# Synthesis and photophysical properties of 6-aryl-2,4-bis(2'-hydroxyphenyl) pyrimidines ligands and their boron fluoride complexes

Clément Diguet, Amparo Navarro, M. Paz Fernández-Liencres, Sonia B.

Jiménez-Pulido, Nuria A. Illán-Cabeza, Julien Massue, Sebastien Gauthier, Françoise Robin-Le Guen, Sylvain Achelle, Julián Rodríguez-López

► To cite this version:

Clément Diguet, Amparo Navarro, M. Paz Fernández-Liencres, Sonia B. Jiménez-Pulido, Nuria A. Illán-Cabeza, et al.. Synthesis and photophysical properties of 6-aryl-2,4-bis(2'-hydroxyphenyl) pyrimidines ligands and their boron fluoride complexes. Dyes and Pigments, 2025, 236, pp.112660. 10.1016/j.dyepig.2025.112660. hal-04984427

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Submitted on 10 Mar 2025

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# Synthesis and photophysical properties of 6-aryl-2,4-bis(2'-hydroxyphenyl) pyrimidines ligands and their boron fluoride complexes

<figure><img src="image_2.png" /></figure>

Clément Diguet<sup>a</sup>, Amparo Navarro<sup>b, d</sup>, M. Paz Fernández-Liencres<sup>b, d</sup>,
Sonia B. Jiménez-Pulido<sup>c</sup>, Nuria A. Illán-Cabeza<sup>c</sup>, Julien Massue<sup>d, e</sup>, Sébastien Gauthier<sup>a, e</sup>,
Françoise Robin-le Guen<sup>a</sup>, Sylvain Achelle<sup>a, * f</sup>, Julián Rodríguez-López<sup>e, **</sup>

## ABSTRACT

A new family of 6-aryl-2,4-bis(2'-hydroxyphenyl)pyrimidines has been efficiently synthesized and characterized using Suzuki-Miyaura reactions, demonstrating their ability to function as rigid tridentate O'N'O chelating ligands for the preparation of four-coordinate organoboron compounds. Similar to related structures, the uncoordinated compounds do not exhibit fluorescence in either solution or the solid state. The intramolecular hydrogen bonds in their molecular structure promote an excited-state intramolecular proton transfer (ESIPT) process, leading to an excited keto tautomer that deactivates through non-radiative decay. Protonation and boron complexation can successfully inhibit the ESIPT process, resulting in fluorescence recovery, which can be finely tuned by modifying the substituents on the aryl group. Advanced techniques, including X-ray diffraction, density functional theory (DFT), and time-dependent DFT (TD-DFT), provide critical insights into the electronic structure and emission properties. This work offers a deeper understanding of the role of boron chelation in modulating emission properties, opening new avenues for the design of functional fluorescent materials.

## 1. Introduction

Molecular materials capable of emitting light in response to external stimuli have been a focal point of extensive research over the last few decades [1–5]. In this context, push-pull organic chromophores, characterized by the presence of electron donor and acceptor groups in a functional conjugated structure (D-π-A structure), have garnered significant attention [6–10]. The emission properties of these chromophores are intrinsically related to intramolecular charge transfer (ICT), a phenomenon that can be finely tuned by manipulating the molecular environment. These compounds hold potential applications as fluorescent probes and in bioimaging [11–13].

In particular, push-pull pyrimidine chromophores stand out for their robust emission properties [14–19]. Their sensitivity to the surrounding environment is noteworthy, given that the electron acceptor nature of the pyrimidinyl moiety can be easily modulated through interactions with the lone electron pairs of the nitrogen atoms, such as complexation, alkylation, hydrogen bonding, or reversible protonation [15,20]. This

inherent adaptability has led to the utilization of these emitters in various applications, including the sensing of polarity, pH, metal cations, nitroaromatics, and diverse bio-organic species [15,20–22].

Nevertheless, 2-(2'-hydroxyphenyl)pyrimidine derivatives with aryl or arylvinylene substituents at the 4- and 6- positions of the pyrimidine ring have recently demonstrated minimal to negligible luminescence in both solution and the solid state [23]. These compounds present short OH...N intramolecular hydrogen bonds, which leads to the occurrence of a straightforward excited-state intramolecular proton transfer (ESIPT) from the OH group to the nitrogen atoms upon photoexcitation. ESIPT is a four-level photophysical process involving enol-keto phototautomerism that leads to emission from the keto tautomer. ESIPT induces significant molecular motions in the backbone of organic fluorophores, resulting in pronounced fluorescence quenching [24–26]. However, the ESIPT process can be partially or fully inhibited through protonation, thereby reversibly restoring the fluorescence response.

2,4-Bis(2'-hydroxyphenyl)pyrimidine analogs not only emerge as ESIPT-capable molecules, where ESIPT abilities can be modulated

<footnote><sup>a</sup> Univ. Rennes, CNRS, Institut des Sciences Chimiques de Rennes (ISCR), UMR 6226, F-35000, Rennes, France</footnote>

<footnote><sup>b</sup> Universidad de Jaén, Dpto. de Química Física y Analítica, Facultad de Ciencias Experimentales, Campus Las Lagunillas, 23071, Jaén, Spain</footnote>

<footnote><sup>c</sup> Universidad de Jaén, Dpto. de Química Inorgánica y Orgánica, Facultad de Ciencias Experimentales, Campus Las Lagunillas, 23071, Jaén, Spain</footnote>

<footnote><sup>d</sup> Institut de Chimie et Procédés pour l'Energie, l'Environnement et la Santé (ICPEES), UMR CNRS 7515, ECPM, 25 Rue Becquerel, 67087, Strasbourg, France</footnote>

<footnote><sup>e</sup> Universidad de Castilla-La Mancha, Area de Química Orgánica, Facultad de Ciencias y Tecnologías Químicas, Avda. Camilo José Cela 10, 13071, Ciudad Real, Spain</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>** Corresponding author.</footnote>

<footnote>E-mail addresses: sylvain.achelle@univ-rennes.fr; sylvain.achelle@univ-rennes1.fr (S. Achelle), julian.rodriguez@uclm.es (J. Rodríguez-López).</footnote>
through protonation/deprotonation, but also as intriguing O'N'O ligands for constructing tetracoordinated boron complexes. Furthermore, in four-coordinate organoboron compounds, the coordinative saturation of the boron center imparts increased chemical stability and rigidity, often accompanied by significant fluorescence enhancement [27–31]. As a result of chelation, the π-conjugation of the structure is reinforced, and the π-π* electronic transitions of the ligand or the ICT transitions from the aryl groups to the ligand typically account for the luminescence properties. All these features have been recently demonstrated by some of us in a series of non-emissive 6-arylvinylene-2,4-bis(2'-hydroxyphenyl)pyrimidines [32]. The incorporation of boron atoms restored fluorescence in both solution and the solid state, enabling effective fine-tuning of the emission color by modifying the substituent on the arylvinylene moiety. Similarly, four-coordinate fluoroboron compounds incorporating the structurally related 2,6-bis(2'-hydroxyphenyl)pyridine moiety have been successfully employed as emitting materials for the fabrication of high-performance white-light electroluminescent devices [33,34]. Additionally, this tridentate ligand has been used in the design of a novel class of efficient thermally activated delayed fluorescence (TADF) emitters for OLED applications, demonstrating long operational stabilities [35].

Building on this foundation, here we present a new family of 6-aryl-2,4-bis(2'-hydroxyphenyl)pyrimidine ligands and their boron fluoride complexes, with the aim of elucidating the impact of modifying the conjugated system on their structural and emissive properties. Similar to their 6-arylvinylene counterparts and the 2-(2'-hydroxyphenyl)pyrimidine derivatives, these compounds exhibit non-emissive behavior due to an ESIPT process from the OH groups to the nitrogen atoms of the pyrimidine ring upon photoexcitation, which is effectively inhibited upon protonation and boron complexation, thereby restoring fluorescence that can be finely tuned. Using advanced photophysical, computational, and crystallographic techniques, we provide deeper insights into the design principles governing these fluorophores, paving the way for their application in responsive materials and optoelectronic devices.

## 2. Results and discussion

**Synthesis and characterization.** The synthetic route for obtaining 2,4-bis(2'-hydroxyphenyl)pyrimidines **2a-e** and their corresponding organoboron complexes **3a-e** is outlined in Scheme 1. The aryl group at the 6-position was initially introduced via a Suzuki-Miyaura reaction between 2,4,6-trichloropyrimidine and one equivalent of the appropriate 4-substituted phenylboronic acid, yielding the dichloro derivatives **1a-e**. Notably, the chlorine atoms at the C4/C6 positions of the pyrimidine ring are more reactive than the chlorine atom at the C2 position in cross-coupling reactions [36,37]. Subsequently, these intermediates were reacted with two equivalents of 2-hydroxyphenylboronic acid in a second step. Finally, the synthesis of the corresponding organoboron complexes **3a-e** was achieved by treatment with BF₃.Et₂O in the presence of Cs₂CO₃.

