Fig. 3: Photophysical properties of hetero-ligand MOF nanocrystals.

a Normalized absorption (dashed line) and photoluminescence (PL, solid line) spectra of the hetero-ligand Zr-DPT:DPA-1% nanocrystals (0.1 mg mL⁻¹) in tetrahydrofuran (THF) compared with homo-ligand MOFs based on DPA (Zr-DPA, 0.1 mg mL⁻¹) and DPT (Zr-DPT, 0.1 mg mL⁻¹). The excitation wavelength is 355 nm. The pink solid line is the excitation photoluminescence spectrum of the Zr-DPT:DPA-1% dispersion recorded at 540 nm. b PL spectrum of the Zr-DPT:DPA-1% dispersion at different dilution ratios under 355 laser excitation and c, d PL intensity decay with time recorded at 430 nm (c) and 540 nm (d) at the same dilution ratios of Zr-DPT:DPA-1% in comparison with that of Zr-DPA and Zr-DPT reference nanocrystals, respectively. e Energy transfer efficiency ϕ_ET between DPA and DPT co-ligands as a function of their relative molar ratio in the MOF framework. The theoretical ϕ_ET (solid line) is calculated considering a diffusion-mediated Förster energy transfer with interaction radius of 2.8 nm derived from ligands properties. The fit of experimental data (dashed line) results a characteristic interaction radius of 3.0 nm. Error bars show the standard deviation calculated for reported values. f Normalized PL spectra of the hetero-ligand nanocrystals dispersions series (0.1 mg mL⁻¹) and g time-resolved PL spectra recorded at 430 nm. Solid lines are the fit of the data with multi-exponential decay function. h Absorption (dashed line) and PL (solid line) spectrum of Zr-DPT:DPA-8% nanocrystals in THF (0.1 mg mL⁻¹, optical path 1 cm). The excitation wavelength is 355 nm. The inset is a digital picture of the dispersion in the quartz cuvette under 355 nm excitation.