kinetics, depending on the nature of the anchor group. Our results suggest that the naphthalimide group “stabilizes” empty deep trap states due to the carbonyl group capacity to act as both a σdonor and a π-acceptor toward cations. In addition, the thiolate group can induce the location of electron density at shallow trap states 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) exhibit high fluorescence, which makes them attractive for biological and medical applications, among others.1,2 The semiconductor bulk material possesses defect states that originate from impurities, divacancies, or surface reactions as a result of their synthesis. The ratio between the defect states and the number of atoms increases in the semiconductor nanoparticle due to its high surface-to-volume ratio. The efficiency of the QD bandedge luminescence decreases through the implication of additional transition levels caused by defect states in the forbidden band of the QD. Energy relaxation and recombination dynamics in QDs strongly depend on surface passivation. Hence, in core−shell (CS) CdSe/ZnS QDs, the ZnS shell plays a crucial role in their emissive properties, enhancing the chemical stability and photostability of the core. However, the shell is far from perfect at the surface, and if tunneling of charges through the shell occurs, these defects could also serve as trap sites due to the presence of dangling bonds.3,4 The fluorescence of CdSe/ZnS QDs exhibits multiexponential decays; this phenomenon has been attributed, in addition to differences between the individual nanoparticles in the colloidal solution, to a recombination of (i) delocalized carriers in the internal core states and (ii) localized carriers at the heterointer© 2013 American Chemical Society
face of the QDs. The localization of electrons or holes may be generated at the heterointerface of the QDs due to interface roughness or defects.5 Furthermore, there are two likely locations for the trapped charges to reside on the CdSe/ZnS QDs: (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 organic solvents and water, plays a key role for the photophysical properties of the QDs. The spherical QD nanoparticle can incorporate 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 transfer implicating the QD electron (e−)/hole (h+) or energy transfer to the ligand, has been extensively studied.6−9 However, the nature of the group anchoring the ligand to the QD surface atoms is of special relevance for the QD emissive properties, since that group can increase the coordination number of the surface atoms and, as a consequence, reduce dangling bond energy states within the band gap, which otherwise may act as Received: December 29, 2012 Revised: March 16, 2013 Published: March 20, 2013 7365
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nonradiative centers.3,4 Alternatively, the anchoring group can influence the tunneling of the electron of the photogenerated exciton and its localization in trap states. For example, it has been reported that certain electron-accepting organic ligands, such as pyridine, attached to the QD surface can trap the photogenerated exciton and decrease the emission efficiency as a result of creating new surface or mid-band-gap states that are characterized by a fast nonradiative relaxation.10 However, QD capping with electron-donating moieties can result in a nearly complete suppression of QD blinking.11−13 The drastic decrease of blinking after addition of short-chain thiols (such as β-mercaptoethanol) to streptavidin-coated CdSe/ZnS QDs has been attributed to electron donation from the thiol moiety to the surface electron traps.11 However, the thiol effect on CdSe/ZnS QD blinking dynamics can be more complex, either increasing or decreasing the frequency of blinking.14 Reduction of the QD blinking has been related with an increase in the radiative rate constant accompanied by a reduction in the nonradiative rate constant.12 For example, propyl gallate has been proven to influence the radiative and nonradiative excited state decay of CdSe/ZnS QDs, yielding highly luminescent QDs. Inversion of the excited energy level structure of the QDs and preferential thermal population of the bright exciton state or the partial mixing of bright character into the dark exciton have been suggested as plausible explanations for the increased radiative rate constant induced by the organic ligand. Especially with respect to the understanding of unconventional quenching, not involving electron or energy transfer, it seems highly desirable to further investigate the dependence of the photophysical properties of the CdSe/ZnS QDs on the nature of the ligand anchor group. Here we report on the optical properties of CdSe/ZnS QDs capped with ligands which possess a mercapto or an amino group and a naphthalimide (NI) group as chromophore unit, linked by a short ethylene chain (CS@S−NI and CS@H2N−NI, respectively; Figure 1). The organic ligands HS−NI and H2N−NI (Figure 1) may act as bidentate ligands, causing a drastic decrease in the QD emission, in particular in the case of CS@S−NI. The interaction between the anchor groups and the QD surface had different consequences on the radiative and nonradiative kinetics, depending on the nature of the anchor group. Steadystate and time-resolved absorption and emission spectroscopies as well as voltammetric (CV) data are in accordance with the interpretation that the naphthalimide group “stabilizes” empty deep trap states and that the sulfur anchor group induces the localization of electron density at shallow trap states close to the conduction band edge. As a consequence, the QD−ligand interaction through the thiol sulfur and the naphthalimide provides two different channels of controlling the CdSe/ZnS fluorescence quenching.
