Journal of Materials Chemistry C This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online PERSPECTIVE View Journal | View Issue Perspectives for polychlorinated trityl radicals Cite this: J. Mater. Chem. C, 2021, Imma Ratera, *a Jose Vidal-Gancedo, a Daniel Maspoch, bc Open Access Article. Published on 14 July 2021. Downloaded on 12/13/2022 9:20:42 AM. 9, 10610 Stefan T. Bromley, cd Núria Crivillers *a and Marta Mas-Torrent *a An organic free radical is a molecule with one or more unpaired electrons. Although most free radicals are highly reactive, chemists have developed a few families of so-called persistent organic radicals with high kinetic stabilities. Polychlorinated trityl radicals, also known as polychlorotriphenylmethyl (PTM) radicals, are particularly chemically stable due to the high steric hindrance provided by the chlorine atoms in ortho positions which protect their single unpaired electron localised on the central carbon atom. In addition to their inherent magnetic spin due to the unpaired electron, PTMs exhibit other Received 12th May 2021, appealing properties such as a rich electrochemistry and characteristic optical properties (absorption and Accepted 14th July 2021 emission). Moreover, it has been shown that these properties can be tuned through the preparation of a large DOI: 10.1039/d1tc02196f library of PTM-based derivatives. Here, we review recent developments employing PTM radicals, which include their implementation in molecular electronic junctions/switches, as building blocks for the preparation of rsc.li/materials-c magnetic networks and opto-electronic devices/materials and their exploitation in bio-applications. 1. Introduction rings. The inter-chloro steric strain forces the molecule to adopt a propeller-like conformation. Thus, PTM radicals are inert to An organic free radical is a molecule with one or more unpaired oxygen and many aggressive reagents and even stable under electrons and is, generally, highly reactive. However, there are a light in the solid state. As a consequence, PTM radicals can few families of persistent organic radicals that have been withstand standard synthetic procedures and, hence, a large developed over the last few decades that are stable at ambient variety of materials4 have been prepared exhibiting fascinating conditions.1 These species have attracted great interest for a magnetic properties, such as polyradicals with high spin ground wide spectrum of applications including their use as building states.5,6 The magnetic properties of PTM derivatives in crystals blocks for the preparation of magnetic and multifunctional and supra-molecular assemblies have also been a subject of materials, for obtaining structural, dynamical or chemical intense research.7–9 information from other systems (i.e. spin labelling, spin trapping), In addition to their inherent magnetic moment, PTM radicals in catalysis and polymerisation processes, as antioxidants and as possess a rich electrochemistry. PTM is a very good electron polarizing agents in Dynamic Nuclear Polarisation (DNP) to acceptor and can be easily chemically or electrochemically con- enhance Nuclear Magnetic Resonance (NMR) sensitivity.2 verted to its corresponding anion form in a reversible manner. One particularly stable and intensively investigated family of Moreover, PTM radicals can also be oxidised to the cation species, persistent organic free radicals is that of polychlorinated trityl although this process occurs at higher voltages (Fig. 1b). Accordingly, radicals, also known as polychlorotriphenylmethyl (PTM) radicals. in solution PTM radicals have been extensively studied as molecular PTMs are composed of three totally or partially chlorinated phenyl switches,10 as acceptor units in donor–acceptor dyads to investigate rings connected to a central carbon atom with a sp2 hybridization intramolecular charge transfer processes11–14 and non-linear where the spin is localised (Fig. 1a).3 These radicals are chemically optical properties,15 and as charge reservoirs in the preparation and thermally stable due to the steric hindrance at the central of mixed-valence systems to gain information about the trans- carbon atom caused by the ortho chlorine atoms on the phenyl port properties of molecular wires.16,17 In most of these cases, the use of Electronic Paramagnetic Resonance (EPR) has been a Institut de Ciència de Materials de Barcelona (ICMAB-CSIC) and CIBER-BBN, key to monitor the radical species and thus to follow the Campus de la UAB, 08193 Bellaterra, Spain. E-mail: iratera@icmab.es, electronic processes taking place (Fig. 1c). More recently, PTMs ncrivillers@icmab.es, mmas@icmab.es have also been immobilized on solid supports in a step forward b Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and the Barcelona towards more advanced devices.18,19 Institute of Science and Technology, Campus UAB, 08193 Bellaterra, Spain c Institució Catalana de Recerca i Estudis Avançats (ICREA), 08010 Barcelona, Spain PTM radicals offer a unique combination of magnetic, electrical d Departament de Ciència de Materials i Quı́mica Fı́sica & Institut de Quı́mica Teòrica i and optical properties, which make these species hugely attractive Computacional (IQTCUB), Universitat de Barcelona, 08028 Barcelona, Spain for numerous ongoing investigations where they have the potential 10610 | J. Mater. Chem. C, 2021, 9, 10610–10623 This journal is © The Royal Society of Chemistry 2021 View Article Online Perspective Journal of Materials Chemistry C This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 14 July 2021. Downloaded on 12/13/2022 9:20:42 AM. Fig. 1 (a) Molecular structure of the PTM radical and its corresponding oxidation and reduction to the cation and anion, respectively. (b) Representative cyclic voltammogram of a PTM radical in CH2Cl2, with 0.1 M n-Bu4NPF6 (vs. Ag/AgCl). Adapted from ref. 4 Copyright 2012 Royal Society of Chemistry. (c) EPR spectrum of a PTM radical recorded at room temperature in deoxygenated CH2Cl2 (g = 2.0023). In grey the same spectrum recorded with a higher power to better appreciate the 13C hyperfine coupling. to renew or transform research fields. In this work, we give an multi-channel switch. The absorption and emission bands overview of recent work employing PTM radicals