Photoexcited intramolecular electron transfer

In a general description of intramolecular electron-transfer (ET) processes one has to differentiate between charge separation in donor/acceptor (D/A) systems via the formation of photoexcited states and a charge-transfer or charge-shift reaction that is thermally activated (Cannon, 1980 Fox and Chanon, 1988 Meyer, 1978). 

Photoinduced intramolecular interaction of t-S and tertiary amine moieties linked with a polymethylene chain has also been studied24. The photoexcitation of fraws-stilbene in which a tertiary amine is attached to the ortho position with a (CH2)i-3 linker leads to fluorescent exciplexes by intramolecular electron transfer, and results in no more than trans-cis isomerization. The failure to give adducts from the intramolecular exciplexes could arise from the unfavourable exciplex geometry to undergo the necessary bond formation. 

Following our recent report on the first intramolecular electron transfer within a photoexcited SWNT and ferrocene moieties covalently linked to it (Scheme 9.19),68 a... 

Intramolecular photoinduced electron transfer reactions of homonaphthoquinones are also made possible by the presence of Mg(C104)2 in MeCN [212]. As shown in Scheme 27, the photoexcitation of 10 in the presence of Mg2 + results in intramolecular electron transfer due to the complexation of Mg2 + with the semiquinone anion moiety, which can accelerate the photoinduced electron transfer and at the same time may retard the back electron transfer [212], No reaction occurred in the absence of Mg2+ or in the dark at ordinary temperature [212], The generated radical ion I undergoes ring... 

In view of the photoinduced processes seen in exTTF-oPPE -C6o (see Sect. 9.1.1), the replacement of exTTF in the presented FUP/ZnP-oPPE -Cgo (15a-c), leads only to minor alteration in the overall reaction pattern. Upon photoexcitation the systems undergo intramolecular electron transfer from the electron donating ZnP/H2P to the... 

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) ... 

As a leading example for short-spaced dyads, a n-n stacked porphyrin-fullerene dyad (ZnP-Ceo) 21 should be mentioned, which was probed in light of their electron transfer and back electron transfer dynamics [361, 362], The close van der Waals contact ( 3.0A) is responsible for pronounced electronic interactions in the ground state between the two 7t-chromophores. For example, the ZnP Soret-and Q-bands in the n-n stacked dyad 21 show a bathochromic shift and lower extinction coefficients compared to free ZnP [361], In the n-n stacked dyad 21 the linkage of the two bridging units occurs in the trans-2 position at the fullerene. A charge-separated radical pair evolves from a rapid intramolecular electron transfer k 35 ps) between the photoexcited metalloporphyrin and the fullerene core in a variety of solvents (i.e., ranging from toluene to benzonitrile). Remarkably, the lifetimes in tetrahydrofuran (t = 385 ps) and DCM (t = 122 ps) are markedly increased relative to the more polar solvents dichloromethane (r = 61 ps) and benzonitrile (t = 38 ps) [362]. This dependency prompts to an important conclusion ... 

Pulse Radiolysis Measurements of Intramolecular Electron Transfer with Comparisons to Laser Photoexcitation... 

Marcus theory [69] predicts that the rate of non-adiabatic electron transfer with weak exergonicity shows a bell-shaped behavior with respect to the free energy difference (-AG) between electron donor and acceptor giving a maximum rate when AG equals the reorganization energy of the medium. Extensive studies have been published especially on intramolecular electron transfer in this regard, and we shall not review this work here. Instead, this section will review intermolecular electron transfer between a photoexcited redox molecule and the ground state one incorporated in a polymer. 

Sun et al. reported tris(bipyridine) ruthenium(II) complex covalently linked to a manganese(II) complex (without catalytic activity for water oxidation) and showed that after initial electron transfer from the photoexcited Ru(II) to an external electron acceptor, methylviologen (MV " "), intramolecular electron transfer took place from the Mn(II) to the photogenerated Ru(III) in an acetonitrile solution [124,125]. 

A common feature of all these C6o-[Ru(bpy)3] " systems is that upon photoexcitation a long-lived charge-separated state, C6o -[Ru(bpy)3], evolves from an intramolecular electron transfer quenching of the (MLCT) state. Owing to the diverse topologies of these dyads, the lifetimes of the C6o -[Ru(bpy)3] turn out to be quite different. For example, in dichloromethane solutions the rigidly spaced Ceo-androstane- KvL( o-py)if and Ceo-hexapeptide- Kvi(y> i)y)if dyads yield lifetimes of 304 ns and 608 ns, respectively, while no appreciable lifetime was noted for the flexibly spaced C6o-po/> g/yco/-[Ru(bpy)3] analogue. ... 

---