Omima M. I. Adly*,
Ali Taha,
Shery A. Fahmy and
Magdy A. Ibrahim
Department of Chemistry, Faculty of Education, Ain Shams University, Roxy, Cairo, Egypt. E-mail: omima_adly@edu.asu.edu.eg
First published on 5th September 2023
2-Aminochromone-3-carboxaldehyde (ACC) and its hydrazones (ACMHCA and ACMNPHTCA) with semicarbazide hydrochloride and N-phenylthiosemicarbazide were synthesized and characterized by elemental analysis and spectral studies. The solvatochromic behavior of the title compounds in various solvents showed distinct bathochromic shifts on going from nonpolar to polar solvents, suggesting intramolecular-charge-transfer (ICT) solute–solvent interactions. The ground and excited state dipole moments of ACC, ACMHCA, and ACMNPHTCA were determined experimentally by the solvatochromic shift method using the Bilot–Kawski, Lippert–Mataga, Bakhshiev, Kawski–Chamma–Viallet functions, and a microscopic Reichardt's solvent polarity parameter (ENT). All the investigated molecules showed a substantial increase in the dipole moment upon excitation to the emitting state. The experimental results were generally consistent with the values obtained by the TD-DFT, B3LYP/6-311G++(d,p) method. Molecular electrostatic potential (MEP) mapping and natural charge and natural bonding orbital (NBO) analysis were performed and the results were discussed. The 1H NMR chemical shifts of the prepared compounds were simulated by the gage independent atomic orbital (GIAO) method and compared with their experimental chemical shift values. The biological activity data were correlated with the frontier molecular orbitals. The photovoltaic behavior of the title compounds showed there was sufficient electron injection.
Dipole moments for short-lived species provide important information about the excited states, also providing useful information on their emission energy as a function of solvent polarity.11 Various methods have been reported for the determination of the dipole moments of fluorescent molecules.14–18 Among these, the solvatochromic shift method has been widely accepted for the determination of ground and singlet excited dipole moments because of the linear correlation between the spectral parameters and solvent polarity functions.19–21 Since, not many literature reports can be found based on the determination of the ground and singlet excited state dipole moments for the titled molecules (Scheme 1), the present study focused on the solvatochromic shift for the determination of the ground and singlet excited state dipole moments of the synthesized molecules, namely 2-aminochromone-3-carboxaldehyde (ACC) and its hydrazones, with semicarbazide hydrochloride and N-phenylthiosemicarbazide yielding 2-[(2-aminochromon-3-yl)methylidene]hydrazine carboxamide (ACMHCA) and 2-[(2-aminochromon-3-yl)methylidene]-N-phenylhydrazine carbothioamide (ACMNPHTCA) (Scheme 1).22,23 The specific and nonspecific interactions between the solvent polarity parameters and solute molecules were studied with the current molecules based on the linear solvation free energy relationship (LSFER).24 The Gaussian 09 program was used based on TD-DFT calculations at the B3LYP/6-311G++(d,p) level to optimize the present compounds to complement the experimental results.25 In addition, estimations of the effect of the structural parameters data on the antimicrobial activity were performed.
The current hydrazones may exist in various tautomeric forms, such as keto–enol forms for ACMHCA and thione-thiol forms for ACMNPHTCA, as shown in Scheme 3 and Scheme 4.
Step 1: formation of chromone-3-carboxaldehyd-oxime.
A mixture of chromone-3-carboxaldehyde (1.74 g, 10 mmol) and hydroxylamine hydrochloride (0.77 g, 11 mmol) in 95% ethanol (15 mL) was heated under reflux for 15 min. The white crystals obtained during heating were filtered and recrystallized from DMF/H2O; yield: 1.60 g (85%), m.p.: 209–210 °C.
Step 2: Rearrangement of chromone-3-carboxaldehyd-oxime.
A mixture of the previous oxime (1.87 g, 10 mmol) and 0.05 M sodium hydroxide solution (15 mL) was stirred at 70 °C for 2 h. Water was added (50 mL) and the solid obtained was filtered and recrystallized from AcOH as pale yellow crystals; yield: 1.10 g (58%), m.p.: 252–253 °C.
IR (KBr, ν/cm−1): 3305, 3175 (NH2), 1671 (COaldehyde), 1636 (COγ-pyrone), 1590 (CC). UV/vis (in DMF, λmax/nm): 268, 288, and 302 nm. 1H NMR (DMSO-d6, δ/ppm, 300 MHz): 7.40 (t, 1H, H-6, J = 7.5 Hz), 7.55 (d, 1H, H-6, J = 7.5 Hz), 7.79 (t, 1H, H-7, J = 7.5 Hz), 8.04 (d, 1H, H-5, J = 7.5 Hz), 9.60 (br, 2H, NH2 exchangeable with D2O), 10.14 (s, 1H, CHO). Anal. Found (Calcd.) for C11H9N4O2 (M.Wt): 187.18; % C: 64.38 (64.36); % H: 5.40 (5.35); N: 6.83 (6.80).