systems including characteristics from the two redox states (i.e., PTM radical: molecular electronic junctions, optical devices, active units in labs = 365 nm, lem = 688 nm/PTM anion: labs = 515 nm, no molecular switches, building blocks for magnetic networks and emission) changed simultaneously in opposite directions whilst bio-applications. applying write/erase cycles in a completely reversible way. In turn, the EPR signal observed in the PTM radical SAM disap- peared upon reduction to the anionic form and appeared again 2. Surface confined molecular after further oxidation, corroborating that the magnetic output switches based on PTM radicals was also a suitable read-out tool to determine the state of the switch. In analogous SAMs on gold, it was additionally proved The functionalisation of surfaces with redox-active molecules that the modification of the redox state of the PTM SAM had allows for the modulation of the substrate properties upon the also a strong impact on the surface wetting properties.26 The application of an electrical stimulus. Such hybrid systems have water contact angle of the highly hydrophobic PTM SAM was been focus of major interest for the development of switches reported to be 102  61, but changed to 73  31 after reducing and memories.20,21 In this sense, it has been shown that PTM the radical due to the formation of the more polar anionic state. radicals are appealing building blocks since they can be electrically Recently, SAMs of PTM have also been prepared on Si to interconverted between two easily accessible redox states (i.e. the fabricate a light-triggered capacitance switch under electro- PTM radical and PTM anion), which reveal distinct magnetic, chemical conditions.25 The electronic properties of Si can be tuned optical and chemical properties. For this purpose, great efforts by modifying the density and the nature of the charge carriers have been devoted over the last few years to synthesise PTM under illumination.27 In this way, the SAMs on Si in the dark moieties bearing appropriate grafting groups to prepare chemically showed negligible oxidation and reduction currents, whereas bonded surface self-assembled monolayers (SAMs) on Au, quartz, under illumination through a red filter, the radical/anion redox Si, and indium tin oxide (ITO) substrates.22–25 In particular, SAMs process could be observed in the CV voltammograms. To obtain of a PTM radical bearing a silane group on ITO were reported by further insights into the light dependence of the redox process in Rovira and co-workers to lead to robust electrochemical switches this PTM SAM, differential capacitance measurements were per- (Fig. 2a).24 The electrochemical characteristics of these PTM formed. Accordingly, in the dark, the measured capacitance values radical SAMs were investigated by cyclic voltammetry (CV). As were small and did not exceed 1 mF cm2, whereas under red light expected, one reversible redox wave was observed with an oxidation a much more intense capacitance peak (enhancement by a factor peak at +53 mV and a reduction peak at 32 mV at a scan rate of 10) was found at the formal potential of the surface bound PTM. 100 mV s1 (vs. Ag wire). The optical absorption, emission and The electrochemical switching ability of PTM radicals has magnetic responses were employed to read the state of this also been implemented in other materials, such as in TiO2 and This journal is © The Royal Society of Chemistry 2021 J. Mater. Chem. C, 2021, 9, 10610–10623 | 10611 View Article Online Journal of Materials Chemistry C Perspective This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 14 July 2021. Downloaded on 12/13/2022 9:20:42 AM. Fig. 2 (a) Scheme of a self-assembled monolayer of a PTM radical on ITO and monitoring of the optical absorption (bottom left) and EPR signal (bottom right) outputs upon the application of write/erase voltage cycles to convert the PTM radical to the anion form. The optical absorption plot corresponds to the monitoring of the PTM radical absorption band at 365 nm (red) and the anion one at 515 nm (purple). In the EPR monitoring, red lines correspond to the radical SAM and the purple lines to the anion form. Reprinted with permission from ref. 24. Copyright 2011 Nature Group. (b) Current–voltage curve extracted by EC-STS from a PTM SAM on Au and schematic energy diagram of a two-step electron transfer process mediated by a redox active radical molecule. Reprinted with permission from ref. 30. Copyright 2016 Royal Society of Chemistry. (c) Top: Scheme of the neutral D–A and zwitterionic species and corresponding energy levels upon excitation. Bottom: Shift of the surface potential differences of PTM–Fc SAMs in the dark and under 950 nm illumination measured by KPFM. Reprinted with permission from ref. 35. Copyright 2019 Royal Society of Chemistry. SiO2 porous thin films consisting of tilted nanocolumns sup- between the substrate and the tip close to the equilibrium ported on an ITO electrode.28 More recently, the modulation potential of the PTM radical/PTM anion redox pair, resembling of the optical absorption and magnetic properties of three a transistor like functionality (Fig. 2b).30 Such behaviour is generations of PTM radical dendrimers has also been reported, typical of a redox mediated electron tunnelling (RMET) in which up to 24 redox active units were incorporated.29 This process,33,34 which is rationalised by a two-step electron trans- work opens the door to the future possibility to gain control on fer. In the first one, the electron transfers from the electrode the exact number of electrons transferred during the switching with a higher Fermi level to the single-unoccupied molecular process, which could lead to a multistate switch. orbital (SUMO). Afterwards, the energy of the now occupied The redox activity of the PTM SAMs have also been investi- SUMO* decreases due to vibrational relaxation until the second gated at the nanoscale by electrochemical scanning tunnelling electron transfer occurs towards the second electrode (Fig. 2b). spectroscopy (EC-STS) in a ionic liquid medium.30,31 Electro- Importantly, the high on/off current ratio of 13 (i.e., tunnelling chemical STS measurements in the constant bias mode were increase ratio at the peak position) showed that, in