
==== Front
J Mol Model
J Mol Model
Journal of Molecular Modeling
1610-2940
0948-5023
Springer Berlin Heidelberg Berlin/Heidelberg

39287699
6138
10.1007/s00894-024-06138-1
Original Paper
Cu(β-diketonato)2 bathochromic shifts from the ultraviolet towards the visible region
http://orcid.org/0000-0001-8104-7684
Conradie Marrigje M. conradiemm@ufs.ac.za

https://ror.org/009xwd568 grid.412219.d 0000 0001 2284 638X Chemistry Department, University of the Free State, Bloemfontein, Republic of South Africa
17 9 2024
17 9 2024
2024
30 10 33620 8 2024
5 9 2024
© The Author(s) 2024
2024
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Context

The DFT-calculated ultraviolet/visible properties of 11 different Cu(β-diketonato)2 complexes are presented. The selected β-diketonato ligands on the Cu complex contain none, one or two aromatic rings. The experimentally measured absorbance maxima range of the ultraviolet/visible is observed at 295–390 nm, and the calculated range is 302–425 nm, for the 11 complexes in this study. More aromatic rings on the ligand lead to bathochromic shifts of the experimentally measured absorbance maxima from the ultraviolet towards the visible region. Absorbance maxima of the Cu(β-diketonato)2 complexes with no aromatic rings on the ligand are found to be predominantly ligand-to-metal charge transfer excitations, whereas introducing one or two aromatic rings shifts the excitations to predominantly ligand-to-ligand charge transfer.

Methods

DFT calculations were conducted on the neutral molecules with multiplicity 2, using the PBEh1PBE functional and the aug-cc-pVDZ basis set as implemented in the Gaussian 16 package. The selected solvent was acetonitrile, the solvent in which most of the experimental UV/Vis are reported. The molecules were all optimized in the solvent phase, using the IEFPCM. The initial coordinates for the compounds were generated using Chemcraft.

Highlights

TDDFT of 11 different Cu(β-diketonato)2 complexes follow the experimental trend.

Aromatic rings on the ligand lead to Bathochromic shifts of UV/Visible spectra.

No aromatic rings on the ligand lead to ligand-to-metal charge transfer excitations.

Aromatic rings on the ligand lead to ligand-to-ligand charge transfer excitations.

Graphical abstract

Bathochromic shifts in eco-friendly Cu(β-diketonato)2.

Supplementary information

The online version contains supplementary material available at 10.1007/s00894-024-06138-1.

Keywords

DSSC
TDDFT
Copper(II)
β-Diketone
South African National Research Foundation108960 Conradie Marrigje M. University of the Free StateOpen access funding provided by University of the Free State.

issue-copyright-statement© Springer-Verlag GmbH Germany, part of Springer Nature 2024
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pmcIntroduction

The increase in the global demand for electricity, propelled by a rising population, is challenging due to the reliance on diminishing fossil fuel resources [1]. Solar energy, particularly through silicon-based solar cells, presents an eco-friendly alternative, though it faces issues like complex manufacturing, hazardous materials and high costs [2]. This has led to the evolution of low-cost dye-sensitized solar cells (DSSCs), which use dyes, electrolytes, photoanodes and counter electrodes to convert sunlight into electricity [3, 4]. DSSCs, a third-generation photovoltaic technology, are sustainable and reduce greenhouse gas emissions, offering energy independence and solutions for remote areas. They efficiently operate under various lighting conditions and utilize dyes that absorb and generate charged particles, categorized into natural, organic metal-free and inorganic metal-containing dyes [5].

The efficiency of DSSCs relies heavily on the dye’s capability to absorb sunlight. UV/Vis spectroscopy measures the dye’s absorbance across the UV and visible light spectrum, indicating how effectively the dye can harvest light energy. The UV/Vis properties aid in understanding the excitation of electrons within the dye molecules. When light energy is absorbed by the dye in the UV/Vis range, it generates excited electrons, which are then transferred to the semiconductor, initiating the process of electricity generation; see Fig. 1.Fig. 1 “Diagram showing the basic operation of a DSSC containing a TiO2 semi-conductor and an I−/I3− redox electrolyte.” from reference [6] (open access)

