Formation of the Excited State

The rate constant for formation of excited-state S02 was measured by Halstead and Thrush to be 1.7 X 10 12 exp(-2100/T) cm3 molec1 s 1 [28-31], while the overall rate constant for the loss of SO is 3.6 x 10 12 exp(-1100/T) cm3 molec-1 s 1 [11], Thus, at room temperature, S02 constitutes —1.6% of the product channels. This figure can only be considered approximate, however, since mixing of states within the singlet manifold affects the interpretation of the rate constant for formation of the excited state [32],... 

This mechanism has been formulated in analogy to the known electrochemiluminescence, in which radical-ion annihilation generated at opposite electrodes leads to the formation of the electronically excited state (Scheme 2) . The difference between the CIEEL mechanism and electrochemiluminescence is that, in the former, the radical ions—whose annihilation is responsible for the formation of the excited state—are formed chemically by electron transfer to high-energy peroxides and subsequent bond cleavage or rearrangements. 

A further question raised by the electrochemical preannihilative emission process is what is the oxidant that oxidizes the anion or the reductant that reduces the cation before the potential at which the counterion is produced. Only two explanations are available ( ) impurities, and (2) heterogeneous electron transfer at the electrode.2 The difficulties involved in invoking an impurity mechanism for a phenomenon observed in both oxidation and reduction processes in a wide variety of solvents have been mentioned before. Unfortunately, if the emission is ascribed to heterogeneous electron transfer at the electrode, the problems are also severe. Marcus83 has shown that theoretically unless the formation of the excited state of an ion by electron transfer to or from the electrode involves a very small free energy barrier, it should not compete with the process which results in formation of the ground state of the ion and an excited state of the electrode. He has suggested... 

What can be seen from the foregoing examples is that one can use the photochemistry of small model compounds to predict the photochemistry of a polymeric material, provided that certain structural features are included and that one has some idea of the free volume required for the conformational or other motions necessary for the formation of the excited state and rearrangements or disproportionation into products. It can be concluded that... 

In the formation of the excited state, an electron is transferred from donor to acceptor. Az a consequence the ionic form becomes the predominant contributor in the excited state... 

Figure 1 shows that after photoexcitation the ferric complexes HM-H2S, Hbl-N3, Hbl-NO and metHbl exhibit an absorption transient formation near -435 nm, which were formed in -300 fs. Time decays varies from 4.5 ps to 6 ps. More than 95 % of signal recovery was observed within 20 ps after photoexcitation. These transients appear to be similar to those observed and assigned to a reduced species of ferrous hemeprotein complexes [6]. An absorption transient formation at 455 nm was also observed and corresponds to formation of the excited states Hbn [3]. As with the ferrous Hbl complexes, the species formed was observed immediately... 

The change in the absorbance of the solution, AA(t) at t, with the formation of the excited state is obtained (Equation 6.49) with some minimum manipulation of Equation 6.48 ... 

In the bulk, the low concentration of ground-state pairs excludes their observation by absorption. The formation of the excited-state complex, termed exciplex, is a collisional process electronic excitation of either the acceptor or the donor leads to the formation of a locally excited state (for instance, in hydrocarbon molecules, it is a nn state). During the lifetime of this state, a collision with the other partner (which is in the ground state) leads to the formation of the exciplex. This mechanism is compatible with the fact that the absorption and fluorescence excitation spectra of the system are identical with those obtained by superimposing the spectra of the individual components. At the same time, the fluorescence emission spectrum changes drastically—a broad band, red shifted with respect to the bare molecule s emission spectrum, appears. It is usually devoid of vibrational structure, and is shifted to longer wavelengths as the solvent polarity increases [1],... 

Fig. 18. Values of the kc jk-i ratio (evaluated from the experimental efficiencies of formation of the excited state) as a function of the solvent longitudinal relaxation time Ti. Data for 4-(9-anthryl)-V,V,3,5-tetra-methylaniline in acetonitrile (ACN), pro-pionitrile (PN), butyronitrile (BN), propylene carbonate (PC), sulfolane (TMS) and 7-butyrolactone (BL) solutions. Adapted from [153].

Fig. 25. ECL efficiencies (squares), luminescence quantum yields (circles) and efficiencies of formation of the excited state (diamonds) for Ru(bpy)3 in acetonitrile (open symbols) and Ru(dph)j in butyroni-trile solutions (solid symbols) as a function of the temperature (F). Data from [180] and [189].

Fig. 30. Corrected (normalized) efficiency of formation of the excited state ( es/ as as a function of the reaction exothermicity AGe for ECL system M06CIJ4/D+ ( ), Mo6Clr4/A-(0) and Mo6Clr4/ R (0) in acetonitrile solutions. AGe values are calculated fix>m the difference in the standard redox potentials corrected for the Coulombic interactions between reactants and product. The shape of the parabolic curve corresponds to the outer reorganization energy Ao = 0.8 eV.

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