Electron Affinity Of Acceptor Research Articles

Effects of Electron Affinity and Steric Hindrance of the Trifluoromethyl Group on the π-Bridge in Designing Blue Thermally Activated Delayed Fluorescence Emitters.

To explore the correlation of the acceptor electron affinity and the molecular conformation to the thermally activated delayed fluorescence (TADF) feature, a series of d-π-A molecules were designed and synthesized with triazine (Trz) as the acceptor (A) and carbazole (Cz) or tert-butylcarbazole (BuCz) as the donor (D). On the phenylene bridge between D and A, methyl or trifluoromethyl was incorporated close either to D or to A to tune the molecular conformation and the electron-withdrawing ability of acceptor. Both the twist angles and the singlet and triplet energy difference (ΔEST ) were observed strongly dependent on the type and position of the substituent on the π-bridge. Only those molecules with trifluoromethyl locating close to the D side, namely TrzCz-CF3 and TrzBuCz-CF3 , exhibit TADF feature, verifying that both sufficient electron affinity of the A unit and large dihedral angle between D and the π-bridge are necessary to ensure the occurrence of TADF. The blue organic light-emitting diodes fabricated with TrzCz-CF3 and TrzBuCz-CF3 achieved external quantum efficiencies of 9.40 % and 14.22 % with CIE coordinates of (0.19, 0.23) and (0.18, 0.29) respectively. This study provides practical design strategy for blue TADF materials particularly when planar and less crowded group is used as donor.

Suppression of Recombination Losses in Polymer:Nonfullerene Acceptor Organic Solar Cells due to Aggregation Dependence of Acceptor Electron Affinity

AbstractHere, it is investigated whether an energetic cascade between mixed and pure regions assists in suppressing recombination losses in non‐fullerene acceptor (NFA)‐based organic solar cells. The impact of polymer‐NFA blend composition upon morphology, energetics, charge carrier recombination kinetics, and photocurrent properties are studied. By changing film composition, morphological structures are varied from consisting of highly intermixed polymer‐NFA phases to consisting of both intermixed and pure phase. Cyclic voltammetry is employed to investigate the impact of blend morphology upon NFA lowest unoccupied molecular orbital (LUMO) level energetics. Transient absorption spectroscopy reveals the importance of an energetic cascade between mixed and pure phases in the electron–hole dynamics in order to well separate spatially localized electron–hole pairs. Raman spectroscopy is used to investigate the origin of energetic shift of NFA LUMO levels. It appears that the increase in NFA electron affinity in pure phases relative to mixed phases is correlated with a transition from a relatively planar backbone structure of NFA in pure, aggregated phases, to a more twisted structure in molecularly mixed phases. The studies focus on addressing whether aggregation‐dependent acceptor LUMO level energetics are a general design requirement for both fullerene and NFAs, and quantifying the magnitude, origin, and impact of such energetic shifts upon device performance.

Theoretical study on the charge transport and metallic conducting properties in organic complexes.

The charge transfer process between substrate molecular and dopant always appears in doped organic semiconductors, so that molecular doping is a common method to improve the electrical properties by combining appropriate complexes of electron acceptor and donor molecules. At the interface of the doped complexes, the amount of charge-transfer based on the charge analysis method could be affected by various factors, including the stacking structure, the HOMOD-LUMOA gaps, the offset defined by the donor ionization potential and the acceptor electron affinity IPD-|EAA|, and the strength of the intermolecular orbital interaction. To better understand the charge transport properties in complex crystals, reasonable mobility values were calculated by combining semi-classical Marcus-Hush theory with molecular dynamics simulation, in which the mobility values were on the same order of magnitude as experimental values. The largest and average room-temperature mobility were 4.59 and 0.21 cm2 V-1 s-1 for TTF-TCNQ based on the anisotropic transport properties and random-walk schemes of the charge diffusion coefficient. The interface of the TTF-TCNQ crystal possesses metallic conducting properties with a predicted resistance of 4.43 kΩ. Charge-transfer complexes exhibit larger mobility and higher conductivity compared to the constituent donor and acceptor molecules.

