Donor-acceptor Architecture Research Articles

Dissociation of charge-transfer states at donor–acceptor interfaces of organic heterojunctions

The dissociation of charge-transfer (CT) states into free charge carriers at donor–acceptor (DA) interfaces is an important step in the operation of organic solar cells and related devices. In this paper, we show that the effect of DA morphology and architecture means that the directions of CT states (where a CT state’s direction is defined as the direction from the electron to the hole of the CT state) may deviate from the direction of the applied electric field. The deviation means that the electric field is not fully utilized to assist, and could even hinder the dissociation process. Furthermore, we show that the correct charge carrier mobilities that should be used to describe CT state dissociation are the actual mobilites at DA interfaces. The actual mobilities are defined in this paper, and in general are not the same as the mobilities that are used to calculate electric currents which are the mobilites along the direction of the electric field. Then, to correctly describe CT state dissociation, we modify the widely used Onsager–Braun (OB) model by including the effect of DA morphology and architecture, and by employing the correct mobilities. We verify that when the modified OB model is used to describe CT state dissociation, the fundamental issues that concern the original OB model are resolved. This study demonstrates that DA morphology and architecture play an important role by strongly influencing the CT state dissociation as well as the mobilites along the direction of the electric field.

Controlling Singlet–Triplet Energy Splitting for Deep‐Blue Thermally Activated Delayed Fluorescence Emitters

AbstractThe development of efficient metal‐free organic emitters with thermally activated delayed fluorescence (TADF) properties for deep‐blue emission is still challenging. A new family of deep‐blue TADF emitters based on a donor–acceptor architecture has been developed. The electronic interaction between donor and acceptor plays a key role in the TADF mechanism. Deep‐blue OLEDs fabricated with these TADF emitters achieved high external quantum efficiencies over 19.2 % with CIE coordinates of (0.148, 0.098).

Controlling Singlet-Triplet Energy Splitting for Deep-Blue Thermally Activated Delayed Fluorescence Emitters.

The development of efficient metal-free organic emitters with thermally activated delayed fluorescence (TADF) properties for deep-blue emission is still challenging. A new family of deep-blue TADF emitters based on a donor-acceptor architecture has been developed. The electronic interaction between donor and acceptor plays a key role in the TADF mechanism. Deep-blue OLEDs fabricated with these TADF emitters achieved high external quantum efficiencies over 19.2 % with CIE coordinates of (0.148, 0.098).

Effect of hexyl substituent groups on photophysical and electrochemical properties of the poly[(9,9‐Dioctyluorene)−2,7‐diyl‐alt‐(4,7‐bis (3‐Hexylthien‐5‐Yl)−2,1,3‐Benzothiadiazole)−2′,2″‐diyl

ABSTRACTThe effect of the presence of hexyl group in thiophene on the photophysical and electrochemical properties of poly[(9,9‐dioctyluorene)−2,7‐diyl‐alt‐(4,7‐bis(3‐hexylthien‐5‐yl)−2,1,3‐benzothiadiazole)−2′,2″‐diyl] (F8TBT) is investigated. The copolymers present electron donor–acceptor architecture and are synthesized by Suzuki coupling reaction. The UV/Vis spectra show absorption maximum in the wavelength range of blue and orange, which are associated with different segments of the polymer backbone. Addition of hexyl substituent groups has a positive effect on the molar absorptivity and increases the emission and absorption intensities due to fluorene and thiophene‐benzothiadiazole‐thiophene (TBT) units, although an increment in the bandgap is observed. Cyclic voltammetry study of the polymer films reveal irreversible reduction and oxidation processes of the TBT units in the polymer chain and the HOMO and LUMO energy levels suggest ambipolar character for the polymers, while the electrochemical bandgaps are consistent with the absorbance measurements. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1975–1982

Controlled donor-accepter ratio for application of organic photovoltaic cells by alternative intermittent electrospray co-deposition

