Photochromic pyridylfulgenates and their cadmium complexes

Abstract

In this work, two novel photochromic pyridylfulgenate molecules, Fulg1 and Fulg2, were successfully synthesized as photosensitive ligands. Their further reactions with Cd²⁺ species resulted in four interesting cadmium–fulgenate frameworks, namely, [Cd₂(Fulg1)₄(DMF)]_n (1), [Cd(Fulg1′)(HCO₂)]_n (2), [Cd(Fulg1′)₂(H₂O)₂]_n (3), and {[Cd(Fulg2)₂(H₂O)]·H₂O}_n (4). It was observed that the molecular packing of both Fulg1 and Fulg2 was a 1D infinite zigzag supramolecular chain linked via hydrogen bonding interactions. X-ray crystallographic studies revealed the crystal packing of the four cadmium–fulgenate frameworks as follows: a 2D 4,4-linked topological network for both 1 and 4, a 1D fishtrap-like cage chain with an inner cavity diameter of 6.3 Å for 2, and a 1D metallalooper-like chain for 3. Upon irradiation using a UV lamp, either Fulg1 or Fulg2 exhibited an excellent photochromic property with a rapid crystalline color conversion from colorless to blue for Fulg1 and to pale purple for Fulg2, which were confirmed by spectroscopic investigation and theoretical calculations. In contrast, their cadmium complexes (compounds 1–4), exhibited extremely low photosensitivity. The experimental observations demonstrated that the inflexible conformation of chromophore units caused by the coordinated cadmium centers and the resulting diverse structural features of these pyridylfulgenate compounds are mainly responsible for their significant differences in the photosensitivity.