Molecular Coplanarity Research Articles

Regulating Molecular Structure of S,N‐Heteroacene Non‐fullerene Acceptors to Achieve Efficient Photoelectric Properties†

Comprehensive SummaryThe ternary strategy has demonstrated its efficacy in improving charge transport in organic solar cells (OSCs). Here, three novel non‐fullerene acceptors, SN6C9‐4F, SN6C9‐4Cl and SN6C10‐4F, based on S,N‐heteroacene linear backbone were designed and synthesized. The three acceptors exhibit excellent molecular coplanarity, high crystallinity and possess a deep‐lying lowest unoccupied molecular orbital energy level, which is beneficial for charge transport and injection in organic field‐effect transistors (OFETs). Notably, the OFET devices based on all three acceptors achieved impressive electron mobilities, with SN6C10‐4F achieving up to 0.73 cm2·V–1·s–1, which is one of the highest values among A‐D‐A type small molecules. In addition, the OSCs device based on PBDB‐T:SN6C9‐4F exhibited the best power conversion efficiency of 12.07% owing to its optimal morphology and enhanced charge transport. Moreover, the incorporation of SN6C9‐4F into the efficient PM6:L8‐BO binary system to form ternary OSCs resulted in extended absorption range, enhanced donor crystallization, improved and more balanced charge transport, ultimately leading to an improvement of PCE from 17.78% to 18.32%. This study highlights the potential of developing acceptors with distinct structures from Y‐series acceptors to broaden absorption and regulate donor crystallization, providing a novel approach to enhance the PCE of OSCs.

D-π-A type fluorescent dyes: Effect of π-bridge units on optical and G4 DNA binding properties

Fluorescent solvatochromic dyes that are sensitive to the nature of local microenvironmental, have been explored as probes in applications ranging from the imaging biomolecules to understanding of basic biomolecule functions. To expand the scope of fluorescent solvatochromic dyes for G-quadruplex (G4) DNA structures, and to illustrate the relationship between structure and properties, three newly designed D-π-A type fluorescent dyes were synthesized by introducing diarylimidazole to carbazole skeleton linked to benzene, furan or thiophene π-conjugated bridge and connected with pyridinium acceptor, respectively. Their structural characteristics, optical properties, and G4 DNA binding properties were discussed in detail. In general, the incorporation of furan and thiophene as π-conjugated bridges leads the better conjugation and molecular coplanarity with more efficient intramolecular charge transfer (ICT) effect compared with benzene bridge. The fluorescence intensities induced upon interaction were found that TP-6 with thiophene π-conjugated bridge had the strongest response toward G4 DNAs. In addition, the application of this dye as a fluorescent agent for living cell imaging was also demonstrated.

Theoretical perspective for structural isomerism effect on photoelectric properties of organic room temperature phosphorescence molecules

Purely organic room temperature phosphorescence (RTP) materials have garnered extensive attentions in anti-counterfeiting and encryption, information display, biological imaging and organic light emitting diodes (OLEDs) due to the features of long lifetimes, low toxicity and good biocompatibility. Nevertheless, both the species and amounts of the efficient RTP molecules are far from meeting the requirements for practical applications, and the quantitative relationship between molecular structures and optoelectronic properties needs full elucidated. Herein, the photophysical properties and properties of three isomers (o-BA, m-BA and p-BA) are explored based on the first-principles calculations coupled with thermal vibration correlation function (TVCF) method and kinetic Monte Carlo simulations. The substituent effects on the molecular structures, intermolecular interactions, transition properties, Huang-Rhys factors and reorganization energies, excited state dynamics as well as transfer integrals and charge carrier mobilities are investigated in detail to clarify the structure-property relationships. Results show that o-BA achieves fast radiative rate on account of impressive transition dipole moment and oscillator strength, but equally large non-radiative decay rate leads to low RTP efficiency. Promisingly, the highest holes mobility is determined for o-BA due to the largest transfer integrals of holes originating from the strong intermolecular interactions. In addition, compared with o-BA and p-BA, m-BA could achieve fast intersystem crossing (ISC) rate with large spin orbit coupling constant and low non-radiative decay rate with small geometric structure changes. Relatively balanced holes and electrons mobilities of m-BA have been demonstrated, which contributes to the recombination of excitons and the improvement of luminescence efficiency. For p-BA, the strong intermolecular hydrogen bonding restricts the free rotation of single bonds and promotes the molecular coplanarity, which contributes to efficient emission. Furthermore, the charge mobility is calculated and the temperature dependence is investigated, the hopping mechanisms for studied isomers are illustrated. This work reveals the relationship among molecular structures, crystal packing modes, photophysical properties and charge transport properties of isomers, and provides an insight for molecular design and property prediction.

