Connecting the Dots of Carbon Nanodot: Excitation- (In)dependency

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C: Physical Processes in Nanomaterials and Nanostructures

Connecting the Dots of Carbon Nanodot: Excitation(in)Dependency and White-Light Emission in One-Step Aritra Nandy, Amrit Kumar, Shivam Dwivedi, Surja Kanta Pal, and Debashis Panda J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02428 • Publication Date (Web): 08 May 2019 Downloaded from http://pubs.acs.org on May 8, 2019

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The Journal of Physical Chemistry

Connecting the Dots of Carbon Nanodot: Excitation(In)dependency and White-light Emission in One-Step

Aritra Nandy†, Amrit Kumar†, Shivam Dwivedi, Surja Kanta Pal, Debashis Panda* Rajiv Gandhi Institute of Petroleum Technology, (An Institute of National Importance) Jais, Uttar Pradesh, INDIA

RECEIVED DATE () †

Equal Contribution

* To whom correspondence should be addressed. Email: [email protected] Phone: +91 9455196041.

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Abstract:

The synthetic origin of nanocarbon and its role on the luminescence properties of carbon nanodot, in particular, citric acid derived ones remain an enigma till date. We report here, for the very first time that citrazinic acid alone builds nanocarbon structure upon incubation in dimethyl formamide at room temperature. The emission properties of incubated fluorophore resemble to that of nanodot. The dispersion of H-bonded cluster’s size originated from citrazinic acid only is the cause of excitation-dependence emission. We have shown that the steric hindrance caused by the presence of alkyl chain of butyl amine restricts such dispersions, resulted in excitationindependent emission, a molecular behavior. On the other hand, for achieving white-light emission through one-step method, the solvothermal reaction of citric acid with ammonium thiocyanate has been performed. The ground-state heterogeneity and luminescence properties get strongly influenced by the solvent polarity. We identify the existence of blue, green, and red-emissive fluorophores in a product yielded from solvothermal reaction. The computed vertical excitations of the molecular fluorophore predicted by the reaction mechanism are in good agreement with the experimental observations. Unprecedently, product embedded PVA/PVP film exhibit white light emission under irradiation of UV light, 365 nm. The mechanism of white light emission is attributed to effective energy transfer among fluorophores.


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1. Introduction: Since its discovery, carbon nanodots (CNDs) have been investigated with great zeal due to the easy one-pot preparative method, tunability of their optoelectronic properties and compatibility for bio-imaging applications.1–6 However, the term nanocarbon is perhaps juxtaposed owing to the observation of graphitic domain in nanodot under high resolution transmission electron microscopy, but its contribution in the overall properties of nanodot is still speculative.7 The proposed emission from the graphitic domain, in particular, at the red end of the emission spectra has been negated very recently.8,9 The aggregation of fluorophores contributing to the blueemission of nanodot are proposed to be responsible for the red-edge emission.10,11 The most debatable subject in this nanodot research is the observation of excitationdependent photoluminescence. A significant effort has been made to rationalize the origin of such excitation-dependent emission.12–15 Among the proposed hypotheses of excitation-dependent emission, the one suggesting the formation of different kind of fluorophores under hydrothermal condition is gathering steam, however, it requires complete validations- at least, observation of distinct emission spectra corresponding to each of the components. And the hypothesis of aggregation demands the understanding of the aggregation in homogenous solvent, specifically in spectroscopic terms. Nevertheless, such understanding cannot remain indifferent from the emergence of nanodot exhibiting excitation-independent emission, a typical molecular behavior.16–18 The excitation dependency is found to be highly dependent on precursors. Thus, the breaking of the Kasha-Vavilov rule seems to be dependent on precursors chosen for the synthesis, not on the existence of carbon lattice spacing generally observed under electron microscope. It therefore raises pertinent questions on the feasibility of excitation dependent emission for a



fluorophore and most importantly, observation of nanocarbon-like feature from the same fluorophore, that even at room temperature. On the other hand, the effect of doping on the augmentation of luminescence properties of nanodot has been reported with renewed fervor.19–21 Various molecular dopants have been used in hydrothermal method to yield predominantly nitrogen and sulphur –doped CNDs. Surprisingly, how a molecular precursor does the atomic manipulation in a graphitic domain, that even in hydrothermal condition at temperature 160-200oC remains as elusive as ever. And even, the role of solvents in promoting elemental doping has not been clearly understood yet.22,23 It is therefore highly intrigued us to explore the fundamental science underneath. Moreover, the development of a class of luminescent nanostructure demands an in-depth molecular understanding for the targeted applications. In this article, first, we have made an attempt to show that the room-temperature incubation of a single fluorophore can yield nanocarbon and also exhibit excitation-dependent emission, thus mimicking nanodot properties (Scheme-1). The fluorophore selected in this case is citrazinic acid which has been reported to get synthesized under hydrothermal condition on the reaction of citric acid and primary amine based precursors, and is the origin of blue-emission for nanodot. The hydrothermal reaction is reportedly carried out in the temperature range 160-220oC. The dielectric constant,  of water at 160oC is 42.8, which closer to that of a common solvent, dimethyl formamide (DMF) at 20oC.24 The citrazinic acid has hence been incubated in DMF for 8 weeks at room temperature. The steady-state and time-resolved studies of incubated citrazinic acid in DMF have thus been performed. The vertical excitations of clusters have been computed to substantiate the role of intermolecular H-bonding among citrazinic acid molecules in dictating the emission dynamics. To further authenticate the hypothesis on H-bonding, a primary amine having aliphatic


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chain, n-butyl amine is chosen as a precursor. The butyl chain of n-butyl amine plays a crucial role to block the H-bonding interactions among derivative of citrazinic acid molecules, resulting in excitation-independent emission.

