Chiang Mai Journal of Science

Investigation of the Structural and Photophysical Properties of Cyclometalated Platinum(II) Complexes as Singlet Oxygen Photosensitizers for Photodynamic Therapy: A DFT Computational Study

1. INTRODUCTION

Oxygen is essential for all living organisms, given its presence in the air, water, blood, cells, and tissues. It plays a critical role in various biological and medical processes, including metabolism [1]. Further, it is involved in the production of adenosine triphosphate (ATP) during oxidative phosphorylation in mitochondria [2] and is required in photodynamic therapy (PDT), a two-stage treatment that utilizes light energy, oxygen, and light-activated compounds (photosensitizers) to induce the destruction of cancerous and precancerous cells through phototoxicity. In PDT, oxygen molecules can be divided into two types: Type I, which generates radicals or radical ions, and Type II, which produces singlet oxygen (¹O₂) [3, 4]. Identifying and detecting oxygen levels is strategically essential in many fields, including medical diagnostics [5-11], bioreactors [12, 13], and environmental management [14, 15]. Various detection methods are available; for example, optical methods based on luminescence techniques (fluorescence and phosphorescence) involve quenching with molecular oxygen. Generally, sensors comprise two components-organic and inorganic materials- and are called oxygen-sensitive probes (OSPs). Dynamic quenching occurs via collisions between triplet molecular oxygen and the excited states of OSPs, resulting in reduced emission intensities and shorter excited-state lifetimes. This nondestructive and reversible process enables continuous oxygen monitoring.

Currently, optical oxygen sensors constitute essential tools for real-time oxygen detection, given their distinct advantages over traditional techniques, such as zero oxygen consumption during measurements, complete reversibility, and excellent precision and accuracy [16-19]. Substances commonly used to produce OSPs include complexes of Ir(III) [20], Ru(II) [21], Pd(II) [22], and Pt(II). Among the materials used to create OSPs, cyclometalated platinum(II) complexes are particularly notable for their excellent properties, including long phosphorescent lifetimes and high quantum yields. They typically display planar molecular geometries, which contribute to their highly emissive excited states, and possess other favorable photophysical properties, such as a high color purity and tunable emission energies [22-25].

Cyclometalated complexes are typically built upon tridentate N^C^N or N^N^C ligand frameworks, often accompanied by halogen, pseudo-halogen, or acetylide ancillary ligands. The N^C coordination motif is commonly derived from 2-phenylpyridine (ppy)-type ligands, which are known to produce high-energy phosphorescence. The formation of an anionic metal-carbon bond in N^C-type systems enhances the ligand field strength, elevating the nonemissive metal-centered d-d states and, thereby, minimizing thermal quenching. Furthermore, these ligands can be easily modified with functional groups possessing different electronic and steric properties, allowing for the precise tuning of the photophysical properties of N^C-chelate metal complexes. They generally include bidentate co-ligands, such as acetylacetonate (acac), due to their high triplet energy and excellent solubility [26, 27]. Brooks et al. synthesized a collection of cyclometalated Pt(II) complexes and discovered that several exhibit strong spin–orbit coupling, which enables the mixing of ¹MLCT states with ligand-centered ³π-π* states. This mixing allows for the typically forbidden radiative relaxation of these states to occur efficiently, achieving quantum yields of up to 0.25 at ambient temperatures. In part, halide ions are widely recognized as suitable ligands for stabilizing transition metals. Moving down the halide group, the ionic radius, σ-bond donation capability, trans effect, and bond strength with low-oxidation-state metals progressively increase. Conversely, the π-bond donation ability and bond strength with high-oxidation-state metals decrease from fluorine to iodine [28]. Hu et al. conducted a comparative study on the heavier halogen derivatives, demonstrating a decreasing trend in the emission quantum yield, following the order of F > Cl > Br > I. The complexes containing fluorine exhibit the highest quantum yield of 53.1% [29]. The unique properties of fluorine, including its high electronegativity and relatively small atomic size, make fluoro substituents an ideal choice for tuning the electronic characteristics of a compound. Further, the strong and highly stable C-F bond allows fluorinated groups to stabilize organometallic complexes [30, 31] effectively. Moreover, fluorinated matrices exhibit high oxygen permeability and excellent resistance to irradiation, properties that arise from fluorine’s strong electronegativity [32, 33].

