Aryl ring rotation in porphyrins. A carbon-13 NMR spin-lattice relaxation time study

Overall tumbling and internal rotational motions in porphyrins bearing meso aryl substituents and, in some cases, flanking alkyl groups at the beta-pyrrolic positions have been determined using C-13 spin-lattice relaxation time measurements. In deuteriochloroform solution at 303 K, the overall reorientation of all three porphyrins investigated occurs with diffusion coefficients of similar to 1 x 10(9) s(-1). In porphyrins with only hydrogen at the beta-pyrrolic positions, the meso phenyl rings undergo rotations about their single bonds to the porphyrin with diffusion coefficients of similar to 4 x 10(9) s(-1). Introduction of methyl substituents at the beta-pyrrolic positions adjacent to the phenyl rings reduces these motions, but only to similar to 1 x 10(9) s(-1). Thus, significant internal motions are present in both types of molecules. These motions occur on the time scale of many photoinduced electron and energy transfer processes in porphyrins covalently linked to electron or energy donors or accepters through meso aryl groups. Thus, the internal librational motions may affect rates of photoinduced electron and energy transfer, even in relatively ''rigid'' molecular constructs.