Dimer Radical Cation Research Articles

Ultrafast spectroscopy reveals singlet fission, ionization and excimer formation in perylene film

Singlet exciton fission (SF) is a spin-allowed process whereby two triplet excitons are created from one singlet exciton. This phenomenon can offset UV photon energy losses and enhance the overall efficiency in photovoltaic devices. For this purpose, it requires photostable commercially available SF materials. Excited state dynamics in pure perylene film, ease of commercial production, is studied by time-resolved fluorescence and femtosecond transient absorption techniques under different photoexcitation energies. In film, polycrystalline regions contain perylene in H-type aggregate form. SF takes place from higher excited states of these aggregates in ultrafast time scale < 30 fs, reaching a triplet formation quantum yield of 108%. Moreover, at λex = 450 nm singlet fission was detected as a result of two-quantum absorption. Other competing relaxation channels are excimer (1 ps) and dimer radical cation formation (< 30 fs). Excimer radiatively relaxes within 19 ns and radical cation recombines in 3.2 ns. Besides, exciton self-trapping by crystal lattice distortions occurs within hundreds of picosecond. Our results highlight potential of simple-fabricated perylene films with similar properties as high-cost single crystal in SF based photovoltaic applications.

The Simplest Model for Doped Poly(3,4‐ethylenedioxythiophene) (PEDOT): Single‐crystalline EDOT Dimer Radical Cation Salts

Invited for the cover of this issue is the group of Tomoko Fujino and Hatsumi Mori at the University of Tokyo. The image depicts the structural information of doped PEDOT uncovered by the single-crystalline EDOT dimer model. Read the full text of the article at 10.1002/chem.202005333.

The Simplest Model for Doped Poly(3,4-ethylenedioxythiophene) (PEDOT): Single-crystalline EDOT Dimer Radical Cation Salts.

Although doped poly(3,4-ethylenedioxythiophene) (PEDOT) is extensively used in electronic devices, their molecular-weight distributions and inadequately defined structures have hindered the elucidation of their underlying conduction mechanism. In this study, we introduce the simplest discrete oligomer models: EDOT dimer radical cation salts. Single-crystal structural analyses revealed their one-dimensional (1D) columnar structures, in which the donors were uniformly stacked. Band calculations identified 1D metallic band structures with a strong intracolumnar orbital interaction (band width W≈1 eV), implying the origin of the high conductivity of doped PEDOT. Interestingly, the salts exhibited semiconducting behavior reminiscent of genuine Mott states as a result of electron-electron repulsion (U) dominant over W. This study realized basic models with tunable W and U to understand the conduction mechanism of doped PEDOT through structural modification in oligomers, including the conjugation length.

Viologen radical stabilization by molecular spectators for aqueous organic redox flow batteries

Aqueous organic redox flow battery (AORFB) using viologens suffers from poor cycling life especially at high concentrations due to the dimerization of viologen radical cations. Here, we report a molecular spectator approach, exploiting α-cyclodextrin (α-CD) with suitable cavity size as the spectator, to weaken the intermolecular interaction of viologen radicals and suppress dimerization in AORFBs. UV–vis spectroscopy reveals that α-CD effectively suppresses the dimerization of viologen radicals. Applying α-CD as the spectator, a symmetric static cell (0.1 M EV-0.01 M α-CD) demonstrated a significantly lower capacity decay rate of 0.075% per day (20 days, 100% capacity utilization) compared to the sole EV electrolyte (0.31% per day). Asymmetric flow cells (1.0 M EV-0.1 M α-CD) coupling with a ferrocene derivative demonstrated stable cycling over 500 cycles during 26 days at 80% capacity utilization. This work provides a facile and effective strategy to stabilize the viologen radicals for AORFBs applications. • A molecular spectator approach was developed to stabilize viologen radical cations. • A lower capacity decay was achieved with spectators (0.075% per day) compared to without (0.31% per day) at 100% capacity utilization. • An asymmetric full flow cell using spectators demonstrated a stable volumetric capacity of 21.4 Ah L –1 negolyte over 26 days.

Experimental and theoretical study of photo-dissociation spectroscopy of pyrene dimer radical cations stored in a compact electrostatic ion storage ring.

