Absolute asymmetric photocyclization of isopropylbenzophenone derivatives using a cocrystal approach involving single-crystal-to-single-crystal transformation

Absolute asymmetric photocyclization of isopropylbenzophenone derivatives was achieved by means of a cocrystal approach. Three chiral salt crystals formed by carboxylic acid derivatives with achiral amines could be prepared by spontaneous crystallization. In the M-crystal of 4-(2,5-diisopropylbenzoyl)benzoic acid with 2,4-dichlorobenzylamine, a twofold helical arrangement occurs in a counterclockwise direction to generate the crystal chirality. Conversely, the clockwise helix exists alone in the P-crystal. Irradiation of the M-crystal at >290 nm caused highly enantioselective Norrish type II cyclization to give the (R,R)-cyclopentenol, (R)-cyclobutenol, and (R)-hydrol in a 6:3:1 molar ratio, resulting in successful absolute asymmetric synthesis, while irradiation at around 350 nm afforded the (R,R)-cyclopentenol as the sole product. The reaction proceeded via single-crystal-to-single-crystal transformation, and therefore the reaction path producing the (R,R)-cyclopentenol could be traced by X-ray crystallographic analysis before and after irradiation.     

Diarylethene microcrystals make directional jumps upon ultraviolet irradiation

Microcrystals of a diarylethene {1,2-bis[5'-methyl-2'-(2"-pyridyl)thiazolyl]perfluorocyclo-pentene} undergo jumps upon photoirradiation. These photochromic crystals present molecular structural changes upon irradiation with ultraviolet light because of reversible photocyclization reactions. When the energy absorbed by crystals reaches about 10 microJ, the uniaxial stress induced in the crystal lattice relaxes through directional jumps. If one prevents crystals from jumping, then parallel, equidistant cracks appear on crystal surfaces. These photomechanical effects could result from a Grinfeld surface instability.     

Polymorphism of 1,2-Bis(2-methyl-5-p-methoxyphenyl-3-thienyl)perfluorocyclopentene and Photochromic Reactivity of the Single Crystals

Four polymorphic crystals were obtained by recrystallization of 1,2-bis(2-methyl-5-p-methoxyphenyl-3-thienyl)perfluorocyclopentene (1 a) from hexane. All crystals underwent photochromic reactions upon alternate irradiation with ultraviolet (lambda=370 nm) and visible light (lambda>500 nm). The photocyclization quantum yields were found to be close to unity irrespective of the crystal types, while the photocycloreversion quantum yields were different as much as four times depending on the conformation of the closed-ring isomers in the crystals.     

Control of the Orientation and Photoinduced Phase Transitions of Macrocyclic Azobenzene

Photoinduced phase transitions caused by photochromic reactions bring about a change in the state of matter at constant temperature. Herein, we report the photoinduced phase transitions of crystals of a photoresponsive macrocyclic compound bearing two azobenzene groups ( 1 ) at room temperature on irradiation with UV (365 nm) and visible (436 nm) light. The trans/trans isomer undergoes photoinduced phase transitions (crystal–isotropic phase–crystal) on UV light irradiation. The photochemically generated crystal exhibited reversible phase transitions between the crystal and the mesophase on UV and visible light irradiation. The molecular order of the randomly oriented crystals could be increased by irradiating with linearly polarized visible light, and the value of the order parameter was determined to be ?0.84. Heating enhances the thermal cis‐to‐trans isomerization and subsequent cooling returned crystals of the trans/trans isomer.     

Multiple Anti‐Counterfeiting Guarantees from a Simple Tetraphenylethylene Derivative – High‐Contrasted and Multi‐State Mechanochromism and Photochromism

Herein the novel tetraphenylethylene (TPE) derivative 1 was designed with an integration of aggregation‐induced emission (AIE), multi‐state mechanochromism and self‐recovery photochromism. The molecule was susceptible to grinding, heating and vapor fuming and showed corresponding transition of its emission colors. The heated powder or single crystal of 1 exhibited reversible photochromism. After a short period of UV irradiation, it showed a bright red color, but recovered to its original white appearance within 1 min. The photochromism is due to the formation of photocyclization intermediates upon UV irradiation, while the eversible mechanochromism is attributed to the weak molecular interactions derived from head‐to‐tail stacking of the molecules. This reversible multi‐state, high‐contrasted and rapid responsive mechanochromic and photochromic property cooperatively provide double enhancement of a multimode guarantee in advanced anti‐counterfeiting.     