IR (KBr, ν/cm−1): 3472, 3279, 3146 (NH, NH2), 1721 (COamide), 1659 (COγ-pyrone), 1613 (CN), 1533 (CC). UV/vis (in DMF, λmax/nm): 259, 286, and 316 nm. 1H NMR (DMSO-d6, δ/ppm, 300 MHz): 6.38 (br, 2H, NH2 exchangeable with D2O), 7.36–7.41 (m, 2H, H-6 and H-8), 7.66 (t, 1H, H-7, J = 7.5 Hz), 7.98 (d, 1H, H-5, J = 7.2 Hz), 8.38 (s, 1H, CHN), 8.64 (br, 2H, NH2 exchangeable with D2O), 9.80 (br, 1H, NH, exchangeable with D2O). 13C NMR (DMSO-d6, δ, 75 MHz): 92.6 (C-3), 116.9 (C-8), 122.1 (C-4a), 125.2 (C-7), 125.5 (C-6), 133.2 (C-5), 138.7 (CHN), 152.9 (C-8a), 156.9 (CO as amide), 162.0 (C-2), 173.4 (CO as C-4). MS (m/z); 246. Anal. Found (Calcd.) for C11H10N4O3 (M.Wt): 246.22; % C: 53.35 (53.65); % H: 3.93 (4.09); % N: 22.54 (22.75).
IR (KBr, ν/cm−1): 3343, 3219 (NH, NH2), 1640 (COγ-pyrone), 1605 (CN), 1559 (CC). UV/vis (in DMF, λmax/nm): 289 and 355 nm. 1H NMR (DMSO-d6, δ/ppm, 300 MHz): 7.17 (t, 1H, H-6, J = 7.8 Hz), 7.33–7.47 (m, 6H, Ph–H and H-8), 7.69 (t, 1H, H-7, J = 7.5 Hz), 8.00 (d, 1H, H-5, J = 7.2 Hz), 8.73 (s, 1H, CHN), 8.75 (br, 2H, NH2 exchangeable with D2O), 9.89 (br, 1H, NH, exchangeable with D2O), 11.45 (s, 1H, NH, exchangeable with D2O). 13C NMR (DMSO-d6, δ, 75 MHz): 91.8 (C-3), 116.4 (C-8), 121.4 (C-4a), 123.5 (C-7), 124.8 (Ph–C), 125.2 (C-6), 126.6, 127.9, 128.2, 128.6 (Ph–C), 133.0 (C-5), 139.5 (CHN),142.7 (Ph–C), 152.5 (C-8a), 161.7 (C-2), 173.4 (CO as C-4) 174.9 (CS). MS (m/z); 338. Anal. Found (Calcd.) for C17H14N4O2S (M.Wt); 338.39. % C: 60.30 (60.34); % H: 4.12 (4.17); % N: 16.23 (16.56); % S: 9.50 (9.48).
The test for the antimicrobial activity was performed on medium potato dextrose agar (PDA), which contained an infusion of 200 g potatoes, 6 g dextrose, and 15 g agar. Uniform-sized filter paper disks (6 mm diameter, 3 disks per compound) were impregnated by equal volume (10 μl) from the concentrations of 500 and 1000 μg mL−1 of the dissolved compounds in dimethylformamide (DMF) and carefully placed on inoculated agar surface. After incubation for 36 h at 27 °C in the case of the bacteria and for 48 h at 24 °C in the case of the fungi, the obtained results were recorded for each tested compound as the average diameter of inhibition zones of the bacteria and fungus around the disks in mm at the concentrations of 500 and 1000 μg mL−1.33
Furthermore, the dynamic equilibrium between the various tautomers of the current compounds (Schemes 5–7) was emphasized by the positive slope of the relationship of their νCOγ–pyrone stretching vibrations that appeared at 1636, 1659, and 1640 cm−1 versus their corresponding amino group in position 2 yielding: νNH2/cm−1 = −8326.9 + 7.113 νCOγ–pyrone/cm−1, r = 0.999, n = 3 (ESI Fig. 4†).
The signals observed at δ 9.60, 8.64, and 8.75 ppm for ACC, ACMHCA, and ACMNPHTCA, respectively, were due to the NH2 protons at position 2 of the chromone moiety, while these signals disappeared in the presence of D2O. Subsequently, the NH2 protons of ACMHCA were in a more electron dense environment related to ACC and ACMNPHTCA (ACMHCA > ACMNPHTCA ≫ ACC). This conclusion was supported by the negative slope of the linear relationship of the highest occupied molecular orbital energy (EHOMO) that measures the electron-donating extent of the molecule (vide infra) versus the chemical shift of δNH2 nmr of the current compounds; whereby EHOMO/eV = 4.948–1.1995 δNH2 nmr/ppm, r = 0.99, n = 3. Further evidence could be provided from the chemical shift values of the H-5 signal in the chromone rings that appeared as a characteristic doublet at δ 8.04, 7.98, and 8.00 ppm for ACC, ACMHCA, and ACMNPHTCA, respectively.
Chemical shifts of the azomethine protons of ACMHCA and ACMNPHTCA were observed at δ 8.38 and 8.73 ppm. This difference agreed with the IR spectral data and could be attributed to the higher electronegativity of the carbonyl of ACMHCA compared to that of the thione of ACMNPHTCA. This interpretation was further evidenced by shifts of the amidic protons (NH2 and NH) at δ 6.38, 9.80 for ACMHCA and δ 9.89, 11.45 for ACMNPHTCA. These signals also disappeared in the presence of D2O.
Since the tautomeric equilibria strongly depend on the polarity of solvent media, so the emission spectra of 1 × 10−5 mol dm−3 of the present compounds were recorded in 11 selected solvents with various polarities.
The electronic spectral data, recorded in DMF solutions, showed electron transition bands at 268, 288, 302 nm for ACC, while its organic hydrazones showed electronic bands at 259, 286, 316 nm for ACMHCA and 261, 289, 355 nm for ACMNPHTCA.
