From Perylene Dimide Polymers to Fused-Ring Electron Acceptors:A 15-Year Exploration Journey of Nonfullerene Acceptors

Fullerene derivatives are classic electron acceptor materials for organic solar cells (Oscs) but possess some intrinsic drawbacks such as weak visiblelightabsorption,limitedoptoelectronic property tunability,dificult purification and photochemical/morphological instability.Fullereneacceptors area bottleneck restricting further development of this field. Ourgroup pioneered the exploration of novel nonfulerene acceptors in China in 2006,andinitiated the research of two representative acceptor systems, rylene dimide polymer and fused-ring electron acceptor (FREA).FREA breaks the theoreticalefficiencylimit of fullerene-based OsCs (-13%) and promotes the whole field to an unprecedented prosperity with efficiency of 20%, heraldinga nonfullerene era for OsCs.In this review, we revisit 15-year nonfullerene exploration journey,summarize the design principles,molecular engineeringstrategies, physical mechanisms and device applications of these two nonfullerene acceptor systems, and propose some possible researchtopics in the nearfuture.

Enhancing Photovoltaic Performance of Asymmetric Fused-Ring Electron Acceptor by Expanding Pyrrole to Pyrrolo[3,2-b]pyrrole

We propose a strategy to improve performance of unidirectionally extended fused-ring electron acceptors by using pyrrolo[3,2-b]pyrrole to replace pyrrole ring, and design two asymmetric nonfullerene acceptors 1PIC and 2PIC. Replacing pyrrole in 1PIC with pyrrolo[3,2-b]pyrrole remarkably red-shifts absorption peak by 109 nm, elevates the HOMO and LUMO energy levels, and improves electron mobility. The photovoltaic devices based on blend of PM6 donor and 2PIC acceptor exhibit power conversion efficiency as high as 12.6%, which is much higher than that of PM6:1PIC (3.53%), due to more efficient exciton generation and dissociation, faster and more balanced carrier transport and less charge recombination.

Low-cost polymer acceptors with noncovalently fused-ring backbones for efficient all-polymer solar cells

The polymerization of fused-ring acceptors(FRAs) to afford their corresponding polymeric acceptors for high-performance all-polymer solar cells(all-PSCs) has achieved remarkable progress in the past few years.However,due to the high degree of synthetic complexity for the monomer,the high-cost of these polymeric acceptors may limit their commercial applications.Thus,it is urgent to develop inexpensive and high-performance polymeric acceptors for all-PSCs.Herein,two novel polymeric acceptors(PBTzO and PBTzO-2F) have been designed and synthesized by copolymerization of noncovalently fused ring acceptors(NFRAs),which were employed in all-PSCs for the first time.Upon introducing the “noncovalently conformational locks(NoCLs)” in the backbone and selective fluorination of the end-group,photophysical and electrical properties,and solidstate packing properties of the NFRAs have been rationally tuned.As a result,the PBDB-T:PBTzO-2F based devices presented an excellent power conversion efficiency(PCE) of 11.04%,much higher than that of PBTzO based ones due to the increased charge generation and extraction,improved hole transfer and carrier mobilities,and reduced energy loss.More importantly,PBTzO-2F exhibited a much lower synthetic complexity(SC) index and higher figure-of-merit(FOM) values than the high-performance fused-ring acceptor based polymer acceptors(FRA-PAs) due to the simpler structures and more effective synthesis.This contribution provided a novel idea to achieve low-cost and high-performance all-PSCs.

