Chiral Fluorophores Research Articles

Control of Dynamic Chirality in Donor-Acceptor Fluorophores.

Very recently, the control of dynamic chirality has emerged as a powerful strategy to design chiral functional materials. In this context, we describe herein a molecular design in which a tethered configurationally stable binaphthyl chiral unit efficiently controls the dynamic chirality of donor-acceptor fluorophores, involving diverse indolocarbazoles as electron donors and terephthalonitrile as an electron acceptor. The high conformational discrimination in such a molecular system suggested by density functional theory calculations is experimentally probed using electronic and vibrational circular dichroism and confirmed by the crystallization of these chiral molecules in gel and their single crystal X-ray diffraction analysis. In addition to extending the scope of dynamic chirality control to donor-acceptor fluorophores, this work also highlights the positive effect of the configurationally stable chiral inductor on the magnitude of the dissymmetry factors of the active dynamically chiral fluorophores, both in ground and excited states, through chiral perturbation.

Control of Dynamic Chirality in Donor‐Acceptor Fluorophores

Very recently, the control of dynamic chirality has emerged as a powerful strategy to design chiral functional materials. In this context, we describe herein a molecular design in which a tethered configurationally stable binaphthyl chiral unit efficiently controls the dynamic chirality of donor‐acceptor fluorophores, involving diverse indolocarbazoles as electron donors and terephthalonitrile as an electron acceptor. The high conformational discrimination in such a molecular system suggested by density functional theory calculations is experimentally probed using electronic and vibrational circular dichroism and confirmed by the crystallization of these chiral molecules in gel and their single crystal X‐ray diffraction analysis. In addition to extending the scope of dynamic chirality control to donor‐acceptor fluorophores, this work also highlights the positive effect of the configurationally stable chiral inductor on the magnitude of the dissymmetry factors of the active dynamically chiral fluorophores, both in ground and excited states, through chiral perturbation.

Chiral Bay-Alkynylated Tetraazaperylenes: Photophysics and Chiroptical Properties.

Fully bay-alkynylated octaazaperopyrene dioxide (OAPPDO) derivatives were accessible through Stille cross coupling reaction of the corresponding bay-chlorinated derivatives. This steric congestion of the bay area led to helically chiral fluorophores, and chiral resolution of two derivatives allowed the investigation of their chiroptical properties as well as their kinetics of enantiomerization and the related thermodynamic parameters depending on the size of the terminal alkynyl substituent. An increase of the latter resulted in stable OAPPDO atropisomers at room temperature. The dynamics of the photoexcited states of two of the OAPPDO derivatives were investigated by transient absorption (TA) and time-resolved photoluminescence (tr-PL) spectroscopy.

Rhodium-Catalyzed Two-Fold, Regioselective and Enantioselective C-H Activation: an Efficient Strategy to Chiral Single-Benzene-Based Fluorophores.

A Rh-catalyzed two-fold, regioselective and enantioselective C-H activation via chiral transient directing group strategy has been demonstrated in moderate to good yields with commendable enantioselectivities. The newly synthesized chiral fluorophores exhibit favorable photophysical properties, including large Stokes shifts, good fluorescence quantum yields, aggregation-induced emission in aqueous solution, and intense emission and circularly polarized luminescence in the solid state, indicating great potential applications as chiral fluorescent probes or optoelectronic materials.

Chiral Nanographene‐Based Near‐Infrared Fluorophore with Self‐Blinking Properties

AbstractNear‐infrared (NIR) fluorophores are highly desirable for super‐resolution fluorescence microscopy; however, their optical properties are typically relatively poor, with low photo‐intensity (mean photon number below 1000), and unfavorably high on‐off blinking duty cycle. In this work, the synthesis and characterization of tetrabenzodinaphthopyranthren (TBDNP) derivative as a novel nanographene that exhibits NIR emission in the range of 800–1000 nm with a photoluminescence quantum yield of 14% are reported. TBDNP demonstrated excellent self‐blinking properties, in particular, mean photon number of >3000 and a low on–off‐duty cycle (≈10−4), which are superior to reported NIR dyes, and can be successfully employed for the NIR super‐resolution imaging. Moreover, TBDNP serves also as a chiral fluorophore, showing notable chiroptical activities from 300 to 850 nm with a high dissymmetry factor (gabs) of 0.032 at 574 nm.

