Resolving the Multiple Emission Centers in Carbon Dots: From

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Spectroscopy and Photochemistry; General Theory

Resolving the Multiple Emission Centers in Carbon Dots: From Fluorophore Molecular States to Aromatic Domain States and to Carbon-core States Mojtaba Shamsipur, Ali Barati, Avat Arman Taherpour, and Morteza Jamshidi J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b02043 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 13, 2018

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The Journal of Physical Chemistry Letters

Resolving the Multiple Emission Centers in Carbon Dots:

1

From Fluorophore Molecular States to Aromatic Domain States

2

and to Carbon-core States

3 4

Mojtaba Shamsipur*, Ali Barati*, Avat (Arman) Taherpour, Morteza Jamshidi

5

Department of Chemistry, Razi University, Kermanshah, Iran

6

Corresponding Authors

7

Mojtaba Shamsipur, Fax: +982166908030; Tel: +982166908032; E-mail: [email protected]

8

Ali Barati, Fax: +982166908030; Tel: +982166908032; E-mail: [email protected]

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ACS Paragon Plus Environment

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The Journal of Physical Chemistry Letters 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

Abstract

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1

Despite many efforts focused on the emission origin of carbon dots (CDs), it is still a

2

topic of debate. This is mainly due to the complex structure of these nanomaterials. Here, we

3

developed an innovative method to evaluate the number and spectral characterizations of

4

various emission centers in CDs. We monitored the photostability of a series of column-

5

separated CDs under UV irradiation to obtain three-dimensional data sets and resolve them

6

using multivariate decomposition methods. The obtained results clearly revealed the presence

7

of three different types of emission centers in CDs, including molecular states, aromatic

8

domain states, and carbon-core states so that their single or coexisting appearance found to be

9

deeply depended on the reaction temperature. Furthermore, the density functional theory and

10

time-dependent density functional theory were used to investigate the electronic and optical

11

properties of some different aza-polycyclic and corannulene molecules as possible polycyclic

12

aromatic hydrocarbons responsible for the above mentioned aromatic domain states.

13 14 15 16

TOC image

17 18 19 20 Keywords

21

Carbon dots, Fluorescence, Photoluminescence mechanism, Formation mechanism,

22

Multivariate decomposition method

23 24


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Carbon dots (CDs) are a new class of heavy metal-free and low-toxic nanomaterials

1

that have attracted a rapidly growing interest in recent years.1-3 This class contains various

2

types of nanomaterials including carbon nanodots, carbon quantum dots, graphene quantum

3

dots, and even some polymer dots with more and less similar photoluminescence (PL)

4

6

5

Compare to other fluorescence nanomaterials, CDs display many advantages including cost-

6

efficient

of

7

functionalization. These unique advantages have resulted in widespread applications of CDs

8

in various filed form imaging to sensing, catalysis and energy conversion.7-12 However,

9

despite considerable efforts made and various techniques and methods used,13-32 there are still

10

many questions remained about the intrinsic mechanisms governing the PL of CDs.

11

properties such as blue/green emission color and typical excitation-dependent PL behavior. and

easy

preparation,

water

solubility,

biocompatibility,

and

ease

2

Traditionally, CDs are considered as nano-sized materials with sp -hybridized 3

12

domains in their carbogenic cores passivated by a sp -hybridized matrix of oxygen/nitrogen-

13

containing surface functional groups. By considering this structure, the respective proposed

14

models for PL origin of CDs can be summarized into I) the quantum confinement effect or

15

conjugated π-domains determined by carbon-core and II) the surface states determined by

16

hybridization of the carbon backbone and connected surface functional groups.18 However,

17

among different kinds of CDs, those prepared through bottom-up carbonization of citric acid

18

and amine-containing compounds (citric acid-based CDs) display ultrahigh PL quantum

19

yields (QYs) and normally excitation-independent PL behaviors.

12, 13, 19, 20, 27, 28, 33-37

These

20

superior PL properties are attributed to an additional emission center called as molecular

21

states corresponding to small molecular fluorescent species in CDs.

