Synthesis and Carbon-13 NMR Longitudinal Relaxation Time Studies of Porphyrin Dimers Possessing Conformational Fluctuation

Abstract

The synthesis and carbon-13 NMR longitudinal relaxation times (T₁) studies on the calixarene-substituted porphyrin dimers possessing conformational fluctuation were carried out for the first time. The T₁ values were measured under proton-noise-decoupling conditions by the inversion-recovery technique. The T₁ values for a calix[4]arene bridged porphyrin dimer 1 decrease with the increase in the solvent polarity. This implies that the T₁ values reflect the diffusion rate. As to a motion of the phenol unit, there are two different motions for phenol units in 1 and 2: a seesaw motion around a C-2 to C-2’ axis (A) and a rotational motion around a C-1 to C-4 axis (B). The T₁ values indicate that in 1 motion (B) is predominant over motion (A). This shows that motion (A) is specifically suppressed because of the hydrogen-bonding interactions in 1.