A. Experimental part: general General Chemicals. IR-780 iodide (99%), 4-mercaptobenzoic acid (99%), yttrium(III) oxide (Y 2 O 3 , 99.99%), ytterbium (III) oxide (Yb 2 O 3, 99.9%), erbium (III) oxide (Er 2 O 3, 99.9%), trifluoroacetic acid (CF 3 COOH, 99%), sodium trifluoroacetate (CF 3 COONa, 98%), N,Ndimethylformamide (DMF, anhydrous, 99.8%), and chloroform (ACS spectrophotometric grade, ≥99.8%, with amylenes as stabilizer) were obtained from Aldrich. Oleylamine (C 18-content 80-90%) was purchased from Acros. Dichloromethane (DCM, AR grade) and diethyl ether (AR grade) were obtained from LAB-SCAN. Ethanol (absolute) was purchased from Merck. All chemicals were used as received. Lanthanide trifluoroacetate trihydrates (RE(CF 3 COO) 3 •3H 2 O) were prepared by a method described in the literature. [1] Instrumentation. 1 H and 13 C NMR spectra were recorded on a Varian AMX400 (400 and 100.59 MHz, respectively) using CDCl 3 as solvent at room temperature. Spectra were referenced to the solvent line (CHCl 3 : 7.26 ppm for 1 H, 77.0 ppm for 13 C) relative to tetramethylsilane. Data are reported as follows: chemical shifts, multiplicity (s = singlet, d = doublet, dd = doublet of doublets, t = triplet), coupling constants (Hz), and integration. FT-IR spectra were recorded on a Nicolet Nexus FT-IR spectrometer, using the SMART iTR for ATR measurements (diamond). Mass spectra were recorded on a LTQ Orbitrap XL (ESI+). Transmission electron microscopy (TEM) was performed on a Philips CM10 transmission electron microscope operating at an accelerating voltage of 100 kV. Images were recorded on a Gatan slow-scan CCD camera. UV-Vis absorption spectra were measured on a Perkin/Elmer Lambda 900 UV-Vis-NIR Spectrometer in chloroform in a quartz cuvette with a path length of 1 cm. Standard CW fluorescence measurements of solutions in chloroform were performed on a Fluorolog 3 (Jobin Yvon Horiba). For the up-conversion luminescence measurements, a Ti:Sapphire laser system (MIRA-900-F) was used as the excitation source. Luminescence spectra and lifetime measurements were performed in the cw and pulsed mode, respectively. In the latter case, the repetition rate was 76 MHz. The excitation light was focused into a 1 mm thick sample cell by a 75 mm focal length lens. This resulted in a focal spot of 120±10 μm full-width at half-maximum level. The excitation power was controlled by a gradient neutral density filter and set at 2 mW for all experiments with the exception of the power-dependence measurements. The emission was collected at the right-angle geometry via an f/2 collimating lens and subsequently focused by an f/4 lens onto the slit of the spectrograph. The (time-resolved) emission detection was performed by a streak camera system equipped with a spectrograph (Hamamatsu C5680) running with the vertical time axis sweep off while recording cw emission spectra.