Coordination‐Driven Macrocyclization for Locking of Photo‐ and Thermal cis→trans Isomerization of Azobenzene

Both trans and cis isomers of azobenzene‐linked bis‐terpyridine ligand L1 were incorporated in rigid macrocycles linked by Fe^II(tpy)₂ (tpy: terpyridine) units. The complex of the longer trans‐ L1 is dinuclear [(trans‐ L1 )₂ ? Fe^II₂], whereas the complex of the shorter cis‐ L1 is mononuclear [cis‐ L1? Fe^II]. The complex cis‐ L1? Fe^II was not only thermally stable but also photochemically inactive. These results indicate a perfectly locked state of cis‐azobenzene. The stable macrocyclic structure of cis‐ L1? Fe^II causes locking of the isomerization. To the best of our knowledge, this is first example of dual locking of photo‐ and thermal isomerization of cis‐azobenzene.     

Dihydroazulene-Azobenzene-Dihydroazulene Triad Photoswitches

Photoswitch triads comprising two dihydroazulene (DHA) units in conjugation with a central trans-azobenzene (AZB) unit were prepared in stepwise protocols starting from meta- and para-disubstituted azobenzenes. The para-connected triad had significantly altered optical properties and lacked the photoactivity of the separate photochromes. In contrast, for the meta-connected triad, all three photochromes could be photoisomerized to generate an isomer with two vinylheptafulvene (VHF) units and a cis-azobenzene unit. Ultrafast spectroscopy of the photoisomerizations revealed a fast DHA-to-VHF photoisomerization and a slower trans-to-cis AZB photoisomerization. This meta triad underwent thermal VHF-to-DHA back-conversion with a similar rate of all VHFs, independent of the identity of the neighboring units, and in parallel thermal cis-to-trans AZB conversion. The experimental observations were supported by computation (excitation spectra and orbital analysis of the transitions).     

Solid‐State Synthesis of Two Different Polymers in a Single Crystal: A Miscible Polymer Blend from a Topochemical Reaction

The topochemical synthesis of a miscible polymer blend is described. The azide‐ and alkyne‐decorated tetrol 1 crystallizes as two different conformers. Both conformers exhibit self‐sorted head‐to‐tail alignment with proximally placed reacting groups such that topochemical polymerization yields two types of polymer chains, each containing only one type of conformer. The orientation of complementary reactive groups in one of the head‐to‐tail‐arranged conformers favors the formation of cis‐triazole linkages, and the other favors the trans‐triazole linkages. Crystals of 1 on heating gave a perfect polymer blend containing equal amounts of cis‐triazole‐linked and trans‐triazole‐linked polymers. As each conformer is H‐bonded to four conformers of the other kind, the polymerization yields a perfect polymer blend wherein each polymer chain is surrounded by chains of the other type. Thus, the molecular ordering in the prepolymerized state in a crystal is utilized to create a polymer blend.     

Azobenzene‐bridged diradical janus nucleobases with photo‐converted magnetic properties between antiferromagnetic and ferromagnetic couplings

We computationally design a series of azobenzene (AB)‐bridged double radicalized nucleobases, a novel kind of diradical Janus‐type nucleobases, and explore their spin coupling characteristics. Calculations prove that such diradical Janus‐bases not only normally match with their complementary bases, but also exhibit well‐defined diradical character with photo‐convertible intramolecular magnetic couplings (antiferromagnetic vs. ferromagnetic). Combination of four radical nucleobases (rG, rA, rC, rT) and photoswitch AB can yield 10 diradical Janus‐bases with different magnetic characteristics in which AB functions a bridge to mediate the spin coupling between two radical bases. The trans‐form supports mild antiferromagnetic couplings with the spin coupling constants (J) ranging from −153.6 cm⁻¹ to −50.91 cm⁻¹ while the cis‐form has weak magnetic couplings with ferromagnetic (0.22–8.50 cm⁻¹) for most of them or antiferromagnetic (−0.77, −1.73, −3.30 cm⁻¹) properties for only three. Further structural examination and frontier molecular orbital analyses indicate that the extended π conjugation for better spin polarization provides an effective through‐π‐bond pathway to mediate the spin coupling in the trans conformation while nonplanarity of the cis conformation weakens the through‐bond coupling and causes a competitive through‐space pathway and as an overall result inhibits the spin coupling between two spin moieties. Meanwhile, we also find that the J values of the cis conformation vary with their angle between the radical base and its linked phenylene. Furthermore, the magnetic properties of the diradical Janus‐bases can be significantly increased by interacting with metal ions. They also maintain a good UV absorption characteristics and there is a clear redshift compared with AB. This work provides a promising strategy for the rational design of photo‐convertible Janus‐base magnets as the magnetism‐tunable DNA building blocks. © 2018 Wiley Periodicals, Inc.     

