Photoinduced intermolecular electron transfer

Wang 0, Akhremitchev B and Walker G 0 1997 Femtosecond infrared and visible spectroscopy of photoinduced intermolecular electron transfer dynamics and solvent-solute reaction geometries Coumarin 337 in dimethylaniline J. Rhys. Chem. A 101 2735-8... 

Transient absorption spectra of the CeoN cluster (C6oN" ) -MePH system following laser excitation at 355 nm indicate that the photoinduced intermolecular electron-transfer from the triplet excited state of PH to the QqN cluster (CfioN) occurs as shown in Figure 15.9a. 

Figure15.9 (a) Reaction scheme of photoinduced intermolecular electron-transfer in C oN cluster- MePH system, (b) MFEs on A = Abs(BT)/Abs(0T) in CeoN -MePH inTHF-HjO (2 1) mixed solvent.

Crystalline mixtures of the dimers (150-152) with tetracyanoquinodi-methane (153) undergo photoinduced intermolecular electron transfer when irradiated at >350 nm. Cleavage of the C-C bond and the formation of the monomers then occurs. The photochromic properties of the chromene (154) have been studied.The ring opening kinetics using 357 nm were measured and ring closure was effected by irradiation at 422 nm. The quantum yields of the forward and back processes were also measured. Kaneko and coworkers have studied the photochemical cyclization of the enediyne (155) which forms the aromatic compound (156). The best yields are obtained using hexane as the solvent. 

PCAB 99] Kabatc J., Kucybala Z., Pietrzak M. et al, Free radical polymerization initiated via photoinduced intermolecular electron transfer process kinetic study . Polymer, vol. 40, p. 735, 1999. 

A. Chakraborty, D. Chakrabarty, P. Hazra, D. Seth, and N. Sarkar, Chem. Phys. Lett., 382, 508 (2003). Photoinduced Intermolecular Electron Transfer between Coumarin Dyes and Electron Donating Solvents in Cetyltrimethylammonium Bromide (CTAB) Micelles Evidence for Marcus Inverted Region. 

Electron-Transfer Reactions. It is well known that thermal and photochemical electron-transfer reactions exhibit characteristic pressure dependences and associated volumes of activation (see Sections II, III, and VI). It is therefore realistic to expect that photoinduced thermal electron-transfer reactions will also exhibit a characteristic pressure dependence that should reveal mechanistic information on the nature of the reaction. Recent interest in the mechanistic understanding of long-distance electron-transfer reactions prompted an investigation of the effect of pressure on intramolecular electron transfer in ruthenium-modified cytochrome c [151] (a typical example of a closely related intermolecular electron-transfer reaction was... 

For a realistic value of X = 0.5 eV and at room temperature this gives ket = 3.6xl08(HDA)2(ket in s" in cm ). Practical lower limits to kg for detection of intramolecular electron-transfer are either set by the lifetime of the electronically excited state from which it occurs (i.e. for photoinduced electron-transfer) or by competition of intermolecular electron- transfer, which is often occurring at a diffusion controlled rate. The former typically limits the time window for observation of kgj to < 10 ns, while at high dilution ( 10" mol/1) the latter does not produce problems if the intramolecular electron transfer proceeds significantly within < 10 ps. These constraints thus require kg values 10 s and S 10 s" corresponding to > 0.5 cm" and S 0.017 cm for photoinduced and thermal electron-transfer respectively, while the latter can be expanded to still much lower values if diffiisional encounter is avoided e.g. in solid matrices or for D/A pairs encapsulated in large protein envelopes. 

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... 

The photoinduced -elimination of 1,2,3-triazole from 1-(A,A-bisacyl)amino-l,2,3-triazoles (142), itself formed from the photochemical isomerization of triazoles (141), proceeds either via an intra-or intermolecular hydrogen abstraction or electron-transfer mechanism followed by homolytic cleavage of the A,A-bond (path a) or via t -assisted )8-cleavage of the same weak bond (path b). The composition of the products suggests that in all cases a c-type 1,2,3-triazolyl radical (143) is eliminated which is further quenched by hydrogen abstraction as shown in Scheme 24 <93JHC1301>. 

Electron transfer (ET) reactions play a key role in both natural (photosynthesis, metabolism) and industrial processes (photography, polymerisation, solar cells). The study of intermolecular photoinduced ET reactions in solution is complicated by diffusion. In fact, as soon as the latter is slower than the ET process, it is not anymore possible to measure km, the intrinsic ET rate constant, directly [1], One way to circumvent this problem, it is to work in a reacting solvent [2]. However, in this case, the relationship between the observed quenching rate constant and k T is not clear. Indeed, it has been suggested that several solvent molecules could act as efficient donors [3]. In this situation, the measured rate constant is the sum of the individual ksr-... 

Photoinduced electron-transfer reactions generate the radical ion species from the electron-donating molecule to the electron-accepting molecules. The radical cations of aromatic compounds are favorably attacked by nucleophiles [Eq. (5)]. On the contrary, the radical anions of aromatic compounds react with electrophiles [Eq. (6)] or carbon radical species generated from the radical cations [Eq. (7)]. In some cases, the coupling reactions between the radical cations and the radical anions directly take place [Eq. (8)] or the proton transfer from the radical cation to the radical anion followed by the radical coupling occurs as a major pathway. In this section, we will mainly deal with the intermolecular and intramolecular photoaddition to the aromatic rings via photoinduced electron transfer. 

Photoinduced electron transfer from the amine to C6o to yield a radical ion pair is suggested to be the initial step for the formation of 54a-b. This is followed by deprotonation of the amine cation by the fullerene anion to give an a-aminoalkyl and HC6o radical pain [134], Subsequent combination of the radical pair leads to the final product. Formation of 55 is likely to be initiated by PET from 54b to C6o. This is then followed by successive intermolecular proton transfer, hydrogen abstraction, and ring closure to give l,2-H2C6o and 55 (Scheme 21). 

Photoinduced electron transfer occurs through excitation of the 400-nm absorption bands of the donor chromophores based on the aminonapthalene-dicarboximide derivatives. The tails of the dopants absorption bands extend to at least 500 nm, which allows for the use of an Ar+ laser. Figure 8 illustrates the ground-state absorption spectra of the donor and acceptor for both the intramolecular and intermolecular charge transfer dopants in toluene. The spectra are similar for all of the dopants, with the exception of 2, which has a 50-nm red-shifted absorption band. The inset illustrates the broadened spectra in the liquid crystalline environment. The extinction coefficient at 457 nm varies from approximately 1000 M-1 cm-1 for 4, 2000 M-1 cm-1 for 1, 5000 M-1 cm-1 for 3, and 10,000 M-1 cm-1 for 2. 

Switching systems based on photochromic behavior,I29 43,45 77-100 optical control of chirality,175 76 1011 fluorescence,[102-108] intersystem crossing,[109-113] electro-chemically and photochemical induced changes in liquid crystals,l114-119 thin films,170,120-1291 and membranes,[130,131] and photoinduced electron and energy transfer1132-1501 have been synthesized and studied. The fastest of these processes are intramolecular and intermolecular electron and energy transfer. This chapter details research in the development and applications of molecular switches based on these processes. 

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