Functionalized cationic [4]helicenes with unique tuning of absorption, fluorescence and chiroptical properties up to the far-red range

Cyclic Azahelicene Dimers Showing Bright Circularly Polarized Luminescence and Selective Fluoride Recognition

Although helicenes are promising molecules, the synthetic difficulty and tediousness have often been problems, and only small amounts of optically pure helicenes have been obtained by using chiral HPLC in most cases. Herein, aza[7]helicenes or closed-aza[7]helicenes with (1R)-menthyl substituents were selectively synthesized via the intramolecular Scholl reaction, and the diastereomeric pairs were separated by silica gel column chromatography. The optically pure helicenes were further transformed into the corresponding cyclic dimers, and the chiroptical properties were investigated. The rigid π-frameworks of the dimers led to the high molar extinction coefficients and fluorescence quantum yields, while the twisted helicene moieties induced clear Cotton effects and CPL in the visible region, and the high CPL brightness (BCPL) was achieved. Furthermore, the cyclic dimers were found to have the macrocyclic cavity with the two NH groups suitable for the selective binding of a fluoride anion, which induced significantly redshifted fluorescence and CPL in the red region.

Achieving Circularly Polarized Phosphorescence through Noncovalent Clipping of Metallotweezers

Circularly polarized phosphorescent materials, based on host-guest complexation, have received significant attention due to their outstanding emission performance in solutions. Recent studies have primarily focused on macrocyclic host-guest complexes. To broaden the scope of this research, there is a keen pursuit of developing novel chiral phosphorescent host-guest systems. Metallotweezers with square-planar d8 transition metal complexes emerge as promising candidates for achieving this objective. Specifically, metallotweezers, comprising platinum(II) terpyridine and gold(III) diphenylpyridine pincers on a diphenylpyridine scaffold, have been designed and synthesized. Due to the preorganization effect rendered by the diphenylpyridine scaffold, the resulting metallotweezers are capable of complexing with each other and forming quadruple stacking structures. The phosphorescent emission is enhanced owing to the synergistic rigidifying and shielding effects. Meanwhile, the steric effect of chiral (1R) pinene units on the platinum(II) terpyridine pincers results in a stereospecific twist for the quadruple stacking structures. Thus, the chirality transfers from the molecular to the supramolecular level. By a combination of phosphorescent enhancement and supramolecular chirality for the clipping complex, circularly polarized phosphorescent emission is achieved. Overall, noncovalent clipping of metallotweezers exemplified in the current study presents a novel and effective approach toward solution-processable circularly polarized phosphorescent materials.

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.

Triple para-Functionalized Cations and Neutral Radicals of Enantiopure Diaza[4]helicenes

Modulation of absorbance and emission is key for the design of chiral chromophores. Accessing a series of compounds absorbing and emitting (circularly polarized) light over a wide spectral window and often toward near-infrared is of practical value in (chir)optical applications. Herein, by late-stage functionalization on derivatives bridging triaryl methyl and helicene domains, we have achieved the regioselective triple introduction of para electron-donating or electron-withdrawing substituents. Extended tuning of electronic (e.g., E1/2red -1.50 V → -0.68 V) and optical (e.g., emission covering from 550 to 850 nm) properties is achieved for the cations and neutral radicals; the latter compounds being easily prepared by mono electron reductions under electrochemical or chemical conditions. While luminescence quantum yields can be increased up to 70% in the cationic series, strong Cotton effects are obtained for certain radicals at low energies (λabs ∼ 700-900 nm) with gabs values above 10-3. The open-shell electronic nature of the radicals was further characterized by electron paramagnetic resonance revealing an important spin density delocalization that contributes to their persistence.

Dibenzoindolo[1,8]naphthyridines: Synthesis and Characterization of X-Shaped Aza[4,6]helicenes

This report describes a one-pot multi-step procedure to obtain double azahelicenes via nucleophilic fluorine substitution of 2,2-di(2-bromophenyl)-1,1-difluoroalkenes and palladium-catalysed ring closing reaction. The developed synthesis approach allows easy diversification of substituents at all four fragments of the obtained X-shaped aza[4,6]helicene entity. Yields range from 20 % to 60 % among 12 product examples. X-ray single crystal analysis reveals formation of (P,P) and (M,M) enantiomer mixture of products. Optical and electrochemical properties of selected products were studied by performing UV/Vis absorption, photoluminescence and cyclic voltammetry measurements. Experimental results are supported by (TD)-DFT, NICS and NICS2BC calculations.

