Stilbenes, photoisomerization

Nonvertical Energy Transfer. The concept of nonvertical energy transfer was introduced by Hammond and Saltiel to explain the observation that low energy sensitizers would photoisomerize stilbene, despite the fact that energy... 

According to Kramers model, for flat barrier tops associated with predominantly small barriers, the transition from the low- to the high-damping regime is expected to occur in low-density fluids. This expectation is home out by an extensively studied model reaction, the photoisomerization of tran.s-stilbene and similar compounds [70, 71] involving a small energy barrier in the first excited singlet state whose decay after photoexcitation is directly related to the rate coefficient of tran.s-c/.s-photoisomerization and can be conveniently measured by ultrafast laser spectroscopic teclmiques. 

Figure A3.6.5. Photoisomerization rate constant of (ran.s -stilbene m n-pentane versus inverse of the self-diflfrision coefficient. Points represent experimental data, the dashed curve is a model calculation based on an...

Figure A3.6.8. Isothemis of k ) for tran.s-stilbene photoisomerization in n-hexane at temperatures between...

Figure A3.6.9. Viscosity dependence of photoisomerization rate constants of tran.s -stilbene (open circles) and E,E-diphenylbutadiene (fiill circles) in n-butanol. The broken line indicates a -dependence of k [96],...

Schroeder J, Tree J and Vdhringer P 1995 Photoisomerization of f/ a/rs-stilbene in compressed solvents Kramers turnover and solvent induced barrier shift Z. Phys. Chem. 188 287... 

Schroeder J, Schwarzer D, Troe J and Voss F 1990 Cluster and barrier effects in the temperature and pressure dependence of the photoisomerization of trans.stilbene J. Chem. Phys. 93 2393-404... 

Syage J A, Felker P M and Zewail A H 1984 Picosecond dynamics and photoisomerization of stilbene in supersonic beams. I. Spectra and mode assignments J. Chem. Phys. 81 4685-705... 

Nikowa L, Schwarzer D, Troe J and Schroeder J 1992 Viscosity and solvent dependence of low barrier processes photoisomerization of c/s-stilbene in compressed liquid solvents J. Chem. Phys. 97 4827... 

Poliak E 1987 Transition state theory for photoisomerization rates of f/ a/rs-stilbene in the gas and liquid phases J. Chem. Phys. 86 3944... 

The easiest method for creating many vibrational excitations is to use convenient pulsed visible or near-UV lasers to pump electronic transitions of molecules which undergo fast nonradiative processes such as internal conversion (e.g. porjDhyrin [64, 65] or near-IR dyes [66, 62, 68 and 62]), photoisomerization (e.g. stilbene [12] or photodissociation (e.g. Hgl2 [8]). Creating a specific vibrational excitation D in a controlled way requires more finesse. The easiest method is to use visible or near-UV pulses to resonantly pump a vibronic transition (e.g. 

Category 4. Photoisomerization. The most common reaction in this category is photochemical cis-trans isomerization." For example, cw-stilbene can be... 

Electronic excited states are often reactive intermediates in many photochemical reactions. In a number of cases, the excited state may undergo energy relaxation. The photoisomerization reaction of fra i-stilbene provides a well-studied... 

Evidence that eliminates the triplet mechanism as the mode for the cis-trans isomerization of stilbene upon direct photolysis has been provided by azulene quenching studies.(48) Using the experimentally determined decay ratio a/(l — a) and the triplet mechanism, it is possible to calculate what the effect of azulene is upon the pss. The predicted and observed azulene effects on the direct photoisomerization are shown in Figure 9.6. The failure of the triplet mechanism in predicting the very small changes observed in the pss provides a crucial test that is the basis for rejecting the triplet mechanism. 

Fig. 27 (a) Optimized graphene sheet for the realization of a half-adder. Each logical input noted a and [i controls the photoisomerization state of one of the two stilbene groups. Depending on this isomerization state, the overall conductance of the molecule between the three electrodes is modified, (b) Current intensity calculated in the two output electrodes depending on the conformation of the stilbene groups... 

