Acceptor Strength Research Articles

Effect of acceptor strength on optical, electrochemical and photovoltaic properties of phenothiazine-based small molecule for bulk heterojunction organic solar cells

Two new acceptor–π(donor)–donor–π(donor)-acceptor small molecules SM1 and SM2 with benzodithiophene (BDT) donor core unit linked via phenothiazine (PTZ) donor units with 1,3–indanedione and malononitrile end capping units, respectively, have been designed and synthesized and used as electron donor along with PC71BM as acceptor for solution-processed bulk heterojunction organic solar cells. The influence of these end-capping groups on their thermal, optical, electrochemical properties and photovoltaic performance was investigated. After the optimization of the donor to acceptor weight ratio and solvent vapor annealing, the organic solar cells based on SM1:PC71BM and SM2:PC71BM active layers showed the power conversion efficiency (PCE) of 6.20% and 7.45%, respectively. Hence, the end-capping acceptor units for the two small molecules revealed a significant influence on the PCE of the organic solar cells, which resulted from differences in film absorption, molecular packing and charge transport properties.

Prediction of the equivalent alkane carbon number (EACN) of aprotic polar oils with COSMO-RS sigma-moments

The Equivalent Alkane Carbon Numbers (EACNs) of 16 aprotic polar hydrocarbon oils are measured by the fish-tail method. These values, supplemented by 54 already reported EACNs of other hydrocarbon oils are used, as a training set, to build up a QSPR model, based on COSMO-RS σ-moments as oil descriptors. It provides a reliable model requiring only three meaningful molecular parameters i.e. the surface area (M0oil), the overall polarity (M2oil) and the hydrogen bond acceptor strength (Maccoil) of the oil. The robustness of this predictive model is verified with a validation set including various polar oils such as esters, ketones, acetates or nitriles. Furthermore the model is also valid to predict EACN values of weakly polar oils, such as, (cyclo)alkenes, terpenes, aromatics, alkynes and chloroalkanes, as well as bifunctional compounds, namely ethyl oleate, terpenic acetates, dialkyldiethers and hexylmethacrylate.

Effect of acceptor strength in new acceptor–donor–acceptor-type molecules on their miscibility with donor polymers for bulk-heterojunction fullerene-free solar cells

We designed and synthesized two kinds of spirobifluorene (SP)-based acceptor–donor–acceptor (A–D–A)-type acceptor materials, denoted by SP-IC and SP-TCF, which have an A–D–A structure consisting of a 3D tetrathienospirobifluorene donor core and 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC) or 2-(3-cyano-4,5,5-trimethylfuran-2(5H)-ylidene)malononitrile (TCF) acceptors as terminal units, respectively. Their LUMO levels (−3.91 eV for SP-IC and −3.97 eV for SP-TCF) are suitable as n-type molecules for PTB7-Th-based polymer solar cells. However, their morphologies in blend films with a conjugated donor polymer, Poly([2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene]{3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7-Th), exhibited considerable differences. The PTB7-Th:SP-IC blend film exhibited a better-defined nanophase-separated morphology than PTB7-Th:SP-TCF blend film. The corresponding bulk heterojunction polymer solar cell based on PTB7-Th:SP-IC showed four times higher power conversion efficiency (PCE = 4.50%) than that of PTB7-Th:SP-TCF. The higher PCE was well explained by investigating the morphology, which was affected by the surface energy of the acceptor molecules at the donor–acceptor interface.

Unsymmetrical and Symmetrical Push-Pull Phenothiazines.

A series of unsymmetrical and symmetrical push-pull phenothiazines (3-7) were designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction and subsequent [2 + 2] cycloaddition-retroelectrocyclization reaction with tetracyanoethylene (TCNE) and 7,7,8,8-tetracyanoquinodimethane (TCNQ). The effect of systematic variation of the number and nature of cyano-based acceptor TCNE and TCNQ units on the photophysical, electrochemical, and computational studies was investigated. The single-photon absorption on phenothiazines 3-7 reveals that substitution of 1,1,4,4-tetracyanobutadiene (TCBD) and a cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD unit results in strong intramolecular charge transfer and lowering of the LUMO energy level. The TCBD-linked and cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD-linked phenothiazines 3-7 exhibit multiredox waves. The computational studies on phenothiazines 3-7 exhibit substantial stabilization of the LUMO with the increase in acceptor strength, which results in lowering of the HOMO-LUMO gap.

