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C: Physical Processes in Nanomaterials and Nanostructures

Solvatochromic Response of Carbon Dots : Evidence of Solvent Interaction with Different Types of Emission Centres Nabaruna Basu, and Debabrata Mandal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04601 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 19, 2018

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The Journal of Physical Chemistry

Solvatochromic Response of Carbon Dots : Evidence of Solvent Interaction with Different Types of Emission Centres

Nabaruna Basu and Debabrata Mandal* Department of Chemistry, University of Calcutta, 92, APC Road, Kolkata 700009, India E-Mail: [email protected]

Abstract The fluorescence emission of Carbon dots (CDs) of ~ 4 nm diameter were 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 centres – the edge-states at the periphery of the sp2-hybridized CD core, and the surface fluorophores. Emission from both these centres are sensitive to solvent polarity and H-bonding capacity, and undergo prominent red-shift with solvent ET(30) polarity parameter, which reveals comparable polar character of these centres and their strong interaction with solvent molecules. However, between these two centres, the emission of surface fluorophores is more drastically quenched at high ET(30) solvents, suggesting the opening up of additional non-radiative relaxation channels. This distinct solvatochromic response of these two emission centres also enables the CDs to perform as ratiometric probes for solvent polarity.

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Introduction 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 Based on 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 sp2hybridized C atoms dispersed amid a matrix of diamond-like 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, while 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 < 300 nm induces ππ∗like absorptions localized within the core. On the other hand, optical excitation of the functional groups at the edge promotes transfer of electrons from the heteroatom-based non-bonding orbitals into the core π∗ orbitals, leading to nπ∗-like absorptions typically covering 300 - 400 nm. These two constitute the two principal absorption mechanisms in the CDs, and are characterized as core and edge state absorptions, respectively. Both give rise to hole – electron excitons.17 Krysman 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 holeelectron 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 red-shift 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 sub-band gap levels. In fact, single molecule spectroscopy of CDs has demonstrated that the recombination occurs overwhelmingly at centres outside the core, and that it is coupled with phonon modes of


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the CD lattice. The spectral dispersion of the fluorescence is then a consequence of the random distribution of the fluorescence centres.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 CDs, such as charge-transfer transitions spanning the core, the edge-states and the surface fluorophores.17 The characterization of CD fluorescence gets complicated due to its dependence on two factors: (i) excitation wavelength,11,15-21 and (ii) nature of solvent.11,27-29 In general, a red-shift in the excitation wavelength is known to induce a red-shift in the emission spectrum, giving rise to the red edge excitation shift (REES).10,16 Some authors have attributed this to quantum confinement effect similar to the case of semiconductor nanoparticles

18,30

, which implies that a

heterogeneous core size distribution causes a range of different emission wavelengths. Others however tend to interpret it instead in terms of the surface fluorophore emission.14-17,19-21 Since these fluorophores are localized at the surface, they are accessible to solvent molecules, so that strong interactions with solvent molecules can have a significant impact upon the fluorescence characteristics. Indeed, solvatochromism in CDs have been reported by several authors.11, 27-29 In a study involving water and DMSO as solvents, the emission from edge-states and surface fluorophores were found to be modulated by the polarity and H-bonding capacity of the solvent.17 In another recent study, it has been claimed that the H-bonding capacity of the solvent is primarily responsible for the emission behavior of the CDs.29 In short: the problem of CD fluorescence still requires closer examination. This motivated us to undertake the present study, where the fluorescence properties of CDs were investigated in a number of solvents covering a broad range of polarity and H-bonding capacity. The emission spectra were analyzed in order to separately identify the major sources of the fluorescence, particularly based on the framework elaborated in Ref 17. This allowed us to determine how the fluorescence response from different centres are affected by variations in (i) excitation wavelength, and (ii) nature of solvent, providing an insight into the overall fluorescence mechanism. The present study also includes measurements of the fluorescence