All new compounds were fully characterized by ¹H and ¹³C NMR spectroscopy as well as mass spectrometry (see Supplementary data, Figs. S1–S30). The most relevant characteristic of the ¹H NMR spectra of **3a-e** was the disappearance of the hydroxylic protons (OH) from the corresponding precursor ligands **2a-e**, confirming the formation of the desired boron complexes. These complexes were also characterized by ¹⁹F and ¹¹B NMR spectroscopy. While the ¹⁹F NMR spectra displayed poorly resolved signals, except for **3c** where a quartet is clearly evident, the ¹¹B NMR spectra showed doublets with one-bond ¹¹B–¹⁹F coupling constants around 30–35 Hz.

**Crystal structure.** To further confirm the structures of the organoboron complexes and investigate the solid-state packing features, single crystals of **3e** were successfully obtained by slow evaporation from a CH₂Cl₂/CH₃CN solution. Crystallographic data are provided in Table S1. X-ray crystallographic analysis reveals that the (pyrimidine)BF moiety closely resembles that of similar arylvinyl boron complexes previously reported by some of us [32]. The boron atom exhibits four-coordinate bonding and adopts a typical tetrahedral geometry (τ = 0.94). It is chelated in a tridentate O'N'O fashion, resulting in the formation of two six-membered rings, with a fluorine atom occupying the remaining coordination site (Fig. 1).

<figure><img src="image_3.png" /><figcaption>**Scheme 1.** Synthesis of 2,4-bis(2'-hydroxyphenyl)pyrimidines **2a-e** and their corresponding organoboron complexes **3a-e**.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Molecular structure of 3e extracted from X-ray analysis with labeling scheme (thermal ellipsoids at 50 % probability level). Selected bond lengths (Å): B–F 1.408 (3), B–O2A 1.438 (3), B–O2B 1.440 (3), and B–N3 1.579 (3).</figcaption></figure>

The B–O and B–N bond distances and angles are comparable to those found in other four-coordinate boron compounds [38,39]. The B–N bond length of 1.579 (3) Å, while typical for these compounds, is slightly longer than a standard B–N single bond (~1.45 Å), indicating weaker bonding at this site. Angles formed by the boron center and donor atoms range from 106.52° to 111.38°, which is close to the ideal tetrahedral angle of 109°.

The coordination promotes a moiety containing five rings, and the conformational analysis shows a planar heterocyclic core with some flexibility in the aromatic and boron substituents, with torsion angles ranging between 7.88° to 15.08°. The aromatic ring defined by plane C11–C61 forms a moderate angle of 26.18° with the pyrimidine ring, while the N-phenyl groups are nearly perpendicular to each other, with an angle of 86.49° between their planes.

Examination of intermolecular interactions in the crystal packing reveals significant π-π stacking along the [100] direction (Fig. 2).

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Crystal packing structure of 3e showing the distances of the π-π interactions.</figcaption></figure>

Molecules are arranged in an antiparallel disposition, forming dimers with the boron atoms facing outward and short interplanar distances of 3.313 (1) Å between (pyrimidine)BF moieties. In addition, these units stack through π-π interactions that occur between the pyrimidine ring and one of the phenol rings of the neighboring molecule, with a distance of 3.607 Å. Both of these distances are well below the maximum distance of 3.8 Å for these interactions [40]. The slippage values of 0.28 Å and 0.990 Å, respectively, indicate near-perfect alignment. While classical hydrogen bonds are absent, the fluorine atom, although not forming significant bonds, plays a crucial role in crystal cohesion by participating in a weak interaction through the C61–H61···F bond (d = 3.3474 Å and ∠ = 174°). Solvent molecules contribute to structural stabilization through weak C–H···Cl bonds.

**Experimental photophysical properties.** The optical properties of compounds **2** and **3** were examined by UV/vis and fluorescence spectroscopy in CH₂Cl₂ solution at room temperature, as well as in the solid state. A summary of the collected data is presented in Table 1. Ligands **2a-d** display a similar absorption profile, with an absorption band centered at 343–348 nm, attributed to a π → π* transition (Fig. 3a). However, for the diphenylamino derivative **2e**, which exhibits stronger ICT character due to the more pronounced electron-donating nature of the amino group, the main absorption band is red-shifted to 405 nm.

All boron complexes **3a-e** show intense absorption bands in the higher energy region at 342–352 nm and low-energy absorption bands at 375–440 nm (Fig. 3b). According to the results obtained for related 2,6-bis(2'-hydroxyphenyl)pyridine structures [35], the high-energy absorption bands do not exhibit significant shifts when the aryl group is changed. These bands originate from a ligand-centered π → π* transition of the (pyrimidine)BF moiety, which are also observed in the absorption spectra of the free ligands **2a-e**. The low-energy absorption bands are less intense (except for **3b** and **3e**) and are likely due to an intra-molecular charge transfer (ICT) transition from the aryl group to the (pyrimidine)BF acceptor moiety.

As observed in similar structures [23,32], compounds **2** exhibited no fluorescence both in solution and in the solid state, except for a subtle emission in **2e**. The intramolecular hydrogen bonds present in the molecular structure favor an ESIPT process, resulting in an excited keto tautomer that deactivates through radiationless decay. These results are consistent with our previous findings and can be justified in the same way as in our earlier works [23,32].

As previously mentioned, the ESIPT process can be inhibited by protonation. Thus, the presence of trifluoroacetic acid (TFA, 0.1 M) in the CH₂Cl₂ solutions resulted in the appearance of new red-shifted absorption bands. Additionally, the acidic solutions exhibited emission bands ranging from the blue to the red regions of the optical spectrum (Fig. 4). However, it should be noted that for compounds **2c-e**, the fluorescence quantum yields were very low (Φ<sub>F</sub> < 1 %) at this acid concentration (Table 1). For complexes **2c** and **2d**, this may be due to moderate ICT caused by the absence of a strong donor, as well as incomplete protonation. In contrast, for **2e**, the low quantum yield can be attributed to an excessively intense ICT. Indeed, as shown in Figs. S31–S32, strong emission solvatochromism is observed in acidified solvents, with compound **2e** exhibiting significantly higher photoluminescence quantum yield (PLQY) in lower polarity solvents (0.21 in heptane, 0.27 in toluene).

Unlike the dark 2,4-bis(2'-hydroxyphenyl)pyrimidines **2**, their corresponding boron complexes **3** are intensely emissive in the visible light region, both in solution and in the solid state (Table 1, Fig. 5).

Upon excitation, complexes **3** exhibit structureless emission bands in CH₂Cl₂ solution, which can tentatively be assigned to a singlet ICT excited state origin (Fig. 6). However, it is worth noting that a clear trend in the shifts of emission spectra is not observed based on the electronic nature of the aryl group. The largest red-shifts were found for compounds **3d** (λ<sub>max</sub> = 511 nm) and **3e** (λ<sub>max</sub> = 606 nm), which have a strong electron-acceptor CF₃ and a potent electron-donating NPh₂ substituent, respectively. Compared to their arylvinyl analogs, complexes **3**
<table><caption>**Table 1**
UV/vis and photoluminescence data (PL) of compounds 2 and 3.ª</caption><thead><tr><th rowspan="2">Compd</th><th colspan="3">CH<sub>2</sub>Cl<sub>2</sub></th><th colspan="3">CH<sub>2</sub>Cl<sub>2</sub> + TFA<sup>b</sup></th><th colspan="2">Solid, powder</th></tr><tr><th>UV/vis λ<sub>max</sub>, nm (ε, mM<sup>-1</sup>·cm<sup>-1</sup>)</th><th>PL λ<sub>max</sub>, nm</th><th>Φ<sub>F</sub><sup>c</sup></th><th>UV/vis λ<sub>max</sub>, nm (ε, mM<sup>-1</sup>·cm<sup>-1</sup>)</th><th>PL λ<sub>max</sub>, nm</th><th>Φ<sub>F</sub><sup>c</sup></th><th>PL λ<sub>max</sub>, nm<sup>d</sup></th><th>Φ<sub>F</sub><sup>c</sup></th></tr></thead><tbody><tr><td>2a</td><td>345 (24.7)</td><td>–</td><td>–</td><td>355 (32.7), 387 (36.1)</td><td>444</td><td>0.03</td><td>–</td><td>–</td></tr><tr><td>2b</td><td>344 (27.6)</td><td>–</td><td>–</td><td>368 (22.8), 409 (30.1)</td><td>510</td><td>0.12</td><td>–</td><td>–</td></tr><tr><td>2c</td><td>343 (18.3)</td><td>–</td><td>–</td><td>336 (22.0), 374 (16.5)</td><td>452</td><td>&lt;0.01</td><td>–</td><td>–</td></tr><tr><td>2d</td><td>348 (15.8)</td><td>–</td><td>–</td><td>339 (14.1), 375 (11.4)</td><td>466</td><td>&lt;0.01</td><td>–</td><td>–</td></tr><tr><td>2e</td><td>348 (14.8), 405 (28.7)</td><td>531</td><td>&lt;0.01</td><td>483 (32.2)</td><td>667</td><td>&lt;0.01</td><td>–</td><td>–</td></tr><tr><td>3a</td><td>347 (46.6), 377 (26.4)</td><td>464</td><td>0.09</td><td></td><td></td><td></td><td>455, 517 (482)</td><td>0.03</td></tr><tr><td>3b</td><td>352 (18.9), 375 (20.3)</td><td>476</td><td>0.28</td><td></td><td></td><td></td><td>475, 585 (512)</td><td>0.01</td></tr><tr><td>3c</td><td>342 (13.6), 379 (9.8)</td><td>484</td><td>0.05</td><td></td><td></td><td></td><td>458, 517<sup>e</sup> (497)</td><td>0.23</td></tr><tr><td>3d</td><td>344 (11.3), 389 (7.8)</td><td>511</td><td>0.02</td><td></td><td></td><td></td><td>474, 517<sup>e</sup> (493)</td><td>0.13</td></tr><tr><td>3e</td><td>338 (13.4), 440 (38.5)</td><td>606</td><td>0.88</td><td></td><td></td><td></td><td>550 (591)</td><td>0.33</td></tr></tbody></table>

