**4. Conclusion**

The reaction between NBD-Cl and morpholine, piperidine or pyrrolidine, as nucleophile group, proceeds normally by a SNAr attack affording 4.7-di-substituted benzofurazans. They were thoroughly analyzed electrochemically and optically by means of Cyclic Voltammetry (CV) and UV–Vis and time resolved photoluminescence (TR-PL), respectively.

Complementary study based on density function theory (DFT) and its extent TD-DFT were also conducted to unravel the electronic structure, the simulated optical spectra and to further understand the structure property relationships of the investigated compounds. The used procedure based theoretical calculations allows deducing the electronic parameters related to the HOMO and LUMO energy levels and their energy differences. These levels are accessible by measuring the peak potentials of the oxidation and reduction reactions. It is found that the measured energy levels are in close agreement with the values computed from DFT method. Specific nucleophile-electrophile interactions in SNAr reactions were expected to be governed by three main types of non-covalent bonds (O—H, N—O and N—H). Importantly, the compounds absorb visible light at longer wavelengths. In addition, the SNAr attack leads to intra-molecular charge transfer of formed compounds and thus provides a remarkably slow non-radiative decay from the excited state. Investigation of photo-physical properties revealed importance of intra-molecular twisting on excited (S1) states.

As verified by experimental and calculated results, the obtained molecules, by SNAr reactions, were modulated by intra-molecular non-covalent interactions that force the co-planarity and the rigidity of the structures yielding an efficient intramolecular charge transfer (ICT). Thus, the nucleophile substituents acting as electron-donating groups in SNAr reaction afford to obtain materials with interesting nonlinear optical (NLO) response. In addition, the photo-optical data corroborated with TD-DFT approach were discussed in correlation with the structural architecture of each compound. We have shown the high efficiency fluorescent NBD-Morph compound that emits green to orange color at wavelength maximum of 559-603 nm.

The theoretical optimized parameters have been found to be in good agreement with the corresponding experimental data and results in the literature. Satisfactory linear correlation has been demonstrated between the parameters *E* and the global electrophilicity index w for the electrophilic reactivity at the C-X position of nitrobenzofurazans **1a-d**. Accordingly, structure–property relationships were found to be able to evaluate the unknown electrophilicity parameter *E* of 7-X-4 nitrobenzofurazans **1d-f**. Alternatively, the validity of Eqs. (7) and (8) has been satisfactorily verified by comparison of calculated and previously reported, in the literature, of the Hammett constant σ<sup>p</sup> values of HNOH, HNNH2, HNH and HNCH3 groups.
