Vishesh
Kumar
a,
Sunil Kumar
Patel
b,
Ved
Vyas
a,
Deepak
Kumar
a,
E. Siva
Subramaniam Iyer
*b and
Arindam
Indra
*a
aDepartment of Chemistry, Indian Institute of Technology (BHU), Varanasi, 221005, UP, India. E-mail: arindam.chy@iitbhu.ac.in
bSchool of Chemical and Materials Sciences, Indian Institute of Technology Goa, Ponda, Goa, India. E-mail: essiyer@iitgoa.ac.in
First published on 25th July 2024
Photoredox catalysis involving perovskite quantum dots (QDs) has gained enormous attention because of their high efficiency and selectivity. In this study, we have demonstrated CsPbBr3 QDs as photocatalysts for the C–N bond formation reaction. The introduction of Ni(dmgH)2 (dmgH = dimethyl glyoximato) as a cocatalyst with CsPbBr3 QDs facilitates photocatalytic C–N coupling to form a wide variety of amides. The optimized interaction between the cocatalyst and photocatalyst enhances charge transfer and mitigates charge recombination, ultimately boosting photocatalytic performance. The photocatalytic activity is notably influenced by the variation in the amount of cocatalyst and 7 wt% Ni(dmgH)2 produces the best yield (92%) of amide. Femtosecond transient absorption spectroscopy reveals that the dynamics of the trap states of QDs are affected by cocatalyst. Further, Ni(dmgH)2 facilitates molecular oxygen activation to form superoxide radicals, which further initiates the radical pathway for the C–N coupling.
In this context, CsPbBr3 QDs have been explored as the photoredox catalyst for various organic reactions.14,15 The kinetic energy associated with the CB electrons of QDs and the favorable reduction potential enable them to overcome the energy barriers and facilitate charge transfer, ultimately favoring photocatalytic activity. High light-absorption efficiency, low exciton binding energy, high carrier mobility, and optimized electron–hole diffusion length make them ideal for photoredox catalysis.16,17 Further, the long-lived active carriers of CsPbBr3 QDs promote photocatalytic organic reactions.18
For example, CsPbBr3 QDs have been employed to selectively oxidize benzyl alcohols to benzaldehydes under visible light irradiation.19 Similarly, photoredox catalysis of CsPbBr3 QDs has been explored for the oxidative coupling of thiols to disulfides, cross-coupling of phosphonates with tertiary amines, α-alkylation of aldehydes, etc.20 Recently, Yan groups explored the photoredox properties of CsPbBr3 QDs for organic transformation reactions such as C–C, C–O, and C–N coupling reactions.21 CsPbBr3 QDs have been also explored by other groups for facilitating coupling reactions including C–C,22,23 N–N,24 C–S,25 or C–P25 bond formation processes.
On the other hand, the photocatalytic production of amide by the reaction of aldehyde and amine has been explored by different groups.26,27 Amides serve as the foundational components of natural peptides and crucial intermediates in the production of polymers, agrochemicals, and pharmaceuticals.27–30 While simple, the traditional thermal condensation of carboxylic acids and amines is limited to substrate scope because of the harsh reaction conditions.31 Similarly, traditional approaches involving derivatives of activated carboxylic acids and amines, such as the Beckmann rearrangement, Schmidt and Staudinger reaction often result in a substantial waste production.32 This ongoing challenge of achieving efficient amide synthesis while minimizing waste has prompted the exploration of novel photocatalytic strategies from both industrial and academic standpoints.
However, most of the photochemical methods for amide synthesis in the homogeneous medium suffer from drawbacks like costly catalysts, use of non-oxygen oxidizing agents, and limited tolerance towards secondary amines. Simpler methodologies using photocatalysts like phenazine salt, Rose Bengal, and aminoanthraquinone derivatives were found to be promising.26 Further, heterogeneous photoredox catalysts (Ag/g-C3N4, Ni/g-C3N4, Ag2O/P–C3N4, TiO2, and Fe3O4/PDA/CdS) have been demonstrated for oxidative amination of aldehydes.26
In this context, we have explored the photoredox properties of CsPbBr3 QDs for the formation of a series of structurally versatile amides by the reaction of aldehydes and amines. To improve the charge transfer dynamics and access catalytic active sites, [Ni(dmgH)2] was used as the cocatalyst with CsPbBr3 QDs. The combination of [Ni(dmgH)2] and CsPbBr3 produced high efficiency for the oxidative C–N bond formation by the reaction of secondary amines and aldehydes to form structurally diverse amides (highest yield = 92%). The catalyst can be recycled four times with a minimum loss of initial activity. Further, the transient absorption spectroscopic studies have revealed that [Ni(dmgH)2] acts as an electron funnel, enabling fast electron transfer from QDs to the catalytic centers.
