N-Heterocyclic Carbenes as Reversible Exciton-Delocalizing Ligands for Photoluminescent Quantum Dots

Abstract

Delocalization of excitons within semiconductor quantum dots (QDs) into states at the interface of the inorganic core and organic ligand shell by so-called “exciton-delocalizing ligands (EDLs)” is a promising strategy to enhance coupling of QD excitons with proximate molecules, ions, or other QDs. EDLs thereby enable enhanced rates of charge carrier extraction from, and transport among, QDs and dynamic colorimetric sensing. The application of reported EDLs–which bind to the QDs through thiolates or dithiocarbamates–is however limited by the irreversibility of their binding and their low oxidation potentials, which lead to a high yield of photoluminescence-quenching hole trapping on the EDL. This article describes a new class of EDLs for QDs, 1,3-dimethyl-4,5-disubstituted imidazolylidene N-heterocyclic carbenes (NHCs), where the 4,5-substituents are Me, H, or Cl. Postsynthetic ligand exchange of native oleate capping ligands for NHCs results in a bathochromic shift of the optical band gap of CdSe QDs (R = 1.17 nm) of up to 111 meV while the colloidal stability of the QDs is maintained. This shift is reversible for the MeNHC-capped and HNHC-capped QDs upon protonation of the NHC. The magnitude of exciton delocalization induced by the NHC (after scaling for surface coverage) increases with the increasing acidity of its π system, which depends on the substituent in the 4,5-positions of the imidazolylidene. The NHC-capped QDs maintain photoluminescence quantum yields of up to 4.2 ± 1.8% for shifts of the optical band gap as large as 106 meV.

Graphical Abstract

INTRODUCTION

Exciton-delocalizing ligands (EDLs) are a class of molecules that couple quantum-confined electrons within semiconductor nanocrystals to their immediate environments.¹ The orbitals of an EDL with appropriate symmetry and energy mix with conduction or valence band orbitals of the nanocrystal and thereby create new interfacial states into which the nanocrystal’s exciton delocalizes. In a series of papers from 2010 to 2013, we introduced phenyldithiocarbamate (PTC) and its para-substituted derivatives as EDLs that decrease the optical band gaps of Cd and Pb chalcogenide quantum dots (QDs) by up to 1 eV without changing the physical structures of their cores.^2–5

Our mechanistic studies on the QD-PTC complexes led to the conclusion that mixing of the σ-character orbitals of PTC and the Cd²⁺ or Pb²⁺ s orbitals of the QD conduction band and mixing of the π-character orbitals of PTC and the chalcogenide p orbitals of the QD valence band relax the quantum confinement of the exciton, primarily the excitonic hole.^3,4,6–9 This “through-bond” mechanism for exciton delocalization was substantiated and elaborated by a series of papers that combine experimental data with density functional theory calculations.^{2–4,6,10–12} By an increase in the permeation of the excitonic wave function through the dielectric interface separating the inorganic core and the organic ligand of the colloid, EDLs greatly enhance our ability to, for example, extract the otherwise highly localized excitonic hole from a QD^13–15 (allowing us to achieve the fastest photo-reduction of a metal chalcogenide QD recorded and the first photo-reduction of the biexdtonic state of a QD)¹⁶ transport excitons through QD films,^17–19 and perform multihole photo-oxidative reactions with QDs.^16,20

Much of the physics of the EDL effect is fairly straightforward, but thiolates and PTC derivatives are problematic as EDLs due to their specific chemical structures. First, when it is bound to a metal cation on the QD surface, PTC is very chemically stable, even in air and light, but when it freely diffuses as an acid or salt, PTC degrades within minutes to hours to the corresponding aniline, thiourea, and phenyl isothiocyanate.⁶ Although these degradation products do not themselves delocalize the QD exciton, and therefore do not contribute to the observed spectral shift, the degradation makes ligand exchange inefficient and complicates quantification of the yield of ligand exchange. Second, PTC is effectively an irreversible binder; once it is bound to Cd²⁺ or Pb²⁺, we have not found a way to displace it without also etching the QD core. The tight binding of PTC (and thiolates) makes their mechanism of delocalization difficult to prove because it is not straightforwardly reversible. Finally, and perhaps most importantly, FTC and thiolates adsorb in more than one configuration on the Qp surface, and some of these binding modes trap the excitonic hole on the ligand rather than delocalizing it through the ligand.^7,21–23 Hole trapping, when it occurs in high yield, efficiently quenches the photoluminescence (PL) of the QP ensemble by limiting electron–hole coupling. Sensing; photovoltaic, and photocatalytic applications of QDs require high photoluminescence quantum yields (QYs); or at least accessible exritonic charge carriers.

