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 β-pyrrolic positions have been determined using ¹³C spin-lattice relaxation time measurements. In deuteriochloroform solution at 303 K, the overall reorientation of all three porphyrins investigated occurs with diffusion coefficients of ∼1 × 10⁹ s^-1. In porphyrins with only hydrogen at the β-pyrrolic positions, the meso phenyl rings undergo rotations about their single bonds to the porphyrin with diffusion coefficients of ∼4 × 10⁹ s^-1. Introduction of methyl substituents at the β-pyrrolic positions adjacent to the phenyl rings reduces these motions, but only to ∼1 ×10⁹ 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 acceptors through meso aryl groups. Thus, the internal librational motions may affect rates of photoinduced electron and energy transfer, even in relatively "rigid" molecular constructs.