Exciton Self-Trapping in sp2

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials 2

Exciton Self-Trapping in sp Carbon Nanostructures Induced by Edge Ether Groups Shunwei Chen, Naeem Ullah, and Rui-Qin Zhang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01972 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 7, 2018

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The Journal of Physical Chemistry Letters

Exciton Self-trapping in sp2 Carbon Nanostructures Induced by Edge Ether Groups Shunwei Chen1,2, Naeem Ullah1,3, Ruiqin Zhang1,3* 1 Department of Physics, City University of Hong Kong, Hong Kong SAR, China 2 Shenzhen Research Institute, City University of Hong Kong, Shenzhen, China 3 Beijing Computational Science Research Center, Beijing 100193, China ABSTRACT: Recent experiments have suggested that exciton self-trapping plays an important role in governing the optical properties of graphene quantum dots (GQDs) and carbon dots (CDs), while the molecular structures related to this phenomenon remain unclear. This theoretical study reports exciton self-trapping induced by edge-bonded ether (C-O-C) groups in graphene nanosheets. Density functional theory (DFT) and time-dependent DFT calculations show that the initially delocalized electron and hole are trapped in the vicinity of the edge ether groups on graphene nanosheets upon excited-state (S1) relaxation, accompanied by structural planarization of the seven-membered cyclic ether rings in the same region. The resulted significant structural deformation leads to large Stokes shift energies, which are comparable to the magnitudes of the notably large emission shift observed in experiments. This study provides a feasible explanation of the origin of exciton self-trapping and offers guidance for experiments to investigate and engineer exciton self-trapping in relevant materials.

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Graphene quantum dots (GQDs) and carbon dots (CDs),1-2 featuring tunable photoluminescence, high photostability, good biocompatibility, and low costs, have emerged in recent years as superior fluorescent materials for a variety of optical applications, such as bioimaging and biosensoring.3-5 Despite the clear understanding of quantum size effect6 and the great progress made in regard to these materials, the origin of their photoluminescence is still being debated, which significantly hinders further development in this field.7-8 To understand the underlying photophysics following photoexcitation, the charge carrier dynamics (relaxation) in GQDs and CDs have been extensively studied.9-12 As a result of the electron and hole excitations, exciton self-trapping,13 a phenomenon that the excited electron and hole are simultaneously trapped (localized) to some part of the structure due to self-induced structural deformation in excited state de-excitation process, 14-15 was recently suggested as an important phenomenon in GQDs and CDs, governing their optical behaviors.8, 16-18 In particular, exciton self-trapping has been proposed as the reason for the induction of the large Stokes shift (ST) in CDs.8, 19 Several studies have suggested aromatic molecular aggregates (H-aggregates) as the molecular origin of this trapping phenomenon.8, 20 Given that the π-π stacking of conjugated molecules generally quenches light emission, which is known as aggregation-caused quenching (ACQ),21-22 this is seemingly contradictory to the fact that experimentally synthesized GQDs and CDs frequently exhibit high levels of emission intensity and quantum efficiency.23-24 Further studies have suggested that individual chemical bonds may be responsible for this exciton self-trapping phenomenon.17 So far, no unanimous 2 ACS Paragon Plus Environment

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agreement has been reached in regard to the molecular origin of the exciton self-trapping phenomenon in GQDs and CDs. In this work, by employing a combination of density functional theory (DFT) and time-dependent DFT (TDDFT) calculations, we examine the optical (absorption and emission) properties of a series of graphene nanosheets and reveal that exciton self-trapping can be induced by edge-bonded ether (C-O-C) groups in these structures. The formation of these edge-oxidized ethers is found to be energetically favorable, thus this exciton self-trapping phenomenon is very likely to be common in GQDs and CDs, which contain graphitic carbon nanostructures in the experimentally produced samples. Upon excitedstate structural relaxation, the initially delocalized electron and hole are self-trapped in the vicinity of one or two ether O atoms in the lowest excited singlet (S1) state. This exciton localization is accompanied by the structural planarization of the cyclic ether rings (formed by an ether O atom with a benzene ring) in the same region. The significant structural relaxation associated with the exciton selftrapping leads to a large loss of energy in regard to light emission (S1→S0) after photoexcitation (S0→S1). The ST energies are computed as varying from 0.53 to 1.16 eV, comparable to the magnitudes of the large emission shifts observed in experiments regarding GQDs and CDs. In particular, the edge-oxidized graphene nanosheets with an exciton self-trapping character are predicted to possess much higher fluorescence efficiencies than the structures (pristine and center-oxidized graphene nanosheets) without a self-trapping character. This study provides a feasible explanation for the appearance of exciton self-trapping in GQDs and CDs. The disclosed molecular structural information is valuable for experiments in regard to synthesizing appropriate molecules in order to systematically investigate and utilize the exciton self-trapping phenomenon in relevant materials. The ground- and excited-state properties of the pristine graphene nanosheets and their ether oxides were examined by the DFT and TDDFT methods, respectively, as implemented in the Gaussian 09 package.25 The range-separated ωB97XD hybrid function,26 the accuracy of which has recently been 3 ACS Paragon Plus Environment