In Mott–Schottky (MS) plot, both CsPbBr3 QDs and [Ni(dmgH)2] exhibited positive slopes within the frequency range of 0.5–1.5 kHz, signifying the formation of an n–n-type heterojunction. Based on MS plots, we have determined the flat band potentials as −0.99 eV and −0.59 eV vs. NHE (normal hydrogen electrode) for CsPbBr3 and [Ni(dmgH)2], respectively (Fig. 1b and S4†).38–40 As the conduction band minimum (CBM) of an n-type semiconductor is about 0.1 or 0.2 eV higher than the flat band potential, the CBMs of CsPbBr3 QDs and [Ni(dmgH)2] are determined to be −1.09 eV and −0.69 eV, respectively. The valence band maximum (VBM) of CsPbBr3 QDs is calculated to be 1.19 eV vs. NHE. Therefore, the Fermi level energy (Ef) of CsPbBr3 QDs is high enough to transfer the photogenerated electrons from the CB of CsPbBr3 to the LUMO of [Ni(dmgH)2] (Fig. 1c).41
Photoluminescence (PL) spectrum of CsPbBr3 QDs showed the emission peak at 517 nm (Fig. 1d).42 The emission peak intensity is gradually decreased with the increasing amount of cocatalyst as the introduction of [Ni(dmgH)2] into CsPbBr3 solution significantly reduced the charge recombination process (Fig. 1d and S5†). In [Ni(dmgH)2], the strong π-acceptor conjugation in the ligand backbone facilitates the electron transfer from CsPbBr3 QDs to the cocatalyst, minimizing the charge recombination process.43 The trend in the PL intensity is also reflected in the photocatalytic C–N coupling reaction, where the best activity is observed with 7% [Ni]-CsPbBr3 (Table S4†).
For a more comprehensive understanding, we recorded the PL of [Ni(dmgH)2] as well (Fig. S5†). PL reveals that [Ni(dmgH)2] has emission peaks at 507 nm and 562 nm.38 A slight shift in the two fluorescence peaks was observed in 7% [Ni]-CsPbBr3 compared to pure [Ni(dmgH)2], which can be attributed to the charge transfer from CsPbBr3 QDs to the cocatalyst (Fig. S5†).
Further, the photocurrent response of the catalyst systems was recorded under on/off cycling of the light (Fig. 1e and S6†). Compared to CsPbBr3 QDs, [Ni]-CsPbBr3 photocatalysts showed a substantial improvement in the photocurrent density, and the best photocurrent response was achieved with 7% [Ni]-CsPbBr3, indicating efficient separation of charges after cocatalyst addition. The [Ni(dmgH)2] captures the photogenerated electrons from CsPbBr3 QDs, effectively minimizing charge recombination and enhancing the charge transfer.42 The effective electron transfer was revealed through electrochemical impedance spectroscopy (EIS) measurements (Fig. S7†). The Nyquist plots of [Ni]-CsPbBr3 exhibited noticeably smaller diameters in comparison to that of CsPbBr3 QDs and 7% [Ni]-CsPbBr3 showed the lowest charge-transfer resistance (Rct) value (Fig. 1f).44,45
The HOMO–LUMO energy gap of the cocatalyst was determined using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in the presence of light and in the dark (Fig. 2a and S8†). In ground state, the energy gap between CB of CsPbBr3 QDs and LUMO of [Ni(dmgH)2] is high. In the presence of light, the HOMO of [Ni(dmgH)2] shifts to a more positive potential value while the LUMO moves to a further negative potential. This leads to a reduction in the energy gap between the conduction band potential of CsPbBr3 and the LUMO of [Ni(dmgH)2], facilitating the transfer of the photogenerated CB electrons to the cocatalyst. As a result, O2 molecules are faster reduced to superoxide, improving the rate of photocatalytic C–N coupling reaction (see later).
Fig. 2 (a) Cyclic voltammogram of CsPbBr3 QDs and 7% [Ni]-CsPbBr3 in the dark and light. XPS spectra of CsPbBr3 and 7% [Ni]-CsPbBr3 (b) Cs 3d, (c) Pb 4f, (d) Br 3d. |
The X-ray photoelectron spectroscopy (XPS) study has also shown strong electronic interaction between CsPbBr3 QDs and cocatalyst (Fig. 2b–d). The comparison of Cs 3d XPS of CsPbBr3 QDs and 7% [Ni]-CsPbBr3 showed a positive shift (0.33 eV) in the binding energies of Cs 3d5/2 (723.82 eV) and Cs 3d3/2 (737.79 eV) peaks after the cocatalyst incorporation, indicating electron transfer from the CB of photocatalyst to cocatalyst.