Chiroptical detection of self-aggregating fluorescent rhodamine conjugates: Mistakes and prospects.

Fluorescent conjugation can be considered as the chromophoric derivatization of the target, and as such, it may provide additional structure-related information available by using circular dichroism (CD) spectroscopy. In this essay, peculiar CD spectroscopic data reported earlier for thyroid hormone-rhodamine conjugates have been re-evaluated. Contrary to the original proposal on the intramolecular folding of the labelled hormone, the bisignate motif observed in the CD spectrum is a clear evidence of dye-dye intermolecular chiral exciton coupling, indicating supramolecular self-association of the conjugates. This anomalous solution behaviour undermines the credibility of experimental results reported with such conjugates still being used in the laboratory practice. The extension of routine far-ultraviolet (UV) CD spectroscopic scans of chiral fluorophore conjugates into the near-UV and visible spectral region is strongly recommended.

Hindered Tetraphenylethylene Helicates: Chiral Fluorophores with Deep‐Blue Emission, Multiple‐Color CPL, and Chiral Recognition Ability

AbstractNew hindered tetraphenylethylene (TPE) helicates with substitution at 2,6‐position of phenyl rings were designed and synthesized. Due to the increased hindrance, the TPE helicates emit strong deep‐blue to violet fluorescence both in the solid state and in solution, and could be resolved into enantiomers that emit strong and multicolor circularly polarized luminescence (CPL), and exhibit a high enantioselective recognition of chiral tartaric acid and its derivatives. Surprisingly, the derived helicate tetramines possess amino groups with an unpredented planar structure and sp2‐hybridized nitrogen, arousing the change between AIE effect and ACQ phenomenon through photoinduced electron transfer (PET). With advantages of short synthetic route, many modification positions, deep‐blue to violet emission, wide CPL tuning, and high chiral recognition ability, the hindered TPE helicates show broad prospects as chiral materials.

Hindered Tetraphenylethylene Helicates: Chiral Fluorophores with Deep-Blue Emission, Multiple-Color CPL, and Chiral Recognition Ability.

New hindered tetraphenylethylene (TPE) helicates with substitution at 2,6-position of phenyl rings were designed and synthesized. Due to the increased hindrance, the TPE helicates emit strong deep-blue to violet fluorescence both in the solid state and in solution, and could be resolved into enantiomers that emit strong and multicolor circularly polarized luminescence (CPL), and exhibit a high enantioselective recognition of chiral tartaric acid and its derivatives. Surprisingly, the derived helicate tetramines possess amino groups with an unpredented planar structure and sp2 -hybridized nitrogen, arousing the change between AIE effect and ACQ phenomenon through photoinduced electron transfer (PET). With advantages of short synthetic route, many modification positions, deep-blue to violet emission, wide CPL tuning, and high chiral recognition ability, the hindered TPE helicates show broad prospects as chiral materials.

Regulating Circularly Polarized Luminescence of Axially Chiral Anthracene Derivatives through Solvatochromism and Supramolecular Self‐assembly

AbstractTwo axially chiral fluorophores derived from anthracene were designed and synthesized, and the chiroptical behaviour were thoroughly investigated including solvent‐dependent emission and circularly polarized luminescence (CPL). Interestingly, unexpected supramolecular self‐assembly of the chiral molecules driven by strong π−π interaction can enable significant hypsochromic shift and fluorescence intensity enhancement. Meanwhile, the luminescence dissymmetry factor of CPL activity exhibited significant enhancement through self‐assembly process. This work provides a novel approach to achieve the supramolecular CPL materials possessing both high luminescence quantum efficiency and high circular polarization.

