Templated bilayer self-assembly of fully conjugated π-expanded macrocyclic oligothiophenes complexed with fullerenes

Fully conjugated macrocyclic oligothiophenes exhibit a combination of highly attractive structural, optical and electronic properties, and multifunctional molecular thin film architectures thereof are envisioned. However, control over the self-assembly of such systems becomes increasingly challenging, the more complex the target structures are. Here we show a robust self-assembly based on hierarchical non-covalent interactions. A self-assembled monolayer of hydrogen-bonded trimesic acid at the interface between an organic solution and graphite provides host-sites for the epitaxial ordering of Saturn-like complexes of fullerenes with oligothiophene macrocycles in mono- and bilayers. STM tomography verifies the formation of the templated layers. Molecular dynamics simulations corroborate the conformational stability and assign the adsorption sites of the adlayers. Scanning tunnelling spectroscopy determines their rectification characteristics. Current–voltage characteristics reveal the modification of the rectifying properties of the macrocycles by the formation of donor–acceptor complexes in a densely packed all-self-assembled supramolecular nanostructure.

While several aspects of this study have in one way or the other been reported before (not necessarily for the same system), e.g. the combination provided in this study is of interest, of high quality, and complex. The data (STS) and data interpretation are supported by semi-quantitative modelling, based on concepts demonstrated by the Rabe group before.
While I favour publication, a number of aspects should be addressed in detail. In particular, more details should be given (in the supporting information). At this stage, not enough information is given to evaluate the statistical relevance of the STS data. In addition, certain statements seem not very well supported by the experimental evidence (e.g. difference in rectification ratio between 1 ML and 2 ML) -Concentration / composition of the solutions (for STS and STM topographs).
-Details on the STS o Fig. 3: what are the error bars for positive biases? o In general: What is the initial setpoint (current, voltage) to record the STS curves? (provide in Caption Figure 4, not in Figure 3). o Fig. 3: Why a "saturation" at -1,2 V? What is meant by "saturation"? Do you mean "levels off"? o Fig Fig S9b: rectification ratio: based upon these data, it is dangerous to claim there is a significant difference in the rectification ratio between 1 ML and 2 ML, ad the data are very scattered.
The article by Rabe et al. builds upon a previous study (reference 30) which describes the synthesis and characterisation of the 'Saturn-like' complexes formed between oligothiophenes and C60. In the current study a series of additional surface based characterisation is reported including STS etc.. The work is well carried out and it is difficult to criticise the quality of the science.
The authors should make it clearer that the E,E-8-mer.C60 is stable/favourable in solution and this has been established in the previous paper (reference 30). From the current study it isn't clear how stable this complex is and this would help the reader's understanding of the deposition process and whether it is surprising that the complex is deposited in the form described. I don't think it is surprising.
Indeed the results are not particularly significant in moving beyond what has been previously described by the same team. I don't find the paper to be a significant step beyond what they have already published (e.g. reference 30) and therefore I cannot see the justification for publishing the article in a journal such as Nature Communications. If there is a case it certainly hasn't been made in the manuscript and therefore I cannot recommend publication of the article.

Reviewer #3 (Remarks to the Author):
This contribution contains some interesting results related to the self-assembly of π-"expanded" macrocyclic oligothiophenes hosting fullerenes at the liquid-solid interface of a TMA-modified HOPG surface. Scanning tunneling microscopy and spectroscopy have been employed to analyze the characteristics of the ordered monolayers formed by the different components and molecular dynamic simulations have been as well performed. The manuscript lacks however of the degree of originality, importance, and interest, which is necessary for publication in Nature Communication. Concerning the validity of the conclusions extracted from the experiments, major and deep relevant doubts emerge and a comment follows below. I cannot recommend the manuscript for publication in Nature Communication.
The paper based on the adsorption of complex 3 onto the TMA-modified HOPG surface but no evidence of adsorption of the pristine complex can be obtain neither from the STS results nor from the STM images. Those show namely the adsorption of the macrocyclic oligothiophenes without fullerenes. The crystal structure of 3 bases on weak Van der Waals interactions and packing effects but is absolutely no proof for a stable complex formation at room temperature in heptanoic acid solution. The authors have to revise deeply the assumptions based on this preliminary key point over the entire manuscript. There is too much speculation about the results and data interpretation, which is not the best scientific approach that can be done with the experimental results presented in this communication. On another level, the authors claim several times about the need of "control" of the self-assembling and about supramolecular "engineering", but fairly speaking the paper show no more, no less, than basic observation of a process We appreciate that the reviews expressed constructive and positive comments. In order to improve this work, corrections based on comments from the reviewers are included in the revised version of the manuscript and supplemental information. Please find below our detailed responses to all concerns raised by the reviewers.