Similarly, Pb 4f and Br 3d XPS exhibited positive shifts in the binding energies in 7% [Ni]-CsPbBr3 compared to that of CsPbBr3. In contrast, a negative shift of Ni 2p3/2 and 2p1/2 peaks was detected in 7% [Ni]-CsPbBr3 compared to that in [Ni(dmgH)2] due to the electron accumulation in the LUMO of the cocatalyst (Fig. S9†).38 The C 1s, N 1s, and O 1s XPS spectra of 7% [Ni]-CsPbBr3 are presented in Fig. S9.† Consequently, these findings provide strong evidence of electron transfer from CsPbBr3 QDs to [Ni(dmgH)2].
Based on the above studies, it is clear that a cocatalyst should possess an optimized reduction potential and improve the photogenerated charge separation and transport process of CsPbBr3 QDs, enabling it to accept electrons from the CsPbBr3 CB and relay the electron to atmospheric molecular oxygen (from air) to form superoxide radicals. It should be mentioned here that CsPbBr3 QDs can also activate molecular oxygen to form superoxide radicals. However, the cocatalyst [Ni(dmgH)2] helps in the relay of the photogenerated electrons from the CB of CsPbBr3 to O2 because of the suitable energy levels and redox potentials.
To understand the process, we have conducted a comparison of the energy levels and reduction potentials of the three components: (i) CB position of CsPbBr3 QDs, (ii) single electron reduction potential of [Ni(dmgH)2], and (iii) reduction potential of O2 to O2˙−. The MS study confirms that the reduction potential of [Ni(dmgH)2] (−0.59 V vs. NHE) is higher enough to reduce O2 to O2˙− (−0.33 V vs. NHE) (Fig. 1c and S4†). As the superoxide radical is responsible for the activation of the aldehyde, the rate-determining step is not the initial photoexcitation or charge injection but the subsequent conversion of O2 to O2˙−. Therefore, the optimized energy levels of the photocatalyst CB and cocatalyst LUMO can enhance the overall photoredox performance.
a Reaction conditions: aldehyde (0.5 mmol), amine (1 mmol), 7% [Ni]-CsPbBr3 (10 mg), THF (2.5 mL), 15 W blue LED, air, temperature: 35 ± 3 °C, time: 18 h. *In the case of 3r, aldehyde and pyrrolidine were taken 0.5 mmol and 2 mmol, respectively. In all the cases, isolated yield of the product was reported and the products were characterized by 1H and 13C NMR (Table S5). |
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In contrast, benzaldehydes having electron-withdrawing groups (–NO2, and –CF3) in the phenyl ring produced high yields, ranging from 82% to 92% (Table 1, 3f–3i). Further, the position of the substituent (ortho-, meta-, para-nitro) in the phenyl ring of benzaldehydes has shown a significant effect on the yield of the product (Table 1, 3f–3h). Even, 4-bromobenzaldehyde produced a good yield of amide (89.5%) under the similar reaction conditions (Table 1, 3e). The reaction of morpholine with different benzaldehyde derivatives produced slightly lower yield of amide than the corresponding piperidine derivative (Table 1, 3j–3l).
The reaction of pyrrolidine with different substituted benzaldehydes produced a wide variety of amides (Table 1, 3m–3p). A similar trend for the substitution in the phenyl ring of benzaldehyde was observed when 5-membered pyrrolidine was used instead of 6-membered piperidine. However, in all the cases, a slightly lower yield (74–89%) was attained with pyrrolidine compared to piperidine because of increased ring strain in the cationic amine radical of pyrrolidine (see later). Interestingly, the process can even tolerate hydroxyl group at the ortho-position of benzaldehyde to produce a yield of 56% of amide (Table 1, 3q). The reaction of terephthaldehyde with pyrrolidine produced 69% yield of 3q (Table 1, 3r). The heterocyclic aldehydes (thiophene-2-carbaldehyde and pyridine 2-carbaldehyde) also produced >80% yield of amides (Table 1, 3s–3t).
A wide variety of amines have also been studied to form amides with moderate to high yield. The effect of the variation of amine (with a similar structure) is not so pronounced in the yield of amides. For example, when 6-membered piperidine was replaced by morpholine, the yield was not significantly affected (Table 1, 3a, and 3k). Similarly, 6-membered piperidine and 5-membered pyrrolidine produced similar yields (Table 1, 3a, and 3m). However, the reaction of 2-oxolidone with benzaldehyde lowered the yield of the amide (Table 1, 3u).