Design and Synthesis of BODIPY-Hetero[5]helicenes as Heavy-Atom-Free Triplet Photosensitizers for Photodynamic Therapy of Cancer

Designing heavy-atom-free triplet photosensitizers (PSs) is a challenge for the efficient photodynamic therapy (PDT) of cancer. Helicenes are twisted polycyclic aromatic hydrocarbons (PAHs) with an efficient intersystem crossing (ISC) that is proportional to their twisting angle. But their difficult syntheses and weak absorption profile in the visible spectral region restrict their use as heavy-atom-free triplet PSs for PDT. On the other hand, boron-containing PAHs, BODIPYs are highly recognized for their outstanding optical properties. However, planar BODIPY dyes has low ISC and thus they are not very effective as PDT agents. We have designed and synthesized fused compounds containing both BODIPY and hetero[5]helicene structures to develop red-shifted chromophores with efficient ISC. One of the pyrrole units of the BODIPY core was also replaced by a thiazole unit to further enhance the triplet conversion. All the fused compounds have helical structure, and their twisting angles are also increased by substitutions at the boron centre. The helical structures of the BODIPY-hetero[5]helicenes were confirmed by X-ray crystallography and DFT structure optimization. The designed BODIPY-hetero[5]helicenes showed superior optical properties and high ISC with respect to [5]helicene. Interestingly their ISC efficiencies increase proportionally with their twisting angles. This is the first report on the relationship between the twisting angle and the ISC efficiency in twisted BODIPY-based compounds. Theoretical calculations showed that energy gap of the S1 and T1 states decreases in BODIPY-hetero[5]helicene as compared to planar BODIPY. This enhances the ISC rate in BODIPY-hetero[5]helicene, which is responsible for their high generation of singlet oxygen. Finally, their potential applications as PDT agents were investigated, and one BODIPY-hetero[5]helicene showed efficient cancer cell killing upon photo-exposure. This new design strategy will be very useful for the future development of heavy-atom-free PDT agents.

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.

Applications of red light photoredox catalysis in organic synthesis

Photoredox catalysis has emerged as an efficient and versatile approach for developing novel synthetic methodologies. Particularly, red light photocatalysis has attracted more attention due to its intrinsic advantages of low energy, few health risks, few side reactions, and high penetration depth through various media. Impressive progress has been made in this field. In this review, we outline the applications of different photoredox catalysts in a wide range of red light-mediated reactions including direct red light photoredox catalysis, red light photoredox catalysis through upconversion, and dual red light photoredox catalysis. Due to the similarities between near-infrared (NIR) and red light, an overview of NIR-induced reactions is also presented. Lastly, current evidence showing the advantages of red light and NIR photoredox catalysis is also described.

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.

Chemo- and Regioselective Multiple C(sp2 )-H Insertions of Malonate Metal Carbenes for Late-Stage Functionalizations of Azahelicenes

Chiral quinacridines react up to four times, step-by-step, with α-diazomalonates under Ru^II and Rh^II catalysis. By selecting the catalyst, [CpRu(CH₃ CN)₃ ][PF₆ ] (Cp=cyclopentadienyl) or Rh₂ (oct)₄ , chemo and regioselective insertions of derived metal carbenes are achieved in favor of mono- or bis-functionalized malonate derivatives, respectively, (r.r.>49 : 1, up to 77 % yield, 12 examples). This multi-introduction of malonate groups is particularly useful to tune optical and chemical properties such as absorption, emission or Brønsted acidity but also cellular bioimaging. Density-functional theory further elucidates the origin of the carbene insertion selectivity and also showcases the importance of conformations in the optical response.

Triple Regioselective Functionalization of Cationic [4]Helicenes via Iridium-Catalyzed Borylation and Suzuki Cross-Coupling Reactivity

In essentially one-pot, using Ir- and Pd-catalysis, tris(arene)-functionalized cationic [4]helicenes are synthesized with full regioselectivity and enantiospecificity starting from a trivial precursor (17 examples). This poly-addition of aryl groups improves key optical properties, that is, fluorescence quantum yields and lifetimes. Electronic circular dichroism and circularly polarized luminescence signatures are observed up to the far-red domain, in particular with additional arenes prone to aggregation.