45‐2: Invited Paper: Highly Efficient Acridine‐based TADF Emitters

Organic molecules with efficient thermally activated delayed fluorescence (TADF) are emerging as attractive emitters in OLEDs. In this talk, some of our recent works on acridine‐triazine hybrid TADF materials will be highlighted. One of this series TADF materials exhibits nearly 100% photoluminescence quantum yield, excellent thermal stability, and a horizontal dipole ratio of 83%, leading to extremely efficient blue organic EL with external quantum efficiency (EQE) of ~37%. In addition, efficient and tunable blue‐green to yellow TADF emitters capable of generating OLED EQEs of >31% are developed adopting acridine as donor unit and CN‐substituted benzene, pyridine and pyrimidine as acceptor units are reported. These materials permit one to systematically probe the influence of different acceptor strengths and also the influence of tunable conformations (twist angles) within the acceptor moieties through controlling the orientation of asymmetric heteroaromatic ring relative to the donor component.

Mediating Charge Shift Reactions in Electron Transport Chains

In Nature, energy and electron transfer processes in the photosynthetic machinery following light absorption have been perfectionated. To mimic the aforementioned, that is, efficiently utilizing light to separate charges and, in turn, to generate solar fuels, a great variety of multicomponent artificial photosynthetic systems have been designed and synthesized. Incentives for the design build around molecules have been taken from Nature: For example, implementing suitable electrochemical gradients across arrays of several molecular building blocks have enabled the unidirectional transformation of solar energy into electrical energy. A key parameter is the right balance between the reduction / oxidation strengths of the individual building blocks and their spatial arrangements. Both affect the thermodynamic driving forces for each step and, in turn, the overall efficiency. Notably, acceptor-donor1-donor2 or acceptor-donor1-donor2-donor3 types of artificial photosynthetic systems have been reported. Besides an initial charge transfer, charge shifts take place via oxidative electron transfers from donor1 to donor2 and, eventually, to donor3. Considering the effectiveness of multicomponent systems, in which photoexcitation triggers a cascade of short range electron transfers, we report on two sets of novel multicomponent DA1A2s – Figure 2. These DA1A2 types feature electron transport chains with C60 and C70 of different electron acceptor strengths to create an electrochemical gradient. As a complement to electron accepting C60 / C70 either zinc porphyrins (ZnP) and zinc phthalocyanines (ZnPc) were employed. The motivation for our pump-probe investigations was to establish the role of ZnPc or ZnP as light harvester and electron donor to power unidirectional (reductive) charge shift reactions along a variety of electron transport chains. In summary, we will demonstrate the control over unidirectional charge shift along electron transport chains en route towards highly energetic, long-lived charge separated states. Key is, however, a charge shift mediating state, whose relative energy determines the overall efficiency. If the energy of the mediating state, on one hand, is higher than the charge separated state, resonance conditions enable efficiencies as high as 50%. If the energy, on the other hand, is lower than the charge separated state, deactivation to the ground state limits the efficiency to 10%.

Systematic Tuning of 2,1,3-Benzothiadiazole Acceptor Strength by Monofunctionalization with Alkylamine, Thioalkyl, or Alkoxy Groups in Carbazole Donor–Acceptor Polymers

A simple route to the preparation of alkylamine, thioalkyl, and alkoxy monofunctionalized 4,7-di(thiophen-2-yl)-2,1,3-benzothiadiazole) based monomers is reported from a common fluorinated precursor. Copolymerization with a carbazole comonomer under Suzuki conditions yielded a series of analogous donor–acceptor copolymers in which the only difference was the nature of the heteroatom (N, O, or S) on the benzothiadiazole core. This was shown to have a significant impact on the wavelength and intensity of the intramolecular charge transfer (ICT) absorption peak due to a combination of electronic and steric factors. Substantial differences were also observed in the solar cell performance of blends with PC71BM, with the octylamino substituted polymer exhibiting significantly lower performance than the other two polymers. This polymer also exhibited a reversible change in the optical spectra upon exposure to acid, suggesting potential as a sensing material.

