Tuning Transition Electric and Magnetic Dipole Moments: [7]Helicenes Showing Intense Circularly Polarized Luminescence

Synthesis and resolution of multi-chiral carbonyl-N embedded hetero[7]helicenes for efficient circularly polarized luminescence

Novel carbonyl-N embedded hetero[7]helicene diastereomers incorporating axially chiral binaphthyl were facilely synthesized and separated. The separated homochiral hetero[7]helicenes exhibit intense green photoluminescence and circularly polarized luminescence (CPL) with luminescence dissymmetry factors (glum) of 1.4 × 10-3 due to the intrinsic helical multiple-resonance skeleton.

Carbonyl mediated fluorescence in aceno[n]helicenones and fluoreno[n]helicenes

Helicenes are very attractive chiral non-planar polycyclic aromatic hydrocarbons possessing strong chiroptical properties. However, most of the helicenes absorb light mainly in the ultraviolet region, with only a small segment in the blue part of the visible spectrum. Furthermore, carbo[n]helicenes exhibit only weak luminescence that limits their utilization. Herein, we demonstrate that peripheral decoration of the helicene backbone with an aryl-carbonyl group shifts the absorption to the visible region and simultaneously improves their fluorescence quantum yields. We thus show that the carbonyl group, commonly considered as detrimental to emission, has the capability of improving optical and photophysical properties. Two different families, aceno[n]helicenones and fluoreno[n]helicenes, are presented with comprehensive spectrochemical characterization. TD-DFT calculations were implemented to clarify their electronic profiles. We show that increasing the helical length in aceno[n]helicenes increases absorption onset, g abs and g lum. Extension of the peripheral aromatic part in fluoreno[n]helicenes leads to a blue shift in both absorption and emission.

Enhancing Circularly Polarized Electroluminescence through Energy Transfer within a Chiral Polymer Host

Organic light-emitting diodes (OLEDs) that are able to emit high levels of circularly polarized (CP) light hold significant promise in numerous future technologies. Such devices require chiral emissive materials to enable CP electroluminescence. However, the vast majority of current OLED emitter classes, including the state-of-the-art triplet-harvesting thermally activated delayed fluorescence (TADF) materials, produce very low levels of CP electroluminescence. Here a host-guest strategy that allows for energy transfer between a chiral polymer host and a representative chiral TADF emitter is showcased. Such a mechanism results in a large amplification of the circular polarization of the emitter. As such, this study presents a promising avenue to further boost the performance of circularly polarized organic light-emitting diode devices, enabling their further development and eventual commercialization.

Supramolecular Polymerization as a Tool to Reveal the Magnetic Transition Dipole Moment of Heptazines

Heptazine derivatives have attracted significant interest due to their small S1-T1 gap, which contributes to their unique electronic and optical properties. However, the nature of the lowest excited state remains ambiguous. In the present study, we characterize the lowest optical transition of heptazine by its magnetic transition dipole moment. To measure the magnetic transition dipole moment, the flat heptazine must be chiroptically active, which is difficult to achieve for single heptazine molecules. Therefore, we used supramolecular polymerization as an approach to make homochiral stacks of heptazine derivatives. Upon formation of the supramolecular polymers, the preferred helical stacking of heptazine introduces circular polarization of absorption and fluorescence. The magnetic transition dipole moments for the S1 ← S0 and S1 → S0 are determined to be 0.35 and 0.36 Bohr magneton, respectively. These high values of magnetic transition dipole moments support the intramolecular charge transfer nature of the lowest excited state from nitrogen to carbon in heptazine and further confirm the degeneracy of S1 and T1.

Circularly polarised luminescence from intramolecular excimer emission of bis-1,8-naphthalimide derivatives

Chiral excimers exhibit unique photophysical behaviour. However, further molecular design is required along with systematic studies on the effect of spacer groups and solvent polarity. In this study, we prepared four circularly polarised luminescence (CPL)-active molecules that exhibit intramolecular excimer emission. Bis-1,8-naphthalimide (bNI) derivatives D-LybNI, L-LybNI, D-LyMebNI, and L-LyMebNI were prepared with chiral backbones and alkyl linkages between the NI rings with chain lengths of five carbon atoms, suitable for excimer fluorescence. The fluorescence properties were investigated experimentally and theoretically using density functional theory. The molecules exhibited intramolecular excimer fluorescence in polar organic solvents. Mirror-image circular dichroism and CPL spectra were obtained for the D and L forms. D- and L-LyMebNI exhibited relatively large luminescence dissymmetry factors (|glum|) in acetonitrile of 1.9 × 10-3 and 1.6 × 10-3, respectively. Thus, this study demonstrates chiral bNI derivatives with simple synthesis procedures that emit intramolecular excimer fluorescence and have effective CPL properties. These molecules are promising for developing organic molecular systems with bright, highly polarised emission.

Simultaneous discovery of chiral and achiral dyes: elucidating the optical functions of helical and flag-hinged boron tetradentate complexes

The construction of novel complexes can lead to the manifestation of unexpected structures and properties, thereby making chemical exploration in experiments a potential source for novel discoveries. In this study, by reacting 6,6'-dihydrazineyl-2,2'-bipyridine with acyl chlorides and subsequently coordinating with boron trifluoride, two different boron-tetradentate ligand complexes were simultaneously generated. One of these complexes exhibited a unique structure in which tetra-BF2 moieties coordinated to all four coordination sites of the ligand molecule, forming a flag-hinged structure around the bipyridine part. The second complex featured a helical structure formed by the hybridization of two BF2 and one B-O-B moieties, representing a highly unusual form of the complex. The structures of these two boron complexes were consistently observed when various substituted acyl chlorides were employed. Furthermore, it was found that enhancing electron-donor properties could induce a redshift in emissions. Utilizing the dimethylamino group as the proton receptor promoted a yellow-to-blue fluorescence switch in the tetra-BF2 complex and an OFF/ON fluorescence in the B-O-B bridged complex upon protonation. The helical chirality observed in the latter complex resulted in stable (P)/(M)-enantiomers after optical resolution. This complex exhibited circular dichroism with a |gabs| of up to 1.2 × 10-2 and circularly polarized luminescence with a |glum| on the order of 10-3 in solution and polymer film.