dynamics of CDs in the time-scale of ~100 ps to several ~10 ns. Since solvent relaxation/ reorganization in most polar solvents is complete within ~10 ps, our choice of time-scale allows us to analyze the observed time-resolved fluorescence data without having to consider the effects of solvation dynamics. Experimental details Chemicals and sample preparation: CDs were synthesized following a method adapted from that previously reported by Dhenadhayalan et al.15 Briefly, an aqueous solution of citric acid was irradiated in a microwave oven operated at 540 W for 8 min, leading to a pale brown coloration, indicative of carbon dot formation. The resultant pale yellow solution was dispersed into water and purified by centrifugation (10,000 rpm, 20 min) and dialysis to remove large sized particles. The purified CDs were isolated as a brown powder after solvent removal and thorough drying. The shape and size of the CDs were characterized in Transmission Electron Microscopy (TEM), conducted in a 200 kV instrument (JEOL JEM 2100 HR) at the Centre for Research in Nanoscience and Nanotechnology (CRNN), University of Calcutta. Spectroscopy: Solutions for spectroscopy were prepared by dispersing the CD powder in different solvents, and checking by trial and error, whether stable dispersions could be obtained. In this way, we finally obtained a limited set of suitable solvents as listed in Table 1. Absorption and fluorescence spectra of CD solutions were recorded using a Hitachi UV spectrophotometer (U3501) and Perkin Elmer (LS 55) spectrofluorimeter respectively. Picosecond fluorescence dynamic studies of the fluorophore solutions was performed using a time correlated single photon counting (TCSPC) system by employing a picosecond diode laser operating at λex = 375 nm and 450 nm (pulsewidth of ~ 60 ps), and nanosecond diode laser operating at λex = 290 nm (pulsewidth of ~ 400 ps).

Results and discussion TEM analysis and size distribution Representative TEM images (at two different magnifications) in Figure 1 show that the CDs are well dispersed and spherical in shape. The EDS spectrum is also appended in Figure 1.


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The particle size histogram, constructed from pictorial data obtained from several TEM images, conforms to a distribution that is almost symmetric with an average diameter of ~ 4.5 nm. Steady-state spectroscopy Spectroscopic data were recorded for CD solutions prepared in the solvents listed in Table 1. From the list, it is evident that a fairly wide range of solvent polarity 31 and H-bonding capacity32 have been explored in this work. It may be noted that while the CDs could not be dispersed in non-polar solvents like alkanes, stable dispersions were obtained in solvents ranging from weakly polar dioxane to strongly polar and protic water and formamide. The normalized fluorescence emission spectra of the CDs in all solvents are displayed in Figure 2, for excitation wavelengths 300 nm, 370 nm and 450 nm. We used excitation wavelengths (λex) over a range from 300 nm to 620 nm. However, between λex = 450 nm and λex = 620 nm, the emission spectra were identical, except that the intensities steadily decreased. Beyond λex = 620 nm, emission intensity was almost nil. Two important features are found in Figure 2. Firstly, for a given excitation wavelength, the emission spectra depend on the nature of the solvent, as is expected of a solvatochromic system. Secondly, within a given solvent, the emission peak positions undergo an apparent red-shift as the excitation wavelength is increased from 300 nm to 450 nm. This illustrates the Red Edge Excitation Shift or REES, mentioned above. The spectra in Figure 2 are fairly broad, and have prominent minor peak-like features especially for λex = 300 nm and 370 nm. In order to resolve the individual emission peaks, we attempted to deconvolute each emission spectrum by fitting it with a lognormal function, which is widely accepted as a reliable model for replicating the emission response of fluorophores33,34. In general, we used a lognormal sum function L(ε) given by: L(ε) = ∑ Li(ε) where Li(ε) is the lognormal tem representing the intensity of the ith component emission peak with relative amplitude hi, peak emission energy εi,max, asymmetry parameter γi and bandwidth ∆i. Specifically, Li(ε) = hi × exp[−ln(2){ln(1 + 2γi(ε – εi,max)/∆i)/γi}2]

for

2(ε – εi,max)/∆i > −1

for

2γi(ε – εi,max)/∆i < −1.

