Two π-Electrons Make the Difference: From BODIPY to BODIIM Switchable Fluorescent Dyes

Abstract

(aza-)BODIPY dyes (boron dipyrromethene dyes) are well-established fluorophores due to their large quantum yields, stability, and diversity, which led to promising applications including imaging techniques, sensors, organic (opto)electronic materials, or biomedical applications. Although the control of the optical properties in (aza-)BODIPY dyes by peripheral functional groups is well studied, we herein present a novel approach to modify the 12 π-electron core of the dipyrromethene scaffold. The replacement of two carbon atoms in the β-position of a BODIPY dye by two nitrogen atoms afforded a 14 π-electron system, which was termed BODIIM (boron diimidazolylmethene) in systematic analogy to the BODIPY dyes. Remarkably, the BODIIM dye was obtained with a BH₂ -rigidifying entity, which is currently elusive and highly sought after for the BODIPY dye class. DFT-Calculations confirm the [12+2] π-electron relationship between BODIPY and BODIIM and reveal a strong shape correlation between LUMO in the BODIPY and the HOMO of the BODIIM. The modification of the π-system leads to a dramatic shift of the optical properties, of which the fluorescent emission is most noteworthy and occurs at much larger Stokes shift, that is, ≈500 cm^-1 in BODIPY versus >4170 cm^-1 in BODIIM system in all solvents investigated. Nucleophilic reactivity was found at the meso-carbon atom in the formation of stable borane adducts with a significant shift of the fluorescent emission, and this behavior contrasts the reactivity of conventional BODIPY systems. In addition, the reverse decomplexation of the borane adducts was demonstrated in reactions with a representative N-heterocyclic carbene to retain the strongly fluorescent BODIIM compound, which suggests applications as fully reversible fluorescent switch.

Keywords: BODIIM; BODIPY; bisimidazoles; fluorescent dyes; switchable fluorescence.

Scheme 1

Structures of compounds 1–6.

Scheme 2

Key conditions and reagents. i) 1 equiv. nBuLi, THF, −30 °C, 30 min, then 0.45 equiv. benzoyl chloride, 0 °C to rt, 1 h, 65 %. ii) 0.5 equiv. BH₂Cl⋅SMe₂, DCM, 0 °C to rt, 15 min, then THF, 15 min, then hexanes, 95 %. iii) 2.05 equiv. nBuLi, THF, −78 °C to rt, overnight, then 1.05 equiv. methyl benzoate, rt, 1 h, aqueous work‐up with brine, 95 %. iv) 1.05 equiv. Na[N(SiMe₃)₂], THF, −78 °C to rt, 3 h, then 3 equiv. MeI, rt, 12 h, aqueous workup with brine, 96 %. v) 4 equiv. KC₈, THF, 0 °C, 15 min, 95 %.

Figure 1

Molecular structures of compounds 6 (left) and 10 (right). The phenyl entity is represented in the wireframe model. Thermal ellipsoids are drawn at the 50 % probability level. Solvent molecules (chloroform) in the structure of 10 are omitted. Selected bond lengths (Å) and bond angles (°) for 6: B1−N1 1.546(1), B1−N3 1.550(1), C8−N3 1.356(1), C1−C8 1.413(1), C1−C2 1.417(1), C1−C14 1.490(1), C2−N1 1.357(1), C8‐C1‐C14 121.95(9), C2‐C1‐C14 122.73(9), C2‐C1‐C8 115.17(9); for 10: B1−N1 1.553(3), B1−N3 1.553(3), C8−N3 1.326(3), C1−C8 1.512(3), C1−C2 1.513(3), C1−C14 1.534(3), O1−C1 1.412(3), C2−N1 1.323(3), C8‐C1‐C14 107.90(18), C2‐C1‐C14 109.06(18), C2‐C1‐C8 108.56(18).

Figure 2

Selected molecular orbitals for BODIPY (A, B) and BODIIM (C, D) systems as obtained by DFT calculations (B3LYP‐D3/TZVP/COSMO(THF)). The addition of 2 π‐electrons to the LUMO in BODIPY systems retains the HOMO in the novel BODIIM system with preservation of the characteristic atomic orbital contributions.

Figure 3

Comparison of the optical properties of BODIPY reference compound B (data reported in Ref. 17, recorded in toluene) and BODIIM 6 (recorded in toluene). i) Solution of compound 6 in toluene at ambient light. ii) Solution of compound 6 in toluene with UV‐lamp excitation (λ≈366 nm). λ _abs,max: wavelength at the maximum of absorbance, λ _em,max: wavelength at the maximum emission intensity, Φ_F: fluorescence quantum yield.

Figure 4

Experimental absorption spectra of 6. Insert: Calculated absorption spectra of 6 using empirically corrected CAM‐B3LYP/cc‐pVDZ TD‐DFT excitation energies at PBE0/def2‐TZVP structures and a Gaussian broadening of FWHM=0.25 eV.

Figure 5

Experimental fluorescence spectra of compound 6. Insert: Calculated emission spectra of 6 at wB97XD/def2‐TZVP TD‐DFT level of theory. Oscillatory strengths were normalized to 1.0 at λ _max in THF.

Scheme 3

Key conditions and reagents. i) 1.3 equiv. BH₃⋅SMe₂ or MesBH₂, toluene, rt, 30 min, 75 % (12) or 70 % (14). ii) 1 equiv. NHC, C₆D₆, rt, 5 % (12) or quantitative (14). Mes=2,4,6‐Me₃C₆H₂.

Figure 6

Molecular structure of compounds 12 (left) and 14 (right). The phenyl and mesityl entity are represented in the wireframe model. Thermal ellipsoids are drawn at the 50 % probability level. Carbon bound hydrogen atoms are omitted. The illustrated molecule for 12 is located on a crystallographic mirror plane containing B1, C1, B2 and the phenyl entity. Selected bond lengths [Å] and bond angles [°] for 12: B1−N2 1.5443(16), C1−C2 1.4975(14), C2−N2 1.3330(15), C1−B2 1.679(2), C2‐C1‐C2' 108.16(13), C2‐C1‐B2 106.34(9), N2‐B1‐N2' 105.03(14); for 14: B1−N2 1.5519(17), B1−N4 1.5543(18), C1−B2 1.734(2), C2−N2 1.3368(16), C8−N4 1.3340(16), C1−C2 1.5033(18), C1−C8 1.4989(17), N2‐B1‐N4 104.24(10), C8‐C1‐C2 107.40(10).

Figure 7

Upon irradiation (UV‐lamp, λ≈366 nm) NMR‐solutions of compound 6 and 14 (0.1 m in C₆D₆) show intense green fluorescence (6) and weak blue fluorescence (14).