Acceptor Strength Research Articles

Multicomponent synthesis of 7-(diethylamino)coumarin–pyrrolo[3,4-b]pyridin-5-one conjugates and modulation of their twisted intramolecular charge transfer (TICT) processes

By coupling an Ugi-Zhu three-component reaction to a cascade sequence (aza Diels-Alder cycloaddition/N-acylation/decarboxylation/dehydration) into a one-pot process, seven new 7-(diethylamino)coumarin-pyrrolo[3,4-b]pyridin-5-one bis-heterocyclic conjugates were synthesized in moderate overall yields (15–38 %). Next, the photophysical properties of all products were determined using UV–Vis and fluorescence spectroscopy. These products exhibited fluorescence quantum yields (FQY) ranging from 17 % to 25 %. In contrast, one of their precursors, the free 7-(diethylamino)-3-carbaldehyde displayed a low FQY of 2 %, and, also exhibited dual emission across all solvents used for the measurements. The increase in quantum yield observed upon incorporation of the coumarin moiety into MCR products was attributed to the inhibition of the twisted intramolecular charge transfer (TICT) process. This one naturally occurs in the free coumarin but was suppressed in the products due to a decrease in the acceptor strength of the substituent at position 3 of the coumarin moiety. Furthermore, the electronic structure of the compounds was computed by DFT/TD-DFT calculations allowing to determine which molecular orbitals (MO) were involved in the electronic transitions.

Fast Collective Hydrogen-Bond Dynamics in HexafluoroisopropanolRelated to its Chemical Activity.

Using fluorinated mono-alcohols, in particular hexafluoro-isopropanol (HFIP), as a solvent can enhance chemical reaction rates in a spectacular manner. Previous work has shown evidence that this enhancement is related to the hydrogen-bond structure of these liquids. Here, we investigate the hydrogen-bond dynamics of HFIP and compare it to thatof its non-fluorinated analog, isopropanol. Ultrafast infrared spectroscopy show that the dynamics of individual hydrogen-bonds is about twice as slow in HFIP as in isopropanol. Surprisingly, from dielectric spectroscopy we find the opposite behavior for the dynamics of hydrogen-bonded clusters: collective rearrangements are3 times faster in HFIP than in isopropanol. This difference indicates that the hydrogen-bonded clusters in HFIP are smaller than in isopropanol. The differences in cluster size can be traced to changes in the hydrogen-bond donor and acceptor strengths upon fluorination. The smaller cluster size canboostreaction rates in HFIP by increasing the concentration of reactive, terminal OH-groups of the clusters, whereas the fast collective dynamics can increase the rate of formation of hydrogen bonds with the reactants. The longer lifetime of the individual hydrogen bonds in HFIP can enhance the stability of the hydrogen-bonded clusters, and so increase the probability of reactant-solvent hydrogen bonding.

Role of electron withdrawing moieties in phenoxazine–oxadiazole-based donor–acceptor compounds towards enriching TADF emission

Herein, we reported the effect of electron withdrawing units, such as trifluoromethyl (CF3) and cyanide (CN), substitution on a thermally activated delayed fluorescent (TADF) molecule (10-(4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl)-10H-phenoxazine (PXZ-OXD)). The addition of electron withdrawing units has increased the acceptor strength and reduced the interaction between PXZ and OXD, thus reducing the gap between the singlet and triplet states (ΔEST) of excitons. To understand the variation in the acceptor strength of PXZ-OXD as a function of its molecular structure and density of states, density functional theory (DFT) and time-dependent DFT (TDDFT) were conducted. Transition density matrix investigations revealed that the addition of −CF3 and –CN to PXZ-OXD triggers CT excitons, while inducing lower ΔEST values. Particularly, the lowest ΔEST (0.05 eV) with a notable red shift in the emission spectrum was observed with 4′-CF3PXZOXD. The delayed component lifetime of PXZOXD is found to be reduced after the substitution of −CNwhm and −CF3. Despite the weak charge transfer transitions, the substitution of conjugative –CN group is found to be beneficial in improving the HOMO-LUMO overlap with a moderate decrease in reverse intersystem crossing (KRISC), which attained enhancement in the photoluminescent quantum yield. Additionally, the substitution of the above electron withdrawing units on PXZ-OXD yielded a red shift in the electroluminescence spectrum. Furthermore, the external quantum efficiency (at 100 cd/m2, i.e., EQE100) of the 4′-CNPXZOXD-based organic light-emitting diode is found to improve by 21.13 % against the PXZOXD (16.9 %).