but Li(ε) = 0



A single lognormal fit suffices for the emission spectra at λex = 450 nm, while double lognormal fit is required for spectra at both λex = 300 nm 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 SF1 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 nm, 370 nm and 450 nm. For a given solvent, the emission peak components clearly fall into two categories: one at 440 – 470 nm (for λex = 300 nm and 370 nm) and one at ≥510 nm (for all excitation wavelengths). It is noted that the ≥510 nm low-energy peak remains nearly invariant in a given solvent, irrespective of excitation wavelength, while the high energy peak undergoes a minor shift in some solvents as the excitation wavelength is changed from 300 nm to 370 nm. Fluorescence excitation spectra of the CDs in any given solvent were recorded at emission wavelengths corresponding to the peaks of all the emission spectra. Figure 4 displays the excitation spectra in all solvents recorded at four different emission wavelengths : 400 nm, 460 nm, 520 nm and 580 nm, that span the major part of the emission spectra (as in Figure 2). The following features are to be noted. Firstly, in all the solvents, long wavelength emissions (λem ≥ 520 nm) originate mostly from excitations at 400 – 450 nm. Secondly, emission in the 400 nm – 500 nm window derive 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 nm – 500 nm emissions (corresponding to the highenergy peak in Figure 3) correlate with a broad range of edge-state excitation, while the λem ≥ 500 nm emissions (corresponding to the low-energy peak in Figure 3) correlate with excitation of surface fluorophores. Thirdly, many of the excitation spectra display a small peak at < 300 nm, especially in low to moderately polar solvents. This peak matches well with the core absorption band, as can be seen from the absorption spectra of some selected solvents (dioxane, methanol and water) displayed in Figure 4. In other words, core excitation also contributes to the edgestate 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


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wavelengths, if the radiative excitonic recombination occurs exclusively inside the core. In such a situation, a 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 upto 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 sub-populations 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 nm, 370 nm and 450 nm, the emission peak positions display red-shift with increasing ET(30) value. 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 nm an 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. While solvatochromism of surface fluorophore emission has been well-documented,

28,29

nothing similar has been reported for edge-state emission. In Figure 5, the red-shift of both highand low-energy emission components with increasing ET(30) in reveals that both the edge-state emitters and surface fluorophores possess highly polar character, and that both are sufficiently accessible to the solvent molecules to be affected by them. At the same time, the variation of the ILO:IHI ratios shown in the insets of Figure 5 also highlight a striking difference between the edge-state emitters and the surface fluorophores. In case of the latter, increase in solvent polarity promotes additional non-radiative relaxation channels, which causes a far more drastic reduction in their fluorescence intensity. We further applied a Kamlet-Taft multivariate regression analysis on the emission peak positions. In this method, the emission peak energy is assumed to be a linear function of three



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independent solvent properties – the π∗ polarity, the H-bond acidity (α), and the H-bond basicity (β) : ε = ε0+ a×α + b×β + p×π∗

(1)

Here the coefficients a, b and p represent the relative contributions of the solvent parameters α, β and π∗, respectively, towards determining the emission peak energy of the CD. The fit parameters are listed in Table 2, while the calculated emission peak energies (obtained by using Eq. 1) are plotted against the experimental emission peak energies (obtained by lognormal fitting of emission spectra) in Figure 6. The linear relationship (Eq. 1) seems to hold better for the low energy peaks, as indicated by the regression coefficient values. In all instances, the a, b and p have negative values, i.e., α, β and π∗ all induce red-shift in the emission energy, implying that the fluorescence centres in the CDs are sensitive to solvent polarity as well as H-bond capacity. Time-resolved fluorescence spectroscopy Time-resolved emission was monitored for the CDs using three different excitation wavelengths: λex = 290 nm, 375 nm, and 450 nm, in order to predominantly excite the core, the edge-states, and the surface fluorophores, respectively. We chose the solvents DMF, formamide, methanol and dioxane which straddled a wide range of polarity - nearly 20 ET(30) units. On the one hand, none of DMF (α=0, β=0.69) or dioxane (α=0, β=0.37) is a H-bonding acid but behave as strong to moderate H-bonding bases. On the other hand, both methanol (α=0.98, β=0.66) and formamide (α=0.71, β=0.48) are strong H-bonding acids, as well as strong to moderate Hbonding bases. Emission was collected over a range of emission wavelengths placed 10 nm apart. Emission time-profiles collected at some emission wavelengths are displayed in Figures 7 and 8. The curves generally exhibit non-exponential decay, the exact nature depending on the emission wavelength. Irrespective of the nature of the solvent, the curves for λex = 290 nm and 375 nm undergo faster decay at shorter emission wavelengths, and slower decay at longer emission wavelengths. The curves were fitted with a multi-exponential decay function: F(t) = Ʃ ai exp (-t/τi) Detailed fitting parameters are given in Table S2 and Table S3 of the Supporting Information section.