Here, we introduce a new class of BDL: 1,3-dimethyl-4,S-disubstituted imidazolyfcdene N-heterocyclic carbene (NHC) ligands, with hydrogen (H), methyl (Me), or chloride (Cl) substituents (Scheme l), that address many of the issues with dithiocarbamate and thiolate EDLs. We targeted NHCs as BDLs because, much like dithiocarbamates and thiolates, NHCs interact through both s- and p-type orbitals located on the carbene carbon.^24–26 which has the potential to bind to either cadmium^27–29 or selenium^30–32 or both on the QP surface, and because there is literature precedent for NHCs as ligands for metal dusters^33–35 and metal nanopartides.^36–41 The imidazolylidene NHCs in particular are partially aromatic; thus, modification of substituents at the 4,5-positions affects their π acidity and should affect the degree of esrciton delocalization enabled by the molecule.⁴² We find that all three NHC derivatives studied in this work delocalize the QD exciton; as expected for an EDL, the magnitude of the apparent increase in excitonic radius (ΔR) per bound NHC ligand depends on the substituent Unlike PTC-capped QDs, (i) HNHC- and MeNHC-capped QDs maintain PL QYs of 0.6 ± 0.1% and 4.8 ± 1.7%, respectively, at an optical shift of 92 meV and (ii) the exciton delocalization induced by HNHC and MeNHC is reversible upon protonation and subsequent desorption of the NHC.

Scheme 1. Ligand Exchange Procedure^a.

^aVarying amounts (up to 120 equiv, depending on X) of XNHC dimer were added to 10 μM oleate (OA)-capped CdSe QDs in toluene, and the mixture was stirred at room temperature under an N₂ atmosphere. The cartoon of the NHC-capped Qp is not to scale.

The spectral shift induced by C1NHC is not reversible, and its adsorption does not preserve the PL of the QDs (for reasons explained in the text). We nonetheless include it in our data set, because comparison of its EDL effect to those of HNHC and MeNHC allows us to correlate the EDL effect of an NHC ligand with its π acidity, a correlation that is consistent with our proposed mechanism for exciton delocalization by dithiocarbamates and thiolates.

RESULTS AND DISCUSSION

We synthesized CdSe QDs with an average radius of 1.17 nm and oleate capping ligands according to a previously published procedure⁴³ and purified the oleate-capped QDs by sequential precipitation with acetone followed by methanol (see the Supporting Information for details). We synthesized die MeNHC and HNHC precursor salts according to a patented procedure⁴⁴ and die CINHC precursor salt according to a published procedure (see the Supporting Information for details).⁴⁵ Under an N₂ atmosphere, we deprotonated the 1,3-dimethyl-4,5-disubstituted imidazolium salts with a minimum of 1.2 equiv of sodium hydride and 3 mol % cesium carbonate in anhydrous THF.⁴⁶ After the reaction was allowed to proceed for at least 4 h for the ClNHC and HNHC derivatives or between 13 and 18 h for the MeNHC derivative, we filtered the solution through Celite and washed with additional THF. The resulting NHC derivatives exist predominantly as dimers in THF (see Figure S1 of the Supporting Information).^47,48 All optical measurements of the NHC-capped QDs were performed using air-free cuvettes unless otherwise indicated, and all NMR measurements were performed using J. Young tubes to ensure an air- and water-free environment

Scheme 1 depicts the procedure that we used to exchange the native oleate ligands on the QPs for each type of NHC ligand. Under an N₂ atmosphere, we added an aliquot of the prescribed concentration of the XNHC dimer in a THF stock solution to an anhydrous toluene solution of 10 μM CdSe QDs and allowed the ligand exchange to proceed for 12–18 h (overnight) at room temperature. Upon addition of the NHC derivatives, a population of freely diffusing oleate (or oleic add) appeared in the ¹H NMR spectra of the samples; some residual QD-bound oleate remained (see Figure S2 of the Supporting Information). It appears that MeNHC and CINHC react with the double bond of oleate, which prevented us from accurately quantifying the remaining coverage of oleate ligands (see Table S1 of the Supporting Information). This reaction, which we have not characterized, does not appear to occur with HNHC, and we can determine that each added HNHC displaces 2.3 ± 0.3 oleate ligands from the QD surface (see Table S2 of the Supporting Information). We did not wash out free oleate or unbound NHC dimers prior to characterization of the NHC-capped QDs to avoid stripping bound NHC ligands from the QD surface during washing. The resulting MeNHC-, HNHC-, and ONHC-capped QDs are colloidaHy stable in toluene under an N₂ atmosphere for at least 2 days (see Figure S3 of the Supporting Information).