In this paper, we present an experimental and theoretical study of the photo-dissociation of free-flying dimer radical cations of pyrene (C16H10)2+. Experimentally, the dimers were produced in the plasma of an electron cyclotron resonance ion source and stored in an electrostatic ion storage ring, the Mini-Ring for times up to 10 ms and the photo-dissociation spectrum was recorded in the 400 to 2000 nm range. Two broad absorption bands were observed at 550 (2.25 eV) and 1560 nm (0.79 eV), respectively. Theoretical simulations of the absorption spectrum as a function of the temperature were performed using the Density Functional based Tight Binding approach within the Extended Configuration Interaction scheme (DFTB-EXCI) to determine the electronic structure. The simulation involved all excited electronic states correlated asymptotically with the five lowest excited states D1-D5 of the monomer cation and a Monte Carlo exploration of the electronic ground state potential energy surface. The simulations exhibit three major bands at 1.0, 2.1 and 2.8 eV respectively. They allow assigning the experimental band at 1560 nm to absorption by the charge resonance (CR) excited state correlated with the ground state of the monomer D0. The band at 550 nm is tentatively attributed to dimer states correlated with excited states D2-D4, in the monomer cation. Simulations also show that the CR band broadens and shifts towards longer wavelength with increasing temperature. It results from the dependence on the geometry of the energy gap between the ground state and the lowest excited state. The comparison of the experimental spectrum with theoretical spectra at various temperatures allows us to estimate the temperature of the stored (C16H10)2+ in the 300-400 K range, which is also in line with the expected temperatures of the ions deduced from the analysis of the natural decay curve.

Enhancement of deformation of redox-active hydrogel as an actuator by increasing pendant viologens and adding filler or counter-charged polymer

Strategies for enhancement of redox-driven deformation of a hydrogel, viologen pendant poly-L-Lysine based glutaraldehyde cross-linked hydrogel (PLLV-GA-gel), were pursued. To surpass the previously reported fastest reductive contraction, a 93 % volume decrease within 100 s on dithionite reduction, and to accelerate expansion on oxidation, we examined the pendant rate dependence and the effect of the addition of conductive fillers or an anionic polymer, poly(styrene sulfonate) (PSS). In testing deformation of water-floated PLLV-GA-gels, the contraction of a 25 % pendant rate gel was greater and faster than others. The initial rate of reductive contraction of the gels with fillers was in the order of graphene nano-platelet (GNP) > gold nanoparticle > carbon nanotube (CNT) > no-filler. The initial rates of swelling of GNP and CNT gels on oxidation were faster than the no-filler gel. PSS incorporation enhanced the swelling rate and amplitude but slowed down the contraction. All the PLLV-based samples on a Au electrode showed largely diffusion-controlled redox reactions of viologen in voltammograms and radical cation dimer rich light-absorption features in electroreflectance spectra. The guide for the choice of conductive filler and coexistence of anionic polymer described herein should be applicable to enhance the deformation of many redox-driven hydrogel actuators.

Formation of intramolecular dimer radical ions of diphenyl sulfones

Dimer radical ions of aromatic molecules in which excess charge is localized in a pair of rings have been extensively investigated. While dimer radical cations of aromatics have been previously produced in the condensed phase, the number of molecules that form dimer anions is very limited. In this study, we report the formation of intramolecular dimer radical ions (cations and anions) of diphenyl sulfone derivatives (DPs) by electron beam pulse radiolysis in the liquid phase at room temperature. The density functional theory (DFT) calculations also showed the formation of the dimer radical ions. The torsion barrier of the phenyl ring of DPs was also calculated. It was found that the dimer radical ions show the larger barrier than the neutral state. Finally, stability of the dimer radical anion is dependent on not only the inductive effect of the sulfonyl group but the conjugation involving the d-orbital of the S atom and the phenyl rings.

Balancing Density Functional Theory Interaction Energies in Charged Dimers Precursors to Organic Semiconductors.

