Hypsochromism excitation

The meso carbon atom should present a carbenium structure with a low TT electron density in the ground state, in the excited state this carbon possesses the carbeniate structure (C ) with a high tt electron density (119). An electron-donating group in such a position should stabilize the ground state and rise the excited state to the highest level hypsochromic shift results as a whole. 

The hypsochromic shift indicates, however, that the excited state (Sj) of Ar —NJ is destabilized relative to S0 in the complex as a consequence of the charge shift (Skrabal, 1994). 

To qualify the environment into which the colorant molecule is embedded, the actual fluorescence spectrum is compared with the one under standard conditions. If the fluorescence emission spectrum is shifted to longer wavelengths (bathochromic shift), it can be concluded that the molecular enviromnent is of a more polar nature or is polarized by the excited fluorophore. Conversely, a fluorescence shift to shorter wavelengths (hypsochromic shift) indicates a transfer of the fluorophore from a polar... 

In contrast to normal spiropyrans, in spironaphthooxazine series, as the polarity of the solvent decreases, a hypsochromic shift of the of the colored form is observed, except for spiropiperidinonaphthooxazine.79 For example, the of 33 shifts to shorter wavelength of ca. 20-60 nm in less polar solvents, such as toluene and cyclohexane, compared to ethanol. This result may suggest that the ground state of the photomerocyanine form in spironaphthooxazine is less polar than the excited state and the neutral quinoid form largely contributes to the photomerocyanine form in the ground state. 

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]. 

In some transitions, the polarity of the chromophore is weaker after absorption of radiation. One case of this is the n — jt absorption due to the carbonyl present in ketones in solution. Before absorption, the C+-0 polarisation stabilises in the presence of a polar solvent whose molecules will be clustered around the solute because of electrostatic effects. Thus, the n —> -rr electronic transition will require more energy and its maximum will be displaced towards a shorter wavelength, contrary to what would be observed in a nonpolar solvent. This is the hypsochromic effect. Because the excited state is readily formed, the solvent shell around the... 

Absorption maxima of the open-ring form in benzene, THF, and acetonitrile were observed in the wavelengths ranging from 335 to 340 nm. Although the maximum showed a small hypsochromic shift in hexane, the solvent shift in the absorption spectrum was rather small. On the other hand, the fluorescence spectra showed remarkable Stokes shifts depending on the solvent polarity. The maximum at 488 nm in hexane shifted to 560 nm in THF. At the same time, the intensity decreased. The fluorescence intensity in acetonitrile was <1% of the intensity in hexane. The results indicate that the excited state of the open-ring form has a polar structure with a large dipole moment. 

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. 

The effects of increased solvent polarity on the light absorption energy from the ground to the (Franck-Condon) excited state can be hypsochromic, if the ground state is more stabilized than the excited state or bathochromic if the opposite relative stabilization takes place. 

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