Emission spectra of the three MOFs PCN-224, Pt(II)PCN-224 and Pd(II)PCN-224 are shown in Fig. . In case of PCN-224 the emission is attributed to fluorescence due to decay time in the nanosecond time domain ( = 6.7 ns in the absence of O2). In contrast, the MOFs based on Pt(II) and Pd(II) porphyrins show phosphorescence ( = 18.7 and 390 µs, respectively). As can be seen, the emission maximum for the PCN-224 in free powder is located at significantly longer wavelength (711 nm) than expected and only a small shoulder at around 660 nm is visible. Such distortion can be attributed to the inner-filter effect that is particularly strong in the MOF due to high concentration of the porphyrin and strong overlap between fluorescence and the Qx (0,0) absorption band. The same phenomenon is well known for concentrated solutions of fluorescent emitters including porphyrins. In accordance to the properties of Pt(II) and Pd(II) porphyrins in solutions, the emission maxima of Pt(II)PCN-224 is located at shorter wavelength (max = 645 nm) compared to the corresponding Pd(II) MOF (max = 675 nm). The structure of PCN-224-based MOFs already suggests that individual porphyrin molecules are isolated and their aggregation, which causes pronounced changes of the optical properties of the materials, is mitigated. Indeed, the excitation spectra recorded for Pt(II)PCN-224 (in aqueous dispersion) and the respective building block Pt(II)TCPP (in aqueous solution) are very similar (Fig. ). Thus, the arrangement of porphyrin molecules in MOF appears to be an efficient strategy to obtain systems with high concentration of the luminophore without its aggregation.