A donor–acceptor substituted derivative 2,6-diamino-4-(3,4,5-trimethoxy-phenyl)-pyridine-3,5-dicarbonitrile (DTPP) has been synthesized and its photophysical properties have been studied. Effect of solvent on the photophysical features of DTPP has been undertaken by steady state absorption and emission techniques. Strong solvatochromic emission has been observed due to intramolecular charge transfer characteristics, upon changing the solvent polarity, revealing the highly polar character of the excited state. Dipole moment changes between the excited and ground state have been estimated by using the theory of solvatochromism from Lippert–Mataga and Reichardt’s correlations. The geometrical parameters for ground and excited states, conformational flexibility, and NLO behavior of the molecule have been theoretically investigated. The electronic distributions of DTPP in HOMO and LUMO were also investigated using density functional theory (DFT) methods at B3LYP/6-31 G** level. The correlation of experimental results with theoretical predictions obtained via DFT substantiates the presence of polarity dependent of the emission spectra.
Research on adaptable and responsive systems has endorsed considerable interest in recent years and has led to great effort in the chemistry of photochromic and thermochromic organic materials. Applications for photo-induced responsive organic luminescent dyes range from energy production, chemical sensing, molecular actuators, to biological systems [1,2,3,4,5]. To achieve long-lasting organic dyes with tunable electronic properties and variable efficiency of electron-hole combination, researchers have been examining multi donor–acceptor systems. Intramolecular charge transfer (ICT)  is a phenomenon that occurs when π-conjugated organic molecules with donor–acceptor groups and strong electron delocalized systems along their backbone are exposed to photon; consequently, the majority of research on this class of compounds focuses on how to inhibit or take advantage of the ICT effect. Furthermore, fluorophores bearing the ICT properties have been extensively utilized in the applications of bio-imaging and organic light-emitting diodes [7,8].
The degree of electronic distribution in a molecule can be determined by understanding the dipole moment. The redistribution of charges caused by photonic excitation of an electron in a molecule causes conformational changes in the excited state and modifies the dipole moment of the excited state (μ e) relative to the ground state (μ g). A molecule’s excited-state dipole moment is very helpful in revealing important information about its electronic and geometrical structures, which aids in understanding photophysical and photochemical processes. Understanding the excited state dipole moment is important for evaluating the effectiveness of quantum chemical wave function derivations and electron correlation treatments. This is especially true in respect to the design of organic compounds with substantial hyperpolarizabilities [9,10,11,12]. The present stratagem aims at prediction of photophysical parameters of a multi donor–acceptor type pyridine derivative having π-conjugated flat rigid planar structure by applying solvent shift method, and in order to compare the electrical configurations with experimental data, theoretical computations were performed.
2.1 Procedure for the synthesis of DTPP
The desired compound was synthesized by reacting 3,4,5-trimethoxy benzaldehyde with malononitrile in the presence of ammonia (Scheme 1 and the detailed synthetic and experimental part is explained in Supplementary information).
3 Results and discussion
3.1 Solvent effect on absorption and emission characteristics
The steady state absorption and emission spectra of the DTPP (∼10−5 M) are recorded in non-polar, polar aprotic, and polar protic solvents. Figures 1 and 2 depict the absorption and emission spectra recorded in some selective solvents and the detailed spectral data are summarized in Table 1. Barring toluene and acetonitrile, spectral absorption band maxima for all solvents lie within the range of 349–352 nm with broad spectrum that can be assigned to S0 → S1 transition and is indicative of its being a π–π* type transition . Interestingly, the absorption spectra in toluene and acetonitrile undergo a red shift of ∼20 nm on increasing solvent polarity from toluene to alcoholic solvents and this kind of shift could be observed due to greater degree of stabilization of the ground state in non-polar environments [14,15].
|Solvents||λ abs (nm)||λ em (nm)||(cm−1)||ε (M −1 cm)−1||Φ f||f||μ 12 Debye||ET (30) kcal mol−1||Δf(D,n)|
Solvent effect on the emission spectral characteristics demonstrates how the protic nature of the solvent and the various solvent polarity have varied effects on the fluorescence spectra. As inferred from Figure 2, on excitation at 365 nm, the emission spectrum undergoing a remarkable red shift of 63 nm is observed on going from toluene to propanol with a reduction in the emission intensity, demonstrating that photo-induced ICT from the electron-donating methoxy group to the electron-accepting pyridine ring, with a greater dipole moment in the excited state than in the ground state, is involved in the singlet excited state . Furthermore, the size of the Stokes shift also suggests ICT, and significant Stokes shifts suggest that the fluorophores can adopt twisted ICT state in the solution. The later results conclude that solvent polarizability is the most significant contributor in the case of absorption. In contrast, solvent polarizability and solvent polarity are important in the case of fluorescence. Figure 3 shows the solvent polarity (Δf) dependent changes in the absorption and fluorescence peak energies in cm−1 and the abs and fl values follow reasonably good linear relationship with the solvent polarity parameter Δf, suggesting that the ground and excited state nature of DTPP dye involved in the absorption and fluorescence processes essentially remain unchanged in all the solvents studied [17,18]. The slight negative slopes suggest that both absorption and fluorescent states are strongly dipolar in nature due to strong ICT from the donor to the acceptor moieties of the dye.