Fig. 1 depicts the electronic and emission spectra of the synthesized compounds in DMF (1 × 10−4 M). The electronic spectra exhibited three main absorption bands, with the first band observed in the range 289–268 nm, which could be attributed to π → π* transition over the whole conjugated system (K-band).16 The second band was observed in the range 355–288 nm and might arise from enhanced n → π* transitions (R-band), resulting from nitrogen, oxygen, and/or sulfur atoms.16 However, the third band revealed the extent of enhancement of the n → π* transition (302, 316, 355 nm) for ACC, ACMHCA, and ACMNPHTCA, respectively, which depended on the ability of the lone pairs to transfer to the π* energy level. This conclusion was further confirmed by the same order of Egap (energy gap) values as those obtained from the theoretical data (Egap/eV: 4.836, 4.295, 3967), thus, vide infra.
Fluorescence offers additional means for characterizing molecular motions and interactions within an investigated compound, thus offering an estimation of its polarity. The fluorescence spectra of 10−5 M solutions of ACC, ACMHCA, and ACMNPHTCA were recorded in 11 solvents with different polarities at room temperature. Fig. 2(a) and (b) depict the fluorescence spectra for ACC, ACMHCA, and ACMNPHTCA in DMF solution, and their emission peak values in different solvents are listed in Table 1.
Fig. 2 (a). Fluorescence spectra for ACC, ACMHCA, and ACMNPHTCA in DMF. (b) Fluorescence spectra for ACC in different solvents. |
No. | Solvent | ACC, λex = 268 | 2-APy | ϕ ACC | ACMHCA, λex = 370 nm | 2-APy | ϕ ACMHCA | ACMNPHTCA, λex = 395 nm | 2-APy | ϕACMNPHTCA | |||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
λem/nm | ε × 106 | Fst × 106 | λem/nm | ε × 106 | Fst × 106 | λem/nm | ε × 106 | Fst × 106 | |||||
a Two standards used, namely 2-APy and QBS, to cover the range of emission wavelengths. | |||||||||||||
1 | 1,4-Diox | 350.53 | 3.505 | 8.59 | 0.090 | 426.80 | 4.272 | 1.95 | 0.548 (0.128) | 436.70 | 4.411 | 1.32 | 0.153 (0.205) |
2 | Benzene | 362.94 | 3.629 | 9.89 | 0.079 | 419.17 | 4.195 | 2.55 | 0.411 (0.199) | 430.40 | 4.347 | 1.68 | 0.119 (0.223) |
3 | CHCl3 | 381.43 | 3.814 | 8.15 | 0.109 (0.784)a | 420.12 | 4.205 | 2.49 | 0.419 (0.158) | 436.10 | 4.405 | 1.31 | 0.154 (0.209) |
4 | Etac | 380.38 | 3.803 | 8.34 | 0.098 (0.929) | 426.47 | 4.269 | 2.00 | 0.529 (0.130) | 479.60 | 4.845 | 0.24 | 0.859 (0.243) |
5 | THF | 432.13 | 4.321 | 1.55 | 0.597 (0.102) | 418.17 | 4.185 | 2.85 | 0.364 (0.181) | 475.60 | 4.804 | 0.28 | 0.786 (0.243) |
6 | DCM | 355.82 | 3.558 | 9.53 | 0.080 | 422.82 | 4.232 | 2.29 | 0.458 (0.174) | 445.50 | 4.500 | 0.91 | 0.227 (0.191) |
7 | 2-PrOH | 329.37 | 3.294 | 2.39 | 0.295 | 424.48 | 4.249 | 2.00 | 0.527 (0.137) | 446.30 | 4.509 | 0.89 | 0.232 (0.192 |
8 | Me2CO | 408.52 | 4.085 | 3.80 | 0.230 (0.275) | 425.14 | 4.255 | 2.04 | 0.517 (0.134) | 495.20 | 5.002 | 0.12 | (0.328) |
9 | EtOH | 427.58 | 4.276 | 1.85 | 0.495 (0.106) | 426.47 | 4.269 | 2.00 | 0.529 (0.13) | 495.30 | 5.003 | 0.12 | (0.327) |
10 | MeOH | 426.33 | 4.263 | 2.00 | 0.457 (0.114) | 426.47 | 4.269 | 2.00 | 0.529 (0.13) | 468.70 | 4.734 | 0.33 | 0.657 (0.216) |
11 | DMF | 432.13 | 4.321 | 1.55 | 0.597 (0.100) | 436.42 | 4.368 | 1.30 | 0.833 (0.11) | 442.63 | 4.471 | 0.99 | 0.207 (0.196) |
Also, the Stokes shifts calculated for the present compounds in diverse polar solvents are listed in Table 2.13,22,23 The magnitude of the Stokes shifts of the current organic compounds showed divergent values in different solvents exposing the ranges 7219.8, 1000.0, and 3045.232 cm−1 for ACC, ACMHCA, and ACMNPHTCA, respectively. The observed Stokes shifts could be due to changes in the polarizability between the excimer and the dissociative ground state. The magnitudes of the shifts in ACC excimer were larger than for its hydrazones, indicating that the ACC excimer was more polarizable.