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validated for the study of graphene molecules,27-28 together with the 6-311G(d,p) basis set,29 was chosen for all the calculations. The lowest absorption energy (∆Eabs) was determined as the first vertical excitation energy calculated by the TDDFT method on the basis of optimized ground-state (S0) geometry; the emission energy (∆Eemi), following the Kasha’s rule,30 was determined as the S1→S0 transition energy by optimizing the S1 geometry. The ST energy (∆EST) was computed by taking the energy difference between the lowest absorption and the emission energy.31 The electron-hole distributions were analyzed using the Multiwfn program (version 3.5).32-33 To understand the optical properties of pure sp2 carbon nanostructures, the absorption and emission properties of several pristine graphene nanosheets (C6-C96, named according to the number of contained carbon atoms) were first examined, considering the size and edge/shape effects. As shown in Figure 1, the lowest excitation and emission energy of these structures exhibit a negative dependence on their physical dimensions and are deviated as the edge/shape changes. This trend is essentially determined by the quantum confinement effect, as previously reported.6, 34-35 As a result of energy loss due to excited-state structural relaxation, the emission red-shifts in energy, as compared to the first excitation energy. The ST energies in these structures are computed to vary from 0.16 to 0.28 eV, the extents of which are in line with the values reported in experiments for pristine graphene nanostructures.36-37 The experimentally synthesized GQDs and CDs are mostly characterized by very large ST values (for example, >1 eV);7, 19, 38 these relatively small ST values thus indicate that the large ST energy loss observed in the experiments is not an intrinsic property of the individual graphene nanosheets with perfect sp2 carbon networks.7 It should be noted that the experimentally detected absorption and emission may also involve other excited states,39 thus the ST values computed in this work by taking as the energy difference of the lowest absorption and emission level can only be roughly compared with the experimental results in terms of excited state energy loss.40 Given that exciton selftrapping generally induces large ST by noticeably deforming the electronic and physical structures, the computed small ST values also signify the absence of exciton self-trapping in these pristine graphene 4 ACS Paragon Plus Environment

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nanosheets.14 Then, the HOMO-LUMO orbitals of these nanosheets at the S0 and S1 geometries were inspected and only small differences in the energy levels and in the spatial distributions were found, as exemplified in Figure 1b, further indicating the absence of exciton self-trapping (see Figure S1 for the results of the other graphene nanosheets). Consistent with previous reports, these results suggest that the structurally-perfect individual graphene nanostructures possess a rather delocalized exciton, rather than a trapped exciton.28, 35 The ether group is a kind of common species in the experimentally synthesized GQDs and CDs.3, 5 To understand its role in trapping the electron and the hole, we have mainly investigated the optoelectronics of a series of C54 ethers (Figure 2a). These ethers are formed by attaching oxygen atoms to the C54 surface, with the O atom bridging two adjacent carbon atoms after breaking the C=C bond.41 It should be noted that we also examined the edge-oxidized ethers for the other two graphene nanosheets (C24 and C42) and two C54 oxides which have oxygen-containing groups binding at both the center and edge positions. Among all these structures, self-trapped exciton (STE) was found in each structure after excited-state (S1) relaxation; the results can be seen in Figure S2. It is predictable that exciton selftrapping at the cyclic ether rings will occur as long as the resulted structural planarization provides sufficient stabilization to the excited state, even with the presence of other chemical species on the structure. To determine the favorable positions in regard to chemisorbing the ether groups on the graphene nanosheets, we computed the reaction energy for the formation of the C54 ethers from the reaction of C54 with one or more triplet oxygen atoms, using the formula ∆Erec=E C54

