Exploring Spin-Crossover Cobalt(II) Single-Ion Magnets as Multifunctional and Multiresponsive Magnetic Devices: Advancements and Prospects in Molecular Spintronics and Quantum Computing Technologiesâ€
, Rafael Ruiz-GarcÃa 1 and Joan Cano 1,*Round 1
Reviewer 1 Report
Comments and Suggestions for Authors
This is an interesting and timely review, dedicated to M. Julve and F. Lloret in occasion of their retirement and now sadly to the memory of the former. It focuses on the use of mononuclear six-coordinate Co(II) Spin Crossover complexes showing slow relaxation of the magnetization appropriately functionalized to form metal molecular junctions or metal-organic frameworks (MOFs) or metal-covalent organic frameworks (MCOFs), and the perspective possibility measure the single-molecule electron transport and quantum coherence properties, bith relevant topics in the field of molecular nanoscience. The breadth of the subjects touched upon by the review makes it well suited for the readership of Magnetochemistry.
However, as it is quite obvious in such an ample manuscript, some improvements are required before publication is suggested:
1- Even if some of them are adapted from other publications. the graphic quality of the figures must be largely improved. This is particularly important for Figures 1,2,4,11.
2- As a general comment, I would avoid identifying LS Cobalt(II) as SIMs, since these are systems where no anisotropy barrier is present and slow relaxation is due to processes different from Orbach one. Slow relaxing molecules would be a more appropriate choice of words.
3- Authors refer a few times to the classic 1959 Feynman’s talk at Caltech, but uses a wrong sentence (and a wrong title of the corresponding paper in the reference): the correct sentence (and title of the talk transcript) is “there is plenty of space at the bottom”, not “in the bottom”.
4- On l. 125, when [Fe(TERPY-PhSAc)(TERPY-PhXY)]2+ is discussed, the full name should be given in brackets. In the actual form of the manuscript it is not clear from the text that each terpyridyl ligand is disubstituted -allowing grafting on the two metal end of a junction - and one can only get this information from the figure.
5- I find paragraph 1.2 not much fitting in the review. The first example reported there is related to observation of Kondo resonance, whereas the second reports on the grafting of a TbPc2 derivative to SWCNT and the subsequent characterization of the transport properties of these systems. While interesting, these results are not much relevant to the perspective authors report in the following, which mostly concern Cobalt(II) systems with SCO behaviour. I suggest dropping this or finding more appropriate examples.
6- On l. 233 authors identifies LS Co(II) as a “magnetically isotropic” S=1/2 system. This is not actually the case, since the complexes of this ion show relevant g-anisotropy. The difference with respect to a SIM is that these systems are S=1/2 so they do not have a barrier. This point should be corrected.
7- On l. 305 authors state that they demonstrated that a proton can switch the redox, luminescent, and spin dynamic properties of a single molecule. Even if the reference to the original literature is reported, some more detail on the way this occurs should also be reported here.
8- On l. 417-418 authors suggest that in the potential devices based on the molecular systems herein described, the electrical current flow would be governed by a magnetic field rather than by an electric one. However, this is currently considered a disadvantage rather than an advantage, given the difficulty in focusing the magnetic field at the nanoscale compared to the electric field. A comment should be added here.
9- On l. 433 authors correctly indicated that diamagnetic LS CoIII ion would in general lack any selectivity toward spin-polarized electrons. However, this might be different in the case of chiral systems, since in that case CISS effect may be effective. Authors should briefly comment on this point which is gaining increasing relevance in the field.
10- The discussion on the potential use of Co(II) based systems as molecular qubits should indicate that it is expected that any decohernece, if presence, is expected to rapidly collapse on increasing temperature, due to the strong SOC contribution which leads to a T1 rapidly decreasing on increasing temperature.
11- L. 568 ps should be changed in ns
12- All along the manuscript, some important symbols in Greek letters were not correctly printed: mu (before s in the discussion of decoherence times), alfa and beta when discussing spin states.
Author Response
Comments 1: Even if some of them are adapted from other publications. the graphic quality of the figures must be largely improved. This is particularly important for Figures 1,2,4,11.
