Charge Transfer Complex between O-Phenylenediamine and 2, 3-Dichloro-5, 6-Dicyano-1, 4-Benzoquinone: Synthesis, Spectrophotometric, Characterization, Computational Analysis, and its Biological Applications.

UV-vis electronic absorption spectroscopy was used to investigate the new molecular charge transfer complex (CTC) interaction between electron donor O-phenylenediamine (OPD) and electron acceptor 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ). The CTC solution state analysis was carried out by two different polarities. The stoichiometry of the prepared CTC was determined by using Job's, photometric, and conductometric titration methods and was detemined to be 1:1 in both solvents (at 298 K). The formation constant and molar extinction coefficient were determined by applying the modified (1:1) Benesi-Hildebrand equation. The thermodynamic parameter ΔG° result indicated that the charge transfer reaction was spontaneous.The stability of the synthesized CTC was evaluated by using different spectroscopic parameters like the energy, ionization potential, oscillator strength, resonance energy, dissociation energy, and transition dipole moment. The synthesized solid CTC was characterized by using different analytical methods, including elemental analysis, Fourier transform infrared, nuclear magnetic resonance, TGA-DTA, and powder X-ray diffraction. The biological evolution of the charge transfer (CT) complex was studied by using DNA binding and antibacterial analysis. The CT complex binding with calf thymus DNA through an intercalative mode was observed from UV-vis spectral study. The CT complex produced a good binding constant value (6.0 × 105 L.mol-1). The antibacterial activity of the CT complex shows notable activity compared to the standard drug, tetracycline. These results reveal that the CT complex may in future be used as a bioactive drug. The hypothetical DFT estimations of the CT complex supported the experimental studies.

Introduction

The interaction of an electron donating precursor with an electron accepting substrate can produce a new molecular aggregate in the ground state called CTC (electron transfer donor–acceptor complex).[1,2] The quantum chemistry of CTCs based on the formation of colored complexes through the electronic interaction between donor and acceptor was proposed by Mulliken in the 1950s.[3] Foster also examined that colored complexes arise from Lewis acid–base interactions.[4] The CT systems create weaker bonds between donor and acceptor[5,6] which play a valuable role in our day-to-day life through their various material applications. They have great significance in the field of DNA binding, drug receptors, and antimicrobial and antibacterial mechanisms.[7,8] CTCs[9−14] have been used in various categories of science like optics, solar physics, photonic science, chemosensors for sensing hazardous materials, materials chemistry, and catalysis.

The acceptor DDQ, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone, is an important substrate for many chemical and biochemical processes.[15−21] Free extreme anion is acquired during donor–acceptor electronic interaction.

The multisite donor, O-phenylenediamine (OPD), is also called 1,2-diaminobenzene. It has many applications in organic and inorganic chemistry and biochemistry. It is used in identification of materials, analytical chemistry, ELISA procedures, detection of transforming growth factor, etc. The reactions of OPD with π-electron acceptors were explored due to their peculiar properties.[22−26] The reaction between donor and acceptor resulted in a donor–acceptor CT complex. The intercalation binding mode of a radical ion pair of the complex is observed through a DNA binding study.

In general, the hydrogen-bonded complexes, which have hydrogen bonds of positively charged hydrogen atoms, interact with an electron donor. They are the strongest intermolecular interactions. Similarly, the halogen bond complexes which generate halogen bonds contain a halogen atom, usually iodine, and act in a similar fashion to hydrogen bonds. In a three-component system, there can be competition between the formation of hydrogen bonds and halogen bonds.[27] The main objective of this work is to verify the CTC stoichiometry with the mechanism and stability in the solution state, as well as to investigate the spectroscopic characterization of the [(OPD)(DDQ)] complex to elucidate the nature of the multisite donor–acceptor contact with the thermal analysis techniques (TGA and DTA) in the solid state.

Results and Discussion

Observation of Charge Transfer Band

Figure a describes the UV–vis spectra of the OPD, DDQ, and CTC (OPD, 1 × 10–4 mol L–1 + DDQ, 1 × 10–4 mol L–1) in AN and MeOH, respectively. The electronic CT-bands (λmax > 400 nm) are observed from the reaction of donor and acceptor in AN and MeOH, which do not exist in individual spectra. The UV–vis spectrum of the solid state CT-complex also shows two absorption bands at 368 and 504 nm (Figure b). Results are also shown for calculated UV–vis spectrum from TD DFT B3LYP+6-31G (Figure S10).

Figure 1

(a) CT-band observation plots in polar solvents. (b) Observation of CT-bands of solid CT complex.

The new multiple (three) bands were centered at λmax 434, 542, and 588 nm in AN and later shifted to λmax 415 nm (Figure S1), which could be observed from a color change from red–brown (<25 °C) to yellow (>25 °C), attributed to the interaction of multisite donor with acceptor as showed in Scheme . This was because of the electronic path interchange of CTC in AN medium at 298 K. In methanol, a single stable CT-band formed at λmax 463 nm (i.e., yellow-colored solution appeared at 298 K). The shifting of the position of the CT-band (as bathochromic shift) reflects the sensitivity of the [(OPD)(DDQ)] complex to the solvent medium polarity.

