In situ characterization of post-synthetic metalation in porous salt thin films

Abstract

Post-synthetic metalation and metathesis chemistry are central to rational synthesis of metal-organic frameworks (MOFs) that are unavailable by direct self-assembly. The inherent microcrystallinity and heterogeneous nature of many MOFs renders characterization of the rate, extent, and distribution of post-synthetic modifications challenging. Here we describe the deposition of optically transparent, permanently porous thin films comprised of peripherally carboxylated free-base porphyrins and cationic porous molecular cages. The films are assembled via layer-by-layer growth controlled by coulombic charge pairing, which allows for systematic control over film thickness. The obtained thin films are optically transparent monoliths that retain the permanent porosity of the corresponding porous salts. Post-synthetic metalation of these films with Mn(HMDS)₂ affords the corresponding Mn(ii) porphyrin-based materials (HMDS = hexamethyldisilazide). Access to thin films with systematically varied thickness (and thus optical density), combined with in situ spectroscopy, enables the kinetics and extent of metalation to be directly monitored. We demonstrate both structure- and thickness-dependence on metalation kinetics. These results provide a unique window into the molecular-scale mechanisms that underpin materials synthesis.

Fig. 1. Optically transparent porous salt thin films are highly tunable as their bulk properties can be modified by judicious selection of porous cage (top), chromophore (middle), and surface treatment, including several factors such as plasma etching and silylation (bottom).

Fig. 2. UV-vis absorption spectra of a [ZrmBDC][H₂(tcpp)] thin film grown to 5 (blue), 10 (green), 15 (red), and 20 (black) deposition cycles. After every 5 cycles the film was exposed to the solution containing the target anion 10 additional times and a UV-vis spectra was collected (dashed line = additional 10 exposures).

Fig. 3. Plot of Soret band absorbance versus deposition cycles deposited for films grown on a plasma treated (black) glass slide as compared to an untreated (red) glass slide.

Fig. 4. UV-vis spectra of optically transparent porous thin films grown to 5 deposition cycles using 0.5 mM solutions (blue), 1.0 mM solutions (red), and 2.0 mM solutions (black) of [ZrmBDC][OTf]₄ and [HNEt₃]₄[H₂(tcpp)] in methanol.

Fig. 5. Deposition cycles vs. absorbance plots for films grown pairing [ZrmBDC]⁴⁺ with H₂[H₂(dcpp)] (green), H₄[H₂(tcpp)] (red), and H₈[H₂(ocpp)] (black).

Fig. 6. UV-vis spectra of 10 deposition cycle thin films comprised of [ZrMe₂BDC][H₂(tcpp)] (black), [ZrFDC][H₂(tcpp)] (red), and [ZrmBDC][H₂(tcpp)] (blue).

Fig. 7. Time-dependent UV-vis spectra collected every 10 minutes during the metalation of 10 deposition cycle [ZrmBDC][H₂(tcpp)] (top), [ZrMe₂BDC][H₂(tcpp)] (middle), and [ZrFDC][H₂(tcpp)] (bottom) films with Mn(HMDS)₂. The change in freebase Soret absorbance as a function of time illustrates a marked cage dependence of metalation.

Fig. 8. Time-dependent UV-vis spectra collected every 10 minutes during the metalation of 10 (top), 20 (middle), and 30 (bottom) deposition cycle [ZrmBDC][H₂(tcpp)] films with Mn(HMDS)₂.

Fig. 9. Change in freebase Soret band absorbance as a function of time illustrates the thickness dependence of metalation where the first ∼10 deposited layers of each film display rapid metalation while subsequent metalation is sluggish as a result of diffusion limitations in the thicker films.