Scheme-1: [A] Schematic representation of the emission properties of commonly-observed nanodots synthesized in hydrothermal method. [B] Highlighting the need of room temperature method for mimicking nanodot properties.

Secondly, citric acid and ammonium thiocyanate have been used as precursors for the synthesis of so-called doped nanodot. The reason for choosing ammonium thiocyanate as a molecular dopant is that recently we have identified its role for yielding the molecular heterogeneity.21 Solvothermal in DMF has been done to decipher the role of solvents in regulating the product yields, resulting color gamut. A direct comparison has been made with the products synthesized in the hydrothermal reaction condition and remarkable similarities have been highlighted. Time-resolved emission studies point out the striking difference in temporal properties of nanodots. To answer a specific question, quite frequently raised on nanodot discussion, the nonappearance of either distinct 5 ACS Paragon Plus Environment


emission spectra or spectral-broadening of fluorophores present, we have studied the emission properties of as-synthesized product embedded in polyvinyl alcohol/polyvinyl pyrrolidone (PVA/PVP) film. The hydrogel system having extensive hydroxyl-groups can offer similar interactions, like water does with nanodot, thus selected as a model system. No more gradual shift of emission peak is noticed, rather both distinct emission spectra and spectral-broadening have been clearly observed. Presence of ground state heterogeneities- blue, green and red emissive fluorophores turn the PVA/PVP film white under UV-light. Time-resolved studies have also been done on films to understand the origin of white-light emission.


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4. Materials and Methods: Materials: Citrazinic acid from Alfa Aesar, citric acid and n-butyl amine from Fisher Scientific, and polyvinyl pyrolidone from SRL are purchased. Ammonium thiocyanate, polyvinyl alcohol, and silica are obtained from SDFCL company. All chemicals used in the experiments of analytical grade and used without any purification. Solvents such as dimethyl formamide (DMF), ethyl acetate (EA), Petroleum ether are received from Finar Chemicals. Water from ELGA LAB Waterpurifier (18.2 mΩ) is used throughout the experiments. Reaction of Citric Acid with Ammonium thiocyanate: In a typical synthesis, 0.4 g of citric acid and 0.4 g of ammonium thiocyanate are dissolved in respective solvents- water for hydrothermal and DMF for solvothermal. The transparent solution is then transferred into a Teflon-coated stainless steel autoclave (purchased from M/S Shilpa Enterprises, Nagpur, India) at 160ºC. For hydrothermal reaction, the incubation time is 5 hours, whereas the same is kept for 24 hours for solvothermal reaction. After cooling to room temperature, the color of the solution obtained from solvothermal reaction is changed to blood red from colorless, whereas the same is turned greenish for hydrothermal reaction. The colored solution is centrifuged again at 13000 rpm for 20 minutes to remove large particles. Thereafter, 2 mL of as-synthesized product has been added in 200 mL of ethyl acetate-pet ether mixture (50:50) and thoroughly mixed for 20 minutes. The insoluble part has been extracted by adding 2 mL of water. The solution has been passed through 0.2 µM syringe membrane filter prior subjected to characterization methods- mass spectrometry, optical spectroscopic and electron microscopic methods. Preparation of fluorescent PVA/PVP Film: Solvent evaporation method is employed to prepare the luminescent PVA/PVP film. In brief, 1.0 g of PVA is dissolved in 20 mL of water at 90°C for 4 hours; subsequently, 1.0 g of PVP is added. The PVA/PVP solution is cooled down to 50oC and



then 1 mL of freshly prepared and purified products obtained from solvothermal method is added to it and mixed for 30 minutes. After that, the mixture is poured into a polyethylene petridish and kept it in room temperature for 7 days to cast the composite films. The dried films are used for optical properties of nanodots. Instrumentation: The steady state absorption spectra are recorded using a LAB India UV-Vis 3200 spectrophotometer. Varian Cary Eclipse fluorescence spectrophotometer has been used to record the steady state photoluminescence (PL) spectra. The Perkin Elmer Spectrum Two FT-IR spectrometer is used to record the FT-IR spectra of the vacuum dried samples. The samples are recorded using “attenuated total reflectance” (ATR) mode. The PIKE MIRacleTM single reflection horizontal ATR accessory is used for recording the FT-IR spectra. Time-resolved emission measurements are performed using Time Correlated Single Photon Counting (TCSPC) system, from IBH, UK. Excitation sources used in this experiment are- pulsed diode lasers of 375 nm and 440 nm with instrument response function (IRF) near about 300 ps and a nanoled diode laser of 560 nm with IRF of 1 ns. Emissions from products at various emission wavelengths are collected at right angles to the direction of the excitation beam, at magic angle polarization to nullify the rotational anisotropy effect. The decay traces for each of the excitation are analyzed with IBH DAS v6.2 data analysis. Using the iterative re-convolution method, the data were fitted to the sum of multi-exponential functions as shown below: 𝑡 𝐼(𝑡) = 𝐼(0) ∑ 𝐴𝑖 exp⁡(− ) 𝜏𝑖 𝑖

