Extension Of Π-conjugation Research Articles

Varied π-Conjugated Extension Directions in B←N Coordinated Benzodipyrrolidone (BDP) Polymer Semiconductors: Effects on n-Type Organic Field-Effect Transistors (OFETs)

Varied π-Conjugated Extension Directions in B←N Coordinated Benzodipyrrolidone (BDP) Polymer Semiconductors: Effects on n-Type Organic Field-Effect Transistors (OFETs)

Near-IR Emissive B-N Lewis Pair-Functionalized Anthracenes via Selective LUMO Extension in Conjugated Dimer and Polymer.

Acenes are attractive as building blocks for low gap organic materials with applications, for example, in organic light emitting diodes, solar cells, bioimaging and diagnostics. Previously, we have shown that modification of dipyridylanthracene via B-N Lewis pair fusion (BDPA) strongly redshifts the emission, while facilitating self-sensitized reactivity toward O2 to reversibly generate the corresponding endoperoxides. Herein, we report on the further expansion of the π-system of BDPA to a vinyl-substituted monomer, vinylene-bridged dimer, and a polymer with an average of 20 chromophores. The extension of π-conjugation results in largely reduced band gaps of 1.8 eV for the dimer and 1.7 eV for the polymer, the latter giving rise to NIR emission with a maximum at 731 nm and an appreciable quantum yield of 7 %. Electrochemical and computational studies reveal efficient delocalization of the lowest unoccupied molecular orbital (LUMO) along the pyridyl-anthracene-pyridyl axis, which results in effective electronic communication between BDPA units, selectively lowers the LUMO, and ultimately narrows the band gap. Time-resolved emission and transient absorption (TA) measurements offer insights into the pertinent photophysical processes. Extension of π-conjugation also slows down the self-sensitized formation of endoperoxides, while significantly accelerating the thermal release of singlet oxygen to regenerate the parent acenes.

Effect of multi-donor and extended π-conjugation in ferrocene appended indanedione chromophores for aggregation induced emission enhancement (AIEE) and nonlinear optics†

A new (D-D’π-A) type ferrocene and methoxyphenyl (D-D′) conjugated indanedione (A) based linear and π-extended multi-donor-π-acceptor chromophores 1–4 were synthesized and characterized. The chromophores 2–4 were further investigated by single-crystal analysis through X-ray diffraction, revealing centrosymmetric space groups with non-covalent interactions (inter, intramolecular hydrogen bonding, and C–H…π interactions) and these molecular packing interactions favor SHG efficiencies. Electrochemical studies show the one-electron charge transfer (Fe2+⇌Fe3+) and observed greater redox potential of chromophore 4 than the parent ferrocene due to the substitution effect. The solvatochromic studies in emission spectra resulted in redshift (56–109 nm) for the chromophores 1–4 by increasing the solvent polarities, which indicates higher excited state stabilization relative to the ground state, results effective intramolecular charge transfer process within the chromophores. The occurrence of ferrocene moieties lowers the fluorescence intensities of the chromophores 1–4, which was overcome through aggregation induced emission enhancement (AIEE) studies at 60 % (THF/H2O). The AIEE state also shows 23 times higher quantum yield than the THF solution for chromophore 4. In addition, a thin film using polymethyl methacrylate as a binder shows a significant blue shift with higher emission intensity than those obtained in the solution due to the intermolecular interactions and enhancement of viscosity. The SHG efficiency studied using the Kurtz and Perry powder technique results in a higher value for chromophore 4 (2 times higher than the reference KDP) attribute to the multi donors, acceptor, and extended π conjugation reducing the anti-parallel alignment in the solid state. The computational calculations were done to validate the experimental results using B3LYP/6-31+G** level of theory.

Thiopyran-Fused Polycyclic Aromatic Compounds Synthesized via Pt(II)-Catalyzed One-Pot Ring-Expansion and 6-endo Cyclization Reactions.