Boron-containing helicenes as new generation of chiral materials: opportunities and challenges of leaving the flatland

Increased interest in chiral functional dyes has stimulated activity in the field of boron-containing helicenes over the past few years. Despite the fact that the introduction of boron endows π-conjugated scaffolds with attractive electronic and optical properties, boron helicenes have long remained underdeveloped compared to other helicenes containing main group elements. The main reason was the lack of reliable synthetic protocols to access these scaffolds. The construction of boron helicenes proceeds against steric strain, and thus the methods developed for planar systems have sometimes proven ineffective in their synthesis. Recent advances in the general boron chemistry and the synthesis of strained derivatives have opened the way to a wide variety of boron-containing helicenes. Although the number of helically chiral derivatives is still limited, these compounds are currently at the forefront of emissive materials for circularly-polarized organic light-emitting diodes (CP-OLEDs). Yet the design of good emitters is not a trivial task. In this perspective, we discuss a number of requirements that must be met to provide an excellent emissive material. These include chemical and configurational stability, emission quantum yields, luminescence dissymmetry factors, and color purity. Understanding of these parameters and some structure-property relationships should aid in the rational design of superior boron helicenes. We also present the main achievements in their synthesis and point out niches in this area, e.g. stereoselective synthesis, necessary to accelerate the development of this fascinating class of compounds and to realize their potential in OLED devices and in other fields.

Impact of helical elongation of symmetric oxa[n]helicenes on their structural, photophysical, and chiroptical characteristics

The adjustment of the main helical scaffold in helicenes is a fundamental strategy for modulating their optical features, thereby enhancing their potential for diverse applications. This work explores the influence of helical elongation (n = 5-9) on the structural, photophysical, and chiroptical features of symmetric oxa[n]helicenes. Crystal structure analyses revealed structural variations with helical extension, impacting torsion angles, helical pitch, and packing arrangements. Through theoretical investigations using density functional theory (DFT) calculations, the impact of helical extension on aromaticity, planarity distortion, and heightened chiral stability were discussed. Photophysical features were studied through spectrophotometric analysis, with insights gained through time-dependent DFT (TD-DFT) calculations. Following optical resolution via chiral high-performance liquid chromatography (HPLC), the chiroptical properties of both enantiomers of oxa[7]helicene and oxa[9]helicene were investigated. A slight variation in the main helical scaffold of oxa[n]helicenes from [7] to [9] induced an approximately three-fold increase in dissymmetry factors with the biggest values of|glum| of oxa[9]helicene (2.2 × 10-3) compared to|glum|of oxa[7]helicene (0.8 × 10-3), findings discussed and supported by TD-DFT calculations.

Synthesis and Characterization of a Cyclic Trimer of a Chiral Spirosilabifluorene

A chiral shape-persistent macrocyclic compound (Si-[3]), designed by the C/Si substitution in the spiro-atom of spirobifluorene in the cyclic trimer (C-[3]), has been successfully synthesized in this study. The C/Si substitution made the spiro-conjugation and energy levels of HOMO and LUMO decrease. Due to the silicon substitution, the macrocyclic compound Si-[3] was able to be degraded by fluoride ions, but its reaction rate was slower than that of the unsubstituted spirosilabifluorene, showing the chemical stability of Si-[3]. Furthermore, the chiroptical properties of Si-[3] with D3-symmetric macrocyclic structure were investigated, and (P,P,P)-Si-[3] showed a high emission quantum yield (Φf=80 %) and moderate dissymmetry factor of circularly polarized luminescence (CPL) (glum,exp=-1.2×10-3). According to the time-dependent density-functional theory (TD-DFT) calculations using polarizable continuum model (PCM), the bright CPL from Si-[3] was explained by a planarization of one bisilafluorenyl moiety at the excited state, which is responsible for the almost fully-allowed radiative transition with a short emission lifetime of τf=1.89 ns.

Expanded Azahelicenes with Large Dissymmetry Factors

Expanded azahelicenes, as heteroanalogues of helically chiral helicenes, hold significant potential for chiroptical materials. Nevertheless, their investigation and research have remained largely unexplored. Herein, we present the facile synthesis of a series of expanded azahelicenes NHn (n=1-5) consisting of 11, 19, 27, 35, and 43 fused rings, mainly by Suzuki coupling followed by Bi(OTf)3-mediated cyclization of vinyl ethers. The structures of NH2, NH3 and NH4 were confirmed through X-ray crystallography analysis, and their (P)- and (M)- enantiomers were also isolated with chiral high performance liquid chromatography. The enantiomers exhibit large absorption (abs) and luminescence (lum) dissymmetry factors, with |gabs|max=0.044; |glum|max=0.003 for NH2, |gabs|max=0.048; |glum|=0.014 for NH3, and |gabs|max=0.043; |glum|max=0.021 for NH4, which are superior to their respective all-carbon analogues.

