Impact of substituents in the N∧N ligand on the emission wavelength of Cu(I) complexes: Insight from experimental and theoretical approach

Three mononuclear Cu(I) complexes [Cu(bop)(PPh3)2][BF4], [Cu(fop)(PPh3)2][BF4] and [Cu (pop)(PPh3)2][BF4] were synthesized, where 2-(5-tert-butyl-1,3,4-oxadiazol-2-yl)pyridine (bop), 2-(5-(trifluoromethyl)-1,3,4-oxadiazol-2-yl)pyridine (fop) or 2-(5-phenyl-1,3,4-oxadiazol-2-yl)pyridine (pop) was used as N∧N chelate ligand and triphenylphosphine was used as ancillary ligand. Several substituents with different electronic effects, such as tert-butyl (t-Bu), trifluoromethyl (CF3) and phenyl (Ph) groups, were introduced into the 1,3,4-oxadiazole moiety of the N∧N chelate ligands. The photophysical properties of the complexes were examined by UV–vis absorption and photoluminescence (PL) spectroscopies. The complex [Cu(fop)(PPh3)2][BF4] with a CF3 group in the N∧N chelate ligand exhibited the lowest energy absorption and emission band. Electrochemical analyses combined with density functional theory (DFT) calculations established that the introduction of electron withdrawing group (CF3) decreases the HOMO–LUMO energy gap, and the introduction of electron donating group (t-Bu) into the 1,3,4-oxadiazole moiety has a similar effect on the emission wavelength as that of the introduction of a phenyl group with a π-conjugation.