The pursuit of an increasingly accurate description of intermolecular interactions within the framework of Kohn-Sham density functional theory (KS-DFT) has motivated the construction of numerous benchmark databases over the past two decades. By far, the largest efforts have been spent on closed-shell, neutral dimers for which today the interaction energies and geometries can be accurately reproduced by various combinations of dispersion-corrected density functional approximations (DFAs). In sharp contrast, charged, open-shell dimers remain a challenge as illustrated by the analysis of the OREL26rad benchmark set, composed of π-dimer radical cations. Aside from the methodological aspect, achieving a proper description of radical cationic complexes is appealing due to their role as models for charge carriers in organic semiconductors. In the interest of providing an assessment of more realistic dimer systems, we construct a data set of large radical cationic dimers (CryOrel9) and jointly train the 19 parameters of a dispersion corrected, range-separated hybrid density functional (ωB97X-dDsC). The main objective of ωB97X-dDsC is to provide the maximum balance between the treatment of long-range London dispersion and reduction of the delocalization error, which are essential conditions to obtain accurate energy profiles and binding energies of charged, open-shell dimers. The performance of ωB97X-dDsC, its parent ωB97X functional series, and a selection of wave function-based methods is reported for the CryOrel9 data set. The robustness of the reoptimized variant (ωB97X-dDsC) is also tested on other GMTKN30 data sets.

The Mechanism, Kinetic Study and Non-Linear Optical Properties (NLO) of Para-Chloroaniline Using DFT Approach

The synthesis of poly-p-chloroaniline (PpCA) by oxidative chemical polymerization using potassium dichromate as oxidizing agent was carried out. The optimum conditions for the polymerization reaction and the order of reactions and thermodynamic activation parameters were investigated. A molecular mechanism for the oxidation of p-chloroaniline using potassium dichromate is proposed. The TGA analysis and spectroscopic studies IR, UV-vis and elemental analysis have evidenced the structure of polymeric chain. The surface morphology of the obtained polymer was characterized by X-ray diffraction and transmission electron microscopy (TEM). Moreover, determinations of dielectric properties of the prepared polymer were carried out. The a.c conductivity (σac) of (PpCA) was investigated as a function of frequency and temperature. The microscopic conduction mechanism of charge carries over the potential barrier in polymer backbone was found classical hopping model. The electronic structure of neutral PpCA, radical cation and dimer radical cation are investigated theoretically at the B3LYP/6-311G** level of theory. The mechanism of the polymerization process is discussed and analyzed. The calculated EHOMO and ELUMO energies of the studied compounds can be used to calculate the global properties; chemical hardness (η), softness (S) and electronegativity (χ). The calculated nonlinear optical parameters (NLO); polarizibilty (α), anisotropy of the polarizibility (Δα) and first order hyperpolarizibility (β) of the studied compounds show promising optical properties. 3D-plots of the molecular electrostatic potential (MEP) for neutral monomer and radical cation dimer are investigated and analyzed showing the distribution of electronic density of orbitals describing the electrophilic and nucleophilic sites of the neutral monomer and radical cation dimer.

Sulfonated Pyrene as a Photoregulator for Single-Stranded DNA Looping.

Controlling the conformation and function of biomolecules through photoregulators holds great promise as a spatiotemporally controllable tool for disease control. In addition, introducing photoregulators into biomolecules has also found applications in constructing smart nanomaterials. In spite of the astonishing advances that have been made in the past few years, realizing highly controllable and efficient regulation over the conformation and function of biomolecules under physiological conditions is still challenging. Herein, sulfonated pyrene SPy was synthesized and used as a photoregulator to control the looping of single-stranded DNAs (ssDNAs) in aqueous solution. Due to its water solubility, SPy merits use in the study of biomolecules in aqueous solution. The looping of the doubly SPy-modified ssDNAs is stimulated by irradiation and regulated by SPy. Photoionization generates the radical cation of SPy (SPy.+ ). The association of SPy.+ with its neutral counterpart, SPy, gives rise to the dimer radical cation of SPy (SPy2 .+ ). During the association process, the stabilization energy released to form SPy2 .+ provides a driving force for the looping of ssDNAs. Conversely, the formed loop conformations were trapped by the formation of SPy2 .+ , and this allowed the looping dynamics to be investigated. The results reported herein suggest potential of SPy as a photoregulator for controlling the conformation and function of biomolecules under physiological conditions.