S-Shaped Helical Singlet Diradicaloid and Its Transformation to Circumchrysene via a Two-Stage Cyclization

Polycyclic hydrocarbons with diradical and polyradical characters usually display unique reactivities in ring-cyclization reactions. However, such reactions are rarely used to construct π-extended polycyclic aromatic hydrocarbons. Here, we describe the synthesis of an S-shaped doubly helical singlet diradicaloid compound and its facile transformation into an unprecedented circumchrysene via a two-stage ring cyclization, which includes: (1) an eletrocylization from diradicaloid precursor and (2) a Scholl reaction. The reaction mechanism was investigated through in situ spectroscopic studies, assisted by theoretical calculations. This reaction sequence yields an optically resolved π-extended [5]helicene derivative with a fluorescence quantum yield up to 85% and a circularly polarized luminescence brightness up to 6.05 M-1 cm-1 in the far-red to near-infrared regions. This sequence also yielded a highly delocalized circumchrysene molecule, exhibiting large electron delocalization, moderate fluorescence quantum yield, and multistage redox properties.

Unraveling the Remarkable Influence of Substituents on the Emission Variation and Circularly Polarized Luminescence of Dinuclear Aluminum Triple-Stranded Helicates

This study explored the development of functional dyes using aluminum, focusing on aluminum-based dinuclear triple-stranded helicates, and examined the effects of substituent variations on their structural and optical properties. Key findings revealed that the modification of methyl groups to the pyrrole positions significantly extended the conjugation system, resulting in a red shift in the absorption and emission spectra. Conversely, the modification of methyl groups at the methine positions due to steric hindrances increased the torsion angle of the ligands, leading to a blue shift in the absorption and emission spectra. A common feature across all complexes was that in the excited state, one of the three ligands underwent significant structural relaxation. This led to a pronounced Stokes shift and minimal spectra overlap with high photoluminescence behaviors. Moreover, our research extended to the optical resolution of the newly synthesized complexes by analyzing the chiroptical properties of the resulting enantiomers, including their circular dichroism and circularly polarized luminescence. These insights offer valuable contributions to the design and application of novel aluminum-based functional dyes, potentially influencing a range of fields, from materials science to optoelectronics.

One-pot synthesis of azabora[6]helicene by a Schiff base forming reaction

Azabora[6]helicene as a new heterohelicene analogue was synthesized by a one-pot reaction of commercially available 2,6-diaminopyridine and benzo[c,d]indole-2(1H)-one and subsequent boron coordination. While the single-crystal X-ray diffraction analysis elucidated a helical structure in the solid state, a dynamic helicity inversion was observed in solution.

Circularly polarized luminescence and high photoluminescence quantum yields from rigid 5,10-dihydroindeno[2,1-a]indene and 2,2'-dialkoxy-1,1'-binaphthyl conjugates and copolymers

5,5,10,10-Tetramethyl-5,10-dihydroindeno[2,1-a]indene (COPV1(Me)) was installed into either the 3,3'- or 6,6'-positions of chiral 2,2'-dioctyloxy-1,1'-binaphthyl to afford 2 : 1 conjugates (monomeric compounds) and 1 : 1 copolymers. These compounds showed high photoluminescence quantum yields of >0.95 whilst also exhibiting circular dichroism (CD) and circularly polarized luminescence (CPL). The dissymmetry factors of CPL (gCPL) for the 3,3'- and 6,6'-monomeric compounds in THF were 6.6 × 10-4 and 3.3 × 10-4, respectively. The 3,3'-isomer has a higher g value than the 6,6'-isomer, which was attributed to the difference in the extent of π-conjugation and the angle between electronic and magnetic transition moments. The gCPL values of the 3,3'-linked and 6,6'-linked copolymers were 1.1 × 10-3 and 6.8 × 10-4, respectively. The structural rigidity of the COPV units is beneficial to achieve relatively high g values whilst maintaining a photoluminescence quantum yield that is close to unity by using a single type of fluorophore.

Highly Luminescent Chiral Double π-Helical Nanoribbons

Unveiling the mechanism behind chirality propagation and dissymmetry amplification at the molecular level is of significance for the development of chiral systems with comprehensively outstanding chiroptical performances. Herein, we have presented a straightforward Cu-mediated Ullmann homocoupling approach to synthesize perylene diimide-entwined double π-helical nanoribbons encompassing dimer, trimer, and tetramer while producing homochiral or heterochiral linking of chiral centers. A significant dissymmetry amplification was achieved, with absorption dissymmetry factors (|gabs|) increasing from 0.009 to 0.017 and further to 0.019, and luminescence dissymmetry factors (|glum|) rising from 0.007 to 0.013 and eventually to 0.015 for homochiral double π-helical oligomers. The disparity of magnetic transition dipole moment (m) densities in homochiral and heterochiral tetramers by time-dependent density functional theory calculations confirmed that homochiral oligomerization can maximize the total m, which is favorable for achieving ever-increasing g factors. Notably, these double π-helices exhibited exceptional photoluminescence quantum yields (ΦPL) ranging from 83 to 95%. The circularly polarized luminescence brightness (BCPL) eventually reached a remarkable 575 M-1 cm-1 for the homochiral tetramer, which is among the highest values reported for chiral small molecules. This kind of linearly extended double π-helices offers a platform for a comprehensive understanding of the mechanism behind chirality propagation and dissymmetry amplification.

Hydrophilic Cocrystals with Water Switched Luminescence

Utilizing water molecules to regulate the luminescence properties of solid materials is highly challenging. Herein, we develop a strategy to produce water-triggered luminescence-switching cocrystals by coassembling hydrophilic donors with electron-deficient acceptors, where 1,2,4,5-Tetracyanobenzene (TCNB) was used as the electron acceptor and pyridyl benzimidazole derivatives were used as the electron donors enabling multiple hydrogen-bonds. Two cocrystals, namely 2PYTC and 4PYTC were obtained and showed heat-activated emission, and such emission could be quenched or weakened by adding water molecules. The cocrystal structure exhibited the donor molecule that can form multiple hydro bonds with water and acceptor molecules due to the many nitrogen atoms of them. The analyses of the photophysical data, powder X-ray diffraction, and other data confirmed the reversible fluorescence "on-off" effects were caused by eliminating and adding water molecules in the crystal lattice. The density functional theory calculations indicate that the vibration of the O-H bond of water molecules in the cocrystal can absorb the excitation energy and suppress fluorescence. Furthermore, the obtained cocrystals also showed temperature, humidity, and H+ /NH4 + responsive emission behavior, which allows their applications as thermal and humidity sensors, and multiple information encryptions. This research paves the way for preparing intelligent hydrophilic organic cocrystal luminescent materials.