No. | Solvent | ACC, a = 37313 cm−1 | ACMHCA, a = 26882 cm−1 | ACMNPHTCA, a = 25317 cm−1 | |||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
f | a − f | a + f | (a + f)/2 | f | a − f | a + f | (a + f)/2 | f | a − f | a + f | (a + f)/2 | ||
1 | 1,4-Diox | 28528 | 8785 | 65842 | 32921 | 23431 | 34520 | 50312 | 25156 | 22901 | 24160 | 48217 | 24109 |
2 | Benzene | 27552 | 9761 | 64866 | 32433 | 23856 | 30250 | 50738 | 25369 | 23234 | 20820 | 48550 | 24275 |
3 | CHCl3 | 26217 | 11096 | 63530 | 31765 | 23802 | 30780 | 50.684 | 25342 | 22933 | 23830 | 48249 | 24124 |
4 | Etac | 26289 | 11024 | 63602 | 31801 | 23448 | 34330 | 50330 | 25165 | 20848 | 44670 | 46165 | 23082 |
5 | THF | 23141 | 14172 | 60454 | 30227 | 23913 | 29680 | 50795 | 25397 | 21027 | 42880 | 46344 | 23172 |
6 | CH2Cl2 | 28104 | 9209 | 65417 | 32708 | 23650 | 32300 | 50532 | 25266 | 22449 | 28670 | 47765 | 23882 |
7 | 2-PrOH | 30361 | 6952 | 67674 | 33837 | 23558 | 33230 | 50439 | 25219 | 22404 | 29120 | 47720 | 23860 |
8 | Me2CO | 24479 | 12835 | 61792 | 30896 | 23521 | 33600 | 50403 | 25201 | 20193 | 51230 | 45509 | 22754 |
9 | EtOH | 23387 | 13926 | 60700 | 30350 | 23448 | 34330 | 50330 | 25165 | 20188 | 51270 | 45505 | 22752 |
10 | MeOH | 23456 | 13857 | 60769 | 30384 | 23448 | 34330 | 50330 | 25165 | 21337 | 39790 | 46653 | 23326 |
11 | DMF | 23141 | 14172 | 60454 | 30227 | 22913 | 39680 | 49795 | 24897 | 22592 | 27240 | 47080 | 23954 |
However, fluorescence is very sensitive to the polarity of the surrounding environment, showing a clear bathochromic shift in all cases here as the solvent polarity increased. Among the three molecules, ACC showed the maximum red-shift (82 nm) and ACMNPHTCA also exhibited a red-shift (65 nm). On the other hand, ACMHCA had a lower tendency for solvent interactions, resulting in a lesser red-shift of the fluorescence band (18 nm). These results confirmed that the singlet excited state energy level was more stabilized than the ground state energy level. The large fluorescence spectral shifts suggest the titled molecules were more polarized in the singlet excited state than in the ground state.18 The solvent effect will be discussed in detail in the solvatochromic section.
This interpretation was further confirmed by the negative slopes of ΔλStoke/nm versus either the theoretical calculated dipole moments of the ground states (μg D−1) or the stretching frequencies of the carbonyl and amino groups attached with the chromone moiety (νNH2/cm−1): ΔλStoke/nm = 381.28 − 50.999 μg D−1, r 0.997, n = 3, ΔλStoke/nm = 4472.5 − 2.6854 νCO/cm−1, r = 0.995, n = 3, and ΔλStoke/nm = 1331.5 − 0.3784 νNH2/cm−1, r = 0.999, n = 3.
Moreover, the absorption and fluorescence data of the investigated compounds in different solvents are given in Table 2. Upon increasing the solvent polarity, the Stokes shift values were found to vary from 8785.2 to 14172.24, 2968 to 3968, and 2082 to 5127 cm−1 for ACC, ACMHCA, and ACMNPHTCA, respectively. These great differences in Stokes shift values (5387, 1000, and 3045 cm−1) for ACC, ACMHCA, and ACMNPHTCA indicated a bathochromic shift and ICT due to π → π* transition and confirmed the molecules were more stabilized in polar solvents than in nonpolar ones. This arose from the strong interaction of the singlet excited state with polar solvents; thus, there was a large charge distribution between ground and singlet excited state of the solute molecules, i.e., ACC showed a more pronounced red-shift than its hydrazones ACMHCA and ACMNPHTCA. This finding could be deduced as due to the greater electronic delocalization of ACC than its hydrazones.
The quantum yields (ϕ) of ACC, ACMHCA, and ACMNPHTCA were determined using a single point method using 2-aminopyridine as a standard reference, such as in the reported method.38 The quantum yields determined were 0.10, 0.11, and 0.196 in higher polar DMF solution, and 0.08, 0.199, and 0.223 in the least polar benzene solution, for ACC, ACMHCA, and ACMNPHTCA, respectively, as shown in Table 1. These lower quantum yield values revealed the low lifetime of the excited molecules of the current compounds, whereby they could not be fully protected from collisions with quenchers, in addition to the formation of excimers and/or exciplex. This finding is discussed in detail below.
Furthermore, the ICT of the present compounds depended on the type of solute–solvent interactions. Increasing the polarity of the solvent quenched the fluorescence emission, as evidenced by the negative slopes of the quantum yield (ϕ) with the specific solvent parameters, such as DN, AN, and HBA (β). In contrast, the nonspecific interactions, such as π*, enhanced the fluorescence emission, as indicated from the positive slope of (ϕ) with π* (ESI Fig. 8†).
The solvatochromic data of the present compounds were analyzed by the linear solvation energy relationships (LSERs) depending on various solvent parameters, such as DN, AN, ET, refractive index (n), dielectric constant (ε), and Kamlet–Taft parameters (π*, β, and α). The LSER method was carried out using the solvent parameter as the independent variable and Stokes shift as the dependent variable; first correlated individually with each one of the solvent parameters to assess them for their ability to provide a reasonable explanation.