Diastereoselective and enantioselective photoredox pinacol coupling promoted by titanium complexes with a red-absorbing organic dye

The pinacol coupling reaction, a reductive coupling of carbonyl compounds that proceeds through the formation of ketyl radicals in the presence of an electron donor, affords the corresponding 1,2-diols in one single step. The photoredox version of this transformation has been accomplished using different organic dyes or photoactive metal complexes in the presence of sacrificial donors such as tertiary amines or Hantzsch's ester. Normally, the homo-coupling of such reactive ketyl radicals is neither diastereo- nor enantio-selective. Herein, we report a highly diastereoselective pinacol coupling reaction of aromatic aldehydes promoted by 5 mol% of the non-toxic, inexpensive and available Cp₂TiCl₂ complex. The key feature that allows the complete control of diastereoselectivity is the employment of a red-absorbing organic dye in the presence of a redox-active titanium complex. Taking advantage of the well-tailored photoredox potential of this organic dye, the selective reduction of Ti(iv) to Ti(iii) is achieved. These conditions enable the formation of the d,l (syn) diastereoisomer as the favored product of the pinacol coupling (d.r. > 20 : 1 in most of the cases). Moreover, employing a simply prepared chiral SalenTi complex, the new photoredox reaction gave a complete diastereoselection for the d,l diastereoisomer, and high enantiocontrol (up to 92% of enantiomeric excess).

A metallaphotoredox, diastereoselective and enantioselective pinacol coupling reaction promoted by titanium complexes with the use of a red-absorbing organic dye was developed.

Broad-band Chiral Absorbance of Visible Light

The amplification of chiral absorbance and emission is a primary figure of merit for the design of chiral chromophores. However, for dyes to be practically relevant in chiroptical applications, they must also absorb and/or emit chiral light over broad wavelength ranges. We investigate the interplay between molecular symmetry and broad-band chiral absorbance in a series of [6]helicenes. We find that an asymmetric [6]helicene containing two distinct chromophores absorbs chiral light across a much wider wavelength range than the symmetric [6]helicenes investigated here. Chemically reducing the helicenes shifts the absorption edge of the ECD spectra into the near-infrared wavelength range while preserving broad chiral absorption, producing a [6]helicene that absorbs a single handedness of light across the entire visible wavelength range.

Acetylene Derivatives of Cationic Diazaoxatriangulenes and Diaza [4]Helicenes ‐ Access to Red Emitters and Planar Chiral Stereochemical Traits

Cationic triangulenes, and related helicenes, constitute a rich class of dyes and fluorophores, usually absorbing and emitting light at low energy, in the orange to red domains. Recently, to broaden the scope of applications, regioselective late‐stage functionalizations on these core moieties have been developed. For instance, with the introduction of electron‐donating groups (EDGs), important bathochromic shifts are observed pushing absorptions towards or in the near‐infrared (NIR) spectral domain while emissive properties disappear essentially completely. Herein, to upset this drawback, acetylene derivatives of cationic diazaoxa triangulenes (DAOTA) and [4]helicenes are prepared (16 examples). Contrary to other EDG‐functionalized derivatives, C≡C− functionalized products remain broadly fluorescent, with red‐shifted absorptions (Δλ_abs up to 25 nm) and emissions (Δλ_em up to 73 nm, Φ_PL up to 51 %). Quite interestingly, a general dynamic stereoisomerism phenomenon is evidenced for the compounds derived from achiral DAOTA cores. At low temperature in ¹H NMR spectroscopy (218 K), N−CH₂ protons become diastereotopic with chemical shifts differences (Δδ) as high as +1.64 ppm. The signal coalescence occurs around 273 K with a barrier of ∼12 kcal mol⁻¹. This phenomenon is due to planar chiral conformations (S_p and R_p configurations), induced by the geometry of the alkyl (n‐propyl) side‐chains next to the acetylenic substituents. Ion pairing studies with Δ‐TRISPHAT anion not only confirm the occurrence of the chiral conformations but evidence a moderate but definite asymmetric induction from the chiral anion onto the cations. Finally, DFT calculations offer a valuable insight on the geometries, the corresponding stereodynamics and also on the very large difference in NMR for some of the diastereotopic protons.

Recent Advances in Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds

This review surveys recent progress in the chemistry of polycyclic heteroaromatic molecules with a focus on structural diversity and synthetic methodology. The article covers literature published during the period of 2016-2020, providing an update to our first review of this topic (Chem. Rev. 2017, 117 (4), 3479-3716).