A series of thiopyran-fused polycyclic aromatic hydrocarbons (PAHs) have been straightforwardly synthesized from 2,5-di(1-en-3-ynyl)thiophene-containing precursors via one-pot ring-expansion and 6-endo cyclization reactions. The reaction monitoring and the density function theoretical calculation suggest that the ring-expansion reaction occurs prior to 6-endo cyclization. Moreover, the absorption profiles of the thiopyran-fused PAHs suggest that the π-conjugation extension on the side of the cyclopentadiene ring in the cyclopenta[b]thiochromene core is predominant in prolonging the effective conjugation length, while the effect from extension on the other side is negligible. Furthermore, all of the thiopyran-fused PAHs exhibit halochromic properties. Upon the addition of trifluoromethanesulfonic acid, fluorescence "off-on" switches can be found for these thiopyran-fused PAHs. Therefore, this work not only provides a new synthetic approach for one-pot ring-expansion and 6-endo cyclization reactions but also expands the diversity of thiopyran-fused PAHs.

Theoretical study on the photophysical properties of thiophene-fused-type BODIPY series molecules in fluorescence imaging and photodynamic therapy.

As a class of photosensitizers (PSs) with dual functions of photodynamic therapy (PDT) and fluorescence imaging, the relationship between the structure and dual-function of thiophene-fused-type BODIPY dyes has not been studied in depth before. We found that the thiophene-fused-type BODIPY triplet photosensitizer is produced according to the energy level matching rule and the introduction of the thiophene ring significantly reduces the energy gap ΔEST between singlet and triplet states, as revealed by our investigation of the excited state structures and energies of thieno-fused BODIPY dyes. At the same time, a tiny ΔEST also results in a greatly enhanced intersystem crossing (ISC) rate, kISC. The kISC value of MeO-BODIPY, having the highest singlet oxygen quantum yield (ΦΔ), is the largest. Substitution with a strong electron donor N,N-dimethylaminophenyl (DMA) leads to the vertical configuration in the T1 state. The small ΔE (0.0029 eV) between the HOMO and HOMO-1 triggers the photo induced electron transfer (PET) of inhibiting ISC and fluorescence. When thieno-fused BODIPYs react with pyrrole, the increase of π-conjugation and smaller ΔEHOMO-LUMO explain the redshift in emission wavelength of thieno-pyrrole-fused BODIPY. The more planar configuration of the S1 state and the stronger oscillator intensity reflect a higher fluorescence quantum yield (ΦF). The extension of π-conjugation can cause molecules to transition to higher-level singlet excited states (Sn states, n ≥ 1) after absorbing energy and reduce the energy level of the excited state, resulting in multiple channels and favoring 1O2 production for thieno-pyrrole-fused BODIPYs with electron-withdrawing groups at the para-position of the phenyl groups. Due to ΔES0-T1 < 0.980 eV, the substitution of electron-donating groups cannot produce 1O2. In this work, we have revealed the mechanism of ISC and the fluorescence emission process in the thiophene-fused-type BODIPY dye, which has provided a theoretical foundation and guidance for the future design of BODIPY-based heavy-atom-free PSs for molecular applications in PDT.

Recent progress in multi-resonance thermally activated delayed fluorescence emitters with an efficient reverse intersystem crossing process.

Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters have become an active research topic at the forefront of organic light-emitting diodes (OLEDs) owing to their excellent photophysical properties such as high efficiency and narrow emission characteristics. However, MR-TADF materials always exhibit slow reverse intersystem crossing rates (kRISC) due to the large energy gap and small spin-orbit coupling values between singlet and triplet excited states. In order to optimize the RISC process, strategies such as heavy-atom-integration, metal perturbation, π-conjugation extension and peripheral decoration of donor/acceptor units have been proposed to construct efficient MR-TADF materials for high-performance OLEDs. This article provides an overview of the recent progress in MR-TADF emitters with an efficient RISC process, focusing on the structure-activity relationship between the molecular structure, optoelectronic feature, and OLED performance. Finally, the potential challenges and future prospects of MR-TADF materials are discussed to gain a more comprehensive understanding of the opportunities for efficient narrowband OLEDs.