A comparison technique was used to evaluate the fluorescence quantum yield (ϕ f) (equation (S1)) values in various solvents and solvent mixtures, and the results are reported in Table 1. The polarity and hydrogen bonding capacity of the solvents have a significant impact on the fluorescence quantum yield of DTPP, with the latter showing a very rapid reduction with increasing solvent polarity. The decrease in ϕ f could be attributed to effective internal conversion by significant mixing between the closely spaced singlet states of π–π* and n–π* . The efficiency decreases with the increase in the solvent polarity and strong hydrogen bonding characteristics, as shown in Table 1.
3.2 Estimation of dipole moment
Transitions between ground and excited electronic states cause the Stokes shift of molecules with absorption and fluorescence spectra and is expected to follow a linear relationship with solvent orientation polarizability (Δf), as given by Lippert–Mataga equation (equations (S2) and (S3)) [20,21] and can be used to determine difference in ground and excited state dipole moments.
Though there exist some solute–solvent hydrogen bonding interactions, the (Stokes’ shift) values were seen to correlate reasonably well with ∆ƒ; the solvent orientation polarizability parameter [22,23] are as inferred from Table 1 and Figure 4. From the slope of the plot, the change in dipole moment is estimated to be 9.54 D suggesting strong ICT in the S1 state of the dye.
Further, the above method is compared with spectral shift method, introduced by Reichardt , given by the equations (equations (S4)–(S7)), where the dipole moment changes are calculated by plotting the Stokes shift of the fluorophore to (dimensionless microscopic solvent polarity parameter), as shown in Figure 5, and the value for Δμ is found to be 4.32 D.
The experimental oscillator strength values for DTPP in various solvents and transition dipole moment (μ 12) were calculated using equations (1) and (2) and the values of f and μ 12 are listed in Table 1 [25,26].
where, E max is the energy maximum absorption in cm−1, is the numerical value of the wave number measured in cm−1, and f is the oscillator strength which shows the active number of electrons whose transition from ground to excited state contributes to the absorption area of the electronic spectrum. The values of f and μ 12 are listed in Table 1.
4 Theoretical investigation
Computational studies were performed to evaluate the nature of electronic transitions in the molecules in ground and excited state. The geometry optimization of ground state (S0) equilibrium structure of DTPP was performed without symmetry constraints in the gas phase and in solution using density functional theory (DFT) and time dependent density functional theory, with Gaussian 16 package . The ground state (S0) and excited state (S1) geometries have been computed at B3LYP/6-31G** level of theory in acetonitrile. The optimized geometrical parameters bond lengths (Å), bond angles, and dihedral angles (degrees) for ground and excited states are labelled and depicted in Figure 6 and selected bond angles and bond lengths are depicted in Table S1. The results revealed that bond length increased from ground state to excited state in C1–C2, C1–O1, C4–C5, C5–O3, C11–C10, C10–N1, C9–N1, and C9-N3 as 0.063, 0.011, 0.0046, 0.0154, 0.0086, 0.0294, 0.0294, and 0.0263 Å, respectively, while bond length shortened from ground state to excited state in, C2–C3, C3–C4, C7–C11, C7–C8, C8–C9, C8–C12, C12–N4, C11–C13, C13–N5, O1–C14, O2–C15, and O3–C16 as 0.0026, 0.0023, 0.0051, 0.0043, 0.0086, 0.0073, 0.0026, 0,007, 0.0026, 0.01, 0.0088, and 0.0082 Å, respectively.
4.1 Conformational analysis
In order to describe conformational flexibility of the title compound, the energy profile as a function of C4–C3–C7–C11 torsion angle was performed with B3LYP method using 6-31 G(d,p) basis set (Figure 7). Using the scan option, the dihedral angle C4–C3–C7–C11, is varied in steps of 10° for the angle −180 to +180° to get the stable geometry of the molecule. The four stable conformers A, B, C, and D are observed for the title compound molecule having energies 6.01, 0.033, 1.16, and 8.49 (kcal mol−1), respectively. The minimum energy is observed for the conformer B with stability of the conformers in the order D > A > C > B.