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According to the discussion on steady-state spectroscopic data, both core and edge-state excitations generate emission from the (higher energy) edge-state and (lower energy) surface fluorophores. Core excitation produces hole-electron excitons that may diffuse into the edge and recombine at sub – band gap energy levels, causing edge-state emission. But how the excitons induce fluorescence from the surface fluorophores is not clear. One possibility may be energy transfer, whereby surface fluorophores are generated by accepting energy from the decaying excitonic pairs. In that case, the time-profiles in Figure 7 and 8 for λex = 290 nm and 375 nm should exhibit definite growth-like feature at longer emission wavelengths (> 500 nm) where the surface fluorophores are expected to contribute most. However, neither the time-profiles nor the fitting parameters offer any such evidence. One reason may be that the transfer occurs in an ultra-fast 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 ultra-fast dynamics can be probed. For the present, we utilized the fitting data to construct the time-resolved area normalized emission spectra (TRANES) of the CDs at λex = 290 nm 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 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 nm 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, welldefined 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 considerably red-shifted than that in the previous case. This is apparent when we compare the TRANES for λex = 290 nm and 375 nm in Figure 9. Evidently, the fluorescence centres 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, 3 and 4. In addition to the edgestate emission, a second, lower energy emission peak is observed for both λex = 290 nm 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 nm and 375 nm. Thus, while the emission at all delay times consists of a high-energy edge-state and low-energy surface fluorophore emission, excitation at λex = 290 nm and 375 nm probably generates emission from different sub-sets of the edge-state fluorescence centres. 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 nm and 375 nm (as listed in Table 3) does not show any systematic variation with the nature of the solvent. Nevertheless, the edge-state emission peak of the TRANES curves in Figure 9 are noticeably broader in 1,4-dioxane and DMF, compared to that in methanol and formamide. The latter are distinguished by their stronger H-bond 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.

Conclusion The fluorescence of CDs have so far been assigned to a variety of sources of which the three principal ones are: radiative recombination of photo-induced 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 solution state rather than as solids, it is imperative to understand how and why the solvent might modulate its fluorescence properties. Solvatochromism of CDs have been reported in literature, but most previous studies are either


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empirical in nature, or involve only a few solvents. Moreover, the interpretation of results is often 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). 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 peak-like 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 those 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. 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; while 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 sub-bandgap energies, producing edge-state emission. This process constitutes the non-radiative decay of the excited core into the edge-states. In addition, the excited core also non-radiatively 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. 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 is all solvents. According to the results displayed in Figure 3, surface fluorophores show little or no REES. On the other hand, while the edge-state emitters in some solvents do show some fluorescence shift depending on the excitation wavelength, its 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 centres 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, red-shift 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 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 non-radiative relaxation channels are likely to open up.

Conflicts of Interest There are no conflicts of interest to declare.

Supporting Information: Table S1. Fitting parameters for time-resolved emission curves of CDs in (i) dioxane and (ii) DMF Table S2. Fitting parameters for time-resolved emission curves of CDs in (i) methanol and (ii) formamide Figure S1. Lognormal fitting of emission spectra of the CDs in dioxane recorded with different excitation wavelengths, showing the deconvoluted emission components (broken curves)

Acknowledgements


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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. thanks University Grants Commission, India, for awarding research fellowship.



