Solvatochromic Response of Carbon Dots : Evidence of Solvent

Article Cite This: J. Phys. Chem. C 2018, 122, 18732−18741

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Solvatochromic Response of Carbon Dots: Evidence of Solvent Interaction with Different Types of Emission Centers Nabaruna Basu and Debabrata Mandal* Department of Chemistry, University of Calcutta, 92, APC Road, Kolkata 700009, India

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S Supporting Information *

ABSTRACT: The fluorescence emission of carbon dots (CDs) of ∼4 nm diameter was studied in a large number of solvents ranging from the weakly polar 1,4-dioxane to the strongly polar and protic water and formamide. The emission spectra in all solvents comprise of two major components, attributed to two distinct populations of emission centers, the edge-states at the periphery of the sp2-hybridized CD core and the surface fluorophores. Emission from both these centers are sensitive to solvent polarity and Hbonding capacity and undergo prominent redshift with solvent ET(30) polarity parameter, which reveals a comparable polar character of these centers and their strong interaction with solvent molecules. However, between these two centers, the emission of surface fluorophores is more drastically quenched at high ET(30) solvents, suggesting the opening up of additional nonradiative relaxation channels. This distinct solvatochromic response of these two emission centers also enables the CDs to perform as ratiometric probes for solvent polarity.

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INTRODUCTION

core and edge state absorptions, respectively. Both give rise to hole−electron excitons.17 Krysmann et al. have demonstrated that the carbon core is predominantly formed only at higher pyrolysis temperatures.25 They proposed that the principal source of fluorescence emission of CDs synthesized at higher temperatures is the radiative recombination of the hole−electron excitonic pairs. If the recombination occurs completely inside the core and across the CD band gap, it is susceptible to the quantum confinement effect, implying that the emission energy undergoes a redshift as the size of the core is enlarged. However, the charge carriers may diffuse into the edge and surface states, allowing recombination to take place at subband gap levels. In fact, single-molecule spectroscopy of CDs has demonstrated that the recombination occurs overwhelmingly at centers outside the core and that it is coupled with phonon modes of the CD lattice. The spectral dispersion of the fluorescence is then a consequence of the random distribution of the fluorescence centers.13 The other important source of fluorescence in CDs is the surface region comprising of small molecule fluorophore units, which behaves essentially as an array of independent emitters like a dye-doped polymer. In fact, according to several reports, the surface fluorophores seem to account for most of the CD fluorescence.9,14,16,19,20,27−29 This may be particularly true for CDs synthesized at lower temperatures where a well-defined core is absent.25 Apart from these, other additional relaxation pathways may also contribute to the fluorescence response of

The fluorescence optical properties of carbon dots (CDs) have generated immense interest in recent years.1−7 As more information accumulates about the optical properties of CDs, there have been vigorous efforts to understand the origin of these properties.5−10 On the basis of the data of a large number of experimental11−21 as well as theoretical12,22−24 studies, a broad consensus has slowly emerged on how to associate the electronic states and transitions with the structural features of the CD. A CD is essentially made up of a carbon core surrounded by a peripheral surface region. The carbon core is composed of graphene-like domains of sp2-hybridized C atoms dispersed amid a matrix of diamondlike sp3-hybridized C atoms. The core is terminated at the edge by heteroatomic functional groups that constitute the surface region. In pure CDs, these groups are mostly −OH, −CO, and −COOH, whereas in N- or S-doped CDs, amine and thiol-like groups are also present. Distinct from both the core and the edge is the surface region of the CD, which comprises of functionalized units behaving like small organic molecule fluorophores.25,26 These are created during the formation of the carbon core through the dehydration and carbonization of the precursor molecules.25 Optical excitation of the graphene-like core at UV wavelengths −1

