Harnessing Fluorescence versus Phosphorescence Ratio via Ancillary Ligand Fine-Tuned MLCT Contribution

A series of gold(I) alkynyl-diphosphine complexes (XC₆H₄C₂Au)PPh₂ - spacer - PPh₂(AuC₂C₆H₄X); spacer = - C₂(C₆H₄)_nC₂ - (A1, n = 2, X = CF₃; A2, n = 2, X = OMe; A3, n = 3, X = CF₃; A4, n = 3, X = OMe), - (C₆H₄)_n - (B5, n = 3, X = OMe; B6, n = 4, X = OMe) were prepared, and their photophysical properties were investigated. The luminescence behavior of the titled compounds is dominated by the diphosphine spacer, which serves as an emitting ππ∗ chromophore. The complexes exhibit dual emission, comprising low and high energy bands of triplet (phosphorescence) and singlet (fluorescence) origins, respectively. The electron-donating characteristics of ancillary groups X significantly affect the LLCT/MLCT contribution of both alkynyl ligand and the metal center into the lowest lying excited state. In turn, it substantially influences the rate of intersystem crossing k_isc S₁ → T_m (m ≥ 1) that allows for tuning the ratio fluorescence vs phosphorescence without alteration of the chromophore core. Quantum chemical analysis of the excited states at the CC2/TZVP level of theory supports the experimental observations. Crystalline complexes A2 and B5, which exhibit dominating phosphorescence emission, for the first time for the unsupported gold luminophores were probed as molecular oxygen sensors (max K_SV1 = 63 atm^-1).