Position isomers of Ru(II) polypyridine complexes with tunable photophysical properties, aggregation-induced phosphorescence enhancement and application in triplet-triplet annihilated upconversion

Abstract

Three Ru(II) polypyridine complex isomers (Ru-2-DP, Ru-3-DP and Ru-5-DP) have been synthesized using phenanthroline-based isomers containing 4-(2,2-diphenyl-vinyl)-phenyl unit. Their photophysical properties are systematically investigated. Notably, three coordination isomers exhibit much longer triplet lifetimes compared to the model complex [Ru(bpy)2(Phen)]2+. The density function theory calculations reveal that the extension of triplet lifetimes is attributed to the establishment of an excited-state equilibrium between the intraligand (3IL) state and the metal-to-ligand charge-transfer (3MLCT) state. In aggregates, Ru-2-DP shows an interesting aggregation-induced phosphorescence enhancement (AIPE) property. The phosphorescence intensity of Ru-2-DP increases by 16.2-fold from acetonitrile solution to aggregated state. However, Ru-3-DP and Ru-5-DP are AIPE-inactive. Single-crystal analyses show substituent position has a dramatic effect on intramolecular steric hindrance, leading to different molecular conformation and packing pattern. Using three coordination isomers as triplet sensitizers for triplet-triplet annihilated (TTA) upconversion, Ru-3-DP and Ru-5-DP display obvious upconversion properties, but it's quite the opposite for Ru-2-DP. Experimental data demonstrate 3- and 5-positions of phenanthroline, especially 3-position, are beneficial to enhance intersystem crossing and triplet-triplet energy transfer and for the resulting upconverted efficiency enhancement. This work definitely suggests that minor structural change may have major effects upon the solid-state spectroscopic properties and TTA upconversion performances, which provides a rational basis for designing excellent solid phosphorescent materials and triplet sensitizers.