# Impact of iodine-induced nonplanarity on the vibrational and electronic properties of a triphenylene derivative: A combined experimental and DFT study

Ahmed Hewail, David Guérin, Christophe Krzeminski, Eric Faulques,
Nataliya Kalashnyk

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Ahmed Hewail, David Guérin, Christophe Krzeminski, Eric Faulques, Nataliya Kalashnyk. Impact of iodine-induced nonplanarity on the vibrational and electronic properties of a triphenylene derivative: A combined experimental and DFT study. Journal of Molecular Structure, 2026, 1358, pp.145178. 10.1016/j.molstruc.2025.145178. hal-05476586

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# Impact of iodine-induced nonplanarity on the vibrational and electronic properties of a triphenylene derivative: A combined experimental and DFT study

Ahmed Hewail ID, David Guérin ID, Christophe Krzeminski ID, Eric Faulques * ID,
Nataliya Kalashnyk ID

Univ. Lille, CNRS, Centrale Lille, Univ. Polytechnique Hauts-de-France, UMR 8520 -IEMN – Institut d'Electronique, de Microélectronique et de Nanotechnologie, F-59000 Lille, France

## ARTICLE INFO

**Keywords:**

Triphenylene derivatives
Infrared and Raman spectra
UV-visible spectra
Density functional theory (DFT) and time dependent DFT calculations
Vibrational analysis

## ABSTRACT

We present a molecular study combining Raman, infrared, and UV-visible absorption spectroscopy with density functional theory (DFT) calculations to investigate triphenylene and its iodinated derivative, 1,5,9-triiodotriphenylene. The results provide a detailed comparison of their vibrational and electronic properties, revealing how iodine substitution affects molecular geometry and electronic structure. Iodination induces nonplanarity in the molecular backbone, leading to shifts in vibrational frequencies as well as modifications of π-π* transitions in the UV-visible spectra. Complementary analyses of natural bond orbitals, polarizabilities, and Hirshfeld surfaces further elucidate electronic delocalization and intermolecular interactions. These findings could enable the rational design of triphenylene-based precursors optimized for on-surface reactions, facilitating controlled formation of nanocarbon architectures with tailored structural and electronic properties.

## 1. Introduction

Polycyclic aromatic hydrocarbons (PAHs) are a large class of chemically stable organic molecules composed of two or more fused aromatic rings. Structurally, PAHs exhibit remarkable versatility, adopting configurations such as planar forms like triphenylene (TP) and pyrene, bowl-shaped curved structures like corannulene and sumanene, and helical geometries [1–3]. Their stability, resulting from the extensive conjugation of π-electrons across the benzenoid rings [4], makes them prevalent across diverse environments such as in the interstellar medium [5–7]. Despite their utility in fields like materials science, where their rigidity and π-conjugated systems make them valuable for electronic and optical devices [8–10], PAHs are significant environmental pollutants [11]. On Earth, PAHs are naturally found in coal, oil, and tar and are also widely produced during incomplete combustion, appearing in flames, soot, and related clusters [12]. These soot particles, largely composed of PAHs, are recognized as major contributors to greenhouse effects [13]. Many PAHs are known to be carcinogenic, mutagenic, and teratogenic, highlighting the dual challenge of leveraging their technological potential while addressing their

environmental and health impacts [14,15]. Vibrational frequency data for PAHs is particularly valuable for identifying these molecules in interstellar and terrestrial environments through infrared, Raman, and optical spectroscopy [16,17]. While the former two techniques provide detailed vibrational signatures, optical spectroscopy can be used to probe electronic transitions, offering complementary insights into the molecular structure and behavior of PAHs. These techniques are crucial for the detection and analysis of PAHs in various astrophysical and atmospheric contexts.

Among PAHs, triphenylene and its derivatives have attracted particular interest for their discotic structure, which enables them to self-assemble into one-dimensional columnar arrays [18,19]. These π-stacked molecules form liquid crystalline phases facilitating efficient charge transport. They are promising candidates for organic semiconductors, photovoltaic cells, organic light-emitting diodes (OLEDs), and other optoelectronic devices [20–22]. Moreover, triphenylene and its derivatives are promising key molecular building blocks in on-surface synthesis, due to their potential to act as precursors for extended carbon-based nanostructures. Through cyclodehydrogenative C-C coupling, these compounds can form different architectures, including

<footnote>* Corresponding author.
E-mail address: eric.faulques@univ-lille.fr (E. Faulques).</footnote>
fullerenes ($C_{60}$), as well as planar and curved nanographenes. Notably, while triphenylene exhibits self-assembly on single-crystal substrates without undergoing further reactions [23], its derivatives display distinct reactivity [24–29], including formation of 2D metal-organic frameworks (MOF) and nanomesh-like structures. These differences arise from variation in molecular geometry, substitutions (identity and positioning), as these factors critically influence on-surface reactions pathways [30]. In this context, vibrational spectroscopy proves useful: infrared (IR) spectroscopy, in particular, has long been recognized as a valuable tool for characterizing aromatic compounds [31], especially PAHs. However, while spectroscopic data for triphenylene are available, no detailed vibrational data exist for its derivative, 1,5,9-triiodotriphenylene.

Functionalizing triphenylene leads to tuning electronic properties. Bromination of triphenylene is reported to reduce the optical gap and red-shifts main bands in UV-vis spectra [32]. Previous studies have shown substituents can distort its structure and alter its vibrational spectra [33]. Such studies illustrate the general principle that substitutions can change electronic properties, bond lengths, vibrational features, and lower symmetry in a PAH.

In this work, we have studied triphenylene (TP) and 1,5,9-triiodotriphenylene (TITP) by combining Raman, infrared, and UV-visible absorption spectroscopy with density functional theory (DFT) calculations of the molecular structures and vibrations. We focused on three primary objectives: (i) to establish reliable consistency between experimental and theoretical results obtained on the molecules, (ii) to assign all molecular vibrational frequencies through theoretical mode visualizations, and (iii) to determine the HOMO-LUMO gap and explore the electronic properties of these molecules. This study addresses the lack of comprehensive investigations on iodinated triphenylene and emphasizes the consequence of halogen substitution. By directly comparing TP and TITP, we show how iodine-induced nonplanarity modifies vibrational and electronic signatures, offering detailed insights into structure-property relationships that are relevant for molecular design in nanocarbon materials.

## 2. Methods

### 2.1. Experimental

1,5,9-Triiodotriphenylene (95 % purity) was obtained from Chem-Shuttle (USA) and further locally purified by flash chromatography on silica gel using hexane as the eluent, since a noticeable change in color was observed in the as-received compound. The purified compound was checked by NMR (see Fig. S1). Triphenylene (95 % purity) was purchased from BLDPharm. Both purified compounds were obtained as white microcrystalline powders. Fourier-transform infrared (FTIR) spectra were recorded in the 500–4000 cm⁻¹ range using attenuated total reflection (ATR) mode. A PerkinElmer Spectrum GX spectrometer equipped with a Golden Gate Specac diamond ATR accessory was used for data acquisition with 4 cm⁻¹ resolution. UV-Visible absorption spectra were recorded in hexane solution using a PerkinElmer Lambda 365+ spectrophotometer in the 190–1000 nm range.

Molecular powders deposited on stainless steel or aluminum plates were analyzed using a Horiba XploRA one confocal Raman microscope operating with a 532 nm (2.33 eV) excitation wavelength and a 900 gr/mm grating. The system was equipped with a motorized translation stage and a × 50 objective, providing a spatial resolution of approximately 1 µm. Spectra were collected between 30 and 4000 cm⁻¹ with an acquisition time of 5 s per scan, laser power on samples was kept below 500 µW. Complementary Raman measurements were performed using a mini-Raman system, comprising an OceanOptics QEPro Peltier-cooled spectrometer coupled with a 785 nm (1.58 eV) laser source, mounted on a compact optical table. The laser beam and scattered signal were transmitted and collected via 1-meter-long optical fibers, achieving a 150 µm spot size on the sample with a 7.5 mm working distance. The

system is non-confocal and includes a 3D micrometer translation stage for precise sample positioning. Raman spectra were recorded between 200 and 3600 cm⁻¹ with an exposure time of 5 s. Laser power onto samples was 80 mW. The Raman spectrometers were calibrated using the 520.5 cm⁻¹ Raman band of silicon or the 1332 cm⁻¹ band of diamond. All measurements were carried out at room temperature under ambient conditions. Sample integrity was assessed before and after each measurement using either the camera-integrated microscope of the confocal setup or a binocular microscope equipped with a camera to detect potential laser-induced thermal degradation.

### 2.2. Simulations

Density functional theory (DFT) calculations were performed using the Gaussian software [34]. For geometry optimizations and vibrational analyses of triphenylene, we employed the hybrid B3LYP functional with the 6-311G(d,p) and 6-311++G(d,p) basis sets.

