Electronic tuning of tetranuclear Cu(I)-pyrazolate macrocycles: anti-Kasha emission through high-lying triplet states of a C2F5-substituted complex.

The phosphorescence of tetranuclear Cu(I)-pyrazolate macrocycles (Cu4(R-pz)4; pz = 3,5-di-tBu-pyrazolyl) can be influenced by electronic as well as steric factors of the R group at C4, however separating their effects can be challenging. This issue has been addressed in the study of a Cu(I)-pyrazolate tetramer with C4 pentafluoroethyl groups (Cu4(C2F5-pz)41), a homolog of Cu4(CF3-pz)42. C2F5-pz was synthesized by a novel route and combined with CuPF6 to form macrocycle 1; X-ray crystal analysis at 150 K revealed all peripheral C2F5 groups adopt endo conformations and impart similar torsional strain as the CF3 groups in 2. The CxFy units have a strong inductive effect on the luminescent behavior of thin-film samples. Excitation of 1 at 270 nm (∼4.6 eV) produces bimodal emission with peaks centred at 450 and 520 nm and a marked increase in intensity of the higher-energy (HE) band at low temperatures, whereas 2 exhibits the opposite trend. TD-DFT analysis of Cu4(C2F5-pz)41 reveals cluster-centred (CC) orbitals mixed with MLCT character for most excited states, with spin-orbit coupling (SOC) and singlet-triplet energy gap (ΔEST) analysis indicating favourable intersystem crossing (ISC) between S1 and high-lying Tn states. The dual HE/LE emission is ascribed to a large energy gap between high- and low-lying states within the triplet manifold.