**2. Experimental details**

#### **2.1 Materials**

The 7-chloro-4-nitrobenzofurazan (NBD-Cl) and the tetraethyl ammonium tetrafluoroborate (TEAF) were commercial products (Aldrich, Darmstadt-Germany). Samples of 7-methoxy-4-nitrobenzofurazan (NBD-OCH3) and 7 phenoxy-4-nitrobenzofurazan (NBD-OC6H5) were prepared according to the standard methods described in the literature [27–29]. The secondary cyclic amines (morpholine, piperidine and pyrrolidine) were commercial products (Aldrich, Darmstadt-Germany) and were redistilled before use whenever necessary.

#### **2.2 Apparatus**

Cyclic Voltammetry (CV) measurements were carried out under nitrogen gas with a Voltalab 10 apparatus from Radiometer driven by the Volta Master software at a potential scan rate of 0.1 V/s, at room temperature, in 25 mL of CH3CN solution containing TEAF (0.1 M) as a supporting electrolyte. The cell used three electrodes. The working electrode was a 2 mm diameter platinum disk (Tacussel type EDI) and a platinum (Pt) wire as a counter electrode. The concentrations of the substrates were 10<sup>3</sup> M. The potentials EOx and ERed give us the information on HOMO and LUMO energy levels of studied materials.

The UV–visible absorption spectra of compounds have carried out on a Shimadzu UV–visible model 1650 spectrometer. The samples were dissolved in acetonitrile medium. The optical properties of the sample are secondly investigated by photoluminescence spectroscopy. The 514.5 nm line of the continuous-wave argon Ar + laser is used as an excitation source. Spectral analysis of the PL was performed using a Jobin Yvon iHR320 monochromator through the lock-in amplifier technique.

For the Time-Resolved PL (TRPL) study, the sample was illuminated by a pulsed laser diode with photon energy of 1.58 eV with a repetition rate of 80 MHz, using the time correlated photo counting technique. The details experiment was reported elsewhere [34].

#### **2.3 Computational methods**

Ground-state (S0) optimized geometries of the studied compounds were performed by means of DFT at B3LYP level of theory [35, 36] in conjunction with the 6-31+ g(d,p) basis set. No constraints were used and all structures were free to optimize in an acetonitrile solution by applying the conductor polarizable continuum model (CPCM) with the Gaussian 09 program [37] and the output files were visualized with the Gauss View (5.0.8) molecular visualization program [38]. To predict the optical absorption properties of the studied compounds, UV–Vis absorption spectra were simulated at the time-dependent TD-DFT level with the same basis set. The electronic properties such as highest occupied molecular orbital (εHOMO) and lowest unoccupied molecular orbital (εLUMO) energy levels and dipole moment have been extracted from calculations.

The semi-empirical quantum-chemical ZINDO level was used to predict the optical emission spectra on S1 optimized structures. Electronic transitions assignment and oscillator strengths were also calculated using the same method of calculations.

### **3. Results and discussion**

### **3.1 Electrochemical analysis**

Electrochemical comparative studies were undertaken using Cyclic Voltammetry (CV) for the six compounds 4.7 di-substituted benzofurazan (See **Figures 3** and **4**). The CVs profiles were performed in the potential range of 2.0 to +3.0 V.

The potentials (EOX and ERed) peaks can be used to determine the HOMO and LUMO energy levels of the studied compounds. Indeed, the value of their energy gaps can be estimated from the potentials difference.

The highest occupied molecular orbital (HOMO) energy and the lowest unoccupied molecular orbital (LUMO) energy were estimated from the first oxidation and reduction potentials, respectively. The energy levels were calculated according to an empirical method [39] and by assuming that the energy level of the ferrocene/ ferrocenium is 4.8 V below the vacuum level.

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

**Figure 3.**

*Oxidation and reduction voltammograms of NBD-Cl, NBD-OCH3 and NBD-OC6H5 products (10*�*<sup>3</sup> M) recorded in acetonitrile.*

**Figure 4.**

*Oxidation and reduction voltammograms of NBD-Pyrr, NBD-pip and NBD-Morph products (10*�*<sup>3</sup> M) recorded in acetonitrile.*

$$\mathbf{e}\_{\text{HOMO}} = -\mathbf{e} \left[ \mathbf{E}\_{\text{OX}} - \mathbf{E}\_{\text{Fc/Fc}^+}^0 \right] - \mathbf{4.8} \tag{1}$$

$$\mathbf{e}\_{\text{LUMO}} = -\mathbf{e} \left[ \mathbf{E}\_{\text{Red}} - \mathbf{E}\_{\text{Fc/Fc}^+}^0 \right] - \mathbf{4.8} \tag{2}$$

Here, EOx and ERed are the potential of the oxidation peak and reduction peak, respectively. These values were obtained from the CV curve for each compound;

E0 Fc*=*Fc<sup>þ</sup> is 0.59 eV, the average of oxidation and reduction peak potentials of ferrocene measured under the same experimental condition; 4.8 eV is the energy difference between the energy level of ferrocene and the vacuum. Note that when the oxidation/reduction peak of the studied compound was reversible, the Eox/red was replaced in the equation by E°ox/red determined experimentally.

