Enhancement of triplet-triplet annihilation upconversion in organically modified clay colloids

Controlling the spatial distribution of energy donors and acceptors within constrained microenvironments remains a fundamental challenge in the development of efficient triplet-triplet annihilation upconversion (TTA-UC) and advanced photofunctional materials. Herein, we report the rational modulation of TTA-UC in an (R)-limonene-swollen organoclay (Sumecton-SAN) system using four iridium(III) complexes as donors: two cationic species, [Ir(ppy)₂(phen)]PF₆ (Hppy = 2-phenylpyridine, phen = 1,10-phenanthroline, Ir1-PF6) and [Ir(piq)₂(phen)]PF₆ (Hpiq = 1-phenylisoquinoline, Ir2-PF6), and two neutral analogues, [Ir(ppy)₂(acac)] (Hacac = acetylacetone, Ir3) and [Ir(piq)₂(acac)] (Ir4). We initially demonstrate that cationic Ir(III) sensitizers suffer from severe UC quenching in clay colloids due to spatial segregation; specifically, the cationic donors are electrostatically anchored to the polar clay surface via ion exchange, while the hydrophobic 9,10-diphenylanthracene (DPA) acceptors preferentially partition into the (R)-limonene-rich, hydrophobic interlayer domains. To overcome this microenvironmental mismatch, we employed the neutral Ir(III) complexes Ir3 and Ir4. In these neutral systems, UC emission was significantly enhanced when the iridium complexes were loaded at a low density (corresponding to 5% of the cation exchange capacity (CEC) of the clay). Conversely, at a higher loading density corresponding to 50% of the CEC, Ir3 exhibited a marked decrease in UC emission, whereas the sterically bulkier Ir4 sustained its enhanced emission compared to the clay-free homogeneous solution. We attribute this difference to the aggregation-induced self-quenching of Ir3 on the clay surface at high concentrations, which was effectively suppressed by the steric hindrance of Ir4. Furthermore, excited-state lifetime measurements of Ir2-PF6 and Ir4 in the presence and absence of the clay revealed contrasting behavior: the lifetime of Ir2-PF6 was shortened (the excited state of Ir2+ was destabilized) in the presence of the clay, whereas that of Ir4 was prolonged (the excited state of Ir4 was stabilized). These results suggest that both the neutral iridium donors and the DPA acceptors co-localize on the organically modified hydrophobic clay surfaces. Consequently, their molecular diffusion is effectively restricted to two dimensions, leading to amplified UC emission compared to that in a homogeneous solution. Based on these findings, we highlight the crucial role of tailoring the hydrophobic microenvironment on 2D clay sheet surfaces to facilitate efficient TTA-UC.