Evidence of Enhanced Conjugation in <i>ortho</i>-Arylene Ethynylenes with Transition Metal Coordination

The effective conjugation of <i>ortho</i> and <i>ortho-alt-para</i>-arylene ethynylenes, with appropriately positioned pyridine and pyrazine heterocycles, increases upon binding to Ag­(I) and Pd­(II) cations. Significant bathochromic shifts in the electronic spectra, witnessed upon introduction of these metal bridges, are consistent with enhanced electron delocalization in the unsaturated backbone. Control studies suggest that this electronic behavior is attributable exclusively (in the case of Ag­(I)) or partially (in the case of Pd­(II)) to conformational restrictions of the conjugated backbones