Takuya Nakashima

Circularly Polarized Luminescence in Chiral Molecules and Supramolecular Assemblies

Circularly polarized luminescence, or CPL, is a luminescence phenomenon that provides the differential emission intensity of right and left circularly polarized light, thereby providing information on the excited state properties of the chiral molecular systems. In recent years, there has been a growing interest toward the development of organic chromophores capable of circularly polarized emission due to their potential applications in sensors, asymmetric synthesis as well as display and optical storage devices. The major drawback with organic molecules is the low dissymmetric factors exhibited by these systems. One of the recent strategies adopted for the improvement in luminescence dissymmetry of organic systems is through the controlled self-assembly of chromophores. In this Perspective, we highlight the recent experimental and theoretical developments in the field of chiral organic chromophoric systems and their self-assembly, that has produced promising results toward the enhancement of glum values in CPL.

Optical activity and chiral memory of thiol-capped CdTe nanocrystals.

CdTe nanocrystals (NCs) passivated with chiral ligands d- and l-cysteinemethylester hydrochloride were prepared and observed to exhibit characteristic CD profiles with symmetrical mirror images depending on the chirality of the capping ligand. The optical activity of CdTe NCs is shown to originate from the distorted CdS shell including chiral capping molecules. The chirality of the NC surface is maintained even after ligand exchange with an achiral thiol, providing an unprecedented chiral memory effect.

Photon‐Quantitative Reaction of a Dithiazolylarylene in Solution

In the cascade of mammalian vision, the cis-to-trans photoisomerization reaction of the retinal chromophore bound to the opsin apoprotein acts as the first trigger for the conversion of light signals into electrical pulses, which are finally transmitted to the brain. One of the specific chemical characteristics of retinal chromophore is its high quantum yield of cis-to-trans photoisomerization, Fcis–trans = 65 %, which affords the high sensitivity of the rod rhodopsin, an opsin–retinal complex required for night vision. Interestingly, the Fcis–trans value of retinal decreased to less than 30% in solution without the protein. This observation suggests that the environmental conditions of the binding pocket in rhodopsin may be responsible for the high photochemical reactivity of retinal. The hydrophobic pocket in rhodopsin is thought to favor appropriate noncovalent bonding to regulate the geometry of the guest molecule in preparation for the photoisomerization reaction. With the aim of developing highly efficient and sensitive photochromic molecules on the basis of the opsin–retinal system, we designed an organic photochromic molecule with a terarylene structure and demonstrated its photon-quantitative reaction. Photochromic terarylenes are composed of three aromatic rings connected to form a hexatriene backbone, which is converted into a cyclohexadiene structure upon UV irradiation, as similarly observed for the photochromism of diarylethenes. Recent studies on terarylenes have highlighted their high photocyclization quantum yields, which exceed 60% 5] and are apparently higher than those of conventional diarylethenes. Since the photoelectrocyclic reactions typically occur in less than 200 fs, the photocyclization quantum yield is generally determined by the electronic structure in the excited state, which is directly interpreted as the molecular conformation in the ground state. Diarylethenes are known to possess two conformations in solution, with the two rings in mirror symmetry (parallel conformation) or C2 symmetry (antiparallel conformation). As the photocyclization reaction proceeds only from the C2-symmetrical conformation, the quantum yield is about 50% at most. In contrast, some diarylethenes undergo a quantitative photocyclization reaction in the crystalline state, when their conformation is fixed in the antiparallel conformation and the distance between reactive carbon atoms is sufficiently short: typically less than 0.4 nm. These observations indicate that the ground-state conformation plays a crucial role in the photoinduced pericyclization reaction. Therefore, various attempts have been made to regulate the ground-state geometry of diarylethenes so that they have C2 symmetry. [7] We herein propose a concept for the design of highly reactive photochromic hexatriene molecules that is inspired by the design of foldamers; thus, multiple intramolecular interactions were used to stabilize the photoreactive conformation of a terarylene. The 2,3-dithiazolylbenzothiophene 1a was designed as a representative photochromic molecule whose conformation is expected to be stabilized in the photoreactive conformation through intramolecular interactions, as depicted by dotted lines in Scheme 1. The rotation of one side-chain thiazolyl

Observation of Chiral Aggregate Growth of Perylene Derivative in Opaque Solution by Circularly Polarized Luminescence

The concentration-dependent enhancement of the luminescence dissymmetry factor has suggested that pi-conjugated chiral perylene derivatives in a high concentrated opaque solution grow to aggregated structures with higher optical chirality.

Remarkable Luminescence Properties of Lanthanide Complexes with Asymmetric Dodecahedron Structures

The distorted coordination structures and luminescence properties of novel lanthanide complexes with oxo-linked bidentate phosphane oxide ligands--4,5-bis(diphenylphosphoryl)-9,9-dimethylxanthene (xantpo), 4,5-bis(di-tert-butylphosphoryl)-9,9-dimethylxanthene (tBu-xantpo), and bis[(2-diphenylphosphoryl)phenyl] ether (dpepo)--and low-vibrational frequency hexafluoroacetylacetonato (hfa) ligands are reported. The lanthanide complexes exhibit characteristic square antiprism and trigonal dodecahedron structures with eight-coordinated oxygen atoms. The luminescence properties of these complexes are characterized by their emission quantum yields, emission lifetimes, and their radiative and nonradiative rate constants. Lanthanide complexes with dodecahedron structures offer markedly high emission quantum yields (Eu: 55-72 %, Sm: 2.4-5.0 % in [D(6)]acetone) due to enhancement of the electric dipole transition and suppression of vibrational relaxation. These remarkable luminescence properties are elucidated in terms of their distorted coordination structures.

