Optically active luminescent materials based on lanthanide ions attract significant attention due to their unique spectroscopic properties, nonlinear optical activity, and the possibility of application as contactless sensors. Lanthanide metal-organic frameworks (Ln-MOFs) that exhibit strong second-harmonic generation (SHG) and are optically active in the NIR region are unexpectedly underrepresented. Moreover, such Ln-MOFs require ligands that are chiral and/or need multistep synthetic procedures. Here, we show that the NIR pulsed laser irradiation of the noncentrosymmetric, isostructural Ln-MOF materials (MOF-Er3+ (1) and codoped MOF-Yb3+/Er3+ (2)) that are constructed from simple, achiral organic substrates in a one-step procedure results in strong and tunable SHG activity. The SHG signals could be easily collected, exciting the materials in a broad NIR spectral range, from ≈800 to 1500 nm, resulting in the intense color of emission, observed in the entire visible spectral region. Moreover, upon excitation in the range of ≈900 to 1025 nm, the materials also exhibit the NIR luminescence of Er3+ ions, centered at ≈1550 nm. The use of a 975 nm pulse excitation allows simultaneous observations of the conventional NIR emission of Er3+ and the SHG signal, altogether tuned by the composition of the Ln-MOF materials. Taking the benefits of different thermal responses of the mentioned effects, we have developed a nonlinear optical thermometer based on lanthanide-MOF materials. In this system, the SHG signal decreases with temperature, whereas the NIR emission band of Er3+ slightly broadens, allowing ratiometric (Er3+ NIR 1550 nm/SHG 488 nm) temperature monitoring. Our study provides a groundwork for the rational design of readily available and self-monitoring NLO-active Ln-MOFs with the desired optical and electronic properties.

This work was supported by the National Science Centre, Poland (SONATA grant UMO-2020/39/D/ST4/01182); from the budget for science in 2018-2021, as a part of the Polish Ministry of Science and Higher Education project, Grant No. 0088/DIA/2018/47, in the frame of the "Diamond Grant" programme; Spanish Ministerio de Economía y Competitividad (MINECO) under the National Program of Sciences and Technological Materials (PID2019-106383GB-44); by Spanish Research Agency (AEI) under projects MALTA Consolider Team network (RED2018-102612-T); and by EU-FEDER funds. A.G. is a scholarship holder of the Polish Ministry of Education and Science for outstanding young scientists. D.M. is a scholarship holder of the Adam Mickiewicz University Foundation for the academic year 2021/2022. M.R. acknowledges support from Fondo Social Europeo and Agencia Estatal de Investigación (RYC2020-028778-I/AEI/10.13039/501100011033).