Scheme 1

Possible Pathways of the Radical Ion Pair Formation

The occurrence of stable CT bands specifies electron transfer from OPD to DDQ.[35−37] The electron transfer corresponds to the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) interaction.[38] Further, the color change is the primary indication for the CT-complex. The lone pair (N-electrons) and ring π-electrons of OPD are responsible for the participation in the electronic interaction in the formation of CTC. At low temperatures, a lone pair of N atoms actively participated in the donation. The π-electrons were involved in electron donation toward acceptor by increasing the temperature, and this was noticed by a visible color change (from red–brown to yellow).[39] Finally, a single absorption band (monowavelength) was monitored and changed to a yellowish color. The chosen polar solvents were utilized for the verification of the electron transfer mechanism of CTC.[40] The CTC was monitored as stable for more than 1 h due to the strong interaction in solution phase.

Stoichiometric Relation and Polarity Effect

Three methods were used for the identification of the stoichiometric relation in the CT-complex in chosen polar solvents. The sum of the moles of acceptor and donor solutions were kept constant in the Job’s continuous variation (first) method. Job’s method was published in 1928. He called it a method of continuous variation, a method used in analytical chemistry to determine the stoichiometry of a binding result. In his work, he plotted UV absorbance of Tl(NO3)/NH3 against the mole fraction of Tl(NO3). He produced a graph which provided equilibrium complexes present in solution. So, this method is used to determine the amount of donor binding to acceptor. By applying this method, the absorptions of the CT-complex vs mole fraction of DDQ graphs are shown in Figure S2. From this figure, the maximum absorbance (peak point) observed at 0.5 mole fraction (DDQ) represents the 1:1 composition for the CTC in a polar solvent medium. In the photometric titration (second) method, the two lines were produced which intersecting at a 1:1 (D:A) molar ratio (Figure S2).

Similarly, the maximum conductance observed from the titration between 5.0 mL of electron donor and 5.0 mL of acceptor in (third) conductometric method indicates 1:1 composition of CTC (Figure S3). Therefore, the 1:1 (donor: acceptor) composition was confirmed at room temperature. The electron transfer progressed with the amplifying polarity of the solvent medium. In polar media, a medium of maximum dielectric constant and the resulting CTC may dissociate into radical ions which gives appreciable conductivity.

Absorbance versus Time

The CT-complex spectrum depends on the time as well as the media. The 1.0 × 10–4 mol L–1 of OPD was mixed to 1.0 × 10–4 mol L–1 of DDQ for the observation of electronic spectra at room temperature. In MeOH, the absorption band of the CTC enhanced was with time, and the band was shifted to a lower wavelength, i.e., hypsochromic at λ463.

Similarly, the absorption band was increased at λ434 and decreased at λ588 with time by using the AN solvent (Figure and Figure S4). This was because of the orientation of the electron transfer and was understood as the CTC being strong.[41]

Figure 2

Variation of absorbance with time in solvents (A) AN and (B) MeOH.

Determination of Stability Constant (KCTC)

The formation constant or stability constant (KCTC) and molar extinction coefficient (εCTC) were estimated by the 1:1 Benesi–Hildebrand method provided at 298 K in two chosen polar solvents.[42]where Ca and Cd were correspondingly denoted as concentrations of the DDQ and the OPD substrates. A denoted the absorbance at different polarities.

In AN, the absorbance values are taken after 5 min of mixing of both acceptor and donor solutions in stoichiometric ratio at 415 nm. The absorbance of [(OPD)(DDQ)] complexes was enhanced by mixing of 5.0 × 10–4 mol L–1 DDQ substrate with dissimilar concentrations of the OPD from 1.0 × 10–4 mol L–1 to 5.0 × 10–4 mol L–1 in both polar solvents. It revealed the CT band to be stable at all concentrations in two polarities. In plots of (CaCd/A) versus (Cd + Ca), straight lines were fitted for understanding of the formation of 1:1 stable CT complexes. The slope and intercept denote the (1/ε) and (1/KCTCε), in that order. The B–H graphical representations in AN and MeOH are expressed in Figure . The descriptors of Ca, Cd, A, (Ca + Cd), and CaCd/A are listed in Table S1.

Figure 3

Observation of B–H plots for the CTC in AN and MeOH.

From the Table S1, we observed that the stability constant KCTC and molar absorptivity ε values were high positive values in AN than in methanol. These results showed that the observed CT complex was more stable in acetonitrile than in methanol due to resonance of N electrons and π–π* interaction between OPD and DDQ. This led to a high formation constant and molar absorptivity in AN compared with MeOH. In addition, by observing the KCTC value, the CT complex in AN is 125.09 × 102 L mol–1 and the CT complex in MeOH is 56.50 × 102 L mol–1, and dielectric constant of AN (37.5) is greater than that of MeOH (32.7). It is confirmed that the CT complex is more stable in AN than MeOH.

Evaluation of the Spectroscopic Physical Parameters

Band Gap Energy (ECTC) of the CT-Complex

The energy of the CTC (ECTC) value obtained[43,44] by the following relation:where denotes the wavelength (nm) at CT-absorption band observed from the abs versus wavelength plot. The ECTC supports the strength of the donor–acceptor complex.The values of ECTC are shown in Table .

