Photoisomerization of cis,cis- to trans,trans-1,4-diaryl-1,3-butadienes in the solid state: the bicycle-pedal mechanism

The photoisomerizations of crystalline or powdered cis,cis-1,4-diphenyl- and 1,4-di(o-tolyl)-1,3-butadienes (cc-DPB and cc-DTB) to the trans,trans isomers were studied at room temperature. The progress of the reactions was monitored by fluorescence spectroscopy, powder X-ray diffraction, (1)H NMR, and high-performance liquid chromatography. Conversions to the trans,trans isomers were as high as 90% for cc-DPB and 20% for cc-DTB. Formation of the cis,trans isomers, the sole products obtained in solution and in very viscous glassy media at 77 K, is completely suppressed in the solid state. The observed two-bond photoisomerizations are explained by the bicycle-pedal (BP) photoisomerization mechanism. X-ray structure determinations show that o-methyl substitution causes a widening of the phenyl/diene dihedral angles from 40 degrees to 56 degrees and decreases the number of conformers in the crystal from two in cc-DPB to one in cc-DTB. The two conformers of cc-DPB molecules exist in crystals in edge-to-face alternating arrays, one of which has the two phenyls in parallel planes and the other in roughly perpendicular planes. The edge-to-face relationship is maintained in cc-DTB, but only the conformer with the o-tolyl groups in parallel planes is present. The time evolutions of fluorescence spectra measured in the course of the photoreaction show remarkable similarities, despite the different molecular conformations and crystal packing arrangements. Principal component analyses of the spectral matrices indicate the formation of discrete components, suggesting that the two-bond photoisomerizations proceed in stages involving molecules in different microcrystal environments. The structureless appearances of the initial fluorescence spectra show that the reactions are in part diabatic. The BP mechanism can account for the observations if the bicycle-pedal motion began in the excited state, S(1), and were completed in the ground state, S(0). Analysis of void spaces in the crystal lattice reveals much less compact packing of cc-DPB than of cc-DTB molecules, possibly explaining the much higher conversions to photoproduct from cc-DPB.     

Bis(1 degree-amino)cyclodistib(III)azanes: the first structural characterization of cis and trans isomers of a single cyclodipnict(III)azane

The dichlorocyclodistib(III)azane [ClSb(mu-NtBu)]2 (1) has been shown to exist as the cis isomer in the solid state. A series of bis(1 degree-amino)cyclodistib(III)azanes [R'NHSb(mu-NtBu)]2 (2, R' = tBu; 3, R' = Dipp; 4, R' = Dmp) has been prepared by the reaction of 1 with 2 equiv. of LiNHR'. On the basis of NMR solution spectra, all three derivatives are formed as a mixture of cis and trans isomers. In the case of 3, the structures of both the cis and trans isomers have been determined by X-ray crystallography; cis-3 adopts an endo, endo arrangement for the amido protons of the DippNH groups. Isomerization of trans-3 into cis-3 occurs slowly in solution. Deprotonation of 2 with 2 equiv. of nBuNa or trans-3 with nBuLi produces [Na2Sb2(mu-NtBu)4] (5) and [Li2Sb2(mu-NtBu)2(mu-NDipp)2] (6), whose solvated cubane structures were established by X-ray crystallography. In contrast, the reaction of cis-3 with 2 equiv. of nBuLi produces the tricyclic compound [Li2Sb(mu-NtBu)2(mu-NDipp)(mu-NHDipp)] (7).     

Photoswitchable luminescence of rhenium(i) tricarbonyl diimines

The synthesis, characterization, and X-ray crystal structures of [Re(diimine)(CO)(3)(dpe)](PF(6)) (dpe = 1,2-di(4-pyridyl)ethylene) compounds are reported. The cis-dpe complexes exhibit yellow luminescence after UV excitation, whereas the trans-dpe counterparts are nonluminescent. The luminescence quantum yields of the cis-dpe complexes are strongly dependent on the identity of the diimine ligand. Irradiation (350 nm) of the trans-dpe complexes induces trans --> cis dpe-ligand isomerization with quantum yields on the order of 0.2, and this process leads to an on-switching of yellow luminescence. After long 350-nm irradiation times, a steady state composed of roughly 70% cis- and 30% trans-dpe complexes is reached. The reverse cis --> trans photoisomerization reaction is induced by irradiating the cis-dpe complexes at 250 nm, switching off the yellow luminescence. For 250-nm excitation, photodecomposition of the [Re(diimine)(CO)(3)(dpe)](+) complexes competes efficiently with photoisomerization.     