The influence of heteroatoms on the circularly polarized luminescence performance of [7]helicene derivatives: aromatic vs. non-aromatic five-membered rings

Helicenes are promising candidates for circularly polarized luminescence (CPL) materials, although the performance is poor due to the unsatisfactory dissymmetric factor (glum) and fluorescence quantum efficiency (ΦF). Herein, the influence of heteroatoms (C, Si, Ge, O, S and Se) on the electronic structures and chiroptical properties of [7]helicene derivatives is systematically investigated using density functional theory (DFT) and time-dependent DFT calculations combined with the thermal vibration correlation function theory. The results reveal that the non-radiative energy consumption processes for helicene systems are closely related to the variation of bond length upon electronic excitation. Moreover, by introducing five-membered rings and heteroatoms, the dipole-forbidden S1 → S0 emission of [7]helicene changes to dipole-allowed transition due to the rearrangement of occupied orbitals and lifting of the nearly degenerate orbitals, resulting in an enhancement of ΦF. As the heteroatomic radius increases, ΦF decreases while the glum increases. Compared with the derivatives containing aromatic five-membered rings ([7]H-O, [7]H-S, and [7]H-Se), the non-aromatic counterparts ([7]H-C, [7]H-Si, and [7]H-Ge) exhibit a balance in ΦF and glum values. The present study helps to clarify the relationship between structures and chiroptical properties and offers a feasible strategy for the future design of helicene-based CPL materials.

Crystalline, Porous Helicene Covalent Organic Frameworks

Helicenes are a class of fascinating chiral helical molecules with rich chemistry developed continuously over the past 100 years. Their helical, conjugated, and twisted structures make them attractive for constructing molecular systems. However, studies over the past century are mainly focused on synthesizing helicenes with increased numbers of aromatic rings and complex heterostructures, while research on inorganic, organic, and polymeric helicene materials is still embryonic. Herein, we report the first examples of helicene covalent organic frameworks, i.e., [7]Helicene sp2 c-COF-1, by condensing [7]Helicene dialdehyde with trimethyl triazine via the C=C bond formation reaction under solvothermal conditions. The resultant [7]Helicene sp2 c-COF-1 exhibits prominent X-ray diffraction peaks and assumes a highly ordered 2D lattice structure originated from the twisted configuration of [7]Helicene unit. The C=C linked [7]Helicene sp2 c-COF-1 materials exhibited extended π conjugation and broadly tuned their absorption, emission, redox activity, photoconductivity, and light-emitting activity, demonstrating rich multifunctionalities and great potentials in developing various applications. This work opens a way to a new family of COFs as well as helicene materials, enabling the exploration of unprecedented π architectures and properties.

Synthesis and Photochromic Properties of Diaryl [5]Helicene Derivatives

Photoinduced tuning of aromaticity and correlative molecular properties has attracted enormous interest in recent years both for modulating photochromism properties and designing novel photochromic materials. Here, we report the synthesis and photochemical characterization of diaryl[5]helicene-based diarylethene molecular switches. 3,4-Bis(2,4-dimethyl-5-phenylthiophen-3-yl)dibenzo[c,g]phenanthrene derivative 1 a showed no photochromic reaction, whereas 3,4-bis(2-methyl-5-phenylthiophen-3-yl)dibenzo[c,g]phenanthrene derivative 2 a and 3,4-bis(5-methyl-2-phenylthiazol-4-yl)dibenzo[c,g]phenanthrene derivative 3 a exhibited reversible photochromism in different aprotic solvents with specific light irradiation. Meanwhile, the diarylethene compounds 2 a and 3 a underwent turn-off mode fluorescence photoswitching together with photoisomerization upon light irradiation. Remarkably, the photoinduced changes in the aromaticity of [5]helicene as a central ethene bridge along with the relative smaller activation energy and higher frequency factor facilitated the thermal bleaching rates of diarylethene switches 2 and 3 in solution. This research provides new insight for designing aromatic diarylethene photoswitches for reversible fluorescence switching, photoinduced changes in aromaticity and further fast thermal back reactions.

Push-pull [7]helicene diimide: excited-state charge transfer and solvatochromic circularly polarised luminescence

In this communication we describe a helically chiral push-pull molecule named 9,10-dimethoxy-[7]helicene diimide, displaying fluorescence (FL) and circularly polarised luminescence (CPL) over nearly the entire visible spectrum dependent on solvent polarity. The synthesised molecule exhibits an unusual solvent polarity dependence of FL quantum yield and nonradiative rate constant, as well as remarkable gabs and glum values along with high configurational stability.

Chiral perturbation on single benzene-based fluorophores: A structure/(chir)optical activity relationship study

In this paper, we describe the synthesis and the (chir)optical properties of a novel series of circularly polarized luminescent emitters. These molecules involve a compact single benzene-based donor-acceptor fluorophore composed of two cyclic alkylamines as electron donors and a phthalonitrile moiety as electron acceptor linked to a configurationally stable BINOL acting as a chiral perturbation unit. These new compounds display fair quantum yields (up to 66%) with emission maxima around 500 nm in toluene solutions, and the study of their chiroptical properties has shown that the cyclic alkylamine's ring size affects significantly the luminescence dissymmetry factors, reaching 2.2 × 10-3 for the larger cyclic alkylamine moieties.