π-Conjugation effects on excited-state intermolecular proton-transfer reactions of anthracene-urea derivatives in the presence of acetate anions.

This study investigated emissive urea compounds with an anthryl moiety on one side and a substituent group (biphenyl, naphthyl, benzyl, or cyclohexyl) on the other side across from the urea group. This was performed to determine the contribution of π-conjugation on a substituent group to excited-state intermolecular proton-transfer (ESPT) reactions in the presence of acetate anions. Fluorescence lifetime measurements revealed that the rate constant of the ESPT reaction from the normal form to the tautomer form increased with the length of the π-conjugation. Considering that there were a few differences among the wavelengths of the fluorescence maxima for the anthracene-urea derivatives in the presence of acetate anions, we observed that the extension of π-conjugation promoted tautomer formation. This maintained the energy levels of the normal and tautomer forms in the excited state. Furthermore, an anthracene-urea derivative without π-conjugation did not undergo a reverse ESPT reaction, implying that π-conjugation is considerably involved in the reverse ESPT reaction from the tautomer form to the normal form.

Multi-arm quinoxaline-based acceptors formed by π-conjugation extension for efficient organic solar cells.

Manipulating the conjugated backbone of small molecule acceptors (SMAs) is of particular importance in developing efficient organic solar cells (OSCs). Recently, trimers and other multi-arm SMAs have been found to be able to provide more intermolecular interaction, demonstrating excellent molecular stacking and device performance. However, the synthesis of this type of SMA usually relies on tristin or polystin compounds. Instead, expanding multiple arms in the central cores of SMAs is relatively simple and not restricted by tin compounds. Based on the quinoxaline core, two kinds of multi-arm SMAs, FQx-IC and TQx-IC with 4 and 3 arms, have been developed in this work. Compared to FQx-IC, TQx-IC exhibits an ordered face-on molecular orientation, appropriate film-forming process, and more favorable phase separation morphology and balanced charge transport. When blended with the polymer donor D18, OSCs based on TQx-IC achieve a power conversion efficiency (PCE) of 17.36%, which is superior to the device based on D18:FQx-IC (16.24%). In addition, using the ternary strategy of incorporating the TQx-IC into the D18:Y6 system, an excellent PCE of 18.82% is achieved. Therefore, this multi-arm molecular design strategy has great potential for regulating molecular stacking, absorption, and the corresponding device performance.

Rational design of a lysosome-targeted fluorescent probe for monitoring the generation of hydroxyl radicals in ferroptosis pathways.

Ferroptosis is a newly discovered iron-dependent form of regulated cell death associated with high levels of hydroxyl radical (˙OH) production. Meanwhile, lysosome dysfunction has been shown to be one of the causes of ferroptosis. Although a variety of ˙OH-responsive fluorescent probes have been developed for detecting intracellular ˙OH in living cells, there are still only few lysosome-targeted probes to monitor the variation in lysosomal ˙OH levels during ferroptosis. Herein, we report a novel ˙OH-specific fluorescent probe HCy-Lyso, which is composed of the hydrocyanine and morpholine moiety. Upon treatment with ˙OH, its hydrocyanine unit was converted to the corresponding cyanine group, thus leading to a large π-conjugation extension of HCy-Lyso, accompanied by a significant fluorescence off-on response. Moreover, after reacting with ˙OH in an acidic environment, the protonation product of HCy-Lyso exhibits a higher fluorescence enhancement, which is suitable for detecting lysosomal ˙OH variation. HCy-Lyso has been utilized for imaging endogenous ˙OH in living cells under phorbol myristate acetate (PMA) stimuli and monitoring the changes in lysosomal ˙OH levels during ferroptosis. Thus, our study proposes a new strategy to design lysosome-targeted and ˙OH-responsive fluorescent probes to investigate the relationship between lysosomes and ferroptosis.

Small is Beautiful: Electronic Origin and Synthetic Evolution of Single-Benzene Fluorophores.