Synthesis and ring-opening metathesis of tetraalkoxy-substituted [2.2]paracyclophane-1,9-dienes

Tetraalkoxy-substituted [2.2]paracyclophane-1,9-dienes can be prepared in three steps from dithia[3.3]paracyclophanes. A mixture of pseudo-geminal and pseudo-ortho diastereomers is produced and the pure compounds can be separated by fractional crystallization. The solid state structures of these diastereomers reveal strongly distorted aromatic rings consistent with high levels of ring strain. Reaction of these diastereomers with the second generation Grubbs catalyst shows that only the pseudo-geminal isomer can be ring opened to give cis,trans-distrylbenzenes. The origin of this selectivity is discussed and the photoisomerization of the as-formed cis,trans-product to the all trans isomer is demonstrated.     

Photoisomerization of cis,cis-1,4-diphenyl-1,3-butadiene in glassy media at 77 K: the bicycle-pedal mechanism

The cis-trans photoisomerization of cis,cis-1,4-diphenyl-1,3-butadiene in a soft isopentane glass at 77 K gives significant two-bond photoisomerization in contrast to solution and hard glassy media where only one-bond photoisomerization takes place.     

Probing highly efficient photoisomerization of a bridged azobenzene by a combination of CASPT2//CASSCF calculation with semiclassical dynamics simulation

Mechanism of phototriggered isomerization of azobenzene and its derivatives is of broad interest. In this paper, the S(0) and S(1) potential energy surfaces of the ethylene-bridged azobenzene (1) that was recently reported to have highly efficient photoisomerization were determined by ab initio electronic structure calculations at different levels and further investigated by a semiclassical dynamics simulation. Unlike azobenzene, the cis isomer of 1 was found to be more stable than the trans isomer, consistent with the experimental observation. The thermal isomerization between cis and trans isomers proceeds via an inversion mechanism with a high barrier. Interestingly, only one minimum-energy conical intersection was determined between the S(0) and S(1) states (CI) for both cis → trans and trans → cis photoisomerization processes and confirmed to act as the S(1) → S(0) decay funnel. The S(1) state lifetime is ～30 fs for the trans isomer, while that for the cis isomer is much longer, due to a redistribution of the initial excitation energies. The S(1) relaxation dynamics investigated here provides a good account for the higher efficiency observed experimentally for the trans → cis photoisomerization than the reverse process. Once the system decays to the S(0) state via CI, formation of the trans product occurs as the downhill motion on the S(0) surface, while formation of the cis isomer needs to overcome small barriers on the pathways of the azo-moiety isomerization and rotation of the phenyl ring. These features support the larger experimental quantum yield for the cis → trans photoisomerization than the trans → cis process.     

Tetrathiafulvalenenaphthalenophanes: planar chirality and cis/trans photoisomerization

