Norimitsu Tohnai

Regulation of π‐Stacked Anthracene Arrangement for Fluorescence Modulation of Organic Solid from Monomer to Excited Oligomer Emission

The construction and precise control of the face-to-face π-stacked arrangements of anthracene fluorophores in the crystalline state led to a remarkable red shift in the fluorescence spectrum due to unprecedented excited oligomer formation. The arrangements were regulated by using organic salts including anthracene-1,5-disulfonic acid (1,5-ADS) and a variety of aliphatic amines. Because of the smaller number of hydrogen atoms at the edge positions and the steric effect of the sulfonate groups, 1,5-ADS should prefer face-to-face π-stacked arrangements over the usual edge-to-face herringbone arrangement. Indeed, as the alkyl substituents were lengthened, the organic salts altered their anthracene arrangement to give two-dimensional (2D) edge-to-face and end-to-face herringbone arrangements, one-dimensional (1D) face-to-face zigzag and slipped stacking arrangements, a lateral 1D face-to-face arrangement like part of a brick wall, and a discrete monomer arrangement. The monomer arrangement behaved as a dilute solution even in the close-packed solid state to emit deep blue light. The 1D face-to-face zigzag and slipped stacking of the anthracene fluorophores caused a red shift of 30-40 nm in the fluorescence emission with respect to the discrete arrangement, probably owing to ground-state associations. On the other hand, the 2D end-to-face stacking induced a larger red shift of 60 nm, which is attributed to the excimer fluorescence. Surprisingly, the brick-like lateral face-to-face arrangement afforded a remarkable red shift of 150 nm to give yellow fluorescence. This anomalous red shift is probably due to excited oligomer formation in such a lateral 1D arrangement according to the long fluorescence lifetime and little shift in the excitation spectrum. The regulation of the π-stacked arrangement of anthracene fluorophores enabled the wide modulation of the fluorescence and a detailed investigation of the relationships between the photophysical properties and the arrangements.

Systematic investigation of molecular arrangements and solid-state fluorescence properties on salts of anthracene-2,6-disulfonic acid with aliphatic primary amines.

Organic salts of anthracene-2,6-disulfonic acid (ADS) with a wide variety of primary amines have been fabricated, and their arrangements of anthracene molecules and solid-state fluorescence properties investigated. Single-crystal X-ray studies reveal that the salts show seven types of crystal forms and corresponding molecular arrangements of anthracene moieties depending on the amine, while anthracene shows only one form and arrangement in the solid state. Depending on the molecular arrangements, the ADS salts exhibit various solid-state fluorescence properties: spectral shift (30 nm) and suppression and enhancement of the fluorescence intensity. Especially the ADS salt with n-heptylamine (nHepA), which shows discrete anthracene moieties in the crystal, exhibits the highest quantum yield (Phi(F)=46.1+/-0.2%) in the series of ADS salts, which exceeds that of anthracene crystal (Phi(F)=42.9+/-0.2%). From these systematic investigations on the arrangements and the solid-state properties, the following factors are essential for high fluorescence quantum yield in the solid state: prevention of contact between pi planes of anthracene moieties and immobilization of anthracene rings. In addition, such organic salts have potential as a system for modulating the molecular arrangements of fluorophores and the concomitant solid-state properties. Thus, systematic investigation of this system constructs a library of arrangements and properties, and the library leads to remarkable strategies for the development of organic solid materials.

A C3‐Symmetric Macrocycle‐Based, Hydrogen‐Bonded, Multiporous Hexagonal Network as a Motif of Porous Molecular Crystals

A C3-symmetric π-conjugated macrocycle combined with an appropriate hydrogen bonding module (phenylene triangle) allowed the construction of crystalline supramolecular frameworks with a cavity volume of up to 58%. The frameworks were obtained through non-interpenetrated stacking of a hexagonal sheet possessing three kinds of pores with different sizes and shapes. The activated porous material absorbed CO2 up to 96 cm(3) g(-1) at 195 K under 1 atm.

