Organic Donor-acceptor Research Articles

Covalent Radical Pairs as Spin Qubits: Influence of Rapid Electron Motion between Two Equivalent Sites on Spin Coherence.

Ultrafast photodriven electron transfer reactions starting from an excited singlet state in an organic donor-acceptor molecule generate a radical pair (RP) in which the two spins are initially entangled and, in principle, can serve as coupled spin qubits in quantum information science (QIS) applications, provided that spin coherence lifetimes in these RPs are long. Here we investigate the effects of electron transfer between two equivalent sites comprising the reduced acceptor of the RP. A covalent electron donor-acceptor molecule (D-C-A24+) including a p-methoxyaniline donor (D), a 4-aminonaphthalene-1,8-imide chromophoric primary acceptor (C), and a m-xylene bridged cyclophane having two equivalent phenyl-extended viologens (A24+) as a secondary acceptor was synthesized along with the analogous molecule having one phenyl-extended viologen acceptor and a second, more difficult to reduce 2,5-dimethoxyphenyl-extended viologen in a very similar cyclophane structure (D-C-A4+). Photoexcitation of C within each molecule results in subnanosecond formation of D+•-C-A23+• and D+•-C-A3+•. The spin dynamics of these RPs were characterized by time-resolved EPR spectroscopy and magnetic field effects on the RP yield in both CH3CN and CD3CN. The data show that rapid electron hopping within A23+• promotes spin decoherence in D+•-C-A23+• relative to D+•-C-A3+• having a monomeric acceptor, while the interaction of the RP electron spins with the nuclear spins of the solvent have little or no effect on the spin dynamics. These observations provide important information for designing and understanding novel molecular assemblies of spin qubits with long coherence times for QIS applications.

Aggregation-Induced Emission with Long-Lived Room-Temperature Phosphorescence from Methylene-Linked Organic Donor-Acceptor Structures.

Aggregation-caused quenching (ACQ), where excited-state and/or ground-state electronic structures are altered to exhibit an increased proclivity for non-radiative decay for the aggregates, is largely responsible for the lack of fluorescence and phosphorescence in molecular solids in general. Here we show that ACQ could be effectively circumvented by constructing an aromatic system with a methylene-linker, where the system exhibits typical aggregation-induced emission (AIE) with long-lived room-temperature phosphorescence, since the tetrahedral structure in the solid state may significantly reduce strong intermolecular interactions contributing to ACQ.

Stack the Bowls: Tailoring the Electronic Structure of Corannulene‐Integrated Crystalline Materials

AbstractWe report the first examples of purely organic donor–acceptor materials with integrated π‐bowls (πBs) that combine not only crystallinity and high surface areas but also exhibit tunable electronic properties, resulting in a four‐orders‐of‐magnitude conductivity enhancement in comparison with the parent framework. In addition to the first report of alkyne–azide cycloaddition utilized for corannulene immobilization in the solid state, we also probed the charge transfer rate within the Marcus theory as a function of mutual πB orientation for the first time, as well as shed light on the density of states near the Fermi edge. These studies could foreshadow new avenues for πB utilization for the development of optoelectronic devices or a route for highly efficient porous electrodes.

Overcoming the Complex Excited-State Dynamics of Colloidal Cadmium Selenide Nanocrystals Involved in Energy Transfer Processes

Semiconductor nanocrystals are often characterized by complex excited-state dynamics which reflect the inhomogeneous character of ensemble of nanocrystals. A new hybrid inorganic–organic donor–acceptor system involving CdSe nanocrystals and paramagnetic nitronyl nitroxide free radicals is shown to lead to efficient Forster (dipolar) resonance energy transfer. This transfer process, which is monitored by steady-state and time-dependent photoluminescence quenching experiments, occurs on a time scale similar to that of the intrinsic recombination in CdSe nanocrystals, allowing to unravel some of the complexity associated with the excited-state photophysics of semiconductor nanocrystals. A Stern–Volmer formalism that can handle the multiexponential nature of the time-dependent excited-state kinetics of CdSe nanocrystals is developed, leading to excellent agreement between steady-state and time-dependent photoluminescence data when a log-normal distribution model is used for the intrinsinc recombination rate c...