The keto→enol photoisomerization of N‐salicilydenemethylfurylamine: Nonadiabatic ab initio dynamics simulation

The photoinduced isomerization of cis‐keto and trans‐keto isomers in N‐salicilydenemethylfurylamine has been studied using the surface‐hopping approach at the CASSCF level of theory. After the cis‐keto or trans‐keto isomer is excited to S₁ state, the molecule initially moves to a excited‐state local minimum. The torsional motion around relative bonds in the chain drives the molecule to approach a keto‐form conical intersection and then nonadiabatic transition occurs. According to our full‐dimensional dynamics simulations, the trans‐keto and enol photoproducts are responsible for the photochromic effect of cis‐keto isomer excited to S₁ state, while no enol isomer was obtained in the photoisomerization of trans keto on excitation. The cis keto to enol and cis keto to trans keto isomerizations are reversible photochemical reactions. It is confirmed that this aromatic Schiff base is a potential molecular switch. Furthermore, the torsion of C N bond occurs in the radiationless decay of trans‐keto isomer, while it is completely suppressed by an intramolecular hydrogen bonding interaction in the dynamics of cis‐keto form. Moreover, the excited‐state lifetime of cis keto is longer than that of trans‐keto form due to the O···H N hydrogen bond.     

Formation of interstellar vinyl alcohol via simple radical processes: Theoretical study

Density functional theory calculations at the B3LYP/6‐31++G(d,p) level of theory were employed to verify whether the formation of vinyl alcohol (VA) in the interstellar medium can be explained by reactions of common and abundant interstellar species such as acetylene, radicals C₂, HC? C · , · OH, and H · . Several reaction sequences are possible. They include radical combinations, which proceed without activation barriers and are highly exothermic. The reactions of closed‐shell species acetylene and HC?C‐OH with hydrogen atoms, as well as H‐transfer and OH‐rotation processes, might require activation energy. Nevertheless, either the corresponding transition states lay below the reactant level or there are alternative routes that involve no transition states at all, such as the reaction sequences C₂ → HC?C · → HC?CH → trans‐syn‐HC( · )?CH‐OH → syn‐VA; C₂ → HC?C · → syn‐?C?CH‐OH → (trans or cis)‐syn‐HC( · )?CH‐OH → syn‐VA; and C₂ → · C?C‐OH → syn‐:C?CH‐OH → (trans or cis)‐syn‐HC( · )?CH‐OH → syn‐VA. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

A CASSCF study on the lowest π→π* excitation of urocanic acid

The photochemical cis/trans isomerization of urocanic acid (UCA, (E)‐3‐(1′H‐imidazol‐4′‐yl)propenoic acid) was investigated using complete active space SCF (CASSCF) ab initio calculations. The singlet ground state and the triplet and the singlet manifolds of the lowest‐lying π→π* (HOMO→LUMO) excitation of the neutral and the anionic UCA were calculated using the 6‐31G* and the 6‐31+G* basis sets, respectively. The torsional barrier of the double bond of the propenoic acid moiety in UCA is observed to be considerably lower in the T₁ and S₁ excited states of the neutral UCA and in the T₁ but not in the S₁ excited state of the anionic UCA, as compared to the S₀ state of the respective protonation form. The cis‐isomer of both the neutral and the anionic UCA is lower in energy than the trans‐isomer in the S₀, T₁, and S₁ states. This energy difference is larger in the excited states than in the ground state, probably due to strengthening of the intramolecular hydrogen bond of cis‐UCA as the molecule is excited. The results of the calculations, interpreted in terms of the idea that UCA is deprotonated upon electronic excitation, led to construction of a new model for the photoisomerization mechanisms of UCA. According to this model, the trans‐to‐cis isomerization proceeds via both the triplet and the singlet manifolds in the deprotonated form of UCA. This isomerization may occur in the S₀ state of the neutral UCA as well. The cis‐to‐trans isomerization is suggested to proceed only in the S₀ state of the neutral UCA. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 25–37, 1999     

