Theoretical study of switching characteristics of molecular tweezers based on bis(Zn-salphen)

A series of novel tweezers based on bis(Zn-salphen) complex is theoretically studied. Density functional theory (DFT) method is used to investigate the switchable properties of terpy/bis(Zn-salphen) complex (1, terpy=2,2':6',2″-terpyridine) and Br-phtpy/bis(Zn-salphen) complex (2, Br-phtpy=4'-bromophenyl-2,2':6',2″-terpyridine). In this study, the free tweezers 1 and 2 can be converted from a "W" open form to a "U" closed form upon Ru(III) coordination. The switching performances were characterized by ¹H NMR and absorption spectra. DFT calculations were carried out using a B3LYP-D3 functional and def2-SVP basis set for all atoms. ¹H NMR spectra showed that terpyridine protons had an obvious upfield shift during complexation with RuCl₃. The absorption spectrum was observed in the closed tweezers with a significant red shift and a decreased oscillator strength. In addition, the tweezers were reopened by introducing molecule pyrazine in the "U"-shaped conformation to form a host-guest system. The recognition ability of two Zn-salphen complexes was studied by geometrical optimization and absorption spectra.