Solvatochromism Unravels the Emission Mechanism of Carbon

Letter pubs.acs.org/JPCL

Solvatochromism Unravels the Emission Mechanism of Carbon Nanodots Alice Sciortino,†,‡ Emanuele Marino,§ Bart van Dam,§ Peter Schall,§ Marco Cannas,† and Fabrizio Messina*,† †

Dipartimento di Fisica e Chimica, Università degli Studi di Palermo, Via Archirafi 36, 90123 Palermo, Italy Dipartimento di Fisica e Astronomia, Università degli Studi di Catania, Via Santa Sofia 64, 95123 Catania, Italy § Van der Waals−Zeeman Institute, Universiteit van Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands ‡

S Supporting Information *

ABSTRACT: High quantum yield, photoluminescence tunability, and sensitivity to the environment are hallmarks that make carbon nanodots interesting for fundamental research and applications. Yet, the underlying electronic transitions behind their bright photoluminescence are strongly debated. Despite carbon-dot interactions with their environment should provide valuable insight into the emitting transitions, they have hardly been studied. Here, we investigate these interactions in a wide range of solvents to elucidate the nature of the electronic transitions. We find remarkable and systematic dependence of the emission energy and kinetics on the characteristics of the solvent, with strong response of the photoexcited dots to hydrogen bonding. These findings suggest that the fluorescence originates from the radiative recombination of a photoexcited electron migrated to surface groups with holes left in the valence band of the crystalline core. Furthermore, the results demonstrate the fluorescence tunability to inherently derive from dot-to-dot polydispersity, independent of solvent interactions.

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evidence of such distributions remain scarce,24 and recent reports have fundamentally questioned this view.25 If surface states are involved in the emission mechanism, the interactions of CDs with the surrounding solvent should be essential in determining their optical properties. Conversely, solvation could be used to interrogate the photophysics of CDs and clarifying the characteristics of their fluorescent transitions, especially if site-specific interactions are involved. Despite the explosion of studies on CDs, this route remains hardly explored. Only few papers have reported CD emission in a nonaqueous environment,17,26−30 and only a handful openly discussed their solvatochromism.27,29 In this paper, we address these ideas and explore CD− solvent interactions to elucidate the nature of their tunable fluorescence. We find a systematic dependence of the emission energy and decay kinetics on solvent properties, directly highlighting the crucial role of surface moieties. Our results point to a model in which absorption and emission transitions couple core and surface states through a transfer of electronic charge from the crystalline core to surface moieties and back. This model explains the strong and regular solvent sensitivity exhibited by these CDs, especially with respect to hydrogen bonding (HB) interactions, and allows to understand their fluorescence tunability as a consequence of polydispersity.

arbon nanodots (CDs) are a novel class of carbon-based nanoparticles discovered in 2004,1 intensely studied because of their bright fluorescence and appealing characteristics,2,3 such as biocompatibility, high sensitivity to metal ions, photocatalytic properties, low toxicity, and ease of synthesis. The CD core is a few nanometers in size, and its structure can be tuned from crystalline to amorphous,4,5 whereas the surface is covered with polar moieties, making them highly watersoluble.2 CDs are largely composed of carbon, oxygen and hydrogen, whereas nitrogen-doping6−9 strongly enhances their quantum yield (QY),7,10,11 and can change the very crystalline structure.12,13 The hallmark of CDs is their intense and tunable photoluminescence, and considerable effort has been devoted to clarifying the underlying emission mechanisms, likely more complex and well-distinguished from classical organic fluorophores, and the characteristics of the transitions responsible for them. This aspect was addressed by analyzing, for example, the role of surface groups on CD emission,14 the charge transfer dynamics to surrounding molecules or ions,15,16 or the pHdependence of the fluorescence.17 Yet, the emission mechanisms are still debated: some authors interpreted CD fluorescence as an electronic transition within a small, quantum-confined, crystalline core,18,19 whereas many other works linked the emission to surface-localized states,20,21 as suggested by the decisive role of surface passivation in producing highly fluorescent CDs.3,4 Also, the origin of the characteristic fluorescence tunability of CDs remains debated: although several authors attributed it to a distribution of different surface chromophores or sizes,22,23 direct experimental © 2016 American Chemical Society

