Fine-Tuning the Luminescence and HOMO–LUMO Energy Levels in Tetranuclear Gold(I) Fluorinated Amidinate Complexes

Tetranuclear gold­(I) fluorinated amidinate complexes
have been
synthesized and their photophysical properties and structures described.
DFT calculations were carried out to illustrate how a minor change
in the ligand resulted in a loss of emission in the perfluorophenyl
amidinate complex compared with nonfluorinated phenyl amidinate complexes
reported previously. The fluorinated complexes reported here [Au­(ArN)2C­(H)]4 (1, Ar = 4-FC6H4; 2, 3,5-F2C6H3; 3, 2,4,6-F3C6H2; 4, 2,3,5,6-F4C6H) emit in the blue-green
region at 470, 1, 478, 2, 508, 3, and 450 nm, 4, by excitation at ca. 375 nm at room
temperature with nanosecond lifetimes. The emissions observed at 77
K in the solid state show structured emission for complexes 1 and 2, with a vibrational spacing of ca. 1200
and 1500 cm–1, corresponding to the vibrational
modes of the amidinate ligand. The pentafluorophenyl derivative 5, Ar = C6F5, shows no photoluminescence
in the solid state nor in the solution. This result is different from
results in which the pentafluorophenyl group is attached to a phenylpyridine
ligand in an Ir­(III) complex and other organics. This quenching appears
to be related to a nonradiative de-excitation process caused by the
ππ*−πσ* crossover in the excited state
of the pentafluorophenyl amidinate ligand. With increasing numbers
of fluorine atoms, there is a progressive decrease in the contribution
of the amidinate ligands to the corresponding HOMO orbital. There
also is a slight decrease in the ligand contribution to the LUMO with
increased numbers of fluorine atoms and an exchange of the character
of the orbitals of the gold centers.