# Insights into the structure-property relationship of difluoro boron dipyridomethene derivatives

Camille Chériaux, Thomas Papineau, Pascal Retailleau, Carlotta Figliola,

Denis Jacquemin, Gilles Ulrich

► To cite this version:

Camille Chériaux, Thomas Papineau, Pascal Retailleau, Carlotta Figliola, Denis Jacquemin, et al. Insights into the structure-property relationship of difluoro boron dipyridomethene derivatives. ChemPhotoChem, In press, 10.1002/cptc.202400395. [https://doi.org/10.1002/cptc.202400395](https://doi.org/10.1002/cptc.202400395)

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# Insights into the Structure-Property Relationship of Difluoro Boron Dipyridomethene Derivatives

Camille Chériaux, Thomas V. Papineau, Pascal Retailleau, Carlotta Figliola,* Denis Jacquemin,* and Gilles Ulrich*

To elucidate the molecular structure of the difluoro dipyridomethene system and to establish structure-property relationships, an extensive synthetic work is reported. Four classes of novel fluorescent BF₂-DIPYR complexes are prepared via Suzuki–Miyaura cross-coupling reaction from the corresponding mono- and bisbrominated dyes in moderate to good yields. The strategy allows

the introduction of a large panel of electron-donating and -withdrawing groups on the meso, 3 and/or 7 position of the aromatic BF₂-DIPYR core. The new symmetric and asymmetric mono-, bis-, and tris substituted dyes are thoroughly characterized by X-ray diffraction analysis and spectroscopic measurements. Furthermore, the latter are rationalized by TD-DFT calculations.

## 1. Introduction

The design and synthesis of new organic fluorescent dyes is an active research area due to their diverse applications in materials science, biology, and medicine.[1-7] Among the current plethora of chromophores, tetracoordinated organoboron complexes remain at the forefront of the current research in the field owing to their accessible synthesis, excellent optical properties, biocompatibility, and nontoxicity.[3,8,9] Indeed, they have been developed as fluorescent trackers in bioimaging,[5,6,10,11] sensors in diagnostics and therapeutics,[1,2,5,10,11] as well as dyes in optoelectronic devices.[3,7] When chelating aromatic or heterocyclic π-conjugated ligands to the boron atom, the corresponding

bora-cycles of 5, 6, and 7 member-ring feature increased rigidity combined with improved chemical and photostability as well as large fluorescence quantum yields.[12-14] The 6-membered class encompasses the majority of the tetracoordinated boron complexes and, among them, the most popular and widely exploited are undoubtedly the 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) and their aza derivatives thanks to their chemical, photophysical, and electrical exceptional properties.[3,15-20] However, some drawbacks are linked, on the one hand, to the synthesis of these complexes, i.e., the need for inert conditions, long reaction times, and toxic solvents and reagents and, on the other hand, to their highly planar and π-conjugated structure quenching the solid-state fluorescence and limiting the solubility in aqueous media.[17,21,22] Consequently, inspired by the BODIPY skeleton, several research groups have investigated the photophysical properties of parent structures featuring O,O-, [6,23-26] N,N-, [9] N,N-, [9,15,27-30] and N,C-based ligands.[31,32] In particular, desymmetryzation of the complexes has been shown to be an efficient strategy for increasing the Stokes shifts (ΔSS), which is of interest for biological applications.[33,34] Recent examples include push-pull dioxaborines, which have been reported to have good fluorescence in solution and excellent sensitivity to the microenvironment,[6,35,36] and β-ketoimine with different substituents,[37] which feature emissive properties in solution, in the solid state, and as aggregates.[38-40] Furthermore, Schiff bases are an emerging class of fluorescent boron ligands for labeling biological and multi-response material.[8,41-45] Asymmetric tetracoordinated boron complexes featuring benzoindole, carbazole, and β-carboline also have excellent photoluminescent properties in solution and in the solid state with color tunability according to the peripheral substituents.[46-52] Desymmetrized 6-membered boron complexes with a chelating pyridine and its derivatives have also been used for their large ΔSS[33,53-58] in bioimaging, solar energy, organic light emitting diode (OLED), and laser applications.[9,27,34,46,57,59-66]

In contrast to BODIPYs, dipyridomethene boron complexes (BF₂-DIPYR) have been scarcely investigated. The first synthesis of the unsubstituted dye and some of its derivatives was

C. Chériaux, C. Figliola, G. Ulrich
Institut de Chimie pour l'Energie
l'Environnement et la Santé (ICPEES)

UMR CNRS 7515
Université de Strasbourg
25 rue Becquerel, 67087 Strasbourg Cedex 02, France