The analysis results of the LSERs for the Stokes shifts data of the investigated compounds with various solvent parameters are summarized in ESI Table 1.†21 The results presented in ESI Table 1† can be evaluated upon the sign of the slopes, the correlation coefficient (r), the number of points that obey the straight line, and the coefficient value of the variables solvent parameter to provide the following information:
1- The linear relationship of νStoke/cm−1 versus DN (Lewis basicity) of the solvents (ESI Fig. 9†) for ACC, ACMHCA, and ACMNPHTCA reveals the acidic character of the investigated compounds at N+H2 (tautomer B) as indicated from the positive slopes. Furthermore, the trend of the coefficient values (162.5 > 27.7 < 91.3) reflected the weakest acidity of ACMHCA, i.e., the degree of acidity of ACC was ∼6 fold that of ACMHCA and 1.8-fold that of ACMNPHTCA.
2- However, the positive slope of νStoke/cm−1 versus (AN + DN), where AN refers to the Lewis acidity (ESI Fig. 10†), indicates the presence of a basic center, such as the lone pairs on nitrogen, oxygen, and sulfur, besides the carbanion (C–O−) in tautomer B, beside the acidic centers. The extent of the basicity could be deduced from their coefficients: 91 > 6.62 <45.12 in the ratio of 14:2:1 for ACC, ACMHCA, and ACMNPHTCA, respectively. This result agreed well with that concluded from their 1H-NMR data and its correlation with EHOMO/eV given above.
3- The relationship of νStoke/cm−1 versus ENT shown in ESI Fig. 11† indicates that ACMHCA had the least polarity of the investigated compounds as designated from their slope's coefficient order: 256.8 > 20.65 < 77.8 for ACC and ACMHCA, and ACMNPHTCA, respectively. The positive slope revealed that the polarity degree as inferred from their coefficients for ACC was ∼12.4 fold that of ACMHCA and 3.3-fold that of ACMNPHTCA.
4- The relationship of νStoke/cm−1 versus α (solvent hydrogen-bond donor, HBD) presented in ESI Fig. 12† illustrates that the HBD was more prominent in ACC and ACMNPHTCA than ACMHCA as indicated from the coefficient of their slopes (3391 ≫ 418.81 ≪ 3155), respectively. The positive slope revealed that the extent of hydrogen-bond-acceptor centers of ACC was about 8 times that of its hydrazone ACMHCA, indicating the consumption of most acceptor centers during the formation of the hydrazones.
5- Whereas, the relationship of νStoke/cm−1 versus β (hydrogen-bond acceptor, HBA) presented in ESI Fig. 13,† shows the effects of two opposite groups for ACC and ACMNPHTCA (- & + slopes) referring to the dual nature of these compounds, and a poor correlation in the case of ACMHCA. The negative slope indicated hypsochromic shifts occurred with increasing the solvent hydrogen bonding effects. The strength of the HBD ability could be deduced from their coefficient values of β (6241.4 > very bad < −4618.5, +4149.1) for ACC, ACMHCA, and ACMNPHTCA, respectively. Thus, the hydrogen-bond-donor property of ACC was ∼1.5-fold that of ACMNPHTCA.
6- Finally, the relationship of νStoke/cm−1 versus π* (dipolarity/polarizability) demonstrated in ESI Fig. 14† verifies the role of nonspecific solute–solvent interactions in the changes of the emission spectra of the current compounds with two opposite groups for ACC and ACMNPHTCA, but the relationship was very bad for ACMHCA. The extent of the role of the nonspecific interactions can be deduced from their coefficients of π* (+13859, −13859 > 2849.7 < −6299.9) for ACC, ACMHCA, and ACMNPHTCA, respectively. The presence of two groups with opposite signs of the slope in the cases of ACC and ACMNPHTCA refers again to the dual nature of these compounds depending on the type of solvent used.
Here, the absorption maximum was almost independent of the nature of the solvent, while the emission maxima were controlled by the solvent polarity. The absolute values of the α, β, and π* coefficients of ACC were 3391, 6241.4, and 13859; (the ratio was nearly 1:2:4), respectively. Consequently, the effect of the dispersion–polarization forces was greater than the effect of the orientation–induction interactions. Also, it was observed that the β coefficient was significantly higher than the α coefficient. Thereby, the H-bond-accepting ability was more powerful than the H-bond-donating ability. The high absolute value of the β coefficient compared to the α coefficient revealed the tendency of a bathochromic effect.39
As mentioned above, most the linear relationships of ACMHCA were bad, which might be attributed to the presence of CO, which is harder than CS in the other hydrazone. So, multilinear regression (MLR) was used instead, yielding: νStoke/cm−1 ACMHCA = 24107 cm−1 + 412 α − 1168 β − 382 π*, r = 0.912, n = 10. The negative slopes for the β (HBA) and π*, and positive for α (HBD), correlations suggest that the HBD (specific interaction) enhances the red-shift, but HBA (specific interaction) acts in the opposite direction (causes a blue-shift) in addition to the effect of the nonspecific interaction (π*). Finally, it could be concluded that ACMHCA has weak solvatochromic behavior in comparison with ACC and ACMNPHTCA.
Furthermore, the weight of each solvent parameter on the solvatochromic behavior could be deduced by considering the MLR yields: νStoke/cm−1 ACMNPHTCA = 23105 cm−1 + 736 α − 4451 β + 187 π*, r = 0.90, n = 8. The positive slopes for α and π*, but negative for β correlations suggest the nonspecific interactions could be measured using the solvent's dielectric parameters, such as π*, whereas specific interactions could be measured using α (hydrogen-bond-donor strength) and β (hydrogen-bond-acceptor strength) in addition to the DN (Lewis basicity, AN (Lewis acidity), and ET parameters.
The ratios between the contributions of the solvatochromic parameters for the current compounds given above show that most of the solvatochromism was due to the solvent dipolarity/polarizability rather than to the solvent acidity and basicity. These results could be explained by the effect of the positive charge on the nitrogen atom in the N+H2 and hydrazone tautomers (ACMHCA and ACMNPHTCA).