comparison performed.32 In this mode, the potentials of the substrate ES with other redox-active moieties, the PTM radical is among and the tip ET were swept simultaneously while keeping the bias the most efficient redox mediators in RMET, demonstrating voltage Ebias = ET  ES constant. The STS results demonstrated its applicability as active component in electronic devices of a significant enhancement of the tunnelling current flowing nanoscale size. 10612 | J. Mater. Chem. C, 2021, 9, 10610–10623 This journal is © The Royal Society of Chemistry 2021 View Article Online Perspective Journal of Materials Chemistry C Finally, PTMs have also been exploited in light-triggered In this regard, PTM radicals have shown to be fascinating switches without the need of using electrochemical stimuli. In candidates for preparing functional molecular junctions. Due to this case, the PTM was employed as the acceptor group in a the open-shell nature of the PTM radicals, the most accessible donor–acceptor (D–A) dyad, where D was a ferrocene (Fc) molecular levels are the single-occupied molecular orbital This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. moiety.35 The near-infrared (NIR) irradiation of PTM–Fc SAMs (SOMO) and the SUMO. In the case of PTM radicals, the close on gold in air led to the formation of a metastable zwitterionic energetic level alignment between the SUMO and the Fermi state due to a light driven intramolecular charge transfer from level of the gold contacts has been proven to significantly the Fc to the PTM radical unit (Fig. 2c). This phenomenon had enhance the conductivity of the junction in comparison with a significant impact on the metal work-function (WF). Hence, the closed-shell hydrogenated PTM counterpart. This phenomenon Kelvin Probe Force Microscopy (KPFM) was used to measure the was demonstrated for PTM radicals incorporated in mechanically contact potential difference changes (Fig. 2c). When switching controlled break junctions (MCBJ)44,45 as well as by top-contacting Open Access Article. Published on 14 July 2021. Downloaded on 12/13/2022 9:20:42 AM. cycles were applied, it was found that the WF increased when PTM-based SAMs on gold with an atomic force microscopy (AFM) the sample was illuminated, while it decreased in the darkness tip (i.e., conducting-AFM)46,47 or with eutectic gallium–indium alloy in a reversible way.36 The value of the WF shift was reported to (EGaIn).25,48,49 Importantly, the high chemical stability of the PTM equal 250 meV, which was slightly higher than the largest shift radicals allowed the preparation of derivatives with different anchor- reported for photoswitchable SAMs where structural isomerisations ing groups for gold (thiophene, thiol, disulfide and terminal alkyne) are place instead of electronic reorganisations.37 and with different linkers. In order to experimentally demonstrate Considering all the above, clearly the high reversibility and the charge transport mechanism through PTM SAMs on Au, a stability of the PTM redox process that occurs at very low voltage, family of PTM radical derivatives and their corresponding makes these materials very appealing for the development of a closed-shell form bearing long non-conjugated alkyl tether wide range of switches. chains of different lengths were synthesized ensuring localization of the molecular frontier orbitals on the PTM moiety (Fig. 3a).49 The experimental results indicated that the charge transport mechanism 3. PTM radicals as active electronic across the junctions was SUMO-assisted coherent non-resonant components in molecular junctions tunnelling. This observation was further supported by a temperature independent transport behavior. Further, the transport through The incorporation of molecular species between two electrodes to PTM-based D–A dyad SAMs having ferrocene48 or tetrathiaful- evaluate their transport properties, primarily as molecular wires, has valene50 as donor units were also investigated, demonstrating the been pursued in the field of molecular electronics over the last few possibility to modulate the current rectification ratio between the decades. Beyond the understanding of the charge transport radical and the non-radical (i.e., closed-shell) derivative. mechanisms in non-functional molecules such as alkanethiols Beyond their relevance as molecular wires in the solid state, of different lengths38 and molecules with different degrees organic radicals are very appealing in the field of molecular of conjugation,39 the study of more complex systems showing spintronics because they show an intrinsic magnetic moment. photo-,40 redox-41 and/or magnetic-activity42,43 has also attracted In particular, compared to transition-metal-based magnetic much attention. Such intrinsic molecular characteristics bring compounds, they have low spin–orbit coupling and low hyper- additional functionalities to the electrode/molecule/electrode fine interactions suggesting relatively long spin lifetimes. The structure since the conductivity can be modulated by external magnetic state of PTM radicals has been demonstrated to be stimuli. stable in solid state single molecule junctions. Frisenda et al. Fig. 3 (a) Scheme of the Au/S-alkyl-PTM SAM/GaOx/EGaIn and the corresponding energy level diagram. Reprinted with permission from ref. 49. Copyright 2016 Nature Group. (b) Left: Structure and magnetism schematics of the neutral diradical investigated. The red dots and the dashed lines are the radical spins (2-spin system as synthesised, S = 0) and the exchange interactions, respectively. Right: Differential conductance (red) and corresponding redox center spin values (blue) as a function of gate voltage. The added electron is marked as the blue dot (3-spin system, S = 1/2). Reprinted with permission from ref. 52. Copyright 2017 ACS Publications. This journal is © The Royal Society of Chemistry 2021 J. Mater. Chem. C, 2021, 9, 10610–10623 | 10613 View Article Online Journal of Materials Chemistry C Perspective reported the first work on the incorporation of a thiophene- attention over the past 20 years. The frameworks of reticular terminated PTM radical in a single molecular junction.44 Here, materials can be endowed with magnetism mainly by two the magnetic character of the PTM radical was undoubtedly