Terpolymerization and Regioisomerization Strategy to Construct Efficient Terpolymer Donors Enabling High-Performance Organic Solar Cells.

For the first time, we combined terpolymerization and regioisomerization strategies to develop novel polymer donors to overcome the difficulty of improving organic solar cells (OSCs) performance. Two novel isomeric units, TTO and TTI, were obtained and incorporated into the PM6 backbone via random copolymerization to form a series of terpolymers. Interestingly, we found that different chlorine (Cl) substituent positions can significantly change the molecular planarity and electrostatic potential (ESP) owing to the steric hindrance effect of the heavy Cl atom, which leads to different molecular aggregation behaviors and miscibility between the donor and acceptor. The TTO unit features a higher number of multiple S⋅⋅⋅O non-covalent interactions, more positive ESP, and fewer isomer structures than TTI. As a result, the terpolymer PM6-TTO-10 exhibited a much better molecular coplanarity, stronger crystallinity, more obvious aggregation behavior, and proper phase separation in the blend film, which are conducive to more efficient exciton dissociation and charge transfer. Consequently, the PM6-TTO-10:BTP-eC9-based OSCs achieved a champion power conversion efficiency (PCE) of 18.37% with an outstanding fill factor of 79.97%, which are among the highest values reported for terpolymer-based OSCs. This work demonstrates that terpolymerization combined with Cl regioisomerization is an efficient approach for achieving high-performance polymer donors. This article is protected by copyright. All rights reserved.

Acyclic 2-Ethynylnaphthalene-Modified 8-Aza-3,7-dideaza-2′-deoxyadenosine Allows Thymine Discrimination by Probing the DNA Minor Groove Environment

AbstractTwo novel acyclic environment-sensitive fluorescent nucleosides named ac37zA and an37zA are synthesized and their photophysical properties are investigated. Both ac37zA and an37zA exhibit dual fluorescence emission depending upon molecular coplanarity. Oligodeoxynucleotide probes containing ac37zA clearly discriminate perfectly matched thymine in target DNA strands by strong charge transfer (CT) emission in the longer wavelength region and by strong quenching of emission from the twisted conformation in the mismatched case.

Synthesis, Crystal Structures, and Molecular Properties of Three Nitro-Substituted Chalcones

Three functionalized chalcones containing combinations of nitro functional groups have been synthesized via Claisen-Schmidt condensation between 2-nitroacetophenone and nitrobenzaldehyde, and the crystal structures obtained ((E)-1,3-bis(2-nitrophenyl)prop-2-en-1-one, 1a, (E)-1-(2-nitrophenyl)-3-(3-nitrophenyl)prop-2-en-1-one, 1b and (E)-1-(2-nitrophenyl)-3-(4-nitrophenyl)prop-2-en-1-one, 1c), C15H10N2O5, are reported. Compounds 1a and 1c crystallized in the triclinic centrosymmetric space group P1¯, whereas compound 1b crystallized in the orthorhombic space group Pbca. The X-ray analysis reveals that structures 1a and 1b exhibits s-trans conformation, whereas structure 1c exists in s-cis conformation, concerning the olefinic double bonds. In addition, the results show that the position of the nitro substituent attached to the aromatic B-ring has a direct effect on the molecular coplanarity of these compounds. The Hirshfeld surface analysis suggests that the non-covalent π-π stacking interactions are the most important contributors for the crystal packing of 1a and 1b. In 1c, the crystal packing is mainly stabilized by weak intermolecular C―H···O interactions due to the planar nature of the molecule.

