Donor-acceptor Molecules Research Articles

Enhanced efficiency in nondoped, blue organic light-emitting diodes utilizing simultaneously local excition and two charge-transfer exciton from benzimidazole-based donor–acceptor molecules

The simultaneous utilization of all charge-transfer excitons and local excitons is the pathway to obtain the high efficiency fluorescent organic light-emitting diodes (FOLEDs). Here, a twisted intramolecular charge transfer state (TICT-state), a planar intramolecular charge transfer state (ICT-state), and a locally excited state (LE-state) are demonstrated to enhance the occurrence of singlet excitons in the fluorescent emitters, which are based on benzimidazole and triphenylamine donor–acceptor derivatives. The synthesis, photophysics and electroluminescent (EL) performance are studied systematically. The fluorescence emitters (TPABBBI and TPABBI) with the special TICT and ICT characteristics realize the electron–hole (e–h) recombination via intramolecular conversion from charge-transfer excitons to radiative singlet exciton. The devices based on them show high efficiency (5.1 cd/A, 5.77 lm/W, 5.66% of EQEm for TPABBBI and 3.56 cd/A, 3.11 lm/W, 4.23% for TPABBI), low efficiency roll-off at high luminance and stable blue emission.

Synthesis and optoelectronic properties of donor-acceptor molecules containing pyromellitic diimide chromophore

Three donor-acceptor molecules containing pyromellitic diimide chromophore were designed and synthesized. The synthetic route towards the target compounds was systematically investigated and discussed in detail. The resulted organic semiconductors have shown promising optoelectronic properties as further revealed by UV-Vis absorption spectroscopy, cyclic voltammetry, and theoretic calculation.

Anticorrelation between the Evolution of Molecular Dipole Moments and Induced Work Function Modifications.

We explore the limits of modifying metal work functions with large molecular dipoles by systematically increasing the dipole moment of archetype donor–acceptor molecules in self-assembled monolayers on gold. Contrary to intuition, we find that enhancing the dipoles leads to a reduction of the adsorption-induced change of the work function. Using atomistic simulations, we show that large dipoles imply electronic localization and level shifts that drive the interface into a thermodynamically unstable situation and trigger compensating charge reorganizations working against the molecular dipoles. Under certain circumstances, these are even found to overcompensate the effect that increasing the dipoles has for the work function.

Enhancing the coplanarity of the donor moiety in a donor-acceptor molecule to improve the efficiency of switching phenomenon for flash memory devices

Two conjugated small molecules were synthesized by Heck coupling reaction, wherein 1,8-naphthalimide acted as an electron acceptor, while either carbazole or triphenylamine, which contributed the different coplanarity, acted as electron donors, respectively. The devices based on both materials show non-volatile flash memory characteristics, and the effect of molecular coplanarity of the donor groups on the device performance was precisely studied. The results indicated that the reproducibility of the switching phenomenon for the memory device based on the carbazole containing naphthalimide derivative was much better than that based on the triphenylamino based naphthalimide due to the rigid carbazole moiety which improved the surface morphology as revealed by atomic force microscopy measurement. Therefore, the significance of the coplanarity of the donor moiety on improving reproducibility of switching phenomenon for memory device applications was revealed.

Correlation between Particle Deformation Kinetics and Polymer Interdiffusion Kinetics in Drying Latex Films

Using an experimental setup which determines the turbidity of the sample and the efficiency of Förster resonance energy transfer (FRET) at the same time, we have correlated the particle deformation kinetics in a drying latex film, quantified by light scattering with the kinetics of polymer interdiffusion. Interdiffusion was quantified making use of energy transfer (FRET) between donor molecules and acceptor molecules, bound to polymer chains on different particles. When the chains cross the interparticle boundaries, the rate of energy transfer increases. The latex was prepared by miniemulsion polymerization. The amount of emulsifier employed during polymerization had a pronounced effect on the relative timing of interdiffusion and particle deformation. Increasing the amount of emulsifier delayed the onset of interdiffusion relative to the time when the film became transparent. This is mostly the consequence of a size effect, as opposed to surfactant acting as a barrier for transport.

