Quantum yield stilbene isomerization

An analogous cation radical chain process has been proposed for cis to trans isomerization of N-methyl-4-(6-stryl)-pyridinium ions via electron-transfer sensitization by Ru(bpy)-j2+ and metalloporphyrins (145). Quantum yields for isomerization are substantially higher in aqueous anionic micelles versus homogeneous solution due to the higher concentration of cis-styrylpyridinium ions. A radical cation chain mechanism may also account for previous reports of selective cis to trans sensitized photoisomerization of stilbene (25,26). 

On benzopheneone sensitization the quantum yields of cis-trans isomerization of stilbene, d>c, t and t- c. were 0.55 and 0.38, respectively, and the value (1-45) ([c]/[t])pss ( 1.48) [77,78]. Detailed experiments revealed that with sensitizers having excitation energies much higher than those of trans-and cw-stilbene there is a variation in the photostationary state isomer composition as well as in the quantum yields of Isomerization ([c]/[t])psj varies from... 

Sandros and Sundahl [127] have synthesized all six isomers of 4,4-bis (3,5-di-fert-butylstyryl) stilbene (Exhibit 16) to study photoisomerization. Isomerization is preferred from cis-to-trans conversion and single isomerizations are more dominant. The increase in the quantum yield of isomerization resulting from an increase in the solvent polarity is interpreted to mean that a twisted singlet excited (p ) state is polar in nature and its interaction with polar medium lowers its energy, thereby facilitating the isomerization process. They have found that 103 with central double bond in cis configuration displayed adiabatic cis-to-trans isomerization. 

For nitrostilbenes (Table 5d) in nonpolar solvents at ambient temperature the quantum yields of isomerization are comparable to those of stilbene itself. However, 4-nitro-4 -R-stilbenes show a different dependence on solvent properties and temperature. On increasing the solvent polarity, decreases strongly for polar substituents for example, R = OCH3, NH2, and N(CH3)2, 4,4 -NMS, 4,4 -NAS, and 4,4 -NDS, respectively. nonpolar solvents (cyclohexane and benzene) are... 

The effect of temperature on the quantum yields of isomerization and fluorescence was determined for frons-stilbene and for certain halogen-substituted frans-stilbenes (Dyck and McClure, 1962 Malkin and Fischer, 1962). It was found that as the temperature is decreased (below about 200°K) the trans cis quantum yield decreases but that the fluorescence quantum yield increases. These results suggest that an activated process (i.e., internal conversion or intersystem crossing) competes with fluorescence from S - and that this process is an essential step in the mechanism for isomerization. The results for intersystem crossing into T. ... 

The cyclization product is thermally unstable relative to Z-stilbene and reverts to starting material unless trapped by an oxidizing agent. The extent of eyclization is solvent-dependent, with nonpolar solvents favoring cyclization more than polar ones. ° Whereas the quantum yield for Z-E isomerization is nearly constant at about 35%, the cyclization... 

Table 9.1. Quantum Yields for Sensitized Stilbene Isomerization...

The initial quantum yields for cis- to tam-stilbene isomerization (O0 T) and for trans to cis isomerization (4>T-.C) are consistent with Hammond s postulate that isomerization takes place from a common state, most likely the twisted or phantom triplet state ... 

If fluorescence and cis-trans isomerization (9.26)-(9.29) are the main competing reactions upon direct excitation, then inhibition of rotation about the central bond should produce an increase in the fluorescence quantum yield. The rigid systems (3) and (4) both have fluorescence quantum yields of 1.0 at room temperature.<44,52) While the fluorescence of /rmy-stilbene is a... 

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. 

The 9,10-dicyanoanthracene sensitized irradiation of c/i-stilbene results in nearly quantitative isomerization (>98%) to the trans isomer with quantum yields greater than unity. Therefore, the isomerization was formulated as a free radical cation chain mechanism with two key features (1) rearrangement of the c/i-stilbene radical cation and (2) electron transfer from the unreacted cis-olefin to the rearranged (trans-) radical cation. 

In all cases studied the E,Z-isomerization has the higher quantum yield, so it is of no importance from which isomer the photoreaction starts. In analogy to the primary product from stilbene the cyclization products from other diaryl olefins will be denoted by DHP. The lifetime of such DHP s can vary from some milliseconds to several hours I2). 

The direct photoisomerization of substituted stilbenes has also received attention. Several 4,4 -disubstituted stilbenes in which one substituent is electron withdrawing and the other electron donating, such as 14, have quantum yields for cis -> trans isomerization similar to that of m-stilbene, but exhibit very low quantum yields for trans -> cis isomerization in hydrocarbon solvents and zero quantum yields in ethanol.250 Likewise, certain salts of 4 -amino-2-styrylpyridine, such as 15, do not undergo direct trans -> cis photoisomerization.251 The strong interactions between the ring systems in the ground states of 14 and 15 are probably increased in the excited states. Consequently the planar... 

Platinum porphyrin complexes can be prepared by reaction with PtCl2(PhCN)2. Purification of the final complex is by medium pressure liquid chromatography on alumina. The strongly phosphorescent platinum(II) porphyrin complexes are efficient sensitizers for stilbene isomerization. The quantum yields for the cis to trans process are greater than unity because of a quantum chain process in which the metalloporphyrin serves both as an energy donor and an acceptor.1110 Picosecond laser spectroscopy has been used to obtain time-resolved excited-state spectra of platinum octaethylporphyrin complexes, and to probe the excited-state energy levels.1111 Tetrabenzoporphyrin complexes have been prepared for platinum in both the divalent and tetravalent oxidation states. The divalent complex shows strong phosphorescence at 745 nm.1112... 