Consistent with the effects of dithiocarbamate^2–4 and thiolate^7,22,23 EDLs, ligand exchange from oleate to XNHC bathodiromicaUy shifts the absorbance spectrum of the QD ensemble; die shift corresponds to a decrease in die energy of the QD’s first exdtonic state by an amount (ΔR) that depends on (i) the coverage of XNHC and (ii) the substituent, X. Figure 1A shows the maximum achievable bathochromic shift of the ground-state absorbance spectrum of 1.2 μM CdSe QDs (physical radius of 1.17 nm) while maintaining colloidal stability, upon treatment of oleate-capped QDs (black) with MeNHC (red, ΔE =115 meV), CINHC (green, ΔE = 48 meV), or HNHC (blue, ΔE =111 meV), The emission spectra, for the data shown in Figure 1A show a corresponding bathochromic shift except for the ClNHC-capped QPs, which are nonemissive (see Figure S4 of the Supporting Information). At >80 added equiv, the MeNHC-capped QDs precipitated from solution (see Figure S5A of the Supporting Information), as has been seen with Pd nanop articles capped with NHCs with short-ehain aliphatic groups in the 4- and 5-positions of the imidazolylidene.⁴⁹ For the ClNHC-capped QDs, precipitation occurred at >6 equiv added, while for the HNHC-capped QDs, precipitation did not occur until >120 equiv were added (see Figure S5B,C of the Supporting Information).

Figure 1.

(A) Ground-stale absorbance spectra of 10 μM oleate-capped CdSe QDs In toluene (black) and of those same QDs after treatment with 6 equiv of ClNHC (green), 78 equiv of MeNHC (red), or 120 equivof HNHC (blue). All spectra are normalized to the first absorption maximum. We occasionally observed minor shifts in absorbance baseline due to a fraction of the QDs aggregating during ligand exchange; the Supporting Information describes how we manually subtract those baselines from the spectra before analyzing any spectral shift. Inset: a plot of the apparent change in the average exritonic radius of the QD ensemble, “ΔR”, vs added equivalents of each NHC derivative. The data sets include measurements on at least three separately prepared sets of samples for each NHC derivative. (B) ¹H NMR spectra of each NHC derivative in d_g-toluene with and without 6.6 μM CdSe QDs (45 equiv of MeNHC/QD), 7.6 μM QDs (77 equiv of HNHC/QP), or 6.9 μM QDs (2 equiv of C1NHC/QP). The quintet at 2.18 ppm (top) is a satellite peak of residual THF. The peaks at 4.02 ppm (middle) axe due to impurities introduced during the deprotonation of the HNHC.

We translate these ΔE values into “ΔR”, defined as die apparent increase in average exdtonic radius of die QDs within the ensemble due to the EDL effect. The ΔR parameter is a convenient means of comparing EDL effects among different QD materials, ligand structures, and coverages. Figure 1A (inset) is a plot of ΔR versus added equivalents of each NHC derivative over the linear region of this plot (Le., before colloidal stability is compromised; see Figure S5 of the Supporting Information). HNHC produces the largest ΔR without the loss of the QD colloidal stability, 0.165 nm; this value is on the order of the maximum ΔR value (0.175 nm) achieved by treating CdSe QDs with unsubstituted PTC.³ Addition of 50 equiv of the protonated forms of MeNHC or ClNHC, or 400 equiv of the protonated form of HNHC, which should not bind to the QD surface, results in no shift of the absorbance or emission of the QDs (see Figure S6 of the Supporting Information).