Because co-deposition method has been utilized in a conventional thermal evaporation process to realize graded donor-acceptor architectures, we investigated an alternative intermittent electrospray co-deposition method for solution-processed organic photovoltaic cells. In this method, two solutions of poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C61 butyric acid methyl ester (PCBM) were alternatively deposited using high-voltage pulse. Thus, the P3HT:PCBM blend thin film could be deposited even in a vacuum-free experimental setup. The optimum pulse width was found to be greater than 6 s to avoid an unexpected charge to the adjacent glass capillary, which causes the instable electrospray. The P3HT molecular ordering estimated from Raman spectroscopy and grazing incidence X-ray diffraction patterns was comparable to that estimated from the spin-coated device. In addition, the P3HT:PCBM ratio of the deposited thin film could be controlled by changing the ratio of the pulse width for the P3HT and PCBM solutions and was evaluated from the ultraviolet–visible absorption spectrum. Finally, a two layered bulk heterojunction structure with P3HT:PCBM was successfully demonstrated, leading to a maximum photoconversion efficiency of 3.1%. This value was 1.4-fold higher than that of the uniformly mixed bulk heterojunction device because of the high carrier-collection efficiency.

Full‐Color Delayed Fluorescence Materials Based on Wedge‐Shaped Phthalonitriles and Dicyanopyrazines: Systematic Design, Tunable Photophysical Properties, and OLED Performance

Purely organic light‐emitting materials, which can harvest both singlet and triplet excited states to offer high electron‐to‐photon conversion efficiencies, are essential for the realization of high‐performance organic light‐emitting diodes (OLEDs) without using precious metal elements. Donor–acceptor architectures with an intramolecular charge‐transfer excited state have been proved to be a promising system for achieving these requirements through a mechanism of thermally activated delayed fluorescence (TADF). Here, luminescent wedge‐shaped molecules, which comprise a central phthalonitrile or 2,3‐dicyanopyrazine acceptor core coupled with various donor units, are reported as TADF emitters. This set of materials allows systematic fine‐tuning of the band gap and exhibits TADF emissions that cover the entire visible range from blue to red. Full‐color TADF‐OLEDs with high maximum external electroluminescence quantum efficiencies of up to 18.9% have been demonstrated by using these phthalonitrile and 2,3‐dicyanopyrazine‐based TADF emitters.

Indicator displacement assay using an in situ generated polymeric system in water: exploiting donor–acceptor interactions

Indicator displacement assays (IDAs) using an electron-rich polymer as the receptor for electron-deficient guests are described in this study. The electron-rich water-soluble polymeric system possesses hydrophobic pockets, which were generated by the reaction of 1 (a Michael donor) and 1,3,5-triacryloylhexahydro-1,3,5-triazine, 5 (TAT, a Michael acceptor), in water at pH 8.2 in 2 min at 85oC. When electron-deficient fluorescent indicators are added to the polymeric systems, large changes in fluorescent intensity of the indicators occur. This donor–acceptor architecture between polymer and indicator was in turn explored for displacement of the indicator with electron-deficient analytes such as TNT.

Triphenylamine–thiazolothiazole–benzodithiophene based conjugated copolymers for polymer solar cells

Two novel conjugated copolymers (P1 and P2) with donor–acceptor architecture, based on triphenylamine, benzodithiophene and thiazolothiazole (TTz), have been prepared by the Stille coupling reaction. Compared with alternative copolymer P1 with the single electron-withdrawing unit TTz, random copolymer P2 introduced another stronger electron-withdrawing unit benzothiadiazole intramolecularly. Incorporation of two different electron-withdrawing units on the backbone was proved to have a significant influence on the optical and electrical properties. Compared with P1, copolymer P2 has the wider absorption spectrum, lower bandgap, and therefore the better device performances.

Synthesis and evaluation of styrylpyran fluorophores for noninvasive detection of cerebral β-amyloid deposits

The development of amyloid-specific fluorophores allows the visualization of cerebral β-amyloid deposits using optical imaging technology. In the present study, a series of smart styrylpyran fluorophores with compact donor–acceptor architecture were designed and evaluated for noninvasive detection of cerebral β-amyloid deposits. Spectral behavior of the fluorophores changed significantly (optical turn-on) upon binding to β-amyloid aggregates. Computational studies were conducted to correlate the experimental Kd values with calculated binding energies, speculating the relationship between fluorophore structure and β-amyloid affinity. In vivo studies demonstrated that PAD-2 could discriminate APP/PS1 transgenic mice from wild type controls, with specific labeling of cerebral β-amyloid deposits confirmed by ex vivo observation. Collectively, these styrylpyran fluorophores could provide a new scaffold for the development of optical imaging probes targeting cerebral β-amyloid deposits.

