Polarized absorption spectroscopy of trans-azobenzene and trans-stilbene in stretched polyethylene films

The linear dichroism spectra of trans-azobenzene and trans-stilbene aligned in stretched polyethylene films are reported for the u.v.-vis region. On the basis of the results the u.v. transition moment directions with respect to the long molecular axis are determined. Infrared data indicate that trans-azobenzene has a non-planar geometry while trans-stilbene is more planar in the polyethylene matrix. The polarization direction of the nπ* transition in azobenzene is different from that of the first ππ* absorption band, which indicates a vibrational borrowing mechanism corresponding to a nonplanar geometry.     

A CNDO/S study of trans-indanylideneindane. Implications to the trans-stilbene electronic structure

CNDO calculations have been performed on chiral derivatives of trans-1-(indan-1-ylidene)-indane, a stiff trans-stilbene analogue. The results are in complete agreement with the recently published UV and CD spectra of these compounds and allow state assignments down to 225 nm. The calculations are relevant to the trans-stilbene electronic structure which is under current dispute.     

Is solvated trans-azobenzene twisted or planar?

To arbitrate between previous conflicting theoretical and experimental results, a quantum-mechanical investigation of the ground-state geometry and UV spectrum of trans-azobenzene has been performed by using the Møller–Plesset approach and time-dependent density functional theory. It turns out that gas-phase trans-azobenzene is consistently predicted to be planar by the most accurate approaches. In solvent, the energy difference between planar and twisted trans-azobenzene is also negligible, although the twisted form tends to be favoured by a polar surrounding medium. These predictions are supported by comparisons between measured and predicted excitation energies.     

Normal coordinate analysis of trans-stilbene and tolane

The vibrational frequencies and modes of trans-stilbene and tolane have been calculated using a simplified overlay valence force field previously developed for a series of smaller conjugated molecules. The force constants have been directly transferred assuming that the large molecules can be regarded as weakly coupled systems. Several unexpected changes in frequencies and modes on isotopic substitution are discussed, the most interesting ones occurring in the Raman spectrum of α,α′-¹³C substituted trans-stilbene. On the basis of the calculations many revisions in the assignments have been made. The rms frequency deviations are 8.0 cm⁻¹ and 6.9 cm⁻¹ for trans-stilbene and tolane, respectively.     

一种合成反式-1,2-二苯乙烯的新方法

采用Grubbs's催化剂,以苯乙烯为原料,CH2Cl2为溶剂进行烯烃置换作用,合成出反式-1,2-二苯乙烯。为合成反式-1,2-二苯乙烯提供了一种实验简单、方法可靠的新方法。实验证明,用2(mol)%Grubbs's催化剂,在40℃,6h内反应产率可达70%。产物的结构由1^HNMR、MS等确定。     

ESR of the lowest triplet state of trans-stilbene

ESR of the lowest triplet state of trans-stilbene has been studied in glassy media at 77 K. This is the first report of ESR of a triplet polyene.     

Dynamics of the isomerization of trans-stilbene in n-alcohols studied by ultraviolet picosecond absorption recovery

We have used a simple At⁺ synchronously pumped and cavity-dumped dye-laser system to generate UV picosecond pulses with energies sufficiently high for absorption recovery experiments. With these pulses, we have studied the dynamics of the isomerization of trans-stilbene in n-alcohols as a function of viscosity and temperature. It is concluded that the excited-state barrier of trans-stilbene in n-alcohols is less than 1 kcal mol^?1 and that the trans → cis isomerization yield is 0-54 ± 0.05.     

The influence of sodium dodecyl sulfate/benzyl alcohol/H2O system on the photoisomerization of trans-stilbene

Photoisomerization of trans-stilbene (TS) was investigated in sodium dodecyl sulfate (SDS)/benzyl alcohol (BA)/H(2)O systems in order to establish the relationship between the reaction yields and the compositions and structures of molecular organized assemblies. The results show that, in SDS/BA/H(2)O systems with the structures of oil in water microemulsions, bicontinuous microemulsions, and hexagonal liquid crystals, the reaction yields are much larger than those in benzyl alcohol, both with and without the photosensitizer 9,10-anthracenedicarbonitrile. However, SDS/BA/H(2)O water in oil microemulsion and lamellar liquid crystal can obviously catalyze the photoisomerization of TS only when the photosensitizer is not present. The yield is increased with the increase in H(2)O content or with the decrease in BA content.     

Photochemical reactions of 7-aminocoumarins. 5. [2+2]-Cycloadducts with trans-stilbene

The photocycloaddition of trans-stilbene to 7-diethylaminocoumarin, 4-methyl-7-diethylaminocoumarin, and 4-(N-morpholino)-7-diethylaminocoumarin was investigated. Adducts involving stereospecific (2+2)-cycloaddition at the 3–4 bond were isolated. On the basis of the PMR spectra and an x-ray diffraction study, it was established that the phenyl groups in the cycloadducts occupy 1-endo and 2-exo positions.For Communication 4, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1022–1027, August, 1990.     

Observation of transient resonance raman spectra of the S1 state of trans-stilbene

Transient resonance Raman spectra of trans-stilbene in n-hexane have been obtained using two pulsed lasers at 266 and 585 nm. The former was used to pump the molecule to the first excited singlet state (S₁) and the latter to proble spontaneous Raman scattering in resonance with the S_n ← S₁ electronic transition. The dependence of the pump and probe laser power, the temporal behavior, and the excitation profile of the spectra clearly indicate that they are due to the S₁ state of trans-stilbene.     