22

In this context, Giannelis et al. was the first group that demonstrated both molecular

23

states and carbon-core states in CDs prepared from citric acid and ethanolamine.13

24

Interestingly, through some valuable reports, Yang’s group have recently revealed the

25

structure of a high fluorescent molecule (imidazo[1,2-a]pyridine-7-carboxylic acid, 1,2,3,5-

26

tetrahydro-5-oxo, IPCA) beside of carbon nanostructures in CDs solutions synthesized from

27

citric acid and ethylenediamine, that was found to be responsible for the high blue emission

28

12, 33

of these nanomaterials.

They introduced IPCA as an independent fluorescent molecule

29

that might be free in solution, connected on the surface of CDs, or incorporated inside of the

30

CD structure. Similar small molecules with high fluorescence QYs have also been detected

31

from the reaction between comparable starting materials.35, 36, 38-40 More recently, it has been

32

claimed that small molecular fluorophores and polycyclic aromatic domains are two most

33


3


Page 4 of 23

prevalent emission centers that can explain the PL of CDs.25, 30, 41 Urban and coworkers even

1

tried to mimic the optical properties of CDs through three different types of fluorescence

2

41

polycyclic aromatic hydrocarbons. Beside of all these valuable successes, PL mechanism of

3

CDs is still unclear and requires further investigations.

4

An important but less considered aspect of CDs is their highly different PL stabilities under UV irradiation.42,

43

5

Although, most of CDs show no meaningful PL reduction after

6

continuous exposure to excitation, a dramatic decrease in the PL intensity of the citric acid-

7

based CDs is reported.33,

This is because the main PL of citric acid-based CDs is

8

originated from their molecular states, which results in lower photostability, compared to the

9

carbon-core states.

10

34, 44

In this work, we have tried to determine all the possible emission centers in CDs, and

11

extract the pure excitation and emission spectra of each emission center, through monitoring

12

the photostability of the CDs followed by decomposition of the recorded data using

13

multivariate decomposition methods. For this purpose, the CDs synthesized through pyrolysis

14

of citric acid and ethylenediamine at four different temperatures were first subjected to size-

15

exclusion chromatography to separate possible coexisted CD species in the as-prepared

16

solutions. Then, the separated CD fractions were exposed to a high power UV irradiation and

17

the regular decrease in their PL intensity was monitored over time. This procedure provided

18

us the ability to record a series of excitation-emission matrices (EEMs) for each separated

19

45

fraction. Parallel factor analysis (PARAFAC)

as the most simple and efficient multi-way

20

decomposition model was then used to decompose the recorded PL data sets into individual

21

components (emission centers) that contribute to the total PL of CDs. To estimate the

22

possible responsible aromatic domains in the PL properties of CDs and elucidate the

23

influence of nitrogen doping, the electronic and optical properties of computationally

24

polycyclic aromatic hydrocarbons containing nitrogen atoms were also investigated using

25

density functional theory(DFT) and time-dependent density functional theory(TD-DFT).

26

The CDs prepared through pyrolysis of citric acid and ethylenediamine at four

27

different temperatures 150 oC (CDs150), 200 oC (CDs200), 250 oC (CDs250), and 300 oC

28

(CDs300) were selected to study the evolution and chemical nature of the emission centers

29

during the synthesis process. Consistent with previous reports,13, 34 the CDs prepared at lower

30

temperatures exhibited an excitation-independent PL behavior, while those synthesized at

31

higher temperatures presented the common excitation-dependent PL behavior (Figure S1).

32

Interestingly, the fluorescence QYs of CDs deceased from 75% to 12% when the temperature

33

o

increased from 150 to 300 C.

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The TEM images of CDs are shown in Figure 1. As is clearly seen, while the TEM

1

image of CDs150 showed large-sized polymeric structures with diameters in the range of

2

100-200 nm without any distinctable nanoparticles (Figure 1a), the image of CDs200

3

presented some disturbed polymer-like structures containing some new small particles with

4

average diameters 10-15 nm (Figure 1b). Moreover, the TEM images of CDs derived at 250

5

and 300 oC (Figure 1c,d) showed well dispersed and nearly spherical nanoparticles with

6

average sizes of about 5 and 4 nm, respectively. These size variations are in high compliance

7

with the formation mechanism of CDs, including dehydration, polymerization, aromatization,

8

and carbonization.46, 47 Accordingly, in the primary formation steps, dehydration of starting

9

26

materials produces photoluminescent components with cross-linked polymer-like structures.

