Molecular and Nanoaggregation in Cyclometalated Iridium(III) Complexes through Structural Modification

New terpyridyl ligands TP1 , TP2 and cyclometalated iridium(III) complexes 1 and 2 based on these ligands have been synthesized. The ligands and complexes have been characterized by elemental analysis and spectroscopic studies (ESI‐MS, 1 H and 13 C NMR, UV/Vis, fluorescence). The molecular structure of 1 has been verified by X‐ray single‐crystal analysis. It has been unambiguously established that variation of the substituents on 1 and 2 leads to molecular aggregation in 1 , while 2 remains nonaggregated. Furthermore, complexes 1 and 2 have been successfully utilized as capping agents for the stabilization of gold nanoparticles (AuNPs). It is of note that 1 forms discretely, while 2 aggregates AuNPs through the assemblage of ultrasmall nanoparticles. It has been affirmed by 1 H NMR titration studies that – N H groups from 1 and 2 are involved in the capping of AuNPs. The role of simple structural variations in directing molecular and nanoaggregation has been clearly established for the first time by spectroscopic (UV/Vis, fluorescence, 1 H NMR titration) and morphological studies [SEM, TEM, EDX (energy‐dispersive X‐ray), DLS (dynamic light scattering)].