Photoinduced absorption-detected electron

Photoinduced absorption (PA) measurements provide useful information about relaxation processes of photocarriers and tail and gap states in a-Si H [Tauc, (1982) see also Chapter 9 by Tauc in Volume 21B]. Photoinduced absorption also depends on the recombination processes of those trapped electrons and holes that are created under optical excitation. Thus it is expected that PA will be spin-dependent, as is luminescence. Hirabayashi and Morigaki (1983a,b) have observed, for the first time, spin-dependent PA at 2°K in a-Si H, monitoring the PA intensity while irradiating the sample with microwaves at 9.6 GHz. This is called photoinduced absorption-detected ESR (PADESR), which provides complementary information to the conventional ODMR (luminescence-detected ESR). However, since the PADESR technique can be applied to nonluminescent materials, it may provide a more general means for highly sensitive detection of ESR than conventional ODMR. 

The data discussed so far indicate the bipolaronic nature of the excitations responsible for the two photoinduced peaks of polyCPDA. For polyDCHD, in contrast, PA measurements show the presence of different types of electronic excitations. In this case indeed the PA spectrum (Fig. 3) shows three sharp and well resolved peaks at 0.80, 0.95 and 1.24 eV, and a shoulder at 1.04 eV. There is also an indication of a rising band which peaks below the detection threshold of our experimental set up. Preliminary photoinduced absorption measurement in the infrared region indicate a value of... 

Radical anions are produced in a number of ways from suitable reducing agents. Common methods of generation of radical anions using LFP involve photoinduced electron transfer (PET) by irradiation of donor-acceptor charge transfer complexes (equation 28) or by photoexcitation of a sensitizer substrate (S) in the presence of a suitable donor/acceptor partner (equations 29 and 30). Both techniques result in the formation of a cation radical/radical anion pair. Often the difficulty of overlapping absorption spectra of the cation radical and radical anion hinders detection of the radical anion by optical methods. Another complication in these methods is the efficient back electron transfer in the geminate cation radical/radical anion pair initially formed on ET, which often results in low yields of the free ions. In addition, direct irradiation of a substrate of interest often results in efficient photochemical processes from the excited state (S ) that compete with PET. 

Finally, it is important to note (Section 5.3.6) that electrochemistry and UV-Vis absorption spectra of molecular dyads or triads based on metal polypyridines show that electronic interactions between the components of the systems discussed above are too small to influence ground-state behavior. Nevertheless, they are sufficient to allow for very fast intramolecular electron transfer when electronically excited. In fact electronic coupling of 0.002-0.005 eV would be quite enough, but hardly detectable electrochemically. Detailed studies of electrochemistry and spectroscopy of these supramolecular systems and their components are, nevertheless, essential for the understanding of the energetics of photoinduced intramolecular electron and energy transfer reactions. 

The rate constant for photoinduced electron transfer k4 and the charge recombination rate constant ks are directly observed experimentally. The reciprocal of the 3-ps time constant detected in the transient absorption experiments, equals kn, 3 X 10 s. This assignment is verified by the results for a model P-C6o dyad, where the same value was obtained for the rate constant for photoinduced electron transfer. The charge recombination of (Pzp)3-Pzc-P -C6o is associated with the 1330-ps decay component observed in transient absorption, as demonstrated by the spectral signature of the fullerene radical anion with absorption in the 1000-nm region. This lifetime is within a factor of 2.5 of the lifetime observed for the P" -C6o in a model dyad (480 ps). 

In the previous sections, it has been shown how powerful the time-resolved fluorescence techniques are in real time probing of photoinduced processes and in allowing the determination of reaction rates from fluorescence lifetimes. The present section is devoted to the method of UV/vis transient absorption spectroscopy, which is a key method in probing non emissive species and is thus crucial to detect photoreaction products or intermediates following optical excitation of molecules in their electronic excited states. When carried out on short time scales, i.e. with femtosecond to subnanosecond excitation sources, fluorescent species can also be detected by their stimulated emission. Combining time-resolved fluorometry and transient absorption spectroscopy is ideal for the study of photochemical and photophysical molecular processes. 

Both phase transitions can be triggered by optical pulses [16]. The crystal is excited by an 80 fs pulse in the region of its CT absorption at temperatures between 77 and 105 K, thus near the transition temperature. The phase transition is detected by the changes in the reflection spectrum of the crystal. It is found that the phase transitions occur with a delay of 500 to 800 fs. Evidently it requires this time for an optically-excited local CT state to be converted into a macroscopic ionic I or neutral N phase. Here, cooperative electron-electron and electron-lattice interactions are presumably the driving forces for these photoinduced phase transitions. This is demonstrated by the dependence of the transition on the hght intensity. The phase transitions are shown schematically in Fig. 12.8. For further details of this process, which is still not understood in all its aspects, we refer the reader to the original literature [16]. 

Irradiation of iron and cobalt porphyrins (13 Fe(TPP), 14 Co(TPP), H2TPP = 5,10,15,20-tetraphenyl-21/f,23/f-porphyrin) in the presence of triethylamine (TEA) using > 320-nm tight caused the photocatalytic reduction of CO2 [15, 16, 27-31]. When 13 was used as a photocatalyst, CO was detected with TNco = 70 after 180-h irradiation [15]. Formic acid was the main product when 14 was employed as a photocatalyst [16]. The reaction mechanism proposed on the basis of UV-vis absorption changes during photolysis and radiolysis, and electrochemical measurements are shown in Scheme 3. M (TPP) is reduced to M (TPP) by photoinduced electron transfer from TEA, which subsequently disproportionates to M°(TPP), the proposed catalyticaUy-active species. 

One problem that is still consistently recurrent in the study of porphyrin assemblies stems from very similar absorption and excitation properties of the species involved in the charge separation or the energy transfer processes. Very often, it is difficult to selectively generate the excited state of a zinc porphyrin donor without partial excitation of the free base porphyrin acceptor, even though no electronic coupling can be detected in ground state absorption spectra. This also stands for excitation of the electron acceptor or hole donor free base with a zinc electron donor or hole acceptor. It is thus useful to connect the multiporphyrin assembly to a nonporphyrinic electron/energy donor or acceptor, or both, that can be selectively excited in order to initiate the photoinduced process. This class of assemblies will now be examined. 

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