Received: July 19, 2016 Accepted: August 15, 2016 Published: August 15, 2016 3419

DOI: 10.1021/acs.jpclett.6b01590 J. Phys. Chem. Lett. 2016, 7, 3419−3423

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The Journal of Physical Chemistry Letters These results also imply that CDs can be used as nanoprobes of the polarity of the surrounding environment, generalizing their known capability to sense neighboring ions and molecules.31,32 We synthesized highly N-doped CDs by decomposition of organic precursors, as described in ref 12, where we thoroughly analyzed their structure. These CDs are carbon nitride nanocrystals with an average size of 3 nm, surface-functionalized with amide, carboxylic and hydroxyl groups, similar to other N-doped CDs in the literature.6,8,9 They exhibit an absorption band at 3.05 eV; the corresponding luminescence undergoes a redshift with decreasing excitation energy (see Figure 1), reflecting the typical tunability of CDs. Notably, the

Figure 1. Optical properties of CDs in H2O: normalized absorption spectrum (cyan), normalized steady-state fluorescence excited at 2.82 eV photon energy (440 nm wavelength, purple trace), 2.75 eV (450 nm, blue), 2.64 eV (470 nm, pale blue), 2.53 eV (490 nm, green), 2.43 eV (510 nm, orange), 2.34 eV (530 nm, red), and 2.25 eV (550 nm, dark red). Figure 2. (a) Normalized emission of CDs excited at 2.82 eV photon energy (440 nm wavelength) in different solvents. Spectra were vertically shifted for the sake of clarity. Acronyms are defined in the experimental section. (b) Emission peak of spectra from (a) as a function of ENT solvent polarity parameter; the line is a linear fit of the data. (c) Spectra from panel (a) in selected solvents: dioxane (pale blue continuous line), acetone (red dashed line), acetonitrile (black dashed line), 2-propanol (red continuous line), methanol (black continuous line). (d) Single-molecule fluorescence of CDs deposited on a quartz substrate and excited at 2.54 eV (488 nm).

observation of a well-defined absorption band at energies below band gap transitions of the crystalline core (above ∼3.5−4.0 eV, see Figure S1 and12,33) is strongly suggestive of midgap electronic states. To investigate CD−solvent interactions, we dispersed the dots in a wide range of solvents with different solvation capabilities (Table S1). The resulting emission spectra exhibit a strong solvent-dependent shift as shown in Figure 2a. The fluorescence red-shifts with increasing solvent polarity from 2.57 eV in toluene, the bluemost fluorescence we detect, to 2.32 eV in H2O, therefore showing a maximum shift of 0.25 eV, or ≈2000 cm−1, associated with the stabilization of the excited state by solvation. Similar results are observed at any excitation energy. The trend in Figure 2a is systematic and without substantial changes of the emission shape, differently from the few previous solvatochromic studies, where no clear trend of the emission peak was found27 or rather dramatical shape changes were observed.29 To a certain extent, these CDs hence behave similarly to solvent-sensitive molecules used as polarity indicators, which mostly display solvatochromic absorption or emission shifts of a few thousands cm−1 between a weakly polar and a strongly polar solvent.34 On the other hand, the photoluminescence properties of CDs are well distinguished from those of organic fluorophores in that the latter do not show fluorescence tunability effects. We also find that CD solvatochromism can be effectively described by the so-called ENT polarity scale,34 that is a normalized parameter measuring the overall solvation capability of solvents as a combination of

