Spin-allowed excitation

One of the crucial points in the recent developments of coordination compound photochemistry has been the debate concerning the identity of the excited state(s) responsible for the photosolvation reactions that are obtained by irradiating Cr(III) complexes in their ligand field bands (5,8), Direct photolysis experiments revealed that the most likely candidates (see e.g,. Figures 2 and 3) are the lowest spin-allowed excited state ( T2ff in octahedral symmetry) and the lowest spin-forbidden excited state ( Eg). Such experiments, however, did not warrant any definite conclusion about the actual role of each of these two states (5). 

For donors with triplet energies below 52 kcal/mol only doublet acceptor states are energetically accessible hence only the doublet encounter pair provides a spin-allowed excitation-transfer channel with a maximum rate constant of At higher donor energies excitation to a quartet excited state of the acceptor is also possible and both quartet and doublet encounter pairs can lead to excitation transfer. The combined maximum quenching rate constant when both doublet... 

For the ions listed we survey the most recent spectroscopic investigations and present a critical compilation of values of the fitting parameters, Y>q, and B. For the d system, TiF, only the Dq value can, of course, be obtained, and it is similarly difficult to estimate B for MnF and CoF since in these cases essentially only one spin-allowed excitation can be identified with confidence. Assignments are also complicated by the operation of significant Jahn-Teller effects for the Mn, Co, and Ni complexes, with smaller influences encoimtered for the Ti and V compounds. 

All orbitals up to and including t2g(P) are occupied. Spin-allowed excitations must therefore be from the t2g(P) to the e iP) orbitals. Mapping the ground and spin-allowed d—>d excited states onto Hcf yields... 

Figure 2 shows a schematic Jablonski diagram for a typical molecule where deactivation by chemical reaction and quenching processes are not represented. Light absorption by the ground state molecule, A, leads mostly to the spin-allowed excited states A(aj), A(a2), A(a3), etc. The spin-forbidden excited states, A(fj), A(f3>, etc., can be populated by... 

In an ociiihcOraJ field (he free-ion ground F lerm of a d ion is split into an A and two T terms which, along with the excited T(P) term (Fig. A), give rise to the possibility of three spin-allowed d-d transitions of which (he one of lowest eneigy is a direct measure of the ciystal field splitting, A or 10 Dq ... 

On the right side of Fig. 3-9 are represented the relative energies of the two Tig terms, the and 2g. The ground term is the from the 2g configuration. Spin-allowed electronic transitions (those between terms of the same spin angular momentum - but see also Sections 3.6, 3.7 and Chapter 4) now take place upon excitation from -> A2g, The d-d spectra of octahedrally... 

Occasionally, some bands which might otherwise be expected to be weak are observed to be quite strong. Two examples are shown in Fig. 4-4. The first shows the electronic spectrum of a solution containing [CoC ] ions in nitromethane. For this cT system, we expect three spin-allowed transitions and these are observed at roughly 3500, 7000 and 14,000 cm h They correspond (see Chapter 3) to the excitations M2 —> Ti F) and T P) respectively. Note, however, that the... 

Alternatively, optical excitation of the LS state in a solid metal complex which is involved in a thermally driven spin-state transition may result in the formation of a trapped HS state, the study of the kinetics of the HS -> LS relaxation being then possible [134]. This process is initiated, e.g., in an iron(II) complex, by irradiation into the spin-allowed absorption band at... 

Fe(ptz)6] (BF4)2 and [Zni Fej.(ptz)6] (BF4,)2. The solid iron(II) complex of the unidentate ligand ptz = 1-propyltetrazole shows a reasonably sharp spin-state transition at about 130 K [112]. Optical excitation into the spin-allowed -> absorption band produces via LIESST the HS T2 state which remains trapped at temperatures below 50 K. Relaxation of the metastable J2 state to the LS state has been studied by following the changes of... 

In addition, it can be shown for the concentration range of the 4,4 -BPDC used, assuming each molecule occupies a spherical volume, the average radius of this volume is about 108 a. This calculation predicts, on the average, the probability of an excited DMT molecule having a 4,4 -BPDC molecule within the required 15 A for energy transfer to occur by the exchange mechanism, which would be spin allowed, is small. 

Expressions have been given by Sohn, Hendrickson, and Gray (48) for the excitation energies of the three spin-allowed one-electron d-d transitions, and for the corresponding spin-forbidden transitions. Since these partial results already include all the singly excited states for d6, Cocv, systems, the use of the full matrices (Table 5) will usually have only a minor effect on the fitting parameters. It should be noted that the one-electron excitation... 

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