10

With the progress of the process, dehydration and aromatization of the polymer structure

11

yield some embedded nanoparticle nuclei that are finally converted into individual

12

nanoparticles through further aromatization and carbonization.

13

The X-ray photoelectron spectroscopy (XPS) analyses showed the existence of similar

14

elements in the CDs. The full scan XPS spectra showed the existence of carbon (C1s, 285 eV),

15

nitrogen (N1s, 402 eV) and oxygen (O1s, 532 eV) in all as-prepared CDs (Figure 1a’-d’). De-

16

convolution of the C1s spectra revealed three peaks at 284.6, 285.9, and 288.1 eV, which are

17

assigned to C-C/C=C, C-O/C-N, and C=O/C=N groups, respectively. The surface area ratios

18

of these three peaks changed from 35:49:16 for CDs150, 51:32:17 for CDs200, 74:19:7 for

19

CDs250, and 80:14:6 for CDs300. Moreover, the XRD profiles exhibited no signal for

20

CDs150, but instead a broad and weak peak at 2θ of about 23°, corresponding to a disordered

21

graphitic structure for CDs200 was observed, the intensity of which being increased

22

remarkably for the cases of CDs250 and CDs300 (Figure S2). The thermal gravimetric

23

analysis results (Figure S3) clearly showed that the CDs synthesized at higher temperatures

24

present a better thermal stability, indicating the dehydration starting materials with heating.

25

All these results clearly revealed the improved dehydration and aromatization of precursors

26

with increasing temperature.

27 28 (Figure 1)

29 30

As mentioned before, the difference in the photostability of CDs is associated with

31

their different PL origins. Therefore, it would be quite reasonable to expect that monitoring of

32

the PL spectral changes of CDs during UV irradiation, accompanied with a multivariate data

33

analysis method, could be a promising method to resolve the various emission centers of

34


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CDs. Figure S4 shows the gradual PL spectral changes of the as-prepared CDs (at excitation

1

wavelength 300 nm) under continues irradiation with a high power UV light. As seen, while

2

no spectral shape change or shift was observed in the course of the photobleaching of

3

CDs150, the PL spectra for CDs250 and CDs300 revealed apparent shape changes. As a

4

primary result, it could be concluded that more than one emission center might contribute to

5

the overall PL of as-prepared CDs, at least for those prepared at higher temperatures.

6

However, the key question here is if whether the multiple emission centers belong to a single

7

CD particle or they result from individual CDs containing different types of emitters. Of

8

course, a separation step would be required to answer this question.

9

Considering the high potential of column chromatography methods in separation of 48-50

CDs with different sizes, charges, and more important with PL properties,

10

in the next

11

step, Silica gel G60, Sephadex gel G25, and Sephadex gel G50 were tested as stationary

12

phases to possibly separate the coexisted CD species within the initial as-prepared solutions.

13

Since the use of Sephadex gel G25 resulted in the largest differences in the PL of the

14

separated fractions, all initial solutions were subjected to this column before any

15

photobleaching investigation. The PL excitation and emission spectra of the resulting

16

separated fractions 1, 4, and 7 of CDs are presented in Figure 2. Accordingly, it is seen that

17

all fractions of CDs150 possess an excitation-independent PL behavior with maximum

18

intensities at about 456 nm.