Physicochemical properties of binary mixtures of cation-fluorinated ionic liquids with their non-fluorinated analogues

This work reports an investigation of the physicochemical properties (density, viscosity, refractive index and thermal stability) of four binary mixtures having a common acetate ([OAc]¯), trifluoromethanesulfonate ([OTf]¯) or bis(trifluoromethylsulfonyl)imide ([NTf2]¯) anion with either cation-fluorinated and non-fluorinated methylimidazolium (MIm) or pyridinium (Py) cations respectively, at various temperatures. The mixing behaviour of the binary mixtures was analysed by 1H and 19F Nuclear magnetic resonance (NMR) spectrometry, and the excess properties of the mixtures were calculated from the physicochemical data. The binary mixtures of fluorinated ionic liquids (FILs) with their non-fluorinated or alkyl-substituted analogues (AILs) showed an increase in density and viscosity but a decrease in refractive index with an increase in mole fraction of the pure FILs. The thermal stability of the AILs was higher than the FILs, however, an increase in the mole fraction of the pure FILs in the binary mixtures made a negligible impact on the thermal stability of the binary mixtures. These binary mixtures were found to deviate from ideal behaviour for the different physicochemical properties tending to vary between the extremes of the pure components. The large differences in properties of FILs compared with their non-fluorinated analogues were shown to be associated with the size of the fluorine atoms in the fluoro-alkyl chain, the tendency of the fluoro-alkyl chain to form aggregates with fluorinated anions, and the hydrogen bond acceptor strength of the anions. The excess properties were fitted to the Redlich-Kister polynomial equation, and no significant deviation was obtained. This study shows that the physicochemical properties of the fluorinated ionic liquids can conveniently be adjusted for specific applications by simply varying the amount of the cation-fluorinated component in binary mixtures.

An All‐Rounder for NIR‐II Phototheranostics: Well‐Tailored 1064 nm‐Excitable Molecule for Photothermal Combating of Orthotopic Breast Cancer

AbstractThe second near‐infrared (NIR‐II, 1000–1700 nm) light‐activated organic photothermal agent that synchronously enables satisfying NIR‐II fluorescence imaging is highly warranted yet rather challenging on the basis of the overwhelming nonradiative decay. Herein, such an agent, namely TPABT‐TD, was tactfully designed and constructed via employing benzo[c]thiophene moiety as bulky electron donor/π‐bridge and tailoring the peripheral molecular rotors. Benefitting from its high electron donor–acceptor strength and finely modulated intramolecular motion, TPABT‐TD simultaneously exhibits ultralong absorption in NIR‐II region, intense fluorescence emission in the NIR‐IIa (1300–1500 nm) region as nanoaggregates, and high photothermal conversion upon 1064 nm laser irradiation. Those intrinsic advantages endow TPABT‐TD nanoparticles with prominent fluorescence/photoacoustic/photothermal trimodal imaging‐guided NIR‐II photothermal therapy against orthotopic 4T1 breast tumor with negligible adverse effect.

