Quinoline-Based Materials: Spectroscopic Investigations as well as DFT and TD-DFT Calculations
Quinoline derivatives such as 15,15-difluoro-[1,3,2] diaazaborininodiquinoline (DDP) and 15,15-difluoro-[1,3,2] diaazaborininodiquinoline acetonitrile (DDPA) have a range of biological and medical activities. So, it is vital to shed light on these compounds in terms of their optical properties supported by quantum calculations. The absorption and emission spectra of studied compounds were measured within the laboratory, whereas the quantum calculations were performed utilizing the density functional theory (DFT) calculations. Additionally, the time-dependent density functional theory (TD-DFT) was applied for the comparison of some sensible results with the theoretical ones. The molecular structures of these compounds were presented via applying chemical analysis techniques. The electronic absorption spectra of DDP and DDPA molecular structures were monitored through an experiment in hosts such as carbon tetrachloride (CCl4), chloroform (CHCl3), methylene dichloride (CH2Cl2), acetone, and dimethyl sulfoxide (DMSO). Also, the influence of pH on the absorption spectra of the DDP molecule was studied. The molecular structures of these quinoline derivatives have been optimized via utilizing the B3LYP/6-31G (d) level of theory. The electronic absorption and emission spectra of the DDP compound in gas, THF, and DMSO have been calculated utilizing TD-DFT at the CAM-B3LYP/6-31G ++(d, p) level.
Quinoline is a heterocyclic compound with few chemical applications on its own. However, several of its derivatives have numerous applications such as pharmaceutical preparations , agricultural chemistry , materials and dyes , as well as in medicine. Recently, the nucleus of quinoline and plenty of its derivatives grasped interest to chemists and biologists owing to their many and necessary applications within the field of medicine as a number of them are used as antituberculosis [1, 4, 5], antimalarial , anticancer , antibiotic , hypotensive, antiviral , antioxidant , and antifungal . Given the wide importance of quinoline derivatives, it is absolutely necessary to shed light on them during this analysis.
Liquid quinoline dye lasers have significantly high potency, low optical device threshold, wide calibration varying from the near-ultraviolet to near-infrared spectral region, and long life [11, 12]. None of the opposite lasers will replace dye lasers for their strength, gain, low price, and big selection of viability . However, these lasers cannot enter the industrialized market due to their inherent defects such as massive size, inflammable and cytotoxic solvents, dye deterioration, heating, and triplet state formation . The dye optical device resolution ought to flow through the optical device cavity to keep up stable gain and beam quality. This needs an outside variety of dye flow systems and reservoirs, and an outside quantity of dye is dissolved in typically inflammable and cytotoxic solvents [11–13, 15–17].
The electronic absorption spectra of DDP dye at completely different pH values show clear isosbestic points in their absorption spectra indicating the existence of two substances in equilibrium with each other [12, 18–21] showing an isosbestic point. However, the presence of associated degree isosbestic point is not a continuous proof of the existence of solely two compounds because the third absorbing species might have zero absorptivity at this specific wavelength .
The main objective of the study in this manuscript is the spectroscopic investigations and computational calculations of compounds under study using the DFT method followed by the determination of the accurate functional as well as the basis set using TD-DFT to arrive at the accurate theoretical results and comparing them with the experimental ones.
2. Materials and Methods
2.1. Materials’ Preparation
15,15-Difluoro-[1,3,2] diaazaborininodiquinoline (DDP) and 15,15-difluoro-[1,3,2] diaazaborininodiquinoline acetonitrile (DDPA) were provided by Professor Dr. Ewald Daltrozzo of Konstanz University, Germany, and used without further purification. All solvents employed in this work were of chemical analysis grade and were preliminary checked for the absence of absorbing or fluorescent impurities among the scanned spectral ranges. UV-Vis electronic absorption spectra were obtained utilizing a Shimadzu UV-Vis 1650-PC spectrophotometer, and steady-state fluorescence spectra were presented employing a quartz cuvette of 0.2 cm path length to minimize the reabsorption of emitted photons; the emission was monitored at 90° geometry using a Jasco FP-8200 spectrofluorometer, excitation bandwidth 5 nm and emission bandwidth 5 nm, with a Xe lamp light source.
2.2. Methodology and Theoretical Calculations
Upon usage of density functional theory (DFT) and time-dependent density functional theory (TD-DFT) strategies, the ground electronic structure and excited state structures of the probe dye area unit were improved using the Gaussian 09 package . The molecular structures under study have been optimized employing [24–26] the DFT/B3LYP/6-31G (d) level of theory. The electronic absorption and emission spectra of the quinoline derivatives in numerous solvents have been calculated exploiting the TD-DFT  at the CAM-B3LYP/6-31G ++(d, p) level.
