Photocatalytic Reduction of Fluorescent Dyes in Sunlight by Newly Synthesized Spiroindenoquinoxaline Pyrrolizidines

[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.