Self-Assembled Architectures with Segregated Donor and Acceptor Units of a Dyad Based on a Monopyrrolo-Annulated TTF-PTM Radical

Self-Assembly of Discrete Multi-Chromophoric Systems

Self-assembly of chromophoric systems is a prerequisite to create well-ordered, processable nanomaterials with multiple functionalities. In the past two decades, the field of functional organic materials has primarily focused on systems featuring only one type of dye/π-conjugated unit. Consequently, many reports with mechanistic insights on the self-assembly of the dyes featuring different molecular packing have been reported. Subsequently, we have witnessed several attempts to organize the multi-chromophoric systems in solution and solid-state via different approaches using self-assembly as a tool. Incorporation of more than one dye is important in creating materials with tuneable optoelectronic properties. Consequently, self-assembly of more than one chromophoric systems have been investigated to some extent. This review aims to discuss the self-assembled materials derived from discrete π-conjugated systems comprising more than one dye units connected through covalent bonding (multi-chromophoric systems). Molecular design of various multi-chromophoric systems leading to the formation of crystals, liquid crystals and supramolecular polymers have been correlated with corresponding properties. We envisage that classification of self-assembled multi-chromophoric systems, with a note on tuneable optoelectronic properties, can provide a deeper understanding on the molecular design strategies, which is important in the fabrication of functional organic materials with optimum performances.

Nanothermometer Based on Polychlorinated Trityl Radicals Showing Two-Photon Excitation and Emission in the Biological Transparency Window: Temperature Monitoring of Biological Tissues

Nanothermometers are emerging probes as biomedical diagnostic tools. Especially appealing are nanoprobes using NIR light in the range of biological transparency window (BTW) since they have the advantages of a deeper penetration into biological tissues, better contrast, reduced phototoxicity and photobleaching. This article reports the preparation and characterization of organic nanoparticles (ONPs) doped with two polychlorinated trityl radicals (TTM and PTM), as well as studies of their electronic and optical properties. Such ONPs having inside isolated radical molecules and dimeric excimers, can be two-photon excited showing optimal properties for temperature sensing. Remarkably, in TTM-based ONPs the emission intensity of the isolated radical species is unaltered increasing temperature, while the excimer emission intensity decreases strongly being thereby able to monitor temperature changes with an excellent thermal absolute sensitivity of 0.6-3.7% K-1 in the temperature range of 278-328 K. The temperature dependence of the excimeric bands of ONPs are theoretically simulated by using electronic structure calculations and a vibronic Hamiltonian model. Finally, TTM-doped ONPs as ratiometric NIR-nanothermometers are tested with two-photon excitationwith enucleated pig eye sclera, as a real tissue model, obtaining a similar temperature sensitivity as in aqueous suspensions, demonstrating their potential as NIR nanothermometers for bio applications.

Exploring the Luminescence, Redox, and Magnetic Properties in a Multivariate Metal-Organic Radical Framework

Persistent neutral organic radicals are excellent building blocks for the design of functional molecular materials due to their unique electronic, magnetic, and optical properties. Among them, triphenylmethyl radical derivatives have attracted a lot of interest as luminescent doublet emitters. Although neutral organic radicals have been underexplored as linkers for building metal-organic frameworks (MOFs), they hold great potential as organic elements that could introduce additional electronic properties within these frameworks. Herein, we report the synthesis and characterization of a novel multicomponent metal-organic radical framework (PTMTCR@NR-Zn MORF), which is constructed from the combination of luminescent perchlorotriphenylmethyl tricarboxylic acid radical (PTMTCR) and nonemissive nonradical (PTMTCNR) organic linkers and Zn(II) ions. The PTMTCR@NR-Zn MORF structure is layered with microporous one-dimensional channels embedded within these layers. Kelvin probe force microscopy further confirmed the presence of both organic nonradical and radical linkers in the framework. The luminescence properties of the PTMTCR ligand (first studied in solution and in the solid state) were maintained in the radical-containing PTMTCR@NR-Zn MORF at room temperature as fluorescence solid-state quenching is suppressed thanks to the isolation of the luminescent radical linkers. In addition, magnetic and electrochemical properties were introduced to the framework due to the incorporation of the paramagnetic organic radical ligands. This work paves the way for the design of stimuli-responsive hybrid materials with tunable luminescence, electrochemical, and magnetic properties by the proper combination of closed- and open-shell organic linkers within the same framework.

