Formation of Naphthalene Diimide Derivatives in NTCDA Solution in EMIM-DCA/EMIM-Ac Ionic Liquid Mixture: Identification of Product Structures Using EPR, DFT and Detection of Light-Induced NTCDA Radical Anion in EMIM-DCA by 1H NMR

In this study, two low-concentration solutions of 1,4,5,8-Naphthalene tetracarboxylic dianhydride (NTCDA) in two imidazolium-based ionic liquids (IL), EMIM-DCA and EMIM-Ac, at room temperature were mixed; after around ten days of storage, the formation of two variants of electron-withdrawing molecules of Naphthalene tetracarboxylic diimide NTCDI-R derivatives were detected. Their radical anions (RA) were registered by EPR via irradiating the resulting mixture of both solutions with visible or UV light. Chemical shift recording and precise concentration control of NTCDA in ionic liquid solutions were accomplished using 500 MHz and 700 MHz 1H NMR. The formation of the NTCDA•− upon light illumination was also confirmed by NMR, which showed paramagnetic broadening of the 1H line due to the electron chemical exchange between NTCDA and NTCDA•− after light-on and light-off modes. The time-dependent line broadening is in quantitative agreement with the corresponding EPR data for the light-on-/light-off-dependent concentration of NTCDA•−. The putative structure of R in NTCDI-R was calculated by DFT. The possibility of using NTCDI-R•− as a spin probe for studying molecular diffusion in high viscous ionic liquid is assessed.