No abstract

Photoexcitation at a certain wavelength in the near-infrared (NIR) region followed by luminescence at a shorter wavelength in the visible domain is called near-infrared-to-visible photon upconversion. This is a rather unusual process as lowenergy photons are "converted" into higher energy photons: two or three photons of NIR light are required to generate one photon of visible light.[1] Nonlinear optics (NLO) is another mechanism that can lead to the generation of light at a frequency that is either twice or integral-multiples of the incident light.[2] The phenomena of two-or three-photon upconversion, and second or third harmonic generation (SHG or THG) on organic materials have received considerable attention since fluorescence detection became popular in applications such as photodynamic therapy, optical data storage, and microfabrication. [3] Multiphoton excitation allows molecules that typically absorb in the UV region to be excited with red or NIR light. The photoluminescence processes from organolanthanide complexes are usually induced by an organic chromophoric ligand that absorbs incident light and transfers its resultant excitation energy to the lanthanide ion.[4] Until now, only a few multiphoton processes in organolanthanide complexes have been observed owing to the limitations of experimental measurements.[5] Although there have been reports on lanthanide complexes that display second harmonic generation, these have been limited and focused only on glassy doped lanthanide materials.[6] In recent years, our group [7] and others [8] have investigated several high-order multiphoton processes from organic ligands and metal complexes. Herein we report, to the best of our knowledge, the first observation of rare multiphoton upconversion emission from a lanthanide complex as well as SHG and THG transmission phenomena, which occur simultaneously from the same excitation source in the IR region at l % 1.26 and 1.34 mm.The N-tripodal ligand L was treated with terbium nitrate to give the complex [Tb(L)(NO 3 ) 3 ] (1, see Figure 1), which is thermally stable up to 300 8C, as shown by thermogravimetric analysis (see Supporting Information). Single crystals of 1 were isolated through the slow evaporation of a solution of 1 in THF/MeCN at room temperature over a few days. The crystal structure of 1 was determined by X-ray crystallography and is shown in Figure 2. The coordination geometry of the metal center in 1 can be described as a tricapped trigonal prism, in which each Tb 3+ ion is coordinated by three carbonyl (amide) groups from three separate but identical tripodal ligands, and doubly coordinated by three nitrate ions. Upon complexation, the tripodal ligand adopts a polymetallic dendritic form, as the branches contain oxygen groups for binding with lanthanide ions. The ensemble of the tripodal ligand and its connectivity with the lanthanide ion, which itself is preorganized as a result of its coordination to three chromophores, forms a metallodendrimer. The metal takes on a "metal as ligand, metal as co...