[2.2]Paracyclophane-Bridged Mixed-Valence Compounds: Application of a Generalized Mulliken−Hush Three-Level Model

Tuning Electron Transfer Coupling and Exchange Interaction in Bis-triarylamine Radical Cations and Dications by Bridge Electron Density

The influence of the electron density of a bridge connecting two redox centers on both the intervalence hole transfer and the magnetic superexchange was investigated in a series of bridged bis-triarylamine mono- and dications. In this series, the bridge was 2,7-fluorenyl, where the bridge electron density was modified by substituents at the 9-position. For the mixed-valence monocations, the observation of both an intervalence charge transfer (IVCT) band and an absorption band associated with an electron transfer from the bridging fluorene to the triarylamine radical cation centers allowed determination of the electron transfer couplings in the framework of the three-state generalized Mulliken-Hush theory. Comparison of the derived couplings with those obtained from a classical two-state approach demonstrates an enhancement of the electronic coupling which increases with decreasing bridge state energy. For the dicationic diradical counterparts, the singlet-triplet gap (exchange interaction) was determined both experimentally and by quantum chemical methods. Hereby, an increase of antiferromagnetic coupling with a lowering of the bridge state energy by electron donating substituents was observed. Analysis of the involved molecular orbitals suggests that the ferromagnetic coupling is inversely proportional to the square of the bridge energy, which is also supported by the experimental findings. This influence of the bridge state energy on both types of interactions, electron transfer and magnetic exchange, provides a design guideline for fine-tuning the properties of electronically coupled organic redox dyads by variation of the bridge electron density.

Bright Luminescence by Combining Chiral [2.2]Paracyclophane with a Boron-Nitrogen-Doped Polyaromatic Hydrocarbon Building Block

Novel BN-doped compounds based on chiral, tetrasubstituted [2.2]paracyclophane and NBN-benzo[f,g]tetracene were synthesized by Sonogashira-Hagihara coupling. Conjugated ethynyl linkers allow electronic communication between the π-electron systems through-bond, whereas through-space interactions are provided by strong π-π overlap between the pairs of NBN-building blocks. Excellent optical and chiroptical properties in racemic and enantiopure conditions were measured, with molar absorption coefficients up to ϵ=2.04×105  M-1  cm-1 , fluorescence quantum yields up to ΦPL =0.70, and intense, mirror-image electronic circular dichroism and circularly polarized luminescence signals of the magnitude of 10-3 for the absorption and luminescence dissymmetry factors. Computed glum,calcd. values match the experimental ones. Electroanalytical data show both oxidation and reduction of the ethynyl-linked tetra-NBN-substituted paracyclophane, with an overlap of two redox processes for oxidation leading to a diradical dication.

Pre-Planarized Triphenylamine-Based Linear Mixed-Valence Charge-Transfer Systems

Three linear dimers with two redox-active planarized triphenylamines were synthesized and their structures verified by X-ray crystallography. Their radical cations, which exhibit electron self-exchange between the two redox centers, are of great interest. This process was thoroughly investigated by means of electron paramagnetic resonance spectroscopy, absorption spectroscopy, and (time-dependent) density functional theory calculations. A comparison of the key parameters of electron transfer with non-planarized nitrogen-centered building blocks emphasizes the impact of using redox centers with low internal reorganization energies. However, the distance-dependence attenuation factor of the super-exchange mechanisms remains similar.

Rutheniumethynyl-Triarylamine Organic-Inorganic Mixed-Valence Systems: Regulating Ru-N Electronic Coupling by Different Aryl Bridge Cores

Four rutheniumethynyl-triarylamine complexes 1-4 with different aryl bridge cores were prepared. The solid structures of complexes 2-4 were fully confirmed by X-ray single-crystal diffraction analysis. Two consecutive one-electron oxidation processes of complexes 1-4 were attributed to the ruthenium and nitrogen centers, as revealed by cyclic voltammetry and square-wave voltammogram. Results also showed decreasing potential difference ΔE of complexes 1, 3, and 4, with the largest value for 2. Upon chemical oxidation of complexes 1-4 by 1.0 eq oxidation reagents FcPF₆ or AgSbF₆ , the mixed-valence complexes, except for 2⁺ , show characteristic broad NIR absorptions in the UV-vis-NIR spectroscopic experiments. NIR multiple absorptions were assigned to NAr₂ →RuCp*(dppe) intervalence charge transfer (IVCT) and metal-to-ligand charge transfer transitions by TDDFT calculations. Coupling parameter (H_ab ) from Hush theory revealed that increasing electronic communication in 1⁺ , 3⁺ , and 4⁺ . Electron density distribution of the HOMO for neutral molecules (1, 3, and 4) and spin density distribution of the corresponding single-oxidized states (1⁺ , 3⁺ , and 4⁺ ) increases progressively on the bridge as the size of the aromatic system increases, proving incremental contributions from bridge cores during oxidation.

