All chemicals used in this study were commercially purchased and used as received. Hydrazine monohydrate (79%), Rhodamine 6G, Chrome-3-carboxaldehyde, 3-Formyl-6-nitrochromone, 6-Chloro-3-formyl-7-methylchromone, 3-Formyl-6-methylchromone, acetonitrile, ethanol, DMSO (d₆), and nitrate salts were used in this experiment. Microwave irradiation reactions were carried out in a CEM reactor. The CEM microwave irradiation system is developed to enhance the capacity to perform reactions under controlled conditions. The reactions were performed safely at a maximum temperature of 100˚C. However, reactions can safely be performed at pressures up to 20 bar and temperatures ranging from 40˚C to 300˚C. ¹H-NMR and ¹³C-NMR spectra were recorded at 400 MHz NMR on a Bruker Avance spectrometer with tetramethyl silane (TMS) as an internal standard and DMSO-d₆ as solvent. MALDI-HRMS analysis was recorded using Bruker 12T solarix FT-ICR-MS. All optical experiments were recorded using Agilent Cary 60 Ultraviolet -Visible (UV-Vis’s) spectrometer and Varian Cary Eclipse fluorescence spectrophotometer respectively.

Rhodamine 6G hydrazide intermediate was synthesized according to Yang’s method [18] . A mixture of Rhodamine hydrazide intermediate (100 mg, 0.219 mmol), Chrome-3-carboxaldehyde (22 mg, 0.0406 mmol) and 2 ml of ethanol was placed in a microwave vessel. The resulting mixture was stirred and placed in a reactor. The reaction vessel was then run under pressure and irradiation at a specific temperature and time shown in Table 1 and then the reaction vessel was cooled in ice bath. The reaction mixture was filtered out and washed three times with cold ethanol. After drying, the solid product was isolated, and obtained a higher yield ranging from 77% - 80%. ¹H-NMR (d₆-DMSO), δ (ppm): 8.59 (s, 1H), 8.41 (s, 1H), 8.00 (m, 1H), 7.90 (m, 1H), 7.71 (m, 1H), 7.60 (m, 3H), 7.50 (m, 1H), 7.00 (d, 1H), 6.40 (s, 2H), 6.26 (s, 2H), 5.01 (s, 2H, -NH), 3.14 (q, 4H, NCH₂CH₃), 1.87 (s, 6H, -CH₃), 1.21 (t, 6H, NCH₂CH₃). ¹³C-NMR (DMSO), δ (ppm): 165.23, 152.07, 151.33, 147.35, 132.31, 129.48, 128.00, 127.01, 123.43, 122.13, 117.79, 104.99, 95.85, 64.96, 55.99, 37.45, 18.53, 17.06, 14.20. HRMS (MALDI): m/z Calcd for RD1 (M + 1): 585.2496; Found: 585.2504.

A mixture of Rhodamine hydrazide derivative (100 mg, 0.219 mmol), 3-Formyl-6-nitrochromone (22 mg, 0.0511 mmol) and 2 ml of solvent was placed in a microwave vessel. The resulting mixture was stirred and placed in a reactor. The reaction vessel was then run under pressure and irradiation at a specific temperature and time as shown in Table 2. The reaction mixture was filtered out and washed three times with cold ethanol. After drying, the solid product was isolated, and obtained a higher yield ranging from 64% - 84%. ¹H-NMR (d₆-DMSO), δ (ppm): 8.68 (s, 1H), 8.50 (m, 2H), 7.90 (m, 2H), 7.62 (m, 2H), 7.00 (m, 2H), 6.27 (s, 2H), 6.10 (s, 2H), 5.11 (s, 2H, -NH), 3.14 (q, 4H, NCH₂CH₃), 1.86 (s, 6H, -CH₃), 1.22 (t, 6H, NCH₂CH₃). ¹³C-NMR (DMSO), δ (ppm): 164, 153, 152, 145, 1321, 130, 127.02, 127.01, 122.43, 120.23, 116.79, 103.99, 94.85, 63.96, 54.99, 36.44, 18.43, 17.05, 14.20. HRMS (MALDI): m/z Calcd for RD2 (M + 1): 630.2347; Found: 630.2364.

A mixture of Rhodamine hydrazide derivative (100 mg, 0.219 mmol), 6-Chloro-3-formyl-7-methylchromone (22 mg, 0.0519 mmol) and 2 ml of ethanol was placed in a microwave vessel. The resulting mixture was stirred and placed in a reactor. The reaction vessel was then run under pressure and irradiation at a specific temperature and time as shown in Table 3 The reaction mixture was filtered out and washed three times with cold ethanol. After drying, the solid product was isolated, and obtained a higher yield ranging from 78% - 84%. ¹H-NMR (d₆-DMSO), δ (ppm): 8.62 (s, 1H), 8.46 (s, 1H), 7.88 (m, 2H), 7.60 (s, 1H), 7.58 (m, 2H), 7.00 (m, 1H), 6.40 (s, 2H), 6.23 (s, 2H), 5.04 (s, 2H, -NH), 3.15 (q, 4H, NCH₂CH₃), 1.89 (s, 6H, -CH₃), 1.22 (t, 6H, NCH₂CH₃). ¹³C-NMR

(DMSO), δ (ppm): 165.23, 152.07, 151.33, 147.35, 132.31, 129.48, 128.00, 127.01, 123.43, 122.13, 117.79, 104.99, 95.85, 64.96, 55.99, 37.45, 18.53, 17.06, 14.20. HRMS (MALDI): m/z Calcd for RD3 (M + 1): 633.2263; Found: 633.2263.

