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...