Effect Of Substitution Position Research Articles

Accessing chemically recyclable polyamides via geminal dimethyl substitution

The development of new chemically recyclable polymers could serve as a means of reducing the plastic pollution and alleviating global energy crisis and monomer design strategy has driven the innovation in this field. In this contribution, α-dimethyl substituted caprolactam (α-DMCL) and γ-dimethyl substituted caprolactam (γ-DMCL) were prepared to evaluate the substitution position effect on chemical recyclability and material performance in polycaprolactam (PCL) system. The introduction of geminal dimethyl substitution to ε-caprolactam (CL) at α and γ position could change the thermal and mechanical properties of the resulting polyamides P(α-DMCL) and P(γ-DMCL). Remarkably, thermal depolymerization of P(γ-DMCL) in presence of potassium caprolactamate (CL-K) could be carried out at 200 °C and converted to γ-DMCL in 91% yield. The recovered γ-DMCL was capable of repolymerization to P(γ-DMCL) without a decrease in reactivity, demonstrating the proof-of-concept recyclability of P(γ-DMCL).

Rational molecular design of efficient yellow-red dendrimer TADF for solution-processed OLEDs: a combined effect of substitution position and strength of the donors

AbstractThe development of high-performance solution-processed red organic light-emitting diodes (OLEDs) remains a challenge, particularly in terms of maintaining efficiency at high luminance. Here, we designed and synthesized four novel orange-red thermally activated delayed fluorescence (TADF) dendrimers that are solution-processable: 2GCzBP, 2DPACzBP, 2FBP2GCz and 2FBP2DPACz. We systematically investigated the effect of substitution position and strength of donors on the optoelectronic properties. The reverse intersystem crossing rate constant (kRISC) of the emitters having donors substituted at positions 11 and 12 of the dibenzo[a,c]phenazine (BP) is more than 10-times faster than that of compounds substituted having donors substituted at positions 3 and 6. Compound 2DPACzBP, containing stronger donors than 2GCzBP, exhibits a red-shifted emission and smaller singlet-triplet energy splitting, ΔEST, of 0.01 eV. The solution-processed OLED with 10 wt% 2DPACzBP doped in mCP emitted at 640 nm and showed a maximum external quantum efficiency (EQEmax) of 7.8%, which was effectively maintained out to a luminance of 1,000 cd m−2. Such a device∙s performance at relevant display luminance is among the highest for solution-processed red TADF OLEDs. The efficiency of the devices was improved significantly by using 4CzIPN as an assistant dopant in a hyperfluorescence (HF) configuration, where the 2DPACzBP HF device shows an EQEmax of 20.0% at λEL of 605 nm and remains high at 11.8% at a luminance of 1,000 cd m−2, which makes this device one of the highest efficiency orange-to-red HF SP-OLEDs to date.

Effect of alkoxy substitution position of phenyl side chains on the photovoltaic performance of nonfused acceptors

Side chain configuration plays a significant role in intermolecular interaction and aggregation morphology of organic small molecules. In this work, by altering the position of the hexyloxy substitution on the phenyl side chains, three non-fused acceptors were designed and synthesized, e.g. o-2T2Se-F with 2,3-di(hexoxy)phenyl substituents, m-2T2Se-F with 2,4-di(hexoxy)phenyl substituents and p-2T2Se-F with 2,5-di(hexoxy)phenyl substituents. In comparison with its isomeric counterpart 2T2Se-F with 2,6-di(hexoxy)phenyl substituents, these three acceptors exhibit twisted backbones, which result in blue-shifted absorption and weak intermolecular interaction and molecular aggregation. When paired with donor PM6, all three blend films display weak crystallinity, leading to poor photovoltaic performance. The devices based on o-2T2Se-F, m-2T2Se-F and p-2T2Se-F obtained power conversion efficiencies (PCEs) of 4.43%, 1.77%, and 1.30%, respectively, which are significantly lower than that of 2T2Se-F-based devices (12.17%). The results reveal that manipulating the alkoxy-chain attaching point can significantly alter the molecular geometry, optoelectronic properties and molecular aggregation properties, thereby affecting the photovoltaic performance of OSCs.

Tailoring the Position of Ester Group on N-Alkyl Chains of Benzotriazole-based Small Molecule Acceptors for High-Performance Organic Solar Cells.