A cyclophane incorporating one 1,5-dioxynaphthalene ring system and one tetrathiafulvalene (TTF) unit bridged by [SCH(2)CH(2)O] linkages has been synthesized. In this cyclophane, the TTF unit can adopt either cis or trans configurations. In addition, the 1, 5-dioxynaphthalene ring system imposes one element of planar chirality on this cyclophane. A second element of planar chirality is introduced by the trans form of the TTF unit. Thus, the cyclophane exists in diastereoisomeric forms as three pairs of enantiomers. The enantiomeric pairs associated with the cis form of the TTF unit, as well as one of those associated with the trans form, have been isolated by crystallization, and their structures assigned in the solid state by single-crystal X-ray analyses. In solution, cis/trans isomerization occurs when either the cis or the trans form of the cyclophane is exposed to light. The photoisomerization reaction can be followed by (1)H NMR and UV-vis spectroscopies, as well as by HPLC. The photoisomerization quantum yield has been measured at two different excitation wavelengths (406 and 313 nm). In both cases, the trans --> cis process (Phi = 0.20 at 406 nm) is much more efficient than the reverse cis --> trans process (Phi = 0.030 at 406 nm). Since the absorption spectra of the trans and cis isomers are different and the quantum yield of the trans --> cis photoisomerization reaction depends on the excitation wavelength, the mole fraction of the two diastereoisomers present at the photostationary state depends on the wavelength of the exciting light. No isomerization occurs when the solutions, regardless of the mole fraction of the two diastereoisomers, are stored in the dark.     

Synthesis and structures of cis- and trans-bis(alkyneselenolato)platinum(II) complexes

The reaction of 2 equiv of LiSeCC-n-C(5)H(11) (1) with cis-PtCl(2)(Ph(3)P)(2) (2) gives a mixture of the cis and trans isomers of Pt(Ph(3)P)(2)(SeCC-n-C(5)H(11))(2) (3), which slowly isomerizes in CH(2)Cl(2) to the preferred trans form trans-3. The closely related cis-[Pt(dppf)(2)(SeCC-n-C(5)H(11))(2)] (4) (dppf = bis(diphenylphosphino)ferrocene) was prepared by a similar metathetical reaction using the platinum chloride complex of the chelating dppf to impose the cis geometry. The structures of the cis and trans complexes have been investigated in solution by heteronuclear NMR ((31)P, (77)Se, and (195)Pt) and, in the cases of trans-3 and 4, characterized in the solid state by single-crystal X-ray diffraction. Changing the coordination geometry from cis to trans induces significant changes in the structural and spectroscopic parameters, which do not comply with the previously anticipated donor-acceptor properties of selenolate ligands.     

Stereoselectivity reversal of a photochemical reaction in the solid state

The stereoselectivity of Norrish type II cyclobutanol formation resulting from photolysis of α-adamantyl-p-methoxyacetophenone is altered in favor of the more hindered cis isomer as the reaction medium is changed from isotropic liquid phases (benzene or acetonitrile) to the pure crystal. Based on the reactant X-ray crystal structure, it is suggested that the intermediate 1,4-biradical in the solid state is born in, and restricted to, a conformation which is ideal for direct closure to the more hindered product. In the relatively unrestricted solution environment, however, conformational isomerism of the biradical is faster than closure, thus leading to a predominance of the less hindered trans cyclobutanol.     

Synthesis of heteroaryl imines: theoretical and experimental approach to the determination of the configuration of C=N double bond

The reaction between an iminophosphorane with furan-2-carbaldehyde, thiophene-2-carbaldehyde, furan-3-carbaldehyde, and thiophene-3-carbaldehyde at 60 degrees C gives the corresponding trans imines in 53-84% yields, while the same reaction at 100 degrees C gives a mixture of the corresponding trans and cis imines. Whether the iminophosphorane reacted with 5-nitrofuran-2-carbaldehyde or 5-nitrothiophene-2-carbaldehyde only the trans imines were obtained in 85-89% yields. The irradiation of the imines obtained from thiophene-2-carbaldehyde and thiophene-3-carbaldehyde gave the corresponding photocyclization products. Cis/trans stereochemistry of the imines can be assigned simulating the UV-vis spectra. In the case of the imine from furan-2-carbaldehyde the computed spectra are characterized by an intense absorption at 361 and 357 nm respectively for the trans-1 and trans-2 structures. No other absorptions of comparable intensity have been predicted: the agreement with the experimental spectrum can be considered good. Furthermore, the experimental weak peaks at 280 and 270 nm can be associated to the computed transitions at 278 and 260 nm for the trans-1 isomer. Several minima of the energy surface can be assigned to the cis isomer, and they all present a very similar energy. The structures of the cis-1 and cis-2 isomers present quite coincident computed electronic spectra. In both cases, the computed spectrum shows two principal features. For the cis-1 structure, the first characteristic absorption is located at 414 nm and the second one at 284 nm. For the cis-2 structure, the first feature is located at 412 nm and the second one at 286 nm. The second transition is computed somewhat more intense. The experimental spectrum could be the consequence of similar populations of the planar cis structure (cis-3) and nonplanar cis structures (cis-1, cis-2, and their enantiomers).     