Focusing on inorganic metal-containing dyes, copper, a plentiful and non-toxic metal, has emerged as a promising and cost-effective alternative to ruthenium in DSSCs [7, 8]. Various Cu complexes including phenanthroline or bipyridine ligand substituents have been experimentally evaluated [9–15] and examined theoretically [7, 16] as potential dye sensitizers. This study aims to systematically investigate 11 different Cu(β-diketonato)2 complexes, as indicated in Scheme 1. The effect of different groups on the β-diketonato ligand on the UV/Vis spectra will be investigated, to establish which substituents will lead to bathochromic (red) shifts. Theoretical insights from this study could contribute to the development and evaluation of high-efficiency dyes for DSSCs, which is the main motivation behind this computational investigation.Scheme 1 The structure and sequencing of the Cu(β-diketonato)2 complexes presented in this study, including abbreviation used for the β-diketonato ligand

Theoretical calculations

Density functional theory (DFT) calculations were conducted on the neutral molecules with multiplicity 2, using the PBEh1PBE [17] functional and the aug-cc-pVDZ [18, 19] basis set as implemented in the Gaussian 16 package [20]. The selected solvent was acetonitrile, the solvent in which most of the experimental UV/Vis are reported [21, 22]. This functional/basis set combination previously showed the best performance of optimization and TDDFT calculations for related Cu(β-diketonato)2 complexes [23]. The molecules were all optimized in the solvent phase, using the integral equation formalism polarizable continuum model (IEFPCM) [24, 25]. The initial coordinates for the compounds were generated using Chemcraft [26].

Several properties important for the effective functioning of DSSC can be calculated theoretically [6] and are calculated here for (1)–(11) using Eqs. 1–4 below. These properties are determined as outlined in the literature [16] for a dye in a cell having TiO2 as a semiconductor and I−/I3− as redox electrolyte, as often used when theoretically evaluating dyes for application in DSSCs [27–35].

Light harvesting efficiency (LHE) can be obtained by [36–40]:1 LHE=1-10-f

Here, f is the TDDFT calculated oscillator strength of the absorption bands.

The excited state lifetime (τ) of the excitation at λ of dyes can be obtained by [8]:2 τins=1.499fEλ2

Here, Eλ is the calculated transition energy (cm−1), and f is the calculated oscillator strength of the excited state corresponding to the specified wavelength λ, obtained from TDDFT calculations [23].

The driving force of electron injection (ΔGinject) and the driving force of dye regeneration (ΔGregenerate) can be obtained from oxidation potentials by [8, 32]:3 ΔGinject=Edye∗-ECB=EHOMO-Eλmax-ECB

4 ΔGregenerate=EI-/I3--Edye

Here, Edye is the oxidation potential of the dye (estimated by EHOMO). Edye∗ is the oxidation potential of the excited dye (estimated by EHomo-Eλmax). ECB is the reduction potential of the conduction band edge of TiO2 (− 4.0 eV vs vacuum or − 0.5 eV vs NHE [41]). Eλmax is the electronic vertical transition energy corresponding to λmax. EI-/I3- is the redox potential of the electrolyte I−/I3− redox couple (− 4.8 eV vs vacuum or 0.3 eV vs NHE) [23, 42].

Results and discussion

Geometry and electronic structure

Cu(β-diketonato)2 complexes are d9 copper(II) with spin ½, and thus, 5 alpha and 4 beta predominantly copper-based occupied molecular orbitals (MOs) under the frontier MOs, with only 1 unoccupied copper-based MO, that is the LUMO in the case of Cu(acac)2 [23]. The β-diketones form a square geometry planar around the copper(II) centre in the optimized geometries, as illustrated in Fig. 2.Fig. 2 Optimized geometry of selected Cu(β-diketonato)2 complexes. Bond lengths (Å) and angles (°) are as illustrated. Atoms are coloured as follows: Cu (orange), C (grey), O (red), F (green), S (yellow) and H (white)

The molecular structure of the unsymmetrical Cu(β-diketonato)2 complexes (2), (4)–(7), (9) and (11), can theoretically be a cis- or a trans-isomer. Since only the trans-isomers were experimentally isolated [43–45] and the trans-isomers have lower energies than cis-isomers [22, 43], calculations were performed using only the trans-isomers.