High brightness NIR-Ⅱ nanofluorophores based on fused-ring acceptor molecules

It is challenging to develop molecular fluorophores in the second near-infrared(NIR-Ⅱ)window with long wavelength emission and high brightness,which can improve the performance of biological imaging.Herein,we report a molecular engineering approach to afford NIR-Ⅱ fluorophores with these merits based on fused-ring acceptor(FRA)molecules.Dioctyl 3,4-propylenedioxy thiophene(PDOT-C8)is utilized as the bridging donor to replace 3-ethylhexyloxy thiophene(3-EHOT),leading to more than 20 times enhancement of brightness.The nanofluorophores(NFs)based on the optimized CPTIC-4F molecule exhibit an emission peak of 1,110 nm with a fluorescence quantum yield(QY)of 0.39%(QY of IR-26 is 0.050%in dichloroethane as reference)and peak absorption coefficient of 14.5 x 10^4 M^-1·cm^-1 in aqueous solutions,which are significantly higher than those of 3-EHOT based COTIC-4F NFs.It is found that PDOT-C8 can weaken intermolecular aggregation,enhance protection of molecular backbone from water,and decrease backbone distortion,beneficial for the high brightness.Compared with indocyanine green with same injection dose,CPTIC-4F NFs show 10 times higher signal-to-background ratio for whole body vessels imaging at 1,300 nm long pass filters.

Recent progress in low-cost noncovalently fused-ring electron acceptors for organic solar cells

The power conversion efficiencies(PCEs)of organic solar cells(OSCs)have improved considerably in recent years with the development of fused-ring electron acceptors(FREAs).Currently,FREAs-based OSCs have achieved high PCEs of over 19%in single-junction OSCs.Whereas the relatively high synthetic complexity and the low yield of FREAs typically result in high production costs,hindering the commercial application of OSCs.In contrast,noncovalently fused-ring electron acceptors(NFREAs)can compensate for the shortcomings of FREAs and facilitate large-scale industrial production by virtue of the simple structure,facile synthesis,high yield,low cost,and reasonable efficiency.At present,OSCs based on NFREAs have exceeded the PCEs of 15%and are expected to reach comparable efficiency as FREAs-based OSCs.Here,recent advances in NFREAs in this review provide insight into improving the performance of OSCs.In particular,this paper focuses on the effect of the chemical structures of NFREAs on the molecule conformation,aggregation,and packing mode.Various molecular design strategies,such as core,side-chain,and terminal group engineering,are presented.In addition,some novel polymer acceptors based on NFREAs for all-polymer OSCs are also introduced.In the end,the paper provides an outlook on developing efficient,stable,and low-cost NFREAs for achieving commercial applications.

Enhancing photovoltaic performance via aggregation dynamics control in fused-ring electron acceptor

A new fused-ring electron acceptor FNIC3 with dynamics controlled aggregation behavior was synthesized.FNIC3 shows strong absorption in 600–900 nm,HOMO/LUMO energy levels of−5.59/−4.04 eV,and electron mobility of 1.2×10^(−3) cm^(2) V^(−1) s^(−1).The aggregation of FNIC3 shows strong dependency on film formation time.Prolongation of film formation time promotes the crystallization of FNIC3,leading to improved crystallinity and enlarged aggregate sizes.Aggregation of FNIC3 significantly influences the photovoltaic device parameters.Appropriate aggregation red-shifts the absorption and improves the mobilities of the blend,which contributes to high photocurrent and fill factor thus high power conversion efficiency(PCE).Overaggregation leads to increased nonradiative energy loss and insufficient charge generation,resulting in decreased open-circuit voltage and short-circuit current density.The blends based on PM6:FNIC3 fabricated under proper film formation time exhibit a PCE of 12.3%,higher than those fabricated under short and long film formation time(10.0–10.5%).

Effects of Thieno[3,2-b]thiophene Number on Narrow-Bandgap Fused-Ring Electron Acceptors

We synthesize and compare four near-infrared absorbing fused-ring electron acceptors named as nTTIC(n=2,3,4,and 5),based on different number of thieno[3,2-b]thiophene(TT)unit as the electron-donating core.With increasing the TT unit,absorption spectrum of the TTIC series red shifts,and the highest occupied molecular orbital(HOMO)upshifts notably.It is worth noting that 4TTIC and 5TTIC exhibit absorption edges approaching 1100 nm,which is the photoresponse limit of solar cells based on crystal silicon.When the TTIC series acceptors are blended with polymer donor PM6,the binary-blend organic solar cells based on 3TTIC show the best power conversion efficiency(PCE)of 13.1%.In contrast,2TTIC-based devices exhibit relatively lower PCE of 8.32%,mainly caused by the larger energy loss and blue-shifted absorption.Due to insufficient driving force of charge separation caused by very high HOMO,4TTIC and 5TTIC show poor PCEs lower than 3%.