Mechanistic insights into the cis-trans isomerization of ruthenium complexes relevant to catalysis of olefin metathesis

The mechanism of the trans to cis isomerization in Ru complexes with a chelating alkylidene group has been investigated by using a combined theoretical and experimental approach. Static DFT calculations suggest that a concerted single‐step mechanism is slightly favored over a multistep mechanism, which would require dissociation of one of the ligands from the Ru center. This hypothesis is supported by analysis of the experimental kinetics of isomerization, as followed by ¹H NMR spectroscopy. DFT molecular dynamics simulations revealed that the variation of geometrical parameters around the Ru center in the concerted mechanism is highly uncorrelated; the mechanism actually begins with the transformation of the square‐pyramidal trans isomer, with the Ru?CHR bond in the apical position, into a transition state that resembles a metastable square pyramidal complex with a Cl atom in the apical position. This high‐energy structure collapses into the cis isomer. Then, the influence of the N‐heterocyclic carbene ligand, the halogen, and the chelating alkylidene group on the relative stability of the cis and trans isomers, as well as on the energy barrier separating them, was investigated with static calculations. Finally, we investigated the interconversion between cis and trans isomers of the species involved in the catalytic cycle of olefin metathesis; we characterized an unprecedented square‐pyramidal metallacycle with the N‐heterocyclic carbene ligand in the apical position. Our analysis, which is relevant to the exchange of equatorial ligands in other square pyramidal complexes, presents evidence for a remarkable flexibility well beyond the simple cis–trans isomerization of these Ru complexes.     

Regiospecific cationic polymerization of spiroorthoesters with different cyclic ether ring sizes

Spiroorthoesters (SOEs), cis‐2,3‐tetramethylene‐1,4,6‐trioxaspiro[4,5]decane ( I ) and cis‐2,3‐tetramethylene‐1,4,6‐trioxaspiro[4,6]undecane ( II ), with different cyclic ether ring sizes were synthesized, and their stereostructure and steric energy were determined. With steric‐hindrance‐sensitized 9‐phenyl‐9,10‐dihydro‐anthracen‐10‐ylium cation as an initiator, I and II underwent regiospecific polymerization to yield trans form of stereoregular poly(ether esters)—poly(trans‐2‐oxycyclohexyl pentanoate) (? [trans‐2‐OCHP]_n? ) ( III ) and poly(trans‐2‐oxycyclohexyl hexanoate) (? [trans‐2‐OCHH]_n? ) ( V ), respectively. With SnCl₄ as another initiator, I and II underwent regiospecific polymerization through different mechanisms to afford cis form poly(cis‐2‐oxycyclohexyl pentanoate) (? [cis‐2‐OCHP]_n? ) ( IV ) and trans form (? [trans‐2‐OCHH]_n? ) ( VI ) stereoregular poly(ether esters). The polymerization mechanisms of SOEs proceeded in the regiospecific manner were determined by the relationship among the sterostructures of SOEs and its subsequently formed polymers, the steric energy of monomers, and the free energy difference in the transition state of reaction. Owing to the conversion of cis substitution at C‐2 and C‐3 in I or II to the trans form during polymerization, polymers III , V , and VI exhibited a higher volume of expansion during polymerization than IV , which showed high volume shrinkage. Group contributions of divalent trans‐ and cis‐1.2‐cyclohexyl groups were derived and confirmed by measuring the densities of the corresponding stereoregular polymers. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Structure‐Dependent cis/trans Isomerization of Tetraphenylethene Derivatives: Consequences for Aggregation‐Induced Emission

