Impact of substituents in the NN 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.