A mixture of Rhodamine hydrazide derivative (100 mg, 0.219 mmol), 3-Formyl-6-methylchromone (22 mg, 0.0439 mmol) and 2 ml of solvent was placed in a vessel. The resulting mixture was stirred and placed in a reactor. The reaction vessel was then run under pressure and irradiation at a specific temperature and time as shown in Table 4. The reaction mixture was filtered out and washed three times with cold ethanol. After drying, the solid product was isolated, and obtained a higher yield ranging from 81% - 89%. ¹H-NMR (d₆-DMSO), δ (ppm): 8.62 (s, 1H), 8.40 (s, 1H), 7.88 (m, 1H), 7.70 (s, 1H), 7.56 (m, 4H), 7.02 (m, 2H), 6.37 (s, 2H), 6.20 (s, 2H), 5.08 (s, 2H, -NH), 3.20 (q, 4H, NCH₂CH₃), 2.40 (s, 3H, -CH₃), 1.86 (s, 6H, -CH₃), 1.21 (t, 6H, NCH₂CH₃). ¹³C-NMR (DMSO), δ (ppm): 165.23, 152.07, 151.33, 147.35, 132.31, 129.48, 128.00, 127.01, 123.43, 122.13, 117.79, 104.99, 95.85, 64.96, 55.99, 37.45, 18.53, 17.06, 14.20. HRMS (MALDI): m/z Calcd for RD4 (M + 1): 599.2652; Found: 599.2655.

Novel green microwave-assisted synthesis methods using ethanol as a solvent for the synthesis of rhodamine 6G-Chromone imines have been established. A total of four rhodamine 6G derived imine derivatives were synthesized using a controlled CEM microwave heating reactor under closed-vessel conditions. The microwave system is equipped with a magnetic stirrer as well as temperature and power controls. Microwave-assisted synthesis of Rhodamine hydrazide with various substituted chromone compounds such as Chrome-3-carboxyaldehyde, 3-formyl-6-Nitrochromone, 6-Chloro-3-formyl-7-methylchromone, 3-formyl-6-methylchromone, and 2-amino-3-formylchromone in ethanol results all target products in high yield, as illustrated in Scheme 1. In the first phase, the intermediate compound was synthesized by microwave irradiation of Rhodamine 6G and excess hydrazine hydrate in ethanol, and in the second phase, the target products (RD1 - RD4) were synthesized by microwave irradiation and condensation of the resulting rhodamine 6G hydrazone and corresponding aldehydes in

1:1 molar ratio in ethanol. The microwave-assisted synthesis results in a green and fast synthesis time, minimum solvent use, easy operation, and scalability. The reaction conditions for the microwave-assisted synthesis of rhodamine 6G imine derivatives RD1 - RD4 are summarized in Tables 1-4. The optimized conditions for each rhodamine 6G-Chromone based imine derivatives were determined by conducting extensive temperature and time studies. It was observed that microwave-assisted reaction with temperature 80˚C afforded the best result with 82% - 89% product yield for 20 - 30 minutes (Tables 1-4, entry 3 and 4). The results indicate that microwave irradiation method accelerates synthesis of Rhodamine 6G-Chromone imine products in minutes. Due to our interest in green chemistry, minimum solvent conditions were mainly exploited. The target products required no rigorous purification, and pure solid products were isolated from the reaction mixture. Therefore, this irradiation method offers an easy and practical access for the synthesis of a series of rhodamine 6G-based imine products. Structural analysis of these rhodamine 6G-Chromone imine derivatives were performed with ¹H-NMR, ¹³C-NMR, and high-resolution mass spectrometry, and all data are in accordance with the proposed structure (Figures S1-S12).

The rhodamine 6G component has been demonstrated as an attractive sensor with emission turn on effect [18] [19] . In this work, chrome-3-carboxaldehyde, 3-Formyl-6-nitrochromone, 6-Chloro-3-formyl-7-methylchromone, and 3-Formyl-6-methylchromone were used to modify rhodamine component, hoping to optimize a suitable structure for sensing application. All the photophysical studies were performed in aqueous acetonitrile in which rhodamine 6G-chromone compounds RD1 - RD4 formed colorless solutions. In addition, the absorption spectra showed no peak above 400 nm due to the predominant ring-closed spirolactum (Figure 1(a)). This was further confirmed by the ¹³C NMR signal around δ 66 corresponding to bridging carbon, C-1 (Figure S2). Similarly, compounds were very weakly fluorescent at 565 nm (λ_ex = 488 nm) in the absence of any analyte due to the predominant ring-closed spirolactum. The fluorescence spectrum of target compounds showed a peak at 565 nm upon the addition of Cu²⁺ corresponding to the delocalization in the xanthene moiety of rhodamine 6G (Figure 2). Several studies show that substituents can affect the optical properties of compounds [20] - [25] . As shown in Figure 2(a), Rhodamine 6G derivative RD1 with no substituent on the chromone unit exhibits excellent selectivity and sensitivity towards Cu²⁺. Rhodamine 6G derivatives RD3 and RD4 were designed to study the effect of an electron-donating group at the chromone ring. These compounds showed less fluorescence emission upon binding with copper (II) ion and some interference from Ni²⁺ and Pb²⁺ ions. The presence of a strong electron-withdrawing group on the chromone unit in compound RD2 can significantly affect the solubility and optical properties. The results indicate different electronic distributions among the compound’s structure have an influence on their optical sensing properties. Furthermore, Job’s plot experiment was used to investigate the stoichiometry of rhodamine-copper (II) ion binding. In an aqueous acetonitrile solution, the rhodamine derivative RD1 is coordinated to copper (II) ion with a 1:1 stoichiometry, Figure 3. As demonstrated in Scheme 2, a monomeric system forms a 1:1 complex, which is a reversible process.