Comparison of high-performance polyimide copolymer fibers containing pyrimidine moieties based on coplanar structures

The effects of molecular coplanar structure on crystallinity, orientation, and overall performances of polyimide (PI) copolymer fibers, especially mechanical properties, were investigated. In detail, three diamines with similar structures, including 4,4′-diaminobenzophenone (DABP), 4,4′-diaminobenzanilide (DABA), and benzidine (Bz), were introduced as third monomers into the classical heterocyclic 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA)/2,5-bis(4-aminophenyl)pyrimidine (PRM) system, eventually the high-performance PI fibers being fabricated successfully via dry-jet wet spinning process. The conformation simulations illustrated the different coplanar levels found in three similar diamines: DABP (worst), DABA (better), and Bz (best). Thus, the BPDA/Bz dimer showed excellent coplanar structures, and the dihedral angle of the farthest benzene rings was only 4.42°. The wide-angle X-ray diffraction patterns indicated that the crystallinity and orientation of fibers increased with the increase of the coplanarity degree and molecular packing. Interestingly, the Bz-PI fiber possessed the strongest mechanical properties, i.e., fracture strength of 4.01 GPa and initial modulus of 137.34 GPa, which was primarily contributed by optimal molecular coplanarity and highest orientation.

Fluorination on electron-deficient units of benzothiadiazole-based donor-acceptor conjugated polymers for novel fullerene-based organic solar cells

Three new donor-acceptor (D-A) conjugated polymers based on benzo[1,2-b:4,5-b′]dithiophene (BDT) and benzo[c][1,2,5]thiadiazole (BT), namely, PB-BT, PB-BTf and PB-BTPf, were synthesized with different substituents (Hydrogen atoms, Fluorine atoms and fluorophenyl groups) on BT unit. Experimental results and theoretical calculations indicate that molecularly tuning of side chains on BT moeity simultaneously influences the energy levels and intermolecular packing of the resulted polymers by modulating their electron affinity and molecular coplanarity. The polymer solar cells (PSCs) based on a blend of PB-BTf/[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) exhibits the best photovoltaic performance among the three copolymers, with a open-circuit voltage (Voc) of 0.79 V and a power conversion efficiency (PCE) of 8.43%. Furthermore, polymer PB-BTPf containing fluorophenyl groups shows a higher Voc of 0.99 V due to its more low-lying highest occupied molecular orbital (HOMO) compared with other two structural polymeric analogues. The results here provide further fundamental insights into the relationship between the fluorination on electron-withdrawing moiety and the photovoltaic performance for the conjugated polymers applied in fullerene organic solar cells.

Triphenylamine dyes bearing 5-phenyl-2-(arylthiophen-2-yl)thiazole bridge for dye sensitized solar cells

Three triphenylamine dyes were synthesized with 5-phenyl-2-(arylthiophen-2-yl)thiazole as π bridge and cyanoacrylic acid as the acceptor. Three dyes differ in the aryl ring binding with the acceptor in the π bridge, which leads to different photophysical properties, molecular planarity, energy level and accordingly the photoelectric conversion efficiency. The insertion of furan and thiophene units in π bridge attain broader absorption spectra and better molecular coplanarity, contributing to the distinct improvement of JSC and hence the photovoltaic performance. On the other hand, the presence of benzene unit raises the LUMO level and increases the drive force for the electron injection into TiO2, which enhance VOC. Among three dyes, dye bearing two thiophene units sensitized DSSC achieves a cell efficiency 6.90% due to better JSC (14.09 mA cm2) and VOC (0.73 V).