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Absorption spectra of (a) ligands 2 and (b) boron complexes 3 in CH<sub>2</sub>Cl<sub>2</sub>.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** (a) Absorption and (b) normalized emission spectra (B) of ligands 2.H⁺ in acidified CH<sub>2</sub>Cl<sub>2</sub> (TFA 0.1 M).</figcaption></figure>

exhibit, as expected, blue shifted emission [32]. Surprisingly, moderately electron-donating groups in 3a (λ<sub>max</sub> = 464 nm) and 3b (λ<sub>max</sub> = 476 nm) lead to the most energetic emissions. A PLQY of up to 0.88 was measured for diphenylamino complex 3e, significantly higher than that of its arylvinyl analogue [32].

The solvatochromic shifts of the emission bands can be used to evaluate the ICT upon excitation. Therefore, the emission spectra of

compounds 3 were recorded in a series of aprotic solvents with varying polarity (Fig. 7 and S33-S36). As is typical for emissive push-pull derivatives, an increase in solvent polarity led to a red shift in the emission maxima, accompanied by a successive decrease in fluorescence intensity. This positive solvatofluorochromism is particularly pronounced in compound 3e, which contains the strongest electron-donating group (NPh<sub>2</sub>), and results from the stabilization of a highly polar emitting state

<footnote>a All spectra were registered at room temperature (c = ~10⁻⁵ M).</footnote>

<footnote>b Data in the presence of TFA (0.1 M).</footnote>

<footnote>c Fluorescence quantum yield determined relative to that of 9,10-bis(phenylethynyl)anthracene in cyclohexane (Φ<sub>F</sub> = 1).</footnote>

<footnote>d λ<sub>max</sub> for samples embedded in a KBr matrix (1 wt%) in parentheses.</footnote>

<footnote>e Fluorescence quantum yield measured with a Jasco ILF-835/100 mm integrating sphere.</footnote>

<footnote>f Small shoulder.</footnote>
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** From top to bottom: color photographs of 3a-e under 365 nm UV light in CH₂Cl₂ solution, as solid samples embedded in a KBr matrix (1 wt%), and as powdered samples. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Normalized solution-state emission spectra of complexes 3 in CH₂Cl₂.</figcaption></figure>

[41–43]. The phenomenon is easily discernible by the naked-eye when 3e is exposed to UV light in different solvents, displaying a color change from green in cyclohexane to red in acetonitrile solution. As expected, the solvatochromic range for 3a-d was lower than for their diphenylamino analog 3e, indicating a less significant ICT.

Solid samples (powders) of compounds 3a-e exhibit broad emission bands accompanied with red-shifted shoulders (Fig. 8a, see also Figs. S37–S41), which can be tentatively ascribed to emission arising from π-π stacked aggregated forms (see below). This hypothesis aligns with the identification of adjacent molecules in close proximity in the crystalline structure of 3e. These shoulders completely vanish when boron complexes 3 are dispersed in a KBr matrix with a concentration of 1 wt% (Fig. 8b). Under these conditions, where each molecule should be isolated, greenish-blue to orange fluorescence emissions specifically attributable to the excited monomers are observed. The varying

intensities of the shoulders highlight the significant role played by the aryl substituent in aggregate formation. Notably, this effect was more pronounced for 3a and 3b, both featuring moderate electron-donating groups. Boron-containing fluorophores are known to form aggregates/excimers in concentrated solution and in the solid state [44–46] and their emitting properties are greatly influenced by the intermolecular interactions present in the crystal [47]. Complexes 3c and 3e are the most emissive, with PLQY of 0.23 and 0.33, respectively.

**DFT and TD-DFT computational insights.** Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) calculations were performed to rationalize the experimental photo-physical properties of boron compexes 3a-3e using the B3LYP, PBE0, M06-2X, CAM-B3LYP, and ωB97xD functionals, in combination with the 6-31+G** basis set in CH₂Cl₂ solution (see SI, Computational Details section). Solvent effects were included using the polarizable continuum model (PCM) to account for implicit solvation. Figs. S42 and S43 show the optimized molecular structures in CH₂Cl₂ solution for both the ground and first excited states using PBE0 and M06-2X. One of the most interesting geometric parameters to discuss is the dihedral angle between the aryl group at the 6-position and the boron platform, for which significant twisting is predicted in the ground state for all compounds, regardless of the functional used. The X-ray crystal structure of 3e shows a value of 24.2°. Table 2 indicates that this dihedral angle ranges from 12° to 26° in the ground state and decreases significantly in the excited state S₁, reaching the lowest values for derivatives 3b and 3e. The lower value predicted for 3b and 3e in S₁ compared to the other compounds may be due to the higher electronic delocalization caused by the methoxynaphthyl and diphenylamino groups after photoexcitation. M06-2X, CAM-B3LYP, and ωB97xD predict values close to zero in S₁, while B3LYP and PBE0 yield values in the range of 2-4°.

The vertical electronic transitions and oscillator strength (f) were also calculated in CH₂Cl₂ solution for compounds 3a-3e (Table 3 and Table S2). PBE0 yields the best agreement with the experimental absorption data (see Table 3). The experimental absorption maximum has been assigned to the lowest energy transition S₀→S₁. This transition is predicted to be the strongest for compounds 3b and 3e, with a significant HOMO→LUMO contribution and, therefore, ICT character.

The HOMO and LUMO molecular orbitals are illustrated in Fig. 9 and S44. For compounds 3b and 3e, the HOMO is mainly localized on the methoxynaphthyl and triphenylamino groups, consistent with their high electron-donating character, while the LUMO is extended over the boron platform. In contrast, both the HOMO and LUMO are extended over the entire molecule in 3a, whereas in 3c and 3d, the HOMO is mainly localized on the boron platform and the LUMO is delocalized over the whole molecule. The theoretical HOMO and LUMO energy levels are listed in Table S3. The narrowest HOMO-LUMO gap is predicted for 3b and 3e, in agreement with the red shift observed for these two compounds in the absorption spectra compared to the other compounds studied.