Rational Molecular Design via Cyanobenzene Integration for Constructing Efficient Yellow‐Orange Thermally Activated Delayed Fluorescence Emitters

AbstractDeveloping efficient long‐wavelength thermally activated delayed fluorescence (TADF) emitters is a challenging issue due to inherent limitations of the energy‐gap law. In this contribution, a new molecular design of cyanobenzene‐decorated quinoxaline acceptor, combining two or four cyanobenzene acceptors and a rigid carbazole donor, is successfully utilized to construct long‐wavelength TADF emitters (tCzQx2CN and tCzQx4CN). The two additional cyanobenzene units at the 5‐ and 8‐positions of the quinoxaline acceptor are presumed to interlock the molecular rotations and improve the acceptor strength. Compared with tCzQx2CN, tCzQx4CN exhibits orange‐red emission with a considerable bathochromic shift (>35 nm), as expected. Owing to its enhanced charge transfer and well‐separated highest‐occupied and lowest‐unoccupied molecular orbitals, tCzQx4CN exhibits reduced singlet‐triplet energy splitting and improves reverse intersystem crossing. An organic light–emitting diode (OLED) of tCzQx4CN manifestes bright orange‐red emission (λEL ≈581 nm), a superior external quantum efficiency (EQE) of ≈23.7% (compared with that of tCzQx2CN), and a satisfactory operational lifetime. Furthermore, a tCzQx4CN‐sensitized red‐hyperfluorescent OLED device exhibits excellent performance with an EQE of 16.0% and a CIE(x,y) of (0.62, 0.37). This design would potentially pave the way for constructing red/deep‐red TADF emitters by using a diverse combination of donor/acceptor units.

Multi‐Responsive Thermally Activated Delayed Fluorescence Materials: Optical ZnCl2 Sensors and Efficient Green to Deep‐Red OLEDs

AbstractThermally activated delayed fluorescence (TADF) is an emission mechanism whereby both singlet and triplet excitons can be harvested to produce light. Significant attention is devoted to developing TADF materials for organic light‐emitting diodes (OLEDs), while their use in other organic electronics applications such as sensors, has lagged. A family of TADF emitters, TPAPyAP, TPAPyBP, and TPAPyBPN containing a triphenylamine (TPA) donor and differing nitrogen‐containing heterocyclic pyrazine‐based acceptors is developed and systematically studied. Depending on the acceptor strength, these three compounds emit with photoluminescence maxima (λPL), of 516, 550, and 575 nm in toluene. Notably, all three compounds show a strong and selective spectral response to the presence of ZnCl2, making them the first optical TADF sensors for this analyte. It is demonstrated that these three emitters can be used in vacuum‐deposited OLEDs, which show moderate efficiencies. Of note, the device with TPAPyBPN in 2,8‐bis(diphenyl‐phoshporyl)‐dibenzo[b,d]thiophene (PPT) host emits at 657 nm and shows a maximum external quantum efficiency (EQEmax) of 12.5%. This electroluminescence is significantly red‐shifted yet shows comparable efficiency compared to a device fabricated in 4,4′‐bis(N‐carbazolyl)‐1,1′‐biphenyl (CBP) host (λEL = 596 nm, EQEmax = 13.6%).

Rational Design of AIE Photosensitizers with Full‐Visible Color Emission and Efficient ROS Generation

AbstractOrganic photosensitizers (PSs) with aggregation‐induced emission (AIE) characteristics have shown several advantages in photodynamic therapy (PDT). However, how to design PSs with the required optical and biological properties is a formidable challenge. In this work, a series of AIE active pyridine‐ and pyridinium‐functionalized carbazole with different substituents are designed and synthesized. Compared to other analogs, BK‐Pys‐BP containing benzophenone moiety on the pyridinium unit produces reactive oxygen species (ROS) efficiently in the presence of light due to its large spin‐orbit coupling constant to promote efficient intersystem crossing. Based on its structure, three molecules with enhanced donor–acceptor strength and conjugation are further investigated to realize emission covering the whole visible region. The cationic nature and excellent ROS generation ability of benzophenone‐containing AIE photosensitizers allows for specific imaging of mitochondria, killing cancer cells, and as a singlet oxygen donor to catalyze the formation of endoperoxides. Thus, the present work presents an effective strategy for creating AIE photosensitizers with efficient ROS generation for wide biological applications.