Effect of aromatic π-bridges on molecular structures and optoelectronic properties of A-π-D-π-A small molecular acceptors based on indacenodithiophene

Investigation on the relationship between molecular structure and device performance is of great important to develop highly efficient A-π-D-π-A small molecular acceptors (SMAs). However, there is still lack of a complete and in-depth study on effects of π-bridge on molecular structure, optoelectronic properties and photovoltaic performances. Herein, we reported the design, synthesis and photovoltaic application of four A-π-D-π-A type SMAs, denoted as IDT-Py-IC , IDT-Fu-IC , IDT-Th-IC , and IDT-Ph-IC , which possess an identical central D unit of indacenodithiophene and the terminal A group of 3-(dicyanomethylidene)indol-1-one, linked by various aromatic π-bridges of pyrrole, furan, thiophene, and benzene, respectively. The impact of the different aromatic π-bridge on molecular structures, optoelectronic and photovoltaic properties as well as active layer morphologies was comprehensively explored. Results show that both molecular co-planarity and electron-donating ability of aromatic π-bridges distinctly affect optical bandgaps ( E g opt ) and HOMO/LUMO levels of these SMAs. The poor backbone planarity of pyrrole-bridged IDT-Py-IC observed by theory calculation leads to a blue-shifted absorption and up-shifted HOMO/LUMO levels. The E g opt of these SMAs is gradually increased and HOMO levels are gradually down-shifted with the decrease of the electron-donating ability of aromatic π-bridges. Polymer solar cells (PSCs) based on these SMAs exhibit a high V oc over 0.93 V, especially for PBDB-T: IDT-Py-IC -based PSCs, producing a rather high V oc up to 1.06 V due to the high-lying LUMO level. After optimizations, the PBDB-T: IDT-Th-IC -based PSC outperforms the other three SMAs with a high PCE up to 8.72% mainly due to the large J sc and FF, which could be ascribed to better absorption characteristics, higher and more proportional carrier mobility, efficient exciton dissociation and charge collection, reduced bimolecular recombination and superior active layer morphology. This finding demonstrates that the π-bridge plays a crucial role in tailoring molecular structures, optoelectronic properties and device performance of A-π-D-π-A type SMAs. • Four A-π-D-π-A SMAs with the same D and A groups, but various π-bridge were developed. • Effect of aromatic π-bridges on structures and optoelectronic properties of SMAs was studied. • Pyrrole-bridged IDT-Py-IC showed a blue-shifted absorption and a high V oc over 1.0 V. • IDT-Th-IC -based PSCs outperformed other three SMAs, along with a high PCE up to 8.72%.

The effect of intramolecular hydrogen bond on the ultraviolet absorption of bi‐aryl Schiff bases

AbstractTwo kinds of Schiff bases, 85 samples of N‐(benzylidene)‐anilines (ZBAY) and 83 samples of N‐(phenyl‐ethylene)‐anilines (ZAPEY), were used as model compounds, in which the ZBAY contains 13 compounds with 2‐OH and the ZAPEY contains 35 compounds with 2‐OH (synthesized by this work). The quantitative correlation analysis ultraviolet absorption spectra of ZBAY and ZAPEY were performed, and the effect of intramolecular hydrogen bond on their wave number vmax (cm−1) of the maximum absorption wavelength λmax (nm) was investigated. The results show that (a) the factors affecting the vmax of ZBAY and ZAPEY are roughly the same, but their intensities are different. (b) The vmax move caused by intramolecular hydrogen bond is all red shift for both ZBAY and ZAPEY, but the red shift value (2,381) of the ZBAY is more than twice than that (850) of ZAPEY. (c) The effect of intramolecular hydrogen bond on vmax is only dominated by the parent structure unit of Schiff base, rather than the substituents in the molecule. For those compounds with the same parent structure unit, their red shift values of vmax are at fixed value, but their red shift values of λmax are unequal. Generally, the red shift value caused by the intramolecular hydrogen bond is larger for the compound with a larger λmax value. The above observed phenomena are discussed from the view of molecular coplanarity.

