Calculated HOMO and LUMO energy levels and extracted chemical reactivity descriptors for studied compounds.
In this work we seek to understand and to quantify the reactivity of benzofurazan derivatives toward secondary cyclic amines, like pyrrolidine, piperidine and morpholine, acting as nucleophile groups in SNAr reactions. For this aim, physico-chemical and structural descriptors were determined experimentally and theoretically using the DFT/B3LYP/6-31+ g (d,p) methodology. Thus, different 4-X-7-nitrobenzofurazans (X = OCH3, OC6H5 and Cl) and products corresponding to the electrophilic aromatic substitution by pyrrolidine, piperidine and morpholine, were investigated. Particularly, the HOMO and LUMO energy levels of the studied compounds, determined by Cyclic Voltammetry (CV) and DFT calculations, were used to evaluate the electrophilicity index (ω). The latter was exploited, according to Parr’s approach, to develop a relationship which rationalizes the kinetic data previously reported for the reactions of the 4-X-7-nitrobenzofurazans with nucleophiles cited above. Moreover, the Parr’s electrophilicity index (ω) of these benzofurazans determined in this work were combined with their electrophilicity parameters (E), reported in preceding papers, was found to predict the unknown electrophilicity parameters E of 4-piperidino, 4-morpholino and 4-pyrrolidino-7-nitrobenzofurazan. In addition, the relationship between the Parr’s electrophilicity index (ω) and Hammett constants σ, has been used as a good model to predict the electronic effect of the nucleophile groups. Finally, we will subsequently compare the electrophilicity index (ω) and the electrophilicity parameters (E) of these series of 7-X-4-nitrobenzofurazans with the calculated dipole moment (μ) in order to elucidate general relationships between E, ω and μ.
- Structure–property correlations
- Optical properties
- Mayr’s approach
- Parr’s approach
- Voltammetry Cyclic
- NLO properties
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, 2, 3, 4, 5, 6, 7] They have been also used as an effective electron-withdrawing building block for organic solar cell materials , and have been discovered to be nonlinear optical (NLO) materials.  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, 11, 12, 13] and some of 4,7 di-substituted benzofurazan compounds to generate weakly or non-fluorescents products. [14, 15, 16, 17, 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 .
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, 25, 26]
Recently, we reported a kinetic study for SNAr reactions of NBD with secondary amines. [27, 28, 29, 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, 28, 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.
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 energy of LUMO, εLUMO) and ionization potentials (i.e., the energy of HOMO, εHOMO).  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]
2. Experimental details
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, 28, 29] The secondary cyclic amines (morpholine, piperidine and pyrrolidine) were commercial products (Aldrich, Darmstadt-Germany) and were redistilled before use whenever necessary.
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 10−3 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. 
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  and the output files were visualized with the Gauss View (5.0.8) molecular visualization program.  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  and by assuming that the energy level of the ferrocene/ferrocenium is 4.8 V below the vacuum level.
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; 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).
|Compounds||Cyclic voltammetry (CV)|
|(V vs. FC/FC+)||(V vs. FC/FC+)||Energy levels||Chemical reactivity descriptors|
|(eV)||(eV)||μ (eV)||η (eV)||WCV (eV)|
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 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).
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 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, 2, 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  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 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, 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.  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.
|Intra-molecular Short Contacts||Covalent contacts||Dihedral Angle||Dipole Moment (D)|
|D1 O---H (Å)||D2 N---O (Å)||D3 N---H (Å)||dC-N (1) (Å)||dC-N (2) (Å)||ϕ (°)|
|NBD-Pyrr||2.39 (2.38)||2.78 (2.74)||2.47 (2.53)||1.41 (1.37)||1.33 (1.32)||3.47|
|NBD-Pip||2.39 (2.38)||2.78 (2.74)||2.11 (2.25)||1.41 (1.37)||1.34 (1.33)||9.66 (16.76)||16.84|
|NBD-Morph||2.39 (2.38)||2.78 (2.74)||2.11 (2.22)||1.41 (1.37)||1.35 (1.33)||5.16 (12.34)||13.57|
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 electro-generation of a radical cation or radical anion in solution involves other thermodynamic (salvation…) and kinetic effects. Consequently, it is expected that the peak potential will be lowered due to fast chemical reactions following the primarily electron transfer generating chemically reactive radical ions.
