key: cord-1015253-xrzjzjnl
authors: Kumari, Renu; Singh, Man
title: Photocatalytic Reduction of Fluorescent Dyes in Sunlight by Newly Synthesized Spiroindenoquinoxaline Pyrrolizidines
date: 2020-09-01
journal: ACS Omega
DOI: 10.1021/acsomega.0c02976
sha: ce68a8885791185cb9200f1243e91f2668815af5
doc_id: 1015253
cord_uid: xrzjzjnl

[Image: see text] Spiroindenoquinoxaline pyrrolizidines (SIQPs)—7-nitro-2′-phenyl-5′,6′,7′,7a′-tetrahydrospiro[indeno[1,2-b]quinoxaline-11,3′-pyrrolizine]-1′,1′(2′H)-dicarbonitrile (SIQP I), 2′-(4-cyanophenyl)-7-nitro-5′,6′,7′,7a′-tetrahydrospiro[indeno[1,2-b]quinoxaline-11,3′-pyrrolizine]-1′,1′(2′H)-dicarbonitrile (SIQP II), and 2′-(4-methoxyphenyl)-7-nitro-5′,6′,7′,7a′-tetrahydrospiro[indeno[1,2-b]quinoxaline-11,3′-pyrrolizine]-1′,1′(2′H)-dicarbonitrile (SIQP III)—have been synthesized through a one-pot cascade Knoevenagel condensation reaction in acetonitrile (ACN) with 91, 98, and 87% yields, respectively. Structures are characterized by (1)H NMR and (13)C NMR spectroscopy, nuclear Overhauser enhancement spectroscopy (NOESY), Fourier transform infrared (FT-IR) and UV–vis spectroscopy, thermogravimetric analysis (TGA), high-resolution mass spectroscopy (HRTEM), fluorescence and Raman spectroscopy, and energy-dispersive analysis by X-ray (EDX) spectroscopy. SIQPs in ACN photocatalyzed methylene blue (MB) but not phenolphthalein (HIn). SIQPs distinguished the quaternary atoms and dipoles of the fluorescent dye (MB) contrary to the quinonoid HIn structure. In sunlight, SIQPs without electricity input acted as a photonic sensor to detect fluorescent dyes in waste effluents of textile, paper, dyes, and other industries. Activation energy (E(a)), enthalpy (ΔH), entropy (ΔS), and Gibbs free energy (ΔG) calculated from UV–vis absorption spectra show photocatalytic reduction (PCR) activities in the order SIQP II > III > I. The N-atom of pyrrolizidine and −NO(2) of nitro-indenoquinoxaline (NIQ) induced the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) electrodynamics to enable the SIQPs to catalyze biochemical activities.