Quantum dots–ionic liquid hybrids: efficient extraction of cationic CdTe nanocrystals into an ionic liquid

Water soluble CdTe nanocrystals covered with cationic thiol derivatives are efficiently transferred into a hydrophobic ionic liquid, in which they show enhanced photoluminescence.

Photomodulation of Ionic Interaction and Reactivity: Reversible Photoconversion between Imidazolium and Imidazolinium

An interconversion system between imidazolium and imidazolinium has been proposed for the first time. Imidazolium and imidazolinium cations exhibit different reactivity due to the difference in the aromaticity and charge localization structure, which is successfully controlled by means of photoirradiation in the present system. A 4,5-dithiazolylimidazolium salt was prepared and studied as a new class of photochromic materials modulating electrostatic interactions and chemical reactivities. The photochromic 4,5-dithiazolylimidazolium showed reversible photoconversions between imidazolium open-form and imidazolinium closed-form upon successive irradiation with UV and visible light. The imidazolinium closed-form exhibited characteristic solvato- and ionochromisms in which the absorption maximum shifted by more than 80 nm depending on the solvent polarity and counteranions, whereas the imidazolium open-form showed no such solvent-dependent property. Because the corresponding nonionic 4,5-dithiazolylimidazole also did not exhibit the solvent-dependent absorption profile both in open- and closed-forms, the appearance of these chromisms in imidazolinium closed-form was attributed to the change in the extent of ionic interaction, which was brought about by the photoconversion of imidazolium to imidazolinium. The photoderived strong ionic interaction of imidazolinium with counteranion was further applied to the photocontrolled nucleophilic reaction system. Whereas the imidazolium open-form was inert to nucleophiles such as sodium methoxide, the imidazolinium closed-form was reactive to the nucleophilic reaction, demonstrating a photogated reaction system.

Photo-, solvent-, and ion-controlled multichromism of imidazolium-substituted diarylethenes.

Cationic diarylethenes with an imidazolium ring are synthesized for the first time. The imidazolium cationic moiety is connected directly to the ethene unit as one of the aryl units that take part in the photoinduced pericyclization reaction. The imidazolium-substituted diarylethenes undergo reversible photochromic reactions in a variety of organic media, including ionic liquids, even though they have a delocalized cationic charge in one of the five-membered aromatic rings. The closed-ring isomer shows solvatochromism depending on the solvent donor numbers. Addition of some tetraalkylammonium salts, such as tetrabutyl ammonium nitrate, into the colored organic solution of diarylethene also causes a color change, indicating its ionochromic property. These solvato- and ionochromic properties are considered in connection with the shift of chemical equilibrium between the closed-ring isomers, one with an extended pi-conjugation system and one with limited pi-conjugation due to the strong interaction with solvent molecules and anions with high donor number.

Self-Discriminating Termination of Chiral Supramolecular Polymerization: Tuning the Length of Nanofibers†

Directing the supramolecular polymerization towards a preferred type of organization is extremely important in the design of functional soft materials. Proposed herein is a simple methodology to tune the length and optical chirality of supramolecular polymers formed from a chiral bichromophoric binaphthalene by the control of enantiomeric excess (ee). The enantiopure compound gave thin fibers longer than a few microns, while the racemic mixture favored the formation of nanoparticles. The thermodynamic study unveils that the heterochiral assembly gets preference over the homochiral assembly. The stronger heterochiral binding over homochiral one terminated the elongation of fibrous assembly, thus leading to a control over the length of fibers in the nonracemic mixtures. The supramolecular polymerization driven by π-π interactions highlights the effect of the geometry of a twisted π-core on this self-sorting assembly.

Circularly Polarized Luminescence in Supramolecular Assemblies of Chiral Bichromophoric Perylene Bisimides

Chiral bichromophoric perylene bisimides are demonstrated as active materials of circularly polarized emission. The bichromophoric system exhibited circularly polarized luminescence with dissymmetry factors typical of that of similar organic chiral chromophoric systems in the monomeric state. Variation in solvent composition led to the formation of stably soluble helical aggregates through intermolecular interactions. A large enhancement in the dissymmetry of circularly polarized luminescence was exhibited by the aggregated structures both in the solution and solid states. The sum of excitonic couplings between the individual chromophoric units in the self-assembled state results in relatively large dissymmetry in the circularly polarized luminescence, thereby giving rise to enhanced dissymmetry factors for the aggregated structures. The spacer between chiral center and chromophoric units played a crucial role in the effective enhancement of chiroptical properties in the self-assembled structures. These materials might provide opportunities for the design of a new class of functional bichromophoric organic nanoarchitectures that can find potential applications in the field of chiroptical memory and light-emitting devices based on supramolecular electronics.