Photochromic effect

Obvious applications of photochromism include glasses which darken according to the intensity of the surrounding light, and such glasses have now been available for some time both in the form of plate glass for windows and in the form of lenses for sunglasses. Their action is very slow, it takes indeed many minutes for the photostationary state to be established. This is due to the fact that the photochromic effect takes place at the molecular level... 

During the last decade, the fascinating phenomena of the photochromic effect became a hot problem of molecular structure and spectroscopy due to its promising application in the design of new memory storage systems at the molecular level. This phenomenon has been discovered and studied in several nitrosyl complexes of transition metal ions [1-6]. At low temperatures, the most extensively studied sodium nitroprusside Na2[Fe(CN)5(NO)] 2H2Q (SNP) demonstrates two excited... 

The shapes of the adiabatic potential sheets are governed by eight parameters - V, v1, ih. A1, A2, t, coBi, coAi. In order to elucidate the conditions that favor the photochromic effect, first we shall try to discuss the values of the above-mentioned parameters. For the usually accepted value of k = 105 dyn cm-1 the frequencies o>Bi and coAl are estimated as 340 and 400 cm-1, respectively. The energy gaps A2 and Ai can be approximately identified with those between the HOMO and the split components of LUMO in the MS2 state. These values are calculated in paper [ 17] by the DFT method and prove to be equal to A2 = 21185 cm-1 and A = 15850 cm-1. 

The degree of light-induced photoyellowing measured on mechanical pulps depends to some extent on the time between the actual irradiation and the brightness measurement. For example, if a sheet is kept for a certain time after accelerated irradiation the brightness increases slowly to reach a new upper limit. If the irradiated and stared sheet is irradiated a second time, the brightness decreases again, i.e. the pulp shows a photochromic effect. The extent of the photochromic effect depends both on the type of mechanical pulp used and on the irradiation time. In this study we have tried to evaluate which factors affect the photochromic effect and to identify the chemical structures responsible. 

We have evaluated some of the factors affecting the photochromic effect and tried to identify responsible chemical structures. 

The peroxide-bleached groundwood pulp was also severely oxidized with Fremy s salt Although the pulp was almost stabilized against any further brightness decrease during irradiation, die photochromic effect remained (Table HI). 

The observed photochromic effect can be due to a triplet-triplet absorption which is the simplest photochromic system in materials containing molecules in the stable ground state and in a metastable photochemically excited state. 

The observed phenomenon, which we interpret as a photochromic effect, can in principle result in an instant photoyellowing of high-yield pulps upon exposure to ultraviolet irradiation. 

The photochromic effect was observed after both UV-irradiation and thermal ageing, but it was not dependent on the storage temperature. Extraction of the pulp had a slight effect. The most pronounced decrease was obtained if the pulp was impregnated with ascorbic acid, which probably works as a radical scavenger. The brightness increase follows the decrease in radicals in the pulp upon storage as monitored by ESR. 

Highfield JG, Pichat P. Photoacoustic study of the influence of platinum loading and bulk doping with chromium III ions on the reversible photochromic effect in titanium dioxide, correlation with photocatalytic properties. New J Chem 1989 13 61-66. 

Photochromic Effects. To estimate the maximum refractive-index change achievable with a photochromic transition, we consider separately the normalized index change (defined by eq. 12), and the net number density of the active species that undergoes the transition. 

The peak shift due to aggregation is observed not only in LBK films containing azobenzene chromophores, but also for other chromophores with extended Ji-systems, such as viologen polymers. For monolayers of the poly(p-phenylene sulfonate) 9/ dioctadecyldimethylammonium bromide (DODA) complex, the peak shift due to aggregation results in a piezo-chromic effect—that is, upon compression of the monolayer, a significant shift of the poly(p-phenylene sulfate) A band is observed (see Figure 6.9). This photochromic effect has been shown to be based on the improved 7t-Jt interaction upon compression of the monolayer. ... 

A model of the photochromic effect based on the concept of the pseudo JTE was suggested in [ 196- 197]. This vibronic model allows us to examine the microscopic physical origin of the nuclear rearrangement and the underlying mechanism of the photochromic effect at the electronic level. At the same time the pseudo JTE approach can be considered as a background for the dynamical (quantum-mechanical solution) of the problem of light absorption and emission including the crucial question of interpretation of the anomalous lifetimes for the excited states and shape-functions of the optical bands. 

Studies of radical 39 in solution further proved that the naphthoxyl radical is not involved in the photochromic process. The radical, whose EPR spectrum consisted of a 1 2 1 triplet (a2H = 1.75 G) due to interaction of the unpaired electron with two protons, identified as those in positions 5 and 7 of naphthalene on the basis of semiempirical calculations, was found to exist in equilibrium with its dimer 40.43 Radical 39 or its dimer 40, was also found to absorb at 305 nm with e < 100 whereas the product responsible for the photochromic effect absorbed at 530 nm and was characterized by a much higher extinction coefficient. Strong indications have instead been gathered indicating that the dissociation of 38 is a heterolytic process and that the carbenium ion 41 is most likely responsible for the photochromism of P-TKN ... 

S. Schneider, Investigation of the photochromic effect of spiro[indolino-naphthoxazine] derivatives by time-resolved spectroscopy, Z. Phys. Chem. N. F. 154, 91-119 (1987). 

Biochrom-BR film Biochrom-4KBR film (Erasable optical image storage) photochromic effect (ground state-M state transition) BR (wildtype and modified chromophore) [40-42]... 

D random access optical memory photochromic effect (two-photon absorptivity) BR (wild type) [33,52-54]... 

Fig. 12.1 The photochromic effect a molecule in the configuration A is transformed to configuration B by light of wavelength Light of wavelength Xg brings the molecule back to the initial state A.

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