Synthesis of N-(2-Aminoethyl)-1,8-naphthalimide (H2N− NI). H2N−NI was synthesized according to a reported procedure (see Figure S1 in the Supporting Information).16 The 1H NMR spectrum of the herein prepared sample is identical to reported data. 1H NMR (400 MHz, CDCl3): δ 8.61 (d, J = 8.4 Hz, 2H), 8.22 (d, J = 9.2 Hz, 2H), 7.76 (t, J = 7.8 Hz, 2H), 4.29 (t, J = 6.6 Hz, 2H), 3.08 (t, J = 6.6 Hz, 2H) ppm. Synthesis of N-(2-Mercaptoethyl)-1,8-naphthalimide (HS− NI). 1,8-Naphthalic anhydride (396 mg, 2.0 mmol) was placed in a round-bottomed flask with a magnetic stirrer and reflux condenser, and 15 mL of absolute ethanol and cysteamine (193 mg, 2.5 mmol) were added. The solution was heated to reflux with constant stirring for 6 h. After cooling to room temperature, 30 mL of water was added and the precipitate was filtered. The solid was washed with water, 1:1 ethanol/ water, ethanol, and vacuum-dried to yield 546 mg of a 90:10 disulfide/thiol mixture. The mixture was dissolved in 20 mL of CHCl3, and DL-dithiothreitol (660 mg, 4.28 mmol) was added. The suspension was warmed to 37 °C for 3 days. After cooling to room temperature 30 mL of water was added, and the compound was extracted with ethyl acetate (2 × 15 mL). The organic phase was washed with brine (2 × 15 mL) and saturated NH4Cl solution (2 × 15 mL) and dried over anhydrous Na2SO4. Removal of the solvent resulted in a solid which was recrystallized from (1:1) chloroform/acetonitrile. After vacuum drying, 246 mg of a slightly yellowish solid was obtained (48% yield over the two steps). The 1H NMR spectrum of the herein prepared sample is identical to reported data (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 the commercial QD and 0.296 mL of HS-D (molar ratio between QD:ligand is 1:5000) were added in a flask and heated to reflux in 40 mL of chloroform for 48 h, under N2 flow in the absence of light. Then, the mixture was cooled to room temperature. For the purification, the nanocrystals were precipitated from MeOH several times. Finally, CS@S−D was dissolved in toluene. Synthesis of CdSe/ZnS QDs Capped with HS−NI or H2N− NI (CS@2HN−NI and (CS@S−NI). For the ligand exchange procedure, 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 and heated to reflux in 40 mL of chloroform for 48 h, under N2 flow in the absence of light. Then, the mixture was cooled down to room temperature. For the purification, the nanocrystals were precipitated three times from dioxane. Finally, the QDs were dissolved in 3 mL of toluene. Characterization. UV−vis spectra of the samples were recorded with a UV−vis spectrophotometer (Agilent 8453E). The average diameter value of the nanoparticles was estimated following the procedure published by Peng et al.17 Steady-state fluorescence spectra were measured on a spectrofluorometer PTI, equipped with a lamp power supply (LPS-220B), motor driver (MD-5020), and Brytebox PTI, and working at room temperature. The fluorescence quantum yield was calculated by following the procedure of Resch-Genger et al.18 The emission lifetime measurements were done by time-correlated singlephoton-counting (Edinburgh Instruments FLS 920) using a picosecond pulsed UV-LED (EPL 445, λ = 442.2 nm) as
