Synthesis, Spectroscopic Characterization, and DFT/TD-DFT Investigation of a Novel Nano-Sized Azo Dye Ligand Derived from 2-Amino-6-methoxybenzothiazole and Its Pt(IV) Nano-Complex

INTRODUCTION
Roughly 60% of all industrially produced dyes are azo dyes; some surveys put the share closer to 70% [1]. No other class of synthetic colorant comes close. The name refers to the –N=N– unit joining two aromatic or heteroaromatic rings, and this bridge is also the chromophore: conjugation through it gives strong bands throughout the visible region. Textile work is only part of the story now. A benzothiazole, benzimidazole or pyridine ring attached to the azo group brings additional N and S donor atoms, and the dye then behaves as a multidentate chelator toward transition metals [2,3]. Reported applications for the resulting complexes include catalysis, drug candidates, treatment of contaminated water, chemical sensing and solar cells [4,5].
Interest has grown lately in metal complexes whose crystallites are of nanometer size. The bulk and nano forms of the same compound can behave quite differently. Small crystallites expose more surface, and quantum confinement together with surface effects alters the optical response; biological activity often improves as well [6]. Nano-sized azo dye complexes in particular have given good antimicrobial and anticancer results [7]. There is a practical difficulty, though: the particles like to agglomerate. Controlled preparation, followed by careful characterization, is therefore essential in this field [8].
Pt(IV) occupies a special position among the platinum-group ions. It is a d⁶ ion, kinetically inert, and it takes six-coordinate octahedral geometry almost without exception. Pt(IV) prodrugs built on these properties are under study as replacements for cisplatin; oral dosing becomes possible and off-target toxicity drops [9]. Reports that combine Pt(IV) with benzothiazolyl azo dyes at the nanoscale are nevertheless rare, and calculations describing the electronic consequences of that combination are rarer. One cannot design such compounds rationally without knowing how charge moves across the –N=N– bridge from the heterocyclic donor to the metal [10].
DFT is the standard tool for questions of this sort. The frontier orbital energies give the ionization potential and the electron affinity at once; Koopmans’ theorem turns them into electronegativity (χ), chemical hardness (η), softness (σ) and the electrophilicity index (ω), the usual global measures of reactivity [11]. TD-DFT handles the excited states, which lets one compare computed UV-Vis transitions with the recorded spectrum [12]. Mulliken population analysis assigns a charge to each atom, showing where donor–acceptor interactions will occur and which sites are most reactive [13]. Calculations of this kind, run beside the usual spectroscopic and morphological measurements, tell more about a new azo dye–metal system than either would on its own.
Here we describe a new tridentate azo dye, 2-[2′-(6-methoxybenzothiazolyl)azo]-3,4-dimethylbenzoic acid (6-MBTAMB), and the Pt(IV) nano-complex derived from it. The two compounds were characterized by elemental analysis, UV-Vis, FT-IR and ¹H-NMR spectroscopy, mass spectrometry, powder XRD and FESEM imaging; special care was taken to establish that the prepared complex is indeed nano-dimensional. The experimental work is accompanied by a DFT/TD-DFT study of the free ligand at the B3LYP level covering the HOMO–LUMO energies, the reactivity descriptors, the Mulliken charges, the molecular electrostatic potential and a simulated absorption spectrum. Our aim was to connect the calculated electronic properties with what the spectra actually show, and in doing so to learn how a benzothiazolyl azo framework binds a heavy-metal center at the nanoscale.

MATERIALS AND METHODS
Chemicals and Instrumentation
Reagents were used as received. 2-Amino-6-methoxybenzothiazole, sodium nitrite (NaNO₂), hydrochloric acid, 3,4-dimethylbenzoic acid, potassium hexachloroplatinate(IV) (K₂PtCl₆) and the common solvents were analytical-grade materials (BDH/Aldrich). Electronic spectra were run in ethanol (10⁻³ M) between 200 and 1100 nm on a Shimadzu UV-1800 spectrophotometer, and FT-IR spectra were taken from KBr discs on a Shimadzu IRAffinity-1 instrument over 4000–400 cm⁻¹. ¹H-NMR measurements employed a Bruker 400 MHz spectrometer, with the samples in DMSO-d₆ and TMS as the internal standard. An Agilent 5975 GC-MS system supplied the mass spectra. Powder XRD patterns were collected on a Shimadzu XRD-6000 diffractometer with Cu Kα radiation (λ = 1.5406 Å) at a scan rate of 2° min⁻¹, and FESEM images were taken on a TESCAN MIRA3 field-emission scanning electron microscope to examine the morphology, particle size and surface features of the ligand and its Pt(IV) complex.