Tuning the optical absorption of potential blue emitters

The quest for more efficient blue emitters to be applied in organic light-emitting diodes is one of the challenging tasks of contemporary nanotechnologies. An approach to enhance substantially the intrinsic efficiency of luminescent organic molecules is the so-called thermally activated delayed fluorescence. A prerequisite for its occurrence is a vanishing energy separation between the first singlet and triplet excited states. A series of donor–acceptor molecules is investigated theoretically within this study in order to validate a molecular model for design of efficient organic blue emitters with closely spaced singlet and triplet excited states. The model is based on meta-linkage of the donor and acceptor residues to a spacer ensuring frontier molecular orbitals partitioning. The optimal geometries of the molecules are obtained with density functional theory (B3LYP/6-31G*) and the singlet and triplet absorption spectra are simulated within the time-dependent density functional framework. The excited singlet-triplet energy gap is estimated and correlated to structural and energetic characteristics of the donors and acceptors. Several requirements for achieving high-energy triplet states at the molecular level in such donor–acceptor systems are outlined, the main being disjoint character of the molecular orbitals on the spacer and sufficient energy separation of the two topmost occupied orbitals. It is shown that by variation of the acceptor moiety the optical absorption transitions of the compounds can be fine-tuned in a systematic fashion. Molecules with degenerate singlet and triplet first excited states are proposed, combining bisdimethylaminotriphenylamine or phenoxazine as donors with diphenyloxadiazole or diphenyl-2,2′-bipyridine as acceptors. Bipolar molecules derived from this model could be used as prospective building blocks for efficient emissive materials in blue organic light-emitting diodes.

Permanent dipole moments and energies of excited states from density functional theory compared with coupled cluster predictions: Case of para-nitroaniline

Different ways to extract properties of excited states from time-dependent density functional theory (TD-DFT) calculations are compared to ab initio results obtained with the Equation of Motion Coupled Cluster approach. The recently proposed a posteriori Tamm–Dancoff approximation (ATDA) predicts the permanent dipole moments to be underestimated by 25% on average, close to the results of the relaxed density TD-DFT formalism, quadratic response formalism, and numerical energy derivatives, while the unrelaxed density approximation results are less accurate (40% overestimate). We also propose a correction for TD-DFT excitation energies, which are known to be problematic for charge transfer states. The static DFT energies evaluated on the relaxed densities of the excited states are found to be more accurate than TD-DFT excitation energies (RMSD is 0.7eV vs. 1.1eV, while maximum deviation is −1.0eV vs. −2.0eV). This validates ATDA for description of nonlinear optical properties of donor–acceptor molecules, exemplified by para-nitroaniline, and extends this method to improve the excitation energy predictions.

T-Shaped Donor–Acceptor Molecules for Low-Loss Red-Emission Optical Waveguide

A series of T-shaped polycyclic molecules with high fluorescence were developed as optical waveguide materials. Their emissions covered almost the whole visible range from 450 to 800 nm. Compound 3-1 showed an optical loss coefficient about 0.29 dB/μm in red-emission waveguide. Our investigations demonstrated that these molecules held great potential for organic optical waveguide due to the high fluorescence quantum efficiency and large Stokes' shift.

A new exploration on the substantial improvement of rectifying behaviors for a donor–acceptor molecular diode by graphene electrodes

Sandwiching a donor–acceptor molecule between two graphene nanoribbon (GNR) electrodes to construct a particular molecular diode, our calculations from the first-principles method predict that such a configuration can lead to dramatically different and unexpected effects. When the GNR electrodes are semiconducting, the rectifying behaviors for this molecule can be enhanced greatly. The maximum rectification ratio rises sharply with an increase of the bandgap size of GNR electrodes and can reach a value more than 4000 as the bandgap is about 0.8eV, which is of a very significant improvement as compared with previously investigated results for such a molecule coupled to bulk metal electrodes. Therefore, our studies imply that using graphene with a large bandgap as electrodes might be a novel effective pathway to greatly raise rectifying behaviors of a donor–acceptor molecule.