Unconventional Synthesis of a CuI Rotaxane with a Superacceptor Stopper: Ultrafast Excited‐State Dynamics and Near‐Infrared Luminescence

A CuI bis-phenanthroline rotaxane was prepared by using the [2+2] cycloaddition-retroelectrocyclization (CA-RE) reaction to graft a bulky dicyanoquinodimethane (DCNQ) stopper. The electronic properties were investigated with electrochemical and photophysical techniques, in parallel with three reference compounds, namely, the DCNQ derivative alone, the DCNQ-based phenanthroline ligand, and an analogue CuI complex lacking the DCNQ moiety. In all the systems containing the DCNQ unit, the lowest electronic excited states are centered thereon, with the singlet level (S1 ) located at about 1.0 eV, as suggested by TDDFT calculations. Accordingly, in the DCNQ-equipped rotaxane, the typical metal-to-ligand charge-transfer luminescence of the CuI center is totally quenched. Ultrafast transient absorption and emission studies show that, in the rotaxane, the final sink of photoinduced processes is the lowest singlet state of the DCNQ moiety (S1 ), which exhibits strong charge-transfer character and a lifetime of 0.4 ps. Its deactivation leads to population of another excited state with a lifetime of 1.3 ps, which can be the related triplet state (T1 ) or a vibrationally hot level (hot-S0 ). Notably, S1 also shows stimulated fluorescence in the near-infrared (NIR) region between 1100 and 1500 nm, corroborating the TDDFT prediction. This unusual finding opens up the study of ultrashort-lived NIR luminescence in organic donor-acceptor systems.

Guanidines as Reagents in Proton-Coupled Electron-Transfer Reactions and Redox Catalysts

Redox-active guanidines are ideal proton-coupled electron-transfer (PCET) reagents, since they combine a high Brønsted basicity with a low and tunable redox potential. In this article, the development of redox-active guanidines (especially guanidino-functionalized aromatics, GFAs) in the last ten years is summarized, and their properties compared to other organic Brønsted bases and organic electron donors. First, some applications in organic chemistry that purely use the redox activity (formation of organic donor–acceptor materials and photochemical reductive C–C coupling reactions) are presented. Then, reactions that involve both proton and electron transfer are reviewed. In stoichiometric reactions, redox-active guanidines are used for the dehydrogenative coupling of thiols and phosphanes. The first redox catalytic applications are discussed, using dioxygen as green oxidizing reagent.1 Introduction2 Redox-Active Amines and Guanidines3 Brønsted Basicity of Amines and Guanidines4 Variations of GFA Compounds5 GFA Compounds in Organic Donor–Acceptor Materials and as Reducing Reagents in Organic Synthesis6 Stoichiometric Dehydrogenative Coupling Reactions with Redox-Active Guanidines7 Guanidines as Redox Catalysts8 Conclusions and Outlook

Variable charge transfer state energies at nanostructured pentacene/C60 interfaces

While it has recently been recognized that organic donor-acceptor charge transfer (CT) state energies can vary substantially under different interfacial morphologies, this behavior is under-appreciated in the context of organic singlet fission solar cells where a specific alignment between the triplet state of the fission material and the CT state of the donor-acceptor interface is necessary to the function of the device. In this work, we demonstrate that the CT state energy of a prototypical pentacene-C60 singlet fission system is around 1 eV in most systems, but can vary over 300 meV depending on the composition and morphology of the interface. Moreover, we show that the inclusion of a poly(3-hexylthiophene-2,5-diyl) underlayer, which commonly serves as a triplet blocker and hole collector in pentacene/C60 solar cells, helps promote active layer morphologies with stabilized, low energy CT states. These trends in the interfacial energetics are correlated with structural characterization of the films by atomic force microscopy and x-ray diffraction.

Charge Transfer and Collection in Dilute Organic Donor-Acceptor Heterojunction Blends.

Experimental and theoretical approaches are used to understand the role of nanomorphology on exciton dissociation and charge collection at dilute donor-acceptor (D-A) organic heterojunctions (HJs). Specifically, two charge transfer (CT) states in D-A mixed HJs comprising nanocrystalline domains of tetraphenyldibenzoperiflanthene (DBP) as the donor and C70 as the acceptor are unambiguously related to the nanomorphology of the mixed layer. Alternating DBP:C70 multilayer stacks are used to identify and control the optical properties of the CT states, as well as to simulate the dilute mixed heterojunctions. A kinetic Monte Carlo model along with photoluminescence spectroscopy and scanning transmission electron microscopy are used to quantitatively evaluate the layer morphology under various growth conditions. As a result, we are able to understand the counterintuitive observation of high charge extraction efficiency and device performance of DBP:C70 mixed layer photovoltaics at surprisingly low (∼10%) donor concentrations.

Role of the Open-Shell Character on the Pressure-Induced Conductivity of an Organic Donor-Acceptor Radical Dyad.