Side chain engineering plays a vital role in exploring high-performance small molecule acceptors (SMAs) for organic solar cells (OSCs). In this work, we designed and synthesized a series of A-DA'D-A type SMAs by introducing different N-substituted alkyl and ester alkyl side chains on benzotriazole (BZ) central unit and aimed to investigate the effect of different ester substitution positions on photovoltaic performances. All the new SMAs with ester groups exhibit lower the lowest unoccupied molecular orbital (LUMO) energy levels and more blue-shifted absorption, but relatively higher absorption coefficients than alkyl chain counterpart. After blending with the donor PM6, the ester side chain-based devices demonstrate enhanced charge mobility, reduced amorphous intermixing domain size and long-lived charge transfer state compared to the alkyl chain counterpart, which are beneficial to achieve higher short-circuit current density (Jsc ) and fill factor (FF), simultaneously. Thereinto, the PM6 : BZ-E31 based device achieves a higher power conversion efficiency (PCE) of 18.33 %, which is the highest PCE among the OSCs based on the SMAs with BZ-core. Our work demonstrated the strategy of ester substituted side chain is a feasible and effective approach to develop more efficient SMAs for OSCs.

Tailoring the Position of Ester Group on N‐Alkyl Chains of Benzotriazole‐based Small Molecule Acceptors for High‐Performance Organic Solar Cells

Side chain engineering plays a vital role in exploring high‐performance small molecule acceptors (SMAs) for organic solar cells (OSCs). In this work, we designed and synthesized a series of A‐DA’D‐A type SMAs by introducing different N‐substituted alkyl and ester alkyl side chains on benzotriazole (BZ) central unit and aimed to investigate the effect of different ester substitution positions on photovoltaic performances. All the new SMAs with ester groups exhibit lower the lowest unoccupied molecular orbital (LUMO) energy levels and more blue‐shifted absorption, but relatively higher absorption coefficients than alkyl chain counterpart. After blending with the donor PM6, the ester side chain‐based devices demonstrate enhanced charge mobility, reduced amorphous intermixing domain size and long‐lived charge transfer state compared to the alkyl chain counterpart, which are beneficial to achieve higher short‐circuit current density (JSC) and fill factor (FF), simultaneously. Thereinto, the PM6:BZ‐E31 based device achieves a higher power conversion efficiency (PCE) of 18.33%, which is the highest PCE among the OSCs based on the SMAs with BZ‐core. Our work demonstrated the strategy of ester substituted side chain is a feasible and effective approach to develop more efficient SMAs for OSCs.

Effect of Number and Position of Chlorine Atoms on the Photovoltaic Performance of Asymmetric Nonfullerene Acceptors.

It has been well proved that the introduction of halogen can effectively modify the optoelectronic properties of classic symmetric nonfullerene acceptors (NFAs). However, the relevant studies for asymmetric NFAs are limited, especially the effect of halogen substitution number and position on the photovoltaic performance is not clear. In this work, four asymmetric NFAs with A-D-A1-A2 structure are developed by tuning the number and position of chlorine atoms on the 1,1-dicyanomethylene-3-indanone end groups, namely, A303, A304, A305, and A306. The related NFAs show progressively deeper energy levels and red-shifted absorption spectra as the degree of chlorination increases. The PM6:A306-constructed organic solar cells (OSCs) give a champion power conversion efficiency (PCE) of 13.03%. This is mainly ascribed to the most efficient exciton dissociation and collection, suppressed charge recombination, and optimal morphology. Moreover, by alternating the substitution position, the PM6:A305-based device yielded a higher PCE of 12.53% than that of PM6:A304 (12.05%). This work offers fresh insights into establishing excellent asymmetric NFAs for OSCs.

Ring-opening polymerisation of alkyl-substituted ε-caprolactones: kinetic effects of substitution position

Alkyl-functional polycaprolactones were prepared via direct and well-controlled ring-opening polymerisation and the relationship between the substituent position and polymerisation rate was investigated.

