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

• Published in: Advanced functional materials 2007-02, Vol.17 (3), p.413-424
• Main Authors: Oh, J. B., Nah, M.-K., Kim, Y. H., Kang, M. S., Ka, J.-W., Kim, H. K.
• Format: Article
• Language: English
• Subjects: Dendrimers; Energy transfer; Europium complexes; Phosphorescence; Photoluminescence; Porphyrins
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.200600451

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. A series of ErIII‐cored dendrimer complexes that exhibit good emission in the near‐IR region are synthesized. The emission intensity is significantly enhanced with increasing generation number of the dendritic ligands because of the light‐harvesting (LH) and site‐isolation (SI) effects depicted in the figure. In these complexes, the latter seems to be dominant over the former.