In nanoscience and technology, multifunctional spirochiral molecules have been in high demand due to their multiple localized chemical environments fitting within the framework of intramolecular multiple force theory (IMMFT) with multiple tentropic activities for efficient photocatalysis, biochemical activities, phase extraction, and others. Spirochirals are abundantly manufactured in nature, but their physicochemical potential like photocatalyzing activities has never been elucidated. Researchers are engaged in developing smart chiral structures with diverse constituents with extraordinary intramolecular activities having remarkable electron spins and rotational, vibrational, and translational motions. These features lead to a shorter band gap in organic chemistry, termed the highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) gap, and allow semiconductor activities, which have never been reported before for reducing the methylene blue (MB) to leucomethylene blue (LMB) dye. For the first time, Huisgen et al. 1 reported a class of spiroheterocyclics through the 1,3dipolar cycloaddition reaction with complicated methods and mechanisms. Spiroheterocyclics play a vital role in biological, medicinal, and materials sciences 2, 3 and in the synthesis of many quinoxalines. Thus, much attention has been paid to the study of spiroheterocyclics due to their unique spirostructures with wider biological and other activities to explore their photocatalytic (PC) potential. Huang et al. 4 synthesized pharmacological agents, but synthesizing spiroheterocyclics with a minimum number of steps in a shorter time using green chemistry has been a major challenge. A literature search of spiroheterocyclics shows that no attempt has been made to synthesize them using green routes despite their huge chemical relevance. Shahrestani et al. 5 synthesized spiroheterocyclics with ortho−para directing groups using 4,5-dimethyl benzene-1,2-diamine as a starting material with ninhydrin. However, they could not introduce a meta-directing (MD) group (to increase activity and sites). Thus, the absence of the MD group limited their solubility and applications. To address this problem and to develop green alternatives, we replaced 4,5- Table 1 . PC Activity Comparison for Reported Spiroheterocyclic Compounds and SIQPs ACS Omega http://pubs.acs.org/journal/acsodf Article dimethyl benzene-1,2-diamine with 4-nitrobenzene-1,2-diamine with ninhydrin for a new series of indenoquinoxalines (IQs). This is in contrast to researchers who conducted the reactions with ninhydrin and derivatives of phenyl 1,2diamines. As per the literature survey, no IQ synthesis with the MD −NO 2 group has been reported yet. Thus, a series of enantiomerically pure, novel chiral heterocyclic spiroindenoquinoxaline pyrrolizidines (SIQPs) with high yields via a onepot, three-component 1,3-dipolar cycloaddition reaction under normal conditions using moieties of highly efficient azomethine ylides have been synthesized. For SIQP synthesis at RT, −NO 2 was introduced in IQ, saving 80% resources with new properties and activities contrary to the IQs reported elsewhere. 6, 7 Introducing the −NO 2 group to IQ, i.e., nitroindenoquinoxaline (NIQ) using methanol (CH 3 OH) and acetic acid (CH 3 COOH) solvents, the SIQPs showed increased affinity toward green solvents with low activation energy (E a ), thus opening alternative ways for their applications. Efforts are initiated to conduct a similar reaction in a 10% aq CH 3 OH and CH 3 COOH medium separately for a greener synthesis. Hence, −NO 2 of NIQ could catalyze the reaction, and it was complete within 3 h, giving ∼87−98% yields. NIQ with an electron-withdrawing group (EWG) extended the structural delocalization that reduced the synthesis time and E a compared to the reported IQ. Saragi et al. 8 have reported basic physiochemical and electronic properties of spiroheterocyclics, and Mahns et al. 9 have reported their electronic properties, but no study has initiated their photocatalytic (PC) activities and vibrational splitting. Hongyan Xia et al. 10 have reported advanced spiropyrans and their applications with fluorescent materials as per fluorescence resonance energy transfer (FRET), but their PC activities have not been discussed yet (Table 1) . 11−14 Barkov et al. 15 reported regio-and stereoselective syntheses of spiroheterocyclics using the 1,3-dipolar cycloaddition reaction. In this study, phenyl (C 6 H 5 −), cyanophenyl (Ph-CN), and methoxyphenyl (Ph-OCH 3 ) were introduced to the pyrrolizidine ring of SIQPs to enhance their interaction and PC abilities (Figures S1−S3). Such a mechanism has never been reported yet. However, Zheng et al. 16 had synthesized a few spiro compounds with restricted solubility but with many drawbacks, which encouraged us to study the solubility and PC of SIQPs. The SIQPs attain manifold physicochemical activities with stabilities in the order of SIQP II > III > I, derived from weight loss from thermogravimetric analysis (TGA). Their λ max was III λ max > II λ max > I λ max . SIQPs act as sensors at the minimum inhibition concentration (MIC) and exhibit similar other activities. Pardasani et al. 17 had reported many chiral molecules to explore photocatalysis, which failed to materialize; thus, we studied the SIQPs with active chiral centers and developed a new experiment to photocatalyze the MB to LMB in visible light (Figures 1−3) and phased out the phenolphthalein (HIn) to develop a liquid−liquid phase ( Figure 4 ). Thus, SIQPs were synthesized with various functional groups. The −CN and −OCH 3 induced hydrophilic and hydrophobic interacting activities, respectively, to widen the solubility needed for drugs, biochemical dispersion, and therapeutic agents. 18 SIQPs with these unique electrophobic and electrophilic constituents have never been reported. NIQ reacted with the derivate of benzylidene malononitrile and Lproline in acetonitrile (ACN) at ∼100°C, while other researchers conducted the same reaction at a higher temperature for a longer period of time. SIQPs with quantized electron clouds lead to an extraordinary electronic transition that synchronize the PC potential. Various studies have synthesized spiroheterocyclics with varieties of chiralities to be used as drugs, but SIQPs act as green photocatalytic potential alternatives of semiconductor nanomaterials like graphene oxide (GO), TiO 2 , and SiO 2 . 20 restricts π electron delocalization due to interrupted hyperconjugation (σ−π bond interactions). The σ−π interactions immobilize the activities of spiroheterocyclics. During IQ synthesis, the −NH 2 groups at 1,2 positions of benzene also hamper π delocalization due to the lone pair electron (LPE) of N-atoms. Comparatively, these electronic orientations and shifts with the σ−π and LPE−π interactions reduced the solubilization and produced a low yield in >60 min, contrary to <40 min for NIQ reported in this work ( Figure S4 ). These electronic restrictions were tuned by placing EWG and −NO 2 at the para position of NIQ to prevent interruption of hyperconjugation. It enhanced the extended delocalization, which photocatalyzed the MB to LMB, contrary to that reported for spiroheterocyclics. The −NO 2 exponentially integrates the electron cloud of quinoxaline that catalyzed the partial charge on ketonic atoms (−C + − O − ) of the indeno part of NIQ. The −C + −O − attracts LPE of N and H of L-proline through Coulombic interactions. Unlike previous methodologies, 20,21 −NO 2 electronically catalyzes azomethine ylide intermediate formation through the 1,3dipolar cycloaddition reaction for SIQP synthesis. Thus, our strategy seems robust to synthesize chiral exponentially active SIQPs as sensors. It could further be improved by replacing Lproline with indoline-2-carboxylic acid, N-Boc-cis-4-hydroxy-Dproline methyl ester, and trans-4-hydroxy-L-proline amino acids for a new series of stable azomethine ylides as a precursor for SIQPs. However, no creative and novel routes of synthesis for extraordinary SIQP structures have been reported yet. A one-pot three-component reaction mechanism was initiated in MeOH, dichloromethane (DCM), and ACN (Table 2) to study the solvent effect on enhancing yield. ACN produced ∼87−98% yields, in contrast to other solvents. A triple bond of ACN (sp C-atom) has fascial orientation, which favors the where S indicates (number of reacting species is 4) benzylidene malononitrile, NIQ, L-proline, and ACN as dispersing agents, P (number of phases) is 1 (homogenous), and R (number of intermediate species) is 1 (azomethine ylide) at 100°C reaction temperature T. Putting the values of S, P, R, and T coordinates, the F becomes

F = 4 depicts that the SIQP synthesis is controlled by four variables, i.e., the compositions of reacting species including the solvent (benzylidene malononitrile, NIQ, L-proline, and ACN) in a 1.0:1.0:1.0:8.0 ratio at constant T for SIQP I, II, and III syntheses with benzylidene malonotrile, cyano, and methoxybenzylidene malonotriles, respectively. The reactions with various compositions were studied at different temperatures, but the yields and durations were haphazard. Based on our reaction variables, the above-mentioned ratios are the most accurate and optimized with minimum time duration and maximum yield for a series of spiroheterocyclic syntheses. The reaction temperature remains constant; L-proline acted as the bridging agent, but 1.0 mmol of l-proline gave 98% yield, in contrast to 65% with 1.5 mmol. MeOH and DCM polar solvents produced a lower yield compared to ACN, which may be due to the poor solubility of the reactant, especially of Lproline. The dipole moments for ACN, MeOH, and DCM solvents show lesser values for DCM and MeOH and greater values for ACN. ACN with an electron-releasing group (ERG) (−CH 3 ) and LPE of the N-atom expeditiously monodispersed the reacting species. ACN as a solvent robustly dispersed and induced a favorable mechanism in a reacting orientation to produce the maximum yield. Therefore, ACN was found to be a better solvent. Higher Δλ values were found for SIQPs I and II (140) and the lowest for III (139 nm). Changes in enthalpy (ΔH), entropy (ΔS), activation energy (E a ), and Gibbs free energy (ΔG) thermodynamic parameters at 298.15 K were obtained from the binding constant. 22 The SIQP molecules haphazardly moved from one point to another as they gained the kinetic energy that generated entropic disorders to favorably orient the reacting species. These orientations align the positive (h + ) and negative (e − ) charges for counterbalancing them in a reduction process at the cost of ΔG (J/mol). The higher entropy is associated with a lower ΔG. It indicates that the enthalpy released on reaction was not fully occupied, and hence it was expressed in terms of entropy. Thus, entropy, enthalpy, and other physical parameters are directly correlated with the PC reduction (PCR).

