Photoinduced Charge Transfer Reactions

Photoinduced charge-transfer reactions have been extensively investigated in many fields of science for more than 15 years. Especially, in the now fast-developing field of photovoltaics it is of fundamental interest to understand the photophysics and photochemistry of excited states in organic molecules [61]. Photosynthetic energy conversion in green plants serves herein as the ultimate prototype [62]. 

To fully understand the relaxation pathways for photoinduced charge-transfer reactions in solutions we need to take solvent effects into account. For that reason it is necessary to recall some basic principles of the classical Marcus Theory for electron-transfer reactions in solution. 

Importantly, all photoinduced processes share some common features. A photochemical reaction starts with the ground state structure, proceeds to an excited state structure and ends in the ground state structure. Thus, photochemical mechanisms are inherently multistep and involve intermediates between reactants and products. In the course of a photoinduced charge transfer reaction the molecule passes through several energy states with different activation barriers. This renders the electron transfer pathway quite complex. 

To summarize, quantum mechanical investigations of the above systems provide evidence for the occurrence of effective photoinduced charge-transfer reactions. Under these aspects, we will now move on to the photophysical characterization of the proposed processes and verify the charge-separation features by means of steady-state and time-resolved spectroscopic techniques. 

The interconversion of the various oxidation states of Mn in natural waters is influenced by UVR through its effects on reactions involving ROS [Chapter 8] and natural phenols, photoinduced charge transfer reactions, and microbial processes. The oxidation of Mn + is slow at pH < 8.5 in the absence of a catalyst. The oxidation of Mn(ii) is faster on metal oxide surfaces than in homogeneous solution in the pH range of 8 to 9 [217], and its oxidation also can be biologically mediated in the environment [153]. In comparison to bacteria-free waters, the oxidation rate of Mn(ii) in seawater is increased dramatically by catalysis on bacterial surfaces. However, even with such catalysis, its half-life still is of the order of weeks to months in open ocean waters [153]. 

We estimate that the IRAVs are photogenerated within 2 ps in the 10% Cgo-doped film, in contrast to the time constant of about 50 fs in 50% Ceo-doped films [160]. This indicates that the photoinduced charge transfer reaction in Ceo-doped films is actually limited by the exciton diffusion toward Qo molecules close to the polymer chains. Consequently, the exciton wavefiinc-tion in MEH-PPV films is not as extended as previously thought. The same conclusion was drawn for Ceo-doped DOO-PPV films, where it was estimated [161] that the exciton diffusion constant to reach the Cgo molecules is of the order of 10 crcd/s. 

Photoinduced charge-transfer reaction. In this scheme the absorbed photon induces the harpoon reaction, e.g. 

The photoinduced charge-transfer reactions between N or Q and 1,4-dicyanobenzene (DB) in aceto-nitrile/methanol lead to adducts 17 and 18, photoinduced nucleophile-olefin-combination-aromatic-substitution (NOCAS) products 19-22 and two acetonitrile adducts, 23 and 24, which are formed only from N. ... 

Irradiation of an ITIES by visible or UV light can give rise to a photocurrent, which is associated with the transfer of an ion or electron in its excited state. Alternatively, the photocurrent can be due to transfer of an ionic product of the photochemical reaction occurring in the solution bulk. Polarization measurements of the photoinduced charge transfer thus extend the range of experimental approaches to... 

Recently, photochemical and photoelectrochemical properties of fullerene (Cto) have been widely studied [60]. Photoinduced electron-transfer reactions of donor-Qo linked molecules have also been reported [61-63]. In a series of donor-Cfio linked systems, some of the compounds show novel properties, which accelerate photoinduced charge separation and decelerate charge recombination [61, 62]. These properties have been explained by the remarkably small reorganization energy in their electron-transfer reactions. The porphyrin-Qo linked compounds, where the porphyrin moieties act as both donors and sensitizers, have been extensively studied [61, 62]. 

These are photoinduced electron transfer reactions between two ions. The closed-shell ions then form free radicals which can be charged or neutral, these primary photochemical products being very reactive. One example of this process is the electron transfer between a uranyl cation and a nitrate anion... 

As already outlined in the previous parts, the basic description of photoinduced charge transfer between a donor D and an acceptor A considers different steps. For the following discussion we will assume the following reaction sequence ... 

Understanding the theoretical principles of light induced nonadiabatic reactions is therefore crucial for the comprehension of the photo-driven processes that lead to photoinduced charge transfer and energy transfer reactions which will be discussed later on in this thesis. 

The photoinduced electron transfer reaction rates depend strongly on the molecular structures of the donor and acceptor entities [16]. The nature of the linker between them influences the charge or energy transfer, which is facilitated considerably when both partners are connected by bridging ligands [13,17,31],... 

Fig. 25 The series of dyads, 29(n), possessing the oligo-p-phenylenevinylene bridge that were used to investigate the switchover from superexchange characteristics to molecular wire behaviour in the photoinduced electron transfer reaction, from the locally excited state of tetracene (TET) donor to the pyromellitimide (PI) acceptor.148 Also, shown are a schematic of the photoinduced charge separation rate versus, donor-acceptor distance (lower left-hand side) and the LUMO energies of TET and the various bridges (lower right-hand side).

In summary, the stereoselectivity was certainly observed in the photoin-duced electron transfer reactions of chiral ruthenium(II) complexes with chiral viologen and Co(III) complexes. However, not only the photoinduced electron transfer reaction but also the charge separation in the encounter complex and the reverse reaction between the ruthenium(III) complex and Co(acac)2 + acac participate in the stereoselection. In the reactions between the ruthenium(II) and Co(III) complexes, the energy transfer also contributes to the quenching reaction, which makes difficult the observation of stereoselectivity in the quenching reaction. 

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