Consequently, the effect of the dispersion–polarization forces was greater than the effect of the orientation–induction interactions. However, the H-bond-donating ability dominated over the ability to accept H-bonding, as evidenced from the α coefficient being greater than the β coefficient, indicating a tendency for a bathochromic effect.39 Accordingly, the solvent-dependent shift of the fluorescence spectra of the present compounds could be attributed to various factors, such as:
(1) Dipole–dipole interactions between the solvent and solute.
(2) Change in the nature of the emitting state induced by the solvent.
(3) Specific solvent–solute interactions, such as H-bonding.
The shifts of the emission peaks with the solvent polarity changes were more pronounced than the shifts of the absorption peaks. This indicates that Δμ = μ* − μ, was positive, i.e., the dipole moments of all the systems studied here increased upon excitation. The calculated values of the correlation coefficients of the Stokes shifts versus the bulk solvent polarity functions F(ε,n) were analyzed using the proposed linear correlation methods13,22,23 reported by Bilot–Kawski, Lippert–Mataga, Bakhshiev, and Kawski–Chamma–Viallet, as well as by the microscopic solvent polarity parameter ENT40–42 (ESI Table 2†).
The ground state (μ) and excited state (μ*) dipole moments and Onsager cavity radii (a) of ACC, ACMHCA, and ACMNPHTCA were calculated as reported22,23 and were found to be 2.99, 3.47, and 4.42 Å, respectively (Table 3). ESI Table 3† presents the slopes obtained from the plots of the Stokes shift and averages ((a + f)/2) versus the bulk solvent polarity functions. The changes in dipole moment, μ, values derived from the obtained slopes (m) are given in ESI Table 1.† The theoretically calculated ground state dipole moments showed the highest value for the hydrazone ACMHCA, which may be due to the presence of electron-donating and electron-withdrawing groups located at the extreme ends, which result in maximum charge separation, compared to ACC, which has the minimum charge separation, as shown in Scheme 5, whereas ACMNPHTCA had the highest dipole moment in the excited state. The disagreement between the experimental and theoretical values of μ may be attributed to the stronger specific solute–solvent interaction in the solvent media, which are absent in the vapor phase. It was noticed that the μ values obtained from the Lippert–Mataga method were relatively large compared to the values obtained by the other methods, especially for ACMHCA, since it does not consider the polarizability effect of the solute. The change in dipole moment upon excitation can be considered a result of the nature of the emitting state and charge transfer.
Compound | a | μga | aμ* | μgb | bμ* | μgc | cμ* | Δμd | μ*/μg |
---|---|---|---|---|---|---|---|---|---|
a Ground state and excited state dipole moments estimated by Gaussian09 using DFT software.b Calculated from Bakhshiev's (F1, F2) equation.c Calculated from Kawski–Chamma–Viallet's (F2, F3) equation.d Calculated from ENT.e Using F1, F2 results. | |||||||||
ACC | 2.99 | 5.91 | 7.34 | 2.34 | 4.48 | 0.367 | 4.19 | 2.485 | 1.915 |
ACMHCA | 3.465 | 0.88 | 2.66 | 1.798 | 3.392 | 3.071 | 4.484 | 1.918 | 1.89 |
ACMNPHTCA | 4.42 | 0.87 | 2.21 | 2.337 | 5.49 one gp | 1.389 | 6.359 | 2.544 | 2.35 |
— | — | — | 1.69 | 5.904 gp1 | — | — | 3.034 | 3.49 | |
— | — | — | 2.58 | 5.020 gp2 | — | — | 2.540 | 1.946 |
It could be seem that the experimental dipole moment changes (μ) (Table 3) were smaller than the theoretical estimates for all cases. The ratios of the singlet excited state dipole moment and ground state dipole moment (μ*/μ) experimentally and calculated were found to be 1.915, 1.89, and 2.35 (Table 3).
Linear graphs of the Stokes shift (νa − νf) versus F1 (FL-M), F2 (FB), and ENT, as well as average absorption and fluorescence (νa + νf)/2 versus F3, were plotted for ACC, ACMHCA, and ACMNPHTCA. The solvatochromic plots for ACC are shown in Fig. 3(a–d).
The ground (μ) and singlet excited state dipole moments (μ*) were found to be 2.34 D, 4.48 D (for ACC), 1.798 D, 4.48 D (for ACMHCA), and 2.17 D, 5.42 D (for ACMNPHTCA), respectively, and are tabulated in Table 3. As seen in this table, significant differences were found for the singlet excited state dipole moment values of the studied compounds, depending on the type of solvatochromic relationship. These differences in the estimated dipole moment values were very probably due to a number of assumptions and approximations that were made concerning the validity of their use; for example, (a) the dipole moments in the ground and excited states were supposed to be approximately collinear43 and (b) the solute molecules were considered to be spherical and consequently, the dipole moment was supposed to remain constant in the Onsager cavity21 as pointed out in the literature.44,45
The difference in dipole moment between the ground and singlet excited states suggests that the current molecules had considerable charge distribution in the singlet excited state, which plays an important role in the ICT process. The singlet excited state dipole moment values were found to be higher than that of the ground state, thereby confirming that the molecules were more polarized in the singlet excited state than in the ground state. This also suggests that the molecules have strong solute–solvent interactions, causing a large charge distribution in the singlet excited state.
