# Design of Heavy-Atom Free Dipyridomethene Boron Complexes for Singlet Oxygen Emission with Induced Chirality

Carlotta Figliola, Camille Chériaux, Nicolas Oppé, Lilian Estaque, Pascal Retailleau, Nicolas Vanthuyne, Grégory Pieters, Denis Jacquemin, Gilles Ulrich

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Carlotta Figliola, Camille Chériaux, Nicolas Oppé, Lilian Estaque, Pascal Retailleau, et al. Design of Heavy-Atom Free Dipyridomethene Boron Complexes for Singlet Oxygen Emission with Induced Chirality. Chemical Communications, In press, Online ahead of print. 10.1039/D5CC06879G. hal-05416931

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# Design of Heavy-Atom Free Dipyridomethene Boron Complexes for Singlet Oxygen Emission with Induced Chirality

Carlotta Figliola,<sup>*a</sup> Camille Chériaux,<sup>a</sup> Nicolas Oppé,<sup>a</sup> Lilian Estaque,<sup>b</sup> Pascal Retailleau,<sup>c</sup> Nicolas Vanthuyne,<sup>d</sup> Grégory Pieters,<sup>b</sup> Denis Jacquemin,<sup>*e,f</sup> Gilles Ulrich<sup>*a</sup>

The straightforward synthesis of the first difluoro dipyridomethene boron complexes as heavy-atom free photosensitizers is described. These molecules show good singlet oxygen generation ($\Phi_{\Delta}= 10$ to 47%) and significant nonplanar arrangement inducing axial and boron-centered chirality.

Efficient population the triplet excited-state finds application in many fields, including photochemistry,<sup>1,2</sup> light-emitting and electrical devices,<sup>3</sup> and photodynamic therapy (PDT).<sup>4</sup> Functionalization of organic fluorophores with transition metals or halogens<sup>5,6</sup> is the most popular strategy to favor the intersystem crossing (ISC) from the lowest singlet state ($S_1$) to a triplet ($T_n$) excited state by enhancing the spin orbital coupling (SOC).<sup>7</sup> Beside this, other approaches to obtain efficient heavy-atom free photosensitizers (PS) have been developed. Theoretical and experimental studies confirm that nonplanar molecular arrangements,<sup>8</sup> as in twisted units in a dimer<sup>9</sup> or helix in polyaromatic systems,<sup>10,11</sup> has a fundamental role in enhancing the SOC in organic dyes.<sup>12</sup> This approach is especially valid in the difluoroboron dipyridomethene (BF<sub>2</sub>-DIPYR) dyes. Boron (B) complexation induces a distortion of the central six-membered boracycle adopting a boat-like conformation, which is detrimental to the fluorescence but favorable to the phosphorescence emission (Figure 1).<sup>13,14</sup> Similarly, several BF<sub>2</sub>-DIPYR dimers have shown very small $S_1-T_n$ gap ($\Delta E_{ST}$) and efficient ISC (Figure 1).<sup>15</sup>

<figure><img src="image_3.png" /><figcaption>Figure 1. Literature examples of BF<sub>2</sub>-DIPYR populating the triplet excited-state and molecular targets studied in the present work.</figcaption></figure>

In an effort to develop tetracoordinated BF<sub>2</sub> complexes as efficient PDT agents,<sup>16,17</sup> cognizant that heavy atoms reduce significantly the compatibility of the PS with medical applications,<sup>18</sup> we report herein the first meso-CN BF<sub>2</sub>-DIPYR with axial (**1a-b**) and stereogenic boron-centered or central (**2a-b**) chirality (Figure 1). In our study, the nonplanar molecular arrangement was designed strategically to increase the distortion of DIPYR central core and to favor the population of the nearest $T_n$ state and its emission. These two classes were fully characterized by X-ray diffraction, enantiomeric resolution, photophysical and chiroptical analyses, and *ab initio* investigations.

Complex **1a** was prepared according to literature (see experimental section in the ESI¹).<sup>19,16</sup> Its fused analogue **2a** is obtained by an *in-situ* Suzuki-Miyaura cross coupling on ligand **3** followed by BF<sub>2</sub> complexation (Scheme 1). Under the same conditions, ligands **4-OMe** and **4-MOM** were first obtained and afforded subsequently **1b** and **2b** in high yields (Scheme 1).

