Spectral and dynamical properties of a Zr-based MOF†
We report on the spectra and dynamics of a Zr-naphthalene dicarboxylic acid (Zr-NDC) MOF in different diluted solvent suspensions and in a concentrated tetrahydrofuran (THF) one. In a diluted diethyl ether (DE) suspension, we observed intraparticle excimer formation between neighboring naphthalene organic linkers, leading to a red-shifted broad band in the emission spectrum and to a dynamics composed of three components τ1 = 650 ps, τ2 = 3.7 ns and τ3 = 13.9 ns, assigned to the excimer photoproduction, monomer and excimer lifetimes, respectively. Furthermore, both absorption and emission spectra show a blue shift in more polar solvents characterized by the solvent polarity function f(ε,n). We also observed changes in the excimer formation time (490–840 ps) probably due to a variation in the MOF structural fluctuation induced by solvent filling. The global fluorescence quantum yield of these suspensions is around 0.30 ± 0.05. At higher concentrations of the MOF particles, we observed the absorption and emission signals of aggregates having an intercrystal excimer formation in ∼5 ps in a THF suspension, ∼100 times shorter than that observed in a diluted one. Our results give the spectral and dynamical properties of a Zr-NDC MOF in solvent suspensions, opening the way to further studies of these kinds of MOFs interacting with fluorescent dyes for possible photonic applications.