Rich-colour mechanochromism of a cyanostilbene derivative with chiral self-assembly

The tricolored fluorescence switching was realized in a novel chiral fluorophore. The fabrication of a helical assembly was proposed as a candidate strategy for attaining an additional metastable state, which contributed to enriched PL colors via pairwise excimer emission.

Synthesis of optically active 2,7-disubstituted naphthalene derivatives and evaluation of their enantioselective recognition ability

The palladium-catalyzed amination was used to synthesize 2,7-diamino derivatives of naphthalene containing two chiral substituents and fluorophore groups (dansyl, 7-methoxycoumarin, 6,7-dimethoxycoumarin, 6-aminoquinoline). The synthesized compounds were studied by UV absorption and fluorescence spectroscopy in the presence of individual enantiomers of amino alcohols and salts of 21 metals. The possibility of using these compounds as fluorescent detectors for optically active compounds and metals was examined. In the presence of (S)-leucinol, the diquinoline derivative showed enhanced emission with a maximum at shorter wavelengths, which is not typical of its (R) isomer. This fact can be used for the recognition of these enantiomers. A number of naphthalene derivatives can be considered as potential fluorescent chemosensors for CuII cations due to the total fluorescence quenching in the presence of this metal.

Electrically controlled fluorescence in a nematic liquid crystal doped by a chiral fluorophore

We have discovered a strong electric field sensitivity of the fluorescence intensity while studying the diffusion processes of a chiral fluorescent molecule (CFM) in a nematic liquid crystal (NLC) host. The experimental and theoretical study of this phenomenon indicates that the alignment of the CFM along two privileged orientation directions (with asymmetric distribution with respect to the NLC’s optical axis) is in the origin of this phenomenon. As a result, the obtained guest–host system demonstrates noticeable dichroism. Thus, the application of an electric field allows the reorientation of the anisotropy axis of the host, the change of CFM’s absorption (at the wavelength of excitation) and the dynamic electric control of its fluorescence intensity. The study of these phenomena allows also the identification of the angular distribution of guest CFMs suggesting that the elastic energy of orientation of the host molecules might be in the origin of the asymmetric angular distribution of CFM.

Synthesis, Structure, and Photophysical/Chiroptical Properties of Benzopicene-Based π-Conjugated Molecules.

The convenient synthesis of substituted benzopicenes and azabenzopicenes has been achieved by the cationic rhodium(I)/H8-BINAP or BINAP complex-catalyzed [2+2+2] cycloaddition under mild conditions. This method was applied to the synthesis of benzopicene-based long ladder and helical molecules. The X-ray crystal structure analysis revealed that the benzopicene-based helical molecule is highly distorted and the average distance of overlapped rings is markedly shorter than that in the triphenylene-based helical molecule. Photophysical and chiroptical properties of these benzopicene and azabenzopicene derivatives have also been examined. With respect to photophysical properties, substituted benzopicenes and azabenzopicenes showed red shifts of absorption and emission maxima compared with the corresponding triphenylenes and azatriphenylenes. With respect to chiroptical properties, the CPL spectra of the benzopicene-based helical molecule showed two opposite peaks, and thus the value of the CPL was smaller than that of the triphenylene-based helical molecule presumably due to the presence of two chiral fluorophores.

Solid-state circularly polarised luminescence and circular dichroism of viscous binaphthyl compounds

The chiroptical properties of two viscous chiral fluorophores, (R)-2,2′-hepthoxy-1,1′-binaphthyl [(R)-1] and its antipode [(S)-1], dispersed in inorganic (KBr and KCl) and organic polymer (PMMA and PS) matrices are studied in the photoexcited and ground states. Both (R)-1 and (S)-1 emit circularly polarised luminescence (CPL) with a high quantum efficiency in these solid matrices (15–58%) and in chloroform solution (14%).