19

The deconvolution of the PL excitation spectra of these fractions exhibited two

20

absorptive transition peaks appeared at 240 and 352 nm. The peak at 352 nm with energy

21

value slightly larger than that of PL emissions is ascribed to n → π* (C=O and C=N)

22

transitions, and the short wavelength peak at 240 nm is believed to result from n → σ* (C-

23

OH) transitions.51-53 Similar PL properties were observed for fractions 1 to 3 of CDs200,

24

while the excitation-dependent PL behavior regularly increased for the later fractions. This

25

behavior was accompanied with appearing a new peak in the excitation spectra of these

26

fractions at about 290 nm, indicating the emerging of a new emission center. This

27

intermediate-wavelength peak is associated with the π → π* transitions in the aromatic sp2

28

(C=C) domains, and was detected in all fractions of CDs prepared at higher temperatures. It

29

should be noted that some reports have attributed the peak at 240 nm to π → π* transitions

30

from the carbon network.20, 54 However, even by considering this assumption, the origin of

31

2

the peak at 240 nm and that at 290 nm is certainly different sp domains.

32

All separated fractions of CDs250 showed an excitation-dependent PL behavior with

33

three peaks in their excitation spectra. From fraction 3, an emission peak at about 520 nm

34


6

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start appearing, and its intensity significantly increased in Fraction 7. In addition, another

1

peak with lower intensity was also detected at wavelength lower than 400 nm. This later

2

emission peak was completely dominant in the emission spectra of all fractions of CDs300. It

3

should be noted that, although the excitation spectra of fractions of CDs300 contained the

4

three mentioned peaks, the peak attributed to the n→π* energy level was considerably

5

broadened, and covered a wide wavelength range from 200 to 400 nm, particularly for the

6

later fractions.

7

The absorption spectra of all fractions of CDs150 and CDs200 displayed two

8

absorption bands centered at about 240 nm and 360 nm (Figure S5). It has been recently

9

proved that these absorbance bands are related to the highly fluorescent molecule IPCA

10

synthesized from dehydration of citric acid and ethylenediamine, Scheme S1.

12, 33

Thus,

11

similar to the de-convoluted excitation spectra, these absorbance bands could be related to the

12

n→σ* and n→π* transitions. Moreover, an obvious additional peak at about 290 nm was also

13

appeared in the absorption spectra of some fractions of CDs250 and CDs300, in which

14

parallel to intermediate-wavelength peaks in the excitation spectra, can be attributed to π→π*

15

transitions.51-53 For the later fractions, the absorption spectra became similar to those

16

observed for totally carbonized precursors without any appearing peaks.

17 18

(Figure 2)

19 20

To investigate the possible emission centers in CDs, the solution of separated

21

fractions 1, 4, and 7 of CDs, as model systems, were subjected to continuous UV irradiation.

22

For each sample, a three dimensional tensor array (data set) was obtained by augmenting the

23

EEMs of the sample recorded at various irradiation times. The PARAFAC models of data

24

sets were developed using different number of components and the percentage of the core

25

consistency values and the explain variance percentages were used to select the correct

26

number of involved components.

27

The PARAFAC decomposition results of the fractions of CDs150 for one to four

28

components (Table S1) indicated that two components are necessary to have the best models

29

for resolving these data sets with core consistency values close to 100%. The involvement of

30

more components led to large decreases in the core consistency values of the PARAFAC

31

models. However, more than 99.8% of variances of the data sets were explained by the first

32

component, and the second component explained only a negligible percentage of total

33

variances. Moreover, the excitation and emission spectral profiles of the second component

34


7


Page 8 of 23

were very similar to the inherent spectra of the mixture solution of the non-reacted citric acid

1

and ethylenediamine (Figure S6). In addition, no decrease in the emission intensity of this

2

component was observed during the irradiation time. Thus, we attributed it to the weak

3

fluorescence of the carboxyl and amino groups of the initial precursors that remained in the

4

structure of the prepared photoluminescent products and, consequently, omitted this

5

component from the resolved components. It should be noted that this component was also

6

detected in all fractions of CDs200, but not in those prepared at higher temperatures.