Synthesis of the Azo Dye Ligand (6-MBTAMB)
The ligand was obtained by the usual diazotization and coupling sequence [14,15]. A sample of 2-amino-6-methoxybenzothiazole (1.80 g, 10 mmol) was taken up, with gentle warming, in concentrated HCl (5 mL) diluted with deionized water (35 mL). After the solution had been cooled to 0–5 °C in an ice bath, a pre-cooled solution of NaNO₂ (0.69 g, 10 mmol in 10 mL H₂O) was run in dropwise over 15 min with stirring; the mixture was then held below 5 °C for a further 30 min to complete formation of the diazonium salt. Separately, 3,4-dimethylbenzoic acid (1.50 g, 10 mmol) was dissolved in NaOH solution (10 mL, 10%). The alkaline coupling component was added gradually to the diazonium solution with vigorous stirring at 0–5 °C. The resulting deep-orange precipitate was collected by filtration, washed thoroughly with cold water, and recrystallized from hot ethanol. Yield: 78%, m.p. 110–112 °C.
The synthetic route for the preparation of the azo dye ligand 6-MBTAMB is outlined in Fig. 1.

Synthesis of the Pt(IV) Nano-Complex
The Pt(IV) nano-complex was prepared by dissolving the ligand 6-MBTAMB (0.10 g, 0.29 mmol) in 20 mL of hot ethanol. This solution was added with stirring to a stoichiometric amount of K₂PtCl₆ dissolved in 20 mL ethanol. The resulting mixture was refluxed for 3 h at 78 °C with continuous stirring. The dark reddish-purple precipitate that formed was collected by filtration, washed with cold ethanol and diethyl ether, and dried in a vacuum desiccator. Yield: 76%, decomp. 190–192 °C. The non-electrolytic character was confirmed by a low molar conductance value (Λₘ < 15 S cm² mol⁻¹ in DMF), ruling out ionic species [16]. The analytical data for both compounds are summarized in Table 1.

Computational Details
All quantum chemical calculations were performed using the PySCF program package (version 2.4) [17] interfaced with the RDKit cheminformatics toolkit for initial three-dimensional coordinate generation via the ETKDG algorithm followed by MMFF94 force-field pre-optimization. Ground-state electronic structure calculations for the free ligand employed the restricted Kohn–Sham (RKS) formalism with the B3LYP hybrid density functional and the 3-21G basis set (250 basis functions for 178 electrons) in the gas phase. The SCF procedure was converged to a threshold of 10⁻⁸ Hartree using the default DIIS algorithm with a maximum of 200 cycles. Frontier molecular orbital (FMO) energies were used to derive global reactivity descriptors via Koopmans’ theorem [18]: ionization potential (I = –E_HOMO), electron affinity (A = –E_LUMO), electronegativity (χ = (I + A)/2), chemical potential (μ = –χ), chemical hardness (η = (I – A)/2), chemical softness (σ = 1/2η), electrophilicity index (ω = χ²/2η), and maximum charge transfer index (ΔN_max = –μ/η).
Mulliken population analysis was performed on the converged SCF density to quantify atomic charge distribution. Time-dependent DFT (TD-DFT) calculations were carried out at the B3LYP/STO-3G level (139 basis functions) using the Tamm–Dancoff approximation (TDA) for the lowest 10 singlet excited states. Oscillator strengths were computed in the dipole-length gauge. The molecular electrostatic potential (MEP) surface was constructed from the Mulliken charge distribution projected onto the molecular framework. Throughout, the molecule was treated as a neutral closed-shell singlet (charge = 0, spin = 0), the appropriate ground state for the free ligand [19].

