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.