These platinum(II) complexes are electrically neutral, air stable, and sublimable, making them highly suitable for applications in optoelectronic devices. Their wide range of excited-state energies and redox potentials further enhances their versatility as tunable photooxidants and photo-reductants [34]. Xing’s group [35] studied the impact of fluorophenyl substitution in cyclometalated platinum(II) complexes on photostability for oxygen sensing; notably, these complexes are of significant interest for further study. Therefore, our research aims to investigate the photophysical properties of cyclometalated platinum(II) complexes featuring fluorophenyl ligands at the meta and para positions and provide theoretical insights that support the development of efficient photosensitizers for PDT.

2. COMPUTATIONAL DETAILS

All computational results presented herein were obtained using density functional theory (DFT) and time-dependent DFT (TD-DFT), as implemented in Gaussian 16 [36]. The geometries of the ground state (S₀), first singlet excited state (S₁), and first triplet excited state (T₁) of all complexes (shown in Figure 1) were fully optimized via the DFT and TD-DFT methods [37]. The Perdew-Burke-Ernzerhof correlation functional (PBE0) was employed throughout; it was identified as the most appropriate choice based on comparisons with related results [38-42], considering geometries, photophysical properties, photoelectronic properties, quenching with oxygen, and other parameters. All calculations employed a mixed 6-31G(d,p)/LanL2DZ basis set, where 6-31G(d,p) was used for C, H, N, and O atoms, and LanL2DZ for the Pt(II) atom. Solvent effects of dichloromethane (CH₂Cl₂) were incorporated using the polarizable continuum model (PCM).

To gain deeper insights into the radiative decay process, TD-DFT/PBE0 calculations with spin-orbit coupling (SOC) based on perturbation theory were performed using the Amsterdam Density Functional (ADF 2024.1) [43] at the optimized T₁ geometries. The five lowest scalar relativistic singlet and triplet excitations were included. An all-electron triple-zeta plus polarization basis set was used for Pt, while a double-zeta plus polarization version was applied for all other atoms. The environmental effects of CH₂Cl₂ were taken into account using the conductor-like screening model (COSMO) [44, 45].

The phosphorescence quantum yield is governed by the rates of intersystem crossing (k_ISC), radiative decay (k_r), and nonradiative decay (k_nr). The intersystem crossing (ISC) rate can be evaluated using Equation 1 [37, 46, 47]:

$k_{\mathrm{ISC}}^n=\sum _m^{}\frac{\text{2π}}{h}{⟨S_n⟩}^2\text{×FCWD}$, (1)

where $⟨S_n⟩$ represents the SOC matrix element between the pure spin states S_n and T_m, and FCWD denotes the Franck–Condon weighted density of states, which was evaluated within the framework of Marcus–Leich–Jorter theory, as shown in Equation 2:

$\text{FCWD=}\frac{1}{\sqrt{\text{4πλ}{k}_{B}T}}\sum _{\text{n=0}}^{\infty }\mathrm{\text{exp⁡}}\text{(-S)}\frac{S^n}{\text{n!}}\exp \left[-\frac{{\text{(∆E+nℏω+λ)}}^2}{\text{4λ}{k}_{B}T}\right]$ (2)

In this context, λ refers to the Marcus reorganization energy, ΔE is the energy gap between the singlet (S_n) and triplet (T_m) states, ћω corresponds to the energy of an effective nonclassical vibrational mode involved in the transition, and S is the corresponding Huang–Rhys factor. The FCWD of structurally related complexes generally lies within a similar order of magnitude [48]. Therefore, the main difference in the FCWD results from the ΔE. In other words, k_ISC is primarily governed by the SOC matrix elements and the energy gap between the S_n and T_m states [44].

During the radiative decay process, based on the Born-Oppenheimer approximation and perturbation theory, interactions between the excited triplet state and nearby singlet states can induce SOC. The radiative rate constant (k_r) for the T_m → S₀ transition can be determined using Equation 3 [49-51]:

$k_r^{\alpha }\left(T_m\text{→}{S}_0\right)\text{=}\frac{{\eta }^2}{1.5}E{\left(T_m\right)}^3{\left\{\sum _n^{}\frac{⟨T_m^{\alpha }⟩}{E\left(S_n\right)\text{-E}\left(T_m\right)}\right\}}^2\text{×}\frac{f_n}{E\left(S_n\right)}$ (3)

Here, η represents the refractive index of the medium, which is set to 1.424 (dichloromethane). $⟨T_m^{\alpha }⟩$denotes the SOC matrix element between the m-th triplet excited state (T_m) and the n-th singlet excited state (S_n). The SOC interaction becomes significant when the T_m and S_n states share the same unoccupied orbital distributed over the ligand. In contrast, their occupied orbitals correspond to different Pt d orbitals, typically associated with ¹MLCT and ³MLCT transitions. f_n is the oscillator strength of the corresponding singlet state, and α refers to the spin sublevel of the triplet state T_m (x, y, or z).

