Protonation-induced chromism of pyridylethynyl-appended [core+exo]-type Au8 clusters. Resonance-coupled electronic perturbation through π-conjugated group.

A series of [core+exo]-type Au8 clusters bearing two alkynyl ligands on the exo gold atoms ([Au8(dppp)4(C≡CR)2](2+), 2-6) were synthesized by the reaction of [Au8(dppp)4](2+) (1) with alkynyl anions. Although the C≡C moieties directly attached to the Au8 units did not affect the optical properties arising from intracluster transitions, the pyridylethynyl-bearing clusters (4-6) exhibited reversible visible absorption and photoluminescence responses to protonation/deprotonation events of the terminal pyridyl moieties. The chromism behaviors and proton-binding constants of these clusters were highly dependent on the relative position of the pyridine nitrogen atom, such that the 2-pyridyl (4) and 4-pyridyl (6) isomers showed more pronounced responses than the 3-pyridyl isomer (5). These results suggest that the resonance-coupled movement of the positive charge upon protonation is involved in the optical responses, where the formation of extended charged resonance structures causes significant perturbation effects on the electronic properties of the Au8 unit and also contributes to the high binding affinities.