An n-Type All-Fused-Ring Molecule with Narrow Bandgap

All-fused-ringπ-conjugated molecules have received considerable attention because of their unique electronic structures,low conformation disorder,and excellent optoelectronic properties.Most all-fused-ring molecules are p-type organic semiconductors and possess medium bandgaps.In this work,we design and synthesize an all-fused-ring molecule(FM1)with an n-type property and narrow bandgap,which is a 10-fused-ring system composed of one electrondeficient benzotriazole core,two electron-rich thienopyrrole bridging units,and two electron-deficient malononitrile-functionalized end-cappers.FM1 exhibits low-lying highest occupied molecular orbit/lowest unoccupied molecular orbit energy levels of−5.77 eV/−3.89 eV,high electron mobility of 6.0×10^(−4)cm^(2)V^(−1)s^(−1),an optical bandgap of 1.50 eV,and a maximum absorption wavelength of 769nm.Because of the all-fused-ring skeleton,FM1 shows superior photostability and chemical stability.We use FM1 as an electron acceptor and successfully construct organic solar cell(OSC)devices with a decent power conversion efficiency(PCE)of 10.8%.Most importantly,the intrinsic stability of FM1 leads to its excellent OSC device stability.After irradiation with simulated solar light for 16 h,while control of the OSC device of the state-of-the-art small molecule electron acceptor shows a 46%decrease of PCE,the FM1’s unencapsulated OSC device exhibits only a 9%decrease of PCE.

Enhancing performance of tin-based perovskite solar cells via fused-ring electron acceptor

The performance of tin-based perovskite solar cells has been substantially hampered by voltage loss caused by energy level mismatch,charge recombination,energetic disorder,and other issues.Here,a fused-ring electron acceptor based on indacenodithiophene(IDIC)was for the first time introduced as a transition layer between a tin-based perovskite layer and a C 60 electron transport layer,leading to better matched energy levels in the device.In addition,coordination interactions between IDIC and perovskite improved the latter's crystallinity.The introduction of IDIC raised the power conversion efficiency from 8.98%to 11.5%and improved the device's stability.The decomposition mechanism of tin-based perovskite was also revealed by detecting the optical properties of perovskite microdomains through innovative integration of confocal laser scanning microscopy and photoluminescence spectroscopy.

Optimizing side chains on different nitrogen aromatic rings achieving 17% efficiency for organic photovoltaics

Balancing charge generation and low energy loss(E_(loss)), especially in the wide spectral absorption region is critical to obtain high-performance organic photovoltaics(OPVs). Therefore, Y11-M and Y11-EB are designed and synthesized through modifying alkyl chains on different nitrogen aromatic rings of the reported non-fullerene acceptor Y11. Although all the molecules have almost similar low band-gap(around 1.30 e V), Y11-M and Y11-EB exhibit wider absorption in 410–870 nm region. Eventually, the conventional devices based on Y11-M and Y11-EB possess more efficient charge generation with low Eloss(around 0.44 e V). In addition, outstanding efficiencies of 16.64% and 17.15% with the fill factor of 76.15% and 74.73% are obtained in PM6:Y11-M and PM6:Y11-EB-based devices, both higher than Y11:PM6. The results highlight the importance of rational alkyl chains optimization, and a good structureproperty relationship is established as well.

Organic Host-Guest Materials with Bright Red Room-Temperature Phosphorescence for Persistent Bioimaging

Organic room-temperature phosphorescence(RTP)materials have attracted immense attention in bioimaging due to their long emission lifetime and large Stokes shift.RTP materials with long emission wavelength can improve the penetration depth for bioimaging.However,the design of red persistent RTP materials is still challenging.In this study,a fused-ring structure has been proposed to effectively decrease the triplet energy level,thus extending the emission wavelength of phosphorescence.In addition,the fused-ring structure exhibits a high molar extinction coefficient(ɛ)and high luminescence efficiency due to the rigid structure.A new class of crystalline hosts(iminodibenzyl,IDB)are developed to stabilize the triplet excitons that are generated from the fused-ring molecules.The maximum RTP wavelength of doping materials can reach 635 nm with a lifetime of 9.35 ms.Water-disperse nanoparticles are successfully prepared for in vivo time-resolved bioimaging,which eliminates the background fluorescence interference from biological tissues.These reveal a delicate design strategy for the construction of long-wavelength emissive RTP materials for high-resolution bioimaging.

