Striking Similarities in the Fluorescence Behavior Between Carbon

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Striking Similarities in the Fluorescence Behavior Between Carbon Dots and Ionic Liquids: Towards Understanding the Fluorescence Behavior of Carbon Dots Subhasis Roy, Naupada Preeyanka, Debashis Majhi, Sudipta Seth, and Moloy Sarkar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03859 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on May 4, 2018

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The Journal of Physical Chemistry

Striking Similarities in the Fluorescence Behavior Between Carbon Dots and Ionic Liquids: Towards Understanding the Fluorescence Behavior of Carbon Dots Subhasis Roy,a Naupada Preeyanka,a Debashis Majhi,a Sudipta Sethb and Moloy Sarkar*a

a

School of Chemical Sciences, National Institute of Science Education and Research, HBNI,

P.O. Jatni, Khurda 752050, Bhubaneswar, Odisha, India b

School of Chemistry, University of Hyderabad, Hyderabad – 500046, India.

___________________________________________________________ *Corresponding author, Tel: +91-674-2494190 Fax: +91-674-2494004. [email protected]

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Abstract Despite several studies, convincing explanation for the fluorescence of CDs and its excitation wavelength dependence behavior has not yet been emerged. It may be noted that direct structure-property correlation can be misleading based on solely TEM micrographs as it does not fully reveal the possibility of heterogeneous nature of the samples in a sense that it does not fully reveal the possibility of having both carbonaceous nanoparticles as well as small organic molecular based systems. The present work is undertaken specifically to address this issue. A detailed spectroscopic investigation comprising steady state absorption, emission, time-resolved fluorescence and fluorescence correlation spectroscopy (FCS) studies have been carried out on CDs, synthesized from two different sources. Similar investigations have also been carried out on the systems such as aromatic and aliphatic ionic liquids (ILs), which are known to be fluorescent in their neat conditions. Interestingly, the fluorescence behavior of CDs are observed to be very similar to neat ILs. Recent studies by Kim and co-workers have categorically demonstrated that fluorescence from neat IL can originate from associated structures of ILs. In the present work, the excitation wavelength dependent fluorescence measurements, emission wavelength dependent radiative recombination and FCS studies on CDs and other systems including ILs have established that the presence of energetically different associated structures (in the ground state) in CDs solution is primarily responsible for the fluorescence behavior of CDs. Dynamic light scattering (DLS) measurements and dilution studies through FCS have also provided evidence in favour of associated structures in CD solution. Excitation wavelength dependent fluorescence behavior of CDs can also be explained on the basis of energetically different associated structures that are formed in CDs solution during its synthesis. Essentially the present investigations have revealed that carbon dots are not inherently fluorescent, rather fluorescence from CD solution arises due to the presence of associated/networked structures similar to what has been observed in systems such as neat ILs.


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1. Introduction In recent times, fluorescent carbon dots (CDs) have attracted considerable attention from the scientific community owing to their simple method of preparations and useful optoelectronic properties.1,2 Interestingly, photoluminescent CDs are also thought to be more attractive candidate in comparison to usual semiconductor quantum dots and organic dyes in terms of high

solubility

in

aqueous

medium,

chemical

inertness,

optical

stability,

easy

functionalization, low toxicity and good biocompatibility.3-5 Even though photoluminescence of CDs are being used extensively in many applications such as bioimaging,6 light harvesting,7 optical materials8 etc., the origin of photoluminescence behavior of CDs is not yet understood fully and thus this issue merits further investigations. Several research groups have made an attempt to unravel the origin of photoluminescence in CDs in recent times. These reports summaries the following possibilities for the origin of fluorescence in CDs: (a) presence of conjugated π-systems in carbon core,9,10 (b) functional groups present on the main carbon backbone of CDs, also known as surface states,11,12 (c) presence of fluorescent molecules connected to the surface or inside of CDs,13,14 (d) enhanced emission due to crosslinking15,16 and (e) electron-hole recombination.17,18 The several propositions with regard to the origin of fluorescence behavior of CDs are self indicative of the fact that there is no convincing explanation for the fluorescence behavior of CDs. However, of late, several reports have described that inherently CDs are not fluorescent rather a fluorescent polycyclic molecule (produced during the synthesis of CDs) is responsible for the emission of CDs. Please note that many of the above studies depicted that carbon dots are inherently fluorescent. In majority of these works the structure-property correlation, in particular, structure-optical property correlation, was based on standard characterization technique such as transmission electron microscopy (TEM) and spectroscopic data from spectrometer in