Determination of Nanoscale Structural Parameters
Crystallographic quantities were derived from the powder XRD patterns by the standard routes. Bragg’s law (Eq. 1) gave the interplanar spacings; the volume-weighted mean crystallite size D came from the Debye–Scherrer equation (Eq. 2); the dislocation density δ followed from the Williamson–Smallman relation (Eq. 3); and the lattice microstrain ε was estimated with the tangent formula (Eq. 4):

nλ = 2d sinθ                                                              

D = Kλ / (β cosθ)                                                        

δ = 1 / D²                                                                     

ε = β / (4 tanθ)                                                          

Here λ = 1.5406 Å (Cu Kα), K = 0.9, β is the FWHM of the reflection in radians and θ is the Bragg angle. Each observed reflection was treated separately; the tables list D and δ per line together with their arithmetic means [29,30,52]. For the FESEM sizes we measured at least 50 well-resolved particles per image in ImageJ and report the arithmetic mean [33].

RESULTS AND DISCUSSION
Elemental Analysis and Physical Properties
Analytical and physical data appear in Table 1. 6-MBTAMB forms reddish-orange crystals, m.p. 110–112 °C, in 78% yield. The Pt(IV) product is a reddish-purple powder obtained in 76% yield; it shows no melting point and decomposes at 190–192 °C instead. The gap between the two temperatures is large, roughly 80 degrees, and it reflects what coordination does here: Pt–N and Pt–O bonds lock the ligand into an octahedral frame that resists heating far better than the free molecule.
Microanalysis matches the proposed formulae for both compounds. Ligand C₁₇H₁₅N₃O₃S (MW = 341.39 g/mol): C, 59.81 (60.19); H, 4.43 (4.54); N, 12.31 (12.82); S, 9.39 (9.70). Complex C₁₇H₁₆Cl₃N₃O₄PtS (MW = 659.84 g/mol): C, 30.95 (31.46); H, 2.44 (2.54); N, 6.37 (6.91); S, 4.86 (5.25); Pt, 29.56 (29.82). Platinum is the decisive figure. Found 29.82% versus calculated 29.56%: a 1:1 metal-to-ligand ratio, with one lattice water included in the formula, reproduces it almost exactly [14,15]. The found C and N values sit a little above the calculated ones, within the ±0.5–1.0% tolerance customary for heterocyclic azo compounds.

Molar Conductance Measurements
The complex was dissolved in DMF (10⁻³ M) and its molar conductance read at room temperature. We measured Λₘ < 15 S cm² mol⁻¹. Non-electrolytes fall below Λₘ < 30 S cm² mol⁻¹, so the compound gives off no ions in solution [16]. This result confirms that all three chloride ligands are coordinated directly to the Pt(IV) center within the inner coordination sphere, rather than existing as dissociated counter-ions. The non-electrolytic nature is fully consistent with the proposed neutral molecular formula [Pt(L)Cl₃]·H₂O, in which the water molecule occupies a lattice position outside the coordination sphere and the ligand coordinates as a monoanionic tridentate (N,N,O) donor after deprotonation of the carboxylic acid group.

Magnetic Susceptibility Measurements
The magnetic susceptibility of the Pt(IV) nano-complex was measured at room temperature (Table 2). The complex exhibited diamagnetic behavior (μeff = 0 B.M.), consistent with a d⁶ low-spin electronic configuration in which all six electrons are paired in the t₂g set of orbitals under the influence of the strong octahedral ligand field. This observation is characteristic of Pt(IV) complexes, where the large crystal field splitting energy (Δoct) of the 5d orbitals invariably produces a low-spin arrangement [21,49]. The diamagnetic nature, combined with the electronic spectral data discussed in Section 3.4, supports the assignment of an octahedral geometry with d²sp³ hybridization at the platinum center in the nano-complex.

Electronic Absorption Spectra
In ethanol, the free ligand 6-MBTAMB shows two absorption bands (Table 2, Fig. 2). The higher-energy band at 328 nm (30488 cm⁻¹) belongs to the π→π* transition of the conjugated aromatic and heterocyclic framework. The second band, at 451 nm (22173 cm⁻¹), arises from the n→π* transition of the azo (–N=N–) group; it is this absorption that gives the free ligand its reddish-orange color [20].
For the Pt(IV) nano-complex, three bands appear, at 496 nm (20576 cm⁻¹), 441 nm (22676 cm⁻¹) and 348 nm (28736 cm⁻¹). We assign them to the d–d transitions ¹A₁g→¹T₁g(F), ¹A₁g→¹T₂g(F) and ¹A₁g→¹T₁g(P), the pattern expected of a d⁶ ion held in an octahedral ligand field. Together with the diamagnetism of the compound, these assignments point to low-spin octahedral Pt(IV) with d²sp³ hybridization [21]. The ligand bands themselves move to longer wavelength on complexation, a bathochromic shift that we put down to extended conjugation and to charge transfer between the ligand π-system and the platinum center in the nano-complex.

