Zn(II)-induced ground-state pi-deconjugation and excited-state electron transfer in N,N-bis(2-pyridyl)amino-substituted arenes

The synthesis and X-ray crystal structures of two N,N-bis(2-pyridyl)amino (dpa)-substituted aromatic systems (Ar-dpa) 1 (Ar = 4,4'-disubstituted trans-stilbene) and 2 (Ar = 1,4-disubstituted benzene) and their ZnCl(2) complexes (1/ZnCl(2) and 2/ZnCl(2)) are reported. The fluoroionophoric behavior of 1-2 in response to Zn(II) in acetonitrile also has been investigated. In addition, compound 3DPA has been prepared and served as a pi-deconjugated model for 1DPA. The observed crystal structures for 1/ZnCl(2) and 2/ZnCl(2) could be divided into two distinct types, the planar and the twisted forms, depending on the aryl-dpa (C(ph)-NC(3)) dihedral angle. The twisted form is more favorable for these complexes unless the arene has a strong "push-pull" character. Nonetheless, the degree of pi-conjugation between the N-pyridyl and the N-aryl group is reduced in both complex forms when compared with the free ligands. Such a Zn(II)-induced pi-deconjugation not only directly affects the internal charge transfer (ICT) fluorescence of the dpa-substituted stilbenes but also facilitates the occurrence of photoinduced electron transfer (PET) from the stilbene donor to the dpa/Zn(II) acceptor. The PET process is particularly important in accounting for the observed Zn(II)-induced fluorescence quenching for 1DPA as well as 3DPA.