The isomerization and optical properties of the cis and trans isomers of tetraphenylethene (TPE) derivatives with aggregation‐induced emission (AIEgens) have been sparsely explored. We have now observed the tautomerization‐induced isomerization of a hydroxy‐substituted derivative, TPETH‐OH, under acidic but not under basic conditions. Replacing the proton of the hydroxy group in TPETH‐OH with an alkyl group leads to the formation of TPETH‐MAL, for which the pure cis and trans isomers were obtained and characterized by HPLC analysis and NMR spectroscopy. Importantly, cis‐TPETH‐MAL emits yellow fluorescence in DMSO at ?20 °C whereas trans‐TPETH‐MAL shows red fluorescence under the same conditions. Moreover, the geometry of cis‐ and trans‐TPETH‐MAL remains unchanged when they undergo thiol–ene reactions to form cis‐ and trans‐TPETH‐cRGD, respectively. Collectively, our findings improve our fundamental understanding of the cis/trans isomerization and photophysical properties of TPE derivatives, which will guide further AIEgen design for various applications.     

Efficient Separation of cis‐ and trans‐1,2‐Dichloroethene Isomers by Adaptive Biphen[3]arene Crystals

Reported here is the highly efficient separation of industrially important cis‐ and trans‐1,2‐dichloroethene (cis‐DCE and trans‐DCE) isomers by activated crystalline 2,2′,4,4′‐tetramethoxyl biphen[3]arene (MeBP3) materials, MeBP3α. MeBP3 can be synthesized in excellent yield (99 %), and a cyclic pentamer is also obtained when using 1,2‐dichloroethane as the solvent. The structure of MeBP3 in the CH₃CN@MeBP3 crystal displays a triangle‐shape topology, forming 1D channels through window‐to‐window packing. Desolvated crystalline MeBP3 materials, MeBP3α, preferentially adsorb cis‐DCE vapors over its trans isomer. MeBP3α is able to separate cis‐DCE from a 50:50 (v/v) cis/trans‐isomer mixture, yielding cis‐DCE with a purity of 96.4 % in a single adsorption cycle. Single‐crystal structures and powder X‐ray diffraction patterns indicate that the uptake of cis‐DCE triggers a solid‐state structural transformation of MeBP3, suggesting the adaptivity of MeBP3α materials during the sorption process. Moreover, the separation can be performed over multiple cycles without loss of separation selectivity and capacity.     

Two‐dimensional liquid chromatography analysis of all‐trans‐, 9‐cis‐, and 13‐cis‐astaxanthin in raw extracts from Phaffia rhodozyma

An effective two‐dimensional liquid chromatography method has been established for the analysis of all‐trans‐astaxanthin and its geometric isomers from Phaffia rhodozyma employing a C18 column at the first dimension and a C30 column in the second dimension, connected by a 10‐port valve using the photo‐diode array detector. The regression equation of astaxanthin calibration curve was established, and the precision and accuracy values were found to be in the range of 0.32–1.14% and 98.21–106.13%, respectively. By using two‐dimensional liquid chromatography, it was found that day light, ultrasonic treatment, and heat treatment have significant influence on the content of all‐trans‐astaxanthin in the extract from P. rhodozyma due to the transformation of all‐trans‐astaxanthin to cis‐astaxanthin. The day light and ultrasonic treatments more likely transform all‐trans‐astaxanthin to 9‐cis‐astaxanthin, and the thermal treatment transforms all‐trans‐astaxanthin to 13‐cis‐astaxanthin. These results indicate that the two‐dimensional liquid chromatography method can facilitate monitoring astaxanthin isomerization in the raw extract from P. rhodozyma. In addition, the study will provide a general reference for monitoring other medicals and bioactive chemicals with geometric isomers.     

Photocontrol of the Catalytic Activity of a β‐Cyclodextrin Bearing Azobenzene and Histidine Moieties as a Pendant Group

An azobenzene group was linked to β‐cyclodextrin via a histidine spacer ( 1 ) to produce a photoresponsive catalyst. The ester hydrolysis of p‐nitrophenyl acetate, Boc‐L ‐alanine‐p‐nitrophenyl ester and Boc‐D ‐alanine‐p‐nitrophenyl ester was examined in the presence of trans‐ 1 or cis‐ 1 . In the case of cis‐ 1 , the cyclodextrin cavity was used as the substrate binding site during imidazole‐catalyzed ester hydrolysis. This was not possible in the case of trans‐ 1 due to the inclusion of the trans‐azobenzene moiety in the cyclodextrin cavity. Consequently, the catalytic mechanism switches in an on‐off fashion on UV irradiation, associated with the conversion of the azobenzene moiety of 1 from trans to cis.     

