Oscillating absorbance patterns

Because of the competition between the pH-dependent rate of the reaction leading from the uncolored Cc form to the colored AH+ and A species and the pH-indepen-dent cis- trans back-isomerization, the amount of colored species formed upon light excitation of the Ct solution depends on pH. In other words, the pH plays the role of a tap for the color intensity generated by light excitation. 41 This also means that this system can be viewed as a light-switchable pH indicator. The color-tap effect is larger in the case of 4, 7-dihydroxyflavylium than in that of 7-hydroxyflavylium. 

For the 4,-hydroxyflavylium compound in the presence of micelles, 18 starting from Ct at pH 5.0 and taking the formation of the AH+ absorption at 436 or 450 nm as an output, the tmth table for the effect of the three inputs (pH-jump to 1.0, addition of SDS, light excitation) shows a peculiar pattern (Table 3) corresponding to an OR function, which is activated only in the presence of the third input (enabled OR). 

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Fig. 7. Binary oscillations of H -upfake and absorbance change at 320 nm in chloroplasts exposed to train of flashes. The 320-nm change reflects the formation and disappearance of semiquinone. Figure adapted from Clayton (1980) Photosynthesis Physical Mechanisms and Chemical Patterns, p 207. Cambridge Univ Press.

Most single bonds absorb at similar frequencies and hence the vibrations couple. The observed, pattern will depend on the carbon skeleton, and the resulting bands will originate from the oscillation of large parts of the skeleton, or the skeleton and the attached functional groups. In addition C—C stretching frequencies may also couple with C—bending vibrations (discussed in more detail in Section 4.4.4). 

Ito, Kubiak and co-workers have reported properties of their ruthenium cluster complexes that differ from this general pattern. For these complexes an absorption band, assigned as an intervalence absorbance, is reported at about 12 x 10 cm (a larger energy than expected for Xr in dichloromethane) with —(3-4) x lO cm and (10-16) x 10 cm f The arguments presented above predict such large oscillator strengths to be characteristic of MMCT absorption... 

ATR-FTIR spectrum of pure PVDF film which was annealed at 50°C, from 550-3500 cm-i is also shown in Fig.2. The observed pattern originates from oscillations of large parts of the skeleton and/or the skeleton and attached functional groups. Below 1500 cm i, most single bonds absorb at similar frequencies, and the vibrations couple. 

The absorbance changes evoked by subsequent flashes show different patterns depending on the wavelength below 270 nm, uniform turnover (ferrocinium dominates) and above 300 nm, in-phase oscillation (due to semiquinone) can be detected. At 288 nm, however, out-of-phase, binary oscillation is observed (Fig.l right) odd flashes (1 and 3) induce very small absorbance increase compared to those after even flashes (2 and ). Vermeglio also obtained out-of-phase oscillation with reduced cytochrome c as external electron donor [ ]. These results confirm that the absorbance change at 288 nm is due to the appearance of fully reduced quinone. 

Sion close to the absorption lines causes interference patterns behind the Mach-Zehnder interferometer, which can be displayed behind a spectrograph as a function of the wavelength X. This dispersed pattern has the form of hooks (see Fig.2.24, also Fig.4.33), which gave the method its name. With simultaneous detection of absorption and dispersion the density N. of absorbing molecules can be determined. This allows the oscillator strength to be derived from one experiment without additional information [2.17]. Such combined techniques have considerably increased the accuracy and relevance of the hook method. 

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