Role of the Trichlorostannyl Ligand in Tin–Ruthenium Arene Complexes: Experimental and Computational Studies

A set of neutral and ionic ruthenium arene trichlorostannyl complexes are reported herein. The tin(II) compounds L1SnCl {1; L1 = [2‐(CH2NEt2)‐4,6‐(tBu)2C6H2]–} and [L2SnCl][SnCl3] {2; L2 = 2,6‐[(CH3)C=N(C6H3‐2,6‐iPr2)2]C5H3N} showed a rather different reactivity towards the ruthenium complex [(η6‐cymene)RuCl]2(η‐Cl)2. As a consequence, the neutral complex [Ru(η6‐cymene)(L1SnCl)Cl2] (4) and the ionic compound [L2SnCl][Ru(η6‐cymene)(SnCl3)2Cl] (8) were isolated. The insertion reaction of 4 with SnCl2 provided the neutral trimetallic ruthenium complex [Ru(η6‐cymene)(L1SnCl)(SnCl3)Cl] (6). Analogous ruthenium complexes [Ru(η6‐cymene)(L3PPh2)Cl2] (5) and [Ru(η6‐cymene)(L3PPh2)(SnCl3)Cl] (7) containing the phosphane ligand L3PPh2 {3; L3 = [2,6‐iPr2‐(C6H3)NH]–} were also prepared to evaluate the donor–acceptor strength of the tin(II)‐ and phosphorus‐containing ligands. The structural characterization and DFT calculations of the above‐mentioned complexes suggest a strong influence of the [SnCl3]– moiety on the Ru–E interaction (E = Sn, P). The influence of the trichlorostannyl ligand on the Ru–E interaction in the complexes 4–7 was further evaluated by means of a distortion/interaction analysis.

Excellent Long-Term Stability of Power Conversion Efficiency in Non-Fullerene-Based Polymer Solar Cells Bearing Tricyanovinylene-Functionalized n-Type Small Molecules.

New small molecules having modified acceptor strength and π-conjugation length and containing dicyanovinylene (DCV) and tricyanovinylene (TCV) as a strongly electron-accepting unit with indacenodithiophene, IDT(DCV)2, IDT(TCV)2, and IDTT(TCV)2, were synthesized and studied in terms of their applicability to polymer solar cells with PTB7-Th as an electron-donating polymer. Intriguingly, the blended films containing IDT(TCV)2 and IDTT(TCV)2 exhibited superior shelf life stabilities of more than 1000 h without any reduction in the initial power conversion efficiency. The low-lying lowest unoccupied molecular orbital energy levels and robust internal morphologies of small TCV-containing molecules could afford excellent shelf life stability.

Simulation study on the effects of chemical structure and molecular size on the acceptor strength in poly(3-hexylthiophene)-based copolymer with alternating donor and acceptor for photovoltaic applications