𝐼(𝑡) and 𝐼(0) are the emission intensities at time t and zero after excitation by the source. 𝐴𝑖 is the amplitude of the i th component. Liquid chromatography–mass spectrometry (LC-MS/MS) is performed to determine the molecular weights of the products using Waters ACQ-TQD-QBB1152 instrument which is coupled to a 8 ACS Paragon Plus Environment

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MassLynx triple quadrupole MS detector. After injection into the column, Thermo C18 column the sample is eluted by the water:acetonitrile (95:5) solvent mixture pumped by Waters ACQUITY UPLC pump through solvent manager system. The samples are imaged by a transmission electron microscope (TEM) (HR-TEM FEI Titan G2 60) mounted with field emission gun FEG TEM at 300 kV accelerating voltage. The samples for TEM are prepared on amorphous carbon films supported on a copper//nickel grids. The average values are expressed as mean ± standard deviation (SD). Quantum Chemical Calculations: The ground state geometries of citrazinic acid, its H-bonded clusters and proposed fluorophores are optimized using the density functional theory method with the Becke3LYP functional as implemented in GAUSSIAN 09w software package.25,26 The default options for the self-consistent field (SCF) convergence and threshold limits in the optimization are used. The Time dependent density functional theory (TDDFT) calculations are performed on the gas phase optimized geometry of the ground state (S0) and vertical excitations are carefully analyzed. The polarized continuum model using Integral Equation Formalism (IEFPCM) is invoked in both cases- optimization and vertical excitation energy calculations.27



3. Results & discussion: Disentangling the Excitation Dependent/Independent Emission: Identifying the origin of excitation-dependent emission and nanocarbon feature from a single fluorophore at room temperature is the subject of investigation here. Most of the synthetic method for nanodot synthesis are based on bottom-up chemical approach- hydrothermal reaction of precursors. The dielectric constant of water is highly dependent on the temperature; hence, at temperature, 160°C-180oC, the water has a dielectric constant around 42.8, which is equivalent to that of DMF (Fig. S1).24 Notwithstanding the structural difference, both DMF and water molecules possess strong H-bond accepting ability. We have thus chosen citrazinic acid as a fluorophore and DMF as solvent to provide a direct correlation with the emission behavior of nanodot.

Figure 1. Steady-state and time resolved fluorescence properties of incubated citrazinic acid in DMF: [A] Fluorescence spectra at different excitation wavelengths, [B] The emission maximum as a function of the excitation wavelength, [C] Fluorescence decay traces, and [D] Average fluorescence lifetime (solid blue circle) over emission wavelengths; at 375 nm excitation, Solid grey lines show the representative emission spectra recorded at exc=375 nm. 10 ACS Paragon Plus Environment

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The incubated citrazinic acid in DMF shows two distinct absorption peaks, one at 378 nm and another, relatively broader at 580 nm. A closer analysis of absorption spectrum reveals the presence of a shoulder peak at 405 nm (Fig. S2A). Hydrodynamic diameter of 142.8 nm measured by the dynamic light scattering is clearly indicative of molecular clusters (Fig. S2B). Surprisingly, the emission behavior of incubated citrazinic acid becomes excitation dependent, phenomenologically similar to observation of nanodots (Fig. 1A). The emission peak is gradually getting red-shifted, near about 45 nm for an excitation range- 340 to 450 nm. (Fig. 1B). No noticeable emission has been found for the absorption peak 580 nm, indicating the archaic polaritydependent aggregation for citrazinic acid.28 The fluorescence decay traces show two components- one, dominating long component and a gradually appearing short-component with the red-shift of emission wavelength (Fig. 1C). The decay traces are well fitted with biexponential up to 480 nm emission wavelengths, beyond that triexponential for 375 nm excitation (Table S1). Analysis of decay traces reveals the presence of two components across emission wavelengths: a short one with a lifetime (τ1) of ~ 2 ns and a longer component (τ2), ~10.5 ns. An ultrafast component (τ3), ~ 0.25 ns is observed for the emission wavelengths- 510 and 550 nm. These observations are consistent with the temporal dynamics of nanodots. Since the decays are multiexponential, it is pertinent to use an average decay time. The average fluorescence lifetimes, for an emission range 450-510 nm, remain unchanged, suggesting molecular behavior (Fig. 1D).29 The gradual emergence of short component in this case, is attributed to the aggregation-caused quenching. The high-resolution transmission electron microscopy (HRTEM) is employed to understand the morphology and size of the incubated fluorophore. TEM images clearly show the typical particle formation (Fig. 2A). The average diameter of the particles deposited is measured to be