Understanding the g-tensors of perchlorotriphenylmethyl and Finland-type trityl radicals

The 285 GHz EPR spectra of perchlorotriphenylmethyl and tetrathiatriarylmethyl radicals in frozen solution have been accurately measured. The relationship between their molecular structures and their g-tensors has been investigated with the aid of DFT calculations, revealing that the degree of spin density delocalization away from the central methylene carbon is an important determining factor of the g-anisotropy. In particular, the small amount of spin densities on the Cl or S heteroatoms at the 2 and 6 positions with respect to the central carbon have the strongest influence. Furthermore, the amount of spin densities on these heteroatoms and thus the anisotropy can be modulated by the protonation (esterification) state of the carboxylate groups at the 4 position. These results provide unique insights into the g-anisotropy of persistent trityl radicals and how it can be tuned.

Influence of the donor unit on the rectification ratio in tunnel junctions based on donor-acceptor SAMs using PTM units as acceptors

Dyads formed by an electron donor unit (D) covalently linked to an electron acceptor (A) by an organic bridge are promising materials as molecular rectifiers. Very recently, we have reported the charge transport measurements across self-assembled monolayers (SAMs) of two D-A systems consisting of the ferrocene (Fc) electron-donor linked to a polychlorotriphenylmethane (PTM) electron-acceptor in its non-radical (SAM 1) and radical (SAM 2) forms. Interestingly, we observed that the non-radical SAM 1 showed rectification behavior of 2 orders of magnitude higher than its radical analogue dyad 2. In order to study the influence of the donor unit on the transport properties, we report herein the synthesis and characterization of two new D-A SAMs in which the electron-donor Fc unit is replaced by a tetrathiafulvalene (TTF) moiety linked to the PTM unit in its non-radical (SAM 3) and radical (SAM 4) forms. The observed decrease in the rectification ratio and increased current density for TTF-PTM based SAMs 3 and 4 in comparison to Fc-PTM based SAMs 1 and 2 are explained, supported by theoretical calculations, by significant changes in the electronic and supramolecular structures.

Donor-Donor'-Acceptor Triads Based on [3.3]Paracyclophane with a 1,4-Dithiafulvene Donor and a Cyanomethylene Acceptor: Synthesis, Structure, and Electrochemical and Photophysical Properties

Donor-donor'-acceptor triads (1, 2), based on [3.3]paracyclophane ([3.3]PCP) as a bridge, with electron-donating properties (D') using 1,4-dithiafulvene (DTF; TTF half unit) as a donor and dicyanomethylene (DCM; TCNE half unit) or an ethoxycarbonyl-cyanomethylene (ECM) as an acceptor were designed and synthesized. The pulse radiolysis study of 1 a in 1,2-dichloroethane allowed the clear assignment of the absorption bands of the DTF radical cation (1 a^.+ ), whereas the absorption bands due to the DCM radical anion could not be observed by γ-ray radiolysis in 2-methyltetrahydrofuran rigid glass at 77 K. Electrochemical oxidation of 1 a first generates the DTF radical cation (1 a^.+ ), the absorption bands of which are in agreement with those observed by a pulse radiolysis study, followed by dication (1 a²⁺ ). The ESR spectrum of 1 a^.+ showed a symmetrical signal with fine structure and an ESR simulation predicted that the spin of 1 a^.+ is delocalized over S and C atoms of the DTF moiety and the central C atom of the trimethylene bridge bearing the DTF moiety. Pulse radiolysis, ESR, and electrochemical studies indicate that the DTF radical cation of 1 a^.+ is more stable than that of 6^.+ , and the latter shows a strong tendency to dimerize. This result indicates that the [3.3]PCP moiety as a bridge can stabilize the DTF radical cation more than the 1,3-diphenylpropane moiety because of kinetic stability due to its rigid structure and the weak electronic interaction of DTF and DCM moieties through [3.3]PCP.