This research assessed the effects of various chemical structures and molecular sizes on the simulated geometric parameters, electron structures, and spectroscopic properties of single-chain complex alternating donor–acceptor (D–A) monomers and copolymers that are intended for use as photoactive layer in a polymer solar cell by using Kohn–Sham density functional theory with B3LYP exchange-correlation functional. The 3-hexylthiophene (3HT) was selected for electron donor, while eight chemicals, namely thiazole (Z), thiadiazole (D), thienopyrazine (TP), thienothiadiazole (TD), benzothiadiazole (BT), thiadiazolothieno-pyrazine (TPD), oxadiazole (OXD) and 5-diphenyl-1,2,4-triazole (TAZ), were employed as electron acceptor functional groups. The torsional angle, bridge bond length, intramolecular charge transfer, energy levels, and molecular orbitals were analyzed. The simulation results reveal that the geometry and electron structure of donor–acceptor monomer and copolymer are significantly impacted by heterocyclic rings, heteroatoms, fused rings, degree of steric hindrance and coplanarity of the acceptor molecular structure. Planar conformation was obtained from the D copolymer, and a pseudo-planar structure with the TD copolymer. The TAZ acceptor exhibited strong steric hindrance due to its bulky structure and non-planarity of its structure. An analysis of the electron structures indicated that the degree of intramolecular electron-withdrawing capability had the rank order TAZ < Z < D < TPD < OXD < TP < BT < TD. The TD is indicated as the most effective acceptor among those that were simulated. However, the small energy gaps of TD as well as TPD copolymer indicate that these two copolymers can be used in transparent conducting materials. The copolymer based on BT acceptor exhibited good intramolecular charge transfer and absorbed at 656 nm wavelength which is close to the maximum flux of solar spectrum. Hence, the BT acceptor functional group provides a compromise in the characteristics of a donor–acceptor copolymer, useful in a polymeric candidate material for the photoactive layer in a polymer solar cell.

An insight into the communication between β-olefin/phenyl olefin-mediated acceptors and porphyrin π-system: a way to establish porphyrin based chemodosimeters and chemosensors.

The synthesis of three new classes of mixed β-pyrrole substituted tetraphenylporphyrins, MTPP(R), MTPP(Br)2(R) (where R = -CH[double bond, length as m-dash]C(CN)2, -CH[double bond, length as m-dash]C(CN)(COOC2H5), -CH[double bond, length as m-dash](CN)(COOH) and M = 2H, Ni(ii), Cu(ii)) and MTPP(NO2)(Ph-p-R')2 (R' = -CHO and -CH[double bond, length as m-dash]C(CN)2 and M = 2H, Ni(ii), Cu(ii)) have been reported. The crystal structures of CuTPP(Br)2-ECA (7a), NiTPP(NO2)(Ph-p-CHO)2 (9) and NiTPP(NO2)(Ph-p-CH[double bond, length as m-dash](CN)2)2 (10) are highly nonplanar among β-trisubstituted porphyrins reported to date as evidenced from the mean displacement of β-pyrrole carbon (ΔCβ) in the range ±(0.39-0.674) Å. The olefin mediated ethyl cyanoacetate is in plane with the porphyrin core in CuTPP(Br)2-ECA (the dihedral angle relative to the pyrrole NC4-mean plane is 39.81°) while dicyanovinyl stays aside for NiTPP(NO2)(Ph-p-CH[double bond, length as m-dash](CN)2)2 (dihedral angles relative to the pyrrole NC4-mean plane are 75.03° and 67.47°). NiTPP-MN (2), NiTPP-ECA (3), NiTPP(Br)2-MN (6) and NiTPP(Br)2-ECA (7) act as chemodosimeters for toxic CN- ions whereas NiTPP-CAA (4) and NiTPP(Br)2-CAA (8) act as chemosensors to detect toxic ions such as CN-, F- and OAc- depending on the acceptor strength and an obstacle in the conjugation pathway. The dicyanovinyl group in the phenyl olefinic-mediated porphyrin NiTPP(NO2)(Ph-p-CH[double bond, length as m-dash](CN)2)2 (10) also acts as chemodosimeter for CN- ions but no vivid changes are observed via different spectroscopic methods.

Understanding the colorimetric properties of quinoxaline-based pi-conjugated copolymers by tuning their acceptor strength: a joint theoretical and experimental approach

We report the synthesis and characterization of donor–acceptor π-conjugated copolymers leading to blue-to-green coloration according to acceptor strength.

Charge-transfer states in triazole linked donor-acceptor materials: strong effects of chemical modification and solvation.