As can be noted in Table 3, the excited state dipole moments (4.48, 3.39, and 5.49 D) were found to be higher than the ground state dipole moments (2.34, 1798, and 2.337 D). The differences ranged from about 2.14–3.15 D, indicating that the studied molecules were significantly more polar in their excited state than in their ground state. Therefore, the solvent–solute interactions should be stronger in the excited state than in the ground state, demonstrating an important redistribution of charge densities between both electronic states.
The ground and excited state dipole moments of the current compounds were also estimated assuming that they were parallel such as previously reported.46–50 The estimated values are depicted in Table 3. The difference in values of μg and μe were compared to the respective values obtained from other methods (ESI Table 3†) and suggest that μg and μe were not parallel. This prompted us to estimate the angle between μg and μe as reported previously23 and the values were found to be 65.07°, 66.51°, and 76.71°, showing that μg and μe were not parallel in all the investigated compounds.
Therefore, these angles suggest that there was charge movement across the molecules. The direction of the dipole moment vector in a molecule depends on the centers of the positive and negative charges. Although, non-parallelism between the ground and excited state dipole moments hinders charge migration across the molecule in the excited state more than in the ground state, the angles between them were in the range 65.07°–76.71°, which is an additional reason for the decrease in the quantum yields.
From Table 3, it could be observed that the dipole moment of the present organic compounds was higher in the first-excited state compared to in the ground state. The dipole moment increased almost two times upon excitation. This indicates the existence of a more relaxed excited state, due to ICT being favored by the cooperative effects of the 2-aminochromone moieties as donors and the carbonyl/thione groups as acceptors, and suggesting that the present compounds can serve as good nominee components of nonlinear optical materials.51
The discrepancies between the experimental and theoretically obtained data using DFT results, which were 5.914, 7.13, and 6.14 D for ACC, ACMHCA, and ACMNPHTCA, respectively, could be attributed to the fact that the theoretical calculations involved the gaseous phase of an isolated molecule while the experimental results were related to solutions.
Fig. 4 Contour plots of the HOMO and LUMO of ACC, ACMHCA, and ACMNPHTCA obtained at the B3LYP/6-311G(dp) level of theory. |
The energy gap values between the HOMO and LUMO energy levels for the reported molecules (ESI Table 4†) can help describe the chemical reactivity, thermal and kinetic stability, optical polarizability, as well ICT transition in the molecules, as concluded in the following approaches. The HOMO–LUMO energy gap for ACC had the highest value, 4.836 eV, whereas ACMNPHTCA had the lowest value, 3.976 eV, as indicated in ESI Table 4.† A low value for the energy gap suggests that a molecule is more reactive, soft, and has easier π−π* electronic transition. Moreover, the Egap values followed the order: ACC > ACMHCA > ACMNPHTCA; subsequently, ACC was the most stable and ACMNPHTCA the most reactive. The EHOMO values followed the order: ACC < ACMHCA ∼ ACMNPHTCA, while the ELUMO values followed the order: ACC < ACMNPHTCA < ACMHCA.
The bond lengths of CO15, CO27, CN18, CN19, and N18–N19 were computed and found to be 1.25984, 1.25033, 1.30903, 1.39021, and 1.39306 Å, respectively. Thus, the higher electronegativity (more charge density) of N19 related to N18 was antithetical and this was reflected by an elongation in the CN18 (1.30903 Å) and CN19 (1.39021 Å) bond lengths (ESI Table 4†).
The data in ESI Table 4† demonstrate that ACC was the hardest and most stable (less reactive) as it had the highest η value, as opposed to ACMNPHTCA, which was the softest and most reactive.
According to the data reported in ESI Table 4,† the electrophilicity index (ω) values followed the order: ACC > ACMNPHTCA> ACMHCA; which was the same order as the increasing electrophilic character, which controlled the chemical reactivity of the system.
Since, the electronic chemical potential (μ) is defined as the tendency of electrons to escape from the equilibrium state, so ACC was the most reactive, while ACMHCA was the least reactive as it had the lowest μ value. This outcome can be supported by the positive slope of the linear relationship of chemical potential versus the 1H NMR data, μ = −3.8242 + 0.83181 δ/ppm NH2, r = 0.998, n = 3, as illustrated in ESI Fig. 18.†
Compound | Charges | Bond lengths | ||||||||
---|---|---|---|---|---|---|---|---|---|---|
O14 | O27 | S24 | N21 | N22 | CO15 | CS24 | CN18 | CN19 | N–N | |
ACC | −0.599 | — | — | — | 1.258 | — | — | — | — | |
ACMHCA | −0.624 | −0.633 | — | −0.312 | −0.458 | 1.259 | 1.798 | 1.309 | 1.390 | 1.393 |
−0.457 | −0.464 | −0.545 | −0.329 | |||||||
ACMNPHTCA | −0.503 | — | −0.021 | −0.309 | −0.398 | 1.259 | 1.717 | 1.309 | 1.386 | 1.412 |
−0.457 | — | −0.062 | −0.338 | −0.525 |
Voc = EHOMO (comp) − ELUMO (PCBM/TiO2) |
As summarized in ESI Table 4,† the VOC values were 2.567–1.326 eV and 2.876–1.626 eV according to the first formula, but the second formula gave ranges of 2.814–2.26 and 2.514–0.994, with respect to TiO2 and PCM, respectively. These values are adequate to ensure an efficient electron injection, suggesting our molecules would be good applicants for organic solar cells.
