Organic Donor-acceptor Complexes Research Articles

Organic Donor-Acceptor Complexes As Potential Semiconducting Materials.

In this review article, the synthesis, characterization and physico-chemical properties of the organic donor-acceptor complexes are highlighted and a special emphasis has been placed on developing them as semiconducting materials. The electron-rich molecules, i. e., donors have been broadly grouped in three categories, namely polycyclic aromatic hydrocarbons, nitrogen heterocycles and sulphur containing aromatic donors. The reactions of these classes of the donors with the acceptors, namely tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE), tetracyanobenzene (TCNB), benzoquinone, pyromellitic dianhydride and pyromellitic diimides, fullerenes, phenazine, benzothiadiazole, naphthalimide, DMAD, maleic anhydride, viologens and naphthalene diimide are described. The potential applications of the resulting DA complexes for physico-electronic purposes are also included. The theoretical investigation of many of these products with a view to rationalise their observed physico-chemical properties is also discussed.

Energy Level, Crystal Morphology and Fluorescence Emission Tuning in Cocrystals via Molecular-Level Engineering.

Organic donor-acceptor complexes as new organic semiconductor class have attracted wide attention, due to their potential applications in functional optoelectronics. Herein, we present two new charge transfer cocrystals of di-cyanodiazafluorene -perylene (DCPE) and di-cyanodiazaflfluorene-pyrene (DCPY) through a rational cocrystal-engineering strategy. Although they are both 1 : 1 mixed stacking cocrystals with similar chemical structures, the DCPE cocrystal possesses a non-centrosymmetric space group and narrower band gap compared to DCPY cocrystal, because of the non-covalent bonding variation. The electrostatic potential accumulated in the lateral facets leads to highly twisted DCPE nanobelts, and the small band gap causes near infrared fluorescence. Meanwhile, the DCPY crystals with centrosymmetric space groups and weaker intermolecular interactions exhibited an untwisted morphology and red emission. This study will be helpful for the design and understanding of functional cocrystal materials that can be used in flexible micro/nano-mechanics, mechanical energy, and optical devices.

A molecular roadmap towards organic donor-acceptor complexes with high-performance thermoelectric response

As a unique class of molecular electronic materials, organic donor–acceptor complexes now exhibit tantalizing prospect for heat–electricity interconversion. Over the past decades, in design of these materials for thermoelectric applications, consistent efforts have been made to synthesize a wide variety of structures and to characterize their properties. However, hitherto, one of the paramount conundrums, namely lack of systematic molecular design principles, has not been addressed yet. Here, based on ab initio calculations, and by comprehensively examining the underlying correlation among thermoelectric power factors, non-intuitive transport processes, and fundamental chemical structures for 13 prototypical organic donor–acceptor complexes, we establish a unified roadmap for rational development of these materials with increased thermoelectric response. We corroborate that the energy levels of frontier molecular orbitals in the isolated donor and acceptor molecules control the charge transfer, electronic property, charge transport, and thermoelectric performance in the solid-state complexes. Our results demonstrate that tailoring a suitable energy-level difference between donor’s highest occupied molecular orbital and acceptor’s lowest unoccupied molecular orbital holds the key to achieving an outstanding power factor. Moreover, we reveal that the charge-transfer-caused Coulomb scattering governs the charge and thermoelectric transport in organic donor–acceptor complexes.

Supramolecular Design of Donor-Acceptor Complexes via Heteroatom Replacement toward Structure and Electrical Transporting Property Tailoring.

A feasible strategy relies on using heteroatom replacement, namely, chemical modification of an organic compound. Here we present this design concept for donor-acceptor complexes, which involves introducing nitrogen atoms to the middle ring of donor molecules to promote short contacts and reduce steric effect of the mixed framework. These nitrogen-modified complexes can possess a shorter molecular distance besides the mixed-stacking pathway, enlarged π-π interactions, or even a scarce 1:2.5 molar ratio through extra acceptor insertion. As a result, the unique 1:2 complex with nitrogen atoms on the different sides demonstrated stable electron field-effect mobility performance, whereas the binary system with no nitrogen replacement or N atoms on the identical sides displayed poor ambipolar properties. These results confirmed that heteroatom replacement was a powerful molecular design tool to fine-tune the molecular packing of organic donor-acceptor complexes and their corresponding electronic properties.

Recent progress on organic donor–acceptor complexes as active elements in organic field-effect transistors

The recent progress on the methods to prepare donor (D)–acceptor (A) co-crystals and their potential applications in OFETs has been reviewed.

Organic Donor-Acceptor Complexes as Novel Organic Semiconductors.