nonspecific (e.g., dipole−dipole) and specific (HB) interactions.34 As shown in Figure 2b, the emission peak position displays an approximate linear dependence (r = 0.95) on solvent polarity ENT , confirming the trend in Figure 2a. These results underline the possible employment of these CDs as nanoprobes of the local polarity. In contrast, the photoluminescence excitation spectra (Figure S2) do not shift and almost coincide among different polar solvents, all slightly blueshifted from apolar dioxane. This indicates that unlike the excited state that depends sensitively on the solvent environment, the ground-state of CDs is much less affected by the solvent. In principle, both dipole−dipole and HB interactions can contribute to the observed redshifts of the emission peak. Nevertheless, careful comparison of the emission spectra in selected solvents (Figure 2c) shows that the major solvatochromic effect arises from HB interactions, and precisely the HB donating capability of protic solvents, quantified by the well-known α Kamlet−Taft parameter.35 In Figure 2c, the emission of CD dispersed in nonpolar dioxane (ϵ = 2.3, α = 0) 3420

DOI: 10.1021/acs.jpclett.6b01590 J. Phys. Chem. Lett. 2016, 7, 3419−3423

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The Journal of Physical Chemistry Letters

spectrum. This interpretation contrasts with recent papers that rather appeal to solvent interactions to explain fluorescence tunability of CDs or graphene oxide.25,38 To obtain deeper insight into the fluorescence mechanism, we finally investigated the emission kinetics by time-resolved fluorescence measurements (Figure 3). During the decay in any

at 2.56 eV is considered as a reference point. In other aprotic solvents (very low α) having larger dielectric constants, the fluorescence shifts only about 0.01 eV in acetone (ϵ = 21, α = 0.08), or 0.04 eV in acetonitrile (ϵ = 37.5, α = 0.19), very small red shifts if we consider the substantial increase in polarity. In contrast, protic solvents with almost the same two dielectric constants, 2-propanol (ϵ = 18, α = 0.76) and methanol (ϵ = 33, α = 0.98), induce an additional emission red shift, much larger than the first (0.10 eV in 2-propanol and 0.12 eV in methanol). Hence, the strongest solvation effect on CDs can be clearly attributed to the donation of HB from protic solvents to the CD. Conversely, the surface of CDs may also donate a proton to the solvent, although this interaction barely influences the emission, which hardly shifts (3.50 eV.12,33 Because the dots appear much more solvent-sensitive in the excited state (Figure 2a) than in ground-state (Figure S2), this means that the HB accepting capability of CDs is strongly enhanced by photoexcitation, likely due to an increase of the negative charge localized on surface HB-accepting groups. Indeed, similar situations where an increase the HB propensity of a certain site is caused by the charge transfer character of the electronic transition are common for optically active systems.41,42 Therefore, our results lead to attribute the lowest-energy absorption at 3.05 eV of these CDs to a transition of an electron from the valence band of the core to the manifold of midgap (π*) empty states localized on surface carboxylic or amide groups, whereas the fluorescence arises from the radiative recombination of the photoexcited electron with the hole left in the core. The inhomogeneous distribution of fluorescence energies (Figure 2d) then results from the size distribution of these dots (from 1 to 5 nm, with an average of 3 nm12): the polydispersity affects the core bandgap and, in turn, the relative alignment between the valence band and the surface accepting level, that ultimately determines the absorption/emission energies. This interpretation contrasts with studies where fluorescence tunability was rather interpreted as a violation of Kasha’s rule due to an unusually slow relaxation of polar solvents around photoexcited CDs;25 here, these effects can be ruled out because fluorescence tunability is observed even in the most apolar solvents (as exemplified in Figure S6), and we observe no spectral migration

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b01590. Experimental procedures, characteristics of the solvents used in our study, additional steady-state absorption, excitation spectra and normalized emission spectra, comparison of CD emission in water and D2O, microphotograph of a CD sample on a quartz substrate. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the LAMP group (www.unipa.it/lamp) at University of Palermo for support and stimulating discussions and G. Napoli for technical assistance in optical measurements.

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