3. RESULTS AND DISCUSSION

3.1 Geometry of Cyclometalated Pt(II) Complexes

The present work aimed to predict the photophysical properties of cyclometalated Pt(II) complexes with the preferred substitution of a fluorophenyl group on the 2-phenylpyridine ligand. Figure 1a presents a molecular scheme representing our substitution preferences, at the para and meta positions of the ligand, and the number of fluorine atoms participating in the substituent. To better describe the charge transfer mechanism, the molecular geometry of the studied complexes in various electronic states (S₀, S₁, and T₁ states) was optimized using DFT calculations. The significant geometrical features in all optimized structures are presented in Table 1. In short, the optimum geometry for all studied complexes involves a square planar complex of a Pt(II) core with a d⁸ electron configuration. Typically, the Pt(II) ion is coordinated with two oxygen atoms from the acac ligand, while nitrogen and carbon atoms from the 2-phenylpyridine ligand introduce two further coordinated covalent bonds (Figure 1a). The variation in the geometry upon fluorophenyl substitution was investigated, considering changes in the bond length, bond angle, and dihedral angle, to construct the square planar Pt(II) complex (see Table 1). No significant differences in molecular geometry arise among the studied complexes in each respective electronic state (S₀, S₁, and T₁).

The apparent strengthening of the Pt-N and Pt-C bonds in the S₁ and T₁ states does not imply a formal change in the oxidation state of the Pt center. In square-planar Pt(II) complexes, the ground state (S₀) is well described as a d⁸ closed-shell singlet. Upon photoexcitation, the system typically accesses excited states with significant metal-to-ligand charge transfer (MLCT) character. In this process, electron density is partially redistributed from metal-centered d orbitals to ligand-centered π* orbitals; however, this redistribution is inherently delocalized and does not correspond to a full one-electron oxidation of Pt(II) to Pt(III). Although the excited-state electronic configuration may be qualitatively described as “d⁷-like” due to depopulation of a metal-based orbital, such a description is only formal and should not be interpreted as a true oxidation state change. Instead, the excited state is better characterized as a mixed metal-ligand state, in which both charge and spin densities are shared between the Pt center and the coordinated ligands.

3.2 Frontier Molecular Orbital Properties

Frontier molecular orbitals play a pivotal role in governing both absorption and emission processes. Photoluminescence properties are determined mainly by the nature of the molecular orbitals (MOs) that dominate the electronic ground state and the lowest excited state. Figure 2 presents key details on the energy levels of the frontier orbitals and the corresponding energy gaps, which are critical parameters for evaluating the luminescent behavior of the complexes.

In this study, the molecular orbital energy levels were modulated by introducing a fluorophenyl group into the cyclometalated Pt(II) complex. The Pt complex with a meta-substituted monofluorophenyl group (PtM1) exhibits a HOMO energy level of -5.82 eV, slightly higher than that of the parent Ptppy complex (-5.98 eV). In contrast, the para-substituted analog (PtP1) maintains a HOMO energy level of -5.97 eV, comparable to that of Ptppy. Additionally, the PtM1 and PtP1 analogs possess LUMO energy levels of -1.68 and -1.75 eV, respectively, which are lower than the LUMO level of the pristine form (-1.63 eV). Therefore, one can draw a vague conclusion regarding energy gap (ΔE_HOMO–LUMO) engineering through fluorophenyl substitution. The results demonstrate that introducing a fluorophenyl group into the meta or para positions leads to a slightly deeper LUMO level and an elevated HOMO level, resulting in a narrower energy gap for the substituted Pt(II) complexes. Furthermore, Figure 2a reveals the effect of multiple fluorine atoms on the substituent. Considering samples with similar substituted positions, the addition of a fluoride ion can shift the HOMO and LUMO energy levels of the complexes downward while maintaining the energy gap value. As illustrated in Figure 2a, most meta-substituted analogs show an energy band gap of 4.16 eV (PtM1 and PtM2), whereas a value of 4.21 eV is generally observed for para-substituted samples (PtP1, PtP2, and PtP3). Among all the modified Pt(II) complexes, the PtM3 complex represents an exceptional case of a large energy gap (4.20 eV), which is comparable to that of the para-substituted samples. Multiplying the fluorine atom causes a downward shift of the band position by approximately 0.08 eV.