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separate manner. We would like to recall the recent investigation by Ferrante and coworkers19 where they have categorically mentioned that direct structure-property correlation can be misleading based on solely TEM micrographs as it does not fully reveal the possibility of heterogeneous nature of the samples in a sense that it does not fully reveal the possibility of having both carbonaceous nanoparticles as well as small organic molecular based systems. Please also note that TEM results won’t be able to tell whether the emission is originating from carbonaceous particles or small or oligomeric molecular based system. In this context we would also like to note that very recently Baker and coworkers20 have shown that during the synthesis of carbon dots, molecular fluorophore (potentially oligomeric and polymeric in nature) may form which predominantly contribute in carbon dot emission. One more interesting aspect related to the same issue that is observed is the excitation wavelength dependent emission of CDs. As per Kasha’s rule21,22 emission maximum of a given fluorophore should be independent of excitation wavelength. However, in case of some CDs, upon increasing the excitation wavelength the emission maximum is observed to shift towards longer (red) wavelength region.14,23 This aspect also needs to be understood in conjunction with the usual fluorescence behavior of CDs. In this context it is noteworthy to mention that a recent work by Kumbhakar and co-workers,23 have shown the presence of various type of aggregates even at very dilute solution of CDs that are found to be responsible for the formation of various multiple discrete electronic states which causes the excitation wavelength dependent emission in CDs. While discussing the fluorescence behavior of CDs, we would like to take a serious note on the recent observations that have emerged in explaining the fluorescence behavior of neat ionic liquids.24-26 It has been recently demonstrated that associated structures of molecules of ILs can be responsible for fluorescence behavior of a given sample.27-32 It has


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also been shown that π-conjugate systems are not essential, rather non π-conjugated systems such as pyrrolidinium ILs can also exhibit fluorescence.31 At this point, it is interesting to note that some of the earlier reports on neat ILs,33,34 suggested that neat ILs itself cannot be fluorescent, and their fluorescence originates from the chemical impurities. However, recent work by Kim and co-workers through FCS studies have demonstrated that fluorescence from neat ILs originates from molecular aggregates in the neat ILs.27,28 This particular study by Kim’s group and earlier work by Samanta and coworkers29,30 have categorically demonstrated that the fluorescence from neat ILs is not due to any fluorescence impurity rather associated structures of the IL. This is a very interesting observation in the context of research regarding CDs’ fluorescence where presence of fluorescence impurity is thought to be one of the main reasons for emission of CDs. It may be noted here that the excitation wavelength dependent fluorescence behavior of neat ILs are also explained on the basis of associated structures that are formed in neat ILs. In fact, it has been demonstrated by Paul and co-workers30,35 that excitation wavelength depended fluorescence which is also known as ‘red-edge effect’ (REE) in neat ILs can arise due to the existence of energetically different associated structures in the ground state and the inefficiency of the excitation energy transfer process among these structures. These recent important studies unambiguously established that the fluorescence behavior of neat ILs can be best explained on the basis of associated structure but not just on the fluorescence impurity present within the system. We also note here that while searching for literature on fluorescence from neat chemical systems we could find that systems having networked structure like dendrimer can fluoresce on its own.36,37 Here we again note that very recently Baker and Coworkers20 have depicted that during synthesis of carbon dots, molecular fluorophore, potentially oligomeric and polymeric in nature, may form which predominantly


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contribute in carbon dot emission. The above literature reports and outcome of the work on the fluorescence behavior of neat ILs demand a re-look at the origin of fluorescence for CDs. Since, it is also known that synthetic processes such as hydrothermal and microwave syntheses can give rise to network structures,38 it is highly likely that during the synthesis of CDs through similar synthetic routes, some kind of associated/networked structures are formed instead of a specific fluorescent molecule, and these associated structures can cause fluorescence

similar

to

what

has

been

observed

for

ILs.

Since,

associated

structures/molecular aggregates of ILs are known to be primarily responsible species emission behavior of ILs, it is expected to be worthwhile objective to investigate the optical behaviour of CDs along with neat ILs. Keeping all these in mind, in the present work we have carefully studied steady state absorptions, emissions, temperature dependent emissions and time-resolved florescence decay, in CDs synthesized from different carbon sources along with ILs. Particularly, CDs synthesized from citric acid (CA-CD) and L-tartaric acid (TA-CD), one aromatic IL, 1Propyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (PMIMTFMSI), and one aliphatic

IL,

1-Methyl-1-propylpyrollidinium-

bis(trifluoromethylsulfonyl)imide

(MPPLTFMSI) have been used for this study. Citric acid and tartaric acid based CDs are used in the present study as these CDs are explored quite extensively for several works. The outcome of the present work is interesting in a sense suggests that associated structure of molecules produced during the synthesis of carbon dots can cause fluorescence of CDs similar to what has been observed for ILs. Present study is expected to provide a new pathway towards understanding the origin of fluorescence in CDs.


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N

N

CH3

N H3C

PMIM

PMPL

H3C O

F3C

S

CH3

O N

O

S

CF3

O

TFMSI

Scheme 1 Molecular structures of different ILs. 2. Experimental section 2.1 Materials 1-Propyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide),

1-Propyl-1-

methylpyrolidinium- bis(trifluoromethyl-l-sulfonyl)imide) were obtained from Merck, Germany (>99% purity) and used as received. The water and halide contents of the ILs were

Striking Similarities in the Fluorescence Behavior Between Carbon

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