Controlled assembly of a bicyclic porphyrinoid and its 3-dimensional boron difluoride arrays

Abstract

A fully conjugated cryptand-like bicyclic porphyrinoid ligand 4, incorporating three carbazole linkages and four dipyrrin moieties, was prepared via the acid-catalysed condensation of an extended 2,2'-bipyrrole analogue containing a central carbazole moiety and 3,4-diethyl-2,5-diformylpyrrole in 79% isolated yield. This new cryptand-like system acts as an effective ligand and allows for complexation of BF₂ (boron difluoride) subunits. Three BODIPY arrays, containing two, three, and four BF₂ subunits, namely 4·2BF₂, 4·3BF₂ and 4·4BF₂, could thus be isolated from the reaction of 4 with BF₃·Et₂O in the presence of triethylamine at 110 °C, albeit in relatively low yield. As prepared, bicycle 4 is characterized by a rigid C ₂ symmetric structure as inferred from VT NMR spectroscopic analyses. In contrast, the three BODIPY-like arrays produced as the result of BF₂ complexation are conformationally flexible and unsymmetric in nature as deduced from similar analyses. All four products, namely 4, 4·2BF₂, 4·3BF₂ and 4·4BF₂, were characterized by means of single crystal X-ray diffraction analyses. Tetramer 4·4BF₂ gives rise to a higher extinction coefficient (by 2.5 times) relative to the bis- and tris-BODIPY arrays 4·2BF₂ and 4·3BF₂. This was taken as evidence for stronger excitonic coupling in the case of 4·4BF₂. All three BODIPY-like arrays proved nearly non-fluorescent, as expected given their conformationally mobile nature. The efficiency of reactive oxygen species (ROS) generation was also determined for the new BODIPY arrays of this study.

Scheme 1. Synthesis of the figure-of-eight monocyclic porphyrinoid 3 and the bicyclic porphyrinoid 4 and its corresponding BODIPY arrays (4·2BF₂, 4·3BF₂, and 4·4BF₂). Reaction conditions: a) TFA, DDQ, DCM, RT; b) BF₃·Et₂O, TEA, toluene, 110 °C. β-Pyrrolic ethyl groups are omitted in 3, 4, and the BF₂ derivatives for clarity.

Fig. 1. Comparative ¹H NMR spectra (aromatic region) of ligand 4 recorded in THF-d₈ at a) −70 °C and b) 25 °C and the ¹⁹F NMR spectra of c) 4·4BF₂, d) 4·3BF₂, and e) 4·2BF₂ recorded in THF-d₈ at −80 °C.

Fig. 2. Single crystal X-ray structures of 4. a) Top and b) side views. Atom colour key: carbon (light grey), nitrogen (light purple), hydrogen (white). Hydrogen atoms that are not involved in intramolecular interactions, β-pyrrolic alkyl groups and solvent molecules have been omitted for clarity. c) and d) Molecular packing arrangement in stick and space-filling form, respectively, showing six methanol and two THF molecules encapsulated within a self-assembled dimer.

Fig. 3. Single crystal X-ray structures of BODIPY arrays. Atom colour key: carbon (light grey), nitrogen (light purple), hydrogen (white), boron (light pink), fluorine (yellow green). Hydrogen atoms that are not involved in intramolecular interactions, β-pyrrolic alkyl groups and solvent molecules have been omitted for clarity. a) 4·2BF₂, b) 4·3BF₂, and c) 4·4BF₂.

Fig. 4. UV-Vis absorption and emission spectra of ligand 4, 4·2BF₂, 4·3BF₂, and 4·4BF₂ recorded in toluene at room temperature.

Fig. 5. Plots of the change in absorbance of DPBF at 414 nm against irradiation time (λ_irr = 520 nm) seen in the presence of the BODIPY arrays of this study in CH₂Cl₂ at room temperature. RB in methanol was used as a refence standard. These plots were used to calculate the singlet oxygen (¹O₂) quantum yields (cf. eqn S1 in the ESI†).