Locally excited triplets

The proposition that locally excited triplet states can be formed from back electron transfer within a doublet-doublet radical ion pair has firm theoretical (88) and experimental support. For example, with time-resolved Resonance Raman spectroscopy, one can directly monitor the chemical fate of the exciplex, solvent separated ion pair, and doublet free radical ion pairs formed between stilbene and amines. As might be expected from the above discussion, adduct formation is observed from the exciplex or contact ion pairs, whereas enhanced intersystem crossing ensues from the solvent separated ion pairs, producing spectroscopically observable stilbene triplets. This back electron transfer process, eq. 30 (89),... 

The excited fraws-stilbene/fumaronitrile complex produces a locally excited triplet state, which is considered to be responsible for isomerization of substituted stilbenes. Similarly, the CT complexes between aromatic hydrocarbons and fumaronitrile produce the isomerization of fumaronitrile to malonitrile34. 

It is interesting to consider also the triplet states. We have shown that T, lies close to Sj for TICT states of a simple dative bond but locally excited triplets certainly lie well above locally excited singlets. Therefore, a very strong donor-acceptor combination in the case of the complex dative bond will be needed in order to obtain minimum in T, for twisted geometries. 

The sequence of reactions (31)—(33), illustrated in Fig. 35a for triplet ET occurring from the locally excited triplet acceptor, constitutes a potentially efficient... 

Hviid and co-workers have recently found a way around this limitation by using the method of intermolecular triplet sensitisation to generate the locally excited triplet state of the donor chromophore in a D-A dyad.222 The sensitiser scheme is described by equations (35)-(38) and is also illustrated graphically in Fig. 36 for the case of triplet sensitisation of a donor chromophore (S is the sensitiser molecule). The principal advantage of this method is that the locally excited triplet state of one of the chromophores (in this case, the donor) is populated without invoking its... 

This method has been applied to the rigid 3-bond dyad, DNM[3]M, (Fig. 37), in which the maleate ester group (M) serves as the acceptor (the DCV acceptor group cannot be used because the energies of the singlet and triplet +DMN[3]DCV CS states all lie above the 3[ DMN[3]DCV] locally excited triplet state, see Fig. 29). Benzophenone (BP) was used as the sensitiser, and a highly polar solvent... 

Exciplex phosphorescence can be studied with complexes which are present in the ground state [118]. Equation (43) and its corollaries with respect to variations in A are applicable also to exciplex phosphorescence. For example, positive deviations (A > 0.18 eV) which have been found in the phosphorescence of complexes with (E — E d) > 2.75 eV are ascribed to the stabilizing interaction between the triplet CT state and energetically higher locally excited triplet states [108,119],... 

Figure 1. JabJonski-type diagram of the lowest energy levels of electron donor-acceptor molecules formally linked by a single bond which show dual fluorescence phenomenon. D-A, (D A), (D+-A ), (D -A ) and (D-A) denote the ground state, the primary excited and charge-transfer (CT) singlet states, and CT and locally excited triplet states, respectively. The arrows correspond to the radiative (absorption, A, fluorescence, F, and phosphorescence, Ph) and the radiationless (internal conversion, IC, and intersystem crossing, ISC) processes.

Fig. 20. Molecular orbital description for the electron transfer reactions within ECL processes of intramolecular donor-acceptor systems A-D to give (A) excited singlet intramolecular charge-transfer state (B) excited triplet intramolecular charge-transfer state (C) locally excited triplet and (D) ground state. Adapted from [138],...

Lim et al have presented evidence that singlet-triplet intersystem crossing from the charge-transfer state of electron-donor-acceptor complexes is efficient only when a locally excited triplet state of a component molecule (donor or acceptor) lies below the charge-transfer singlet state. The influence of starting conformations on intramolecular exciplex formation in (o-phenyl-a,-yV,/V-dimethylaminoalkanes has been extensively investigated. ... 

Figure 1. The dimer (ZnOEP)2Ph fluorescence intensity quenching with increasing amounts of extra-ligand H2P(p-Pyr)2 and the triad (ZnOEP)2Ph H2P(p-Pyr)2 formation. Schematic energy levels diagram for low-lying locally excited singlet states of the dimer (ZnOEP Ph, the extra-ligand H2P (Lig), locally excited triplet state of H2P (T), radical ion pair charge transfer state (CT), and rate constants of deactivation pathways (toluene, 295 K).

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