<footnote><sup>*a</sup> 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 and gulrich@unistra.fr</footnote>

<footnote><sup>b</sup> Département Médicaments et Technologies pour la Santé (DMTS), Université Paris-Saclay, CEA, INRAE, SCBM, Gif-sur-Yvette 91191, France</footnote>

<footnote><sup>c</sup> Service de Cristallographie Structurale, ICSN-CNRS Université Paris-Saclay, 1 Avenue de la Terrasse, Bât.27, 91198 Gif-sur-Yvette Cedex, France</footnote>

<footnote><sup>d</sup> Aix-Marseille Univ, CNRS, Centrale Med, FSCM, Chiropole, Marseille, France</footnote>

<footnote><sup>e</sup> Nantes Université, CNRS, CEISAM UMR 6230, F-44000 Nantes, France. E-Mail: denis.jacquemin@univ-nantes.fr</footnote>

<footnote><sup>f</sup> Institut Universitaire de France (IUF), F-75005 Paris, France</footnote>

<footnote>*Supplementary Information available. See DOI: 10.1039/x0xx00000x</footnote>
<figure><img src="image_4.png" /><figcaption>Scheme 1. Synthesis of chiral complex **2a** and **1-2b** with the insert showing the ORTEP view of the molecular structure of **2b**.</figcaption></figure>

All products were fully characterized by NMR spectroscopy and HRMS spectrometry. ¹⁹F and ¹¹B NMR spectra of **1a** show two doublets of quartets (¹⁹F) and two doublets (¹¹B), suggesting a distinct coupling of each fluorine atom with the B atom (Figure S1-S2, ESI†). In the ¹³C NMR spectrum the signal multiplicities for C(6), C(12) and C(16) indicate a through-space coupling with one F atom. Indeed, as observed in the X-ray structures, the intramolecular distances F-C(6), F-C(12) and C(16) are within the van der Waals radius (for H···F, 2.67 Å, and for C···F, 3.17 Å).²⁰ The ¹³C and ¹⁹F spectra of **1b** also show similar through-space couplings between the two F atoms and the naphthalene system (Figure S1-S2, ESI†). These results indicate that, upon complexation to BF₂, the bulky -OMe hinders the rotation around the C-C axis and induces an axial chirality making the two fluorine atoms enantiotopic and leading to the formation of two atropisomers. Because of the presence of only one F atom bound to the B atom, the ¹⁹F and ¹¹B NMR spectra of complexes **2a-b** show one quartet and one doublet, respectively (ESI†). These data suggest the formation of a racemic mixture for both **2a** and **2b** because the nucleophilic substitution of the hydroxyl group on the neighbouring BF₂ is expected to give two molecules with the same connectivity but opposite three-dimensional shape.

The molecular structures of the racemic mixture **1a** and **2b** were obtained by X-ray diffraction after slow evaporation of CHCl₃ and octane/CH₂Cl₂, respectively (ESI†). The central six-hetero-membered ring adopts an intermediate conformation between 'screw-boat' and 'boat' with the -BF₂ lying outside the aromatic cores in both **1a** and **2b** structures.¹³,¹⁷,¹⁶ The dihedral angle between the two pyridines is more pronounced in **1a** than in **2b** (15.4° vs 12.8°). In **1a**, the planar anisole substituent is slightly repelled by the B atom, but it remains quasi-orthogonal to the DIPYR core (88.7°), with the OMe group pointing away from the concavity. In **2b**, the naphthalene core makes a dihedral angle of 23.2° with the DIPYR core and it is slightly twisted (8.2°).

The racemic mixture of **1a-b** and **2a-b** were resolved by HPLC on a chiral stationary-phase under isocratic conditions. The optical activity of the enantiomers was measured, and large specific rotation values of equal degrees and opposite signs were obtained for each pair of enantiomers (ESI†). The resolution of the racemates **1a-b** allowed the isolation of these heteroaromatic atropisomers.²¹ The rotational barrier in **1a** was evaluated by DFT at 100 kJ.mol⁻¹, whereas for **1b**, the rotation involves very large deformation of the naphthyl and yields a significantly larger barrier. Consistently, racemization of **1a** occurred rapidly at 25°C and the half-life time was evaluated as 16 min in a CH₂Cl₂/n-heptane mixture. With a bulkier substituent,²² **1b** was resolved and enantiomers did not racemize even after several days at 25°C. Complexes **2a-b** were also resolved by chiral HPLC and their absolute configuration was assigned by comparison of experimental and calculated ECD spectra (ESI†). Notably, the first eluted enantiomer of *rac-2a* and *rac-2b* was assigned as the (*S*)-enantiomer and the first eluted atropisomer of **1b** as *aS*, considering the BF₂ centre of **1b** as prochiral to the B atom of **2b**.

