Solar-light-induced green conversion of amines into imines by lemon derived heteroatoms-doped GQDs as a green photocatalyst

Abstract

Graphene is one of the amazing present encroachments in current research area of science and one of the utmost fascinating materials for relevance in cutting-edge research. Herein, we designed lemon-derived heteroatoms-doped graphene quantum dots (S, N-GQDs) based photocatalyst for the first time. For the integrating reactions of amines in aerobic conditions under solar light by S, N-GQDs photocatalyst exhibit utmost higher photocatalytic activity than simple oxygen-doped graphene quantum dots (O-GQDs) due to slow recombination charges. The mechanisms accountable for the drastically increased photocatalytic activity of S, N-GQDs in solar light responsive integrating reactions of amines in aerobic conditions into the corresponding derivative of imines are also completely scrutinized.

Superstar graphene derivatives quantum dots are zero quantum-confined materials with exceptional chemical and physical properties that permit them to be utilized in different fields such as catalysis, biosensing, conversion of solar light [1(a-d)], and hydrogen evolution [2, 3]. Superstar graphene quantum dots (GQDs) are structurally different from dots of carbon; GQDs usually have higher feature ratios between the tangential size and the height. In addition, basically, GQDs have higher crystallinity than dots of carbon [4, 5]. However, one of the assuring routes to optimize the fundamental physicochemical properties of GQDs is doping with heteroatom organic and inorganic [6, 7] species. Insertion of doping agent has resulted in GQDs that have higher activity and molar extinction coefficient [8, 9]. Due to these extraordinary properties, GQDs doped have been extensively utilized in phototherapy and bioimaging [8(a-b)]. In contrast, as per the reported paper, many researchers have been finding few investigations exhibiting that doping of heteroatom except oxygen (O) in GQDs can modify their catalytic/electrocatalytic activity [10]. Compared to the many electrical, mechanical, medical, and biological studies of GQDs based doped materials there is a lack of research articles on the effect of various dopants contents on the catalytic and electrocatalytic activity of GQDs. In this context, there have been a small number of efficient studies on how affects doped heteroatom on GQDs during the photocatalytic conversions of organic molecules [10 –14]. It is of immense significance and interest to transform the activity of GQDs photocatalyst by inserting dopants and to understand the cause of doping with respect to kind dopants, creation, and content of the activity of photocatalyst via an organized synthetic manner. For the clear-cut preparation of doped metal and non-metal GQDs, a precursor organic and inorganic molecule is necessary to be satisfactorily dynamic to be polymerized, carbonized, and condensed without any additive agents under heating condition. In addition, it is beneficial if natural and synthetic precursors are able to form various elements-doped GQDs with comparable morphologies, microstructure, and nanostructures [15]. Thus, the enlargement of a synthetic approach employing green and synthetic precursors for the synthesis of heteroatoms doped GQDs under simple heating and refluxing under conditions is essential. This pathway may provide a path to best understanding the doping effect during photocatalysis.

Derivatives of imines are very significant intermediate ingredients for the preparation of organic dyes, pharmaceutical drugs, and solar fine chemicals [15]. One effectual approach for the generation of imine derivatives is the oxidation of amines in aerobic conditions. Nevertheless, this reaction needs expensive metal-based catalysts. In this context, the conventional technique for the oxidation of amines in aerobic conditions exhibits low yield and conversion of amines into the matching derivative of imines [16]. Oxidative integration of amines in the aerobic condition is too much hard to under-reported synthetic procedures [16]. Currently, catalytic approaches have been projected for oxidative integration in the aerobic condition of derivative of amines under solar light irradiation [17, 18]. The reported photocatalysts, nonetheless, exhibited relatively low photocatalytic activity to generate imines under the irradiation of solar light. Hence, more effectual and expensive photocatalysts such as metal-free are future needs for the solar light-active integrating reactions of amines in aerobic conditions. Herein, we account glucose and thiourea based synthetic pathways to generate various heteroatom-doped GQDs [17, 18]. These pathways allowed us to efficiently transform the GQDs photocatalytic activity in the aerobic condition for integrating reactions of derivative of amines under solar light. Herein, we synthesized sulfur and nitrogen-codoped GQDs (S, N-GQDs) photocatalyst by green lemon, and thiourea precursors under thermal and refluxing conditions (Scheme 1).

