Stepwise Adsorption of Alkoxy‐Pyrene Derivatives onto a Lamellar, Non‐Porous Naphthalenediimide‐Template on HOPG

Abstract The development of new strategies for the preparation of multicomponent supramolecular assemblies is a major challenge on the road to complex functional molecular systems. Here we present the use of a non‐porous self‐assembled monolayer from uC33‐NDI‐uC33, a naphthalenediimide symmetrically functionalized with unsaturated 33 carbon‐atom‐chains, to prepare bicomponent supramolecular surface systems with a series of alkoxy‐pyrene (PyrOR) derivatives at the liquid/HOPG interface. While previous attempts at directly depositing many of these PyrOR units at the liquid/HOPG interface failed, the multicomponent approach through the uC33‐NDI‐uC33 template enabled control over molecular interactions and facilitated adsorption. The PyrOR deposition restructured the initial uC33‐NDI‐uC33 monolayer, causing an expansion in two dimensions to accommodate the guests. As far as we know, this represents the first example of a non‐porous or non‐metal complex‐bearing monolayer that allows the stepwise formation of multicomponent supramolecular architectures on surfaces.

Abstract: The development of new strategies for the preparation of multicomponent supramolecular assemblies is am ajor challengeo nt he road to complex functional molecular systems. Here we present the use of a non-porous self-assembled monolayer from uC 33 -NDI-uC 33 ,anaphthalenediimide symmetrically functionalized with unsaturated 33 carbon-atom-chains, to prepareb icomponent supramolecular surfaces ystems with as eries of alkoxy-pyrene (PyrOR)d erivativesa tt he liquid/HOPG interface. While previous attempts at directly depositing many of these PyrOR units at the liquid/HOPG interface failed, the multicomponent approach throught he uC 33 -NDI-uC 33 template enabled control over molecular interactions and facilitated adsorption. The PyrOR deposition restructured the initial uC 33 -NDI-uC 33 monolayer,c ausing an expansion in two dimensions to accommodate the guests. As far as we know,t his represents the first example of a non-porous or non-metal complex-bearing monolayer that allows the stepwise formation of multicomponent supramolecular architectures on surfaces.
All the chemical structures investigated are shown in Figure 1, whereas their syntheses and characterization are presented in the Supporting Information (Supporting Information, pages S7-S14). Previous investigation highlighted the possibility to deposit pyrene moieties at the liquid/HOPG interface, providedt he simultaneous presence of other extended aromatic moieties in the chemical design. [35,50] Congruously,a ll our attempts at the "untemplated" deposition of PyrOMe, PyrSMe, PyrOEt and PyrOPr failed. The only exception was PyrOBu, which forms self-assembled monolayers at the 1-PO/HOPG interface (Supporting Information, Figure S4). Hence, we highlight the use of our uC 33 -NDI-uC 33 template as at ool to adsorb and organize with nanometer precision alkoxy-pyrene derivatives on HOPG.
We recently showed that long carbon-chain NDIs (C n -NDI-C n ) featuring internal double bonds along the carbon-chain self-assemble at the 1-PO/HOPG interface into lamellar monolayers. They compriseo fa lternating areas of NDI cores and carbon tails, which appear as bright protrusions and darker regions, respectively,i nt he STM images. [49] The internal double bonds along the carbon chains could also be imaged in the darker areas as additional bright protrusions symmetrically placed with respect to the aromatic cores. [49] The simultaneousp resence of EE-, EZ-and ZZ-configured isomerso fuC 33 -NDI-uC 33 did not impede the formation of long-range ordered lamellar domains,a lso due to the preferred physisorption for the less abundant EE-configured molecules. [49] Preparation of uC 33 -NDI-uC 33 monolayers confirmed our previous results, [49] as shown by the STM image and unit cell lattice parameters shown in Figure 2. Right after having obtained the monolayer,w er insed the modified HOPG with n-octanoic acid (OA) to remove the excess of uC 33 -NDI-uC 33 that did not adhere to the substrate and checked the preservation of the monolayera fter the rinsing step by STM (SupportingI nformation, Figure S1).
Next, the rinsed uC 33 -NDI-uC 33 monolayer was treated with a PyrOMe solution in OA. STM imaging of the newly treated surfacer evealed the presence of new protrusions placed in a zipper-like fashion alongside the NDI cores of the original lamellar morphology (Figure 3a). These new protrusions were more extended andb righter than the internal double bonds of uC 33 -NDI-uC 33 of Figure 2.