Engineering Switchable Rotors in Molecular Crystals with Open Porosity

The first example of a porous molecular crystal containing rotors is presented. The permanently porous crystal architecture is sustained by rotor-bearing molecular rods which are connected through charge-assisted hydrogen bonds. The rotors, as fast as 10(8) Hz at 240 K, are exposed to the crystalline channels, which absorb CO2 and I2 vapors at low pressure. The rotor dynamics could be switched off and on by I2 absorption/desorption, showing remarkable change of material dynamics by the interaction with gaseous species and suggesting the use of molecular crystals in sensing and pollutant management.

Total Synthesis of (−)-Martinellic Acid via Radical Addition−Cyclization−Elimination Reaction

The asymmetric total synthesis of martinellic acid, the first pyrrolo[3,2-c]quinoline alkaloid found in nature, is described. Three key steps in our synthesis of (-)-martinellic acid are the Bu(3)SnH-promoted radical addition-cyclization-elimination (RACE) reaction of an oxime ether with an alpha,beta-unsaturated ester to generate the pyrrolo[3,2-c]quinoline core, a chemoselective lactam carbonyl reduction, and guanidinylation under Mitsunobu reaction conditions. The key radical cyclization has also been investigated by using SmI(2). (-)-Martinellic acid was synthesized from commercially available methyl 4-bromo-3-methylbenzoate in fewer steps than previous syntheses and in an improved overall yield.

Guest-Responsive Fluorescence of Inclusion Crystals with π-Stacked Supramolecular Beads†

Lattice inclusion host compounds entrap guest molecules of various sizes into their flexible cavities formed by the host framework. The inclusion phenomena have been extensively investigated because of the potential application of these compounds in many fields such as separation, storage, catalysis, and chemical sensing. Considerable efforts have demonstrated that bulky and rigid molecules, or awkwardly shaped molecules, tend to include guest molecules because they do not pack easily without guests. For example, crystals of anthracene derivatives bearing diphenylphosphanyl groups at the 9and 10-positions include toluene molecules and show on/off fluorescence switching by absorption and desorption of the guest. Inclusion of various guests into fluorescent hosts will afford a multitude of fluorescent colors as if doping strongly affects the colors of jewels, as seen in sapphires and rubies. However, chemical affinity of their inclusion spaces often limits the appropriate guest molecules (i.e. polar or nonpolar molecules). Therefore, it still remains challenging to design new lattice host compounds that efficiently incorporate and respond to a wide range of guests. Previously, we readily prepared bulky and rigid supermolecules from simple molecules: triphenylmethylamine (TPMA) and a variety of sulfonic acids. Four TPMA molecules and four monosulfonic acid molecules formed cubic hydrogen-bonding (H-bonding) networks completely covered with their substituents (Figure 1a). Herein, in order to create fluorescent materials responsive to various guests, we have designed a new host supermolecule that possesses two aromatic fluorophores at the periphery. The fluorescent host forms one-dimensional (1D) p-stacked

Octadehydrodibenzo[12]annulene‐Based Organogels: Two Methyl Ester Groups Prevent Crystallization and Promote Gelation

Recently, p-conjugated cyclic systems involving benzene rings and acetylene units, the dehydrobenzoannulenes (DBAs), have attracted substantial interest from the viewpoint of acting as building blocks of supramolecular assemblies, along with their optoelectronic functionality, rigid, well-defined molecular structures, and extensively delocalized p electrons. To date, a handful of superstructures based on triangular DBAs have been reported. They include onedimensional (1D) fibrous superstructures in films from Diederich, Nielsen et al. and Iyoda et al., micelles and liquid crystals from Tew et al., and two-dimensionally ordered networks at the liquid–solid interface from Tobe, De Feyter et al. More recently, we also demonstrated that a single crystal composed of one-dimensionally p-stacked DBA exhibited highly anisotropic charge-carrier mobility. However, superstructures of DBAs have not been extensively reported except for the case of crystals, and there has been no report on organogels based on DBAs. One of the reasons for the lack of DBA superstructures is that the affinity interactions between DBA rings are weaker than those of other systems, such as discotic graphene-like molecules, owing to weaker p–p interactions of DBA rings containing sphybridized carbon atoms. Indeed, for gelators, strong and highly directional intermolecular interactions are required. Herein, we describe the construction and structural characterization of an organogel derived from boomerangshaped DBA 1 with methyl ester groups, which was designed according to the strategy detailed below. For construction of organogels, it is generally recognized that gelators have to satisfy the following three requirements: 1) formation of 1D fibrous aggregates through self-complementary and unidirectional intermolecular interactions,