Synthesis and photovoltaic properties of a star-shaped molecule based on a triphenylamine core and branched terthiophene end groups

A new star-shaped donor-acceptor molecule has been synthesized for application as the donor material in solution-processed bulk-heterojunction organic solar cells (OSCs). The molecule consists of a triphenylamine (TPA) unit as the core and a donor unit with three arms containing benzo[1,2,5]thiadiazole (BT) acceptor units and 5,5″-dihexyl-2,2′:3′,2″-terthiophene (tTh) end groups. The molecule, denoted S(TPA-BT-tTh), exhibits a broad absorption band in the wavelength range 300–650 nm and high hole mobility of 1.1×10−4 cm2 V−1 s−1. An OSC device based on S(TPA-BT-tTh) as donor and [6,6]-phenyl C71-butyric acid methyl ester (PC70BM) as the acceptor (1:3, w/w) exhibited a power conversion efficiency of 2.28% with a short circuit current density of 6.39 mA/cm2 under illumination of AM.1.5, 100 mW/cm2.

Photophysical properties and energy transfer mechanism of PFO/Fluorol 7GA hybrid thin films

Photophysical properties of poly (9,9′-di-n-octylfluorenyl-2.7-diyl) (PFO)/2-butyl-6- (butylamino)benzo [de] isoquinoline-1,3-dione (Fluorol 7GA) and energy transfer between them have been investigated. In this work, both PFO and Fluorol 7GA act as donor and acceptor, respectively. Based on the absorption and luminescence measurements, the photophysical and energy transfer properties such as fluorescence quantum yield (Φf), fluorescence lifetime (τ), radiative rate constant (kr), non-radiative rate constant (knr), quenching rate constant (kSV), energy transfer rate constant (kET), energy transfer probability (PDA), energy transfer efficiency (η), critical concentration of acceptor (Co), energy transfer time (τET) and critical distance of energy transfer (Ro) were calculated. Large values of kSV, kET and Ro suggested that Förster-type energy transfer was the dominant mechanism for the energy transfer between the excited donor and ground state acceptor molecules. It was observed that the Förster energy transfer together with the trapping process are crucial for performance improvement in ITO/(PFO/Fluorol7GA)/Al device.

Near-infrared and multicolored electrochromism of solution processable triphenylamine-anthraquinone imide hybrid systems

A series of novel donor-acceptor molecules (TPA-AQI, TPA-AQI2 and TPA-AQI3) with various number of electron-accepting anthraquinone imide (AQI) arms connected to the electron-donating triphenylamine (TPA) core were synthesized and characterized by UV–vis absorption spectroscopy, cyclic voltammetry and spectroelectrochemical measurements. The geometries of these molecules in ground-state as well as the electronic absorption properties on the basis of the optimized geometries were investigated by theoretical calculations. For all the three molecules, there are two main absorptions in the range of 300–450nm and 450–700nm, respectively. The former corresponds to ππ* transitions and the latter a nature of intramolecular charge transfer (ICT). Hypsochromic shift of the ICT absorption was observed from the single armed molecule TPA-AQI to the tribranched molecule TPA-AQI3, which may result from the increased steric hindrance between the donor and acceptor segments. Electrochemical studies revealed the ambipolar properties of these molecules and three redox couples, with two quasi-reversible redox couples appearing in the cathodic regime and one redox couple in the anodic regime, on CV curves. CV results also indicated the stabilization of the HOMO levels by increasing AQI arms. Spectroelectrochemical measurements demonstrated the near-infrared (NIR) and multicolor electrochromism of these molecules. When reduced to radical anions or oxidized to radical cations, intense NIR absorptions were observed accompanied with color changes of the solutions. As for TPA-AQI, there existed four different colors at different redox states: Indian-red at the neutral state, bluish green at the radical cationic state, olive green at the radical anionic state, and dark blue at the dianion state. The detailed transitions of the observed NIR absorptions were also discussed.

Charge Photogeneration in Donor–Acceptor Conjugated Materials: Influence of Excess Excitation Energy and Chain Length

We investigate the role of excess excitation energy on the nature of photoexcitations in donor-acceptor π-conjugated materials. We compare the polymer poly(2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[1,2-b;3,4-b']dithiophene)-4,7-benzo[2,1,3]thiadiazole) (PCPDTBT) and a short oligomer with identical constituents at different excitation wavelengths, from the near-infrared up to the ultraviolet spectral region. Ultrafast spectroscopic measurements clearly show an increased polaron pair yield for higher excess energies directly after photoexcitation when compared to the exciton population. This effect, already observable in the polymer, is even more pronounced for the shorter oligomer. Supported by quantum chemical simulations, we show that excitation in high-energy states generates electron and hole wave functions with reduced overlap, which likely act as precursors for the polaron pairs. Interestingly, in the oligomer we observe a lifetime of polaron pairs which is one order of magnitude longer. We suggest that this behavior results from the intermolecular nature of polaron pairs in oligomers. The study excludes the presence of carrier multiplication in these materials and highlights new aspects in the photophysics of donor-acceptor small molecules when compared to polymers. The former are identified as promising materials for efficient organic photovoltaics.

