Molecular Motions of .beta.-Carotene and a Carotenoporphyrin Dyad in Solution: A Carbon-13 NMR Spin-Lattice Relaxation Time Study

Internal rotational motions in carotenoid polyenes and porphyrins are of interest because they can mediate energy and electron transfer between these two moieties when the pigments are joined by covalent bonds. Such internal motions can affect the performance of synthetic model systems which mimic photosynthetic antenna function, photoprotection and photoinduced electron transfer. Analysis of C-13 NMR spin-lattice relaxation times (T-1) yields information concerning both overall tumbling of molecules in solution and internal rotations about single bonds. Relaxation time and nuclear Overhauser effect data have been obtained for beta-carotene and two related molecules, squalane and squalene, for zinc meso-tetraphenylporphyrin, and for a dyad consisting of a porphyrin covalently linked to a carotenoid polyene through a trimethylene bridge, Squalane and squalene. which lack conjugated double bonds, behave essentially as limp string, with internal rotations at least as rapid as overall isotropic tumbling motions. In contrast, beta-carotene reorients as a rigid rod, with internal motions which are too slow to affect relaxation times. Modeling it as an anisotropic rotor yields a rotational diffusion coefficient for motion about the major axis which is 14 times larger than that for rotation about axes perpendicular to that axis. The porphyrin reorients more nearly isotropically and features internal librational motions about the single bonds to the phenyl groups. The relaxation time data for the carotenoporphyrin are consistent with internal motions similar to those of a medieval military flail, consisting of a rigid, rod-like carotenoid and ball-like porphyrin linked by a flexible chain of single bonds. Internal reorientation about this linkage is approximately 100 times faster than triplet-triplet energy transfer from the porphyrin to the carotenoid, which is mediated by such motions.