For 1,5,9-triiodotriphenylene vibrations, we compared in the first instance three combinations of hybrid functionals and basis sets: B3PW91/def2-TZVP, B3LYP/DGDZVP, and B3LYP/def2-TZVP. The B3PW91 functional was selected as a primary choice due to its established ability to yield improved agreement with experimental bond lengths [35], particularly in systems involving π-conjugation and heavy atoms [36]. This functional incorporates an exchange component distinct from B3LYP, specifically enhancing the treatment of electron correlation. It is well-suited for elements like iodine, where the impact of relativistic and core-valence effects cannot be ignored [37]. Furthermore, B3PW91 has been shown to possibly outperform B3LYP in the prediction of harmonic vibrational frequencies, in case of halogenated aromatics [38,39]. The def2-TZVP basis set was employed because it includes effective core potentials (ECPs) that efficiently account for scalar relativistic effects and reduce computational cost, while maintaining accuracy for heavy atoms like iodine [40]. In parallel, the B3LYP/DGDZVP combination was also retained for comparison because it has previously demonstrated very good performance in reproducing IR vibrational modes of halogen-carbon bonds, including C-I stretches, particularly in organoiiodine systems [41] (see Supplementary Material and Table S1). All structures were fully optimized without symmetry constraints using analytical gradients (Fig. S2). Convergence was achieved when the maximum and root mean square (RMS) forces and displacements satisfied the default Gaussian thresholds. The final electronic energies at the optimized geometries were −693.338021147 Ha (−1.8204 × 10⁶ kJ·mol⁻¹, B3LYP) and −1584.82186609 Ha (−4.1609 × 10⁶ kJ·mol⁻¹, B3PW91). Subsequent harmonic vibrational frequency calculations confirmed that the optimized structures correspond to true minima on the potential energy surface.

Time-dependent density functional theory (TD-DFT) calculations were performed with 15 excited states to compute UV-visible absorption spectra without and with solvent effect to assess whether this latter approach enhance the accuracy of excitation energies. The solvent effects were included with the conductor-like polarizable continuum model (CPCM) using n-hexane as the dielectric medium.

Calculations performed without a solvent model are treated as gas-phase simulations, corresponding to the isolated molecule in vacuum.

The vibrational assignments in this work were performed by direct visualization of the normal modes using Jmol and Avogadro [42] programs. This procedure, which is customary applied in vibrational spectroscopy studies of polyaromatic hydrocarbons and their derivatives [33,43], allows sufficient reliable mode identification without the need for a potential energy distribution (PED) analysis [44]. The major spectral features, such as C-H stretching, C = C ring stretching, and C–I stretching vibrations, are unambiguous by inspection and consistent with reported trends. Additionally, the GaussSum [45] graphical user interface was used for the comparison of vibrational and UV-visible computed spectra with experimental data.

The simulated IR and non-resonant Raman spectra of TP and TITP
were scaled using appropriate scaling factors (SFs), which provided a good agreement with experimental data in terms of both intensity and frequency. Alecu *et al.* reported a scaling factor of 0.972 at the B3PW91/6-31G(d) level based on benchmark calculations for a set of small and organic molecules in the gas phase [46]. In the present work, we optimized the scaling factor by minimizing the root-mean-square deviation (RMSD) between the experimental and computed frequencies of the most intense triphenylene bands in the 400–1800 cm⁻¹ range. This procedure yielded a value of 0.971, which lies well within the typical 0.97–0.98 range reported for hybrid functionals with triple-ζ basis sets by Zapata Trujillo *et al.*, [47]. More importantly, it is also worth noting that B3PW91 is a hybrid functional closely related to B3LYP, and the value of 0.971 agrees closely with the factors reported by Bauschlicher *et al.* [48] for B3LYP/6-31G(d,p) (0.9730) and B3LYP/6-311G(d,p) (0.9757) and Onchoke *et al.* (0.980) [33] in their study of the vibrational spectra of PAHs such as naphthalene and triphenylene. For the C–H stretching vibrations, a scaling factor of 0.960 was applied for both molecules, which accurately reproduced the most intense C–H stretching bands. This choice is consistent with the values compiled by Bauschlicher *et al.* [48] for C–H stretches and with the comprehensive study of Laury *et al.* [49], who reported distinct scaling factors for low- and high-frequency regions. In contrast, the theoretical UV-visible spectra were not scaled, and calculated without and with solvent effect.

Polarizabilities were determined, and a natural bond orbital (NBO) analysis was performed as a continuation of the above calculations, also using Gaussian. Finally, Hirshfeld surfaces of triphenylene molecular crystal were computed using the experimental crystal structure of TP.

### 3. Results and discussion

#### 3.1. Structural analysis

Table S2 and Fig. 1 presents the theoretically optimized geometry and parameters for both unsubstituted triphenylene and its iodinated derivative, highlighting the structural modifications induced by iodine substitution. The structural model assumes that each iodine atom is substituted on a separate peripheral phenyl ring, consistent with the expected 1,5,9-triiodotriphenylene isomer, as specified by the chemical supplier, after its purification. The introduction of iodine atoms leads to notable perturbations in the molecular geometry, including significant variations in bond lengths, bond angles, and a reduction in overall molecular symmetry. These structural modifications directly impact the electronic properties and steric interactions within the molecule. While triphenylene belongs to the D₃h symmetry group, maintaining a highly symmetric, planar structure with nearly uniform bond lengths and

angles, characterized by well-defined infrared (IR) and Raman-active vibrational modes, triiodotriphenylene exhibits distortions due to the bulky iodine atoms, reducing the symmetry to C₁. The most noticeable deviations occur in C–I bond lengths, C–C bonds adjacent to iodine, and internal ring angles, where steric and electronic effects disrupt the uniformity seen in the parent structure.

The non-planarity of the TITP molecule was evaluated by performing several geometry optimizations starting from an artificially constrained planar structure that was optimized with the MOPAC PM7 Hamiltonian. Although the molecule remained planar, the calculation failed to converge to a true ground-state minimum. When this planar TITP geometry was relaxed using the Avogadro molecular-mechanics tools, the optimization spontaneously distorted toward a non-planar conformation. A total-energy scan at the B3PW91/def2-TZVP level, performed by incrementally varying one particular dihedral angle, also identified this non-planar geometry as a minimum-energy structure (Fig. S3).

In triphenylene, the C–C bond lengths display a high degree of uniformity, typically ranging between 1.380 Å and 1.466 Å. This consistency results from the delocalized π-electron system, which distributes electron density evenly across the conjugated framework. This uniform charge distribution supports the molecule’s symmetrical and planar nature, reinforcing its stability. However, in triiodotriphenylene, the incorporation of iodine atoms disrupts this structural regularity. The electron-withdrawing inductive effects of iodine lead to elongation of C–C bonds adjacent to the iodine-substituted carbons. Conversely, C–C bonds positioned further from the iodine-substituted site may undergo slight contraction. Additionally, the C–I bonds, measuring approximately 2.115–2.118 Å, are significantly longer than C–C and C–H bonds due to iodine’s large atomic radius and low electronegativity. This bond elongation leads to distortions in adjacent C–C bonds, which exhibit increased variability in length, deviating from the uniform pattern observed in triphenylene. As a result, planarity is compromised, and the π-system becomes more distorted, impacting conjugation and electronic properties.

The bond angles in triphenylene molecule are remarkably consistent, with values centered around 120°, which reflects the planar hexagonal symmetry of its benzene rings. This high degree of symmetry promotes uniform electron delocalization, thereby ensuring both structural stability and electronic homogeneity. The calculated C–C and C–H bond lengths, together with the C–C–C angles, are in very good agreement with the crystallographic structure of triphenylene reported by Ahmed and Trotter in 1963 (see Table S2). A slight discrepancy between the calculated and experimental C–C–H angles is observed, which we attribute to solid-state effects.

In triiodotriphenylene, however, bond angle variations become

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** The optimized equilibrium geometries of (A) triphenylene and (B) 1,5,9-triiodotriphenylene, obtained using DFT calculations at the B3LYP/6-311-G(d,p) and B3PW91/def2tzvp levels respectively. (C) Side-view of 1,5,9-triiodotriphenylene.</figcaption></figure>
pronounced due to the steric influence of iodine atoms. The internal ring angles, which are tightly grouped at 120° in triphenylene, now range from 117.185° to 119.147° indicating a localized strain within the central ring. Further distortions arise in the C–C–I bond angles, which expand significantly due to steric repulsion from iodine. For instance, the C1–C14–I1 bond angle expands to 126.621°, a significant deviation from the 119.862° observed in triphenylene when hydrogen occupies the same position instead of iodine. This increase in bond angle pushes adjacent carbon atoms apart, further disrupting the molecular symmetry. Conversely, bond angles distant from the iodine atoms remain relatively unchanged, typically falling within the range of 119.5° to 121.5°. The minimum bond angle, observed at C3–C18–I9, compresses to 113.615°, reflecting the localized steric effects near the substitution sites. Since no experimental crystallographic structure of TITP has been reported to date, no direct comparison with the calculated geometry can be made. The non-planarity of TITP was quantified using dihedral angles defined by the C–I bonds and adjacent aromatic carbon atoms. Two iodine substituents show pronounced out-of-plane distortions (|θ| ≈ 17–24°), whereas the third remains nearly coplanar (|θ| ≈ 0.2°), confirming that iodination distorts the triphenylene framework from planarity.