Exploring Structure-Property Relationships in a Bio-Inspired Family of Bipodal and Electronically-Coupled Bistriphenylamine Dyes for Dye-Sensitized Solar Cell Applications

A bio-inspired family of organic dyes with bichromic-bipodal architectures were synthesized and tested in dye-sensitized solar cells (DSSC). These dyes are comprised of a D-π-D-A motif with two triphenylamine (TPA) units acting as donors (D) and two cyanoacetic acid acceptors (A) capable of binding to a titania semiconductor. The role of the thiophene π-spacer bridging the two TPA units was examined and the distal TPA (relative to TiO2) was modified with various substituents (-H, -OMe, -SMe, -OHex, -3-thienyl) and contrasted against benchmark L1. It was found that the two TPA donor units could be tuned independently, where π-spacers can tune the proximal TPA and R-substituents can tune the distal TPA. The highest performing DSSCs were those with -SMe, 3-thienyl, and -H substituents, and those with one spacer or no spacers. The donating abilities of R-substituents was important, but their interactions with the electrolyte was more significant in producing high performing DSSCs. The introduction of one π-spacer provided favourable electronic communication within the dye, but more than one was not advantageous.

Intramolecular Charge Transfer in Kekulé- and Non-Kekulé-Bridged Bis(triarylamine) Radical Cations: Missing Key Compounds in Organic Mixed-Valence Systems

From the viewpoint of para-meta topological bridging effect on the electronic coupling in organic mixed-valence (MV) systems, the optically induced and thermally assisted intramolecular charge/spin transfer (ICT/IST) processes have been investigated for three bis(triarylamine) (BTA) radical cations as missing key compounds in very basic BTA MV systems. In contrast to the case of p- and m-dinitrobenzene radical anions, the difference in the strength of electronic coupling (V) was not so large for the present BTA MV radical cations, although they still fall within the paradigm of strong V for para-linkage and weak V for meta-linkage. Unexpectedly, it has been found that meta-phenylenediamine radical cation has an electronic coupling comparable to those in the para-conjugated BTA-based MV species, and the ICT/IST rate exceeds the ESR time-scale. This finding is very encouraging considering that sufficient electronic communication can be ensured even when the redox-active centers are linked directly by the meta-phenylene bridge, thus broadening the selection of π-bridging units for molecule-based optoelectronics.

Efficient and stable perovskite solar cells based on perfluorinated polymers

Novel perfluorinated semiconductor compounds were introduced into the perovskite layer as additives and stable and efficient perovskite-based devices were achieved.

Mixed-Valent Molecular Triple Deckers

Two phenothiazine (PTZ) moieties were connected via naphthalene spacers to a central arene to result in stacked PTZ-arene-PTZ structure elements. Benzene and tetramethoxybenzene units served as central arenes mediating electronic communication between the two PTZ units. Based on cyclic voltammetry, UV/Vis-NIR absorption, EPR spectroscopy, and computational studies, the one-electron oxidized forms of the resulting compounds behave as class II organic mixed-valence species in which the unpaired electron is partially delocalized over both PTZ units. The barrier for intramolecular electron transfer depends on the nature of the central arene sandwiched between the two PTZ moieties. These are the first examples of rigid organic mixed-valent triple-decker compounds with possible electron-transfer pathways directly across a stacked structure, and they illustrate the potential of oligo-naphthalene building blocks for long-range electron transfer and a future molecular electronics technology.

Bis(aminoaryl) Carbon-Bridged Oligo(phenylenevinylene)s Expand the Limits of Electronic Couplings

Carbon-bridged bis(aminoaryl) oligo(para-phenylenevinylene)s have been prepared and their optical, electrochemical, and structural properties analyzed. Their radical cations are class III and class II mixed-valence systems, depending on the molecular size, and they show electronic couplings which are among the largest for the self-exchange reaction of purely organic molecules. In their dication states, the antiferromagnetic coupling is progressively tuned with size from quinoidal closed-shell to open-shell biradicals. The data prove that the electronic coupling in the radical cations and the singlet-triplet gap in the dications show similar small attenuation factors, thus allowing charge/spin transfer over rather large distances.