Synthesis, structure, ultrafast kinetics, and light-induced dynamics of CuHETPHEN chromophores.

Five heteroleptic Cu(i)bis(phenanthroline) chromophores with distinct variation in the steric bulk at the 2,9-phenanthroline position were synthesized using the HETPHEN method, and their ground and excited state properties are described. Analysis of the crystal structures reveals a significant distortion from tetrahedral geometry around the Cu(i) centre which is attributed to favourable aromatic interactions between the two phenanthroline ligands. Ultrafast and nanosecond transient optical spectroscopies reveal that the excited state lifetime can be tuned across two orders of magnitude up to 74 nanoseconds in acetonitrile by changing the 2,9-substituent from hydrogen to sec-butyl. X-ray transient absorption spectroscopy at the Cu K-edge confirmed Cu(i) oxidation to Cu(ii) and revealed a decrease of the Cu-N bond lengths in the excited state. The ground and excited state characterization presented here will guide the integration of CuHETPHEN chromophores into complex electron donor-acceptor architectures.

Exquisite 1D Assemblies Arising from Rationally Designed Asymmetric Donor-Acceptor Architectures Exhibiting Aggregation-Induced Emission as a Function of Auxiliary Acceptor Strength.

One-dimensional nanostructures with aggregation-induced emission (AIE) properties have been fabricated to keep the pace with growing demand from optoelectronics applications. The compounds 2-[4-(4-methylpiperazin-1-yl)benzylidene]malononitrile (PM1), 2-{4-[4-(pyridin-2-yl)piperazin-1-yl]-benzylidene}malononitrile (PM2), and 2-{4-[4-(pyrimidin-2-yl)piperazin-1-yl]benzylidene}malononitrile (PM3) have been designed and synthesized by melding piperazine and dicyanovinylene to investigate AIE in an asymmetric donor-acceptor (D-A) construct of A'-D-π-A- topology. The synthetic route has been simplified by using phenylpiperazine as a weak donor (D), dicyanovinylene as an acceptor (A), and pyridyl/pyrimidyl groups (PM2/PM3) as auxiliary acceptors (A'). It has been established that A' plays a vital role in triggering AIE in these compounds because the same D-A construct led to aggregation-caused quenching upon replacing A' with an electron-donating ethyl group (PM1). Moreover, the effect of restricted intramolecular rotation and twisted intramolecular charge transfer on the mechanism of AIE has also been investigated. Furthermore, it has been clearly shown that the optical disparities of these A'-D-π-A architectures are a direct consequence of comparative A' strength. Single-crystal X-ray analyses provided justification for role of intermolecular interactions in aggregate morphology. Electrochemical and theoretical studies affirmed the effect of the A' strength on the overall properties of the A'-D-π-A system.

From Dark to Light to Fluorescence Resonance Energy Transfer (FRET): Polarity-Sensitive Aggregation-Induced Emission (AIE)-Active Tetraphenylethene-Fused BODIPY Dyes with a Very Large Pseudo-Stokes Shift.

The work presented herein is devoted to the fabrication of large Stokes shift dyes in both organic and aqueous media by combining dark resonance energy transfer (DRET) and fluorescence resonance energy transfer (FRET) in one donor-acceptor system. In this respect, a series of donor-acceptor architectures of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) dyes substituted by one, two, or three tetraphenylethene (TPE) luminogens were designed and synthesised. The photophysical properties of these three chromophore systems were studied to provide insight into the nature of donor-acceptor interactions in both THF and aqueous media. Because the generation of emissive TPE donor(s) is strongly polarity dependent, due to its aggregation-induced emission (AIE) feature, one might expect the formation of appreciable fluorescence emission intensity with a very large pseudo-Stokes shift in aqueous media when considering FRET process. Interestingly, similar results were also recorded in THF for the chromophore systems, although the TPE fragment(s) of the dyes are non-emissive. The explanation for this photophysical behaviour lies in the DRET. This is the first report on combining two energy-transfer processes, namely, FRET and DRET, in one polarity-sensitive donor-acceptor pair system. The accuracy of the dark-emissive donor property of the TPE luminogen is also presented for the first time as a new feature for AIE phenomena.