The contribution of the frontier orbitals to the Ptppy and modified Pt(II) complexes was studied to describe electron delocalization during the molecular excitation process. The results presented in Figure 2b and Table 2 reveal the frontier orbital contributions in terms of the molecular constituents. For instance, a composition of 47% π-orbital on the 2-phenylpyridine ligand, 35% d-orbital on the Pt(II) core, and another 18% π-orbital on acac contribute to the ground state (S₀) HOMO of the pristine cyclometalated Pt(II) complex. Meanwhile, introducing the fluorophenyl group to the 2-phenylpyridine ligand induces a change in the HOMO attribute of the modified complexes. For example, the PtM1 sample gains an additional electron from the para-substituted fluorophenyl group. In this system, the electron density is predominantly localized on the fluorophenyl ring, contributing approximately 16% of the π-orbital character to the overall complex. In contrast, the para-substituted analog (PtP1) exhibits enhanced electron delocalization due to both the additional electron contribution and the extended π-conjugation introduced by the substituent, leading to a more delocalized HOMO across all molecular fragments. Moreover, fluorine substitution on the fluorophenyl ring significantly influences the electronic distribution. As the number of fluorine atoms increases, the HOMO becomes progressively delocalized from the ppy ligand toward the Pt(II) center and the acac ligand. This effect occurs independently of the substitution position.

In addition, the variation in the LUMO attribution was investigated. In most cases, a significant contribution (>80%) of the LUMO is attributed to the 2-phenylpyridine constituent, suggesting its essential role as the chromophore of the cyclometalated Pt(II) complex. The 5d orbital of Pt(II) and the π-orbitals of the acac ligands are associated with a π*-orbital contribution in the LUMO of the ppy ligand. Furthermore, an additional contribution of the LUMO π*-orbital in the fluorophenyl constituent was noted for all para-substituted samples (PtP1–PtP3). In these systems, the para-substituted fluorophenyl ring extends the π-conjugation, resulting in an increased contribution from the π* orbital. The degree of π* orbital participation correlates with the electron affinity of the fluorophenyl ring, which is determined by the number of fluorine substituents. Increasing the number of fluorine atoms enhances both the electron affinity and π* orbital contribution.

This behavior demonstrates the fine-tuning of the electronic structure, particularly the energy levels and frontier orbital characteristics. It reveals that the energy levels, energy gaps, and orbital nature of the Pt(II) complexes are susceptible to the position of fluorophenyl substitution. Moreover, these observations highlight how different substitution patterns, coupled with extended π-conjugation, modulate the electronic properties of cyclometalated Pt(II) complexes.
 

3.3 Electronic Absorption and Emission Spectra

To elucidate the electronic transitions of the studied Pt(II) complexes, absorption spectra were calculated using TD-DFT. This study aimed to demonstrate the electronic transitions and delocalization mechanism using optimized models of the complexes in the S_n and T₁ states. Additionally, the interfragment charge transfer was investigated for the complexes using the Multiwfn program, as presented in Table 3.

Table 3 reveals the electronic transitions upon the excitation and emission of the studied Pt complexes. Upon molecular excitation, the S₀-to-S₁ transition occurs. Absorption for all modified complexes based on this transition mode was calculated to appear within the range of 420–434 nm, depending on the energy gap discussed previously. In the case of the modified Pt(II) complexes, the absorption spectra exhibit a redshift with respect to the pristine form upon substitution and an increase in the fluorine number. Regarding the pristine Pt(II) complex, absorption spectra at 411 nm were obtained, with an oscillator strength of 0.0630. In this case, the frontier orbital contribution changes from the HOMO (S₀ state), consisting of contributions from states above, toward the LUMO (S₁ state), in which the LUMO consists of 92% of the π*-orbital in 2-phenylpyridine. In this particular case, frontier orbital migration, which depicts the HOMO-LUMO transition in the pristine Pt(II) complexes, was used as a representative example for all studied complexes (see Figure 3). The contribution of the 5d orbital of Pt(II) and the π-orbital of the acac ligand of the HOMO (S₀ state) migrates toward the π*-orbital of 2-phenylpyridine of the LUMO (S₁ state) upon molecular excitation. Therefore, two important mechanisms, metal-to-ligand charge transfer (MLCT, d(Pt) → π*(ppy)) and ligand-to-ligand charge transfer (LLCT, π(acac) → π*(ppy)), take place as the main processes. These two mechanisms can describe charge migration in the modified Pt(II) complexes, as a high HOMO-LUMO transition (>90%) was also noted.