UV/Vis properties

Experimental and calculated ultraviolet/visible spectra of Cu(β-diketonato)2 complexes (1)–(11) in this study are given in Fig. 3, with the absorbance maxima (λA,max) summarized in Table 1. The spectra of the Cu(β-diketonato)2 have a strong absorbance peak (λA,max) in the 250–450 nm region. A good correlation is found between experimental and calculated spectra, with an average deviation (AD) of 13 nm. A bathochromic shift is observed for Cu(β-diketonato)2 complexes with no aromatic groups ((1)–(3), 295–310 nm experimental), to complexes with one aromatic group ((4)–(7), 325–340 nm experimental), to complexes with two aromatic groups ((8)–(10), 360–363 nm experimental). This shift was observed for both the experimental and calculated complexes. The highest bathochromic shift was observed for Cu(di-tfth)2 ((11) 390 nm experimental), which contains an oligothiophene chain of two thiophene groups on one side of the β-diketone and a CF3 group on the other side. The complexes that were not experimentally analyzed in CH3CN also followed these trends that is also shown by the calculated spectra, all simulated in CH3CN.Fig. 3 Ultraviolet/Visible spectra of (1)–(11), in the 250–500 nm region. For calculated spectra, Gaussian broadening with FWHM = 30 in CH3CN was used. Experimental solvent as indicated in Table 1

Table 1 Experimental (solvent indicated) and TDDFT calculated wavelengths (in CH3CN), related to the absorbance maximum (λA,max) in the UV\Vis region for (1)–(11)

No	Complex	λA,max (calc) (nm)	λA,max (exp) (nm)	Difference calc-expa (nm)	Exp solvent	Exp reference	
1	Cu(acac)2	304	295	9	CH3CN	[21]	
2	Cu(tfaa)2	302	300	2	CH3CN	[21]	
3	Cu(hfaa)2	295	310	15	CH3CN	[21]	
4	Cu(ba)2	313	325	12	CH3CN	[21]	
5	Cu(tfba)2	316	325	9	CH3CN	[21]	
6	Cu(tffu)2	331	335	4	CH3CN	[21]	
7	Cu(tfth)2	333	340	7	CH3CN	[21]	
8	Cu(dbm)2	336	360	24	C2H6OS	[21]	
9	Cu(thfph)2	352	362	10	CH2Cl2	[45]	
10	Cu(dtm)2	352	363	11	CH2Cl2	[44]	
11	Cu(di-tfth)2	425	390	35	CH3CN	[22]	
aAverage deviation (AD) from the difference between the calculated and experimental is 13 nm

By studying the charge transfer (CT) bands with the support of TDDFT (time-dependent density functional theory), more insight into this bathochromic shift can be obtained. In Table 2, the molecular orbitals (MOs) contributing to the maximum absorbance transitions are summarized. Complexes (1)–(3), which have no aromatic groups on the β-diketone, involve excitation from occupied MOs that are ligand-based to MOs that are predominantly metal-based, hence, predominantly ligand-to-metal charge transfer (LMCT). These three complexes have 98% (1), 95% (2) and 98% (3) orbital contributions from the HOMO-5 to the LUMO. The remainder of the complexes (4)–(11) involves excitation from occupied MOs that are ligand-based to MOs that are predominantly ligand-based, hence, predominantly ligand-to-ligand charge transfer (LLCT). For the metal component involved in the excitation of (4)–(7), which have one aromatic group on the β-diketone, the orbital contributions to the metal-based LUMO show a decrease from 28 (4), 8 (5) and 9 (6) to 7% (7). The introduction of two aromatic groups in complex (8) splits the metal-based UMO between the LUMO and the LUMO + 4, with a total of 9% contribution to the Eλmax excitation. For the remaining complexes (9)–(11), the metal component remains on the LUMO + 4 and decreases from 3 (9) and 2 (10) to 1% (11) contribution to the Eλmax excitation. As the metal component in the excited state character decreases, a bathochromic shift is observed. This is ascribed to the engagement of the aromatic substituent groups, amplifying the π-conjugations throughout the β-diketonato ligand leading to the LLCT bands. Table 2 PBEh1PBE/aug-cc-pVDZ calculated energy (Eλmax), absorbance maximum wavelength (λA,max), oscillator strengths (f) and orbitals contributing to the transitions in the indicated excitation peaks of copper(II) complexes (1)–(11)

