Electrons ground and excited states

Figure 48. Ab initio transition dipole moments between the electronically ground and excited states. Taken from Ref. [126].

Ultraviolet/visible absorption, fluorescence, infrared and Raman spectroscopies are useful for studying structures (configuration, conformation, symmetry etc.) of electronically ground and excited states of linear polyenes, which have attracted much attention of... 

Valuable findings on the electronic ground and excited states of clusters have been derived from laser-induced multi-photon ionization (MPl) investigations, such as laser-induced fluorescence (LIF) and REMPI. This latter technique is particularly promising since it enables mass selection of cluster species and their spectral and thermochemical characterization. The complex is excited from its electronic ground state from a photon and then ionized by a second photon of equal or different frequency, near threshold to avoid cluster fragmentation. ... 

Fig. 3 Schematic potential energy diagram illustrating alternative decarboxylation pathways of carbonyloxy radicals Ri-COj. f(E) denotes the initial internal energy distribution of the carbonyloxy radical, k(E) is the specific rate constant for decarboxylation of the intermediate radical, AE denotes the energy separation of electronic ground and excited state of the carbonyloxy radical, and ArE is its dissociation energy into CO and product radical R(. For further details see Ref. [3].

The total absorption spectrum appears as the simple addition of the individual absorbance of all chromophores in the CC. The trace is reduced to the vibrational wave packet overlap in the electronic ground and excited state (averaged with respect to the chromophore electronic ground-state vibrational equilibrium, described by the density operator Rmg Hma = Tm + Uma). 

Metal-Metal Bonds at Electronic Ground and Excited States... 

The practical limitations in the Z-value approach can be overcome by using pyridinium A -phenolate betaine dyes such as (44) as the standard probe molecule. They exhibit a negatively solvatochromic n n absorption band with intramolecular charge-transfer character cf. discussion of this dye in Section 6.2.1, its UV/Vis spectrum in Fig. 6-2, and its dipole moment in the electronic ground and excited states mentioned in Table 6-1, dye no. 12. 

Schmitt, M Muller, H., Henrichs, U., Gerhards, M., Perl, W., Deusen, C., and Kleinermanns, K Structure and vibrations of phenol-CH(CDjOD) in the electronic ground and excited state, revealed by spectral hole burning and dispersed fluorescence spectroscopy, J. Chem. Phys. 103, 584-594 (1995). 

Here the integral represents the spectral overlap, Hsa the interaction Hamiltonian and y> and y > are the electronic ground- and excited-state functions, respectively, with j = 1,2. Here 1 refers to S and 2 to A. The distance dependence depends on the interaction mechanism. 

The amount of research performed and literature published on electron-transfer reactions of metal-polypyridine complexes is enormous. Several excellent reviews [42, 74, 93-97] and books [38, 62, 98, 99] deal with polypyridine eomplexes, their redox chemistry, photochemistry, and applications. Hereinafter, the most prominent aspects of electron transfer reactivity of mononuclear metal-polypyridine eomplexes will be surveyed without attempting to cover exhaustively the vast original literature. Instead, the main purpose of this chapter is to single out the structural, thermodynamic, and kinetic factors which enable and eontrol the special and diverse electron-transfer behavior of metal-polypyridine complexes in their electronic ground and excited states. Although supramolecular eleetron-transfer chemistry of metal-polypyridines is not discussed here in detail, beeause it is covered in Volume 3 of this monograph, links connecting the redox behavior of mononuclear polypyridine eomplexes and their supramolecular counterparts will be briefly outlined. 

Transition metal polypyridine complexes are highly redox-active, both in their electronic ground- and excited states. Their electron transfer reactivity and properties can be fine-tuned by variations in the molecular structure and composition. They are excellent candidates for applications in redox-catalysis and photocatalysis, conversion of light energy into chemical or electrical energy, as sensors, active components of functional supramolecular assemblies, and molecular electronic and photonic devices. 

Figure 1. Schematic energy level diagram of an organic dye molecule illustrating radiative (full line) and radiationless transitions (dotted line). S0 and. S, are the electronic ground and excited states, T, is the triplet state and /cjsc is the intersystem crossing rate, t, and t3 are the life times of the states S, and Tj, respectively.

In the case of totally symmetric modes, the product of the integrals t u) and (u y) in Eq. 21.2 is finite due to nonorthogonality of the vibrational wave functions at the electronic ground and excited states. On the other hand, these wave functions are nearly orthogonal for nontotally symmetric vibrations. Thus, only totally symmetric modes can derive Raman intensities via the A term. Nontotally symmetric and totally symmetric modes can gain Raman intensity via the B term as long as the ( u)(u <2a y)/(i —E ) in Eq. 21.3 is nonzero. In general, the B term contribution is small relative to the A term due in part to the additional denominator E —E. ... 

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