Spectroscopic site selective excitation

Hole burning The photohleaching of a feature, normally a narrow range, within an inhomogeneous broader absorption or emission band. The holes are produced by the disappearance of resonantly excited molecules as a result of photophysical or photochemical processes. The resulting spectroscopic technique is site-selection spectroscopy. 

In addition to the absorption, PL spectra were also analysed to disentangle the nature of the lowest excited state. The general luminescence properties of thin T6 films were quite poor and a typical example is shown in Fig. 12 [149, 150]. By going from solution to thin films the PL quantum yields decreases by three orders of magnitude [84] and apparently broad emission lines dominate the spectra. Even at very low temperatures the resolution of the optical spectra is rather poor (several 100 cm ) and spectroscopic details are smeared out. In most cases a considerable red-shift between the absorption and PL onsets and multiple different PL-components could be found within the spectra (Fig.l2) [149, 151]. The main radiative decay channels were attributed to deep trap levels [149] or aggregates [8], which are strongly depending on the preparation conditions and film thicknesses (Fig. 12) [149]. By site selective PL-spectroscopy the positions of at least three trap levels could be located which are up to 2 000 cm" lower than the absorption onset... 

It is highly instructive to compare spectroscopic properties of [Os(bpy-h8)2(bpy-d8)], [Os(bpy-h8)(bpy-d8)2]. and [Os(bpy-d8))3] + to those of [Os(bpy-h8)3]. All of these compounds can be doped into [Zn(bpy-118)3] (0104)2, iid one always obtains highly resolved emission spectra[98, 104]. Those of the two partially deuterated compounds represent superpositions of spectra of different sites if nonselectively excited. In [104] the occurrence of three sites A, B, and C for both complexes is reported, and it is shown that it is suitable to investigate sites B in more detail. These specific sites are easily singled out with the methods of site-selective spectroscopy. The spectra obtained are compared in Fig. 31 to those of the two per-complexes. 

Of the several less common spectroscopic methods to combine with electrochemical intermediate generation such as luminescence, Raman, NMR, or X-ray absorption spectroscopy, the EPR method is presented here because of its relative simplicity and pronounced selectivity. Only paramagnetic compounds with a certain, not too rapid relaxation rate from the spin-excited state give detectable signals for EPR spectroscopy, which helps to disregard many simultaneously present species. On the other hand, the rather slow time frame (At 10 s) and the sensitivity of the EPR method to electronic influences from the participating atoms via g-factor shift and hyperfine interaction can render EPR a very valuable method to determine the site of electron transfer (ligand or metal) as well as the spin and thus valence distribution. 

Chromoproteins are characterized by an electronic absorption band in the near-UV, visible or near-IR spectral range. These bands may arise from Jt Jt" transitions of prosthetic groups or from charge-transfer transitions of specifically bound transition metal ions. Thus, chromoproteins which may serve as electron transferring proteins, enzymes or photoreceptors, are particularly attractive systems to be studied by RR spectroscopy since an appropriate choice of the excitation wavelength readily leads to a selective enhancement of the Raman bands of the chromo-phoric site. Moreover, these chromophores generally constitute the active sites of these biomolecules so that RR spectroscopic studies are of utmost importance for elucidating structure-function relationships. 

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