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

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 BF(2) (boron difluoride) subunits. Three BODIPY arrays, containing two, three, and four BF(2) subunits, namely 4·2BF(2), 4·3BF(2) and 4·4BF(2), could thus be isolated from the reaction of 4 with BF(3)·Et(2)O 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 BF(2) complexation are conformationally flexible and unsymmetric in nature as deduced from similar analyses. All four products, namely 4, 4·2BF(2), 4·3BF(2) and 4·4BF(2), were characterized by means of single crystal X-ray diffraction analyses. Tetramer 4·4BF(2) gives rise to a higher extinction coefficient (by 2.5 times) relative to the bis- and tris-BODIPY arrays 4·2BF(2) and 4·3BF(2). This was taken as evidence for stronger excitonic coupling in the case of 4·4BF(2). 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.