Assessing the potential of para-donor and para-acceptor substituted 5-benzylidenebarbituric acid derivatives as push–pull electronic systems: Experimental and quantum chemical study
Graphical abstract
Introduction
Barbituric acid and its synthetic derivatives are getting great attention from researchers due to their unique structural properties and important technological and pharmaceutical applications. Various derivatives of barbituric acid were obtained by introducing the substituents in the C5 and N1/N3 positions of the barbituric acid ring, yielding compounds with a broad spectrum of biological activities such as antibacterial, antioxidant, as inhibitors of enzyme tyrosinase [1], [2], [3], and as selective oxidizing agents [4] for the synthesis of unsymmetrical disulfide [5], [6]. Moreover, 5-ethylidenepyrimidine-2,4,6(1H,3H,5H) trione moiety has a broad range of applications in the design of dyes [7] and pigments [8] with multifunctional properties and as organic nonlinear optical materials [9], [10].
Electron-donating substituent (D - push) covalently bonded to electron-accepting substituent (A - pull) via a π-conjugated bridge (linker) represent a class of molecules known as push–pull electronic systems (D-π-A). These compounds have attracted a great attention as organic materials with promising electronic and optical properties applied in non-linear optics (NLO) [11], [12], [13], [14], dye-sensitized solar cells (DSSCs) [15], [16], organic light-emitting diodes (OLEDs) [17], colorimetric pH sensors, and ions detection [18], [19]. The structural motifs, important for the potential application of push–pull barbituric acid derivatives as NLO chromophores, are summarized in a brief article of Ikeda et al [20]. Some of the main features of push–pull systems are low energy intramolecular charge-transfer (ICT) band in absorption spectra, low HOMO-LUMO gap, and high value for first molecular hyperpolarizability. By varying three key elements (donor, π-linker, and acceptor) one can finely tune the HOMO-LUMO gap of the compound and modulate corresponding charge transfer interactions, which can influence its optoelectronic properties [21]. The computational chemistry methods of today can predict and explain the push–pull properties of a molecule. High level ground state quantum-chemical calculations can accurately predict the HOMO-LUMO gap, while time-dependent density functional theory (TDDFT) calculations of the excited state allow us to estimate the potential for the ICT transition in the various organic π-systems, as well as electronic properties of push–pull chromophores [22], [23], [24], [25]. Also, based on the calculated electron density difference between ground and excited state charge-transfer distance (DCT), which represents the distance between the location of the departing electron from the ground state and the locations of the arriving electron in the excited state, and the amount of charge transferred upon excitation (QCT) can be calculated [22].
The structure and properties of the barbituric acid moiety indicate that pseudoaromatic pyrimidine-2,4,6-trione ring can act as electron-donating or electron-accepting substituent in push–pull systems [26]. Electron-accepting abilities of barbituric acid moiety are well documented in the literature [7], [27], [28], [29], [30]. However, barbituric acid as electron-donor moiety in the molecules of the push–pull type is not yet confirmed [19], [31], [32], [33], [34].
In this study, a series of eleven 5-benzylidenebarbituric acid derivatives with different substituents were synthesized (Fig. 1) with the purpose of experimental and theoretical analysis on the nature of compounds and intermolecular interactions. Based on electron-donating/accepting properties the substituents can be roughly divided into four groups: group I is composed of strong electron-donors i.e. substituents with strong positive resonant effect (R) and weak negative inductive effect (I); group II contains weak electron-donors (alkyl substituents) with weak positive R and negligible I; in group III – (halogen group) are weak electron-acceptors, with large negative I and weak positive R and in group IV are strong electron-acceptor substituents with negative R and negative I. Also, an unsubstituted compound (compound 7) was added for comparison purposes. A list of all substituents, together with their Hammett σp+ parameters, as the good indicator of electron-donating/accepting ability, is shown in Fig. 1.
The nature of the ground and excited state of synthesized compounds were characterized and investigated using spectrochemical, solvatochromic (including LFER and LSER analysis), and computational chemistry approaches including optimization the molecular geometry, time-dependent DFT calculations of excited states, and the difference in charge distribution between the ground and excited state. Besides, the push–pull character of investigated molecules was quantified through parameters such as chemical shift differences, absorption spectra, HOMO-LUMO gap, hole-electron distribution analysis, and intramolecular charge transfer (ICT) analysis of excitations.
Section snippets
General method for synthesis of 5-benzylidenebarbituric acid derivatives
In this study, a series of substituted 5-benzylidenebarbituric acid derivatives were synthesized by Knoevenagel condensation [35], [36] of barbituric acid with the corresponding benzaldehyde. 3 mmol of barbituric acid was dissolved in 20 ml hot distilled water and aromatic aldehyde (3 mmol) and 10 ml of 95% ethanol were added. The solution was stirred at room temperature between 0.5 h and 2 h, depending on the added aldehyde. The crude product was filtered off, purified by recrystallization
Computational methods
Optimal geometries of all investigated molecules were obtained by using the second-order Møller-Plesset (MP2) method [41] with the 6-311G(d,p) basis set. Absorption spectra were calculated using the TDDFT method, more specifically B3LYP functional which includes the Becke three-parameter exchange [42] and the Lee, Yang, and Parr correlation functionals [43] with a 6-311G(d,p) basis seton MP2/6-311G(d,p) optimized geometries. For absorption spectra calculations PCM (polarizable continuum model)
Experimental 1H/13C NMR study
5-Benzylidenebarbituric acid derivatives are structurally versatile compounds that might exist in several isomeric/tautomeric forms in solution (Fig. S1). The 1H NMR spectra of compounds 1–12 found triketo form as the dominant tautomeric species in DMSO, as concluded by the presence of two –NH signals at ~11.3 ppm and the absence of signals of enolic –OH protons. Also, three distinct signals from carbonyl groups (165–163 ppm, 162.7–161.5 ppm, and 150 ppm) are found in 13C NMR. These results
Conclusion
A systematic experimental and theoretical investigation of electron interactions in different substituted 5-benzylidenebarbituric acid derivatives has been done. The tautomeric preferences of investigated compounds have been studied by NMR spectra, spectrophotometric determination of the pKa, and quantum chemical calculations, and our results confirm that the most stable tautomer of the barbituric acid moiety is the triketo form.
MP2 optimizations suggest that the most stable keto isomers of
CRediT authorship contribution statement
Ivana N. Stojiljkovic: Investigation, Writing - original draft, Writing - review & editing, Formal analysis, Project administration. Milica P. Rančić: Conceptualization, Validation. Aleksandar D. Marinković: Supervision, Resources, Funding acquisition. Ilija N. Cvijetić: Investigation, Formal analysis. Miloš K. Milčić: Conceptualization, Methodology, Writing - original draft, Writing - review & editing, Supervision, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by the Ministry of Education, Science and Technological Development of Republic of Serbia (Contract number: 451-03-9/2021-14/200168).
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