Organic donor-acceptor (DA) complexes have attracted wide attention in recent decades, resulting in the rapid development of organic binary system electronics. The design and synthesis of organic DA complexes with a variety of component structures have mainly focused on metallicity (or even superconductivity), emission, or ferroelectricity studies. Further efforts have been made in high-performance electronic investigations. The chemical versatility of organic semiconductors provides DA complexes with a great number of possibilities for semiconducting applications. Organic DA complexes extend the semiconductor family and promote charge separation and transport in organic field-effect transistors (OFETs) and organic photovoltaics (OPVs). In OFETs, the organic complex serves as an active layer across extraordinary charge pathways, ensuring the efficient transport of induced charges. Although an increasing number of organic semiconductors have been reported to exhibit good p- or n-type properties (mobilities higher than 1 or even 10 cm2 V-1 s-1), critical scientific challenges remain in utilizing the advantages of existing semiconductor materials for more and wider applications while maintaining less complicated synthetic or device fabrication processes. DA complex materials have revealed new insight: their unique molecular packing and structure-property relationships. The combination of donors and acceptors could offer practical advantages compared with their unimolecular materials. First, growing crystals of DA complexes with densely packed structures will reduce impurities and traps from the self-assembly process. Second, complexes based on the original structural components could form superior mixture stacking, which can facilitate charge transport depending on the driving force in the coassembly process. Third, the effective use of organic semiconductors can lead to tunable band structures, allowing the operation mode (p- or n-type) of the transistor to be systematically controlled by changing the components. Finally, theoretical calculations based on cocrystals with unique stacking could widen our understanding of structure-property relationships and in turn help us design high-performance semiconductors based on DA complexes. In this Account, we focus on discussing organic DA complexes as a new class of semiconducting materials, including their design, growth methods, packing modes, charge-transport properties, and structure-property relationships. We have also fabricated and investigated devices based on these binary crystals. This interdisciplinary work combines techniques from the fields of self-assembly, crystallography, condensed-matter physics, and theoretical chemistry. Researchers have designed new complex systems, including donor and acceptor compounds that self-assemble in feasible ways into highly ordered cocrystals. We demonstrate that using this crystallization method can easily realize ambipolar or unipolar transport. To further improve device performance, we propose several design strategies, such as using new kinds of donors and acceptors, modulating the energy alignment of the donor (ionization potential, IP) and acceptor (electron affinity, EA) components, and extending the π-conjugated backbones. In addition, we have found that when we use molecular "doping" (2:1 cocrystallization), the charge-transport nature of organic semiconductors can be switched from hole-transport-dominated to electron-transport-dominated. We expect that the formation of cocrystals through the complexation of organic donor and acceptor species will serve as a new strategy to develop semiconductors for organic electronics with superior performances over their corresponding individual components.

Description of electron transfer in the ground and excited states of organic donor-acceptor systems by single-reference and multi-reference density functional methods.

Electron transfer in the ground and excited states of a model donor-acceptor (D-A) system is investigated using the single-reference and multi-reference density functional theory (DFT) methods. To analyze the results of the calculations, a simple two-site multi-reference model was derived that predicts a stepwise electron transfer in the S0 state and a wave-like dependence of the S1 electron transfer on the external stimulus. The standard single-reference Kohn-Sham (KS) DFT approach and the time-dependent DFT (TDDFT) method failed to describe the correct dependence of the S0 and S1 electron transfer on the external electric field applied along the donor-acceptor system. The multi-reference DFT approach, the spin-restricted ensemble-referenced KS (REKS) method, was able to successfully reproduce the correct behavior of the S0 and S1 electron transfer on the applied field. The REKS method was benchmarked against experimentally measured gas phase charge transfer excitations in a series of organic donor-acceptor complexes and displayed its ability to describe this type of electronic transitions with a very high accuracy, mean absolute error of 0.05 eV with the use of the standard range separated density functionals. On the basis of the calculations undertaken in this work, it is suggested that the non-adiabatic coupling between the S0 and S1 states may interfere with the electron transfer in a weakly coupled donor-acceptor system. It is also suggested that the electronic excitation of a D(+)-A(-) system may play a dual role by assisting the further electron transfer at certain magnitudes of the applied electric field and causing the backward transfer at lower electric field strengths.

Co-grafting of porphyrins and fullerenes on ZnO nanorods: Towards supramolecular donor–acceptor assembly

This work presents the synthesis and physico-chemical characterization of a novel artificial photosynthetic design, using anisotropic semiconducting nanorods as scaffolds to assemble organic donor–acceptor complexes on their surface. These hierarchical hybrid D–A assemblies were obtained by the co-grafting of porphyrins and fullerenes on the ZnO nanorods. Polarity of the solvent and porphyrin to fullerene ratios were investigated to be markedly influencing the donor–acceptor interaction under the co-grafted conditions on ZnO nanorods. Fourier transform infrared spectroscopy, cyclic voltammetry, electronic absorption and fluorescence spectroscopic techniques were used to characterize the formation and investigate the optoelectronic properties of porphyrin–fullerene complexes on the surface of ZnO. To the best of our knowledge, this is the first example of highly interacting porphyrin–fullerene complexes on ZnO nanorods, which may allow generating efficient nanosystems for artificial photosynthesis and harvesting of solar energy.