Intriguedb yt his preliminaryr esult, we repeated the experiment with PyrSMe,the sulfur analogue of PyrOMe,toexamine the possible consequences on both adsorption and imaging contrast deriving from the substitution of the heteroatom in the (thio)ether linkage. Similar STM images werer ecorded, with the new brightd ots ascribed to PyrSMe staying next to the NDI cores in az ipper-like fashion (Figure 3b). Replacing OA with 1-PO or n-tetradecane (TD) consistently afforded the same resultsw ith both PyrOMe (Figure 3c)a nd PyrSMe (Figure 3d), suggesting no influence of the solvent(s)c hosen to form the bicomponent system and the robustness of the multicomponent co-assembly. Time monitoring of the sequential adsorption of PyrOMe/PyrSMe onto the uC 33 -NDI-uC 33 template suggested the occurrence of nucleation processes:l ow   coverages by PyrOMe/PyrSMe werei maged at the initial stages of the process (5 min), with clear surface regionss howing only the initial uC 33 -NDI-uC 33 monolayer (Supporting Information, Figure S2). Prolonged exposure (3-4 hours) of the uC 33 -NDI-uC 33 template to the PyrOMe/PyrSMe solutionsr esulted in quantitative coverage (Figure 3).
Having demonstrated the consistency of our systems over a broad scope of solvents (OA, 1-PO and TD), we introduced subtle structural modificationsi nthe PyrOR molecular design. Extending the length of the alkyl chain by one (PyrOEt)o rt wo (PyrOPr)c arbon atoms resultedi nt he same outcomeo bserved with PyrOMe and PyrSMe,namely the zipper-like deposition of the pyrene derivativesa longside the NDI cores of uC 33 -NDI-uC 33 (Figure 4a for PyrOEt,F igure 4b for PyrOPr). In stark contrast, PyrOBu offered only modest signs of adsorption onto the uC 33 -NDI-uC 33 template at ca. 10 À2 m concentrations (Supporting Information, Figure S4). At en-fold increasei nc oncentration resulted in the displacement of uC 33 -NDI-uC 33 and subsequentf ormation of an ew monolayer uniquelyf ormed by PyrOBu,i nstead( Supporting Information, Figure S4). Such observation suggested that the competition between stepwise adsorption onto the uC 33 -NDI-uC 33 template and alkoxypyrene monolayer formation is controlled by the subtle interplay of different parameters, whichcertainly include concentration and adsorption energy of the alkoxy-pyrenes on HOPG. This balanceb ecomes particularly evident in the case of PyrOBu,w hich features al onger alkyl chain and likely enhanced van der Waals interactions, since the other PyrOR investigatedd on ot form monolayers at the liquid/HOPG interface. Indeed, the use of more concentrated (saturated) solutions of PyrOMe, PyrSMe, PyrOEt and PyrOPr did not affect their adsorption onto the uC 33 -NDI-uC 33 template.
In addition, we tested other potentialt emplates from the unsaturated and saturated C n -NDI-C n [49,51] family to elucidate essential structural parameters (the complete list of C n -NDI-C n tested is reported in Figure S5). Surprisingly enough, although these compounds are structurally very similart ouC 33 -NDI-uC 33 ,t heir assistance towards subsequenta dsorption of the PyrOR compounds was extremely modest. The case of C 33 -NDI-C 33 [49] (Figure S5) , which only differs from uC 33 -NDI-uC 33 by the absence of the internal double bonds in the chemical structure, is particularly remarkable. Currently, the reason for the high selectivity of the uC 33 -NDI-uC 33 monolayer is not understood, but it certainly highlights a" unique" character for this particulart emplate. Importantly,a ll the experimental results on co-assemblyo btained with the uC 33 -NDI-uC 33 monolayer were highly consistent and reproducible.
Further insights into the system were obtained by analyzing the profile plots of the STM images obtained duringt he early stages( 5min) of the PyrSMe adsorption ( Figure 5). The coverage of the uC 33 -NDI-uC 33 adlayer by PyrSMe was not quantitative during the initial stages of the process. Thus, the STM images displayedt he simultaneous presence of the uC 33 -NDI-uC 33 lamellae similart oF igure 2, and additional protrusions positioned in az ipper-like fashion with respect to the NDI cores as in Figures 3a nd 4( STM image of Figure 5). Profile plots measured over lengths of approximately 42 nm in both surfacea reas, that is, with and without adsorbed PyrSMe, highlighted remarkable differencesi nt he periodical organizations. The underlying uC 33 -NDI-uC 33 template was highly regular and consistent with an about 5nmp eriodical lamellar morphologyi na ccordance with our previousr eport [49] (orange trace in Figure 5, bottom). The profile plot of the surface area featuring the adsorbed PyrSMe showed less regularity,a lthoughac lear increase in the distance between the maxima was noticeable (cyan trace in Figure 5, top). The presence of 7 maximai n4 2nmr esulted in a6nm average distance between parallel arrays in the surfacea reasw ith PyrSMe.T his suggested that the parallel NDI cores increased their distance by ap-  proximately 1nmt oh ost PyrSMe and form the bicomponent system.