Cycloaddition is a singlet state reaction, triplet quenching yielding only stilbene isomerization. In the limit of high t-1 concentration, the quantum yield for the formation of 89 and 90 is 0.66 and no c-1 is formed. Nonradiative exciplex decay is proposed to occur by partitioning at the pericyclic minimum (Fig. 2) between products and reactants. In the limit of high c-1 concentration, 91 is formed with a quantum yield of 0.05 and the predominant exciplex decay pathway is dissociation to yield f-c, which decays to a mixture of t-1 and c-1. 

Reaction of singlet cyanoanthracenes with t-1 or c-1 in polar solvents results in the formation of stilbene cation radicals (see Section VII.A). In the absence of oxygen, the t-1 cation radical decays by back electron transfer to the cyanoanthracene anion radical without undergoing isomerization. In contrast, the c-1 cation radical undergoes isomerization with concentration dependent quantum yields which can exceed 1.0 to yield a photostationary state consisting of 99% t-1 and 1% c-1 (27). The selective isomerization of c-1 but not t-1 is... 

The general features of the isomerization are compatible with a free radical cation chain mechanism, featuring electron transfer from unreacted olefin to rearranged radical cation. This chain mechanism was firmly established in several other isomerizations by the observation of quantum yields greater than unity. Thus, the dicyanoanthracene sensitized irradiation of m-stilbene results in nearly quantitative isomerization (> 98%) to the trans-isomer. In this system, the quantum yield increases with increased ds-stilbene concentration, solvent polarity, salt concentration, as well as decreasing light intensity [159]. 

A surfactant stilbene (76) has been synthesized as a viscosity probe of micellar systems 76). The stilbene surfactant undergoes photochemical trans-cis isomerization with quantum yields very similar to stilbene. Like stilbene, the quantum yield (quantum yield of fluorescence (ft) increases. For example, decreases from 0.5 in CH2C12 to 0.39 in HDTBr, while the f, increases from 0.04 in CH2C12 to 0.18 in HDTBr. This probe has also been used to study bilayer vesicle systems. Cyanine dyes become more photostable and have higher organic solvents 77). This is also due to an increase in viscosity. 

Solvent reorientation and isomerization of trans-stilbene in alkane solutions has been studied by ps time scale anisotropic absorption and polarization239 Coupling of solute and solvent decreases as the size of the solvent molecules increases. The applicability of currently favoured models for the activated barrier crossing in the photoisomerization of stilbene is discussed, A method for measuring quantum yields in the photoisomerization of trans-stilbene gives high accuracy without use of a chemical actinometer . Evidence has been found for dynamic solvent effects on the photoisomerization of 4,4 -dimethoxystilbene in which the effects of temperature and hydrostatic pressure were made in n-alkane and n-alkyl alcohol. A ps laser time-resolved study fits frequency dependent solvent shifts but gives results inconsistent with the free volume model. Photophysical and theoretical studies of trans and 9-... 

Physical details relating to the isomerization of the stilbenes (12) have been determined.A study of the photophysical properties of the styrylstiIbenes (13) has shown that quantum yields for trans-cis isomerism are low from the singlet but high from the triplet state. The photochemical isomerization of the alkene (14) in an ethanol glass affords the trans-isomer with high efficiency even at liquid helium temperatures. Photochemical cis-trans-isomerization of cis-1,2-di-l-naphthylethylene (14) has also been studied in the crystalline phase. A study of the photochemical isomerization of a series of styryl phenanthrenes has been reported. The mechanism of the reaction involved was discussed. ... 

Isomerization can be induced by light in both directions or by heat in the Z — E direction. The reverse thermal reaction is not observed at normal temperatures. Any one of the elementary reactions can be missing. Z-azobenzene in solution has a thermal Z E activation enthalpy AH 96 kJ moT and a half life time of 2 to 3 days at room temperature. Thus, the thermal reaction is irrelevant for the photoisomerization at usual irradiation intensities (for comparison Z-stilbene has Eg 180 kJ moT is liquid, and is kinetically stable). On the other hand, one of the photoreactions may not be active (e.g., when an irradiation wavelength is selected where one form does not absorb or when the quantum yield is too small). Inspection of Figure I.IB shows that E- and Z-azobenzene have virtually no spectral region without overlapping absorption. 

The photoisomerization of all types of azobenzenes is a very fast reaction on either the singlet or triplet excited-state surfaces according to the preparation of the excited state, with nearly no intersystem crossing. Bottleneck states have lifetimes on the order of 10 ps. The molecules either isomerize or return to their respective ground states with high efficiency. So photoisomerization is the predominant reactive channel, and the azobenKnes are photochemically stable. Only aminoazobenzene-type molecules and pseudo-stilbenes have small quantum yields of photodegradation. 

Figure 7.4. Temperature dependence of the quantum yield for trans-cis isomerization (A), of the quantum yield p of fluorescence (o), and of the fluorescence lifetime ( ) of stilbene (by permission from Saltiel and Charlton, 1980).

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