Quantum yields isomerization, photo

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. 

For all Azo-PURs, the quantum yields of the forth, i.e., trans—>cis, are small compared to those of the back, i.e., cis—>trans, isomerization—a feature that shows that the azo-chromophore is often in the trans form during trans<->cis cycling. For PUR-1, trans isomerizes to cis about 4 times for every 1000 photons absorbed, and once in the cis, it isomerizes back to the trans for about 2 absorbed photons. In addition, the rate of cis—>trans thermal isomerization is quite high 0.45 s Q 1 shows that upon isomerization, the azo-chromophore rotates in a manner that maximizes molecular nonpolar orientation during isomerization in other words, it maximizes the second-order Legendre polynomial, i.e., the second moment, of the distribution of the isomeric reorientation. Q 1 also shows that the chromophore retains full memory of its orientation before isomerization and does not shake indiscriminately before it relaxes otherwise, it would be Q 0. The fact that the azo-chromophore moves, i.e., rotates, and retains full orientational memory after isomerization dictates that it reorients only by a well-defined, discrete angle upon isomerization. Next, I discuss photo-orientation processes in chromophores that isomerize by cyclization, a process that differs from the isomeric shape change of azobenzene derivatives. 

Polarized light absorption orients both isomers of photisomerizahle chromo-phores, and quantified photo-orientation both reveals the symmetrical nature of the isomers photochemical transitions and shows how chromophores move upon isomerization. Photo-orientation theory has matured by merging optics and photochemistry, and it now provides analytical means for powerful characterization of photo-orientation by photoisomerization. In azobenzenes, it was found that the photochemical quantum yields and the rate of the cis—>trans thermal isomerization strongly influence photo-... 

The procedure for determining QYs is summarized as follows. On the one hand, both trans and cis isomers may be excited by the same irradiation and interconverted, and an equilibrium of the two isomers, called the photo-stationary state is reached. On the other hand, thermal cis->trans isomerization moves this equilibrium in favor of the trans isomer. So, the first part of the experiment consists of eliminating the effect of thermal isomerization on the equilibrium by determining the absorbance of hypothetical photo-stationary states of the photoisomerization reactions only, by extrapolating the irradiating intensity to infinity for combinations of two irradiation and analysis wavelengths (Rau s method). In the second part of the experiment, the obtained extrapolated values of the absorbance are used (in Fisher s method) to determine the extinction coefficients e i.e., the absorbance spectrum, of the cis isomer, and the determination of the quantum yields is straightforward. 

Photoresponsive polymers so far described change their properties in proportion to the number of photons that they absorb. Thus, when they contain more photo-chromic chromophores, which undergo an isomerization by absorbing a definite number of photons depending on the quantum yield, their properties change more. To make a sensitive photoresponsive polymer, i.e. one which responds more effidently to a fewer photons, we have to introduce an amplification mechanism into the system. A convenient way to achieve this end is to utilize the phase transition of polymers. 

The photochromism of 1 is achieved by atom rearrangement into the intramolecular oxygen-atom insertion product 2, in contrast to the frequently reported organic photochromic systems based on photo-induced cyclization, cis/trans isomerization, or H atom transfer [1]. The quantum yield of the photoreaction from 1 to 2 at 509 nm in acetonitrile without 02 is 0.14 0.01. In solution, photoreaction of 1 causes the oxidation reaction by atmospheric oxygen, resulting in a mixture of 2 and further oxidation products such as [(RhCp )2((i-CH2)2( i-S03)] and [(RhCp )2 ( r-CH2)2( t-S04)]. In contrast, in the crystalline state, the photochromic system between 1 and 2 is stable and repeatable with essentially 100% interconversion ratio. 

Another rhodium(III) system for which both quantum yield and lifetime pressure effects have been measured under analogous conditions is the bis(bipyridine) complex, ds-Rh(bpy)2Cl2 in aqueous solution [86]. The AF , values have also been reported for the photosubstitution quantum yields of Rh(NH3)e + [87] and for the concomitant photoaquation/photo-isomerization reactions of the tetraammine complexes cis- and trans-Rh(NH3)4X2 (X = Cl or Br) [81]. These data are summarized in Table 3. 

Poly butadiene in the solid state undergoes photochemical cis—trans isomerization when irradiated in vacuo at 123.6 nm or 253.7 nm. Quantum yields have been estimated to be, respectively, 0.25 [48] and 0.036 [47] at these two wavelengths. The reaction proceeds in both cases towards a photostationary cis—trans ratio of about 60 40 (Fig. 12) [48]. Photo-isomerization at 253.7 nm was first assumed to be sensitized by... 

Effects of Quenchers. The cis trans photoisomerization of 1,2-diarylethylenes can be influenced by properties of the medium, nature of the sensitizer (Section III), and also several additives acting as quenchers [192-202], The effect of quenchers on the quantum yields and the photo-stationary trans/cis ratio has been used as a mechanistic probe for exploration of isomerization pathways. Since quenching can occur with singlet as well as with triplet states, measurements on various systems have been performed under direct and sensitized excitation conditions. 

In recent years, optical dichroism and birefringence based on photo-induced trans-cis-trans isomerization of azobenzene groups has been observed with preoriented liquid-crystalline polymers [31-35] at temperatures above the glass transition temperature, and also with various amorphous polymers at temperatures well below the glass transition temperature. In the case of a polyimide (see Chart 5.7), a quasi-permanent orientation can be induced [36-38]. Here, the azobenzene groups are rather rigidly attached to the backbone and photoisomerization occurs at room temperature, i.e. 325 °C below the glass transition temperature, Tg = 350°C. This behavior is in accordance with the fact that the isomerization quantum yields of azobenzene compounds are very similar in solution and in polymer matrices 0 trans cis) 0,1 and 0(cis trans) 0.5. 

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