The effects of nitro substituents on the cis-trans isomerization of stilbenes has been reviewed70 (equation 63). The trans-to-cis isomerization occurs from a triplet excited state, whereas the reverse cis-to-trans isomerization occurs through a main route which bypasses the triplet state. A nitro substituent usually causes a significant enhancement of the quantum yield of the intersystem crossing. Nitro substituent effects on the photoisomerization of trans-styrylnaphthalene71 (equation 64), trans-azobenzenes72 and 4-nitrodiphenylazomethines73 (equation 65) have been studied for their mechanisms. 

In calcium chelators Indo-1 (17) and Fura-3 (18b) (Figure 2.9),(18) the fluoropho-res have donor-acceptor stilbene-like structures rigidified so as to avoid photoisomerization. Based on the same principle, Fura-2 (18a)a8) is one of the most popular calcium indicator for microscopy of individual cells because, in contrast to Quin-2 (see Section 2.2.5.), the excitation spectrum is blue shifted on cation binding, thus allowing intensity-ratio measurements. On the other hand, there is almost no shift of the emission spectrum, which can be interpreted along the same line as DCM-crown (see earlier in this section). 

Sensitivity to the magnetic held strength has also been observed in the process of photoisomerization of tran -stilbene (tS) into c -stilbene (cSt) in the presence of pyrene (P). The reaction was run in a solution with acetonitrile, DMSO, or hexafluorobenzene being a solvent. During the reaction, spin polarization was established (Lyoshina et al. 1980). The spin polarization was explained by the following sequence of the elementary acts ... 

Stilbenes and Stilbene-Like Molecules, Cis-Trans Photoisomerization of... 

Diphenylethene (Stilbene). This molecule has been the subject of many photophysical and photochemical investigations and the subject of several reviews. It is the prototypical alkene for studies of photoisomerization. Transient spectroscopic measurements in the picosecond time domain have been performed on electronically excited tran -stilbene in a wide range of environments. Selections from these studies are described here. 

Diphenyl-1,3-butadiene. The excited-state behavior of this diene differs significantly from stilbene and is the subject of a review. Unlike tS in which the lowest vertical excited singlet state is the 1 B state and S2 is the 2 Ag state in solution, these two excited states lie very close to each other in all-trans-1,4-diphenyl-1,3-butadiene (DPB). The additional carbon-carbon double bond introduces a new conformational equilibrium involving the s-trans and s-cis rota-mers. Most spectroscopic studies in solution have concluded that the l B state is S. The DPB compound has a low quantum yield for photoisomerization, so the use of DPB in time-resolved spectroscopic studies on photoisomerization, especially those that monitor only fluorescence decay, needs to be considered cautiously and critically. 

Diphenyl-1,3,5-hexatriene. The quantum yield for photoisomerization of fllTfrani-l,6-diphenyl-l,3,5-hexatriene (DPH) is much lower than that of fra 5-stilbene. ° The DPH compound is the first in the series of vinylogous stil-benes for which the 2 A state is lower than the 1 B state. Early picosecond fluor-... 

The excited-state behavior of 1,1,2,2-tetraphenylethene (TPE) has been studied by means of picosecond fluorescence, absorption, and Raman spectroscopies and picosecond optical calorimetry. It has been shown that, like stilbene, TPE derivatives substituted with minimally perturbing stereochemical labels such as methyl groups undergo efficient photoisomerization. However, unlike stilbene, strong spectroscopic evidence exists for the direct detection of the twisted excited singlet state, 5ip herein but traditionally designated as of TPE. 

The only recent example of Forster transfer of photochemical importance is the demonstration by Saltiel163 that the ability of azulene to increase the photostationary transjcis ratio in direct photoisomerization of the stilbenes is due entirely to radiationless transfer of excitation from traw.y-stilbene singlets to azulene. As expected for Forster transfer, this azulene effect did not depend upon solvent viscosity. The experimental value of R0, the critical radius of transfer in Forster s formula,181 was 18 A, in good agreement with the value calculated from the overlap of stilbene emission and azulene absorption. 

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