2. EXPERIMENTAL SECTION Materials. All reagents were commercially available and used as received: decanethiol, 1,8-naphthalic anhydride, DLdithiothreitol, cysteamine (Sigma-Aldrich); ethylenediamine, NH4Cl (Fluka); chloroform, ethyl acetate, ethanol, acetonitrile (Scharlau); Na2SO4 (Panreac). Core−shell QDs capped with a long-chain primary amine were purchased from Ocean NanoTech. Solvents were of reagent grade and used without further purification. HS−NI and H 2 N−NI are known compounds.15,16 The synthetic procedure of HS−NI was modified (see below). The NMR spectroscopic data matched the reported ones. 7366
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NI and CS@H2N−NI, Figure 1B). Typically, a chloroform solution of the QDs and the naphthalimide ligand, [ligand]/
excitation source. Deconvolution analysis of the kinetic traces yielded the luminescence lifetimes. The instrument response function was recorded with a light-scattering Ludox solution. For multiexponential decays the amplitude-averaged emission lifetime τav was stated. In this equation, τi are the decay times and αi represent the amplitudes of the components. Their relationship with the fractional amplitude (ai), intensity (f i), and quantum yield (Φi) of each component is also presented: α ai = n i ∑i = 1 αi n
τav =
n
∑ aiτi
with
∑ ai = 1
i=1
fi =
i=1
a iτi n ∑i = 1 aiτi
Φi = fi Φf =
=
aiτi τav
aiτi Φf τav
Laser flash photolysis (LFP) studies were performed with a pulsed Nd:YAG laser, using 355 nm as excitation source. The pulse width was ca. 10 ns, and the energy was ca. 15 mJ/pulse. A xenon lamp was employed as the detecting light source. The photomultiplier-amplified output signal was transferred to a personal computer. The optical density of the QD samples (3 mL) was adjusted to ca. 0.3 at 355 nm, and they were bubbled with N2 for 10 min prior to the measurements. IR measurements were performed on a Bruker Equinox 55/IRScope II equipped with a multiple reflection unit for attenuated total reflectance (ATR) measurements on solids. Usually 1−2 drops of QD solutions were placed on the sample holder, and after drying, spectra were collected. 1H NMR spectra were recorded on Bruker Avance DPX400 spectrometer equipped with a QNP 1H/13C/19F/31P probe. Standard Bruker software was used for acquisition and processing routines. Chemical shifts are given in ppm and internally referenced to TMS. Images of the QDs were obtained by high-resolution tunneling microscopy (HRTEM) on a FEI Tecnai G2 F20 at an accelerating voltage of 200 kV. Samples were prepared by dropping the colloidal solution on a Lacey Formvar/carboncoated copper grid. The digital analysis of the HRTEM micrographs was done by using digital Micrograph TM 1.80.70 for GMS by Gatan. Images were treated with the ImageJ software. Voltammetric measurements were carried out in a conventional three-electrode cell using BAS 660I equipment. Glassy carbon (GCE) and gold working electrodes were complemented with a Pt-wire auxiliary electrode and a Pt disk pseudoreference electrode. The potentials are stated relative to the ferrocenium/ferrocene couple (0.1 mM). These measurements were performed with ca. 5 μM solutions of the different QD systems in 50/50 (v/v) toluene/MeCN (0.10 M Bu4NPF6) which were deaerated by bubbling with Ar. Blank experiments were performed with 1.0 mM solutions of the different capping reagents in the same electrolyte.
Figure 1. (A) Ligand structures and (B) proposed binding modes of naphthalimide ligands to the CdSe/ZnS QDs used in this report.