7

The decomposed spectral profiles (i.e., the irradiation time and excitation and

8

emission modes) of the first component provided by the PARAFAC model are shown in

9

Figure 3. The excitation spectra of the resolved component showed two similar peaks at

10

about 240 and 350 nm. Although the resolved emission spectral profiles of different fractions

11

of CDs150 were highly similar, their peak position shifted by about 5 nm from CDs150-F1

12

(λem= 455 nm) to CDs150-F7 (λem= 460). The PL intensity of this component in all fractions

13

showed an almost similar and significant decrease upon irradiation time. Based on the TEM

14

images and the resolved optical properties, this component can be attributed to the “molecular

15

state” of highly fluorescent IPCA molecules in a polymer-like structure. Appearing strong

16

peaks in accordance with the molecular weights of IPCA and its fragments in the mass

17

spectrum of CDs150 clearly confirmed that IPCA is the major molecular structure existed in

18

these CDs (Figure S7). Since the CDs solutions were first filtered through the dialysis bags,

19

this resolved component cannot be related to the individual molecules. However, the solution

20

outside the dialysis bag showed similar emission spectra with that remaining in the bag,

21

clearly revealing the fact that in the fluorescent emission centers of the molecules passed

22

from the bag and those polymer-like structures inside the bag are identical.

23

The PARAFAC results obtained from the decomposition of CDs200-F1 data set were

24

similar to those obtained from the fractions of CDs150, showing one component with similar

25

excitation and emission spectra (see Table S2 and Figure 3). However, the decomposition

26

results of CDs200-F4 and CDs200-F7 data sets showed an additional component possessing

27

an emission maximum at 475 nm and a completely different excitation spectrum with a new

28

peak at about 300 nm. The photobleaching rate of this component was also quite high so that,

29

after 20 min of UV irradiation, its emission peak diminished completely, while the PL

30

intensity of the first component was only decreased by about 50%. The excitation spectrum

31

of this component was appeared in the same wavelength range observed for π→π‫ ٭‬transition

32

peak resulted from the de-convolution of the excitation spectra of these fraction (Figure 2).

33


8

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The core consistency diagnostic for the fractions of CDs250 showed the presence of

1

even more number of components, Table S3 and Figure 3. The first component in all

2

fractions showed the excitation and emission spectra similar to those for the molecular

3

component detected in CDs prepared at lower temperatures. A new component with an

4

emission maximum at about 400 nm and an excitation spectrum covering the whole range of

5

the presented wavelengths was also detected in all fractions. Because of the high resistance of

6

this component to the photobleaching and its low contribution to the overall PL data set, we

7

may attribute this component to direct electron-hole recombination in the carbon-core states

8

of CDs. In fact, this assumption agrees well with the previously reports demonstrating that,

9

compared to the molecular states, the carbon-core states usually emit at shorter wavelengths

10

and exhibit much lower fluorescence QYs.

13

11

The third shared component in the resolved profiles showed an emission spectrum at

12

515 nm. Moreover, for CDs250-F4, an additional component with an emission maximum at

13

about 430 nm was also detected. Both of these two components had almost similar excitation

14

spectra with obvious peaks at around 290-300 nm, which were comparable to the second

15

component resolved for CDs200. We suggest that the second component in the CDs200, and

16

two later components in CDs250 are probably attributed to the presence of polycyclic

17

2

aromatic hydrocarbons of different sizes, compared with the conjugated sp domains

18

produced from the dehydration of polymeric structure.

19

However, the best PARAFAC models for data sets of CDs300 were obtained by using

20

two components (Table S4). The resolved profiles for CDs300-F1 and CDs300-F7 data sets

21

are presented in Figure 3. The component with the shorter emission spectrum can be safely

22

assigned to the carbon-core state of CDs, similar to that resolved in fractions of CDs250. The

23

second component with an emission maximum about 450 nm presented a broad spectrum.

