Pyrimidine Ring Motion Correlated with Electron Transfer at the Copper(II)/(I) Coordination Site Immobilized on Au Electrode Surface

Introduction Repetitive molecular motion controllable around room temperature is a promising approach for the molecular device, especially when it is correlated with electron transfer. We employed copper(I) complexes containing an asymmetric pyrimidyl moiety as a ligand, which takes two coordination forms, inner (i) and outer (o) isomers, derived from the inversion of the pyrimidine ring (Figure 1). We have focused on the differences in the redox potential of the two isomers, and succeeded in control electron transfer through ON/OFF switching of the pyrimidine ring inversion. As we can deduce the rotational trajectory of this system, the dynamics of isomerization can be tuned by steric effect around the rotating pyrimidine. Also, we can extend the steric effect on the isomerization by tethering various sizes of substituent on pyrimidine. In these aspects, our system has rich designable factors, whereas it has a rather simple structure compared with previous models of molecular machines. In this research, we focused on the pyrimidine ring motion in the molecule fixed on the electrode surface, to clarify the following points: i) can our system work in a molecularly separated phase in a heterogeneous system? ii) can the motion be detected by the gold electrode through redox of Cu(II/I)? and iii) does any specific effect exist on dynamics by immobilization? For this purpose, we prepared two kinds of copper(II/I)-pyrimidine complexes, 1•PF6 and 2•PF6 with a thiolate unit to fix them on the gold electrode surface. I evaluated their dynamics in solution and on the electrode surface mainly by electrochemical methods.