Electronic absorption spectral shifts

Of these four properties, spectral shifts are the most sensitive to environmental changes and also the most readily measured. As a result the majority of investigations into electronic absorption spectral changes resulting from surface adsorption have been confined to measurements of spectral shifts. While the shift of the 0-0 bands is the most meaningful measurement to make, these 0-0 bands are not always discernible, especially when the molecules are adsorbed on polar surfaces, so it has become common practice simply to measure the shift of the absorption maximum. In most cases this measurement would correspond to the shift of the 0-0 band, in others, however, adsorption processes can produce unequal displacement of the ground and excited state potential curves, resulting in a different vibronic band shape. 

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. 

Similar vivid colorations are observed when other aromatic donors (such as methylbenzenes, naphthalenes and anthracenes) are exposed to 0s04.218 The quantitative effect of such dramatic colorations is illustrated in Fig. 13 by the systematic spectral shift in the new electronic absorption bands that parallels the decrease in the arene ionization potentials in the order benzene 9.23 eV, naphthalene 8.12 eV, anthracene 7.55 eV. The progressive bathochromic shift in the charge-transfer transitions (hvct) in Fig. 13 is in accord with the Mulliken theory for a related series of [D, A] complexes. 

The x-band in malachite green arises from an NBMO—>n transition, so that 3- and 4-substituents affect the energy of the excited state only and bring about spectral shifts of the first absorption band which vary linearly with the appropriate Hammett substituent constants. Thus, electron-withdrawing groups cause bathochromic shifts of the x-band whereas donor substituents cause hypsochromic shifts (Table 6.6) [64,67]. The 3-band arises from a n—>n transition [68] so that substituent effects are less predictable. As the donor strength of the 4-substituent increases, however, the 3-band moves bathochromically and eventually coalesces with the x-band - at 589 nm in the case of crystal violet (6.164), which possesses two NBMOs that are necessarily degenerate [69]. 

With durene an orange coloration develops and a clear bright red solution results from hexamethylbenzene. The quantitative effects of the dramatic colour changes are manifested in the spectral shifts of the electronic absorption bands that accompany the variations in aromatic conjugation and substituents. The progressive bathochromic shift parallels the decrease in the arene ionization potentials (IP) in the order benzene 9.23 eV, naphthalene 8.12eV, and anthracene 7.55 eV, much in the same manner as that observed with the tropylium acceptor (Takahashi et al.,... 

The extinction features at energies where water is transparent are rapidly squelched in the ultraviolet as the onset of electronic transitions greatly increases bulk absorption. In the infrared, however, vibrational absorption bands in water are carried over into similar bands in extinction (dominated by absorption if a A) by a water droplet. Unlike MgO there are no appreciable spectral shifts in going from the bulk to particulate states. The reason for this lies in the strength of bulk absorption and will be discussed more thoroughly in Chapter 12. 

The carbonyl stretching frequencies of a number of heteroannular ferro-cenecarboxylic acids have been measured (41). The spectral shifts vary in the direction expected from the electronic effects of the substituent groups, and parallel the effects of these substituents on the acid constants. Little and Clark have also demonstrated resonance interaction of the ferrocenyl group in ferro-cenylazobenzenes by measuring shifts in the absorption maxima in the visible and ultraviolet spectra (49). 

In the unsymmetrical cyanines such as (122) the direction of the spectral shift compared with the two possible symmetrical dyes depends upon the electron releasing ability of the two nitrogen atoms. The dye (122) can be regarded as a hybrid of the two dyes (123) and (124), which absorb at 490 and 610 nm respectively. The unsymmetrical cyanine (122) absorbs at 553 nm in the same solvent (nitromethane) which is close to the average of the two former values and indicates the electronic symmetry of the dye. In cases where the basicity of the terminal nitrogens differs markedly then electronic symmetry,is lost and the dyes exhibit an absorption maximum deviation to shorter wavelength than the averaged value of the two parent symmetrical dyes. This difference is known as the Brooker deviation. 

Very few experiments have been performed on vibrational dynamics in supercritical fluids (47). A few spectral line experiments, both Raman and infrared, have been conducted (48-58). While some studies show nothing unique occurring near the critical point (48,51,53), other work finds anomalous behavior, such as significant line broadening in the vicinity of the critical point (52,54-60). Troe and coworkers examined the excited electronic state vibrational relaxation of azulene in supercritical ethane and propane (61-64). Relaxation rates of azulene in propane along a near-critical isotherm show the three-region dependence on density, as does the shift in the electronic absorption frequency. Their relaxation experiments in supercritical carbon dioxide, xenon, and ethane were done farther from the critical point, and the three-region behavior was not observed. The measured density dependence of vibrational relaxation in these fluids was... 

---