Balancing DFT Interaction Energies in Charged Dimers Precursors to Organic Semiconductors

Accurately describing intermolecular interactions within the framework of Kohn-Sham density functional theory (KS-DFT) has resulted in numerous benchmark databases over the past two decades. By far, the largest efforts have been spent on closed-shell, neutral dimers for which today, the interaction energies and geometries can be accurately reproduced by various combinations of dispersion-corrected density functional approximations (DFAs). In sharp contrast, charged, open-shell dimers remain a challenge as illustrated by the analysis of the OREL26rad benchmark set consisting of pi-dimer radical cations. Aside from the methodological aspect, achieving a proper description of radical cationic complexes is appealing due to their role as models for charge carriers in organic semiconductors. In the interest of providing an assessment of more realistic dimer systems, we construct a dataset of large radical cationic dimers (CryOrel) and jointly train the 19 parameters of a dispersion corrected, range-separated hybrid density functional (wB97X-dDsC), with the objective of providing the maximum balance between the treatment of long-range London dispersion and reduction of the delocalization error. These conditions are essential to obtain accurate energy profiles and binding energies of charged, open-shell dimers. Comparisons with the performance of the parent wB97X functional series and state-of-the-art wavefunction based methods are provided.

New Insight into an Old Problem: Analysis, Interpretation, and Theoretical Modeling of the Absorption and Magnetic Circular Dichroism Spectra of Monomeric and Dimeric Zinc Phthalocyanine Cation Radical.

The chemically or spectroelectrochemically generated formation and aggregation of zinc(II) tetra-tert-butylphthalocyanine cation radical [ZnPctBu]+•, which was highly soluble in common organic solvents, were investigated using UV-vis and magnetic circular dichroism (MCD) spectroscopies with an emphasis on the influence of the axial ligand on the fingerprint (∼500 nm) and NIR (720∼1000 nm) spectral envelopes. MCD spectroscopy is suggestive that the NIR band at ∼1000 nm observed for the antiferromagnetically coupled cation radical dimer, [ZnPctBu]22+, has no degeneracy, the monomer-dimeric equilibrium is temperature dependent, and higher degree aggregates can be formed at specific conditions. Sixteen different exchange-correlation functionals were tested to accurately predict the energies, intensities, and profiles of the UV-vis and MCD spectra of the phthalocyanine cation radical monomer and dimer. It was found that the M05 exchange-correlation functional (along with several other functionals that include 27-42% of Hartree-Fock exchange) provided an excellent agreement (∼0.1 eV for the degenerate excited states observed by MCD spectroscopy) between theory and experiment for the phthalocyanine cation-radical monomer and dimer. Not only did time-dependent density functional theory (TDDFT) calculations with M05 exchange-correlation functional correctly predict the nondegenerate NIR charge-transfer band at ∼1000 nm, all degenerate excited states, monomer and dimer energies, and oscillator strengths, but also they correctly described the nature of the experimentally observed at ∼500 nm MCD B-term (fingerprint band) detected for both the monomeric and dimeric phthalocyanine cation radicals. The TDDFT data explain the similarities in the UV-vis and MCD spectra of the monomeric and dimeric species observed between the UV and fingerprint spectral envelopes as well as correctly predicted the antiferromagnetic coupling between the two singly oxidized phthalocyanine macrocycles in the dimer.

Delocalization of positive charge in aromatic liquids studied by subnanosecond near-infrared transient absorption spectroscopy

Transient absorption spectra of 1,2,4,5-tetracyanobenzene (TCNB) in aromatic liquids (benzene, toluene, m-xylene, and mesitylene) were measured in subnanosecond time resolution. Upon photoexcitation at 355 nm, exciplexes between TCNB and the aromatic liquids formed quickly, and we observed characteristic absorption bands due to a TCNB radical anion in the visible wavelength range and charge resonance (CR) absorption bands due to aggregate radical cations of the donor molecules in the near-IR wavelength range. The peak positions of the CR bands for the exciplexes differed significantly from those of the dimer radical cations of the corresponding donor molecules, indicating that positive charge was delocalized among more than two donor molecules. The TCNB radical anion decomposed efficiently within the lifetime of the exciplexes, and the positive charge remained in the aromatic liquids after the decomposition. Thus, we could observe CR bands due to positive charge (holes) in the aromatic liquids. From the peak positions of the CR bands, we were able to draw some conclusions regarding the delocalization of positive charge in aromatic liquids.