Table 1

Spectroscopic Physical Descriptors of CT-Complex in Two Different Solvents

Media	λmax (nm)	IP-ID (eV)	ECTC (eV)	W (eV)	–ΔG° (kJ mol–1)	RN (eV)	f	μ (D)
AN	434.0	9.262	2.865	4.447	23.37	0.81	1.94	13.3
	542.0	8.564	2.294	4.32
	588.0	8.345	2.115	4.28
MeOH	463.0	9.043	2.686	4.407	21.40	0.76	1.27	11.2

Oscillator Strength (f) and Transition Dipole Moment (μ in Debye)

The charge migration probability is expressed by the oscillator strength (f), which is a dimensionless variable. We can bring out the f values from the spectrum of CT absorption and transition dipole moments (μ in Debye) of the donor–acceptor complex[45−48] were estimated from the subsequent relationswhere denotes the extinction coefficient of the CT-peak and denotes the width of the half CT-band at maximum absorption value.where is the half bandwidth (in cm–1), and εmax and are the extinction coefficient and wavenumber at the maximum absorption of the CT-band, respectively. The results have been presented in Table .

Standard Free Energy Change

For the complex [(OPD)(DDQ)], ΔG° standard free energy change (units taken in kJ mol–1) values were calculated by using following expression.[49]where ΔG° is the change free energy of CT-complex, R referred as gas constant (8.3144 J mol–1 K–1), T denotes temperature in Kelvin, KCTC is the stability or formation constant of the [(OPD)(DDQ)] complex. The ΔG° values for the CT-complex are given in Table . The sign of ΔG° discloses that the [(OPD)(DDQ)] complex formation is via a spontaneous process. The values of ΔG° turn out to be highly negative when the bonds between the components become stronger. Thus, the OPD donor and DDQ acceptors are mainly subjected to less physical strain or increase of freedom so that interacted reaction proceeds forward side to reach equilibrium.

Ionization Energy

Ionization potential[50] of multisite donator OPD in the donor–acceptor complex was estimated with an equation given by Aloisi and Pignataro.where νCT is the wavenumber (in cm–1) equivalent to the CT-absorption band. The electron donating capacity of an OPD is measured by its ID,which gives the energy required for an electron removal from the outer/highest occupied MO-HOMO (Table ).

Resonance Energy

Resonance energy (RN) is theoretically interpreted by J. Czekalla and G. Briegleb who also derived a relationship to the resonance energy change of the formed donor–acceptor complex.[51]Equation is derived for RN as given below:where νCT and RN denote the frequency (cm–1) of the CT-band and the ground state resonance interaction energy (eV) of the intermolecular donor–acceptor system, respectively, and a notable point is that the RN descriptor contributes to the KCTC of the [(OPD)(DDQ)] complex. The CT descriptor denotes the extinction coefficient of the [(OPD)(DDQ)] at the maximum absorption point of the UV–vis CT band peak.

Dissociation Energy

The energy (W) of the dissociation parameter for the CTC indicates the nature of the [(OPD)(DDQ)] complex, and it was estimated with the help of energy (ECT), ionization energy referring to the OPD (ID), and electron affinity (EA) referring to the DDQ molecule[52] (Table ).

FT ATR Infrared Spectral Study

The FT-IR attenuated total reflectance (ATR) spectroscopy is a surface sensitive, uncomplicated, and fast technique to distinguish the CTC in the range 600–4000 cm–1, and the experimental data and theoretical information are assembled in Table , and the spectral graphs are reported in Figure .

Table 2

Characteristic FT-IR ATR/Theoretical Data (cm–1) and Peak Assignmentsa

OPD	DDQ	OPD-DDQ (FT-ATR) (cm–1)	OPD-DDQ Computational IR (cm–1)	assignments
	1654 s	1652 mw	1612	υ(C=O)
3373 ms		3207 br, ms	3235	υ(N+–H)
1256 ms	1270 mw	1293 ms	1273	υ(C–N)
	2235 ms	2185 ms	2278	υ(C≡N)
	758 ms	736 ms	752	υ(C–Cl)
1493 ms	1516 ms	1450 ms	1405	υ(C=C)
1142 mw	1145 s	1172 s	1193	υ(C–C)
1748 ms		1741 vw	1722	δ N–H
3043 m		2809 br, m	3197	υ(C–H)
921 mw		989 mw	1057	δ (CH) deformation
769 ms		787 ms	775	δ (CH) oop

Abbreviations: s - strong intense, m - medium intense, w - weak intense, υ - stretching mode, δ - bending mode.

Figure 4

FT ATR infrared spectra of the OPD, DDQ, and CT-complex in the range from 4000 to 600 cm–1.

The N atoms are recognized as the source of donation many times. The electron donation[53−55] from the donor to the πe-acceptor occurred either from the nonbonded pair of the −NH2 group or from the aromatic phenyl group. The FT-IR ATR spectrum of the product has been distinguished with the stretching vibration mode of ν(N+–H) of donor is ca. 3207 cm–1. In the theoretical (B3LYP) IR spectral graph presented in Figure S5, this ν(N+–H) was observed at 3235 cm–1. Furthermore, the ν(C–N) stretching modes corresponding to reactants at 1270 and 1256 cm–1 are shifted to lower value 1293 cm–1 (from DFT, it is observed at 1273 cm–1), while the (C=C) bands at 1493 and 1516 cm–1 were shifted to a lower wavenumber, i.e., 1450 cm–1 (from DFT, it is observed at 1405 cm–1). From the above data of FT-IR, one can say that the formed molecular CTC is based on π–π* and n−π* charge migration.