strategies: by linking appropriate magnetically-active metal identified by the presence of fully screened spin – 12 Kondo ions/clusters and/or MBBs; and by introducing magnetic guest This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. features, which were determined to be insensitive to the species in the pores.63 To generate reticular materials with electrode displacement and background conductance. Further- cooperative magnetic properties specifically, a particularly exotic more, later on Bejarano et al. investigated the role of the strategy is to connect magnetically-active metallic nodes via binding motif in the electrical response of room temperature magnetically-active organic-radical MBBs.62 MBBs based on PTM MCBJ.45 With a PTM radical having terminal alkynes, the radicals are ideally suited to this application. The structure of the formation of C–Au s-bonds was demonstrated leading to more PTM molecule defines a rigid backbone that resembles those of robust junctions with a better-defined geometry of the contact many MBBs used to synthesize iconic MOFs and COFs.65,66 Open Access Article. Published on 14 July 2021. Downloaded on 12/13/2022 9:20:42 AM. and a reduction in the variability of the conductance values. Another advantage of PTM-radical MBBs is that the meta and/or Remarkably, the tuning of the magnetic state of PTM-based para positions (relative to the central carbon) of each phenyl ring molecular junctions could be achieved by employing neutral can be functionalized with moieties that can coordinate to metal organic di- and triradical molecules embedded in a three- ions (e.g. COOH) or that can be linked via dynamic covalent bonds terminal device.51,52 The redox state of a PTM diradical could (e.g. NH2 or CHO). This versatility provides countless potential be reversibly modulated by charging the molecule with the combinations to design MBBs with different connectivities, topol- application of an electric field. This induced the on and off ogies and functionalities. For example, two or three carboxylic acid switch of a parallel exchange path between the two radical spins. It groups could be introduced at the para position of the phenyl rings was shown that the added electron led to the formation of a spin to generate the (4,4 0 -dicarboxytridecachlorotriphenyl)methyl system with three antiferromagnetically coupled spins (Fig. 3b).52 radical (PTMDC)67 or the (4,40 ,400 -tricarboxydodecachlorotriphenyl)- In the case of the triradical system, a ferro-to-antiferromagnetic methyl radical (PTMTC),68 respectively. Accordingly, the PTMDC transition was observed and assigned to structural distortions. radical can act as a two-connected (2-c) bent MBB, whereas PTMTC Additionally, the exchange coupling could also be tuned by can act as a 3-c triangular MBB. Finally, PTM-radical MBBs are electrical gating.51 In summary, all these findings constitute the stable in the solid state up to relatively high temperatures. bases of a promising future for integrating PTM radicals in Seeking to exploit the aforementioned advantages of PTM- electronic and spintronic nanodevices. radical MBBs, Veciana and co-workers synthesized the first-ever magnetic MOFs built from organic-radical MMBs. To this end, 3-c triangular PTMTC was reacted with either Cu(II) or Co(II) to 4. PTM radicals as molecular building obtain two 2-D MOFs exhibiting an underlying 3-c hcb topology blocks for reticular materials (Fig. 4a).69,70 The first, a Cu(II)-based MOF (called MOROF-1; where MOROF = metal–organic radical open-framework), com- Reticular materials are crystalline open-framework materials bines very large pores (diameter: 2.8 nm to 3.1 nm) with bulk assembled from judiciously-designed molecular building blocks magnetic ordering at low temperature (Fig. 4b).69 Remarkably, (MBBs) that are linked by strong bonds.53,54 Among the main MOROF-1 was among the first reported breathing MOFs and families of reticular materials are metal–organic frameworks sponge-like magnets: upon loss and reuptake of solvent molecules, (MOFs) and covalent–organic frameworks (COFs):55,56 MOFs are it transforms between a crystalline and an amorphous phase, a 2D and 3D porous frameworks formed by joining organic MBBs process that leads to a reversible variation in the critical ordering to metal MBBs, whereas COFs exclusively comprise organic temperature (Fig. 4b and c). Later on, 3-c PTMTC was also MBBs. All these materials share ultrahigh porosity (500 m2 g1 combined with two different 6-c Tb(III) clusters to obtain two 3-D to 10 000 m2 g1), high thermal stability (at up to 200 1C to MOFs: one with an underlying 3,6-c spn topology and one with a 500 1C), and good chemical stability in organic and aqueous 3,6-c 3,3,3,5T33 topology, both showing antiferromagnetic media, including acids and bases.57 Based on these properties, interactions.71–73 The versatility of PTM-MBBs was further reticular materials offer great potential for myriad applications, demonstrated by adding six carboxylic acid groups at the meta including storage of pollutants and/or hazardous gases (e.g. CO positions of the three phenyl rings to obtain a new MBB: a 3,3-c and CO2); fuel applications (e.g. with H2 or CH4); catalysis; MBB.74 Reaction of this MBB with Cu(II) and 4,4 0 -bypiridine biomedical applications; and gas/liquid separation (e.g. CO2/ afforded a two-interpenetrated 3,3,3-c MOF. Finally, a new 3-c CH4, and xylene and alkane isomers).58–60 Beyond their inherent PTM-MBB built up with three pyridyl groups, tris(3,5-dichloro-4- porosity, reticular materials offer rich chemical and structural pyridyl)methyl radical (trisPyM), was recently reacted with Zn(II) versatility, allowing them to be conferred with additional functional to synthesize a 2-D luminescent MOF also exhibiting an under- properties such as magnetism and electrical or optical properties— lying 3-c hcb topology.75 typically, via fine-designing their frameworks.61–64 Such properties Another option for making network materials of linked can in turn be modulated by using the intrinsic porosity of the PTMs is to join them together using only covalent bonding and frameworks to create devices such as sensors.64 organic linkers. Joining PTMs (and/or similar triarylmethyls – TAMs) The design of magnetic reticular