Porphyrin based metal–organic frameworks: highly sensitive materials for optical sensing of oxygen in gas phase

Abstract

Luminescence of PCN-224-based MOFs is efficiently quenched by molecular oxygen making them promising for optical sensing of oxygen in the gas phase.

Luminescence quenching in epitaxially grown PCN-224 type metal–organic frameworks

“…In contrast, PCN-224 exhibits fluorescence which is quenched around 4-fold at 20 kPa. 3 The quenching behavior of all other MOFs is strikingly different (Fig. 2B, left) with the sensitivity much below that of the physically mixed MOF and similar to that of the pure PCN-224.…”

“…Due to longer phosphorescence decay time of the palladium porphyrin, Pd(II)PCN-224 is more sensitive to oxygen than Pt(II)PCN-224 and responds in the ultra-low range (0-20 ppm) in gas phase. 3 Emission wavelengths of Pt(II)PCN-224 and Pd(II)PCN-224 are well separated from each other (650 nm and 680 nm, respectively), allowing simultaneous readout with the same excitation wavelength. Calibration of ultra-trace sensing materials is challenging, thus only emission spectra in oxygen-free atmosphere and at 0.2 kPa pO 2 were recorded (Fig.…”

“…the classical Stern–Volmer equation (eqn (S1), ESI†) or the non-linear equation from the so called “two-site model” 16 (eqn (S2), ESI†) that assumes the location of the indicator in two environments characterized by significantly different oxygen accessibility. Since the one component MOFs already show significant nonlinearity in their Stern–Volmer plots, 3 a further extension of the model is necessary. Although the best fit ( R 2 = 0.99999) is obtained with the model assuming 4 different Stern–Volmer constants, the complexity of such a model is very high.…”

“…2 The PCN-224 MOFs are characterized by efficient absorption of visible light and variable luminescence decay times (nanoseconds to hundreds of microseconds) depending on the use of the free or differently metalated porphyrin linkers. 3 Accordingly, PCN-224-based materials show tunable dynamic ranges making them suitable for e.g. ultra-trace sensing of oxygen in the gas phase 3 or sensing of ambient oxygen concentrations in water.…”

“…3 Accordingly, PCN-224-based materials show tunable dynamic ranges making them suitable for e.g. ultra-trace sensing of oxygen in the gas phase 3 or sensing of ambient oxygen concentrations in water. 4 Although such MOF-based oxygen sensor materials evolved as an interesting alternative to conventional polymer based sensors, 5 insights in their working principle are hardly available.…”