2.4. Fluorescence Spectra Analysis. Fluorescence emitted wavelength (λ em ) values for SIQPs I, II, and III are 490, 490, and 489 nm ( Figures S5f, S6f , and S7f), respectively, at 350 nm excited wavelength (λ exc ). Fluorescence intensity (I f ), Stokes shift (Δλ), and quantum yield (Φ) for SIQPs were calculated (Table 3) . SIQP III has lower I f and Δλ values compared with SIQPs I and II due to the ERG attached at the para position. The para position of phenyl connected to pyrrolizidine synergizes the pyrrolizidine and NIQ units with maximum HOMO → LUMO populations. ERG is unable to influence, whereas the electron−electron repulsion (EER) of −CN creates electron-deficient sites (EDSs) to compensate the EDS of NIQ and the pyrrolizidine generates maximum oscillations. The EWG and ERG both affect SIQPs II and III as sensors to identify the fluorescent activities. SIQP II with −CN produced a fluorescence intensity (I f ) of 13 995 au, but SIQP III (ERG −OCH 3 ) produced 65 au. Electrons of Table 3 . UV−Vis and Fluorescent Spectral Study of SIQPs in ACN (1. 

and for SIQP III (eq 6), Φ is 

The highest Φ value (71.6%) is seen for SIQP III due to the greater structural rigidity (two LPEs of OMe) ( Table 3) . The I f increases with EWR like −CN. Both I and III, compared to SIQP II, have negligible I f values as the −CN initiates enormous π → π* and n → π* (Figure 7a −c). The solvent affected the absorption maxima (λ max ) and the molar absorption coefficient (ε max ). The high polarity of ACN caused bathochromic shifts of bands π → π* and hypsochromic shifts of bands n → π*, 10 associated with groups capable of binding a free electron pair with hydrogen bonds. SIQPs possessed bathochromic shifts (Δλ = λ 350 < λ em ), strong λ em , and I f (Table 3) , which could widen the optical applications. 23 Thus, the quantum yield, emitted wavelength (λ em ), and I f fluorescence spectroscopy widened our research work to approve and initiate the PC activities with a unique theory, especially with the high I f value of SIQP II compared to I and III. did not affect the ν, proving that the two LPEs of the −O− atom between −CH 3 and the phenyl ring minimized the electron-releasing action due to EER. As a result, SIQP III occupies less surface energy as it has a circular shape. SIQP III may interconnect with its other similar molecular units through van der Waals forces as it is heteroatomic and has a highly asymmetric structure. The electron-rich 2-CN bonding with pyrrolizidine generates EER and electron−nuclei attraction to stabilize the SIQPs by gaining a higher cohesive energy with the least surface energy. Either an electrostatic dipolar interaction between −C + − and N − of the respective −CN groups or their delocalization seems operative. The −CN produced 13 995 au intensity with fluorescence. The Φ C and Φ N that develop Ψ CN through a quantum dot mechanism quantized an electron cloud as it appeared in the HRTEM image ( Figure 6c ). Selected area electron diffraction (SAED) patterns show orbital electron distribution as no specific functional units induce the directing factors to quantize SIQP III. SIQP II seems to delocalize electronic charge throughout the molecular plan and appeared as a threadlike structure. SIQP I has an unsubstituted and electronically optimized spherical shape. Haphazard patterns of SAED depict that SIQP I does not have any set pattern of electron distribution similar to metallic nanoparticles. The constituents of SIQPs are asymmetric as the spiro center atom (SCA) adjoined the basic units of SIQPs (substituted phenyl ring, pyrrolizidine ring, NIQ). The effects of the constituents with different electronic configurations are expressed in SAED. HRTEM images are very similar to optical fibers.

2.6. TGA Analysis. Onset temperatures in TGA for weight loss are the lowest for SIQP III and the highest for SIQP II, which point to the role of ERG and EWG due to the comparatively stronger interactions of −OCH 3 and weaker interactions of −CN, respectively (glass-transition temperatures (T g ) for SIQPs I, II, and III are at 110.55, 204.84, and 92.88°C, respectively) ( Figures S5g, S6g , and S7g). It predicts a smaller stability of SIQP II than SIQP III due to hydrophobic−hydrophobic interactions, which is supported by the photocatalyst properties. The lowest weight loss of SIQP III at 92.88°C shows the stability of its pyrrolizidine ring and SIQP I at 110.55°C, while SIQP II at 204.84°C has the highest weight loss at the first onset temperature due to its transitory crystal structure induced by an additional −CN group as it had developed an extended delocalization. It further opens a window for advanced research by introducing ERGs and EWGs. Nonuniform rodlike surface structures are observed for SIQPs I and III, whereas a randomly oriented larger platelike surface morphology is observed for SIQP II as they reorient and align in a 1D geometry due to the −NO 2 of NIQ and the −CN and −OCH 3 at the terminal of the phenyl ring. Hence, an extended delocalization induced by −CN regulates the overall geometry, which shows their ability to develop a film by maintaining the substrate temperature. Since we explored the −NO 2 position contrary to other studies, it was essential and fundamental to determine the thermal stability and degradation pattern for the study of the photocatalyst activity. TGA analysis was required so that applications could be explored based on temperature. 2.7. Activation Energy (E a ) Calculation. The Arrhenius equation, eq 7, where A is the frequency or pre-exponential factor to determine the activation energy as the UV−vis photons interact to activate SIQP molecules to be adsorbed, is as follows:

where R = 8.314 J/(mol K) is the gas constant, T here is RT, and C is the SIQP concentration that adsorbed the photons. abs is a function of C, so plots of ( 

The ΔH, ΔG, and ΔS are calculated from E a values (Table 4) with eqs 9 and 10.