The photophysical properties of **1a-b** and **2a-b** were obtained by UV/vis absorption and emission spectroscopy in both solution and the solid state, first on the racemic mixtures (Table 1 and Table S1) and, on each enantiomer (Table S2). Complexes **1a-b** and **2a-b** absorb all in the blue region (450-480 nm in CH₂Cl₂) of the visible spectrum, with extinction coefficients (ε) of the main band, corresponding to the S₀-S₁ transition, of similar intensity (1.5-3.0 x 10⁴ M⁻¹cm⁻¹ in CH₂Cl₂). The increase of the solvent polarity (from *p*-xylene to acetonitrile) induces a small hypsochromic shift of the λₐbs₅, independently of the structure (from 6 to 23 nm, Table S1). This negative solvatochromism might be explained by a decrease of the dipole moment magnitude in going from the ground to the excited state, as observed in similar complexes.¹⁶ TD-DFT calculations indeed predict such changes for all dyes here (ca. -1.5 to -1.7 D). The main absorption band of the fused systems **2a-b** is bathochromically shifted and less defined compared to that of **1a-b**, consistent with an improved π-delocalization in the former dyes (Figure S5, S7, S9 and S11). Furthermore, for these systems, the ε values are found as approximately half of those of their axially chiral counterparts.

Concerning the emission properties, complexes **1a-b** and **2a-b** are all emissive in solution with quantum yields (Φ<sub>F</sub>) values dropping in polar solvents (Table S1). The fluorescence emission band of **2a-b** appears as the symmetric mirror-image of the absorption band, and it becomes broader and less resolved in going to more polar environments. In contrast, **1a-b** display a large and unstructured fluorescence band, suggesting a significant geometrical relaxation in the excited state. Consistent with the emissive
properties of this class of molecules,¹⁶ **1a-b** and **2a-b** fluoresce in the solid state with maxima in the 500–600 nm range and moderate $\phi_F$ (Table S1 and Figure S12).

<table><caption>Table 1. Photophysical data of racemic mixtures of **1a-b** and **2a-b** in CH₂Cl₂.</caption><thead><tr><th></th><th>λ<sub>Abs</sub><br>(nm)</th><th>ε<sub>max</sub><br>(M<sup>-1</sup>·cm<sup>-1</sup>)</th><th>λ<sub>Em</sub> (nm)</th><th>ΔSS (cm<sup>-1</sup>)</th><th>Φ<sub>F</sub><br>(%)<sup>[a]</sup></th><th>Φ<sub>Δ</sub><br>(%)<sup>[b]</sup></th><th>τ<sub>F</sub><br>(ns)<sup>[c]</sup></th><th>τ<sub>ρ</sub><br>(μs)<sup>[d]</sup></th><th>k<sub>f</sub> (s<sup>-1</sup>)</th><th>k<sub>nr</sub> (s<sup>-1</sup>)</th></tr></thead><tbody><tr><td>1a</td><td>446</td><td>2.5 x 10<sup>4</sup></td><td>542<sup>[e]</sup><br>541<sup>[f]</sup>, 451(S<sub>1</sub>)<sup>[g]</sup>, 515(T<sub>1</sub>)<sup>[h]</sup></td><td>3971</td><td>13</td><td>10</td><td>2.3</td><td>94</td><td>5.65 x 10<sup>7</sup></td><td>3.78 x 10<sup>8</sup></td></tr><tr><td>2a</td><td>460</td><td>1.8 x 10<sup>4</sup></td><td>520<sup>[e]</sup><br>514<sup>[f]</sup>, 495<sup>[g]</sup>, 532<sup>[h]</sup></td><td>2508</td><td>27</td><td>28</td><td>6.3</td><td>n.d.</td><td>4.29 x 10<sup>7</sup></td><td>1.16 x 10<sup>8</sup></td></tr><tr><td>1b</td><td>449</td><td>2.9 x 10<sup>4</sup></td><td>511<sup>[e]</sup><br>506<sup>[f]</sup>, 452(S<sub>1</sub>)<sup>[g]</sup>, 515(T<sub>1</sub>)<sup>[h]</sup></td><td>2802</td><td>28</td><td>18</td><td>4.4</td><td>58</td><td>6.38 x 10<sup>7</sup></td><td>1.64 x 10<sup>8</sup></td></tr><tr><td>2b</td><td>486</td><td>1.5 x 10<sup>4</sup></td><td>529<sup>[e]</sup><br>528<sup>[f]</sup>, 505<sup>[g]</sup>, 513<sup>[h]</sup></td><td>1673</td><td>34</td><td>47</td><td>4.7</td><td>n.d.</td><td>7.25 x 10<sup>7</sup></td><td>1.41 x 10<sup>8</sup></td></tr></tbody></table>