Relationship of Ionization Potential and Oxidation Potential of Organic Semiconductor Films Used in Photovoltaics

Ultraviolet photoelectron spectroscopy (UPS) and cyclic voltammetry (CV) are employed to measure energy levels for charge transport in organic semiconductor films. A series of classical molecules/polymers used in organic bulk heterojunction solar cells are deposited on platinum substrates/electrodes to form thin films and a linear relationship of vertical ionization potential (IP) measured by UPS and relative oxidation potential (Eox) obtained by CV is found, with a slope equal to unity. The intercept varies with the different reference redox couples and repeated potential sweep numbers during experiment processes. The relationship provides for an easy conversion of values obtained by the two techniques and correlates well with device parameters. The precision in the CV‐derived IP values is not sufficient, however, to enable precise design of energy level alignment at heterojunction and the approach does not improve upon the current “best practice” for obtaining donor ionization potential–acceptor electron affinity gaps at heterojunctions.

Electron transfer in biologically important systems: Polycyclic aromatic hydrocarbons, DNA bases and free radicals

Occurrence of electron transfer was studied for different combinations of polycyclic aromatic hydrocarbons (PAHs) and DNA bases as electron donors or acceptors and free radicals only as electron acceptors. Geometries of all the molecules and radicals were optimized in aqueous medium employing the polarizable continuum model. Single electron transfer (SET) and sequential proton loss electron transfer mechanisms were investigated employing Gibbs free energies of the appropriate neutral, anionic and cationic species. Barrier energies involved in these phenomena were calculated using the Marcus theory. The SET barrier energies were found to be linearly correlated with [Formula: see text] (Electron affinities of acceptors – Ionization potentials of donors). SET barrier energies from the DNA bases to the PAHs follow the order Cy [Formula: see text] Th [Formula: see text] Ad [Formula: see text] Gu, whereas SET barrier energies from the PAHs to the DNA bases follow the order Gu [Formula: see text] Ad [Formula: see text] Th [Formula: see text] Cy. Thus, guanine, among the DNA bases, is the best electron donor to the PAHs and worst electron acceptor from the same.

Charge Transfer Dynamics in Donor–Acceptor Complexes between a Conjugated Polymer and Fluorene Acceptors

We report on ground and excited state charge transfer in charge-transfer complexes in films formed between a semiconducting polymer, MEH-PPV (poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene]), and a series of fluorene electron acceptors. The latter were designed to vary systematically the electron affinity (EA) over ∼1.5 eV by attachment of various electron withdrawing groups to the fluorene core. The EAs of the acceptors are determined by cyclic voltammetry and compared with those from density functional theory calculations. The charge-transfer dynamics are studied using an ultrafast visible-pump–IR-probe photoinduced absorption technique. We demonstrate that the acceptor EA is the key—but not the only—parameter that governs charge recombination rates that scale exponentially with the acceptor EA. From the time-resolved data we deduced a model that describes charge dynamics for acceptors with low and high EAs. The two opposite trends—higher acceptor EA increases the driving force for charge separation but also inevitably increases the rate of undesirable charge recombination—should be carefully counterbalanced in designing novel polymer–fullerene bulk heterojunctions.

Low-Energy Inverse Photoemission Study on the Electron Affinities of Fullerene Derivatives for Organic Photovoltaic Cells

The lowest unoccupied molecular orbital (LUMO) levels of six fullerene derivatives frequently used for organic photovoltaic cells are examined in the solid state. We employed a new experimental technique, low-energy inverse photoemission spectroscopy (LEIPS), which allows us to determine the electron affinities of solid samples within the uncertainties of 0.1 eV. Using the precisely determined electron affinities, the energy difference between the ionization energy of donor and the electron affinity of acceptor is compared with the open circuit voltage for various polymer/fullerene combinations taken from the literature. The result suggests that the well-known relation between the energy difference and the open circuit voltage should be reconsidered.

Trap‐Assisted Recombination via Integer Charge Transfer States in Organic Bulk Heterojunction Photovoltaics

Organic photovoltaics are under intense development and significant focus has been placed on tuning the donor ionization potential and acceptor electron affinity to optimize open circuit voltage. Here, it is shown that for a series of regioregular‐poly(3‐hexylthiophene):fullerene bulk heterojunction (BHJ) organic photovoltaic devices with pinned electrodes, integer charge transfer states present in the dark and created as a consequence of Fermi level equilibrium at BHJ have a profound effect on open circuit voltage. The integer charge transfer state formation causes vacuum level misalignment that yields a roughly constant effective donor ionization potential to acceptor electron affinity energy difference at the donor–acceptor interface, even though there is a large variation in electron affinity for the fullerene series. The large variation in open circuit voltage for the corresponding device series instead is found to be a consequence of trap‐assisted recombination via integer charge transfer states. Based on the results, novel design rules for optimizing open circuit voltage and performance of organic bulk heterojunction solar cells are proposed.