Figure 1B shows portions of the ¹H NMR. spectra of each freely diffusing NHC in d₈-toluene and the same compound upon addition of 7–8 μM QDs. We scale the NMR spectra, with and without QDs to correct for dilution (see the Supporting Information for details). We know that the peaks in these six: spectra are attributable to the dimer of each XNHC from ¹³C NMR (see Figure S7 of the Supporting Information), because their peak positions are not consistent with those previously reported for the monomeric species.⁴⁶ The top spectra show the MeNHC dimer peak for the backbone methyl groups at 226 ppm with QDs and at 2.20 ppm without QDs. In d₈-toluene, the wingtip protons are buried beneath residual THF at 3.54 ppm (see Figure S8 of the Supporting Information). The middle spectrum shows the HNHC dimer peak for the wingtip protons at 4.00 ppm with QDs and at 3.95 ppm (and broadened) without QDs. The broadening of the peak for the freely diffusing HNHC dimer is due to the low solubility of this molecule in d₈-toluenej the QPs, and associated oleate ligands, help to bring the HNHC into solution. The bottom spectrum in Figure 1B shows the ClNHC dimer peak at 3.20 ppm with QDs and 3.19 ppm without QDs. A downfield shift and slight broadening of the peak upon addition of QDs is consistent with dynamic exchange of the molecule (through the monomer species) on and off the QD surface on a time scale faster than that of the NMR experiment^49–52 We suspect, but have not proven, that the difference in chemical shift between the resonances for the bound monomer and freely diffusing dimer of ClNHC is very small because this shift has a large contribution from NHC–NHC interactions on the Qp surface, and we added only 2 equiv/QD of ClNHC (as opposed to the tens of equivalents of MeNHC and HNHC) in this experiment.

The inset of Figure 1A demonstrates that ΔR increases with increasing added XNHC but does not account for the different binding equilibria of the various XNHCs and therefore does not allow us to connect the molecular structure of each XNHC to the magnitude of its EDL effect. To probe the mechanism of exciton delocalization by NHCs, we need the value of ΔR per bound XNHC, which we denote ΔR_b. NMR is not useful for quantifying the number of XNHCs bound because excess MeNHC and ClNHC dimer appear to react with oleate (see Table S1 of the Supporting Information) and because the freely diffusing HNHC dimer is only marginally soluble in d₈-toluene. We instead determine the value of ΔR_b by monitoring the position of the first extitonic peak of the QDs upon addition of various concentrations of p-toluenesulfonic add (TsOH), which protonates XNHC molecules on die QD surface, induces their desorption, and causes the QD band gap to shift back toward its original value (that corresponding to oleate-coated QDs).

This experiment relies on the reversibility of the EDL effect of the XNHCs; the lack of reversibility of the optical shift of dithiocarbamate-treated QDs was a notable weakness of this class of EDL ligands. Figure 2A,B shows that the optical shift induced by 15 equiv of MeNHC is completely reversible upon addition of 30 μL of 40 mM TsOH solution (50 equiv of TsOH/QD) in acetonitrile to 6 μM QDs and that the shift induced by addition of 64 equiv of HNHC is completely reversible upon addition of 100 μL of 40 mM TsOH solution (122 equiv of TsOH/QD). The optical band gap of the QDs returns to its value before NHC treatment; NMR evidence that this reversion is due to protonation of the NHC off the surface of the QD is given in Figures S9 and S10 of the Supporting Information. The number of equivalents of MeNHC and HNHC we added in this experiment was limited by the amount of TsOH the QDs would tolerate before their emission was quenched, not the degree to which their EDL effect is reversible (see Figure S11 of the Supporting Information). The shift in the QDs’ optical band gap by C1NHC is not reversible upon addition of TsOH (Figure 2C), presumably due to stripping of both the C1NHC and residual oleate from the QD surface, which results in aggregation and precipitation of the QDs after the addition of 36 equiv of TsOH per QD (see Figure S12 of the Supporting Information).

Figure 2.

Normalized and baseline-subtracted absorbance spectra for 10 μM CdSe QDs treated with IS equiv of MeNHC (A), 64 equiv of HNHC (B), or 4 equiv of C1NHC (C) upon addition of various equivalents per QD (up to SO equiv for MeNHC-capped QDs, 122 equiv for HNHC-capped QDs, and 36 equiv for ClNHC-capped QDs) of 40 mM p-toluenesulfonic acid (TsOH) in acetonitrile solution. The ClNHC-capped QDs precipitate upon addition of TsOH, as described in the text.