## 3.2. Vibrational analysis

Triphenylene is a non-linear molecule consisting of $N = 30$ atoms, thus possessing a total of 90 degrees of freedom (3 $N$) which encompass translational, rotational, and vibrational motions. The molecule exhibits three translational and three rotational modes, resulting in 84 vibrational modes (normal modes), as given by $3N-6$ which are classified as follows according to the irreducible representations (irreps) of the D3h point group to which belongs triphenylene:

$$ \Gamma_{\text{vib}} = 10A'_{1} + 19E' + 9A'_{2} + 4A''_{2} + 9E'' + 5A''_{1} $$

The six non-vibrational modes are distributed as $\Gamma_{\text{trans}} = E' + A''_{2}$ for translational motions and $\Gamma_{\text{rot}} = A'_{2} + E''$ for rotational motions. The irreps $A'_{1}$, $E'$, and $E''$ are associated with changes in the polarizability tensor and correspond to 66 Raman-active modes in total. The irreps $A''_{2}$

and $E'$ correspond to changes in the dipole moment, yielding a total of 42 infrared-active modes. The five $A''_{1}$ and nine $A''_{2}$ modes are neither infrared nor Raman active and are therefore considered silent. In the nonsymmetric 1,5,9-triiodotriphenylene molecule (point group $C_{1}$) optimized in this work, all 84 vibrational modes belong to the irrep $A$, making them possibly observable in both infrared and Raman spectra [50].

A vibrational analysis of the mid-infrared and Raman spectra of triphenylene and 1,5,9-triiodotriphenylene was performed in three main regions: (i) C–H stretching vibrations between 3200 and 2800 cm$^{-1}$, (ii) in-plane vibrations between 1600 and 1000 cm$^{-1}$, and (iii) out-of-plane and mixed vibrations below 1000 cm$^{-1}$ (Fig. 2). It should be emphasized that experimental measurements performed in the solid state inherently account for intermolecular interactions and anharmonic effects. In contrast, theoretical calculations are typically carried out for an isolated molecule in vacuum, under the harmonic approximation, thereby neglecting these influences. This simplification often leads to an overestimation of vibrational frequencies, with calculated bands appearing at higher wavenumbers than those observed experimentally. To correct for this systematic discrepancy, empirical scaling factors (SFs) were applied to the DFT-computed spectra to achieve better agreement with the experimental data [39].

In the following sections we evaluate the RMS deviation between theoretical and experimental vibrational frequencies $\omega$ defined as follows:

$$ RMSD = \sqrt{\frac{\sum_{i=1}^{N} \Delta_{i}^{2}}{N}} $$

where $\Delta = \omega_{\text{th}} - \omega_{\text{exp}}$, and $N$ is the number of lines considered.

### 3.2.1. IR study of triphenylene and its derivative

The infrared (IR) spectrum of triphenylene has been previously reported [51], with its most intense band theoretically predicted at 740.8 cm$^{-1}$ and experimentally observed at 743.6 cm$^{-1}$. In the present study, this band is observed at 736 cm$^{-1}$ in the experimental spectrum and calculated at 736 cm$^{-1}$, as shown in Table 1. The simulated spectra exhibit a good agreement with the experimental data in Fig. 3A,

<figure><img src="image_4.png" /><figcaption>Fig. 2. Selected normal-mode examples for (A) Triphenylene and (B) 1,5,9-triiodotriphenylene obtained from DFT calculations. The theoretical vibrational frequencies have been scaled. Only a subset of modes is listed in the tables.</figcaption></figure>
<caption>**Table 1**</caption>

Selected Experimental and Scaled DFT-Calculated IR Frequencies of Triphenylene (in cm⁻¹), Extracted from Measured and Simulated Spectra, with Corresponding Vibrational Mode Assignments. B3LYP/6-311G(d,p). SF = 0.9714. SF = 0.960 for CH bands > 3000cm⁻¹. Δ: frequency difference between model and experiment, RMSD: root mean square deviation.

<table><thead><tr><th>Assignment</th><th>Experiment</th><th>Model</th><th>Δ</th></tr></thead><tbody><tr><td>ν(CH) + β(CH) scissor + β(CC) def of external cycles</td><td>617 (m)</td><td>624</td><td>7</td></tr><tr><td>ω(CH)</td><td>736 (s)</td><td>736</td><td>0</td></tr><tr><td>γ(CH)</td><td>772 (m)</td><td></td><td></td></tr><tr><td>β(CH) + (ν,β(CC))</td><td>778 (w)</td><td></td><td></td></tr><tr><td>τ (CH)</td><td>935 (w)</td><td>936</td><td>1</td></tr><tr><td>τ (CH)</td><td>949 (w)</td><td></td><td></td></tr><tr><td>ν(CH) + ρ(CC) of central cycle + β(CC) def external rings</td><td>994 (w)</td><td>996</td><td>2</td></tr><tr><td>β(CH) rocking+scissor + ν(CC)</td><td>1050 (w)</td><td>1051</td><td>1</td></tr><tr><td>β(CH) scissor + ν(CC)</td><td>1061 (w)</td><td></td><td></td></tr><tr><td>β(CH) rocking + ν(CC)</td><td>1108 (w)</td><td>1108</td><td>0</td></tr><tr><td>β(CH) scissor + ν(CC)</td><td>1161 (w)</td><td>1163</td><td>2</td></tr><tr><td>β(CH) scissor + ν(CC)</td><td>1242 (m)</td><td>1240</td><td>2</td></tr><tr><td>β(CH) rock+strong ν(CC) + deformation of central cycle</td><td>1298 (w)</td><td>1288</td><td>10</td></tr><tr><td>β(CH) rock + strong ν(CC)</td><td>1431 (m)</td><td>1432</td><td>1</td></tr><tr><td>β(CH) rock + scissor + strong ν(CC)</td><td>1496 (m)</td><td>1496</td><td>0</td></tr><tr><td>RMSD for low-frequency modes</td><td></td><td></td><td></td></tr><tr><td>ν(CH)</td><td>3020 (w)</td><td></td><td></td></tr><tr><td>ν(CH)</td><td>3057 (w)</td><td>3055</td><td>-2</td></tr><tr><td>ν(CH)</td><td>3085 (w)</td><td>3087</td><td>2</td></tr><tr><td>RMSD for all assigned bands</td><td></td><td></td><td></td></tr><tr><td>RMSD (&lt;2000 cm⁻¹)</td><td></td><td></td><td></td></tr><tr><td>RMSD (&lt;2000 cm⁻¹)</td><td></td><td></td><td></td></tr><tr><td>Average % Deviation (all bands)</td><td>—</td><td>0.26 %</td><td></td></tr><tr><td>Average % Deviation (&lt;2000 cm⁻¹)</td><td>—</td><td>0.26 %</td><td></td></tr><tr><td>Average % Deviation (&gt;2000 cm⁻¹)</td><td>—</td><td>0.05 %</td><td></td></tr></tbody></table>

ν, β, γ, ρ, ω and τ denote stretching, in plane bending, out of plane bending, rocking, wagging and twisting, respectively. w: weak, m: medium, s/vs: strong/very strong.

particularly in positions and intensity. Using either the B3LYP/6-311G (d,p) or B3LYP/6-311++G(d,p) level of theory yields identical IR spectra, with no variation in the calculated vibrational frequencies. For frequencies below 1000 cm⁻¹, a mixture of in-plane and out-of-plane vibrations is identified. C-H out-of-plane vibrations are observed with frequencies at 949, 935, 772, and 736 cm⁻¹, consistent with the literature [33,52]. The peak at 994 cm⁻¹ is ascribed to C—C in-plane deformation, consistent with an earlier work [52]. Additionally, at this frequency, rocking of the central ring occurs with minor contributions from C-H stretching. The bands observed at 1496, 1431, 1242, 1161, 1108, and 1050 cm⁻¹ match perfectly with those reported in previous studies [33], and are attributed to slight C-C stretching and C-H in-plane bending motions. Moreover, significant phenyl ring deformation coupled with C-H in-plane bending is observed at 1298 cm⁻¹. However, a few vibrational bands present in the experimental spectrum are absent in the theoretical IR spectrum. This can be attributed to differences in selection rules between gas-phase calculations and solid-state measurements. In solid state, intermolecular interactions and crystal packing effects can lower molecular symmetry, leading to the activation of forbidden modes in the gas phase. For example, the experimental IR peak at 850 cm⁻¹ (not reported in Table 1) is not reproduced in our DFT calculations. This finding is consistent with a previous study on triphenylene vibrations by Onchoke et al [33], who similarly could not reproduce this band theoretically. However, this 850 cm⁻¹ band appears clearly as a weak feature in the DFT Raman spectrum (Fig. 4A), suggesting that it corresponds to a Raman-active or symmetry-forbidden mode in the gas phase that becomes experimentally IR-active in the solid state.