Utilizing Selective Chlorination to Synthesize New Triangulenium Dyes

Functionalization of new sites on the triangulenium structure has been achieved by early-stage chlorination with N-chlorosuccinimide (NCS), giving rise to two new triangulenium dyes (1 and 3). By introducing the chlorine functionalities in the acridinium precursor, positions complementary to those previously obtained by electrophilic aromatic substitution on the final dyes are accessed. The chlorination is selective, giving only one regioisomer for both mono- and dichlorination products. For the monochlorinated acridinium compound, a highly selective ring-closing reaction was discovered, generating a single regioisomer of the cationic [4]helicene product. Further investigations into the mechanism of the [4]helicene formation lead to the first isolation of the previously proposed intermediate of the two-step S_NAr reaction, key to all aza-bridged triangulenium and helicenium systems. Late-stage functionalization of DAOTA⁺ with NCS gave rise to a different dichlorinated compound (2). The fully ring closed chlorinated triangulenium dyes 1, 2, and 3 show a redshift in absorption and emission, while maintaining relatively high fluorescence quantum yields of 36%, 26%, and 41% and long fluorescence lifetimes of 15, 12.5, and 16 ns, respectively. Cyclic voltammetry shows that chlorination of the triangulenium dyes significantly lowers reduction potentials and thus allows for efficient tuning of redox and photoredox properties.

Circularly polarized luminescence of Eu(III) complexes with chiral 1,1'-bi-2-naphtol-derived bisphosphate ligands

The interest for lanthanide circularly polarized luminescence (CPL) has been quickly growing for 10 years. However, very few of these studies have involved correlation between the dissymmetry factor (g_lum ) and the chemical modifications in a series of chiral ligands. Four polymeric compounds of Eu(III) were prepared by using a series of binaphtyl derivatives for which the size of the π system as well as the number of stereogenic elements (i.e., the binaphtyl moiety) are modulated. The resulting {[Eu(hfac)₃ ((S)/(R)-L^x )]}_n (x = 1 and 3) and {[Eu(hfac)₃ ((S,S,S)/(R,R,R)-L^x )]}_n (x = 2 and 4) have been characterized by powder X-ray diffraction by comparison with the X-ray structures on single crystal of the Dy(III) analogs. In solution, the structure of the complexes is deeply modified and becomes monomeric. The nature of the ligand induces change in the shape of the CPL spectra in CH₂ Cl₂ solution. Furthermore, a large |g_lum | = 0.12 of the magnetic-dipole transition for the [Eu(hfac)₃ ((S,S,S)/(R,R,R)-L² )] complex involving the ligand with three stereogenic elements and an extended 𝜋 system has been measured. This report also shows CPL measurements in solid state for the series of {[Eu(hfac)₃ ((S)/(R)-L^x )]}_n (x = 1 and 3) and {[Eu(hfac)₃ ((S,S,S)/(R,R,R)-L^x )]}_n (x = 2 and 4) polymers.

Helical donor-acceptor platinum complexes displaying dual luminescence and near-infrared circularly polarized luminescence

A series of chiral platina[5]helicenes displaying dual luminescence, i.e., fluorescence between 450 and 600 nm and red/NIR phosphorescence between 700 and 900 nm, has been synthesised, characterised and studied by first-principle calculations. This unusual behavior has been attributed to limited electronic interactions between the d orbitals of the metal and the π-orbitals of the organic ligand on the excited-state. Accordingly, the electron richness of the donor group on the helical ligand does not affect the energy of the phosphorescence process but does play a role on its efficiency. Interestingly, near-infrared circularly polarized luminescence can be obtained for the three complexes with dissymmetry factors up to 3 × 10^-3 at 750 nm.

Enantioenriched Ruthenium-Tris-Bipyridine Complexes Bearing One Helical Bipyridine Ligand: Access to Fused Multihelicenic Systems and Chiroptical Redox Switches

The synthesis and photophysical and chiroptical properties of novel aza[n]helicenes (6a-d, 10a,b, n = 4-7) substituted with one or two 2-pyridyl groups are described. The preparation was performed via an adapted Mallory reaction using aromatic imines as precursors. The obtained novel class of helical 2,2'-bipyridine ligands was then coordinated to Ru(bipy)₂²⁺ units, thus affording the first diastereomerically and enantiomerically pure [RuL(bipy)₂]²⁺ (11a,c, L = 6a,c) or [Ru₂L'(bipy)₄]⁴⁺ (12, L' = 10b) complexes. The topology and stereochemistry of these novel metal-based helical architectures were studied in detail, notably using X-ray crystallography. Interestingly, the coordination to ruthenium(II) enabled the preparation of fused multihelical systems incorporating aza- and ruthena-helicenes within the same scaffold. The photophysical, chiroptical, and redox properties of these complexes were examined in detail, and efficient redox-triggered chiroptical switching activity was evidenced.

