Polymorphism and Thermally Induced Structural Transformation in Semiconducting Cd(II) Coordination Polymers

Three polymorphs of a semiconducting Cd(II)-thiolate coordination polymer with the formula [Cd(2-SNPh)₂]_<i>n</i> (2-HSNPh = 2-naphthalenethiol) have been reported. Single-crystal X-ray structural analysis revealed that <b>KGF-51</b> adopts a one-dimensional (1D) structure composed of single-strand (-Cd-S-)_<i>n</i> ladder chains, whereas <b>KGF-71</b> adopts a 1D architecture with double-strand (-Cd-S-)_<i>n</i> chains assembled through S···S interactions. In contrast, microcrystal electron diffraction analysis demonstrated that <b>KGF-79</b>, obtained through thermal treatment of <b>KGF-51</b> and <b>KGF-71</b>, exhibits the formation of a 1D structure comprising triple-strand (-Cd-S-)_<i>n</i> chains, also formed via S···S interactions. This thermally induced, irreversible structural transformation to <b>KGF-79</b> is accompanied by a transition from a nonpolar structure to a polar structure, leading to the emergence of second harmonic generation activity. Furthermore, time-resolved microwave conductivity measurements and first-principles calculations demonstrated that all three polymorphs exhibit photoconductivity, which is attributed to charge transport along the inorganic (-Cd-S-)_<i>n</i> chains via a hopping mechanism.