Insight into the Oriented Growth of Surface-Attached Metal–Organic Frameworks: Surface Functionality, Deposition Temperature, and First Layer Order

Abstract

The layer-by-layer growth of a surface-attached metal-organic framework (SURMOF), [Cu2(F4bdc)2(dabco)] (F4bdc = tetrafluorobenzene-1,4-dicarboxylate and dabco = 1,4-diazabicyclo-[2.2.2]octane), on carboxylate- and pyridine-terminated surfaces has been investigated by various surface characterization techniques. Particular attention was paid to the dependency of the crystal orientation and morphology on surface functionality, deposition temperature, and first layer order. For the fully oriented deposition of SURMOFs, not only a suitable surface chemistry but also the appropriate temperature has to be chosen. In the case of carboxylate-terminated surfaces, the expected [100] oriented [Cu2(F4bdc)2(dabco)] SURMOF can be achieved at low temperatures (5 °C). In contrast, the predicted [001] oriented SURMOF on pyridine-terminated surface was obtained only at high deposition temperatures (60 °C). Interestingly, we found that rearrangement processes in the very first layer determine the final orientation (distribution) of the growing crystals. These effects could be explained by a surprisingly hampered substitution at the apical position of the Cu2-paddle wheel units, which requires significant thermal activation, as supported by quantum-chemical calculations.

Mechanistic Insights into Growth of Surface‐Mounted Metal‐Organic Framework Films Resolved by Infrared (Nano‐) Spectroscopy

“…Surface‐anchored films of metal‐organic frameworks (SURMOFs) can be synthesized in a straightforward manner by using a liquid phase layer‐by‐layer (LbL) synthesis approach 8, 9. The benefits of this method include the growth of highly homogeneous and oriented films at room temperature,8, 10 with diverse topologies,11, 12 enabling at the same time controlled mechanistic studies of MOF assembly13, 14 and guest‐induced framework changes 15, 16, 17…”

“…Subsequently, alternately immersing the SAM/Au substrates in separate metal and linker solutions, with solvent washing steps in between, assures controlled growth. Resulting SURMOF orientation is dictated by the terminating groups of the SAMs 14, 18, 19. The most studied example of a SURMOF is [Cu 3 (btc) 2 (H 2 O) n ], commonly known as HKUST‐1, which typically includes copper acetate (Cu(CH 3 COO) 2 , abbreviated here as CuAc 2 ) and 1,3,5‐benzenetricarboxylate (btc) as precursors 8, 20…”

“…Attempts to unravel LbL growth mechanisms in the initial stages by using spectroscopic tools have been previously performed, for example, by Zhuang et al. for the Cu 2 (F 4 bdc) 2 ‐(dabco) SURMOF (dabco=1,4‐diazabicyclo[2.2.2]octane), but so far no conclusive results have been found for HKUST‐1 14. Therefore, fundamental understanding, and as a result further synthetic improvements, are needed in order to justify advantages of the LbL approach over other synthetic strategies 19, 24, 25, 26, 27…”

“…Zhuang et al. have previously shown the potential of IRRAS to disclose initial stages of SURMOF growth at different synthesis temperatures 14. However, IRRAS does not provide microscopic insights into the topology and local nanoscale chemistry of SURMOFs.…”

Triptycene-terminated thiolate and selenolate monolayers on Au(111)

“…Recently, it has been shown that the growth of HKUST-1 on –COOH-terminated triptycene (Trp)-based SAMs proceeds along a different crystallographic axis than the growth on SAMs with a higher surface density of –COOH-functions, indicating that the surface density of –COOH units is a factor to liquid phase epitaxy of SURMOFs [20]. The temperature has another crucial impact on the efficiency and the direction of MOF growth on surfaces [3,21]. The strength of the chalcogenate–gold bond represents an upper limit to the MOF deposition temperature.…”