Hybrids of cationic [4]helicene and N-heterocyclic carbene as ligands for complexes exhibiting (chir)optical properties in the far red spectral window

Synthesis, electronic and structural properties of a chiral NHC bearing a N-bonded cationic [4]helicene moiety are reported. This ligand is used to construct Au^I, Au^III and Rh^I complexes exhibiting far-red (chir)optical properties regardless of the metal.

Chiral Near-Infrared Fluorophores by Self-Promoted Oxidative Coupling of Cationic Helicenes with Amines/Enamines

In one pot, tertiary alkyl amines are oxidized to enamines by cationic dioxa[6]helicene, which further reacts as electrophile and oxidant to form mono or bis donor-π-acceptor coupling products. This original and convergent synthetic approach provides a strong extension of conjugation yielding chromophores that absorb intensively in far-red or NIR domains (λ_max up to 791 nm) and fluoresce in the NIR as well (λ_max up to 887 nm). Intense ECD properties around 790 nm with a |Δϵ| value up to 60 M^-1  cm^-1 are observed.

Achieving high circularly polarized luminescence with push-pull helicenic systems: from rationalized design to top-emission CP-OLED applications

While the development of chiral molecules displaying circularly polarized luminescence (CPL) has received considerable attention, the corresponding CPL intensity, g lum, hardly exceeds 10-2 at the molecular level owing to the difficulty in optimizing the key parameters governing such a luminescence process. To address this challenge, we report here the synthesis and chiroptical properties of a new family of π-helical push-pull systems based on carbo[6]helicene, where the latter acts as either a chiral electron acceptor or a donor unit. This comprehensive experimental and theoretical investigation shows that the magnitude and relative orientation of the electric (μe ) and magnetic (μ m ) dipole transition moments can be tuned efficiently with regard to the molecular chiroptical properties, which results in high g lum values, i.e. up to 3-4 × 10-2. Our investigations revealed that the optimized mutual orientation of the electric and magnetic dipoles in the excited state is a crucial parameter to achieve intense helicene-mediated exciton coupling, which is a major contributor to the obtained strong CPL. Finally, top-emission CP-OLEDs were fabricated through vapor deposition, which afforded a promising g El of around 8 × 10-3. These results bring about further molecular design guidelines to reach high CPL intensity and offer new insights into the development of innovative CP-OLED architectures.

Photophysical Properties and Redox Potentials of Photosensitizers for Organic Photoredox Transformations

Photoredox catalysis has proven to be a powerful tool in synthetic organic chemistry. The rational design of photosensitizers with improved photocatalytic performance constitutes a major advancement in photoredox organic transformations. This review summarizes the fundamental ground-state and excited-state photophysical and electrochemical attributes of molecular photosensitizers, which are important determinants of their photocatalytic reactivity.

Trioxatriangulenium (TOTA+) as a robust carbon-based Lewis acid in frustrated Lewis pair chemistry

We report the reactivity between the water stable Lewis acidic trioxatriangulenium ion (TOTA+) and a series of Lewis bases such as phosphines and N-heterocyclic carbene (NHC). The nature of the Lewis acid-base interaction was analyzed via variable temperature (VT) NMR spectroscopy, single-crystal X-ray diffraction, UV-visible spectroscopy, and DFT calculations. While small and strongly nucleophilic phosphines, such as PMe3, led to the formation of a Lewis acid-base adduct, frustrated Lewis pairs (FLPs) were observed for sterically hindered bases such as P( t Bu)3. The TOTA+-P( t Bu)3 FLP was characterized as an encounter complex, and found to promote the heterolytic cleavage of disulfide bonds, formaldehyde fixation, dehydrogenation of 1,4-cyclohexadiene, heterolytic cleavage of the C-Br bonds, and interception of Staudinger reaction intermediates. Moreover, TOTA+ and NHC were found to first undergo single-electron transfer (SET) to form [TOTA]·[NHC]˙+, which was confirmed via electron paramagnetic resonance (EPR) spectroscopy, and subsequently form a [TOTA-NHC]+ adduct or a mixture of products depending the reaction conditions used.

