Photoinduced electron transfer intramolecular charge-separation

We have demonstrated in two independent studies that electrolyte effects can have a strong influence on the energetics of a charge-separated species and on the dynamics of intramolecular electron transfer in moderately polar media. In the case of weakly exoergic ET, the ion dynamics can effectively control the transfer rate. The developed spectroscopic probes indicate that association with ions can stabilize a photoinduced charge-separated species by as much as 1 eV. Further investigation of electrolyte effects in photoinduced electron transfer, particularly in the "inverted region, is needed. 

Molecular dyads of ruthenium(ii)- or osmium(ii)-bis(terpyridine) chromophores and expanded pyridinium acceptors have been used to demonstrate the effect of the bridge and the metal ions to the photophysical properties of linear systems. In particular, via ultrafast transient absorption spectroscopy, an equilibration between MLCT and photo-induced charge-separated excited states has been observed demonstrating that intramolecular photoinduced electron transfers can occur within multicomponent systems in spite of driving forces virtually approaching zero. ... 

W. Rettig, Photoinduced charge separation via twisted intramolecular charge transfer states, in Topics in Current Chemistry, Vol. 169 Electron Transfer I (J. Mattay, ed.), pp. 253-299, Springer Verlag, Berlin (1994). 

Photoinduced intramolecular electron transfer in the donor-acceptor complex 87 (R = H) generates transient charge-separated open-shell species with the remarkably long lifetime of about 75 ps [89]. Dyads that contain Tt-extended tetrathiafulvalene units also form stable cationic species upon oxidation [90]. The dumbbell shaped triad 91 [91-93] (Scheme 4.13) was obtained by carrying out the reaction with the in situ generated bis-diene at room temperature, in the dark and in o-dichlorobenzene as a solvent in 50% yield. The product is thermally unstable and easily undergoes a retro-Diels-Alder reaction [91]. 

As reported earlier (Oevering et al., 1987 Warman et al., 1986) intramolecular electron transfer following photoexcitation occurs for all values of n and in a variety of solvents. This electron transfer results in quenching of the typical dimethoxynaphthalene fluorescence for l(n) as compared to that for the isolated donor 2. Determination of the rate constant (k) of this photoinduced charge-separation was achieved in a variety of solvents (Oevering et al., 1987) by comparison of the lifetime of the residual donor fluorescence in l(n) for n= 8, 10, 12 with that of the reference system 2 via eq.(2) ... 

T -C5Me5) Ir (CO) (n -NCCgH4Cl) in hydrocarbon solvents results in activation of the solvent C-H bonds,and a study of the photoactivation of methane matrices by complexes of the type (M(n -C5Rs)(CO)2l (M Ir or Rh, R H or Me) has been reported.Photoinduced charge separation and recombination kinetics have been measured for the dimeric Ir(I) complexes [ Ir ( i-pz ) (CO) (PPh2 o(CH2)2R) ) I2 IP 3,5-dime thy Ipyrazolyl R pyridine (py), 4-phenylpyridine]. The results suggest that these complexes are well adapted to detailed studies of Intramolecular electron-transfer reactions. 

Electronically excited phthalimides can act as good electron acceptors and carboxylic acids are documented to serve as electron donors in photoinduced SET reactions. It is likely then, that in the excited state, an electron transfer process between the phthalimide system and the carboxylic acid would occur, leading to charge-separated diradical 6. Proton transfer from the carboxylic acid function (iT" is an electrofugal group) would form a carboxy radical 7, which could undergo rapid decarboxylation to azomethyne ylide 8. This reactive species is transformed into the final decarboxylated product 3 by intramolecular proton transfer (Scheme 16.4). 

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