¹H-NMR Spectral Analysis
Fig. 3 shows the ¹H-NMR spectrum of the ligand. The carboxylic acid proton (–COOH) gives a signal at δ = 9.47 ppm; it vanishes on shaking with D₂O, as a labile proton should. Aromatic protons of the benzothiazole unit fall as a multiplet between δ = 7.72 and 6.91 ppm. The methoxy group (–OCH₃) resonates at δ = 3.85 ppm, and the two ring methyls of the benzoic acid fragment appear as singlets at δ = 2.27 and 2.32 ppm [22].
Fig. 4 shows the complex. Aromatic protons again appear between δ = 7.72 and 6.91 ppm, now displaced a little downfield; the metal deshields nearby protons on coordination. The –COOH line changes more: it broadens and moves to higher frequency, the behavior expected once the carboxylate oxygen is bound to platinum. Lattice water announces itself as a new broad signal at δ = 3.33 ppm. Methyl and methoxy resonances are essentially where they were in the free ligand, so neither group takes part in metal coordination [23].

Mass Spectrometry
6-MBTAMB gives its molecular ion at m/z = 343 [M+2]⁺ (Fig. 5), in accord with the formula C₁₇H₁₅N₃O₃S and a molecular weight of 341.39 g/mol. Fragments at m/z = 328 [M–CH₃]⁺, 301 and 192 record the stepwise loss of substituents; the base peak, at m/z = 149, comes from the benzothiazolyl fragment. This breakdown route is the one normally seen for heterocyclic azo compounds (Fig. 6, Table 3) [24].
For the Pt(IV) nano-complex the molecular ion appears at m/z = 603 [M–Cl–2H₂O]⁺ (Fig. 7). Successive losses of chloride, water and pieces of the organic ligand account for the remaining peaks (Fig. 8, Table 3), and the pattern as a whole supports the proposed molecular formula and the 1:1 metal-to-ligand ratio [25].

FT-IR Spectral Analysis
The FT-IR spectra of the free ligand and its Pt(IV) nano-complex provide direct evidence for the coordination mode (Table 4, Figs. 9 and 10). In the ligand spectrum, the broad band at 2834 cm⁻¹ is attributed to ν(O–H) of the carboxylic acid group. The ν(C=O) stretching vibration appears at 1689 cm⁻¹, and the thiazole ring ν(C=N) is observed at 1604 cm⁻¹. The azo bond ν(N=N) stretching frequency appears at 1490 cm⁻¹. Bands at 3088 and 2934 cm⁻¹ correspond to aromatic and aliphatic C–H stretching vibrations, respectively [26].
Upon formation of the Pt(IV) nano-complex, several diagnostic shifts are observed. A broad band at 3446 cm⁻¹ corresponds to ν(O–H) of the lattice water molecule. The ν(C=O) band shifts from 1689 to 1686 cm⁻¹, and the ν(C=N) from 1604 to 1605 cm⁻¹, while the ν(N=N) shifts from 1490 to 1486 cm⁻¹. Asymmetric and symmetric COO⁻ stretching bands appear at 1563 and 1404 cm⁻¹ in the complex. Critically, new bands at 517 cm⁻¹ and 441 cm⁻¹ are assigned to ν(Pt–N) and ν(Pt–O) stretching modes, confirming metal–ligand bond formation. A further band at 400 cm⁻¹ belongs to ν(Pt–Cl), as expected when three chloride ligands remain bound in the octahedral nano-complex [27,28].

