In this work, the photophysical characteristics of [Cu(N^N)₂]⁺ and [Cu(N^N)(P^P)]⁺ complexes were described. The concept of thermally activated delayed fluorescence (TADF) and its development throughout the years was also explained. The importance of ΔE (S₁–T₁) and spin-orbital coupling (SOC) values on the TADF behavior of [Cu(N^N)₂]⁺ and [Cu(N^N)(P^P)]⁺ complexes is discussed. Examples of ΔE (S₁–T₁) values reported in the literature were collected and some trends were proposed (e.g. the effect of the substituents at the 2,9 positions of the phenanthroline ligand). Besides, the techniques (or calculation methods) used for determining ΔE (S₁–T₁) values were described. The effect of SOC in TADF was also discussed, and examples of the determination of SOC values by DFT and TD-DFT calculations are provided. The last chapter covers the applications of [Cu(N^N)₂]⁺ and [Cu(N^N)(P^P)]⁺ TADF complexes and the challenges that are still needed to be addressed to ensure the industrial applications of these compounds.

A novel diimine ligand (N^N_b) and the corresponding heteroleptic diimine–diphosphine Cu(I) complex (Cu–N^N_b) have been synthesized and their structural properties characterized. The N^N_b ligand is [2,9-bis-[4-(dimethylamino)phenyl]ethenyl]-4,7-diphenyl-1,10-phenanthroline, which has a bulky structure designed to retain the tetrahedral geometry of the corresponding Cu(I) complex in the excited state in order to avoid potential quenching by flattening distortion. The corresponding Cu(I) complex comprises the mentioned diimine ligand and bis-[2-(diphenylphosphino)-phenyl]ether (POP) as the diphosphine ligand. The electrochemical and photophysical properties, suitably complemented by theoretical calculations, have been studied and compared to those of a reported heteroleptic Cu(I) complex including POP and bathocuproine as the diphosphine and diimine ligands, respectively. According to both experimental and computational data, the new complex Cu–N^N_b is characterized by an unexpected luminescence behaviour that markedly differs from those typically encountered in heteroleptic diimine–diphosphine Cu(I) complexes and stems from the direct, spin-allowed radiative decay (fluorescence) of an intraligand charge transfer excited state on the bulky diimine ligand. The emission quantum yield is also surprisingly high (35%), possibly inspiring potential and alternative applications of this novel complex in luminescent devices or imaging.

With an appropriate mixture of cyclometalating and ancillary ligands, based on simple structures (commercial or easily synthesized), it has been possible to design a family of eight new Ir(III) complexes (1A, 1B, 2B, 2C, 3B, 3C, 3D and 3E) useful as luminescent materials in LEC devices. These complexes involved the use of phenylpyridines or fluorophenylpyridines as cyclometalating ligands and bipyridine or phenanthroline-type structures as ancillary ligands. The emitting properties have been evaluated from a theoretical approach through Density Functional Theory and Time-Dependent Density Functional Theory calculations, determining geometric parameters, frontier orbital energies, absorption and emission energies, injection and transport parameters of holes and electrons, and parameters associated with the radiative and non-radiative decays. With these complexes it was possible to obtain a wide range of emission colours, from deep red to blue (701–440 nm). Considering all the calculated parameters between all the complexes, it was identified that 1B was the best red, 2B was the best green, and 3D was the best blue emitter. Thus, with the mixture of these complexes, a dual host–guest system with 3D-1B and an RGB (red–green–blue) system with 3D-2B-1B are proposed, to produce white LECs.

A series of heteroleptic Cu(I) complexes of type [Cu(N^N)(POP)]⁺, with POP: bis-[2-(diphenylphosphino)-phenyl]ether and different phenanthroline-based N^N ligands were investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT). The complexes comprise N^N ligands with different bulky substituents at the 2,9 positions, and the addition of phenyl substituents at the 4,7 positions is also investigated. Furthermore, in the complex with the bulkier N^N ligand, the effect of unsaturated hydrocarbon chains added and the presence of dimethylamine electron donor groups were analyzed. The composition/energies of the frontier orbitals and the emission energies were determined for all complexes. Besides, we evaluated the steric effects induced by different N^N ligands in electronic (Pauli repulsion energy) and structural (root-mean-square deviation) terms to determine their influence on the photoluminescence efficiency comparing the properties of the proposed Cu(I) complexes with four reported complexes. The analyzed data allow us to find simple relationships to predict the radiative (k_r) and non-radiative (k_nr) constants based on the steric effects that each complex undergoes according to the type of N^N ligand used, obtaining the best balance (higher k_r and lower k_nr) in complexes with bulkier substituents. Therefore, these complexes could be proposed as suitable luminescent materials for optical applications.