but Li(ε) = 0 for 2γi(ε − εi ,max )/Δi < −1

A single lognormal fit suffices for the emission spectra at λex = 450 nm, whereas double lognormal fit is required for spectra at both λex = 300 and 370 nm, implying the presence of two major emissive components at these excitation wavelengths. An example of the lognormal fitting is given in Figure S1 of the Supporting Information section for the emission spectra of CDs in solvent dioxane. The situation is summarized in Figure 3 where the peak positions of the deconvoluted emission components are plotted against the excitation energies corresponding to λex = 300, 370, and 450 nm. For a given solvent, the emission peak components clearly fall into two categories: one at 440−470 nm (for λex = 300 and 370 nm) and one at ≥510 nm (for all excitation wavelengths). It is noted that the ≥510 nm lowenergy peak remains nearly invariant in a given solvent, irrespective of excitation wavelength, whereas the high-energy peak undergoes a minor shift in some solvents as the excitation wavelength is changed from 300 to 370 nm. Fluorescence excitation spectra of the CDs in any given solvent were recorded at emission wavelengths corresponding to the peaks of all emission spectra. Figure 4 displays the excitation spectra in all solvents recorded at four different emission wavelengths: 400, 460, 520, and 580 nm, which span the major part of the emission spectra (as in Figure 2). The

∑ Li(ε)

where Li(ε) is the lognormal term representing the intensity of the ith component emission peak with relative amplitude hi, peak 18734

DOI: 10.1021/acs.jpcc.8b04601 J. Phys. Chem. C 2018, 122, 18732−18741

Article

The Journal of Physical Chemistry C

Figure 4. Normalized excitation spectra of the CD in different solvents (as indicated) monitored at different emission wavelengths: 400 nm (blue), 460 nm (light blue), 520 nm (green), and 580 nm (red). For 1,4-dioxane, methanol and water, the normalized absorption spectra (black dotted curve) are also appended.

water) displayed in Figure 4. In other words, core excitation also contributes to the edge-state and surface fluorophore emission. As noted above, electronic transitions within the CD core are expected to be sensitive to the quantum confinement effect: CDs with larger core sizes should absorb and emit at longer wavelengths, if the radiative excitonic recombination occurs exclusively inside the core. In such a situation, size heterogeneity of the core diameter might account for REES in CDs.30 However, according to Figures 2 and 3, the emission peaks in our experiments extend at most up to 2.95 eV (∼420 nm), which is far lower than the band gap for core absorption (≤300 nm). Hence, the emission cannot be attributed to radiative excitonic recombination across the core band gap. In other words, size heterogeneity is not the reason why the emission peaks shift as a function of excitation energy. Instead, we might recall that according to Figure 3, emission shifts are more prominent among the edge-state component peaks than among the surface fluorophore component peaks. Moreover, the excitation spectra reveal that the edge-state emission peaks in any given solvent originate from a broad range of excitation energies in the near UV. Thus, using two different excitation wavelengths λex = 300 nm and λex = 370 nm, different subpopulations of emissive edge states are generated, causing the emission peak shifts in Figure 3. Next, the peak positions of the emission components were plotted against the ET(30) solvent polarity parameter in Figure 5. For all three λex = 300, 370, and 450 nm, the emission peak positions display redshift with increasing ET(30) values. Further, the insets in Figure 5 show the intensity ratio of the low-energy and high-energy emission peak component (ILO/ IHI) plotted against the ET(30) for λex = 300 and 370 nm. The ILO/IHI is found to decrease sharply with solvent ET(30), indicating that the CDs perform as efficient ratiometric polarity probes for the solvents under study. Although solvatochromism of surface fluorophore emission has been well-documented,28,29 nothing similar has been

Figure 3. Emission peak positions (in eV and nm units) of the CDs in different solvents plotted as a function of the excitation energies corresponding to λex = 300, 370, and 450 nm.

following features are to be noted. First, in all solvents, longwavelength emissions (λem ≥ 520 nm) originate mostly from excitations at 400−450 nm. Second, emission in the 400−500 nm window derives mostly from excitations at 350−400 nm. Here, a dependence on excitation wavelength is observed even within a given solvent: the λem ∼ 460 nm and λem ∼ 400 nm peaks derive mostly from longer and shorter excitation wavelengths, respectively. Thus, it can be safely concluded that the λem = 400−500 nm emissions (corresponding to the high-energy peak in Figure 3) correlate with a broad range of edge-state excitation, whereas the λem ≥ 500 nm emissions (corresponding to the low-energy peak in Figure 3) correlate with excitation of surface fluorophores. Third, many of the excitation spectra display a small peak at 500 nm) where the surface fluorophores are expected to contribute the most. However, neither the time profiles nor the fitting parameters offer any such evidence. One reason may be that the transfer occurs in an ultrafast time scale, too short to be captured with the ≥100 ps time resolution of our TCSPC system. Thus, the exact mechanism can be ascertained only if the ultrafast dynamics can be probed. For the present, we utilized the fitting data to construct the time-resolved area normalized emission spectra (TRANES) of

