Excited-state charge recombination

As seen in Sections 4.1 and 4.2, a Ru(II) - Cr(III) intervalence state is present in all the chromophore-luminophore systems. On the other hand, the study of the photophysics originating from this state is precluded by the presence of more intense, overlapping MLCT absorption bands of the chromophore. A system designed to allow selective intervalence transfer excitation is the trinuclear complex shown in Fig. 17 [88]. In this system, the Ru(II) center has similar oxidation potential as in the previously studied cases (and thus similar intervalence transfer transitions are expected), but the Ru(II)-based component is not chromophoric as it lacks of the polypyridine ligands responsible for MLCT absorption. Actually, a single prominent band, of intervalence transfer type, is present at 338 nm (aqueous 

Excitation into this band gives rise, in a subnanosecond time scale and with appreciably unit efficiency, to the typical phosphorescent emission from the doublet state of the Cr(cyclam)(CN)2 component. This result is rather interesting from several viewpoints. 

As it occurs in ordinary chemiluminescent reactions, a competition between the electron transfer process leading to the excited product (eq 31) and the analogous process leading to the ground state (eq 33) should in 

X = 8000 cm ). It is seen that the charge recombination process leading to the Cr(ni) doublet state is expected to be in the nearly activationless regime, whereas that leading to the ground state is likely to lie deep into the inverted region of electron transfer (section 1.3.2). In terms of radiationless transition theory, the excited-state charge recombination is favored by its 

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Excitation of the NI anion radical within ANI+-NI -PI with a 480 nm laser pulse rapidly forms the ANI+ NI -PI excited doublet state. Its electronic configuration is analogous to that in Figure 4(b). There are two pathways for decay from this excited state charge recombination back to yield the ANI excited state or charge shift to PI. The driving force for each of these paths is nearly identical, and... 

Fig. 18 Photophysical processes in NC-Cr(cyclam)-CN-Ru CN) -NC-Cr(cyclam)-CN IT excitation (1), excited-state charge recombination (2), localized emission (3).

It should be remarked that since the charge transfer reaction (at least for q > 3) selectively populates a small number of excited states, the recombination spectra resulting from (1.3) differs considerably from that produced by radiative or dielectronic recombination. There is thus a great need for reliable cross-sections in low energy collisions (thermal to 10 eV). 

In oligomers and polymers, neutral excited states, or excitons, can be produced by photo-excitation or charge recombination (capture of electrons and holes in LEDs). These can either decay radiatively, as desired for light-emitting diodes or nonradia-tively, with the possibility of yielding mobile charge carriers, for photoconductive and photovoltaic cells. 

In alicyclic hydrocarbon solvents with aromatic solutes, energy transfer (vide infra) is unimportant and probably all excited solute states are formed on neutralization of solute cations with solute anions, which are formed in the first place by charge migration and scavenging in competition with electron solvent-cation recombination. The yields of naphthalene singlet and triplet excited states at 10 mM concentration solution are comparable and increase in the order cyclopentane, cyclohexane, cyclooctane, and decalin as solvents. Further, the yields of these... 

Since the values of i/ depend on several factors noted above, in the absence of additional data such as the temperature dependence of the electron transfer rate constants for i-2 it is difficult to analyze the apparent difference between i/ for the charge separation reaction and that of the radical ion pair recombination reaction. However, the difference between these two values of u is not unreasonable given that the charge separation involves oxidation of an excited state of the donor, while radical ion pair recombination involves two ground state radicals. Small changes in the nuclear coordinates of the donor and acceptor for these two reactions should be sufficient to produce the observed difference in i/. The electronic coupling factor between ZnTPP and AQ should be different than that between ZnTPP " and AQ". 

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