Synthesis of coumarin-containing conjugated polymer for naked-eye detection of DNA and cellular imaging

Three novel coumarin-containing conjugated polyelectrolytes (P1–P3) containing coumarin, carbazole, fluorene and phenylene moieties were successfully synthesized by Suzuki coupling reaction. The narrow bandgap coumarin–carbazole units and the wide bandgap fluorene–phenylene units in P1–P3 might form energy donor–acceptor molecule architectures, in which coumarin–carbazole units serve as an acceptor of the fluorescent resonance energy transfer (FRET). The addition of calf thymus DNA resulted in efficient FRET from fluorene–phenylene (energy donor) to coumarin–carbazole (energy acceptor) unit for P1, which caused the solution fluorescent color to change from blue to light green. In addition, P1 can be utilized as a reliable fluorescent probe for cellular imaging of human adult skin fibroblast cells, where blue and green fluorescence were observed in the cytoplasm by use of different excitation wavelength. Subsequently, the water-soluble and fluorescent P1 were used as high sensitive DNA sensor and effective fluorescent cell labeling agents with dual emission pathways.

New Donor–Acceptor–Donor Molecules with Pechmann Dye as the Core Moiety for Solution-Processed Good-Performance Organic Field-Effect Transistors

In this paper, we report the synthesis and characterization of two new D-A-D molecules (E)-5,5′-bis(5-(benzo[b]thiophen-2-yl)thiophen-2-yl)-1,1′-bis(2-ethyl- hexyl)-[3,3′-bipyrrolylidene]-2,2′(1H,1′H)-dione (BTBPD) and (E)-5,5′-bis- (5-(benzo[b]furan-2-yl)thiophen-2-yl)-1,1′-bis(2-ethylhexyl)-[3,3′-bipyrrolylidene]-2,2′(1H,1′H)-dione (BFBPD). They entail bipyrrolylidene-2,2′(1H,1′H)-dione (BPD, known as Pechmann dye) as the electron-accepting core that is flanked by two benzo[b]thiophene moieties and two benzo[b]furan moieties, respectively. Crystal structures of BTBPD and BFBPD provide solid evidence for the intermolecular donor–acceptor (D-A) interactions, which are favorable for improving charge transport performance. Organic field-effect transistors (OFETs) were prepared based on thin films of BTBPD and BFBPD through solution-processed technique. OFETs of BTBPD exhibit relatively high hole mobility up to 1.4 cm2 V–1 s–1 with high on/off ratio up to 106. In comparison, the hole mobility of OFETs with BFBPD (0.14 cm2 V–1 s–1) is relatively low, because of the poor thin-film morphology and low molecular ordering, even after annealing. Thin-film morphological and XRD studies were carried out to understand the variation of hole mobilities after annealing at different temperatures. The present studies clearly demonstrate the potentials of BPD that is planar and polar as the electron-acceptor moiety to build D-A molecules for organic semiconductors with good performance.

A Dramatic Odd–Even Oscillating Behavior for the Current Rectification and Negative Differential Resistance in Carbon‐Chain‐Modified Donor–Acceptor Molecular Devices

AbstractThe donor–acceptor molecule is the only molecule that features a real intrinsic rectification. However, all investigations in the last decades showed that rectification behaviors of such molecules are not promising since their rectification ratio is only on the order of 10. Use of carbon chains Cn to serve as spacers is reported, along with attempts to modulate electrical behavior of the donor–acceptor molecule. Calculations using the first‐principles method show that electrical behavior is indeed altered substantively, and a particular regularity can be clearly observed, i.e., a dramatic odd–even oscillation for electronic behavior with increasing carbon‐chain length n. For models with even‐n carbon chains, the rectification ratio is small (30), and no negative differential resistance (NDR) behavior is detected, but the rectifying performance of models with odd‐n carbon chains is tremendously improved and rectification ratios on the order of 50 to 400 can be achieved, alongside a large NDR. This study thus suggests that using a suitable spacer might be an effective way to significantly boost electrical characteristics, including rectifying performance, of the donor–acceptor molecule.