2.8 0.5 nm (Fig. 2A, Inset). Organic fluorophores are expected to form aggregates (dark spots) under high vacuum condition in electron microscopic studies, but do not show any nanocarbon features. Unprecedently, high-resolution TEM images presented in Fig. 2B & S3 show twodimensional nanosheets (graphene-like) with the appearance of lattice fringes. The observed lattice spacing (Fig. 2B, Inset) of 0.22 nm is generally referred to the [020] facet of graphitic carbon.30,31 If the aggregation is the only reason for the observation of nanocarbon, then it must remain restricted to size of dark spots. Surprisingly, the nanocarbon features are observed beyond aggregation spot (Fig. S3). Thus it clearly points out that observation of lattice spacing and aggregation are two separate entities. It therefore brings us to hypothesize that the H-bonded molecular clusters of citrazinic acid play a critical role in forming such lattice fringes.

Figure 2. [A] TEM image of incubated citrazinic acid; Inset - particle size distribution, [B] HRTEM image with lattice spacing of approx. 0.22 nm (Inset) of incubated citrazinic acid in DMF. [C] Computed absorption spectra and corresponding oscillator strengths of citrazinic acid (Mo) and its Dimer (Di) and tetramer (Te); Mo- Green lines, Di- blue lines, and Te- red lines. [D] The ground state optimized geometries of citrazinic acid and its clusters (Table S2). [E] Absorption spectrum, [F] Emission spectra of product obtained from the hydrothermal treatment of citric acid and n-butyl amine.


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Corroborative Evidence: The mimicking of nanocarbon feature through H-bonding interactions demands planar arrangement among citrazinic acid molecules. The carboxylic groups are known to form planar H-bonding interaction among themselves.32 However, the propagation of such interactions desires the involvement of another end, pyrido of citrazinic acids. The ground state geometries of monomer (Mo), dimer (Di) and tetramer (Te) of citrazinic acid have been optimized using density functional theory (DFT). The computed absorption spectra of monomer, dimer and tetramer have been plotted in Fig. 2C. The computed absorption maxima of citrazinic acid is 350.04 nm (3.542 eV) which is in excellent agreement with the experimental one, 345 nm in water. The absorption peak maximum of dimer gets blue-shifted, whereas the tetramer shows expected bathochromic shift. The oscillator strengths follow a decreasing trend, which fortifies the experimental observation of gradual decrease in emission intensity over emission wavelengths. TDDFT calculations predict that there are a three vertical transitions involved for dimer, while two for tetramer. The predicted wavelengths in the lower energy regions are- 358.87 nm (3.4549 eV), 342.15 nm (3.6237 eV) and 318.06 nm (3.8981 eV) for dimer; 366.53 nm (3.3827 eV), and 366.52 nm (3.3828 eV) for tetramer (Table S3). It should be mentioned here that a shift of 30 nm in the absorption peak has been noticed between the dimer and tetramer only. Therefore, the gradual redshifted excitation spectra may thus be assigned to the H-bonded clusters (Fig. 2D). The feasibility of such clusters formation solely depends on the alternate two and three Hbonding interactions among citrazinic acid molecules. To provide a direct experimental evidence to the involvement of pyrido group of citrazinic acid in H-bonded clusters, we have chosen butyl amine as one of the precursor in synthesis along with citric acid. The breaking of such cluster formations is expected due to the introduction of steric hindrance through aliphatic chain.33,34 The absorption and emission spectra of product obtained hydrothermal reaction of citric acid and butyl



amine are shown in Figure 2E-F respectively. A very sharp absorption peak in water is observed at 333 nm. Its emission spectra do not show any signature of excitation-dependent emission behavior, rather a typical molecular behavior, excitation-independent emission. Steric hindrance caused inaccessibility retards the intermolecular interactions among fluorophores, which is perhaps the driving factor for the excitation-independent emission which is also observed for cysteine, glutathione as one of the precursors. Therefore, the study clearly provides a rationale for switching of the excitation-dependency.

Chameleon Character through Insertion of Molecular dopant: The ammonium thiocyanate is used a molecular dopant here for understanding the impact of thiocyanate in dictating the structure and thereby luminescence properties of fluorophores contributing to the photoluminescence of nanodots. In order to decipher the role of solvent in autoclave reaction condition, we have chosen water and DMF here. Surprisingly, the resultant solution from solvothermal treatment turns deep red (product, P1) whereas the same from hydrothermal method becomes green (product, P2) in color (Fig. 3A). FT-IR spectroscopy is first employed for the identification of functional groups present in the synthesized products obtained from the methods (Fig. 3B). Both the products exhibit nearly equal number of vibrations, except, a visible difference for bands appeared at 1650 cm-1 and 1400 cm-1. From 3700 to 2700 cm−1, the stretching vibrations of CH, NH, OH, and COOH contribute to the broad IR band. The stretching vibration at 2058 cm-1 is assigned to SCN bonding.35 The strong FTIR bands at 1700 cm-1, 1650 cm−1 and 1550 cm−1 belong to C=O, C=N and C=C vibrations respectively. At 1393 cm−1, a strong C−N stretching has also been observed.36 Notably, the bands observed for products in hydrothermal treatment is relatively broader in nature.