Interestingly, aniline derivatives of pyridine and quinoline (2-aminopyridine, 2-(2-pyridyl)ethylamine, and 7-aminoquinoline) also produced a high yield of amides when reacted with benzaldehyde (Table 1, 3v–x). However, the reaction of aniline with benzaldehyde led to the formation of a mixture of products, which cannot be separated by column chromatography. Even benzylamine produced 72% yield of amide reacting with benzaldehyde (Table 1, 3y). However, the yield of the amide was decreased when aliphatic aldehyde was reacted with benzylamine (Table 1, 3z). Octanal showed only 61% conversion under similar reaction conditions with a 55% yield of amide and 6% yield of N-octylidene-1-phenylmethanamine as a byproduct following the hydrogen atom transfer (HAT) mechanism (Table S5,†3z′).46 A decrease in the yield of amides was also observed when benzaldehyde reacted with open-chain aliphatic amines (Table 1, 3aa, and 3bb).
Further, the catalytic recyclability test was performed four times with a minimum loss of activity (Fig. S10†). The recovered catalyst did not show any change in the UV-visible spectrum (Fig. S11†).
The TA of 7% [Ni]-CsPbBr3 showed similar spectral features for the photobleach bands and the lower wavelength ESA band (PB3, PB4, and ESA3) (Fig. S12h–n†). The excited absorption band, akin to ESA2 in CsPbBr3, was not detected in 7% [Ni]-CsPbBr3. Photo-induced absorption ESA3 appeared at 510 nm while the photo bleach band PB3 was red-shifted (526 nm) compared to that of CsPbBr3 (Fig. S12h–n†). Although several spectral features were observed to be similar in QDs and 7% [Ni]-CsPbBr3, the rates at which the spectra evolve are significantly different. This suggests that the same electronic states are involved but their formation rates are different.
The decay dynamics at different wavelengths are monitored by fitting the kinetic traces to a sum of exponentials. The kinetic trace at 400 nm corresponds to the bleach of the ground state to the higher energy states (Fig. 3a(1)). The 550 nm kinetics belongs to the recovery of bleach of ground state as this wavelength matches the lowest energy band in the steady state spectrum. A negative signal in the TA can be assigned to bleach as well as stimulated emission. The hot electrons undergo intra-band relaxation processes. Had the signal at 390 nm to 430 nm been exclusively from the stimulated emission of hot electrons, then this band would have evolved along with ESA2. However, as noted from TA spectra (Fig. S12†) this blue band exists well beyond ESA2 absorption. ESA2 rises and falls within 800 fs, while the PB2 signatures exist even in the 2 ns spectra. The 7% [Ni]-CsPbBr3 spectra also showed the negative band in the same region, however, the ESA2-type band was not observed. Based on the electrochemical spectroscopic studies, an ultrafast electron transfer from perovskite to cocatalyst can be proposed. If the negative band is originated from the stimulated emission of hot electrons, it should not be detected in 7% [Ni]-CsPbBr3. As the higher energy negative band was detected even in the presence of cocatalyst, we assigned it as a photobleach band.
The comparison of the kinetic traces at 500 nm has shown that the rise and decay of the signal of CsPbBr3 is faster than 7% [Ni]-CsPbBr3 (Fig. 3a(2)). The rise times were calculated to be 98 ± 5 ps for CsPbBr3 and 41 ± 2 ps for 7% [Ni]-CsPbBr3. The recovery of bleach (550 nm) was found to be faster for 7% [Ni]-CsPbBr3 than pristine QDs (Fig. 3a(3)). The same inference was drawn from 400 nm traces as well. The lifetimes at 550 nm decreased from 60 ± 8 ps and 817 ± 165 ps for CsPbBr3 to 31 ± 3 ps and 293 ± 32 ps for 7% [Ni]-CsPbBr3. Similar observations were obtained for kinetic traces at 650 nm (Fig. 3a(4)) and the time constants are listed in Table S3.† The decay-associated spectra (DAS) revealed three ultrafast components for both systems (Fig. 3b and c).47 The global lifetimes are obtained as 0.8 ± 0.01 ps, 60 ± 7 ps, and 758 ± 89 ps for CsPbBr3 and 1 ± 0.01 ps, 39 ± 3 ps, and 530 ± 43 ps for 7% [Ni]-CsPbBr3.