Figure 8. Fluorescence time profiles of the CD in solvents methanol and formamide, using excitation wavelengths 290, 375, and 450 nm. Time profiles recorded at different emission wavelengths are vertically offset for visual comparison. 18737


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Figure 9. TRANES of the CD at different time intervals, recorded in four selected solvents using two different excitation wavelengths: 290 and 375 nm. The dotted curve in each panel represents the corresponding steady-state emission spectrum.

the CDs at λex = 290 and 375 nm. Following this method, the time-resolved emission spectra (TRES) were first constructed for a given excitation wavelength at different time delays. Next, each of the TRES curves was normalized with respect to the area covered under it, producing the corresponding TRANES. The TRANES curves for different time delays, obtained in this method, are then plotted simultaneously. If the curves exhibit a unique isoemissive point, this indicates the presence of two emitting species in the excited state.35,36 The TRANES method involves no a priori assumption about the nature of the fitting parameters and treats them merely as numerical figures devoid of any physical significance. Thus, the method is independent of any model or mechanism chosen for interpreting the experimental data. Moreover, it is valid for the existence of any two emitter species, regardless of whether

they are independent of each other or are connected by a reversible/irreversible interconversion process. The TRANES curves of the CDs at λex = 290 and 375 nm are displayed in Figure 9. Over a time scale from ∼100 ps to 10 ns, the TRANES in each solvent exhibits a unique, well-defined isoemissive point. Clearly, the result corroborates the inferences we drew from the steady-state spectroscopic data. Thus, when the CDs are subjected to λex = 290 nm, the core is excited preferentially, generating a hole−electron exciton across the band gap. The exciton rapidly escapes into the edge state where it undergoes radiative recombination, producing fluorescence at energies substantially lower than the band gap. When the CDs are subjected to λex = 375 nm, nπ*-like absorptions involving the edge states are predominant. The subsequent radiative recombination also occurs in the edge states, but the fluorescence in this instance is 18738


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An important part of this work was to pinpoint the sources of the CD fluorescence, since in most cases, the emission spectra were broad and possessed multiple peaklike features. For this, we took recourse of a spectral deconvolution method, which allowed us to break down the overall response into a sum of two distinct individual emissive components. Comparing these results with the excitation and absorption spectra, the components were identified as emissions from the edge state and the surface fluorophores. This helped us to trace out the photophysical mechanism of the CDs, which is summarized in the simple Jablonski diagram depicted in Figure 10.

considerably red-shifted than that in the previous case. This is apparent when we compare the TRANES for λex = 290 and 375 nm in Figure 9. Evidently, the fluorescence centers in the edge region span a very broad and heterogeneous distribution of energy levels, a point that was made earlier while discussing the results depicted in Figures 2−4. In addition to the edgestate emission, a second, lower energy emission peak is observed for both λex = 290 and 375 nm, which is attributable to surface fluorophores. The isoemissive points listed in Table 3 typically show differences of ≥ 10 nm between λex = 290 and Table 3. Isoemissive Peak Positions (in eV and nm units) in the TRANES Curves of the CDs in Different Solvents solvent 1,4-dioxane DMF methanol formamide

λex = 290 nm 2.72 2.81 2.74 2.64

eV eV eV eV

(457 (441 (453 (471

nm) nm) nm) nm)

λex = 375 nm 2.66 2.70 2.66 2.64

eV eV eV eV

(467 (460 (467 (471

nm) nm) nm) nm)

375 nm. Thus, although the emission at all delay times consists of a high-energy edge state and low-energy surface fluorophore emission, excitation at λex = 290 and 375 nm probably generates emission from different subsets of the edge-state fluorescence centers. The role of the solvent in the time-resolved emission does not appear to be significant. Isoemissive points are found in the TRANES in all solvents. Moreover, the differences in isoemissive points for λex = 290 and 375 nm (as listed in Table 3) do not show any systematic variation with the nature of the solvent. Nevertheless, the edge-state emission peak of the TRANES curves in Figure 9 is noticeably broader in 1,4dioxane and DMF, compared to that in methanol and formamide. The latter are distinguished by their stronger Hbond acidity, which the former lack completely. In other words, the high-energy edge states are more susceptible to quenching due to H-bond donation of the solvent.