24

Despite some differences, spectral profiles of this component still share many common

25

features with those of IPCA resolved at lower temperatures. As, the correlation coefficients

26

of the emission spectra of this component in CDs300-F1 and CDs300-F7 with the emission

27

spectrum of IPCA was still 0.969 and 0.975, respectively. However, for the later factions, the

28

emission spectrum of this component significantly broadened and its excitation spectrum

29

becomes more and more different (see blue components in the resolved emission spectra of

30

CD300 in Figure 3). Thus, this component might also be an average emission spectrum

31

resulted from multiple polycyclic aromatic hydrocarbons with very close structures. More

32

importantly, from the comparable photobleaching resistance of this component and the

33

carbon-core component, it can be concluded that the origin of this component is more

34


9


Page 10 of 23

possibly incorporated inside of the CDs structure rather than their surfaces. These results

1

indicated that the CDs prepared at 300 oC still contain structurally changed or close

2

derivatives of the IPCA molecules and/or variety of polycyclic aromatic hydrocarbons

3

incorporated inside of the carbon network matrix of CDs.

4

In order to estimate the structures of the possible aromatic domains associated with

5

the resolved PL spectra, the optical properties of some nitrogen-containing polycyclic

6

aromatic hydrocarbons were studied using the DFT and TD-DFT calculations. The molecular

7

structures were first optimized using the DFT-B3LYP/6-31G* method. The UV-Vis

8

absorption and fluorescence emission spectra were calculated by using the CAM-B3LYP

9

method at the same conditions. In the case of calculated emission spectra, the longest

10

emission peak was extracted and the results are shown in Figure 3.

11 12

(Figure 3)

13 14

As mentioned above, the IPCA molecule is probably the first fluorescent component

15

produced during the synthesis process of CDs. The calculated absorption and emission

16

spectra of this molecule showed a distinct absorption peak at 314 nm, and an emission

17

spectrum appeared at 406 nm, which is associated with the electron transfer between the

18

corresponding HOMO and LUMO orbitals with energies levels of 5.32 and -1.49 eV,

19

respectively (see Figure 4A). Despite of some small deviations in the peak positions, the

20

resulting calculated spectra are in good agreement with those obtained for the first component

21

in the resolved spectra of the experimental data (see blue components in Fig. 3). However,

22

negligible deviations observed between the calculated and experimental obtained spectra of

23

IPCA molecule might be related to the fact that the theoretical spectra are related to a

24

separated molecule in the gas phase.

25

To estimate the possible structures of the fluorescent component related to aromatic

26

domain states, the optical properties of some different aza-polycyclic aromatic hydrocarbons

27

were also investigated. The structures, orbital energies, and absorption and emission spectra

28

of some of these molecules are shown in Figure 4B-E. The pyracylene molecule with two

29

adjacent heterocyclic structures containing nitrogen atoms at the graphitic and pyridinic

30

positions (Figure 4B) showed an emission peak at 385 nm. The emission spectra of the aza-

31

benzoanthracene derivative (Figure 4C) and aza-perylene (Figure 4E) molecules were very

32

similar and appeared at about 439 nm, and that of aza-pyrene molecule (Figure 4D) was

33


10

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appeared at 336 nm. It should be noted that the pyrene-based molecules have already been

1

studied in more detail in some of the previous reports to investigate the optical properties of

2

CDs.

24, 41, 54, 55

These reports have indicated the strong influence of the added nitrogen atoms

on the emission wavelength of the nitrogen-containing pyrene derivatives.

3 4 5

(Figure 4)

6 7

Moreover, the pyrolysis of different precursors has already been used to synthesize a

8

variety of curved polycyclic aromatic hydrocarbons identified as fullerene fragments, which

9

56

10

In this regard, corannulene (C20H10) is considered as the smallest subunit of C60. Therefore,

11

the formation of corannulene-based fullerene fragments or defective intermediate opened-

12

cage fullerene structures, and even perfect fullerene molecules during the synthesis of CDs,

13

especially at high temperatures, cannot be far from reality. Interestingly, the mass spectrum

14

of CDs300 showed notable signals at m/z values of 720 and 840, respectively (Figure S8),

15

which are corresponded to C60 and C70. It is worth mentioning that these two signals were not

16

detected in the mass spectra of CDs prepared at lower temperatures (Figure S7). Moreover,

17

the toluene-extracted solution of CDs300 showed a new absorbance spectrum very similar to

18

can finally result in production of fullerenes through intramolecular condensation processes. 57

that observed for the corannulene-based fullerene fragments, Figure S9. Thus, based on the previous related reports56,