In Situ Spectroelectrochemical Investigations of RuII Complexes with Bispyrazolyl Methane Triarylamine Ligands

The synthesis and characterization of two triarylamine ligands, 4-(di(1H-pyrazol-1-yl)methyl)-N-(4-(di(1H-pyrazol-1-yl)methyl)phenyl)-N-phenylaniline (TPA-2bpm) and tris(4-(di(1H-pyrazol-1-yl)methyl)phenyl)amine (TPA-3bpm), containing the bispyrazolylmethane moiety and its RuII terpyridine complexes are presented. The redox properties of the ligands and RuII complexes are explored in detail through cyclic and square-wave voltammetry in addition to in situ UV-vis-near infrared, electron paramagnetic resonance, and fluorescence spectroelectrochemistry. It was demonstrated that the triarylamine radical cation was able to be generated, and further, TPA-2bpm underwent an electrochemically induced dimerization process.

Effect of multi-armed triphenylamine-based hole transporting materials for high performance perovskite solar cells

A series of hole-transporting materials (HTMs) based on [2,2]paracyclophane and triphenyl-amine (TPA) was synthesized. We studied the effect of the chemical structure of the HTM on the photovoltaic performance of perovskite solar cells by varying the number of TPA charge transporting components in the HTM. Tetra-TPA, in which four TPAs are incorporated into the [2,2]paracyclophane core, exhibited better hole transport properties than di-TPA and tri-TPA, which contain two and three TPAs, respectively. In particular, incorporation of the TPA group with a multi-armed structure effectively enhanced the conductivity of the HTM layer in the out-of-plane direction in the solar cell device. Due to the improved charge transport and appropriate molecular energy levels of tetra-TPA, the perovskite solar cell based on the tetra-TPA HTM achieved higher Jsc and FF values than the devices based on di-TPA and tri-TPA HTMs, with a high solar cell efficiency (17.9%).

Synthesis of Functionalized 1,4-Azaborinines by the Cyclization of Di-tert-butyliminoborane and Alkynes

Di-tert-butyliminoborane is found to be a very useful synthon for the synthesis of a variety of functionalized 1,4-azaborinines by the Rh-mediated cyclization of iminoboranes with alkynes. The reactions proceed via [2 + 2] cycloaddition of iminoboranes and alkynes in the presence of [RhCl(PiPr3)2]2, which gives a rhodium η(4)-1,2-azaborete complex that yields 1,4-azaborinines upon reaction with acetylene. This reaction is compatible with substrates containing more than one alkynyl unit, cleanly affording compounds containing multiple 1,4-azaborinines. The substitution of terminal alkynes for acetylene also led to 1,4-azaborinines, enabling ring substitution at a predetermined location. We report the first general synthesis of this new methodology, which provides highly regioselective access to valuable 1,4-azaborinines in moderate yields. A mechanistic rationale for this reaction is supported by DFT calculations, which show the observed regioselectivity to arise from steric effects in the B-C bond coupling en route to the rhodium η(4)-1,2-azaborete complex and the selective oxidative cleavage of the B-N bond of the 1,2-azaborete ligand in its subsequent reaction with acetylene.

Regioselective Catalytic and Stepwise Routes to Bulky, Functional-Group-Appended, and Luminescent 1,2-Azaborinines

The regioselective syntheses of 1,2-azaborinines is achieved using an unsymmetrical iminoborane through both catalytic and stepwise modular routes. The 1,2-azaborinine ring can be selectively functionalized in the 4- and/or 6-position through control of the stepwise reaction sequence, allowing access to vinyl-functionalized and redox-active, luminescent, donor-functionalized 1,2-azaborinines. The electrochemistry and photochemistry of a tetraarylamine-substituted 1,2-azaborinine are studied. Cyclic voltammetry of this compound, relative to a non-B,N-substituted reference molecule, showed an additional oxidation wave assigned to the oxidation of the azaborinine ring, while emission spectroscopy indicated that the azaborinine was significantly more fluorescent than the reference.

Redox-switchable intramolecular π-π-stacking of perylene bisimide dyes in a cyclophane

Molecular actuation by stepwise electrochemical reduction is demonstrated for a cyclophane that exhibits a pronounced conformational transition from a closed cavity with cofacially stacked PBIs in the neutral state to an expanded open cavity in the three- and fourfold reduced state.