[QD] = 500, was heated to reflux under a nitrogen atmosphere for 48 h in the dark (see Experimental Section). The reaction mixture was cooled to room temperature, and the nanocrystals were precipitated from dioxane. Finally, the QD was dissolved in toluene, and they remained stable for more than 3 months. High-resolution transmission electron microscopy (HRTEM) images showed the QDs maintained the size and the crystallinity 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 of comparison, QDs capped with decanethiol (CS@S-D) were also synthesized using the same methodology, except that they were precipitated from methanol (for detailed characterization data 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 deuterated toluene (Figure S4 in Supporting Information), evidenced the attachment of the ligand to the QD surface. As expected, the spectrum showed considerable broadening of the ligand signals but also evidenced a considerable shift of its aliphatic and aromatic protons to a lower field and partial disappearance of the CH2−S signal. These facts suggest that the ligand could act as a bidentate ligand, involving not only the mercapto but also the naphthalimide group, the latter presumably through the carbonyl group (Figure 1B). The infrared (IR) spectrum of CS@S−NI was also registered to corroborate the interaction between the naphthalimide group and the QD surface. Figure 2 shows the comparison between the strong absorption bands of νCO (1620−1720 cm−1) of CS@S−NI and HS−NI. The IR spectrum of HS−NI shows the characteristic features of naphthalimides:19,20 four bands in the 1600−1800 cm−1 region. The bands at 1695 and 1657 cm−1 are assigned to the asymmetric and symmetric C
3. RESULTS AND DISCUSSION Synthesis of the QDs. Commercial (Ocean Nanotech) core−shell (CS) CdSe/ZnS QDs (CS518: λem = 518 nm and CS524: λem = 524 nm at λex = 400 nm; the only specification by the supplier is that octadecylamine is the surface ligand) were used as precursors of the naphthalimide-capped QDs (CS@S− 7367
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Figure 2. IR spectrum of CS@S−NI (red) and HS−NI (black) in the 1570−1720 cm−1 region. The spectra have been scaled to the intensity of the ν(CO) band at ∼1700 cm−1.
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).
O stretching modes, respectively, while the other two bands at 1624 and 1603 cm−1 originate from aromatic ring vibrations (ν(Ar)). The changes of the CO bands were smaller in the case of CS@H2N−NI, suggesting a less effective interaction between the naphthalimide group and the nanoparticle surface (see comparison between H2N−NI and CS@H2N−NI in Figure S5). The presence of a strong, broad band at ca. 1550 cm−1 for CS@H2N−NI (not detected for the free ligand) can be attributed to the NH2 scissoring mode of H2N−NI bound to the shell.21 Photophysical Characterization of the QDs. Here we only 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 exchange of the CS518 ligands by H2N−NI, HS−NI, and HS−D (Figure 1) led to a red-shift of the exciton peak by ca. 4 nm. An average number of 57 ligands per QD was estimated for CS@S−NI using the molar absorption coefficient of the ligand (3535 M−1 cm−1 at 355 nm). Figure 3 shows the comparison between the spectra of CS@S−NI, CS518, and the free ligand.
emission (by 2 and 1 nm, respectively) compared with that of CS518. The QD emission quantum yields (Φf) followed the order CS@S−NI (0.06) < CS@S−D (0.08) < CS@H2N−NI (0.18) < CS518 (0.45). Consequently, the reduced emission of the thiol-capped QDs appeared to be mainly related to the attachment of the organic ligand through the sulfur atom, but apparently the naphthalimide moiety also contributed to the decrease of the luminescence of the naphthalimide-capped QDs. It has to be taken into account that the thiolate group can bind to the surface of CdSe/ZnS QDs and give rise to an enhancement or a drastic decrease of the QD fluorescence, depending on the QD ligand and conditions used for the ligand exchange.22−25 Photoluminescence excitation (PLE) spectroscopy has been used by Bawendi and co-workers for mapping the electronic states of CdSe QDs.26,27 The comparison between the excitation spectra (from 500 to 400 nm, monitored at the emission maximum of the QD) of CS@S−NI, CS@S−D, CS@ NH2−NI, and CS518 normalized at 446 nm showed that the ligand exchange did not produce a significant shift in the energy of the electronic states (Figure 5).
