Evidence of Enhanced Conjugation in ortho-Arylene Ethynylenes with Transition Metal Coordination

Abstract

The effective conjugation of ortho and ortho-alt-para-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.

Conjugated metallorganic macrocycles: opportunities for coordination-driven planarization of bidentate, pyridine-based ligands

“…These "tails" span into the visible region, giving the complexes a colored appearance. As has been observed for 1,2-bis(2′-pyridinylethynyl)benzene-based systems, 20 fluorescence in ligands 1 and 2 is quenched gradually with the addition of AgOTf and abruptly upon introduction of PdCl 2 (PhCN) 2 .…”

“…In a previous study, we have shown that introduction of transition metals to 1,2-bis(2′-pyridinylethynyl)benzene-based systems can lead to significant electronic changes in a conjugated backbone. 20 These types of changes likely require significant differences between the most favorable conformation of the ligand and the planar complex. While this can be achieved in longer conjugated systems, small, conjugated molecules such as ligand 1, ligand 2, and 1,2-bis(2′-pyridinylethynyl)benzene are unlikely to display dramatic, conformation-driven electronic shifts upon metal coordination.…”

“…Metal complexation in each of these examples provides coordination-enforced planarity. This behavior can be used to alter the properties of conjugated oligomers/polymers in which these structures are incorporated 20 and in the development of transition metal sensors. 21 1,2-Bis(2′-pyridinylethynyl)benzenes are a particularly interesting study in geometry.…”

Locking the Coplanar Conformation of π‐Conjugated Molecules and Macromolecules Using Dynamic Noncovalent Bonds

“…As demonstrated in the UV‐vis titration experiments, the addition of Ag + led to a red shift in the absorption spectrum of oligomer 44 , indicating an elongated conjugation length resulted from the coplanar conformation (Figure b). This strategy was further employed by Bowling and co‐workers to enhance the conjugation of ortho ‐arylene ethynylene derivatives …”

Effects of Halogen and Hydrogen Bonding on the Electronics of a Conjugated Rotor

“… , In most solid-state materials, where p -PEs are employed for their efficient charge-transport properties − or sensing abilities, − bond rotation is minimized by close packing. Coplanarity of the conjugated backbone can be encouraged via a number of tactics, such as intramolecular hydrogen bonding or metal coordination. − Such features can be exploited in solution for the development of sensors or molecular switches . Conversely, steric impediments, covalent bridges, or intramolecular attractions that force twisting of the backbone can prevent access to the coplanar conformation. − Additionally, efforts have been made to design materials in which crystal packing tendencies and low barriers to bond rotation are exploited to provide freely rotating units that function as molecular rotors in the solid state. , …”