Modular Assembly of FTO|Chromophore-Catalyst Hierarchical Films Based on Strong Dipole Interactions

We have designed and characterized modular self-assembled hierarchical films containing a molecular catalyst tethered to an anchoring molecule by means of dipole–induced dipole interactions. In order to do so, two new Co^III-based molecular catalyst candidates were designed, namely, [Co^IIIL¹(pyrr)₂]ClO₄ (Co1) and [Co^IIIL²(pyrr)₂]ClO₄ (Co2), where L¹ and L² are the respective deprotonated forms of N,N′-[4,5-bis(dodecyloxy)-1,2-phenylene]dipicolinamide and N,N′-[4,5-bis(methoxyethoxy)-1,2-phenylene]dipicolinamide and were characterized by electrochemical, electronic, and film formation properties. Species Co1 and Co2 were deposited onto an anchor molecule such as octylphosphonic acid (OPA) or the chromophoric [Ru^II(bpy^PO3H)₂(bpy^C7)]Cl₂ (Ru) previously attached onto conductive fluorine-doped tin oxide (FTO). Four hierarchical films of the form substrate|anchor-catalyst were obtained, namely, FTO|OPA-Co1, FTO|OPA-Co2, FTO|Ru–Co1, and FTO|Ru–Co2, and the role of dipole–dipole interactions between anchor and catalyst modules was assessed. These newly synthesized hierarchical films were characterized by a host of surface-specific methods that include X-ray photoelectron spectroscopy, ellipsometry, X-ray fluorescence, and water contact angle, thus enabling an unprecedented level of analysis. Compared to the weak C–H van der Waals interactions exhibited by Co1, the presence of alkoxy chains in Co2 ensures stronger dipole–dipole interactions with the alkyl chain of the anchors due to O···H formation. The persistence of their redox properties, which include metal oxidation, and directionality of electron transport were probed suggesting direct relevance to catalytic processes such as water oxidation.