materials, including those via organic linkers to form open-shell 2D COFs (or covalent with cooperative or molecular properties, has attracted much organic radical frameworks – CORFs) was proposed theoretically 10614 | J. Mater. Chem. C, 2021, 9, 10610–10623 This journal is © The Royal Society of Chemistry 2021 View Article Online Perspective Journal of Materials Chemistry C This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 14 July 2021. Downloaded on 12/13/2022 9:20:42 AM. Fig. 4 (a) Schematic representation of the formation of hcb MOROF-1 using the 3-c PTMTC and Cu(II), and its breathing behaviour. (b) Value of wT as a function of the temperature for MOROF-1 (filled circle, MOROF-1; open circle, evacuated and amorphous MOROF-1 phase). (c) Breathing phenomenon of a single crystal of MOROF-1 when exposed to the air (top) and re-exposed to ethanol liquid (bottom). (b and c) Adapted from ref. 69. Copyright 2003 Nature Group. in 2017.76,77 Hexagonally connected planar lattices of TAM show diamagnetic quinoidal closed-shell, and graphene-like radical MBBs are of particular interest for making 2D CORFs semimetallic behaviour.77 Soon after these predictive modelling that could possess a range of energetically low-lying electronic studies, the first PTM-based 2D COF was synthesised (see Fig. 5a). states. Specifically, at low temperatures hexagonal 2D CORFs This material (PTM-CORF) is based on layers of hexagonally ordered will tend to exhibit antiferromagnetic (AFM) semiconducting/ PTM MBBs linked together via acetylenic bridges and was found to insulating states, but, in principle, also have the potential to be an AFM semiconductor.78,79 Interestingly, PTM-CORF was also Fig. 5 (a) Synthesised PTM-CORF. Adapted from ref. 78. Copyright 2018 Wiley-VCH Verlag GmbH & Co. (b) Mott-like transition in a ring-sharing hexagonal 2D CORF upon out-of-plane pressure. Reprinted from ref. 80. Copyright 2021 Wiley-VCH Verlag GmbH & Co. (c) Transition between open-shell AFM state and closed-shell quinoidal state in a ring-sharing 2D CORF induced by in-plane strain. Reprinted from ref. 84. Copyright 2021 Nature Group. This journal is © The Royal Society of Chemistry 2021 J. Mater. Chem. C, 2021, 9, 10610–10623 | 10615 View Article Online Journal of Materials Chemistry C Perspective found to be highly active electrochemically for the oxygen reduction synthesizing them without chlorine atoms at the meta positions. reaction which was attributed to the energetically low lying This will reduce the steric effects allowing the carboxylate groups unoccupied electronic levels of the regularly spaced and acces- to participate in the formation of metallic clusters typically found sible radical centres which favours the acceptance of electrons in MOFs (e.g. Cu(II) paddle-wheel clusters, Fe(III) oxo clusters, etc.) This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. from oxygen.78 and therefore, enhance the possibility to synthesize magnetic Although COFs, due to their covalent bonding, tend to be MOROFs with permanent porosity. In CORFs, their impressive structurally more rigid than MOFs, they are still more flexible electrocatalytic properties strongly suggest that a suitably opti- and stretchable than most inorganic materials. To access mised 2D CORF could have technological promise for fuel cell different electronic states in hexagonal TAM-based 2D CORFs, applications. Generally, the theoretically predicted electronic/ it has been proposed that out-of-plane compression could help structural responsiveness of TAM-based 2D CORFs to in-plane induce a crossover from the parent open-shell AFM insulating strain and out-of-plane compression shows that these materials Open Access Article. Published on 14 July 2021. Downloaded on 12/13/2022 9:20:42 AM. state towards a more delocalised graphene-like semimetallic could eventually find applications in the burgeoning fields of state80 (see Fig. 5b). This Mott-like transition81 is favoured by straintronics and biosensors. To progress towards these goals the pressure-induced flattening (with respect to the plane of the new TAM-based CORFs should aim to enhance the spin–spin CORF) of the aryl rings in each TAM centre, which leads to coupling between radical centres through chemical design (e.g. increased inter-radical conjugation. This behaviour is predicted TAM functionalisation, linker choice, aryl ring-sharing). to be enhanced in hexagonal 2D CORFs in which the TAMs share aryl rings. Although ring-sharing CORFs have not been yet synthesised, molecules with six ring-sharing TAMs have 5. Optical applications for PTM been reported,82 which potentially could be used as CORF radicals MBBs. As the AFM insulating state is a correlated electronic state83 (i.e., due to strong inter-site electron–electron interactions) PTMs have a number of promising optical properties associated the ability to tune this state via external pressure implies that 2D with their doublet electronic configuration, like their emission at CORFs could provide a new class of tuneable low dimensional long wavelengths without an elongated p-system, large Stokes’ correlated electronic material.81 shift, and emission lifetimes of the order of tens of nanoseconds. An alternative means to externally induce changes in the However, PTMs also possess three principal drawbacks that have electronic/chemical properties of TAM-based hexagonal 2D limited their use in optical applications for many years: (i) low CORFs is to apply in-plane stress to strain the network. When values of luminescent quantum yield, fF = 0.02, especially in polar uniaxially strained, the aryl rings in TAM-based 2D CORFs are solvents, (ii) low photostability in solution giving the fluorenyl affected in an anisotropic manner such that some rings flatten radical derivative88 and (iii) non-fluorescent characteristics in solid and some twist more out-of-plane.77,84,85 In hexagonal 2D state despite being completely photostable. The absorption CORFs this structural response can lead to pairing of electrons spectrum of the PTM radical presents a weak absorption in between neighbouring TAM centres and the transition from the visible region, due to a symmetry forbidden transition the open-shell multi-radical ground state to a diamagnetic SOMO - SUMO (D0 - D1), and an intense