ether

– EC54 – nEO (n=1-6).42

Figure 2b shows the reaction energies for eight C54 mono-ethers. As shown, binding one ether oxygen atom at the edge p7 position (C54-Op7) is most energetically favorable, with an exothermic energy as high as 66.6 kcal/mol. To compare this with experiments, we calculated the reaction energy for two C54-Op7 analogues that have been experimentally synthesized, dibenz[b,f]oxepin and dinaphth[1,85 ACS Paragon Plus Environment


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bc:1',8'-ef]oxepine, using the above formula;41, 43 the exothermic energies are 75.6 and 71.6 kcal/mol, respectively, close to the reaction energy calculated for C54-Op7 (66.6 kcal/mol). Thus, it can be predicted, from the energy point of view, that binding an ether group at the p7 position is quite feasible in experiments. Based on this finding, C54 ethers bonded with 2-5 ether O atoms at the edges were further studied by successively attaching one O atom to the p7 site; the reaction energies are shown in Figure 2c. In addition to this, we considered cases with 2-5 ether O atoms binding at the C54 center positions, with the reaction energies shown in Figure 2d. In both cases, the exothermic energies are positively proportional to the number of the binding ether O atoms. As indicated by the exothermicity of these reactions, all these C54 ether configurations are thermodynamically stable, while forming the edge-oxidized C54 ethers is about two times more exothermic than forming the center-oxidized counterparts, imping the edge-oxidized configurations are more energetically favorable, consistent with the findings revealed in experiments.44-45 In addition, previous studies revealed that the chemisorbed epoxy or ether groups are readily to migrate on the graphene surface after overcoming moderate reaction barriers.46 As an example, we examined the energy profile of an oxygen migration pathway which leads to the formation of a seven-membered ether ring on the C24 basal plane; in accordance with previous findings, such oxygen migration reactions are predicted to be energetically favorable (see Figure S3).46-48 Thereby, such edge-oxidized configurations studied in this work, owing to their great relative stability, might widely exist in sp2 carbon materials, especially in GQDs and CDs. In the following section, we thus mainly focus on the edge-oxidized C54 ethers, in which the exciton selftrapping phenomenon is found; for the completeness of the study, the results for the center-oxidized C54 ethers are also demonstrated.


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Figure 1. a) Lowest absorption, emission, and corresponding ST energies of the pristine graphene nanosheets with different sizes and edges/shapes. b) Energy levels and spatial distributions of the HOMO-LUMO orbitals of C54 at the S0 and S1 geometries.

Figure 2. a) Schematic presentation for binding one or more ether oxygen atoms onto the C54 basal plane: 1. Binding one ether O atom onto eight different sites (C54 mono-ethers); 2. Binding more ether 7 ACS Paragon Plus Environment


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O atoms at the C54 edges (edge-oxidized C54 ethers); 3. Binding more ether O atoms at the C54 center positions (center-oxidized C54 ethers). b) Reaction energy for the C54 mono-ethers. c) Reaction energy for the edge-oxidized C54 ethers. d) Reaction energy for the center-oxidized C54 ethers. Figure 3a shows the lowest absorption and emission energies, as well as the corresponding ST energies for the C54 ethers that are bonded with 1-6 ether O atoms at the edge or center positions, which indicates a functionalization-dependence of optical properties of graphene nanostructures.35, 49 For the edge-oxidized C54 ethers (i.e., C54-Op7 and C54-mOe (m=2-5)), the lowest absorption energies increase as the binding O atom number increases, whereas the emission energies show an almost opposite trend. As a result, the computed ST energies increase from 0.53 to 1.16 eV (see Table 1 for these values). By contrast, the center-oxidized C54 ethers show no recognizable trend in regard to the changes in their absorption and emission energies; in addition to this, their ST energies (0.21-0.35 eV) are much smaller than those of the edge-oxidized structures. The apparent difference in the ST energies in the two cases signifies their different excitonic characters; that is, exciton self-trapping exists in the edge-oxidized C54 ethers, while it is absent in the center-oxidized ones (see below). Due to electronvibration coupling, the self-trapped exciton leads to a much more potent change in the physical and electronic structures of the edge-oxidized C54 ethers than those of the center-oxidized C54 ethers; thus, the edge-oxidized C54 ethers exhibit more ST loss after structural relaxation in the S1 state. According to our calculations, functional groups on the surface of GQDs and CDs are crucial contributors to increase the ST loss because the disruption of the intrinsic π-electron system upon chemisorbing of chemical species may raise the degree of excited-state relaxation.50 Whereas, the extent of resulted energy loss notably depends on the binding positions of the chemical groups and the number of binding groups exhibits less prominent influence. In line with previous suggestions, this exciton self-trapping phenomenon can be used to explain the origin of the large emission shifts (for example, >100 nm) observed in GQDs and CDs.8, 19 One important reason for this is that the computed ST energies (0.531.16 eV) are comparable to the experimentally observed huge shifts.8, 38, 51