To estimate the average number of XNHC ligands that desorb from each QD for each added amount of TsOH—and therefore determine the number of bound XNHC ligands that correspond to a certain shift in the absorption spectrum, ΔE—we measure the number of freely diffusing XNHC molecules that are protonated upon addition of various equivalents of TsOH (in the absence of QDs). We measure this number through the disappearance of the dimer signal in the NMR spectra in d₈-toluene of each XNHC (see Figure S13 of the Supporting Information), since the protonated NHCs with p-toluenesulfonate counterions are insoluble in toluene. We then assume that the pK_a, value of the QD-associated NHC derivatives is similar to that of the free NHC dimers in solution, since the species are in equilibrium, such that we can convert “equivalents of TsOH added” to an XNHC-capped QD sample to “equivalents of XNHC desorbed” for that sample. We then determine the energy by which the band gap of the QDs increases (decreases) upon desorption (adsorption) of a given number of XNHC ligands and plot ΔR vs equivalents of MeNHC and HNHC bound to the QD surface in Figure 3. The slope of this line is ΔR_b the value of ΔR per bound XNHC.

Figure 3.

Change in the apparent excitonic radius of the QDs, “ΔR”, versus the number of equivalents of MeNHC (red squares), C1NHC (green triangles), and HNHC (blue circles) per QD. For MeNHC and HNHC, the x axis is “Equivalents of NHC bound per QD”, as determined from the TsOH titration described in the text. For CINHC, the x axis is “Equivalents of NHC added per QD”. The data are fit with lines with slopes of ΔR_b, defined as ΔR per equivalent of bound NHC (or added, in the case of ClNHC). Since the equivalents of CINHC added per QD are upper bounds for CINHC bound per QD, the value of ΔR_b, for CINHC listed in the caption is a lower limit for the actual value of ΔR_b. We can therefore conclude that $\text{Δ}{R}_{\text{b}}(\text{ClNHC})\gtrsim \text{Δ}{R}_{\text{b}}(\text{MeNHC})>\text{Δ}{R}_{\text{b}}(\text{HNHC})$. “σ_R” is the Hammett resonance parameter for the correspondingly substituted benzoic acid: $\left|{\sigma }_{\text{R}}\right|(\text{ClNHC})\gtrsim \left|{\sigma }_{\text{R}}|(\text{MeNHC})>\right|{\sigma }_{\text{R}}|(\text{HNHC})$. The data sets include measurements on at least three separately prepared sets of samples for each NHC derivative.

We cannot do such an analysis for C1NHC because its binding is not reversible under conditions where the QDs remain colloidally stable, due to the initial displacement of the majority of die oleate ligands during the ligand exchange with ClNHC (see Figure S2 of the Supporting Information). We therefore plot in Figure 3 ΔR vs equivalents of ClNHC added. The value of ΔR_b extracted from this plot is a lower limit for the actual ΔR_b value for this ligand.

From Figure 3, we conclude that (i) ΔR_b for XNHC depends on X, a result consistent with the dependence of ΔR_b of para-substituted phenyldithiocarbamate ligands on their substituents,⁴ and (ii) $\text{Δ}{R}_{\text{b}}(\text{ClNHC})\gtrsim \text{Δ}{R}_{\text{b}}(\text{MeNHC})>\text{Δ}{R}_b(\text{HNHC})$. NHC ligands are strong σ donors hut may also show significant π acidity depending on the nature of the backbone substituents.⁵³ Density functional theory calculations of the orhital energies of cadmium complexes of each XNHC suggest that exdton delocalization could occur through ground-state electron transfer from the QP valence hand to the π-type LUMO+1 of the Cd²⁺-bound XNHC (see Figures S14 and S15 of the Supporting Information). Such a mechanism would imply that the strongest π-acceptor NHC of the series should also he the most strongly exdton delocalizing ligand (and have the largest ΔR_b). One measure of π acidity is the Hammett resonance parameter, σ_R, for a given substituent. The values of σ_R for the three substituents in our study are given in the caption of Figure 3; $\left|{\sigma }_{\text{R}}\right|(\text{Cl})\gtrsim \left|{\sigma }_{\text{R}}\right|(\text{Me})>\left|{\sigma }_{\text{R}}\right|(\text{H})$.^54,55 This trend explains our result: $\text{Δ}{R}_{\text{b}}(\text{ClNHC})\gtrsim \text{Δ}{R}_{\text{b}}(\text{MeNHC})>\text{Δ}{R}_{\text{b}}(\text{HNHC})$. Additionally, ⁷⁷Se NMR studies of Se–XNHC adducts, where the NHC is a H- or Cl-disubstituted imidazolylid ene with dipp wingtip groups, support our conclusion that ClNHC is a stronger π-donor than HNHC.³⁰ There has been no report comparing the X acidities of HNHC, ClNHC, and MeNHC; methods involving changes in the CO stretching frequency trans to an NHC ligand for a metal center do not isolate the influence of the π acidity of the NHC ligand on this frequency.