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Figure 3. [A] Solvothermal method employed for the synthesis of fluorescent products. [B] ATR FT-IR spectra of products- (i) P1 (red line), (ii) P2 (green-line). [C] TEM image of Product P1; Inset - particle size distribution. [D] Distribution of the mass of the products- (i) P1 (red square), (ii) P2 (green-circle).

We have used here also the electron microscopy to characterized the particles formation in vacuum condition. At lower concentration, it does not show any particle formation, the TEM images recorded at high concentration is shown in Fig. 3C. The average diameter of the aggregated particles is measured to be 19.1 5.5 nm (Fig. 3C, Inset). Moreover, high-resolution TEM images (Fig. S4) show a typical (graphene-like) with the appearance of lattice fringes. The observed lattice spacing of 0.22 nm is consistent with the lattice spacing observed in incubated citrazinic acid. One of the lacuna of nanodot studies is the mutated response on the molecular weight. To provide a direct answer to this question, we have performed the mass spectrometry, LC-MS/MS to characterize the products.37 LC-MS/MS method is a leading tool to identify the mass of molecules present in the corresponding peaks in chromatogram (Fig. S5-6). It is generally observed that the



mass spectra of nanodot samples contains number of peaks, hence a conclusion becomes cumbersome. To overcome this, we have constructed a distribution the mass observed in all the mass spectra recorded for a sample (Fig. 3D). The mass of products ranges from 120-700 m/z, which is much lower than the size of the nanodot, often depicted. The chameleon property can be attributed to the relatively greater yield of molecules having m/z 350 or higher in range. Therefore, it can be emphasized that the distribution of mass is the origin of color of the products, rather elemental doping in carbon core.

Luminescence Dynamics of Carbon Nanodots: The absorption spectra of products obtained from reactions show presence of multiple peaks, a sharp at 337 nm for both and shoulder peaks at 455 nm, 555 nm for product, P1 and 445 nm, 595 nm for P2 respectively (Fig. S7). The chromophoric units for P1 in absorbing wavelength region, 420-570 nm are relatively greater in abundant in comparison to that of P2. Such distribution is also indicated by the LC-MS/MS studies mentioned above. The absorption spectrum of reported N, S-doped nanodot which was synthesized by using citric acid and thiourea as precursors in hydrothermal condition, show a striking similarities with that of product P2, in particular, a common peak at near 600 nm.19 The steady-state emission spectra of products are recorded at different excitation wavelengths shown in Fig. 4A for P1 and Fig. S8 for P2, respectively. A strong excitation-dependent emission has been observed for both the cases. There exists similarity and expectedly, dissimilarity for emission properties of both the products. A distinct isoemissive point has been noticed at 530 nm for P2, which is indicative of effective emissions from two fluorophores- blue and green emissive. Aside isoemissive point at 525 nm, a strong emission at 585 nm has also been observed for product, P1 signaling the ground state heterogeneity as well. To the best of our understanding, the ground state heterogeneities has been clearly shown for very first time, for the products yielded from the 16 ACS Paragon Plus Environment

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reaction of citric acid and a molecular dopant under solvothermal reaction condition. The plot of emission peak maxima against the excitation wavelengths (Fig. 4B) of the products can be categorized into three distinct excitations regions- first, the linear dependence (340 - 400 nm) which is very similar to the aggregated citrazinic acid, the plateau region 1 (430 – 470 nm) and plateau region 2 (500-580 nm) for product, P1. The sharp change is visible for P2 in the plateau region 2. The emission beyond 600 nm are primarily feeble in nature. Interestingly, the emission maxima at 585 nm exhibit a typical molecular behavior, i.e. excitation independent. Therefore, one can reasonably state that the origin of excitation-dependence lies solely with blue-emissive fluorophores, such as citrazinic acid. The temporal properties of products have been characterized by picosecond time-resolved measurements. The emission lifetimes of the products are measured at three different excitations375 nm, 440 nm and 560 nm. The entire emission wavelengths, 480 nm – 650 nm have been mapped for both the products. The decay traces of product, P1 measured in water become faster at the higher emission wavelength. The traces of P1 are well fitted with triexponential in water, while biexponential in DMF for the same excitation, 440 nm (Table S4). An ultrafast component does not appear in DMF, thus suggestive of specific fluorophore-water interaction. Fig. 4D illustrates not only the dependence of fluorescence lifetime of P1 on emission wavelengths for an excitation wavelength, also excitation wavelengths and solvent, DMF. The emission lifetime of product, P2 does not exhibit strong emission wavelength-dependence for an excitation source (Fig. S9). The emission lifetimes of products in DMF for an emission window 480-550 nm are remarkably matching up to the temporal properties of incubated citrazinic acid. The average lifetime of P1 seems to be remain greatly unaffected over emission wavelengths in both the solvents. However, the fluorescence lifetime of product, P2 is relatively lower than that of P1 in water at 440 nm