Effects of substitution position with different charge characteristics groups on mechanofluorochromic behaviors of the pyridine-based difluoroboron compounds

Two donor-acceptor type pyridine-based difluoroboron compounds (3F-Py-TPA BF2 and TPA-Py-3F BF2) were designed and successfully synthesized, which including triphenylamine unit acted as the electron donor and difluoroboron moiety and 3,4,5-trifluorophenyl group served as the electron acceptors. Combined with the results of the photophysical properties, it was found that compound 3F-Py-TPA BF2 with 4-position of pyridine skeleton substituted by 3,4,5-trifluorophenyl unit and triphenylamine unit location at the end of the bridged phenyl displayed more excellent intramolecular charge transfer character and solvatochromic effect. Moreover, the solid emission spectrum of 3F-Py-TPA BF2 was red-shifted from 550 to 594 nm, that of TPA-Py-3F BF2 was red-shifted from 526 to 546 nm under external mechanical stimulation, which caused by phase transformation between the ordered crystalline and disordered amorphous states according to the powder X-ray diffraction measurements. Based on the results of theoretical calculations and X-ray single-crystal diffraction analysis, we speculated that TPA-Py-3F BF2 with 4-position of pyridine skeleton substituted by triphenylamine unit and 3,4,5-trifluorophenyl unit location at the end of the bridged phenyl had higher molecular polarity and too loose molecular packing mode, resulting in the mechanofluorochromic behavior was worser than that of 3F-Py-TPA BF2. This work presented a strategy for regulating mechanofluorochromic behaviors of the pyridine-based difluoroboron molecular materials.

Investigation of the Substitution Site Effect on o-Carborane-Based Chromophores by Anthracene Introduction at the B(3) Position

Abstract o-Carborane-based emitters have been greatly developed over the last ten years. From the viewpoint of molecular symmetry, o-carborane has one type of equivalent carbon and four types of equivalent boron atoms. However, in contrast to the vast research of aryl-modified o-carboranes on the carbon atom of o-carborane, substitution on the boron atom is less investigated. Herein, we introduced an anthracene unit on the B(3) position of o-carborane and explored the substitution position effect by comparison with the previously reported C(1)-substituted regioisomer. Single-crystal structures, optical measurements, and quantum chemical calculation revealed the significant impact of the substitution position, especially in the intramolecular charge transfer (ICT) process.

Effect of substitution position of carbazole based conjugated polymers on the photocatalytic hydrogen evolution activities of conjugated polymer/g-C3N4 heterojunction catalysts

Conjugated polymers/Graphitic carbon nitride (g-C3N4) based heterojunctions have been widely investigated in photocatalytic hydrogen Production (PHP). How to design high-performance conjugated polymers matched with g-C3N4 is the critical issue. Here, two linear D-A type conjugated polymers (BSO-S27 and BSO-S36) were synthesized by Suzuki coupling reaction between dibenzothiophene-5,5-dioxide and N-methylcarbazole units. The substitution sites of N-methylcarbazole are 2,7- positions for BSO-S27 and 3,6- positions for BSO-S36, respectively. The different substitution position leads to that BSO-S27 and BSO-S36 exhibit different optical absorption, energy level, and charge transport ability. Through the composition with g-C3N4 nanosheets (CN), BSO-S27/CN and BSO-S36/CN heterojunctions were formed. PHP results demonstrate that both BSO-S27 and BSO-S27/CN exhibit higher hydrogen evolution rate (HER) than BSO-S36 and BSO-S36/CN photocatalysts (BSO-S27: 1783.5 μmol g−1 h−1; BSO-S36: 1112.5 μmol g−1 h−1; BSO-S27/CN: 23078.2 μmol g−1 h−1; BSO-S36/CN: 15732.6 μmol g−1 h−1). And the hydrogen evolution mechanism of BSO-S27/CN and BSO-S36/CN are discussed deeply. These results demonstrate that regulating substitution position is a feasible method to design high-performance conjugated polymers or polymer heterojunctions as photocatalysts.

Effect of substitution position of dibenzofuran-terminated robust hole-transporters on physical properties and TADF OLED performances

A four dibenzofuran-terminated hole-transporter, T1DBFBP, realizes high-efficiency and highly stable green TADF OLEDs with an EQE of over 20% and a long operation lifetime LT50of 30 000 h at 1000 cd m−2.

Substitution Effect on Self-Assembly and the Resulting Fluorescence Efficiency of Triangular Azo Chromophores

To investigate the effect of substituents and substitution positions on absorption and fluorescence spectroscopic properties, and self-assembled structures, we designed triangular azo chromophores (1-3) in which three azo groups are connected to a central ring. 1 and 2 where electron-donating methoxy (MeO) groups are attached to para and meta positions, respectively, had different melting temperatures and colors. A dilute para-MeO (1) solution was redder than 3 with electron-withdrawing bromo (Br) groups and liquefied at lower temperatures. The elongated structure formed as a result of the one-dimensional (1D) assembly of 1 showed little fluorescence, whereas the chopstick-shaped structure and the entangled fiber assembled from 2 and 3, respectively, were brightly red fluorescent. Our investigation suggest that the molecular geometry and steric effects play important roles in self-assembly and the resulting fluorescence characteristics.