Photoresponsive rolling and bending of thin crystals of chiral diarylethenes

Dithienylhexafluorocyclopentene with (R)- or (S)-N-phenylethylamide substituents formed rod-like and 0.2-1.0 microm-thick platelike crystals by sublimation; upon UV irradiation, the crystals bent concavely to the incident light and finally rolled crystals were obtained; the bent crystals were reconverted to flat crystals by visible light irradiation.     

Photo-Arbuzov rearrangements of 1-arylethyl phosphites: stereochemical studies and the question of radical-pair intermediates.

The direct UV irradiation of the 1-arylethyl phosphites 7, 8, and 9 was carried out in acetonitrile, benzene, and cyclohexane, as was the triphenylene-sensitized reaction of 9. Dimethyl 1-phenylethyl phosphite, 7, gives the photo-Arbuzov rearrangement product, dimethyl 1-phenylethylphosphonate (10), in 67% average yield and minor amounts (2%) of 2,3-diphenylbutane (11a) in quantum yields of 0.32 and 0.02, respectively. The photorearrangement of optically active, predominantly (R)-1-phenylethyl phosphite 7 (R/S = 97/3; 94% ee), at 35-40 degrees C proceeds with a high degree of stereospecificity at the stereogenic migratory carbon to give predominantly (R)-10 (R/S = 86/14, 72 +/- 2% ee). Use of the nitroxide radical trap TEMPO affords phosphonate 10, presumably all cage product, from predominantly (R)-7 (R/S = 97/3; 94% ee) in 64% yield (80% ee, R/S = 90/10). By contrast, the 1-(4-acetylphenyl)-ethyl phosphite, predominantly (S)-8 (S/R = 98/2, 96% ee), on direct irradiation gives the corresponding phosphonate (12) in only 20% yield along with dimer 11b in 40% accountability yield. Phosphonate 12 is nearly racemic (R/S = 52/48). Direct irradiation of predominantly (R)-9 (R/S = 98/2, 96% ee), a 1-(1-naphthyl)ethyl phosphite, results in a product distribution similar to that from predominantly (R)-7, but with a somewhat higher degree of retention of configuration in the product phosphonate 13 (R/S = 93/7, 86 +/- 3 ee). By contrast, the triplet triphenylene-sensitized photorearrangement of largely (R)-9 (R/S = 98/2, 96% ee) leads to product distributions similar to those from direct irradiation of predominantly (S)-8 and is accompanied by almost total loss of stereochemistry in its product phosphonate, 13 (R/S = 51/49). The partial loss of stereochemistry on direct irradiation of 7 and 9 provides evidence for radical pair formation. Furthermore, these stereochemical results are diagnostic of the multiplicity of the initial radical pair formed. Values for kcomb/krot for the proximate free radical pairs from 7 and 9, derived experimentally, are severalfold larger than those for the proximate singlet pair from Ph2C=C=N-CHPhMe, corrected to 35 degrees C. The possibility that kcomb is increased for the pairs from 7 and 9 is proposed.     

Photoinduced swing of a diarylethene thin broad sword shaped crystal: a study on the detailed mechanism

We report a swinging motion of photochromic thin broad sword shaped crystals upon continuous irradiation with UV light. By contrast in thick crystals, photosalient phenomena were observed. The bending and swinging mechanisms are in fact due to molecular size changes as well as phase transitions. The first slight bending away from the light source is due to photocyclization-induced surface expansion, and the second dramatic bending toward UV incidence is due to single-crystal-to-single-crystal (SCSC) phase transition from the original phase I to phase II_UV. Upon visible light irradiation, the crystal returned to phase I. A similar SCSC phase transition with a similar volume decrease occurred by lowering the temperature (phase III_temp). For both photoinduced and thermal SCSC phase transitions, the symmetry of the unit cell is lowered; in phase II_UV the twisting angle of disordered phenyl groups is different between two adjacent molecules, while in phase III_temp, the population of the phenyl rotamer is different between adjacent molecules. In the case of phase II_UV, we found thickness dependent photosalient phenomena. The thin broad sword shaped crystals with a 3 μm thickness showed no photosalient phenomena, whereas photoinduced SCSC phase transition occurred. In contrast, large crystals of several tens of μm thickness showed photosalient phenomena on the irradiated surface where SCSC phase transition occurred. The results indicated that the accumulated strain, between isomerized and non-isomerized layers, gave rise to the photosalient phenomenon.