E-mail: figliola@unistra.fr
gulrich@unistra.fr

T. V. Papineau, D. Jacquemin
CNRS
CEISAM UMR 6230

Nantes Université
F-44000 Nantes, France
E-mail: denis.jacquemin@univ-nantes.fr

P. Retailleau
Service de Cristallographie Structurale
ICSN-CNRS Université Paris-Saclay

D. Jacquemin
Institut Universitaire de France (IUF)
F-75005 Paris, France

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Supporting information for this article is available on the WWW under https://doi.org/10.1002/cptc.202400395

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© 2025 The Author(s). ChemPhotoChem published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
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published in the 1990s by Douglass and Boyer,[67,68] while the description of the physical and spectroscopic properties was reported from 2010 in the context of optoelectronic applications.[69,70] Meso-H quinoline and isoquinoline BF₂ complexes have been shown to feature similar properties as BODIPYs, i.e., high molar absorption, good photostability, and efficient green fluorescence emission in solution (Figure 1).[69] Replacing the meso carbon of the dipyridomethene scaffold with a nitrogen (Figure 1) induces an hypsochromic shift of the absorption/emission wavelengths,[71–75] which has been exploited for white organic light-emitting diodes (WOLEDs).[72,75] Meso-CN DIPYR BF₂-complexes, published first by Kubota et al. (Figure 1),[76] have shown dual-state fluorescence (both in solution and in the solid state), and they have been applied in solar cells and as chemosensors for hydrazine.[77,78] Regarding the meso-CH and meso-N BF₂-DIPYR, structure diversification of the efficient derivatives has remained limited.

As part of our ongoing interest in the tetracoordinated boron complexes,[79] we herein report the structure–property relationship for BF₂-DIPYR complexes *via* the substitution of the 3, 7, and 10 positions in accordance with the numbering of the anthracene structure (Figure 1).[69] Our synthetic approaches to functionalize the DIPYR ligands are described, and the challenging preparation of four series of new boron complexes is obtained in good to high yields. The new symmetric and asymmetric mono-, bis- and tris-substituted dyes were thoroughly characterized, including by X-ray diffraction analysis and spectroscopic measurements. The experimental results were all rationalized using *ab initio* calculations.

## 2. Results and Discussion

To establish structure–property relationship, the electron-rich nature of the BF₂-DIPYR was tuned by the introduction of

different electron-donating and -withdrawing aryl groups on the aromatic core, notably on the meso-, 3-, and 7-position.

### 2.1. Modification of the Meso Position

To obtain the targeted BF₂-DIPYR complexes I bearing different aryl substituents at the meso position, three main synthetic approaches were explored (Scheme 1). The first strategy (A) involves the nucleophilic addition of different Grignard reagents to the commercially available dipyridylketone 1[80] followed by reductive dehydroxylation to obtain ligands IV and by the BF₂-complexation (Scheme 1, approach A). Approach B requires the reduction of 1 to the corresponding dipyridylmethanol 2 and halogen substitution to allow cross-coupling reaction (Scheme 1, approach B).[81] According to the approach C, the synthesis of the dipyridylmethene VI is followed by the nucleophilic aromatic substitution of the meso carbon onto different fluoroaryl compounds (Scheme 1, approach C).[82]

The first two synthetic strategies were discarded because neither the reductive dihydroxylation (approach A) nor the cross-coupling reactions (approach B) were accomplished successfully (see the Supporting Information (SI)). However, substitution of the meso position with halogens afforded ligand 3 (R=Cl) and 4 (R=Br)[81,83] in good to excellent yields. Subsequent reaction with BF₃ OEt₂ and Hunig's base in dichloroethane (DCE) at refluxing temperature gave the corresponding complexes 5 and 6 in 79% and 44% yield, respectively, according to previous reports (Scheme 2).[69] Meso-iodo BF₂-DIPYR 10 was also synthetized, but it could not be characterized nor further used in coupling reactions because of the instability of the product (see the SI).

Consequently, we turned our attention toward the synthetic approach C (Scheme 1). Deprotonation of the meso position in 7a–b with LDA followed by nucleophilic aromatic substitution

<figure><img src="image_6.png" /><figcaption>**Figure 1.** Literature and herein targeted BF₂-DIPYR complexes for a detailed structure–property relationship study on these compounds.</figcaption></figure>
<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>**Scheme 1.** Retrosynthetic strategies toward meso-substituted complexes I.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Scheme 2.** Synthesis of complexes 5 and 6.</figcaption></figure>

onto the 2-fluoro pyridine and 4-fluoronitrobenzene successfully yielded ligands **8a** (15%) and **8b** (44%), respectively.[69,82] Following BF₂-complexation dyes **9a–b** were synthesized in 55% and 56% yield, respectively (**Scheme 3**).[69]

