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

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 a monomer L while a low-energy band at 450 nm is the emission of a 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 the complex 1; A’, B’ and C’ for the 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 the complex 1 at λex = 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.