NIR-Absorbing 1,1,4,4-Tetracyanobuta-1,3-diene- and Dicyanoquinodimethane-Functionalized Donor-Acceptor Phenothiazine Derivatives: Synthesis and Characterization.

A series of symmetrical and unsymmetrical donor-acceptor type phenothiazine derivatives 1-18 were designed and synthesized via Pd-catalyzed Sonogashira cross-coupling and [2 + 2] cycloaddition-retroelectrocyclization reactions. The incorporation of cyano-based acceptors 1,1,4,4-tetracyanobutadiene (TCBD) and dicyanoquinodimethane (DCNQ) in the phenothiazine derivatives resulted in systematic variation in the photophysical, thermal, and electrochemical properties. The electronic absorption spectra of the phenothiazine derivatives with strong acceptors 2, 3, 5, 6, 8, 9, 11, 12, 14, 15, 17, and 18 show red-shifted absorption as compared to phenothiazine derivatives 1, 4, 7, 10, 13, and 16 in the near-IR region due to a strong intramolecular charge transfer (ICT) transition. The electrochemical analysis of the phenothiazine derivatives 2, 3, 5, 6, 8, 9, 11, 12, 14, 15, 17, and 18 reveals two reduction waves at low potential due to the TCBD and DCNQ acceptors. The mono-TCBD-functionalized phenothiazine 2 shows higher thermal stability compared to other phenothiazine derivatives. The computational studies on phenothiazines 1-18 reveal the LUMO is substantially stabilized as acceptor strength increases, which lowers the HOMO-LUMO gap.

Tuning the Photophysical Properties of Acceptor-Donor-Acceptor Di-2-(2-oxindolin-3-ylidene) Malononitrile Materials via Extended π-Conjugation: A Joint Experimental and Theoretical Study.

Many optoelectronic applications require organic semiconductor (OSC) materials with high electron affinity. In this work, a series of novel acceptor-donor-acceptor (A-D-A) materials with low-lying LUMO energy levels were designed and characterized. In this strategy, two acceptor dyes, bis-isatin and di-2-(2-oxindolin-3-ylidene) malononitrile, were connected by various π-bridges (benzene ring, benzo[c][1,2,5]thiadiazole, monothiophene, trithiophene). We varied the length of the π-conjugation of the central core and the linkage position of the acceptor core (4- vs. 6-position of the phenyl ring) to investigate the effect on the optical and electrochemical properties of the materials. We performed density functional theory (DFT) and time-dependent DFT (TD-DFT) studies to gain insight into the dyes' electronic properties by determining the energy levels. Our findings demonstrate that with increasing acceptor strength and π-conjugation length of the core, the wavelength of the longest absorption maximum as well as their respective extinction coefficients are enhanced, which results in band-gap reduction either by lowering the LUMO and/or raising the HOMO energy level of the molecules. The potential practical utility of these materials as electron-transport materials for perovskite solar cells (PSCs) has been demonstrated.

Transformation of organic afterglow mechanism from room-temperature phosphorescence to thermally-activated delayed fluorescence in intramolecular charge transfer systems

Organic afterglow materials have emerged as an important class of luminescent materials. These afterglow systems feature small rate constants of key photophysical processes, such as phosphorescence decay and nonradiative decay of triplet excited states. When coupled to other photophysical processes, these afterglow systems would give rise to intriguing photophysical behaviors. Here we report the transformation of afterglow mechanism from room-temperature phosphorescence (RTP) to thermally-activated delayed fluorescence (TADF) in intramolecular charge transfer (ICT) systems. Detailed studies reveal that the increase of the acceptor strength of ICT molecules can reduce singlet-triplet splitting energy and consequently open reverse intersystem crossing (RISC) pathway with moderate kRISC of 1–10 s−1 to harvest triplet energies, leading to the RTP-to-TADF afterglow transformation. Such observations provide deep understanding on the behaviors of organic triplets with small photophysical rate constants. Besides, these organic ICT molecules have been incorporated into aqueous emulsion systems to exhibit promising applications.