A series of 1,2,3-triazole linked donor-acceptor chromophores are prepared by Click Chemistry from ene-yne starting materials. The effects of three distinct chemical variations are investigated: enhancing the acceptor strength through oxidation of the sulphur atom, alteration of the double bond configuration, and variation of the triazole substitution pattern. A detailed photophysical characterization shows that these alterations have a negligible effect on the absorption while dramatically altering the emission wavelengths. In addition, strong solvatochromism is found leading to significant red shifts in the case of polar solvents. The experimental findings are rationalized and related to the electronic structure properties of the chromophores by time-dependent density functional theory as well as the ab initio algebraic diagrammatic construction method for the polarization propagator in connection with a new formalism allowing to model the influence of solvation onto long-lived excited states and their emission energies. These calculations highlight the varying degree of intramolecular charge transfer character present for the different molecules and show that the amount of charge transfer is strongly modulated by the conducted chemical modifications, by the solvation of the chromophores, and by the structural relaxation in the excited state. It is, furthermore, shown that enhanced charge separation, as induced by chemical modification or solvation, reduces the singlet-triplet gaps and that two of the investigated molecules possess sufficiently low gaps to be considered as candidates for thermally activated delayed fluorescence.

Phospholipid bilayer affinities and solvation characteristics by electrokinetic chromatography with a nanodisc pseudostationary phase.

Phospholipid bilayer nanodiscs composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine and synthetic maleic acid-styrene copolymer belts have been introduced as a pseudostationary phase (PSP) in electrokinetic chromatography and demonstrated good performance. The nanodiscs provide a suitable migration range and high theoretical plate counts. Using this nanodisc pseudostationary phase, the affinity of the bilayer structure for probe solutes was determined and characterized. Good correlation is observed between retention factors and octanol water partition coefficients for particular categories of solutes, but the general correlation is weak primarily because the nanodiscs show stronger affinity than octanol for hydrogen bond donors. This suggests that a more appropriate application of this technology is to measure and characterize interactions between solutes and lipid bilayers directly. Linear solvation energy relationship analysis of the nanodisc-solute interactions in this study demonstrates that the nanodiscs provide a solvation environment with low cohesivity and weak hydrogen bond donating ability, and provide relatively strong hydrogen bond acceptor strength.

Substituent Effects on Drug-Receptor H-bond Interactions: Correlations Useful for the Design of Kinase Inhibitors.

Investigation of troponin I-interacting kinase (TNNI3K) as a potential target for the treatment of heart failure has produced a series of substituted N-methyl-3-(pyrimidin-4-ylamino)benzenesulfonamide inhibitors that display excellent potency and selectivity against a broad spectrum of protein kinases. Crystal structures of prototypical members bound to the ATP-binding site of TNNI3K reveal two anchoring hydrogen bond contacts: (1) from the hinge region amide N-H to the pyrimidine nitrogen and (2) from the sulfonamide N-H to the gatekeeper threonine. Evaluation of various para-substituted benzenesulfonamides defined a substituent effect on binding affinity resulting from modulation of the sulfonamide H-bond donor strength. An opposite electronic effect emerged for the hinge NH-pyrimidine H-bond interaction, which is further illuminated in the correlation of calculated H-bond acceptor strength and TNNI3K affinity for a variety of hinge binding heterocycles. These fundamental correlations on drug-receptor H-bond interactions may be generally useful tools for the optimization of potency and selectivity in the design of kinase inhibitors.

Structure-Property Relationships in Click-Derived Donor-Triazole-Acceptor Materials.

To shed light on intramolecular charge-transfer phenomena in 1,2,3-triazole-linked materials, a series of 1,2,3-triazole-linked push-pull chromophores were prepared and studied experimentally and computationally. Investigated modifications include variation of donor and/or acceptor strength and linker moiety as well as regioisomers. Photophysical characterization of intramolecular charge-transfer features revealed ambipolar behavior of the triazole linker, depending on the substitution position. Furthermore, non-centrosymmetric materials were subjected to second-harmonic generation measurements, which revealed the high nonlinear optical activity of this class of materials.