The molecular geometry of the first excited state S₁ was optimized to estimate the S₁→S₀ transition energy using the B3LYP, M06-2X, CAM-B3LYP, PBE0, and ωB97xD functionals. In general, the best agreement between experimental data and theoretical values was obtained using PBE0/6-31+G** and the linear response (LR) approach, as shown in Table 4. In addition, M06-2X/6-31+G** with the state-specific (SS) approach yields good results only for compounds 3b and 3e (see Table S4 for B3LYP, CAM-B3LYP, PBE0, and ωB97xD). PBE0 predicts the highest oscillator strength values for 3b (f = 0.93) and 3e (f = 0.55), which aligns with the large quantum yield measured for these compounds: 28 % for 3b and 88 % for 3e. These values are predicted to be even higher with M06-2X, specifically 1.40 and 1.31 for 3b and 3e, respectively. PBE0 also predicts very low values for f in the range of 0.05–0.07 for 3a, 3c, and 3d in CH₂Cl₂ solution, reflecting the weak fluorescence emission observed for these boron complexes (see Table 4). At this point, it is important to clarify that the energy values for the S₁→S₀ transition estimated with M06-2X and PBE0 in Table 4 are not
<figure><img src="image_10.png" /><figcaption>**Fig. 7.** (a) Normalized emission spectra and (b) color changes observed for **3e** in various solvents upon irradiation with UV light (365 nm). From left to right: cyclohexane, toluene, dioxane, THF, CH₂Cl₂, and acetonitrile. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Normalized solid-state emission of complexes **3**: (a) as powder and (b) in a KBr matrix (1 wt%).</figcaption></figure>

<table><caption>**Table 2**

Theoretical dihedral angle (in degrees) between the aryl group at the 6-position and the boron platform at various TD-DFT/6-31+G** levels of theory.</caption><thead><tr><th rowspan="2">Compd</th><th colspan="2">M06-2X</th><th colspan="2">PBE0</th><th colspan="2">B3LYP</th><th colspan="2">CAM-B3LYP</th><th colspan="2">ωB97xD</th></tr><tr><th>S<sub>0</sub></th><th>S<sub>1</sub></th><th>S<sub>0</sub></th><th>S<sub>1</sub></th><th>S<sub>0</sub></th><th>S<sub>1</sub></th><th>S<sub>0</sub></th><th>S<sub>1</sub></th><th>S<sub>0</sub></th><th>S<sub>1</sub></th></tr></thead><tbody><tr><td>3a</td><td>-17.6</td><td>-5.4</td><td>-14.2</td><td>-6.1</td><td>-13.5</td><td>-5.8</td><td>-16.2</td><td>-4.7</td><td>-19.8</td><td>-6.9</td></tr><tr><td>3b</td><td>-18.1</td><td>-0.4</td><td>-15.2</td><td>-2.1</td><td>-14.9</td><td>-4.5</td><td>-16.9</td><td>-0.4</td><td>-19.8</td><td>-0.5</td></tr><tr><td>3c</td><td>-22.8</td><td>-9.4</td><td>-21.4</td><td>-4.9</td><td>-21.4</td><td>-4.6</td><td>-22.5</td><td>-8.3</td><td>-24.6</td><td>-13.7</td></tr><tr><td>3d</td><td>-24.4</td><td>-7.9</td><td>-22.8</td><td>-3.2</td><td>-22.9</td><td>-3.0</td><td>-24.3</td><td>-6.8</td><td>-26.4</td><td>-10.0</td></tr><tr><td>3e</td><td>-17.4</td><td>0.03</td><td>-12.7</td><td>-2.3</td><td>-12.1</td><td>-2.8</td><td>-14.4</td><td>-0.5</td><td>-19.1</td><td>-0.5</td></tr></tbody></table>

directly comparable because they are calculated using different solvation methods: SS for M06-2X and LR for PBE0. Table S4 provides the results obtained with both functionals using the same solvation methodology (SS or LR). As expected, M06-2X consistently predicts higher energies than PBE0 when comparing results obtained using the same solvation method. Additionally, it is worth noting the difference in oscillator strength values predicted by M06-2X and PBE0 in Table 4. As discussed in previous studies, these differences arise from the varying HF exchange percentages in the functionals: 25 % in PBE0 and 54 % in M06-2X. In general, functionals with a larger HF exchange percentage show improved performance [48–50]. Table S4 further demonstrates that B3LYP and PBE0, with lower HF exchange, predict much smaller oscillator strengths compared to M06-2X, CAM-B3LYP, and ωB97XD, which have higher HF exchange percentages.

In addition, the highest dipole moment predicted for **3e** in the ground state (10.9 D) with M06-2X increases in S₁ (12.3 D) due to the redistribution of electron density upon excitation (see Table S5). This is in agreement with the more pronounced solvatochromism measured for this compound (Fig. 7) and the largest Stokes shift experimentally observed (10700 cm⁻¹).

Fig. 10 displays the correlation between the experimental emission and the calculated S₁→S₀ transition energy using different functionals in the SS approach. B3LYP and PBE0 underestimate the calculated emission, whereas M06-2X, CAM-B3LYP, and ωB97xD overestimate it. In any case, the theoretically predicted data are quite far from the experimental emission measurements, except for compounds **3b** and **3e**, for which a reasonable agreement is found. Unlike the SS approach, a significant improvement is observed using the LR approach with PBE0 for
<caption>**Table 3**</caption>

Experimental maximum absorption wavelengths ($\lambda_{ab}^{\text{exp}}$) and calculated vertical electronic transitions ($\lambda_{\text{vert-ab}}^{\text{calc}}$), oscillator strength ($f$), and main components of the $S_0 \rightarrow S_n$ transitions (% contribution) at the TD-PBE0/6-31+G** level of theory in CH₂Cl₂ solution.

<table><thead><tr><th>Compd</th><th>λ<sub>ab</sub><sup>exp</sup> nm (eV)</th><th>λ<sub>vert-ab</sub><sup>calc</sup> nm (eV)</th><th>Transition</th><th>f</th><th>% Contribution</th></tr></thead><tbody><tr><td rowspan="2">3a</td><td>377 (3.29)</td><td>379 (3.27)</td><td>S<sub>0</sub>→S<sub>1</sub></td><td>0.28</td><td>H → L (94)</td></tr><tr><td>347 (3.57)</td><td>348 (3.57)</td><td>S<sub>0</sub>→S<sub>2</sub></td><td>0.66</td><td>H-1 → L (89)</td></tr><tr><td rowspan="2">3b</td><td>375 (3.31)</td><td>405 (3.06)</td><td>S<sub>0</sub>→S<sub>1</sub></td><td>0.71</td><td>H → L (95)</td></tr><tr><td>352 (3.52)</td><td>332 (3.74)</td><td>S<sub>0</sub>→S<sub>5</sub></td><td>0.36</td><td>H-1 → L+1 (83)</td></tr><tr><td rowspan="2">3c</td><td>379 (3.27)</td><td>384 (3.23)</td><td>S<sub>0</sub>→S<sub>1</sub></td><td>0.16</td><td>H → L (97)</td></tr><tr><td>342 (3.63)</td><td>343 (3.61)</td><td>S<sub>0</sub>→S<sub>2</sub></td><td>0.11</td><td>H-1 → L (97)</td></tr><tr><td rowspan="2">3d</td><td>389 (3.19)</td><td>397 (3.13)</td><td>S<sub>0</sub>→S<sub>1</sub></td><td>0.14</td><td>H → L (98)</td></tr><tr><td>344 (3.60)</td><td>336 (3.69)</td><td>S<sub>0</sub>→S<sub>3</sub></td><td>0.26</td><td>H → L+1 (94)</td></tr><tr><td rowspan="2">3e</td><td>440 (2.82)</td><td>454 (2.73)</td><td>S<sub>0</sub>→S<sub>1</sub></td><td>0.96</td><td>H → L (97)</td></tr><tr><td>338 (3.67)</td><td>376 (3.30)</td><td>S<sub>0</sub>→S<sub>3</sub></td><td>0.10</td><td>H-1 → L (95)</td></tr></tbody></table>

compounds **3a**, **3c**, and **3d** (see also Table S4). As described in previous works, [51] the LR approach yields suitable results for emissive excited states when a large change in electron distribution is not induced during the electronic transition, while the SS approach is more suitable when the transition exhibits a stronger intramolecular charge transfer character.

Consequently, the best agreement between experimental and theoretical emission using M06-2X in the SS approach is found for derivatives **3b** and **3e**, which is consistent with the larger differences

predicted between the dipole moments for these compounds in $S_0$ and $S_1$: 2.4 D and 1.4 D, respectively (see Table S5). This is also related to the ICT character of the transition from HOMO to LUMO, which are spatially separated into the donor and acceptor units of the molecule (see Fig. 9 and S44). However, the discrepancies between experimental and theoretical data using M06-2X in the LR approach for **3b** and **3e** are similar to those found for **3a**, **3c**, and **3d** (see Table S4). Unlike M06-2X, a significant improvement is obtained for **3a**, **3c**, and **3d** using PBE0 in the LR approach, despite no significant changes being observed in the localization of HOMO and LUMO using either M06-2X or PBE0. In addition, the differences in dipole moments between $S_0$ and $S_1$ calculated with PBE0 are similar to those for **3b** and **3e**. The oscillator strength for the $S_1 \rightarrow S_0$ transition is almost zero with PBE0, while M06-2X predicts significant values for these compounds and nearly double values for **3b** and **3e**.