excitations. The emission lifetimes of products in DMF are relatively higher, ~ 2.5-3.5 ns than that in water, revealing its polarity dependence. At 560 nm excitation, the emission measured for P1 at 625 nm of emission wavelength in water is found to be single exponential with emission lifetime, 2.05 ns (Fig. S10). The average lifetime for the same emission wavelength region for 440 nm excitations is much higher, ~ 5 ns, suggesting significant contribution from blue and greenemitting fluorophores as well.

Figure 4: [A] Emission spectra of product, P1 in aqueous solution at different excitation wavelengths. [B] The emission maximum as a function of the excitation wavelength: P1 in aqueous solution (red circle), P2 in aqueous solution (green circle), incubated citrazinic acid in DMF (blue circle). [C] The fluorescence decays of P1 in aqueous solution recorded over emission wavelengths at exc=440 nm. [D] Average luminescence lifetime of P1 over emission wavelengths at different excitation wavelengths: in water (hollow red circle) and DMF (solid red square) at exc=440 nm. In aqueous solution at exc=560 nm (green star). [E] The peak-normalized components of time-resolved emission spectra of P1 in aqueous medium. With the passage of time, the spectra move toward lower energies. [F] The time-resolved area-normalized emission spectra (TRANES) of P1 in water, between time 0 and 20 ns.

To understand the different origins of temporal properties of product, P1 we have performed a time-resolved area-normalized emission spectroscopic analysis of the data. The analysed emission 18 ACS Paragon Plus Environment

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decay components and their corresponding weightages of P1 in aqueous medium at excitation wavelength 440 nm are used for construction of Time-Resolved Emission Spectra (TRES) and Time-Resolved Area Normalized Emission Spectra (TRANES).38 Periasamy and co-workers have developed the TRANES technique and have established that an isoemissive point in TRANES indicates the presence of two distinct emissive species.39 A small time dependent Stokes shift (TDSS), 900 cm-1 is observed for the emission over a period of 20 ns (Figure 4D). A clear isoemissive point is obtained in TRANES at 24781 cm-1 (Figure 4E). It therefore suggests that there are predominantly two emissive species for the excitation wavelength 440 nm.

White Light Emission from PVA/PVP Film: It is often unaccounted the role of solvent, water molecules in excitation-dependence emission spectra. In order to directly address this, we have used polymers, such as PVA/PVP primarily for its offering of water-like environment due to the presence of large number of hydroxyl groups.40,41 PVA/PVP film impregnated with product, P1 remains transparent and still reddish in color (Fig. 5A). Absorption spectrum of film indicates that the polymer matrix does not interact the strong interaction with the product, rather provide a support system. However, a peak observed in aqueous medium at 555 nm is redshifted to 572 nm and a shoulder peak gets resolved at 610 nm (Fig. S11). Surprisingly, it illuminates as a white emitter under irradiation in UV lamp (365 nm) (Figure 5A, Inset). The 1931 colour matching functions are used to calculate the chromaticity index which comes out to be 0.22, 0.25 (Fig. S12).42 To understand the origin of white light emission, the emission spectra of the film are scanned over the entire range of excitation wavelengths. The spectra are presented in Fig. 5B for better clarity. Four distinct peaks in the emission spectrum have been clearly resolved, even excitation at 340



nm, which are generally not observed in aqueous medium. Instead of a gradual decreasing trend in emission intensity with excitations often observed in aqueous medium, we have shown that when the excitation wavelengths fall in the absorbing range of a fluorophore, the emission intensity of that fluorophore dominates over others. The plot of emission peak maxima vs. excitation shows four distinct regions (Fig. 5C). The emission peaks are appeared at 435 nm, 530 nm, 585 nm and 625 nm respectively. Except 435 nm, others do not exhibit excitation-dependent emission. It therefore conclusively proves that the generally observed excitation-dependence is emanating from the blue-emissive fluorophore. A small difference, 40 nm of emission peaks contributing to red emission and disappearance of that in aqueous medium suggest plausible vibronic interactions in the fluorophore.43,44 The differences among the emission peaks suggest that there are effectively three fluorophores.

Figure 5: [A] PVA/PVP film impregnated with P1 under ambient light. Inset: under UV excitation (365 nm). [B] Emission properties of P1 in the dried PVA/PVP film. [C] The emission maximum as a function of the excitation wavelength. [D] Fluorescence decays of P1 the dried film.