We report a swinging motion of photochromic thin broad sword shaped crystals upon continuous irradiation with UV light.     

Chiral helicenoid diarylethene with highly diastereoselective photocyclization

A novel photochromic helicenoid diarylethene (R)-1-[1-(1-methoxymethoxyethyl)-2-naphtho[2,1-b]thienyl]-2-(2,4,5-trimethyl-3-thienyl)hexafluorocyclopentene was synthesized enantioselectively. It showed highly diastereoselective photocyclization (90% de) and a large change (950 degrees) in the specific optical rotation value at 633 nm upon UV light irradiation in ethyl acetate.     

Photosalient Phenomena that Mimic Impatiens Are Observed in Hollow Crystals of Diarylethene with a Perfluorocyclohexene Ring

A diarylethene with a perfluorocyclohexene ring formed hollow crystals by sublimation under normal pressure. Upon UV irradiation of the crystals, they showed remarkable photosalient phenomena and scattered into small pieces. The speed of the flying debris released from the crystal by UV irradiation exceeded several meters per second. To clearly show a photosalient effect resembling the scattering behavior of Impatiens on a smaller scale, small fluorescent beads (1‐μm diameter) were inserted into the hollow crystal. Consequently, scattering of the beads was observed as UV irradiation caused deformation and bursting of the hollow structure. This phenomenon is unique to hollow crystals, and the ability to effectively induce remarkable photosalient phenomena is similar to the behavior of hollow‐structured Impatiens in nature.     

Synthesis and Crystal Structure of 1-(2-Methyl-5-formyl-3-thienyl)-2-(2-methoxylphenyl)perfluorocyclopentene

A new family of photochromic diarylethene compounds, 1-(2-methyl-5-formyl-3- thienyl)-2-(2-methoxylphenyl)perfluorocyclopentene (1o) having an unsymmetrically substituted hexafluorocyclopentene unit, was synthesized and its structure was determined. The crystal belongs to the monoclinic system, space group P21/c with a = 15.4866(5), b = 9.0744(9), c = 12.6906(3), β = 90.1480(10)°, Z = 4, V = 1783.4(3)3, Dc = 1.513 mg/m3, μ = 0.25, F(000) = 824, the final R = 0.0579 and wR = 0.1566 for 2584 observed reflections (I > 2σ(I)). Interestingly, a colorless compound 1o undergoes photocyclization upon irradiation of UV light to give the blue isomer diarylethene. Upon irradiation with visible light with wavelength greater than 510 nm, the blue compound can return to its initial colorless state.     

Photochemical Studies on 5‐Methylbicyclo[1.1.1]pentane Derivatives: p‐Orbital Overlap Controlled Enantioselectivity

The first example of the p‐orbital overlap controlled enantioselectivity of Norrish type II photocyclization reaction was described. Irradiation of 5‐methyl bicyclo[1.1.1]pentanyl ketone with UV in the solid state as well as in the acetonitrile solution afforded the Norrish/Yang photocyclization compound as the sole product. Solid‐state asymmetric photochemical studies using ionic chiral auxiliary technique led to the enantioselectivity as high as 60%. The results were rationalized by X‐ray single crystal structure.     