ConspectusSingle-benzene fluorophores (SBFs) are small molecules that produce visible light by using only one benzene ring as the sole aromatic core. This Account centers around the chemistry of a new class of SBF that we accidentally discovered but rationally developed and refined afterward. In a failed experiment that took an unintended reaction pathway, we encountered the bright green fluorescence of ortho-diacetylphenylenediamine (o-DAPA). Despite its uninspiring look, reminiscent of textbook examples of simple benzene derivatives, this molecule had neither been synthesized nor isolated before. This discovery led to our studies on the larger DAPA family, including isomeric m-DAPA and p-DAPA. Remarkably, p-DAPA is the lightest red fluorophore, with a molecular weight of only 192. While o- and p-DAPA are emissive, m-DAPA rapidly undergoes internal conversion, facilitated by sequential proton transfer reactions in the excited state.Leveraging the synthetic utility of the amine group, we carried out straightforward single-step modifications to create a full-color SBF library from p-DAPA as the common precursor. During the course of the investigation, we made another fortuitous discovery. With increasing acidity of the N-H group, the excited-state intramolecular proton transfer reaction is promoted, opening up additional pathways for emission to occur at even longer wavelengths. Tipping the balance between the two excited-state tautomers enabled the first example of a single-benzene white-light emitter. We demonstrated the practical utility of these molecules in white light-emitting devices and live cell imaging.According to the particle-in-a-box model, it is difficult to expect a molecule with only one small aromatic ring to produce long-wavelength emission. SBFs rise to this challenge by exploiting electron donor-acceptor pairs around the benzene core, which lowers the energy of light absorption. However, this answers only half of the question. Where do the exceptionally large spectral shifts in the light emission of SBFs originate from? Chemists have long been curious about the molecular mechanisms underlying the dramatic spectral shifts observed in SBFs. Prevailing paradigms invoke the charge transfer (CT) between electron donor and acceptor groups in the excited state. However, without a large π-skeleton for effective charge separation, how could benzene support a CT-type excited state? Our experimental and theoretical studies have revealed that large excited-state antiaromaticity (ESAA) of the benzene core itself is responsible for this remarkable phenomenon. The core matters, not the periphery. With appropriate molecular design, large and extended π-conjugation is no longer a prerequisite for long-wavelength light emission.

Improved Description of Intra- and Intermolecular Interactions through Dispersion-Corrected Second-Order Møller-Plesset Perturbation Theory.

ConspectusThe quantum chemical modeling of organic crystals and other molecular condensed-phase problems requires computationally affordable electronic structure methods which can simultaneously describe intramolecular conformational energies and intermolecular interactions accurately. To achieve this, we have developed a spin-component-scaled, dispersion-corrected second-order Møller-Plesset perturbation theory (SCS-MP2D) model. SCS-MP2D augments canonical MP2 with a dispersion correction which removes the uncoupled Hartree-Fock dispersion energy present in canonical MP2 and replaces it with a more reliable coupled Kohn-Sham treatment, all evaluated within the framework of Grimme's D3 dispersion model. The spin-component scaling is then used to improve the description of the residual (nondispersion) portion of the correlation energy.The SCS-MP2D model improves upon earlier corrected MP2 models in a few ways. Compared to the highly successful dispersion-corrected MP2C model, which is based solely on intermolecular perturbation theory, the SCS-MP2D dispersion correction improves the description of both inter- and intramolecular interactions. The dispersion correction can also be evaluated with trivial computational cost, and nuclear analytic gradients are computed readily to enable geometry optimizations. In contrast to earlier spin-component scaling MP2 models, the optimal spin-component scaling coefficients are only mildly sensitive to the choice of training data, and a single global parametrization of the model can describe both thermochemistry and noncovalent interactions.The resulting dispersion-corrected, spin-component-scaled MP2 (SCS-MP2D) model predicts conformational energies and intermolecular interactions with accuracy comparable to or better than those of many range-separated and double-hybrid density functionals, as is demonstrated on a variety of benchmark tests. Among the functionals considered here, only the revDSD-PBEP86-D3(BJ) functional gives consistently smaller errors in benchmark tests. The results presented also hint that further improvements of SCS-MP2D may be possible through a more robust fitting procedure for the seven empirical parameters.To demonstrate the performance of SCS-MP2D further, several applications to molecular crystal problems are presented. The three chosen examples all represent cases where density-driven delocalization error causes GGA or hybrid density functionals to artificially stabilize crystals exhibiting more extended π-conjugation. Our pragmatic strategy addresses the delocalization error by combining a periodic density functional theory (DFT) treatment of the infinite lattice with intramolecular/conformational energy corrections computed with SCS-MP2D. For the anticancer drug axitinib, applying the SCS-MP2D conformational energy correction produces crystal polymorph stabilities that are consistent with experiment, in contrast to earlier studies. For the crystal structure prediction of the ROY molecule, so named for its colorful red, orange, and yellow crystals, this approach leads to the first plausible crystal energy landscape, and it reveals that the lowest-energy polymorphs have already been found experimentally. Finally, in the context of photomechanical crystals, which transform light into mechanical work, these techniques are used to predict the structural transformations and extract design principles for maximizing the work performed.