1H NMR Spectrum of [(OPD)(DDQ)] Donor–Acceptor Complex

The 1H NMR data of the [(OPD)(DDQ)] was observed in DMSO (Figure ), and the data was compared with free OPD (Figure S6). Using the 1H NMR study data (Table S2), the nature of electronic interaction was observed between OPD and DDQ in [(OPD)(DDQ)] complex. The peaks observed between 0.9 and 2.2 ppm denote the solvent peaks. The signal peak at δ 3.72 ppm represents the −NH2 group protons of the complex.

Figure 5

1H NMR of formed CT-complex.

The signal peak intensity was high at δ 7.28 ppm due to the two −Ar–H protons. The peak at δ 6.75 ppm represents the other two aromatic −C–H protons of OPD donor.[56]Table S2 also expressed the free OPD NMR data in which three signals were observed at 4.85 ppm (N–H), 6.72(Ar–H), and 7.28 ppm (Ar–H) chemical shifts.

TGA-DTA Analysis

The thermogravimetric analysis of [(OPD)(DDQ)] complex presented in Figure .

Figure 6

TGA-DTA plots of [(OPD)(DDQ)] CT-complex.

Two analyses, thermogravimetric (TG) and differential thermogravimetric analysis (DTA),[57] were used in the 0–1000 °C range using 1.899 mg to examine the thermal stability of the [(OPD)(DDQ)]. The thermogram of the [(OPD)(DDQ)] decomposed at 95.3°C (endothermic/step 1) with weight loss of 15.35% (ΔH = −4281.20 mJ/g) and another decomposition (endothermic/step 2) at 259.86 °C (ΔH = −1.57 × 105mJ/g) with a weight loss of ∼37.04%. The remaining [(OPD)(DDQ)] decomposition (step 3) with weight loss of 47.61% was probably due to the residual carbon. These results have characterized the thermal stability of the [(OPD)(DDQ)] complex.

Morphology-EDX Analysis

The microstructural analysis of the synthesized CT-complex was scanned by scanning electron microscope (SEM),[58] and recorded images are shown in Figure a. The EDX spectrum of elemental analysis is shown in Figure b.

Figure 7

(a) CTC microstructural SEM images. (b) EDX analysis of CT-complex.

The microfiber/needle-like structural morphology of [(OPD)(DDQ)] also shows that assembly of donor/acceptor is an energetically favorable superstructure. The size of the CTC structure was inexact nanometer range expected from microstructural morphology.

XRD Analysis of [(OPD)(DDQ)] Complex

From the pattern of XRD, we have analyzed the crystallite size of the obtained [(OPD)(DDQ)] complex; this pattern was observed in the range of 50° > 2θ > 5° for the OPD, DDQ, and CT-complex, and the recorded pattern is shown in Figure .

Figure 8

XRD patterns of (a) OPD, (b) DDQ, and (c) CT-complex.

Figure explores the crystal size investigations based on high-intensity scattering peaks occurring in the range of 20° to 40°. Based on this investigation, the sharp peaks at 2θ angles recognize the crystalline nature of the OPD, DDQ, and [(OPD)(DDQ)] complex. The average crystallite or grain sizes of OPD, DDQ, and [(OPD)(DDQ)] are calculated from XRD patterns corresponding to high-intensity peak values with the Debye–Scherrer relation[59] shown in eq :where Davg denotes the average crystallite size, K is a constant (0.94 for the Cu grid), and λ denotes the employed wavelength (0.15406 nm)—it is half the Bragg’s diffraction angle and is the full width at half-maximum (fwhm) of the X-ray diffraction peak in radians.

Table S3 shows the XRD data for the reactants and complex together with the Bragg angle (2θ) values, the fwhm (β, fwhm) of the important peaks, and the average crystallite size (D) in nm. The average particle size of the complex (∼15.64 nm) was different from the reactants (22.53 nm, 17.98 nm) according to the highest peak intensities. The values were confirming the sizes of crystals located within the nanoscale size.

Biological Applications

CT-DNA Binding Study of [(OPD)(DDQ)] Complex

The UV electronic spectral method is one of the main supportive techniques for binding analysis of CT-DNA with donor–acceptor complexes. The CTC binding through intercalation frequently results in hypochromic shift with or without bathochromic or hypsochromic shift, because the intercalation influences the strong interaction among the different chromophores and the DNA base pairs (Figure ).

Figure 9

Absorption spectra of CT-complex in the absence and presence of increasing amounts of CT-DNA. Condition: [CT-DNA] = 0–4 μM. Downward arrow shows the absorbance changes upon increasing CT-DNA concentration. Insets: linear plot for calculation of Kb.