Sam P. de Visser

Sam P. de Visser

Structures of benzonitrile dimer radical cation and the protonated dimer: Observation of hydronium ion core solvated by benzonitrile molecules.

The recent discovery of benzonitrile (C6H5CN), one of the simplest nitrogen-bearing polar aromatic molecules, in the interstellar medium motivates structural characterization of the benzonitrile-containing molecular ions as potential precursors for nitrogen-containing complex organics in space. Herein, we present mass-selected ion mobility measurements combined with density functional theory (DFT) calculations to reveal, for the first time, the structures of the benzonitrile dimer radical cation, the protonated dimer, and the protonated hydrated small clusters in the gas phase. The measured collision cross sections of the investigated ions in helium are in excellent agreement with the calculated values of the lowest energy DFT structures. Unlike the dimer radical cations of nonpolar aromatic molecules which adopt parallel sandwich configurations, the (C6H5CN)2 ·+ displays a symmetrically planar geometry with a double hydrogen bond formed between the nitrogen and hydrogen atoms. The protonated dimer has the structure of a proton-bound dimer (C6H5CNH+NCC6H5) where the bridging proton connects the nitrogen atoms in the two benzonitrile molecules resulting in a calculated collision cross section of 101.1 Å2 in excellent agreement with the measured value of 103.3 Å2. The structure of the hydrated protonated trimer consists of a hydronium ion core solvated by three benzonitrile molecules. By locating the proton on the lower proton affinity water molecule, the resulting hydronium ion can be fully solvated by forming three ionic hydrogen bonds with the benzonitrile molecules. These unique structural motifs could be useful for the molecular design and recognition involving charged aromatic systems and also for the search of nitrogen-containing complex organics in space.

Formation of the Charge-Localized Dimer Radical Cation of 2-Ethyl-9,10-dimethoxyanthracene in Solution Phase.

Although dimer radical ions of aromatic molecules in the liquid-solution phase have been intensely studied, the understanding of charge-localized dimers, in which the extra charge is localized in a single monomer unit instead of being shared between two monomer units, is still elusive. In this study, the formation of a charge-localized dimer radical cation of 2-ethyl-9,10-dimethoxyanthracene (DMA), (DMA)2 .+ is investigated by transient absorption (TA) and time-resolved resonance Raman (TR3 ) spectroscopic methods combined with a pulse radiolysis technique. Visible- and near-IR TA signals in highly concentrated DMA solutions supported the formation of non-covalent (DMA)2 .+ by association of DMA and DMA.+ . TR3 spectra obtained from 30 ns to 300 μs time delays showed that the major bands are quite similar to those of DMA except for small transient bands, even at 30 ns time delay, suggesting that the positive charge of non-covalent (DMA)2 .+ is localized in a single monomer unit. From DFT calculations for (DMA)2 .+ , our TR3 spectra showed the best agreement with the calculated Raman spectrum of charge-localized edge-to-face T-shaped (DMA)2 .+ , termed DT.+ , although the charge-delocalized asymmetric π-stacked face-to-face (DMA)2 .+ , termed DF3.+ , is the most stable structure of (DMA)2 .+ according to the energetics from DFT calculations. The calculated potential energy curves for the association between DMA.+ and DMA showed that DT.+ is likely to be efficiently formed and contribute significantly to the TR3 spectra as a result of the permanent charge-induced Coulombic interactions and a dynamic equilibrium between charge localized and delocalized structures.