References: 1. Baker, S. N.; Baker, G. A. Luminescent Carbon Nanodots: Emergent Nanolights. Angew. Chem. Int. Ed. 2010, 49, 6726-6744. 2. Li, H.; Kang, Z.; Liu, Y.; Lee, S.-T. Carbon Nanodots: Synthesis, Properties and Applications. J. Mater. Chem. 2012, 22, 24230–24253. 3. Hola, K.; Zhang, Y.; Wang, Y.; Giannelis, E. P.; Zboril, R.; Rogach A. L. Carbon Dots Emerging Light Emitters for Bioimaging, Cancer Therapy and Optoelectronics. Nano Today 2014, 9, 590-603. 4. Fu, M.; Ehrat, F.; Wang, Y.; Milowska, K. Z.; Reckmeier, C.; Rogach, A. L.; Stolarczyk, J. K.; Urban, A. S.; Feldmann, J. Carbon Dots: A Unique Fluorescent Cocktail of Polycyclic Aromatic Hydrocarbons. Nano Lett. 2015, 15, 6030-6035. 5. Zhu, S.; Song, Y.; Zhao, X.; Shao, J.; Zhang J.; Yang, B. The Photoluminescence Mechanism in Carbon Dots (Graphene Quantum Dots, Carbon Nanodots, and Polymer Dots): Current State and Future Perspective. Nano Res. 2015, 8, 355-381. 6. Cayuela, A.; Soriano, M. L.; Carrillo-Carrio´n, C.; Valca´rcel, M. Semiconductor and Carbon-based Fluorescent Nanodots: the Need for Consistency. Chem. Commun. 2016, 52, 1311-1326. 7. Kozák, O.; Sudolská, M.; Pramanik, M.; Cígler, P.; Otyepka, M.; Zbořil, R. Photoluminescent Carbon Nanostructures. Chem. Mater. 2016, 28, 4085-4128. 8. Cushing, S. K.; Li, M.; Huang, F.; Nianqiang Wu, N. Origin of Strong Excitation Wavelength Dependent Fluorescence of Graphene Oxide. ACS Nano 2014, 8, 1002–1013. 9. Wang, L.; Zhu, S.-J.; Wang, H.-Y.; Qu, S.-N.; Zhang, Y.-L.; Zhang, J.-H.; Chen, Q.-D.; Xu, H.-L.; Han, W.; Sun, H.-B. Common Origin of Green Luminescence in Carbon Nanodots and Graphene Quantum Dots. ACS Nano 2014, 8, 2541–2547. 10. Deng, Y.; Zhao, D.; Chen, X.; Wang, F.; Song, H.; Shen, D. Long Lifetime Pure Organic Phosphorescence Based on Water Soluble Carbon Dots. Chem. Commun. 2013, 49, 57515753. 11. Pan, D.; Zhang, J.; Li, Z.; Wu, C.; Yan, X.; Wu, M. Observation of pH-, Solvent-, Spin-, and Excitation-dependent Blue Photoluminescence from Carbon Nanoparticles. Chem. Commun. 2010, 46, 3681-3683.


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12. Strauss, V.; Margraf, J. T.; Dolle, C.; Butz, B.; Nacken, T. J.; Walter, J.; Bauer, W.; Peukert, W.; Spiecker, E.; Clark, T.; Guldi, D. M. Carbon Nanodots: Toward a Comprehensive Understanding of Their Photoluminescence. J. Am. Chem. Soc. 2014, 136, 17308-17316. 13. Ghosh, S.; Chizhik, A. M.; Karedla, N.; Dekaliuk, M. O.; Gregor, I.; Schuhmann, H.; Seibt, M.; Bodensiek, K.; Schaap, I. A. T.; Schulz, O.; Demchenko, A. P.; Enderlein, J.; Chizhik, A.

I.

Photoluminescence

of

Carbon

Nanodots:

Dipole

Emission

Centers

and

Electron−Phonon Coupling. Nano Lett. 2014, 14, 5656-5661. 14. Dekaliuk, M. O.; Viagin, O.; Malyukin, Y. V.; Demchenko, A. P. Fluorescent Carbon Nanomaterials: ‘‘Quantum Dots’’ or Nanoclusters? Phys. Chem. Chem. Phys. 2014, 16, 16075-16084. 15. Dhenadhayalan, N.; Lin, K.-C.; Suresh, R.; Ramamurthy, P. Unravelling the Multiple Emissive States in Citric-Acid-Derived Carbon Dots. J. Phys. Chem.C 2016, 120, 1252−1261. 16. Sharma, A.; Gadly, T.; Gupta, A.; Ballal, A.; Ghosh, S.K.; Kumbhakar, M. Origin of Excitation Dependent Fluorescence in Carbon Nanodots. J. Phys. Chem. Lett. 2016, 7, 3695−3702. 17. Reckmeier, C. J.; Wang, Y.; Zboril, R.; Rogach, A. L. Influence of Doping and Temperature on Solvatochromic Shifts in Optical Spectra of Carbon Dots. J. Phys. Chem. C 2016, 120, 10591−10604. 18. Thiyagarajan, S.K.; Raghupathy, S.; Palanivel, D.; Raji, K.; Ramamurthy. P. Fluorescent Carbon Nano Dots from Lignite: Unveiling the Impeccable Evidence for Quantum Confinement. Phys. Chem. Chem. Phys. 2016, 18, 12065-12073. 19. Gude, V.; Das, A.; Chatterjee, T.; Mandal, P. K. Molecular Origin of Photoluminescence of Carbon Dots: Aggregation-induced Orange-red Emission. Phys. Chem. Chem. Phys. 2016, 18, 28274-28280. 20. Hsu, Y.-F.; Chen, Y.-H.; Chang, C.-W. The Spectral Heterogeneity and Size Distribution of the Carbon Dots Derived from Time-resolved Fluorescence Studies. Phys. Chem. Chem. Phys. 2016, 18, 30086-30092. 21. Das, A.; Gude, V.; Roy, D.; Chatterjee, T.; De, C. K.; Mandal, P. K. On the Molecular Origin of Photoluminescence of Nonblinking Carbon Dot. J. Phys. Chem. C 2017, 121, 9634-9641.