Antimony(V) as Acceptors

Lewis acidity of antimony(V) pentahalides and their organo substituted derivatives is well known. SbX5 acts as a strong acceptor towards halide ion as compared to antimony trihalide due to presence of two more electronegative halogen atoms in antimony pentahalide makes it better acceptor. Hexacoordinated organoantimony(V) complexes including chelating ligands are also known and hexacoordination and heptacoordination environment have been suggested for such compounds. The substitution of hydrocarbon groups bound to antimony by trifluoromethyl or pentafluorophenyl groups enhances the Lewis acidity of antimony facilitating the formation of neutral adducts. Such properties have mainly been investigated with organoantimoy(V) halides. As expected successive replacement of an organic group by halogen atom increases acceptor strength of antimony(V) compounds.Academic Voices Vol.5 2015: 34-39

Molecular Design Principles for Near-Infrared Absorbing and Emitting Indolizine Dyes.

Desirable components for dye-sensitzed solar cell (DSC) sensitizers and fluorescent imaging dyes include strong donating building blocks coupled with well-balanced acceptor functionalities for absorption beyond the visible range. We have evaluated the effects of increasing acceptor strengths and incorporation of dye morphology controlling groups on molar absorptivity and absorption breadth with indolizine donor-based dyes. Indolizine-based D-A and D-π-A sensitizers incorporating bis-rhodanine, tricyanofuran (TCF), and cyanoacrylic acid functionalities were analyzed for performance in DSC devices. The TCF derivatives were also evaluated as near-infrared (NIR)-emissive materials with the AH25 emissions extending past 1000 nm.

Efficient and Tunable Thermally Activated Delayed Fluorescence Emitters Having Orientation‐Adjustable CN‐Substituted Pyridine and Pyrimidine Acceptor Units

A series of twisted D–π–A type emitters based on the acridine donor unit and CN‐substituted pyridine, pyrimidine, and benzene acceptor units are studied. They not only allow one to systematically probe the influence of different acceptor strengths, but also permit one to intriguingly probe the influence of tunable conformations (twist angles) within the acceptor moieties through controlling the orientation of asymmetric heteroaromatic ring relative to the donor component. Intramolecular charge‐transfer transitions are observed in all these compounds and emission wavelengths are widely tunable from deep blue to yellow not only by the general acceptor strength due to the characters of heteroarene and CN‐substitution pattern but also by the subtle control of in‐acceptor conformation (twist angles). Small triplet‐to‐singlet energy gaps (ΔEST) and significant thermally activated delayed fluorescence (TADF) characteristics are obtained in a series of D–π–A compounds with sufficient acceptor strengths and tunable in‐acceptor conformation, yielding a series of efficient blue‐green to yellow TADF emitters with promisingly high photoluminescence quantum yields of 90%–100%. Highly efficient blue‐green to yellow TADF organic light‐emitting diodes (OLEDs) having external quantum efficiencies of up to 23.1%–31.3% are achieved using these efficient TADF emitters, which are among the most efficient TADF OLEDs ever reported.

Modulating the strength of hydrogen bond acceptors to achieve low Caco2 efflux for oral bioavailability of PARP inhibitors blocking centrosome clustering

During the lead generation and optimization of PARP inhibitors blocking centrosome clustering, it was discovered that increasing hydrogen bond acceptor (HBA) strength improved cellular potency but led to elevated Caco2 and MDR1 efflux and thus poor oral bioavailability. Conversely, compounds with lower efflux had reduced potency. The project team was able to improve the bioavailability by reducing efflux through systematic modifications to the strength of the HBA by changing the electronic properties of neighboring groups, whilst maintaining sufficient acceptor strength for potency. Additionally, it was observed that enantiomers with different potency showed similar efflux, which is consistent with the promiscuity of efflux transporters. Eventually, a balance between potency and low efflux was achieved for a set of lead compounds with good bioavailability which allowed the project to progress towards establishing in vivo pharmacokinetic/pharmacodynamic relationships.