Population Saturation in Trivalent Erbium Sensitized by Organic Molecular Antennae

We investigate sensitization efficiency of near-infrared emission and population saturation of trivalent erbium in erbium-quinolinolato complexes photoexcited into the absorption band of the organic sensitizer. At low-excitation levels, we find high (∼80%) sensitization efficiencies. We observe excited-state population saturation at inversion threshold under subnanosecondpumping at the level of one injected photoexcitation per complex. SECTION Kinetics, Spectroscopy T he increasing demand of larger bandwidth in a local area network can be met by adopting novel optical technologies with cost-effective materials. Molecular materials hold great potential in this context. The high processability of molecular materials, combined with the large tunability of their optical properties, have boosted a rapid development of low-cost photonic systems, e.g., optical waveguides and routers. Near-infrared organic light-emitting diodes have also been demonstrated. In these devices, emission takes place from trivalent erbium (Er) ions coordinated to organic molecules (ligands) to form coordination complexes. Erbium complexes can also be exploited as gain materials for signal regeneration in the third telecom band at 1.54 μm. Population inversion in Er is typically achieved only by intense laser pumping, owing to the low absorption crosssection of Er 4f intrashell transitions. The use of erbium complexes can provide significant advantages, i.e., strong ligand-centered absorption, and efficient and spatially homogeneous lanthanide-ion sensitization. Coordination complexes have also the necessary solubility to reach high concentrations (up to∼10 cm) in polymeric media without the drawback of concentration quenching. To date, very little is known about the response of these systems at high excitation fluences, where excited-state population saturation occurs; in particular, population inversion in Er sensitized by molecular optical antennae has not yet been demonstrated. In this letter we report on experimental assessment of infrared emission sensitization and emission saturation in model Er coordination complexes with 8-quinolinolato (Q) and its dichloro-substitute as the organic ligands, photoexcited in the ligand absorption spectrum. We study the trimetallic complex Er3Q9 and the partially halogenated,monometallic complex [Er(5,7ClQ)2(H5,7ClQ)2Cl] (H5,7ClQ = 5,7-dichloro-8-quinolinol; 5,7ClQ = deprotonated form of H5,7ClQ), where Er is coordinated to four ligands and one chloride (ErClQ4). Materials synthetic procedures, chemical, structural and photophysical charaterization are described in recent reports. The molecular structures of ErClQ4 and Er3Q9 are shown in Figure 1. Linear optical measurements are performed to establish ligand-centered and ion-centered absorption strengths and emission spectra of erbium complexes. Ultraviolet-visibleinfrared (UV-vis-IR) absorption cross-section and emission spectra are depicted inFigure 2. Cross-sections on the orderof 5 10 cm are found in both complexes for the ligandcentered absorption in the 350-380 nm wavelength range, while values of ∼1 10 cm are measured for Er absorption at the 1.54 μm peak, relating to the I13/2 T I15/2 transition manifold. Erbium lifetimes (τ), ∼2 μs in both complexes (see Table 1), have been previously ascribed to nonradiative deactivation induced by C-H stretching vibrations. As a first step to determine Er sensitization efficiency, we measure the quantum yields (ΦIR) of the Er III IR emission by the relativemethod using tris(2,20-bipyridyl)ruthenium(II) ion (Ru(bipy)3 2þ; 2.40 10 M) in deaerated water as the reference standard (ΦR=4.2%, τ=0.58 μs). 7 The quantum yields is determined as ΦIR 1⁄4 nS 2 nR IS IR FRAR FSAS ΦR where nS(R) is the refractive index (nDMSO = 1.4785, nH2O = 1.333), IS(R) is the integrated emission rate (in photons/s), FS(R) is the incident pump rate (in photons/s), and AS(R) is the fraction of pump radiation absorbed in the sample, Received Date: October 2, 2009 Accepted Date: November 4, 2009