State distortion

Semiconductivity in oxide glasses involves polarons. An electron in a localized state distorts its surroundings to some extent, and this combination of the electron plus its distortion is called a polaron. As the electron moves, the distortion moves with it through the lattice. In oxide glasses the polarons are very localized, because of substantial electrostatic interactions between the electrons and the lattice. Conduction is assisted by electron-phonon coupling, ie, the lattice vibrations help transfer the charge carriers from one site to another. The polarons are said to "hop" between sites. 

The discriminatory emission properties between two-coordinate d ° gold(I) complexes and their respective three-coordinate counterparts have been demonstrated in the literature [6, 10-13]. As discussed in the later sections, Che and coworkers have rationalized that the extraordinarily large Stokes shift of the visible emission of [Au2(diphosphine)2] from the [5da 6pa] transition is due to the exciplex formation ofthe excited state with solvent or counterions [6]. Fackler [14—16] reported the photophysical properties of monomeric [AUL3] complexes, which show visible luminescence with large Stokes shifts (typically lOOOOcm ), suggesting significant excited-state distortion. Gray et al. [10] examined the spectroscopic properties of... 

The ground-state distortion (Table I) from the normal octahedral geometry, i.e., the displacement of the central metal atom by up to 0.34 A toward the oxo from the plane formed by the four cyano carbon atoms, is in further agreement with this reasoning, indicative of more reactive complex with an increased metal-oxygen bond length. 

Tetragonal distortion from octahedral symmetry often occurs even when ail six ligands of a complex are the same Two L groups that are irans to each other are found to be either closer to or farther from ihe metal ion than are the other four ligands. A distortion of this type actually is favored by certain conditions described by the Jahn-Teller theorem. The theorem stales that for a nonlinear molecule in an electronically degenerate state, distortion must occur to lower the symmetry, remove the degeneracy, and lower the energy.47 We can determine which octahedral complexes... 

Values of the radiative rate constant fcr can be estimated from the transition probability. A suggested relationship14 57 is given in equation (25), where nt is the index of refraction of the medium, emission frequency, and gi/ga is the ratio of the degeneracies in the lower and upper states. It is assumed that the absorption and emission spectra are mirror-image-like and that excited state distortion is small. The basic theory is based on a field wave mechanical model whereby emission is stimulated by the dipole field of the molecule itself. Theory, however, has not so far been of much predictive or diagnostic value. 

The theoretical background that will be needed to calculate the excited state distortions from electronic emission and absorption spectra is discussed in this section. We will use the time-dependent theory because it provides both a powerful quantitative calculational method and an intuitive physical picture [7-11]. In this section we will concentrate on the physical picture and on the ramifications of the theory. 

Starting with (t = 0), the evolution of with time can be calculated to high accuracy. The most important components that determine the excited state potential are the excited state distortions, i.e. the position of the potential minimum along the configurational coordinates of the vibrational modes involved. Only modes with a displacement in the excited state contribute to vibronic structure in an allowed transition and therefore only these modes need to be considered in our calculation. 

Both Qj and Ax are in dimensionless units and therefore the force constants kj are equal to the excited state vibrational energies in wavenumbers. The latter are obtained from the spectrum (Fig. 10 and Table 1). The At denote the excited state distortions and are the parameters to be determined. The values of both E00 (22613 cm-1) and F(19 cm-1) are obtained from the spectrum in Fig. 10. The initial wavepacket 0 is calculated by using the literature values for the ground state vibrational energies along the 3 modes. 

Fig. 14. Excited state distortions of Pt(hfacac)2 in the molecular beam...

A hypsochromic shift in the emission band would be observed if the amount of excited-state distortion were lower in a rigid matrix than in fluid solution. Such an interpretation has been recently suggested for Cu4l4py4 and related copper clusters, and this constitutes an unusual case of molecules that exhibit rigidochromism but not solvatochromism in its emission from a cluster-centered excited state [115]. It is likely that both excited-state destabilization and distortion arise during the polymerization processes in the tungsten and rhenium organometallic systems. 

Excited State Distortions Determined by Electronic and Raman Spectroscopy... 

Excited State Distortions of W(C0)5pyridine and W(C0)5piperldlne from Time-Dependent Theory, Pre-resonance Raman Spectroscopy, and Electronic Spectroscopy... 

ZINK ET AL. Electronic and Raman Spectroscopy Excited States Distortions 41... 

Calculation of Excited State Distortions and Electronic Spectra from Raman Intensities... 

Calculation of Excited State Distortions of W(C0)rL. The emission spectrum discussed earlier, the theory discussed above and pre-resonance Raman data will now be used in concert to calculate the multi-mode distortions. The relative intensities of the peaks in the pre-resonance Raman spectra were determined by integrating the peaks. All of the peaks in the experimental spectrum having intensities greater than three percent of that of the most intense peak were measured and used in the calculations. 

Correlations between Excited State Distortions and Photochemical Reactivity... 

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