Photoisomerization quantum yield

Therefore, the quantum yield for photoisomerization approximates that for nitrogen formation and both arc typically ca 0.5. Where the cis isomer is thermally stable, quantum yields for initiator disappearance are low (>... 

The validity of the above conclusions rests on the reliability of theoretical predictions on excited state barriers as low as 1-2 kcal mol . Of course, this required as accurate an experimental check as possible with reference to both the solvent viscosity effects, completely disregarded by theory, and the dielectric solvent effects. As for the photoisomerization dynamics, the needed information was derived from measurements of fluorescence lifetimes (x) and quantum yields (dielectric constant, where extensive formation of ion pairs may occur [60], the observed photophysical properties are confidently referable to the unperturbed BMPC cation. Figure 6 shows the temperature dependence of the... 

Recently, a photoisomerization reaction of azoferrocene was found to proceed in polar solvents such as benzonitrile and DMSO through both a 7t it transition of the azo-group with a UV light (365 nm) and the MLCT transition with a green light (546 nm) (Fig. 6) (Scheme 1) (153). The quantum yields of the photo-isomerization reaction at 365 nm and 546 nm were estimated to be 0.002 and 0.03, respectively. The transformation into the cis form causes the higher field shift of Cp protons in the 1H-NMR spectrum and an appearance of u(N = N) at 1552 cm-1. The cis form is greatly stabilized in polar media, and dilution of the polar solution of cis-25 with less polar solvents resulted in a prompt recovery of the trans form. 

Photo-isomerism is also found in this group of compounds. Cis-Pt(NH3)2(H20)l+ photo-isomerizes to trans with a quantum yield of about 0.1 at 363 nm (40). Photoisomerism of Pt (glycine) 2 from cis to trans, but not its reverse is also reported. 

This transformation was demonstrated (14) by evaluating changes in the UV, IR and NMR spectra and comparing these to the spectra of authentic samples of each cluster (15., 16.). Quantum yields for the photoisomerization depicted in Equation 17 were found to be notably dependent both on the CO concentration and on the A rr. Although the resulting optical changes were the same for different A rr, the quantum yields in CO saturated cyclohexane ranged from < 10 at 405 run to 4.9 x 10 at 313 nm. Furthermore, 2 varied linearly from 1.2 x 10" at Pqq " 0.0 to 4.9 x 10 at P q - 1.0 atm for 313 nm photolysis in cyclohexane. 

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. 

Quantum yields for direct E,Z-photoisomerization of conjugated polyenes... 

In solution, an initial photoequilibrium is established between the Z- and -isomers, while the rearrangement products 117 and 118 are formed along with traces of cyclohexadiene (CHD) over much longer irradiation times (equation 46). In solution, the major products are 3-vinylcyclobutene (117) and bicyclo[3.1.0]hex-2-ene (118) Z-l,2,4-hexatriene (119), which is a major product in the gas phase176,211, is formed in relatively low yields. The quantum yields for ,Z-photoisomerization of Z- and -l,3,5-hexatriene in pentane solution (265 nm excitation) are /, r = 0.034 and E—Z = 0.016, respectively188. 

QUANTUM YIELD PHOTOCHROMISM PHOTO-CROSS-LINKING PHOTOAFFINITY LABELING PHOTODIMERIZATION PHOTOISOMERIZATION PHOTOLYSIS FLASH PHOTOLYSIS... 

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-... 

A second point of contrast between the thermal and the photochemical behavior is that, while the cis isomers are all thermodynamically favored, and consequently all the trans forms appear more labile, the situation on photolysis is varied. The quantum yields for C are ten to twenty times those for T HT is more sensitive than HC (but only towards photoisomerization), and H2C is more photosensitive than H2T. 

It might be thought that photoisomerization and photoaquation could be competitive towards a common precursor excited state, but this explanation fails on quantitative testing. Briefly, the quantum yield for photoaquation is too high to accommodate its negative apparent activation energy. Photoaquation must then be competitive with some other process such as deactivation, or return to the original cis species. 

Next is the contrast between the 370 m/x to 550 m/x behavior and that for wave lengths greater than 680 m/x. We are now comparing the consequence of exciting a quartet band and a doublet band. The activation energy for photoisomerization has increased to about 13 kcal., and there is a dramatic shift in the aquation behavior which now shows 20 kcal. apparent activation energy and quantum yields approaching unity at low temperatures. The minimal conclusion here is that irradiation of... 

In the triplet counting technique the substance whose intersystem crossing yield is to be determined is used as a sensitizer for the photoisomerization of piperylene (1,3-pentadiene). Under conditions where essentially all of the sensitizer triplets are intercepted by diene, the quantum yield of isomerization is directly proportional to the intersystem crossing yield of the sensitizer. If the diene interferes with sensitizer singlets however, the yield of triplets will fall, resulting in an anomalously low value for 0,. 

Camphorate complexes of chromium (III) have been studied. The four possible isomers of the tris complex of (+ )-3-acetylcamphorate (173) were isolated,752,753 and absolute configurations were tentatively assigned. The photoisomerization of these complexes has been investigated 754 quantum yields of the order of 10-3 were obtained with visible or ultraviolet radiation at temperatures around 100 °C. Bond-breaking processes were held to be important in the reactivity of cis isomers. 

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... 

Biacetyl sensitized photoisomerization of l,2-di-9-anthrylethane 7a does not lead to the 4n + 4n cyclomer 8a but yields exclusively the An + 2n cycloadduct 26 with a quantum yield of 0.1 [72]. Since the phosphorescence of biacetyl is quenched by dianthrylethane 7a at nearly diffusion controlled rate, the photochemical Diels-Alder reaction is explicable by triplet energy transfer from biacetyl to 7a. The photochemical isomerization of 10-benzoyl-l,2-di-9-anthrylethane 27 also proceeds exclusively by An + 2n cycloaddition and gives cycloadduct 28 with a quantum yield of 0.005 [73], The low fluorescence quantum yield of 27 (excited triplet state. Biacetyl sensitization of 27 leads to 28... 

Formula 392 (173). The quantum yield for the formation of toluene in the vapor phase increases with decreasing pressure, the extrapolated value at zero pressure being one within experimental error (173). It has been suggested that the photoisomerization of cycloheptatriene to toluene involves a vibrationally excited ground state while the photoisomerization to the bicyclic product involves an electronically excited state (173). 

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). 

If a system reaches the point where further irradiation under constant conditions causes no further change in composition, it is said to have reached its photostationary state (pss). The (cis/trans)PSB ratio in olefin isomerization depends on the relative extinction coefficients of the cis and trans isomers (i.e., their relative opportunities to isomerize) and on the quantum yields for both directions of the photoisomerization, as shown in Equation 13.35. 

The photodimerization is a reaction competing with the previously discussed photoisomerization (Section III). The quantum yields of photodimerizations are affected by the concentration of the photosubstrate (c > 0.1 molL-1) and the polarity of the... 

Table IV. Photoisomerization Quantum Yields for SNA and MSNA in Different Media at 25°C.

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