The discussed behavior of the emission of these compounds suggests that the dihedral angle between the aryl group at the 6-position and the boron platform could play a key role in their photophysical properties. Fig. 11 shows the relaxed energy barrier for compounds **3a-3e** in the excited state $S_1$ as this dihedral angle varies. When analyzing the energy barrier starting from a dihedral angle far from the minimum (from left to right), it decreases gradually for compounds **3a**, **3c**, and **3d** until reaching the minimum corresponding to the most stable conformation in $S_1$, while for compounds **3b** and **3e**, the drop is more abrupt. This result aligns with the observation that a larger geometric change is required for compounds **3b** and **3e** to transition from the ground state to the excited state $S_1$. Consequently, the electronic relaxation from the bright first

<figure><img src="image_12.png" /><figcaption>**Fig. 9.** Molecular orbital representation (isocontour plots 0.02 a.u.) calculated for compounds **3a-3e** in CH₂Cl₂ solution at the PBE0/6-31+G** level of the theory.</figcaption></figure>
<caption>**Table 4**</caption>

Experimental ($\lambda_{\text{exp-em}}$) and calculated ($\lambda_{\text{calc-vert-em}}$) maximum emission wavelengths for the S<sub>1</sub>→S<sub>0</sub> transition at the TD-M06-2X/6-31+G** (SS) and TD-PBE0/6-31+G** (LR) levels of theory in CH<sub>2</sub>Cl<sub>2</sub> solution.

<table><thead><tr><th rowspan="2">Compd</th><th rowspan="2">λ<sup>exp</sup><sub>em</sub> nm (eV)</th><th rowspan="2">Φ<sub>F</sub></th><th colspan="3">TD-M06-2X/6-31+G** (SS)</th><th colspan="3">TD-PBE0/6-31+G** (LR)</th></tr><tr><th>λ<sup>calc</sup><sub>vert-em</sub> nm (eV)</th><th>f</th><th>% Contr.</th><th>λ<sup>calc</sup><sub>vert-em</sub> nm (eV)</th><th>f</th><th>% Contr.</th></tr></thead><tbody><tr><td>3a</td><td>464 (2.67)</td><td>0.09</td><td>384 (3.23)</td><td>0.80</td><td>H → L (92)</td><td>466 (2.66)</td><td>0.07</td><td>H → L (98)</td></tr><tr><td>3b</td><td>476 (2.61)</td><td>0.28</td><td>478 (2.60)</td><td>1.40</td><td>H → L (91)</td><td>446 (2.78)</td><td>0.93</td><td>H → L (97)</td></tr><tr><td>3c</td><td>484 (2.56)</td><td>0.05</td><td>398 (3.12)</td><td>0.51</td><td>H → L (89)</td><td>485 (2.56)</td><td>0.06</td><td>H → L (99)</td></tr><tr><td>3d</td><td>511 (2.43)</td><td>0.02</td><td>417 (2.98)</td><td>0.43</td><td>H → L (88)</td><td>518 (2.40)</td><td>0.05</td><td>H → L (99)</td></tr><tr><td>3e</td><td>606 (2.05)</td><td>0.88</td><td>553 (2.24)</td><td>1.31</td><td>H → L (89)</td><td>554 (2.24)</td><td>0.55</td><td>H → L (98)</td></tr></tbody></table>

<figure><img src="image_13.png" /><figcaption>**Fig. 10.** Calculated S<sub>1</sub>→S<sub>0</sub> transition energy at the TD-DFT/6-31+G** level of theory in CH<sub>2</sub>Cl<sub>2</sub> solution versus experimental emission for compounds 3a-3e.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Fig. 11.** Potential energy barrier for the first excited state (S<sub>1</sub>) at the M06-2X/6-31+G** level of theory for compounds 3a-3e.</figcaption></figure>

excited state to the ground state in 3b and 3e would involve greater twisting, resulting in a longer S<sub>1</sub> state lifetime and higher emission compared to the other compounds. These findings are consistent with the higher quantum yield observed for 3b and 3e in solution.

On the other hand, if the viscosity of the medium increases or if the molecule is in a solid state or forms aggregates, rotation around the dihedral angle would be restricted, hindering the molecule from achieving the predicted S<sub>1</sub> geometry. This restriction would directly affect fluorescent emission and is known as the Restriction of Intramolecular Motion (RIM) effect. For compounds with larger geometric changes, such as 3b and 3e, their photophysical properties are expected to be more sensitive to these conditions compared to those of compounds with smaller dihedral angle variations. In summary, these results suggest that the RIM effect could have a greater impact on the emission of complexes 3b and 3e than on that of compounds 3a, 3c, and 3d.

In order to delve into the fluorescent emission of compound 3e in the solid state, for which the X-ray structure is available, TD-DFT calculations were performed on the crystal structure using the ONIOM methodology. Fig. 12 shows the ONIOM model clusters containing 36 molecules extracted from the experimental X-ray crystal structure to mimic the crystalline environment. To further investigate the effect of the aggregation on the observed emission in the solid state, two possibilities were explored: i) the ground and first excited states were optimized for one central active molecule (high level with PBE0/6-31G**), and ii) two central active molecules were optimized (high level with PBE0/6-31G**). In the ground and first excited states. In both cases, the crystallographic positions of the surrounding molecules were kept frozen (low level with PM7). The cluster size was determined by ensuring that the central active molecules were sufficiently surrounded by neighboring molecules to avoid edge effects and to allow all intermolecular interactions to be represented in the model cluster, while also keeping the computational cost manageable.

Table 5 shows the calculated S<sub>1</sub>→S<sub>0</sub> energy transition using both cluster models, along with the experimental emission in solid state for comparison. As observed, the two central active molecules model predicts a blue shift with respect to the one central active molecule model, from 593 nm to 496 nm. This result evidences the strong electronic couplings attributed to π-π stacking between the boron platforms, as described in the crystallographic section. The band observed at 550 nm in the powdered sample can be assigned to the S<sub>1</sub>→S<sub>0</sub> transition calculated at 496 nm for the two central active molecules. On the other hand, the band observed at 591 nm in the KBr matrix can be assigned to the S<sub>1</sub>→S<sub>0</sub> transition calculated at 593 nm using the one central active molecule, since the intermolecular interactions and electronic couplings with the neighboring molecules are minimized when the molecules are dissolved in the KBr matrix. Table 1 shows that a blue shift occurs from solution (606 nm) to solid state (550 nm) in complex 3e, along with a decrease in the quantum yield from 88 % to 33 %, respectively. This experimental observation, along with the theoretical calculations discussed above, suggest that the packing in compound 3e could be governed by H-type aggregation as a consequence of the proximity and orientation of boron platforms within the crystal structure.
<figure><img src="image_15.png" /><figcaption>**Fig. 12.** QM/MM model clusters a) one active molecule and b) two active molecules for compound 3e. The active molecules (in red) are treated at a high level (PBE0/6-31G**) and the surrounding molecules (in green) at a low level (PM7). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

**Table 5**

Experimental emission ($\lambda_{em}^{exp}$) and calculated S₁→S₀ transition ($\lambda_{vert-em}^{calc}$) in the solid state using the ONIOM level of theory (TD-PBE0/6-31G**/PM7) for compound 3e.

<table><thead><tr><th>Active molecules</th><th>λ<sub>emt</sub><sup>exp</sup> nm (eV)</th><th>λ<sub>vert-em</sub><sup>calc</sup> nm (eV)</th><th>f</th><th>% Contr.</th></tr></thead><tbody><tr><td>1</td><td>591 (2.10)</td><td>593 (2.09)</td><td>0.01</td><td>H → L (100)</td></tr><tr><td>2</td><td>550 (2.25)</td><td>496 (2.50)</td><td>0.02</td><td>H → L (98)</td></tr></tbody></table>

**3. Experimental section**

**General procedure for the synthesis of 2,4-dichloro-6-arylpyrimidines 1.** Reactions were carried out under an inert atmosphere. A suspension of the appropriate boronic acid (1 mmol) and 2,4,6-trichloropyrimidine (459 mg, 2.5 mmol) in 10 mL of toluene was prepared in a Schlenk flask. Next, 1 M aqueous Na₂CO₃ solution (1 mL), ethanol (1 mL), and Pd(PPh₃)₄ (58 mg, 0.05 mmol) were added, and the reaction was allowed to stir at 110 °C overnight. After cooling, water was added, and the mixture was extracted with ethyl acetate (×3). Finally, the combined organic extracts were dried (MgSO₄) and the solvent was evaporated. The crude products were purified by column chromatography.

2,4-Dichloro-6-(4'-methoxyphenyl)pyrimidine (1a). Purified by column chromatography (silica gel, petroleum ether/EtAcO, 9:1). Pale yellow solid (89 mg, 35 %). ¹H NMR (CDCl₃, 300 MHz) δ: 3.89 (s, 3H, OCH₃), 7.01 (A of ABq, 2H, *J* = 9.0 Hz, ArH), 7.59 (s, 1H, PyrH), 8.06 (B of ABq, 2H, *J* = 9.0 Hz, ArH). NMR data match those reported in the literature [52,53].