Mechanistically transparent models for predicting aqueous solubility of rigid, slightly flexible, and very flexible drugs (MW

Yalkowsky's General Solubility Equation (GSE), with its three fixed constants, is popular and easy to apply, but is not very accurate for polar, zwitterionic, or flexible molecules. This review examines the findings of a series of studies, where we have sought to come up with a better prediction model, by comparing the performances of the GSE to Abraham's Solvation Equation (ABSOLV), and Random Forest regression (RFR) machine-learning (ML) method. Large, well-curated aqueous intrinsic solubility databases are available. However, drugs may be sparsely distributed in chemical space, concentrated in clusters. Even a large database might overlook some regions. Test compounds from under-represented portions of space may be poorly predicted, as might be the case with the 'loose' set of 32 drugs in the Second Solubility Challenge (2020). There appears to be still a need for better coverage of drug space. Increasingly, current trends in predictions of solubility use calculated input descriptors, which may be an advantage for exploring properties of molecules yet to be synthesized. The risk may be that overall prediction approaches might be based on accumulated uncertainty. The increasing use of ML/AI methods can lead to accurate predictions, but such predictions may not readily suggest the strategies to pursue in selecting yet-to-be-synthesized compounds. Based on our latest findings, we recommend predictions based on both 'grouped' ABSOLV(GRP) and 'Flexible Acceptor' GSE(Φ,B) models with the provided best-fit parameters, where Φ is the Kier molecular flexibility index and B is the Abraham H-bond acceptor strength. For molecules with Φ < 11, the prudent choice is to pick the Consensus Model, the average of ABSOLV(GRP) and GSE(Φ,B). For more flexible molecules, GSE(Φ,B) is recommended.

The Mechanism of the Photodimerization of [60]Fullerene Derivatives: It is Still a Charge Separation Followed by a Back Transfer!

AbstractHerein, we present a systematic study of the photochemical behavior of a set of fullerene compounds bearing various organic addends attached to the fullerene cage. It has been shown that the yield of photodimerization products strongly depends on the structure and electronic properties of the material, primarily the electron affinity (acceptor strength) of the modified fullerene core. This finding being inconsistent with the conventional [2+2]cycloaddition mechanism and pointed to an alternative pathway. This proposed pathway involves photoinduced charge separation followed by back charge transfer events leading to the efficient generation of triplet excitons, which are responsible for the formation of dimerized species. The light‐induced charge separation pathway was confirmed by electron spin resonance spectroscopy and was supported by theoretical calculations. The revealed mechanism allows one to control the photochemical behavior of the fullerene derivatives via rational structural engineering: some of the compounds were shown to be fully resistant to photodimerization, which makes them promising candidates in the development of stable organic photovoltaics. Furthermore, this approach could be used to suppress back charge transfer in donor‐acceptor blends and hinder the formation of triplet excitons which represents one a major energy loss channel in the current generation of organic solar cells.

Enantioselective Switch and Potential Applications in Biocatalysis.

Enantioselectivity has always been a key feature of enzymatic synthesis. In some cases, when enzymes are not strictly enantioselective, by tuning the reaction conditions it is possible to induce an enantioselective switch. A transaminase from Halomonas elongata (ω-HeWT), while generally S-selective, could be shifted towards generating the R-enantiomer at higher concentrations of amino acceptor or ionic strength, for example. Other enzymes are reported to have a similar behavior, and here we discuss some of them and their potential applications.