Donor-Acceptor Complexes in Copolymerization. XXXV. Alternating Diene-Dienophile Copolymers. 3. Copolymerization of cis- and trans-1, 3-Pentadiene with Maleic Anhydride and Acrylonitrile… AlEt1.5CI1.5

Abstract

The copolymerization of the cis or trans isomers of 1,3-pentadiene with maleic anhydride in the presence of a peroxide catalyst yields identical equimolar, alternating copolymers in which the pentadiene units have a cis-1, 4 configuration (IR, NMR). The copolymerization of the cis or trans isomers of 1, 3-pentadiene with acrylonitrile in the presence of ethyl aluminum sesquichloride yields identical equimolar, alternating copolymers in which the pentadiene units have a trans-1,4 configuration (IR, NMR). Although the trans isomer forms cyclic adducts with both maleic anhydride and acrylonitrile, the cis isomer does not undergo the Diels-Alder reaction with these dienophlles. The formation of identical copolymers from cis- and trans-1, 3-pentadiene is attributed to isomerization of the diene-dienophile charge transfer complex in the excited state, resulting in the generation of the same homopolymerizable exciplex from both isomers.     

Synthesis, characterization, electrochemistry, electronic structure, and isomerization of mononuclear oxo-molybdenum(V) complexes: the serine gate hypothesis in the function of DMSO reductases

Crystal structures of DMSO reductases isolated from two different sources and the crystal structure of related trimethylamine-N-oxide reductase indicate that the angle between the terminal oxo atom on the molybdenum and the serinato oxygen varies significantly. To understand the significance of this angular variation, we have synthesized two isomeric compounds of the heteroscorpionato ligand (L1OH) (cis- and trans-(L1O)Mo(V)OCl(2)), where the phenolic oxygen mimics the serinato oxygen donor. Density functional and semiempirical calculations indicate that the trans isomer is more stable than the cis. The lower stability of the cis isomer can be attributed to two factors. First, a strong antibonding interaction between the phenolic oxygen with molybdenum d(xy) orbital raises the energy of this orbital. Second, the strong trans influence of the terminal oxo group in the trans isomer places the phenol ring, and hence the bulky tertiary butyl group, in a less sterically hindered position. In solution, the cis isomer spontaneously converts to the thermodynamically favorable trans isomer. This geometric transformation follows a first-order process, with an enthalpy of activation of 20 kcal/mol and an entropy of activation of -9 cal/mol K. Computational analysis at the semiempirical level supports a twist mechanism as the most favorable pathway for the geometric transformation. The twist mechanism is further supported by detailed mass spectral data collected in the presence of excess tetraalkylammonium salts. Both the cis and trans isomers exhibit well-defined one-electron couples due to the reduction of molybdenum(V) to molybdenum(IV), with the cis isomer being more difficult to reduce. Both isomers also exhibit oxidative couples because of the oxidation of molybdenum(V) to molybdenum(VI), with the cis isomer being easier to oxidize. This electrochemical behavior is consistent with a higher-energy redox orbital in the cis isomer, which has been observed computationally. Collectively, this investigation demonstrates that by changing the O(t)-Mo-O(p) angle, the reduction potential can be modulated. This geometrically controlled modulation may play a gating role in the electron-transfer process during the regeneration steps in the catalytic cycle.     

