Molecular design and photophysical criteria for lanthanide emission enhancement in erbium(III)-cored complexes based on dendritic ligands for information technology

Abstract

Recently, luminescent lanthanide complexes have considerable interest because of their academic interests and potential utility in a wide variety of photonic applications, such as planar waveguide amplifiers, plastic lasers, lightemitting diodes, and luminescent probes. The 4f electrons in lanthanide (Ln) ions are slightly perturbed by the effects of lattice phonons and static strain fields in the coordination environment of ions, since the f-electrons are shielded by the outer 5s and 5p electrons. It leads to the sharp spectral line-like emission bands. Also, the forbidden 4f -4f n electronic transitions renders the low absorption and emission cross-section of lanthanide ions, while luminescent lifetime is relatively long. To overcome these shortcomings, recently, luminescent ligands are being used to excite Ln ions via an energy transfer from the luminescent ligands to the Ln ions. In most cases, the luminescent Ln ions are usually coordinated to the organic luminescent ligands, acting as sensitizers or antenna chromophores, which efficiently absorb and transfer light to excite Ln ions via energy transfer process. This sensitization process is much more effective than the direct excitation of Ln ions, since the absorption coefficients of organic chromophores are many orders of magnitude larger than the intrinsically low molar absorption coefficients (typically 1-10 M cm) of Ln ions. Recently, several research groups have focused on developing the efficient artificial light-harvesting (LH) lanthanide complexes, in which the use of dendrimers for light harvesting systems has been widely demonstrated. The encapsulation of luminescent Ln ions into a luminescent dendrimer can lead to a system capable of shielding central Ln ion from nonradiative environment and efficiently transferring excited energy from the peripheral chromophores to the focal point of the dendrimer. For example, Frechet et al. have reported the site isolation and antenna effects on luminescent properties of spherical lanthanide(III)-cored dendrimer complexes. Although its spectral overlap integral (J) between the emission band of peripheral antenna and the absorption band of Ln ions was not satisfactorily large to obtain the effective energy transfer, the Frechet arylether typed dendrons were widely used as light-harvesting antenna. Moreover, very recently, to enhance the near-infrared (NIR) emission intensity and maintain the effective energy transfer process, our research efforts have been focused on developing stable and inert Er(III)-encapsulated complexes with artificial light-harvesting systems using dendritic luminescent ligands based on metalloporphyrins, naphthalenes, and anthracenes bearing the Frechet aryl-ether dendrons, namely, (Er3+-[Gn-Pt-Por]3(terpy), Er 3+-[Gn-Naph]3(terpy) and Er3+-[Gn-An]3(terpy)). We observed that the NIR emission intensity of the lanthanide complexes was dramatically enhanced with increasing the generation number (n) of the Frechet aryl-ether dendrons, due to the site-isolation and light-harvesting effects. Two possible energy transfer (ET) pathways for the sensitized emission in luminescent Ln(III) complexes have been suggested, as schematically illustrated in Scheme I. It is well-believed that in general only energy transfer from the