Topological Classification and Supramolecular Chirality of 21‐Helical Ladder‐Type Hydrogen‐Bond Networks Composed of Primary Ammonium Carboxylates: Bundle Control in 21‐Helical Assemblies

The supramolecular chirality of 1D ladder-type hydrogen-bond networks composed of primary ammonium carboxylates was determined based on topological considerations. Chirality in such networks is based on the absolute configuration of the primary ammonium cation, which arises from discrimination between the two oxygen atoms of the carboxylate anion. The configurations of the cations and anions generate topological diversity in the networks, which are classified into six subgroups. In the Cambridge Structural Database, salts based on ladder type 1 constitute over 70 % of salts with a 1D-ladder-type network. Ladder type 1, based on a 2(1)-axis, is not superimposable on its mirror image, which leads to the first definition of right- or left-handedness of 2(1)-helicity on the basis of supramolecular tilt chirality. Helical assemblies of 2(1)-type with triaxial chirality can be assembled in various ways to yield chiral bundles and crystals. On the basis of these considerations, we constructed clay mimic structures with several bundle patterns by connecting the hydrogen-bond networks by using bifunctional molecules. These results open up the possibility of in-depth crystal engineering based on hydrogen-bond topology.

Linkage control between molecular and supramolecular chirality in 2₁-helical hydrogen-bonded networks using achiral components.

Chiral molecules preferentially form one-handed supramolecular assemblies that reflect the absolute configuration of the molecules. Under specific conditions, however, the opposite-handed supramolecular assemblies are also obtained because of flexibility in the bond length and reversibility of non-covalent interactions. The mechanism of the handedness selectivity or switching phenomenon remains ambiguous, and most phenomena are observed by chance. Here we demonstrate the construction of chiral hydrogen-bonded twofold helical assemblies with controlled handedness in the crystalline state based on crystallographic studies. Detailed investigation of the obtained crystal structures enabled us to clarify the mechanism, and the handedness of the supramolecular chirality was successfully controlled by exploiting achiral factors. This study clearly reveals a connection between molecular chirality and supramolecular chirality in the crystalline state.

Fluorescence Spectroscopic Properties of Nitro-Substituted Diphenylpolyenes: Effects of Intramolecular Planarization and Intermolecular Interactions in Crystals

The steady-state absorption and fluorescence properties of (E,E,E)-1,6-diaryl-1,3,5-hexatrienes (2, aryl = 2-nitrophenyl; 3, aryl = 3-nitrophenyl; 4, aryl = 4-nitrophenyl) have been investigated in solution and in the crystalline state. The solid-state absorption spectra of 2-4 shifted to longer wavelengths than those in solution. A combination of theoretical calculations and single-crystal X-ray structure analyses shows considerable planarization of molecules in the solid state, which is mainly responsible for the spectral red shifts. The effects of intermolecular interactions on the absorption spectra appeared to be relatively small in these crystals. This is consistent with the monomeric origin of the solid-state emission. Molecule 2 was nonfluorescent in all solvents studied, probably due to the efficient nonradiative deactivation from ionic species produced by excited-state intramolecular proton transfer (ESIPT) along the C-H...O-type hydrogen bonds. The fluorescence of 3, observed only in medium polar solvents, originated from an intramolecular charge transfer (ICT*) state, while that of 4 derived from locally excited (LE*) and/or ICT* states depending on the solvent polarity. All three molecules exhibited LE* fluorescence in the solid state. No observation of ICT* emission in crystals strongly suggests the twisted geometries for ICT* (TICT) of 3 and 4 in solution. The measurable fluorescence from crystal 2 can be attributed to the restricted torsional motions in the solid excited state.