Response 1: Following the reviewer’s suggestion, the quality of Figures 1, 2, 3, 4, 7, and 11 have been appropriately improved. Some original figures in the bibliography needed definitions in some parts, so we decided to reconstruct them partially. We have also reviewed other figures and improved them if we considered it appropriate.
Comments 2: As a general comment, I would avoid identifying LS Cobalt(II) as SIMs, since these are systems where no anisotropy barrier is present and slow relaxation is due to processes different from Orbach one. Slow relaxing molecules would be a more appropriate choice of words.
Response 2: The first SIMs reported by Ishikawa in 2003 in the form of the mononuclear double-decker bis(phthalocyaninato)terbium(III) and dysprosium(III) complexes, abbreviated as TbPc2 and DyPc2, possess indeed large anisotropy barriers for the spin reversal which are responsible for the Orbach mechanism of slow magnetic relaxation in these systems. However, there is now plenty of lanthanide-based SIMs and particularly, those concerning the magnetically isotropic GdIII ion, that do not follow an Orbach mechanism but a resonant phonon trapping through a phonon-bottleneck mechanism. This situation is particularly true for the vast majority of transition metal-based SIMs with easy-plane (D > 0) magnetic anisotropy, where alternative Raman or direct mechanisms are operative. In fact, this type of dynamic magnetic behavior was unexpected in mononuclear complexes. Therefore, SIM was accepted to differentiate it from the acronym SMM used for polynuclear complexes. We agree with the reviewer's comment and have acted accordingly in previous publications where we only discussed one or a few compounds. However, we must review a broad set of compounds in a wide set of publications.
Indeed, using new terms and acronyms is convenient but also leads to ambiguous situations when approaching boundaries. Thus, for practical reasons, we believe it is not convenient to modify the text according to the reviewer's suggestion since either this class would exclude such a large number of slow relaxation systems previously identified as SIM in recent literature, or it would imply also considering those based on polynuclear complexes.
As a matter of fact, the slow magnetic relaxation behavior along with this family of SCO cobalt(II)-TERPY and PDI complexes herein described is dominated by single or double Raman mechanisms, with no participation of Orbach one, regardless of their LS or HS nature and type of magnetic anisotropy in the latter case, either easy-axis (D< 0) or easy-plane (D > 0), and, occasionally, with additional temperature-independent Intra-Kramer [LS or HS (D > 0)] or QTM mechanisms [HS (D < 0)] (see refs. [106] and [107]). We have discussed this point in the revised version (see section 2.3.1).
Comments 3: Authors refer a few times to the classic 1959 Feynman’s talk at Caltech, but uses a wrong sentence (and a wrong title of the corresponding paper in the reference): the correct sentence (and title of the talk transcript) is “there is plenty of space at the bottom”, not “in the bottom”.
Response 3: Following the reviewer's suggestion, the wrong sentence in the manuscript and the title in reference [2] for Feynman‘s talk have been rectified.
Comments 4: On l. 125, when [Fe(TERPY-PhSAc)(TERPY-PhXY)]2+ is discussed, the full name should be given in brackets. In the actual form of the manuscript it is not clear from the text that each terpyridyl ligand is disubstituted -allowing grafting on the two metal end of a junction - and one can only get this information from the figure.
Response 4: The authors already used the nomenclature for the [Fe(TERPY-PhSAc)(TERPY-PhXY)]2+ complex in the original paper. We have clarified the reviewer's concern by identifying the nature of each TERPY ligand as TERPY-PhSAc and TERPY-PhXY.
Comments 5: I find paragraph 1.2 not much fitting in the review. The first example reported there is related to observation of Kondo resonance, whereas the second reports on the grafting of a TbPc2 derivative to SWCNT and the subsequent characterization of the transport properties of these systems. While interesting, these results are not much relevant to the perspective authors report in the following, which mostly concern Cobalt(II) systems with SCO behaviour. I suggest dropping this or finding more appropriate examples.