Effect of metal cation on photoisomerization of azobenzenes in zeolite nanocavities

In the photostationary state, the cis/trans isomer ratio of azobenzene and 3,3′-dimethylazobenzene adsorbed in zeolite NaY increases significantly to ca. 90:10, in contrast to the reaction in cyclohexane. However, for azobenzene-4,4′-dicarboxylic acid diethyl ester the formation of the cis isomer is remarkably suppressed to ca. 30%. On the basis of ab initio MO calculations, it is suggested that electrostatic interaction between these azobenzenes and the metal ions in zeolite nanocavities regulates the cis-trans photoisomerization process. In addition, it is found that the photoisomerization behavior of azobenzenes adsorbed on silica gel is similar to that in NaY.     

1.1](3,3')-azobenzenophane: novel crystal structure and cis-trans isomerization of distorted azobenzene

[formula: see text] [1.1](3,3')-Azobenzenophane, in which two azobenzenes are cyclically connected by -CH2- chains at the meta positions, has been synthesized. The crystal structures of all isomers have been revealed. This is the first report on the crystal structure of the cis isomer of macrocyclic azobenzenes. The trans,trans isomer was slightly distorted, the trans,cis isomer highly deformed, and the cis,cis isomer unstrained. The thermal stability of cis isomers in solutions are deducible from the crystal structures.     

2,2-二取代-1,3-环戊二酮的Wittig反应研究

对2,2-二取代-1,3-环戊二酮进行Wittig反应,发现用NaOH做碱时,给出了与Grignard反应相同的加成产物一醇,而不是烯烃,分离了其中的一对顺反异构体并对顺式异构体培养了单晶,经X-射线衍射进一步确定了其构型。对此新颖的Wittig反应结果提出了可能的反应机理。     

Trans --> cis isomerization of trans-[(dppm)2Ru(H)(L)][BF4] (L = P(OR)3) complexes: preparation of cis-[(dppm)2Ru(eta2-H2)(L)][BF4]2

A series of new dicationic dihydrogen complexes of ruthenium of the type cis-[(dppm)(2)Ru(eta(2)-H(2))(L)][BF(4)](2) (dppm = Ph(2)PCH(2)PPh(2); L = P(OMe)(3), P(OEt)(3), PF(O(i)Pr)(2)) have been prepared by protonating the precursor hydride complexes cis-[(dppm)(2)Ru(H)(L)][BF(4)] (L = P(OMe)(3), P(OEt)(3), P(O(i)Pr)(3)) using HBF(4).Et(2)O. The cis-[(dppm)(2)Ru(H)(L)][BF(4)] complexes were obtained from the trans hydrides via an isomerization reaction that is acid-accelerated. This isomerization reaction gives mixtures of cis and trans hydride complexes, the ratios of which depend on the cone angles of the phosphite ligands: the greater the cone angle, the greater is the amount of the cis isomer. The eta(2)-H(2) ligand in the dihydrogen complexes is labile, and the loss of H(2) was found to be reversible. The protonation reactions of the starting hydrides with trans PMe(3) or PMe(2)Ph yield mixtures of the cis and the trans hydride complexes; further addition of the acid, however, give trans-[(dppm)(2)Ru(BF(4))Cl]. The roles of the bite angles of the dppm ligand as well as the steric and the electronic properties of the monodentate phosphorus ligands in this series of complexes are discussed. X-ray crystal structures of trans-[(dppm)(2)Ru(H)(P(OMe)(3))][BF(4)], cis-[(dppm)(2)Ru(H)(P(OMe)(3))][BF(4)], and cis-[(dppm)(2)Ru(H)(P(O(i)Pr)(3))][BF(4)] complexes have been determined.     

Photoisomerization of cis-1-(3-methyl-2-naphthyl)-2-phenylethene in glassy methylcyclohexane at 77 K