Figure 10. Simplified Jablonski diagram depicting the probable mechanism of fluorescence originating from different regions of the CD.

CONCLUSIONS The fluorescence of CDs have so far been assigned to a variety of sources of which the three principal ones are: radiative recombination of photoinduced charge carriers (i) inside the CD core or (ii) at the edge states bordering the core and (iii) fluorescence from functionalized units at the surface behaving as if they are discrete molecular fluorophores. Since in a large number of real and potential applications, CDs are used in the solution state rather than as solids, it is imperative to understand how and why the solvent might modulate its fluorescence properties. Solvatochromism of CDs has been reported in the literature, but most previous studies are either empirical in nature or involve only a few solvents. Moreover, the interpretation of results is often made complicated by important interfering factors like REES, which is quite common in CDs. In this work, we have tried to address the problem of solvent dependence in a large number of diverse solvents. In fact, the only factor that acted as a constraint in this work was the solubility: we chose only those solvents that gave a stable dispersion of CDs. Thus, our study included solvents covering a wide range of polarity and H-bonding capacity, ranging from dioxane (low polar, weak H-bond base, not H-bond acid) to formamide and water (protic and high polar, strong H-bond base and strong H-bond acid).

According to Figure 10, optical excitation of the CD involves three separate possibilities: using high excitation energy (λex < 400 nm), the relevant transitions are: (1) ππ*-like absorption across the core band gap and (2) nπ*-like absorptions involving the edge and the core, whereas (3) using low excitation energy (λex > 400 nm) leads to absorption by surface fluorophores. These initial excited states subsequently decay through a variety of pathways described below. First, the ππ*-exciton in the core diffuses out rapidly into the edge states, where it undergoes radiative recombination at subband gap energies, producing edge-state emission. This process constitutes the nonradiative decay of the excited core into the edge states. In addition, the excited core also nonradiatively decays into the surface fluorophore levels, producing surface fluorescence. On the other hand, radiative recombination of the ππ*-exciton within the core itself is ruled out, since we do not observe any emission corresponding to core band gap. Second, the nπ*-exciton undergoes radiative recombination at the edge states, also producing edge-state emission. As before, a fraction of the excited edge states also nonradiatively decays into the surface fluorophore levels, producing surface fluorescence. Third, the surface fluorophores excited with λex > 400 nm produce surface fluorescence following the same mechanism as in conventional small-molecule fluorophores.

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Apart from these, other minor relaxation processes may also be operative, as suggested by previous workers.17 Spectral deconvolution also revealed that, contrary to previous reports, the individual emission components do not all consistently exhibit REES in all solvents. According to the results displayed in Figure 3, surface fluorophores show little or no REES. On the other hand, although the edge-state emitters in some solvents do show some fluorescence shift depending on the excitation wavelength, their magnitude is small and the direction of the shift is not well-defined. In any case, the dependence on excitation wavelength indicates that the fluorescence centers responsible for edge-state emission form a heterogeneous distribution over a broad range of energies. The effect of solvent polarity and H-bonding capacity on the emission properties was remarkable. In a departure from previous reports, redshift was observed for both edge-state as well as surface fluorophore emission, as the solvent ET(30) was increased. This indicates that the excited emissive states in both cases have a marked polar character and both are susceptible to interactions with solvent molecules. Surface fluorophore emission is, however, quenched to a greater extent in more polar solvents, where additional nonradiative relaxation channels are likely to open up.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b04601. Fitting parameters for time-resolved emission curves of CDs in (i) dioxane and (ii) DMF (Table S1); fitting parameters for time-resolved emission curves of CDs in (i) methanol and (ii) formamide (Table S2); lognormal fitting of emission spectra of the CDs in dioxane recorded with different excitation wavelengths, showing the deconvoluted emission components (broken curves) (Figure S1) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Debabrata Mandal: 0000-0002-4945-8317 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support of this work was obtained from DST-SERB, India (Project No. EMR/2017/000387). Time-resolved picosecond spectroscopic experiments were carried out at the University of Calcutta with the TCSPC instrument purchased under the DST (FIST Program), India. N.B. acknowledges University Grants Commission, India, for awarding research fellowship.

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REFERENCES

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Solvatochromic Response of Carbon Dots : Evidence of Solvent

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