Synthesis of phenothiazine/anthracene–p–phenylene–maleimide compounds

From a chemical perspective, maleimide and its derivatives serve as Michael acceptors or efficient dienophiles. On the other hand, the quantum yield of fluorescence for N-substituted maleimides is extremely low due to rapid intersystem crossing – a characteristic feature of this class of compounds. Meanwhile, the adducts of nucleophilic reagents or dienes to the maleimide double bond – succinamides – are quite strong acceptors and exhibit intense fluorescence via an intramolecular charge-transfer mechanism after combining in a single molecule with donor fragments. These unique chemical and photophysical properties of maleimides are applied in the design of new TURN/OFF sensors, fluorescent labels for biological targets, live-cell imaging processes, and for solving other practically significant tasks. Additionally, maleimide derivatives are widely used in materials science, including in self-healing materials, functional polymers and copolymers, as well as optoelectronics. Our study aims to develop practical approaches to synthesizing new maleimides, linked by a para -phenylene bridge to donor carbocyclic or heterocyclic systems, such as henothiazine or anthracene. The design strategy for the desired target compounds involved the preliminary synthesis of 4-donor-substituted anilines and cyclization into the maleimide at the final stage. Thus, the reaction of phenothiazine with 4-fluoronitrobenzene produced 10-(4-nitrophenyl)-10 H -phenothiazine. Using the Gomberg-Bachmann-Hey reaction between anthracene and 4-nitrophenyldiazonium tetrafluoroborate, 9-(4-nitrophenyl)anthracene was synthesized. The resulting nitrobenzenes with phenothiazine and anthracene substituents were reduced by the NaBH 4 /NiCl 2 system to the corresponding amines, which reacted with maleic anhydride to yield the corresponding acid amides. It was found that the resulting monoamides of maleic acid smoothly cyclize to maleimides in acetic anhydride in the presence of dry sodium acetate.The structure of synthesized final maleimides were proved by 1 H and 13 C NMR spectroscopy and for compound 9a single crystal XRD analysis have been performed. From the XRD data, it was found that the phenothiazine unit is in a bent conformation. Keywords : maleimides, phenothiazine, anthracene, cyclization, donor-acceptor compounds.