Solvent and temperature effects on the fluorescence and competitive photoreactions of cis-9-styrylanthracene

A fluorimetric and photochemical study of cis-9-styrylanthracene as a function of temperature has been carried out in different solvents with the aim of answering some open questions about the behavior of its first excited singlet state, S₁. In non-polar solvents, a parallel photoreactive pathway, leading to a cyclization adduct, was found to compete with fluorescence and isomerization already at 200 K, its contribution increasing markedly with temperature. The cis » trans photoisomerization occurs prevalently by a triplet mechanism, a detectable contribution of diabatic and adiabatic isomerization in S₁ being operative in these solvents from room temperature upwards. In polar solvents, the main deactivation pathway competitive with fluorescence is isomerization to trans, which occurs prevalently through a mixed singlet mechanism with a major diabatic and a minor adiabatic components.     

Synthesis, structure, photochromism and DFT calculations of copper(I)-triphenylphosphine halide complexes of thioalkylazoimidazoles

[Cu(SRaaiNR′)(PPh₃)X] complexes are synthesized by the reaction of CuX (X = Cl, Br, I), triphenylphosphine and 1-alkyl-2-[(o-thioalkyl)phenylazo]imidazole (SRaaiNR′). The single crystal X-ray structure of [Cu(SEtaaiNH)(PPh₃)I] (SEtaaiNH = 2-[(o-thioethyl)phenylazo]imidazole) shows a distorted tetrahedral geometry of the copper center with bidentate, N(azo), N(imidazole) chelation of SEtaaiNH and coordination from PPh₃ and iodine. These complexes show a trans-to-cis isomerization upon irradiation with UV light. The reverse transformation, cis-to-trans isomerization, is very slow with visible light irradiation and is thermally accessible. The quantum yields (?_t→c) of the trans-to-cis isomerization of [Cu(SRaaiNR′)(PPh₃)X] are lower than the free ligand values. This is due to the increased mass and rotor volume of the complexes compared to the free ligand data. The rate of isomerization follows the order: [Cu(SRaaiNR′)(PPh₃)Cl] < [Cu(SRaaiNR′)(PPh₃)Br] < [Cu(SRaaiNR′)(PPh₃)I]. The activation energy (E_a) of the cis-to-trans isomerization is calculated by a controlled temperature reaction. DFT computation of representative complexes has been used to determine the composition and energy of the molecular levels.     

The isomerization of cis,trans-1,5-cyclodecadiene by RhCl3 · 3H2O in ethanol

The isomerization of cis,cis-1,6-cyclodecadiene to cis,trans-1,5-; and cis,cis-1,5-cyclodecadiene in the presence of their rhodium complexes seems to indicate an isomerization for which no thermal conversion has been reported. Evidences are presented for the functioning of ethanol as a hydride source in these polymerizations.     

Quantum mechanical study of energetics and mechanisms of cis–trans isomerization of some four-membered heterocycles

Ab initio quantum mechanical calculations using density functional (B3LYP) method and 6-311G** basis set have been performed on two cis and trans conformers of 2,4-diphenyl thietane dioxide (DPTD), 2,4-diphenyl thietane (DPT), 2,4-diphenyl azetidine (DPA) and 2,4-diphenyl oxetane (DPO). The calculated stability energy for cis–trans isomerization in gas phase and in different solvents such as benzene, DMSO, water and methanol indicated that the cis conformer is more stable than trans in all above-mentioned compounds about 11–2 kcal mol^?1. In the next step, a transition states for cis–trans inter-conversion for all four-membered heterocycles (DPTD, DPT, DPA and DPO) were proposed in methanol as solvent. Thermodynamic functions such as standard enthalpies of isomerization (?Hº_iso), standard entropy of isomerization (?Sº_iso) and standard Gibbs free energy of isomerization (?Gº_iso) for all studied compounds were also evaluated. The calculation showed that the conversion of trans to cis isomer is exothermic and spontaneous. In all calculations, solvent effects were considered using a polarized continuum model.     

