Molecular dynamics of covalently-linked multi-porphyrin arrays

The molecular dynamics of a series of diphenylethyne- or diphenylbutadiyne-linked porphyrin arrays in solution at 296 K have been investigated using high-field proton NMR spectroscopy. Measurements of the direct dipolar interactions between vicinal proton pairs located on the pyrrole and phenylene moieties provide information about the flexibility of the para-substituted diphenylethyne linker. The magnitude of the 2-fold rotational barrier is about 3.3 kJ/mol, with the most stable form having coplanar phenylene moieties. This is in rough agreement with the behavior reported for diphenylacetylene. The diphenylethyne linker undergoes large bending motions, causing the phenylene proton-proton axis to be bent out of the plane of the more remote porphyrin ring by an average angle of 26 degrees. This large bending motion presumably is distributed at sites over the entire linker, including bending at the ethyne and possible out-of-plane bending of the phenylene group relative to the attached porphyrin. The diphenylbutadiyne linker is bent by an average angle of 31 degrees and has a much lower rotational barrier.