Cyclic Carbaporphyrin Arrays

Abstract

Two cyclic carbaporphyrin arrays (trimer <b>6</b> and tetramer <b>7</b>) were synthesized from a dibrominated carbaporphyrin precursor (<b>5</b>) via a one-pot Yamamoto-type coupling. Single-crystal X-ray diffraction analyses revealed that <b>6</b> and <b>7</b> contain three and four covalently linked carbaporphyrin (formally dicarbacorrole) units, respectively. Trimer <b>6</b> adopts a roughly planar conformation and tetramer <b>7</b> adopts an up-and-down zig-zag conformation. Both <b>6</b> and <b>7</b> contain a [n]cyclo-meta-phenylene ([n]CMP) core, namely, [6]- and [8]CMP for <b>6</b> and <b>7</b>, respectively. Transient absorption (TA) anisotropy and pump-power-dependent excited-state decay studies provided evidence for excitation energy transfer (EET) within both trimer <b>6</b> and tetramer <b>7</b>. The exciton energy hopping (EEH) times were estimated to be 18 and 35 ps for <b>6</b> and <b>7</b>, respectively, as inferred from pump-power-dependent TA measurements. Since the center-to-center distances between adjacent carbaporphyrin units are similar in <b>6</b> and <b>7</b>, the different EEH times are attributed to differences in the orientation of the transition dipoles in these two congeneric arrays. The orientation factor κ₂, the key parameter defining the Förster resonance energy transfer efficiency, was calculated to be 2.15 and 1.03 for <b>6</b> and <b>7</b>, respectively, a finding that supports the shorter excitation energy hopping time seen in the case of trimer <b>6</b>. 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.