The experimentally obtained IR bands of 1,5,9-triiodotriphenylene exhibit slight deviations from the theoretical values, with both being nearly identical and fitting well overall in terms of peak positions (Fig. 3B, Fig. S4). While several relative intensities are consistent with

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** FTIR spectra of (A) triphenylene, recorded experimentally in ATR and simulated at the B3LYP/6-311G(d,p) level of theory; (B) 1,5,9-triiodotriphenylene, measured experimentally and computed at the B3PW91/def2-TZVP level of theory; and (C) a direct comparison of the experimental FTIR spectra of triphenylene and 1,5,9-triiodotriphenylene, highlighting their spectral differences. Stripes indicate agreement between experiment and theory. The C-I stretching line around 650 cm⁻¹ is highlighted.</figcaption></figure>

the DFT predictions, some slight discrepancies in band profiles and intensities can appear in this compound. These differences are attributed to the fact that the experimental ATR spectrum has not been corrected for wavelength-dependent effects. Interestingly, the distinct experimental IR peak observed at 649 cm⁻¹ can be attributed to the C-I stretching vibration. The corresponding theoretical values were calculated to be 646–647 cm⁻¹ using the B3LYP/DGDZVP (Table S3) and the
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Raman spectra of (A) triphenylene, measured experimentally and simulated at the B3LYP/6-311G(d,p) level of theory; (B) 1,5,9-triiodotriphenylene, recorded experimentally and calculated at the B3PW91/def2-TZVP level of theory; and (C) a direct comparison of the experimental spectra of triphenylene and 1,5,9-triiodotriphenylene, highlighting their spectral differences. The DFT simulations are displayed here in terms of Raman intensity and are also available in terms of Raman activity. SF = 0.9714. Stripes indicate either agreement between experiment and theory or common experimental bands shared by both molecules.</figcaption></figure>

B3PW91/def2-TZVP methods (Table 2). This close agreement between experimental and theoretical results shows the reliability of these computational methods in predicting vibrational frequencies. A similar C–halogen stretching peak has been reported experimentally in the literature at 658 cm⁻¹ [53]. Additionally, the C–I stretching vibrations

<caption>**Table 2**</caption>

Selected Experimental and Scaled DFT-Calculated IR Frequencies of 1,5,9-triiodotriphenylene (in cm⁻¹), Extracted from Measured and Simulated Spectra, with Corresponding Vibrational Mode Assignments. B3PW91/def2-TZVP. SF = 0.9714. SF = 0.960 for CH bands > 3000cm⁻¹. Δ: frequency difference between model and experiment, RMSD: root mean square deviation.

<table><thead><tr><th>Assignment</th><th>Experiment</th><th>Model</th><th>Δ</th></tr></thead><tbody><tr><td>γ(CC) + γ(CH)</td><td>569 (w)</td><td>569</td><td>0</td></tr><tr><td>strong ν(CI) + γ(CH) + β(CH) + β(CC)</td><td>649 (s)</td><td>647</td><td>-2</td></tr><tr><td>ω(CH) + τ(CC)</td><td>719 (s)</td><td></td><td></td></tr><tr><td>ω(CH)</td><td>749 (vs)</td><td>745</td><td>-4</td></tr><tr><td>ω(CH) + γ(CC) ring def</td><td>774 (s)</td><td>773</td><td>-1</td></tr><tr><td>ω(CH) + γ(CC) ring def</td><td>788 (s)</td><td>795</td><td>7</td></tr><tr><td>ω(CH) + τ(CC)</td><td>809 (s)</td><td></td><td></td></tr><tr><td>ν(CI) + ν(CC) + breathing of one external ring</td><td>1008 (m)</td><td>1004</td><td>-4</td></tr><tr><td>weak ν(CI) + β(CH) scissoring + ν(CC)</td><td>1081 (s)</td><td></td><td></td></tr><tr><td>β(CH) scissoring + ν(CC) + weak ν(CI)</td><td>1093 (s)</td><td>1092</td><td>-1</td></tr><tr><td>β(CH) rocking + strong ν(CC)</td><td>1395 (s)</td><td>1396</td><td>1</td></tr><tr><td>β(CH) rocking + strong ν(CC)</td><td>1459 (m)</td><td>1454</td><td>-5</td></tr><tr><td>β(CH) + strong ν(CC)</td><td>1543 (s)</td><td>1553</td><td>10</td></tr><tr><td>β(CH) + very strong ν(CC)</td><td>1576 (m)</td><td>1576</td><td>0</td></tr><tr><td>ν(CH) (many small peaks there)</td><td>2869 (w)</td><td></td><td></td></tr><tr><td>ν(CH)</td><td>2912 (w)</td><td></td><td></td></tr><tr><td>ν(CH)</td><td>2954 (w)</td><td></td><td></td></tr><tr><td>ν(CH)</td><td>3044 (w)</td><td>3062</td><td>18</td></tr><tr><td>ν(CH)</td><td>3086 (w)</td><td>3086</td><td>0</td></tr><tr><td>ν(CH)</td><td>3108(vw)</td><td>3118</td><td>10</td></tr><tr><td>RMSD for all assigned bands</td><td></td><td>4.4</td><td></td></tr><tr><td>RMSD (&lt;2000 cm⁻¹)</td><td></td><td>4.0</td><td></td></tr><tr><td>RMSD (&gt;2000 cm⁻¹)</td><td></td><td>6.0</td><td></td></tr><tr><td>Average % Deviation (all bands)</td><td></td><td>0.18 %</td><td></td></tr><tr><td>Average % Deviation (&lt;2000 cm⁻¹)</td><td></td><td>0.16 %</td><td></td></tr><tr><td>Average % Deviation (&gt;2000 cm⁻¹)</td><td></td><td>0.26 %</td><td></td></tr></tbody></table>

ν, β, γ, ρ, ω and τ denote stretching, in plane bending, out of plane bending, rocking, wagging and twisting, respectively.

are IR active at 1008 cm⁻¹, in agreement with the theoretical prediction, with weaker contributions also observed at 1081 and 1093 cm⁻¹.

The peaks observed between 1000 and 1600 cm⁻¹ are primarily attributed to C–H in-plane bending and C–C stretching deformations of the aromatic rings, consistent with reported data of TP. Out-of-plane C–H vibrations appear at frequencies of 809, 788, 774, 749, 719, 649 and 569 cm⁻¹.

A comparison of IR spectra for the two molecules, as illustrated in Fig. 3C, reveals shifts in the vibrational bands resulting from structural modifications induced by iodine substitution. TP exhibits in-plane vibrations at 617 cm⁻¹, while TITP displays out-of-plane vibrations at 649 cm⁻¹. Moreover, TP shows in-plane C–H vibrations and aromatic rings deformations at 778 cm⁻¹, whereas in TITP, C–H wagging and out-of-plane deformation of the aromatic rings are observed at 788 cm⁻¹. Notably, although some frequencies for TP are associated primarily with C–H motions, the corresponding vibrations in TITP are predominantly coupled with out-of-plane deformations of the phenyl rings. This behavior is attributed to the molecule’s loss of planarity caused by iodine substitutions at three positions.

It is also well established from similar studies that environmental factors and neighboring molecular interactions can significantly influence vibrational frequencies in IR and Raman spectra, often resulting in shifts relative to those calculated for isolated molecules in vacuum [54].

### 3.2.2. Raman study of triphenylene and its derivative

The theoretical Raman spectra of TP and TITP, calculated using the DFT levels of theory listed in Table S1, are in excellent agreement with the experimental results. Examination of the data presented in Fig. 4A for TP, together with the peak assignments provided in Table 3, reveals that only a limited number of Raman-active peaks correspond to out-of-plane vibrations. These are experimentally observed at 700, 418, and 283 cm⁻¹, with closely matching theoretical values at 696, 416, and 272 cm⁻¹. Peaks with frequencies above 1000 cm⁻¹ are attributed to in-plane vibrations, consistent with previously reported studies [55,56]. In
<caption>**Table 3**</caption>

Selected Experimental and Scaled DFT-Calculated Raman Frequencies of Triphenylene (in cm⁻¹), Extracted from Measured and Simulated Spectra, with Corresponding Vibrational Mode Assignments. Force constants relative to specific intense lines are listed (1 mdyn/Å = 10² N/m). B3LYP/6-311G(d,p). SF = 0.9714 for lines < 2000cm⁻¹. SF = 0.960 for CH bands > 3000cm⁻¹.

<table><thead><tr><th>Assignment</th><th>Experiment</th><th>Model</th><th>Δ</th><th>Calculated force constant (mdyn/Å)</th></tr></thead><tbody><tr><td>τ (CH) + τ (CC)</td><td>283 (w)</td><td>272</td><td>-11</td><td></td></tr><tr><td>ω (CH) + ω (CC)</td><td>418 (m)</td><td>416</td><td>-2</td><td>0.74</td></tr><tr><td>ν(CH) + β(CH) + β(CC) def of rings</td><td>620 (w)</td><td>624</td><td>4</td><td></td></tr><tr><td>τ(CH) + τ(CC) slight</td><td>700 (m)</td><td>696</td><td>-4</td><td>1.66</td></tr><tr><td>β(CH) + (ν,β(CC)) def of rings</td><td>775 (w)</td><td>776</td><td>1</td><td></td></tr><tr><td>ν(CH) + ρ(CC) of central cycle + β(CC) def external rings</td><td>1005 (w)</td><td>1000</td><td>-5</td><td></td></tr><tr><td>β(CH) scissoring + strong ν(CC)</td><td>1061 (s)</td><td>1064</td><td>3</td><td>1.77</td></tr><tr><td>β(CH)) scissoring + weak ν(CC)</td><td>1161(w)</td><td>1160</td><td>-1</td><td></td></tr><tr><td>β(CH) scissoring + ν(CC)</td><td>1181 (w)</td><td>1176</td><td>-5</td><td></td></tr><tr><td>β(CH) scissoring + ν(CC)</td><td>1229 (m)</td><td>1216</td><td>-13</td><td>2.82</td></tr><tr><td>β(CH) rocking + strong ν(CC)</td><td>1339 (vs)</td><td>1328</td><td>-11</td><td>9.70</td></tr><tr><td>β(CH) rocking + ν(CC)</td><td>1458 (s)</td><td>1440</td><td>-18</td><td>3.39</td></tr><tr><td>β(CH) scissoring + strong ν(CC) + breathing of aromatic rings</td><td>1546(w)</td><td>1544</td><td>-2</td><td></td></tr><tr><td>β(CH) scissoring + strong ν(CC)</td><td>1603 (m)</td><td>1608</td><td>5</td><td>8.30</td></tr><tr><td>ν(CH)</td><td>3032(w)</td><td>3038</td><td>6</td><td></td></tr><tr><td>ν(CH)</td><td>3052(w)</td><td>3054</td><td>2</td><td></td></tr><tr><td>ν(CH)</td><td>3086(s)</td><td>3086</td><td>0</td><td></td></tr><tr><td>ν(CH)</td><td>3097(s)</td><td></td><td></td><td></td></tr><tr><td>RMSD for all assigned bands</td><td></td><td>7.9</td><td></td><td></td></tr><tr><td>RMSD (&lt;2000cm⁻¹)</td><td></td><td>7.8</td><td></td><td></td></tr><tr><td>RMSD (&gt;2000cm⁻¹)</td><td></td><td>3.6</td><td></td><td></td></tr><tr><td>Average % Deviation (all bands)</td><td></td><td>-0.41 %</td><td></td><td></td></tr><tr><td>Average % Deviation (&lt;2000 cm⁻¹)</td><td></td><td>-0.41 %</td><td></td><td></td></tr><tr><td>Average % Deviation (&gt;2000 cm⁻¹)</td><td></td><td>0.12 %</td><td></td><td></td></tr></tbody></table>