Response 5: We find the reviewer’s comment very appropriate indeed. A few examples exist in the recent literature concerning the deposition of appropriately ligand-functionalized Co SIMs on different inorganic and organic supports (silicon or graphene) while preserving their singular SIM behavior. Still, their single-molecule electron transport and quantum coherence properties were not investigated (see new refs. [125-129]). Thus, the present perspective review precisely attempts to fill this gap by proposing the families of cobalt(II)-TERPY and PDI SCO-SIM complexes as suitable candidates for spin quantum nanodevices in SMS.
The vast majority of reports concerning the use of SIMs as spin quantum nanodevices in SMS are restricted to the well-known [Tb(PC)2] complex and its ligand functionalized derivatives (see refs. [69-71]). Even if they are not the best examples within the framework of the present perspective review, the alternative of dropping this part, albeit initially contemplated, was finally discarded. Indeed, these pioneering studies serve to illustrate how the addressing of Ln SIMs over gold surfaces and SWCNTs (see Figures 3 and 4) can be conveniently translated to our family of cobalt(II)-PDI SCO-SIM complexes when appropriately functionalized with pyrene substituents (see Figure 12). Anyway, the further rediscussion of these studies in section 2.2.3 has been conveniently reduced, as suggested by reviewer 3.
Comments 6: On l. 233 authors identifies LS Co(II) as a “magnetically isotropic” S=1/2 system. This is not actually the case, since the complexes of this ion show relevant g-anisotropy. The difference with respect to a SIM is that these systems are S=1/2 so they do not have a barrier. This point should be corrected.
Response 6: In response to the reviewer’s suggestion, a brief discussion on the optical (luminescent) and redox switching behavior in these benzoic/benzoate-substituted CoII-TERPY complexes is now provided (see section 2.1.2).
Comments 7: On l. 305 authors state that they demonstrated that a proton can switch the redox, luminescent, and spin dynamic properties of a single molecule. Even if the reference to the original literature is reported, some more detail on the way this occurs should also be reported here.
Response 7: In response to the reviewer’s suggestion, a brief discussion on the optical (luminescent) and redox switching behavior in these benzoic/benzoate-substituted CoII-TERPY complexes is now provided (see section 2.1.2).
Comments 8: On l. 417-418 authors suggest that in the potential devices based on the molecular systems herein described, the electrical current flow would be governed by a magnetic field rather than by an electric one. However, this is currently considered a disadvantage rather than an advantage, given the difficulty in focusing the magnetic field at the nanoscale compared to the electric field. A comment should be added here
Response 8: The use of electric or magnetic fields to govern the electric current flow in EFE-SQTs and MFE-SQTs are not excluding, but complementary. Even if the electric field could be more timing and positioning precise, to our knowledge, there are no such difficulties in focusing the magnetic field over the molecule placed between the source and drain electrodes of the molecular junction. From an operational viewpoint, the magnetic field would be provided by a magnet generating up to 1 T, like in related spin valve molecular conductance measurements (see ref. [73]).
Comments 9: On l. 433 authors correctly indicated that diamagnetic LS CoIII ion would in general lack any selectivity toward spin-polarized electrons. However, this might be different in the case of chiral systems, since in that case CISS effect may be effective. Authors should briefly comment on this point which is gaining increasing relevance in the field.
Response 9: Following the reviewer's suggestion, we have included a short paragraph and three additional references discussing the CISS effect and some of its applications related to the topic of this perspective review.
Comments 10: The discussion on the potential use of Co(II) based systems as molecular qubits should indicate that it is expected that any decohernece, if presence, is expected to rapidly collapse on increasing temperature, due to the strong SOC contribution which leads to a T1 rapidly decreasing on increasing temperatura.
Response 10: A brief sentence has been added to discuss the role of SOC in the quantum decoherence times, as suggested by the reviewer (see section 2.3.1). Besides, the temperature values associated with the decoherence times, whenever lacking, have been now included within the text for reasons of completeness (see section 2.3). As discussed now in the text, 1 ms is a good relaxation time at 2 K in terms of magnetic coherence performance.
Comments 11: L. 568 ps should be changed in ns.
Response 11: The error in the time units has been corrected as the reviewer suggested.
Comments 12: All along the manuscript, some important symbols in Greek letters were not correctly printed: mu (before s in the discussion of decoherence times), alfa and beta when discussing spin states.