For the electrophile compounds (**Figure 3** and **Table 1**), the peaks appeared at the reduction potentials of about: -0.440 V (NBD-Cl), �0.716 (NBD-OCH3) and � 0.610 V (NBD-OC6H5). They correspond to LUMO energy levels at �3.77 eV, �3.49 eV and � 3.60 eV, respectively. In the same way, the oxidation potentials located at: 2.90 V, 2.28 V and 2.40 V led to HOMO energy level values of �7.11 eV, �6.49 eV and � 6.61 eV for NBD-Cl, NBD-OCH3 and NBD-OC6H5, respectively. The electrochemical energy gaps deduced from these measurements were: 3.34 eV (NBD-Cl), 3.00 eV (NBD-OCH3) and 3.01 eV (NBD-OC6H5).

For the three first compounds used as electrophile groups, they were oxidized and reduced at relatively distinct potentials. Indeed, the NBD-Cl is oxidized and reduced at higher potentials than methoxy and phenoxy substituted NBD.

For the amino-substitued NBD (**Figure 4** and **Table 1**), the oxidation potential peak values were detected nearly 1.66 V, 1.58 V and 1.55 V for morpholine, piperidine and pyrrolidine substituted NBD, respectively. They correspond to HOMO energy levels at �5.87 eV, �5.79 eV and � 5.76 eV, respectively. However, the reduction peaks of the studied compounds were reversible and thus the ERed was replaced in the equation by E0 Red determined experimentally. Accordingly, the estimated LUMO energy levels are of about: �3.62 eV (NBD-Morph), �3.55 eV (NBD-Pip) and � 3.41 eV (NBD-Pyrr).


**Table 1.**

*Calculated HOMO and LUMO energy levels and extracted chemical reactivity descriptors for studied compounds.*

#### **3.2 Photo-physical properties**

#### *3.2.1 UV-Vis optical absorption and emission (PL) analysis*

The optical absorption spectra of the four substituted benzofurazans (NBDs) compounds are illustrated in **Figure 5**. The NBD-Cl, as starting material, has two distinct absorption bands (262 nm and 337 nm). The spectral characteristics of NBD-Cl were maintained and appeared as the same general features for

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

#### **Figure 5.**

*Experimental UV–vis optical absorption spectra of studied compounds as well as experimental PL spectrum of NBD-Morph.*

amino-NBD derivatives. For the later systems, a new optical band in the range 478-488 nm was assigned to the formation of amino-NBD derivatives by SNAr reactions. In fact, typical three distinct absorption bands are presented for NBD substituted with pyrrolidine, piperidine or morpholine, similar to that previously reported NBD-amino derivatives [1–3]. Besides, the maximum absorption wavelengths of amino-NBD derivatives shifted more bathochromically than that of NBD-Cl.

According to assignments reported previously by Heberer and coworkers for NBD-amino derivatives [40] the short-wavelength band is associated to the aromatic benzofurazan compounds. The middle band at around 340 nm was attributed to a π ! π\* electronic transition. However, the band with lower energy in the range of 478-487 nm has ascribed to ICT arranged between the electron-donor and the nitro (NO2) electron-withdrawing groups within the molecule) (NBD-Cl, free band). Importantly, the maxima for the charge transfer band in these systems are detected in the visible region of the spectra.

In addition, similar profiles of optical absorption spectra for amino-NBD were obtained. Thus, the excitation corresponding to the lowest energy band of title compounds was used in order to investigate the excitation process from the ground (S0) to the first excited state (S1).

Experimentally, the 4,7-disubstitued benzofurazan compound, NBD-Morph was excited at the maximum absorption wavelength and show two distinct fluorescence bands (See **Figure 5**). The PL spectrum shows of two dominant features peaking at approximately 2.21 eV (559 nm) and 2.05 eV (603 nm), thus, a broad green to orange emission band has been identified for the tested NBD-Morph compound.