Strong intercalative binding ability observed for the CT-complex has been identified by changes in the absorbance of CTC after the addition of CT-DNA. Upon adding the high amount of CT-DNA, a significant hypochromism of the CT-band (at λmax = 423 nm) is monitored. This can be attributed to the strong electron contact between DNA base pairs and CTC via intercalation. The intrinsic binding constant (Kb) can be evaluated[60,61] from the subsequent equation which was derived by Wolfe and Shimer.where εa, εf, and εb are denoted as (Abs/[OPD-DDQ]), the extinction coefficients of the free [(OPD)(DDQ)], and the [(OPD)(DDQ)] complex when fully bound with CT-DNA, respectively. [DNA] denotes the concentration in mol/lit of calf thymus DNA solution. From the graph (Figure ), intrinsic binding constant Kb (in L mol–1) was evaluated by utilizing the ratio of slope to intercept. Similarly, individual reactants have external contact with DNA due to electrostatic binding, i.e., hyperchromic shift. The magnitude of Kb value for CTC is approximately 6.0 × 105 L mol–1. From the binding results, the main CTC has planar phenyl rings and its π–e– values are responsible for DNA intercalation. It is considered a primary indication for DNA interaction.

In Vitro Antibacterial Activity Results

The [(OPD)(DDQ)] complex was dissolved in DMSO solvent. The zones of diameter were calculated giving actual information about the antibacterial activity of the complex.

Checking the antibacterial capacity of the [(OPD)(DDQ)] complex by the in vitro disc diffusion technique against the chosen bacteria/gram + ve (B. subtilis and S. aureas) and bacteria/gram – ve (E. coli and P. aeruginosa) at the chosen moles of the complex substance. Data comparing antibacterial detection with the drug tetracycline used as the standard is shown in Table .

Table 3

Antibacterial Inhibition Capacity (mm) of [(OPD)(DDQ)] Complex

test organism	[(OPD)(DDQ)]	tetracycline (mm)
S. aureus	9 mm	22
B. subtilis	14 mm	25
E. coli	13 mm	26
P. aeruginosa	8 mm	23

The [(OPD)(DDQ)] complex had significant inhibition against the growth of the selected bacterial strains, and results specify that the substrate has a major influence against the B. subtilis and E. coli bacterial strains which are listed in Table .

Computational Analysis

The density functional study in gas-phase/PCM includes B3LYP/6-31G (d,p) for optimization of molecular geometries and evaluation of electronic properties. The optimized total energies, dipole moments, bond lengths, bond angles, electrostatic potential[62] (ESP) characterizations, important FMOs, and Mulliken and natural atomic charges were reported. The optimized figures of OPD, DDQ, and CTC are shown in Figure .

Figure 10

Optimized structures of OPD, DDQ, and [(OPD)(DDQ)] complex.

Initially, the distance between two reactants was around ∼6.65 Å, but after optimization, it was observed as ∼3.2 Å. The decrease in distance between reactants leads to strong electronic interaction. The total optimization energy values observed are −342, −1484, and −1827 Ha for the donor, acceptor, and CTC, respectively. The dipole moment ranges of OPD, DDQ, and CTC were observed as 1.8–2.5, 3.4–4.2, and 5.5–14.5 D, respectively.

The structural bond lengths (B3LYP; optimized) are presented in Table S4. The oxygen–carbon bond lengths change in CTC, i.e., C2=O8 and C5=O7 of the DDQ authenticating the electron transfer from OPD (−NH) nitrogen toward the C=O/C≡N of e-acceptor. In gas-phase B3LYP, the C2–O8 bond length had been increased from 1.24 to 1.2581 Å. In AN/MeOH solvation models, it had been changed from 1.2412 to 1.2612 and 1.2569 Å. The C5–O7 bond lengths 1.24 Å (gas phase) and 1.2412 Å (AN and MeOH) were increased to 1.2486 Å (gas phase), 1.2676 Å (AN), and 1.2590 Å (MeOH) indicating the electron transfer enhancing the bond lengths of acceptor. In the case of C9–N13 and C10–N14 bond lengths of DDQ, slight increments had been observed. The additional report from Table S4 was that one of the −NH groups and the π-ring electrons of OPD were found to be oriented toward the DDQ giving the resonating structure of CTC. This verifies the radical anion of DDQ in CTC composition, but the bond lengths of OPD decreased in the [(OPD)(DDQ)] complex as compared to the free optimized OPD donor. The optimized CTC had indicated the n- and π-electron migration from the important FMOs of OPD to the π* FMO’s of C=O/C≡N containing the DDQ moiety.

The CTC formation between OPD and DDQ was verified from the modified structural bond angles as compared to the free OPD and DDQ (Table S5). The OPD radical cation was confirmed from the increased angles of C3–C4–N11 and C4–C3–N14 (from 117.96° to 118.26°and from 117.96° to 118.72°) in CTC.

The bond angles of C2–C3–N14 and C5–C4–N11 of CTC were also decreased (122.74° to 122.18° and 122.74° to 122.71°) indicating the strain relief of the CT-complex. The free radical anion of DDQ was also supported from the change in bond angles of C1–C2–O8 and C4–C5–O7 of CTC (122.396° to 122.427° and 120.251° to 121.178°). Moreover, the C1–C2–C3 (DDQ) bond angle had been decreased from 117.3524° to 116.2816° indicating the electronic acceptance from a donor molecule. Similar results were observed from the PCM-solvation methods (Table S5). From the OPD molecular entity, n−π*/π–π* electronic migration was confirmed from the highest occupied MO’s (donor) to the lowest unoccupied MO’s (acceptor) of the CTC.