Single-component conductors based on neutral organic radicals have received a lot of attention due to the possibility that the unpaired electron can serve as a charge carrier without the need of a previous doping process. Although most of these systems are based on delocalized planar radicals, we present here a nonplanar and spin localized radical based on a tetrathiafulvalene (TTF) moiety, linked to a perchlorotriphenylmethyl (PTM) radical by a conjugated bridge, which exhibits a semiconducting behavior upon application of high pressure. The synthesis, electronic properties, and crystal structure of this neutral radical TTF-Ph-PTM derivative (1) are reported and implications of its crystalline structure on its electrical properties are discussed. On the other hand, the non-radical derivative (2), which is isostructural with the radical 1, shows an insulating behavior at all measured pressures. The different electronic structures of these two isostructural systems have a direct influence on the conducting properties, as demonstrated by band structure DFT calculations.

Simple solution processable carbazole-oxadiazole hybrids for un-doped deep-blue OLEDs

Development of new efficient luminescent materials is one of the active research areas of interest due to its promising optoelectronic applications including in organic light emitting devices (OLEDs). Among the various luminescent systems, emitters of deep blue electroluminescence with excellent color purity have great deal of importance as these are one of the important components for fabricating white OLEDs. In this context, we have designed and synthesized novel solution processable organic donor-acceptor dyads 1 and 2 having electron donating carbazole and accepting oxadiazole groups. The substitution pattern at the phenylene linker has been changed to understand the intramolecular electron communication between the donor and acceptor moieties of these dyads. The photophysical properties of these dyads were studied in both solution and film state. The emission maxima of the dyads red shifted with the increase in solvent polarity, which could be attributed to the intramolecular charge transfer (ICT) process. In the film state, dyads 1 and 2 showed deep blue emission having λmax at 415 nm and 409 nm, respectively. To understand the structure and band gap of these dyads, density functional theory (DFT) calculations have been performed. In line with the design strategy adopted, high thermal stability and formation of solution processed homogeneous film were obtained. Furthermore, we have fabricated the solution processed un-doped devices based on these dyads 1 and 2, which exhibited deep blue electroluminescence in near ultraviolet (NUV) region having λmax at 410 and 408 nm with a narrow full width at half maximum (FWHM) of 54 and 49 nm and CIE coordinates of (0.16, 0.07) and (0.16, 0.05), respectively, indicating thereby their potential in possible optoelectronic applications.

Graphene Field-Effect Transistor as a High-Throughput Platform to Probe Charge Separation at Donor-Acceptor Interfaces.

In organic and low-dimensional materials, electrons and holes are bound together to form excitons. Effective exciton dissociation at interfaces is essential for applications such as photovoltaics and photosensing. Here, we present an interface-sensitive, time-resolved method that utilizes graphene field effect transistor as an electric-field sensor to measure the charge separation dynamics and yield at donor-acceptor interfaces. Compared to other interface-sensitive spectroscopy techniques, our method has a much reduced measurement time and can be easily adapted to different material interfaces. Hence, it can be used as a high throughput screening tool to evaluate the charge separation efficiency in a large number of systems. By using zinc phthalocyanine/fullerene interface, we demonstrate how this method can be used to quantify the charge separation dynamics and yield at a typical organic donor-acceptor interface.

Rational Design of Dithienopicenocarbazole-Based Dyes and a Prediction of Their Energy-Conversion Efficiency Characteristics for Dye-Sensitized Solar Cells

A series of metal-free organic donor–acceptor (D–A) derivatives (ME01–ME06) of the known dye C281 were designed using first-principles calculations in order to evaluate their potential for applications in dye-sensitized solar cells (DSSCs). Their physical and electronic properties were calculated using density functional theory (DFT) and time-dependent density functional theory (TD-DFT). These include molecular properties that are required to assess the feasibility of a dye to function in DSSCs: UV–vis absorption spectra, light-harvesting efficiency (LHE), and driving forces of electron injection (ΔGinj). ME01, ME02, and ME04 are predicted to exhibit broad absorption optical spectra that cover the entire visible range, rendering these three dyes promising DSSC prospects. Device-relevant calculations on these three short-listed dyes and the parent dye C281 were then performed, whereupon the dye molecules were adsorbed onto anatase TiO2 surfaces to form the DSSC working electrode. Associated DSSC device cha...

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.

The crucial role of a spacer material on the efficiency of charge transfer processes in organic donor-acceptor junction solar cells.

