High-resolution structure and conformational dynamics of rigid, cofacially aligned porphyrin-bridge-quinone systems as determined by NMR spectroscopy and ab initio simulated annealing calculations

The high-resolution solution structure and dynamics of a cofacially aligned porphyrin-phenylene-quinone compound have been determined using ¹H NMR spectroscopy and simulated annealing calculations. Members of this class of π-stacked assemblies feature a 1,8-naphthyl pillaring motif that enforces sub van der Waals interplanar separations between juxtaposed porphyryl, aromatic bridge, and quinonyl components of the donor-spacer-acceptor compound; this structural motif gives rise to a comprehensive set of structurally significant NOE signatures that can be used as constraints in quantitative structural calculations. Examination of such data using ab initio simulated annealing analytical methods shows that 5-[8′-(4″-[8‴-(2″″,5‴-benzoquinonyl) -1‴-naphthyl]-1″-phenyl)-1′-naphthyl]-10, 20-diphenylporphyrin displays an unusual degree of conformational homogeneity in the condensed phase, and represents a rare example where such an analysis determines unequivocally a single unique structure in solution.