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 5. Comparative excitation spectra (λem at 516 nm) of deaerated toluene solutions of CS518 (black), CS@S−NI (red), CS@H2N−NI (blue), and CS@S−D (green). The spectra were normalized at 446 nm.
The fluorescence spectrum of CS@S−NI was recorded at λex = 400 nm, where the ligand does not absorb. It should be noted that the QDs showed no deep trap emission, which would manifest at longer wavelengths than observed for the exciton emission. For the sake of comparison, Figure 4 shows the emission 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
At high excitation photon energies, the PLE signal of the QD decreased drastically compared with the absorption signal. It has been postulated that high excitation photon energies create holes with higher energy than that of the core−shell barrier. Such holes become more exposed to the shell surface defects and are likely to be lost through nonradiative pathways.28 7368
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Table 1. Average Fluorescence Lifetime (τav) and Quantum Yield (Φf), Radiative (kr), and Nonradiative (knr) Rate Constants of Deaerated Toluene Solutions of CS518, CS@S−NI, CS@H2N−NI, and CS@S−D in the Presence and in the Absence of Dodecylamine (DDA)a CS518 DDA 0 h DDA 72 h CS@S−NI DDA 0 h DDA 72 h CS@H2N−NI DDA 0 h DDA 72 h CS@S−D DDA 0 h DDA 72 h
Φf
τav/ns
kr × 10−7/s
0.45 0.43 0.45 0.06 0.031 0.074 0.18 0.165 0.217 0.08 0.038 0.091
24.7 24.8 23.3 15.8 18.6 22.2 21.9 22.6 22.5 38.8 38.3 38.4
1.82 1.74 1.93 0.38 0.17 0.33 0.82 0.73 0.96 0.21 0.10 0.24
Δkr (%)
6
−13
17
14
knr × 10−7/s 2.23 2.30 2.36 5.94 5.21 4.17 3.73 3.69 3.47 2.37 2.51 2.37
Δknr (%)
6
−30
−7
0
The samples were excited at λex = 446 nm (where the naphthalimide does not absorb), and the decay was registered at λem = 525 nm for all QDs except for CS@S−NI (λem = 529 nm).
a
Figure 6. Fractional amplitudes (A), lifetimes (B), and fractional quantum yields (C) of the three components (short-, medium-, and long-lived) of CS@S−D, CS@S−NI, CS@H2N−NI, and CS518 in the absence and in the presence of dodecylamine.
emission quenching, was 0.56 for the case of CS@S−NI (see Supporting Information).29 Moreover, we investigated the fluorescence lifetime of the QDs to gain further insight into the effect of naphthalimide group on the emissive properties of the QDs when they are excited at a wavelength (λ ex = 446 nm) where the naphthalimide does not absorb (see Table 1 and Table S2).