The 25.99 J/mol E a for SIQP I is higher than SIQPs II and III, which shows its lesser activity with a lower surface area due to centralized delocalization of phenyl−phenyl packing ( Figure  7a− 

Both calculations and experiments were made at constant temperature and atmosphere, respectively, where dT = 0 and dP = 0. Putting these values in eq 11, we get

Both μ and ∂G are in the order III > I > II, despite generating them computationally and experimentally with a UV−vis spectrophotometer, respectively. These are supported by the reverse trends (II > I > III) of ΔS values as per thermodynamic activities (eq 12). These retrieve their intrinsic nature and validate a choice of our adopted research methodologies computationally and experimentally both ( Table 5 ). The −NO 2 , −CN, and N constituents develop hydrogen bonding with water, which further photocatalyzed π → π* and n → π* transitions to produce the h + and e − holes. The holes overcome the HOMO−LUMO energy gap to enhance the PC activities. The solvent surrounds the MB to enhance the permittivity of holes to S + − and Cl − to reduce the MB to LMB. The photons of sunlight strike the LPE of the SIQPs that generate the h + and e − holes. The e − holes approach the −S + (Cl) chemical constituent of the MB as e − + −S + (Cl) → −S− + Cl − (reduction step), and the h + holes approach the Cl − ion as 2Cl − + 2h + → Cl 2 (oxidation step). The reduction of −S + (Cl) produced the colorless MB. From reduction and oxidation steps, turning blue to colorless MB indicated the scavenging or reduction of Cl − into Cl 2 gas. The Cl 2 gas generated a pressure in a lid-fitted reaction vessel, resulting in opening of the lid due to Cl 2 generation. This experiment was repeated several times to ensure lid opening by Cl 2 generation. The Cl 2 production was also chemically analyzed by bringing a moistened NH 4 OH glass rod near the open mouth of the reaction vessel. It produced white dense fumes and a white precipitate on a glass rod as the NH 4 OH + Cl 2 → NH 4 Cl↓ + HCl↑. The operating string is materialized in situ where the SIQPs were highly monodispersed along with the monodispersion of MB. The SIQPs released the h + and e − holes visa-vis MB. The e − holes reduced −S + (Cl) into colorless −S− MB. Therefore, it was the Cl − ion that acted as a reducing radical for the reduction reaction in sunlight. Also, HIn did not produce the PCR because it has no Cl − ion in its molecular structure. Therefore, the generation of the Cl − radical reduced the MB, but no color change was noticed with HIn. It constitutes an advanced greener and nanoreduction model. 24 Protic solvents like CH 3 OH, n-butanol, and nitromethane could exponentially catalyze photocatalysis. 2.8. Mechanism of Photocatalysis. With increasing concentrations, the SIQPs linearly increased the absorption without blue or red shifts, which confirms the nondegradation of SIQP in sunlight ( Figure S5e ). The increase in abs is perfectly fitted in the reaction abs = εcl. First, we studied the SIQPs PC effect in the absence of a dye with ACN, aq ACN, MeOH, and aq MeOH solvents, which resulted in no change. It can be explained by the availability of LPE in the solvent itself, which did not induce adequate transitions. Thus, no h + and e − hole formation occurred and no reduction occurred with these solvents. Hence, there was no change in PCR in the absence of MB. Since the SIQPs (1.5 ppm) and MB (18 ppm) were taken at fixed concentration for PCR study. The PCR experiments were conducted in sunlight, where the photons were exposed to SIQP molecules, since with time, the molecules acquired E a and explored their PC activity. Therefore, with time, the molecules of SIQPs get activated, which generates the h + and e − holes to reduce the MB. Therefore, as per the law of mass action, the concentration did not increase with time. Thus, the molecules of SIQPs that were considered are fully activated with time and reduced the MB. The PC kinetics with MB reduction was studied. The concentrations of both SIQPs and MB were fixed so that each molecule becomes activated and participated in the redox cycle as a = a o e −E a /k B T , where "a" is concentration at time t, "a 0 " is the initial concentration, k B is the Boltzmann distribution constant, T is the reaction temperature, and E a is the activation energy. Had their concentration been increased, then there would have been the lowest activation of these molecules in fractions. For PCR kinetics, the 1.5 ppm SIQPs and 18 ppm MB were separately prepared in ACN. Next, 0.01 mL of MB was added to each sample solution of SIQPs at RT. The UV− vis absorption (λ max ) and images were separately recorded at initial and final PC reductions. In sunlight, initially, the intense blue color of MB with SIQP II was reduced to a pale yellow within ∼10 min, and then it became colorless, whereas the disappearance of the blue color with SIQPs I and III took a longer time. SIQPs on absorbing sunlight generated h + and e − holes, which interacted with −S +  and Cl − species of MB. LPE of the N-atom of −NO 2 in NIQ had further repelled out the sp 2 electrons to expeditiously develop the e − holes on electron−electron repulsion (EER) induced by the LPE of the N-atom as per the Born−Oppenheimer approximation discussed below. The PC rate of SIQP II for MB reduction is greater than that of SIQP III (Table 5 ) as no such EWG mechanism exists with SIQPs I and III, so its sp 2 electrons visa-vis the redox cycle may not promote the valance-band (VB)to-conduction-band (CB) shift expeditiously. SIQPs further quantize the sp 2 electrons to reach the e − holes due to the LPE of the N-atom of NO 2 . Had there not been LPE, the sp 2 (13) where ζ VB-CB is the ζ potential, ε (e − )×(h + ) is the dielectric constant, ε medium is the medium permittivity, η SIQP−MB is the shear stress, and μ (e − )×(h + ) is the electron mobility. Equation 13 reflects the dynamics of h + and e − holes vis-a-vis their generation, mobility, solvent contribution, alignment, and interactions with S + − and Cl − of MB as oxidizing agents. The solvents play a critical role in establishing a higher medium permittivity to shift the respective holes from SIQPs to MB polar species. The LPE of 2N atoms of NIQ inhibits the delocalization of sp 2 electrons so that they move to initiate the sp 2 → sp 3 (h + to e − ) holes and then orient toward the −S +  and Cl − , respectively, as a medium has moderate shear stress. Rotational, vibrational, and transitional motions of the pyrrolizidine due to SCA seem to orient the NC−Ph− and NC− toward quinoxaline and indeno units, which further catalyze the sp 2 → sp 3 (h + to e − ) holes. The ζ VB−CB promotes the electrokinetics of h + and e − holes despite the Fermi energy. The shear in GO for h + and e − holes is higher that lowers the value of μ (e − )×(h + ) from sp 2 → sp 3 . Thus, the frequencies of the redox cycle, i.e., sp 2 → sp 3 for h + to e − holes and vice versa for SIQP II, are more (Scheme 1) due to the higher μ (e − )×(h + ) from sp 2 → sp 3 electron clouds. The MB photocatalytic initiation and efficiency depend on a mutual shift of quantized electron clouds. Thus, −NO 2 further creates an efficient sp 2 → sp 3 transfer mechanism to quickly reorient the holes vis-a-vis the sp 2 → sp 3 shift of NIQ. The −NO 2 promotes photocatalysis. sp 2 → sp 3 and sp → sp 2 populations are generated, which greatly support the VB → CB shifts for expeditious MB reduction, which are missing in GO. The SIQPs supersede the use of GO by acting as a green photocatalyst for fluorescent dye reduction. 25 SIQP I with the phenyl ring reduced the MB by 60%, as compared to 90% by SIQP II with 90−60 = 30% more MB PC reduction. SIQP II with sp 2 → sp 3 and sp → sp 2 electronic shifts generated exponential holes due to a terminal CN. The MB photocatalysis with SIQP III is 70%, which a lower than the reduction with SIQP II. Thus, compared to ERG, EWG increased photocatalysis ( Table 5 ) as EER of ERG inhibits the expeditious shift from sp 2 → sp 3 , which lowers the photocatalysis by 90−70 = 20%. These shifts are prompt but in GO are generated by exfoliating its sheets through sonication, which weaken its intersheet van der Waals forces. The functional defects or edges that generate sp 2 → sp 3 are mechanically induced in GO in contrast to being naturally available in SIQP structures. Functional edges of GO are induced by exfoliating GO sheets on sonication using suitable solvents. However, SIQP II does not require sonication to generate such functional edges (Figure 7b ). These edges already exist in SIQP structures and act as superactive sites with their intrinsic intramolecular entropy noted as tentropy.