Application of cinchona-sulfonate-based chiral zwitterionic ion exchangers for the separation of proline-containing dipeptide rotamers and determination of on-column isomerization parameters from dynamic elution profiles

The interconversion of cis and trans isomers of dipeptides containing C-terminal proline was studied by dynamic chromatography on zwitterionic chiral stationary phases at temperatures ranging from −15 °C to +45 °C The cis–trans isomers could be separated below 0 °C and above 0–10 °C plateau formation and peak coalescence phenomena occurred, which is characteristic for a dynamic process at the time-scale of partitioning. At and above room temperature, full coalescence was observed, which allowed separations of enantiomers without interference from interconversion effects. Analysis of the dynamic elution profiles of the interconverting peptides allowed the determination of isomerization rate constants and thermodynamic activation parameters (isomerization enthalpy, entropy and activation energy). In accordance with established results, isomerization rates and thermodynamic parameters were found to depend on the nature of the N-terminal amino acid. Isomerization barriers were only slightly lower than values determined with other methods but significant differences in the relative contributions of the activation enthalpy and entropy as well as isomerization rates pointed toward selector-moderated isomerization dynamics.     

Hydrogen bonding in cis and trans 1-t butylcyclohexan-4-ol

The homogeneous autoassociation of cis and trans 1-t-butylcyclohexan-4-ol has been investigated by means of IR spectroscopy. The temperature and concentration dependences of this interaction have been examined in CCl₄ as solvent. The results demonstrate a lower degree of association in the case of the cis compound, especially because of the steric hindrance of the acceptor function of this substance. The enthalpy of associatio-n-ΔH is found at about 13 [kJ/mole] for the trans compound and may be somewhat lower for the cis alcohol. The absorbance of the associated OH band of the trans compound is constant with varying concentration and/or temperature, but for the cis alcohol it increases with increasing concentration and decreasing temperature.     

Unusually facil cis to trans isomerization of a polypropiolate bearing a long alkyl chain polymerized using a [Rh(norbornadiene)Cl]2 catalyst: An ESR study

Alkylpropiolate having n-hexadecyl moiety in the ester group was stereospecifically polymerized with a [Rh(norbornadiene)Cl]₂ complex catalyst in alcohol to give rise to cis-transoid polyacetylenes in high yields. The unusually facile cis to trans isomerization for the polymer was found to be induced when the polymer was warmed to even less than ca. 57°C. Under these conditions ESR spectra bearing hyperfine structures due to the radicals generated by the rotational scission in the cis–π bond were observed. This isomerization induced a large g value shift in the ESR spectra together with an increase of the radical concentration during the isomerization, indicating formation of planar conjugated trans sequences that stabilize mobile unpaired electrons as solitons. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. A Polym. Chem. 36: 2457–2461, 1998     

Derivatographic studies on transition metal complexes. XII. Thermal isomerization of trans-[CrBr2en2]Br·H2O and cis-[CrBr2tn2]Br·2 H2O in the solid phase

The thermal trans-to-cis isomerization of trans-[CrBr₂en₂]Br·H₂O and cis-to-trans isomerization of cis-[CrBr₂tn₂]Br·2 H₂O were studied by means of derivatographic and isothermal measurements. In both cases isomerization took place in anhydrous state after the complete dehydration. The activation energies for dehydration of the above two complexes were evaluated isothermally to be 61 and 38 kJ mole^?1, respectively, and for their isomerization to be 420 and 275 kJ mole^?1, respectively. These data were compared with those for the corresponding chloro complexes and the ease of isomerization between them was discussed.     

Radiation-induced Cis to Trans isomerization of poly(n-butylpropiolate) prepared with a [Rh(norbornadiene)Cl]2 complex as a stereospecific catalyst

Normal alkylpropiolate, i.e., n-butylpropiolate: HCCCOOnC₄H₉ was polymerized with [Rh(norbornadiene)Cl]₂ catalyst in methanol solvent to produce poly(n-butylpropiolate), (PBPA) polymer called polyacetylene esters in relatively high yields. The obtained polymer was characterized in detail using ¹HNMR, gel permeation chromatography (GPC), diffuse reflective UV methods. Consequently, the data showed that the resulting polymers selectively have cis-transoid form. We also found that γ-ray irradiation of the cis isomer resulted in isomerization to the trans isomer even at room temperature in air to generate the extended trans sequences which can stabilize mobile unpaired electrons as π-radicals called solitons.     