2,4-Dichloro-6-(6'-methoxynaphthalen-2-yl)pyrimidine (1b). Purified by column chromatography (silica gel, petroleum ether/EtAcO, 9:1). Yellow solid (55 mg, 18 %). ¹H NMR (CDCl₃, 300 MHz) δ: 3.96 (s, 3H, OCH₃), 7.16-7.25 (m, 2H, ArH), 7.77 (s, 1H, PyrH), 7.87 (t, 2H, *J* = 9.0 Hz, ArH), 8.08 (dd, 1H, *J* = 9.0 Hz, *J* = 1.9 Hz, ArH), 8.58 (d, 1H, *J* = 1.4 Hz, ArH).

¹³C{¹H} NMR and JMOD (CDCl₃, 75 MHz) δ: 167.2 (C), 161.8 (C), 158.7 (C), 135.9 (C), 129.9 (CH), 128.1 (C), 127.5 (CH), 126.8 (CH), 123.1 (CH), 119.1 (CH), 113.9 (CH), 104.8 (CH), 54.4 (CH). Two quaternary carbon peaks could not be observed. HRMS (ESI) *m/z*: [M+H]⁺ Calcd for C₁₅H₃₅Cl₂N₂O 305.0243; Found: 305.0243 (0 ppm).

2,4-Dichloro-6-phenylpyrimidine (1c). Purified by column chromatography (silica gel, petroleum ether, 9.5:0.5). Colorless solid (115 mg, 51 %). ¹H NMR (CDCl₃, 300 MHz) δ: 7.49-7.61 (m, 3H, ArH), 7.68 (s, 1H, PyrH), 8.07 (d, 2H, *J* = 9.0 Hz, ArH). NMR data match those reported in the literature [52].

2,4-Dichloro-6-[(4'-trifluoromethyl)phenyl]pyrimidine (1d). Purified by column chromatography (silica gel, petroleum ether/EtAcO, 9:1). Pale yellow solid (94 mg, 32 %). ¹H NMR (CDCl₃, 300 MHz) δ: 7.71 (s, 1H, PyrH), 7.78 (A of ABq, 2H, *J* = 9.0 Hz, ArH), 8.19 (B of ABq, 2H, *J* = 9.0 Hz, ArH). NMR data match those reported in the literature [54].

2,4-Dichloro-6-(4'-diphenylaminophenyl)pyrimidine (1e). Purified by column chromatography (silica gel, hexanes/EtAcO, 1:1). Yellow solid (130 mg, 33 %). ¹H NMR (CDCl₃, 300 MHz) δ: 7.07 (A of ABq, 2H, *J* = 8.3 Hz, ArH), 7.14-7.33 (m, 10H, ArH), 7.52 (s, 1H, PyrH), 7.90 (B of ABq, 2H, *J* = 9.0 Hz, ArH). NMR data match those reported in the literature [55].

**General procedure for the synthesis of 2,4-bis(2'-hydroxyphenyl)-6-arylpyrimidines 2.** Reactions were carried out under an inert atmosphere. A suspension of the appropriate dichloropyrimidine derivative 5 (1 mmol) and 2-hydroxyphenylboronic acid (349 mg, 2.5 mmol) in 15 mL of toluene was prepared in a Schlenk flask. Next, 1 M aqueous Na₂CO₃ solution (2 mL), ethanol (2 mL), and Pd(PPh₃)₄ (116 mg, 0.1 mmol) were added, and the reaction was allowed to stir at 110 °C for 48 h. After cooling, water was added, and the mixture was extracted with ethyl acetate (×3). Finally, the combined organic extracts were dried (MgSO₄) and the solvent was evaporated. The crude products were purified by column chromatography.

2,4-(2'-hydroxyphenyl)-6-(4'-methoxyphenyl)pyrimidine (2a). Purified
by column chromatography (silica gel, petroleum ether/EtAcO, 9:1). Pale yellow solid (100 mg, 27 %). ¹H NMR (CDCl₃, 300 MHz) δ: 3.88–3.94 (m, 3H, OCH₃), 6.95–7.15 (m, 6H, ArH), 7.46 (t, 2H, *J* = 7.7 Hz, ArH), 7.95 (dd, 1H, *J* = 8.0 Hz, *J* = 1.7 Hz, ArH), 7.98 (s, 1H, PyrH), 8.03 (B of ABq, 2H, *J* = 8.0 Hz, ArH), 8.21 (dd, 1H, *J* = 8.0 Hz, *J* = 1.7 Hz, ArH), 13.73 (s, 1H, OH), 13.96 (s, 1H, OH). ¹³C{¹H} NMR (CDCl₃, 75 MHz) δ: 165.5, 163.1, 162.7, 162.5, 161.2, 161.1, 134.0, 133.8, 128.9, 128.3, 127.9, 127.1, 119.5, 119.4, 119.0, 118.3, 117.7, 117.2, 114.8, 107.6, 55.5. HRMS (ESI) *m/z*: [M+H]⁺ Calcd for C₂₃H₁₉N₂O₃ 371.1390; Found: 371.1392 (0 ppm).

**2,4-(2'-hydroxyphenyl)-6-(6'-methoxynaphthalen-2-yl)pyrimidine (2b).**
Purified by column chromatography (silica gel, petroleum ether/EtAcO, 9:1). Yellow solid (189 mg, 45 %). ¹H NMR (CDCl₃, 300 MHz) δ: 3.94 (s, 3H, OCH₃), 7.01 (t, 2H, *J* = 7.5 Hz, ArH), 7.07–7.14 (m, 3H, ArH), 7.19 (dd, 1H, *J* = 8.9 Hz, *J* = 2.4 Hz, ArH), 7.45 (td, 2H, *J* = 7.2 Hz, *J* = 1.6 Hz, ArH), 7.84 (dd, 2H, *J* = 8.8 Hz, *J* = 4.3 Hz, ArH), 7.92–8.08 (m, 3H, ArH and PyrH), 8.17 (d, 1H, *J* = 7.9 Hz, ArH), 8.40 (s, 1H, ArH), 13.74 (s, 1H, OH), 13.93 (s, 1H, OH). ¹³C{¹H} NMR and JMOD (CDCl₃, 75 MHz) δ: 165.5 (C), 163.1 (C), 162.9 (C), 161.2 (C), 161.1 (C), 159.4 (C), 136.4 (C), 134.0 (CH), 133.8 (CH), 130.6 (CH), 130.5 (C), 128.6 (C), 128.3 (CH), 128.0 (CH), 127.6 (CH), 127.2 (CH), 124.0 (CH), 120.0 (CH), 119.5 (CH), 119.4 (CH), 119.0 (CH), 118.3 (CH), 117.7 (C), 117.2 (C), 108.2 (CH), 105.7 (CH), 55.4 (CH). HRMS (ESI) *m/z*: [M+H]⁺ Calcd for C₂₇H₂₁N₂O₃ 421.1547; Found: 421.1543 (1 ppm).

**2,4-(2'-hydroxyphenyl)-6-phenylpyrimidine (2c).**
Purified by column chromatography (silica gel, petroleum ether/EtAcO, 9:1). Colorless solid (51 mg, 15 %). ¹H NMR (CDCl₃, 300 MHz) δ: 6.96–7.06 (m, 2H, ArH), 7.10 (ddd, 2H, *J* = 8.3 Hz, *J* = 4.5 Hz, *J* = 1.2 Hz, ArH), 7.42–7.49 (m, 2H, ArH), 7.59 (dd, 2H, *J* = 5.0 Hz, *J* = 2.0 Hz, ArH), 7.95 (dd, 1H, *J* = 8.1 Hz, *J* = 1.6 Hz, ArH), 8.02–8.08 (m, 4H, PyrH and ArH), 8.21 (dd, 1H, *J* = 8.0 Hz, *J* = 1.7 Hz, ArH), 13.60 (s, 1H, OH), 13.89 (s, 1H, OH). ¹³C{¹H} NMR and JMOD (CDCl₃, 75 MHz) δ: 164.9 (C), 162.3 (C), 162.1 (C), 160.2 (C), 160.1 (C), 134.8 (C), 133.1 (CH), 132.9 (CH), 130.7 (CH), 128.4 (CH), 127.3 (CH), 126.3 (CH), 126.2 (CH), 118.6 (CH), 118.5 (CH), 118.1 (CH), 117.4 (CH), 116.6 (C), 116.1 (C), 107.7 (CH). HRMS (ESI) *m/z*: [M+H]⁺ Calcd for C₂₂H₁₇N₂O₂ 341.1285; Found: 341.1288 (1 ppm).