Acid/Base-Triggered Photophysical and Chiroptical Switching in a Series of Helicenoid Compounds

A series of molecules that possess two quinolines, benzoquinolines, or phenanthrolines connected in a chiral fashion by a biaryl junction along with their water-soluble derivatives was developed and characterized. The influence of the structure on the basicity of the nitrogen atoms in two heterocycles was examined and the photophysical and chiroptical switching activity of the compounds upon protonation was studied both experimentally and computationally. The results demonstrated that changes in the electronic structure of the protonated vs. neutral species, promoting a bathochromic shift of dominant electronic transitions and alternation of their character from π-to-π* to charge-transfer-type, when additionally accompanied by the high structural flexibility of a system, leading to changes in conformational preferences upon proton binding, produce particularly pronounced modifications of the spectral properties in acidic medium. The latter combined with reversibility of the read-out make some of the molecules in this series very promising multifunctional pH probes.

Impact of Truncation on Optoelectronic Properties of Azaborole Helicenes

Herein, we report configurationally stable singly-truncated (ST) and structurally flexible doubly-truncated (DT) helically chiral compounds derived from azabora[7]helicenes by a hypothetical removal of a single or two C=C double bonds. The singly-truncated constitutional isomers were synthesized from either benzoisoquinoline (BIQ) or phenantherene building blocks and the corresponding biaryls in excellent yields to give azabora[5]helicenes with a pendant phenyl ring at a sterically hindered position. These systems highlight the electronic impact of the nitrogen donor substitution position. The compounds with a disrupted BIQ moiety (STN) possess remarkable photoluminescence quantum yields of up to 0.53 in the solid state and a blue emission in solution with dissymmetry factors of up to ca. 3×10-3 . Upon cooling to 79 K all compounds exhibit phosphorescence with lifetimes of up to ca. 0.5 s. A methyl complex of azabora[7]helicene showing excellent configurational stability was used as a chiral inducer embedded in an emissive polymer (F8BT) to produce circularly polarized organic light emitting diodes with an electroluminescence dissymmetry factor gEL of up to 0.54.

Substitution Effects on the Photophysical and Photoredox Properties of Tetraaza[7]helicenes

A series of substituted derivatives of tetraaza[7]helicenes were synthesized and the influence of the substitution on their photophysical and photoredox-catalytic properties was studied. The combination of their high fluorescence quantum yields of up to 0.65 and their circularly polarized luminescence (CPL) activity results in CPL brightness values (BCPL ) that are among the highest recorded for [7]helicenes so far. A sulfonylation/hetarylation reaction using cyanopyridines as substrates for photoinduced electron transfer (PET) from the excited helicenes was conducted to test for viability in photoredox catalysis. DFT calculations predict the introduction of electron withdrawing substituents to yield more oxidizing catalysts.

Metal Halide Perovskite Nanocrystals for Near-Infrared Circularly Polarized Luminescence with High Photoluminescence Quantum Yield via Chiral Ligand Exchange

Using ligand exchange on FAPbI₃ perovskite nanocrystals (PNCs) surface with chiral tridentate l-cysteine (l-cys) ligand, we successfully prepared chiral FAPbI₃ PNCs that show circularly polarized luminescence (CPL) (dissymmetry factor; g_lum = 2.1 × 10^-3) in the near-infrared (NIR) region from 700 to 850 nm and a photoluminescence quantum yield (PLQY) of 81%. The chiral characteristics of FAPbI₃ PNCs are ascribed to induction by chiral l/d-cys, and the high PLQY is attributed to the passivation of the PNCs defects with l-cys. Also, effective passivation of defects on the surface of FAPbI₃ PNCs by l-cys results in excellent stability toward atmospheric water and oxygen. The conductivity of the l-cys treated FAPbI₃ NC films is improved, which is attributed to the partial substitution of l-cys for the insulating long oleyl ligand. The CPL of the l-cys ligand treated FAPbI₃ PNCs film retains a g_lum of -2.7 × 10^-4. This study demonstrates a facile yet effective approach to generating chiral PNCs with CPL for NIR photonics applications.

Pivotal role of transition density in circularly polarized luminescence

Realizing high luminescence dissymmetry factor (g) in circularly polarized luminescence (CPL) materials remains a big challenge, which necessitates understanding systematically how their molecular structure controls the CPL. Here we investigate representative organic chiral emitters with different transition density distributions and reveal the pivotal role of transition density in CPL. We rationalize that to obtain large g-factors, two conditions should be simultaneously satisfied: (i) the transition density for the S₁ (or T₁)-to-S₀ emission must be delocalized over the entire chromophore; and (ii) the chromophore inter-segment twisting must be restricted and tuned to an optimal value (∼50°). Our findings offer molecular-level insights into the CPL of organic emitters, with potential applications in the design of chiroptical materials and systems with strong CPL effects.

Noncovalent tailoring of coordination complexes by resorcin[4]arene-based supramolecular hosts

Molecular recognition of guest molecules in a confined cavity is one of the important phenomena in biological and artificial molecular systems. When the guest is trapped within an artificial nano-space, its conformation is fixed in an unusual fashion by noncovalent interactions with host frameworks, and also the guest is kept away from the bulk solvent by the steric effect of the host. Therefore, host-guest formations lead to the effective modulation of the chemical and physical properties of guests via noncovalent interactions. In contrast to the many examples of organic guests, the examples of host-guest formation using coordination complex guests have been less explored. This is simply due to the size and shape complementarity problem between small hosts and large coordination complex guests. Resorcin[4]arene-based supramolecular hosts have been shown to provide internal cavities that are large enough to fully accommodate coordination complexes within the internal spaces via effective molecular interactions. In this article, we focus on supramolecular strategies to control the chemical and physical properties of the coordination complex guests within resorcin[4]arene-based supramolecular hosts. By the careful selection of the host and guest complexes, these combinations can produce a new supramolecular system, showing unusual structures, redox, catalytic, and photophysical properties derived from the entrapped coordination complexes.