22. Sk, M.A.; Ananthanarayanan, A.; Huang, L.; Lim, K.H.; Chen, P. Revealing the Tunable Photoluminescence Properties of Graphene Quantum Dots. J. Mater. Chem. C 2014, 2, 6954-6960. 23. Margraf, J. T.; Strauss, V.; Guldi, D. M.; Clark, T. The Electronic Structure of Amorphous Carbon Nanodots. J. Phys. Chem.B 2015, 119, 7258-7265. 24. Sudolská, M.; Dubecký, M.; Sarkar, S.; Reckmeier, C. J.; Zbořil, R.; Rogach, A. L.; Otyepka, M. Nature of Absorption Bands in Oxygen-Functionalized Graphitic Carbon Dots. J. Phys. Chem.C 2015, 119, 13369-13373. 25. Krysmann, M. J.; Kelarakis, A.; Dallas, P.; Giannelis, E. P. Formation Mechanism of Carbogenic Nanoparticles with Dual Photoluminescence Emission. J. Am. Chem.Soc. 2012, 134, 747-750. 26. Song, Y.; Zhu, S.; Zhang, S.; Fu, Y.; Wang, L.; Zhao, X.; Yang, B. Investigation from Chemical Structure to Photoluminescent Mechanism: a Type of Carbon Dots from the Pyrolysis of Citric acid and an Amine. J. Mater. Chem. C 2015, 3, 5976-5984. 27. Kumar, P.; Bohidar, H. B. Observation of Fluorescence from Non-functionalized Carbon Nanoparticles and its Solvent. J. Lumin. 2013, 141, 155-161. 28. Sciortino, A.; Marino, E.; van Dam, B.; Schall, P.; Cannas, M.; Messina, F. Solvatochromism Unravels the Emission Mechanism of Carbon Nanodots. J. Phys. Chem. Lett. 2016, 7, 34193423. 29. Mukherjee, S.; Prasad, E.; Chadha, A. H-Bonding Controls the Emission Properties of Functionalized Carbon Nano-dots. Phys. Chem. Chem. Phys. 2017, 19, 7288-7296. 30. Li, H.; He, X.; Kang, Z.; Huang, H.; Liu, Y.; Liu, J.; Lian, S.; Tsang, C.A.; Yang, X.; Lee, S.-T. Water-Soluble Fluorescent Carbon Quantum Dots and Photocatalyst Design. Angew. Chem. Int. Ed. 2010, 49, 4430-4434. 31. Reichardt, C. Solvatochromic Dyes as Solvent Polarity Indicators. Chem. Rev. 1994, 94, 2319-2358. 32. Marcus, Y. The Properties of Organic Liquids that are Relevant to their Use as Solvating Solvents. Chem. Soc. Rev. 1993, 93, 409-416. 33. Suda, K.; Terazima, M.; Sato, H.; Kimura, Y. Excitation Wavelength Dependence of Excited State Intramolecular Proton Transfer Reaction of 4′-N,N-Diethylamino-3-Hydroxyflavone in


Page 16 of 31

Page 17 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Room Temperature Ionic Liquids Studied by Optical Kerr Gate Fluorescence Measurement. J. Phys. Chem. B 2013, 117, 12567-12582. 34. Ghosh, D.; Batuta, S.; Das, S.; Begum, N. A.; Mandal, D. Proton Transfer Dynamics of 4′N,N-Dimethylamino-3-hydroxyflavone Observed in Hydrogen-Bonding Solvents and Aqueous Micelles. J. Phys. Chem. B 2015, 119, 5650-5661. 35. Koti, A. S. R.; Krishna, M. M. G.; Periasamy, N. Time-Resolved Area-Normalized Emission Spectroscopy (TRANES): A Novel Method for Confirming Emission from Two Excited States. J. Phys. Chem. A 2001, 105, 1767-1771. 36. Koti, A. S. R.; Periasamy, N. Application of Time Resolved Area Normalized Emission Spectroscopy to Multicomponent Systems. J. Chem. Phys. 2001, 115, 7094-7099.