Electrostatic Potential Surface Maps

The ESPs quantify the repulsive/attractive region of the molecule or complex.[63] The ESP surface potential values were observed from the DFT (gas phase/PCM) methods, and the basis set 6-31G was applied to geometrical optimization of structures (Figure ).

Figure 11

ESP surface maps of OPD, DDQ, and [(OPD)(DDQ)] complex.

These results were well correlated with experimental reports. The DDQ acceptor molecule ESP 3D-diagram described a positive zone, which was mainly observed at the center (3D surface map positive region: +0.06302 a.u. to +0.07782 a.u.), and the more negative charge was distributed at oxygen atoms of the C=O group (−0.0272 a.u. to −0.0296 a.u.) and C–N groups (−0.0381 a.u. to −0.0451 a.u.). In the donor (OPD), the major negative region was distributed on both N’s (−0.0017 a.u. to −0.0368 a.u.) and the phenyl ring (−0.0370 a.u. to −0.0459 a.u.). When the donor (OPD) interacted with the acceptor (DDQ), the positive surface map value of OPD is decreased (+0.0372 a.u. to +0.0264 a.u.) and the electron density on N atoms was also decreased (i.e., +0.0093 a.u. to +0.0838 a.u.). All the information above confirms the electron transfer from n-electrons of the −NH2 group and π-electrons of the Ph-ring to carbonyl and cyano groups of DDQ.

HOMO–LUMO Gap Energies

The FMOs illustrate the composed p-atomic orbitals[64,65] and HOMO–LUMO of CTC in the ground state observed from DFT (B3LYP) 6-31G(d,p), which is shown in Figure S7.

From Figure S7, the DDQ moiety acted as a LUMO because unoccupied orbitals are delocalized on its moiety, while the HOMOs were localized on the OPD only. The HOMOs are observed as n and π orbitals and LUMO is a π* orbital. Therefore, the electron transfer can be assigned as n−π* and π–π*. The experimentally observed electronic absorption energies (in eV), the theoretically reported HOMO–LUMO transition energies (eV), and the dispensing of electronic transitions are displayed in Table S6. The DFT-PCM values are less when compared with the gas-phase energy values due to the effect of polarity.

The HOMOs and LUMOs of OPD, DDQ, and CTC are presented in Table S7. From the data, the LUMO level of [(OPD)(DDQ)] complex [B3LYP; −0.1616 (GAS)/–0.1537 (AN)/–0.1537 (MeOH) Ha) compared with the LUMO of DDQ [B3LYP; −0.2002 (GAS)/–0.1856 (AN)/–0.1857 (MeOH) Ha], while HOMO energy [B3LYP; −0.2337 (GAS)/–0.2256 (AN)/–0.2261 (MeOH) Ha] of the [(OPD)(DDQ)] complex is close to the HOMO energy level of OPD [B3LYP; −0.1741 (GAS)/–0.1810 (AN) /–0.1810 (MeOH) Ha]. These results had been observed from the gas phase and PCM optimization methods. Experimentally, electronic charge transfer bands gave multiple bands.

Therefore, HOMO–1 and HOMO–2 have also been involved in the electronic migration reaction (Figure ). The main reason for the localization of frontier MO’s of the CT-complex is very similar to the literature charge transfer donor–acceptor composite system. Hence, orbital contact energy arises principally due to the electronic transition from occupied MO’s to unoccupied MO.

Figure 12

Important MO’s of CT-complex calculated by DFT (B3LYP).

Atomic Charge Distribution Analysis

Mulliken Atomic Charge Distribution Analysis

The atomic or Mulliken charges are charges based on the local charge density observed from the DFT computations of the chemical systems.

The Mulliken atomic charges (gas phase/PCM) of donor, acceptor, and CTC are revealed in Table S8 and Table S9. The comparison of Mulliken atomic charges[66] of initial reactants and CTC were shown in Figure S8.

The increasing negative charge on the acceptor moiety implies the electronic transition from OPD to DDQ. The large changes were observed on O7 (from −0.3145 to −0.3621), O8 (from −0.3145 to −0.4301), N11 (from −0.7931 to −0.7639; OPD), and N14 (from −0.7931 to −0.7796; OPD), and ring carbon atoms indicate that the HOMO–LUMOs localized mainly on these atomic moieties, which can make sense of the discharging/charging of these centers in the observed radical ion pair of [(OPD)(DDQ)] complex (Scheme ).

Natural Atomic Charge Distribution

The natural atomic charges (through NPA analysis) of OPD, DDQ, and CTC are disclosed in Table S8 and Table S9. The comparison of the natural charge distribution of CTC[67] and their reactants is shown in Figure S9.

Huge charge changes were observed. The atomic charge migration was observed on O7 (from −0.426 to −0.4643; DDQ), O8 (from −0.426 to −0.4696; DDQ), N11 (from −0.8937 to −0.8472; OPD), N14 (from −0.8937 to −0.9332; OPD), and Ph-ring unsaturated carbon atoms. This information strongly recommended the HOMO–LUMOs oriented on given atomic moieties of CTC (Scheme ). Similar results have been observed in solvent phase optimization studies.