Cuproptosis accompanied by obvious •OH generation revealed with an ultrasensitive NIR fluorescence probe

Cuproptosis is a recently discovered copper-dependent pathway of regulated cell death, and may play a role in a number of physiological and pathological processes; however, the variation of reactive oxygen species such as OH level is unclear during cuproptosis. Herein, we have developed an ultrasensitive near-infrared (NIR) fluorescence probe CDHQ for characterizing cuproptosis, which reveals an obvious hydroxyl radical (OH) generation during cuproptosis. The specific hydrogen abstraction of OH causes a large π-conjugation extension and spectral shift of CDHQ, accompanied by a high-contrast fluorescence off–on response. CDHQ can detect trace OH that is undetectable by other methods such as electron spin resonance during autoxidation of low-valent metals (Fe2+, Ti3+, V3+ and especially Cu+) in the absence of H2O2. Using CDHQ, we have found that OH is significantly generated in the copper-overloaded cells of cuproptosis, even before cell death. Moreover, CDHQ is capable of monitoring the generation of OH in living mice, and thus may be expected to be useful for studying the in vivo change of OH in more biosystems.

Development of functional dye with redshifted absorption based on Knoevenagel condensation at 1-site in phenyl[b]-fused BODIPY

Integrating ring-fused modification with π-conjugated extension is an effective approach for designing, synthesizing, and application for novel borondipyrromethene (BODIPY) structures. In this work, based on phenyl[b]-fused BODIPY, we made reasonable modification of the methyl group at 1-site to generate dye NBDP. NBDP possessed near-infrared region (NIR) absorption and emission properties, and the intramolecular charge transfer (ICT) resulted in low fluorescence. Whereas, heat energy is evidently released in the presence of light, which can be exploited for intracellular photothermal therapy via the cell apoptosis process, reducing the inflammatory side-effects induced by necrosis. This research provides a crucial foundation for the novel molecule via BODIPY multi-directional alteration and its potential application in anti-tumor phototherapy.

Styryl-NIR Mitochondrial Probes with Rapid HOCl Detection in Live-Cells.

Hypochlorous acid (HOCl) is critical for maintaining immune system balance, but it can harm mitochondria by hindering enzyme activity, leading to decreased ATP and increased cell death. In this study, we have designed a fluorophore with a pyridinium scaffold for selective staining of the mitochondria and to detect hypochlorite. The fluorophore exhibits strong solvatochromic emission due to intramolecular charge transfer and excellent sub-cellular localization in the mitochondria. Additionally, it shows a rapid response to HOCl with high selectivity among different reactive oxygen/nitrogen compounds with a detection limit of 2.31 μM. Moreover, it is also utilized for the exogenous and endogenous detection of HOCl in live cells, which may help study the role of hypochlorite in organelles at the cellular level. DFT and TDDFT calculations have been carried out to understand the relationship between the structure and properties of the cationic probes with respect to the α-cyano substitution and extension of π-conjugation. The selective detection of HOCl by C4 over other cationic probes has also been well-demonstrated, showing how the binding of HOCl affects the electronic properties of C4 through the analysis of non-bonding orbitals (NBO) population, electrostatic potential surface (ESP), and density of states (DOS) projected DOS investigations.