The moment photons strike these superactive electronic sites, sp 2 → sp 3 The −S +  and Cl − PC active ionic constituents of MB differently respond to the UV− vis light vis-a-vis h + and e − holes. Similar to GO, the SIQPs upon absorbing photons from solar radiation produced h + and e − holes, which were transferred from their conduction band (sp 2 ) to valence bands (sp 3 ) (Scheme 1). As per Le Chatelier's law, the h + and e − holes interact with −S +  and Cl − species, respectively, to counterbalance their charges within the Lennard-Jones potential that reduce the MB. Patterns of UV−MB interactions with SIQP II effectively reduced the MB in ACN as ACN brings the SIQP II and MB together. The UV−vis absorption is reduced to a negative value as the electronic transition of SIQP II is diverted to interact with electronic transitions of MB. Thus, before reaching LUMO states, the electrons are captured by MB so the MB undergoes a reduction as ACN electronically exists as CH 2 + −CN − . The MB synchronizes and reorients within the SIQP semiconductor mechanism, which differs from GO as it has a hexagonal structural sheet bound together with van der Waals forces. The GO expresses the symmetric h + and e − hole alignments to develop a double layer with positive and negative charges with the ζ potential, but SIQPs do not have symmetric sheet arrangements, so the h + and e − hole alignment may not be symmetric. Thus, the electron−electron cyclic attraction could be disrupted and SIQPs of electrons move toward MB to reduce the S + , which is counterbalanced by h + as they move toward Cl − to reduce MB (Scheme 1, step 3). SIQPs have manifold nucleophilic domains, which result from the electrophilic domain of MB. These domains also support the lowest absorption as these electrophilic and nucleophilic domains of MB and SIQPs, respectively, bring them together through Coulombic interactions reported in the following equation15 

The Coulombic mechanism results in a closer distance, where h + and e − reached S + and Cl − , respectively. ACN with 3.92 D dipole moment acted as a dispersion medium ( Figure S8i ,ii). The solvent engages the H + , which was produced due to absorption of the photon via an electron-release mechanism. The solvents created extra spontaneity to engage the H + . The H + -release mechanism is supported by SIQPs. Since PCR was conducted in sunlight in November 2019 from 11 am to 1 pm, the samples were kept in an open box with a rigid body so that air fluctuations were minimized and solar radiations in almost equal amounts with time were absorbed by the sample that tuned the intensity. PCR under similar experimental conditions with similar stoichiometries of SIQP and MB was conducted at the same time in sunlight. Each experiment generated h + and e − holes, which reduced a similar amount of MB with ±0.1% variation. Hence, the reproducibility of PCR was determined by conducting authentic experiments that reproduced the same results.

2.9. Adsorption Activity of SIQPs with MB. The MB reduction rate by SIQPs is determined with eq 16, 25 and the highest rate with SIQP II was found ( (17) where C o is the initial MB concentration at time t = 0, and C t is the reduced MB concentration at time t min.