Pre-regulation of the planar chirality of pillar[5]arenes for preparing discrete chiral nanotubes

Regulating the chirality of macrocyclic host molecules and supramolecular assemblies is crucial because chirality often plays a role in governing the properties of these systems. Herein, we describe pillar[5]arene-based chiral nanotube formation via pre-regulation of the building blocks' chirality, which is different from frequently used post-regulation strategies. The planar chirality of rim-differentiated pillar[5]arenes is initially regulated by chiral awakening and further induction/inversion through stepwise achiral external stimuli. The pre-regulated chiral information is well stored in discrete nanotubes by interacting with a per-alkylamino-substituted pillar[5]arene. Such pre-regulation is more efficient than post-regulating the chirality of nanotubes.

Near-infrared dyes for two-photon absorption in the short-wavelength infrared: strategies towards optical power limiting

The recent advances in the field of two-photon absorbing chromophores in the short-wavelength infrared spectral range (SWIR 1100–2500 nm) are summarized, highlighting the development of optical power limiting devices in this spectral range.

Tunable carbocation-based redox active ambiphilic ligands: synthesis, coordination and characterization

The synthesis of novel redox active ambiphilic ligands L1-L3 and their coordination chemistry to first-row late transition metal halides (M = Co and Ni) is reported. The heterocyclic carbocation scaffolds act as Lewis acid moieties while the pyridine anchor acts as the coordinating Lewis base. The high synthetic tunability of this ligand scaffold allows for control of its rigidity and electronic properties. Anion exchange and coordination of a chloride anion to the metal center was observed resulting in the formation of [MCl3]- metallate. Upon coordination to the pyridine anchor, the metallate centers adopt a canonical tetrahedral geometry, resulting in an overall neutral complex best described as a zwitterionic metallate trichloride bound to a cationic ligand. Characterization techniques including single crystal X-ray diffraction, cyclic voltammetry, and UV-Vis absorption spectroscopy were employed to better understand the structural and chemical properties of the ligands and metal complexes. A possible weak interaction between one of the chlorides and the carbenium moiety in the ligand is observed in crystals of both of the Co(ii) and Ni(ii) complexes with ligand L1. Density functional theory (DFT) calculations support that this electrostatic interaction for complexes 2a and 2b exists only in the solid state.

What is Best Strategy for Water Soluble Fluorescence Dyes?-A Case Study Using Long Fluorescence Lifetime DAOTA Dyes*

The lipophilic nature of organic dyes complicates their effectiveness in aqueous solutions. In this work we investigate three different strategies for achieving water-solubility of the diazaoxatriangulenium (DAOTA⁺ ) chromophore: hydrophilic counter ions, aromatic sulfonation of the chromophore, and attachment of charged side chains. The long fluorescence lifetime (FLT, τ_f =20 ns) of DAOTA⁺ makes it a sensitive probe to analyze solvation and aggregation effects. Direct sulfonation of the chromophore was found to increase solubility drastically, but at the cost of greatly reduced quantum yields (QYs) due to enhanced non-radiative deactivation processes. The introduction of either cationic (4) or zwitterionic side chains (5), however, brings the FLT (τ_f =18 ns) and QY (ϕ_f =0.56) of the dye to the same level as the parent chromophore in acetonitrile. Time-resolved fluorescence spectroscopy also reveals a high resistance to aggregation and non-specific binding in a high loading of bovine serum albumin (BSA). The results clearly show that addition of charged flexible side chains is preferable to direct sulfonation of the chromophore core.

Synthesis and properties of chiral fluorescent helicene-BODIPY conjugates

A series of chiral fluorescent helicene-BODIPY conjugates was prepared by the regioselective formylation of aza[4]helicene precursors and then an efficient one-pot two-step BODIPY synthesis (13 examples, 28-82%). Fused conjugates exhibit absorption and fluorescence properties (ΦF 30-45%) in the red visible domain, and a CPL signature could be measured at 605 nm (glum ±5 × 10-4). Photophysical and electronic properties were investigated and rationalized through first principles.

Persistent, highly localized, and tunable [4]helicene radicals

Persistent organic radicals have gained considerable attention in the fields of catalysis and materials science. In particular, helical molecules are of great interest for the development and application of novel organic radicals in optoelectronic and spintronic materials. Here we report the syntheses of easily tunable and stable neutral quinolinoacridine radicals under anaerobic conditions by chemical reduction of their quinolinoacridinium cation analogs. The structures of these [4]helicene radicals were determined by X-ray crystallography. Density functional theory (DFT) calculations, supported by electron paramagnetic resonance (EPR) measurements, indicate that over 40% of spin density is located at the central carbon of our [4]helicene radicals regardless of their structural modifications. The localization of the charge promotes a reversible oxidation to the cation upon exposure to air. This unusual reactivity toward molecular oxygen was monitored via UV-Vis spectroscopy.