Interactions of Molecules with cis and trans Double Bonds: A Theoretical Study of cis‐ and trans‐2‐Butene

Noncovalent interactions of cis‐ and trans‐2‐butene, as the smallest model systems of molecules with cis and trans double bonds, were studied to find potential differences in interactions of these molecules. The study was performed using quantum chemical methods including very accurate CCSD(T)/CBS method. We studied parallel and displaced parallel interactions in 2‐butene dimers, in butane dimers, and between 2‐butene and saturated butane. The results show the trend that interactions of 2‐butene with butane are the strongest, followed by interactions in butane dimers, whereas the interaction in 2‐butene dimers are the weakest. The strongest calculated interaction energy is between trans‐2‐butene and butane, with a CCSD(T)/CBS energy of ?2.80 kcal mol^?1. Interactions in cis‐2‐butene dimers are stronger than interactions in trans‐2‐butene dimers. Interestingly, some of the interactions involving 2‐butene are as strong as interactions in a benzene dimer. These insights into interactions of cis‐ and trans‐2‐butene can improve understanding of the properties and processes that involve molecules with cis and trans double bonds, such as fatty acids and polymers.     

Lanthanide Coordination Polymers Constructed from Infinite Rod‐Shaped Secondary Building Units and Flexible Ligands

Three lanthanide coordination polymers constructed from infinite rod‐shaped secondary building units (SBUs), [Nd₂(H₂O)₂(cis‐chdc)₂(trans‐chdc)]?2H₂O ( 1 ), Nd₂(H₂O)₄(trans‐chdc)₃ ( 2 ), and [Sm₂(H₂O)₂(cis‐chdc)(trans‐chdc)₂]?4H₂O ( 3 ) (chdcH₂=1,4‐cyclohexanedicarboxylic acid), were hydrothermally synthesized and structurally characterized. The structures of 1 – 3 are modulated by different ratios of the cis and trans configurations of chdc^2? ligands, which was achieved by temperature control in the hydrothermal reactions. Crystal‐structure analysis revealed that 1 is a four‐connected pcu‐type rod packing network built from cross‐linking of rod‐shaped neodymium–oxygen SBUs by cis‐ and trans‐chdc^2? ligands in a 2:1 ratio, 2 displays a complicated six‐connected hex‐type rod packing structure built by connection of rod‐shaped neodymium–oxygen SBUs and trans‐chdc^2? ligands, and 3 features an unprecedented five‐connected rod packing pattern constructed from rod‐shaped samarium–oxygen SBUs and cis‐ and trans‐chdc^2? ligands in a 1:2 ratio.     

Synthesis of the Active Stilbenoids by Photooxidation Reaction of trans-ε-Viniferin

Two new stilbenoids, cis‐?‐viniferin (3) and 2b, 14b‐dehydro‐bisresveratrol (4) were synthesized by photooxidation reaction of trans‐?‐viniferin (2) prepared from tram‐resveratrol (1). Pentamethoxyl trans‐?‐viniferin (5) and pentamethoxyl cis‐?‐viniferin (6) were also obtained by methylation of trans‐?‐viniferin (2) with (MeO)₂SO₂. Their structures were elucidated on the basis of spectral evidence. Compounds 3 and 4 showed potent inhibition of TNF‐α at concentrations of 10^?‐⁵ mol.L^?1 with inhibitory ratios of 51.43% and 36.64%. respectively.     

Three Polymorphic CdII Coordination Polymers Obtained from the Solution and Mechanochemical Reactions of 3‐Cyanopentane‐2,4‐dione with CdII Acetate