Self-assembly of a tripyrenylboron molecule towards solid sensor for fluoride anions

We have succeeded in the synthesis of a novel triarylboron compound TPB, in which an electron-accepting B atom is connected to three electron-donating pyrene molecules. Through the method of supramolecular self-assembly, this donor–acceptor molecule can easily form solid supermolecular structures, such as 1D nanowires, nanofibers, nanorods and highly orderly-patterned microslice arrays, which all showed blue-fluorescence. Interestingly, a unique feature was observed: the solid fluorescence of these supermolecular aggregates could be quenched by fluoride anions. This result indicated that these solid superstructural materials have a potential application in the detection and separation of fluoride anions.

Controlled transition dipole alignment of energy donor and energy acceptor molecules in doped organic crystals, and the effect on intermolecular Förster energy transfer

The orientation factor κ(2) ranging from 0 to 4, which depends on the relative orientation of the transition dipoles of the energy donor (D) and the energy acceptor (A) in space, is one of the pivotal factors deciding the efficiency and directionality of resonance energy transfer (RET) in a D-A molecular system. In this work, tetracene (Tc) and pentacene (Pc) are successfully doped in a trans-1,4-distyrylbenzene (DSB) crystalline lattice to form definite D-A mutually perpendicular transition dipole orientations. The cross D-A dipole arrangement results in an extremely small orientation factor, which is about two orders smaller than that in the disordered films. The energy transfer properties from the host (DSB) to the guest (Tc/Pc) were investigated in detail by steady-state as well as time-resolved fluorescence spectroscopy. Our experimental research results show that the small value of κ(2) allows less or partial energy transfer from the host (DSB) to the guest (Tc) in a wide range of guest concentration, with the Förster distance of around 1.5 nm. By controlling the doping concentrations in the Tc and Pc doubly doped DSB crystals, we demonstrate, as an example, for the first time the application of the restricted energy transfer by D-A cross transition dipole arrangement for preparation of a large-size, white-emissive organic crystal with the CIE coordinates of (0.36, 0.37) approaching an ideal white light. In contrast, Tc is also doped in an anthracene crystalline lattice to form head-to-tail D-A transition dipole alignment, which is proved to be highly effective to promote the intermolecular energy transfer. In this doped system, the orientation factor is relatively large and the Förster distance is around 7 nm.

Impact of 2,6-connectivity in azulene: optical properties and stimuli responsive behavior

The possible incorporation of the redox active, small donor acceptor molecule azulene as a core structure for potential optoelectronic applications has been evaluated. The synthesis and characterization of different isomers of di(phenylethynyl)azulene 1–4 have been successfully carried out. The photophysical properties as well as their stimuli responsive behavior reflecting their corresponding electronic properties were also investigated. The experimental observations show that 2,6-connection (3) exhibits intense luminescence and shows the strongest stimuli responsive behavior upon acid treatment. DFT calculations of 3 reveal the highest contributions of the alkyne substituents at the 2- and 6-position of the azulene to the ground- and excited state that makes the 2,6-connectivity at azulene a very promising candidate for low band-gap conjugated materials.

Synthesis of donor–acceptor molecules based on isoxazolones for investigation of their nonlinear optical properties

A series of donor–acceptor molecules that incorporate N,N-dimethylaniline or carbazole as the common electron donor and 3-phenyl-5-isoxazolone as the electron acceptor moiety have been synthesized and their structural and optical properties and self-assembly behaviors were investigated. All these compounds showed the property of aggregation induced emission (AIE). These compounds with different donors and different conjugation could be self-assembled into different superstructures such as ribbon-like architectures, hollow nanospheres, multilayer plates or rhombic microplates by phase transfer methodology or solvent-vapor techniques. Furthermore, the nonlinear optical (NLO) properties of the four D–A molecules were studied by the Z-scan technique and all of these compounds exhibited third-order nonlinear absorption effects.