The pseudobinary phase diagram cis-/trans-azobenzene and the cis → trans isomerization in various states of aggregation

The pseudobinary phase diagram of trans-azobenzene and the thermally instable cis-isomer was examined by thermomicroscopical and DSC-measurements. Two eutectic temperatures have been determined (at 41.4 C and 36.7 C). The latter occurs only in the presence of a metastable polymorph of cis-azobenzene after quenching a melt, that contained as well cis-as trans-azobenzene, presumable under conditions of heterogeneous nucleation. In the melt and in solution the thermally induced cis-trans-isomerization follows first order kinetics. Below the eutectic temperature the rate is controlled by three-dimensional nuclei growth to reaction extent 0.20. Between the eutectic temperature and the melting points of product respectively educt the kinetics follow the PROUT-TOMPKINS law. The reason is the increasing isomerization rate with increasing fraction of melt.

---

Zusammenfassung Das pseudobinäre Phasendiagramm von trans-Azobenzol und des thermisch instabilen cis-Isomeren wurde mit Hilfe thermomikroskopischer und DSC-Messungen untersucht. Es wurden zwei Eutektika bestimmt /bei 41.4 °C und 36.7 °C/, wobei das zweite Eutektikum nur in Anwesenheit einer metastabilen Modifikation von cis-Azobenzol nach Abschrecken einer Schmelze, die gleichzeitig eis- und trans-Azobenzol enthielt, vermutlich bei heterogener Keimbildung entstand. In der Schmelze und in Lösung verläuft die thermische cis-trans-Isomerisierung nach einem Geschwindigkeitsgesetz erster Ordnung. Unterhalb der eutektischen Temperatur ist die Festkörperreaktion bis zu Umsätzen von 0.20 durch dreidimensionales Keimwachstum bestimmt. Zwischen eutektischer Temperatur und Schmelzpunkt lässt sich die Reaktion durch die Gleichung von PROUT-TOMPKINS beschreiben, was auf die zunehmende Isomerisierungsgeschwindigkeit mit zunehmende Anteil Schmelze zurückzuführen ist.

---

- -. 41,4 36,7°. - , -, -, , , . - . 0,20. , -. .

---

---

We thank the Deutsche Forschungsgemeinschaft for financial support.     

Planar chirality of twisted trans-azobenzene structure induced by chiral transfer from binaphthyls

The absolute configuration of a binaphthyl-azobenzene dyad 2b, which has a chiral axis and a chiral plane, was determined by comparing the experimental circular dichroism (CD) spectra with the theoretical CD spectra calculated by the time-dependent (TD)-DFT method. The CD signals of the trans-azobenzene moiety indicated that the two benzene rings of this moiety are twisted unidirectionally. It is suggested that these dyads with shorter linkers may be suitable for use as chiroptical switches.     

Chiral separation of trans-stilbene oxide through cellulose acetate butyrate membrane

An enantioselective membrane was prepared using cellulose acetate butyrate as a membrane material. The flux and permselective properties of membrane using 50% ethanol solution of (R,S)-trans-stilbene oxide as feed solution were studied. The top surface and cross-section morphology of the resulting membrane were examined by scanning electron microscopy. The resolution of over 92% enantiomeric excess was achieved when the enantioselective membrane was prepared with 15 wt % cellulose acetate butyrate and 30 wt % N,N-dimethylformamide in the casting solution of acetone, 10 °C temperature of water bath for the gelation of the membrane, and the operating pressure and the feed concentration of the trans-stilbene oxide were 3 kgf/cm² and 5.2 mmol/L, respectively. Since the cellulose acetate butyrate contained a large amount of asymmetric carbons on the backbone structure, it was possible to form helical structure, this was considered to be the reason for the enantioselectivity of the membrane.     

Ab initio spectroscopy and photoinduced cooling of the trans-stilbene molecule

We present a theoretical study of the S(0)-->S(1) and S(0)<--S(1) vibronic spectra for trans-stilbene. Franck-Condon spectra in the harmonic approximation are generated for the complete system with 72 degrees of freedom by means of an analytic time-dependent approach accounting for Dushinsky rotations and thermal effects. The force fields are computed by means of density functional theory (DFT) and time-dependent DFT, on the one hand, and ab initio complete active space self-consistent field theory, on the other hand. The B3LYP functional shows that almost planar potential energy surface minima are found for the S(0) and S(1) state. Imposing C(2h) symmetry constraints, we obtain low-temperature high-resolution Franck-Condon spectra for both absorption and emission which are in reasonably good agreement with the experimental spectra measured by Syage et al. [J. Chem. Phys. 81, 4685 (1984)] in supersonic jets. Due to thermal population of low-energy modes, the room temperature absorption spectrum is very broad. An almost structureless band which extends over several thousand cm(-1) is obtained from the present theory and agrees with the experimental absorption band shape. Finally, within the harmonic model, we study the effect of photoexcitation on the energy distribution in the excited S(1) state. We find noticeable cooling of approximately 20 K within a frequency interval spanning from -400 to 200 cm(-1) around the 0-0 transition. This indicates that photoinduced cooling must be taken into account when considering the dynamics of the photoinduced isomerization of trans-stilbene. Moreover, this is not the final word, as anharmonicity of the low frequency modes must be taken into account to obtain a full picture which would explain both the energy dependence of the isomerization rates as well as the dependence on the pressure of an external buffer gas.     

Excitation energy effects in the photophysics of trans-stilbene in solution

Excitation of trans-stilbene with 306 nm pulses generates a narrower transient spectrum than with 265 nm. These spectra do not evolve in the same manner, suggesting that the spectral changes with 265 nm result from vibrational cooling. Spectral differences at longer times suggest there is a slow cross-relaxation component in the inhomogeneous distribution.