Rapid Exciton Transport and Structural Defects in Individual Porphyrinic Metal Organic Framework Microcrystals

Abstract

To date, spectroscopic characterization of porphyrin-based metal organic frameworks (MOFs) has relied almost exclusively on ensemble techniques, which provide only structurally averaged insight into the functional properties of these promising photochemical platforms. This work employs time-resolved pump-probe microscopy to probe ultrafast dynamics in PCN-222 MOF single crystals. The simultaneous high spatial and temporal resolution of the technique enables the correlation of spectroscopic observables to both inter- and intracrystal structural heterogeneity. The pump-probe measurements show that significant differences in the excited state lifetime exist between individual PCN-222 crystals of an ensemble. On a single PCN-222 crystal, differences in excited state lifetime and photoluminescence quantum yield are found to correlate to microscale structural defects introduced at crystallization. Pump probe microscopy also enables the direct measurement of excited state transport. Imaging of exciton transport on individual MOF crystals reveals rapid, but subdiffusive exciton transport which slows on the 10s of ps time scale. Time-averaged exciton diffusion coefficients over the first 200 ps span a range of 0.27 to 1.0 cm²/s, indicating that excited states are rapidly transported through the porphyrin network of PCN-222 before being trapped. Together, these single-particle-resolved measurements provide important new insight into the role played by structural defects on the photochemical functionality of porphyrin-based MOFs.

Figure 1

PCN-222 microrods. SEM images of a straight (A) and curved (B) rod. (C) Powder XRD pattern and crystal structure viewed along [001] and [010]. Pattern analysis shows a = 41.56 Å and c = 16.1 Å.

Figure 2

Power dependence and diffusion in individual PCN-222 rods. (A) Pump–probe image of a single rod. (B) Normalized power dependent decay kinetics. (C, D) Representative MSD vs Δt for two PCN-222 rods. The blue line is a fit to eq 1 with 90% confidence intervals in gray. (E, F) Histogram showing distribution of α (see eq 1) and Δt-averaged diffusion constants.

Figure 3

(A) Transient transmission images of rods (i), (ii), (iii). (B) Normalized kinetics collected from marked locations in panel (A).

Figure 4

Spectroscopic comparison of straight and bent rods. (A, B) Transient transmission images of two rods. The boxed region in each is magnified in panels (C) and (D). Panels (E) and (F) show PL images of the straight and bent rods, respectively. (G) and (H) show transient transmission kinetics collected from the locations indicated in panels (C) and (D).