ν, β, γ, ρ, ω and τ denote stretching, in plane bending, out of plane bending, rocking, wagging and twisting, respectively.

addition, characteristic breathing modes of aromatic ring are identified at 1546 and 1181 cm⁻¹. Raman spectra calculated for TP using B3LYP/6-311G(d,p) and B3LYP/6-311++G(d,p) levels of theory are nearly identical. The most notable difference is a slight downshift – approximately 8 cm⁻¹ – of the highest-frequency C = C stretching mode ~1600 cm⁻¹ when diffuse functions are included with the 6-311++G(d,p) basis set.

It is important to note that experimental Raman spectra of triphenylene were successfully acquired using a green laser at 532 nm, whereas attempts with a red laser at 785 nm were unsuccessful. This difficulty likely stems from the presence of fluorescent impurities interfering with the Raman signal, as well as from the intrinsic photoluminescence of aromatic rings [57]. Both excitation wavelengths fall outside the absorption bands of the TP molecule, resulting in non-resonant Raman conditions. Nevertheless, the spectra obtained with the green laser were sufficiently intense when the molecular powders were deposited on either aluminum or stainless-steel surface.

Table 4 summarizes the Raman-active modes of TITP, including both experimental frequencies and theoretical values calculated at the B3PW91/def2-TZVP level, along with their corresponding vibrational assignments (Fig. 4B). In contrast to TP, the Raman spectra of TITP were successfully recorded using both green (532 nm) and red (785 nm) laser

<caption>**Table 4**</caption>

Selected Experimental and Scaled DFT-Calculated Raman Frequencies of 1,5,9-triiodotriphenylene (in cm⁻¹), Extracted from Measured and Simulated Spectra, with Corresponding Vibrational Mode Assignments. Force constants relative to specific intense lines are listed. B3PW91/def2-TZVP. SF = 0.9714 for lines <2000cm⁻¹. SF = 0.960 for CH bands > 3000cm⁻¹.

<table><thead><tr><th>Assignment</th><th>Experiment</th><th>Model</th><th>Δ</th><th>Calculated force constant (mdyn/Å)</th></tr></thead><tbody><tr><td>γ(CI) + γ(CH) + def of aromatic rings γ(CC)</td><td>178 (s)</td><td>176</td><td>-2</td><td></td></tr><tr><td>γ(CI) + τ (CH) + def of aromatic rings γ(CC)</td><td>191 (s)</td><td>192</td><td>1</td><td>0.16</td></tr><tr><td>γ(CI) + τ(CH) + def of aromatic rings γ(CC)</td><td>218 (w)</td><td>210</td><td>-8</td><td></td></tr><tr><td>γ + β(CI) + γ(CH) + def of aromatic rings γ(CC)</td><td>266 (s)</td><td>264</td><td>-2</td><td>0.27</td></tr><tr><td>γ(CI) + γ(CH) + def of aromatic rings γ(CC)</td><td>297 (m)</td><td>288</td><td>-9</td><td></td></tr><tr><td>γ(CI) + γ(CH) + def of aromatic rings γ(CC)</td><td>340 (m)</td><td>336</td><td>-4</td><td></td></tr><tr><td>γ(CI) + γ(CH) + def of aromatic rings γ(CC)</td><td>408 (s)</td><td>408</td><td>0</td><td>0.69</td></tr><tr><td>β(CI) + γ(CH) + def of aromatic rings β, γ(CC)</td><td>441 (w)</td><td>440</td><td>-1</td><td></td></tr><tr><td>γ(CC) + τ(CH)</td><td>570 (m)</td><td>576</td><td>6</td><td></td></tr><tr><td>γ(CC) + τ(CH)</td><td>722 (w)</td><td>728</td><td>6</td><td></td></tr><tr><td>ω(CH) + γ(CC)</td><td>780 (w)</td><td>784</td><td>4</td><td></td></tr><tr><td>strong ν(CI) + ν(CC) + breathing of one external ring</td><td>994 (m)</td><td>984</td><td>-10</td><td>4.93</td></tr><tr><td>strong ν(CI) + ν(CC) + breathing of one external ring</td><td>1010 (m)</td><td>1012</td><td>2</td><td>5.04</td></tr><tr><td>strong ν(CI) + β(CH) scissoring + ν(CC)</td><td>1082 (m)</td><td>1088</td><td>6</td><td>2.25</td></tr><tr><td>β(CH), scissoring + ν(CC)</td><td>1190 (w)</td><td>1184</td><td>-6</td><td></td></tr><tr><td>β(CH) + ν(CC)</td><td>1218 (m)</td><td>1217</td><td>-1</td><td></td></tr><tr><td>ρ (CH) + ν(CC), breath central ring</td><td>1265 (s)</td><td>1248</td><td>-17</td><td>3.15</td></tr><tr><td>β + ρ(CH) + ν(CC)</td><td>1277 (s)</td><td>1273</td><td>-4</td><td></td></tr><tr><td>β(CH) + ν(CC)</td><td>1292 (vs)</td><td>1288</td><td>-4</td><td>8.42</td></tr><tr><td>ρ (CH) + ν(CC)</td><td>1390 (s)</td><td>1384</td><td>-6</td><td>3.69</td></tr><tr><td>ρ (CH) + ν(CC)</td><td>1402 (s)</td><td>1400</td><td>-2</td><td>2.90</td></tr><tr><td>β(CH) + ν(CC)</td><td>1459 (w)</td><td>1457</td><td>-2</td><td>4.86</td></tr><tr><td>β(CH) + ν(CC)</td><td>1520 (m)</td><td>1530</td><td>10</td><td>7.09</td></tr><tr><td>β(CH) + ν(CC)</td><td>1577 (s)</td><td>1584</td><td>7</td><td>9.73</td></tr><tr><td>ν(CH)</td><td>3049 (m)</td><td>3104</td><td>55</td><td>6.53</td></tr><tr><td>ν(CH)</td><td>3067 (m)</td><td>3062</td><td>-5</td><td>6.65</td></tr><tr><td>ν(CH)</td><td>3080 (m)</td><td>3086</td><td>6</td><td>6.75</td></tr><tr><td>ν(CH)</td><td>3110 (w)</td><td>3110</td><td>0</td><td></td></tr><tr><td>ν(CH)</td><td>3155(w)</td><td></td><td></td><td></td></tr><tr><td>RMSD for all assigned bands</td><td></td><td>4.4</td><td></td><td></td></tr><tr><td>RMSD (&lt; 2000 cm⁻¹)</td><td></td><td>3.2</td><td></td><td></td></tr><tr><td>RMSD (&gt; 2000 cm⁻¹)</td><td></td><td>5.8</td><td></td><td></td></tr><tr><td>Average % Deviation (all bands)</td><td></td><td>-0.41 %</td><td></td><td></td></tr><tr><td>Average % Deviation (&lt; 2000 cm⁻¹)</td><td></td><td>-0.45 %</td><td></td><td></td></tr><tr><td>Average % Deviation (&gt; 2000 cm⁻¹)</td><td></td><td>0.46 %</td><td></td><td></td></tr></tbody></table>

ν, β, γ, ρ, ω and τ denote stretching, in plane bending, out of plane bending, rocking, wagging and twisting, respectively.

excitations. No significant spectral differences were observed between the two excitations wavelengths, except that the green laser offered enhanced peak resolution and enabled the detection of low-frequency vibrational modes. It is well established that out-of-plane deformations of aromatic rings and C-H out-of-plane bending vibrations typically occur at frequencies below 1000 cm⁻¹, which is also consistent with our findings for the halogenated precursor. A notable difference in the Raman spectrum of TITP with respect to that of TP is the presence of a C-I out-of-plane bending mode appearing at frequencies below 500 cm⁻¹, along with a strong C-I stretching band observed at 1082 cm⁻¹. Meanwhile, C-H in-plane bending modes are observed at frequencies
above 1000 cm⁻¹, accompanied by contributions from in-plane de-formations of the aromatic rings.