Mechanochemical-induced cis-to-trans isomerization of poly(2-ethynyl-3-n-octylthiophene) prepared with a Rh complex catalyst

Highly stereoregular polymerization of 2-ethynyl-3-n-octylthiophene was successfully performed with a [Rh(norbornadiene)Cl]₂ catalyst to produce the corresponding polymers in fairly high yields by using triethylamine or a mixture of it with other solvents as the polymerization solvent. We found that the obtained polymer using CHCl₃ was a mixture of cis-transoid form, ca. 68% and trans-transoid form, ca. 32% unlike our previous conjecture. Further, we found that the cis-to-trans isomerization can be also induced when the pristine predominant cis polymer was subjected to mechanochemical grinding (MCG) treatment at 77 K using a mortar filled with liquid nitrogen to decrease the cis content from ca. 68% to ca. 7%. The polymers obtained before and after the MCG treatment were characterized in detail using ¹H NMR, laser Raman, solution UV-vis, diffuse reflective UV-vis, and ESR methods in order to determine the geometry of the main-chain CC bonds in the polymer. The data showed that the polymer obtained by the treatment has a fairly distorted trans conjugation length, i.e., bent trans structure in which less mobile unpaired electrons generated by the rotational scission of the original cis CC bonds are stabilized.     

Chromatographic study of thermal reactions of cis- and trans-[Coen2 (OCOC6H4X)2]NO3 complexes in the solid phase

Thermal reactions of some cis- and trans-[Coen₂(OCOC₆H₄X)₂]NO₃ complexes mixed with ammonium or sodium chloride, nitrate and sulphate are reported. Reflectance spectra have shown a cis to trans thermal isomerization and, in the presence of NH₄Cl, the formation of trans-[Coen₂Cl₂]Y as final product. For the cis-isomers, the most interesting result is the chromatographic evidence of a stepwise reaction, resulting in the replacement of one carboxylato ligand by chloride and the formation of an intermediate species, characterized as trans-[Coen₂(OCOC₆H₄X)Cl]Y. These intermediate species, are hardly detectable in the corresponding reactions of the trans-isomers. These results may mean that in the trans-carboxylato compounds the reaction rate of the second step is higher than that of the first, while the reverse can be supposed in the corresponding reaction of the cis-isomers.     

Degradation and isomerization of 1,2-dichloroethenes by photocatalytic reactions

The photocatalytic degradation and isomerization of trans- and cis-1,2-dichloroethenes (1,2-DCEs) by TiO₂ photocatalyst have been investigated using gas chromatography. The reaction half-life of 1,2-DCEs in nitrogen was longer than in dry air and oxygen, and the initial concentration of them affects the extent of the isomerization. The results indicate that the oxygen molecule and chlorine atoms play an important role in the degradation and isomerization of 1,2-DCE. It is also apparent that the photocatalytic degradation of 1,2-DCEs occurs on the TiO₂ surface.     

Isolation of the retinal isomers from the isomerization of all-trans-retinal by flash countercurrent chromatography

The isolation of the retinal isomers from all-trans-retinal was performed by flash countercurrent chromatography. In each separation, isomerization reaction solution of 200 mg all-trans-retinal could be loaded on a 1200 mL of high-speed countercurrent chromatographic column with 5 mm bore, eluted by a mobile phase flow rate of 25 mL/min, resulting in 63 mg of 11-cis-retinal, 24 mg of 13-cis-retinal and 26 mg of 9-cis-retinal with purities more than 95%. n-Hexane–acetonitrile (3:1) was used as the solvent system which possesses the advantages of simplicity, re-use of the solvent and multiple injections. This method could be used to prepare 13-cis-retinal, 11-cis-retinal and 9-cis-retinal for the photoisomerization investigation, such as the effect of 11-cis-retinal in the visual system.     