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Figure 3. a) Lowest absorption, emission, and corresponding ST energies of the C54 ethers binding with 1-6 ether O atoms at the C54 edge and center positions. b) HOMO-LUMO gap energies of the C54 ethers at the S0 and S1 state geometries. c) HOMO-LUMO distributions of three reprehensive C54 ethers at the S0 and S1 state geometries. Table 1. Energy and oscillator strength (OS) for the lowest absorption and emission levels, together with the corresponding ST energy (∆EST) and electronic gap decrease (∆EH-L) due to excited-state relaxation for C54 and its ether oxides. The values in parentheses show the corresponding ST wavelengths. Structure C54 C54-Op7 C54-2Oe C54-3Oe C54-4Oe C54-5Oe C54-6Oe C54-Op3

∆ES0→S1 [eV] 2.68 2.74 2.79 2.84 2.95 3.04 3.15 2.62

OSS0→S1 [a.u.] 0.000 0.003 0.009 0.003 0.027 0.068 0.001 0.002

∆ES1→S0 [eV] 2.5 2.21 2.16 1.92 1.98 1.97 1.98 2.37

OSS1→S0 [a.u.] 0.000 0.135 0.087 0.032 0.060 0.056 0.022 0.001

∆EST [eV (nm)] 0.18 (33) 0.53 (109) 0.63 (130) 0.93 (211) 0.96 (204) 1.07 (223) 1.16 (231) 0.25 (51)

∆EH-L [eV] 0.19 0.59 0.56 0.82 0.71 0.74 1.02 0.26



C54-2Oc C54-3Oc C54-4Oc C54-5Oc C54-6Oc

2.46 2.44 2.42 2.29 2.41

0.004 0.010 0.007 0.009 0.008

2.25 2.13 2.09 1.96 2.06

0.005 0.009 0.005 0.003 0.0001

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0.21 (47) 0.31 (75) 0.32 (79) 0.33 (91) 0.35 (86)

0.14 0.29 0.30 0.31 0.33

Previous experiments have revealed that pristine (un-functionalized) graphene nanostructures generally exhibit low levels of photoluminescence efficiency, but this efficiency can be largely enhanced via chemical modifications.36 This phenomenon is presumably due to the fact that the radiative transition in pristine graphene nanostructures is optically forbidden, whereas surface functionalization may alter the optical transition selection rule.49 As can be seen in Table 1, binding ether groups on the C54 basal plane activates the otherwise forbidden S1→S0 transition (non-zero OS). Surprisingly, the OS values in the edge-oxidized C54 ethers are larger by one or two orders of magnitude than those of the center-oxidized C54 ethers, suggesting that exciton self-trapping plays a predominant role in increasing the radiative decay rate.13-14, 52 As such, the edge-oxidized C54 ethers are expected to possess much higher levels of photoluminescence efficiency than the center-oxidized C54 ethers, as well as pristine C54.13 These findings support the notion that the experimentally synthesized GQDs and CDs with exciton self-trapping characters may exhibit notably high levels of photoluminescence efficiency. A similar phenomenon, in which ether groups enhance photoluminescence efficiency by trapping the exciton, has been reported for single-walled carbon nanotubes.53-54 To interpret the different optical features in these two types of C54 ethers (the edge-oxidized and center-oxidized C54 ethers), the associated electronic structures are compared. Figure 3b shows the electronic gaps of these ether oxides at the S0 and S1 geometries; the gap changes after excited-state relaxation, signifying the extent of the structural deviation (see Table 1 for detailed gap values).55 As shown, the electronic gaps of the edge-oxidized ethers always possess much larger decreases after excited-state relaxation than those of the center-oxidized ethers, which means that the edge-oxidized ethers exhibit more structural deviation from the ground10 ACS Paragon Plus Environment