Page 18 of 31

Table 1. Solvent parameters of the solvents used

Solvent

ET(30)

π∗

α

β

1,4-dioxane

36

0.55

0

0.37

DMF

43.8

0.88

0

0.69

DMSO

45

1

0

0.76

Acetonitrile

45.6

0.75

0.19

0.4

Butanol

50.2

0.4

0.79

0.84

Propanol

50.7

0.52

0.84

0.9

Ethanol

51.9

0.54

0.86

0.75

Methanol

55.4

0.6

0.98

0.66

Ethylene

56.3

0.92

0.9

0.52

Formamide

56.6

0.97

0.71

0.48

Glycerol

57

0.62

1.21

0.51

H2O

63.1

1.09

1.17

0.47

glycol


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Table 2. Kamlet-Taft multivariate linear regression parameters and corresponding regression coefficients (R) for the emission peak energies of CD solutions.

Excitation wavelength

ε0 (eV)

p (eV)

a (eV)

b (eV)

R

300 nm (high energy emission peak)

3.17

-0.23

-0.08

-0.33

0.80

370 nm (high energy emission peak)

2.95

-0.09

-0.08

-0.22

0.78

300 nm (low energy emission peak)

2.70

-0.15

-0.07

-0.15

0.90

370 nm (low energy emission peak)

2.61

-0.06

-0.10

-0.11

0.95

450 nm

2.56

-0.05

-0.08

-0.07

0.98



Table 3. Isoemissive peak positions (in eV and nm units) in the TRANES curves of the CDs in different solvents.

Solvent

λex = 290 nm

λex = 375 nm

1,4-dioxane

2.72 eV (457 nm) 2.81 eV (441 nm) 2.74 eV (453 nm) 2.64 eV (471 nm)

2.66 eV (467 nm) 2.70 eV (460 nm) 2.66 eV (467 nm) 2.64 eV (471 nm)

DMF Methanol Formamide


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Intensity

Page 21 of 31

Energy (keV)

Figure 1. Representative TEM images, size distribution histogram and EDS spectrum of the CDs.



Figure 2. Normalized emission spectra of the CD in different solvents (as indicated) and with different excitation wavelengths : 300 nm (∗), 370 nm (o) and 450 nm (•).


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



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.


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Figure 5. Emission peak positions and (inset) intensity ratio of low-energy to high-energy emission peaks of the CD as function of the ET(30) polarity parameter of the solvents. The corresponding excitation wavelength in indicated in each panel



Figure 6. Calculated emission peak energies of the CD solutions obtained by Kamlet-Taft regression analysis, plotted against the observed emission peak energies recorded in various solvents. The R-value in parantheses indicates Pearson’s linear correlation coefficient for the corresponding fit. (R = ±1 implies complete correlation while R=0 implies no correlation)


Page 26 of 31

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Figure 7. Fluorescence time-profiles of the CD in solvents 1,4-Dioxane and DMF, using excitation wavelengths 290 nm, 375 nm and 450 nm. Time-profiles recorded at different emission wavelengths are vertically offset for visual comparison.



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


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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 nm and 375 nm. The dotted curve in each panel represent the corresponding steady-state emission spectrum



-O H

-C H O O

-CO OH

-OH

Core

π∗

π∗

Edge

Edge Surface

Surface

n

3

2

π

Edge-state

1 H -C OO

-O H

-O H

H

Surface Fluorophore

-C OO H

Surface

-C OO

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 31

1 ⇒ ππ*-like absorption (across core band-gap) 2 ⇒ nπ*-like absorption (edge-to-core) 3 ⇒ Surface Fluorophore absorption

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


H

OH

O

-O H -CO

Area Normalized Em. Int.

TOC Graphic: O -C Emission energy (eV)

Core

π∗

π∗

-C OO H

300 nm

Surface

n

Edge-state -C OO H -O H

OH

-C O

π Area Normalized Em. Int.

450 nm

Surface Fluorophore

Surface

370 nm

Edge

Edge Surface

-O H

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-OH

Page 31 of 31

Emission energy (eV)


Solvatochromic Response of Carbon Dots ... - ACS Publications

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