Reactivity Descriptors

The parameters of reactivity [electron affinity (EA), ionization potential (I), chemical potential (μ), hardness (η), electrophilicity index (ω), and softness (σ)] were estimated from significant FMO energy values which had been proposed for understanding the chemical conversions.[68−71] The electronic action of both acceptor substrate and donor precursor was evident from the data in Table S10. From the HOMO–LUMO values, molecules with higher HOMO energies tend to be major donors. On the contrary, the molecules that have lower LUMO energies behaved as major acceptors. From this we could judge that the DDQ is distinguished from OPD by a lower ELUMO value, whereas OPD has a higher EHOMO than the DDQ molecule and is thus expected to be a donor. In addition, the chemical potential (μ) represents the path of electron transfer among the donor–acceptors.

Electronic charge flow takes place from the substance with an upper μ to the other with lower μ. From this approach, OPD substance has a higher μ than DDQ. The electrophilicity index ω assesses the e-likeness of a substance. Since the e-acceptor DDQ has ω, the former substance is a better electrophile than the latter one. The softness σ values and all results reveal that the OPD is a donor, whereas DDQ is an acceptor. If these values are compared with PCM, then we can see electrons shifting toward the lower side.

Conclusions

A new charge transfer complex interaction was studied between the electron donor OPD and the electron acceptor DDQ by using absorption spectroscopy in acetonitrile and methanol media at 25 °C. The new multi CT bands were observed at λmax 434, 542, and 588 nm in acetonitrile and a single CT band observed at 463 nm in methanol. The molecular stoichiometry was confirmed as a 1:1 ratio by jobs, photometric, and conductometric titration methods in both AN and MeOH solvents. The stability constant and molecular absorptivity coefficient parameters were evaluated by the modified 1:1 Benesi–Hildebrand equation, and these results reveal that the CT complex produced is highly stable in acetonitrile compared to methanol. Other spectroscopic physical parameters were also evaluated, and these results also support the stability of the CT complex. The synthesized solid CT complex was confirmed by different characterization techniques like FT-IR ATR, NMR, TGA-DTA, SEM-EDX, and powdered XRD analysis. The CT complex biological activity was estimated by DNA binding studies and antibacterial activity. The synthesized solid CT complex exhibits excellent antibacterial activity and binds with CT-DNA through intercalative mode. Further, computational analysis was carried out by DFT/B3LYP-6-31G (d,p) gas phase and PCM analysis. The optimized bond parameters (lengths, angles), electrostatic potential maps, MO energies, Mulliken atomic/natural atomic charge distributions, comparison of reactivity descriptors from HOMO–LUMOs in gas-phase/PCM calculations were estimated. These results fully supported the experimental studies of the CT complex.

Methods

Materials, Instrumentation, and Solid Synthesis of CT Complex

The materials were purchased from different chemical sources. OPD (98% Avra Chemicals) and DDQ (98% Sigma-Aldrich) were commercially available. The OPD and DDQ are soluble in organic polar solvents like DMSO (≥99.9% Merck), AN (99.8% purity-Merck), and MeOH (≥99% purity-Merck), and stored in the light protected area. Gloves, eye shields, and dust masks were used throughout all experimental work. The spectra of complex and initial reactants were recorded with the help of UV-2600: SHIMADZU, UV–vis spectrophotometer in the region of 700–200 nm. The solid CTC spectrum was recorded using UV-3600: SHIMADZU, UV spectrophotometer. The cell holder temperature was controlled with a SHIMADZU CORP. 01929 (±0.2 °C). The ELICO conductivity meter was used for measuring the conductance values during the titration between donor and acceptor. FTIR attenuated total reflectance (ATR) spectra of reactants and CTC were detected in the 4000–500 cm–1 range on the SHIMADZU PerkinElmer IR model. 1H NMR of the [(OPD)(DDQ)] complex was observed on a 400 MHz BRUKER NMR instrument using DMSO-d6 as an internal reference. The melting temperature of the CTC was observed on a POLMON device (model no. MP-96). The thermogravimetric analysis (TGA) was accomplished in a dynamic N2 environment with a heat rate of 10 °C/min using TGA-50H SHIMADZU in the temperature range of 20–1000 °C. The morphology was identified by SEM (scanning electron microscopy on Zeiss evo18). The CTC quantitative chemical composition was analyzed through an energy dispersive X-ray spectrometer (EDS/EDX) connected with the scanning electron microscope. The X-ray diffractometer was used for the analysis of XRD patterns and sizes of crystals.

The CTC (red–brownish solution) of OPD with DDQ was synthesized by adding the saturated 1 equiv of OPD solution prepared in MeOH with a saturated solution of 1 equiv of DDQ in the chosen solvent. The solution composition was stirred constantly for 2 to 3 h and left standing at 298 K for 7–8 days. A red–brown-colored crystalline product of CTC was formed. It was used for further analysis without washing. This product was dried completely in a container containing anhydrous CaCl2. The product of CTC was C14H8N4Cl2O2 (MW 335.144 g, mp = 275–280 °C).

Stock Solutions

Standard OPD Stock Solution Preparation

The standard solution of OPD (10–2 mol L–1) was prepared by adding 0.011 g in a graduated flask (10 mL) with polar solvent. The different concentrations of solutions (from 1.0 × 10–4 mol L–1 to 5.0 × 10–4 mol L–1) were prepared in different volumetric flasks by diluting 0.01 mol·L–1 standard stock of OPD with the chosen solvent.