Apart from excitation, molecular emission can also be explained considering the results in Table 3. In this case, the emission spectra of the considered complexes were determined starting from the optimized geometries of the Pt(II) complexes in the T₁ state, imitating an excited molecule experiencing thermal relaxation before photoemission. Phosphorescence results from the T₁ → S₀ electronic transition. Phosphorescence is inherently weak and long-lived because it originates from a spin-forbidden electronic transition between the lowest triplet excited state (T₁) and the singlet ground state (S₀). According to spin selection rules (ΔS = 0), transitions involving a change in spin multiplicity are formally forbidden, resulting in a negligible transition dipole moment and, consequently, an extremely low oscillator strength. The emission spectra of all modified Pt complexes show a redshift compared to the case of the pristine form, owing to the presence of an extended π-π conjugated system. The electronic transition from the T₁ to the S₀ state proceeds well in the para-substituted complexes due to a significant contribution (approximately 90%) from the LUMO-HOMO transition. The lowest energy emissions of complexes Ptppy-PtP3 occur at 476, 546, 543, 541, 549, 552, and 554 nm, respectively, corresponding to the experimental emission peaks at 486, 499, 496, 492, 525, 526, and 526 nm, respectively [26]. Overall, the experimental emission wavelengths agree with the calculated results, neglecting the solvent effect during practical work. The details of all complexes were presented in the supporting information.

3.4 Role of Cyclometalated Pt(II) Complex in Type II PDT

Type II PDT is a medical treatment that uses a photosensitizer (PS), light, and oxygen to generate singlet oxygen (denoted as ¹O₂). The PS absorbs light and transfers its energy to molecular oxygen (denoted as ³O₂, in the triplet spin state), creating a highly reactive ¹O₂. This energy transfer mechanism differs from Type I PDT, which relies on electron transfer reactions to generate other reactive oxygen species (ROS). Figure 3 demonstrates the energy transfer mechanism between a cyclometalated Pt(II) complex and a ³O₂ molecule using the Ptppy complex as a representative PS. It is worth mentioning that the singlet-to-triplet ISC of the PS is a key step that occurs on the singlet-state excited PS (¹PS*, electronic state = S₁) to generate the triplet-state excited PS (denoted as ³PS*, electronic state = T_n). The ³PS* molecule gradually releases its energy to reach the lowest triplet state (T₁) via thermal relaxation before engaging in a photoemission event (T₁→S₀ transition), also called “phosphorescence”. Subsequently, the released photon triggers another ISC process in molecular oxygen, generating ¹O₂, which then damages nearby cellular components and leads to cell death.

Herein, effective photoemission from ³PS* is the key capability of a candidate PS. Firstly, the released photon from the ³PS* must provide sufficient energy for ³O₂ ISC. The minimum energy for the oxygen singlet-triplet transition (O₂ ³Σg⁻→¹Δg transition) has been experimentally reported as 0.98 eV [7]. Therefore, this value has become a standard qualification for candidate PSs. As shown in Table 3, phosphorescence of the ³Ptppy* complex releases a photon with energy corresponding to the difference between the T₁ and S₀ electronic states. Similarly, the phosphorescence of other studied samples was examined via the T₁→S₀ transition. Table 3 presents the energy released from each studied Pt(II) complex. The ³Ptppy* complex emits phosphorescence with a photon energy of 2.60 eV. Meanwhile, the emitted photon energy of the modified Ptppy complexes ranges between 2.24 and 2.29 eV. Hence, the results confirm that all Pt(II) complexes in this study could trigger Type II PDT via a phosphorescence mechanism.

Secondly, the effectiveness of photoemission relies on the quantum yield, which, in turn, determines the rate of ³O₂ conversion. In particular, the ISC of PSs, a collection of ³PS* species before photoemission, is a key foundational step and can be predicted computationally for candidate screening [7, 10]. Herein, significant values associated with an ideal PS candidate, such as the SOC and the ISC energy barrier of ΔE(S₁-T_n), were calculated using relativistic quantum chemical methods with the ADF program. Specifically, a low energy barrier and a high SOC strength facilitate effective ISC for a PS. Additionally, the radiative decay rate (k_r), a constant that determines the successful photoemission of a PS, can be estimated. Table 4 presents the calculated ISC energy barriers for the studied Pt(II) complexes, which fall within the narrow range of 0.07-0.10 eV. The Ptppy complex represents the PS with the highest ISC energy barrier (0.10 eV). Meanwhile, fluorophenyl substitution can slightly decrease the energy barrier (by 0.1-0.3 eV; see Table 4). However, the reduction in the energy barrier appears to be independent of the substituted position and the number of fluorine atoms.