Scheme 1

Synthesis of S, N- Doped GQD photocatalyst.

The S, N-Doped GQD photocatalyst was prepared by using citric acid (obtained from lemon juice) and thiourea as a building block. Initially, thiourea (2 g) was mixed well into 185 mL of citric acid followed by continuous stirring at room temperature till a homogenous solution is obtained (Scheme 1). After that, the homogenous solution was refluxed at 140°C for 48 hours. The reaction is progressed with time deep color was observed. After completing the reaction, filter the refluxed solution. After filtration, brown solid precipitate of S,N-Doped GQD was obtained. The activity of photocatalytic was examined in the oxidative integrating reactions in aerobic condition of derivative amines under solar light illumination after characterization of the S, N-GQDs material.

This sulfur and nitrogen-codoped GQDs (S, N-GQDs) photocatalyst by green lemon, and thiourea precursors under thermal and refluxing conditions were characterized by UV-visible spectroscopy (Fig. 1a, b), both thiourea (TU) and S, N-GQDs exhibited strong and weak absorption in the ultraviolet and visible range. The molar absorptivity coefficients of S, N-GQDs is higher in comparison to TU, however, were absorption ability much larger than those of TU, explained to the hefty segment of π-conjugations in graphitic C = C along with sulfur and lone-pair electrons in the S, N-GQDs photocatalyst. This broader absorptive (Fig. 1) in the visible range could have an effect on the utmost useful electron-hole or HOMO (highest occupied molecular orbital)-LUMO (lowest unoccupied molecular orbital) partition, which would inform excellent photocatalytic ability to GQDs in the same reaction medium. In other words, S, N-GQDs photocatalyst has new bands come into view near about at 450, 475, and 525 nm due to doping effect [19] (Fig. 1b), which alters the surface-active sites [19] of GQDs photocatalyst. The bands at 475, and 525 nm may be explained to the n-> π*, and π-> π* of C = N/C-N and C = S respectively [18].

Fig. 1

UV absorption spectroscopy of (a) TU (green line), (b) S, N-Doped GQD (blue line).

FT-IR spectra was utilized to determine the functional groups presenting on TU (Fig. 2a) [20] surface. As per reported spectra of TU, the different types of vibrations are found in the range of 3156 cm^-1 to 3371cm^-1, which indicates that, the various types of NH₂ functional groups happening out of tautomeric form. The few additional peaks as per reported papers [20] are exhibiting thioamide (C = S) asymmetric stretching vibration, C-S vibrations, and ammonium & iminium salt like structure [20]. Additionally, the sharp peak near about at 729 cm^-1, 627 cm^-1, and 493 cm^-1 is allocated to the S-H, C-N, and NH bending vibrations. Furthermore, the wide absorption bands of S, N-GQDs photocatalyst at 3100–3500 cm-¹ are designated to N-H and O-H stretching vibrations in Fig. 2(b), due to these stretching vibrations S, N-GQDs have excellent hydrophilic property. The bands near about at 1710–1670 cm^-1 are ascribed to C = O vibrational absorption band in S, N-GQDs photocatalyst. While the bands near about at 1400 to 1600 cm^-1 are from the C-N and C = C bending vibrations, respectively. The C = S stretching vibration could appear in a broad range near about 1400 to1025 cm^-1. The peaks near about at 1080 cm^-1 in TU and S, N-GQDs exhibited to the C = S bond. Weak stretching C-S peak was also determined near about at 630 cm^-1 [21].

Fig. 2

FTIR Spectra of (a)TU (green line), (b) S, N-Doped GQD (blue line) and, Zeta potential distribution graph of (c) TU (–3.58 mV) and, (d) S, N-GQDs (–5.87 mV) respectively.