The B3PW91/def2-TZVP level of theory demonstrates excellent agreement between simulated and experimental Raman spectra (Fig. 4B), confirming its reliability for modeling this class of halogenated aromatic compounds. By contrast, additional calculations using B3LYP/def2-TZVP with scaling factors of 0.963 and 0.9714 did not improve spectral reproduction. Alternative calculations performed with B3LYP/DGDZVP (SF = 0.978) also showed a similarly high level of consistency with the experimental data (Fig. S5 and Tables S3, S4).

In all calculations, C-H stretching frequencies in the IR and Raman spectra were scaled by 0.960 to correct for systematic overestimations associated with the harmonic approximation and the neglect of solid-state effects. The RMSD values in Tables 1–4 indicate very good agreement between simulated and experimental Raman spectra for both triphenylene and 1,5,9-triiodotriphenylene. Below 2000 cm⁻¹, deviations remain small, typically < 8 cm⁻¹, with an average relative error < 0.45 %. Comparison of the Raman spectra (Fig. 4C) shows a redshift of major bands from 1603, 1458, and 1339 cm⁻¹ to 1577, 1390, and 1292 cm⁻¹, respectively, upon replacing hydrogen with iodine, a heavier electron-withdrawing halogen that lowers vibrational frequencies.

### 3.3. Effective force constants

The force constants derived from DFT calculations are summarized in Tables 3 and 4 for the most intense Raman-active modes. These constants are obtained by diagonalizing the mass-weighted Hessian matrix, which contains the second derivatives of the potential energy with respect to the nuclear Cartesian coordinates. They express the stiffness of the corresponding normal vibrational modes.

At low frequencies (below ≈ 400 cm⁻¹), the vibrations are dominated by relatively soft bending, rocking, or wagging motions, particularly involving the iodine atoms in TITP, and resulting in moderate force constants typically below 0.8 mdyn/Å. As the frequency increases, the force constants rise accordingly. Above 1200 cm⁻¹, a sharp increase is observed for both TP and TITP, consistent with in-plane C-C and C = C stretching modes within the aromatic rings, yielding very strong force constants exceeding 8 mdyn/Å. High force constants have been associated with aromatic C-C stretching vibrations in simpler model compounds [58]. In TITP, the C-I stretching vibrations also contribute significantly, with calculated force constants ranging from approximately 2 to 5 mdyn/Å. The intense IR-active mode observed at 649 cm⁻¹, attributed to a C-I stretching vibration, corresponds to a computed force constant of 2.1 mdyn/Å. This value is consistent with the C-I force constants obtained from the analysis of Raman-active modes, as summarized in Table 4. Finally, C-H stretching force constants are in the range of 6.5–6.7 mdyn/Å for both compounds.

### 3.4. UV- visible study of triphenylene and its derivative

The incorporation of iodine atoms in TITP leads to significant differences in its electronic properties compared to unsubstituted TP. These modifications are evident in their absorption spectra, oscillator strengths, and canonical frontier molecular orbitals (FMOs, i.e. HOMO and LUMO). The iodine substitution alters the electronic structure, affecting both electronic and optical behavior, notably breaking the orbital degeneracy present in TP and reducing the overall molecular planarity and symmetry.

The experimental absorption spectrum of TP displays strong peaks primarily in the 245–260 nm range, with two prominent bands at 249 and 257 nm (4.98 and 4.81 eV), accompanied by two well-defined shoulders at 273 and 285 nm, and a weaker one near 242 nm, in line with previous studies [32]. This spectral pattern is also consistent with literature reports that identify the most intense absorption band at 260 nm [59]. The observed features correspond to π-π* transitions characteristic of polycyclic aromatic hydrocarbons.

#### 3.4.1. Calculations in the gas phase

Theoretical predictions using the B3LYP functional show a redshift (towards higher wavelengths) relative to the experimental spectrum, which could be due to the limitations of the calculation method and the absence of solvent effects, as the simulations are performed in vacuum while the measurements are taken in solution. Further analysis of the TP absorption spectrum (Table 5) by using the B3LYP/6-311++G(d,p) method, which includes diffuse functions to better estimate the HOMO-LUMO gap, reveals multiple electronic excitations. The corresponding oscillator strength values reflect the probability of these electronic transitions, with higher values indicating greater transition propensities [60]. In the spectrum shown in Fig. 5A, each oscillator strength peak corresponds to two degenerate electronic transitions, reflecting the molecule's high symmetry. Several transitions occur at nearly identical wavelengths and intensities, further emphasizing this degeneracy arising from its highly symmetrical, planar structure. This phenomenon results from near-degenerate orbitals within the delocalized π-system, where electronic states split into pairs of energetically equivalent or nearly equivalent transitions. The most intense transition appears at 269.7 nm (4.6 eV), mainly involving HOMO-1 → LUMO+1 (27 %) and HOMO → LUMO (27 %), highlighting the extensive delocalization of TP's electronic structure, where multiple states contribute to absorption. The HOMO is located at orbital with an energy of -6.17 eV, while the LUMO is at orbital with an energy of -1.44 eV, giving a HOMO-LUMO gap of 4.73 eV. Using the CAM-B3LYP functional, the theoretical spectrum predicts an absorption peak at 246 nm, at the high-energy edge of the experimental band.

In contrast, the experimental absorption spectrum of TITP (Fig. 5B) displays broader bands with a primary absorption peak at 276 nm (4.49 eV) and a shoulder around 323 nm (3.84 eV). This red-shift relative to triphenylene is consistent with previous observations in brominated derivative, where halogen substitution leads to similar spectral shifts [32]. The broadening can be attributed to the non-planarity induced by

<table><caption>**Table 5**
The electronic transitions involved in UV-visible spectrum for triphenylene.</caption><thead><tr><th>Wavelength (nm)</th><th>Oscillator Strength</th><th>Major contributions</th><th>Minor contributions</th></tr></thead><tbody><tr><td>322.6</td><td>0</td><td>H-1→LUMO (42%), HOMO→L + 1 (42%)</td><td>H-1→L + 1 (8%), HOMO→LUMO (8%)</td></tr><tr><td>307.2</td><td>0</td><td>H-1→L + 1 (42%), HOMO→LUMO (42%)</td><td>H-1→LUMO (8%), HOMO→L + 1 (8%)</td></tr><tr><td>298.5</td><td>0.0011</td><td>H-2→LUMO (17%), HOMO→L + 2 (78%)</td><td>H-2→L + 1 (2%)</td></tr><tr><td>298.5</td><td>0.0011</td><td>H-2→L + 1 (17%), H-1→L + 2 (78%)</td><td>H-2→LUMO (2%)</td></tr><tr><td>269.7</td><td>0.3631</td><td>H-2→L + 1 (11%), H-1→LUMO (27%), H-1→L + 1 (13%), HOMO→LUMO (13%), HOMO→L + 1 (27%)</td><td>π→π*</td></tr><tr><td>269.7</td><td>0.3644</td><td>H-2→LUMO (11%), H-1→LUMO (13%), H-1→L + 1 (27%), HOMO→LUMO (27%), HOMO→L + 1 (13%)</td><td>π→π*</td></tr><tr><td>262.5</td><td>0.0002</td><td>H-2→L + 2 (98%)</td><td>H-1→L + 1 (6%), H-1→L + 2 (4%)</td></tr><tr><td>261.5</td><td>0.3376</td><td>H-2→LUMO (64%), HOMO→L + 2 (14%)</td><td>HOMO→LUMO (6%)</td></tr><tr><td>261.5</td><td>0.3388</td><td>H-2→L + 1 (64%), H-1→L + 2 (14%)</td><td>H-1→LUMO (6%), HOMO→L + 1 (6%), HOMO→L + 2 (4%)</td></tr><tr><td>232.5</td><td>0</td><td>HOMO→L + 3 (97%)</td><td>H-1→L + 5 (6%), HOMO→L + 4 (6%)</td></tr><tr><td>232.5</td><td>0</td><td>H-1→L + 3 (97%)</td><td>H-1→L + 5 (6%), HOMO→L + 4 (6%)</td></tr><tr><td>225.5</td><td>0</td><td>H-2→L + 3 (21%), H-1→L + 4 (33%), HOMO→L + 5 (33%)</td><td>H-1→L + 5 (6%), HOMO→L + 4 (6%)</td></tr><tr><td>224.2</td><td>0.0256</td><td>H-1→L + 5 (20%), HOMO→L + 4 (63%), HOMO→L + 5 (11%)</td><td>H-1→L + 4 (5%)</td></tr><tr><td>224.1502</td><td>0.0017</td><td>H-1→L + 5 (70%), HOMO→L + 4 (26%)</td><td>H-1→L + 5 (70%), HOMO→L + 4 (26%)</td></tr><tr><td>224.1461</td><td>0.0001</td><td>H-1→L + 4 (50%), HOMO→L + 5 (46%)</td><td>H-1→L + 4 (50%), HOMO→L + 5 (46%)</td></tr></tbody></table>
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Experimental (blue curves), calculated in the gas phase (red curves), and calculated in the solvent (green curves) UV–Vis spectra of (A) triphenylene, (B) 1,5,9-triiodotriphenylene, and (C) comparison between experimental spectra of triphenylene and 1,5,9-triiodotriphenylene. Vertical bars represent oscillator strengths.</figcaption></figure>

iodine substitution, which reduces molecular symmetry. Similar effects have been observed in other non-planar PAHs, where reduced symmetry results in less structured absorption spectra [59,61,62].