3. Results and Discussion
3.1. DFT Calculations
The optimized molecular structures for DDP and DDPA molecules within the gaseous state area unit were obtained by the utilization of the B3LYP/6-31G (d) level of theory. The obtained results are shown in Figure 1. Some necessary calculated and optimized geometrical parameters such as bond length (BL) in Å and bond angle (BA) and dihedral angle (DA) in degree (°) were computed for DDP and DDPA in gas at the ground (S0) and initial excited state (S1) using B3LYP/6-31G (d) level of theory; the obtained results are tabulated in Table 1. The labeling scheme for the studied molecular structures is shown in Figure 1. DFT was used to obtain the ground state (G) molecular structure of DDPA and DDP in the gaseous state. Also, TD-DFT was applied to obtain electronic excited state (Exc.) molecular modeling structures for DDP and DDPA in the gaseous phase. The optimized electronic ground and excited molecular structures were obtained via applying the B3LYB/6-31G (d) level of theory. As presented in Table 1 and Figure 1, the BLs (C2-C3 and C1-C31) for the DDPA compound and (C3-C4 and C13-N32) for DDP increase from G to Exc., and the difference values are 0.014 and 0.001 Å for DDPA and 0.021 and 0.006 Å for DDA. On the contrary, (C2-N38 and N37-C18) BLs for DDPA and (C3-N34 and N33-C19) BLs for DDA were shortened from S0 to S1 state. The difference values are 0.014 and 0.015 Å for DDPA and DDA, respectively. The DAs (C12-C7-C2-C38, C31-C28-N37-C18, C27-C24-C19-C20 and B32-N37-C18-C19) for DDPA and (C1-C2-C3-N34, C19-N33-C28-N32, C29-C25-C18-C17 and B31-N33-C19-C18) for DDP are 179.99o in the G and Exc. states. This is referring to the planar molecular structures for DDPA and DDP compounds in both G and Exc. states. The BAs (C11-C12-C7, C7-C2-C3 and N38-B32-N37) for DDPA are increased via 0.04, 0.2, and 1.26, respectively, upon excitation from S0 to S1 state. On the contrary, the BAs (C2-N38-C1 and N38-C1-C31) for DDPA and (C6-C1-C2, C2-C3-C4, C3-N34-C13 and N34-C13-N32) for DDP are shortened upon excitation from S0 to S1 state. The BAs for studied molecular structures in G and Exc. states indicate sp2 hybridization.
The DDP and DDPA molecular modeling graphical presentation of the molecular orbitals HOMO (H)/LUMO (L), H − 1/L + 1, H − /L + together with their respective energy gaps (Egs) in the gaseous state were made via applying the B3LYP/6-31G (d) level of theory. The energy gaps go as follows: H and L (Eg1), H − 1 and L + 1 (Eg2), and H − 2 and L + 2 (Eg3) in the gaseous state via applying the B3LYP/6-31G (d) level of theory. The obtained results are presented in Figure 2. The energy values of H (EH), L (EL), H − 1 (EH − 1), L + 1 (EL + 1), H − 2 (EH − 2), and L + 2 (EL + 2) are presented in Figure 2. Also, the Eg values among the following, H and L (Eg1), H − 1 and L + 1 (Eg2), and H − 2 and L + 2 (Eg3), in the gaseous state are listed in Figure 2. Due to hyperconjugation of the studied molecular structures, the HOMO MOs and lone pairs of electrons of nitrogen and fluoride atoms are delocalized over the whole DDP and DDPA molecular structures. On the contrary, the LUMO MOs are delocalized over the whole DDP and DDPA molecular modeling structures, while the lone pairs of electrons of fluoride and nitrogen (N35 for DDPA and N32 for DDP) atoms are not contributed. The energy gap (Eg1) value is calculated by applying the difference (EL − EH). The calculated Eg1 value for DDP and DPPA compounds increases in the order DDP < DDPA indicating that the DDP molecular modeling structure is more reactive compared to the DDPA molecular structure. When the magnitude of Eg increases, the kinetic stability of the molecule increases, and chemical reactivity decreases . Hence, the kinetic stability of the DDPA molecule is higher than DDP in the gaseous phase as shown in Figure 2.