Deep blue thermally activated delayed fluorescent emitters based on methyl-modified acridin-9(10H)-one acceptor

Developing deep blue thermally activated delayed fluorescence (TADF) materials with high efficiencies and good color purities is still a formidable challenge. Here, two novel deep blue TADF emitters, namely MePhCz-AD and MePhCz-AD-Py, were synthesized by incorporating two 3,6-diphenyl-9H-carbazole donors onto the methyl-modified acridin-9(10H)-one (AD) acceptor, respectively. The high rigidity of AD acceptor effectively suppressed non-radiative transition and vibration relaxation of excited states, which is favorable for a good emitting quantum yield and narrow spectral width. The presence of methyl groups at 2,7-sites of AD ring slightly reduced the acceptor strength and contributed to the deep blue emission color. Incorporation of phenyls at 3,6-sites of carbazole ring delocalized the distribution of the highest occupied molecular orbital (HOMO), promoting the radiative decay process. As a result, these two emitters showed deep blue TADF with fast radiative decay rates kR > 107 s−1. In addition, with the introduction of pyridyl (Py) group at 10-site of AD ring instead of phenyl, both the spin-orbital coupling (SOC) constant between high-lying triplet states and the lowest singlet excited state (S1) and thus the reverse intersystem crossing rate constant (kRISC) were dramatically promoted for MePhCz-AD-Py. The organic light-emitting diode with MePhCz-AD-Py as doped emitter exhibited a maximum external quantum efficiency (EQEmax) of 15.4% with a relatively narrow FWHM of 60 nm and the deep-blue color coordinates of (0.15, 0.15).

Dative quadruple bonds between d10 transition metals and beryllium in BeM(PMe3)2 and BeM(CO)2 (M = Ni, Pd, and Pt) complexes: Transition metal fragments as six‐electron donor and two‐electron acceptor

AbstractThe structure, chemical bonding, and reactivity of neutral 16 valence electrons (VE) transition metal complexes of beryllium, BeM(PMe3)2 (1M‐Be) and BeM(CO)2 (2M‐Be, M = Ni, Pd, and Pt) were studied. The molecular orbital and EDA‐NOCV analysis suggest dative quadruple bonds between the transition metal and beryllium, viz., one Be→M σ bond, one Be←M σ bond, and two Be←M π bonds. The strength of these bonding interactions varies based on the ligands coordinated to the transition metal. The Be←M σ bond is stronger than the Be→M σ bond when the ligand is PMe3, whereas the reverse order is observed when the ligand is CO. This is attributed to the higher π acceptor strength of CO as compared to PMe3. Since these complexes have M–Be dative quadruple bonds, the beryllium center is susceptible to ambiphilic reactivity, as indicated by high proton and hydride affinity values.

Thermoelectric properties of self-assembled thiophene derivatives: effect of molecular structure on doping efficiency and Fermi level alignment

Organic materials have emerged as promising candidates for thermoelectric (TE) applications, offering distinct advantages over their inorganic counterparts. While polymers have received considerable attention in this context, small organic semiconductors have not been widely explored due to the challenge of obtaining high conducting films. Advancing this field necessitates the development of novel materials and a deeper understanding of their structure-dependent TE properties. The present study reports how slight modifications to the molecular structure affect the TE characteristics of two thiophene-based small molecules (OT1 and OT2), which differ only in their end substitution with varying acceptor strengths. This difference led to distinct morphologies when self-assembled. OT2 doped with FeCl3 displayed high electrical conductivity and power factor (52.0 μW/mK2), while OT1 exhibited significantly lower conductivity and power factor (1.6 μW/mK2) under identical conditions. The weaker acceptor strength of the end group in OT2 resulted in higher doping efficiency. Improved charge carrier transport in OT2 was due to a low-lying highest occupied molecular orbital (HOMO) energy level and enhanced density of states around the HOMO after doping. Maintaining its self-assembly after doping was also crucial for better electrical properties. These findings emphasize the importance of molecular engineering in developing high-performance TE materials.