The ECD spectra of (S)/(R)-**2a**, (aS)/(aR)-**1b** and (S)/(R)-**2b** were recorded in acetonitrile (ESI†). All ECD spectra share mirror symmetry across the entire region of electromagnetic radiation and overlap well with the absorption spectra (ESI†). Enantiomers (S)/(R)-**2a-b** show mirror-imaged circularly polarized luminescence (CPL) properties with dissymmetry factors ($g_{lum}$) ranging from 10⁻⁵ to 10⁻⁴ (Figure S19), which are relatively small compared other BF₂ complexes.²³

<figure><img src="image_5.png" /><figcaption>Figure 2. A) S₁ and T₁ emission spectra of dyes **1a** and **1b** in MeTHF at 298K and 77K; B) EDD plot of **2b**; C) table reporting the $\Delta_{ST}$ of **2b** computed at SCS-CC2 levels and M06-2X SOC elements. In the EDD plot the blue and red regions correspond to decrease and increase of density upon absorption, respectively.</figcaption></figure>

The fluorescence lifetimes are in ns range, yet the nonradiative decay is one order of magnitude faster (10⁷ vs 10⁸).¹³,¹⁹,¹⁷,¹⁶ This along with the low $\phi_F$ values (10-30%, Table 1 and S1) prompted us to search for the triplet excited-state population. Because the photophysical properties of each enantiomer do not differ from those of the racemic mixture (Table S2, Figure S13-15), phosphorescence emission and singlet oxygen generation (¹O₂) were done on the latter. The steady-state spectrum at 77K of complexes **1a-b** in a degassed and aerated methyl-tetrahydrofuran (MeTHF) solution show both S₁ and T₁ emissions. As expected, the S₁ emission band at low temperature is shifted hypsochromically and it is more structured than at 298K. The T₁ emission band is shifted bathochromically (≈60 nm) and it reproduces the structural profile of the S₁ (Figure 2 and ESI†). Furthermore, the phosphorescence spectra of **1a-b** as well as their capacity of generating ¹O₂ ($\phi_Δ$) were obtained. By setting a delay of 0.1 ms before recording the emission spectra, both complexes in a degassed MeTHF solution show a well-defined band resembling that of the steady-state T₁ emission. The lifetime of this emission is found in the µs range, that is 94 (1a) and 58 µs (1b) and, consequently, it was assigned to the phosphorescence emission (Figure 2 and Table 1). Furthermore, in an aerated chloroform solution, they generate ¹O₂ in 10% and 18% yield, respectively (Table 1 and ESI†). Under the same experimental conditions, the steady-state emission of **2a-b** does not show two distinct bands for the emissions from S₁ and T₁ and the recorded phosphorescence band is