The calculated SOC matrix elements, which measure the interaction between an electron’s spin and orbital motion, are reported in Table 4. Typically, the stronger the SOC, the more efficient the ISC process and, hence, the higher the probability of phosphorescence and the higher the overall quantum yield of a PS. From the SOC matrix of the Ptppy complex (Table 4), one can deduce that S₁→T₃ is a significant transition mode, accompanied by a SOC strength of 348 cm^-1. On the other hand, all fluorophenyl-substituted complexes take the S₁→T₂ mode as a central transition channel and display a stronger SOC when compared with the unmodified Pt(II) complex (see Table 4). Therefore, fluorophenyl substitution represents a simple and effective modification approach for cyclometalated Pt(II) complexes, as it improves the k_r rate constant of phosphorescence. The meta position is the most preferable substitution site for cyclometalated Pt(II) complexes since the SOC strength of the PtM1-PtM3 samples (ca. 900 cm^-1) is roughly 1.3-fold higher than that of the PtP1-PtP3 samples (689 cm^-1). Furthermore, the multiplication of fluorine atoms is significant for the SOC strength and shows site-dependent behavior. As shown in Table 5, multiple fluorine atoms are more beneficial for the meta-substituted samples than the para-substituted ones. Consequently, the most effective modification was observed in the case of the PtM3 sample, characterized by strong SOC (921 cm^-1) and a large k_r constant (1.72 × 10⁵ s^-1), which were the optimal values across all samples. The information derived from these results provides a basis for the rational design of new compounds for PDT applications.

4. CONCLUSIONS

In summary, this study examined the effects of fluorophenyl substitution on the structure and photophysical properties of cyclometalated platinum(II) complexes. Optimized geometries revealed that all complexes adopt a square-planar structure, with subtle variations in bond lengths and angles depending on ligand positions and substitution patterns. Frontier molecular orbital analysis revealed that meta-positioned fluorophenyl substituents enhance electron transfer by reducing the HOMO-LUMO energy gap, rendering the complexes superior to para-substituted complexes for charge transport. This molecular modification also resulted in a redshift in the UV-Vis absorption spectra of the meta-substituted complexes, thereby enhancing their phosphorescence efficiency. Emission spectral analysis further demonstrated that both the substituent position and the presence of fluorine atoms significantly influence phosphorescence properties, with meta-substituted complexes exhibiting more pronounced redshifts in their emission peaks. Additionally, an analysis of the ISC rates and decay processes revealed that meta-substitution increases both k_ISC and k_r, particularly in complexes with a higher number of fluorine atoms, resulting in higher phosphorescence quantum yields and improved photophysical characteristics for oxygen sensing. In conclusion, the complexes favored Type II oxygen quenching, efficiently producing singlet oxygen through energy transfer. Overall, the studied complexes exhibit excellent photophysical characteristics, highlighting their potential as effective PSs for PDT applications.

ACKNOWLEDGEMENTS

This work was funded by the Faculty of Science, Kasetsart University, under the Graduate Fellowship Program. R.P.P. thanks the King Mongkut’s University of Technology Thonburi (KMUTT), Thailand Science Research and Innovation (TSRI), and National Science, Research and Innovation Fund (NSRF) Fiscal year 2027. We also acknowledge the Department of Chemistry, Faculty of Science, Kasetsart University, for their continuous assistance and support.

AUTHOR CONTRIBUTIONS

Wilaiwan Rosungnoen: Computation, Writing - Original draft preparation. Wanchai Deeloed: Writing - Reviewing and Editing. Supa Hannongbua: Writing - Reviewing and Editing. Rungtiva P. Poo-arporn: Writing - Reviewing and Editing. Songwut Suramitr: Supervision, Conceptualization, Methodology, Project administration.

CONFLICT OF INTEREST STATEMENT

All authors have contributed significantly to the research and have read and approved the manuscript. We have no conflicts of interest to declare.

FUNDING

This work was funded by the Faculty of Science, Kasetsart University, under the Graduate Fellowship Program. R.P.P. thanks the King Mongkut’s University of Technology Thonburi (KMUTT), Thailand Science Research and Innovation (TSRI), and National Science, Research and Innovation Fund (NSRF) Fiscal year 2027.

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