Some significant quantum parameters such as dipole moment (μ), chemical potential (ρ), electronegativity (χ), and chemical hardness (η) were calculated from the energy values of Lumo (EL) and Homo (EH) molecular orbitals by applying the following equations: [23, 24], [23, 25]. Generally, once the chemical structure possesses high dipole moment, it acquires a large asymmetry in the electric charge distribution, and then it can be highly sensitive due to the change of molecular structure and electronic properties under an external electric field. Thus, as shown in Table 2, the µ value of the compound DDPA is high compared to the DDP compound; therefore, this compound is more active compared to the DDP molecular structure. As conferred in Table 2, the ρ value of the DDP molecular structure is lower compared to the opposite DDPA compound. This indicates that the escaping electrons from the DDP molecular structure are low compared to the DDPA molecule. Also, the high χ value for the DDP molecular structure compared to the DDPA compound (see Table 2) results in the power of this compound to draw in electrons from different compounds . On the contrary, the η value for the DDP molecular structure is high compared to DDPA (see Table 2). This means that the DDP compound is very difficult to liberate electrons, whereas the DDPA molecular structure is a good candidate to give electrons to another acceptor molecule.
3.2. Experimental UV-Vis Absorption Spectra
Experimental (Exp.) normalized absorption spectra (1 × 10−5 M) for DDP (Figure 3(a)) and DDPA (Figure 3(b)) in several solvents are obtained; the obtained results are shown in Figures 3(a) and 3(b). The Exp. maximum absorption wavelength λabs (Exp.) in several solvents for DDP and DDPA molecular structures’ area unit is tabulated in Table 3. The two and three maximum absorption wavelengths of DDP and DDPA molecular structures were obtained by applying completely different solvents such as CCl4, CHCl3, CH2Cl2, acetone, and DMSO as presented in Figures 3(a) and 3(b) because of electronic transition and increasing electronic vibrations in the S1 state. The values of maximum absorption wavelengths of DDP and DDPA molecular structures modified from one solvent to another are different because of solute/solvent interactions.
3.3. TD-DFT Calculations
Via applying the Gaussian 09 program, the best DFT functional of the DDP molecule is studied using the 6-31G (d) basis set. Also, the best basis set of the same molecule was investigated utilizing CAM-B3LYP DFT functional. Therefore, the best DFT functional and the basis set for calculating the optical properties of the DDP and DDPA molecules were CAM-B3LYP/6-311G++ (d, p). Upon utilizing the TD-DFT technique and CAM-B3LYP/6-311G++ (d, p) level of theory, the calculated UV-Vis absorption and emission spectra of the DDP molecule in different solvents such as gaseous phase, THF, and DMSO are illustrated and compared with the obtained spectra recorded in Figure 4. The electronic calculated UV-Vis absorption spectrum of the DDP molecule in various solvents has three maximum absorption wavelengths (see Figure 4). The three maximum absorption wavelengths (λmax.) of the DDP dye in the gaseous phase are 210, 265, and 391 nm. In the THF solvent, the three calculated λmax. values of the DDP molecule exist at 211, 265, and 398 nm. And in DMSO, the four λmax absorption values are 183, 211, 265, and 398 nm matching the three and four maximum absorption wavelengths of the DDP molecular structure by applying gas, THF, and DMSO being of electronic transition at increasing electronic vibrational in the S1 state. Depending on the polarity of solvents, the absorption maximum bands for the DDP molecule in THF and DMSO are red-shifted compared to the gas state as shown in Figure 4.
For DDP dye in different solvents, the calculated electronic emission spectra have only one band of maximum emission wavelength (λmax.) between 413 and 438 nm, depending on the polarity of the solvents. Because of the high polarity of DMSO compared to THF and gas phase, the λmax emission of the DDP molecule in DMSO is red-shifted (438 nm) compared to that in THF (437 nm) and gas phase (413 nm). Due to the overlap between calculated absorption and emission spectra of the DDP molecule, a part of photons from the emission spectrum of the DDP molecule is reabsorbed via the inner filter effect. As presented in Table 4, the calculated electronic absorption and emission spectra of the DDP molecule in numerous solvents via victimization CAM-B3LYP/6-311G++ (d, p) level of theory are in reasonable agreement with the experimental observation. The electronic transitions in THF were reconsidered according to previous data available to us. The calculated electronic absorption spectrum of DDP in THF is available at six possible electronic transitions at 398 (f = 0.5143), 330 (f = 0.0219), 314 (f = 0.0448), 310 (f = 0.0354), 265 (f = 0.4749), and 211 (f = 0.5159) nm. Accordingly, three of the new values agree with experimental results. This was quoted in Table 2 showing an agreement between theoretical and experimental results. The first, second, and third electronic transitions correspond to the experimental peaks at 398, 330, and 314 nm arising from HOMO ⟶ LUMO, HOMO ⟶ LUMO + 1, and HOMO − 1⟶ LUMO transitions.
