ErIII-Cored Complexes Based on Dendritic PtII–Porphyrin Ligands: Synthesis, Near-IR Emission Enhancement, and Photophysical Studies

Authors


  • This research work was financially supported by the Korea Ministry of Science and Technology and KOSEF through the National Research Laboratory Program at Hannam University.

Abstract

A series of stable and inert complexes with ErIII cores and dendritic PtII-porphyrin ligands exhibit strong near-IR (NIR) emission bands via highly efficient energy transfer from the excited triplet state of the PtII-porphyrin ligand to Er3+ ions. The NIR emission intensity of thin films of ErIII complexes at 1530 nm, originating from 4f–4f electronic transitions from the first excited state (4I13/2) to the ground state (4I15/2) of the Er3+ ion, is dramatically enhanced upon increasing the generation number (n) of the aryl ether dendrons because of site-isolation and light-harvesting (LH) effects. Attempts are made to distinguish the site-isolation effect from the LH effect in these complexes. Surprisingly, the site-isolation effect is dominant over the LH effect in the Er3+-[Gn-PtP]3(terpy) (terpy: 2,2′:6′,2″-terpyridine) series of complexes, even though the present dendrimer systems with ErIII cores have a proper cascade-type energy gradient. This might be due to the low quantum yield of the aryl ether dendrons. Thus, the NIR emission intensity of Er3+-[G3-PtP]3(terpy) is 30 times stronger than that of Er3+-[G1-PtP]3(terpy). The energy transfer efficiency between the PtII-porphyrin moiety in the dendritic PtII-porphyrin ligands and the Ln3+ ion increases with increasing generation number of the dendrons from 12–43 %. The time-resolved luminescence spectra in the NIR region show monoexponential decays with a luminescence lifetime of 0.98 μs for Er3+-[G1-PtP]3(terpy), 1.64 μs for Er3+-[G2-PtP]3(terpy), and 6.85 μs for Er3+-[G3-PtP]3(terpy) in thin films of these complexes. All the ErIII-cored dendrimer complexes exhibit excellent thermal stability and photostability, and possess good solubility in common organic solvents.

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