Charge Transfer in Molecular Complexes with 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ): A Density Functional Theory Study

Molecular doping is a charge-transfer process intended to improve the electrical properties of organic semiconductors and the efficiency of organic electronic devices, by incorporation of a complex-forming, strong molecular electron acceptor or donor. Using density functional theory methods with dispersion corrections, we seek to monitor charge transfer and estimate its amount via calculations of experimental observables. With 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) as a p-dopant (electron acceptor) and an array of π-conjugated molecules as hole-transport materials (donors), the amount of charge transfer is seen to be a non-monotonic function of the offset defined by the donor ionization potential (IP) and the acceptor electron affinity (EA), IP – |EA|. Interestingly, a well-defined, linear relationship between the amount of charge transfer and IP – |EA| is obtained when the IP and EA values are adjusted to reflect intramolecular geometric changes in the final form of the complex. This study offers a straightforward way to match donor–acceptor pairs with desired doping effects and to estimate the resulting charge density in organic semiconductors.

Triplet Formation in Fullerene Multi‐Adduct Blends for Organic Solar Cells and Its Influence on Device Performance

AbstractIn organic solar cells, high open circuit voltages may be obtained by choosing materials with a high offset between the donor highest occupied molecular orbital (HOMO) and acceptor lowest unoccupied molecular orbital (LUMO). However, increasing this energy offset can also lead to photophysical processes that compete with charge separation. In this paper the formation of triplet states is addressed in blends of polyfluorene polymers with a series of PCBM multi‐adducts. Specifically, it is demonstrated that the formation of such triplets occurs when the offset energy between donor ionization potential and acceptor electron affinity is ∼1.6 eV or greater. Spectroscopic measurements support a mechanism of resonance energy transfer for triplet formation, influenced by the energy levels of the materials, but also demonstrate that the competition between processes at the donor–acceptor interface is strongly influenced by morphology.

Periodic Trends in Organic Functionalization of Group IV Semiconductor Surfaces

Organic functionalization of group IV semiconductor surfaces provides a means to precisely control the interfacial properties of some of the most technologically important electronic materials in use today. The 2 x 1 reconstructed group IV (100) surfaces in ultrahigh vacuum, in particular, have a well-defined surface that allows adsorbate-surface interactions to be studied in detail. Surface dimers containing a strong sigma- and weak pi-bond form upon reconstruction of the group IV (100) surfaces, imparting a rich surface reactivity, which allows useful analogies to be made between reactions at the surface and those in classic organic chemistry. To date, most studies have focused on single substrates and a limited number of adsorbate functional groups. In this Account, we bring together experimental and theoretical results from several studies to investigate broader trends in thermodynamics and kinetics of organic molecules reacted with group IV (100)-2 x 1 surfaces. By rationalizing these trends in terms of simple periodic properties, we aim to provide guidelines by which to understand the chemical origin of the observed trends and predict how related molecules or functionalities will react. Results of experimental and theoretical studies are used to show that relative electronegativities and orbital overlap correlate well with surface-adsorbate covalent bond strength, while orbital overlap together with donor electronegativity and acceptor electron affinity correlate with surface-adsorbate dative bond strength. Using such simple properties as predictive tools is limited, of course, but theoretical calculations fill in some of the gaps. The predictive power inherent in periodic trends may be put to use in designing molecules for applications where controlled attachment of organic molecules to semiconductor surfaces is needed. Organic functionalization may facilitate the semiconductor industry's transition from traditional silicon-based architectures to other materials, such as germanium, that offer better electrical properties. Potential applications also exist in other fields ranging from organic and molecular electronics, where control of interfacial properties may allow coupling of traditional semiconductor technology with such developing technologies, to biosensors and nanoscale lithography, where the functionality imparted to the surface may be used directly. Knowledge of thermodynamic and kinetic trends and the fundamental basis of these trends may enable effective development of new functionalization strategies for such applications.