absorption in the closed-shell quinoidal state86 (see Fig. 5c). Low-lying open- near ultraviolet due to the SOMO - LUMO+1 transition at shell radical and closed-shell quinoidal states are also found 380 nm. PTM shows low room temperature fluorescence at in di-radical molecules.87 Forming 2D hexagonal CORFs from 520 nm (SUMO - SOMO) but becomes more intense at low TAM radical MBBs provides a means with which to reversibly temperature (Fig. 6a and b). control this fundamental chemical property. Recently, it has been shown that thanks to their doublet Like MOFs, COFs are also porous materials, which also spin-configuration, organic radicals can avoid the formation of enhances their structural flexibility. Hexagonal 2D CORFs such triplet excitons that limit the electroluminescence efficiency of as PTM-CORF possess regularly spaced arrays of radially sym- non-radical emitters. Thus, organic radicals turned out to be a metric micropores with diameters 41 nm. By joining PTMs new solution to obtain new organic deep-red emitters able to together with triazine-based linkers, a 2D CORF (PTMR-CTF) was exhibit internal quantum efficiencies of 100%. In this regard, in made with pores having both radially symmetric and elongated 2006, Julià and co-workers reported the PTM-carbazole derivative micropores with sizes of 1.1 nm and 1.2 nm diameter, (TTM-1Cz) as possible molecular material for a deep-red organic respectively.86 Although the relatively large distance between light emitting diode (OLED) (Fig. 6c).89 TTM-1Cz presents an PTM MBBs in PTMR-CTF leads to a paramagnetic electronic emission in cyclohexane at 628 nm with a fF = 0.64 due to an ground state, the ability to change micropore geometry via increase of the oscillator strength of the SOMO - SUMO different chemical connectivities will be important for tailoring transition.90,91 In 2015, Li et al. reported the development of the future multifunctionality of 2D CORFs. the first doublet emitter OLED with deep red emission based on To date, only a few PTM-based MOROFs and two PTM-based an organic radical using a blend containing TTM-1Cz (Fig. 6d).92 2D CORFs have been synthesised and thus there is huge Recently, it has been showed that non-alternant systems with a potential for further exploration. For example, the use of PTM conjugated hydrocarbon with an odd-membered ring., i.e. like radicals functionalized with carboxylic acid groups at the para TTM-1Cz, are necessary to lift the degeneracy of the lowest energy position of the phenyl rings as MBBs may be optimized by orbital excitations enhancing the oscillator strength of the emitter. 10616 | J. Mater. Chem. C, 2021, 9, 10610–10623 This journal is © The Royal Society of Chemistry 2021 View Article Online Perspective Journal of Materials Chemistry C This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 14 July 2021. Downloaded on 12/13/2022 9:20:42 AM. Fig. 6 (a) Schematic representation of the frontier orbitals of PTM radicals. (b) Absorption and emission of PTM in THF at room temperature (fF = 0.02) and 2-methyl THF at 77 K (fF = 0.72). (c) Molecular structure of the TTM-1Cz. (d) Device architecture of the OLED based on TTM-1Cz. The inset shows a picture of the TTM-1Cz-based OLED operating at 7 V. Reprinted with permission from ref. 92. Copyright 2015 Wiley-VCH. (e) TEM image of PTMd-ONPs. (f) Normalized emission spectra of TTMd-ONPs. Reprint with permission from ref. 108. Copyright 2017 Royal Society of Chemistry. In fact, the TTM radical’s low emission is typical of alternant which exhibited low emission at 585 nm in dichloromethane. hydrocarbon systems in which conjugated atoms are divided The emission increased to 0.81 at 77 K and 0.26 when dispersed into two groups and atoms of the same group are not directly in a polymeric film.98 Its luminescence could also be increased connected. In these systems, the HOMO–SOMO and SOMO– from 0.02 to 0.2 by substituting the chlorine atoms in ortho by LUMO gaps are identical. This is the reason why it is necessary fluorine and making a pyridyl Au(I) complex.99,100 Also, cation- to covalently attach electron-rich, non-alternant hydrocarbon responsive turn-on fluorescence without heavy atoms101,102 and groups to TTM or PTM units to form non-alternant donor– increased photostability has been reported for a radical containing acceptor radicals to achieve stable and luminescent radicals. two pyridyl groups.103 Radicals should be synthesized with a broken alternacy symmetry Electron-donating units quench the luminescence of PTM which, was not considered by the early theories on radical radicals due to the appearance of a charge transfer (CT)90 electronic structure. Thus, aza substitution of the Cz moiety phenomena given by the HOMO - SUMO transition. This boosts the radical emission to above 90%. OLEDs based on these behaviour is observed in PTM-triphenylamine (PTM-TPA) deri- molecules showed pure-red emission with more than 27% exter- vatives, which exhibit high luminescence in the NIR region nal quantum efficiency, the highest reported value for deep-red (760 nm) when dissolved in non-polar solvents (fF = 0.38 in and infrared OLEDs. These insights have been beneficial for the cyclohexane) but when electron donating groups are added to the rational design and discovery of new highly luminescent doublet TPA moiety, the luminescence is quenched.104,105 In addition, it emitters (Fig. 7).93–95 In fact, with the right choice of the hole- has been found that PTM radicals linked to electron-donating injection and hole-transporting layers, it is possible to generate units (D-PTM) do not follow the Aufbau principle because the the 100% of doublet excitons, achieving in this way an internal SOMO is found to lie below the HOMO. These D–A radicals have quantum efficiency of 100%.96 Finally, it is worth mentioning a strong emission yield (up to 54%) and high photostability. that the first discotic all-organic liquid crystal with the magnetic OLEDs based on them show deep-red emission with a maximal spin source placed in the core is also based on a para carbazolyl external quantum efficiency of 5.3%.106 Recently, Brédas and substituted dichlorotriphenyl methyl system.97 collaborators have reported on the impact of chemical substitution In 2014, Nishihara et al. reported a PTM derivative