Polymer/Polymer Blend Solar Cells Using Tetraazabenzodifluoranthene Diimide Conjugated Polymers as Electron Acceptors

Two n-type semiconducting polymers with alternating arylene (thiophene or selenophene)–tetraazabenzodifluoranthene diimide (BFI) donor–acceptor architecture have been investigated as new electron acceptors in polymer/polymer blend solar cells. The new selenophene-linked polymer, PBFI-S, has a significantly smaller optical band gap (1.13 eV) than the thiophene-linked PBFI-T (1.38 eV); however, both polymers have similar HOMO/LUMO energy levels determined from cyclic voltammetry. Blends of PBFI-T with the thiazolothiazole–dithienylsilole donor polymer (PSEHTT) gave a 2.60% power conversion efficiency (PCE) with a 7.34 mA/cm2 short-circuit current. In contrast, PBFI-S:PSEHTT blends had a 0.75% PCE with similarly reduced photocurrent and external quantum efficiency. Reduced free energy for charge transfer and reduced bulk electron mobility in PBFI-S:PSEHTT blends compared to PBFI-T:PSEHTT blends as well as significant differences in bulk film morphology are among the reasons for the large loss in efficiency in PBFI-S:PSEHTT blend solar cells.

Optical properties of disilane-bridged donor-acceptor architectures: strong effect of substituents on fluorescence and nonlinear optical properties.

A series of disilane-bridged donor-acceptor architectures 1-9 containing strong electron-donating and -withdrawing substituents were designed and synthesized in acceptable yields. The substituents substantially affected the fluorescence and nonlinear optical properties of the compounds. In the solid state, the compounds showed purple-blue fluorescence (λ(em) = 360-420 nm) with high quantum yields (up to 0.81). Compound 3, which had p-N,N-dimethylamino and o-cyano substituents, exhibited optical second harmonic generation (activity 2.9 times that of urea, calculated molecular hyperpolarizability β = 1.6 × 10(-30) esu) in the powder state. Density functional theory calculations for the ground and excited states indicated that both the locally excited state and the intramolecular charge transfer excited state make important contributions to the luminescence behavior.

Tuning the reorganization energy of electron transfer in supramolecular ensembles--metalloporphyrin, oligophenylenevinylenes, and fullerene--and the impact on electron transfer kinetics.

Oligo(p-phenylenevinylene) (oPPV) wires of various lengths featuring pyridyls at one terminal and C60 moieties at the other, have been used as molecular building blocks in combination with porphyrins to construct a novel class of electron donor-acceptor architectures. These architectures, which are based on non-covalent, directional interactions between the zinc centers of the porphyrins and the pyridyls, have been characterized by nuclear magnetic resonance spectroscopy and mass spectrometry. Complementary physico-chemical assays focused on the interactions between electron donors and acceptors in the ground and excited states. No appreciable electron interactions were noted in the ground state, which was being probed by electrochemistry, absorption spectroscopy, etc.; the electron acceptors are sufficiently decoupled from the electron donors. In the excited state, a different picture evolved. In particular, steady-state and time-resolved fluorescence and transient absorption measurements revealed substantial electron donor-acceptor interactions. These led, upon photoexcitation of the porphyrins, to tunable intramolecular electron-transfer processes, that is, the oxidation of porphyrin and the reduction of C60. In this regard, the largest impact stems from a rather strong distance dependence of the total reorganization energy in stark contrast to the distance independence seen for covalently linked conjugates.

Evaluation of molecules based on the electron donor-acceptor architecture as near-infrared β-amyloidal-targeting probes.

