Optical density changes

The term electrochromism was apparently coined to describe absorption line shifts induced in dyes by strong electric fields (1). This definition of electrocbromism does not, however, fit within the modem sense of the word. Electrochromism is a reversible and visible change in transmittance and/or reflectance that is associated with an electrochemicaHy induced oxidation—reduction reaction. This optical change is effected by a small electric current at low d-c potential. The potential is usually on the order of 1 V, and the electrochromic material sometimes exhibits good open-circuit memory. Unlike the well-known electrolytic coloration in alkaU haUde crystals, the electrochromic optical density change is often appreciable at ordinary temperatures. 

J. D. Ellis, K. L. Scott, R. K. Wharton, and A. G. Sykes, Inorg. Chem. 11, 2565 (1972), who observe optical density changes when 1 M acid solutions are mixed with water in a Durrum-Gibson stopped-flow apparatus. Such traces could be incorrectly assigned to chemical reactions. 

Stability tests performed in sandwich-type cells containing the dyes adsorbed on Sn02/Sb electrodes demonstrated a high stability, with optical density changes lower than 2% after cycling the electrochromic device 20000 times between -0.5 and +0.5 V. 

We have used a Reticon photodiode array and an Optical Multi-channel Analyzer to measure optical density changes Induced in photovisual materials using this technique. The Reticon is a device which incorporates a linear high density monolithic array of silicon photodiodes with integrated scanning circuits for serial readout. The array consists of 128... 

In addition to the shock wave velocity it is necessary, in a system where the density does not uniquely determine the composition (and this includes all but the very simplest chemical systems of interest), to measure some concentration function in order to follow the reaction. This is one of the greatest experimental difficulties associated with the method, since the changes occur so rapidly. Where possible, the concentration change is followed spectrophotometrically. This concentration monitoring is the second function of the observation points in Fig. 3. Especially for species with line spectra, the small optical density change, coupled with the fast response-time necessary, excludes the use of a conventional spectrophotometer. An example of a detection system which has been used for the hydrogen/oxygen... 

Figure 3. Optical density change Dt, after one photolyzing light flash as a function of the flash energy. (Reprinted from Ref. 28 with permission of Editions Scientiflques et Medicates Elsevier.)...

A sample of human DNA is subjected to increasing temperature until the major fraction exhibits optical density changes due to disruption of its helix (melting or denaturation). A smaller fraction is atypical in that it requires a much higher temperature for melting. This smaller, atypical fraction of DNA must contain a higher content of... 

Figure 2.25. Kinetic of optical density change in CDA-film containing azomethines at corresponding Amax l-330nm (BA), 2-272nm (XXXIV), 3-368nm (XXXV) in the process of irradiation by mercury-quartz lamp.

If k2 is known, the value of k.Gh may be determined from the optical density change. 

Fig. 3. Transient difference spectrum of photosystem I. The optical density changes observed can be ascribed almost entirely to the oxidized photoactive chlorophyll a of photosystem I. (From Ref. 19.)...

Figure 1 shows a schematic diagram of the apparatus used in our laboratory, which incorporates all of these features. Quite often the data are presented after analysis in the form of optical density change (AA) vs time in picoseconds (Fig. 2) for a selected region of the spectrum. It is... 

The net optical density changes (corrected to the values expected for a 1-cm light path) at 365 and 650 nm are 0.0120 and 0.0125 optical density, respectively. The insert to this figure compares the spectrum of NADH free in solution with the spectrum of enzyme-bound NADH. Taken from Ref. (68) with permission... 

Fig. 10. The relative J-peak optical density change depending on the relative MQ18 content...

The optical density change of the Stark spectra AA varied quadratically with the applied electric field in accordance with theory. 

Coloration efficiency (electrochromic efficiency). This is the amount of electronic charge (2d) which is necessary to produce an optical density change (AOD). The coloration efficiency (/ ) of an electrochromic material can be defined at the absorbing wavelength and is given by... 

In a comparable study in which unsaturated dicarboxylic acids (H2A) were used as substrates, activation enthalpies were observed for the formation of the intermediates remarkably similar to those above in reactions of both H2A and HA with oxidant. The authors in this paper, however, chose to describe the ester intermediate as a manganese(v) complex, an observation which has been rejected on spectroscopic grounds. In the reaction with crotonic acid, the optical density changes at 525 nm and 420 nm paralleled those described above. The spectra both of Mn and of the intermediates formed by the reactions... 

Temperature interval (°C) over which 2/3 of the total optical density change occurs. 

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