X-ray Powder Diffraction (XRD) Analysis
Powder XRD patterns were recorded for solid 6-MBTAMB and for its Pt(IV) nano-complex, partly to check crystallinity and phase purity, and above all to measure the crystallite size (Table 5, Fig. 11). Both patterns contain sharp reflections; the compounds are therefore well-ordered crystalline solids. For every diffraction line the interplanar spacing (d-spacing) was calculated from Bragg’s law, nλ = 2d sinθ [29].
Mean crystallite sizes came from the Debye–Scherrer equation, D = Kλ / βcosθ, with K = 0.9 (shape factor), λ = 1.5406 Å (Cu Kα radiation), β the full width at half maximum (FWHM) and θ the Bragg angle; the dislocation density δ = 1/D² was evaluated as well, as a measure of lattice imperfection [30]. Both compounds turn out to be nanocrystalline. The free ligand averages D = 29.25 nm, and the Pt(IV) nano-complex averages a smaller D = 21.71 nm. Complexation therefore shrinks the coherent domains: a reasonable reading is that the bulky PtCl₃ unit interferes with the packing the organic molecule would otherwise adopt, leaving smaller nano-sized crystalline regions [31,32]. The dislocation density moves in the same direction, from δ = 6.36 × 10⁻³ nm⁻² on average for the ligand to δ = 1.51 × 10⁻² nm⁻² for the complex, meaning the lattice of the complex carries more strain and disorder. In both compounds the crystallites measure well under 100 nm; the nano-crystalline description of the Pt(IV) complex is thus supported directly by the diffraction data.
Lattice microstrain was evaluated for each reflection from Eq. (4). Averaged over all observed lines, the free ligand exhibits ε = 6.66 × 10⁻³, whereas the Pt(IV) nano-complex shows a value almost twice as large, ε = 1.16 × 10⁻². The simultaneous rise in microstrain and dislocation density on passing from the ligand to the complex is the expected signature of smaller and more defective coherent domains, and furnishes an independent, quantitative confirmation of the crystallite-size reduction inferred from the Scherrer analysis [30,52]. It should be noted that the Debye–Scherrer treatment used here attributes the whole of the observed line broadening to size effects and does not deconvolute the instrumental contribution; the crystallite sizes in Table 5 are therefore lower-bound estimates, and the microstrain values are correspondingly upper-bound estimates [31,32,34].

Field-Emission Scanning Electron Microscopy (FESEM)
FESEM analysis was performed to evaluate the surface morphology, particle size distribution, and nanoscale features of the ligand and its Pt(IV) complex (Fig. 12). The surface properties of the compounds were studied in terms of particle dimensions, shape, and the degree of aggregation between neighboring crystallites, which are critical parameters for assessing nanoscale character [33].
The FESEM image of the benzothiazolyl azo ligand (6-MBTAMB) revealed agglomerated spherical crystalline nanoparticles with an average particle size of 68.89 nm. Its surface looks fairly smooth, and neighboring particles cling together to a moderate degree, much as one would expect where carboxylic acid and methoxy groups allow intermolecular hydrogen bonding. Even before complexation, then, the particle size of the free ligand sits well inside the nanoscale regime (< 100 nm) [34].
FESEM tells a different story for the complex. The particles are irregular rather than spherical, they aggregate less, the surface is more porous, and the mean size drops to 40.50 nm against 68.89 nm for the ligand. XRD had already recorded the same trend in the crystallites, 29.25 nm falling to 21.71 nm. One explanation covers both observations: the bulky PtCl₃ fragment gets in the way of the packing the free molecule prefers. Every size measured here is below 100 nm, which places both solids among nanostructured materials [35,36]. Small dimensions of this kind tend to raise surface reactivity, and often biological activity as well, compared with the bulk compound; follow-up biomedical and catalytic studies of the complex therefore seem worthwhile.

DFT Computational Study of the Free Ligand
Ground-State Energy and Convergence
SCF convergence at B3LYP/3-21G presented no problems. The total electronic energy of the neutral closed-shell singlet came to −1437.1389 Hartree, i.e. −39107.13 eV. There were 178 electrons in the calculation and 250 contracted Gaussian functions to describe them, sufficient for a π-system that extends from the benzothiazole across the azo bridge and into the substituted benzoic acid ring. With the wavefunction settled at this level, the orbital energies and charge analyses discussed next stand on firm ground [37].

