Studying physisorption processes and molecular friction of cycloparaphenylene molecules on graphene nano-sized flakes: role of π⋯π and CH⋯π interactions

We theoretically study, by means of dispersion-corrected and cost-effective methods, the strength of non-covalent interactions between cyclic organic nanorings (i.e. [8]cycloparaphenylene molecule) and nano-sized (e.g. C96H24) graphene flakes acting as substrates. Both CH⋯π and π⋯π driven interactions are investigated, according to the relative orientation between the two weakly interacting monomers, whose potential energy profiles are accurately calculated in both cases. These configurations provide different physisorption curves, with the CH⋯π interaction leading to a larger well depth, and are found to slightly depend on edge effects of the nano-sized graphene flakes. Additionally, we fit the energy profiles to a compact (analytical) potential function, and study the atomic-scale friction between the molecule and the surface in the search of mechanisms for new molecular machines.This work is supported by the “Ministerio de Economía y Competitividad” of Spain and the “European Regional Development Fund” through project CTQ2014-55073-P

Formation of stimuli-responsive cyclophanes by self-assembly: the case of carbazole-based biradicals

Dynamic covalent bonds has recently received lot of attention because of their unique feature to become reversible under mild conditions.[1] In this context, π-conjugated biradical compounds has emerged as essential building blocks.[2] For instance, we have demonstrated that 2,7-dicyanomethylene-9-(2-ethylhexyl)carbazole biradical reversibly converts to a macrocycle cyclophane upon soft stimuli (temperature, pressure, light), showing strong chromic effects.[3] We now extent this study towards longer conjugated carbazole backbone (i.e., indolocarbazole shown in Figure 1), aiming at investigating how the elongation of the conjugated backbone impacts on the formation of stimuli-responsive cyclophanes. The self-assembly process is investigated both in solution and solid state by linking theory and experiments.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Structure and Charge Transport Properties of Cycloparaphenylene Monolayers on Graphite

The nanoscale organization of cycloparaphenylene molecules when physisorbed on a graphite surface is theoretically investigated by means of atomistic molecular dynamics simulations employing a tailored and benchmarked force field. The landing of a single molecule is first considered, to progressively deposit more molecules to finally reach the full coverage of the surface. This protocol allows to study, consequently, the mechanism and structural pattern of their self‐aggregation. The interfacial morphologies obtained are then analyzed in terms of the electronic coupling between neighboring molecules, allowing thus to provide information about the associated charge‐transfer phenomena which could take place in these highly organized monomolecular layers.A.J.P.J. and J.C.S.G. acknowledge the project CTQ2014–55073-P from the Spanish Government (MINECO/FEDER) and the project AICO/2018/175 from the Regional Government (GVA/FSE). L.M. acknowledges funding from MIUR–PRIN 2015XJA9NT (Molecular Organization in Organic Thin Films via Computer Simulation of their Fabrication Processes)

Carbazole-based Diradicals for Dynamic Covalent Chemistry

Dynamic covalent chemistry (DCC) is focused on the creation of structural scaffolds based on chemical components that interact through strong but reversible bonds. In fact, dynamic covalent bonds receive lot of attention because of their unique feature to become reversible under mild conditions.1 conjugated diradical compounds has emerged as essential building blocks in DCC.2 In this work, we will review our most recent works on the formation of stimuli-responsive cyclophanes by self-assembly of carbazole-based diradicals. To this end, we use a combined experimental and theoretical approach that links vibrational spectroscopy with DFT calculations. In this sense, it is interesting to note that we have recently demonstrated the potential of a para-substituted carbazole with terminal dicyanomethylene (DCM) groups to act as building blocks in DCC.3 This quinoid carbazole monomer transforms to a macrocycle cyclophane upon soft external stimuli (temperature, pressure, light), showing strong chromic features. In addition, we have also recently explored how the different DCM substitution position affects the interesting chromoactive properties of carbazole compounds.4 Finally, we are currently exploring the effect of the elongation of the carbazole backbone on the formation of stimuli-responsive cyclophanes.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Multiresponsive chromic soft materials: formation of strongly coupled σ-dimers from IndoloCarbazole-based biradicaloids