<footnote>[a] $\phi_F$ were calculated by using Rhodamine 6G as a reference ($\lambda_{exc} = 488$ nm, $\phi_F = 0.88$ in EtOH); [b] $\phi_Δ$ were obtained by direct measurement of the singlet oxygen emission in CHCl₃ and with phenalenone ($\phi_Δ = 0.95$ in CHCl₃) as reference; [c] Fitted (biexponential, average weighted) fluorescence lifetimes with $\chi^2$ in the range of 0.9-1.2; [d] Fitted (average weighted with $\chi^2$ in the range of 0.9-1.2) phosphorescence lifetime in degassed MeTHF solution at 77K with a delay before measure of 50 ns and emission wavelength at 520 nm; [e] Fluorescence $\lambda_{Em}$ in CH₂Cl₂ solution at 298 K; [f] Fluorescence $\lambda_{Em}$ in MeTHF solution at 298 K; [g] Steady-state $\lambda_{Em}$ of S₁ and/or T₁ in both degassed and aerated MeTHF solution at 77 K; [h] Phosphorescence $\lambda_{Em}$ at 77 K in degassed MeTHF solution with a delay before spectra recording of 0.5 ms.</footnote>
shifted only by a few nanometers (7 and 18 nm, respectively, ESI†) resembling the steady-state spectra. In these circumstances, the lifetime of this emission could not be obtained with sufficient confidence. Beyond this, complexes **2a-b** generate ¹O₂ in higher yields than **1a-b** 28% and 47% yields, respectively (Table 1 and ESI†).

To obtain additional insights *ab initio* calculations were performed. The first interesting result is the minimal geometries of the compounds. The anisole is nearly perpendicular to the DIPYR core S₀ state of **1a** preventing effective conjugation, yet significant geometric relaxation takes place in both S₁ and T₁, in which the anisole significantly rotates becoming more conjugated with the dye core, consistent with rather large Stokes shift (ESI†). In contrast, the structure of **2a** is less flexible, and the torsion of the side ring is essentially state-independent. The use of a methoxynaphthyl substituent in **1b** increases significantly the steric hindrance, inducing deformation. The electron density difference (EDD) plots for absorption are consistent with the above analysis (Figure 2 and ESI). The excited state is more delocalized in **2a** than in both **1a-b**, whose substituent does not affect the absorption process. Additionally, the EDD indicates a slightly more delocalized excited state in **2b** than in **2a**. This suggests that **1a-b** should have alike absorption wavelength, whereas small additive redshifts should be observed for **2a** and next **2b**, which fits the measured trends (Table 1 and Table S1). The fact that the ordering of the emission wavelengths differs from that of the absorption is related to the above-described geometric effects. It is also valuable to obtain more quantitative theory-experiment comparisons, which can be achieved through comparisons of 0-0 energies and band shapes. Theory provides accurate 0-0 energies, with a mean absolute error of 0.04 eV as compared to the experiment, a remarkable result (ESI†). To investigate the efficiency of the ISC process which is related to ¹O₂ production, the Sₙ-Tₙ energy gaps and SOC have been computed on the optimal S₁ structure. TD-DFT and SCS-CC2 deliver significantly different absolute values for the gaps, but similar trends (ESI†). Considering the most refined SCS-CC2 values, S₁-T₁ gaps are almost constant in the series (~0.2 eV), and the related SOCs are rather small (<0.10 cm⁻¹) but in **2b** (0.19 cm⁻¹, Figure 2). The S₁-T₂ data indicate that the second triplet is higher than the lowest singlet, but with significant changes in the series. Indeed, for **1a**, this second triplet is 0.36 eV higher with a 0.24 cm⁻¹ SOC, hinting that the ISC in that compound is likely limited to the S₁-T₁ process. In both **2a** and **1b**, the gap decreases to 0.20 eV, with moderate SOC (0.32 and 0.37 cm⁻¹), whereas in **2b**, the second triplet is rather close (0.15 eV only) with the largest SOC of the series (0.43 cm⁻¹) hinting a significant contribution from the S₁-T₂ process due to thermal activation. In short, according to theory ISC is the most favored in **2b**, and the least probable in **1a**, which fits and might explain the experimental φ<sub>Δ</sub>.

In conclusion, we prepared two novel classes of meso-CN BF₂-DIPYR complexes featuring axial and central chirality, respectively. Besides the straightforward and high-yielding synthesis, the structural and photophysical characterization confirmed that the chirality-inducing nonplanar molecular arrangement of the aromatic DIPYR plane has been proved a valuable strategy to promote the population of the excited triplet state and the ¹O₂ generation. As both singlet (in solution and solid state) and photosensitizers, these dyes are currently tested in imaging-guided PDT application for antimicrobial treatment.

**Conflicts of interest**

There are no conflicts to declare.

**Acknowledgement**

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. D.J. thanks the HPC resources from the GLiCID Computing Facility (Ligerien Group for Intensive Distributed Computing, 10.60487/glicid, Pays de la Loire, France).

**Data availability**

The data supporting this article have been included as part of the electronic supplementary information (ESI).

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