Organic photovoltaic donor-acceptor junction devices composed of π-conjugated polymer electron donors (D) and fullerene electron acceptors (A) show greatly increased performance when a spacer material is inserted between the two layers (W. Y. Nie, G. Gupta, B. K. Crone, F. L. Liu, D. L. Smith, P. P. Ruden, C. Y. Kuo, H. Tsai, H. L. Wang, H. Li, S. Tretiak and A. D. Mohite, Adv. Sci., 2015, 2, 1500024.). For instance, experimental results reveal significant improvement of photocurrent when a terthiophene oligomer derivative is inserted in between π-conjugated poly(3-hexylthiophene-2,5-diyl) (P3HT) donor and C60 acceptor. These results indicate favorable charge separation dynamics, which is addressed by our present joint theoretical/experimental study establishing the beneficial alignment of electronic levels due to the specific morphology of the material. Namely, based on the experimental data we have constructed extended structural interface models containing C60 fullerenes and P3HT separated by aligned oligomer chains. Our time-dependent density functional theory (TD-DFT) calculations based on a long-range corrected functional, allowed us to address the energetics of essential electronic states and analyze them in terms of charge transfer (CT) character. Specifically, the simulations reveal the electronic spectra composed of a ladder of excited states evolving excitation toward spatial charge separation: an initial excitonic excitation at P3HT decomposes into charges by sequentially relaxing through bands of C60-centric, oligomer → C60 and P3HT → C60 CT states. Our modeling exposes a critical role of dielectric environment effects and electronic couplings in the self-assembled spacer oligomer layer on the energetics of critical CT states leading to a reduced back-electron transfer, preventing recombination losses, and thus rationalizes physical processes underpinning experimental observations.

TTF-TCNQ Solid Contact Layer in All-Solid-State Ion-Selective Electrodes for Potassium or Nitrate Determination

Nowadays, organic donor-acceptor molecules have attracted a lot of scientific attention because of their unique properties and potential applications in various fields. Although tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) is well known charge transfer salt for a long time, the first application of this material as ion-to-electron transducer in ion-selective electrodes is presented. As a proof of concept, potassium-selective and nitrate-selective solid-state electrodes were constructed. Moreover, TTF-TCNQ intermediate layers were fabricated using two organic solvents. Developed potassium sensors displayed a good Nernstian response with a slope of 58.52 mV/decade (10−6–10−1 M K+), whereas the nitrate-selective electrodes showed a sensitivity of −58.47 mV/decade (10−5–10−1 M NO-3). Reproducible standard potentials and low detection limits were observed. Potential stability of studied electrodes was evaluated using current-reversal chronopotentiometry. The capacitance of TTF-TCNQ-contacted electrodes is 255 μF and 629 μF for K+ and NO−3 sensors. The results of measurements conducted reveal that TTF-TCNQ has an appropriate effect on potentiometric sensors performance.

(Oligo-)Thiophene Functionalized Tetraazaperopyrenes: Donor-Acceptor Dyes and Ambipolar Organic Semiconductors.

Tetraazaperopyrenes (TAPPs) have been functionalized with thiophene and terthiophene units of different architecture resulting in a variety of organic donor-acceptor (D-A) compounds. The influence of the connection of the thiophenes to the TAPP core on their structural, photophysical and electrochemical properties has been studied in detail by a combination of X-ray crystallography, UV-vis and fluorescence spectroscopy as well as cyclic voltammetry, which allowed the establishment of structure-property relationships. The HOMO-LUMO gap is significantly decreased upon substitution of the TAPP core with electron-donating thiophene units, the extent of which is strongly influenced by the orientation of the thiophene units. The latter also crucially directs the molecular packing in the solid. Linkage at the α-position allows both inter- and intramolecular N···S interaction, whereas linkage in the β-position prevents intramolecular N···S interaction, resulting in a less pronounced conjugation of the TAPP core and the thiophene units. The new TAPP derivatives were processed as semiconductors in organic thin-film transistors (TFTs) that show ambipolar behavior. The insight into band gap and structure engineering may open up new possibilities to tailor the electronic properties of TAPP-based materials for certain desired applications.

