Spontaneous small molecule migration via reversible Michael reactions

Small molecule walkers developed to date take advantage of the reversibility of dynamic covalent bond formation to transport molecular fragments along molecular tracks using both diffusion processes and ratchet mechanisms. However, external intervention (the addition of chemical reagents and/or irradiation with light) is required to mediate each step taken by the walker unit in systems reported so far. In this Thesis, the first synthetic small molecule able to walk back-and-forth upon an oligoethylenimine track without external intervention via intramolecular Michael and retro- Michael reactions is described. The 1D random walk is highly processive and exchange takes place between adjacent amine groups in a stepwise fashion. The walker is used to perform a simple task: quenching of the fluorescence of an anthracene group situated at one end of the track as a result of the walking progress. In the presence of excess of base, the molecule preferentially ‘walks’ towards the favoured final foothold of tracks of increasing length and it is possible to monitor the population of all or a few positional isomers over time. In each case the molar fraction of walkers reaching the final foothold is determined quantitatively by 1H NMR. Control over the rate of exchange is achieved by varying the amount of base added. The dynamic migration of a small molecule upon the track is a diffusion process limited to one dimension and as such can in principle be described using the one dimensional random walk. Chapter I identifies a set of fundamental walker characteristics and includes an overview of the DNA-based and small molecule transporting systems published to date. Chapter II describes the inspiration for this work and model studies which lay the groundwork for the research presented in this thesis. The initial track architecture and optimisation of reaction conditions are demonstrated using a simple model compound which then led to the development and a detailed investigation of a first synthetic small molecule able to walk upon an oligoethylenimine track without external intervention. Chapter III presents a modified synthetic route towards the desired walker-track architectures and a comprehensive investigation of the dynamic properties of a series of tracks of increasing length upon which the walker migrates in a unidirectional fashion. The Outlook contains closing remarks about the scope and significance of the presented work as well as ideas for the design of novel small-molecule walkers, some of which are well under way in the laboratory. Chapter II (with the exception of model studies included at the beginning of the chapter) is presented in the form of article that has recently been published. No attempt has been made to re-write this work out of context other than merging content of the article with the supplementary information published together with the article. Chapter II is reproduced in the Appendix in its published format

Organic free radicals as circularly polarized luminescence

Chiroptical properties of two chiral atropisomers of propeller‐like trityl‐based radical derivatives have been analyzed. A new absolute configuration (AC) assignment has been made, according to the combination of experimental and theoretical data. In this sense, their ACs have been determined through the comparison of the Cotton effects recorded by electronic circular dichroism (ECD) with the theoretical ECD of the open shell structures obtained by TD‐DFT calculations. Finally, their circularly polarized luminescence (CPL) responses have been addressed. Remarkably, this is the first description of organic free radicals as intrinsic CPL emitters. Opposite signed CPL has been detected for each pair of conformers, with acceptable luminescent dissymmetry factors (|glum|≈0.5–0.8×10−3) considering their pure organic nature. In fact, highly efficient chiral emissions have been demonstrated, according to the comparison of |glum| with their respective absorption anisotropy factors (|gabs|). This pioneering study lays the foundations for the optimization of new magnetically active organic chiral emitters.The authors are grateful for the financial support received from: MOTHER (MAT2016-80826-R) granted by the DGI (Spain), GenCat (2017-SGR-918) financed by DGR (Catalunya) and the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-2015-0496) and through the “Proyecto interdisciplinar de frontera“, FIP-2018 HECTIC-PTM. We acknowledge the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (ERC-2015-STG-677023). This study has been also supported by the Networking Research Center on Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN), an initiative funded by the VI National R&D&I Plan 2008-2011, Iniciativa Ingenio 2010, Consolider Program, CIBER Actions and financed by the Instituto de Salud Carlos III with assistance from the European Regional Development Fund. P. M. B. gratefully acknowledges financial support from the Juan de la Cierva-Formación 2015 programme (FJCI-2015-23577) supported by MINECO. A. G. C. also thanks RyC-2013-12943 contract from MINECO. We also acknowledge Prof. O. Trapp for kindly suppling the DCxplorer software

An Enantiopure Propeller‐Like Trityl‐Brominated Radical: Bringing Together a High Racemization Barrier and an Efficient Circularly Polarized Luminescent Magnetic Emitter