Frontier Molecular Orbital Analysis
Much of the chemistry of the nano-ligand system can be read off its frontier orbitals. Table 6 and Fig. 13 place the HOMO of 6-MBTAMB at −5.9529 eV and the LUMO at −2.5220 eV, so the gap is ΔE = 3.4309 eV. A gap of this size is typical for a conjugated heterocyclic azo compound: larger than in reactive open-shell species (ΔE < 2 eV), smaller than in kinetically inert saturated molecules (ΔE > 6 eV). The LUMO is fairly low at −2.52 eV, which means the molecule accepts electron density without difficulty; that fits the ease with which it was found to bind Pt(IV) and form the nano-complex. Similar gaps have been published for related chromophores, 2.85–3.08 eV for benzothiazolyl azo-pyrazolone dyes at B3LYP/6-31+G(d) [38] and 3.42 eV for a heterocyclic azo dye ligand at B3LYP/6-311G(d,p) [39], so the value of 3.4309 eV obtained here falls where this class of compound would put it.

Global Reactivity Descriptors
Table 6 and Fig. 14 list the global reactivity parameters obtained from the orbital energies through Koopmans’ theorem. Oxidation of 6-MBTAMB is only moderately easy (I = 5.9529 eV); accepting electrons comes more naturally to it (A = 2.5220 eV). Electronegativity is high for an organic molecule, χ = 4.2375 eV. Three withdrawing groups act on one π-system here, azo plus thiazole plus carboxylic acid, and the pull accumulates. Hardness lands at η = 1.7154 eV. In HSAB terms that is a soft molecule, and a soft donor suits a soft acceptor; Pt(IV) is exactly that [40].
ω = 5.2336 eV is a large electrophilicity index; anything above 1.5 eV already counts as a strong electrophile [41]. Electron density therefore flows toward this ligand whenever a nucleophilic partner is available. Experiment agrees: complexation with Pt(IV) went quickly under mild conditions and gave stable nanoparticles. The charge-transfer estimate ΔN_max = 2.4702 says the ligand could take up about 2.5 electrons in an ideal donor–acceptor pair, and a tridentate (N,N,O) attachment, the mode the spectra indicate, is consistent with that number [42].

Mulliken Population Analysis
Mulliken charges are collected in Table 7 and drawn in Fig. 15; the coordination chemistry can almost be read straight off them. S22, the benzothiazole sulfur, holds the largest positive charge in the molecule, +0.7668 e, which comes from its place in the aromatic π-system. The carboxylic carbon C13 follows at +0.6302 e. On either side of C13 sit two negative oxygens, O14 (C=O) at −0.4846 e and O15 (OH) at −0.5501 e. A charge like that on O15 is what qualifies it as a donor atom toward the metal [43].
N9 and N10, the azo nitrogens, carry −0.3152 e and −0.3022 e; π-density from the –N=N– bond accounts for both values. The thiazole nitrogen N7 is more negative still, −0.5129 e, the most nucleophilic nitrogen in the molecule and an obvious σ-donor toward platinum. O2 of the methoxy group also holds substantial charge, −0.5588 e, but it points away from the binding region. Nucleophilic character thus gathers at N7, N9/N10 and O15, and these are the very atoms the NNO tridentate arrangement uses. The stability of the resulting Pt(IV) nano-complex follows naturally [44].

Molecular Electrostatic Potential Surface
Projecting the Mulliken charges onto the molecular surface gives the MEP map of Fig. 16. Red, negative regions sit on the carboxylate oxygens, the methoxy oxygen and the thiazole nitrogen: the electron-donating positions. Blue, positive regions surround the sulfur and follow the carbon skeleton. Nothing in the map contradicts the point-charge analysis; it presents the same conclusion spatially. Platinum binding through the N and O donors, and not through other positions, is what one would predict from this picture, and it is what happens in the stable nano-complex [45].