Response 12: We have checked all possible errors in the Greek letters. They all appear correctly, even in the PDF file we generated. We think the error arises from the manipulation of the software used by the publisher to build the final file submitted to the reviewers.
Reviewer 2 Report
Comments and Suggestions for Authors
Molecular materials have played an important role in the field of material science. However, molecular spintronics and quantum computing technologies based on molecular materials are still face significant challenges, particularly in terms of how to achieve their applications. In this paper, the authors summarized the strategy using mononuclear six-coordinate Co(II) SCO-SIM complexes to prepare single-molecule devices. At present, there is indeed a need for such a summary in relevant fields, so that readers can set research ideas based on the development situation in this area. So, I recommend it to be published in Magnetochemistry.
Questions as following:
1. The authors focus spin-crossover and single-ion magnets as the main properties of materials to review the progress of single-molecule prototype devices, giving the impression that it should include the relationship between two properties in device fabrication. But there is no such example in the main text. In fact, even in the work reported previously, we did not pay attention to combining the two. The terms SCO and SIM appear in the title. Can the authors make some adjustments to reduce readers' misunderstanding of this expression? If it's difficult, then in its current form.
2. Review articles often use many abbreviations, and this article is no exception. I often forget the full names of abbreviations when reading this article, but it is not very convenient to search, so I suggest the authors to create a list of abbreviations before the Introduction to facilitate reading.
3. The most representative prototypes in quantum computing research are the IBM Q System One, Google's Sycamore, and Jiuzhang by Jian-Wei Pan (Science 2020, 370, 1460; Phys. Rev. Lett. 2020, 125, 210502). The authors have discussed the IBM and Google systems on lines 57-60, but the Jiuzhang system should also be included.
4. Some important symbols are missing. Please check. For example, on lines 154, 188, 191, 274.
4. Figures 5b, 6b and 14 cannot intuitively show that the two ligand planes are perpendicular. Regardless of the original image in the reference, please redraw a more three-dimensional image. Maybe, the authors can refer to Figure 19a to add two colors.
Author Response
Comments 1: The authors focus spin-crossover and single-ion magnets as the main properties of materials to review the progress of single-molecule prototype devices, giving the impression that it should include the relationship between two properties in device fabrication. But there is no such example in the main text. In fact, even in the work reported previously, we did not pay attention to combining the two. The terms SCO and SIM appear in the title. Can the authors make some adjustments to reduce readers' misunderstanding of this expression? If it's difficult, then in its current form.
Response 1: We agree with the reviewer about searching for some relationship (synergy) between the SCO and SIM properties, but we assume this point does not justify changing the title. While finding synergy is challenging given the different temperature regions for each phenomenon, other alternatives exist that we are just exploring along this family of SCO cobalt(II)-TERPY and PDI complexes. Hence, both SCO and SIM behaviors are sensible to other stimuli besides temperature, whether chemical (pH or analytes) or physical (light or electron flow), which allows the control of the spin state and spin relaxation dynamics simultaneously so that a spin modulation of the spin relaxation dynamics is made possible. This critical point has been further discussed in the revised version, both within the respective individual sections and in the conclusion (see sections 2.1.2, 2.2.1, and 2.3.1), to clarify the possible misunderstanding when using together the SCO and SIM expressions in the title of the present perspective review.
So, for instance, light has been used at length to promote LS-HS transformations at low temperatures (where the SIM should also work) through the well-known LIEEST and LD-LISC processes, which we are currently exploring (particularly the latter one) in this perspective review by synthesizing photo-active TERPY ligands (see Figure 9). Likewise, the photo-switching magnetic behavior between LS and HS forms found in the parent CoII-TERPY complex upon MLCT excitation inspired us to propose synthesizing related opto-active PDI ligands where SCO and optical properties converge, but also SIM ones (see Figure 14). In truth, different spin relaxation dynamics can be presumed for the ground LS (ON) and excited HS (OFF) states along this family of cobalt(II)-TERPY and PDI complexes after light excitation.