The results in Figure 3 are consistent with our picture of the interfacial electronic structure of phenyldithiocarbamate-capped QDs,⁴ in that the σ interaction between the BDL and a metal cation on the QP surface allows for adsorption of the EDL, while raising of orbitals within the valence band of the QD and a π-type orbital or orbitals on the ligand mediates the ground state charge transfer-type interaction necessary for ezdton delocalization. The degree of this charge transfer, dictated by the electron-accepting ability of those ligand π-type orbitals, determines the magnitude of the EDL effect and die degree to which the optical spectrum of the QD shifts.

Unlike with PTC ligands, we detect PL of the NHC-capped QDs without the addition of a cadmium salt, which, in the FTC case, was needed to precipitate excess PTC that would otherwise bind as a hole trapping ligand.²¹ Figure 4 shows the photoluminescence quantum yield (PL QY) of the QDs as a function of increasing equivalents of each XNHC bound to die QD surface, plotted as the ΔR value for each sample.⁵⁶ We observe an initial increase in the PL QY upon the addition of MeNHC and HNHC, followed by a decrease. Addition of ClNHC results in quenching of the PL QY even upon addition of <1 equiv. We suspect this quenching is due to decreased surface passivation upon oleate desorption and subsequent aggregation of these particles (see Figures S2 and S5B of the Supporting Information).

Figure 4.

Fractional photoluminescence quantum yields (PL QY) of a set of samples of QDs in tohiene, relative to that of quinine sulfate in 0.1 M aqueous H₂SO₄ after treatment with various concentrations of MeNHC, ClNHC, or HNHC, vs the values of ΔR for those samples. The excitation wavelength used to acquire the emission spectra, which are given in Figure S16 of the Supporting Information, is 4S0 nm.

CONCLUSIONS

In summary, we demonstrate the use of l,3-dimethyl-4,5-disubstituted imidazolylidene XNHC (X = H, Me, Cl) ligands as exciton-delocalizing ligands (EDLs) for CdSe QDs; the magnitude of the delocalization effect, which appears as a bathochromic shift of the absorption and emission spectra of the QDs of up to 0.111 eV, increases with the π acidity of the XNHC ligand. This trend is consistent with the mechanism proposed for exdton delocalization by phenyldithiocarbamate and thiolate EDLs in that the σ interaction between die ligand and a surface Cd²⁺ enables binding of the NHC ligands to the surface, but the mixing of π orbitals on the carbene carbon with the QD valence band enables the ezdton delocalization to occur.

This work broadens the scope of colloidal nanopartide applications of NHCs, which until now has been limited to NHC-capped metal nanoparticles^36–41 and dusters^33–35 and the synthetic precursors of coinage-metal chalcogenide QDs.⁵⁷

It also broadens the scope of EDLs beyond those containing thiolate binding groups and simultaneously overcomes two of the major limitations of sulfur-containing EDLs: the lack of reversibility of exdton delocalization (due to lack of reversible binding to the QD surface) and the of lack of PL without the addition of excess cadmium salts to the solution following ligand exchange. In contrast, titration of MeNHC- and HNHC-capped QDs with TsOH results in full reversibility of the bathochromic shift induced upon ligand exchange with these NHC derivatives, which we attribute to protonation and subsequent desorption of the MeNHC and HNHC ligands from the QD surface. The NHC-capped QDs maintain photoluminescence quantum yields of up to 42 ± 1.896 for shifts of the optical band gap as large as 106 meV. The conservation of PL of the QDs at coverages of MeNHC and HNHC that display a change in PL peak position which is 87% of the PL line width suggests that NHC-capped QPs may be useful as colorometric sensors. The lack of hole trapping by these EDLs also suggests that they may greatly enhance the rates of direct charge carrier extraction to NHC-linked molecular acceptors in, for instance, photo catalytic applications.