**1. Introduction**

4-Nitrobenzofurazan (NBD) is neutral 10π electron-deficient heteroaromatic substrates. Organic materials containing the NBD moiety have been tested in

potential biomedical and bio-analytical applications [1–7]. They have been also used as an effective electron-withdrawing building block for organic solar cell materials [8], and have been discovered to be nonlinear optical (NLO) materials [9]. In addition, the strong electron withdrawing properties of nitrobenzofurazan (NBD) derivatives make it an exceptional electrophilic product of the nucleophilic aromatic substitution (SNAr).

In addition, the activated chlorine atom in 4-chloro-7-nitrobenzofurazan (NBD chloride, namely NBD-Cl) can undergo electrophilic aromatic substitution by phenoxide, primary or secondary cyclic amines [10–13] and some of 4,7 di-substituted benzofurazan compounds to generate weakly or non-fluorescents products [14–18].

A previous kinetic study of amino-substituted NBD showed that methoxy and aryloxy substituent's are better leaving groups than chloro and that a red-colored Meisenheimer complex immediate was formed during the reactions [19].

On the other hand, the frontier molecular orbital descriptors including HOMO and LUMO energy levels play major roles in governing many chemical reactions as function of reactivity and stability of related molecules [20, 21]. Alternatively, contacts from intra-molecular interactions have been identified as an important driving force on the stabilization of various planar molecular structures [22, 23]. However, these interactions lead to intra-molecular charge transfer of formed compounds and thus provides a non-radiative decay from the excited state [24–26].

Recently, we reported a kinetic study for SNAr reactions of NBD with secondary amines [27–30]. The present chapter is focused on the elucidation of the combined experimental–theoretical investigation relationships between the optical (UV–Vis, PL and time resolved photoluminescence (TR-PL), electrochemical (CV) properties and the chemical structures of 4.7-di-substituted benzofurazans (NBDs), based on the kinetic reactivity process of compounds. Parr's approach was employed to develop a relationship which rationalizes the kinetic data previously reported for the reactions of 4-X-7-nitrobenzofurazans with various nucleophiles [27–29].

Then, the substituent effects were particularly limited only to morpholine, piperidine and pyrrolidine groups. Thus, we seek to study the structure–property relationships for molecules containing 4-nitrobenzofurazan moiety for a better understanding of their optical behavior and their chemical reactivity. For this aim, theoretical calculations using quantum chemical calculations within density functional theory (DFT) and its extension TD-DFT in the 6-31 + g(d,p) basis set have been developed to facilitate the in-depth understanding of structure–property relationships.

The Molecular structures of the investigated compounds are illustrated in **Figures 1** and **2**.

To the best of our knowledge, no similar study has been published on electrochemical and photo-physical properties of the studied compounds. Cyclic Voltammetry (CV) is used to estimate the band gaps, electron affinities (i.e. the

**Figure 1.** *Chemical structures of the studied electrophile compounds.*

*Structure-Property Relationships in Benzofurazan Derivatives: A Combined Experimental… DOI: http://dx.doi.org/10.5772/intechopen.99246*

**Figure 2.**

*Reaction mechanism between NBD-Cl and three examples of nucleophiles (morpholine, piperidine and pyrrolidine) groups.*

energy of LUMO, εLUMO) and ionization potentials (i.e., the energy of HOMO, εHOMO) [31]. Besides, the computed results from DFT and TD-DFT calculations enable the access to the structure geometry, the Mullikan charge distribution and the dipole moment of the 4,7 di-substituted benzofurazan at the ground- (S0) and excited (S1)-states [32, 33].