In addition to the above three components, a long-lived component, associated with the charge recombination process, that did not decay within the timescale of our measurement, was also observed (Fig. S13†). The DAS at 0.8 ± 0.01 ps showed a strong positive band (533 nm) corresponding to ESA2. The strong positive band in the DAS at 60 ± 7 ps matched with ESA1. The amplitude at the peak of ESA1 of the 760 ps DAS is stronger than that observed in the 60 ps DAS. This is in line with the observation that the ESA1 becomes stronger over the range of 500 ps and then decays. The DAS has a longer lifetime having a positive band at the same wavelength 506 nm associated with the ESA1 and PB1 bands (Fig. S12†). PB1 is attributed to the depopulation of the ground state, as its peak position aligns with the lowest energy steady-state absorption of the CsPbBr3 (Fig. S12a†).48,49 The 0.8 ps rise in PB1 signal resulting from 370 nm excitation is attributed to the gradual transition of the high energy excitons towards the band edge excitonic state through intra-band relaxation.
The loading of [Ni(dmgH)2] on CsPbBr3 results in faster kinetics in the TAS (Fig. 3d). A faster decay arises from the increased rate of depopulation of the involved states by an excited state process due to the migration of the charge from CsPbBr3 to [Ni(dmgH)2]. This is manifested as a decrease in lifetime in 7% [Ni]-CsPbBr3 and is a reflection of the formation of a trap state from CsPbBr3. Earlier works on CsPbBr3 have also indicated that the exciton bleach in the picosecond to nanosecond time scales serves as a reliable indicator of electron trapping.48,50,51
The electrochemical studies showed that light-induced charge transfer was facilitated from CsPbBr3 to [Ni(dmgH)2] (Fig. 2a and S8†). We take the aid of TA measurements to estimate the rate of this migration. The lifetimes of exciton formation and cooling are affected by cocatalyst. The inverse of lifetime corresponds to the rate constant of the associated process. Hence the difference in rate constants of [Ni(dmgH)2] loaded and pristine CsPbBr3 will serve as a measure of the electron transfer process (eqn S1†). The formation time of the trap state decreases from 60 ± 8 ps for CsPbBr3 to 31 ± 3 ps for 7% [Ni]-CsPbBr3. Substituting these lifetimes yields a rate constant of ket = 1.6 × 1010 s−1. The observed electron transfer rate agrees with the rate constant typically observed in other semiconductor metal systems where electrons have relaxed to the bottom of the conduction band.51,52
In the presence of [Ni(dmgH)2], faster electron transfer from the CB of CsPbBr3 to cocatalyst takes place, followed by the reduction of surface-bound O2, producing superoxide radicals. Considering these findings, we have proposed a mechanism for the photo-induced amide formation reaction (Fig. 3e). Under visible light irradiation, [Ni]-CsPbBr3 is initially photoexcited, promoting an electron from the VB to CB. This photogenerated electron in the CB is relayed to atmospheric oxygen by cocatalyst to form a superoxide radical (O2˙−). Concurrently, the holes (h+) in the VB are transferred to amine (2), resulting in the formation of amine radical cation (2a). The superoxide radical abstract proton from 4-nitrobenzaldehyde forms acyl radical (1a) along with a hydroperoxyl radical (˙OOH).26,53–58 Interestingly, intermediate 1a and 2a were trapped by TEMPO and identified using NMR spectroscopy (Fig. S15a†) and mass spectrometry (Fig. S15b†).59 The radical coupling of intermediates 1a and 2a produces cationic species 3#, which is further oxidized by ˙OOH to final product 3.53,54 As a result, H2O2 was formed as the reduction product of O2. Interestingly, we were able to detect the formation of H2O2 (Fig. S16†).60,61
The higher efficiency of O2 reduction by 7% [Ni]-CsPbBr3 compared to CsPbBr3 was also confirmed (Fig. S16a†). Further, an increase in H2O2 concentration was observed with increasing reaction time (Fig. S16b†). Therefore, it is clear that 7% [Ni]-CsPbBr3 is more effective in the activation of molecular oxygen compared to CsPbBr3. In addition, superoxide radical was also detected by UV-visible spectroscopy (Fig. S17†).62,63 A higher amount of O2˙− formation was also observed for 7% [Ni]-CsPbBr3 compared to the pristine QDs.
Therefore, in this study, we are able to detect the reaction intermediates (1a, 2a) by mass-spectroscopy and NMR, active radical spices (O2˙−) and H2O2 by UV-visible spectroscopy (Fig. S15–S17†). Based on the previous literature, the formation of ˙OOH radicals and 3# has been proposed to complete the catalytic cycle.60–63
Footnote |
† Electronic supplementary information (ESI) available: Experimental details, including synthesis, characterization, spectroscopic and photoelectrochemical measurement, characterization, catalytic studies, 1H NMR, and 13C NMR spectroscopy. See DOI: https://doi.org/10.1039/d4sc03023k |
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