The DLS technique was utilized to determine the average particle size (d) and zeta potential (ξ) for each TU (Fig. 3(a) and S, N-GQDs photocatalyst (Fig. 3(b). The particle size of S, N-GQDs photocatalyst is smaller compared with the TU (Fig. 3(a), which indicates the high photocatalytic activity and more stability. According to ‘ξ’ measurements, the individual peaks show sparingly dispersed stabilities for the prepared samples. Moreover, a comparatively higher deviation that contributed to enhanced scattering of the signal was shown in S, N-GQDs photocatalyst, in which the average values were achieved at –3.58 and –5.87 mV for the TU and the S, N-GQDs photocatalyst, respectively (Fig. 2c and Fig. 2d). Therefore, these values confirmed the formation and stabilization of the, S, N-GQDs photocatalyst without agglomeration or sedimentation. In other words, higher negative zeta potential values for S, N-GQDs photocatalyst were confirmed the increased volume to facilitate the surface-active sites [22]. The synthesized S, N-GQDs photocatalyst was immediately dispersible in aqueous ethanol which was credited to their electron-rich surface (promoted by sulfur and nitrogen doping), and small size [23]. To probe this hypothesis, the particle size and electron-rich surface charge on S, N-GQDs photocatalyst were estimated using zeta potential measurements and dynamic light scattering (DLS), respectively. The size of S, N-GQDs photocatalyst in aqueous ethanol was found to be lower than TU (Fig. 2a,) while the surface potential was measured more negative in comparison to TU (Fig. 2b) which confirms the formation S, N-GQDs photocatalyst. Also demonstrates the designed S, N-GQDs photocatalyst is more active and stable than that of TU [24]. SEM images are also confirmed the formation of S, N-GQDs (Fig. 3c and Fig. 3d) photocatalyst [25].

Fig. 3

A particle size distribution graph of (a)TU, (b) S, N-Doped GQD photocatalyst and, FE-SEM morphologies of (c) TU and (d) S, N-Doped GQD photocatalyst.

To study the control experiment, optimization, and conversion of amines to imines by the S, N-GQDs photocatalyst, amine integrating was elected as the model reaction in the presence of solar light (Table 1). Oxidation is not happened in absence of solar light or S, N-GQDs photocatalyst (Entry 7, Table 1). With S, N-GQDs, solar light, and O₂, the yield obtained N-benzylidene-benzylamine 99% (Table 1). Among the solvent, acetonitrile is the better solvent scrutinized (Entries 1–6, Table 1). Hence, under optimized reaction conditions (Entry 7, Table 1), total conversion of amine derivatives into N-benzylidene-benzylamine (imines) was received (Entries 5, Table 1). The oxidation of few amines was also scrutinized under these similar reaction manners, and the yields are also scheduled in Table 2. The substituted amine derivatives with an electron-releasing group (OCH₃) could undergo oxidative integrating to the corresponding imines along with good yields and high selectivity as per reported literature methods [26].

Table 1

S, N-GQDs photocatalyst was used for the oxidative integrating of benzylamines under solar light

Table 2

Transformation of amines into the corresponding derivative of imines by S, N-GQDs photocatalyst under solar light

Reaction conditions: Benzylamines (1 mmol), photocatalyst (5 mg), solvent (10 mL) under solar light irradiated for about 12 h in the aerobic condition.

The association between the photocatalytic ability/activity and the incident solar light wavelength was examined. The results exhibited the photocatalytic activity of S, N-GQDs as per the reported photocatalyst [27 –29]. In this context, the activity of S, N-GQDs photocatalyst was evaluated for the conversion of various amino compounds [27 –29]. As exposed in Table 2, the electron-releasing substituted benzylamine derivative (entry 2) was converted into the corresponding (E)-N-(4-methoxybenzylidene)-1-(4-methoxyphenyl) methanamine with good yield (97%) and selectivity (97%). This results advocate that S, N-GQDs photocatalyst are utmost effective for the oxidative integrating reactions in aerobic condition of few amines under solar light. It needs to highlight that the catalyst is very active in this photocatalysis reaction over GQDs. The synthesized organic compound was further confirmed by ¹H-NMR (Fig. 4 a, b).