4.2 Electronic properties
Figure 8 shows the electronic distributions of DTPP and the estimated pattern of the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals. The oxygen atom has a single pair of electrons and a delocalized HOMO on its left side. The LUMO has an antibonding nature with π* and is dispersed across the entire system. The orbital energy level analysis at the B3LYP/6-31 G* level showed HOMO energy (E HOMO) and LUMO energy (E LUMO) and HOMO–LUMO energy gap (E gap) was found to be 4.5 eV which has been used as an indicator of kinetic stability of the molecule. Since adding electrons to a high-lying LUMO or removing electrons from a low-lying HOMO is energetically unfavorable, a big HOMO–LUMO gap predicts a high kinetic stability and low chemical reactivity.
4.3 The molecular electrostatic potential (MESP)
The electronic density and the prevalence of either negative or positive charge contributions determine the MESP. Consequently, electrophilic and nucleophilic attack sites can be identified through examination of the MESP surface. The MESP map for title molecule, as shown in Figure 9, was calculated at the B3lyp/6-31 G** method which displays the reactivity according to the colors of the region for electrophilic attack and the regions for nucleophilic attack. The MESP measurements show that it is largely concentrated at the oxygen atoms within the molecule and is reflected as a yellowish lobe. Because MESP correlates with electronegativity and partial charges, this outcome is expected. Nucleophilic attack regions were associated with the negative (red) sections of MESP map, while nucleophilic reactivity was associated with the positive (blue) regions (electrophilic attack regions). The red region of greatest negative electrostatic potential illustrates the relationship between the high nucleophilic reactivity of the ligand and the high electronegativity of the OMe and CN groups.
4.4 Non-linear optical properties
The first hyperpolarizabilities (β 0) of the studied compound have been calculated at B3LYP/6-31 G**, B3LYP/6-31+G**, and B3LYP/6-31++G** levels of theories. The β 0 is third rank tensor that can be described by 3 × 3 × 3 matrix. The 27 components of the 3D matrix can be reduced to 10 components due to the Kleinman symmetry . The β 0 can be evaluated by using x, y, z components as
The calculated β 0 values at all the levels of theories have been presented in Table 2. The β 0 value of chloramphenicol at HF/6-31G* level of theory is half of that computed at DFT level of theory. The first hyperpolarizability (β) value was found to be 66.7 × 10−31 esu, which is 8.5 times greater than that of urea which indicates the molecule as NLO material (For urea β = 7.803 × 10−31 esu obtained by B3LYP/6-311++G(d,p) method. The hyperpolarizability dominated by the longitudinal components of β xxx = −1445.27, and the values are listed in Table 2.
|B3LYP/6-31 G**||B3LYP/6-31 + G**||B3LYP/6-31 + + G**|
|β 0 × 10−30 esu||54.0 × 10−31||68.5 × 10−31||66.7 × 10−31|
A multi donor–acceptor substituted pyridine derivative was developed, characterized, and its photophysical activity has been investigated both experimentally and theoretically using quantum chemical calculations. Different organic solvents with differing degrees of polarity are studied for their effects on the absorption and emission spectra. Due to ICT and intermolecular hydrogen bonding between the solute and solvent, the emission spectra of DTPP exhibits a red shift as the solvent polarity is increased. The change in dipole moments (Δμ) associated with the first excited singlet state is calculated by solvatochromic Stokes shift method from Lippert–Mataga and Reichardt equations. The higher value of (Δμ) suggests that the emissive state of DTPP is more polar than the ground state. The geometrical parameters, conformational flexibility, and NLO behavior of the molecule have been theoretically investigated. The electronic distributions of DTPP in HOMO and LUMO were also investigated using DFT method.
Funding information: The Deanship of Scientific Research at King Khalid University is greatly appreciated for funding this work under Grant number R.G.P.-2/179/43.
Author contributions: Saleh S. Alarfaji: conceptualization, methodology, and investigation; AbdullahG. Al-Sehemi: formal analysis and conceptualization; Mehboobali Pannipara: conceptualization, supervision, writing – review & editing.
Conflict of interest: The authors declare that there is no Conflict of interest.
Ethical approval: The conducted research is not related to either human or animal use.
Data availability statement: Data sharing is not applicable to this article as no datasets were generated or analyzed during the current study.
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