Coordinate bond- and hydrogen bond-assisted electron transfer strategy towards the generation of photochromic metal phosphites

Abstract

Grafting conjugated dipyridine derivatives, BPB (1,4-bis(pyrid-4-yl)benzene) and BPBP (4,4′-di(pyridin-4-yl)-1,1′-biphenyl), to a metal phosphite system yields two hybrid zincophosphites [Zn₂(HPO₃)₂(BPB)]·0.5DMF (1, DMF = N,N-dimethylformamide) and [H₂BPBP]₃·[Zn₅Cl₈(HPO₃)₄]·2H₂O (2). 1 shows a hybrid layer with the inorganic zincophosphite chains as building blocks. 2 shows isolated pentanuclear Zn clusters in which four phosphite moieties as a bridging ligand, together with eight chloride ions as terminal ligands, bond with five Zn ions to produce anionic inorganic clusters. The negative charge was compensated by the protonated dipyridine derivative species, which was located in the intercluster voids and interplays with the adjacent clusters via N–H⋯O–P hydrogen bonds. Interestingly, 1 and 2 show photochromism driven by the coordinate bond- and hydrogen bond-assisted electron transfer (ET) under the stimulus of Xe lamp irradiation in ambient conditions. Although 1 and 2 had similar constituents in terms of electron donors (EDs) and electron acceptors (EAs), they exhibited distinct photochromic mechanism with ligand-to-ligand ET (LLET) for 1 and proton-coupled ET (PCET) for 2. Unlike the majority of previous hybrid photochromic materials supported by photo-responsive species, the photochromism in 1 and 2 was based on the ET between phosphite and non-photochromic dipyridine derivative units. Our work provides a general strategy towards the design of photochromic hybrid phosphites by integrating conjugated dipyridine- or polypyridine derivative units with metal phosphites.