Near infrared light emission quenching in organolanthanide complexes

Abstract

We investigate the quenching of the near infrared light emission in Er 3+ complexes induced by the resonant dipolar interaction between the rare-earth ion and high frequency vibrations of the organic ligand. The nonradiative decay rate of the lanthanide ion is discussed in terms of a continuous medium approximation, which depends only on a few, easily accessible spectroscopic and structural data. The model accounts well for the available experimental results in Er 3+ complexes, and predicts an ϳ100% light emission quantum yield in fully halogenated systems. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2177431͔The research on low-cost materials emitting in the near infrared ͑NIR͒ is receiving a great deal of renewed interest for their potential in local and premise optical communication networks, and imaging and sensing applications. Among these materials, organolanthanides are very promising candidates as they combine the well-established NIR emission properties of Ln 3+ ions with the unique optical 1,2 and electrical response 3 of organic semiconductors, coupled with their easy processability.Excitation processes in lanthanide complexes differ considerably from those in inert glasses doped with transition metals. Due to the large absorption cross section of the allowed − * optical transitions, the organic ligand acts as an efficient light harvester in the ultraviolet-visible spectral window. From the organic photonic antenna the electronic excitations are quickly transferred to the rare-earth ion. The twostep excitation process permits the achievement of a large excited-state population using light fluences four to five orders of magnitude lower than those required for bare ions. Ligands also prevent deleterious formation of metal clusters, allowing the deposition of thin films with Ln 3+ densities as large as 10 21 ions/ cm 3 . All these properties make organolanthanides very attracting for the development of low-cost light sources and infrared amplifiers to integrate in planar photonic circuits for optical communications, where light signals can be generated, amplified, and processed. [1][2][3][4][5] The major drawback of these materials is related to the presence of efficient nonradiative deactivation channels, which shorten the erbium population lifetime from milliseconds to microseconds. 1,2 NIR emission quenching mainly results from the coupling of the excited state of the Ln 3+ ion with high frequency vibrations of CH and OH groups. 6 Substitution of hydrogen with heavier halogen atoms lowers the vibration frequencies and, hence, represents a possible strategy to reduce the induced emission quenching. Investigations on halogenated systems show an unambiguous improvement of the NIR light emission. 7,8 The best performances have been obtained in perfluorinated ͑PF͒ systems, which show nonradiative decay times in the submillisecond timescale. 9 NIR emission quantum yield has been drastically enhanced, but remains, however, rather low, around a few percent. Whether these results can be further impro...

Quenching of Er(III) luminescence by ligand C–H vibrations: Implications for the use of erbium complexes in telecommunications

“…These results demonstrate that the nearest neighbor CH oscillators are by far the biggest contributor to the quenching of erbium ions in organic hosts which corresponds to the R min value in the theory proposed by Quochi et al, 6 the role of the more distant oscillators is only of secondary importance. These results also show that to produce erbiumcontaining molecules where the lifetime of the erbium is not being dominated by quenching to hydrogen-related vibrational modes in the surrounding ligands requires the removal of all hydrogen from a distance of at least 20 Å from the Er 3+ ions, for a 100 s lifetime.…”

“…5 Nd͑III͒ has been a subject of particular study; however, there have been little data in actually quantifying the extent of the quenching. A recent paper by Quochi et al 6 has proposed a model for calculating the degree of quenching based on the assumption of a continuous distribution of CH oscillators which makes good predictions of the degree of quenching for systems where there are large amounts of CH or OH bonds present. As the authors state, the recent work by Mancino et al 7 where it has been shown that an evaporated thin film of Er͑F-tpip͒ 3 , where F-tpip is the perfluorinated imidodiphosphinate ͓͑C 6 F 5 ͒ 2 P͑O͔͒ 2 N − , having a luminescence lifetime of 224 s does not fit with their model and they suggest that the discrepancy is probably due to residual hydrogen-containing impurities.…”

Near-infrared photoluminescence of erbium tris(8-hydroxyquinoline) spin-coated thin films induced by low coherence light sources

“…[1][2][3][4][5][6] The main advantage of these compounds lies in the large absorption cross section of the allowed -* optical transitions in the organic ligands, that acts as an efficient light harvesting antenna. [7][8][9] This property enables optical excitation by means of low coherence and low-cost light sources, such as light emitting diodes ͑LEDs͒. Some organolanthanides have shown electroluminescence in the near-infrared range.…”

Population Saturation in Trivalent Erbium Sensitized by Organic Molecular Antennae

“…Recently, halogenation of the ligands and dispersion of the complexes in IR transmitting media, e.g., perfluorinated polymers, has been shown to be effective in reducing vibrational quenching induced by C-H groups, with consequent increase in Er III lifetime by 2 orders of magnitude. [14][15][16] If τ reached the radiative limit, threshold pump intensity would be decreased to values less than 5 W/cm 2 that are practical for indirect pumping by semiconductor light-emitting diodes. 17 In conclusion, we show experimental evidence for transient Er III population saturation near inversion threshold in a monometallic coordination complex exhibiting very efficient lanthanide-ion sensitization.…”