Spectrophotometric study of the charge-transfer and ion-pair complexation of methamphetamine with some acceptors

The charge-transfer (CT) complexes of methamphetamine (MPA) as a n-donor with several acceptors including bromocresolgreen (BCG), bromocresolpurple (BCP), chlorophenolred (CPR), picric acid (PIC), and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) have been studied spectrophotometrically in chloroform solutions in order to obtain some information about their stoichiometry and stability of complexation. The oscillator strengths, transition dipole moments and resonance energy of the complex in the ground state for all complexes have been calculated. Vertical ionization potential of MPA and electron affinity of acceptors were determined by ab initio calculation. The acceptors were also used to utilize a simple and sensitive extraction-spectrophotometric method for the determination of MPA. The method is based on the formation of 1:1 ion-pair association complexes of MPA with BCG, BCP and PIC in chloroform medium. Beer's plots were obeyed in a general concentration range of 0.24–22 μg ml −1 for the investigated drug with different acceptors. The proposed methods were applied successfully for the determination of MAP in pure and abuse drug with good accuracy and precision.

Offset energies at organic semiconductor heterojunctions and their influence on the open-circuit voltage of thin-film solar cells

Organic semiconductor heterojunction (HJ) energy level offsets are modeled using a combination of Marcus theory for electron transfer, and generalized Shockley theory of the dark current density vs voltage $(J\text{\ensuremath{-}}V)$ characteristics. This model is used to fit the $J\text{\ensuremath{-}}V$ characteristics of several donor-acceptor combinations commonly used in thin film organic photovoltaic cells. In combination with measurements of the energetics of donor-acceptor junctions, the model predicts tradeoffs between the junction open-circuit voltage $({V}_{\mathrm{OC}})$ and short-circuit current density $({J}_{\mathrm{SC}})$. The ${V}_{\mathrm{OC}}$ is found to increase with light intensity and inversely with temperature for 14 donor-acceptor HJ materials pairs. In particular, we find that ${V}_{\mathrm{OC}}$ reaches a maximum at low temperature $(\ensuremath{\sim}175\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ for many of the heterojunctions studied. The maximum value of ${V}_{\mathrm{OC}}$ is a function of the difference between the donor ionization potential and acceptor electron affinity, minus the binding energy of the dissociated, geminate electron-hole pair: a general relationship that has implications on the charge transfer mechanism at organic heterojunctions. The fundamental understanding provided by this model leads us to infer that the maximum power conversion efficiency of double heterostructure organic photovoltaic cells can be as high as 12%. When combined with mixed layers to increase photocurrent and stacked cells to increase ${V}_{\mathrm{OC}}$, efficiencies approaching 16% are within reach.

Photoinduced Intramolecular Charge Separation at the Repetition Units of Light-Emitting Alternating Copolymers

Light-emitting nitrogen-containing poly- (phenylene vinylene) PPV-related copolymers are synthesized by the well-known Wittig-Horner reaction. The alternating copolymers have hole-transporting moieties such as triphenylamine (TPA) and conjugated aromatic units such as 1,4-phenylene, 1,4- or 1,5-naphthylene and 9,10-anthrylene. The dipole moments within the repetition units of the copolymers in the excited state are estimated by measuring the solvent effect on absorption and fluorescence emission spectra, indicating that charge separation is present. The dipole moment values are in agreement with the electron affinities of acceptors, i. e. aromatic units. The evidence can help to elucidate the photophysical behavior, particularly the fluorescence quantum efficiencies.

Photoinduced electron transfer reactions in mixed films of π-conjugated polymers and a homologous series of tetracyano-p-quinodimethane derivatives

Near-steady-state photoinduced absorption (PIA) and photoluminescence studies are presented on photoinduced electron transfer reactions from poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and poly[3-(2-(3-methylbutoxy)ethyl)thiophene] (P3MBET) as donors (D) onto a homologous series of tetracyano-p-quinodimethane (TCNQ) derivatives containing fused aromatic rings as acceptors (A) to systematically study the effect of acceptor electron affinity. We observe that composite films of these D/A couples give rise to long-lived charge separated states upon photoexcitation, as evidenced from the formation of polaron bands (radical cation absorption) in the PIA and the concomitant loss of the metastable triplet PIA and photoluminescence that are observed in pristine MEH-PPV and P3MBET. We find that the efficiency of the photoinduced electron transfer reaction correlates with the reduction potential of the acceptors.