DDQ Stock Preparation

The stock of the DDQ (10–2 mol L–1) is prepared by dissolving 0.023 g of DDQ in a 10 mL standard flask using the same polar solvent. The prepared (1 × 10–3 mol L–1) stock solution was poured in a 25 mL graduated flask and dilution to 10–2 mol L–1 solution with the solvent. Dry conditions were maintained for the whole experimental work. The stock solutions were protected from EMR radiation.

Stoichiometric Relation

Job’s Continuous Plot of Variation Technique

Job’s procedure[28] was followed for the stoichiometric determination of the charge transfer reaction involving OPD and DDQ molecules in selected polar solvents. The highest absorbance at 0.5 mole fraction/DDQ value indicates the development of 1:1 [(OPD)(DDQ)] complex.

Photometric Titration Technique

The photometric titration technique[29] was employed in chosen polar solvents at 463 and 588 nm wavelengths. The concentration ratios {[D]/[A]} were chosen to be 0.166, 0.33, 0.5, 0.667, 0.833, 1.00, 1.166, 1.33, 1.5, 1.66, 1.833, and 2.00. The acceptor volume for every addition was kept constant, i.e., 3 mL. The concentration of DDQ (Ca) was set at 1.0 × 10–4 mol L–1, while the concentrations of the OPD varied. The molecular stoichiometry 1:1 of CTC was determined by the usual photometric molar ratio technique.

Conductometric Titration Technique

The CTC composition was also determined by the popular conductometric titration plot method.[30] The conductometric titrations were carried out by titrating 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, and 10 mL aliquots of a standard solution of the donor (5 × 10–4 mol L–1) with 5.0 mL of DDQ (5 × 10–4 mol L–1) in polar solvents. The conductance values were increased up to 1:1 titration, and this ratio was slowly increased after the 1:1 ratio. The two lines intersected at 1:1 CTC, which was observed from the graph.

CT-DNA Binding Ability of [(OPD)(DDQ)] Complex

The binding mode between CT-DNA (calf thymus deoxyribonucleic acid) and CTC was inspected by SHIMADZU UV-2600 visible operator utilizing 1 cm quartz microcuvettes. The binding mode of the CTC with DNA was carried out with tris(hydroxymethyl)-aminomethane (Tris, 5 mM) and NaCl (50 mM) at pH = 7.2. The purity of the CT-DNA was surveyed from the UV absorbance ratio proportion of about 1.80–1.90 at 260.0 and 280.0 nm. It reveals the absence of contamination in DNA solution.[31] The DNA concentration was determined by UV–vis spectroscopy using the molar absorptivity coefficient (6600 Lit mol–1 cm–1) at λ260 nm. CTC was dissolved in 5% AN and 95% trishydrochloric acid buffer for all the experiments. The stock solution was kept near 5°C and utilized for 1 week. The titrations were carried out by using different concentrations of DNA (0–4 μM) at a constant concentration of CTC.

Antibacterial Activity of [(OPD)(DDQ)] Complex

Four microbial strains in which Gram +ve bacteria (Staphylococcus aureus and Bacillus subtilis) and Gram −ve bacteria (Pseudomonas aeruginosa and Escherichia coli) strains were used to test the in vitro antibacterial activity of [(OPD)(DDQ)]complex in DMSO solvent. The agar well assay method was included for testing.[32] The sterile Petri dishes were filled with 20 mL of Muller Hinton agar and allowed to solidify. Before pouring the plates, media were inoculated with the appropriate bacterial culture; after the agar solidified, 5.0 mm (diameter) wells were punched in the medium utilizing a sterile borer. The wells were labeled, and 0.1 mL of the extract was directly added to the well made on the surface of Muller Hinton agar containing bacterial culture; the wells containing solvent alone were maintained as the negative control. The inoculated plates were incubated overnight at 310 K to allow the growth of the bacteria, and after 24 h, the diameter of the inhibition zone thus formed was measured (in mm). Screening was performed at concentrations of 0.25, 0.5, 1.0, and 2.0 mg/mL of the [(OPD)(DDQ)] complex under investigation. For the control, the drug tetracycline (30 μg/mL, Hi-Media) was used.

Growth of bacterial strains was observed after the incubation period (24 h) at 310 K. The maximum diluted [(OPD)(DDQ)] complex in DMSO required for the inhibition of bacterial growth was denoted as the minimum inhibitory concentration (MIC) of the complex. To obtain the zone diameter, volumes of 0.1 mL each were distributed on agar plates.

Theoretical Analysis

The theoretical DFT[33] computational optimizations of the DDQ, OPD, and CTC were carried out using the Gaussian 09 program in the gas phase and in the polarizable continuum model (PCM) using MeOH and AN polarities. Gauss View 5.0.8[34] was used for illustration and drawing of molecular arrangements. The basis set 6-31G(d,p) was used for the geometry optimization. The Becke 3-Parameter Lee–Yang–Parr hybrid exchange-correlation (B3LYP) functional was employed for the optimization of geometries. The geometrical descriptors (like bond lengths, bond angles, electrostatic potential maps (ESPs), comparison of Mulliken atomic charges and natural atomic charges through natural population analysis, and depiction of frontier molecular orbital surfaces) of CTC have been computed for theoretical identification of the mechanism of the charge transfer reaction. TD DFT (B3LYP 6-31G+(d,p)) was applied for estimation of calculated UV–vis spectra.