Attempts to introduce electron-rich aryl groups at the meso position using the reaction conditions shown in Scheme 3 failed due to the occurrence of many side reactions. However, exploiting the reactivity of the nitro (NO₂) group, complex **9b** was first reduced to the corresponding primary amine **10** with hydrazine on Pd/C, then to the mono- (**11**) and bis-alkylated (**12**) derivative using bromohexane and K₂CO₃ in acetonitrile at reflux (**Scheme 4**).[84]

## 2.2. Modification of the 3 and 3,7 Positions

After attempting the functionalization of the meso position of the DIPYR ligands, the introduction of various substituents on the 3 and 3,7 positions was targeted. Following the literature,[69] nucleophilic aromatic substitution of the organolithium derivative from picoline **13** onto the 2-fluoro-5-bromo-pyridine afforded the mono-brominated dipyridyl ligand **14** in 24% yield, along with

the side-product **15** in 23% yield. Boron complexation of **14** afforded complex **16** in moderate yield (36%) under the same reaction conditions used above (**Scheme 5**). The structural and electronic diversification of **16** was achieved by introducing different aryl groups via Pd-catalyzed Suzuki–Miyaura coupling. As a result, both electron-rich (**17a–b**) and electron-deficient (**17c–d**) BF₂-DIPYR complexes were obtained in 20–97% yields (**Scheme 5**).

The corresponding 3,7-dibromo ligand **19** was obtained from the reduction of ketone **18** under Wolff–Kishner reaction conditions in 75% yield (**Scheme 6**).[83] Following boron complexation, Suzuki–Miyaura cross-coupling on **20** ensured the synthesis of the final compounds **21a–d** in good yields (17–60%) featuring both electron-donating and electron-withdrawing substituents.

## 2.3. Modification of the Meso as well as the 3 and/or 3,7 Positions

Ligand **15**, synthesized as shown in Scheme 5, was complexed to BF₂ and the corresponding symmetric **22** and asymmetric **23** dyes were obtained in 35% and 29% yields, respectively (**Scheme 7**).
<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>Scheme 3. Synthesis of complexes 9a–b.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Scheme 4. Synthesis of complexes 13–15.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Scheme 5. Synthesis of complexes 17a–d.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Scheme 6. Synthesis of complexes 21a–d.</figcaption></figure>
<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /><figcaption>**Scheme 7.** Synthesis of complexes 22 and 23.</figcaption></figure>

Under the Suzuki–Miyaura cross-coupling reaction, complexes **22** and **23** afforded the functionalized derivatives **24a–b** and **25a–c** with boronic acids of different electronic nature in good to excellent yields (**Scheme 8**). The corresponding *p*-CN-phenyl substituted **24c** could not be purified from the reaction impurities and, consequently, it was not further analyzed nor reported in the SI.

The final structures of complexes **5**, **6**, **9a–b**, **10–12**, **16**, **17a–d**, **20**, **21a–d**, **22**, **23**, **24a–b**, and **25a–c** and all the synthetic intermediates were confirmed by NMR spectroscopy (¹H, ¹³C, ¹¹B, and ¹⁹F) and HRMS (see the SI).

Single crystals of **9b**, **21a**, **24a**, and **25a** suitable for X-ray diffraction analysis were obtained from slow recrystallization out of a CH₂Cl₂/octane mixture, and their molecular structures are shown in **Figure 2**. The crystal packings with the corresponding discussion are available in the SI. These complexes adopt C₂ or C₅ point group symmetries along the B'–C10 direction, because of the substitution in C3 and C7 (**21a** and **24a**) and C10 (**9b** and **24a**). The substitution of both the C3 and the *meso* position produces the asymmetry of complex **25a**.

Substitution of the *meso* (**9b**) or both the C3 and C7 positions (**21a**) with a phenyl group does not (or slightly) affect the planar conformation of the DIPYR core, respectively. The combination of the *meso* with one or two substituents on the DIPYR core (**24a** and **25a**) increasingly twists the ligand plane (rmsd from 0.01 to 0.17 Å, Table S5, Supporting Information). Regarding the

central six-membered boracycle, it appears flat in **21a** featuring the boron atom within the mean plane of the ring and quasi-planar in **9b** (Figure 2). The puckering of the borocyclic ring is particularly noticeable in **24a** and **25a**, where the BF₂ group is lying above the aromatic DIPYR plane (0.26 and 0.65 Å, respectively), and the *meso* carbon lies in the mean plane of the central ring in all structures. Consequently, in these two structures, the central ring adopts an intermediate conformation between screw-boat (s.b) and boat (b) (see the results of the puckering analysis in the Table S6, Supporting Information [q close to 112.5° (s.b.) and j close to k x 60° (b)] (Figure 3)).

