Solvatochromic effects/shift

Solvent Influence. Solvent nature has been found to influence absorption spectra, but fluorescence is substantiaHy less sensitive (9,58). Sensitivity to solvent media is one of the main characteristics of unsymmetrical dyes, especiaHy the merocyanines (59). Some dyes manifest positive solvatochromic effects (60) the band maximum is bathochromicaHy shifted as solvent polarity increases. Other dyes, eg, highly unsymmetrical ones, exhibit negative solvatochromicity, and the absorption band is blue-shifted on passing from nonpolar to highly polar solvent (59). In addition, solvents can lead to changes in intensity and shape of spectral bands (58). 

To estimate how many dye molecules fit into the dendritic micelles, UV-titra-tion experiments have been employed. In comparison with the spectra of a pure pinacyanol chloride solution in water, the peaks of the absorption maxima of the dye in the presence of the dendrimer are shifted bathochromically due to solvatochromic effects, which indicates the incorporation of the dye within the branches of the dendrimer. At dye-to-dendrimer molar ratios higher than 4 1, in addition to the bathochromic shifts, hypsochromically shifted peaks start to appear, indicating that the dendrimer is not incorporating further dyes. We interpret this as an incorporation of up to four dyes within the branches of the dendrimer. This observation correlates with the calculated available space within the dendrimer, obtained from the molecular simulations. Further studies of the interactions of the dyes within the dendritic micelle are in progress. 

A thermochromic shift is the displacement of an absorption or emission band with the temperature of the solvent. These displacements result from the change in solvent polarity with temperature, the general rule being that the polarity decreases as the temperature increases. These shifts are small compared with solvatochromic effects and are unlikely to lead to state inversion (Figure 3.52). 

As previously noted (see Section II.B.l), the A emission exhibits an important solvatochromic effect due to the appearance of a large dipole moment in the TICT state. The polar interactions between the solute molecule and the polar environment lead to the reorientation of the solvent molecules and to a relaxation of the electronic energy of the TICT state whose manifestation is a spectral shift during the lifetime of the excited state. The competition between the energy relaxation, whose dynamics is strongly viscosity dependent, and the deactivation of the TICT state has been made evident for DMABN78,89 ... 

Only the nitro-substituted oligothiophenes display large bathochromic shifts, large Stokes shifts, high fluorescent quantum yields, and long lifetimes for excited states. As for the other substituents, the trend is mostly noticeable for the short oligomers like terthiophenes and seems to disappear for sexithiophenes. As can be inferred from their solvatochromic effect, an intramolecular charge transfer takes place in the excited states of these molecules. 

A further property associated with the radial displacement of charge associated with CT electronic transitions is a change in the dipolar moment of the molecule. If the electronic transition causes, for example, an increase in the dipolar moment, the energy of the CT excited state will decrease (other factors aside) with the polarity of the solvent. Therefore, the CT absorption bands will experience solvatochromic shifts of tens of nanometers. Related solvatochromic effects will be detected in the emission spectrum of CT excited states. While the solvatochromism of absorption bands is a tool for the assignment of CT transitions in the absorption spectrum of complexes, the rationalization of such effects in terms of the solvent properties, for example, the dielectric constant, is not always possible. 

These data have been tabulated <66AHC(7)39>. In the parent ion (4) only two absorptions are evident, at 202 and 285 nm. The long wavelength band shows a slight solvatochromic effect. The 3-phenyl and 4-phenyl compounds exhibit two absorptions each, at 287 and 356, and 242 and 345 nm, respectively. Electron releasing substituents in the 3-position provide a greater bathochromic shift than those at the 4-position. 

Molecular EDA complexes as well as charge-transfer ion pairs show (negative) solvatochromism [128], i.e. the charge-transfer absorption maxima (2cx) undergo hypsochromic shifts with increasing solvent polarity. The solvatochromic effect is readily explained on the basis of the Marcus correlation for charge-transfer energies in solution [129], (Eq. 9) ... 

Prabhumirashi and Kunte [181] have proposed a new procedure employing Bakhshiev s equation for solvatochromic frequency shifts for excited-state dipole moments and specific solute -solvent interaction energies based on absorption spectra only, without using emission spectra. Suppan [182] has expanded upon the solvatochromic shift method and discussed the effect of the medium on the energies of electronic states and Ghoneim and Suppan [183] discussed solvatochromic shifts of non-dipolar molecules in polar solvents. 

The proposed calculation procedure will be first tested by analysing in detail the effects of the solvent polarity on the structure and electronic spectra of the simple merocyanine Ml. Afterwards, the selected calculation procedure will be applied to the more complex dyes M2 and M3, characterized by equal length of the conjugated path connecting the donor and acceptor group, but exhibiting opposite solvatochromic effects. To be precise, the acyclic merocyanine M2 shows, like the simpler chromophore Ml, positive solvatochromism [25] (i.e. bathochromic shift of the first absorption band on increasing solvent polarity),... 

The solvatochrome effect may also appear in the infrared spectrum. The acceptor strength scale of Kagiya [Ka 68] is based on the solvent dependence of the C==0 vibration band for acetophenone. As for his donor strength scale, the reference solvent is benzene. The acceptor strength is denoted by the band shift,... 

The solute descriptors such as Jt, a, and P can be extracted from the hterature [2-5] and are in most cases empirically determined by spectroscopic measurements. Their calculation is based on the shift of absorption bands of the given solutes due to varying solvatochromic effects when the dipolar or hydrogen-bond donor /acceptor properties of the solvent mixture used for spectroscopy are altered. Hence, they are not true thermodynamic data. Abraham and co-workers also determined descriptors from GC retention data and octanol/water partition coefficients [6-9]. With the help of such descriptors, the so-called solvation equation (Eq. 5) can be set out [10,11] ... 

---