Cyclic Carbaporphyrin Arrays.

Two cyclic carbaporphyrin arrays (trimer and tetramer ) were synthesized from a dibrominated carbaporphyrin precursor () via a one-pot Yamamoto-type coupling. Single-crystal X-ray diffraction analyses revealed that and contain three and four covalently linked carbaporphyrin (formally dicarbacorrole) units, respectively. Trimer adopts a roughly planar conformation and tetramer adopts an up-and-down zig-zag conformation. Both and contain a [n]cyclo-meta-phenylene ([n]CMP) core, namely, [6]- and [8]CMP for and , respectively. Transient absorption (TA) anisotropy and pump-power-dependent excited-state decay studies provided evidence for excitation energy transfer (EET) within both trimer and tetramer . The exciton energy hopping (EEH) times were estimated to be 18 and 35 ps for and , respectively, as inferred from pump-power-dependent TA measurements. Since the center-to-center distances between adjacent carbaporphyrin units are similar in and , the different EEH times are attributed to differences in the orientation of the transition dipoles in these two congeneric arrays. The orientation factor κ, the key parameter defining the Förster resonance energy transfer efficiency, was calculated to be 2.15 and 1.03 for and , respectively, a finding that supports the shorter excitation energy hopping time seen in the case of trimer . To our knowledge, this is the first time that covalently linked cyclic carbaporphyrin arrays were synthesized using a single carbaporphyrin as the starting point and that EET between carbaporphyrin subunits constrained within a well-defined polycyclic framework has been correlated with structural differences.