Solid-state circularly polarized luminescence measurements: Theoretical analysis

Because a circularly polarized luminescence (CPL) spectrophotometer is a polarization-modulation instrument, artifacts resulting from optical anisotropies that are unique to the solid state necessarily accompany CPL signals. A set of procedures for obtaining the true CPL signal has been derived based on the Stokes–Mueller matrix method. Experiments on chiral fluorophore single crystals of benzil with larger and smaller optical anisotropies have shown that our method can eliminate parasitic artifacts to obtain the true CPL signal, even in cases where optical anisotropies are substantial.

Polymorphic supramolecular organic fluorophore composed of 2-naphthalenecarboxylic acid and benzylamine

By using achiral fluorescent 2-naphthalenecarboxylic acid with achiral benzylamine, three types of polymorphic supramolecular organic fluorophores with different optical properties are created. These include a racemic supramolecular fluorophore and chiral fluorophores. These polymorphic supramolecular fluorophores can be controlled by changing the crystallization method.

Defocused Emission Patterns from Chiral Fluorophores: Application to Chiral Axis Orientation Determination

Experimental defocused fluorescence emission patterns from single chiral molecular systems are compared to semiclassical simulations as a means for a priori determination of chiral axis orientation in single-molecule systems. Using a coupled-oscillator model as a radiation source, we show that the basic features of defocused emission patterns from chiral fluorophores can be recovered and suggests the feasibility of chiral axis orientation determination (within some limits) in single-molecule systems.

A 2D Layered Chiral Supramolecular Organic Fluorophore Composed of 1‐Amino‐2‐indanol and Carboxylic Acid Derivatives

AbstractA chiral anthracenecarboxylic acid/amine supramolecular organic fluorophore composed of a 2D layered network structure was developed by combining 1‐amino‐2‐indanol and 2‐anthracenecarboxylic acid molecules. This 2D, chiral fluorophore exhibits circularly polarized luminescence (CPL) without quenching the fluorescence in the solid state.

Multi‐Input/Multi‐Output Molecular Response System Based on the Dynamic Redox Behavior of 3,3,4,4‐Tetraaryldihydro[5]helicene Derivatives: Reversible Formation/Destruction of Chiral Fluorophore and Modulation of Chiroptical Properties by Solvent Polarity

3,3,4,4-Tetaaryldihydro[5]helicenes (1) and 1,1'-binaphthyl-2,2'-diylbis(diarylcarbenium)s (2(2+)) can be reversibly interconverted upon electron transfer, which is accompanied by a vivid color change (electrochromism) as well as by the formation/cleavage of a C-C bond ("dynamic redox behavior"). Because only the neutral donor 1 exhibits strong fluorescence, electrochemical input can further modify the fluorescent properties of the pair. Due to the configurational stability of the helicity in 1 and axial chirality in 2(2+), the redox reaction of optically pure material proceeds stereospecifically, which induces a chiroptical change such as circular dichroism (CD) as an additional output. The CD spectra of dications 2(2+) exhibit solvent dependency (chiro-solvatochromism), which is accompanied by solvatochromic behavior based on the pi-pi interaction of the two cationic chromophores as well as coordinative interaction of the Lewis basic solvent to the Lewis acidic triarylcarbenium moieties. Thus, the present system is endowed with multi-input functionality for modifying multiple output signals.

Limits on Fluorescence Detected Circular Dichroism of Single Helicene Molecules

Fluorescent imaging of single helicene molecules is applied to study the optical activity of chiral fluorophores. In contrast to the previous report by Hassey et al. (Science 2006, 314, 1437), the dissymmetry factors of single chiral fluorophores are found not to differ significantly from the bulk value of |g| < 10(-4) at 457 nm. Linear dichroism and birefringence of the dichroic mirror inside the fluorescence microscope change the polarization state of the incoming laser beam significantly; i.e., circular polarized light sent into the microscope becomes highly elliptically polarized after reflection from the dichroic mirror. Compensation for this effect should be made to avoid artifacts brought by linear dichroism in single immobilized molecules.