A versatile bis-porphyrin tweezer host for the assembly of noncovalent photoactive architectures: a photophysical characterization of the tweezers and their association with porphyrins and other guests.

A bis(Zn(II)-porphyrin) tweezer host with anthracene components as apex and side-arms has been synthesized. Mono- (pyridine) and bidentate (4,4'-bipyridine) guests were used as models for single and double axial coordination inside the cavity, respectively. A series of dipyridylporphyrin guests with different substitution patterns and excited-state energy levels have association constants with the tweezers that are of the order of 10(6) M(-1), which is indicative of complexation with the inside of the cavity. This complexation can only occur upon an important distortion of the cavity that opens the bite by about 30 %. This characteristic, in conjunction with their ability to reduce the bite distance by rotation around single bonds, makes these porphyrin tweezers amongst the most versatile so far reported, with tuning of the bite distance in the range of approximately 5-20 Angstroms. Energy transfer to the free-base guest within the triporphyrin complex is nearly quantitative (95-98 %) and the rates of transfer are consistent with a Förster mechanism that is characterized by a reduced orientation factor.