Dynamic covalent chemistry is focused on the creation of structural scaffolds based on chemical components that interact through strong but reversible bonds. In fact, dynamic covalent bonds receive lot of attention because of their unique feature to become reversible under mild conditions.1 π-conjugated biradical compounds has emerged as essential building blocks in DCC (dynamic covalent chemistry).2 We have recently demonstrated the potential of a para-substituted carbazole with terminal dicyanomethylene groups to act as building blocks in DCC.3 In fact, this quinoid carbazole monomer transform to a macrocycle cyclophane upon soft external stimuli (temperature, pressure, light), showing strong chromic features. Here, we explore the effect of the elongation of the carbazole backbone on the formation of stimuli-responsive cyclophanes by self-assembly. To this end, we use a join experimental and theoretical approach that links vibrational spectroscopy (Raman and IR) with DFT calculationsUniversidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

High-frequency photothermal processing of commercial polymers under femtosecond laser irradiation for waveguide writing

The processing of three commonly used commercial polymer films (poly(vinyl chloride) (PVC), poly(ethylene terephthalate) (PET) and polypropylene (PP)) with different thermal properties under femtosecond (450 fs) irradiations at high frequency (1 kHz-1 MHz) multi-pulse (N=10-18000) laser at λ=515 (1.34 J/cm2, radius 9 μm) is analysed in order to have knowledge of which material and laser conditions are more suitable to write waveguides. Thermal and ablative effects are observed after laser irradiations. Heat accumulation effects of successive pulses impinging are simulated through a photothermal model in order to explain the results of irradiating these materials. Thermal analyses (Modulated Differential Scanning Calorimetry (MDSC) and Thermogravimetry (TG)) are performed and used to explain the different behaviour of each polymer. Three different regimes (non-thermal, thermal and saturation) are identified and explained from the model and experimental results. A connection between ablation depth and simulated reached temperature is established. A study of which number of pulses/spot area and frequency are appropriate for a better shape and ablation depth for writing waveguides on these polymers is performed.The work was supported by “Generalitat Valenciana” (IDIFEDER/2021/014 cofunded by FEDER program, project PROMETEO/2021/006, and INVEST/2022/419 financed by Next Generation EU), “Ministerio de Ciencia e Innovación” of Spain (projects PID2021-123124OB-I00; PID2019-106601RB-I00) and UATALENTO18-10 by “Universidad de Alicante”

Theoretical Study of Cyclic Pyrene Oligomers and Their Resemblance with Cyclic Paraphenylenes: Disclosing Structure–Property Relationships for Cyclic Nanorings

We theoretically discuss here the relationships between the structure of recently synthesized nanorings, dubbed as cyclo-2,7-pyrenylene (CPY) and formed upon bending and bonding a finite number of pyrene units until self-cyclation, and a set of chemically relevant properties such as the induced structural and energetical strain, the electronic and optical properties, or the response to charge injection, as well as their transport mechanism through a concerted migration of charge-carriers. We also compare these properties, and their evolution with the number of pyrene-linked units, with those obtained for the closely related cycloparaphenylene (CPP) compounds, trying to disclose the underlying structure–property guidelines. To do it, we always employ dispersion-corrected DFT methods to systematically include the key effects affecting all the properties tackled. A correct match with some available experimental results, for the [4]CPY compound (the only one synthesized so far), anticipates the accuracy of the calculations done for the rest of compounds. Finally, since this kind of systems are envisioned as possible precursors for the fine-tuned and controlled synthesis of carbon nanotubes, we also address the stability of the dimers found in their crystalline structure, and the associated cohesive energy, which may drive the synthesis of the corresponding nanotubes after an adequate dehydrogenation reaction.This work is supported by the “Ministerio de Economía y Competitividad’’ of Spain and the “European Regional Development Fund” through Project CTQ2014-55073-P

The role of topology in organic molecules: origin and comparison of the radical character in linear and cyclic oligoacenes and related oligomers

We discuss the nature of electron-correlation effects in carbon nanorings and nanobelts using an analysis tool known as fractional occupation number weighted electron density (ρFOD) and the RAS-SF method, revealing for the first time significant differences in static correlation effects depending on how the rings (i.e. chemical units) are fused and/or connected until closing the loop. We choose to study in detail linear and cyclic oligoacene molecules of increasing size, and relate the emerging differences with the difficulties for the synthesis of the latter due to their radicaloid character. We finally explore how minor structural modifications of the cyclic forms can alter these results, showing the potential use of these systems as molecular templates for the growth of well-shaped carbon nanotubes as well as the usefulness of theoretical tools for molecular design.A. J. P. J. and J. C. S. G acknowledge the “Ministerio de Economa y Competitividad” of Spain and the “European Regional Development Fund” through the project CTQ2014-55073-P, and E. S. F. M. through the project FIS2015-64222-C2-2-P. D. C. is thankful to Eusko Jaurlaritza and the Spanish Government MINECO/FEDER (projects IT588-13 and CTQ2016-80955). M. E. S.-S. acknowledges CONACyT-México for a PhD fellowship (ref. 591700)

