Strategy for an Effective Eco-Optimized Design of Heteroleptic Cu(I) Coordination Polymers Exhibiting Thermally Activated Delayed Fluorescence

The new series of copper(I) coordination polymers [Cu(N-N)(mu-PTA)](n)[PF6](n) {N-N = dmbpy (1), bpy (2), ncup (3), and phen (4)} were generated by straightforward reaction in solution or through a mechanochemical route, of [Cu(MeCN)(4)][PF6] with 1,3,5-triaza-7-phosphaadamantane (PTA) and the corresponding polypyridines, namely, 5,5 '-dimethyl-2,2 '-bipyridine (dmbpy), 2,2 '-bipyridine (bpy), 2,9-dimethyl-1,10-phenanthroline (ncup), and 1,10-phenanthroline (phen). The compounds were obtained as air-stable solids and fully characterized by IR, NMR spectroscopy, and elemental analyses. The molecular structures were confirmed by single-crystal X-ray diffraction analysis (for 1, 2, and 4), revealing infinite one-dimensional (1D) linear chains driven by mu-PTA N,P-linkers. All tested Cu(I) polymeric compounds show emission at room temperature, which was attributed to thermally activated delayed fluorescence (TADF). Evidence of the involvement of the excited singlet state in the emission process is presented. Comparing the photophysical properties of 1 and 2 as well as 3 and 4, of which 1 and 3 have a stiffened structure, by introducing a methyl group to one of the ligands, we demonstrate how TADF properties depend on molecular rigidity. It is shown that stiffening of the structure reduces the flattening distortion around the Cu(I) center in the (MLCT)-M-3 state. As a result, the Delta E(S-1-T-1) energy gap becomes smaller and the fluorescence quantum yield increases without significantly extending the emission lifetime. In particular, the Delta E(S-1-T-1) values for complexes 1 and 3 are among the shortest reported in the scientific literature, 253 and 337 cm(-1), and the TADF lifetimes are tau(300 K) = 5.7 and 4.2 mu s, respectively. The fluorescence quantum yields for these complexes are measured to be Phi(PL)(300 K) = 70 and 80%.