In terms of bond and angle distances (Table S6, Supporting Information), the C—C and the C—N bond lengths as well as the distorted tetrahedral geometry around the boron atom are consistent with what was observed for anthracene, BODIPY, and previous BF₂-DIPYR complexes.[69,76,79,85]

## 2.4. Photophysical Properties

The photophysical properties of the boron complexes **5**, **6**, **9a–b**, **10–12**, **16**, **17a–d**, **20**, **21a–d**, **22**, **23**, **24a–b**, and **25a–c** were characterized by UV/Vis absorption and emission spectroscopy (**Table 1** as well as Table S2, S3 and S4, Supporting Information). Examples of UV/Vis and emission spectra are shown in **Figure 4** (9a).

<figure><img src="image_17.png" /><figcaption>**Scheme 8.** Synthesis of complexes **24a–b** and **25a–c**.</figcaption></figure>
<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /><figcaption>**Figure 2.** Ortep views of 24a, 21a, 25a, and 9b (from left to right). Ellipsoids of non-H atoms are drawn at 30% of probability. Hydrogen atoms are displayed as small spheres of arbitrary radius. Orthogonal views of the corresponding structures are displayed as capped sticks on their right-hand side.</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>**Figure 3.** Overlay of the four BF₂-DIPYR structures displayed as capped sticks. Atoms of 24a are in yellow, in grey for 21a, in pink for 25a, and in green for 9b.</figcaption></figure>

Meso-substituted BF₂-DIPYR complexes 5, 6, 9a–b, and 10–12 all display well-structured absorption and emission profiles. The main absorption band corresponding to the S₀–S₁ transition is shown in the blue region of the spectrum with extinction coefficients of similar intensity (ε = 10⁴ M⁻¹ cm⁻¹, Table 1). For all compounds within the series, an increase in the solvent polarity induces a small hypsochromic shift of both vibronic transitions, while the intensity of these transitions varies independently of the meso-substituent or solvent nature (Figure 4, Figure S1–S3, Supporting Information).

All complexes of the meso-substituted series show weak green emission with fluorescence quantum yields (φ<sub>F</sub>) of 2–15%. The fluorescence emission of the NO₂-substituted complex 9b is completely quenched due to a change in the nature of the electronic states (vide infra). In contrast, the corresponding (reduced) amino-substituted complexes 10–12 show traces of fluorescence emission in nonpolar solvents. This hints that an intramolecular charge transfer (ICT) takes place in the most polar solvents, leading to a broader band, shifted to higher wavelengths with an increase in the the ΔSS, as compared to nonpolar solvents in which a well-resolved symmetric mirror image of the absorption is obtained (Table 1 and S2, Supporting Information). Compared to the photophysical properties of the unsubstituted BF₂-DIPYR complex in I,<sup>[69]</sup> the introduction of different functional groups at the meso position does not induce a significant change in the absorption profile nor the position of its maximum. However, it induces a significant bathochromic shift in the fluorescence maximum emission, i.e., from 518–523 nm (5 and 6) to 540–568 nm (9a–b and 10–12), while the low φ<sub>F</sub> values are consistent with those previously reported (Table 1 and S2, Supporting Information).<sup>[69]</sup>

Dyes 16, 17a–d, 20, and 21a–d also exhibit the S₀–S₁ transition in the blue region of the visible light with ε in the 10⁴ M⁻¹ cm⁻¹ intensity order, which is doubled going from the mono- to the bis-substituted complexes (Table 1 and S3, Supporting Information). Except for the 3-bromo substituted complex 16 showing a broad absorption profile, complexes
<figure><img src="image_21.png" /></figure>

