Theoretical Investigation of the Effect of N Substitution in C^{^}N and N^{^}N Heteroaromatic Ligands on the Photophysical Properties of Two Series of Iridium(III) Carbene Complexes

A DFT/TDDFT investigation has been conducted on two series of cyclometalated iridium(III) complexes to shed light on the effects of structure and steric factors on the photophysical properties. The results reveal that the nature of the N substitution can influence the electron density distributions of the frontier molecular orbitals and their energies, resulting in a change in transition character and emission color. The absorption spectra of the complexes are similar to each other, with 2a-5a and 2b-5b presenting a slight redshift compared with their parent complexes. Again, on the basis of the calculations, the higher radiative rate constants (k_r) of 1a-5a with respect to 1b-5b can be explained by a larger metal contribution to the excited states (MLCT%) and larger S₀-S₁ transition electric dipole moments (μ S 1).