MOLECULAR-DYNAMICS OF COVALENTLY-LINKED MULTI-PORPHYRIN ARRAYS

The molecular dynamics of a series of diphenylethyne- or diphenylbutad iyne-linked porphyrin arrays in solution at 296 K have been investigat ed using high-field proton NMR spectroscopy. Measurements of the direc t dipolar interactions between vicinal proton pairs located on the pyr role 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 t he behavior reported for diphenylacetylene. The diphenylethyne linker undergoes large bending motions, causing the phenylene proton-proton a xis to be bent out of the plane of the more remote porphyrin ring by a n 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 relat ive to the attached porphyrin. The diphenylbutadiyne linker is bent by an average angle of 31 degrees and has a much lower rotational barrie r.