The % MB reduction by SIQP II in sunlight for 30 min in ACN is MB reduction to transform it into LMB has the order SIQP II (85.5) > SIQP III (80.9) > SIQP I (41.3%); their rates are calculated in terms of percentage (%) ( Table 5 ). The higher % of MB reduction of SIQP II compared to SIQPs I and III in a shorter duration made it more effective. It can be explained by the illustrated mechanism. The photons interact with the sp 2 electron orbitals of SIQPs forming h + and e − holes, i.e., from valance (VB) and conduction (CB) bands, respectively. These holes tend toward MB to induce its sp 2 electrons, which reduce the MB to LMB. Therefore, other molecules of MB dyes could easily be reduced to their leuco state. The process was conducted in ACN, aq ACN, and aq EtOH (variable dipole moment) solutions with different reduction powers (Chart 1), where H 2 , O 2 , and Cl 2 gases were formed along with MB reduction to LMB (colorless). High polarity of the solvent medium hindered the PCR due to the resistance faced by strong dipolar interactions of h + and e − holes. Thus, the polarity and PCR are inversely proportional. SIQP II is better than SIQPs I and III for the PC studies based on the rate of reaction for photocatalysis (% MB reduction). Since the MB has quaternary N, it was extraordinarily active and highly sensitive toward light. However, SIQPs I, II, and III produced electrons (e − ), which reduced the MB. So these activities tend to stabilize the MB via reduction. It is observed that the solution is monodispersed and is not agglomerated in the solution, and it does not consume oxygen in the solution. It is merely participating in the interacting modes with the chemical process in the reaction medium. If there is an adequate amount of MB to consume the released e − holes, then there is no effect of the presence of oxygen because oxygen transfers into the oxide ion with e − holes, which consumes more E a compared to MB. However, the situation is different when the MB is in less amount and oxygen is present; then, excess e − holes may react as O 2 + 4e − → 2O 2− ; 2O 2− → O 2 + 4e − . This depends on the concentration of MB. Since h + and e − holes are active and the oxygen gas molecules also respond to the freely available e − holes, when the MB is completely reduced, then e − approaches the oxygen gas. To balance the h + and e − holes, the O 2 → O 2−

and O 2− → O 2 redox cycles keep going on. Hence, SIQP is fully balanced with h + and e − holes. There is the possibility that ACN generates CH 2 CN − and H + and develops the neutral H 2 O molecule as 2H + + O 2− → H 2 O. However, it responds to pH and the chemical potential. Each reacting molecule individually is dispersed in the solvent without undergoing coagulation or coalescence vis-a-vis the solvent or medium. Thereby, a reduction prominently occurs rather than a coagulation so the SIQPs remained monodispersed, which facilitates kinetic orientation to align the reacting species favoring a reaction. Therefore, any salt of monovalent, divalent, trivalent, or transition metals if added could induce robust PC activities for MB reduction. After conducting valid experiments for PCR of MB, a similar PCR experiment setup was extended for HIn. However, it was not reduced as it does not have any quaternary atom in its structure, which could accept the h + and e − charges individually. In fact, with time, HIn developed two liquid phases and it remained dispersed in the bottom layer. Thus, the usability of SIQP in sunlight is for those dyes that have quaternary atoms to furnish the dipoles to be neutralized by charges of holes generated by SIQP (Scheme 2). Therefore, in terms of PC activities and efficiency, SIQP favorably generated h + and e − holes; these holes by SIQPs with photons photocatalyzed the MB to LMB through the redox cycle. These activities electronically transform the MB to colorless LMB. Thus, the reduction of MB using the generated h + and e − holes is called PC efficiency, i.e., the quantum yield for reducing the % of MB to LMB. Thus, the PC activity leads to PC efficiency.

2.10. Raman Analysis. Vibrational frequencies (ν) and intensity of Raman spectra elucidate the molecular structure ( Figure 8 ). Raman frequencies for SIQP I are 1129 cm −1 (ν(CC aromatic ring stretching)) and 1508 (ν(NO)) as well as intensity shifts for D and G bands are 81 127 au, 1506 cm −1 and 78 191 au, 1514 cm −1 , respectively. Raman frequencies for SIQP II are 1487 (ν(CC aromatic ring stretching)), 1654 (ν(NO)), 1958 (ν(CN)), and 2052 cm −1 (ν(CN)) as well as intensity shifts for D and G bands are 285 523 au, 1654 cm −1 and 279 899 au, 1658 cm −1 , respectively. Raman frequencies for SIQP III are 1181 (ν(CC aromatic ring structure)), 1314 (ν(C−O)), 1564 (ν(NO)), and 2222 cm −1 (ν(CN)) as well as intensity shifts for D and G bands are 44 955 au, 1555 cm −1 and 44 028 au, 1566 cm −1 , respectively ( Figure 8 ). SIQP II has the highest Raman intensity due to para-substituted EWG like −CN compared to SIQP III with ERG like −OMe, and hence it is a Raman-active compound. Thus, the −CN group acts as a sensor to depict a population of phonon generations contrary to phenyl and −OMe. Thus, SIQP II may be categorized as an upconversion nanoparticle (UCNP). The Raman intensities for SIQPs I and III are comparable due to their hydrophobic nature (Chart 2). The functional-group-dependent Raman intensity differentiates the nature of SIQPs to act as sensors. The D and G bands like GO showed disordered and synergistic structures, respectively. The intensities of the D and G bands have ratios 1.8:6.3:1 and 1.7:6.5:1 ( Table 6) , showing that the phenyl ring alone and the −CN at the para position (SIQP II) induced the maximum electronic disorder compared to the para-OMe in SIQP III. Comparatively, the lower electronic disruption of SIQP III depicts the EER due to −OCH 3 as ERG and the −O-atom that holds the two LPEs. The noticeable impacts of phenyl, −CN, and −OCH 3 at the para position have almost a similar ratio of intensities. The G band frequencies are comparatively higher than the D ones as the disruption of the electronic structure inhibits the phonon generation and its frequency. Since the G band depicts the reordering, its electronic transition optimizes the frequency and hence a higher frequency indicates the same. Raman studies confirm the impact of −CN at the para position of phenyl of pyrrolizidine and −NO 2 of NIQ because their EWG SIQP II substantially catalyzed the semiconductor h+ and e − hole-generating abilities. Thus, h + and e − hole generation with EWG at terminal positions of NIQ might have affected the Fermi energy (E F ) level ( Figure S9 ) with a shorter HOMO− LUMO gap (band gap) in the case of specially SIQP II. The Fermi−Dirac distribution (eq 20) 20) where E is the energy, T is the temperature, k B is the Boltzmann distribution constant, and ν Raman is the Raman frequency. Using the equation in Table 6 The E − E F value is negligible (eqs 20 and 20a−c; Table 6 ) for SIQPs, and hence the semiconductor mechanism effectively works for PCR conjugated and quaternary atom holding dye but not for dyes that lack a quaternary atom in structures like HIn and so on (Scheme 2). The mechanistic path followed by SIQPs for acting as reliable sensors to distinguish and separate conjugated dyes with quaternary atoms in their structures, e.g., MB, by photocatalytic reduction was studied (Scheme 3a and Table 7 ). These SIQPs also separated dyes like HIn, which do not have a quaternary atom in their structures (Scheme 3b and Chart 3). The SIQPs may minimize the challenges of global warming, resulting from a mixture of fluorescent dyes present in industrial effluents, whereas the reduced dyes do not cause global warming. The scope of SIQP was further extended by engineering the probable structure of NIQ as depicted in Table  8 with the derivatives of 1,2-diaminebenzaldehyde for the probable products. The IQ unit could be modified by substituting EWG groups for synthesizing the products in desired yields (2a) ( 