<table><caption>Table 1. Photophysical data of 5, 6, 9a–b, 10–14, 20a–d, 21a–d, 22, 23, 24a–b, and 25a–c complexes in CH₂Cl₂ at room temperature.</caption><thead><tr><th></th><th>Abs<sub>Max</sub> [nm]</th><th>ε<sub>max</sub> [M<sup>-1</sup> cm<sup>-1</sup>]</th><th>Em<sub>Max</sub> [nm]</th><th>ΔSS [cm<sup>-1</sup>]</th><th>Φ<sub>F</sub> [%]<sup>a)</sup></th><th>τ [ns]</th><th>k<sub>r</sub> [s<sup>-1</sup>]<sup>b)</sup></th><th>k<sub>nr</sub> [s<sup>-1</sup>]<sup>c)</sup></th></tr></thead><tbody><tr><td>5</td><td>490</td><td>1.9 × 10<sup>4</sup></td><td>523</td><td>1287</td><td>2</td><td>1.2</td><td>1.67 × 10<sup>7</sup></td><td>8.17 × 10<sup>8</sup></td></tr><tr><td>6</td><td>488</td><td>1.8 × 10<sup>4</sup></td><td>518</td><td>1187</td><td>3</td><td>1.1</td><td>2.73 × 10<sup>7</sup></td><td>8.82 × 10<sup>8</sup></td></tr><tr><td>9a</td><td>491</td><td>2.2 × 10<sup>4</sup></td><td>548</td><td>2118</td><td>8</td><td>1.3</td><td>6.15 × 10<sup>7</sup></td><td>7.08 × 10<sup>8</sup></td></tr><tr><td>9b</td><td>485</td><td>2.6 × 10<sup>4</sup></td><td>n.d.<sup>d)</sup></td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td>10</td><td>494</td><td>2.0 × 10<sup>4</sup></td><td>540</td><td>2895</td><td>&lt;1</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td>11</td><td>494</td><td>2.1 × 10<sup>4</sup></td><td>553</td><td>2160</td><td>&lt;1</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td>12</td><td>495</td><td>1.8 × 10<sup>4</sup></td><td>567</td><td>2596</td><td>&lt;1</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td>16</td><td>472</td><td>3.1 × 10<sup>4</sup></td><td>519</td><td>1919</td><td>1</td><td>0.4</td><td>2.50 × 10<sup>7</sup></td><td>2.48 × 10<sup>9</sup></td></tr><tr><td>20</td><td>470</td><td>1.2 × 10<sup>4</sup></td><td>516</td><td>1896</td><td>1</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td>17a</td><td>491</td><td>2.0 × 10<sup>4</sup></td><td>506</td><td>2052</td><td>20</td><td>1.9</td><td>1.05 × 10<sup>8</sup></td><td>4.21 × 10<sup>8</sup></td></tr><tr><td>21a</td><td>504</td><td>4.0 × 10<sup>4</sup></td><td>520</td><td>611</td><td>19</td><td>2.6</td><td>7.31 × 10<sup>7</sup></td><td>3.12 × 10<sup>8</sup></td></tr><tr><td>17b</td><td>496</td><td>3.2 × 10<sup>4</sup></td><td>515</td><td>2078</td><td>16</td><td>1.6</td><td>1.03 × 10<sup>8</sup></td><td>5.25 × 10<sup>8</sup></td></tr><tr><td>21b</td><td>520</td><td>5.9 × 10<sup>4</sup></td><td>536</td><td>504</td><td>20</td><td>1.1</td><td>1.82 × 10<sup>8</sup></td><td>7.27 × 10<sup>8</sup></td></tr><tr><td>17c</td><td>487</td><td>2.7 × 10<sup>4</sup></td><td>502</td><td>1947</td><td>17</td><td>1.7</td><td>1.00 × 10<sup>8</sup></td><td>4.88 × 10<sup>8</sup></td></tr><tr><td>21c</td><td>505</td><td>5.0 × 10<sup>4</sup></td><td>517</td><td>460</td><td>19</td><td>2.5</td><td>7.60 × 10<sup>8</sup></td><td>3.24 × 10<sup>8</sup></td></tr><tr><td>17d</td><td>489</td><td>2.2 × 10<sup>4</sup></td><td>505</td><td>648</td><td>19</td><td>1.4</td><td>1.36 × 10<sup>8</sup></td><td>5.79 × 10<sup>8</sup></td></tr><tr><td>21d</td><td>503</td><td>6.2 × 10<sup>4</sup></td><td>517</td><td>538</td><td>25</td><td>2.0</td><td>1.25 × 10<sup>8</sup></td><td>3.75 × 10<sup>8</sup></td></tr><tr><td>22</td><td>505</td><td>2.3 × 10<sup>4</sup></td><td>527</td><td>827</td><td>&lt;1</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td>23</td><td>479</td><td>2.3 × 10<sup>4</sup></td><td>527</td><td>1901</td><td>&lt;1</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td>24a</td><td>510</td><td>4.8 × 10<sup>4</sup></td><td>526</td><td>596</td><td>1</td><td>0.06</td><td>1.67 × 10<sup>8</sup></td><td>1.50 × 10<sup>10</sup></td></tr><tr><td>25a</td><td>496</td><td>3.3 × 10<sup>4</sup></td><td>516</td><td>781</td><td>9</td><td>0.1</td><td>9.00 × 10<sup>8</sup></td><td>9.10 × 10<sup>9</sup></td></tr><tr><td>24b</td><td>527</td><td>5.9 × 10<sup>4</sup></td><td>545</td><td>627</td><td>12</td><td>1.0</td><td>1.20 × 10<sup>8</sup></td><td>8.80 × 10<sup>8</sup></td></tr><tr><td>25b</td><td>504</td><td>4.2 × 10<sup>4</sup></td><td>523</td><td>721</td><td>&lt;1</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td>25c</td><td>498</td><td>4.8 × 10<sup>4</sup></td><td>515</td><td>663</td><td>&lt;1</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr></tbody></table>