**2,4-(2'-hydroxyphenyl)-6-[(4'-trifluoromethyl)phenyl]pyrimidine (2d).**
Purified by column chromatography (silica gel, petroleum ether/EtAcO, 9:1). Pale yellow solid (151 mg, 37 %). ¹H NMR (CDCl₃, 300 MHz) δ: 6.96–7.07 (m, 2H, ArH), 7.11 (td, 2H, *J* = 8.6 Hz, 1.2 Hz, ArH), 7.41–7.55 (m, 2H, ArH), 7.86 (A of ABq, 2H, *J* = 8.2 Hz, ArH), 7.96 (dd, 1H, *J* = 8.1 Hz, *J* = 1.6 Hz, ArH), 8.06 (s, 1H, PyrH), 8.16 (B of ABq, 2H, *J* = 8.1 Hz, ArH), 8.22 (dd, 1H, *J* = 8.0 Hz, *J* = 1.7 Hz, ArH), 13.28 (s, 1H, OH), 13.76 (s, 1H, OH). ¹³C{¹H} NMR and JMOD (CDCl₃, 75 MHz) δ: 166.4 (C), 163.6 (C), 161.8 (C), 161.3 (C), 161.0 (C), 139.3 (C), 134.5 (CH), 134.2 (CH), 128.4 (CH), 127.7 (CH), 127.2 (CH), 126.4 (q, *J* = 3.7 Hz, CH), 119.8 (CH), 119.6 (CH), 119.2 (CH), 118.5 (CH), 117.4 (C), 116.9 (C), 109.2 (CH). Two quaternary carbon peaks could not be observed. HRMS (ESI) *m/z*: [M+H]⁺ Calcd for C₂₃H₁₆N₂O₂F₃ 409.1158; Found: 409.1163 (1 ppm).

**2,4-(2'-hydroxyphenyl)-6-(4'-diphenylaminophenyl)pyrimidine (2e).**
Purified by column chromatography (silica gel, petroleum ether/EtAcO, 6:4). Yellow solid (274 mg, 54 %). ¹H NMR (CDCl₃, 300 MHz) δ: 6.96–7.24 (m, 12H, ArH), 7.30–7.39 (m, 4H, ArH), 7.40–7.50 (m, 2H, ArH), 7.89–7.97 (m, 4H, ArH and PyrH), 8.22 (dd, 1H, *J* = 8.0 Hz, *J* = 1.7 Hz, ArH), 13.78 (s, 1H, OH), 14.02 (s, 1H, OH). ¹³C{¹H} NMR (CDCl₃, 75 MHz) δ: 165.4, 163.1, 162.4, 161.2, 161.1, 151.3, 146.6, 133.9, 133.7, 129.6, 128.3, 128.3, 127.5, 127.0, 125.8, 124.6, 121.3, 119.5, 119.4, 119.0, 118.4, 117.7, 117.3, 107.5. HRMS (ESI) *m/z*: [M+H]⁺ Calcd for C₃₄H₃₆N₃O₂ 508.2020; Found: 508.2026 (1 ppm).

**General procedure for the synthesis of boron complexes 3.** Reactions were performed in screw-cap glass flasks that were placed inside metal heat transfer blocks. Boron trifluoride diethyl etherate was added to a mixture of the corresponding 2,4-bis(2'-hydroxyphenyl)pyrimidine, cesium carbonate, and chloroform. The reaction mixture was stirred at

80 °C for 6 h. After cooling, it was diluted with dichloromethane and the insoluble cesium carbonate was removed by filtration. The solvent was evaporated under vacuum. Trituration in boiling methanol yielded a solid, which was collected by filtration and then dried under vacuum.

**Complex 3a.** Prepared from 50 mg (0.13 mmol) of 2,4-bis(2'-hydroxyphenyl)-6-(4'-methoxyphenyl)pyrimidine (2a), 212 mg (0.65 mmol) of cesium carbonate, and 0.1 mL (0.8 mmol) of boron trifluoride diethyl etherate in 5 mL of chloroform. Colorless solid (43 mg, 80 %). ¹H NMR (CDCl₃, 500 MHz) δ: 3.95 (s, 3H, OCH₃), 7.07–7.12 (m, 4H, ArH), 7.22 (dd, 1H, *J* = 8.5 Hz, *J* = 1.0 Hz, ArH), 7.24 (dd, 1H, *J* = 8.5 Hz, *J* = 1.0 Hz, ArH), 7.55-7.61 (m, 2H, ArH), 7.94 (dd, 1H, *J* = 8.0 Hz, *J* = 1.5 Hz, ArH). ¹³C{¹H} NMR and DEPT (CDCl₃, 125 MHz) δ: 166.3 (C), 163.8 (C), 157.9 (C), 157.4 (C), 156.0 (C), 153.9 (C), 136.1 (CH), 135.9 (CH), 130.0 (CH), 128.4 (CH), 127.5 (C), 125.7 (CH), 121.3 (CH), 120.8 (CH), 120.5 (CH), 120.1 (CH), 117.0 (C), 115.3 (CH), 114.7 (CH), 105.7 (CH), 55.6 (CH₃). ¹⁹F NMR (CDCl₃, 471 MHz) δ: -136.0 (m). ¹¹B NMR (CDCl₃, 160 MHz) δ: 1.54 (d, *J* = 36 Hz). MALDI-TOF MS (dithranol) *m/z*: 379.4 [M - 19]⁺. HRMS (ESI) *m/z*: [M+Na]⁺ Calcd for C₂₃H₁₆N₂O₃F¹¹BNa 421.1130; Found: 421.1140 (2 ppm).

**Complex 3b.** Prepared from 50 mg (0.12 mmol) of 2,4-bis(2'-hydroxyphenyl)-6-(6'-methoxynaphthalen-2-yl)pyrimidine (2b), 195 mg (0.60 mmol) of cesium carbonate, and 0.1 mL (0.8 mmol) of boron trifluoride diethyl etherate in 5 mL of chloroform. Yellow solid (44 mg, 82 %). ¹H NMR (DMSO-d₆, 500 MHz) δ: 3.95 (s, 3H, OCH₃), 7.15–7.18 (m, 2H, ArH), 7.21–7.24 (m, 2H, ArH), 7.33 (dd, 1H, *J* = 8.5 Hz, *J* = 2.5 Hz, ArH), 7.49 (d, 1H, *J* = 2.5 Hz, ArH), 7.67–7.69 (m, 2H, ArH), 8.07 (d, 1H, *J* = 8.5 Hz, ArH), 8.12 (d, 1H, *J* = 9.0 Hz, ArH), 8.63 (dd, 1H, *J* = 8.5 Hz, *J* = 1.5 Hz, ArH), 8.67 (dd, 1H, *J* = 8.5 Hz, *J* = 2.0 Hz, ArH), 8.70 (dd, 1H, *J* = 8.0 Hz, *J* = 1.5 Hz, ArH), 8.95 (s, 1H, PyrH), 9.27 (s, 1H, ArH). ¹³C{¹H} NMR and DEPT (DMSO-d₆, 125 MHz) δ: 166.4 (C), 159.6 (C), 157.1 (C), 156.4 (C), 154.7 (C), 153.5 (C), 137.0 (C), 136.3 (CH), 136.0 (CH), 131.2 (CH), 129.8 (CH), 129.7 (C), 128.4 (CH), 128.1 (C), 128.0 (CH), 127.6 (CH), 125.0 (CH), 120.8 (CH), 120.7 (CH), 120.0 (CH), 119.8 (CH), 119.4 (CH), 116.8 (C), 115.4 (C), 108.3 (CH), 106.2 (CH), 55.5 (OCH₃). ¹⁹F NMR (CDCl₃, 471 MHz) δ: -135.9 (m). ¹¹B NMR (CDCl₃, 160 MHz) δ: 1.58 (d, *J* = 32 Hz). MALDI-TOF MS (dithranol) *m/z*: 429.4 [M - 19]⁺. HRMS (ESI) *m/z*: [M+Na]⁺ Calcd for C₂₇H₁₈N₂O₃F¹¹BNa 471.1287; Found: 471.1287 (0 ppm).