The photoisomerization of retinal

Abstract— –Quantum efficiencies have been measured for the photoisomerization of four stereoisomers of retinal (all-trans, 13-cis, 11 cis, and 9-cis) in two solvents at different wavelengths of irradiation and at various temperatures. In heane at 25°C the quantum efficiencies for isomerization at 365 nm are: 9-cis to trans, 0.5; 13-cis to trans, 0.4; 11-cis to trans, 0.2; all-trans to monocis isomers, 0.2-0.06, depending upon assumptions made regarding the stereo-isomeric composition of the product. These values vary somewhat with the wavelength of the irradiating light. The quantum efficiency for the photoisomerization of all-trans retinal in hexane decreases by a factor of 30 when the temperature is lowered from 25° to – 65°C; the activation energy for this photoisomerization is about 5 kcal/mole. The quantum efficiencies for the isomerization of the monocis isomers to all-trans retinal in hexane are virtually independent of temperature. In ethanol the rates of photoisomerization from trans to cis or cis to trans depend only slightly on the temperature between 25° and – 65°C. The photosensitivities of the stereoisomers of retinal are of the same order of magnitude as those of the retinylidene chromophores of rhodopsin (11 -cis), metarhodopsin I (all-trans), and isorhodopsin (9-cis); but it is not yet possible to derive the photochemistry of rhodopsin uniquely and quantitatively from that of retinal.     

Kinetic analysis of cis-trans thermal isomerization of polyacetylene

A single broad exothermic peak due to cis-trans isomerization appears near 140°C in the differential scanning calorimetry (DSC) curve of polyacetylene (PA) films, polymerized by a nonsolvent method using a high-temperature-aged catalyst. This exothermic peakis described by a two-state model, in which population probabilities for cis and trans states are assumed to change through the forward and backward reactions. The enthalpy difference between the two states is 1.69 kcal/mol, which was derived from the slope of the plot of heat of isomerization versus cis content. The activation energy was determined experimentally from the heating-rate dependence of the peak temperature. A single activated process with an activation energy of 28 kcal/mol was observed for the PA film. Other parameters such as the preexponential factor were estimated by comparing theoretical and experimental DSC curves.     

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.     

Stereochemistry of reaction products of 1,3-dimethylthymine epoxide with amines

Reaction of 1,3-dimethylthymine epoxide with amines afforded cis and trans adducts, stereostructures of which were elucidated and treatment of trans adduct with boron trifluoride etherate afforded cis adduct; mechanism for the isomerization is presented.     

Triazolopeptides: chirospecific synthesis and cis/trans prolyl ratios of structural isomers

As cis/trans prolyl isomerization plays a crucial role in various biological processes, peptide mimics capable of modifying the cis/trans Xaa-Pro ratio are of particular interest. A practical approach toward proline derived triazolopeptides employing [3+2] azide-alkyne cycloadditions as the key reaction step and the analysis of their cis/trans prolyl ratios are reported. Structural investigations indicated the adjustability of both the cis-percentage and the conformational stability toward intramolecular H-bonding effects.     

Determination of the geometric structure of three substituted polyacetylenes: Polymethylacetylene,polypentylacetylene, and poly(tbutylacetylene)

The geometric structure of polymethylacetylene (PMA), polypentylacetylene (PPA), and poly(t-butylacetylene) (PTA) was investigated by ¹H NMR, ¹³C NMR, and IR spectroscopies. It was shown that both NMR techniques can be used to determine the trans isomer content of PPA and PTA, whereas the ¹H NMR and IR methods can be used for PMA. A calibration curve was constructed by using the 965- and 720-cm^?1 bands of the IR spectrum of PPA, and could be used in future work for the same purpose if the samples had molecular weights similar to that of the one used in this study. The isomerization kinetics of PTA was investigated and cis^→ trans activation energies of 88 and 121 kJ/mol were calculated in solution and in the solid state, respectively. Heat treatment of the PMA and PPA samples always leads to a cis^→ trans isomerization with a 100% trans content under extreme conditions. Moreover, a cis^→ trans isomerization of PTA was induced in CCl₄, CDCl₃, toluene, and benzene, but a trans^→ cis isomerization was induced in decalin. The reversible isomerization of PTA covered a trans isomer concentration ranging form 25 to 60%.     

A force field study of the chemical reaction between ethylene and cis-N2H2

The fully optimized geometry of the activated complex which occurs as an intermediate in the concerted H-transfer reaction between C₂H₄ and cis-N₂H₂ has been determined using the ab initio FORCE method of Pulay. The activation energy for the synchronous transfer of two hydrogen atoms from cis-N₂H₂ to ethylene is found to be 18.8kcal/mol, i.e. substantially lower than the previously estimated energy barrier of around 60 kcal/mol. The same method applied to trans-N₂H₂ and semilinear N₂H₂ gave an isomerization energy of 49.7 kcal/mol indicating that the isomerization of trans-N₂H₂ to the cis form might be the overall rate-controlling step.