57

58

19

and the obtained theoretical and

20

experimental results in this work, the optical properties of corannulene as a possible

21

polycyclic aromatic hydrocarbon formed during the pyrolysis of initial materials was also

22

investigated. The absorption and emission spectra of this molecule and some of its nitrogen-

23

containing derivatives are shown in Figure 4F-L. As is quite obvious, while the emission

24

peak of corannulene was appeared in the UV region at 314 nm, all nitrogen-containing

25

corannulene molecules showed red-shifted emission peaks mostly appeared in visible region

26

with different emission maxima, depending on the number and the position of added nitrogen

27

atoms. Compared to the spectrum of the corannulene base molecule (Figure 4F), the addition

28

of pyridinic nitrogen (Figure 4G) caused the smallest changes in the optical spectra. The

29

maximum emission of this derivative with a slight red shift was appeared at 318 nm. On the

30

other hand, the addition of nitrogen atom to the pentagonal ring (Figure 4H) and the addition

31

of graphitic nitrogen at the valley position (Figure 4I) led to significant red-shifts in both

32

absorption and emission spectra. The emission maxima of these two derivatives significantly

33


11


Page 12 of 23

shifted to 526 and 716 nm, respectively, compared to 314 nm of the corannulene base

1

molecule. This is in fair agreement with previous reports demonstrating that the introduction

2

of graphitic nitrogen atom into carbon lattice compare to pyridinic, pyrrolic and even amino

3

nitrogen atoms induces the highest red-shift effect on the emission spectra of CDs. Such

4

24, 55,

5

strong red-shift effect of graphitic nitrogens is attributed to their strong n-doping effect. 59

6 Figures 4J-L show the results obtained for some corannulene derivatives containing

7

one nitrogen atom at the pentagonal ring and one graphitic nitrogen atom at three different

8

positions. These results clearly revealed that the addition of the second nitrogen atom to the

9

corannulene structure increased the number of peaks in absorption spectra. Meanwhile, the

10

emission spectra of these derivatives almost similarly appeared at around 500 nm, showing

11

large red-shifts relative to that calculated for corannulene base molecule, but with small

12

changes compared to the emission spectrum of the corannulene derivative containing one

13

nitrogen atom at the pentagonal ring.

14

Besides, the carbogenic core of CDs is generally considered as compact sp2 graphitic

15

networks. In the case of crystalline carbon-cores, the quantum confinement effect is

16

suggested as the origin of the PL of carbon-cores. However, most of CDs consist

17

amorphous/disordered graphitic sp

2

structures without quantum confinement effect.

18

Basically, the near-UV-to-blue emission of these disordered carbon structures is attributed to

19

the recombination of photogenerated electron–hole pairs within finite-sized isolated sp2

20

carbon clusters embedded in the parent structure.4, 13, 14, 60 As, Eda and coworkers theoretically

21

demonstrated that sp2 clusters with a diameter of about 3 nm consist of >100 aromatic rings

22

and have energy gaps of around 0.5 eV that cannot be responsible for the observed blue

23

emission from these nanoparticles.

60

2

They concluded that much smaller sp clusters of few

24

aromatic rings or of some other sp configuration of similar size are likely to be responsible

25

for the observed blue emission.

26

2

The formation of amorphous carbon-cores in the synthesized CDs especially at

27

reaction temperatures 250 and 300 oC can be clearly detected from the XRD spectra of CDs

28

(Figure S2). Accordingly, we suggest that carbon-cores contain isolated small sp2 carbon

29

2

clusters that are responsible for their emission. These small sp clusters are likely contain

30

fewer aromatic rings with lower content of nitrogen atoms (based on the elemental analysis

31


12

Page 13 of 23 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


results of CDs, Table S5) compared to the aromatic domain states. Some of these possible

1

small sp2 clusters emitting at near-UV-to-blue wavelength region can be found in Figure S10.

