Excitation wavelength dependent emission of silver(i) complexes with a pyrimidine ligand

Abstract

Silver(I) complexes with 2-amino-4-phenyl-6-methylpyrimidine (L), [Ag₃L₂(H₂O)_0.55(NO₃)₃]_n (1) and [Ag₂L₂(PPh₃)₂(NO₃)₂] (2), have been synthesized and structurally characterized. In the solid state the free ligand L demonstrates dual fluorescence: a high energy band at 375 nm is the emission of the monomer L while a low-energy band at 450 nm can be attributed to the emission of the tautomeric dimer, L₂-imino, formed in the excited state double proton transfer (ESDPT) process. In the solid state the complexes demonstrate excitation wavelength dependent photoluminescence originating from three emission mechanisms (hereafter abbreviated as A, B and C for complex 1; A′, B′ and C′ for complex 2). The spectroscopic and computational results reveal that these emission mechanisms can be assigned to metal + nitrate anion to ligand L charge transfer phosphorescence (mechanisms A and A′), excimer ¹π–π* fluorescence (mechanism B), fluorescence of L₂-imino dimeric units formed due to ESDPT (mechanism B′) and ligand-centered ¹π–π*/¹n–π* fluorescence (mechanisms C and C′). The luminescence of complex 1 at λ_ex = 340 nm shows thermochromic behavior; the emission color dramatically changes from blue-green at 77 K to yellowish green at 175 K and white at 300 K.