No	Complex	Eλmax	λA,max	f	Orbital contributions	Character	
(eV)	(nm)	From	To	(%)	
(1)	Cu(acac)2	4.1	304	0.27	HOMO-5	LUMO	98	LMCT	
(2)	Cu(tfaa)2	4.1	302	0.24	HOMO-5	LUMO	95	LMCT	
(3)	Cu(hfaa)2	4.2	295	0.26	HOMO-5	LUMO	98	LMCT	
(4)	Cu(ba)2	4.0	313	0.69	HOMO-3	LUMO + 2	6	LLCT	
HOMO-1	LUMO + 3	28	LLCT	
HOMO-17	LUMO	5	LMCT	
HOMO-12	LUMO	13	LMCT	
HOMO-11	LUMO	8	LMCT	
HOMO-6	LUMO	2	LMCT	
HOMO-2	LUMO + 2	4	LLCT	
HOMO	LUMO + 4	32	LLCT	
(5)	Cu(tfba)2	3.9	316	0.86	HOMO-3	LUMO + 1	8	LLCT	
HOMO-1	LUMO + 3	35	LLCT	
HOMO-17	LUMO	4	LMCT	
HOMO-13	LUMO	4	LMCT	
HOMO-2	LUMO + 2	4	LLCT	
HOMO	LUMO + 4	44	LLCT	
(6)	Cu(tffu)2	3.7	331	1.37	HOMO-3	LUMO + 1	25	LLCT	
HOMO-1	LUMO + 3	20	LLCT	
HOMO-17	LUMO	2	LMCT	
HOMO-9	LUMO	7	LMCT	
HOMO-2	LUMO + 2	21	LLCT	
HOMO	LUMO + 4	24	LLCT	
(7)	Cu(tfth)2	3.7	333	1.22	HOMO-3	LUMO + 1	19	LLCT	
HOMO-1	LUMO + 3	26	LLCT	
HOMO-17	LUMO	2	LMCT	
HOMO-13	LUMO	5	LMCT	
HOMO-2	LUMO + 2	16	LLCT	
HOMO	LUMO + 4	31	LLCT	
(8)	Cu(dbm)2	3.7	336	0.97	HOMO-3	LUMO + 1	8	LLCT	
HOMO-1	LUMO + 2	38	LLCT	
HOMO-25	LUMO	1	LMCT	
HOMO-25	LUMO + 4	2	LMCT	
HOMO-2	LUMO	3	LMCT	
HOMO-2	LUMO + 4	3	LMCT	
HOMO	LUMO + 3	43	LLCT	
(9)	Cu(thfph)2	3.5	352	1.29	HOMO-4	LUMO + 2	2	LLCT	
HOMO-3	LUMO	16	LLCT	
HOMO-1	LUMO + 2	31	LLCT	
HOMO-25	LUMO + 4	2	LMCT	
HOMO-2	LUMO + 1	12	LLCT	
HOMO-2	LUMO + 4	1	LMCT	
HOMO	LUMO + 3	34	LLCT	
(10)	Cu(dtm)2	3.5	352	1.42	HOMO-3	LUMO	16	LLCT	
HOMO-1	LUMO + 2	31	LLCT	
HOMO-25	LUMO + 4	2	LMCT	
HOMO-2	LUMO + 1	14	LLCT	
HOMO	LUMO + 3	35	LLCT	
(11)	Cu(di-tfth)2	2.9	425	2.05	HOMO-3	LUMO	31	LLCT	
HOMO-2	LUMO + 2	18	LLCT	
HOMO-18	LUMO + 4	1	LMCT	
HOMO-1	LUMO + 1	29	LLCT	
HOMO	LUMO + 3	19	LLCT	

DSSC application

To be considered an effective dye in DSSCs, a complex should have a high molar extinction coefficient and absorb light efficiently over a broad range of the sunlight spectrum, particularly in the near ultraviolet and visible region (ca 300–800 nm). The experimental spectra for the Cu(II) complexes (1)–(11) are 295–390 nm, placing these complexes in the near UV range. The energies of the frontier MOs are crucial, as the HOMO and LUMO levels are associated with the molecule’s stability and reactivity. Furthermore, the LUMO and HOMO energies of a dye molecule should align favourably with the conduction band (CB) potential (E) of the semi-conductor in the DSSC (such as the TiO2 example in this study) and the redox potential (Eredox) of the electrolyte (such as the I−/3− example in this study) used in the DSSC [42]. To determine if the Cu(II) complexes in this study are suitable to be tested as dyes in DSSC, computationally derived properties of these prosperous dyes are summarized in Table 3. Table 3 PBEh1PBE/aug-cc-pVDZ calculated values related to the indicated absorbance maximum excitation of Cu(II) complexes (1)–(11). Calculations for a DSSC containing a −/I3− electrolyte and a TiO2 semi-conductor