in which and electronic-state hybridization on the luminescence properties one phenyl ring was substituted with a 3,5-dichloropyridyl one, of D-PTM radicals.107 These results highlight that in order to This journal is © The Royal Society of Chemistry 2021 J. Mater. Chem. C, 2021, 9, 10610–10623 | 10617 View Article Online Journal of Materials Chemistry C Perspective This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 14 July 2021. Downloaded on 12/13/2022 9:20:42 AM. Fig. 7 (a) Chemical structures of TTM, TTM-3NCz and TTM-3PCz; (b) electroluminescence (photoexcitation wavelength, 375 nm) spectra; (c) external quantum efficiency (EQE)–current density curves for TTM-3NCz (red) and TTM-3PCz (black) LEDs. The inset shows the TTM-3NCz device layout; the labels give the energy levels in electronvolts and the thickness of layers in nanometres. For the best TTM-3NCz LED architecture, SOMO = 3.7 eV and HOMO = 6.0 eV. Reprint with permission from ref. 95. Copyright 2018 Nature. understand the emissive properties of D–A radicals, the electronic optics.111–113 Moreover, in a recent pioneering study, PTMs have hybridization of the CT states with both the ground and the low been described as the first organic radicals showing chiral excitation states should be properly considered. This information emission with intrinsic Circularly Polarized Luminesce (CPL). will be very useful to design a new generation of efficient D–A Opposite signed CPL signs for each pair of conformers (the right radicals with high-energy emission. and left-handed propeller-like PTM structures) have been Veciana and co-workers reported the luminescence properties obtained with acceptable luminescent dissymmetry factors.114,115 of PTM-doped polymeric films and organic nanoparticles (PTMd- This study opens the foundations for the optimization of new ONPs)108 (Fig. 6e). The confinement of radicals into organic rigid magnetically active organic chiral emitters and the emergence hosts decreases the non-radiative interactions, which affect the of a new field in optoelectronics and spintronics, where chiral, radicals in solid state enhancing their luminescence and photo- magnetic and luminescent properties can be exploited in a stability. ONPs exhibited uniform size distribution and high single device. colloidal stability in water showing life times and fF much higher The exploitation of the luminescent properties of PTM radicals than PTMs in solution. When increasing the radical doping, a new in devices is still in its infancy. However, considering the pre- excimeric band at the NIR region, which varied with temperature, viously mentioned promising results, progress in this direction appeared (Fig. 6f). Their water compatibility together with the will undoubtedly bring new insights in the field of organic emission in the first biological transparency window make these optoelectronics. materials promising candidates for optoelectronics and also for bio-sensing and bio-imaging applications.108 The intensity of these monomer and excimeric organic radical emissions were 6. PTM radicals for bio-applications modulated with a magnetic field (18 T at 4.2 K) for the first time.109 Further, the excited-state dynamics of such luminescent PTM radicals are appealing in bio-applications as polarizing radicals doped in a host crystal has been measured using optically agents in dynamic nuclear polarization (DNP), oxygen sensors detected magnetic resonance (ODMR) and time-resolved emission or in spin labelling thanks to their high stability, chemical spectroscopies corroborating the excimeric emission.110 versatility and narrow EPR linewidth. Interestingly, PTMs also exhibit high values of hyperpolarizability DNP is used for the enhancement of nuclear magnetic resonance opening the possibility to be explored in the field of non-linear (NMR) signals both in liquids (dissolution DNP, dDNP) and 10618 | J. Mater. Chem. C, 2021, 9, 10610–10623 This journal is © The Royal Society of Chemistry 2021 View Article Online Perspective Journal of Materials Chemistry C solids (magic angle spinning DNP, MAS-DNP). DNP allows for the obtained was reported to be mediated by the Cl nuclei and it intrinsically large spin polarization of electrons to be transferred was dependent on the position of the chlorine substituents on to nuclei, leading to a NMR signal enhancement of up to more the trityl skeleton. An interesting reported example was the use than four orders of magnitude. DNP has permitted NMR studies of the biradical PTM-TEMPO [i.e., (trans)-4-(2,2,6,6,-tetramethyl- This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. of previously unreachable systems, such as material surfaces or 1-piperidinyloxyl)-2,3,5,6-(tetrachlorophenyl)bis(pentachloro-phenyl)- low concentration biological samples. methyl biradical] (Fig. 8c).116 In this work, the DNP efficiency of The nature of the polarizing agent has a crucial role in the this biradical was compared with two model systems representing efficiency of DNP, as it determines the polarization-transfer the individual radical species: the monoradicals 4-oxo-TEMPO and mechanism. Supramolecular interactions take place between (trans)-4-(2,2,6,6,-tetramethyl-1-piperidine)-2,3,5,6-(tetrachloro- the radical, the glassing solvent, and the molecule undergoing phenyl)bispentachlorophenyl)methyl radical PTM-TEMPE polarization. In the case of dDNP, efforts have been focused on (Fig. 8c). A mixture of the two monoradicals at the same optimal Open Access Article. Published on 14 July 2021. Downloaded on 12/13/2022 9:20:42 AM. monoradicals, while biradicals have raised more interest as concentration was also studied. Interestingly, it was observed polarizing agents mainly for MAS-DNP experiments. In particular, that the polarization achieved with the biradical was much the contribution of trityl and PTM radicals to DNP has been higher than with the individual monoradicals or their mixture, significant in solid state and in solution, using organic solvents116 demonstrating the high efficiency of the biradical (Fig. 8d). and even water by synthesising suitably soluble derivatives. The With the PTM-TEMPO biradical almost 50% more polarization most soluble trityl radicals in aqueous media are those based on the was achieved when compared to the