Isomerism: Minor Changes in the Bromine Substituent Positioning Lead to Notable Differences in Photovoltaic Performance

An isomerism strategy was employed to develop single,end‐group bromine-substituted non‐fullerene two isomeric acceptors,2,2′-((2Z,2′Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2,"3′′:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(4-bromo-3-oxo-2,3-dihydro-1H-inden-1-ylidene)dimalononitrile(BTIC-2Br-β)and 2,2′-((2Z,2′Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2,"3′′:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5-bromo-3-oxo-2,3-dihydro-1Hinden-1-ylidene)dimalononitrile(BTIC-2Br-γ).

Red-Emissive Organic Room-Temperature Phosphorescence Material for Time-Resolved Luminescence Bioimaging

Organic room-temperature phosphorescence(RTP)materials have been used in high-resolution imaging.However,the development of long-wavelength-emis sion RTP materials in aqueous solution remains a challenge.Here,we report red-emissive RTP materials via integration of the ring-fusing effect and host–guest interaction.

Construction of Angularly Fused Tricyclic Compounds via Photo-induced Radical Cascade Cyclization

Photo-induced electron transfer (PET) of silyl enol ethers has been employed to synthesize several fused ring systems. However, the method has limited applicability due to its narrow substrate scope, low product yields, unsatisfactory stereo- and regioselectivity. Herein, we report a PET-triggered cascade reaction of silyl enolates that leads to the formation of angularly fused tricyclic scaffolds. The reaction exhibits broad substrate scope and excellent stereoselectivity. The regio- and stereoselectivity of this cascade reaction is elucidated via DFT calculation and conformational analysis.

Derivation of Benzothiadiazine-1,1-dioxide Scaffolds via Transition Metal-Catalyzed C-H Activation/Annulation

Comprehensive Summary Benzothiadiazine-1,1-dioxide scaffold is bioactive framework with wild utility and applications.Synthesis of benzothiadiazine-fused isoquinoline derivatives and spiro benzothiadiazine derivatives through transition metal-catalyzed C—H activation/annulation was reported.3-Phenyl-2H-benzothiadiazine-1,1-dioxide was used as the reaction substrate,and vinylene carbonate and 4-diazopyrazolone were used as the coupling reagents,respectively.This strategy provides straightforward access to complex N-heterocycles in a highly efficient and simple manner.

Recent Advances in Domino Synthesis of Fused Polycyclic N-Heterocycles Based on Intramolecular Alkyne Hydroamination under Copper Catalysis

Fused polycyclic N-heterocycles are very important scaffolds in biomedicinal chemistry and materials science.Intramolecular alkyne hydroamination is a powerful method for the construction of N-heterocycles.In the last two decades,copper-catalyzed domino reactions based on intramolecular alkyne hydroamination has emerged as a robust strategy for assembling various fused polycyclic N-heterocycles.Great progress has been achieved in this area.This short review covers the advances made in copper-catalyzed domino synthesis of fused polycyclic N-heterocycles based on the strategy from 2008 to 2023,and will hopefully serve as an inspiration towards the exploration of new copper-catalyzed versions of the transformation.The domino transformations are introduced and discussed from five aspects according to the different key processes involved in these reactions.

Structure-Property Relationship of Conjugated Polymers Utilizing Isomerized Fused Benzothiadiazole-Based Building Blocks

Only a few fused benzothiadizole(BTz)based conjugated polymers have been applied in organic field-effect transistors(OFETs),and their performances were low,mainly due to the lack of available building blocks and in-depth study of the relationship between structure and performance.Herein,we report two isomeric fused BTz-based building blocks(M1 and M2)and their copolymers PBTzVBTz-T and PBTzVBTz-TT,PTVT-T and PTVT-TT.DFT study showed all polymers had nearly planar backbone geometries,and PBTzVBTz-TT exhibited the most linear backbone.PTVT-T and PTVT-TT exhibited red-shift absorption spectra and deeper LUMO level than those of PBTzVBTz-T and PBTzVBTz-TT.Thin film microstructure study demonstrated PBTzVBTz-TT and PTVT-TT had edge-on molecular orientation,while the others had the mixed molecular orientation of edge-on and face-on.Furthermore,PBTzVBTz-TT had the longest coherence length in thin film.All these features of PBTzVBTz-TT resulted in its highest carrier mobility of 0.1 cm^(2)·V^(–1)·s^(–1) among these polymers.