Synthesis and Excited-State Properties of Donor-Acceptor Azahelical Coumarins

A set of three donor-acceptor azahelical coumarins (DA-AHCs), namely, H-AHC, Me-AHC, and Ph-AHC, were rationally designed and synthesized, and their excited-state properties were comprehensively investigated. All three DA-AHCs are shown to display very high fluorosolvatochromic shifts as a result of significant intramolecular charge transfer in their excited states. The para-quinoidal forms of the latter apparently contribute predominantly to large dipole moments in their excited states. By virtue of the fact that these helical systems structurally incorporate a highly fluorescent coumarin dye, they exhibit high quantum yields in both solution and solid states. Indeed, their emission behaviors in the crystalline media are shown to be remarkably correlated with their respective crystal packings. Incisive analyses demonstrate (i) strengthening of hydrogen bonding in the excited state promotes quenching (H-AHC), (ii) efficient crystal packing promotes high emission (Me-AHC) by precluding deactivations via vibrational motions, and (iii) loose crystal packing contributes to excited-state deactivation to account for low quantum yields of emission (Ph-AHC).

Naphthalimide-Annulated [n]Helicenes: Red Circularly Polarized Light Emitters

Two [n]heliceno-bis(naphthalimides) 1 and 2 (n = 5 and 6, respectively) where two electron-accepting naphthalimide moieties are attached at both ends of helicene core were synthesized by effective two-step strategy, and their enantiomers could be resolved by chiral stationary-phase high-performance liquid chromatography (HPLC). The single-crystal X-ray diffraction analysis of enantiopure fractions of 1 and 2 confirmed their helical structure, and together with experimental and calculated circular dichroism (CD) spectra, the absolute configuration was unambiguously assigned. Both 1 and 2 exhibit high molar extinction coefficients for the S₀-S₁ transition and high fluorescence quantum yields (73% for 1 and 69% for 2), both being outstanding for helicene derivatives. The red circularly polarized luminescence (CPL) emission up to 615 nm for 2 with CPL brightness (B_CPL) up to 66.5 M^-1 cm^-1 demonstrates its potential for applications in chiral optoelectronics. Time-dependent density functional theory (TD-DFT) calculations unambiguously showed that the large transition magnetic dipole moment |m| of 2 is responsible for its high absorbance dissymmetry (g_abs) and luminescence dissymmetry (g_lum) factor.

A C-H activation-based enantioselective synthesis of lower carbo[n]helicenes

The three-dimensional structure of carbohelicenes has fascinated generations of molecular chemists and has been exploited in a wide range of applications. Their strong circularly polarized luminescence has attracted considerable attention in recent years due to promising applications in new optical materials. Although the enantioselective synthesis of fused carbo- and heterohelicenes has been achieved, a direct catalytic enantioselective method allowing the synthesis of lower, non-fused carbo[n]helicenes (n = 4-6) is still lacking. We report here that Pd-catalysed enantioselective C-H arylation in the presence of a unique bifunctional phosphine-carboxylate ligand provides a simple and general access to these lower carbo[n]helicenes. Computational mechanistic studies indicate that both the C-H activation and reductive elimination steps contribute to the overall enantioselectivity. The observed enantio-induction seems to arise from a combination of non-covalent interactions and steric repulsion between the substrate and ligand during the two key reductive elimination steps. The photophysical and chiroptical properties of the synthesized scalemic [n]helicenes have been systematically studied.

Circularly polarized luminescence of helicenes: A data-informed insight

Helicenes are an interesting scaffold for chiroptical properties and in particular circularly polarized luminescence (CPL). In this short review, we collect the luminescence (g_lum ) and absorption (g_abs ) dissymmetry factors associated to the first Cotton effect of the electronic circular dichroism (ECD) spectrum. Considering the data for 170 [n]-helicenes (n = 4-11), overall we found reasonable correlations between g_lum and g_abs . Despite a few notable exceptions, this would confirm a similarity in the stereochemistry of the ground and emitting excited states for most helicenes. These results may be useful in rationalizing chiroptical data and help chemists in designing new helicene structures with the desired CPL properties.

Effect of Terminal Fluorination on Chiroptical Properties of Carbo[5-8]helicenes: A Systematic Computational Study at the RI-ADC(2) Level

In this article, effects of di-, tetra-, and octafluorination on the structural and chiroptical properties of carbo[5-8]helicenes are reported. Three fluorinated derivatives are designed from each parent carbohelicene by substituting either one, two, or four hydrogens at each terminal ring with fluorine atoms. Excited states properties such as UV-vis and CD spectra of all the six fluorinated carbohelicenes are computed at the ADC(2)/def2-TZVP level, and the results are compared against the results of their respective parent carbohelicene. In addition, CPL properties are also computed at the same level of theory. In the case of carbo[5]helicene (5H), g_CPL decreases with an increase in the degree of fluorination. A similar observation is made in carbo[6]helicene (6H) too, although the value for tetrafluorinated 6H is slightly larger than for difluorinated 6H. Di- and tetrafluorination in carbo[7]helicene (7H), and all types of considered fluorination in carbo[8]helicene (8H) produce improved g_CPL results. Results for fluorescence rate constants are also shown. Results are analyzed in terms of the transition dipole moment vectors and the angles between those.