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state geometry, in line with the trend in the computed ST values. Consistent with the trend in the energy level changes, the spatial distributions of the HOMO and LUMO orbitals show more deviations in the edge-oxidized C54 ethers. As demonstrated in Figure 3c, both the HOMO and LUMO orbitals of the edge-oxidized ethers show a trend of localizing toward the edge ether groups after excited-state relaxation. In addition to this, the ether (C-O-C) moieties increase their contribution to the π-conjugation (see the shadowed areas). These results are indicative of exciton self-trapping induced by the edge ether groups (see Figure S4 for the HOMO-LUMO distributions of the other edge-oxidized C54 ethers).13 In contrast to this, the exciton selftrapping character is not found in the center-oxidized C54 ethers; their HOMO-LUMO orbital distributions remain essentially delocalized and show negligible changes, as demonstrated by the case for C54-6Oe (see Figure S5 for the HOMO-LUMO distributions of the other centeroxidized C54 ethers). To further verify the nature of exciton self-trapping in these edge-oxidized C54 ethers, the electron and hole distributions before and after excited-state relaxation are plotted in Figure 4. In contrast to this, the electron and hole in the center-oxidized C54 ethers and in C54 show no selftrapped exciton character (see Figure S6).32-33 As shown, in these the edge-oxidized ethers, both the electron and the hole initially delocalize over the sp2 carbon moiety, away from the ether O atom(s), while, after excited-state relaxation, both the electron and the hole are significantly localized to one or two cyclic ether rings. This is firm evidence for exciton self-trapping.13-14, 56 Accordingly, a physical picture appears; that is, in the graphene nanosheets, the initially mobile electron and hole generated upon photoexcitation can diffuse to the edge ether sites, where they are stabilized by the local potential minimum.14, 57 As such, it is clear that edge ether groups may greatly affect the charge carrier dynamics and thus the photoluminescence performance of GQDs

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and CDs.53 It should be mentioned that a very recent experiment suggests that the C-O bond may be involved in the exciton self-trapping in CDs.17 It is thus of great importance that further experiments ascertain how general this exciton self-trapping (induced by edge ether groups) is in GQDs and CDs.

Figure 4. Electron-hole distributions of the edge-oxidized C54 ethers at the S0 and S1 state geometries (blue = electron; orange = hole). An inspection of the structural features revealed that the localization of the excited electron and hole in the edge-oxidized C54 ethers is accompanied by structural planarization in the same region. This local deformation of the structure is the character of exciton self-trapping, resulted from the electron-vibrational coupling.14, 58-59 As depicted in Figure 5a, the nonplanar groundstate cyclic ether ring (an oxepin ring, formed by incorporating an ether O atom into a benzene ring) changes to a planar configuration upon S1 relaxation via the elongation of carbon-carbon distance and the integration of an ether O atom into the main sp2 carbon plane.41 To estimate this structural planarization, which is the predominant structural changes in the excited state, the CC-O-C dihedral angle (absolute values) at the cyclic ether rings of these edge-oxidized structures at the S0 and S1 geometries were measured; as shown in Figure 5b, one or two of the dihedral angles at each structure were found to significantly decrease from ~60° at the ground-state


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geometry to ~10° at the relaxed S1 geometry (see Figure S7 for the equilibrium S0 and S1 structures of two representative ethers along with the information of the measured dihedral angle and Figure S8 for distance changes of the carbon-carbon atoms of the ether moiety after excited state relaxation). It should be noted that, in each of the edge-oxidized C54 ethers, only one or two ether groups are involved in this structural planarization, while the others remain relatively unchanged. Thereby, all the edge-oxidized C54 ether geometries in the S1 state are globally nonplanar, except for C54-Op7 (see Figure S7). This structural planarization is associated with the fact that a planar configuration at the S1 state geometry favors the enlargement of the πconjugation network, which provides stabilization to the excited state.41, 60-61 In comparison, this local planarization was not found for the center-oxidized C54 ethers (see Figure S9 for distance changes of the carbon-carbon atoms of the ether moiety after excited state relaxation); as can be expected, the cyclic ether rings embedded by the outmost edge sp2 carbon network are significantly restricted to achieving a planar configuration during excited-state relaxation. In fact, the relaxed S1 geometry of each center-oxidized C54 ether only shows a slight structural difference from the ground-state geometry, mainly involving slight distance changes in regard to the C-C bonds.