Structurally Regulated Carbazole-Pyridine Derivatives Based on Space-Crowded Theory for Efficient Narrowband Ultraviolet Nondoped Organic Light-Emitting Diodes from the High-Lying Reverse Intersystem Crossing Process.

Achieving ultraviolet and narrowband emission simultaneously in nondoped organic light-emitting diodes (OLEDs) remains a tremendous challenge. Here, a "space-crowded donor-acceptor-donor" molecular design strategy is proposed for developing ultraviolet pure organic fluorophores by the nearby substituted positions at the phenyl linker between carbazole and pyridine units. Benefitting from the large steric hindrance effect, multiple intramolecular interactions, and low-frequency vibronic coupling dominated excited state property, all the emitters exhibit excellent fluorescence efficiencies at the solid state as well as the narrow full width at half maximums (FWHMs). Moreover, the effect of different substitution positions of pyridine on the structure-property relationship is also revealed. Consequently, the nondoped OLEDs exhibit an electroluminescence emission peak of 397 nm with FWHMs of 17 and 22 nm. Due to the high-lying reverse intersystem crossing process, external quantum and exciton utilization efficiencies of 3.6 and 54.55%, respectively, have been achieved based on the emitter with para-linkage. These findings may pave an avenue for the development of high-performance narrowband ultraviolet materials and OLEDs.

Different positions of cyano substitution controlled directionality of ESIPT processes with two asymmetric proton acceptors system: A TD-DFT study

In order to systematically investigate the effect of substitution position on excited state intramolecular proton transfer (ESIPT) process and photophysical properties, three 3-(benzo[d]thiazol-2-yl)-2-hydroxy-5-methoxy-benzaldehyde (BTHMB) derivatives (BTHMB-1, BTHMB-2 and BTHMB-3) were designed by introducing the strong electron-withdrawing group (–CN) at different positions. Based on the analyses of configurations, electron densities and infrared vibrational frequencies, the intramolecular hydrogen bond (IHB) of BTHMB derivatives are all enhanced in the excited state, which would promote the ESIPT process. All the BTHMB-1, BTHMB-2 and BTHMB-3 compounds can undergo ESIPT process via two possible pathways. The potential energy curves (PECs) showed that the ESIPT barriers decrease along the order of BTHMB-2-O > BTHMB-3-O > BTHMB-1-O, and BTHMB-3-N > BTHMB-2-N > BTHMB-1-N. Due to the lower energy barrier, the ESIPT process in BTHMB-2 tends to happen to the direction of proton acceptor O atom, while the ESIPT processes in BTHMB-1 and BTHMB-3 tend to happen to the direction of proton acceptor N atom. No matter where the substituted position was, the introduction of –CN group resulted in the obvious red-shift of absorption and fluorescence peaks. For BTHMB-1-O/BTHMB-2-O/BTHMB-3-O and BTHMB-1-N/ BTHMB-2-N/BTHMB-3-N, the red-shift values of absorption and fluorescence peaks in BTHMB-1-O and BTHMB-3-N are the largest, respectively. The more red-shift the absorption and emission peaks show, the smaller the S0 and S1 energy gap between HOMO and LUMO are.

Substitution position effects of an electron-withdrawing group on the tautomer fluorescence of Coumarin–urea derivatives with an acetate anion

To improve the fluorescence properties of coumarin–urea derivatives having a phenyl urea group bound at the 7-position of the coumarin ring (7CU derivatives), the substitution position effects of the electron-withdrawing group, CF3, on the photochemical behaviors of 7CU derivatives were investigated in the absence and presence of an acetate anion (AcO−). Both o- and m-CF3-7CU were prepared besides p-CF3-7CU, all of which bear a CF3 group at different positions on a phenyl ring opposite to the coumarin ring (o-, m-, and p-isomers, respectively). These three CF3–7CU derivatives underwent excited-state intermolecular proton transfer (ESPT) in the presence of AcO− followed by emissive tautomer formation, resulting in dual fluorescence. To elucidate the fluorescence state of tautomer, we compared the spectroscopic results of CF3–7CU derivatives including the rate constants for ESPT with regard to the inductive and resonance effects. The results revealed that the emission yield of tautomer of o-CF3-7CU was an order of magnitude larger than those of both m- and p-CF3-7CU due to the effective inductive effect of a CF3 group; this observation indicates that o-CF3-7CU is a suitable anion sensor. Our findings will contribute to the development of new molecular designs for fluorescent probes with excellent visibility.