Theoretical spectra of TITP calculated at the B3PW91/def2-TZVP and B3LYP/DGDZVP levels predict a redshifted peak near 295 nm, likely influenced by solvent effects and vibronic coupling. Despite the shift, the overall spectral profile is well reproduced, with the experimental shoulder at 323 nm reflected as a distinct feature around 341 nm. The incorporation of iodine atoms in TITP also stabilizes the LUMO, narrowing the HOMO–LUMO gap and shifting absorption into the lower-energy UV region, enabling additional allowed transitions. Consequently, broader peaks around 300 nm emerge, reflecting lower-energy electronic transitions. The CAM-B3LYP functional does not improve the UV-visible spectral simulation, as it produces a narrower band, redshifted to 256 nm, significantly below the experimental maximum, and yields an optical gap of ≈ 4.84 eV. The presence of iodine atoms in TITP induces spin–orbit coupling (SOC), promoting singlet–triplet mixing, spectral broadening, and redshifted absorption. Oscillator strength analysis reveals a greater number of allowed transitions in the lower-energy UV range, driven by iodine-induced perturbations of the molecular orbitals. The 280–300 nm (≈ 4.4 eV) region corresponds to π→π* transitions characteristic of the conjugated system, while the lower-energy shoulder around 323 nm likely arises from SOC-mediated mixing of electronic states involving iodine orbitals.

Analysis of the theoretical absorption spectrum of TITP (Table 6) reveals multiple low-energy electronic excitations introduced by iodine substitution. Compared to TP, TITP exhibits a higher number of transitions and broader absorption features, due to the disruption of orbital degeneracy, reduced molecular symmetry, and enhanced heavy-atom effects (Fig. 5C). The distribution of oscillator strengths across multiple wavelengths indicates that many transitions originate from mixed electronic states, highlighting complex interactions among perturbed molecular orbitals.

Frontier molecular orbitals (FMOs) were computed using functionals listed in Table S1, namely B3LYP, CAM-B3LYP, and B3PW91. Since these functionals yield nearly identical electron density distributions for both TP and TITP molecules, only the FMOs obtained with B3LYP and B3PW91 are shown (Figs. 6 and 7). Notably, CAM-B3LYP predicts a substantially larger HOMO–LUMO gap – approximately 7.0 eV for TP and 6.7 eV for TITP – consistent with its known tendency to overestimate orbital energy differences due to its long-range exchange correction.

Fig. 6 shows that in triphenylene, both the HOMO and LUMO are highly delocalized across the π-system, indicating extensive conjugation. Symmetry and delocalization of HOMO indicate a low localization on the hydrogen atoms or on the individual carbon positions. The LUMO lobes are more localized and alternate in phase. In contrast, iodine substitution in TITP alters this electronic distribution, as illustrated in Fig. 7. The orbitals are more perturbed and show increasing disorder due

to loss of symmetry. The HOMO becomes more localized around the iodine atoms, denoting contributions from iodine lone pairs, while the LUMO shows increased electron density near the substituted regions, suggesting perturbed π* orbitals. This redistribution implies that iodine plays a key role in stabilizing the LUMO and in promoting charge-transfer character. The broader absorption profile observed for TITP is consistent with its more complex electronic structure, compared to the well-defined features in TP. The most intense electronic transition in TITP occurs at 307.12 nm (4.04 eV), primarily involving HOMO → LUMO+1 (20 %) and HOMO → LUMO+2 (20 %). The HOMO has an energy of –6.42 eV, while the LUMO lies at –2.12 eV, yielding a HOMO–LUMO gap of 4.30 eV – smaller than that of triphenylene. This reduced gap, along with the presence of additional low-energy transitions, suggests enhanced electron delocalization induced by iodine substitution.

### 3.4.2. Calculations in the solvent

For both molecules in n-hexane, we employed the long-range corrected CAM-B3LYP functional together with the triple-ζ basis set with polarization functions def2-TZVP. The calculated spectra are reported in Figs. 5A and 5B with peaks at 245 nm for TP and at 260 and 310 nm for TITP. These values are lower than the experimental ones, in contrast to the solvent-free case discussed above. In TP, the experimental band at 254 nm is well reproduced, whereas the one at 257 nm is not. For TITP, the inclusion of solvent improves the reproduction of the shoulder around 323 nm, yielding a narrower spectral profile.

The calculated HOMO–LUMO gaps are 7.25 eV for TP and 6.72 eV for TITP, close to those obtained without solvent with the same functional. Fig. S6 shows the FMOs obtained with this model. For TP, the HOMO is more delocalized over the central ring, whereas the LUMO remains unchanged relative to the absence of solvent. In TITP, both the HOMO and LUMO exhibit the same characteristics as without solvent.

### 3.5. Static polarizability trends

The calculated polarizabilities at the B3LYP/6-311++G(d,p) and B3LYP/DGDZVP levels of theory reveal how iodine substitution modulates the electronic response of the triphenylene framework when subjected to an external electric field. Our calculations show that the unsubstituted triphenylene (TP) molecule exhibits a negligible dipole moment (μ ≈ 0 D) due to its high planar symmetry. The static polarizability components ($α_{xx} \approx 277.9$ a.u., $α_{yy} \approx 277.8$ a.u., $α_{zz} \approx 100.3$ a.u.) give an isotropic polarizability of $α_{iso} \approx 219$ a.u reflecting the delocalized π-electron system of TP in addition to the fact that $α_{xx} \approx α_{yy} \gg α_{zz}$ indicates strong in-plane π-conjugation. The presence of three iodine atoms in 1,5,9-triiodotriphenylene (TITP) breaks down the molecular symmetry with consequences of giving a small dipole moment (μ ≈ 0.45 D) consistent with asymmetric iodine substitution. With respect to TP,
<table><caption>**Table 6**
The electronic transitions involved in UV-visible spectrum for 1,5,9-triiodotriphenylene (B3PW91/def2-TZVP).</caption><thead><tr><th>Wavelength (nm)</th><th>Oscillator Strength</th><th>Major contributions</th><th>Minor contributions</th></tr></thead><tbody><tr><td>346.0</td><td>0.0065</td><td>H-1→LUMO (66%), HOMO→L + 1 (20%)</td><td>HOMO→LUMO (8%)</td></tr><tr><td>336.4</td><td>0.0444</td><td>HOMO→LUMO (78%)</td><td>H-1→L + 1 (5%), HOMO→L + 1 (3%)</td></tr><tr><td>327.1</td><td>0.0018</td><td>H-2→LUMO (74%)</td><td>H-2→L + 2 (2%), H-1→L + 1 (3%), HOMO→LUMO (3%), HOMO→L + 1 (5%), HOMO→L + 2 (5%)</td></tr><tr><td>317.8</td><td>0.0271</td><td>H-2→L + 1 (12%), H-1→L + 1 (14%), HOMO→L + 1 (20%), HOMO→L + 2 (19%)</td><td>H-2→L + 3 (7%), H-1→LUMO (3%), H-1→L + 3 (7%), HOMO→L + 3 (4%), HOMO→L + 5 (2%)</td></tr><tr><td>311.9</td><td>0.0013</td><td>H-2→L + 2 (39%), HOMO→L + 2 (22%), HOMO→L + 3 (10%)</td><td>H-2→LUMO (6%), H-2→L + 4 (4%), H-2→L + 5 (4%), H-1→L + 2 (4%), H-1→L + 4 (2%)</td></tr><tr><td>307.1</td><td>0.0655</td><td>H-2→L + 2 (12%), H-1→L + 1 (15%), HOMO→L + 1 (20%), HOMO→L + 2 (20%)</td><td>H-2→L + 1 (4%), H-2→L + 4 (7%), H-1→LUMO (8%), HOMO→L + 3 (4%), HOMO→L + 4 (3%)</td></tr><tr><td>306.3</td><td>0.0124</td><td>H-1→L + 2 (27%), HOMO→L + 3 (23%)</td><td>H-2→LUMO (9%), H-2→L + 2 (2%), H-2→L + 3 (5%), H-1→L + 1 (9%), H-1→L + 4 (7%), HOMO→L + 1 (5%), HOMO→L + 5 (2%)</td></tr><tr><td>304.5</td><td>0.0627</td><td>H-2→L + 1 (69%), HOMO→L + 1 (11%)</td><td>H-1→LUMO (4%), H-1→L + 2 (2%), H-1→L + 3 (5%)</td></tr><tr><td>301.8</td><td>0.0481</td><td>H-1→L + 1 (34%), H-1→L + 2 (19%), HOMO→L + 2 (21%)</td><td>H-2→L + 2 (5%), H-2→L + 4 (2%), H-1→L + 3 (3%), HOMO→LUMO (2%)</td></tr><tr><td>290.7</td><td>0.0447</td><td>H-1→L + 2 (24%), HOMO→L + 3 (19%)</td><td>H-3→LUMO (2%), H-2→L + 1 (3%), H-2→L + 2 (8%), H-2→L + 3 (9%), H-1→LUMO (2%), H-1→L + 3 (7%), H-1→L + 4 (2%), HOMO→L + 4 (6%), HOMO→L + 5 (2%)</td></tr><tr><td>289.6</td><td>0.078</td><td>H-1→L + 3 (43%), HOMO→L + 4 (10%)</td><td>H-3→LUMO (3%), H-2→L + 2 (3%), H-2→L + 4 (4%), H-1→L + 1 (8%), H-1→L + 4 (3%), HOMO→LUMO (3%), HOMO→L + 1 (2%), HOMO→L + 5 (5%)</td></tr><tr><td>285.5</td><td>0.0621</td><td>H-1→L + 3 (16%), HOMO→L + 3 (10%), HOMO→L + 4 (47%)</td><td>H-2→L + 3 (4%), H-1→L + 2 (4%)</td></tr><tr><td>284.1849</td><td>0.0412</td><td>H-2→L + 3 (13%), HOMO→L + 3 (12%), HOMO→L + 5 (24%)</td><td>H-5→LUMO (5%), H-4→LUMO (3%), H-2→L + 2 (7%), H-2→L + 4 (7%), H-1→L + 4 (4%), HOMO→L + 1 (2%), HOMO→L + 2 (4%)</td></tr><tr><td>280.4121</td><td>0.0131</td><td>H-7→LUMO (13%), H-5→LUMO (11%), H-3→LUMO (21%)</td><td>H-6→LUMO (6%), H-4→LUMO (9%), H-2→L + 2 (2%), H-2→L + 3 (9%), H-1→L + 4 (3%), H-1→L + 5 (5%), HOMO→L + 3 (3%), HOMO→L + 4 (4%)</td></tr><tr><td>277.9665</td><td>0.005</td><td>H-1→L + 2 (10%), H-1→L + 4 (26%), H-1→L + 5 (18%)</td><td>H-6→LUMO (5%), H-6→L + 3 (3%), H-5→LUMO (6%), H-1→L + 3 (5%), HOMO→L + 3 (3%), HOMO→L + 5 (5%)</td></tr></tbody></table>