Voltammetric and mass spectrometry investigation of methionine oxidation

The electro-oxidation mechanism of free methionine and bound within different peptide sequences was investigated by voltammetry, at glassy carbon electrode, and mass spectrometry. It is proposed that the electro-oxidation of free methionine occurs in two steps, each involving the transfer of one electron and turns pH-independent from mild acid to mild alkaline electrolytes. The first oxidation reaction leads to the formation of a cation radical stabilized either through the amino group resulting in the dehydromethionine intermediate, or by interaction with a neutral methionine molecule leading to production of a dimer cationic radical. The dehydromethionine hydrolysis gave methionine sulfoxide as final oxidation product, whereas a future oxidation of methionine dimer cation radical, i.e. the second electro-oxidation step, results in a methionine dimer dication. Moreover, at high acid media, the protonated amino group influence the electro-oxidation process to take place via proton transfer mechanism. The presence of methionine sulfoxide and of the dimer cationic radical as oxidation products of methionine was confirmed by mass spectroscopy.

An aryl-fused redox-active tetrathiafulvalene with enhanced mixed-valence and radical-cation dimer stabilities

Molecular recognition of stable organic radicals is a relatively novel, but important structural binding motif in supramolecular chemistry. Here, we report on a redox-switchable veratrole-fused tetrathiafulvalene derivative VTTF which is ideally suited for this purpose and for the incorporation into stimuli-responsive systems. As revealed by electrochemistry, UV/Vis measurements, X-ray analysis, and electrocrystallisation, VTTF can be reversibly oxidised to the corresponding radical-cation or dication which shows optoelectronic and structural propterties similar to tetrathiafulvalene and tetrakis(methylthio)tetrathiafulvalene. However, theoretical calculations, variable temperature EPR, and NIR spectroscopy indicate that the dispersion-driven binding in the mixed-valence dimer (VTTF2)˙+ (KMV = 69 M-1 in CH2Cl2) and the radical-cation dimer (VTTF˙+)2 (KRC = 38 M-1 in CH3CN) is significantly enhanced by the additional veratrole π-surface in comparison to pristine tetrathiafulvalene.

Dynamics of Radical Ions of Hydroxyhexafluoroisopropyl-Substituted Benzenes.

Fluorination of resist materials is an effective method used to enhance the energy deposition of extreme ultraviolet (EUV) light in the fabrication of next-generation semiconductor devices. The dynamics of radical ions are important to understand when considering the radiation-chemistry of the resist materials using EUV and electron beam lithography. Here, the dynamics of the radical anions and cations of benzenes with one or two 2-hydroxyhexafluoroisopropyl groups (HFABs) were studied using radiolysis techniques. The formation of dimer radical cations was observed only in the monosubstituted benzene solutions of 1,2-dichloroethane. If the compound contained more than two substituents, it was found to hinder the necessary π-π overlapping. Pulse radiolysis of HFABs in tetrahydrofuran showed a characteristic spectral shift of the radical anion within the region of several hundred nanoseconds. From the results of low-temperature spectroscopy and density functional calculations, it is suggested that excess electrons of the 2-hydroxyhexafluoroisopropyl group of the radical anions cause dissociation into neutral radicals.

Reactive pathways in the bromobenzene-ammonia dimer cation radical: Evidence for a roaming halogen radical

Photoinitiated reactions in van der Waals complexes provide a means to examine reactive pathways from well-defined initial geometries. In recent work, we re-examined reactive pathways following resonant two-photon ionization (R2PI) of the chlorobenzene-ammonia (PhCl⋯NH3) dimer. The dimer cation radical reacts primarily via Cl atom loss, and additional channels corresponding to HCl and H atom loss were identified. The structure of the reactive complex was confirmed as an in-plane σ-type, and computational studies of the dimer cation radical potential energy landscape revealed two nearly isoenergetic arenium ion intermediates (or Wheland intermediates). The intermediate produced from ipso addition was not stable with respect to either Cl or HCl loss, and the relative branching observed in experiment was well reproduced by microcanonical transition state theory (TST) calculations. Here, we report experimental and computational studies of the related PhBr⋯NH3 dimer, examined for the first time. We present evidence that the dimer structure is also an in-plane σ-type. However, in contrast to the PhCl⋯NH3 system, calculations predict that the structure of the reactive intermediate corresponds to a distonic ion-radical complex, where the radical density is largely localized on the bromine atom. The calculated barrier to HBr loss is sufficiently high to render this channel nearly insignificant (less than 1% branching) in TST calculations, yet experiment shows a sizable (37%) branching into this channel. We rationalize these results in terms of a roaming Br radical mechanism for HBr formation.