No	Complex	Eλmax	λA,max	f	LHE	τ	ΔGinject	ΔGregenarate	Egap	VOCa	
(eV)	(nm)	(ns)	(eV)	(eV)	(eV)	(V)	
(1)	Cu(acac)2	4.1	304	0.27	0.467	5	1.4	1.9	4.5	1.9	
(2)	Cu(tfaa)2	4.1	302	0.24	0.421	6	0.9	2.4	4.5	1.3	
(3)	Cu(hfaa)2	4.2	295	0.26	0.453	5	0.4	3.0	4.5	0.7	
(4)	Cu(ba)2	4.0	313	0.69	0.796	2	1.4	1.8	4.4	1.8	
(5)	Cu(tfba)2	3.9	316	0.86	0.861	2	0.8	2.3	4.4	1.3	
(6)	Cu(tffu)2	3.7	331	1.37	0.957	1	0.8	2.1	4.2	1.3	
(7)	Cu(tfth)2	3.7	333	1.22	0.940	1	0.8	2.2	4.3	1.3	
(8)	Cu(dbm)2	3.7	336	0.97	0.893	2	1.1	1.8	4.3	1.7	
(9)	Cu(thfph)2	3.5	352	1.29	0.948	1	1.0	1.7	4.1	1.6	
(10)	Cu(dtm)2	3.5	352	1.42	0.962	1	1.1	1.7	4.1	1.6	
(11)	Cu(di-tfth)2	2.9	425	2.05	0.991	1	0.6	1.5	3.5	1.2	
aApproximation from analytical relationship

The excitation energy (Eλmax), which is correlated to the absorbance maximum wavelength (λA,max), steadily decreases as the wavelength of the series (1)–(11) increases. A longer wavelength (such as (11)) is more favourable for DSSCs, since it is closer to the visible spectrum area. The LHE is correlated to f. Within the framework of DSSCs, LHE relates to the process of harnessing and transforming energy obtained from sunlight into electricity. This process is fundamental to the functioning of DSSCs, as it represents the first step where light energy is absorbed and transformed into electrical power. The photoactive dye is crucial in this process, as it absorbs and collects light, enabling the semiconductor to produce electricity (Fig. 1). The LHE contributes directly to the short-circuit current density (Jsc) in DSSC; see Eq. 5 [31, 32, 46, 47]. It is a key parameter that represents the current density (current per unit area) generated by the DSSC when the cell’s terminals are shorted together, meaning there is no external voltage applied across the cell and is important for improving the performance of DSSCs.5 Jsc=∫λLHEλϕinjectηcollectdλ

ηcollect is the charge collection efficiency (generally constant for a specific DSSC). ϕinject is the electron injection efficiency, which is closely related to ΔGinject calculated by Eq. (3) [8, 32, 46–49].

In this study, the complexes with no aromatic rings, (1)–(3), with a maximum absorbance LMCT band, have a distinct lower LHE (0.451–0.467) than the remaining complexes, (4)–(11), with maximum absorbance LLCT bands, that contains one or more aromatic rings on the β-diketonato ligand (0.796–0.991). The calculated excited state lifetime (τ) of the Cu(II) complexes in this study ranges from 1 to 6 ns. This is significantly lower than the values reported for the established dyes such as YD2-o-C8 (Zn–porphyrin type, 12 ns [50]) and CYC-B11 (Ru-bipyridine type, 27 ns [51]). The half-time for regeneration (the time required for the dye to recover its original state after undergoing a photo-induced reaction) of another established dye, cis-Ru(dcbpy)2(NCS)2, ranges from 100 ns to 10 µs [52]. This suggests that the excited state of the complexes could be long enough to adequately decelerate the charge recombination process necessary for an efficient DSSC.

