Molecular Orientation Controls Triplet Exciton Dynamics in Organic Molecules Coupled to Lanthanide-Doped Nanoparticles

Hybrid organic-inorganic materials, which combine the unique properties of organic semiconductors (OSCs) and inorganic nanoparticles, show great promise for optoelectronic applications. Understanding structure-function relationships in these nanohybrids is crucial for understanding the mechanisms governing their excited state dynamics, particularly those controlling triplet excitons, which are key to their performance. While previous studies focused primarily on triplet energy transfer (TET) across the interface, we study the full triplet exciton dynamics in OSCs coupled with various lanthanide-doped nanoparticles (LnNPs), with an emphasis on the impact of molecular orientation. We examine three anthracene carboxylic acid (ACA) isomers, differing in the position of the carboxylic acid group (1-, 2-, and 9-position) on the anthracene core. These isomers all exhibit similar triplet dynamics and low triplet yields (∼10%) when uncoordinated, but adopt distinct binding geometries on the LnNP surface, making them ideal for studying how coordination geometry influences triplet exciton dynamics. Using time-resolved optical spectroscopy, we observe significant variations in triplet generation rates, yields, lifetimes, and TET rates between the ACA isomers upon coordination onto the LnNPs. Triplet generation rates and yields are consistently highest in 1-ACA (up to 86%) and lowest in 2-ACA. TET rates are fastest for 9-ACA (up to 1.1 × 10⁸ s^-1) and slowest for 2-ACA. Notably, in the absence of TET, triplet exciton lifetimes exceed 0.1 ms for all LnNP@ACA nanohybrids. These results quantitatively describe how positional isomerism governs the triplet exciton dynamics in LnNP@OSC nanohybrids and highlight the pivotal role of molecular orientation in mediating interfacial photophysics.