<figure><img src="image_22.png" /><figcaption>Figure 4. Absorption (left, conc. = 3.4 × 10⁻⁵ M) and emission (right, $λ_{exc} = 430$ nm, conc. = 1.7 × 10⁻⁶ M) spectra of complex 9a in different solvents.</figcaption></figure>

17a–d, 20 and 21a–d display a structured main absorption band with well-defined vibronic transitions as above, whose intensity is not affected by the dipole moment of the solvent nor the nature

of the substituents. As in the previous series, the absorption maximum exhibits a small hypochromic shift when increasing the solvent polarity. However, the presence of two substituents on

<footnote>a) $Φ_F$ were calculated by using Rhodamine 6 G as a reference ($λ_{exc} = 488$ nm, $Φ_F = 0.88$ in EtOH). b) $k_r$ was calculated following the equation $k_r = Φ_F/(τ * 10^{-9})$. c) $k_{nr}$ was calculated following the equation $k_{nr} = (1-Φ_F)/(τ * 10^{-9})$. d) "n.d." is used when the measurements are not appropriate or the results not reliable.</footnote>
<figure><img src="image_23.png" /></figure>

the BF₂DIPYR core induces a small bathochromic shift of the main absorption band as compared to their mono-substituted counterparts (Figure S4–S6, Supporting Information). Complexes **16**, **17a–d**, **20**, and **21a–d** are all weakly luminescent in solution with $\phi_F$ of 6–32%, large $\Delta SS$ (Table 1 and S3, Supporting Information) increasing with the solvent dipole moment, as well as fluorescence emission band being a well-resolved symmetric mirror-image of its absorption counterpart (Figure S4–S6, Supporting Information). With the exception of the brominated dyes **16** and **20**, substitution of the 3 and/or 3,7 positions induces a bathochromic shift in both the absorption and emission profiles along with similar $\phi_F$ values, which are relatively high compared to complexes **9a–b**.[69] This aspect, which is not specific to DIPYR, but it appear in many fluoroborates,[86] is attributed to a different access to the conical intersection. Complexes **22**, **23**, **24a–b**, and **25a–c** do not differ from this general trend (Table 1 and Figure S7–S8, Supporting Information). Compared to complexes **22** and **24a–b**, the corresponding asymmetric compounds **23** and **25a–c** exhibit a hypochromic shift in both the absorption and emission maximum with a remarkably low $\phi_F$ values (Table 1 and S4, Supporting Information). These results are consistent with those previously recorded for the mono-substitution of the dipyridyl core as well as its meso position (Table 1). Lifetimes ($\tau$) for all the three series of complexes are in the nanosecond order with a significant nonradiative component

(10⁸ s⁻¹) compared to the radiative one (10⁷ s⁻¹, Table 1 and S2–S4, Supporting Information). These values are consistent with those expected for the organic nature of the presented complexes and their low $\phi_F$ values. Regarding the bromo-substituted complexes **6**, **16**, **20**, **22**, and **23**, the low $\phi_F$ values (<1%) are all consistent with the heavy atom substitution enhancing the inter-system crossing (ISC).

## 2.5. Theoretical Calculations

To further characterize the excited state of all synthesized compounds, we have used theoretical calculations, with a protocol detailed in the SI. Key results can be found in Table 2. First, the theoretical 0–0 energies fit reasonably well the measured absorption-emission crossing point. For instance, this crossing point is located at 2.40 eV for **22**, and theory predicts 2.45 eV. This nice match is found for most compounds. The same holds for the band shapes. Illustratively, we display band topologies computed through vibronic calculations for **9a** and **17a** in Figure 5. As can be seen the presence of two peaks in both absorption and fluorescence is nicely reproduced by the calculations, with the relative heights of the two maxima evolving between the two species, though the 0–1 band remains relatively

