Chromophores, electronic absorption bands

Table 7.9 Electronic Absorption Bands for Representative Chromophores Table 7.10 Ultraviolet Cutoffs of Spectrograde Solvents Table 7.11 Absorption Wavelength of Dienes Table 7.12 Absorption Wavelength of Enones and Dienones Table 7.13 Solvent Correction for Ultraviolet-Visible Spectroscopy Table 7.14 Primary Bands of Substituted Benzene and Heteroaromatics Table 7.15 Wavelength Calculation of the Principal Band of Substituted Benzene Derivatives...

TABLE 7.9 Electronic Absorption Bands for Representative Chromophores... 

A few typical examples having electronic absorption bands for various representive chromophores are provided in the following Table 21 1 ... 

This section discusses some simple chemical reactions which convert a chiral nonracemic compound containing no electron absorption band in an easily accessible spectral range into a derivative with absorption in either the visible or quartz ultraviolet region. This is a useful operation if a reliable correlation exists between absolute configuration (conformation) and chiroptical properties. A collection of useful chromophoric systems is found in reference 167. 

Resonance Raman (RR) spectroscopy is a powerfiil and versatile technique for the study of both vibrational and electronic structures of chromophoric molecular systems. RR spectra are obtained by irradiation of the sample with a monochromatic light source whose energy is close to that of an electric-dipole-allowed electronic absorption band. Most of the Raman bands are attenuated by the absorption, but some bands may be greatly enhanced. This effect arises from a coupling of the electronic and vibrational transitions, and the vibrational modes that do show enhancement are localized on the chromophore, that is, on the group of atoms that give rise to the electronic transition. 

Resonance enhancement of the Raman scattering will occur when the laser excitation is within the electronic absorption band of either a prosthetic group or one of the amino acid residues. The resonance enhancement can be as large as 10, dramatically enhancing the vibrational modes of the parts of the system that are coupled to the electronic excitation. When the excitation is resonant with a chromophoric prosthetic... 

An important practical outcome of the resonance Raman effect is that the accompanying intensity enhancement allows one to obtain the Raman spectrum of molecules with suitable electronic absorption bands at low concentration, e.g. in the 10 M range in solution. At the same time the resonance Raman effect enables the spectroscopist to selectively and specifically pick out the Raman spectrum of an absorbing molecule in a complex environment which only contributes a weak normal Raman spectrum. This property is used to great effect in biological Raman studies since the chromophore is often found at the site of biological activity. 

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. 

When the laser wavelength used to excite the Raman effect lies under an intense electronic absorption band of a chromophore, this condition will lead to a considerable resonance errhancement of the Raman signal by a factor of 10 to 10. With resonance Raman in the absence of fluorescence, detection limits for an individual component can be less than one femtogram [25]. 

The reduced symmetry of the chromophore, which still contains 187t-electrons and is therefore an aromatic system, influences the electronic spectrum which shows a bathochromic shift and a higher molar extinction coefficient of the long-wavelength absorption bands compared to the porphyrin, so that the photophysical properties of the chlorins resulting from this structural alteration render them naturally suitable as pigments for photosynthesis and also make them of interest in medical applications, e.g. photodynamic tumor therapy (PDT).2... 

The UV-visible spectra of the H- and nifro-azobenzene dendrimers in chloroform solution showed strong absorption bands within the visible region due to the transitions of azobenzene chromophores (Table 2). Because of the stronger delocalization of n-electrons in nitro-azobenzene, the maximum absorption band is at a longer wavelength compared with that for H-azoben-zene. There was little spectral shift of the absorption maximum for dendrimers with different numbers of azobenzene units, indicating that dendrimers did not form any special intermolecular aggregates. 

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