Base-Free Synthesis and Photophysical Properties of New Schiff Bases Containing Indole Moiety

Abstract

Schiff bases represent an essential class in organic chemistry with antitumor, antiviral, antifungal, and antibacterial activities. The synthesis of Schiff bases requires the presence of an organic base as a catalyst such as piperidine. Base-free synthesis of organic compounds using a heterogeneous catalyst has recently attracted more interest due to the facile procedure, high yield, and reusability of the used catalyst. Herein, we present a comparative study to synthesize new Schiff bases containing indole moieties using piperidine as an organic base catalyst and Au@TiO 2 as a heterogeneous catalyst. In both methods, the products were isolated in high yields and fully characterized using different spectral analysis techniques. The catalyst was reusable four times, and the activity was slightly decreased. The presence of Au increases the number of acidic sites of TiO 2 , resulting in C=O polarization. Yields of the prepared Schiff bases in the presence of Au@TiO 2 and piperidine were comparable. However, Au@TiO 2 is an easily separable and recyclable catalyst, which would facilitate the synthesis of organic compounds without applying any hazardous materials. Furthermore, the luminescence behavior of the synthesized Schiff bases exhibited spectral shape dependence on the substituent group. Interestingly, the compounds also displayed deep-blue fluorescence with Commission Internationale de l’Éclairage (CIE) coordinates of y < 0.1. Thus, these materials may contribute to decreasing the energy consumption of the emitting devices.

Ag@TiO2 Nanocomposite as an Efficient Catalyst for Knoevenagel Condensation

“…Figure 2 c shows the high resolution (HR)TEM image, where the d spacing of 0.23 and 0.34 nm were attributed to the d spacing of Ag (111) and anatase TiO 2 (101), respectively. 19 , 42 The polycrystallinity of Ag@TiO 2 due to the presence of Ag, anatase TiO 2 , and rutile TiO 2 nanoparticles was also indicated from the SAED image illustrated in Figure S2 . The EDX mapping of TiO 2 and Ag(5%)@TiO 2 components is illustrated in Figure S3 , where the distribution of titanium, oxygen, and silver elements is shown.…”

“…The FTIR spectra of the as-prepared and recycled Ag@TiO 2 catalyst are illustrated in Figure S5 , where the absorption bands at 1400 and <700 cm –1 were ascribed to Ti–O–Ti (TiO 2 ). 19 , 69 Bands at 1615 and 3413 cm –1 were attributed to the absorbed water. 19 , 69 No significant changes were observed after recycling.…”

“…At 27.3°, 35.9°, 41.1°, 54.9°, 56.5°, 68.9°, and 70.0°, characteristic peaks were observed and indexed to (110), (101), (111), (211), (220), (301), and (112) crystallographic rutile TiO 2 planes, respectively. 19 , 37 , 38 The characteristic peaks at 25.1°, 37.6°, 47.8°, 53.7°, 62.7°, 75.0°, and 82.5° were indexed to (101), (004), (200), (105), (204), (215), and (303) crystallographic anatase TiO 2 planes, respectively. 19 , 38 , 39 After the hydrothermal process in the presence of AgNO 3 , new characteristic peaks at 44.2°, 64.3°, and 77.3° were observed in Figure 1 a, which were indexed to the (200), (220), and (311) planes of Ag nanoparticles, respectively.…”

“… 19 , 37 , 38 The characteristic peaks at 25.1°, 37.6°, 47.8°, 53.7°, 62.7°, 75.0°, and 82.5° were indexed to (101), (004), (200), (105), (204), (215), and (303) crystallographic anatase TiO 2 planes, respectively. 19 , 38 , 39 After the hydrothermal process in the presence of AgNO 3 , new characteristic peaks at 44.2°, 64.3°, and 77.3° were observed in Figure 1 a, which were indexed to the (200), (220), and (311) planes of Ag nanoparticles, respectively. 40 − 42 Figure 1 b shows the XRD spectra of normalized Ag(5%)@TiO 2 after subtracting the normalized XRD spectrum of TiO 2 to resolve the overlapping between the characteristic peaks of Ag and TiO 2 .…”

“…15 , 68 The mechanism can be explained as, in the first step, the acidic centers of Ag@TiO 2 would activate the carbonyl group of aldehydes to coordinate between these centers and the oxygen of carbonyl groups. 15 , 19 , 24 The active hydrogen species would attach to the carbon of the carbonyl group, which then protonated and underwent water elimination, resulting in the product. The coordination environment surrounding the Ag centers is anticipated to prevent the sterically congested Z-type product.…”