On the other hand, the dual metal- and ligand-centered capacitor-like behavior exhibited by this family of cobalt(II)-PDI complexes could be the foundations for a unique ligand-based molecular spin quantum electro-switch (see Figure 11), which complements the already mentioned metal-based one (see Figure 7), where SCO and redox properties converge, but also SIM ones. Indeed, we can infer different spin relaxation dynamics for the (LS/HS)-[CoII(PDI)2]2+ (ON) and (LS/HS)-[CoII(PDI·–)2] pairs (OFF) after chemical (or electrochemical) ligand reduction (see Figure 11), like for the slow-relaxing paramagnetic LS CoII ion (ON) and the diamagnetic LS CoIII ion (OFF) after chemical (or electrochemical) metal oxidation (see Figure 7).
Comments 2: Review articles often use many abbreviations, and this article is no exception. I often forget the full names of abbreviations when reading this article, but it is not very convenient to search, so I suggest the authors to create a list of abbreviations before the Introduction to facilitate reading.
Response 2: Following the reviewer’s suggestion, a list of abbreviations (in alphabetical order) was included before the introduction, just after the keywords, for easy reading. In any case, we define each abbreviation the first time it appears in the text.
Comments 3: The most representative prototypes in quantum computing research are the IBM Q System One, Google's Sycamore, and Jiuzhang by Jian-Wei Pan (Science 2020, 370, 1460; Phys. Rev. Lett. 2020, 125, 210502). The authors have discussed the IBM and Google systems on lines 57-60, but the Jiuzhang system should also be included
Response 3: The Jiuzhang photonic quantum computer developed by Pan et al. is now discussed in the introduction (see new refs. [7] and [8]), as suggested by the reviewer.
Comments 4: Some important symbols are missing. Please check. For example, on lines 154, 188, 191, 274.
Response 4: Please, read the response to the twelfth comment by the first reviewer.
Comments 5: perpendicular. Regardless of the original image in the reference, please redraw a more three-dimensional image. Maybe, the authors can refer to Figure 19a to add two colors.
Response 5: Following the reviewer’s suggestion, Figures 5b, 6b, 7, 8, 9, and 14 have been redrawn by changing the image orientation and using different black and gray colors for the backbone of the two distinct perpendicularly oriented ligands.
Reviewer 3 Report
Comments and Suggestions for Authors
The perspective review by Cano and co-workers focusses on cobalt(II) complexes displaying SIM and/or SCO behaviour as core components of future devices for spintronics, quantum computation, and related applications. Frankly speaking, I fully agree with the opinion the authors express in the last part of the text, namely that "...most proposals concerning the scaling and addressing of spin-crossover cobalt(II) molecular nanomagnets as prototypes of single-molecule quantum spintronics and quantum computing devices feel more like daydreams or aspirations than tangible achievements", and I include this review in the list of papers emerging more in terms of fundamental science than of realistic applications.
The manuscript is reasonably well written, especially in the very first part of the Introduction and in the Conclusions and Epilog.
Though, I have some concerns on the way the paper is organized.
Sections 1.1 and 1.2 provide an introduction based on selected examples from the literature. The cited articles (59-71) are mostly old works, with only one paper from the last 5 years. Furthermore, most of the presented compounds do not contain cobalt(II) but are based on Fe or Tb. It is thus unclear why the authors used these specific examples rather than focussing on selected, relevant work on cobalt(II) complexes. There is nothing wrong in comparing different metals, but this part should remain focussed on the main topic of the review. Note: some of this excessive "extra" material is also re-discussed later in the article at the beginning of paragraph 2.2.3.
The manuscript also contains a number of inaccuracies or unclear points.
a) On page 12, Figure 10 presents ligands with X-X = CH2-CH2 and CH=CH. It is impossible to understand how the latter linkage is introduced using the depicted reactions. The same figure (panel b) is cited in the discussion on chemo- and photo switching using alkali-metal ions and vinyl groups, respectively. A clearer description of these processes would require additional panels.
b) All acronyms (e.g. FMR and SMR on page 14) must be defined.
c) Among the bullet points in paragraph 2.2.2, only (iii) is written as a complete sentence.
d) Many symbols do not appear correctly (e.g. spin-up and spin-down on page 4, azimuthal angle and orbital type on page 6, time units in section 2.3)
e) On page 18, the sentence "Examples of this feature..." is badly constructed.
f) On page 22, the name of the main author of ref. 152 contains a misprint.
g) The dedication should obviously be revised after the premature death of prof. Julve this year.