Novel photocatalyzing chiral SIQPs have been synthesized via a one-pot three-reacting species with 87−98% yields. 1 H NMR, 13 C NMR, NOESY, FT-IR, UV−vis, fluorescence, Raman spectroscopy, liquid chromatography-mass spectrometry (LC-MS), TGA, HRTEM, and energy-dispersive analysis with X-ray spectroscopy (EDX) processes have confirmed the structures and relation with photocatalytic activity. E a , ΔH, ΔS, and ΔG thermodynamic parameters calculated from UV−vis spectrophotometric absorption established their optical and interacting activities in various solvents. SIQPs photocatalytically reduced the fluorescent MB dye but salted out the HIn to a newly developed liquid phase. Our PC methodology is being extended to several other fluorescent dyes like malachite green, mitosensor green, octadecyl rhodamine B chloride, and rhodamine 123. SIQPs act as an adsorbent for several toxic heavy metals in industrial waste effluents and various techniques. 27 In place of the phenyl ring, several other superactive molecules could be used to explore competent constitutional units and in vitro activities for biological applications 28, 29, 26 in the future.

4.1.. Materials. Materials used in this study were thin-layer chromatography (TLC) plates, analytical-grade hexane (Sigma-Aldrich, ≥99%), ethyl acetate (Sigma-Aldrich, ≥99%), dichloromethane (DCM) (Sigma-Aldrich, ≥99.8%), dimethylformamide (DMF) (Sigma-Aldrich, ≥99.8%), acetone (Sigma-Aldrich, ≥99.9%), MeOH (Sigma-Aldrich, ≥99.9%), and ACN (Sigma-Aldrich, ≥99.8%). Solvents were redistilled and used. Benzaldehyde, para-cyanobenzaldehyde, paramethoxybenzaldehyde, ninhydrin, malononitrile, lithium bromide (LiBr), acetic acid (CH 3 COOH), methanol (CH 3 OH), L-proline, magnetic beads, and Whatman filter paper were used as received.

4.2. Characterization Methods. Structures were analyzed with 1 H NMR, 13 C NMR, and NOESY (mixing time 0.5 s) (500 MHz, Bruker Avance spectrometer) in CDCl 3 and dimethyl sulfoxide (DMSO)-d 6 at 500 and 125 MHz using tetramethylsilane (TMS) as the internal standard; FT-IR spectra from 200 to 800 cm −1 with KBr pellets on the PerkinElmer TL8000 TG-IR interface; mass spectra on an Agilent Technology 6545 Q-TOF LC-MS mass spectrometer operating at 70 eV; UV−vis spectra from 190 to 1100 nm with UV-1800 SHIMADZU (UV spectrophotometer) in the electrospray ionization (ESI) mode; high-resolution transmission electron microscopy (HRTEM) with a JEOL JEM-2100 electron microscope at 200 kV operating voltage; thermal gravimetric analysis (TGA) with an intercooler PerkinElmer TGA-6000 thermometer for ∼25−500°C; fluorescent spectra with Edinburgh Instruments Mark McCallum (λ max = 200−800 nm, counts 0−10 7 ); and Raman spectroscopy and EDX with EDX QUANTA FEG 250 SEM.

4.3. Central Theme for the Interacting Mechanism and Activities. SCA interconnects NIQ and pyrrolizidine together through covalent bonds for SIQP synthesis. Pyrrolizidine (2-phenylhexahydro-1H-pyrrolizine-1,1-dicarbonitrile) was prepared by mixing 2-benzylidene malononitrile and L-proline with NIQ. 4-Cyano and 4-methoxy 2benzylidene malononitriles separately replaced 2-benzylidene malononitrile for SIQPs II and III. SIQPs with a unique electronic configuration and favorable HOMO−LUMO gap Table 8 . Reaction Scope (Scheme 7) a a Reaction conditions: All reactions were carried out with ninhydrin (10 mmol, 1.0 equiv, 1.8 g) (Scheme 7) by stirring in a solvent mixture of CH 3 COOH (10 mL) and CH 3 OH (30 mL) with a 1:3 ratio for 30 min at RT. Isolated yields are reported for 2a. (Table S1 ), 4cyano-benzaldehyde (100 mmol, 1.0 equiv, 13.1 g) (Scheme 5) (Table S2) , 4-methoxy benzaldehyde (100 mmol, 1.0 equiv, 13.6 mL) (Scheme 6), and malononitrile (100 mmol, 1.0 equiv, 6.6 g) (Table S3) with LiBr (catalytic amount) in DMF solvent were separately taken in RB and stirred for 1 h at RT. This synthesis did not work with NaBr but worked with the LiBr catalyst due to its smaller cationic size. Starting materials remained unreacted with NaBr (studied from TLC). The main reaction was monitored with TLC, and the product was filtered and dried under vacuum. Structures were analyzed with 1 H NMR.

4.5. Synthesis of 7-Nitro-11H-indeno[1,2-b]quinoxalin-11-one (NIQ). 4-Nitrophenylene-1, 2-diamine (11 mmol, 1.1 equiv, 1.7 g) (Scheme 7) and ninhydrin (10 mmol, 1.0 equiv, 1.8 g) were stirred in a CH 3 COOH (10 mL) (Figures S6a−g, 5b, and 6b) and CH 3 OH (30 mL) mixture for 30 min at RT (Table S4) . The reaction was monitored by TLC. A pale-yellow product was obtained, which was filtered and washed with CH 3 OH (unreacted CH 3 COOH was washed out with chilled water) and dried under vacuum. Structures were analyzed with 1 H NMR.