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Figure 5. Predominant structural deviation of the edge-oxidized C54 ethers during excited-state structural relaxation. a) Schematic presentation of the structural planarization of the cyclic ether rings. b) Evolution of the C-C-O-C dihedral angles at the cyclic ether rings upon excited state relaxation. In conclusion, by performing a combination of DFT and TDDFT calculations, we revealed that exciton self-trapping can be induced by the edge-bonded ether groups in graphene nanosheets. It is found that the initially delocalized electron and hole are trapped in one or two seven-membered cyclic ether rings (oxepin rings) after excited-state (S1) relaxation, accompanied by structural planarization in the same region. The binding of the ether groups at the edges of graphene nanosheets is predicted to be highly energetically favorable; thus, this exciton self-trapping phenomenon is very likely to be common in GQDs and CDs. The significant structural deformation associated with this self-trapped exciton causes a great deal of ST energy loss (0.53-1.16 eV), which is a plausible explanation for the very large red-shifts of


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emission in experiments involving GQDs and CDs. The structural, light absorption/emission, and excitonic features studied in this work further the understanding of exciton self-trapping in GQDs and CDs. The revealed molecular structural information provides useful guidance for precise experimental syntheses, allowing for a deep study and in turn the utilization of exciton self-trapping phenomenon in relevant materials.

ASSOCIATED CONTENT Supporting Information Available: HOMO-LUMO distributions of the other six graphene nanosheets at the S0 and S1 geometries (Figure S1); electron-hole distributions of the C24 monoether, the C42 mono-ether, and the center and edge concurrently oxidized C54 oxides at the S0 and S1 geometries (Figure S2); a reaction pathway leading to the formation of a sevenmembered ether ring on the C24 structure via oxygen migration reactions (Figure S3); HOMOLUMO distributions of the other four edge-oxidized C54 ethers at the S0 and S1 geometries (Figure S4); HOMO-LUMO distributions of the other five center-oxidized C54 ethers at the S0 and S1 geometries (Figure S5); electron-hole distributions of the center-oxidized C54 ethers at the S0 and S1 geometries (Figure S6); and ground- and excited-state geometry of two representative edge-oxidized C54 ethers (Figure S7); distances between the carbon-carbon atoms of the ether groups at the S0 and S1 geometries of the edge-oxidized and center-oxidized C54 ethers (Figure S8 and Figure S9, respectively).

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] ORCID Shunwei Chen: 0000-0002-2536-5589 Naeem Ullah: 0000-0002-1428-8557 Ruiqin Zhang: 0000-0001-6897-4010 Notes The authors declare no competing financial interests. ACKNOWLEDGMENT