Facile Synthesis of S-Fused Multi-Membered Polycyclic Heterocycles:A Construction Strategy towards Thermally Stable Thiepine Derivatives

S-fused heterocycles have become popular building blocks to construct functional polycyclic compounds.In contrast to the abundant synthetic methodologies for thiophene-fused aromatics,the synthesis of S-heterocycles containing six-membered thiopyran and seven-membered thiepine rings is much less reported owing to their unfavorable synthetic protocols and the thermal instabilities.Herein,a series of thiepine-containing polycyclic S-heterocycles have been successfully synthesized via different synthetic routes which involve initial construction of sulfur bridges and final ring-closure reactions.Therefore,the dilithium intermediates are ex-cluded,which facilitates the fusion on the thiepine ring with different S-heterocycles,including thiophene and thiopyran derivatives.Typically,a S-fused multi-membered polycyclic compound simultaneously involving thiophen,thiopyran,and thiepine rings has been successfully prepared.Interestingly,nucleus-independent chemical shift calculations reveal that the incorporated thiopyran and thiepine rings demonstrate aromatic and nonaromatic characteristics,respectively.Moreover,the thermal stabilities of the thiepine derivatives have been tremendously improved after the fusion on the three vinyl groups in the thiepine unit,which is attributed to the enhancements of the activation energies for the S-extrusion reactions,as revealed by density functional theory calculations.Therefore,our findings not only provide a facile synthetic methodology for S-fused multi-membered polycyclic heterocycles,but also furnish a novel construction strategy towards thermally stable thiepine derivatives.

Synthesis and Characterization of Planar Five-Ring-Fused Dithiophene-dione

A series of new organic semiconductors based on s-indaceno[1,2-b：5,6-b＇]dithiophene-4,9-dione was success fully synthesized and characterized. The electron withdrawing carbonyl group lowers the LUMO energy levels, leading to increased electronegativities, which is beneficial for high photo-stability in air. The n-alkyl substituted compounds, lc and ld, crystallize with the rigid coplanar systems packed into slipped face-to-face n-stacks. Inter- estingly, lc and ld also show liquid crystalline behaviors, which give highly ordered molecular packing over large area.

High-Efficiency and Low-Energy-Loss Organic Solar Cells Enabled by Tuning Conformations of Dimeric Electron Acceptors

Dimeric fused-ring electron acceptors(DFREAs)have attracted much attention due to the combined advantages of their monomeric and polymeric acceptors,including a well-defined molecular structure,excellent repeatability,and stable morphology.However,the additionally introduced single-bonds during dimerization may result in a twisted backbone of DFREAs,which is detrimental to intermolecular packing and charge transport.Herein,three DFREAs are designed and synthesized,in which DFREA conformations were systematically tuned via adjusting the intensities of intramolecular noncovalent interactions(INIs)to achieve high-performance organic solar cells(OSCs).Theoretical and experimental results show that the gradual introduction of S…F INIs can continuously improve molecular planarity and rigidity,resulting in reduced reorganization energies,ordered packing mode,and enhanced crystallization of DFREAs.Benefiting from the incorporation of fourfold S…F INIs,DYF-TF-based binary OSCs show a record high efficiency of 18.26%with an extremely low energy loss(0.493 eV)for DFREAbased OSCs.In addition,DYF-TF-based OSCs exhibited good long-term stability with a T_(80%)lifetime of 2681 h,and the power conversion efficiency of the DYF-TF-based ternary device is further enhanced to 18.73%.This contribution demonstrates the great potential of the INIs strategy in achieving excellent DFREAs materials.