Strategy for the Co-Assembly of Co-Axial Nanotube-Polymer Hybrids

Nanostructured materials having multiple, discrete domains of sorted components are particularly important to create efficient optoelectronics. The construction of multicomponent nanostructures from self-assembled components is exceptionally challenging due to the propensity of noncovalent materials to undergo structural reorganization in the presence of excipient polymers. This work demonstrates that polymer-nanotube composites comprised of a self-assembled nanotube wrapped with two conjugated polymers could be assembled using a layer-by-layer approach. The polymer-nanotube nanostructures arrange polymer layers coaxially on the nanotube surface. Femtosecond transient absorption (TA) studies indicated that the polymer-nanotube composites undergo photoinduced charge separation upon excitation of the NDI chromophore within the nanotube.

Twisted intramolecular charge transfer in a carbazole-based chromophore: the stable [(4-N-carbazolyl)-2,3,5,6-tetrachlorophenyl]bis(2,3,5,6-tetrachlorophenyl)methyl radical

A stable radical adduct 1 with carbazole as an electron-donor and polychlorotriphenylmethyl radical as an electron-acceptor shows interesting properties.

TTF–PTM dyads: from switched molecular self assembly in solution to radical conductors in solid state

Dyads formed by tetrathiafulvalene (TTF) linked to perchlorotriphenylmethyl (PTM) radicals exhibit interesting physical properties such as bistability in solution or conductivity in solid state.

Excimers from stable and persistent supramolecular radical-pairs in red/NIR-emitting organic nanoparticles and polymeric films

For the first time, using a carbon free-radical, excimeric emission from stable and persistent supramolecular radical-pairs has been observed.

Identification of a new electron-transfer relaxation pathway in photoexcited pyrrole dimers

Photoinduced electron transfer is central to many biological processes and technological applications, such as the harvesting of solar energy and molecular electronics. The electron donor and acceptor units involved in electron transfer are often held in place by covalent bonds, π-π interactions or hydrogen bonds. Here, using time-resolved photoelectron spectroscopy and ab initio calculations, we reveal the existence of a new, low-energy, photoinduced electron-transfer mechanism in molecules held together by an NH⋯π bond. Specifically, we capture the electron-transfer process in a pyrrole dimer, from the excited π-system of the donor pyrrole to a Rydberg orbital localized on the N-atom of the acceptor pyrrole, mediated by an N-H stretch on the acceptor molecule. The resulting charge-transfer state is surprisingly long lived and leads to efficient electronic relaxation. We propose that this relaxation pathway plays an important role in biological and technological systems containing the pyrrole building block.

Covalent non-fused tetrathiafulvalene–acceptor systems

The main families of non-fused TTF–acceptors are discussed with a special focus on their characteristics and properties.

Structural, Magnetic and DFT studies on a Charge-Transfer Salt of a Tetrathiafulvalenepyridyl-(1,5-diisopropyl) verdazyl Diradical Cation

A new tetrathiafulvalene (TTF) donor covalently appended with a 1,5-diisopropylverdazyl radical through a cross-conjugated pyridyl linker (3) has been prepared and characterised. Reaction of 3 with tetracyanoquinonedimethane (TCNQ) afforded the 2:1 charge-transfer complex (3)₂ ⋅TCNQ (4), in which the IR and structural data are consistent with 0.25 e^- charge transfer from the TTF donor (D) to the TCNQ acceptor (A). The TTF and TCNQ molecules adopt a mixed-stack D⋅⋅⋅D⋅⋅⋅A arrangement that does not facilitate conduction. A solution EPR spectrum of 4 comprises a broad featureless singlet, which is consistent with the presence of a TCNQ radical anion. Theoretical studies were performed to probe the exchange interactions within selected fragments of 4 with and without charge transfer. In the absence of charge transfer, DFT calculations reveal weak antiferromagnetic exchange between verdazyl radicals within the (3)₂ monoradical unit. However, partial oxidation of the dimer (3)₂ to the diradical cation leads to an S= 1 / 2 ground state, in which the verdazyl radical spins are now aligned co-parallel as a consequence of antiferromagnetic exchange to the additional delocalised TTF-based spin containing unit. The magnetic properties of 4 are consistent with a net S= 1 / 2 spin state per formula unit with dominant antiferromagnetic interactions between spin-bearing building blocks.

Organic radicals for the enhancement of oxygen reduction reaction in Li–O2 batteries

Carbon free-radicals are used as soluble redox mediators to catalyze ORR and enhance the discharge of Li–O₂ batteries.