Overall, this manuscript looks adequate for publication in Magnetochemistry after revision.
Author Response
Comments 1: Though, I have some concerns on the way the paper is organized. Sections 1.1 and 1.2 provide an introduction based on selected examples from the literature. The cited articles (59-71) are mostly old works, with only one paper from the last 5 years. Furthermore, most of the presented compounds do not contain cobalt(II) but are based on Fe or Tb. It is thus unclear why the authors used these specific examples rather than focusing on selected, relevant work on cobalt(II) complexes. There is nothing wrong in comparing different metals, but this part should remain focused on the main topic of the review. Note: some of this excessive "extra" material is also re-discussed later in the article at the beginning of paragraph 2.2.3.
Response 1: This reviewer has some concerns about how the paper is organized. He/she particularly questions the selected examples in sections 1.1 and 1.2 from the introduction, mostly from old works on iron(II)-based SCO and terbium(III)-based SIM compounds. In the following, we will try to briefly justify our choice (see also our response to reviewer 1's fifth comment).
The selected Fe SCO and Tb SIM provide archetypical (text-book) examples of spin quantum nanodevices, such as transistors, valves, and filters, for potential applications in SMS. Unfortunately, as far as we know, there are no such relevant reports of Co SCO or Co SIM functioning as spin quantum nanodevices in SMS. A remarkable exception, however, is the family of thiol-functionalized cobalt(II)-TERPY SCO complexes (see refs. [61] and [62]), which are also included in this perspective review as prototypes of spin quantum transistors (see the second paragraph of section 1.1). A few examples exist in the recent literature during the last five years, however, concerning the deposition of appropriately ligand functionalized Co SCO or Co SIM on different inorganic and organic supports (silver, silicon, or graphene) while preserving their singular SCO or SIM behavior. Still, their single-molecule electron transport and quantum coherence properties were not investigated (see new refs. [125-130]). Following the lead of our response to one of the first reviewer's comments, we attempt here to contribute to fill this gap by suggesting this family of cobalt(II)-TERPY and PDI SCO-SIM complexes as suitable candidates for spin quantum nanodevices in SMS and QIP.
Comments 2: On page 12, Figure 10 presents ligands with X-X = CH2-CH2 and CH=CH. It is impossible to understand how the latter linkage is introduced using the depicted reactions. The same figure (panel b) is cited in the discussion on chemo- and photo switching using alkali-metal ions and vinyl groups, respectively. A clearer description of these processes would require additional panels.
Response 2: The proposal of the [2]catenane vinyl-derivative and its photoswitch were removed from the text once we could not find a reliable methodology. Furthermore, a new scheme was introduced in Figure 10 to clarify the proposed chemo-switch of the magnetic properties triggered by the presence of alkali ions in the medium.
Comments 3: All acronyms (e.g. FMR and SMR on page 14) must be defined.
Response 3: Please, read the response to the second comment by the reviewer 2.
Comments 4: Among the bullet points in paragraph 2.2.2, only (iii) is written as a complete sentence.
Response 4: The first and second bullet points in section 2.2.2. were rewritten as a complete sentence.
Comments 5: Many symbols do not appear correctly (e.g. spin-up and spin-down on page 4, azimuthal angle and orbital type on page 6, time units in section 2.3).
Response 5: Please, read the response to the twelfth comment by the first reviewer.
Comments 6: On page 18, the sentence "Examples of this feature..." is badly constructed.
Response 6: The sentence has been rewritten.
Comments 7: On page 22, the name of the main author of ref. 152 contains a misprint.
Response 7: The author’s name has been corrected.
Comments 8: The dedication should obviously be revised after the premature death of prof. Julve this year.
Response 8: The dedication in the title footnote has been conveniently revised to account for the premature death of Prof. Julve this year.
Round 2
Reviewer 3 Report
Comments and Suggestions for Authors
The authors have amended their manuscript in a satisfactory way based on my previous comments. I have no further objection against publication of this manuscript in Magnetochemistry.