#### *3.2.2 Time-resolved photoluminescence (TR-PL) decay kinetics*

The performed time-resolved photoluminescence (TR-PL) decay kinetics is analyzed and illustrated by experiment (**Figure 6**). A particular attention is devoted to the thorough analysis of non-exponential decay kinetics. However, the TR-PL decay recorded at longer wavelength emission was relatively affected. It should be mentioned that lifetimes are inaccessible from the decay process by the use

#### **Figure 6.**

*Time-resolved decay curves recorded at emission energy of 2.21 eV (560 nm) and 2.05 eV (603 nm) for NBD-Morph film.*

experimental equipment and could be related to an efficient intra-molecular charge transfer occurred in NBD-Morph molecule. The result was supported by the ICT characteristic optical band for a compound that had a lower intensity compared to those of analogue compounds (See **Figure 5**). The flat TR-PL spectrum means that the photo-carriers have a long lifetime (more than the nanosecond regime), leading to an important diffusion length.

#### **3.3 Quantum chemistry computation results**

Complete geometry optimizations of studied compounds were firstly carried out at DFT using B3LYP functional and the polarized 6-31 g+(d,p) basis set. The optimized molecular structures are illustrated in **Figure 7**. Particularly, the title amino-NBD compounds contain mainly three types of chemical bonds, namely O—H, N—H and N—O which are associated to an intra-molecular non covalent interactions.

The computed geometrical parameters related to the short contacts at ground (S0) and excited (S1) states are given in **Figure 7** and tabulated in **Table 2**. The S0 calculated short contact O—H was 2.39 Å (versus 2.38 Å, S1) in all systems,

**Figure 7.** *Molecular optimized structures with intra-molecular short contacts.*

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


**Table 2.**

*Calculated non covalent interactions for O—H N—O and N—H participating in short intra-molecular contacts within molecular at ground (S0) and excited (S1) states. The values presented in parentheses are obtained at S1.*

considerably smaller than the sum of the van der Waals radii (2.72 Å). The N—O short contact is fond 2.78 Å versus 2.74 Å, S1) significantly too smaller than the sum of the van der Waals radii (3.07 Å). In addition, the third short contact of N—H (smaller than the sum of the van der Waals radii (2.75 Å)) is allowed to vary from 2.11 Å -2.47 Å (S0) to 2.22 Å-2.53 Å (S1). The later (N—H) contacts are sensitive to amino substituted groups. As a result, these contacts have been identified as an important driving force on the stabilization of the studied co-planar molecular structures [41]. The major difference in the ground and excited state is the change in torsional angle ϕ and in the dipole moment. Calculations reveal that the existence of the amino groups significantly modifies the properties of their excited states. While the ground state is predicted to be planar, the excited-state geometry is twisted by about 2-7°. Due to conformational changes, the singlet excited-state dipole moment was found to be greater than ground-state dipole moment, unexpected for NBD-Pip (16.84 D (S1) versus 16.53 D (S0)). Importantly, the C-NO2 bond lengths indexed as (1) decreases by 0.4 Å upon excitation, due the nitro strong electron-withdrawing group effect.

Note that the band gap values estimated using the electrochemical method may be different from those calculated by optical or theoretical methods (see **Table 3**). This result could be explained by the fact that the oxidation/reduction process at the electrode are reactions generating species in ground states, whereas the optical electron transition leads to the formation of excited states. Moreover, the electrogeneration of a radical cation or radical anion in solution involves other thermodynamic (salvation … ) and kinetic effects. Consequently, it is expected that the peak


#### **Table 3.**

*Calculated HOMO and LUMO energy levels and extracted chemical reactivity descriptors for studied compounds using DFT//B3LYP/6-31+ g(d,p) in acetonitrile.*

potential will be lowered due to fast chemical reactions following the primarily electron transfer generating chemically reactive radical ions.

To better understand the optical responses, we used the time-dependent density functional theory (TD-DFT) to simulate the optical absorption spectra (See **Figure 8**). Their corresponding electronic transition assignments are given in **Table 4**. The maximum absorption wavelengths and the molecular extinction coefficients (ε) of the chloro- and amino-substituted NBD, in acetonitrile, are also given.

It is worthwhile mentioning that the extensive DFT and TD-DFT calculations show a good correlation between observed absorption and theoretical vertical excitation.

For the emission properties, it should be noted that the ZINDO semi-empirical quantum chemistry [42] was used to predict the emission spectra (See **Figure 9**). **Table 4** lists the emission optical bands of the amino substituted benzofurazan and their assignments. The emission properties are mainly from LUMO and LUMO+1 to HOMO.

