Nanoscale Metal–Organic Frameworks and Metal–Organic\nLayers with Two-Photon-Excited Fluorescence

Abstract

Metal–organic\nframeworks (MOFs) based on 9,10-diphenylanthracene-derived\nligands had been reported to exhibit upconverted fluorescence through\ntriplet–triplet annihilation. We found that zirconium MOFs\nbased on 9,10-diphenylanthracene can also give upconverted fluorescence\nvia two-photon absorption without adding a triplet photosensitizer\nwhen a femtosecond pulsed laser is used as the excitation source.\nBy tuning the synthetic condition, we obtained nanoscale MOFs of UiO\nstructure in both octahedral and hexagonal nanoplate shapes, as well\nas a hexagonal nanoplate of MOFs of hcp-UiO structure and two-dimensional\nmetal–organic layers. All of them, as well as a homogeneous\nsolution of the 9,10-diphenylanthracene ligand, exhibit upconverted\nfluorescence upon excitation using a laser pulse of 60 fs with a pulse\nenergy of ∼1.1 × 10⁶ nJ/cm² (unfocused).\nMoreover, we observed different emission spectra by two-photon excitation\ncompared to those by one-photon excitation, which indicates access\nto a unique initial excited state via two-photon excitation. This\nphenomenon is not observed for a homogeneous solution of the ligand.\nThese nanoscale MOFs may find application in two-photon fluorescence\nimaging.