|Compounds||Energy levels||Chemical reactivity descriptors||IP(eV)||EA(eV)|
|μ (eV)||η (eV)||W (eV)|
|X = Cl||−7.648||−3.929||−5.788||1.859||9.009||7.481||4.195|
|X = OCH3||−6.988||−3.561||−5.274||1.713||8.117||6.806||3.830|
|X = OC6H5||−6.985||−3.684||−5.334||1.650||8.620||6.629||4.041|
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.
|Compounds||(nm)||Osc. strength||Major contribs||Minor contribs|
|NBD-Cl||341||0.2729||H → L (99%)||—|
|255||0.0709||H-4 → L (79%)||H → L + 2 (10%)|
H → L + 1 (6%)
|: 262 nm, 337 nm|
|NBD-Pyrr||227||0.0181||H → L + 3 (94%)||H → L + 5 (3%)|
|341||0.2781||H → L + 1 (87%)||H-1 → L (6%)|
H → L (5%)
|438||0.4065||H → L (94%)||H → L + 1 (5%)|
|: 268 nm, 349 nm, 484 nm|
|NBD-Pip||230||0.0012||H-7 → L (78%)|
H-4 → L + 1 (13%)
|H-9 → L (3%)|
H-4 → L(2%)
|343||0.2083||H-1 → L (30%)|
H1 → L + 1 (59%)
|H-1 → L + 1 (7%)|
H → L (2%)
|452||0.4129||H → L (94%)||H → L + 1 (5%)|
|: 272 nm, 347 nm, 487 nm|
|NBD-Morph||237||0.0859||H → L + 2 (64%)|
H-3 → L + 1 (15%)
H-6 → L (12%)
|H-6 → L + 1 (2%)|
|341||0.2608||H → L + 1 (89%)||H-1 → L (5%)|
H → L (4%)
|451||0.4209||H → L (95%)||H → L + 1 (4%)|
|: 274 nm, 340 nm, 478 nm|
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  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.
|Compounds||(nm)||Osc. strength||Major contribs||Minor contribs|
|NBD-Pyrr||418||0.0498||L + 1 → H (95%)||—|
|623||0.6989||L → H (97%)||—|
|NBD-Pip||418||0.0564||L + 1 → H (95%)||—|
|638||0.6806||L → H (97%)||—|
|NBD-Morph||418||0.0552||L + 1 → H (95%)||—|
|631||0.6747||L → H (97%)||—|
|: 559 nm, 603 nm|
4-amino substituted NBD is recognized among to the broad family of intra-molecular 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 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).
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
3.4.2 Correlation between electrophilicity parameter (
E) and global electrophilicity index ( w)
Based on the data listed in Table 6, the plot of electrophilicity parameters
It is interesting to note that the relationships parameters
It should be noted that these predicted values of electrophilicity parameters
3.5 Effect of substituents X on electrophilicity:
Evs. σand wvs. σcorrelations
Examination of the data in Tables 6 and 8 show that the electrophilicity parameter (
In order to test the applicability of Eq. (7) for the prediction of the Hammett’s constants σp of other groups, we have calculated the global electrophilicity index (
Most importantly, the satisfactory correlations shown in Figure 14 can be employed to obtain more information about the unknown Hammett’s substituent constants (σ) of morpholine, piperidine and pyrrolidine groups.
Substitution of the
3.6 Correlation between dipole moment (μ) and Hammett’s substituent constants (σ)
As shown in Table 8 and Figure 15, the dipole moments (μ) calculated at DFT//B3LYP/6-31+g(d,p) for these series of benzofurazans
On the other hand, comparing the electrophilicity parameter
3.7 Correlation between global electrophilicity index wCV and Hammett’s substituent constants (σp)
To further extend the generality of the structure–property relationships to the global electrophilicity index wCV calculated from the Cyclic Voltammetry (CV) listed in Table 1, we examined correlations wCV vs. σp, wCV vs.
Similar behaviors have also been obtained when the global electrophilicity index wCV values were plotted against the electrophilicity parameter
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 intra-molecular 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
Conflict of interest
The authors declare no conflict of interest.