golden standard OX63 Finland trityl radical family like OX63, the benchmark radical in this under the same conditions. field (Fig. 8a). Most of the current advances have been developed Trityl radicals also have been found to be the most sensitive using these types of systems, especially those more soluble in soluble EPR oximetric probes and with good stability in vivo. The aqueous media, which have been applied for the sensitivity improve- EPR-based method, known as ‘‘EPR oximetry’’, is becoming a ment of a wide range of techniques (solid-state NMR, DNP, high fundamental tool in biology and medicine to monitor changes field 13C DNP, EPR).117–122 Therefore, the current trend is to in the pO2. It uses oxygen-sensing spin probes whose EPR lines are increasingly use DNP techniques in the study of complex biological broadened by molecular oxygen.127 It is even possible to perform systems both in vitro and in vivo. Especially promising is the use of in vivo oxygen mapping in animals,128,129 or to use two techniques DNP in nuclear magnetic resonance imaging (MRI), for example at the same time such as dissolution dynamic nuclear polarization in metabolic imaging applications of 13C-labelled metabolites and Overhauser-enhanced magnetic resonance imaging (OMRI) in vivo.123 for simultaneously visualizing tissue oxygen concentration and Remarkably, PTM functionalized with three (1) or six carboxylic microvascular permeability.130 acids (2) and their corresponding sodium salt (Fig. 8b) have been Trityl organic radicals can be used as spin labels to measure show to be efficient DNP 13C polarizers.124–126 The polarization distances in bio-macromolecules. Using pulsed dipolar EPR Fig. 8 (a) OX63. (b) Two different PTM derivatives functionalized with three (1) and six (2) carboxylate groups.124 (c) Molecular structures of biradical PTM-TEMPO and monoradical PTM-TEMPE. (d) Polarization building curves (3.35 T, 1.4 K) of biradical PTM-TEMPO (90 mM) at 94.067 GHz, monoradical PTM-TEMPE (90 mM) at 94.08 GHz, 4-oxo-TEMPO (90 mM) at 94.120 GHz, and a mixture of monoradical PTM-TEMPE and 4-oxo-TEMPO both at 90 mM.116 This journal is © The Royal Society of Chemistry 2021 J. Mater. Chem. C, 2021, 9, 10610–10623 | 10619 View Article Online Journal of Materials Chemistry C Perspective spectroscopy (PDS) in conjunction with site-directed spin labelling molecular quantum bits,139,140 where PTMs derivatives have (SDSL) techniques, these species have enabled distances measure- already shown promise.141 In the area of energy, PTM radicals ments to be performed in DNA and proteins for structural biology can also play a key role. PTMs can act as redox mediators studies. This approach has allowed structure and dynamics of favouring the oxygen reduction reaction and thus, improving This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. these bio-molecules to be studied in order to better understand the capacity and potential of discharge of non-aqueous Li–O2 their biomolecular functions under native conditions.131–135 batteries.142 These findings open new ways to enhance the Finally, it has been shown that trityl radicals are also suitable for performance of batteries using organic radicals like PTM, with EPR imaging. Regarding this application, carboxy-substituted trityl synthetically tuneable properties. radicals have proved to be highly appealing for in vivo probes. The Considering all the above, we expect that PTM radical based narrow spectral widths they exhibit facilitate spectral–spatial rapid systems will continue to be key in many new and established scan electron paramagnetic resonance imaging.136 areas of pure and applied research for many years to come. Open Access Article. Published on 14 July 2021. Downloaded on 12/13/2022 9:20:42 AM. 7. Future innovative perspectives for Conflicts of interest PTM radicals There are no conflicts to declare. Even though PTM radicals have been known for a few decades, it is clear that they continue to be highly attractive species for a Acknowledgements wide range of research fields. The unique electronic, magnetic and optical properties of PTMs, combined with their chemical The authors thank Prof. Jaume Veciana and Prof. Concepció Rovira and thermal stability and their amenability to functionalisation, for their continuous support along all these years. This work was have led to their innovative use in many systems of both funded by the Spanish Ministry of Science and Innovation with fundamental and technological importance. the projects GENESIS PID2019-111682RB-I00, Mol4Bio PID2019- As mentioned, the high reversibility of PTM redox processes 105622RB-I00, RTI2018-095622-B-I00 and RTI2018-095460-B-100, that occur at low voltage has allowed the exploitation of these and through the ‘‘Severo Ochoa’’ Programme for Centers of materials as active components in a variety of electrochemical Excellence in R&D (FUNFUTURE CEX2019-000917-S and Grant switches. Particularly appealing in the future might be the No. SEV-2017-0706) and the Spanish Structures of Excellence Marı́a synthesis of macromolecules containing a large number of de Maeztu program (MDM-2017-0767). The authors also thank PTM units to achieve high charge storage molecular systems the Generalitat de Catalunya (2017-SGR-918, 2017SGR13 and and multistate switches. Furthermore, PTM radicals have been 2017SGR238). It was also funded by the CERCA Program/Generalitat integrated in molecular junctions. The presence of the SOMO de Catalunya. and SUMO energy levels has been shown to have a strong impact on the electronic transport properties. Remarkably, the inter- References action of the transported electrons with the unpaired spin offers great opportunities in the area of molecular spintronics. 1 Stable Radicals. Fundamentals and Applied Aspects of Odd- PTMs have been employed as molecular building blocks for Electron Compounds, ed. R. G. Hicks, John Wiley & Sons, the fabrication of extended materials such as COFs and MOFs, Chichester, 2010. which can show potential in a variety of applications in energy (i.e., 2 R. G. Hicks, Org. Biomol. Chem., 2007, 5, 1321–1338. fuel cells and electrocatalysis). Additionally, having 2-dimensional 3 O. Armet, J. Veciana, C. Rovira, J. Riera, J. Castaner, E. 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