Note: The solvent system in a 1:3 ratio of CH 3 COOH (10 mL) and CH 3 OH (30 mL) was used to maintain the pH to maximize the yield. Here, CH 3 COOH acted as a source of H + to remove water. (Table S5) , 2-(4cyanobenzylidene) malononitrile (1.0 mmol, 1.0 equiv, 131.1 g) (Scheme 10) (Table S6) , and 2-(4-methoxybenzylidene) (Scheme 11) (Table S7) were separately added to NIQ (1.0 mmol, 1.0 equiv, 277 g) and L-proline (1.0 mmol, 1.0 equiv, 126.5 g) in ACN (10 mL) solvent. Reacting mixtures were stirred and refluxed for 3 h. The reaction was monitored by TLC on aluminum plates coated with silica gel-G F 254 (ACME) with 0.25 mm thickness and visualized with UV− vis light and iodine. Excess solvent was removed by the rota evaporator. The crude product was washed with DCM and lime water (∼3 times) for purification and isolated by column chromatography (ACME, 60−120 mesh) with 70% petroleum ether + 30% ethyl acetate eluting mixture and then dried under a vacuum. Products SIQPs I, II, and III were synthesized. The condensation reactions depicted in Schemes 9−11 proceeded linearly with time, and since no undesired product was developed, they were first-order reactions. The PC undergoes the semiconductor mechanism. The MB reduction was directly proportional because h + and e − holes are produced continuously with time until SIQPs saturate vis-a-vis the photon-receiving ability.

■ ASSOCIATED CONTENT * sı Supporting Information

The Supporting Information is available free of

Email: mansingh50@hotmail.com; Fax

1,3-Dipolar Cycloadditions. Past and Future

Review of Synthesis and Various Biological Activities of Spiro Heterocyclic Compounds Comprising Oxindole and Pyrrolidine Moities

Efficient and Regioselective Synthesis of Novel Functionalized Dispiropyrrolidines and Their Cytotoxic Activities

Cycloaddition Reaction for the Synthesis of Spiro[Indoline-3,3′-Pyrrolidines] and Evaluation of Cytotoxicity towards Cancer Cells

Asymmetric Synthesis Approach of Enantiomerically Pure Spiro-Indenoquinoxaline Pyrrolidines and Spiro-Indenoquinoxaline Pyrrolizidines

Dipolar Cycloaddition: Regio and Diastereoselective Synthesis of Spiropyrrolidinyl Indenoquinoxaline Derivatives

One-Pot Diastereoselective Synthesis of New Spiro Indenoquinoxaline Derivatives Containing Cyclopropane Ring

Compounds for Organic Optoelectronics

Electronic Properties of Spiro Compounds for Organic Electronics

Synthesis and Fluorescent Properties of Novel Isoquinoline Derivatives

III) (Figures S7a−g, 5c, and 6c) Synthesis of Spirooxindole-Pyran Derivatives in Aqueous Ethyl Lactate

One-Pot, Catalyst-Free Synthesis of Spiro[Dihydroquinoline-Naphthofuranone] Compounds from Isatins in Water Triggered by Hydrogen Bonding Effects

Base-Free and Aqueous Synthesis of Quinolin-2(1H)-Ones under Ambient Conditions

Synthesis of Spiro-and Fused Heterocycles via (4+4) Annulation of Sulfonylphthalide with o-Hydroxystyrenyl Derivatives

Regio-and Stereoselective 1,3-Dipolar Cycloaddition of Indenoquinoxalinone Azomethine Ylides to β-Nitrostyrenes: Synthesis of Spiro

The Use of Spirocyclic Scaffolds in Drug Discovery

Theoretical and Synthetic Approach to Novel Spiroheterocycles Derived from Isatin Derivatives and L-Proline via 1,3-Dipolar Cycloaddition

Pyrrolidinyl-Spirooxindole Natural Products as Inspirations for the Development of Potential Therapeutic Agents

Synthesis of the Functionalized Spiro

Asymmetric Synthesis Approach of Enantiomerically Pure Spiro-Indenoquinoxaline Pyrrolidines and Spiro-Indenoquinoxaline Pyrrolizidines

An Expedient Sequential One-Pot Four Component Synthesis of Novel Steroidal Spiro-Pyrrolidine Heterocycles in Ionic Liquid

Situ Growth of Low-Dimensional Silver Nanoclusters with Their Tunable Plasmonic and Thermodynamic Behavior

Synthesis and Photophysical Characterization of Several 2,3-Quinoxaline Derivatives: An Application of Pd(0)/PEG Nanoparticle Catalyst for Sonogashira Coupling

Extra Elements Detection in Organic Compounds by Nonbreakable Sodium Ignition Apparatus (NOSIA)

Metallic Sulfide Nanoparticles Anchored Graphene Oxide: Synthesis, Characterization and Reduction of Methylene Blue to Leuco Methylene Blue in Aqueous Mixtures

Photocatalytic Degradation of Methyl Orange by Magnetically Retrievable Supported Ionic Liquid Phase Photocatalyst

Optimization and Characterization of NiO Thin Films Prepared via NSP Technique and Its P-N Junction Diode Application

Key Green Chemistry Research Areas from a Pharmaceutical Manufacturers' Perspective Revisited

A Facile One-Pot Construction of Succinimide-Fused Spiro[Pyrrolidine-2,3′-Oxindoles] via 1,3-Dipolar Cycloaddition Involving 3-Amino Oxindoles and Maleimides

Complete contact information is available at: https://pubs.acs.org/10.1021/acsomega.0c02976

The authors declare no competing financial interest.

The authors are thankful to the Central University of Gujarat, India, for infrastructural support; CSIR Bhavnagar for EDX, fluorescent, and mass spectroscopy; SNU for Raman spectroscopy; and specially thankful to Nakul Kumar, a research scholar, SCS, Central University of Gujarat.

Renu Kumari − School of Chemical Sciences, Central University of Gujarat, Gandhinagar 382030, India