Nonfullerene acceptors with cyano-modified terminal groups for organic solar cells: Effect of substitution position on photovoltaic properties

Two kinds of nonfullerene acceptors based on cyano-modified 1,1-dicyanomethylene-3-indanone (IC) as end groups and the dithienothiophen[3,2-b]-pyrrolobenzothiadiazole (BTP) unit as a core are developed. Both electron acceptors exhibit strong intramolecular charge transfer absorption with the maximum absorption peak at 832 nm, and the absorption edge extends to 953 nm, corresponding to the narrow bandgap down to 1.30 eV. Comparing with the electron acceptor with cyano groups at mixed γ/δ position, the acceptor with cyano substituted at the β position shows more twisted structure and slightly higher energy level. When a wide bandgap polymer PM6 is adopted as the donor, the blend film with β-cyano substituted acceptor shows more suitable phase separation morphology and more balanced charge transport. The device based on the β-cyano substituted acceptor achieves a larger open-circuit voltage (VOC) of 0.88 V than the acceptor with cyano groups at mixed γ/δ position (VOC = 0.78 V), and the energy loss was calculated to be 0.45 eV, leading to a higher power conversion efficiency (PCE) of 12.41% comparing with the devices with the γ/δ-cyano substituted acceptor (PCE = 11.08%).

Monomeric carbazolylcyanobenzenes as thermally activated delayed fluorescence emitters: effect of substitution position on photoluminescent and electroluminescent properties

In this study, two new donor-acceptor type thermally activated delayed fluorescence materials using carbazole as a donor and cyano substituted benzene as an acceptor were synthesized. Structural parameters, molecular orbital densities, photophysical characteristics were determined. By changing the substitution position of the cyano groups on the phenyl rings, the relationship between structure and properties was also be disclosed. These materials were tested as emissive species for the fabrication of organic light-emitting diodes. The blue devices showed EL emission peaks at 464 and 488 nm with the CIE coordinates of (0.17, 0.18) and (0.21, 0.31), respectively.

The effect of alkyl substitution position of thienyl outer side chains on photovoltaic performance of A–DA′D–A type acceptors

Two isomeric A–DA′D–A type SMAs o-TEH and m-TEH were designed and synthesized, and the PCE of the OSC based on PBQ6:m-TEH reached 18.51%.

Halogen bond-directed self-assembly in bicomponent blends at the solid/liquid interface: effect of the alkyl chain substitution position.

The fabrication of well-organised molecular assemblies on surfaces is fundamental for the creation of functional molecular systems applicable to nanoelectronic and molecular devices. In this study, we investigated the effect of substitution positions of alkyl chains on the formation of halogen-bonded molecular networks. For this purpose, building blocks with different head groups (i.e., pyridine (Py) or tetrafluoro-iodobenzene (FI)) were substituted with hexadecyloxy chains at either the 3,4- or the 3,5-positions. The two-dimensional assembly of each compound as a single-component system was studied using scanning tunnelling microscopy (STM) at the highly oriented pyrolytic graphite (HOPG)/1-phenyloctane interface. All compounds displayed linear structures in which the alkyl chains were aligned along one of the HOPG axes. In the exceptional case of FI bearing hexadecyloxy chains at the 3,5-positions (denoted as FI-3,5), hexagonal arrays were tentatively formed owing to the triangular molecular arrangement induced by halogen bonding. A bicomponent blend of Py-3,4/FI-3,5 (1 : 1 molar ratio) enabled the formation of a honeycomb structure, whereas that of Py-3,5/FI-3,4 (1 : 1 molar ratio) produced a rectangular assembly that was periodically arranged in a zig-zag fashion. Finally, based on the observed blend ratio dependence, the formation of these different two-dimensional structures by variation in the substitution positions of the alkyl chains was discussed in terms of molecule-molecule and molecule-substrate interactions.

Isomer engineering of dipyrido[3,2-a:3′,4′-c]phenazine-acceptor-based red thermally activated delayed fluorescent emitters

Study of the donor substitution position effect on the dipyrido[3,2-a:2′,3′-c]phenazine acceptor based red thermally activated delayed fluorescent emitters was carried out using to isomers.