Nonlinear optical properties of the novel kind of organic donor-acceptor thiophene derivatives with click chemistry modification

Two new classes of organic third-order optical chromophores, D-π-A-D and D-π-A-D-A-π–D, were designed and synthesized using a Sonogashira coupling reaction and the high-yielding [2 + 2] click reaction. The photophysical and click chemistry were characterized by means of UV/Vis absorption spectroscopy. The electrochemical properties were characterized by means of cyclic voltammetry. Moreover, the third-order nonlinear properties, including the nonlinear refraction and the nonlinear absorption, were investigated by using Z-scan techniques. The chromophores with a D-π-A-D-A-π-D motif, Bt1-X and Bt2-X, exhibited the larger third-order susceptibility χ(3) value than the analogues (D-π-A-D), St1-X and St2-X, whereas the bithiophene chromophores, St2 and Bt2, exhibited a larger χ(3) value than their single thiophene counterparts, St1 and Bt1. Upon the introduction of electron acceptors, third-order nonlinear absorption of the products Stn-X showed a typical SA-RSA transition. Interestingly, the chromophores with a D-π-A-D-A-π-D motif, Btn-X, exhibited more diversified NLO phenomenon. All of the click compounds showed the considerable third-order susceptibility χ(3) values. The results of these studies could provide new molecular-design strategies for third-order nonlinear optical materials for optoelectronic device components.

Prediction of the lowest charge-transfer excited-state energy at the donor-acceptor interface in a condensed phase using ground-state DFT calculations with generalized Kohn-Sham functionals.

Knowledge of the charge (electron) transfer process at the donor-acceptor interface is required to understand the working mechanisms of different organic photovoltaic materials. Investigating the lowest charge-transfer state in organic donor-acceptor solar cells is important as it allows the destruction/formation of excitons and polarons to be studied, and is directly related to the open circuit voltage. By performing low-cost and feasible calculations of ground-state electronic structures using the Mulliken rule as well as the optimally tuned range-separated hybrid (OTRSH) density functional and a regular long-range corrected functional, the lowest charge-transfer (CT) state energies of a series of dimers containing functionalized anthracene (the donor) and tetracyanoethylene (the acceptor) were obtained. The jumping distances of excited electrons during CT were calculated. The polarizable continuum model was applied to account for the effects of the solvent methylene chloride (CH2Cl2) on the lowest CT state energies obtained from gas-phase calculations. The calculated lowest CT state energies of the dimers were close to the corresponding experimental results, with a root mean square deviation (RMSD) of 0.22eV.

Naphthalene Diimide-Based n-Type Polymers: Efficient Rear Interlayers for High-Performance Silicon-Organic Heterojunction Solar Cells.

Silicon-organic heterojunction solar cells suffer from a noticeable weakness of inefficient rear contact. To improve this rear contact quality, here, two solution-processed organic n-type donor-acceptor naphthalene diimide (NDI)-based conjugated polymers of N2200 and fluorinated analogue F-N2200 are explored to reduce the contact resistance as well as to passivate the Si surface. Both N2200 and F-N2200 exhibit high electron mobility due to their planar structure and strong intermolecular stacking, thus allowing them to act as excellent transporting layers. Preferential orientation of the polymers leads to reduce contact resistance between Si and cathode aluminum, which can enhance electron extraction. More importantly, the substitution of fluorine atoms for hydrogen atoms within the conjugated polymer can strengthen the intermolecular stacking and improve the polymer-Si electronic contact due to the existence of F···H interactions. The power conversion efficiencies of Si-PEDOT:PSS solar cells increased from 12.6 to 14.5% as a consequence of incorporating the F-N2200 polymer interlayers. Subsequently, in-depth density functional theory simulations confirm that the polymer orientation plays a critical role on the polymer-Si contact quality. The success of NDI-based polymers indicates that planar conjugated polymer with a preferred orientation could be useful in developing high-performance solution-processed Si-organic heterojunction photovoltaic devices.

A Perylene-Based Polycyclic Aromatic Hydrocarbon Electron Donor for a Highly Efficient Solar Cell Dye.

The continuing efforts to develop novel polycyclic aromatic hydrocarbons and exploit them as building blocks to create organic donor-acceptor (D-A) dyes with impressive excited-state features should offer an excellent means by which to improve the power conversion efficiency (PCE) of dye-sensitized solar cells (DSSCs). To this end, a nonacyclic aromatic hydrocarbon, N-annulated benzoindenopentaphene (NBIP) was tethered with multiple solubilizing groups, including NBIPs with one 2-hexyldecyl, with one 2-hexyldecyloxy, and with four 4-hexylphenyl substituents. The side- and end-chain-functionalized NBIPs can be conveniently prepared in excellent yield, and further cross-coupled with 4-(7-ethynylbenzo[c][1,2,5]thiadiazol-4-yl)benzoic acid to afford a metal-free D-A dye, which achieves a high power conversion efficiency of 12.6 % under AM1.5G illumination in DSSCs without need for any coadsorbent.