ACC | ACMHCA | ACMNPHTCA | ||||||
---|---|---|---|---|---|---|---|---|
Atoms | Calculated | Experimental | Atoms | Calculated | Experimental | Atoms | Calculated | Experimental |
18-H | 5.537718 | 9.60 | 25-H | 3.774216 | 6.38 | 17-H | 3.480961 | 8.75 |
6-H | 7.571464 | 7.55 | 26-H | 4.05817 | 6.38 | 25-H | 5.778975 | 11.45 |
12-H | 7.653417 | 7.40 | 17-H | 5.454761 | 8.64 | 34-H | 7.367763 | 7.17 |
13-H | 7.931241 | 7.79 | 6-H | 7.551962 | 7.36 | 6-H | 7.447364 | 7.33 |
11-H | 8.410165 | 8.04 | 12-H | 7.572793 | 7.41 | 12-H | 7.631333 | 7.35 |
19-H | 10.21059 | 9.60 | 28-H | 7.589662 | 9.80 | 38-H | 7.753892 | 7.37 |
20-H | 10.5051 | 10.14 | 13-H | 7.862116 | 7.66 | 32-H | 7.824341 | 7.39 |
11-H | 8.424244 | 7.98 | 37-H | 7.84938 | 7.44 | |||
19-H | 8.604212 | 8.38 | 36-H | 7.851485 | 7.47 | |||
18-H | 9.574084 | 8.64 | 13-H | 7.92449 | 7.69 | |||
11-H | 8.18711 | 8.00 | ||||||
26-H | 8.449107 | 9.89 | ||||||
19-H | 8.893963 | 8.73 | ||||||
18-H | 9.775394 | 8.75 |
The data in Table 5 displayed a chemical shift value of the aldehyde proton of the ACC compound, which appeared as a singlet signal at δ 10.505 ppm, which was higher than the analogous experimental value of δ 10.14 ppm. Similarly, the experimental singlet signals of CHazomethine at δ 8.38 and 8.73 ppm were lower than those calculated at δ 8.604 and 8.893 ppm for ACMHCA and ACMNPHTCA compounds, respectively. In contrast, the NH protons theoretically had a lower chemical shift than that observed in the experimental spectra in Table 5. The divergence between the theoretical and experimental data might arise from the differences between a single molecule and the solution medium, respectively. The correlations between the observed and calculated chemical shift values were predicted for the target ACC compound, as δ/ppm (exp) = 0.774 + 0.877 δ/ppm (calc), R2 = 0.99. As concluded from the slope, the agreement between the theoretical and experimental 1H NMR spectral data was ≈ 88%.
Organism | Mean of zone diameter, nearest whole mm | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Gram-positive bacteria | Gram-negative bacteria | Yeasts and fungi | ||||||||||
Staphylococcus aureus (ATCC 25923) | Bacillus subtilis (ATCC 6635) | Salmonella typhimurium (ATCC 14028) | Escherichia coli (ATCC 25922) | Candida albicans (ATCC 10231) | Aspergillus fumigatus | |||||||
Concentration | 1 mg ml−1 | 0.5 mg ml−1 | 1 mg ml−1 | 0.5 mg ml−1 | 1 mg ml−1 | 0.5 mg ml−1 | 1 mg ml−1 | 0.5 mg ml−1 | 1 mg ml−1 | 0.5 mg ml−1 | 1 mg ml−1 | 0.5 mg ml−1 |
Sample | ||||||||||||
ACC | 22 I | 19 H | 31 H | 22H | 29 H | 26 H | 24 I | 20 H | 33 H | 22 H | 25 I | 20 H |
ACMHCA | 6 L | 3 L | 5 L | 2 L | 4 L | 2 L | 4 L | — | 7 L | 3 L | 5 L | 3 L |
ACMNPHTCA | 7 L | 3 L | 8 L | 2 L | 5 L | 2 L | 6 L | 2 L | 6 L | 3 L | 4 L | 2 L |
Control c | 35 | 26 | 35 | 25 | 36 | 28 | 38 | 27 | 35 | 28 | 37 | 26 |
Increasing the electronegativity of the present compounds (opposite to the chemical potential, μ) enhanced their antimicrobial activity against all the screened microorganisms as deduced from ω the negative slopes of the linear relationships of μ versus the normalized biological activity data: G + 1 = −2.7152 − 0.83828 μ, r = 0.999, n = 3, F = 11637; G − 1 = −3.001 − 0.9138 μ, r = 0.99, n = 3, F = 107.88; Y1 = −3.089 − 0.953 μ, r = 0.98, n = 3, F = 57.30; F1 = 3.334 − 1.007 μ, r = 0.982, n = 3, F = 54.32.
Moreover, the role of the chemical potential on the antimicrobial activity followed the order: G + 1 < G − 1 < Y1 < F, as deduced from their slope's values. Whereas, the positive slope of the normalized G + 1 versus electrophilicity, ω, revealed an enhancement with the increasing electrophilicity: G + 1 = −1.830 + 0.710 ω, r = 0.90, n = 3, F = 8.2.
However, the contribution of two structural parameters could be obtained using the MLRA: G + 1 = −0.2620 − 0.3017 dipole + 1.165 hardness, r = 1.0; G −1 = −0.9218 − 0.2801 dipole + 1.399 hardness, r = 1.0; Y1 = −1.175 − 0.2712 dipole + 1.516 hardness r = 1.0; and F1 = −1.339 − 0.2843 dipole + 1.607 hardness, r = 1.0.
The increase in dipole moment thus deactivates the biological activity of the investigated compounds against all the screened microorganisms, and vice versa for the effect of hardness parameter as concluded from their opposite slopes of their linear relationships. The weight of each parameter was deduced from the analogous coefficient, which revealed the dipole moment has a similar effect on all the screened organisms, but the hardness effect takes this order: G + 1 < G − 1 < Y1 < F1.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ra05081e |
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