Efficient Narrowband Circularly Polarized Light Emitters Based on 1,4-B,N-embedded Rigid Donor-Acceptor Helicenes

Narrow-band emission is essential for applicable circularly polarized luminescence (CPL) active materials in ultrahigh-definition CP-OLEDs. One of the most promising classes of CPL active molecules, helicenes, however, typically exhibit broad emission with a large Stokes shift. We present, herein, a design strategy capitalizing on intramolecular donor-acceptor interactions between nitrogen and boron atoms to address this issue. 1,4-B,N-embedded configurationally stable single- and double helicenes were synthesized straightforwardly. Both helicenes show unprecedentedly narrow fluorescence and CPL bands (full width at half maximum between 17-28 nm, 0.07-0.13 eV) along with high fluorescence quantum yields (72-85 %). Quantum chemical calculations revealed that the relative localization of the natural transition orbitals, mainly on the rigid core of the molecule, and small values of root-mean-square displacements between S₀ and S₁ state geometries, contribute to the narrower emission.

The effect of axial and helical chirality on circularly polarized luminescence: lessons learned from tethered twistacenes

The effect of axial and helical twisting on the circularly polarized luminescence of acenes was studied both experimentally and computationally, using four series of tethered twisted acenes. We find that the combination of axial and helical chirality yields the highest anisotropy factors, and that the ratio between the absorption and emission anisotropy factors is an intrinsic property for twistacenes.

Helical β-isoindigo-Based Chromophores with B-O-B Bridge: Facile Synthesis and Tunable Near-Infrared Circularly Polarized Luminescence

It is essential to create organic compounds that exhibit circularly polarized luminescence (CPL) in the near-infrared (NIR) range. Helicene-type emitters possess appealing chiroptical features, however, such NIR molecules are scarce due to a paucity of synthetic strategies. Herein, we developed a series of helical β-isoindigo-based B-O-B bridged aza-BODIPY analogs that were synthesized conveniently. The reaction of diimino-β-isoindigo with a heteroaromatic amine produced a restricted ligand cavity, which triggered off the generation of a B-O-B bridge. The B-O-B bridge led to distorted conformations that satisfy the helical requirements, resulting in excellent spectroscopic and chiroptical properties. Tunable CPL with the highest luminescence dissymmetry factor (g_lum ) of 1.3×10^-3 and a CPL brightness (B_CPL =11.5 M^-1  cm^-1 ) in the NIR region was achieved. This synthetic approach is expected to offer a new opportunity to chiral chemistry and increase flexibility for chiroptical tuning.

Di-2,7-pyrenidecaphyrin(1.1.0.0.0.1.1.0.0.0) and Its Bis-Organopalladium Complexes: Synthesis and Chiroptical Properties

A non-aromatic expanded carbaporphyrinoid, incorporating two built-in 2,7-pyrenylene moieties was synthesized. The intrinsically labile structure was demonstrated by proton-triggered conformational changes between the figure-of-eight and quasi-Möbius conformers. Upon treatment with Pd(OAc)₂ , the reaction produces two bis-Pd^II complexes with distinct coordination modes. Metal coordination serves to fix the macrocyclic frameworks with the net result that both bis-Pd^II complexes could be resolved by high performance liquid chromatography (HPLC) on a chiral stationary phase. The isolated enantiomers showed persistent chiroptical properties as evidenced by the intense response in the circular dichroism (CD) spectra and the record high absorption dissymmetry factors (g_abs of up to 0.038) seen in the near-infrared spectral region. Moreover, the mutual interconversion of these two Pd^II complexes was found to be stereospecific and to favor the more stable isomers under weakly acidic conditions.

Symmetry-breaking host-guest assembly in a hydrogen-bonded supramolecular system

Bio-inspired self-assembly is invaluable to create well-defined giant structures from small molecular units. Owing to a large entropy loss in the self-assembly process, highly symmetric structures are typically obtained as thermodynamic products while formation of low symmetric assemblies is still challenging. In this study, we report the symmetry-breaking self-assembly of a defined C₁-symmetric supramolecular structure from an O_h-symmetric hydrogen-bonded resorcin[4]arene capsule and C₂-symmetric cationic bis-cyclometalated Ir complexes, carrying sterically demanding tertiary butyl (^tBu) groups, on the basis of synergistic effects of weak binding forces. The flexible capsule framework shows a large structural change upon guest binding to form a distorted resorcin[4]arene hexameric capsule, providing an asymmetric cavity. Location of the chiral guest inside the anisotropic environment leads to modulation of its Electric Dipole (ED) and Magnetic Dipole (MD) transition moments in the excited state, causing an increased emission quantum yield, longer emission lifetime, and enhancement of the dissymmetry factor (g_lum) in the circularly polarized luminescence.

Enhanced Optical Properties of Azaborole Helicenes by Lateral and Helical Extension

The synthesis and characterization of laterally extended azabora[5]-, -[6]- and -[7]helicenes, assembled from N-heteroaromatic and dibenzo[g,p]chrysene building blocks is described. Formally, the π-conjugated systems of the pristine azaborole helicenes were enlarged with a phenanthrene unit leading to compounds with large Stokes shifts, significantly enhanced luminescence quantum yields (Φ) and dissymmetry factors (g_lum ). The beneficial effect on optical properties was also observed for helical elongation. The combined contributions of lateral and helical extensions resulted in a compound showing green emission with Φ of 0.31 and |g_lum | of 2.2×10^-3 , highest within the series of π-extended azaborahelicenes and superior to emission intensity and chiroptical response of its non-extended congener. This study shows that helical and lateral extensions of π-conjugated systems are viable strategies to improve features of azaborole helicenes. In addition, single crystal X-ray analysis of configurationally stable [6]- and -[7]helicenes was used to provide insight into their packing arrangements.