Synthesis, Resolution, Configurational Stability, and Properties of Cationic Functionalized [5]Helicenes

A straightforward approach to the synthesis of two different series of cationic [5]helicenes has been achieved including, in dioxa series, the possibility to introduce aromatic functional groups at the periphery of the helical structure. While photophysical study highlights that the introduction of aryl substituents at position 23 of the helical moieties has a negligible impact on the optical properties, styryl substituents allow a welcoming extension of the conjugation pathways. Finally, a red shift of the optical properties was evidenced upon introduction of nitrogen atoms in the helicene scaffold, leading to particularly good fluorescence efficiencies in the red domain for a helicenic dye. Detailed information on racemization kinetics was collected for the most stable species upon direct high-performance liquid chromatography (HPLC) resolution or, when configurational lability was too high, through VT-HPLC analysis on the chiral stationary phase (ΔG^‡ values ranging from 85.0 to 137.1 kJ·mol^-1 and above).

Simultaneous Control of Central and Helical Chiralities: Expedient Helicoselective Synthesis of Dioxa[6]helicenes

An expedient synthesis of a new family of configurationally stable dioxa[6]helicenes was established using a sequential helicoselective organocatalyzed heteroannulation/eliminative aromatization via enantioenriched fused 2-nitro dihydrofurans featuring both central and helical chiralities. Starting from simple achiral precursors, a broad range of these previously unknown chiral heterocyclic scaffolds were obtained with good efficiency, and their aromatization proceeded with very high enantiopurity retention in most cases.

Elucidation of the Intersystem Crossing Mechanism in a Helical BODIPY for Low-Dose Photodynamic Therapy

Intersystem crossing (ISC) of triplet photosensitizers is a vital process for fundamental photochemistry and photodynamic therapy (PDT). Herein, we report the co-existence of efficient ISC and long triplet excited lifetime in a heavy atom-free bodipy helicene molecule. Via theoretical computation and time-resolved EPR spectroscopy, we confirmed that the ISC of the bodipy results from its twisted molecular structure and reduced symmetry. The twisted bodipy shows intense long wavelength absorption (ϵ=1.76×105  m-1  cm-1 at 630 nm), satisfactory triplet quantum yield (ΦT =52 %), and long-lived triplet state (τT =492 μs), leading to unprecedented performance as a triplet photosensitizer for PDT. Moreover, nanoparticles constructed with such helical bodipy show efficient PDT-mediated antitumor immunity amplification with an ultra-low dose (0.25 μg kg-1 ), which is several hundred times lower than that of the existing PDT reagents.

Self-enhanced multicolor electrochemiluminescence by competitive electron-transfer processes

Controlling electrochemiluminescence (ECL) color(s) is crucial for many applications ranging from multiplexed bioassays to ECL microscopy. This can only be achieved through the fundamental understanding of high-energy electron-transfer processes in complex and competitive reaction schemes. Recently, this field has generated huge interest, but the effective implementation of multicolor ECL is constrained by the limited number of ECL-active organometallic dyes. Herein, the first self-enhanced organic ECL dye, a chiral red-emitting cationic diaza [4]helicene connected to a dimethylamino moiety by a short linker, is reported. This molecular system integrates bifunctional ECL features (i.e. luminophore and coreactant) and each function may be operated either separately or simultaneously. This unique level of control is enabled by integrating but decoupling both molecular functions in a single molecule. Through this dual molecular reactivity, concomitant multicolor ECL emission from red to blue with tunable intensity is readily obtained in aqueous media. This is done through competitive electron-transfer processes between the helicene and a ruthenium or iridium dye. The reported approach provides a general methodology to extend to other coreactant/luminophore systems, opening enticing perspectives for spectrally distinct detection of several analytes, and original analytical and imaging strategies.