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Frontier molecular orbitals (HOMO and LUMO) for triphenylene (B3LYP/6-311++G(d,p)).</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Frontier molecular orbitals (HOMO and LUMO) for 1,5,9-triiodotriphenylene. (B3PW91/def2-TZVP).</figcaption></figure>

there is a significant increase of the isotropic in-plane polarizability to $\alpha_{iso} \approx 300$ a.u. (($\alpha_{xx} \approx 371.5$ a.u., $\alpha_{yy} \approx 361.8$ a.u., $\alpha_{zz} \approx 165.8$ a.u.). This enhancement shows that heavy-atom substitution reinforces the electronic response, and potentially improves nonlinear optical behavior relative to the parent molecule.

### 3.6. Natural bond orbital analysis

Unlike canonical frontier molecular orbitals, which are delocalized over the whole molecule, natural bond orbitals (NBOs) provide a localized description of bonds and lone pairs. In TP, the NBO analysis (for B3LYP/6-311++G(d,p)) discloses a significant degree of $\pi$-electron delocalization, with $\approx 10$ electrons (e$^-$) assigned to delocalized (non-Lewis) orbitals while 110 electrons are in localized $\sigma$ bonds and core orbitals. This is characteristic of a strongly resonance-stabilized aromatic system with a fused $\pi$-conjugated framework. The natural population analysis (NPA) results show negative charges on the aromatic carbon atoms ($\approx -0.03$ to $-0.20$ e$^-$) and positive charges on the peripheral hydrogen atoms ($\approx +0.20$ e$^-$). There is a small Rydberg population ($< 0.4$%) indicating that the excited electrons stay in normal orbitals and do not spread out in diffuse orbitals which is consistent with a predominantly valence-type electronic distribution.

The NBO analysis of TITP (for B3LYP/DGDZVP) reveals also a localized electronic structure ($\approx 266$ e$^-$ in Lewis orbitals), with a small delocalized population ($\approx 10$ e$^-$), and shows additionally electron-donating interactions or $\sigma-\pi$ overlap effects induced by the iodine substituents. The NPA indicates that the three iodine atoms carry positive charges ($+0.17$ e$^-$), while the adjacent aromatic carbon atoms in C-I bonds are correspondingly negative ($-0.17$ to $-0.21$ e$^-$).

Therefore, compared to TP, TITP exhibits a noticeable polarization of electron density toward the $\pi$-framework, with minor $\sigma(C-I) \to \pi$ delocalization and with more negative aromatic character. Additionally, $\pi \to \sigma(C-I)$ interactions occur, reflecting complementary donor-acceptor interactions triggered by the heavy iodine substituents. The very small Rydberg population ($< 0.3$%) confirms the primarily valence character of the electronic structure.
### 3.7. Hirshfeld analysis

Raman and infrared spectra were acquired on crystalline powders, motivating an evaluation of intermolecular interactions within the molecular crystals. The earliest orthorhombic P2<sub>1</sub>2<sub>1</sub>2<sub>1</sub> crystalline structure of TP was reported by Ahmed and Trotter [63] and later refined by Collings et al. [64]. The latter data are available in the Cambridge Structural Database (CSD) and were used here for Hirshfeld analysis with the Crystal Explorer software [65]. In contrast, no experimental crystal structure of TITP is currently available, so a reliable comparison with TP cannot be performed at this stage. Hirshfeld surfaces were determined at the B3LYP/6-311G(d,p) level of theory, and Hirshfeld surface together with 2D fingerprint analyses were employed to characterize the intermolecular environment of TP.

In Hirshfeld fingerprint plots, $d_i$ denotes the distance to the nearest atom inside the molecule, while $d_e$ denotes the distance to the nearest atom in a neighboring molecule. The 2D fingerprint plot reveals the relative contributions of close contacts governing the crystal packing. In triphenylene crystal $d_i \approx d_e \approx 1.8$ Å in the central region which is consistent with the dominance of van der Waals contacts expected for aromatic systems. The plot is confined between 1.2 and 2.4 Å, reflecting homogeneous hydrocarbon interactions and a regular packing arrangement (Fig. 8). Complementary Hirshfeld descriptors support this interpretation (Fig. 9). In the first map, $d_{norm}$ represents the normalized contact distance that characterizes intermolecular interactions by scaling $d_i$ and $d_e$ distances relative to van der Waals radii. The $d_{norm}$ map shows only minor contact compression. In other maps, the shape index surface shows red patches in ring centers ($\pi...\pi$ stacking regions), blue convex areas on carbons and hydrogens, and green neutral regions elsewhere, consistent with weak, nondirectional intermolecular contacts and indicative of regular $\pi-\pi$ stacking. The electrostatic potential map shows electron-rich peripheral rings (red), nearly neutral regions directly at the hydrogen periphery (white), and a surrounding blue belt reflecting electron-deficient zones, consistent with weak, diffuse potentials and the absence of strong directional intermolecular forces.

### 4. Conclusion

This study provides a comprehensive analysis of the effects of iodine substitution on the vibrational and electronic properties of triphenylene. The introduction of iodine atoms induces pronounced structural distortions, lowering the molecular symmetry, which leads to observable shifts in the vibrational spectra. These shifts reflect electronic reorganization driven by the inductive effects of iodine and illustrate the close coupling between molecular geometry and vibrational behavior. Combined IR and Raman analyses demonstrate that DFT methods accurately reproduce vibrational bands for both triphenylene and its iodinated derivative, with the best agreement for 1,5,9-triiodotriphenylene obtained using B3PW91/def2-TZVP. UV-visible absorption calculations reveal that iodine substitution induces a redshift in the spectrum and reduces the HOMO-LUMO gap, reflecting LUMO stabilization and enhanced low-energy electronic transitions. By explicitly comparing TP and TITP, this work highlights how halogen-induced nonplanarity and electronic effects modulate vibrational and optoelectronic properties. NBO and polarizability analyses reveal enhanced donor-acceptor delocalization in TITP, while Hirshfeld analysis of the experimental molecular crystal of triphenylene provides complementary evidence of electron density redistribution. These molecular-level insights provide a useful basis for the design of functionalized PAHs as precursors in on-surface synthesis and for potential applications in nanocarbon architectures and molecular electronics.

### CRediT authorship contribution statement

**Ahmed Hewail:** Writing – original draft, Visualization, Investigation, Formal analysis, Data curation. **David Guérin:** Methodology,

<figure><img src="image_10.png" /><figcaption>Fig. 8. Hirshfeld 2D fingerprint of the triphenylene crystal.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 9. Hirshfeld surface descriptors of the triphenylene crystal.</figcaption></figure>

Investigation, Data curation. **Christophe Krzeminski:** Methodology, Formal analysis. **Eric Faulques:** Writing – review & editing, Validation, Supervision, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Nataliya Kalashnyk:** Writing – review & editing, Validation, Supervision, Resources, Project administration, Investigation, Funding acquisition, Conceptualization.

### 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.

### Acknowledgements

DFT calculations were performed both at the Centre de Calcul Intensif des Pays de la Loire (CCIPL) and on the IEMN cluster, where the study was conducted. During the review process, part of this work was granted access to the HPC resources and to the Gaussian16 software of IDRIS (allocation [D010813508R1], DARI). We gratefully acknowledge Dr. T. Mélin for his support of this work.

### Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.molstruc.2025.145178.
# Data availability

Data will be made available on request.

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