TD-DFT UV-Vis Absorption Spectrum
Ten singlet excited states emerge from the TD-DFT calculation at the TDA-B3LYP/STO-3G level, spread over 260–528 nm (Table 8, Fig. 17). Strongest among them is S₄ at λ = 328.1 nm, with oscillator strength f = 0.3919. Its character is mainly π→π*: the electron leaves occupied π-orbitals on the benzothiazole ring system for an antibonding π*-orbital spread across the azo bridge. Transitions at 289.8 nm (S₆, f = 0.2329) and 278.1 nm (S₈, f = 0.1864) carry appreciable strength too, and together these build the broad UV absorption envelope [46].
The lowest-energy excitation (S₁, λ = 527.9 nm, f = 0.0092) corresponds to the n→π* transition of the azo chromophore, its very small oscillator strength being characteristic of a symmetry-forbidden excitation. Because the calculation was performed on the uncomplexed molecule, the computed spectrum must be compared with the experimental spectrum of the free ligand rather than with that of the complex. On this basis the agreement for the principal band is excellent: the computed S₄ transition at 328.1 nm coincides with the experimental π→π* maximum of 6-MBTAMB at 328 nm (Table 2). The calculated n→π* transition (527.9 nm) is overestimated by roughly 77 nm relative to the experimental value of 451 nm, a well-documented shortcoming of TD-DFT for n→π* excitations that is further amplified here by the minimal STO-3G basis set and by the neglect of solvent (ethanol) effects in the gas-phase model [46,47]. The visible band of the Pt(IV) nano-complex at 496 nm is not directly comparable with either value, since it arises from the metal-centred d–d manifold that is by construction absent from the free-ligand calculation. Notwithstanding the quantitative offset for the forbidden transition, TD-DFT reproduces the overall spectral topology correctly a weak n→π* band in the visible region and intense π→π* transitions at higher energy and provides reliable assignments for the observed absorptions.

Density of States Analysis
The density of states (DOS) diagram (Fig. 18) provides a complementary view of the electronic structure. The occupied DOS shows a dense manifold of states from approximately −12 eV up to the HOMO at −5.95 eV. Above the LUMO at −2.52 eV the virtual states climb steeply. Between the two regions the diagram stays empty, so the 3.43 eV HOMO–LUMO gap can be seen at a glance. The DOS view also anticipates what coordination will do to the spectrum: metal d-orbital states enter the gap when the nano-complex forms, and it is these that permit the LMCT and d–d transitions recorded experimentally [48].

Proposed Structure of the Pt(IV) Nano-Complex
Putting the spectroscopic, morphological and computational evidence side by side leads to the following assignment for the Pt(IV) nano-complex. 6-MBTAMB binds as a tridentate (N,N,O) ligand, using the thiazole nitrogen (N7), one azo nitrogen (N9) and the carboxylate oxygen (O15); these three donors take up one face of the octahedron, and three chloride ligands fill the remaining positions. A single water molecule sits in the crystal lattice, which is what the broad O–H stretch in the IR spectrum indicates. The non-electrolytic molar conductance, the diamagnetism and the d–d band pattern all agree on a low-spin octahedral d⁶ Pt(IV) center with d²sp³ hybridization. FESEM and XRD, finally, show the compound to be nanoparticulate, with a mean crystallite size of 21.71 nm and a mean particle size of 40.50 nm. On this evidence the formula is [Pt(L)Cl₃]·H₂O, where L = 6-MBTAMB bound as a monoanionic tridentate ligand [49]. The proposed structure is depicted in Fig. 19.

Comparison with Related Azo Dye Ligands and Their Metal Complexes
To place the present results in context, Table 9 collects frontier-orbital and nanostructural data reported for structurally related azo dye ligands and their metal complexes. Three observations follow. First, the HOMO–LUMO gap computed here for 6-MBTAMB (3.4309 eV) falls within the narrow band of 2.85–3.42 eV reported for benzothiazolyl and other heterocyclic azo chromophores [38,39], indicating that the 6-methoxybenzothiazole–azo–benzoic acid framework possesses the electronic character expected of this family notwithstanding the modest basis set employed. Second, the nanoscale dimensions obtained here are closely paralleled by the nano-azo system of Mohamed et al. [36], in which SEM particle sizes fell from 64–67 nm for the free ligand to 20–32 nm for the Cd(II) complex, and XRD crystallite sizes from 33.78 to 26.77 nm the same direction and very nearly the same magnitude of change as observed here (FESEM 68.89 → 40.50 nm; XRD 29.25 → 21.71 nm). Contraction of the crystalline domain upon coordination therefore appears to be a general characteristic of this class of azo dye complex rather than a peculiarity of the present preparation. Third, benzothiazole-derived ligands are consistently reported to yield nano-dimensioned complexes with platinum-group and other heavy metals [14,53], which supports the structural assignment advanced above.
A further trend evident in Table 9 is that coordination almost invariably narrows the HOMO–LUMO gap of the free ligand, frequently by 1–2 eV [39,50]. Although the corresponding calculation was not performed for [Pt(L)Cl₃]·H₂O in the present work (see Section 3.13), the pronounced bathochromic shift of the ligand bands observed experimentally upon complexation (328 → 348 nm and 451 → 496 nm) is entirely consistent with such a contraction of the frontier-orbital separation.