**Complex 3c.** Prepared from 50 mg (0.15 mmol) of 2,4-bis(2'-hydroxyphenyl)-6-phenylpyrimidine (2c), 244 mg (0.75 mmol) of cesium carbonate, and 0.1 mL (0.8 mmol) of boron trifluoride diethyl etherate in 5 mL of chloroform. Colorless solid (28 mg, 50 %). ¹H NMR (CDCl₃, 500 MHz) δ: 7.10–7.14 (m, 2H, ArH), 7.23 (dd, *J* = 8.0 Hz, *J* = 1.0 Hz, ArH), 7.27 (dd, *J* = 8.0 Hz, *J* = 1.0 Hz, ArH), 7.57–7.70 (m, 5H, ArH), 7.98 (dd, *J* = 8.5 Hz, *J* = 1.0 Hz, ArH), 8.13 (s, 1H, PyrH), 8.33-8.35 (m, 2H, ArH), 8.60 (dd, *J* = 8.0 Hz, *J* = 1.5 Hz, ArH). ¹³C{¹H} NMR and DEPT (CDCl₃, 125 MHz) δ: 167.1 (C), 157.9 (C), 157.5 (C), 156.3 (C), 154.5 (C), 136.4 (CH), 136.1 (CH), 135.3 (C), 132.9 (CH), 129.4 (CH), 128.5 (CH), 128.1 (CH), 125.9 (CH), 121.4 (CH), 120.9 (CH), 120.7 (CH), 120.2 (CH), 116.9 (C), 115.2 (C), 106.9 (CH). ¹⁹F NMR (CDCl₃, 471 MHz) δ: -135.7 (q, *J* = 33 Hz). ¹¹B NMR (CDCl₃, 160 MHz) δ: 1.59 (d, *J* = 38 Hz). MALDI-TOF MS (dithranol) *m/z*: 349.3 [M - 19]⁺. HRMS (ESI) *m/z*: [M+Na]⁺ Calcd for C₂₂H₁₄N₂O₂F¹¹BNa 391.1025; Found: 391.1024 (0 ppm).

**Complex 3d.** Prepared from 50 mg (0.12 mmol) of 2,4-bis(2'-hydroxyphenyl)-6-[(4'-trifluoromethyl)phenyl]pyrimidine (2d), 199 mg (0.61 mmol) of cesium carbonate, and 0.1 mL (0.8 mmol) of boron trifluoride diethyl etherate in 5 mL of chloroform. Pale yellow solid (21 mg, 40 %). ¹H NMR (CDCl₃, 500 MHz) δ: 7.09–7.14 (m, 2H, ArH), 7.23 (dd, 1H, *J* = 8.5 Hz, *J* = 1.0 Hz, ArH), 7.26 (dd, 1H, *J* = 8.5 Hz, *J* = 1.0 Hz, ArH), 7.59–7.62 (m, 2H, ArH), 7.88 (A of ABq, 2H, *J* = 8.5 Hz, ArH), 7.96 (dd, 1H, *J* = 8.0 Hz, *J* = 1.5 Hz, ArH), 8.13 (s, 1H, PyrH), 8.43 (B of ABq, 2H, *J* = 8.0 Hz, ArH), 8.55 (dd, 1H, *J* = 8.0 Hz, *J* = 2.0 Hz, ArH). ¹³C{¹H} NMR and DEPT (CDCl₃, 125 MHz) δ: 165.6 (C), 158.0 (C), 157.7 (C), 156.6 (C), 155.1 (C), 138.5 (C), 136.9 (CH), 136.5 (CH), 134.2 (q, *J*
= 30.9 Hz, C), 128.5 (CH), 128.4 (CH), 126.3 (q, J = 3.6 Hz, CH), 125.9 (CH), 123.6 (q, J = 271.3 Hz, C), 121.4 (CH), 121.1 (CH), 120.8 (CH), 120.3 (CH), 116.7 (C), 114.9 (C), 107.4 (CH). ¹⁹F NMR (CDCl₃, 471 MHz) δ: -63.0, -135.1 (m). ¹¹B NMR (CDCl₃, 160 MHz) δ: 1.59 (d, J = 35 Hz). MALDI-TOF MS (dithranol) *m/z*: 417.5 [M - 19]⁺. HRMS (ESI) *m/z*: [M+Na]⁺ Calcd for C₂₃H₁₃N₂O₂F¹¹BNa 459.0898; Found: 459.0904 (1 ppm).

**Complex 3e.** Prepared from 50 mg (0.10 mmol) of 2,4-bis(2'-hydroxyphenyl)-6-[(4'-diphenylamino)phenyl]pyrimidine (2e), 163 mg (0.50 mmol) of cesium carbonate, and 0.1 mL (0.8 mmol) of boron trifluoride diethyl etherate in 5 mL of chloroform. Yellow solid (42 mg, 80 %). ¹H NMR (CDCl₃, 500 MHz) δ: 7.06-7.10 (m, 2H, ArH), 7.15 (A of ABq, 2H, J = 9.0 Hz, ArH), 7.19-7.25 (m, 8H, ArH), 7.36-7.40 (m, 4H, ArH), 7.54-7.58 (m, 2H, ArH), 7.92 (dd, 1H, J = 8.0 Hz, J = 1.5 Hz, ArH), 7.97 (s, 1H, PyrH), 8.19 (B of ABq, 2H, J = 9.0 Hz, ArH), 8.53 (dd, 1H, J = 8.0 Hz, J = 1.5 Hz, ArH). ¹³C{¹H} NMR and DEPT (CDCl₃, 125 MHz) δ: 165.9 (C), 157.8 (C), 157.3 (C), 155.9 (C), 153.5 (C), 152.4 (C), 146.2 (C), 135.9 (CH), 135.8 (CH), 129.7 (CH), 129.4 (CH), 128.4 (CH), 126.6 (C), 126.1 (CH), 125.6 (CH), 125.0 (CH), 121.3 (CH), 120.8 (CH), 120.5 (CH), 120.3 (CH), 120.1 (CH), 117.1 (C), 115.4 (C), 105.3 (CH). ¹⁹F NMR (CDCl₃, 471 MHz) δ: -136.4 (m). ¹¹B NMR (CDCl₃, 160 MHz) δ: 1.52 (d, J = 29 Hz). MALDI-TOF MS (dithranol) *m/z*: 535.5 [M]⁺, 516.5 [M - 19]⁺. HRMS (ESI) *m/z*: [M+Na]⁺ Calcd for C₃₄H₂₃N₃O₂F¹¹BNa 558.1760; Found: 558.1765 (1 ppm).

## 4. Conclusions

In this study, we successfully synthesized a series of 6-aryl-2,4-bis(2'-hydroxyphenyl)pyrimidine ligands and their corresponding boron fluoride complexes. The synthesis involved a Suzuki-Miyaura coupling to introduce the aryl group at the 6-position, followed by the formation of the 2,4-bis(2'-hydroxyphenyl) structure and final coordination with boron trifluoride. As observed in similar structures, the ligands exhibited non-emissive behavior due to an ESIPT process from the OH groups to the nitrogen atoms of the pyrimidine ring. This process can be inhibited by protonation, which restores the emission properties. Upon complexation with boron, the ESIPT process was also effectively inhibited. This inhibition, along with the increased rigidity and planarity of the complexes, led to the recovery of fluorescence in both solution and the solid state, allowing fine-tuning of the emission color by modifying the substituent on the aryl group.

Comprehensive techniques such as X-ray diffraction analysis, DFT, and TD-DFT were critical in interpreting the experimental results, providing key insights into the electronic structure, confirming electronic delocalization, and accurately predicting emission energies. The optimized geometries in both ground and excited states, including dihedral angles and electronic transitions, helped to explain the fluorescence behavior observed experimentally.

In summary, the results demonstrate that boron complexation is an effective strategy to modulate the emission properties of these ligands, opening up new possibilities for their application in optoelectronic devices and fluorescent sensors.

### CRediT authorship contribution statement

**Clément Diguet:** Writing – original draft, Investigation, Data curation. **Amparo Navarro:** Writing – original draft, Investigation, Data curation, Conceptualization. **M. Paz Fernández-Lienres:** Investigation, Data curation. **Sonia B. Jiménez-Pulido:** Investigation, Data curation. **Nuria A. Illán-Cabeza:** Investigation, Data curation. **Julien Massue:** Writing – original draft, Validation, Investigation, Conceptualization. **Sébastien Gauthier:** Validation, Supervision. **François Robin-le Guen:** Supervision, Funding acquisition. **Sylvain Achelle:** Writing – review & editing, Writing – original draft, Validation, Supervision, Funding acquisition, Conceptualization. **Julián Rodríguez-López:** Writing – review & editing, Writing – original draft, Funding acquisition, Conceptualization.

### Declaration of competing interest

There is no conflict of interest.

### Acknowledgments

The following acknowledgments are expressed for the EUR LUMOMAT project and the Investments for the Future program ANR-18-EURE-0012, as well as for the funding received from the Junta de Comunidades de Castilla-La Mancha/FEDER (project SBPLY/21/180501/000042). We also extend our gratitude to the Junta de Andalucía (PAIDI-FQM-337) and the Universidad de Jaén/FEDER UJA 2020 (projects 2021/00627/001 and 2020/1380695) for their support of the research presented in this article. Additionally, we acknowledge the essential role played by the Centro de Servicios de Informática y Redes de Comunicaciones (CSIRC, Universidad de Granada) in providing the necessary computer resources that enabled this work.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.dyepig.2025.112660.

### Data availability

Data will be made available on request.

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