From cyclic nanorings to single-walled carbon nanotubes: disclosing the evolution of their electronic structure with the help of theoretical methods

We systematically investigate the relationships between structural and electronic effects of finite size zigzag or armchair carbon nanotubes of various diameters and lengths, starting from a molecular template of varying shape and diameter, i.e. cyclic oligoacene or oligophenacene molecules, and disclosing how adding layers and/or end-caps (i.e. hemifullerenes) can modify their (poly)radicaloid nature. We mostly used tight-binding and finite-temperature density-based methods, the former providing a simple but intuitive picture about their electronic structure, and the latter dealing effectively with strong correlation effects by relying on a fractional occupation number weighted electron density (ρFOD), with additional RAS-SF calculations backing up the latter results. We also explore how minor structural modifications of nanotube end-caps might influence the results, showing that topology, together with the chemical nature of the systems, is pivotal for the understanding of the electronic properties of these and other related systems.A. J. P. J. and J. C. S. G acknowledge the project CTQ2014-55073-P from the Spanish Government (MINECO/FEDER) and the project AICO/2018/175 from the Regional Government (GVA/FSE). J. F. R. acknowledges the projects MAT2016-78625 from the Spanish Government (MINECO/FEDER) and projects No. PTDC/FIS-NAN/4662/2014 and No. PTDC/FIS-NAN/3668/2014 from the Portuguese Government (Fundaçao para a Ciencia e Tecnologia). D. C. is thankful to projects IT588-13 (Eusko Jaurlaritza) and CTQ2016-80955 from the Spanish Government (MINECO/FEDER). M. E. S.-S. acknowledges CONACyT-México for a PhD fellowship (ref. 591700). R. O. C. acknowledges “Generalitat Valenciana” and “Fondo Social Europeo” for a PhD fellowship (ACIF/2018/198)

Formation of Cyclophane Macrocycles in Carbazole-Based Biradicaloids: Impact of the Dicyanomethylene Substitution Position

We have recently demonstrated that carbazole-based biradicaloids are promising building blocks in dynamic covalent chemistry. To elucidate their intriguing dynamic covalent chemical properties, it is necessary to understand the physical origin of their biradical nature. To this end, here we focus on two quinoid carbazole systems substituted with dicyanomethylene (DCM) groups via para (p-Cz-alkyl) or meta positions (m-Cz-ph), which are able to form cyclophane macrocycles by the formation of long C–C bonds between the bridgehead carbon atoms linked to the DCM groups. We aim at exploring the following questions: (i) How is the biradicaloid character of a quinoid carbazole affected by the substitution position of the DCM groups? (ii) How is the stability of the resulted cyclophane aggregate attained? (iii) How is the dynamic interconversion between the carbazole-based monomers and cyclophane aggregates affected by this subtle change in the substitution pattern position? Density functional theory-based calculations reveal that both p-Cz-alkyl and m-Cz-ph are open-shell biradicals in the ground electronic state, with the DCM substitution in the meta position resulting in a more pronounced biradical character. In contrast, the derivatization via the nitrogen of the carbazole unit is not predicted to affect the biradicaloid character. The spontaneous nature of the cyclophane-based macrocycle formation (i.e., the cyclic tetramer in p-Cz-alkyl and the cyclic trimer and the tetramer in m-Cz-ph) is supported by the negative relative Gibbs free energies calculated at 298 K. Interestingly, cyclic oligomers in which the DCM groups are inserted in the meta position tend to adopt folded conformations with attractive π–π interactions resulting in more stable aggregates; in contrast, note that an extended ring-shaped conformation is acquired for (p-Cz-alkyl)4. In addition, the larger spin density on the bridgehead carbon atom in the meta-substituted system strengthens the bridging C–C bond in the aggregate forms, hampering its dissociation. In fact, the C–C bond dissociation of (m-Cz-ph)4 and (m-Cz-ph)3 was suppressed in solution state, although it was achieved in solid state in response to soft external stimuli (i.e., temperature and grinding). In summary, we report a very comprehensive study aiming at elucidating the challenging chemical properties of carbazole-based biradicaloid systems.The work at the University of Málaga was funded by the MINECO (CTQ2015-66897) and Junta de Andalucía (P09-FQM-4708) projects. The work at the University of Alicante was supported by the MINECO (CTQ2014-55073) and Generalitat Valenciana (AICO/2018/175) projects