<table><caption>**Table 2.** Main theoretical results: vertical absorption and emission wavelengths [nm], 0–0 energies [eV], $S_1-T_1$ and $S_1-T_2$ gaps [eV] and related SOC [cm⁻¹].</caption><thead><tr><th></th><th>λ<sup>vert-a</sup> [nm]</th><th>λ<sup>vert-f</sup> [nm]</th><th>ΔE<sup>0-0</sup> [eV]</th><th>ΔS<sub>1</sub>-T<sub>1</sub> [eV]</th><th>SOC [cm<sup>-1</sup>]</th><th>ΔS<sub>1</sub>-T<sub>2</sub> [eV]</th><th>SOC [cm<sup>-1</sup>]<sup>a</sup></th></tr></thead><tbody><tr><td>9a</td><td>431</td><td>479</td><td>2.549</td><td>-0.415</td><td>0.041</td><td>0.102</td><td>0.283</td></tr><tr><td>9b</td><td>428 (462)</td><td>1079</td><td>-</td><td>-0.346</td><td>0.486</td><td>-0.044</td><td>0.785</td></tr><tr><td>10</td><td>432</td><td>508</td><td>2.483</td><td>-0.419</td><td>0.044</td><td>0.152</td><td>0.256</td></tr><tr><td>11</td><td>433</td><td>521</td><td>2.446</td><td>-0.420</td><td>0.045</td><td>0.173</td><td>0.249</td></tr><tr><td>12</td><td>433</td><td>526</td><td>2.430</td><td>-0.420</td><td>0.041</td><td>0.178</td><td>0.250</td></tr><tr><td>16</td><td>426</td><td>489</td><td>2.533</td><td>-0.437</td><td>0.426</td><td>0.102</td><td>1.127</td></tr><tr><td>20</td><td>436</td><td>478</td><td>2.536</td><td>-0.430</td><td>0.450</td><td>0.029</td><td>0.792</td></tr><tr><td>17a</td><td>432</td><td>473</td><td>2.578</td><td>-0.423</td><td>0.037</td><td>0.083</td><td>0.347</td></tr><tr><td>17b</td><td>438</td><td>487</td><td>2.500</td><td>-0.447</td><td>0.024</td><td>0.074</td><td>0.363</td></tr><tr><td>17c</td><td>432</td><td>475</td><td>2.576</td><td>-0.428</td><td>0.045</td><td>0.111</td><td>0.319</td></tr><tr><td>17d</td><td>431</td><td>473</td><td>2.581</td><td>-0.424</td><td>0.037</td><td>0.081</td><td>0.349</td></tr><tr><td>21a</td><td>447</td><td>492</td><td>2.482</td><td>-0.391</td><td>0.040</td><td>0.135</td><td>0.372</td></tr><tr><td>21b</td><td>460</td><td>518</td><td>2.338</td><td>-0.406</td><td>0.017</td><td>0.137</td><td>0.506</td></tr><tr><td>21c</td><td>449</td><td>495</td><td>2.479</td><td>-0.395</td><td>0.040</td><td>0.184</td><td>0.316</td></tr><tr><td>21d</td><td>446</td><td>492</td><td>2.490</td><td>-0.393</td><td>0.035</td><td>0.145</td><td>0.377</td></tr><tr><td>22</td><td>447</td><td>499</td><td>2.452</td><td>-0.394</td><td>0.479</td><td>0.131</td><td>0.807</td></tr><tr><td>23</td><td>437</td><td>490</td><td>2.495</td><td>-0.387</td><td>0.509</td><td>0.122</td><td>1.185</td></tr><tr><td>24a</td><td>457</td><td>503</td><td>2.422</td><td>-0.359</td><td>0.029</td><td>0.198</td><td>0.312</td></tr><tr><td>24b</td><td>471</td><td>528</td><td>2.270</td><td>-0.378</td><td>0.033</td><td>0.200</td><td>0.405</td></tr><tr><td>25a</td><td>443</td><td>488</td><td>2.502</td><td>-0.387</td><td>0.045</td><td>0.182</td><td>0.299</td></tr><tr><td>25b</td><td>449</td><td>499</td><td>2.429</td><td>-0.410</td><td>0.039</td><td>0.179</td><td>0.281</td></tr><tr><td>25c</td><td>448</td><td>499</td><td>2.472</td><td>-0.376</td><td>0.077</td><td>0.216</td><td>0.344</td></tr><tr><td>5</td><td>434</td><td>479</td><td>2.537</td><td>-0.384</td><td>0.092</td><td>0.127</td><td>0.321</td></tr><tr><td>6</td><td>432</td><td>476</td><td>2.553</td><td>-0.367</td><td>0.450</td><td>0.140</td><td>0.640</td></tr></tbody></table>

<footnote>a) See the SI for the various processes used to determine these values.</footnote>
<figure><img src="image_24.png" /></figure>

<figure><img src="image_25.png" /><figcaption>**Figure 5.** Computed absorption and emissions for **9a** (left) and **17a** (right). The green and blue curves correspond to absorption and emission, respectively.</figcaption></figure>

<figure><img src="image_26.png" /><figcaption>**Figure 6.** EDD plots for selected compounds. The blue and red lobes represent regions of decrease and increase of density, respectively. Contour $1 \times 10^{-3}$.
See the SI for all molecules.</figcaption></figure>

too weak for **9a**, as compared to experiment. Nevertheless, these nice matches between theory and experiment provide confidence that the selected level of theory is suited for our purposes.