A Photo- and Thermo-Driven Azoarene-Based Circularly Polarized Luminescence Molecular Switch in a Liquid Crystal Host

The development of chiral optical active materials with switchable circularly polarized luminescence (CPL) signals remains a challenge. Here an azoarene-based circularly polarized luminescence molecular switch, (S, R, S)-switch 1 and (R, R, R)-switch 2 , are designed and prepared with an (R)-binaphthyl azo group as a chiral photosensitive moiety and two (S)- or (R)-binaphthyl fluorescent molecules with opposite or the same handedness as chiral fluorescent moieties. Both switches exhibit reversible trans/cis isomerization when irradiated by 365 nm UV light and 520 nm green light in solvent and liquid crystal (LC) media. In contrast with the control (R, R, R)-switch 2 , when switch 1 is doped into nematic LCs, polarization inversion and switching-off of the CPL signals are achieved in the resultant helical superstructure upon irradiation with 365 nm UV and 520 nm green light, respectively. Meanwhile, the fluorescence intensity of the system is basically unchanged during this switching process. In particular, these variations of the CPL signals could be recovered after heating, realizing the true sense of CPL reversible switching. Taking advantage of the unique CPL switching, the proof-of-concept for “a dual-optical information encryption system” based on the above CPL active material is demonstrated.     

四苯基乙烯衍生物光物理和光化学研究

设计合成了一个四苯基乙烯衍生物Model-TPE,该化合物具有典型的聚集诱导发光（AIE）特性。¹HNMR、质谱以及高效液相色谱研究表明,有氧条件下Model-TPE经紫外光照射发生关环反应,生成二苯基取代菲衍生物,取代和未取代苯环侧关环反应得到异构体产物,提出了光氧化关环反应机理。本工作对TPE衍生物在发光及传感领域的应用有一定的指导意义。     

Mechanism of preferential enrichment,an unusual enantiomeric resolution phenomenon caused by polymorphic transition during crystallization of mixed crystals composed of two enantiomers

The mechanism of Preferential Enrichment, an unusual enantiomeric resolution phenomenon observed upon recrystallization of a series of racemic crystals which are classified as a racemic mixed crystal with fairly ordered arrangement of the two enantiomers, has been studied. On the basis of the existence of polymorphs and the occurrence of the resulting polymorphic transition during crystallization from solution, the mechanism has been accounted for in terms of (1) a preferential homochiral molecular association to form one-dimensional chain structures in the supersaturated solution of the racemate or nonracemic sample with a low ee value, (2) a kinetic formation of a metastable crystalline phase retaining the homochiral chain structures in a process of nucleation, (3) a polymorphic transition from the metastable phase to a stable one followed by enantioselective liberation of the excess R (or S) enantiomers from the transformed crystal into solution at the beginning of crystal growth to result in a slight enrichment (up to 10% ee) of the opposite S (or R) enantiomer in the deposited crystals, together with an enantiomeric enrichment of the R (or S) enantiomer in the mother liquor, and (4) a chiral discrimination by the once formed S (or R)-rich stable crystalline phase in a process of the subsequent crystal growth, leading to a considerable enantiomeric enrichment of the R (or S) enantiomer up to 100% ee in the mother liquor. The processes (3) and (4) are considered to be directly responsible for an enrichment of one enantiomer in the mother liquor. The association mode of the two enantiomers in solution has been investigated by means of (i) the solubility measurement and (ii) the number-averaged molecular weight measurement in solution by vapor pressure osmometry, together with (iii) the molecular dynamics simulation of oligomer models. The polymorphic transition during crystallization has been observed visually and by means of the in situ FTIR technique and DSC measurement. Both metastable and stable crystals have been obtained, and their crystal structures have been elucidated by X-ray crystallographic analysis of their single crystals.     

Photoinduced CC‐coupling Reactions of Rigid Diastereomeric Benzophenone‐Methionine Dyads

The reactions of ketone/methionine systems are widely used as efficient and selective sources of biorelevant radical species. In this study, we address intramolecular variants of this couple with respect to its photosynthetic utility and as a mechanistic model of underlying elementary reaction steps of biological importance, especially with respect to the study of photoinitiated electron transport in complex peptides. The outcomes of this study are two‐fold: (1) steady‐state irradiation of sterically constrained benzophenone/methionine dyads afforded stable photocyclization products with high yield and product selectivity. (2) Mechanistic insights into the triplet‐triggered product formation were obtained from an analysis of the flash photolysis results and the molecular structure of the stable product formed upon irradiation. Time‐resolved experiments identified (net) hydrogen‐atom transfer from the methionine as the mechanism of the triplet quenching and the resulting biradicals as the major precursor of the isolated stable product. Both the analyses of triplet quenching and stable‐product formation in the diastereomeric pairs point to effects of chiral center configuration, i.e., significant stereoselectivity is observed for all elementary steps. The underlying stereochemical restraints were quantitatively addressed by means of molecular dynamics simulations.     