It is found that compounds emit at appreciably higher wavelengths in the range 620-640 nm. It should be noted also that bathochromic effect is due to the NBD moiety and the amino substituted groups. It should be noted that NBD-Morph present absorption at wavelength maximum of 478 nm and emit orange light at 559-603 nm. This provides a large apparent Stokes shift of 81 nm attributed to the ICT process (**Table 5**). The combined experimental and computed results suggest that the new NBD-Morph compound is still considered fluorescent. Then, considerable efforts have been undertaken to test the other analogue compounds.

4-amino substituted NBD is recognized among to the broad family of intramolecular charge transfer (ICT) complexes. The ICT characteristics are identified by the presence of both electron donor and withdrawing acceptor substituent moieties within the same molecule. Thus, the photo-initiated electron transfer from the donor to the acceptor sites yields two kinds positive and negative charges within separated functional parts of the molecule. Herein, we have extracted the atomic

#### **Figure 8.**

*Simulated optical absorption spectra of Cl-NBD and its related amio-substituted NBD by means of DFT// B3LYP/6-31+ g(d,p) level of theory, in acetonitrile.*


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

#### **Table 4.**

*The vertical excited energies (nm) and their oscillator strengths (f) for the ground (S0* ! *S1) states of studied compounds by TD//B3LYP/6-31+ g(d,p) level of theory.*

Mulliken charges from both ground- and excited-geometry structures. The results are illustrated in **Figure 10**. In fact the 4-amino substituted NBD belongs to the broad family of ICT molecules, with the amino group acting as an electron donor upon photo-excitation, and the nitro (NO2) group as an electron acceptor.

As illustrated in **Figure 11**, we have presented the frontier molecular orbitals (FMOs) to obtain insight into the molecular structure and optoelectronic changes from the ground (S0) to excited (S1) States. We can observe that for all systems, the nitro and the 4-amino substituent groups significantly contribute to the optoelectronic properties. Particularly, the NBD-Morph differs from the other two compounds, where the oxygen atom did not contribute exclusively in excited state. This could explain the electron transfer from the nitrogen group of the electron donor moiety.

#### **3.4 Relationships of chemical structure and reactivity properties**

It is relevant to note that good accuracy of computational predictions of optical properties is already attainable. Accordingly, we utilize this approach for accurate prediction of the global reactivity descriptors including chemical potential (μ), Chemical hardness (η) and electrophilicity index (ω) (see **Table 5**).

Parr's electrophilicity ω values are calculated according to Eq. (3) [43, 44] based on μ and η which can be evaluated using Eqs. (4) and (5).

#### **Figure 9.**

*Simulated emission spectra of amino-substituted NBD compounds (a,b,c). Superposition of emission spectra with experimental data for NBD-Morph is given (Figure 9D).*


#### **Table 5.**

*The vertical excited energies (nm) and their oscillator strengths (f) for the S1* ! *S0 states of studied compounds obtained by ZINDO method. The wavelengths excitation was ranging from 424 to 435 nm.*

#### **Figure 10.**

*Mulliken charge distribution of ground- and excited (bold values) states optimized structures of studied compounds.*

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

#### **Figure 11.**

*Frontier molecular orbitals (FMOS) obtained at the ground- (S0) and excited- (S1) states of the optimized geometries of studied compounds.*

$$w = \frac{\mu^2}{2\eta} \tag{3}$$

$$
\mu \approx \frac{1}{2} (\varepsilon\_{\text{HOMO}} + \varepsilon\_{\text{LUMO}}) \tag{4}
$$

$$\eta \approx \frac{1}{2} \left( \varepsilon\_{LUMO} - \varepsilon\_{HOMO} \right) \tag{5}$$

#### *3.4.1 Correlation analysis*

In a recent study, we showed that the second-order rate constants for the reactions of 7-X-4-nitrobenzofurazans **1** (**1a**, X = Cl, **1b**, X = OC6H5 and **1c**: X = OCH3) with secondary cyclic amines **2** (**2a**, morpholine, **2b**, piperidine and **2c**, pyrrolidine) in acetonitrile at 20°C [27, 29] can be described by the linear free

energy relationship *log k = sN(N+ E)* [45–48], and we determined the *E* parameters of these electrophiles **1a-c** which are collected in **Figure 12**. We have now used these parameters *E* in order to elucidate the relationship between Mayr's electrophilicities *E* [45] and Parr's global electrophilicity index *w* values [49, 50]. As will be seen, satisfactory correlations between experimentally electrophilic reactivities *E* and other properties of these series of para-substituted nitrobenzofurazans **1a-c** are found and discussed. On the other hand, we discuss how the structure–property relationships can be used to evaluate electrophilicity parameter *E* and Hammett constant *σ* values which are not directly accessible.