In DSSCs, ΔGinject and ΔGregenerate are determined by comparing the calculated energies of EHOMO and ELUMO (or the higher energy UMOs involved in the excitation), with the conduction band potential of the semiconductor (ECB =  − 4.0 eV relative to vacuum; − 0.5 eV relative to NHE for TiO2) and the redox potential of the electrolyte (− 4.8 eV relative to vacuum; 0.3 eV relative to NHE for the commonly used I−/I3− electrolyte), as shown in Eqs. 3 and 4 and Fig. 4. A more negative EHOMO relative to the redox potential of the electrolyte indicates a faster regeneration of oxidized dyes, while a higher ELUMO compared to ECB ensures efficient electron injection from the excited state. Analyzing ΔGinject and ΔGregenerate is vital in both experimental and theoretical research on redox mediators and dye sensitizers, as it helps to identify suitable candidates for DSSCs. Larger values of ΔGinject and ΔGregenerate facilitate easier charge transfer between the semiconductor’s conduction band and the electrolyte. For a DSSC to operate efficiently, ΔGinject should be greater than 0.2 eV [53, 54]. In this study, all complexes exhibit ΔGinject values exceeding 0.2 eV, as shown in Table 3. The calculated energy gap (Egap) for complexes (1) to (11) decreases progressively with the addition of aromatic groups to the β-diketonato ligand, accompanied by an increase in the maximum absorbance wavelength. Given that the solar spectrum peaks around 500 nm (approximately 2.50 eV) [23], a smaller Egap is beneficial for achieving larger photocurrents in DSSCs.Fig. 4 MO energy levels of Cu(β-diketonato)2 complexes (1)–(11). For each complex, the left lines are alpha orbitals and the right lines are beta orbitals. The thicker bottom lines are OMO and the thinner top lines are UMO. Also shown: the charge band (CB) at − 4.0 eV and valence band (VB) at − 7.2 eV of TiO2 and redox potential at − 4.8 eV of the I−/I3− electrolyte

Figure 4 presents the energies of the alpha and beta MOs for (1)–(11). The thicker bottom lines represent the occupied molecular orbitals (OMOs), while the thinner top lines indicate the unoccupied molecular orbitals (UMOs). These MO energies are associated with the conduction band and valence band potentials of the TiO2 semiconductor, as well as the redox potential (Eredox) of the I−/I3− electrolyte. All UMOs of compounds (1)–(11) exhibit higher energies than the conduction band edge of TiO2 (ECB(TiO2)), ensuring a sufficient driving force for electron injection. Additionally, the energies of the HOMOs of these complexes are below that of the redox potential of the I−/I3− system, providing a substantial driving force for the regeneration of the dye.

Voc represents the open-circuit voltage in a DSSC. It is a crucial parameter that represents the maximum voltage difference between the anode and cathode of the DSSC when no external circuit is connected (i.e. when the circuit is open and no current is flowing). Voc is a measure of the energy conversion efficiency of the cell. It reflects the difference in energy between the Fermi level of the semiconductor and the redox potential of the electrolyte (ECB − Eredox) and can be estimated by Eq. (6) [30, 55–57].6 eVoc≈ELUMO-ECB

Improving Voc is one of the strategies for enhancing the performance and efficiency of dye-sensitized solar cells. Equation (6) indicates that a more positive ELUMO value enhances Voc, thus improving the energy conversion efficiency of the DSSC. From Table 3, Voc for (1)–(11) are all above 1, with complexes (1), (4) and (8) exhibiting the highest Voc and complex (3) the lowest.

Conclusions

The introduction of an aromatic ring at the groups on the β-diketonato ligand on Cu(β-diketonato)2 improved the DSSC properties of the complexes in this study by enhancing the oscillator strength, LHE and bathochromic shift of the maximum absorbance band into the visible region of the UV/vis spectrum. The frontier MOs (HOMO and LUMO) energies of complexes (1)–(11) are all favourable for electron injection into the TiO2 semiconductor and regeneration of the dye by the I−/I3− electrolyte. With modification to a further bathochromic shift of the maximum absorbance band Cu(β-diketonato)2 complexes could be thought of as an eco-friendly and affordable dye for DSSCs.

Supplementary Information

Below is the link to the electronic supplementary material.Supplementary file1Optimized coordinates of the DFT calculations.(DOCX 197 KB)

Acknowledgements

The high-performance computing (HPC) facility of the UFS of RSA is acknowledged for computer time.

Author contribution

Marrigje M. Conradie: Conceptualization, DFT calculations, Formal analysis, Interpretation, Resources, Writing- Reviewing and Editing, Funding acquisition.

Funding

Open access funding provided by University of the Free State. This work has received support from the South African National Research Foundation (NRF, grant number 108960) and the Central Research Fund (CRF) of the University of the Free State (UFS), Bloemfontein, RSA.

Data availability

All data is provided within the article and supplementary information.

Declarations

Ethics approval

This work does not require any ethical statement.

Competing interests

The authors declare no competing interests.

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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