Influence of rigid polymer matrices on the reversible photochemical reaction of α-p-dimethylaminophenyl-N-m-nitrophenylnitrone

The thermally reversible photocyclization of α-p-dimethylaminophenyl-N-m-nitrophenylnitrone (I) to the corresponding oxaziridine (II) was studied in solution and in rigid polymer matrices. Irradiation with light of wavelength above 380 nm causes a clean transformation of (I) to (II) in solution as well as in polymer matrices. The quantum yield of the photoreaction in solution is about 0.06, the value being almost independent of the nature of the solvent and the irradiation time. In poly-(methyl methacrylate) (PMMA) and in polystyrene (PSt) films, the photocyclization shows initial quantum yields of 0.1 and 0.23, respectively, which decrease with irradiation time. In solution in the dark, II reverted mainly to I with formation of side product(s) in a first-order reaction. The recovery of I during the dark reaction (60–90%) as well as the rate constant of the reaction are strongly affected by the solvent. The kinetics of the thermal return in the two polymer matrices does not follow the first-order rate law. The initial reaction rate as well as the extent of the recovery of I in PMMA film are greater than those in ethyl acetate.     

Photochemical Studies on Bicyclo[2.1.1]hexyl Derivatives: Chemical Behavior and Asymmetric Induction

The photochemical behavior of bicyclo[2.1.1]hexyl derivatives was investigated by irradiation with a 450 W medium‐pressure mercury lamp in acetonitrile solution. The irradiation of methyl bicyclo[2.1.1]hexane‐5‐carbonylbenzoate ( 1a ) led to both Norrish type II cyclization and cleavage products with a molar ratio of 1:2.2, whereas the irradiation of methyl 5‐methylbicyclo[2.1.1]hexane‐5‐carbonylbenzoate ( 1b ) afforded the only Norrish/Yang photocyclization compound as the sole product. Such results were illustrated by several geometric parameters for Norrish/Yang photoreaction as ?₁, ?₄ and β obtained from the crystal structures. Furthermore, asymmetric photochemical studies using ionic chiral auxiliary technique were also conducted in the solid state.     

Enhanced regioselectivity of Yang photocyclization in the crystalline state

[reaction: see text] In contrast to the solution state, where Yang photocyclization of ketones of general structure 1 leads to equal amounts of cyclobutanols 2 and 3 (by abstraction of HA and HB, respectively), irradiation in the crystalline state is much more regioselective, favoring either 2 or 3 depending on the nature of the substituent X. X-ray crystallography and molecular mechanics calculations reveal the source of this remarkable solid-state regioselectivity.     

Thickness-dependent molecular chain and lamellar crystal orientation in ultrathin poly(di-n-hexylsilane) films

The molecular chain and lamellar crystal orientation in ultrathin films (thickness < 100 nm) of poly-(di-n-hexylsilane) (PDHS) on silicon wafer substrates have been investigated by using transmission electronic microscopy, wide-angle X-ray diffraction, atomic force microscopy, and UV absorption spectroscopy. PDHS showed a film thickness-dependent molecular chain and lamellar crystal orientation. Lamellar crystals grew preferentially in flat-on orientation in the monolayer ultrathin films of PDHS, i.e., the silicon backbones were oriented along the surface-normal direction. By contrast, the orientation of lamellar crystals was preferentially edge-on in ultrathin films thicker than ca. 13 nm, i.e., the silicon backbones were oriented parallel to the substrate surface. We interpret the different orientations of molecular chain and lamellar crystal as due to the reduction of the entropy of the polymer chain near the substrate surface and the particularity of the crystallographic (001) plane of flat-on lamellae, respectively. A remarkable influence of the orientations of the silicon backbone on the UV absorption of these PDHS ultrathin films was observed due to the one-dimensional nature of sigma-electrons delocalized along the silicon backbone. With the silicon backbones perpendicular or parallel to the surface of the substrate, the UV absorbance increased or decreased with an increase of the angle between the incident UV beam direction and direction normal to the thin film, respectively.