Fig. 4

(a)¹ H NMR spectrum of synthesis organic compounds (a) N- benzyl-1-phenylmethanimine and, (b) N-(4-methoxybenzyl)-1-(4-methoxyphenyl)methanimine.

(a) N- benzyl-1-phenylmethanimine: ¹ H NMR (500 MHz, DMSO-D₆) δ 8.46 (s, 1 H), 7.75 (d, J = 4.9 Hz, 2 H), 7.42 (d, J = 1.8 Hz, 3 H), 7.29 (s, 5 H), 7.22 (ddt, J = 9.4, 6.5, 4.2 Hz, 1 H), 4.73 (s, 3 H).

(b) N-(4-methoxybenzyl)-1-(4-methoxyphenyl) methanimine: 1 H NMR (500 MHz, DMSO-D6) δ 8.32 (s, 1 H), 7.67 (d, J = 8.7 Hz, 2 H), 7.19 (d, J = 8.5 Hz, 2 H), 6.95 (d, J = 8.7 Hz, 2 H), 6.85 (d, J = 8.6 Hz, 2 H), 4.61 (s, 2 H), 3.74 (s, 6 H).

On account of the reported above final outcomes, the planned plausible reaction pathways is showed in Scheme 2. Similar to few reported light harvesting materials that encourage photosynthesis, [16] the solar light reaction is instigated by hole (h+) and electron (e^-) pairs by solar light generated reported photocatalyst. The solar light-generated electron reduces singlet oxygen such as molecular oxygen to generate^.O₂^-, as per reported literature [30]. The oxidative integrating of derivatives of amines then proceeds via multiple steps. At first, the amine drops an electron most probably, thus generating the intermediate radical. The radical anion superoxide then withdraws a hydrogen atom and a proton to produce the corresponding imines, and appears to be the oxidation reaction [30, 31]. As we not at all observe molecular hydrogen peroxide (H₂O₂), we expect the presence of a second, a like two-electron cycle on S, N-GQDs that leads to the generation of an imines and water [32]. The hole positively charged can harmonize to imines and thereby create it more prone to attack of nucleophilic by the amine to form the aminal. Eventually, the aminal group will then go through positively charged hole-assisted elimination of ammonia (NH₃) to obtain the coupled final product (Scheme 1).

Scheme 2

Plausible mechanism of the photocatalytic oxidative integrating of amines.

In this article, we demonstrated a green and eco-friendly strategy for the lemon derived heteroatoms-doped GQDs as a green photocatalyst i.e., S, N-GQDs photocatalyst for the oxidative coupling of amines to imines. The photocatalytic performance and coupling reactions are supported by UV-visible, FTIR, zeta potential, particle size, and ¹H-NMR spectroscopy. In this context, the mechanism accountable for the S, N-GQDs -photocatalyzed oxidative integrating reaction in aerobic condition of amines was determined. We anticipate that the utilization of S, N-GQDs can be extended to various photocatalytic reactions from other organic transformations to the carbon-carbon bond activation, carbon-hydrogen activation, carbon-sulfur, and hydrogen evolution reaction. This aerobic oxidation metal-free photocatalytic system is very cheap, eco-friendly, and efficient and can be easily activated/operated. Finally, S, N-GQDs photocatalyst showed outstanding activity for the oxidative integrating in aerobic condition reactions of amines under solar light compared to without heteroatom doped GQDs. This gives imminent into the effect of doping materials on the photocatalytic activity in this photocatalytic transformation in aerobic condition.

Footnotes

Acknowledgments

We are very thankful to Department of Chemistry and Environmental Science of Madan Mohan Malviya University of Technology, Gorakhpur -273010, U.P., India for their financial support and data analysis.

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