Excited-State Turn-On of Aurophilicity and Tunability of Relativistic Effects in a Series of Digold Triazolates Synthesized via iClick

iClick reactions between Au­(I) acetylides PPh₃Au–CCR, where R = nitrophenyl (PhNO₂), phenyl (Ph), thiophene (Th), bithiophene (biTh), and dimethyl aniline (PhNMe₂), and Au­(I)–azide PPh₃AuN₃ provide digold complexes of the general formula R-1,5-bis-triphenylphosphinegold­(I) 1,2,3-triazolate (Au₂-R). Within the digold triazolate complexes the Au­(I) atoms are held in close proximity but beyond the distance typically observed for aurophilic bonding. Though no bond exists in the ground state, time-dependent density functional theory interrogation of the complexes reveals excited states with significant aurophilic bonding. The series of complexes allows for tuning of the excited-state “turn-on” of aurophilicity, where ligand to metal charge transfer (LMCT) induces the aurophilic bonding. Complexes containing ligand-localized excited states, however, do not exhibit aurophilicity in the excited state. As a control experiment, a monogold complex was synthesized. The computed excited state of the monogold species exhibited LMCT to the gold ion as in the dinuclear cases, but without a partnering gold ion only a distinct N–Au–P bending occurs, revealing a potential mechanism for the excited-state turn-on of aurophilic bonding. Analysis of the steady-state electronic spectra indicates that LMCT states are achievable for compounds with sufficiently strong electron-donating ligands, and in digold complexes this is associated with enhanced fluorescence, suggestive of an aurophilic interaction.