A2-D-A1-D-A2-type small molecule acceptors incorporated with electron-deficient core for non-fullerene organic solar cells

Two A2-D-A1-D-A2-type small molecule acceptors (SMAs), DFB-dIDT and BT-dIDT with 2,5-difluorobenzene (DFB) or benzothiadiazole (BT) as electron-withdrawing core (A1) and a derivative of indanone as A2 units, were prepared for applications in organic solar cells (OSCs). The results indicate that BT unit is more beneficial to forming multiple noncovalent conformational locks of N⋯S and N⋯H between BT and IDT unit than DFB in the core, so BT-dIDT showed better molecular coplanarity, higher-lying HOMO energy level, more red-shifted spectrum, superior molar absorption coefficient (1.60 × 105 M−1 cm−1 at 696 nm), more complementary absorption spectrum with PBDB-T and better photovoltaic performance than DFB-dIDT. As a result, the BT-dIDT-based OSCs blending with PBDB-T exhibited higher power conversion efficiency (PCE) value of 10.52% with higher Jsc of 18.59 mA cm−2 than that of the DFB-dIDT-based devices (PCE of 6.71% with Jsc of 15.58 mA cm−2). These results demonstrate that the A2-D-A1-D-A2-type SMAs incorporated with a suitable electron-deficient core are promising candidates for high performance OSCs.

Development of s-tetrazine-based polymers for efficient polymer solar cells by controlling appropriate molecular aggregation

Two conjugated polymers PTTTz and PHTTz have been designed and synthesized by introducing s-tetrazine as electron-withdrawing moieties. Compared with PTTTz with 2-octyldodecyl-substituted tetrathiophene derivative as electron-donating moieties, PHTTz with hexyl- and 2-octyldodecyl-substituted hexathiophene derivative possesses relatively lower molecular coplanarity, larger stacking distance but more orderly molecular stacking, more red-shifted absorption, more suitable microphase separation, more efficient exciton dissociation, higher hole mobility, which lead to higher short-circuit current density (11.54 mA cm−2), fill factor (69.0%) and power conversion efficiency (PCE = 6.69%) values in PSCs. The PTTTz-based PSC shows an open circuit voltage of 1.02 V for its lower-ling HOMO energy level though the PCE of the PSC is only 5.16%. Moreover, the ternary PSC based PHTTz/PC71BM/ITIC exhibits a higher PCE value of 7.88% than the binary PSCs for more efficient exciton dissociation and complementary absorption spectrum in ternary blend film, which is one of the highest PCE values in s-tetrazine-based polymers.

Benzothienoisoindigo-based polymers for efficient polymer solar cells with an open-circuit voltage of 0.96 V

Two conjugated polymers have been designed and synthesized by introducing benzothienoisoindigo as electron-withdrawing moieties on side chains. PBDTT-TBID based on alkylthienyl-substituted benzo[1,2-b:4,5-b’]dithiophene (BDT) derivative as electron-donating moieties possesses a low-lying highest occupied molecular orbital energy level of −5.38 eV, so its polymer solar cells (PSCs) shows a high open-circuit voltage (Voc) of 0.96 V, which is one of the highest Voc values among benzothienoisoindigo-based polymers. PBDT-TBID based on alkoxyl-substituted BDT derivative possesses better molecular coplanarity, stronger intramolecular charge transfer, stronger intermolecular π-π stacking, more suitable microphase separation with PC71BM, more effective exciton dissociation, and higher hole mobility than PBDTT-TBID, which lead to higher short-circuit current density (Jsc = 11.81 mA cm−2), fill factor (FF = 0.58) and power conversion efficiency (PCE = 5.87%) values than those based on PBDTT-TBID. The PCE value is still higher than those of the reported benzothienoisoindigo-based polymers though it lags behind those of recent non-fullerene PSCs.

Di-2-(2-oxindolin-3-ylidene)malononitrile Derivatives for N-Type Air-Stable Organic Field-Effect Transistors

The nitrogenization of phenyl rings on DIM derivatives not only enhances molecular coplanarity but also stabilizes molecular LUMO levels, favoring charge transfer and improving air stability. Therefore, n-type organic field-effect transistors (OFETs) that are based on DIM-N2C8 with nitrogen atoms on both sides of the phenyl rings exhibit a moderate electron mobility of 0.059 cm2 V-1 s-1 under ambient conditions.