The cis-trans photoisomerizations of cis-1-(3-methyl-2-naphthyl)-2-phenylethene (c-3-MPE) was studied in methylcyclohexane (MCH) glass at 77 K. The fluorescence spectra of c- and t-3-MPE are excitation wavelength (λ(exc)) independent because the steric requirement of the methyl group restricts the conformational space of each isomer to a single conformer. Photocyclization, the dominant reaction pathway of c-3-MPE in solution, is entirely suppressed in MCH glass at 77 K. The only reaction on 313 nm irradiation of c-3-MPE in MCH glass is cis-trans isomerization. As the reaction progresses, the structureless fluorescence of c-3-MPE is replaced by the vibronically resolved fluorescence of the stable conformer of the trans isomer. The results are consistent with photoisomerization by the conventional one bond twist (OBT) pathway. Previously reported results on the photoisomerization of cis-1-(2-naphthyl)-2-(o-tolyl)ethene (c-NTE) are reinterpreted. Calculated geometries and energy differences for c- and t-3-MPE and c- and t-NTE [DFT using B3LYP/6-311+G(d,p)] are consistent with the interpretation of the experimental results.     

The solid-state isomerization of cis- and trans-(η5-C5H4Me)Mo(CO)2(P(OiPr)3)I

The cis- and trans-(η⁵-C₅H₄Me)Mo(CO)₂(P(OⁱPr)₃)I complexes undergo a bi-directional thermal ligand isomerization reaction to yield an equilibrium mixture of isomers (30/70 cis/trans ratio, 90 °C, < 80 min) in the solid state. The activation energy barrier for the cis-trans isomerization reaction (80–100 °C) was found to be 68 ± 10 kJ mol^–1. In benzene (reflux, 2 h) this isomer ratio was found to be 70:30 cis/trans. DSC and powder XRD studies have revealed reactions that occur in the solid state entailing decomposition and isomerization. DSC experiments did not reveal the presence of the cis–trans isomerization reaction.     

Molecular mechanisms of the photostability of indigo

The photophysics of indigo as well as of bispyrroleindigo, the basic chromophore of indigo, has been investigated with ab initio electronic-structure calculations. Vertical electronic excitation energies and excited-state potential-energy profiles have been calculated with the CASSCF, CASPT2 and CC2 methods. The calculations reveal that indigo and bispyrroleindigo undergo intramolecular single-proton transfer between adjacent N-H and C=O groups in the (1)ππ* excited state. The nearly barrierless proton transfer provides the pathway for a very efficient deactivation of the (1)ππ* state via a conical intersection with the ground state. While a low-lying S(1)-S(0) conical intersection exists also after double-proton transfer, the latter reaction path exhibits a much higher barrier. The reaction path for trans→cis photoisomerization via the twisting of the central C=C bond has been investigated for bispyrroleindigo. It has been found that the twisting of the central C=C bond is unlikely to play a role in the photochemistry of indigo, because of a large potential-energy barrier and a rather high energy of the S(1)-S(0) conical intersection of the twisted structure. These findings indicate that the exceptional photostability of indigo is the result of rapid internal conversion via intramolecular single-proton transfer, combined with the absence of a low-barrier reaction path for the generation of the cis isomer via trans→cis photoisomerization.     

Phenylene vinylene oligomers studied by theoretical methods: Joint analysis of computational and x-ray results of the configurational isomers of 1,4-bis[2-(3,4,5-trimethoxyphenyl)ethenyl]benzene

The configurational isomers of 1,4-bis[2-(3,4,5-trimethoxyphenyl)ethenyl]benzene have been investigated by ab initio and MOPAC-AM1 semiempirical methods. The calculations were guided by and compared with single crystal X-ray results of the trans, trans-isomer (taken from the literature) and of the cis,cis-isomer (reported here). Using 4-21G-based ab initio calculations, free state geometries, deviations from coplanarity, and barriers to rotation of the central and peripheral rings were evaluated. Such barriers were also enumerated for the solid state of the cis,cis- and trans,trans-isomers. A single-molecule cluster surrounded by point charges sufficed to rationalize observed solid state properties in the trans,trans-isomer, including the quasi-free rotation of the central ring. A multimolecule cluster, however, was required to rationalize the restricted rotation of the rings in the cis,cis-isomer. MOPAC-AM1 methods were used to calculate geometries and energies of rotameric forms on the singlet photoisomerization path cis,cis → cis,trans → trans,trans. Finally, UV absorption wavelengths and oscillator strengths were calculated and the electronic structure of the states discussed. © 1996 by John Wiley & Sons, Inc.