ErIII-cored complexes based on dendritic PtII-porphyrin ligands:: Synthesis, near-IR emission enhancement, and photophysical studies

A series of stable and inert complexes with Er-III cores and dendritic Pt-II-porphyrin ligands exhibit strong near-IR (NIR) emission bands via highly efficient energy transfer from the excited triplet state of the Pt-II-porphyrin ligand to Er3+ ions. The NIR emission intensity of thin films of Er-III complexes at 1530 nm, originating from 4f-4f electronic transitions from the first excited state (I-4(13/2)) to the ground state (I-4(15/2)) 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 Er-III cores have a proper cascade-type energy gradient. Ibis 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 Pt-II-porphyrin moiety in the dendritic Pt-II-porphyrin ligands and the Ln(3+) 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 mu s for Er3+-[G1-PtP](3)(terpy), 1.64 mu s for Er3+-[G2-PtP](3)(terpy), and 6.85 mu s for Er3+-[G3-PtP](3)(terpy) in thin films of these complexes. All the Er-III-cored dendrimer complexes exhibit excellent thermal stability and photostability, and possess good solubility in common organic solvents.