Molecular Engineering of Efficient Organic Sensitizers Containing Diyne-Bridge for Dye-Sensitized Solar Cells

Diyne unit containing organic dyes coded as FSD105 and FSD106 were designed and synthesized to investigate their light harvesting capabilities and the DSSC performances. Through tuning the geometrical structures of dye molecules by introducing an extra ethynylene, FSD105 and FSD106 showed an obviously improved coplanar geometry, and thus the significantly bathochromic shifted absorption spectra, IPCE spectra and larger Γ values, leading to the enhancement of Jsc which guaranteed the highest η (3.82%) for FSD106-based DSSC. In order to avoid the dye aggregation, CHCl3/EtOH was adopted as the sensitizing solvents instead of CHCl3 only or CDCA as coadsorbent was added to reduce the Γ value of FSD106-based DSSC. As a result, the Jsc and the η value increased significantly. Finally, FSD106/CDCA-based DSSC showed the highest η value (4.76%), which is 1.53-fold as the previous reported FSD101-based DSSC (3.12%) by introducing an extra ethynylene to improve the molecular coplanarity.

Strain-released method to enhance the photovoltaic performance in solution-processed organic solar cells

To release the steric-strain in largely fused molecular backbone and achieve the much planar molecules for efficient organic solar cells, a facile method was proposed by dividing the rigidly fused dibenzosexithiophene (DBST) units in FT3 with large molecular skeletons to two smaller building blocks to construct the electron donor-acceptor type compound FT4. As observed, much improved molecular co-planarity was achieved for FT4 in contrast to that of FT3, then leading to the higher charge carrier mobilities up to 6.7 × 10−4 cm2V−1s−1 by SCLC method. Meanwhile, FT4-based BHJ solar cells also exhibited much enhanced photovoltaic performance with the maximum PCE value of 3.07% and higher Jsc value of 8.23 mA/cm2, which is only 2.13% and 5.67 mA/cm2 for FT3-based solar cells. The results here indicated that the strain-released method, especially in the highly fused molecules, is beneficial to further improve their co-planarity, then leading to the high charge carrier mobilities and much enhanced photovoltaic performance in solar cells.

Tuning the solid-state fluorescence of chalcone crystals via molecular coplanarity and J-aggregate formation

Eleven crystals of chalcone were obtained. Polymorphism and their solid-state fluorescent properties mainly affected by molecular coplanarity and J-aggregate formation are investigated.

Molecular Coplanarity and Self-Assembly Promoted by Intramolecular Hydrogen Bonds.

Active conformational control is realized in a conjugated system using intramolecular hydrogen bonds to achieve tailored molecular, supramolecular, and solid-state properties. The hydrogen bonding functionalities are fused to the backbone and precisely preorganized to enforce a fully coplanar conformation of the π-system, leading to short π-π stacking distances, controllable molecular self-assembly, and solid-state growth of one-dimensional nano-/microfibers. This investigation demonstrates the efficiency and significance of an intramolecular noncovalent approach in promoting conformational control and self-assembly of organic molecules.

Diketopyrrolopyrrole based highly crystalline conjugated molecules for application in small molecule donor-polymer acceptor nonfullerene organic solar cells

In order to specifically investigate the low efficiency of small molecule donor-polymer acceptor (M-P) nonfullerene organic solar cells, we have successfully modify the synthesis of a series of D-π-A-π-D conjugated molecules containing diketopyrrolopyrrole (DPP) and different end groups. By incorporation of end group with different size of π-conjugation (benzene, naphthalene and pyrene), we further improved the fill factor (FF) and short current density (Jsc) of the donors molecule. Our experimental results and theoretical calculations have proven that the size of the end groups can influence the molecule crystallinity, mobility and intermolecular packing by altering the molecular coplanarity. As the result of improved crystallinity, morphology and fine-tuned mobilities, we demonstrated an increased FF, a high Jsc of ∼4.5 mA/cm2 and a power conversion efficiency of 2.05%, which is among the highest efficiency reported for M-P nonfullerene solar cells. Our results provide opportunities and possibilities of achieving higher performance M-P nonfullerene solar cells in the future.