Chiral Multi-Resonance TADF Emitters Exhibiting Narrowband Circularly Polarized Electroluminescence with an EQE of 37.2

Highly efficient circularly polarized luminescence (CPL) emitters with narrowband emission remain a formidable challenge for circularly polarized OLEDs (CP-OLEDs). Here, a promising strategy for developing chiral emitters concurrently featuring multi-resonance thermally activated delayed fluorescence (MR-TADF) and circularly polarized electroluminescence (CPEL) is demonstrated by the integration of molecular rigidity, central chirality and MR effect. A pair of chiral green emitters denoted as (R)-BN-MeIAc and (S)-BN-MeIAc is designed. Benefited by the rigid and quasi-planar MR-framework, the enantiomers not only display mirror-image CPL spectra, but also exhibit TADF properties with a high photoluminescence quantum yield of 96 %, a narrow FWHM of 30 nm, and a high horizontal dipole orientation of 90 % in the doped film. Consequently, the enantiomer-based CP-OLEDs achieved excellent external quantum efficiencies of 37.2 % with very low efficiency roll-off, representing the highest device efficiency of all the reported CP-OLEDs.

Highly Fluorescent Bipyrrole-Based Tetra-BF2 Flag-Hinge Chromophores: Achieving Multicolor and Circularly Polarized Luminescence

This study reports the facile syntheses of tetra-boron difluoride (tetra-BF₂ ) complexes, flag-hinge-like molecules that exhibit intense green-to-orange luminescence in solution and yellow-to-red emission in the solid states. Single-crystal structure analysis and density functional theory calculations suggested a bent structure of this series of compounds. The complexes also exhibited excellent optical properties, with quantum yields reaching 100 % and a large Stokes shift. These properties were attributed to the altered bending angle of the molecule in the S₁ excited state. As the rotational motion was suppressed around the 2,2'-bipyrrole axis, atropisomers with axial chirality were formed, which are optically resolvable into (R) and (S)-enantiomers through a chiral column. The atropisomers thus function as circularly polarized luminescent (CPL) materials, in which the color (green, green-yellow, and yellow) can be varied by controlling the aggregation state. This rational design of multi-BF₂ complexes can potentially realize novel photofunctional materials.

Boosting Circularly Polarized Luminescence Performance by a Double π-Helix and Heteroannulation

Design challenges in the development of circularly polarized luminescence (CPL) materials are focused on balancing the luminescence dissymmetry factor (g_lum) and photoluminescence quantum yield (Φ_PL) by regulating the electric (μ) and magnetic (m) transition dipole moment vectors. Aiming at designing efficient CPL emitters and clarifying the chiroptical variation mechanism, herein, we present a double π-helix based on a cyclooctatetraene-embedded perylene diimide dimer that combines chirality with molecular entanglement and very high barriers for racemization. Through finely regulating the magnitudes of μ and m, the maximal dissymmetry factors |g_abs| and |g_lum| can be boosted to 0.035 and 0.030, respectively, as revealed by circular dichroism (CD) and CPL spectra. The results indicate a 3-fold improvement of g values and a modulated Φ_PL from 1a, 4, to 5 by nitrogen heteroannulation at the bay region. The CPL brightness (B_CPL) of 5 reaches a recorded value of up to 573.4 M^-1 cm^-1, among the highest values of chiral small molecules reported so far. This work has provided a comprehensive insight into a new class of chiral materials with high CPL activities, further laying molecular fundamentals for chiral optoelectronics.

The contribution of intermolecular spin interactions to the London dispersion forces between chiral molecules

Dispersion interactions are one of the components of van der Waals (vdW) forces that play a key role in the understanding of intermolecular interactions in many physical, chemical, and biological processes. The theory of dispersion forces was developed by London in the early years of quantum mechanics. However, it was only in the 1960s that it was recognized that for molecules lacking an inversion center, such as chiral and helical molecules, there are chirality-sensitive corrections to the dispersion forces proportional to the rotatory power known from the theory of circular dichroism and with the same distance scaling law R^-6 as the London energy. The discovery of the chirality-induced spin selectivity effect in recent years has led to an additional twist in the study of chiral molecular systems, showing a close relation between spin and molecular geometry. Motivated by it, we propose in this investigation to describe the mutual induction of charge and spin-density fluctuations in a pair A-B of chiral molecules by a simple physical model. The model assumes that the same fluctuating electric fields responsible for vdW forces can induce a magnetic response via a Rashba-like term so that a spin-orbit field acting on molecule B is generated by the electric field arising from charge density fluctuations in molecule A (and vice versa). Within a second-order perturbative approach, these contributions manifest as an effective intermolecular exchange interaction. Although expected to be weaker than the standard London forces, these interactions display the same R^-6 distance scaling.

[5]Helicene-based chiral triarylboranes with large luminescence dissymmetry factors over a 10-2 level: synthesis and design strategy via isomeric tuning of steric substitutions

Constructing chiral luminescent systems with both large luminescence dissymmetry factor (g_lum) and high luminous efficiency has been considered a great challenge. We herein describe a highly efficient approach to sterically stabilize the helical configurations of carbo[5]helicenes for improved CPL properties in a series of π-donor and π-acceptor substituted [5]helicenes (1, 2, 3, 4 and 5). Enabled by the ortho-installation of methyl groups as well as the steric effects of triarylamine (Ar₃N) and triarylborane (Ar₃B) handles in meta-substituted [5]helicenes, their optical resolution into enantiomers has been accomplished using preparative chiral HPLC. The molecular chirality of [5]helicenes can be transferred to Ar₃B and Ar₃N as light emitters, which allowed further investigations of their chiroptics, including optical rotation, circular dichroism (CD) and circularly polarized luminescence (CPL). Remarkably, 4 has been demonstrated to display dramatically enhanced CPL performance with a much larger g_lum (>1.2 × 10^-2) and an increased emission quantum efficiency (Φ_S = 0.75) compared with the other analogues, as a result of the isomeric tuning of substitutions with differential steric and electronic effects. These experimentally observed CPL activities were rationalized by TD-DFT computations for the angle (θ_μ,m) between electric and magnetic transition dipole moments in the excited states. In addition, the conspicuous intramolecular donor-acceptor charge transfer led to thermal responses in the emissions of 2 and 4 over a broad temperature range.