Reviewer #1
This study is one of the very few reports on the supramolecular organization of While I favour publication, a number of aspects should be addressed in detail. In particular, more details should be given (in the supporting information).
Question/comment 1. At this stage, not enough information is given to evaluate the statistical relevance of the STS data. In addition, certain statements seem not very well supported by the experimental evidence (e.g. difference in rectification ratio between 1 ML and 2 ML) Answer/correction 1. Regarding the statistical relevance of the STS data, we re-evaluated the data and calculated the average (arithmetic mean) of the rectification ratios, R(V 0 ), at the maximum bias value V 0 for all the curves, instead of the previously formulated average rectification ratio, R av . Changed have been made in the section "Results and discussion" of the manuscript and in the supplemental information.

Question/comment 2. Concentration / composition of the solutions (for STS and STM topographs).
Answer/correction 2. As mentioned in the method section, all solutions used heptanoic acid as solvent. Concentration has been added to the method section.

-Details on the STS
In addition, there are several minor typos that should be corrected.

Reviewer #2
The article by Rabe et al. builds upon a previous study (reference 30)

which describes the synthesis and characterisation of the 'Saturn-like' complexes formed between oligothiophenes and C60. In the current study a series of additional surface based characterisation is reported including STS etc.. The work is well carried out and it is difficult
to criticise the quality of the science.

Question/comment 1. The authors should make it clearer that the E,E-8-mer.C60 is stable/favourable in solution and this has been established in the previous paper (reference 30). From the current study it isn't clear how stable this complex is and this would help the reader's understanding of the deposition process and whether it is surprising that the complex is deposited in the form described. I don't think it is surprising.
Answer/correction 1. In the previous publication, we investigated the stability of the complex in toluene solution as part of its crystal characterization (Shimizu, H. et al. J. Am. Chem. Soc. 137, 3877 (2015)). In the present study we use a different solvent, heptanoic acid, which might cause a different in-solution stability of the complex. However, we believe this is not a key issue for the final outcome of the adsorption process.
In fact, the formation of the self-assembled monolayer may follow two pathways: (1) Two-step adsorption: The E,E-8mer·C 60 complex is first dissociated in solution, and C 60 is adsorbed on TMA sites to afford A. Then, E,E-8-mer forms E,E-8mer·C 60 complex on TMA sites, and the complex forms a 2D network B.
(2) One-step adsorption: The E,E-8mer·C 60 complex is adsorbed on TMA sites to form the bilayer self-assembly B.
These two pathways afford the templated bilayer self-assembly of the E,E-8mer·C 60 complex, although it is difficult to determine which process is favored. In the case of the first process, the stability of the E,E-8mer·C 60 complex in heptanoic acid is not a serious problem. In the Griessl et al. study (Griessl, S. J. H. et al. J. Phys. Chem. B 108, 11556-11560 (2004)), C 60 is adsorbed on TMA sites to afford A, although with low density of adsorbed C 60 due to the low solubility of C 60 in heptanoic acid. Therefore, the low solubility of C 60 in heptanoic displaces the equilibrium towards the second process.
We have added a short discussion about the dynamics of the adsorption process in the section "Results and discussion" of the manuscript.

Question/comment 2. Indeed the results are not particularly significant in moving beyond
what has been previously described by the same team. I don't find the paper to be a significant step beyond what they have already published (e.g. reference 30)… Answer/correction 2. In the present manuscript we report on the formation of a highly complex target structure, namely a templated bilayer of the E,E-8mer·C 60 complex. This goes significantly beyond the self-assembly of the sole E,E-8mer macrocycle. In fact the complex does not assemble by itself on graphite, it requires a specific molecular template (TMA). In order to achieve this goal we had to combine: 1. Modification of the HOPG surface using a TMA network 2. Effective (high density) host-guest complexation of C 60 into TMA adsorption sites.
4. Further self-assembly of a second monolayer of complexes on top of the first one with a similar arrange to its crystal phase.
5. Verification of the templated mono and bilayer by bias depended imaging, STS and MD simulations.
… and therefore I cannot see the justification for publishing the article in a journal such as Nature Communications. If there is a case it certainly hasn't been made in the manuscript and therefore I cannot recommend publication of the article.

Reviewer #3
This contribution contains some interesting results related to the self-assembly of π- Answer/correction 1. Our experiments using bias dependent STM imaging showed two kinds of patterns for different bias values: in the experiments with one monolayer of complexes on TMA, we resolve the TMA honeycomb structure for negative sample bias, and macrocycles for the opposite bias.
Macrocycle and C 60 form a donor-acceptor complex where the HOMO (LUMO) is located in the macrocycle (C 60 ). Using a model that has been previously described (Seifert, C. et al. Phys. Rev. B 80, 245429 (2009)), we interpret the invisibility of fullerene C 60 to the fact that the LUMO energy cannot be reached within our accessible bias range.
Furthermore, we performed experiments using TMA and macrocycle without C 60 and never got the double layer structure (detailed in the Supplemental Information section); therefore we conclude fullerene C 60 plays a decisive role in the formation of the double layer.
Finally, the STS measurements of E,E-8mer·C 60 complex on TMA-modified HOPG have features only reported before on fullerenes C 60 adsorbed on thiophene macrocycles (Mena-