The direct current electrical conductivity of the charge transfer complexes of some thiazoles and benzothiazoles with certain di- and trinitrobenzene derivatives

The DC electrical conductivity of 2-aminothiazole ( I), 2-aminobenzothiazole ( II), 2-amino-4-methylbenzothiazole ( III) and 2-amino-6-nitrobenzothiazole ( IV) and their charge transfer (CT) complexes with the acceptors, 1,3-dinitrobenzene ( 1), 3,4-dinitrobenzoic acid ( 2) and picric acid ( 3) were investigated. The positive temperature coefficients of the electrical conductivity of these thiazoles suggested their semiconducting characteristics. The CT complexes were classified into two main groups. The first group included the complexes I 1 , I 2 and IV 1,2,3 . All these complexes had negative temperature coefficients of resistance and revealed conventional semiconducting behaviour. Two different activation energies for the complexes I 2 , IV 2 and IV 3 indicated the presence of more than one conduction mechanism. The second group contained the complexes I 3 , II 1,2,3 . and III 1,2,3 . The thermal conductivity plots exhibited curves with two inflections and three regions. The slope of the line of a region and also its temperature range were found to depend upon the nature and structure of the complex. No correlation was found between the activation energies of the CT complexes and either the electron affinities of acceptors or the ionization potentials of donors. This was explained by assuming that the geometrical, and not the electronic, structure of the complexes was the determining factor.

Charge transfer complexes of some thiazoles and benzothiazoles with certain nitrobenzene derivatives

Abstract Interaction of some thiazole and benzothiazole derivatives as donors with certain di- and trinitrobenzene derivatives as acceptors results in the formation of 1:1 molecular species. The infrared, NMR and ultraviolet analysis of the complexes with non-acidic acceptors reveals the presence of π-π* interaction from a HOMO of the thiazole nucleus or the phenyl moiety of the benzothiazoles to a LUMO of the benzene ring of the acceptors. The existence of this type of interaction is supported by HMO calculations on the donor molecules. On the other hand, the molecular complexes derived from acidic acceptors are stabilized, in addition to the π-π* interaction, by proton transfer from the hydroxyl or carboxylic group of the acceptor to the amino group of the aminothiazole donors. The ionization potentials of donors, electron affinities of acceptors as well as the energy of the CT complexes were computed from their u.v. and visible spectra.

CHARGE TRANSFER COMPLEXES OF PHEOPHYTIN a WITH NITROAROMATICS. ELECTRON TRANSFER FROM EXCITED SINGLET OF PHEOPHYTIN a TO NITROAROMATICS

Abstract— The interaction of pheophytin a (Pheo) with seven nitroaromatic acceptors of varying ring sizes and electron acceptor abilities has been studied both in the ground and excited states. The ground state association constants (K) of the 1: 1 complexes of donor (Pheo) and acceptors were found to increase with increasing electron affinities of the different acceptors. All the nitroaromatic compounds efficiently quench the singlet emission of Pheo and the quenching follows the Stern‐Volmer (SV) relationship. The SV constants (Ksv) for different quenchers follow the same order as that of the K values. The reduction potential of Pheo+/Pheo* obtained from the quenching data agrees well with the theoretically predicted value. A charge transfer interaction between the singlet excited state of Pheo and the nitroaromatics is suggested from the dependence of quenching rate constants on the electron affinities of the acceptors.

Interaction of aliphatic amines with substituted quinones—a conductometric study

Interactions of aliphatic amines with chloranil and 2,3-dichloro-5,6-dicyano- p-benzoquinone have been examined by employing a conductometric technique. The stoichiometries, σ M and σ p values of the complexes are reported. The σ p values reflect electron affinity of acceptors with a fixed electron donor and the donor strength of donors with a fixed electron acceptor. σ M values do not exhibit a definite order. Effect of solvent and temperature on the σ p values of the complexes has been studied.

Difluorotetracyanoquinodimethane: electron affinity cut-off for ‘metallic’ behaviour in a tetrathiafulvalene salt

The synthesis and properties of 2,5-difluorotetracyanoquinodimethane and its tetrathiafulvalene salt whose electrical conductivity is limited by the enhanced acceptor electron affinity are described.