A Review on Fullerene Derivatives with Reduced Electron Affinity as Acceptor Materials for Organic Solar Cells

Organic solar cells (OSCs) represent a promising emerging photovoltaic technology offering such benefits as light weight, mechanical flexibility, semitransparency, environmental friendliness and aesthetic design of solar panels. Furthermore, organic solar cells can be produced using scalable and high-throughput solution-based printing and coating technologies, which are expected to lead to very low product costs. Fullerene derivatives have been used as acceptor materials in virtually all efficient organic solar cells for more than two decades, following the demonstration of the first proof-of-concept devices in the middle of 1990s. Still, the power conversion efficiencies of fullerene-based organic solar cells became stuck at around 12% due to the suboptimal optoelectronic properties of conventional fullerene acceptors. Therefore, the latest efficiency records (&gt;18%) for organic solar cells were set using different types of non-fullerene acceptor (NFA) materials with tailorable properties. However, NFA materials appeared to be very sensitive to light, thus impairing the operational stability of OSCs. On the contrary, there is growing evidence that rationally designed fullerene-based acceptors enhance the photostability of conjugated polymers and also NFAs, when used in ternary blends. Hence, a renaissance of fullerene-based materials is currently expected in the context of their use in multicomponent organic solar cells (e.g., as stabilizers) and also lead halide perovskite solar cells, where they play an important role of electron transport materials. The success in both of these applications requires the tunability of optoelectronic characteristics of fullerene derivatives. In particular, electron affinity of the fullerene cage has to be reduced in many cases to match the energy levels of other absorber material(s). Herein, we present a systematic review of different strategies implemented to reduce the acceptor strength of the fullerene derivatives and the results of their performance evaluation in OSCs with model conjugated polymers. Particular attention is paid to correlations between the chemical structure of organic addends and their influence on the electronic properties of the fullerene core. We believe this review would be valuable to researchers working on the rational design of new fullerene-based materials with tailored properties for photovoltaic and other electronic applications.

Achieving narrowband emissions with tunable colors for multiple resonance-thermally activated delayed fluorescence materials: effect of boron/nitrogen number and position.

The boron/nitrogen (B/N)-based multiple resonance-thermally activated delayed fluorescence (MR-TADF) materials with tunable colors have attracted widespread attention owing to their great potential in next-generation display, white lighting, and imaging applications. Numerous MR-TADF emitters with different B/N number and position have been reported to realize full-color narrowband emissions. To gain a better understanding of the effect of B/N number and position on the photo-electronic properties, geometric and electronic properties, Huang-Rhys factors and reorganization energies, charger transfer and absorption/emission properties were analyzed in detail to determine the structure-property relationship for the investigated molecules. The calculated results show that the molecules with para-atoms having the same electronic characteristics (para-B-π-B/para-N-π-N) exhibited smaller structural relaxations upon excitation, and the molecules with increased B/N atoms showed more obvious short-range charge transfer (SRCT) properties. Besides, the para-B-π-N and para-B-π-B/para-N-π-N substructures could reduce and enhance the donor and acceptor strengths, respectively, leading to tunable HOMO-LUMO gaps and emission colors. Such theoretical insights well rationalize the experimental results, revealing that the small reorganization energy and dominant SRCT property should be two key factors in realizing narrowband emissions of MR-TADF materials. These findings and understandings could give an in-depth insight into the structure-property relationship, providing molecular design strategies for the exploration of narrowband MR-TADF materials with tunable emission colors.

Dicyanomethylene-functionalized s-indacene-based D-π-A-π-D dyes exhibiting large near-infrared two-photon absorption cross-section

A series of centrosymmetric chromophores with D-π-A-π-D molecular architecture has been synthesized and characterized with respect to their third-order nonlinear optical (NLO) properties. The investigated compounds feature novel central electron acceptor fragments composed of derivatives of s-indacene-1,3,5,7(2H,6H)-tetraone (Janus dione), in which the carbonyls have been substituted by either two or four dicyanomethylene groups. Due to the increased electron acceptor strength, up to twofold increase in two-photon absorption (2 PA) cross-section is observed for the dyes in comparison to the structural analogues based on the parent compound. The best performing dye exhibits intensive 2 PA absorption band in 1000–1300 nm range with a peak cross-section value of 11,000 GM. Optical limiting was successfully demonstrated using the compound, marking the presented chemical design as a promising direction for optical power-limiting applications in the important near-infrared (NIR) spectral region.Electronic Supplementary Information (ESI) available: experimental procedures, results of quantum chemical calculations, NMR spectra.