A new persistent organic free radical has been synthetized with Br atoms occupying the ortho‐ and para‐positions of a trityl core. After the isolation of its two propeller‐like atropisomers, Plus (P) and minus (M), their absolute configurations were assigned by a combination of theoretical and experimental data. Remarkably, no hints of racemization were observed up to 60 °C for more than two hours, due to the higher steric hindrance imposed by the bulky Br atoms. Therefore, when compared to its chlorinated homologue (t1/2=18 s at 60 °C), an outstanding stability against racemization was achieved. A circularly polarized luminescence (CPL) response of both enantiomers was detected. This free radical shows a satisfactory luminescent dissymmetry factor (|glum(592 nm)|≈0.7×10−3) despite its pure organic nature and low luminescence quantum yield (LQY). Improved organic magnetic CPL emitters derived from the reported structure can be envisaged thanks to the wide possibilities that Br atoms at para‐positions offer for further functionalization.The authors are grateful for the financial support received from: MOTHER project (MAT2016-80826-R) granted by the DGI (Spain), GenCat (2017-SGR-918) financed by DGR (Catalunya) and the Spanish Ministry of Economy and Competitiveness (PGC2018-095808-B-I00 and PGC2018-101181-B-I00 projects) and through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-2015-0496) and through the “Proyecto Interdisciplinar de Frontera“, FIP-2018 HECTIC-PTM. We acknowledge the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (ERC-2015-STG-677023). This study has been also supported by the Networking Research Center on Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN), an initiative funded by the VI National R&D&I Plan 2008-2011, Iniciativa Ingenio 2010, Consolider Program, CIBER Actions and financed by the Instituto de Salud Carlos III with assistance from the European Regional Development Fund. P. M. B. gratefully acknowledges financial support from the Juan de la Cierva-Formación 2015 programme (FJCI-2015-23577) supported by MINECO and, together with J.V, A. G. C. also thanks RyC-2013-12943 contract from MINECO. We also thankthe Servei de Ressonància Magnètica Nuclear, Universitat Autònoma de Barcelona, for allocating instrument time to this project

Circularly Polarized Luminescence of [6]Helicenes through Excited-State Intramolecular Proton Transfer

We present the concept of combining circularly polarized luminescence (CPL) and excited-state intramolecular proton transfer (ESIPT) features into a single molecule as a strategy to generate high-performance ESIPT-based CPL materials. For this purpose, a [6]helicene bearing two ESIPT structural units was synthesized using a double Suzuki–Miyaura reaction and a double C(sp2)−H hydroxylation approach. The photophysical properties of the doubly hydroxylated [6]helicene were studied in parallel with a non-hydroxylated [6]helicene control compound, revealing that the presence of a chiral [6]helicene unit results in a strong CPL response and the presence of the ESIPT units in a considerable red shift. The red-shifted emission along with the outstanding glum (≈10−2) and a large Stokes shift makes the doubly hydroxylated [6]helicene a promising candidate for use in optoelectronics

Vinyl sulfonyl chemistry-driven unidirectional transport of a macrocycle through a [2]rotaxane

This work has been financially supported by FEDER(EDRF)/Junta de Andalucia-Consejeria de Transformacion Economica, Industria, Conocimiento y Universidades (P18-FR-2877), grant PID2020-112906GA-I00 funded by MCIN/AIE (/10.13039/501100011033) and Ministerio de Economia y Competitividad (MINECO, Spain) (CTQ2014-55474-C2-2-R and CTQ2017-86125P, co-financed by FEDER funds). Funding for open access APCs provided by Universidad de Granada through a Paid by Read & Publish agreement with RSC.By applying a combination of the coupling-and-decoupling (CAD) chemistry of the vinyl sulfonate group with the click thia-Michael addition to the vinyl sulfone group (MAVS) we performed the irreversible unidirectional transportation of the ring through the linear component in a [2]rotaxane by a chemically and pH-driven flashing energy ratchet mechanism. The design is based on a monostoppered thread precursor bearing a sulfonate stopper, a vinyl sulfone group on the unstoppered end and a dibenzylammonium unit as recognition site for the dibenzo-24-crown-8 macrocycle. First, the ring enters from the vinyl sulfone side and the rotaxane is capped through a thia-Michael addition reaction. Then, the cleavage of the sulfonate group of the opposite stopper using MgBr2 as chemical stimulus and subsequent addition of base (Et3N) promoted the controlled and directional release of the macrocycle into the bulk under mild conditions. The efficiency of the system allowed the in situ operation as demonstrated by NMR and HRMS techniques.FEDER(EDRF)/Junta de Andalucia-Consejeria de Transformacion Economica, Industria, Conocimiento y Universidades - MCIN/AIE P18-FR-2877Ministerio de Economia y Competitividad (MINECO, Spain) (FEDER funds) CTQ2014-55474-C2-2-R; CTQ2017-86125PUniversidad de GranadaRSCMCIN/AIE PID2020-112906GA-I0

On-Surface Thermal Stability of a Graphenic Structure Incorporating a Tropone Moiety

On-surface synthesis, complementary to wet chemistry, has been demonstrated to be a valid approach for the synthesis of tailored graphenic nanostructures with atomic precision. Among the different existing strategies used to tune the optoelectronic and magnetic properties of these nanostructures, the introduction of non-hexagonal rings inducing out-of-plane distortions is a promising pathway that has been scarcely explored on surfaces. Here, we demonstrate that non-hexagonal rings, in the form of tropone (cycloheptatrienone) moieties, are thermally transformed into phenyl or cyclopentadienone moieties upon an unprecedented surface-mediated retro–Buchner-type reaction involving a decarbonylation or an intramolecular rearrangement of the CO unit, respectivel

Single-Molecule Conductance of 1,4-Azaborine Derivatives as Models of BN-doped PAHs

