Oligophosphine-thiocyanate Copper(I) and Silver(I) Complexes and Their Borane Derivatives Showing Delayed Fluorescence

The series of chelating phosphine ligands, which contain bidentate P2 (bis[(2-diphenylphosphino)phenyl] ether, DPEphos; 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene, Xantphos; 1,2-bis(diphenylphosphino)benzene, dppb), tridentate P3 (bis(2-diphenylphosphinophenyl)phenylphosphine), and tetradentate P4 (tris(2-diphenylphosphino)phenylphosphine) ligands, was used for the preparation of the corresponding dinuclear [M(μ2-SCN)P2]2 (M = Cu, 1, 3, 5; M = Ag, 2, 4, 6) and mononuclear [CuNCS(P3/P4)] (7, 9) and [AgSCN(P3/P4)] (8, 10) complexes. The reactions of P4 with silver salts in a 1:2 molar ratio produce tetranuclear clusters [Ag2(μ3-SCN)(t-SCN)(P4)]2 (11) and [Ag2(μ3-SCN)(P4)]22+ (12). Complexes 7–11 bearing terminally coordinated SCN ligands were efficiently converted into derivatives 13–17 with the weakly coordinating –SCN:B(C6F5)3 isothiocyanatoborate ligand. Compounds 1 and 5–17 exhibit thermally activated delayed fluorescence (TADF) behavior in the solid state. The excited states of thiocyanate species are dominated by the ligand to ligand SCN → π(phosphine) charge transfer transitions mixed with a variable contribution of MLCT. The boronation of SCN groups changes the nature of both the S1 and T1 states to (L + M)LCT d,p(M, P) → π(phosphine). The localization of the excited states on the aromatic systems of the phosphine ligands determines a wide range of luminescence energies achieved for the title complexes (λem varies from 448 nm for 1 to 630 nm for 10c). The emission of compounds 10 and 15, based on the P4 ligand, strongly depends on the solid-state packing (λem = 505 and 625 nm for two crystalline forms of 15), which affects structural reorganizations accompanying the formation of electronically excited states.