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This work was financially supported by the grants from the Research Grants Council of the Hong Kong SAR [Project No. CityU 11334716], the Basic Research Programs in Shenzhen, China (Project No. JCYJ20160229165210666) and the Guangdong-Hong Kong Technology Cooperation Funding Scheme (Project No. 2017A050506048). REFERENCES (1) Ponomarenko, L. A.; Schedin, F.; Katsnelson, M. I.; Yang, R.; Hill, E. W.; Novoselov, K. S.; Geim, A. K. Chaotic Dirac Billiard in Graphene Quantum Dots. Science 2008, 320, 356-358. (2) Xu, X. Y.; Ray, R.; Gu, Y. L.; Ploehn, H. J.; Gearheart, L.; Raker, K.; Scrivens, W. A. Electrophoretic Analysis and Purification of Fluorescent Single-Walled Carbon Nanotube Fragments. J. Am. Chem. Soc. 2004, 126, 12736-12737. (3) Cayuela, A.; Soriano, M. L.; Carrillo-Carrion, C.; Valcarcel, M. Semiconductor and Carbon-Based Fluorescent Nanodots: The Need for Consistency. Chem. Commun. 2016, 52, 1311-1326. (4) Hutton, G. A. M; Martindale, B. C. M.; Reisner, E. Carbon Dots as Photosensitisers for Solar-Driven Catalysis. Chem. Soc. Rev. 2017, 46, 6111-6123. (5) Georgakilas, V.; Perman, J. A.; Tucek, J.; Zboril, R. Broad Family of Carbon Nanoallotropes: Classification, Chemistry, and Applications of Fullerenes, Carbon Dots, Nanotubes, Graphene, Nanodiamonds, and Combined Superstructures. Chem. Rev. 2015, 115, 4744-4822. (6) Zhang, R. Q.; Bertran, E.; Lee, S. T. Size Dependence of Energy Gaps in Small Carbon Clusters: The Origin of Broadband Luminescence. Diamond Relat. Mater. 1998, 7, 1663-1668. (7) Li, X. M.; Rui, M. C.; Song, J. Z.; Shen, Z. H.; Zeng, H. B. Carbon and Graphene Quantum Dots for Optoelectronic and Energy Devices: A Review. Adv. Funct. Mater. 2015, 25, 49294947. (8) Bhattacharyya, S.; Ehrat, F.; Urban, P.; Teves, R.; Wyrwich, R.; Döblinger, M.; Feldmann, J.; Urban, A. S.; Stolarczyk, J. K. Effect of Nitrogen Atom Positioning on The Trade-Off Between Emissive and Photocatalytic Properties of Carbon Dots. Nat. Commun. 2017, 8, 1401. (9) Mueller, M. L.; Yan, X.; Dragnea, B.; Li, L.-S. Slow Hot-Carrier Relaxation in Colloidal Graphene Quantum Dots. Nano Lett. 2010, 11, 56-60. (10) Volk, C.; Neumann, C.; Kazarski, S.; Fringes, S.; Engels, S.; Haupt, F.; Müller, A.; Stampfer, C. Probing Relaxation Times in Graphene Quantum Dots. Nat. Commun. 2013, 4, 1753. (11) Li, Q. J.; Zhou, M.; Yang, M. Y.; Yang, Q. F.; Zhang, Z. X.; Shi, J. Induction of LongLived Room Temperature Phosphorescence of Carbon Dots by Water in Hydrogen-Bonded Matrices. Nat. Commun. 2018, 9, 734.


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TOC GRAPHICS 49x49mm (600 x 600 DPI)



Figure 1. a) Lowest absorption, emission, and corresponding ST energies of the pristine graphene nanosheets with different sizes and edges/shapes. b) Energy levels and spatial distributions of the HOMOLUMO orbitals of C54 at the S0 and S1 geometries. 73x34mm (300 x 300 DPI)


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Figure 2. a) Schematic presentation for binding one or more ether oxygen atoms onto the C54 basal plane: 1. Binding one ether O atom onto eight different sites (C54 mono-ethers); 2. Binding more ether O atoms at the C54 edges (edge-oxidized C54 ethers); 3. Binding more ether O atoms at the C54 center positions (center-oxidized C54 ethers). b) Reaction energy for the C54 mono-ethers. c) Reaction energy for the edgeoxidized C54 ethers. d) Reaction energy for the center-oxidized C54 ethers. 131x101mm (300 x 300 DPI)



Figure 3. a) Lowest absorption, emission, and corresponding ST energies of the C54 ethers binding with 1-6 ether O atoms at the C54 edge and center positions. b) HOMO-LUMO gap energies of the C54 ethers at the S0 and S1 state geometries. c) HOMO-LUMO distributions of three reprehensive C54 ethers at the S0 and S1 state geometries. 120x85mm (300 x 300 DPI)


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Figure 4. Electron-hole distributions of the edge-oxidized C54 ethers at the S0 and S1 state geometries (blue = electron; orange = hole). 56x18mm (600 x 600 DPI)



Figure 5. Predominant structural deviation of the edge-oxidized C54 ethers during excited-state structural relaxation. a) Schematic presentation of the structural planarization of the cyclic ether rings. b) Evolution of the C-C-O-C dihedral angles at the cyclic ether rings upon excited state relaxation. 88x98mm (300 x 300 DPI)


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Exciton Self-Trapping in sp2

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