Rapid time-resolved Circular Polarization Luminescence (CPL) emission spectroscopy

Circular polarisation luminescence (CPL) emission spectroscopy is a powerful tool for probing the fundamental chiroptical features of optically emissive chiral molecular systems. However, uptake of CPL spectroscopy has been impeded by the limitations of conventional scanning monochromator (SM) CPL spectrometers, which are costly to acquire and maintain, and typically require tens of minutes to acquire a typical CPL spectrum. Here, we demonstrate a design of CPL spectrometer which uses rapid readout solid state (SS) spectrometer detectors and a dual channel optical layout to acquire CPL spectra in as little as 10 milliseconds. We validate and demonstrate equivalent CPL measurement by measuring CPL spectra of two reference europium(III) complexes. Further, we demonstrate time-gated CPL acquisition, enabling long-lived CPL luminescence to be distinguished from short-lived emission of other fluorescent species. We anticipate that SS-CPL spectrometers will enable flexible, rapid, and relatively low-cost CPL spectroscopy for diverse applications.

Circular polarization luminescence (CPL) spectroscopy is a tool to study chiroptical systems, but the measurement process is generally very slow. The authors introduce a CPL technique with much faster acquisition, demonstrating meaningful time-dependent measurements and enabling new applications.

Near-infrared electrochemiluminescence in water through regioselective sulfonation of diaza [4] and [6]helicene dyes

A series of water-soluble helicene dyes generating intense electrochemiluminescence (ECL) signal in physiological conditions is reported.

Solvent-sensitive circularly polarized luminescent compounds bearing a 9,9′-spirobi[fluorene] skeleton

Chiral 9,9′-spirobi[fluorene] derivatives with a donor–π–acceptor system were prepared and found to exhibit solvent-sensitive circularly polarized luminescence.

Merging polyacenes and cationic helicenes: from weak to intense chiroptical properties in the far red region

A series of helical tetracenes and pentacenes was synthesized from cationic [6] and [4]helicene precursors. These colorful acenes fluoresce in the far red region. While [4]helicene-based pentacenes exhibit chiroptical properties mainly in the UV region, [6]helicene-derived tetracenes show enhanced ECD in the visible range, in addition to clear CPL responses. This difference is rationalized using first principles.

Coumarin-Annelated Regioisomeric Heptahelicenes: Influence of Helicity on Excited-State Properties and Chiroptical Properties

Two regioisomeric pairs of heptahelical mono- and biscoumarins that are differentiated by "inward" and "outward" disposition of the pyran-2-one moiety have been synthesized and investigated to understand the influence of helicity on excited-state and chiroptical properties. A slight variation in the helicities is found to manifest in contrasting excited-state properties of coumarin-annelated heptahelicenes; in addition to the intramolecular charge transfer, structural relaxation in the excited state is shown from theoretical calculations to cause decrease in the fluorescence quantum yield for a system with higher helicity. The optically pure enantiomers of heptahelical coumarins exhibit helicity-dependent chiroptical properties, namely, specific rotations, molar ellipticities, Cotton effects, and anisotropic dissymmetry factors. Theoretical calculations point to factors that are not readily explicable.

Stereochemical significance of O to N atom interchanges within cationic helicenes: experimental and computational evidence of near racemization to remarkable enantiospecificity

Oxygen atoms of cationic dioxa and azaoxa [6]helicenes can be exchanged by amino groups to form azaoxa and diaza [6]helicenes respectively. The mild reaction conditions developed herein allow the construction of libraries of derivatives with sensitive and/or functionalized side chains. Using enantioenriched dioxa or azaoxa helicene precursors, these exchanges lead to either near racemization (es 3%) or to a remarkable enantiospecificity (es up to 97%). This unusual behavior is fully characterized via experimental and computational mechanistic evidence. Based on these investigations, the enantiospecificity of the first transformation can be improved to 57-61%.

Structural analysis of helicene molecules adsorbed on symmetric surfaces

Helicenes are chiral polyaromatic hydrocarbon molecules which self-assemble into ordered monolayers on solid substrates, and are of current interest in the study of supramolecular systems and the development of smart materials. In this work we investigate the geometry of helicene monomers and stacked dimers on (111) facets of coinage metals. The geometry of the adsorbed molecules is shaped by the coupling of intermolecular dispersive forces, intramolecular steric repulsion between end rings and surface-molecule interactions. Thus, binding and stereospecificity outcomes vary broadly depending on the identity of molecule/surface pair. Overall, homochiral interactions are found to be more effective than heterochiral stacking, due to a better fit between the helical structures in like dimers. On a surface, this effect is enhanced by the flattening of surface-proximal molecular rings. However, our results show that the "sandwich" effect of the second molecular layer increases molecular footprints in the first layer, with potentially large implications in monolayer organization and surface commensuration.