The single–molecule conductance of a series of BN-acene-like derivatives has been measured by using scanning tunneling break-junction techniques. A strategic design of the target molecules has allowed us to include azaborine units in positions that unambiguously ensure electron transport through both heteroatoms, which is relevant for the development of customized BN-doped nanographenes. We show that the conductance of the anthracene azaborine derivative is comparable to that of the pristine all-carbon anthracene compound. Notably, this heteroatom substitution has also allowed us to perform similar measurements on the corresponding pentacene-like compound, which is found to have a similar conductance, thus evidencing that B–N doping could also be used to stabilize and characterize larger acenes for molecular electronics applications. Our conclusions are supported by state-of-the-art transport calculations.This work has been supported by the MICINN projects PID2019-105458RB-I00, PID2019-106732GB-I00, PGC2018-101873-A-I00, CTQ2017-85454-C2-1-P, FIS2016-77889-R and MAT2017-88693-R, the European Research Council (ERC) (677023), the FEDER/Junta de Andalucía project A-FQM-221-UGR18 and the Comunidad de Madrid project NanoMagCOST (CM S2018/NMT-4321). We also thank Severo Ochoa Programme for Center of Excellence in R&D (SEV-2016-0686) and María de Maeztu Programme for Units of Excellence in R&D (CEX2018-000805-M). We acknowledge the allocation of computer time by the Red Española de Supercomputación and the Centro de Computación Científica at the Universidad Autónoma de Madrid (CCC-UAM). J.G.F. thanks the PFI program of the MICINN co-financed by the European Social Fund. I.R.M. thanks MICINN for a Personal Técnico de Apoyo contract (PTA2017-13681-I). E.L. thanks the Comunidad de Madrid grant Atracción de Talento 2019-T1/IND-16384

Nanografenos imperfectos: perfectos en la búsqueda de nuevas propiedades quirópticas

Graphene has become the rising star in the field of organic materials by virtue of their outstanding chemico-physical pro-perties. Hand in hand with graphene, a novel current has emerged, based on the synthesis of nanometric graphene-like compounds with well-defined structures and high in-solution processability. Therefore, in this review we sample the nanographene outlook tac-kling the last bottom up synthetic strategies in the search of both endo- and exoskeletal defects, focusing on structures containing heptacycles and helicenes, evaluating the structure-property relationships, and in particular their chiroptical properties.El grafeno se ha convertido en la joven promesa en el campo de los materiales orgánicos gracias a sus excelentes pro-piedades químico-físicas. De la mano del grafeno, ha surgido una corriente emergente basada en la síntesis de compuestos tipo grafeno de dimensión nanométrica con estructuras bien definidas y elevada procesabilidad en disolución. Por ello, en este artículo de revisión muestreamos el panorama de los nanografenos abordando las últimas estrategias sintéticas en disolución, en busca de la incorporación de defectos endo- y exoesqueléticos, centrándonos en estructuras con heptaciclos y helicenos, evaluando la relación estructura propiedades, y en particular sus propiedades quirópticas

π-System Bistability Determines the Circularly Polarized Luminescence in Helicene para-Phenylenes

We resolve the origin of the anomalous circularly polarized luminescence (CPL) observed in helicene para-phenylenes (HPPs), a new class of chiral macrocyclic nanocarbons reported recently. We show by synthesis of HPPs that embed [5]helicene unit that the presence of two emission bands in the CPL, each with the opposite handedness, relates to the topological bistability of their π-electron system. This bistability allows the HPPs to adopt Möbius and Hückel conformations that differ in the orientability of their π-system. We demonstrate that the bistability can be controlled by molecular strain and it can be effectively used to turn off one of the emissions. Our work is thus the first to report that the topological bistability of π-electron system can dramatically impact the chiroptical properties in single-stranded Möbius molecular nanocarbons. In addition, we demonstrate that the parity of the number of π-electrons in the delocalization paths in the doubly-oxidized HPPs with [5]helicenes permits induction of global ring currents, i.e., these compounds may display global (anti)aromaticity

Atomically Precise Distorted Nanographenes: The Effect of Different Edge Functionalization on the Photophysical Properties down to the Femtosecond Scale

Nanographenes (NGs) have been attracting widespread interest since they combine peculiar properties of graphene with molecular features, such as bright visible photoluminescence. However, our understanding of the fundamental properties of NGs is still hampered by the high degree of heterogeneity usually characterizing most of these materials. In this context, NGs obtained by atomically precise synthesis routes represent optimal benchmarks to unambiguously relate their properties to well-defined structures. Here we investigate in deep detail the optical response of three curved hexa-peri-hexabenzocoronene (HBC) derivatives obtained by atomically precise synthesis routes. They are constituted by the same graphenic core, characterized by the presence of a heptagon ring determining a saddle distortion of their sp2 network, and differ from each other for slightly different edge functionalization. The quite similar structure allows for performing a direct comparison of their spectroscopic features, from steady-state down to the femtosecond scale, and precisely disentangling the role played by the different edge chemistry.European Research Council (ERC) 677023MCIN/AEI PID2021-127521NB-I00ERDF A way of making EuropeConsejeria de Transformacion Economica, Industria, Conocimiento y Universidades de la Junta de Andalucia POSTDOC_21_0013