True- to Sky-Blue Emitters Bearing the Thiazolo[5,4-d]thiazole Electron Acceptor for Single and Tandem Organic Light-Emitting Diodes

A series of emitters displaying efficient photoluminescence (PL) and electroluminescence (EL) into the true- to sky-blue regions is herein described and employed as electroluminescent materials for the fabrication of efficient blue organic light-emitting diodes (OLEDs). The compounds possess linear, yet a twisted, donor−π–acceptor−π–donor (D−π–A−π–D) architecture, where D and A are a (substituted) carbazole (Cz) and thiazolo[5,4-d]thiazole (TzTz) moiety as the electron donor and accepting unit, respectively. D = 9H-carbazol-9-yl and 3,6-di-tert-butyl-9H-carbazol-9-yl for compounds TzTz-PCz2 and TzTz-PtbCz2, respectively. In dilute CH2Cl2 solutions, both compounds display intense (PL quantum yield = 60–74%) and short-lived (τ = ca. 0.8–2.0 ns) luminescence arising from an intramolecular charge-transfer (1ICT) excited state and falling into the sky-blue to the greenish-blue region (λem = 448–502 nm). Compared to the previously reported TzTz-TPA parental emitters that features two triphenylamine (TPA) donor groups, the highly twisted nature and less extended π-conjugation of the donor–acceptor TzTz-PCz2 and TzTz-PtbCz2 counterparts herein presented enable to widen the HOMO–LUMO energy gaps and warrant blue shifting of the emission. Finally, OLED devices fabricated by employing the herein proposed emitters are presented as well with both single- and tandem-device architectures. The so-prepared OLEDs display true-blue to sky-blue EL peaking at λEL,max = 460 nm (TzTz-PCz2) and 467–470 nm (TzTz-PtbCz2) and achieve maximum external quantum efficiency and luminance of 10.2% and 44,851 cd m–2 (for TzTz-PtbCz2) in the case of a tandem device, respectively, with a mitigated roll-off efficiency. These results demonstrate that the tandem device employing these TzTz donor–acceptor blue emitters raises the feasibility of fluorescent compounds to compete with other potential emitters in practical applications.

Novel deep red to near-infrared phosphorescent iridium(III) complexes bearing pyrenyl: Syntheses, structures and modulation of the photophysical properties

The extension of π-conjugation and the introduction of substituents are efficient channel for achieving low-energy emitting phosphors. To investigate the influence of substituent effect on the deep-red or near-infrared iridium(III) complexes containing great π-conjugation cyclometalating ligands, we synthesized four pyrenyl-based heteroleptic Ir(III) complexes bearing electron-withdrawing or -donating groups. The four Ir(III) complexes comprise 5-R-2-(1-pyrenyl)pyridine (R = H, CF3, F, CH3) as the main ligands and 2,2,6,6-tetramethyl-heptane-3,5-dione as ancillary ligands. Their structures and coordination geometries are determined by single crystal X-ray diffraction. It is found that the emission profiles of four complexes are located on deep red to near-infrared region and -CF3 substitution leads to a bathochromic shift of 11 nm relative to the unsubstituted complex. From density functional theory calculation results as well as electrochemical analysis, their HOMO and LUMO energy levels fall in the order of substituents -CF3 < -F < -H < -CH3. Intriguingly, the electron density distributions of LUMOs of both -CF3 and -CH3 substituted complexes are more susceptible than their HOMOs, while the HOMO of the -F substituted complex is more susceptible. Our work showcases the vital significance of substituent effect on modulating the electronic structure of deep-red or near-infrared iridium complexes cyclometalated by great π-conjugation cyclometalating ligands.

Near-Infrared Emissive Hypervalent Compounds with Germanium(IV)-Fused Azobenzene π-Conjugated Systems.