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Emission lifetimes of P1 impregnated film are also measured at 440 nm and 560 nm excitations respectively. Unprecedently, the decay traces recorded at 440 nm excitation show an emerging rise time at red end of the emission. (Fig. 5D). The emission lifetime has been gradually increased, distinctly different from the observation in aqueous medium. The decay parameters of P1 in dried PVA/PVP film are listed in Table S5. The rise time is estimated to be 1.44 ns at the emission wavelength, 635 nm. It can be mentioned here that the observation of rise times for a single fluorophore system are assigned to its slow solvation, however, in this case, we assign this to energy transfer among the fluorophores present in matrix, primarily for two reasons- strong overlap in their energy bands and omission of solvent polarity-directed quenching resulted in strong emission in polymer matrix.45,46 Fluorescence lifetimes at 560 nm excitations found be more-or-less remain constant at varied emission wavelengths and are found be higher, by 1 ns than that quantified in aqueous medium.

Predicting the molecular structure of fluorophores: The formation mechanism of fluorophores contributing in nanodot emission dynamics remain obscured due to the complexity attached with one-pot solvothermal method. Moreover, the heterogeneity existing with organic nature of the products restricts the crystallization for resolving the molecular geometry. Thus we have made an attempt to predict the fluorophores which has been proposed through reaction mechanism. First, the reaction of weak organic acids with ammonia results in the formation of citrazinic acid (precursor- citric acid). Thereafter, isothiocyanate reacts with citrazinic acid giving rise to fluorophores (Scheme S1). The proposed reaction mechanism is based on the recent reports on crystal structures of fluorophores synthesized by N, S containing molecular dopants such as cysteine, cysteamine.47 It can be stated here that the previous studies show a good agreement



between X-ray Photoelectron spectroscopic (XPS) studies on N, S-doped CNDs and fluorophores.19,47 Apart from molecular weights observed in LC-MS/MS studies, a due consideration has also been given while drawing the reaction mechanism so that the molecules must contain the pyridinic, pyrrolic nitrogens and thiophenic sulfur as well. The primary objective is to make a statement that even organic N, S containing fluorophores can cover the entire emission region. The ground state geometries of the molecules have been optimized in the water phase using the density functional theory (DFT) at 6-311++G (d, p) basis set with IEFPCM method (Table S6).

Figure 6: [A] Spectral decomposition of the absorption spectrum of P1 in aqueous solution. Absorption spectrum of P1 (red, solid line), Fitted data (dotted grey line), Components with peak maxima 338 nm (blue, solid line), 457 nm (cyan, solid line) and 555 nm (orange, solid line) respectively. [B] In aqueous solution, the peak-normalized excitation spectra of P1 at different emission wavelengths (ems) - (i) 450 nm (blue dotted line), (ii) 550 nm (green dotted line), (iii) 600 nm (dotted red line). The molecular structures of proposed fluorophores. An analysis of the absorption spectrum of P1 identifies three distinct peaks at 338 nm, 457 nm and 555 nm respectively (Figure 6A), suggesting efficacy of these chromophoric units in dictating the overall luminescence dynamics. The absorption peak at 338 nm has been attributed to the citrazinic 22 ACS Paragon Plus Environment

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acid molecule. The optimized molecular geometries and Cartesian coordinates of the atoms in proposed molecules- pyrido-triazin-thione (M1) and pyrido-pentazin-thione (M2) are presented in Table S6. The vertical excitations in the lower energy region computed at the TDDFT/B3LYP/6311++G (d, p) // B3LYP /6-311++G (d, p) level for both the molecules in the water phase employing IEFPCM model (Table S7). The excitation spectra recorded at three different emissions wavelengths- 450 nm, 550 nm and 600 nm are shown in Figure 6B. Corresponding to each emission, a clearly resolved excitation peak maximum is noticed, appeared at 360 nm, 453 nm and 555 nm respectively. Except blue-emissive one, the excitation peaks of contributing fluorophores find strong correlation with the absorption peaks. TDDFT calculations predict that a single vertical transition is involved for fluorophore, M2, whereas, three such transitions are associated with M3. The predicted wavelengths in the lower energy regions are- 491.9 nm (2.505 eV) for M2; 588.1 nm (2.108 eV), 515 nm (2.407 eV) and 493.2 nm (42.514 eV) for M3 respectively (Table-S7). The predicted wavelengths for low energy absorption are found to be in fairly agreement with experimental excitation spectrum and thus satisfactorily explain the gradual appearance of shoulder peak at 515 nm in the excitation spectra of red emissive fluorophores. In both cases, the highest intensity transitions are found to be predominantly between the HOMO and LUMO. However, it should be mentioned here that the difference of 30-40 nm (0.32 - 0.23 eV) in peak maxima in lower energy region is due to the omission specific interactions such as H-bonding, and presence of sulphur atoms might be the reason for the underestimation of computed vertical transitions. 48,49



