To understand the change in the emission property for different species (1c and 1c .-) a detailed theoretical calculation was performed using density functional theory (DFT) at CAM-B3LYP/6-311G(d) level. The optimized geometries of 1c, 1c .-and 1o were obtained from the PCM model with DMF as a solvent. Molecular orbital analysis on the optimized 1c predicts the HOMO to be centred over the DHP and the electron-deficient NDI core contributed most to the LUMO (Figure ). The electrostatic potential surface (ESP) analysis of 1c revealed the electrondeficient nature of the NDI core (Figure ). Therefore, the electron flow from DHP to NDI in 1c resulted in a low emission. Upon exposure to 370 nm light, 1c was transformed to 1c.-(vide supra) and DFT calculation showed the NDI core to become electron-rich as the HOMO is shifted from DHP unit to the NDI in 1c .-(Figure ). Also, the electronegative charge density mostly positioned over NDI (Figure ). Spin density calculation further revealed the delocalization of the unpaired electron over NDI moiety in 1c .-(Figure ). Therefore, the electron flow from DHP to NDI was restricted in 1c .-and the emission at 658 nm was gradually intensified upon radical formation. Moreover, the radical formation in the open form (1o .-) could not be observed as the anionic radical in the open form was found to be higher in energy by 30.7 kcal/mol compared to the closed form (1c .-) (Figure ). This is in line with the experimental observation of the decrease in emission intensity upon 525 nm irradiation.