A novel molecular design for showing near-infrared (NIR) emission is still required for satisfying growing demands for NIR-light technology. In this research, hypervalent compounds with germanium (Ge)-fused azobenzene (GAz) scaffolds were discovered that can exhibit NIR emission (λPL =690∼721 nm, ΦPL =0.03∼0.04) despite compact π-conjugated systems. The unique optical properties are derived from the trigonal bipyramidal geometry of the hypervalent compounds constructed by combination of Ge and azobenzene-based tridentate ligands. Experimental and theoretical calculation results disclosed that the germanium-nitrogen (Ge-N) coordination at the equatorial position strongly reduces the energy level of the LUMO (lowest unoccupied molecular orbital), and the three-center four-electron (3 c-4 e) bond in the apical position effectively rises the energy level of the HOMO (highest occupied molecular orbital). It is emphasized that large narrowing of the HOMO-LUMO energy gap is achieved just by forming the hypervalent bond. In addition, the narrow-energy-gap property can be enhanced by extension of π-conjugation. The obtained π-conjugated polymer shows efficient NIR emission both in solution (λPL =770 nm and ΦPL =0.10) and film (λPL =807 nm and ΦPL =0.04). These results suggest that collaboration of a hypervalent bond and a π-conjugated system is a novel and effective strategy for tuning electronic properties even in the NIR region.

Perpetual Extension of Conjugation of Fluorene-Based Donor–Acceptor Dyads Yield Diminished Nonlinear Optical Response

A fundamental limit of π-conjugation seems to dictate the maximum nonlinear optical (NLO) response of fluorene-based “push–pull” (D−π–A) chromophores SPH1–SPH5 and the precursor aldehyde 4. Comparison of the first hyperpolarizability (βHRS) and intrinsic first hyperpolarizability (βint) values of the fluorene-based chromophores having different lengths and/or pathways of π-conjugation between donor and acceptor units, measured using the hyper-Rayleigh scattering (HRS) technique, revealed that the extension of π-conjugation beyond the limit (the horizontal limit!) does not necessarily improve the NLO response, which is determined by the combined influence of red shift of the intramolecular charge transfer (ICT) bands, optical band gap, and dipole moment of these chromophores. Additionally, the spatial distance between the donor and the acceptor units also constitutes an important attribute influencing the βHRS.

A nonfullerene acceptor with π-conjugation extended end groups to achieve enhanced photovoltaic performance

The π-conjugation extension on terminal unit enables high-efficiency photovoltaic performance, benefiting from the enhancement of Jsc and FF.

Integrating the atomically separated frontier molecular orbital distribution of two multiple resonance frameworks through a single bond for high-efficiency narrowband emission.

Atomically separated frontier molecular orbital (FMO) distribution plays a crucial role in achieving narrowband emissions for multiple resonance (MR)-type thermally activated delayed fluorescence emitters. Directly peripherally decorating a MR framework with donor or acceptor groups is a common strategy for developing MR emitters. However, this approach always induces bonding features and thus spectral broadening as a side effect. How direct donor/acceptor decoration enhances atomic FMO separation while avoiding bonding features has not been explored. For this aim, two MR derivatives are synthesized by integrating two MR frameworks at different sites. Following resonance alignment, DOBNA-m-CzBN avoids breaking nonbonding FMO features at the single connecting bond and shows enhanced MR characteristics, with a sharp emission at 491 nm and a full width at half maximum (FWHM) of 24 nm/118 meV. Conversely, DOBNA-p-CzBN emerges as a bonding feature due to its continuous π-conjugation extension, with a broadened FWHM of 26 nm/132 meV peaking at 497 nm. Impressively, both emitters exhibit outstanding external quantum efficiencies of 37.8-38.6% in organic light-emitting diodes (OLEDs), demonstrating improved performance with rigid acceptor decoration. Distinctly, the electroluminescence of DOBNA-m-CzBN shows a narrower FWHM than that of DOBNA-p-CzBN. This work for the first time reports the enhancement of atomic FMO separation for MR emitters via peripheral decoration through a single bond and provides a more comprehensive illustration for further development of MR emitters.