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 7369
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larger Φ1, and (iii) CS@H2N−NI showed an intermediate behavior, with a smaller decrease of Φ2 and Φ3, but presenting the highest value for Φ1. It should be noted that besides the discussed dynamic quenching, also the occurrence of static quenching of the QD emission by the ligand is supposed to proceed due to the preorganized nature of the ligands on the QD surface. Laser flash photolysis (LFP, Nd:YAG, 355 nm, 10 ns pulse, 15 mJ/pulse) studies of the QDs were also performed, aiming to further reveal the role of naphthalimide in the excited state behavior of the herein investigated QDs. Time-resolved transient spectra of deaerated solutions of CS518 showed a strong bleaching of the QD absorption assigned to the CdSe 1s3/21se transition (see Figure 7). However, excitation of CS@
The decay of the emission was registered at λem = 525 nm for all QDs except for CS@S−NI (λem = 529 nm). Time-resolved fluorescence experiments showed that the average fluorescence lifetime (τav) of CS518 was ca. 25 ns, and those of the others followed the order CS@S−D > CS@H2N−NI > CS@S−NI (values of ca. 38, 22, and 16 ns, respectively). Taking into account the QD emission quantum yield (Φf) and the expression Φf = kr/(kr + knr), where kr and knr refer to the radiative and nonradiative rate constants, respectively, the variation of the kr and knr values with the capping ligand nature was analyzed. Both CS@S−NI and CS@S−D exhibited kr values much smaller than that of CS518, indicating that the sulfur atoms slow down the radiative process. This result is in accordance with the previous finding of smaller radiative constants for thiol-capped CdSe/ZnS QDs as compared to primary-aminecapped QDs.6 This can be related with the reported capacity of the thiolate anchor group to donate electrons to the QD surface electron traps, thereby making them incapable of accepting electrons from the QD.11 Interestingly, CS@S−NI showed a much higher nonradiative rate constant compared to that of the CS518 precursor. This cannot be attributed to the sulfur atoms at the QD surface, since knr of CS@S−D was very similar to that of CS518, but it should be ascribed to the naphthalimide moiety. In accordance, CS@H2N−NI exhibited a greater knr than CS518, though its value was considerably smaller than that of CS@S−NI. The involvement of the naphthalimide moiety in electron-transfer processes should be ruled out, since its LUMO is at ca. 0.6 eV higher energy than that of the QD conduction band (see voltammetric measurements below). Therefore, the enhanced nonradiative QD emission decay may be attributed to an active trapping 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 details in the Supporting Information. In addition, Figure 6 shows the changes of the lifetime (τ1, τ2, and τ3), the fractional emission amplitude (a1, a2, and a3), and quantum yield (Φ1, Φ2, Φ3) of the three (short, medium, long) components induced by ligand exchange of CS518 by NH2−NI, HS−NI, and HS−D (see Experimental Section for the relation between these variables). Noticeably, important differences or similarities were appreciated for the three fractional amplitudes for the different QDs. Thus, the relative contribution of the three emission components were similar for CS@H2N−NI and CS518, being the medium component strongly dominant (Figure 6A). However, both the short and medium components had an important contribution (a1 and a2 > 20%) in CS@S−NI fluorescence, while the medium and long components were dominant for CS@S−NI (a2 and a3 > 20%). With respect to the lifetimes, the medium component lifetime (τ2) decreased in CS@S−NI and CS@H2N−NI compared with that of CS518 (Figure 6B), while the lifetime of the long component (τ3) of CS@S-D was the largest. Finally, the fractional quantum yield of each component of the prepared QDs was compared with that of CS518, whose medium and long components were the largest. Thus, (i) the quantum yield of the three components decreased drastically in CS@S−D, (ii) Φ2 was even smaller in CS@S−NI than in CS@S−D, while CS@S−NI exhibited a
Figure 7. Transient absorption spectra of deaerated toluene solutions of CS518 0.05 μs (black), 0.07 μs (red), and 0.12 μs (dark blue) after the 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 trace of the transient absorption of CS@S−NI registered at 620 nm.
S−NI gave rise to an absorption band with a maximum at 470 nm, which can be ascribed to the naphthalimide triplet excited state,31 and a broad absorption from 520 to 700 nm (Figure 7). The latter could be attributed to a combination of surfacecharge trapping and excited-state absorption;32 the naphthalimide group played a key role in both its formation and considerable long lifetime (ca. 70 ns). The same species was detected when the measurements were performed with CS518/ H2N−NI (1:100 molar ratio) mixtures. However, in the case of CS@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 in Figure 7) was detected in the LFP of CS@S−NI; this may indeed be tentatively ascribed to the radical anion. A possible explanation is the homo-electron-transfer between excited- and ground-state naphthalimides which is favored by their close mutual interaction on the quantum dot surface. This phenomenon has been discussed for bisnaphthalimide dyads.31 The time scale (70 ns after the pulse) of the appearance of the radical anion feature makes it in principle less credible that this transient has its origin in excited singlet naphthalimides, which are known to be much shorter lived (