A novel class of near-infrared molecules based on the donor-acceptor architecture were synthesized and evaluated as Aβ imaging probes. In vivo imaging studies suggested that MCAAD-3 could penetrate the blood-brain barrier and label Aβ plaques in the brains of transgenic mice. Computational studies could reproduce the experimental trends well.

Smart Near-Infrared Fluorescence Probes with Donor–Acceptor Structure for in Vivo Detection of β-Amyloid Deposits

The deposition of β-amyloid (Aβ) plaques in the parenchymal and cortical brain is accepted as the main pathological hallmark of Alzheimer's disease (AD); however, early detection of AD still presents a challenge. With the assistance of molecular imaging techniques, imaging agents specifically targeting Aβ plaques in the brain may lead to the early diagnosis of AD. Herein, we report the design, synthesis, and evaluation of a series of smart near-infrared fluorescence (NIRF) imaging probes with donor-acceptor architecture bridged by a conjugated π-electron chain for Aβ plaques. The chemical structure of these NIRF probes is completely different from Congo Red and Thioflavin-T. Probes with a longer conjugated π system (carbon-carbon double bond) displayed maximum emission in PBS (>650 nm), which falls in the best range for NIRF probes. These probes were proved to have affinity to Aβ plaques in fluorescent staining of brain sections from an AD patient and double transgenic mice, as well as in an in vitro binding assay using Aβ(1-42) aggregates. One probe with high affinity (K(i) = 37 nM, K(d) = 27 nM) was selected for in vivo imaging. It can penetrate the blood-brain barrier of nude mice efficiently and is quickly washed out of the normal brain. Moreover, after intravenous injection of this probe, 22-month-old APPswe/PSEN1 mice exhibited a higher relative signal than control mice over the same period of time, and ex vivo fluorescent observations confirmed the existence of Aβ plaques. In summary, this probe meets most of the requirements for a NIRF contrast agent for the detection of Aβ plaques both in vitro and in vivo.

Spin-coated P3HT:Aminated carbon microsphere composite films for polymer solar cells

Abstract

Resonance energy transfer-based fluorescent probes for Hg2+, Cu2+and Fe2+/Fe3+ions

Resonance energy transfer (RET) between donor-acceptor architecture is an important physical mechanism which is used enormously for the development of fluorescent probes. The unique advantage of RET is its ability to transfer energy non-radiatively between molecules over biologically relevant distances. The distance dependency of RET makes this approach suitable for bioanalysis such as distances between biomolecules and molecular level interactions, both in vitro and in vivo. In addition, RET is a proficient approach for the development of fluorescent probes with ratiometric measurements. In the recent years, resonance energy transfer has been extensively applied for the design of fluorescent sensors for different types of analytes such as metal ions, anions, reactive oxygen species and molecules of biological interest. In this review, our aim is to highlight the applications of resonance energy transfer mechanisms, i.e. Förster or fluorescence resonance energy transfer (FRET) and through-bond energy transfer (TBET) for the development of fluorescent probes, mainly for Hg(2+), Cu(2+) and Fe(2+)/Fe(3+) ions.

Modeling of photoactive conjugated donor-acceptor copolymers: the effect of the exact HF exchange in DFT functionals on geometries and gap energies of oligomer and periodic models.

Conjugated copolymers with an alternating donor-acceptor (D-A) architecture are exploited as low-bandgap and high-hole-mobility materials in organic electronics. However, several of the presently available modeling methods predict different geometries and electronic properties for the same copolymer. In this work, the effect of the amount of exact Hartree-Fock (HF) exchange in density functionals on the planarity of the geometry and the electronic properties of the single oligomer chains of one benzodithiophene- and benzotriazole-based donor-acceptor copolymer and two fluorene-, thiophene-, and benzotriazole-based donor-acceptor copolymers is assessed. The functionals are B3LYP, PBE, PBE0, HSE06, LC-ωPBE, ωB97XD, M06, M06L, M062X, M06HF, and the optimally tuned OT-ωB97X. Benchmarking of the methods is useful for understanding the results of a particular functional and allows, to a certain degree, comparison between results obtained with different functionals. Additionally, the applicability of the one-dimensional periodic boundary condition (PBC) for modeling the D-A copolymers with long constitutional repeating units (CRUs) is evaluated.