2 3

(Scheme 1)

4 5

In summary, in this study, we used the intrinsically different photostability of

6

emission centers in CDs to find the number of possible emission centers and to resolve the

7

pure excitation and emission spectra of each center. Our novel proposed method for

8

collecting three-dimensional PL data sets along with a suitable multivariate decomposition

9

method established a high potential application to extract and monitor the emission centers in

10

CDs during the synthesis process. For the first time, we clearly showed the existence of three

11

different emission centers including molecular states, aromatic domain states, and carbon-

12

core states in CDs synthesized through pyrolysis of citric acid and ethylenediamine.

13

Based on both experimental and theoretical results thus obtained, the possible

14

mechanism of formation of CDs from starting materials to carbogenic CDs is schematically

15

explained in Scheme 1. As seen, at low synthesis temperatures, through dehydration process,

16

both cyclization and polymerization of starting materials produced highly fluorescent

17

polymeric-like CDs with high fluorescence intensities. The PL property of these CDs can be

18

originated from molecular states of the fluorophore molecules incorporated in polymer

19

structures. By increasing the temperature, further dehydration reactions lead to the growth of

20

higher conjugated aromatic structures within the polymeric structures; these include

21

polycyclic aromatic hydrocarbons, aza-polycyclic aromatic hydrocarbons, corannulene-

22

based fullerene fragments, intermediate opened-cage fullerene structures, and even more

23

complex aromatic structures. These new polycyclic aromatic hydrocarbons constructed some

24

aromatic domain states exhibiting new PL peaks emitting at different wavelengths depend on

25

their structures. Further carbonization produced individual nano-sized CDs with carbon-core

26

states, but still containing fluorophore molecules and various aromatic domains. However,

27

most of the aromatic domains and fluorophore molecules are finally turned into carbogenic

28

cores. Observing multiple emission spectra from different emission centers in individual CDs

29

is also an explanation for the typical excitation-dependent PL behavior of CDs. Moreover,

30

while we investigated the synthesis progress and emission centers evaluations by increasing

31

the reaction temperature, it could be expected that prolonging the reaction time at a constant

32


13


Page 14 of 23

temperature produce similar result. We hope these results provide a better insight into both

1

PL origin and formation mechanism of citric acid-based CDs.

2 3

Supporting Information

4

Information regarding apparatus, preparation of CDs, separation of as-prepared CDs by size

5

exclusion column chromatography, experimental procedure for UV irradiation and data

6

recording, XRD profiles, thermogravimetric analysis, absorption spectra, mass spectra,

7

PARAFAC result tables, additional DFT and td-DFT results, and other supplementary

8

figures.

9

References:

10 11

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17


Figure 1. TEM images and XPS spectra of CDs prepared at temperature 150 oC (a and a’), 200 oC (b and b’), 250 oC (c and c’), and 300 oC (d and d’). Insets show the high-resolution XPS spectra of C1s of CDs.


18

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1 2 3 4 5 6 7

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1 2 Figure 2. Excitation and emission spectra of fractions 1, 4, and 7 of CDs prepared at temperature 150 oC, 200 oC, 250 oC, and 300 oC.

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18


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1 Figure 3. PARAFAC decomposition results of data sets of different fractions of CDs prepared at temperatures 150 oC, 200 oC, 250 oC, and 300 oC: UV irradiation mode, excitation mode, and


20

2 3

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emission mode. (Blue, red, and green profiles are respectively assigned to the molecular, aromatic domains, and carbon core states)

1 2 3 4 5 6 7

8

Figure 4. The HOMO and LUMO band gap, orbital shape, and the UV-Vis absorption (red) and emission (blue) spectra of IPCA (A), some different aza-polycyclic molecules: aza-pyracylene molecule (B), an aza-benzoanthracene derivative (C), aza-pyrene (D), aza-perylene (E), corannulene (F), and some nitrogen-containing corannulene derivatives (G-L).

9 10 11 12 13 14 15


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1 2 3 4

5 6 7 Scheme 1. Schematic representation of the formation of CDs from citric acid and ethylenediamine through pyrolysis process

8 9 10 11


22

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Graphical Abstract:


Resolving the Multiple Emission Centers in Carbon Dots: From

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