Luminescent Platinum(II) Complexes of NN-N Amido Ligands with Benzannulated N-Heterocyclic Donor Arms: Quinolines Offer Unexpectedly Deeper Red Phosphorescence than Phenanthridine

A platform for investigating the impact of pi-Cextension in benzannulated, anionic pincer-type N<^>N-<^>N-coordinating amido ligands and their Pt(II) complexes is presented. Based on bis(8-quinolinyl)amine, symmetric and asymmetric proligands bearing quinoline or pi-extended phenanthridine (3,4-benzoquinoline) units are reported, along with their red-emitting, phosphorescent Pt(II) complexes of the form (N<^>N-<^>N)PtCl. Comparing the photophysical properties of complexes of (quinolinyl)amido ligands with those of pi-extended (phenanthridinyl)amido analogues revealed a counterintuitive impact of site-selective benzannulation. Contrary to conventional assumptions regarding pi-extension, and in contrast to isoenergetic lowest energy absorption bands and a red shift in fluorescence from the organic proligands, a blue shift of nearly 40 nm in the emission wavelength is observed for Pt(II) complexes with more extended bis(phenanthridinyl) ligand pi-systems. Comparing the ground state and triplet excited state structures optimized from density functional theory (DFT) and time-dependent-DFT calculations, we trace this effect to a greater rigidity of the benzannulated complexes, resulting in a higher energy emissive triplet state, rather than to a significant perturbation of orbital energies caused by pi-extension.