Combining all the information obtained, we formulate a schematic pictorial representation for the PyrOMe/uC 33 -NDI-uC 33 bicomponent system ( Figure 6). Although the clear-cut image in Figure 5f eatures PyrSMe,t he qualitative modelw as built for PyrOMe due to the larger statistics availablef or such PyrOR.W hile the cartoon depicts ar egulars ystem, we would like to stress that analysiso ft he STM images revealed fluctuations in the molecular arrangement.H ence, the illustration in Figure 6s hould be taken as aq ualitative description rather than an unambiguous unit cell of the PyrOMe/uC 33 -NDI-uC 33 monolayer.M oreover,t he orientation of the alkoxy-pyrene moieties was arbitrarily chosen in our qualitative model due to the lack of precise information obtained from the STM images. Ap revious model to explain the deposition of pyrenes in poroust emplates followed the same approach. [52] Statistical analysiso nanumber of STM images of the PyrOMe/uC 33 -NDI-uC 33 system allowed to estimate an a PyrOMe value as high as 6.05 AE 0.15 nm (averaged value for the assemblies in 1-PO and OA), whereas the a unit cell parameter for the original uC 33 -NDI-uC 33 monolayer is 5.27 AE 0.08 nm (a is shown in Figure 2). [49] The enhancemento ft he lateral distance between two rows of NDIs in the bicomponent system (a PyrOMe )p ointed to al ateral expansiono ft he initial uC 33 -NDI-uC 33 monolayer to accommodate PyrOMe.S ince the unsaturated carbon chains of uC 33 -NDI-uC 33 have as trongp reference for interdigitation, [49] we hypothesize that such lateral expansion necessarily pulls the carbon chains apart and createse xposed pockets at well-defined distances on the underlying HOPG substrate, while partially retaining the interdigitation of the template molecules. The lattice expansion alongt he a vector appears to reach am aximum limit, which explainst he decreased adsorption of PyrOR upon chain extension. Afurthere xpansion along the b direction is also necessary to favor the adsorption of PyrOMe,m odifying the b unit cell parameter from 0.94 AE 0.06 nm to an average value 1.84 AE 0.22 for b PyrOMe (obtained from the statistical analysis of an umber of STM images 1-PO and OA). The two-dimensional expansion of the uC 33 -NDI-uC 33 templatei si nl ine with the lack of preorganization of the initial monolayer.I ndeed, the latter must adapt to the presenceo f the PyrOR species and create the pockets fort heir adsorption. Additional stabilizing intermolecular interactions between pyr-enes and NDI cores cannotb ee xcluded within the bicomponent system. However, ah ypothesis on the exact geometry would be speculative due to the unknown orientation of the pyrene derivatives and awaits in depth computational studies. Nevertheless, it should be emphasized that the positioning of alkoxy-pyrenes alongside the NDI cores, and not in the typical stacked donor-acceptor configuration occurringi nc hargetransfer solution-phase supramolecular systems, [53] is au nique aspect of our system.
In conclusion, we presentedt he stepwise adsorption of alkoxy-pyrene derivatives PyrOMe, PyrSMe, PyrOEt, PyrOPr and PyrOBu onto al amellar,n on-porous uC 33 -NDI-uC 33 template on HOPG.T he deposition of these alkoxy-pyrenes brings to the formation of ab icomponent system that appears dramatically different from the initial monolayer upon STM imaging. New bright protuberances (the alkoxy-pyrenes) are imaged in az ipper-like fashion alongside the NDI cores. Moreover,t he formation of ab icomponents ystem significantly alters the organization of the initial uC 33 -NDI-uC 33 template, causing an expansion in both directions of the original unit cell directions to accommodate the guest in the template. To the best of our knowledge, the uC 33 -NDI-uC 33 template is the first self-assembled monolayer that allows for the stepwise construction of on-surfacem ulticomponent supramolecular architectures without resortingt ot he preorganization of porous architectures or host-guesti nteractions with metal-complexes. This offers au nique approach to establish future directions of supramolecular surface chemistry through stepwise multicomponent assembly.C urrente fforts of our joined research program focus on the use of the uC 33 -NDI-uC 33 template as am olecular track for the controlled unidirectionalt ranslation of light-driven molecular motors on surfaces. Figure 6. Qualitative representation of the PyrOMe/uC 33 -NDI-uC 33 bicomponent system with related a PyrOMe and b PyrOMe distances (6.05 AE 0.15 and 1.84 AE 0.22 nm, respectively). The bluer ectangle showni nF igure 3a sg uide to the eye is also displayed.