Nanoindentation as a Probe for Mechanically‐Induced Molecular Migration in Layered Organic Donor–Acceptor Complexes

Nanoindentation and scratch experiments on 1:1 donor-acceptor complexes, 1 and 2, of 1,2,4,5-tetracyanobenzene with pyrene and phenanthrene, respectively, reveal long-range molecular layer gliding and large interaction anisotropy. Due to the layered arrangements in these crystals, these experiments that apply stress in particular directions result in the breaking of interlayer interactions, thus allowing molecular sheets to glide over one another with ease. Complex 1 has a layered crystal packing wherein the layers are 68° skew under the (002) face and the interlayer space is stabilized by van der Waals interactions. Upon indenting this surface with a Berkovich tip, pile-up of material was observed on just one side of the indenter due to the close angular alignment of the layers with the half angle of the indenter tip (65.35°). The interfacial differences in the elastic modulus (21%) and hardness (16%) demonstrate the anisotropic nature of crystal packing. In 2, the molecular stacks are arranged in a staggered manner; there is no layer arrangement, and the interlayer stabilization involves C-H···N hydrogen bonds and π···π interactions. This results in a higher modulus (20%) for (020) as compared to (001), although the anisotropy in hardness is minimal (4%). The anisotropy within a face was analyzed using AFM image scans and the coefficient of friction of four orthogonal nanoscratches on the cleavage planes of 1 and 2. A higher friction coefficient was obtained for 2 as compared to 1 even in the cleavage direction due to the presence of hydrogen bonds in the interlayer region making the tip movement more hindered.

Unusual supramolecular donor—acceptor complexes of bis(crown)stilbenes and bis(crown)azobenzene with viologen analogs

The results of the comprehensive study of novel supramolecular donor-acceptor complexes of bis(crown)stilbenes and bis(crown)azobenzene with viologen analogs are generalized. The original methodology for self-assembling of the organic donor-acceptor complexes possessing a very high thermodynamic stability is described. The hydrogen bonds between the peripheral fragments of the donor and acceptor play the key role in the self-assembling. The influence of different structural factors on the thermodynamic stability of the supramolecular donor-acceptor complexes and the efficiency of charge-transfer interactions between the donor and acceptor are discussed. The driving forces of the reaction leading to exotic trimolecular charge-transfer complexes are considered.

Glassy Protein Dynamics and Gigantic Solvent Reorganization Energy of Plastocyanin

We report the results of molecular dynamics simulations of electron-transfer activation parameters of plastocyanin metalloprotein involved as an electron carrier in natural photosynthesis. We have discovered that slow, non-ergodic conformational fluctuations of the protein, coupled to hydrating water, result in a very broad distribution of donor-acceptor energy gaps far exceeding those observed for commonly studied inorganic and organic donor-acceptor complexes. The Stokes shift is not affected by these fluctuations and can be calculated from solvation models in terms of the linear response of the solvent dipolar polarization. The non-ergodic character of large-amplitude protein/water mobility breaks the strong link between the Stokes shift and the reorganization energy characteristic of equilibrium (ergodic) theories of electron transfer. This mechanism might be responsible for fast electronic transitions in natural electron-transfer proteins characterized by low reaction free energy.

Equilibrium charge distribution in organic donor-acceptor complexes: the role of coupling to intramolecular vibrations

The donor-acceptor molecular dimer is used for modeling the neutral-ionic transition. Charge transfer interaction and electron coupling to intramolecular vibrations are taken into account explicitly. A way of calculating the adiabatic potential as a function of charge transfer between donor and acceptor molecules is proposed. The relative significance of transfer integral and small-polaron binding energies of donor and acceptor molecules is discussed with respect to the feasibility of a nano-scale neutral-ionic transition.

Magnetic gap and partial charge transfer in mixed donor—acceptor stacks

The ground state charge transfer γ( z) and magnetic gap Δ E m( z) of organic donor—acceptor complexes with one dimensional⋯D +γA −γD +γA +γ⋯ stacks are computed for arbitrary ratios of z = δ/2 1 2 | t|where | t| is the charge-transfer integral and −2δ is the excitation energy for a D +A − electron—hole pair. Valence bond techniques give exact results for rings of N = 4, 6, 8, and 10 sites, which suffice to show that the neutral—ionic transition and Δ E m( z) → 0 occur at z c = 0.53 ± 0.01 with γ( z c) = 0.68 ± 0.01.