A bis(terpyridine)ruthenium complex with three redox-active amine sites: electrochemical, optical, and computational studies

Two ruthenium complexes, [Ru(NN)(ttpy)](2+) and [Ru(NN)(daatpy)](2+), have been designed and prepared, where NN is bis(amine) ligand 4'-tolyl-5,5"-bis(di-p-anisylamino)-2,2':6',2"-terpyridine, ttpy is 4'-tolyl-2,2':6',2"-terpyridine, and daatpy is 4'-di-p-anisylamino-2,2':6',2"-terpyridine. Complex [Ru(NN)(daatpy)](2+) contains three redox-active amine groups and has been characterized by single-crystal X-ray analysis. These two complexes display much-enhanced light absorption capabilities with respect to the prototype compound [Ru(tpy)(2)](2+) (tpy = 2,2':6',2"-terpyridine), which has been rationalized on the basis of time-dependent density functional theory calculations. Electrochemical and optical studies showed that there was little electronic coupling between two amine sites in complex [Ru(NN)(ttpy)](2+). On the other hand, a ligand-to-ligand (N → N'(•+)) charge-transfer band has been observed at 1430 nm for singly and doubly oxidized forms of [Ru(NN)(daatpy)](2+), and an electronic coupling parameter of 1000 cm(-1) was derived using the Hush formula. This band is interpreted as a charge transfer from the neutral amine of the daatpy ligand to oxidized aminium units in the NN ligand.

Intervalence charge transfer in mixed valence compound modified by the formation of a supramolecular complex

The electrochemical and spectroelectrochemical properties of N,N-diphenyl-1,4-phenylenediamine (PDA) were investigated in the absence and in the presence of 18-crown-6-ether (18C6) or dibenzo 24-crown-8-ether (DB24C8), in a solution of tetrabutylammonium hexafluorophosphate (TBAPF6) in acetonitrile and in the presence of trifluoroacetic acid (TFA) only for 18C6. In neutral acetonitrile, PDA undergoes two reversible oxidation processes, which lead first to the formation of the cation-radical considered as mixed valence (MV) compound, and then to the dicationic species. When 18C6 is added in the medium and depending on 18C6 concentration, cyclic voltammetry shows a marked shift to more cathodic potentials of the current waves of the second redox process only. This is attributed to a strong interaction between the PDA(+2) dication and two 18C6 molecules, leading to the formation of a supramolecular complex with an association constant value K(a) = 7.0 × 10(7) M(-2). The interaction of 18C6 with PDA(+2) dication has a direct effect on the PDA(+.) cation-radical corresponding to a decrease in the lifetime of the MV compound and of the intramolecular electron transfer rate when 18C6 is present. Indeed, it results in a large decrease in the intervalence charge transfer (IV-CT) between the two amine centers in the MV compound (k(th) = 1.35 × 10(10) s(-1) in 18C6-free neutral solution containing 5.0 × 10(-4) M PDA, and k(th) = 3.6 × 10(9) s(-1) in the same medium at [18C6]/[PDA] = 20/1). And the comproportionation constant K(co) falls from 6.0 × 10(6) in 18C6-free solution to 1.6 × 10(3) at [18C6]/[PDA] = 20/1. In acidified acetonitrile and when TFA concentration is increased, PDA still shows the two successive and reversible oxidation processes, but both are shifted to more anodic potentials. However, when 18C6 is added, the two oxidation waves shift to more cathodic potentials, indicating an interaction of all protonated PDA redox states with 18C6, resulting in the formation of supramolecular complexes. In the presence of TFA, the value of K(co) is decreased to 4.3 × 10(4), but it remains unchanged when 18C6 is added, indicating no change in the lifetime of the MV compound. In this medium, IV-CT in the MV compound is greater with 18C6 (k(th) = 2.3 × 10(10) s(-1) for [18C6]/[PDA] = 20/1) than without (k(th) = 1.4 × 10(9) s(-1)), which indicates a more important IV-CT rate when 18C6 is present. The results show for the first time that is it possible to control the IV-CT rate, through the lifetime and the potential range where the MV compound is the most important. This control is not obtained as usual by chemical modification of the structure of the starting molecule, but by varying either the acidity or the 18C6 concentration as external stimuli, which lead to reversible formation/dissociation of a supramolecular complex species. Moreover, we also studied the electrochemical properties of PDA in the presence of wider crown ether such as DB24C8. We showed that PDA undergoes the same electrochemical behavior with DB24C8 than with 18C6 in neutral organic medium (K(a) = 2.9 × 10(3) M(-1)). This result suggests that the complexation between the electrogenerated PDA(+2) dication and the crown ethers may occur through face-to-face mode rather than rotaxane mode even with DB24C8 which is supposed to form inclusion complexes.