Experimental and spin-orbit coupled TDDFT predictions of photophysical properties of three-coordinate mononuclear and four-coordinate binuclear copper(I) complexes with thioamidines and bulky triarylphosphanes

Abstract

Abstract The reactions of copper halides CuX with the N-heterocyclic thioamidine 5-methyl-1,3,4-thiadiazole-2-thione (mtdztH) in the presence of the sterically demanding tri(o-tolyl)phosphane (totp), depending on the halide, affords either the mononuclear compound [CuX(totp)(mtdztH)] (1), when X = I, with the thione acting as a terminal S-bound ligand or the symmetrical, thione-S-bridged binuclear compound [CuX(totp)(μ-S-mtdzt)]2 (2), when X = Cl. The steric effects of the large cone-angle phosphane ligand are further demonstrated by the distorted coordination environments of the copper centers in the solid-state structures of the two complexes. The two compounds are photoluminescent in the solid state at ambient temperature, with their emission maxima located in the blue region of the electromagnetic spectrum and influenced by the type of halide present in each case. Density Functional Theory (DFT) and Time-Dependent DFT (TDDFT) calculations were employed in order to elucidate the mechanism of the photoluminescence of the two Cu(I) complexes. The emissive T1 state exhibits a 3MLCT character and minor structural distortions compared to the S0 state. Employing self-consistent spin-orbit coupling TDDFT (SOC-TDDFT) calculations the radiative lifetimes and the zero-field splitting (ZFS) were estimated to be in the ranges 21–375 μs and 1–4 cm−1 respectively. The highest values for the Spin - Orbit matrix elements, calculated with the SOC-TDDFT method, are the and for 1 and the mononuclear counterpart of 2 (2′) indicating a thermal singlet–singlet deactivation path followed by ISC from S1 to T1. In contrast, the highest value SOC matrix element for 2 is the and therefore this complex should exhibit a direct ISC from S1 to T1 without a preceding thermal deactivation. The very small values of the energy difference between S1 and T1 states (ΔE(S1-T1)) calculated to be in the range 1–9 kcal/mol for these complexes, should also favor the ISC process between these excited states.