The electron density difference (EDD) plots are displayed for selected compounds in **Figure 6**. To analyze the experimental outcome, let us start by **9b**, which presents a very specific behavior. In **9a**, the EDD topology is the expected one, with a $\pi-\pi^*$ excitation localized on the core of the dye, with a significant loss of density for the meso carbon atom. In contrast for **9b**, one notices a clear CT character with an electron transfer from the core of the dye to the nitro-phenyl unit. This transition computed at 462 nm (Table 2) has a low oscillator strength, the main band being located at 428 nm and possessing a more standard topology (not shown). This weak absorption can be seen in the experimental spectrum (Figure S2, Supporting Information) that shows a tail at long wavelengths not seen in other compounds. The presence of this specific transition is a clear explanation for the total quenching of the emission in **9b**, since the relaxation of this ICT band would indeed yield a very low-lying state (computed at >1000 nm). This "pure ICT" state was not found in any other compound. Illustratively, we also show the EDD plots of **17b** and **21b** in Figure 6. The topology of the excited state is essentially conserved as compared to **12a**, explaining why all compounds show reasonably similar absorption position and band shapes. Nevertheless, in **17b**, one notices a small contribution from

the thienyl moiety, which induces a slight asymmetry in the excited state. In **21b**, symmetry is logically restored and there is again a slight delocalization of the excited state onto the lateral groups, but this effect remains moderate, consistent with the measured small bathochromic shifts as compared to the parent **16** and **20**.

Table 2 also lists the singlet-triplet gaps as well as the associated spin-orbit couplings, determined on the relaxed excited-state ($S_1$) geometry. The lowest triplet is systematically located ≈0.4 eV below the singlet, but the SOC vary significantly in the series, being rather large (≈0.5 cm⁻¹) for **9b**, **16**, **20**, **22**, **23**, and **6**. These are the species bearing either a nitro group (**9b**) or bromine atom(s) (the others), and this result is therefore unsurprising, since one indeed expects these dyes to be especially prone to ISC. The Cl-bearing **5** displays an intermediate SOC of ≈0.1 cm⁻¹, whereas it is smaller (≈0.05 cm⁻¹) for all other compounds.

### 3. Conclusions and Outlook

In the present work, we report the synthesis of four new classes of tetracoordinated boron complexes based on the $BF_2-DIPYR$ scaffold. To date, only few examples of these dyes have been studied, and no structure-property relationship have been established.
<figure><img src="image_27.png" /></figure>

Although the structural modifications of the DIPYR ligands are challenging, we successfully introduced various aryl substituents on the meso, 3 and 7 positions for the first time and studied their influence on the photophysical properties of the corresponding complexes. Three new synthetic approaches toward the brominated derivatives (16, 20, 21, 23) are described, each with the objective of obtaining starting molecules, which could be efficiently functionalized efficiently through Pd-catalyzed Suzuki–Miyaura cross coupling. The X-ray diffraction of complexes 9b, 21a, 24a, and 25a, featuring a phenyl group (except for 9b with a p-NO₂-phenyl group) on each targeted position, allowed us to evaluate the evolution of the conformation of the BF₂-DIPYR core upon substitution. Indeed, while the addition of a phenyl group on the meso position or at both the 3 and 7 positions does not significantly affect the planar conformation of the BF₂-DIPYR core, the combination of the meso with one or two substituents on the DIPYR core results in a twisting of the aromatic plane inducing the six-membered boracycle to assume boat conformation. Each structural modification was thoroughly characterized by spectroscopic measurements, which were all confirmed using *ab initio* calculations. Regardless the position on the aromatic core and the electronic nature of the substituent, all the four classes of BF₂-DIPYR complexes exhibit comparable photophysical properties (absorption and emission profiles, ε values, φ<sub>F</sub>, etc.). This behavior can be attributed to the conservation of the excited state on the BF₂-DIPYR core, which is largely unaffected by the substituents, always assuming a twisted conformation. This likely impedes any contribution to the emissive excited state.

## Acknowledgements

C.F. thanks the Interdisciplinary Institute HiFunMat for support of the SMARTY starting grant. This work of the Interdisciplinary Institute HiFunMat, as part of the ITI 2021–2028 program of the University of Strasbourg, CNRS and Inserm, was supported by IdEx Unistra (ANR-10-IDEX-0002) and SFRI (STRAT'US project, ANR-20-SFRI-0012) under the framework of the French Investments for the Future Program. The Nantes and Strasbourg teams are indebted to the ANR for support in the framework of the BiBiFlu grant (ANR-21-CE43-0019). C.F. thanks the CNRS for additional financial support. This research used resources of the GLiCID Computing Facility (Ligerien Group for Intensive Distributed Computing, 10.60487/glicid, Pays de la Loire, France).

## Conflict of Interest

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.

**Keywords:** boron complexes · molecular engineering · photophysical analysis · structure–property relationship · TD-DFT calculation

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