Photoisomerizations, experimental

J. Frederick, Y. Fujiwara, J. H. Penn, Y. Yoshihara, and H. Petek, Models for stilbene photoisomerization Experimental and theoretical studies ofthe excited-state dynamics of 1,2-diphenycycloalk-enes, J. Phys. Chem. 95, 2845-2858 (1991). 

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...

The photoisomerization of 3-hydroxyisoxazoles gave the corresponding 2-ox-azolones without the isolation of the azirine (Scheme 21) (67HCA137). Also in this case calculations are in agreement with the experimental results [99H(50)1115]. 

The remainder of the present article is divided into two parts. The first one reviews the main points of our combined theoretical-experimental approach. The second one reports an application of it to the study of the mechanism and dynamics of trans-cis photoisomerization of bisdimethylaminopentamethine cyanine (BMPC). ... 

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... 

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. 

BR from H. salinarum is a light-driven proton pump, which is triggered by the photoisomerization of retinal covalently linked to its Lys216. It consists of a single polypeptide of 248 amino-acid residues, including seven a-helical TM chains A-G and interconnecting loops, as schematically illustrated in Figure 23. BR is one of the most intensively studied membrane proteins. A variety of experimental techniques have shown it to be... 

Cyclophanes are naturally suited for MMPI (15b) calculations. The results ofsuch calculations regarding the structures and electronic spectra of the [m] paracyclophanes (n = 5-10) agreed well with the experimental data (169). Attempted X-ray analyses of [2.4]- and [2.5](9,10)-anthracenophanes (46) encountered serious disorder in the ahphatic bridges. MMPI calculations of all possible conformers of these molecules revealed four and six energy minima for 46a and 46b, respectively. Comparison of the calculated C10 C10 distances and bridge conformations with X-ray information unambiguously identified two conformations each for 46a and 46b as the final solutions. These and the calculated structures of photoisomer 47 were highly useful in the interpretation of fluorescence spectra and photoisomerization processes of 46 (170). 

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. 

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). 

Calculation results on a possible unified approach fit the experimental data (Figure 2) (99H(50)1115). Ab initio calculations showed a different level in the Dewar thiophene (02JPP(A)(149)31). More recently, the possible presence of a cage thiophene structure into the photoisomerization has been studied in order to explain some results that cannot be rationalized by using the above-described approach (09H(78)737). 

Investigations of organic reactions in supercritical solvents are subject to several constraints, one attributable to supercritical fluid properties and others imposed for interpretive and experimental simplicity. Because supercritical fluid properties are affected by changes in temperature, a reaction should be selected which does not require heat for initiation and is not highly exothermic. Additionally, for experimental simplicity and clarity of interpretation, a clean, well-understood reaction should be chosen and one should expect an experimentally observable response to changes in pressure. Finally, a unimolecular reaction which produces a single product obviates the complication of controlling the concentrations of two reactants and simplifies product analysis. The photoisomerization of trans-stilbene meets these requirements. 

Su has published a high order computational study of the photoisomerization of arsenin to possible Dewar-type valence isomers <2007JPC971>. The reaction has yet to be observed experimentally. 

We demonstrated how the photoisomerization hypothesis can be supported by accurate quantum chemical calculations (103). The experimental infrared and resonance Raman study of complex 5 led to the first determination of normal modes and force constants of diazene coordinated to a metal fragment. Isotope substitution yielding 15N- and 2H-isotopomers permitted the assignment of diazene normal modes in the experimental spectrum. Moreover, the spectra of these three isotopomers indicated that a laser-induced photoisomerization occurred in the Raman sample. However, a detailed assignment of the split bands was not possible in the experiment. 

In contrast with complex 5, the two isomers 7(A) and 7(B) have been isolated and their structure could be characterized by X-ray analyses (106,107). A comparison of the experimental structures and our BP86/RI/TZVP optimized structures, which are in very good agreement, was given in Ref. (105). Since vibrational spectra are not yet available for complex 7, a photoisomerization process as... 

In the triarylallyl carbanion study and in the absence of accepting stilbene, EjZ photoisomerization of the irradiated carbanion was observed and the effect of changing the substituent at position 2 was examined. Starting from the zero coefficient at carbon-2 in the non bonding MO of the allyllic system, Tolbert considers that the substituent at the 2 position does not affect the energy of the MO. If an electron transfer mechanism governs the reactivity, the substituent on this position will not modify the process in a significant way. The experimental results were very dependent on the C-2 substitution and this reaction was therefore considered as relevant of the intrinsic photochemistry of the anion [145]. This view has been confirmed in other studies on 1,3-diphenylallyl carbanion the kinetic parameters of the photoisomerizations were found to be inconsistent with an electron transfer mechanism [146, 147]. 

Figure 10 Asymmetric photoisomerization of 48 ( — II) and 49 (<— I) (see Scheme 11) with r-cpl and 1-cpl. Points experimental lines calculated with g (49) = —0.0074 and g (48) = + 0.005 for r-cpl and g factors of opposite sign for 1-cpl. (From Ref. 117. Copyright The Royal Society of Chemistry.)...

Huppert et al. have recently applied picosecond and nanosecond flash spectroscopy to methanol solutions of 11-cis PRSB (196). Monitoring at 485 nm which is within the main absorption band of the molecule, they observe a fast (<10 ps) depletion, followed by a slow (tr 11 ns) recovery of the absorbance to a final permanent value consistent with a net (11-cis) -+ (all-trans) interconversion. In the red, between 550 and 660 nm, they observe a fast rise in absorbance (<10 ps) due to an unidentified long-lived (Td 0.5 ns) transient (X). Although unclear as to whether Tr = Td they suggested that X is an (excited-state or ground-state) isomer precursor of the final all-trans photoproduct. It was thus concluded that photoisomerization is a relatively slow process occurring in the nanosecond range. Independent of the identity of X, this conclusion does not seem to be a unique interpretation of the experimental observations noted above. In fact, these are consistent with either one of two mechanisms ... 

Because the amount of decomposition of a well-characterized photochemical reaction depends on the photochemical quantum yield (a constant) and the amount of radiation absorbed by the sample, the intensity of unknown irradiation sources can be determined very accurately by measuring the amount of decomposition, if the quantum yield of the photoreaction is known. Among the large number of well-characterized photoreactions, only a few are suitable for actinometry. Thus, only photoreactions with very simple mechanisms are less sensitive to the experimental conditions of the irradiation. Well-defined experimental conditions and easy monitoring are important requirements for a suitable actinometric system if reproducible results are to be obtained. Examples of acceptable reaction types include photodegradations, photoisomerizations, photooxidation, etc., as discussed later in detail (vide infra). 

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