Steric Control over Supramolecular Polymer Formation in trans-1,2-Bis(4-pyridyl)ethylene Adducts of Zinc Xanthates

Steric control over supramolecular aggregation is demonstrated for a series of adducts formed between Zn(S₂COR)₂ and trans-1,2-bis(4-pyridyl)ethylene, whereby supramolecular zigzag polymers are found when R is small, that is, Et (1) and n-Bu (2), but only bimetallic aggregates could be formed when R=Cy (3). This results in different coordination geometries: fourcoordinateN₂S₂ zinc in 1 and 2, and five-coordinateNS₄ zinc in 3, a feature which greatly influences photophysical responses in the solid state.When excited in the UV region, {Zn(S₂COEt)₂L}∞ (1) and {[Zn(S₂COCy)₂]₂L} (3) produce broad luminescence in the visible region. Five-coordinate 3 produces more broad and intense luminescence than four-coordinate 1. The configuration interaction singles (CIS) and post-Hartree-Fock (HF) calculations for 1 and 3 indicate the A-band excited state is responsible for the observed luminescence which is strongly associated with charge transfer from S and Zn.