Photoinduced response

The relation between inhibition of the photoinduced responses in isolated chloroplasts and the expression of phytotoxicity remains to be identified positively. Any hypothesis proposed to account for the mode of action must take into consideration that (a) phytotoxic symptoms develop only in the light, and severity is proportional to light intensity (b)... 

To date, almost all attention has been paid to the nmacroscopic behavior at a centimeter level (film area and surface pressure) obtained with a Langmuir film balance. The photoinduced response at a molecular level, on the other hand, is obtainable by UV-visible absorption spearoscopy and surface-potential measurements. In these contexts, our attention recently has been paid to microscopic observations that cover the ranges between the previously cited scale levels using 6Azn-PVA monolayers. 

The photoinduced properties of azobenzene systems are strongly related to their polymeric architecture. The way in which azobenzene units are bound to the polymeric chain and the nature, either amorphous or liquid crystalline, of the final material strongly affects the magnitude and stability of the photoinduced response (Natansohn and Rochon,2002). 

Comparison of the spectral response and of the power efficiency of these first conjugated polymer/fullerene bilayer devices with single layer pure conjugated polymer devices showed that the large potential of the photoinduced charge transfer of a donor-acceptor system was not fully exploited in the bilayers. The devices still suffer from antibatic behavior as well as from a low power conversion efficiency. However, the diode behavior, i.e. the rectification of these devices, was excellent. 

The cationie dye was assoeiated with the anion (7,8,9,10,11,12 Brg-l-CBnHg) in order to dissolve it in the organie phase. The polarizable window available for photoinduced electron transfer in this system extended over 100 mV for the eonditions specified in Fig. 13. The photoeurrent responses were measured under ehopped light and lock-in detection at 8.4 Hz. Figure 13(b) shows that some photoresponses oeeur only in the presence of the dye speeies, whieh the authors attributed to the transfer of Ru(bpy)3" to the aqueous phase as a result of interfaeial polarization induced by the ehopped light [130]. Upon addition of in the aqueous phase, an inerease in the amplitude of the photo-... 

Another point subject to criticism from this work is the apparent absence of DC photocurrents upon constant illumination. Although the responses in Fig. 13(c) appear effectively in phase at the chopping frequency, it is possible that slow photoinduced perturbation of accumulated charge at the interface can be connected with these photoresponses, e.g., photoionization of the dye [72]. In this respect, clearer evidence of photo-induced ET was introduced by Brown et al. for the reaction [49]... 

Although the reports by Brown et al. provided strong evidence of photoinduced heterogeneous ET responses [49], the rather slow rising times observed in the transients of Fig. 14(a) are difficult to rationalize on the basis of the mechanism in Fig. 11. Another puzzling point is the potential dependence of the photocurrent obtained under chopped... 

Between 0.20 and 0.30 V, a decay of the initial photocurrent and a negative overshoot after interrupting the illumination are developed. This behavior resembles the responses observed at semiconductor-electrolyte interfaces in the presence of surface recombination of photoinduced charges [133-135] but at a longer time scale. These features are in fact related to the back-electron-transfer processes within the interfacial ion pair schematically depicted in Fig. 11. 

Under potentiostatic conditions, the photocurrent dynamics is not only determined by faradaic elements, but also by double layer relaxation. A simplified equivalent circuit for the liquid-liquid junction under illumination at a constant DC potential is shown in Fig. 18. The difference between this case and the one shown in Fig. 7 arises from the type of perturbation introduced to the interface. For impedance measurements, a modulated potential is superimposed on the DC polarization, which induces periodic responses in connection with the ET reaction as well as transfer of the supporting electrolyte. In principle, periodic light intensity perturbations at constant potential do not affect the transfer behavior of the supporting electrolyte, therefore this element does not contribute to the frequency-dependent photocurrent. As further clarified later, the photoinduced ET... 

The first estimations of for photoinduced processes were reported by Dvorak et al. for the photoreaction in Eq. (40) [157,158]. In this work, the authors proposed that the impedance under illumination could be estimated from the ratio between the AC photopotential under chopped illumination and the AC photocurrent responses. Subsequently, the faradaic impedance was calculated following a treatment similar to that described in Eqs. (22) to (26), i.e., subtracting the impedance under illumination and in the dark. From this analysis, a pseudo-first-order photoinduced ET rate constant of the order of 10 to 10 ms was estimated, corresponding to a rather unrealistic ket > 10 M cms . Considering the nonactivated limit for adiabatic outer sphere heterogeneous ET at liquid-liquid interfaces given by Eq. (17) [5], the maximum bimolecular rate constant is approximately 1000 smaller than the values reported by these authors. 

Despite the potential impact of novel photosynthetic routes based on these developments, the most ambitious application remains in the conversion of solar energy into electricity. Dvorak et al. showed that photocurrent as well as photopotential response can be developed across liquid-liquid junctions during photoinduced ET reactions [157,158]. The first analysis of the output power of a porphyrin-sensitized water-DCE interface has been recently reported [87]. Characteristic photocurrent-photovoltage curves for this junction connected in series to an external load are displayed in Fig. 22. It should be mentioned that negligible photoresponses are observed when only the platinum counterelectrodes are illuminated. Considering irradiation AM 1, solar energy conversions from 0.01 to 0.1% have been estimated, with fill factors around 0.4. The low conversion... 

The ability to switch a molecular unit on and off is a key component of an efficient molecular device, since it allows modulation of the physical response of such a device by external physical or chemical triggers. A molecular device, based on a trinuclear metal complex, shown in Figure 59, functions as an electroswitchable-photoinduced-electron-transfer (ESPET) device.616 Electrochemical switching of the redox state of a spacer intervening between a donor-acceptor pair can dictate the type of the observable charge separation and the lifetime of the resulting ion pair.616... 

The first photophysical investigation performed on stereochemically pure metal-based dendrimers having a metal complex as the core is that concerning the tetranuclear species based on a [Ru(tpphz)3]2+ core (tpphz=tetrapyrido[3,2-a 2, 3 -c 3",2"-h 2",3"j]phenazine) [67]. Dendrimer 45 is an example of this family. In this compound, two different types of MLCT excited states, coupled by a medium- and temperature-dependent photoinduced electron transfer, are responsible for the luminescence behavior. However, the properties of all the optical isomers of this family of compounds are very similar. This finding is also in... 

The second group of intermolecular reactions (2) includes [1, 2, 9, 10, 13, 14] electron transfer, exciplex and excimer formations, and proton transfer processes (Table 1). Photoinduced electron transfer (PET) is often responsible for fluorescence quenching. PET is involved in many photochemical reactions and plays... 

Photoinduced electron transfer reactions between surface bound dye molecules and semiconductor electrodes are important for practical as well as fundamental reasons. Absorption of light by the dye can improve the spectral response of the semiconductor and these systems are models for the photographic process (47-511. MDC surfaces are excellent substrates for studying electron injection into the conduction band of the semiconductor. 

Photoinduced electron transfer (PET) is often responsible for fluorescence quenching. This process is involved in many organic photochemical reactions. It plays a major role in photosynthesis and in artificial systems for the conversion of solar energy based on photoinduced charge separation. Fluorescence quenching experiments provide a useful insight into the electron transfer processes occurring in these systems. 

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