Scope and Limitations of the Computational Model
Several caveats should be borne in mind when the computational results presented above are interpreted. (i) The DFT and TD-DFT calculations were performed on the free ligand only; the Pt(IV) nano-complex itself was not modelled, because a reliable description of a third-row transition metal requires a relativistic effective core potential (e.g. LANL2DZ or SDD) combined with a polarised valence basis set for the light atoms, which lies beyond the scope of the present study. No calculated HOMO–LUMO gap, Mulliken charge distribution or MEP surface is therefore reported for the complex, and the d–d transitions observed at 496, 441 and 348 nm are assigned on the basis of ligand-field theory rather than from first principles.
(ii) The 3-21G and STO-3G basis sets employed for the ground- and excited-state calculations respectively are of minimal to split-valence quality and carry neither polarisation nor diffuse functions; absolute orbital energies and excitation energies obtained at this level are subject to systematic errors of several tenths of an electronvolt, so the descriptors listed in Table 6 are best read as internally consistent trends rather than as quantitatively converged values [43,47]. (iii) All calculations were carried out in the gas phase on a geometry pre-optimised at the MMFF94 force-field level, whereas the experimental spectra were recorded in ethanol; solvent stabilisation of the more polar excited states typically red-shifts π→π* bands and blue-shifts n→π* bands, which accounts for part of the residual discrepancy discussed in Section 3.10.6. (iv) The MEP surface was reconstructed from Mulliken atomic charges projected onto the molecular framework rather than from the full converged electron density, and is consequently qualitative; Mulliken charges are themselves known to be basis-set dependent [13,43].
A fully relativistic DFT and TD-DFT treatment of [Pt(L)Cl₃]·H₂O at the B3LYP/LANL2DZ ∪ 6-311G(d,p) level, incorporating a continuum solvation model and complemented by molecular docking, represents the natural next step and would allow the coordination-induced changes in electronic structure inferred here to be quantified directly.

CONCLUSION
A novel heterocyclic azo dye ligand, 6-MBTAMB, was successfully synthesized and coordinated with Pt(IV) to form a mononuclear octahedral nano-complex [Pt(L)Cl₃]·H₂O. Elemental analysis together with FT-IR, UV-Vis, ¹H-NMR, mass spectrometry, powder XRD and FESEM established the identity and purity of both compounds and fixed the binding mode of the ligand as tridentate (N,N,O). FESEM put the mean particle size at 68.89 nm for the ligand and 40.50 nm for the complex, and the XRD crystallite sizes, 29.25 nm and 21.71 nm respectively, told the same story. By either measure the prepared complex is a nano-sized material.
The B3LYP/3-21G calculations gave a quantitative account of the frontier orbitals of the free ligand. The gap, 3.4309 eV, points to moderate kinetic stability; the LUMO lies low, so the molecule takes up electrons readily. By the Koopmans measures (ω = 5.23 eV, η = 1.72 eV) 6-MBTAMB is soft and strongly electrophilic, properties that suit binding to the equally soft Pt(IV) ion. Donor sites identified by the Mulliken charges, namely the thiazole nitrogen, the azo nitrogens and the carboxylate oxygen, coincide with the coordination mode the spectra gave. The TD-DFT spectrum has the right shape as well; its main π→π* band appears at 328 nm. The nanoscale morphology, combined with the favorable electronic properties revealed by DFT analysis, makes this Pt(IV) nano-complex a promising candidate for further investigation in biomedical, catalytic, and materials science applications.
The lattice microstrain and dislocation density averaged over all observed reflections (ε = 6.66 × 10⁻³ and δ = 6.36 × 10⁻³ nm⁻² for the ligand; ε = 1.16 × 10⁻² and δ = 1.51 × 10⁻² nm⁻² for the complex) independently corroborate the contraction and increased defectiveness of the crystalline domains upon coordination. The computational component of this work is confined to the free ligand; a relativistic DFT treatment of the complex itself, together with an evaluation of its antimicrobial and cytotoxic profile, constitutes the natural continuation of the study.

ACKNOWLEDGMENTS
The authors gratefully acknowledge the Department of Chemistry, College of Science, University of AL-Qadisiyah for providing laboratory facilities and computational resources.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.