3. Conclusion: The ground and excited state properties of incubated citrazinic acid in DMF are greatly resemblance to that of citric-acid derived nanodot. The origin of excitation-dependent emission in this case is attributed to the fluorescent clusters. Formation of nanocarbon from room temperature incubation of citrazinic acid is an unprecedented observation. Introduction of sterically crowded amine precursor, n-butyl amine in hydrothermal synthesis turns off the excitation-dependent emission behavior. One-pot synthesis of citric acid and ammonium thiocyanate clearly resolves the ground state heterogeneity. The mass spectrometry quantifies the molecular weights of the products. We observe both excitation-dependent and independent emission with distinct emission peaks for both the products. The emission properties of products are strongly influenced by the excitations wavelengths (ground-state heterogeneities), solvent and emission wavelengths as well. Remarkably, a white light emission is observed for PVA/PVP film which is impregnated with product obtained from solvothermal method. The emission property of the film suggest that the excitation-dependence is restricted to only blue-emissive one and other contributing fluorophores exhibit a typical molecular behavior. The quantum chemical calculations have been used to compute the vertical excitations of molecular clusters of citrazinic acid and proposed fluorophores, which are in good agreement with the experimental observations.


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Supporting Information Available: Steady state and time-resolved fluorescence data, Mass Spectrometric data, HRTEM images, molecular geometries and computed vertical excitations are included.



Acknowledgements: The authors greatly acknowledge Council of Scientific and Industrial Research (CSIR), Govt. of India (Project No.: 01(2954)/18/EMR-II), and RGIPT for the financial support. Authors express sincere gratitude to Prof. Anindya Datta, IIT Bombay for availing the facility of timeresolved emission measurements. Authors are thankful to Sucheta Banerjee, Souradip Dasgupta and Farogh Abbas for their help in time-resolved experimentations. Authors acknowledge Central Drug Research Institute, Lucknow for mass spectrometry, Advanced Imaging Centre at IIT Kanpur for Electron Microscopic Characterizations.


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Figures and Captions:

Scheme 1. [A] Schematic representation of the emission properties of commonly-observed nanodots synthesized in hydrothermal method. [B] Highlighting the need of room temperature method for mimicking nanodot properties.

Figure 1. Steady-state and time resolved fluorescence properties of incubated citrazinic acid in DMF: [A] Fluorescence spectra at different excitation wavelengths, [B] The emission maximum as a function of the excitation wavelength, [C] Fluorescence decay traces, and [D] Average fluorescence lifetime (solid blue circle) over emission wavelengths; at 375 nm excitation, Solid grey lines show the representative emission spectra recorded at exc=375 nm.

Figure 2. [A] TEM image of incubated citrazinic acid; Inset - particle size distribution, [B] HRTEM image with lattice spacing of approx. 0.22 nm (Inset) of incubated citrazinic acid in DMF. [C] Computed absorption spectra and corresponding oscillator strengths of citrazinic acid (Mo) and its Dimer (Di) and tetramer (Te); Mo- Green lines, Di- blue lines, and Te- red lines. [D] The ground state optimized geometries of citrazinic acid and its clusters (Table S2). [E] Absorption spectrum, [F] Emission spectra of product obtained from the hydrothermal treatment of citric acid and n-butyl amine.

Figure 3. [A] Solvothermal method employed for the synthesis of fluorescent products. [B] ATR FT-IR spectra of products- (i) P1 (red line), (ii) P2 (green-line). [C] TEM image of Product P1; Inset - particle size distribution. [D] Distribution of the mass of the products- (i) P1 (red square), (ii) P2 (green-circle).



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Figure 4: [A] Emission spectra of product, P1 in aqueous solution at different excitation wavelengths. [B] The emission maximum as a function of the excitation wavelength: P1 in aqueous solution (red circle), P2 in aqueous solution (green circle), incubated citrazinic acid in DMF (blue circle). [C] The fluorescence decays of P1 in aqueous solution recorded over emission wavelengths at exc=440 nm. [D] Average luminescence lifetime of P1 over emission wavelengths at different excitation wavelengths: in water (hollow red circle) and DMF (solid red square) at exc=440 nm. In aqueous solution at exc=560 nm (green star). [E] The peak-normalized components of time-resolved emission spectra of P1 in aqueous medium. With the passage of time, the spectra move toward lower energies. [F] The timeresolved area-normalized emission spectra (TRANES) of P1 in water, between time 0 and 20 ns.

Figure 5: [A] PVA/PVP film impregnated with P1 under ambient light. Inset: under UV excitation (365 nm). [B] Emission properties of P1 in the dried PVA/PVP film. [C] The emission maximum as a function of the excitation wavelength. [D] Fluorescence decays of P1 the dried film.

Figure 6: [A] Spectral decomposition of the absorption spectrum of P1 in aqueous solution. Absorption spectrum of P1 (red, solid line), Fitted data (dotted grey line), Components with peak maxima 338 nm (blue, solid line), 457 nm (cyan, solid line) and 555 nm (orange, solid line) respectively. [B] In aqueous solution, the peak-normalized excitation spectra of P1 at different emission wavelengths (ems) - (i) 450 nm (blue dotted line), (ii) 550 nm (green dotted line), (iii) 600 nm (dotted red line). The molecular structures of proposed fluorophores.


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Scheme-1:

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Connecting the Dots of Carbon Nanodot: Excitation- (In)dependency

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