The optimal DFT functional and the basis set for calculating the optical properties of the DDPA molecule were CAM-B3LYP/6-311G++ (d, p). Hence, the calculated electronic UV-Vis absorption and emission spectra of the DDPA molecule in different solvents such as CCl4, CH3Cl, CH2Cl2, acetone, and DMSO were compared with the CAM-B3LYP/6-311G++(d, p) level of theory. The obtained spectra are shown in Figure 5. The UV-Vis absorption spectra have two maximum absorption wavelengths assigned to and as shown in Figure 5. Depending on the polarity of various solvents, the two maximum absorption wavelengths of DDPA are shifted to a longer wavelength (red-shifted) once the polarity of the solvents is decreased as shown in Figure 5 and Table 5. For the DDPA molecular structure in numerous solvents, the emission spectra show only one band of maximum emission wavelength (λmax.) between 461 and 481 nm, depending on the polarity of the solvents. Owing to the high polarity of DMSO compared to alternative solvents and gaseous phase, the λmax emission of the DDPA molecule in DMSO is red-shifted (482 nm) compared to other solvents and gaseous phase. Due to overlapping between calculated electronic absorption and emission spectra of the DDPA molecule, inner filter effect leads to reabsorption of photons from the short-wavelength component of the emission spectrum. By comparing the experimental results of DDPA (Table 3) with the theoretical results for the same compound (Table 5), we find that the theoretical results agree with the experimental ones. By the comparison the absorption spectrum of compounds under study, as its absorption spectrum ranges (300 to 500 nm) that match previously reported quinoline derivatives that have been studied in literature [30, 31] of showing absorbance in the range 280 to 510 nm. So, there is an agreement between the theoretical calculations in this manuscript and the other previous research studies. Hence, the CAM-B3LYP/6-311G++(d, p) level of theory is best for calculating UV-Vis absorption spectra of quinoline derivatives.
3.4. Molecular Electrostatic Potential (MESP) Maps
The molecular electrostatic potential (MESP) maps [27, 28] for DDP and DDPA molecular structures via applying the B3LYP/6-31G (d) [29, 32] level of theory are obtained. The graphical MESP maps are shown in Figure 6. At the surface of the studied molecular structure, the MESP is characterized via totally different colors as shown in Figure 6. In the MESP map, the red color regions correspond to negative MESP (electrophilic reactivity), the blue color sites refer to positive MESP (nucleophilic reactivity), and the green color parts relate to zero MESP. Therefore, the negative potential site in DDP is the BF2 group, as well as the positive potential regions in the DDP molecule are around the hydrogen atoms as bestowed in Figure 6. On the contrary, the negative potential site in DDPA is the C = N cluster, as well as the positive potential regions within the same molecule are around the hydrogen atoms as bestowed in Figure 6.
3.5. Studying the Effect of pH on the DDP Molecule in Methanol
It was noted during the study that the quinoline derivatives are affected by the acidic medium, so it was important to study the effect of the acidic medium on one of these compounds as an example to assess the extent of the effect of pH on the quinoline family. For the DDP molecule in methanol, the experimental electronic absorption spectra at different pH values are studied; the obtained spectra are shown in Figure 7. Due to protonation of (=N) within the DDP molecule utilizing the acidic medium, the maximum absorption bands at 366 and 382 nm are decreased. At the same time, the maximum absorption bands at 403 and 426 nm are increased. The isosbestic point of the DDP molecule occurs at 391 nm. According to the half-height technique and upon utilizing UV-spectrophotometric titration (Figure 7 inset), the ground-state protonation constant (pKa) was determined. From electronic absorption spectra of the DDP molecule, the magnitude of pKa is 4.72. The excited-state protonation constant is calculated by using the following equation: , where refers to the difference between the wavenumbers (cm−1) of the electronic transition of the pure acidic and conjugate base. The value was found to be −0.95 indicating high basicity of the DDP molecule. Figure 8 shows the molecular structures of DDP and DDPA.
The molecular structures of the quinoline derivatives were optimized via applying the B3LYP/6-31G (d) level of theory. The electronic absorption and emission spectra of the DDP compound in gas, THF, and DMSO were calculated exploiting the time-dependent density functional theory (TD-DFT) at the CAM-B3LYP/6-31G ++(d, p) level. The calculated electronic absorption and emission spectra for DDP agree with experimental results. The experimental electronic absorption spectra for the DDP molecule in methanolic solutions of different pH values are studied. The molecular electrostatic potential (MESP) maps for DDP and DDPA molecular structures via applying the B3LYP/6-31G (d) level of theory are obtained.
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare no conflicts of interest.
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