We previously reported that monomeric and polymeric metal complexes are obtained from solution and mechanochemical reactions of 3‐cyano‐pentane‐2,4‐dione (CNacacH) with 3d metal acetates (M=Mn^II, Fe^II, Co^II, Ni^II, Cu^II, and Zn^II). A common feature found in all complexes was that their structural base is trans‐[M(CNacac)₂]. Here, we report that the reactions of CNacacH with Cd^II acetate in the solution and solid states afford different coordination polymers composed of trans‐[Cd(CNacac)₂] and cis‐[Cd(CNacac)₂] units, respectively. From a methanol solution containing CNacacH (L) and Cd(OAc)₂ ? 2 H₂O (M), a coordination polymer ( Cd‐1 ) in which trans‐[Cd(CNacac)₂] units are three‐dimensionally linked was obtained. In contrast, two different coordination polymers, Cd‐2 and Cd‐3 , were obtained from mechanochemical reactions of CNacacH with Cd(OAc)₂ ? 2 H₂O at M/L ratios of 1:1 and 1:2, respectively. In Cd‐2 , cis‐[Cd(CNacac)₂] units are two‐dimensionally linked, whereas the units are linked three‐dimensionally in Cd‐3 . Furthermore, Cd‐1 and Cd‐2 converted to Cd‐3 by applying an annealing treatment and grinding with a small amount of liquid, respectively, in spite of the polymeric structures. These phenomena, 1) different structures are formed from solution and mechanochemical reactions, 2) two polymorphs are formed depending on the M/L ratio, and 3) structural transformation of resulting polymeric structures, indicate the usability of mechanochemical method in the syntheses of coordination polymers as well as the peculiar structural flexibility of cadmium‐CNacac polymers.     

A Light‐Controlled Molecular Brake with Complete ON–OFF Rotation

A light‐controlled molecular machine based on cyclic azobenzenophanes consisting of a dioxynaphthalene rotating unit and a photoisomerizable dioxyazobenzene unit bridged by methylene spacers is reported. In compounds 1 and 2 , 1,5‐ and 2,6‐dioxynaphthalene moieties, respectively, are linked to p‐dioxyazobenzene by different methylene spacers (n=2 in 1 a and 2 ; n=3 in 1 b ), whereas a 1,5‐dioxynaphthalene moiety is bonded to m‐dioxyazobenzene by bismethylene spacers in 3 . In 1 b and 2 , the naphthalene ring can rotate freely in both the trans and cis states at room temperature. The rotation speed can be controlled either by photoinduced reversible trans–cis (E–Z) isomerization of the azobenzene or by keeping the system at low temperature, as is evident from its NMR spectra. Furthermore, for the first time, we demonstrate a light‐controlled molecular brake, wherein the rotation of the naphthalene moiety through the cyclophane is completely OFF in the trans isomer of compound 3 due to its smaller cavity size. Such restricted rotation imparts planar chirality to the molecule, and the corresponding enantiomers could be resolved by chiral HPLC. However, the rotation of the naphthalene moiety is rendered ON in the cis isomer due to its increased cavity size, and it is manifested experimentally by the racemization of the separated enantiomers by photoinduced E–Z isomerization.     

Ab initio static and molecular dynamics study of 4-styrylpyridine.

We report an in‐depth theoretical study of 4‐styrylpyridine in its singlet S₀ ground state. The geometries and the relative stabilities of the trans and cis isomers were investigated within density functional theory (DFT) as well as within Hartree–Fock (HF), second‐order Møller–Plesset (MP2), and coupled cluster (CC) theories. The DFT calculations were performed using the B3LYP and PBE functionals, with basis sets of different qualities, and gave results that are very consistent with each other. The molecular structure is thus predicted to be planar at the energy minimum, which is associated with the trans conformation, and to become markedly twisted at the minimum of higher energy, which is associated with the cis conformation. The results of the calculations performed with the post‐HF methods approach those obtained with the DFT methods, provided that the level of treatment of the electronic correlation is high enough and that sufficiently flexible basis sets are used. Calculations carried out within DFT also allowed the determination of the geometry and the energy of the molecule at the biradicaloid transition state associated with the thermal cis?trans isomerization and at the transition states associated with the enantiomerization of the cis isomer and with the rotations of the pyridinyl and phenyl groups in the trans and cis isomers. Car–Parrinello molecular dynamics simulations were also performed at 50, 150, and 300 K using the PBE functional. The studies allowed us to evidence the highly flexible nature of the molecule in both conformations. In particular, the trans isomer was found to exist mainly in a nonplanar form at finite temperatures, while the rotation of the pyridinyl ring in the cis isomer was incidentally observed to take place within ≈1 ps during the simulation carried out at 150 K on this isomer.